Bubble resistance in near-zero-gap alkaline water electrolysis

Abstract

Modern alkaline water electrolysers for hydrogen production often use a zero-gap configuration in which electrodes are pressed directly against the separator. Counterintuitively, the inclusion of a small electrode-diaphragm gap was previously shown to reduce the cell potential significantly. This work aims to understand, quantify, and model this effect, and take first steps towards optimisation of the gap width. We present experimental measurements and simulations of the cell potential and kinetic and ohmic losses for expanded metal electrodes. We find that the configuration with our smallest used gap, created using a 60µm spacer, yields the lowest cell potential, while the zero-gap configuration incurs additional voltage losses of approximately 80mV at a current density of 10⁴A/m². This can be explained by bubbles and gas films in between the electrode and diaphragm, which block part of the diaphragm and electrode area. We introduce an analytical model that predicts the vertical gas fraction and current density distribution in the electrode-diaphragm gap, which is in good agreement with experimental and simulation results. For a maximum gas fraction below 0.7, the model can explain why there is no optimal gap width based solely on the gap resistance. Instead, the gap allows gas to escape, which mitigates the additional zero-gap overpotential. Our findings confirm, explain, and quantify that intentionally adding a small gap can be an effective way to improve the performance of alkaline electrolysers with perforated plate-like electrodes.