Extracting lignin with superior photothermal performance from wood in molten salt hydrate for preparation of solar-driven gradient evaporator.

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 revealed that elevated reaction temperatures (170°C) promoted lignin depolymerization (Mw=1206) and increased phenolic hydroxyl content (3.5 mmol/g), enhancing π-π stacking interactions to achieve a photothermal conversion efficiency of 36.31%. Structural analyses through 2D-HSQC NMR confirmed β-O-4 bond cleavage and demethylation, while fluorescence quenching validated reduced radiative losses. Leveraging this lignin, a gradient evaporator was fabricated by integrating polyvinyl alcohol (PVA)-modified melamine foam (MF) with a hydrophobic lignin-polyvinylidene fluoride (PVDF) photothermal layer. The evaporator exhibited hierarchical wettability, enabling gravity-guided water transport (2.8 kg m-2 h-1) under 0.1 W/cm²) and environmental heat harvesting. It demonstrated robust performance in hypersaline water (1.85 kg m-2 h-1 for 10.5 wt% brine) and dye removal (>99.98% rejection). Additionally, lignin-coated thermoelectric devices generated stable power (27.69 W/m²) via 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.