Interplay of Homogeneous Reactions Mass Transportand Kinetics in Determining Selectivity of the Reduction of CO2 on Gold Electrodes

摘要

Gold electrocatalysts have been a research focus due to their ability to reduce CO2 into CO, a feedstock for further conversion. Many methods have been employed to modulate CO2 reduction (CDR) vs hydrogen evolution reaction (HER) selectivity on gold electrodes such as nano-/mesostructuring and crystal faceting control. Herein we show that gold surfaces with very different morphologies (planar, leaves, and wires) lead to similar bell-shaped CO faradaic efficiency as a function of applied potential. At low overpotential (E > −0.85 V vs standard hydrogen electrode (SHE)), HER is dominant via a potential quasi-independent rate that we attribute to a rate limiting process of surface dissociation of competent proton donors. As overpotential is increased, CO faradaic efficiency reaches a maximal value (near 90%) because CO production is controlled by an electron transfer rate that increases with potential, whereas HER remains almost potential independent. At high overpotential (E < −1.2 V vs SHE), CO faradaic efficiency decreases due to the concurrent rise of HER via bicarbonate direct reduction and leveling off of CDR as CO2 replenishment at the catalystsurface is limited by mass transport and homogeneous coupled reactions.Importantly, the analysis shows that recent attempts to overcome masstransport limitations with gas diffusion electrodes confront low carbonmass balance owing to the prominence of homogeneous reactions coupledto CDR. The comprehensive kinetics analysis of the factors definingCDR vs HER on gold electrodes developed here provides an activation-drivingforce relationship over a large potential window and informs on thedesign of conditions to achieve desirable high current densities forCO2 to CO conversion while maintaining high selectivity.