Active hydrogen tuning by copper–cobalt bimetal catalysts for boosting ammonia electrosynthesis from simulated wastewater

Abstract

The electrochemical nitrate reduction reaction (NO₃RR) represents a promising approach to balance the nitrogen cycle, converting environmental pollutant NO₃⁻ to valuable ammonia (NH₃). However, the whole reaction involves complex proton-coupled electron transfer processes, requiring the development of efficient catalysts. Owing to unique d-orbitals, Cu-based catalysts exhibit excellent performance. Here, we design a Cu₅–Co₅ bimetal nanocomposite that achieves a high FE_NH3 of 94.1%, a yield rate of 14.8 mg h⁻¹ cm⁻² and great stability over twenty hours. The yield rate can be enhanced in a flow cell and reach 30.9 mg h⁻¹ cm⁻². We test the performance of the Cu₅–Co₅ catalyst for simulated wastewater treatment, exhibiting a yield rate of 6.7 mg h⁻¹ cm⁻² at −100 mA cm⁻². Furthermore, in situ ATR-SEIRAS and Raman spectra reveal the reaction pathway on the Cu₅–Co₅ catalyst. The Cu can adsorb NO₃⁻ and convert to *NO₂⁻, while Co(OH)₂ derived from metallic Co can promote water spillover and facilitate the subsequent *NO₂⁻-to-NH₃ conversion.

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1 Introduction

The natural nitrogen cycle is regarded as one of the most essential biochemical cycles, closely related to human beings and ecological environments.^1,2 Versatile nitrogen-containing species within the nitrogen cycle can be mutually interconverted to maintain this balance.³ However, this natural nitrogen balance has been disrupted by excessive human activity, such as excessive use of fertilizer, emission of substandard industrial waste and combustion of fuels. Nitrate (NO₃⁻) is widespread in agricultural and industrial wastewater as the primary nitrogenous pollutant, which poses a significant hazard. The over-accumulation of NO₃⁻ in water will affect water quality to damage ecosystems and induce cancer to endanger human health.^4,5 The current treatment methods are biological denitrification and ion exchange, which are inefficient for high-concentration pollutants.⁶ Biological denitrification is an important route for removing NO₃⁻: denitrifying bacteria in nature can convert N in harmful NO₃⁻ to harmless nitrogen (N₂).⁷ Ammonia (NH₃), which is a fundamental and adaptable nitrogen-containing feedstock for fertilizers, pharmaceuticals and chemicals, is a higher value-added product compared to the worthless product N₂.⁸ Meanwhile, NH₃ with high hydrogen (H₂) storage capacity and zero emission is also regarded as a next-generation energy carrier.⁹ The annual demand for ammonia exceeded 150 million tons in 2023 and its annual production is expected to exceed 200 million tons globally in 2030 and 560 million tons in 2050.¹⁰ The Haber–Bosch method is the current method for industrial NH₃ production, requiring harsh reaction conditions (over 400–600 K and 200–400 atm) and causing serious pollution (releasing more than 1.2% of greenhouse gas).¹¹ Therefore, exploring a green and sustainable approach to highly efficient NH₃ production is very desirable. The conversion of NO₃⁻ to NH₃ driven by renewable electricity is a promising alternative to the conventional NH₃ manufacturing process and an effective method for the removal of nitrogen-containing wastewater. Hence, achieving a high faradaic efficiency (FE) of NH₃ (>90%) under high current density (>200 mA cm⁻²) is necessary.¹²

The electrochemical NO₃⁻ reduction reaction (NO₃RR) is an eight-electron process, involving a series of by-products (NO₂⁻, N₂, N₂H₄, etc.) and competitive hydrogen evolution reaction (HER) with a two-electron transfer.^13,14 The catalyst is the key factor in the whole reaction. Thus, designing an efficient catalyst for active NO₃RR is meaningful and imperative. Based on theoretical and experimental results, copper (Cu)-based catalysts can inject electrons into the N–O bond of NO₃⁻ to adsorb NO₃⁻ and activate NO₃⁻ to *NO₂⁻.^15,16 Meanwhile, the NO₃RR is also a complicated nine-proton coupled process. The following *NO₂⁻-to-*NH₃ conversion is a multi-step deoxygenation and hydrogenation process, while Cu-based catalysts have a weak ability to generate active hydrogen (*H) from water dissociation.^17,18 The moderate ability of water spillover to provide *H plays an important role, where insufficient *H supply may produce by-product NO₂⁻ and the excessive accumulation of *NO₂⁻ will deactivate Cu sites leading to sluggish reaction kinetics and inferior selectivity.¹⁹ Sufficient *H supply can promote the *NO₂⁻ conversion to achieve high performance for NH₃ generation.²⁰

Enhancement of FE and reaction kinetics is important for catalyst design.²¹ Various design strategies have been adapted for Cu-based catalysts (alloying, heteroatom doping, amorphous structure, oxygen vacancy defects, etc.).^22,23 Li et al. have introduced amorphous CeO_x to regulate the electronic structure of Cu (Cu/a-CeO_x), achieving a dynamic balance between *H generation and consumption.²⁴ Li et al. have anchored Cu in MnO₆ octahedral molecular sieve (Cu-OMS), where dispersed Cu adsorbs NO₃⁻ and one-dimensional OMS provides enough *H for sequent protonation.²⁵ However, metal oxide catalysts suffer from poor stability (mostly less than 10 hours) due to reconstruction and active metal site leaching.²⁶ Compounding with high HER active elements is another feasible strategy for tuning of *H.^27,28 Zeng et al. have introduced Co₃O₄ with Cu₁–N–C, where Cu₁–N–C sites adsorb NO₃⁻ to generate *NO₂⁻ and adjacent Co₃O₄ sites can accelerate *NO₂⁻ reduction to NH₃.²⁹ Sun et al. have alloyed nickel (Ni) with solid-solution Cu to produce a Janus Cu@Ni catalyst. Ni sites can accelerate H₂O dissociation to provide *H for the hydrogenation of *NO_x, which is generated on the adjacent Cu sites.³⁰

Cobalt-based catalysts are regarded as active catalysts for HER and NO₂⁻ reduction reaction (NO₂RR). Inspired by the above, we have successfully introduced Co to synthesize Cu–Co bimetal catalysts (denoted as Cu_x–Co_y) through annealing CuCo benzene-1,3,5-tricarboxylate (CuCo-BTC) precursors in a H₂/Ar atmosphere. The Cu₅–Co₅ catalyst exhibits a remarkable NO₃RR performance, FE_NH3 > 80% from −0.2 to −0.8 V versus RHE (reversible hydrogen electrode; the following potentials have been converted into reversible hydrogen electrode values unless otherwise stated), reaching a maximum FE_NH3 of 94.1% and a yield rate of 14.8 mg h⁻¹ cm⁻². Furthermore, the NH₃ FE reaches 94.8% and the yield rate can be enhanced to 30.9 mg h⁻¹ cm⁻² in a flow cell. Besides, the Cu₅–Co₅ catalyst exhibits excellent performance in stimulated wastewater where different concentrations of NO₃⁻ and NO₂⁻ coexist. In 30 mM NO_x⁻ electrolyte, the yield rate reaches a maximum of 6.7 mg h⁻¹ cm⁻². In situ Raman and XRD results show the metallic Co can transfer Co(OH)₂ during the reaction. Besides, in situ ATR-SEIRAS (in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy) combined with in situ Raman results are used to reveal the reaction mechanism and pathway of the NO₃RR on the Cu₅–Co₅ catalyst. The introduction of Co can facilitate the reduction of *NO₂⁻ and promote the dissociation of water to provide enough *H for subsequent hydrogenation, improving NH₃ generation and exhibiting excellent NO₃RR performance. This work provides an efficient catalyst for green NH₃ generation through recycling pollutant NO₃⁻.

2 Discussion

2.1 Synthesis and characterization

Various ratios of Cu–Co catalysts are synthesized via a two-step strategy: hydrothermal reaction and calcination post-treatment. Briefly, CuCo-BTC precursors are synthesized through the hydrothermal method and the precursors are subsequently calcined under a H₂/Ar atmosphere to obtain Cu–Co nanocomposites (see details in Supporting Information†). A series of characterizations were conducted to investigate the composition and morphology of the obtained catalyst. XRD (X-ray diffraction) patterns of pure Cu and Co are respectively ascribed to metallic Cu and metallic Co (Fig. S1a and S2a†). The peaks of pure Cu at 74.0°, 50.4°, and 43.3° are ascribed to (220), (200) and (111) planes of FCC (face-centered cubic) Cu (JCPDS no. 04-0836). In addition, the diffraction peaks of Co at 75.9°, 51.5° and 44.2° are indexed to (220), (200) and (111) planes of FCC Co (JCPDS no. 15-0806). The XRD pattern of Cu₅–Co₅ in Fig. 1a demonstrates the purity of the obtained catalyst and there are only normal diffraction peaks belonging to metallic Cu and Co and no obvious peaks for their oxides, implying the successful synthesis of the composite of Cu and Co. The results of ICP-OES (inductively coupled plasma optical emission spectrometry) can prove that the molar ratio of Cu : Co is close to 5 : 5 (Table S1†). The morphology and structure of the catalysts were investigated through SEM (scanning electron microscopy) and TEM (transmission electron microscopy). The SEM images show the nanoparticle structure of the pure Cu and pure Co (Fig. S1b and S2b†). In the TEM images, the 0.208 nm lattice spacing of pure Cu corresponds to metallic Cu(110) and the 0.180 nm lattice spacing of pure metallic Co corresponds to Co(110) (Fig. S1c, d and S2c, d†). The SEM results of the Cu₅–Co₅ catalyst also exhibit nanoparticles and HRTEM images show that Cu₅–Co₅ has a great crystallinity, the lattice spacings measured to be 0.208 nm and 0.180 nm being respectively ascribed to the (111) facets of metallic Cu and the (200) facets of metallic Co (Fig. 1b and Fig. S3†). The EDX (energy-dispersive X-ray) elemental mapping images demonstrate that Cu and Co are uniformly dispersed (Fig. 1c). The results match well with the XRD observation, proving the successful fabrication of Cu–Co bimetal composition. XPS (X-ray photoelectron spectroscopy) is used to analyze the chemical state of surface elements of the as-prepared Cu₅–Co₅ catalyst (Fig. S4a†). The measured C 1s high-resolution spectra show two peaks at 284.8 and 285.7 eV, attributed to C–C and C–O respectively (Fig. S4b†).^31,32 No obvious peak ascribed to O–C=O is observed, demonstrating the transformation of carbon after the calcination treatment.³³ The Co 2p high-resolution spectrum exhibits the presence of metallic Co and oxidized Co²⁺ and Co³⁺ (Fig. 1d). The superficial oxidized Co^x+ originates from the oxidation of the surface exposed to air.³⁴ A similar result is observed in the Cu 2p high-resolution spectrum and AES (Auger electron spectroscopy, Fig. 1e and f) of Cu LMN. A set of Cu 2p satellite feature peaks can be observed, which shows that Cu²⁺ and Cu⁺ originate from the partial oxidation of the catalyst surface.^35,36

Fig. 1 Morphological and structural characterization of the Cu₅–Co₅ nanocomposite. (a) XRD patterns. (b) HRTEM image. (c) HAADF-STEM image and corresponding EDX mappings. (d) XPS peaks of Co 2p. (e) XPS peaks of Cu 2p. (f) Cu LMN spectrum.

2.2 Electrocatalytic NO₃RR performance

The NO₃RR performance of as-prepared Cu–Co catalysts has been investigated in an H-type electrolytic reactor. We initially confirm the NO₃RR activity through linear scanning voltammetry (LSV) in 1 M KOH with and without NO₃⁻ (Fig. 2a). The LSV curves exhibit that the Cu₅–Co₅ catalyst possesses the highest current density while Cu possesses the lowest current density, suggesting the incorporation of Co can improve the intrinsic activity of Cu (Fig. S5a†). The reduction peak (S1) near 0 V is ascribed to the adsorption of NO₃⁻ (NO₃⁻ to *NO₃⁻) and the peak (S2) at −0.3 V is accorded to the reduction of *NO₃⁻ (*NO₃⁻ to *NO₂⁻).^37,38 The peaks of Cu located at approximately −0.2 V and −0.6 V indicate that the composition of Co can promote the reaction to occur at lower potentials. Besides, various atom ratios of Cu–Co catalysts display better current densities than pure Cu and Co, showing that the introduction of Co can improve the NO₃RR performance (Fig. S5b†).

Fig. 2 Electrochemical performances. (a) LSV curves of the Cu₅–Co₅ catalyst at a scan rate of 10 mV s⁻¹ in 1 M KOH with and without 0.1 M NO₃⁻ (the shading S1 and S2 representing peaks near 0 V and −0.3 V). (b) NH₃ FEs and yield rates under different potentials (−0.2 V to −0.8 V). (c) NH₃ FEs and yield rates of Cu, Co and Cu₅–Co₅ catalysts under −0.6 V. (d) Curves of capacitance Δj as a function of different scan rates. (e) Nyquist comparison plots at −0.3 V. (f) Tafel slopes. (g) Stability of the Cu₅–Co₅ catalyst during cyclic reduction tests at −0.6 V.

Chronoamperometry measurements are performed to test the NO₃RR activity of the Cu₅–Co₅ catalyst under various applied potentials. The liquid products (NO₂⁻, NH₃ and N₂H₄) are quantified using various colorimetric methods (Fig. S6†). The gaseous products (H₂ and N₂) generated during the reaction are detected by gas chromatography. Trace amounts of N₂ and N₂H₄ are detected, which demonstrate that no N–N coupling products are generated during the whole reaction. In a wide potential window (from −0.2 V to −0.8 V), FE_NH3 are all over 80% which exhibits a volcano-like shape and reaches the highest FE_NH3 of 94.1% under −0.6 V (Fig. 2b and Fig. S7†). Besides, the yield rate gradually increases and reaches a maximum of 14.8 mg h⁻¹ cm⁻², outperforming greatly pure Cu and Co (Fig. 2c). Besides, the Cu₅–Co₅ catalyst shows the highest FE and yield rate compared to Cu–Co catalysts of other atomic ratios as shown in Fig. S8.† As the percentage of Co increases, the yield rate also increases and then decreases, with the best performance at Cu : Co = 5 : 5 ratio. We have further compared the Cu₅–Co₅ catalyst with physically mixed Cu and Co (mixed Cu₅–Co₅), where Cu₅–Co₅ exhibits better performance (Fig. S9†). Due to intrinsic retarded kinetics for spillover, pure Cu exhibits the lowest yield and yield increases with the introduction of Co. The introduction of Co sites can promote water dissociation to provide sufficient hydrogen while excessive hydrogen will cause the HER to be the dominant reaction.³⁹ The above results suggest that the presence of Co can improve the NO₃RR performance of Cu-based catalysts. We have also synthesized and tested the performance of Cu₅Co₅ alloy (see details in Supporting Information†). The results demonstrate that the Cu₅–Co₅ composition exhibits better performance (Fig. S10†).

The Cu₅–Co₅ catalyst is utilized for further intrinsic electrocatalytic activity analysis. We measure electrochemical double-layer capacitance to evaluate the electrochemical active surface area (ECSA).⁴⁰ Pure Cu exhibits the lowest ECSA value (0.02 mF cm⁻²) while Co has a slightly higher ECSA value (0.74 mF cm⁻²), which suggests that metallic Cu possesses inferior kinetics.¹⁶ The Cu₅–Co₅ catalyst has the highest ECSA value (5.89 mF cm⁻²), much higher than that of metallic Co and Cu (Fig. 2d and Fig. S11†). The results of ECSA measurements indicate that the addition of Co can expose more active sites. The charge-transfer resistance is compared by electrochemical impedance spectroscopy (EIS) measurements conducted from 10⁵ to 10⁻¹ Hz. The charge-transfer resistance (R_ct) and the series resistance (R_s) are obtained through an equivalent circuit fitting, showing that the charge-transfer resistance of the Cu₅–Co₅ catalyst is smaller than that of metallic Cu and Co (Fig. 2e and Table S2†). Furthermore, the reaction kinetics of catalysts are also analyzed through Tafel slopes. The smallest value for Cu₅–Co₅ reflects the faster intrinsic kinetics compared to pure Cu and Co, which is favorable for the NO₃RR and is consistent with chronoamperometry results (Fig. 2f). The above results imply that the rapid reaction kinetics benefits from the synergy effect between metallic Cu and Co. The above results imply that combining Co with Cu can increase the acceleration of the electron transfer, which can greatly enhance the entire NO₃RR performance.⁴¹

Consecutive electrolysis cycles under constant potential (−0.6 V) are performed to test the stability of the Cu₅–Co₅ catalyst. The results display great stability of the obtained catalyst, FE_NH3 exceeding 90% and yield rate reaching 10 mg h⁻¹ cm⁻² before the penultimate cycle (Fig. 2g). The gradual increase of yield during the initial cycles originates from the activation under reduction potential. The Cu₅–Co₅ catalyst can be operated stably for close to 20 cycles. XPS and XRD are also used to analyse the Cu₅–Co₅ catalyst after consecutive stability testing (Fig. S12a–c†). The results imply that the decay stems from leaching of Cu in alkaline and air environments, which demonstrates that Cu acts as the predominant sites for the high performance of the NO₃RR.^42,43 The Co(OH)₂ appearing after the reaction originated from the oxidation of metallic Co by NO₃⁻ during the reaction (Fig. S12d†).⁴⁴ Besides, the stability of the Cu₅–Co₅ catalyst outperforms that of most reported Cu-based catalysts.^45–47

Based on the excellent performance of the Cu₅–Co₅ catalyst in the H-cell, a flow electrolytic cell assembled with an NH₃ capture chamber is used to meet the requirements of industrialization for higher current densities (Fig. S13†). In the LSV curve (Fig. 3a), approximately quadruple current densities can be achieved in the flow system due to lower ohmic resistances and faster mass transfer. Chronopotentiometry was used to test the NO₃RR performances and the results show that FE_NH3 reaches up to 94.8% (Fig. 3b and Fig. S14†), while still maintaining high NH₃ selectivity. Besides, the yield rate increases with the current density, reaching 30.9 mg h⁻¹ cm⁻² under 600 mA cm⁻². These results show the promising potential of the Cu₅–Co₅ catalyst for NO₃RR application at industrial current density levels. Considering the coexistence of NO₃⁻ and NO₂⁻ in wastewater, we stimulate and test the performance of NO_x⁻-to-NH₃ conversion in wastewater at different concentrations (varying from 10 mM to 100 mM NO_x⁻). The results display a curve with a volcano shape under different concentrations, with a maximum yield of 6.7 mg h⁻¹ cm⁻² at 30 mM NO_x⁻ concentration (Fig. 3c). Low concentration will cause the HER to become the dominant reaction while high concentration will cause diffusion problems.⁴⁸ The performance of Cu₅–Co₅ in the NO_xRR implies the potential ability for the conversion of N-containing wastewater to value-added NH₃.

Fig. 3 Electrochemical performances. (a) LSV curves of the Cu₅–Co₅ catalyst in H-cell and flow cell. (b) NH₃ yield rates under different current densities in the flow cell. (c) NH₃ FEs and yield rates in various concentrations of NO_x⁻. (d) ¹H NMR spectra of produced ¹⁵NH₄⁺ and ¹⁴NH₄⁺. (e) Comparison of the Cu₅–Co₅ catalyst with other recently reported electrocatalysts.

To investigate whether there is external NH₃ interference, ¹⁵N isotope labeling experiments are conducted to confirm the true nitrogen origin of NH₃. In the ¹H nuclear magnetic resonance spectra (¹H NMR) of the electrolyte after 1 h reaction, a triplet coupling peak attributed to ¹⁴NH₄⁺ can be observed while a double peak appears using K¹⁵NO₃ as the reagent (Fig. 3d). These results demonstrate the generated NH₃ originates from the electroreduction of NO₃⁻ ions rather than from environmental contamination. Meanwhile, we have also utilized a control experiment to confirm the results, which is performed in 1 M KOH without NO₃⁻ as the catholyte. The UV-visible spectrum shows that there is a trace amount of NH₃, further precluding external interference and demonstrating the reliability of our experiments (Fig. S15a and b†). Furthermore, ion chromatography is also utilized to prove the accuracy and reliability of the colorimetric method, showing that the quantitative results of both methods are close and indicating the accuracy and reliability of the indophenol blue method (Fig. S15c†). Fig. 3e and Table S3† compare the NO₃RR performance of the Cu₅–Co₅ catalyst with those of other catalysts, its performance being superior to those of most recently reported catalysts.^49–56

2.3 Reaction pathway

The generation of *H during the reaction can be detected through EPR (electron paramagnetic resonance, Fig. S16a and b†). DMPO (5,5-dimethyl-1-pyrroline-N-oxide) as a trapping agent can combine with *H to form DMPO-H. Besides, the increased signal intensity of Cu₅–Co₅ demonstrates the introduction of Co can facilitate water dissociation to generate *H. The reaction mechanism is investigated through in situ ATR-SEIRAS spectra in the same electrolyte. ATR-SEIRAS spectra are measured ranging from 0.3 V to −0.6 V (Fig. 4a). The peak near 1350 cm⁻¹ is ascribed to the N–O bond of *NO₃⁻ asymmetric stretching vibration and the peak near 1220 cm⁻¹ is attributed to the anti-symmetric stretching vibration of *NO₂⁻.⁵⁷ The negative trends indicate that NO₃⁻ is adsorbed on the Cu₅–Co₅ surface to generate *NO₃⁻ and then reduces to *NO₂⁻. The appearance of N=O scaling mode of *NO at approximately 1550 cm⁻¹ shows that consumed *NO₂⁻ is converted to *NO.⁵⁸ Then, the distinct peaks at 1280 cm⁻¹ of *NH, 1150 cm⁻¹ of *NH₂ and 1440 cm⁻¹ of NH₃ can demonstrate the following deoxygenation and hydrogenation to generate *NH₄⁺.⁵⁹ The obvious *NO₂⁻ and *NO peaks in the spectrum of pure Cu imply the weak ability for *NO₂⁻ reduction and hydrogenation, which leads to NO₂⁻ as the main by-product (Fig. 4b). The peaks at approximately 3600 cm⁻¹ are ascribed to weak hydrogen bonds of surface adsorption of H₂O, which is considered the key to providing *H (Fig. 4c). The peaks shift to lower wavenumber as the applied potential gradually increases while no obvious change is observed for pure Cu (Fig. 4d), showing the introduction of Co sites can improve the adsorption and cleavage of surface H₂O to generate more *H to promote hydrogenation steps.⁶⁰ Consequently, the whole reaction can be regarded as a preliminary deoxygenation and subsequent hydrogenation process. Furthermore, time-dependent ATR-SEIRAS spectra of the Cu₅–Co₅ catalyst show increasingly negative peaks of *NO₂⁻ and increasingly positive peaks of *NH₂, indicating the continuous *NO₂⁻ consumption and reduction to generate *NH₂ (Fig. S16c†). These results demonstrate that *NO₂⁻ and *NH_x are essential intermediates in the whole reaction.

Fig. 4 (a–d) In situ ATR-SEIRAS spectra of NO₃RR on Cu₅–Co₅ and Cu catalysts under different potentials. (e) Scheme of the reaction mechanism on the Cu₅–Co₅ catalyst.

Furthermore, in situ Raman spectra are also used to investigate the surface of the catalyst and reaction mechanism. For the pure Cu catalyst, the characteristic peak of CuO at 290 cm⁻¹ and the characteristic peak of Cu₂O at 620 cm⁻¹ due to partial oxidation are observed, which are consistent with the XPS results.²⁴ The peaks gradually shrink while a peak at 690 cm⁻¹ ascribed to Cu-OH is observed, demonstrating the oxide layer on the surface is gradually reduced to metallic Cu (Fig. S17a†).⁵⁷ For the pure Co catalyst, the characteristic peak of CoO_x at 680 cm⁻¹ is observed, which decreases in intensity under the reduction potential and then metallic Co is oxidized to Co(OH)₂ by NO₃⁻ (Fig. S17b†).⁴⁴ The oxidized Cu and Co are also observed on the Cu₅–Co₅ catalyst, proving the negligible effect of the oxide species on the performance (Fig. S17c†). Besides, a similar phenomenon of reconstruction is also observed for the Cu₅–Co₅ catalyst, which is consistent with the XRD results after reaction (Fig. S12d†). In the Raman spectra measured from OCP to −0.3 V, the characteristic peaks at 1325 cm⁻¹ of NO₂⁻, 1528 cm⁻¹ of *NO and 1610 cm⁻¹ of *NH₂ can indicate the N-intermediate reaction pathway, which is consistent with the results of in situ ATR-SEIRAS spectra.^24,51

Based on the in situ ATR-SEIRAS and in situ Raman spectra, the NO₃RR reaction mechanism on the Cu₅–Co₅ catalyst can be considered as a series of deoxygenation and proton-coupled electron transfer steps with the following pathway: NO₃⁻ → *NO₃⁻ → *NO₂⁻ → *NO → *NH → *NH₂ → *NH₃ → NH₃. We propose a possible reaction mechanism over the Cu₅–Co₅ catalyst. NO₃⁻ can be adsorbed on Cu sites to generate *NO₂⁻, while the Co(OH)₂ sites facilitate the conversion of *NO₂⁻ to *NO and the generation of moderate protons to drive subsequent hydrogenation of *NO through water spillover (Fig. 4e).

3 Conclusions

In summary, we have successfully prepared bimetal Cu–Co nanocomposites. The obtained Cu₅–Co₅ catalyst can exhibit an outstanding performance for the NO₃RR to produce valuable NH₃, which shows great selectivity for NH₃ in a wide potential window ranging from −0.2 V to −0.8 V. Besides, the Cu₅–Co₅ catalyst can reach a maximum FE of 94.8% and a maximum yield rate of 30.9 mg h⁻¹ cm⁻². It also shows great stability of NH₃ electrosynthesis for 20 h without obvious decay, outperforming most of the recently reported catalysts. The Cu₅–Co₅ catalyst can also exhibit a great performance in simulated wastewater electrolytes containing different NO_x⁻ concentrations, displaying the potential to convert pollutants to value-added NH₃. In situ Raman spectra and XRD reveal that metallic Co can produce Co(OH)₂ during the reaction. Based on the in situ ATR-SEIRAS and Raman results, we propose a possible reaction pathway on the Cu₅–Co₅ catalyst. The Cu sites activate NO₃⁻ to form *NO₂⁻. The incorporation of Cu can not only promote the reduction of *NO₂⁻ but also facilitate the water dissociation to provide sufficient *H for subsequent hydrogenation, which is beneficial for NH₃ generation. Our work can provide a promising and available strategy for the development of efficient NO₃RR catalysts, aiming to deal with the treatment of nitrogenous pollutants and maintenance of the nitrogen cycle.

Author contributions

C. L. and Y. L. conceived, designed and supervised the project. C. Y. prepared the catalysts, performed the characterizations, conducted electrochemical tests and wrote the paper. C. L., J. Z., Z. Y. and A. C. assisted in experiments and characterization. All authors discussed the results and contributed to the manuscript.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22322805, 22178104, U22B20143), the Shanghai Municipal Science and Technology Major Project, the Shanghai Scientific and Technological Innovation Project (22dz1205900), the “Fundamental Research Funds for the Central Universities”, and the Shanghai Rising-Star Program (23QA1402200).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4gc04120h

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