Preparation Of Porous Carbon Derived From Biomass And The Supercapacitance Performance

The main problems hindering the development of today’s society are the exhaustion of fossil energy and the environmental pollution caused by its usage.The development and utilization of clean energy is an effective way to solve the above problems.However,the utilization of renewable clean energy,such as solar,wind,and tidal energy,is affected by natural conditions.Therefore,it is needed to convert the renewable clean energy into electrical energy for storage before usage.Now,a variety of energy storage and conversion devices,such as solar cells,ion-type batteries,zinc-air batteries,and supercapacitors have been developed.Among them,supercapacitors have attracted great attention due to the advantages of high efficiency,long cycle life,high power density,environmental protection,high safety,and so on.The electrochemical performance of supercapacitors depends on its electrode material.including carbon materials,conductive polymers and metallic oxides.Due to low cost and long cycling life,the commercial supercapacitors are mainly based on carbon materials.However,the low specific capacitance and energy density limits the further development and popularization of carbon-based supercapacitors.To improve the electrochemical performance of carbon-based supercapacitors,several methods can be adopted,such as:（1）increasing the specific surface area of carbon materials;（2）fabricating hierarchical porous carbon materials containing micropores,mesopores and macropores;（3）co-doping heteroatoms of O,N,S,P in carbon network to increase the pseudocapacitance and improve the hydrophilicity of the carbon material.Biomass such as flowers,grass,trees,wood,hair,feces,etc,have the advantages of abundant yield,renewable,environment friendly,and low cost,accompanying with unique structure and inherent heteroatoms,which can be adoped to derive porous carbon for high performance supercapacitors.In addition,the utilization of biomass waste not only solve the energy crisis faced by today’s society,but also solve the problems of environmental pollution that caused by biomass waste.In this work,biomass materials such as soybeans,soybean roots,and flixweed seeds are used as precursors to derive porous carbon materials by a simple carbonization and chemical activation process.Scanning Electron Microscope（SEM）,N₂ adsorption-desorption,X-ray diffraction（XRD）,X-ray Photoelectron Spectrom（XPS）,Raman spectroscopy and infrared spectroscopy are used to characterize the morphology,structure,chemical composition and pore structure of porous carbon.In addition,electrochemical performance is tested to explore the practical application capabilities of biomass-derived porous carbon materials.（1）Soybeans were used as precursors to derive HNPC co-doped with O,N,S,P by a low-temperature carbonization,hydrothermal treatment and potassium hydroxide activation treatment.As a comparison,NPC was obtained by the similar process with HNPC only without hydrothermal treatment.It is demonstrated that the hydrothermal reaction can stabilize the structure of HNPC,increasing the content of O and N elements.The results of electrochemical test show that HNPC-600 displays specific capacitance of685.1 F g^-1(0.5 A g^-1)and 439.5 F g^-1(1 A g^-1)in 2 M KOH and 1 M H₂SO₄,respectively,which are the highest among all samples and higher than most reported biomass derived carbons.In addition,after 13,000 cycles of constant current charge and discharge tests,the specific capacitance of HNPC-600 retains 80%of its initial value in 2 M KOH,suggesting its excellent cycle stability.A hybrid supercapacitor of NiCo-S//HNPC-600 with HNPC-600 and nickel cobalt sulfide（NiCo-S）as the negative and positive electrodes,respectively,is assembled,and the result shows that NiCo-S//HNPC-600 exhibits a high energy density of 41.8 Wh kg^-1 at a power density of 750 W kg^-1.It shows that porous carbon derived from biomass（soybeans）have the prospect of applicant.（2）HSRC co-doped with O,N,S was derived from soybean roots by a process of low-temperature carbonization,hydrothermal treatment and potassium hydroxide activation treatment.As a result,HSRC has the hierarchical porous structure which contains microporous,mesoporous and microporous.HSRC-700 and HSRC-800 exhibit high specific surface areas of 3107 m² g^-1 and 4490 m² g^-1,respectively.The results of electrochemical test show that HSRC-700 displays specific capacitances of 486.8 F g^-1(0.5A g^-1),and 240 F g^-1(100 A g^-1),showing good rate performance in 2 M KOH.In addition,HSRC-700 exhibits excellent cycle stability.After 18400 cycles of constant current charge and discharge tests,the specific capacitance retains 92%of the first cycle.In 1 M Li PF₆,HSRC-700//HSRC-700 supercapacitor shows a high energy density of 59.4 Wh kg^-1(1500W kg^-1),which maintains 42.5 Wh kg^-1(15,000 W kg^-1)and 33.3 Wh kg^-1(30,000 W kg^-1)respectively.The results show that the supercapacitor based on HSRC-700 can provide high power density and energy density at the same time.This work lays a foundation for soybean root-derived porous carbon in the practical application.（3）Porous carbon co-doped with O,N,S（DPC）was derived from flixweeds seeds by a process of low-temperature carbonization and potassium hydroxide activation treatment.It is demonstrated that DPC-600 delivers a superior specific capacitance of 662.4 F g^-1(0.5A g^-1)in 2 M KOH.In addition,a superior cycle stability delivered by DPC-600 with a retention of 86.6%after 12,000 cycles of constant current charge and discharge tests.In 1M Na₂SO₄,DPC-600//DPC-600 supercapacitor exhibits a high specific capacitance of 245F g^-1(1 A g^-1),and a high energy density of 34 Wh kg^-1(1050 W kg^-1).In addition,the DPC-600//DPC-600 supercapacitor keeps a high energy density of 11.7 Wh kg^-1 at 20,000 W kg^-1,showing its excellent electrochemical performance.The experimental results prove that bio-waste flixweeds seeds has broad application prospects for energy storage and conversion devices.