Advancing near-neutral pH water electrolysis: The rising bubble layers govern pressure-induced voltage reduction

Water electrolysis at near-neutral pH offers an alternative technology for hydrogen production, well suited for designing small domestic units or stand-alone electrolysers for applications in remote locations. In this study, a prototype with electrodes of 80 cm 2 surface area was operated using phosphate or carbonate electrolytes, in galvanostatic mode, under pressures of up to 7 bar. The cell voltage was high, around 5 V at 500 A m −2 , but tended to decrease under pressure. Due to the narrow compartments (15 mm) and the absence of forced convection, a bubble-induced looped circulation developed within each compartment: bubbles rose along the electrode, while the electrolyte flowed downwards along the separator. Hydrodynamic modelling with an Euler-Euler approach (COMSOL Bubbly Flow software) fully featured this behaviour and aligned well with the various gas fraction patterns observed experimentally. Thermodynamically, increasing pressure is expected to increase cell voltage. In addition, the model predicted that increasing pressure would alter the bulk gas fraction in a way that further increases voltage. However, at 500 A m −2 , the opposite trend was observed experimentally: the cell voltage decreased with increasing pressure. Empirical correlations were used to account for the effect of bubbles adhering to the surface of the electrodes and of the film of bubbles sliding upwards along them. These correlations identified the sliding bubble layer as the key factor responsible for the observed cell voltage decrease under pressure. Based on these findings, potential avenues for reducing the energy cost of water electrolysis at near-neutral pH are discussed. • A water electrolysis cell was operated at near-neutral pH under various pressures • Increasing the pressure up to 7 bar reduced the cell voltage • Modelling showed that gas fraction increased as bubble size decreased • The bubbles dispersed in the electrolyte were not a controlling factor • The compact bubble layer against the electrode was the controlling step