ABSTRACT
Supercapacitors can be classied into two categories, namely, electrical double-layer capacitors (EDLCs) with carbon materials as electrodes and pseudocapacitors with transition metal oxides or conducting polymers as electrodes.1-3 However, one major limitation of carbon-based EDLCs is the lower specic energy density. Most of the commercial products available in the market have a specic energy density lower than 10 Whkg−1. This specic energy density value is still relatively lower than the lowest gures for batteries, that is, 35-40 Whkg−1 (lead acid battery). Metal oxides provide an alternative solution as an electrode material because of their high specic capacitance (SC; farad per gram) at low resistance, thereby making it easier to construct high energy and high power density supercapacitors. Several transition metal oxides with various oxidation states represent attractive
CONTENTS
5.1 Introduction .............................................................................................. 119 5.2 Ruthenium Oxide ..................................................................................... 121 5.3 Manganese Oxide ..................................................................................... 125 5.4 Nickel Oxide ............................................................................................. 137 5.5 Cobalt Oxides ........................................................................................... 138 5.6 Molybdenum Oxides ............................................................................... 140 5.7 Vanadium Pentoxide ............................................................................... 140 5.8 Tin Oxide ................................................................................................... 141 5.9 Indium Oxide ............................................................................................ 142 5.10 Bismuth Oxide .......................................................................................... 143 5.11 Iron Oxide .................................................................................................. 143 5.12 Lesser Used Oxides .................................................................................. 144 5.13 Conclusion ................................................................................................ 145 References ............................................................................................................. 146
materials for supercapacitor electrodes owing to their excellent structural stability and high SC. For the transition metal oxides, the reversible redox reactions are primarily responsible for the energy storage in addition to the electric double-layer storage. This makes metal oxides with pseudocapacitance a predominant part in charge storage process in supercapacitors. There have been plenty of researches on utilization of these metal oxides in different nanostructure forms to enhance the overall performance of the supercapacitors. The metal oxides that have been explored as supercapacitor electrodes, including RuO2, NiO, MnO2, Co2O3, IrO2, FeO, TiO2, SnO2, and V2O5, that exhibit high SC values.4,5
As we know, nanotechnology can manipulate particles on both atomic and molecular scales, the nanometer products such as metal oxides used in supercapacitors can be processed by two techniques: Top-down or Bottom-up approach (Figure 5.1). The top-down approach starts with a bulk material, which breaks down into smaller fragments. So this approach makes use of larger (initial) structures, which is controlled externally in the processing of nanostructures, for example, ball milling or attrition and lithography (etching through a mask).
