Electrochemical Kinetics of Hydrogen Oxidation and Evolution on Platinum in Alkaline Electrolyte: Mechanistic Plurality and the Role of pH

High Resolution Image Download MS PowerPoint Slide The alkaline hydrogen reaction on platinum is of great interest for alkaline fuel cell and electrolyzer systems. However, its mechanism is still debated. In this work, we performed extensive rotating disk electrode measurements at different H 2 -partial pressures and bulk-pH values in highly alkaline electrolytes to determine the reaction parameters and identify the reaction mechanism in different potential regions, namely, at high overpotentials and close to equilibrium, for polycrystalline platinum electrodes. Based on the precise kinetic parameters determined in this study, it was shown that the alkaline H 2 -evolution/oxidation reaction (HER/HOR) follows different mechanisms in different potential regions: from a Volmer-limited Tafel−Volmer mechanism around equilibrium shifting to a Heyrovsky-limited Heyrovsky−Volmer mechanism at higher HOR overpotentials. This conclusion is supported by the values of the transfer coefficients and H 2 -reaction orders determined by fitting a generalized electrochemical rate model across the comprehensive set of experimental data under various H 2 pressures and solution pH values. The determined reaction orders in OH −, however, do not match any of the alkaline HER/HOR mechanisms and rather suggest an acidic pathway involving H + instead of OH − as the active species, which is counter-intuitive given the highly alkaline pH of the bulk electrolyte solution. EIS experiments for quantifying the charge-transfer resistance and capacitance of the H upd process (Volmer reaction) under HOR limiting current conditions support this picture and revealed an acidic reaction environment due to protons generated by the HOR. These findings reconcile previous hypotheses on the mechanism of the HER/HOR in alkaline electrolytes and reveal a surprisingly acidic character of the local reaction environment under reaction conditions.