Fluorene-Reinforced Polyamide Hybrid Proton Exchange Membranes with Ultralow Methanol Permeability and High Selectivity for Direct Methanol Fuel Cells

Proton exchange membranes (PEMs) for direct methanol fuel cells (DMFCs) must simultaneously achieve high proton conductivity, low methanol permeability, and robust stability; however, these properties are intrinsically coupled in hydrocarbon-based systems. Here, we report a fluorene-reinforced polyamide hybrid membrane that decouples proton transport from methanol crossover through a rational dual-phase design. A highly sulfonated phenylenediamine-based polyamide (PDP) is integrated with a rigid fluorene-containing polyamide (PFP), forming continuous proton-conducting domains confined within a mechanically robust framework. Strong acid–base interactions between sulfonic acid and amide groups generate a dynamically cross-linked network, which effectively suppresses swelling and methanol transport while preserving efficient proton conduction. As a result, the optimized DF-20 membrane exhibits a high proton conductivity of 0.27 S/cm, an ultralow methanol permeability of 1.57 × 10^–7 cm²/s, and a selectivity nearly 1 order of magnitude higher than that of Nafion 117. More importantly, this intrinsic performance translates into a 57% enhancement in maximum power density in DMFC operation. This work establishes a fluorene-enabled hybridization strategy that provides a general pathway for designing high-selectivity hydrocarbon PEMs.