Microwave Synthesis Of Mental Oxide/Graphite Composites For Supercapacitors

Supercapacitors,as a new electrical energy storage device between secondary battery and conventional capacitors,have drawn considerable research due to their high power densities,rapid charge and discharge rate,long cycle life and environmentally benign.The excellent electrochemical performances of supercapacitors were determined by electrode materials.RuO₂,as a representative of metal oxides,was initially investigated on account of its low resistivity and superior specific capacitance.Nevertheless,the high cost and toxicity of RuO₂ seriously hinders its practical applications.Thus,tremendous research works have subsequently been developed to other mental oxides as substitutes.MnO₂ and NiO,based on their natural abundance,low cost,environmentally friendly and excellent electrochemical properties,have become promising candidates for supercapacitors.However,owing to the poor conductivity,superior electrochemical properties of Ni O and MnO₂ have been severely restricted for practical application.In addition,microwave method can be carried out at room temperature in air within 60 s,which is much easier and fasterthantraditionalhydrothermalprocess,chemicalprecipitation,electrochemical oxidation and so on.Hence,in this paper,based on high conductivityofGraphitewepreparedMnO₂-NW@Graphiteand NiO/MnO₂@Graphite composites via microwave solid-state approach.The main contents are as follows:1.We successfully prepared MnO₂ nanowires?MnO₂-NW?@Graphite compositebymicrowavesolid-stateapproach,whichemployed Mn?NO₃?₂·4H₂O as a precursor and Graphite as microwave absorbing material.Various parameters such as material mass ratio,microwave power,reaction time,have been investigated to optimize the conditions to have control over MnO₂nanowires growth.According to SEM images,the ratio of substrate to precursor at 50:20,the microwave power of 800 W and microwave heating time of 60 s are the optimal experimental conditions for the Mn O₂-NW@Graphite system.In addition,electrochemical performances of MnO₂-NW@Graphite composites were evaluated using 1 M KOH as electrolyte.The prepared electrodes exhibit maximum specific capacitance of 380.4 F/g at the current density of 1 A/g.It shows high rate capability and good long-term stability with about 80%?303.57F/g?of capacitance retention after 2000 cycles.2.We successfully manufactured NiO@Graphite composite via microwave method,which used Ni?NO₃?₂·6H₂O as a precursor and Graphite as microwave absorbing material.To obtain high performance electrochemical electrode materials,many experimental conditions were studied such as material mass ratio,microwave power and reaction time.According to the experimental results,when the mass ratio of substrate to precursor at 50:40,the microwave power of 800 W and microwave heating time of 60 s,the electrode performances were the best.The maximum specific capacitance was 455 F/g at 0.5 A/g.When the current density increased to 10 A/g,the specific capacitance was still 327 F/g indicating favorable rate capability.However,the cycling stability of NiO@Graphite electrode was poor due to the agglomeration of Ni O fine nanoparticles.3.In order to improve the cycle stability of NiO@Graphite,we have prepared Ni O/MnO₂@Graphite composite by adding another precursor Mn?CH₃COO?₂·4H₂O based on Graphite and Ni?NO₃?₂·6H₂O.According to SEM results,the addition of MnO₂ effectively prevented the agglomeration of NiO nanoparticles improving the nanostructure stability of composite.The maximum specific capacitance of NiO/MnO₂@Graphite electrode was reached to 740 F/g at 1 A/g.The NiO/MnO₂@Graphite showed excellent cycling stability with capacitance retention of 140%?592 F/g?of the initial value after1600 cycles.