Dynamics and Hysteresis of Hydrogen Intercalation and Deintercalation in Palladium Electrodes: A Multimodal In Situ X-ray Diffraction, Coulometry, and Computational Study

 Understanding hydrogen intercalation and deintercalation in palladium is the key to utilizing palladium-based materials for hydrogen storage, hydrogen separations, and electrochemical hydrogen evolution and CO2 reduction catalysis. Here, we combine in situ synchrotron X-ray diffraction and coulometry measurements with density functional theory calculations to provide complementary insights on the dynamics of hydrogen intercalation and deintercalation under electrochemical conditions. By employing multimodal in situ characterization, we demonstrate that the interplanar d-spacing and the hydrogen/palladium ratio are decorrelated under certain conditions. Additionally, there is a clear hysteresis in the electrode potentials where the β-phase of palladium hydride forms and disappears. Computed energetics of hydrogen intercalation and deintercalation predict this hysteresis. These calculations indicate that the potential-driven absorption of subsurface hydrogen during intercalation and oxidation of surface hydrogen during deintercalation could contribute to the observed hysteresis. These results suggest that surface processes during hydrogen intercalation and deintercalation are important, providing additional mechanistic understanding that is complementary to bulk phase transition theory. This multimodal in situ characterization and computational study provides new insights into hydrogen intercalation and deintercalation in palladium electrodes, which could lead to improvements in palladium-based materials needed in a sustainable energy economy. The computational data can be found at https://www.catalysis-hub.org/publications/LandersUnderstanding2020