Fig. 3. XPS analysis of NirGO3 material: (a) complete XPS survey spectrum, deconvoluted spectra of (b) C1s, (c) O1s, and (d) Ni2p.
Electrochemical performance of supercapacitor cells
Cyclic voltammetry
As seen from the voltammograms, the capacitive performance of the reference rGO electrode (containing binder) was limited because of its relatively high resistance and low surface area (Fig. 4a). When CB was added to the mixture, a decrease in the capacitance was observed because of the increase in the surface area of the electrode. Besides, as it is well known, the square-like shape of the voltammogram indicates an ideal capacitive behaviour in a device. However, the rGO-CB graph was not square, indicating high resistance related to the binder. When the voltammograms of the binder-free NirGO foamed electrodes are examined, it is noticed that the graphs are relatively closer to the square-like shape than those of rGO and rGO: CB. The oxidation peak of Ni(OH)2 at 0.3 V, which was expected in NirGO voltammograms, could not be observed very clearly. This is due to a phenomenon called the surface-based faradaic process. In this process, a capacitive faradaic activity is exhibited on the surface thanks to the delocalized electrons on the electrode. Therefore, square-like voltammograms were obtained at [40]. Since there was no binder in these electrodes, the resistance in the electrode was relatively lower. In addition, the square-like graphs showed that the pore sizes and ion sizes were compatible.
The capacitive performance of NirGO3 is comparable to that of rGO:CB based electrode (900 F g─1). The results regarding porosity and surface areas were in agreement with the results obtained from SEM studies. Furthermore, voltammograms were recorded at different scanning rates (Fig. 4(b), (c) and Fig. S2) to compare pore size-ion diameter match and resistances at the electrolyte-electrode interface. When the graphs of rGO:CB and NirGO3, which showed the relatively best performance, were examined, it was observed that the voltammograms of NirGO3 obtained at different scan rates were less distorted than those of rGO:CB as the speed increased (Fig. 4(b)). In the device prepared with NirGO3 electrode material, this shows that ions move more efficiently during entry and exit into the pores than in devices prepared with other electrode materials.