Preparation Of ZnCo₂O₄-based Composite Materials For Supercapacitor Electrodes And Their Investigation Of Electrochemical Performance

With the continuous development of energy storage equipment,supercapacitor,as a new type of energy storage device,has advantages in power density,charging speed and service life,and has broad scope in the fields of electric vehicles and microelectronics.In recent years,supercapacitor has become one of the research hotspots in scientific research field However,the main disadvantage of the current supercapacitor is that the energy density is not high,which limits its application range.In general,the performance of supercapacitors is largely influenced by the electrode material.Therefore,how to regulate and optimize the electrode materials is our research emphasis and challenge.Transition metal oxides/hydroxides possess the advantages of simple preparation procedure,good electrical conductivity,affordable price and environmental protection.Especially ternary metal oxides have various valence state and can transfer more electrons when redox reactions happen.Therefore,ternary metal oxides have better electrical conductivity.However,single-component electrode materials still have pretty low energy density.Thus,the synergistic effect is used to combine different complementary electrode materials to form a specific morphological structure.For example,an electrode material having a core-shell structure can obtain a larger specific surface area,and thus more reactive sites can be obtained to rapidly transport electrons and ions in the electrolyte,thereby achieving a leap in electrochemical performance.In this thesis,a series of composite electrode materials based on ZnCo₂O₄ were prepared by hydrothermal method or electrodeposition method and its relationship between morphology and electrochemical properties was explored.The main contents are as follows:1.The core-shell structured ZnCo₂O₄/ZnWO₄ nanowires were prepared by a simple two-step hydrothermal method and post calcination procedure.The crystal phase structure and composition of the as-prepared composites were studied by means of XRD and XPS.Furthermore,SEM and TEM were used to study the morphology,a series of tests were performed to evaluate their electrochemical performance.The result shows that the electrochemical performance of ZnCo₂O₄/ZnWO₄ hybrid electrode is significantly improved and its areal capacity can reach 13.4 F cm^-2 under the current density of 4 mA cm^-2,while ZnCo₂O₄ electrode can only reach 1.5 F cm^-2under the same condition.In addition,we used the as-prepared ZnCo₂O₄/ZnWO₄ composite electrode to assemble an asymmetric supercapacitor.The result shows the maximum of the as-assembled ASC's energy density can reach 24 W h kg^-1,and its capacity retention rate was up to 98.5%after 5000 cycles.Finally,we made two as-assembled ASCs connected in series and it can light a small white LED for 30 minutes,which indicates that the ASC has certain application prospects.2.The core-shell structured ZnCo₂O₄/Co?OH?₂ nanowires were prepared by hydrothermal method and subsequent electrodeposition method,and their structures were tested by XRD,XPS,SEM,and the electrochemical performance were tested through an electrochemical workstation.As expected,the results indicate that the electrochemical performance of ZnCo₂O₄/Co?OH?₂ hybrid electrode is obviously improved and its specific capacity can reach 995 F g^-1 at 1 A g^-1,while ZnCo₂O₄electrode and Co?OH?₂ electrode can only reach 350 F g^-1 and 820 F g^-1 respectively under the same condition.Additionally,the as-prepared ASC has a maximum energy density of 29.6 W h kg^-1 and it can still retain the specific capacity of 84.1%after 5000cycles.The as-assembled ASC in series can illuminate a white LED,which indicates that the ASC is a promising energy storage device.3.The ZnCo₂O₄/Ni?OH?₂ nanosheets with core-shell structure were synthesized by hydrothermal method and subsequent electrodeposition method,and their composition and corresponding morphologies were studied and their electrochemical properties were tested.Consequently,the results turn out that the as-prepared ZnCo₂O₄/Ni?OH?₂hybrid electrode has outstanding electrochemical performance after combination.Moreover,the specific capacity of the hybrid electrode is as high as 2100 F g-1at 1 A g^-1 and the energy density of as-assembled ASC is 36 W h kg-1.After 5000 continuous charge/discharge cycles,the ASC can still retain 98.61%of the initial capacity.Finally,two ASCs in series can illuminate three white LED in parallel,presenting excellent practical application potential.4.The hierarchical nanostructure of the ZnCo₂O₄/Co₃O₄ was prepared by hydrothermal method and post-annealing process.The specific capacity of ZnCo₂O₄/Co₃O₄ electrode is greatly improved compared with pure ZnCo₂O₄ electrode.Specifically,the specific capacity of as-prepared hybrid electrode at can reach up to1115 F g^-1 at 1 A g^-1.Besides,the as-assembled ASC has an energy density of up to38.2 W h kg^-1,and the ASC can still retain 91.1%of the initial capacity after 5000continuous charge/discharge cycles.Connecting two as-assembled ASCs in series can illuminate three blue LED in parallel,which tells that the device possess certain practical application prospects.