Transients in Electrochemical CO Reduction Explained by Mass Transport of Buffers

Pulsed electrolysis has shown promise to improve CO(2) reduction activity and steer selectivity by potential pulsing. Nevertheless, a detailed mechanistic understanding of the transient activity upon potential pulsing is still lacking. Utilizing electrochemical mass spectrometry, we demonstrate highly active but short-lived methane and hydrogen transients for pulsed electrochemical CO reduction in phosphate buffer. Compared with the absence of transients in borate buffer, we conclude that methane and hydrogen transients arise from an initial presence and local depletion of phosphate ions, acting as facile proton donors. We further support our conclusion by mass transport modeling, including homogeneous buffering reactions. Our result stresses the importance of the proton donor nature and its local concentration for proton-coupled electron transfer-limited reactions. In this paper, buffer anions improve methane and hydrogen activities transiently by up to one order of magnitude. Similar strategies can be of importance for the selective transformation of more complex biomass molecules and electrosynthesis.