Sonochemical Synthesis Of Metal Oxide/Graphene Composites And Their Electrochemical Performance

Focusing on the existing energy crisis and environmental pollution problems in modern society,it is urgent to develop new energy storage technologies to improve these problems.In recent years,supercapacitors and Li-ion batteries as the electrical energy storage systems have attracted growing attention due to their high performance,cost effective,easy-to-manufacture,and environmentally efficient.The performances of supercapacitors and Li-ion batteries depended on their electrode materials,metal oxide/graphene nanocomposiets had been recognized as the most promising electrode materials,because they can combine the high energy density of metal oxides with the high specific surface area and outstanding electric conductivity of graphene.In this paper,a facile sonochemical strategy was developed,a series of metal oxide/graphene composites,MnO2/graphene,Co3O4/graphene,and SnO2/graphene composites were synthesized by controlling the ultrasonic time,calcination temperature,and feeding ratio.The micromorphology and crystal structure of metal oxide/graphene composites were adjusted to improve their electrochemical performances.The detailed contents are indicated as follows:(1)MnO2/graphene composites were synthesized via the sonochemical method by using MnSO4 as the Mn source in the presence of graphene oxide(GO).The sonochemical reaction time were adjusted to control the density and micromorphology of MnO2,it was found that MnO2 nanosheets densely and uniformly grown on the surface of GO sheets after three hours of sonochemical reaction.This hybrid structure not only prevented the restack of graphene sheets to enhance the specific surface area,but also increased transport channels for electrolyte ions.As a supercapacitor electrode material,the resulted MnO2/graphene composite achieved a highest specific capacitance up to 292.9 F/g at a 5 mV/s and 156.1 F/g at 100 mV/s,and it also retained 91.5%capacitance after 1,000 charge/discharge cycles at 5 A/g.(2)Co3O4/graphene composite also prepared through the sonochemical method with a subsequent heat treatment process.The crystal structure and particle size of Co3O4 were regulated by adjusting the calcination temperature from 190°C to 280°C.The crystallization of metal oxide and thermal reduction of GO can take place simultaneously at a proper calcination temperature(220℃),Co3O4 nanoparticles with a diameter～10 nm were uniformly coated on the surface of reduced graphene oxide(RGO),thus increasing the specific surface area for full usage of the active material,and enhancing intimate interactions for efficient charge transport between Co3O4 and conductive RGO.As a supercapacitor electrode material,this Co3O4/graphene composite achieved a high specific capacity of 524.7 F/g at 5 mV/s and 243.1 F/g at 100 mV/s,it also showed a good cycling stability with negligible capacitance degradation after 1,000 charge/discharge cycles at 5 A/g.(3)SnO2/graphene composites with various component ratios were prepared via controlling of the feeding ratios of GO and SnCl2 in the sonochemical reaction systems.SnO2 nanoparticles with different densities and particle sizes were tightly wrapped on the surface of RGO sheets.The electrochemical properties of SnO2/graphene composites are varied with the RGO content,the composite with 18.4 wt%of RGO was found to reach the highest first-discharge capacity up to 1615 mAh/g at 50 mA/g,and retained 33.9%at 500 mA/g,the retention rate up to 76.2%when back to the initial current density,the first coulombic efficiency of 45.8%was achieved.