Preparation And Electrical Properties Of Biomass-based Electrode Materials For Supercapacitors

With the development of the economy,the energy crisis becomes more and more obvious.The development of renewable clean green energy has become an important way to alleviate the current energy crisis,which requires efficient and stable energy storage equipment to achieve.Supercapacitor is a new type of energy storage device,which has the characteristics of fast charging and discharging speed,good stability and high power density.At present,carbon materials are the most widely used electrode materials for supercapacitors.There are many kinds of carbon materials,such as graphene and carbon nanotubes,which are difficult to achieve industrial production due to their high cost and low yield.Biomass resources have the advantages of abundant yield,renewable and low price,and are an ideal resource for preparing carbon electrode materials for supercapacitors.However,there are still some problems,such as the lack of mesoporous and macroporous structures,the elemental composition is too single,and many biomass materials do not have a unique structure and morphology.Aiming at the above problems,this paper uses natural biomass as raw materials（agricultural and forestry waste and natural wood）to prepare a variety of high-performance biomass-based porous carbon materials with special structural characteristics and electrode materials with natural biomass structures.The new green preparation methods of material-based electrode materials and their application and development in supercapacitors are studied.The details are as follows:1.Using forestry waste poplar wood powder as raw material,a hydrothermal carbon（HTC）with regular spherical morphology was prepared by one-step hydrothermal method,and this was used as a precursor,and CO₂/H₂O mixed gas was used as an activator.Porous carbon electrode materials with microporous and mesoporous structures.Compared with the reference group,the specific surface area and pore volume of the activated electrode material were significantly improved,which were 56.46 m² g^-1 and 0.0393 cm³ g^-1,respectively.Under the condition of a current density of 1 A g^-1,the specific capacitance of the electrode material was tested to be 146.34 F g^-1,which was 50%higher than that of the reference group,and showed good Coulomb efficiency,rate performance and cycling.stability.The as-prepared electrode materials were assembled into a button-type symmetrical supercapacitor CH-HTC-850//CH-HTC-850 with a maximum energy density of 5.1 Wh kg^-1 and a maximum power density of4900 W kg^-1,showing excellent energy storage properties.It can be seen that the CO₂/H₂O mixed gas activation method is an effective method to improve the capacitance performance of biomass carbon materials.This method does not use any chemical reagents,and the preparation process is environmentally friendly and pollution-free.However,most of the pores produced by it are micropores,while the number of mesopores and macropores is very small,so the overall porosity has not been greatly improved,and the specific capacitance of the obtained electrode material is limited.2.In view of the low porosity of the physical activation method using CO₂/H₂O mixed gas as the activator,this study adopted the hard template method,using nano-silica as the template agent,and forestry waste poplar wood powder as the carbon source.A spherical carbon electrode material with a microporous-mesoporous-macroporous hierarchical channel structure was prepared.At the same time,in order to increase the specific capacitance of the electrode material,N and S heteroatoms were successfully doped with L-cysteine as the donor.The specific surface area and pore volume of the final product have been greatly improved(88.46m² g^-1,0.2312 cm³ g^-1),and the effect is significantly better than the physical activation method with CO₂/H₂O mixed gas as the activator(56.46 m² g^-1,0.0393 cm³ g^-1).At the same time,the surface of the obtained electrode material contains various active functional groups,such as-NH₂,-SH,-NO₂ and-SO₂ and so on.From the electrochemical performance results of the three-electrode test,it is concluded that the intercalation of N and S heteroatoms not only provides a part of the pseudocapacitance for the electrode material,but also effectively reduces the internal resistance of the carbon material itself.Since the material itself has a multi-level pore structure of micropores,mesopores and macropores,it is beneficial to the adsorption,transport and storage of electrolyte ions,and exhibits excellent electrochemical performance.At a current density of 1 A g-1,the specific capacitance reaches 183.66 F g-1,which is much higher than that of the electrode material prepared by the CO₂/H₂O mixed gas activation method(146.34 F g^-1).The assembled symmetric supercapacitors N/S-HTCs//N/S-HTCs have a maximum energy density of 8.0 Wh kg^-1 and a maximum power density of 5030 W kg^-1,showing excellent energy storage properties.3.Aiming at the problem of lack of unique natural structure and low utilization of biomass resources with low heteroatomic content,a hollow-out with a large number of micropores and mesopores was prepared from the coffee grounds of agricultural waste Arabica coffee beans as raw materials.Spherical porous carbon electrode material.Since coffee grounds are an amorphous carbon material that is difficult to pyrolyze,it is difficult to undergo morphological changes under normal conditions.In order to overcome this problem,in this study,the method of vacuum impregnation of sodium hydroxide was used to prepare coffee grounds-based hydrothermal carbon with regular spherical morphology by one-step hydrothermal method,and zinc chloride was used as the precursor.Activator,an ideal hollow spherical electrode material with a large number of micropores and mesopores is prepared after high temperature activation and carbonization.Under the condition of current density of 0.5 A g^-1,the specific capacitance of the electrode material was tested to be 191.32 F g^-1,which was150%higher than that of the reference group,and showed good rate performance and cycle stability.sex.The obtained electrode material can use sodium sulfate solution as electrolyte,and the assembled Zn-COFs//Zn-COFs supercapacitor can be charged and discharged at a wide operating voltage of 0～2.0 V,and the power density is 461 W kg^-1.,its energy density reaches a maximum of 17.4 Wh kg^-1,and its highest power density can reach 9130 W kg^-1,which is much higher than that of electrode materials prepared from most carbon materials.However,the carbon electrode material prepared by this method has a low coulombic efficiency of only79%,and the preparation process is complex and energy-intensive.4.In view of the complex process and high energy consumption of coffee grounds-based hollow spherical electrode materials prepared by Zn Cl₂ activation method,this study used Co（NO₃）₂ and Ni（NO₃）₂ to pretreat coffee grounds,and then used KOH,In order to activate the pretreated coffee grounds at high temperature by the activator,a foamed electrode material with interconnected micropore-mesoporous-macropore hierarchical pores was prepared,and the material itself has ultra-low internal resistance（0.31 ohm）.The method has simple technological process and less energy consumption.Through the catalysis of Co²⁺and Ni²⁺,the activated material has a relatively ordered lattice structure,so the internal resistance becomes small and the electrical conductivity increases.KOH has a strong pore-forming function and is an excellent activator.The interconnected structure of the final product is mainly due to the activation function of KOH,but at the same time,with the increase of the amount of KOH used,the degree of graphitization of the material also increases.will decrease.The specific capacitance of CGC-2 measured at a current density of 1 A g^-1 is 302.65 F g^-1,which is much higher than that of the coffee grounds-based hollow spherical electrode material(191.32 F g^-1)prepared by the Zn Cl₂ activation method.The material with the pore-channel interconnection structure not only exhibits good rate performance and cycling stability,but also has a Coulombic efficiency of nearly 100%.The high performance exhibited by the final product can be attributed to the highly ordered hierarchical porous interconnected structure and high graphitization,the highest energy density of the assembled CGC-1//CGC-1 supercapacitor is6.4 Wh kg^-1,the highest The power density is 5050 W kg^-1,showing excellent energy storage properties.5.In order to make full use of the unique natural pore structure of biomass materials,a solid wood-type all-solid-state solid wood material with a Electrode material with structural properties of natural wood.Its multi-dimensional vertically arranged longitudinal conduit channels and transverse multi-level pit channel structures provide convenience for the storage and transport of electrolyte ions.The purpose of further creating pores on the fiber wall can be achieved by vacuum impregnation with potassium hydroxide solution.Through co-doping with melamine,not only heteroatomic nitrogen was introduced to improve the capacitive performance of the material,but also the dimensional stability of the material was increased.The electrochemical performance of NKBW800 was tested by the three-electrode system,and NKBW800 showed good capacitance performance,mainly in high specific capacity(8.47 F cm^-2),high rate performance（capacitive loss rate was only 14.7%）and high cycle stability（After 5000 charge-discharge cycles,the specific capacitance remained at 94.2%）.By assembling a simple symmetric supercapacitor and testing its energy storage performance,it has high energy density(up to 8.6 Wh m^-2)and high energy power density(up to 243 W m^-2),and shows excellent performance Cyclic stability.These conditions create a theoretical possibility for it to be a symmetric or hybrid supercapacitor electrode（anode）material.