Mechanistic Switching by Hydronium Ion Activity for Hydrogen Evolution and Oxidation over Polycrystalline Platinum Disk and Platinum/Carbon Electrodes

Fundamental electrochemical reactions, namely the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR), are re‐evaluated under various pH conditions over polycrystalline Pt disk electrodes and Pt/C electrodes to investigate the overpotential and Tafel relations. Kinetic trends are observed and can be classified into three pH regions: acidic (1–5), neutral (5–9), and alkaline (9–13). Under neutral conditions, in which H 2 O becomes the primary reactant, substantial overpotential, which is not affected by pH and the supporting electrolyte type, is required for electrocatalysis in both directions. This ion independence, including pH, suggests that HER/HOR performance under neutral conditions solely reflects the intrinsic electrocatalytic activity of Pt in the rate determining steps, which involve electron transfer with water molecules. A global picture of the HER/HOR, resulting from mechanistic switching accompanied by change in pH, is detailed.