Stable branched polybenzimidazole high temperature proton exchange membrane: Crosslinking and pentaphosphonic-acid doping lower fuel permeability and enhanced proton transport

Branched polybenzimidazole (PBI) derivatives with large free volume and deformation rate exhibit unexpected properties in high temperature proton exchange membranes (HTPEMs) applications. However, the weak chain entanglement of branched structure implies poor membrane-forming performance. To effectively improve the comprehensive performance of HTPEMs, a branched tqPBI polymer is prepared by direct polymerization of mPBI polymers with tetra-acids (TCAQ), and crosslinked interpenetrating networks are constructed by thermal crosslinking with brominated polyimide (dBPEI) crosslinker. The networks can completely wrap the synthesized cerium pentaphosphonic acid (CePMP) by ionic and hydrogen bonds to achieve high doping levels of proton conductors. Meanwhile, the good mechanical property, chemical stability, and dimensional stability of tqPBI-dBPEI/CePMP membranes are acceptable. The permeability coefficient of tqPBI-dBPEI/CePMP membranes is 1–2 orders of magnitude lower for gas (H2 & O2) and liquid (methanol) phase fuel than Nafion membrane. At 180 °C, the proton conductivity of tqPBI-dBPEI(8)/CePMP(40) is 0.143–0.048 S/cm from 100% RH to anhydrous conditions. More importantly, after the proton-conducting durability test for 96 h, the conductivity of tqPBI-dBPEI(8)/CePMP(40) decayed by 1.9% at 100% and 1.0% at anhydrous conditions. The results demonstrate the effectiveness of crosslinking and doping strategy in the branched membrane, which is suitable for HTPEMS applications.