Electrochemical impedance spectroscopy (EIS) study was
carrier out to analyze the electrochemical properties of the
prepared supercapacitors electrodes materials. The charge transfer
resistance (Rct) value and conductivity of the carbon nanostructured materials (T-GNS and HB-GNS) were calculated from the EIS
over the frequency range 0.01 Hz–100 kHz as shown in Fig. 9(a).
The EIS discusses the important parameters such as resistance and
capacitance of supercapacitors. Nyquist plot of T-GNS and HB-GNS
was recorded in the frequency range from 0.01 Hz to 100 kHz using
three electrode systems in 0.5 M H2SO4 solution using sinusoidal
wave amplitude of 10 mV at high frequency region as shown in
Fig. 9(b). Both T-GNS and HB-GNS curves are similar at high
frequency region composed of semicircle, which gives solution
resistance (Rs) and charge transfer resistance (Rct). Warburg
impedance (WZ) was observed from the project line (45) at the
low frequency region and it shows the diffusive resistance of
electrolyte ions in host materials. The Rct values of T-GNS and
HB-GNS were calculated from EIS (Fig. 9) and found to be 7.22 V
and 3.52 V respectively. The Rct value of HB-GNS is lower than the
T-GNS, so there is free charge transfer in HB-GNS. EIS study shows good electrochemical behavior for HB-GNS compared to reduced
graphene (T-GNS).
Fig. 9(c) represents current densities versus specific capacitance (Csp) for T-GNS and HB-GNS. Both materials have linear plots
with decreased current density and increased the specific
capacitance. The maximum specific capacitance (Csp) values of
HB-GNS and T-GNS materials are 113 F/g and 52 F/g for the current
density 1 A/g respectively. HB-GNS showed almost twice the
specific capacitances of T-GNS. The boron doped graphene shows
the good capacitive performance for better supercapacitor energy
storage applications [42,43].
Fig. 9(d) depicts the Ragone plots for T-GNS and HB-GNS
samples to evaluate the energy density and powder density for
supercapacitor applications. The energy density (ED) and power
density (PD) was calculated using the following equations:
E = 1/2 CV2 (A.2)
P = E/t (A.3)
Where, E is the energy density (Wh/ kg), C is the specific
capacitance (F/g), V is the discharge potential (V), P is the power
density (KW/ kg) and t is the discharging time (s). The Ragone plot
was plotted from ED and PD, which were calculated from Eqs. (A.2)
and (A.3) respectively. It was observed that the T-GNS material has
high ED values in the range of 1.25–0.484 Wh/kg, while the PD
values are in the range of 0.05–0.22 kW/kg. The HB-GNS supercapacitor can deliver a high energy density (ED) in the range of
5.96–4.64 Wh/kg and power density (PD) in the range of 0.16–
0.97 kW/kg. Hence, the HB-GNS is a promising electrode material
with good capacitance and energy density for supercapacitor
applications.