Functionalized clay nanocomposites for enhanced proton conductivity in polyether-based fuel cell membranes

This review critically examines the recent progress in the development of sulfonated polyether sulfone (SPES) and its derivatives (including sulfonated polyether sulfone with open sulfonic acid groups (SPESOS)) as functional matrices for next-generation proton exchange membrane (PEM) fuel cells. SPES-based membranes offer a tunable backbone, excellent thermal and mechanical stability, and chemical resistance, yet they often suffer from reduced proton conductivity and hydration stability under low-humidity or high-temperature conditions. To overcome these limitations, the integration of functionalized clay nanomaterials such as montmorillonite (Mt), layered double hydroxides (LDHs), and sepiolite (Sep) has emerged as a promising strategy. We discuss how these clays improve ionic domain morphology, water uptake, and mechanical reinforcement through molecular-level interactions with sulfonic acid sites. The review highlights scalable fabrication routes (e.g., solution casting, electrospinning, additive manufacturing), performance under realistic operating conditions, and comparisons with commercial Nafion membranes. Particular attention is given to structure property relationships, transport mechanisms, and interfacial engineering strategies that enable defect-free, high-conductivity membranes. Finally, challenges and future perspectives are addressed to guide sustainable, cost-effective membrane design for advanced electrochemical energy systems.