Why the activity of the hydrogen oxidation reaction on platinum decreases as pH increases

Platinum is a very effective electrode for the hydrogen evolution and oxidation reactions (HER/HOR) in acidic media. However, the activity for the HOR on platinum falls two orders of magnitude from acidic to alkaline media, which has not been completely understood yet. Here, we provide an explanation for that. Both the HER and the HOR were investigated on the three basal planes of platinum in a pH range near neutral pH conditions in buffered solutions in the absence of anion specific adsorption for guaranteeing the protons availability. Whereas changes in the pH from acid to neutral values produced negligible effects on the HER, the HOR was found to be pH sensitive, even under near neutral pH conditions. From these results, it can be consistently reasoned that the drastic fall in the activity of the HOR on platinum from acidic to alkaline media is an effect of the charge on the electrode, which is more negative as the pH increases. With the aid of density functional theory calculations, kinetic arguments explaining the unfavorable effect that negative charge on the electrode has on the HOR are provided. (C) 2020 Elsevier Ltd. All rights reserved.