How symmetry factors cause potential- and facet-dependent pathway shifts during CO2 reduction to CH4 on Cu electrodes

Abstract

The deactivation of copper electrodes is a serious problem that can affect the scalability and deployment of CO2 electrolyzers. The effect is generally attributed to the cathodic deposition of Fe and Zn impurities from the electrolyte. Herein, an experimental-theoretical study shows the existence of potential- and facet-dependent pathways for CO2 reduction to CH4 on Cu. The small-overpotential pathway deactivates the electrodes, while the large-overpotential pathway does not. Theoretical modeling traces the origin of the deactivation to *COH and *CHO, the two *CO hydrogenation products. *COH, which reduces to *C (precursor to coke), is more stable than *CHO around the equilibrium potential, but its symmetry factor is smaller. Hence, the *COH-based coking pathway opens first until the potential is negative enough for the *CHO-based pathway to dominate. This highlights the often-neglected role of symmetry factors in electrocatalysis design and suggests that small increases in *CHO's symmetry factor can mitigate Cu deactivation.