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Abstract

The hydrogen evolution reaction (HER) is a complex reaction involving many interdependent physicochemical steps. Highly ordered carbon nitride-based materials, such as Na-PHI and K-PHI, display some of the highest activities for H2 evolution among the carbon nitride-based materials, due to their electronic properties, but also the presence of cyanimide terminations, which favors the charge transfer for the Pt cocatalyst nanoparticles (NPs). For such highly optimized semiconductor structures, the necessity to control and improve other steps of the photocatalytic process becomes essential, in particular the poor electron transfer from the Pt NPs to the protons in solution over the Helmholtz or Stern layer. Taking highly ordered Na-PHI as a test material, the influence of water-dissolved alkali cations on the HER is systematically studied and it is experimentally verified that the electron transfer from the Pt NPs to the protons in solution limits the efficiency of heterogeneous carbon nitride-based catalysts. This paper explains how hydrated alkali cations influence electron transfer and are able to boost the H₂ evolution rate of the same Na-PHI from 2401 up to 5330 µmol h^-1 g^-1 with an apparent quantum yield of 13% at 420 nm.