(2) The HOR/HER proceeds either via the Tafel-Volmer (T-V) path or the Heyrovský-Volmer (H-V) path, described here shortly in the HOR direction. In the T-V path, molecular hydrogen is split by adsorptive dissociation on two reaction sites forming adsorbed hydrogen atoms (Tafel step, Eq. 27a). The adsorbed hydrogen is then oxidized in the Volmer step (Eqs. 27c) and leaves the reaction site. In the H-V path, molecular hydrogen adsorbs on a reaction site and splits into one proton taken by a proton acceptor and one adsorbed hydrogen atom in the Heyrovský step (Eq. 27b). This is followed by a Volmer step analogous to the T-V path.