Hydrogen Oxidation and Evolution Reaction (HOR/HER) on Pt Electrodes in Acid vs. Alkaline Electrolytes: Mechanism, Activity and Particle Size Effects

In this study, we provide an overview regarding our recent finding on the mechanism, activity and particle size effect for the hydrogen oxidation and evolution reaction (HOR/HER) on Pt-group metal electrodes, under both acid and alkaline conditions. We show that there is an activity decrease of about two orders of magnitude when going from acid to base conditions on electrodes which are able to form a H-UPD layer like Pt, Ir, Pd and Rh. Similarities in the HOR/HER process between acid and base conditions have been found: the rate determining step, which has been identified by means of electrochemical impedance spectroscopy measurements to be the Volmer reaction, remains the same and there is no particle size dependency on the reactivity of Pt electrodes. In the light of these finding, Volcano plots of the HOR/HER activities in acid and base have been proposed.