Hierarchical Co-Based Materials:Synthesis And Their Applications In Supercapacitors

The electrochemical supercapacitor is an emerging technology that promises to play an important role in meeting the demands of electronic devices and systems both now and in the future. This thesis traces the history of the development of the technology, explores the principles, theory of operation, supercapacitors in applications and supercapacitor electrode materials. For that reasons, the quest for higher energy and power for supercapacitors to complement or compete with batteries has led to a major effort in developing advanced electrode materials. The criterion for designing supercapacitor electrode include high performance (energy stored per unit mass, volume, or area of active materials), large rate capability (capacitance retention at high scan rate or current density), and high cycle stability. In addition, the toxicity and cost of the active materials used in an electrode design should be taken into account as well. Many recent researches focused on hierarchical nanoarchitecture and porous structure, both of them can provide advantages for improving the electrochemical performance in energy storage electrodes. Here we report a novel strategy to synthesize new electrode materials, hierarchical Co-based porous layered double hydroxide (PLDH) arrays derived via alkali etching from Co(OH)2@CoAl LDH nanoarrays. This structure not only has the benefits of hierarchical nanoarrays including short ion diffusion path and good charge transport, but also possesses a large contact surface area owing to its porous structure which lead to a high specific capacitance (23.75 F cm-2 or 1734 F g-1 at 5 mA cm-2) and excellent cycling performance (over 85% after 5000 cycles). To access the feasibility such as hierarchical Co-based and Porous Structure (PLDH) arrays electrode, an asymmetric supercapacitor device were demonstrated by using optimum as-prepared sample Co(OH)2@PLDH-18 as a positive electrode material and activated carbon (AC) as the negative electrode. The enhanced electrode material is a promising candidate for supercapacitors in future application. Other, hierarchical Co3O4@MnO2 core-shell arrays on Ni foam have been fabricated by using a simple, low-cost, template-free and environment-friendly method. In this structure with the high conductivity of the mesoporous Co3O4 nanowire arrays served as the "core" and the ultrathin branch MnO2 nanosheets is the "shell" layer. By virtue of the synergetic contribution from individual constituents and the sophisticated configuration, the resulting Co3O4@MnO2 core-shell arrays exhibit a higher capacitance 21.5 F cm-2 or 977 F g-1 at current density of 5 mA cm-2 and excellent cycling ability (94% retention) after 5000 cycles. Expected to meet all requirements for the improvement of electrochemical performance of the materials and open the door to more investigated and a promising to candidate for supercapacitors in future application...