High specific surface area α-Fe2O3 nanostructures as high performanceelectrode material for supercapacitors

S. Shivakumara n, Tirupathi Rao Penki, N. MunichandraiahDepartment of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

a r t i c l e i n f o

Article history:Received 8 October 2013Accepted 25 May 2014Available online 2 June 2014

Keywords:α-Fe2O3 nanostructuresSol–gel preparationPorous materialsSupercapacitors

a b s t r a c t

Porous α-Fe2O3 nanostructures have been synthesized by a simple sol–gel route. The α-Fe2O3 nano-structures are poorly crystalline and porous with BET surface area of 386 m2 g�1. The high dischargecapacitance of α-Fe2O3 electrodes is 300 F g�1 when the electrodes are cycled in 0.5 M Na2SO3 at acurrent density of 1 A g�1. The capacitance retention after 1000 cycles is about 73% of the initialcapacitance at a current density of 2 A g�1. The high discharge capacitance of α-Fe2O3 in comparisonwith the literature reports are attributed to high surface area and porosity of the iron oxide prepared inthe present study. As the iron oxides are inexpensive, the capacity of α-Fe2O3 is expected to be ofpotential use for supercapacitor application.

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1. Introduction

Energy storage has emerged to be one of the major issues duringthe development of alternative energy sources. In this view, electro-chemical capacitors (ECs) also known as supercapacitors haverecently attracted strong interests because of their higher powerdensity than batteries and higher energy density than conventionaldielectric capacitors, which make them probably the most importantnext generation energy storage device [1–3]. Various electrodematerials including porous carbon materials, conducting polymersand transition metal oxides have been widely considered for ECs.Among the transition metal oxides iron oxide has been consideredto be one of the attractive candidate due to its low cost, abundantresources, environmentally friendly nature, and better capacitorperformance [4–10]. So far, the specific capacitance (SC) of Fe2O3-or Fe3O4-based electrodes is generally reported to be within5–138 F g�1 [4–12]. It is noted that electrode materials play a keyrole in the development of high performance supercapacitors interms of morphology, specific surface area, porosity and so forth. Toenhance the desired properties, porous nanostructured materialshave been intensely investigated [7–9]. The porous nanostructuredmaterials have several advantages in electrochemical energy storagesystem, such as Li-ion batteries and ECs [7–9,13]. They are consid-ered to facilitate fast redox reactions, high specific surface areas andshort diffusion paths for electrons and ions, hence leading to highcharge/discharge rates [14]. These distinguished features of porous

nanostructured materials are expected to play an essential role inECs. Until now, α-Fe2O3 nanostructures have been prepared byvarious synthetic methods, among these methods sol–gel methodis suitable in mass production of porous α-Fe2O3 nanostructures.Compared to the conventional techniques the sol–gel route offersnumber of advantages for the production of materials can be usedfor specific applications [15,16]. The main potential fact that inor-ganic solids can be produced and processed at temperatures, whichare considerably lower than those required in conventional method.This allows the incorporation of organic molecules and polymers,leading to materials with added functionalities. The major advantageof the sol–gel method is that it produces porous materials whosepore-size distributions can be controlled, both by the chemicalcomposition of starting material, as well as by the processingconditions.

In the present work, we report a cost-effective approach toprepare porous α-Fe2O3 nanostructures by a simple sol–gelmethod. The obtained α-Fe2O3 nanostructures electrode exhibit ahigher specific capacitance with good cycling stability.

2. Experimental section

Synthesis of α-Fe2O3 nanostructures (nano-Fe2O3): All chemicalsused were of analytical grade and they were used as received.The porous α-Fe2O3 nanostructures was prepared by the sol–gelmethod. In a typical synthesis procedure, 3 g of FeCl3 �6H2Owas dissolved in 100 mL of double distilled water under magneticstirring at room temperature. To the above solution, 5 cc ofethylene glycol was added slowly with continued stirring for 1 h,

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.05.1600167-577X/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ91 80 22932240.E-mail address: elessk@gmail.com (S. Shivakumara).

Materials Letters 131 (2014) 100–103

a red transparent colloidal suspension appeared. To this solution1 M NaOH solution was added drop-wise (pH 9). The solution wasaged for 6 h on stirring at ambient conditions. The solution washeated on a hot plate at 120 1C for three hours. The resulted gelwas washed with distilled water for several times followed byethanol and dried in air at 60 1C overnight. The bulk Fe2O3 was

prepared conventionally by heating FeCl3 �6H2O at 600 1C in afurnace for 3 h under air atmosphere.

Formation mechanism of Fe2O3 nanostructures: The Fe2O3 nanos-tructures was prepared by using ethylene glycol and FeCl3 �6H2Oas starting materials. During the hydrolysis process, ethyleneglycol reacts with hydrochloric acid, which is one of the products

Fig. 1. (a) XRD patterns and (b) XPS spectrum of nano and bulk Fe2O3.

Fig. 2. SEM image of (a) bulk Fe2O3, (b) nano-Fe2O3, (c) TEM image and SAED pattern (inset) of nano-Fe2O3, and (d) typical N2 adsorption–desorption isotherm and BJH pore-size distribution curve (inset) of nano-Fe2O3.

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and it polymerized to polyethylene glycol (PEG) [15,16]. The by-product PEG, prevents the Fe2O3 nanoparticles from agglomerat-ing, just as other surfactants do. The uniform Fe2O3 nanostructureswere obtained after heating at 120 1C for 3 h.

Materials characterization: The structures of the samples wereexamined by X-ray diffraction (XRD, Bruker D8 Advance diffract-ometer) and X-ray photoelectron spectra (XPS, AXIS ULTRA). Themorphology of the samples are observed by scanning electronmicroscopy (SEM, FEI Co. model Sirion) and transmission electronmicroscope (TEM model JEOL JEM 2100F). Surface area and poresize distribution of the samples were measured using Micromeri-tics surface area analyzer (model ASAP 2020).

Electrode preparation and electrochemical measurements: Forfabrication of electrodes, Fe2O3 (75 wt%), conductive carbon (Ket-jen black, 20 wt%) and polyvinylidene fluoride (5 wt%) were mixedand ground in a mortar. Few drops of N-methyl pyrolidinone wereadded to form syrup. The syrup was coated on the pre-treatedstainless steel (SS) foil of (1 cm�1 cm) and dried at 100 1C underreduced pressure for 12 h. Coating and drying steps were repeateduntil the mass of active material 0.5–0.6 mg cm�2 is obtained.Electrochemical cell was assembled using Fe2O3 coated SS, Pt foilsand saturated calomel electrode (SCE) as the working, counter andreference electrodes, respectively in a glass container. The electro-lyte was aqueous 0.5 M Na2SO3 solution. Cyclic voltammetry (CV)and galvanostatic charge–discharge cycling were measured by aBiologic SA multichannel potentiostat/galvanostat model VMP3.The electrochemical impedance spectroscopy (EIS) measurementswere carried out (at OCP) in the frequency ranging from 0.01 Hz to100 kHz with an ac perturbation of 5 mV by Solatron frequencyresponse analyzer model 1255B and Solatron electrochemicalinterface model 1287.

3. Results and discussion

Fig. 1a represents powder XRD patterns of the nano-Fe2O3 andbulk Fe2O3 samples. The XRD patterns suggest that nano and bulkFe2O3 are amorphous and crystalline respectively, and thesepatterns match with that of α-Fe2O3 (JCPDS 33-0664) [13]. TheXPS spectrum of both nano-Fe2O3 and bulk Fe2O3 samples (Fig. 1b)shows the Fe 2p photoelectron peaks appeared around 710 and723.5 eV with a shake up satellite peak at 718.5 eV. It is thecharacteristic of Fe3þ in α-Fe2O3 [13,17].

Fig. 2a and b reveals the morphology of bulk and nano-Fe2O3

respectively. The bulk Fe2O3 sample has microsized particles whereasnano-Fe2O3 sample (Fig. 2b) is composed of numerous nearlyspherical nanoparticles with diameter of around 5–15 nm. TEM imageand the corresponding SAED pattern of the nano-Fe2O3 sample areshown in Fig. 2c, SAED pattern shows diffused pattern characteristicsof an amorphous phase. The BET measurement reveals that the bulkFe2O3 sample has a very low surface (0.234 m2 g�1) without havingany pore size distribution. On the other hand nano-α-Fe2O3 samplehas a BET surface area of 386 m2 g�1 (Fig. 2d) with a dominant poresize distribution of 9.2 nm based on BJH desorption (Fig. 2d inset),which likely benefits the electrochemical performance.

Fig. 3a compares the cyclic voltammograms (CV) of nano-Fe2O3

and bulk Fe2O3 electrodes in the potential range from �0.8 to0.0 V vs SCE at 25 mV s�1. The charge storage capacity of thenano-Fe2O3 electrode was significantly higher than that of the bulkFe2O3 electrode. The high performance of nano-Fe2O3 is attributedto the porous and high surface area. The capacitance of Fe2O3 inNa2SO3 aqueous solution may arise from the surface redox reac-tion of sulfur in the form of sulfate and sulfite anions [10].Furthermore, the possible redox reaction between FeII and FeIII is

Fig. 3. (a) Cyclic voltammograms at scan rate of 25 mV s�1, (b) specific capacitance as a function of different discharge current densities, (c) cyclic performance at a currentdensity of 2 A g�1, and (d) Nyquist plot of nano and bulk Fe2O3 electrodes.

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accompanied by intercalation of sulfite ions for charge balance theextra charge within the iron oxide layer [11,12]. Fig. 3b comparesthe SC of the two electrodes derived from the discharging curvesat different current densities. The nano-Fe2O3 electrode delivereda SC of 300 F g�1 at the current density of 1 A g�1. This value ismuch higher than that of the bulk Fe2O3 (40 F g�1 at currentdensity of 1 A g�1). The SC values of nano-Fe2O3 electrode arereasonably high in comparison with the values reported in theliterature [7–9]. Thus, specific capacitance of about 300 F g�1

measured in the present study for nano-Fe2O3 is quite attractive.We believe that the high specific capacitance retained at highcharging rate is attributed to the porous, high surface area α-Fe2O3

nanostructures, which can provide a large surface area, a shortdiffusion path to ions and electrons. The electrochemical stabilityof Fe2O3 electrodes was examined by conducting galvanostaticcharge–discharge cycling for 1000 cycles at a specific current2 A g�1 (Fig. 3c). The capacitance retention nano-Fe2O3 electrodeafter 1000 cycles is about 73% indicating the good cycling stabilityand high degree of reversibility on repetitive charge–dischargecycling. Fig. 3d shows the Nyquist plots of nano and bulk Fe2O3

electrodes. The semicircle at the high frequency region representsthe charge transfer resistance (Rct) and the spike at low frequencyregion corresponds to Warburg impedance (Zw), which representsthe electrolyte diffusion in the porous electrode. The size of thesemicircle was clearly smaller for nano-Fe2O3 electrode (4 Ω)compared to that of bulk Fe2O3 electrode (9.5 Ω). The large surfacearea of the nano-Fe2O3 electrode could be the main reason for thereduction of Rct.

4. Conclusions

The porous α-Fe2O3 nanostructures were synthesized by asimple sol–gel route. Based on the electrochemical results, theporous and high surface area nano-Fe2O3 sample exhibits excellent

supercapacitance properties than that of low surface area bulkFe2O3 sample. The nano-Fe2O3 sample exhibits higher dischargecapacitance and good cyclic stability, highlighting their promisingfunctionality as a supercapacitor electrode.

Acknowledgments

One of the authors, SS acknowledges the financial support fromthe University Grant Commission (UGC), Government of India,under Dr. D.S. Kothari fellowship programme [Ref. no. F.4-2/2006(BSR)/13-626/2012(BSR)].

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