Research On The Performance Of The Doped Micro/Mesoporous Carbons For Supercapacitors

Supercapacitor is an important energy storage device that fills the gap between the traditional capacitors and batteries. Undoubtedly, the properties of the electrode materials are the key factors to determine the performance of the supercapacitors. Compare with the metal oxide and conductive polymer, carbons are the earliest and the most widely used electrode materials. Whereas how to improve the energy density without compromise the power density and reduce the cost is the challenge.The controllable synthesis of carbons rich in heteroatom surface functional groups based on different kind of carbon precursors, and the fundamental research of their capacitive performance were investigated. The porous structure and the surface modification can be tuned by simple adjusting the synthetic process of carbon materials. Carbon materials were characterized by nitrogen and carbon dioxide adsorption, SEM, Raman spectrum, XPS, elemental analysis, and the electrochemical evaluation methods, which including cyclic voltammetry, galvanostatic charge/discharge, impedance test and wide potential window tests. Furthermore, the physico-chemical properties of those carbon materials are correlated to their electrochemical properties, and obtained some inspiration results. This paper mainly includes the following aspects:In chapter 3, phosphorus-rich carbons (PCs) were prepared by phosphoric acid activation of waste coffee grounds in different impregnation ratios. The porous structure and, XPS results indicate that the activation step not only creates a porous structure, but also introduces various phosphorus and oxygen functional groups to the surface of carbons. As evidenced by the electrochemical test, a supercapacitor constructed from PC-2 (impregnation ratio of 2), with the highest phosphorus content, can operate very stably in 1 M H2SO4 at 1.5 V with a retention ratio of 82% after 10000 cycles at a current density of 5 A g-1. Due to the wide electrochemical window, a supercapacitor assembled with PC-2 has a high energy density of 15 Wh kg-1 at a power density of 75 W kg-1. The possibility of widening the potential window above the regular potential window (0?1.0 V) is attributed to reversible electrochemical hydrogen storage in narrow micropores and the positive effect of phosphorus-rich functional groups can inhibit the oxidation of the carbon surface. In addition, using the optimized carbon as the electrode material, a batch of simulation capacitor cells (20 cm×10 cm) were prepared and preliminary tested the capacitive performance.In chapter 4, a series of phosphorus and oxygen enriched carbons prepared by phosphoric acid activation of the lignocellulosic waste, fruit stones, are studied as the electrodes of supercapacitor in 1M H2SO4 electrolyte. Cyclic voltammetry, galvanostatic charge/discharge and wide potential window test are carried out to evaluate the electrochemical performances of the carbon electrodes. In addition, statistical analysis is employed to confirm the limitation of the operation potential of the supercapacitors. The results show that the sample with balanced porous structure and higher phosphorus content exhibits a specific capacitance of 165 F g-1, delivers an energy density of 13 Wh kg-1 at the power density of 75 W kg-1 and an operation window of 1.5 V, and shows stable cycling performance at 1.0 V with a retention ratio of 99% even after 20000 cycles.In chapter 5, polyimide copolymer was prepared by using 4,4-bis(maleimidodiphenyl) methane and divinylbenzene as comonomer via suspension polymerization method. A series of phosphorus-oxygen-nitrogen co-doped carbon have been obtained by activation of this copolymer by phosphoric acid at the temperatures of 400?1000?. As evidenced by the electrochemical test in 1 M H2SO4, the carbon derived by the activation of phosphoric acid at the temperatures of 800? (P800) shows the best performance with a moderate specific surface area (595 m2g-1). It shows a capacitance of 157 F g-1 at the current density of 0.05 A g-1, with the existence of phosphorus functionalites, it can be operated very stably under the voltage of 1.5 V and deliver an energy density of 12 Wh kg-1 at the power density of 75 W kg-1, what's more, the capacitance can maintain 84% after 10000 cycles at high current density of 5 A g-1. It was attributed to the appropriate proportion of heteroatom (phosphorus, nitrogen and oxygen), the nitrogen and oxygen heteroatom can contribute to some part of pseudocapacitance, while the existence of phosphorus atoms can inhibit the occurrence of adverse reactions which is helpful to keep the cycle stability of supercapacitors.In chapter 6, four porous carbons with similar porosities but visible differences in their surface functionalities were investigated as supercapacitor electrodes in 1 M H2SO4. The objective was to monitor the changes to the phosphorus, oxygen and nitrogen functionalities of the carbons upon a three-electrode test and the effect of these changes on the energy storage capacity in a real two-electrode supercapacitor setup. The results indicated that the pretreatment of the three-electrode test in an acidic electrolyte are beneficial to the overall specific capacity with different extent. This can be attributed to the increment of the wettability, the pores become more accessible to the electrolyte ions; the increment of quinone type oxygen and prrolic nitrogen which contribute to the pseudocapacitive, and the favorable changes of the nitrogen, oxygen and phosphorous functionalities in carbons. Thus, the total capacitance increases as a result of the improved the double-layer capacitance as well as the pseudocapacitance.