Lignin nanoparticle-reinforced chitosan hydrogels for enhanced solar evaporation performance

Solar-driven interfacial evaporation requires photothermal materials that combine efficient solar absorption, continuous water transport, and resistance to salt accumulation. Herein, a lignin nanoparticle-reinforced chitosan hydrogel evaporator (LPCH) was constructed by integrating alkali lignin nanoparticles into a chemically cross-linked chitosan network. The uniformly dispersed lignin nanoparticles functioned as photothermal and nanoreinforcing domains, enhancing light harvesting and increasing the surface temperature to 39.5 °C under one sun irradiation. Interactions between the hydroxyl-rich lignin nanoparticles and chitosan regulated the water state within the hydrogel network, increasing the intermediate-water-to-free-water ratio from 0.35 for the chitosan hydrogel (CS) to 1.24 for LPCH and lowering the equivalent evaporation enthalpy to 2085 J g^-1. The optimized LPCH evaporator achieved an evaporation rate of 2.77 kg m^-2 h^-1 under one sun irradiation, together with an apparent solar-to-vapor efficiency of 152.4%. It retained an evaporation rate of 2.29 kg m^-2 h^-1 in 10 wt% saline water and maintained continuous operation over 10 h in simulated seawater. Outdoor tests using real seawater produced a peak evaporation rate of 3.47 kg m^-2 h^-1 and a daytime average rate of 2.38 kg m^-2 h^-1. These results demonstrate a sustainable strategy for combining lignin-derived photothermal conversion, water-state regulation, and structural reinforcement in biomass-based solar evaporators.