Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions

Palladium is an increasingly investigated electrocatalyst for the electrochemical reduction of carbon dioxide due to its unique ability to yield carbon monoxide or formate with large selectivities at high vs low overpotentials (i.e., similar to-0.5 to similar to-1.0 vs similar to-0.1 to similar to-0.4 V vs the reversible hydrogen electrode), respectively. While this behavior has been described multiple times on different Pd-electrocatalysts, previous studies disagree with regard to palladium's ability to form a hydride phase (PdHx) under CO2 reduction reaction (CO2RR) conditions, as well as on the influence of this PdHx on its CO2RR-selectivity. These inconsistencies are partially related to the known poisoning of the Pd-surface during the CO2RR with adsorbed CO, whose precise influence on the formation of PdHx and corresponding effects on the CO2RR-mechanism and product selectivity remain poorly understood. With this motivation, herein we used an unsupported Pd-aerogel to investigate CO adsorption and PdHx formation at CO2RR potentials in electrolytes with different compositions (i.e., in the presence or absence of CO2 and bicarbonate) and as a function of the applied potential and duration of the potential hold. The results of these electrochemical measurements unveiled the strong influence of surface-adsorbed CO on the formation rate of Pd-hydride, for which the final stoichiometry was nevertheless found to be independent of the presence of CO2 or bicarbonate. This finding was supported by in situ X-ray absorption spectroscopy (XAS) measurements that confirmed the complete formation of beta-phase PdHx (with x asymptotic to 0.6) at all applied potentials under CO2RR conditions, thus shedding light on the contradictory results in this regard reported in previous operando XAS studies.