Ladder-type sulfonated poly(arylene perfluoroalkylene)s for high performance proton exchange membrane fuel cells†
Sulfonated poly(arylene perfluoroalkylene)s containing a sulfone-bonded ladder structure (SPAF-P-Lad) were synthesized by treating the precursor SPAF-P polymers with oleum as a novel proton exchange membrane for fuel cells. SPAF-P-Lad membranes had excellent solubility in polar organic solvents and high molecular weight (Mn = 145.4–162.9 kDa, Mw = 356.9–399.1 kDa) to provide bendable membranes with ion exchange capacity (IEC) ranging from 1.76 to 2.01 meq. g−1. SPAF-P-Lad membranes possessed higher proton conductivity than that of the precursor SPAF-P membranes because of the stronger water affinity. Compared with SPAF-P membranes (Tg: 72–90 °C, Young's modulus: 0.08–0.42 GPa; yield stress: 5.7–15.1 MPa), SPAF-P-Lad membranes showed better mechanical stability to humidity and temperature and improved tensile properties (Young's modulus: 0.51–0.59 GPa; yield stress: 23.9–29.6 MPa). The selected membrane, SPAF-mP-Lad, exhibited improved fuel cell performance, in particular, under low humidity with air; the current density at 0.5 V was 0.56 A cm−2, while that for SPAF-pP was 0.46 A cm−2. The SPAF-mP-Lad membrane endured an open circuit voltage hold test for 1000 h with average decay of as small as 70 μV h−1. A series of post-analyses including current–voltage characteristics, molecular structure, molecular weight, and IEC suggested very minor degradation of the membrane under the accelerated testing conditions.