Perfluorosulfonic acid proton exchange membrane with double proton site side chain for high-performance fuel cells at low humidity

Structural optimization of ionomers is an effective strategy for achieving high-performance proton exchange membranes (PEMs) under low relative humidity (RH) conditions. In this study, sulfonimide group and trifluoromethanesulfonate acid (TFSA) ionic liquids were introduced to the perfluorosulfonic acid (PFSA) side chain, resulting in polymer membranes with varying chain lengths (i.e., PFC2-TF-SI, PFC4-TF-SI, and PFC5-TF-SI). This dual proton-conducting structure extended the length of the hydrophilic side chain and enhanced the hydrophobic-hydrophilic phase separation, aiding in the formation of proton transport channels. Notably, the proton conductivity of PFC5-TF-SI and PFC2-TF-SI membranes reached 7.1 and 10.6 mS/cm at 30% RH and 80 °C, respectively, which were approximately 29.1% and 92.7% higher than that of the pristine PFC5-SA membrane (5.5 mS/cm). Furthermore, the maximum power density of the PFC5-TF-SI and PFC2-TF-SI membranes from the built single fuel cell achieved 649 and 763 mW/cm2 at 30% RH and 80 °C, respectively, which were higher than that of the pristine PFC5-SA membrane (567 mW/cm2) by about 14.5% and 34.6%, respectively. Thus, this study provides a strategy for PEM design under low RH conditions.