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.