Addressing Mass Transport Limitation and Gas Crossover Behavior in Zero-Gap Porous Separator Configurations for Alkaline Water Electrolysis

Traditional alkaline water electrolysis (AWE) is an established technology for green hydrogen production but suffers from high overpotentials and balance of plant costs. Efforts to allow high current densities have included a zero-gap design but so far neglected the parallel advancements in related membrane electrolysis technologies. This study therefore employs a 5 cm² zero-gap AWE setup with a porous diaphragm, nanostructured electrodes, and mild electrolyte concentrations of 1 M KOH. Initially, the system is bound to low current densities by mass transport limitation. By variation of the compression and comparison of different porous transport layers (PTLs), a structure-performance relationship is developed. The gas purity and attained hydrogen flux at the cathode are analyzed simultaneously. Configurations with low overpotentials are identified to commonly bear high crossover rates so that optimization toward efficient hydrogen production can be achieved only when electrochemical and gas crossover analyses are paired. A potential solution is found when the path of the liquid electrolyte within the cell is modified. By forcing convective transport through the PTL, the gas bubbles are removed efficiently. Ultimately, the system is able to reach current densities above 2.5 A cm^-2 at 2.3 V while keeping adequate gas purity.