Molybdenum carbides show great potential to replace platinum for electrocatalytic hydrogen evolution reaction (HER) to resolve the problem of hydrogen production, due to their high reserves, stability, low cost, and structural diversity. However, the effect of atomic configurations of different surfaces on HER is still lacking theoretical insights. In this work, the HER activity on 29 surfaces of seven phases is systematically explored by density functional theory, taking into account water effect. The exchange current for each surface is also given. Totally, there are nine surfaces which own high exchange current (>0.1 mA/cm2), especially the hydroxylated (014)-C and (010) of TiP-MoC, (110) of β-Mo2C, and (100)-C of α-Mo2C (1.410, 0.835, 0.687, and 0.464 mA/cm2, respectively). Combining with the stabilities of the surfaces for each phase, the phases with high HER activity could be also screened out. The electronic properties, including electron transfer to adsorbed hydrogen and the shift of the electronic states coupled by oxygen and adsorbed hydrogen orbitals, are applied to uncover the termination of surface and water effect on HER. Our results are expected to contribute to the understanding of the HER on different surfaces of molybdenum carbides and give some evidence for control synthesis of high HER activity surfaces.

The introduction of electronegative non-metallic heterostructures is promisingly desired to develop Ru-based oxide catalysts, which have higher activity and robust stability for acidic oxygen evolution reaction (OER). Developing scalable synthetic strategies and enlightening the mechanism understanding of the enhancement is imperative but still challenging. Herein, a facile selenium-assisted reduction approach is reported to fabricate the partially reduced nano-sized Ru with mixed-valence Se species immobilization, directly from commercial RuO 2 through ball milling and annealing procedures. The obtained Ru/Se-RuO 2 composites demonstrate superior electrocatalytic performance toward acidic OER with small overpotentials of 190 and 240 mV at 10 and 100 mA cm −2 , a Tafel slope of only 43.7 mV dec −1 , and no detectable activity decay under operation for 24 h. Experimental results further demonstrate a synergistic effect including enhanced electron transfer interaction by the formation of Ru/RuO 2 heterostructures and more available active sites due to Se doping. Theoretical calculations indicate that slight Se doping and metallic Ru loading can significantly decrease the free energy change for the formation of *OOH intermediate, and thus dramatically promotes the OER performance. This strategy offers a promising strategy for the development of non-metallic elements incorporated high-performance catalysts and related water electrocatalytic devices for sustainable energy applications.

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Tungsten carbide (WC) is an established model electrocatalyst for the hydrogen evolution reaction (HER) in aqueous solutions. In spite of extensive interest in and work on this material, systematic atomistic understanding of the combined role of faceting and hetero-atom doping for hydrogen evolution on WC electrocatalysts in aqueous solution is not available. To fill this knowledge gap, here we explore the interplay of these parameters for the electrocatalytic performance of WC by means of Density Functional Theory (DFT) simulations. In our simulations, we explicitly account for solvent (water) effects and related electrochemical, potential dependent oxygen or hydroxyl (co-)adsorption on the WC surfaces as determined by analysis of surface Pourbaix diagrams. Among the several low-index WC surfaces screened, the DFT simulations indicate that the W-terminated (-100) and (111) surfaces of WC show comparable exchange currents of 3.908 and 0.133 mA/cm 2 , respectively. The simulations also suggest that V or N doping can substantially increase the HER activity of some WCsurfaces to levels comparable to commercial Pt/C electrocatalysts, with current densities as large as 10.555, 1.730, and 0.636 mA/cm 2 for the V-doped W-terminated (001), V-doped (101), and N-doped (101) WC-surfaces, respectively. The improved HER activities are rationalized in terms of the d-band or p-band center, which are both shown to be related to the capability of tuning the hydrogen adsorption, thence HER performance. The calculated strong dependence of the HER activity on the presence of electrochemical adsorption of oxygen and hydroxyls reiterates the importance of modeling realistic electrochemical conditions for WC pristine or doped surfaces. We expect the DFT results presented to provide timely and original guidelines for further theoretical and experimental studies of HER activity on WC-related electrocatalysts.

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