Fast synthesis and electrochemical performance of hollow NiCo₂O₄ flowerlike microstructures

Abstract

In the present work, a two-step route was designed for fast synthesis of hollow NiCo₂O₄ flowerlike microstructures. A flowerlike precursor containing Ni and Co was firstly prepared via a fast microwave-assisted hydrothermal route. Then, hollow NiCo₂O₄ flowerlike microstructures were successfully obtained through pyrolyzing the above precursor. Experiments showed that the pyrolysis temperature could strongly affect the performance of the final product. The product prepared at 280 °C exhibited a bigger BET surface area and higher specific capacitance than that prepared at 400 °C. The as-prepared NiCo₂O₄ microstructures were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), (high resolution) transmission electron microscopy (HRTEM/TEM) and energy dispersive X-ray spectrometry (EDS).

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1. Introduction

As one of promising candidates for energy storage, electrochemical supercapacitors (ESCs) are drawing extensive research interest because of some advantages including higher power density, longer cycle life, shorter charging time, faster recharging capability, and so forth.^1–3 In general, according to the charge-storage mechanism supercapacitors can be mainly classified into two types: electrical double-layer capacitors (EDLCs) and pseudocapacitors.^4–6 It has been found that RuO_x is a prominent electroactive material for pseudocapacitors since it shows remarkably large specific capacitance and excellent reversibility.⁷ In practical applications, however, its use is limited due to the expensive price, the rareness of Ru and the toxic nature of the hydrated RuO_x. In order to replace RuO_x as the electrode material for pseudocapacitors, some inexpensive transitional metal oxides have been studied, such as MnO₂,^8,9 Ni/Co(OH)₂,¹⁰ Fe₃O₄ (ref. 11) and SnO₂.¹² However, it is still of great importance to explore new electrode materials with higher power performance and longer cycle life for pseudocapacitors.¹³

As a double-metal oxide with the spinel structure, NiCo₂O₄ was found to bear better electrochemical activity and higher electronic conductivity than NiO and Co₃O₄.¹⁴ Therefore, NiCo₂O₄ micros/nanostructures have been paid much attention in the new-energy-source field as electrode materials for supercapacitors and Li-ion batteries.^15,16 Many methods have been developed for the synthesis of NiCo₂O₄, including hydroxide decomposition, thermal treatment, hydrothermal synthesis, sol–gel method,¹⁷ cation exchange method,¹⁸ and so on. Employing the above methods, various NiCo₂O₄ micro/nano-structures have been successfully obtained, such as nanoplates,¹⁹ mesoporous structures,²⁰ nanotubes,²¹ urchin-like sphere,²² nanorods,²³ nanowires²⁴ and nanoflowers.²⁵ However, long reaction durations were usually needed in the above approaches. This is unfavorable for saving the energy.

Recently, flowerlike micro/nano-structures have attracted increasing interest owing to their bigger surface areas compared with other shapes and wide applications in many fields.²⁶ Some flowerlike micro/nano-structures have been successfully fabricated in our group in the past years, including Ni–P, Ni–NiO, copper quinoline sulfate, Ni/Ni(OH)₂ microstructures, and so on.²⁷ In this paper, we designed a two-step route for rapid preparation of hollow flowerlike NiCo₂O₄ microstructures, employing NiCl₂·6H₂O, CoCl₂·6H₂O and urea as the initial reactants in the presence of trisodium citrate. The flowerlike precursor constructed by nanosheets was firstly obtained by a microwave-assisted hydrothermal route under the power of 800 W at 160 °C for 10 min. Then, hollow NiCo₂O₄ flowerlike microstructures were prepared through calcining the precursor in air at 280 or 400 °C for 2 h. The investigations showed that the pyrolysis temperature could not markedly change the morphology of the final product. However, some performances of the final product could be affected by the pyrolysis temperature. Experiments found that NiCo₂O₄ microstructures prepared at 280 °C for 2 h presented the bigger BET surface area, the smaller pore size, and the higher specific capacitance.

2. Experimental section

All reagents and chemicals are analytically pure, bought from Shanghai Chemical Company and used without further purification. The deionized water was used throughout experiments.

2.1 Synthesis of hollow flowerlike NiCo₂O₄ microstructures

In a typical experiment, 1 mmol of NiCl₂·6H₂O and 2 mmol of CoCl₂·6H₂O were firstly dissolved into deionized water of 20 mL. After magnetic-stirring for 10 min, 0.25 mmol of trisodium citrate was added into the mixed solution. Continuous stirring for 30 min, 24 mmol of urea was added. After stirring for another 10 min, the above solution was transferred into the Teflon container of the double-walled vessel of a microwave system (WX-4000) and irradiated under the power of 800 W at 160 °C for 10 min. After the system was cooled to room temperature naturally, the solid precipitates were collected, washed several times with deionized water and absolute ethanol in turn, and dried in a vacuum oven at 60 °C for 6 h. Finally, the as-prepared precursor was calcined at 280 and 400 °C in air for 2 h, respectively. The obtained black powders were called as C280 and C400, respectively.

2.2 Characterization

The powder X-ray diffraction (XRD) patterns of the product were recorded on a Japan Shimadzu X-ray diffractometer (XRD-6000) equipped with graphite monochromated Cu Kα radiation (λ = 0.15406 nm), using a scanning rate of 0.02° s⁻¹ in 2θ ranges from 10° to 80°. Scanning electron microscopy (SEM) images and energy dispersive spectrometry (EDS) of the product were obtained on a Hitachi S-4800 Field emission scanning electron microscope with an accelerating voltage of 5 kV or 15 kV (15 kV for EDS). (High-resolution) transmission electron microscopy (HRTEM/TEM) and electron diffraction (ED) pattern of the final product were taken on JEOL 2010 transmission electron microscope, equipping an acceleration voltage of 200 kV. Thermogravimetric analysis (TGA) was performed on a Shimadzu DTG-60A thermal analyzer under the protection of Argon gas with a ramping rate of 10 °C min⁻¹. N₂ adsorption measurements were performed on a Micromeritics ASAP 2020 M + C volumetric adsorption equipment at 77 K using Barrett–Emmett–Teller (BET) calculations for surface area.

2.3 Electrochemical performance

Electrochemical experiments were performed with CHI 660C electrochemical analyzer (ChenHua Corp., Shanghai, China) with a three electrode experimental setup. The working electrode was prepared by pasting a mixture of NiCo₂O₄, acetylene black and polytetrafluoroethylene (PTFE) binder (the weight ratio of 80 : 15 : 5) onto a piece of nickel foam and dried under vacuum at 60 °C for 6 h. The platinum wire and standard calomel electrode (SCE) were used as the counter and reference electrodes in 3 M KOH, respectively. Cyclic voltammetry (CV) curves were measured within a potential range of 0–0.45 V at a scan rate from 5 to 100 mV s⁻¹. Galvanostatic charge–discharge (CD) was tested at current densities of 1, 2, 4, 6, 8, 10 A g⁻¹, respectively. The specific capacitance (C_s) of the electrode can be evaluated according to the following equation:²⁸where C (F g⁻¹) is the specific capacitance of the electrode based on the mass of active materials, I (mA) is the current during discharge process, Δt (s) is the discharge time, ΔV (V) is the potential window (here ΔV = 0.45 V) and M (mg) is the mass of active materials.

3. Results and discussion

3.1 Structure and morphology characterization

Fig. 1a shows a typical SEM image of the precursor obtained by the microwave-assisted hydrothermal route under the power of 800 W at 160 °C for 10 min. The precursor presents porous flowerlike structures constructed by abundant nanosheets. The XRD analysis exhibited the poor crystallinity of the precursor (see Fig. 1b). EDS analysis shown in the inset of Fig. 1b proved the presences of C, O, Co, Ni and Cu elements. The Cu peak should be attributed to the copper support. Based on the calculation of the peak areas, the Co/Ni atomic ratio was 8.36/4.16, which was very close to 2/1. This is in good agreement with the original molar ratio of Ni/Co. Fig. 1c depicts TG-DTA curves of the precursor. A dramatic weight loss is found at the temperature range from 200 to 280 °C in TG curve, implying the decomposition of the precursor. Accordingly, an obvious endothermic peak appears at the same temperature range in DTA curve. Therefore, 280 °C was chosen as the pyrolyzing temperature of the precursor for the preparation of NiCo₂O₄. Simultaneously, to investigate the influence of the pyrolyzing temperature on the morphology and performance of the final product, 400 °C was also employed as the pyrolyzing temperature of the precursor.

Fig. 1 (a) A typical SEM image, (b) XRD pattern and EDS analysis, and (c) DT-TGA curve of the precursor prepared by the present microwave-hydrothermal route under the power of 800 W at 160 °C for 10 min.

Fig. 2a depicts XRD patterns of the final products prepared by pyrolyzing the precursor at 280 and 400 °C, respectively. No obvious difference is found in two diffraction patterns. By comparison with the standard data of JCPDS card files no. 73-1702, all diffraction peaks can be indexed as the face-centered cubic NiCo₂O₄. No impurity peak is detected, indicating that the final products obtained under the above two temperatures are pure. The XRD results show that the precursor has been successfully transformed into pure spinel ternary nickel cobaltite. Fig. 2b exhibits the EDS analyses of the final products obtained under two pyrolyzing temperatures. Two similar curves are clearly visible. Markedly, strong Ni, Co and O peaks should come from the final product. Also, compared with the EDS analysis of the precursor, the C peak obviously weakens, which indirectly confirms the decomposition of the precursor.

Fig. 2 (a) XRD pattern and (b) the EDS analyses of the final product prepared by pyrolyzing the precursor at 280 °C and 400 °C for 2 h in air, respectively.

Fig. 3a–d display representative SEM images of C280 and C400, respectively. The final products prepared at different pyrolyzing temperatures present similar morphologies. Low magnification SEM images show the hollow flowerlike microstructures of the products constructed by abundant nanoplates (see Fig. 3a and c). The sizes of two flowerlike NiCo₂O₄ microstructures are in turn 1000–1200 nm and 600–800 nm. C280 has a larger size than C400, which should be caused by the shrink of the product during the pyrolysis of the precursor under the higher temperature. Obviously, enhancing the pyrolyzing temperature hardly changes the morphology of the final NiCo₂O₄ microstructures, but can cause the decrease of the particle size. High resolution SEM images given in Fig. 3b and d discover that nanoplates are porous structures and ∼10 nm in thickness. Fig. 3e gives a typical TEM image of C400. The porous structures of the nanoplates can be clearly seen, which proves the result of SEM observations. A SAED pattern is depicted in Fig. 3f. Concentric rings indicate the polycrystalline crystals of nanoplates. Five bright rings are assigned to the (220), (222), (400), (440) and (531) planes from inside to outside, respectively, which confirm the result of the XRD analysis.

Fig. 3 Electron micrographs of the products prepared by pyrolyzing the precursor at different temperatures: (a) a representative low-magnification SEM image and (b) a high-magnification SEM image of C280; (c) a representative low-magnification SEM image, (d) a high-magnification SEM image, (e) a typical TEM image and (f) a SAED pattern of C400.

Fig. 4 depicts N₂ adsorption–desorption isotherms of C280 and C400, and the corresponding pore size distributions calculated by BJH method (see the insets in Fig. 4). Two isotherms exhibit the characteristics of type-IV isotherms with a pronounced hysteresis loop according to the IUPAC classification. In two isotherms, sharp capillary condensation steps can be observed at relative high pressures of P/P₀ > 0.75 (Fig. 4a) and 0.9 (Fig. 4b), respectively. These imply the high porosities of two samples. BET surface areas of two samples are in turn ∼107.8 m² g⁻¹ (C280) and 48.5 m² g⁻¹ (C400). C400 owns the smaller BET surface area than C280, which is the result of the shrink of the final product under the higher temperature. Moreover, two samples also exhibit the different BJH pore size distribution curves. As shown in the insets of Fig. 4, both samples present two peaks. Besides the same peak at 2.4 nm, the second peak is obviously different in two samples. The second peak range is 10–40 nm in C280 and 5–90 nm in C400, respectively. The mean pore sizes are in turn 17.8 and 33.9 nm. Namely, the sample treated under the higher temperature has bigger pore size. To conveniently compare the influence of pyrolyzing temperature on the BET surface area and pore size of the final product, Table 1 lists characteristic surface areas and pore structures of C280 and C400. One can easily find that lower pyrolyzing temperature lead to higher BET specific surface areas and total pore volumes. According to Duan's report, the second peak of each sample should be caused by crack on the surface of hollow hierarchical 3D flowerlike NiCo₂O₄.²⁹ The above porous structures are favorable for supercapacitors because mesoporous structure with a large surface area and a small pore size can facilitate the electrolyte ion diffusion, enhance the charge transport, and provide more electroactive sites for fast energy storage at large current densities.³⁰

Fig. 4 The nitrogen adsorption–desorption isotherms and pore size distribution curves (inset) of the final products: (a) C280 and (b) C400.

Table 1 Characteristic surface areas and pore structures of the samples obtained under different pyrolyzing temperatures

Sample	BET surface area cm^2 g^−1	BJH pore volume cm^3 g^−1	BJH mean pore size (nm)
C280	107.8	0.57	17.8
C400	48.5	0.45	33.9

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3.2 Electrochemical performance

The electrochemical performances of the hollow NiCo₂O₄ flowerlike microstructures were investigated for their potential application in electrochemical energy storage. Fig. 5a and b show the CV curves of two samples in 3 M KOH solution within a potential range of 0–0.45 V at a scan rate of 5, 10, 20, 50, 100 mV s⁻¹, respectively. The peak currents increase with the increase of the scan rate and simultaneously, the separations between the anodic peak potential (E_pa) and the cathodic peak potential (E_pc) in the CV curves increase, too. A pair of redox peaks (vs. SCE) is clearly observed, which corresponds to the reversible reactions of Co³⁺/Co⁴⁺ and Ni²⁺/Ni³⁺ transitions associated with anions OH⁻.^31,32 The redox reactions in the alkaline electrolyte are given as follows:

NiCo₂O₄ + OH⁻ + H₂O ↔ NiOOH + 2CoOOH + e⁻ (2)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (3)

Fig. 5 The CV curves of hollow hierarchical 3D flowerlike NiCo₂O₄ microstructures at various scan rates: (a) C280; (b) C400; (c) the CV curves of C280 and C400 at 50 mV s⁻¹.

Fig. 5c compares the CV curves of C280 and C400 at a scan rate of 50 mV s⁻¹, from which one can clearly find that the specific capacitance of C280 is much higher than that of C400. This may be attributed to different crystallite sizes, surface areas as well as pore volumes that affect the hydroxyl ions to participate in the electrochemical reactions.

Fig. 6a and b depict the galvanostatic charge–discharge measurements of two samples under various current densities. The nonlinear charge–discharge profiles further verified the pseudo-capacitance behavior. With the increase of the current density, the charge–discharge time decreased. Fig. 6c shows the galvanostatic charge–discharge curves of two samples at the current density of 1 A g⁻¹. Markedly, the sample obtained at the higher temperature exhibits the faster charge–discharge rate. However, the calculation displayed that the specific capacitances of C280 and C400 were separately 387 and 260 F g⁻¹. Namely, the sample obtained under the lower temperature had bigger specific capacitance than the one obtained under the higher temperature. This result is consistent with the CV results shown in Fig. 5c. It is well known that the specific capacitance of an electrode material is highly dependent on their specific surfaces area and pore structures. The N₂ adsorption–desorption results showed that C280 had a much higher specific surface area and a bigger pore volume (see Table 1). Thus, it was understandable that C280 exhibited better specific capacitance than C400. Fig. 6d depicts the specific capacitance–current density plots of two samples. Although the specific capacitances of two samples gradually decreased with the increase of current density from 1 to 10 A g⁻¹, the capacitance loss of C280 was less than that of C400. At the current density of 10 A g⁻¹, the capacitance of C280 still retained 64% and that of C400 only 55%. The abovementioned facts indicated that the higher calcining temperature was unfavorable for the improvement of the electrochemical performance of the final product. Furthermore, the cycling stability measurement also proved the above conclusion. As shown in Fig. 7, the specific capacitances of two samples increased during the first 1000 cycles, which resulted from the activation process for the NiCo₂O₄ electrode material. Similar phenomena have also been reported by other groups.^20,33 After the subsequent 4000 cycles, the specific capacitance of C280 decreased from 387 F g⁻¹ to 372 F g⁻¹. Namely, the capacitance of 96.1% was retained. However, only 92.3% capacitance was maintained for C400. To further investigate the electrochemical behavior of two samples, the electrochemical impedance spectra (EIS) of two samples in 3 M KOH aqueous solution with a frequency ranging from 100 kHz to 0.01 Hz were measured. Fig. 8 shows the typical Nyquist plots of the electrodes prepared by two samples in 3 M KOH aqueous solution at the current density of 1 A g⁻¹ after 1, 1000 and 5000 cycles, respectively. All the curves present a depressed semicircle in the high frequency region and a straight line in the low frequency region. Distinctly, C280 presented smaller the semicircular arc than C400 under the same cycle number, which revealed the lower charge-transfer resistance in the electrode reaction. These results further confirmed that a lower calcining temperature availed to achieve NiCo₂O₄ material with better electrochemical performance.

Fig. 6 Galvanostatic charge–discharging curves of two samples at various current densities: (a) C280 and (b) C400; (c) galvanostatic charge–discharge curves of C280 and C400 measured at a current density of 1 A g⁻¹; (d) specific capacitance comparison of two samples under different discharge current densities.

Fig. 7 The long-term stability curves of two samples at the current density of 1 A g⁻¹.

Fig. 8 Nyquist plots of C280 (solid line) and C400 (dot line). Black: the 1^st cycle; red: the 1000^th cycle and blue: the 5000^th cycle.

4. Conclusions

In summary, hollow NiCo₂O₄ flowerlike microstructures have been successfully prepared through a facile microwave-hydrothermal method and subsequent calcinations. Experiments showed that flowerlike precursors were firstly obtained by the microwave hydrothermal route at 160 °C for 10 min and then, could be transferred into hollow flowerlike NiCo₂O₄ by annealing in air at 280 °C and 400 °C for 2 h, respectively. It was found that the product obtained by the calcinations at 280 °C presented the bigger particle size, the higher BET surface area and the bigger pore volume, and the better electrochemical performance than that prepared at 400 °C. The abovementioned phenomena should be attributed to the shrink of the final product under the higher calcining temperature, implying that the higher treating temperature is unfavorable for the performance improvement of the final product. The present rapid synthetic route is an environment-friendly and cost-effective method, which can be further developed for the synthesis of other ternary oxide micro-/nano-structures with certain shapes.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21171005) and Key Foundation of Chinese Ministry of Education (210098) for the fund support.

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