Electrochemical Preparation Of Graphene And Its Application In Energy Storage And Conversion

The excellent properties and application demands of graphene in many fields continue to inspire and promote the research on its preparation methods.Among the numerous methods explored,the electrochemical method is regarded as one of the most promising approaches for industrialization due to its mild conditions,controllable operations,and considerable yields.This dissertation focuses on the experimental conditions in electrochemical exfoliation,such as electrolyte,voltage,and electrodes,aiming to develop low-cost and high-yield electrochemical methods for preparing graphene with different properties.In addition,the energy storage,electrothermal and photothermal properties of graphene are also studied.In the second chapter of this dissertation,a novel green solvent,that was threecomponent deep eutectic solvent(3-DES,Zn(NO3)2-CO(NH2)2-HCONH2),was firstly prepared.Then the 3-DES was used as electrolyte,and the graphite paper was subjected to electrochemical anodic exfoliation at 5 V voltage.Finally,after purification and separation,the few-layer graphene was obtained with a yield of about 33%.The whole experiment process was simple,safe,and green.The prepared graphene sheets accounted for 77%in the range of 1～3 layers,and 64%in the lateral dimension reaching micron level,with D band and G band intensity ratio ID/IG of about 0.68 and the oxygen content of 10.7%,and can be stably dispersed in NMP for 1 month.In addition,the recycling of 3-DES was carried out,and the feasibility of its recycling was proved by observing the graphene obtained from the utilization of 3-DES,which further reduced the cost of this green solvent in the electrochemical preparation of graphene.In the third chapter of this dissertation,the water-dispersible graphene was prepared by one-step electrochemical anodic exfoliation of graphite paper in molybdate aqueous solution(Na2MoO4 and(NH4)6Mo7O24),with the highest yield of about 45%.Characterization of the prepared graphene showed that 76%of the graphene sheets owned layer numbers not exceeding 5,94%of the sheets had lateral sizes of more than 1 μm,and the ID/IG was 0.98.Also,the graphene had a stable dispersion period of up to 6 months in water.At the same time,the relationship between the applied voltage and oxidation degree of graphene during the electrochemical process was explored.Finally,the graphene electrothermal film prepared by vacuum filtration exhibited a high steadystate temperature of 147℃ and a maximum heating rate of 11.8℃ s-1 at 10 V voltage.This exfoliation method provides a new reference for the anodic preparation of graphene.In the fourth chapter of this dissertation,a new two-step dual-electrode preparation of graphene was demonstrated,by selecting quaternary ammonium phosphorus hexafluoride(PF6)/boron tetrafluoride(BF4)as the electrolytes and wrapped graphite flakes as the electrodes.In the first step,graphite flakes were electrochemically intercalated on the anode and cathode simultaneously at 5 V voltage,and the products after intercalation were expanded and exfoliated under microwave irradiation in the second step.At least 90%of the graphene sheets obtained from anode and cathode had no more than 5 layers,and 94%of the sheets possessed micron-level lateral size.The oxygen content,ID/IG of anodic and cathodic products were 6.1%and 1.9%,0.214 and 0.284,and the maximum yields of anode and cathode were 81%and 76%,respectively.In addition,the recovery and reuse of 60%electrolytes were realized,which provides prominent advantages over traditional electrochemical exfoliation routes for the largescale production of high-quality graphene.In the fifth chapter of this dissertation,the composite of reduced graphene oxide and poly(3,4-ethyl-enedioxythiophene):polystyrenesulfonate(PEDOT:PSS)was firstly prepared by the hydrothermal method.By adjusting the ratio of GO and PEDOT:PSS,the composite(rGO/PEDOT:PSS)with the best electrochemical performance was obtained.This composite aerogel possessed excellent photothermal properties,and its surface temperature can reach 337℃ under the irradiation of 1.05 W cm-2 power density(808 nm),with a photothermal conversion efficiency of about 62.0%.The symmetric supercapacitor assembled by the composite as electrode material can achieve the improvement of electrochemical properties under laser irradiation.Under the irradiation with a power density of 0.18 W cm-2,the specific capacitance and energy density of the supercapacitor were increased to 1.6 times and 1.8 times of those under the dark condition,respectively.Finally,by direct heating,it was verified that the improvement of supercapacitor performance under laser irradiation was due to photothermal heating.The design of this supercapacitor provided a new idea for the development of the new generation of energy storage devices.