Design, Scalable Synthesis And Application In Supercapacitors Of Carbon-Based Nanocomposites

In the twenty-first century, developing sustainable clean energy and advanced energy storage technology are unquestionably great challenges. Supercapacitors (also called electrochemical capacitors), a unique energy storage device, can provide energy density several orders of magnitude higher than conventional capacitors, and also greater power density and better cyclicity than batteries. Thus, supercapacitors, as the bridge between conventional capacitors and batteries, have attracted intensive attention in recent years. Generally, it is considered that electrode materials play the most important role in the supercapacitors. Among various electrode materials, carbon-based nanocomposite material attracts increasingly attention because of its abundance, diversity, and good conductivity. Currently, a certain progress has been made in developing electrode materials. However, expensive equipments or severe, complicated, and time-consuming reaction routines or hypertoxic chemical reagents are often utilized in the preparation of these materials, which limits its practical application. Therefore, it is urgent to design a environmental, simple, and green method to prepare carbon-based composite materials with a good capacitive performance. All the above is the major motivation of this thesis. And we carried out a relatively systematic study as the following contents:1. The introduction summaries the latest achievements of electrode materials with high capacitance performances. We conclued several representative ways of improving the properties of carbon-based supercapacitors, including introducing heteroatoms, developing appropriate porous structure, compositing them with new kinds of metal oxides, and designing new synthetic methods for carbon materials. From the view points of experiment and theory, there is still a long way ahead of us for preparing high-performance electrode materials. However, we believe that new kinds of carbon-based electrode material with large capacitance can be prepared via skillful fabrication techniques, such as, incorporating heteroatoms, optimizing sub-nanopores, and compositing with classical metal oxides.2. A new kind of nitrogen-doped porous carbon nanofiber with high electrochemical capacity has been prepared on a large scale without using activators, that is, loading polypyrrole on the scale-up carbon nanofibers, and then pyrolyzing them at a certain temperature. The specific capacitance value of this material based capacitors can be up to202.0F g-1(at a current density of1.0A g-1in6.0mol L-1KOH aqueous solution electrolyte). Meanwhile, it exhibits high rate capacity and high power density of89.57kW kg-1.3. A green, cost-effective, and efficient method is carried out to prepare high-performance electrode materials. Immersing pyrolyzed bacterial cellulose (p-BC)in0.1M KMnO4/0.1M K2SO4aqeous solution, the MnO2layer with pseudo-capacitive property is deposited on the surface of p-BC obtainning the p-BC@MnO2as positive electrode materials. Furthermore, using a simple hydrothermal reaction beteen urea and p-BC, the p-BC/N is prepared as negative electrode materials. We ultilize the two kinds of electrode materials and design an asymmetric supercapacitor without any adhesive, using p-BC@MnO2as the cathode, p-BC/N as the anode, and Na2SO4solution as the electrolyte. Because the two electrode materials have good synergistic effect, the optimal asymmetric capacitor exhibits excellent electrochemical properties with maximum power density of32.91Wh kg-1, energy density of284.63kW kg-1, and a long-term durability (retain95.4%capacitance after2000cycles).4. Nitrogen-doped pyrolyzed bacterial cellulose is successfully synthesized via a low-cost, environment-friendly, and large-scale hypothermal hydrothermal approach. Furthermore, a new kind of flexible all-solid-state supercapacitor is constructed with this electrode material. The as-prepared device can provide a highest power density of390.53kW kg-1. Also, it remains95.9%initial specific capacitance after5000cycles, showing it posesses good supercapacitive performances.5. Fe2O3nanoneedles coating carbon cloth (CC@Fe2O3) prepared by an easy hydrothermal method is reported, meanwhile, with this electrode material and2.0M Li2SO4aqueous solution electrolyte, a new type of high-energy-density symmetrical supercapacitor has been designed. This as-constructed device possesses a high energy density of11mWh cm-3and good circle durability. Therefore, this kind of high-energy-density supercapacitor is suitable for commerical application.6. Fluorine doped with iron oxide coating the carbon cloth (CC@Fe2O3-F) is synthesized as the electrode materials of supercapacitors. At a current density of1mA cm-2, area capacitance of the electrode material can reach1.12F cm-2, about twice as much as the carbon cloth without fluoride. Moreover, when the current density is increased from1mA cm-2to50mA cm-2and100mA cm-2, respectively, the area capacitance can retain the initial87.5%and83%. Meanwhile, the supercapacitor has a high power density of11.11W cm-3and energy density of1.85mWh cm-3, and high cyclic stability (at different current densities after5000cycles without obvious loss of capacitance). When the capacitive area is enlarged, the capacitance of this device is almost linear and can reach6.84F (area is8cm2), suggesting that it is able to fabricate such a capacitor with a large area of electrode material.7. A simple, eco-friendly, green, general and effective way to synthesize3D free-standing heteroatom-doped carbon nanofiber on a large scale has been reported. In the method, bacterial cellulose immersed with H3PO4, NH4H2PO4, and H3BO3/H3PO4aqueous solution, successively, is pyrolyzed in in the inert gas. Moreover, the as-prepared N,P-co-doped carbon nanofiber exhibits good supercapacitive performances.