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Electrochemical performances of iron-cobalt oxides nanoparticles loaded crumpled graphene for supercapacitor
Accepted Manuscript Electrochemical performances of iron-cobalt oxides nanoparticles loaded crumpled graphene for supercapacitor Chongmin Lee, Sun Kyung Kim, Ji-Hyuk Choi, Hankwon Chang, Hee Dong Jang PII:
S0925-8388(17)34175-0
DOI:
10.1016/j.jallcom.2017.11.393
Reference:
JALCOM 44086
To appear in:
Journal of Alloys and Compounds
Received Date: 13 October 2017 Revised Date:
25 November 2017
Accepted Date: 30 November 2017
Please cite this article as: C. Lee, S.K. Kim, J.-H. Choi, H. Chang, H.D. Jang, Electrochemical performances of iron-cobalt oxides nanoparticles loaded crumpled graphene for supercapacitor, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.11.393. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Electrochemical performances of iron-cobalt oxides nanoparticles loaded crumpled graphene for supercapacitor
Chongmin Lee
[a, b]
, Sun Kyung Kim
[a]
, Ji-Hyuk Choi
[a]
, Hankwon Chang
[a, b]
, Hee Dong
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Jang *[a, b]
[a] C. Lee, Dr. S. K. Kim, Dr. J. H. Choi, Prof. H. Chang, Prof. H. D. Jang
Resources Utilization Research Center, Korea Institute of Geoscience and Mineral Resources,
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Yuseong-gu, Deajeon 34132, (Republic of Korea)
[b] C. Lee, Prof. H. Chang, Prof. H. D. Jang
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Department of Nanomaterials Science and Engineering, University of Science & Technology, Yuseong-gu, Deajeon 34113, (Republic of Korea)
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* Corresponding author. E-mail: [email protected] (Hee Dong Jang)
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ACCEPTED MANUSCRIPT Abstract Mixed transition metal oxides (MTMOs) have been explored as attractive electrode materials for supercapacitors owing to their higher electronic conductivity and larger specific capacitance compared with simple transition metal oxides. Among MTMOs, cobalt-based binary metal oxides such as Fe-Co oxides have emerged as alternative electrode materials for
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supercapacitors because they have the advantages of low cost, natural abundance, and environmental friendliness. Recently, graphene-based Fe-Co oxides composites have become of particular interest for their improved electrochemical performance of supercapacitors due to the synergetic effect of both materials. In this study, we present three-dimensional (3D)
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crumpled graphene (CGR) decorated with Fe-Co oxides nanoparticles to determine which molar ratio of Fe/Co can exhibit higher electrochemical performance. The Fe-Co oxides/CGR
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composites were synthesized in different molar ratios of Fe/Co via aerosol spray pyrolysis and post heat treatment. Sizes of Fe-Co oxides nanoparticles ranged from 5 to 10 nm when loaded onto 500 nm CGR. The electrochemical properties of the Fe-Co oxides/CGR composites with different molar ratios of Fe/Co were examined. The Fe-Co oxides/CGR electrodes fabricated at the Fe/Co ratio of 0.1 showed the highest performances in the
Fe/Co.
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capacitance and electrical conductivity among those electrodes with different molar ratios of
KEYWORDS Fe-Co oxides/CGR composites; aerosol spray pyrolysis; supercapacitor;
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electrochemical performance
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ACCEPTED MANUSCRIPT 1. Introduction Supercapacitors have attracted widespread research interest as an important energy storage devices due to their long cycle life, high power densities, and fast recharge capability.[1-3] The performance of supercapacitors relies on electrochemical properties of the electrode materials.[4] There are three main materials which are used as electrode materials such as
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carbonaceous materials, transition metal oxides, and conducting polymer materials.[5] Among them, transition metal oxides such as Co3O4, Fe3O4, NiO, MnO2 and RuO2 have generally been reported potential pseudocapacitor electrode materials due to their high specific capacitance.[6-8] Unfortunately, single metal oxides exhibit low electrochemical
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performances such as poor rate performance and cycling stability.[9]
Mixed transition metal oxides (MTMOs) have been explored as attractive electrode
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materials owing to their higher electronic conductivity and larger specific capacitance than those of simple transition metal oxides.[10] It is well known that the various combinations of metal cations and the tunable compositions in MTMOs provide opportunities to manipulate the physical and chemical properties.[11] Among the various MTMOs investigated for use as supercapacitors, Fe-Co binary oxides are of interest because both oxides have nearly identical physical and chemical properties. The combination of the Co content with Fe is expected to
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result in an increase in the electrical conductivity and electrochemical activity.[12] In addition, it has also been shown to be important for reducing the toxicity and cost of the final electrode material. Furthermore, Fe-Co oxides are of great concern due to its natural abundance, environmental friendliness, and low cost.[13]
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In addition, these metal oxides will need to be combined with other carbonaceous materials in order to further improve the electrochemical benefits of Fe-Co oxides.[14] It is of great
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importance to improve the kinetics of ion and electron transport in the electrodes and at the electrode-electrolyte interface; this sort of improvement can ensure that a sufficient amount of electro active species is exposed on the surface of the electrode to allow for a sufficient Faradaic redox reaction.[15] Among various carbonaceous materials, graphene (GR) has been considered as an outstanding candidate for combining with those metal oxides due to its high surface area, good electrical conductivity, high flexibility, and mechanical strength.[16] In previous studies, Xiao et al. reported CoFe2O4-GR nanocomposites that exhibited high Li-ion storage capacity as anode materials in Li-ion batteries, suggesting that the material can offer high-charge storage capacity through redox reactions which could also be beneficial as 3
ACCEPTED MANUSCRIPT potential electrodes in supercapacitors.[17] He et al. prepared CoFe2O4-GR composites via a hydrothermal method for supercapacitor application, showing that the specific capacitance (123 F g-1) of the composite was improved compared with that of GR and CoFe2O4.[18] Even though previous researchers have reported enhanced capacitance of GR loaded with FeCo2O4 or CoFe2O4, the previous results only suggested the potential application for pseudocapacitors
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using Fe-Co oxides/GR composites synthesized at simple experimental conditions.[19]
It is noted that the specific capacitance of MTMOs can be enhanced compared to those of the single metal oxides if the molar ratio of two kinds of metal ion in the initial reactants is tailored.[20] However, none of the previous studies has investigated the effect of the Fe/Co
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molar ratio on the electrochemical performance of 3D CGR loaded with Fe-Co oxides. Therefore, it is imperative to study the influence of the Fe/Co molar ratio on the
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microstructure and performance of 3D CGR loaded with the Fe-Co oxides in depth. In this study, we synthesized 3D crumpled graphene loaded with Fe-Co mixed oxides with different components by systematically changing the Fe/Co molar ratio via an aerosol spray pyrolysis (ASP) process and post heat treatment. We employed 3D CGR as supporting material because 3D CGR has been showing distinguishable improvement of electrochemical performance of supercapacitors,[21-23] biosensors,[24-25] and fuel cell catalysts,[26-27]
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respectively. The Fe-Co oxides nanoparticles were loaded into the 3D CGR by ASP and post heat treatment. ASP shows many advantages for the synthesis of 3D CGR-based composites because it is a very fast, simple, and continuous process that can be used to fabricate selfassembled composites via a one-step method.[25] Here, we investigated the effects of the
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Fe/Co molar ratio in the Fe-Co oxides/CGR composites on the particle morphology and diffraction patterns. We measured the electrical performance using the as-prepared Fe-Co
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oxides/CGR composites. Furthermore, we verified the relationships between the composite containing different Fe/Co molar ratio, and electrochemical performance.
2. Experimental section
2.1 Synthesis of Fe-Co oxides/CGR composites Colloidal graphene oxides (GO) was prepared by the oxidation of graphite powder (Alfa Aesar, 99.9%) using a modification of the Hummers' method.[28] For the synthesis of the FeCo oxides/CGR composites, a colloidal mixture solution as a precursor was prepared using the as-prepared GO colloid, Fe(NO3)3·9H2O, and Co(NO3)2·6H2O. The molar ratios of Fe and 4
ACCEPTED MANUSCRIPT Co in the colloidal mixture were 0:1, 1:10, 1:2, 1:1, 2:1, 10:1, and 1:0, respectively. The experimental apparatus for the aerosol spray pyrolysis (ASP) process consisted of an ultrasonic atomizer, an electrical tubular furnace, and a filter sampler. The ultrasonic atomizer was used to generate micron-sized sprayed droplets of the colloidal mixture solution precursor. The droplets were then carried into the furnace using a flow of 10 L min-1 of argon.
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The residence time of precursor was 0.5 min.
The evaporation of water, the thermal reduction of GO into GR, and the self-assembly between GO and the Fe-Co hydroxide nanoparticles were carried out in series in a tubular furnace. The fabricated Fe-Co hydroxides/CGR composites were then collected using a
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Teflon membrane filter. The operating temperature was 400 oC. Resultant powder was heated in air at 300 C for 2 h to obtain Fe-Co oxides in the composites. A schematic diagram of the
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synthesis of the Fe-Co oxides/CGR composites is shown in Fig. 1.
2.2 Analysis
The morphologies and elemental compositions of the as-prepared Fe-Co oxides/CGR composites were observed with field emission scanning electron microscopy (FE-SEM; Sirion, FEI), transmission electron microscopy (TEM; JEM-ARM200F, JEOL), and energy
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dispersive X-ray spectroscopy (EDS; Quantax 400, Bruker, UK). The crystal structures of the composites were analyzed by X-ray diffractometry (XRD; SmartLab, Rigaku Co.). The molecular species of the samples were determined using Raman spectra (Lambda Ray, LSI
532 nm laser.
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Dimension P1) at wave numbers ranging from 1,000 to 3,000 cm-1 and with excitation of a
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2.3 Electrochemical measurement The electrochemical behavior of the as-prepared Fe-Co oxides/CGR composites was characterized by constant current charge-discharge and impedance measurements with two symmetric electrodes in an HS FLAT CELL (HOHSEN Corp., Japan) using an electrochemical interface instrument (VSP, Bio-Logics, USA). Fabrication of the electrodes for the supercapacitors was conducted as follows. For each electrode, the samples were mixed with polyvinylidene difluoride (PVDF) binder (mass ratio of composites: PVDF=9:1), and then the correct amount of N-methyl-2-pyrrolidone (NMP) was added. After 20 min of mixing, a uniform suspension was obtained; this suspension was dried at 80 oC for 2 h in a 5
ACCEPTED MANUSCRIPT vacuum. The electrodes were prepared by cutting out ~ 2.0 cm2 areas and then stacking them to achieve a mass loading of 4.0~5.0 mg per electrode. A KOH solution (5.0 M) was used as the electrolyte; a piece of filter paper (Waterman, GF/C) was used as the separator.
3. Results and discussion
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The synthetic process to Fe-Co oxides/CGR composites is depicted in Fig. 1. At first, Fe3+ and Co2+ ions were incorporated onto GO nano-sheets by adding Fe(NO3)3 and Co(NO3)2 colloidal solution under stirring of precursor solution. Then, the Fe(OH)3-Co(OH)2/GO was obtained as shown in part a. Subsequently, the obtained Fe(OH)3-Co(OH)2/GO solution was
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nebulized to generate micron-sized sprayed droplets of the colloidal mixture solution precursor, which were carried into the furnace using a flow of 10 L min-1 of argon at 400 °C.
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During the ASP process, Fe(OH)3-Co(OH)2/GO nanosheets in the aerosol droplets became self-assembled into a crumpled morphology by capillary compression due to the rapid evaporation of H2O. Then, Fe(OH)3-Co(OH)2/GO nanosheets were then reduced to Fe(OH)3Co(OH)2/GR nanosheets by the thermal reduction. At this point, the Fe(OH)3-Co(OH)2 nanoparticles are locally crystallized on the surface of CGR. During the following post heat treatment progress at 300 °C, the deposited Fe(OH)3-Co(OH)2 was converted into Fe-Co the surface of CGR.
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oxides nanoparticles. As a result, Fe-Co oxides nanoparticles were successfully decorated on
Fig. 2 shows FE-SEM and TEM images of as-fabricated CGR and Fe-Co oxides/CGR composite (Fe:Co=1:10). The morphology of the CGR (Fig. 2a-c) and composite (Fig. 2d-g)
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both shared a 3D crumpled paper ball-like architecture (diameter ~500 nm). It was considered that this unique structure is beneficial to diffuse electrolyte ions into the porous structure and
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provides an ideal morphology for enhancing electrochemical performances. The SEM images (Fig. 2d-f) indicate Fe-Co oxides nanoparticles are successfully decorated on the surface of the CGR, as is further confirmed by element mapping of a Fe-Co oxides/CGR composite (Fig. 2g). Fig. 2e-f shows Fe-Co oxides nanoparticles have an average size of 5~10 nm and are uniformly dispersed on the surface of the CGR. The well-dispersed Fe-Co oxides nanoparticles are expected to afford high electrochemical performance of Fe-Co oxides/CGR composites. We then synthesized a series of Fe-Co oxides/CGR composites with varying Fe/Co ratios via aerosol spray pyrolysis and post heat treatment. The molar ratio of the Fe and Co atoms in 6
ACCEPTED MANUSCRIPT the Fe-Co oxides/CGR composites was controlled by adjusting the molar ratio of the Fe and Co ion source in the precursor. The crystallite phases of the composite samples were characterized by X-ray diffraction (XRD) analysis. Fig. 3 and Fig. S1 show the XRD patterns of the Fe-Co oxides/CGR composites prepared under different Fe/Co molar ratios. The Co oxides/ CGR composites synthesized from cobalt nitrate and GO without iron content
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(Fe:Co=0:1) can be indexed to the cubic phase of Co3O4 (JCPDS PDF # 43-1003). The sample without cobalt content (Fe:Co=1:0) exhibits a crystal structure of rhombohedral Fe2O3 (JCPDS PDF # 33-0664). The other samples exhibited a lower degree of crystallinity than the pure Fe and Co oxides nanoparticles loaded on the surface of graphene.
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To check the real Fe/Co molar ratios in the as-prepared Fe-Co oxides/CGR, EDS point measurements were performed on the products, as shown in Table S1, in which the given data
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were average values. The Fe/Co molar ratios for the samples Fe:Co=10:1, Fe:Co=5:1, Fe:Co= 2:1, Fe:Co=1:1, Fe:Co=1:2, Fe:Co=1:10 were determined to be 0.89:0.11, 0.83:0.17, 0.63:0.37, 0.48:0.52, 0.32:0.68, and 0.07:0.93, respectively. Obviously, not all of these values were consistent with the designed Fe/Co molar ratios. The measured Fe/Co molar ratios were higher than the set values. This suggests a displaced co-precipitation process, which may be attributed to the fact that the solubility constant (Ksp) of Fe(OH)3 (Ksp = 2.79 × 10-39) is far
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less than that of Co(OH)2 (Ksp = 5.92 × 10-15).[29]
Fig. S2 shows the raman spectra of the as-fabricated Fe-Co oxides/CGR composites. For the CGR and CGR based composites, there are two peaks appearing at about 1350 cm-1 and 1590 cm-1, which are known as the D-band and the G-band.[30] The D-band is a first-order
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zone boundary phonon mode associated with defects in the GR or the GR edge, while the Gband is a radial C-C stretching mode of sp2 bonded carbon.[31] The intensity ratio of the D-
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band and G-band (ID/IG) reflects the degree of defects in the GR or GR edge. It was found that the ID/IG ratio of composites were higher than that of CGR. We then noticed the more defects are generated through the incorporation in the composite.[32] Fig. 4 shows FE-SEM and TEM images of the Fe-Co oxides/CGR composites fabricated at different molar ratios of Fe and Co ions; the images reveal that the Fe-Co oxides nanoparticles had sizes of 5-10 nm, and are uniformly decorated on the CGR. Moreover, with increasing content of Co ion, the Fe-Co oxides nanoparticles on the surface of the CGR become more agglomerated. The electrochemical performances of the electrodes made with Fe-Co oxides/CGR 7
ACCEPTED MANUSCRIPT composites were measured by cyclic voltammetry (CV) using a two-electrode symmetric system. Fig. 5a shows typical CV curves of Fe-Co oxides/CGR composite electrodes, obtained using a scanning rate of 10 mV s-1 in 5 M KOH within a potential window between 0 and 1.0 V. The CV curves of the Fe-Co oxides/CGR composites showed slight distortions from the quasi-rectangular shape, which indicated good capacitive behavior of the composites.
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Here, the redox peaks of pseudocapacitor electrode materials are not clearly observed, which is normally seen in the case of two-electrode systems.[33-35] For the composites, it is found that the electrochemical performance is associated with their component content.
The galvanostatic charge-discharge curves of the Fe-Co oxides/CGR composites in the
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potential window of 0-1.0 V are shown in Fig. 5b. Galvanostatic charge-discharge experiments were performed at current densities of 0.1 A g-1. The specific capacitance of the
ܥ௦ = 2(
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electrode materials was calculated using the following equation:[36] ݐ∆ܫ ) ݉∆ܸ
where m is the mass of the active material (g), ∆V is the range of charge-discharge voltage (V), I refers to the discharge current (A), and ∆t represents the discharge time (s). When we compare capacitances of composite containing binary oxides (Fe-Co oxides/CGR) and single
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oxides (Fe2O3/CGR and Co3O4/CGR), we can see that the Fe-Co oxides/CGR composites exhibited higher capacitance than sample Co:Fe=0:1 (Fe2O3/CGR) and sample Co:Fe=1:0 (Co3O4/CGR) in Fig. 5b. The calculated capacitance was 310, 325, 286, and 223 F g-1 for Fe:Co=0:1, Fe:Co=1:10, Fe:Co=1:1, and Fe:Co=1:0, respectively, further indicating the
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advantage of Fe-Co oxides/CGR composites as a supercapacitor electrode material. The capacitance was measured at a different current density to estimate the rate performance of
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the Fe-Co oxides/CGR composite electrode (Fig. 5c and S2a). The capacitance of Fe-Co oxides/CGR composites (Co:Fe=10:1) was 325, 302, 298, 293, 290, and 286 F g-1 at current densities of 0.1, 0.5, 1, 2, 3, and 4 A g-1; 86% capacitance was retained from 0.1 A g-1 to 4 A g-1 (Fig. 6c). The above electrochemical measurements demonstrate higher supercapacitive behavior of the Fe-Co oxides/CGR composites (Fe:Co=1:10) than sample CGR-Fe2O3 (Fe:Co=1:0) and sample CGR-Co3O4 (Fe:Co=0:1), in terms of their rate capability and specific capacitance. The Fe-Co oxides/CGR composite (Fe:Co=1:10) electrode exhibited outstanding electrochemical performance. These specific capacitance values are higher than the reported literature values for the symmetric supercapacitor, such as the Fe2O3/GR composites (254 F g-1 at 0.5 A g-1), Co3O4/GR composites (241 F g-1 at 0.1 A g-1), and 8
ACCEPTED MANUSCRIPT CoFe2O4/GR sheets (123 F g-1 at 1.0 A g-1) in the three-electrode system.[37-39] EIS measurement was conducted to investigate the resistance of the electrodes. Fig. 5d shows the Nyquist plots which consist of a line in the low frequency region and a semicircle at high frequencies. The intercepts of the Nyquist plots on the real axis are solution resistance (Rs) measured as about 0.6 Ω, suggesting the favorable conductivity of the electrolyte and
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low internal resistance for all electrodes. The interfacial charge-transfer resistance (Rct) was evaluated by the diameter of the semicircle. The Rct of Fe-Co oxides/CGR composites was smaller than those of sample CGR-Fe2O3 (Fe:Co=1:0) and sample CGR-Co3O4 (Fe:Co=0:1). In particular, the mixed metal oxides showed smaller resistance than the single metal oxides.
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The cycling performance of the Fe-Co oxides/CGR composite electrodes was evaluated by 2,000 repeated charging-discharging measurements at a constant current density of 4 A g-1. As
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shown in Fig. 6a, after 2,000 charge-discharge cycles, 86%, 80%, and 71% specific capacitances was retained for Fe:Co=0:1, Fe:Co=1:10, and Fe:Co=1:0, respectively, indicating their excellent cycling properties of Fe:Co=1:10. Based on the above analysis, FeCo oxides/CGR composite electrodes exhibited outstanding electrochemical performance. CGR has positive effects on the capacitance and cyclic performance when it acts as a matrix. The functional groups on the GO surface can anchor Fe-Co oxides nanoparticles to form a
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homogeneous structure. It will make active material contact with electrolytes more efficient and sufficient. CGR also could make up the poor intrinsic conductivity of metal oxides due to its good electron transport ability. It is noted that the homogeneous structure, increased active sites, and improved charge transfer property could contribute to a significant improvement in
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cycling performance of the Fe-Co oxides/CGR composite electrode.[40] Furethermore, when Fe-Co oxide nanoparticles were fabricated, a higher number of charge carrier derived from
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the replacement of a divalent (Co2+) cation by a trivalent one (Fe3+) paired with a free electron (e–) was generated compared to the single metal oxide nanoparticles. [41] Because the formation of charge carriers by Fe-Co oxides particles could contribute to the high electrical conductivity of the composites as well, Fe-Co oxide nanoparticles loaded CGR could result in the improved electrochemical performance compared to the single metal oxides/CGR composites. Moreover, the energy density and power density of the Fe-Co oxides/CGR composite electrodes were shown in the Ragone plot from Fig. 6b. The energy density of symmetric supercapacitor was calculated to evaluate their potential in practical applications. The Fe-Co 9
ACCEPTED MANUSCRIPT oxides/CGR composite electrodes showed higher energy density than CGR-Fe2O3 (Fe:Co=1:0) and CGR-Co3O4 (Fe:Co=0:1), which can be easily explained by their higher capacitance and lower resistance. In particular, it is exhibited that the Fe-Co oxides/CGR composite (Fe:Co=1:10) electrode could deliver a high energy density in the range of 11.29.5 W h kg-1 when the power density was in the range of 50-2,000 W kg-1. These results
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indicated that mixed metal oxides loaded on the surface of CGR could deliver a high energy density.
4. Conclusions
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The systematic fabrication of Fe-Co oxides/CGR composites via aerosol spray pyrolysis and post heat treatment was successfully investigated along with different Fe/Co molar ratios
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for supercapacitor application. The composites, which have different crystal phases and sizes of Fe-Co oxides nanoparticles, were synthesized via the variation of the molar ratio of the Fe and Co. Fe-Co oxides/CGR composites were composed of Fe-Co oxides nanoparticles with sizes in the range of 5-10 nm loaded onto the 500 nm CGR; these composites were prepared in order to compare the electrochemical performance of composite electrodes. The highest capacitance of the as-fabricated Fe-Co oxides/CGR composite electrode (Co:Fe=10:1) was
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325 F g-1 at current density 0.1 A g-1; this electrode showed higher values of capacitance and electrical conductivity than those of the as-fabricated Fe-Co oxides/CGR composite electrode. This was the result of the synergistic effect of the rapid direct electron transfer of Fe-Co oxides nanoparticles, as well as being the result of the high conductivity of the CGR of the
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composite. Considering the feasible fabrication of electrode materials when using this process, both Fe-Co oxides/CGR composites are good prospective materials for energy storage in
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supercapacitors.
Acknowledgements
This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT and Future Planning.
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graphene-based
platinum-gold
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composites as superior electrocatalysts for direct methanol fuel cells, Carbon, 93 (2015) 869-877.
[27] J. Luo, H.D. Jang, J. Huang, Effect of Sheet Morphology on the Scalability of Graphene-Based Ultracapacitors, Acs Nano, 7 (2013) 1464-1471. [28] Y.X. Xu, K.X. Sheng, C. Li, G.Q. Shi, Self-Assembled Graphene Hydrogel via a One-Step Hydrothermal Process, Acs Nano, 4 (2010) 4324-4330. [29] M.A. Garakani, S. Abouali, B. Zhang, C.A. Takagi, Z.L. Xu, J.Q. Huang, J.Q. Huang, J.K. Kim, Cobalt Carbonate/and Cobalt Oxide/Graphene Aerogel Composite Anodes for High 12
ACCEPTED MANUSCRIPT Performance Li-Ion Batteries, Acs Appl Mater Inter, 6 (2014) 18971-18980. [30] Y.P. Zhai, Y.Q. Dou, D.Y. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon Materials for Chemical Capacitive Energy Storage, Adv Mater, 23 (2011) 4828-4850. [31] E. Frackowiak, F. Beguin, Carbon materials for the electrochemical storage of energy in capacitors, Carbon, 39 (2001) 937-950. [32] H. Nishihara, T. Kyotani, Templated Nanocarbons for Energy Storage, Adv Mater, 24
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(2012) 4473-4498.
[33] J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P.L. Taberna, Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer, Science, 313 (2006) 1760-1763. [34] T.C. Chou, R.A. Doong, C.C. Hu, B.S. Zhang, D.S. Su, Hierarchically Porous Carbon with Manganese Oxides as Highly Efficient Electrode for Asymmetric Supercapacitors,
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Chemsuschem, 7 (2014) 841-847.
[35] C.Z. Yuan, B. Gao, L.F. Shen, S.D. Yang, L. Hao, X.J. Lu, F. Zhang, L.J. Zhang, X.G. Zhang,
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Hierarchically structured carbon-based composites: Design, synthesis and their application in electrochemical capacitors, Nanoscale, 3 (2011) 529-545.
[36] C.M. Chen, J.Q. Huang, Q. Zhang, W.Z. Gong, Q.H. Yang, M.Z. Wang, Y.G. Yang, Annealing a graphene oxide film to produce a free standing high conductive graphene film, Carbon, 50 (2012) 659-667.
[37] Z.S. Wu, W.C. Ren, L. Wen, L.B. Gao, J.P. Zhao, Z.P. Chen, G.M. Zhou, F. Li, H.M. Cheng, Graphene Anchored with Co3O4 Nanoparticles as Anode of Lithium Ion Batteries with
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Enhanced Reversible Capacity and Cyclic Performance, Acs Nano, 4 (2010) 3187-3194. [38] C.M. Chen, Q. Zhang, M.G. Yang, C.H. Huang, Y.G. Yang, M.Z. Wang, Structural evolution during annealing of thermally reduced graphene nanosheets for application in supercapacitors, Carbon, 50 (2012) 3572-3584. [39] H. Kim, D.H. Seo, S.W. Kim, J. Kim, K. Kang, Highly reversible Co3O4/graphene hybrid
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anode for lithium rechargeable batteries, Carbon, 49 (2011) 326-332. [40] Y.X. Xu, H. Bai, G.W. Lu, C. Li, G.Q. Shi, Flexible graphene films via the filtration of
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water-soluble noncovalent functionalized graphene sheets, J Am Chem Soc, 130 (2008) 5856-5857.
[41] C. Zhang, J. Wei, L.Y. Chen, S.L. Tang, M.S. Deng, Y.W. Du, All-solid-state asymmetric supercapacitors based on Fe-doped mesoporous Co3O4 and three-dimensional reduced graphene oxide electrodes with high energy and power densities, Nanoscale, 9 (2017) 15423-15433.
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ACCEPTED MANUSCRIPT Figure Captions
Figure 1. Schematic illustration of the formation of Fe-Co oxides/CGR composites from colloidal mixture of Fe and Co precursor and GO via aerosol spray pyrolysis and post heat
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treatment.
Figure 2. FE-SEM and TEM images of the (a, b, c) CGR and (d, e, f) Fe-Co oxides/CGR composites at the mass ratio of Fe:Co=1:10 (Reaction temperature: 400 oC, gas flow rate: 10
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L min-1, post heat treatment: 300 oC), and (e) EDS elemental mapping analysis of Fe-Co
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oxides/CGR composites.
Figure 3. X-ray diffraction patterns of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co (Reaction temperature: 400 oC, gas flow rate: 10 L min-1, post heat treatment: 300 oC, 2h).
Figure 4. FE-SEM and TEM images of the Fe-Co oxides/CGR composites prepared at
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different weight ratios of Fe and Co composites (Reaction temperature: 400 oC, gas flow rate: 10 L min-1, post heat treatment: 300 oC, 2h).
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Figure 5. (a) CV curves, (b) GCD curves, (c) Specific capacitances and (d) EIS curves of the CGR/Fe-Co oxides composites prepared at different weight ratios of Fe and Co which
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measured using a symmetric two-electrode system.
Figure 6. (a) Cycling test and (b) Ragone plot of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co which measured using a symmetric two-electrode system.
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Figure 1. Schematic illustration of the formation of Fe-Co oxides/CGR composites from
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colloidal mixture of Fe and Co precursor and GO via aerosol spray pyrolysis and post heat
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Figure 2. FE-SEM and TEM images of the (a, b, c) CGR and (d, e, f) Fe-Co oxides/CGR composites at the mass ratio of Fe:Co=1:10 (Reaction temperature: 400 oC, gas flow rate: 10
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oxides/CGR composites.
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L min-1, post heat treatment: 300 oC), and (e) EDS elemental mapping analysis of Fe-Co
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Figure 3. X-ray diffraction patterns of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co (Reaction temperature: 400 oC, gas flow rate: 10 L min-1,
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post heat treatment: 300 oC, 2h).
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Figure 4. FE-SEM and TEM images of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co composites (Reaction temperature: 400 oC, gas flow rate: 10 L min-1, post heat treatment: 300 oC, 2h).
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Figure 5. (a) CV curves, (b) GCD curves, (c) Specific capacitances and (d) EIS curves of the CGR/Fe-Co oxides composites prepared at different weight ratios of Fe and Co which
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measured using a symmetric two-electrode system.
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Figure 6. (a) Cycling test and (b) Ragone plot of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co which measured using a symmetric two-electrode
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ACCEPTED MANUSCRIPT Highlights > 3D Fe-Co oxides/CGR composites were synthesized in different molar ratios of Fe/Co
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> The composites were decorated of oxides nanoparticles on the CGR.
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> The highest electrochemical performance was found at the Fe/Co ratio of 0.1.
S0925-8388(17)34175-0
DOI:
10.1016/j.jallcom.2017.11.393
Reference:
JALCOM 44086
To appear in:
Journal of Alloys and Compounds
Received Date: 13 October 2017 Revised Date:
25 November 2017
Accepted Date: 30 November 2017
Please cite this article as: C. Lee, S.K. Kim, J.-H. Choi, H. Chang, H.D. Jang, Electrochemical performances of iron-cobalt oxides nanoparticles loaded crumpled graphene for supercapacitor, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.11.393. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Electrochemical performances of iron-cobalt oxides nanoparticles loaded crumpled graphene for supercapacitor
Chongmin Lee
[a, b]
, Sun Kyung Kim
[a]
, Ji-Hyuk Choi
[a]
, Hankwon Chang
[a, b]
, Hee Dong
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Jang *[a, b]
[a] C. Lee, Dr. S. K. Kim, Dr. J. H. Choi, Prof. H. Chang, Prof. H. D. Jang
Resources Utilization Research Center, Korea Institute of Geoscience and Mineral Resources,
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Yuseong-gu, Deajeon 34132, (Republic of Korea)
[b] C. Lee, Prof. H. Chang, Prof. H. D. Jang
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Department of Nanomaterials Science and Engineering, University of Science & Technology, Yuseong-gu, Deajeon 34113, (Republic of Korea)
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* Corresponding author. E-mail: [email protected] (Hee Dong Jang)
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ACCEPTED MANUSCRIPT Abstract Mixed transition metal oxides (MTMOs) have been explored as attractive electrode materials for supercapacitors owing to their higher electronic conductivity and larger specific capacitance compared with simple transition metal oxides. Among MTMOs, cobalt-based binary metal oxides such as Fe-Co oxides have emerged as alternative electrode materials for
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supercapacitors because they have the advantages of low cost, natural abundance, and environmental friendliness. Recently, graphene-based Fe-Co oxides composites have become of particular interest for their improved electrochemical performance of supercapacitors due to the synergetic effect of both materials. In this study, we present three-dimensional (3D)
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crumpled graphene (CGR) decorated with Fe-Co oxides nanoparticles to determine which molar ratio of Fe/Co can exhibit higher electrochemical performance. The Fe-Co oxides/CGR
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composites were synthesized in different molar ratios of Fe/Co via aerosol spray pyrolysis and post heat treatment. Sizes of Fe-Co oxides nanoparticles ranged from 5 to 10 nm when loaded onto 500 nm CGR. The electrochemical properties of the Fe-Co oxides/CGR composites with different molar ratios of Fe/Co were examined. The Fe-Co oxides/CGR electrodes fabricated at the Fe/Co ratio of 0.1 showed the highest performances in the
Fe/Co.
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capacitance and electrical conductivity among those electrodes with different molar ratios of
KEYWORDS Fe-Co oxides/CGR composites; aerosol spray pyrolysis; supercapacitor;
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electrochemical performance
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ACCEPTED MANUSCRIPT 1. Introduction Supercapacitors have attracted widespread research interest as an important energy storage devices due to their long cycle life, high power densities, and fast recharge capability.[1-3] The performance of supercapacitors relies on electrochemical properties of the electrode materials.[4] There are three main materials which are used as electrode materials such as
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carbonaceous materials, transition metal oxides, and conducting polymer materials.[5] Among them, transition metal oxides such as Co3O4, Fe3O4, NiO, MnO2 and RuO2 have generally been reported potential pseudocapacitor electrode materials due to their high specific capacitance.[6-8] Unfortunately, single metal oxides exhibit low electrochemical
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performances such as poor rate performance and cycling stability.[9]
Mixed transition metal oxides (MTMOs) have been explored as attractive electrode
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materials owing to their higher electronic conductivity and larger specific capacitance than those of simple transition metal oxides.[10] It is well known that the various combinations of metal cations and the tunable compositions in MTMOs provide opportunities to manipulate the physical and chemical properties.[11] Among the various MTMOs investigated for use as supercapacitors, Fe-Co binary oxides are of interest because both oxides have nearly identical physical and chemical properties. The combination of the Co content with Fe is expected to
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result in an increase in the electrical conductivity and electrochemical activity.[12] In addition, it has also been shown to be important for reducing the toxicity and cost of the final electrode material. Furthermore, Fe-Co oxides are of great concern due to its natural abundance, environmental friendliness, and low cost.[13]
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In addition, these metal oxides will need to be combined with other carbonaceous materials in order to further improve the electrochemical benefits of Fe-Co oxides.[14] It is of great
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importance to improve the kinetics of ion and electron transport in the electrodes and at the electrode-electrolyte interface; this sort of improvement can ensure that a sufficient amount of electro active species is exposed on the surface of the electrode to allow for a sufficient Faradaic redox reaction.[15] Among various carbonaceous materials, graphene (GR) has been considered as an outstanding candidate for combining with those metal oxides due to its high surface area, good electrical conductivity, high flexibility, and mechanical strength.[16] In previous studies, Xiao et al. reported CoFe2O4-GR nanocomposites that exhibited high Li-ion storage capacity as anode materials in Li-ion batteries, suggesting that the material can offer high-charge storage capacity through redox reactions which could also be beneficial as 3
ACCEPTED MANUSCRIPT potential electrodes in supercapacitors.[17] He et al. prepared CoFe2O4-GR composites via a hydrothermal method for supercapacitor application, showing that the specific capacitance (123 F g-1) of the composite was improved compared with that of GR and CoFe2O4.[18] Even though previous researchers have reported enhanced capacitance of GR loaded with FeCo2O4 or CoFe2O4, the previous results only suggested the potential application for pseudocapacitors
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using Fe-Co oxides/GR composites synthesized at simple experimental conditions.[19]
It is noted that the specific capacitance of MTMOs can be enhanced compared to those of the single metal oxides if the molar ratio of two kinds of metal ion in the initial reactants is tailored.[20] However, none of the previous studies has investigated the effect of the Fe/Co
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molar ratio on the electrochemical performance of 3D CGR loaded with Fe-Co oxides. Therefore, it is imperative to study the influence of the Fe/Co molar ratio on the
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microstructure and performance of 3D CGR loaded with the Fe-Co oxides in depth. In this study, we synthesized 3D crumpled graphene loaded with Fe-Co mixed oxides with different components by systematically changing the Fe/Co molar ratio via an aerosol spray pyrolysis (ASP) process and post heat treatment. We employed 3D CGR as supporting material because 3D CGR has been showing distinguishable improvement of electrochemical performance of supercapacitors,[21-23] biosensors,[24-25] and fuel cell catalysts,[26-27]
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respectively. The Fe-Co oxides nanoparticles were loaded into the 3D CGR by ASP and post heat treatment. ASP shows many advantages for the synthesis of 3D CGR-based composites because it is a very fast, simple, and continuous process that can be used to fabricate selfassembled composites via a one-step method.[25] Here, we investigated the effects of the
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Fe/Co molar ratio in the Fe-Co oxides/CGR composites on the particle morphology and diffraction patterns. We measured the electrical performance using the as-prepared Fe-Co
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oxides/CGR composites. Furthermore, we verified the relationships between the composite containing different Fe/Co molar ratio, and electrochemical performance.
2. Experimental section
2.1 Synthesis of Fe-Co oxides/CGR composites Colloidal graphene oxides (GO) was prepared by the oxidation of graphite powder (Alfa Aesar, 99.9%) using a modification of the Hummers' method.[28] For the synthesis of the FeCo oxides/CGR composites, a colloidal mixture solution as a precursor was prepared using the as-prepared GO colloid, Fe(NO3)3·9H2O, and Co(NO3)2·6H2O. The molar ratios of Fe and 4
ACCEPTED MANUSCRIPT Co in the colloidal mixture were 0:1, 1:10, 1:2, 1:1, 2:1, 10:1, and 1:0, respectively. The experimental apparatus for the aerosol spray pyrolysis (ASP) process consisted of an ultrasonic atomizer, an electrical tubular furnace, and a filter sampler. The ultrasonic atomizer was used to generate micron-sized sprayed droplets of the colloidal mixture solution precursor. The droplets were then carried into the furnace using a flow of 10 L min-1 of argon.
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The residence time of precursor was 0.5 min.
The evaporation of water, the thermal reduction of GO into GR, and the self-assembly between GO and the Fe-Co hydroxide nanoparticles were carried out in series in a tubular furnace. The fabricated Fe-Co hydroxides/CGR composites were then collected using a
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Teflon membrane filter. The operating temperature was 400 oC. Resultant powder was heated in air at 300 C for 2 h to obtain Fe-Co oxides in the composites. A schematic diagram of the
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synthesis of the Fe-Co oxides/CGR composites is shown in Fig. 1.
2.2 Analysis
The morphologies and elemental compositions of the as-prepared Fe-Co oxides/CGR composites were observed with field emission scanning electron microscopy (FE-SEM; Sirion, FEI), transmission electron microscopy (TEM; JEM-ARM200F, JEOL), and energy
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dispersive X-ray spectroscopy (EDS; Quantax 400, Bruker, UK). The crystal structures of the composites were analyzed by X-ray diffractometry (XRD; SmartLab, Rigaku Co.). The molecular species of the samples were determined using Raman spectra (Lambda Ray, LSI
532 nm laser.
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Dimension P1) at wave numbers ranging from 1,000 to 3,000 cm-1 and with excitation of a
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2.3 Electrochemical measurement The electrochemical behavior of the as-prepared Fe-Co oxides/CGR composites was characterized by constant current charge-discharge and impedance measurements with two symmetric electrodes in an HS FLAT CELL (HOHSEN Corp., Japan) using an electrochemical interface instrument (VSP, Bio-Logics, USA). Fabrication of the electrodes for the supercapacitors was conducted as follows. For each electrode, the samples were mixed with polyvinylidene difluoride (PVDF) binder (mass ratio of composites: PVDF=9:1), and then the correct amount of N-methyl-2-pyrrolidone (NMP) was added. After 20 min of mixing, a uniform suspension was obtained; this suspension was dried at 80 oC for 2 h in a 5
ACCEPTED MANUSCRIPT vacuum. The electrodes were prepared by cutting out ~ 2.0 cm2 areas and then stacking them to achieve a mass loading of 4.0~5.0 mg per electrode. A KOH solution (5.0 M) was used as the electrolyte; a piece of filter paper (Waterman, GF/C) was used as the separator.
3. Results and discussion
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The synthetic process to Fe-Co oxides/CGR composites is depicted in Fig. 1. At first, Fe3+ and Co2+ ions were incorporated onto GO nano-sheets by adding Fe(NO3)3 and Co(NO3)2 colloidal solution under stirring of precursor solution. Then, the Fe(OH)3-Co(OH)2/GO was obtained as shown in part a. Subsequently, the obtained Fe(OH)3-Co(OH)2/GO solution was
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nebulized to generate micron-sized sprayed droplets of the colloidal mixture solution precursor, which were carried into the furnace using a flow of 10 L min-1 of argon at 400 °C.
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During the ASP process, Fe(OH)3-Co(OH)2/GO nanosheets in the aerosol droplets became self-assembled into a crumpled morphology by capillary compression due to the rapid evaporation of H2O. Then, Fe(OH)3-Co(OH)2/GO nanosheets were then reduced to Fe(OH)3Co(OH)2/GR nanosheets by the thermal reduction. At this point, the Fe(OH)3-Co(OH)2 nanoparticles are locally crystallized on the surface of CGR. During the following post heat treatment progress at 300 °C, the deposited Fe(OH)3-Co(OH)2 was converted into Fe-Co the surface of CGR.
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oxides nanoparticles. As a result, Fe-Co oxides nanoparticles were successfully decorated on
Fig. 2 shows FE-SEM and TEM images of as-fabricated CGR and Fe-Co oxides/CGR composite (Fe:Co=1:10). The morphology of the CGR (Fig. 2a-c) and composite (Fig. 2d-g)
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both shared a 3D crumpled paper ball-like architecture (diameter ~500 nm). It was considered that this unique structure is beneficial to diffuse electrolyte ions into the porous structure and
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provides an ideal morphology for enhancing electrochemical performances. The SEM images (Fig. 2d-f) indicate Fe-Co oxides nanoparticles are successfully decorated on the surface of the CGR, as is further confirmed by element mapping of a Fe-Co oxides/CGR composite (Fig. 2g). Fig. 2e-f shows Fe-Co oxides nanoparticles have an average size of 5~10 nm and are uniformly dispersed on the surface of the CGR. The well-dispersed Fe-Co oxides nanoparticles are expected to afford high electrochemical performance of Fe-Co oxides/CGR composites. We then synthesized a series of Fe-Co oxides/CGR composites with varying Fe/Co ratios via aerosol spray pyrolysis and post heat treatment. The molar ratio of the Fe and Co atoms in 6
ACCEPTED MANUSCRIPT the Fe-Co oxides/CGR composites was controlled by adjusting the molar ratio of the Fe and Co ion source in the precursor. The crystallite phases of the composite samples were characterized by X-ray diffraction (XRD) analysis. Fig. 3 and Fig. S1 show the XRD patterns of the Fe-Co oxides/CGR composites prepared under different Fe/Co molar ratios. The Co oxides/ CGR composites synthesized from cobalt nitrate and GO without iron content
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(Fe:Co=0:1) can be indexed to the cubic phase of Co3O4 (JCPDS PDF # 43-1003). The sample without cobalt content (Fe:Co=1:0) exhibits a crystal structure of rhombohedral Fe2O3 (JCPDS PDF # 33-0664). The other samples exhibited a lower degree of crystallinity than the pure Fe and Co oxides nanoparticles loaded on the surface of graphene.
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To check the real Fe/Co molar ratios in the as-prepared Fe-Co oxides/CGR, EDS point measurements were performed on the products, as shown in Table S1, in which the given data
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were average values. The Fe/Co molar ratios for the samples Fe:Co=10:1, Fe:Co=5:1, Fe:Co= 2:1, Fe:Co=1:1, Fe:Co=1:2, Fe:Co=1:10 were determined to be 0.89:0.11, 0.83:0.17, 0.63:0.37, 0.48:0.52, 0.32:0.68, and 0.07:0.93, respectively. Obviously, not all of these values were consistent with the designed Fe/Co molar ratios. The measured Fe/Co molar ratios were higher than the set values. This suggests a displaced co-precipitation process, which may be attributed to the fact that the solubility constant (Ksp) of Fe(OH)3 (Ksp = 2.79 × 10-39) is far
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less than that of Co(OH)2 (Ksp = 5.92 × 10-15).[29]
Fig. S2 shows the raman spectra of the as-fabricated Fe-Co oxides/CGR composites. For the CGR and CGR based composites, there are two peaks appearing at about 1350 cm-1 and 1590 cm-1, which are known as the D-band and the G-band.[30] The D-band is a first-order
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zone boundary phonon mode associated with defects in the GR or the GR edge, while the Gband is a radial C-C stretching mode of sp2 bonded carbon.[31] The intensity ratio of the D-
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band and G-band (ID/IG) reflects the degree of defects in the GR or GR edge. It was found that the ID/IG ratio of composites were higher than that of CGR. We then noticed the more defects are generated through the incorporation in the composite.[32] Fig. 4 shows FE-SEM and TEM images of the Fe-Co oxides/CGR composites fabricated at different molar ratios of Fe and Co ions; the images reveal that the Fe-Co oxides nanoparticles had sizes of 5-10 nm, and are uniformly decorated on the CGR. Moreover, with increasing content of Co ion, the Fe-Co oxides nanoparticles on the surface of the CGR become more agglomerated. The electrochemical performances of the electrodes made with Fe-Co oxides/CGR 7
ACCEPTED MANUSCRIPT composites were measured by cyclic voltammetry (CV) using a two-electrode symmetric system. Fig. 5a shows typical CV curves of Fe-Co oxides/CGR composite electrodes, obtained using a scanning rate of 10 mV s-1 in 5 M KOH within a potential window between 0 and 1.0 V. The CV curves of the Fe-Co oxides/CGR composites showed slight distortions from the quasi-rectangular shape, which indicated good capacitive behavior of the composites.
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Here, the redox peaks of pseudocapacitor electrode materials are not clearly observed, which is normally seen in the case of two-electrode systems.[33-35] For the composites, it is found that the electrochemical performance is associated with their component content.
The galvanostatic charge-discharge curves of the Fe-Co oxides/CGR composites in the
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potential window of 0-1.0 V are shown in Fig. 5b. Galvanostatic charge-discharge experiments were performed at current densities of 0.1 A g-1. The specific capacitance of the
ܥ௦ = 2(
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electrode materials was calculated using the following equation:[36] ݐ∆ܫ ) ݉∆ܸ
where m is the mass of the active material (g), ∆V is the range of charge-discharge voltage (V), I refers to the discharge current (A), and ∆t represents the discharge time (s). When we compare capacitances of composite containing binary oxides (Fe-Co oxides/CGR) and single
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oxides (Fe2O3/CGR and Co3O4/CGR), we can see that the Fe-Co oxides/CGR composites exhibited higher capacitance than sample Co:Fe=0:1 (Fe2O3/CGR) and sample Co:Fe=1:0 (Co3O4/CGR) in Fig. 5b. The calculated capacitance was 310, 325, 286, and 223 F g-1 for Fe:Co=0:1, Fe:Co=1:10, Fe:Co=1:1, and Fe:Co=1:0, respectively, further indicating the
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advantage of Fe-Co oxides/CGR composites as a supercapacitor electrode material. The capacitance was measured at a different current density to estimate the rate performance of
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the Fe-Co oxides/CGR composite electrode (Fig. 5c and S2a). The capacitance of Fe-Co oxides/CGR composites (Co:Fe=10:1) was 325, 302, 298, 293, 290, and 286 F g-1 at current densities of 0.1, 0.5, 1, 2, 3, and 4 A g-1; 86% capacitance was retained from 0.1 A g-1 to 4 A g-1 (Fig. 6c). The above electrochemical measurements demonstrate higher supercapacitive behavior of the Fe-Co oxides/CGR composites (Fe:Co=1:10) than sample CGR-Fe2O3 (Fe:Co=1:0) and sample CGR-Co3O4 (Fe:Co=0:1), in terms of their rate capability and specific capacitance. The Fe-Co oxides/CGR composite (Fe:Co=1:10) electrode exhibited outstanding electrochemical performance. These specific capacitance values are higher than the reported literature values for the symmetric supercapacitor, such as the Fe2O3/GR composites (254 F g-1 at 0.5 A g-1), Co3O4/GR composites (241 F g-1 at 0.1 A g-1), and 8
ACCEPTED MANUSCRIPT CoFe2O4/GR sheets (123 F g-1 at 1.0 A g-1) in the three-electrode system.[37-39] EIS measurement was conducted to investigate the resistance of the electrodes. Fig. 5d shows the Nyquist plots which consist of a line in the low frequency region and a semicircle at high frequencies. The intercepts of the Nyquist plots on the real axis are solution resistance (Rs) measured as about 0.6 Ω, suggesting the favorable conductivity of the electrolyte and
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low internal resistance for all electrodes. The interfacial charge-transfer resistance (Rct) was evaluated by the diameter of the semicircle. The Rct of Fe-Co oxides/CGR composites was smaller than those of sample CGR-Fe2O3 (Fe:Co=1:0) and sample CGR-Co3O4 (Fe:Co=0:1). In particular, the mixed metal oxides showed smaller resistance than the single metal oxides.
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The cycling performance of the Fe-Co oxides/CGR composite electrodes was evaluated by 2,000 repeated charging-discharging measurements at a constant current density of 4 A g-1. As
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shown in Fig. 6a, after 2,000 charge-discharge cycles, 86%, 80%, and 71% specific capacitances was retained for Fe:Co=0:1, Fe:Co=1:10, and Fe:Co=1:0, respectively, indicating their excellent cycling properties of Fe:Co=1:10. Based on the above analysis, FeCo oxides/CGR composite electrodes exhibited outstanding electrochemical performance. CGR has positive effects on the capacitance and cyclic performance when it acts as a matrix. The functional groups on the GO surface can anchor Fe-Co oxides nanoparticles to form a
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homogeneous structure. It will make active material contact with electrolytes more efficient and sufficient. CGR also could make up the poor intrinsic conductivity of metal oxides due to its good electron transport ability. It is noted that the homogeneous structure, increased active sites, and improved charge transfer property could contribute to a significant improvement in
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cycling performance of the Fe-Co oxides/CGR composite electrode.[40] Furethermore, when Fe-Co oxide nanoparticles were fabricated, a higher number of charge carrier derived from
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the replacement of a divalent (Co2+) cation by a trivalent one (Fe3+) paired with a free electron (e–) was generated compared to the single metal oxide nanoparticles. [41] Because the formation of charge carriers by Fe-Co oxides particles could contribute to the high electrical conductivity of the composites as well, Fe-Co oxide nanoparticles loaded CGR could result in the improved electrochemical performance compared to the single metal oxides/CGR composites. Moreover, the energy density and power density of the Fe-Co oxides/CGR composite electrodes were shown in the Ragone plot from Fig. 6b. The energy density of symmetric supercapacitor was calculated to evaluate their potential in practical applications. The Fe-Co 9
ACCEPTED MANUSCRIPT oxides/CGR composite electrodes showed higher energy density than CGR-Fe2O3 (Fe:Co=1:0) and CGR-Co3O4 (Fe:Co=0:1), which can be easily explained by their higher capacitance and lower resistance. In particular, it is exhibited that the Fe-Co oxides/CGR composite (Fe:Co=1:10) electrode could deliver a high energy density in the range of 11.29.5 W h kg-1 when the power density was in the range of 50-2,000 W kg-1. These results
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indicated that mixed metal oxides loaded on the surface of CGR could deliver a high energy density.
4. Conclusions
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The systematic fabrication of Fe-Co oxides/CGR composites via aerosol spray pyrolysis and post heat treatment was successfully investigated along with different Fe/Co molar ratios
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for supercapacitor application. The composites, which have different crystal phases and sizes of Fe-Co oxides nanoparticles, were synthesized via the variation of the molar ratio of the Fe and Co. Fe-Co oxides/CGR composites were composed of Fe-Co oxides nanoparticles with sizes in the range of 5-10 nm loaded onto the 500 nm CGR; these composites were prepared in order to compare the electrochemical performance of composite electrodes. The highest capacitance of the as-fabricated Fe-Co oxides/CGR composite electrode (Co:Fe=10:1) was
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325 F g-1 at current density 0.1 A g-1; this electrode showed higher values of capacitance and electrical conductivity than those of the as-fabricated Fe-Co oxides/CGR composite electrode. This was the result of the synergistic effect of the rapid direct electron transfer of Fe-Co oxides nanoparticles, as well as being the result of the high conductivity of the CGR of the
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composite. Considering the feasible fabrication of electrode materials when using this process, both Fe-Co oxides/CGR composites are good prospective materials for energy storage in
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supercapacitors.
Acknowledgements
This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT and Future Planning.
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ACCEPTED MANUSCRIPT Figure Captions
Figure 1. Schematic illustration of the formation of Fe-Co oxides/CGR composites from colloidal mixture of Fe and Co precursor and GO via aerosol spray pyrolysis and post heat
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treatment.
Figure 2. FE-SEM and TEM images of the (a, b, c) CGR and (d, e, f) Fe-Co oxides/CGR composites at the mass ratio of Fe:Co=1:10 (Reaction temperature: 400 oC, gas flow rate: 10
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L min-1, post heat treatment: 300 oC), and (e) EDS elemental mapping analysis of Fe-Co
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oxides/CGR composites.
Figure 3. X-ray diffraction patterns of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co (Reaction temperature: 400 oC, gas flow rate: 10 L min-1, post heat treatment: 300 oC, 2h).
Figure 4. FE-SEM and TEM images of the Fe-Co oxides/CGR composites prepared at
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different weight ratios of Fe and Co composites (Reaction temperature: 400 oC, gas flow rate: 10 L min-1, post heat treatment: 300 oC, 2h).
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Figure 5. (a) CV curves, (b) GCD curves, (c) Specific capacitances and (d) EIS curves of the CGR/Fe-Co oxides composites prepared at different weight ratios of Fe and Co which
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measured using a symmetric two-electrode system.
Figure 6. (a) Cycling test and (b) Ragone plot of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co which measured using a symmetric two-electrode system.
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Figure 1. Schematic illustration of the formation of Fe-Co oxides/CGR composites from
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colloidal mixture of Fe and Co precursor and GO via aerosol spray pyrolysis and post heat
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Figure 2. FE-SEM and TEM images of the (a, b, c) CGR and (d, e, f) Fe-Co oxides/CGR composites at the mass ratio of Fe:Co=1:10 (Reaction temperature: 400 oC, gas flow rate: 10
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oxides/CGR composites.
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L min-1, post heat treatment: 300 oC), and (e) EDS elemental mapping analysis of Fe-Co
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Figure 3. X-ray diffraction patterns of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co (Reaction temperature: 400 oC, gas flow rate: 10 L min-1,
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Figure 4. FE-SEM and TEM images of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co composites (Reaction temperature: 400 oC, gas flow rate: 10 L min-1, post heat treatment: 300 oC, 2h).
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Figure 5. (a) CV curves, (b) GCD curves, (c) Specific capacitances and (d) EIS curves of the CGR/Fe-Co oxides composites prepared at different weight ratios of Fe and Co which
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Figure 6. (a) Cycling test and (b) Ragone plot of the Fe-Co oxides/CGR composites prepared at different weight ratios of Fe and Co which measured using a symmetric two-electrode
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ACCEPTED MANUSCRIPT Highlights > 3D Fe-Co oxides/CGR composites were synthesized in different molar ratios of Fe/Co
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> The composites were decorated of oxides nanoparticles on the CGR.
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> The highest electrochemical performance was found at the Fe/Co ratio of 0.1.