Research On Flexible Supercapacitors Based On Metallic Oxide And Hydroxide Electrodes

Supercapacitor is a new type of energy storage device between traditional capacitor and battery.It has the advantages of high power density,high energy density,long service life,and is cost-effective as well as environment friendly.With the rapid development of wearable,foldable and flexible electronic products,thin flexible supercapacitor which can provide energy for them has become one of the research hotspots in the current industry and academia.At present,the research focuses on electrode materials and electrolytes,but there is very little research on flexible structures and interfaces.In addition,commercial carbon cloth and nickel foam are mostly used as the current collector of electrodes,but it is difficult to achieve good electrochemical and mechanical properties simultaneously.Therefore,this paper proposes the development of flexible supercapacitors based on metal oxide and hydroxide electrode,in order to both ensure the electrochemical performance of the device and improve its flexibility.First of all,a kind of flexible conductive substrate with special structure is designed as the current collector,and a conductive nickel layer with nano tip array is uniformly grown on the light and soft chemical fiber fabric by a chemical method similar to "silver mirror reaction".A large area of flexible conductive substrate with surface resistance between 0.3 ? and 0.5 ? is obtained,which has excellent performance of bending and washing.It can be used not only in current collector of electrodes,but also in the field of electromagnetic shielding and flexible electronics.Based on this flexible current collector,metal oxides and hydroxides with excellent electrochemical performance are selected as electrode materials,including Co?OH?₂,NiO and NiCo₂O₄.The materials are synthesized in situ on the flexible collector by electrodeposition and heat treatment.The three electrodes show good morphology and electrochemical performance.The specific capacitance of Co?OH?₂,NiO and NiCo₂O₄ electrodes are 880.1 F g^-1,589.1 F g^-1 and 725.7 F g^-1 at the current density of 1 A g^-1,respectively.Use the fabricated Co?OH?₂,NiO and NiCo₂O₄ as cathode electrode,respectively,activated carbon as negative electrode and PVA/KOH as gel electrolyte to assemble into flexible solid supercapacitors.After testing and characterization,the three supercapacitors show excellent electrochemical and mechanical properties.The specific capacitance of Co?OH?₂//AC,NiO//AC and NiCo₂O₄//AC supercapacitors are 170.3 m F cm^-2(64.0 F g^-1),265.6 m F cm^-2(89.4 F g^-1)and 219.8 m F cm^-2(60.2 F g^-1),respectively,with the maximum energy density of 1.24 m W h cm^-3,1.63 m W h cm^-3 and 1.35 m W h cm^-3 at the current density of 1 m A cm^-2.The specific capacitance maintain 63.0%,233.3% and 91.9% after 3000 cycles of charge and discharge,and 92.8%,94.4% and 95.1% after 1000 cycles of bending for the three supercapacitors,respectively.In order to explore the action mechanism of this flexible collector,four different current collectors are compared respectively,including self-made nickel fabric,commercial electromagnetic shielding fabric,commercial carbon cloth and commercial foam nickel.Co?OH?₂ materials are synthesized on these collectors by the same process,respectively.By comparison,the self-made nickel fabric not only has outstanding micro morphology,but also can provide large specific surface area,good conductivity,excellent interface characteristics and stable mechanical properties.The electrode also has advantages in electrochemical performance and bending performance.By analyzing its equivalent circuit diagram in electrode reaction,we find that it is similar to carbon cloth,with small electrode resistance and charge transfer resistance,and the electrode surface is in a relatively ideal state.Therefore,the flexible conductive substrate has many advantages when it is used for current collector of electrodes,and the prepared flexible all-solid-state supercapacitor has excellent electrochemical and mechanical properties.