which is well fulfilled by our experimentally determined parameters. As for the HOR, the H 2 reaction order of the HER on the overpotential scale assumes a fractional value that is influenced by the respective transfer coefficient, thus concealing the intuitive picture of H 2 -independent HER kinetics revealed on the fixed potential scale. Both for the HOR and HER, we thus find that the reaction order defined on a fixed potential scale (m HOR/HER , Eq. ( )) provides a comprehensible picture of the reaction kinetics, whereas the reaction order defined under constant overpotential (γ HOR/HER , Eq. ( )) is biased by the Nernst shift of the thermodynamic equilibrium potential, which is opposite to the terminology used by some authors. The results discussed so far provide an intuitive picture of the HOR and HER. The HOR kinetics are directly proportional to the hydrogen concentration (partial pressure) with a reaction order of essentially one, whereas the HER kinetics are independent of the hydrogen concentration. This behavior could be intuitively expected, because hydrogen is the reactant and product of the HOR and HER, respectively. Moreover, the apparent linearity of kinetic current curves on a logarithmic scale observed at larger overpotentials suggests that the HOR and HER can be described by the two exponential branches of a Butler-Volmer-type kinetic model,