Global and Local Connectivities Describe Hydrogen Intercalation in Metal Oxides.

We introduce global connectivity, Ω, which quantifies the detailed connectivity of a material and accurately captures reactivity behavior. We demonstrate that combining global and local connectivity describes how metal oxides interact with hydrogen. Blending density functional theory, graph theory, and machine learning we built a reactivity model which accurately predicts hydrogen intercalation potentials of different metal oxides experimentally measured in the lab. The use of global connectivity can accelerate materials design through the development of novel structure-property relationships.

The Sensitivity of Metal Oxide Electrocatalysis to Bulk Hydrogen Intercalation: Hydrogen Evolution on Tungsten Oxide.

Metal oxides are attracting increased attention as electrocatalysts owing to their affordability, tunability, and reactivity. However, these materials can undergo significant chemical changes under reaction conditions, presenting challenges for characterization and optimization. Herein, we combine experimental and computational methods to demonstrate that bulk hydrogen intercalation governs the activity of tungsten trioxide (WO3) toward the hydrogen evolution reaction (HER). In contrast to the focus on surface processes in heterogeneous catalysis, we demonstrate that bulk oxide modification is responsible for experimental HER activity. Density functional theory (DFT) calculations reveal that intercalation enables the HER by altering the acid-base character of surface sites and preventing site blocking by hydration. First-principles microkinetic modeling supports that the experimental HER rates can only be explained by intercalated HxWO3, whereas nonintercalated WO3 does not catalyze the HER. Overall, this work underscores the critical influence of hydrogen intercalation on aqueous cathodic electrocatalysis at metal oxides.

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 (WO3). 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 WO3 to H0.625WO3, 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 H0.500WO3 as a maximum stable stoichiometry for the bronzes that arises from competition with hydrogen evolution rather than the inability of WO3 to accommodate additional hydrogen. Our experimental results further indicate hydrogen insertion in WO3 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.