Sulfonated Terpolymers Containing Alkylene and Perfluoroalkylene Groups: Effect of Aliphatic Groups on Membrane Properties and Interface with the Catalyst Layers

Two series of terpolymers (SPA-A, -B, and -C) composed of perfluoroalkylene, alkylene, and sulfonated phenylene (SP) groups were prepared to investigate the effect of aliphatic groups and their compositions onto the membrane properties for proton exchange membrane fuel cells. The composition of perfluoroalkylene (m) and alkylene groups (n) in SPA terpolymers was controlled to be m/n = 0.5/0.5 for SPA-A, 0.65/0.35 for SPA-B, and 1/0 for SPA-C. SPA terpolymers provided thin and bendable membranes with ion exchange capacity (IEC) ranging from 1.61 to 3.18 mequiv g^-1. Introducing alkylene groups into the polymer main chain was effective in achieving high IEC values. SPA-B membranes with lower alkylene group content showed slightly more developed phase-separated morphology than that of SPA-A membranes with higher alkylene group content. The developed phase separation with interconnected ionic channels resulted in high proton conductivity for SPA-B membranes. The alkylene groups in the main chain also contributed to improving mechanical properties as suggested by stress versus strain curves, in which SPA membranes exhibited higher Young's moduli and higher yield strength than those of copolymer (SPA-C) membranes with no alkylene groups. An H₂/O₂ (or air) fuel cell with SPA-B membrane exhibited high open circuit voltage (OCV, 0.99 V at 100% RH with O₂), low ohmic resistance (0.05 cm² at 100% RH with O₂), and good current/voltage performance, reflecting the properties of SPA-B membrane. However, interfacial compatibility with the catalyst layers was somewhat deteriorated with SPA-B membrane to cause lower mass activity (70 A g^-1 at 100% RH) of the cathode compared to that with SPAF membrane (102 A g^-1 at 100% RH). SPA-B membrane was durable in OCV hold test for 1000 h with slight degradation in alkylene groups.