ELECTROCATALYSTS, PREPARATION THEREOF, AND USING THE SAME FOR AMMONIA SYNTHESIS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a Bypass Continuation of PCT Patent Application No. PCT/IL2024/050327, having International filing date of Mar. 28, 2024, which claims the benefit of priority of U.S. Provisional Patent Application No. 63/455,263 filed Mar. 29, 2023, the contents of which are incorporated herein by reference in their entirety.

FIELD OF INVENTION

[0002]The present invention, in some embodiments thereof, relates to metal-based catalysts and uses thereof for ammonia synthesis.

BACKGROUND OF THE INVENTION

[0003]Ammonia is extensively produced by using the Haber-Bosch process developed in the nineteenth century which requires very high pressure and temperature. Industries produce annually more than 200 million tons of ammonia from this method and the majority of it is used for the production of fertilizers. The hydrogen required for this process is generated from steam reformation, which consumes three to five percent of total natural gas production and releases a huge quantity of greenhouse carbon dioxide gas to the atmosphere.

[0004]Therefore, alternative greener, energy-efficient and mild conditional ammonia synthesis is one of the major global challenges.

SUMMARY OF THE INVENTION

[0005]According to an aspect of some embodiments of the present invention there is provided an electrocatalyst comprising a transition metal oxide; wherein the transition metal oxide is characterized by an electrocatalytic activity, and wherein the electrocatalytic activity comprises any one of (i) reduction of nitrogen to ammonia; (ii) reduction of nitrate, or nitrite to ammonia, or both (i) and (ii); and wherein transition metal oxide is devoid of iron oxide and of TiO2, and is further devoid of an elemental state metal, or a salt thereof.

[0006]In one embodiment, the transition metal oxide is in contact with (i) a conductive material; (ii) a co-catalyst, or both (i) and (ii); and wherein a concentration of the transition metal oxide within the electrocatalyst is between 1 and 90% w/w.

[0007]In one embodiment, the conductive material comprises a carbon material, a metal, a conductive metal oxide, or any combination thereof; and wherein the co-catalyst comprises a metal-phthalocyanine dyc.

[0008]In one embodiment, the carbon material comprises carbon black, activated carbon, graphite, carbon nanotube, graphene, and any combination thereof.

[0009]In one embodiment, the transition metal oxide comprises a transition metal selected from (i) Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, a lanthanide, and Au including any combination thereof; and (ii) a combination of iron oxide and (i).

[0010]In one embodiment, the transition metal in the transition metal oxide is selected from Ru, Fc, Co, Ni, Cu, Mo, Mn, V, Ag, Pt, Pd, and Pt or any combination thereof.

[0011]In one embodiment, the transition metal oxide is selected from RuO₂, cerium oxide-iron oxide and PdO.

[0012]In one embodiment, the outer surface of the transition metal oxide comprises a plurality of reactive oxygen species; optionally wherein the reactive oxygen species are selected from singlet oxygen, a peroxide, a superoxide, a hydroxyl radical or any combination thereof.

[0013]In another aspect, there is provided an electrode comprising a conductive substrate and the electrocatalyst of the invention, wherein the electrocatalyst is deposited on a surface of the conductive substrate; and wherein the electrode is configured to electrochemically induce ammonia synthesis from composition comprising at least one of nitrogen, nitrogen oxide, nitrite, and nitrate, including any salt or any combination thereof.

[0014]In one embodiment, the electrocatalyst is in a form of a layer.

[0015]In one embodiment, the conductive substrate comprises a carbon substrate, a metal substrate, a conductive metal oxide substrate, or any combination thereof.

[0016]In one embodiment, the electrode is a cathode.

[0017]In another aspect, there is provided an electrochemical cell comprising a working electrode in operable communication with a chamber configured to contain an aqueous electrolyte comprising a nitrate salt or a nitrite salt; the electrochemical cell further comprises an additional electrode; wherein: the additional electrode and the working electrode are in connectable to a power source; the electrochemical cell is configured to generate ammonia under operable conditions; the working electrode is (i) the electrode the invention; or (ii) comprises an electrocatalyst deposited on a conductive substrate, wherein the electrocatalyst is selected from iron oxide-TiO₂ composite, a noble metal, or a noble metal composite.

[0018]In one embodiment, the noble metal composite comprises a first noble metal and a second noble metal, wherein a molar ratio of the first noble metal to the second noble metal is in the range of 1:9 to 9:1.

[0019]In one embodiment, the noble metal is selected from Ru, Rh, Pt, Pd, Ag, Re, Ir, and Au.

[0020]In one embodiment, the molar ratio of the first noble metal to the second noble metal is in the range of about 2:1 to about 1:2.

[0021]In one embodiment, the noble metal composite is a multi-layered material, and wherein each of the first noble metal and the second noble metal are in a form of a distinct layer.

[0022]In one embodiment, the iron oxide-TiO₂ composite comprises (i) Fe₂O₃, Fe₃O₄ and/or Fe₂O₃FeO and (ii) TiO₂.

[0023]In one embodiment, a weight ratio between Fe and Ti within the iron oxide-TiO2 composite is between about 4:1 and 1.5:1, and wherein the iron oxide-TiO₂ composite is a layered material comprising a first layer in contact with a second layer, wherein the first layer comprises or consists essentially of iron oxide and the second layer comprises or consists essentially of TiO₂.

[0024]In one embodiment, the operable conditions comprise a temperature in a range from 5° C. to 150° C.; and application of a voltage.

[0025]In one embodiment, the voltage is between 0.3V and −2V.

[0026]In one embodiment, the aqueous electrolyte is an alkaline electrolyte solution; and wherein a concentration of the nitrate ion within the aqueous electrolyte is between 0.01 and 5M; optionally wherein the aqueous electrolyte is a supersaturated solution.

[0027]In one embodiment, the aqueous electrolyte is pressurized with a gas comprising nitrogen.

[0028]In another aspect, there is provided an electrochemical cell comprising a working electrode and an additional electrode; wherein the working electrode and the additional electrode are in operable communication with a chamber configured for containing a supersaturated alkaline electrolyte solution pressurized with a gas comprising nitrogen; the working electrode is (i) the electrode of the invention; or (ii) comprises an electrocatalyst deposited on a conductive substrate, wherein the electrocatalyst is selected from iron oxide-TiO₂ composite, and a noble metal composite; the working electrode and the additional electrode are in electrical communication with a power source; and wherein the electrochemical cell is configured to generate ammonia under operable conditions comprising applying a cathodic electric potential.

[0029]In one embodiment, the cathodic electric potential comprises: (i) a positive electric potential above 0.5V relative to SHE, sufficient for oxidizing nitrogen to a nitrogen oxide; and (ii) a negative electric potential of at least −0.05V relative to SHE, sufficient for generating ammonia from the nitrogen oxide.

[0030]In one embodiment, the positive electric potential is between 1 and 2V; and wherein the negative electric potential is between −0.1 and −0.5V.

[0031]In one embodiment, the operable conditions further comprise a temperature in a range from 5° C. to 150° C.

[0032]In one embodiment, the electrochemical cell is characterized by at least one of: configured to synthesize ammonia at a rate of at least 1×10⁻¹¹ mol cm⁻² s⁻¹; and has a faradaic efficiency of above 30% at an electric potential of between about −0.1 and about −0.3 V.

[0033]In one embodiment, the supersaturated alkaline electrolyte solution comprises nitrate salt, nitrite salt, or both.

[0034]In another aspect, there is provided a method of synthesizing ammonia, comprising: providing the aqueous electrolyte comprising a nitrate salt or a nitrite salt to the electrochemical cell of the invention, or providing the supersaturated alkaline electrolyte solution optionally comprising a nitrate salt or a nitrite salt and a gas to the electrochemical cell of the invention; and applying a negative electric potential to the electrochemical cell of the invention, under conditions suitable for generating ammonia; wherein the gas comprises air, nitrogen, a mixture of oxygen and nitrogen or a mixture of hydrogen and nitrogen; and wherein the negative electric potential is of at least −0.05V.

[0035]In one embodiment, the conditions comprise a temperature between 5° C. and 150° C.; wherein the negative electric potential is between −0.1V and −0.5V; and wherein the aqueous electrolyte is a supersaturated solution having a pH between 10 and 14.

[0036]In one embodiment, the method further comprises a preliminary step of activating the working electrode by (i) contacting the working electrode with an oxidizing solution; or (ii) applying to the working electrode in contact with a gas comprising oxygen an electric potential of at least 0.5V; and wherein the preliminary step is performed prior to performing step (i) or step (ii).

[0037]In one embodiment, the method is characterized by (i) a rate of ammonia production is in the range of 1×10⁻¹¹ mol s⁻¹ cm⁻² to 1×10⁻⁶ mol s⁻¹ cm⁻²; (ii) faradaic efficiency of at least 5% at an electric potential of about −0.3 V, or both (i) and (ii).

[0038]In one embodiment, the step (i), the step (ii), or both comprise introducing the gas into the aqueous electrolyte.

[0039]Further embodiments and the full scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

[0040]Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0042]Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced.

[0043]In the drawings:

[0044]FIGS. 1A-1B are graphs showing nitrate formation rate and FE (Faradaic Efficiency) % calculated for each CoPc/RuO2 composition at 0.1 V vs. RHE in air saturated 0.1 M Na2SO4 solution (1A), and comparison of remaining peroxide and formed nitrate at different RuO2 content of CoPc/RuO2 composite (1B).

[0045]FIGS. 2A-2E Are X-ray Diffraction patterns of (2A)100% Ce, (2B)100% Fe (2C)75% Ce:25% Fe, (2D) 50% Ce:50% Fe and (2E)25% Ce:75% Fe.

[0046]FIGS. 3A-3D Are scanning electron microscopy microgram of as synthesized 50Fe:50Ce composite catalyst at 20Kx magnification (3A), and EDS mapping of oxygen (3B), iron (3C) and cerium (3D) on as synthesized 50Fe:50Ce composite catalyst under same magnification

[0047]FIG. 4 is a scanning electron microscopy micrograph of as-synthesized NiCo₂S₄.

[0048]FIGS. 5A-5D Are bar graphs showing production rate and Faradaic efficiency of (5A) NH₃; (5B) NO₂⁻; and (5C) NH₂OH from NO₃⁻ reduction at selected applied potentials for NiCo₂S₄ catalyst mixed with 20 wt. % Vulcan carbon in Ar-saturated 0.1M KOH containing 0.5M NO₃⁻ electrolyte; (5D) Selectivity of the NiCo₂S₄ as a function of Faradaic efficiency contribution of the products NH₃, NO₂⁻, NH₂OH and other by-products, possibly H_2y.

[0049]FIGS. 6A-6E present XRD, SEM and TEM micrographs of diffraction od Rh/C catalysts. FIG. 6A is an X-ray diffraction pattern of as-synthesized carbon and 5 wt. % Rh/C catalysts. FIGS. 6B-6C are scanning electron microscopy (SEM) micrographs at two different magnifications. FIGS. 6D-6E are scanning transmission electron microscopy (TEM) images of a 5 wt. % Rh/C sample at two different magnifications. Inset in FIG. 6E shows the particle size distribution of Rh.

[0050]FIG. 7 presents a non-limiting schematic illustration of electrochemical cell used for nitrogen reduction reaction.

[0051]FIG. 8 presents a non-limiting schematic illustration of electrochemical cell used for nitrogen reduction reaction.

[0052]FIGS. 9A-9B present non-limiting schematic illustrations of an electrochemical cell used for electrochemical reduction of the nitrogen to ammonia.

DETAILED DESCRIPTION OF THE INVENTION

[0053]The present invention, in some embodiments thereof, relates to a catalyst (e.g. electrocatalyst) comprising a transition metal oxide as disclosed herein, and uses thereof for electrochemical reduction of nitrogen to ammonia, or for electrochemical reduction of nitrogen oxide (e.g. a nitrate salt) to ammonia. The instant invention in some embodiments thereof is based on a surprising finding that transition metal oxides (such as Ru oxide, or PdO) are capable of electrochemically reducing nitrogen and nitrogen oxide to ammonia.

[0054]In some embodiments, the electrocatalyst of the invention has high electrocatalytic activity towards nitrogen reduction. In some embodiments, the electrocatalytic nitrogen reduction is carried out at ambient pressure and temperature in aqueous media by using either air or pure nitrogen as nitrogen sources.

[0055]Furthermore, the present invention in some embodiments thereof is based on a surprising finding that electrochemical reduction of nitrogen to ammonia catalyzed by an electrocatalyst comprising (i) the transition metal oxide catalyst of the invention, (ii) an iron oxide-TiO₂ based catalyst, or (iii) a noble metal alloy (e.g. RuPt) catalyst is accompanied by the formation of nitrate as an intermediate. In some embodiments, the electrocatalyst of the invention is capable of reducing nitrogen oxide (e.g. nitrate or nitrite) to ammonia.

[0056]Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings and/or the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways.

The Electrocatalyst

1. Metal Oxide Catalyst

[0057]According to some embodiments, the present invention provides an electrocatalyst comprising a transition metal oxide (also termed herein as “metal oxide catalyst”); wherein the transition metal oxide is characterized by an electrocatalytic activity comprising any one of (i) reduction of nitrogen to ammonia (termed herein as “NRR activity”); (ii) reduction of nitrogen-based specie to ammonia (termed herein as “NO₃RR activity”), (iii) oxidation of nitrogen to nitrogen-based specie (termed herein as “NOR activity”), wherein the nitrogen-based specic is selected from nitrogen oxide, nitrate, and nitrite including any combination and any salt thercof, or any combination of (i)-(iii). The term “electrocatalytic activity” is well known in the art and refers to the ability of a catalyst material to catalyze (i.e. to initiate, induce, enhance and/or promote) a redox reaction upon application of voltage (i.e. to a working electrode containing the catalyst material). The reaction (also termed as “electrochemical reaction”) occurs on the surface of a working electrode containing the catalyst material and being in contact with an electrolyte, wherein the electrolyte (i.e. a liquid electrolyte) contains one or more species (i.e. a gas, an ion, and/or a molecule dissolved or dispersed in the electrolyte) which is/are reduced or oxidized during the reaction.

[0058]In some embodiments, the electrocatalyst of the invention comprises an electrocatalytically active material consisting essentially of the transition metal oxide. In some embodiments, at least 95%, at least 90%, at least 97%, between 90 and 99%, between 90 and 99.9%, between 95 and 99%, between 95 and 99.9% by weight of the electrocatalytically active material consists of one or more transition metal oxide specie(s). In some embodiments, the electrocatalytically active material consisting essentially of a single transition metal oxide specie. In some embodiments, the electrocatalytically active material is devoid of iron oxide; TiO₂; an elemental state metal or an alloy thereof; an oxide of any one of Sn, Pb, Bi, Hg, and Cd, or any combination thereof.

[0059]In some embodiments, the transition metal oxide is or consists essentially of ruthenium oxide (e.g.RuO₂), CeO₂ or PdO. In some embodiments, the metal oxide catalyst is ruthenium oxide.

[0060]The term “devoid of” as used herein encompasses that the material may comprise only trace amounts of the specific element or of the specific material, so that the specific element or of the specific material doesn't contribute to any electrocatalytic activity.

[0061]In some embodiments, the term “electrocatalytically active material” encompasses a portion of the electrocatalyst being capable of inducing electrocatalysis. For example, the electrocatalyst of the invention may be in a form of a first layer in contact with a second layer facing the ambient, and the second layer is the transition metal oxide layer, as disclosed herein. Thus, the electrocatalysis occurs on the surface of the second layer and it is referred to herein as the “electrocatalytically active material”.

[0062]In some embodiments, the transition metal oxide is devoid of iron oxide as the sole metal oxide. In some embodiments, the transition metal oxide is devoid of TiO₂. In some embodiments, the transition metal oxide is devoid of a composite comprising iron oxide and TiO₂. In some embodiments, the transition metal oxide is further devoid of an elemental state metal, or a salt thereof. In some embodiments, the transition metal oxide is further devoid of an oxide of any one of Sn, Pb, Bi, Hg, and Cd.

[0063]In some embodiments, the electrocatalyst is in a form of a solid material (e.g., a powder, a film, a particle, a bulk material, a fiber, a porous matrix, a plurality of aggregated particles, etc.) having at least one average dimension of at least 1 um, at least 100 um, at least 1 mm, at least 1 cm, at least 10 cm, including any range between. In some embodiments, the electrocatalyst is a particle (e.g. substantially spherical nano-size or micro-sized particle). In some embodiments, the particle is characterized by an average particle size between 10 and 500 nm, between 10 and 100 nm, between 50 and 500 nm, between 200 and 500 nm, between 1 and 500 um, between 10 and 500 um, between 1 and 50 um, between 1 and 100 um, including any range between.

[0064]In some embodiments, the electrocatalyst is in a form of a film, a sheet, a bulk material, a fiber, a porous matrix, characterized by at least one average dimension (e.g. thickness, length, width) between 10 and 500 nm, between 10 and 100 nm, between 50 and 500 nm, between 200 and 500 nm, between 1 and 500 um, between 10 and 500 um, between 1 and 50 um, between 1 and 100 um, including any range between.

[0065]In some embodiments, the transition metal oxide comprises or consists essentially of a transition metal selected from a lanthanide-series element (e.g. Ce), an actinide-series element, Fe, Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, and Au, including any combination thereof. In some embodiments, the transition metal is selected from Ru, Pd, Co, Ni, Ce, Cu, Zn, Ru, Rh, Pd, Re, Ir, including any combination thereof. In some embodiments, the transition metal oxide comprises NiOx/Ni(OH)₂, Co3O4, and mixed spinel Ni-Co oxides (e.g. NiCoO₄). In some embodiments, the transition metal oxide is selected from ruthenium oxide (e.g.RuO₂) and PdO, or both.

[0066]In some embodiments, the metal oxide catalyst is a composite. In some embodiments, the composite comprises a first transition metal oxide and a second transition metal oxide. In some embodiments, the first transition metal oxide and the second transition metal oxide are homogeneously distributed within the composite. In some embodiments, the first transition metal oxide and the second transition metal oxide are mixed within the composite.

[0067]In some embodiments, the composite is in a form of a layered material. In some embodiments, the composite comprises an outer layer on top of a core. In some embodiments, the composite comprises a shell enclosing the core.

[0068]In some embodiments, the outer layer consists essentially of the transition metal oxide. In some embodiments, the core comprises or consists essentially of the transition metal and/or a salt thereof. In some embodiments, the core comprises or consists essentially of the transition metal oxide. In some embodiments, the outer layer comprises transition metal oxide nano-particles.

[0069]In some embodiments, the core is in a form of a particle (e.g. 1 nano-size or micro-sized particle). In some embodiments, the core is in a form of a substantially spherical particle. In some embodiments, the core is in a form of a film, a bulk material, a fiber, a porous matrix, a plurality of aggregated particles, etc.

[0070]In some embodiments, the term “shell”, refers to the coating domain surrounding the core. By “coated by a shell” it is meant to refer to a composition of two or more entities, namely an entity that defines an enclosure (the enclosing entity, i.e. the shell) and the entity (or entities) that is being at least partially enclosed therein. In some embodiments, the coating may be conformal with the exact contour of the core. In some embodiments, the core comprises or is made of a plurality of particles.

[0071]Particle(s) coated by a shell may be characterized by a discrete inner and outer surface wherein the inner surface constitutes the boundary of the enclosed area or space. The enclosed area or space may be secluded from the exterior area of space which is bounded only by the outer surface.

[0072]In the context of the present invention, the closure of the enclosing entity may depend on the size, shape, and chemical composition of the entity that is being enclosed therein, such that the enclosing entity may be “closed” for one entity and at the same time be “open” for another entity. For example, structures presented herein are closed with respect to certain chemical entities which cannot pass through their enclosing shell, while the same “closed” structures are not closed with respect to other entities.

[0073]In some embodiments, the thickness of the outer layer is between 1 nm and 100 um, between 1 nm and 10 um, between 1 nm and 100 nm, between 10 nm and 100 nm, between 10 nm and 500 nm, between 10 nm and 1000 nm, between 100 nm and 1000 nm, between 1 nm and 1 um, between 1 and 100 um, between 1 and 10 um, between 10 and 100 um, including any range between.

[0074]In some embodiments, the composite (also used herein as “iron oxide-TiO₂ composite”) comprises or consists essentially of iron oxide (e.g. Fe^II oxide, and/or Fe^III oxide) and TiO₂, wherein the iron oxide forms the outer layer and TiO₂ is in the core. In some embodiments, iron oxide is in a form of hematite. In some embodiments, iron oxide is in a form of nanoparticles on top of a micro-particular TiO₂ core; and wherein a molar ratio between Fe and Ti within the composite is between about 1:1 and 10:1, between about 1:1 and 5:1, between about 1.2:1 and 10:1, between about 1.5:1 and 10:1, between about 1.2:1 and 5:1, between about 1.2:1 and 4:1, between about 2:1 and 10:1, between about 2:1 and 5:1, between about 2:1 and 4:1, including any range between.

[0075]In some embodiments, iron oxide-TiO₂ composite comprises (i) Fe₂O₃, Fe₃O₄ and/or Fe₂O₃FeO and (ii) TiO₂. In some embodiments, the iron oxide-TiO₂ composite comprises Fe₂O₃, Fe₃O₄ or Fe₂O₃FeO as the first metal component and TiO₂ as the second metal component, in a molar ratio in the range of 1:9 to 9:1. In some embodiments, the molar ratio of the first metal component to the second metal component are in the range of 1:9 to 9:1, 2:9 to 9:1, 3:9 to 9:1, 4:9 to 9:1, 5:9 to 9:1, 6:9 to 9:1, 7:9 to 9:1, 8:9 to 9:1, 9:9 to 9:1, 1:9 to 9:2, 1:9 to 9:3, 1:9 to 9:4, 1:9 to 9:5, 1:9 to 9:6, 1:9 to 9:7, 1:9 to 9:8, or 1:9 to 9:9, including any range therebetween.

[0076]In some embodiments, the composite comprises or consists essentially of (i) iron oxide (e.g. FeO, and/or Fe₂O₃) and/or TiO₂ and optionally (ii) an additional transition metal and/or an additional transition metal oxide which is not iron oxide or TiO₂. In some embodiments, the additional transition metal is or comprises a lanthanide-series metal (e.g. oxophilic lanthanides, such as Ce). In some embodiments, the additional transition metal oxide is or comprises a lanthanide-series metal oxide (e.g. Ce oxide). In some embodiments, a molar ratio between the additional transition metal and Fe is about 1:1, or between 10:1 and 1:10, including any range between.

[0077]In some embodiments, the composite comprises or consists essentially of iron oxide (e.g. FeO, and/or Fe₂O₃) and/or TiO₂; and (i) the additional transition metal and/or the additional transition metal oxide which is not iron oxide or TiO₂, and/or (ii) the co-catalyst, wherein the co-catalyst is as described herein (such as a catalyst capable of catalyzing reduction of oxygen to peroxide (H₂O₂); or a catalyst capable of catalyzing reduction of nitrate to ammonia).

[0078]In some embodiments, the composite is a Ce-oxide Fe-oxide composite. In some embodiments, the Ce-oxide Fe-oxide composite comprises CeFeO₃. In some embodiments, the Ce-oxide Fe-oxide composite comprises or consists essentially of (i) CeO₂, (ii) CeFeO₃ and optionally of (iii) iron oxide (e.g. Fe₃O₄, Fe₂O₃). In some embodiments, a molar ratio between Ce and Fe within the Ce-oxide Fe-oxide composite is between 80:20 and 30:70, between 80:20 and 30:70, between 80:20 and 30:70, between 75:25 and 30:70, between 60:40 and 30:70, between 60:40 and 50:60, between 55:45 and 45:55, or about 50:50, including any range between. In some embodiments, a molar ratio between Ce and Fe within the Ce-oxide Fe-oxide composite is about 50:50. In some embodiments, a weight ratio between Ce and Fe within the Ce-oxide Fe-oxide composite is between 7:1 and 1.5:1, between 7:1 and 2:1, between 6.5:1 and 2:1, between about 6:1 and 1.5:1, between about 5:1 and 2:1, between about 4:1 and 2:1, between about 3:1 and 2:1, or about 2.5:1, including any range between.

[0079]In some embodiments, a weight portion of CeFeO₃ in the Ce-oxide Fe-oxide composite is between 40 and 95%, between 40 and 90%, between 40 and 85%, between 40 and 80%, between 45 and 90%, between 50 and 95%, between 50 and 80%, including any range between.

[0080]In some embodiments, a weight ratio between CeFeO₃ and CeO₂ within the Ce-oxide Fe-oxide composite is between about 0.8:1 and 10:1, between 1:1 and 8:1, between 1:1 and 6:1, between about 1:1 and 5:1, between about 5:1 and 2:1, between about 4:1 and 2:1, between about 3:1 and 2:1, or about 5:1, including any range between.

[0081]In some embodiments, the Ce-oxide Fe-oxide composite is characterized by XRD pattern comprising at least 3, at least 4, at least 5 or the entire peaks of: 22.7°, 32.3°, 39.8°, 46.3°, 57.8°, and 67.6° corresponding to CeFeO₃.

[0082]In some embodiments, the Ce-oxide Fe-oxide composite is in a form of micro-particles. In some embodiments, the Ce-oxide Fe-oxide composite is characterized by an average particle size (based on SEM measurements) between 0.5 and 100 μm, between 0.5 and 25 μm, between 1 and 50 μm including any range between.

[0083]In some embodiments, the Ce-oxide Fe-oxide composite is a porous matrix. In some embodiments, the Ce-oxide Fe-oxide composite is characterized by porosity (e.g. as determined by BET method) of between 20 and 90, between 50 and 90, between 50 and 80%, including any range between.

[0084]In some embodiments, the outer surface of the metal oxide catalyst of the invention comprises a plurality of reactive oxygen species (ROS), also used herein as “activated electrocatalyst”. The presence of ROS on the can be determined spectroscopically, such as by Surface Enhanced Raman Spectroscopy (SERS), X-ray photoemission spectroscopy (XPS) XRD, FTIR, Raman.

[0085]In some embodiments, ROS are selected from singlet oxygen, a peroxide (e.g. H₂O₂), a superoxide, a hydroxyl radical or any combination thereof. In some embodiments, the ROS are in contact with the outer surface of the metal oxide. In some embodiments, the ROS are absorbed (e.g. physisorbed) to the metal oxide. In some embodiments, the ROS are covalently bound to the metal oxide surface.

[0086]In some embodiments, the metal oxide catalyst comprises or consist essentially of RuO₂, characterized by NO₃RR activity.

[0087]In some embodiments, the metal oxide catalyst comprises or consist essentially of Ce-oxide Fe-oxide composite, characterized by NO₃RR activity and by NOx (i.e. nitrate and/or nitrite) to hydroxylamine reduction activity.

[0088]In some embodiments, the metal oxide catalyst comprises or consist essentially of iron oxide-TiO₂ composite, and is characterized by NRR activity and/or by NOR activity. In some embodiments, the metal oxide catalyst comprises or consist essentially of Ni oxide, Co oxide, and mixed spinel Ni-Co oxide and is characterized by NOR activity.

2. Metal Chalcogenide Catalyst

[0089]According to another embodiment, the present invention provides an electrocatalyst comprising a transition metal chalcogenide (also termed herein as “metal chalcogenide catalyst”); wherein the electrocatalyst is characterized by NO₃RR activity. In some embodiments, the transition metal chalcogenide comprises copper sulfide, nickel sulfide, or NiCO₂S₄. In some embodiments, the transition metal chalcogenide consists essentially of NiCo₂S₄.

[0090]In some embodiments, the transition metal chalcogenide is in a form of nanoparticles (e.g. nanoparticular agglomerate). In some embodiments, transition metal chalcogenide is a crystalline material characterized by a crystallite size of about 40 nm.

[0091]In some embodiments, transition metal chalcogenide is characterized by XRD pattern, as disclosed herein.

[0092]In some embodiments, transition metal chalcogenide is further characterized by NOx to hydroxylamine reduction activity (e.g. nitrite to hydroxylamine reduction activity). In some embodiments, under predefined conditions (e.g. about 1M hydroxide within the liquid electrolyte) the transition metal chalcogenide is substantially devoid of NOx to hydroxylamine reduction activity.

3. Rh-catalyst

[0093]According to another embodiment, the present invention provides an electrocatalyst comprising Rh (also termed herein as “Rh catalyst”); wherein the electrocatalyst is characterized by NOR activity, NO₃RR and/or by NRR activity. In some embodiments, the Rh catalyst consists essentially of a carbon material (e.g. carbon matrix) dopped with Rh (i.e. elemental Rh). In some embodiments, Rh is embedded or incorporated into the carbon material. In some embodiments, a weight content of Rh within the Rh catalyst is between 1 and 30%, between 3 and 30%, between 3 and 25%, between 4 and 20%, between 3 and 15%, between 3 and 10%, between 3 and 7%, or about 5%, including any range between.

[0094]In some embodiments, the Rh catalyst is characterized by XRD pattern, as disclosed herein.

[0095]In some embodiments, the carbon material of the Rh catalyst is in a form of microparticles. In some embodiments, the microparticles are in a form of sheets or flakes. In some embodiments, the microparticles are characterized by an average dimension (width and/or length dimension) between 1 and 100 um, between 10 and 100 um, including any range between (as determined by SEM).

[0096]In some embodiments, Rh is in a form of nanoparticles (NPs) within the Rh catalyst. In some embodiments, Rh NPs are homogenously distributed within the carbon microparticles. In some embodiments, Rh NPs are characterized by an average particle size (as determined by STEM) between 5 and 30 nm, between 5 and 28 nm, between 5 and 15 nm, between 8 and 15 nm, between 10 and 15 nm, including any range between.

[0097]In some embodiments, the Rh catalyst is characterized by a selective NOR activity resulting in a selective nitrate formation (optionally wherein selective NOR activity comprises at least 60% w/w, at least 70, at least 80%, or at least 90% w/w of nitrate from the total NOx species generated during NOR).

[0098]In some embodiments, the electrocatalyst of the invention consist essentially of a crystalline material. In some embodiments, at least 70%, at least 80%, at least 90%, at least 95%, between 70 and 100%, between 70 and 99%, between 70 and 97%, between 70 and 95%, between 70 and 90%, between 70 and 85% by weight of the electrocatalyst is a crystalline material.

[0099]In some embodiments, at least 70%, at least 80%, at least 90%, at least 95%, between 70 and 100%, between 70 and 99%, between 70 and 97%, between 70 and 95%, between 70 and 90%, between 70 and 85% of the metallic content of the electrocatalyst is in a crystalline state.

Co-Catalyst

[0100]In another aspect, there is provided a composite material comprising the electrocatalyst (e.g. metal oxide catalyst disclosed above) and a co-catalyst. In some embodiments, the composite material is characterized by NOR activity.

[0101]The term “electrocatalyst” as used herein encompasses any one of the Rh-catalyst, metal oxide catalyst and metal chalcogenide catalyst disclosed above and the composite material disclosed hereinbelow.

[0102]In some embodiments, the co-catalyst is in contact or mixed with the electrocatalyst in the composite material. In some embodiments, the co-catalyst is located on the outer surface of the electrocatalyst (e.g. the metal oxide catalyst) or is embedded within the matrix formed by the electrocatalyst. In some embodiments, the co-catalyst is in a form of a layer on top of the electrocatalyst. In some embodiments, the composite material comprises a weight excess of the co-catalyst relative to the electrocatalyst. In some embodiments, the composite material consists essentially of the electrocatalyst (e.g. metal oxide catalyst) and the co-catalyst. In some embodiments, the electrocatalyst suitable for use in the composite material is an H2O2 disproportionation catalyst (optionally a H2O2 disproportionation metal oxide).

[0103]In some embodiments, a w/w ratio between the co-catalyst and the electrocatalyst within the composite material of the invention is between about 20:1 and about 1:1, between about 10:1 and about 1:1, between about 10:1 and about 2:1, between about 5:1 and about 1:1, between about 5:1 and about 2:1, between about 3:1 and about 1:1, including any range between.

[0104]In some embodiments, a w/w ratio between the co-catalyst and the metal oxide catalyst (e.g. RuO₂, Ce-oxide, Fe-oxide) within the composite material of the invention is between about 1:1 and about 10:1, between about 1:1 and about 5:1, between about 1:1 and about 3:1, between about 2:1 and about 10:1, between about 2:1 and about 5:1, between about 2:1 and about 3:1, including any range between. In some embodiments, a w/w ratio between the co-catalyst (e.g. macrocyclic ring-transition metal complex) and the metal oxide catalyst (e.g. RuO₂) within the composite material of the invention is between about 2:1 and about 4:1. In some embodiments, the composite material consists essentially of (i) Co-phthalocyanine (as the co-catalyst) and RuO₂ (as the metal oxide catalyst), at a co-catalyst:metal oxide catalyst weight ratio of about 3:1.

[0105]In some embodiments, the composite material consists essentially of the co-catalyst and RuO₂ is characterized by NOR activity. In some embodiments, a working electrode (e.g. cathode) comprising the composite material is configured for generating nitrate from nitrogen (or a gas comprising same, such as air) in a liquid electrolyte (e.g. aqueous electrolyte) being substantially devoid of nitrate.

[0106]In some embodiments, the co-catalyst is capable of catalyzing reduction of oxygen to peroxide (H₂O₂) upon activation thereof. In some embodiments, the co-catalyst is capable of generating ROS (e.g. H₂O₂) in-situ upon exposure thereof to oxygen and activation of the co-catalyst. In some embodiments, the activation of the co-catalyst is performed via applying thereto an electrical potential. In some embodiments, the electrical potential sufficient for activation of the co-catalyst is between −1 and 1V, between −1 and 0.8V, between −0.8 and 0.8V, between −0.5 and 0.6V, between −1 and 0.6V, between −0.2 and 0.6V versus NHE, including any range between. In some embodiments, the activation of the co-catalyst is performed via light excitation by irradiating the co-catalyst with light at a wavelength within the absorbance range of the co-catalyst (usually between about 300 and about 700 nm). In some embodiments, the activation of the co-catalyst is performed by contacting thereof with an oxidizer (e.g. H₂O₂, hypochlorite, per-acid, organic/inorganic peroxide and/or hydrogen peroxide precursor such as a percarbonate).

[0107]In some embodiments, the co-catalyst is a carbon catalyst capable of catalyzing oxygen reduction to H₂O₂ or to water. In some embodiments, the co-catalyst is a metal oxide capable of catalyzing oxygen reduction to H₂O₂ or to water. In some embodiments, the co-catalyst is a fluorophore. In some embodiments, the co-catalyst is ROS generating dye. In some embodiments, the co-catalyst comprises a macrocyclic ring. In some embodiments, the macrocyclic ring is selected from a phthalocyanine dye, a cyanine dye, a porphyrin dye, or any combination thereof. In some embodiments, the co-catalyst is or comprises a macrocyclic ring-transition metal complex. In some embodiments, the co-catalyst is selected from a phthalocyanine dye, a cyanine dye, a porphyrin dye, including any transition metal complex, any salt, or any combination thereof. Additional ROS generating dyes are known in the art (such as xanthene (e.g. Rose-Bengal), curcumin-based dyes, etc.).

[0108]In some embodiments, the co-catalyst (i.e. a catalyst capable of catalyzing oxygen reduction to H₂O₂ or to water) is a metal-phthalocyanine complex. In some embodiments, the metal-phthalocyanine complex comprises a transition metal cation. In some embodiments, the transition metal cation is a divalent or a trivalent metal cation (e.g. Fe, Zn, Ni, Cu, Co, Pd, Pt, cation). In some embodiments, the co-catalyst is Co-phthalocyanine.

[0109]In some embodiments, a w/w ratio between the co-catalyst and the electrocatalyst (e.g. RuO₂) disclosed above, is a synergistic effective ratio. In some embodiments, the synergistic effective ratio refers to NOR activity of the composite material.

[0110]The term “synergism”, or any grammatical derivative thereof, is defined as the simultaneous action of two or more compounds (e.g. co-catalyst and the electrocatalyst) in which the electrochemical activity (e.g. NOR activity) of the combination is greater than the sum electrochemical activity of the individual components.

[0111]The term “greater” encompasses at least 20%, at least 50%, at least 100%, at least 3 fold, at least 5 fold, at least 10 fold, at least 50 fold enhanced electrochemical activity, as compared to the control (i.e. electrochemical activity of the individual components), including any range between.

Ink Composition

[0112]In another aspect, there is provided a composition, comprising the electrocatalyst (i.e. Rh-catalyst, metal oxide catalyst and metal chalcogenide catalyst, or the composite material disclosed above) in contact with a conductive material. In some embodiments, the composition consists essentially of the electrocatalyst (and optionally the co-catalyst) and the conductive material. In some embodiments, the composition is a solid or a powderous composition. In some embodiments, the transition metal oxide and conductive material and optionally additional constituents of the composition are homogenously mixed within the composition. In some embodiments, the composition is a coating composition or ink composition for coating an electrode with the electrocatalyst.

[0113]In some embodiments, the composition comprises the electrocatalyst and the conductive material in a form of particles (e.g. nano-particles, and/or micro-particles). In some embodiments, the composition is a mixture.

[0114]In some embodiments, the composition (i.e. upon application thereof on an electrode surface) is an electrically conductive material. In some embodiments, the composition is characterized by electrical conductivity of at least 10⁻¹ S/m, at least 1 S/m, at least 10 S/m, at least 10² S/m, at least 10³ S/m, between 10⁻¹ and 10¹⁰ S/m, between 1 and 10¹⁰ S/m, between 10 and 10¹⁰ S/m, between 10 and 10⁸ S/m, between 10² and 10¹⁰ S/m, including any range between.

[0115]In some embodiments, a weight percentage of the electrocatalyst within the composition (i.e. relative to the total weight of the dry constituents of the composition) is between 1 and 90% w/w, between 50 and 90% w/w, between 70 and 90% w/w, between 1 and 5% w/w, between 1 and 10% w/w, between 5 and 90% w/w, between 5 and 20% w/w, between 5 and 10% w/w, between 1 and 30% w/w, between 1 and 40% w/w, between 1 and 50% w/w, between 5 and 50% w/w, between 10 and 90% w/w, between 10 and 50% w/w, between 20 and 90% w/w, between 20 and 70% w/w, including any range between. In some embodiments, a relative weight percentage of the conductive material (i.e. which is not the electrocatalyst or the co-catalyst) within the composition is between 1 and 50% w/w, between 1 and 5% w/w, between 1 and 10% w/w, between 5 and 50% w/w, between 5 and 20% w/w, between 5 and 30% w/w, between 5 and 10% w/w, between 1 and 30% w/w, between 1 and 40% w/w, between 1 and 50% w/w, between 5 and 50% w/w, between 10 and 50% w/w, between 10 and 30% w/w, between 20 and 50% w/w, between 20 and 40% w/w, including any range between.

[0116]In some embodiments, the composition is a fluid (liquid) ink further comprising a solvent. In some embodiments, the solvent comprises a polar organic solvent (e.g. a water-miscible organic solvent, such as an alcohol or a lower alcohol), and optionally further comprising an aqueous solvent. In some embodiments, a weight portion of the solid constituents (i.e. any one of: the electrocatalyst, the co-catalyst and the conductive material) within the fluid ink is between 5 and 50%, between 10 and 50%, between 5 and 30%, between 20 and 70%, including any range between.

[0117]In some embodiments, the fluid ink is characterized by a viscosity suitable for deposition thereof on at least one surface of a substrate. In some embodiments, the viscosity of the fluid ink is between 0.1 and 1.000.000, between 0.1 and 100.000, between 0.1 and 1000, between 0.1 and 100 Pa*s at 25° C., including any range between.

[0118]In some embodiments, the conductive material comprises a carbon material, a metal (e.g. metal mesh, metal particle or metal foam), a conductive metal oxide, a conductive ceramic material, a conductive polymer (e.g. a ionomer such as Nafion, a sulfonated fluropolymer), or any combination thereof. In some embodiments, the conductive material is in a form of nano- and/or micro-particles.

[0119]In some embodiments, the carbon material comprises carbon black, activated carbon, graphite, carbon nanotube, graphene, and any combination thereof. Carbon black may be selected from, without being limited thereto, Vulcan XC-72, Black Pearls 700, Black Pearls 800, Black Pearls 2000, Vulcan XC-605, Regal 350, Regal 250, Black Pearls 570, and Vulcan XC-68, or any combination thereof.

[0120]Herein throughout, the expression “deposited on at least one surface” is also referred to herein, for simplicity, as a coating on a substrate, or surface of a substrate.

[0121]In some embodiments, the term “coating”, or any grammatical derivative thereof, is defined as a coating that (i) is positioned above the substrate, (ii) is in contact with the substrate, and (iii) does not necessarily completely cover the substrate. In some embodiments, the term “coating”, or any grammatical derivative thereof encompasses a single layer coating or a plurality of coating layers.

[0122]In some embodiments, the electrocatalyst or the composition of the invention is deposited on the substrate. In some embodiments, deposition is performed by printing, liquid coating, casting, molding, pressure shaping, extrusion, electroless deposition, etc. including any combination thereof.

Electrode

[0123]In another aspect of the invention, there is provided an electrode comprising the electrocatalyst or the composition of the invention in contact with a substrate. In some embodiments, the electrocatalyst is embedded on or within the substrate. In some embodiments, the composition of the invention is in a form of a coating bound or adhered (stably attached) to a surface of the substrate. In some embodiments, the coating comprises dry constituents of the composition disclosed above. In some embodiments, the coating is in a form of a continuous layer. In some embodiments, the electrode consists essentially of the electrocatalyst in contact with the substrate.

[0124]In some embodiments, the electrocatalyst or the composition of the invention is in a form of a layer (e.g. continuous layer) or coating on top of the substrate. In some embodiments, the coating has a thickness of between 10 nm and 10 mm, between 1 and 1000 um, between 0.1 and 1000 um, between 0.1 and 100 um, between 1 and 100 um, between 100 nm and 10 um, between 100 nm and 1 um, between 100 nm and 100 um, between 100 nm and 1 mm, between lum and 10 mm, between 1 um and 1 mm, between lum and 500 um, including any range between. In some embodiments, the layer is characterized by a porosity between 10 and 80%, between 10 and 60%, between 10 and 50%, between 10 and 30%, between 10 and 20%, including any range between.

[0125]The term “substrate”, as used herein encompasses any material (i.e. electrically conductive material) utilized for fabrication of electrodes. In some embodiments, the substrate comprises or consists essentially of organic or inorganic electrically conductive surface(s).

[0126]In some embodiments, the substrate is a conductive substrate. In some embodiments, the substrate (and/or the substrate material) is characterized by electrical conductivity of at least 1 S/m, at least 10 S/m, at least 10² S/m, at least 10³ S/m, between 10⁵ and 10¹⁰ S/m, between 1 and 10¹⁰ S/m, between 10 and 10¹⁰ S/m, between 10 and 10⁸ S/m, between 10² and 10¹⁰ S/m, between 10³ and 10¹⁰ S/m, between 10⁴ and 10¹⁰ S/m, between 10⁵ and 10¹⁰ S/m, between 10² and 10⁸ S/m, including any range between.

[0127]In some embodiments, the substrate is or consists essentially of a material selected from, but is not limited to, carbon material (e.g. carbon fiber, carbon black, activated carbon, graphite, carbon nanotube, graphene, and any combination thereof), a conductive metal substrate (comprising a metal or a metalloid in an elemental state, or a conductive metal oxide), a conductive polymer (e.g. a conductive organic polymer such as PANI, polyacetylene; polyphenylene vinylene; polypyrrole, polythiophene, polyphenylene sulfide), or any combination thereof. In some embodiments, the conductive metal substrate is in a form of a sheet, a rod, a foil, a wire, etc.) or a porous material such as: metal mesh (woven or non-woven), metal foam (e.g. Ni foam, Sn foam, etc.).

[0128]In some embodiments, the carbon material is a woven or non-woven carbon-fiber based material (e.g. Toray paper, carbon cloth, carbon paper).

[0129]In some embodiments, the substrate is configured to facilitate a gas flow through at least one dimension thereof (e.g. length and/or cross section). In some embodiments, the substrate is a porous substrate. In some embodiments, the substrate is characterized by a porosity between 50 and 95%, between 50 and 90%, between 50 and 80%, between 60 and 90%, including any range between. In some embodiments, the substrate is a porous electrode (e.g. a mesh or foam). In some embodiments, the substrate is a non-porous substrate characterized by a porosity below 20%, below 10%, below 5%, including any range between.

[0130]In some embodiments, the electrocatalyst or the composition of the invention is present at a concentration of between 0.1% and 99%, between 5% and 99%, between 0.1% and 5%, between 0.1% and 10%, between 0.1% and 20%, between 10% and 99%, between 2% and 10%, between 2% and 20%, between 2% and 30%, between 10% and 99%, between 10% and 90%, between 10% and 80%, between 20% and 99%, between 20% and 90%, between 20% and 80%, between 50% and 99%, between 50% and 90%, by total weight of the electrode, including any range between.

[0131]In some embodiments, the electrode is characterized by an electrocatalytically effective loading of the electrocatalyst of between 0.5 and 10 mg/cm², between 1 and 10 mg/cm², between 1 and 5 mg/cm², between 2 and 10 mg/cm², between 2 and 5 mg/cm², or about 4 mg/cm² including any range between.

[0132]In some embodiments, the electrode comprises the electrocatalyst or the composition of the invention stably bound to at least one surface of the substrate. In some embodiments, the term stably bound refers to the ability of the electrode to maintain its structural integrity (i.e. to maintain its shape, chemical composition, loading, and/or electrocatalytic activity, and is further devoid of disintegration of the composition from the substrate) under operable conditions of the electrocatalytic ammonia generation, as disclosed herein. Further, the electrode to maintain its structural integrity for at least 3, at least 5, at least 10, at least 100, at least 500, at least 1000, at least 10,000 operation cycles, including any range between.

[0133]In some embodiments, the electrode described herein is characterized by an electrocatalytic activity selected from: (i) reduction of nitrogen to ammonia (NRR activity); (ii) reduction of nitrogen-based specie to ammonia (NO₃RR activity), (iii) NOR activity, (iv) NOx (i.e. nitrate and/or nitrite) to hydroxylamine reduction activity or any combination of (i) to (iv). In some embodiments, the electrode is a working electrode. In some embodiments, the electrode described herein comprises the electrocatalytically effective loading of the electrocatalyst characterized by an electrocatalytic activity selected from: (i) reduction of nitrogen to ammonia (NRR activity); (ii) reduction of nitrogen-based specie to ammonia (NO₃RR activity), (iii) NOR activity, (iv) NOx (i.e. nitrate and/or nitrite) to hydroxylamine reduction activity or any combination of (i) to (iv).

[0134]In some embodiments, the electrode is a cathode. In some embodiments, the cathode is characterized by NRR activity and comprises or consist essentially of the electrocatalyst characterized by NRR activity. In some embodiments, the electrocatalyst characterized by NRR activity comprises any of (i) RuO₂, (ii) Ce-oxide Fe-oxide composite, or a composite material comprising (i) or (ii) and the co-catalyst (e.g. CoPc). In some embodiments, the NRR activity cathode is operable at a negative potential. In some embodiments, the NRR activity cathode is operable at a negative potential of between −0.2 and −0.4V, between −0.25 and −0.35V, about −0.35V (e.g. for CoPc/RuO₂) vers. RHE.

[0135]In some embodiments, the NRR activity cathode is operable with the liquid electrolyte disclosed herein. In some embodiments, the liquid electrolyte for NRR further comprises (i) one or more peroxide species (e.g. H₂O₂) and (ii) a nitrate anion or a salt thereof dissolved therewithin.

[0136]In some embodiments, a concentration of the one or more peroxide species within the liquid electrolyte (e.g. liquid alkaline electrolyte) is between 1 and 1000 uM, between 50 and 1000 uM, between 10 and 200 uM, between 50 and 200 uM, between 50 and 500 uM, including any range between.

[0137]In some embodiments, a concentration of nitrate within the liquid electrolyte is in the range between 500 uM and 2M, of 0.01M to 2M, of 0.1M to 2M, of 0.2M to 1M, of 0.1M to 1M, of 0.2M to 0.5M, or about 0.5M.

[0138]In some embodiments, the liquid electrolyte for NRR is further saturated with a mixture of nitrogen and oxygen (e.g. oxygen content of between 5 and 30%, as disclosed above) and optionally one or more additional gas(es). In some embodiments, the liquid electrolyte for NRR is further saturated with air.

[0139]In some embodiments, the cathode is characterized by NO₃RR activity and comprises or consist essentially of the electrocatalyst characterized by NO₃RR activity. In some embodiments, the electrocatalyst characterized by NO₃RR activity comprises the metal oxide catalyst (e.g. RuO₂, Ce-oxide Fe-oxide composite), the composite metal oxide catalyst and the co-catalyst (e.g. RuO₂ co-catalyst such as CoPc), or the metal chalcogenide catalyst (c.g. NiCo₂S₄). In some embodiments, the NO₃RR activity cathode is operable at a negative potential of between 0 and −0.6V, between −0.05 and −0.45V, between −0.3 and −0.5V, about −0.3V (e.g. for NiCo₂S₄), about −0.45V (e.g. for Ce-oxide Fe-oxide composite), about −0.35V (e.g. for RuO₂-CoPc), or about −0V (e.g. RuO₂) vers. RHE. The operable cathodic potential may vary, depending on the pH of the electrolyte.

[0140]In some embodiments, the NO₃RR activity cathode is operable with a liquid electrolyte comprising between about 0.1 and 1M of metal hydroxide (and/or characterized by a pH between 10 and 14). In some embodiments, the NO₃RR activity cathode is operable with a liquid electrolyte comprising between about 0.001 and 1M, or between 0.1 and 1M of metal sulfate (and is optionally characterized by a pH value of about 7). In some embodiments, the electrocatalyst is NiCo₂S₄ and a concentration of the metal hydroxide in the liquid electrolyte is about 1M. In some embodiments, liquid electrolyte further comprises between about 0.1 and 1M of KCl. In some embodiments, liquid electrolyte comprises at least 10, at least 100, at least 400, at least 500, at least 1000 uM nitrate, between 0.1 mM and 1M, between 0.1M and 1M, including any range between. In some embodiments, the liquid electrolyte is further saturated with nitrogen.

[0141]In some embodiments, the NO₃RR activity cathode comprising the metal oxide catalyst (e.g. RuO₂) has a nitrate generation rate of about 170 μg h⁻¹ cm⁻² with a Faradaic efficiency (FE) of about 17%. In some embodiments, the NO₃RR activity cathode comprising the metal oxide catalyst (e.g. Ce-oxide Fe-oxide composite) has a nitrate generation rate of about 21000 μg h⁻¹ cm⁻² with a Faradaic efficiency (FE) of between 80 and 100%.

[0142]In some embodiments, the cathode comprising NiCo₂S₄ as the electrocatalyst is characterized by nitrite to ammonia reduction activity.

[0143]In some embodiments, the cathode comprising RuO₂/co-catalyst (e.g. CoPc) iron oxide-TiO₂ composite, Ni oxide, Co oxide, and mixed spinel Ni-Co oxide is characterized by NOR activity.

[0144]In some embodiments, the cathode is characterized by NOx to hydroxylamine reduction activity and comprises or consist essentially of the electrocatalyst characterized by NOx to hydroxylamine reduction activity. In some embodiments, the electrocatalyst characterized by NOx to hydroxylamine reduction activity comprises Ce-oxide Fe-oxide composite, or NiCo₂S₄. In some embodiments, the NOx to hydroxylamine reduction activity cathode is operable at a negative potential of between −0.1 and −0.5V, between −0.05 and −0.45V, between −0.3 and −0.5V, about −0.3V, −0.5V, about −0.45V (e.g. for Ce-oxide Fe-oxide composite) vers. RHE.

[0145]In some embodiments, the NOx to hydroxylamine reduction activity cathode is operable with a liquid electrolyte comprising between about 0.1 and 1M of metal hydroxide. In some embodiments, liquid electrolyte comprises at least 100, at least 500, at least 1000 uM, between 0.1 mM and 1M, between 0.1M and 1M nitrate or nitrite, including any range between.

[0146]In some embodiments, the NOx to hydroxylamine reduction activity cathode comprising the metal oxide catalyst (e.g. Ce-oxide Fe-oxide composite) has a hydroxylamine generation rate of about 1000-1300 μg h⁻¹ cm⁻² with a Faradaic efficiency (FE) of about 12%.

[0147]In some embodiments, the NOx to hydroxylamine reduction activity cathode consist essentially of NiCo₂S₄ as the electrocatalyst and is operable at a negative potential of about −0.3V vers. RHE (for NO₂⁻ to hydroxylamine reduction activity), at about −0.5V vers. RHE or at about −0.25V vers. RHE (for NO₃⁻ to hydroxylamine reduction activity).

[0148]In some embodiments, the NO₂⁻ to hydroxylamine reduction activity cathode comprising NiCo₂S₄ has a hydroxylamine generation rate of about 3000 μg h⁻¹ cm⁻² with a Faradaic efficiency (FE) of about 18%.

[0149]In some embodiments, the electrode is an anode. In some embodiments, the anode is characterized by NOR activity and comprises or consist essentially of the electrocatalyst characterized by NOR activity. In some embodiments, the electrocatalyst characterized by NOR activity comprises the Rh catalyst or the composite material.

[0150]In some embodiments, the anode is operable at a potential of between 0.1 and 2.0V, between 1.6 and 1.8V, between 1.5 and 1.7V, about 0.1V (for RuO₂/CoPc) or about 1.7V (for Rh/C) vers. RHE, including any range between. In some embodiments, the anode comprising the Rh catalyst has a nitrate generation rate of about 20 and 50 μg h⁻¹ cm⁻² with a Faradaic efficiency (FE) of between about 20 and 30%.

[0151]In some embodiments, the anode is operable with a liquid electrolyte comprising between about 0.1 and 1M of metal hydroxide (OH⁻) or metal sulphate (SO₄²⁻). In some embodiments, the liquid electrolyte is further saturated with air or with a nitrogen oxygen mixture, wherein oxygen content of the mixture is between 5 and 30%, between 15 and 30, between 15 and 25%, including any range between.

The Apparatus

[0152]In another aspect of the invention, there is provided an apparatus (e.g. electrochemical cell) comprises a first chamber(s) and a second chamber(s) each configured to contain a liquid electrolyte; wherein (i) one of the first chamber and the second chamber comprises a working electrode and another one comprises a counter electrode; or (ii) wherein the first chamber and the second chamber comprises a working electrode; and wherein the working electrode is the electrode disclosed hereinabove. In some embodiments, the apparatus is configured for performing, inducing or catalyzing an electrochemical reaction. In some embodiments, the electrochemical reaction is selected from NRR, NOR, NO₃RR, nitrate/nitrite reduction to hydroxylamine and nitrite reduction to ammonia, or any combination thereof.

[0153]In some embodiments, the working electrode is the cathode disclosed above, operable at a negative potential. In some embodiments, the negative potential ranges between 0 and −0.6, or between about −0.05 and −0.5V, including any range between.

[0154]In some embodiments, the working electrode is the anode disclosed above, operable at a positive working electrode potential. In some embodiments, the positive working electrode potential ranges between 0.1 and 2.0V, or about 1.7V, including any range between.

[0155]In some embodiments, the liquid electrolyte (also termed herein as “electrolyte solution”) is or comprises an aqueous solution. In some embodiments, the liquid electrolyte comprises an organic solvent and further comprises an oxidizer (e.g. H₂O₂, hypochlorite, per-acid, organic/inorganic peroxide and/or hydrogen peroxide). The terms “liquid electrolyte” and “electrolyte” or “electrolyte solution” are used herein interchangeably.

[0156]In some embodiments, the liquid electrolyte comprises between about 0.1 and 1M, or about 0.1M, between about 0.001M to 5M, between about 0.05M to 1M, between about 0.05M to 5M of a metal hydroxide (e.g. alkali metal hydroxide, such as KOH, NaOH, or LiOH), including any range between. In some embodiments, the liquid electrolyte comprises between about 0.1 and 1M, or about 0.1M, between about 0.001M to 5M, between about 0.05M to 1M, between about 0.05M to 5M of a metal sulphate (SO₄²⁻). In some embodiments, the liquid electrolyte comprises between about 0.5 and 1M of KCl. In some embodiments, the liquid electrolyte is nitrogen saturated. In some embodiments, the liquid electrolyte is saturated by nitrogen/oxygen mixture (as disclosed below) or with air. In some embodiments, the liquid electrolyte is a supersaturated solution.

[0157]In some embodiments, the liquid electrolyte is a liquid alkaline electrolyte. In some embodiments, the liquid alkaline electrolyte has a pH value of at least pH 11, or at least pH 12, or at least pH 13. In some embodiments, the liquid alkaline electrolyte is in the pH value between 10 and 14, between 11 and 14, or between 11 and 13, including any range between.

[0158]In some embodiments, liquid alkaline electrolyte (e.g. for NRR) further optionally comprises (i) a nitrate salt or a nitrite salt and/or one or more peroxide species dissolved therewithin, wherein a concentration of (i) and/or (ii) is as described herein.

[0159]In some embodiments, the liquid electrolyte comprises a gas dissolved therewithin, wherein the gas comprises nitrogen, or a combination of nitrogen and oxygen (e.g. oxygen content of between 5 and 30%, as disclosed above) and optionally one or more additional gas(es). In some embodiments, the gas is air. In some embodiments, the concentration of the gas within the liquid electrolyte is the maximum concentration of the specific gas, predetermined by the solubility of the specific gas within the specific liquid electrolyte, and by the temperature of the liquid electrolyte. In some embodiment, the liquid electrolyte is air saturated.

[0160]In some embodiments, the liquid electrolyte comprises a salt selected from a nitrate salt and a nitrite salt. In some embodiments, the liquid electrolyte is an alkaline electrolyte further comprising a nitrate salt or a nitrite salt dissolved therewithin. In some embodiments, the liquid electrolyte is super saturated with the nitrate salt or nitrite salt.

[0161]FIG. 9A presents a schematic illustration of an apparatus (e.g. electrochemical cell) used for electrochemical reduction of the nitrogen to ammonia.

[0162]Apparatus 100 comprises an anode 101 located within a first chamber or a container 110, and a cathode 102 located within a second chamber or a container 120. The first and second chambers or containers 110 and 120 are configured to contain a liquid electrolyte 130. The length of the anode 101 and of the cathode 102 is sufficient to be immersed within the liquid electrolyte 130. Optionally, each of the anode 101 and the cathode 102 is configured for being in contact with the liquid electrolyte (e.g. such that at least 90% of the electrocatalyst coated electrode surface is in contact with or immersed into the liquid electrolyte). The cathode 102 and of the anode 101 are electrically connected to each other and are further connectable to a power source. The first chamber 110 is configured to contain a gas pressurized liquid electrolyte (i.e. electrolyte pressurized with a gas, wherein gas is as disclosed herein). The first chamber 110 and optionally the second chamber 120 is configured to held a pressure ranging between 500 to 5000 mm Hg, or between 500 and 2000 mm Hg.

[0163]In some embodiments, the apparatus 100 is an electrochemical cell, wherein the first chamber 110 is a first half-cell and the second chamber 120 is a second half-cell of the electrochemical cell, and wherein the first chamber 110 is in liquid (e.g. fluid) communication with the second chamber 120. In some embodiments, the power source is configured to generate a current (e.g. direct current or alternating current ranging) at predefined voltage sufficient for operation of the apparatus 100.

[0164]In some embodiments, the apparatus is operable at an electric potential ranging between 1 and 3V, between 1.5 and 2.5V, between 1.7 and 2.3, between 1.5 and 2.3V, including any range between.

[0165]In some embodiments, the power source is configured to generate a current (e.g. direct current). In some embodiments, the current is between 1 mA and 10 A, between 1 mA and 10 A, between 1 mA and 5 A, including any range between.

[0166]In some embodiments, the anode 101 is the NOR activity anode disclosed hereinabove. In some embodiments, the anode 101 comprises or consists essentially of the electrocatalyst characterized by NOR activity (e.g. the Rh catalyst, or metal oxide catalyst as disclosed hereinabove). In some embodiments, the electrocatalyst characterized by NOR activity comprises any one of: Rh/C, iron oxide-TiO₂ composite Rh/C, iron oxide-TiO₂ composite, Ni oxide, Co oxide (e.g. Co₃O₄), and mixed spinel Ni-Co oxide as disclosed above.

[0167]The first chamber 110 may have a valve or a pump 116 (also referred to as “first gas inlet”) configured to induce gas flow into the first chamber. First gas inlet 116 may be in fluid communication with the liquid electrolyte 130 via a channel 117, so as to induce flow of a gas (e.g. nitrogen, air, or a mixture of oxygen and nitrogen) into the liquid electrolyte 130 to generate the gas pressurized liquid electrolyte.

[0168]In some embodiments, the gas comprises a nitrogen oxygen mixture, wherein oxygen content of the mixture is between 5 and 30%, between 15 and 30, between 15 and 25%, including any range between. In some embodiments, the gas is or comprises air.

[0169]First gas inlet 116 may be located at an upper part of the first chamber 110. First gas inlet 116 may include a pipe of various shapes and sizes, connected to, attached to, or integrally formed with the first chamber 110. Apparatus 100 may be connected to a gas supply unit (not shown) allowing to supply gas via first gas inlet.

[0170]The first chamber 110 and/or second chamber 120 may be equipped with an overpressure control valve (optionally positioned at the upper part of the first/second chamber).

[0171]The first chamber 110 may have a valve or a pump (also referred to as “first liquid inlet”) 115 configured to induce a flow of a liquid inside the first chamber 110. The first chamber 110 may be filled with a predefined volume of the liquid electrolyte via the liquid inlet 115. In some embodiments, the predefined volume of the liquid electrolyte is so that the anode 101 is immersed therewithin.

[0172]The first chamber 110 is in fluid connection with the second chamber 120 via a channel 140. Channel 140 may be located below the surface of the predetermined volume of the liquid electrolyte 130 in the first chamber 110 and in the second chamber 120. Channel 140 is configured to support a flow of the liquid electrolyte from the first chamber 110 into the second chamber 120. Channel 140 may comprise a unidirectional flow element (e.g. valve or pump) 145 configured to generate flow of the liquid electrolyte from the first chamber 110 into the second chamber 120. In some embodiments, unidirectional flow element 145 is located downstream the first chamber 110 and upstream the second chamber 120 at any location within the channel 140. In some embodiments, unidirectional flow element 145 is a unidirectional pump.

[0173]Apparatus 100 may comprises a gas exchanger (or degassing unit) downstream the first chamber 110 and upstream the second chamber 120. The gas exchanger may be located at any location within the channel 140, such as downstream to unidirectional flow clement 145. The gas exchanger is configured to substantially remove a gas (e.g. oxygen) dissolved in the liquid electrolyte form the first chamber before entering the second chamber. The gas exchanger may be in a form of a vacuum-based degasser (configured to remove the gas dissolved in the liquid electrolyte) or a gas purging unit configure to feed an inert gas (such as nitrogen or argon) to the liquid electrolyte thus replacing the gas (e.g. nitrogen and oxygen mixture, such as air) in the liquid electrolyte with the inert gas.

[0174]In some embodiments, the cathode 102 is the NO₃RR activity cathode disclosed hereinabove. In some embodiments, the cathode 102 comprises the electrocatalyst characterized by NO₃RR activity, such as the metal oxide catalyst (e.g. RuO₂, Ce-oxide Fe-oxide composite) or the metal chalcogenide catalyst (e.g. NiCo₂S₄), disclosed above.

[0175]The apparatus 100 is configured to generate ammonia via simultaneous or subsequent reactions: NOR at the anode 101 in the first chamber 110 to convert the nitrogen (from the gas fed into the liquid electrolyte 130 via the first gas inlet 116) to nitrate; and NO₃RR at the cathode 102 in the second chamber 120 to generate ammonia from the nitrate obtained in the first chamber 110. The apparatus 100 is configured for transferring the nitrate generated during the NOR in liquid electrolyte 130 in the first chamber 110 to the second chamber 120, where the nitrate undergoes NO₃RR at the cathode 102 to generate ammonia. The apparatus 100 is configured for generating a flow of the liquid electrolyte 130 within channel 140 for transferring the nitrate from the first chamber 110 to the second chamber 120. The flow may be generated via the unidirectional flow element 145.

[0176]The second chamber 120 contain a heating or cooling element and a temperature controller configured to maintain a predetermined temperature of the liquid electrolyte 130 in the second chamber 120.

[0177]The second chamber 120 may have an ammonia outlet 126. Ammonia outlet 126 may be located the upper part of second chamber 120. Ammonia outlet 126 may allow ammonia (and/or H₂) exiting the second chamber 120. Ammonia outlet 126 may further allow other gasses involved in the ammonia synthesis, e.g., nitrogen, hydrogen exiting the second chamber 120. Ammonia outlet 126 may be in fluid communication (e.g. via a channel or a pipe) with the first liquid inlet 115. The apparatus 100 may be configured to generate a gas flow from

[0178]Ammonia outlet 126 may be in fluid communication with an ammonia capturing unit. In some embodiments, the ammonia capturing unit may have an ammonia trap. Ammonia trap may be in the form of a container configure to contain an acid (e.g., sulfuric acid). Ammonia trap may have an inlet, allowing a gas (e.g., ammonia and other gasses) exiting ammonia outlet 126 to enter ammonia trap. Ammonia trap may have an outlet allowing gasses (e.g., nitrogen and hydrogen) to exit therefrom. In some embodiments, the ammonia capturing unit may have a gas separation unit configured to separate (e.g. via a membrane, sorption, cryogenic distillation apparatus, etc.) between ammonia and additional gas(es) such as N2 and H2. In some embodiments, the ammonia capturing unit may be further in fluid communication with a gas tank.

[0179]The ammonia capturing unit may be in fluid communication (e.g. via a channel or a pipe) with the first liquid inlet 115. The apparatus 100 may be configured to generate a gas flow from the ammonia capturing unit via the first liquid inlet 115 into the first chamber 110. The apparatus 100 may be configured to feed a gas obtained in the ammonia capturing unit (e.g. H2, N2, or both) into the first chamber 110, to obtain the liquid electrolyte 130 saturated with the gas (e.g. H2).

[0180]The second chamber 120 may have a unidirectional flow element 125 (e.g. valve or pump) configured to allow the liquid electrolyte 130 exiting second chamber 120. The valve or pump 125 may be further in fluid communication with a channel 160 configured for allowing the liquid electrolyte 130 to reenter the first chamber 110 via valve/pump 115. Channel 160 may be in a form of a pipe supporting a unidirectional flow of the liquid electrolyte 130 from the second chamber 120 into the first chamber 110. Channel 160 may be in fluid communication with ammonia separating unit 150 located downstream to unidirectional flow element 125 and upstream to valve/pump 115.

[0181]Ammonia separating unit 150 is configured to separate the liquid electrolyte from ammonia, to obtain a liquid electrolyte having an ammonia weight content below 0.1%, below 0.01%, below 1000 ppm, below 100 ppm, or below 10 ppm. Ammonia separating unit 150 may have a heating element and optionally a condenser. Ammonia separating unit 150 may be further in fluid communication with the ammonia capturing unit and gas separation unit described above.

[0182]FIG. 9B presents an alternative configuration of the apparatus. As presented in FIG. 9B, first gas inlet 116 may be in fluid communication and upstream to anode 101. The apparatus 100 may be configured to generate a gas flow from the first gas inlet 116 via the anode into the liquid electrolyte 130. Anode 101 may be composed of a porous material configured to support gas flow (e.g. metal mesh, metal foam, or a porous carbon material) from. Anode 101 may be configured to support gas flow through at least one dimension (e.g. length, width) thereof.

[0183]FIG. 7 presents a schematic illustration of an apparatus (e.g. electrochemical cell) used for the electrochemical reduction of the nitrogen-based specie to ammonia, wherein the nitrogen-based specie from nitrogen, nitrogen oxide, nitrate, and nitrite including any combination and any salt thereof.

[0184]Apparatus 100 may have a working electrode (e.g., a cathode) 110 and an anode 120. Apparatus 100 may have a chamber or a container 105 configured to contain the liquid electrolyte. Working electrode 110 and anode 120 may be disposed separately from each other in chamber 105.

[0185]In some embodiments, the thickness of the cathode is greater than the thickness of the anode, wherein greater thickness comprises 2 times, 5 times, 10 times, 20 times, 50 times, 100 times greater thickness, including any range between. Increased thickness of the cathode is essential for increasing the electrocatalytic surface in contact with the gas, and thereby improving electrocatalytic performance of the instant apparatus.

[0186]In some embodiments, the thickness of the cathode is between 0.1 mm and 100 cm, between 0.2 mm and 20 cm, between 0.2 mm and 1 m, including any range between.

[0187]In some embodiments, the width or the cross-section dimension of the electrochemical cell, and may vary between several centimeters to several meters.

[0188]In some embodiments, ammonia may be synthesized at the outer surface of the working electrode 110, wherein the outer surface faces the liquid electrolyte.

[0189]Apparatus 100 may have an electrode separation membrane 125. Electrode separation membrane 125 may be disposed in chamber 105. Electrode separation membrane 125 may be disposed in chamber 105 between cathode 110 and anode 120 and may electrically separate cathode 110 and anode 120, e.g., by dividing chamber 105 to do define a cathode zone 112 and an anode zone 114.

[0190]Chamber 105 may have a gas inlet 130 (also referred to as “first gas inlet”). Gas inlet 130 may be located at a lower part of the chamber 105 e.g., in cathode zone 112. Gas inlet 130 may include a pipe of various shapes and sizes, connected to, attached to, or integrally formed with chamber 105. Gas inlet 130 may allow gas (e.g., nitrogen, air, or a mixture of oxygen and nitrogen) to enter chamber 105. Optionally, the gas may be humidified with water vapor.

[0191]Apparatus 100 may be connected to a gas supply unit 150 allowing to supply gas via gas inlet 130. Gas may enter gas supply unit 150, and, optionally, the gas may be humidified in gas supply unit 150.

[0192]Apparatus 100 may have an ammonia outlet 160. Ammonia outlet 160 may be located in chamber 105, e.g., in cathode zone 112. Ammonia outlet 160 may allow ammonia exit chamber 105. Ammonia outlet 160 may further allow hydrogen gas generated to exit chamber 105. Ammonia outlet 160 may further allow other gasses involved in the ammonia synthesis, e.g., nitrogen, air or water, to exit chamber 105.

[0193]Apparatus 100 may have an ammonia trap 170. Ammonia trap 170 may be in the form of a container configure to contain an acid (e.g., sulfuric acid). Ammonia trap 170 may have an inlet 180, allowing a gas (e.g., ammonia and other gasses) exiting ammonia outlet 160 to enter ammonia trap 170. Ammonia trap 170 may have a first outlet 190 allowing ammonia exit therefrom. First outlet 190 may be located at a lower part of ammonia trap 170. First outlet 190 may include a valve allowing to control the rate of ammonia flow exiting outlet 190. Ammonia trap 170 may have a second outlet 200 allowing gasses (e.g., nitrogen and hydrogen) to exit therefrom. Outlet 200 may be located at an upper part of ammonia trap 170.

[0194]Chamber 105 may have another gas inlet 210 (also referred to as “second gas inlet”). Inlet 210 may be located at a lower part of the chamber 105 e.g., in anode zone 114. Gas inlet 210 may include a pipe of various shapes and sizes, connected to, attached to, or integrally formed with chamber 105. Gas inlet 210 may further allow gasses (e.g. nitrogen and hydrogen) exiting second outlet 200 to reenter, or recirculate to, chamber 105.

[0195]Apparatus 100 may have a gas outlet 220. Gas outlet 220 may be located in chamber 105, e.g., in anode zone 114. Gas outlet 220 may allow gasses (e.g., gasses involved in the ammonia synthesis, such as nitrogen or water) exit chamber 105. Gas outlet 220 may include a pipe of various shapes and sizes, connected to, attached to, or integrally formed with chamber 105. Optionally, gasses exiting chamber 105 via gas outlet 220, may be allowed to reenter, or recirculate, to chamber 105 via gas inlet 130.

[0196]Apparatus 100 may have various components of the apparatus disclosed herein, such as any of the valves, sensors, weirs blowers, fans, dampers, or pumps, etc.

[0197]In some embodiments apparatus 100 is an electrochemical cell and is configured to synthesize ammonia at a rate (in mol cm⁻² s⁻¹) of 1×10⁻⁸, at least 5×10⁻⁸, at least 1×10⁻⁹, 2×10⁻⁹, at least 3×10⁻⁹, 4×10⁻⁹, at least 5×10⁻⁹, including any value and range therebetween, e.g., at a pressure of 1 atm. According to an aspect of some embodiments of the present invention there is provided an electrolysis cell having an electrocatalyst comprising the disclosed composition in an embodiment thereof. In some embodiments, the electrocatalyst is the cathode.

[0198]The term “electrochemical cell” or “cell” as used herein refers generally to a device that converts chemical energy into electrical energy, or electrical energy into chemical energy. Generally, electrochemical cells have two or more electrodes and an electrolyte, wherein electrode reactions occurring at the electrode surfaces result in charge transfer processes. Examples of electrochemical cells include, but are not limited to, batteries and electrolysis systems.

[0199]In some embodiments, the electrochemical cell is configured to synthesize ammonia at a rate of 1×10⁻¹² mol s⁻¹cm⁻² to 1×10⁻⁶ mol s⁻¹ cm⁻² at 1 atm N₂.

[0200]In some embodiments, the electrochemical cell is configured to synthesize ammonia (either from an aqueous nitrite or nitrate electrolyte solution, or from a liquid electrolyte comprising nitrogen) at a rate of 1×10⁻¹² mol s⁻¹cm⁻² to 1×10⁻⁶ mol s⁻¹cm⁻², 5×10⁻¹² mol s⁻¹cm⁻² to 1×10⁻⁷ mol s⁻¹cm⁻², 10×10⁻¹¹ mol s⁻¹cm⁻² to 1×10⁻⁷ mol s⁻¹cm⁻², 1×10⁻¹⁰ mol s⁻¹cm⁻² to 1×10⁻⁷ mol s⁻¹cm⁻², 10×10⁻¹⁰ mol s⁻¹cm⁻² to 1×10⁻⁷ mol s⁻¹cm⁻², 1×10⁻¹¹ mol s⁻¹cm⁻² to 1×10⁻⁸ mol s⁻¹cm⁻², or 1×10⁻¹⁰ mol s⁻¹cm⁻² to 1×10⁻⁸ mol s⁻¹cm⁻², at least 6×10⁻¹¹ mol s⁻¹cm⁻², at least 6.5×10⁻¹¹ mol s⁻¹cm⁻², including any range therebetween.

[0201]In some embodiments, electrochemical cell is configured to synthesize ammonia a faradaic efficiency at a cathodic electric potential in the range of 4% to 50%, 1% to 25%, 1% to 20%, 1% to 15%, 1% to 10%, 3% to 30%, 5% to 30%, 5% to 25%, 5% to 20%, 5% to 15%, or 5% to 10%, including any range therebetween.

[0202]In some embodiments, the faradaic efficiency of the electrochemical cell is at least 4%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 30% or at least 40%, including any range between wherein the electric potential is between 0.3 and −0.5 V with respect to standard hydrogen reference electrode (SHE).

[0203]In some embodiments, the faradaic efficiency of the electrochemical cell is at least 4%, at least 5%, at least 10%, at least 30%, wherein the electric potential is between about −0.2 and about −0.3 V with respect to standard hydrogen reference electrode (SHE).

[0204]The apparatus 100 may be used for synthesizing hydrogen.

[0205]The apparatus 100 may be used for synthesizing ammonia by NRR using the liquid electrolyte disclosed herein. In some embodiments, the liquid electrolyte for NRR further comprises (i) one or more peroxide species (e.g. H₂O₂) and (ii) a nitrate anion or a salt thereof dissolved therewithin.

[0206]In some embodiments, a concentration of the one or more peroxide species within the liquid electrolyte is between 1 and 1000 uM, between 50 and 1000 uM, between 10 and 200 uM, between 50 and 200 uM, between 50 and 500 uM, including any range between.

[0207]In some embodiments, a concentration of nitrate within the liquid electrolyte is in the range 0.0001M to 0.1M, of 0.01M to 2M, of 0.001M to 5M, of 0.001M to 0.1M, of 0.05M to 1M, of 0.1M to 1M, of 0.05M to 5M, of about 0.1M to about 1M, including any range between.

[0208]In some embodiments, the liquid electrolyte for NRR is further saturated with In some embodiments, the liquid electrolyte comprises a gas dissolved therewithin, wherein the gas comprises nitrogen, or a combination of nitrogen and oxygen (e.g. oxygen content of between 5 and 30%, as disclosed above) and optionally one or more additional gas(es). In some embodiments, the gas is air.

[0209]In some embodiments, the apparatus 100 is devoid of gas inlet 130 and/or devoid of gas supply unit 150. In some embodiments, the apparatus 100 is devoid of gas inlet 130 and/or devoid of gas supply unit 150, and the liquid electrolyte is an alkaline electrolyte further comprising a nitrate salt or a nitrite salt dissolved therewithin and is substantially devoid of nitrogen.

[0210]In some embodiments, the working electrode 110 (e.g., cathode) is the electrode of the invention. In some embodiments, the working electrode 110 (e.g., cathode) is selected from iron oxide-TiO₂ composite, or a noble metal composite comprising at least two noble metals.

[0211]In some embodiments, the noble metal composite comprises a first noble metal and a second noble metal, wherein a molar ratio of the first noble metal to the second noble metal is in the range of 1:9 to 9:1, 2:9 to 9:1, 3:9 to 9:1, 4:9 to 9:1, 5:9 to 9:1, 6:9 to 9:1, 7:9 to 9:1, 8:9 to 9:1, 9:9 to 9:1, 1:9 to 9:2, 1:9 to 9:3, 1:9 to 9:4, 1:9 to 9:5, 1:9 to 9:6, 1:9 to 9:7, 1:9 to 9:8, or 1:9 to 9:9, including any range therebetween.

[0212]In some embodiments, the noble metal composite is a layered material comprising a first layer in contact with a second layer, wherein the first layer comprises or consists essentially of first noble metal, and wherein the second layer comprises or consists essentially of second noble metal. In some embodiments, the noble metal composite is an alloy.

[0213]In some embodiments, the noble metal composite is in a form of a coating, wherein the coating is as described herein. In some embodiments, the noble metal composite is in a form of particles the particles have a size in the range of 1 nm to 50 μm, 3 nm to 50 μm, 5 nm to 50 μm, 10 nm to 50 μm, 25 nm to 50 μm, 50 nm to 50 μm, 100 nm to 50 μm, 250 nm to 50 μm, 500 nm to 50 μm, 1 nm to 900 nm, 1 nm to 800 nm, 1 nm to 500 nm, 1 nm to 250 nm, or 1 nm to 100 nm, including any range therebetween.

[0214]In some embodiments, the noble metal comprises Ru, Rh, Pd, Ag, Re, Ir, Pt, and Au, or any combination thereof. In some embodiments of the present invention, noble metal comprises Ruthenium (Ru) and Platinum (Pt), wherein the molar ratio of the Ru to the Pt is in the range of 1:10 to 10:1, respectively.

[0215]In some embodiments, the Ru: Pt molar ratio within the noble metal composite is 10:1, 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10, respectively, including any value and range therebetween.

[0216]In exemplary embodiments, the noble metal composite consists essentially of Ru and Pt, wherein the molar ratio of the Ru to the Pt is about 1:1 (±20%).

[0217]Non-limiting exemplary anode 120 comprise nickel (Ni), iron, zinc, cobalt, chromium, titanium, or any oxide or a combination thereof.

[0218]In some embodiments, apparatus 100 is configured to synthesize ammonia. In some embodiments, apparatus 100 is configured to synthesize ammonia from the liquid electrolyte comprising a nitrate salt or a nitrite salt dissolved therewithin. In some embodiments, apparatus 100 is configured to synthesize ammonia from the supersaturated liquid electrolyte comprising a nitrate salt or a nitrite salt dissolved therewithin by reducing the nitrate salt or a nitrite salt on the working electrode 110 (e.g., cathode); wherein the working electrode 110 comprises an electrocatalyst selected from (i) the electrocatalyst of the invention, (ii) iron oxide-TiO₂ composite, or (iii) the noble metal composite, as described hereinabove. In some embodiments, apparatus 100 is configured to synthesize ammonia by applying a voltage to the working electrode.

[0219]In some embodiments, the voltage is between 0.5 and 0, between 0.5 and −2V, between 0.4 and −2V, between 0.3 and −2V, between 0.1 and −2V, between 0.3 and −0.5V, including any range between. In some embodiments, the voltage is a negative voltage.

[0220]In some embodiments, the negative voltage is at least −0.05V, at least −0.1V, at least −0.15V, at least −0.2V, between −0.05 and −2V, between −0.1 and −2V, between −0.1 and −1V, between −0.05 and −0.5V, between −0.1 and −0.5V, between −0.1 and −0.7V, including any range therebetween with respect to standard hydrogen reference electrode (SHE).

[0221]In some embodiments, working electrode 110 is an activated electrode, wherein the activated electrode is as described hereinabove. In some embodiments, working electrode 110 undergoes activation before each operation cycle. In some embodiments, working electrode 110 is activated by contacting thereof with an oxidizing solution comprising an effective amount of an oxidizer (e.g. between 100 ppm and 20% w/w) such as a hypochlorite, a peroxide (such as H₂O₂), a peracid, or a precursor thereof. In some embodiments, the oxidizing solution comprises a ROS (e.g. singlet oxygen, superoxide, hydroxyl radical, etc.). In some embodiments, working electrode 110 is activated by exposing thereof to oxygen, or air and by applying to the working electrode a potential above 0.5V, above 0.7V, above 0.8V, above 0.9V, above 1V, between 0.5 and 2V, between 0.7 and 2V, between 0.8 and 2V, between 1 and 2V, including any range between.

[0222]In some embodiments, working electrode 110 further comprises the co-catalyst. In some embodiments, working electrode 110 is activated by activating the co-catalyst, as disclosed herein.

[0223]The dimensions of each component of the apparatus are selected to be sufficient, for a given desired fluidization and to provide sufficient contact time to provide e.g., a desired level of water/nitrogen consumption and/or ammonia regeneration.

[0224]Conditions may be monitored using any suitable type monitoring devices e.g., a computer-implemented system. Variables that may be tracked include, without limitation, pH, temperature, electric potential, conductivity, turbidity, rate of the gas flow in each inlet or outlet, concentration of the alkaline solution. These variables may be recorded throughout apparatus 100.

[0225]A monitoring device, a control unit, or a controller (e.g., computer) may also be used to monitor, control and/or automate the operation of the various components of the apparatus disclosed herein, such as any of the valves, sensors, weirs, blowers, fans, dampers, pumps, etc.

[0226]The present apparatus of the invention may be a system, a method, and/or a computer program product. The computer program product may comprise a computer-readable storage medium. The computer-readable storage medium may have program code embodied therewith. The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an crasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

[0227]Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.

[0228]Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention.

Ammonia Synthesis

[0229]In some embodiments, ammonia may be synthesized by using the electrolytic cell disclosed herein.

[0230]In one aspect, there is provided a process of synthesizing ammonia from a gas comprising nitrogen, the process comprising: (i) providing a liquid electrolyte (e.g. the aqueous electrolyte disclosed herein) to the apparatus of the invention (e.g. electrochemical cell as depicted in FIGS. 9A-B) comprising the anode and the cathode; (ii) generating a flow of the gas into the electrolyte in contact with the anode; and (iii) generating a positive electric potential at the apparatus; thereby obtaining the ammonia; wherein the anode is has the NOR activity and the cathode has the NO3RR activity, as disclosed herein.

[0235]In some embodiments, the flow of the gas is generated by feeding a pressurized gas via the first gas inlet, or by using a pump (e.g. as the first gas inlet 116) in fluid communication with a container comprising the gas and configured to feed the gas from the container into the first chamber. In some embodiments, the method comprises generating a flow of the gas into the first chamber to saturate the liquid electrolyte in the first chamber with the gas. In some embodiments, the flow of the gas is directed into the volume of the liquid electrolyte (e.g. via a pipe entering the liquid electrolyte volume).

[0236]In some embodiments, the flow of the gas is generated via the first gas inlet in fluid communication with the anode, wherein the anode comprises a porous material configured to support gas flow therethrough.

[0237]In some embodiments, the liquid flow is generated via the unidirectional flow element 145 (e.g. a liquid pump). In some embodiments, the gas further comprises between 5 and 30%, between 10 and 30%, between 10 and 25% v/v of oxygen, including any range between. In some embodiments, the gas is or comprises an oxygen nitrogen mixture, wherein the oxygen content of the gas is as described hereinabove. In some embodiments, the gas is air.

[0238]In some embodiments, the electrical current is generated by the power source. In some embodiments, the electrical current is a direct current. In some embodiments, the apparatus is operated at positive electric potential. In some embodiments, the positive electric potential is between land 3V, between 1.5 and 3V, between 1.5 and 2.5V, between 1.7 and 3V, between 1.7 and 2.5V, between 1.7 and 2.3V, between 1.7 and 2V, including any range between.

[0239]In some embodiments, the liquid flow is at a flow rate of between 0.01 and 5 ml/min, between 0.01 and 1 ml/min, between 0.01 and 0.5 ml/min, between 0.1 and 1 ml/min, between 0.1 and 0.5 ml/min, including any range between.

[0240]In some embodiments, the gas flow is performed at a flow rate between 1 ml/min and 1000 L/min, between 1 ml/min and 100 L/min, between 0.1 L/min and 10 L/min, including any range between.

[0241]In some embodiments, the step of generating a flow of the gas and the step of generating a liquid flow are performed simultaneously or subsequently (e.g. gas flow is generated before the liquid flow). In some embodiments, the step of generating a flow of the gas is performed simultaneously with the step of applying the electric current (and prior to the step of liquid flow generation), wherein the gas flow is maintained for a time sufficient for generating a predetermined nitrate concentration. In some embodiments, the predetermined nitrate concentration is at least 300, at least 400, at least 500 uM, or between 0.01 and 2M nitrate, including any range between.

[0242]In some embodiments, the step of applying the electric current and the step of generating the liquid flow are performed simultaneously or subsequently. In some embodiments, the step of applying the electric current is performed prior to the step of generating the liquid flow.

[0243]In some embodiments, the liquid electrolyte further comprises an ion selected from hydroxide, halide (e.g. chloride), sulfate, nitrite and nitrate, including any combination thereof. In some embodiments, the liquid electrolyte is an aqueous electrolyte comprising between 0.01 and 2M of at least one ion selected from hydroxide and sulfate, and optionally further comprises any of halide (e.g. chloride), nitrite and nitrate.

[0244]In another aspect, there is provided a process of synthesizing ammonia from nitrate or nitrite via NO3RR, the process comprises: (i) providing a liquid electrolyte (e.g. aqueous electrolyte) comprising a nitrate salt or a nitrite salt to the electrochemical cell of the invention comprising the working electrode (having NO3RR activity) and the counter-electrode, as disclosed herein; and (ii) applying a electric potential to the electrochemical cell of under conditions suitable for generating ammonia; thereby obtaining the ammonia.

[0245]In some embodiments, there is provided a process of synthesizing ammonia, the process comprising: (i) providing the liquid electrolyte to the electrochemical cell, wherein a working electrode comprises the electrocatalyst of the invention (comprising the transition metal oxide which is not iron oxide and/or TiO₂), and simultaneously or subsequently contacting the liquid electrolyte with a gas comprising or consisting essentially of air, nitrogen, or a mixture of oxygen and nitrogen; and (ii) applying an electric potential to the electrochemical cell under conditions suitable for generating ammonia; thereby obtaining the ammonia.

[0246]In some embodiments, there is provided a process of synthesizing ammonia, the process comprising: (i) providing the supersaturated alkaline electrolyte solution to the electrochemical cell of the invention comprising the cathode as the working electrode and simultaneously or subsequently contacting the supersaturated alkaline electrolyte with a gas comprising or consisting essentially of air, nitrogen, or a mixture of oxygen and nitrogen; and (ii) applying a cathodic electric potential to the electrochemical cell under conditions suitable for generating ammonia; thereby obtaining the ammonia.

[0247]In some embodiments, the electric potential comprises a negative cathodic potential of at least −0.05V, at least −0.1V, or between −0.05 and −0.6V, between −0.05 and −0.5V, between −0.05 and −0.4V relative to RHE, including any range therebetween. In some embodiments, the synthesis of the ammonia is performed at low electric potential. In some embodiments, the low electric potential is required to avoid hydrogen evolution competing reaction.

[0248]In some embodiments, the electric potential comprises a positive cathodic potential of at least 0 V, at least 0.05V, or between 0.05 and 0.3V, between 0.05 and 0.2V, or about 0.1V with respect to RHE, including any range therebetween.

[0249]In some embodiments, the electric potential comprises a positive anodic potential (e.g. for NOR using Rh/C as the electrocatalyst) of at least 1 V, at least 1.5V, or between about 1.5 and about 2V, between about 1.5 and about 1.9V, between about 1.5 and about 1.8V, between about 1.5 and about 1.7V, between 1.6 and 1.7V, or about 1.7V with respect to RHE, including any range therebetween.

[0250]The ammonia is synthesized by using the liquid electrolyte as described above. In some embodiments, the synthesis of the ammonia from the gas comprising or consisting essentially of air, nitrogen, a mixture of oxygen and nitrogen, or a mixture of nitrogen and hydrogen and optionally oxygen, and is performed by (i) activating the working electrode, as disclosed hereinbelow thereby generating a nitrogen oxide (e.g. comprising nitrite, and/or nitrate) in the liquid electrolyte (e.g. supersaturated alkaline electrolyte); and (ii) subsequently applying a negative electric potential to the working electrode thereby reducing the nitrogen oxide to generate ammonia.

[0251]In some embodiments, the synthesis of the ammonia from the gas comprising or consisting essentially of air, nitrogen, or a mixture of oxygen and nitrogen is performed by (i) applying a positive electric potential to the working electrode thereby generating a nitrogen oxide (e.g. comprising nitrite, and/or nitrate); and (ii) subsequently applying a negative electric potential to the working electrode thereby reducing the nitrogen oxide to generate ammonia.

[0252]In some embodiments, step (i) is performed under conditions (e.g. normal pressure, temperature as disclosed herein, etc.) and for a time period sufficient for converting at least 5%, at least 10%, at least 20%, at least 50%, at least 70%, at least 80%, at least 90%, at least 95% of the initial nitrogen content of the electrochemical cell into nitrogen oxide, including any range between. In some embodiments, the positive electric potential of step (i) is at least 0.5V, at least 1V, at least 2V, between 0.5 and 5V, between 1 and 5V, between 1 and 2V, between 0.5 and 3V, between 1 and 3V with respect to RHE, including any range therebetween.

[0253]In some embodiments, step (ii) is performed under conditions (e.g. normal pressure, temperature as disclosed herein, etc.) and for a time period sufficient for converting at least 20%, at least 50%, at least 70%, at least 80%, at least 90%, at least 95% of the nitrogen oxide generated in step (i), including any range between. In some embodiments, the negative electric potential of step (ii) is as described hereinabove.

[0254]In some embodiments, the synthesis of the ammonia is performed at a temperature of from 5° C. to 150° C., 5 to 80° C., 5° C. to 10° C., 5° C. to 20° C., 5° C. to 30° C., 30° C. to 70° C. 30° C. to 65° C., or 30° C. to 150° C., including any range therebetween.

[0255]In some embodiments, the synthesis of the ammonia is performed at a temperature of from 10 to 30° C. In some embodiments, the synthesis of the ammonia is performed at a temperature of from 10 to 50° C.

[0256]In some embodiments, the synthesis of the ammonia is performed at a pressure of 500 to 2000 mm Hg. In some embodiments, the synthesis is performed at a pressure of 500 to 1000 mm Hg.

[0257]In some embodiments, the synthesis of the ammonia is performed at an ambient temperature. In some embodiments, the synthesis of the ammonia is performed at an ambient pressure. In some embodiments, the synthesis of the ammonia is performed at an ambient pressure and at an ambient temperature.

[0258]In some embodiments, the term “ambient pressure” is intended to mean approximately 740 mm Hg to about 780 mm Hg.

[0259]In some embodiments, the method further comprises a preliminary step of activating the working electrode, wherein the preliminary step is performed prior to performing step i. In some embodiments, the preliminary step is performed prior to performing step ii. In some embodiments, the preliminary step is performed under conditions suitable for obtaining the activated electrode. The presence of radical species at the surface of the activated electrode can be determined as disclose hereinabove.

[0260]In some embodiments, preliminary step of activating the working electrode comprises contacting thereof with an oxidizing solution comprising an effective amount of an oxidizer (e.g. between 100 ppm and 20% w/w) such as a hypochlorite, a peroxide (such as H₂O₂), a peracid, or a precursor thereof. In some embodiments, the oxidizing solution comprises a ROS (e.g. singlet oxygen, superoxide, hydroxyl radical, etc.), or an agent configured to generate ROS.

[0261]In some embodiments, preliminary step of activating the working electrode comprises exposing the working electrode to oxygen, or a gas mixture comprising oxygen (e.g. air) applying to the working electrode a potential above 0.5 V, above 0.7 V, above 0.8 V, above 0.9 V, above 1 V, between 0.5 and 2 V, between 0.7 and 2 V, between 0.8 and 2 V, between 1 and 2 V, including any range between. In some embodiments, applying is for a time period between 1 s and 2 h, including any range between. In some embodiments, applying is at a temperature between 5 and 100° C. including any range between. In some embodiments, exposing the working electrode to oxygen is performed by introducing a gas comprising oxygen (e.g. air, or pure oxygen) into the liquid electrolyte.

[0262]In some embodiments, the preliminary step and the subsequent steps i-ii are performed sequentially, consecutively or concurrently. In some embodiments, the preliminary step and the subsequent steps i-ii are performed sequentially, or consecutively forming an operation cycle. In some embodiments, the process of the invention comprises concurrently performing a plurality of operation cycles.

[0263]The dimensions of each component of the apparatus are selected to be sufficient, for a given desired fluidization and to provide sufficient contact time to provide e.g., a desired level of nitrate or nitrite/nitrogen consumption and/or ammonia generation. Ammonia generation can be determined by measuring the volume of gas emerging the apparatus of the invention, or alternatively via colorimetric UV-Vis spectroscopy. The concentration of nitrate or nitrite within the liquid electrolyte can be determined spectroscopically (e.g. by measuring UV absorption of the aqueous electrolyte solution).

[0264]In some embodiments, the synthesis of ammonia is characterized by a faradaic efficiency at a cathodic (e.g. negative) electric potential in the range of 4% to 50%, 1% to 25%, 1% to 20%, 1% to 15%, 1% to 10%, 3% to 30%, 5% to 30%, 5% to 25%, 5% to 20%, 5% to 15%, or 5% to 10%, including any range therebetween.

[0265]In some embodiments, the synthesis of the ammonia is characterized by a faradaic efficiency of at least 4%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 30% or at least 40%, including any range between wherein the electric potential is between −0.1 and −0.5 V.

[0266]In some embodiments, the synthesis of the ammonia is characterized by a faradaic efficiency of at least 4%, at least 5%, at least 10%, at least 30%, wherein the electric potential is between about −0.2 and about −0.3 V.

[0267]In some embodiments, the synthesis of the ammonia is characterized by a rate of ammonia production is in the range of 1×10⁻¹¹ mol s⁻¹cm −² to 1×10⁻⁷ mol s⁻¹cm⁻², 5×10⁻¹¹ mol s⁻¹cm⁻² to 1×10⁻⁷ mol s⁻¹cm⁻², 10×10⁻¹¹ mol s⁻¹cm⁻² to 1×10⁻⁷ mol s⁻¹cm⁻², 1×10⁻¹⁰ mol s⁻¹cm⁻² to 1×10⁻⁷ mol s⁻¹cm⁻², 10×10⁻¹⁰ mol s⁻¹cm⁻² to 1×10⁻⁷ mol s⁻¹cm⁻², 1×10⁻¹¹ mol s⁻¹cm⁻² to 1×10⁻⁸ mol s⁻¹cm⁻², or 1×10⁻¹⁰ mol s⁻¹cm⁻² to 1×10⁻⁸ mol s⁻¹cm⁻², including any range therebetween.

General

[0268]The terms “comprises”, “comprising”, “includes”, “including”, “having” and their conjugates mean “including but not limited to”. The term “consisting of means “including and limited to”. The term “consisting essentially of” means that the composition, method or structure may include additional ingredients, steps and/or parts, but only if the additional ingredients, steps and/or parts do not materially alter the basic and novel characteristics of the claimed composition, method or structure.

[0269]The word “exemplary” is used herein to mean “serving as an example, instance or illustration”. Any embodiment described as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments and/or to exclude the incorporation of features from other embodiments.

[0270]The word “optionally” is used herein to mean “is provided in some embodiments and not provided in other embodiments”. Any particular embodiment of the invention may include a plurality of “optional” features unless such features conflict.

[0271]As used herein, the singular form “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a compound” or “at least one compound” may include a plurality of compounds, including mixtures thereof.

[0272]Throughout this application, various embodiments of this invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0273]Whenever a numerical range is indicated herein, it is meant to include any cited numeral (fractional or integral) within the indicated range. The phrases “ranging/ranges between” a first indicate number and a second indicate number and “ranging/ranges from” a first indicate number “to” a second indicate number are used herein interchangeably and are meant to include the first and second indicated numbers and all the fractional and integral numerals therebetween.

[0274]As used herein the term “method” refers to manners, means, techniques and procedures for accomplishing a given task including, but not limited to, those manners, means, techniques and procedures either known to, or readily developed from known manners, means, techniques and procedures by practitioners of the chemical, and electrochemical arts.

[0275]In those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.).

[0276]It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

[0277]It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

EXAMPLES

[0278]Reference is now made to the following examples, which together with the above descriptions illustrate some embodiments of the invention in a non-limiting fashion.

Material Characterizations

[0279]Powder X-ray diffraction (XRD) analysis of the samples was performed using a Rigaku Smart Lab SE diffractometer. For the Fourier-transform infrared (FT-IR) spectra of the synthesized catalysts, the KBr pellet method was employed, utilizing a Bruker vertex70 instrument. The Raman spectroscopic analysis was conducted using a Kratos AXIS-HS spectrometer with a 532 nm laser and a monochromatized Al Kα source, as well as the XploRA ONE™ micro-Raman system from Horiba Scientific, Japan. To determine the iron content in the electrolyte, Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) was carried out using a Spectro Arcos optical emission spectrometer. For X-ray Photoelectron Spectroscopy (XPS) analysis, which provides insight into the electronic/chemical states and elemental composition of the catalyst, a Nexsa spectrometer equipped with a monochromatic Al Kα X-ray source was used. The JEOL JSM-35 CF model scanning electron microscope (SEM) was utilized for the morphological analysis of the materials. Electrochemical measurements

Nitrate/Nitrite Quantification

[0280]The nitrate/nitrite concentration in the electrolyte was determined using a UV-Visible spectrophotometer. The absorbance at 300 and 350 nm was measured for nitrate (NO₃⁻) and nitrite (NO₂⁻) respectively. A calibration curve, based on known nitrate/nitrite concentrations in 0.1 M KOH solution was generated. The rate of nitrate formation and its Faradaic efficiency were calculated using Equations 3 and 4.

$\begin{array}{cc}N{O}_{3^{-}}\mathrm{or}{\mathrm{NO}}_{2^{-}}\mathrm{formation}\mathrm{rate}=\left(C\times V\right)/\left(t\times A\right)& \left(3\right)\end{array}$$\begin{array}{cc}\mathrm{FE}\%=\left(3\times F\times C\times V\right)/\left(Q\times MW\right)& \left(4\right)\end{array}$

[0281]C is Concentration NO₃⁻ or NO₂⁻ in μM; V is sample volume in mL; t is electrolysis time in hour; A is electrode active area in cm²; Q is applied charge in coulomb; MW is molecular weight of NO₃⁻ (62.004 g mol⁻¹) or NO₂⁻ (46.005 g mol−¹); and F is the Faraday constant 96485 C mol⁻¹.

Ammonia Quantification

[0282]Concentration of ammonia in the samples was determined using a UV-Visible spectrophotometer, employing the indophenol blue method. Initially, approximately 200 μL of cach sample was diluted to a final volume of 2.0 mL. This dilution was performed using a 0.1 M potassium hydroxide (KOH) solution, which contained 5 wt. % salicylic acid and 5 wt. % sodium citrate dissolved in a 1 M sodium hydroxide (NaOH) solution. In the subsequent step, 1 mL of 0.05 M sodium hypochlorite (NaOCl) followed by 0.2 mL of 1 wt. % sodium nitroprusside (Na₂[Fe(CN)₅NO]) were added to the mixture. The reaction mixture was then stored in a dark place for 1 hour to ensure the completion of the reaction before measuring the absorbance at 655 nm. For calibration purposes, a series of ammonia standards with known concentrations were prepared in 0.1 M KOH solution. These standards were subjected to the same procedure to construct a calibration curve. The ammonia formation rate and Faradaic efficiency (FE) were calculated using equations 3 and 4, respectively. These calculations incorporated the molecular weight of ammonia, which is 17.031 g/mol.

Example 1

[0283]The present invention in some embodiments thereof is directed to electrocatalytic nitrogen reduction to ammonia under ambient reaction conditions with the detailed reaction mechanism via nitrate or NOx formation. Further feature of the present invention is to provide the suitable electrochemical cell setup preferably with high efficiency and low cost. This invention comprises an electrochemical cell comprised of an electrochemical cell, a water based electrolyte solutions with different ionic strengths (free water molecules), and specific catalysts on which the NRR proceeds at low overpotentials. The same system is constructed for the production of NOxs followed by the formation of ammonia. This provides the ability to store the NOx's and produce the ammonia on demand. The catalysts such as platinum-ruthenium alloy (PtRu), Ruthenium (Ru), Palladium (Pd) and Iron (Fe) based oxides (RuOx, PdOx and FeOx) and the water based electrolytes with less number of free water molecules thus facilitates the N₂ solubility with limited HER activity is employed in the present invention. The preliminary electrochemical tests were carried out in a standard three-electrode cell comprising the catalyst coated carbon paper as working electrode, metallic/graphitic counter electrode and a suitable reference electrode. Water-based electrolytes such as 0.1M KOH were used for the preliminary studies and high pure Nitrogen gas was purged during the reaction. The cell's outlet was connected to an acid-containing trap to collect the formed ammonia during the eNRR.

Materials and Methods

[0284]The required amount of a commercial PdO or RuO₂ was ultrasonically dispersed (30 minutes) in isopropyl alcohol (IPA) and water mixture (1:1 v/v) with the addition of 15 wt. % Nafion ionomer to prepare the catalyst ink. The required amount of prepared catalyst ink was drop-casted on teflonized Toray carbon electrode support followed by drying to fabricate the working electrode with a catalyst loading of 4 mg/cm².

[0285]Electrochemical technique such as linear sweep voltammetry and chronoamperometry were employed to optimize the potential effect and to achieve maximum ammonia production and NOx formation for each catalyst. NO₃RR activity of these catalysts with optimization of applied potential also studied in detail by directly using the commercially available NOx sources such as nitrate salts employing the same electrochemical setup described above. From the commercial aspects, the results were also verified by carrying out the electrochemical test in a three-electrode proto-type electrochemical reactor comprising an anode and the cathode compartment separated by a suitable ion-conducting membrane with a suitable reference electrode. The formation of the NOx and ammonia was verified and quantified by UV-visible spectrophotometry analysis. Nitrate from the electrolyte was directly analyzed by UV and the absorption value at 3000 nm was considered for the NOx quantification. Ammonia from both the electrolyte and trap was analyzed by salicylate method and cumulative value reported as the total ammonia production.

[0286]The NRR activity is verified by lincar sweep voltammetry (LSV) analysis. The detailed electrochemical study of NH₃ generation was evaluated by the combination of LSV and chrono-amperometry (CA) electrochemical techniques with the commercial RuPt-black electrocatalyst. The currents measured in N₂-saturated electrolytes were higher than that in Ar-saturated electrolytes due to the contribution of the NRR. Hence, the NRR performance was assessed in that region of −0.045 V to −0.85V. The chrono-amperometry measurement were conducted for 1 hour at various potential (−0.045, −0.25, −0.45, −0.65 and −0.85V vs NHE). In CA it was observed that the current increased with external potential. The extent of NH₃ production and Faradaic efficiency (FE) was assessed by UV-vis absorption spectroscopy.

[0287]The rate of NH₃ formation was increased with an increase in negative potential. The maximum NH3 production achieved was 3.4 μgh⁻¹ cm⁻² at −0.25V vs. NHE above, which the rate is decreased due to the competitive formation of hydrogen evolution reaction (HER). The FE at −0.25 V is 4.3. %

[0288]In the next stage, the electrode was oxidized at 0.65V and then verified the NRR activity at different potentials i.e.; −0.045, −0.25, −0.45V vs NHE. After the electrode oxidation, the NH₃ formation and FE rate were slightly increased. The rate of NH₃ formation was increased to 4.2 μgh⁻¹ cm⁻² at −0.25V vs. NHE with a FE 5.7. %

[0289]The inventors further demonstrated that it is possible to significantly increase ammonia production rate using the electrode with 4-5 mg/cm² electrocatalyst loading by: (i) first generating NOx species in-situ by applying a positive electrode potential (about +1.8 V vs. RHE for one hour) and (ii) subsequently applying a negative electrode potential of only −0.1 V vs. RHE to reduce NOx species to ammonia. The reduction rate is of 2^nd step was about 147 ug/h/cm², which is more than ten times faster than the reaction rate of less than 10 ug/h/cm² obtained by direct reduction of N₂ to NH₃. The observed reduction rate is of 1^st step was about 50 ug/h/cm².

[0290]NO₃RR activity of the RuOx is evaluated in Ar saturated 0.1M KOH containing 0.1M NaNO₃ at wider potential window in a standard three-electrode cell configuration. Chronoamperometry (CA) experiment was performed at wider potential window for 30 min and the ammonia formation after CA at each potential was quantified by salicylate method. RuOx electrocatalyst delivered a maximum ammonia production rate of 1680 μgh⁻¹ cm⁻² with FE of 35% at −0.35V vs RHE.

[0291]The formation of NOx during NRR was evidenced with both the RuOx and PdOx. After the NRR activity evaluation of the RuOx and PdOx in N₂ saturated 0.1M KOH electrolyte, the sample was collected from the electrolyte and analyzed for the formation of NOx. Each of RuOx and PdOx leads to the formation of NOx as supported by a quite high absorbance value for the electrolyte sample than that of the blank 0.1M KOH. This interesting finding add the new insights to the NRR mechanism and facile electrochemical ammonia synthesis with less energy investment.

Example 2

[0292]10 mg of RuO₂ was added to 12 mL of 0.1M KOH followed by N₂ purging for 3 h. The resultant nitrate-containing 0.1M KOH electrolyte was used for ammonia synthesis. This experiment was carried out under Ar atmosphere to avoid contribution from NRR. Catalyst loading is 4.4 mg cm⁻².

NO 3 RR Activity of RuO 2 in 0.1M Na 2 SO 4

[0293]The effect of electrolysis time for NO₃RR (also termed NitRR) was studied with the aim of limiting the energy required for the reaction. CA experiments were performed in 0.1 M KOH containing the 0.1 M NaNO₃ at −0.25V vs. RHE employing the RuO₂ catalyst. The optimum electrolysis time is 20 min above which no effect on Faradaic efficiency of ammonia formation is observed. After 30 min of electrolysis, the remaining nitrate concentration was calculated to be 24.4 mM. NH₃ production rate (μgh⁻¹ cm⁻²) of about 1500-4650 and FE of 10-32% were observed.

[0294]The NOx formation during NRR was also observed in RuO₂. This interesting finding clearly demonstrates the formation of oxygenated nitrogen species (nitrate) by applying a very small over potential (50 mV).

[0295]RuO₂ NO₃RR activity in 0.1M Na₂SO₄ containing 0.1 M NaNO₃ is described in details in Example 7.

Electrochemical NOR Employing RuO 2 Catalyst

[0296]This experiment was performed with a catalyst loading of 3.7 mg cm⁻² in 0.1 M KOH. Reported nitrate concentration is after CA experiment for 30 min at each potential.

Nitrate (Chemical) Formation Under Ambient Conditions Employing RuO 2 Catalyst

[0297]5 mg of RuO₂ was dispersed in a known volume of 0.1 M KOH in separate containers and then purged with the N₂/Air/N₂+Air/N₂ with the addition of peroxide. Then the sample was analyzed for nitrate. Upon additional of H₂O₂, the resulting nitrate concentration was about 51 mM.

[0298]A vigorous reaction was observed immediately after adding the RuO₂ into 0.1 M KOH containing the 200 μM of peroxide, validating the surface reaction with H₂O₂. The possible reaction is a decomposition of peroxide to water and oxygen through a hydroxyl radical mechanism initiated on the RuO₂ surface. This process may leave activated oxygen moieties that facilitate nitrogen oxidation to nitrate.

[0299]In summary, the electrochemical reduction of Nitrogen to ammonia was demonstrated using PtRu, RuOx, Fe₂O₃-TiO₂ and PdOx catalysts in an electrolyte (supper saturated salt solution) with no free water molecules through a mechanism that is associated with the formation of nitrate (NO₃⁻). In addition, the evolution of hydrogen (HER) is also suppressed by the use of electrolytes with less amount of water molecules.

Example 3

H 2 O 2 Mediated NOR-NO 3 RR of RuO 2 and RuO 2 /CoPc Composite

[0300]Cobalt (II) phthalocyanine with a minimum purity of 97%, Sodium nitroprusside dihydrate, salicylic acid, and sodium hypochlorite solution with an available chlorine content ranging from 11% to 15% by weight, as well as Sulfuric acid with a w/w of 96%, were acquired from Sigma-Aldrich. The chemicals used in this study, namely isopropyl alcohol (2-propanol), ruthenium oxide (RuO₂), sodium sulfate (Na₂SO₄), and Nafion ionomer, were obtained from Bio-Lab Ltd-Jerusalem, Strem Chemicals, Riedel-De Haen Ag Seelze-Hannover, and Ion power, respectively. The sodium citrate dihydrate (>99 wt %) and sodium nitrate were acquired from Merck. The Teflonised Toray carbon sheet was acquired from the Fuel Cell Store. All compounds utilized in this study were of analytical quality and employed without undergoing additional purification. The high pure deionized water with a resistivity of 18.2 M Ωcm was used for all the experiments.

Preparation of the Working Electrode

[0301]The CoPc/RuO₂ ink was prepared by mixing the 1 mg of RuO₂ with 3 mg of CoPc and 15 wt. % Nafion ionomers in a glass vial, 400 μL of water and isopropyl alcohol (1:1 v/v) added to the mixture. This slurry was sonicated for 30 mins to ensure homogeneity and coated on a pre-weighed Teflonised Toray Carbon sheet (active area 1×1 cm²). The electrode was dried at 80° C. for 1 hour. The total catalyst loading calculated from electrode weight was ˜4 mg cm⁻².

Peroxide Induced Nitrate Formation on RuO 2

[0302]Ruthenium oxide activity in oxidizing nitrogen to nitrate chemically in the presence of peroxide was evaluated in 0.1 M Na₂SO₄ solution. The experiment was done by continues purging of air at 100 mL/min flow into 5 mg RuO₂ dispersed in 200 μM peroxide solution containing 0.1 M Na₂SO₄ at room temperature for 5 hours solution. For comparison, a control experiment was run without a ruthenium oxide catalyst. Nitrate concentration of 49.51 and 65.6 μM were obtained under N₂ atmosphere. The concentration of nitrate increased to 140.1 +6.3 μM when exposed to a mixture of 75% N₂ and 15% O₂. When the oxygen concentration in the mixture was adjusted to 5% and 10%, the resulting nitrate concentrations were 75.3 μM and 101.3 μM, respectively. Although the mechanism of chemical nitrate formation involves peroxide disproportionation reaction on RuO₂ surface, this nitrate yield variation in oxygen levels indicate the impact of oxygen on chemical nitrate formation, via oxygen radical anion O₂⁻*. At the same time, in the presence of air, it gives 179.8±15.7 μM. The inventors observed a linear correlation between the quantity of RuO₂ and nitrate forming rate under the air atmosphere a study of first order with respect to the number of functional catalytic active sites controlling the peroxide disproportionation and N₂ adsorption succeeding the nitrate formation.

Proposed Reaction Pathways

[0303]Based on the obtained experimental results, mechanistic insights into the promotion of eNOR by in-situ formed peroxide and the successive electro-reduction of the formed nitrate to ammonia are proposed. As presented in Equations 6 to 16, the overall reaction pathway can be better described in three individual reactions with multiple elementary reaction steps.

Step 1: Formation of Peroxide (Electrochemical Process)

[0304]Oxygen reduction to peroxide can be induced using various metal centered phthalocyanine. The Co-metal center of the macrocyclic rings interacts with the O₂ present in the air, forming an adduct in the first step (equation 6). Successive electron transfer from Co^II metal atom to oxygen leads the formation of an oxidized Co^III metal center (equation 7).

[0305]Finally, the charge transfer from the Coot metal atom to the O₂^δ− occurs with a sequential addition of protons from the electrolyte thus results the H₂O₂ generation electrochemically as shown in equation 8. The rate of electron/proton transfer process is enhanced by the N present on the macrocyclic ring.

Step 2A: Disproportionation of Peroxide (Autocatalytic Process)

[0306]Although the peroxide formed at the CoPc surface is stable enough in neutral pH, it can undergo disproportionation on contact with a catalyst like RuO₂ (e.g. Ru-based salts or Ru pincer complexes). Hence it might end up in reactions forming hydroxyl ions which involve deactivation of the catalyst as shown in Equation 9. Based on these studies, the possible peroxide disproportionation can occur by following Heber-Weiss mechanism, as follows:

[0307]According to Herber-Weiss, the peroxide can easily form peroxyl ions and protons as shown in Equation 9. In the next step this formed peroxyl ion further reacts with another neighboring peroxide present in the electrolyte ending up with hydroxyl radical, water, and oxygen (equation 10). The inventors presume that RuO₂ induces a catalytic reaction of hydroxyl radical and molecular N₂.

Step 2B: Formation of RuO(OH) 2 (Autocatalytic Process)

[0308]The highly reactive hydroxyl radical formed on the RuO₂ surface via peroxide disproportionation, interacts back with the free reactive sites of ruthenium oxide which results in the double bond cleavage of Ru-O bond. Thus, forming ruthenium oxyhydroxide [RuO(OH)₂] as depicted in equation 11.

Step 2 C: Formation of Nitrate (The *Denote The Surface Adsorbed Species)

[0309]The subsequent reaction of the formed ruthenium oxyhydroxide (RuO(OH)₂) with neighboring N₂ adsorbed on the Ru site of RuO₂ yields the *NNOH and RuOOH (equation 12). This RuOOH phase further oxidizes the *NNOH leads the formation of *N, *NO, metallic Ru, and water (equation 13). The formation of metallic ruthenium was confirmed using XRD analysis of the electrode after NOR. In the next step the *NO species reacts with new ruthenium oxyhydroxide ((RuO(OH)₂) converting it into *NOOH (equation 14). In continuation, the RuOOH further oxidizes the *NOOH to *NOOOH with a simultaneous reduction Ru⁴⁺ to metallic Ru. 12-15 are repeated for the conversion another *N formed in the reaction Equation 13 to NO₃⁻. Overall chemical fixation of N₂ to NO₃⁻ is shown below,

[0310]Accordingly, any peroxide generating co-catalyst together with peroxide disproportionation metal catalyst (e.g. Ru-based salt or a complex thereof, such as Ru-pincer complex) may be utilized in the composited material characterized by NOR activity, as described herein.

Peroxide Formation Activity of CoPc, RuO 2 and Composites

[0311]The peroxide versus water formation in oxygen reduction reaction was (ORR) measured on CoPc/RuO₂ selected composite and its components in a potential window of 1.0-0.0 V vs. RHE using rotating ring disk electrode (RRDE). the observed peroxide yield of CoPc is 82.5% at 0.1 V Vs. RHE which is higher than RuO₂ (60.8%). The CoPc/RuO₂ composite catalyst with the composition of 25% RuO₂ with 75% CoPc yielded a peroxide percentage of 36.4% at the same potential. The ORR current of the selected composite is higher than CoPc and RuO₂ of the same loading, but the same couldn't be detected in the Pt ring. this indicates that the formed peroxide is reacting with N₂ present in the air before being getting oxidized and detected at the Pt ring.

[0312]The inventors concluded that the peroxide concentration produced by the composite electrocatalyst is less than a corresponding peroxide concentration produced by sole RuO₂ electrocatalyst, implying that about 30% of the formed peroxide participates in the proposed radical-assisted mechanism of electro-oxidative fixation of N₂ to NO₃⁻.

[0313]To study the composition effect on NOR yield, we optimized catalyst RuO₂ to CoPc ratio and measured the corresponding NO₃⁻ formation rate in air-saturated 0.1 M Na₂SO₄ electrolyte under selected potentials. The nitrate formation rate and FE corresponding to each composition are shown in FIG. 1A. For 100% CoPc, the Nitrate yield rate of 12.1±1.0 μg h⁻¹ cm⁻² with FE of 1.3% is measured. In composite containing 25 wt. % RuO₂ in CoPc, a significant increase to 71.1±4.2 μg h⁻¹cm⁻² was observed while the rate of 100% RuO₂ catalyst was only 4.1 μg h⁻¹ cm⁻². This supports the synergistic effect of the CoPc and RuO₂.

[0314]To complete the picture, we quantified residual hydrogen peroxide post-nitrogen oxidation reaction using a Pt ring and spectrophotometry, comparing it with the measured nitrate concentration in FIG. 1B. It found that solely using CoPc resulted in a high leftover peroxide concentration of 135.3 μM and a minimal concentration of nitrate (21.0 μM), aligning with CoPc's limitations in peroxide disproportionation which leading to nitrate formation. In contrast, adding 25% RuO₂ to CoPc significantly improved performance, leaving only 38.4 μM of peroxide and increasing nitrate concentration to 127.4 μM. Consequently, the study identifies a 25% RuO₂ and 75% CoPc mix as the optimal composition which effectively produce peroxide and oxidize nitrogen to nitrate.

Potential Effect on Nitrate Formation

[0315]The influence of the applied potential on nitrate formation was studied using the optimized catalyst composition of a 25% RuO₂ and 75% CoPc The potentiodynamic was recorded from LSV measurements under Ar and air atmosphere. Higher current observed under air is attributed mostly to the ORR with an onset potential of 0.42 V vs. RHE. Chronoamperometry (CA) electrolysis experiments performed at selected applied potentials from 0.4 to 0.0 V vs. RHE for 1 h. Oxygen reduction currents start at 0.35 V vs. RHE under Ar. The maximum nitrate yield rate of 73.6 μg h⁻¹ cm⁻² was obtained at 0.1 V vs. RHE with FE of 2.0%. This higher yield is attributed to a peroxide production of 36.4% at the same potential. Considering the highest nitrate formation rate, the applied potential of 0.1 V vs. RHE was selected as the optimized for further electrochemical stability measurements.

Reduction of Formed Nitrate to Ammonia

[0316]The formed nitrate via in-situ peroxide induced pathway, further converted into high value ammonia by its electro-reduction using RuO₂ catalyst-based electrode (loading was 4 mg/cm²). To optimize the applied potential of eNO₃RR, chronoamperometry studies were carried out at selected applied potentials in 1000 μM NaNO₃ containing 0.1 M Na₂SO₄ solution for 30 mins under Ar atmosphere. The maximum ammonia yield rate of 168.4 μg·h⁻¹ cm⁻² with 17.6% FE was attained at an optimized potential of −0.35 V vs. RHE. The nitrate is accumulated using CoPc/RuO₂ composite electrode (electrode area of 2×2 cm²) for 1 hour at 0.1 V vs. RHE under air atmosphere.

[0317]The total nitrate concentration is quantified as 1081.7 μM and this resultant nitrate was subjected to electro reduction to ammonia on RuO₂ electrode at −0.35 V vs. RHE for 30 mins under Ar atmosphere. The calculated ammonia formation rate is 147.2±13.7 μg·h⁻¹ cm⁻² with 13.8±1.7% FE, which is 30 times higher than direct eNRR using RuO₂ catalyst in N₂ saturated 0.1 M Na₂SO₄ at −0.35 V vs. RHE (4.9 μg h⁻¹ cm⁻²). In addition, the change in nitrate concentration of 227.3 μM after eNO₃RR well matches with the generated ammonia concentration of 223.4 μM which indicates efficient conversion of formed nitrate into ammonia.

[0318]In summary, the chemical nitrate formation of ruthenium oxide catalyst was studied in 0.1 M Na₂SO₄ solution under an air atmosphere in a peroxide environment. Peroxide forming ability of Cobalt Phthalocyanine (CoPc) at lower potential is utilized by combining it with RuO₂ catalyst to perform in-situ chemical NOR. The nitrate-forming ability of this mixture in the peroxide region via the peroxide radical mechanism was experimentally validated in detail. This catalyst mixture of 25% RuO₂ with 75% CoPc showed 36.4% peroxide production at 0.1 V Vs. RHE in the neutral medium under an air atmosphere. The same mixture showed a nitrate formation rate of 71.11±4.21 μg h⁻¹ cm⁻² with 2.14% FE at a very lower potential of 0.1 V vs. RHE under air. The nitrate formation under this very low applied potential is attributed to the CoPc induced in-situ peroxide formation which is further disproportionate and forms nitrate on the RuO₂ surface in turn converting RuO₂ to metallic Ru. This formed nitrate is converted into more valuable ammonia using RuO₂ catalyst with a 147.23±13.71 μg h⁻¹ cm⁻² ammonia formation rate and 13.76±1.68% FE at −0.35 V vs. RHE. In addition, the CoPc/RuO₂ composite catalyst showed stability up to 5 consecutive cycles of nitrate formation.

Example 4

Effect of N 2 in Electrochemical Nitrate Reduction Reaction

[0319]The inventors further aimed to study nitrogen reduction to ammonia in the presence of nitrate in the electrolyte solution. It was observed that N₂ reduction rate in the presence of nitrate (0.01M to 1.0M) and under air atmosphere (at 1 atm and at 20 C) was 3 order of magnitude higher than a similar reaction utilizing electrolyte solution devoid of nitrate and about 50% higher than the reduction under Ar. The authors hypothesize that reduction on NO₃⁻ result in formation of metal nitride provides N₂ reduction sites in Mars-van Krevelen mechanism NRR.

Example 5

Cerium Ferrite Composites

[0320]Cerium (III) nitrate hexahydrate (99.9%+Ce) and Iron (III) nitrate nonahydrate (98+%) were acquired from Strem Chemical Inc. Poly(vinylpyrrolidone) (PVP) along with potassium hydroxide, sodium citrate dihydrate (over 99 wt. %), sodium nitroprusside dihydrate, ammonium chloride, and salicylic acid, were sourced from Sigma Aldrich. Sulfuric acid (95-98 wt. %), isopropyl alcohol, and sodium hypochlorite solution (containing 11-15 wt. % available chlorine) were procured from Honeywell, Bio-Lab Ltd. (Jerusalem), and Thermo Scientific, respectively. Vulcan XC-72 carbon was obtained from Cabot Corporation. Both the Nafion® 115 membrane and Nafion® ionomer (a 5 wt. % solution in a mix of lower aliphatic alcohols and water) were purchased from the Fuel Cell Store. Ultra-high-purity water with resistivity of 18 M Ωcm, was used for all experiments. All chemicals were used in their received state without any further processing.

Synthesis of CeO 2 /CeFeO 3 Composite Catalyst

[0321]The synthesis of CeO₂/CeFeO₃ composites was carried out using a polyol-assisted microwave method, chosen for its inherent polarity that facilitates rapid and localized heating at the molecular level. The polyvinylpyrrolidone (PVP) was used as a capping agent to polydisperse Ce and Fe uniformly throughput the composite mixture. An aqueous solution of PVP (MW=31,000) was prepared by dissolving 4 g of PVP in 100 ml of deionized water at 80° C. Different molar ratio of iron nitrate and cerium nitrate were added to a specific volume of water, mixed well, and added to the polymer solution, followed by continuous stirring for 2 hours with a magnetic stirrer. This mixture was microwave treated at 1000 W power for 15 minutes until complete evaporation of water. The resultant dried solid was washed with water until no more nitrate content was detected in UV-Visible analysis. Resultant powder was calcined under Ar atmosphere at 500° C. for 3 hours thus resulted a desired composite CeO₂/CeFeO₃ catalysts. Pure cerium oxide and iron oxide catalysts were synthesized using the same methodology without addition of iron nitrate or cerium nitrate salt.

[0322]Electrochemical measurements were conducted in a double chamber H-type cell using a standard three-electrode configuration. A Mercury/Mercurous oxide (Hg/HgO) electrode served as the reference, while a nickel strip (with 99.9% purity) was employed as the counter electrode.

[0323]To prepare the working electrode, 80 wt. % of the CeO₂/CeFeO₃ composite catalyst was mixed with 20 wt. % Vulcan carbon XC-72. This admixture was then added to 2 mL of a 1:1 isopropanol (IPA): water solution and 15 wt. % Nafion® ionomer. The resulting mixture subjected to sonication for 30 minutes to ensure homogeneity. The homogenized catalyst dispersion was then drop-cast onto Teflonized Toray carbon paper, with an active area of 1×1 cm². After drying, this paper was used as the working electrode, maintaining a total catalyst (catalyst+Vulcan carbon) loading of 2 mg cm⁻² for all experiments. The electrochemical tests were carried out using a BioLogic potentiostat/galvanostat workstation (VSP/VMP 3B-20). The electrochemical nitrate reduction reaction (eNO₃RR) activity of the catalyst was assessed in an argon-saturated 0.1 M KOH electrolyte containing 0.1 M KNO₃. The 0.1 M KOH solution was used as an anolyte. Pretreated Nafion®115 membrane was used as a separator. Argon gas was purged for 30 minutes prior to each experiment. To collect ammonia, a 1 mM H₂SO₄ acid trap was connected to an outlet of electrochemical cell. All potentials reported were referenced against the reversible hydrogen electrode (RHE), with pH adjustments made using the Nernst equation, as described in Equation 2

$\begin{array}{cc}E\left(\mathrm{RHE}\right)=E\left(\mathrm{Hg}/\mathrm{HgO}\right)+\left(0.059\times \mathrm{pH}\right)& \left(2\right)\end{array}$

Physio-chemical Characterization of CeO 2 /CeFeO 3 Composite Catalysts

[0324]The CeO₂/CeFeO₃ composite catalyst with three different atomic ratios of cerium/iron, pure cerium oxide and iron oxides were analysed by XRD to determine their crystallinity and phase purity. The phase distribution in these composites was examined using Expert High Score Plus software. For the 100 wt. % cerium catalyst (FIG. 1a), the XRD pattern revealed the presence of a CeO₂ phase with characteristic diffraction peaks (JCPDS 01-018-0792). These peaks were observed at 2θ values of 28.5°, 33.1°, 47.4°, 56.3°, 59.1°, 69.4°, 76.7°, 79.1°, and 88.4°, corresponding to the (111), (200), (311), (222), (400), (331), (420), and (422) planes of a cubic structure, respectively. Introducing 25% iron to pure CcO₂ (FIG. 1c) resulted in the formation of 51% CeFeO₃ with an orthorhombic phase (JCPDS 00-022-016). This was confirmed by diffraction patterns at 2θ values of 22.7°, 32.3°, 39.8°, 46.3°, 57.8°, and 67.6°, which align with the (110), (112), (202), (004), (024), and (224) planes. FIGS. 2D and 2E illustrate the XRD patterns for composites with 50 and 75% iron, leading to 75% and 32% of the CeFeO₃ phase, along with 15% and 29% CeO₂, respectively. A decrease in cerium content reduced the CeFeO₃ phase formation to 43%, with a corresponding 19% increase in the magnetite (Fe₃O₄) phase of iron oxide. The catalyst containing cerium, depicted in FIG. 2B, consisted of a mixture of maghemite (Fe₂O₃), magnetite (Fe₃O₄), and hematite (Fe₁₂O₁₈), correlating with the JCPDS files 00-039-1346, 01-088-0315, and 96-901-5504. Among these iron oxides, the maghemite phase was predominant, constituting 72%, with the remaining 38% being a combination of magnetite and hematite phases. The average crystallite sizes calculated using Scherer's formula for the 100% Ce, 75% Ce: 25% Fe, 50% Ce: 50% Fe, 25% Ce: 75% Fe, and 100% Fe composite catalysts are 20.4 nm, 141.4 nm, 89.2 nm, 157.1 nm, and 127.5 nm, respectively.

[0325]The Raman spectroscopy study investigated the local crystal symmetry and defects/disorders in the metal oxides of the composite catalysts. Raman spectra were recorded over a range of 50 to 2000 cm⁻¹ using a 532 nm laser. For the 100 wt. % cerium catalyst, a single Raman shift was observed at 452 cm⁻¹, which originated from the T2g symmetric vibration of the Ce-O-Ce bond. In contrast, the catalyst without cerium (100% Fe) displayed distinct Raman bands, including A1g and Eg bands of Fe₂O₃ at 197 (shift from 226 cm⁻¹) and 264 cm⁻¹ (shift from 249 cm⁻¹), respectively, 19 and T2g bands at 373 cm⁻¹ (shift from 365). Furthermore, bands observed at 268, 755, and 1288 cm⁻¹ in catalysts containing 50, 75, and 100% iron were attributed to the Eg and Alg bands of magnetite, corroborating the findings from the XRD analysis. Notably, these bands were absent in the 25% Fe catalyst, aligning with expectations based on the XRD analysis. The FT-IR spectra, further confirmed the formation of cerium ferrite composites within the mixture. Absorption bands at 550 and 696 cm⁻¹, associated with metal-oxygen (Fe-O) stretching, were evident in the 100% Fe sample which has two different Fe-O bond one from Fe₂O₃ and other Fe₃0₄. The peak at 975 cm⁻¹ corresponded to the characteristic Ce-Fe bond in the CeFeO₃ phase, particularly prominent in the 50% Fe addition. Additionally, peaks at 3453 and 1623 cm⁻¹ indicated the presence of adsorbed water molecules, while the peak at 1384 cm⁻¹ was attributed to C=C stretching, likely originating from to a trivial amount of carbon due to thermal oxidative removal of polymer PVP during high temperature calcination.

[0326]The Scanning Electron Microscopy (SEM) micrographs of the as-synthesized catalysts, reveal a sponge-like morphology across all composite catalysts. The distinctive morphology observed in the composite is largely due to the inclusion of PVP in the synthesis process. PVP plays a critical role as a capping agent, not only facilitating the uniform dispersion of particles but also stabilizing them, which contributes significantly to the unique structural characteristics of the composite. The homogeneous distribution of metal oxides within the composites is further confirmed by Energy-dispersive X-ray Spectroscopy (EDS) mapping. This is particularly evident in the 50Fe:50Ce samples, as illustrated in FIG. 3.

[0327]For the 100% Ce catalyst, the elemental composition of 84.5% Ce and 15.5% O closely aligns with the theoretically calculated composition of 81.4% Ce and 18.6% O. Similarly, the 100% Fe catalyst exhibits an average elemental composition of 69.7% Fe and 30.3% O, which is in good agreement with the theoretical values of 70.3% Fe and 29.7% O, based on XRD data. The inclusion of 25% Fe results in a composition of 70.3% Ce, 11.4% Fe, and 18.4% O. This closely matches the calculated figures of 69.2% Ce, 11.7% Fe, and 19.1% O, corresponding to the phase mixtures of 51% CeFeO₃ and 49% CeO₂. Furthermore, the addition of 50 and 75% Fe yields compositions of 56.9% Ce, 32.1% Fe, and 20.1% O, and 42.9% Ce, 35.4% Fe, and 21.8% O, respectively. These values also correspond well with the calculated compositions for their respective oxide composite mixtures.

[0328]Chronoamperometric electrolysis was performed on various catalyst systems for one hour across seven chosen potentials from 0.05 V to −0.55 V vs. RHE in an argon-saturated 0.1M KOH containing 0.1 M KNO₃ solution. With the pure CeO₂ catalyst (FIG. 6e), the ammonia formation rate sharply increased to 4040.5 μg h⁻¹ cm⁻² at −0.45 V vs. RHE, nearing saturation, while the faradaic efficiency started high at 75.2% at 0.05 V vs. RHE and declined to 48.2% at −0.55 V vs. RHE. The maximum ammonia yield rate observed was 4132.5 μg h⁻¹ cm⁻² with a Faradaic efficiency of 48.2% at −0.55 V vs. RHE.

[0329]Introducing 25% Fe to the catalyst, which consists of 51% CeFeO₃ and the remainder CeO₂, resulted in an increased ammonia yield rate at higher applied potentials. Differing from CeO₂, the Faradaic efficiency peaking at 81.2% at −0.35 V vs. RHE which 31% higher than the pure CeO₂ system. The maximum rate for this composite was 3693.9 μg h⁻¹ cm⁻² at −0.55 V vs RHE, only 10.6% lower than that of pure CeO₂, suggesting that the CeFeO₃ phase significantly boosts the Faradaic efficiency for ammonia with minimal impact on the yield rate. Additionally, this composite exhibited a high hydroxylamine rate at lower potentials (below—0.25 V vs. RHE), diminishing to 71.0 μg h⁻¹ cm⁻² at −0.25 V and then increasing at higher potentials. Faradaic efficiencies for nitrite and hydroxylamine were higher at lower potentials and decreased as the potential rose. Given the relatively high yield rates and efficiencies, −0.45 V vs. RHE was determined to be the optimized potential.

[0330]The 50Ce:50Fe composite catalyst, achieved a peak ammonia yield rate of 3911.5 μg h⁻¹ cm⁻² at −0.55 V vs. RHE. Its highest FE of 89.1% was reached at −0.35 V vs. RHE, slightly surpassing the efficiency at −0.25 V vs. RHE. When compared to the 75Ce:25Fe mixture, the inclusion of 50% Fe led to the best faradaic efficiency of 89.1% at −0.35 V vs. RHE and ammonia yield, likely due to the 75% CeFeO₃ phase in the composition. On considering the relative rate and FE the potential of −0.45 V vs. RHE was considered as optimized. In this ratio, hydroxylamine efficiency rose by 30% at lower potentials, and its yield rate doubled at −0.55 V vs. RHE, a shift attributable to the 10% presence of the new Fe₃O₄ phase in the mixture.

[0331]For the 25Ce:75Fe catalyst system, the ammonia yield rate dropped to 3101.6 μg h⁻¹ cm⁻² at −0.55 V vs. RHE, which is 20.7% less than the 50Ce:50Fe mixture, reflecting the reduced CeFeO₃ phase content. The absence of Ce in the composite led to the lowest ammonia yield rate of 1912.8 μg h⁻¹ cm⁻² at −0.55 V vs. RHE compared to other catalyst ratios, underscoring Ce's role in enhancing ammonia production. Conversely, this composite achieved the highest nitrite rate of 3146.7 μg h⁻¹ cm⁻² at −0.55 V vs. RHE, indicating a preference for nitrite formation in the Fe₂O₃/Fe₃O₄ phases.

[0332]Ammonia rate distribution and overall faradaic efficiency of the catalyst systems at −0.45 V vs. RHE was tested. The CeO₂ and CeFeO₃ phases exhibited higher ammonia yield rates, as evidenced by the increased yields with 25-100% Ce in the mixture. Additionally, the formation of the CeFeO₃ phase was highly selective towards ammonia, with the 50Ce:50Fe composite mixture, containing 75% CeFeO₃ phases, achieving a maximum ammonia FE of 88.5%. Furthermore, the presence of iron (Fe₂O₃/Fe₃O₄) shifted selectivity towards hydroxylamine from ammonia, as the FE % of hydroxylamine increased with additional Fe. Considering the highest ammonia efficiency and significant CeFeO₃ content, the 50% Ce and 50% Fe composite mixture was chosen for stability studies at −0.45 V vs. RHE.

[0333]It is postulated that the superior catalytic performance of the CeFeO₃ composite is largely due to the intrinsic defects within its perovskite structure, especially oxygen vacancies. In the ABO₃-type perovskite oxides, such as CeFeO₃, these oxygen vacancies are not mere imperfections but act as critical enablers of enhanced catalytic activity. The mixed valency of iron—where Fe²⁺ and Fe³⁺ states coexist—leads to the formation of these vacancies, as the material compensates for charge imbalances. These vacancies effectively serve as active catalytic sites that facilitate various electron transfer reactions. In the context of nitrate reduction to ammonia, oxygen vacancies provide sites for adsorption and activation of nitrate ions, thereby lowering activation energies and enabling improved kinetics for the conversion process. This mechanism underscores the importance of structural defects in optimizing the efficiency and selectivity of catalysts in electrochemical reactions.

[0334]The inventors confirmed stability of CeO₂/CeFeO₃ based electrode for at least 25 repetitive cycles of eNO₃RR, resulting in an almost constant ammonia generation rate.

[0335]Additional stability tests indicate that the CeO₂/CeFeO₃ composite maintains stable performance for 25 hours in eNO₃RR cycles. Online mass spectroscopy, confirmed the absence of gases like N₂O, NO and N₂, with only H₂ being evolved during the reaction. To this end, various ratios of CeO₂/CeFeO₃ composite catalysts were synthesized using a microwave-assisted method and comprehensively characterized by XRD, FT-IR, Raman, SEM, EDS, and XPS techniques. The characterization results indicated that a 50% Ce to 50% Fe ratios resulted in a composite with a predominant 75% CeFeO₃ phase, complemented by 15% CeO₂ and 10% Fe₃O₄. Electrochemical evaluations of nitrate reduction for the synthesized composite catalysts demonstrated that the introduction of Fe, forming the 75% CeFeO₃ phase, notably enhanced the ammonia Faradaic efficiency to 88.2%, alongside an appreciable yield rate of 3223.9 μg h⁻¹ cm⁻². This CeFeO₃ phase also restrict the parasitic HER to 3.1% while the remaining goes for nitrite and hydroxylamine.

Effect of N 2 in Electrochemical Nitrate Reduction Reaction

[0336]The CeFeO₃ catalyst along with 20% Vulcan carbon XC-72 was coated on the Teflonized Toray carbon electrode and used as working electrode. The preliminary experiments were carried out in H-type electrochemical cell using 0.1 M KOH containing 0.1 M KNO₃ solution as electrolyte. Hg/HgO and Ni strip (99.9% pure) were used as reference and counter electrode respectively. Pre-treated Nafion® 115 membrane was used as a separator. Both Ar and N₂ gases were purged for 30 mins prior to corresponding experiment.

[0337]The chronoamperometry was carried out in 8 different potentials from −0.05 V Vs. RHE. As the potential increases, the rate tends to increase further with a sudden decrease in efficiency. Interestingly, the same experiment was carried out under a nitrogen atmosphere (the CeFeO₃ has no activity for NRR), and the FE tends to be stable with increasing rate.

[0338]The rate under N₂ atmosphere was found to be 35% higher than the Ar atmosphere with relatively stable FE % at −0.45 V vs. RHE in 0.1 M KOH containing 0.1 M NO₃⁻.

[0339]Further, we varied the concentration of the nitrate from 0.01 M to 0.5 M in 0.1 M KOH solution by keeping the applied potential constant at −0.45 V vs. RHE. The ammonia formation rate increases with increase in the nitrate concentration but the faradaic efficiency reaches a saturation after 0.1 M KNO₃ under N₂ atmosphere. The same trend also observed under Ar atmosphere. As observed before, around 47% higher ammonia formation was observed under N₂ atmosphere using 0.5 M KNO₃ solution in 0.1 M KOH.

Effect of N 2 in eNO 2 RR and eNH 2 OHRR

[0340]As we observed the higher ammonia yield in nitrate reduction under N₂ atmosphere, we tried the same with Nitrite and hydroxylamine also. Here, the ammonia formation rate observed under N₂ atmosphere was almost equal in Ar atmosphere.

Effect of Hydroxyl Ion Concentration (in The Presence of N 2 /Ar)

[0341]Ammonia formation rate increases 4.19 times on increasing the OH— concentration from 0.01 M to 0.1 M. On increase the concentration further to 1.0 M the ammonia formation rate increased to 1.28 times then it becomes saturated. The same trend was observed with NO₂⁻ formation rate. The hydroxylamine formation rate was increased upto 0.1 M hydroxyl concentration then decreases slowly. These results shown the selectivity changes of the process highly influenced by changing hydroxyl ion concentration.

[0342]The inventors observed that increasing the supporting electrolyte concentration supressing the HER and enhancing the ammonia formation. Here the change of HER mechanism from Volmer to Hyrovsky can also be possible on increasing the ⁻OH concentration. Due to the higher ammonia formation rate the 1 M KOH concentration was optimized.

Cation Effect on Supporting Electrolyte

[0343]The inventors tested the influence of different chloride salts added to the electrolyte (at the final concentration of 1M) on the rate of ammonia formation during NO₃RR at Ar and N₂ atmosphere. Catholyte: 0.5 M KNO₃ in 1.0 M LiCl, 1.0 M KCl, 1.0 M NaCl (35.0 mL); Anolyte: 1.0 M LiCl, 1.0 M KCl, 1.0 M NaCl solution (35.0 mL).

[0344]The ammonia formation rate was found to be increasing with increasing the size of the cation of the supporting electrolyte. The ammonia formation rate increases more than five times in NaCl than LiCl which reflected in the conductivity measurement (71 mS for NaCl and 13.7 mS for LiCl). Greatest ammonia formation rate was observed in KCl under N₂ atmsopehere of 8272.95 μg/h/cm² and FE of about 78%.

Example 6

Electrochemical Reduction of Nitrate to Ammonia under Ambient Condition Using NiCo 2 S 4 Nanoparticles

Materials

[0345]Cobalt (II) acetate tetrahydrate (Co(OCOCH₃)₂·4H₂O) and sulphur (99+%) were purchased from Strem Chemicals. Nickel (II) acetate tetrahydrate (Ni(OCOCH₃)₂·4H₂O, 98%), potassium hydroxide, isopropyl alcohol, sulfuric acid (96 wt %), sodium nitroprusside dihydrate, salicylic acid, hydroxylamine hydrochloride, hydrochloric acid (37 wt. %), and sodium hypochlorite solution (11-15 wt % available chlorine) were procured from Sigma Aldrich. Sodium citrate dihydrate (>99 wt %) and sodium hydroxide was obtained from Merck. Nafion® 115 membrane and Nafion® ionomer (5 wt. % solution in a mixture of lower aliphatic alcohols and water) was purchased from Fuel cell store. Vulcan XC-72 carbon (Cabot) was purchased from Cabot. All the chemicals used in this work were analytical grade and the ultra-high pure water (resistivity is 18.2 M Ωcm) was used for all the experiments.

Synthesis of Spinel Trimetallic Nickel Cobalt Sulfide (NiCo 2 S 4 )

[0346]10 mmol of Nickel (II) acetate tetrahydrate, 20 mmol of cobalt (II) acetate tetrahydrate and 40 mmol of sulphur was added to 60 mL of water. The resultant admixture was transferred to a Teflon beaker of a hydrothermal reactor and hydrothermal reaction was performed at 160° C. for 8 h. The catalyst was obtained after centrifugation, washing three times with water, drying at 50° C. overnight and finally calcination at 400° C. (heating ramp was 5° C. min⁻¹) for 2 h under Argon atmosphere.

[0347]Catalyst coated Teflonized Toray carbon paper, a Nickel strip (99.9% pure) and Hg/HgO was used as working, counter and reference electrodes, respectively. Required weight of as-synthesized catalyst (80 wt. %) and Vulcan XC-72 carbon (20 wt. %) was dispersed in 1:1 v/v ratio of IPA and water mixture by ultra-sonication. The resultant homogeneous catalyst ink was drop-cast on 1 cm² active area of Toray carbon and dried to prepare the working electrode. The BioLogic potentiostat/galvanostat workstation (VSP/VMP 3B-20) used to perform the electrochemical tests. The eNO₃RR activity analysis of the prepared catalysts was tested in 0.1M KOH containing 0.5M KNO₃ under Ar-atmosphere.

Nitrite Analysis

[0348]The sample was analysed for nitrite (NO₂⁻) using UV-Vis spectrophotometer and the absorbance at 350 nm was noted to estimate the NO₂⁻ concentration. A standard calibration curve was made using standard sample of 0.1M KOH containing a series of known NO₂⁻ concentration. Rate of NO₂⁻ and the corresponding FE was estimated applying the concentration and molecular weight (46.0055 g mol⁻¹) of NO₂⁻ in equation (2) and .(3)

Hydroxylamine Analysis

[0349]A colorimetric method based on a chemical reaction of potassium ferricyanide (K₃Fe(CN)₆) with hydroxylamine (NH₂OH) under alkaline pH was used to quantify the NH2OH formed during eNO₃RR following a slightly modified procedure reported elsewhere. 1.5 mL of 25 wt. % of KOH solution and 2 mL of 3 mM of K₃Fe(CN)₆ prepared using 0.1 M potassium chloride was added to 1 mL of electrolyte sample and UV-Vis spectra was recorded after 15 minutes of wait. The absorbance value at 425 nm was noted to quantify the NH₂OH concentration. The yield rate and FE was obtained by applying the concentration and molecular weight (33.03 g mol⁻¹) of NH₂OH in equation (1) and (2), respectively.

[0350]The XRD pattern of the as-synthesized NiCo₂S₄ powder was obtained to scrutinize its phase purity and the crystallinity. diffraction peaks at 2θ values of 16.26°, 26.70°, 26.70°, 31.47°, 36.01°, 38.28°, 47.49°, 50.44°, 55.12°, 59.85°, 62.47°, 65.07°, 69.16°, 77.96 and 88. 84 is assigned to the (111), (220), (311), (222), (400), (422), (511), (440), (531), (620), (533), (444), (731) and (800) planes which are characteristics to the cubic phase of NiCo₂S₄ and the observed diffraction peaks is matching with JCPDS no. 20-0782. The presence of trivial amount of CoSz phase is evidenced from the existence of two additional diffraction peaks at 2θ values of 39.37° and 45.84° are assigned to the (211), and (220) planes of cubic phase of CoS₂ (JCPDS no. 9008393). The Scherrer equation (D=kλ/B cose) was applied to the (311) plane of NiCo₂S₄ located at 31.47° to calculate the crystallite size. Crystallite size of as-synthesized NiCo₂S₄ is calculated to be 39.73 nm.

[0351]Morphological characteristics of the NiCo₂S₄ was analyzed by SEM as shown in FIG. 4. SEM micrographs exposed the nano-sized cubical morphology of the of the NiCo₂S₄ sample. The elemental composition was analysed by EDX spectroscopy. EDX analysis revealed that only Ni, Co and S present in the sample with weight percentage of 23.0, 39.9 and 37.0% 37%, respectively. In addition, the elemental composition was also analyzed by ICP-OES and results (Ni (19.8 wt %), Co (33.3 wt. %), and S (47 wt, %)) corroborates the EDX analysis. Combined XRD, the Raman, SEM and ICP-OES analysis evidenced the formation of NiCo₂S₄ nanostructure from material synthesis route adopted in the present study.

[0352]Prior to the NO₃RR activity assessment, the electrochemical active surface area (ECSA) of the NiCo₂S₄ catalyst mixed with 20 wt. % Vulcan carbon was quantified double-layer capacitance (C_d1). ECSA represents the extent of the availability of electrochemically active catalytic sites and is an important parameter determines the activity of the catalysts. This analysis exhibited an ECSA of 0.622 cm² and 0.317 cm² for the working electrode constituted with 80 wt. % NiCo₂S₄+20 wt. % Vulcan carbon (4 mg cm⁻²) and 20 wt. % Vulcan carbon (0.8 mg cm⁻²) electrode, respectively. In the 80 wt. % NiCo₂S₄+20 wt. % Vulcan carbon working electrode, an ECSA of 0.305 cm² is due to the presence of active NiCo₂S₄ catalyst and is available for the NO₃RR.

[0353]To further investigate the NO₃RR activity and product selectivity of this catalyst, the constant potential electrolysis was performed in a potential range of −0.05 V to −0.50 V vs. RHE. (FIG. 3b). The observed current response is relatively stable at all applied potential which can be attributed to the electrochemical stability of this catalysts. Generally, the rate of NH₃ formation increase with the applied potential. The FE reaches a maximum of 64% with an NH₃ yield rate of 3793.0 μg h⁻¹ cm⁻² at −0.3V vs. RHE. More interestingly, only a minor deviation is observed in FE irrespective of the applied potential. This is attributed to the NO₃RR as fast as competing HER even at higher applied potential on this catalyst. As discussed previously, the eNO₃RR is a complex process with multiple proton-coupled electron transfer (PCET) steps which occurs vis more than one intermediates and by-products. A careful analysis of the electrolyte sample post electrolysis reveals the formation of NO₂⁻ and NH₂OH in addition to NH₃. As shown in FIG. 5B, the formation of NO₂⁻ spiked at −0.25 V vs. RHE with a yield rate of 9411 μg h⁻¹ cm⁻² (FE 17.6%) and the rate remains nearly constant below −0.25 V vs. RHE but the FE drops down to 7.5% at −0.50 V vs. RHE. Similarly, the NH₂OH quantification showed a sharp increase in FE with a value of 10.0% (Yield rate is 1286 μg h⁻¹ cm⁻²) at −0.25 V vs. RHE as presented in FIG. 5C. Interestingly, the lowest rate of 425.0 μg h⁻¹ cm⁻² with lowest FE of 2.8% was obtained at an applied potential of −0.30 V vs. RHE.

[0354]Based on the above results, the product selectivity analysis was performed as a function of FE and the corresponding results presented in FIG. 5D. This analysis clearly depicts that higher than 50% (average of 59.85) of the total applied charge is utilized for the final product of NH₃. However, this analysis may be misleading, because the rest of the charge is not wasted. Unlike, the most expected cathodic competing side reaction that produces evolving H₂ gas, NO₂⁻ and NH₂OH are essential byproducts in this multistep reaction, expected to proceed to ammonia under prolong reduction. Thus indicates the suitability of the NiCo₂S₄ nanostructured catalyst for eNO₃RR. An average 12.82 and 5.28% of the applied electrical charge is utilized for NO₂⁻ and NH₂OH formation, respectively at the tested wider potential window. In addition an average charge of 22% is utilized for the formation of other than NH₃, NO₂⁻ and NH₂OH, most likely for HER. Specifically, at −0.3V vs. RHE, FE of 64.01, 13.65, 2.77, and 19.55 was achieved for NH₃, NO₂⁻, NH₂OH and other products, respectively. In other words, the FE of total nitrate reduction products is 80.43% which is signify the NO₃RR performance of NiCo₂S₄ electrocatalysts. To conform that H₂ is the only gaseous side-product released during the NO₃RR, the online mass spectrometry analysis during NO₃⁻ electrolysis at −0.3V was carried out. Various possible gaseous products including the N₂O, N₂, NO₂, NO and H₂ was monitored and found that only H₂ is being released due to the competing HER during NO₃RR over the time. Considering the highest FE for NH₃ and lowest FE for NH₂OH with a moderate NH₃ yield rate of 3793 μg h⁻¹ cm⁻², the −0.3V vs. RHE is proposed to be optimized an applied potential for longer eNO₃RR electrolysis using the NiCo₂S₄.

[0355]We have studied the effect of NO₃⁻ and OH⁻ concentration on the NO₃RR activity of NiCo₂S₄ catalysts at an optimized applied potential of −0.3V vs. RHE. The current increases with increasing the NO₃⁻ concentration in 0.1M KOH supporting electrolyte, thus indirectly signify the NO₃RR performance dependency on NO₃⁻ concentration. The ammonia formation rate increases with NO₃⁻ concentration until highest concentration tested and a maximum NH₃ formation rate of 3793.3 μg h⁻¹ cm⁻² was achieved with 0.5M NO₃⁻. The FE reaches a maximum of 67.25% with 0.5M NO₃⁻ and almost remains constant beyond that concentration. The corresponding NO₂⁻ and NH₂OH formation rate is presented in Table 2. This analysis shows that the NO₃RR is mass transport controlled with respect to NO₃⁻ concentration. Increasing trend in the current, ammonia formation rate and FE is also seen with increasing the OH⁻ concentration keeping nitrate ion concentration constant (0.5M NO₃⁻) (FIG. 5c and d). A maximum ammonia formation rate of 8513.0 μg h⁻¹ cm⁻² with FE of 73.21% was achieved using 1 M KOH containing 0.5M NO₃⁻. This is 66% higher compared to ammonia formation rate of 0.1 M KOH containing 0.5M NO₃⁻. Moreover, as shown in Table 2, the formation of NH₂OH is avoided 1 M KOH thus increases the selectivity of NO₃⁻ reduction to NH₃. The higher performance with 1 M KOH could be due to the suppressed HER.

Nitrite (NO 2 ) Reduction Using NiCo 2 S 4

[0356]Nitrite reduction of the NiCo₂S₄-based electrode has been tested using 0.5M of KNO₂ in 0.1M KOH electrolyte (under Ar atmosphere). The results are presented in table below:

[0357]As evident form the table above, hydroxylamine generation rate is higher for NO₂RR than that of NO₃RR.

[0358]To this end, the NiCo₂S₄ nanostructure was constructed for electrochemical nitrate reduction under ambient conditions. Systematic experimental results revealed the suitability of the NiCo₂S₄ for efficient electrochemical conversion of NO₃⁻ with high product selectivity towards desired NH₃. A maximum ammonia yield rate of 3793 μg h⁻¹ cm⁻² with a FE of 64% was achieved at −0.3 V vs. RHE using the developed NiCo₂S₄ catalysts.

Example 7

Electrochemical Nitrogen Oxidation Reaction (eNOR) Using Rh Supported on Carbon Nanosheets (Rh/C) Catalyst

Materials

[0359]Rhodium (III) acetylacetonate 97% and Ruthenium (IV) oxide anhydrous (99.9+% Ru) were purchased from Strem Chemical INC Inc and used as received. Anhydrous Phloroglucinol (99%) was procured from Acros Organics. Potassium hydroxide, sodium citrate dihydrate (>99 wt %), sodium nitroprusside dihydrate, ammonium chloride, and salicylic acid were purchased from Sigma Aldrich. Sulfuric acid (95-98 wt %), isopropyl alcohol, and sodium hypochlorite solution (11-15 wt % available chlorine) were purchased from Honeywell, Bio-Lab Ltd. (Jerusalem), and Thermo Scientific, respectively. Vulcan XC-72 was purchased from Cabot. Ultra high-pure water with a resistivity of 18 M Ωcm was used in all experiments. All the chemicals were used as received.

Synthesis of a Carbon-Supported Rhodium Catalyst

[0360]Rhodium (III) acetylacetonate with different required weight percentages is was mixed well with Phloroglucinol using by use of a mortar and pestle. The obtained mixture was calcined at 700° C. for 2 hrs with a temperature ramping of 10° C. per minute under an Ar atmosphere. The calcined samples were labeled as 5 wt. % Rh/C, 20 wt. % Rh/C, and 40 wt. % Rh/C with respectively added Rh content of 5 wt. %, 20 wt. %, and 40 wt. %. In addition to this, a pure carbon sample was also synthesized, without Rhodium acetylacetonate, by adhering to the same procedure. All the electrochemical measurements were performed in a single chamber cell with a regular three-electrode configuration. Mercury/Mercurous oxide (Hg/HgO) and Nickel foil (99.9% pure) were used as reference and counter electrodes, respectively. The required weight of Rh/C catalyst was added with 2 mL of a 1:1 (v/v) IPA: water mixture and 15 wt. % of Nafion® ionomer. This mixture was sonicated for 30 mins to attain homogencity. This catalyst dispersion was drop cast in Teflonized Toray carbon paper with an 1×1 cm² active arca, dried and used as a working electrode. A total catalyst (Rh/C) loading of 4 mg cm⁻² was maintained throughout this study. The BioLogic potentiostat/galvanostat workstation (VSP/VMP 3B-20) was used to perform the electrochemical tests. The eNOR activity of the catalyst was measured in a 0.1 M KOH electrolyte in Ar, N₂ and air atmospheres. The respective gas was purged 30 mins prior to cach experiment. An additional acid trap was used for the eNO₃RR to collect ammonia.

Physico-Chemical Characterization of the as-synthesized Rh/C Catalysts

[0361]Three different Rh/C catalysts were synthesized with varying Rh content. ICP-OES clemental analysis of these catalysts revealed the contents of Rh as 5. 6, 20.7, and 40.5 wt. % and were designated as 5 wt. % Rh/C, 20 wt. % Rh/C, and 40 wt. % Rh/C, respectively. The XRD pattern of the as-synthesized carbon and 5 wt. % Rh/C was recorded to assess the phase purity and crystallinity. As shown in FIG. 6, the XRD pattern of carbon exhibited only two diffraction peaks characteristic to the graphitic carbon, thus conforming the synthesis of a pure phase of carbon. On the other hand, the 5 wt. % Rh/C catalysts exhibited a series of diffraction peaks at 2θ values of 40.01°, 49.20°, 70.0°, 84.3°, and 89.2°. These were assigned to the (111), (200), (220), (311), and (222) planes which are characteristics of the face-centred cubic phase of Rh. In addition, a broad diffraction peak centered at 24.8° was assigned to the hexagonal (002) plane of the conducting carbon support. The XRD analysis confirms the structure of a pure phase Rh/C prepared in this work.

[0362]SEM micrographs presented in FIGS. 6B and 6C reveal the sheet-like morphology of the carbon support. Morphological analysis also exhibited a uniform dispersion of Rh nanoparticles over the carbon matrix. This uniform dispersion was expected to provide better exposure of active sites for N₂-adsorption with full utilization of the metallic active sites. This could result in higher NO₃⁻ formation per gram of the active precious metal Rh. Only Rh and Care were detected in the EDX elemental analysis of these catalysts with 5.8 and 94.2%, thus matching well with the ICP-OES analysis.

[0363]The scanning transmission electron microscope (STEM) images were are displayed in FIGS. 6D and 6E at different magnifications. These images further validate the carbons layered structures in which the Rh metals were uniformly dispersed. The particle size of the Rh metal is shown in FIG. 6E, and the particle's size distribution histogram is included (see insert). The average particle size of Rh was found to be 12.6 nm which matches with the crystallite size of 13.2 nm calculated from the XRD pattern of Rh/C using the (111) plane of Rh. Electrochemical Nitrogen oxidation reaction activity of Rh/C

[0364]The inventors observed a 48% higher current for LSV recorded in a N₂ atmosphere with a positive shift in the current above 1.5 V vs. RHE compared to the voltammogram of the same electrode in an Ar atmosphere. The observed higher currents under N₂ are assigned to eNOR activity on Rh/C. Chronoamperometry (CA) electrolysis was performed at each selected potential to find an optimum potential for a longer electrolysis span. A maximum NO₃⁻ formation rate of 21.3 μg h⁻¹ cm⁻² with a FE of 28.2% was obtained at 1.7 V vs. RHE. An applied potential of 1.7 V vs. RHE is considered optimum owing to the maximum ammonia formation rate despite the highest FE of 37.2% obtained at 1.6 V vs. RHE.

[0365]The eNOR activity of the other two compositions (20 and 40 wt. % Rh supported on carbon) was also studied to scrutinize the optimal Rh content. The current increases in the wider potential region with an increase in Rh content from 5 to 20 and 40 wt. % supported on C. Hence, the Rh is directly related to the rise of the current via a larger number of active catalytic sites, promoting eNOR.

[0366]The potential optimization of both the 20 and 40 wt. % Rh/C revealed the maximum NO₃⁻ formation rate of 29.4 μg h⁻¹ cm⁻² (FE 13.9%) and 24.1 μg h⁻¹ cm⁻² (FE 4.5%) at 1.7 V vs. RHE as observed for 5 wt. % Rh/C. This teaches us that irrespective of the Rh content, the optimum potential for longer electrolysis is 1.7 V vs. RHE. This observation is attributed to the undesired competitive oxygen evolution reaction (OER) dominance over eNOR at an anodic applied potential higher than 1.7 V vs. RHE.

[0367]5 wt. % Rh/C catalysts offered effective utilization of the Rh with the highest NO₃⁻ formation rate of 94.9 μg h⁻¹ mgRh⁻¹ at 1.7 V vs. RHE compared to the other two catalysts, i.e., 20 wt. % Rh/C (36.2 μg h⁻¹ mgRh⁻¹) and 40 wt. % Rh/C (14.8 μg h⁻¹ mgRh⁻¹). Moreover, the FE was also higher for this catalyst. To this end, the 5 wt. % Rh/C was considered as optimal.

[0368]In view of understanding the eNOR mechanistic pathway and the possibility of utilizing the nitrogen present in the air without the need for pure N₂, the eNOR activity of 5 wt. % Rh/C was assessed in air-saturated 0.1 M KOH. A positive shift in the current was observed in an air atmosphere, which signifies eNOR kinetics. At 1.7 V vs. RHE, the maximum NO₃⁻ formation rate of 47.0 μg h⁻¹ cm⁻² (FE 30.9%) was achieved. This is about 50% higher than results attained in N₂ under identical operating conditions. This improved eNOR kinetics in the presence of air is attributed to the combined oxygen and hydroxide mediated conversion of N₂ to NO₃⁻ over Rh/C catalysts.

[0369]The stability of 5 wt. % Rh/C was assessed in the air and in N₂-saturated 0.1 M KOH at the optimized potential of 1.7 V vs. RHE. Each cycle represents a 1 h of eNOR electrolysis and fresh electrolytes were used in each cycle. Stable NO₃⁻ formation rate of ˜20 and ˜45 μg h⁻¹ cm⁻² was observed in N₂ and air atmosphere, respectively, throughout the five consecutive cycles. The XRD pattern recorded for the electrode before and after electrolysis displayed no phase change, oxide, or nitride formation. ICP analysis of these solutions didn't show signs of Rh after the continuous polarization.

Conversion of Electrochemically Generated Nitrate Via eNOR to NH 3 via eNO 3 RR

[0370]In the second step of the proposed route of ammonia synthesis, the electrochemically generated NO₃⁻ was converted to NH₃ via eNO₃RR. In order to find a suitable eNO₃RR catalyst, the eNO₃RR activity of the 5, 20 and 40 wt. % Rh/C catalysts was measured in Ar-saturated 0.1 M KOH containing simulated 1.0 mM KNO₃ electrolyte at selected applied potentials. The NH₃ formation rate and FE of the Rh/C catalysts was compared with the eNO₃RR performance of the RuO₂ catalyst under similar testing conditions.

[0371]Among the three different Rh/C catalysts, the 40 wt. % Rh/C catalyst exhibited a higher ammonia formation rate of 25.4 μg h⁻¹ cm⁻² while the rate of 20 wt. % Rh/C and 5 wt. % Rh/C catalysts exhibited lower ammonia formation rates of 11.9 and 3.7 μg h⁻¹ cm⁻², respectively at 0 V vs. RHE. However, the RuO₂ catalyst resulted in an NH₃ formation rate of 48.2 μg h⁻¹ cm⁻² at 0 V Vs. RHE surpassing the eNO₃RR performance of all three Rh/C catalysts. Additionally, the Faradaic efficiency of Rh/C of the 40 wt. % was found to be three times lower than that of RuO₂ (11.9%) at 0 V vs. RHE when tested with the same metal catalyst loading (1.6 mg cm⁻²). Hence, the RuO₂ catalyst was selected as a suitable catalyst for the reduction of electrochemically gencrated nitrate at an optimum applied potential of 0 V vs. RHE.

[0372]eNOR was performed at different durations such as 1, 3, 5, 10, and 24 h at 1.7 V vs. RHE using a 5 wt. % Rh/C catalyst in air saturated 0.1 M KOH solutions. Nitrate concentrations were recorded as 79.9, 206.6, 347.4, 428.2 and 496.0 μM during the applied selected electrolysis time. At longer eNOR duration, the nitrate accumulates in the electrolyte. After 10 h of eNOR, the detected nitrate concentration was 428.2 μM, which was slightly increased to 496.0 μM after 24 h. Before and after eNO₃RR the nitrite concentration decreased from 490.9 to 283.2 μM, matching with the produced ammonia concentration (283.6 μM). These results depict the successful quantitative indirect electrochemical conversion of N₂ to NH₃ via NO₃⁻ synthesis. Ammonia formation rate shown above is 2.73 higher than direct N₂ reduction via eNRR on RuO₂ accompanied by a 22.3 times improvement in the FE (16.5 μg h⁻¹ cm⁻² and FE of 0.26%). This demonstrates the advantage of a combined cycle of eNOR and eNO₃RR approach demonstrated in this work for large-scale ammonia synthesis under ambient conditions.

Continuous NH 3 Synthesis by Coupling of eNOR and eNO 3 RR

[0373]The electrochemical conversion of N₂ to NH₃ via the formation of NO₃⁻ was demonstrated in two steps, i.e., oxidation of N₂ to NO₃⁻ in the first step via eNOR followed by the reduction of the formed NO₃⁻ to NH₃ via eNO₃RR. However, this two-step process may not be suitable for practical application due to energy consumption in each step separately, due to high energy consumption and HER and OER during the eNOR and eNO₃RR.

[0374]Accordingly, the inventors propose an alternative electrochemical cell for ammonia synthesis from nitrogen suitable for continuous operation mode (depicted in FIG. 9). FIG. 9 presents a single electrochemical cell configured for producing ammonia at the cathode and generating nitrate at the anode, which is pumped into the cathode side. The required applied voltage is similar to the voltage designated to each reaction (˜2 V). After a sufficient time period of electrolysis, the N₂/Air saturated supporting electrolyte (containing the electrochemically formed NO₃⁻) from the first anode chamber is fed into the second cathode chamber, where it undergoes reduction to ammonia. This combined approach to the synthesis of ammonia will be energetically superior over current eNRR thus being suitable for an alternative industrial ammonia synthesis.

[0375]Furthermore, the inventors designed and successfully implemented an exemplary cell for an efficient synthesis of ammonia from nitrogen. Since nitrogen reduction is accompanied by massive generation of hydrogen, the proposed cell design efficiently recycles the resulting hydrogen. The inventors surprisingly observed that the introduction of a nitrogen/hydrogen mixture into the cell enhanced the reaction rate and as a consequence the overall FE of the process.

[0376]A non-limiting exemplary apparatus of the invention (electrochemical cell) as designed and implemented by the inventors is presented in FIG. 8.

[0377]FIG. 8 presents a non-limiting setup of a cell manufactured by the inventors and incorporating a 3-D/thick electrode for cathode in a centrifuge tube. A disc shaped Ni foam coated with Pt-Black is used as anode. O-rings are used as a spacer between anode and cathode. It also helps to direct the flow of gases through the center rather than sides.

[0378]A cylindrical carbon felt coated with catalyst is used as cathode and a Ni foam is placed below as current collector. Connections from anode and cathode to potentiostat terminals have been manufactured using Ni wires. Springs are introduced to induce compression to the carbon felt.

[0379]The nitrogen purged electrolyte is circulated through the cell using a peristaltic pump. External nitrogen is also flown through the cell along with the electrolyte via inlet. Inside the cell, during NRR NH₃ and H₂ are produced and are carried to 1 mM H₂SO₄ trap via gas outlet, where NH₃ is trapped. H₂ and excess N₂ are recirculated back to the cell.

[0380]An optional gas flow within the cell is as follows: initially nitrogen flow is induced through the cell where it undergoes reduction to ammonia at the cathode. The protons in the electrolyte also get reduced forming hydrogen. Anode can oxidize some hydrogen back to protons. Through the gas outlet the resulting gases, excess nitrogen, formed ammonia and hydrogen is directed to an ammonia trap, where the ammonia gets trapped, and the rest of the gases are recirculated to the cell.

[0381]The inventors observed that the addition of hydrogen (e.g. external or recycled hydrogen from the NRR or form NO3RR) to the gas stream (comprising nitrogen) entering the supersaturated electrolyte solution (in this case 50 wt. % CaBr₂) substantially enhances rection rate and efficiency of the electrochemical reaction.

[0382]The reaction conditions were as follows: Cathode=Pt-Ru on carbon cloth, 4.09 mg/cm²; Anode=Pt black on Teflonized Toray carbon paper, 3.6 mg/cm²; N₂:H₂ flow rate=2:1 (100 ccpm:50 ccpm).

[0383]Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

[0384]All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. To the extent that section headings are used, they should not be construed as necessarily limiting.