Characterising the Charge Storage Mechanisms in Electrochemical Capacitors Using a Combination of Electrochemical Impedance Spectroscopy (EIS) and Step Potential Electrochemical Spectroscopy (SPECS)

Abstract

Electrochemical capacitors are energy storage devices which have been demonstrated to exhibit both high specific capacitance and high specific power. Electrochemical capacitors are a promising energy storage technology due to their unique combination of specific energy and power output in addition to being relatively inexpensive and environmentally friendly. The performance of electrochemical capacitors is largely influenced by the electrode material properties. The specific properties of the electrode will determine both the nature and magnitude of the charge storage processes occurring within the electrode, such as double layer capacitance (non-faradaic) and redox reactions (faradaic; pseudo-capacitance). For example, activated carbons store charge almost exclusively via double layer capacitance, whereas metal oxides, such as ruthenium oxide and manganese oxide, will store charge via a combination of double layer and pseudo-capacitance. In metal oxides, pseudo-capacitance arises due to the reduction and oxidation of the material via proton insertion (and de-insertion), with ruthenium oxide and manganese dioxide exhibiting different charge storage characteristics. Understanding how the charge storage mechanism is influenced by material (and electrolyte) properties is vital for designing electrodes with optimised performance characteristics. However, this requires an understanding of how each charge storage mechanism contributes to capacitive performance. In terms of evaluating electrode performance, conventional electrochemical methods, such as cyclic voltammetry and constant current charge-discharge, cannot differentiate the capacitance contributions from charge storage processes involved. Characterising the different charge storage contributions from double-layer charge storage (non-faradaic) and pseudo-capacitive redox processes (faradaic) is a vital step in relating electrode performance to its material properties. In this work, both electrochemical impedance spectroscopy (EIS) and step potential electrochemical spectroscopy (SPECS) have been applied to electrochemical capacitor electrodes as a performance analysis method to determine the charge storage contributions from different processes. EIS has a number of advantages over other techniques, namely, the different time constants of electrochemical processes can be utilized to separate the different mechanisms occurring at the electrode. The SPECS experiment uses the same principle as EIS, i.e. separating the electrochemical mechanisms based on their different time constants. The SPECS procedure involves applying a small (±25 mV) potential step to the working electrode followed by a long equilibration time (300 s). This process is repeated over and entire charge-discharge cycle. By scanning at such a slow rate, the electrode has time to equilibrate at each potential, and the maximum charge storage capabilities of the electrode can be accessed. Each of the different charge storage processes occurring at the electrode has a unique time- dependent current response, and hence each potential step profile can be fitted to a model describing each of these processes. From this, values for series resistance (RS), double layer capacitance (CDL), diffusion limited capacitance (CD) and residual capacitance (CR) can be extracted. When the potential is stepped over an entire capacitor cycling range, contributions from each process can be determined at each point in the cycle. Additionally, by varying the equilibration time over which the current response is analysed, the scan rate can be effectively increased therefore the electrode behaviour can be analysed over a range of scan rates. This allows the development of a Ragone diagram for the different charge storage processes, indicating how specific charge storage mechanisms contribute to the power and energy characteristics of different electrode materials. This technique has been applied to a range of commonly used electrochemical capacitor systems including activated carbon, manganese dioxide and ruthenium oxides.