Evaluating the experimental Tafel plot of the hydrogen evolution reaction over a polycrystalline platinum electrode by microkinetics and machine learning to extract the free adsorption energy of the reactive hydrogen species, it turns out that adsorbed hydrogen is bound weakly rather than thermoneutrally. Comparing this outcome to a hypothetical thermoneutral electrocatalyst, it becomes evident that the optimum binding energy of the reaction intermediate shifts with increasing overpotential from thermoneutral to weak bonding, in agreement with the extended Sabatier principle.
Binding energies of reaction intermediates are largely used to comprehend activity trends in a class of electrode materials. For a two-electron process, such as the hydrogen evolution reaction (HER), it is a well-established paradigm that the optimum electrocatalyst binds adsorbed hydrogen thermoneutrally at zero overpotential. While this picture was challenged recently by means of density functional theory (DFT) calculations and microkinetic considerations, reporting a shift of the optimum binding energy to strong or weak bonding with increasing overpotential, now experiments show further evidence for this theory. This perspective article juxtaposes the different views of the optimum binding energy for the HER by means of the Sabatier principle, microkinetic considerations, DFT calculations, and experiments, and provides an outlook of potential future investigations by the combination of experiments with DFT aiming at sustainable materials development for the hydrogen electrocatalysis.Zum Volltext