Efficient ether-free poly(p-terphenyl-isatin-dimethylfluorene) for proton exchange membrane water electrolysis

Proton exchange membrane (PEM) is an important component that affects the cost and cell performance of PEM water electrolysis (PEMWE). In this study, a series of ether-free poly(p-terphenyl-isatin- dimethylfluorene) (PID-x) polymers were synthesized by superacid-catalyzed Friedel-Crafts alkylation. The transparent PEMs of sulfonated poly(p-terphenyl-isatin- dimethylfluorene) (SPID-x) were prepared by the solution casting method. Introduction of dimethylfluorene groups enhances proton conductivity of PEMs while also preserving their high dimensional stability. The results indicate that the synthesized PID-x has excellent solubility in non-protic polar solvents. SPID-5, prepared with 5 % dimethylfluorene in the backbone structure exhibits a high conductivity of 0.176 S·cm⁻¹ at 80 °C (compared to 0.155 S·cm⁻¹ at 80 °C for Nafion115) and excellent dimensional stability, with a swelling ratio of only 10 % at 80 °C (compared to 20 % at 80 °C for Nafion115). In the water electrolysis cell performance test, SPID-25 achieved a current density of 167 mA·cm⁻² at 2.5 V, outperforming the commercial Nafion 211 (151 mA·cm⁻²) at a similar membrane thickness.