Quantum Proton Tunneling in Multi-electron/-proton Transfer Electrode Processes
Quantum proton tunneling (QPT) in the two representative multi-electron/-proton transfer electrode processes, i.e. hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), was investigated by using polycrystalline platinum (pcPt) and gold (pcAu) electrodes at 298 kelvin (K). In order to observe the quantum effect in the electrode processes, the hydrogen/deuterium kinetic isotope effect constant ratio (≡ KH/D) was measured in a variety of conditions. For the HER in both an acidic and an alkaline conditions, it was concluded that the pcPt shows a negligible QPT evident by a small value of KH/D (1 < KH/D < 3), which indicates that the semiclassical transition state theory (sc-TST) scheme dominates the rate-determining step (RDS) and there is a small or negligible contribution of QPT. In case of the pcAu in alkaline condition, the KH/D was small value showing ca. 1 at a low η region around 0.2 V, however at a high η region > 0.6 V, a high KH/D ( > 13) was obtained. This result suggests that there is a transition of the electrode process from sc-TST to a full QTP in the RDS on increasing the overpotential. For the ORR with pcPt, a KH/D higher than the theoretical maximum in sc-TST was observed in alkaline condition at a low overpotential region. A primitive but robust theoretical analysis suggests that the QPT governs the rate-determining step of ORR in this condition. However this full QPT path transits to the classical in a higher overpotential region. Therefore, on the contrary to the HER on pcAu in alkaline, the electrode process shows a transition from a full QPT to sc-TST on increasing the overpotential. No QPT in ORR on a pcPt electrode was observed in an acidic condition. In this Contribution, it is shown that the QPT in surface electrochemical system is highly affected by a choice of system. Although several systems show a clear manifestation of QPT in the electrode processes and primitive interpretations can be given to these observations, it is still challenge to derive a fine molecular-level picture on the results including several complicated effects. However, based on the observations, a selection of a full QPT path may be strongly affected by the different microscopic proton transfer mechanisms, i.e. proton transfer from hydronium ion or water molecule.
- This article is part of the themed collection: Quantum effects in complex systems