A Fish-Gill-Inspired Biomimetic Multiscale-Ordered Hydrogel-Based Solar Water Evaporator for Highly Efficient Salt-Rejecting Seawater Desalination

Solar energy-driven steam generation is a renewable, energy-efficient technology that can alleviate the global clean water shortage through seawater desalination. However, the contradiction between resistance to salinity accretion and maintaining high water evaporation properties remains a challenging bottleneck. Herein, we have developed a biomimetic multiscale-ordered hydrogel-based solar water evaporator for efficient seawater desalination. The as-prepared solar water evaporator consists of highly ordered ultralong hydroxyapatite (HAP) nanowires as a supporting backbone and heat insulator, MXene as a sunlight absorber, and hydrophilic hyaluronic acid methacryloyl (HAMA) as an interfacial bonding agent, and a modifier to reduce the water evaporation enthalpy. The MXene/ultralong HAP nanowires/HAMA (MHH) photothermal hydrogel evaporator with the multiscale-ordered hierarchical structure mimics the fish-gill structure. The highly ordered alignment of ultralong HAP nanowires is realized at multiple scales, from the nanoscale to the microscale to the macroscale and from 1D to 2D to 3D in the as-prepared photothermal hydrogel evaporator. The high-performance MHH photothermal hydrogel water evaporator exhibits high efficiency of photothermal conversion, low water evaporation enthalpy, excellent heat management capability, and high solar water evaporation performance. The water evaporation enthalpy decreases from 2431 J g^-1 (pure water) to 1113 J g^-1 using the MHH photothermal hydrogel evaporator. As a result, the high-performance MHH hydrogel water evaporator can realize a high water evaporation rate of 6.278 kg m^-2 h^-1 under one sun illumination (1 kW m^-2). Moreover, the as-prepared MHH hydrogel evaporator is able to achieve a water evaporation rate of 4.931 kg m^-2 h^-1 using the real seawater sample, exhibiting excellent salt-rejecting performance. It is expected that the as-prepared MHH hydrogel evaporator has promising applications in high-performance seawater desalination and wastewater purification using the sustainable solar energy.