(b) Plots of specific capacitance and its retention ratio vs. voltage scan rate. (c) Galvanostatic charge–discharge curves at a current density of 2 A g−1. (d) Plots
of specific capacitance and its retention ratio vs. current density. In addition, the current density at each scan rate in H2SO4 electrolyte is higher than that in KOH electrolyte, which indicates that oxygen-containing groups exhibit more pseudocapacitance in acid electrolyte. Therefore, as shown in Figure 4b, the specific capacitance calculated from CV curves displays that RGOA possesses larger capacitance in H2SO4 electrolyte when the scan rates are lower than 100 mV s−1. However, RGOA maintains a higher capacitance in KOH electrolyte selleck compound when the scan rates exceed 100 mV s−1, which is probably due to the higher ionic concentration of KOH electrolyte than that of H2SO4 electrolyte. The galvanostatic charge–discharge curves of RGOA in different electrolytes are composed of two parts: the first part is within the potential window of 0.0 ~
−0.3 V in KOH electrolyte and 0.6 ~ 1.0 V in NVP-BEZ235 H2SO4 electrolyte, which is attributed to the electric double-layer capacitance. The other part exhibits a longer duration time, indicating the existence of pseudocapacitance besides the electric double-layer capacitance. As shown in Figure 4d, capacitance retention ratios of RGOA remain 74% and 63% in KOH and H2SO4 electrolytes when current density increases from 0.2 to 20 A g−1, exhibiting a
high-rate capacitive performance. This high-rate performance is mainly attributed to the three-dimensional structure, which is beneficial for the ionic diffusion of electrolyte to the inner pores of bulk material. As shown in Figure 4d, Ribose-5-phosphate isomerase the specific capacitances are calculated to be 211.8 and 278.6 F g−1 in KOH and H2SO4 electrolytes at the current density of 0.2 A g−1. The specific capacitances per surface area are calculated to be 25.5 and 33.6 μF cm−2 in KOH and H2SO4 electrolytes, respectively, indicating more pseudocapacitance in H2SO4 electrolyte. These results coincide well with the cyclic voltammetry measurements. EIS is adopted to investigate the chemical and physical processes occurring on the electrode surface. The Nyquist plots of RGOA in different electrolytes are shown in Figure 5a. Within the low-frequency region, the curve in KOH electrolyte is more parallel to the ordinate than that in H2SO4 electrolyte, indicating a better capacitive behavior in KOH electrolyte. The intersection of the curve with the abscissa represents equivalent series resistance [40]. This value is due to the combination of the following: (a) ionic and electronic charge-transfer resistances, (b) intrinsic charge-transfer resistance of the active material, and (c) diffusive as well as contact resistance at the active material/current collector interface [41]. It can be seen from the inset in Figure 5a that these resistance values are 0.30 and 0.
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