Evaluation of Stability of Performance of Electrochemically Exfoliated Graphene for Electrochemical Double-Layer Capacitors

Abstract

Energy storage devices are known as supercapacitors. One highlighted characteristic of them is high specific power and cycling long-life. These properties are associated with charging and discharging in short periods of time, especially useful for high demanding systems, such as regenerative technologies, strong acceleration in mobility and radical maneuvers in aeronautics. So, market demands for supercapacitors have been growing fastly. The performance improvement and cost reduction of supercapacitors are essential for the popularization of electromobility all over the world. For this aim, graphene and carbon nanotubes, among others, are materials widely used to such proposes due to their high specific surface area and excellent pseudocapacitive and electrochemical double layer properties. In this work, the use of graphene oxide obtained by electrochemical exfoliation and polyvinylpyrrolidone as active mass of supercapacitor electrodes was studied. The electrodes were prepared using the dip coating technique. To characterize the samples obtained, cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD) were carried out in a 3-electrode cell with a 1 M sulfuric acid solution. CV was used to define the potential window, estimate capacitive profile and observe possible faradic phenomena. After that, GCD was used to study the material’s stability during 1000 operation cycles, at three specific current densities: 1.0; 1.5; and 2.0 A g-1. The voltammograms obtained are shown in Figure 1. It was possible to observe a symmetrical voltammogram with discrete quasi-reversible faradic phenomena (pseudocapacitive effects) defined by the cathodic peak at 0.48 V and anodic peak at 0.22 V. These peaks may be related to the redox reactions of graphene oxide. GCD was carried out to evaluate coulombic efficiency and charge retention such as shown in Figure 2. There was no great damage during the cycles; there was a slight positive change (increase) during 1000 cycles for all current densities. Furthermore, an increase in charge retention was observed for all current densities, this effect can be related to elongation of the polymeric chain due to electrostatic interaction of the binder in acidic media, leading to a higher effect of double-layer. For the 1 A g-1 current density, the increase of charge retention and coulombic efficiency were more relevant. These increases may be associated with gradual and discrete pseudocapacitive processes also observed in cyclic voltammetry measurements. The determined values of capacitance, energy and power densities are shown in Table 1. The values obtained at 1 A g-1 specific current density showed a higher coulombic efficiency and a significant increase in charge retention, despite having presented a lower power density. to this material, it is possible to assert to be a working condition better than others. At specific current densities of 1.5 A g-1 and 2 A g-1 higher values of energy and power density were determined, but lower coulombic efficiency. Figure 1