Impact of palladium/palladium hydride conversion on electrochemical CO₂ reduction via in-situ transmission electron microscopy and diffraction

Electrochemical conversion of CO₂ offers a sustainable route for producing fuels and chemicals. Pd-based catalysts are effective for converting CO₂ into formate at low overpotentials and CO/H₂ at high overpotentials, while undergoing poorly understood morphology and phase structure transformations under reaction conditions that impact performance. Herein, in-situ liquid-phase transmission electron microscopy and select area diffraction measurements are applied to track the morphology and Pd/PdH_x phase interconversion under reaction conditions as a function of electrode potential. These studies identify the degradation mechanisms, including poisoning and physical structure changes, occurring in PdH_x/Pd electrodes. Constant potential density functional theory calculations are used to probe the reaction mechanisms occurring on the PdH_x structures observed under reaction conditions. Microkinetic modeling reveals that the intercalation of *H into Pd is essential for formate production. However, the change in electrochemical CO₂ conversion selectivity away from formate and towards CO/H₂ at increasing overpotentials is due to electrode potential dependent changes in the reaction energetics and not a consequence of morphology or phase structure changes.