Side chain engineered ether-free polybenzimidazole membranes with enhanced proton transport and stability for PEM water electrolysis

Abstract

Polymer electrolyte membranes used in proton exchange membrane water electrolysis (PEMWE) must simultaneously exhibit high proton conductivity, chemical durability, and structural stability under strongly oxidative operating conditions. However, many hydrocarbon-based membranes contain ether linkages that are susceptible to radical-induced degradation, limiting their long-term stability. Herein, an ether-free polybenzimidazole (PBI) framework containing fluorinated hexafluoroisopropylidene units was engineered to improve oxidative robustness while maintaining membrane processability. To enhance proton transport, the polymer backbone was functionalized via side-chain grafting of 1,4-butane sultone, introducing sulfonated hydrophilic domains capable of forming hydrogen bonded water network for proton-conduction. The resulting membranes (FPBI-g-BS-x) were systematically investigated in terms of physicochemical, thermal, and electrochemical properties. Among the fabricated membranes, FPBI-g-BS-43 exhibited the highest ion-exchange capacity (2.28 ± 0.21 meq g⁻¹), water uptake (40.6 ± 2.08), and proton conductivity (5.22 ± 0.54 mS cm⁻¹ at 40 °C). Morphological studies revealed well-developed nanophase separation with interconnected ionic clusters facilitating proton transport. Importantly, the ether-free PBI backbone demonstrated excellent thermal stability (>400 °C) and strong resistance to oxidative degradation, highlighting the structural advantage of this architecture. In a single-cell PEM water electrolyzer operating at 80 °C, FPBI-g-BS-43 delivered a current density of ∼649 mA cm⁻² at 1.8 V, outperforming Nafion® 117 under identical conditions. These results demonstrate that side-chain engineering of ether-free PBI membranes provides an effective strategy to balance proton conductivity and chemical durability, offering an efficient hydrocarbon-based alternative to perfluorinated membranes for PEMWE applications.