Industrial Kraft Lignin-Functionalized Cellulose Nanofluidic Membrane for Enhanced Osmotic Energy Conversion

Cellulose nanofiber (CNF)-based nanofluidic membranes are emerging as effective materials for osmotic energy harvesting, thanks to their tunable morphology and facile surface chemical modification. Despite their potential, these fibers face notable challenges, including poor water stability and limited output power density, largely due to the swelling behavior in aqueous environments and restricted ionic flux resulting from fibers entanglement. Herein, we report an advanced nanofluidic membrane composed of TEMPO-oxidized cellulose nanofibers (TCNF), industrial kraft lignin nanoparticles (KL), and poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS). The hydrophobic nature of KL substantially improves the water stability of the membrane. Additionally, KL minimizes the horizontal packing and entanglement of TCNF, facilitating increased ion flux. The inclusion of PEDOT:PSS imparts dual ionic and electronic conductivity, amplifying the membrane ionic conductivity and reducing internal resistance. Due to the interaction between the pore structures formed by TCNF and KL and the conductive network of PEDOT:PSS, the ionic conductivity of the KL/TCNF-PEDOT:PSS membrane is 0.02 S/cm at a low KCl concentration (pH 11). Notably, at a 100-fold KCl concentration gradient, it exhibits an osmotic energy conversion efficiency of 36% and cation selectivity of 0.92. In a 50-fold KCl concentration gradient, the membrane delivers a high output power density of 1.37 W/m 2 . When connected in series across 18 units, the KL/TCNF-PEDOT:PSS nanofluidic devices generate sufficient power to operate electronic devices such as a bulb and a calculator. This study highlights the potential of industrial sulfate lignin in designing stable, high-performance nanofluidic membranes for sustainable energy applications.