Research Article pubs.acs.org/journal/ascecg
X‑ray Absorption Spectroscopic Study on Interfacial Electronic Properties of FeOOH/Reduced Graphene Oxide for Asymmetric Supercapacitors Han-Wei Chang,† Chung-Li Dong,*,† Ying-Rui Lu,†,‡,§ Yu-Cheng Huang,†,‡,§ Jeng-Lung Chen,‡ Chi Liang Chen,‡ Wu-Ching Chou,⊥ Yu-Chen Tsai,*,∥ Jin-Ming Chen,‡ and Jyh-Fu Lee‡ †
Department of Physics, Tamkang University, 151 Yingzhuan Road, Tamsui 25137, Taiwan National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu 30076, Taiwan § Program for Science and Technology of Accelerator Light Source, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan ∥ Department of Chemical Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan ⊥ Department of Electrophysics, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan
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S Supporting Information *
ABSTRACT: The effects of growth time and interface between the iron oxyhydroxide (FeOOH) and carbon materials (carbon nanotubes (CNT) and reduced graphene oxide (RGO)) to form an asymmetric supercapacitor was studied by X-ray absorption spectroscopy (XAS) and electrochemical measurements. FeOOH/CNT (FCNT) and FeOOH/RGO (FRGO) were successfully synthesized by a simple spontaneous redox reaction with FeCl3. The RGO functions as an ideal substrate, providing rich growth sites for FeOOH, and it is believed to facilitate the transport of electrons/ions across the electrode/electrolyte interface. FRGO has been identified as a supercapacitor and found to exhibit significantly greater capacitance than FCNT. To gain further insight into the effects of growth times and the interface of FeOOH for FCNT and FRGO, the electronic structures of FCNT and FRGO with various FeOOH growth times were elucidated by XAS. The difference between the surface electronic structures of CNT and RGO yields different nucleation and growth rates of FeOOH of FeOOH. RGO with excellent interface properties arises from a high degree of covalent functionalization, and/or defects make it favorable for FeOOH growth. FRGO is therefore a promising electrode material for use in the fabrication of asymmetric supercapacitors. In this work, coupled XAS and electrochemical measurements reveal the electronic structure of the interface between FeOOH and the carbon materials and the capacitance performance of asymmetric supercapacitors, which are very useful in the fields of nanomaterials and nanotechnology, especially for their applications in storing energy. KEYWORDS: Synchrotron X-ray absorption spectroscopy, Electronic structure, Asymmetric supercapacitor, Reduced graphene oxide, FeOOH/RGO, FeOOH/CNT
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INTRODUCTION The supercapacitor is one of the most promising energy storage devices owing to its high power density, excellent reversibility, high charge/discharge rate, and long-term stability.1−4 As specified by the equation E = 0.5 CΔV2, the energy density (E) of a supercapacitor depends on its potential window (ΔV) and/ or its specific capacitance (C). However, the low energy density of existing supercapacitors limits their use as primary energy storage and conversion systems. Most recent research in the field has focused on the development of asymmetric supercapacitors with improved specific capacitance and increased operating potential windows.5−7 An asymmetric supercapacitor can be feasibly fabricated by two electrode materials with © 2017 American Chemical Society
different operating potential windows and therefore advances the working potential windows in the aqueous electrolyte, resulting in excellent capacitive behavior. Accordingly, substantial efforts have been made to prepare electrode materials with various transition metal oxides to evaluate and compare the achieved enhancements in capacitive performance when they are used in asymmetric supercapacitors. One current research theme is the design and synthesis of iron oxide-based materials as negative electrodes to improve the performance of Received: December 7, 2016 Revised: February 16, 2017 Published: March 8, 2017 3186
DOI: 10.1021/acssuschemeng.6b02970 ACS Sustainable Chem. Eng. 2017, 5, 3186−3194
Research Article
ACS Sustainable Chemistry & Engineering
All chemical solutions were prepared using a Milli-Q water purification system (Milli-Q, USA). Preparation of FeOOH/Carbon Materials. CNTs were produced from a mixture of concentrated sulfuric acid/nitric acid (3:1, v/v). GO was synthesized from graphite powder by a modified Hummers method. Ten milligrams of GO was dispersed in 10 mL of deionized water and ultrasonicated for about 15 min to yield a uniform GO solution. GO was converted to RGO under mild conditions using AA. Following synthesis, 100 mg of AA was added to 50 mL of aqueous GO dispersion solution (0.1 mg mL−1) for 3 days. CNT and RGO-modified electrodes were prepared by mixing and dispersal (CNT or RGO) to produce a more homogeneous paste. This was cast on the electrode to yield a uniform sheet. FeOOH grew uniformly on the surface of CNT, and RGO-modified electrodes were prepared via a simple spontaneous redox reaction in aqueous FeCl3 at 90 °C. The electrodes were then dried for 2 h in a hot-air oven at 90 °C. The samples were collected at various growth times (3, 6, and 24 h) in various growth stages; they were denoted as FCNT-x and FRGO-x (x = 3, 6, and 24). MnO2/RGO (MRGO) electrodes were synthesized using a procedure that was presented elsewhere.23 MnO2 was spontaneously deposited on the surfaces of RGO by the simple immersion of RGO-modified electrodes in 50 mL of 0.001 M KMnO4 with stirring at 80 °C for 40 min, followed by drying in a hot-air oven at 90 °C for 2 h. An MRGO//FRGO asymmetric supercapacitor was fabricated using MRGO and FRGO-modified graphite electrodes as the positive and negative electrodes. The obtained samples were washed with deionized water to remove the remaining reagents and collected for subsequent characterization. Characterization. The morphology was characterized using field emission scanning electron microscopy (FESEM, JEOL, JSM-7410F, Japan). The chemical structure and composition were determined by X-ray photoelectron spectroscopy (XPS, PHI-5000 Versaprobe, ULVAC-PHI, Japan). Electrochemical measurements were made using a two- and three-electrode system with an electrochemical analyzer (Autolab, model PGSTAT30, Eco Chemie, Netherlands). The conventional three-electrode system comprised as-prepared samples with a modified glassy carbon working electrode (GCE, 3 mm diameter), a platinum wire counter electrode, and an Ag/AgCl (3 M KCl) reference electrode in 1 M Li2SO4. The conventional twoelectrode system consisted of two MRGO and FRGO-modified graphite electrodes as the positive and negative electrodes in 1 M Li2SO4. The mass loading of the active materials in the electrode was carefully evaluated from the difference between the mass before and that after casting, obtained using a microbalance with a resolution of 0.01 mg (Model HM-202, A&D Company Ltd., Japan). The mass loading of each electrode is 0.5 mg cm−2. Synchrotron XAS spectra (C, O K-edges, and Fe L-edges) were obtained by recording total electron yield at beamline BL20A of the National Synchrotron Radiation Research Center (NSRRC), Taiwan, operated with an energy of 1.5 GeV with a maximum stored current of 300 mA.
asymmetric supercapacitors. For example, Tang et al. synthesized an asymmetric supercapacitor with a positive electrode of NiO nanoflakes and a negative electrode of Fe2O3 nanowires on carbon fiber paper. The cell potential could be reversibly charged/discharged with a maximal operating potential window of 1.8 V and excellent cycling performance.8 Wang et al. demonstrated an efficient approach to the fabrication of an asymmetric supercapacitor based on combining the potential windows of two electrodes, comprising CuCo2O4/CuO nanowire and Fe2O3/RGO composites, with a high operating potential. The enhanced asymmetric supercapacitor performances have been observed and attributed to their unique structural features.9 Chen et al. developed Co−Ni double hydroxides (Co−Ni-DH) and FeOOH for use as positive and negative electrodes, respectively, in asymmetric supercapacitors and thus achieved high rate capability and cycling stability.10 On the basis of the above discussion, iron oxides such as FeOOH, Fe2O3, and Fe3O4 are promising supercapacitor electrode materials owing to their high reversible capacity, wide operating potential windows, ease of preparation, environmental friendliness, and low cost.11−13 Most transition metal oxides have a low electrical conductivity and, therefore, a poor rate capability, which limits their use as a supercapacitor electrode materials. Various conductive materials such as carbon-based materials and metals can be used instead of transition metal oxide-based electrode materials to eliminate the problems of low electrical conductivity. The decoration of the surfaces of carbon-based materials by hybrid electrodes that consist of transition metal oxides (FeOx, MnOx, CuOx, and CoOx) facilitates rapid electron transport between the active materials and electrolyte, resulting in favorable capacitance performance.14−17 In particular, graphene-based materials are promising for use in supercapacitors, storing charge by a reversible ion adsorption/desorption process and the redox reactions of surface functional groups at the electrode/ electrolyte interface.18,19 The capacitance of electrode materials depends strongly on the local electronic and chemical environments of the active element. The electronic structure near the Fermi level governs the chemical properties of the electrode material. Understanding the electronic structures of a material at various stages in its growth therefore supports the control and engineering of the functional materials. X-ray absorption spectroscopy (XAS) involves the transition of electrons from their core levels to unoccupied electronic states; it is a chemically sensitive and element-selective approach to study the local chemical environment of the constituent element of a material, providing the insights on the chemical reaction and growth mechanism.20−22 Hence, XAS was utilized herein to elucidate the growth process and capacitance of electrode materials. MnO2/RGO (MRGO) and FRGO as the positive and negative electrodes, respectively, form an asymmetric supercapacitor (denoted as MRGO//FRGO), which was examined in detail using both XAS and electrochemical measurements.
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RESULTS AND DISCUSSION The structure and surface morphology of the FCNT and FRGO were determined by FESEM analysis. Figure 1 compares FESEM images of FCNT and FRGO at different growth stages. Before the growth of FeOOH, the CNTs were about a few micrometers long and 40−90 nm in diameter. The 2-D RGO sheet in the FESEM image exhibited typical wrinkle and crumple morphology (Figure 1e). The images indicate that the FeOOH rods gradually grew in the surfaces of CNT and RGO. When FeOOH grew for 3 h, determining whether the FeOOH had grown on both the CNT and the RGO substrates was difficult. FeOOH nanorods with lengths of approximately 100 nm were clearly observed on the surface of FRGO after 6 h. Notably, the amount of FeOOH nanorods at FCNT-6 was significantly less than that at FRGO-6. During a growth period of 24 h, the surface of FRGO-24 formed a large amount of FeOOH nanorods, whereas that of FCNT-24 formed much
EXPERIMENTAL SECTION
Reagents. Potassium permanganate (KMnO4), lithium sulfate monohydrate (Li2SO4·H2O), urea, graphite, poly(vinyl alcohol) (PVA), and ascorbic acid (AA) were obtained from Sigma−Aldrich. FeCl3 was obtained from Merck. CNTs were used as received and had outer diameters in the range 40−90 nm and lengths of up to several micrometers (Mitsui & Co., Ltd., Japan). All chemicals used were of analytical grade and were used as received without further purification. 3187
DOI: 10.1021/acssuschemeng.6b02970 ACS Sustainable Chem. Eng. 2017, 5, 3186−3194
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The chemical structure and composition of FCNT and FRGO were verified from the XPS measurements. Figure 2 compares the XPS spectra of FCNT and FRGO before and after the growth of FeOOH. Figure 2a presents XPS survey spectra from which the chemical species near the surfaces of the materials are specified; Fe, C, and O elements are all present at the surface of both FCNT and FRGO. While no trace of Fe is obtained from bare CNT or RGO before the growth of FeOOH, Fe was present in both FCNT and FRGO, possibly revealing that FeOOH was successfully grown on the surfaces of both CNT and RGO. Fe 2p XPS spectra of FCNT-24 and FRGO-24 included two obvious peaks that were centered at ∼725.0 and 711.4 eV, which can be attributable to Fe 2p1/2 and Fe 2p3/2, respectively, with a spin−orbital separation of ∼13.6 eV (Figure 2b). A shakeup satellite at ∼719.4 and 733.6 eV was characteristic of Fe3+ in FeOOH, matching the relevant reported values.24 Figure 2c and d compare the C 1s and O 1s XPS spectra of FCNT and FRGO before and after 24 h of growth of FeOOH. Clear variations in the peak intensity of both C and O 1s XPS reflect modulations of atomic and electronic structures during FeOOH growth. The C 1s XPS spectra include four main peaks at ∼284.4, 285.0, 286.3, and 288.4 eV, which correspond to C−C (sp2), C−C (sp3), C−O, and CO groups, respectively.25 The C 1s XPS spectrum of FCNT-24 (FRGO-24) is weaker than that of CNT (RGO), implying the growth of FeOOH on the CNT (RGO). Notably, the intensity of the C 1s XPS spectra of FRGO-24 was much lower than that of FCNT-24. Figure 2d shows the deconvoluted O 1s XPS spectra, fitted using four peaks at ∼530.0, 531.7, 533.1, and 534.3 eV, are attributable to Fe−O, CO, HO-CO, and C−O−C groups, respectively.26 The peak width of the O 1s XPS spectra of FCNT-24 and FRGO-24 become broader as the peak positions shift slightly to lower binding energies. These changes are caused by the formation of Fe−O bonds upon the growth of FeOOH by a spontaneous reaction between FeCl3 and carbon materials (CNT and RGO). The peak in Figure 2d that is indicated by an arrow is associated with Fe−O bonds, which were present when the reaction time exceeded 24 h. However, the intensity of the peak that was associated with the Fe−O bonds in FRGO-24 increased more than did that of Fe−O bonds in FCNT-24, indicating the different FeOOH growth rates that originate
Figure 1. FESEM images of (a−d) CNT, FCNT-3, FCNT-6, and FCNT-24, (e−h) RGO, FRGO-3, FRGO-6, and FRGO-24.
fewer nanorods, indicating that the FeOOH nanorods had different growth rates on the surfaces of CNT and RGO, as verified by the quantitative elemental analysis of the XPS survey (see Figure S1 in Supporting Information). Table S1 (in Supporting Information) summarizes the atomic percentage as determined by XPS. The Fe/C mass ratios in RGO systems increase over time more than do those in CNT systems. This finding reflects the fact that different carbon materials lead to different rates of nucleation and different growth of FeOOH, resulting in differences between FCNT and FRGO in capacitive performance. XPS and XAS measurements characterize in greater detail FCNT and FRGO in various growth stages.
Figure 2. (a) Full-survey-scan XPS spectra and (b) Fe 2p XPS spectra of FCNT-24 and FRGO-24; (c) C 1s XPS spectra and (d) O 1s XPS spectra of CNT, FCNT-24, RGO, and FRGO-24. 3188
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evidence that RGO exhibits greater functionalization or has more defects than CNT. The top inset in Figure 3a magnifies the region of feature B following the background has been subtracted. For ease of comparison, the peak intensities of feature B in the top inset are normalized such that features B from both samples have the same height and energy. Clearly, the RGO exhibits a broader peak than do the CNT, suggesting that the RGO has more functionalization or defect sites than the CNT. Figure 3b displays the O K-edges of CNT and RGO. Feature E3 at ∼532.8 eV has a π* characteristic, and it depends strongly on the chemical environment that is associated with the CO states.28,29 The rather broad feature F at ∼540 eV is attributed to the σ* transitions of a mixture of O−H, C−O, or CO states. The presence of an additional weak feature D at ∼530.5 eV is attributable to the π* transitions of C−O states from carbon−oxygen species and disordered carbon. The O Kedge XAS spectrum that was obtained from RGO revealed obvious broadening (top inset) and shifting (bottom inset) of feature E relative to that of the CNT, consistent with the behavior of feature B (inset in Figure 3a), indicating the formation of a strong covalent bond between carbon and oxygen.30 Consequently, the high degree of covalent functionalization or/and defects in both CNT and RGO is expected and likely to favor the growth of FeOOH rods.31 However, feature D of RGO and the broadening of the features B3 and E3, associated with carbon and oxygen species, suggest that RGO has more defects and a greater variety of oxygen functional groups than do CNT; such differences would be expected to result in more anchoring sites or nucleation sites for the growth of FeOOH rods. Figure 4 shows the effect of the growth time on the electronic structure of FeOOH by presenting the C K-edge
from the different surface characteristics of CNT and RGO. Table S1 and Figure S1 (see Supporting Information) show the atomic percentages and XPS spectra of various chemical elements on the surfaces of FCNT and FRGO with various growth times of FeOOH, as measured by XPS analysis. XPS analysis revealed that the atomic Fe/C ratio in FRGO increased faster than that in FCNT as the FeOOH growth time increased, as was especially evident in the 24 h samples of FCNT and FRGO. These results arise from the different interface properties in RGO and CNT and reflect the fact that FeOOH nanorods had different growth rates on the surfaces of CNT and RGO. The results herein suggest that RGO has a unique architecture that favors FeOOH growth. To gain further insight into the growth process of FeOOH for FCNT and FRGO, the electronic structures of FCNT and FRGO following various FeOOH growth times were characterized using XAS. Before the effects of growth time of FeOOH on various carbon materials are considered, the electronic structures of bare carbon materials (CNT and RGO) are characterized because they prevail during the initial growth stage. Figure 3
Figure 3. (a) C K-edge and (b) O K-edge XAS spectra of CNTs and RGO. Insets in a and b magnify regions of features B3 and E3, respectively.
displays the XAS spectra at the C and O K-edge of CNT and RGO. The C K-edge XAS spectrum is associated with the electronic transition from the C 1s core level to the C 2p unoccupied states. The C K-edge XAS spectra of CNT and RGO, presented in Figure 3a, have three main features (A, B, and C). Features A and C at ∼284.6 and 290.9−291.9 eV are associated with the transitions from C 1s to the unoccupied π* and σ* orbitals, respectively. Feature B3 in the energy range 285−290 eV originates in the chemically functionalized carbon atoms and/or defects in carbon-based materials. The pronounced C−C π* in both CNT and RGO reveal the presence of a graphitic crystalline structure (sp2-type bonding), indicating the favorable electric conductivity of both samples.21,27 In addition, since peak C is associated with the long-range order of the σ* states of the carbon material, the fact that peak C from RGO is less pronounced than that from CNT suggests that the former exhibits more disorder, providing
Figure 4. C K-edge XAS spectra of (a) CNT, FCNT-3, FCNT-6, and FCNT-24, and (b) RGO, FRGO-3, FRGO-6, and FRGO-24. Top left and bottom right insets show the range 282−286 eV and the range 285−290 eV, respectively.
XAS spectra of FCNT and FRGO with various growth times of FeOOH. The intensities of C−C π* and C−C σ* features (A4 and C4) of FCNT and FRGO become increasingly lower than those of CNT and RGO as the deposition time increases, owing to modification of their atomic/electronic structures and the chemical environment during FeOOH growth. The change in the intensity of these features of FRGO was obviously greater than that of FCNT, suggesting that RGO favors the growth of FeOOH because its higher density of functional 3189
DOI: 10.1021/acssuschemeng.6b02970 ACS Sustainable Chem. Eng. 2017, 5, 3186−3194
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ACS Sustainable Chemistry & Engineering groups provide more anchoring or nucleation sites. The retention of intense C−C π* and C−C σ* features of both FCNT and FRGO following FeOOH growth reveals that FCNT and FRGO maintain their graphitic crystalline structure. Accordingly, the atomic environment and electronic structure of these carbon materials undergo slight modulation, indicating that the growth of FeOOH did not affect the graphitic characteristics of these carbon materials. The favorable graphitic crystalline structures of both FCNT and FRGO are critical to their good capacitive performance and therefore may make them suitable for use in supercapacitors. The bottom right-hand insets in Figure 4a and b magnify the areas of the C K-edge XAS spectra of FCNT and FRGO with various deposition times of FeOOH, in the energy range 285− 290 eV. Clearly, FRGO exhibits a greater spectral evolution with increasing FeOOH deposition time than does FCNT, revealing that these carbon materials have different surface electronic structures. The intensity of B significantly increases with the deposition times of FeOOH in FRGOs, suggesting a strong interfacial interaction between FeOOH rods and RGO. The new peak from the FRGO samples at 286.5 eV in Figure 4b may be direct evidence of the fact that FeOOH interacts more strongly with RGO than with CNT because the FCNT samples do not yield such a peak. The appearance of this new peak is likely to arise from the presence of a greater variety of surface functional groups or defect sites on the surface of RGO. The larger number of oxygen functional groups in RGO is believed to favor a strong interfacial interaction between FeOOH and RGO. The strong anchoring of FeOOH onto RGO makes FRGO a good material for use in a supercapacitor. The top left insets in Figure 4a and b display the evolution of feature A from FCNT and FRGO, respectively, magnifying the graphitic C−C π* states in the energy range 282−286 eV. The intensities of feature A of both FCNT and FRGO decline as the growth time increases, owing to the strong interaction between the FeOOH and carbon materials; this effect is attributed primarily to formation of the Fe−O−C bonds between FeOOH and the carbon material.32 The intensity of feature A of FRGO declines with increasing growth time of FeOOH more than does that of FCNT, and the density of the unoccupied states with π* character in FRGO-24 is clearly lower than that of RGO-24. Notably, feature A of FRGO-24 is at a lower photon energy than those of other FRGO-based materials. The above results suggest that FeOOH is electrondonating and responsible for a larger charge transfer to the C 2p-derived π* state in RGO than in CNT.33,34 Therefore, FeOOH rods grow on CNT and RGO at different growth rates and to form different interfaces between the FeOOH and carbon materials (CNT and RGO). FRGO yields increasingly intense feature B with increasing growth time of FeOOH to an extent greater than FCNT because RGO provides both an active interface and nucleation sites, supporting FeOOH growth. Notably, the decrease in the intensity of peak C as a function of the growth time of FeOOH is greater for FRGO than that for FCNT, revealing that the long-range order σ* states are affected more strongly in FRGO. Clearly, after the addition of FeOOH to RGO, the disorder increased with growth time. Thus, the decrease in the intensity of peak C suggests greater functionalization or more defects in FRGO systems, allowing a stronger interaction between FeOOH and RGO. Figure 5 shows the O K-edge XAS spectra of FCNT and FRGO following the growth of FeOOH for various periods.
Figure 5. O K-edge XAS spectra of (a) CNT, FCNT-3, FCNT-6, and FCNT-24, and (b) RGO, FRGO-3, FRGO-6, and FRGO-24. Insets in a and b magnify and compare prepeak regions.
Feature E is associated with a π* character and depends strongly on the chemical environment that represents CO states.28,29 Notably, features G, H, and I were observed from both CNT and RGO after the growth of FeOOH, and their intensities increased with growth time. Features G and H arise from the transition to O 2p orbitals, which are hybridized with the Fe 3d t2g and 3d eg orbitals, respectively. Peak I results from the excitation of the O 2p of hydroxide to Fe 3d states. The systematic spectral evolutions strongly suggest the successful deposition of FeOOH on the surfaces of CNT and RGO. Comparing the spectra with the reference spectrum of FeOOH in Figure 5a and b demonstrates enhancements of the intensity of peaks G, H, and I, indicative of the presence of FeOOH. Notably, the ratio of the intensity of feature H to that of feature G for both FRGOs and FCNT becomes increasingly similar to that for FeOOH as the growing time increase. However, the ratio of the intensity of feature H to that of feature G for FRGO-24 is much closer to that for FeOOH than that for FCNT-24. These findings suggest that RGO and CNT differ in interface properties, resulting in different nucleation and growth mechanisms of FeOOH. The greater variation of the ratio of the intensity of feature H to that of G for FRGO demonstrates that the growth of FeOOH is favored on RGO, which therefore has potential as an active material for use in supercapacitors. To further clarify from the viewpoint of the Fe site, Figure 6 presents the Fe L2,3-edge spectra of FCNT and FRGO with various growth times of FeOOH. The Fe L2,3-edges arose from electron transitions from the Fe 2p core hole to the 3d unoccupied states. The spin−orbit interaction of the Fe 2p core hole (Fe 2p3/2 and 2p1/2) splits the spectrum into two wellseparated multiplets: L3 (∼710 eV) and L2 (∼723 eV) edges. As the growth time of FeOOH increases, the spectral profiles of the Fe L2,3-edges of FCNT and FRGO do not differ, suggesting 3190
DOI: 10.1021/acssuschemeng.6b02970 ACS Sustainable Chem. Eng. 2017, 5, 3186−3194
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Figure 6. (a) Fe L-edge XAS spectra of FCNT and FRGO with various growth times of FeOOH. (b) XAS spectra of FCNT and FeOOH. (c) XAS of FRGO and FeOOH. (d) XAS of FRGO-24, FCNT-24, and FeOOH.
that the Fe valence in FCNT and FRGO during the growth process remains Fe3+. In Figure 6a, the intensity of the Fe L-edge in FRGO is higher than that in FCNT, and increases with the growth time. These observations are likely to follow from the redistribution of charge from FeOOH to the C 2p-derived π* state. Recall that feature B4 (Figure 4) is associated with C−O and may be indicative of an interaction between the carbon material and FeOOH. The interaction between RGO and FeOOH was stronger in FRGO than in FCNT so the enhanced intensity of the Fe L-edge of the former was greater because more charge is redistributed from FeOOH to RGO than to the CNT, as revealed by the drop in the intensity of the C 2p-derived π* states (feature A4). Figure 6b and c display the changes of FCNT and FRGO with growth time; the intensity of the Fe L3 feature preserves its original intensity scale to compare the absolute change in intensity. Notably, the variation in the intensity of FRGO with the duration of growth of FeOOH (Figure 6c) is greater than that of FCNT (Figure 6b). Furthermore, Figure 6d reveals that the intensity of both FRGO-24 and FCNT-24 are more intense than FeOOH, confirming the charge transfer from FeOOH to carbon materials (CNT or RGO), and that the interaction between FeOOH and RGO is stronger than that between FeOOH and CNT. The above findings suggest that FRGO is a good material for reversible capacity with strong electronic correlations and intimate coupling between FeOOH and RGO, facilitating the formation of Fe−O−C bonds. As a result, the growth process of FeOOH is believed to depend strongly on the interfacespecific properties of carbon materials. This provides strong support for the claim that the excellent interface properties of RGO arise from the wide range of oxygen functional groups or/ and defects, which make it suitable for growing FeOOH. FRGO is also expected to be a good material for the negative electrode in a supercapacitor. To gain a detailed understanding of the supercapacitive performance that is achieved by the growth of FeOOH on CNT and RGO with different surface characteristics, the capacitive properties of FCNT and FRGO were examined by making CV and galvanostatic charges/discharge measurements. Figure 7a and b compares the CV curves of FCNT and FRGO electrodes at different scan rates with a potential window of −0.8 to 0 V versus Ag/AgCl in 1 M Li2SO4 solution. Both
Figure 7. Cyclic voltammetry curves of (a) FCNT and (b) FRGO modified glassy carbon electrodes at various scan rates; galvanostatic charge/discharge curves of (c) FCNT and (d) FRGO modified glassy carbon electrodes at various current densities; (e) galvanostatic charge/discharge of FCNT and FRGO electrodes at different current densities, and (f) cycling stability of FCNT and FRGO electrodes in a three-electrode system in 1 M aqueous Li2SO4.
FCNT and FRGO exhibited an obvious increase of current with scan rate, revealing favorable capacitive behavior. Both FCNT and FRGO electrodes yield quasi-rectangular CV curves, suggesting good capacitive behavior and the facilitation of charge/ion transport in the electrode materials. The CV curve of FRGO has a larger included area than that of FCNT, indicating that FRGO has a higher specific capacitance. The FRGO electrode has an almost quasi-rectangular CV curve even at a high scan rate of 200 mV s−1, which is consistent with the pseudocapacitance contribution of FRGO. The interface functional groups and/or defects in the nanostructured RGO architecture are believed to favor the formation of FeOOH. Charge redistribution between FeOOH and carbon materials (CNT and RGO) and a strong interaction between FeOOH and RGO are revealed, suggesting that RGO has a better interconnectivity with FeOOH than CNT does, providing better electron/ion paths and faster transport, further favoring capacitive performance. To elucidate further the difference between FCNT and FRGO, the enhanced capacitive performances of the FCNT and FRGO electrodes were verified by making galvanostatic charge/discharge measurements. Figure 7c and d compare the galvanostatic charges/discharges of FCNT and FRGO electrodes at various current densities. The charge curves of FCNT and FRGO were almost symmetrical and triangular with a slight curvature, owing to the combined contributions of carbon materials and FeOOH to the 3191
DOI: 10.1021/acssuschemeng.6b02970 ACS Sustainable Chem. Eng. 2017, 5, 3186−3194
Research Article
ACS Sustainable Chemistry & Engineering electrochemical double layer capacitor (EDLC) and pseudocapacitance.35 This finding is consistent with the CV curves. The specific capacitances of the FCNT and FRGO electrodes are calculated using the formula C = (I Δt)/(ΔVm), where C is specific capacitance (F g−1); I is the charge/discharge current (A); Δt is the discharge time (s); ΔV is the potential change during discharge (V), and m is the mass of the active materials in the electrode (g). At a current density of 1 A g−1, the specific capacitances of FCNT and FRGO electrodes are 80.0 and 142.0 F g−1, respectively. FRGO exhibited a much higher specific capacitance than the RGO electrodes at the same current density because the RGO functions as an ideal growth substrate, providing many sites for FeOOH and facilitating the transportation of electrons/ions across the electrode/electrolyte interface, causing it to have a significantly higher capacitance than FCNT. Figure 7e plots the results of the galvanostatic charge/discharge of FCNT and FRGO electrodes at various current densities. The specific capacitances of FCNT and FRGO gradually decrease as the current density increases. Even at a high current density of 40 A g−1, FRGO retains 90% of its original capacitance retention, more than that retained by FCNT (80%), suggesting that FRGO exhibits both a good rate capability and structural stability owing to its interfacial electronic properties. The long-term cycling performances of FCNT and FRGO electrodes were assessed with galvanostatic charge/discharge cycling at a high current density (Figure 7f). After more than 1000 cycles, both electrodes retained ∼90% of their initial specific capacitance. These results demonstrate the long-term electrochemical stability of both electrodes, so FRGO and FCNT, especially FRGO, can be regarded as promising materials for the fabrication of supercapacitor devices. The low energy density of existing supercapacitors limits their range of applications, relative to that of batteries. According to the equation E = 0.5CΔV2, the energy density of supercapacitors can be increased in two ways: improving the specific capacitance and increasing the operating potential. Such an asymmetric supercapacitor may exhibit excellent capacitive behavior. Therefore, asymmetric supercapacitors with positive and negative electrodes of different materials can exhibit better charge storage performance than others, with a larger operating potential window in aqueous electrolyte. MnO2 is a highly promising material for use in a positive electrode in a pseudocapacitor owing to its advanced redox activity and high theoretical specific capacitance. Hence, MnO2 in the positive electrode and FeOOH in the negative electrode were fabricated and combined to form an asymmetric FRGO// MRGO supercapacitor, which exhibits an extended operating potential window up to 1.6 V, favoring its energy density. MRGO electrodes were synthesized following a procedure that was presented in earlier work.23 To estimate the stable potential window of the fabricated asymmetric supercapacitor, the CV curves (50 mV s−1) and galvanostatic charge/discharge curves (4 A g−1) of FRGO and MRGO electrodes were measured in a three-electrode system in 1 M aqueous Li2SO4, as presented in Figure 8. The curves reveal that the stable potential windows of FRGO and MRGO are in the ranges of −0.8−0 V and 0−0.8 V, respectively. The overall operating potential window is expressed as the sum of the potential ranges of FRGO and MRGO electrodes. Accordingly, the asymmetric supercapacitor that comprises FRGO and MRGO exhibits a stable operating potential window of 1.6 V.
Figure 8. (a) Cyclic voltammetry curves (50 mV s−1) and (b) galvanostatic charge/discharge curves (4 A g−1) of FRGO and MRGO electrodes in a three-electrode system in 1 M aqueous Li2SO4.
To evaluate the electrochemical performance of an asymmetric supercapacitor that is composed of FRGO and MRGO, a two-electrode system was fabricated. Figure 9a and c plots the CV curves (10 mV s−1) and galvanostatic charge/ discharge curves (1 A g−1) of the FRGO//MRGO asymmetric supercapacitor with different operating potential windows, indicating that the device can be operated with an operating potential window of up to 1.6 V. As the operating potential
Figure 9. (a) Cyclic voltammetry curves of FRGO//MRGO asymmetric supercapacitor (a) with various potential windows (10 mV s−1) and (b) with different scan rates; galvanostatic charge/ discharge curves of an FRGO//MRGO asymmetric supercapacitor (c) with different potential windows (1 A g−1) and (d) with different current densities; (e) galvanostatic charge/discharge tests of an FRGO//MRGO asymmetric supercapacitor at different current densities; (f) Ragone plot (power density vs energy density) of an FRGO//MRGO asymmetric supercapacitor in a two-electrode system in 1 M aqueous Li2SO4. 3192
DOI: 10.1021/acssuschemeng.6b02970 ACS Sustainable Chem. Eng. 2017, 5, 3186−3194
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window is increased, the CV curves in Figure 9a remain rectangular with two redox peaks, suggesting a favorable capacitive response of the asymmetric supercapacitor. The redox peaks are dominated by the Faradaic redox reactions that arise from the pseudocapacitance contribution of FeOOH and MnO2. The CV curves in Figure 9b exhibit no obvious distortion as the scan rate is increased, revealing the highly reversible system and fast diffusion of the electrolyte ions toward the electrode/electrolyte interface. As expected, the galvanostatic charge/discharge curves of the FRGO//MRGO asymmetric supercapacitor were obtained at current densities from 1 to 10 A g−1, as presented in Figure 9d and e. The charge/discharge curves have a slightly distorted triangular shape and exhibit only a modest IR drop, verifying near-perfect capacitive behavior of the device. The specific capacitance is calculated to be 45 F g −1 at a low current density of 1 A g−1, corresponding to a specific capacitance of a single electrode of 180 F g−1 (which is four times the calculated specific capacitance). Figure 9f presents the Ragone plot of the FRGO//MRGO asymmetric supercapacitor that was calculated from the galvanostatic charge/discharge results. The energy density (E) and the power density (P) were calculated using the formulas E = 0.5CΔV2 and P = EΔt−1. Accordingly, the device exhibits an energy density of 16 Wh kg−1 at a power density of 0.6 kW kg−1 of and 12 Wh kg−1 at a power density of 7.92 W kg−1, which are comparable with values obtained in previous investigations of FeOx-based supercapacitors, including Fe3O4//MnO2 (8.8 Wh kg−1 at 0.25 kW kg−1),36 Fe3O4− graphene//Fe3O4−graphene (11 Wh kg−1 at 0.2 kW kg−1 and 9 Wh kg−1 at 3 kW kg−1),37 and FeOOH/graphene/CNT// MnO2/graphene (7.4 Wh kg−1 at 8.6 kW kg−1).38 These results thus obtained demonstrate that such as-made electrode materials have commercial potential for use in supercapacitor devices and may support alternative means of large-area manufacturing.
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CONCLUSION
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ASSOCIATED CONTENT
Research Article
AUTHOR INFORMATION
Corresponding Authors
*(C.-L.D.) Tel: +886-3-5780281 ext. 7106. Fax: +886 3 5789816. E-mail:
[email protected]. *(Y.-C.T.) Tel: +886-4-22857257. Fax: +886-4-22854734. Email:
[email protected]. ORCID
Chung-Li Dong: 0000-0002-4289-4677 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was financially supported by the National Science Council of Taiwan under contracts MoST 104-2112-M-032008-MY3 and 104-2923-M-032-001-MY3.
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REFERENCES
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In this study, the supercapacitor-related properties of FCNT and FRGO were examined using element-selective XAS and by making electrochemical measurements. The growth process of FeOOH depended strongly on the interfacial functional groups and electronic properties of the carbon material on which it was grown. Analytical results suggest that RGO has excellent interface properties that make it favorable for FeOOH growth. Finally, an asymmetrical supercapacitor was fabricated with FRGO and MRGO as the positive and negative electrodes, respectively, and it exhibited excellent capacitive performance. The designed FRGO//MRGO asymmetric supercapacitor could open up the operating potential window and is promising for use in energy storage devices.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02970. XPS as a function of growth time; mass ratios determined from XPS results (PDF) 3193
DOI: 10.1021/acssuschemeng.6b02970 ACS Sustainable Chem. Eng. 2017, 5, 3186−3194
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ACS Sustainable Chemistry & Engineering
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