Innovative experimental design for investigating side chain structure regulation and battery performance of functionalized SPEEK proton exchange membranes

Vanadium redox flow batteries (VRFBs) have emerged as one of the most promising technologies for large-scale renewable energy storage owing to their high safety, long cycle life, and tunable energy capacity. The performance of the proton exchange membrane (PEM) plays a critical role in determining the energy efficiency and operational stability of VRFBs. Commercial Nafion membranes suffer from high cost and severe vanadium-ion permeability. Although sulfonated poly(ether ether ketone) (SPEEK) was a low-cost alternative, its performance is highly dependent on the degree of sulfonation. Increasing the degree of sulfonation enhances proton conductivity but inevitably increases water uptake and membrane swelling, leading to high vanadium permeability and reduced dimensional stability. To overcome this bottleneck, a side-chain covalent grafting strategy was proposed in this study. Different functional groups, including imidazole, benzimidazole, and phosphonic acid, were grafted onto the SPEEK backbone. Acid–base pairs or hydrogen bonding networks formed between the sulfonic acid groups on the polymer backbone and the grafted side-chain functional groups were utilized to construct non-water-dependent Grotthuss proton hopping pathways, thereby enhancing proton conductivity. Meanwhile, the rigid side chains suppressed membrane swelling through steric hindrance and improved chain packing, resulting in a synergistic enhancement of ion selectivity.

SPEEK with a sulfonation degree of 65% was first prepared via post-sulfonation, followed by activation with N,N^'-carbonyldiimidazole and amidation with imidazole-, benzimidazole-, and phosphonic acid–based precursors. The resulting modified membranes, designated mSPEEK, BSPEEK, and PSPEEK, were fabricated via solution casting. Successful side-chain grafting was confirmed using Fourier transform infrared and ¹H nuclear magnetic resonance spectroscopy, while membrane morphology was characterized using scanning electron microscopy. Water uptake, swelling ratio, proton conductivity (σ), vanadium permeability (P), and ion selectivity (σ/P) were systematically compared. Finally, single-cell VRFB performance was assessed by measuring charge–discharge efficiency at current densities of 60–200 mA·cm⁻² and cycling stability over 300 cycles at 150 mA·cm⁻².

Compared with the rough, layered, and fractured cross-section of pristine SPEEK, the modified membranes exhibited smoother and denser cross-sectional morphologies. Among them, BSPEEK exhibited the most uniform structure, and its swelling ratio decreased from 12.5% for pristine SPEEK to 10.9%. Regarding proton conductivity, BSPEEK achieved 24.8 mS·cm⁻¹, representing a 51% increase over pristine SPEEK (16.4 mS·cm⁻¹), while its vanadium permeability was only 20.8 × 10⁻⁷ cm²·min⁻¹, much lower than those of Nafion 212 (94.3 × 10⁻⁷ cm²·min⁻¹) and pure SPEEK (42.4 × 10⁻⁷ cm²·min⁻¹). Consequently, the ion selectivity of BSPEEK reached 11.9 × 10³ S min·cm⁻³, which was three times that of pristine SPEEK and four times that of Nafion 212. After 300 charge–discharge cycles at 150 mA·cm⁻², the BSPEEK membrane maintained an energy efficiency of above 70%, while its coulombic efficiency was consistently higher than 95%. By contrast, Nafion 212 failed after 200 cycles because of capacity decay.

These results demonstrate that the two nitrogen atoms in the benzimidazole side chain exhibit strong basicity and readily form stable acid–base pairs with the sulfonic acid groups of SPEEK. In addition, the rigid conjugated benzimidazole structure enhances interchain interactions through π–π stacking. Together, these effects synergistically construct efficient Grotthuss proton-hopping pathways while effectively suppressing membrane swelling, thereby simultaneously improving proton conductivity and ion selectivity. The BSPEEK membrane developed in this study outperformed commercial Nafion 212 while offering significantly lower material cost, providing a theoretical basis for the molecular design of low-cost, high-performance proton exchange membranes for VRFB applications.