Highly Conductive and Ultra Alkaline Stable Anion Exchange Membranes by Superacid-Promoted Polycondensation for Fuel Cells

Abstract

A series of anion exchange membranes (4-QPPAF-TMA) were prepared by a metal-free, superacid-promoted polymerization reaction. The polymers were obtained with high molecular weight (Mn = 9.5–19.6 kDa, Mw = 44.9–622.5 kDa). 4-QPPAF-TMA membranes exhibited high hydroxide ion conductivity (up to 115 mS cm–1) at 80 °C, reasonable water absorbability (45% water uptake at 30 °C for 1.7 meq g–1), low to moderate dimensional swelling (5–15% at 30–80 °C for 1.7 meq g–1), and mechanical robustness (12.8 MPa maximum stress and 32% elongation at break for 1.7 meq g–1). Furthermore, 4-QPPAF-TMA membranes exhibited excellent alkaline stability in 8 M KOH at 80 °C for 1000 h, maintaining high conductivity (105 mS cm–1, 97% remaining). density functional theory (DFT) calculations suggested that the unique molecular configuration of the pendant ammonium head groups was responsible for high resistivity to the hydroxide ion attack. A fuel cell was operated with the 4-QPPAF-TMA membrane and an ionomer using a non-PGM cathode catalyst (Fe–N–C) to achieve a peak power density of 215 mW cm–2 accountable for 860 mW mg–1 Pt at a 590 mA cm–2 current density and 0.40 V (Pt–C cathode achieved 370 mW cm–1 at 810 mA cm–2 and 0.50 V). The fuel cell was operated at constant current density (15 mA cm–2) for 240 h with −0.79 mV h–1 average cell voltage decay. The postdurability analyses revealed that the membrane did not deteriorate while the degradation of the cathode catalysts/ionomer caused the performance loss.