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Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors
international journal of hydrogen energy xxx (xxxx) xxx
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Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors Qinfang Wu a,c, Yunhe Zhao a, Jing Yu a,*, Dalei Song a, Rongrong Chen a,c, Qi Liu a, Rumin Li a, Meiqing Fan b,** a
Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, China b College of Food Engineering, Jilin Engineering Normal University, Changchun, 130052, China c Institute of Advanced Marine Materials, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, 150001, China
highlights A core-shell structure of Co3O4@FeCo2O4 is prepared as supercapacitor electrode. The hierarchical structures electrode exhibits remarkable specific capacitance and cycling stability. Such enhanced properties owning to the judiciously engineer the interface with unique hierarchical structures.
article info
abstract
Article history:
In recent years, the tenable design and synthesis of the core/shell heterostructure as
Received 14 July 2019
electrode for the supercapacitor, have attained a huge attention and concerns. In this
Received in revised form
article, the three-dimensional heterostructure consisting of FeCo2O4 ultrathin nanosheets
28 September 2019
grown on the space of vertical Co3O4 nanowires has been designed and synthesized onto
Accepted 17 October 2019
nickel foam (NF) for pseudocapacitive electrode applications. According to previous
Available online xxx
research, the NF@ FeCo2O4 electrodes can only exhibit specific capacity of 1172 F g1 at a current density of 1 A g1. In addition, although the capacity of the NF@Co3O4 electrodes
Keywords:
can reach to 1482 F g1 and it has the disadvantage of agglomeration, which restricts the
Supercapacitor
diffusion of ions and has a negative effect on the progress of electrochemical reactions.
Transition metal oxides
Therefore, a core-shell nanostructure is fabricated by an improved two-step hydrothermal
Core-shell heterostructures
process, which improves the probability of ion reaction with more efficient charge transfer.
Co3O4
Furthermore, in as-prepared unique core/shell heterostructure, the resultant electrode
FeCo2O4
possesses the merits of large capacitance of 1680 F g1 at a current density of 1 A g1, an excellent rate capability of 70.1% at 20 A g1 and only 9.8% loss of initial capacitance at a high charge/discharge current density after 2000 cycles. These results demonstrate that this kind of distinct electrode has potential utilization for supercapacitor. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yu), [email protected] (M. Fan). https://doi.org/10.1016/j.ijhydene.2019.10.119 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Introduction Now a days, the development of energy storage devices and conversion has attracted a lot of attention in today’s human society due to the depletion of non-renewable resources and environmental pollution caused by the excessive use of fossil energy [1e5]. Therefore, developing a new kind of energy storage equipment with high efficiency, stability and environmental friendliness has become the focus of research to reduce the dependence on traditional fossil fuels [6,7]. Currently, energy storage devices including lithium-ion batteries, fuel cell and supercapacitors (SCs) can store and convert chemical energy by electrochemical energy storage technology. Among them, SCs are known as a promising candidate for portable and mobile based applications [8e10]. SCs are also known as electrochemical capacitors, ultracapacitors and power capacitors. As a sort of energy storage container, SCs are well known due to good energy storage performance, high specific capacity, moderate energy density, high power density and long cycle life, which have been widely used in our daily life applications such as notebook computers, automobiles and other industries [11,12]. Owing to the ability of charging and discharging in a short time and outputting high current pulses under high power requirements, SCs have become an indispensable part of energy storage equipment such as spare batteries and storage batteries. From the macro-point of view, all above advantages are attributed to the thinner dielectrics and large specific surface area of the working electrodes of SCs, which results a high electron/ion transfer rate and increase the electrochemical active surface area. It is believed that the physical and chemical properties of the working electrodes acts as key sources for excellent performance of supercapacitors [13,14]. SCs overcome the low energy density more as comparison with traditional capacitors, but it is still lower than that of rechargeable batteries and storage batteries. Therefore, energy density can be improved by using energy density formula E ¼ 1/2CV2, through increasing the specific capacitance and working voltage. Furthermore, exploring a proper electrode material for the SCs also can greatly enlarge the active contact surface for redox reactions and offer higher specific capacitance than traditional electrode materials [15e17]. Transition metal oxides have been explored as supercapacitor electrode materials since 1975. Excepting electronic conductivity, they contain metal which have more than two valence states, so phase transition and irreversible structural changes will not occur and protons can be embedded and exited from oxygen lattices freely [18,19]. As pseudocapacitive materials, their electrochemical reaction not only occurs at the electrode/electrolyte interface, but also takes part in adjacent areas of electrode surface as a result of the diffusion of electrolyte ions in the electrolyte [20e22]. Cobalt oxide, as an important pseudocapacitive material, has been widely investigated by many research groups due to its plenty of accessible sites for faradaic reaction and strong adhesion ability on the surface of substance. However, the experiment showed that the available specific capacitance of cobalt oxide was smaller than that of
predicted theoretical value owing to the limited conductivity of electrons and electrolyte contact limitations [23e25]. At the same time, ternary metal oxides (TMOs) have attracted much attention because they contain several metal elements and the synergistic effect of multiple oxides, which can produce more abundant redox reactions and better conductivity. Owing to its variable valence state of redox reaction and the presence of active Fe2þ, ferric cobalt oxide is also a promising electrode material. But the single iron cobalt oxide electrode has limited diffusion length at the reaction interface and low ion conversion rate, which also shows unsatisfactory performance. Recently, the three-dimensional heterostructure combined with pseudocapacitive materials has attracted a huge attention. Constructing a core-shell structure with metal oxide can produce the synergistic effect with pseudocapacitive material considerably, which is an effective way to overcome the above drawbacks. Core-shell electrode materials include a great variety of morphologies and structures such as nanowires, nanotubes, nanosheets, nanospheres and nanotubes [26]. Generally, core materials have more effects on the performance of the electrode materials as the main active components, while the shell materials can be served as a protective layer to improve the stability of internal materials, this architectural design can take advantage of the each components and offer the valid paths for electrolyte diffusion, resulting in the rapid surface redox reactions for the better electrochemical properties. In addition, the precise control of the morphology and structure of the electrode components can be achieved to meet the specific performance requirements by adjusting the coreshell type materials, molar mass ratio and the thickness of shell. According to previous research, Bai et al. [27] fabricated hierarchical structure Co3O4@Ni(OH)2 core-shell binder-free electrode in a simple hydrothermal method, which present high specific capacitance of 1306 F g1 and good cycling performance (retaining about 90.5% after 5000 cycles). Kong et al. [28] reported a cost-effective way to design and synthesize Co3O4@MnO2 hierarchical nanoneedle arrays on nickel foam, which demonstrated exceptional specific capacitance of 1639.2 F g1 and long-term cycling stability. As mentioned above, special structure of the core-shell material facilitates the diffusion of ions in the electrolyte and accelerates the progress of the faraday reaction. At the same time, choosing a kind of material for the shell structure also plays an essential role to increase the active contact surface of the redox reaction with high energy storage. Most importantly, such hierarchies can hold well volume expansion and contraction during charge-discharge cycles to attain a good cycling stability [29e32]. Herein, we have introduced a facile approach to synthesize core-shell structure based on Co3O4 as core scaffold and FeCo2O4 ultrathin nanosheets as shell material. The specific experiment concludes that the electrochemical performance of core-shell electrode materials is obviously superior to single-component material. In SCs electrochemical test, the obtained electrode exhibits a high specific capacitance of 1680 F g1 (at the current density of 1 A g1) and excellent cycling stability.
Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Experimental Preparation of Co3O4 arrays First, the porous foamed nickel (4 2.5 cm2) was ultrasonically washed with HCl solution, acetone and ethanol. Ultrasonic cleaning of HCl solution was used to remove the oxide film existing on the surface of the foamed nickel. The function of acetone was to remove the oily stain on the surface of the porous foamed nickel. The treated foamed nickel was placed in a vacuum drying oven at 60 C for overnight. 0.59 g of NH4F, 1.18 g of Co(NO3)2$6H2O and 0.4 g of CON2H4 were dissolved in 50 mL of deionized water under magnetic stirring for 0.5 h to prepare a homogenous and clear solution and then the cleaned foam nickel was placed in the reaction vessel as to be completely immersed in the solution, then was transferred to a Teflon-lined autoclave at temperature of 120 C for 5 h. Later the reactor was naturally cooled at room temperature in order to grow the as-prepared Co3O4 on the substrate and was washed with deionized water and ethanol. In last, hydrothermal treated nickel foam sample was placed in a vacuum drying oven at 60 C for 4 h and the dried sample was placed in a muffle furnace for calcination at 350 C for 2 h.
Preparation of FeCo2O4 ultrathin nanosheets arrays on nickle foam Firstly, a piece of foamed nickel was rinsed with DI water, then 0.58 g of Co(NO3)2$6H2O, 0.18 g of Fe (NO3)2$6H2O and 0.90 g of CH4N2O were dissolved in 36 mL deionized water under magnetic stirring for 0.5 h. Then the liquid and the treated nickel foam were transferred into a Teflon lined stainless steel autoclave at temperature of 100 C for 6 h and later it was cooled at room temperature. In last, the sample of FeCo2O4 was rinsed and dried at 60 C for 12 h, followed by the calcination at 350 C for 2 h in muffle furnace.
Synthesis of Co3O4@ FeCo2O4 core-shell nanosheet arrays In a typical synthesis, FeCl2$4H2O and CoCl2$6(H2O) were mixed at a ratio of 2: 1 and dissolved in deionized water. Then appropriate amount of CH4N2O and NH4F were added in the reagent solution for 4 h. The solution was transferred to the stainless steel autoclave with a piece of as obtained Co3O4 on the nickel foam. Afterward, the autoclave was maintained at 100 C for 6 h and cooled at room temperature. Finally, the dried sample was placed in a muffle furnace for calcination at 350 C for 2 h.
Material characterization The microstructures of Co3O4 nanowires and Co3O4@FeCo2O4 core-shell nanosheet arrays were examined by the Scanning electron microscope(SEM) (Hitachi SU-8010, Tokyo, Japan). Transmission electron microscopy (TEM) picture, high resolution transmission electron microscopy (HRTEM) picture and selected-area electron diffraction (SAED) images were recorded via using TEM (JEOL JEM-2010) with a field emission gun operating at 200 kV. The spectra of specimens were
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investigated by X-ray photoelectron spectroscopy (XPS), PHI5700 ESCA spectrometer with Al KR radiation. The crystal phase and structure information of the samples were studied by X-ray powder diffraction (XRD) diffractometer (Rigaku TTRIII). Nitrogen adsorptionedesorption isotherms were obtained using a micromeritics ASAP 2010 apparatus to evaluate surface area and porosity of the as prepared products.
Electrochemical measurements Electrochemical impedance spectroscopy (EIS) is an electrochemical measurement using small amplitude sinusoidal potential (or current) as interference signal. Electrochemical measurements were conducted by an electrochemical work station (CHI 660D, Shanghai, China), which used the threeelectrode mode in a 2 M KOH at the potential window range from 0 to 0.5 V. A piece of Pt foil and saturated calomel electrode (SCE) were acted as the counter electrode and the reference electrode, respectively, while the heterostructures Co3O4@FeCo2O4 on nickel foam (1 cm2 in area) was directly used as the working electrodes. The specific capacitance of Co3O4@FeCo2O4 electrodes was computed using the following calculation formula： C¼
It mV
where I represents discharge current; V is the potential range upon discharging; and m means the mass of active materials, t is the discharge time derived from charge/discharge measurement.
Results and discussion Characterization of Co3O4@ FeCo2O4 core/shell The Co3O4@FeCo2O4 core-shell electrode materials were grown on the substrate via a stepwise bottom-up assembly synthesis process, which is shown in Fig. 1. Firstly, abundant cross-linked Co3O4 nanowires precursors were synthesized by the hydrothermal method. The core nanowires were deposited on nickel foam involved an uncomplicated hydrothermal and post-annealing process. The formation of Co3O4 phase could be confirmed by the corresponding XRD pattern in Fig. S1 with the PDF card of 80e1534. In the reaction process, the addition of urea could increase the content of OH and 2þ and Fe2þ reacted with them at a certain ratio CO2 3 , then Co to form precursor nanoparticles (FexCo2x(OH)6xCO3), which attached to Co3O4 nanowires, the precursor nanoparticles can be successfully converted into FeCo2O4, the FeCo2O4 ultrathin nanosheets were constructed on the surface of Co3O4 nanowires by the second hydrothermal process, forming the hierarchical Co3O4@FeCo2O4 core/shell arrays [33,34]. This is confirmed by the peaks observed in the XRD pattern (Fig S2). In the diagram, the three strongest diffraction peaks are the diffraction peaks of the foam nickel substrate. Other diffraction peaks can be indexed as the FeCo2O4 spinel phase (JCPDS No.71-0816) and Co3O4(JCPDS 42-1467). As shown in Fig. 2, the morphology of Co3O4, FeCo2O4 and Co3O4@FeCo2O4 were studied by SEM, respectively.
Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Fig. 1 e Schematic illustration of the formation processes of Co3O4@ FeCo2O4 on foam nickle. Fig. 2A exhibits the in-situ growth of the Co3O4 nanowires on the substrate. The SEM images as shown in Fig. 2B display that large number of connected nanowires are deposited on the surface of substrate, which can be used as a skeleton to continue the growth of FeCo2O4 nanosheets. Fig. 2C presents the low-magnification SEM image of the FeCo2O4 nanosheets, which completely covered the substrate. The SEM image in Fig. 2D reveales that the forest-like FeCo2O4 nanosheets were successfully grown on nickel foam and they were interconnected with each other as a staggered sheet-like structure that is radially distributed. Clearly, the average length of the nanosheet is about 4e5 mm, with FeCo2O4 nanosheet morphology assembles together to form a loose but irregular structure. Fig. 2E shows SEM image of Co3O4@FeCo2O4 core-shell nanoflakes.
It’s obvious that nanowires are surrounded by a layer of nanosheets, Further magnification (Fig. 2F) reveals the heterostructure was structured like many florets. The as prepared unique core-shell structure can effectively reduce the electrode contact resistance and greatly increase the active site of the electrode material [35e37]. This unique core-shell structure can increase the active specific surface area of the material by reducing the internal impedance of the electrode [38]. Detailed morphologies and structural information of the Co3O4@FeCo2O4 core-shell materials were also investigated via the TEM analysis. The TEM image is shown in Fig. 3; the microstructure of Co3O4 nanowires and core-shell Co3O4@FeCo2O4 nanowires can be seen more clearly by TEM and HRTEM. Fig. 3A and B presents TEM diagrams of a single
Fig. 2 e SEM images of (AeB) Co3O4, (CeD) FeCo2O4, (EeF) Co3O4@ FeCo2O4 core-shell nanoflakes arrays on nickel foam. Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Co3O4 nanowire. It can be observed from Fig. 3B that the diameter of Co3O4 nanowires is about 50e100 nm, which is composed of nanoparticles of 20e50 nm in size. There are a lot of mesoporous structures among these nanoparticles. Fig. 3C shows the high resolution TEM image of Co3O4 nanowires. From the image, we can find that the lattice fringes of Co3O4 nanowires are clearly distinguishable, which indicates that the prepared Co3O4 nanowires have high crystallinity. The spacing of lattice fringes is about 0.28 nm. In Fig. 3D and E, it is evident that Co3O4 nanowires are surrounded by ultrathin FeCo2O4 nanosheets, forming a three-dimensional core-shell structure. At the same time, Fig. 3F obtained the high resolution transmission of FeCo2O4 nanosheets. The lattice fringe spacing of FeCo2O4 nanosheets is about 0.295 nm, which corresponds to the (220) crystal plane of FeCo2O4 with spinel structure [39,40]. The corresponding SAED patterns appear well-defined rings (inset of Fig. 3F), indicating the polycrystalline nature of the FeCo2O4. The results are consistent with the TEM results of single FeCo2O4 on foam nickel, as shown in Fig S2, where the TEM and EDS results confirm the successful preparation of FeCo2O4. It is noteworthy that the core-shell structures of Co3O4 nanowires and ultra-thin FeCo2O4 nanosheets have a mesoporous structure, which can promote the transport of electrolyte ions in the electrode and improve the utilization of electrode material. The specific surface area and pore size distribution of samples prepared by hydrothermal method were investigated by nitrogen adsorption/desorption test. The nitrogen adsorption/desorption curves and pore size distribution curves of Co3O4@FeCo2O4 are shown in Fig. 4. Compared with other samples (Fig S4 and Fig S5), the core-shell structure of Co3O4@FeCo2O4 has larger Brunauer-EmmetTeller (BET) specific surface area (120.32 m2/g) due to the existence of micropore and mesopore, whereas that of
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Co3O4 and FeCo2O4 are 19.68 and 58.34 m2/g, respectively. The adsorption/desorption curve can be classified as type IV, which can be proved by strong internal adsorption/ desorption effect. The existence of wide hysteresis loops in the curves of 0.4 < P/P < 0.8 proves the existence of micropore in sample species. It is noteworthy that the pore size distribution of the core-shell nanostructures is narrow and single peak as shown in Fig. 4. Most of the micropore diameters are less than 10 nm and mainly concentrated at 3.8 nm. The existence of well dispersed mesoporous in the sample can provide a path for the rapid transfer of electrolyte ions, which is conducive to the acquisition of high performance supercapacitors. The XPS spectra were employed to further investigate the chemical compositions and valence state of element in the Co3O4@ FeCo2O4 samples as shown in Fig. 5. The full-surveyscan spectrum in Fig. 5A demonstrates that the elemental composition of Ni, Co, Fe，C and O obtained in the samples. The Co 2p 3/2 peak of Co3O4 nanowire arrays is fitted with peaks located at 780.5 and 794.7 eV (Fig. 5C)，which correspond to Co2þ and Co3þ in Co3O4. The XPS spectrum of Fe 2p is shown in Fig. 5B, which can be decomposed into two shakeup satellites and spineorbit doublets. They are centered at 711.5 and 724.6 eV, which are associated with Fe2þ and Fe3þ ions. According to the fitted data, a conclusion can be drawn that the existence of Fe2þ, Fe3þ, Co2þ, Co3þ, and oxygen element within the obtained samples. The above XPS results reveal that Co2þ, Co3þ, Fe3þ, Fe2þ, and Ni2þ are presented in the samples, consisting with the results of EDS [41e43].
Electrochemical performance of the Co3O4@FeCo2O4 electrode The electrochemical performance test is a significant method for testing the properties of synthetic materials [44]. The
Fig. 3 e (A, B) TEM and (C) HRTEM images of Co3O4. (D, E) TEM and (F) HRTEM images of Co3O4@ FeCo2O4，inset in (F) is the corresponding SAED pattern. Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Fig. 4 e The nitrogen (N2) adsorption/desorption isotherms of Co3O4@FeCo2O4, inset is pore size distribution curves.
Fig. 5 e XPS spectra of (A) survey, (B) Fe 2p, (C) Co 2p, (D) O 1s for Co3O4@ FeCo2O4, herein, the satellite is denoted as “sat.” in these figures.
Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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electrochemical performance of Co3O4, FeCo2O4 and Co3O4@FeCo2O4 hybrid electrodes were evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) in a three-electrode cell with 2 M KOH aqueous as the electrolyte [45,46]. Fig. 6A shows the typical CV of Co3O4, FeCo2O4 and Co3O4@FeCo2O4 with a potential range of 0 and 0.5 V (vs SCE). A pair of redox peaks are observed at a scan rate of 5 mV s1. That is relevant to the reversible faradaic reactions of MeO/ MeOeOH, where m refers to Ni, Co, Fe. It is noteworthy that the peak currents of the Co3O4@FeCo2O4 are much higher than that of the pristine FeCo2O4 or Co3O4, Meanwhile, the Co3O4@FeCo2O4 hybrid electrodes exhibits the largest enclosed area in CV curves, demonstrating that the hybrid electrode possesses higher electrochemical reaction activity and higher specific capacitance compared with the single material. Fig. 6C presents the CV curves of the Co3O4, FeCo2O4 and Co3O4@FeCo2O4 electrodes at various scan rates ranged from 5 to 50 mV s1. With the increased scan rates, the oxidation and reduction peaks shift toward higher and lower potential. The separation of potential is considered to be related with charge diffusion polarization within the electrode material, indicating the weakening of the quasireversible properties of redox pairs [47]. The GCD curves of the Co3O4, FeCo2O4 and Co3O4@FeCo2O4 electrodes at the constant current density of 1 A g1 are shown in Fig. 6B. Compared with other electrodes, Co3O4@FeCo2O4 electrode
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has longer charging and discharging time. In addition, the GCD tests of the Co3O4@ FeCo2O4 electrode is implemented at various current densities from 1 to 20 A g1 (Fig. 6D). Through the observation and analysis of these curves, it is not difficult to find that the GCD curves are almost perfectly symmetric at all current densities, suggesting a high coulombic efficiency of the hybrid electrode in the charge-discharge process. As shown in Fig. 7 A, the corresponding specific capacitances at various discharge current densities for Co3O4, FeCo2O4 and Co3O4@ FeCo2O4 hybrid electrodes are calculated. At a current density of 1 A g1, the hybrid Co3O4@ FeCo2O4 electrodes exhibited high specific capacitances of 1680 F g1. When the current density increases to 20 A g1, the specific capacitances of the hybrid electrodes is still up to 1192 F g1, which corresponds to capacitance retentions of 70.1%, while that of the pristine Co3O4 and FeCo2O4 electrodes are 65.0% and 62.8% respectively. Besides specific capacitance, the cyclic stability of electrodes is also one of the important performance parameters. We carried out 2000 cycles with the chargedischarge curves test at a current density of 1 A g1. As demonstrated in Fig. 7B, the Co3O4 and FeCo2O4 electrodes exhibit a specific capacitance of about 89.3% retention and 88.1% retention after 2000 cycles, while the capacity retentions of the Co3O4@FeCo2O4 hybrid electrodes are 90.2%. We attributed the excellent cycling stability to the unique core/shell architecture with robust interconnected networks
Fig. 6 e (A) Cyclic voltammetry curves of Co3O4, FeCo2O4 and core-shell Co3O4@ FeCo2O4; (B) charge-discharge curves of three samples tested at 1A g¡1; (C) CV curves of Co3O4@FeCo2O4 at different scan rate from 5 to 50 mVs¡1; (D) GCD plots of Co3O4@FeCo2O4 at various current density. Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Fig. 7 e (A) Charge-discharge cycling test of Co3O4, FeCo2O4 and core-shell Co3O4@ FeCo2O4 electrodes; (B) Specific capacitance of Co3O4@ FeCo2O4 at different current densities. and good strain accommodation, which provides more reactive sites and greatly improves the utilization of the hybrid electrode materials. Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range from 0.01 Hz to 100 kHz, Fig. 8 presents the corresponding Nyquist plots. Based on the equivalent electrical circuit, the Nyquist plots are analyzed by the Z view software simulation method. This plot can be divided into two parts, high frequency and low-frequency. In the high-frequency region, the intersection of the real axis represents internal resistance (Rs), and the radius of the semicircle is related to charge transfer resistance (Rct). The internal resistance (Rs) of Co3O4, FeCo2O4 and Co3O4@FeCo2O4 are 0.46, 0.52 and 0.78 U respectively. The charge transfer resistance (Rct) is mainly related with the electronic and ionic resistances, the morphology and conductivity of electrode [48,49]. In the low-frequency region, the Co3O4@ FeCo2O4 hybrid electrode shows a more vertical line, suggesting the lower diffusion resistance. The impedance spectroscopy (EIS)
measurement results indicate that the electrolyte ion can easily diffuse into the hybrid electrode [50,51].
Conclusion To sum up, we have synthesized the hierarchical Co3O4@FeCo2O4 core/shell hybrid structures by two-step method. The Co3O4 nanowires can be served as the “core materials”, and the interconnect nanosheets play the part of “shell materials”. The as-fabricated Co3O4@FeCo2O4 hybrid electrode manifests a high specific capacitance of 1649 F g1 at a current density of 1 A g1, desirable rate capability with 70.3% retention of the initial capacitance when the current density is increased from 1 to 20 A g1 and superior cycling stability (90.6% capacitance retention after 2000 cycles). Such enhanced properties are owing to the judiciously engineer the interface with unique hierarchical core/shell structures. As proofeofeconcept application in devices with high energy and power densities, this study presented a novel material for designing a composite electrode for supercapacitors.
Acknowledgements This work was supported by National Key R&D Program of China (2016YFE0202700), National Natural Science Foundation of China (NSFC 51603053), the Application Technology Research and Development Plan of Heilongjiang Province (GX16A008), Fundamental Research Funds of the Central University, International Science & Technology Cooperation Program of China (2015DFA50050) and Defense Industrial Technology Development Program (JCKY2016604C006, JCKY2018604C011).
Appendix A. Supplementary data
Fig. 8 e Nyquist plots of Co3O4, FeCo2O4 and Co3O4@FeCo2O4
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.119.
Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors Qinfang Wu a,c, Yunhe Zhao a, Jing Yu a,*, Dalei Song a, Rongrong Chen a,c, Qi Liu a, Rumin Li a, Meiqing Fan b,** a
Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, China b College of Food Engineering, Jilin Engineering Normal University, Changchun, 130052, China c Institute of Advanced Marine Materials, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, 150001, China
highlights A core-shell structure of Co3O4@FeCo2O4 is prepared as supercapacitor electrode. The hierarchical structures electrode exhibits remarkable specific capacitance and cycling stability. Such enhanced properties owning to the judiciously engineer the interface with unique hierarchical structures.
article info
abstract
Article history:
In recent years, the tenable design and synthesis of the core/shell heterostructure as
Received 14 July 2019
electrode for the supercapacitor, have attained a huge attention and concerns. In this
Received in revised form
article, the three-dimensional heterostructure consisting of FeCo2O4 ultrathin nanosheets
28 September 2019
grown on the space of vertical Co3O4 nanowires has been designed and synthesized onto
Accepted 17 October 2019
nickel foam (NF) for pseudocapacitive electrode applications. According to previous
Available online xxx
research, the NF@ FeCo2O4 electrodes can only exhibit specific capacity of 1172 F g1 at a current density of 1 A g1. In addition, although the capacity of the NF@Co3O4 electrodes
Keywords:
can reach to 1482 F g1 and it has the disadvantage of agglomeration, which restricts the
Supercapacitor
diffusion of ions and has a negative effect on the progress of electrochemical reactions.
Transition metal oxides
Therefore, a core-shell nanostructure is fabricated by an improved two-step hydrothermal
Core-shell heterostructures
process, which improves the probability of ion reaction with more efficient charge transfer.
Co3O4
Furthermore, in as-prepared unique core/shell heterostructure, the resultant electrode
FeCo2O4
possesses the merits of large capacitance of 1680 F g1 at a current density of 1 A g1, an excellent rate capability of 70.1% at 20 A g1 and only 9.8% loss of initial capacitance at a high charge/discharge current density after 2000 cycles. These results demonstrate that this kind of distinct electrode has potential utilization for supercapacitor. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yu), [email protected] (M. Fan). https://doi.org/10.1016/j.ijhydene.2019.10.119 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Introduction Now a days, the development of energy storage devices and conversion has attracted a lot of attention in today’s human society due to the depletion of non-renewable resources and environmental pollution caused by the excessive use of fossil energy [1e5]. Therefore, developing a new kind of energy storage equipment with high efficiency, stability and environmental friendliness has become the focus of research to reduce the dependence on traditional fossil fuels [6,7]. Currently, energy storage devices including lithium-ion batteries, fuel cell and supercapacitors (SCs) can store and convert chemical energy by electrochemical energy storage technology. Among them, SCs are known as a promising candidate for portable and mobile based applications [8e10]. SCs are also known as electrochemical capacitors, ultracapacitors and power capacitors. As a sort of energy storage container, SCs are well known due to good energy storage performance, high specific capacity, moderate energy density, high power density and long cycle life, which have been widely used in our daily life applications such as notebook computers, automobiles and other industries [11,12]. Owing to the ability of charging and discharging in a short time and outputting high current pulses under high power requirements, SCs have become an indispensable part of energy storage equipment such as spare batteries and storage batteries. From the macro-point of view, all above advantages are attributed to the thinner dielectrics and large specific surface area of the working electrodes of SCs, which results a high electron/ion transfer rate and increase the electrochemical active surface area. It is believed that the physical and chemical properties of the working electrodes acts as key sources for excellent performance of supercapacitors [13,14]. SCs overcome the low energy density more as comparison with traditional capacitors, but it is still lower than that of rechargeable batteries and storage batteries. Therefore, energy density can be improved by using energy density formula E ¼ 1/2CV2, through increasing the specific capacitance and working voltage. Furthermore, exploring a proper electrode material for the SCs also can greatly enlarge the active contact surface for redox reactions and offer higher specific capacitance than traditional electrode materials [15e17]. Transition metal oxides have been explored as supercapacitor electrode materials since 1975. Excepting electronic conductivity, they contain metal which have more than two valence states, so phase transition and irreversible structural changes will not occur and protons can be embedded and exited from oxygen lattices freely [18,19]. As pseudocapacitive materials, their electrochemical reaction not only occurs at the electrode/electrolyte interface, but also takes part in adjacent areas of electrode surface as a result of the diffusion of electrolyte ions in the electrolyte [20e22]. Cobalt oxide, as an important pseudocapacitive material, has been widely investigated by many research groups due to its plenty of accessible sites for faradaic reaction and strong adhesion ability on the surface of substance. However, the experiment showed that the available specific capacitance of cobalt oxide was smaller than that of
predicted theoretical value owing to the limited conductivity of electrons and electrolyte contact limitations [23e25]. At the same time, ternary metal oxides (TMOs) have attracted much attention because they contain several metal elements and the synergistic effect of multiple oxides, which can produce more abundant redox reactions and better conductivity. Owing to its variable valence state of redox reaction and the presence of active Fe2þ, ferric cobalt oxide is also a promising electrode material. But the single iron cobalt oxide electrode has limited diffusion length at the reaction interface and low ion conversion rate, which also shows unsatisfactory performance. Recently, the three-dimensional heterostructure combined with pseudocapacitive materials has attracted a huge attention. Constructing a core-shell structure with metal oxide can produce the synergistic effect with pseudocapacitive material considerably, which is an effective way to overcome the above drawbacks. Core-shell electrode materials include a great variety of morphologies and structures such as nanowires, nanotubes, nanosheets, nanospheres and nanotubes [26]. Generally, core materials have more effects on the performance of the electrode materials as the main active components, while the shell materials can be served as a protective layer to improve the stability of internal materials, this architectural design can take advantage of the each components and offer the valid paths for electrolyte diffusion, resulting in the rapid surface redox reactions for the better electrochemical properties. In addition, the precise control of the morphology and structure of the electrode components can be achieved to meet the specific performance requirements by adjusting the coreshell type materials, molar mass ratio and the thickness of shell. According to previous research, Bai et al. [27] fabricated hierarchical structure Co3O4@Ni(OH)2 core-shell binder-free electrode in a simple hydrothermal method, which present high specific capacitance of 1306 F g1 and good cycling performance (retaining about 90.5% after 5000 cycles). Kong et al. [28] reported a cost-effective way to design and synthesize Co3O4@MnO2 hierarchical nanoneedle arrays on nickel foam, which demonstrated exceptional specific capacitance of 1639.2 F g1 and long-term cycling stability. As mentioned above, special structure of the core-shell material facilitates the diffusion of ions in the electrolyte and accelerates the progress of the faraday reaction. At the same time, choosing a kind of material for the shell structure also plays an essential role to increase the active contact surface of the redox reaction with high energy storage. Most importantly, such hierarchies can hold well volume expansion and contraction during charge-discharge cycles to attain a good cycling stability [29e32]. Herein, we have introduced a facile approach to synthesize core-shell structure based on Co3O4 as core scaffold and FeCo2O4 ultrathin nanosheets as shell material. The specific experiment concludes that the electrochemical performance of core-shell electrode materials is obviously superior to single-component material. In SCs electrochemical test, the obtained electrode exhibits a high specific capacitance of 1680 F g1 (at the current density of 1 A g1) and excellent cycling stability.
Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Experimental Preparation of Co3O4 arrays First, the porous foamed nickel (4 2.5 cm2) was ultrasonically washed with HCl solution, acetone and ethanol. Ultrasonic cleaning of HCl solution was used to remove the oxide film existing on the surface of the foamed nickel. The function of acetone was to remove the oily stain on the surface of the porous foamed nickel. The treated foamed nickel was placed in a vacuum drying oven at 60 C for overnight. 0.59 g of NH4F, 1.18 g of Co(NO3)2$6H2O and 0.4 g of CON2H4 were dissolved in 50 mL of deionized water under magnetic stirring for 0.5 h to prepare a homogenous and clear solution and then the cleaned foam nickel was placed in the reaction vessel as to be completely immersed in the solution, then was transferred to a Teflon-lined autoclave at temperature of 120 C for 5 h. Later the reactor was naturally cooled at room temperature in order to grow the as-prepared Co3O4 on the substrate and was washed with deionized water and ethanol. In last, hydrothermal treated nickel foam sample was placed in a vacuum drying oven at 60 C for 4 h and the dried sample was placed in a muffle furnace for calcination at 350 C for 2 h.
Preparation of FeCo2O4 ultrathin nanosheets arrays on nickle foam Firstly, a piece of foamed nickel was rinsed with DI water, then 0.58 g of Co(NO3)2$6H2O, 0.18 g of Fe (NO3)2$6H2O and 0.90 g of CH4N2O were dissolved in 36 mL deionized water under magnetic stirring for 0.5 h. Then the liquid and the treated nickel foam were transferred into a Teflon lined stainless steel autoclave at temperature of 100 C for 6 h and later it was cooled at room temperature. In last, the sample of FeCo2O4 was rinsed and dried at 60 C for 12 h, followed by the calcination at 350 C for 2 h in muffle furnace.
Synthesis of Co3O4@ FeCo2O4 core-shell nanosheet arrays In a typical synthesis, FeCl2$4H2O and CoCl2$6(H2O) were mixed at a ratio of 2: 1 and dissolved in deionized water. Then appropriate amount of CH4N2O and NH4F were added in the reagent solution for 4 h. The solution was transferred to the stainless steel autoclave with a piece of as obtained Co3O4 on the nickel foam. Afterward, the autoclave was maintained at 100 C for 6 h and cooled at room temperature. Finally, the dried sample was placed in a muffle furnace for calcination at 350 C for 2 h.
Material characterization The microstructures of Co3O4 nanowires and Co3O4@FeCo2O4 core-shell nanosheet arrays were examined by the Scanning electron microscope(SEM) (Hitachi SU-8010, Tokyo, Japan). Transmission electron microscopy (TEM) picture, high resolution transmission electron microscopy (HRTEM) picture and selected-area electron diffraction (SAED) images were recorded via using TEM (JEOL JEM-2010) with a field emission gun operating at 200 kV. The spectra of specimens were
3
investigated by X-ray photoelectron spectroscopy (XPS), PHI5700 ESCA spectrometer with Al KR radiation. The crystal phase and structure information of the samples were studied by X-ray powder diffraction (XRD) diffractometer (Rigaku TTRIII). Nitrogen adsorptionedesorption isotherms were obtained using a micromeritics ASAP 2010 apparatus to evaluate surface area and porosity of the as prepared products.
Electrochemical measurements Electrochemical impedance spectroscopy (EIS) is an electrochemical measurement using small amplitude sinusoidal potential (or current) as interference signal. Electrochemical measurements were conducted by an electrochemical work station (CHI 660D, Shanghai, China), which used the threeelectrode mode in a 2 M KOH at the potential window range from 0 to 0.5 V. A piece of Pt foil and saturated calomel electrode (SCE) were acted as the counter electrode and the reference electrode, respectively, while the heterostructures Co3O4@FeCo2O4 on nickel foam (1 cm2 in area) was directly used as the working electrodes. The specific capacitance of Co3O4@FeCo2O4 electrodes was computed using the following calculation formula： C¼
It mV
where I represents discharge current; V is the potential range upon discharging; and m means the mass of active materials, t is the discharge time derived from charge/discharge measurement.
Results and discussion Characterization of Co3O4@ FeCo2O4 core/shell The Co3O4@FeCo2O4 core-shell electrode materials were grown on the substrate via a stepwise bottom-up assembly synthesis process, which is shown in Fig. 1. Firstly, abundant cross-linked Co3O4 nanowires precursors were synthesized by the hydrothermal method. The core nanowires were deposited on nickel foam involved an uncomplicated hydrothermal and post-annealing process. The formation of Co3O4 phase could be confirmed by the corresponding XRD pattern in Fig. S1 with the PDF card of 80e1534. In the reaction process, the addition of urea could increase the content of OH and 2þ and Fe2þ reacted with them at a certain ratio CO2 3 , then Co to form precursor nanoparticles (FexCo2x(OH)6xCO3), which attached to Co3O4 nanowires, the precursor nanoparticles can be successfully converted into FeCo2O4, the FeCo2O4 ultrathin nanosheets were constructed on the surface of Co3O4 nanowires by the second hydrothermal process, forming the hierarchical Co3O4@FeCo2O4 core/shell arrays [33,34]. This is confirmed by the peaks observed in the XRD pattern (Fig S2). In the diagram, the three strongest diffraction peaks are the diffraction peaks of the foam nickel substrate. Other diffraction peaks can be indexed as the FeCo2O4 spinel phase (JCPDS No.71-0816) and Co3O4(JCPDS 42-1467). As shown in Fig. 2, the morphology of Co3O4, FeCo2O4 and Co3O4@FeCo2O4 were studied by SEM, respectively.
Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Fig. 1 e Schematic illustration of the formation processes of Co3O4@ FeCo2O4 on foam nickle. Fig. 2A exhibits the in-situ growth of the Co3O4 nanowires on the substrate. The SEM images as shown in Fig. 2B display that large number of connected nanowires are deposited on the surface of substrate, which can be used as a skeleton to continue the growth of FeCo2O4 nanosheets. Fig. 2C presents the low-magnification SEM image of the FeCo2O4 nanosheets, which completely covered the substrate. The SEM image in Fig. 2D reveales that the forest-like FeCo2O4 nanosheets were successfully grown on nickel foam and they were interconnected with each other as a staggered sheet-like structure that is radially distributed. Clearly, the average length of the nanosheet is about 4e5 mm, with FeCo2O4 nanosheet morphology assembles together to form a loose but irregular structure. Fig. 2E shows SEM image of Co3O4@FeCo2O4 core-shell nanoflakes.
It’s obvious that nanowires are surrounded by a layer of nanosheets, Further magnification (Fig. 2F) reveals the heterostructure was structured like many florets. The as prepared unique core-shell structure can effectively reduce the electrode contact resistance and greatly increase the active site of the electrode material [35e37]. This unique core-shell structure can increase the active specific surface area of the material by reducing the internal impedance of the electrode [38]. Detailed morphologies and structural information of the Co3O4@FeCo2O4 core-shell materials were also investigated via the TEM analysis. The TEM image is shown in Fig. 3; the microstructure of Co3O4 nanowires and core-shell Co3O4@FeCo2O4 nanowires can be seen more clearly by TEM and HRTEM. Fig. 3A and B presents TEM diagrams of a single
Fig. 2 e SEM images of (AeB) Co3O4, (CeD) FeCo2O4, (EeF) Co3O4@ FeCo2O4 core-shell nanoflakes arrays on nickel foam. Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Co3O4 nanowire. It can be observed from Fig. 3B that the diameter of Co3O4 nanowires is about 50e100 nm, which is composed of nanoparticles of 20e50 nm in size. There are a lot of mesoporous structures among these nanoparticles. Fig. 3C shows the high resolution TEM image of Co3O4 nanowires. From the image, we can find that the lattice fringes of Co3O4 nanowires are clearly distinguishable, which indicates that the prepared Co3O4 nanowires have high crystallinity. The spacing of lattice fringes is about 0.28 nm. In Fig. 3D and E, it is evident that Co3O4 nanowires are surrounded by ultrathin FeCo2O4 nanosheets, forming a three-dimensional core-shell structure. At the same time, Fig. 3F obtained the high resolution transmission of FeCo2O4 nanosheets. The lattice fringe spacing of FeCo2O4 nanosheets is about 0.295 nm, which corresponds to the (220) crystal plane of FeCo2O4 with spinel structure [39,40]. The corresponding SAED patterns appear well-defined rings (inset of Fig. 3F), indicating the polycrystalline nature of the FeCo2O4. The results are consistent with the TEM results of single FeCo2O4 on foam nickel, as shown in Fig S2, where the TEM and EDS results confirm the successful preparation of FeCo2O4. It is noteworthy that the core-shell structures of Co3O4 nanowires and ultra-thin FeCo2O4 nanosheets have a mesoporous structure, which can promote the transport of electrolyte ions in the electrode and improve the utilization of electrode material. The specific surface area and pore size distribution of samples prepared by hydrothermal method were investigated by nitrogen adsorption/desorption test. The nitrogen adsorption/desorption curves and pore size distribution curves of Co3O4@FeCo2O4 are shown in Fig. 4. Compared with other samples (Fig S4 and Fig S5), the core-shell structure of Co3O4@FeCo2O4 has larger Brunauer-EmmetTeller (BET) specific surface area (120.32 m2/g) due to the existence of micropore and mesopore, whereas that of
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Co3O4 and FeCo2O4 are 19.68 and 58.34 m2/g, respectively. The adsorption/desorption curve can be classified as type IV, which can be proved by strong internal adsorption/ desorption effect. The existence of wide hysteresis loops in the curves of 0.4 < P/P < 0.8 proves the existence of micropore in sample species. It is noteworthy that the pore size distribution of the core-shell nanostructures is narrow and single peak as shown in Fig. 4. Most of the micropore diameters are less than 10 nm and mainly concentrated at 3.8 nm. The existence of well dispersed mesoporous in the sample can provide a path for the rapid transfer of electrolyte ions, which is conducive to the acquisition of high performance supercapacitors. The XPS spectra were employed to further investigate the chemical compositions and valence state of element in the Co3O4@ FeCo2O4 samples as shown in Fig. 5. The full-surveyscan spectrum in Fig. 5A demonstrates that the elemental composition of Ni, Co, Fe，C and O obtained in the samples. The Co 2p 3/2 peak of Co3O4 nanowire arrays is fitted with peaks located at 780.5 and 794.7 eV (Fig. 5C)，which correspond to Co2þ and Co3þ in Co3O4. The XPS spectrum of Fe 2p is shown in Fig. 5B, which can be decomposed into two shakeup satellites and spineorbit doublets. They are centered at 711.5 and 724.6 eV, which are associated with Fe2þ and Fe3þ ions. According to the fitted data, a conclusion can be drawn that the existence of Fe2þ, Fe3þ, Co2þ, Co3þ, and oxygen element within the obtained samples. The above XPS results reveal that Co2þ, Co3þ, Fe3þ, Fe2þ, and Ni2þ are presented in the samples, consisting with the results of EDS [41e43].
Electrochemical performance of the Co3O4@FeCo2O4 electrode The electrochemical performance test is a significant method for testing the properties of synthetic materials [44]. The
Fig. 3 e (A, B) TEM and (C) HRTEM images of Co3O4. (D, E) TEM and (F) HRTEM images of Co3O4@ FeCo2O4，inset in (F) is the corresponding SAED pattern. Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Fig. 4 e The nitrogen (N2) adsorption/desorption isotherms of Co3O4@FeCo2O4, inset is pore size distribution curves.
Fig. 5 e XPS spectra of (A) survey, (B) Fe 2p, (C) Co 2p, (D) O 1s for Co3O4@ FeCo2O4, herein, the satellite is denoted as “sat.” in these figures.
Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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electrochemical performance of Co3O4, FeCo2O4 and Co3O4@FeCo2O4 hybrid electrodes were evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) in a three-electrode cell with 2 M KOH aqueous as the electrolyte [45,46]. Fig. 6A shows the typical CV of Co3O4, FeCo2O4 and Co3O4@FeCo2O4 with a potential range of 0 and 0.5 V (vs SCE). A pair of redox peaks are observed at a scan rate of 5 mV s1. That is relevant to the reversible faradaic reactions of MeO/ MeOeOH, where m refers to Ni, Co, Fe. It is noteworthy that the peak currents of the Co3O4@FeCo2O4 are much higher than that of the pristine FeCo2O4 or Co3O4, Meanwhile, the Co3O4@FeCo2O4 hybrid electrodes exhibits the largest enclosed area in CV curves, demonstrating that the hybrid electrode possesses higher electrochemical reaction activity and higher specific capacitance compared with the single material. Fig. 6C presents the CV curves of the Co3O4, FeCo2O4 and Co3O4@FeCo2O4 electrodes at various scan rates ranged from 5 to 50 mV s1. With the increased scan rates, the oxidation and reduction peaks shift toward higher and lower potential. The separation of potential is considered to be related with charge diffusion polarization within the electrode material, indicating the weakening of the quasireversible properties of redox pairs [47]. The GCD curves of the Co3O4, FeCo2O4 and Co3O4@FeCo2O4 electrodes at the constant current density of 1 A g1 are shown in Fig. 6B. Compared with other electrodes, Co3O4@FeCo2O4 electrode
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has longer charging and discharging time. In addition, the GCD tests of the Co3O4@ FeCo2O4 electrode is implemented at various current densities from 1 to 20 A g1 (Fig. 6D). Through the observation and analysis of these curves, it is not difficult to find that the GCD curves are almost perfectly symmetric at all current densities, suggesting a high coulombic efficiency of the hybrid electrode in the charge-discharge process. As shown in Fig. 7 A, the corresponding specific capacitances at various discharge current densities for Co3O4, FeCo2O4 and Co3O4@ FeCo2O4 hybrid electrodes are calculated. At a current density of 1 A g1, the hybrid Co3O4@ FeCo2O4 electrodes exhibited high specific capacitances of 1680 F g1. When the current density increases to 20 A g1, the specific capacitances of the hybrid electrodes is still up to 1192 F g1, which corresponds to capacitance retentions of 70.1%, while that of the pristine Co3O4 and FeCo2O4 electrodes are 65.0% and 62.8% respectively. Besides specific capacitance, the cyclic stability of electrodes is also one of the important performance parameters. We carried out 2000 cycles with the chargedischarge curves test at a current density of 1 A g1. As demonstrated in Fig. 7B, the Co3O4 and FeCo2O4 electrodes exhibit a specific capacitance of about 89.3% retention and 88.1% retention after 2000 cycles, while the capacity retentions of the Co3O4@FeCo2O4 hybrid electrodes are 90.2%. We attributed the excellent cycling stability to the unique core/shell architecture with robust interconnected networks
Fig. 6 e (A) Cyclic voltammetry curves of Co3O4, FeCo2O4 and core-shell Co3O4@ FeCo2O4; (B) charge-discharge curves of three samples tested at 1A g¡1; (C) CV curves of Co3O4@FeCo2O4 at different scan rate from 5 to 50 mVs¡1; (D) GCD plots of Co3O4@FeCo2O4 at various current density. Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Fig. 7 e (A) Charge-discharge cycling test of Co3O4, FeCo2O4 and core-shell Co3O4@ FeCo2O4 electrodes; (B) Specific capacitance of Co3O4@ FeCo2O4 at different current densities. and good strain accommodation, which provides more reactive sites and greatly improves the utilization of the hybrid electrode materials. Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range from 0.01 Hz to 100 kHz, Fig. 8 presents the corresponding Nyquist plots. Based on the equivalent electrical circuit, the Nyquist plots are analyzed by the Z view software simulation method. This plot can be divided into two parts, high frequency and low-frequency. In the high-frequency region, the intersection of the real axis represents internal resistance (Rs), and the radius of the semicircle is related to charge transfer resistance (Rct). The internal resistance (Rs) of Co3O4, FeCo2O4 and Co3O4@FeCo2O4 are 0.46, 0.52 and 0.78 U respectively. The charge transfer resistance (Rct) is mainly related with the electronic and ionic resistances, the morphology and conductivity of electrode [48,49]. In the low-frequency region, the Co3O4@ FeCo2O4 hybrid electrode shows a more vertical line, suggesting the lower diffusion resistance. The impedance spectroscopy (EIS)
measurement results indicate that the electrolyte ion can easily diffuse into the hybrid electrode [50,51].
Conclusion To sum up, we have synthesized the hierarchical Co3O4@FeCo2O4 core/shell hybrid structures by two-step method. The Co3O4 nanowires can be served as the “core materials”, and the interconnect nanosheets play the part of “shell materials”. The as-fabricated Co3O4@FeCo2O4 hybrid electrode manifests a high specific capacitance of 1649 F g1 at a current density of 1 A g1, desirable rate capability with 70.3% retention of the initial capacitance when the current density is increased from 1 to 20 A g1 and superior cycling stability (90.6% capacitance retention after 2000 cycles). Such enhanced properties are owing to the judiciously engineer the interface with unique hierarchical core/shell structures. As proofeofeconcept application in devices with high energy and power densities, this study presented a novel material for designing a composite electrode for supercapacitors.
Acknowledgements This work was supported by National Key R&D Program of China (2016YFE0202700), National Natural Science Foundation of China (NSFC 51603053), the Application Technology Research and Development Plan of Heilongjiang Province (GX16A008), Fundamental Research Funds of the Central University, International Science & Technology Cooperation Program of China (2015DFA50050) and Defense Industrial Technology Development Program (JCKY2016604C006, JCKY2018604C011).
Appendix A. Supplementary data
Fig. 8 e Nyquist plots of Co3O4, FeCo2O4 and Co3O4@FeCo2O4
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.119.
Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119
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Please cite this article as: Wu Q et al., Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.119