  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.
