An interesting observation in Fig. is the convergence of the HER curves on the fixed potential scale at more negative potentials, where the HOR contribution becomes negligible. This is a direct consequence of the fact that the HER is virtually independent of the hydrogen concentration, i.e., the respective reaction order is essentially zero, m HER ≈ 0, as shown in Fig. . The analysis on the fixed potential scale thus provides the intuitively expected result that the HER rate is independent of the concentration of the product. 4c shows the kinetic currents re-plotted on the overpotential scale. Here, the H 2 -pressure-dependent Nernst shifts of the respective reference potentials open up effective gaps between the kinetic current curves at more negative overpotentials. As a consequence, the corresponding reaction order, γ HER , defined under constant overpotential, assumes a non-zero value of about γ HER ≈ 0.4, as shown in Fig. . Again, using partial differential calculus, an analogous relationship to Eq. ( ) is obtained,