Electrochemical Capacitors
Novel Iron Oxyhydroxide Lepidocrocite Nanosheet as Ultrahigh Power Density Anode Material for Asymmetric Supercapacitors Ying-Chu Chen, Yan-Gu Lin,* Yu-Kuei Hsu,* Shi-Chern Yen, Kuei-Hsien Chen, and Li-Chyong Chen*
A
simple one-step electroplating route is proposed for the synthesis of novel iron oxyhydroxide lepidocrocite (γ-FeOOH) nanosheet anodes with distinct layered channels, and the microstructural influence on the pseudocapacitive performance of the obtained γ-FeOOH nanosheets is investigated via in situ X-ray absorption spectroscopy (XAS) and electrochemical measurement. The in situ XAS results regarding charge storage mechanisms of electrodeposited γ-FeOOH nanosheets show that a Li+ can reversibly insert/desert into/from the 2D channels between the [FeO6] octahedral subunits depending on the applied potential. This process charge compensates the Fe2+/Fe3+ redox transition upon charging–discharging and thus contributes to an ideal pseudocapacitive behavior of the γ-FeOOH electrode. Electrochemical results indicate that the γ-FeOOH nanosheet shows the outstanding pseudocapacitive performance, which achieves the extraordinary power density of 9000 W kg−1 with good rate performance. Most importantly, the asymmetric supercapacitors with excellent electrochemical performance are further realized by using 2D MnO2 and γ-FeOOH nanosheets as cathode and anode materials, respectively. The obtained device can be cycled reversibly at a maximum cell voltage of 1.85 V in a mild aqueous electrolyte, further delivering a maximum power density of 16 000 W kg−1 at an energy density of 37.4 Wh kg−1.
1. Introduction Electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, have recently attracted growing
interest as efficient energy-storage devices owing to their high power density, short charging time, and long durability. These benefits enable them highly promising for use in electric vehicles, hybrid electric vehicles, and other high power
Y.-C. Chen Karlsruhe Institute of Technology (KIT) Institut für Anorganische Chemie Engesserstraße 15, D-76131, Karlsruhe, Germany Dr. Y.-G. Lin National Synchrotron Radiation Research Center Hsinchu 30076, Taiwan E-mail: [email protected] Prof. Y.-K. Hsu Department of Opto-Electronic Engineering National Dong Hwa University Hualien 97401, Taiwan E-mail: [email protected]
Prof. S.-C. Yen Department of Chemical Engineering National Taiwan University Taipei 10617, Taiwan Dr. K.-H. Chen Institute of Atomic and Molecular Sciences Academia Sinica Taipei 10617, Taiwan Dr. K.-H. Chen, Dr. L.-C. Chen Center for Condensed Matter Sciences National Taiwan University Taipei 10617, Taiwan E-mail: [email protected]
DOI: 10.1002/smll.201400597 small 2014, DOI: 10.1002/smll.201400597
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energy sources.[1,2] Numerous efforts have been devoted to developing active electrode materials, where the transitionmetal oxides, conducting polymers, and carbon materials dominate mainly the landscape of ECs.[3] Metal oxides, using fast and reversible redox reactions at the surface of active materials, are typical examples of pseudocapacitive materials, which could offer high power density as well as high energy density. Although the metal oxides are well-established for electrochemical energy-storage application, there are only a few reports on the study of anode materials due to the unsatisfactory capacitive performance. Iron oxides are a class of materials that are potentially advantageous as negative electrode materials for ECs owing to their low cost and low toxicity.[4,5] A typical redox system involving a single-electron transfer is responsible for iron oxide pseudocapacitive behavior. However, the low electrical conductivity and low ion diffusion constant of conventional iron oxides can result in a low specific capacitance and poor rate capability, which is a major bottleneck for their application in ECs. Although blending iron oxide with carbon materials to form composites can effectively increase the charge transfer, the restack and/or the aggregation of carbon materials have limited the electrochemical performance of the devices.[5b,e,6] Consequently, it is necessary and significant to intrinsically change the textural property of iron oxide material, which facilitates electronic and ionic transport and increases electrochemical performance. It is well known that the process of the charge storage involves the insertion/extraction of protons or ions in the first few nanometers on the surface of electrode materials for pseudocapacitors. Iron oxyhydroxide lepidocrocite (γ-FeOOH) with open two-dimensional (2D) permeable channels may be favorable for ECs, as we recently described for birnessite-type MnO2 with layered nanostructures to facilitate the ion transportation and exhibit excellent capacitive performance.[2d] γ-FeOOH with orthorhombic crystal structure consists of infinite layer of [FeO6] octahedra, which elongates perpendicularly to the direction of b axis and linked by hydrogen bondings with an interlayer spaces of b = 12.52 Å.[7] Particularly, the 2D layered structure with good interlayer conductivity of γ-FeOOH could greatly benefit ECs. It is therefore anticipated that the design of anode materials based on γ-FeOOH with layered characteristics would be an ideal microstructural solution to definitely decrease the diffusion length of ions, increase the contact area with electrolyte as well as improve active material utilization, which leads to an enhanced electrochemical performance. Quite surprisingly, γ-FeOOH with 2D layered channels as negative electrode materials for ECs are never reported thus far. On the other hand, the rational design of nanoarchitectures with specific functions in energy storage applications is also a key requisite to increase energy and power density in electrical storage systems. Definitely, energy extracted from the electrode material strongly depends on the textural and morphological effects at the electrolyte/material interface. Some of recent reports have emphasized the structureactivity relationship of electrode material which accentuates the importance of surface morphology on the charge storage performance.[3d,8] 2D nanosheets (NSs) with one or several layers of atom or crystallites have been investigated
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extensively as a new class of nanomaterials. To the best of our knowledge, γ-FeOOH-based electrodes with 2D morphologies and characteristic pseudocapacitance behavior for ECs remain scarce. Thus, it would be of significance to design γ-FeOOH with the desirable nanoarchitecture and crystal structure for an improved electrochemical performance. In this work, for the first time, novel γ-FeOOH NSs were prepared via a simple one-step electroplating route without the assistance of any templates or surfactants. The need for binders or conducting additives, which add extra contact resistance and weight, can be eliminated in this case. Moreover, we have studied the relationship between the charge-storage characteristics of γ-FeOOH anodes and the corresponding NS morphology. An asymmetric supercapacitor was further realized using the MnO2 NS as the positive electrode and the γ-FeOOH NS as the negative electrode. The obtained device can be operated reversibly at a maximum cell voltage of 1.85 V in a mild aqueous electrolyte, delivering both high power density and good rate performance.
2. Results and Discussion 2.1. Characterization of γ-FeOOH NS/CC as Anode Material Our attempts employing simple one-step electroplating approach for synthesis of FeOOH nanostructures on the carbon cloth (CC) resulted in formation of radiating NSs as shown in Figure 1a. These NSs are smooth and homogeneous with an average thickness of 30∼50 nm and an average length of ≈1.4 µm. The crystal structure of these NSs was studied by X-ray diffraction (XRD) analysis (Figure 1b), which matched with the JCPDS (No. 76-2301) of orthorhombic-type γ-FeOOH. A high-resolution transmission electron microscopy (HRTEM) image of the FeOOH NSs also shows that the lattice fringe spacing between two adjacent crystal planes of the particle is of 0.238 nm, corresponding to the (111) lattice plane of these nanocrystals (Figure S1, Supporting Information). The Brunauer-Emmett-Teller (BET) surface area of γ-FeOOH NS/CC nanocomposites is 170 m2 g−1 which stands in contrast to that of pure CC (26 m2 g−1) in the absence of γ-FeOOH NS. The corresponding X-ray photoelectron spectroscopy (XPS) provides further information about the chemical states of the surface elements of the products, as shown in Figure 1c,d. It is noted that there are two distinct peaks at binding energies of ≈711.4 eV for Fe 2p3/2 and ≈725 eV for Fe 2p1/2 with a shake-up satellite at ≈719.5 eV in the Fe 2p core level spectrum. This is characteristic of Fe3+ in FeOOH.[9,10] Figure 1d shows the O 1s spectrum of the FeOOH NS/CC electrode, which could be fitted into three main constituent peaks corresponding to different oxygen containing species, viz. Fe–O–Fe bond, Fe–O–H bond, and H–O–H bond, respectively. Notably, the H–O–H bond corresponds to water molecule, which confirms the FeOOH NS to be in hydrated form. Electrochemical characteristics of the γ-FeOOH NS/ CC electrodes prepared for different deposition periods were evaluated by using cyclic voltammetry (CV) and galvanostatic charge/discharge methods. Figure 2a presents the
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small 2014, DOI: 10.1002/smll.201400597
Iron Oxyhydroxide Lepidocrocite Nanosheet as Anode Material for Asymmetric Supercapacitors
Figure 1. a) SEM images and b) XRD patterns (reference: JCPDS card no. 76-2301) of γ-FeOOH NS/CC hybrid electrodes (deposition time of 30 min). XPS spectra of c) Fe 2p and d) O 1s transitions for γ-FeOOH NS/CC hybrid electrodes (deposition time of 30 min).
CV curves for various electrodes, measured in 1 m Li2SO4 electrolyte with a potential scan rate of 50 mV s−1. Notably, the voltammetric current response increased substantially with increase in deposition time from 30 to 90 min. This is related to the increase in the loading amount of γ-FeOOH NSs during this deposition period, as illustrated in Figure 2b. However, degradation in the gravimetric capacitance of γ-FeOOH NS/CC electrodes, as calculated by the specific capacitance relationship reported in the literature,[2e] was observed at deposition time over 30 min, presumably due to the poor utilization of electroactive species and reduced active surface area of γ-FeOOH NSs at the thicker thickness (Figure S2, Supporting Information). Furthermore, the CV results of γ-FeOOH NS/CC electrode are almost one order of magnitude higher than that of bare CC recorded in the same condition. This improvement in the capacitive performance of γ-FeOOH NS/CC electrode is, hence, believed to result from the γ-FeOOH NS not the CC. As can be seen from the SEM images, deposition of γ-FeOOH NS uniformly covers the entire surface of CC. In this case, the weight of γ-FeOOH NS on the CC is considered in the calculation of specific capacitance for γ-FeOOH NS/CC electrode in order to deduce a meaningful capacitive performance. Besides, the specific capacitance of iron oxide-based electrodes is generally reported to be within 22–166 F g−1 and is strongly influenced by the preparation methods. The need for binders or small 2014, DOI: 10.1002/smll.201400597
conducting additives, which add extra contact resistance and weight, can be eliminated in our case. More significantly, the optimum specific capacitance achieved in this study is as high as 310.3 F g−1, which is higher than many of the data reported in the literature,[3b,4d,e,5a,d,g,h,6b,c] suggesting that the simple electrochemical process for γ-FeOOH NS deposition is highly promising for fabrication of γ-FeOOH NS/CC electrodes with superior pseudocapacitive performance. Figure 2c shows that the current density also increases with an increase in scan rate. This implies that the redox reaction is not kinetically limited at least within the scan rates of 2–100 mV s−1, thus, indicating a significant rate capability. Moreover, the inset of Figure 2d shows the chargedischarge curves of γ-FeOOH NS/CC electrodes at different discharge currents within a potential window of −0.8 to −0.1 V. The gravimetric capacitance of the electrode can be seen to decrease with increasing charge-discharge current (Figure 2d). The capacitance retention is about 70% with the increase in discharge current from 0.13 to 12.6 A g−1. It should be noted that the gravimetric capacitance of γ-FeOOH NS/CC electrodes at the ultrahigh charge/discharge current of 12.6 A g−1 remains 219.5 F g−1, indicating a high rate capability. To the best of our knowledge, this value of weight-normalized capacitance is the highest reported to date for the iron oxide-based anodes or even higher than that of many transition metal oxide anode materials.[4c,d,5a,11]
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Figure 2. a) Cyclic Voltammogramms of γ-FeOOH NS/CC hybrid electrodes with different deposition times at a scan rate of 50 mV s−1. b) Actual γ-FeOOH loading amount and gravimetric capacitance as a function of deposition time. c) Cyclic Voltammogramms of γ-FeOOH NS/CC hybrid electrodes at different scan rates. d) The gravimetric capacitances as functions of charge-discharge specific current. Inset: Charge-discharge tests within the potential window of −0.8 to −0.1 V vs Ag/AgCl.
From the above measurements, a maximum power density of 9000 W kg−1 at the energy density of 29.8 Wh kg−1 could be achieved, so far the highest value of the reported anode materials in pseudocapacitors. Therefore, the superb performance of γ-FeOOH NSs/CC obtained here confirms the advantages of 2D layered channels of γ-FeOOH NS that facilitate the fast penetration of the electrolytes through γ-FeOOH NSs to allow rapid electron-transfer for charge storage and delivery.
2.2. Investigation of Pseudocapacitive Mechanism To elucidate the mechanism of energy storage and the variation of Fe oxidation state of the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte during charge/discharge cycles, systematic in situ X-ray absorption spectroscopy (XAS) studies were carried out in Figure 3. In situ X-ray absorption near-edge structure (XANES) spectra of the γ-FeOOH NS/CC electrodes are presented in Figure 3a together with that of Fe oxides of known Fe oxidation states serving as reference compounds. Although all the spectra do not exhibit considerable difference in shape, revealing a similarity in the structural characteristics of γ-FeOOH, a clear energy shift of the adsorption peak, toward higher energy with varying applied potential, can be recognized. The absorption threshold energy (E0), which is obtained from the first inflection point on the absorption edge, is linearly
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correlated with the oxidation state of Fe in the γ-FeOOH NS/ CC electrodes (Figure 3b). According to the E0 derived from the in situ XANES spectra during charge/discharge cycles, the average oxidation state of Fe of the γ-FeOOH NS/CC electrodes was determined in the sequence shown in Figure 3c. The results clearly confirm a large electronic and ionic conductivity for the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte and a continuous and reversible faradic redox transition of γ-FeOOH NS occurred that contributes the superior capacity performance observed in Figure 2. Figure 3d shows the Fourier-transformed (FT) magnitude of Fe K-edge EXAFS spectra of the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte measured at several potentials in a sequence. The first FT maximum about 1.5 Å is attributed to an interaction of central Fe atoms with six coordinated oxygen atoms in the first coordination shell (i.e. Fe-O bond within a [FeO6] octahedral unit). The second FT maximum located near 2.7 Å corresponds to the nearest Fe atoms between neighboring [FeO6] octahedral sites (i.e., Fe-Fe interatomic distance between neighboring [FeO6] units). The interatomic distance of the Fe-O bond progressively decreases with varying applied potential from −0.8 to −0.1 V, originating from an oxidation from Fe2+ to Fe3+. Meanwhile, a decreased distance of the Fe–Fe could also be observed, indicating a contraction between the [FeO6] octahedral units. Here, Li+ is the primary working species that can intercalate into the 2D layered
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Iron Oxyhydroxide Lepidocrocite Nanosheet as Anode Material for Asymmetric Supercapacitors
Figure 3. a) In situ Fe K-edge XANES spectra of the γ-FeOOH NS/CC electrodes in Li2SO4 electrolyte with respect to applied potential from −0.8 to −0.1 V. b) Relation between edge position and Fe average oxidation state for γ-FeOOH NS/CC electrodes at different applied potentials. c) Variation of Fe oxidation state with respect to applied potential. d) In situ Fe K-edge EXAFS spectra of the γ-FeOOH NS/CC electrodes in Li2SO4 electrolyte with respect to applied potential from −0.8 to −0.1 V.
channels between the [FeO6] units, the Fe–Fe distance would be expected to increase. On the contrary, oxidation of the electrode led to a clear decrease of the Fe–Fe distance. This can be attributed to the desertion of Li+ from the γ-FeOOH NS structure. In a Li+ operative electrolyte, it could be noted that both the Fe–O and Fe–Fe peaks shift in the same direction; that is, toward longer Fe–O and Fe–Fe interatomic distances upon lithiation and back to shorter distances upon delithiation. Moreover, as noted from Figure 3d, the intensity of the FT Fe-O maximum increases with varying applied potential from −0.8 to −0.1 V. This indicates that the relief of distortion in γ-FeOOH NS under deintercalation process could restore a symmetric oxygen distribution in a [FeO6] octahedral unit. The redox transition during the fast charge/discharge cycling is hence charge compensated by the reversible insertion and desertion of Li+ into and from 2D layered channels between the [FeO6] octahedral units. Accordingly, this pseudocapacitive mechanism of the γ-FeOOH NS/CC electrode, based on Li+ as working ions in Li2SO4 electrolyte, is proposed: Fe ( III) OOH + Li + ⇔ LiFe ( II) OOH
(1)
Therefore, our superior electrochemical performance of the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte small 2014, DOI: 10.1002/smll.201400597
could be ascribed to the following factors. A unique 2D and mesoporous architecture of γ-FeOOH NS provides a largearea contact for the electrode and electrolyte and enables accommodation of the large volume change and release of the associated strain generated during rapid charge and discharge cycling. Meanwhile, the 2D layered channel and mesoporous texture of the γ-FeOOH NS also enhance the electrolyte penetration and ion migration by decreasing the resistance and contracting the diffusion path for ion, consequently increasing the effective utilization of the γ-FeOOH NSs. The highly electrically conductive CC facilitates transport of electrons to γ-FeOOH, producing a rapid faradaic reaction. Moreover, Li+ as the primary working species reversibly insert into and leave the channels between the [FeO6] octahedral subunits and cause the large variations of the oxidation state of Fe of the electrode. To support our explanation, quantitative electrochemical impedance spectroscopy (EIS) analysis, which is a powerful tool to study the electrochemical characteristics of the electrode, was further performed. Figure 4 shows the Nyquist plot of the γ-FeOOH NS/CC electrode at different applied voltages with a perturbation of 5 mV in a frequency range from 100 kHz to 0.1 Hz. The electrode can be analyzed by the complex nonlinear least square (CNLS) fitting method based on a transmission line model, which is a classical model of porous electrode with large
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of Figure 4). Ci (i = 1,2,…5) represents double-layer capacitance and/or pseudocapacitance and Ri (i = 1,2,…5) represents resistance in redox transition of γ-FeOOH NS and/or the ion penetration/intercalation into the pores/channels between infinite [FeO6] layers. Excellent fitting can be obtained with the five element transmission line model, and the fitting values of these elements can be found in inset of Figure 4. According to the time constant (RiCi), all the capacitance can be achieved in a very short period of time (less than 3 s), meaning that the electrode with the unique nanoarchitecture has smooth and short channels to be quickly accessed by electrolyte. This is why γ-FeOOH NS/CC electrodes can deliver high specific capacitance at the high charging/discharging current densities. 2.3. Asymmetric Supercapacitor based on γ-FeOOH NS/CC Anode and MnO2 NS/CC Cathode
Figure 4. The Nyquist plots of the γ-FeOOH NS/CC electrodes at different applied potentials. Inset: The equivalent circuit and the fitting results from the simulation of EIS data.
surface area. Here, the equivalent circuit of the five-element transmission line model, containing distributed capacitors and resistors is used to simulate the EIS curve of the electrode (inset
As mentioned above, with ultrahigh power density, excellent rate performance and improved specific capacity, the as-prepared γ-FeOOH NS/CC nanocomposites should be the promising anode materials for high-power ECs. In order to further investigate this potential superiority, we designed a hybrid supercapacitor in a mild aqueous electrolyte, consisting of a negative electrode of γ-FeOOH NS/ CC and a positive electrode of MnO2 NS/CC. Figure 5a
Figure 5. a) Comparison of Cyclic Voltammogramms curves of MnO2 NS electrodes and FeOOH NS electrodes in a three-electrode cell at a scan rate 5 mV s−1. b) Cyclic Voltammogramms of MnO2 NS-CC//FeOOH NS-CC hybrid supercapacitors at different scan rates from 2 to 500 mV s−1. c) Galvanostatic charge/discharge curves at different specific current from 0.4 to 8.4 A g−1. d) Ragone plots of MnO2 NS-CC//FeOOH NS-CC hybrid supercapacitors compared with other reported asymmetric supercapacitors and commercial electronic energy storage devices. Reproduced with permission.[3f] Copyright 2008, Nature Publishing Group.
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small 2014, DOI: 10.1002/smll.201400597
Iron Oxyhydroxide Lepidocrocite Nanosheet as Anode Material for Asymmetric Supercapacitors
compares CV curves of the MnO2 NS/CC electrode and the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte under a three-electrode cell at a scan rate of 5 mV s−1. Clearly, the sum of the potential ranges of these two electrodes is 1.85 V, indicating that they can potentially be used in a high-voltage asymmetric supercapacitor. Figure 5b shows the CV curves of the full-sized hybrid supercapacitors (MnO2 NS-CC// FeOOH NS-CC) in various scan rates. The results show that the current density increases with an increase in scan rate. This implies that the redox reaction is not kinetically limited at least within the scan rates of 2–500 mV s−1, thus, indicating an improved rate capability. Galvanostatic charge/ discharge measurements were also made at various current densities to elucidate the electrochemical behaviors of the MnO2 NS-CC//FeOOH NS-CC hybrid supercapacitors with Li2SO4 electrolyte at a cell voltage of 1.85 V, as presented in Figure 5c. The charge curves are clearly symmetrical with the corresponding discharge curves, revealing that the iR drop in this hybrid system under the charge/discharge process is really small. Particularly, compared with the performance of other reported various hybrid supercapacitors, it can be concluded that the MnO2 NS-CC//FeOOH NS-CC hybrid supercapacitor reaches the highest power density with an improved energy density, which is summarized in Figure 5d. Quite surprisingly, our hybrid supercapacitor can deliver a maximum power density of 16 000 W kg−1 at an energy density of 37.4 Wh kg−1, which is the highest value among various reported iron oxide-based hybrid supercapacitors. This result is even superior to current asymmetric hybrid electrochemical cells (AHEC) based on transition metal oxides as anode materials.[3i,j,4d,6a,11] It is also worthy to note that for a packaged cell, the MnO2 NS-CC//FeOOH NS-CC hybrid device has much higher energy density than the commercial ECs and can be even competitive with commercial lithium-ion batteries (LIBs),[3f] while the power density can also keep an extra high value, meaning a faster charge-discharge rate than commercial LIBs.
3. Conclusions The efficient energy-storage system was successfully developed based on layered γ-FeOOH NS anodes. For the first time, the microstructural influence on the pseudocapacitive performance of the obtained γ-FeOOH NSs was investigated via in situ X-ray absorption spectroscopy and electrochemical measurement. Most importantly, the γ-FeOOH NS showed the outstanding pseudocapacitive performance, which achieved the extraordinary power density of 9000 W kg−1 with good rate performance, so far the highest value of the reported anode materials. Last but not least, the asymmetric supercapacitors with excellent electrochemical performance were further realized by using 2D MnO2 and γ-FeOOH NSs as cathode and anode materials, delivering a maximum power density of 16 000 W kg−1 at an energy density of 37.4 Wh kg−1. The work demonstrated here will bring more insight for the design and optimization of high-performance pseudocapacitors. small 2014, DOI: 10.1002/smll.201400597
4. Experimental Section Materials: Carbon cloth (B-1, Designation A: plain-weave, 116 g m−2, 0.35 mm thickness; no wet-proofing) works as the substrate, which was purchased from E-TEK Division (USA). Ferrous ammonium sulfate (Fe(NH4)2(SO4)2·6H2O) and sodium acetate (CH3COONa) worked as the precursor solution were all obtained from Sigma-Aldrich. All the chemicals used in this study were of analytical grade and were used as received without any further purification. All solutions were prepared with deionized water of resistivity not less than 18.2 MΩ cm. Preparation of γ-FeOOH Nanosheets/CC Electrodes: Carbon cloth (CC) worked as supporting material was first attached by a copper wire at the edge and then the junction was covered by the silver paste to ensure a well conducted circuit for the following electrodeposition. The connecting pad was further coated with epoxide in order to forbid unwanted side reactions and allow an exposure of carbon cloth with a geometrical area ≈1 cm2 to the precursor solution. The well prepared carbon cloth was then immersed into the precursor solution, which contains 0.01 M ferrous ammonium sulfate and 0.04 M sodium acetate, and nanostructured γ-FeOOH was electrodeposited onto the current collector under a constant potential of 0.7 V for 30, 60, and 90 min, respectively. The as-prepared specimens were then rinsed with distilled water for washing free of the deposition solution. The loading amount of electroactive materials on the carbon cloth were determined by the inductively coupled plasma-optical emission spectroscopy (ICP-OES, PerkinElmer ICP-OES Optima 125 3000). The deposition per unit area was found to be between 0.8–1.8 mg cm−2. Preparation of MnO2 Nanosheets/CC Electrodes: The detailed procedure for the fabrication of MnO2/CC electrode could be found elsewhere, but is described here briefly.[2d,3c] The deposition of MnO2 onto carbon cloth was performed by an anodizing method in an electrolyte solution containing manganese sulfate (MnSO4·5H2O) and sulfuric acid (H2SO4) at a constant current density of 0.5 mA cm−2 for 30 min. The as-prepared specimens were then rinsed with distilled water for washing free of the deposition solution. The loading amounts of MnO2 on the carbon cloth were determined by the inductively coupled plasma-optical emission spectroscopy as well. Characterizations of γ-FeOOH Nanosheets/CC Electrodes: The nanostructures and morphological evolution of γ-FeOOH/CC electrodes with different preparation conditions were examined using field-emission scanning electron microscopy (FESEM, JEOL6700). The X-ray diffraction spectroscopy (Brucker D8 Advanced diffractometer) was applied to identify the crystal phase of the specimens. The chemical compositions of the fabricated composites were collected by the X-ray photoluminescence spectroscopy using a Perkin Elmer PHI 1600 ESCA system. A potentio/galvanostat electrochemical workstation (Solartron electrochemical test system 1470E) was employed to evaluate the electrochemical performances of the electrodes. The measurements were conducted at ambient temperature using a conventional three-electrode system, where the electroactive materials work as the working electrode, a square Pt foil as the counter electrode and an Ag/AgCl electrode in 3 M KCl as reference electrode. Cyclic voltammetry (CV), galvanostatic charge/discharge method and AC impendence analysis were applied to investigate the capacitive performance of composited electrode. All potentials reported in this article are with respect
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to that of Ag/AgCl (3 M KCl, 0.207 V vs SHE) reference electrode. Posterior to the above mentioned characterizations, the oxidation state and crystal-structural environment of FeOOH/CC electrodes under various applied potentials were then investigated with in situ X-ray absorption spectroscopy (XAS) under fluorescence mode. A sealed spectro-electrochemical cell with a fluorescencetransparent Kapton tape window was employed. Prior to collecting the XAS spectra at a given applied potential, the specimen was kept at the designated potential for at least 15 min for ensuring the electroactive materials to reach a steady state. The XAS experiments were performed on beamline 17C at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. The storage ring was operated with electron energy of 1.5 GeV and a current between 100 and 200 mA. A Si (111) double-crystal monochromator was employed for energy selection. The X-ray absorption energy was calibrated using the first inflection point in the Fe K-edge main absorption region of a metallic Fe foil, which was measured before every XAS scan. Assembly of Asymmetric Capacitors: An asymmetric supercapacitor was built by adopting MnO2/CC as the cathode material and γ-FeOOH/CC as the anode material in a two-electrode system, where the 1 M Li2SO4 mild aqueous solution as the electrolyte. A separator permeable to ion transport was placed between the electrodes. CV and galvanostatic charge/discharge analyses were applied to investigate the electrochemical performance of hybrid capacitor. All the operating current densities were calculated based on the total mass of the system, viz. the total weight of the positive and negative electroactive materials.
[3]
[4]
[5]
Supporting Information [6]
Supporting Information is available from the Wiley Online Library or from the author. [7]
Acknowledgements
[8]
This work was supported by the Ministry of Science Technology, Ministry of Education, Taiwan, and AOARD under AFOSR, US. The authors gratefully thank MOST, NSRRC, IAMS, and NTU for financial support in this project.
[1] B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Techological Applications Kluwer Academic/Plenum Press, New York 1999. [2] a) X Zhao, B. M. Sanchez, P. J. Dobson, P. S. Grant, Nanoscale 2011, 3, 839; b) J. R. Miller, P. Simon, Science 2008, 321, 651; c) C. C. Hu, K. H. Chang, M. C. Lin, Y. T. Wu, Nano Lett. 2006, 6, 2690; d) Y. C. Chen, Y. K. Hsu, Y. Y. Horng, C. C. Chen, L. C. Chen, K. H. Chen, Electrochim. Acta 2011, 56, 7124; e) Y. Y. Horng,
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Received: March 5, 2014 Revised: April 21, 2014 Published online:
small 2014, DOI: 10.1002/smll.201400597
Novel Iron Oxyhydroxide Lepidocrocite Nanosheet as Ultrahigh Power Density Anode Material for Asymmetric Supercapacitors Ying-Chu Chen, Yan-Gu Lin,* Yu-Kuei Hsu,* Shi-Chern Yen, Kuei-Hsien Chen, and Li-Chyong Chen*
A
simple one-step electroplating route is proposed for the synthesis of novel iron oxyhydroxide lepidocrocite (γ-FeOOH) nanosheet anodes with distinct layered channels, and the microstructural influence on the pseudocapacitive performance of the obtained γ-FeOOH nanosheets is investigated via in situ X-ray absorption spectroscopy (XAS) and electrochemical measurement. The in situ XAS results regarding charge storage mechanisms of electrodeposited γ-FeOOH nanosheets show that a Li+ can reversibly insert/desert into/from the 2D channels between the [FeO6] octahedral subunits depending on the applied potential. This process charge compensates the Fe2+/Fe3+ redox transition upon charging–discharging and thus contributes to an ideal pseudocapacitive behavior of the γ-FeOOH electrode. Electrochemical results indicate that the γ-FeOOH nanosheet shows the outstanding pseudocapacitive performance, which achieves the extraordinary power density of 9000 W kg−1 with good rate performance. Most importantly, the asymmetric supercapacitors with excellent electrochemical performance are further realized by using 2D MnO2 and γ-FeOOH nanosheets as cathode and anode materials, respectively. The obtained device can be cycled reversibly at a maximum cell voltage of 1.85 V in a mild aqueous electrolyte, further delivering a maximum power density of 16 000 W kg−1 at an energy density of 37.4 Wh kg−1.
1. Introduction Electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, have recently attracted growing
interest as efficient energy-storage devices owing to their high power density, short charging time, and long durability. These benefits enable them highly promising for use in electric vehicles, hybrid electric vehicles, and other high power
Y.-C. Chen Karlsruhe Institute of Technology (KIT) Institut für Anorganische Chemie Engesserstraße 15, D-76131, Karlsruhe, Germany Dr. Y.-G. Lin National Synchrotron Radiation Research Center Hsinchu 30076, Taiwan E-mail: [email protected] Prof. Y.-K. Hsu Department of Opto-Electronic Engineering National Dong Hwa University Hualien 97401, Taiwan E-mail: [email protected]
Prof. S.-C. Yen Department of Chemical Engineering National Taiwan University Taipei 10617, Taiwan Dr. K.-H. Chen Institute of Atomic and Molecular Sciences Academia Sinica Taipei 10617, Taiwan Dr. K.-H. Chen, Dr. L.-C. Chen Center for Condensed Matter Sciences National Taiwan University Taipei 10617, Taiwan E-mail: [email protected]
DOI: 10.1002/smll.201400597 small 2014, DOI: 10.1002/smll.201400597
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energy sources.[1,2] Numerous efforts have been devoted to developing active electrode materials, where the transitionmetal oxides, conducting polymers, and carbon materials dominate mainly the landscape of ECs.[3] Metal oxides, using fast and reversible redox reactions at the surface of active materials, are typical examples of pseudocapacitive materials, which could offer high power density as well as high energy density. Although the metal oxides are well-established for electrochemical energy-storage application, there are only a few reports on the study of anode materials due to the unsatisfactory capacitive performance. Iron oxides are a class of materials that are potentially advantageous as negative electrode materials for ECs owing to their low cost and low toxicity.[4,5] A typical redox system involving a single-electron transfer is responsible for iron oxide pseudocapacitive behavior. However, the low electrical conductivity and low ion diffusion constant of conventional iron oxides can result in a low specific capacitance and poor rate capability, which is a major bottleneck for their application in ECs. Although blending iron oxide with carbon materials to form composites can effectively increase the charge transfer, the restack and/or the aggregation of carbon materials have limited the electrochemical performance of the devices.[5b,e,6] Consequently, it is necessary and significant to intrinsically change the textural property of iron oxide material, which facilitates electronic and ionic transport and increases electrochemical performance. It is well known that the process of the charge storage involves the insertion/extraction of protons or ions in the first few nanometers on the surface of electrode materials for pseudocapacitors. Iron oxyhydroxide lepidocrocite (γ-FeOOH) with open two-dimensional (2D) permeable channels may be favorable for ECs, as we recently described for birnessite-type MnO2 with layered nanostructures to facilitate the ion transportation and exhibit excellent capacitive performance.[2d] γ-FeOOH with orthorhombic crystal structure consists of infinite layer of [FeO6] octahedra, which elongates perpendicularly to the direction of b axis and linked by hydrogen bondings with an interlayer spaces of b = 12.52 Å.[7] Particularly, the 2D layered structure with good interlayer conductivity of γ-FeOOH could greatly benefit ECs. It is therefore anticipated that the design of anode materials based on γ-FeOOH with layered characteristics would be an ideal microstructural solution to definitely decrease the diffusion length of ions, increase the contact area with electrolyte as well as improve active material utilization, which leads to an enhanced electrochemical performance. Quite surprisingly, γ-FeOOH with 2D layered channels as negative electrode materials for ECs are never reported thus far. On the other hand, the rational design of nanoarchitectures with specific functions in energy storage applications is also a key requisite to increase energy and power density in electrical storage systems. Definitely, energy extracted from the electrode material strongly depends on the textural and morphological effects at the electrolyte/material interface. Some of recent reports have emphasized the structureactivity relationship of electrode material which accentuates the importance of surface morphology on the charge storage performance.[3d,8] 2D nanosheets (NSs) with one or several layers of atom or crystallites have been investigated
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extensively as a new class of nanomaterials. To the best of our knowledge, γ-FeOOH-based electrodes with 2D morphologies and characteristic pseudocapacitance behavior for ECs remain scarce. Thus, it would be of significance to design γ-FeOOH with the desirable nanoarchitecture and crystal structure for an improved electrochemical performance. In this work, for the first time, novel γ-FeOOH NSs were prepared via a simple one-step electroplating route without the assistance of any templates or surfactants. The need for binders or conducting additives, which add extra contact resistance and weight, can be eliminated in this case. Moreover, we have studied the relationship between the charge-storage characteristics of γ-FeOOH anodes and the corresponding NS morphology. An asymmetric supercapacitor was further realized using the MnO2 NS as the positive electrode and the γ-FeOOH NS as the negative electrode. The obtained device can be operated reversibly at a maximum cell voltage of 1.85 V in a mild aqueous electrolyte, delivering both high power density and good rate performance.
2. Results and Discussion 2.1. Characterization of γ-FeOOH NS/CC as Anode Material Our attempts employing simple one-step electroplating approach for synthesis of FeOOH nanostructures on the carbon cloth (CC) resulted in formation of radiating NSs as shown in Figure 1a. These NSs are smooth and homogeneous with an average thickness of 30∼50 nm and an average length of ≈1.4 µm. The crystal structure of these NSs was studied by X-ray diffraction (XRD) analysis (Figure 1b), which matched with the JCPDS (No. 76-2301) of orthorhombic-type γ-FeOOH. A high-resolution transmission electron microscopy (HRTEM) image of the FeOOH NSs also shows that the lattice fringe spacing between two adjacent crystal planes of the particle is of 0.238 nm, corresponding to the (111) lattice plane of these nanocrystals (Figure S1, Supporting Information). The Brunauer-Emmett-Teller (BET) surface area of γ-FeOOH NS/CC nanocomposites is 170 m2 g−1 which stands in contrast to that of pure CC (26 m2 g−1) in the absence of γ-FeOOH NS. The corresponding X-ray photoelectron spectroscopy (XPS) provides further information about the chemical states of the surface elements of the products, as shown in Figure 1c,d. It is noted that there are two distinct peaks at binding energies of ≈711.4 eV for Fe 2p3/2 and ≈725 eV for Fe 2p1/2 with a shake-up satellite at ≈719.5 eV in the Fe 2p core level spectrum. This is characteristic of Fe3+ in FeOOH.[9,10] Figure 1d shows the O 1s spectrum of the FeOOH NS/CC electrode, which could be fitted into three main constituent peaks corresponding to different oxygen containing species, viz. Fe–O–Fe bond, Fe–O–H bond, and H–O–H bond, respectively. Notably, the H–O–H bond corresponds to water molecule, which confirms the FeOOH NS to be in hydrated form. Electrochemical characteristics of the γ-FeOOH NS/ CC electrodes prepared for different deposition periods were evaluated by using cyclic voltammetry (CV) and galvanostatic charge/discharge methods. Figure 2a presents the
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small 2014, DOI: 10.1002/smll.201400597
Iron Oxyhydroxide Lepidocrocite Nanosheet as Anode Material for Asymmetric Supercapacitors
Figure 1. a) SEM images and b) XRD patterns (reference: JCPDS card no. 76-2301) of γ-FeOOH NS/CC hybrid electrodes (deposition time of 30 min). XPS spectra of c) Fe 2p and d) O 1s transitions for γ-FeOOH NS/CC hybrid electrodes (deposition time of 30 min).
CV curves for various electrodes, measured in 1 m Li2SO4 electrolyte with a potential scan rate of 50 mV s−1. Notably, the voltammetric current response increased substantially with increase in deposition time from 30 to 90 min. This is related to the increase in the loading amount of γ-FeOOH NSs during this deposition period, as illustrated in Figure 2b. However, degradation in the gravimetric capacitance of γ-FeOOH NS/CC electrodes, as calculated by the specific capacitance relationship reported in the literature,[2e] was observed at deposition time over 30 min, presumably due to the poor utilization of electroactive species and reduced active surface area of γ-FeOOH NSs at the thicker thickness (Figure S2, Supporting Information). Furthermore, the CV results of γ-FeOOH NS/CC electrode are almost one order of magnitude higher than that of bare CC recorded in the same condition. This improvement in the capacitive performance of γ-FeOOH NS/CC electrode is, hence, believed to result from the γ-FeOOH NS not the CC. As can be seen from the SEM images, deposition of γ-FeOOH NS uniformly covers the entire surface of CC. In this case, the weight of γ-FeOOH NS on the CC is considered in the calculation of specific capacitance for γ-FeOOH NS/CC electrode in order to deduce a meaningful capacitive performance. Besides, the specific capacitance of iron oxide-based electrodes is generally reported to be within 22–166 F g−1 and is strongly influenced by the preparation methods. The need for binders or small 2014, DOI: 10.1002/smll.201400597
conducting additives, which add extra contact resistance and weight, can be eliminated in our case. More significantly, the optimum specific capacitance achieved in this study is as high as 310.3 F g−1, which is higher than many of the data reported in the literature,[3b,4d,e,5a,d,g,h,6b,c] suggesting that the simple electrochemical process for γ-FeOOH NS deposition is highly promising for fabrication of γ-FeOOH NS/CC electrodes with superior pseudocapacitive performance. Figure 2c shows that the current density also increases with an increase in scan rate. This implies that the redox reaction is not kinetically limited at least within the scan rates of 2–100 mV s−1, thus, indicating a significant rate capability. Moreover, the inset of Figure 2d shows the chargedischarge curves of γ-FeOOH NS/CC electrodes at different discharge currents within a potential window of −0.8 to −0.1 V. The gravimetric capacitance of the electrode can be seen to decrease with increasing charge-discharge current (Figure 2d). The capacitance retention is about 70% with the increase in discharge current from 0.13 to 12.6 A g−1. It should be noted that the gravimetric capacitance of γ-FeOOH NS/CC electrodes at the ultrahigh charge/discharge current of 12.6 A g−1 remains 219.5 F g−1, indicating a high rate capability. To the best of our knowledge, this value of weight-normalized capacitance is the highest reported to date for the iron oxide-based anodes or even higher than that of many transition metal oxide anode materials.[4c,d,5a,11]
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Figure 2. a) Cyclic Voltammogramms of γ-FeOOH NS/CC hybrid electrodes with different deposition times at a scan rate of 50 mV s−1. b) Actual γ-FeOOH loading amount and gravimetric capacitance as a function of deposition time. c) Cyclic Voltammogramms of γ-FeOOH NS/CC hybrid electrodes at different scan rates. d) The gravimetric capacitances as functions of charge-discharge specific current. Inset: Charge-discharge tests within the potential window of −0.8 to −0.1 V vs Ag/AgCl.
From the above measurements, a maximum power density of 9000 W kg−1 at the energy density of 29.8 Wh kg−1 could be achieved, so far the highest value of the reported anode materials in pseudocapacitors. Therefore, the superb performance of γ-FeOOH NSs/CC obtained here confirms the advantages of 2D layered channels of γ-FeOOH NS that facilitate the fast penetration of the electrolytes through γ-FeOOH NSs to allow rapid electron-transfer for charge storage and delivery.
2.2. Investigation of Pseudocapacitive Mechanism To elucidate the mechanism of energy storage and the variation of Fe oxidation state of the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte during charge/discharge cycles, systematic in situ X-ray absorption spectroscopy (XAS) studies were carried out in Figure 3. In situ X-ray absorption near-edge structure (XANES) spectra of the γ-FeOOH NS/CC electrodes are presented in Figure 3a together with that of Fe oxides of known Fe oxidation states serving as reference compounds. Although all the spectra do not exhibit considerable difference in shape, revealing a similarity in the structural characteristics of γ-FeOOH, a clear energy shift of the adsorption peak, toward higher energy with varying applied potential, can be recognized. The absorption threshold energy (E0), which is obtained from the first inflection point on the absorption edge, is linearly
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correlated with the oxidation state of Fe in the γ-FeOOH NS/ CC electrodes (Figure 3b). According to the E0 derived from the in situ XANES spectra during charge/discharge cycles, the average oxidation state of Fe of the γ-FeOOH NS/CC electrodes was determined in the sequence shown in Figure 3c. The results clearly confirm a large electronic and ionic conductivity for the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte and a continuous and reversible faradic redox transition of γ-FeOOH NS occurred that contributes the superior capacity performance observed in Figure 2. Figure 3d shows the Fourier-transformed (FT) magnitude of Fe K-edge EXAFS spectra of the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte measured at several potentials in a sequence. The first FT maximum about 1.5 Å is attributed to an interaction of central Fe atoms with six coordinated oxygen atoms in the first coordination shell (i.e. Fe-O bond within a [FeO6] octahedral unit). The second FT maximum located near 2.7 Å corresponds to the nearest Fe atoms between neighboring [FeO6] octahedral sites (i.e., Fe-Fe interatomic distance between neighboring [FeO6] units). The interatomic distance of the Fe-O bond progressively decreases with varying applied potential from −0.8 to −0.1 V, originating from an oxidation from Fe2+ to Fe3+. Meanwhile, a decreased distance of the Fe–Fe could also be observed, indicating a contraction between the [FeO6] octahedral units. Here, Li+ is the primary working species that can intercalate into the 2D layered
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Iron Oxyhydroxide Lepidocrocite Nanosheet as Anode Material for Asymmetric Supercapacitors
Figure 3. a) In situ Fe K-edge XANES spectra of the γ-FeOOH NS/CC electrodes in Li2SO4 electrolyte with respect to applied potential from −0.8 to −0.1 V. b) Relation between edge position and Fe average oxidation state for γ-FeOOH NS/CC electrodes at different applied potentials. c) Variation of Fe oxidation state with respect to applied potential. d) In situ Fe K-edge EXAFS spectra of the γ-FeOOH NS/CC electrodes in Li2SO4 electrolyte with respect to applied potential from −0.8 to −0.1 V.
channels between the [FeO6] units, the Fe–Fe distance would be expected to increase. On the contrary, oxidation of the electrode led to a clear decrease of the Fe–Fe distance. This can be attributed to the desertion of Li+ from the γ-FeOOH NS structure. In a Li+ operative electrolyte, it could be noted that both the Fe–O and Fe–Fe peaks shift in the same direction; that is, toward longer Fe–O and Fe–Fe interatomic distances upon lithiation and back to shorter distances upon delithiation. Moreover, as noted from Figure 3d, the intensity of the FT Fe-O maximum increases with varying applied potential from −0.8 to −0.1 V. This indicates that the relief of distortion in γ-FeOOH NS under deintercalation process could restore a symmetric oxygen distribution in a [FeO6] octahedral unit. The redox transition during the fast charge/discharge cycling is hence charge compensated by the reversible insertion and desertion of Li+ into and from 2D layered channels between the [FeO6] octahedral units. Accordingly, this pseudocapacitive mechanism of the γ-FeOOH NS/CC electrode, based on Li+ as working ions in Li2SO4 electrolyte, is proposed: Fe ( III) OOH + Li + ⇔ LiFe ( II) OOH
(1)
Therefore, our superior electrochemical performance of the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte small 2014, DOI: 10.1002/smll.201400597
could be ascribed to the following factors. A unique 2D and mesoporous architecture of γ-FeOOH NS provides a largearea contact for the electrode and electrolyte and enables accommodation of the large volume change and release of the associated strain generated during rapid charge and discharge cycling. Meanwhile, the 2D layered channel and mesoporous texture of the γ-FeOOH NS also enhance the electrolyte penetration and ion migration by decreasing the resistance and contracting the diffusion path for ion, consequently increasing the effective utilization of the γ-FeOOH NSs. The highly electrically conductive CC facilitates transport of electrons to γ-FeOOH, producing a rapid faradaic reaction. Moreover, Li+ as the primary working species reversibly insert into and leave the channels between the [FeO6] octahedral subunits and cause the large variations of the oxidation state of Fe of the electrode. To support our explanation, quantitative electrochemical impedance spectroscopy (EIS) analysis, which is a powerful tool to study the electrochemical characteristics of the electrode, was further performed. Figure 4 shows the Nyquist plot of the γ-FeOOH NS/CC electrode at different applied voltages with a perturbation of 5 mV in a frequency range from 100 kHz to 0.1 Hz. The electrode can be analyzed by the complex nonlinear least square (CNLS) fitting method based on a transmission line model, which is a classical model of porous electrode with large
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of Figure 4). Ci (i = 1,2,…5) represents double-layer capacitance and/or pseudocapacitance and Ri (i = 1,2,…5) represents resistance in redox transition of γ-FeOOH NS and/or the ion penetration/intercalation into the pores/channels between infinite [FeO6] layers. Excellent fitting can be obtained with the five element transmission line model, and the fitting values of these elements can be found in inset of Figure 4. According to the time constant (RiCi), all the capacitance can be achieved in a very short period of time (less than 3 s), meaning that the electrode with the unique nanoarchitecture has smooth and short channels to be quickly accessed by electrolyte. This is why γ-FeOOH NS/CC electrodes can deliver high specific capacitance at the high charging/discharging current densities. 2.3. Asymmetric Supercapacitor based on γ-FeOOH NS/CC Anode and MnO2 NS/CC Cathode
Figure 4. The Nyquist plots of the γ-FeOOH NS/CC electrodes at different applied potentials. Inset: The equivalent circuit and the fitting results from the simulation of EIS data.
surface area. Here, the equivalent circuit of the five-element transmission line model, containing distributed capacitors and resistors is used to simulate the EIS curve of the electrode (inset
As mentioned above, with ultrahigh power density, excellent rate performance and improved specific capacity, the as-prepared γ-FeOOH NS/CC nanocomposites should be the promising anode materials for high-power ECs. In order to further investigate this potential superiority, we designed a hybrid supercapacitor in a mild aqueous electrolyte, consisting of a negative electrode of γ-FeOOH NS/ CC and a positive electrode of MnO2 NS/CC. Figure 5a
Figure 5. a) Comparison of Cyclic Voltammogramms curves of MnO2 NS electrodes and FeOOH NS electrodes in a three-electrode cell at a scan rate 5 mV s−1. b) Cyclic Voltammogramms of MnO2 NS-CC//FeOOH NS-CC hybrid supercapacitors at different scan rates from 2 to 500 mV s−1. c) Galvanostatic charge/discharge curves at different specific current from 0.4 to 8.4 A g−1. d) Ragone plots of MnO2 NS-CC//FeOOH NS-CC hybrid supercapacitors compared with other reported asymmetric supercapacitors and commercial electronic energy storage devices. Reproduced with permission.[3f] Copyright 2008, Nature Publishing Group.
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small 2014, DOI: 10.1002/smll.201400597
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compares CV curves of the MnO2 NS/CC electrode and the γ-FeOOH NS/CC electrode in Li2SO4 electrolyte under a three-electrode cell at a scan rate of 5 mV s−1. Clearly, the sum of the potential ranges of these two electrodes is 1.85 V, indicating that they can potentially be used in a high-voltage asymmetric supercapacitor. Figure 5b shows the CV curves of the full-sized hybrid supercapacitors (MnO2 NS-CC// FeOOH NS-CC) in various scan rates. The results show that the current density increases with an increase in scan rate. This implies that the redox reaction is not kinetically limited at least within the scan rates of 2–500 mV s−1, thus, indicating an improved rate capability. Galvanostatic charge/ discharge measurements were also made at various current densities to elucidate the electrochemical behaviors of the MnO2 NS-CC//FeOOH NS-CC hybrid supercapacitors with Li2SO4 electrolyte at a cell voltage of 1.85 V, as presented in Figure 5c. The charge curves are clearly symmetrical with the corresponding discharge curves, revealing that the iR drop in this hybrid system under the charge/discharge process is really small. Particularly, compared with the performance of other reported various hybrid supercapacitors, it can be concluded that the MnO2 NS-CC//FeOOH NS-CC hybrid supercapacitor reaches the highest power density with an improved energy density, which is summarized in Figure 5d. Quite surprisingly, our hybrid supercapacitor can deliver a maximum power density of 16 000 W kg−1 at an energy density of 37.4 Wh kg−1, which is the highest value among various reported iron oxide-based hybrid supercapacitors. This result is even superior to current asymmetric hybrid electrochemical cells (AHEC) based on transition metal oxides as anode materials.[3i,j,4d,6a,11] It is also worthy to note that for a packaged cell, the MnO2 NS-CC//FeOOH NS-CC hybrid device has much higher energy density than the commercial ECs and can be even competitive with commercial lithium-ion batteries (LIBs),[3f] while the power density can also keep an extra high value, meaning a faster charge-discharge rate than commercial LIBs.
3. Conclusions The efficient energy-storage system was successfully developed based on layered γ-FeOOH NS anodes. For the first time, the microstructural influence on the pseudocapacitive performance of the obtained γ-FeOOH NSs was investigated via in situ X-ray absorption spectroscopy and electrochemical measurement. Most importantly, the γ-FeOOH NS showed the outstanding pseudocapacitive performance, which achieved the extraordinary power density of 9000 W kg−1 with good rate performance, so far the highest value of the reported anode materials. Last but not least, the asymmetric supercapacitors with excellent electrochemical performance were further realized by using 2D MnO2 and γ-FeOOH NSs as cathode and anode materials, delivering a maximum power density of 16 000 W kg−1 at an energy density of 37.4 Wh kg−1. The work demonstrated here will bring more insight for the design and optimization of high-performance pseudocapacitors. small 2014, DOI: 10.1002/smll.201400597
4. Experimental Section Materials: Carbon cloth (B-1, Designation A: plain-weave, 116 g m−2, 0.35 mm thickness; no wet-proofing) works as the substrate, which was purchased from E-TEK Division (USA). Ferrous ammonium sulfate (Fe(NH4)2(SO4)2·6H2O) and sodium acetate (CH3COONa) worked as the precursor solution were all obtained from Sigma-Aldrich. All the chemicals used in this study were of analytical grade and were used as received without any further purification. All solutions were prepared with deionized water of resistivity not less than 18.2 MΩ cm. Preparation of γ-FeOOH Nanosheets/CC Electrodes: Carbon cloth (CC) worked as supporting material was first attached by a copper wire at the edge and then the junction was covered by the silver paste to ensure a well conducted circuit for the following electrodeposition. The connecting pad was further coated with epoxide in order to forbid unwanted side reactions and allow an exposure of carbon cloth with a geometrical area ≈1 cm2 to the precursor solution. The well prepared carbon cloth was then immersed into the precursor solution, which contains 0.01 M ferrous ammonium sulfate and 0.04 M sodium acetate, and nanostructured γ-FeOOH was electrodeposited onto the current collector under a constant potential of 0.7 V for 30, 60, and 90 min, respectively. The as-prepared specimens were then rinsed with distilled water for washing free of the deposition solution. The loading amount of electroactive materials on the carbon cloth were determined by the inductively coupled plasma-optical emission spectroscopy (ICP-OES, PerkinElmer ICP-OES Optima 125 3000). The deposition per unit area was found to be between 0.8–1.8 mg cm−2. Preparation of MnO2 Nanosheets/CC Electrodes: The detailed procedure for the fabrication of MnO2/CC electrode could be found elsewhere, but is described here briefly.[2d,3c] The deposition of MnO2 onto carbon cloth was performed by an anodizing method in an electrolyte solution containing manganese sulfate (MnSO4·5H2O) and sulfuric acid (H2SO4) at a constant current density of 0.5 mA cm−2 for 30 min. The as-prepared specimens were then rinsed with distilled water for washing free of the deposition solution. The loading amounts of MnO2 on the carbon cloth were determined by the inductively coupled plasma-optical emission spectroscopy as well. Characterizations of γ-FeOOH Nanosheets/CC Electrodes: The nanostructures and morphological evolution of γ-FeOOH/CC electrodes with different preparation conditions were examined using field-emission scanning electron microscopy (FESEM, JEOL6700). The X-ray diffraction spectroscopy (Brucker D8 Advanced diffractometer) was applied to identify the crystal phase of the specimens. The chemical compositions of the fabricated composites were collected by the X-ray photoluminescence spectroscopy using a Perkin Elmer PHI 1600 ESCA system. A potentio/galvanostat electrochemical workstation (Solartron electrochemical test system 1470E) was employed to evaluate the electrochemical performances of the electrodes. The measurements were conducted at ambient temperature using a conventional three-electrode system, where the electroactive materials work as the working electrode, a square Pt foil as the counter electrode and an Ag/AgCl electrode in 3 M KCl as reference electrode. Cyclic voltammetry (CV), galvanostatic charge/discharge method and AC impendence analysis were applied to investigate the capacitive performance of composited electrode. All potentials reported in this article are with respect
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to that of Ag/AgCl (3 M KCl, 0.207 V vs SHE) reference electrode. Posterior to the above mentioned characterizations, the oxidation state and crystal-structural environment of FeOOH/CC electrodes under various applied potentials were then investigated with in situ X-ray absorption spectroscopy (XAS) under fluorescence mode. A sealed spectro-electrochemical cell with a fluorescencetransparent Kapton tape window was employed. Prior to collecting the XAS spectra at a given applied potential, the specimen was kept at the designated potential for at least 15 min for ensuring the electroactive materials to reach a steady state. The XAS experiments were performed on beamline 17C at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. The storage ring was operated with electron energy of 1.5 GeV and a current between 100 and 200 mA. A Si (111) double-crystal monochromator was employed for energy selection. The X-ray absorption energy was calibrated using the first inflection point in the Fe K-edge main absorption region of a metallic Fe foil, which was measured before every XAS scan. Assembly of Asymmetric Capacitors: An asymmetric supercapacitor was built by adopting MnO2/CC as the cathode material and γ-FeOOH/CC as the anode material in a two-electrode system, where the 1 M Li2SO4 mild aqueous solution as the electrolyte. A separator permeable to ion transport was placed between the electrodes. CV and galvanostatic charge/discharge analyses were applied to investigate the electrochemical performance of hybrid capacitor. All the operating current densities were calculated based on the total mass of the system, viz. the total weight of the positive and negative electroactive materials.
[3]
[4]
[5]
Supporting Information [6]
Supporting Information is available from the Wiley Online Library or from the author. [7]
Acknowledgements
[8]
This work was supported by the Ministry of Science Technology, Ministry of Education, Taiwan, and AOARD under AFOSR, US. The authors gratefully thank MOST, NSRRC, IAMS, and NTU for financial support in this project.
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: March 5, 2014 Revised: April 21, 2014 Published online:
small 2014, DOI: 10.1002/smll.201400597
