Unraveling Complex Electrode Processes by Differential Electrochemical Mass Spectrometry and the Rotating Ring-Disk Electrode Technique

The competition between the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) on a polycrystalline Pt (pc-Pt) electrode in weakly acidic solutions (pH ≈ 3) under the condition when both reactions are thermodynamically allowed is investigated using the rotating ring disk electrode (RRDE) technique and differential electrochemical mass spectrometry (DEMS). The partial currents of HER and ORR producing H₂O₂ or H₂O are quantified. The results show that (i) ORR occurs preferentially in a diffusion-limited manner (either limited by the mass transfer of O₂ or H⁺) and only the excess H⁺ not being consumed by the ORR can be converted to H_ad and H₂ and (ii) at potentials negative of the onset of H_ad formation, the H₂O₂ production rate first increases and then decreases with the increasing overpotential. Possible origins for the H₂O₂ yield in the HER region are suggested. Present results demonstrate that combining DEMS and RRDE techniques can be powerful for deriving the intrinsic kinetics of individual processes in a complex electrochemical system.