Poly((biphenyl)m-(aryl methylpiperidine)n-(dibenzothiophene)p)-Based Proton Exchange Membrane for High-Temperature Fuel Cell Application

Abstract

The development of proton exchange membranes (PEMs) with sustained performance above 180 °C remains a critical challenge for high-temperature PEM fuel cells (HT-PEMFC) due to the rapid degradation and phosphoric acid (PA) leaching of conventional PA-doped polybenzimidazole (PBI) membranes. Here, a terpolymer of poly((biphenyl)_m-(aryl methylpiperidine)_n-(dibenzothiophene)_p) (P-BTSAM) membrane is synthesized via superacid-catalyzed Friedel–Crafts polymerization using 4-N-methylpiperidone, dibenzothiophene (BTS), and biphenyl (BP). DBT substitution was optimized by adjusting the biphenyl monomer ratio, profoundly affecting the membranes’ physicochemical properties through spectroscopic and thermal characterization. The membranes showed exceptional thermal stability, retaining 60% of their weight above 500 °C and maintaining structural integrity over 380 °C. The thiophene ring-conjugated structure increased charge density, significantly enhancing phosphoric acid (PA) affinity and forming a hydrogen-bonded network that improved proton mobility. Proton conductivity measurements indicated that the P-BTSAM₂₀ membrane showed an impressive conductivity value of 9.2 × 10^–2 S cm^–1 at 180 °C. In H₂/O₂ HT-PEMFC tests at 180 °C, P-BTSAM₂₀ delivered a peak power density of 0.76 W/cm², surpassing commercial PA-doped PBI membranes (0.52 W/cm²). The membrane showcased durability over 160 h with a fixed current density of 0.4 A/cm² at 160 °C. These findings indicate that P-BTSAM₂₀ extends PEM operational limits beyond 180 °C while maintaining stability. Theoretical studies confirmed that the P-BTSAM structure possesses excellent chemical stability. Moreover, the straightforward synthesis positions this membrane as a practical alternative for high-temperature PEM fuel cell (HT-PEMFC) applications, overcoming performance and economic challenges to commercialization.