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• Ammonia (NH₃), as an important feedstock chemical, is indispensable in manufacturing agricultural fertilizers, industry extensively products, and carbon-free condensed fuels [1, 2]. As well known, biological ammonia synthesis can only support a fraction of today's global NH₃ requirement [3]. Since the nitrogen reduction reaction (NRR) catalyst emerged in 1807, the Haber–Bosch method has been regarded as one of the most important strategies for N₂ fixation [4-7]. However, the Haber-Bosch N₂ fixation process has to be performed under high pressure and high temperature (around 400 ℃, 15-25 MPa), which usually suffers from severe equipment requirements and causes serious environmental issues [8-11]. Although N₂ is abundant in the atmosphere, NRR has proven to be extremely challenging under mild conditions due to its high bond energy of N≡N (940 kJ/mol) and non-polarizable N₂ molecule [12-15]. It is highly attractive to explore alternative strategies for N₂ fixation under mild conditions for sustainable energy solutions [16-21].

  Up to date, high-performance NRR catalysts mostly depend on noble metals (Pt, Pd, Au, Ru, etc.) and lanthanide ions [22, 23]. In this filed, Yang et al. combined Au and nanoporous hydrophobicity zeolitic imidazole framework (ZIF) as NPG@ZIF-8 to achieve a NH₃ yield of 28.7 mg h⁻¹ cm⁻² [24]. Ling and coworkers developed a Pt/Au electrocatalyst modified by ZIF, which shows satisfactory Faradaic efficiency (FE) of 44% and NH₃ yield of 161.9 µmol mg_cat⁻¹ h⁻¹ [25]. Meanwhile, Zhao and co-workers introduced Pb into Pd nanosponges to promote the adsorption of N₂, resulting a NH₃ yield of 25.68 µg mg_cat⁻¹ h⁻¹ with 5.79% FE [26]. However, the high cost and limited reserves of the noble metals seriously limit their large-scale application [27-29]. Recently, low-cost transition metals, such as Ti, Cu, Fe and Mn, have been used to construct NRR catalysts [30-33]. In this field, Ti³⁺ ion in Ti₂O₃ nanoparticles can be used as the active sites for promoting the NRR activity to achieve a NH₃ yield of 26.01 µg h⁻¹ mg_cat⁻¹ with a FE of 9.16% [31]. The Fe based C₁₈@Fe₃P/CP was also explored to activate the N₂ with a NRR performance of 2.16 × 10⁻¹¹ mol s⁻¹ cm⁻² [33]. However, their NRR performance still need to be further improved to achieve satisfactory NH₃ yield. Therefore, it is necessary to innovate in the direction of high-performance, economical, and environmentally friendly nitrogen fixation strategies with high efficiency.

  Molybdenum (Mo) in nitrogenase has been recognized as the active center for N₂ fixation under mild conditions. In nitrogenases, Mo centers donate unpaired electron to antibonding orbitals of N₂ and accept lone electron pairs from N₂ to facilitate the dissociation of N≡N bonds [34-38]. Moreover, Mo located at the top of the NRR theoretical volcano plot, which induces Mo-based catalysts standing out from a series of metal catalysts for high-performance NRR electrocatalysts [39-42]. However, the reaction overpotential of Mo-based electrocatalysts are still higher than 0.5 V, leading to excessive energy consuming [43, 44]. Recently, Du and co-workers reported that atomically dispersed tungsten (W) catalysts exhibited optimal performances for NRR with a low onset potential of 0.25 V according to the calculating theory and experimental results [45, 46]. It is evidenced that Mo cooperates with W derivative to provide optimal adsorption of N₂ to improve NRR and impede the hydrogen evolution reaction [36]. Thus, it is reasonable to expect that heteronuclear W/Mo electrocatalysts would optimize the NRR activity and selectivity, which are not observable in their individual monometallic systems [47]. In this field, Liang and co-workers reported a Mo-PTA@CNT electrocatalyst by doping Mo into the phosphotungstic acid (PTA) [36]. In Mo-PTA@CNT, Mo and W display strong interaction with N₂ to achieve a high NH₃ yield rate of 51± 1 µg h⁻¹ mg_cat⁻¹. Accordingly, it is expected that the W/Mo synergy in the uniformly dispersed dual-metal catalysts can maximize the atom utilization and improve the NRR activity. However, the synthesis of bimetal W/Mo-derived catalyst remains a huge challenge owing to the lack of uniform distribution and interaction techniques at an atom-level under the harsh synthesis condition. Hence, it is highly desirable to controllable synthesis of uniform-distributed W/Mo catalyst to provide in-depth insight into the synergetic mechanism for NRR. Polyoxometalates (POMs), with W/Mo-enriched structure and attractive electron/proton reservoir properties [48, 49], are considered as excellent matrixes for construction of W/Mo derived co-catalysis.

  Herein, we developed a W/Mo@rGO electrocatalyst via anchoring the heteronuclear H₃PW₆Mo₆O₄₀ (PW₆Mo₆) POM on reduced graphene oxide (rGO) following a calcining process. Further, trace amount of Pt was transplanted on the surface of W/Mo@rGO via in situ electroplating treatment as Pt-W/Mo@rGO-6 to further improve the performance for NRR at ambient conditions. It is noted that the W/Mo synergy effect obviously promotes the NRR efficiency with the NH₃ yield of 79.2 µg h⁻¹ mg_cat⁻¹ at −0.3 V vs. RHE. The NRR performance of heteronuclear POM-derived Pt-W/Mo@rGO-6 was much superior to that of the catalysts derived from mono-metal POMs (Pt-W@rGO-6 and Pt-Mo@rGO-6). This work demonstrates POMs as efficient precursors for the construction of heterometallic composites to act as advanced NRR catalysts.

  In nitrogenase, Mo centers can donate unpaired electron to antibonding orbitals of N₂ and accept lone electron pairs from N₂, which can facilitate the dissociation of N≡N bond. Accordingly, Mo doping represents an effective approach to supply catalytic active sites and modulate electronic structures to achieve N₂ fixation under mild conditions [50]. Inspired by nitrogenase, we selected Mo as the dopant to potentially lower the energy barrier of the dissociation of N≡N bond. The doped Mo species possess different d-electron structure from W, which allows tuning the electronic structure of W-based matrix for high efficient NRR [51, 52]. In this work, PW₆Mo₆ was chosen and synthesized as the model POM for the construction of W/Mo catalyst. Powder X-ray diffraction (PXRD) illustrates the successful synthesis of PW₆Mo₆ (Fig. S1 in Supporting information). Then, according to the PXRD and Raman spectra, graphene oxide (GO) was produced successfully for further application (Fig. S2 in Supporting information) [53, 54]. The detailed synthetic process of Pt-W/Mo@rGO-X electrocatalyst was schematically illustrated in Fig. 1. Firstly, a precursor solution was prepared by ultrasonic dissolution of PW₆Mo₆ in pure water in the presence of rGO. With the assistance of poly(ethylenimine) (PEI), PW₆Mo₆ was loaded onto the GO nanosheets to obtain PW₆Mo₆@PEI-GO. The PW₆Mo₆@PEI-GO precursor was then subjected to a calcining treatment to produce W/Mo oxide species on the surface of rGO to construct the W/Mo@rGO. To further improve the NRR catalytic activity, trace amount of Pt was electroplated for 6 h to form Pt-W/Mo@rGO-6. For comparison, the Mo-based Pt-Mo@rGO-6 and W-based Pt-W@rGO-6 electrocatalysts were further synthesized under the similar conditions.

  Figure 1

  

  Figure 1.  Schematic synthesis of Pt-W/Mo@rGO-X (X means the electroplating time).

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  The morphology and structure of Pt-W/Mo@rGO-6 were initially investigated by PXRD, high resolution transmission electron microscopy (HRTEM). As revealed in Fig. 2a, the XRD patterns of W/Mo@rGO displays characteristic signals located at 25.9° (110), 37.4° (002) and 53.5° (022), corresponding to characteristic peaks of WO₂ (PDF#71-0614) and MoO₂ (PDF#73-1249). TEM images reveal that W/Mo@rGO and rGO have the similar morphologies with obvious wrinkles, which illustrates unchangeable ultrathin rGO sheets before and after the fixation of W/Mo oxides (Figs. S3 and S4 in Supporting information). As shown in Fig. S4 in Supporting information, W/Mo oxide nanoparticles can be obviously observed on W/Mo@rGO with average particle size of ca. 31 nm (Fig. S4 in Supporting information). HRTEM image exhibits the ordered lattice fringes with a spacing of 2.4 Å, which corresponds to the (002) plane of WO₂ and the (200) plane of MoO₂. Further, energy dispersive X-ray (EDX) elemental mappings prove the uniform distribution of Mo and W elements on the rGO (Fig. S5 in Supporting information).

  Figure 2

  

  Figure 2.  (a) PXRD patterns of W/Mo@rGO and the simulated MoO₂ and WO₂. XPS spectra of (b) W/Mo@rGO, (c) W 4f, (d) Mo 3d and (e) O 1s. (f) AFM image of Pt-W/Mo@rGO-6.

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  Further, X-ray photoelectron spectroscopy (XPS) was carried out to gain insight into the chemical states and components of W/Mo@rGO. As exhibited in Fig. 2b, XPS survey spectrum confirms the existence of W, Mo, N, O and C elements, which consists with the EDX elemental mapping results. As shown in Fig. 2c, the spin orbit split doublet W 4f_7/2 and W 4f_5/2 peaks were detected at 35.5 and 37.6 eV, which indicates the valence of W species between W⁴⁺ and W⁵⁺ [17, 48]. Further, the Mo 3d_5/2 and 3d_3/2 peaks corresponding to Mo⁶⁺ in W/Mo@rGO can be observed at 232.2 and 235.7 eV; meanwhile the peak locations of 230.7 and 232.8 eV can confirm the existence of Mo⁴⁺ (Fig. 2d) [53]. It can be speculated that the surface oxidized Mo/W species impede further oxidation of WO₂ and MoO₂ in W/Mo@rGO, and the high valence W/Mo species can only be observed here by XPS because of its surface detection feature. To further demonstrate the reliability of the surface protection theory, the content of Mo⁶⁺ under Ar and atmosphere were detected via XPS spectra. There are approximate 52.4% Mo⁶⁺ on the surface of W/Mo@rGO under Ar, which was similar to that under atmosphere (54.0%) (Fig. 2d and Fig. S6a in Supporting information). Meanwhile, the spin orbit split doublet W 4f_7/2 and W 4f_5/2 peaks still locate at 35.5 and 37.6 eV without further oxidation under atmosphere (Fig. S6b in Supporting information). The WO₂ and MoO₂ in W/Mo@rGO observed in PXRD pattern can keep stable under atmosphere (Fig. S6c in Supporting information). All above results demonstrate the protection ability of the high valence W and Mo oxides on the surface of the catalysts to prevent the further oxidation of W⁴⁺ and Mo⁴⁺ inside the W/Mo@rGO. Hence, WO₂ and MoO₂ both exist in abundance in the W/Mo@rGO catalyst, which consists with the PXRD patterns. In the O 1s spectrum for W/Mo@rGO, the characteristic peaks at 530.5 eV and 532.1 eV belong to W-O and Mo-O, respectively (Fig. 2e). The C-O bond located at 533.6 eV is assigned to the characteristic peak of rGO [55]. In the Fourier transform infrared (FTIR) spectra of Pt-W/Mo@rGO-6, rGO and PW₆Mo₆, the characteristic peaks of PW₆Mo₆ located at 878.4, 976.4 and 1074.9 cm⁻¹ disappeared in the FTIR spectra of Pt-W/Mo@rGO-6, which indicates the destruction of the POM (Fig. S7 in Supporting information) [56]. Atomic force microscopy of Pt-W/Mo@rGO-6 reveals that the average thickness of rGO nanosheets is around 7.0 nm to support about 1.2 nm W/Mo oxide nanoparticles (Fig. 2f).

  After Pt electroplating, HRTEM images reveal that WO₂ and MoO₂ still keep the crystal lattice in Pt-W/Mo@rGO-6 with 3.4 Å lattice space, which corresponds to (110) facet of WO₂ and MoO₂ (Figs. 3a and b). Simultaneously, the HAADF-TEM image of Pt-W/Mo@rGO-6 clearly reveals the existence of WO₂ and MoO₂ nanoparticles anchored on rGO sheets (Fig. 3c). Further, the EDX elemental mapping exhibits the existence and uniform distribution of C, N, O, W, Mo and Pt in the composite catalyst of Pt-W/Mo@rGO-6 (Figs. 3d-i). The Pt content was determined to be 0.05 wt% via inductively coupled plasma mass spectrometry. The W and Mo contents on Pt-W/Mo@rGO-6 were determined to be 9.7% and 2.0%, respectively. As a comparison, Pt-W@rGO-6 and Pt-Mo@rGO-6 were synthesized with PW₁₂ and PMo₁₂ as the precusors, respectively, which were used as the control samples for further studying the NRR performance. From FTIR spectra, Mo or W oxide was successfully loaded on the rGO following a 6 h electroplating process to form Pt-W@rGO-6 and Pt-Mo@rGO-6 (Figs. S8 and S9 in Supporting information).

  Figure 3

  

  Figure 3.  (a) TEM, (b) HRTEM, (c) HADDF-TEM and (d-i) EDX elemental mapping images of Pt-W/Mo@rGO-6.

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  The NRR performance was determined at room temperature and atmospheric pressure with N₂-saturated electrolyte (0.05 mol/L H₂SO₄) in gas-tight two-compartment electrochemical cell. Firstly, linear sweeping voltammetry (LSV) curves were carried out to determine the electroplating time. As exhibited in Fig. 4a, the current density of Pt-W/Mo@rGO-6 enhances obviously comparing with that of W/Mo@rGO, Pt-W/Mo@rGO-2 and Pt-W/Mo@rGO-12. Hence, Pt-W/Mo@rGO-6 was chosen for further research of the NRR efficiency in this work. As shown in Fig. S10 (Supporting information), the LSV curves were recorded from 0.2 V to −0.75 V with saturated N₂ or Ar, respectively. The current density in N₂ atmosphere was much higher than that in Ar atmosphere, especially for the potential exceeding −0.3 V, which indicates high activity of Pt-W/Mo@rGO-6 toward electrocatalytic NRR. The NH₃ yield enhenced with increasing the potentials from −0.1 V to −0.3 V with the maximum yield of 79.2 µg h⁻¹ mg_cat⁻¹ at −0.3 V vs. RHE, which is 4 times higher than that of W/Mo@rGO (19.7 µg h⁻¹ mg_cat⁻¹) (Fig. 4b, Figs. S11 and S12 in Supporting information). After 12 h electroplating, the NH₃ yield of Pt-W/Mo@rGO-12 decreased at −0.3 V vs. RHE (Fig. 4b) compared to that of Pt-W/Mo@rGO-6. With increasing the electrolysis potential from −0.1 V to −0.3 V, the current density and the yield of NH₃ increased significantly, however the NH₃ yield decreased significantly at −0.4 V due to the competitive hydrogen evolution reaction in the aqueous solution.

  Figure 4

  

  Figure 4.  (a) LSV curves of Pt-W/Mo@rGO-X (X = 2, 6, 12) with different electroplating time. (b) Corresponding NH₃ yields for W/Mo@rGO, Pt-W/Mo@r GO-6 and Pt-W/Mo@rGO-12. (c) The NH₃ yields of Pt-W/Mo@rGO-6, Pt-W@rGO-6 and Pt-Mo@rGO-6 under different electrolysis potentials. (d) Recycle experiments with Pt-W/Mo@rGO-6 as the catalyst at −0.30 V vs. RHE.

  DownLoad: Full-Size Img PowerPoint

  Further, LSV curves of Pt-W/Mo@rGO-6, Pt-W@rGO-6 and Pt-Mo@rGO-6 were investigated to understand the synergy effect of Mo and W in depth and attest the superiority of Pt-W/Mo@rGO for promoting the NRR. As shown in Fig. S13 (Supporting information), the current density of Pt-W/Mo@rGO-6 from 0 to −0.7 V is much higher than that of Pt-W@rGO-6 and Pt-Mo@rGO-6. Notably, the NH₃ yield of Pt-W/Mo@rGO-6 was 7 and 3 times higher than that of Pt-W@rGO-6 and Pt-Mo@rGO-6, respectively. Additionally, the performance of Pt-Mo@rGO-6 was more than two times higher than that of the W-catalyst Pt-W@rGO-6. These results indicate the important role of Mo element in NRR and the fatal influence of Mo doping in improving the NRR performance (Fig. 4c). The outstanding performance of Pt-W/Mo@rGO is superior to that of most the state-of-the-art W/Mo-based catalysts (Table S1 in Supporting information).

  Meanwhile, hydrazine as the possible by-product was detected by Watt and Chrisp method [57]. As shown in Fig. S14 (Supporting information), no obvious hydrazine can be detected in the NRR system, indicating a high selectivity of Pt-W/Mo@rGO-6 for the synthesis ammonia from N₂. After 24 h electrolysis, negligible attenuation of current density raised during NRR (Fig. S15 in Supporting information), and the NH₃ yield tended to be stable after five cycles of NRR at −0.3 V vs. RHE (Fig. 4d). The PXRD pattern of Pt-W/Mo@rGO-6 exhibits neglect signal change after five electrocatalytic cycles (Fig. S16 in Supporting information). All these results imply excellent stability of Pt-W/Mo@rGO-6 for electrocatalytic NRR. Additionally, a series of control experiments were carried out to confirm the reliable NRR detection. Firstly, no NH₃ can be observed in Ar-saturated solution rather than N₂-saturated electrolyte (Fig. S17a in Supporting information). Then, N₂ was used as a feedstock gas but under an open-circuit potential condition, where no NH₃ can be obtained (Fig. S17b in Supporting information). During the ¹⁵N₂-labeling experiments, ¹H NMR spectrum shows the doublet coupling signals of ¹⁵NH₄⁺ (Fig. S18 in Supporting information). However, only triplet coupling signal for ¹⁴NH₄⁺ can be detected with ¹⁴N₂ as feeding gas. All those results confirm N₂ as the N source of NH₃ rather than any nitrogenous species in catalyst or pollutants.

  In conclusion, we have developed a highly efficient electrocatalyst Pt-W/Mo@rGO-6 with PW₆Mo₆ as the W/Mo precursor, which can promote NRR under mild conditions. In Pt-W/Mo@rGO-6, the synergetic effect of Mo and W species plays an important role on N₂ activation and dissociation for high efficiency NRR. Meanwhile, trace amount of electroplating Pt can further improve the performance of electrocatalytic NRR. The resulting Pt-W/Mo@rGO-6 catalyst shows a high NH₃ yield of 79.2 µg h⁻¹ mg_cat⁻¹ at a low potential of −0.3 V vs. RHE, which is superior to that of most the state-of-the-art W/Mo-based catalysts. This finding opens a new avenue to construct highly efficient NRR electrocatalyst by employing mixed-metal POMs as the precursors.

  Declaration of competing interest

  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.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 92161103, U21A20286, 22071180), Natural Science Foundation of Tianjin City of China (No. 18JCJQJC47700).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.03.060.

  
  
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  Figure 1  Schematic synthesis of Pt-W/Mo@rGO-X (X means the electroplating time).

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  Figure 2  (a) PXRD patterns of W/Mo@rGO and the simulated MoO₂ and WO₂. XPS spectra of (b) W/Mo@rGO, (c) W 4f, (d) Mo 3d and (e) O 1s. (f) AFM image of Pt-W/Mo@rGO-6.

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  Figure 3  (a) TEM, (b) HRTEM, (c) HADDF-TEM and (d-i) EDX elemental mapping images of Pt-W/Mo@rGO-6.

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  Figure 4  (a) LSV curves of Pt-W/Mo@rGO-X (X = 2, 6, 12) with different electroplating time. (b) Corresponding NH₃ yields for W/Mo@rGO, Pt-W/Mo@r GO-6 and Pt-W/Mo@rGO-12. (c) The NH₃ yields of Pt-W/Mo@rGO-6, Pt-W@rGO-6 and Pt-Mo@rGO-6 under different electrolysis potentials. (d) Recycle experiments with Pt-W/Mo@rGO-6 as the catalyst at −0.30 V vs. RHE.

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