Modified Needle Coke/PVA Composite Hydrogel For Solar-driven Interfacial Evaporation

The whole world is faced with the crisis of drinking water,and the desalination of seawater by solar-driven interface evaporation technology can effectively solve this problem.With advantages of low cost and high light absorption rate,carbon materials are widely used in the design of photothermal conversion materials.However,the high energy consumption in the evaporator that causes the water to evaporate prevents the evaporation rate from reaching a high level.At the same time,in the practical application of desalination,salt ions will impede water transmission in the evaporator,and inevitably cause salt particles to deposit on the surface,which ultimately has a bad effect on evaporation rate.In this paper,the modified petroleum needle coke was used as the photothermal conversion material,and the methods of chemical crosslinking and physical crosslinking are used to gel the composite with polyvinyl alcohol（PVA）,and a 3D network structure hydrogel evaporator was prepared.The hydrophilic groups on the PVA skeleton can activate water molecules and greatly reduce the enthalpy of evaporation of water.The interaction between salt ions in seawater and PVA skeleton can promote the evaporation of water,and effectively inhibit the rapid crystallization of salt ions on the evaporation surface,which provides a new idea for the design of evaporator in the practical application of efficient and sustainable solar-driven interface desalination.The main research contents are as follows:The lattice spacing of the modified needle coke（MNC）was increased from 0.35 nm to 0.69 nm by high temperature calcination and chemical modification using petroleum coke as raw material.The larger lattice spacing enhances the vibration amplitude between carbon atoms of the lattice and improves the photothermal conversion ability.Further,a composite hydrogel evaporator（CH）was prepared by chemically crosslinking MNC with PVA.After optimizing the ratio of MNC to PVA,the CH3 evaporator can achieve the evaporation rate of 2.66 kg m^-2h^-1under one sunlight,and maintain the stability in 10 hours of continuous evaporation.Under the same conditions,the evaporation rate of 2.52 kg m^-2h^-1was realised by CH3 in simulated seawater.The presence of salt ions affects the evaporation capacity of the evaporator.In order to solve this problem,a composite hydrogel evaporator（PH）was prepared by physical crosslinking of MNC and PVA by freeze-melting method.Due to the Hofmeister effect between Cl^-and PVA,a PH2 evaporator under sunlight exhibits a higher evaporation rate in 3.5 wt.%Na Cl solution than in pure water(3.18 kg m^-2h^-1vs.2.53 kg m^-2h^-1).The interaction between Cl^-and PVA was investigated by constructing the model of PVA molecules in the environment of salt ions.The simulation results were consistent with the test characterization results.