Is the higher current density, the better performance for CO2 electrochemical reduction reaction?

Abstract

Higher current density and faradaic efficiency are always the goals for CO2 electrochemical reduction reaction (CO2RR) over the years. However, in our latest experiments, we inadvertently observe a sharp increase of current density of CO2RR in a zero-gap electrolyzer operating at a constant cell voltage. Based on rigorous investigation, it is found that the high current density results from an undesirable oxygen reduction reaction on cathode, due to the oxygen crossover from anode. As oxygen crossover is inevitable for zero-gap electrolyzers, obviously, the pseudo-high current density may lead to an inaccurate evaluation of CO2RR performance. To comprehensively illuminate the oxygen crossover behavior and effect, we qualitatively measure the O2 permeation flux and O2 permeability, and systematically investigate the influence of materials, electrode designs, and operating conditions. The results suggest that oxygen crossover is a critical issue that should be reflected in CO2RR.