(Invited) Redox Contributions to the Behaviour of Electrochemical Capacitors

Abstract

The use of electrochemical capacitors is expanding rapidly in the electronics industry due to their excellent performance characteristics, including high specific power and cyclability. However, their specific energy, and hence cost per unit energy, is somewhat limited, thus prompting significant global research to address this shortcoming. Energy storage in these systems is a combination of electrical double layer formation, or charge separation at the electrode-electrolyte interface, and pseudo-capacitance, which is based on facile redox reversible reactions at this interface. One of the key characteristics of these systems is the relative contributions that these processes make to overall charge storage. Much ado has been made about the contributions that redox processes made to pseudo-capacitive electrode performance. Many apparently similar materials such as RuO2, MnO2 and NiOOH contribute charge through redox processes at the electrode surface; however, their performance can be quite different depending on the electrode material itself and the conditions under which it is cycled. The typical response for an electrochemical capacitor electrode is a box-like voltammogram; however, materials such as these have a basis of such behaviour, but may also include distinct redox peaks within the voltammogram. Here we will examine the role that the redox mechanism for these pseudo-capacitive materials has on electrode performance, as well as investigate its impact on electrode properties, in particular crystal size. Outcomes will allow further differentiation of materials used in electrochemical capacitors.