Compressible-Gas Invasion into Liquid-Saturated Porous Media: Application to Polymer-Electrolyte-Membrane Electrolyzers

Understanding gas transport in liquid-saturated porous media is crucial for reducing mass transport-related inefficiencies in polymer-electrolyte-membrane (PEM) electrolyzers. While incompressible fluid-fluid displacement in porous media has been studied extensively, transport behavior with high compressibility effects remains poorly understood. Here, we investigate the impact of compressibility on gas transport in porous media via experiments in patterned micromodels. Macroscopically, we find that the displacement pattern follows the classical transition from capillary to viscous fingering as capillary number increases, despite the compressed state of the injected gas. Microscopically (i.e., pore scale), we find that the displacement occurs via discrete bursts in the form of Haines jumps. We demonstrate that in the presence of compressibility, the pore throat size exerts fundamental control over the burst velocity. Furthermore, we show that the inclusion of a thin, low-porosity region with small pore throats at the inlet of the micromodel increases the burst velocity of gas into the bulk of the micromodel, leading to significantly reduced gas saturation in the bulk. Our work provides a mechanistic explanation of the previously reported performance improvement due to the addition of microporous layers in PEM electrolyzers.