Lignin with enhanced photothermal performance for the preparation of a sustainable solar-driven double-layer biomass evaporator

The undesirable photothermal conversion performance of lignin limits the potential for directing use as a photothermal conversion material, and no strategies have been found to enhance the photothermal conversion efficiency of lignin. Herein, three modified lignin of corn stover alkali lignin (DEHL), pine alkali lignin (DAL) and birch alkali lignin (DTAL) with excellent photothermal conversion efficiency and fast temperature response was prepared by a one-step iodocyclohexane (ICH) activation strategy. The maximum temperature of the DEHL, DAL and DTAL can reach to 138 °C, 157 °C, and 172 °C in 12 min under 0.15 W/cm2 compared to the native lignin. Moreover, the photothermal conversion efficiency of the DEHL, DAL and DTAL was calculated to be 36.52 %, 36.19 % and 43.19 % by ICH modification. The photothermal enhancement mechanism of the modified lignin was determined by various characterizations and simulation calculations, which can be summarized that the activation of lignin by ICH breaks the molecular structure and reduces the methoxy content of the lignin, which enhances the intermolecular bonding, and the hydrogen bonding between the phenolic hydroxyl groups strengthens the π-π intermolecular interactions between the benzene rings. In addition, a double-layer biomass aerogel evaporator (DAGS) with cost-friendly and superior evaporation performance was synthesized by chemical cross-linking using DTAL as the photothermal agent. The evaporation rate of 4DAGS can be as high as 2.063, 2.024, 2.259 and 2.447 kg/(m2∙h) in water, 3.5 wt% seawater, and 200 mg/L of MG or MB wastewater under 1sun. And no salt was observed on the surface of the 4DAGS after a long period of 36 h of operation in 3.5 wt% seawater, suggesting the outstanding salt-reject performance of the aerogel. Furthermore, there is no structural damage and chemical decomposition found on 4DAGS after 24 h immersion in NaOH and HCl solutions, demonstrating the excellent acid and alkali resistance of the synthesized aerogels. This work may offer a novel strategy for the photothermal performance enhancement of lignin and expand the potential application in solar-driven evaporation materials.