The effect of grafted alkyl side chains on the properties of poly(terphenyl piperidinium) based high temperature proton exchange membranes

High temperature proton exchange membrane fuel cells (HT-PEMFCs) operating at elevated temperatures above 120 ℃ take advantage of feasible anode fuels and simplified water/heat management. A high temperature polymer electrolyte membrane (HT-PEM) is the core material for HT-PEMFCs. In this work, a series of phosphoric acid (PA) doped HT-PEMs based on poly(terphenyl piperidine) (PTP) tailored with alkyl groups are synthesized. Five different pendant alkyl groups (including methyl, propyl, pentyl, heptyl and decyl) are grafted onto the piperidine group through the Menshutkin reaction between PTP and alkyl halides. Compared with PTP and methyl grafted PTP (PTP-C1) membranes, the PTP-Cx membranes with long alkyl side chains exhibit improved PA doping contents and conductivities. The optimized pentyl-substituted PTP membrane (PTP-C5) possessed a reasonable PA doping content (202% after immersing in 85 wt% PA at 60 ℃), high proton conductivity (96 mS cm⁻¹ at 180 ℃) and good tensile strength (4.6 MPa at room temperature). A H₂–air single cell equipped with PTP-C5/PA consequently achieved a high peak power density of 676 mW cm⁻² at 210 ℃ without any humidification or backpressure. Thus, this work provides a simple method for preparing high-performance HT-PEMs.