Mixed-phase bismuth ferrite nanoflake electrodes for supercapacitor application
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Nanoflake bismuth ferrite thin film was synthesized by means of electrodeposition technique at room temperature. The morphology and phase evaluation of the synthesized electrode were analyzed using scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and surface wettability techniques. Specifically, the bismuth ferrite nanoflake electrode exhibited high specific capacitance of 72.2 F g−1 at a current density of 1 A g−1, and high rate capability with 37 % retention of capacitance even up to 20A g−1, and excellent cycling stability with 82.8 % retention of the initial capacitance after 1500 charge/discharge cycles, supporting that the bismuth ferrite thin-film electrode could be a potential candidate for supercapacitor application.
KeywordsElectrodeposition Nanostructure Ferrite Electrochemical supercapacitor
Now-a-days, fast-growing market for portable electronic devices such as mobile phones, laptops, notebook computers, uninterruptible power supply, and the development of hybrid electric vehicles, etc., is increasing and there is an urgent demand for environmental-friendly high-power energy storage resources. Also rapid growth of research in the field of electrochemical energy storage systems has been driven by the increased development of nanostructured materials. Electrochemical supercapacitors have attracted great attention because of their ability to deliver high power and energy densities with outstanding cycling stability. A supercapacitor is an assuring energy storage device, which can function as a gap-bridging device between batteries and conventional capacitors (Chu and Majumdar 2012; Winter and Brodd 2004; Zhai et al. 2011). Supercapacitive performance of a material either in the form of a thin-film or of a pallet, used as an electrode can be assayed by cyclic-voltammetry (CV) and galvanostatic charge–discharge measurements. An electrode is judged by its specific capacitance value and the number of charge–discharge cycles it withstands, maintaining the constancy of capacitance. This brings forth the search for a wide variety of materials. In general, transition metal oxides, including RuO2, MnO2, NiO, Co3O4, SnO2, ZnO, TiO2, V2O5, CuO, Fe2O3, and WO3 (Lang et al. 2011; Athoue et al. 2008; Yan et al. 2010; Chen et al. 2012; Luo et al. 2014; Liu et al. 2014; Sathiya et al. 2011; David et al. 2014; Brezesinski et al. 2009; Wang et al. 2013; Liang et al. 2010; Xia et al. 2014; Kang et al. 2013; Yu et al. 2014), etc., have demonstrated high specific capacitance values with their redox reaction behaviors.
It is difficult to develop the pure phase of BFO, i.e., BiFeO3, as while forming BFO, other thermodynamically more stable phases such as Bi2Fe4O9, Bi46Fe2O72, Bi25FeO40, Bi24Fe2O39, and Bi36Fe2O57, etc, are generally dominant.
In general, the platinum crucible is needed for its preparation, which can be used only for three or four times. This increases the sample preparation cost, and,
during the deposition, the substrate must be placed at the elevated temperatures; therefore, this causes restrictions on the selection of the substrate material.
Single-phase BFO powder was prepared by oxide-mixing technique followed by leaching with dilute nitric acid to eliminate unreacted impurity phases (Kumar et al. 2000). Palkar and Pinto (2002) have reported the synthesis of highly resistive thin film of pure BFO phase using pulsed laser-deposition method. Epitaxial growth of BFO phase, i.e., Bi3Fe5O12, has been prepared by a reactive ion beam sputtering technique by Adachi et al. (2000). BFO nanostructures including nanofibers (Mohan and Subramanian 2013), nanowires (Das et al. 2013; Gao et al. 2006), nanorods (Xie et al. 2008), nanoparticles (Chen et al. 2006; Biasotto et al. 2011; Jadhav et al. 2013), microcubes, and nanoplates (Waghmare et al. 2012) have already been reported. These nanostructures are expected to offer better efficiency owing to their large surface areas giving rise to high values of specific capacitance. Electrodeposition is a powerful and interesting methodology that can be applied in numerous fields for synthesizing thin metal oxide/chalcogenide thin films or coatings. It has been used for the preparation of thin and thick films of iron group metal oxides at relatively low temperatures (Sartale et al. 2004; Mo et al. 2000; Gujar et al. 2006). Lokhande et al. (2007) obtained BFO films with a specific capacitance of 81 F g−1 using electrodeposition method. Dutta et al. (2013) reported remarkably a high value of specific capacitance of 450 F g−1 for BFO nanorods prepared by a wet chemical template method.
In the present study, BFO structures of mixed phases were synthesized onto conducting substrate using electrodeposition technique at room temperature. These electrodes were further envisaged for their structural elucidation and morphological confirmation studies. Different preparative parameters including the effective potential window, the pH of bath composition, and the temperature effects were optimized so as to get optimal supercapacitive performance. The mechanisms of reduction and oxidation reactions in the electrolyte were studied for different current densities and CV curves. The effects of bismuth nitrate and ferric nitrate concentrations on structural, morphological, surface wettability, and supercapacitive properties were explored. The effects of electrolyte concentration and scan rate on electrochemical properties of BFO were also investigated. In addition, charging–discharging, Raman analysis, and impedance characteristics of BFO electrodes were studied.
Sample synthesis and characterizations
BFO thin film electrodes were prepared using analytic grade chemicals and double-distilled water. The bath solution was made up of 0.2 M bismuth (III) nitrate (Bi (NO3)3·5H2O), and 0.2 M ferric (III) nitrate (Fe (NO3)3) with 0.2 M tartaric acid (C4H6O6) as a complexing agent, and the pH value of ~12 was maintained through the addition of sodium hydroxide (NaOH) aqueous solution. BFO thin films were electrodeposited onto precleaned SS substrate under galvanostatic mode using CP at 3 mA for 300 s. At room temperature, electrodeposited sample was black in color. These films were then air annealed at 400, 500, and 600 °C temperatures for 2 h. Reddish films were seen with naked eyes on the SS substrate after annealing at 400 and 500 °C. At 600 °C, the sample again turned black in color. The mass of active BFO electrode material was around 1 mg cm−2.
Mixed-phase BFO electrodes were examined for their morphologies and structures. X- ray diffractometer (Rigaku D/MAX 2500 V, Cu Kα, λ=1.5418 Å) was used for obtaining X-ray diffraction (XRD) spectra, and for evaluating surface morphologies, field-emission scanning electron microscopy (FE-SEM, Hitachi S-4200) images were recorded. Raman shift measurement was taken in order to confirm ferrite phase transformation with the increasing temperature. Contact angle measurements on as-deposited film electrode surfaces were carried out by sessile drop method, in which water drop was observed through a microscope-coupled goniometer (Phoenix 150, Surface Electro Optics, Korea). For electrochemical measurements, 1 × 1 cm2 was defined as the active electrode surface area for each measurement in the presence of platinum (Pt) as counter electrode and Ag/AgCl as reference electrode in 1 M NaOH. A frequency range of 0.01 Hz–1.5 MHz and an amplitude of 50 mV were applied during the EIS analysis.
Results and discussion
Cyclic voltammetry (CV) measurement was employed to explore the BFO electrodes for electrochemical supercapacitor application with NaOH electrolyte of different concentrations in a conventional three-electrode system.
To further investigate electrochemical performances of the nanoflake BFO electrode, we perform galvanostatic charge–discharge (GCD) curves at various current densities in an electrochemical window from −0.8 to 0.2 V (Fig. 7b). There was a nontriangular symmetry and linear slopes, consolidating the good pseudocapacitive behavior. The capacitance within the voltage region mainly resulted from the reduction of Fe3+ species to Fe2+ and the presence of oxygen vacancies for charge compensation (Qin et al. 2011). The capacitance values of two electrodes at various current densities were calculated based on their corresponding GCD curves and are plotted in Fig. 7c. The specific capacitance values were 72.2, 54.0, 41.5, 36.3, 33.0, 30.0, and 26.7 F g−1 at the current densities of 1, 2, 3, 5, 10, 15, and 20 A g−1, respectively. These excellent performances could be attributed to the nanoflakes and high specific surface areas which facilitated fast ion transfer by enhancing redox faradic reactions. This indicates that about 37 % of capacitance still remained as the current density increased from 1 to 20 A g−1, suggesting the superb rate capability. Figure 7d shows the cycling stability of the as-prepared BFO nanoflake-type electrodes by conducting charge/discharge tests at a current density of 5 A g−1 for 1500 cycles. The specific capacitance of the BFO electrode maintained 82.8 % of its initial value, indicating a good stability. The charge–discharge curves of the final 10 cycles are shown in the inset of Fig. 7d, showing almost the same symmetric shape, which implies that the BFO electrode remain unchanged for significant structural change during the charge/discharge processes.
The EIS measurement of the BFO electrode obtained after air annealing at 600 °C is presented in Fig. 7e. Capacitance can be interpreted as a measure for electrode activity, surface coverage, and the state-of-charge of an energy storage. The frequency response of capacitance reflects the amount of the surface area accessible to the electrolyte. Capacitance at high-frequency region shows the outer surface, which may depend on grain boundaries and other interparticle phenomena. Capacitance at low frequencies highlights the inner surface, which is primarily determined by the pore-size distribution and the speed of ion transport through the porous electrode (Qin et al. 2011). The plot at the high-frequency side was parallel to the X-axis whereas the subsequent regions were inclined to the X-axis suggesting that the charge transfer resistance rates at high-frequency regions was faster than those at low-frequency regions which could be due to an inherent property of ferrites.
In this study, we have demonstrated a simple and economical electrodeposition process to synthesize bismuth ferrite (BFO) for supercapacitor application. By changing the electrodeposition composition/condition, structure and morphology were greatly altered. Effects of annealing on BFO electrodes for 400, 500, and 600 °C were studied for evaluation of phase and surface morphology, if there is any. The cyclic voltammetry parameters of BFO electrodes were significantly enhanced in NaOH electrolytes than in other aqueous electrolytes. The maximum specific capacitance was obtained in 2 M NaOH electrolyte; therefore the same electrode was employed in the rest of the measurements. At 600 °C, BFO electrodes were superhydrophilic in nature, with the lower contact angle being beneficial for achieving enhanced specific capacitance. The effect of scan rate on supercapacitive properties of annealed BFO thin-film electrode was reported. The maximum specific capacitance of electrode annealed at 600 °C using galvanostatic charge–discharge curve was found to be 72.2 F g−1 at 1 A g−1 in 2 M NaOH electrolyte.
The authors wish to thank the Department of Science and Technology (DST), New Delhi, India, and the NRF, South Korea for selecting them in Indo-Korean Research Internship (IKRI) project to carry out the research for 12 months in Pusan National University, Busan, South Korea. Authors, R. S. Mane and M. Naushad, extend their gratitude to the Visiting Professor (VP) Unit of King Saud University (KSU) for the financial support.
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