Photothermal polypyrrole-cellulose nanofiber membranes as advanced platforms for osmotic energy conversion

Using nanofluidic membranes to harvest osmotic energy at the interface between seawater and river water is a promising solution for mitigating energy shortages. However, the development of stimuli-responsive ion transport into nanofludic membranes remains largely underexplored. Herein, we design a photothermally responsive nanofluidic membrane by integrating polypyrrole (PPY), TEMPO-oxidized cellulose nanofibers (CNF), and lignosulfonate (LS) to achieve enhanced osmotic energy conversion. PPY, a conductive polymer with excellent photothermal conversion capability, enables light-driven modulation of ion transport. The CNF framework, enriched with -COOH groups, imparts high surface charge density and mechanical robustness, while LS effectively modulates the nucleation of PPY nanoparticles, yielding improved uniformity and elevated charge density. As a result, the CNF-PPY-LS membrane demonstrates an ionic conductivity of 4.51 × 10^-4 S cm^-1, 9.8 times greater than that of its CNF-PPY counterpart. Its ion selectivity and energy conversion efficiency increase from 0.74 and 11.19% to 0.85 and 25.18%, respectively. The power density of the CNF-PPY-LS nanofluidic membrane was measured to be 1.87 W/m² under a 50-fold KCl concentration gradient, It further increased to 2.38 W/m² under 100 mW/cm² photoirradiation. This work proposes a novel strategy for the design of photo-responsive, biomass-based membranes for tunable ion transport and superior osmotic energy conversion performance.