Hierarchical NiCo2O4 Nanosheets@halloysite Nanotubes with

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Hierarchical NiCo2O4 Nanosheets@halloysite Nanotubes with Ultrahigh Capacitance and Long Cycle Stability As Electrochemical Pseudocapacitor Materials Jin Liang,† Zhaoyang Fan,† Sheng Chen,† Shujiang Ding,*,† and Guang Yang*,‡ †

State Key Laboratory for Mechanical Behavior of Materials and MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter and Department of Applied Chemistry, School of Science, and ‡Electronic Materials Research Laboratory, Key Laboratory Of The Ministry Of Education & International Center For Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China ABSTRACT: One-dimensional hierarchical nanostructure of NiCo2O4 nanosheets@halloysite nanotubes was successfully prepared through a facile coprecipitation method followed by a thermal annealing treatment. The microstructure and chemical composition of NiCo2O4 nanosheets@halloysite nanotubes are investigated by SEM, TEM, HRTEM, XRD, and XPS. The specific capacitance of the unique NiCo2O4 nanosheets@ halloysite nanotubes is 1728 F g−1 at the end of 8600 cycles when the charge−discharge current density is 10 A g−1, leading to only 5.26% capacity loss. Broadly, the as-obtained NiCo2O4 nanosheets@halloysite nanotubes reveal ultrahigh capacitance and remarkable cycling stability in virtue of the ultrathin and hierarchical nanosheets and intense cation/anion exchange performance of halloysite.

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INTRODUCTION With the increasing energy demand in high power electric devices and electric vehicles, supercapacitors have attracted much research interest because of their higher power density, faster charge/discharge process, longer lifespan, low maintenance cost, and minimal safety concerns than other conventional rechargeable devices.1−7 Supercapacitors can be classified into two types based on the charge-storage mechanism: electrical double-layer capacitors and pseudocapacitors.8 The pseudocapacitance can be generated from electroactive materials possessing multiple oxidation states.9 Recently, transition metal oxides belonging to this class of materials have attracted a great deal of attention for this application. Among various transition metal oxides, RuO2 and IrO2 exhibit remarkable electrochemical performances as pseudocapacitive electrode materials, due to their higher specific capacitance than carbon-based materials and better cycling behavior than conductive polymers.10−12 Nevertheless, they are still limited in industrial supercapacitor applications because of the disadvantages of high cost and a toxic nature. Consequently, it is extremely urgent to explore inexpensive and environmentally friendly electrode materials with high specific capacitance and preferable cycling stability. Among various materials investigated, nickel- and cobalt-based materials, including NiO, CoOx, Ni(OH)x, and Co(OH)x, are of great interest as cheap and environmental friendly replacements for ruthenium and iridium oxide-based supercapacitors.13−18 However, it is also noted that these materials are too insulating to support fast electron transport, thus leading to inferior rate capability and reversibility. Fortunately, it has been reported © 2014 American Chemical Society

that ternary nickel−cobalt metal oxides such as nickel cobaltite (NiCo2O4), have greater electronic conductivity by at least 2 orders of magnitude, and electrochemical activity than nickel and cobalt based materials.19−21 NiCo2O4 adopts a spinel structure in which nickel occupies the octahedral sites and cobalt distributes over both octahedral and tetrahedral sites.8 The spinel NiCo2O4 offers many intriguing advantages such as low cost, abundant resources and environmental friendliness;22 More significantly, it is expected to offer richer redox chemistry than the two single-component oxides because of the combined contributions from both nickel and cobalt ions.23 The possible redox reactions can be described by eqs 1 and 2 NiCo2O4 + OH− + H 2O ↔ NiOOH + CoOOH + 2e− (1)

CoOOH + OH− ↔ CoO2 + H 2O + e−

(2)

It is apparent that the rate capability of NiCo2O4 is mainly determined by the kinetics of hydroxyl ion diffusion and electronic conductivity. It is well-known that kinetics of processes impact the participation degree of the electrode materials occurring in the electrode of supercapacitors and thus has influence on the capacitance and stability of supercapacitors. In view of this point, we can adopt substrate possessing abundant OH− to grow spinel NiCo2O4 to promote kinetics of process, thereby increasing participation degree to Received: March 5, 2014 Revised: July 21, 2014 Published: July 21, 2014 4354

dx.doi.org/10.1021/cm500786a | Chem. Mater. 2014, 26, 4354−4360

Chemistry of Materials

Article

Synthesis of NiCo2O4 Nanosheets@HA. Ten mg of Halloysite (HA) nanotubes was well dispersed in 40 mL of deionized water by sonication treatment. Then, 0.25 mmol of Ni(NO3)2·6H2O, 0.5 mmol Co(NO3)2·6H2O, 0.25 mmol of hexamethylenetetramine and 0.025 mmol of citric acid trisodium salt dehydrate were dissolved into the above dispersion to form a light orange solution. The resulting solution was transferred into a 100 mL flask and heated to 90 °C in an oil bath for 6 h with slow stirring. After the solution was cooled down to room temperature naturally, the product was collected by centrifugation and washed with deionized water and ethanol several times and then dried at 60 o C for 12 h under vacuum. Finally, the powder was heated with a heating ramp of 1 o C min−1 to a temperature of 350 o C for 3.5 h under an air atmosphere. Materials Characterization. X-ray diffraction (XRD) patterns were collected on a SHIMADZU Lab X X-6000 Advanced X-ray Diffract meter. The chemical states of the products were studied using the X-ray photoelectron spectroscopy (XPS) measurement performed on an Axis Ultra, Kratos (UK) at monochromatic Al K a radiation (150 W, 15 kV and 1486.6 eV). The vacuum in the spectrometer was 1 × 10−9 Torr. Binding energies were calibrated relative to the C 1s peak (284.6 eV). Field-emission scanning electron microscopy (FESEM) images were obtained on a HITACHI SU-8010 microscope. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were taken on a JEOL JEM-2100F microscope. Thermogravimetric analysis (PerkinElmer TGA 7) was carried out under a flow of air with a temperature ramp of 10 °C min−1 from room temperature to 800 o C. Electrochemical Measurements. For electrochemical measurements, the working electrode is consisted of an active material, carbon black (Super C65), and a polymer binder in a weight ratio of 70:20:10. The polymer binder is self-prepared by dissolving commercial PVDF powder into NMP solvent with a weight of 7:93. The as-prepared slurry was then pasted onto graphite paper and then dried at 60 °C at oven for 8h and at 120 °C overnight under vacuum. The electrochemical test was conducted with a CHI 660D electrochemical workstation in an aqueous KOH electrolyte (2.0 M) with a threeelectrode cell where Pt foil serves as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode.

enhance the electrochemical performance of supercapacitors. HA, a kind of this substrate, is one-dimensional tubular clay mineral and has versatile features, such as high porosity, tunable surface chemistry and low cost.24−26 Particularly, it possesses excellent cation/anion exchange capacity and abundant OH− on the surface and interlayer, and it is highly possible to enhance the diffusion rate of OH−, thus promoting the kinetics of process of NiCo2O4 based materials. In this work, we design an environmentally friendly lowtemperature solution method to construct 1D NiCo 2O4 nanosheets@HA nanotubes hybrid nanostructures followed by a thermal annealing treatment. The synthetic process is illustrated in Scheme 1. The as-prepared NiCo2O4@HA Scheme 1. Schematic Illustration of the Synthetic Procedure of NiCo2O4 Nanosheets@HA Nanotube Hybrid Nanostructures

composites are unusually suitable as the electrode materials for supercapacitors for at least four reasons: (i) all materials are inexpensive, environmental friendly, owing abundant resources and the preparation method is facile, green and low energy consumption; (ii) the standing structure may help to the separation of neighboring nanosheets, thus restraining the active material from aggregating; (iii) these hierarchical and ultrathin NiCo2O4 nanosheets could offer rich accessible electroactive sites, short ion transport pathways, and superior electron collection efficiency and buffer the large volume change during the fast charging−discharging process; (iv) the remarkable cation/anion exchange capacity of HA nanotubes can offer quick and sustainable supply or removal of participant ions during the charge−discharge process, which can improve the reaction rate and increase the participation degree of NiCo2O4 nanosheets. Although HA itself has low capacitance, it could combine with NiCo2O4 nanosheets to produce a phenomenon seeming as synergistic effect to tremendously promote the electrochemical performance. The specific capacitance of the NiCo2O4 nanosheets@halloysite nanotubes is 1728 F g−1 at the end of 8600 cycles when the charge− discharge current density is 10 A g−1. To date, the synthesis of NiCo2O4 nanosheets@halloysite nanotubes hierarchical nanostructures and excellent performance of NiCo2O4 based materials remain unreported.

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RESULTS AND DISCUSSION A commercial halloysite nanotube was selected as substrate for preparing the 1D NiCo2O4 nanosheets@HA nanostructures. Morphology and structure of HA nanotubes have been characterized with SEM and TEM. As shown in Figure 1A, the halloysite predominately consists of cylindrical nanotubes

EXPERIMENTAL SECTION

Materials. The halloysite (HA) nanotubes were purchased from Imerys Tableware Asia Limited. All reagents were analytical grade and used without further purification. The chemicals, polyvinylidene fluoride (PVDF) and N-methyl-2-pyr-rolidone (NMP), are purchased from Sigma-Aldrich and directly used without further purification.

Figure 1. (A) SEM and (B) TEM image of the hallysite nanotubes; (C) SEM and (D) TEM images of NiCo2O4 without HA support. 4355



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50−150 nm in diameter and 1−2 μm in length.25 The empty limen structure of HA is revealed by TEM. As shown in Figure 1B, the average inner diameter of the HA is in the range of 40− 60 nm. The aggregates of NiCo2O4 without HA support are prepared as compared samples in the similar procedure, and the morphology is shown in Figure 1C, D. It is apparent that NiCo2O4 aggregates possess a chain morphology composed of irregular spheres with a diameter of several micrometers. By close observation, it is found that those irregular spheres are composed of thin nanosheets, as seen in Figure 1D. Figure 2A

HA nanotubes are further analyzed by energy-dispersive X-ray spectroscopy (EDS) and scanning transmission electron microscopy (STEM) with the results shown in Figure 3. The

Figure 3. (B−F) EDS elemental mapping images of (A) an individual NiCo2O4 nanosheets@halloysite nanotubes.

elemental mapping images show that the Al and Si elements formed the inner of the NiCo2O4 nanosheets@HA nanotubes, the outer is consist of Co and Ni elements, respectively. It gives a direct proof for the uniform surface of NiCo2O4 nanosheets on HA nanotubes. In order to understand the weight loss with the temperature during the calcination process, and to determine the calcining temperature of the sample, TGA data in air was obtained in Figure 4. Curve (I) Shows the TGA curve of pure HA nanotubes, adsorbed water and interlayer water are removed in the first stage of this process (up to 60 °C), the thermal decomposition of HA is completed at 500 °C. This weight loss

Figure 2. (A) SEM and (B) TEM images of NiCo-precursor@HA; (C) SEM and (D) TEM images, (E) SAED pattern, and (F) HRTEM image of NiCo2O4 nanosheets@HA nanotubes.

shows the SEM image of NiCo-precursor@HA. Large amount of uniform 1D nanostructures are obtained with a hierarchical architecture and these nanostructures are composed of uniform NiCo-precursor nanosheets grown surrounding the surface of HA nanotubes. Figure 2B shows the corresponding TEM images of the NiCo-precursor nanosheets @HA. It can be clearly observed that ultrathin NiCo-precursor nanosheets are grown uniformly on the HA nanotubes to form the 1D nanostructures. After annealing, the NiCo-precursor nanosheets can be converted to crystallized NiCo2O4 nanosheets. SEM and TEM images of the NiCo2O4 nanosheets@HA (Figure 2C, D) reveal that the ultrathin nanosheet morphology of the NiCo-precursor is well retained after the thermal conversion and the thickness is approximate 4 nm (the inset in Figure 2D). The SAED pattern (Figure 2E) indicates the polycrystalline nature of the NiCo2O4 nanosheets and can be readily indexed to (200), (311), (400), (420), and (440) crystal planes of the NiCo2O4 phase,27 which is consistent with latter XRD characterization. A representative high-resolution TEM image is shown in Figure 2F. The measured interplanar distance is 0.24 nm, which matches well to the (311) plane of spinel NiCo2O4.28 The detailed elemental composition and the unique nanostructure of the as formed NiCo2O4 nanosheets@

Figure 4. TGA analysis of HA nanotubes (I) and NiCo2O4precursor@HA (II) under air flow with a temperature ramp of 10 °C min−1. 4356



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NiCo2O4 and NiCo2O4@HA. Evidently, seven well-defined diffraction peaks (curve II) are observed at 2θ values of 18.9, 31.1, 36.7, 44.6, 55.4, 59.1° and 64.9 o. All of these peaks including their peak positions can be successfully indexed to the (111), (220), (311), (400), (422), (511), and (440) plane reactions of the spinel NiCo2O4 crystalline structure (JCPDF no. 20−0781). Meanwhile, some crystal orientation of halloysite nanotubes, such as (002), (110), (003), (211) at 24.84, 35.02, 37.98, and 54.34° appear (curve I). Curve (III) indicates that the hybrid materials synchronously possess the characteristic diffraction peaks of HA and NiCo2O4, confirming the composition of the hybrid materials. The more detailed elemental composition and the oxidation state of the asprepared NiCo2O4 nanosheets@HA nanostructures are further characterized by XPS measurements and the corresponding results are presented in Figure 6A−D. Figure 6A shows the survey XPS spectrum of the NiCo2O4 nanosheets@HA, mainly including carbon (C 1s), oxygen (O 1s), nickel, and cobalt species. The halloysite nanotubes (HA) is primarily composed of Si, O, Al and C.25 Two kinds of Co species (Co2+ and Co3+) have been detected. The binding energies at 779.3 and 794.3 eV are ascribed to Co3+. The binding energies for Si 2p, Al 2p, O 1s, and C 1s are 104.4, 70.4, 530, and 285 eV, respectively. In the Co 2p spectra (Figure 6B), another two fitting peaks at 781.0 and 795.7 eV are ascribed to Co2+.29 In the Ni 2p spectra (Figure 6C), two kinds of nickel species containing Ni2+ and Ni3+ can also be observed. The fitting peaks at 854.0 and 871.7 eV are in dexed to Ni2+, while the fitting peaks at 855.9 and 873.8 eV are indexed to Ni3+.30 The satellite peak at around 861.0 and 879.4 eV are two shakeup type peaks of nickel at the high binding energy side of the Ni 2p 3/2 and Ni 2p 1/2 edge.31 The high resolution spectrum for O 1s (Figure 6D)

can be vested as a continuous thermal depletion of deeptrapped hydroxyl groups and inherent moisture. Curve (II) displays the thermal decomposition of NiCo2O4-precursor@ HA hybrid nanostructures in air. The thermogram shows three stages of weight loss. The first weight loss stage (up to 100 °C) may be attributed to the removal of chemisorbed and occluded water in the NiCo2O4-precursor@HA samples during heating process. The major weight loss occurred between 250 and 300 °C could be due to dehydration of the oxide hydrate. The final weight loss occurrs at 400−500 °C can be the thermal decomposition of HA. The X-ray diffraction (XRD) is used to further confirm the crystal structure and composition of the hybrid structure. Figure 5 shows the XRD patterns of HA,

Figure 5. XRD patterns of HA (I), NiCo2O4 (II), and NiCo2O4 nanosheets@HA nanotubes (III).

Figure 6. XPS spectra of NiCo2O4 nanosheets@HA nanotubes: (a) full spectrum; (B) Co 2p; (C) Ni 2p; (D) O 1s. 4357



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Figure 7. Electrochemical characterizations of the NiCo2O4 nanosheets@HA nanotubes: (A) CV curves at various scan rate ranging from 10 to 80 mV s−1; (B) charge/discharge voltage profiles at various current densities ranging from 6 to 30 A g−1; (C) calculated capacitance as a function of current density according to the data in B; (D) capacitance cycling performance at current density of 10 A g−1.

shape of the CV curves basically remains unchanged except for the small shift of the peak position, thus indicating the excellent electrochemical reversibility and outstanding high-rate performance.36 To get more information about their potential application in supercapacitor, we carried out galvanostatic charge−discharge measurements in 2 M KOH solution between 0 and 0.5 V (vs SCE) at various current densities ranged from 6 to 30 A g−1, as shown in Figure 7B. The typical CP plots suggest the desirable supercapacitive performance. The specific capacitance of the NiCo2O4 nanosheets@HA nanostructures are calculated from the CP curves in Figure 6B, according to eq 3

shows three oxygen species marked as O 1, O 2, and O 3. According to previous reports, the fitting peak of O 1 at a binding energy at 529.6 eV is a typical metal−oxygen bond, O 2 at a binding energy of 531.1 eV is usually associated with oxygen in OH− groups, indicating the presence of the NiCo2O4 material is hydroxylated to some extent as a result of either surface hydroxide or substitution of oxygen atoms at the surface by hydroxyl groups.32 O 3 at a binding energy of 532.5 eV can be a mixed composition containing Co2+, Co3+, Ni2+ and Ni3+. Thus, the formula of the proposed NiCo2O4 can be generally expressed as follows: Co2+1−xCo3+x [Co3+Ni2+xNi3+1+x]O4 (0 < x < 1). These results are consistent with previous reports.33,34 The solid redox couple of Ni2+/Ni3+ and Co2+/Co3+ can afford enough active sites for methanol oxidation, which may be one of the important factors contributing to the high electrocatalytic performance of NiCo2O4. For the sake of evaluating the electrochemical characteristics of the NiCo2O4 nanosheets@ HA nanostructures, cyclic voltammetry (CV) and chronopotentiometry (CP) are employed to characterize their electrochemical capacitance in a three-electrode cell. Figure 7A presents the representative CV curves of the NiCo 2O4 nanosheets@HA nanostructures electrode in 2 M KOH aqueous electrolyte at various scan rates ranged from 10 up to 80 mV s−1. Apparently, well-defined redox reaction peaks within 0 to 0.65 V (vs SCE) are visible in all CV curves, indicating that the electrochemical capacitance of the NiCo2O4 nanosheets@HA nanostructures electrode is distinct from that of electric double-layer capacitance with common rectangular shape. And these peaks mainly originate from Faradaic redox reactions related to MO/MOOH, where the M represents Ni and Co ions.35 Obviously, with the increasing scan rate, the

Cm = I Δt /(ΔVm)

(3)

where C, I, t, and ΔV are the specific capacitance (F g−1) of NiCo2O4 nanosheets@HA electrode, the discharging current density (A g−1), the discharging time (s), and the discharging potential range (V), respectively. The specific capacitance of the NiCo2O4 nanosheets@HA nanostructures at various current densities can be calculated and the typical data are plotted in Figure 7C. The unique superstructures electrode delivers good pseudocapacitances of 1886.6, 1860.8, 1824, 1731, 1688, 1600, and 1500 F g−1 at 6, 8, 10, 15, 20, 25, and 30 A g−1, respectively. This also suggests that ca. 80% of the capacitance is still retained when the charge−discharge rate changes from 6 to 30 A g−1. Therefore, the electrode not only exhibits large specific capacitance but also maintains them well at higher current densities. The good electrochemical performance could result from unique structural features with numerous hierarchical, ultrathin nanosheets and fascinating synergetic properties with HA, which reduces the diffusion 4358



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length for the electrolyte ions, offers rich accessible electroactive sites, guarantees enough electrolyte ions to rapidly contact the large surfaces of the electroactive NiCo2O4 nanosheets@HA nanostructure with good electronic conductivity, ultrathin nanosheets also greatly enhance the participation degree of NiCo2O4 nanosheets@HA in the charge− discharge process to obtain desirable cycle life and high rate performance. HA nanotubes owning abundant OH− ions could offer quick and sustainable supply or removal during the charge−discharge process and ensure sufficient Faradaic reactions to take place at high current densities for energy storage. Therefore, the unique NiCo2O4 nanosheets@HA electrode can get high electrochemical utilization even at large current densities. The cycling performance of any electroactive material is one of the most significant parameters for its practical applications. The cycle life of NiCo2O4 nanosheets@HA nanostructures is carried out at a current density of 10 A g−1. As demonstrated in Figure 7D, the specific capacitance increases up to 1904 F g−1 after the materials are fully activated through electrochemical reactions during charge−discharge. After cycling for 8600 cycles, the specific capacitance gradually decreases to 1728 F g−1 and the capacitance loss is only 5.3%. To make a further comparison, we have tested that the specific capacitance of bulk NiCo2O4 is about 896 F g−1 at the discharge current densities of 10 A g−1 after 2000 cycles. Clearly, HA could increase the participation degree of active materials, thereby enhancing the capacitance and stability. In previous work by Yuan et al., they prepared hierarchical porous network-like spinel NiCo2O4 framework as electrode material and the capacitances were 587, 569, 560, 545, and 518 F g−1 at the current densities of 2, 4, 6, 10, and 16 A g−1, respectively.37 Besides, they also synthesized mesoporous hollow NiCo2O4 submicrospheres as electrode material and they exhibited pseudocapacitance of 678, 660, 648, 647, 630, 612, 576, and 540 F g−1 at current densities of 1, 2, 3, 4, 5, 6, 8, and 10 A g−1, respectively.38 Such data further highlights the capability of the NiCo2O4 nanosheets@HA nanostructures electrode to meet the requirements of both long cycling performance and good rate capability, which are important merits for the practical energy storage devices.

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported partially by the National Natural Science Foundation of China (51273158, 21303131); Natural Science Basis Research Plan in Shaanxi Province of China (2012JQ6003, 2013KJXX-49); Ph.D. Programs Foundation of Ministry of Education of China (20120201120048); Program for New Century Excellent Talents in University (NCET-130449). The authors are grateful to the Fundamental Research Funds for the Central Universities for financial support.

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REFERENCES

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CONCLUSIONS In summary, we have developed a facile method to produce NiCo2O4 nanosheets@HA nanostructures. The unique nanostructures possess hierarchical, standing and ultrathin nanosheets thus the products can offer more contact area between active materials and the electrolyte ions, enhance the participation degree of the electrode material and buffer the large volume change during the fast charging−discharging process simultaneously. Furthermore, HA could increase the participation degree of the NiCo2O4 nanosheets occurring in the electrode of supercapacitors, thus to promote the capacitance and stability of supercapacitors. These advantages could enhance the electrochemical performance of the NiCo2O4 nanosheets@HA nanotubes as an electrode material for supercapacitors. Naturally, it reveals that these superior NiCo2O4 nanosheets@HA nanomaterials have been manifested with ultrahigh capacitance and remarkable cycling stability compared to other electrode materials for supercapacitors. This method could also be used to prepare other HA-based metal oxide such as Co3O4, CoO, and NiO. These related works are in progress. 4359



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Hierarchical NiCo2O4 Nanosheets@halloysite Nanotubes with

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