Solvent-assisted thermal annealing of disulfonated poly(arylene ether sulfone) random copolymers for low humidity polymer electrolyte membrane fuel cells

Abstract

Fundamental studies of a new thermal annealing strategy for maximizing proton conductivity and single cell performance of glassy sulfonated hydrocarbon fuel cell membrane materials under both fully and partially hydrated conditions are reported. Directly copolymerized disulfonated poly(arylene ether sulfone) random copolymers (BisA-XX, XX is the mole percent of hydrophilic moieties, XX = 20 to 40) with 2,2′-isopropylidene diphenol swivel ((CH₃)₂–C) units in their polymer backbone were cast from N,N-dimethylacetamide (DMAc) and dried under two different protocols; one set was dried at 60 °C (BisA-XX_60 °C), and the other at 150 °C under vacuum immediately after the initial drying (BisA-XX_150 °C). Small amounts of DMAc solvent remained during the second drying step which depressed the glass transition temperatures (T_g) of the BisA copolymers lower than 150 °C. BisA-XX_150 °C samples were essentially thermally annealed when dried at 150 °C. This increased the density of BisA copolymer chains; their T_g values were increased. Moreover, T₁ values of the protons in BisA-XX aromatic phenylene rings measured via solid-state NMR technique were lower as a result of the improved ¹H–¹H dipolar interaction. Interestingly, proton conductivity was improved after thermal annealing, possibly because the sulfonic acid density in the fully hydrated state (i.e., IEC_v(wet)) was enhanced. The synergistic effect of reduced water uptake and more developed hydrophilic–hydrophobic nanochannel formation may be important. The thermal annealing significantly influenced proton conductivity particularly at a low humidity (30 to 80% relative humidity (RH)). As a result, BisA-XX_150 °C samples derived from identical materials exhibited electrochemical polymer electrolyte membrane fuel cell (PEMFC) performances similar and superior to Nafion^® 112 and BisA-XX_60 °C samples measured at 65% RH at 80 °C, respectively.