The influence of comonomer structure on properties of poly(aromatic pyridine) copolymer membranes for HT-PEMFCs

The development of high temperature proton exchange membranes (HT-PEMs) is crucial for the advancement of high temperature proton exchange membrane fuel cells (HT-PEMFCs). The conventional use of phosphoric acid (PA) doped polybenzimidazole (PBI) membranes in HT-PEMFCs is associated with challenges such as a demanding synthetic procedure and the need for carcinogenic monomer. Therefore, the quest for cost-effective and readily synthesizable HT-PEMs with satisfactory physicochemical properties and commendable fuel cell performance is of great importance. In this work, a series of pyridine-containing aromatic copolymers are synthesized using a straightforward Friedel-Crafts hydroxyalkylation reactions. Five different aryl derivates including biphenyl, m-terphenyl, dibenzo-18-crown-6, 9,9-dimethylfluorene, 1,2-diphenylethane are adopted as the comonomers along with p-terphenyl to copolymerize with 4-acetylpyridine. Thus, the effects of different backbone structures with rigid, flexible or bulky moieties on the resulting membrane properties is investigated systematically. The PA absorption capacity and conductivity of the copolymer membranes increases, attributed to the introduction of twisted, hydrophilic or bulky moieties in the copolymer structures. One of the membranes, the P(TP-co-DMF) membrane with 9,9-dimethylfluorene groups, achieve a good PA uptake of 194% when immersed in 85 wt% PA at 30 °C, which also exhibit a moderate tensile strength of 6.5 MPa and a high conductivity of 0.075 S cm−1 at 180 °C. Consequently, when tested in the H2–O2 fuel cell under 180 °C and no backpressure condition, the P(TP-co-DMF)/194%PA membrane delivers a high peak power density of 947 mW cm−2. This performance highlights the significant potential of these copolymers for application in HT-PEMFCs.