Sulfonated aromatic polymers containing hexafluoroisopropylidene groups: a simple but effective structure for fuel cell membranes

Abstract

Partially fluorinated and sulfonated aromatic copolymers (SBAFs) were designed and synthesized as chemically stable, highly proton conductive membranes for fuel cell applications. The effect of hexafluoroisopropylidene groups on the membrane properties was investigated in detail by comparing with those of our previous sulfonated polyphenylene ionomer (SPP-QP) membranes. SBAFs were obtained by a Ni(0)-promoted coupling reaction with high molecular weight (M_w > 158 kDa and M_n > 42 kDa) to provide thin membranes with three different ion exchange capacities (IECs), 1.5, 2.5, and 3.0 mequiv. g⁻¹. SBAF membranes showed higher water uptake and proton conductivity than those of SPP-QP membranes with comparable IEC values. The stress/strain curves and DMAs revealed that SBAF and SPP-QP membranes were similarly mechanically stable. The chemical stability evaluated by Fenton's test suggested that isopropylidene groups were intact under harsh oxidative conditions. In an H₂/O₂ fuel cell, SBAF membranes exhibited better performance (in particular, at low humidity) and durability than those of SPP-QP membranes. During an open circuit voltage (OCV) hold test, the SBAF fuel cell retained a high OCV value over 0.9 V with minor loss up to 1000 h (the average decay was −40 μV h⁻¹).