Controllable Growth of Hierarchical NiCo2O4 Nanowires and Nanosheets on Carbon Fiber Paper and their Morphology-Dependent Pseudocapacitive Performances

Electrochimica Acta 133 (2014) 382–390

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Controllable Growth of Hierarchical NiCo2 O4 Nanowires and Nanosheets on Carbon Fiber Paper and their Morphology-Dependent Pseudocapacitive Performances Fangze Deng, Lin Yu ∗ , Ming Sun, Ting Lin, Gao Cheng, Bang Lan, Fei Ye School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China

a r t i c l e

i n f o

Article history: Received 8 February 2014 Received in revised form 10 April 2014 Accepted 13 April 2014 Available online 21 April 2014 Keywords: Spinel nickel cobaltate Nanowires Nanosheets Solvothermal Supercapacitor

a b s t r a c t Hierarchical NiCo2 O4 nanowires and nanosheets have been grown on carbon fiber paper (CFP) using a facile solvothermal route allowed by heat treatment. The two morphologies of NiCo2 O4 can be easily controlled using solvents with different physicochemical properties (water or metanol) and their pseudocapacitive performances have been evaluated for supercapacitors applications. The results revealed that the CFP supported NiCo2 O4 nanosheets exhibits high value of specific capacitance (690 F g−1 ) at current density of 16 A g−1 and the retained capacitance was 87.1% after 2400 cycles at the discharge current density of 10 A g−1 . The much improved capacity and cycling stability of NiCo2 O4 nanosheets may be attributed to the unique hierarchical nanosheets array structures, which has higher specific surface area, providing more sites for the active species and facilitating the fast penetration of electrolyte. Moreover, it also favors better accommodation of strain during the cycle. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, the intense interest in supercapacitors stems from their properties like high power density, excellent reversibility, good pulse charge-discharge characteristics and long cycle life [1–3]. In general, supercapacitors can be classified into electrical double-layer capacitors (EDLCs) and pseudocapacitors based on their charge-storage mechanism [4]. Pseudocapacitors, utilizing in principle fast and reversible electrochemical redox reactions from materials such as RuO2 [5], MnO2 [6–8], NiO [9,10], Ni(OH)2 [11,12], Co3 O4 [10,13,14], Co(OH)2 [15,16] often have very high theoretical capacitance. However, they usually suffer from disadvantages such as poor stability, low conductivity and large volume change during the charge/discharge processes [17]. Therefore, there is currently an impending need to improve the electrodes performance to meet the increasing urgent demand for energy storage. Recently, spinel NiCo2 O4 has been a hotspot among various ‘pseudoactive’ materials because of its greater electronic conductivity and electrochemical activity than NiO and Co3 O4 [18–20]. Note that electrode materials with high specific surface area, good electronic conductivity and a fast anion or cation intercalation/de-intercalation process are the requirements to the manufacture of high-performance

∗ Corresponding author. E-mail address: [email protected] (L. Yu). http://dx.doi.org/10.1016/j.electacta.2014.04.070 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

supercapacitors. In attempts to further improve the electrochemical performance of NiCo2 O4 -based electrodes, considerable research interest has been focused on designing of additive/binderfree electrode architectures to avoid the ‘dead surface’, which make for more efficient charge exchange and electrolyte penetration [18,21–25]. In such case, 3D nickel foam was widely used as the current collector owing to its huge supporting area and high electrical conductivity. However, experimental evidence assumes that using nickel foam as current collector can bring about substantial errors to the specific capacitance of electrode materials, especially when a small amount of electrode active material is used in the measurement [26,27]. On the other hand, low-cost carbon fiber paper (CFP), a network of microsized carbon fibers with good electric conductivity, high porosity, large surface area, has been reported to be a promising current collector and backbone for conformal coating of transition metal oxides [13,28–30]. Up to now, two primary shapes of NiCo2 O4 , namely nanowires [22,23,29,31]/nanorods [32] and nanosheets [21,31,32], have been synthesized by hydrothermal/solvothermal route together with a post-heating treatment. It has been reported that the control of different morphologies and microstructures of Co3 O4 can lead to substantial differences in electrochemical performance due to dissimilarities in the electrode/electrolyte interface properties and ion transfer rates during the charge storage processes [13,33]. As is similar with Co3 O4 , it is very interesting and imperative to study the relationship between the shape of NiCo2 O4 and its

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electrochemical properties. Based on this idea, Zhang and David Lou [32] fabricated NiCo2 O4 nanorods and nanosheets and evaluated their electrochemical properties, finding that NiCo2 O4 nanosheets exhibits higher capacitance and much better cycling stability. However, Wang et al. [31] grew NiCo2 O4 nanowires and nanosheets on carbon cloth and they showed that nanowires morphology has higher specific capacitance and better cycling performance. This contradictory result may be interpreted from the inconsonant synthesis method. Specifically, they adopt hexamethylenetetramine as alkali source and capping agent to synthesize nanosheets. While, in order to get nanowires, they both substitute the urea for hexamethylenetetramine. It seems that the kind of alkali source (or capping agent) plays a vital role in determining the morphology of the product. However, herein, we present different results and conclusion. We first grow NiCo2 O4 nanosheets and nanowires on CFP, and further display that the reaction solvent instead of alkali source matters a lot! To be specific, whether we use hexamethylenetetramine or urea as capping agent, the morphology of NiCo2 O4 can be easily controlled to be nanosheets or nanowires by just changing the solvent from methanol to water. In another word, the kind of solvent used is the critical factor to the morphology of NiCo2 O4 . When the two hierarchical NiCo2 O4 array structural are evaluated as electrode materials for supercapacitors, the NiCo2 O4 nanosheets/CFP composite electrodes exhibits better electrochemical performance than NiCo2 O4 nanowires/CFP composite electrodes, revealing a morphology-property dependent relationship in electrochemical energy storage. 2. Experimental

electron microscopy (SEM, Hitachi S-3400 N) and transmission electron microscopy (TEM, JEOL-2100). The high-resolution TEM (HRTEM) images were recorded using a JEOL-2100 microscope. The composition of the samples was analyzed by an energy-dispersive X-ray spectroscope (EDX) attached to the SEM instrument. The nitrogen sorption measurements were performed using an ASAP 2020 instrument at 77 K. 2.3. Electrochemical Measurements Cyclic voltammetry (CV) curves and galvanostatic chargedischarge were tested on Autolab PGSTAT302 N Electrochemical Workstation in a three-electrode configuration using a 2 M KOH aqueous solution as electrolyte at room temperature. The CFP supported electroactive materials (∼1 cm2 in area) serves directly as the working electrode. Pt foil and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. Before electrochemical tests, 30 CV cycles were performed in 2 M KOH to activate each NiCo2 O4 electrode [34]. The electrochemical impedance spectroscopy (EIS) measurements were performed by applying an AC voltage with 5 mV amplitude in a frequency range from 100 kHz to 0.01 Hz. The specific capacitance is calculated according to the following equations[12,35], where C (F g−1 )was specific capacitance, m (g) is the mass of the electroactive materials in the electrodes, (V s−1 ) is the potential scan rate, Vc -Va is the applied potential window (Va to Vc ) of the voltammetric curve, I(V) is the response current density (A). In equation (2), I(A)represented discharge current and m(g), V(V), and t (s) designated mass of active materials, potential drop during discharge and total discharge time, respectively.

2.1. Materials and methods C= Synthesis of NiCo2 O4 with different morphologies on CFP: All the chemicals were of analytical grade and were used without further purification. In a typical synthesis, commercial CFP (approximately 1 cm × 4 cm) were cleaned by ultrasonication in 5 M HCl aqueous solution, absolute ethanol and deionized water for 30 min each. 0.5 mmol of Co(NO3 )2 ·6H2 O and 0.25 mmol of Ni(NO3 )2 ·6H2 O were dissolved into a mixed solvent of DI water (8 mL) and methanol (4 mL) to form a transparent pink solution, followed by the addition of 1.5 mmol of hexamethylenetetramine. In order to control the morphology of the precursor, the amount of methanol was varied in the range of 0–12 mL while keeping the total volume of 12 mL. The solution was transferred into a 25 mL Teflon-lined stainless steel autoclave. Then, a piece of the pre-treated CFP was put vertically in the above autoclave, heated to 120 ◦ C, and kept for 12 h. After it was cooled down to room temperature, the substrate was taken out and then ultrasonically cleaned for several times with DI water and ethanol, dried at 60 ◦ C for 12 h. In order to get crystallized NiCo2 O4 nanostructures, the samples were annealed at 350 ◦ C in air for 3 h. The weight of NiCo2 O4 deposit was accurately calculated from the difference in the weight of the substrate before solvothermal process and after calcinations using an analytical micro balance. (BT25S, Sartorius, max 22 g, 0.01 mg of resolution). On average, the mass loading of the NiCo2 O4 nanosheets or NiCo2 O4 nanowires on CFP was around 0.8 mg cm−2 . When study the influence of alkali source or capping agent, the 1.5 mmol of hexamethylenetetramine was replaced by 0.75 mmol of urea and all the other synthesis conditions kept the same with the above. 2.2. Materials Characterization The crystalline structure of the products was examined by X˚ ray diffraction (XRD, Rigaku, D/MAX-Ultima IV, Cu K␣, ␭=1.5406 A). The morphology of the samples was characterized by scanning

383

C=

1 mv(Vc − Va)



Vc

I(V ) dV

(1)

Va

It mV

(2)

3. Results and discussion 3.1. Structural analysis The fabrication process of the NiCo2 O4 nanosheets and nanoswires on CFP is shown in Fig. 1. The synthesis strategy involves two steps: the solvothermal process to fabricate arrays of Ni-Co precursor on the CFP substrate and then a post-calcination treatment to generate the corresponding hierarchical NiCo2 O4 structure. The morphologies of the Ni-Co precursor are first characterized by scanning electron microscope (SEM) and the results are shown in Fig. 2. When the reaction solvent is pure water, onedimensional (1D) Ni-Co precursor nanowires (denoted as NCO-0) are uniformly grown on the carbon nanofibers (CNFs) to form highly aligned nanowire arrays with the diameter of 40-60 nm. Interestingly, the tangled nanowires seem to constitute the mesh array architectures through careful observation, which may have better mechanical strength than individual nanowires array structures. When methanol was introduced to the water and the volumetric ratio of methanol/water reaches 4/8 (the NCO-4), as shown in Fig. 2b, nanosheets begin to grown on the carbon fibers and a few nanowires still exist and even cover the surface of nanosheets. If the solvent is obtained by mixing equal volumes of methanol and water (v/v = 6/6, the NCO-6), less nanowires are found in the product. With the further increase of methanol in the mixed solvent (v/v = 8/4, the NCO-8), nanosheets seems to dominate the product and they interconnect with each other to form a dense wall-like structure with little nanowires attached (Fig. 2d). When the reaction solvent becomes pure methanol, the NCO-12 sample

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Fig. 1. Schematic diagram illustrate the processes for growth NiCo2 O4 nanowires and nanosheets on CFP.

owns an ultrathin nanosheet structure with loose connection. Significantly, when we kept other reaction conditions unchanged and just replaced hexamethylenetetramine with urea, we can also find that the morphology of Ni-Co precursor can change gradually from nanowires to nanosheets with increasing the amount of methanol (Fig. S1, Supporting Information). This directly indicates that it is the solvent rather than capping agent that plays a key role in controlling the morphology of the products. Similar nanostructures transitions are obtained using Ni foam as the substrate or in the absence of any substrate (Fig. S2, Supporting Information), so we can conclude that it is the solvent compositions that induce the

variation in morphology of these nanostructures. Coincidentally, similar phenomenon has been reported by Zhou et al. [36] and they discovered that cobalt hydroxide with nanorod morphology and nanosheet morphology can be controlled fabricated by using water and a mixed solution of water-methanol as solvents, respectively. As we all have known, solvents with different physicochemical properties, such as coordinating ability, polarity, viscosity, saturated vapor pressure, and steric hindrance et al., have a pronounced effect on the final crystallinity and morphology [37–39]. Here, we study the effect of the dielectric constant of solvent on the morphology of the Ni-Co precursor [40,41]. Time-dependent

Fig. 2. SEM images of Ni-Co precursor structures grown on CFP with different methanol/water volume ratios: (a) NCO-0, V(MeOH/H2 O) = 0/12 mL; (b) NCO-4, V(MeOH/H2 O) = 4/8 mL; (c) NCO-6, V(MeOH/H2 O) = 6/6 mL; (d) NCO-8, V(MeOH/H2 O) = 8/4 mL; (e) NCO-12, V(MeOH/H2 O) = 12/0 mL and (f) magnified image of (e).

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experiments are first carried out in order to further understand the growth process and the results were shown in Fig. S3 (Supporting Information). As the reaction proceeds, it can be clearly seen that the nanosheets gradually split into nanowires in water and the similar phenomenon has been reported before [42]. However, in methanol, the nanosheets morphology remains unchanged with increasing duration. Why the nanosheets morphology is thermodynamically unstable in water and transform into nanowires, while stable in methanol? It is presumed that the formation of nanowires in water can be expressed as a kinetically controlled nucleationdissolution-recrystallization mechanism [43,44], because the high dielectric constant of water features with high solubility, and thus the nanosheets may be dissolved to form new nuclei. While, in methanol, the new nuclei is difficult to form because of its low dielectric constant leads to a low solubility, so the nanosheets is stable with increasing the duration. When we use the mixed solvent of water and methanol, its dielectric constant will located between 78.5 for water and 32.7 for methanol, thus having higher solubility than methanol and lower solubility than water, therefore, only part of nanosheets can transform into nanowires, as a result, nanosheets and nanowires will coexist eventually as shown in Fig. 2. Furthermore, DMF is selected as solvent to replace methanol, because it has similar dielectric constant (36.7) as that of methanol, and the morphology of the product is consistent with our expectations: nanosheets (Fig. S4, Supporting Information). Based on the above results, there is reason to believe that the dielectric constant of the solvent can guide morphological variation to a large extent in this reaction system. The XRD patterns of the precursors from NCO-0 and NCO-12 were shown in Fig. S5 (Supporting information). All the reflection peaks of the Ni-Co precursor nanowires can be well indexed to nickel cobalt hydroxide Ni0.988 Co0.012 (OH)2 (JCPDS 59-0461) (Fig.S5a). However, the reflection peaks of the Ni-Co precursor nanosheets were hard to well index, which may ascribed to the small organic molecules involving in the reaction to form the precursor [45]. More, To better understand the transformation process of the Ni-Co precursor, TG analysis of the as-grown Ni-Co precursor nanowires and nanosheets on the CFP were conducted the typical results are shown in Fig. S6 (Supporting information). The gradual weight loss from room temperature to ca.250 ◦ C represents the evaporation of physically adsorbed water of the precursor. The obvious mass loss from 250 to 400 ◦ C is ascribed to the thermal decomposition of the precursor. Therefore, The TG results show that it is reasonable to set the heat temperature at 350 ◦ C. In the latter case, for the convenience of the discussion, we focus on two composite oxides obtained from NCO-0 and NCO12 as the representative products for the hierarchical structures of nanowires and nanosheets. The two obtained oxides are first analyzed by XRD to determine their crystallographic structures. In order to reduce the strong impact of the CFP substrate on the XRD peak signals, the oxides powder is scratched from CFP for XRD analysis. As shown in Fig. 3, all of the reflections of hierarchical nanowires and nanosheets obtained by calcining NCO-0 and NCO12 could be assigned to NiCo2 O4 (JCPDF file no. 20-0781; space group: F*3 (202)) and no other phases or impurities can be detected. Further, we can also observe that intensities of the peaks for the two samples are relative weak, indicating a very small crystallite size. The energy dispersive X-ray spectroscope (EDX) result indicates that the atoms percent of Co and Ni are different in the two samples, unexpectedly, the Ni/Co atomic ratio of the nanosheets and nanowires are both about 1:2.16 (Fig. S6, Supporting Information), which fits well with the theoretical value. The nitrogen adsorption and desorption isotherms of the NiCo2 O4 nanowires and nanosheets are shown in Fig. S7 (Supporting Information) and the calculated Brunauer-Emmett-Teller (BET) surface area are 97

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Fig. 3. XRD patterns of (a) hierarchical NiCo2 O4 nanowires (from NCO-0) and (b) NiCo2 O4 nanosheets (from NCO-12) scratched from CFP.

and 231 m2 g−1 , respectively. The pore size distribution of the two sample calculated by desorption isotherm using Barrete-JoynereHalenda (BJH) method are also shown in Fig. S7 (inset), indicating a pore size range of 3-5 nm for nanosheets and 3-7 nm for nanowires. Evidently, the nanosheets morphology has much higher BET surface area than the nanowires morphology, which indicates that the nanosheets may exhibit better electrochemical performance because it can provide more active sites for electrochemical reaction [13,21]. SEM and TEM studies provide further insights into the morphologies and detailed geometrical structures of the hierarchical NiCo2 O4 nanowires and nanosheets. As shown in Fig. 4, after the heat treatment with a slow heating rate of 1 ◦ C min−1 , the overall morphologies of these Ni-Co precursors can be well preserved without significant aggregation or collapse. The hierarchical NiCo2 O4 nanowires inherit 1D feature from the precursor with a similar diameter and length. The NiCo2 O4 nanowires are highly porous and composed of 10-20 nm nanocrystallites with pores of 2-4 nm in diameter (Fig. 4c). The lattice spacing of 0.244 nm corresponds to the (311) crystal plane of NiCo2 O4 , as seen from the HRTEM image (Fig. 4d). The selected area of electron diffraction (SAED) pattern (Fig. 4d, inset) demonstrates a polycrystalline structure and the diffraction rings can be indexed to the (220), (311), (511) and (440) planes of the NiCo2 O4 phase. Meanwhile, from the SEM image in Fig. 4e, the well-interconnected ultrathin nanosheet morphology of NCO-12 is perfectly retained after the heat treatment, which could be ascribed to the robust support of carbon nanofibers and the slow heating rate. The low-magnification image (Fig. 4f) indicates that the nanosheets are continuous with a smooth surface. The magnified image (Fig. 4g) clearly shows that pores around 5 nm are uniformly distributed throughout the whole surface of nanosheets. The lattice fringes shown in Fig. 4h can be readily indexed to the (111) crystal planes of the NiCo2 O4 . The SAED pattern also shows that the NiCo2 O4 nanosheets are polycrystalline. 3.2. Electrochemical characterization It is well known that the integrated electrodes have many interesting structural features, such as favorable diffusion channels for electrolyte penetration, reduced ion and electron transport path, good electrical contact of electroactive materials with the current collector [21,46,47], so we believed the CFP supported NiCo2 O4 nanowire or nanosheet electrodes may exhibit excellent electrochemical performance. However, considering thedifferences in

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Fig. 4. SEM and TEM images of the NiCo2 O4 nanowires and NiCo2 O4 nanosheets: (a) SEM and (b-d) TEM of the NiCo2 O4 nanowires (bottom-right inset is the SAED pattern of NiCo2 O4 nanowiress); (e) SEM and (f-h) TEM of the NiCo2 O4 nanosheets (top-right inset shows the SAED pattern of NiCo2 O4 nanosheets).

morphologies and microstructures, the charge storage efficiency of the two electrodes construction may express more or less disparity. In order to identify which architecture is favorable for high-rate capacitive energy storage, we first perform cyclic voltammetry (CV) measurements in 2 M KOH aqueous solution under different operating conditions. Representative cyclic voltammograms for

nanowires and nanosheets morphologies at different scan rates are shown in Fig. 5a and b. As observed from Fig. 5a, cyclic voltammograms of NiCo2 O4 nanowires grown over CFP substrate show a distinct pair of redox peaks during the positive and negative sweeps at different scan rates. The shape of the CV curves reveals the pseudocapacitive characteristics derived from Faradaic reactions and

Fig. 5. The CV curves of the (a) NiCo2 O4 nanowires electrode and (b) NiCo2 O4 nanosheets electrode at different scan rates of 5, 10, 20, 40, 60 and 80 mV s−1 ; (c) comparison of the NiCo2 O4 nanowires electrode and NiCo2 O4 nanosheets electrode at the same scan rate of 20 mV s−1 ; (d) specific capacitance of the NiCo2 O4 nanowires and NiCo2 O4 nanosheets as a function of the scan rates based on the respective CV curves.

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Fig. 6. Galvanostatic current charge-discharge curves of (a) NiCo2 O4 nanowires electrode and (b) NiCo2 O4 nanosheets electrode at different current densitys; (c) the specific capacitance as a function of current density of both NiCo2 O4 nanowires electrode and NiCo2 O4 nanosheets electrode; (d) the cycling performance at constant current densitys of 10 A g−1 of the two electrodes.

agrees well with previous reports on NiCo2 O4 in KOH solution [23,48,49]. Compared with the CV curves of the NiCo2 O4 nanowires, the NiCo2 O4 nanosheets behaves a more obvious pair of redox peaks. For the same mass loading, it is well-known that specific capacitance is proportional to the area of the CV curve. To better compare the two samples, CV property of the NiCo2 O4 nanowires and NiCo2 O4 nanosheets are shown in Fig. 5c. Obviously, the nanosheets shows higher current density and CV curve area than that of the nanowires at the same scan rate, indicating the NiCo2 O4 nanosheets has higher capacitances and electrochemical activity. In addition, it is noteworthy that variations in redox peaks positions can be observed from the two samples, which may be ascribed to the difference in electrode polarization behaviors during CV tests. As depicted, the polarization behavior is closely related to the physical morphology of the electrode material [32,35]. The contribution of the carbon fiber paper to the capacitance is negligible (Fig. S8a, Supporting Information), therefore, the total capacitance results mainly from the redox pseudocapacitance of the loaded NiCo2 O4 . Fig. 5d displays the variation in the specific capacitance of NiCo2 O4 nanowires electrode and NiCo2 O4 nanosheets electrode as a function of scan rate. The specific capacitance of the NiCo2 O4 nanowires and nanosheets electrode reaches 410 and 691 F g−1 at a scan rate of 5 mV s−1 , respectively. When increase the scan rate to 80 mV s−1 , the specific capacitance of the two samples reduce to 285 and 511 F g−1 , respectively. Compared with the NiCo2 O4 nanowires electrode, the NiCo2 O4 nanosheets electrode shows a better performance in terms of both specific capacitance and rate capability. The difference may be attributed to the nanosheets array structure with higher specific surface area, which can provide more sites for the active species and also facilitate the fast penetration of electrolyte [13].

To further evaluate the capacitive performance of the NiCo2 O4 nanowires electrode and NiCo2 O4 nanosheets electrode, we carried out galvanostatic charge-discharge test. Fig. 6a and b show galvanostatic charge-discharge curves of the both electrodes at various current densitys. The constant current charge discharge curves of NiCo2 O4 nanosheets are more symmetric at high current densitys, implying the NiCo2 O4 nanosheets morphology with higher charge-discharge coulombic efficiency and lower polarization [13,18,28]. Fig. 8Sb (Supporting Information) shows the comparison of galvanostatic charge discharge curves for the two samples at a constant current density of 1 A g−1 . From which it is evident that the discharge time and hence specific capacitance of NiCo2 O4 nanosheets electrodes are higher than those of NiCo2 O4 electrodes with nanowires morphology, which is well consistent with the results measured by cyclic voltammetry. The specific capacitance of the two samples can be calculated by equation (2) and the specific capacitance is shown as a function of the current density in Fig. 6c. Specially, the specific capacitance of the NiCo2 O4 nanowires is 471, 462, 447, 424, and 396 F g−1 at the current densitys of 1, 2, 4, 8, and 16 A g−1 , respectively. In contrast, at the same current densitys with the counterpart, the specific capacitance of the NiCo2 O4 nanosheets is 799, 782, 773, 755, 690 F g−1 . In general, ions can penetrate into the inner-structure of electrode material at lower current density, thus making use of almost all available pores of the electrode. However, only the outer surface of electrodes can be effective utilized at higher current density. This difference can result in the reduction of specific capacitance with the current density increasing, which is the same as the one observed in the case of increasing scan rate. Of particular note is that there is still around 86% initial capacitance retention even when the current density increases to 16 A g−1 , indicating the relatively better

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Fig. 7. (a) Nyquist plots of the EIS of the NiCo2 O4 nanowires at different bias potentials; (b) Nyquist plots of the EIS of the NiCo2 O4 nanosheets and NiCo2 O4 nanowires electrode at bias potential of 0.35 V and the equivalent circuit of diagram (the inset).

high-rate capability for NiCo2 O4 nanosheets electrodes (86%) than for NiCo2 O4 nanowires electrodes (84%). The long-term stability of the electrodes was examined by galvanostatic charge-discharge cycling at a current density of 10 A g−1 and the results are presented in Fig. 6d for both electrodes. For the NiCo2 O4 nanosheets electrode, the capacitance decreased relatively fast at the first 500th cycle, and then declined quite slowly after additional 1900 cycles. The total capacitance loss after 2400 cycles is only ∼12.9% at a high current density of 10 A g−1 . To the best of our knowledge, the cycling performance of the NiCo2 O4 nanosheets electrode is less than that reported for mesoporous NiCo2 O4 nanocrystals [50] and single-crystalline NiCo2 O4 nanoneedle [22], but higher than previously reported values for the NiCo2 O4 electrodes [20,49,51]. In comparison, at the same test conditions, the capacitance loss for NiCo2 O4 nanowires electrode is ∼19.6%. It can be clearly seen that the NiCo2 O4 nanosheets electrode exhibits better electrochemical performance with both higher capacitance and better cycling stability. This might be understood by considering the following factors. First, the NiCo2 O4 nanosheets have a higher specific surface area compared to NiCo2 O4 nanoswires (231 m2 g−1 for nanosheets vs 97 m2 g−1 for nanowires). This will ultimately make NiCo2 O4 nanosheets electrode offer more active sites to facilitate electrochemical reactions and make the electrolyte penetrate more efficient into the electroactive materials [21,29]. Besides, the ultrathin nanosheet morphology is advantageous for efficient ion and electron transport since the transport path of the nanosheets is much shorter than nanowires [29,32]. What’s more, the nanosheets morphology can better accommodate the volume alteration because it can release stress that produced during the electrochemical reaction in two dimensions [52]. These structural characteristics will undoubtedly results in the better electrochemical performance of NiCo2 O4 nanosheets electrode over the NiCo2 O4 nanowires electrode. It is note that the cycling stability and rate performance are highly related to the interfacial charge-transfer process and ion diffusion. To gain further insight into the transport kinetics of the electrochemical reaction process, we first measured the electrochemical impedance spectra (EIS) of the nanowires electrode at different potentials in the frequency range between 0.01 Hz and 100 kHz (Fig. 7a), from which we can see that the Nyquist plots at different potential display almost similar capacitive properties, except that the straight line part in the low frequency region leans more toward the imaginary axis with increasing the potential biases. Fig. 7b shows the EIS of the nanowires and nanosheets electrodes at a biased potential of 0.35 V and the equivalent circuit

for the impedance analysis is also shown in Fig. 7b (the inset). Where, the intercept on the real axis in the high frequency range provides the equivalent series resistance (Rs), which includes the intrinsic resistances of the electroactive material, bulk resistance of electrolyte, and contact resistance at the interface between electrolyte and electrode. Rct, Cdl and Cps represents charge-transfer resistance, double-layer capacitance and pseudocapacitance, respectively [53]. Warburg behavior (W) ascribes the impedance due to highly distributed anion and/or cation diffusion within the oxide [53]. It is noted that the Rs of the two systems are both estimated to be 1.36 , indicating that the interfacial contact between the NiCo2 O4 deposits and the carbon fibers were consistent. However, the smaller diameter of the semicircle in the impedance spectrum of the nanosheets electrode was an indication of lower charge-transfer resistance[13,28]. Furthermore, The phase angle for the impedance plot of the two samples were observed to be higher than 45◦ in the low frequencies, suggesting that the electrochemical capacitive behavior of the two electrode is not controlled by diffusion process [53,54]. Therefore, the EIS result further proves that NiCo2 O4 nanosheets electrode has better electrochemical performance compared to its competitor. 4. Conclusions In summary, we have prepared hierarchical different nanostructures (nanowires and nanosheets) using a solvothermal route allowed by heat treatment. Our results demonstrated that the solvent plays a key role in control the morphology of NiCo2 O4 . When the CFP supported NiCo2 O4 nanosheets and nanowires are evaluated as integrated electrode materials for supercapacitors, NiCo2 O4 nanosheets electrode exhibits higher capacitance and much better cycling stability compared to NiCo2 O4 nanowires electrode. This difference in electrochemical performance derives from the fact that the nanosheet morphology is advantageous for efficient ion and electron transport and can better accommodate the volume changes. This comparison through different morphology reveals a morphology-property relationship in electrochemical energy storage and offers strategies to enhance the performance of supercapacitor electrodes. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21306026), Natural Science Foundation of Guangdong Province (10251009001000003, S2012010009680),

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the Scientific Program of Guangdong Province (2012A030600006), the Fund of Higher Education of Guangdong Province (cgzhzd1104) and Foundation for Distinguished Young Talents in Higher Education of Guangdong(2013LYM0024).

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2014.04.070.

References [1] P.J. Hall, M. Mirzaeian, S.I. Fletcher, F.B. Sillars, A.J.R. Rennie, G.O. Shitta-Bey, G. Wilson, A. Cruden, R. Carter, Energy storage in electrochemical capacitors: designing functional materials to improve performance, Energy Environ. Sci 3 (2010) 1238. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845. [3] B.G. Choi, M. Yang, W.H. Hong, J.W. Choi, Y.S. Huh, 3D Macroporous Graphene Frameworks for Supercapacitors with High Energy and Power Densities, ACS Nano 6 (2012) 4020. [4] J.R. Miller, P. Simon, Materials science. Electrochemical capacitors for energy management, Science 321 (2008) 651. [5] C.-C. Hu, K.-H. Chang, M.-C. Lin, Y.-T. Wu, Design and tailoring of the nanotubular arrayed architecture of hydrous RuO2 for next generation supercapacitors, Nano Lett. 6 (2006) 2690. [6] R. Liu, S.B. Lee, MnO2 /Poly(3,4-ethylenedioxythiophene) coaxial nanowires by one-step coelectrodeposition for electrochemical energy storage, J. Am. Chem. Soc. 130 (2008) 2942. [7] Q. Li, Z.-L. Wang, G.-R. Li, R. Guo, L.-X. Ding, Y.-X. Tong, Design and Synthesis of MnO2 /Mn/MnO2 Sandwich-Structured Nanotube Arrays with High Supercapacitive Performance for Electrochemical Energy Storage, Nano Lett. 12 (2012) 3803. [8] W. Chen, R.B. Rakhi, L. Hu, X. Xie, Y. Cui, H.N. Alshareef, High-Performance Nanostructured Supercapacitors on a Sponge, Nano Lett. 11 (2011) 5165. [9] G. Zhang, L. Yu, H.E. Hoster, X.W. Lou, Synthesis of one-dimensional hierarchical NiO hollow nanostructures with enhanced supercapacitive performance, Nanoscale 5 (2012) 877. [10] X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X.-b. Zhao, H.J. Fan, High-Quality Metal Oxide Core/Shell Nanowire Arrays on Conductive Substrates for Electrochemical Energy Storage, ACS Nano 6 (2012) 5531. [11] J. Ji, L.L. Zhang, H. Ji, Y. Li, X. Zhao, X. Bai, X. Fan, F. Zhang, R.S. Ruoff, Nanoporous Ni(OH)2 Thin Film on 3D Ultrathin-Graphite Foam for Asymmetric Supercapacitor, ACS Nano 7 (2013) 6237. [12] H.B. Li, M.H. Yu, F.X. Wang, P. Liu, Y. Liang, J. Xiao, C.X. Wang, Y.X. Tong, G.W. Yang, Amorphous nickel hydroxide nanospheres with ultrahigh capacitance and energy density as electrochemical pseudocapacitor materials, Nat. Commun. 4 (2013) 1894. [13] R.B. Rakhi, W. Chen, D. Cha, H.N. Alshareef, Substrate Dependent SelfOrganization of Mesoporous Cobalt Oxide Nanowires with Remarkable Pseudocapacitance, Nano Lett. 12 (2012) 2559. [14] J. Xu, L. Gao, J. Cao, W. Wang, Z. Chen, Preparation and electrochemical capacitance of cobalt oxide (Co3 O4 ) nanotubes as supercapacitor material, Electrochim. Acta 56 (2010) 732. [15] D. Tai Dam, J.-M. Lee, Ultrahigh pseudocapacitance of mesoporous Ni-doped Co(OH)2 /ITO nanowires, Nano Energy 2 (2013) 1186. [16] D. Ghosh, S. Giri, C.K. Das, Preparation of CTAB-Assisted Hexagonal Platelet Co(OH)2 /Graphene Hybrid Composite as Efficient Supercapacitor Electrode Material, ACS Sustainable Chem. Eng. 1 (2013) 1135. [17] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797. [18] C. Yuan, J. Li, L. Hou, X. Zhang, L. Shen, X.W. Lou, Ultrathin Mesoporous NiCo2 O4 Nanosheets Supported on Ni Foam as Advanced Electrodes for Supercapacitors, Adv. Funct. Mater. 22 (2012) 4592. [19] T.Y. Wei, C.H. Chen, H.C. Chien, S.Y. Lu, C.C. Hu, A cost-effective supercapacitor material of ultrahigh specific capacitances: spinel nickel cobaltite aerogels from an epoxide-driven sol-gel process, Adv. Mater. 22 (2010) 347. [20] H. Wang, Q. Gao, L. Jiang, Facile Approach to Prepare Nickel Cobaltite Nanowire Materials for Supercapacitors, Small 7 (2011) 2454. [21] G. Zhang, X.W. Lou, General Solution Growth of Mesoporous NiCo2 O4 Nanosheets on Various Conductive Substrates as High-Performance Electrodes for Supercapacitors, Adv. Mater. 25 (2013) 976. [22] G.Q. Zhang, H.B. Wu, H.E. Hoster, M.B. Chan-Park, X.W. Lou, Single-crystalline NiCo2 O4 nanoneedle arrays grown on conductive substrates as binder-free electrodes for high-performance supercapacitors, Energy Environ. Sci. 5 (2012) 9453. [23] Q. Wang, X. Wang, B. Liu, G. Yu, X. Hou, D. Chen, G. Shen, NiCo2 O4 nanowire arrays supported on Ni foam for high-performance flexible all-solid-state supercapacitors, J. Mater. Chem. A 1 (2013) 2468.

389

[24] H.B. Wu, H. Pang, X.W. Lou, Facile synthesis of mesoporous Ni0.3 Co2.7 O4 hierarchical structures for high-performance supercapacitors, Energy Environ. Sci. 6 (2013) 3619. [25] C. Yuan, H.B. Wu, Y. Xie, X.W. Lou, Mixed Transition-Metal Oxides: Design, Synthesis, and Energy-Related Applications, Angew. Chem., Int. Ed. 53 (2014) 1488. [26] W. Xing, S. Qiao, X. Wu, X. Gao, J. Zhou, S. Zhuo, S.B. Hartono, D. HulicovaJurcakova, Exaggerated capacitance using electrochemically active nickel foam as current collector in electrochemical measurement, J. Power Sources 196 (2011) 4123. ´ M. Alsabet, G. Jerkiewicz, Surface Science and Electrochemical Anal[27] M. Grden, ysis of Nickel Foams, ACS Appl. Mater. Interfaces 4 (2012) 3012. [28] L. Yang, S. Cheng, Y. Ding, X. Zhu, Z.L. Wang, M. Liu, Hierarchical Network Architectures of Carbon Fiber Paper Supported Cobalt Oxide Nanonet for HighCapacity Pseudocapacitors, Nano Lett. 12 (2012) 321. [29] L. Huang, M. Liu, Nickel-cobalt hydroxides nanosheets coated on NiCo2 O4 nanowires grown on carbon fiber paper for high-performance pseudocapacitors, Nano Lett. 13 (2013) 3135. [30] Y. Luo, J. Jiang, W. Zhou, H. Yang, J. Luo, X. Qi, H. Zhang, D.Y.W. Yu, C.M. Li, T. Yu, Self-assembly of well-ordered whisker-like manganese oxide arrays on carbon fiber paper and its application as electrode material for supercapacitors, J. Mater. Chem. 22 (2012) 8634. [31] H. Wang, X. Wang, Growing Nickel Cobaltite Nanowires and Nanosheets on Carbon Cloth with Different Pseudocapacitive Performance, ACS Appl. Mater. Interfaces 5 (2013) 6255. [32] G. Zhang, X.W. Lou, Controlled Growth of NiCo2 O4 Nanorods and Ultrathin Nanosheets on Carbon Nanofibers for High-performance Supercapacitors, Sci Rep. 3 (2013) 1470. [33] Y. Wang, Z. Zhong, Y. Chen, C. Ng, J. Lin, Controllable synthesis of Co3 O4 from nanosize to microsize with large-scale exposure of active crystal planes and their excellent rate capability in supercapacitors based on the crystal plane effect, Nano Res. 4 (2011) 695. [34] C.-C. Hu, C.-T. Hsu, K.-H. Chang, H.-Y. Hsu, Microwave-assisted hydrothermal annealing of binary Ni–Co oxy-hydroxides for asymmetric supercapacitors, J. Power Sources 238 (2013) 180. [35] L. Mei, T. Yang, C. Xu, M. Zhang, L. Chen, Q. Li, T. Wang, Hierarchical mushroomlike CoNi2 S4 arrays as a novel electrode material for supercapacitors, Nano Energy 3 (2014) 36. [36] E. Hosono, S. Fujihara, I. Honma, H. Zhou, Fabrication of morphology and crystal structure controlled nanorod and nanosheet cobalt hydroxide based on the difference of oxygen-solubility between water and methanol, and conversion into Co3 O4 , J. Mater. Chem. 15 (2005) 1938. [37] L. Yu, L. Zhang, H.B. Wu, G. Zhang, X.W. Lou, Controlled synthesis of hierarchical Cox Mn3-x O4 array micro-/nanostructures with tunable morphology and composition as integrated electrodes for lithium-ion batteries, Energy Environ. Sci. 6 (2013) 2664. [38] Z. Wang, F. Su, S. Madhavi, X.W. Lou, CuO nanostructures supported on Cu substrate as integrated electrodes for highly reversible lithium storage, Nanoscale 3 (2011) 1618. [39] M. Salvalaglio, T. Vetter, M. Mazzotti, M. Parrinello, Controlling and Predicting Crystal Shapes: The Case of Urea, Angew. Chem., Int. Ed. 52 (2013) 13369. [40] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and Properties of Nanocrystals of Different Shapes, Chem. Rev. 3 (2005) 1025. [41] J. Wu, X. Lü, L. Zhang, F. Huang, F. Xu, Dielectric Constant Controlled Solvothermal Synthesis of a TiO2Photocatalyst with Tunable Crystallinity: A Strategy for Solvent Selection, Eur. J. Inorg. Chem. 2009 (2009) 2789. [42] X.Y. Liu, Y.Q. Zhang, X.H. Xia, S.J. Shi, Y. Lu, X.L. Wang, C.D. Gu, J.P. Tu, Self-assembled porous NiCo2 O4 hetero-structure array for electrochemical capacitor, J. Power Sources 239 (2013) 157. [43] G. Xi, K. Xiong, Q. Zhao, R. Zhang, H. Zhang, Y. Qian, Nucleation-DissolutionRecrystallization: A New Growth Mechanism for t-Selenium Nanotubes, Cryst. Growth Des. 6 (2006) 577. [44] L. Yan, R. Yu, J. Chen, X. Xing, Template-Free Hydrothermal Synthesis of CeO2 Nano-octahedrons and Nanorods: Investigation of the Morphology Evolution, Crystal Cryst. Growth Des. 8 (2008) 1474. [45] K. Cheng, D. Cao, F. Yang, Y. Xu, G. Sun, K. Ye, J. Yin, G. Wang, Facile synthesis of morphology-controlled Co3 O4 nanostructures through solvothermal method with enhanced catalytic activity for H2 O2 electroreduction, J. Power Sources 253 (2014) 214. [46] J. Liu, J. Jiang, C. Cheng, H. Li, J. Zhang, H. Gong, H.J. Fan, Co3 O4 Nanowire@MnO2 ultrathin nanosheet core/shell arrays: a new class of high-performance pseudocapacitive materials, Adv. Mater. 23 (2011) 2076. [47] J.-H. Zhong, A.-L. Wang, G.-R. Li, J.-W. Wang, Y.-N. Ou, Y.-X. Tong, Co3 O4 /Ni(OH)2 composite mesoporous nanosheet networks as a promising electrode for supercapacitor applications, J. Mater. Chem. 22 (2012) 5656. [48] Y. Chen, B. Qu, L. Hu, Z. Xu, Q. Li, T. Wang, High-performance supercapacitor and lithium-ion battery based on 3D hierarchical NH4 F-induced nickel cobaltate nanosheet-nanowire cluster arrays as self-supported electrodes, Nanoscale 5 (2013) 9812. [49] L. Yu, G. Zhang, C. Yuan, X.W. Lou, Hierarchical NiCo2 O4 @MnO2 core–shell heterostructured nanowire arrays on Ni foam as high-performance supercapacitor electrodes, Chem. Commun. 49 (2013) 137. [50] C.-T. Hsu, C.-C. Hu, Synthesis and characterization of mesoporous spinel NiCo2 O4 using surfactant-assembled dispersion for asymmetric supercapacitors, J. Power Sources 242 (2013) 662.

390

F. Deng et al. / Electrochimica Acta 133 (2014) 382–390

[51] J. Du, G. Zhou, H. Zhang, C. Cheng, J. Ma, W. Wei, L. Chen, T. Wang, Ultrathin porous NiCo2 O4 nanosheet arrays on flexible carbon fabric for highperformance supercapacitors, ACS Appl. Mater. Interfaces 5 (2013) 7405. [52] L. Hu, H. Zhong, X. Zheng, Y. Huang, P. Zhang, Q. Chen, CoMn2 O4 spinel hierarchical microspheres assembled with porous nanosheets as stable anodes for lithium-ion batteries, Sci Rep. 2 (2012) 986.

[53] C.-C. Hu, K.-H. Chang, T.-Y. Hsu, The Synergistic Influences of OH- Concentration and Electrolyte Conductivity on the Redox Behavior of Ni(OH)2 /NiOOH, J. Electrochem. Soc. 155 (2008) F196. [54] S.K. Meher, P. Justin, G.R. Rao, Pine-cone morphology and pseudocapacitive behavior of nanoporous nickel oxide, Electrochim. Acta 55 (2010) 8388.