Graphene and metal oxide nanomaterials for high-performance supercapacitors

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Energy storage systems (ESSs) play a critical role in plenty of applications including renewable energy systems, power systems for electric vehicles (EVs) and hybrid electric vehicles (HEVs), and electrical power grids for improving reliability and overall use of the entire system. Currently, there are several types ESSs dominated the energy storage. Each kind of ESSs has their own operation mechanism, energy efficiency, energy density, power density, cycle life, charge and discharge capability, cost efficiency, operating temperature. The common ESS is based on lead acid battery which stores electrical energy in the form of chemical energy. However, if the batteries are overdischarged or kept at a discharged state, its capability will be irreversibly undermined because the sulfate crystals become larger and more difficult to break up during recharge. Since the first NiCd battery was created by Waldemar Jungner in 1899, even though NiCd battery technologies have experienced a series of evolutionary developments, its demerits are obvious including 1) shorter life cycle; 2) memory effect; 3) toxicity of Cd; 4) lower energy density; and 5) limited negative temperature coefficient. Based on the development of NiCd battery technology, nickel metal hydride (NiMH) batteries was proposed by researchers which possess better performance than NiCd batteries in cycle life, energy density and charge&discharge rates. Lithium ion is the preferred chemistry, having a superior specific energy and power density to nickel metal hydride. More lithium per gram stored in the electrodes contributes to higher energy density and power density. In addition to chemical battery system, researchers recently proposed some new sorts of ESSs including flywheel, compressed air energy storage (CAES), superconductive magnetic energy storage (SMES), etc. All of them can provide super energy density and power density. But they are more or less blocked ether in complex mechanical construction or cooling device. Supercapacitor has emerged to be an exciting energy storage device, which is able to provide high specific power, charge and discharge up to million times, have long lifetime and broad range of working temperature. Even though supercapacitor has been widely seen as a promising energy storage candidate to replace the traditional chemical batteries, it also suffer its drawback that the low energy density (the energy stored in per unit of volume and weight), high equivalent series resistance (ESR) and its high cost associated with its performance. Therefore, this PHD thesis project aims to address these drawbacks of supercapacitor by designing different nanotechnologies and fabrication methods to synthesize advance materials with better performance than that of conventional supercapacitor. A Series of designed structures and materials were fabricated by designed methods. All the materials were also investigated by using X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) observation techniques, Brunauer–Emmett–Teller (BET) surface area measurement and electrochemical testing. A facile and effective hydrothermal treatment that is able to control the condensation speeds of precursors in the solution along the <010>, <100> and <001> directions was designed to fabricate vanadium oxide nanoribbon used for the electrode of supercapacitor. It was achieved by controlling the hydrothermal reaction time and the weight ratio to synthesize the ultralong vanadium oxide nanoribbon with controlled width. It has high specific capacitance of 453 F g⁻¹ at the scam rate of 2 mV s⁻¹ in 2 M NaCl electrolyte, and it still maintained a high capacitance of 201 F g⁻¹ at a higher scan rate of 50 mV s⁻¹, attributing to the easy ion insertion and electronic transport along the a-b plane rather through the layers of the c-axis. Vanadium oxide nanotubes were synthesized by a revised hydrothermal treatment with high-speed stirring. The preparation involved dissolution of V₂O₅ into H₂O₂ and high-speed stirring (10000 r/min) with hexadecylamine. The product was characterized by scanning electron microscopy, transmission electron microscope, X-ray diffraction and thermogravimetric analysis. The electrochemical properties of the materials as electrodes for electrochemical capacitors were evaluated by cyclic voltammetry in a three electrode system consisting of a saturated calomel electrode as reference electrode, platinum as a counter electrode and the active materials as the working electrode. A high capacitance of 148.5 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹ in 2M KCl. The electrode maintained a high capacitance of 105 F g⁻¹ at a higher scan rate of 50 mV s⁻¹ in 2M KCl electrolyte. 3D mesoporous hybrid NiCo₂O₄@graphene nanoarchitectures were successfully synthesized by a combination of freeze drying and hydrothermal reaction. Field-emission scanning electron microscopy (FESEM) and TEM analyses revealed that NiCo₂O₄@graphene nanostructures consist of a hierarchical mesoporous sheet-on-sheet nanoarchitecture with a high specific surface area of 194 m² g⁻¹. Ultrathin NiCo₂O₄ nanosheets, with a thickness of a few nanometers and mesopores ranging from 2 to 5 nm, were wrapped in graphene nanosheets and formed hybrid nanoarchitectures. When applied as electrode materials in supercapacitors, hybrid NiCo₂O₄@graphene nanosheets exhibited a high capacitance of 778 F g⁻¹ at the current density of 1 A g⁻¹, and an excellent cycling performance extending to 10000 cycles at the high current density of 10 A g⁻¹. We also presented a rational, large-scale and general method, called controllable freeze casting (CFC), to fabricate a high-densely assembled and aligned free-standing NiCo₂O₄@graphene 3D foam by vacuum filtration and air compress pressure assembly method. In the designed method, the amount of water is controllable, therefore controlling the size and the shape of the ice when the material was introduced into freeze drying system, finally achieving controllable pore size and aligned structure. This free-standing foam retains the intrinsic properties of graphene sheet, such as high surface area and high electrical conductivity. In the foam, the graphene sheets build the high conductive skeletons. And the skeletons with high surface areas support the uniform distribution of NiCo₂O₄ nanoparticles on the graphene sheets. By controlling the amount of water in the precursor, it is possible to fabricate 3D NiCo₂O₄@graphene foams with a wide range of thickness and pore size. This dense NiCo₂O₄@graphene material exhibited a high capacitance of 790 F g⁻¹ at a current density of 2 A g⁻¹, and an excellent cycling performance at a high current density of 10 A g⁻¹. The compression test revealed that the 3D NiCo₂O₄@graphene foam exhibited strong mechanical property which is able to support 20,000 times its own weight without structure collapsing. The novel synthesis method of such 3D foam with excellent properties paves the way to explore the application of lamellar materials like graphene in a self-supporting, metal oxide deposition and 3D foam.
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