The Effects of Duty Cycle on the Step Potential Electrochemical Spectroscopy (SPECS) and Electrochemical Impedance Spectroscopy (EIS) Analysis of Manganese Oxides for Electrochemical Capacitors

Abstract

Step potential electrochemical Spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS) have been developed for characterising the performance of electrochemical capacitors. However the effects of the different duty cycle on the behaviour and analysis of electrochemical methods still need to be investigated. In SPECS method a small potential step is applied with a sufficient rest time to allow equilibrium between each step throughout an applied potential window. This small scan rate enables an electrode to reach to its maximum charge storage capabilities. More importantly, it allows separation of the charge storage mechanisms such as electrical double layer charge storage and diffusional processes. EIS method also can provide the different information about the electrochemical cell such as double layer capacitance, series resistance, charge transfer resistance and mass transport using a small AC excitation signal. However, the AC excitation signal can be large in magnitude for the electrochemical capacitors due to the cyclability over a large potential window. In this work, the combined SPECS and EIS methods have been applied to electrolytic manganese dioxide (EMD) in 0.5 M Na2SO4 between 0.0 – 0.8 V (vs SCE) at different duty cycles to examine the changes in charge storage mechanisms. Hence, the different scan rates and equilibrium times have been applied for the same electrode ink and electrolyte. Firstly, the same duty cycle with the same effective scan rate has been examined with increasing the step size and equilibrium time which is constituted of the scan rate of 10 mV in 120 s, 25 mV in 300 s and 50 mV in 600 s. Then, the longer duty cycle with slower discharge rate such as 25 mV in 10 min has been applied to compare the results of different duty cycles. So, nine different experiments have been run using the different step size of 10 mV, 25 mV and 50 mV and different equilibrium times ranging of 120 s, 300 s and 600 s. The outcome of this work indicates the systematic effects of different scan rate on the charge storage mechanisms. Also, it can be seen that, the average capacitance of the electrochemical cell from cycling voltammetry (CV), increased with decreasing the scan rate of the duty cycle.