Hydrogen electrocatalysis revisited : Weak bonding of adsorbed hydrogen as the design principle for active electrode materials

Hydrogen electrocatalysis has been spurred by theoretical predictions, using simple ab initio thermodynamic considerations, in that the free-binding energy of adsorbed hydrogen has been applied in a heuristic fashion to search for sustainable electrocatalysts as a replacement for scarce platinum in electrolyzers and fuel cells. The original volcano model of Norskov et al. is given in [14] purports that the optimum hydrogen-evolution catalyst binds adsorbed hydrogen thermoneutrally at zero overpotential, a paradigm based on pure thermodynamic considerations. Recently, the Sabatier principle was revisited by factoring the applied overpotential and kinetics into the analysis. The extended Sabatier principle suggests that the optimum hydrogen-evolution catalyst binds adsorbed hydrogen weakly rather than thermoneutrally. This notion is corroborated by the fact that the most active hydrogen-evolution catalysts, Pt, MoS2, or Mo2C, indeed bind hydrogen weakly by about (100–200) meV rather than thermoneutrally at zero overpotential.