Electrocatalysis of the HER in acid and alkaline media

Trends in the HER were studied on selected metals (M = Cu, Ag, Au, Pt, Ru, Ir. and Ti) in acid and alkaline environments. It was found that with the exception of Pt, Ir and Au, due to high coverage by spectator species on non-noble metal catalysts, the experimentally established positions of Cu, Ag, Ru and Ti in the observed volcano relations are still uncertain. It was also found that while the M-H-upd binding energy most likely controls the activity trends in acidic solutions, the trends in activity in alkaline solutions are controlled by a delicate balance between two descriptors: the M-H-ad interaction and the energetics required to dissociate water molecules. The importance of the second descriptor was confirmed by introducing bifunctional catalysts such as M modified by Ni(OH); e.g., while the latter serves to enhance the catalytic decomposition of water, the metal sites are required for collecting and recombining the produced hydrogen intermediates.