Preparation Of Porous Carbon Based On Different Dimensional Precursors With Structural Orientational Modulation And Electrochemical Performance

As an important energy storage device,supercapacitor has the advantages of high power density,long cycle life and high safety,and acts as an irreplaceable role of secondary battery in many fields.However,the low energy density compared to secondary batteries is the main reason limiting the large-scale application of supercapacitors.Improving the energy density of supercapacitors focuses on the structural design of electrode materials and synergistic effect between electrode and electrolyte.Porous carbon electrode is the optimal candidate due to its low production cost and stable physicochemical properties.The difficulty in improving the electrochemical performance of porous carbon lies in the orientational regulation of pore structure and optimization of surface properties for different electrolytes.Based on this,granular hazelnut shells,discarded flaky polyimide films and linear aramid fibers as precursors of different dimensions were employed to fabricate porous carbon.The H₃PO₄-assisted KOH stepwise activation strategy was used to directionally regulate hierarchical pore framework of porous carbon.And the constitutive relationships between the pore framework,surface chemistry and electrochemical performance of porous carbon,and the influence of morphological characteristics of porous carbon on electrochemical properties were investigated by means of modern analytical characterization.The assembled supercapacitor devices were matched with suitable electrolytes to evaluate the performance for applications.The specific results of the study are as follows.（1）The three-dimensional granular hazelnut shell was used as the precursor to achieve the orientational regulation of the pore structure of porous carbon with the help of H₃PO₄-assisted KOH stepwise activation strategy and optimization of the preparation process.H₃PO₄ destroyed the internal compact texture of hazelnut shell and constructed a widely distributed micro-mesopores,and KOH achieved the orientational regulation of the pore structure based on the inner and outer pore walls of the formed pore structure to construct interconnected hierarchical pore framework.The prepared porous carbon obtained a specific surface area of 3469 m² g^-1with a pore volume of 2.320 cm³ g^-1 and a wide distribution of micro-mesopores in the range of 0.5～4 nm,where the micro-and mesopores were concentrated near 0.7 nm and 2 nm,respectively,and the functional groups on the surface or edges of the carbon matrix were dominated by C=O,C-O,and O-C=O.The pore structure and surface properties of porous carbon had a significant effect on its electrochemical properties.The developed micropores near 0.7 nm provided abundant energy storage sites,the interconnected hierarchical pore framework with a gradient distribution provided a fast diffusion channel for the electrolyte,and the heteroatoms on the surface or edge of the carbon matrix promoted the increase of pseudocapacitance.Benefitting from the reasonable ratio of electric double layer capacitance to pseudocapacitance,the porous carbon maintained high capacitance retention while increasing specific capacitance.In the three-electrode system,the highest specific capacitance of 338.0 F g^-1 was achieved at a current density of 0.2 A g^-1,and the capacitance retention rate was 86.3%at 10 A g^-1.The assembled symmetrical supercapacitor exhibited a high energy density of 22.46 Wh kg^-1 at a power density of 450 W kg^-1 in a Na₂SO₄ gel electrolyte,as well as an excellent cycling stability of 133%over the initial capacity after 10,000 cycles at a voltage window of 1.8 V.（2）The porous carbon was fabricated from two-dimensional flaky polyimide films using the above activation strategy.Compared with three-dimensional granular hazelnut shells,polyimide films based porous carbon activated by H₃PO₄ formed micropores below 1.5 nm due to the two-dimensional flaky structure and the different chemical structure（containing amide or subamide groups）,and introduced a large number of heteroatoms,providing abundant contact sites for KOH activation.The KOH activation achieved the orientational regulation of pore structure and constructd a hierarchical pore framework with a connected gradient distribution.The resulting porous carbon exhibited a high specific surface area and pore volume of 3758 m² g^-1 and 2.530 cm³ g^-1,respectively,as well as a high heteroatom content（8.6 at.%）with phosphorus-containing groups in addition to oxygen and nitrogen functional groups.The high heteroatom content on the surface or edges of the carbon matrix allowed the pseudocapacitance contribution of the porous carbon to reach 26.03%and the specific capacitance to reach 404.6 F g^-1 at 0.2 A g^-1.The symmetrical supercapacitor delivered a high energy density when Na₂SO₄ and SBPBF₄/PC were employed as electrolytes.In particular,the energy density in the SBPBF₄/PC electrolyte was as high as 70.2 Wh kg^-1 at a power density of320 W kg^-1 and possessed over 90%capacitance retention after 10,000 cycles.（3）One-dimensional linear aramid fibers were employed as precursors to prepare porous carbon using the above activation strategy.Based on the linear morphology of aramid fibers and the more abundant subamide groups,the porous carbon was prepared by the interaction with H₃PO₄ to form a micro-mesoporous structure dominated by micropores.And the connected hierarchical porous carbon was achieved by KOH activation on the basis of the formed pore structure.The porous carbon was dominanted by micro-mesoporous,and obtained a specific surface area of up to 3917 m² g^-1 and a pore volume of 2.358 cm³ g^-1.The heteroatoms on the surface of the carbon matrix were mainly oxygen-containing functional groups.Since the aramid fiber-based porous carbon did not have macropores and micron-level pore channels,its effective pores mainly existed on the outer surface of the porous carbon,which effectively shortened the diffusion path of the electrolyte and was more conducive to the efficient utilization of the energy storage sites,thus obtaining the highest specific capacitance of 434.8F g^-1 at 0.2 A g^-1.The symmetrical supercapacitor matched with Na₂SO₄ electrolyte exhibited a wide voltage window of 1.8 V with higher energy density than hazelnut shell based and polyimide based supercapacitors,reaching 25.26 Wh kg^-1 at a power density of 450 W kg^-1.