English

• INTRODUCTION

  The excessive exploitation and heavy consumption of fossil fuels result in increasingly serious problems such as environmental pollution and resource exhaustion, which has aroused wide concern to transit the traditional energy infrastructure by seeking clean renewable sources to address the above dilemma.^[1] Hydrogen has been widely regarded an ideal energy carrier with outstanding energy conversion efficiency and zero carbon dioxide (CO₂)-emission. Water electrolysis technology is known as one of the most promising pathways for producing hydrogen due to its high efficiency, strong reproducibility and the ability to produce high-purity hydrogen on a large scale.^[2] Noble metal platinum (Pt)-based materials have been taken for a long time as the most advanced electrocatalysts for HER.^[3-5] However, their high cost, poor stabi-lity and scarcity are the main drawbacks that limit the industrial application.^{[6, 7]} Hence excavating alternatives with abundant resources, low cost and high catalytic efficiency has been put on the agenda.^[8-10]

  Transition metal nitrides (TMNs) have attracted extensive attention in recent years due to their high catalytic activity and glorious electrical conductivity resulting from their unique d-band electronic structure.^{[11, 12]} Specifically, the integration of nitrogen atom into the parent metal interstitial sites leads to the contraction of metal d-band, causing a great density of states (DOS) near the Fermi level.^[12] Such DOS redistribution in TMNs leads to Pt-like electronic structure, thus facilitating HER process by modulating the adsorption of hydrogen.^[13] However, single-phase TMNs usually display undesired activity and poor stability under strong acid or alkaline chemical conditions.^[11] Various strategies (e.g., designing bimetallic or polymetallic nitride, creating heterostructure and boundary) have been exploited to further modify the activity of TMNs.^[14-16] For example, Ma et al. developed an interfacial Co/CoMoN heterostructure supported on nickel foam (NF) (Co/CoMoN/NF).^[17] The heterostructure between low valence state Co metal and CoMoN substrate leads to the enhanced HER activity. Ma et al. reported a high-quality molybdenum nitride/graphene (MoN/G) heterostructure as a pH-universal HER electrocatalyst, in which graphene acts as a Mo diffusion barrier for the growth of 2D thin MoN crystals and facilitates the charge transfer for higher efficiency and stability.^[18] Chen group synthesized an electrocatalyst consisting of Ni nanoparticles embedded metallic MoN microrods cultured on roughened nickel sheet (Ni/MoN/rNS), and the constructed Ni/MoN interface contributes to the high intrinsic catalytic activity toward overall water splitting.^[19] Chen et al. designed a N-doped carbon-coated Ni₃Mo₃N micro-rods anchored on NF (NC/Ni₃Mo₃N/NF) using a hydrothermal reaction and subsequent nitriding process.^[20] The porous NC/Ni₃Mo₃N/NF microrods display efficient catalytic activity for HER due to the synergistic effect between the catalyst and the N dopant. Despite the remarkable pioneering achievements, great efforts are still needed to develop a facile strategy to produce TMN-based hetero-geneous electrocatalyst for HER in alkaline medium.^[21-24]

  In this work, we report an efficient and durable Co/MoN hetero-geneous domains/nitrogen-doped carbon (Co/MoN/NC) HER electrocatalyst by a facile melamine-assisted nitriding-carbonization strategy. Thermal treatment leads to the generation of Co/MoN heterogeneous domains, and the electronic interaction at the heterogeneous interface optimizes the adsorption of hydrogen. Meanwhile, the high conductive MoN and NC induce increased electron transfer rate, thus resulting in the promotion of HER performance. As a result, Co/MoN/NC exhibits a low overpotential of 29 mV to reach the current density of 10 mA cm^-2 in alkaline solution, as well as excellent durability at 20 mA cm^-2 for 90 h. The melamine-assisted nitriding-carbonization treatment in this work provides a new strategy for designing highly efficient TMNs-based electrocatalysts.

  RESULTS AND DISCUSSION

  The Co/MoN/NC was prepared by nitriding-carbonization treatment of CoMoO₄ precursors grown on NF via a hydrothermal process. The digital photos of samples under different synthetic stages are shown in Figure S1. Obviously, the scanning electron microscopy (SEM) image in Figure 1a that the CoMoO₄ nano-sheet arrays are evenly distributed on the NF substrate. After the following thermal annealing process with melamine, the porous Co/MoN/NC nanosheets were successfully obtained. As shown in Figure 1b, the width of the nanosheets is about 700-900 nm and the surface seems to be much rougher. In the transmission electron microscopy (TEM) image (Figure 1c), the melamine-assisted thermal treatment ensured that plenty of nanoparticles were formed on the rough nanosheet, thus increasing the specific surface area and exposing more active sites. X-ray diffraction (XRD) characterization was used to analyze the phase constitutions. The peaks situated at 37.1, 43.1 and 62.7° are in accordance with those of MoN (JCPDS No. 04-007-6077). Since the peaks of Co (JCPDS No. 89-4307) and Ni (JCPDS No. 87-0712) always coincide, we cannot distinguish their affiliation at 44.6, 51.9 and 76.4° from XRD data only. In the high-resolution TEM (HRTEM) image (Figure 1e), the typical lattice spacing of 0.20, 0.14 and 0.24 nm is in consistent with Co (111), MoN (220) and MoN (111) planes, respectively. The interface between Co and MoN marked with red dashed lines can be clearly found in the HRTEM image. The diffraction rings in the selected area electron diffraction (SAED) figure (Figure 1f) corresponds to (111), (220) and (200) planes of Co and (111) and (220) planes of MoN, which is in agreement with HRTEM results. The energy-dispersive X-ray spectroscopy (EDX) images show that the Co/MoN/NC nano-sheets are composed of Co, Mo, C and N elements (Figure 1g-k). It can be preliminarily concluded from the combination of XRD, HRTEM and SAED data that the as-obtained electrocatalyst is Co/MoN/NC. Especially, the agglomerated Co, Mo and N elements in EDX further demonstrate the creation of Co/MoN heterogeneous domains.^[25] This is also in accordance with the previous reports that the NC layer could affect the metal agglomeration process during thermal treatment, which contributes to the formation of numerous Co/MoN heterostructures.^{[26, 27]}

  Figure 1

  

  Figure 1.  SEM images of (a) CoMoO₄ and (b) Co/MoN/NC. (c) TEM image, (d) XRD patterns, (e) HRTEM image and (f) SEAD of Co/MoN/NC. (g-k) Elemental mapping images of Co/MoN/NC.

  DownLoad: Full-Size Img PowerPoint

  X-ray photoelectron spectroscopy (XPS) tests were performed to further investigate the compositions and chemical states of different elements in Co/MoN/NC. As shown in Figure S2, the dis-tinctive signals corresponding to Ni, Co, O, N, C and Mo can be found in the XPS survey spectrum. The characteristic peaks located at 778.4, 793.4 eV and 781.1, 797.0 eV can be ascribed to Co⁰ and Co²⁺, respectively (Figure 2a). The appearance of Co²⁺ is probably due to the surface oxidation in air. Figure 2b shows the high-resolution spectra of Mo 3d, which could be deconvolved into six subpeaks. The peaks at 229.5 and 232.6 eV can be assigned to Mo³⁺. The other peaks can be attributed to Mo⁴⁺ (230.4 and 233.7 eV) and Mo⁶⁺ (232.3 and 235.5 eV), respectively, which are probably induced by the surface oxidation of MoN. The formation of NC layer can be supported by the appearance of C-N/C=N bond at 285.4 eV (Figure 2c). In the N 1s XPS spectra, the specific peaks located at 395.8, 397.6, and 399.5 eV can be allocated to Mo 3p, metal-nitride (M-N) and pyrrolic-N (Figure 2d). The generation of M-N configurations can lead to the contraction of Mo d-band and effectively tune the DOS of the parent metal Mo towards Fermi level, optimizing the adsorption energy of hydrogen to facilitate HER process.^{[28, 29]}

  Figure 2

  

  Figure 2.  XPS survey spectra of Co/MoN/NC in regions of (a) Co 2p, (b) Mo 3d, (c) C 1s and (d) N 1 s and Mo 3p.

  DownLoad: Full-Size Img PowerPoint

  The HER performance of Co/MoN/NC and the corresponding control samples were characterized in 1.0 M KOH solution by a typical three-electrode configuration. As can be observed from linear sweep voltammetry (LSV) curves (Figure 3a), Co/MoN/NC shows a low overpotential of 29 mV at the current density of 10 mA cm^-2, which is comparable to Pt/C/NF (η₁₀ = 17 mV) and much lower than those of MoN/NC (η₁₀ = 91 mV), CoN/NC (η₁₀ = 150 mV), CoMoO₄ (η₁₀ = 216 mV) and NF (η₁₀ = 249 mV). The lower overpotential indicates better HER catalytic activity, which can be attributed to the abundant Co/MoN heterostructure domains.^[28-32] The HER performance of Co/MoN/NC is comparable to numerous recently reported HER electrocatalysts (Table S1). The electron transfer between Co and MoN at the Co/MoN interface could efficiently tune the oxidation state of Mo atom, accelerate the water dissociation in the alkaline media, and optimize the adsorption energy of H*, thereby improving the capability of catalyzing hydrogen evolution.^[25] Tafel slope was calculated to further display the superior reaction kinetics of Co/MoN/NC (Figure 3b). On one hand, the Tafel slope of Co/MoN/NC is 89.28 mV dec^-1, which is prone to the Volmer-Heyrovsky reaction mechanism during HER process. On the other hand, the Tafel slope of Co/MoN/NC is significantly lower than those of MoN/NC (138.95 mV dec^-1), CoN/NC (201.73 mV dec^-1), CoMoO₄ (169.34 mV dec^-1) and NF (108.71 mV dec^-1), indicating the faster reaction kinetics of Co/MoN/NC.^[34] Furthermore, electrochemical impedance spectroscopy (EIS) was conducted and the resulting Nyquist plots are shown in Figure 3c. Calculated from the equivalent circuit mode, the charge transfer resistances (R_ct) increasing order of various samples is as follows: Co/MoN/NC (R_ct = 6.4 Ω) < MoN/C (R_ct = 20.4 Ω) < CoN/NC (R_ct = 52.63 Ω) < CoMoO₄ (R_ct = 103.3 Ω) < NF (R_ct = 162.7 Ω). The lower R_ct indicates faster charge transfer rate and HER reaction kinetics, ^{[33, 34]} which can be attributed to the high electric conductive components of MoN and NC.^{[24, 34]}

  Figure 3

  

  Figure 3.  (a) HER polarization curves of Co/MoN/NC, CoMoO₄, MoN/NC, CoN/NC, Pt/C/NF and NF in 1.0 M KOH; (b) the corresponding Tafel plots, and (c) The Nyquist plots, and the inset is the equivalent circuit. (d) The capacitive currents as a function of scan rates. (e) HER performance comparison of different samples in terms of overpotentials at 10 mA cm^-2 and Tafel slopes.

  DownLoad: Full-Size Img PowerPoint

  The melamine-assisted thermal annealing temperature imposes great effect on the material composition, which is decisive to the HER performance.^[24] Therefore, the samples treated under 400 and 600 ℃ were obtained for comparison, which are named as CoMoN/NC-400 and CoMoN/NC-600, respectively. It can be found in the XRD patterns that the thermal treatment temperature has a tremendous influence on the nitriding degree (Figure S3). Specifically, CoMoN/NC-400 is composed of Mo₃N₂ and CoMoO₄, verifying the incomplete nitriding of the CoMoO₄ precursor. By contrast, only Mo₅N₆ can be detected in the XRD patterns of CoMoN/NC-600. Figure S4 compares the electrocatalytic performance of samples obtained at different annealing temperatures in terms of overpotential and Tafel slope, while Figure S5 compares the R_ct. It is clear to deduce that 500 ℃ would be the optimal nitriding temperature, the generated Co/MoN/NC under which behaves better HER performance than CoMoN/NC-400 and CoMoN/NC-600.

  The cyclic voltammogram (CV) tests at various scan rates in the non-faradaic region were conducted to measure the double layer capacitance (C_dl) of different samples (Figure 3d, Figure S6, 7 and S8), which is positively correlated with electrochemical surface area (ECSA).^{[1, 37]} The C_dl value of Co/MoN/NC is 10.63 mF cm^-2, which is much larger than those of MoN/NC (C_dl = 5.21 mF cm^-2), CoN/NC (C_dl = 5.34 mF cm^-2), CoMoO₄ (C_dl = 3.43 mF cm^-2) and NF (C_dl = 1.21 mF cm^-2). The calculated ECSAs of Co/MoN/NC, MoN/NC, CoN/NC, CoMoO₄ and NF are 66.22, 32.56, 33.38, 21.44 and 7.56, respectively, suggesting a larger surface area and more exposed active sites of Co/MoN/NC which may be derived from the abundant heterostructure domains as well as the porous nanosheet structure. It can be more intuitively summarized from the statistical data in Figure 3e that the Co/MoN/NC exhibits the lowest overpotential and Tafel slope among the MoN-based samples in this work.

  Stability is another important parameter to evaluate the performance of electrocatalysts.^[33] Firstly, the accelerated electrochemical aging test was carried out (1000 continuous CV cycles between -0.2146 and 0.1854 V vs. RHE with the scan rate of 100 mV s^-1). As shown in Figure 4a, there is no significant decay for the LSV curves before and after 1000 CV cycles. Secondly, Co/MoN/NC shows a stable trend for over 90 h with minor decline of 12 mV after the long-term chronoamperometry test, indicating an excellent HER durability (Figure 4b). Post characterizations were further accomplished to prove the structural stability of Co/MoN/NC. As can be seen from the SEM image (Figure S9), the Co/MoN/NC still retains clear and complete geometric nano-sheet structure after long-term HER test. Besides, the typical lattice spacing of Co and MoN is well reserved in the HRTEM characterization (Figure S10), further proving the excellent stability of the as-formed Co/MoN/NC electrocatalyst.

  Figure 4

  

  Figure 4.  (a) HER polarization curves of Co/MoN/NC before and after 1000 cycles of voltammetry scanning and (b) the stability test at the current density of -20 mA cm^-2.

  DownLoad: Full-Size Img PowerPoint

  CONCLUSION

  In summary, Co/MoN/NC was successfully designed and prepared by thermal annealing CoMoO₄ with melamine in this work. The as-obtained Co/MoN/NC exhibits excellent HER performance with low overpotential of 29 mV at 10 mA cm^-2, together with satisfactory durability with stable potential at 20 mA cm^-2 for 90 h. Such outstanding HER performance can be attributed to the generation of abundant Co/MoN heterostructure domains, which can optimize the adsorption of hydrogen toward HER. Additionally, the conductive MoN and NC are beneficial to enhance the charge transfer rate and thus improve reaction kinetics. The excellent HER catalytic performance of Co/MoN/NC combined with its facile synthesis process implies the promising prospects for industrial application.

  EXPERIMENTAL

  Preparation of CoMoO₄. NF was pretreated by ultrasonication in 3.0 M HCl solution, ethanol, and deionized water. 1.5 mmol cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O) and 1.5 mmol sodium molybdate dihydrate (NaMoO₄·2H₂O) were dissolved in 150 mL deionized water. After magnetic stirring for 30 min, the mixture solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave, and then the pretreated NF (2.5 × 3.5 cm) was immersed into the mixture solution. The autoclave was sealed and maintained at 140 ℃ for 10 h. After cooling down at room temperature, the obtained CoMoO₄ was taken out and cleaned with deionized water and ethanol and dried at 60 ℃ for 12 h.

  Preparation of Co/MoN/NC. The prepared CoMoO₄ was transferred into one porcelain boat and 200 mg melamine was placed in another porcelain boat. Both two were putted in tubular furnace and annealed in Ar atmosphere at 500 ℃ for 2 h (the melamine was purposely placed at the upstream). The Co/MoN/NC was obtained after natural cooling at room temperature.

  Preparation of CoMoN/NC-400 and CoMoN/NC-600. CoMoN/NC-400 and CoMoN/NC-600 were prepared using the same method of Co/MoN/NC except the annealing temperature was set as 400 and 600 ℃, respectively.

  Preparation of Pt/C Electrode. The Pt/C electrocatalyst ink was prepared by dispersing 5 mg commercial Pt/C (20 wt%) powder into the mixture containing 950 ul water-ethanol (water: ethanol = 1:1) and 50 ul Nafion (5 wt%). After a sonication for 20 minutes, the prepared Pt/C ink was directly deposited onto NF (2 mg cm^-2).

  Materials Characterization. Scanning electron microscopy (SEM) experiments were performed on a Zeiss Sigma 500 microscope. Transmission electron microscopy (TEM) images were acquired on an FEI Tecnai G2 F30 microscope. Powder X-ray diffraction (XRD) patterns were obtained on Rigaku Smartlab equipment. X-ray photoelectron spectroscopy (XPS) tests were performed on a Thermos Scientific spectrometer.

  Electrochemical Measurements. The electrochemical test was performed on a VSP-300 (BioLogic, France) electrochemical workstation. A three-electrode system in 1.0 M KOH was used, where the prepared NF-based electrode (0.5 × 0.5 cm) was used as the working electrode, Hg/HgO as the reference electrode, and graphite rod as the counter electrode for HER. The iR correction was applied to all the LSV curves, and all potentials were converted into the RHE (E_RHE = E_Hg/HgO + 0.098 V + 0.0592 pH). Electrochemical impedance spectroscopy (EIS) tests were recorded in the frequency range between 50 mHz and 100 kHz with an amplitude of 5 mV, and tested at the voltage of -0.040 V vs. RHE.

  
  ACKNOWLEDGEMENTS: This study was financially supported by the National Natural Science Foundation of China (No. 22075159), Taishan Scholar Program (No. tsqn202103058), and the Youth Innovation Team Project of Shandong Provincial Education Department (No. 2019KJC023). The authors declare no competing interests.
  COMPETING INTERESTS
  For submission: https://mc03.manuscriptcentral.com/cjsc
  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2021-0144
  ADDITIONAL INFORMATION
  
• 1. [1]

     Yu, Z. Y.; Duan, Y.; Feng, X. Y.; Yu, X.; Gao, M. R.; Yu, S. H. Clean and affordable hydrogen fuel from alkaline water splitting: past, recent progress, and future prospects. Adv. Mater. 2021, 33, e2007100. doi: 10.1002/adma.202007100

  2. [2]

     Yang, H.; Driess, M.; Menezes, P. W. Self-supported electrocatalysts for practical water electrolysis. Adv. Energy Mater. 2021, 11, 2102074. doi: 10.1002/aenm.202102074

  3. [3]

     Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects. Int. J. Hydrogen Energy 2002, 27, 991-1022.

  4. [4]

     Wu, Y. L.; Xie, N.; Li, X. F.; Fu, Z. M.; Wu, X. T.; Zhu, Q. L. MOF-derived hierarchical hollow NiRu-C nanohybrid for efficient hydrogen evolution reaction. Chin. J. Struct. Chem. 2021, 40, 1346-1356.

  5. [5]

     Zhang, Y. Y.; Zhang, N.; Peng, P.; Wang, R.; Jin, Y.; Lv, Y. K.; Wang, X.; Wei, W.; Zang, S. Q. Uniformly dispersed Ru nanoparticles constructed by in situ confined polymerization of ionic liquids for the electrocatalytic hydrogen evolution reaction. Small Methods 2021, 5, 2100505. doi: 10.1002/smtd.202100505

  6. [6]

     Tang, C.; Wang, H. F.; Zhang, Q. Multiscale principles to boost reactivity in gas-involving energy electrocatalysis. Acc. Chem. Res. 2018, 51, 881-889. doi: 10.1021/acs.accounts.7b00616

  7. [7]

     Shang, X.; Tang, J. H.; Dong, B.; Sun, Y. Recent advances of nonprecious and bifunctional electrocatalysts for overall water splitting. Sustain. Energy Fuels 2020, 4, 3211-3228. doi: 10.1039/D0SE00466A

  8. [8]

     Qin, R.; Wang, P.; Lin, C.; Cao, F.; Zhang, J.; Chen, L.; Mu, S. Transition metal nitrides: activity origin, synthesis and electrocatalytic applications. Acta Phys. Chim. Sin. 2020, 37, 2009099.

  9. [9]

     Yan, D. Q.; Zhang, L.; Chen, Z. P.; Xiao, W. P.; Yang, X. F. Nickel-based metal-organic framework-derived bifunctional electrocatalysts for hydrogen and oxygen evolution reactions. Acta Phys. Chim. Sin. 2021, 37, 2009054.

  10. [10]

      Wang, J.; Liao, T.; Wei, Z. Z.; Sun, J. T.; Guo, J. J.; Sun, Z. Q. Heteroatom-doping of non-noble metal-based catalysts for electrocatalytic hydrogen evolution: an electronic structure tuning strategy. Small Methods 2021, 5, 2000988. doi: 10.1002/smtd.202000988

  11. [11]

      Li, P. Y.; Hong, W. T.; Liu, W. Fabrication of large scale self-supported WC/Ni(OH)₂ electrode for high-current-density hydrogen evolution. Chin. J. Struct. Chem. 2021, 40, 1365-1371.

  12. [12]

      Theerthagiri, J.; Lee, S. J.; Murthy, A. P.; Madhavan, J.; Choi, M. Y. Fundamental aspects and recent advances in transition metal nitrides as electrocatalysts for hydrogen evolution reaction: a review. Curr. Opin. Solid State Mat. Sci. 2020, 24, 100805. doi: 10.1016/j.cossms.2020.100805

  13. [13]

      Chen, P.; Xu, K.; Fang, Z.; Tong, Y.; Wu, J.; Lu, X.; Peng, X.; Ding, H.; Wu, C.; Xie, Y. Metallic Co₄N porous nanowire arrays activated by surface oxidation as electrocatalysts for the oxygen evolution reaction. Angew. Chem. Int. Ed. 2015, 54, 14710-14714. doi: 10.1002/anie.201506480

  14. [14]

      Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060-2086.

  15. [15]

      Feng, J. X.; Xu, H.; Dong, Y. T.; Lu, X. F.; Tong, Y. X.; Li, G. R. Efficient hydrogen evolution electrocatalysis using cobalt nanotubes decorated with titanium dioxide nanodots. Angew. Chem. Int. Ed 2017, 56, 2960-2964.

  16. [16]

      Kasian, O.; Grote, J. P.; Geiger, S.; Cherevko, S.; Mayrhofer, K. J. J. The common intermediates of oxygen evolution and dissolution reactions during water electrolysis on iridium. Angew. Chem. Int. Ed. 2018, 57, 2488-2491.

  17. [17]

      Ma, H.; Chen, Z.; Wang, Z.; Singh, C. V.; Jiang, Q. Interface engineering of Co/CoMoN/NF heterostructures for high-performance electrochemical overall water splitting. Adv. Sci. 2022, 9, e2105313.

  18. [18]

      Ma, Y.; Lu, S.; Han, G.; Liu, Y.; Chen, Z. Chemical vapor deposition of two-dimensional molybdenum nitride/graphene van der Waals heterostructure with enhanced electrocatalytic hydrogen evolution performance. Appl. Catal. B-Environ. 2022, 589, 152934.

  19. [19]

      Chen, Y.; Wang, Y.; Yu, J.; Xiong, G.; Niu, H.; Li, Y.; Sun, D.; Zhang, X.; Liu, H.; Zhou, W. Underfocus laser induced Ni nanoparticles embedded metallic MoN microrods as patterned electrode for efficient overall water splitting. Adv. Sci. 2022, 9, e2105869.

  20. [20]

      Chen, Y.; Yu, J.; Jia, J.; Liu, F.; Zhang, Y.; Xiong, G.; Zhang, R.; Yang, R.; Sun, D.; Liu, H.; Zhou, W. Metallic Ni₃Mo₃N porous microrods with abundant catalytic sites as efficient electrocatalyst for large current density and superstability of hydrogen evolution reaction and water splitting. Appl. Catal. B-Environ. 2020, 272, 118956.

  21. [21]

      Wang, Y.; Sun, Y.; Yan, F.; Zhu, C.; Gao, P.; Zhang, X.; Chen, Y. Self-supported NiMo-based nanowire arrays as bifunctional electrocatalysts for full water splitting. J. Mater. Chem. A 2018, 6, 8479-8487.

  22. [22]

      Sun, H.; Yan, Z.; Liu, F.; Xu, W.; Cheng, F.; Chen, J. Self-supported transition-metal-based electrocatalysts for hydrogen and oxygen evolution. Adv. Mater. 2020, 32, e1806326.

  23. [23]

      Wu, A.; Xie, Y.; Ma, H.; Tian, C.; Gu, Y.; Yan, H.; Zhang, X.; Yang, G.; Fu, H. Integrating the active OER and HER components as the heterostructures for the efficient overall water splitting. Nano Energy 2018, 44, 353-363.

  24. [24]

      Yan, D.; Li, Y.; Huo, J.; Chen, R.; Dai, L.; Wang, S. Defect chemistry of nonprecious-metal electrocatalysts for oxygen reactions. Adv. Mater. 2017, 29, 1606459.

  25. [25]

      Sun, J.; Xu, W.; Lv, C.; Zhang, L.; Shakouri, M.; Peng, Y.; Wang, Q.; Yang, X.; Yuan, D.; Huang, M.; Hu, Y.; Yang, D.; Zhang, L. Co/MoN hetero-interface nanoflake array with enhanced water dissociation capability achieves the Pt-like hydrogen evolution catalytic performance. Appl. Catal. B-Environ. 2021, 286, 119882.

  26. [26]

      Du, Y.; Pan, G.; Wang, L.; Song, Y. Co_xNi_yP embedded in nitrogen-doped porous carbon on Ni foam for efficient hydrogen evolution. Appl. Surf. Sci. 2019, 469, 61-67.

  27. [27]

      Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. A metal-organic framework-derived bifunctional oxygen electrocatalyst. Nat. Energy 2016, 1, 15006.

  28. [28]

      Peng, X.; Pi, C.; Zhang, X.; Li, S.; Huo, K.; Chu, P. K. Recent progress of transition metal nitrides for efficient electrocatalytic water splitting. Sustain. Energy Fuels 2019, 3, 366-381.

  29. [29]

      Schwarz, K. Band structure and chemical bonding in transition metal carbides and nitrides. Crit. Rev. Solid State 1987, 13, 211-257.

  30. [30]

      Shah, S. A.; Shen, X.; Xie, M.; Zhu, G.; Ji, Z.; Zhou, H.; Xu, K.; Yue, X.; Yuan, A.; Zhu, J.; Chen, Y. Nickel@nitrogen-doped carbon@MoS₂ nanosheets: an efficient electrocatalyst for hydrogen evolution reaction. Small 2019, 15, e1804545.

  31. [31]

      Gu, Y.; Wu, A.; Jiao, Y.; Zheng, H.; Wang, X.; Xie, Y.; Wang, L.; Tian, C.; Fu, H. Two-dimensional porous molybdenum phosphide/nitride heterojunction nanosheets for pH-universal hydrogen evolution reaction. Angew. Chem. Int. Ed. 2021, 60, 6673-6681.

  32. [32]

      Lin, Y.; Sun, K.; Liu, S.; Chen, X.; Cheng, Y.; Cheong, W. C.; Chen, Z.; Zheng, L.; Zhang, J.; Li, X.; Pan, Y.; Chen, C. Construction of CoP/NiCoP nanotadpoles heterojunction interface for wide pH hydrogen evolution electrocatalysis and supercapacitor. Adv. Energy Mater. 2019, 9, 1901213.

  33. [33]

      Zhang, J.; Liu, Y.; Li, J.; Jin, X.; Li, Y.; Qian, Q.; Wang, Y.; El-Harairy, A.; Li, Z.; Zhu, Y.; Zhang, H.; Cheng, M.; Zeng, S.; Zhang, G. Vanadium substitution steering reaction kinetics acceleration for Ni₃N nanosheets endows exceptionally energy-saving hydrogen evolution coupled with hydra zine oxidation. ACS Appl. Mater. Interfaces 2021, 13, 3881-3890.

  34. [34]

      Zhang, L.; Cao, X.; Feng, C.; Zhang, W.; Wang, Z.; Feng, S.; Huang, Z.; Lu, X.; Dai, F. Interfacial Mo-N-C bond endowed hydrogen evolution reaction on MoSe₂@N-doped carbon hollow nanoflowers. Inorg. Chem. 2021, 60, 12377-12385.

  35. [35]

      Xing, Z.; Li, Q.; Wang, D.; Yang, X.; Sun, X. Self-supported nickel nitride as an efficient high-performance three-dimensional cathode for the alkaline hydrogen evolution reaction. Electrochim. Acta 2016, 191, 841-845.

  36. [36]

      Wu, Y.; Li, G. D.; Liu, Y.; Yang, L.; Lian, X.; Asefa, T.; Zou, X. Overall water splitting catalyzed efficiently by an ultrathin nanosheet-built, hollow Ni₃S₂-based electrocatalyst. Adv. Funct. Mater. 2016, 26, 4839-4847.

  37. [37]

      Jiang, W. J.; Tang, T.; Zhang, Y.; Hu, J. S. Synergistic modulation of non-precious-metal electrocatalysts for advanced water splitting. Acc. Chem. Res. 2020, 53, 1111-1123.

• 

  Figure 1  SEM images of (a) CoMoO₄ and (b) Co/MoN/NC. (c) TEM image, (d) XRD patterns, (e) HRTEM image and (f) SEAD of Co/MoN/NC. (g-k) Elemental mapping images of Co/MoN/NC.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  XPS survey spectra of Co/MoN/NC in regions of (a) Co 2p, (b) Mo 3d, (c) C 1s and (d) N 1 s and Mo 3p.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  (a) HER polarization curves of Co/MoN/NC, CoMoO₄, MoN/NC, CoN/NC, Pt/C/NF and NF in 1.0 M KOH; (b) the corresponding Tafel plots, and (c) The Nyquist plots, and the inset is the equivalent circuit. (d) The capacitive currents as a function of scan rates. (e) HER performance comparison of different samples in terms of overpotentials at 10 mA cm^-2 and Tafel slopes.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) HER polarization curves of Co/MoN/NC before and after 1000 cycles of voltammetry scanning and (b) the stability test at the current density of -20 mA cm^-2.

  下载: 全尺寸图片 幻灯片
•