Investigation On Preparation And Nanostructure Control Of Tubular Fe-Co Oxides Array Electrode Materials For Energy Storage

Energy shortage and environmental pollution are the major challenges for sustainable development.Developing green and high-performance energy storage devices is an effective way to solve this problem.Supercapacitors as a new type of environmentally friendly energy storage devices have attracted extensive research interest due to their fast recharge capability,high power density,and long cycling life.Preparation of electrode materials with favorable architectures allowing large porosity and optimized compositions is the key to push the energy storage performance of supercapacitors.Ternary metal oxides(such as MCo2O4 where M=Ni,Cu,Mn,Fe et al.)owing to their multiple oxidation states and high conductivity have become the ideal electrodes for supercapacitors.Tubular array structures have relatively high specific surface area and facilitate electrolyte diffusion.However,the reported tubular MCo2O4 nanostructures have smooth outer and interior surfaces endowing few electroactive sites and weak mechanical stability.Especially,the electrolyte diffusion into an electrode is limited,which means that most active material is "dead" for electrochemical energy storage,thus resulting in a low utilization ratio of electrode materials.These difficulties have restricted the performance of supercapacitors.To address the problems above,Firstly,FeCo2O4 submicron-tube arrays with good mechanical stability on Ni foam were prepared by chemical deposition followed by thermal treatment.Then,FeCo2O4 tubes were wrapped in MnO2 nanosheets by hydrothermal reaction.This hierarchical structure facilitates electrolyte and electron transport overcoming poor electrical conductivity of metal oxides and maximizing synergies of the components.A general method is proposed:novel hierarchical supercapacitor electrodes consisting of FeCo2S4-tubes with intersecting nanosheets built porous shells via controlled sulfidation were successfully prepared.Nanoporous micron tube arrays constructed by intersecting nanosheets have multiple channels,which will largely increase ion-accessible surface area and effectively avoid the "dead volume."Finally,all-solid-state devices are assembled and researched the practical application.The main contents are summarized as follows:(1)Template-free chemical growth on Ni foam and thermal treatment results in homogeneous FeCo2O4 submicron-tube arrays.Porous array structures increase the specific surface area of electrode material and facilitate electrolyte ion diffusion.the high conductivity of FeCo2O4 and a good connection between the tubes and the Ni endow the arrays with good electric conductivity.Combination between submicron tubes and metal substrate results in good mechanical stability.Owing to conductivity,high porosity,and strong bonding between tubes and Ni foam allow FeCo2O4 electrodes with high capacitance,rate-capability and cycling-stability.Assembled all-solid-state supercapacitors have superior energy density.The areal capacitance of the device is 0.67 F cm-2 at current density of 2 mA cm-2.An energy density of 30.9 Wh kg-1 is achieved at a power density of 1551 W kg-1.(2)In order to further improve electroactive sites of FeCo2O4 submicron-tube arrays,hierarchically MnO2 nanosheets covered FeCo2O4 submicron-tube arrays on Ni foam were prepared by hydrothermal reaction.The covers' thickness can be adjusted by reactant ion concentration.Submicrometer tubes provide efficient spaces for growing MnO2 to optimize the structure in nanoscale.Under the optimal concentration,nanosheets distribute homogeneously on the tubes,building a porous coating via a cross-linked structure,which shortens ion diffusion distance.The specific capacitance of the optimum FeCo2O4@MnO2 composite electrode is 3.30 F cm-2(1.0 mA cm-2)doubling of the bare FeCo2O4-tube forests.A symmetric solid-state supercapacitor made from these electrodes achieves 2.52 F cm-2.The device also exhibits superior rate capability and remarkable cycling stability.(3)Based on the insights into concentration and time dependence on sulfidation reaction kinetics,also from the perspective of controlling sulfidation kinetics,hierarchical electrodes with FeCo2S4 nanosheet-built porous micron tubes were prepared by a controlled sulfidation process.FeCo2S4-tubes with square cross-section and intersecting nanosheets built permeable multiple channels leading to maximized electrolyte ions accesible surface area,and thus effectively overcoming the difficulties of "dead volume".Especially,This research mentality and the preparation method can be generally extended to other ternary metal sulfides.Owing to high electrical conductivity,rich redox reactions and highly porous,the FeCo2S4 electrode achieves a specific capacitance reaching 2411 F g-1 at 5 mA cm-2.An asymmetric solid-state device applying the FeCo2S4 as positive electrode and graphene as negative electrode has a high cell voltage of 1.6 V and thus delivers considerably higher energy density of 76.1 Wh kg-1(at 755 W kg-1).