Dual‐Modified Cellulose Nanofiber Membranes with Boosted Surface Charge for High‐Performance Osmotic Energy Conversion

Despite the promise of nanofluidics in osmotic energy conversion, the low surface charge of nanomaterials leads to inadequate ion selectivity and permeability in membranes, restricting the power output. Herein, we propose a novel dual-modified strategy to develop oppositely highly charged cellulose nanofiber membranes. This approach involves small-molecule functionalization and polymer grafting, successfully overcoming the substitution limit of single-step modifications and achieving substantial surface charge while maintaining optimal nanochannel structure. Systematic investigation reveals that the enhanced surface charge and tailored nanochannel synergistically promote selective ion transport (high t₊ of 0.97 in 0.5/0.01 M), contributing to the boosted power output. The resulting negatively and positively charged membranes achieve power densities of 5.1 W·m⁻² and 4.6 W·m⁻² (0.5/0.01 M), demonstrating 12-fold and 10-fold enhancements over pristine cellulose membranes. Notably, a RED unit combining both membranes delivers an exceptional power density of 12.9 W·m⁻² (5/0.01 M), surpassing most cellulose nanofiber-based nanofluidics. Furthermore, scaling up the system with only 15 membrane pairs generates 2.5 V, demonstrating both performance and practical scalability. This work paves the way for the rational design of sustainable, low-cost cellulose nanofiber membranes with high ion selectivity, advancing their application in efficient and scalable osmotic energy conversion.