Effect of crosslinked chain length in sulfonated polyimide membranes on water sorption, proton conduction, and methanol permeation properties

Crosslinked sulfonated polyimide (XSPI) membranes were synthesized using various crosslinkers (HO–(CH2)n–OH, n = 2–6, 10) with different chain lengths to investigate the effects of crosslinking on water uptake, proton conductivity, and methanol permeability for both polymer electrolyte membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). Differing from a general expectation that crosslinking leads to the reduction of proton conductivity and methanol permeability, XSPI fabricated using longer crosslinkers showed higher proton conductivity (σ = 9.3 × 10−2 S cm−1) and lower methanol permeability (PMeOH = 2.0 × 10−7 cm3 cm cm−2 s−1) in comparison with non-crosslinked SPI membrane (σ = 6.2 × 10−2 S cm−1, and PMeOH = 1.1 × 10−6 cm3 cm cm−2 s−1). Crosslinking between polymer chains also resulted in improved membrane resistances to hydrolytic attack (40–50 days) in deionized water at 80 °C and free radical attack (33–42 h) in Fenton's solution as compared to non-crosslinked SPI having weak durability (1 day and 23 h, respectively). These properties and performances were found to be strongly related with the chain length of the crosslinker, but relationship between chain length and these properties was not linearly dependent. Interestingly, crosslinker with chain length of n = 5–6 was desirable considering overall membrane performances such as proton conductivity, methanol barrier property, and membrane durability. The electrochemical performances using a membrane electrode assembly (MEA) based on the XSPI with crosslinker size of n = 5 (900 mA cm−2 at 0.6 V in PEMFC, and 130 mA cm−2 at 0.4 V in DMFC) were similar or superior to those of Nafion 117 (800 mA cm−2 in PEMFC, and 125 mA cm−2 in DMFC) and non-crosslinked SPI membrane (488 mA cm−2 in PEMFC, and 25 mA cm−2 in DMFC) under the same measurement conditions.