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• H₂, with its high energy density (142,351 kJ/kg) and environmentally friendly nature, is considered the most promising alternative energy for resolving the environmental crisis [1,2]. Taking into account sustainable development, electrocatalytic water splitting has been recognized as a green and efficient method for H₂ production [3-6]. The alkaline hydrogen evolution reaction (HER) has gained recognition over the acidic HER due to its greater stability and cost-effectiveness, making it more suitable for large-scale applications [7].

  Platinum (Pt) is well-known as one of the most effective noble metal catalysts for HER, primarily because of its moderate Pt-H bond [8,9]. However, Pt-based catalysts are expensive and impractical for large-scale applications. Recently, due to the excellent performance and significantly lower cost (approximately Pt~4%), researchers considered ruthenium (Ru) as a promising electrocatalyst for HER [10]. While excessively strong OH adsorption by Ru mono-atoms can lead to desorption difficulties and reduce the HER reaction rate [11]. Therefore, it is crucial to identify a suitable carrier with a strong water dissociation capacity to enhance the HER catalytic activity [12-14].

  Transition metal nitrides (TMNs) are promising materials that can sustain HER activity at low noble metal loadings [15]. Studies have shown that the interaction between nitrogen (N) elements in TMNs with metals affects their electronic structures. The d orbitals of metal atoms will extend after being hybridized with the s and p orbitals of nitrogen atoms. This process gives them d-band characteristics similar to noble metals [16,17]. Compared to other carriers, such as transition metal carbides (TMCs), TMNs offer additional benefits by desorbing with extra nitrogen atoms as N₂ during synthesis, leaving a clean TMN surface for direct interaction with supported metal [18]. However, for TMCs, excess carbon may accumulate on the surface, preventing direct contact with the active metal [19]. In a recent study, Turaczy's team experimentally determined the hydrogen binding energy (HBE) value of molybdenum nitride, demonstrating its potential as a support to reduce noble metal loadings in HER catalysts [18]. The introduction of nitrogen atoms can alter the charge density of Mo atoms, effectively adjusting the strength of metal (Mo-H) bonds and optimizing electrochemical activity [20-22].

  However, current synthesis methods of molybdenum nitrides under N₂ or NH₃ atmosphere typically involve high pressure and toxic gases (Table S1 in Supporting information) [23,24]. These methods often result in low yield and incomplete nitridation [25]. Besides, prolonged high-temperature reactions can lead to severe aggregation of Ru species [26,27]. Therefore, the rapid and safe synthesis of molybdenum nitride carriers with enhanced exposure and dispersion of Ru active sites remains a major challenge. High temperature shock (HTS) provides a simple and efficient strategy through rapid heating. Lattice defects generated by extremely rapid temperature changes can enhance the electronic polarization of elemental surfaces [28,29]. With this method, Hu's team successfully synthesized high-entropy alloys, which effectively addressed the aggregation of active sites and greatly simplified the synthesis path [30]. Similarly, niobium-based oxides have recently been synthesized through ultrafast carbon thermal shock (CTS) by Zhao et al., avoiding the need for time-consuming and energy-intensive annealing processes [31].

  Based on the above considerations, we proposed an ultrafast (1.67 s) method to prepare Mo₂N and simultaneously evenly disperse ultrafine Ru sub-nanoparticles on the Mo₂N carrier. Under vacuum conditions, precursor powder was completely nitrided to Mo₂N by the rapid heating and cooling effects of HTS, and the interaction between Ru and Mo₂N effectively regulated the charge redistribution. The synthesized Ru/Mo₂N catalyst shows excellent HER performance under alkaline conditions (1 mol/L KOH). It exhibited a low overpotential of 66 mV vs. RHE for 10 mA/cm²; it also maintains a significant advantage at an overpotential of 200 mV vs. RHE for 100 mA/cm², which is lower than commercial Pt/C (220 mV).

  The specific synthesis method for Ru/Mo₂N is provided in the supporting information. In the synthesis of precursors, melamine and cyanuric acid have the ability to undergo self-association through hydrogen bonding in solution, while Mo₇O₂₄⁶⁻ and Ru³⁺ can effectively anchor through the metal-N/O covalent bonding coordination effect [32,33]. After filtering and drying, the precursor powder was subjected to HTS for 1.67 s to form Ru/Mo₂N. In terms of structural characterization, we used X-ray diffraction (XRD) for compositional analysis of Ru/Mo₂N. In Fig. 1a, the diffraction peaks at 37.38°, 43.44°, 63.10°, and 75.95° are in good agreement with the characteristic crystal planes (111), (420), (311), and (200) of the cubic γ-Mo₂N (JCPDS card No. 25-1366). The significant bulge around 23° may be attributed to non-crystalline carbon. Meanwhile, Raman spectroscopy shows two distinct peaks of carbon: the D band (1342 cm⁻¹) and the G band (1586 cm⁻¹) (Fig. 1b). The value of I_D/I_G (1.16) further indicates the presence of non-crystalline carbon and numerous lattice defects in Ru/Mo₂N [34]. The morphology and structure of the prepared samples were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 1c, Ru/Mo₂N exhibits a curled nanosheet structure with open and continuous channels, and the specific surface area is 13.85 m²/g. These channels may be derived from decomposable melamine and cyanuric acid, which are expected to enhance the accessibility of active sites [35,36]. Moreover, TEM images reveal a relatively high and uniform dispersion of Ru/Mo₂N (Fig. 1d). Notably, the carbon cladding layers were visualized in the TEM image (Fig. 1e), corresponding to the previously mentioned amorphous carbon. These carbon layers provide structural support to the entire Ru/Mo₂N particle and accelerate charge transfer. Furthermore, the lattice spacing of the Ru/Mo₂N sample was calculated to be 0.240 nm, corresponding to the (111) crystal plane of γ-Mo₂N. The structure of Ru/Mo₂N and the highly homogeneous dispersion of Mo, C, N, and Ru were further demonstrated by high-resolution TEM (HR-TEM) and energy-dispersive X-ray spectroscopy (EDS) mapping (Figs. 1f–j). Additionally, there are no obvious aggregates of Ru or its oxides on the entire nanosheet structure, indicating that Ru is dispersed as ultrafine sub-nanoparticles on the Mo₂N matrix [2].

  Figure 1

  

  Figure 1.  Ru/Mo₂N: (a) XRD pattern. (b) Raman spectra. (c, d) SEM images. (e) TEM image. (f) HR-TEM image and (g-j) EDS mapping images.

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  In order to clarify the valence states of the chemical elements and their composition on the surface, X-ray photoelectron spectroscopy (XPS) was utilized. The XPS survey clearly shows signals of Mo, N, C, and Ru (Fig. 2a). The XPS spectra of Ru 3p (Fig. 2b) exhibits binding energies at 483.1 eV and 460.8 eV corresponding to the Ru 3p_1/2 and Ru 3p_3/2 peaks, respectively. Interestingly, the Ru 3p spectra of Ru/Mo₂N are negatively shifted compared to RuO₂ in other studies (486.3 eV and 464.1 eV), indicating an electron-enriched state in Ru species [37,38]. Fig. 2c shows N 1s XPS spectra in Ru/Mo₂N with two peaks located at 401.9 eV and 398.5 eV, corresponding to graphitic N and pyridinic N, respectively. The higher content of pyridinic N typically exhibits stronger alkalinity and nucleophilicity, making it capable of coordinating with metals. A typical N-Mo peak appears at 395.4 eV, indicating the presence of nitrides. The two peaks mentioned above collectively validate the existence of Mo₂N [39]. Fig. 2d displays the XPS spectrum of Mo 3d, showing three pairs of peaks corresponding to Mo²⁺, Mo⁴⁺, and Mo⁶⁺. The peaks at 227.5 eV and 230.7 eV are attributed to Mo²⁺, mainly derived from Mo₂N. The peaks at 228.3 eV and 233.1 eV correspond to Mo⁴⁺, while the peaks at 231.8 eV and 234.9 eV correspond to Mo⁶⁺. The abundance of high-valence molybdenum attributes to the surface oxidation of Mo [40].

  Figure 2

  

  Figure 2.  The XPS spectra of Ru/Mo₂N. (a) XPS survey spectra of Ru/Mo₂N. (b) Ru 3p. (c) N 1s. (d) Mo 3d.

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  The electrochemical HER performance of Ru/Mo₂N was evaluated in 1 mol/L KOH electrolyte using a standard three-electrode system. All potentials are calculated with an iR correction and reported at a reversible hydrogen electrode (RHE) potential of 10 mA/cm². For comparison, the prepared samples and commercial Pt/C electrocatalysts were all tested. The preparation method for Mo₂N is similar to that of Ru/Mo₂N, except that Ru is not added. Linear sweep voltammetry (LSV) curves and overpotential in Figs. 3a and b show that the required overpotentials at 10 mA/cm² (η₁₀) and 100 mA/cm² (η₁₀₀) for Ru/Mo₂N are 66 mV and 200 mV, respectively, which are significantly lower than those of the contrast samples, Mo₂N (η₁₀ = 222 mV) and commercial Pt/C (η₁₀ = 63 mV, η₁₀₀ = 220 mV). These results demonstrate that the inclusion of a trace amount of Ru significantly improved the electrocatalytic performance. We also studied the performance at different pH values (Fig. S1 in Supporting information). Moreover, the Tafel slope of Ru/Mo₂N in Fig. 3c is 57 mV/dec, which is lower than that of Mo₂N (80 mV/dec), suggesting faster kinetics on Ru/Mo₂N, believed to follow the Volmer-Heyrovsky mechanism [41,42]. In order to obtain a more precise study of Ru/Mo₂N, further calculations were performed on the mass activity (MA) and turnover frequency (TOF) related to the noble metals (Pt or Ru). MA represents the active species generated by the catalyst per unit mass, while TOF denotes the rate of reactant transformations per unit time in a catalyzed reaction. Impressively, the Ru/Mo₂N catalyst exhibits a significantly higher MA than commercial Pt/C (Fig. 3d), indicating a high utilization of noble metals. Additionally, it can be observed that at 50, 75, and 100 mV, the TOF of Ru/Mo₂N is also superior to Pt/C (Fig. 3e). Then, we assessed the electrocatalytic durability of the Ru/Mo₂N catalyst of η₁₀ (Fig. 3f). After a 50-h prolonged test, the decay of the potential of the Ru/Mo₂N catalyst can be neglected, demonstrating its superior stability during the HER catalytic process. A range of previously reported Mo-N-based catalyst performances are demonstrated in Fig. 3g.

  Figure 3

  

  Figure 3.  (a–c) LSV curves, overpotentials and Tafel plots of Mo₂N, Ru/Mo₂N and Pt/C in 1 mol/L KOH. (d, e) Comparison of MA and TOF between Ru/Mo₂N and Pt/C. (f) Chronopotentiometric (CP) curve recorded at a constant current density of −10 mA/cm². (g) The overpotential (η₁₀) of various Mo-N-based catalysts.

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  Furthermore, to validate the advantages of Mo₂N synthesized by HTS, Mo₂N-F and Ru/Mo₂N-F were prepared using the same raw materials but high-temperature tube furnace sintering method. Interestingly, the X-ray diffraction patterns of Mo₂N-F and Ru/Mo₂N-F (Fig. 4a), in addition to coinciding with some of the γ-Mo₂N peaks and also show good correspondence with the characteristic crystal planes (101), (110), (103), and (112) of hexagonal β-Mo₂C (JCPDS card No. 65–8766) at 2θ = 39.58°, 61.87°, 69.76°, and 75.19°, respectively. Meanwhile, the crystal plane spacing of Ru/Mo₂N-F was calculated to be 0.228 nm and 0.236 nm based on the TEM images, corresponding to the (101) and (002) crystal faces of β-Mo₂C, respectively (Fig. S2 in Supporting information). The above results indicate that incomplete nitridation exists in Mo₂N-F and Ru/Mo₂N-F synthesized by high-temperature tube furnace sintering. EDS confirmed the uniform distribution of Mo, C, N, and Ru elements throughout the Ru/Mo₂N-F material (Figs. S3b-e in Supporting information). As for electrochemical HER performance, enormous distinctions were observed, LSV revealed that the overpotential (η₁₀) values of Mo₂N and Ru/Mo₂N (222 mV, 66 mV) were significantly lower than those of Mo₂N-F and Ru/Mo₂N-F (287 mV, 148 mV), as illustrated in Fig. 4b. To obtain a more precise investigation of these distinctions, XPS was employed (Fig. 4c). Compared with Ru/Mo₂N-F, the Ru 3p peak of Ru/Mo₂N shows a slight negative shift, implying a higher degree of electron transfer after HTS [43]. Meanwhile, the N 1s spectra of Ru/Mo₂N shows an obvious positive shift relative to that of Ru/Mo₂N-F (Fig. S4 in Supporting information). The above results suggest that there is an extensive interaction between Mo₂N and Ru species under HTS. To further compare the intrinsic HER activity of these samples, the electrochemical double-layer capacitance (C_dl) of the catalysts was determined by cyclic voltammetry (CV) as a function of scan rate (20–100 mV/s) (Fig. 4d) [44-46]. It is worth noting that the C_dl value of Ru/Mo₂N was 6.82 mF/cm², which was significantly higher than the C_dl value of Ru/Mo₂N-F (1.39 mF/cm²). This also demonstrates that Ru/Mo₂N prepared by HTS disperses Ru more evenly, which possesses more catalytically active sites and exhibits higher catalytic activity.

  Figure 4

  

  Figure 4.  (a) XRD patterns of Ru/Mo₂N and Ru/Mo₂N-F. (b) LSV curves of various samples in 1 mol/L KOH. (c) The XPS spectra of Ru 3p (Ru/Mo₂N and Ru/Mo₂N-F). (d) The electrochemical C_dl of Ru/Mo₂N and Ru/Mo₂N-F.

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  In conclusion, we successfully developed a highly effective electrocatalyst, Ru/Mo₂N, using a rapid heating method, resulting in homogeneously dispersed Ru on Mo₂N. As evidenced by experimental analysis, we propose that the superior performance is attributed to a higher degree of electron transfer exhibited by Ru during the HTS, and a synergistic effect between highly dispersed Ru sub-nanoparticle sites and Mo₂N, promoting abundant metal-support interactions in Ru/Mo₂N. With an overpotential (η₁₀) of 66 mV and outstanding catalytic activity, the prepared Ru/Mo₂N catalyst exhibits its remarkable performance. The catalyst also demonstrates excellent long-term electrochemical stability in 1 mo/L KOH solution. This work provides a rational pathway for the preparation of Mo₂N-based electrocatalysts.

  Declaration of competing interest

  No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my coauthors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

  CRediT authorship contribution statement

  Xinyu Hou: Data curation, Methodology, Validation, Writing – original draft. Xuelian Yu: Supervision, Conceptualization, Formal analysis, Methodology. Meng Liu: Conceptualization, Data curation, Supervision, Validation. Hengxing Peng: Conceptualization. Lijuan Wu: Conceptualization, Software. Libing Liao: Methodology, Supervision. Guocheng Lv: Methodology, Resources, Supervision.

  Acknowledgments

  This work was supported by the Beijing Natural Science Foundation (No. 2232061) and the National Natural Science Foundation of China (No. 42377227).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109845.

  
  
• 1. [1]

     J.Q. Chi, M. Yallg, Y.M. Chai, et al., J. Energy Chem. 48 (2022) 398–423.

  2. [2]

     X.L. Fan, C. Liu, M.Y. Wu, et al., Appl. Catal. B: Environ. 318 (2022) 121867.

  3. [3]

     Y.M. Liu, Chem. Bio. Engin. 3 (2007) 79–81.

  4. [4]

     B.H. Suryanto, Y. Wang, R.K. Hocking, et al., Nat. Commun. 10 (2019) 5599. doi: 10.1038/s41467-019-13415-8

  5. [5]

     S.D. Lu, B.Y. Lu, G.C. Tan, et al., Biosens. Bioelectron. 167 (2020) 112491.

  6. [6]

     L.N. Chen, S.H. Wang, P.Y. Zhang, et al., Nano Energy 88 (2021) 106211. doi: 10.1016/j.nanoen.2021.106211

  7. [7]

     H.Q. Fu, M. Zhou, P.F. Liu, et al., J. Am. Chem. Soc. 144 (2022) 6028–6039. doi: 10.1021/jacs.2c01094

  8. [8]

     J.Y. Yu, A.Z. Wang, W.Q. Yu, et al., Appl. Catal. B: Environ. 277 (2020) 119236.

  9. [9]

     D.Y. Ren, G.W. Wang, L.Y. Li, et al., Chem. Eng. J. 454 (2023) 140557.

  10. [10]

      Y.N. Zhou, F.L. Wang, J. Nan, et al., Appl. Catal. B: Environ. 304 (2022) 120917.

  11. [11]

      J. Zhang, G. Chen, Q. Liu, et al., Angew. Chem. Int. Ed. 61 (2022) e20220948.

  12. [12]

      Z.X. Yang, Z.Z. Wang, J.T. Wan, et al., Environ. Sci. Technol. 56 (2022) 18008–18017. doi: 10.1021/acs.est.2c06571

  13. [13]

      C. Li, H. Jang, M.G. Kim, et al., Appl. Catal. B: Environ. 307 (2022) 121204. doi: 10.1016/j.apcatb.2022.121204

  14. [14]

      H. Song, M. Wu, Z. Tang, et al., Angew. Chem. Int. Ed. 60 (2021) 7234–7244. doi: 10.1002/anie.202017102

  15. [15]

      X. Shi, A.P. Wu, H.J. Yan, et al., J. Mater. Chem. A 6 (2018) 20100–20109. doi: 10.1039/c8ta07906d

  16. [16]

      C.G. Morales-Guio, L.A. Stern, X. Hu, Chem. Soc. Rev. 43 (2014) 6555–6569. doi: 10.1039/c3cs60468c.

  17. [17]

      X. Wang, J. He, B. Yu, et al., Appl. Catal. B: Environ. 258 (2019) 117996.

  18. [18]

      K.K. Turaczy, W.J. Liao, et al., ACS Catal. 13 (2023) 14268–14276. doi: 10.1021/acscatal.3c04063

  19. [19]

      Y.C. Kimmel, D.V. Esposito, R.W. Birkmire, et al., Int. J. Hydrogen Energy 37 (2012) 3019–3024.

  20. [20]

      Y.Y. Chen, Y. Zhang, J. W, et al., ACS Nano 10 (2016) 8851–8860. doi: 10.1021/acsnano.6b04725

  21. [21]

      D.J. Ham, J.S. Lee, Energies 2 (2009) 873–899. doi: 10.3390/en20400873

  22. [22]

      L. Zhe, M. Tahir, X.W. Lang, et al., J. Mater. Chem. A 5 (2017) 20932. doi: 10.1039/C7TA06981B

  23. [23]

      K. Ojha, S. Saha, S. Banerjee, A.K. Ganguli, et al., ACS Appl. Mater. Interfaces 9 (2017) 19455–19461. doi: 10.1021/acsami.6b10717

  24. [24]

      L.M. Dai, F.L. Yao, L. Yu, et al., Adv. Energy Mater. 12 (2022) 2200974.

  25. [25]

      J. Li, Y.N. Zhou, W.J. Tang, et al., Appl. Catal. B: Environ. 285 (2021) 119861.

  26. [26]

      Z. Wei, P. Grange, B. Delmon, Appl. Surf. Sci. 135 (1998) 107–114. doi: 10.3724/j.issn.1000-0518.1998.4.107

  27. [27]

      W.F. Chen, K. Sasaki, C. Ma, et al., Angew. Chem. Int. Ed. 51 (2012) 6131–6135. doi: 10.1002/anie.201200699

  28. [28]

      Z.X. Yang, Y. Li, X.T. Wang, et al., J. Environ. Sci. 142 (2024) 204–214. doi: 10.3390/agronomy14010204

  29. [29]

      Z. Zhao, J. Sun, X. Li, et al., Appl. Catal. B 340 (2024) 123277–123293. doi: 10.1016/j.apcatb.2023.123277

  30. [30]

      Y. Yao, Z. Huang, P. Xie, et al., Science 359 (2018) 1489–1494. doi: 10.1126/science.aan5412

  31. [31]

      Q.L. Wu, Y.H. Kang, G.H. Chen, et al., Adv. Funct. Mater. (2024) 2315248.

  32. [32]

      F. Chen, X.L. Wu, C. Shi, et al., Adv. Funct. Mater. 31 (2021) 2007877. doi: 10.1002/adfm.202007877

  33. [33]

      B.X. Zhou, S.S. Ding, K.X. Yang, et al., Adv. Funct. Mater. 31 (2020) 2009230. doi: 10.1002/adfm.202009230

  34. [34]

      J. Jia, T.L. Xiong, L.L. Zhao, et al., ACS Nano 11 (2017) 12509–12518. doi: 10.1021/acsnano.7b06607

  35. [35]

      C. Tang, H. Zhang, K. Xu, et al., J. Mater. Chem. A 7 (2019) 18030–18038. doi: 10.1039/c9ta04374h

  36. [36]

      L.N. Zhang, Z.L. Lang, Y.H. Wang, et al., Science 12 (2019) 2569–2580. doi: 10.1039/c9ee01647c

  37. [37]

      P. Li, M. Wang, X.L. Duan, et al., Nat. Commun. 10 (2019) 1711. doi: 10.1007/s00192-018-3796-y

  38. [38]

      Y. Hu, G. Luo, L. Wang, et al., Adv. Energy Mater. 11 (2021) 2002816. doi: 10.1002/aenm.202002816

  39. [39]

      A.C. Mtukula, X.J. Boand, L.P. Guo, J. Alloys Compd. 692 (2017) 614–621. doi: 10.1016/j.jallcom.2016.09.079

  40. [40]

      Y.P. Zhu, G. Chen, X.M. Xu, et al., ACS Catal. 7 (2017) 3540–3547. doi: 10.1021/acscatal.7b00120

  41. [41]

      M. Zeng, Y. Li, Mater. Chem. A 3 (2015) 14942–14962. doi: 10.1039/C5TA02974K

  42. [42]

      M. Yao, H. Hu, B. Sun, et al., Small 15 (2019) 1905201. doi: 10.1002/smll.201905201

  43. [43]

      T.T. Chao, W.B. Xie, Y.M. Hu, et al., Energy Environ. Sci. 17 (2024) 1397–1406. doi: 10.1039/d3ee02760k

  44. [44]

      J.S. Li, Y. Wang, C.H. Liu, et al., Nat. Commun. 7 (2016) 11204. doi: 10.1038/ncomms11204

  45. [45]

      K. Qu, Y. Zheng, X. Zhang, et al., ACS Nano 11 (2017) 7293–7300. doi: 10.1021/acsnano.7b03290

  46. [46]

      C. Lu, D. Tranca, J. Zhang, et al., ACS Nano 11 (2017) 3933–3942. doi: 10.1021/acsnano.7b00365

• 

  Figure 1  Ru/Mo₂N: (a) XRD pattern. (b) Raman spectra. (c, d) SEM images. (e) TEM image. (f) HR-TEM image and (g-j) EDS mapping images.

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  Figure 2  The XPS spectra of Ru/Mo₂N. (a) XPS survey spectra of Ru/Mo₂N. (b) Ru 3p. (c) N 1s. (d) Mo 3d.

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  Figure 3  (a–c) LSV curves, overpotentials and Tafel plots of Mo₂N, Ru/Mo₂N and Pt/C in 1 mol/L KOH. (d, e) Comparison of MA and TOF between Ru/Mo₂N and Pt/C. (f) Chronopotentiometric (CP) curve recorded at a constant current density of −10 mA/cm². (g) The overpotential (η₁₀) of various Mo-N-based catalysts.

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  Figure 4  (a) XRD patterns of Ru/Mo₂N and Ru/Mo₂N-F. (b) LSV curves of various samples in 1 mol/L KOH. (c) The XPS spectra of Ru 3p (Ru/Mo₂N and Ru/Mo₂N-F). (d) The electrochemical C_dl of Ru/Mo₂N and Ru/Mo₂N-F.

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