Critical Role of Electrochemical Proton Insertion for the Hydrogen Evolution Reaction on Tungsten Oxides

Abstract

The local electrochemical environment can affect electrochemical reactivity by determining the interactions between the electrode, reactant, reaction intermediates, and products. One method for tuning the local electrochemical environment is through confinement in porous materials. However, experimental evidence of the role of confinement on electrochemical reactivity in porous materials remains elusive due to the challenges of decoupling confinement effects from other variables, such as high surface area. In this work, we compared the structure, morphology and electrochemical reactivity of WO3 ∙H2O and a hybrid organic-inorganic layered material, octylamine-expanded WO3 (OA-WO3). We investigated the electrochemical response in three electrolytes to understand how interlayer modification affects cation adsorption at the outer surface and cation and proton insertion into the bulk of the metal oxide. While the WO3 ∙H2O could reversibly insert protons and lithium ions, OA-WO3 significantly suppressed cation insertion. Interestingly, as OA-WO3 was cycled within a narrow potential window in a sulfuric acid electrolyte, the material transformed to WO3 ∙2H2O, which was evidenced by features in the cyclic voltammograms as well as ex situ Raman spectroscopy and X-ray diffraction. Additionally, we determined that the conversion from OA-WO3 to WO3 ∙2H2O was possible via a chemical reaction by stirring in sulfuric acid – thus resulting in two materials with similar surface areas and particle morphologies. These similar material properties enable the study of how the cation insertion behavior and chemical nature of confined molecules between layers of a transition metal oxide framework influence the electrocatalytic activity toward the hydrogen evolution reaction (HER). Our results showed that the presence of octylamine in the interlayer of WO3 required a ~210 mV higher overpotential compared to WO3 ∙H2O for the HER, and the onset of the HER did not appear until after a reduction reaction associated with proton insertion occurred at a more negative potential compared to proton insertion in WO3 ∙H2O. We discuss three hypotheses for how proton insertion leads to HER activity in WO3 ∙ xH2O: 1) proton insertion changes the electronic band structure of WO3 ∙ xH2O, 2) the presence of bulk protons can influence ∆GH,ads at the surface sites, and 3) inserted protons may participate in the HER mechanism on WO3 ∙ xH2O. Overall, this work demonstrated how tuning the interlayer structure and chemistry of bulk layered materials enables the study of electrochemical reactivity under confinement. Using this materials design approach, our results revealed a correlation between proton insertion capacity and the activity of a transition metal oxide toward the HER, where high proton insertion capacity led to lower HER overpotential.