Abstract:

Hydrogen evolution reaction (HER) is unavoidable in many electrochemical synthesis systems, such as CO₂ reduction, N₂ reduction, and H₂O₂ synthesis. It makes those electrochemical reactions with multiple electron-proton transfers more complex when determining kinetics and mass transfer information. Understanding how HER competes with other electrochemical reduction reactions is crucial for both fundamental studies and system performance improvements. In this study, we employed the oxygen reduction reaction (ORR) as a model reaction to investigate HER competition on a polycrystalline-Au surface, using a rotating ring and disk electrode. It’s proved that water molecules serve as the proton source for ORR in alkaline, neutral, and even acidic electrolytes, and a 4-electron process can be achieved when the overpotential is sufficiently high. The competition from H⁺ reduction becomes noticeable at the H⁺ concentration higher than 2 mmol L^-1 and intensifies as the H⁺ concentration increases. Based on the electrochemical results, we obtained an equivalent circuit diagram for the ORR system with competition from the H⁺ reduction reaction, showing that these reactions occur in parallel and compete with each other. Electrochemical impedance spectroscopy measurements further confirm this argument. Additionally, we discover that the contribution of H⁺ mass transfer to the total H⁺ reduction current is significant and comparable to the kinetic current. We believe this work will deepen our understanding of HER and its competition in electrochemical reduction systems.

Key words: Hydrogen evolution reaction, Oxygen reduction reaction, H⁺ reduction competition, Rotating ring and disk electrode, Proton source