Copper Oxide-based Electrode Materials For Flexible Supercapacitor: Design, Construction And Electrochemical Performance

With the rapid advancement of wearable devices and flexible display technology,the demands for energy storage devices are greatly enhanced.Among them,flexible supercapacitor has been recognized by the markets and favored by consumers because of its excellent energy-power density and long cycle life.Copper oxides（Cu O and Cu₂O,Cu_xO）,as common metal oxides,have raised great concern for their superiorities,including high theoretical capacitance,non-toxicity,abundant raw materials and easy preparation.However,Cu_xO suffers the inherent properties,just as the other metal oxides,such as low conductivity,easy agglomeration as nano-phases and low cycling stability,which limit its application in energy storage field.Although there are many methods to prepare Cu_xO nanomaterials,traditional coating method will cause serious agglomeration and poor actual specific capacitance.Besides,the weak connection between the active materials and electrode will lead to a sharp performance decline after long period of cyclic charge-discharge test.Therefore,preparing Cu_xO composites with uniform morphology,non-agglomeration,long cycle life as well as high conductivity and specific capacitance has become the focus of current researches.In this paper,Cu_xO nanowires with uniform morphology were synthesized by chemical oxidation on the surface of Cu foam（CF）,which works as the Cu source and collector.Moreover,Cu_xO was sequentially modified by using Ag nanoparticles and Mn₂O₃nanosheets to improve its electrochemical energy storage characteristics and broaden its application fields.Cu_xO/CF composites were prepared by chemical surface oxidation and calcination process.Copper foam provides a large number of active sites for the growth of Cu_xO nanowires,and acts as a copper source without the participation of other copper precursors.Cu_xO nanowire arrays are grown evenly and orderly on the copper foam frames without agglomeration.Cu_xO/CF-20,Cu_xO/CF-40,Cu_xO/CF-60 and Cu_xO/CF-80 samples are obtained by adjusting the reaction time,in which Cu_xO/CF-60 possesses the best surface morphology and electrochemical properties.The maximum specific capacitance of Cu_xO/CF-60 delivered at 2 m A cm^-2 is 354.6 m F cm^-2.It still retains93.8%of the initial capacity after 5000 cycles,indicating that the Cu_xO/CF self-growing electrode possesses excellent cycle stability.When the scanning rate expands from 10 m V s^-1 to 100 m V s^-1,the pseudocapacitance contribution increases from 23.5%to 48.7%.The R_sand R_ct of the all-solid-state asymmetric supercapacitor are 2.29Ωand 7.19Ω.The maximum energy density is calculated as 25μWh cm^-2 with a maximum energy density reaching 6 m W cm^-2.To solve the problem of poor conductivity of Cu_xO,Ag nanoparticles were synthesized on Cu_xO/CF by a simple replacement reaction.Ag/Cu_xO/CF-10,Ag/Cu_xO/CF-20,Ag/Cu_xO/CF-40 and Ag/Cu_xO/CF-60 are obtained by adjusting the reaction time.Electrochemical results show that compared with those of Cu_xO/CF,the R_s and R_ct of Ag/Cu_xO/CF-40 are reduced by 28%and 55%,respectively,implying that the electrical conductivity of the composite material is significantly improved.At the current density of 2 m A cm^-2,the specific capacitance of Ag/Cu_xO/CF-40 is 1192 m F cm^-2,which is more than twice that of Cu_xO/CF.The final capacitance after 10000cycles still reaches 1112 m F cm^-2,indicating an excellent cycle life.When the scanning rate expands from 5 m V s^-1 to 100 m V s^-1,the pseudocapacitance contribution increases from 45.1%to 88.1%.A flexible asymmetric supercapacitor is assembled by using Ag/Cu_xO/CF-40 as positive electrode.The R_s and R_ct of the device are 1.53Ωand 0.21Ω,respectively,indicating that the resistance is greatly improved.The maximum energy density and power density are 46.32μWh cm^-2 and 9 m W cm-²,respectively.Two devices are connected in series,which could light a 2 V LED for 90 s.Bending the device for 250 times under the condition of 0-90°,the performance only decreases as6.9%.To further enhance the specific capacitance of the electrode together with the energy density of the device,Mn₂O₃ nanosheets were successfully synthesized on the surface of Cu_xO nanowires by hydrothermal method and the core-shell structure of Mn₂O₃/Cu_xO was successfully constructed.Three samples including Mn₂O₃/Cu_xO-6,Mn₂O₃/Cu_xO-9 and Mn₂O₃/Cu_xO-12 are prepared by adjusting the reaction time and a series of characterizations and electrochemical tests are conducted for comparison.A maximum specific capacitance is obtained by Mn₂O₃/Cu_xO/CF-9(1403.5 m F cm^-2)and it still retains 92.8%capacitance retention after 10000 cycles,which proves that the core-shell structure is effective in stabilizing the material structure and maintaining the cyclic performance.The R_s and R_ct are 0.78Ωand 0.18Ω,respectively,implying the good electrical conductivity.When the scanning rate expands from 5 m V s^-1 to 100 m V s^-1,the pseudocapacitance contribution increases from 59.8%to 86.8%.The R_s and R_ctof the flexible asymmetric supercapacitor are 1.80Ωand 1.53Ω,and the maximum energy density and power density of the flexible asymmetric supercapacitor are 120.2μWh cm^-2 and 6.49 m W cm^-2,respectively,which are greatly improved compared with the Cu_xO//AC and Ag/Cu_xO//AC.Three devices connected in series can light the LED for 4 minutes.In this work,Cu_xO nanowires were synthesized by surface oxidation method on the skeleton of copper foam,which greatly improved the homogenous and cycling life of nanomaterials,and hence the electrochemical performance has been enhanced greatly.Ag nanoparticles further promote electron transfer and exchange of the electrode,which improve the electrical conductivity.Nevertheless,two dimensional Mn₂O₃ nanosheets are synthesized by using hydrothermal method.The specific capacitance is enhanced greatly with excellent cycle performance.Furthermore,the energy density of device has also been improved.Flexible asymmetric supercapacitors have good flexibility and mechanical stability.There is no obvious performance degradation after multiple bending and these devices could light LED continuously.In recent years,the technologies of wearable and flexible display devices have become mature gradually.Therefore,the research on high-performance and flexible energy storage device is of great significance in this situation.