Permeability of water through polymer membranes and porous electrodes

Water sorption and transport through hydrocarbon (HC)-based polymer electrolyte membranes (PEMs) and porous electrodes are vital for water management and performance improvements of fuel cells. This thesis is the culmination of three research projects conducted into an extensive water sorption and transport study, including steady-state permeation, transient diffusion, and sorption isotherm on a series of novel HC-based PEMs. Numerical models, such as the Park model for sorption isotherm and the resistance model for steady-state permeation, were chosen and applied to interpret the membranes' chemical and structural features. Conductive atomic force microscope (AFM) and surface roughness measurement were applied to examine the membranes' physical properties. Collectively, transport measurements, numerical models, and characterizations were integrated to generate an insightful structure-transport correlation. The first project studies sulfonated phenylated poly(phenylene) biphenyl (SPPB) and compares it to a HC-based reference, sulfonated phenylated poly(arylene ether), and the commercial benchmark, Nafion. At thickness > ~30 μm, SPPB is the most permeable due to its lowest internal resistance coefficient. The second research involves four structurally controlled, one-element-variant, pyridyl-bearing sulfonated phenylated polyphenylenes. An increase in the number of pyridyl groups increases the fraction of neutralized protons in –SO3H groups, and decreases polymer's ion exchange capacity, proton conductivity, liquid and vaporous water sorption, dimensional swelling, steady-state water permeability, and transient diffusivity. The third investigation expands the research focus to the catalyst layers incorporated with the novel HC-based ionomers. A lower ionomer content of SPPB in the catalyst layer favors a larger water vapor uptake and faster transient diffusion rate. Specifically, the catalyst layer of 15 wt% ionomer SPPB is found possessing the best electrochemical performance with the most hydrophilic and the roughest surface. Insights obtained in this thesis can direct further tuning of the HC-based polymer's structure for desirable mass transport through both the membrane and the catalyst layers, which subsequently lead to electrochemical performance improvements of the fuel cell.