Enhancing CO2 Electroreduction to Ethanol on Copper–Silver Composites by Opening an Alternative Catalytic Pathway

A fundamental question in the electrochemical CO2 reduction reaction (CO2RR) is how to rationally control the catalytic selectivity. For instance, adding a CO-selective cocatalyst like Ag to Cu shifts the latter's CO2RR selectivity toward C2 products, but the underlying cause of the change is unclear. Herein, we show that, during CO2RR, the abundant CO availability at Cu−Ag boundaries facilitates C-C coupling on Cu to selectively generate ethanol through an otherwise closed pathway. Oxide-derived Cu nanowires mixed with 20 nm Ag particles (Cu:Ag mole ratio of 1:20) catalyzed CO2 reduction to ethanol with a maximum current density of −4.1 mA/cm2 and ethanol/ethylene Faradaic efficiency ratio of 1.1 at −1.1 V vs RHE. These figures of merit are, respectively, 5 and 3 times higher than those for pure oxide-derived Cu nanowires. CO2RR on CuAg composite catalysts with different Ag:Cu ratios and Ag particle sizes reveals that ethanol production scales with the amount of CO evolved from Ag sites and the abundance of Cu–Ag boundaries, and, very interestingly, without significant modifications to ethylene formation. Computational modeling shows selective ethanol generation via Langmuir–Hinshelwood *CO + *CHx (x = 1, 2) coupling at Cu–Ag boundaries and that the formation of energy-intensive CO dimers is circumvented.