(Invited) Identifying the Rate-Determining Step in the Electrochemical CO Reduction Reaction

Abstract

Electrocatalysis powered by renewable energy is expected to play a pivotal role in the global decarbonization process. In particular, electrocatalytic CO2 and CO reduction reactions (CO(2)RR) are widely considered as indispensable components in the valorization of CO2. Much recent research effort has been devoted to the development of efficient and selective catalysts for valuable C2+ products on Cu surfaces, however, many open questions regarding how species in the electrochemical interfaces impact the activity and selectivity remain. In this lecture, we discuss recent efforts in understanding the impact of the surface speciation, adsorbed reaction intermediates and near surface species on the CO(2)RR at the molecular level. The identification of the rate-determining step in the CO(2)RR has been complicated by the lack of rigorous electrokinetic data, from which key mechanistic variables such as the Tafel slopes and reaction orders are determined. Rigorous electrokinetic studies without mass transport limitation show that all C2+ products in the CORR have the common Tafel slope of ~120 mV/dec, and the formation rates are largely invariant with respect to the electrolyte pH on the absolute potential scale. In addition, CO adsorption isotherms determined by in-situ surface enhanced infrared absorption spectroscopy shows that the reaction orders for adsorbed CO are 1st and 0th at low and high partial pressures of CO. Based on these results, we assign the first hydrogenation step of adsorbed CO with adsorbed water, rather than the C-C coupling, as the rate-determining step in the CORR, and propose a reaction mechanism capable of reconciling all experimental observations.