Advances in proton exchange membranes for wide-temperature-range fuel cells

Proton exchange membranes (PEMs) play a central role in determining the efficiency, durability, and operational flexibility of PEM fuel cells (PEMFCs). However, conventional PEMs exhibit strong temperature-dependent proton-transport behavior, which limits their ability to support both rapid start-up at low temperatures and stable operation at elevated temperatures. Water-mediated PEMs show excellent conductivity under low-temperature and high-humidity conditions but suffer from dehydration and structural instability in the high-temperature regime. In contrast, water-independent PEMs, particularly phosphoric-acid-doped systems, conduct protons efficiently under anhydrous high-temperature conditions yet experience acid leaching that hampers room-temperature start-up and long-term durability. This review summarizes the fundamental proton-transport mechanisms that govern temperature-dependent performance and discusses recent advances in materials design aimed at enabling wide-temperature-range PEM operation. For water-mediated membranes, strategies such as incorporating hydrophilic fillers, constructing confined hydrophilic domains, and introducing additional proton-transfer sites have been developed to mitigate water loss and stabilize proton conduction. For water-independent membranes, approaches including strengthening polymer–acid interactions, engineering nanoscale confinement, designing multilayer architectures, and constructing multi–proton-carrier networks effectively improve acid retention and broaden operational temperature windows. Emerging fixed-carrier systems based on phosphonic-acid-grafted polymers, metal–organic frameworks, and covalent organic frameworks offer new pathways for stable anhydrous proton conduction across a wide temperature range. We conclude by outlining key challenges and future research opportunities, including reducing the dependence on volatile or leachable proton carriers, developing adaptive nanochannel architectures, improving anhydrous high-temperature conduction, and establishing scalable membrane fabrication methods. Continued innovation in these directions is expected to enable next-generation wide-temperature-range PEMs capable of flexible, high-efficiency operation from sub-zero to high-temperature conditions.