Functional Architecture And Properties Study Of Cunninghamia Lanceolata-based Electrode Materials For Supercapacitors

Continuous growth of the global economy and population with the environmental pollution and energy crisis caused by the rapid consumption of fossil energy has become a global issue restricting sustainable development.The development of green and renewable energy and new energy storage devices becomes imminent.Supercapacitor is considered one of the most promising applications of biomass-based materials due to the characteristics of fast electronic charge and discharge,high power density,good durability and chemical stability.This study firstly develops activated carbons with three-dimensional network structure from forestry waste Cunninghamia lanceolata bark by a two-step carbonization-activation method and then formulates their supercapacitor electrode materials.B and N-doped Cunninghamia lanceolata bark-based porous carbon materials were prepared by introducing heteroatoms by microwave-assisted hydrothermal method,which further improved the specific capacitance.With the goal of constructing high power density and energy density supercapacitor energy storage devices,metal hydroxide/activated carbon fiber cathode composite and metal sulfide/Ti₃C₂T_x cathode composite materials were further studied.The project finally designs and synthesizes positive and negative electrode materials with high specific capacitance,rate performance and cycling stability by adjusting the composition,structure and microscopic morphology of the materials,and consequently assembles high-performance asymmetrical supercapacitors.The main research work can be summarized as follows:（1）Porous activated carbons with 3D network structure were derived from Cunninghamia lanceolata bark by a two-step carbonization-activation method.The obtained activated carbon material has a high specific surface area of 1377 m² g^-1 and a total pore volume of 0.74 cm³ g^-1.The pores are dominated by micropores,while mesopores and macropores coexist.In addition,there is a certain amount of heteroatom N element（1～1.6 wt%）.The specific capacitance of Cunninghamia lanceolata bark-based activated carbon exhibits good electrochemical performance and rate capability benefiting from the large specific surface area,developed pore structure and heteroatom doping.To further optimize the preparation process of Cunninghamia lanceolata bark-based activated carbon,the central composite design（CCD）and response surface analysis were carried out using the ratio of alkali to carbon and activation temperature as experimental factors,the specific capacitance at a current density of 0.5 A g^-1 as the response value.The results show that the interaction between alkali-carbon ratio and activation temperature has a significant effect on specific capacitance.The optimal preparation process of Cunninghamia lanceolata bark-based activated carbon could be obtained by fitting the quadratic polynomial regression equation:the alkali-carbon ratio is 3.04,the activation temperature is 605°C,and the corresponding mass specific capacitance is 185 F g^-1.Three parallel experiments are conducted to verify the optimal parameters,and the obtained average mass specific capacitance is 185.7 F g^-1,the error is only0.38%,which is basically consistent with the predicted value.（2）To further improve the specific capacitance of carbon materials,B and N co-doped porous carbons were prepared by an efficient microwave-assisted hydrothermal and high temperature carbonization two-step method.The surface wettability of carbon materials is improved by the help of B and N heteroatom,and pseudocapacitance is simultaneously introduced to enhance the overall specific capacitance.The study found that the dopant can participate in the reaction in the high temperature carbonization process,resulting in pore-forming and pore-enlarging effects,it can also make some heteroatoms enter the carbon framework.The specific surface area of Cunninghamia lanceolata bark-based B and N doped porous carbon can be as high as 891 m²g^-1,the pore volume up to 0.46 cm³ g^-1,the N element content is 6.35 wt.%and B element content is 3.30%.Carbon materials exhibit excellent electrochemical performance in electric double-layer capacitors benefiting from the micropore-dominated hierarchical porous structure and high content of B and N atoms.The specific capacitance of the Cunninghamia lanceolata bark-based B and N doped porous carbon is219 F g^-1 at 0.5 A g^-1 in 6 mol L^-1 KOH electrolyte.Compared with the undoped porous carbon(93 F g^-1),the electrochemical performance has improved significantly.After 10000 charge-discharge tests at a current density of 10 A g^-1,the initial capacitance retention rate is 90%,showing excellent cycling stability.（3）To further improve the specific capacitance and energy density of carbon materials,and also solve the deficiencies of fragile structure,poor electrical conductivity and cycle stability of metal compounds,NiCo-LDH@ACF composite electrode materials were prepared by microwave hydrothermal method in this study,and bimetallic hydroxides are in-situ growth on the surface of conductive carbon fibers.It is found that the as-prepared NiCo-LDH@ACF surface is composed of a large number of nanosheets and sea urchin-like microspheres,forming a well-developed hierarchical porous structure,which is beneficial to the penetration and diffusion of electrolyte ions.The specific capacitance of NiCo-LDH@ACF could reach 1453 F g^-1 at 1 A g^-1.When the current density increases to 10A g^-1,the initial capacitance retention rate is 78%,which is much higher than that of pure NiCo-LDH（53.5%）.Furthermore,the NiCo-LDH@ACF//AC asymmetric supercapacitor constructed with NiCo-LDH@ACF as the positive electrode and Cunninghamia lanceolata bark-based activated carbon as the negative electrode exhibits excellent electrochemical performance.The maximum output energy density is as high as 52.2 Wh kg^-1 at a power density of 800 W kg^-1.The initial capacitance retention rate of ASC is 79.8%after 10000 charge-discharge cycles at 10 A g^-1.Compared with pure NiCo-LDH（58.1%）,the cycle stability is significantly improved.This result indicates that ACF plays a better supporting role in NiCo-LDH@ACF and improves the cycling stability of the material.（4）To further improve the electronic conductivity and electrochemical redox activity of cobalt-based metal compounds,ZIF-67/Ti₃C₂T_x and Co₃S₄/Ti₃C₂T_x composite electrode materials were prepared by simple room temperature precipitation and hot solvent method.The study found that the flake Ti₃C₂T_x can provide larger contact surface and more deposition points,which can effectively adjust the size and uniformity of ZIF-67.On the other hand,ZIF-67 nanoparticles can effectively prevent the stacking of Ti₃C₂T_x nanosheets,which is beneficial to the construction of composite structures.In 3 mol L^-1 KOH electrolyte,the specific capacitance of ZIF-67/Ti₃C₂T_x and Co₃S₄/Ti₃C₂T_xat 1 A g^-1 are231 F g^-1 and 602 F g^-1,respectively.The Co₃S₄/Ti₃C₂T_x//AC asymmetric supercapacitor was assembled with Co₃S₄/Ti₃C₂T_xas the positive electrode and Cunninghamia lanceolata bark-based activated carbon as the negative electrode.Benefiting from the matched kinetics and synergy between the positive and negative electrodes,the working voltage of Co₃S₄/Ti₃C₂T_x//AC reaches 1.6 V;a high energy density of 44.9 Wh kg^-1 is achieved at a power density of 800.3 W kg^-1,when the power density increased to 8000 W kg^-1,the energy density could still remain 35.6 Wh kg^-1.The capacity retention of Co₃S₄/Ti₃C₂T_x//AC asymmetric supercapacitors is 88.3%of the initial capacitance after 5000 charge-discharge cycles at 5 A g^-1,showing excellent cycling stability.