Dual‐Functional Sulfonated Nanocellulose for Proton Exchange Membranes With High Proton Conductivity at Low Humidity

Cellulose‐based proton exchange membranes have garnered increasing attention for their abundance, sustainability, and environmental compatibility. However, their intrinsically low proton conductivity has restricted their practical use in fuel cells. Herein, we report a scalable dual‐functional strategy that simultaneously introduces side‐chain sulfonation (–SO 3 H) and nanosizes cellulose fibers, yielding sulfonated nanocellulose for proton exchange membranes with high mechanical robustness, suppressed methanol permeability, and excellent environmental stability. Molecular dynamics and density functional theory simulations reveal that the grafted –SO 3 H groups lower the proton dissociation energy and enable the formation of continuous hydrogen‐bonded networks, facilitating efficient proton transport. Meanwhile, the sidechain induced multibranched nanofiber structure improved both water and proton adsorption. Consequently, sulfonated nanocellulose‐based proton exchange membranes achieve proton conductivities of 128 and 44.2 mS cm −1 at 80 °C under 98% and 33% relative humidity, respectively. Notably, the low‐humidity conductivity exceeds that of the benchmark Nafion 212 membrane (20.3 mS cm −1 ) by more than twofold. This work demonstrates a sustainable and scalable approach to overcoming the intrinsic limitations of cellulose and provides a promising route toward cost‐effective and durable proton exchange membranes for next‐generation fuel cells.