(406b) Comparative Analysis of HxWO3 Bronze Formation through Thermochemical and Electrochemical Routes

Several significant industrial chemical processes make use of thermochemical redox reactions involving the addition or removal of hydrogen. Analogous reactions involving proton-coupled electron transfer (PCET) are also common in electrochemistry, and the ability to apply an electrochemical potential (i.e., manipulate the chemical potential of electrons) affords fine control over the thermodynamics and kinetics of these reactions. This fundamental reliance on transferring hydrogen points toward interesting opportunities to synergistically integrate thermochemical and electrochemical processes for efficient and selective chemical transformations. We are studying tungsten trioxide (WO₃) as a model for this type of reactivity due to its ability to reversibly take up hydrogen (i.e., a proton-electron pair) to form tungsten trioxide hydrogen bronze (H_xWO₃ where x ≤ 0.5). This reaction occurs through two known pathways: via H-spillover from hydrogen gas using a hydrogenation catalyst such as Pt and through uncatalyzed electrochemical hydrogen intercalation in water. As such, WO₃ is particularly suited for the analysis of the relationship between thermochemical and electrochemical processes.

This work focuses on the dynamics of WO₃↔H_xWO₃ interconversion using nanoparticulate thin films. We are employing a combined theoretical and experimental approach to address the hypothesis that H-spillover and H-intercalation proceed by identical reaction mechanisms. The experimental portion of this work centers on the use of in-situ optical microscopy to directly track the propagation of diffusion fronts in WO₃/H_xWO₃ films, where the electrochromic transition from yellow-green to deep blue indicates the transformation from WO₃ to H_xWO₃. This technique allows us to determine the effective reaction rate and/or diffusion coefficient of hydrogen in WO₃/H_xWO₃ under conditions where reduction is instigated through either hydrogen exposure, applied potential, or a combination of the two. Recasting the applied potential as an effective partial pressure of hydrogen (and vice versa) allows us to directly study the congruence between H-spillover and H-intercalation mechanisms. In addition, we use density functional theory (DFT) calculations to study the structural transformational characteristics of the WO₃/H_xWO₃ system. Hydrogen adsorption, intercalation, and migration are studied as a function of hydrogen loading and applied potential. This treatment is based upon the fact that bronze formation is essentially PCET. The significance of this work lies in our resulting ability to directly compare the effects of thermochemical and electrochemical “biases” on the chemistry of redox-active oxides that are of significant relevance for applications in catalysis.