Carbon Nanofibers Composite Electrode For Supercapacitors

Supercapacitors as an important energy storage device with high power density,rapid charge and discharge capacity,long cycle stability have broad application prospects in many fields?such as military,hybrid electronic,mobile devices,etc?.The main challenges of supercapacitor are their relatively low energy density and rate performance.Therefore,the main research of this paper is to improve the conductivity of electrode materials.The electrospinning technology is used to prepare carbon nanofibers,and the electrochemical performance of porous carbon nanofibers prepared with different precursor is studied.Here,carbon nanofibers not only act as physical support to load the active materials,but also as a good conductive channel to facilitate the electron's transportation in the composite materials.At the same time this kind of one dimensional structure also facilitate the contact between the active materials and the electrolyte ions,thus stored energy through the chemical reaction.The concrete research content is as follows:Bacterial cellulose?BC?is selected as precursor of carbon nanofibers,because it has a large aspect ratio to get high specific surface area?SSA?carbon nanofibers.The abundant surface functional groups?hydroxy/carboxylic groups?of BC can serves as an excellent adsorbent materials,the absorbed OPPs molecules are large enough to create loose 3D networks after freezing dry,thus,prevent the raw BC nanofibers from aggregation/conglutination during high-temperature pyrolysis,finally obtained the ultrafine carbon nanofibers with a diameter of20-30 nm.The SSA of carbon nanofibers is controlled by absorbing organic macromolecules with different molecular weight,the best sample present a high SSA(589.2 m² g^-1)and exhibit great potential as electrode materials for supercapacitors.Therefore,in three-electrode system,the sample exhibit a high specific capacitance(509 F g^-1 at 0.5 A g^-1)when compared with the carbonized pure BC(132 F g^-1 at 0.5 A g^-1).Moreover,the assembled symmetric supercapacitors is free standing and delivers an energy density of 7.7 W h kg^-1 at the power density of 325 W kg^-1.However,the small diameter of the fibers is not only reduced the conductivity,thus,decrease the rate performance of the electrode materials,but also affects the mass loading of active materials.In order to obtain controllable morphology of carbon nanofibers,we use polyacrylonitrile?PAN?as a precursor of carbon nanofibers,and then the porous carbon nanofibers?P-CNFs?were fabricated by electrospining technique combining with metal ion-assistant acid corrosion process.The resultant fibers display improved SSA and high conductivity.The cobalt nitrate hexahydrate added in the precursor solution was a key material for the formation of the unique hierarchical pore structure which acted as the porogen and catalyst.In the experiment,the template particle size is controlled by temperature,among them;the sample carbonized at 800 ^oC displayed the largest SSA(468.9 m² g^-1)and the best electrochemical property.In the three electrodes testing system,the resultant P-CNFs electrodes can exhibit a specific capacitance of 104.5 F g^-1(0.2 A g^-1),and capacitance retention of⁹4%after 2000 cycles.Furthermore,the P-CNFs also present excellent flexibility?the capacitance retaind 89.4%after 500 bending cycles?.It seems that this porous carbon nanofibers with good conductivity can used as conductive frame to support active materials.Subsequently,we designed a kind of one-dimensional?1D?hybrid structure with active materials particles embedded in porous carbon nanofibers.Based on previous work,cobalt nitrate and polyacrylonitrile were converted into Co₃O₄ and carbon,the Co₃O₄ in the composite fiber was then converted to cobalt sulfide by hydrothermal.In the experiment,the content of cobalt nitrate was optimized to obtain the best samples CoS_x/C-0.4.As a comparison,the specific capacitance of CoS_x/C nanofibers and Co₃O₄/C nanofibers were tested,at a given current density of 0.5 A g^-1,the specific capacitance of the CoS_x/C nanofibers was 496.8 F g^-1,which is 1.9 times higher than that of the Co₃O₄/C nanofibers(258.8 F g^-1).The CoS_x/C-0.4 nanofibers electrode are also compared to other cobalt sulfide based supercapacitors reported previously which showed superior rate capability of 87.0%when the current density increased from 0.5 to 10 A g^-1,and good cycling stability with over 89.0%specific capacitance remained after 2000 cycles.From these investigations,it is recognized that the carbon coating structure of the composite really enhanced their electrical conductivities,prevented reuniting of active substances and improved the structural stabilities.However,the mechanical properties of the composites were destroyed by the heat treatment.To this end,we adjusted the experimental process and used electrodeposition method to vertically grow a layer of Co-Ni hydroxide nanosheets on the surface of porous carbon nanofibers.These nanosheets and porous carbon nanofibers provide an open and continuous channel for electrons and ions to facilitate the rapid diffusion of electrolyte into the active substance.In this process,the active material growth density was adjusted by the electrodeposition time,so that the Co-Ni hydroxide/porous carbon nanofibers showed a high specific capacitance(1503.5 F g^-1 at 0.5A g^-1)and good cycling stability with over 77.5%specific capacitance remained after 2000 cycles.This work also compares the active material of the Co-Ni hydroxide with ordinary carbon nanofibers and porous carbon nanofibers,which proves that porous carbon fiber is an ideal conductive skeleton.