Synthesis Of Biomass Carbon Materials And Their Applications In Flexible Supercapacitors

With the rapid development of portable and wearable electronic devices,there is an increasing demand for flexible and even foldable high-performance energy storage systems.Flexible supercapacitors have been widely used in flexible electronic equipment on account of their good mechanical properties,high charge/discharge efficiency and high energy density.Biomass is rich in carbon atoms and heteroatom functional groups such as nitrogen and oxygen that can provide pseudocapacitance,and its unique structure can serve as a template for preparing porous carbon materials.Therefore,green biomass can be used as a new carbon source for carbon materials.Generally,the charge accumulation of carbon material is formed by electrostatic interaction at the interface between the electrode and the electrolyte.Therefore,the charge accumulation ability will increase with the enlargement of the specific surface area.While the specific capacitances do not always grow linearly along with the specific surface area.This is because micropores can provide a high specific surface area and it also easily hinder the transmission of electrolyte ions.Mesoporous or macroporous structures can provide fast channels for electrolyte ions transfer.Therefore,carbon materials also require mesoporous or macroporous structures in order to improve specific capacitance.Herein,using biomass as the research material,we selected different biomass carbon precursors and prepared a series of micro/mesoporous electrode materials with high specific surface area,high porosity and abundant heteroatoms by high-temperature carbonization or activation ways.At the same time,scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD),Raman,nitrogen adsorption-desorption technique and X-ray photoelectron spectroscopy(XPS)were used to characterize the structure and composition of the material.The electrochemical performance of electrodes in different electrolytes was studied by electrochemical workstation.The particular works were summarized as follows:1.Wax gourd based porous carbon materials(WGPCs)with developed micro/mesoporous structures has been successfully synthesized from sustainable biomass precursor of wax gourd via carbonization and activation approach under a nitrogen inert atmosphere.The optimized material that is denoted as WGPC-4 exhibits high specific surface area(2919 m2 g-1)with oxygen and nitrogen contents of 17.9 at.%and 1.0 at.%,respectively.In a three-electrode system,the WGPC-4 electrode presents high specific capacitance of 333 F g-1 at a current density of 1 A g-1.Meanwhile,the assembled symmetrical cell using aqueous electrolyte(0.5 M Na2SO4)has high specific capacitance(167 F g-1 at 1 A g-1),stable cycling characteristics(93%capacitance retaining after 5000 runs)and excellent energy density of 19.2 Wh kg-1.These materials with excellent structural characteristics are an ideal candidate for high-performance supercapacitors.2.A flexible electrode was prepared by the method of flexible substrate loading material,in which wax gourd based porous carbon(WGPC-4)with great electrochemical performance was used as the active material and nickel foam was the substrate.The device has a high specific capacitance of 258 F g-1 at a current density of 1 A g-1 and good rate performance(the original 81.9%is retained at 20 A g-1)in KOH/polyvinyl alcohol(PVA)electrolyte.When the power density is 297.5 W kg-1,the device can provide an energy density of 13.0 Wh kg-1.In addition,the device can still maintain stable capacitance performance and good cycle stability under bending conditions.This research has a certain reference value for the electrode design of flexible supercapacitors.3.Using wool textiles as a carbon source,porous carbon fiber materials(WTs)that are provided with micro/mesopore structure were prepared by a simple high-temperature carbonization method.The as-prepared WT-800-1 sample has good flexibility,rich nitrogen and oxygen functional groups,the specific surface area of 501 m2 g-1 as well as good wettability to aqueous solutions.In a 6 M KOH aqueous electrolyte,the areal capacitance of WT-800-1 electrode is up to 1012 mF cm-2.The assembled device possesses areal capacitance of 176 mF cm-2 and an energy density of 35.3 ?Wh cm-2 in PVA/KOH electrolyte.The maximum areal capacitance of the device in PVA/H2SO4 electrolyte is 383 mF cm-2,which is better than some previously reported flexible supercapacitors.When the power density is 0.60 mW cm-2,it has a high energy density of 76.6 ?Wh cm-2.In addition,a tandem device filled with PVA/H2SO4 electrolyte is also capable to light a red LED in different states.The porous carbon fiber material WT-800-1 holds a common preparation method,high electrochemical performance,and good flexibility,thence it has good application potential in flexible energy storage devices.