Impact of palladium/palladium hydride conversion on electrochemical CO 2 reduction via in-situ transmission electron microscopy and diffraction.

Electrochemical conversion of CO 2 offers a sustainable route for producing fuels and chemicals. Pd-based catalysts are effective for converting CO 2 into formate at low overpotentials and CO/H 2 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 CO 2 conversion selectivity away from formate and towards CO/H 2 at increasing overpotentials is due to electrode potential dependent changes in the reaction energetics and not a consequence of morphology or phase structure changes.