Green Preparation Of Biochar With Optimized Pore Structure And Assembly Of Aqueous High Specific Energy Supercapacitors

In order to meet the increasing demand for energy storage in modern society,it is an urgent task to develop an efficient energy storage device.Supercapacitors are widely concerned because of their high power density,fast charge and discharge,long cycle life and so on.Biomass has a natural porous structure evolved from thousands of years,which is enough to ensure the efficient transfer of nutrients and water to maintain their own growth needs.It is of great significance to use biomass as precursors to develop biochar materials with natural pores and assemble supercapacitors with high energy density.In this paper,suaeda glauca bge（SGB）and onion skin were used as precursors,combined with their natural properties,and targeted synthesis strategies were adopted to improve the pore characteristics and surface properties of biochar materials,so as to obtain better performance of supercapacitor.The paper can be divided into the following two parts:The above two preparation methods of graded porous biochar successfully improved the physical properties and surface characteristics of biochar materials,and showed excellent energy storage performance,which fully proved the feasibility of using agricultural waste as precursor system to prepare electrode materials with high performance supercapacitor.It provides more choices and ideas for green preparation of low-cost and high-performance electrode materials for supercapacitors.（1）A mild and recyclable"selective"in-situ activation strategy was proposed to prepare electrically active biochar materials with multi-heteroatomic doping and natural pores.Firstly,chlorophyll in SGB was extracted by anhydrous ethanol and the pores were widened in situ.Then,Na Cl crystals were used to fill/support the pores of SGB,and the pores were carbonized into biochar materials.The obtained biochar SGB-700 has high specific surface area(684 m²g^-1)and abundant N,O,S and Cl heteroatom doping.It shows a mass specific capacitance of638 F g^-1(93μF cm^-2)in 1.0 M H₂SO₄ electrolyte.The assembled symmetric supercapacitor has high mass ratio capacitance(515 F g^-1 at 0.5 A g^-1),excellent rate characteristic(76.1%at20.0 A g^-1),long cycle stability（107.1%after 12,000 cycles）and high energy density(30.2 Wh kg^-1 at 164.0 W kg^-1).（2）Biochar nanocages with hollow inner cavity and mesoscopic surface channels were prepared in one step by a dual-function template with synchronous activation and filling support.The CO₂ and water vapor generated during the pyrolysis process of magnesium acetate are used to form mesoscopic pores（2-5 nm）in the biochar layer.Meanwhile,the generated Mg O can be used as a support to form a hollow inner cavity to provide a fast transport channel for electrolyte ions and improve the performance of the supercapacitor.The N,O and S heteroatoms contained in the biomass itself raise the voltage window to 1.7 V and produce the Faraday active site.The typical sample BNC-700 achieves A high mass specific capacitance of 478.2 F g^-1 in 1.0 M H₂SO₄ and an excellent rate characteristic(72.8%at 20.0 A g^-1).In a symmetric supercapacitor with 1.0 M Mg（Ac）₂（has a higher theoretical voltage window）as electrolyte,the energy density can reach 37.4 Wh kg^-1 at 164.0 W kg^-1.It also provides excellent cycle stability（99.5%after15,000 cycles）.