Green Preparation of Fe2O3 Doped Gum Acacia Derived Porous Carbon/Graphene Ternary Nanocomposite as a Supercapacitor Electrode

*Vijayasree Haridas, Zahira Yaakob and Binitha N. Narayanan*

#### **Abstract**

The extended applications of the supercapacitor are possible with the attainment of a wide potential window since then it can exhibit high energy density too. Thus, organic electrolytes are more feasible in supercapacitors due to the accessibility of wide potential windows and the resultant higher storage/release of energy. A highperformance supercapacitor electrode material is prepared here via an eco-friendly procedure using a combination of Fe2O3, gum acacia derived porous carbon, and a ball-mill synthesized graphene for the first time. The synergistic action of the metal oxide and the carbon materials provided excellent specific capacitance values to the ternary nanocomposite. An appreciable specific capacitance of 433 F/g has been displayed by the composite coated glassy carbon electrode at a current density of 6 A/g in tetraethylammonium tetrafluoroborate—acetonitrile electrolyte at a wide potential window of 2.5 V. The material showed outstanding cyclic stability of 109% of the initial specific capacitance after 5000 repeated cycles.

**Keywords:** ternary nanocomposite, graphene, Fe2O3, acacia, green synthesis, supercapacitor

### **1. Introduction**

Supercapacitors are potential energy storage systems that revolutionize conventional energy storage devices by their exceptional energy density, power density, promising cyclic stability as well as rate capability [1–6]. Based on the charge storage mechanism, supercapacitors are broadly classified into electrochemical double-layer capacitors (EDLC), pseudocapacitors, and hybrid capacitors. The faradaic reaction is present in the pseudocapacitors, while electrostatic interaction acts in the working of EDLCs, and hybrid capacitors are a combination of both of the above [1–7]. The power density and shelf life shown by the supercapacitors are higher compared to the batteries; while the energy density is found to be lower [1–6]. To enhance the

performance, the development of modified electrodes having high specific capacitance and energy density is required without sacrificing the power density and cyclic stability of EDLC, which is a challenge for the scientific community.

The planar covalently bonded hexagonal 2D materials like graphene act as EDLC due to its surprising electrical conductivity, high surface area, etc. [8, 9]. Graphene is a perfect candidate that provides a good conducting network with a theoretical specific capacitance value of 550 F/g [10]. Similarly, porous carbon with a graphitic structure also functions as a good electrode material due to its outstanding electrical conductivity as well as high surface area [11]. The porous nature of the material additionally provides fast movement of the electrolyte ions. The preparations of such porous networks are found to be difficult as they require costly methods like chemical vapor deposition and electro-spinning methods [12–14]. Without disturbing the quality and supercapacitor performance, the development of a porous carbon network in a cost-effective manner is an interesting aspect.

Among the different methods of preparation of graphene, ball-mill-assisted exfoliation of graphite has the advantage of the easiness of preparation under mild conditions [15]. In addition, edge functionalization of graphene with milling agent provides synergistic properties advantageous in various applications. The use of naturally occurring biopolymers as milling agents is highly recommendable due to their low cost, eco-friendly nature, and easy availability [16, 17]. Here we use gum acacia for the ball-mill exfoliation of graphite and in addition, it takes the role of the precursor for porous carbon. Gum acacia, also called gum Arabic, is found in different species of Acacia, for example, Acacia arabica, Acacia babul, etc. [18]. It is a highly branched biopolymer composed mainly of high molecular weight glycosidal acid (Arabic acid) [19]. It has medicinal applications as well as it is used in food as an additive, thickening agent, emulsifier, etc. It is also used as a binder in paints, photography, printmaking, ceramics, etc. [18].

The operating voltage of a supercapacitor increases the energy density since it is related to the square of the operating voltage as evident from the equation, E = ½CV2 , where, E is the energy density, C is the specific capacitance, and V is the operating voltage [20]. Electrolytes are one of the vital parts, deciding the performance of supercapacitors. Organic electrolytes like tetraethylammonium tetrafluoroborate (TEABF4), acetonitrile (AN), propylene carbonate, etc. enhance the operating voltage and provide greater specific capacitance as well as specific energy by avoiding the complications caused by the splitting of water in the aqueous electrolytes [21]. Kesavan and co-workers developed nitrogen-doped graphene for high energy density supercapacitors in 1 M TEABF4/AN, and obtained a specific capacitance value of 103 F g−1 at a current density of 0.5 mA cm−2 [22]. Kovalska et al., performed supercapacitor studies using a gel electrolyte lithium bis(oxalate)borate in propylene carbonate displaying a capacitance of 78 μF/cm<sup>2</sup> using graphene-based supercapacitor [23]. Mostly for commercial purposes, supercapacitors are developed using organic electrolytes. But compared to the aqueous electrolyte, organic electrolytes reduce the electrolyte conductivity proceeding slower diffusion of the electrolyte to the electrode [21]. Therefore, developing a better electrode material with suitable functionalization reform the demerit caused by the organic electrolytes.

Hou et al., synthesized nitrogen-doped porous carbon nanosheets from natural silk and obtained a specific capacitance value of 242 F/g at a current density of 0.1 A/g [24]. Wang and co-workers prepared nitrogen-doped porous carbon from silkworm excrement and utilized it for high-energy-density symmetrical supercapacitor having an energy density of 138.4 Wh kg−1 and lithium-ion hybrid electrochemical capacitors *Green Preparation of Fe2O3 Doped Gum Acacia Derived Porous Carbon/Graphene Ternary… DOI: http://dx.doi.org/10.5772/intechopen.103080*

of energy density 242.2 Wh kg−1 [12]. Liu et al., synthesized graphene-like porous carbon nanosheets from salvia splendens and displayed good capacity retention from 1 to 100 A/g [25].

The introduction of metal oxides in the carbon matrix improves the supercapacitor performance due to pseudocapacitance behavior in addition to the EDLC nature of carbon species [26]. The transition metal oxides like Fe2O3, MnO2, RuO2, NiCo2O4, etc. improve the supercapacitor performance of graphene-like materials by adapting pseudocapacitive behavior [27–30]. The non-toxic nature and low cost together with high theoretical capacitance promote the use of Fe2O3 as a supercapacitor electrode material [31]. The conducting carbon network on Fe2O3 can provide excellent supercapacitor properties to the combination [31, 32].

Herein, we have developed a Fe2O3 decorated biomass-derived porous carbon/ graphene ternary nanocomposite in an eco-friendly and cost-effective manner. Environmentally benign gum acacia is used here for the exfoliation purpose. The composite was characterized and supercapacitor performance studies were done via cyclic voltammetric (CV), galvanostatic charge-discharge (CD), and electrochemical impedance spectroscopic (EIS) studies in 1 M tetraethylammonium tetrafluoroborate-acetonitrile (TEABF4/AN). The ternary composite showed excellent supercapacitor performance with a specific capacitance value of 433 F/g at a current density of 6 A/g and outstanding cyclic stability of 109% of the initial specific capacitance after 5000 repeated cycles.

#### **2. Experimental**

#### **2.1 Materials**

Graphite flake (Sigma Aldrich Chemicals India Pvt. Ltd.), FeCl3.6H2O (Loba Chemie), gum acacia (Loba Chemie), ammonia (Nice Chemicals Pvt. Ltd.) ethanol (98.5%), and polyvinyl alcohol (PVA, Loba Chemie) of reagent grade were used as such without purification. Electrolytic water (using electrolytic water purifierdouble stage water purification system—type II water, W3T324491, EVOQUA Water Technologies) was used throughout the electrochemical experiments. Deionized water was used for the material preparation.

#### **2.2 Preparation of Fe2O3/porous carbon graphene ternary composite**

For the exfoliation of graphite, the planetary ball-mill procedure was used with 9 balls of 1 cm diameter and 5 balls of 2 cm diameter. The ball to powder weight ratio is fixed to be 4:1. Forty-nine gram of gum acacia and 1 g of graphite were mixed well and the resulting mixture is then dry milled for 30 hours. To the ball-milled mixture, 75 ml of water was added and then wet-milled for 3 hours. The mixture was then recovered from the ball mill and 425 ml of water was added which was further sonicated (Bath Sonicator, 6.5 L, PCI Analytics Ltd.) for 1 hour. Centrifugation is carried out to remove unexfoliated graphite. 6.77 g of FeCl3.6H2O dissolved in 50 ml water was then added to the graphene dispersion under sonication. Ammonia solution was then added until basic pH and the obtained solution was kept overnight. Hydrothermal treatment was given to the dispersion in a tightly closed Teflon container at 120°C for 18 hours. The treated solution was transferred to a dialysis membrane (Himedia Dialysis Membrane-50, 14.3 mm diameter, 1.61 ml/cm

approximate capacity) and dialyzed using water by stirring for a day until chloride ions were completely removed. It was dried and calcined for 3 hours in a tubular furnace at 350°C in a crucible closed with aluminum foil. The ternary system is further designated as Fe2O3-PC/graphene. Binary systems without Fe2O3 and without graphene are represented as PC/graphene and Fe2O3-PC, respectively, where PC indicates porous carbon.

#### **2.3 Material characterization**

X-ray diffraction (XRD) measurements of the materials were conducted by an advanced X-ray powder diffractometer (Bruker AXS D8 with CuKα radiation 0.15406 nm) in a 2θ range of 10–90°. Fourier transform infrared (FTIR) spectra were measured using a Perkin Elmer Spectrum TwoL1600300 FTIR Spectrometer. Raman analysis was conducted to analyze the defective nature of the prepared composite using JASCO NRS-4100 Spectrometer of 532 nm wavelength laser light. To study morphology, transmission electro microscopic images were taken using a high-resolution transmission electron microscope (TEM/JEM 2100). To investigate the elemental composition and nature of bonding, X-ray photoelectron microscopic analysis was performed by X-ray photoelectron spectroscopy with Auger electron spectroscopy module (PHI 5000 Versa Prob II, FEI Inc.) with C1s as internal standard.

#### **2.4 Electrochemical measurements**

The electrochemical studies were recorded using CHI-760E Electrochemical Analyzer (CH Instruments, USA) with the techniques cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). All of the measurements were done in 1 M TEABF4/AN electrolyte using a modified glassy carbon electrode (GCE) as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode. The studies were conducted by drop coating the dispersion on GCE as prepared by sonicating 70 wt% composite, 15 wt% carbon black (Phillips Carbon Black Limited), and 15 wt% PVA in 60 vol% ethanol-water mixture. The specific capacitance values were calculated from the following equation based on CD studies [33].

$$\text{Specific capacitance} = \text{I} \Delta \text{t} / \text{A} \Delta \text{V} \tag{1}$$

Where I is the current (mA), t is the discharge time (s), ΔV represents the potential window (V), and A is the area of the electrode of the CD measurement. EIS measurements were conducted with an amplitude of 5 mV in a frequency range of 0.01–10,000 Hz.

#### **3. Results and discussion**

A ternary composite of Fe2O3 with carbon nanostructures is prepared here for its use as a supercapacitor electrode material. The composite preparation was attained via ball-mill-assisted exfoliation of graphite with gum acacia and further hydrothermal treatment of the obtained graphene dispersion with the iron oxide precursor. The as-prepared colloidal mixture is dried and heat-treated for the formation of Fe2O3

#### *Green Preparation of Fe2O3 Doped Gum Acacia Derived Porous Carbon/Graphene Ternary… DOI: http://dx.doi.org/10.5772/intechopen.103080*

embedded porous carbon on the highly conducting graphene sheets. The porous nature of the carbon derived from gum acacia can be a result of the release of gaseous materials such as H2O, CO2, etc. during the heat treatment at 350°C. The different stages in the preparation are schematically illustrated in **Figure 1** and detailed in the experimental section, from where it is clear that the preparation procedure follows a green strategy throughout.

The formation of iron species can be explained as follows. During preparation, to the graphene-gum acacia dispersion FeCl3 is added followed by ammonia. Initially, FeCl3 interacts with basic ammoniacal medium to form Fe(OH)3 precipitate as observed during preparation. In hydrothermal treatment, Fe3+ interacts with polysaccharides in the medium. The alcoholic moieties of polysaccharides exist as

**Figure 1.** *Schematic representation showing the formation of Fe2O3—PC/graphene.*

deprotonated in basic medium and these negative species coordinate with Fe3+ via electrostatic interaction forming complexes. Since, after hydrothermal treatment, a colloidal solution is obtained, the iron can exist as FeOOH covered by the polysaccharide as in agreement with reports [34]. Dialysis and further heat treatment convert the colloidal iron species to Fe2O3 as evident from various characterization techniques. In addition, during calcination, the polysaccharides from gum acacia got transferred to a porous carbon network embedding the Fe2O3 nanoparticles. These materials lay on the conductive graphene sheets to work as an efficient ternary composite electrode material.

#### **3.1 Material characterization**

X-ray diffraction patterns of the ternary composite and Fe embedded carbon shown in **Figure 2** displayed broad peaks centered around 2θ values of 24° and 43° indicating the presence of amorphous carbon [35]. Graphitic (002) diffraction is observed as a sharp band at 26.5° [36]. This band is found to be absent on the sample prepared without graphene. Weak bands around 33°, 35.7°, 54°, 62.5°, and 72° in the ternary composite as well as on iron embedded amorphous carbon indicate diffraction from the (104), (110), (116), (214), and (119) planes of α-Fe2O3 in the composites [37, 38]. The results suggest the coexistence of amorphous carbon, graphene, and α-Fe2O3 in the ternary composite.

FTIR spectra of the samples Fe2O3-PC and Fe2O3-PC/graphene (**Figure 3(a)**) show peaks corresponding to C-OH (~3400 and 1150 cm−1), C-H (2916 cm−1), C=O (1708 cm−1), CH2 (1380 cm−1), C-O-C (1056 cm−1), and Fe-O (~580 cm−1) suggesting the presence of oxygen-containing functionalities on the carbon that can bind

**Figure 2.** *XRD patterns of (a) Fe2O3/-PC graphene nanocomposite. And (b) Fe2O3-PC.*

*Green Preparation of Fe2O3 Doped Gum Acacia Derived Porous Carbon/Graphene Ternary… DOI: http://dx.doi.org/10.5772/intechopen.103080*

#### **Figure 3.**

*(a) FTIR spectra of Fe2O3/-PC graphene nanocomposite and Fe2O3-PC, and (b) Raman spectrum of Fe2O3/-PC graphene nanocomposite.*

with Fe2O3 [39–43]. The presence of aromatic carbon stretching of graphene (C=C, 1629 cm−1) is also indicated in the ternary composite [44].

Raman spectrum of the ternary composite (**Figure 3(b)**) displayed two major peaks corresponding to the D and G bands of carbon matrix at 1350 and 1580 cm−1 [45, 46]. The ID/IG value of 0.71 indicates the presence of disordered graphitic structure in the composite [47]. The broad bands around 700 and 2850 cm−1 are indicative of the hematite and graphitic 2D bands, respectively [45, 48].

The morphology of the ternary composite is investigated from the TEM images (**Figure 4**). The Fe2O3 nanoparticle embedded porous carbon structure on graphene sheets is well evident in the images. HRTEM image of the Fe2O3 nanoparticle embedded carbon displayed the lattice fringe at 0.26 nm corresponding to the (110) plane of α-Fe2O3 [49]. Graphene prepared with the assistance of gum acacia displayed a sheet-like

**Figure 4.** *(a)–(c) TEM images of Fe2O3/-PC graphene nanocomposite.*

graphene structure; in addition, torn carbon sheets are also visible with a porous texture (**Figure 5**).

XPS wide scan spectra indicate the presence of C (74.72 at %), O (22.68 at %), and Fe (2.59 at %) in the ternary composite (**Figure 6(a)**). Deconvoluted C1s (**Figure 6(b)**) show the interaction between Fe2O3 and carbon as evident from the peak at 283.69 eV. The peak at 284.8 eV indicates the sp3 C-C bond, and the aromatic C=C is confirmed by the peak at 283.99 eV [50]. The slight shift in the reported values of sp3 C-C bond may be a result of the interaction of some of the sp3 C with Fe. The presence of oxygen moieties is validated from the peak at 286.89 eV indicating the presence of C-O-C/C-OH functionalities [50]. In the XPS profile of O1s (**Figure 6(c)**), the peaks at 532.57 eV, and 531.95 eV indicate the C-O-C and C-OH groups in the ternary composite [51]. A well-specified band noticed at 530.09 eV corresponds to Fe-O-C indicating binding of Fe with carbon via oxygen [52]. The

**Figure 5.** *(a)–(c) HRTEM of Fe2O3/-PC and (d) of PC/graphene.*

*Green Preparation of Fe2O3 Doped Gum Acacia Derived Porous Carbon/Graphene Ternary… DOI: http://dx.doi.org/10.5772/intechopen.103080*

band at 530.73 eV is indicative of Fe-O in Fe2O3 [53]. The Fe-O binding in Fe2O3 and its Fe-C interaction is additionally evident from the band at 531.88 eV [54]. The peak at 532.66 eV further confirms the C-OH functionalities in the composite [55].

#### **3.2 Supercapacitor performance evaluation**

The supercapacitor performance of the materials is evaluated using CV and CD measurements. CV curves are shown in **Figure 7(a)**, which illustrate both electrochemical double-layer capacitance (EDLC) as well as the pseudocapacitive nature of the materials. The ternary composite displayed a very high current in comparison with GCE, Fe2O3-PC, as well as PC/graphene systems. This indicates a synergistic effect of the ternary system enhancing the electrochemical performance of each of the components. Both the EDLC and redox behavior are displayed in the CV curves [56, 57]. A wide potential window has been attained by the electrodes as a result of the use of organic electrolyte, which can provide high energy density to the system [58].

The charge-discharge curves of GCE, Fe2O3-PC, and PC/graphene are shown in **Figure 7(b)**. The specific capacitance value of Fe2O3-PC (77.4 F/g) was found to be lower than that of PC/graphene (83 F/g) that can be due to the lower conductivity of Fe2O3, whereas the high diffusion of electrolyte on the porous carbon and high

**Figure 6.** *XPS spectra of Fe2O3-PC/graphene nanocomposite (a) wide scan spectra, (b) C 1 s, and (c) O1s.*

conductivity of graphene are the reasons of the enhanced specific capacitance of PC/graphene [59]. The specific capacitance of Fe2O3-PC/graphene (433 F/g) measured at a current density of 6 A/g is found to be excellent as a result of the synergy between the individual components in the hybrid structure. All the systems showed an initial IR drop, but the discharge time for the ternary nanocomposite is found to be slower thereafter. The ternary composite shows good specific capacitance values of 433, 97, 88, 84, 23, 14, and 12 F/g at current densities of 6, 7, 9, 10, 25, 50, and 100 A/g, respectively. The synergistic behavior attained by the ternary composite is composed of high pseudocapacitance of Fe2O3, fast electrolyte diffusion on porous carbon, and the highly conducting high surface area graphene sheets with good EDLC [60]. The galvanostatic CD curves of Fe2O3-PC/graphene-modified electrodes at different current densities are shown in **Figure 7(c)**. As expected, the discharge time and thus the specific capacitance values decrease with an increase in the current density [61].

The stability of the electrode in repeated CD cycles was evaluated at a current density of 10 A/g to check the suitability of present ternary systems in electronic devices (**Figure 8(a)** and **(b)**). The performance of the Fe2O3-PC/graphene was slightly

#### **Figure 7.**

*(a) Cyclic voltammograms of Fe2O3-PC/graphene, Fe2O3-PC, PC/graphene and GCE in 1 M TEABF4/AN, (b) GCD curves of Fe2O3-PC, PC/graphene and GCE, and (c) GCD curves of Fe2O3-PC/graphene at different current densities.*

*Green Preparation of Fe2O3 Doped Gum Acacia Derived Porous Carbon/Graphene Ternary… DOI: http://dx.doi.org/10.5772/intechopen.103080*

#### **Figure 8.**

*(a) Capacitance retention versus the number of charge-discharge cycles, (b) comparison charge-discharge cycles of first and 5001th cycle and EIS spectrum (c), and (d) of Fe2O3-PC/graphene nanocomposite.*

improved to 109% of its initial performance after 5000 repeated runs, which is highly promising and potential quality of a supercapacitor electrode material.

EIS of the ternary composite (**Figure 8(c)**) indicates Warburg impedance and absence of a semicircle at high-frequency region (**Figure 8(d)** is indicative of the absence of charge transfer limitations in the composite electrode as well as its high conductivity [62, 63]. The inclination of the EIS plot towards the Y-axis suggested enhanced diffusion of electrolyte ions, which can be resultant of the porous nature of carbon in the composite [64].

**Table 1** indicates a comparative supercapacitor performance evaluation of the present ternary composite with other carbon-based electrodes using organic electrolytes (details of the electrode materials are given in the appendix **Table A1**). The better specific capacitance of the Fe2O3-PC/graphene is well evident from the data.


#### **Table 1.**

*The specific capacitance of carbon-based graphene nanocomposites using organic electrolytes.*

#### **4. Conclusions**

A perfectly green procedure is reported here for the preparation of a ternary system having Fe2O3, porous carbon, and graphene as constituents. Gum acacia takes the dual role of exfoliating agent to graphite and the precursor of porous carbon. The material characterization revealed the porous nature of Fe2O3 embedded carbon and the strong interaction between the components. Electrochemical studies revealed the electrochemical double-layer capacitance, pseudocapacitance, and the conducting nature of the composite leading to high specific capacitance values. Excellent cyclic stability of 109% is offered by the nanocomposite even at a high current density of 10 A/g after 5000 continuous charge-discharge measurements.

#### **Acknowledgements**

Vijayasree Haridas acknowledges UGC, New Delhi, India for UGC-SRF. The authors thank Sree Neelakanta Govt. Sanskrit College Pattambi and the University of Calicut for providing the facilities to carry out the research work. SAIF KOCHI, India is acknowledged for XRD and TEM analyses. The authors express gratitude to ACMS, IIT, Kanpur, for XPS analysis, and PSG Institute of Advanced Studies, Coimbatore for Raman analysis. Technical support for instrument purchase resulting from FIST-2016 grant of Department of Science & Technology, New Delhi, India is greatly acknowledged.

#### **Conflict of interest**

All authors declare, there are no financial/commercial conflicts of interest associated with this manuscript.


#### **Appendix**

#### **Table A1.**

*Details of various graphene-porous carbon-based modified electrodes (mentioned in Table 1 of the manuscript) used in the supercapacitor performance evaluation studies.*

*Green Preparation of Fe2O3 Doped Gum Acacia Derived Porous Carbon/Graphene Ternary… DOI: http://dx.doi.org/10.5772/intechopen.103080*

#### **Author details**

Vijayasree Haridas1 , Zahira Yaakob2 and Binitha N. Narayanan1 \*

1 Department of Chemistry, Sree Neelakanta Government Sanskrit College Pattambi (Affiliated to University of Calicut), Palakkad, Kerala, India

2 Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia

\*Address all correspondence to: binithann@yahoo.co.in

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 12**
