Microstructure Regulation And Electrochemical Performance Study Of Ultrapure Coal Based Porous Carbon For Supercapacitor Applications

Porous carbon used for supercapacitors is a kind of high-end activated carbon（AC）with high specific surface area,abundant pore volume and low ash,which can be widely used in energy storage devices.Due to its low price and stable source,coal has the inherent advantage of mass production of capacitive carbon in industrialization and show higher cost performance.The development of high-quality coal-based capacitor carbon can not only achieve the high value-added utilization of coal,but also contribute to the efficient and clean use of coal resources.Although predecessors have studied coal-based carbon electrode materials,they often only focus on the improvement of specific capacity in electrochemical applications,and pay less attention to some important performance indicators for energy storage devices,such as rate performance,cyclic stability and leakage current.Furthermore,the relevant basic research is not yet mature.Therefore,in this project,super-pure coal is used as the carbon precursor to prepare the high-end capacitance carbons via regulating the pore structure and surface chemistry of carbon materials by means of physical and chemical methods.The aim of this paper is to study the structure-activity relationship between the microstructure and electrochemical properties of electrode materials.Further,how these functional groups and the bonding configuration of P in a carbon matrix affect the surface chemistry of carbon material and therefore its electrochemical performance in organic electrolyte is explored and the underlying principles are also deeply dissected.The study offers in-depth material chemistry for developing low-cost and high-performance electrode materials for supercapacitors.In the research,after preliminarily deashing and removing impurity from high rank anthracite as the carbon source,ultra-pure coal-based activated carbon is prepared by phosphoric acid activation,potassium hydroxide activation,combined activation,and preoxidation,respectively.Then,the active materials were assembled into electrodes for supercapacitors,and their electrochemical properties was tested in three-electrode alkaline electrolyte system and two-electrode organic system.The effect of the preparation process on the pore structure and electrochemical performance of the porous carbon was compared and investigated,indicating that the comprehensive performance of the porous carbon AC-K-P obtained by chemical combination activation was better than other carbon electrodes.The optimized AC-K-P,with high specific surface area up to 2113 m² g^-1,exhibits good capacitive behaviors and electrochemical stability in both aqueous and organic systems.When evaluated as electrodes in 6 M KOH,the resultant material exhibits a high capacitance of 152F/g at 10A/g and excellent rate capability of 71.82%retention,which are far higher than AC-K electrode.More remarkably,such supercapacitors based on the AC-K-P still show extraordinary cycling stability with 97.72%capacitance retention over 2500 cycles in 1 M TEABF₄/PC electrolyte.After the 2500 cycles,the high-current charge/discharge was performed again,and the specific capacity maintained 54.87%at 20 A g^-1.Such excellent rate performance and cycle stability are related to the change of surface chemistry.The surface of carbon materials becomes more stable by phosphorus-doped modification,which greatly promotes the electrochemical reversibility of electrode materials.Based on the comprehensive research conclusions above,we further explore the influence of H₃PO₄ activation on pore structure and surface chemistry of porous carbon,and deeply analyze structure-activity relationship between the microstructure and electrochemical properties of the electrode material.Heteroatom doping is an effective strategy to modify the surface properties of carbon electrode,thus boosting its electrochemical performance.In this work,phosphorus（P）-doped porous carbons with high specific surface（2133 m²/g）are prepared by KOH pre-activation of ultrapure anthracite and further H₃PO₄ activation.H₃PO₄ post-activation not only generates more mesoporous for a rapid ion diffusion,but also provides potential P-source for the P-doping on carbon scaffold.The increase of P-doped content significantly restrains the formation of unstable quinone and carboxylic groups,and then enhances oxidation stability of P-doped porous carbon.When evaluated as electrodes in 1 M Et₄NBF₄/PC,the resultant material exhibits excellent rate capability of 75%retention at 30 A g^-1,extraordinary stability of 90%capacitance retention after20000 cycles and low leakage current of less than 1.2μA.More importantly,due to the blockage of active oxidation sites by phosphate groups,the P-doped porous carbon can stably operate at higher voltage of 3.0 V in Et₄NBF₄/PC compared to that of undoped porous carbon,so as to deliver a high energy density of 38.65 Wh kg^-1 at1500 W kg^-1.The study offers insightful material chemistry for industrial application of H₃PO₄ activated porous carbon in advanced electrochemical energy storage.