Deconvoluting Capacitive and Pseudo-Capacitive Contributions to Electrochemical Capacitor Electrode Behaviour

Abstract

Generally the capacitive behaviour of the electrochemical capacitors are associated with electrical double layer at the surface of the electrode, and in some materials pseudo-capacitance, which results from faradaic processes. Several electrochemical methods have been developed for determining the contribution of different charge storage mechanisms such as via the electrical double layer and diffusion-limited processes. This includes using a cyclic voltammetry (CV) data at different scan rates to obtain the relationship between a voltammetric current and scan rate (v). In this method, it can be assumed that the total current response for a particular potential is a summation of the current associated with a capacitive processes and a current associated with a diffusion-limited process. While, the capacitive current is proportional to v, and the diffusion limited current is relative to v1/2 (Conway, et al. 1998). The cyclic voltammetry data also can be applied to determine the relationship between a voltammetric charge and scan rate. The charge attributed to the surface capacitive processes is assumed to be constant with scan rate while, the charge attributed to diffusion-limited processes varies with v-1/2 (Trasatti, et al. 1990). Both approaches have been used widely for characterization of the performance of electrochemical systems. However, these methods have some limitations as the relationship between current and v1/2 and also, between charge and v-1/2 is not always linear in a different range of scan rates. Step potential electrochemical spectroscopy (SPECS) also, has been used to effectively differentiate between different charge storage mechanisms. The SPECS method is based on applying a series of equal magnitude potential steps on a working electrode, with sufficient rest time to allow for equilibrium to be established for each step throughout an applied potential window. This slow scan rate enables an electrode to approach its maximum charge storage capabilities. More importantly, it allows separation of the charge storage mechanisms, such as electrical double layer charge storage and diffusion-limited processes. The aim of this study is to compare these three methods experimentally and also to discuss about their limitations and advantages for interpreting the current data and their ability to distinguish between different charge storage mechanisms. Moreover, the relationship between voltammetric current and scan rate has been modified by introducing the constant parameter of residual current. The modified relationship then, can be used over the entire range of potential scan rate. In this work, the combined CV and SPECS methods have been applied to electrolytic manganese dioxide (EMD) in 0.5 M K2SO4 between 0.0 – 0.8 V (vs SCE). Firstly, EMD electrode was cycled for 250 cycles at different scan rates ranged from 1 to 100 mV/s. Subsequently, the SPECS method was applied using 25 mV potential steps and 300 s equilibration time. Eventually, the contribution of capacitive and diffusion-limited processes were obtained using three techniques which were discussed here. The linear dependence of current to v1/2 and the linear dependence of capacity to v-1/2 have not been observed. However, it can be seen that there is a good agreement between the modified model of the scan rate dependence of current and experimental data. Liu, T. C., Pell, W. G., Conway, B. E., & Roberson, S. L. (1998). Behavior of molybdenum nitrides as materials for electrochemical capacitors comparison with ruthenium oxide. Journal of the Electrochemical Society, 145(6), 1882-1888. Ardizzone, S., Fregonara, G., & Trasatti, S. (1990). “Inner” and “outer” active surface of RuO2 electrodes. Electrochimica Acta, 35(1), 263-267.