Development of paper-based flexible supercapacitor: Bismuth ferrite/graphene nanocomposite as an active electrode material
Graphical abstract
Development of BFO/graphene nanocomposite electrode for flexible supercapacitor application. Flexibility of the BFO/graphene nanocomposite electrode (left) and Comparison of GCD curves in flat and bending modes.
Introduction
Supercapacitor has been considered as one of the most promising candidate because of its significant merits such as high power density along with other properties such as much lower maintenance cost, simple geometry and eco-friendly nature [[1], [2], [3], [4]]. Supercapacitor is now being proposed as a suitable energy storage system for flexible solid-state devices [[5], [6], [7], [8], [9]]. In this context, a large number of flexible electrodes have been investigated for supercapacitor [7,9,10]. In addition to their required flexibility, the flexible electrodes should have good electrical conductivity and high surface area.
Electrode materials are categorized into two types according to their charge storage process, 1) electrical double layer capacitors (EDLCs) [11,12] and 2) pseudocapacitors [1,13]. Different type's carbon materials are generally used as electrode materials for EDLCs where the charge storage occurs by the adsorption of electrolyte ions on the electrode surface. Metal oxides (for pseudocapacitors) store charge by faradaic reactions between electrode and electrolyte. Although pseudocapacitive materials exhibit more capacitance, the slow charging/discharging rates affect their frequency response and rate capabilities [14,15]. In other hands, the EDLCs have good charging/discharging rate but they exhibit lower capacitance than pseudocapacitors [15,16].
Many transition metal oxides such as MnO2, Ni(OH)2 and Co3O4 and carbon nanomaterials such as graphene, carbon nanotubes and carbon nanofibers have been studied as electrodes material for supercapacitor [1,[17], [18], [19]]. Recently, the well-known multiferroic material, BFO has also been tested for supercapacitor application with multiple crystallite phases i.e. BiFeO3, Bi2Fe4O9, Bi4Fe2O9,Bi3Fe5O12and Bi46Fe2O72 [20]. Therefore, this material has capability to sustain the changes in its phases during the charging/discharging process and expected to show perhaps better performance. However, comparatively poor cycle-life for transition metal oxides electrode restricts their application in supercapacitor [21,22]. Poor electronic conductivity and strong agglomeration of BFO nanoparticles could be the causes to reduce the device performance [[23], [24], [25]].
In literature, there have been reported two main strategies to improve the capacitive properties of BFO electrodes. The first one is to develop the BFO nanostructures with large surface area. In this context, C. D. Lokhande et al. [26] have prepared the nanocrystalline BFO thin film for supercapacitor in an aqueous electrolyte. The BFO film showed the capacitance of 81 F/g. The BFO nanoflakes have also been studied as electrode for supercapacitor application and a specific capacitance of 72.2 F/g has been obtained for the electrode [27].
In the second approach, the supercapacitor performance was improved by developing BFO based composites with high conductive materials [20,21]. Ayan Sarkar et al. [20] have deposited the BFO film on TiO2 nanotubes in order to improve the capacitance of the electrode. This structure enhanced the specific capacitance in the range of 350–440 F/g. However, the fabrication of this electrode requires complex steps i.e. preparation of TiO2 nanotubes at high temperature and then deposition of BFO film on the nanotubes. Sarma. at al. [21] have increased the specific capacitance by adding titania nanotubes to the bismuth oxide electrode. Thus, BFO electrodes have been prepared using different approaches with excellent capacitive properties in presence of aqueous electrolyte. The above studied have been carried out for non-flexible supercapacitor. Despite these achievements, this is also important to develop high-performance BFO-based composites for flexible supercapacitor.
Increasing the power capability of a supercapacitor without losing its energy density and long term cycle stability is a challenge [16,17,28]. The above issue can be addressed by mixing two materials of different charge storage mechanism to form a single electrode, called as hybrid structures (oxide and carbon) [19,29,30]. Thick electrodes with large mass loadings may increase the device capacitance. However, thick electrode of an oxide material increases the series resistance of the electrode and limits the power density [[31], [32], [33]]. In the present study, these issues are addressed by the synthesis of 3D structure of BFO and graphene nanocomposite. For the resulted flexible electrode, the 3D graphene network works as a highly conductive and mechanical support for BFO nanoparticles. Moreover, synergetic effect from both Bi and Fe ions may enhance redox reactions resulting in higher specific capacitance. However, the ferrite materials have low electronic conductivity, leading to poor performance in supercapacitor. Hence, it is required to synthesis nanocomposite electrode material in which the proper utilization of the combination of nano-sized BFO and graphene can be obtained.
In the present work, flexible electrodes of BFO/graphene nanocomposite have been fabricated by a facile and low temperature synthesis process. First, a sol-gel chemical route was used to synthesize BFO nanoparticles. After that, drop casting method was used to deposit BFO/graphene on a flexible substrate. The graphene sheets would give the flexibility for BFO film on a flexible substrate. The capacitive behavior was investigated in two electrodes configuration in 1 M Na2SO4 electrolyte.
Sol-gel combustion method was used to prepare BFO nanoparticles [34]. Initially, Bismuth nitrate (purity, 99.99%) was mixed in 1 N nitric acid in a glass beaker which was placed on a heating plate at temperature of 60 °C. Iron nitrate (purity, 99.99%) was added to the solution and it was stirred continuously at the same temperature. Citric acid was further added in 2:1 M ratio into the beaker, which turns the solution into light brownish. This beaker containing solution was placed at 60 °C (without stirring) for 10–12 h in order to get a thick gel. The resulted gel was kept inside an oven for a few hours, where the temperature was fixed at 220 °C. The as-synthesized BFO nanopowder was calcined at 500 °C for 3 h.
BFO/graphene electrode for supercapacitor was fabricated by drop casting method, which is relatively simple, one step and room temperature process. First, the freshly prepared BFO nanoparticles were dissolved in DI water and a small amount of polyvinyl alcohol (PVA) was also added to get a well dispersed solution of BFO. The binder (PVA) usually increases the adhesion between the electrode material and the substrate. The solution was heated at 60 °C under constant magnetic stirring. The graphene sheets were added to the solution in a fixed ratio of 1:4. This solution was used for the preparation of electrodes on two filter papers of size 2 cm × 2 cm each (Fig. 1 (a)). Two pieces of stainless steel (SS) were used as current collectors (Fig. 1 (a)). The crystal structure of the electrode was investigated by X-ray diffraction, (PANalyticalX'pert PRO)with CuKα radiations. The surface morphology of the prepared electrodes was observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM, Carl Zeiss). The different phases present in the composite were further confirmed by Raman spectroscopy (Horiba, HR 8000, Argon laser 514.5 nm).
The supercapacitor was fabricated in symmetric configuration (Fig. 1 (c)) having two same electrodes of BFO-graphene nanocomposite. Before placing the electrodes in the device, the electrodes were soaked in the electrolyte of 1 M Na2SO4. Electrochemical measurements were carried out using the Bio-Logic system (Model, SP-300). Cyclic voltammetry and galvanostatic charging/discharging methods were employed to study the capacitive properties of the BFO/graphene electrodes.
Section snippets
Results and discussion
The XRD spectrum of BFO/graphene composite film prepared on a filter paper is shown in Fig. 2. Obviously, the XRD pattern shows the presence of the phases of BFO and graphene sheet. The peak position and the relative intensity of the resultant diffraction peaks for BFO are matched well with the reported data [34,35]. The low intensity diffraction peak (002) was also observed from the disordered stacking of graphene nanosheets [15,30]. The BFO exhibits rhombohedrally distorted perovskite
Conclusion
BFO/graphene nanocomposite has been synthesized for flexible supercapacitor application. The composite of BFO/graphene was directly deposited on a filter paper via a simple and cost-effective process. The BFO/graphene nanocomposite electrode exhibited a specific capacitance of 9 mF/cm2 with a stable electrochemical potential window of 0–0.9 V. The device with BFO/graphene electrode has shown good mechanical stability for flexible supercapacitor and it retained 95% of its initial capacitance
Acknowledgments
We would like to thank Prof. V. S. Raja, Dept. of ME and MS, IIT Bombay, for providing the electrochemical measurement facility in his laboratory. We also acknowledge University of Rajasthan for SEM characterization. Dept. of physics, Siksha O Anusandhan, Deemed to be University, Bhubaneswar is also acknowledged for providing electrodes preparation facility.
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