Chitosan-Based Proton Exchange Membranes for PEMFC and DMFC Applications: Recent Advances, Challenges, and Future Perspectives

Proton exchange or electrolyte membranes (PEMs) are vital parts of low-temperature fuel cells because they govern proton transport, fuel crossover, water management, and the mechanical and chemical durability of the membrane electrode assembly. Although perfluorosulfonic acid membranes, such as Nafion, remain widely employed, their dependence on fluorinated chemistry, hydration-sensitive conductivity, cost, and methanol crossover has stimulated interest in alternative membrane materials derived from renewable resources. Chitosan (CS), a naturally derived polysaccharide containing abundant amino and hydroxyl functional groups, has emerged as a promising platform material for the development of proton-conducting membranes for polymer electrolyte membrane fuel cells (PEMFCs) and direct-methanol fuel cells (DMFCs). This review critically examines recent advances in chitosan-based membranes, with emphasis on cross-linking, inorganic and carbon-based composite formation, incorporation of bio-derived fillers, polymer blending, and ionic liquid modification. The influence of these strategies on proton conductivity, methanol permeability, water absorption, dimensional stability, mechanical characteristics, thermal and chemical stability, and fuel cell performance is discussed in relation to membrane composition and structure. The reviewed studies demonstrate that targeted modification of chitosan can substantially improve individual membrane properties; however, achieving a simultaneous balance between proton transport, fuel-barrier characteristics, mechanical integrity, hydration, and long-term durability remains challenging. Variations in chitosan characteristics, membrane fabrication, testing conditions, and performance evaluation also hinder direct comparison among reported systems. Future research should therefore emphasize standardized characterization, reproducible membrane fabrication, membrane electrode assembly optimization, long-term single-cell testing, and life-cycle assessment. These developments are essential for translating chitosan-based PEMs from being laboratory-scale materials towards durable and practical fuel cell applications.