Gas Diffusion Layers: Experimental and Modeling Approach for Morphological and Transport Properties

Abstract

ConspectusElectrochemical technologies are key to decarbonizing the energy sector. Electrification of the energy sector is underway with battery technologies dominating the light-duty electric vehicles market. It is more challenging to decarbonize historically difficult to decarbonize sectors, such as heavy-duty transportation, planes, ships, and the chemical manufacturing industry (ammonia, cement, steel). Green hydrogen produced via electrolysis will be used as a fuel and a feedstock in some of these processes. At the heart of the hydrogen economy are polymer electrolyte fuel cells (PEFCs), devices that convert hydrogen into electricity. Gas diffusion layers (GDLs) have an integral role in PEFCs, as they are porous carbon layers that transport reactants and products and also remove heat and conduct electricity. To improve the PEFCs’ performance and reduce degradation of materials, an understanding of coupled morphological properties and transport phenomena in the GDLs is needed. In this Account, we emphasize the integration of experimental and modeling approaches to achieve complete understanding of materials and transport properties of the GDLs. Our approach builds in complexity from simpler ex situ experiments to in situ and last to 3-D integrated modeling predictions. GDL morphology is complex, as its fabrication includes several stochastic steps (immersion of GDL in various baths to achieve the desired surface wettability) and only 3-D techniques, such as X-ray computed tomography can capture morphology correctly. Porosity, pore-size distribution, tortuosity, and formation factor are the most important morphological properties of the GDLs. For PEFC applications, water is generated in the catalyst layers and is transported through the GDLs. Therefore, GDL wettability directly impacts water permeability through the GDLs. Using in situ water injection experiments, we directly observe which pores water fill at what liquid pressure. This result provides information about the GDL’s affinity to intake water. GDLs are typically of mixed wettabilities, and internal wettability until recently has been unknown. Having images of water inside the GDL enabled us to track the triple-phase boundary at the fiber–water–air interface to obtain local contact angles in the locations where water was present. The percentage of contact angles that were hydrophilic correlated well to the percentage of surface oxides on the GDL surface using X-ray photoelectron spectroscopy (XPS). We envision many other groups using the method of XPS to determine internal surface wettability of the GDLs, as it is relatively fast. Heat transport and evaporation/condensation of water in the GDL is studied using in situ X-ray CT experiments. These provide direct insight into pore-scale water transport under thermal gradients. Three-dimensional geometries of GDLs are exported for transport simulations using the lattice Boltzmann method (LBM). Similarly, we advocate for building the LBM simulations, from water injection studies first to validate the model only to operando PEFC models later. LBM coupling with a continuum model enables a computational saving, allowing us to map local temperature, reactant, and product distributions in the GDLs.