Extracting Lignin with Superior Photothermal Performance from Wood in Molten Salt Hydrate for Preparation of a Solar-Driven Gradient Evaporator

Abstract

Developing sustainable solar-driven evaporators requires efficient photothermal materials and rational structural design. This study presents a green strategy for extracting lignin with enhanced photothermal performance from wood using molten salt hydrate (MSH) and citric acid under mild conditions. Systematic investigations reveal that elevated reaction temperatures (170 °C) promoted lignin depolymerization (M_w = 1206) and increased phenolic hydroxyl content (3.5 mmol g⁻¹), enhancing π-π stacking interactions to achieve a photothermal conversion efficiency of 36.31%. Structural analyses through 2D-HSQC NMR confirm βO4 bond cleavage and demethylation, while fluorescence quenching validates reduced radiative losses. Leveraging this lignin, a gradient evaporator is fabricated by integrating polyvinyl alcohol (PVA)-modified melamine foam (MF) with a hydrophobic lignin-polyvinylidene fluoride (PVDF) photothermal layer. The evaporator exhibits hierarchical wettability, enabling gravity-guided water transport. It demonstrates robust performance in hypersaline water (1.85 kg m⁻² h⁻¹ for 10.5 wt.% brine) and dye removal (>99.98% rejection) under 0.1 W cm⁻² sun irradiation and environmental heat harvesting. Additionally, lignin-coated thermoelectric devices generate stable power (27.69 W m⁻²) by solar-thermal conversion. This work provides an eco-friendly pathway for lignin valorization and scalable solar evaporation systems, addressing energy-water challenges through biomass resource utilization.