Properties and stability of quaternary ammonium-biphosphate ion-pair poly(sulfone)s high temperature proton exchange membranes for H2/O2 fuel cells

Quaternized poly(sulfone)s based on trimethylamine (PSf-TMA-x) and triethylamine (PSf-TEA-x) are developed to form quaternary ammonium (QA)-biphosphate ion pairs as promising high temperature proton exchange membrane (HT-PEM) in H-2/O-2 fuel cells. The strong QA-biphosphate ion-pair interaction results in high acid absorption and thus high proton conductivity of 78.1 mS cm(-1) for PSf-TEA-110/PA membrane at 160 degrees C in spite of its lower degree of functionalization. Thus, the peak power density of more than 400 mW cm(-2) is achieved at 160 degrees C for the quaternized PSf membranes in H-2/O-2 fuel cell, which is higher than that of the poly(benzeimidazole) (m-PBI) membrane (370 mW cm(-2)). Subsequently, the degradation mechanism of the PSf membranes is demonstrated using NMR technique to be occurred in both the polymer backbone and the QA groups. The stability investigation of model compounds further confirms that the degradation of the membranes resulted from the scission of ether linkage and isopropylidene group and the SN2 nucleophilic substitution of QA at operating conditions. Therefore, the significant performance decrease is observed and the lifetime is only less than 30 h at 200 mA cm(-2) which is much lower than that of m-PBI membrane (>40 h).