Study On Graphene-based Nanocomposite Electrodes With New Architectures For Supercapacitors

Electrochemical supercapacitors are an emerging energy storage technology that will take a key role in charging up the future.So far,the lithium-ion battery occupies the leading position in the market,but its power density is relatively smaller,and the cycle life needs to be improved.Therefore,the supercapacitor has become the key technology and device to solve these problems.Due to its high specific surface area,excellent electrical conductivity,chemical stability and mechanical properties,graphene has a great potential application in the field of supercapacitors,especially in supercapacitor electrodes.For supercapacitor,the performance ability of any composite commonly depends on its surface area,porosity,and agglomeration.Thus,the search for new composite with high surface area and porous structure,as well as nanoparticles with smaller size and low agglomeration effect,will be the hot points in this research area.This dissertation focuses on the fabrication of graphene-based electrodes and the investigations in terms of achieving high surface area,porous structure,and low agglomeration,as well as 3D structure.Major achievements in this dissertation are summarized below.(1)A facile,low-cost and novel method to fabricate 3D vertically aligned Ag nanoplates on nickel foam-graphene(NFG)substrate using electrodeposition with ultrasonication AgNO3 solution.The vertically deposited Ag nanoplates with irregular polyhedron shapes standing on the surface of NFG attributes to enhance the surface area,and sequentially be beneficial for supercapacitor.The novel architecture of composite Ag/NFG prepared with ultrasonication((Ag@NFG)S1)exhibits a specific capacitance(SC)of 900 F/g at an applied current density of 0.5 A/g,a value 3.5 fold higher than that of the composite(Ag@NFG)SO prepared without ultrasonication and six-fold higher than that of NFG.Moreover,the composite(Ag@NFG)S 1 shows good long-term cyclic stability after 5000 cycles and its columbic efficiency is almost 99%of the initial value.(2)A ternary NFG/MnO2/Polyaniline(PANI)nanocomposite has been synthesized using in-situ polymerization.All reactants were dispersed homogeneously in precursor solution in the form of ions and molecules.Consequently,PANI and MnO2 nanoparticles on the NFG contact each other and are arranged alternately in the composite.This structure not only separates PANI and MnO2 nanoparticles and prevents them to form aggregation,but also decreases the particle size of the composite on the surface of NFG.The inter-molecule contact improves the conductivity of the composite.The composite showed excellent SC of 1081 F/g at the scan rate of 1 mV/s and SC of 815 F/g at the current density of 3 A/g,having excellent cycling stability.The current study provides an alternative pathway to improve the rate capability and cycling stability of nanostructured electrodes,by offering a great promise for their applications in supercapacitors.(3)The Graphene,Ag2O/PANI composite has been synthesized by in-situ polymerization.It has been observed that Ag2O nanoparticles exist on the porous spongy background of PANI.The optimized composition of the synthesized composite exhibits an extraordinary SC of 1572 F/g at 0.05 A/g current density and good cyclic stability having a retention of 85%over 3000 charge-discharge cycles.The extraordinary electrochemical performance indicates the positive synergistic effect of PANI,graphene,and Ag2O.The Ag2O nanoparticles might be responsible for improved electrical conductivity,and graphene might contribute in enhancing the electrochemical stability of the PANI electrode.