Separating the Faradaic and Non-Faradaic Charge Storage Mechanisms in Electrochemical Capacitors Using Step Potential Electrochemical Spectroscopy

Abstract

Electrochemical capacitors are valuable energy storage devices due to their high capacitance and high specific power. Electrochemical capacitors have a unique performance characteristics due to their ability to charge and discharge much faster than batteries, but with longer discharge times than conventional capacitors.  However, these devices are currently limited by their low specific energy. In order to increase the potential applications of electrochemical capacitors, their performance needs to be optimised for their application. Electrochemical capacitor performance is largely influenced by the electrode material, as it determines 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). Understanding the mechanism by which an electrode material stores charge is fundamental to the improvement of electrochemical capacitors. However, conventional methods for evaluating performance, such as cyclic voltammetry and constant current charge-discharge, cannot differentiate the capacitance contributions from charge storage processes involved. This is particularly important in pseudo-capacitors, which have both faradaic and non-faradaic processes contributing to charge storage, and the separation of these processes is crucial to understanding their performance. In this work, step potential electrochemical spectroscopy (SPECS) has been applied to electrochemical capacitors as a performance analysis method to determine the charge storage contributions from different processes. The SPECS experiment 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. SPECS has been used to examine the performance of a range of electrode-electrolyte conditions and has been successful in differentiating how the relative contributions to capacitance vary depending on the electrode material. The most commonly used electrode materials (activated carbon, manganese dioxide and ruthenium dioxide) have been examined. These materials were chosen because they each exhibit different charge storage mechanisms which can be differentiated by the SPECS method.