Ammonia (NH₃) as a hydrogen-rich fuel may solve the problem of H₂ transportation and storage [1, 2]. Nevertheless, gray NH₃ generation relies great possibility on the century-old Haber-Bosch process leading to a large carbon footprint [3]. Thus, using renewable energy to achieve electrocatalytic green NH₃ synthesis is a promising path [4]. Although N₂ reduction reaction (NRR) process can produce green NH₃, it is relatively energy-intensive due to the high dissociation energy of N≡N bond and low solubility of N₂ in water [5].

Nitrate (NO₃⁻) as a highly water-soluble N source requires a lower energy of 204 kJ/mol for breaking N=O bond compared with N≡N bond dissociation [6]. In addition, accumulation of NO₃⁻ have caused a water pollutant threating to environment and human health [7-10]. Electrocatalytic NO₃⁻ reduction to NH₃ (NRA) is the most promising path for green NH₃ generation [11, 12]. NH₃ can be easily reclaimed from ammonia aqueous solution via regenerated resins or air stripping [13]. However, the poor adsorption and activation of NO₃⁻ ability and complicated eight-electron process as well as the competitive hydrogen evolution reaction (HER) continue to hinder the NRA [14-16]. Therefore, developing efficient and selective electrocatalysts toward NRA that can improve Faradaic efficiency and activate the NO₃⁻ is urgently needed.

Oxygen vacancies (OVs) are anion vacancy defect that can be used in semiconductor metal oxides. Particularly, tungsten (W) based catalysts with OVs have been widely used to improve the performance of selective NH₃ synthesis. For example, WO_3-x nanowires and WO_3-x nanosheets with OVs could impact on N species adsorption and suppression of the competing HER to some extent [17]. Therefore, it can be reasonably anticipated that OVs in W-based catalysts can effectively capture N species and reduce it. Recently, W-based perovskite oxides nanosheets with the potential to tune OVs have expected their great potential toward NH₃ synthesis owing to ultrathin thickness, high surface-volume ratio, and various metallic oxide composition [18]. However, studies on W-based perovskite oxides nanosheets containing OVs for NRA have rarely been reported.

Herein, the NbWO₆ nanosheets with OVs (NbWO_6-x) were demonstrated to provide large surface area with fully exposed active interface as well as specific interacting d orbital electrons, which perform high NH₃ selectivity and Faradaic efficiency for NRA. ¹⁵N isotope labeling coupled with ¹H nuclear magnetic resonance (¹H NMR) spectra experiments proved that NH₃ originated from NO₃⁻. The function of OVs was revealed by computational studies in NRA. Online differential electrochemical mass spectrometry (DEMS) deduced the pathway of NRA involving the O atom in NO₃⁻ filled in OVs of NbWO_6-x. This work proved the enhanced function of OVs in NRA, providing a novel strategy for selective NH₃ generation.

The NbWO_6-x nanosheets were synthesized via thermal treatment and exfoliation (Fig. 1a). Specifically, LiNbWO_6-x was synthesized by H₂ atmosphere at 800 ℃ calcination, and the typical layered nanostructure is observed by scanning electron microscopy (SEM) (Fig. 1b). Then, as-obtained NbWO_6-x was prepared by proton-exchange in nitric acid (Fig. S1 in Supporting information) and followed by exfoliation in tetrabutylammonium solution. The few-layer NbWO_6-x nanosheets was identified by SEM suggesting the successful exfoliation from layered sample (Fig. 1c). The X-ray powder diffraction (XRD) was performed to determine the crystal phase of LiNbWO_6-x and NbWO_6-x. XRD pattern of the LiNbWO_6-x showed the typical diffraction peaks at 9.51°, 19.16°, and 28.64° corresponding to the (001), (002) and (003) crystal planes, respectively (PDF #41–0377/0378) (Fig. S2 in Supporting information) [19, 20]. While, NbWO_6-x nanosheets revealed (110) crystal planes. The thickness of the nanosheets detected by atomic force microscopy (AFM) is around 3.2 nm, implying the formation of NbWO_6-x few layers (Fig. 1d). A transmission electron microscopy (TEM) image revealed an ultrathin sheet-like morphology (Fig. 1e). A high-resolution TEM (HRTEM) image verified distinct lattice fringes corresponding to the (110) lattice planes of NbWO_6-x (Fig. S3 in Supporting information) and further confirmed by selected area electron diffraction (SAED) pattern (Fig. 1f). Nb, W and O were distributed homogeneously in the nanosheets manifested via energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Figs. 1g and h). For NbWO_6-x, the characterization data show that the presence of OVs hardly change the morphology.

To further confirm the existence of more OVs in NbWO_6-x, NbWO_6-x was further examined by electron paramagnetic resonance (EPR). NbWO_6-x shows a symmetry signal in g = 2.004 (Fig. 2a), indicating the presence of electron trapped OVs [21]. While, the Raman peaks of NbWO_6-x nanosheets displayed broadening and shifted toward higher wavenumber, further yielding the information of OVs in NbWO_6-x (Fig. 2b) [22]. X-ray photoelectron spectroscopy (XPS) is performed to further confirm the existence of more OVs in NbWO_6-x. The Nb (Fig. S4 in Supporting information), W, and O are detected in the full XPS survey of NbWO_6-x and NbWO₆ (Fig. S5 in Supporting information). The O 1s XPS spectra show there are two peaks at 531.4 eV (lattice oxygen) and 533.5 eV (OVs) (Fig. 2c). After H₂ treatment, the area ratio of O^–/O_sum increases from ~9.6% (NbWO₆) to ~18.2% (NbWO_6-x). Moreover, W 4f XPS spectra show two predominant peaks with binding energy values at 36.9 and 39.1 eV, assigned to 4f_7/2 and 4f_5/2 of W⁶⁺, respectively (Fig. 2d). Notedly, only weak peaks originating from W⁵⁺ appeared in NbWO_6-x, which proved that more OVs existed in NbWO_6-x derived from W-Site consisting with Raman and EPR results [23]. XRD patterns revealed that the diffraction peaks of as-synthesized NbWO_6-x are well coincident with NbWO₆ (Fig. S6 in Supporting information). In brief, H₂ treatment at 800 ℃ can create more OVs to generate NbWO_6-x with the maintaining of initial few-layer nanosheets morphology and crystal structure.

Electrocatalytic NRA of NbWO_6-x is executed using an H-type electrolytic cell. The linear sweep voltammetry (LSV) curves of NbWO₆ (Fig. S7a in Supporting information) and NbWO_6-x (Fig. S7b in Supporting information) both showed the obvious current density increase in the presence of NO₃⁻. NbWO_6-x exhibited higher current density compared with NbWO₆ during NRA (Fig. S8 in Supporting information). Different applied potentials from −0.3 V to −0.8 V were applied to investigate the NRA performance of NbWO_6-x with NbWO₆ as comparison (Figs. 3a and b). The variations of NH₃ yield rates (0.021 to 0.068 mmol h⁻¹ mg_cat.^-1) and Faradaic efficiency (28.6% to 85.7%) of NbWO_6-x were higher than those of NbWO₆ (0.019 to 0.049 mmol h⁻¹ mg_cat.^-1 and 25.5% to 63.3%), when the potential shifted from −0.3 V to −0.7 V, indicating the high intrinsic activity of NbWO_6-x. Nevertheless, when the applied potential was further reduced to −0.8 V, it was observed that NH₃ yield rates and Faradaic efficiency were greatly reduced. This phenomenon can be ascribed to the occurrence of excessive HER side reaction [24]. Fig. 3c recorded the typical online DEMS results of NbWO_6-x under −0.7 V in 0.10 mol/L Na₂SO₄. The intensity of the m/z signal at 17 (NH₃) varied with the applied voltage, and the highest value was achieved at −0.7 V. Meanwhile, weaker NH₃-related signal could be detected when NbWO₆ was used as the cathode, in good accordance with the NRA experimental results (Fig. 3d). Moreover, the NO₃⁻ conversion rate and NH₃ selectivity (97.9%, 86.8%) of NbWO_6-x were higher than those of NbWO₆ (78.1%, 68.5%) (Fig. S9 in Supporting information). The concentration of NO₃⁻ continuously decreased while NH₃ concentration was constantly increasing as the reaction time lengthened (Fig. 3e). NO₃⁻ concentration was found to 0.21 mmol/L within 90 min of NbWO_6-x which is much lower than that of NbWO₆ (Fig. S10 in Supporting information). The NH₃ selectivity Faradaic efficiency and NO₃⁻ conversion rate retained more than 80% after 10 cycles of NRA on NbWO_6-x indicating high stability and long-term durability (Fig. 3f and Fig. S11 in Supporting information). Furthermore, the selectivity of NH₃ remained basically unchanged even though NO₃⁻ concentration increased to 14.28 mmol/L, revealing the wide application range of the NbWO_6-x (Fig. S12 in Supporting information). In addition, the performance of NbWO_6-x was comparable with or even better than other previous reported electrocatalysts for NRA (Table S1 in Supporting information) [25-30]. In addition, ¹⁵N isotope labeling experiments via ¹H NMR spectra were conducted to further confirm the source of NH₃ (Figs. S13a and b in Supporting information). The ¹H NMR spectra of Na¹⁵NO₃ as N-source after NRA showed typical double peaks in line with the (¹⁵NH₄)₂SO₄, while ¹H NMR spectra of Na¹⁴NO₃ as N-source after NRA showed typical three peaks in accordance with the (¹⁴NH₄)₂SO₄. Taking into account that the peak area of ¹H NMR is related to the NH₃ content, the NH₃ concentration is further measured with the ¹H NMR standard (Figs. S13c and d in Supporting information). The generated ¹⁵NH₃ and ¹⁴NH₃ calculated by ¹H NMR were very close to the results of colorimetric method using Nesslers reagent (Figs. S14a, b and Table S2 in Supporting information). It is proved quantitatively that the generation of NH₃ comes from NO₃⁻ as well as demonstrated the accuracy of different quantitative methods. The plausible interpretation is that more OVs of NbWO_6-x result in more active sites for adsorption and activation of NO₃⁻. Therefore, the introduction of OVs in NbWO_6-x can not only accelerate electron transfer, but also effectively adsorb and activate NO₃⁻, so as to improve the catalytic activity of NRA.

Computational studies are performed to investigate the OVs-enhanced mechanism over NbWO_6-x. The structural models of NbWO₆ and NbWO_6-x are shown in Fig. S15 (Supporting information) and the NO₃⁻ adsorption models are exhibited in Figs. 4a and b. The density of states (DOS) analysis of NbWO₆ shows that O, Nb, W are the most contributing to the valence band and conduction band (Fig. S16 in Supporting information). Whereas, DOS proves the conduction band of NbWO_6-x has obvious defect energy levels, which can lead to better electron transitions (Fig. 4c). By introducing OVs on the surface, the Fermi level moves into the conduction band minimum due to the occupation of excess 4d electrons of W. This gives rise to the metallic behavior of NbWO_6-x and then improves the conductivity, which is beneficial for electrochemical reduction (Fig. S17 in Supporting information). Furthermore, the NO₃⁻ adsorption energy on NbWO_6-x (−1.27 eV) is much lower than that on NbWO₆ (−0.53 eV) which is more conducive to NO₃⁻ adsorption, thus promoting the NRA (Fig. 4d). The mechanism was verified to W cations with their adjacent OVs and specific interacting d orbital electrons of NbWO_6-x can serve as unsaturated active centers that strengthen the ability to adsorb/activate NO₃⁻, making the NRA more conducive to happening.

DEMS system was used to monitor the volatile intermediates and products deducing the NRA path. When the applied voltages were varied from −0.3 V to −0.9 V, signals at m/z values of 46, 30, 33 and 17 (corresponding to NO₂, NO, NH₂OH and NH₃, respectively) appeared during continuous cycles (Fig. 5a). Thus, the reaction pathway of NRA could be deduced: NO₃⁻ → NO₂⁻ → NO → NH₂OH → NH₃. Figuratively, the possible reaction pathway for NRA on the NbWO_6-x with OVs is proposed and illustrated in Fig. 5b (labeling three O atoms in NO₃⁻ as O¹, O², and O³, respectively). In this pathway, NO₃⁻ adsorption on NbWO_6-x surface with OVs to form NO₃*. The next reduction reaction by proton-electron pair will take O³ in NO₃* and the two-H away to form NO₂* and H₂O³. Next, O² will then be released by the second hydrogenation step to form NO* and H₂O². Subsequently, three proton-electron pairs couple with NO* to form NH₂OH*, leaving O¹ on the OV site. Finally, O¹ will be reduced by two protons to form H₂O¹ and NH₃, recovering the OVs on the surface. Above all, OVs of NbWO_6-x can make the N−O bond weaken via filling with O of NO₃⁻ and hinder the undesirable byproducts consisting with the experimental results.

In summary, NbWO_6-x nanosheets with OVs were successfully prepared for enhancing the electrocatalytic NRA activity. NbWO_6-x nanosheets with OVs exhibited the high activity and excellent durability toward NRA. Experimental data and computational studies revealed OVs were beneficial to electron transitions and lower NO₃⁻ adsorption as well activation energy. DEMS measurements were introduced to detect the production of NO and NH₂OH during the NRA which demonstrated the reaction mechanism and pathway. This work will provide new insight for constructing OVs in perovskite oxides as efficient electrocatalysts toward NRA.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors appreciate the supports from the National Natural Science Foundation of China (NSFC, Nos. 22074104 and 51978491), National Program for Support of Top-notch Young Professionals, the Fundamental Research Funds for the Central Universities (No. 2022-4-ZD-07), "Shuguang Scholar Program" (No. 17SG52) by Shanghai Education Development Foundation and Shanghai Municipal Education Commission, Liaoning Provincial Natural Science Fund Project (No. 2019-ZD-0550), and Scientific Research Foundation of Shanghai Institute of Technology (No. YJ2021-24).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.107862.