While in practical application, polymer electrolyte membrane (PEM) fuel cells and electrolyzers exhibit macroscale length scales spanning centimeters to meters, the inner workings of these energy conversion devices fundamentally rely on microscale and nanoscale transport phenomena and porous materials to facilitate their passive operation. In order to maximize the power density and minimize the cost of the PEM fuel cell or electrolyzer, we must optimize our surface area to volume ratios at the reaction sites; thus, state-of-the-art catalysts exhibit diameters on the order of nanometers. At the same time, we must consider the effective delivery of reactants to these small- catalyst sites, and porous materials provide a means to impart homogeneous distributions of reactants, in theory. In addition to laterally distributing reactants across the planar catalyst layer, porous materials also facilitate multiple pathways for the competing reactant and product pathways to co-exist.

The PEM fuel cell and electrolyzer are composed of multiple porous materials, including the catalyst layer, microporous layer, and substrate. Commercial materials, whether by design or not, typically exhibit highly heterogeneous material and chemical properties. For example, in our past work, we extensively examined the heterogeneous microstructure and hydrophobicity of the gas diffusion layer (GDL). In order to reach cost targets for widespread commercial adoption, we must realize materials that enable more effective multiphase flow phenomena than what currently exists. Mass transport losses in PEM fuel cells and electrolyzers are both prohibitively significant. However, designing these materials requires the a priori knowledge of how the heterogeneous properties of the porous materials and their interfacial contacts influence electrochemical performance. These are factors that are currently not fully understood in the literature. In this work, I will discuss these critical design factors (heterogeneous porous materials and nature of interfacial contacts) and how they influence the flow and mass transport behaviour in PEM fuel cells and electrolyzers. I will also discuss the new materials we have designed and fabricated, informed by in-house numerical modelling and tested through a combination of in operando and ex situ X-ray and neutron beam characterization approaches.