The effect of polymer backbones and cation functional groups on properties of anion exchange membranes for fuel cells

The anion conductive polymers based on the aryl ether-free polyaromatics (poly[(fluorene alkylene)-co-(biphenyl alkylene)] (PFBA), poly(fluorene alkylene) (PFA)) and aryl ether-containing polyaromatics (poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)) having various cation groups (quaternary ammonium (QA) or bulky imidazolium (MIm or PIm)) were designed and prepared to investigate the effect of polymer backbones and cation functional groups on the properties of anion exchange membranes (AEMs) in fuel cells. The PFBA AEM having bulky imidazolium group showed good mechanical properties while the PPO AEM with the same cation break into the pieces. The alkalinity of the bulky imidazoliums have determined to be weaker than that of the typical quaternary ammonium by the simulation. Thus, the high hydroxide conductivities of the AEMs based on the quaternary ammonium group have been observed. Although the alkaline stability testing in 2 mol L−1 NaOH at 80 °C indicated the AEMs based on the PFBA backbone are much more alkaline stable than that of the AEMs from PPO backbone, the PFA-QA having typical quaternary ammonium showed significant degradation signals in 2 mol L−1 NaOD CD3OD/D2O solution at 80 °C for 15 days while PFA-MIm with bulky imidazolium groups showed no degradation signal at the same testing condition. The single H2/O2 fuel cell testing at 60 °C suggested the polymer backbone and cations displayed significant effect on the fuel cell performance. The highest peak power density of 610 mW cm−2 was achieved for PFBA-QA-0.4 which is approximate twice as high as that of the PPO-QA-0.4 (351 mW cm−2) and also the PFBA-MIm-0.4 having bulky imidazolium (32 mW cm−2). The device durability test indicates that the voltage of PFBA-QA-0.4 membrane could be maintained above 0.7 V at 200 mA cm−2 for 74 h. These results suggested that the aryl ether-free polymer backbone would be a good choice for the high performance AEM designation, and the AEMs having alkaline stable bulky imidazoliums needed to be further investigated in detail to achieve excellent fuel cell performance.