Lignin-Derived Sustainable Cationic Polymers for Efficient High-Temperature Proton Exchange Membrane Fuel Cells

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) use phosphoric acid (PA)-doped poly(benzimidazole) (PBI) membranes for proton transport. However, the weak acid–base interaction between PA and PBI leads to PA leaching, which lowers the proton conductivity of the HT-PEMFCs. A promising solution could be introducing an ion-pair interaction to retain PA within the membranes. Achieving stable, high proton conductivity and stability with affordable and sustainable cationic polymers could significantly boost performance while reducing costs. Keeping this in mind, we designed an eco-friendly, cost-effective cationic compound (QAL) from Kraft lignin and incorporated it into PBI membranes. The ion-pair interaction between QAL’s cationic groups and PA’s phosphate anions, stronger than traditional acid–based interactions, increased PA uptake from 377 to 503 wt %. Consequently, the PBI–QAL membranes exhibited 50% higher proton conductivity (264 mS/cm) than pure PBI membranes (174 mS/cm) at 200 °C. Furthermore, the membrane conductivity remained consistently high during a 10 day test at 130 °C, addressing stability concerns. QAL also improved the elastic modulus and enhanced membrane chemical stability through lignin’s radical scavenging capabilities. This highlights lignin’s potential as a promising alternative to synthetic, petroleum-derived cationic materials in HT-PEMFCs, fostering the development of efficient, sustainable, and eco-friendly electrochemical devices leveraging green materials.