Design,Fabrication And Performance Study Of The Flexible Supercapacitor Electrode

People's personalized demand for electronic products has driven the emergence and development of flexible electronic technology,which has changed our ways of thinking and life to some extent in the past few years.Flexible electronic technology has been regarded as one of the most competitive and promising technologies in the21st century.With the development and progress of flexible electronic technology,related flexible electronic products have successively appeared in the fields of flexible display,implantable medical device,portable and wearable device,etc.Different from the common traditional electronic products,flexible electronic products possess a variety of novel features such as bendability,foldability,flexibility,shape deformation,etc.Therefore,flexible energy devices,an energy supply for the flexible electronic products,came into being.Flexible supercapacitor,an important part of the flexible energy devices,exhibits a series of advantages such as fast charge and discharge rate,high power density and long cycle life,and ensures the continuous and stable energy supply under the condition of mechanical deformation.And it can be used as a flexible power supply and embedded in movable and arbitrarily shaped objects,which are unachievable by conventional power sources.However,low energy density and narrow potential window of flexible supercapacitor restrict its practical applications.It is worth noting that the electrochemical performance of the flexible supercapacitor is mainly determined by the flexible electrode.The flexible electrode with high specific capacitance and wide voltage window can effectively improve the final energy density of the device.Therefore,it is particularly important to design and develop flexible electrodes with excellent electrochemical properties.In this paper,we fabricated the flexible supercapacitor electrodes with excellent electrochemical performance by utilizing or preparing a variety of flexible current collectors with high electrical conductivity and then directly growing active materials on these flexible current collectors based on the design of flexible electrode structure.The main works of this paper are summarized as follows:1.Fabrication and performance of flexible supercapacitor electrode materials based on filter paper.Taking advantage of good flexibility,rough surface and rich oxygen containing functional groups of cellulose fibers,we coated the uniform and continuous metal nickel layer on the surface of the cellulose fibers to form a complete conductive path by the traditional electroless plating method that preliminarily changed the conductivity of the filter paper.Then,to further improve the electrical conductivity of flexible substrate,the thick porous metal nickel layer was rapidly coated on the surface of electroless nickel plating layer by subsequent electroplating method,and the flexible current collector with high conductivity was obtained.Finally,the electrochemical active material Co?OH?₂ was coated on the flexible current collector by electrodeposition to fabricate the positive electrode Ni/Co?OH?₂-LFP,and active carbon slurry was coated on the flexible current collector by physical coating to fabricate the negative electrode Ni/AC-LFP.The flexible asymmetrical solid-state supercapacitor was assembled and the solid electrolyte was the mixed slurry of ionic liquid EMIMBF₄ and fumed silica.It exhibits the large volume energy density?0.64 mW·h/cm³?,wide operating voltage window?2 V?and good bending cycle stability.The capacitance value of the flexible supercapacitor reaches up to95.89%of the original capacitance value after 500 bending cycles.2.Preparation and properties of mesoporous carbon hollow spheres/manganese dioxide electrode materials for flexible supercapacitor.Mesoporous carbon hollow spheres?MCHS?were first fabricated by silica template method,and then MnO₂nanosheets were directly grown on the inner and outer surface of MCHS by hydrothermal method to obtain hollow MCHS/MnO₂ microspheres.Then MCHS/MnO₂ and MCHS were respectively used as positive and negative electrode active materials to prepare electrode slurry and the slurry was directly coated on nickel foam to obtain the flexible MCHS/MnO₂-NF and MCHS-NF electrodes,and the asymmetric flexible supercapacitor was assembled with neutral Na₂SO₄ solution as electrolyte.The results show that the flexible supercapacitor presents excellent electrochemical performance,it exhibits high energy density?64.6 W·h/kg?and excellent cycle stability?90.3%of the initial specific capacitance after 6000 GCD cycles?.And the voltage window of flexible supercapacitor device can reach up to 2 V because of the asymmetric assembly.3.Preparation and properties of surface functionalized carbon fiber cloth electrode materials for flexible supercapacitor.The flexible carbon fiber cloth electrodes with excellent electrochemical performance were fabricated by the strong corrosion and oxidation treatment of carbon fiber cloth?OCC?,and a large number of oxygen containing functional groups?O-C=O,C?O and C-O?were successfully introduced on the surface of carbon fibers and the dense and inert carbon fiber surface was activated after treatment.The OCC electrodes'surface is loose and contains active carbon layer,and exhibits super-hydrophilicity.At the same time,the carbon fiber skeleton is intact and not damaged,which ensures the smooth transmission channel for electrons.After 18 h of strong corrosion and oxidation treatment,flexible OCC-18 electrode exhibits large area specific capacitance of 588 mF/cm² at current density of 1 mA/cm²,more importantly,it possesses an ultra wide operating potential window of-1.2-1.2 V in neutral electrolyte of Na₂SO₄ solution,and the assembled symmetric supercapacitor device exhibits a large operating voltage of 2.2 V.What's more,the device presents high energy density of 0.87 mW·h/cm³ and superior cycling stability?86.8%of capacitance retention after 10000 cycles?.4.Preparation and performance of paper based flexible electrode based on MXene conductive material.Taking advantage of the strong hydrogen bond between the cellulose fiber and two-dimensional material MXene,we coated the conductive material MXene on the surface of the cellulose fibers to form the continuous and uniform conductive layer by the method of repeated immersion and vacuum drying,thus initially changing the electrical performance of the filter paper and obtaining the flexible current collector with good electrical conductivity.Then,the MXene layer was rapidly coated with the dense metal nickel layer by electroplating to greatly improve the stability and conductivity of the flexible current collector which can be applied in the field of flexible electronics.Finally,the flexible electrode with superior electrochemical performance was prepared by anodic electrodeposition of MnO₂based on the fabricated flexible current collector.The prepared flexible electrode possesses fast electron transfer rate and ion diffusion rate,small charge transfer resistance and good rate capability,and it exhibits the large specific capacitance of1119.5 mF/cm² at current density of 3 mA/cm²,which can be used as positive electrode in flexible supercapacitors.