Novel guanidinium functionalized poly(pentafluorostyrene): Synthesis and application as ion-pair membrane in PA doped HT-PEMFC

Anion exchange membranes based on poly(pentafluorostyrene) (PPFSt) functionalized with tetramethylguanidinium are described in this study. By incorporating flexible thiohexyl groups onto PPFSt followed by functionalization with tetramethylguanidine and its quaternization with dimethylsulfate, free standing anion exchange membranes were fabricated. The resulting membranes were doped in phosphoric acid (PA) and applied for high temperature proton exchange membrane fuel cells (HT-PEMFCs). The 60 % thiohexylated and 40 % tetramethylguanidinium-functionalized PPFSt membrane (M-PPFSt-TH-TMG) showed higher phosphoric acid doping level (ADL) than meta-PBI (m-PBI) membrane: 13.5 PA/guanidinium to 4.9 PA/imidazole respectively. Conductivity of the M-PPFSt-TH-TMG membrane displayed 322 mS cm⁻¹ (ADL 13.5) and is therefore higher than the one of m-PBI membrane showing 203 mS cm⁻¹ (ADL 4.5) at 160 °C. The fuel cell performance was measured at 160 °C with non-humidified gases and without back pressure on both anode and cathode sides and compared with commercial MEA (Celtec-P 1100W). A PA doped M-PPFSt-TH-TMG MEA showed higher performance than the commercial one exhibiting 793 and 656 mW cm⁻² respectively measured under the same condition. The highest performance was obtained with the membrane M-PPFSt-TH-TMG: this membrane showed a peak power density of 1.31 W cm⁻² in H₂/O₂ FC at 160 °C with 2 bars of backpressure on both anode and cathode sides. A stability test in FC showed no significant decay running at a constant current density of 217 mA cm⁻² at 160 °C with non-humidified gases for 100 h. An accelerated stress test (AST) carried out via thermal cycling between 80 and 160 °C with humidified gases showed rapid decay over the first 20 cycles, followed by stabilization.