Nanostructured Cobalt Hydroxide And Its Composite Material For Supercapacitor Applications

Supercapacitors as an energy conversion and storage system have been extensively studied because of their higher power density compared with lithium ion batteries/fuel cells while higher energy storage than traditional capacitors. Many factors can affect the performance of supercapacitors such as electrode materials, electrolyte, conductors and even the fabrication and manufacturing approaches. Among all of these factors, electrode materials are the most important, and therefore much attention has been focused on investing new electrode materials for high performance supercapacitor. Recently, metal oxides such as ruthenium oxides are considered the most promising material for the next generation of supercapacitor; however, the high cost and low abundance of RuO2 limit its large-scale application. Thus there is a great need to explore alternative material for pseudocapacitance-based supercapacitors. Among the electrochemically active materials, Co(OH)2 is attractive due to its high theoretical capacitance (3470F/g), low cost, and well-defined electrochemical redox activity. There are works on Co(OH)2 that are mainly base on the simple electrodeposition method, which result in the micrometers thin film and show limited energy density. At the same time, nanostructured metal sulphides are important materials used in supercapacitor recently such as CoS and ZnS have been explored as electrode materials in pseudocapacitors, of which a high specific capacitance of 508 F/g for CoS nanowires has been achieved. Thus a new material field of metal sulphides has attracted attention. Bi2S3 is a semiconductor with layer-structure and it is easy to synthesis into various morphologies, such as nanorods, nanonetwork, and nanoflowers, etc., and nanorods-structured Bi2S3 has potential use for supercapacitors.In this MS thesis, we reviewed the basic information of supercapacitors and synthesized the electrode materials with new methods. A lot modern analyses were conducted to characterize their morphologies, surface areas, etc., while some electrochemical tests were performed to measure their capacitive behaviors. The result shows that big surface area is important for supercapacitor electrode material; in addition, modification art is also a good way to improve the properties of electrode material. The main sections of this thesis are summarized as follows:(1) Review the background knowledge and application filed of this work including fundamentals, the research status of electrode materials and challenges. The novelty and main achievements are briefly introduced.(2) The electrochemical experimental and characterization methods are employed in this thesis to investigate the capacitive behaviors and evaluate the performance of electrodes are described, while the conditions of all these methods are specified. In addition, some nanomaterials synthesis methods employed in this thesis are briefly introduced.(3) Co(OH)2 hollow structures with different morphologies including cube, octahedron and flower are synthesized via a simple, fast and one-step Cu2O template etching method. The characterization and evaluation of the as-prepared materials show that the obtained various Co(OH)2 inherits the size and shape of the Cu2O templates but with an inside hollow. In addition, the three differently nanostructured Co(OH)2 have different capacitive behaviors. The flower Co(OH)2 has the largest specific capacitance of 1350F/g, while octahedron Co(OH)2 has the smallest one of 986.4F/g. This is mainly because the flower Co(OH)2 has the largest available surface area according to both the theory calculation and BET data. In a word, surface area is important for the electrode material, which is in turn determined by morphology.(4) Nanostructured Bi2S3 has been used in various energy conversion and storage systems but it alone doesn’t possess significant capacitance and never been investigated. A modification art in nanoscales is used to create a structure composed of Bi2S3 nanorods and a-Co(OH)2 nanosheets, which is controllably synthesized by a facile seed-layer assistant hydrothermal method to produce the former followed by electrochemical deposition of the latter. The Co(OH)2-modified Bi2S3 nanorods significantly boost the pseudocapacitance of Bi2S3 for high specific capacitance while improving the cycle life stability. Results reveal that the controlled Co(OH)2 modification process promotes a networked, highly porous structure for a much larger surface to allow the electrolyte-accessing, thus enhancing the pseudocapacitance to result in high specific capacitance.(5) The summarization and conclusion of this thesis are made, and the prospections on electrode material researches are proposed.