Controlled Preparation Of MgCo₂O₄ Electrode Materials And Their Applications In Supercapacitors

Over the past few years,supercapacitor,an electrochemical energy storage(EES)system,has received increasing attention due to its widespread applications in satellite communication systems,microgrids,and hybrid electric vehicles.Especially,the hybrid supercapacitors(HSCs)or asymmetric supercapacitors(ASCs)assembled with battery-type and capacitive electrodes,combine the advantages of high capacity of battery-type cathodes and wide voltage window of electric double layer capacitive(EDLC)anodes,and they generally exhibit excellent electrochemical performance.However,the poor structural stability and low energy density of electrode materials may limit their practical applications in some fields requiring high rate and long-life energy storage.As a binary transition metal oxide,MgCo₂O₄ has attracted extensive attention in supercapacitors due to its high theoretical capacity,relatively good environmental friendliness,and abundant natural reserves of elemental magnesium.In this thesis,we use different methods to synthesize magnesium cobaltate electrode materials with various structures,and the detailed content is described as follows:(1)Microsphere-like MgCo₂O₄(MCO-1)and nanosheet-like MgCo₂O₄(MCO-2)containinga small number of microspheres were synthesized by one-and two-step solvothermal reactions at 190 ?,respectively.Both MCO-1 and MCO-2 materials possessed high specific surface area of 69.4 and 52.8 m~2 g^-1,respectively.The MCO-2 delivered a specific capacity of 375.5 C g^-1at 1 A g^-1 along with a capacity retention of 74.9%at 10 A g^-1.The specific capacities of MCO-1 at 1 A g^-1 and 10 A g^-1 were calculated to be 276.3 and 216.2 C g^-1,respectively.HSCs were assembled with MCO-2(MCO-1)as the cathode and activated carbon(AC)as the anode,and these HSCs could be operated over a voltage window of 1.75 V.The MCO-2//AC HSC possessed a high energy density of 35.4 W h kg^-1 at a power density of 950.6 W kg^-1,and it could still hold 25.8 W h kg^-1 at 8905.0 W kg^-1.The specific capacity of MCO-1//AC HSC and MCO-2//AC HSC did not show any decay after 6000 cycles at a high constant current density of 6 A g^-1.(2)MgCo₂O₄ nanoflakes(NFs)and MgCo₂O₄ nanowires(NWs)electrode materials were direct grown on stainless-steel foil by a simple hydrothermal method.Both MgCo₂O₄ materials exhibited a mesoporous structure,and the electrochemical tests demonstrated their battery-like properties.The MgCo₂O₄ NFs powder peeling off from the stainless-steel foil exhibited a high specific capacity of 424.98 C g^-1 at 1 A g^-1.The specific capacity of MgCo₂O₄ NWs was calculated to be 351.17 C g^-1 at the same current density.An ASC was assembled with MgCo₂O₄NFs(NWs)as cathode and AC as anode.The MgCo₂O₄ NFs//AC ASC exhibited excellent electrochemical properties,such as a specific capacity as high as 146.10 C g^-1,long-life cycling stability with a capacity retention of 108.28%after 5000 cycles at 6 A g^-1,a high energy density of 41.10 W h kg^-1 at a power density of 1012.80 W kg^-1.As for the MgCo₂O₄ NWs ASC,it delivered an energy density of 36.97 W h kg^-1 at a power density of 1063.16 W kg^-1.(3)MgCo₂O₄ NWs and micro-cubes(MCs)were successfully grown on the nickel foam via a hydrothermal method at 140 ? for 9 h with a subsequent calcination.This synthetic method avoided the usage of conductive agents and binders during the working electrode fabrication process.When the current density was increased from 1 to 10 A g^-1,the MgCo₂O₄ NWs@NF electrode exhibited a specific capacity of 389.0 C g^-1 and a capacity retention of 86.2%.At the same conditions,the MgCo₂O₄ MCs@NF possessed a specific capacity of 273.2 C g^-1 and 64.4%rate performance.The HSC was assembled with AC as anode,and after 5000 cycles at 6 A g^-1,the MgCo₂O₄ NWs@NF//AC HSC exhibited a long-term cycling stability without any capacity decay,and delivered an energy density of 37.9 W h kg^-1at a power density of 958.1 W kg^-1.In contrast,the energy density of MgCo₂O₄ MCs@NF//AC HSC was calculated as 30.4 W h kg^-1at 887.2 W kg^-1.