Facile preparation of commercial Bi2O3 nanoparticle decorated activated carbon for pseudocapacitive supercapacitor applications

In this work, a facile method to prepare Bi2O3 decorated activated carbon (Bi2O3@AC) composites with high pseudocapacitive properties was presented. The inorganic–organic composites synthesized using commercial Bi2O3 and active carbon with different weight ratio. The composites were assessed using spectroscopic, microscopic, and diffractive techniques. Our assessments confirmed that active carbons were successfully doped with commercial Bi2O3 nanoparticles with different dopant rates. The electrochemical performance of as-prepared materials was researched by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy in 6 M KOH electrolyte. Higher specific capacitance was achieved for increased Bi2O3 nanoparticle in composites. The 20%-Bi2O3@AC had a maximum specific capacitance of 565 F/g at a current density of 1 A/g. In addition, the symmetrically assembled supercapacitor delivers a high energy density (23.0 Wh kg−1). Moreover, 67% of the initial capacitance is maintained after 1500 cycles at 200 mV/s, suggesting good cycling stability. Due to the synergistic effect of compositing a promising electrochemical performance was observed that was not obtained by bare AC or Bi2O3. As a result, the electrochemical properties of 20%-Bi2O3@AC composite is promising and it may be used as potential electrode for supercapacitor.


Introduction
Renewable energy sources have attracted great attention to meet the increasing energy needs [1][2][3][4][5]. However, the majority of renewable energy sources such as solar energy, wind energy, etc. have an intermittent nature, which is ideally to be stored in devices [6]. As an electrochemical energy-storage devices, supercapacitors were demanded for several reasons such as their relatively high-power densities and energy densities compared to most of batteries and conventional capacitors [7][8][9]. Charge storage mechanism of supercapacitors can be classified as electrochemical double layer capacitors (EDLCs) and pseudocapacitors [10]. EDLCs store charges by electrostatic charge separation on the surface of the materials, whereas pseudocapacitors are based on a redox reaction process [11]. Pseudocapacitors provide a supercapacitor with good specific capacitances and better energy densities than EDLCs [12]. Activated carbons (AC) have displayed great potential as efficient electrodes for EDLC type supercapacitors due to high-micro/mesoporosity, their high specific unique pore structure for efficient ion diffusion and low cost [13]. Although porous activated carbon as a supercapacitor has a high-power density and a long cycle life it has relatively low specific capacitance [14]. Heteroatoms in the porous carbon networks improve surface wettability of the electrode and contribute additional pseudo-capacitance to the total capacitance in the AC derivatives [14][15][16].
Owing to their large capacitance and high-energy density as a result of reversible redox reactions different pseudocapacitive materials were designed extensively as electrode materials for supercapacitors such as metal oxides, metal sulphides and metal selenides [17].
Nanoparticles illustrate outstanding electrical, electronical, optical and magnetic properties with good electron affinity [18][19][20]. Metal oxides nanoparticles such as Co 3 O 4 , CuO, and NiO are also considered pseudocapacitive electrodes in aqueous electrolytes [9,[21][22][23][24]. They are also researched extensively in batteries as electrode active materials [25]. Bi based nanoparticles were found to be promising which can be used in possible medical applications (as contrast agent and/or theranostic agents), electrochemical applications, supercapacitor applications, etc. Among the pseudocapacitive materials for supercapacitors, bismuth oxide has been largely investigated because of its good electrochemical properties, stabilities and relatively high power [26][27][28][29][30]. As a result, the fabrication of AC composites with pseudocapacitive materials is a very effective method to enhance the specific capacitance of supercapacitors [31,32].
In this work, we report a facile and straightforward method to synthesize a nanocomposite from commercial bismuth oxide and plant based AC materials for supercapacitor applications. The composite was characterized by FE-SEM (field emission scanning electron microscope), PXRD (powder X-ray diffraction), FTIR (Fourier transform infrared spectra), thermogravimetric analysis. The electrochemical properties of the nanocomposites were measured in 6 M KOH electrolyte using a three-electrode system. The as-prepared samples exhibit a good performance where a maximum specific capacitance of 565 F/g at a current density of 1 A/g was obtained.

Measurements and materials
Bi 2 O 3 nanoparticles were obtained from Alfa Aesar TM (Bismuth (III) oxide, nanopowder, 99.9%). SETARAM LABSYS evo TG/DTA thermal analyser was used to record simultaneous TGA (thermogravimetric analysis), and DTA (differential thermal analysis) curves in the dry air atmosphere at a heating rate of 10°C min -1 in the temperature range 30-800°C using alumina crucibles. JASCO FTIR-6700 spectrum was used in the FTIR assessments in the range between 400 and 4000 cm -1 . Hitachi SU3500 SEM with EDX apparatus was used in the SEM investigations. RI˙GAKU miniflex600 X-ray diffractometer was used in the XRD investigations.

Electrode preparation and electrochemical measurements
For 30 min, the active material (80%) and acetylene black (10%) were ground and mixed in an agar mortar. Then, PVDF (10%) was added to the mixture, and the resultant product, it was ground for 15 min. A few drops of N-methylpyrrolidone were added to the slurry mixture. The mixture was then drop cast onto Ni foam (10 9 10 cm), dried in an oven at 60°C for 24 h and pressed at a pressure of 5 MPa. Finally, the pressed electrodes were left to soak in 6 M KOH solution for overnight. The mass loading of active material on working electrode is about 2-3 mg. A leakage free Ag/AgCl electrode and platinum plate (1 9 1 9 0.2 cm) were used as the reference and the counter electrodes, respectively. The two-electrode system which was assembled with a home-made device using two nearly identical electrodes. Cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy techniques (EIS) were used to characterize the electrochemical capacitance properties of the materials using a Gamry Reference 600 potentiostat/galvanostat/ZRA. In the electrochemical measurements 6 M KOH was used as electrolyte.

Preparation of composites
Ripe black locust seed pods derived activated carbon (AC) was prepared according to our previous report [33].  Fig. 2. EDX results confirm that Bi 2 O 3 nanoparticles and AC is in bare structure where no contamination were seen in the EDX peaks. EDX spectra in Fig. 2 illustrate that decorating AC with Bi 2 O 3 nanoparticles were successful, since Bi, O and C peaks can be identified in Fig. 2c and d. It was also seen that increased doping rate enhanced the amount of measured Bi in the structure. It was expected to see no Bi and O structure in Fig. 2b Figure 3 illustrates EDX mapping results where EDX maps of bare Bi 2 O 3 nanoparticles, bare AC, Bi 2 O 3 nanoparticle-doped AC structures were illustrated. EDX maps of the samples show no contamination. In Fig. 3a, Bi and O structures were seen. In Fig. 3b, C and O structures were seen on the surface. In Fig. 3c, C, O and Bi were seen where oxygen covers almost whole surface, but Bi nanoparticles can also be seen. Figure 4a also supports the data in Fig. 3a in both data only Bi and O elements were seen. Surface is mostly covered with oxygen; in addition, Bi and C structures can be identified. Such a map may be an indication of oxidized nanoparticles.
FTIR spectra of bare Bi 2 O 3 nanoparticles, bare AC, Bi 2 O 3 nanoparticle-doped AC structures were presented in Fig. 4. FTIR spectra of samples were given in Fig. 4. Spectra show bands appearing at 1125, 1565, and 3600 cm -1 which correspond to the stretching frequencies of oxygen in C=O, aromatic C=C and O-H, respectively [34]. Bi 2 O 3 nanoparticle doped AC structures clearly illustrates that the peaks around 860-1358 cm -1 are related with metal oxygen bond and 510-600 cm -1 which corresponds to Bi-O stretching frequencies [35]. Figure 5 illustrates XRD diffraction pattern of nanoparticles and nanocomposites. Two bumps can be seen in the diffraction pattern of AC where no apparent peak was seen, it is possible that AC is in amorphous form or in low crystallinity. Therefore, no apparent peak can be observed.
where h D is the Bragg angle, W f is the width at half maximum intensity of the Bragg reflection, k is the wavelength of the used X-ray radiation (k Cu-= 1.54056 Å ), b is the width of the x-ray peak on the 2h axis D is the average crystallite size, the value of K was taken as 0.9. The value of K depends on the crystal geometry and size distribution. Since we do not know the exact value of K for the present material system, K= 0.

Thermal analysis
To study the experimental Bi 2 O 3 content in the composites, thermogravimetric analyses (TGA) was performed on composites, AC and commercial Bi 2 O 3 . AC is stable up to around 500°C and thermal decomposition of AC occur in a single stage and decompose into gas products without remaining any ash. It was determined that commercial Bi 2 O 3 does not show any mass loss up to 750°C. Therefore, the amount of Bi 2 O 3 in the composites was calculated by considering the remaining mass at 750°C. According to the results, the amounts of Bi 2 O 3 in 10%-Bi 2 O 3 @AC and 20%-Bi 2 O 3 @AC are %9 and 18%, respectively (Fig. 6).

Electrochemical measurements
Electrochemical behaviours of Bi 2 O 3 , AC, 10%-and 20%-Bi 2 O 3 @AC were measured in a three-electrode system in 6 M KOH. Cyclic voltammetry (CV) was performed for at scan rates of 1, 10, 25, 50 and 100 mV s -1 in the potential range of 0.25-0.45 V vs. Ag/AgCl. The cyclic voltammetry curves of ACbased electrode materials have typically rectangular shape and it indicates that the materials has ideal double-layer capacitance behaviour [41]. In our study, low temperature pyrolyzed of AC exhibits the distinctive redox peaks, that is, while the scan rates are increased, the CV curves turn into fish shape which suggests redox reactions occur onto the AC with 6 M KOH electrolyte (Fig. 7). It might be assumed that the pseudocapacitive behaviour of AC related to faradaic reactions, which has plenty of heteroatoms such as nitrogen, oxygen, sulphur on the pore surface of AC [15,42]. The weak peaks at 0.42 and 0.29 V in the CV curve of the AC might be oxidation and reduction of functional groups on the surface of the AC. The commercial Bi 2 O 3 nanoparticle exhibited lower current response in comparison to AC, determined by CV curves (Fig. 7b). The peak currents of samples were observed to be enhanced after inclusion of Bi 2 O 3 into the AC. Moreover, the higher specific capacitance of 20%-Bi 2 O 3 @AC was obviously determined from the peak current of CV curves (Fig. 7c-e). A small peak at 0.35 V can be seen in CV curves of the Bi 2 O 3 @AC composites. This may be caused by faradaic reactions of Bi 2 O 3 . The specific capacitance of the electrodes can be calculated using cyclic voltammetry according to the following equation [43].
where V is the potential, I is the current density (A/ cm 2 ), t is the potential scan rate (mV/s) and m (g) is the amount of the active material in the electrode. The C s at different scan rates is illustrated in Table 1. Obviously, the composites yield a higher specific capacitance values compared to bare AC electrode (147 F/g at 10 mV/s) at the same scan rates. Furthermore, 20%-Bi 2 O 3 @AC electrode shows 451 F/g at 10 mV/s while 10%-Bi 2 O 3 @AC electrode exhibits 285 F/g at the same rate. Based on the cyclic voltammetry analysis, it can be resulted that Bi 2 O 3 @AC composites show better electrochemical performance compared to the bare AC. The reason of better electrochemical performance might be ascribed to commercial nano-Bi 2 O 3 particles effectively reduce the diffusion distance through the charge and discharge processes, the AC exhibits a highly open and irregular micro/mesopore structure that can provide space for nano-Bi 2 O 3 particles and The chronopotentiometry analyses were used to determine the specific capacitance of the composites. Figure 8 illustrates the chronopotentiometry curves of the samples at different current density of 1, 5, 8 and 10 A/g. The shape of the discharge curves suggests the electrodes have pseudocapacitive behaviours. Regarding the discharge curves of the composites, the specific capacitance was assessed using the following equation: [43].
where Dt is the discharge time (s), I is the discharge current (A), DV is the voltage window (V) and m is the mass of the loaded active material (g) present in working electrode. Based on the Eq. 3, increased discharge time lead to the increased specific  [44,45]. A detailed comparison of Bi 2 O 3based composites with 20%-Bi 2 O 3 @AC is given in Table 2.
The electrochemical impedance spectroscopy (EIS) for electrodes was tested in an open circuit potential over a frequency range of 10 MHz-100 kHz. The Nyquist plots for the electrodes in 6 M KOH electrolyte were illustrated in Fig. 9. The electrochemical parameters were determined by fitting the impedance data to proposed equivalent circuit as seen in  Fig. 9. The determined parameters of corresponding conceptional circuit model are illustrated in the Table 3. The fitting parameters involve the electrolyte resistance (R S ), constant phase element (CPE). charge transfer resistance (R CT ), Warburg impedance (W), capacitance of electrode (C L ) and leakage resistance (R L ) [15,16,50]. Resistance of the electrodes (R s ) was determined from the EIS model. The R S of Bi 2 O 3 was determined as 4.63 X, however, the calculated R s decrease in the composites to under 1.0 X. The charge   [16]. The capacitance of electrode was also proposed by EIS model and the capacitance of samples can be ordered as Bi 2 O 3 \ AC \ 10%-Bi 2 O 3 @-AC \ 20%-Bi 2 O 3 @AC. R L for electrode was also determined by EIS measurement, which is associated with C L and it is a non-ideality factor in the capacitance. Although AC and Bi 2 O 3 exhibited very high leakage resistance, the R L for nanocomposites were relatively lower. The impedance measurement results support the cyclic voltammetry and chronopotentiometry data and the electrochemical properties of the electrodes increases with decreasing of the electrochemical resistance. The electrochemical analysis of the 20%-Bi 2 O 3 @AC electrode was also evaluated in a symmetrically assembled cell. The symmetric cell was composed of two identical electrodes separated by a filter paper soaked with a 6 M KOH electrolyte. The symmetric cell was investigated using CV, GCD and EIS methods. In the range of -1.5-0 V vs. Ag/AgCl of the CV curve display EDLC behaviour, however, in the 0-1.5 V vs. Ag/AgCl, Bi 2 O 3 part of the electrode dominates exhibiting faradic behaviour. CV curves of the symmetric cell collected at various scan rates ranging from 10 to 100 mV/s are shown in Fig. 10. Moreover, with the increase of the scan rates, the redox peaks are almost the same. It showed the Fig. 10 CV curves at different scan rates in the potential range from a -1.5-0 and b 0-1.5 V vs Ag/AgCl; c plot of capacity retention by CV at 200 mV/s; and d chronopotentiometry curves at different current densities for two-electrode system electrode has good rate capability. To evaluate the cycling performance of the symmetric cell, the electrodes were subjected to 1500 CV cycles at a current density of 200 mV/s of which the 1st and 1500th cycles are shown in Fig. 7c. From Fig. 10c, it can be seen that the cell has excellent cycling stability with 67% capacity retention.
The chronopotentiometry (CP) curves of the cell are shown in Fig. 10d at 1 to 10 A/g, which were exhibited successive potential steps, confirms that the faradic behaviour of the electrode in the 0-1.5 V vs. Ag/AgCl window in agreement with the CV curves. In the two-electrode setup, the specific capacitance of a single electrode was calculated from the CP values according to the following equation: In Fig. 10, the symmetric cell possessed the specific capacitance of 74 F/g at 1 A/g and the very good rate performance (rate capability 63% at from 1 to 10 A/g).
The energy density (E, Wh/kg) and the power density (P, W/kg) were obtained as follows: The energy density of symmetric cell is as high as 23.0 W h/kg at a power density of 1.56 kW/kg and reaches 14.6 W h /kg at a power density of 13.49 kW/kg. The Ragone plot of a symmetrical supercapacitor of the 20%Bi 2 O 3 @AC is shown in Fig. 11.
Electrochemical impedance spectroscopy (EIS) is an important method for understanding of the electrochemical performance of the symmetric cell of 20%Bi 2 O 3 @AC//20%Bi 2 O 3 @AC. Nyquist plots were obtained from the results of the EIS experiments in the frequency range between 10 MHz and 100 kHz. The Nyquist plot was fitted using an equivalent circuit model consisting of R S, R CT , CPE, C dl and W, where they were referred as resistance of the electrolyte, charge transfer resistance, constant phase element, capacitance and Warburg impedance. Figure 12 exhibits that the line in the low-frequency region is approximately perpendicular to the real axis, providing the good capacitive behaviour of the cell. The R S and R CT of the cell were calculated to 2.017 and 1.143 X, respectively. The specific capacitance of electrodes is calculated as 55.5 F/g by EIS model, which is in agreement with the results of chronopotentiometry.

Conclusion
In summary, we have studied a facile method to prepare Bi 2 O 3 @AC nanocomposites. Experimental methods illustrate that Bi 2 O 3 nanoparticles can successfully be doped on AC. Electrochemical properties of nanocomposites were investigated using cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS). The measurements show that the electrochemical properties of Bi 2 O 3 @AC composites, which is produced by the synergic contribution of plant based AC and the commercial Bi 2 O 3, has the maximum specific capacitance of 20%-Bi 2 O 3 @AC is about 565 F/g, more than that of bare AC and bare Bi 2 O 3 . Further, the symmetric cell of 20%-Bi 2 O 3 @AC delivers a high energy density (23.0 Wh kg -1 ). Bi 2 O 3 supercapacitor was found to be quite stable, since 67% of the initial capacitance is maintained after 1500 cycles at 200 mV/s. Results were in agreement with the results reported in the literature. Therefore, 20%-Bi 2 O 3 @AC composite would be a potential electrode material for the supercapacitor.