Revisiting basic assumptions of the hydrogen evolution reaction (HER) for water electrolysis at near-neutral pH

The water reduction reaction alone is sufficient to explain the 3 phases of the hydrogen evolution reaction (HER) at near neutral pH, provided that the impact of the pH at the cathode surface is taken into account. • Hydrogen evolution reaction (HER) was modelled with only water as reactant. • Mass transport of protons and buffer species controlled pH at the cathode surface. • Impact of the local pH on water reduction kinetics explained the experimental data. • Model and experiments matched in large range of pH values and buffer concentrations. • It was essential to use activities for equilibrium potential calculations. Water electrolysis at near-neutral pH is attracting increasing attention due to its eco-compatibility, ease of use and lower capital expenditure in comparison to the water electrolysis processes currently on the market. However, the mechanism of hydrogen evolution reaction (HER) at near-neutral pH remains largely debated. The HER kinetics has been explained, in the absence of buffer, by the occurrence of successive proton and water reductions and, in the presence of buffer, by involving a direct electrochemical reaction of the buffer species. These hypotheses were mainly supported by considering the mass-transport limiting currents observed on the current–potential records. Here, a simple one-dimensional numerical model was designed using the sole water reduction reaction and focussing the mass transport of the species involved in pH control. The model perfectly recovered the three-phase shape of the intensity-potential curves. The experimental and numerical results matched well, whether in the absence or presence of buffer and in a large range of pH values. The model also succeeded in explaining experimental findings reported in the literature that compared the efficiency of phosphate and borate buffers for HER. It was thus shown that the single water reduction reaction was sufficient to explain HER at near neutral pH, provided that the impact of the local pH on its kinetics was taken into account. This drastic simplification of the basics will be a major help in simplifying the engineering involved in water electrolysis at near-neutral pH.