(Invited) Differntiating the Behaviour of Electrochemical Capacitor Materials

Abstract

Electrochemical capacitors are rapidly emerging as a viable energy storage technology for many different applications, encompassing a wide range of size scales. Their performance is typified by a high power density and excellent cyclability, as well as a relatively low energy density, improving which has been the focus of considerable research activity. Environmental considerations and cost per unit of energy are also important for widespread commercial acceptance. During the quest for development of an improved electrochemical system many different materials have been examined for electrode use. Most often activated carbon is the material of choice for electrochemical capacitors, in particular when coupled with a non-aqueous electrolyte based on either acetonitrile, propylene carbonate, or perhaps even an ionic liquid. Improving the performance of the activated carbon has been an important avenue of research, in particular optimizing the pore size distribution of the material to enable maximum charge storage in the electrical double layer. Other avenues of research have focused on developing other classes of materials, such as metal oxides and conducting polymers, so as to store charge via pseudo-capacitance, which can be defined as facile, reversible redox reactions occurring at the electrode-electrolyte interface. In this way, the near-surface region of the electrode material can be accessed for what is effectively pseudo-3D charge storage. The study of materials that exhibit pseudo-capacitance has enabled many materials previously considered as battery materials, to make in-roads into electrochemical capacitor applications. While the concept and study of pseudo-capacitive materials is very worthwhile for electrochemical capacitor applications, there is growing interest in highlighting the differences in charge storage mechanisms that occur in these materials, particularly so since the end electrochemical result is very similar even though the choice of materials and notional charge storage mechanisms are very different. This presentation will highlight the differences in materials used, as well as differentiate the charge storage mechanisms that various materials undergo. The presentation will also seek to describe how materials can exhibit battery-like behaviour under one set of conditions, and electrochemical capacitor-like behaviour under others.