Quantifying buffer transport-limited water electrolysis at non-extreme pH by numerical simulation

Abstract

Operating water electrolysis at non-extreme pH is expected to play an essential role in a sustainable society as it significantly broadens the choice of materials for cell components. Buffer species are commonly introduced into the electrolytes to minimize local pH gradient through their buffering action; however, the fundamental limitations imposed by buffer transport remain poorly understood. This study focuses on both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) under non-extreme pH conditions. Limiting current densities associated with mass tranport of buffer species were quantitatively evaluated. The generalized modified Poisson-Nernst-Planck (GMPNP) model was employed to model achievable diffusion limits of buffer ions and their distribution near the electrode surface. While buffer's pKa affects the limiting current of each half-reaction, consideration of coupled HER and OER reveals that diffusion of buffer species plays a decisive role in determining the overall performance. Increasing buffer concentration and temperature, assisted by electrolyte convection, effectively enhances the attainable limiting current density beyond industrially relevant thresholds, demonstrating that electrolyte engineering through control of buffer chemistry and transport enables buffer-based water electrolysis at non-extreme pH.