A 2 nd observation is that the micropolarization exchange current density increases with the partial pressure of hydrogen. This was shown in early works in the first half of the 20 th century both in acid (up to 1 bar) and in base (up to 19 bar) . It was more recently confirmed by the works of Rheinländer et. al. at pH 13 in a three-electrode RDE setup and by Stühmeier et. al. in a PEM fuel cell used as a H2 pump. In both cases a reaction order of approx. 0.5 was deduced for 𝑖 0,𝑚𝑖𝑐𝑟𝑜 at 20-30 °C, which was erroneously associated with a Volmer limited T-V path on the basis of the QE approximation discussed earlier. This observation dismisses in fact a Volmer rds (see Fig. ) but is not sufficient to distinguish between the T-V and H-V reaction paths. The evaluation of 𝑚 𝐻 2 over a broader pH range may nevertheless help reveal the dominant reaction path.