Examining the Charge Storage Mechanisms of Materials Used in Electrochemical Capacitors Using Electrochemical Impedance Spectroscopy

Electrochemical capacitors are a type of energy storage device which are characterised by their high specific energy, fast discharge rates and high cyclability. Electrochemical capacitors store energy in the electrical double layer which is formed at an electrode-electrolyte interface under an applied potential. These electrodes do not undergo any faradaic reactions and as a result, can charge and discharge very quickly and do not degrade significantly during cycling.

Pseudo-capacitors are a type of electrochemical capacitor in which the electrodes exhibit fast, highly reversible redox reactions in addition to double layer charging. This pseudo-capacitance increases the charge storage capabilities of these devices.

Electrodes used for electrochemical capacitors rely on high surface area materials to provide high capacitance, such as activated carbon. Whereas pseudo-capacitors utilise materials which can undergo fast redox reactions without causing any compositional or phase changes in the electrode, such as ruthenium dioxide and manganese dioxide.

Typically, electrochemical performance analysis of electrodes is conducted using potential sweep (cyclic voltammetry) or constant current charge discharge methods. These techniques provide a total capacitance for the electrode but do not allow for extensive analysis of the charge storage mechanisms. This is evident in the similar cycling behaviour of activated carbon, ruthenium dioxide and manganese dioxide, despite these materials having significantly different charge storage mechanisms.

In this work, these methods are used in conjunction with electrochemical impedance spectroscopy (EIS) to separate the capacitance arising from double layer charge storage compared to that stored through pseudo-capacitance. Three commonly used electrode materials were examined, each known to have different capacitance and charge storage mechanisms: (1) activated carbon, which has purely double layer capacitance in the order of ~150 F/g [1], (2) ruthenium dioxide, which exhibits pseudo-capacitance at the surface (due to its high conductivity) resulting in capacitance of ~900 F/g [2] and (3) manganese dioxide, which is reduced and oxidized throughout the bulk of the material leading to extremely high capacitance of 2000 F/g [3] (for electrodeposited thin films) and 260 F/g [4](for powdered electrodes).

1. Beguin, F. and E. Frackowiak, eds. Supercapacitors Materials, Systems, and Applications. ed. M. Lu2013, Wiley-VCH: Weinheim.

2. Long, J.W., et al., Langmuir, 15 (1999) 780-784.

3. Cross, A., et al., Journal of Power Sources, 196 (2011) 7847-7853.

4. Yang, X.-h., et al., Electrochimica Acta, 53 (2007) 752-757.