Asymmetric supercapacitors based on transition metal oxides induced pseudocapacitance
Mak, Wai Fatt
Date of Issue2012
School of Materials Science and Engineering
Pseudocapacitor exhibiting higher capacitance and energy density than electrochemical double layer capacitor (EDLC) and delivering greater power density than batteries represent unique charge storage devices that bridge the gap of EDLC and rechargeable battery. In particular, Ru02 based material has received much attention in past decades. However, they are expensive due to their scarce resources. This study reports on alternative pseudocapacitive material such as iron oxide as an electrode material for supercapacitor (SCs) applications. Also, an asymmetrical device configuration was conceptualized to extend the working voltage so as to achieve higher energy density. The role of nanofibers in electrical double layer formation, pseudocapacitive properties, charge retention and cycling performance has been investigated for various materials. Physical characterization techniques such as Field Emission Scanning Electron Microscopy, X-ray Diffraction, Nitrogen Adsorption Isotherm, and Fourier Transform Infra Red Spectroscopy were used to study the physical properties of various materials. Electrochemical characterization such as cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy were employed to investigate the electrochemical performance of different materials. These studies provide insight into the effect of morphology, surface area, electrode-electrolyte interaction on charge storage and transport mechanisms in SCs. In addition, the effect of material porosity I on capacitance contribution was investigated. Higher content of micropores is found to increase the capacitance as more ions can be accommodated in these micropores. Systematic study on the effect of electrolytes on supercapacitor performance was carried out, including correlation with the size of cations and comparison between aqueous and organic electrolytes and the morphological effect of the electrode material. The size of the ions also plays a substantial part in electrical double layer contribution. With smaller size, higher quantity of charges can be stored and subsequently utilized. Reversible chemical reactions are also found to have overwhelming contribution to the charge storage via pseudocapacitance. Solvated sodium ions have additional pseudocapacitance which greatly contributes to the charge storage capability. Choice of electrode/electrolyte pair is therefore critical to for obtaining higher charge storage/ capacitance in supercapacitors. An asymmetrical device configuration was conceptualized to extend the working voltage so as to achieve higher energy density. Tto realize asymmetrical supercapacitor, different electrodes of nanofibers have been judiciously coupled. The effect of asymmetrical supercapacitor architecture on the voltage window has been investigated. The performance of the devices was then evaluated using Ragone plot. Our studies has shown that such asymmetrical supercapacitors proposed increases the energy density of supercapacitors and would better bridge the gap of EDLC and rechargeable batteries.