Redox Mechanism Contributions to the Behaviour of Electrochemical Capacitor Materials

Abstract

The mechanisms of charge storage in four typical electrochemical capacitor systems are compared and contrasted. These systems are based on activated carbon, ruthenium dioxide, manganese dioxide and nickel hydroxide. Charge storage is discussed in terms of charge delocalization either on the surface or throughout the electrode material. Electrical double layer formation, such as on activated carbon, is considered an example of charge delocalization, with charge distributed over the electrolyte accessible surface irrespective of the applied potential. Ruthenium dioxide also stores delocalized charge, in this case through the reversible Ru(IV)/Ru(III) redox couple. Manganese dioxide is unique in that in alkaline (battery) electrolytes charge is localized in specific structural domains, while in neutral (capacitor) electrolytes charge is delocalized over the material structure. Nickel hydroxide in an alkaline electrolyte is an example of charge localization when redox cycling due to its two-phase redox mechanism. The impact of these differing charge storage mechanisms on electrochemical performance is discussed.