Study Of Nano Carbon And Metal Oxide Composite Electrodes-based Supercapacitors

Supercapacitors, also called electrochemical supercapacitors, are regarded as ideal energy storage devices due to their promising properties, such as high power density and long cycle life. In the 21st century, supply shock-induced energy crisis has influenced the economy and people's life. In response to the crisis, there are movements towards the development of renewable energy, such as wind power and solar energy. Whilst, the solution to the storage of the green sustainable energy has been attached much importance. Supercapacitors as one kind of energy storage devices deserve extensive investigation in both theoretic and application research.In this thesis, carbon nanotubes (CNTs) films were fabricated by both chemical vapor deposition (CVD) and screen-printing method. CNT-based composites as CNT-ZnO and CNT-FexOy were also employed as the electrodes of the experimental supercapacitors. The capacitive properties of the as-made electrodes were investigated. The main contents in this paper are listed as follow:1. Two methods, CVD and screen-printing, were applied to prepare CNT films due to their low cost and large-scaled fabrication process. The capacitive properties of the as-made CNT film in both liquid and gel electrolyte were studied. The CNTs were entangled to form a network structure with many mesopores, which are benefit to the formation of double-layer capacitance. The CNT electrodes exhibited good reversible charge/discharge ability in cyclic voltammetry and charge-discharge tests. Moreover, their capacitive behavior in gel electrolyte is as good as in liquid one, indicating that the gel polymer processes good ion conductivity and potential application in solid electrolyte.2. CNT-ZnO composite was used as the electrode of supercapactiors, and ZnO was deposited by ultrasonic spray pyrolysis (USP). The analysis of electrochemical testing showed that the composite electrode exhibited typical capacitive behavior, and the mechanism of pseudo-capacitance derived from ZnO was discussed. The CNTs with ZnO deposited for 5 min demonstrated the optimized electrochemical capacitive properties and lowest interface charge transfer resistance (Rct), indicating the cooperative effect of the CNTs and ZnO. However, excess ZnO in CNT-ZnO composite will decrease the specific area and conductivity of the electrodes, restrain the redox reactions, and thus deteriorate the capacitive performance.3. CNT-FexOy composite was employed as the electrode of the experimental supercapacitor cell. The FexOy was deposited onto CNT films by USP in various time. The results of electrochemical measurements showed that the electrodes exhibited good capacitive properties in gel electrolyte as well as in liquid solution, suggesting the electrodes working in gel electrolyte is more stable. The sample with FexOy deposited in 10 minutes achieved the highest specific capacitance among the experimental series. Excess FexOy in CNT-FexOy composite will cause agglomeration and deteriorate the capacitive performance by destroying the network structure of the CNT matrix.4. Polypyrrole (PPy) by chemical polymerization was combined with CNTs synthesized by CVD as the electrodes of supercapacitors. The composite materials maintained net-structure with nano-particles of PPy decorated in CNT film. The results of cyclic voltammetry (CV) showed that CNT-PPy exhibited both typical double-layer capacitance and Faradic capacitance, indicating the excellent cooperation of CNTs and PPy. The composite film also exhibited good reversible charge/discharge ability and high energy density in chronopotentiometry test, well consistent with the CV analysis. With the increase of PWA to 50%, the capacitance of the as-made supercapacitor reaches the highest value of 202.5 F/g in the experimental series.5. Graphene as a new kind of carbon-based materials was prepared by a modified Hummers method and hydrazine reduction process, and ZnO was deposited on graphene by USP. Graphene-ZnO composite film was introduced for its potential application in supercapacitors. The electrochemical characteristics of the film were investigated by electrochemical tests. The results showed that graphene-ZnO composite film exhibited an enhanced capacitive behavior with better reversible charging/discharging property and higher capacitance values, by comparison to pure graphene or ZnO electrode.