Biomass-derived Electrode Material With Self-doped N/O And Hierarchically Porous Structure For Supercapacitor

Since the 21st century,the global economy continues to develop rapidly.At the same time,mankind have made great progress in technology and quality of life,which resulted in more consumption of the resources and serious environmental pollution.There is an urgent need for green,efficient,renewable,advanced energy storage devices and conversion technologies.Supercapacitor,as a new energy storage device among traditional capacitors and batteries,has attracted more and more attention due to its high power density,short charging time,long cycle life,wide operating temperature range,and environmental friendly characteristics.It is necessary to combine the advantages of electric double-layer supercapacitors(EDLCs)with faradaic pseudocapacitors.Biomass carbon materials,which is easily accessible,renewable,sustainable and environment friendly,can be simply prepared by carbonization or pyrolysis.The biomass material usually has an orderly structure,and the KOH-activated biomass carbon material can have an ultra-high specific surface area.Moreover,the considerable heteroatomic self-doping(such as N,O,and P)makes it a great candidate for supercapacitor electrode materials.Activated carbon prepared from biomass materials not only has the excellent characteristics of EDLCs such as high power density and high charge and discharge rate,but also has heteroatoms that can provide pseudocapacitance to increase the energy density.In this work,winter-jujube and apple which have excellent pore structure and rich heteroatom content was used to prepare excellent activated carbon materials.And the effects of different KOH dosages on the performance of the electrode materials were analyzed.The main research contents are as follows:(1)Apple-derived carbon materials(AC)were prepared by the same method.We prepared four samples which has different KOH to carbon weight ratio.The properties of AC were characterized by SEM,XPS,XRD,Raman,nitrogen adsorption-desorption isotherms,CV,GCD,and EIS.The as-obtained carbon skeleton possesses hierarchical porous structure,abundant heteroatom groups,and good electrical conductivity.Among these four samples,AC-4.5 which has a KOH to carbon weight ratio of 3.5:1 processes excellent overall performance.AC-4.5 possesses a specific surface area(SSA)of 1603 m2 g-1,and the electrochemistry results show that AC-3.5 exhibits a high specific capacitance up to 323 F g-1 at 0.5 A g-1 and a considerable rate capability of 80%retention when current density increases to 20 A g-1.After 5000 cycles at 5 A g-1,the capacitance still has 97.3%retention.Besides,AC-4.5-based symmetric supercapacitor device possesses energy density of 32.4 kW h-1 at 0.1 A g-1 and power density of 8.9kWat5 Ag-1.(2)Winter-jujube-derived carbon materials(JC)were prepared by pre-carbonization and KOH activation.We prepared four samples which has different KOH to carbon weight ratio.The physical and chemical properties of JC were characterized by field-emission scanning electron microscope(SEM),X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD),Raman spectra,and nitrogen adsorption-desorption isotherms.The electrochemical characteristics are measured by cyclic voltammetry(CV),galvanostatic charge-discharge(GCD),and electrochemical impedance spectroscopy(EIS).The as-obtained carbon skeleton possesses hierarchical porous structure,abundant heteroatom groups,and good electrical conductivity.It is mentionable that the KOH will significantly influence the properties of JC.When the KOH to carbon weight ratio is 3.5:1,the as-prepared sample JC-3.5 possesses an ultra-huge specific surface area(SSA)of 2286.2 m2 g-1.And the electrochemistry results show that JC-3.5 exhibits a high specific capacitance up to 341 F g-1 at 0.5 A g-1 and a considerable rate capability of 76%retention when current density increases to 20 A g-1.After 5000 cycles at 5 A g-1,the capacitance still has 96.5%retention.Besides,JC-3.5-based symmetric supercapacitor device possesses energy density of 30.9 kW h-1 at 0.1 A g-1 and power density of 8.9 kW at 5 A g-1.