Controlled Synthesis And Electrochemical Properties Of MnCo₂O_4.5 Electrode Materials For Supercapacitor Applications

As an electrochemical energy storage device,supercapacitor has drawn considerable attention in the past decades owing to its high power density and excellent cycling stability.However,the relatively low energy density usually limits its further applications.The electrochemical performance of electrode material often plays an important role in determining the charge storage capability of a supercapacitor.As a consequence,the development of high-performance electrode material is a great significance for the practical applications of supercapacitor.The manganese cobalt oxides including Mn Co₂O₄,Co Mn₂O₄and Mn Co₂O_4.5 have been demonstrated to be promising electrode materials in the field of supercapacitor due to their excellent redox activity,high theoretical specific capacity and eco-friendliness.This thesis aims to synthesize Mn Co₂O_4.5 electrode materials with controlled structure and superior electrochemical performance,and the main research work is described as follows.（1）Uniform Mn Co₂O_4.5 nanowires（NWs）and quasi-cubes（QCs）on stainless-steel foil（SSF）were prepared through a facile hydrothermal method with a post-calcination treatment in air.The morphology of Mn Co₂O_4.5 electrode materials prepared at different hydrothermal temperatures was investigated in details.The powdered Mn Co₂O_4.5 NWs were finally obtained by peeling off these NWs from SSF,and such NW powders delivered a high specific capacity of 248.62 C g^-1 at 1 A g^-1 and 179.43 C g^-1 at 8 A g^-1,respectively.In contrast,the Mn Co₂O_4.5 QCs exhibited 177.19 and 111.73 C g^-1,accordingly.To assess the possibility of these NWs and QCs in actual applications,a hybrid supercapacitor（HSC）device was assembled by utilizing the Mn Co₂O_4.5 NWs（QCs）as cathode and activated carbon（AC）as anode.The Mn Co₂O_4.5-NWs//AC HSC exhibited a maximum capacity up to 116.95 C g^-1 and extraordinary cycling durability with only 3.56%capacity loss over 5000 cycles.Besides,the Mn Co₂O_4.5 NWs//AC HSC delivered a maximum energy density of 25.41 W h kg^-1 at a power density of 782.08 W kg^-1.As for the QC-based HSC,it possessed a lower energy density of 20.54 W h kg^-1 at 843.34 W kg^-1.（2）Porous Mn Co₂O_4.5 NWs were directly grown on the nickel foam via an initial hydrothermal route with an extra annealing treatment of precursor,and particle-based Mn Co₂O_4.5 films were also obtained by using different amount of urea in the synthetic process.These binder-free Mn Co₂O_4.5@NF electrodes exhibited battery-type electro-chemical response with superior electrochemical performance.Especially,the Mn Co₂O_4.5-NWs@NF exhibited a huge capacity of 288.47 C g^-1 at 1 A g^-1 along with 78.07%capacity retention at 10 A g^-1.Moreover,an HSC that was assembled using Mn Co₂O_4.5-NWs@NF as cathode and AC as anode could operate over a voltage of 1.7 V.The Mn Co₂O_4.5-NWs@NF//AC HSC delivered an energy density of up to 29.57 W h kg^-1 at the power density of 914.28 W kg^-1,and could still hold 20.82 W h kg^-1 as the power density was improved to8.24 k W kg^-1.In addition,both the Mn Co₂O_4.5 NWs-and film-based HSCs showed an impressive cycling stability over continuous 6,000 cycles at a high current density of 6 A g^-1,and only a little capacity decay was observed.（3）Mn Co₂O_4.5 microspheres（MSs）assembled with thin nanosheets were synthesized through a simple solvothermal approach in the mixed solvent of isopropanol,glycerol and de-ionized water.The Mn Co₂O_4.5 MSs possessed a large specific surface area of 156.85 m~2 g^-1.The specific capacity of Mn Co₂O_4.5 MSs could reach 287.02 C g^-1 at 1 A g^-1,and remained73.3%of initial capacity as the current density was increased to 8 A g^-1.The HSC device with a voltage window of 0～1.75 V was fabricated using Mn Co₂O_4.5 MSs as cathode and AC as anode,respectively.The specific capacity of Mn Co₂O_4.5 MSs//AC HSC retained 103.04%of its original value over 5000 charging-discharging cycles at 6 A g^-1,indicating an excellent cycling durability.In addition,the HSC device exhibited a maximum energy density of 30.33W h kg^-1 at 959.35 W kg^-1,and it still could hold 19.99 W h kg^-1 as the power density was increased to 8.72 k W kg^-1.