Preparation Of Solar-thermal Conversion Membrane Based On Nickel Black And Graphene And Its Application In Interfacial Evaporation

Freshwater is an indispensable resource for human survival.More and more countries have adopted desalination technology to alleviate the shortage of freshwater.However,the traditional desalination technologies often requires high energy consumption,therefore,solar-driven interfacial evaporation technology has gradually attracted attention,and the key to improving its solar energy utilization lies in photothermal conversion materials.Although noble metals materials have good photothermal conversion performance,their prices are very expensive.Therefore,it was considered to use cheap metals to replace noble metal materials and optimize their performance.At the same time,the performance of the photothermal conversion material was not only related to the nature but also the structure.Therefore,it was considered to design a structure that was more favorable for absorbing sunlight.First of all,in view of the high price of noble metal materials,nickel,which also has a plasmon resonance effect under illumination conditions,was used to replace it to achieve the purpose of effectively reducing costs.Nickel black photothermal conversion(C-Ni)membranes were prepared by depositing nickel black on porous nickel foam by an electroless plating method and applied in solar-driven interfacial evaporation.The results showed that the C-Ni membrane exhibited an excellent performance.The light absorption rate can reach 93.87%,the average temperature of the membrane surface can reach48.4?,the evaporation rate can reach 1.266 kg m^-2h^-1 and the solar-vapor conversion efficiency can reach 79.88%.The photothermal conversion performance of the C-Ni membrane was much better than that of the nickel foam,which indicated the nickel black deposited on the surface of the nickel foam by the electroless plating method played a better role in solar-thermal conversion.In the cyclic desalination process,although a large number of salt crystals were deposited on the surface of the membrane after 20 cycles,the evaporation rate can still be stabilized at about 1.15 kg m^-2h^-1,and the rejection rate of Na Cl by the membrane can still reach more than 99%.Secondly,the C-Ni membrane was modified by graphene which with a good photothermal conversion performance.However,common graphene preparation methods were complicated,and often need to dangerous chemicals such as strong acids and strong oxidants.Therefore,a green and environmental-friendly electrochemical exfoliation method was used to prepare the graphene suspension,and then it was naturally deposited on the filter paper to form a graphene photothermal conversion(E-Gr)membrane to test the interfacial evaporation performance.The results showed that when the concentration of ammonium sulfate and ammonia water were 0.1M and 0.001M,respectively,and the voltage was 5V,the interfacial evaporation performance of the E-Gr membrane was the best,the average surface temperature of the membrane can reach 44.1?,and the evaporation rate can reach 1.267 kg m^-2h^-1 and the solar-vapor conversion efficiency reaches 79.65%.Then,the electroless plating method was used to adjust the electroless plating solution with graphene suspension which was prepared by the electrochemical exfoliation method to prepare nickel black-graphene photothermal conversion(C-Ni@Gr)membrane,which was used for the interface evaporation.The results showed that the interfacial evaporation performance of C-Ni@Gr membrane was much higher than that of C-Ni and E-Gr membrane,its evaporation rate could reach 1.620 kg m^-2h^-1,and the solar-vapor conversion efficiency could reach 105.57%,which indicate that graphene and nickel black play a synergistic role in the C-Ni@Gr membrane.Then,inspired by the hierarchical structure of conifers in nature can make full use of sunlight and were evergreen,a conifer-like nickel black solar-thermal conversion(E-Ni)membrane was prepared by electroplating and used for interfacial evaporation.The results showed that the evaporation rate of the E-Ni membrane can reach 1.375 kg m^-2h^-1 under the action of the"light trapping effect"of the hierarchical structure,and the hydrophobicity was also improved(WCA=152°),which was beneficial to the salt resistance of the membrane.During the desalination process,with the increase of the cycle number,the evaporation rate of the E-Ni membrane can be stabilized between 1.200 and 1.400 kg m^-2h^-1,and there was no obvious crystalline salt on the surface of the membrane.It showed that by designing the structure of the membrane,can improve the solar-thermal conversion and salt resistance of the membrane.Finally,compared with the complex process of preparing the nickel black-graphene solar-thermal conversion membrane by the two-step method,a one-step electrochemical method was considered to prepare the nickel black-graphene photothermal conversion(E-Ni@Gr)membrane with the hierarchical structure.This method could make the electrochemical exfoliation and self-assembly of graphene and co-deposited with nickel black to form E-Ni@Gr membranes on the nickel foam.The results showed that the evaporation rate of the E-Ni@Gr membrane could reach 2.050 kg m^-2h^-1 and the solar-vapor conversion efficiency can reach 135.99%during the interfacial evaporation process.With the increase of desalination cycles,the evaporation rate of the E-Ni@Gr membrane could still be stabilized between 2.000 kg m^-2h^-1,the rejection rate of the membrane can still reach more than 99%,and the interfacial evaporation performance of E-Ni@Gr membrane was much higher than that of C-Ni@Gr,E-Ni and C-Ni membranes,indicating that E-Ni@Gr membrane prepared by one-step electrochemical method was benificial to improve the performance of nickel black solar-thermal conversion membrane.This paper was based on the solar-driven interfacial evaporation technology for seawater desalination,and the optimization of the conversion materials to improve the solar-thermal conversion performance.By replacing noble metals with cheap nickel,and modifying them with self-made graphene materials and improving the interfacial evaporation performance of the membrane through design structure,finally,a nickel black-graphite with excellent light-harvesting ability and salt resistance was prepared by one-step electrochemical method.The solar-thermal conversion membranes prepared in this study provide a new possibility for future membrane structure design and replacing of noble metal solar-thermal conversion materials.