Conventional proton exchange membrane fuel cells (PEMFCs) operate at a narrow temperature range, either under low temperature conditions (80-90°C) using fully-humidi ed per uorosulfonic acid (Na on®) membranes or under nonhumidi ed high temperature conditions (140-180°C) using phosphoric acid (PA)-doped membranes to avoid water condensation-induced PA leaching. To allow wide operational exibility over the full spectrum of temperature and humidity ranges, we present an innovative design strategy by using PA-doped intrinsically ultramicroporous membranes constructed from rigid and contorted high free volume polymers. The membranes with an average ultramicropore radius of 3.3 Å showed a signi cant siphoning effect as con rmed by the delocalization of PA in 31 P NMR, thus allowing high retention of PA even under highly humidi ed conditions and presenting more than three orders of magnitude higher proton conductivity retention than conventional dense PA-doped polybenzimidazole membranes (PBI/PA). The resulting PEMFCs display impressive performance over a much broader temperature range from − 20 to 200°C and can accomplish over 100 startup/shut-down cycles even at − 20°C. The broad operational exibility rendered from the high PA-retention can ultimately simplify heat and water management and thereby reduce PEMFC costs.

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