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Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors
Nano Energy (]]]]) ], ]]]–]]]
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Available online at www.sciencedirect.com
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FULL PAPER
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Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors
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Jizhang Chena, Junling Xua, Shuang Zhoua, Ni Zhaoa,n, Ching-Ping Wonga,b,nn
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Department of Electronic Engineering, The Chinese University of Hong Kong, New Territories, Hong Kong School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States
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Received 19 October 2015; received in revised form 25 December 2015; accepted 26 December 2015
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KEYWORDS
Abstract
Aqueous asymmetric supercapacitor; Iron oxyhydroxide; Double hydroxide; Electrodeposition; Nanostructure; Amorphous
Amorphous fish-scale-like FeOOH and flower-like Co–Ni double hydroxides (Co–Ni-DH) have been synthesized through one-step electrodeposition. The unique nanostructures of the hydroxides provide a large number of surface active sites, while the amorphous nature of the material systems facilitates the diffusion and reaction of electrolyte ions and enables an isotropic charging/ discharging process. Because of these advantages, the FeOOH and Co–Ni-DH electrodes exhibit high pseudocapacitances of 1.11 F cm 2/867 F g 1 and 1.48 F cm 2/1201 F g 1, respectively. In addition, high rate capabilities and superior cyclabilities are achieved. By using the FeOOH and Co– Ni-DH as the anode and cathode, respectively, we have assembled an aqueous asymmetric supercapacitor that delivers a high energy density of 86.4 W h kg 1/0.723 mW h cm 3 and a high power density of 11.6 kW kg 1/0.973 mW cm 3. Moreover, the fabrication process presented in this work is facile, scalable, cost-effective, and environmentally benign, offering a feasible solution for manufacturing next-generation high-performance energy storage devices. & 2016 Published by Elsevier Ltd.
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Corresponding author. Tel.: +852 3943 4347. nn Corresponding author at: Department of Electronic Engineering, The Chinese University of Hong Kong, New Territories, Hong Kong. Tel.: +852 3943 8447. E-mail addresses: [email protected] (N. Zhao), [email protected] (C.-P. Wong).
With notable features of rapid energy storage/release and excellent stability, supercapacitors are ideal for high-power applications such as hybrid electrical vehicles, back-up power systems, cranes, and forklifts. However, conventional electrical double-layer capacitors (EDLCs) are limited by their low energy
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http://dx.doi.org/10.1016/j.nanoen.2015.12.029 2211-2855/& 2016 Published by Elsevier Ltd.
Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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density. This problem can effectively be solved by replacing conventional carbon electrode materials with pseudocapacitive materials, which can store and release considerably more charges than EDLC materials through redox reactions [1–4]. Moreover, pseudocapacitive materials can be used to construct aqueous asymmetric supercapacitors that can increase the operating potential of aqueous EDLCs from approximately 1.0 V to approximately 1.6 V. Therefore, several studies have investigated transition-metal oxides [5–13], hydroxides [14–16], sulfides [17–20], as well as conductive polymers [21–23] to achieve medium energy density and high power density. Because pseudocapacitive activities rely on surface or nearsurface redox reactions, the performance of pseudocapacitive materials is kinetically determined by charge transports; therefore, their theoretically predicted high capacitances are seldom obtained in practical experiments. However, nanoengineering is a favorable strategy for enhancing their performance [24–27]. Nanostructures offer several advantages over their bulk counterparts: first, the electrode/electrolyte contact area per unit mass can be increased, resulting in more reaction sites; second, the transport paths for both electrolyte ions and electrons can be shortened, permitting higher capacitance and superior rate performance; and third, the mechanical strain and structural distortion upon redox reactions can be accommodated in a more efficient manner, providing a longer cycle life. Despite considerable achievements through nanoengineering, developing a facile, scalable, and cost-effective method for synthesizing nanostructured pseudocapacitive materials is challenging. Among various pseudocapacitive materials, Fe-based materials (e.g., Fe2O3, Fe3O4, and FeOOH) exhibit great promise as anode materials [28–38], owing to their high capacitance, low working potential, low cost, and environmental benigness. For the cathode material, double hydroxides (DHs) such as Co– Ni-DH, Co–Al-DH, Ni–Al-DH, and Ni–Mn-DH that exhibit rich redox activity and considerable interlayer spacing for charge exchanges, are commonly used [39–43]. In the present study, we used fish-scale-like nanostructured FeOOH as the anode material and flower-like Co–Ni-DH nanosheets as the cathode material, both of which are amorphous in nature. In general, amorphous materials have lower charge carrier mobility than crystalline materials. However, a large number of structural defects, such as vacancies, in amorphous materials facilitate the diffusion and reaction of electrolyte ions [2,44]. In addition, the strain and stress within amorphous materials are isotropic during charging/discharging, beneficial to the long-term electrochemical stability. Therefore for many material systems, including the ones discussed in this work, the amorphous or low-crystalline phase will lead to better performance. Because of their nanoscale and amorphous structures, the FeOOH and Co–Ni-DH together with an ASC device assembled from these materials exhibit low impedances, high capacitances, great rate capabilities, and superior cycling stabilities; hence, they hold considerable potential for commercial applications.
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Experimental section Synthesis of FeOOH and Co–Ni-DH All reagents were purchased from Aldrich, were of analytical grade, and were used as received. The FeOOH and Co–
Ni-DH were electrodeposited through a standard threeelectrode system by using a Ag/AgCl reference electrode and a VMP3 electrochemical workstation (Bio-Logic). The Co–Ni-DH was deposited on Ni foams (pretreated using a HCl solution to remove Ni oxides from the surface) at 1.0 V for 1000 s in an electrolyte containing 100 mM Co2 + and Ni2 + nitrates (Co:Ni= 2:1). Similarly, the FeOOH was deposited for 1800 s in an electrolyte containing 100 mM Fe3 + nitrate. Subsequently, the as-obtained Ni foams were ultrasonically rinsed using water and finally dried in an oven. The mass loadings of the active materials were determined using the weight differences between the Ni foams before and after immersion in the HCl solution through a high-accuracy microbalance. On average, the mass loadings for the FeOOH and Co–Ni-DH on the Ni foams were 1.28 and 1.23 mg cm 2, respectively.
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Characterization and electrochemical measurements
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X-ray diffraction (XRD) patterns were collected using a Rigaku (RU300) diffractometer with a Cu Kα radiation source (λ = 0.1540598 nm). The morphologies and elemental compositions were recorded using a field emission scanning electron microscope (FE-SEM, Quanta F400) configured for energy dispersive X-ray spectroscopy (EDX). The chemical compositions of the surfaces were investigated through Xray photoelectron spectroscopy (XPS, Physical Electronics PHI 5600). Three-electrode measurements were performed using the active electrode, Pt plate, Hg/HgO electrode, and a 3 M KOH aqueous solution as the working electrode, counter electrode, reference electrode, and electrolyte, respectively. Cyclic voltammetry (CV) and galvanostatic charging/ discharging (GCD) tests were conducted at different scan rates and current densities, respectively. Electrochemical impedance spectroscopy (EIS) tests were conducted with frequencies ranging from 10 mHz to 100 kHz with the amplitude set to 5 mV. For the two-electrode measurements, FeOOH, Co–Ni-DH, a 3 M KOH aqueous solution, and thin filter paper served as the negative electrode, positive electrode, electrolyte, and separator, respectively. CV, GCD, and EIS tests were then conducted accordingly.
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Results and discussion
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As illustrated in Figure 1a, the FeOOH and Co–Ni-DH were deposited on the Ni foams through facile and scalable onestep electrodeposition. During electrodeposition, NO3 in the electrolyte accepted electrons from the Ni foam and was reduced to OH on the surface of the Ni foam. Subsequently, the accumulation of OH in the local region led to the deposition of FeOOH or Co–Ni-DH on the Ni foam, as expressed by the following formulas: NO3 + 7H2O+ 8e -NH4+ + 100H 2Co
2+
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+ 30H -Fe(OH)3
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Fe(OH)3-FeOOH + H2O
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Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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Figure 1 (a) Schematic illustration for the fabrications of the FeOOH-1800 and CoNiDH-1000. SEM images of (b–d) FeOOH-1800 and (e–g) CoNiDH-1000.
These processes are cost-effective and environmentally benign; moreover, as-treated Ni foams can be used directly as binder-free electrodes. The obtained FeOOH and Co–NiDH were denoted FeOOH-X and CoNiDH-Y, where X and Y represent the deposition times for FeOOH and Co–Ni-DH, respectively. Figure 1b–d shows the fish-scale-like morphology of the FeOOH-1800 deposited on the Ni foam, whereas Figure 1e–g shows that the CoNiDH-1000 is composed of flower-like nanosheets. These morphologies can increase reaction sites and facilitate charge transports. In addition to analyzing the FeOOH-1800 and CoNiDH-1000, we also evaluated them by using different deposition times (Figures S1 and S2). When the deposition time is short, the FeOOH morphology consists of tiny particles and small nanosheets in the FeOOH-300 and FeOOH-900, respectively. When the time is long, the morphology consists of large sheets/small particles, large particles, and large dense particles for the FeOOH-3600, FeOOH-5400, and FeOOH-7200, respectively. For the Co–Ni-DH, the trend in morphological evolution is similar that longer deposition time results in thicker, larger, and denser nanosheets with a simultaneous increase in the flower size. When the time is 4000 s, the Co–Ni-DH turns to form large particles stacked on large thick sheets. Figure S3 shows XRD patterns of the FeOOH-1800 and CoNiDH-1000. The FeOOH-1800 only exhibits a hump in
accordance with lepidocrocite FeOOH (JCPDS 76-2301), indicating that the FeOOH is generally amorphous, similar to previously reported hydroxides [44–46]. For the CoNiDH1000, no peaks are observed, confirming that it is also amorphous. To determine the elemental compositions and valences in the FeOOH and Co–Ni-DH, X-ray photoelectron spectroscopy (XPS) characterizations were used (Figures S4 and 2). The Fe 2p spectrum of the FeOOH-1800 shown in Figure 2a exhibits two main peaks at 711.1 and 724.8 eV in addition to their shake-up satellite peaks at 719.1 and 732.4 eV, which are characteristic of Fe3 + in FeOOH [29,45–47]. In Figure 2b (deconvoluted O1s spectrum of the FeOOH-1800), the lower binding energy peak at 529.9 eV is associated with an Fe–O–Fe bond, whereas the higher peak at 531.4 eV is due to an Fe–O–H bond [29,46,48]. Another peak at 533.0 eV arising from the absorbed water [29,44–46] is extremely small. In particular, the OH area is approximately 45.7% of the total O 1s peak area, which is consistent with previously reported FeOOH spectra [45,49]. For the CoNiDH-1000, the Co 2p spectrum (Figure 2c) can be fitted with two spin-orbit doublets at 781.3 eV (Co 2p3/2) and 797.1 eV (Co 2p1/2) in addition to two shake-up satellites at 785.02 and 802.4 eV, respectively, corresponding to Co2 + [44,50,51]. Figure 2d depicts the Ni 2p spectrum of the CoNiDH-1000, containing two major peaks at 855.3 eV
Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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Figure 2 1000.
High resolution core level XPS spectra of (a) Fe 2p and (b) O 1s in the FeOOH-1800, (c) Co 2p and (d) Ni 2p in the CoNiDH-
(Ni 2p3/2) and 873.2 eV (Ni 2p1/2) in addition to their shake-up satellite peaks at 861.0 and 879.6 eV, which are attributed to the presence of Ni2 + [52–54]. According to the XPS results, the atomic ratio of Fe:O is estimated to be 1:2.14 in the FeOOH-1800, whereas that of Ni:Co in the CoNiDH-1000 is approximately 1:1.99, verifying the compositions of FeOOH and Co2Ni(OH)6, respectively. According to the EDX spectra in Figure S5, the atomic percentages of Fe and O in the FeOOH-1800 are 4.29% and 9.32%, respectively, which accords with the composition of FeOOH. Moreover, the signal from the Ni foam is intense in the EDX spectra but negligible in the XPS spectra. This phenomenon is caused by the different detection depths of EDX and XPS characterizations. The FeOOH-X and CoNiDH-Y were separately evaluated using cyclic voltammetry (CV) in a KOH electrolyte, and their scan rate-dependent areal and gravimetric capacitances (Ca and Cg, refer to the calculations in Supporting information) are summarized in Figures 3a and b, and S6a and S6b, respectively. When the deposition time is increased, Ca also increases, whereas Cg decreases. This is due to the higher mass loading induced by a longer
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deposition time. A higher mass loading can contribute to a higher Ca; however, this process is accompanied by increases in particle sizes that hinder the redox reactions; therefore, Cg is lowered. Here, the highest values of Ca for the FeOOH and Co–Ni-DH are 1599 mF cm 2 for the FeOOH7200 and 2735 mF cm–2 for the CoNiDH-6000, while the highest values of Cg are 1135.5 F g 1 for the FeOOH-300 and 1767.0 F g 1 for the CoNiDH-200. Considering that the medium deposition time contributes to a balanced performance between the Ca and Cg, we focus on the FeOOH-1800 and CoNiDH-1000, whose CV curves are shown in Figure 3c and d. These two samples exhibit typical redox reaction profiles. The possible reaction mechanisms of the FeOOH and Co–Ni-DH are expressed as follows:
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Ni(OH)2 + OH 2NiOOH + H2O+ e
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The highest values of Ca for the FeOOH-1800 and CoNiDH1000 are 1110 and 1478 mF cm–2 at 5 mV s–1, respectively.
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Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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Figure 3 Areal capacitances of (a) FeOOH and (b) Co–Ni-DH with respect to the different electrodeposition time, plotted vs. the scan rate, and CV curves of (c) FeOOH-1800 and (d) CoNiDH-1000 at various scan rates.
When the scan rate is increased 10-fold at 50 mV s–1, these two electrodes maintain high capacitances of 595 and 702 mF cm–2, manifesting great rate capabilities. These capacitances are considerably higher than those of recently reported Ni–Co–Fe hydroxides (420 mF cm–2) [44], Ni(OH)2 (70.6 mF cm–2) [14], Cu(OH)2 (213 mF cm–2) [55], MoS2 (14.5 mF cm–2) [20], and MnO2/CNT (100 mF cm–2) [56]. In terms of Cg, the FeOOH-1800 and CoNiDH-1000 exhibit 867.3 and 1201.2 F g–1 at 5 mV s–1, respectively. In particular, the performance of the FeOOH-1800 was markedly superior compared with that of recently reported Fe-based pseudocapacitive materials such as porous Fe3O4/C (139 F g–1) [32], graphene/Fe2O3 (347 F g–1) [34], Fe2O3 nanotubes (258 F g–1) [28], Fe2O3 nanorods (89 F g–1, 383 mF cm–2) [36], and FeOOH nanosheets (310 F g–1) [29]. The superior performances of the FeOOH-1800 and CoNiDH-1000 can be ascribed to the following factors. First, the nanostructured morphologies of the FeOOH-1800 and CoNiDH-1000 not only provide sufficient redox reaction sites, but also enable easy accesses for the electrolyte ions and electrons, thereby facilitating charge transports. Second, the amorphous phases are favorable for charge transfers and can therefore promote pseudocapacitances at high rates [2,44]. Third, the FeOOH and Co–Ni-DH are directly connected to the current collector without binders, which allows for rapid electronic transports.
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To further evaluate pseudocapacitive performances of the FeOOH-1800 and CoNiDH-1000, we fabricated an ASC device by using the FeOOH-1800 as the anode and CoNiDH1000 as the cathode, because their potential windows are compatible for delivering high voltages. The working mechanism of this full cell setup is derived from a combination of Eqs. (5)–(7). Figure 4a shows typical CV curves of the ASC at different scan rates in the potential range of 0.3– 1.6 V. These curves reveal redox profiles and a capacitance retention of 47.0% when the scan rate is increased from 10– 100 mV s–1. This retention is high considering the high mass loadings for both the anode and cathode in our ASC. Figure 4b shows cycling performances of the FeOOH-1800 and CoNiDH-1000 at 50 mV s–1 and the ASC at 100 mV s–1. The capacitances decrease relatively rapidly in the initial 200 cycles, due to deactivation of some defect sites in the amorphous FeOOH and Co–Ni-DH. However, after 3000 cycles, the FeOOH-1800, CoNiDH-1000, and ASC maintain 93.3%, 98.5%, and 92.3% of their capacitances at the 200th cycle, respectively, suggesting excellent long-term cycling stabilities. This may benefit from the amorphous nature of the electrode materials, which leads to isotropic strain/ stress upon repeated cycles. Figure 4c shows galvanostatic charging/discharging (GCD) curves of our ASC at various current densities. These curves
Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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Figure 4 Electrochemical performances of the FeOOH-1800//CoNiDH-1000 ASC device: (a) CV curves, (b) capacitance retentions vs. cycle numbers, (c) GCD curves, and (d) Ragone plots of gravimetric energy density vs. average gravimetric power density.
are nonlinearly correlated with the potential, indicating pseudocapacitive behaviors [57,58]. The CASC-a is calculated to be 849 mF cm–2 at a current density of 5 mA cm–2, which declines to 353 mF cm–2 at 50 mA cm–2. In addition, abrupt voltage drops are observed at the beginning of the discharging curves. This is likely due to the equivalent series resistance (RESR) of the ASC device, which consists of the electrical resistances of two electrodes, contact resistances at interfaces, and ionic resistance of the electrolyte. Herein the average RESR is calculated to be 2.22 Ω cm–2 (Figure S7). Energy and power densities are two crucial factors for ASC devices. Figure 4d displays gravimetric Ragone plots of our ASC compared with recently reported advanced ASCs. As the power density increases from 1831.6 to 11 628.5 W kg–1, the specific energy density decreases from 86.4 to 22.8 W h kg– 1 . Notably, such performance is superior to most reported ASCs; for example, NiCo2S4//graphene/C [4], graphene/ MnO2//graphene/Fe2O3 [34], and Co-Al-LDH//graphene/C [43], suggesting that the ASC configuration proposed in the present study is favorable for next-generation energy storage applications. Given that volumetric energy and power densities are crucial for practical applications, we provide these values in Figure S8. A high energy density of 0.7 mW h cm–3 is obtained at a high power density of 15.3 mW cm–3. Note that the volumes of two Ni foams and
the separator were considered when calculating the total volume of our ASC device. To demonstrate the application potential of our ASC, we connected three ASC coin cells in series and used the tandem device to power three blue light-emitting diodes (LEDs), as shown in Figure S9 and the Supplementary video. The LEDs are bright in initial 5 min. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2015. 12.029. Electrochemical behaviors were further investigated using electrochemical impedance spectroscopy (EIS) measurements, and the obtained the Nyquist plots are shown in Figure 5. The intercept of the high-frequency region with the real axis indicates the bulk resistance Rs. The Rs values of the FeOOH-1800, CoNiDH-1000, and ASC (2 cm2) are 3.9, 3.2, and 2.1 Ω, respectively. Following the intercept, a depressed semicircle represents the charge transfer impedance Rct, which is mainly related to the interface properties. The Rct of the CoNiDH-1000 is estimated to be merely 2.0 Ω on the basis of the equivalent circuit in the inset of Figure 5. Such a low Rct likely results from the large interlayer spacing of the Co–Ni-DH, which promotes charge transports. In addition, the Rct values of the FeOOH-1800 and ASC are less than 10 Ω, indicating rapid charge transports in the nanostructured FeOOH. In the low-frequency
Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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Figure 5 Nyquist plots of the FeOOH-1800, CoNiDH-1000, and ASC device. The inset shows the equivalent circuit for fitting these plots.
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Amorphous nanostructured fish-scale-like FeOOH and flower-like Co–Ni-DH were fabricated on Ni foams using facile, scalable, cost-effective, and environmentally benign electrodeposition. Nanostructures offer advantages such as large active sites, favorable electronic and ionic transports, and effective accommodation of strain/distortion. High capacitances of 867 F g–1/1.11 F cm–2 and 1201 F g–1/1.48 F cm–2 were obtained for the anode and cathode electrodes, respectively. The asymmetric supercapacitor fabricated using these two electrodes delivered a high energy density Q4 of 86.4 W h kg–1/0.7 mW h cm–3 when the power density was 1831.6 W kg–1/15.3 mW cm–3. Moreover, this device attained high rate capability and cycling stability as well as low impedance. In addition, our fabrication strategy can be extended to carbon cloth substrates and can be used for constructing all-solid-state flexible and bendable supercapacitors. Therefore, this study provides useful and valuable experimental results for constructing next-generation energy storage devices.
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This work was supported by Research Grants Council of Hong Kong (General Research Fund, No. 417012, GRC-NSFC, No. N_CUHK450/13, and TRS, No. T23-407/13-N).
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Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.12.029.
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Figure 6 CV curves of the unbent and bent ASCs. The insets show their corresponding photographs.
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region, the inclined line is associated with the ionic diffusion on the surface and within the bulk of the active material. In general, the FeOOH-1800, CoNiDH-1000, and ASC all exhibit low impedances with an upper limit of 30 Ω for both Z0 and Z″, which explains the high capacitances and superior rate performances obtained through the CV and GCD tests. We note that the Ni foam is a heavy current collector and adds considerable mass to the device. For practical applications, it is important to search for light-weight current collectors to support the growth of the electrode materials. Consequently, we further used light-weight carbon cloths to replace the Ni foams as the substrate for electrodepositing the FeOOH and Co–Ni-DH. Since solid-state flexible power sources have attracted considerable attention recently [59,60], we also assembled an all-solid-state ASC device consisting of two carbon cloth electrodes and a PVA/KOH gel electrolyte. This ASC device is highly flexible and bendable; even when bent to a strong degree it undergoes negligible
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Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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FULL PAPER
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Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors
Q1
Jizhang Chena, Junling Xua, Shuang Zhoua, Ni Zhaoa,n, Ching-Ping Wonga,b,nn
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a
Department of Electronic Engineering, The Chinese University of Hong Kong, New Territories, Hong Kong School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States
b
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Received 19 October 2015; received in revised form 25 December 2015; accepted 26 December 2015
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KEYWORDS
Abstract
Aqueous asymmetric supercapacitor; Iron oxyhydroxide; Double hydroxide; Electrodeposition; Nanostructure; Amorphous
Amorphous fish-scale-like FeOOH and flower-like Co–Ni double hydroxides (Co–Ni-DH) have been synthesized through one-step electrodeposition. The unique nanostructures of the hydroxides provide a large number of surface active sites, while the amorphous nature of the material systems facilitates the diffusion and reaction of electrolyte ions and enables an isotropic charging/ discharging process. Because of these advantages, the FeOOH and Co–Ni-DH electrodes exhibit high pseudocapacitances of 1.11 F cm 2/867 F g 1 and 1.48 F cm 2/1201 F g 1, respectively. In addition, high rate capabilities and superior cyclabilities are achieved. By using the FeOOH and Co– Ni-DH as the anode and cathode, respectively, we have assembled an aqueous asymmetric supercapacitor that delivers a high energy density of 86.4 W h kg 1/0.723 mW h cm 3 and a high power density of 11.6 kW kg 1/0.973 mW cm 3. Moreover, the fabrication process presented in this work is facile, scalable, cost-effective, and environmentally benign, offering a feasible solution for manufacturing next-generation high-performance energy storage devices. & 2016 Published by Elsevier Ltd.
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n
Corresponding author. Tel.: +852 3943 4347. nn Corresponding author at: Department of Electronic Engineering, The Chinese University of Hong Kong, New Territories, Hong Kong. Tel.: +852 3943 8447. E-mail addresses: [email protected] (N. Zhao), [email protected] (C.-P. Wong).
With notable features of rapid energy storage/release and excellent stability, supercapacitors are ideal for high-power applications such as hybrid electrical vehicles, back-up power systems, cranes, and forklifts. However, conventional electrical double-layer capacitors (EDLCs) are limited by their low energy
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http://dx.doi.org/10.1016/j.nanoen.2015.12.029 2211-2855/& 2016 Published by Elsevier Ltd.
Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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density. This problem can effectively be solved by replacing conventional carbon electrode materials with pseudocapacitive materials, which can store and release considerably more charges than EDLC materials through redox reactions [1–4]. Moreover, pseudocapacitive materials can be used to construct aqueous asymmetric supercapacitors that can increase the operating potential of aqueous EDLCs from approximately 1.0 V to approximately 1.6 V. Therefore, several studies have investigated transition-metal oxides [5–13], hydroxides [14–16], sulfides [17–20], as well as conductive polymers [21–23] to achieve medium energy density and high power density. Because pseudocapacitive activities rely on surface or nearsurface redox reactions, the performance of pseudocapacitive materials is kinetically determined by charge transports; therefore, their theoretically predicted high capacitances are seldom obtained in practical experiments. However, nanoengineering is a favorable strategy for enhancing their performance [24–27]. Nanostructures offer several advantages over their bulk counterparts: first, the electrode/electrolyte contact area per unit mass can be increased, resulting in more reaction sites; second, the transport paths for both electrolyte ions and electrons can be shortened, permitting higher capacitance and superior rate performance; and third, the mechanical strain and structural distortion upon redox reactions can be accommodated in a more efficient manner, providing a longer cycle life. Despite considerable achievements through nanoengineering, developing a facile, scalable, and cost-effective method for synthesizing nanostructured pseudocapacitive materials is challenging. Among various pseudocapacitive materials, Fe-based materials (e.g., Fe2O3, Fe3O4, and FeOOH) exhibit great promise as anode materials [28–38], owing to their high capacitance, low working potential, low cost, and environmental benigness. For the cathode material, double hydroxides (DHs) such as Co– Ni-DH, Co–Al-DH, Ni–Al-DH, and Ni–Mn-DH that exhibit rich redox activity and considerable interlayer spacing for charge exchanges, are commonly used [39–43]. In the present study, we used fish-scale-like nanostructured FeOOH as the anode material and flower-like Co–Ni-DH nanosheets as the cathode material, both of which are amorphous in nature. In general, amorphous materials have lower charge carrier mobility than crystalline materials. However, a large number of structural defects, such as vacancies, in amorphous materials facilitate the diffusion and reaction of electrolyte ions [2,44]. In addition, the strain and stress within amorphous materials are isotropic during charging/discharging, beneficial to the long-term electrochemical stability. Therefore for many material systems, including the ones discussed in this work, the amorphous or low-crystalline phase will lead to better performance. Because of their nanoscale and amorphous structures, the FeOOH and Co–Ni-DH together with an ASC device assembled from these materials exhibit low impedances, high capacitances, great rate capabilities, and superior cycling stabilities; hence, they hold considerable potential for commercial applications.
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Experimental section Synthesis of FeOOH and Co–Ni-DH All reagents were purchased from Aldrich, were of analytical grade, and were used as received. The FeOOH and Co–
Ni-DH were electrodeposited through a standard threeelectrode system by using a Ag/AgCl reference electrode and a VMP3 electrochemical workstation (Bio-Logic). The Co–Ni-DH was deposited on Ni foams (pretreated using a HCl solution to remove Ni oxides from the surface) at 1.0 V for 1000 s in an electrolyte containing 100 mM Co2 + and Ni2 + nitrates (Co:Ni= 2:1). Similarly, the FeOOH was deposited for 1800 s in an electrolyte containing 100 mM Fe3 + nitrate. Subsequently, the as-obtained Ni foams were ultrasonically rinsed using water and finally dried in an oven. The mass loadings of the active materials were determined using the weight differences between the Ni foams before and after immersion in the HCl solution through a high-accuracy microbalance. On average, the mass loadings for the FeOOH and Co–Ni-DH on the Ni foams were 1.28 and 1.23 mg cm 2, respectively.
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Characterization and electrochemical measurements
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X-ray diffraction (XRD) patterns were collected using a Rigaku (RU300) diffractometer with a Cu Kα radiation source (λ = 0.1540598 nm). The morphologies and elemental compositions were recorded using a field emission scanning electron microscope (FE-SEM, Quanta F400) configured for energy dispersive X-ray spectroscopy (EDX). The chemical compositions of the surfaces were investigated through Xray photoelectron spectroscopy (XPS, Physical Electronics PHI 5600). Three-electrode measurements were performed using the active electrode, Pt plate, Hg/HgO electrode, and a 3 M KOH aqueous solution as the working electrode, counter electrode, reference electrode, and electrolyte, respectively. Cyclic voltammetry (CV) and galvanostatic charging/ discharging (GCD) tests were conducted at different scan rates and current densities, respectively. Electrochemical impedance spectroscopy (EIS) tests were conducted with frequencies ranging from 10 mHz to 100 kHz with the amplitude set to 5 mV. For the two-electrode measurements, FeOOH, Co–Ni-DH, a 3 M KOH aqueous solution, and thin filter paper served as the negative electrode, positive electrode, electrolyte, and separator, respectively. CV, GCD, and EIS tests were then conducted accordingly.
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Results and discussion
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As illustrated in Figure 1a, the FeOOH and Co–Ni-DH were deposited on the Ni foams through facile and scalable onestep electrodeposition. During electrodeposition, NO3 in the electrolyte accepted electrons from the Ni foam and was reduced to OH on the surface of the Ni foam. Subsequently, the accumulation of OH in the local region led to the deposition of FeOOH or Co–Ni-DH on the Ni foam, as expressed by the following formulas: NO3 + 7H2O+ 8e -NH4+ + 100H 2Co
2+
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+ 30H -Fe(OH)3
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Fe(OH)3-FeOOH + H2O
(4)
Fe
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Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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Figure 1 (a) Schematic illustration for the fabrications of the FeOOH-1800 and CoNiDH-1000. SEM images of (b–d) FeOOH-1800 and (e–g) CoNiDH-1000.
These processes are cost-effective and environmentally benign; moreover, as-treated Ni foams can be used directly as binder-free electrodes. The obtained FeOOH and Co–NiDH were denoted FeOOH-X and CoNiDH-Y, where X and Y represent the deposition times for FeOOH and Co–Ni-DH, respectively. Figure 1b–d shows the fish-scale-like morphology of the FeOOH-1800 deposited on the Ni foam, whereas Figure 1e–g shows that the CoNiDH-1000 is composed of flower-like nanosheets. These morphologies can increase reaction sites and facilitate charge transports. In addition to analyzing the FeOOH-1800 and CoNiDH-1000, we also evaluated them by using different deposition times (Figures S1 and S2). When the deposition time is short, the FeOOH morphology consists of tiny particles and small nanosheets in the FeOOH-300 and FeOOH-900, respectively. When the time is long, the morphology consists of large sheets/small particles, large particles, and large dense particles for the FeOOH-3600, FeOOH-5400, and FeOOH-7200, respectively. For the Co–Ni-DH, the trend in morphological evolution is similar that longer deposition time results in thicker, larger, and denser nanosheets with a simultaneous increase in the flower size. When the time is 4000 s, the Co–Ni-DH turns to form large particles stacked on large thick sheets. Figure S3 shows XRD patterns of the FeOOH-1800 and CoNiDH-1000. The FeOOH-1800 only exhibits a hump in
accordance with lepidocrocite FeOOH (JCPDS 76-2301), indicating that the FeOOH is generally amorphous, similar to previously reported hydroxides [44–46]. For the CoNiDH1000, no peaks are observed, confirming that it is also amorphous. To determine the elemental compositions and valences in the FeOOH and Co–Ni-DH, X-ray photoelectron spectroscopy (XPS) characterizations were used (Figures S4 and 2). The Fe 2p spectrum of the FeOOH-1800 shown in Figure 2a exhibits two main peaks at 711.1 and 724.8 eV in addition to their shake-up satellite peaks at 719.1 and 732.4 eV, which are characteristic of Fe3 + in FeOOH [29,45–47]. In Figure 2b (deconvoluted O1s spectrum of the FeOOH-1800), the lower binding energy peak at 529.9 eV is associated with an Fe–O–Fe bond, whereas the higher peak at 531.4 eV is due to an Fe–O–H bond [29,46,48]. Another peak at 533.0 eV arising from the absorbed water [29,44–46] is extremely small. In particular, the OH area is approximately 45.7% of the total O 1s peak area, which is consistent with previously reported FeOOH spectra [45,49]. For the CoNiDH-1000, the Co 2p spectrum (Figure 2c) can be fitted with two spin-orbit doublets at 781.3 eV (Co 2p3/2) and 797.1 eV (Co 2p1/2) in addition to two shake-up satellites at 785.02 and 802.4 eV, respectively, corresponding to Co2 + [44,50,51]. Figure 2d depicts the Ni 2p spectrum of the CoNiDH-1000, containing two major peaks at 855.3 eV
Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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Figure 2 1000.
High resolution core level XPS spectra of (a) Fe 2p and (b) O 1s in the FeOOH-1800, (c) Co 2p and (d) Ni 2p in the CoNiDH-
(Ni 2p3/2) and 873.2 eV (Ni 2p1/2) in addition to their shake-up satellite peaks at 861.0 and 879.6 eV, which are attributed to the presence of Ni2 + [52–54]. According to the XPS results, the atomic ratio of Fe:O is estimated to be 1:2.14 in the FeOOH-1800, whereas that of Ni:Co in the CoNiDH-1000 is approximately 1:1.99, verifying the compositions of FeOOH and Co2Ni(OH)6, respectively. According to the EDX spectra in Figure S5, the atomic percentages of Fe and O in the FeOOH-1800 are 4.29% and 9.32%, respectively, which accords with the composition of FeOOH. Moreover, the signal from the Ni foam is intense in the EDX spectra but negligible in the XPS spectra. This phenomenon is caused by the different detection depths of EDX and XPS characterizations. The FeOOH-X and CoNiDH-Y were separately evaluated using cyclic voltammetry (CV) in a KOH electrolyte, and their scan rate-dependent areal and gravimetric capacitances (Ca and Cg, refer to the calculations in Supporting information) are summarized in Figures 3a and b, and S6a and S6b, respectively. When the deposition time is increased, Ca also increases, whereas Cg decreases. This is due to the higher mass loading induced by a longer
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deposition time. A higher mass loading can contribute to a higher Ca; however, this process is accompanied by increases in particle sizes that hinder the redox reactions; therefore, Cg is lowered. Here, the highest values of Ca for the FeOOH and Co–Ni-DH are 1599 mF cm 2 for the FeOOH7200 and 2735 mF cm–2 for the CoNiDH-6000, while the highest values of Cg are 1135.5 F g 1 for the FeOOH-300 and 1767.0 F g 1 for the CoNiDH-200. Considering that the medium deposition time contributes to a balanced performance between the Ca and Cg, we focus on the FeOOH-1800 and CoNiDH-1000, whose CV curves are shown in Figure 3c and d. These two samples exhibit typical redox reaction profiles. The possible reaction mechanisms of the FeOOH and Co–Ni-DH are expressed as follows:
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The highest values of Ca for the FeOOH-1800 and CoNiDH1000 are 1110 and 1478 mF cm–2 at 5 mV s–1, respectively.
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Figure 3 Areal capacitances of (a) FeOOH and (b) Co–Ni-DH with respect to the different electrodeposition time, plotted vs. the scan rate, and CV curves of (c) FeOOH-1800 and (d) CoNiDH-1000 at various scan rates.
When the scan rate is increased 10-fold at 50 mV s–1, these two electrodes maintain high capacitances of 595 and 702 mF cm–2, manifesting great rate capabilities. These capacitances are considerably higher than those of recently reported Ni–Co–Fe hydroxides (420 mF cm–2) [44], Ni(OH)2 (70.6 mF cm–2) [14], Cu(OH)2 (213 mF cm–2) [55], MoS2 (14.5 mF cm–2) [20], and MnO2/CNT (100 mF cm–2) [56]. In terms of Cg, the FeOOH-1800 and CoNiDH-1000 exhibit 867.3 and 1201.2 F g–1 at 5 mV s–1, respectively. In particular, the performance of the FeOOH-1800 was markedly superior compared with that of recently reported Fe-based pseudocapacitive materials such as porous Fe3O4/C (139 F g–1) [32], graphene/Fe2O3 (347 F g–1) [34], Fe2O3 nanotubes (258 F g–1) [28], Fe2O3 nanorods (89 F g–1, 383 mF cm–2) [36], and FeOOH nanosheets (310 F g–1) [29]. The superior performances of the FeOOH-1800 and CoNiDH-1000 can be ascribed to the following factors. First, the nanostructured morphologies of the FeOOH-1800 and CoNiDH-1000 not only provide sufficient redox reaction sites, but also enable easy accesses for the electrolyte ions and electrons, thereby facilitating charge transports. Second, the amorphous phases are favorable for charge transfers and can therefore promote pseudocapacitances at high rates [2,44]. Third, the FeOOH and Co–Ni-DH are directly connected to the current collector without binders, which allows for rapid electronic transports.
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To further evaluate pseudocapacitive performances of the FeOOH-1800 and CoNiDH-1000, we fabricated an ASC device by using the FeOOH-1800 as the anode and CoNiDH1000 as the cathode, because their potential windows are compatible for delivering high voltages. The working mechanism of this full cell setup is derived from a combination of Eqs. (5)–(7). Figure 4a shows typical CV curves of the ASC at different scan rates in the potential range of 0.3– 1.6 V. These curves reveal redox profiles and a capacitance retention of 47.0% when the scan rate is increased from 10– 100 mV s–1. This retention is high considering the high mass loadings for both the anode and cathode in our ASC. Figure 4b shows cycling performances of the FeOOH-1800 and CoNiDH-1000 at 50 mV s–1 and the ASC at 100 mV s–1. The capacitances decrease relatively rapidly in the initial 200 cycles, due to deactivation of some defect sites in the amorphous FeOOH and Co–Ni-DH. However, after 3000 cycles, the FeOOH-1800, CoNiDH-1000, and ASC maintain 93.3%, 98.5%, and 92.3% of their capacitances at the 200th cycle, respectively, suggesting excellent long-term cycling stabilities. This may benefit from the amorphous nature of the electrode materials, which leads to isotropic strain/ stress upon repeated cycles. Figure 4c shows galvanostatic charging/discharging (GCD) curves of our ASC at various current densities. These curves
Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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Figure 4 Electrochemical performances of the FeOOH-1800//CoNiDH-1000 ASC device: (a) CV curves, (b) capacitance retentions vs. cycle numbers, (c) GCD curves, and (d) Ragone plots of gravimetric energy density vs. average gravimetric power density.
are nonlinearly correlated with the potential, indicating pseudocapacitive behaviors [57,58]. The CASC-a is calculated to be 849 mF cm–2 at a current density of 5 mA cm–2, which declines to 353 mF cm–2 at 50 mA cm–2. In addition, abrupt voltage drops are observed at the beginning of the discharging curves. This is likely due to the equivalent series resistance (RESR) of the ASC device, which consists of the electrical resistances of two electrodes, contact resistances at interfaces, and ionic resistance of the electrolyte. Herein the average RESR is calculated to be 2.22 Ω cm–2 (Figure S7). Energy and power densities are two crucial factors for ASC devices. Figure 4d displays gravimetric Ragone plots of our ASC compared with recently reported advanced ASCs. As the power density increases from 1831.6 to 11 628.5 W kg–1, the specific energy density decreases from 86.4 to 22.8 W h kg– 1 . Notably, such performance is superior to most reported ASCs; for example, NiCo2S4//graphene/C [4], graphene/ MnO2//graphene/Fe2O3 [34], and Co-Al-LDH//graphene/C [43], suggesting that the ASC configuration proposed in the present study is favorable for next-generation energy storage applications. Given that volumetric energy and power densities are crucial for practical applications, we provide these values in Figure S8. A high energy density of 0.7 mW h cm–3 is obtained at a high power density of 15.3 mW cm–3. Note that the volumes of two Ni foams and
the separator were considered when calculating the total volume of our ASC device. To demonstrate the application potential of our ASC, we connected three ASC coin cells in series and used the tandem device to power three blue light-emitting diodes (LEDs), as shown in Figure S9 and the Supplementary video. The LEDs are bright in initial 5 min. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2015. 12.029. Electrochemical behaviors were further investigated using electrochemical impedance spectroscopy (EIS) measurements, and the obtained the Nyquist plots are shown in Figure 5. The intercept of the high-frequency region with the real axis indicates the bulk resistance Rs. The Rs values of the FeOOH-1800, CoNiDH-1000, and ASC (2 cm2) are 3.9, 3.2, and 2.1 Ω, respectively. Following the intercept, a depressed semicircle represents the charge transfer impedance Rct, which is mainly related to the interface properties. The Rct of the CoNiDH-1000 is estimated to be merely 2.0 Ω on the basis of the equivalent circuit in the inset of Figure 5. Such a low Rct likely results from the large interlayer spacing of the Co–Ni-DH, which promotes charge transports. In addition, the Rct values of the FeOOH-1800 and ASC are less than 10 Ω, indicating rapid charge transports in the nanostructured FeOOH. In the low-frequency
Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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Amorphous nanostructured FeOOH and Co–Ni double hydroxides
7 change in the CV curve compared with the pristine unbent ASC, as shown in Figure 6.
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Conclusion
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Figure 5 Nyquist plots of the FeOOH-1800, CoNiDH-1000, and ASC device. The inset shows the equivalent circuit for fitting these plots.
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Amorphous nanostructured fish-scale-like FeOOH and flower-like Co–Ni-DH were fabricated on Ni foams using facile, scalable, cost-effective, and environmentally benign electrodeposition. Nanostructures offer advantages such as large active sites, favorable electronic and ionic transports, and effective accommodation of strain/distortion. High capacitances of 867 F g–1/1.11 F cm–2 and 1201 F g–1/1.48 F cm–2 were obtained for the anode and cathode electrodes, respectively. The asymmetric supercapacitor fabricated using these two electrodes delivered a high energy density Q4 of 86.4 W h kg–1/0.7 mW h cm–3 when the power density was 1831.6 W kg–1/15.3 mW cm–3. Moreover, this device attained high rate capability and cycling stability as well as low impedance. In addition, our fabrication strategy can be extended to carbon cloth substrates and can be used for constructing all-solid-state flexible and bendable supercapacitors. Therefore, this study provides useful and valuable experimental results for constructing next-generation energy storage devices.
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Acknowledgments
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This work was supported by Research Grants Council of Hong Kong (General Research Fund, No. 417012, GRC-NSFC, No. N_CUHK450/13, and TRS, No. T23-407/13-N).
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Appendix A.
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Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.12.029.
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Figure 6 CV curves of the unbent and bent ASCs. The insets show their corresponding photographs.
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region, the inclined line is associated with the ionic diffusion on the surface and within the bulk of the active material. In general, the FeOOH-1800, CoNiDH-1000, and ASC all exhibit low impedances with an upper limit of 30 Ω for both Z0 and Z″, which explains the high capacitances and superior rate performances obtained through the CV and GCD tests. We note that the Ni foam is a heavy current collector and adds considerable mass to the device. For practical applications, it is important to search for light-weight current collectors to support the growth of the electrode materials. Consequently, we further used light-weight carbon cloths to replace the Ni foams as the substrate for electrodepositing the FeOOH and Co–Ni-DH. Since solid-state flexible power sources have attracted considerable attention recently [59,60], we also assembled an all-solid-state ASC device consisting of two carbon cloth electrodes and a PVA/KOH gel electrolyte. This ASC device is highly flexible and bendable; even when bent to a strong degree it undergoes negligible
Supplementary material
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Please cite this article as: J. Chen, et al., Amorphous nanostructured FeOOH and Co–Ni double hydroxides for high-performance aqueous asymmetric supercapacitors, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2015.12.029
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