Investigation Of Hierarchical Porous Hydrogels With Controllable Fabrication In Solar-driven Steam Generation

Currently,humanity’s survival and social development are facing an increasingly severe shortage of freshwater resources.Unconventional water sources,which are exemplified by seawater,account for a significant proportion（approximately 96.5%）of the total water resources on Earth.Therefore,effectively utilizing unconventional water sources to address the shortage of freshwater resources is an effective solution.Solar-driven steam generation can capture solar energy to obtain purified water through evaporation and condensation,providing a promising strategy to address the increasingly severe problems such as water scarcity,water pollution,and energy shortage.Among various types of evaporators,the use of hydrogels as the base material for achieving efficient water evaporation has attracted much attention.However,currently reported hydrogel evaporators suffer from several drawbacks,including insufficient solar absorption,excessive thermal losses,low evaporation efficiency,complicated manufacturing processes,and poor mechanical performance.Moreover,the practical application of solar-driven steam generation is constrained by the complicated fabrication processes of micro-nano structure of evaporators,which hinder the concentration of heat and transportation of water to the thermal-localized area.Therefore,the preparation of evaporators for solar-driven steam generation with excellent mechanical properties and high solar-vapor conversion efficiency remains a significant challenge.In this study,two hydrogel evaporators have been designed and fabricated with simple,controllable,and scalable approaches,featuring hierarchical porous structures.The resulting evaporators exhibit significantly improved solar spectral absorption and photothermal conversion efficiency,leading to an enhancement in the overall efficiency of solar-driven steam generation.The main research results are as follows:（1）In this study,a novel hierarchical porous dual-network aerohydrogel（DNAH）was first prepared by utilizing a simple and controllable micropore generation strategy to assemble air and hydrogel into a gas-solid mixture with multi-level pores.The micropore size of the DNAH hydrogel evaporator can be regulated between 99±49μm to 316±58μm by adjusting the assembly conditions,and it exhibits enhanced light absorption and heat localization properties at the air-solid interface.The results indicate that DNAH evaporator achieves high solar spectral absorption（96.2%）in a broad range of wavelengths（200～2500 nm）by utilizing the light-absorbing properties of the chitosan-polyaniline（CP）copolymer and reduction of light reflection to the environment via micropores.Moreover,it improves the transportation ability of water towards the heated evaporation area.The synergistic effect of photothermal conversion and wettability enables the DNAH hydrogel evaporator to achieve a stable evaporation rate of 2.76 kg·m^-2·h^-1 under one sun illumination,resulting in a solar-vapor conversion efficiency of 91.3%.Furthermore,the DNAH hydrogel evaporator demonstrated outstanding evaporation stability and antibacterial properties during continuous evaporation process.Therefore,this research provides a universal and scalable method for the fabrication of hierarchical porous hydrogels,providing an efficient solution for photothermal evaporation water treatment and other engineering applications.（2）Micro-nano structures are widely present in nature,among which the micro-nano structured materials represented by bones exhibit exceptional mechanical strength and good flexibility.Inspired by natural bone structure,this study further uses the micropore generation assisted ice template approach to fabricate a bone-inspired hierarchical porous（BHP）hydrogel.The results showed that BHP hydrogel evaporators possess adjustable micropore sizes in the range of 155±37μm to 298±98μm and excellent mechanical properties,including a compression modulus of 0.05 MPa and a compression strain of 40%.With the combined effect of chitosan-polypyrrole（CS-PPy）copolymer light absorption and reduction of light reflection to the environment via micropores,the BHP hydrogel evaporator has demonstrated a sunlight absorption rate of 98.5%in the broadband spectral range of 200～2500 nm,providing sufficient energy supply for photothermal conversion.Moreover,through excellent photothermal conversion and improved water wettability,the BHP hydrogel evaporator exhibits a high pure water production rate,achieving a stable evaporation rate of 2.74kg·m^-2·h^-1 and a high solar-vapor conversion efficiency of 89.6%under one sun illumination.The improved preparation method for hierarchical porous hydrogels has been found effective in the removal of various organic dyes in wastewater,demonstrating wider applicability.