Design Hierarchical Electrodes with Highly Conductive NiCo2

Jan 17, 2014 - ABSTRACT: We report on the development of highly conductive NiCo2S4 single crystalline nanotube arrays grown on a flexible carbon fiber...
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Design Hierarchical Electrodes with Highly Conductive NiCo2S4 Nanotube Arrays Grown on Carbon Fiber Paper for HighPerformance Pseudocapacitors Junwu Xiao,*,†,‡ Lian Wan,† Shihe Yang,*,‡ Fei Xiao,† and Shuai Wang*,† †

Department of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, Huazhong University of Science and Technology, Wuhan, People’s Republic of China ‡ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: We report on the development of highly conductive NiCo2S4 single crystalline nanotube arrays grown on a flexible carbon fiber paper (CFP), which can serve not only as a good pseudocapacitive material but also as a three-dimensional (3D) conductive scaffold for loading additional electroactive materials. The resulting pseudocapacitive electrode is found to be superior to that based on the sibling NiCo2O4 nanorod arrays, which are currently used in supercapacitor research due to the much higher electrical conductivity of NiCo2S4. A series of electroactive metal oxide materials, including CoxNi1−x(OH)2, MnO2, and FeOOH, were deposited on the NiCo2S4 nanotube arrays by facile electrodeposition and their pseudocapacitive properties were explored. Remarkably, the as-formed CoxNi1−x(OH)2/NiCo2S4 nanotube array electrodes showed the highest discharge areal capacitance (2.86 F cm−2 at 4 mA cm−2), good rate capability (still 2.41 F cm−2 at 20 mA cm−2), and excellent cycling stability (∼4% loss after the repetitive 2000 cycles at a charge−discharge current density of 10 mA cm−2). KEYWORDS: Supercapacitors, NiCo2S4 nanotube arrays, three-dimensional scaffold, core−shell nanostructure, pseudocapacitive materials

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typically too low to support fast electron transport toward high rate capability. To overcome this problem, mounting efforts have been focused on developing three-dimensional (3D) charge conducting nanostructures (carbonaceous, for example) to scaffold the pseudocapacitive materials, which owe their enhanced rate capability to the large surface area and the short diffusion paths for both electron and ions. At the moment, there are a number of outstanding issues in choosing the backbone materials including the following: (1) Fabrication of carbon nanomaterials such as carbon nanotubes and graphene, which limits their practical applications.2,3 (2) Inorganic nanomaterials such as porous Au4 and Ni,5,6 SnO2,7 H-TiO2,8 and ZnO nanorod arrays9−11 are more or less conductive but have almost no specific capacitance in themselves. (3) Typical transition metal oxides such as CoOx and NiO exhibit high pseudocapacity12,13 but the performance of the hybrid materials degrades rapidly with cycling rate due to their poor electron conductivity. Therefore, to further push the pseudocapacitor development it is pressing to create the aforementioned backbone materials with high specific capacitance and good

onductive NiCo2S4 nanotube array scaffold features large surface area and high electric conductivity and its excellent pseudocapacitive performance has been demonstrated for a series of deposited electroactive materials (MOx), such as, CoxNi1−x(OH)2, MnO2, and FeOOH. In recent years, the concerns over environmental pollution and depletion of fossil fuels have galvanized the endeavors to develop alternative energy conversion/storage systems with high specific power and energy. Among the different energy conversion/storage systems in operation or under study, electrochemical capacitors (ECs), also called supercapacitors, are attracting special attention because of their higher specific power and longer cycle-life.1 In general, two major types of electrochemical capacitors exist depending on the underlying energy storage mechanism: electrical double-layer capacitors (EDLCs) and pseudocapacitors. Whereas EDLCs store electrical energy by electrostatic accumulation of charges in the electric double-layer near electrode/electrolyte interfaces, pseudocapacitors make use of reversible Faradaic reactions that occur at the electrode surface, thus offering much higher specific capacitance. Transition metal oxides, hydroxides, and their compounds are being widely explored for high-performance pseudocapacitors because of their low cost, low toxicity, and great flexibility in structure and morphology. However, their conductivity is © 2014 American Chemical Society

Received: November 12, 2013 Revised: December 27, 2013 Published: January 17, 2014 831

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and the deposited pseudocapacitive materials (high specific capacitance). As a result, this unique electrode has shown high specific capacitance, good rate capability, and excellent cycling stability. We first lay out the facile method we proposed and demonstrated for preparing highly conducting NiCo2S4 nanotube arrays on CFP. First, Co2+ and Ni2+ cations reacted with the hydrolysis products of urea (CO32− and OH−) to form bimetallic carbonate hydroxide precursors (Step I in Figure 1 and Figure 2A). The metal carbonate hydroxide precursors can be indexed as (Ni, Co)(CO3)1/2OH·0.11H2O phase, as can be seen from the XRD patterns (Supporting Information Figure S1-A and B). The (Ni, Co)(CO3)1/2OH precursors grew uniformly into a nanorod-shape morphology and were vertically aligned on the CFP (Figure 2A,B). Note that the nanorods have a single crystalline structure extending along the [100] direction (Figure 2C). After continuous vulcanization, thermal treatment, and acid etching processes, the (Ni, Co)(CO3) 1/2OH precursors were transformed into NiCo2S4 (Step II in Figure 1 and Supporting Information Figure S1C).18 As shown in Figure 2D,E, the NiCo2S4 product preserved the general array morphology of the (Ni, Co)(CO3)1/2OH precursors but took a tube structure at the nanoscale, presumably because the interior components were etched out by the acidic solution. Remarkably, the NiCo2S4 nanotubes also have a single crystalline structure growing along the [110] direction, as evidenced by the HRTEM image (Figure 2F). The NiCo2S4 nanotube arrays grown on CFP are an ideal scaffold for loading additional electroactive pseudocapacitive materials, forming electroactive materials (MOx)/NiCo2S4 composites so as to enhance the electrochemical capacitor performance (Step III in Figure 1). Figure 3 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the hybrid electrodes, in which the electroactive materials were deposited on the NiCo2S4 nanotube arrays. Superior to the nanorod scaffold, the hollow structure of NiCo2S4 nanotube array scaffold provided larger surface areas (both interior and exterior) for loading additional pseudocapacitive materials. In a electrolyte containing Co2+ and Ni2+, CoxNi1−x(OH)2 in nanosheet-shape morphology were successfully formed on the NiCo2S4 nanotube arrays, as seen

electric conductivity, as well as the structural hierarchy that imparts high specific surface area and high ionic permeability. Transition metal sulfides such as cobalt sulfides and nickel sulfides have been investigated as a new type electrode material for pseudocapacitors with good performance.14−17 Particularly, NiCo2S4 exhibited an electric conductivity ∼100 times as high as that of NiCo2O4,16,18 although NiCo2O4 is higher than either nickel oxides and cobalt oxides by at least another 2 orders of magnitude in electric conductivity.19,20 With both nickel and cobalt ions, NiCo2S4 can offer richer redox reactions than the corresponding single component sulphides in much the same way as NiCo2O4 to the corresponding single component oxides. In this sense, nanostructured NiCo2S4 is considered to be an excellent backbone material for docking electroactive materials. Herein, we report a significant advance in the design and synthesis of a new and hierarchically structured hybrid of a series of electroactive material coatings (CoxNi1−x(OH)2, MnO2, or FeOOH) on in situ grown NiCo2S4 nanotube arrays on a flexible carbon fiber paper (CFP), as schematically shown in Figure 1. Such a hierarchical electrode design spanning

Figure 1. Nanofabrication flowchart of the hierarchically structured composite electrodes of electroactive materials (MOx)/NiCo2S4.

different material length scales judiciously combine the advantages of the NiCo2S4 nanotube arrays (fast electron transport and ion diffusion as well as large specific surface area)

Figure 2. SEM and TEM images of (A−C) (Ni, Co)(CO3)1/2OH nanorod, and (D−F) NiCo2S4 nanotube arrays grown on the CFP. 832

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Figure 3. SEM and TEM images of the (A−C) CoxNi1−x(OH)2/NiCo2S4, (D−F) MnO2/NiCo2S4, and (G−I) FeOOH/NiCo2S4 composites.

Figure 4. XPS spectra of the (A) Co 2p, (B) Ni 2p, (C) S 2p, (D) Mn 2p, (E) Fe 2p, and (F) O 1s of the NiCo2S4 (black curve), CoxNi(1−x)(OH)2/ NiCo2S4 (red curve), MnO2/NiCo2S4 (blue curve), and FeOOH/NiCo2S4 (pink curve) composites.

from Figure 3A−C. Besides CoxNi1−x(OH)2 nanosheets, MnO2 nanosheets and FeOOH nanoparticles were also explored for being deposited on the NiCo2S4 nanotube arrays via a facile electrodeposition process, which can be seen from SEM and TEM images (Figure 3D−I) and the energy dispersive X-ray (EDX) results (Supporting Information Figure S2). The X-ray

diffraction (XRD) patterns of the CoxNi1−x(OH)2/NiCo2S4, MnO2/NiCo2S4, and FeOOH/NiCo2S4 hybrid materials are almost the same as that of NiCo2S4 (Supporting Information Figure S1−C−F), may since the diffraction peaks of CoxNi1−x(OH)2, MnO2, and FeOOH are so weak that it is difficult to be detected. It also indicates that NiCo2S4 are stable 833

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Figure 5. Electrochemical performance of pristine NiCo2O4 nanorod and NiCo2S4 nanotube array electrodes: (A) CV curves at a scan rate of 10 mV s−1, (B) the galvanostatic charge/discharge curves of NiCo2S4 nanotube array electrode, (C) the discharge areal capacitance performance, and (D) EIS Nyquist plots (inset: equivalent circuit diagram proposed for analysis of the EIS data).

cannot be excluded. As compared with that of NiCo2S4, the shift of the energy bands of Co 2p3/2, Co 2p1/2, Ni 2p3/2, and Ni 2p1/2 in the CoxNi1−x(OH)2/NiCo2S4 hybrid materials support the successful deposition of CoxNi1−x(OH)2 on the NiCo2S4 nanotube arrays. In the MnO2/NiCo2S4 composites, the broad peaks of Mn 2p3/2 and 2p1/2 are located around 642.3 and 653.8 eV, respectively, revealing Mn4+ ions were dominant in the products.30,31 In addition, the difference of the peak separation between Mn 2p3/2 and 2p1/2 is about 11.5 eV, further supporting the above statement, becuase the binding energy difference of Mn 2p3/2 and 2p1/2 can be used to indicate the oxidation state of Mn.31 The observed binding energies for Fe 2p3/2 and Fe 2p1/2 levels in the FeOOH/NiCo2S4 composites were located at 712.2 and 725.2 eV, respectively. The energy difference between Fe 2p3/2 and Fe 2p1/2 levels is 13 eV, which is characteristics of Fe3+ state.32 In the O 1s spectra, the peak at 531.2 and 532.0 eV should be ascribed to −OH and M−O (M = Mn and Fe), respectively. The above results reveal that the electroactive materials (MnO2 and FeOOH) were successfully deposited on the NiCo2S4 nanotube arrays. To demonstrate the electrochemical superiority of the NiCo2S4 nanotube arrays directly grown on CFP, we performed a comparative study with the counterpart of NiCo2O4 nanorod arrays on CFP in a three-electrode configuration using 1.0 M KOH electrolyte. Figure 5A shows the cyclic voltammogram (CV) curves at a scan rate of 10 mV s−1 in a potential window of −0.30 to 0.60 V. The NiCo2O4 nanorod array electrode showed a pair of redox peaks at 0.44 and 0.28 V, which can be attributed to the redox reactions of NiCo2O4 given below33,34

during the electrodeposition process. In a control, CoxNi1−x(OH)2, MnO2, and FeOOH nanomaterials were also deposited on the carbon fibers using the same electrodeposition conditions (Supporting Information Figure S3). To gain further information on the structure and composition of NiCo2S4 and MOx/NiCo2S4 composites, we resort to X-ray photoelectron spectroscopy (XPS) measurement and the results is shown in Figure 4. As regards the Co 2p XPS spectrum of NiCo2S4, it shows a doublet containing a low energy band (Co 2p3/2) and a high energy band (Co 2p1/2) at 778.6 and 793.5 eV (Figure 4A), consistent with the results reported elsewhere.18 The spin−orbit splitting value of Co 2p1/2 and Co 2p3/2 is over 15 eV, suggesting the coexistence of Co2+ and Co3+.21−23 In the Ni 2p XPS spectrum of NiCo2S4, the deconvolution of the Ni 2p peaks shows the atoms in 2p3/2 electronic configuration at 853.0 and 856.8 eV (Figure 4B), suggesting that it was in the divalent and trivalent states.24 The peak at 870.0 eV corresponds to the Ni 2p1/2 band. In the S 2p spectrum of NiCo2S4, the peak at 161.4 eV is characteristics of S2−,25,26 and the component 162.6 eV can be ascribed to the sulphion in low coordination at the surface (Figure 4C).16 The binding energy at 169.4 eV is corresponding to the shakeup satellite. According to the XPS analysis, the near-surface of the NiCo2S4 sample has a composition of Co2+, Co3+, Ni2+, Ni3+, and S2−, which is in good agreement with the NiCo2S4. In the CoxNi1−x(OH)2/NiCo2S4 composites, the energy bands of Co 2p3/2 and Co 2p1/2 of CoxNi1−x(OH)2 shift to 781.4 and 796.8 eV, respectively.27 The energy bands of Ni 2p1/2 and Ni 2p3/2 shift to 873.4 and 855.9 eV, respectively. The main Ni 2p3/2 peaks is close to 854.9 eV for Ni2+ but much lower than 857.1 eV for Ni3+,28,29 suggesting that it was in the divalent state. However the possible existence of trivalent Ni 834

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Figure 6. Electrochemical performance of loaded NiCo2O4 nanorod and NiCo2S4 nanotube array. (A) CV curves at a scan rate of 10 mV s−1, (B) galvanostatic charge/discharge curves of NiCo2S4 nanotube array electrode, (C) the discharge areal capacitance performance, and (D) normalized specific capacitance versus cycle number at a galvanostatic charge−discharge current density of 10 mA cm−2.

calculated from the galvanostatic charge/discharge curves, as shown in Figure 5C. The discharge areal capacitance of NiCo2S4 nanotube array electrodes was much higher than that of NiCo2O4 nanorod array electrodes. For example, the NiCo2S4 nanotube array electrodes exhibited the discharge areal capacitance of 0.87 F cm−2 at 4 mA cm−2, and still 0.58 F cm−2 at 20 mA cm−2, whereas the NiCo2O4 nanorod array electrodes only showed the discharge areal capacitance of 0.52 and 0.40 F cm−2 at 4 and 20 mA cm−2, respectively. To further understand the electrochemical performance characteristics, we resorted to electrochemical impedance spectroscopy (EIS) carried out at open circuit potential with an ac perturbation of 5 mV in the frequency range of 1000 kHz to 0.01 Hz. Figure 5D shows the Nyquist plots thus obtained. The EIS data were fitted based on an equivalent circuit model consisting of bulk solution resistance Rs, charge-transfer resistance Rct, double-layer capacitance Cdl, and Warburg resistance (W), and the result is shown in Figure 5D. The EIS data reveal that the NiCo2S4 nanotube arrays electrodes shows a much smaller Rct (0.8 Ω) in the Nyquist plots as compared to that of the NiCo2O4 electrode (2.3 Ω). From the four point probe resistance measurement (see Supporting Information Table S1), the surface resistivity of NiCo2S4 nanotube arrays grown on the glass substrates (3.41 ohm cm−2) is much smaller than that of NiCo2O4 nanorod arrays formed on the same substrates (498 ohm cm−2). It reveals that NiCo2S4 has much higher electric conductivity in accord with the results report elsewhere.16,18 To recap, the results clearly demonstrate that the NiCo2S4 nanotube arrays display favorable charge-transfer kinetics and fast electron transport and thus exhibit the dramatically enhanced pseudocapacitive performance.

NiCo2O4 + OH− + 2H 2O ⇄ NiOOH + 2CoOOH + H 2O + e−

(1)

CoOOH + OH− ⇄ CoO2 + H 2O + e−

(2)

Similarly, the NiCo2S4 nanotube array electrode gave rise to an anodic and a cathodic peak at 0.44 and 0.30 V, respectively,14−16 corresponding to the redox reactions of NiCo2S4. As is well-known, sulfur is in the same family of oxygen. In addition, the OH− ions play an important role in the electrochemical oxidation and reduction of cobalt sulfides and nickel sulfides as reported previously.15,35,36 Thus, it is believed that the redox reaction mechanism of NiCo2S4 in alkaline electrolyte is via a reversible redox reaction of NiCo2S4 to form NiSOH, CoSOH, and CoSO. The integrated CV area for the NiCo2S4 electrode is significanly larger, even though the amount of material is much smaller due to forming the hollow core via the acidic etching process. This confirms that the NiCo2S4 nanotube based electrodes are superlative for pseudocapacitive devices. Rate capability is a critical parameter of electrochemical capacitors for assessing their application potential. Shown in Figure 5B is the representative galvanostatic charge/discharge plots of NiCo2S4 nanotube arrays electrode at the current densities of 4, 8, 12, 16, and 20 mA cm−2. Consistent with the CV results, the plateaus in the charge/discharge curves indicate the existence of Faradaic processes. These charge/discharge curves are approximately symmetric, indicating that NiCo2S4 nanotube array electrodes have a good electrochemical capacitive characteristic and superior reversible redox reaction. The cycling curves are still symmetrical even at a current density as high as 20 mA cm−2, an indication of very high rate stability. The discharge areal capacitance performance was 835

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Figure 7. (A) CV curves of the FeOOH, NiCo2S4 and FeOOH/NiCo2S4 electrodes at a scan rate of 10 mV s−1. (B) EIS Nyquist plots of the FeOOH and FeOOH/NiCo2S4 electrodes.

discharge curves. Impressively, the CoxNi1−x(OH)2/NiCo2S4 electrode deliver the discharge areal capacitance of high to 2.86 F cm−2 at 4 mA cm−2, and even maintain at 2.41 F cm−2 at 20 mA cm−2, which are much better than CoxNi1−x(OH)2 and NiCo2S4 electrodes. As compared with the CoxNi1−x(OH)2/NiCo2O4 nanorod array electrodes reported previously (2.3 F cm−2 at 2 mA cm−2),37 the CoxNi1−x(OH)2/NiCo2S4 nanotube array electrodes exhibited the largely enhanced pseudocapacitive performance, since NiCo2S4 scaffold not only showed much better pseudocapacitive performance in itself, but also were with higher electrical conductivity and nanotube structure for facilitating electron and electrolyte ion transport, and loading more pseudocapacitive materials. To the best of our knowledge, the discharge areal capacitance performance of the CoxNi1−x(OH)2/NiCo2S4 electrode reported here is also much higher than those of conventional carbonaceous materials, and even higher than most of previously reported MnO2/NiCo2O4 (2.01 F cm−2 at 2 mA cm−2),38 MnO2/Co3O4 (0.56 F cm−2 at 11.25 cm−2),12 NiO/Co3O4 (1.35 F cm−2 at 6 mA cm−2),39 NiO/MnO2 (0.35 F cm−2 at 9.5 mA cm−2),13 and NiO/TiO2 electrodes (3 F cm−2 at 0.4 mA cm−2),40 and so forth. Such high discharge areal capacitance of the CoxNi1−x(OH)2/NiCo2S4 electrodes further proves the great advantages of NiCo2S4 nanotube arrays as the pseudocapacitance materials and the fast electron conductor. Cycle stability is another key parameter in relation to the electrochemical performance of a supercapacitor and was invesitgated at a charge−discharge current density of 10 mA cm−2 in the potential range of 0 to 0.5 V for 2000 repetitive cycles, as shown in Figure 6D. CoxNi1−x(OH)2 nanosheet electrode directly grown on the CFP exhibited poor cycling life, ∼ 24% loss after 2000 cycles. For the NiCo2O4 nanotube array electrodes, the total capacitance loss after 2000 cycles is around 8%. The cycling stability performance of the NiCo2S4 nanotube array electrode is further improved. The NiCo2S4 nanotube array electrode gained the specific capacitance in the initial cycles, perhaps due to activation, and almost suffered no capacitance loss in the subsequent charge−discharge processes. The improvement of the cycling stability is mainly ascribed to the increase of the electrical conductivity in sequence of NiCo2S4 > NiCo2O4 > CoxNi1−x(OH)2. For the hybrid electrode of CoxNi1−x(OH)2/NiCo2S4, ∼ 96% of the initial specific capacitance was retained after the 2000 repetitive cycles, much better than that of CoxNi1−x(OH)2 electrodes directly grown on the CFP, because the NiCo2S4 nanotube

The NiCo2S4 nanotube array electrode can serve multiple functions: it is not only a good pseudocapacitor in itself, but also an electron conducting and an ion permeating high-way network, providing a large specific surface area for loading additional pseudocapacitive materials and facilitating electrolyte ion diffusion, a series of which we have tested include CoxNi1−x(OH)2, MnO2, and FeOOH. The electrochemical performance data of these electrodes is shown in Figure 6. Figure 6A shows the CV curves of the CoxNi1−x(OH)2, CoxNi1−x(OH)2/NiCo2S4, NiCo2S4, and MnO2/NiCo2S4 electrodes. The CoxNi1−x(OH)2 electrode directly deposited on the CFP exhibits a pair of redox peaks at ∼0.5 and 0 V, corresponding to the redox reaction of CoxNi1−x(OH)2 (CoxNi1−x(OH)2 + OH− ⇄ CoxNi1−xOOH + H2O+e−). Such a large difference between the anodic and cathodic peaks reveals the irreversible Faradic process on the CoxNi1−x(OH)2 electrodes. After growing CoxNi1−x(OH)2 nanosheets on the NiCo2S4 nanotube arrays, the difference between the anodic and cathodic peaks decrease to ∼0.25 V and the CV area significantly increase, as compared with CoxNi1−x(OH)2 electrodes. It reveals that the pseudocapactive performance of the CoxNi1−x(OH)2/NiCo2S4 electrode was obviously improved due to the fast electron conduction and large surface area of NiCo2S4. After growing MnO2 on the NiCo2S4 nanotube arrays, a redox peak corresponding to the redox reaction of NiCo2S4 is still found in the CV curve, indicating the efficient utilization of the underlying NiCo2S4 despite covered by MnO2 layer. The integrated CV area increases for the MnO2/NiCo2S4 electrodes, which should be attributed to the additional pseudocapacitance contributed by the MnO2 layer, which can adsorb the cations (K+ and H3O+) on the electrode surface and/or allow their intercalation or deintercalation.12 The galvanostatic charge−discharge measurements are performed in the voltage range of 0−0.5 V at a current density of 4 mA cm−2, as shown in Figure 6B. The CoxNi1−x(OH)2/ NiCo2S4 electrodes exhibited more symmetrical charge/ discharge curves than the CoxNi1−x(OH)2 electrodes directly grown on the CFP, indicating less energy loss during the charge/discharge process. The discharge areal capacitances of the MnO2/NiCo2S4 electrodes showed much higher than that of the MnO2 (Supporting Information Figure S4) and NiCo2S4 electrodes. It suggests that the MnO2 electroactive materials and NiCo2S4 cooperatively contribute the pseudocapactive performance. Figure 6C shows the discharge areal capacitance performance calculated from the galvanostatic charge− 836

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Nano Letters arrays as the scaffold facilitate fast electron transport from the electrode/electrolyte interface and ion diffusion. The electrochemical characteristic of the FeOOH/NiCo2S4 electrodes were investigated in 1.0 M KOH solution, as shown in Figure 7. Figure 7A shows the CV curves at a scan rate of 10 mV s−1 in a potential window of −1.2 to 0 V after the several repetitive cycle stabilizing. The FeOOH electrodes directly deposited on the CFP displayed cathodic peaks at around −1.0 V ascribable to the reduction of Fe3+ and Fe2+ to Fe0, and anodic peaks at ∼−0.84 and −0.75 V resulting from the oxidation of Fe0 to Fe2+ and Fe2+ to Fe3+, respectively. The corresponding redox reactions of FeOOH can be put together as: FeOOH+H2O + e− ⇄ Fe(OH)2 + OH− and Fe(OH)2 + 2e− ⇄ Fe + 2OH−. Calculated from the integrated CV area at a scan rate of 10 mV s−1, we can find that the FeOOH and NiCo2S4 electrodes delivered a discharge areal capacitance of 78.3 and 101.4 mF cm−2, respectively. Of note, the FeOOH/ NiCo2S4 electrodes can have higher discharge areal capacitance (265.2 mF cm−2) than the specific capacitance sum of the FeOOH and NiCo2S4 electrodes obtained under the same testing condition and shift the anodic and cathodic peaks more positive, mainly because the NiCo2S4 nanotube arrays are highly electrically conductive and have a large surface area for loading more FeOOH electroactive materials. The comparison of the electron conductivity for the FeOOH and FeOOH/ NiCo2S4 electrodes can be seen from the EIS results. The EIS results demonstrate that the charge-transfer resistance of the FeOOH/NiCo2S4 electrodes (3.2 Ω) is much smaller than that of the FeOOH electrodes (6.3 Ω). In summary, we have for the first time designed and successfully prepared the coaxial electroactive materials/ NiCo2S4 arrays on a flexible CFP substrate for high-performance pseudocapacitors. In this unique nanoarchitecture, the NiCo2S4 nanotube arrays with an ample core not only act as an excellent pseudocapacitive material by themselves, but also serve as a hierarchical porous scaffold capable of fast electron conduction and ion diffusion for loading a large amount of additional electroactive materials. A series of highly pseudocapacitive materials including CoxNi1−x(OH)2, MnO2 and FeOOH have been tested for demonstration of the principle with excellent performance. Remarkably, the CoxNi1−x(OH)2/ NiCo2S4 nanocomposite electrodes exhibited the highest specific capacitance of 2.86 F cm−2 at 4 mA cm−2, good rate capability, as well as excellent cycling life. We anticipate that such a designer hierarchical architecture will be applicable to more extensive hybrid materials and energy storage and conversion devices such as lithium ion batteries, water splitting cells, and photodetectors.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the HK-RGC General Research Funds (GRF nos. HKUST 605710 and 604809), the Fundamental Research Funds for the Central Universities (Project No. 2013QN158), and Research Fund for the Doctoral Program of Higher Education of China (20130142120024).

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*E-mail: (J.X.) [email protected]. *E-mail: (S.Y.) [email protected]. *E-mail: (S.W.) [email protected]. Notes

The authors declare no competing financial interest. 837

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