Perfluorosulfonic acid (PFSA) based ionomers such as Nafion® are the technological standard for use in state-of-the-art proton exchange membrane water electrolysis (PEMWE) because of its high proton conductivity and excellent mechanical, chemical, and thermal stability. However, increasing environmental concerns with fluorinated polymers and safety issues associated with high gas crossover in PEMWE systems call for alternatives to PFSAs. A wholly hydrocarbon-based PEM sulfonated phenylated polyphenylene biphenyl (sPPB-H+), which has already been employed in proton exchange membrane fuel cell (PEMFC) systems is considered a candidate material for PEMWE as it exhibits high proton conductivity and mechanical robustness. The validation of sPPB-H+ as a membrane and ionomer applied in the catalyst layer in membrane electrode assembly (MEA) structures is evaluated in an electrolyzer cell. When using Nafion D520TM in the catalyst layer, a sPPB-H+ membrane yields better energy efficiency than a reference NafionTM112 membrane. The enhanced energy efficiency is attributed to significantly lower ohmic resistance. The use of sPPB-H+ as ionomer is also investigated, where maximum efficiency is achieved by determining the optimal ionomer content. The stability of MEAs in electrolyzers is studied by monitoring the voltage change at a constant current density. Degradation mechanisms of MEAs are elucidated using polarization curves, EIS and hydrogen gas crossover measurements. Initially, sPPB-H+ membranes yield substantial lower gas crossover compared to the Nafion 112 membrane. As PPB-H+ is prone to radical attack, the membrane develops pinholes allowing increasing gas crossover. The rapid performance decay (i.e., much higher voltage evolution rate) of wholly hydrocarbon sPPB-H+ based MEAs is initially caused by severe catalyst loss and increasing gas crossover due to thinning of the catalyst layers. The results presented here demonstrate sPPB-H+ as a promising material for application in water electrolysis and suggest future research should focus on mitigating chemical degradation and reducing dimensional swelling of the membrane in order to enhance its operational stability.
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Thesis advisor: Holdcroft, Steven
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