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, ( i i ) flow-field open area fraction, ( i i i ) PTL isotropy, ( i v ) 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, ( i i ) the increase of the flow-field open area fraction, ( i i i ) the design of highly anisotropic PTLs with a relatively large pore radius between r p ∼ 10 − 40 μ m , and ( i v ) increasing the homogeneity of the PTL microstructure. • A hybrid two-phase model of mass transport in the anode PTL of a PEMWE is presented. • Gas saturation distribution is validated against in-operando X-ray tomography data. • An optimal thickness to half-width rib exists for mass transport under the rib. • A higher flow-field opening area fraction promotes water transport to the catalyst layer. • PTL anisotropy, moderate pore radius and homogeneity further promote transport under the rib.