The kinetic theory of the H-upd The prevalent explanation of the H-upd pattern is the existence of distinct binding sites with different hydrogen binding energies where atomic hydrogen forms in a Volmer-step-like reaction. Each binding site would exhibit a Langmuir-type adsorption peak that can be derived from the B-V model, extensively discussed in . The characteristic pattern is then a result of the superposition of these peaks further influenced by interactions between sites and with electrolyte species leading to their sharpening (attractive interaction) or broadening (repulsive interaction). We will now investigate the kinetics using the simple first order approximation that the characteristic features marked in Fig. correspond to independent binding sites and originate from the Volmer step. As the H-upd is a pseudocapacitive process, we cannot assume steady-state conditions any longer but must write the kinetics as a differential equation (Eq. 41a). At very low scan rates (Eq. 41b) or fast kinetics (Eq. 41c), the differential equation simplifies, and we obtain the from now on called equilibrium H-upd coverage relationship (Eq. 41d), denoted with the subscript ∞ (properties on the surface at infinitely small scan rates/large rate constants), resembling a Langmuir isotherm.