Nanostructured Metal Oxides for Supercapacitor Applications

Abstract

The fundamental properties of supercapacitors (SCs) with descriptions restricted to the metal oxides systems and the effect on the electrochemical performance and synthesis are described in this chapter. Metal oxides such as manganese oxide (MnO), vanadium oxide (V₂O₅) and ruthenium oxide (RuO) have demonstrated great potential in the field of energy storage due to their structural as well as electrochemical properties, thus attracting huge attention in the past decade and in recent years. The major contributing factor to the electrochemical properties is their capability to achieve relatively high pseudocapacitive performance derived from their theoretical values resulting from their multiple valence state changes. The developments of the metal oxide (MO)-based electrode materials and their composites are being explored from the synthetic point of view as well as their emerging applications as energy storage materials. Therefore, the need to further exploration of MO-based electrodes is motivated by their considerably low-cost and environmentally friendly nature as compared to other supercapacitive electrode materials. This chapter accounts to the overview of various nanostructured metal oxide materials for application as energy storage materials in supercapacitors.

Appendix

Potentiostat – A potentiostat is the electronic hardware required to control electrochemical cell and run most electroanalytical experiments.

Cell potential – The overall electrical potential of an electrochemical cell. It is the sum of the reduction potential of the cathode and the oxidation potential of the anode.

Electrolytic cell – A cell that consumes electrical energy to drive a non-spontaneous redox reaction.

Charge (Q) – The quantity of unbalanced electricity in a body such as an electron or an ion.

Charge density (q) – The measure of charge Q and electrode area A, i.e. q = Q/A.

Chronoamperometry – The techniques and methodology of studying current as a function of time.

Current (I) – The rate of charge flow or passage, i.e. I = (dQ/dt).

Specific current – The measure of current I and electrode mass g, i.e. i = I/g.

Cyclic voltammogram (CV) – A plot of current on the y-axis against potential on the x-axis during a voltammetric experiment in which the potential is ramped twice, once forward to the switch potential and then back again.

Electrochemical area – The area of an electrode; the area that an electrode is ‘perceived’ to have.

Electrochemical cell – An electrochemical cell typically consists of two electronic conductors (also called electrodes) – An ionic conductor (called an electrolyte).

Electrode – A conductor employed either to determine an electrode potential (at zero current, i.e. for potentiometric experiments) or to determine current during a dynamic electroanalytical measurement. The electronic conductivity of most electrodes is metallic.

A reaction is classified as oxidation or reduction depending on the direction of electron transfer. There are two fundamental types of half-cell reactions: – Oxidation reactions – Reduction reactions

Oxidation – Involves the loss of an electron and involves the transfer of electrons from the species to the electrode R = O + ne

Reduction – Involves the gain of an electron and involves the transfer of electrons from the electrode to the species O + ne = R.

Electrode at which the oxidation reaction occurs is called the anode.

Electrode at which the reduction reaction occurs is called the cathode.

Anodic current – An anodic current is the flow of electrical charge (usually carried by electrons) into a working electrode from a second phase (usually an electrolyte solution) as a result of the oxidation of one or more species in the second phase.

Cathodic current – A cathodic current is the flow of electrical charge (usually carried by electrons) out of a working electrode into a second phase (usually an electrolyte solution) leading to the reduction of one or more species in the second phase.

The electrode at which the reaction of interest occurs is called the working electrode.

The electrode at which the other (coupled) reaction occurs is called the counter electrode.

A third electrode, called the reference electrode, may also be used. An ideal reference electrode is one that maintains a constant potential irrespective of the amount of current (if any) that is passed through it.

Electrode potential – The electrode potential for a reaction is derived directly from the free energy change for that reaction ΔG = −NFE. The standard oxidation potential is equal in magnitude but opposite in sign to the std. reduction potential.

Electrolyte – An ionic salt to be dissolved in a solvent or the solution formed by dissolving an ionic salt in a solvent.

Electrolytic processes – Reactions in which chemical changes occur on the passage of an electrical current.

Galvanic or voltaic processes – Chemical reactions that result in the production of electrical energy.