High-performance imidazole-containing polymers for applications in high temperature polymer electrolyte membrane fuel cells

This work focuses on the development of high temperature polymer electrolyte membranes (HT-PEMs) as key materials for HT-PEM fuel cells (HT-PEMFCs). Recognizing the challenges associated with the phosphoric acid (PA) doped polybenzimidazole (PBI) membranes, including the use of carcinogenic monomers and complex synthesis procedures, this study aims to develop more cost-effective, readily synthesized, and high-performance alternatives. A series of superacid-catalyzed polyhydroxyalkylation reactions have been carefully designed between p-terphenyl and aldehydes bearing imidazole moieties, resulting in a new class of HT-PEMs. It is found that the chemical structure of aldehyde-substituted N-heterocycles significantly impacts the polymerization reaction. Specifically, the use of 1-methyl-2-imidazole-formaldehyde and 1H-imidazole-4-formaldehyde monomers leads to the formation of high-viscosity, rigid, and ether-free polymers, denoted as PTIm-a and PTIm-b. Membranes fabricated from these polymers, due to their pendent imidazole groups, exhibit an exceptional capacity for PA absorption. Notably, PTIm-a, carrying methylimidazole moieties, demonstrates a superior chemical stability by maintaining morphology and structural stability during 350 h of Fenton testing. After being immersed in 75 wt% PA at 40 °C, the PTIm-a membrane reaches a PA content of 152%, maintains a good tensile strength of 13.6 MPa, and exhibits a moderate conductivity of 50.2 mS cm⁻¹ at 180 °C. Under H₂/O₂ operational conditions, a single cell based on the PTIm-a membrane attains a peak power density of 732 mW cm⁻² at 180 °C without backpressure. Furthermore, the membrane demonstrates stable cycle stability over 173 h within 18 days at a current density of 200 mA cm⁻², indicating its potential for practical application in HT-PEMFCs. This work highlights innovative strategies for the synthesis of advanced HT-PEMs, offering significant improvements in membrane properties and fuel cell performance, thus expanding the horizons of HT-PEMFC technology.