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 quantum effects in the electrode processes, the hydrogen/deuterium kinetic isotope effect constant ratio (= KH/D) was measured in a variety of conditions. 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.