Fabrication Of Cornstalk Based Porous Carbons And Their Application In Supercapacitors

Hierarchical porous carbon,a three-dimensional material containing micropores,mesopores and macropores,is considered as a promising electrode material in supercapacitors due to its large specific surface area and high mesoporous/microporous ratio.With the increasing awareness of environmental protection,biomass has replaced petrochemical materials to be the ideal carbon precursors because of its abundance and biodegradability,which is in line with the environmental protection concept of clean production and sustainable development.Cornstalk is an important natural renewable resource composed of cellulose,hemicellulose and lignin.As one of the most abundant agricultural and forestry wastes,cornstalk is also considered as an ideal precursor for hierarchical porous carbon owing to its low cost and unique spatial structure.However,compared with the single component such as cellulose,hemicellulose and lignin,the composition and structure of lignocellulose is undoubtedly more complicated,which makes it difficult to regulate the pore structure,further limiting the improvement of specific capacitance and rate performance in supercapacitors.Therefore,finding out a simple and efficient strategy to develop cornstalk derived hierarchical porous carbon with high electrochemical performance has become a great challenge in the development of supercapacitor electrode materials.In this study,cornstalks were used as carbon source（including cornstalk,cornstalk husk and cornstalk pith）to produce cornstalk derived and cornstalk husk derived hierarchical porous carbon.Mild and efficient methods were proposed to construct cornstalk derived hierarchical porous carbon via salt template induced hydrothermal carbonization,activation and pith removal,which can control pore morphology and endow the carbon with outstanding electrochemical performance.Besides,the cornstalk derived hierarchical porous carbon has a promising prospect in supercapacitors for high value utilization of lignocellulose.The main researches are described as follows:1.Two-step synthesis of cornstalk derived porous carbonLignocellulose has a very complicated structure and composition.Particularly,lignin possesses the most intractable molecular structure and forms furanic and aromatic units with oxygen-substituted arene-type moieties under pressurized water and inevitably converts into irregular carbon spheres that aggregate easily and block the pore tunnel,which adversely affects the subsequent activation.Therefore,finding out an efficient method to produce lignin derived hydrochars has become a prerequisite for the synthesis of cornstalk derived hydrochars.Lignin derived hydrochars,namely Pd NP-loaded lignin porous carbonaceous materials have been successfully prepared using lignin/Pd NPs composite as raw material and LiCl as a salt template in hydrothermal carbonization.The as-prepared porous carbonaceous material has excellent catalytic performance in the reduction of p-nitrophenol by sodium borohydride,which reveals the feasibility of salt template induced hydrothermal carbonization and lays a foundation for the synthesis of cornstalk derived hydrochars.Cornstalk derived porous carbon was prepared through two step carbonization method.First,cornstalk derived hydrochars with regulated pores were prepared via hydrothermal carbonization with lithium chloride and zinc chloride as salt templates.Second,the prepared hydrochars were used as carbon source to produce cornstalk porous carbon with potassium oxalate as the activating agent and calcium carbonate as the hard template.The results reveal that the salt template plays an important role in pore regulation and morphology orientation in the hydrothermal process,which is beneficial for hydrochars to be fully mixed with the activating agent and hard template in the subsequent activation process to further increase the porosity of cornstalk derived porous carbon.The cornstalk derived porous carbon obtained by two step carbonization has a high specific surface area of 1934 m²·g^-1,and the assembled symmetric supercapacitor exhibits good electrochemical performance:in 1 mol·L^-1 Na₂SO₄electrolyte at 0.5 A·g^-1,the obtained porous carbon exhibited a high supercapacitance of 216F·g^-1,with a high energy density of 21.6 Wh·kg^-1 at a power density of 295.9 W·kg^-1.2.One-pot synthesis of cornstalk derived porous carbonCornstalk derived porous carbon was prepared through one step carbonization.Corn stalk was used as the carbon precursor with potassium oxalate as the activating agent and calcium carbonate as the hard template.In this study,the tubular cornstalk structure can be reserved,and a large number of pores can be formed on the loose fibre structure after activation.The obtained cornstalk porous carbon has a high specific surface area of 2054m²·g^-1,exhibiting good supercapacitance in the two-electrode system.In 1 mol·L^-1 Na₂SO₄electrolyte,the prepared porous carbon showed high capacitance of 325.36 F·g^-1 at a current density of 1 A·g^-1,and a high energy density of 45.2 Wh·kg^-1 at a power density of 988.9W·kg^-1.Attributed to its stable three-dimensional porous structure,the capacity retention rate of cornstalk porous carbon can reach 87.5%after 10,000 cycles at 10 A·g^-1.3.One-pot synthesis of cornstalk husk derived porous carbonThe study found that the light,loose and short fibre structure of cornstalk pith produce a high proportion of micropores during the activation,leading to limited specific capacitance,relatively poor rate performance and low energy density.Therefore,it is necessary to remove the pith from raw cornstalk.With the removal of pith,cornstalk husk derived porous carbon was successfully obtained using potassium oxalate as the activating agent and calcium carbonate as the hard template.Compared with cornstalk porous carbon,the obtained cornstalk husk derived porous carbon has a high specific surface area of 1910 m²·g^-11 with a high mesopore/total pore ratio of 86.89%,exhibiting good supercapacitance in the two-electrode system.In 1 mol·L^-1 Na₂SO₄ electrolyte,the prepared porous carbon showed high capacitance of 306 F·g^-1 at a current density of 1 A·g^-11 and 240 F·g^-1 at a current density of 20 A·g^-1,and a high energy density of 42.5 Wh·kg^-1 at a power density of 374.1 W·kg^-1.Attributed to its stable three-dimensional porous structure,the capacity retention rate of cornstalk porous carbon can reach 90.4%after 10,000 cycles at 10 A·g^-1.