A defect engineered p-block SnS_2−x catalyst for efficient electrocatalytic NO reduction to NH₃

Abstract

Electrocatalytic NO reduction to NH₃ (NORR) has emerged as an attractive approach for simultaneously realizing NO abatement and ammonia generation. Herein, we demonstrate for the first time that p-block Sn-based materials are promising NORR catalysts. A defect engineering strategy is employed to develop a SnS_2−x catalyst enriched with S-vacancy (V_S) defects. SnS_2−x delivers an exceptional NH₃-faradaic efficiency of 90.3% with an NH₃ yield rate of 78.6 μmol h⁻¹ cm⁻², outperforming most reported d-block NORR catalysts. Theoretical computations unveil that the high NORR performance of SnS_2−x arises from the active Sn-V_S sites which can selectively activate NO and reduce the energy barriers of the NORR pathway.

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Ammonia (NH₃) is an essential chemical and also a clean energy carrier.^1–3 The Haber–Bosch technology for industrial NH₃ production is performed at a high temperature and pressure, resulting in large amounts of energy consumption and CO₂ emissions.^4–6 The electrocatalytic nitrogen reduction reaction (NRR) has been widely deemed as a promising route to replace the Haber–Bosch process for sustainable NH₃ synthesis.^7–15 However, the effective NRR is heavily restricted by the low water solubility of N₂, highly stable N≡N bonds, and the competing hydrogen evolution reaction (HER),^16–24 giving rise to extremely low NH₃ yield rates and poor NH₃-Faraday efficiency (FE_NH3). Alternatively, given the better water solubility of NO and its relatively low N=O bond energy, electrocatalytic NO reduction to NH₃ (NORR) has recently emerged as a more energetically efficient route than the NRR for NH₃ electrosynthesis.^25–33 In the meantime, NO is a common atmospheric contaminant that causes serious environmental issues.³⁴ Thus, the NORR represents an attractive approach for simultaneously realizing NO abatement and ammonia generation.^35–37 Nevertheless, the practical application of the NORR is still hindered by its unsatisfactory reaction kinetics and energetics,^38–40 and thus there is an urgent need to develop efficient catalysts to promote the NORR efficiency.

Until now, an increasing number of d-block metal-based phosphides,³⁶ carbides,²⁵ oxides⁴¹ and sulfides²⁷ have been developed as efficient NORR catalysts, owing to the great advantage of unoccupied d-orbitals in activating NO molecules. Nonetheless, the unoccupied d-orbitals also easily bind with H atoms to favor the competing HER,⁴² leading to the compromised NORR selectivity. Recently, main-group p-block Bi-based materials have been confirmed as effective NORR catalysts with high FE_NH3,^35,37 ascribed to the preferential binding of NO molecules rather than H atoms on Bi during the NORR process,³⁵ highlighting the great potential of p-block metal-based materials as promising catalysts for the NORR. Given that p-block Sn possesses the robust ability to activate nitrogenous molecules and suppress the HER,^43–45 while defect engineering can maximize the exposed active sites,^46–48 we anticipate that defect engineered Sn-based catalysts might be appealing candidates toward the active and selective NORR.

Herein, we employed a defect engineering strategy to develop a p-block SnS_2−x catalyst enriched with S-vacancy (V_S) defects. SnS_2−x delivers an exceptional FE_NH3 of 90.3% with an NH₃ yield of 78.6 μmol h⁻¹ cm⁻², exceeding those of most reported d-block NORR catalysts. Theoretical computations are conducted to further provide atomic-level insight into the NORR mechanism of SnS_2−x.

A hydrothermal approach is first used to prepare SnS₂ nanosheet arrays grown on carbon cloth (CC), which is then subjected to plasma treatment to create V_S defects on SnS₂ and obtain SnS_2−x.^43,44,48 The X-ray diffraction (XRD, Fig. S1†) patterns of both SnS₂/CC and SnS_2−x/CC reveal the diffraction peaks matching well with the crystal planes of the SnS₂ phase (JCPDS no. 23-677). The scanning electron microscopy (SEM, Fig. 1a and b) images of SnS_2−x/CC show that abundant SnS_2−x nanosheets are densely covered on CC, similar to the morphology of SnS₂/CC (Fig. S2†), while the nanosheet morphology of SnS_2−x can be further ascertained by the transmission electron microscopy (TEM, Fig. 1c) image. The high-resolution TEM (HRTEM, Fig. 1d) image of SnS_2−x reveals two d-spacing values of 0.59 and 0.32 nm, corresponding to the (001) and (100) facets of SnS₂. The inverse fast Fourier transform (IFFT, Fig. 1e and f) maps recorded from the marked e and f regions in Fig. 1d show many lattice distortions or dislocations,^49–51 which can be more clearly reflected by their corresponding 3D surface plots (Fig. 1g and h),^52,53 suggesting the existence of abundant lattice defects (i.e. V_S) on SnS_2−x.

Fig. 1 (a and b) SEM images of SnS_2−x/CC. (c) TEM image of SnS_2−x. (d) HRTEM image of SnS_2−x. (e and f) IFFT maps and (g and h) 3D surface plots of the selected regions in (d) and the corresponding line intensity profiles.

X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements are conducted to investigate the chemical states and coordination configurations of SnS_2−x. The XPS Sn 3d spectra of SnS_2−x (Fig. 2a) can be deconvoluted into Sn 3d_5/2 (487.1 eV) and Sn 3d_3/2 (495.6 eV), while the S 2p spectra (Fig. 2b) can be fitted into S 2p_3/2 (161.3 eV) and S 2p_1/2 (162.5 eV). It is apparent that the core-level Sn 3d_5/2 and S 2p_3/2 peaks of SnS_2−x are both negatively shifted compared to those of SnS₂, indicating the decreased Sn/S valence states of SnS_2−x due to the introduction of electron-rich V_S.⁵² This can also be proved by the Sn K-edge X-ray absorption near-edge structure (XANES, Fig. 2c) spectra, displaying the reduced edge energy of SnS_2−x relative to SnS₂. The Sn K-edge extended X-ray absorption fine structure (EXAFS, Fig. 2d) spectra of SnS_2−x reveal two prominent peaks at 2.08 Å and 3.37 Å, which are assigned to Sn–S and Sn–Sn coordination bonds, respectively. Obviously, compared to SnS₂, the Sn–S bond intensity of SnS_2−x is markedly decreased, whereas its Sn–Sn bond intensity remains nearly unchanged, suggesting that the introduced vacancies in SnS_2−x are S-vacancies rather than Sn-vacancies.⁵² Meanwhile, the wavelet transform (WT) contour maps show that in comparison with SnS₂ (Fig. 2e), SnS_2−x presents a noticeable change in the shape of WT profiles (Fig. 2f), which is attributed to the V_S-induced distortion of the Sn–S bond structure. Furthermore, the electron paramagnetic resonance (EPR, Fig. 2g) spectra show that SnS_2−x exhibits a much stronger g signal (2.001) than SnS₂, further confirming that SnS_2−x is enriched with V_S.⁵¹

Fig. 2 (a and b) XPS spectra of SnS₂ and SnS_2−x: (a) Sn 3d, (b) S 2p. (c) Sn K-edge XANES, (d) EXAFS spectra and (e and f) WT profiles of (e) SnS₂ and (f) SnS_2−x. (g) EPR spectra of SnS₂ and SnS_2−x. (h) PDOS profiles of SnS₂ and SnS_2−x. (i) Calculated work functions of SnS₂ and SnS_2−x.

Density functional theory (DFT) calculations are carried out to examine the electronic structures of SnS₂ and SnS_2−x. The projected density of state (PDOS, Fig. 2h) analyses show that, in contrast to semiconducting SnS₂ with a noticeable band gap at the Fermi level, a new defect level appears at the conduction band minimum of SnS_2−x, which reduces the band gap and enhances the conductivity,^54–58 consistent with the electrochemical impedance spectroscopy measurements (Fig. S3†). The enhanced conductivity is favorable for the accelerated electron transfer efficiency and boosts the catalytic NORR kinetics.^59–64 Additionally, the calculated average potential profiles (Fig. 2i, and Fig. S4†) show that SnS_2−x exhibits a lower work function (5.473 eV) than SnS₂ (6.316 eV), which implies the stronger capability of SnS_2−x for the back-donation of electrons to the absorbed NO and reaction intermediates, promoting the NO activation and protonation.⁶ Moreover, ab initio molecular dynamics (AIMD) simulations (Fig. S5†) reveal the high thermodynamic stability of SnS_2−x, in which the energy and temperature can keep the equilibrium states at 700 K, suggesting the stable V_S structure in SnS_2−x.³

Electrochemical NORR measurements are performed in NO-saturated 0.5 M Na₂SO₄ solution using a gas-tight two-compartment electrolytic cell.²⁷ All potentials are calibrated to the reversible hydrogen electrode (RHE). The linear sweep voltammetry (LSV, Fig. 2a) curve of SnS_2−x/CC in the NO-saturated electrolyte shows a noticeably higher current density than that in an Ar-saturated electrolyte, attesting that SnS_2−x/CC is active for the NORR. To quantify the NORR performance of SnS_2−x/CC, chronoamperometry tests are conducted at various potentials for 1 h and the products are analyzed by colorimetric methods (Fig. S6–S8†) and gas chromatography. As shown in Fig. 3b, SnS_2−x/CC presents the maximum NH₃ yield of 78.6 μmol h⁻¹ cm⁻² along with the highest FE_NH3 of 90.3% at −0.7 V, superior to those of most reported d-block NORR catalysts (Table S1†). The reduced NORR performance at potentials higher than −0.7 V is ascribed to the boosted HER (Fig. 3c). Meanwhile, the FEs and partial current densities of N₂ and N₂H₄ by-products are rather low (Fig. 3c and Fig. S9†), suggesting the high NORR selectivity of SnS_2−x/CC toward the NH₃ synthesis. Besides the high NORR activity and selectivity, SnS_2−x/CC also exhibits an excellent stability during the seven consecutive cycles (Fig. 3d) and the long-term chronopotentiometric test for 20 h (Fig. 3e).

Fig. 3 (a) LSV curves of SnS_2−x/CC in Ar-/NO-saturated 0.5 M Na₂SO₄. (b) NH₃ yields and FE_NH3 of SnS_2−x/CC at various potentials. (c) FEs of different products at various potentials. (d) Cycling and (e) long-term tests of SnS_2−x/CC at −0.7 V. (f) Amounts of produced NH₃ over SnS_2−x/CC under different conditions at −0.7 V. (g) ¹H NMR spectra of the ¹⁵NH₄⁺ standard sample and those fed with ¹⁵NO and Ar after NORR electrolysis on SnS_2−x/CC at −0.7 V. (h) Comparison of the NORR performance between SnS₂/CC and SnS_2−x/CC at −0.7 V.

We performed control tests to verify the NH₃ source.²⁶ As shown in Fig. 3f, negligible NH₃ can be detected in the electrolytes under Ar conditions and at the open circuit potential (OCP). The NO-Ar gas switching experiment (Fig. S10†) reveals that NH₃ can only be prominently observed in NO-feeding cycles. In the isotopic labeling ¹H nuclear magnetic resonance (NMR, Fig. 3g) spectra, feeding ¹⁵NO gas leads to a distinct ¹⁵NH₄⁺ signal, whereas the ¹⁵NH₄⁺ signal is absent under Ar conditions. These control experiments validate that the detected NH₃ is derived from the NORR. In addition, NO₃⁻ is barely detected after electrolysis (Fig. S11†), suggesting that our gas-tight cell can effectively restrict NO oxidation during the NORR measurements.³⁵

For comparison, the NORR performance of pristine SnS₂/CC is also evaluated under identical electrolysis conditions (Fig. 3h). The NH₃ yield and FE_NH3 of SnS₂/CC are found to be 2.8 and 1.8 times poorer than those of SnS_2−x/CC, respectively, highlighting the significance of the defect engineering strategy to substantially promote the NORR performance of SnS_2−x. The origin of the enhanced NORR performance of SnS_2−x relative to SnS₂ is then unraveled by theoretical computations as described below.

As shown in Fig. 4a, the NO molecule can hardly be absorbed on pristine SnS₂, showing negligible N=O bond elongation and near-zero SnS₂-to-NO electron transfer. In sharp contrast, NO can be favorably absorbed on SnS_2−x where the N atom of NO is chemically bonded with the V_S-induced unsaturated Sn site (Sn-V_S).⁶⁵ In this scenario, Sn-V_S donates remarkable −0.6 |e| electrons to the anti-bonding orbital of NO and the N=O bond is largely elongated to 1.271 Å.^66–68 Besides, in comparison with the pristine Sn site of SnS₂, the absorbed NO on the Sn-V_S of SnS_2−x shows an enhanced *NO-p/Sn-p orbital hybridization at the Fermi level (Fig. S12†). Hence, Sn-V_S plays a critical role in effectively activating NO and dissociating the N=O bond.

Fig. 4 (a) Electron contour maps of adsorbed NO on SnS₂ and SnS_2−x. Blue: charge depletion. Red: charge accumulation. (b) Snapshots of the final states of the dynamic process of NO adsorption on SnS_2−x and SnS₂ after MD simulations, and the corresponding (c) RDF curves. (d) Free energies of *NO and *H on SnS_2−x. (e) Free energy diagrams of the NORR process on SnS_2−x and SnS₂.

Molecular dynamics (MD) simulations are also performed to investigate the dynamic process of NO absorption. After simulations (Fig. 4b and Fig. S13†), it is obvious that SnS_2−x shows more NO coverage than SnS₂, and the corresponding radial distribution function (RDF, Fig. 4c) curves reveal a stronger NO-SnS_2−x interaction than the NO-SnS₂ interaction,⁶ confirming, once again, the robust ability of SnS_2−x for NO adsorption and coverage. In addition, after examining the competitive adsorption between *NO and *H (Fig. 4d), we found that the Sn-V_S site can selectively absorb *NO (−0.62 eV) rather than *H (0.16 eV), thereby resulting in the suppressed HER and thus the enhanced NORR selectivity of SnS_2−x.

The free energy profiles are then constructed to investigate the entire NORR pathway on SnS₂ and SnS_2−x (Fig. 4e, Fig. S14, S15 and Table S2†).²⁹ Impressively, owing to the strong NO activation enabled by Sn-V_S, SnS_2−x even undergoes an endergonic process for the first protonation step of *NO → *NOH, whereas SnS₂ requires a high energy input of 0.61 eV to overcome *NO → *NOH which is the rate-determining step (RDS). The RDS of SnS_2−x is *NH₂ → *NH₃ with only 0.41 eV uphill, which is lower than that of most previously reported catalysts.^27,28,35 Additionally, the free energies of all the intermediates on SnS_2−x are lower than those on SnS₂, attesting the enhanced protonation energetics of SnS_2−x to promote the NORR pathway.

In conclusion, SnS_2−x has been verified to be an active, selective and durable NORR catalyst. Theoretical computations revealed the critical function of Sn-V_S sites to selectively activate NO and reduce the energy barriers of the NORR pathway. This work highlights the great potential of p-block metal elements for developing high-performance NORR catalysts for NH₃ electrosynthesis.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work is supported by the Natural Science Foundation of Gansu Province (20JR10RA241).

References

1. J. B. Howard and D. C. Rees, Structural basis of biological nitrogen fixation, Chem. Rev., 1996, 96, 2965–2982.
2. J. Liang, Q. Liu, A. A. Alshehri and X. Sun, Recent advances in nanostructured heterogeneous catalysts for N-cycle electrocatalysis, Nano Res. Energy, 2022, 1, e9120010.
3. X. Li, P. Shen, Y. Luo, Y. Li, Y. Guo, H. Zhang and K. Chu, PdFe single-atom alloy metallene for N₂ electroreduction, Angew. Chem., 2022, 134, e202205923.
4. X. Zhao, G. Hu, G. F. Chen, H. Zhang, S. Zhang and H. Wang, Comprehensive understanding of the thriving ambient electrochemical nitrogen reduction reaction, Adv. Mater., 2021, 33, 2007650.
5. K. Tanifuji and Y. Ohki, Metal–sulfur compounds in N₂ reduction and nitrogenase-related chemistry, Chem. Rev., 2020, 120, 5194–5251.
6. K. Chu, Y. Luo, P. Shen, X. Li, Q. Li and Y. Guo, Unveiling the synergy of O-vacancy and heterostructure over MoO_3−x/MXene for N₂ electroreduction to NH₃, Adv. Energy Mater., 2022, 12, 2103022.
7. H. Shen, C. Choi, J. Masa, X. Li, J. Qiu, Y. Jung and Z. Sun, Electrochemical ammonia synthesis: mechanistic understanding and catalyst design, Chem, 2021, 7, 1708–1754.
8. Y. Ren, C. Yu, X. Tan, H. Huang, Q. Wei and J. Qiu, Strategies to suppress hydrogen evolution for highly selective electrocatalytic nitrogen reduction: challenges and perspectives, Energy Environ. Sci., 2021, 14, 1176–1193.
9. Y. Li, H. Wang, C. Priest, S. Li, P. Xu and G. Wu, Advanced electrocatalysis for energy and environmental sustainability via water and nitrogen reactions, Adv. Mater., 2021, 33, 2000381.
10. X. Li, Y. Luo, Q. Li, Y. Guo and K. Chu, Constructing an electron-rich interface over an Sb/Nb₂CT_x-MXene heterojunction for enhanced electrocatalytic nitrogen reduction, J. Mater. Chem. A, 2021, 9, 15955–15962.
11. Q. Li, J. Wang, Y. Cheng and K. Chu, Zn nanosheets: An earth-abundant metallic catalyst for efficient electrochemical ammonia synthesis, J. Energy Chem., 2021, 54, 318–322.
12. K. Chu, H. Nan, Q. Li, Y. Guo, Y. Tian and W. Liu, Amorphous MoS₃ enriched with sulfur vacancies for efficient electrocatalytic nitrogen reduction, J. Energy Chem., 2021, 53, 132–138.
13. K. Chu, X. Li, Q. Li, Y. Guo and H. Zhang, Synergistic enhancement of electrocatalytic nitrogen reduction over boron nitride quantum dots decorated Nb₂CT_x-MXene, Small, 2021, 17, 2102363.
14. K. Chu, W. Gu, Q. Li, Y. Liu, Y. Tian and W. Liu, Amorphization activated FeB₂ porous nanosheets enable efficient electrocatalytic N₂ fixation, J. Energy Chem., 2021, 53, 82–89.
15. K. Chu, Y. Liu, J. Wang and H. Zhang, NiO nanodots on graphene for efficient electrochemical N₂ reduction to NH₃, ACS Appl. Energy Mater., 2019, 2, 2288–2295.
16. V. Kyriakou, I. Garagounis, E. Vasileiou, A. Vourros and M. Stoukides, Progress in the electrochemical synthesis of ammonia, Catal. Today, 2017, 286, 2–13.
17. V. Kordali, G. Kyriacou and C. Lambrou, Electrochemical synthesis of ammonia at atmospheric pressure and low temperature in a solid polymer electrolyte cell, Chem. Commun., 2000, 1673–1674.
18. T. Murakami, T. Nishikiori, T. Nohira and Y. Ito, Electrolytic synthesis of ammonia in molten salts under atmospheric pressure, J. Am. Chem. Soc., 2003, 125, 334–335.
19. G. Zhang, X. Li, P. Shen, Y. Luo, X. Li and K. Chu, PdNi nanosheets boost nitrate electroreduction to ammonia, J. Environ. Chem. Eng., 2022, 10, 108362.
20. K. Chu, Y. Cheng, Q. Li, Y. Liu and Y. Tian, Fe-doping induced morphological changes, oxygen vacancies and Ce³⁺–Ce³⁺ pairs in CeO₂ for promoting electrocatalytic nitrogen fixation, J. Mater. Chem. A, 2020, 8, 5865–5873.
21. K. Chu, Y. Liu, Y. Chen and Q. Li, Synergistic boron-dopants and boron-induce oxygen vacancies in MnO₂ nanosheets to promote electrocatalytic nitrogen reduction, J. Mater. Chem. A, 2020, 8, 5200–5208.
22. P. Shen, G. Wang, K. Chen, J. Kang, D. Ma and K. Chu, Selenium-vacancy-rich WSe₂ for nitrate electroreduction to ammonia, J. Colloid Interface Sci., 2023, 629, 563–570.
23. K. Chu, Y. Liu, Y. Li, H. Zhang and Y. Tian, Efficient electrocatalytic N₂ reduction on CoO quantum dots, J. Mater. Chem. A, 2019, 7, 4389–4394.
24. Y. Luo, Q. Li, Y. Tian, Y. Liu and K. Chu, Amorphization engineered VSe_2−x nanosheets with abundant Se-vacancies for enhanced N₂ electroreduction, J. Mater. Chem. A, 2022, 10, 1742–1749.
25. D. Qi, F. Lv, T. Wei, M. Jin, G. Meng, S. Zhang, Q. Liu, W. Liu, D. Ma, M. S. Hamdy, L. Jun and L. Xijun, High-efficiency electrocatalytic NO reduction to NH₃ by nanoporous VN, Nano Res. Energy, 2022, 1, e9120022.
26. J. Liang, P. Liu, Q. Li, T. Li, L. Yue, Y. Luo, Q. Liu, N. Li, B. Tang, A. A. Alshehri, I. Shakir, P. O. Agboola, C. Sun and X. Sun, Amorphous boron carbide on titanium dioxide nanobelt arrays for high-efficiency electrocatalytic NO reduction to NH₃, Angew. Chem., Int. Ed., 2022, 61, e202202087.
27. L. Zhang, J. Liang, Y. Wang, T. Mou, Y. Lin, L. Yue, T. Li, Q. Liu, Y. Luo, N. Li, B. Tang, Y. Liu, S. Gao, A. A. Alshehri, X. Guo, D. Ma and X. Sun, High-performance electrochemical NO reduction into NH₃ by MoS₂ nanosheet, Angew. Chem., Int. Ed., 2021, 60, 25263–25268.
28. P. Liu, J. Liang, J. Wang, L. Zhang, J. Li, L. Yue, Y. Ren, T. Li, Y. Luo, N. Li, B. Tang, Q. Liu, A. M. Asiri, Q. Kong and X. Sun, High-performance NH₃ production via NO electroreduction over a NiO nanosheet array, Chem. Commun., 2021, 57, 13562–13565.
29. B. He, P. Lv, D. Wu, X. Li, R. Zhu, K. Chu, D. Ma and Y. Jia, Confinement catalysis of a single atomic vacancy assisted by aliovalent ion doping enabled efficient NO electroreduction to NH₃, J. Mater. Chem. A, 2022, 10, 18690–18700.
30. P. Lv, D. Wu, B. He, X. Li, R. Zhu, G. Tang, Z. Lu, D. Ma and Y. Jia, An efficient screening strategy towards multifunctional catalysts for the simultaneous electroreduction of NO₃⁻, NO₂⁻ and NO to NH₃, J. Mater. Chem. A, 2022, 10, 9707–9716.
31. G. Meng, T. Wei, W. Liu, W. Li, S. Zhang, W. Liu, Q. Liu, H. Bao, J. Luo and X. Liu, NiFe layered double hydroxide nanosheet array for high-efficiency electrocatalytic reduction of nitric oxide to ammonia, Chem. Commun., 2022, 58, 8097–8100.
32. X. Peng, Y. Mi, H. Bao, Y. Liu, D. Qi, Y. Qiu, L. Zhuo, S. Zhao, J. Sun, X. Tang, J. Luo and X. Liu, Ambient electrosynthesis of ammonia with efficient denitration, Nano Energy, 2020, 78, 105321.
33. Y. Xiong, Y. Li, S. Wan, Y. Yu, S. Zhang and Q. Zhong, Ferrous-based electrolyte for simultaneous NO absorption and electroreduction to NH₃ using Au/rGO electrode, J. Hazard. Mater., 2022, 430, 128451.
34. Y. Li, C. Cheng, S. Han, Y. Huang, X. Du, B. Zhang and Y. Yu, Electrocatalytic reduction of low-concentration nitric oxide into ammonia over Ru nanosheets, ACS Energy Lett., 2022, 7, 1187–1194.
35. Y. Lin, J. Liang, H. Li, L. Zhang, T. Mou, T. Li, L. Yue, Y. Ji, Q. Liu, Y. Luo, N. Li, B. Tang, Q. Wu, M. S. Hamdy, D. Ma and X. Sun, Bi nanodendrites for highly efficient electrocatalytic NO reduction to NH₃ at ambient conditions, Mater. Today Phys., 2022, 22, 100611.
36. J. Liang, Q. Zhou, T. Mou, H. Chen, L. Yue, Y. Luo, Q. Liu, M. S. Hamdy, A. A. Alshehri, F. Gong and X. Sun, FeP nanorod array: A high-efficiency catalyst for electroreduction of NO to NH₃ under ambient conditions, Nano Res., 2022, 15, 4008–4013.
37. Q. Liu, Y. Lin, L. Yue, J. Liang, L. Zhang, T. Li, Y. Luo, M. Liu, J. You, A. A. Alshehri, Q. Kong and X. Sun, Bi nanoparticles/carbon nanosheet composite: A high-efficiency electrocatalyst for NO reduction to NH₃, Nano Res., 2022, 15, 5032–5037.
38. J. Liang, W.-F. Hu, B. Song, T. Mou, L. Zhang, Y. Luo, Q. Liu, A. A. Alshehri, M. S. Hamdy, L.-M. Yang and X. Sun, Efficient nitric oxide electroreduction toward ambient ammonia synthesis catalyzed by a CoP nanoarray, Inorg. Chem. Front., 2022, 9, 1366–1372.
39. L. Zhang, Q. Zhou, J. Liang, L. Yue, T. Li, Y. Luo, Q. Liu, N. Li, B. Tang, F. Gong, X. Guo and X. Sun, Enhancing electrocatalytic NO reduction to NH₃ by the CoS nanosheet with sulfur vacancies, Inorg. Chem. Front., 2022, 61, 8096–8102.
40. L. Ouyang, Q. Zhou, J. Liang, L. Zhang, L. Yue, Z. Li, J. Li, Y. Luo, Q. Liu, N. Li, B. Tang, A. Ali Alshehri, F. Gong and X. Sun, High-efficiency NO electroreduction to NH₃ over honeycomb carbon nanofiber at ambient conditions, J. Colloid Interface Sci., 2022, 616, 261–267.
41. Z. Li, Z. Ma, J. Liang, Y. Ren, T. Li, S. Xu, Q. Liu, N. Li, B. Tang, Y. Liu, S. Gao, A. A. Alshehri, D. Ma, Y. Luo, Q. Wu and X. Sun, MnO₂ nanoarray with oxygen vacancies: An efficient catalyst for NO electroreduction to NH₃ at ambient conditions, Mater. Today Phys., 2022, 22, 100586.
42. C. Ling, X. Niu, Q. Li, A. Du and J. Wang, Metal-free single atom catalyst for N₂ fixation driven by visible light, J. Am. Chem. Soc., 2018, 140, 14161–14168.
43. K. Chen, Y. Luo, P. Shen, X. Liu, X. Li, X. Li and K. Chu, Boosted nitrate electroreduction to ammonia on Fe-doped SnS₂ nanosheet arrays with rich S-vacancies, Dalton Trans., 2022, 51, 10343–10350.
44. K. Chu, J. Wang, Y. Liu, Q. Li and Y. Guo, Mo-doped SnS₂ with rich S-vacancies for highly efficient electrocatalytic N₂ reduction: the critical role of Mo-Sn-Sn trimer, J. Mater. Chem. A, 2020, 8, 7117–7124.
45. K. Chu, Y. Liu, Y. Li, J. Wang and H. Zhang, Electronically coupled SnO₂ quantum dots and graphene for efficient nitrogen reduction reaction, ACS Appl. Mater. Interfaces, 2019, 11, 31806–31815.
46. D. Yan, H. Li, C. Chen, Y. Zou and S. Wang, Defect engineering strategies for nitrogen reduction reactions under ambient conditions, Small Methods, 2018, 1800331.
47. G. Wang, P. Shen, Y. Luo, X. Li, X. Li and K. Chu, A vacancy engineered MnO_2−x electrocatalyst promotes electroreduction of nitrate to ammonia, Dalton Trans., 2022, 51, 9206–9212.
48. Y. Li, Y. Liu, J. Wang, Y. Guo and K. Chu, Plasma-engineered NiO nanosheets with enriched oxygen vacancies for enhanced electrocatalytic nitrogen fixation, Inorg. Chem. Front., 2020, 7, 455–463.
49. Y. Cheng, X. Li, P. Shen, Y. Guo and K. Chu, MXene quantum dots/copper heterostructure for synergistically enhanced N₂ electroreduction, Energy Environ. Mater., 2022 DOI:10.1002/eem2.12268.
50. Q. Li, Y. Guo, Y. Tian, W. Liu and K. Chu, Activating VS₂ basal planes for enhanced NRR electrocatalysis: the synergistic role of S-vacancies and B dopants, J. Mater. Chem. A, 2020, 8, 16195–16202.
51. Y. Luo, P. Shen, X. Li, Y. Guo and K. Chu, Sulfur-deficient Bi₂S_3−x synergistically coupling Ti₃C₂T_x-MXene for boosting electrocatalytic N₂ reduction, Nano Res., 2022, 15, 3991–3999.
52. P. Shen, X. Li, Y. Luo, Y. Guo, X. Zhao and K. Chu, High-efficiency N₂ electroreduction enabled by Se-vacancy-rich WSe_2−x in water-in-salt electrolytes, ACS Nano, 2022, 16, 7915–7925.
53. P. Shen, X. Li, Y. Luo, N. Zhang, X. Zhao and K. Chu, Ultra-efficient N₂ electroreduction achieved over a rhodium single-atom catalyst (Rh₁/MnO₂) in water-in-salt electrolyte, Appl. Catal., B, 2022, 316, 121651.
54. Q. Li, P. Shen, Y. Tian, X. Li and K. Chu, Metal-free BN quantum dots/graphitic C₃N₄ heterostructure for nitrogen reduction reaction, J. Colloid Interface Sci., 2022, 606, 204–212.
55. Y. Luo, K. Chen, P. Shen, X. Li, X. Li, Y. Li and K. Chu, B-doped MoS₂ for nitrate electroreduction to ammonia, J. Colloid Interface Sci., 2023, 629, 950–957.
56. K. Chu, Q. Li, Y. Cheng and Y. Liu, Efficient electrocatalytic nitrogen fixation on FeMoO₄ nanorods, ACS Appl. Mater. Interfaces, 2020, 12, 11789–11796.
57. D. Ma, Z. Zeng, L. Liu and Y. Jia, Theoretical screening of the transition metal heteronuclear dimer anchored graphdiyne for electrocatalytic nitrogen reduction, J. Energy Chem., 2021, 54, 501–509.
58. D. Wu, B. He, Y. Wang, P. Lv, D. Ma and Y. Jia, Double-atom catalysts for energy-related electrocatalysis applications: a theoretical perspective, J. Phys. D: Appl. Phys., 2022, 55, 203001.
59. K. Chu, X. Li, Y. Tian, Q. Li and Y. Guo, Boron nitride quantum dots/Ti₃C₂T_x-MXene heterostructure for efficient electrocatalytic nitrogen fixation, Energy Environ. Mater., 2022, 5, 1303–1309.
60. W. Gu, Y. Guo, Q. Li, Y. Tian and K. Chu, Lithium iron oxide (LiFeO₂) for electroreduction of dinitrogen to ammonia, ACS Appl. Mater. Interfaces, 2020, 12, 37258–37264.
61. K. Chu, Y. Liu, Y. Li, Y. Guo and Y. Tian, Two-dimensional (2D)/2D interface engineering of MoS₂/C₃N₄ heterostructure for promoted electrocatalytic nitrogen fixation, ACS Appl. Mater. Interfaces, 2020, 12, 7081–7090.
62. S. Javad and W. Guoxiu, Progress and prospects of two-dimensional materials for membrane-based osmotic power generation, Nano Res. Energy, 2022, 1, e9120008.
63. L. Lulu, H. Israr Masood ul Farwa, H. Ruinan, P. Luwei, X. Nengneng, N. Nabeel Khan, Z. Jia-Nan and Q. Jinli, Copper as a single metal atom based photo-, electro-, and photoelectrochemical catalyst decorated on carbon nitride surface for efficient CO₂ reduction: A review, Nano Res. Energy, 2022, 1, e9120015.
64. A. Touqeer, L. Shuang, S. Muhammad, L. Ke, A. Mohsin, L. Liang and C. Wei, Electrochemical CO₂ reduction to C₂₊ products using Cu-based electrocatalysts: A review, Nano Res. Energy, 2022, 1, e9120021.
65. Y. Guo, Y. Cheng, Q. Li and K. Chu, FeTe₂ as an earth-abundant metal telluride catalyst for electrocatalytic nitrogen fixation, J. Energy Chem., 2021, 56, 259–263.
66. Y. Wang, M. Wang, Z. Lu, D. Ma and Y. Jia, Enabling multifunctional electrocatalysts by modifying the basal plane of unifunctional 1T-MoS₂ with anchored transition metal single atoms, Nanoscale, 2021, 13, 13390–13400.
67. Y. Wang, D. Wu, P. Lv, B. He, X. Li, D. Ma and Y. Jia, Theoretical insights into the electroreduction of nitrate to ammonia on graphene-based single-atom catalysts, Nanoscale, 2022, 14, 10862–10872.
68. D. Ma, Y. Wang, L. Liu and Y. Jia, Electrocatalytic nitrogen reduction on the transition-metal dimer anchored N-doped graphene: performance prediction and synergetic effect, Phys. Chem. Chem. Phys., 2021, 23, 4018–4029.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qi02118h

‡ These authors contributed equally to this work.

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