Kinetic origin of the sub-Nernstian cell voltage response in pressurised PEM water electrolysis

Proton-exchange-membrane water electrolysers (PEMWEs) can directly produce pressurised hydrogen, thereby reducing the need for downstream compression. Experimental studies have consistently reported a sub-Nernstian increase in PEMWE operating voltage under elevated pressure; however, existing modelling approaches have struggled to provide a satisfactory explanation for this behaviour. Here, a combined theoretical–experimental study is presented to elucidate the kinetic origin of pressure effects in PEMWE operation. Experiments conducted under differential and balanced pressurisation demonstrate a sub-Nernstian response of the PEMWE polarisation curve governed by cathode pressure, while anode pressurisation shows no kinetic effect. This behaviour is explained using a novel kinetic modelling framework that explicitly accounts for the distinct contributions of the oxygen evolution (OER), hydrogen evolution (HER), and hydrogen oxidation (HOR) reactions to the activation losses, formulated on a fixed reference potential scale. The model accurately reproduces the experimentally observed sub-Nernstian voltage response through simultaneous fitting of the polarisation data acquired at multiple operating pressures. In addition, the model provides a kinetic explanation for the deviations from linear Tafel behaviour observed at intermediate current densities. • Sub-Nernstian voltage shifts are experimentally resolved in PEMWE under differential and balanced pressurisation up to 10 bar. • Cathode pressurisation governs iR-free voltage shifts, while anode pressurisation shows no kinetic effect. • A fixed-reference kinetic model explicitly accounting for HER/HOR and OER/ORR is introduced. • The model reproduces the sub-Nernstian voltage response across multiple pressures without invoking mass-transport limitations. • Pressure effects originate from the competition between HER and HOR near equilibrium.