Electrochemical Properties Study Of Supercapacitors Based On Carbon Fiber And Manganese Dioxide Electrode Materials

With the decrease of non-regenerated energy and the increasing demand for energy globally,the pursuit of regenerated energy and high performance energy storage and conversion devices is increasingly urgent.Supercapacitors,with the advantages of long-term cyclic stability,fast charging/discharging ability?high power density?and low maintenance cost,are widely accepted as energy storage devices.According to the energy storage mechanisms,they can be commonly divided into double-layer capacitors and pseudocapacitors.Double-layer capacitors mainly use porous carbon as electrode material.Its large specific surface area can rapidly accumulate a large amount of charge to store energy and its stable structure can carry out efficient long-term circulation.Pseudocapacitors are given priority to transition metal compounds,where more charge can be stored through Faradic reactions than carbon based material.However,power density of the pseudocapacitors is always inhibited by the poor conductivity of transition metal compounds.Meanwhile,obvious structure changes during cycling usually limit their stability.In this thesis,towards these key scientific issues,carbon and MnO₂ based supercapacitive electrode materials are designed and prepared,then functional devices are assembled using them.High stability and power density together with considerable energy density are realizedthrough aperture adjustment of carbon materials and doping modification of MnO₂.?1?Flexible carbon film with high surface area and high conductivity?HSHC-CFF?is synthesized through controlled heat treatment and served as supercapacitive electrode directly without the using of current collector or any other additives.The HSHC-CFF possesses low resistance and high specific surface area of 951 m² g^-1 with abundant micropores of?0.7 nm in diameter.In aqueous solution of 1 M H₂SO₄,the film exhibits a high capacity of 255 F g^-1(3.32 F cm^-2,127.7 F cm^-3)at 0.5 A g^-1 in a wide potential window of-0.6?1.2 V vs Ag/Ag Cl due to the linear counter-ions'accommodation in the micropores.When considering the weight of the two whole film electrode,a high energy density of 16 Wh kg^-1 is achieved at160 W kg^-1.Importantly,the high performance is maintained and even better along cycling of over 20,000 cycles?118%of the initial capacity is achieved after cycling?.A mechanism of micropore??0.7 nm?induced dehydration process of electrolyte ions and linear counter-ions'accommodation in the pores are proposed,which is supposed to contribute the high areal specific capacitance and wide operation window.?2?As a pseudocapacitor electrode material,the layered-MnO₂ has good cyclic stability due to the adverse effect of H⁺ions on the mechanism of energy storage.However,the crystal structure of?-MnO₂ changes during the long-term cycle resulting in its cyclic stability is not enough for practical use.?-MnO₂-based supercapacitors are still challenging to achieve long-term reversible charge storage/release.In the work here,different molar ratio of Zn is efficiently doped into the?-MnO₂ through controlled addition of Zinc salt during the growth process of MnO₂.It is found that the capacity undergoes a periodic oscillation for the sample with suitable Zn doping while the capacity declines monotonously accompanied by mass loss during cycling for the pure?-MnO₂ electrode.The sample Zn-3 with a Zn:Mn feed molar ratio of 3:100 exhibits best electrochemical performance.An initial specific capacity of 280 F g^-1 at 0.2 A g^-1 and a high retention of?67%at 10 A g^-1 is achieved,indicating good rate performance.During long-term cycling at a discharge current of 10 A g^-1,the capacitance undergoes an oscillation change with a period of about 5,500 cycles:the initial capacity is improved to 117%in the first 500 cycles and then decreases to about 107%in the following?5,000 cycles followed by another round increase and decline,which is accompanied by a same oscillation change of electrolyte,yellow and clear?formation and disappearance of brown deposition?.105%of the initial specific capacity is remained after 20,000 cycles.Similar oscillation can be observed for the sample Zn-1 with less Zn,while the capacitance enhancement is much less.The oscillation period of Zn-5 with more Zn is very short and the enhancement is much smaller than the decline,resulting in obvious decay of the capacitance along cycling.Through careful characterization to the samples before and after cycling,an orientated reattachment mechanism is proposed to explain the capacity change along cycling,where Zn doping is supposed to play a very important role to this process.