Overall reaction (16) These three elementary steps yield two different possible mechanisms on the basis of H2 dissociation: Tafel-Volmer (T-V) and Heyrovsky-Volmer (H-V). Since all steps involve the H* intermediate, ΔGH* has been suggested as the key descriptor for determining the HOR rate. The barrier of the proton-electron transfer has been demonstrated to scale with ΔGH*. Hence, the usual way to improve the HOR rate comes by regulating ΔGH*. According to the Sabatier principle, the optimal HOR active site should have ΔGH* ≈ 0, so that it promotes H* activation on one hand and allows fast H* detachment on the other hand. Too positive ΔGH* means that the Tafel or Heyrovsky step will be hindered, whereas too strong H* adsorption yields a slow Volmer step. Pt is located at the vertex of the volcano plot.