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1-D BiFeO3 nanowire-Reduced Graphene Oxide Nanocomposite as Excellent Supercapacitor Electrode Material Debabrata Moitra, Chayan Anand, Barun Kumar Ghosh, Madhurya Chandel, and Narendra Nath Ghosh ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00097 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Energy Materials

1-D BiFeO3 nanowire-Reduced Graphene Oxide Nanocomposite

as

Excellent

Supercapacitor

Electrode Material Debabrata Moitra,a Chayan Anand,a Barun Kumar Ghosh,a Madhurya Chandel,a and Narendra Nath Ghosh*a

a

Nano-materials Lab, Department of Chemistry, Birla Institute of Technology and Science,

Pilani K.K. Birla Goa Campus, Goa- 403726, India

1 ACS Paragon Plus Environment

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Abstract In this work, we have reported a nanocomposite, composed of BiFeO3 nanowire and Reduced Graphene Oxide (BFO-RGO), as an electrode material for high-performance supercapacitor. A facile hydrothermal method was employed to prepare BFO-RGO nanocomposite. The electrochemical measurements were performed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. The specific capacitance of BFO-RGO nanocomposite was 928.43 Fg-1 at current density 5 Ag-1, which is superior to that of pure BiFeO3. Additionally, this nanocomposite shows good cyclic stability and ~87.51% of specific capacitance is retained up to 1000 cycles. It also exhibits a high charge density of 18.62 W h kg-1 when power density is 950 W kg-1. These attractive results suggest the potential of BiFeO3 nanowire-RGO nanocomposite as an active material to construct highperformance supercapacitor electrode. To the best of our knowledge, this is the first time the application of BiFeO3 nanowire-RGO nanocomposite as supercapacitors has been reported.

Keywords: BiFeO3 nanowire-Reduced Graphene Oxide, hydrothermal method, electrochemical properties, specific capacitance, capacity retention 2 ACS Paragon Plus Environment

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1. Introduction In the last couple of years, development of electrochemical systems for energy storage and conversion has attracted immense attention to the scientists and technologists to overcome the challenges related to exhaustion of fossil fuel reserves, increase of cost of energy, and environmental pollution.

1-2

Therefore, explorations for renewable energy sources, storage of

energy through new technologies have intensified dramatically.2-4 In the current scenario supercapacitors, which are intermediate systems between conventional capacitors and batteries, are becoming attractive due to their excellent characteristics of high power density along with long life cycles.3-8 Generally, as in supercapacitors the charging or discharging process occurs in seconds, they possess lower energy density (~5 W h kg-1) than batteries. However, they are capable of providing higher power density (~10 kW kg-1) for shorter times.1, 3, 9-11 Hence, in the advanced energy storage applications (such as uninterruptible power supplies, load leveling, etc.) supercapacitors can be used as complement or replacement of batteries.1, 12 The physicochemical properties of the electrode materials largely influence the performance of electrochemical supercapacitors.

13

Electrochemical pseudocapacitor electrodes and double layer (EDL)

electrodes are the two major classes of supercapacitors.14-20 The non-faradaic process, where the accumulation of charges occurs at the interface between electrode and electrolyte, is the basic principle of EDL electrodes.3,

8, 13, 21

On the other hand, charge storage occurs at the

pseudocapacitor electrode by the fast reversible faradaic reactions.1 Carbon-based materials are generally used to construct EDL electrodes.8,

21

Recently EDL electrodes are used in some

commercial applications, such as emergency doors on an Airbus A380, etc.21 However, the lower energy density of EDL electrodes than the batteries limits their optimal discharge time to less than a minute.1 This factor limits the wide application of EDL electrodes. Transition metal



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oxides, conducting polymers are commonly used as the active materials in pseudocapacitive electrodes.1,

22-23

Metal oxides, such as RuO2, Fe3O4, and MnO2, have been well studied as

pseudocapacitor.8, 24-30 RuO2 shows relatively high specific capacitance of more than 600 Fg-1 in an aqueous electrolyte. However, the high cost of RuO2 limits its application.1 MnO2 shows specific capacitance of ~ 150 Fg-1 in the neutral aqueous electrolyte with a voltage window of < 1 V.1 However, as MnO2 does not show any oxidation states below 0 V, its use as the electrode material is very limited.1 Recently, spinel ferrites (MFe2O4 where M= Mn, Co, Ni, Zn, and Mg) have been intensively studied as active materials in pseudocapacitive electrodes, because the synergetic effect of Fe and M ions offers richer redox reactions to obtain higher specific capacitance.14,

31-35

However, the poor conductivity of ferrite-based electrodes often adversely

affects to their rate capability and super capacity performance.14 To construct high-performing supercapacitor use of graphene for immobilization of active species is an attractive strategy because graphene provides faster electron transfer paths and improved stability of the entire hybrid system. 4, 7-8, 14, 22, 27, 29-30, 36-42. In the present study, we have fabricated super capacitive electrode using a nanocomposite, which is composed of the BiFeO3 nanowire (BFO) and reduced graphene oxide (RGO), as an active electrode material. Perovskite-type BiFeO3 is an attractive material because of its multiferroic property.43-45 As bismuth iron oxide exists in five crystalline phases (i.e., BiFeO3, Bi2Fe4O9, Bi3Fe5O12, Bi4Fe2O9, and Bi46Fe2O72), this material can sustain the changes in its phase during the electrochemical changes.46 Moreover, the use of BiFeO3 nanowire as supercapacitor has not yet well explored, and only a few reports are available in the literature.4648

As the BiFeO3 thin film (super capacitance of 81 Fg-1)46 and BiFeO3 nanoflakes (super

capacitance of 72 Fg-1)49 exhibited relatively poor super capacitances, attempts have been made


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to prepare various types of BiFeO3 to improve its super capacitance. It has been reported that Cudoped BiFeO3 (568.13 Fg-1),44 BiFeO3 nanorods on porous anodized alumina (450 Fg-1) demonstrated improved super capacitance.50 However, synthesis of pure single phase BiFeO3 is one of the major challenges. In most of the reported methods, some impurity phases also form along with BiFeO3. The presence of impurity phases greatly affects the electrical property of the electrode materials.47-48 In this paper, we report the use of pure BiFeO3 nanowire (BFO) and BiFeO3 nanowirereduced graphene oxide (BFO-RGO) nanocomposite as active materials to construct super capacitive electrodes. In BiFeO3 nanowire, charge transfer occurs more efficiently via a ballistic charge transport mechanism, along with the wire axis than the polycrystalline BiFeO3, where charge transfer occurs through diffusive transport mechanism.47-48, 51-52 As pseudocapacitance is a surface or near-surface phenomena12, 53 and BiFeO3 nanowire makes the electrode surface more conducting by providing a more efficient charge transfer pathway, it has been considered that BiFeO3 nanowire would be a suitable choice as one of the components of the composite material for constructing superior supercapacitor electrode. Moreover, to the best of our knowledge till date, no study on the electrochemical properties of the BiFeO3 nanowire and BiFeO3 nanowireRGO electrode has been reported in the literature. In this work BiFeO3 nanowire-RGO (BFORGO) nanocomposites have been prepared by using a hydrothermal technique. Working electrodes were prepared by using BFO-RGO nanocomposites. Electrochemical measurements were performed by constructing a three-electrode system using cyclic voltammetry. 2. Experimental Section 2.1. Synthesis and Characterization of BiFeO3 nanowire-Reduced graphene oxide (BFORGO) nanocomposite



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We have employed a hydrothermal technique to synthesize pure BFO nanowire and BFO-RGO nanocomposite, which was developed in our lab.47-48 The synthesized BFO nanowire and BFORGO nanocomposite were characterized by using X-ray diffraction (XRD), Fourier TransformInfrared (FT-IR), Raman spectroscopy, Thermo Gravimetric Analysis (TGA), BET surface area analysis, Atomic Force Microscopy (AFM), and FESEM (details of the techniques which were used for the structural characterization are discussed in ESI†). Details of the synthesis procedure and structural characterizations of BFO nanowire and BFO-RGO nanocomposite have been reported elsewhere47-48 and are discussed in brief in ESI†. BFO-RGO nanocomposite, which has been used here for constructing electrode, is composed of 97 wt% BFO nanowire and 3 wt% RGO. From XRD analysis of the BFO-RGO nanocomposite, it has been observed that when the composites were prepared using more than 3 wt% RGO some impurity phases (such as Bi2O2CO3, Bi25FeO40, etc.) have formed.47-48, 54 So in the present study, we have used BFO-RGO nanocomposite which contains 3 wt% RGO. XRD, FT-IR, TGA, and Raman spectra of the synthesized BFO-RGO nanocomposites are shown in Figure S1, Figure S2, Figure S3, and Figure S4 (ESI†) respectively. The micrographs of pure BFO nanowire, pure RGO, and BFO-RGO, which were obtained from FESEM and AFM, are shown in Figure 1. From these micrographs, it was observed that (i) Synthesized BFO particles possess nanowire like structure with 40-200 nm diameter and length varies from hundred nanometers to several microns. (ii) In pure RGO sample agglomeration of nanometer-thin sheets occurs due to π-π interaction. (iii) In BFO-RGO nanocomposite BFO nanowires are immobilized on nanometer thin RGO. The important point noted here is that, during hydrothermal synthesis of BFO-RGO nanocomposites as BFO nanowires grow on the


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surface of RGO, the π-π interaction between RGO sheets becomes weak and this factor reduces the agglomeration of RGO sheets in BFO-RGO nanocomposite.

(A)

(B)

(C)

(D)

(E)

(F)

Figure 1 FESEM micrographs of (A) BFO, (B) individual BFO, (C) individual BFO-RGO nanocomposite, (D) BFO-RGO nanocomposite, and (E) pure RGO. (F) AFM image of BiFeO3 nanowire –RGO nanocomposite. 2.2.

Electrochemical testing

In the present study, to fabricate working electrodes pure BFO, pure RGO, and BFO-RGO nanocomposite were used as active materials. A viscous paste was prepared by mixing 10 wt% polyvinylidene difluoride, 10 wt% acetylene black and 80 wt% active materials in N-methyl-2pyrrolidinone. This paste was cast on (1.5 cm × 1.5 cm) nickel foam (thickness ~0.2 mm). The residual solvent was removed by drying at 100ºC for 12h in a vacuum oven. The weight of the active materials loaded on Ni foam was ~ 3 mg. The electrodes thus fabricated acted as a working electrode in electrochemical testing. The electrochemical measurements were conducted 7 ACS Paragon Plus Environment


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by constructing a three-electrode cell, where the as-prepared electrode, a platinum wire and an Ag/AgCl electrode were used as the working electrode, the counter electrode, and the reference electrode respectively. Electrochemical measurements were performed using (i) 3 M KOH aqueous solution, and (ii) a mixture of 3 M KOH + 0.1 M K4Fe(CN)6 aqueous solution as an electrolyte. Cyclic voltammetry (CV) and Galvanostatic charge-discharge (GCD) measurements were carried out by using a Galvanostat-Potentiostat (Ivium Technologies). All the CVs were measured between 0V to 0.42 V at different scan rate of 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mV.s-1. Galvanostatic charge-discharge measurements were carried out at different current densities of 5, 6, 7, 8, 9 and 10 Ag-1 in the potential range of 0 to 0.42 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 0.01 Hz10000 Hz at an open circuit potential with AC amplitude of 0.01 V. 3. Result and Discussion 3.1. Electrochemical Properties To explore the application of BFO-RGO electrode as a promising supercapacitor, the electrochemical characterizations of pure BFO, pure RGO, and BFO-RGO nanocomposite were performed with cyclic voltammetry (CV) and Galvanostatic charge-discharge (GCD) measurements. As the performance of a supercapacitor also depends on the nature of the electrolyte, in the present study, we have initially chosen 3 M KOH aqueous solution as the electrolyte due to its high ionic concentration and low resistance.40 Figure 2 shows the typical CV curves of BFO, RGO, and BFO-RGO electrodes at a scan rate of 10 mV s-1 in 3 M KOH electrolyte and the working potential window is between 0 to 0.42 V. From Figure 2 following important points were observed: (i) the presence of a pair of distinct redox peaks for pure BFO and BFO-RGO electrodes indicate the fast redox reaction of BFO in the composites. Hence, the


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capacity of BFO mainly originates from pseudocapacitance, (ii) for BFO-RGO electrode both the area under the CV curve and the response of peak current are higher than those of pure BFO and pure RGO. This fact indicates that the specific capacity of BFO-RGO is higher than that of BFO and RGO, which can be attributed to the synergistic contribution of BFO and RGO in the hybrid structure of BFO-RGO nanocomposite.55

Figure 2 Cyclic voltammetry curves of BFO, RGO, and BFO-RGO electrodes at scan rate 10 mV s-1 in 3 M KOH electrolyte. To evaluate the relationship between the scan rate and super capacitive performance of pure BFO and BFO-RGO electrode, the CV measurements were performed in 3 M KOH solution (Figure 3) and different scan rates, ranging from 10 to 100 mV s-1, were used. It was observed that both the peak potential and current are affected by scan rates. The increase in peak current with increasing scan rate indicates an excellent rate capability of pure BFO and BFO-RGO electrodes.7, 56 Moreover, for these electrodes, the increase of current response with increasing scan rate was observed, which might be due to the effect of scan rate on the migration of electrolytic ions and their diffusion into the electrode. At a relatively lower scan rate, a thick diffusion layer grows on the surface of the electrode, and it limits the flux of electrolytic ions towards the electrode. This factor ultimately results in the lower current. Whereas, at a higher 9 ACS Paragon Plus Environment


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scan rate the diffusion layer cannot grow on the electrode surface. Therefore, the enhanced electrolyte flux towards the electrodes leads to increase of current.43 In the CV curve, with an increase of scanning rate the shifting of both the upper and lower peaks towards positive and negative direction respectively was observed. This might be because of the development of overpotential which limits the faradic reactions.43 The Randles-Sevick plot (peak current vs. v1/2 Figure 3(C)) shows a well near linear increasing trend of the peak current when scan rate was progressively increased. This observation suggests the occurrence of a diffusion controlled redox process on the electrode surface (for pure BFO electrode, and BFO-RGO electrode).7, 12.

(A)

(B)

(C)

Figure 3 Cyclic voltammetry curves of (A) BFO and (B) BFO-RGO in 3 M KOH at different scanning rates (10-100 mV.s-1). (C) Randles-Sevick plot for BFO and BFO-RGO nanocomposite in 3 M KOH.


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To determine the performance of the BFO and BFO-RGO electrode as a supercapacitor, they were further analyzed with the help of GCD method. Using GCD profile of the electrodes, their specific capacitance was calculated by using the following equation:57

=

(1)

where i (A) represents the charge or discharge current, ∆ E (V) is the applied potential window, m (g) is the mass of super capacitive material, and ∆ t (s) is the discharge time. To calculate the specific capacitance, charge/discharge for both the electrodes (BFO and BFO-RGO composite) were measured with increasing current densities from 5 to 10 Ag-1 in 3 M KOH electrolyte, and the GCD profiles are shown in Figure 4. These GCD curves indicate that BFO electrode and BFO-RGO electrode possess good pseudo electrochemical character and typical battery like features. These curves also show symmetric nature even at a high current density (10 Ag-1). This fact suggests the high rate performance of the electrodes.58 It was also observed that galvanostatic discharge time for BFO-RGO is significantly higher than that of pure BFO (Figure 4A) as well as pure RGO (Figure S5, ESI†), which also indicates the larger charge capacity of BFO-RGO than that of pure BFO and pure RGO. These results are also consistent with the results obtained from CV scans.



(A)

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(B)

Figure 4 Galvanostatic charge-discharge curves of (A) BFO and (B) BFO-RGO electrodes at different current densities (5 to 10 Ag-1) in 3 M KOH. When pure BFO nanowire was used as an active material to construct the electrode, the highest specific capacity was reached up to 331.71 Fg-1 in 3 M KOH. However, it was observed that when the current density was increased from 5 to 10 Ag-1 the value of specific capacitance was decreased from 333.71 to 65.78 Fg-1 for this electrode. The decreasing rate of diffusion of OH‾ (electrolyte anion) into the electrode surface with increasing current density might cause this decrease of specific capacitance.7,

13

The specific capacitance value of BFO nanowire is

significantly higher than the reported values for the polycrystalline BFO thin film (81 Fg-1 at 20 mV s-1 scan rate)46 and BFO nanoflakes (72 Fg-1 at 1 Ag-1 and 33 Fg-1 at 5 Ag-1 current density).49 The nanowire-like structures of BFO nanowires might be responsible for this significant enhancement of its capacitance because in BiFeO3 nanowire charge transfer occurs more efficiently via ballistic charge transport along the wire axis than the polycrystalline BiFeO3 where charge transfer occurs through diffusive transport mechanism.47,

59

When the electrode

was prepared using pure RGO, the specific capacitance had reached a value of 104.14 Fg-1 when current density was 5 Ag-1, and the value of specific capacitance of RGO was decreased from 104.14 Fg-1 to 71.91 Fg-1 with increasing current density from 5 to 10 Ag-1. According to Stoller et al 5 the theoretical capacitance of a single layer RGO is 550 Fg-1, but in the present study much 12 ACS Paragon Plus Environment

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less value was obtained for synthesized RGO, which is consistent with the reported values by others.7 This decrease in the value of specific capacitance of synthesized RGO might be due to the agglomeration of RGO sheets because of π-π interaction.7 The agglomeration of RGO sheets was also clearly observed in FESEM micrographs (Figure 1(E)). For BFO-RGO electrode, the specific capacitance was found to be increased up to 368.28 Fg-1 at 5 Ag-1, and this value was increased from 268 to 368.28 Fg-1 when the current density was decreased from 10 to 5 Ag-1. It was also observed that the retention of initial capacity was increased from 20% (for pure BFO nanowire) to 78% (BFO-RGO) when BFO was embedded on the surface of RGO sheets in BFORGO nanocomposite. From these electrochemical studies, it was clearly observed that BFORGO electrode exhibits superior specific capacitance than that of pure BFO and pure RGO. This is because in BFO-RGO nanocomposite the BFO nanowires are immobilized on the sheets of RGO and comparatively less aggregation of RGO sheets in the composite, which was also observed in FESEM micrograph (Figure 1(D)). Electrochemical impedance spectroscopy (EIS) is one of the essential tools to determine the performance of the electrode materials as supercapacitors. The data obtained from EIS measurements were used to generate the Nyquist plots for all the synthesized materials and shown in Figure 5.

Figure 5 Electrochemical impedance spectra of RGO, BFO, and BFO-RGO electrodes in 3 M KOH electrolyte. 13 ACS Paragon Plus Environment


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In the Nyquist plot, in the high-frequency region, the presence of a semicircle signifies the charge transfer resistance at the electrode-electrolyte interface. The internal resistance (Rs) of the cell can be determined from the intercept of the semicircle on the X-axis (Z′) at the highfrequency region. Rs originates from the combination of three types of resistance in the cell (i) the ionic resistance of the electrolyte, (ii) intrinsic resistance of the electrode material, and (iii) the contact resistance between the electrode and current collector.60 The charge transfers resistance (Rct) of the electrode and electrolyte interface can be predicted from the diameter of the semicircle. From the Nyquist plots of RGO, BFO, and BFO-RGO electrodes, it was observed that Rct of pure BFO is higher than that of BFO-RGO (Figure 5). The relatively low value of Rct of BFO-RGO indicates the improvement of the charge-transfer ability of BFO-RGO because of the strong interaction between BFO with RGO in the nanocomposite. Recently from Density Functional Theory (DFT) calculation of BFO-RGO nanocomposites, we have also observed that the presence of RGO plays important role in the electronic structure of BFO and hybridization occurs between O 2p and Fe 3d states of BFO and C 2p states of graphene.48 This fact significantly improves the electrical conductivity of BFO-RGO nanocomposite compared to pure BFO.48 The radius of semicircles of BFO-RGO electrode is lower than that of BFO. This improved electrical conductivity signifies the indispensable role of RGO in BFO-RGO nanocomposite. To evaluate the effect of electrolyte on the electrochemical properties of BFO-RGO, we have further studied its electrochemical performance in the presence of an electrolyte, which is a mixture of 3 M KOH and 0.1 M K4[Fe(CN)6]. Figure 6 shows the comparison of CV curves of BFO-RGO electrode when 3 M KOH and 3 M KOH + 0.1 M K4[Fe(CN)6] were used as the electrolyte. From Figure 6 it was observed that (i) a pair of well redox peaks appeared around 14 ACS Paragon Plus Environment

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0.17/0.33 V when 3 M KOH was used as an electrolyte at 10 mV.s-1 scan rate, and the redox peaks appeared at 0.15/0.35 when 3 M KOH + 0.1 M K4[Fe(CN)6] was used as the electrolyte. (ii) the area under the CV curve and the peak current were significantly increased when 0.1 M K4[Fe(CN)6] electrolyte was added into 3 M KOH solution, (iii) CV curves for both the electrolytes indicate the fast faradaic energy storage behavior of BFO-RGO electrode.

(b) (a)

Figure 6 Cyclic voltammetry curves of BFO-RGO electrode in (a) 3 M KOH and (b) 3 M KOH + 0.1 M K4[Fe(CN)6] at scan rate 10 mV s-1. In 3 M KOH + 0.1 M K4[Fe(CN)6] electrolyte, the effect of different scanning rate (10-100 mV s-1) on the current and peak potential was also measured for BFO-RGO electrode (Figure 7 (A)). Here, an increasing trend of peak current was observed when scan rate was increased, which is an indication of good rate capability of BFO-RGO electrode in this electrolyte.7 Figure 7 (B) also shows the peak current was increased when 3 M KOH + 0.1 M K4[Fe(CN)6] was used as the electrolyte instead of 3 M KOH.



(A)

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(B)

Figure 7 At 25ºC, Cyclic voltammetry curves of (A) BFO-RGO electrode in 3 M KOH + 0.1 M K4[Fe(CN)6] at different scanning rates (10-100 mV.s-1). (B) Randles-Sevick plots of BFO-RGO nanocomposite in 3 M KOH, and 3 M KOH + 0.1 M K4[Fe(CN)6]. The specific capacitance of BFO-RGO electrode in 3 M KOH + 0.1 M K4[Fe(CN)6] was determined from GCD analysis (Figure 8). The specific capacitance value of BFO-RGO was increased with decreasing discharge current density. The highest value of specific capacitance was 928.43 Fg-1 at 5 Ag-1 current density. The values of specific capacitance were 928.43, 702, 641.60, 594.10, 560.13 and 528.68 Fg-1 when current density was 5, 6, 7, 8, 9 and 10 A·g-1 respectively. Hence, BFO-RGO electrode can retain ~57% of its initial capacitance in 3 M KOH + 0.1 M K4[Fe(CN)6] electrolyte. The increase in galvanostatic discharge time due to the addition of 0.1 M K4[Fe(CN)6] in 3 M KOH was observed. This addition of K4[Fe(CN)6] was found to be beneficial because (i) in the electrolyte system it provides a complimentary Fe(CN)64-/Fe(CN)63red-ox couple with matching potential (0.20/0.37 V) (Figure. S6, ESI†) and can act as an electron buffer source in the electrochemical reaction at the BFO-RGO electrode /electrolyte interface.12,

61-62

, (ii) as the total pore volume of BFO-RGO was found to be ~0.05 cm3g-1

(obtained from N2 adsorption-desorption isotherm study (Figure S7, ESI†) and the Born radius of 16 ACS Paragon Plus Environment

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Fe(CN)64-/ Fe(CN)63- is ~ 0.4 nm,12 the pores of BFO-RGO electrode is capable to accommodate a significant number of the redox anions. These factors enhance the reversible redox reaction on the surface or near-surface of the BFO-RGO electrode.12 Therefore, the improvement of pseudocapacitance value due to the addition of K4[Fe(CN)6 in KOH solution was observed, which was also noticed in Figure 6.

Figure 8 The Galvanostatic charge-discharge profile of BFO-RGO electrode in 3 M KOH + 0.1 M K4[Fe(CN)6] with changing current density from 5 to 10 Ag-1. Figure 9 shows the change of specific capacitance of BFO and BFO-RGO electrodes with changing current density from 5 to 10 Ag-1. From Figure 9 following important points were observed: (i) In 3 M KOH solution the specific capacitance of BFO-RGO (268.94 Fg-1) is appreciably higher than that of pure BFO (65.78 Fg-1) at 10 Ag-1 current density. (ii) The addition of 0.1 M K4[Fe(CN)6] in 3 M KOH electrolyte solution causes to increase the value of the specific capacitance of BFO-RGO electrode from 268.94 Fg-1 to 526.68 Fg-1 at 10 Ag-1 current



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density. (iii) The significantly high specific capacitance of BFO-RGO electrode in 3M KOH + 0.1 M K4[Fe(CN)6] indicates the pronounced reversibility and sustainability of BFO-RGO electrode in this electrolyte mixture.12

Figure 9 The change of specific capacitance of BFO and BFO-RGO electrodes with changing current density from 5 to 10 Ag-1. As the stability is one of the crucial factors, which influences the electrochemical performance of an electrode material, the cycling behavior of BFO-RGO electrode has been examined at a constant current density of 5 Ag-1 within the potential window of 0 to 0.42 V for 1000 cycles measurements in both the electrolytes (3 M KOH, and 3 M KOH + 0.1 M K4[Fe(CN)6]) (Figure 10). It was observed that the equilibrium was reached after first few cycles, and then a steady capacitance of 812.5 Fg-1 was achieved in 3 M KOH + 0.1 M K4[Fe(CN)6]. This capacitance value was found to be much higher than the value obtained in 3 M KOH (245 Fg-1). The retention of the capacitance for BFO-RGO electrode after 1000 cycles was found to be ~87.51 % and 66% in 3 M KOH + 0.1 M K4[Fe(CN)6] and 3 M KOH respectively. Therefore,


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the addition 0.1 M K4[Fe(CN)6] in 3 M KOH significantly enhances the recycling properties of BFO-RGO electrode as an electrochemical capacitor.

Figure 10 Cyclic stability of BFO-RGO electrode in (A) 3 M KOH + 0.1 M K4[Fe(CN)6] and (B) 3 M KOH showing the capacitance retention after 1000 cycles using a charge /discharge at constant current density 5 Ag-1. The inset is the charge-discharge curves of BFO-RGO electrode in 3 M KOH + 0.1 M K4[Fe(CN)6]. It is a well-known fact that the three electrodes electrochemical measurements are employed to determine the electrochemical behavior of the electrode materials for supercapacitors, but a two electrode test cell measurement provides more reliable data for practical applications because it mimics the cell configuration of commercial supercapacitors.7, 63 However, it is also a known fact that the specific capacitance value of an electrode when obtained from the two electrode system, is usually lower than that obtained from a three electrode system.63 Therefore, the specific capacitance of BFO-RGO electrode was also measured by constructing a twoelectrode electrochemical cell with 3 M KOH + 0.1 M K4[Fe(CN)6] electrolyte, and the value of



Page 20 of 40

specific capacitance for BFO-RGO electrode obtained was 300 Fg-1 at a current density of 5 Ag-1 (Figure 11). (A)

(B)

Figure 11 (A) Cyclic voltammetry curves at different scan rates (10-100 mV.s-1) and (B) Galvanostatic charge-discharge curves with increasing current densities from 5 to 10 Ag-1 of BFO-RGO electrode in 3 M KOH + 0.1 M K4[Fe(CN)6] in two electrode system. As the performance of a supercapacitor can be evaluated by determining the sustainability of its high energy density at high power densities, the energy density (E) and power density (P) of pure BFO and BFO-RGO electrodes were calculated from the GCD by using the following equations:7, 12 (2) (3) where E is the average energy density (W h kg-1), Cs is the specific capacitance based on the mass of the electroactive material (Fg-1), V is the potential window of discharge (V), P is the power density (W kg-1), and ∆t is the discharge time (s).


Page 21 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 12 shows the variation of energy density with changing power density (Ragone plot) for pure BFO and BFO-RGO nanocomposite in both electrolytes (3 M KOH and 3 M KOH + 0.1 M K4[Fe(CN)6]). It was observed that the energy density of BFO decreases from 6.65 to 1.31 W h kg-1 when power density was increased from 950 to 1900 W kg-1 in 3 M KOH. However, in the same range of power density, the energy density of BFO-RGO electrode varies from 7.38 to 5.39 W h kg-1. When 3 M KOH + 0.1 M K4[Fe(CN)6] was used as the electrolyte, the energy density of BFO-RGO electrode significantly enhanced and varies from 18.62 to 10.60 W h kg-1 in the same range of power density. These values are comparable and in many cases superior to those reported in the literature4, 7-8, 14, 22, 29, 36-41, 55, 64-65 and also to the commercial supercapacitors (3-9 W h kg-1 at 3000-10000 W kg-1)12 (Table 1).

Figure 12 Ragone plots of BFO and BFO-RGO electrodes



Page 22 of 40

Table 1: Comparison of different metal oxide and carbon-based nanocomposites with a similar structure for supercapacitor application. Material

Specific

Current

capacitance (Fg-1)

Electrolyte

Power

Energy

Density

Density

Density

(Ag-1)

(W kg-1)

(Wh kg1

Reference

)

163.8

1

6 M KOH

-

-

4

CuFe2O4-GN

576.6

1

3 M KOH

1100

15.8

7

Graphene-NiFe2O4

345

1

1 M Na2SO4

-

-

8

ZnFe2O4/NRG

244

0.5

1 M KOH

3000

6.7

14

NiCo2O4/RGO

947.4

0.5

3 M KOH

-

-

22

Fe3O4@carbon

586

0.5

PVA-KOH

351

18.3

29

400

5.0

36

Co3O4-Reduced Graphene oxide

nanosheet MnFe2O4/Graphene

as electrolyte 120

0.1

(PVA)-H2SO4 gel

767.7

0.1

1 M KOH

178.2

79.7

37

Graphene Oxide/Co3O4

472

0.5

2 M KOH

8300

39.0

38

Manganese

307.2

0.1

1 M KOH

-

13.5

39

Cobalt Ferrite/Graphene/ Polyaniline

ferrite/Graphene/


Page 23 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Polyaniline

ZnCo2O4/Reduced

1256

3

6 M KOH

7492.5

62.8

40

1985

2

1 M KOH

392

54.0

41

77.76

30

0.5 M KOH

4.32

0.89

43

Graphene oxide/NiO Composite Film Porous Nickel HydroxideManganese DioxideReduced Graphene Perovskite Y2NiMnO6

mA g-1

manowire BiFe0.95Cu0.05O3

568.13

1

6 M KOH

-

-

44

5% Ni doped BiFeO3

513.5

1

6 M KOH

-

-

45

Perovskite

81

1 mA

1 M KOH

3.2958

6.6832 J

46

Wg-1

g-1

cm-2

nanocrystalline BiFeO3 Mixed-phase bismuth

72

1

2 M NaOH

-

-

49

440

1.1

0.5 M Na2SO4

1.2 kW

46.5

53

ferrite nanoflakes BiFeO3 Anchored TiO2

kg-1

nanotube array Co(OH)2/Graphene

532

2

6 M KOH

3500

24.3

55

RuO2/Graphitic‑C3N4

704.3

0.5

6 M KOH

4500

2.75

64

@ Reduced Graphene



Page 24 of 40

Oxide Aerogel

3D Fe3O4/rGO hybrids

455

3.6

2 M KOH

2740

80.9

65

Bismuth sulfide

396 F g-1

1

2 M KOH

-

-

66

2 M KOH

-

-

67

1 M H2SO4

-

-

68

-

-

69

nanorods-reduced graphene oxide composites NiCo2O4 nano-needle

1118.6

mA cm-2

arrays MoO2 Nanowire

5.56

140

1 mA cm-2

ultrafine MnO2

321.3

1

0.5 M Na2SO4

933

1 mA

1 M NaCl

nanowire arrays (NWA) directly grew on a Carbon Fiber GO/PPy nanowires

70

cm-2 CuCo2O4 nanowire

714 mF

1 mA

arrays on Ni wire

cm-2

cm-2,

3M KOH

0.4757

94.3 Wh

mW cm-

cm-2

71

2

Perovskite-type

747.75

2

1 M Na2SO4

400

34.8

72

Lanthanum Cobaltate Nanofibers with Sr-


Page 25 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


substitution (i) Graphene–PANI

(i) 724.6

nanowire

(ii) 602.5

1

1.0 M H2SO4

-

-

73

74

(ii) Graphene–PANI nanocone PANI orderly

130.4 mF

0.1

(PVA)-H2SO4

2.74

24.31

nanotubes

cm-2

mA cm-2

gel

mW cm-

mW h

2

.

cm-2

1090

0.1

2 M H2SO4

43

97

75

PANI-CNTs@ZIF-67-

162.5 mF

0.5 mA

3 M KCl

-

-

76

CC

cm-2

cm-2

3D-KSPC/Fe3O4-DCN

285.4

1

2 M KOH

-

-

77

BiFe0.95Co0.05O3

278.2

1

6 M KOH

-

-

78

5

3 M KOH +

950

18.62

Present

KSC/NCNTs/PANI nanocomposite

BFO-RGO

928.43

0.1 M

work

K4[Fe(CN)6]

To demonstrate the real application of BFO-RGO electrode as a supercapacitors device, first for charging two symmetric cells, which are connected in series, was connected to a 9V battery for 10 min. After charging the cell, a red-light-emitting diode (LED) (1.8 V) was connected and was observed to successfully light up the LED for 6 min (Figure13).



(A)

Page 26 of 40

(B)

2 min

0 min

(C)

(D)

4 min

6 min

Figure 13 A red-light-emitting diode (LED) (1.8 V) powered by two BFO-RGO capacitors connected in series. Glowing LED at a different time interval. Several factors are responsible for the high supercapacitive performance of BFO-RGO nanocomposite: (i) the nanowire structure of BFO contributes to the enhancement of charge transportation via ballistic mechanism along with the wire axis, (ii) presence of RGO in BFORGO nanocomposites helps to increase the electrical conductivity and faster electron mobility. It also provides a high surface area matrix to host the BFO nanowires, (iii) as during hydrothermal synthesis of BFO-RGO nanocomposites formation of BFO occurs on the surface of RGO sheets, the π-π interaction between RGO sheets becomes weak and this factor reduces the chance of agglomeration of RGO sheets and increases the interplaner spacing which makes the RGO sheet more accessible to the electrolyte, (iv) moreover, in our previous study

48

where we have

investigated the electronic structures of BFO, graphene, and BFO-RGO nanocomposite by First principles quantum mechanical calculations based on density functional theory, we have 26 ACS Paragon Plus Environment

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observed that a significant amount of orbital overlap exists in the hybridized structure of BFORGO nanocomposites. The inclusion of graphene introduces several new electronic states just above the Fermi level which causes strong electronic interaction in the interfaces between BFORGO in the nanocomposites. Due to these synergistic effects between nanowire and RGO, BFORGO nanocomposites exhibit excellent super capacitive performance. 4. Conclusion In summary, here we have demonstrated the application of BiFeO3 nanowire-RGO nanocomposite as a high-performing supercapacitor. Benefiting from the hierarchical architecture of BFO-RGO nanocomposite, where the BFO nanowires are immobilized on RGO surface, the synthesized nanocomposite exhibits outstanding capacitive and cyclic performance as the electrode material in a supercapacitor. At 10 Ag-1 current density BFO-RGO nanocomposite shows a maximum specific capacitance of 526.68 Fg-1 and up to 1000 consecutive charge-discharge cycles, ~ 87.51% retention of the specific capacitance was also observed. To the best of our knowledge, the supercapacitive property BFO-RGO nanocomposite is reported for the first time. The excellent cyclic stability along with high energy density of 18.62 W h kg-1 at a power density of 950 W kg-1 makes BFO-RGO nanocomposite an excellent electrode material for supercapacitor application. ASSOCIATED CONTENT Supporting Information Figure S1 Room temperature wide angle powder XRD pattern of BFO-RGO nanocomposite, Figure S2 FT-IR spectra of BFO-RGO nanocomposite, Figure S3 TGA curve of BFO-RGO nanocomposite, Figure S4 Raman spectra of BFO-RGO nanocomposite, Figure S5 Galvanostatic charge-discharge curves of RGO electrode at different current densities (5 to 10 Ag-1) in 3 M 27 ACS Paragon Plus Environment


Page 28 of 40

KOH, Figure S6 Cyclic voltammograms of supercapacitor cell constructed with 3 M KOH + 0.1 M K4[Fe(CN)6] as electrolyte using bare Ni foam as working electrode at different scan rates ranging from 10 mVs-1 to 100 mVs-1, Figure S7 N2 adsorption-desorption isotherms and (inset) pore size distribution of synthesized BFO and BFO-RGO nanocomposite, This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Corresponding author. Tel. /fax: +91 832 2580318/2557033. *E-mail address: [email protected] (N. N. Ghosh) ACKNOWLEDGEMENTS Dr. Ghosh is thankful to Central Sophisticated Instrumentation Facility (CSIF) of BITS Pilani K K Birla Goa campus for providing FESEM facility and DST-FIST for providing a fund for AFM facility at the Department of Chemistry, BITS Pilani K K Birla Goa campus. References (1) Simon, P.; Gogotsi, Y., Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7 (11), 845-854. (2) Zhang, S.; Pan, N., Supercapacitors Performance Evaluation. Adv. Energy Mater. 2015, 5 (6), 1401401. (3) Bashir, B.; Shaheen, W.; Asghar, M.; Warsi, M. F.; Khan, M. A.; Haider, S.; Shakir, I.; Shahid, M., Copper Doped Manganese Ferrites Nanoparticles Anchored on Graphene Nano-


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