Examining the mass transport resistance of porous transport layers at the rib/channel scale in polymer electrolyte membrane water electrolyzers: Modeling and design

The porous transport layer (PTL) plays a relevant role in the efficiency of polymer electrolyte membrane water electrolyzers (PEMWE). Extraction of good design guidelines for this porous component is necessary for efficient water/oxygen transport. In this regard, numerical modeling provides a versatile tool to examine large parameter set and determine optimal PTL conditions to be verified experimentally. Here, a hybrid model is presented to analyze two-phase transport of oxygen and water in the anode PTL of a PEMWE. Oxygen capillary transport is modeled with a multi-cluster invasion-percolation algorithm, while water convective transport is modeled with a continuum formulation that incorporates the blockage of gas saturation. The model is validated against in-operando X-ray computed tomography data of the oxygen saturation distribution at the rib/channel scale. Subsequently, a comprehensive parametric analysis is presented, considering the following variables: ( $i$) PTL slenderness ratio, ($ii$) flow-field open area fraction, ($iii$) PTL isotropy, ($iv$) PTL average pore radius, and ($v$) PTL pore-size heterogeneity. Among other conclusions, the results show that the water transport resistance under the rib can lead to non-negligible mass transport losses at high current density. Water transport from the channel to the catalyst layer can be promoted by: ($i$) the use of PTLs with a slenderness ratio, defined as the PTL thickness to rib half-width ratio, around 0.5, ($ii$) the increase of the flow-field open area fraction, ($iii$) the design of highly anisotropic PTLs with a relatively large pore radius between $r_p\sim 10-40\mu m$, and ($iv$) increasing the homogeneity of the PTL microstructure.