Mechanism for acetate formation in electrochemical CO(2) reduction on Cu: Selectivity with potential, pH, and nanostructuring

Nanostructured Cu catalysts have increased the selectivities and geometric activities for high value C-C coupled (C2) products (ethylene, acetate, and ethanol) in the electrochemical CO(2) reduction reaction (CO(2)RR). The selectivity among the high-value C2 products is also altered, where for instance the yield of acetate increases with alkalinity and is dependent on the catalyst morphology. The reaction mechanisms behind the selectivity towards acetate vs. other C2 products remain controversial. In this work, we elucidate the reaction mechanism towards acetate by using ab-initio simulations, a coupled kinetic-transport model, and loading experiments. We find that trends in acetate selectivity can be rationalized from variations in electrolyte pH and the local mass transport properties of the catalyst and not from changes of Cu's intrinsic activity. The selectivity mechanism originates in the transport of ketene, a stable (closed shell) intermediate, away from the catalyst surface into solution where it reacts to acetate. While such a mechanism has not yet been discussed in CO(2)RR, variants of it may explain similar selectivity fluctuations observed for other stable intermediates like CO and acetaldehyde. Our proposed mechanism suggests that acetate selectivity increases with increasing pH, decreasing catalyst roughness and significantly varies with applied potential.