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Facile synthesis of pervoskite type BiYO3 embedded reduced graphene oxide (RGO) composite for supercapacitor applications
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Facile synthesis of pervoskite type BiYO3 embedded reduced graphene oxide (RGO) composite for supercapacitor applications R. Selvarajana, S. Vadivelb, M. Arivanandhana, R. Jayavela,∗ a b
Centre for Nanoscience and Technology, Anna University, Chennai, 25, India Department of Chemistry, PSG College of Technology, Coimbatore, India
A R T I C LE I N FO
A B S T R A C T
Keywords: BiYO3 Graphene Composites Electrochemical studies Supercapacitors
In the present study, BiYO3/RGO composite, is successfully synthesized by simple calcination method without the aid of any surfactants. The morphology, structural, and optical properties of the composite are examined by XRD, FTIR, XPS, SEM, and TEM analyses. The electrochemical properties of BiYO3 incorporated RGO showed higher surface area, good electrical conductivity, and lower resistivity than the pure BiYO3. It is revealed that the RGO facilitated faster ion diffusion pathway for the improved redox activity in the BiYO3/RGO composite. Furthermore, the BiYO3/RGO composite possesses higher specific capacitance of 725 Fg-1 at a current density of 2 Ag-1.
1. Introduction In the past few decades, the development of clean and renewable energy sources is the hot topic among the researchers [1–3]. Due to the rapid development of industries and the invention of smart devices, there is a huge necessity for energy storage devices [4]. With a growing demand for the electrochemical energy storage devices, supercapacitors (SC) is the promising choice [5]. Supercapacitor is the contemporary ilk of energy storage device with fast charging-discharging process. Furthermore, it bridges the energy gap allying the conventional capacitors, and batteries in terms of energy and power density values. SC exhibits exalted power density, rapid charge-discharge potential and prolonged cycle life than ordinary capacitors [6]. The underlying mechanism of energy storage in SC is mainly due to electrical double layer capacitance (EDLC) type or by Faradaic (pseudo capacitance) process [7,8]. Till date, various transition metal oxides and its analogs were studied extensively for pseudo capacitors [9]. The current trend is to select the promising transition metal oxides for pseudo capacitor electrodes and to design the flexible architecture for commercialization. Materials like RuO2 [10], WO3 [11], Mn2O3 [12], and Co3O4 [13] have been reported widely as pseudo capacitor electrodes. The usage of RuO2, WO3, Mn2O3, and Co3O4 in practical application has been restricted due to their exorbitant high cost and toxicity. In order to overcome this obstacle, various bismuth-based electrode materials such as Bi2O3 [14], Bi2S3 [15], BiPO4 [16], BiVO4 [17], BiOI [18] have been exploited as electrodes for supercapacitor due to easy availability and
∗
non-toxicity. Recently a new material has emerged as bismuth analogs, the perovskite-type BiYO3, which has been widely studied for photocatalyst and dielectric applications [19,20]. Generally, bismuth oxide, phosphate and sulfide exhibit faradaic type electrode behavior with limited electrolyte accessibility, poor stability, and lower conductivity. In order to overcome these limitations, the combination of BiYO3 with graphene has been attempted to design composite material. Graphene is a 2D sp2 hybridized honeycomb structured carbon material. It has already been reported that metal oxides incorporated graphene composites exhibit superior energy storage ability and good cyclic life [21]. However as far as the material is considered, there is no report available on the supercapacitor application of BiYO3 and graphene composites. In this study, BiYO3/RGO composite has been blended by simple calcination method and used as an electrode to study reliability for supercapacitor applications. BiYO3/RGO composite revealed higher specific capacitance and exceptional cyclic stability compared to several bismuth analogs. 2. Synthesis of BiYO3/RGO composite The precursor materials from Sigma Aldrich were used without any additional refinement. Modified Hummer's method was practiced to prepare graphene oxide (GO) from natural graphite [22]. In a standard synthesis of BiYO3/RGO composite, graphene oxide (100 mg) was ultrasonically dispersed in diluted HNO3 for 30 min to attain a homogeneous solution. Then, the stoichiometric amount of Bi(NO3)3.5H2O, Y
Corresponding author. Centre for Nanoscience and Technology, Anna University. E-mail address: [email protected] (R. Jayavel).
https://doi.org/10.1016/j.ceramint.2019.10.060 Received 17 July 2019; Received in revised form 1 October 2019; Accepted 7 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: R. Selvarajan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.060
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Scheme: 1. Schematic representation for the synthesis of BiYO3/RGO composite.
(NO3)3.6H2O, and hexamine were dissolved in 20 ml of NH4OH solution, and these solutions were mixed slowly by constant stirring at 70 °C. The obtained pale yellow precipitate was washed and dried at 70 °C for overnight using a vacuum oven. Later calcination was done at 900 °C at a heating rate of 5 °C/min in air atmosphere to obtain the orange colour BiYO3/RGO composite. Pure BiYO3 was prepared by the same strategy without the addition of GO. The schematic representation of the synthesis of BiYO3/RGO composite is shown in Scheme 1. 2.1. Material characterization The crystalline structure and phase purity of the samples were analyzed by XRD using Xpert-Pro-PAN analytical instrument in the 2θ range of 10–80°. FT-IR spectra were recorded using Shimadzu FT-IR 8400 spectrometer. The diffused reflectance spectroscopy (UV-DRS) was carried out using JascoV-750 spectrophotometer with BaSO4 as the reference. The morphology and microstructure were examined using HR-TEM (JEOL-JEM 2100) and SEM (Zeiss-EVO18). The XPS analysis was performed using Omicron X-ray photoelectron spectrometer.
Fig. 1. X-ray diffraction patterns of as-prepared GO, BiYO3 and BiYO3/RGO composite.
2.2. Electrochemical studies The working electrode was prepared by coalescing 75 wt% of functional material, 15 wt % of polyvinylidene difluoride (PVDF) and 10 wt% of activated carbon and grinding with polyethylene glycol in an agate mortar to achieve homogeneous slurry. This slurry was layered on a stainless steel electrode and dried at 80 °C. The electrochemical performance was studied in a three-electrode system at room temperature using CHI660B instrument. For the three-electrode configuration, the working electrode was the stainless steel coated electroactive material with Pt wire being the counter and Ag/AgCl as the reference electrodes. For cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) studies, 6 M KOH solution was used as the electrolyte. 3. Results and discussion 3.1. XRD studies The XRD patterns of GO, pure BiYO3, and BiYO3/RGO are shown in Fig. 1. The diffraction peaks of GO at (2θ = 10.4°) very well matched with the reported literature [22]. The diffraction peaks of BiYO3 correspond to cubic phase, as in thorough agreement with the JCPDS file no. (27–1047). Besides, BiYO3/RGO composite exhibited similar XRD
Fig. 2. FT-IR spectra of as-prepared GO, BiYO3 and BiYO3/RGO composite.
2
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Fig. 3. (a–c) SEM images of RGO, BiYO3and BiYO3/RGO composite.
Fig. 4. (a–b) TEM images of BiYO3/RGO composite at different magnifications(c-f) EDS-mapping of BiYO3/RGO composite.
in Fig. 2. For GO, the characteristic absorption peaks are observed at 3431, 1731, 1616, 1393, 1222, and 1050 cm−1 [24]. The FT-IR spectrum of BiYO3 shows the absorption band around 1643 cm−1 corresponding to O–H bending vibrations of surface adsorbed hydroxyl groups and the band observed at 1520 cm−1 is due to the stretching vibration of Y–O [25]. Additionally, the peaks observed at 1383 and 853 cm−1 correspond to NO3− of bismuth sub-nitrate molecules, which
patterns of the BiYO3, implying that the introduction of graphene has negligible denouement on the BiYO3 crystal structure [19,23]. 3.2. FT-IR analysis The functional groups of GO, BiYO3, and BiYO3/rGO composite have been investigated by FT-IR analysis, and the spectra are presented 3
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Fig. 5. (a) XPS survey spectrum for BiYO3/RGO composite and high resolution spectra of (b) Bi 4f (c) O1S (d) C1s (e) Y3d.
suitable for supercapacitor application. The graphene sheets can act as a bridge for electron transport to facilitate higher charge storage ability. The energy-dispersive X-ray spectroscopy (EDX) mapping obtained for BiYO3 (Fig. 4c–f) clearly indicates the presence of Bi, O, C, and Y atoms. This result confirms the successful formation of BiYO3/RGO hybrid structures.
further confirms that the BiYO3 was successfully synthesized by this method. Compared to BiYO3, there is no apparent change in the spectrum of BiYO3/RGO composite indicating that the structure of BiYO3 has not been affected by graphene incorporation. 3.3. Morphological studies The RGO exhibited a stacked morphology due to the restacking of RGO sheets during the high-temperature calcination process (Fig. 3a). BiYO3/RGO composite showed that several BiYO3 nanoparticles were strongly wrapped on the surface of RGO sheets (Fig. 3b and c). The TEM images reveal that BiYO3 nanoparticles are randomly distributed on the surface of RGO (Fig. 4a and b). These unique features suggest that the as-prepared BiYO3/RGO composite has the appropriate structure
3.4. XPS analysis XPS analysis was carried out to explicate the composition and the chemical oxidation states of BiYO3/RGO composite (Fig. 5). The XPS survey spectrum (Fig. 5a) for BiYO3/RGO composite indicates that the composite mainly comprises of Bi, Y, O, and C atoms, with surface contents of 59.5%, 21.2%, 13.8%, and 5.50% respectively. The results 4
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groups [27]. For C1s spectrum (Fig. 3b), the peak observed at 284.6 eV confirms the existence of RGO in the matrix [28]. As for Y3d, two peaks with binding energies of 156.9 and 160.1 eV correspond to Y3d 3/2 and Y3d 1/2 spins, indicating the existence of Y3d spins with oxidation state +3 [29]. 3.5. N2 adsorption-desorption studies Fig. 6 shows N2 adsorption-desorption isotherms and the pore size distribution curves of the different BiYO3 catalysts. The specific surface area of the BiYO3/RGO nanocomposite was obtained by a multi-point BET method. From Fig. 6, it was observed that the N2 adsorption isotherm of the nanocomposite shows type-III isotherm with an H3-type hysteresis. Initially, at low pressures, the N2 adsorption increased slowly with the increase in relative pressure, which showed that the adsorption is multi-layer adsorption. The BET specific surface area obtained from the N2 adsorption-desorption curves of BiYO3/RGO was 16.32 m2/g. Based on the Barrett-Joyner-Halenda (BJH) method, the pore size and pore volume of BiYO3/RGO nanocomposite were found to be 2.391 nm and 0.059 cm3/g, respectively. Obviously, the surface area and pore volume of BiYO3/RGO will enhance the electrochemical property to a great extent.
Fig. 6. N2adsorption–desorption isotherms for the BiYO3/RGO nanocomposites, Inset shows BJH pore size distribution BiYO3/RGO nanocomposites.
were consistent with the EDS mapping analysis. Fig. 5b shows deconvoluted Bi 4f spectra of BiYO3/RGO with the binding energy of 157.01 and 162.35 eV corresponding to Bi 4f 7/2 and Bi 4f 5/2 spins respectively, connoting that bismuth element remains in the oxidation state of Bi (III) [26]. For the O1s spectrum, two distinct peaks with binding energy at 528.2 eV attributed to lattice oxygen of BiYO3 material while the peak at 534.2 eV is attributed to the surface adhered hydroxyl
3.6. Electrochemical properties The electrochemical properties of the BiYO3 and BiYO3/RGO composite electrodes were evaluated by CV studies. The current-voltage curves of the samples are recorded in the prospective range of 0–1.0 V vs. Ag/AgCl at probe rates of 10–50 mV/s in Fig. 7 (a–c). The shapes of the CV curves imply that the capacitance behavior of pure BiYO3 and
Fig. 7. (a) CV curves of BiYO3 and BiYO3/RGO composite at a probe rate of 10 mV/s. (b–c) CV curves of BiYO3 and BiYO3/RGO composite at different probe rates. 5
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Fig. 8. (a) Comparison of Charge-discharge curves of BiYO3 and BiYO3/RGO at 2 Ag-1 and (b–c) Charge-discharge curves of BiYO3 and BiYO3/RGO at different current densities.
Fig. 9. (a–b) Specific capacitance values of BiYO3 and BiYO3/RGO composites at different current densities,(d)Nyquist plots of BiYO3 and BiYO3/RGO composite at open circuit potential.
the CV curves of BiYO3/RGO composite is apparently larger than BiYO3, indicating that the BiYO3/RGO composite exhibits higher specific capacitance than BiYO3 [30]. The amelioration of current values for BiYO3/RGO composite can be attributed to the introduction of RGO, which increases the conductivity and specific surface area of the host
BiYO3/RGO composite is mainly associated with Faradic nature. All CV curves evince a pair of redox peaks, which originate from the redox reaction between host BiYO3 material and OH− ions on the electrode surface, which is rather dissimilar from the conventional electrical double-layer capacitance (EDLC). Furthermore, the integral extent of 6
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References
BiYO3 [31]. Furthermore, the composite facilitates the contact of inner active substances with electrolytes leading to higher energy storage. The clear insight mechanism behind the oxidation and reduction reaction occurs during the CV process as similar to Bi2O3 [32], BiPO4 [33], and Bi2WO6 [34] materials. Bi
(0)
→ Bi (metallic) ——————————— + e− ———————————
Bi (metallic) → Bi
+
Bi (metallic) → Bi
3+
Bi
3+
−
+ 3 OH
BiO22 –
(1) (2)
———————————
(3)
→Bi(OH)3 ———————————
(4)
Bi(OH)3 →BiOOH + H2O ———————————— −
+ H2O → BiO2 + 4 OH + Bi
(0)
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(5)
—————————— (7)
The galvanostatic charge-discharge (GCD) studies were implemented to calculate the specific capacitance of the electrodes [35]. The GCD curves of BiYO3/RGO composite were recorded at diverse current densities ranging from 2 to 10 A g−1, and the specific capacitance value is calculated in accordance with Eq. (8)
SC =
I × Δt Fg −1 ΔV × m
(8)
The BiYO3/RGO composite electrode delivers a longer discharging time than pure BiYO3 electrode, suggesting enhanced specific capacitance values achieved by successful RGO incorporation. GCD measurements of all the prepared electrodes at different current densities (2–10 A g−1) are shown in Fig. 8. The GCD curves for a current density of 2 A g−1 are compared in Fig. 8a. The curves for different current density values for BiYO3 and BiYO3/RGO are depicted in Fig. 8a and (b) respectively. The maximum capacitance for BiYO3/RGO composite is 696 F g−1 at a current density of 2 A g−1. This value is superior to 500 F g−1 for BiYO3 at the same current density. The specific capacitance values of the BiYO3/RGO composite with increasing current densities were calculated to be 725, 292, 162, 122, and 100 F g−1, suggesting that the specific capacitance values of the electrode materials decrease with increasing current densities as shown in Fig. 9 (a) The electron transferability between the electrode surface and the electrolyte was analyzed by electrochemical impedance spectroscopy (EIS) and represented in Fig. 9 (b). The EIS studies were steered in the frequency range of 100 kHz-0.01 Hz. The Nyquist plots reveal a straight line in the low frequency region that relates to the Warburg impedance and a semicircle at higher frequencies confirming the charge-transfer resistance (Rct) [35]. The intersecting point at the real axis of BiYO3/ RGO composite is lower than pure BiYO3 revealing a reasonably lower inner resistance, due to the amalgamation of graphene nanosheets. From the EIS data, a substantial enhancement of electrochemical performance is observed for BiYO3/RGO composite compared to pure BiYO3 [36,37]. The successful accretion of RGO can provide high electrical conductivity and electron transportation ability to the system.
4. Conclusion Reduced graphene oxide sheets were well festooned by BiYO3 nanoparticles with high surface area by a simple calcination method. The specific capacitance of BiYO3/RGO composite is 696F g−1 at 2 Ag-1, and is far higher than pure BiYO3. Moreover, BiYO3/RGO composite revealed superior extended cycle life with 90.6% specific capacitance retention after 2000 charge-discharge cycles at 2 Ag-1. The BiYO3/RGO composite bids hardy energy storage ability and mesoporous BiYO3 expedites OH ions diffusion for the superior Faradaic reactions. Furthermore, the exceptional supercapacitor functioning of BiYO3/RGO composite could be indorsed to the concerted effect of graphene with large surface area and higher electrical conductivity. 7
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Facile synthesis of pervoskite type BiYO3 embedded reduced graphene oxide (RGO) composite for supercapacitor applications R. Selvarajana, S. Vadivelb, M. Arivanandhana, R. Jayavela,∗ a b
Centre for Nanoscience and Technology, Anna University, Chennai, 25, India Department of Chemistry, PSG College of Technology, Coimbatore, India
A R T I C LE I N FO
A B S T R A C T
Keywords: BiYO3 Graphene Composites Electrochemical studies Supercapacitors
In the present study, BiYO3/RGO composite, is successfully synthesized by simple calcination method without the aid of any surfactants. The morphology, structural, and optical properties of the composite are examined by XRD, FTIR, XPS, SEM, and TEM analyses. The electrochemical properties of BiYO3 incorporated RGO showed higher surface area, good electrical conductivity, and lower resistivity than the pure BiYO3. It is revealed that the RGO facilitated faster ion diffusion pathway for the improved redox activity in the BiYO3/RGO composite. Furthermore, the BiYO3/RGO composite possesses higher specific capacitance of 725 Fg-1 at a current density of 2 Ag-1.
1. Introduction In the past few decades, the development of clean and renewable energy sources is the hot topic among the researchers [1–3]. Due to the rapid development of industries and the invention of smart devices, there is a huge necessity for energy storage devices [4]. With a growing demand for the electrochemical energy storage devices, supercapacitors (SC) is the promising choice [5]. Supercapacitor is the contemporary ilk of energy storage device with fast charging-discharging process. Furthermore, it bridges the energy gap allying the conventional capacitors, and batteries in terms of energy and power density values. SC exhibits exalted power density, rapid charge-discharge potential and prolonged cycle life than ordinary capacitors [6]. The underlying mechanism of energy storage in SC is mainly due to electrical double layer capacitance (EDLC) type or by Faradaic (pseudo capacitance) process [7,8]. Till date, various transition metal oxides and its analogs were studied extensively for pseudo capacitors [9]. The current trend is to select the promising transition metal oxides for pseudo capacitor electrodes and to design the flexible architecture for commercialization. Materials like RuO2 [10], WO3 [11], Mn2O3 [12], and Co3O4 [13] have been reported widely as pseudo capacitor electrodes. The usage of RuO2, WO3, Mn2O3, and Co3O4 in practical application has been restricted due to their exorbitant high cost and toxicity. In order to overcome this obstacle, various bismuth-based electrode materials such as Bi2O3 [14], Bi2S3 [15], BiPO4 [16], BiVO4 [17], BiOI [18] have been exploited as electrodes for supercapacitor due to easy availability and
∗
non-toxicity. Recently a new material has emerged as bismuth analogs, the perovskite-type BiYO3, which has been widely studied for photocatalyst and dielectric applications [19,20]. Generally, bismuth oxide, phosphate and sulfide exhibit faradaic type electrode behavior with limited electrolyte accessibility, poor stability, and lower conductivity. In order to overcome these limitations, the combination of BiYO3 with graphene has been attempted to design composite material. Graphene is a 2D sp2 hybridized honeycomb structured carbon material. It has already been reported that metal oxides incorporated graphene composites exhibit superior energy storage ability and good cyclic life [21]. However as far as the material is considered, there is no report available on the supercapacitor application of BiYO3 and graphene composites. In this study, BiYO3/RGO composite has been blended by simple calcination method and used as an electrode to study reliability for supercapacitor applications. BiYO3/RGO composite revealed higher specific capacitance and exceptional cyclic stability compared to several bismuth analogs. 2. Synthesis of BiYO3/RGO composite The precursor materials from Sigma Aldrich were used without any additional refinement. Modified Hummer's method was practiced to prepare graphene oxide (GO) from natural graphite [22]. In a standard synthesis of BiYO3/RGO composite, graphene oxide (100 mg) was ultrasonically dispersed in diluted HNO3 for 30 min to attain a homogeneous solution. Then, the stoichiometric amount of Bi(NO3)3.5H2O, Y
Corresponding author. Centre for Nanoscience and Technology, Anna University. E-mail address: [email protected] (R. Jayavel).
https://doi.org/10.1016/j.ceramint.2019.10.060 Received 17 July 2019; Received in revised form 1 October 2019; Accepted 7 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: R. Selvarajan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.060
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Scheme: 1. Schematic representation for the synthesis of BiYO3/RGO composite.
(NO3)3.6H2O, and hexamine were dissolved in 20 ml of NH4OH solution, and these solutions were mixed slowly by constant stirring at 70 °C. The obtained pale yellow precipitate was washed and dried at 70 °C for overnight using a vacuum oven. Later calcination was done at 900 °C at a heating rate of 5 °C/min in air atmosphere to obtain the orange colour BiYO3/RGO composite. Pure BiYO3 was prepared by the same strategy without the addition of GO. The schematic representation of the synthesis of BiYO3/RGO composite is shown in Scheme 1. 2.1. Material characterization The crystalline structure and phase purity of the samples were analyzed by XRD using Xpert-Pro-PAN analytical instrument in the 2θ range of 10–80°. FT-IR spectra were recorded using Shimadzu FT-IR 8400 spectrometer. The diffused reflectance spectroscopy (UV-DRS) was carried out using JascoV-750 spectrophotometer with BaSO4 as the reference. The morphology and microstructure were examined using HR-TEM (JEOL-JEM 2100) and SEM (Zeiss-EVO18). The XPS analysis was performed using Omicron X-ray photoelectron spectrometer.
Fig. 1. X-ray diffraction patterns of as-prepared GO, BiYO3 and BiYO3/RGO composite.
2.2. Electrochemical studies The working electrode was prepared by coalescing 75 wt% of functional material, 15 wt % of polyvinylidene difluoride (PVDF) and 10 wt% of activated carbon and grinding with polyethylene glycol in an agate mortar to achieve homogeneous slurry. This slurry was layered on a stainless steel electrode and dried at 80 °C. The electrochemical performance was studied in a three-electrode system at room temperature using CHI660B instrument. For the three-electrode configuration, the working electrode was the stainless steel coated electroactive material with Pt wire being the counter and Ag/AgCl as the reference electrodes. For cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) studies, 6 M KOH solution was used as the electrolyte. 3. Results and discussion 3.1. XRD studies The XRD patterns of GO, pure BiYO3, and BiYO3/RGO are shown in Fig. 1. The diffraction peaks of GO at (2θ = 10.4°) very well matched with the reported literature [22]. The diffraction peaks of BiYO3 correspond to cubic phase, as in thorough agreement with the JCPDS file no. (27–1047). Besides, BiYO3/RGO composite exhibited similar XRD
Fig. 2. FT-IR spectra of as-prepared GO, BiYO3 and BiYO3/RGO composite.
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Fig. 3. (a–c) SEM images of RGO, BiYO3and BiYO3/RGO composite.
Fig. 4. (a–b) TEM images of BiYO3/RGO composite at different magnifications(c-f) EDS-mapping of BiYO3/RGO composite.
in Fig. 2. For GO, the characteristic absorption peaks are observed at 3431, 1731, 1616, 1393, 1222, and 1050 cm−1 [24]. The FT-IR spectrum of BiYO3 shows the absorption band around 1643 cm−1 corresponding to O–H bending vibrations of surface adsorbed hydroxyl groups and the band observed at 1520 cm−1 is due to the stretching vibration of Y–O [25]. Additionally, the peaks observed at 1383 and 853 cm−1 correspond to NO3− of bismuth sub-nitrate molecules, which
patterns of the BiYO3, implying that the introduction of graphene has negligible denouement on the BiYO3 crystal structure [19,23]. 3.2. FT-IR analysis The functional groups of GO, BiYO3, and BiYO3/rGO composite have been investigated by FT-IR analysis, and the spectra are presented 3
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Fig. 5. (a) XPS survey spectrum for BiYO3/RGO composite and high resolution spectra of (b) Bi 4f (c) O1S (d) C1s (e) Y3d.
suitable for supercapacitor application. The graphene sheets can act as a bridge for electron transport to facilitate higher charge storage ability. The energy-dispersive X-ray spectroscopy (EDX) mapping obtained for BiYO3 (Fig. 4c–f) clearly indicates the presence of Bi, O, C, and Y atoms. This result confirms the successful formation of BiYO3/RGO hybrid structures.
further confirms that the BiYO3 was successfully synthesized by this method. Compared to BiYO3, there is no apparent change in the spectrum of BiYO3/RGO composite indicating that the structure of BiYO3 has not been affected by graphene incorporation. 3.3. Morphological studies The RGO exhibited a stacked morphology due to the restacking of RGO sheets during the high-temperature calcination process (Fig. 3a). BiYO3/RGO composite showed that several BiYO3 nanoparticles were strongly wrapped on the surface of RGO sheets (Fig. 3b and c). The TEM images reveal that BiYO3 nanoparticles are randomly distributed on the surface of RGO (Fig. 4a and b). These unique features suggest that the as-prepared BiYO3/RGO composite has the appropriate structure
3.4. XPS analysis XPS analysis was carried out to explicate the composition and the chemical oxidation states of BiYO3/RGO composite (Fig. 5). The XPS survey spectrum (Fig. 5a) for BiYO3/RGO composite indicates that the composite mainly comprises of Bi, Y, O, and C atoms, with surface contents of 59.5%, 21.2%, 13.8%, and 5.50% respectively. The results 4
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groups [27]. For C1s spectrum (Fig. 3b), the peak observed at 284.6 eV confirms the existence of RGO in the matrix [28]. As for Y3d, two peaks with binding energies of 156.9 and 160.1 eV correspond to Y3d 3/2 and Y3d 1/2 spins, indicating the existence of Y3d spins with oxidation state +3 [29]. 3.5. N2 adsorption-desorption studies Fig. 6 shows N2 adsorption-desorption isotherms and the pore size distribution curves of the different BiYO3 catalysts. The specific surface area of the BiYO3/RGO nanocomposite was obtained by a multi-point BET method. From Fig. 6, it was observed that the N2 adsorption isotherm of the nanocomposite shows type-III isotherm with an H3-type hysteresis. Initially, at low pressures, the N2 adsorption increased slowly with the increase in relative pressure, which showed that the adsorption is multi-layer adsorption. The BET specific surface area obtained from the N2 adsorption-desorption curves of BiYO3/RGO was 16.32 m2/g. Based on the Barrett-Joyner-Halenda (BJH) method, the pore size and pore volume of BiYO3/RGO nanocomposite were found to be 2.391 nm and 0.059 cm3/g, respectively. Obviously, the surface area and pore volume of BiYO3/RGO will enhance the electrochemical property to a great extent.
Fig. 6. N2adsorption–desorption isotherms for the BiYO3/RGO nanocomposites, Inset shows BJH pore size distribution BiYO3/RGO nanocomposites.
were consistent with the EDS mapping analysis. Fig. 5b shows deconvoluted Bi 4f spectra of BiYO3/RGO with the binding energy of 157.01 and 162.35 eV corresponding to Bi 4f 7/2 and Bi 4f 5/2 spins respectively, connoting that bismuth element remains in the oxidation state of Bi (III) [26]. For the O1s spectrum, two distinct peaks with binding energy at 528.2 eV attributed to lattice oxygen of BiYO3 material while the peak at 534.2 eV is attributed to the surface adhered hydroxyl
3.6. Electrochemical properties The electrochemical properties of the BiYO3 and BiYO3/RGO composite electrodes were evaluated by CV studies. The current-voltage curves of the samples are recorded in the prospective range of 0–1.0 V vs. Ag/AgCl at probe rates of 10–50 mV/s in Fig. 7 (a–c). The shapes of the CV curves imply that the capacitance behavior of pure BiYO3 and
Fig. 7. (a) CV curves of BiYO3 and BiYO3/RGO composite at a probe rate of 10 mV/s. (b–c) CV curves of BiYO3 and BiYO3/RGO composite at different probe rates. 5
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Fig. 8. (a) Comparison of Charge-discharge curves of BiYO3 and BiYO3/RGO at 2 Ag-1 and (b–c) Charge-discharge curves of BiYO3 and BiYO3/RGO at different current densities.
Fig. 9. (a–b) Specific capacitance values of BiYO3 and BiYO3/RGO composites at different current densities,(d)Nyquist plots of BiYO3 and BiYO3/RGO composite at open circuit potential.
the CV curves of BiYO3/RGO composite is apparently larger than BiYO3, indicating that the BiYO3/RGO composite exhibits higher specific capacitance than BiYO3 [30]. The amelioration of current values for BiYO3/RGO composite can be attributed to the introduction of RGO, which increases the conductivity and specific surface area of the host
BiYO3/RGO composite is mainly associated with Faradic nature. All CV curves evince a pair of redox peaks, which originate from the redox reaction between host BiYO3 material and OH− ions on the electrode surface, which is rather dissimilar from the conventional electrical double-layer capacitance (EDLC). Furthermore, the integral extent of 6
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References
BiYO3 [31]. Furthermore, the composite facilitates the contact of inner active substances with electrolytes leading to higher energy storage. The clear insight mechanism behind the oxidation and reduction reaction occurs during the CV process as similar to Bi2O3 [32], BiPO4 [33], and Bi2WO6 [34] materials. Bi
(0)
→ Bi (metallic) ——————————— + e− ———————————
Bi (metallic) → Bi
+
Bi (metallic) → Bi
3+
Bi
3+
−
+ 3 OH
BiO22 –
(1) (2)
———————————
(3)
→Bi(OH)3 ———————————
(4)
Bi(OH)3 →BiOOH + H2O ———————————— −
+ H2O → BiO2 + 4 OH + Bi
(0)
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(5)
—————————— (7)
The galvanostatic charge-discharge (GCD) studies were implemented to calculate the specific capacitance of the electrodes [35]. The GCD curves of BiYO3/RGO composite were recorded at diverse current densities ranging from 2 to 10 A g−1, and the specific capacitance value is calculated in accordance with Eq. (8)
SC =
I × Δt Fg −1 ΔV × m
(8)
The BiYO3/RGO composite electrode delivers a longer discharging time than pure BiYO3 electrode, suggesting enhanced specific capacitance values achieved by successful RGO incorporation. GCD measurements of all the prepared electrodes at different current densities (2–10 A g−1) are shown in Fig. 8. The GCD curves for a current density of 2 A g−1 are compared in Fig. 8a. The curves for different current density values for BiYO3 and BiYO3/RGO are depicted in Fig. 8a and (b) respectively. The maximum capacitance for BiYO3/RGO composite is 696 F g−1 at a current density of 2 A g−1. This value is superior to 500 F g−1 for BiYO3 at the same current density. The specific capacitance values of the BiYO3/RGO composite with increasing current densities were calculated to be 725, 292, 162, 122, and 100 F g−1, suggesting that the specific capacitance values of the electrode materials decrease with increasing current densities as shown in Fig. 9 (a) The electron transferability between the electrode surface and the electrolyte was analyzed by electrochemical impedance spectroscopy (EIS) and represented in Fig. 9 (b). The EIS studies were steered in the frequency range of 100 kHz-0.01 Hz. The Nyquist plots reveal a straight line in the low frequency region that relates to the Warburg impedance and a semicircle at higher frequencies confirming the charge-transfer resistance (Rct) [35]. The intersecting point at the real axis of BiYO3/ RGO composite is lower than pure BiYO3 revealing a reasonably lower inner resistance, due to the amalgamation of graphene nanosheets. From the EIS data, a substantial enhancement of electrochemical performance is observed for BiYO3/RGO composite compared to pure BiYO3 [36,37]. The successful accretion of RGO can provide high electrical conductivity and electron transportation ability to the system.
4. Conclusion Reduced graphene oxide sheets were well festooned by BiYO3 nanoparticles with high surface area by a simple calcination method. The specific capacitance of BiYO3/RGO composite is 696F g−1 at 2 Ag-1, and is far higher than pure BiYO3. Moreover, BiYO3/RGO composite revealed superior extended cycle life with 90.6% specific capacitance retention after 2000 charge-discharge cycles at 2 Ag-1. The BiYO3/RGO composite bids hardy energy storage ability and mesoporous BiYO3 expedites OH ions diffusion for the superior Faradaic reactions. Furthermore, the exceptional supercapacitor functioning of BiYO3/RGO composite could be indorsed to the concerted effect of graphene with large surface area and higher electrical conductivity. 7
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