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Recently, the bulk electrooxidation of CO on gold or platinum has been used to detect CO produced during CO2 reduction in neutral media. The CO bulk oxidation voltammetry may show two distinct peaks depending on the reaction conditions, which up to now have not been understood. We have used scanning electrochemical microscopy (SECM) to probe CO oxidation and pH in the diffusion layer during CO2 reduction. Our results show that the two different peaks are due to diffusion limitation by two different species, namely, CO and OH-. We find that between pH 7 and 11, CO oxidation by water and OH- gives rise to the first and second peak observed in the voltammetry, respectively. Additional rotating disc experiments showed that specifically in this pH range the current of the second peak is diffusion limited by the OH- concentration, since it is lower than the CO concentration.
Monteiro et al. (Mon,) studied this question.