Understanding the Influence of Electrode Potential on Differently Charged Hydrogen Sources in a Hydrogen Evolution Reaction under Neutral Environments

Electrochemical water splitting is a promising strategy for reducing hydrogen production costs. However, the conventional phosphate buffer (PBS) electrolyte incurs a huge energy penalty in high overpotential ranges. This study investigates the hydrogen evolution reaction (HER) performance using two buffer electrolytes with different charge properties: H2 PO4–/HPO42– (PBS pKa = 6.90) and IMZH+/IMZ (IMZS pKa = 7.03). On Pt microelectrodes, results reveal that the plateau current density of PBS correlates closely with the electrode potential, a trend not observed with IMZS. In addition, when using commercial Pt/C nanocatalysts loaded on carbon paper, the plateau current density phenomenon disappears in the IMZS electrolyte, which is attributed to the preconcentration effect of the positively charged hydrogen source. In situ Raman characterization indicates that the configuration of H2 PO4– at the electrochemical interface changes as the electrode potential becomes progressively negative. In comparison, the adsorption configuration of IMZH+ remains considerably stable across the tested potential range. The grand canonical density functional theory method with both implicit and explicit solvation models was further used to investigate the influence of electrode potential on the dissociation of a differently charged hydrogen source. Results indicate that when the electrode potential becomes progressively negative, H2 PO4– is repelled from the electrode surface owing to electrostatic forces, increasing Gibbs free energy of dissociation accordingly. This phenomenon contributes to the inevitable energy penalty observed in the PBS electrolyte for the HER. In comparison, the Gibbs free energy of dissociation of IMZH+ decreases monotonically with the progressive negative cathode potential. To explain this discrepancy, this study briefly examined the influence of electrode potential in two aspects: theoretical thermodynamics energy and the state of the hydrogen source near the electrode surface.