Participation of electrochemically inserted protons in the hydrogen evolution reaction on tungsten oxides

Understanding the mechanisms by which electrodes undergo the hydrogen evolution reaction (HER) isnecessary to design better materials for aqueous energy storage and conversion. Here, we investigatethe HER mechanism on tungsten oxide electrodes, which are stable in acidic electrolytes and canundergo proton-insertion coupled electron transfer concomitant with the HER. Electrochemicalcharacterization showed that anhydrous and hydrated tungsten oxides undergo changes in HER activitycoincident with changes in proton composition, with activity in the order HxWO3H2O>HxWO3 >HxWO32H2O. We used operando X-ray diffraction and density functional theory to understand thestructural and electronic changes in the materials at high states of proton insertion, when the oxides aremost active towards the HER. H0.69WO3H2O and H0.65WO3 have similar proton composition, structuralsymmetry, and electronic properties at the onset of the HER, yet exhibit different activity. Wehypothesize that the electrochemically inserted protons can diffuse in hydrogen bronzes and participatein the HER. This would render the oxide volume, and not just the surface, as a proton and electronreservoir at high overpotentials. HER activity is highest in HxWO3H2O, which optimizes both the degreeof proton insertion and solid-state proton transport kinetics. Our results highlight the interplay betweenthe HER and proton insertion-coupled electron transfer on transition metal oxides, many of which arenon-blocking electrodes towards protons.