Modeling of Pore-Scale Capillary-Dominated Flow and Bubble Detachment in PEM Water Electrolyzer Anodes Using the Volume of Fluid Method

Fluid dynamics models complement expensive experiments with limited measurement accuracy that investigate the mass transport in PEM water electrolysis. Here, a first-principle microscale model for oxygen transport is successfully validated that accounts for (1) uncertain transport processes in catalyst layers, (2) numerically challenging capillary-dominated two-phase flow and (3) bubble detachments in channels. We developed algorithms for the stochastic generation of geometries and for the coupling of flow and transport processes. The flow model is based on the volume of fluid method and reproduces experimentally measured pressure drops and bubble velocities within minichannels with a 30% and 20% accuracy, respectively, provided that the capillary number is above 2.1 x 10-7. At lower capillary numbers, excessive spurious currents occur. Correspondingly, two-phase flow simulations within the porous transport layers are stable at current densities above 0.5 A cm-2 and match operando gas saturation measurements within a 20% margin at relevant locations. The simulated bubble detachments occur at pore throats that agree with porosimetry and microfluidic experiments. The presented model allows explaining and optimizing mass transport processes in channels and porous transport layers. These were found to be negligibly sensitive to transport resistances within the catalyst layer, providing information on boundary conditions for future catalyst layer models.