Solvothermal synthesis of mesoporous NiCo2O4

Solvothermal synthesis of mesoporous NiCo2O4 spinel oxide nanostructure for high-performance electrochemical capacitor electrode N. Padmanathan & S. Selladurai

Ionics International Journal of Ionics The Science and Technology of Ionic Motion ISSN 0947-7047 Ionics DOI 10.1007/s11581-013-0907-0

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Author's personal copy Ionics DOI 10.1007/s11581-013-0907-0

ORIGINAL PAPER

Solvothermal synthesis of mesoporous NiCo2O4 spinel oxide nanostructure for high-performance electrochemical capacitor electrode N. Padmanathan & S. Selladurai

Received: 14 December 2012 / Revised: 13 April 2013 / Accepted: 14 April 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract NiCo2O4 nanostructure was successfully synthesized via a D-glucose-assisted solvothermal process. Spineltype cubic phase and mesoporous microstructure of the sample for different calcination temperatures were confirmed by X-ray diffraction and transmission electron microscopy. Typical pseudocapacitance feature of the NiCo2O4 treated at different temperatures was then evaluated in aqueous 6 M KOH electrolyte solution. Electrochemical measurements showed that the spinel nickel cobaltite nanostructure heated at 300 °C exhibits maximum specific capacitances of 524 F g−1 at 0.5 A g−1 and 419 F g−1 at 10 A g−1 with excellent cycle stability and only ~9 % of capacitance loss after 2,500 cycles. This demonstrates the potential application of the material for supercapacitors. The attractive pseudocapacitive performance of NiCo2O4 is mainly attributed to the redox contribution of the Ni and Co metal species, high surface area, and their desired mesoporous nanostructure. Keywords D-glucose . Porosity (B) . Spinels (D) . Electrode (E) . Capacitors (E)

Introduction Recently, electrochemical capacitors (ECs) or supercapacitors have received much attention in energy storage applications especially in hybrid electric vehicles and high-power electronics. When compared to batteries, ECs are able to deliver high power density at very short timing and also have extremely long cycle life [1–3]. Based on their charge storage mechanism, N. Padmanathan (*) : S. Selladurai Ionics Laboratory, Department of Physics, Anna University, Chennai-25, India e-mail: [email protected]

ECs are classified into two types such as electric double-layer capacitors (EDLCs) and pseudocapacitors. The former may stores energy due to ion absorption on the electrode/electrolyte interfaces, while the latter stores the charges by fast surface redox reactions [2–5]. Unfortunately, the low specific capacitance [2] (Cs) and energy density of EDLCs limit their applications. This can be overwhelmed by pseudocapacitors as their surface faradaic redox reaction provides very high energy density than EDLCs [6]. Generally, transition metal oxides, sulfides, and conducting polymers has been investigated as an electrode for pseudocapacitors [6–9]. Most efforts have been taken on transition metal oxides, owing to their high theoretical capacitance and multiple oxidation states in favor for redox reaction [8, 9]. Recently, RuO2 [4, 5, 10], NiO [8, 9, 11], Co3O4 [12–15], and MnO2 [16–20] have been widely studied for supercapacitor application because they are of low cost, abundant, and environmentally friendly [8, 11, 13, 17]. When compared to costly RuO2, other metal oxides which are mentioned above show lower Cs [7, 21] and rate capability [14] at high current density. This is mainly due to the fact that they are too insulating to support fast electron transport required by highrate ECs between the electrolyte and electroactive species [7, 21]. To achieve high Cs, the electrode material should possess numerous electrochemically active materials with mesoporous structure [7]. Nanostructured electrode material with high surface area alone is not sufficient for receiving superior electrochemical applications [22]. Gogotsi et al. reported that an ordered porous structure is favorable for electrolyte ion intercalation at the electrode surface than the electrode with a disordered structure [23]. Porous materials, with unique pore diameter, show noteworthy properties in the energy storage applications. Typical electrode materials having a pore size around 2 to 50 nm can enhance the amount of electrolyte intercalation between electrode/electrolyte interfaces and also

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prevent the heavier anions which are present in the electrolyte [10]. Therefore, it is important to synthesize an electrode material with ordered mesoporous structure, thus improving the electrode performance and electrochemical properties [10]. At the same time, high electrical conductivity, fast ion transport rate, and fast electron transfer of the electrode material also take part in the faradaic reaction [7, 21]. The well-known spinel nickel cobaltite (NiCo2O4) offers a high electrical conductivity and rich redox contribution from the Ni2+ and Co2+/3+ ions when compared to monometallic oxides [24–29]. Due to their interesting features, it is expected to be a potential candidate for next-generation supercapacitor electrodes. Various synthesis processes were followed to obtain a porous spinel and other forms of metal oxide nanostructures; commonly, wet chemical routes [25–28] and hydrothermal [7] methods were often used. But these methods have wide particle distribution and lower dispersion in solvents. To overcome these drawbacks, a template was used during synthesis which offers the ordered structure with desired properties. Most commonly alumina, carbon nanotube, surfactants, polymers, and membranes have been employed as the template to produce nanomaterials [30]. Li et al. have reported that the formation of metal oxide hollow sphere nanostructures with carbonaceous polysaccharides as the template [31]. Carp et al. have reported the typical synthetic strategies of metal oxide nanostructure with polysaccharide assistance [32]. From their report, starch, sucrose, and glucose can act as the source for carbon microspheres under hydrothermal conditions, which will act as the soft template for metal oxide hollow sphere formation [33]. It is worthy to note that the formation of hollow sphere depends on the choice of solvent as well as the ratio between the carbohydrate and metal ions for different metal oxides [34]. Only few reports are available on the use of glucose as a template [35, 36]. Yu and Yu have used glucose to synthesize a ZnO hollow sphere [35]. Guan et al. have prepared a size-controlled spinel magnetite (Fe3O4) using sucrose [37]. To the best of our knowledge, there are no reports related to a typical spinel like the AB2O4 structure with glucose assistance published elsewhere. Recently, Li et al. reported a hierarchical porous NiCo2O4 nanowire for high-rate supercapacitors [28]. Further, Yuan et al. have published ultralayered NiCo 2 O 4 nanosheet building blocks as a supercapacitor electrode with high rate capability and cyclability [29]. However, the formation of highly ordered mesoporous NiCo2O4 nanostructures is still challenging. In the present work, we report the synthesis of NiCo2O4 nanostructure with mesoporous structure via a solvothermal process. To our knowledge, this is the first report on the synthesis of NiCo2O4 nanostructure under a nonaqueous polyol medium with inexpensive eco-friendly D-glucose. The synthesized NiCo2O4 nanostructure was characterized, and its potential application for supercapacitor electrode material was investigated.

Experimental techniques Synthesis procedure Analytical grade Co(NO3)2·6H2O and Ni(NO3)2·6H2O with the stoichiometric ratio of 2:1 (Co/Ni) was dissolved in 40 ml of diethylene glycol (DEG) under constant stirring for 30 min. After that, 0.5 M of D-glucose was separately dissolved in 20 ml of DEG and stirred for 30 min at 60 °C. The above two solutions were mixed together, and stirring was continued for 1 h under ambient temperature. Finally, the obtained gray-colored solution was transferred to a double Teflonated stainless steel autoclave and kept at 180 °C for 24 h. The oven was cooled down to room temperature naturally. The light gray color precipitate was separated by centrifugation and washed several times with deionized water, ethanol, and acetone consecutively and finally annealed at temperatures 300 and 400 °C for 6 h under air. The obtained black powder was ground and used directly for characterization. Characterization The phase and structure of the sample were examined by powder X-ray diffraction (XRD, D2 PHASER) using a Cu Kα source (l=0.15406 Å) over a 2θ range of 10°–80°. Fourier transform infrared (FTIR) spectra of the samples were obtained from the PerkinElmer FTIR spectrophotometer in the wavenumber range of 400–4,000 cm−1. Fieldemission scanning electron microscopy equipped with energy-dispersive X-ray analysis (EDAX, Hitachi SU660015KV) and transmission electron microscopy (TEM, FEI TECHNAI system operating at 200 kV) were directly used to identify the morphologies, elemental composition, and microstructure of the sample. The specific surface area of the spinel oxide was performed via the Quantachrome QUADRAWin analyzer version 5.02 with liquid nitrogen (77 K) absorbate. The working electrode was prepared by mixing the electroactive material NiCo2O4 NWs, activated carbon, and polyvinylidene difluoride in a weight ratio of 8:1:1. One drop of 1-methyl-2-pyrrolidinone was added to make the mixture more homogeneous; this was coated onto a nickel foil for the following electrochemical tests. Cyclic voltammetry (CV) and chronopotentiometry (CP) measurements were performed with a CHI (7081C) electrochemical workstation (USA). The typical loading of the electroactive NiCo2O4 is approximately 1 mg cm−2. All the electrochemical measurements were carried out in a three-electrode cell with a working electrode, a platinum wire counter electrode, and Ag/AgCl (3 M KCl solution) reference electrode at room temperature. Here, 6 M KOH aqueous solution was used as the supporting electrolyte [3].

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Results and discussion

Phase analysis

Thermal studies

The XRD patterns of the solvothermally prepared and heat-treated NiCo 2 O 4 nanostructures are shown in Fig. 2(a–c). It can be seen that the as-prepared sample exhibits the mixed hydroxide phases of Ni(OH) 2 / Co(OH)2 (Joint Committee on Powder Diffraction Standards (JCPDS) #74-2075/JCPDS #74-1057) as in Fig. 2(a). Further, the 300 °C temperature is much enough to remove the template, followed by the conversion of single-phase NiCo2O4 nanocrystals as shown in Fig. 2(b, c) [41]. The well-defined diffraction peaks positioned at 2θ values of 18.9°, 31.1°, 36.6°, 44.6°, 59.1°, 64.9°, and 68.3° indicate the formation of a cubic NiCo2O4 spinel nanostructure. Corresponding to the standard JCPDS file (#73-1702), all the diffraction peaks were indexed to (111), (220), (311), (400), (511), (440), and (531) plane reflections of the spinel NiCo2O4 crystalline structure with space group of F*3 (202). Even at 400 °C, there is no change in peak position, which infers the high thermal stability of the sample. The observed broad peaks indicate the formation of nanosize NiCo2O4 crystallites even at 400 °C [41]. Moreover, the relative intensities are increased significantly after being heattreated at 400 °C, suggesting that the change in the physical morphology of NiCo2O4 is due to their heatinduced grain growth [42]. The geometric statistical diameter of the particles is determined using the Scherrer equation D ¼ 0:94l=b cos θ. The average crystallite sizes of porous NiCo2O4 are approximately 9 and 17 nm for thermally treated at 300 and 400 °C, respectively, which substantiates that the crystal size increases significantly with an increase in the annealing temperature.

Figure 1 shows the thermogravimetric/differential thermal analysis (TG/DTA) curves of the NiCo2O4 nanostructure synthesized under the polyol medium recorded from 25 to 1,000 °C. TG/DTA was carried out to find the appropriate crystallization temperature of the NiCo2O4 spinel. The as-prepared sample may contain both intercalated and adsorbed water. The formed intermediate products can vary depending on the experimental conditions and precursors. At different temperatures, these compounds were removed during heat treatment in a multistep process. From the TG curve of the as-prepared mixed oxides, it can be seen that the total weight loss occurred at a typical stepwise decomposition process. The DTA curve shows two exothermic and two endothermic peaks in the temperature range of 25 to 1,000 °C. In the first step, the small exothermic peak positioned at around 60–150 °C is associated to the evaporation of moisture and the formation of metal glyoxalate from the metal nitrates complex within the DEG [38]. Secondly, the endothermic peak observed at 220 °C accompanied by a weight loss of 19 % could be attributed to the decomposition of solvent and conversion of D-glucose. The broad exothermic peak positioned at above 300 °C may be related to the oxidation and formation of a crystalline NiCo2O4 spinel nanostructure. This also concords with TGA of the corresponding weight loss of 41 % at above 300 °C, which indicated the crystallization of a cubic NiCo2O4 spinel structure. It is worthy to note that there is no loss in weight above 400 °C, suggesting the complete decomposition of the glucose template and the high thermal stability of the compound [39, 40]. Hence, it confirms that this temperature is much enough to remove all the solvent, template, and organic moieties completely.

Fig. 1 TG/DTA curve of solvothermally precipitated NiCo2O4

Fig. 2 XRD pattern of as-prepared NiCo2O4 (a) calcined at 300 °C (b) and 400 °C (c)

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FTIR studies Figure 3 illustrates the FTIR spectra of the NiCo2O4 spinel nanostructure recorded in the wavenumber range of 400– 4,000 cm−1 for the sample which was heat-treated at two different temperatures. Both spectra show similar IR activation modes as in the spinel oxides. The presence of two sharp peaks at 653 and 556 cm−1 corresponds to the formation of spinel NiCo2O4 [43]. Enhanced intensity of the above peaks in the sample treated at 400 °C reveals that the crystallinity of the spinel nanostructure is improved significantly [42]. These two characteristic metal oxide bands ascribe the stretching vibration of low-spin Co3+ ions at octahedral site and tetrahedrally coordinated Ni ions [43, 44]. The band positioned at 2,351 cm−1 is assigned to the surface-absorbed CO2 of the NiCo2O4 spinel nanostructure. Absorption bands due to the –CH group in the glucose appeared at the 2,700–2,900- and 1,385-cm−1 regions [45, 46]. These peaks were suppressed almost completely in the spinel oxide after treatment at 400 °C for 6 h. The broad band at around 3,500 cm−1 and the peak at 1,626 cm−1 correspond to the stretching and bending modes of the – OH groups of physically absorbed water molecules [44, 47]. Morphological analysis Surface morphology and mesoporous microstructure of the spinel NiCo2O4 are obtained using SEM and TEM. Typical SEM and TEM images of the sample heat-treated at 300 and 400 °C are shown in Figs. 4a–e and 5a–f, respectively. From the SEM micrographs, a stacked nanostructure with disordered shape was clearly observed. This is possibly due to the intercalation of certain anionic species like –COO− ion during crystallization by changing the surface energy; this

Fig. 3 FTIR spectra of the NiCo2O4 spinel oxides after the calcinations

could induce the typical porous structure [9]. For the sample treated at 300 °C, no appropriate morphology can be visible, thus confirming the presence of template in the sample. When the annealing temperature was increased from 300 to 400 °C, the well-defined morphology [40, 42] of the NiCo2O4 nanostructure can be seen, which ensures the complete removal of glucose template. This is well matched with the TGA result and earlier report [39]. The morphology and average particle size of the NiCo2O4 nanostructures are further confirmed by TEM which are represented in Fig. 5a– f. Observed TEM images suggest that these NiCo2O4 nanostructures are composed of numerous nanocrystals and are stacked together at large quantity. Finer average particle diameter in the range of 5–8 and 12–15 nm was observed for the sample heat-treated at 300 and 400 °C for 6 h, respectively. It is well known that calcination temperature and calcination time strongly influence the crystallization of a spinel nanostructure [26, 40]. The well-defined crystalline structure with less pores depicted in Fig. 5d–f implies the calcination-induced nano/micrograin growth of the spinel oxide. This reduces the electrolyte ion diffusion path considerably, resulting in reduced specific capacitance. The EDAX spectrum shown in Fig. 4e obviously designates the elemental composition of NiCo2O4 with appropriate molar concentration without impurities. BET surface area analysis The presence of mesopores in the NiCo2O4 nanostructure is further investigated with nitrogen adsorption and desorption isotherms at 77 K which is shown in Fig. 6 with the corresponding pore size distribution presented as inset. Typical hysteresis loop of the NiCo2O4 nanostructure shows a soft slope in the relative pressure range of 0.4 to 0.9, which indicates the presence meso- and micropores in the sample [9, 16]. From the inset in Fig. 6, it can be seen that the spinel oxide consists of more mesopores of ~12.5 nm between a broad distribution over 2–50 nm. In the isotherm graph, no considerable change is observed with respect to the calcination temperature. However, it is the change in the pore size and related surface area which confirms the grain growth of the spinel oxide. The spinel oxide heat-treated at 300 and 400 °C offers the specific surface area of 60.8 and 40.2 m2 g−1, respectively, and these values are comparable with the results reported by Chi et al. [40]. The sudden drop in the surface area at 400 °C is associated to the heatinduced grain growth of the nanoparticles and uneven pore distribution. This restricts the electrolyte ion diffusion through pores and thus decreases the specific capacitance. From the N2 isotherm, the average pore diameter and pore volume of the spinel nanostructures are calculated using the Barrett–Joyner–Halenda method, and the obtained values are ca. 13 and 18 nm and 0.19 and 0.16 cm3 g−1 for the

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(a)

(b)

(c)

(d)

(e)

Fig. 4 SEM micrographs of the NiCo2O4 nanostructure heat-treated at 300 °C (a, b) and 400 °C (c, d) and EDAX spectrum (e)

sample annealed at 300 and 400 °C, respectively. Obtained high surface area and mesoporous microstructure of the NiCo2O4 spinel oxide are beneficial for superior electrochemical response as an electrode for supercapacitor [48]. Electrochemical capacitance studies NiCo2O4 spinel as an electrode material for supercapacitor is elucidated by CV and CP measurements. The CV and CP curves of the NiCo 2 O 4 nanostructure electrodes after

calcination at 300 and 400 °C at different scan rates and current densities are shown in Fig. 7a–d. In the CV curves, the distinct broad pair of redox peaks appeared between 0and 0.4-V potential ranges, which correspond to the anodic and cathodic sweeps. This clearly suggests the energy storage mechanism originating from the surface redox reactions of Co2+/Co3+ and Ni2+/Ni3+. However, the two different oxidation and reduction processes are not discernible on the CV curves as like in the earlier reports [7, 26, 49]. CV plot at low scan rates exhibit two different redox processes,

Author's personal copy Ionics Fig. 5 TEM images of the NiCo2O4 nanostructure after the calcinations of 300 °C (a–c) and 400 °C (d–f) at different magnifications

(a)

(b)

(c)

(d)

(e)

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but in the high scan rate, the two peaks are merged together and show only one pair of peaks with larger area. The increasing sweep current with the scan rate concludes the good pseudocapacitive nature of the NiCo2O4 electrodes. High symmetry of the redox peaks further confirms the excellent reversibility of the material. The specific capacitance (Cs) is estimated from the CV curve using Eq. (1) [50]: R i dt Cs ¼ ð1Þ Δv m where i, dt, Δv, and m are the oxidation/reduction current, time differential, potential difference, and mass of the active material, respectively. The possible surface redox reaction is proposed as follows [49]: NiCo2 O4 þ OH þ H2 O $ NiOOH þ 2CoOOH þ 2e

ð2Þ

CoOOH þ OH $ CoO2 þ H2 O þ e

ð3Þ

The optimum Cs of the NiCo2O4 spinel oxides is 519 and 328 F g−1 at a scan rate of 10 mV s−1 for annealing for 6 h at 300 and 400 °C, respectively. The potential application of the mesoporous NiCo2O4 spinel nanostructures for supercapacitor electrode is further evaluated by CP as shown in Fig. 7c, d. Nonlinear charge/discharge curves ascribe the low polarization and dominant pseudocapacitance features of the as-prepared NiCo2O4 electrode material. However, due to the kinetic irreversibility of the OH− ions on the electrode surface, an asymmetry in the charging/discharging curves was observed, which is in good agreement with the CV result as well as with the spectrum obtained with electrochemical impedance spectroscopy (EIS) [9, 51]. Obviously, the linear variation in the CP curve at the −0.2- to 0.1-V range, exactly parallel to the y-axis, represents the contribution of electric double layer capacitance by the charge separation at the electrode/electrolyte interface

Author's personal copy Ionics Fig. 6 N2 adsorption/ desorption isotherms and pore size distribution (inset) of the NiCo2O4 nanostructure annealed at 300 °C (a) and 400 °C (b)

[3, 9]. Above 0.1 V, the rate of charging decreases with time, thus facilitating the faradic redox reaction or electrochemical absorption/desorption at the electrode/electrolyte interface. From the charge/discharge curve, the Cs is determined according to Eq. (4): Cs ¼

It mv

ð4Þ

where I is the discharge current, m is the mass of the active material, V is the potential difference, and t is the discharge

time. The as-prepared NiCo2O4 spinel oxide heat-treated at 300 and 400 °C can reach the maximum specific capacitance of 524 and 320 F g−1, respectively, at a constant current density of 0.5 A g−1. The charge/discharge rate increased from 0.5 to 10 A g−1, and about 80 % of initial capacitance is retained by the spinel oxide nanostructure. The observed maximum capacitance and excellent capacity retention of the sample treated at 300 °C ascribe the highly mesoporous microstructure of the NiCo2O4 spinel oxide. It facilitates the easy access of OH− ion onto the inner active surface due to the short ion diffusion path

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Fig. 7 CV and CP plots of the spinel NiCo2O4 electrode heat-treated at 300 °C (a, c) and 400 °C (b, d)

and the presence of nanopores between the interconnected crystallites. At higher calcination temperature, the heatinduced nano/micrograin growth reduces the porosity and the specific surface area of the material, which reduces the Cs significantly. From the shape of the CV and CP curves, it is clear that the significant contribution of EDLC is the high specific capacitance value. This is associated to the adopted glucose assistance in the synthesis process, which offers the carbonaceous material on the surface of the spinel oxide, thus enhancing the resultant electrode capacitance at lower calcination temperature. At 400 °C, the glucose templates were decomposed and almost completely removed from the sample, thus reducing the EDLC contribution of the electrode. Cyclic stability of the electrode is tested by continuous charging/ discharging over 2,500 cycles at a constant current density of 6 A g−1. The specific capacitance value decreases gradually with increasing cycle number; only about 8 % of capacity loss

is observed for the corresponding NiCo2O4 nanostructures heat-treated at 300 and 400 °C as shown in Fig. 8a–c. This substantiates the excellent electrochemical stability and great columbic efficiency of the electrode material. Electrochemical impedance spectroscopy The observed good electrochemical response of the NiCo2O4 spinel oxide is further confirmed by EIS measurement. As shown in Fig. 8d, the EIS plot consists of a small semicirclelike region at high-frequency and followed by low-frequency spike [49, 50]. Generally, the resistance of the electrode material is the contribution of bulk solution resistance (Rs), and charge-transfer resistance (Rct) is measured from the Nyquist plot by the intersection of semicircle on the x-axis [49]. Charge-transfer resistance is strongly dependent on the specific surface area and electric conductivity of the material [52].

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Fig. 8 Continuous charge/discharge cycles (a, b), coulombic efficiency (c), and EIS spectrum (d) of the NiCo2O4 nanostructures heat-treated at 300 °C (black) and 400 °C (red)

In further analysis, the observed EIS spectra were compared with the suitable equivalent circuit as shown in Fig. 8d inset which is reported earlier [53]. In comparison with this equivalent circuit, the tested cell consists of solution resistance (Rs) added in series to the charge-transfer resistance (Rct) and double-layer capacitance (Cdl) [53]. The negligible semicirclelike region at the high-frequency region represents the low equivalent series resistance of ~0.71 Ω for our material which is due to faradaic redox processes in the system involving the exchange of OH− ions [51]. Further, the low-frequency vertical-like slope with small deviation reveals the nonideal polarization of the electrode [3], which is consistent with the CV and CP results. This suggests the high electrical conductivity of the prepared samples by the solvothermal process. It is possibly due to the presence of carbonaceous material on the surface of the sample. Increasing the calcination

temperature to 400 °C, the Rct value increases significantly, which ascribes the reduced specific surface area of the nanostructure. This may be due to the micro/nanograin growth of the spinel oxide during heat treatment. These results conclude that the prepared NiCo2O4 spinel oxide nanostructures possess better electrochemical activity.

Conclusions In summary, mesoporous NiCo2O4 nanostructure is successfully synthesized through a solvothermal method without any toxic oxidizing agents. The as-obtained nickel cobaltite spinel oxide heat-treated at 300 °C exhibits a maximum capacitance of 524 F g−1 at a constant current density of 0.5 A g−1. The maximum Cs of 320 F g−1 was obtained for

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the sample heated at 400 °C for the same current density. This confirms the physical morphology conversion by the thermally induced grain growth which hinders the surface redox reaction and diffusion process significantly. The observed high specific capacitance and good cyclic stability of the NiCo2O4 nanostructures are probably due to the good electrical conductivity, high surface area, mesoporous microstructure, and contribution of both Ni2+ and Co2+ ions for the redox process. Furthermore, this approach can be applied for synthesizing a wide range of spinel metal oxide with desired nanostructures for electrochemical energy storage applications.

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