CATALYST FOR ELECTROCHEMICAL SYNTHESIS OF AMMONIA, METHOD FOR PREPARING SAME, AND METHOD FOR REGENERATING SAME

Abstract

The present disclosure relates to a catalyst for electrochemical synthesis of ammonia, which includes a metal sulfide, a method for preparing the same and a method for regenerating the same.

Claims

1. A catalyst for electrochemical synthesis of ammonia, comprising a copper-sulfur compound and N-doped carbon.

2. The catalyst for electrochemical synthesis of ammonia according to claim 1, wherein the copper-sulfur compound is represented by the chemical formula Cu.sub.xS.sub.y (wherein x is 1.7-1.8 when y is 1).

3. The catalyst for electrochemical synthesis of ammonia according to claim 1, wherein the copper-sulfur compound is represented by the chemical formula of Cu.sub.9S.sub.5.

4. The catalyst for electrochemical synthesis of ammonia according to claim 1, wherein the copper-sulfur compound and the N-doped carbon are complexed with each other and dispersed throughout the catalyst.

5. The catalyst for electrochemical synthesis of ammonia according to claim 1, wherein a mass ratio of the copper-sulfur compound is 65-85% based on the total mass of the catalyst.

6. The catalyst for electrochemical synthesis of ammonia according to claim 1, wherein the catalyst has a size of 0.1-10 μm.

7. The catalyst for electrochemical synthesis of ammonia according to claim 1, wherein the catalyst has a specific surface area (SSA) of 5-20 m.sup.2g.sup.−1.

8. The catalyst for electrochemical synthesis of ammonia according to claim 1, wherein the catalyst has a total pore volume of 0.001-0.15 cm.sup.3g.sup.−1.

9. The catalyst for electrochemical synthesis of ammonia according to claim 1, wherein the catalyst has an ammonia production yield of 600 nmol/hcm.sup.2 or higher at −0.5 V.sub.RHE.

10. The catalyst for electrochemical synthesis of ammonia according to claim 1, wherein the catalyst can be regenerated using Na.sub.2S.

11. An electrode for electrochemical synthesis of ammonia, comprising the catalyst for electrochemical synthesis of ammonia according to f claim 1.

12. A method for preparing a catalyst for electrochemical synthesis of ammonia, comprising: a step of preparing a mixture of a copper precursor comprising N and a sulfur precursor comprising C and N; and a step of heat-treating the mixture.

13. The method for preparing a catalyst for electrochemical synthesis of ammonia according to claim 12, wherein the copper precursor is Cu(NO.sub.3).sub.2.6H.sub.2O, and the sulfur precursor is CH.sub.4N.sub.2S.

14. The method for preparing a catalyst for electrochemical synthesis of ammonia according to claim 12, wherein, in the step of preparing the mixture, the copper precursor and the sulfur precursor are pulverized and mixed.

15. The method for preparing a catalyst for electrochemical synthesis of ammonia according to claim 12, wherein the step of heat-treating the mixture is performed by a solid-state reaction.

16. The method for preparing a catalyst for electrochemical synthesis of ammonia according to claim 12, wherein the step of heat-treating the mixture is performed at 400-600° C. for 1-5 hours.

17. A method for regenerating the catalyst for electrochemical synthesis of ammonia according to claim 1, comprising: a step of synthesizing ammonia by electrochemical nitrogen reduction reaction (eNRR) in the presence of the catalyst for electrochemical synthesis of ammonia; and a step of regenerating the catalyst for electrochemical synthesis of ammonia after the step of synthesizing ammonia using Na.sub.2S.

18. The method for regenerating a catalyst for electrochemical synthesis of ammonia according to claim 17, wherein the step of regenerating the catalyst for electrochemical synthesis of ammonia is performed by immersing the catalyst for electrochemical synthesis of ammonia in an aqueous solution comprising Na.sub.2S and an electrolyte.

19. The method for regenerating a catalyst for electrochemical synthesis of ammonia according to claim 17, wherein the step of regenerating the catalyst for electrochemical synthesis of ammonia is performed at a potential of 0.05-0.5 V.sub.RHE for 30 minutes to 5 hours.

20. The method for regenerating a catalyst for electrochemical synthesis of ammonia according to claim 17, wherein, after the step of regenerating the catalyst for electrochemical synthesis of ammonia, the step of synthesizing ammonia is repeated using the regenerated catalyst.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0029] FIGS. 1A-1E show a schematic of a Cu.sub.9S.sub.5/NC synthesis process (1A), an XRD pattern of a Cu.sub.9S.sub.5/NC sample (1B), an SEM image of Cu.sub.9S.sub.5/NC (1C), a TEM image of Cu.sub.9S.sub.5/NC (1D) and an EDX elemental mapping of Cu.sub.9S.sub.5/NC (1E). The insert in FIG. 1D shows the enlarged lattice structure, and the larger image in FIG. 1D shows a result of observing the bulk state (low magnification). The insert in FIG. 1D shows the lattice structure of Cu.sub.9S.sub.5/NC and the crystal lattice of Cu.sub.9S.sub.5 observed by HR-TEM. From the TEM image analysis result, it can be seen that a composite structure wherein N-doped carbon is attached to Cu.sub.9S.sub.5 is formed. The interatomic distance corresponding to the (0015) crystal plane of Cu.sub.9S.sub.5 can be confirmed from the high-resolution TEM result.

[0030] FIGS. 2A-2D show a survey scan of Cu.sub.9S.sub.5/NC (2A) and high-resolution XPS results of Cu 2p (2B), S 2p (2C) and N 1s (2D) of Cu.sub.9S.sub.5/NC.

[0031] FIGS. 3A-3F show CV curves of Cu.sub.9S.sub.5/NC under Ar- and N.sub.2-saturated conditions (3A), NH.sub.3 yield and FE (%) of a Cu.sub.9S.sub.5/NC catalyst at different potentials (−0.4V to −0.8V vs RHE) (3B), a chronoamperometric curve Cu.sub.9S.sub.5/NC at −0.5 V for 16 hours (3C), XRD spectra of a Cu.sub.9S.sub.5/NC catalyst 1 hour after CA at −0.5 V in a N.sub.2-saturated electrolyte solution (3D), chronoamperometric curves of Cu.sub.9S.sub.5/NC at −0.5 V under Ar- or N.sub.2-saturated atmosphere (3E), and an XRD result of a Cu.sub.9S.sub.5/NC catalyst after HER at −0.5 V in an Ar-saturated electrolyte solution (3F).

[0032] FIGS. 4A-4F show the NH.sub.3 yield and FE of Cu.sub.9S.sub.5/NC after repeating NRR test for 5 cycles (4A), XRD patterns of Cu.sub.9S.sub.5/NC after NRR at −0.5 V.sub.RHE without Na.sub.2S treatment and Cu.sub.9S.sub.5/NC after an electrochemical regeneration process at +0.1 V in 5 mM Na.sub.2S (4B), NH.sub.3 yield in a test for determining whether regeneration can be achieved at other potentials (4C), NH.sub.3 yield and FE (%) after repeated electrochemical regeneration processes (4D), a .sup.1H (.sup.14N) NMR analysis result of NRR using a catalyst regenerated by a first electrochemical regeneration process at 0.1 V.sub.RHE (4E), and CV curves of a Cu.sub.9S.sub.5/NC catalyst in 0.5 M Na.sub.2SO.sub.4 depending on the presence or absence of 5 mM Na.sub.2S (4F).

[0033] FIG. 5A schematically illustrates electrochemical NRR and regeneration processes including an electrochemical reaction using a Cu.sub.9S.sub.5/NC electrode. FIG. 5B shows the structure of finally produced Cu.sub.9S.sub.5/NC.

[0034] FIGS. 6A-6B show the SEM (6A) and XRD pattern (6B) of a Cu.sub.9S.sub.5 catalyst synthesized by a hydrothermal method.

[0035] FIGS. 7A-7C show a TGA result of Cu.sub.9S.sub.5/NC (7A), N.sub.2 adsorption isotherms of Cu.sub.9S.sub.5/NC and Cu.sub.9S.sub.5 (7B), and BJH plots of Cu.sub.9S.sub.5/NC and Cu.sub.9S.sub.5 (7C).

[0036] FIGS. 8A-8B show chronoamperometric curves of Cu.sub.9S.sub.5/NC at −0.4 to −0.8 V.sub.RHE (8A), and a result of UV-Vis spectroscopy in a 0.5 M electrolyte after NRR test at −0.4 to −0.8 V.sub.RHE (measurement was made by changing potential from −0.4 V to −0.8 V with a step size of 0.1 V) for 1 hour (8B).

[0037] FIGS. 9A-9C show a result of quantifying NH.sub.3 using a NH.sub.4Cl solution with a known concentration (ppm): (9A) UV-Vis spectra, (9B) calibration curve, (9C) images of solutions with increasing ammonia concentrations treated with the indophenol blue indicator.

[0038] FIG. 10 shows the UV-Vis absorption spectra of bare CP, OCV, N.sub.2 and Ar when treated with a Cu.sub.9S.sub.5/NC catalyst and an indophenol indicator at −0.5 V.sub.RHE for 1 hour.

[0039] FIG. 11 shows the .sup.1H (.sup.14N and .sup.15N) NMR spectra of an electrolyte after NRR at −0.5 V.sub.RHE.

[0040] FIGS. 12A-12D show a result of quantifying N.sub.2H.sub.4 using a solution with a known concentration (ppm): (12A) UV-Vis spectra, (12B) calibration curve, (12C) images of solutions with increasing ammonia concentrations subjected to the Watt and Chrisp method, (12D) UV-Vis absorption spectra of an electrolyte evaluated at −0.4 to −0.7 V.sub.RHE for 1 hour by the Watt and Chrisp method.

[0041] FIGS. 13A-13B show the UV-Vis absorption spectra of Cu.sub.9S.sub.5/NC and Cu.sub.9S.sub.5 at −0.5 V.sub.RHE for 1 hour (13A), and chronoamperometric curve of Cu.sub.9S.sub.5 at −0.5 V.sub.RHE (13B).

[0042] FIGS. 14A-14B show the CV curves of Cu.sub.9S.sub.5 (14A) and Cu.sub.9S.sub.5/NC (14B) in the non-faradaic capacitance at a scan rate of 10-200 mV/s, and FIGS. 14C-14D show the electrochemical double-layer capacitance of Cu.sub.9S.sub.5/NC and Cu.sub.9S.sub.5 (14C) and the N.sub.2-TPD profiles of Cu.sub.9S.sub.5/NC and Cu.sub.9S.sub.5 (14D).

[0043] FIGS. 15A-15B show the high-resolution XPS spectra of Cu 2p (15A) and S 2p (15B) of Cu.sub.9S.sub.5/NC after NRR at −0.5 V.sub.RHE.

[0044] FIG. 16 shows the UV-Vis absorption spectra of bare CP, OCV, N.sub.2 and Ar after chronoamperometric analysis at −0.5 V.sub.RHE for 1 hour using the indophenol indicator. Specifically, after conducting chronoamperometry (CA) experiment of monitoring change in current by changing potential from −0.4 V (RHE) to −0.8 V with a step size of 0.1 V while maintaining each potential for 1 hour, the amount of NH.sub.3 produced for 1 hour at each potential was quantified with absorption spectra.

[0045] FIGS. 17A-17B show the CA curves (17A) and UV-Vis absorption spectra (17B) obtained using a catalyst regenerated by electrochemical regeneration at various potentials (−0.3 to −0.9 V.sub.RHE), detected with the indophenol blue reagent.

[0046] FIGS. 18A-18F show the chronoamperometric curves and UV-Vis spectra of Cu.sub.9S.sub.5/NC after an electrochemical regeneration process. The electrochemical regeneration was carried out once (18A, 18D), twice (18B, 18E) or 3 times (18C, 18F).

[0047] FIGS. 19A-19C show the SEM images of Cu.sub.9S.sub.5/NC, before (19A) and after (19B, 19C) NRR at 0.5 VRH. Prior to the NRR test, the prepared Cu.sub.9S.sub.5/NC has a complicated structure in which Cu.sub.9S.sub.5 and NC are mixed uniformly. After the NRR, the structure collapses, resulting in aggregation of Cu.sub.2O (red circles) and scattering of NC (yellow circles).

[0048] FIG. 20 shows the production cycles of an electrochemical regeneration process using the indophenol indicator as well as images depending on potentials.

[0049] FIG. 21 shows the CV curves of a GC electrode and 5 mM Na.sub.2S in a 0.5 M Na.sub.2SO.sub.4 electrolyte.

BEST MODE

[0050] Hereinafter, specific exemplary embodiments of the present disclosure are described in detail referring to the attached drawings.

[0051] The described exemplary embodiments of the present disclosure are provided for the purpose of illustration only. The exemplary embodiments of the present disclosure may also be embodied into other various forms, without being limited to the described exemplary embodiments.

[0052] The present disclosure may be changed variously and may have various exemplary embodiments. The exemplary embodiments are not intended to limit the present disclosure, and should be understood to include all changes, equivalents and substitutes included in the technical idea and scope of the present disclosure.

[0053] In the present disclosure, when a portion is described to “include” a certain element, it does not mean that another element is excluded but means that another element can be included unless specially stated otherwise.

[0054] Catalyst for Electrochemical Synthesis of Ammonia

[0055] In exemplary embodiments of the present disclosure, there is provided a catalyst for electrochemical synthesis of ammonia, which includes a copper-sulfur compound and N-doped carbon.

[0056] The present disclosure discloses a copper-sulfur compound/NC (N-doped carbon) composite which exhibits superior NRR activity and is prepared by a thermal heating process and a solid-state reaction under an Ar atmosphere.

[0057] In an exemplary embodiment, the copper-sulfur compound may be represented by the chemical formula Cu.sub.xS.sub.y (wherein x is 1.7-1.8 when y is 1).

[0058] In an exemplary embodiment, the copper-sulfur compound may be represented by the chemical formula Cu.sub.xS.sub.y (wherein x is 8.5-9 when y is 5).

[0059] In an exemplary embodiment, the copper-sulfur compound may be represented by the chemical formula of Cu.sub.9S.sub.5. The actual structure of the copper-sulfur compound may be the same as (1) in FIG. 5A. When N-doped carbon is present together, a complex may be formed and exist in a dispersed state as shown in FIG. 5B.

[0060] In an exemplary embodiment, the copper-sulfur compound and the N-doped carbon may be complexed with each other and dispersed throughout the catalyst.

[0061] FIG. 5B schematically shows the structure of Cu.sub.9S.sub.5/NC formed according to an exemplary embodiment of the present disclosure. In the figure, the dotted lines means that the illustrated structure is repeated.

[0062] Referring to FIG. 5B, the N-doped carbon serves as a carrier or a support. The copper-sulfur compound may be dispersed on the N-doped carbon, and the N-doped carbon and the copper-sulfur compound may be complexed with each other and dispersed throughout the catalyst as a 3-dimensional structure.

[0063] In an exemplary embodiment, a mass ratio of the copper-sulfur compound may be 65-85% based on the total mass of the catalyst. If the mass ratio is lower than 65%, the apparent activity of an electrode may be low due to insufficient catalyst loading. And, if it exceeds 85%, the mass activity of the catalyst may decrease due to aggregation of excess catalyst and it may be difficult to achieve an NRR electron transfer effect due to low NC content.

[0064] In an exemplary embodiment, the catalyst may have a size of 0.1-10 μm.

[0065] In an exemplary embodiment, the catalyst may have a specific surface area (SSA) of 5-20 m.sup.2g.sup.−1.

[0066] In an exemplary embodiment, the catalyst may have a total pore volume of 0.001-0.15 cm.sup.3g.sup.−1.

[0067] In an exemplary embodiment, the catalyst may have an ammonia production yield of 600 nmol/hcm.sup.2 or higher at −0.5 V.sub.RHE.

[0068] In an exemplary embodiment, the catalyst may be regenerated using Na.sub.2S.

[0069] Another exemplary embodiment of the present disclosure provides an electrode for ammonia synthesis including the catalyst for ammonia synthesis.

[0070] Method for Preparing Catalyst for Electrochemical Synthesis of Ammonia

[0071] Another exemplary embodiment of the present disclosure provides a method for preparing a catalyst for electrochemical synthesis of ammonia, which includes: a step of preparing a mixture of a copper precursor comprising N and a sulfur precursor comprising C and N; and a step of heat-treating the mixture (see FIG. 1A).

[0072] In an exemplary embodiment, the copper precursor may be Cu(NO.sub.3).sub.2.6H.sub.2O and the sulfur precursor may be CH.sub.4N.sub.2S.

[0073] The copper precursor is a hydrate Cu(NO.sub.3).sub.2.H.sub.2O and the sulfur precursor is thiourea (CH.sub.4N.sub.2S). As Cu.sub.9S.sub.5 is formed from the S of the sulfur precursor and the Cu of the copper precursor under a high-temperature condition, and N-doped carbon is synthesized from the remaining C and N sources.

[0074] In an exemplary embodiment, in the step of preparing the mixture, the copper precursor and the sulfur precursor may be pulverized and mixed.

[0075] In an exemplary embodiment, the step of heat-treating the mixture may be performed by a solid-state reaction.

[0076] In an exemplary embodiment, the step of heat-treating the mixture may be performed at 400-600° C. for 1-5 hours.

[0077] In an exemplary embodiment, the step of heat-treating the mixture may be performed under Ar atmosphere.

[0078] Method for Regenerating Catalyst for Electrochemical Synthesis of Ammonia

[0079] Another exemplary embodiment of the present disclosure provides a method for regenerating a catalyst for ammonia synthesis, which includes: a step of synthesizing ammonia by electrochemical nitrogen reduction reaction (eNRR) in the presence of a catalyst for electrochemical synthesis of ammonia; and a step of regenerating the catalyst for electrochemical synthesis of ammonia after the step of synthesizing ammonia using Na.sub.2S.

[0080] Despite the simple synthesis process and remarkable NRR activity of the catalyst for ammonia synthesis described above, the catalyst experiences composition change to Cu.sub.2O during NRR due to the weak durability of the metal sulfide/NC (N-doped carbon). In order to overcome the disadvantage of the metal sulfide-based catalyst, the deteriorated of the catalyst such as Cu.sub.9S.sub.5/NC, i.e., Cu.sub.2O, may be regenerated by an electrochemical regeneration process in between the continuous eNRR reaction. During the electrochemical metal oxide/sulfide redox regeneration cycle, NH.sub.3 and H.sub.2 are produced from dissolved N.sub.2 and H.sub.2O through repeated N.sub.2 fixation reactions.

[0081] Until now, electrochemical NH.sub.3 production using a catalyst regenerated by oxide/sulfide redox cycles mimicking the natural regeneration of a N.sub.2 fixation catalyst has not been reported yet. This unprecedented method for achieving remarkable eNRR reaction provides an insight for an electrochemical N.sub.2 reduction mechanism on the metal sulfide surface. Accordingly, it is expected that regeneration redox cycle of the eNRR catalyst provided in the present disclosure will provide a new method for preparing a material for an effective eNRR catalyst.

[0082] In an exemplary embodiment, the step of regenerating the catalyst for electrochemical synthesis of ammonia may be performed by immersing the catalyst for electrochemical synthesis of ammonia in an aqueous solution containing Na.sub.2S and an electrolyte.

[0083] In an exemplary embodiment, the step of regenerating the catalyst for electrochemical synthesis of ammonia may be performed at a potential of 0.05-1 V.sub.RHE for 30 minutes to 5 hours.

[0084] In an exemplary embodiment, after the step of regenerating the catalyst for electrochemical synthesis of ammonia, the step of synthesizing ammonia may be repeated using the regenerated catalyst.

[0085] Hereinafter, the present disclosure will be described in more detail through examples.

[0086] However, the following examples are for illustrating the present disclosure in more detail, and it will be obvious to those having ordinary skill in the art that the scope and contents of the present disclosure are not reduced or limited by the examples. In addition, it is also obvious that those having ordinary skill in the art that can easily carry out the present disclosure based on the disclosure of the present disclosure even for the matters experimental data of which are not presented and such changes and modifications belong to the scope of the appended claims.

EXAMPLES

[0087] Experimental Methods

[0088] Reagents and Chemicals

[0089] CuCl.sub.2.2H.sub.2O (Sigma-Aldrich), Cu(NO.sub.3).sub.2.2.5H.sub.2O (Sigma-Aldrich), ethylene glycol (99.8%, Sigma-Aldrich), Na.sub.2S (99%, Sigma-Aldrich), p-dimethylaminobenzaldehyde (99%, Sigma-Aldrich), thiourea (99%, Junsei), ethanol (95%, Daejung), Nafion resin solution (5 wt %, Sigma-Aldrich) and isopropyl alcohol (99.5%, Duksan) were used. All chemicals were of analytical grade and used without further purification.

[0090] Synthesis of Cu.sub.9S.sub.5/NC (Catalyst for Electrochemical Synthesis of Ammonia)

[0091] A Cu.sub.9S.sub.5/NC catalyst was prepared by pulverizing Cu(NO.sub.3).sub.2.2.5H.sub.2O and CH.sub.4N.sub.2S powders and then heat-treating the same in a crucible (FIG. 1A).

[0092] Specifically, Cu(NO.sub.3).sub.2.2.5H.sub.2O (0.8 g) and thiourea (1.2 g) were mixed as follows. The precursors were mixed well physically in an agate mortar by agitating for about 5 minutes.

[0093] After transferring to a ceramic crucible, the mixture was heated in a tube furnace at 773 K (500° C.) for 1 hour with a heating rate of 8.3 K/min under Ar gas atmosphere. The prepared catalyst was washed 4 times with deionized water.

[0094] Synthesis of Cu.sub.9S.sub.5

[0095] CuCl.sub.2.2H.sub.2O (1 g) and thiourea (1 g) were added to ethylene glycol (40 mL). After stirring for 30 minutes, the precursors were heated in an oven controlled to 150° C. (423 K) for 4 hours under an autoclave condition. Then, the prepared catalyst was washed several times with deionized water and ethanol.

[0096] NRR Measurement

[0097] For preparation of a working electrode, the wet-dispersed Cu.sub.9S.sub.5/NC or Cu.sub.9S.sub.5 catalyst was sprayed manually onto carbon paper (2.5 cm×2.5 cm, Toray T-120) and then dried in a desiccator at ambient temperature. The loading amount of the catalyst coated on the electrode was set to 1 mg/cm.sup.2. A catalyst ink was prepared by mixing 10 mg of the catalyst powder with 1 mL of an IPA solution. Prior to starting experiment, the working electrode was rinsed with 0.5 M Na.sub.2SO.sub.4 (pH 7) to remove water-soluble pollutants attached to the surface. As an electrolyte for eNRR, 0.5 M Na.sub.2SO.sub.4 (99%, aqueous solution) was used. 10 mM H.sub.2SO.sub.4 was used to trap NH.sub.3 in the electrolysis chamber. The cathode and anode chambers of H-cell were separated using an anion exchange membrane (FAA-3, Fumatech), which were pretreated with 1 M KOH for 24 hours prior to the test, and then further pretreated with DI water for 1 hour. The Bio-Logic SP-300 potentiostat was used for all electrochemical experiments. Electrochemical experiments were conducted at room temperature under atmospheric pressure. For a 3-electrode system, a standard calomel electrode (SCE) and a graphite rod were used as a reference electrode and a counter electrode, respectively, and carbon paper covered with the catalyst powder was used as a working electrode. All potentials were referenced to the reversible hydrogen electrode scale. The activity of electrochemical ammonia synthesis was measured by chronoamperometry (CA) for different potentials from −0.4 to −0.8 V.sub.RHE for an hour. For NRR, .sup.14N.sub.2 (99.999%, Sinyang) and .sup.15N.sub.2 (98 atom %, Sigma) were bubbled for about 20 minutes prior to measurement and then purged into the cathode compartment at a flow rate of 250 ccm and 5 ccm, respectively, during the experiment. The redox characteristics of the catalyst loaded onto a glossy carbon (GC) electrode were investigated in different electrolytes by cyclic voltammetry (CV) at a scan rate of 50 mV/s.

[0098] Regeneration of Metal Chalcogenide Electrode

[0099] During the NRR measurement, the Cu.sub.9S.sub.5/NC electrode was decomposed in an aqueous solution (0.5 M Na.sub.2SO.sub.4) to Cu.sub.20/NC at −0.5 V.sub.RHE. The electrode was regenerated by oxidizing and restoring to Cu.sub.9S.sub.5 via an electrochemical regeneration process. For regeneration, the used electrode was treated for 1 hour at a constant potential of 0.1 V.sub.RHE in an aqueous solution containing 5 mM Na.sub.2S and 0.5 M Na.sub.2SO.sub.4. After completely washing the regenerated electrode with deionized (DI) water, NRR was measured at −0.5 to −0.7 V.sub.RHE as described above. After repeating N.sub.2 reduction and electrochemical sulfurization processes several times, the produced NH.sub.3 was quantified by the indophenol method.

[0100] Physical Properties

[0101] X-ray diffraction (XRD) patterns were obtained on MiniFlex-2 (Rigaku) equipped with Cu Kα radiation (λ=1.5406 Å). Scanning electron microscopy (SEM) images were obtained using the Inspect F50 microscope with an acceleration voltage of 10 kV. TEM images were recorded using a transmission electron microscope (Talos F200X), and X-ray photoelectron spectroscopy (XPS) spectra were measured using the PHI VersaProbe system with a 100-W ALK α X-ray source.

[0102] Low-temperature nitrogen adsorption-desorption isotherms were measured at 77 K (−196° C.) using the volumetric adsorption analyzer BEL (BEL, Inc., Japan). Prior to the measurement, all the samples were deaerated at 173 K (100° C.) for 12 hours under a constant-volume condition in vacuo. Specific surface area was measured from the nitrogen adsorption isotherms by the Brunnauer-Emmet-Teller (BET) method in the relative pressure (P/P.sub.0) range of 0.05-0.20. Pore size distribution (PSD) was measured by the Barrett-Joyner-Halenda (BJH) method. Total pore volume was determined at P/P.sub.0 of 0.99. Temperature-programmed desorption (TPD) was performed on Micromeritics AutoChem II 2920 TPR/TPD.

[0103] Thermogravimetric analysis (TGA) was conducted on SDT Q600 (TA Instruments Inc., New Castle, De, USA).

[0104] NH.sub.3 Detection

[0105] The concentration of the produced ammonia was measured by the indophenol blue method. First, 1 mL of a sample solution was added to an electrochemical cathode reactor. Then, 1 mL of a phenol solution (0.64 M C.sub.6H.sub.5OH, 0.38 M NaOH and 1.3 mM C.sub.5FEN.sub.6Na.sub.2O) and 1 mL of a hypochlorite solution (55 mM NaOCl and 0.75 M NaOH) were added. After reaction at room temperature for 2 hours, the absorbance of the sample was analyzed at 900-350 nm by UV-vis spectroscopy. UV-Vis spectra were obtained using the Cary UV-vis 100 spectrophotometer (Agilent). Absorbance peaks at 633 nm were calibrated by subtracting the background absorbance measured at 875 nm. In the UV-Vis spectra, the background absorbance means the absorbance of the bulk electrolyte solution at 0 ppm (NH.sub.3). In addition, the production of NH.sub.3 was confirmed from the .sup.14N and .sup.15N spectra acquired by .sup.1H NMR (nuclear magnetic resonance) using Bruker Avance III HD 400 MHz.

[0106] N.sub.2H.sub.4 Detection

[0107] The quantity of the byproduct, N.sub.2H.sub.4, produced during the electrochemical NH.sub.3 production was measured by the Watt and Chrisp method. A mixture of p-dimethylaminobenzaldehyde) (6 g), HCl (37%, 30 mL) and ethanol (300 mL) was used as an indicator. Measurement was performed after mixing 2 mL of an electrolyte and 2 mL of the indicator solution for 5 minutes. For measurement of the hydrazine, a standard curve was plotted from a series of N.sub.2H.sub.4.H.sub.2O solutions of different concentrations, diluted with 0.5 M Na.sub.2SO.sub.4. The absorbance of N.sub.2H.sub.4 was measured at 455 nm.

[0108] Calculation of Faradaic Efficiency (FE) and NH.sub.3 Yield

[0109] Assuming that three electrons are required to produce one NH.sub.3, FE is calculated as follows.

FE=(3F×V×C.sub.NH3)/(m.sub.NH3×Q)

[0110] In the above equation, F is the Faraday constant (96485 C mol.sup.−1), V is the volume (mL) of an electrolyte, C.sub.NH3 is the concentration (g/mL) of NH.sub.3 determined 1 hour after CA for UV-Vis measurement, m.sub.NH3 is the molar mass of NH.sub.3 (17 g/mol), and Q is the total charge (C) accumulated for 1 hour during CA.

[0111] The NH.sub.3 yield is calculated as follows.

[0112] (1) Production speed of NH.sub.3 normalized to the mass of catalyst, r.sub.mass=(C.sub.NH3×V)/(m.sub.NH3×m.sub.Cat)

[0113] (2) Production speed of NH.sub.3 per time, normalized to geometrical area, r.sub.area=(C.sub.NH3×V)/(m.sub.NH3×A)

[0114] In the above equations, C.sub.NH3 is the concentration (g/mL) of determined 1 hour after CA for UV-Vis measurement, V is the volume (mL) of an electrolyte in the cathode chamber, m.sub.NH3 is the molar mass of NH.sub.3 (17 g/mol), m.sub.Cat is the mass (mg) of the cathode catalyst, and A is the geometrical area of the electrode (6.25 cm.sup.2).

TEST EXAMPLES

[0115] Observation of Synthesized Catalyst

[0116] The XRD pattern of the prepared catalyst is shown in FIG. 1B. The XRD peaks of Cu.sub.9S.sub.5/NC were observed at 2θ=27.8°, 29.5°, 32.2°, 37.6°, 42.0°, 5.3°, 48.8° and 54.7°, which correspond to the (0015), (107), (1010), (0114), (0117), (0120), (119) and (1115) planes, respectively, of hexagonal-phase Cu.sub.9S.sub.5 (JCPDS No. 26-0476, FIG. 1B). The morphology of the sample was observed by SEM and TEM (FIGS. 1C-1D). A chunk with no uniform shape can be seen from FIG. 1C. The size of the Cu.sub.9S.sub.5/NC composite was confirmed to be 1-5 μm from the SEM image. The structure of the Cu.sub.9S.sub.5/NC composite can be confirmed from the TEM image. The HR-TEM lattice image of Cu.sub.9S.sub.5/NC shows 0.321-nm lattice fringes corresponding to the (0015) plane of Cu.sub.9S.sub.5 and carbon materials attached to Cu.sub.9S.sub.5 (FIG. 1D). The NC-free Cu.sub.9S.sub.5 sample shows a flower-like morphology along with sheet-like plates with a size of 5-10 μm (FIG. 6A). However, the crystal structure was not affected by the presence of NC as can be seen from the XRD pattern (hexagonal Cu.sub.9S.sub.5, JCPDS No. 26-0476, FIG. 6B). The proportion of the active site of the Cu.sub.9S.sub.5/NC catalyst, i.e., the mass ratio of Cu.sub.9S.sub.5, was 74% as estimated from the TGA analysis (FIG. 7A).

[0117] As a result of energy-dispersive X-ray spectroscopy (EDX) mapping, it was confirmed that Cu, S, C and N were distribute uniformly throughout the entire structure of the Cu.sub.9S.sub.5/NC catalyst (FIG. 1E). The XPS survey spectrum confirms the presence of Cu, S, C and N in the composite and is consistent well with the EDX mapping result (FIG. 2A). The Cu 2p spectra show two main peaks at 933.9 and 953.8 eV, which correspond to Cu and Cu.sup.1+ of Cu.sub.9S.sub.5/NC, respectively. A pair of weak peaks at 943.7 and 958.4 eV correspond to Cu and Cu.sup.2+ of Cu.sub.9S.sub.5/NC, respectively. The remaining peaks (at 934.9 and 954.7 eV) are satellite peaks of Cu (FIG. 2B). In the high-resolution S 2p spectra of FIG. 2C, the peaks at 162.3, 163.5, 164.6, 165.7 and 170.3 eV reveal the presence of the covalent bonds Cu—S (162.3, 163.5 eV), S—S (164.6, 165.7 eV) and Cu—S—C (170.3 eV). The high-resolution N 1s spectra are shown in FIG. 2D. The four peaks at 400.1, 401.1, 402 and 406.7 eV correspond to pyridinic N, pyrrolic N, quaternary N and N oxide, respectively.

[0118] The conductive support, i.e., NC, was used in Cu.sub.9S.sub.5/NC to improve the active surface area of the catalyst. For characterization of the catalyst, the textural property of Cu.sub.9S.sub.5/NC and Cu.sub.9S.sub.5 was investigated by N.sub.2 adsorption analysis. From the N.sub.2 adsorption-desorption isotherms, the specific surface area (SSA) and total pore volume of Cu.sub.9S.sub.5/NC and Cu.sub.9S.sub.5 were determined to be 12 m.sup.2g.sup.−1 and 0.084 cm.sup.3g.sup.−1, and 8 m.sup.2g.sup.−1 and 0.051 cm.sup.3g.sup.−1, respectively (FIGS. 7B and 7C).

[0119] As expected, the BJH plots revealed that Cu.sub.9S.sub.5/NC had mesoporosity with a size 2-8 nm larger than Cu.sub.9S.sub.5 (FIG. 7C). Accordingly, it is expected that the larger surface area and porous structure of Cu.sub.9S.sub.5/NC will provide more exposed active sites for a NRR electrocatalyst as compared to the unsupported Cu.sub.9S.sub.5 (FIG. 7C).

[0120] Electrochemical NRR Activity

[0121] The NRR performance of Cu.sub.9S.sub.5/NC was investigated in a N.sub.2-saturated 0.5 M Na.sub.2SO.sub.4 electrolyte using a H-cell under an ambient condition (FIG. 3A). In CV analysis, the current density of N.sub.2-saturated electrolyte solution did not show distinct difference from that observed in an Ar-saturated solution. Whereas it is difficult to find a unique peak current or onset potential for NRR, the onset potential for hydrogen evolution reaction (HER) is clearly detected at about −0.7 V.sub.RHE under N.sub.2 and Ar atmosphere. Instead, the NRR activity of Cu.sub.9S.sub.5/NC can be estimated only from the amount of NH.sub.3 detected in the electrolyte solution after CA for 1 hour at different potentials (FIG. 3B). However, the current observed in CV and CA was changed with time, as can be seen from the changed double layer charging current in the continuous CV cycles (FIG. 3A). This is mainly due to the change in Cu.sub.9S.sub.5 composition under the electrochemical reaction condition (FIG. 4B). If the catalyst is unstable and the NRR activity is limited when compared with HER or degradation current, it is difficult to estimate NRR activity from CV. Accordingly, a longer time period was used in order to investigate the NRR activity and FE of Cu.sub.9S.sub.5/NC from CA at different potentials.

[0122] In order to measure the NRR activity of the Cu.sub.9S.sub.5/NC and Cu.sub.9S.sub.5 catalysts, chronoamperometry (CA) was conducted in a N.sub.2-saturated 0.5 M Na.sub.2SO.sub.4 electrolyte at different potentials in the range from −0.4 to −0.8 V.sub.RHE (FIG. 3B and FIG. 8). The UV-Vis spectra of an indophenol blue solution prepared from the electrolyte solution after the CA experiment are shown in FIG. 9. NH.sub.3 yield was calculated from absorption at 633 nm after subtracting background absorption from a blank solution and comparing with a standard calibration curve (FIG. 9). For the Cu.sub.9S.sub.5/NC surface, the NH.sub.3 production yield reached 11 μg/hcm.sup.2 (or 645 nmol/hcm.sup.2) at −0.5 V.sub.RHE (FIG. 3B). The maximum NH.sub.3 production yield corresponded to a current density of 52 μA/cm.sup.2, which is 35% (FE) of total average current density measured at −0.5 V.sub.RHE. The NRR performance of Cu.sub.9S.sub.5/NC was similar to the highest performance of NRR catalysts reported in previous literatures (see Table 1).

TABLE-US-00001 TABLE 1 Comparison of NRR performance of various samples with previously known data NH.sub.3 yield NH.sub.3 yield Sample Electrolyte FE (%) (μg/hmg.sub.cat) (nmol/hcm.sup.2) Reference Cu.sub.9S.sub.5/NC 0.5M 35 11.0 645 The present Na.sub.2SO.sub.4 11.0 μg/hcm.sup.2 disclosure Fe.sub.SA-N-C 0.1M KOH 56.55 7.48 — Non-patent document 1 SnO.sub.2/RGO 0.1M 7.1 25.6 — Non-patent Na.sub.2SO.sub.4 document 2 Mo.sub.2C/C 0.5M 7.8 11.3 — Non-patent Li.sub.2SO.sub.4 document 3 γ-Fe.sub.2O.sub.3 0.1M KOH 1.96 — 44 Non-patent document 4 Au-Ni 0.05M 67.8 7.4 — Non-patent H.sub.2SO.sub.4 document 5 a-Au/CeOx- 0.1M HCl 8.3 10.1 — Non-patent RGO document 6 ISAS-Fe/NC 0.1M PBS 18.6 62.9 — Non-patent document 7 BiNCs/CB/GC 0.5M 67 — 6.2 × 10.sup.−3 Non-patent K.sub.2SO.sub.4 (mmol/hcm.sup.2) document 8 CoSx/NS-G 0.05M 25.9 25 — Non-patent H.sub.2SO.sub.4 document 9 N,B-FC 0.1M HCl 10.6 16.4 — Non-patent document 10 Au NRs 0.1M KOH 4.02 1.65 — Non-patent document 11 NPC 0.1M HCl 4.2 0.97 — Non-patent document 12 SACs-MoS.sub.2- 0.1M KOH 31.6 91.5 — Non-patent Fe-2.0 (μg/hcm.sup.2) document 13 Ru SAs/N-C 0.05M 29.6 120.9 — Non-patent H.sub.2SO.sub.4 document 14

[0123] Additional controlled CA experiment was conducted to confirm the electrochemical NRR result obtained with the Cu.sub.9S.sub.5/NC catalyst. The conditions of the controlled experiment are as follows: (1) Cu.sub.9S.sub.5/NC-free carbon paper under N.sub.2 atmosphere, (2) Cu.sub.9S.sub.5/NC electrode under N.sub.2 atmosphere, open-circuit potential with no voltage applied, and (3) Cu.sub.9S.sub.5/NC, −0.5 V.sub.RHE in an Ar-saturated 0.5 M Na.sub.2SO.sub.4 electrolyte solution (FIG. 10). Under these conditions, a negligible amount of NH.sub.3 was produced. This experiment confirms that NH.sub.3 was synthesized by electrochemical NRR at controlled potentials in the presence of the supplied N.sub.2 gas. The production of NH.sub.3 was additionally supported .sup.1H (.sup.14N and .sup.15N) NMR analysis and colorimetric observation (FIG. 11). In 1H NMR, the triplet and doublet peaks of .sup.14NH.sub.3 and .sup.15NH.sub.3 were observed. This strongly supports that NH.sub.3 was generated in the electrolyte solution by electrochemical NRR. Although the production of N.sub.2H.sub.4 by side reactions during NRR was investigated by the Watt and Chrisp method, no N.sub.2HA was detected (FIGS. 12C and 12D). Thus, the inventors of the present disclosure concluded that the Cu.sub.9S.sub.5/NC catalyst produces NH.sub.3 via electrochemical fixation of N.sub.2 under ambient conditions. In order to investigate the effect of NC on the NRR activity of Cu.sub.9S.sub.5/NC, the NRR activity of the Cu.sub.9S.sub.5 catalyst was measured in a 0.5 M Na.sub.2SO.sub.4 electrolyte (FIG. 13). The Cu.sub.9S.sub.5 catalyst exhibited an FE of 7.0% and a reaction rate of 7.9 μg/hcm.sup.2 (420 nmol/h cm.sup.2) at −0.5 V.sub.RHE. When compared with Cu.sub.9S.sub.5/NC, the activity was lower by 35% and the selectivity (FE) was lower by 80%. The difference in the NRR activity and the selectivity is partially due to SSA (FIGS. 7B and 7C) and difference in composition. The result of measuring the conductive surface area of Cu.sub.9S.sub.5 and Cu.sub.9S.sub.5/NC also shows clearly that Cu.sub.9S.sub.5/NC has a larger active surface area than Cu.sub.9S.sub.5, which is evidenced by a 24% steeper slope depending on scan rate in the plot of the charging current density (FIGS. 14A-14C). In addition, the N.sub.2-TPD analysis result shows that Cu.sub.9S.sub.5/NC has 28% more N.sub.2-adsorbed sites (which are favorable for NRR) as compared to Cu.sub.9S.sub.5 (FIG. 14D). In addition, since NC is semiconducting and allows facile transport of electrons between Cu.sub.9S.sub.5 and the supporting electrode (carbon paper), unnecessary ohmic drop may be reduced using the electrode. Due to the advantage of the Cu.sub.9S.sub.5/NC composite structure, the degradation and regeneration of the Cu.sub.9S.sub.5 catalyst for NRR were studied further.

[0124] Degradation and Regeneration of Catalyst

[0125] In general, the electrochemical reaction rate (i.e., current) is improved with increased overpotential. The exponential increase of current at applied overpotential is explained by the Butler-Volmer equation using two simple variables of transfer coefficient and overpotential. The NRR and FE of Cu.sub.9S.sub.5/NC decreased greatly at higher negative potentials (see Table 2, FIG. 3B and FIG. 8B).

TABLE-US-00002 TABLE 2 J.sub.NH3, FE and NH.sub.3 yield of Cu.sub.9S.sub.5/NC catalyst depending on potential (vs RHE) Potential j.sub.NH3 FE NH.sub.3 yield NH.sub.3 yield NH.sub.3 yield (vs RHE) (μA/cm.sup.2) (%) (μg/hmg.sub.cat) (nmol/cm.sup.2) (μg/hcm.sup.2) −0.4 V 0.05 0.01 0.8 47 0.8 −0.5 V 52 35 11 645 11 −0.6 V 14 4.3 2.9 173 2.9 −0.7 V 1.8 0.12 0.37 22 0.37 −0.8 V 0 0 0 0 0

[0126] The NH.sub.3 production rate was decreased from 645 to 22 nmol/hcm.sup.2 as the applied potential was increased from −0.5 to −0.7 V.sub.RHE, and then to 0.37 μg/hcm.sup.2 at a potential of 11 V.sub.RHE. The inverse relationship between the NRR activity and the overpotential may be derived from two possible effects: (i) N.sub.2 adsorption may be interrupted on more negative surface, and proton adsorption and reduction may dominate the reduction reaction, or (ii) the catalyst activity may be lost as the surface is actually changed or electrochemically degraded at potential. Although the decrease in reaction rate with increased overpotential was proposed in the inverted Marcus region for electrochemical reactions, it was not considered in the present disclosure. The decrease in NRR activity at higher overpotential for Cu.sub.9S.sub.5/NC was observed as decreased current density, which is commensurate with both HER and NRR in extended CA experiments under N.sub.2 atmosphere (FIG. 3C). The total current density of Cu.sub.9S.sub.5/NC was decreased from −0.4 to −0.1 mA/cm.sup.2 at −0.5 V.sub.RHE during CA for 16 hours. Additionally, the surface degradation of the Cu.sub.9S.sub.5/NC catalyst was investigated by XRD following the CA at −0.5 V.sub.RHE for 16 hours, after the NRR activity measurement (FIG. 3D). The cubic pattern of Cu.sub.2O appeared whereas the intensity of the XRD peak corresponding to the hexagonal Cu.sub.9S.sub.5 was decreased rapidly immediately after the reduction reaction (FIG. 3D). More interestingly, the change in CA and catalyst structure was observed in the N.sub.2-saturated aqueous solution only, whereas the reduction current and crystal structure remained intact after HER in the Ar-saturated electrolyte solution at −0.5 V.sub.RHE for 1 hour. The XRD analysis showed that the structure of Cu.sub.9S.sub.5 was maintained after the HER at −0.5 V.sub.RHE in the Ar-saturated aqueous solution (FIGS. 3E and 3F). This means that the catalyst surface was reconstituted after the NRR, not after the HER. In other words, although the strong adsorption of N.sub.2 onto the Cu—S surface breaks the unstable Cu—S bond and forms Cu.sub.2O during NRR, the decrease in protons does not change the surface structure. The XPS analysis of the Cu.sub.9S.sub.5/NC catalyst after the NRR (FIGS. 15A and 15B) shows that the XPS spectra of Cu 2p are not significantly different from those of Cu.sub.9S.sub.5/NC, because Cu.sub.2O and Cu.sub.9S.sub.5 are similar in oxidation state and electron binding energy. However, the peaks of sulfur anions (S.sup.2− or S.sub.2.sup.2− at 161.5 eV) dissolved during the NRR process appear in the S 2p spectra, together with the peak of SO.sub.4.sup.2− at 170 eV from the Na.sub.2SO.sub.4 electrolyte (FIG. 15B). The peaks of the Cu—S and S—S bonds of Cu.sub.9S.sub.5 were still observed after the degradation. However, whereas the sulfur content was decreased rapidly after the NRR, the oxygen content was increased (see Table 3).

TABLE-US-00003 TABLE 3 Atomic ratio of Cu, S and O of as-prepared Cu.sub.9S.sub.5/NC and after NRR at −0.5 V.sub.RHE estimated from XPS spectra Initially After NRR at Element (as-prepared) (at %) −0.5 V (at %) Cu 61.9 22 S 33.8 9.4 O 4.3 68.8

[0127] Referring to FIG. 4A and FIG. 16, the results of XRD and XPS support the electrochemical degradation of Cu.sub.9S.sub.5/NC, together with the decreased NRR activity due to repeated CA at 0.5 V.sub.RHE. The decrease in NRR activity at higher overpotentials is due to the degradation of the catalyst surface under the electrochemical reaction condition. Under reducing environments, the electrochemical formation of Cu.sub.2O from Cu.sub.9S.sub.5 may occur according to the following half-reaction.

10Cu.sub.1.8S+10H.sub.2O+2e.sup.−.fwdarw.9Cu.sub.2O+10H.sub.2S+O.sup.2−

[0128] Although the faradaic peak current associated with the reductive degradation of Cu.sub.9S.sub.5 was not clearly observed in CV, the structural change and activity loss at more negative potentials than −0.5 V.sub.RHE strongly suggests the electrochemical degradation of the catalyst.

[0129] The inventors of the present disclosure hypothesized that the Cu—S bond is cleaved during NRR and the restoration of the Cu—S bond will recover NRR activity. Therefore, in order to mimic the unstable chemistry of the sulfhydryl group of the FeMo cofactor and restore NRR activity, ex-situ electrochemical sulfurization reaction was used to restore the Cu—S bond on the Cu.sub.9S.sub.5/NC surface. Specifically, a degraded Cu.sub.9S.sub.5/NC electrode was re-oxidized by CA in 5 mM Na.sub.2S and 0.5 M Na.sub.2SO.sub.4 aqueous solutions for 1 hour at a potential in the range from −0.9 to 0.1 V.sub.RHE (FIG. 4B and FIGS. 17A-17B). Then, the electrode was subjected to NRR under N.sub.2 atmosphere in a fresh 0.5 M Na.sub.2SO.sub.4 aqueous solution (FIG. 4C). Importantly, the NRR activity was recovered at the Cu.sub.9S.sub.5/NC electrode which was treated at more positive potential than −0.7 V.sub.RHE in the 5 mM Na.sub.2S solution. The electrochemical regeneration process is described in detail in FIG. 5. The restoration of the Cu.sub.9S.sub.5 structure was confirmed by XRD. The diffraction pattern of hexagonal Cu.sub.9S.sub.5 appeared again in the regenerated electrode, whereas the peaks of Cu.sub.2O decreased significantly (FIG. 4B). Through continuous NRR/regeneration of 4 cycles, the NIH3 production speed was recovered to 68, 43 and 57%, respectively, as compared to the initial NRR of a fresh Cu.sub.9S.sub.5/NC electrode (see FIG. 4D, FIGS. 18A-18F and Table 4).

TABLE-US-00004 TABLE 4 NRR performance after electrochemical regeneration process depending on cycle number Regeneration Potential FE NH.sub.3 yield cycle (vs RHE) (%) (nmol/hcm.sup.2) As-prepared −0.5 V 35 645 Cu.sub.9S.sub.5/NC −0.6 V 4.3 173 −0.7 V 0.12 22 1st regeneration −0.5 V 10.3 406 −0.6 V 3.2 159 −0.7 V — — 2nd regeneration −0.5 V 9.0 308 −0.6 V 4.0 212 −0.7 V — — 3rd regeneration −0.5 V 10.4 365 −0.6 V 5.0 278 −0.7 V — —

[0130] However, the Cu.sub.9S.sub.5/NC composite structure shown in FIG. 1C was disrupted and, Cu.sub.2O and NC were separated during the repeated NRR/regeneration process as observed by SEM (FIGS. 19A-19C). In addition, the NRR activity was not completely recovered to the NRR activity of the original composite structure. NH.sub.3 production by the regenerated electrode was confirmed by .sup.1H NMR and the colorimetric method (FIG. 4E). Both NMR and indophenol blue analyses showed that the NRR performance of the catalyst was recovered by the electrochemical regeneration (FIG. 20). The regeneration of the Cu.sub.9S.sub.5 structure from Cu.sub.2O can proceed by the following oxidation process.

9Cu.sub.2O+10Na.sub.2S+11H.sub.2O.fwdarw.10Cu.sub.1.8S+20NaOH+2H.sup.++2e.sup.−

[0131] It is reported that a Cu substrate is oxidized in an electrolyte solution in the presence of a sulfide at more positive potentials than approximately −0.8 V.sub.RHE to form Cu.sub.xS (Cu.sub.2S and Cu.sub.9S.sub.5). In a previous study, a small bumpy current was recorded at about −0.8 V.sub.RHE in CV for forming Cu.sub.xS from a Cu substrate. In addition, a large oxidation current was not detected at a potential around −0.8 V.sub.RHE from CV in 5 mM Na.sub.2S using a degraded Cu.sub.9S.sub.5 electrode (FIG. 4F). In FIG. 4F, the peak oxidation current observed at −0.3 V.sub.RHE originates from the S.sub.n.sup.2−/S.sup.2− redox reaction on the Cu.sub.9S.sub.5 surface. However, no similar oxidation current was observed when a carbon electrode was used in an electrolyte solution or in the absence of a sulfide (FIG. 4F and FIG. 21).

[0132] The increased NRR activity of the regenerated Cu.sub.9S.sub.5/NC electrode and its crystal structure strongly suggest that the Cu—S bond is regenerated by the electrochemical treatment. The result also suggests that the metal-sulfur bond is very important in the electrochemical NRR process. The Cu—S bond was broken in the NRR process only and was not affected in an aqueous solution in the absence of N.sub.2. In the FeMo cofactor of nitrogenase, the Fe center provides an adsorption site for N.sub.2 adsorption. The Fe—S bond is broken during the N.sub.2 adsorption process, and the atomic hydrogen transferred to the cofactor approaches Fe—N.sub.2 and forms a sulfhydryl group. Then, the sulfhydryl group is involved in hydrogenation processes including reductive removal of H.sub.2 molecules. A similar adsorption mechanism was proposed in the electrochemical N.sub.2 reduction reaction. The N.sub.2Hx intermediate on the electrocatalyst surface was observed through in-situ IR measurement, which supports the related adsorption mechanism under ambient condition. The NRR activity of the metal sulfide catalyst was also reported. However, the role of sulfur in the electrochemical NRR process has not been elucidated yet. The NRR activity observed on the Cu.sub.9S.sub.5/NC surface disappears when the metal-sulfur bond is broken. Importantly, the catalyst activity is recovered through sequential NH.sub.3 production and catalyst regeneration cycles. These results mean that the metal-sulfur bond is important in the electrochemical production of NH.sub.3 and is actively involved in the electrochemical N.sub.2 reduction process.

[0133] In summary, the use of the Cu.sub.9S.sub.5/NC catalyst as a NRR catalyst resulted in significant NH.sub.3 production, activity and selectivity (11.0 μg/h cm.sup.2, 645 nmol/h cm.sup.2 and 35%, respectively). Importantly, the Cu.sub.9S.sub.5 catalyst was degraded to Cu.sub.2O during the NRR process and the catalyst activity was decreased. Subsequently, the Cu.sub.9S.sub.5 surface was reconstructed from the degraded electrode through an electrochemical sulfurization process. The importance of the metal-sulfur bond in NRR was demonstrated by the recovered NH.sub.3 production yield of the recovered Cu.sub.9S.sub.5/NC surface. The degradation and regeneration processes of the metal chalcogenide catalyst for electrochemical NH.sub.3 production are similar to the enzymatic N.sub.2 fixation process discovered in the FeMo cofactor of nitrogenase. In addition, this result strongly suggests that, in order to achieve effective electrochemical NH.sub.3 production under ambient condition, a powerful catalyst that provides an unstable sulfhydryl functional group favored by the metal center for N.sub.2 adsorption is necessary.

[0134] To conclude, in the present disclosure, Cu.sub.9S.sub.5/NC was prepared by a thermal heating reaction using the solid-state method, and an FE of 35% and a yield of 11.0 μg/hcm.sup.2 (or 645 nmol/hcm.sup.2) was achieved in a 0.5 M Na.sub.2SO.sub.4 electrolyte at −0.5 V.sub.RHE for electrochemical NH.sub.3 production. However, the crystal structure of Cu.sub.9S.sub.5/NC was degraded to Cu.sub.2O during the NRR reaction. The regeneration redox cycle of metal sulfide was first demonstrated in the present disclosure. The Cu.sub.9S.sub.5 surface was reconstructed in a 5 mM Na.sub.2S electrolyte via an electrochemical sulfurization process.

[0135] The NRR activity of Cu.sub.9S.sub.5/NC was recovered in the reconstructed electrode, and NH.sub.3 production was activated through repeated NRR and regeneration processes. The important of the metal-sulfur bond in NRR can be explained with the NH.sub.3 yield on the recovered Cu.sub.9S.sub.5/NC surface. The degradation and regeneration processes of the metal chalcogenide catalyst for electrochemical NH.sub.3 production show a mechanism similar to the enzymatic N.sub.2 fixation process discovered for the FeMo cofactor of nitrogenase. The result suggests that, in order to achieve effective electrochemical NH.sub.3 production under ambient condition, a powerful catalyst that provides an unstable sulfhydryl functional group favored by the metal center for N.sub.2 adsorption is necessary. Accordingly, the present disclosure provides a new electrochemical regeneration method for a low-durability catalyst used in continuous NRR.