New Synthesis Method for a Core-Shell Composite Based on α-Bi 2 O 3 @ PPy and its Electrochemical Behavior as Supercapacitor Electrode

New composite based on polypyrrole (PPy) and bismuth oxide (α-Bi2O3) was investigated as supercapacitor electrode. The α-Bi2O3 was obtained by hydrothermal route at 500 C for 2 h. Cyclic voltammetry was used to electropolymerize PPy on graphite electrode (GE) or on GE/α-Bi2O3. The X-ray diffraction profile of Bi2O3 revealed the α-Bi2O3 monoclinic structure with space group P21/c. The scanning and transmission electron microscopy images showed that PPy coated α-Bi2O3. Raman spectra showed that PPy inhomogeneously coated α-Bi2O3, but it was still possible to obtain a α-Bi2O3@PPy core-shell hybrid composite with highly ordered α-Bi2O3 with electronic interaction between the oxide and the polymer chain of the polymer. The GE/α-Bi2O3@PPy composite electrode (type I supercapacitor) displayed a predominantly capacitive profile with low impedance values and good electrochemical stability after 50 charge and discharge cycles. The specific capacitance of α-Bi2O3@PPy composite was found in the range of 634 to 301 F g to gravimetric current of 3 and 10 A g, respectively.


Introduction
Over the last years, core-shell structures based on oxide-oxide and oxide-polymer have been studied as interesting alternatives to produce composite materials with high specific capacitance.2][3][4][5][6] The present work investigated a core-shell structure prepared with two well-known materials: α-Bi 2 O 3 and PPy.
Supercapacitor development is a very important subject in the energy storage field.Supercapacitors have many applications that require high power and an extremely elevated number of discharge/charge cycles.1][22] So far, commercial supercapacitors have been unable to overcome the specific energy of traditional lead batteries (ca.30-50 W h kg -1 ), but core-shell materials could modify this scenario.
This paper reports on the fast and facile electrochemical synthesis of a graphite electrode (GE) containing adhered α-Bi 2 O 3 coated with PPy as a core-shell granulated composite for use as high-performance supercapacitor electrode.In the GE/α-Bi 2 O 3 @PPy electrode, α-Bi 2 O 3 and PPy serve as the "core" and the "shell" layer, respectively.The electrochemical behavior of the GE/α-Bi 2 O 3 @PPy hybrid electrode was evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD), to show that it can be applied as a supercapacitor with high specific capacitance in aqueous electrolytes.

α-Bi 2 O 3 synthesis
All the chemical reagents were used without previous treatment.Typically, a given amount of bismuth(III) nitrate pentahydrate (Bi(NO 3 ) 3 .5H 2 O) was dissolved in ethylene glycol (EG) under constant magnetic stirring for 5 min.Then, the final solution was placed in a 200 mL Teflon-lined stainless steel autoclave at 140 o C for 2 h. 23The liquidsolid mixture was centrifuged, to obtain a solid that was washed with water several times and dried in an oven at 80 o C for 3 h.An amorphous white powder was obtained.To achieve a crystalline material, the white powder was calcined in an oven at 500 o C for 2 h, to afford a yellow powder.The oxide was deposited on a vitreous carbon pellet (graphite) electrode by the dropping of an alcoholic α-Bi 2 O 3 dispersion onto GE before electropolymerization of PPy on α-Bi 2 O 3 .

PPy electrochemical synthesis on GE and GE/α-Bi 2 O 3
PPy was electropolymerized on the GE or GE/α-Bi 2 O 3 surface by CV carried out from -0.30 to 0.90 V vs. Ag (s) /AgCl (s) /Cl - (aq) at 50 mV s -1 in 50 mL of aqueous 1 mol L -1 KCl, containing previously distilled 0.01 mol L -1 pyrrole, up to total charge density of 150 mC cm -2 after 20 cycles for the GE/PPy electrode or after 8 cycles for the GE/α-Bi 2 O 3 @PPy hybrid electrode, respectively.

Structural and morphological characterization
X-ray diffractometry (XRD) was performed on a Shimadzu diffractometer (Model 6000; Cu Kα radiation λ = 1.5406Å, voltage = 40 kV, current = 30 mA, scan rate = 1° min -1 , 2θ ranging from 20 to 70 o ).The unit cell parameters were calculated with the UnitCell Win program. 24The diffractogram peaks had been previously adjusted with the Peak Fit software. 25he Raman spectroscopy measurements were performed on a Raman spectrometer (LabRAM HR Evolution) between 2000 and 200 cm -1 at room temperature, employing He-Ne ion laser of 1 mW with wavelength of 633 nm and charge couple device (CCD) detector with 1 cm -1 resolution.
The GCD tests were carried out by the galvanostatic chronopotentiometry method.The tests were carried out from -0.30 to 0.70 V in 1 mol L -1 Na 2 SO 4 , at 3, 5, 10 and 20 A g -1 .The masses of all electrodes were about 0.10 mg.The mass employed in the determination of specific capacitance was obtained from the average mass of three electrodes prepared in similar conditions.
The EIS measurements were performed at an AC voltage, under open circuit voltage (OCV).The amplitude was 10 mV, and the frequency ranged from 10 4 to 10 -1 Hz.The electrochemical tests were performed at room temperature.An Autolab potentiostat 302 N, interfaced with a microcomputer operating the software NOVA version 1.11, was employed.a white powder (equation 1).Simultaneously, Bi(NO 3 ) 3 hydrolysis obtained white powder of BiONO 3 (equation 2).

Results and Discussion
The mixture of white powders, Bi 2 (OCH 2 CH 2 O) 3 and BiONO 3 , was calcined at 500 o C for 2 h to obtain Bi 2 O 3 , a yellow powder (equation 3).
The Bi 2 O 3 XRD pattern (Figure 1a) displayed wellresolved and highly defined peaks and resembled the XRD pattern of the standard Joint Committee on Powder Diffraction Standards (JCPDS) No. 041-1449, indexed on the basis of a monoclinic structure, which attested that pure monoclinic α-Bi 2 O 3 8 with space group P2 1 /c was formed.Figure 1b shows the α-Bi 2 O 3 unit cell.The XRD of oxide synthesized through the solvothermal method did not have secondary diffraction peaks due to the use of EG, which acted as a chelating agent. 23,26Moreover, EG significantly reduced the mobility of the different ion species in this medium.Additionally, EG served as a soft template that facilitated self-assembly of in situ grown microparticles through hydrogen bond formation, which controlled α-Bi 2 O 3 crystal growth. 27he calculated unit cell parameters agreed with values reported for α-Bi 2 O 3 (Table 1). 28e unit cell parameters calculated herein were slightly higher than the unit cell parameters described in the literature, which was probably a consequence of the synthesis method.Nevertheless, the c / a ratio values (c / a = 1.282) were similar, which demonstrated that the unit cell was not distorted.

PPy electrochemical synthesis on the GE and GE/α-Bi 2 O 3 electrodes
Figure 2 depicts the voltammograms recorded for PPy electrochemical synthesis on the GE and GE/α-Bi 2 O 3 electrodes, as well as the GE/α-Bi 2 O 3 electrode in 1 mol L -1 KCl in the absence of pyrrole monomers.Clearly, Figure 2a suggests a good stability of α-Bi 2 O 3 on surface of GE, since the capacitive charge is the same at several cycles, while the voltammograms in Figures 2b and 2c show the increase of the anodic current density with increase in the number of cycles, evidencing the effective growth of the PPy on the GE and GE/α-Bi 2 O 3 electrodes.In addition, Figure 2c revealed that PPy electrochemical synthesis on the GE/α-Bi 2 O 3 electrode gave rise to a pair of redox peaks, which highlights the presence of a possible interaction between oxidized PPy (as verified on the Raman spectrum) and α-Bi 2 O 3 , which resulted in a material with favored electronic transfer. 29hereby, the voltammograms due to PPy electrochemical synthesis on the GE/α-Bi 2 O 3 electrode showed that fewer cycles (eight cycles) were necessary to achieve total charge as compared to PPy electrochemical synthesis on the GE electrode (20 cycles).

Raman characterization
It is well known in literature that the effect the doping of PPy can be investigated with Raman, such that eventual  electronic interactions between PPy and α-Bi 2 O 3 could be detected when present. 30The GE Raman spectrum (Figure 3) displayed peaks at 1322 and 1568 cm -1 , which referred to the D-band and the G-band, respectively.The Raman spectrum of the GE/α-Bi 2 O 3 electrode presented bands at 283 and 315 cm -1 , attributed to the bending modes of the BiO 5 and BiO 6 units, and bands at 410 and 449 cm -1 , which are due to the vibrational modes associated with a combination of Bi-O bond asymmetric stretching and bending modes. 31,32t is worth to note that the oxide bands derived from GE/α-Bi 2 O 3 @PPy depend on the position of laser on sample.
In the Raman spectrum of the GE/α-Bi 2 O 3 @PPy composite, there are two distinct regions: in one (red dashed line), the bands characteristic of the vibrational modes present in the existing bonds in Bi 2 O 3 and PPy, and in another (black dotted line), the absence of bands of the Bi 2 O 3 , indicating an inhomogeneous coating of Bi 2 O 3 by PPy.
On the other hand, the Raman spectra of the GE/PPy and GE/α-Bi 2 O 3 @PPy electrodes in Figures 3b and 3c, in the region where the PPy bands are present, can be employed to detect the presence or not of expected electronic interaction between PPy and α-Bi 2 O 3 .In fact, the PPy Raman bands up and down shifts, which can be observed (Table 2), were associated with the PPy oxidation state.Thereby, Figure 2 shows that PPy was oxidized in Bi 2 O 3 @PPy.The double bands shown at about 1048 and 1084 cm -1 are initially assigned to be the C-H in plane deformation of oxidized PPy. 33 The Raman spectra presented in Figure 3, and the position of their bands summarized in Table 2, show greater presence of oxidized state (bipolaron) in the composite than in the PPy, suggesting greater conjugation of the polymer chain.Figure 3 also shows that bands at about 1595, 986 and 942 cm -1 , assignable to the reduced form, are still present in the most oxidized state due to the presence of a range of conjugation lengths in the composite film, but slightly displaced. 34The oxide has an electron acceptor metal center  (Lewis acid) from the PPy (Lewis base), weakening the bonds in the polymer chain, whose vibrational modes would be shifted to lower frequencies (superscript a in Table 2).
Raman analysis confirms the formation of p-n junction between PPy and oxide in core-shell structures, where electrons can be exchanged such that electrochemical answer of PPy can be influenced by the type of electronic structure of oxide.4b and 4c, where it is possible to see the interference of α-Bi 2 O 3 on the nucleation of PPy deposits.The PPy coating on GE surface consisted of irregular structures deposits, while in the case of GE/α-Bi 2 O 3 @PPy electrode (Figure 4c) it is possible to observe structures quite different from Figures 4a and 4b.In fact, the PPy deposits on GE/α-Bi 2 O 3 (Figure 4c) by aggregation of several nanometric PPy particles, also with spheroidal shape, unlike the plates detected in Figure 4b.

Morphological characterization by SEM
To demonstrate that the SEM image of GE/α-Bi 2 O 3 @PPy effectively contains α-Bi 2 O 3 particles covered by PPy, it was also collected the energy-dispersive X-ray spectroscopy (EDS) spectra of the electrode (Figure 5), and carried out TEM analysis of material extracted from GE substrate.Clearly, the Figure 5 allows to detect presence of the elements Bi (pink) and O (orange) of α-Bi 2 O 3 under PPy layer, which are recognized by C (purple) and N (yellow).Figure 5 shows, as well, the elements K + (dark green) and Cl -(green) from the electrolyte, which were incorporated into the polymer matrix to neutralize the positive charge density of the PPy nitrogen.
To understand the kind of core-shell structure present into GE/α-Bi 2 O 3 @PPy composite, the Figure 6 is shown.It was found in the TEM grid predominantly nanometric particles of α-Bi 2 O 3 @PPy.The presence of an irregular PPy granular layer growth covering the surface of α-Bi 2 O 3 (dark spot) is resulting from non-uniform current distribution on surface of α-Bi 2 O 3 due to the aggregation of grains.As a matter of fact, the thickness of PPy layer electrodeposited on GE/α-Bi 2 O 3 occurred not in uniform mode as also detected by Raman characterization.Hence, the studied GE/α-Bi 2 O 3 @PPy electrode can be defined as an irregular core-shell structure.

Electrochemical performance measurement by CV and GCD
The electrochemical performances of the GE/PPy, GE/α-Bi 2 O 3 , and GE/α-Bi 2 O 3 @PPy electrodes were analyzed by CV in aqueous 1 mol L -1 Na 2 SO 4 from -0.30 to 0.70 V vs. Ag (s) /AgCl (s) /Cl - (aq) .Figure 7 shows the CV curves recorded for the GE/α-Bi 2 O 3 , GE/PPy, and GE/α-Bi 2 O 3 @PPy electrodes.The curves confirmed the capacitive profile of all the materials due to formation of an electric double layer (2 × Na + and × SO 4 2- ) on the electrode surface.
The CV curve of the GE/α-Bi 2 O 3 @PPy electrode revealed higher total charge (55.60 mC at 25 mV s -1 ) as compared to the GE/α-Bi 2 O 3 (10.65 mC) and GE/PPy (19.50 mC) electrodes.The highest total charge was obtained for GE/α-Bi 2 O 3 @PPy electrode due to the α-Bi 2 O 3 to have favored the redox processes of PPy by the supply of 4 electrons and 6 H + , displaying the electronic conductivity.
According to the galvanostatic tests, the GE/α-Bi 2 O 3 @PPy electrode supported the same current density for a longer time as compared to the GE/α-Bi 2 O 3 electrode, which suggested that PPy contributed predominantly to the energy storage and electronic conductivity of the GE/α Bi 2 O 3 @PPy electrode, due to the cooperation between the double layer capacitance of GE and pseudo-capacitance of the PPy.
In addition, it should be noted that the coating of the α-Bi 2 O 3 particles by PPy resulted in an increase of the electroactive area, which intensified the capacitance values of this material.CV curves were integrated and the galvanostatic tests performed to calculate the specific capacitances (Table 3).
The GE/α-Bi 2 O 3 @PPy electrode presented superior specific capacitances compared to the GE/PPy and GE/α-Bi 2 O 3 electrodes.These results corroborated with the morphological and structural analyses of the GE/α-Bi 2 O 3 @PPy electrode: the α-Bi 2 O 3 redox process   favored PPy polymeric chain protonation during the electrochemical tests, increasing the PPy conductivity in the hybrid material.
On the basis of electrochemical tests the GE/α-Bi 2 O 3 @PPy electrode presented the highest increase in specific capacitance in relation to the increment in current, which suggested that the PPy coating on the α-Bi 2 O 3 surface provided 650.05 F g -1 at 25 mV s -1 and ca.301 F g -1 at 10 A g -1 .The specific capacitance of PPy increases about 95% on surface of α-Bi 2 O 3 when compared to PPy/GE electrode, which suggests that α-Bi 2 O 3 would be a very interesting oxide to improve the performance of PPy.In fact, in the case of PPy@TiO 2 prepared by PPy chemical deposition on surface of TiO 2 , specific capacitance of about 180 F g -1 has been observed, and in the case of PPy@TiN nanotubes and PPy/TiO 2 nanotubes, specific capacitances of 459 and 72 F g -1 at 15 A g -1 , respectively, have been observed. 29,35IS measurements were carried out from 10 MHz to 100 mHz under open circuit potential.Figure 10a shows the Nyquist plots of the GE/α-Bi 2 O 3 , GE/PPy, and GE/α-Bi 2 O 3 @PPy electrodes.Galvanostatic characterization of GE/α-Bi 2 O 3 @PPy in 1 mol L -1 Na 2 SO 4 at different gravimetric current: (a) discharge/charge curves at 20 A g -1 (blue dotted line), 10 A g -1 (red dashed line), 5 A g -1 (green solid line) and 3 A g -1 (black dashed-dotted line); (b) specific capacitance and coulombic efficiency vs. cycle numbers for the GE/α-Bi 2 O 3 @PPy electrode at 3 A g -1 with average mass of 0.10 mg.The Nyquist diagrams of the GE/α-Bi 2 O 3 @PPy electrode showed a predominantly capacitive profile (Figure 10a) with lower impedance values as compared to the impedance of the GE/α-Bi 2 O 3 and GE/PPy electrodes, indicating that PPy, synthesized on the α-Bi 2 O 3 , was in its conductive form and, therefore, the positive charge density of the nitrogen had been neutralized by the chloride ions from the electrolyte, thus characterizing a pseudocapacitance.
Bode plots (Figure 10b) show that GE/α-Bi 2 O 3 @PPy electrode has an electrochemical behavior very similar to the PPy, however, these Bode diagrams revealed increased phase angle for the GE/α-Bi 2 O 3 @PPy electrode at lower frequencies, as well as the disappearance of the second phase angle assigned to the α-Bi 2 O 3 , indicating a complete cover of α-Bi 2 O 3 by PPy, as detected in HRTEM analysis.Besides, it was possible to verify that the charge storage occurs at low frequencies related to charge accumulation in the bulk of the electrode (limit capacitance).
The PPy-α-Bi 2 O 3 hybrid composite is effectively a core-shell structure, where n-p junction is present.The α-Bi 2 O 3 core changes the width of charge distribution within the PPy layer grown on its surface, which explains the specific capacitance behavior observed in the present work.Thus, the different specific capacitances of coreshell GE/α-Bi 2 O 3 @PPy and GE/PPy electrodes can be attributed not only to differences in the PPy electroactive area.

Conclusions
Bi 2 O 3 is a transition oxide with good properties.Its association with PPy can create a composite, which improves the electrochemical behavior of the PPy.The solvothermal synthesis afforded monoclinic α-Bi 2 O 3 , as confirmed by XRD, which showed that Bi 2 O 3 was indexed to the monoclinic structure of α-Bi 2 O 3 with space group P2 1 /c (JCPDS No. 41-1449).SEM, EDS, and Raman characterizations proved that α-Bi 2 O 3 was inhomogeneously coated electrochemically with PPy as a core-shell GE/α-Bi 2 O 3 @PPy hybrid electrode.
The increase in the anodic current density values and consequently the highest charges were obtained for the α-Bi 2 O 3 @PPy electrode due to the α-Bi 2 O 3 having favored the redox processes of PPy by the supply of 4 electrons and 6 H + .Besides, it was noted that the coating of the α-Bi 2 O 3 particles by PPy resulted in an increase of the electroactive area, which intensified the specific capacitance value as compared to the GE/PPy and GE/α-Bi 2 O 3 electrodes.
The Bode diagrams revealed increased phase angle for the GE/α-Bi 2 O 3 @PPy electrode at lower frequencies as well as the disappearance of the second phase angle assigned to the α-Bi 2 O 3 , indicating a complete cover of α-Bi 2 O 3 by PPy.Besides, it was possible to verify that the charge storage occurs at low frequencies related to charge accumulation in the bulk of the electrode (limit capacitance).Therefore, the results indicated that the α-Bi 2 O 3 @PPy is a promising composite material for application in supercapacitors.
α-Bi 2 O 3 synthesis and structural characterization Bi(NO 3 ) 3 .5H 2 O dissolution in EG and heating at 140 o C for 2 h afforded the relatively stable Bi 2 (OCH 2 CH 2 O) 3 ,26

Figures
Figures 4a-4c illustrate the morphology of the thin film of the GE/α-Bi 2 O 3 , GE/PPy, and GE/α-Bi 2 O 3 @PPy electrodes as analyzed by SEM.The SEM images of the GE/α-Bi 2 O 3 electrode (Figure 4a) evidenced particles, which are aggregated in the shape of curved rods ranging from 2 to 5 µm, and individual spheroidal grains with average diameter around 70 nm.The thermal treatment applied on α-Bi 2 O 3 to improve its crystallinity probably provoked a synthesis of α-Bi 2 O 3 nanoparticles, which led to observed α-Bi 2 O 3 microrods, the principal constituent of studied GE/α-Bi 2 O 3 electrode.Another interesting result appears on Figures4b and 4c, where it is possible to see the interference of α-Bi 2 O 3 on the nucleation of PPy deposits.The PPy coating on GE surface consisted of irregular structures deposits, while in the case of GE/α-Bi 2 O 3 @PPy electrode (Figure4c) it is possible to observe structures

Table 1 .
Unit cell parameters calculated for α-Bi 2 O 3 Table 2 summarizes the main vibrational modes of Raman spectra.Both Raman band intensities indicate predominance of the oxidized form of PPy and are related to higher conductivity values, corresponding to doping levels.

Table 3 .
Specific capacitance and coulomb efficiency of 50 th cycle of studied electrodes in 1 mol L -1 Na 2 SO 4 between -0.30 and 0.70 V vs.