Non-planar backbone structure polybenzimidazole membranes with excellent solubility, high proton conductivity, and better anti-oxidative for HT-PEMFCs

Abstract

As an efficient, inexpensive, readily accessible monomer of the polybenzimidazole (PBI) copolymer, the 4,4′-[(4,4′-bipyridine)-2,6-diyl]dibenzoic acid (BPY) containing bipyridine unit was firstly used to synthesize a series of novel non-planar PBI copolymers (BPY–PBI-x) by regulating the ratio with 2,6-pyridinedicarboxylic acid under microwave-assisted conditions. The copolymers exhibit superior solubility in aprotic solvents such as N,N-dimethylacetamide, N,N-dimethylformamide and dimethylsulfoxide than general PBI, and the corresponding BPY–PBI-x serial membranes were prepared by a solution casting method. All the membranes have been characterized by thermal stability, a tension test, scanning electron microscopy, PA-doping ability and swelling ratio, proton conductivity, and a Fenton test. The membranes exhibit excellent comprehensive properties containing high proton conductivity and long lifetime. The conductivity of the doping levels of the BPY–PBI-100% membrane with the highest doping level (342.7%) reaches 78.6 mS cm⁻¹ at 160 °C. The high proton conductivity is attributed to the increase in N atoms of the proton acceptor from the bipyridine monomer and the enhancement of free volume from the existence of a bulky stereostructure BPY unit. The long lifetime is due to the fact that the pyridine unit can quench radicals by the formation of pyridine-N-oxide. The unique anti-oxidation mechanism of the BPY–PBI-x serial membranes, which firstly was pointed out for the application of pyridine heterocyclic rings in HT-PEMs, provides new insights for novel anti-oxidation membrane design. In short, 3-fold objectives, namely, (I) soluble PBI, (II) higher proton conductivity PBI, and (III) anti-oxidative PBI, were achieved in this paper.