Sub-Nernstian Response of the Cell Voltage in Proton Exchange Membrane Water Electrolysis under Elevated Pressure Operation

Proton-exchange-membrane water electrolysers (PEMWEs) can directly produce hydrogen gas under pressurised conditions, which provides an attractive means to reduce post-electrolysis compression costs. Experimental studies have shown a lower-than-expected increase in PEMWE operation voltage under elevated pressure, but existing modelling approaches have difficulties to adequately explain these observations. Herein, a combined theoretical–experimental approach is presented to elucidate the effect of pressure on water electrolysis kinetics in a PEMWE. Experiments, performed under differential and balanced pressure changes, clearly demonstrate a sub-Nernstian response of the PEMWE polarisation curve to pressure increase at both the hydrogen and oxygen electrodes. The kinetic origin of the effect was revealed by a novel modelling strategy accounting for the distinct contributions of the oxygen evolution (OER), hydrogen evolution (HER), and hydrogen oxidation (HOR) reactions to the activation losses, which are defined on a fixed reference potential scale. By simultaneously fitting the experimental data at multiple operating pressures, the sub-Nernstian voltage response is accurately reproduced and shown to arise from the superimposed kinetics of the OER, HER, and HOR. Effectively, increasing the operating pressure thereby transforms part of the irreversible activation losses into useful compression energy, thus enhancing the energy efficiency of the process. The model further demonstrates that deviations from Tafel-line behaviour observed at intermediate current densities are due to a decreased influence of the HOR reverse reaction at the hydrogen electrode. This work is expected to give direction to future developments in high-pressure PEMWE modelling and performance analysis.