Preparation And Electrochemical Properties Of Flexible Asymmetric Supercapacitors Based On Cobalt(Nickel) Electrode Materials

Recently,with the rapid development of various wearable and intelligent electronic devices,it is necessary to develop miniaturized and high-performance flexible energy storage devices to meet the market demands.Among them,the flexible supercapacitor,which not only inherits the excellent electrochemical performance of traditional capacitors,but also possesses the advantages of small size and easy weavability,is considered to be an important choice for the future flexible energy device.However,the practical application of the flexible supercapacitor is seriously restricted due to its low energy density.Improving the energy density without sacrificing its power density is a critical challenge.Transition mental oxides have been investigated extensively as electrodes for supercapacitors,especially nickel and cobalt-based materials,which have attracted much attention due to their high conductivity and electrochemical activity.For the element of Se,which belongs to the same main group in the periodic table with element O,possess much higher conductivity,indicating that metal selenides have higher electron transmission capacity than the corresponding oxides and are expected to be ideal electrode materials.In addition,the synergistic effect of Ni-Co bimetallic ions also contributes to the further improvement of electrochemical properties.Thus,this paper mainly designed the following three kinds of excellent cobalt（nickel）-based materials:Co₃O₄@Ni-Co-S,CoSe₂@PPy and NiCo₂Se₄.And their applications in flexible asymmetric supercapacitors were systematically explored and deeply studied.The main results are as follows:（1）Preparation and electrochemical properties of flexible asymmetric supercapacitors based on Co3O4@Ni-Co-S electrode.The electrode materials with core-shell structure were prepared on a flexible titanium wire substrate by a simple hydrothermal reaction and electrochemical deposition method.The electrochemical test shows that the synergistic effect of the core-shell structure is beneficial to improve the capacitance of electrode materials.Finally,flexible asymmetric supercapacitors are assembled successfully with Co3O4@Ni-Co-S nano-flower as the positive electrode and FeSe2 as negative electrode.The voltage window of this kind of flexible solid state supercapacitor can reach to 1.4 V,which provides the volume specific capacitance of 5.01 F cm^-3 with the excellent electrochemical stability.（2）Preparation and electrochemical properties of flexible asymmetric supercapacitors based on CoSe₂@PPy electrode.The electrode materials with core-shell structure were prepared on a flexible titanium wire substrate by a simple selenization reaction and electrochemical deposition method,and the growth process of nano-flower was investigated by controlling the reaction time of PPy.Experimental results show that the composite electrode coated with highly conductive PPy can significantly improve the reaction speed and ion transmission path of charge transfer,which improve the electrochemical properties.Finally,flexible asymmetric supercapacitors are assembled successfully with CoSe2@PPy as the positive electrode and EACF as negative electrode.And the flexible asymmetric supercapacitors with a working voltage of 1.6 V presented excellent energy storage characteristics.Moreover,a TiO₂@ZnO photodetector can be driven by the two supercapacitors in series,which achieved the energy storage and conversion successfully.（3）Preparation and electrochemical properties of flexible asymmetric supercapacitors based on NiCo₂Se₄ electrode.The electrode materials were successfully synthesized via two-step hydrothermal approach,and the morphology of nanosheets was controlled by the proportion and time of reaction materials.Finally,flexible asymmetric supercapacitors were assembled by taking NiCo₂Se₄ nanosheet as the positive electrode and NiCo₂O₄@PPy as the negative electrode.The voltage window of the supercapacitors can reach to 1.7 V,which provides the volume specific capacitance of 14.2 F cm^-3.The energy density of the device can be maintained at 5.18 mWh cm^-3 under the high power density of 51.6 W cm^-3,indicating the great synergistic effect of Ni-Co bimetallic ions which is beneficial for the improvement of material properties.