Preparation Of Nitrogen-doped Carbon Electrode Materials For Supercapacitor Applications

Sustainable energy technologies have drawn increasing attentions due to the higher consumption rates of fossil fuels. Energy conversion from sustainable resources and effective storage of electrical energy are two main ways to achieve the goals. Supercapacitors are considered to be one of the promising energy storage technologies, as they can provide a higher power density, wider range of working temperature range, better stability and safety and longer cycle life performance than batteries. Supercapacitors are also widely used in various fields. Therefore, they have very large social benefit and practical significance to carry out research work on the development of supercapacitors. Many factors can affect the performance of supercapacitors such as electrode materials, electrolyte and electric conductivity. Among all of these factors, electrode materials are the most important, and thus much attention has been focused on investing new electrode materials for high performance supercapacitor. Herein, we intend to study the electrode materials which are the key component of supercapacitors. Carbon-based materials are chosen as electrode materials. In order to increase the capacitance of the carbon-based electrode materials, we optimize their compositions and microstructures via the introduction of heteroatoms into the skeleton structures and template methods. The morphologies, structures and components of the as-prepared materials were characterized by the scanning electron microscope（SEM）, transmission electron microscope（TEM）, X-ray diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）, Raman spectroscopy and N₂ adsorption. Cyclic voltammetry, galvanostatic charge/discharge, AC impedance and long cycle life tests were used to research electrochemical performance. The detailed work is summarized as follows:1. In the first part of the work, nanostructured polystyrene/polyaniline/reduced graphene oxide（PS–PANI/rGO） hybrid materials based on in situ reduction of graphene oxide with ascorbic acid and self-assembly have been successfully prepared for electrochemical supercapacitor. The supercapacitor based on the PS–PANI/rGO as an electrode material exhibits a specific capacitance of 180 F g-1 at a current density of 0.5 A g-1 from-0.4 to 1.0 V and a high energy density of 49 W h kg-1 at the power density of 352.8 W kg-1. Moreover, the nanostructured PS–PANI/rGO has a good cycle stability（74% capacitance retention after 5000 cycles）, indicating its future potential application in electrochemical energy storage.2. On the second part of the work, nitrogen-enriched carbon sheets were synthesized using egg white as a unique carbon source and expanded perlite as a novel template. Then we got porous carbon materials through hydrothermal treatment of carbon sheets which using potassium hydroxide as the activator. Meanwhile, we obtained optimal nitrogen-doped carbon sheets through changing the carbonization temperature and activation time. Electrochemical test results indicated that a maximum gravimetric specific capacitance of 302 F g-1 at 0.5 A g-1 in a 3-electrode setup for the sample carbonized at 850 °C and activated for 6 h. Moreover, the carbon sheet-based capacitor with 2-symmetric electrodes showed an excellent cycle life（2% loss at 0.1 A g-1 after 10 000 cycles）. The excellent performance of supercapacitors may be attributed to the combination of the 3D carbon structure and the highly concentrated doped nitrogen component from the natural egg source.3. On the third part of the work, we continued to use egg white as the carbon source and nitrogen source. A novel hierarchically structured mesoporous carbon material doped with nitrogen was prepared by using a two-step template method, in which egg white and nanostructured ZnO acted as the precursor for carbon and template respectively. We can obtain the optimal nitrogen-doped mesoporous carbon through altering the mass ratio of template to egg white and the carbonization temperature. The unique structure and component can make it become the promising electrode material. Electrochemical test results revealed that the carbon exhibited a capacitance of 205 F g-1 at a discharge current density of 0.5 A g-1; and, its cyclic performance is dramatically enhanced sustaining greater than 97% of its original capacitance after 5000 charge–discharge cycles for a sample carbonized at 800 °C and megg white : mZnO = 2 : 1. This work displays a simple method to prepare porous carbon electrodes, and provides a vivid example to rationally produce carbon materials from natural sources in capacitors.