Plasma-electrocatalyst method and apparatus for ammonia synthesis

Abstract

A method of producing ammonia includes generating a plasma in contact with an electrolyte solution from a precursor gas comprising nitrogen (N2). NOx species are formed in the plasma, and at least some of the NOx species enter the electrolyte solution, forming nitrates (NO3-) and / or nitrites (NO2-). In the electrolyte solution, the nitrates (NO3-) and / or nitrites (NO2-) are exposed to a counter electrode comprising an electrocatalyst, whereby electrocatalytic reduction of the nitrates and / or nitrites occurs and ammonium (NH4+) is formed in the electrolyte solution. An apparatus for producing ammonia includes a plasma electrode, a container configured to hold an electrolyte solution, a counter electrode comprising an electrocatalyst positioned in the container, and a first power source having a first terminal configured for connection to the plasma electrode.

Description

PLASMA-ELECTROCATALYST METHOD AND APPARATUS FOR AMMONIA SYNTHESIS RELATED APPLICATION

[0001] The present patent document claims the benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No.63 / 668,550, which was filed on July 8, 2024, and is hereby incorporated by reference in its entirety. FEDERALLY FUNDED RESEARCH AND DEVELOPMENT

[0002] This invention was made with government support under N66001-22-2-4033 awarded by the Department of Defense. The government has certain rights in the invention. TECHNICAL FIELD

[0003] This disclosure is related generally to a method of ammonia synthesis and more particularly to a plasma-electrocatalyst method and apparatus for producing ammonia. BACKGROUND

[0004] Ammonia is the second-most produced chemical by mass and an indispensable commodity mainly due to its utilization in fertilizers. Currently, ammonia is predominantly produced at large scale via the Haber-Bosch (H-B) process through a high pressure, high temperature catalytic reaction of nitrogen and hydrogen. Due to using fossil fuels as the source of hydrogen, the synthesis of this “brown” ammonia contributes 15-20% of all greenhouse gas emissions from the chemical sector. Moreover, the H-B process is only economically feasible in centralized plants with significant infrastructure and capital cost and is not compatible with renewable energy sources. For these reasons, alternative approaches that can synthesize “green” ammonia at ambient conditions from renewable sources in a small-scale, decentralized, and on-demand approach have received substantial and growing attention. A major challenge is activating the highly stable triple- bond nitrogen molecule, a process known as nitrogen fixation.

[0005] Among the various methods that have been studied, non-thermal plasmas have shown great potential in nitrogen fixation and ammonia synthesis with high selectivity and faradaic efficiencies. Plasmas are electrical discharges in gases that contain highly energetic electrons capable of activating gases, such as nitrogen (N2) and oxygen (O2), and producing nitrogen-containing compounds at room temperature and pressure. The theoretical minimum energy for nitrogen fixation via a plasma process has been estimated to be over 2.5 times lower than that of the H-B process (e.g., 0.2 MJ / mol N vs.0.5-0.6 MJ / mol N). However, existing plasma processes are plagued by a high energy cost and poor selectivity towards ammonia compared to the Haber-Bosch process. BRIEF SUMMARY

[0006] A plasma-electrocatalyst method and apparatus for ammonia synthesis are described in this disclosure.

[0007] The method includes generating a plasma in contact with an electrolyte solution from a precursor gas comprising nitrogen (N2). NOx species are formed in the plasma, and at least some of the NOxspecies enter the electrolyte solution, forming nitrates (NO3-) and / or nitrites (NO2-). In the electrolyte solution, the nitrates (NO3-) and / or nitrites (NO2-) are exposed to a counter electrode comprising an electrocatalyst, whereby electrocatalytic reduction of the nitrates and / or nitrites occurs and ammonium (NH4+) is formed in the electrolyte solution.

[0008] The apparatus includes a plasma electrode, a container configured to hold an electrolyte solution, a counter electrode comprising an electrocatalyst positioned in the container, and a first power source having a first terminal configured for connection to the plasma electrode. A second terminal of the first power source may be configured for connection to the counter electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG.1 illustrates an example of the plasma-electrocatalyst method and apparatus for batch production of ammonium.

[0010] FIG.2 illustrates an example of the plasma-electrocatalyst method and apparatus for batch or continuous production of ammonium.

[0011] FIG.3 illustrates an example of the plasma-electrocatalyst method and apparatus, where first and second power sources are utilized, and where the plasma electrode functions as an anode.

[0012] FIG.4 illustrates another example of the plasma-electrocatalyst method and apparatus, where first and second power sources are utilized, and where the plasma electrode functions as a cathode.

[0013] FIGS.5A and 5B illustrate additional examples of the plasma-electrocatalyst method and apparatus, where first and second power sources are utilized and one of the electrodes is shared.

[0014] FIG.6 shows, for the apparatus of FIG.2, electrocatalyst potential vs. Ag / AgCl as a function of plasma current for an exemplary electrocatalyst (Co3O4 / CNT on a Ti counter electrode (“Co3O4 / CNT / Ti”) after two hours of operation. DETAILED DESCRIPTION OF THE DRAWINGS

[0015] Described in this disclosure is a plasma-based process that may improve the energy efficiency of ammonia synthesis by integrating a plasma and electrocatalyst into a single reactor. The plasma enables oxidation of molecular nitrogen to NOx species, which form nitrates and / or nitrites in an electrolyte solution, and the electrocatalyst facilitates reduction of the nitrates / nitrites to ammonia. By carrying out oxidation and reduction in a single reactor where the plasma and electrocatalyst are integrated, the overall energy cost and footprint of the apparatus may be minimized, enabling ammonia generation in a cost- effective and portable system.

[0016] Referring to FIG.1, the method includes generating a plasma discharge (or plasma) 102 in contact with an electrolyte solution 104 from a precursor gas comprising nitrogen (N2) (e.g., air) 106. The plasma discharge may contain different species, including electrons, ions, radicals, and molecules at disparate temperatures. The high- energy electrons in the plasma can activate molecules in the precursor gas 106, including nitrogen (N2) and oxygen (O2), and promote chemical reactions between them. These chemical reactions result in the formation of NOx species 108 in the gas phase, such as NO and NO2, as illustrated. At least some of the NOxspecies 108 enter the electrolytesolution 104 and may undergo hydrolysis and / or dissolution, such that nitrates (NO3-) and / or nitrites (NO2-) 110 are formed in the electrolyte solution 104.

[0017] In the electrolyte solution 104, the nitrates and / or nitrites 110 are exposed to a counter electrode 112 comprising an electrocatalyst 114, which facilitates reduction of the nitrates and / or nitrites 110 to ammonium (NH4+). More specifically, electrocatalytic reduction occurs and ammonium ions 116 are formed in the electrolyte solution 104 when the counter electrode / electrocatalyst 112,114 is at a negative potential. The counter electrode 112 and / or electrocatalyst 114 may be partly or fully submerged in the electrolyte solution 104. Oxidation reactions in the plasma 102 and reduction of the nitrates and / or nitrites 110 to ammonium in the electrolyte solution 104 may occur simultaneously.

[0018] As can be seen in FIG.1, the plasma 102 is generated by biasing a plasma electrode 118 with a sufficient electric potential. The atmospheric pressure process may take place at room temperature (e.g., 20-24°C), although the plasma 102 may have a bulk temperature slightly above room temperature. The plasma electrode 118 and the counter electrode 112 are electrically connected to a power source 120, more specifically, to the same power source 120, such that the potential applied to the plasma electrode 118 and the potential applied to the counter electrode 112 are interdependent. In other words, the potentials are not independently controllable. The plasma electrode 118 is configured as an anode, and the counter electrode 112 / electrocatalyst 114 is configured as a cathode. Accordingly, in use, the power supply 120 applies an anodic (positive) potential to the plasma electrode 118 and a cathodic (negative) potential to the counter electrode 112. More specifically, a positive high-voltage direct current (DC) may be applied to the plasma electrode 118. The power source 120 may supply continuous DC or pulsed DC.

[0019] As indicated above, the plasma electrode 118 is exposed to air, molecular nitrogen (N2) or another precursor gas 106 during application of the positive potential, and a plasma discharge 102 is generated in the space between the tip 118a of the plasma electrode 118 and the surface of the electrolyte solution 104. As a result, electric current flows between the counter electrode 112 and the plasma electrode 118, creating what may be referred to as a plasma circuit 150. As shown in FIG.1, the tip 118a may not be in physical contact with the electrolyte solution 104. In other examples, the tip 118a of theplasma electrode 118 may be in contact with or submerged in the electrolyte solution 104, such that the plasma discharge is formed in the electrolyte solution 104.

[0020] As the plasma 102 is being generated, the electrical current may be controlled. Accordingly, the negative potential at the counter electrode 112 (or at the electrocatalyst 114, as these can be assumed to be the same) may vary as a function of time. The negative potential may vary in response to changes in the electrical current and / or to changes in conductivity of the electrolyte solution 104. FIG.6 discussed in the Examples below shows the electrocatalyst potential vs. Ag / AgCl as a function of plasma current for an exemplary electrocatalyst (Co3O4 / CNT on a Ti counter electrode (“Co3O4 / CNT / Ti”) after two hours of operation.

[0021] The electrolyte solution 104 comprises an electrically conductive liquid such as water. The pH, conductivity, and composition of the electrolyte solution 104 may vary during the process and optionally may be controlled. The electrolyte solution 104 may include deionized water or tap water for a neutral pH (e.g., pH ~ 7), one or more acids such as H2SO4for low pH (e.g., pH ≤ 6), and one or more bases such as NaOH for high pH (e.g., pH ≥ 8). Salts such as NaCl or NaHCO3 may be added to influence the conductivity and pH of the electrolyte solution 104. Broadly speaking, additives such as salt(s), acid(s), and / or base(s) may be incorporated into the electrolyte solution 104 over time to regulate the properties of the electrolyte solution 104. Initial data suggest the production rate of both NOx- and NH4+may be higher in acidic or neutral electrolyte solutions. The temperature of the electrolyte solution 104 may be controlled. The temperature regulation may be achieved by directly heating up or cooling down the electrolyte solution 104 and / or by positioning the apparatus 100 in a temperature- controlled environment.

[0022] The electrolyte solution 104 may be held in a container 122 configured for batch or continuous collection of ammonium (NH4+) 116. In batch mode, the container 122 may hold a static volume of the electrolyte solution 104 and the ammonium 116 produced by electrocatalytic reduction may be collected at the conclusion of the process (e.g., when a desired concentration of the ammonium is obtained). Typically, electrocatalytic reduction is carried out for at least about 30 minutes to produce a sufficient amount of ammonium 116 in the electrolyte solution 104. Alternatively, theprocess may be carried out in continuous mode where fresh electrolyte solution 114 is continuously fed into the container 122 and the ammonium-enriched electrolyte solution 114 is continuously delivered out of the container 122. Referring to FIG.2, the container 122 may be a closed container 124 that may alternatively be referred to as a reactor 124.

[0023] If the container 122 holding the electrolyte solution 104 is an open container as shown in FIG.1, then the precursor gas containing nitrogen (e.g., air or molecular nitrogen separated from air) 106 may be readily available from the surrounding environment. In some examples, such as when the container 122 is a closed container 124, the air or other precursor gas 106 may be delivered into the closed container 124 through one or more inlets (“the first inlet”) 126, as discussed below. The precursor gas 106 may comprise air, pure nitrogen (N2), or a mixture of nitrogen (N2) and oxygen (O2) with a ratio different from air. In some examples, the precursor gas 106 may also include an inert gas, such as argon or helium. Other gases, e.g., reactive gases such as carbon dioxide, may be incorporated into the process (e.g., delivered into the reactor 124) for production of chemicals other than ammonia, such as urea (NH2CONH2). The temperature of the precursor gas 106, which is understood to include one or more gases, may be controlled, e.g., by directly heating up or cooling down the precursor gas 106 and / or by positioning the apparatus 100 in a temperature-controlled environment.

[0024] The electrocatalyst 114 comprises a material capable of catalyzing the reduction of nitrates and / or nitrites to ammonium while exposed to a negative electric potential. For example, the electrocatalyst 114 may comprise cobalt (e.g., cobalt oxide), copper, titanium, ruthenium, iron, rhodium, iridium, platinum, palladium, silver, gold and / or carbon (e.g., graphite, carbon nanotubes, etc.). The electrocatalyst 114 may have a size and morphology engineered to provide a high surface area, which may facilitate a higher number of reduction reactions. The electrocatalyst may be micro- or nanostructured. For example, the electrocatalyst 114 may be in particulate form and may have a submicron (< 1 µm) or nanoscale (< 100 nm) particle size. Preferably, particles comprising the electrocatalyst 114 are well-dispersed on the counter electrode 112 and particle agglomeration is avoided. The electrocatalyst 114 may have a multilayer structure. For example, the electrocatalyst 114 may have the form of a particulate coating applied over a high-surface-area template, which may comprise a layer of dispersedmicro- or nanoparticles (e.g., nanotubes or nanowires) supported on the counter electrode 112. In examples described below, the electrocatalyst 114 comprises a layer of cobalt oxide nanoparticles on a layer of carbon nanotubes on a titanium mesh counter electrode 112 (Co3O4 / CNT / Ti). The counter electrode 112 may be constructed from a conductive material such as titanium, platinum, gold, rhodium, or graphite, and may take the form of a mesh, foil, rod, or wire, for example.

[0025] FIGS.1 and 2 illustrate embodiments of an apparatus 100 for producing ammonia. The apparatus 100 includes a plasma electrode 118 and a counter electrode 112 comprising an electrocatalyst 114, and a container 122 configured to hold an electrolyte solution 104. The counter electrode 112 and the electrocatalyst 114 may be positioned in the container 122 (e.g., via a support rod 134, shown in FIG.2). The closed container or reactor 124 of FIG.2 may contain part or all of the plasma electrode 118. In these examples, the apparatus 100 also includes a power source 120 having a terminal (positive terminal (+)) configured for electrical connection to the plasma electrode 118 and a terminal (negative terminal (-)) configured for electrical connection to the counter electrode 112 and the electrocatalyst 114. As illustrated, the support rod 134 may electrically connect the electrocatalyst 114 and counter electrode 112 to the negative terminal (-) of the power source 120. When the plasma electrode 118 and counter electrode / electrocatalyst 112 / 114 are electrically connected to the power supply 120 and a sufficient positive voltage is applied to the plasma electrode 118, the plasma circuit 150 referred to above is formed.

[0026] To promote portability, the container 122 may be a closed container or reactor 124 enclosing some or all of the components of the plasma circuit 150, as illustrated in FIG.2. The reactor 124 may include a first inlet 126 for delivering a precursor gas 106 comprising nitrogen to the plasma electrode 118, a first outlet 128 for removing spent precursor gas and / or venting the reactor 124, a second inlet 130 for introducing or continuously flowing the electrolyte solution 104 into the container 122,124, and / or a second outlet 132 for removing the electrolyte solution (including the ammonium 116 produced) during and / or after electrochemical reduction. Accordingly, the apparatus 100 may be configured for batch and / or continuous collection of ammonium 116. Plasma- treated electrolyte solution 104 may be flowed out of the apparatus 100 either into areservoir for collection, into a second electrochemical reactor (e.g., containing an electrocatalyst and counter electrode) to further reduce NOxspecies, or into a second process / apparatus that utilizes nitrogen products such as ammonium. Gases removed via the first outlet 128 may be flowed into a secondary reservoir, which may contain a solution to capture any remaining NOx or NH3. Both the outlet gases and plasma-treated electrolyte solution 104 may be partially fed back into the apparatus 100 for further plasma treatment.

[0027] As shown in FIG.2, the first inlet 126 may take the form of a tube 138 that surrounds the plasma electrode 118, such that flow of the precursor gas 106 to form the plasma may also assist with cooling the plasma electrode 118. In some examples, the first inlet 126 may take the form of a tube 140 that enters the reactor 124 at a different location, apart from the plasma electrode 118. The tube(s) 138,140 may extend to a position above the electrolyte solution 104, or may extend into the electrolyte solution 104. Flowing the precursor gas 106 directly into the electrolyte solution 104 may produce bubbles that can facilitate chemical reactions under certain conditions. The apparatus 100 may include only one or both of the tubes 138,140, as shown in FIG.2. There are certain situations where using both tubes 138,140 may be advantageous. For example, two gases, such as air and CO2, or O2 and N2, may be flowed into the reactor 124 at the same time. In such cases, one gas may be delivered into the reactor 124 via the inlet / tube 126,138 surrounding the plasma electrode 118, and the other gas may be delivered into the reactor 124 via the other inlet / tube 126,140.

[0028] In some situations, a greater amount of the nitrates and / or nitrates 110 may be produced in the electrolyte solution 104 than can be converted to ammonium 116 by the electrocatalyst 114. In the apparatus 100 illustrated in FIGS.1 and 2, the electric potentials and currents applied to the plasma 102 and the electrocatalyst 114 are interdependent, as mentioned above. The plasma current / potential may determine the production rate of NOxspecies, while the electrocatalyst current / potential may determine the electrocatalyst efficiency, as well as the production rate of NH4+. In order to achieve independent control over the plasma 102 and the electrocatalyst 114, the apparatus 100 may be modified to include two power sources 120,136, e.g., one to operate the plasma102 in the plasma circuit 150, and one to operate a supplemental electrocatalyst 148 in an electrocatalyst circuit 152, as explained in reference to FIGS.3 and 4.

[0029] Referring to FIG.3, the apparatus 100 may further include, in addition to components of the plasma circuit 150, components of an independently-controlled electrocatalyst circuit 152 configured for the reduction of excess nitrates and / or nitrites 110 to ammonium 116 (which are not shown in this figure, but are illustrated in FIGS.1 and 2). The electrocatalyst circuit 152 may include first and second additional electrodes 144,146 along with a second power source 136, where the second additional electrode 146 includes (e.g., is coated with or otherwise contains) a supplemental electrocatalyst 148 capable of catalyzing the reduction of nitrates and / or nitrates 110 to ammonium 116. The second power source 136 has a positive terminal (+) configured for connection to the first additional electrode 144 and a negative terminal (-) configured for connection to the second additional electrode / supplemental electrocatalyst 146,148.

[0030] In use, the first additional electrode 144 and the second additional electrode / supplemental electrocatalyst 146,148 are positioned in the electrolyte solution 104 adjacent to the counter electrode / electrocatalyst 112,114 from the plasma circuit 150. However, the power supplied by the second power source 136 to the electrocatalyst circuit 152 containing the supplemental electrocatalyst 148 is independently controllable from the power supplied by the first power source 120 to the plasma circuit 150. Thus, the electric potential / current applied to the supplemental electrocatalyst 148 may be optimized for reduction of nitrates and / or nitrites to ammonium, and the electric potential / current applied to the plasma electrode 118 may be optimized for the production of NOx species. Since electrocatalytic reduction of the nitrates and / or nitrites 110 in the electrolyte solution 104 may be catalyzed by both the electrocatalyst 114 on the counter electrode 112 and by the supplemental electrocatalyst 148 on the second additional electrode 146, additional ammonium ions 116 may be formed and the reduction efficiency may be increased compared to the apparatus 100 shown in FIG.2. Notably, the first power source 120 that operates the plasma circuit 150 may utilize a higher voltage and lower current than the second power source 136, which operates the electrocatalyst circuit 152. The voltage difference between the two power sources 120,136 may be substantial and thus the energy consumption of the second power source 136 may bemuch lower than that of the first power source 120 (e.g., < 20% of the energy consumption of the first power source 120). Table 4 discussed in the Examples below shows preliminary results for operation of the plasma-electrocatalyst apparatus 100 with one vs. two power sources.

[0031] In the dual power source setup, the main purpose of the plasma circuit 150 is to produce NOxspecies while the main purpose of the electrocatalyst circuit 152 is to reduce nitrates / nitrites to NH4+. Therefore, it is possible to run the plasma 102 in a cathodic or anodic configuration, while operating the supplemental electrocatalyst 148 in a cathodic configuration, as shown in FIG.4. This is in contrast to the single power source setup of FIGS.1 and 2, where the production of NOx species and reduction to NH4+is coupled, such that, if one is cathodic, the other is anodic. In the exemplary apparatus shown in FIG.4, the first power source 120 has a negative terminal electrically connected to the plasma electrode 118 and a positive terminal electrically connected to the counter electrode 112 comprising the electrocatalyst 114; the second power source 136 has a negative terminal electrically connected to the second additional electrode 146 comprising the supplemental electrocatalyst 148 and a positive terminal electrically connected to the first additional electrode 144. The supplemental electrocatalyst 148 is configured as the cathode in order to reduce nitrates and / or nitrites to ammonium.

[0032] It is noted that the description above about the composition and characteristics of the electrocatalyst 114 applies also to the supplemental electrocatalyst 148, which may be the same as or different from the electrocatalyst 114 employed in the plasma circuit 150. Similarly, the first additional electrode 144, which does not include the supplemental electrocatalyst 148, may be the same as or different from the counter electrode 112, and may have any of the characteristics set forth above for the counter electrode 112.

[0033] Alternative embodiments of the plasma-electrocatalyst apparatus are shown in FIGS.5A and 5B. In these examples, the two circuits may share one of the electrodes (either the supplemental catalyst or the first additional electrode), as illustrated. In these configurations, the current and potential of the shared electrode are determined by the synergistic effects of both power supplies, each of which can be independently adjusted for current and voltage.

[0034] Referring first to FIG.5A, the apparatus 100 includes, in addition to the components shown in FIG.2, a second power source 136, and a first additional electrode 144 partially or fully immersed in the electrolyte solution. The apparatus 100 of this example is configured such that the plasma circuit 150 and the electrocatalyst circuit 152 share the electrode 112 comprising the electrocatalyst 114. As illustrated, the first power source 120 has a positive terminal (+) electrically connected to the plasma electrode 118 and a negative terminal (-) electrically connected to the counter electrode 112 comprising the electrocatalyst 114; the second power source 136 has a negative terminal (-) electrically connected to the counter electrode 112 comprising the electrocatalyst 114 and a positive terminal (+) connected to the first additional electrode 144. Accordingly, during the generation of the plasma 102, the first power source 120 and the second power source 136 may be independently adjusted to synergistically determine the electrical current and / or potential applied to the counter electrode / electrocatalyst 112,114.

[0035] Referring to FIG.5B, the apparatus 100 includes, in addition to the components shown in FIG.2, a second power source 136, and a first additional electrode 144 partially or fully immersed in the electrolyte solution. The apparatus 100 of this example is configured such that the plasma circuit 150 and the electrocatalyst circuit 152 share the first additional electrode 114. As illustrated, the first power source 120 has a positive terminal (+) electrically connected to the plasma electrode 118 and a negative terminal (-) electrically connected to the first additional electrode 144; the second power source 136 has a negative terminal (-) electrically connected to the counter electrode 112 comprising the electrocatalyst 114 and a positive terminal (+) connected to the first additional electrode 144. Accordingly, during the generation of the plasma 102, the first power source 120 and the second power source 136 may be independently adjusted to synergistically determine the electrical current and / or potential applied to the first additional electrode 144.

[0036] In some examples, the apparatus 100 may include multiple plasma electrodes capable of forming plasmas. Multiple power supplies, placed near the container or reactor, may be connected to the electrodes with wires for operation of plasmas and electrocatalysts. Each plasma electrode may be connected to one electrocatalyst and onepower supply, forming a circuit, or multiple electrodes and electrocatalysts may be connected to each other and operated with a single power source.

[0037] The plasma-electrocatalyst method and apparatus may find application in agriculture. As described above, the apparatus may include multiple plasma electrodes and electrocatalysts and may be portable. The apparatus may be used by connecting supplies of precursor gas (e.g., air) and electrolyte solution (e.g., water) to the inlets of the apparatus as well as providing electricity (e.g., AC from the grid or DC from renewables) to the power source(s). During operation, water and air may flow into the plasma-electrocatalyst apparatus and ammonia diluted in water may flow out of an outlet of the apparatus (e.g., the “second outlet” described above). The second outlet may be connected to a storage tank or directly to an irrigation system. The plasma-electrocatalyst apparatus and method may allow farmers to produce their own nitrogen fertilizer on-site and on-demand. With this apparatus, farmers can independently produce their own nitrogen fertilizer any time throughout the season. The apparatus may be operated with either renewable energy sources (such as solar and wind) or power from the grid. Since the plasma can turn on / off easily, the operator has the option of operating the apparatus only during off-peak hours or it may be operated continuously over any desired time period.

[0038] Examples

[0039] FIG.6 depicts the relationship between the catalyst potential and plasma current measured in an exemplary plasma-electrocatalyst apparatus 100 utilizing a single power source 120 (e.g., see FIG.2), with H2SO4as electrolyte at pH=1 and cobalt oxide nanoparticles deposited on a carbon nanotube layer supported by a Ti mesh (Co3O4 / CNT / Ti) as the electrocatalyst / counter electrode 114,112. As shown in FIG.6, the electrocatalyst potential shows a linear dependence on electrical current and is between about -0.5 and -3.0 V.

[0040] The following tables present preliminary results of the operation of an exemplary plasma-electrocatalyst apparatus 100 at different conditions utilizing a single power source 120, demonstrating the effects of electrocatalyst material, electrolyte composition, and operating current on the yield of different nitrogen species. In particular, Table 1 shows the production rate of nitrogen species for different catalystmaterials in a plasma-electrocatalyst apparatus with deionized water as the electrolyte and operated at 15 mA of current. Table 2 shows the production rate of nitrogen species at different plasma currents in a plasma-electrocatalyst setup with deionized water as electrolyte and Co3O4 / CNT / Ti catalyst. Table 3 shows the production rate of nitrogen species for various electrolytes in a plasma-electrocatalyst setup with a Co3O4 / CNT / Ti catalyst and operated at 60 mA of current. The production rate of ammonium to NOxspecies (NH4+to NOx ratio) may be at least about 0.01, and is preferably at least about 0.2. Table 1. Production rate of nitrogen species for different catalyst materials. Catalyst material +NH4+to NOx NH4[µmol] NO3- [µmol] NO2- [µmol] ratio Platinum sheet 40 2200 0 0.018 Co3O4 / CNT / Ti 145 2140 0 0.068 Table 2. Production rate of nitrogen species at different plasma currents. Plasma current+4+3- NH4 to NOx NH [µmol] NO [µmol] NO2- [µmol] [mA] ratio 15 145 2140 0 0.068 60 637 3000 0 0.21 Table 3. Production rate of nitrogen species for various electrolytes. NH4+to NOx ElectrolyteNH4+[µmol] NO3- [µmol] NO2- [µmol] ratio Deionized water 637 3000 0 0.21 Tap water 680 3800 12 0.18 H2SO4(pH 1) 670 2400 0 0.28 NaOH (pH 13) 118 360 810 0.10

[0041] Table 4 shows preliminary results for the operation of the plasma- electrocatalyst apparatus with one vs. two power supplies (e.g., comparing the apparatus 100 of FIG.2 with that of FIG.3). The electrolyte is deionized water and both electrocatalysts / counter electrodes is Co3O4 / CNT / Ti. The results show that the use of thesecond power source 136 in addition to the first power source 120 can increase the production of ammonium ions by almost four-fold. Table 4. Comparison of the production rates of nitrogen species in a plasma- electrocatalyst apparatus with one power supply vs. two power supplies. Plasma current+NH4+to NO3- NH4[µmol] NO3- [µmol] NO2- [µmol] [mA] ratio Supply 1: 60 mA 637 3000 0 0.2 Supply 2: - Supply 1: 60 mA 2380 2100 5 1.1 Supply 2: 120 mA

[0042] This disclosure also encompasses the following aspects:

[0043] A first aspect relates to a method of producing ammonia, the method comprising: generating a plasma in contact with an electrolyte solution from a precursor gas comprising nitrogen (N2), whereby NOxspecies are formed in the plasma and at least some of the NOx species enter the electrolyte solution, forming nitrates (NO3-) and / or nitrites (NO2-); and in the electrolyte solution, exposing the nitrates (NO3-) and / or nitrites (NO2-) to a counter electrode comprising an electrocatalyst, whereby electrocatalytic reduction of the nitrates and / or nitrites occurs and ammonium (NH4+) is formed in the electrolyte solution.

[0044] A second aspect relates to the method of the first aspect, wherein generating the plasma comprises: positioning a tip of a plasma electrode adjacent to or within the electrolyte solution; exposing the plasma electrode to the precursor gas comprising nitrogen (N2); and applying an electric potential to the plasma electrode using a first power source, whereby the plasma is generated from the precursor gas and an electrical current flows between the counter electrode and the plasma electrode.

[0045] A third aspect relates to the method of the second aspect, further comprising, as the plasma is being generated, controlling the electrical current.

[0046] A fourth aspect relates to the method of any preceding aspect, wherein an electric potential at the counter electrode varies as a function of time as the plasma is generated.

[0047] A fifth aspect relates to the method of any preceding aspect, wherein the electric potential varies in response to changes in conductivity of the electrolyte solution and / or changes in electrical current.

[0048] A sixth aspect relates to the method of any of the second through the fifth aspects, wherein the tip of the plasma electrode is not in contact with the electrolyte solution.

[0049] A seventh aspect relates to the method of any of the second through the fifth aspects, wherein the tip of the plasma electrode is in contact with the electrolyte solution.

[0050] An eighth aspect relates to the method of any of the second through the seventh aspects, wherein the plasma electrode is electrically connected to a positive terminal of the first power source, the plasma electrode functioning as an anode, and wherein the counter electrode comprising the electrocatalyst is electrically connected to a negative terminal of the first power source, the counter electrode functioning as a cathode.

[0051] A ninth aspect relates to the method of the eighth aspect, further comprising: providing: a second power source; a first additional electrode and a second additional electrode partially or fully immersed in the electrolyte solution, the second additional electrode comprising a supplemental electrocatalyst, wherein the second power source has a positive terminal electrically connected to the first additional electrode and a negative terminal electrically connected to the second additional electrode comprising the supplemental electrocatalyst.

[0052] A tenth aspect relates to the method of the ninth aspect, further comprising, during generation of the plasma: selecting an electrical current and / or voltage of the first power source to optimize formation of the NOxspecies; and selecting an electrical current and / or voltage of the second power source to optimize reduction of the nitrates and / or nitrates to ammonium.

[0053] An eleventh aspect relates to the method of the ninth or tenth aspect, wherein the first power source is operated at a higher voltage and a lower current than the second power source.

[0054] A twelfth aspect relates to the method of the second aspect, further comprising: providing: a second power source; a first additional electrode and a second additionalelectrode partially or fully immersed in the electrolyte solution, the second additional electrode comprising a supplemental electrocatalyst, wherein the first power source has a negative terminal electrically connected to the plasma electrode and a positive terminal electrically connected to the counter electrode comprising the electrocatalyst, wherein the second power source has a negative terminal electrically connected to the second additional electrode comprising the supplemental electrocatalyst and a positive terminal electrically connected to the first additional electrode.

[0055] A thirteenth aspect relates to the method of the twelfth aspect, further comprising, during generation of the plasma: selecting an electrical current and / or voltage of the first power source to optimize formation of the NOx species; and selecting an electrical current and / or voltage of the second power source to optimize reduction of the nitrates and / or nitrates to ammonium.

[0056] A fourteenth aspect relates to the method of the eighth aspect, further comprising: providing: a second power source; a first additional electrode partially or fully immersed in the electrolyte solution, wherein the first power source has a positive terminal electrically connected to the plasma electrode and a negative terminal electrically connected to the counter electrode comprising the electrocatalyst; and wherein the second power source has a negative terminal electrically connected to the counter electrode comprising the electrocatalyst and a positive terminal connected to the first additional electrode.

[0057] A fifteenth aspect relates to the method of the fourteenth aspect, further comprising, during generation of the plasma: independently adjusting the first power source and the second power source to synergistically determine an electrical current and / or potential applied to the counter electrode comprising the electrocatalyst.

[0058] A sixteenth aspect relates to the method of the eighth aspect, further comprising: providing: a second power source; and a first additional electrode partially or fully immersed in the electrolyte solution, wherein the first power source has a positive terminal electrically connected to the plasma electrode and a negative terminal electrically connected to the first additional electrode; wherein the second power source has a positive terminal electrically connected to the first additional electrode and anegative terminal electrically connected to the counter electrode comprising the electrocatalyst.

[0059] A seventeenth aspect relates to the method of the sixteenth aspect, further comprising, during generation of the plasma: independently adjusting the first power source and the second power source to synergistically determine an electrical current and / or potential applied to the first additional electrode.

[0060] An eighteenth aspect relates to the method of any preceding aspect, wherein the precursor gas comprising nitrogen (N2) is selected from the group consisting of pure nitrogen (N2), a mixture of nitrogen (N2) and oxygen (O2), and air.

[0061] A nineteenth aspect relates to the method of any preceding aspect, wherein the precursor gas further comprises an inert gas such as argon or helium, and / or a reactive gas such as carbon dioxide.

[0062] A twentieth aspect relates to the method of any preceding aspect, wherein the electrolyte solution comprises water, such as deionized water.

[0063] A twenty-first aspect relates to the method of any preceding aspect, further comprising incorporating one or more additives selected from the group consisting of salt(s), acid(s), and base(s) into the electrolyte solution during plasma generation, whereby conductivity and / or pH of the electrolyte solution is regulated.

[0064] A twenty-second aspect relates to the method of any preceding aspect, wherein the electrocatalyst comprises cobalt (e.g., cobalt oxide), copper, titanium, ruthenium, iron, rhodium, iridium, platinum, palladium, silver, gold and / or carbon (e.g., graphite, carbon nanotubes).

[0065] A twenty-third aspect relates to the method of any preceding aspect, wherein the electrocatalyst is micro- or nanostructured.

[0066] A twenty-fourth aspect relates to the method of any preceding aspect, wherein the electrocatalyst is in particulate form and has a submicron or nanoscale particle size.

[0067] A twenty-fifth aspect relates to the method of any preceding aspect, being a batch process.

[0068] A twenty-sixth aspect relates to the method of any preceding aspect being a continuous process.

[0069] A twenty-seventh aspect relates to the method of any preceding aspect, wherein a production rate of ammonium, measured as a ratio of NH4+to NOxspecies in the electrolyte solution during or after plasma generation, is at least about 0.01, at least about 0.1, or at least about 0.2.

[0070] A twenty-eighth aspect relates to an apparatus for producing ammonia, the apparatus comprising: a plasma electrode; a container configured to hold an electrolyte solution; a counter electrode positioned in the container, the counter electrode comprising an electrocatalyst; and a first power source having a first terminal configured for connection to the plasma electrode.

[0071] A twenty-ninth aspect relates to the apparatus of the twenty-eighth aspect, wherein the first power source has a second terminal configured for connection to the counter electrode comprising the electrocatalyst.

[0072] A thirtieth aspect relates to the apparatus of the twenty-ninth aspect, wherein the first terminal configured for connection to the plasma electrode is a positive terminal and the second terminal configured for connection to the counter electrode comprising the electrocatalyst is a negative terminal.

[0073] A thirty-first aspect relates to the apparatus of the twenty-ninth aspect, wherein the first terminal configured for connection to the plasma electrode is a negative terminal and the second terminal configured for connection to the counter electrode comprising the electrocatalyst is a positive terminal.

[0074] A thirty-second aspect relates to the apparatus of any preceding aspect, wherein the container is a closed container partially or fully enclosing the plasma electrode and the counter electrode comprising the electrocatalyst.

[0075] A thirty-third aspect relates to the apparatus of the thirty-second aspect, wherein the closed container further comprises: a first inlet for delivery of a precursor gas comprising nitrogen (N2) to the plasma electrode; a first outlet for removal of spent precursor gas or for venting the closed container; a second inlet for introducing the electrolyte solution; and a second outlet for removing the electrolyte solution during or after plasma processing, the electrolyte solution including ammonium.

[0076] A thirty-fourth aspect relates to the apparatus of any preceding aspect, further comprising a second power source and a first additional electrode.

[0077] A thirty-fifth aspect relates to the apparatus of the thirty-fourth aspect, wherein the second power source has a positive terminal configured for connection to the first additional electrode and a negative terminal configured for connection to the counter electrode comprising the electrocatalyst, wherein the first power source further comprises a second terminal configured for connection to the counter electrode comprising the electrocatalyst, whereby the first and second power sources are configured to share the counter electrode comprising the electrocatalyst.

[0078] A thirty-sixth aspect relates to the apparatus of the thirty-fourth aspect, wherein the second power source has a positive terminal configured for connection to the first additional electrode and a negative terminal configured for connection to the counter electrode comprising the electrocatalyst, wherein the first power source further comprises a second terminal configured for connection to the first additional electrode, whereby the first and second power sources are configured to share the first additional electrode.

[0079] A thirty-seventh aspect relates to the apparatus of the thirty-fourth aspect, further comprising a second additional electrode comprising a supplemental electrocatalyst.

[0080] A thirty-eighth aspect relates to the apparatus of the thirty-seventh aspect, wherein the second power source has a positive terminal configured for connection to the first additional electrode and a negative terminal configured for connection to the second additional electrode comprising the supplemental electrocatalyst, wherein the first power source further comprises a second terminal configured for connection to the counter electrode comprising the electrocatalyst, whereby the plasma electrode and the second additional electrode comprising the supplemental catalyst are configured to be independently controllable.

[0081] A thirty-ninth aspect relates to the apparatus of any preceding aspect, wherein the electrocatalyst comprises cobalt (e.g., cobalt oxide), copper, titanium, ruthenium, iron, rhodium, iridium, platinum, palladium, silver, gold and / or carbon (e.g., graphite, carbon nanotubes).

[0082] A fortieth aspect relates to the apparatus of any preceding aspect, wherein the electrocatalyst is micro- or nanostructured.

[0083] A forty-first aspect relates to the apparatus of any preceding aspect, wherein the electrocatalyst is in particulate form and has a submicron or nanoscale particle size.

[0084] A forty-second aspect relates to the apparatus of any preceding aspect, wherein the electrocatalyst has a multilayered structure.

[0085] A forty-third aspect relates to the apparatus of any preceding aspect wherein the electrocatalyst comprises a layer of cobalt oxide nanoparticles on a layer of carbon nanotubes, and wherein the counter electrode comprises titanium.

[0086] A forty-fourth aspect relates to the apparatus of any preceding aspect being portable.

[0087] A forty-fifth aspect relates to a method of producing ammonia using the apparatus of any preceding claim, the method comprising: supplying a precursor gas to the plasma electrode; delivering the electrolyte solution into the container; supplying electricity to the first power source, whereby a plasma is generated by the plasma electrode and electrocatalytic reduction occurs in the electrolyte solution, forming ammonium; and and collecting the ammonium from the electrolyte solution.

[0088] A forty-sixth aspect relates to the method of the forty-fifth aspect, wherein the precursor gas comprises air, and wherein the electrolyte solution comprises water.

[0089] A forty-seventh aspect relates to the method of the forty-fifth or forty-sixth aspect, further comprising producing fertilizer from the ammonium.

[0090] To clarify the use of and to hereby provide notice to the public, the phrases "at least one of , , … and <N>" or "at least one of , , … or <N>" or "at least one of , , … <N>, or combinations thereof" or ", , … and / or <N>" are defined by the Applicant in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted by the Applicant to the contrary, to mean one or more elements selected from the group comprising A, B, … and N. In other words, the phrases mean any combination of one or more of the elements A, B, … or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed. Unless otherwise indicated or the context suggests otherwise, as used herein, "a" or "an" means "at least one" or "one or more."

[0091] While various embodiments have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible. Accordingly, the embodiments described herein are examples, not the only possible embodiments and implementations.

[0092] In addition to the features mentioned in each of the independent aspects enumerated above, some examples may show, alone or in combination, the optional features mentioned in the dependent aspects and / or as disclosed in the description above and shown in the figures.

Claims

CLAIMS 1. A method of producing ammonia, the method comprising: generating a plasma in contact with an electrolyte solution from a precursor gas comprising nitrogen, whereby NOx species are formed in the plasma and at least some of the NOxspecies enter the electrolyte solution, forming nitrates (NO3-) and / or nitrites (NO2-); and in the electrolyte solution, exposing the nitrates (NO3-) and / or nitrites (NO2-) to a counter electrode comprising an electrocatalyst, whereby electrocatalytic reduction of the nitrates and / or nitrites occurs and ammonium (NH4+) is formed in the electrolyte solution.

2. The method of claim 1, wherein generating the plasma comprises: positioning a tip of a plasma electrode adjacent to or within the electrolyte solution; exposing the plasma electrode to the precursor gas comprising nitrogen (N2); and applying an electric potential to the plasma electrode using a first power source, whereby the plasma is generated from the precursor gas and an electrical current flows between the counter electrode and the plasma electrode.

3. The method of claim 2, further comprising, as the plasma is being generated, controlling the electrical current.

4. The method of claim 1, wherein an electric potential at the counter electrode varies as a function of time as the plasma is generated.

5. The method of claim 4, wherein the electric potential varies in response to changes in conductivity of the electrolyte solution and / or changes in electrical current.

6. The method of claim 2, wherein the tip of the plasma electrode is not in contact with the electrolyte solution.

7. The method of claim 2, wherein the tip of the plasma electrode is in contact with the electrolyte solution.

8. The method of claim 2, wherein the plasma electrode is electrically connected to a positive terminal of the first power source, the plasma electrode functioning as an anode, and wherein the counter electrode comprising the electrocatalyst is electrically connected to a negative terminal of the first power source, the counter electrode functioning as a cathode.

9. The method of claim 8, further comprising: providing: a second power source; a first additional electrode and a second additional electrode partially or fully immersed in the electrolyte solution, the second additional electrode comprising a supplemental electrocatalyst, wherein the second power source has a positive terminal electrically connected to the first additional electrode and a negative terminal electrically connected to the second additional electrode comprising the supplemental electrocatalyst.

10. The method of claim 9, further comprising, during generation of the plasma: selecting an electrical current and / or voltage of the first power source to optimize formation of the NOxspecies; and selecting an electrical current and / or voltage of the second power source to optimize reduction of the nitrates and / or nitrates to ammonium.

11. The method of claim 9, wherein the first power source is operated at a higher voltage and a lower current than the second power source.

12. The method of claim 2, further comprising:providing: a second power source; a first additional electrode and a second additional electrode partially or fully immersed in the electrolyte solution, the second additional electrode comprising a supplemental electrocatalyst, wherein the first power source has a negative terminal electrically connected to the plasma electrode and a positive terminal electrically connected to the counter electrode comprising the electrocatalyst, wherein the second power source has a negative terminal electrically connected to the second additional electrode comprising the supplemental electrocatalyst and a positive terminal electrically connected to the first additional electrode.

13. The method of claim 12, further comprising, during generation of the plasma: selecting an electrical current and / or voltage of the first power source to optimize formation of the NOx species; and selecting an electrical current and / or voltage of the second power source to optimize reduction of the nitrates and / or nitrates to ammonium.

14. The method of claim 8, further comprising: providing: a second power source; a first additional electrode partially or fully immersed in the electrolyte solution, wherein the first power source has a positive terminal electrically connected to the plasma electrode and a negative terminal electrically connected to the counter electrode comprising the electrocatalyst; and wherein the second power source has a negative terminal electrically connected to the counter electrode comprising the electrocatalyst and a positive terminal connected to the first additional electrode.

15. The method of claim 14, further comprising, during generation of the plasma: independently adjusting the first power source and the second power source to synergistically determine an electrical current and / or potential applied to the counter electrode comprising the electrocatalyst.

16. The method of claim 8, further comprising: providing: a second power source; and a first additional electrode partially or fully immersed in the electrolyte solution, wherein the first power source has a positive terminal electrically connected to the plasma electrode and a negative terminal electrically connected to the first additional electrode; wherein the second power source has a positive terminal electrically connected to the first additional electrode and a negative terminal electrically connected to the counter electrode comprising the electrocatalyst.

17. The method of claim 16, further comprising, during generation of the plasma: independently adjusting the first power source and the second power source to synergistically determine an electrical current and / or potential applied to the first additional electrode.

18. The method of claim 1, wherein the precursor gas comprising nitrogen (N2) is selected from the group consisting of pure nitrogen (N2), a mixture of nitrogen (N2) and oxygen (O2), and air.

19. The method of claim 1, wherein the precursor gas further comprises an inert gas such as argon or helium, and / or a reactive gas such as carbon dioxide.

20. The method of claim 1, wherein the electrolyte solution comprises water, such as deionized water.

21. The method of claim 1, further comprising incorporating one or more additives selected from the group consisting of salt(s), acid(s), and base(s) into the electrolyte solution during plasma generation, whereby conductivity and / or pH of the electrolyte solution is regulated.

22. The method of claim 1, wherein the electrocatalyst comprises cobalt, copper, titanium, ruthenium, iron, rhodium, iridium, platinum, palladium, silver, gold and / or carbon.

23. The method of claim 1, wherein the electrocatalyst is micro- or nanostructured.

24. The method of claim 1, wherein the electrocatalyst is in particulate form and has a submicron or nanoscale particle size.

25. The method of claim 1 being a batch process.

26. The method of claim 1 being a continuous process.

27. The method of claim 1, wherein a production rate of ammonium, measured as a ratio of NH4+to NOx species in the electrolyte solution during or after plasma generation, is at least about 0.01, at least about 0.1, or at least about 0.

2.

28. An apparatus for producing ammonia, the apparatus comprising: a plasma electrode; a container configured to hold an electrolyte solution;a counter electrode positioned in the container, the counter electrode comprising an electrocatalyst; and a first power source having a first terminal configured for connection to the plasma electrode.

29. The apparatus of claim 28, wherein the first power source has a second terminal configured for connection to the counter electrode comprising the electrocatalyst.

30. The apparatus of claim 29, wherein the first terminal configured for connection to the plasma electrode is a positive terminal and the second terminal configured for connection to the counter electrode comprising the electrocatalyst is a negative terminal.

31. The apparatus of claim 29, wherein the first terminal configured for connection to the plasma electrode is a negative terminal and the second terminal configured for connection to the counter electrode comprising the electrocatalyst is a positive terminal.

32. The apparatus of claim 28, wherein the container is a closed container partially or fully enclosing the plasma electrode and the counter electrode comprising the electrocatalyst.

33. The apparatus of claim 32, wherein the closed container further comprises: a first inlet for delivery of a precursor gas comprising nitrogen (N2) to the plasma electrode; a first outlet for removal of spent precursor gas or for venting the closed container; a second inlet for introducing the electrolyte solution; and a second outlet for removing the electrolyte solution during or after plasma processing, the electrolyte solution including ammonium.

34. The apparatus of claim 28, further comprising a second power source and a first additional electrode.

35. The apparatus of claim 34, wherein the second power source has a positive terminal configured for connection to the first additional electrode and a negative terminal configured for connection to the counter electrode comprising the electrocatalyst, wherein the first power source further comprises a second terminal configured for connection to the counter electrode comprising the electrocatalyst, whereby the first and second power sources are configured to share the counter electrode comprising the electrocatalyst.

36. The apparatus of claim 34, wherein the second power source has a positive terminal configured for connection to the first additional electrode and a negative terminal configured for connection to the counter electrode comprising the electrocatalyst, wherein the first power source further comprises a second terminal configured for connection to the first additional electrode, whereby the first and second power sources are configured to share the first additional electrode.

37. The apparatus of claim 34, further comprising a second additional electrode comprising a supplemental electrocatalyst.

38. The apparatus of claim 37, wherein the second power source has a positive terminal configured for connection to the first additional electrode and a negative terminal configured for connection to the second additional electrode comprising the supplemental electrocatalyst, wherein the first power source further comprises a second terminal configured for connection to the counter electrode comprising the electrocatalyst, whereby the plasma electrode and the second additional electrode comprising the supplemental catalyst are configured to be independently controllable.

39. The apparatus of claim 28, wherein the electrocatalyst comprises cobalt, copper, titanium, ruthenium, iron, rhodium, iridium, platinum, palladium, silver, gold and / or carbon.

40. The apparatus of claim 28, wherein the electrocatalyst is micro- or nanostructured.

41. The apparatus of claim 28, wherein the electrocatalyst is in particulate form and has a submicron or nanoscale particle size.

42. The apparatus of claim 28, wherein the electrocatalyst has a multilayered structure.

43. The apparatus of claim 28, wherein the electrocatalyst comprises a layer of cobalt oxide nanoparticles on a layer of carbon nanotubes, and wherein the counter electrode comprises titanium.

44. The apparatus of claim 28 being portable.

45. A method of producing ammonia, the method comprising: providing the apparatus of claim 28; supplying a precursor gas to the plasma electrode; delivering the electrolyte solution into the container; supplying electricity to the first power source, whereby a plasma is generated by the plasma electrode and electrocatalytic reduction occurs in the electrolyte solution, forming ammonium; and and collecting the ammonium from the electrolyte solution.

46. The method of claim 45, wherein the precursor gas comprises air, and wherein the electrolyte solution comprises water.

47. The method of claim 45, further comprising producing fertilizer from the ammonium.

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