Extracting Lignin with Superior Photothermal Performance from Wood in Molten Salt Hydrate for Preparation of a 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 reveal that elevated reaction temperatures (170 °C) promoted lignin depolymerization (Mw = 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.