Preparation Of C@MnO 2 Composite And Its Supercapacitor Properties

The supercapacitor,also named electrochemical capacitor,is a type of new energy storage device which is discriminated from batteries and traditional electrostatic capacitors.It has be used in various fields such as military,aerospace electronics and new energy vehicles,which is owing to its high power density,long cycle life and fast charging-discharging capacity.Supercapacitors,therefore,have a large potential development.The research hotspot on supercapacitors is mainly focused on electrode materials,including carbon materials,transition metal oxides and conductive polymer.Among series of transition metal oxides,manganese dioxide?MnO₂?,which due to its high specific capacity,environmental affinity,naturally occurring abundance and low operating cost,is considered to be the most promising electrode material.However,MnO₂ has a virtually small specific capacity because of its poor electrical conductivity and remarkable aggregation effect.Thus,it could be expected to acquire an obvious improvement on the specific capacity through combining MnO₂ with carbon nanofibers,which possess of large specific surface area and predominant electrical conductivity.In this study,we prepared a series of samples of carbon nanofibers,including solid carbon nanofibers?CNFs?,hollow carbon nanofibers?HCNFs?and hollow porous carbon nanofibers?PHCNFs?,by electrospinning polyacrylonitrile?PAN?precursor dissolved in DMF solvent.Then,MnO₂ component was introduced in the samples of specified CNFs through hydrothermal treatment on the as-spun fibers.A structure of solid carbon nanofibers coated with manganese dioxide?CNF@MnO₂?were obtained at various temperatures of hydrothermal processing,as was hollow carbon nanofibers coated with manganese dioxide?HCNF@MnO₂?.The preceding samples were characterized by scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?and N2 adsorption method.The electrochemical performance of electrodes,equipped with the preceding CNFs materials via tablet pressing technique,were measured using cyclic voltammetry?CV?,galvanostatic charge/discharge?GCD?and electrochemical impedance spectroscopy?EIS?techniques.The main conclusions were summarized as follows:?1?One-dimensional HCNFs were obtained through combining electrospinning technique and hard template method.Results of XRD shown that HCNFs were composed of amorphous graphite;the specific capacitance?Csp?of HCNFs was up to 124 F·g^-1,which larger then CNFs(31 F·g^-1)at the current density of 1 A·g^-1.?2?We studied the influence of activation time on PHCNFs.Results shown that the BET specific surface area?SBET?of PHCNFs increased firstly and then decreased with the increase of activation time,PHCNFs-60 had the largest Csp(147 F·g^-1)at the current density of 1 A·g^-1 which result from largest SBET(1117 m2·g^-1).?3?Solid carbon nanofibers coated with manganese dioxide?CNF@MnO₂?which possess different microstructures,were prepared by hydrothermal method.Results shown that with the increase of reaction temperature of hydrothermal,the crystal of MnO₂ transformed from K-Birnessite into alpha;the morphology of MnO₂ converted from nanorods into nanosheets and eventually become nanowires;CNF@MnO₂-170 had the largest Csp(164 m1·g^-1)at the current density of 0.5 A·g^-1;experiments of electrochemical stability shown 80%of the initial Csp was still retained after 800 GCD cycles.?4?Hollow carbon nanofibers coated with manganese dioxide?HCNF@MnO₂?were prepared by water bath.Results shown that with the increase of reaction temperature of water bath,the crystal of MnO₂ was K-Birnessite permanently;the load quantity of MnO₂ increased gradually,which also result in the increasement of the Csp for HCNF@MnO₂;HCNF@MnO₂-90 had the largest Csp(267 F·g^-1)at the current density of 0.5 A·g^-1;experiments of electrochemical stability shown 80%of the initial Csp was still retained after 800 GCD cycles.