The porous transport layer (PTL) is a key component in the polymer electrolyte membrane (PEM) electrolyzer for transporting reactant liquid water to the active sites while facilitating oxygen gas removal. Most current PTL designs use an expanded foam structure with uniform porosity. The interactions between the PTL and flow fields typically leads to non-uniform reactant and product distributions due to the necessarily open and closed regions (channels and lands in the flow fields). These non-uniform distributions hinder performance in the cell; therefore, structures tailored to particular flow field geometries would benefit the technology and be of significant interest to the field.

Mesh-type PTLs are promising for their ease of fabrication with controllable structures at low manufacturing costs, yet the nature of two-phase flow within the mesh-type PTLs is not well understood in the literature. Furthermore, mesh structures have not been optimized to be used with conventional flow fields. In this study, the effect of mesh structures on the mass transport properties of a PEM electrolyzer is investigated through customized porous transport layers and synchrotron X-ray imaging. Two porous transport layers were compared: a PTL with straight pores under the channel and the land and another PTL with straight pores under only the channel (Figure 1). The pores under the land were inaccessible for both liquid water and gas, and as a result, reactions were localised under the channel regions. Moreover, the presence of pores under the land region led to a reduced electrical contact area, thereby contributing to increased ohmic resistance. We observed that pores under the land resulted in higher mass transport resistances exhibited by an increase in gas volume in the pores under the channel. Overall, the PTL with pores only under the channel exhibited relatively superior electrochemical performance; thus, we recommend that mesh PTL should be designed with minimal pores under the land.

Figure 1: Schematic of the anode half of PEM electrolyzers installed with a mesh PTL with a) pores under the channel and land regions, and b) pores under only the channel region of the flow fields.