Enhancing the Oxygen Reduction Reaction by Anchoring Proton Close to the Electrode Surface

Protic ionic liquid, when added to the solution in an electrochemical cell, can enhance the electrocatalytic activity for processes involving proton transfer. Impressive promotion has been reported for the sluggish oxygen reduction reaction (ORR), a key step in the fuel cell technology. However, a lack of understanding on the mechanism has hindered the rational molecular design for such enhancement. Herein, we report the measured ORR promotion effects on Pt(111) in acid solution for a series of molecules, each with the same piperidine ring and with only one amine group slightly modified to vary its structure. Comparisons among this series show that the deciding mechanistic factor is the anchoring of proton on the amine bridge, which then acts as a proton source close to the electrode surface for proton coupled electron transfer channels. Our results show a fundamental aspect of solvated proton in electrochemical processes: its mobility in the electrified double layer can slow down or even inhibit the proton transfer to the electrode, especially at high electrode potential. This problem can be addressed on the solution side by using protic ionic liquid as proton anchors.