Supramolecular Engineering of Nanocellulose/COF Composite Membranes: Reconciling Multidimensional Proton Pathways With Framework Anchoring

ABSTRACT Achieving high proton conductivity with dimensional stability remains a critical challenge for nanocellulose‐based proton exchange membranes (NC‐PEMs), as necessary acid functionalization invariably triggers severe swelling. Here we report a supramolecular engineering strategy that integrates sulfonated nanocellulose (SO 3 H‐NC) with a —2D covalent organic framework (TFP‐DABA), yielding a SO 3 H‐NC@TFP‐DABA membrane that overcomes this long‐standing trade‐off. Mechanistically, SO 3 H‐NC acts as a functional intercalator to induce the exfoliation of TFP‐DABA layers, exposing shielded acidic sites and reorganizing the hydrogen‐bond network into a continuous, multidimensional proton‐transport pathway. Simultaneously, TFP‐DABA framework imposes rigid dimensional confinement, effectively constraining the hydration‐induced swelling of the SO 3 H‐NC. Consequently, the optimized SO 3 H‐NC@TFP‐DABA 3/7 membrane delivers record‐breaking proton conductivities of 590.6 and 281.5 mS cm −1 at 80°C under 98% and 33% relative humidity (RH), respectively, corresponding to 13.4‐ and 5.1‐fold enhancements over Nafion 212. The superior transport efficiency facilitates a peak power density of 240 mW cm −2 in single‐cell tests, a threefold performance increase over the commercial standard. Meanwhile, the membrane maintains a swelling ratio of below 5%, nearly four times lower than that of Nafion 212. This work establishes a practical supramolecular design principle for constructing bio‐PEMs that are highly conductive yet dimensionally stable across a wide range of humidity conditions.