The Combined Role of Faceting and Heteroatom Doping for Hydrogen Evolution on a WC Electrocatalyst in Aqueous Solution: A Density Functional Theory Study

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, a systematic atomistic understanding of the combined role of faceting and heteroatom 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 the 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/cm2, respectively. The simulations also suggest that V or N doping can substantially increase the HER activity of some WC surfaces to levels comparable to commercial Pt/C electrocatalysts, with current densities as large as 10.555, 1.730, and 0.636 mA/cm2 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, and thence the HER performance. The calculated strong dependence of the HER activity on the presence of the 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