Electrochemical hydrogen production has been recognized as a promising technique to alleviate energy crisis and environmental degradation.1–41. I. Roger, M. A. Shipman, and M. D. Symes, Nat. Rev. Chem. 1, 003 (2017). https://doi.org/10.1038/s41570-016-00032. F. Yu, H. Zhou, Y. Huang, J. Sun, F. Qin, J. Bao, W. A. Goddard, S. Chen, and Z. Ren, Nat. Commun. 9, 2551 (2018). https://doi.org/10.1038/s41467-018-04746-z3. K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, and K. Domen, Nature 440, 295 (2006). https://doi.org/10.1038/440295a4. Y. Z. Lu, Y. Y. Jiang, X. H. Gao, and W. Chen, Chem. Commun. 50, 8464 (2014). https://doi.org/10.1039/C4CC01841A Much effort has been focused on increasing the insufficient energy density and reducing the high overpotential, caused by the slow proton-coupled electron transfer process in cathodic hydrogen evolution reaction (HER).5–75. D. Gao, J. N. Guo, H. C. He, P. Xiao, and Y. H. Zhang, Chem. Eng. J. 430, 133110 (2022). https://doi.org/10.1016/j.cej.2021.1331106. R. Subbaraman, D. Tripkovic, D. Strmcnik, K. C. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic, and N. M. Markovic, Science 334, 1256 (2011). https://doi.org/10.1126/science.12119347. S. Shen, Z. Wang, Z. Lin, K. Song, Q. Zhang, F. Meng, L. Gu, and W. Zhong, Adv. Mater. 34, 2110631 (2022). https://doi.org/10.1002/adma.202110631 As reported, the appropriate water adsorption energy (ΔEad) and Gibbs free energy of hydrogen adsorption (ΔGH*) on the catalyst's surface played a key role in electrochemical catalytic efficiency.8,98. J. H. Chen, J. W. Liu, J. Q. Xie, H. Q. Ye, X. Z. Fu, R. Sun, and C. P. Wong, Nano Energy 56, 225 (2019). https://doi.org/10.1016/j.nanoen.2018.11.0519. Y. J. Chen, S. F. Ji, C. Chen, Q. Peng, D. S. Wang, and Y. D. Li, Joule 2, 1242 (2018). https://doi.org/10.1016/j.joule.2018.06.019 Nørskov et al. constructed a volcano of exchange currents and hydrogen adsorption energies, demonstrating that Pt possessed optimum ΔEad and ΔGH* and is the most excellent electrocatalyst for HER.1010. J. K. Nørskov, T. Bligaard, A. Logadottir, J. R. Kitchin, J. G. Chen, S. Pandelov, and U. Stimming, J. Electrochem. Soc. 152, J23 (2005). https://doi.org/10.1149/1.1856988 However, its scarcity and high-cost hindered the large-scale commercial applications in HER.11,1211. B. Liu, Y. F. Zhao, H. Q. Peng, Z. Y. Zhang, C. K. Sit, M. F. Yuen, T. R. Zhang, C. S. Lee, and W. J. Zhang, Adv. Mater. 29, 1606521 (2017). https://doi.org/10.1002/adma.20160652112. Y. Zheng, Y. Jiao, Y. Zhu, L. H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec, and S. Z. Qiao, Nat. Commun. 5, 3783 (2014). https://doi.org/10.1038/ncomms4783 Cobalt-based materials with fine HER intrinsic catalytic activity were anticipated to be a promising alternative, as a result of the unique 3d orbital electronic structure and suitable binding energies.1313. B. C. Liu, Y. Cheng, B. Cao, M. H. Hu, P. Jing, R. Gao, Y. P. Du, J. Zhang, and J. H. Liu, Appl. Catal., B 298, 120630 (2021). https://doi.org/10.1016/j.apcatb.2021.120630 Meanwhile, local electronic environments of cobalt-based materials, especially for cobalt–molybdenum oxides with the properties of nonpollution, high temperature resistance, and hydrophilicity, can be modulated by introducing molybdenum, accelerating catalytic activity.14–1814. L. Yuan, Y. Zhang, J. Chen, Y. Li, X. Ren, P. Zhang, L. Liu, J. Zhang, and L. Sun, Nanoscale 14, 2490 (2022). https://doi.org/10.1039/D1NR05941F15. R. Q. Yao, H. Shi, W. B. Wan, Z. Wen, X. Y. Lang, and Q. Jiang, Adv. 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Wang, ACS Catal. 8, 5062 (2018). https://doi.org/10.1021/acscatal.8b00949 According to Yu's report, HER performance of Co2Mo3O8 catalyst has been significantly improved because of the adjusted electronic states from synergistic effect between Co and Mo.2020. K. Zhang, C. L. Liu, N. Graham, G. Zhang, and W. Z. Yu, Nano Energy 87, 106217 (2021). https://doi.org/10.1016/j.nanoen.2021.106217 It is worth mentioning that further increase in intrinsic catalytic activities in cobalt–molybdenum oxides was limited by their poor conductivity, which cannot satisfy industrial applications.It is reported that the conductivity of the materials is closely related to the state of density (DOS) around the Fermi level, that is, the higher the density of electron cloud, the better the conductivity. Until now, several strategies have been proposed to modify the electron cloud distribution of the Fermi level. As in Chen's work, P-doped β-CoMoO4 exhibited more DOS near the Fermi level than β-CoMoO4, resulting in faster charge transfer and higher conductivity.2121. S. Li, N. Yang, L. Liao, Y. Luo, S. Wang, F. Cao, W. Zhou, D. Huang, and H. Chen, ACS Appl. Mater. Interfaces 10, 37038 (2018). https://doi.org/10.1021/acsami.8b13266 Wang et al. applied oxygen defect engineering to enhance the DOS around the Fermi level for CoMoO4 and then accelerate conductivity, which are responsible for stronger water adsorption (ΔEad) in HER and higher catalytic activity.2222. K. Chi, X. Tian, Q. J. Wang, Z. Y. Zhang, X. Y. Zhang, Y. Zhang, F. Jing, Q. Y. Lv, W. Yao, and F. Xiao, J. Catal. 381, 44 (2020). https://doi.org/10.1016/j.jcat.2019.10.025 Compared with CoMoO4 and Co2Mo3O8, DOS of CoMoO4-Co2Mo3O8 heterostructure shows more occupations around the Fermi level, confirming better conductivity and optimized Gibbs free energy of hydrogen adsorption (ΔGH*).2323. Z. Liu, C. H. Zhan, L. K. Peng, Y. Cao, Y. Chen, S. J. Ding, C. H. Xiao, X. Y. Lai, J. W. Li, S. R. Wei, J. Q. Wang, and J. C. Tu, J. Mater. Chem. A 7, 16761 (2019). https://doi.org/10.1039/C9TA04180J Obviously, water adsorption energy (ΔEad) and Gibbs free energy of hydrogen adsorption (ΔGH*) on cobalt–molybdenum oxides are also associated with the DOS around the Fermi level. The higher DOS near the Fermi level means faster electrons transport and more appropriate adsorption energies of molecules and radicals, which determine the kinetics and catalytic efficiency of hydrogen evolution reaction.2424. J. Wu, Q. Zhang, K. Shen, R. Zhao, W. D. Zhong, C. F. Yang, H. Xiang, X. K. Li, and N. J. Yang, Adv. Funct. Mater. 32, 2107802 (2022). https://doi.org/10.1002/adfm.202107802 On the other hand, with high conductivity and unique electronic structure from silver element, numerous studies have been devoted to improving intrinsic catalytic activity by silver nanoparticles loading and silver nanowires compositing, optimizing the DOS, ΔEad, ΔGH* and increasing electrochemical catalytic activity.25–2925. Z. Li, J. Y. Fu, Y. Feng, C. K. Dong, H. Liu, and X. W. Du, Nat. Catal. 2, 1107 (2019). https://doi.org/10.1038/s41929-019-0365-926. K. L. Zhou, C. H. Wang, Z. L. Wang, C. B. Han, Q. Q. Zhang, X. X. Ke, J. B. Liu, and H. Wang, Energy Environ. Sci. 13, 3082 (2020). https://doi.org/10.1039/D0EE01347A27. X. W. Liu, R. C. Wang, Y. He, Z. Y. Ni, N. Su, R. Guo, Y. Zhao, J. H. You, and T. F. Yi, Electrochim. Acta 317, 468 (2019). https://doi.org/10.1016/j.electacta.2019.06.02928. X. Zhao, H. Zhang, Y. Yan, J. Cao, X. Li, S. Zhou, Z. Peng, and J. Zeng, Angew. Chem., Int. Ed. 56, 328 (2017). https://doi.org/10.1002/anie.20160908029. X. Y. Zhang, B. Y. Guo, F. T. Li, B. Dong, J. Q. Zhang, X. Ma, J. Y. Xie, M. Yang, Y. M. Chai, and C. G. Liu, Int. J. Hydrogen Energy 44, 21683 (2019). https://doi.org/10.1016/j.ijhydene.2019.06.116 Xia et al. reported that the DOS near the Fermi level and conductivity of Co(OH)2/Ag/FeP hybrid catalysts were significantly improved, and an optimized overpotential was gained at 118 mV with a current density of 10 mA cm−2.3030. X. Ding, Y. Xia, Q. Li, S. Dong, X. Jiao, and D. Chen, ACS Appl. Mater. Interfaces 11, 7936 (2019). https://doi.org/10.1021/acsami.8b19623 Hu et al. demonstrated that introduction of Ag not only improved conductivity of Co(OH)2 by adjusting the DOS around the Fermi level but also optimized the binding energies with intermediates in reaction.3131. Z. Zhang, X. Li, C. Zhong, N. Zhao, Y. Deng, X. Han, and W. Hu, Angew. Chem., Int. Ed. 59, 7245 (2020). https://doi.org/10.1002/anie.202001703 Gong et al. found that electrons of Ag nanoparticles could transfer to Co and Mo in [email protected] catalyst, and strong electronic interaction between Ag and transition-metal cobalt–molybdenum compounds balanced the adsorption/desorption of intermediates.3232. F. X. Jiao, J. L. Wang, Y. Lin, J. H. Li, X. F. Jing, and Y. Q. Gong, Appl. Surf. Sci. 553, 149440 (2021). https://doi.org/10.1016/j.apsusc.2021.149440 Therefore, it can be expected that Ag-decorated cobalt–molybdenum oxide with improved conductivity will be a significant pH-universal HER catalyst.

In this work, a composite of Ag nanoparticles decorated on Co2Mo3O8 nanosheets was designed (CMO-Ag) to improve the materials' conductivity and catalytic activity of hydrogen evolution reaction. Density functional theory (DFT) calculations first screened the optimum structure of Co2Mo3O8 and Co2Mo3O8-Ag for HER and then disclosed the interface electronic interaction between Ag and Co2Mo3O8. With introduction of silver nanoparticles, the DOS near the Fermi level from CMO-Ag was obviously enhanced. The water adsorption energy (ΔEad) on Co sites decreased from −2.5 to −3.5 eV and Gibbs free energy of hydrogen adsorption (ΔGH*) on Mo sites was optimized from +1.1 to +0.5 eV, significantly promoting HER intrinsic catalytic activity. Inspired by the theoretical calculation results, the CMO-Ag composites were also successfully synthesized experimentally through the following processes. First, the cobalt–molybdenum hydroxide material (Co(OH)2-CoMo) was prepared through hydrothermal and electrochemical deposition processes. Then, the Ag nanoparticles loaded on cobalt–molybdenum hydroxide material (Ag-Co(OH)x-CoMo) was synthesized through a redox process. Finally, the Co(OH)2-CoMo and Ag-Co(OH)x-CoMo materials were heated at 400 °C under N2 for 1 h to obtain the final Co2Mo3O8 (CMO) and Co2Mo3O8-Ag (CMO-Ag) products, respectively (Scheme S1). For the electrochemical catalytic reaction, the CMO-Ag composites present an excellent catalytic performance with a low overpotential of 55.5 mV to achieve 10 mA cm−2 current density for HER in alkaline electrolyte. Meanwhile, superior stability of CMO-Ag catalyst was also demonstrated, as long as 20 h with residual current density of 99.6%. Moreover, the overall water electrolysis system for CMO-Ag‖RuO2 exhibited a cell voltage of 1.50 V at 10 mA cm−2, lower than that of Pt/C‖RuO2 (1.51 V). The combination of theoretical and experimental results disclosed that electron-rich Co and Mo were generated after Ag addition, which reduced the energy barrier of water dissociation adsorption and proton adsorption. Our work supplies a valuable insight for improving conductivity and intrinsic catalytic activity of pH-universal TMO HER catalyst.

As shown in Figs. 1(a)–1(c) and S1(a)–S1(c), three representative surfaces of Co-terminated, Mo-terminated, and O-terminated Co2Mo3O8 (001) are selected to be as calculated models, which have been optimized to reach a stable structure. For the Co-terminated Co2Mo3O8 (001), the DOS results in two spin channels indicate that it is half-metallic [Fig. 1(d)]. When silver nanoparticles were decorated on the surface of Co-terminated Co2Mo3O8 (001), it was converted from half-metallic to metallic, meaning the distinct conductivity improvement of Co2Mo3O8-Ag [Fig. 1(g)]. The DOS in both two spin channels for Mo-terminated and O-terminated Co2Mo3O8 (001) demonstrated their metallic properties [Figs. 1(e) and 1(f)]. Similarly, the Ag nanoparticle-decorated Mo-terminated and O-terminated Co2Mo3O8 (001) were also metallic [Figs. 1(h) and 1(i)]. It is worth noting that the DOS near the Fermi level of Ag nanoparticle-decorated Co-terminated, Mo-terminated, and O-terminated Co2Mo3O8 (001) displayed more occupation than the Co-terminated, Mo-terminated, and O-terminated Co2Mo3O8 (001), indicating the faster electron transfer and higher conductivity after addition of Ag [Figs. S2(a)–S2(f)].33,3433. L. An, J. R. Feng, Y. Zhang, R. Wang, H. W. Liu, G. C. Wang, F. Y. Cheng, and P. X. Xi, Adv. Funct. Mater. 29, 1805298 (2019). https://doi.org/10.1002/adfm.20180529834. L. Wang, Z. J. Li, K. X. Wang, Q. Z. Dai, C. J. Lei, B. Yang, Q. H. Zhang, L. C. Lei, M. K. H. Leung, and Y. Hou, Nano Energy 74, 104850 (2020). https://doi.org/10.1016/j.nanoen.2020.104850 These observations suggested that the addition of Ag nanoparticles can effectively adjust the electronic structure of Co2Mo3O8 nanosheets. The strong electronic interaction between Co2Mo3O8 and Ag nanoparticles can be further revealed by differential charge density. In Fig. 2(a), the addition of Ag has resulted in 0.48 electrons transfer from Ag to Co2Mo3O8, suggesting the formation of electron-rich Co and Mo. This phenomenon will reduce the energy barrier of water dissociation adsorption and proton adsorption. The adjusted electronic structure will boost electron transport, promote molecule adsorption, and enhance conductivity of Co2Mo3O8, thus improving HER kinetics and performance.To disclose the intrinsic catalytic activity of CMO-Ag composites, the adsorption energy of water (ΔEad) on catalysts, playing a vital role in the Volmer reaction step in HER, was calculated.3535. F. T. Luo, X. Z. Shu, X. Jiang, Y. Liu, J. Q. Zhang, X. D. Wang, and S. J. Chen, Appl. Phys. Lett. 121, 013904 (2022). https://doi.org/10.1063/5.0098085 The ΔEad on different sites of CMO-Ag and CMO were investigated using three representative model surfaces. As shown in Fig. 2(c), the calculated ΔEad on Co sites, Mo sites, and O sites were −2.5, −3.1, and −1.4 eV, respectively, suggesting that Mo sites acted as main water adsorption sites in CMO. The ΔEad on Co sites, Mo sites, and O sites of CMO-Ag were, respectively, changed to −3.5, −2.8, and −1.8 eV due to the introduction of Ag. Note that the ΔEad on bare Ag surface is −0.28 eV. The ΔEad on Co sites of CMO-Ag was significantly lower than that on Mo sites from CMO-Ag and CMO (−3.1 eV), hinting that the water adsorption on CMO-Ag was optimized, and the main water adsorption sites have been changed from Mo atoms to Co atoms. It is well-known that Gibbs free energy of hydrogen adsorption (ΔGH*) is a crucial descriptor for HER activity, and ideal value is near zero.2020. K. Zhang, C. L. Liu, N. Graham, G. Zhang, and W. Z. Yu, Nano Energy 87, 106217 (2021). https://doi.org/10.1016/j.nanoen.2021.106217 In Fig. 2(d), the ΔGH* was studied on three representative model surfaces. The highest ΔGH* of Mo sites on CMO implies the hard hydrogen adsorption and poor hydrogen evolution reaction. The ΔGH* from Mo sites of CMO-Ag was optimized to be +0.5 eV, indicating the lowered hydrogen adsorption energy barrier and enhanced interaction between adsorbed hydrogen and Mo active sites.3636. J. W. Zhu, F. J. Xia, Y. Guo, R. H. Lu, L. Gong, D. Chen, P. Y. Wang, L. Chen, J. Yu, J. S. Wu, and S. C. Mu, ACS Catal. 12, 13312 (2022). https://doi.org/10.1021/acscatal.2c03102 Although strong hydrogen adsorption can be achieved on Co and O sites of CMO (−3.2 and −4.9 eV) and CMO-Ag (−2.7 and −4.8 eV), and Ag sites of bare Ag (−5.5 eV), the hydrogen desorption is too difficult. Therefore, Mo sites with ΔGH* near zero for CMO-Ag were the main hydrogen evolution sites. It can be reasonably inferred that the Co sites and Mo sites of CMO-Ag can act as the water adsorption sites and hydrogen evolution sites, respectively [Fig. 2(b)]. The introduction of Ag not only increased the DOS around the Fermi level but also regulated the main water adsorption sites of CMO-Ag composites, significantly optimizing the adsorption/desorption of hydrogen and promoting HER activity.Motivated by theoretical predictions, a CMO-Ag composite was experimentally constructed as a pH-universal HER catalyst through hydrothermal and calcination processes. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the nanosheet morphology of CMO and CMO-Ag samples. As shown in Figs. S3 and 3(a)–3(c), both CMO and CMO-Ag nanosheets displayed a smooth surface. In high resolution transmission electron microscopy image, CMO-Ag nanosheets displayed lattice fringe spacing of 0.22 and 0.23 nm, corresponding to (202) plane of Co2Mo3O8 and (111) plane of Ag, respectively [Fig. 3(d)]. The EDS pattern [Fig. S5(a)] and element mapping [Figs. S5(b)–S5(f)] for CMO-Ag confirmed that the Co, Mo, O, and Ag elements were evenly distributed in CMO-Ag sample. These results suggested that the Co2Mo3O8 nanosheets modified by Ag nanoparticles catalyst were successfully fabricated, which were also demonstrated by XRD and Raman characterizations. As shown in Fig. S6(a), both samples showed distinct diffraction peaks located at 25.2°, 35.9°, 37.1°, 40.4°, and 56.1°, corresponding to the (102), (200), (201), (202), and (213) crystal planes of Co2Mo3O8 (PDF No. 34-0511). The Co2Mo3O8 belonged to hexagonal P63mc space group. The lattice parameters were as follows: a = 5.871, b = 5.871, c = 10.021 Å, α = 90.00°, β = 90°, and ɣ = 120°. The material crystallized in a hexagonal noncentrosymmetric crystal structure of the polar space group P63mc. The Co2+ ions occupied two different cationic sites with tetrahedral and octahedral oxygen coordination.3737. L. Prodan, I. Filippova, A. O. Zubtsovskii, S. Shova, S. Widmann, A. A. Tsirlin, I. Kézsmárki, and V. Tsurkan, Phys. Rev. B 106, 174421 (2022). https://doi.org/10.1103/PhysRevB.106.174421 Compared with pure CMO, an extra peak of 38.1° can be assigned to (111) plane of Ag (PDF No. 04–0783) in CMO-Ag composites. In Fig. S6(b), the same Raman signals from CMO and CMO-Ag nanosheets at 331, 806, and 925 cm−1 belonged to Co–O–Mo, O–Mo–O, and Mo–O bonds, respectively, verifying the presence of Co2Mo3O8 phase.18,3818. W. W. Xie, J. H. Huang, L. Huang, S. P. Geng, S. Q. Song, P. Tsiakaras, and Y. Wang, Appl. Catal., B 303, 120871 (2022). https://doi.org/10.1016/j.apcatb.2021.12087138. G. Kumar Veerasubramani, K. Krishnamoorthy, and S. J. Kim, J. Power Sources 306, 378 (2016). https://doi.org/10.1016/j.jpowsour.2015.12.034The XPS measurement was applied to study the chemical state and electronic structure of CMO and CMO-Ag nanosheets. The XPS survey spectra [Fig. S6(c)] showed the existence of Co, Mo, and O elements in two samples, and only the presence of Ag in CMO-Ag. For Co 2p spectrum of CMO, two couple peaks corresponded to the Co 2p1/2 and Co 2p3/2 [Fig. 3(e)]. The peaks at 782.8 and 798.4 eV were assigned to Co2+, while two peaks of 780.7 and 796.7 eV belonged to Co3+.3939. T. Wang, Q. Pang, B. L. Li, Y. B. Chen, and J. Z. Zhang, Appl. Phys. Lett. 118, 233903 (2021). https://doi.org/10.1063/5.0051233 For CMO-Ag sample, the peak locations of Co 2p1/2 and Co 2p3/2 negatively shifted to 796.2/797.9 and 780.2/782.3 eV, respectively, caused by electron transfer from Ag to Co. This change is consistent with the above theoretical calculation results.40,4140. B. Zhang, G. J. Liu, X. Yao, X. L. Li, B. Jin, L. J. Zhao, X. Y. Lang, Y. F. Zhu, and Q. Jiang, ACS Appl. Nano Mater. 4, 5383 (2021). https://doi.org/10.1021/acsanm.1c0072641. T. T. Wang, P. Y. Wang, W. J. Zang, X. Li, D. Chen, Z. K. Kou, S. C. Mu, and J. Wang, Adv. Funct. Mater. 32, 2107382 (2022). https://doi.org/10.1002/adfm.202107382 The Mo 3d spectrum from CMO sample showed two peaks at 232.2 and 235.3 eV, assigning to Mo 3d5/2 and Mo 3d3/2 of Mo6+, respectively [Fig. 3(f)].4242. T. Ouyang, X. T. Wang, X. Q. Mai, A. N. Chen, Z. Y. Tang, and Z. Q. Liu, Angew. Chem., Int. Ed. 59, 11948 (2020). https://doi.org/10.1002/anie.202004533 The two peaks of Mo 3d5/2 and Mo 3d3/2 for CMO-Ag composites also negatively shifted to 231.9 and 235.1 eV, respectively, manifesting a strong interaction of electrons between Ag nanoparticles and Co2Mo3O8 nanosheets.32,4332. F. X. Jiao, J. L. Wang, Y. Lin, J. H. Li, X. F. Jing, and Y. Q. Gong, Appl. Surf. Sci. 553, 149440 (2021). https://doi.org/10.1016/j.apsusc.2021.14944043. J. H. He, Z. W. Huang, W. Z. Chen, X. Z. Xiao, Z. D. Yao, Z. Q. Liang, L. J. Zhan, L. Lv, J. C. Qi, X. L. Fan, and L. X. Chen, Chem. Eng. J. 431, 133697 (2022). https://doi.org/10.1016/j.cej.2021.133697 As shown in Fig. S6(d), the O1s spectra of both CMO and CMO-Ag exhibited three peaks at 530.0, 531.1, and 532.3 eV, assigning to lattice oxygen (Co–O–Mo), oxygen vacancy, and adsorbed oxygen, respectively.4343. J. H. He, Z. W. Huang, W. Z. Chen, X. Z. Xiao, Z. D. Yao, Z. Q. Liang, L. J. Zhan, L. Lv, J. C. Qi, X. L. Fan, and L. X. Chen, Chem. Eng. J. 431, 133697 (2022). https://doi.org/10.1016/j.cej.2021.133697 The oxygen vacancy ratio of CMO-Ag and CMO was 1:0.96, and slightly increased oxygen vacancies could increase a few active sites (but limited) in CMO-Ag catalyst.44,4544. M. Asnavandi, Y. C. Yin, Y. B. Li, C. H. Sun, and C. Zhao, ACS Energy Lett. 3, 1515 (2018). https://doi.org/10.1021/acsenergylett.8b0069645. S. Y. Guan, L. L. An, S. Ashraf, L. Zhang, B. Z. Liu, Y. P. Fan, and B. J. Li, Appl. Catal., B 269, 118775 (2020). https://doi.org/10.1016/j.apcatb.2020.118775 For Ag 3d XPS spectrum of CMO-Ag [Fig. 3(g)], two typical XPS peaks of Ag 3d5/2 and Ag 3d3/2 were located at 368.1 and 374.1 eV, respectively, which were attributed to the metal Ag. Therefore, the successful load of Ag nanoparticles on Co2Mo3O8 nanosheets was demonstrated again.The HER catalytic activities of prepared CMO-Ag samples were investigated in 1 M KOH electrolyte by a conventional three-electrode system. Compared to that of CMO (132.8 mV), the LSV curve from CMO-Ag sample showed a lower overpotential of 55.5 mV at the current density of 10 mA cm−2, superior to the recently reported catalysts [Table S1 and Fig. 4(a)]. In Fig. 4(b), the overpotential for CMO-Ag was 125.4 mV to deliver a higher current density of 50 mA cm−2, outperforming that of CMO (196.0 mV), Ag-Co(OH)x-CoMo (220.9 mV), Co(OH)2-CoMo (320.8 mV), and Pt/C (127.9 mV). The Tafel slope often represents the electrochemical reaction kinetics. In Fig. 4(c), the CMO-Ag sample exhibited a small Tafel slope value of 63.8 mV dec−1, meaning outstanding HER reaction kinetics. The other samples with poor HER reaction kinetics showed the high Tafel slope values of 82.3 mV dec−1 (CMO), 85.4 mV dec−1 (Ag-Co(OH)x-CoMo), and 120.5 mV dec−1 (Co(OH)2-CoMo). The EIS was applied to further investigate the HER reaction kinetics of prepared samples [Fig. 4(d)], and corresponding fitted results are in Table S2. The charge transfer resistance (Rct) values of CMO-Ag, CMO, Ag-Co(OH)x-CoMo, and Co(OH)2-CoMo were 4.2 Ω, 6.0 Ω, 9.1 Ω, and 10.6 Ω, respectively. The CMO-Ag composites presented the smallest charge transfer barrier, implying the fastest charge transfer rate and excellent HER reaction kinetics. The ECSA was also evaluated to analyze the catalytic activities. As we can see from Fig. 4(e), the calculated Cdl value from CMO-Ag sample was 19.1 mF cm−2, outperforming that of CMO (6.9 mF cm−2), Ag-Co(OH)x-CoMo (2.9 mF cm−2), and Co(OH)2-CoMo (1.9 mF cm−2) [Figs. S9(a)–S9(d)]. This result indicated more exposed catalytic active sites on CMO-Ag, favoring the HER activities. Based on the DFT and XPS results, the increased DOS near the Fermi level of CMO after Ag addition and electrons transfer from Ag to CMO illustrated more electrons around Co and Mo sites of CMO-Ag catalyst. The formation of electron-rich Co and Mo facilitated reactant molecules adsorption. Thus, the original inert Co and Mo sites were effectively activated by the addition of Ag, correspondingly increasing exposed catalytic active sites. In addition, slightly increased oxygen vacancies could increase a few active sites (but limited) in CMO-Ag catalyst. The long-term stability of CMO-Ag was tested by chronoamperometry. As expected, 99.6% of current density remained after 20 h of the stability test, demonstrating the splendid durability for CMO-Ag sample in 1 M KOH electrolyte [inset of Fig. 4(f)]. In addition, negligible quantity of dissolved Ag, Co, and Mo in 1 M KOH electrolyte after the electrochemical stability test (Table S3) demonstrated the outstanding structural stability of CMO-Ag in alkaline electrolyte during electrochemistry. In fact, the LSV curves did not change much before and after 1000 cycles of CV tests [Fig. 4(f)]. Meanwhile, the nanosheets' morphology and micro-structure presented no change in CMO-Ag sample after the stability test (Fig. S10). The XPS results of survey spectrum, Co 2p, Mo 3d, and Ag 3d were also consistent with that of CMO-Ag before the stability test, confirming the high structural stability of CMO-Ag in the alkaline medium during electrochemistry (Fig. S11).

A water–alkali electrolyzer was fabricated to assess practical application of CMO-Ag in hydrogen production, where CMO-Ag was used as cathode and RuO2 acted as anode. Interestingly, the two-electrode system of CMO-Ag‖RuO2 exhibited lower operation voltages (1.50 V) at 10 mA cm−2 than that of Pt/C‖RuO2 (1.51 V) [Fig. S12(a)]. The CMO-Ag‖RuO2 electrolyzer also showed long-time stability with high current density retention (98%) after 12 h of stability test [inset of Fig. S12(b)], and LSV curves did not distinctly change after 1000 cycles of CV tests [Fig. S12(b)]. The above results indicate that loading of Ag nanoparticles on Co2Mo3O8 is an effective approach to achieve high-efficiency electrocatalyst for practical application in alkaline electrolyte.

Moreover, the HER catalytic activities of CMO-Ag sample were also explored in neutral electrolyte (1 M PBS) and acidic electrolyte (0.5 H2SO4). CMO-Ag sample showed low overpotentials of 63.2 and 161.0 mV at 10 and 50 mA cm−2 current density in 1M PBS, respectively [Figs. 5(a) and S13(a)]. Small Tafel slope (81.8 mV dec−1) and Rct (27.2 Ω) for CMO-Ag sample suggested the prominent electrochemical reaction kinetics in neutral electrolyte [Figs. 5(b), S13(b), and Table S4]. Compared with initial LSV curve, the LSV curve showed few changes after 1000 cycles of CV tests [Fig. 5(c)], and 91.4% current density was retained after 16 h of stability measurement [inset of Fig. 5(c)], indicating the excellent stability of CMO-Ag sample in neutral electrolyte. In 0.5 M H2SO4, as shown in Figs. 5(d) and S13(c), CMO-Ag exhibited the overpotentials of 68.2 mV and 146.1 mV under 10 and 50 mA cm−2 in acidic electrolyte. In addition, both Tafel slope and Rct values were small (78.1 mV dec−1 and 72.80 Ω) [Figs. 5(e), S13(d), and Table S5], indicating that the CMO-Ag also possessed outstanding HER reaction kinetics in acidic electrolyte. LSV curves did not change distinctly after 1000 cycles of CV measurements [Fig. 5(f)], and 96.1% current density retention can be observed after 18 h of stability test, confirming the superior durability for CMO-Ag in acidic electrolyte [inset of Fig. 5(f)]. After stability tests of CMO-Ag catalyst in different electrolytes, dissolution amount of metals (Co, Mo and Ag) in acidic electrolyte was significantly higher than that in alkaline and neutral electrolytes (Table S3), which was mainly attributed to the instability of metal oxides in acidic solution. The CMO-Ag catalyst was more stable in alkaline electrolyte than in neutral and acidic electrolytes,31,4631. Z. Zhang, X. Li, C. Zhong, N. Zhao, Y. Deng, X. Han, and W. Hu, Angew. Chem., Int. Ed. 59, 7245 (2020). https://doi.org/10.1002/anie.20200170346. R. Liu, K. Ye, Y. Gao, W. P. Zhang, G. L. Wang, and D. X. Cao, Electrochim. Acta 186, 239 (2015). https://doi.org/10.1016/j.electacta.2015.10.126 guaranteeing more excellent HER performance in alkaline electrolyte. Therefore, CMO-Ag catalyst showed pH dependent HER performance, which was in agreement with other reported works. Some transition-metal oxides also displayed a pH dependency of the ORR performance, and a lower pH resulted in a worse ORR performance.47,4847. L. Zhou, H. Li, Y. Lai, M. Richter, K. Kan, J. A. Haber, S. Kelly, Z. B. Wang, Y. B. Lu, R. S. Kim, X. Li, J. Yano, J. K. Nørskov, and J. M. Gregoire, ACS Energy Lett. 7, 993 (2022). https://doi.org/10.1021/acsenergylett.1c0267348. H. Li, S. Kelly, D. Guevarra, Z. B. Wang, Y. Wang, J. A. Haber, M. Anand, G. T. Kasun, K. Gunasooriya, C. S. Abraham, S. Vijay, J. M. Gregoire, and J. K. Nørskov, Nat. Catal. 4, 463 (2021). https://doi.org/10.1038/s41929-021-00618-wIn this work, three Tafel slope values from CMO-Ag catalysts were between 40 and 120 mV dec−1, indicating that the HER processes on CMO-Ag followed a Volmer–Heyrovsky mechanism in three electrolytes.7,497. S. Shen, Z. Wang, Z. Lin, K. Song, Q. Zhang, F. Meng, L. Gu, and W. Zhong, Adv. Mater. 34, 2110631 (2022). https://doi.org/10.1002/adma.20211063149. N. Xue, Z. Lin, P. K. Li, P. Diao, and Q. F. Zhang, ACS Appl. Mater. Interfaces 12, 28288 (2020). https://doi.org/10.1021/acsami.0c07088 In alkaline and neutral electrolytes, the first Volmer step was for the formation of adsorbed H* on Co sites (H2O + e− + * → H* + OH−). Then, the interaction of adsorbed H*, proton, and electron on Mo sites generated hydrogen molecular via second Heyrovsky step (H* + H2O + e− → H2 + OH−).50,5150. J. Zhang, T. Wang, P. Liu, Z. Q. Liao, S. H. Liu, X. D. Zhuang, M. W. Chen, E. Zschech, and X. L. Feng, Nat. Commun. 8, 15437 (2017). https://doi.org/10.1038/ncomms1543751. H. Y. Yang, M. Driess, and P. W. Menezes, Adv. Energy Mater. 11, 2102074 (2021). https://doi.org/10.1002/aenm.202102074 Due to high energy barrier of water dissociative adsorption process, the Volmer step was the rate determining step (RDS) in alkaline and neutral HER.52,5352. Y. L. Xu, C. Wang, Y. H. Huang, and J. Fu, Nano Energy 80, 105545 (2021). https://doi.org/10.1016/j.nanoen.2020.10554553. Y. Lin, K. Sun, S. J. Liu, X. M. Chen, Y. S. Cheng, W. C. Cheong, Z. Chen, L. R. Zheng, J. Zhang, X. Y. Li, Y. Pan, and C. Chen, Adv. Energy Mater. 9, 1901213 (2019). https://doi.org/10.1002/aenm.201901213 In acidic solution, the first Volmer step was also the formation of adsorbed H* on Co sites (H+ + e− + * → H*), followed by hydrogen evolution reaction on Mo sites as second Heyrovsky step (H* + H+ + e− → H2), which was a RDS in acidic HER.49,51,5449. N. Xue, Z. Lin, P. K. Li, P. Diao, and Q. F. Zhang, ACS Appl. Mater. Interfaces 12, 28288 (2020). https://doi.org/10.1021/acsami.0c0708851. H. Y. Yang, M. Driess, and P. W. Menezes, Adv. Energy Mater. 11, 2102074 (2021). https://doi.org/10.1002/aenm.20210207454. T. F. Li, T. Y. Lu, X. Li, L. Xu, Y. W. Zhang, Z. Q. Tian, J. Yang, H. Pang, Y. W. Tang, and J. M. Xue, ACS Nano 15, 20032 (2021). https://doi.org/10.1021/acsnano.1c07694

In summary, we reported an Ag nanoparticles decorated Co2Mo3O8 nanosheets composite as a pH-universal HER catalyst. Theoretical calculations proved that the introduction of Ag can optimize Gibbs free energy of hydrogen adsorption (ΔGH*) and enhance water adsorption energy (ΔEad) on Co2Mo3O8, indicating the improved intrinsic catalytic activity in HER. Moreover, electronic state and catalytic active sites were also well adjusted by introducing Ag. As expected, prepared CMO-Ag exhibited outstanding HER activities in a wide pH range. Low overpotentials of 55.5 mV (alkaline), 63.2 mV (neutral), and 68.2 mV (acidic) at current density of 10 mA cm−2 could be achieved. Meanwhile, CMO-Ag also displayed superior stability and prominent electrochemical reaction kinetics in alkaline, neutral, and acidic electrolytes. More importantly, water–alkali electrolyzer of CMO-Ag‖RuO2 exhibited a lower cell voltage (1.50 V) at 10 mA cm−2 than that of Pt/C‖RuO2 (1.51 V). This work presents a valuable view for other TMO to achieve high-efficiency pH-universal HER catalysts and their practical application.

See the supplementary material for details regarding preparation, DFT calculations parameters, and characterization of all samples, such as SEM, TEM, XRD, XPS, and EDS, element mapping and Raman characterization, and electrochemical test and overpotential comparison.

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 52073308, 11804395, and 21902186), the Distinguished Young Scholar Foundation of Hunan Province (Grant No. 2015JJ1020), the Central South University Research Fund for Sheng-hua scholars (Grant No. 502033019), the Fundamental Research Funds for the Central Universities of Central South University, the Postgraduate Research and Innovation Project of Central South University (Grant No. 2021zzts0059), and the Open Sharing Fund for the Large-scale Instruments and Equipments of Central South University. This work was carried out in part using computing resources at the High Performance Computing Center of Central South University.

Conflict of Interest

The authors have no conflicts to disclose.

Author Contributions

Yue Zhang: Writing – original draft (equal); Writing – review & editing (equal). Wenzhe Zhou: Investigation (equal); Methodology (equal). Shanzheng Du: Investigation (equal). Qi Zhang: Investigation (equal). Lianwen Deng: Resources (equal). Xiaohui Gao: Writing – review & editing (equal). Fangping Ouyang: Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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