Predicting the Energetics of Hydrogen Intercalation in Metal Oxides Using Acid-Base Properties.

The ability to predict intercalation energetics from first principles is attractive for identifying candidate materials for energy storage, chemical sensing, and catalysis. In this work, we introduce a computational framework that can be used to predict the thermodynamics of hydrogen intercalation in tungsten trioxide (WO). Specifically, using density functional theory (DFT), we investigated intercalation energetics as a function of adsorption site and hydrogen stoichiometry. Site-specific acid-base properties determined using DFT were used to develop linear structure screening models that informed a kernel ridge energy prediction model. These regressions provided a series of hydrogen binding energy predictions across stoichiometries ranging from WO to HWO, which were then converted to equilibrium potentials for hydrogen intercalation. Experimental validation using cyclic voltammetry measurements yielded good agreement with the predicted intercalation potentials. This methodology enables fast exploration of a large geometric configuration space and reveals an intuitive physical relationship between acidity, basicity, and the thermodynamics of hydrogen intercalation. Furthermore, the combination of theoretical and experimental results suggests HWO as a maximum stable stoichiometry for the bronzes that arises from competition with hydrogen evolution rather than the inability of WO to accommodate additional hydrogen. Our experimental results further indicate hydrogen insertion in WO is highly irreversible for low H-stoichiometries, which we propose to be a consequence of the semiconductor-to-metal transition that occurs upon initial H-intercalation. Overall, the agreement between theory and experiment suggests that local acid-base characteristics govern hydrogen intercalation in tungsten trioxide, and this insight can aid the accelerated discovery of redox-active metal oxides for catalytic hydrogenations.