A60 ECS Electrochemistry Letters, 2 (7) A60-A62 (2013) 2162-8726/2013/2(7)/A60/3/$31.00 © The Electrochemical Society Synthesis and Characterization of Porous Flowerlike α-Fe 2 O 3 Nanostructures for Supercapacitor Application S. Shivakumara, z Tirupathi Rao Penki, and N. Munichandraiah * Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India Porous flower-like α-Fe 2 O 3 nanostructures synthesized by an ethylene glycol mediated self-assembly process are crystalline and porous with BET surface area of 64.6 m 2 g -1 . The discharge capacitance is 127 F g -1 when the electrodes are cycled in 0.5 M Na 2 SO 3 at a current density of 1 A g -1 . Capacitance retention after 1000 cycles is about 80% of the initial capacitance. The high discharge capacitance and its retention are attributed to high surface area and porosity of the iron oxide. As the iron oxides are inexpensive, the nano α-Fe 2 O 3 is expected to be of potential use for supercapacitor application. © 2013 The Electrochemical Society. [DOI: 10.1149/2.002307eel] All rights reserved. Manuscript submitted February 20, 2013; revised manuscript received April 2, 2013. Published April 16, 2013. Electrochemical capacitors (ECs) are promising energy storage devices for portable electronics, digital communications, hybrid elec- tric vehicles and renewable energy systems, due to the advantages of high power delivery, high cycling efficiency and almost unlim- ited cycle life. 1–6 To achieve high performance from economic su- percapacitors, the exploration of novel electrode materials with high surface area, high electrical conductivity, and abundant natural re- sources is essential. Pseudocapacitive metal oxides, such as RuO 2 , 7 NiO, 8 Co 3 O 4 , 9 MnO 2 , 10 Fe 3 O 4 , 11–13 and Fe 2 O 3 14,15 have been studied as active electrode materials for supercapacitors. Among these metal oxides, amorphous hydrous RuO 2 is studied extensively because of its high specific capacitance, excellent reversibility, and long cycle life. However, RuO 2 is expensive, and therefore it has limited appli- cation in practical devices. Thus there is a strong need to develop inexpensive and environmental friendly alternative electrode materi- als for supercapacitor applications. Iron oxides are attractive from this prospective. Hematite (α-Fe 2 O 3 ) has been investigated for applications in lithium batteries, sensors, and catalysis. 16–18 It is also one of the promising materials for supercapacitors, because of its high specific capacitance, low cost and low toxicity. 14,15 However, the realized ca- pacitance α-Fe 2 O 3 is considerably lower than the value expected. Extensive efforts have been focused on controlling the nanos- tructures of α-Fe 2 O 3 for improving the capacitance properties. 14,15 However, it is important to develop simple and reliable synthetic methods for nanostructures with designed crystallographic structure and controlled morphology, which strongly influence the properties of nanomaterials. 19 Therefore, exploration of a simple and economical approach is strongly desirable for the fabrication of porous nanostruc- tures. Synthesis of porous nanostructures by simple routes remains a technological challenge. Until now, the metal alkoxide synthesis strategy for porous flower-like α-Fe 2 O 3 nanostructures and their su- percapacitor studies are not reported. In this work, the synthesis of porous flower-like α-Fe 2 O 3 nanos- tructures through iron alkoxide precursor is carried out. The prepared α-Fe 2 O 3 nanostructures consisted of number of pores distributed on the surface of the flowers. The α-Fe 2 O 3 provides a high discharge capacitance and stability over an extended cycle-life study. Experimental All chemicals used were of analytical grade and they were used as received. The iron alkoxide precursor was prepared using ethylene gly- col (EG) as reported elsewhere. 19 In a typical synthesis, FeCl 3 · 6H 2 O (4.4 mmol), urea (90 mmol), and tetrabutylammonium bromide (124 mmol) were added to 180 mL of ethylene glycol in a 250 mL round bottomed flask. The mixture was stirred with a magnetic pad- dle for 10 min to obtain homogeneous red solution. The solution was * Electrochemical Society Active Member. z E-mail: elessk@gmail.com refluxed at 195 ◦ C for 30 min and a green precipitate of iron alkox- ide was formed. After the solution was cooled to room temperature normally, the precipitate was collected by centrifugation and washed with ethanol for several times, and dried in an oven at 60 ◦ C for 12 h. The precursor sample was calcined in a muffle furnace at 300 ◦ C for 3 h in air. The resulting red powder was used for characterization studies. The microscopy studies were conducted by using scanning electron microscopy (SEM, FEI Co. model Sirion) and transmission electron microscopy (TEM, model JEOL JEM 2100F). Surface area and pore size distribution of the samples were measured using Micromeritics surface area analyzer (model ASAP 2020) and powder XRD pattern were recorded using Bruker D8 Advance diffractometer. For fabrication of electrodes, α-Fe 2 O 3 (70 wt%), conductive car- bon (Ketjen black, 20 wt%) and polyvinylidene fluoride (10 wt%) were mixed and ground in a mortar. Few drops of N-methyl pyrolidinone were added to form syrup. The syrup was coated on the pre-treated stainless steel (SS) foil of (1 cm × 1 cm) and dried at 100 ◦ C under reduced pressure for 12 h. Coating and drying steps were repeated until to get the mass of active material 0.5–0.6 mg cm -2 . Electro- chemical cell was assembled using α-Fe 2 O 3 coated SS, Pt foils and saturated calomel electrode (SCE) as the working, counter and ref- erence electrodes, respectively in a glass container. The electrolyte was aqueous 0.5 M Na 2 SO 3 solution. Cyclic voltammetry (CV) and galvanostatic charge-discharge cycling were measured by a Biologic SA multichannel potentiostat/galvanostat model VMP3. Results and Discussion Powder XRD patterns suggested that the product was α-Fe 2 O 3 (JCPDS 33-0664). The SEM image shown in Fig. 1a reveals the flower-like morphology of α-Fe 2 O 3 . The sample is composed of nu- merous flowerlike structures, with 3–4 µm in diameter. The TEM image (Fig. 1b) confirms the flower-like morphology. It is interesting to observe a higly porous structure on a high magnification image (Fig. 1c). There are innumerable pores of a few nanometer distributed on the surface of the petals, which are almost isolated from each other. The high resolution TEM (HRTEM) (Fig. 1d) and selected area electron diffraction (SAED) patterns (inset in Fig. 1d) of the sample indicates the polycrystalline nature. The separation fringes by 0.37 nm corresponds to the (012) plane of α-Fe 2 O 3 . The BET surface area is found to be 64.6 m 2 g -1 and pores are distributed 1–10 nm and centered at 6.44 nm based on BJH desorption. Electrochemical studies of α-Fe 2 O 3 were reported in several elec- trolytes, and it was found that an aqueous solution of Na 2 SO 3 is the most suitable in the view of the maximum specific capacitance measured. 11–13 Accordingly, 0.5 M Na 2 SO 3 solution was used for electrochemical characterization in the present studies also. Typical cyclic voltammograms in a potential window from -0.8 to 0.0 V vs. SCE at different scan rates (5 to 200 mV s -1 ) are shown in Fig. 2a. All voltammograms are nearly rectangular in shape, which is characteris- tic feature of ideal capacitor material. 20 The capacitance of iron oxide ecsdl.org/site/terms_use address. Redistribution subject to ECS license or copyright; see 14.139.128.11 Downloaded on 2013-06-26 to IP