Mitigating phosphoric acid migration in high temperature polymer electrolyte membrane fuel cells with hydrophobic polysilsesquioxane-based binders

Cross-linkable organosilsesquioxanes were synthesized for application as catalyst binders in high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Four different organic functional groups were examined including methyl, phenyl, fluoroalkyl, and fluorophenyl and their chemical, physical, surface, and electrochemical properties were characterized. The effect of surface hydrophobicity on a HT-PEMFC membrane electrode assembly was elucidated, showing that organosilsesquioxanes with lower surface tension or higher hydrophobicity towards water and phosphoric acid could be considered as a key parameter for HT-PEMFC performance. Fuel cell tests showed that the pentafluorophenyl-functionalized organosilsesquioxane showed improved H₂/air performance (a peak power density of 527 mW cm⁻² at 0.4 V) compared to the MEA with PTFE (a peak power density of 425 mW cm⁻² at 0.4 V). Short term durability tests for 500 h showed that membrane electrode assemblies with alternative binders were stable and the developed organosilsesquioxane binders are a viable alternative to PTFE-based binders, all the while having additional advantages in vastly simplified ink slurry preparation through increased dispersibility in alcohol-water mixtures.