(528f) The Role of Hydrogen Intercalation in the Kinetics of Hydrogen Evolution on WO3

Over the past few years, metal oxides have received increased attention as electrocatalysts for the hydrogen evolution reaction (HER). Various techniques of tuning morphology and composition have been investigated to improve their performance and stability. One frequently overlooked parameter is the bulk intercalation of hydrogen into the metal oxide lattice under the cathodic conditions associated with hydrogen evolution. Several acid-stable metal oxides are well known electrochromic materials, forming H_xMO_y compounds known as metal oxide hydrogen bronzes. The formation of these materials frequently occurs positive of the HER equilibrium potential, which motivates the hypothesis that the HER does not occur on the parent metal oxide surface, but rather on the surface of the intercalated bronze.

In this work, we tested this hypothesis and found that bulk hydrogen intercalation in WO₃ induces significant changes in hydrogen surface adsorption, and therefore HER catalysis. First, we used UV-vis spectroscopy and density functional theory (DFT) to confirm the semiconductor-metal transition in WO₃ that results from hydrogen intercalation. We then used constant-potential DFT to determine the activation barriers of Langmuir-Hinshelwood (Volmer-Tafel) and Eley-Rideal (Volmer-Heyrovsky) mechanisms at various stoichiometries of H_xWO₃. We used these constant-potential barriers to construct first-principles current-potential relationships that were directly compared to experimental cyclic voltammograms collected on a rotating disk electrode. The computational results showed that intercalation proceeds as a bulk diffusion-limited process, in agreement with experiment. Our comparisons also showed that non-intercalated WO₃ cannot be responsible for experimentally observed HER current densities. The experimental results are reproduced from first-principles by considering bulk WO₃ hydrogen intercalation.