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 HxWO3*H2O>HxWO3 >HxWO3*2H2O. 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. H0.69WO3*H2O 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 HxWO3*H2O, 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.明晰电极发生析氢反应（hydrogen evolution reaction, HER）的机制，是设计用于水系储能与转化的高性能电极材料的必要前提。本研究针对酸性电解质中稳定、且可在发生析氢反应的同时伴随质子嵌入耦合电子转移的氧化钨电极，探究其析氢反应机制。电化学表征结果表明，无水氧化钨与水合氧化钨的析氢反应活性随质子组成变化发生对应改变，活性顺序为HₓWO₃·H₂O > HₓWO₃ > HₓWO₃·2H₂O。我们采用原位X射线衍射（operando X-ray diffraction）与密度泛函理论（density functional theory），探究了高质子嵌入态下材料的结构与电子结构变化——此时氧化钨的析氢反应活性最高。H₀.₆₉WO₃·H₂O与H₀.₆₅WO₃在析氢反应起始阶段具有相似的质子组成、结构对称性与电子性质，但表现出不同的反应活性。我们提出假设：电化学嵌入的质子可在氢青铜中扩散并参与析氢反应。这意味着在高过电位条件下，氧化钨的整体体积而非仅表面可作为质子与电子的储库。HₓWO₃·H₂O的析氢反应活性最高，其同时优化了质子嵌入程度与固态质子传输动力学。本研究结果揭示了过渡金属氧化物表面析氢反应与质子嵌入耦合电子转移之间的相互作用，而这类氧化物中多数均为质子非阻塞电极。