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A novel Cr 2 O 3 -carbon composite as a high performance pseudo-capacitor electrode material Article in Electrochimica Acta · May 2015 DOI: 10.1016/j.electacta.2015.04.179
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Electrochimica Acta 171 (2015) 142–149
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A novel Cr2O3-carbon composite as a high performance pseudo-capacitor electrode material Shaheed Ullah a , Inayat Ali Khan a , Mohammad Choucair b , Amin Badshah a , Ishtiaq Khan a , Muhammad Arif Nadeem a, * a b
Catalysis and Nanomaterials Lab 27, Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan School of Chemistry, University of Sydney, Sydney, Australia 2006
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 February 2015 Received in revised form 1 April 2015 Accepted 1 April 2015 Available online 7 May 2015
Novel graphitic material containing chromium oxide (Cr2O3) nanoribbons is obtained by carbonizing a mixture of polyfurfuryl alcohol and MIL–101(Cr) at 900 C. Morphological, structural, and chemical analysis of the product is carried out with HR–TEM, SEM, XPS, XRD, and BET surface area. The maximum BET surface area recorded for the nanocomposite is 438 m2 g1. The nanocomposite exhibits a specific capacitance as high as 300 F g1 at 2 mV s1 and 291 F g1 at 0.25 A g1, and presents 95.5% long–term cycling stability over 3000 cycles. The pseudo-capacitive role of Cr2O3 nanoribbons is found to be important towards total capacitance of nanocomposite material. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: capacitance polyfurfuryl alcohol nanoribbons nanocomposite
1. Introduction Electrochemical supercapacitors are energy storage devices that have found wide spread applications in modern electronic equipment due to their high power output and long life cycle compared to modern secondary batteries [1,2]. Supercapacitors work on the principle of double layer capacitance at the electrode– electrolyte interface. Electric charges are compiled along the electrode surface and ions of opposite charge are arranged in the electrolyte solution side. To evaluate the capacitive performance of a capacitor it is necessary to realize the interface properties of the electrode–electrolyte double layer, including the interface structure, charge transport, and modes of ion diffusion. Carbon materials are typically employed as supercapacitor materials due to their high conductivity and high specific surface area and have included; carbon nanotubes (CNTs) [3–8], activated carbon [9], carbon onions [10], carbon fibers [11] and graphene [12,13]. The electrochemical charge storage capacity of the carbonbased electrode materials can be further enhanced by decorating the surface with metal oxides like CuOx, RuO2, NiO, MnO2 [14–19] and conducting polymers [17,20–22]. However, decorating the carbon surface with electro-active species is a tedious, multistep and non-reproducible process in some cases. Very recently, the
* Corresponding author.: +925190642062. E-mail address:
[email protected] (M.A. Nadeem). http://dx.doi.org/10.1016/j.electacta.2015.04.179 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
heat treatment of metal-organic frameworks (MOFs) to convert them into functional materials is area of focus [23]. The heat treatment of MOFs can provide single source for the synthesis of carbon material as well as electroactive specie. In recent years, Zn–based metal–organic frameworks have been used to obtain pure porous carbon materials. In principle, the MOF was used as a ‘template’ which was treated with furfuryl alcohol as the primary ‘carbon precursor’ that ultimately underwent polymerization and carbonization. The selection of different MOFs (MOF–5, ZIF–8, Al–PCP, ZIF–68, ZIF–69, ZIF–70, ZIF–7) [24–27] as templates for the synthesis of porous carbon is often based on the whether pores within the MOF can accommodate the carbon precursor guest molecules. For example, furfuryl alcohol (molecular size 8.43 6.44 4.28 Å3 [24,28]) would permit host-guest interactions with a range of MOFs, including MIL–101(Cr). MIL–101 (Cr) has unique characteristics; with mesoporous architecture, a huge cell aperture (702000 Å3), large free aperture (12 16 14.5 Å3), mesoporous cages (29 Å and 34 Å) and very high surface area (SALangmuir = 5900 300 m2 g1) [29]. It is possible to produce Cr2O3 by the thermal treatment of the MIL– 101(Cr) framework at ca. 400 C, with the sequential removal of the organic linker and oxygen [30]. Mutyala et al. [31] also decomposed MIL -101(Cr) at 300 C to ca. 50 nm particles of Cr2O3 on carbon, which were tested as a catalyst for the dehydrogenation of 1–phenyl ethanol to styrene. However, the electrochemical capacitive performance of Cr2O3–carbon composites remains unexplored [32a], probably due to difficulties in obtaining
S. Ullah et al. / Electrochimica Acta 171 (2015) 142–149
crystalline, conductive and highly ordered mesoporous structures [32b]. Here we synthesize Cr2O3–carbon porous nanocomposites comprised of turbostratic graphite and highly crystalline Cr2O3 nanoribbons by the one-step carbonization of a mixture of MIL– 101(Cr) and furfuryl alcohol. MIL–101(Cr) acts as carbon and Cr2O3 source as well as template to accomodate furfuryl alcohol (FA) as primary carbon source. The FA polymerized into polyfurfuryl alcohol (PFA) inside the template and converted into a crystalline graphitic material through carbonization at elevated temperature [33]. The obtained nanocomposite was characterized by a range of instrumental techniques and evaluated as an electrode material for supercapacitors. Electrochemical performance of the nanocomposite is attributed to high porosity, highly accessible surface area allowing for ion mobility and redox behavior of Cr2O3 nanoribbons present in the nanocomposite material. Given the available range of porous MOFs having electroactive metal oxide units, it is possible for the readers to synthesize and explore the metal oxide decorated carbon materials for future capacitor applications. 2. Materials & Methods 2.1. Syntheses 2.1.1. MIL–101(Cr) Chromium nitrate nonahydrate [Cr(NO3)3.9H2O (2.5mmol)], terephthalic acid [C8H6O4 (2.5 mmol)] and hydrofluoric acid [HF (2.5 mmol)] were dissolved in deionized water [H2O (280 mmol)]. The resulting solution was mixed and introduced into a Teflon lined autoclave. The autoclave was kept at 220 C for 8 h. The product, a green powder, was vacuum filtered and washed several times then dried in an oven at 70 C. The product XRD spectrum matched that of MIL–101(Cr) (Figure S1x) [29]. 2.2. Furfuryl alcohol /MIL –101(Cr) mixture MIL–101(Cr) [Cr3F(H2O)2O[(O2C)C6H4–(CO2)]3.nH2O] was vacuum dried at 200 C and 280 mbar using a Fistreem furnace with Vacuubrand 2008/05 vacuum pump for 12 h to remove the solvent present in the pores. The evacuated MIL–101(Cr) (0.5 g) was mixed with furfuryl alcohol (5 mL) while stirring for 12 h to ensure complete saturation. After filtration, the furfuryl alcohol/MIL-101 (Cr) mixture was washed with absolute ethanol to remove furfuryl alcohol adsorbed on the surface. 2.3. Carbonization of furfuryl alcohol / MIL–101(Cr) mixture The furfuryl alcohol/MIL–101(Cr) mixture was transferred into a ceramic boat which was placed in a quartz tube fixed in a tube furnace (Nabertherm B 180). Initially air was flushed away by continuous flow of Ar for 30 min followed by heating at 150 C for 24 h to carry out the polymerization of furfuryl alcohol inside the pores of MIL–101(Cr) template. The resulting material, polyfurfuryl alcohol/MIL–101(Cr) was kept under Ar atmosphere. Carbonization was carried out at 900 C, for 6 h to obtain Cr2O3–carbon nanocomposite and the resulting sample is from herein referred to as Cr2O3/C 900. The carbonization of MIL–101(Cr) alone at the above mentioned temperature results in the formation of chromium oxide only, as evidenced by PXRD analysis, which confirmed the furfuryl alcohol as major source of carbon in the composite material (Figure S2x).
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a speed of 0.015 s1 with Cu Ka (l = 1.544206 Å) radiation generated at 40 kV and 30 mA. X–ray photoelectron spectroscopy (XPS) measurements were conducted with ESCALAB250Xi (Thermo Scientific, UK) instrument having background vacuum better than 2 109 mbar. Monochromated Al Ka (energy hn = 1486.68 eV) as the radiation source and power of 164 W (10.8 mA and 15.2 kV) with a spot size of 500 mm were used during analysis. The photoelectron take–off angle was 90 and the pass energy was 100 eV for the survey and 20 eV for region scans. Curve fitting was performed using the Scienta ESCA300 data–system software. N2 adsorption/desorption measurements were carried out using the Accelerated Surface Area & Porosimetry System 2020 supplied by Micromeritics Instruments Inc. Approximately 147 mg of sample was loaded into a glass analysis tube and outgassed for 3 h under vacuum at 200 C prior to measurement. The isotherm was measured at 77 K and data was analyzed using Bruner–Emmett–Teller (BET) model to determine the surface area. Scanning electron microscopy (SEM) was performed using JEOL– JSM–6610LV equipped with energy dispersive X–ray (EDX) machine. Transmission electron microscopy (TEM) and SAED analysis were conducted on a JEOL2100 operated at an accelerating voltage of 200 kV.
2.5. Electrochemical studies For capacitance studies the electrode was prepared by mixing Cr2O3–carbon nanocomposite with 5 weight percent (wt.%) Nafion (binder) and the slurry was pressed between two pieces of nickel foam (1 cm 1 cm) under 350 kg cm2 pressure using a hydraulic presser (EQ–HP–88V220) and the electrode was dried at 100 C. The electrode designed for electrochemical studies contained ca.10 mg of active material. The electrode was impregnated with electrolyte solution overnight to ensure the thorough wetting of material with electrolyte. The electrode was subjected to electrochemical studies in a three electrode cell assembly using 6 mol L1 KOH solution as the electrolyte. The cell assembly consist of the sample pressed in nickel foam as the working, platinum wire as the counter and Ag/AgCl as the reference electrodes respectively. The electrochemical measurements were carried out using potentiostate/galvanostate (Biologic SP 300) electrochemical analyzer at room temperature. The following equation was used to calculate the specific capacitance (F g1) from CV results; Csp = (DQ)/ (DV m)
(1)
Where Csp stands for specific capacitance and ‘DQ’ is the charge integrated from the whole voltage range in coulombs, ‘DV’ is the whole voltage difference in volt and ‘m’ is the mass in gram of the material on an electrode. Similarly, from galvanostatic charge/ discharge measurements the following equation was used to calculate the specific capacitance (F g1); Csp = (I Dt)/(DV m)
(2)
Where ‘I’ is the constant discharge current ‘Dt’ is the discharge time ‘DV’ is the voltage difference with in the discharge time Dt and ‘m’ is the mass in gram of the material on electrode.
3. Results and discussion 3.1. Materials Characterization
2.4. Characterization Powder X–ray diffraction (XRD) measurements were carried out using PANalytical X–ray diffractometer (X'Pert PRO 3040/60) at
To check the thermal stability and weight loss of MIL–101(Cr) thermogravimetric analysis (TGA) was carried out and the percentage weight loss with increasing temperature is shown in Figure S3x. The
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TGA curve indicates that all guest molecules (7%), trapped in the cages of MIL–101(Cr) are removed at ca.200 C. The predominant weight change in the TGA curve is ca. 42% between 350–520 C due to the thermal decomposition of the framework [34]. Beyond 650 C there is no weight change with increasing temperature, and only the green Cr2O3 powder remained. The product was characterized by powder XRD and the spectrum contained peaks at 2u positions corresponding to a rhombohedral geometry of Cr2O3 (JCPDS cards file No. 00–002–1362). Decomposition of MIL–101(Cr) was carried out at 900 C and only Cr2O3 was obtained with increasing crystallinity as confirmed by XRD analysis (Figure S2x). The heating of the furfuryl alcohol/MIL–101(Cr) mixture at 900 C resulted in the decomposition to a material appearing predominately of carbon. During heating, the polymerization of furfuryl alcohol occurs within in the 3D channels of MIL–101(Cr) at ca. 150 C [24,25,33] while further increase in temperature beyond 400 C resulted in the decomposition of MIL–101(Cr) and conversion of polyfurfuryl alcohol to carbon material [33]. Powder XRD measurements were carried out to characterize the synthesized materials and their phase purity. PXRD pattern of MIL-101 (Cr) is presented in (Figure S1x) which is consistent with previous reports [30] and the simulated pattern published by Férey et al. [29]. Very sharp PXRD peaks indicate the crystalline nature of the synthesized MIL–101(Cr) and these peaks are attributed to the zeotype structure of MIL–101(Cr) [35]. The PXRD pattern of the composite product obtained at 900 C displays peaks at 2u ¼24.5 , 33.6 , 36.2 , 41.5 , 50.3 , 55 , 63.5 and 65.2 for the formation of rhombohedral geometry(JCPDS cards file No. 00–002–1362) as shown in Fig.1. Fig.1 clearly shows that Cr2O3 was not reduced into Cr metal by carbon at this temperature as evident by the absence of standard Cr metal peaks in PXRD pattern. The Scherer equation [32] was used to calculate the crystallite size (dcrys) of Cr2O3 in the composite material. The obtained calculated dcrys values are 20.08 nm, 40.42 nm and 20.55 nm for (1 0 4), (11 0) and (11 3) peaks respectively. These values clearly suggest that the crystallites are irregular in size and the calculations agree with our TEM observations. XPS analysis was carried out to further investigate the oxidation state of chromium and the composition of the nanocomposite and indicated that the nanocomposite Cr2O3–carbon consists mainly of Cr (23.0 wt.%), O (16.0 wt.%), graphitic carbon (55.9 wt.%) and traces of organic F, as can be seen in Table 1. This roughly corresponded to 30 wt.% Cr2O3, 60 wt.% carbon, 5 wt.% CrO3, 4 wt.% glass impurities, and less than 1 wt.% organic residues. The core Cr3+ 2p3/2 line confirmed the presence of Cr2O3 showing characteristic inflections that appear on high and low binding energy sides of the peak envelope related to the fine structure of Cr2O3 [36],Fig. 2(a). There are two peaks for Cr3+ 2p3/2 at 576.6 eV (A)
Fig. 1. Powder X–ray diffraction pattern of Cr2O3/C 900 obtained after heat treatment of polyfurfuryl alcohol/MIL–101 composite. Standard reference peaks of Cr2O3 and Cr metal are also included.
Table 1 Atomic and total elemental percentage of C, O, Cr and F in the surface analysis of Cr2O3–carbon nanocomposite from XPS measurements. Name
Assignment
Binding Energy (eV)
Atomic (%)
Weight (%)
C 1s * O 1s x O 1s Si 2p F 1s F 1s F 1s Cr 2p3/2 Cr 2p3/2 Cr 2p3/2
C = C/C–C O in Cr Oxides O–Si Si in SiO2 Metal–F –(CHF–CH2)– –(F2C = CF2)– Cr3+ in Cr2O3 Cr3+ in Cr2O3 Cr6+ in CrO3/CrF3
284.5 531.4 533.0 103.1 685.0 686.9 689.4 576.7 577.9 579.6
73.3 15.7 1.8 1.1 0.3 0.6 0.3 1.5 4.6 0.9
55.9 16.0 1.9 1.9 0.3 0.7 0.3 5.0 15.1 2.9
* x
Small amount of oxygen as O = C Small amount of oxygen as O–C
and at 577.9 eV (B). A peak–fit with a single peak at 579.6 eV corresponds to Cr6+. In the presence of carbon and at elevated temperature of 900 C Cr2O3 cannot be reduced to chromium metal (Cr0), that is why elemental analysis indicated that Cr0 was not present. The sample was dominated by Cr3+oxides and to a much lesser extent by the oxidized form containing Cr6+. A PXRD was performed over 6 hours to monitor for the small quantities (
