Asymmetric Supercapacitors Based On α-Fe₂O₃/rGO And α-MnS/N-rGO

Supercapacitor is a kind of device for energy storage,which can be characterized by rapid rates of charging and discharging and high power density.Energy and power densities are two crucial parameters for evaluating the electrochemical performance of supercapacitors.Although the energy density of most of the commercially available supercapacitors is much higher than conventional dielectric capacitors,it is still significantly lower than batteries and fuel cells.It is effective approach to improve energy density through increasing the special capacitance of electrode and operation potential window of the device.Based on the above consideration,the main contents of this paper mainly focus on two important parameters for high energy density supercapacitors,selection and preparation of the appropriate positive and negative electrode materials for building high-performance asymmetric supercapacitors.Firstly,GO was prepared by modified Hummers method and dispersed in different solvent,such as water,ethanol and DMF.The as-prepared GO suspension was subsequently reduced through solvothermal approach.The microstructure and phase constitution results show that the as-prepared GO suspension are reduced into rGO after solvothermal.The XPS results show that the atomic percentage of nitrogen is about 2.58% for rGO reduced in DMF system,verifying the successful nitrogen doping during the solvothermal process.As an electrode material for supercapacitor,the WrGO in water system and A-rGO in ethanol system exhibit a rectangular shape CV,indicating the character of double layer capacitance.N-rGO in DMF system exhibits pseudocapacitance because they contain the certain number of N atoms.The specific capacitances of W-rGO,A-rGO and N-rGO are 101,199 and 268.1 F g-1 at the current density of 1 A g-1,respectively.As the current density increases to 20 A g-1,The specific capacitances of W-rGO,A-rGO and N-rGO are 52,150 and 130 F g-1,respectively.The electrochemical properties of A-rGO and N-rGO are superior to that of W-rGO.Secondly,two-dimensional α-Fe₂O₃/rGO nanocomposites were fabricated by one-pot solvothermal approach using ferric chloride hexahydrate and sodium acetate trihydrate in the GO-ethanol system.Structural and morphological studies show that the as-prepared α-Fe₂O₃/rGO nanocomposites are actually composed of α-Fe₂O₃ nanoplates encapsulated in rGO nanosheets to form two-dimensional nanostructure.As an electrode material for supercapacitor,obvious peaks in CV curves of α-Fe₂O₃/rGO nanocomposites confirm that the capacitance characteristics are mainly governed by Faradaic redox reactions in the range-1.2V^-0.2V.The specific capacitance of α-Fe₂O₃/rGO nanocomposites is 903 F g-1 at the current density of 1 A g-1,which is superior to that of bare α-Fe₂O₃ nanoplates（347 F g-1）at the same current density.The clearly enhenced capacitive performance of α-Fe₂O₃/rGO nanocomposites can be put down to the synergetic effect of higher conductivity of crumpled rGO and mesosporous structure of α-Fe₂O₃ nanoplates.Thirdly,the α-MnS/N-rGO hybrid was fabricated by one-pot solvothermal approach using Mn（Ac）₂·4H₂O and L-cysteinein the GO-DMF system.The SEM and TEM results show that the α-MnS nanoparticles are actually composed of agglomerated α-MnS nanocrystals with averagesize about 5 nm,and abundant α-MnS nanoparticles uniformly anchor on thin,crumpled N-rGO sheets,and α-MnS nanoparticles with irregular shape on the surface of N-rGO nanosheets interconnect to form a network like structure.The atomic percentage of doped nitrogen,characterized by XPS,is about 1%.The CV curve of α-MnS/N-rGO hybrid indicates that the capacitance characteristics mainly result from Faradaic reaction in the range 0-0.65 V,which is based on the surface reversible redox mechanism.The specific capacitance of the α-MnS/N-rGO hybrid reaches as high as 933.6 F g-1 at the current density of 1 A g-1,which is 4 times of bare α-MnS.Moreover,the α-MnS/N-rGO hybrid also exhibits good rate capabilityand good cycling performance.Finally,two asymmetric supercapacitors（ASCs）devices were assembled using the α-MnS/N-rGO hybrid as positive electrode,N-rGO and α-Fe₂O₃/rGO nanocomposites as negative electrode,respectively.The potential window of the fabricated α-MnS/N-rGO hybrid//N-rGO ASCs and α-MnS/N-rGO//α-Fe₂O₃/rGO ASCs can up to 1.6 V in 3M KOH.The specific capacitance of α-MnS/N-rGO hybrid//N-rGO ASCs and α-MnS/N-rGO//α-Fe₂O₃/rGO ASCs are 77.9 F g-1and 140.6 F g-1 at the current density of 1 A g-1,respectively.The highest energy density of α-MnS/N-rGO hybrid//N-rGO ASCs and α-MnS/N-rGO//α-Fe₂O₃/rGO ASCs are 27.7 W h kg-1and 49.9 W h kg-1 at the power density of 800 W kg-1,respectively.