Facile synthesis of three-dimensional NiCo₂O₄ with different morphology for supercapacitors

Abstract

Three-dimensional (3D) hierarchical NiCo₂O₄ with tremella-like, flower-like, urchin-like and pine-needle-like structures has been prepared through a facile hydrothermal method. The key roles of the alkali source and NH₄F in the formation of these nanostructures have been investigated. When applied to supercapacitors as an electrode material, the urchin-like NiCo₂O₄ sample exhibits the optimal electrochemical performance among all the four samples, demonstrating its promising application potential for supercapacitors. Such excellent performance is attributed to the unique mesoporous structure of the superfine nanoneedle-assembled urchin-like NiCo₂O₄ with large specific surface area. This investigation about morphology control of NiCo₂O₄ electrode materials and the relationship between the morphologies and corresponding electrochemical performances also provides a reference for the rational design and synthesis of electrode materials with desirable morphology for high performance supercapacitor applications.

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1. Introduction

With concerns growing over environmental pollution and the energy crisis in the world, clean and renewable energy materials as well as energy storage systems have become hotspots of study.^1–4 Among the different energy storage technologies, electrochemical capacitors (ECs), also called supercapacitors, which have greater energy storage capacity than conventional capacitors, and higher power density, and longer cycle-life than batteries, have drawn increasing attention in recent years.^5,6 Based on the energy storage mechanism, supercapacitors can be classified into two types: electrical double-layer capacitors (EDLCs) and pseudocapacitors.⁷ Pseudocapacitors, owing to the fast and reversible faradaic reactions that occur both on the surface of pseudocapacitor electrodes and in the interior of these electrodes, can provide higher specific capacitance and energy density than EDLCs.^8,9 The electrode, as one part of a full supercapacitor device, is considered to be a key component affecting the electrochemical performance of a supercapacitor.² Therefore, as far as the improvement of pseudocapacitor performance is concerned, it is an important job to choose and design a kind of efficient electrode material.

Transition metal oxides and hydroxides, such as RuO₂,¹⁰ MnO₂,¹¹ Co₃O₄,¹² NiO,¹³ Ni(OH)₂,¹⁴ and NiCo₂O₄,¹⁵ have been extensively studied as electroactive materials for high-performance pseudocapacitors because of their multiple oxidation states capable of rich redox reactions for high specific capacitance and great flexibility in structure and morphology.^5,16 Compared to simple metal oxides, mixed metal oxides exhibit higher electrical conductivity by reason of lower activation energy for electron transfer between cations.¹⁷ Recently, spinel nickel cobaltite (NiCo₂O₄) has drawn intensive research attention in supercapacitors due to the advantages of high theoretical capacitance, environmental friendliness, good electronic conductivity and high electrochemical activity benefiting from richer redox reaction from both nickel and cobalt ions than that from single component nickel oxides or cobalt oxide.^18,19 Up to now, various different types of NiCo₂O₄ morphologies and nanostructures, including nanoparticles,²⁰ nanowires,²¹ nanotubes,²² nanosheets²³ and nanoflowers²⁴ have been prepared and their electrochemical performances have been studied.

It's widely accepted that the morphology, electronic conductivity and porosity of electrode materials play key roles in improving electrochemical performance of supercapacitors.^18,25,26 Especially for morphology, electrode materials with hierarchical/porous structures can increase the specific surface area, improve the contact area between electrode and electrolyte, shorten the diffusion path of electrolyte ion, and enhance the structural stability.²⁷ Three-dimensional (3D) structures can exhibit better cycling performance compared to one-dimensional (1D) and two-dimensional (2D) structures because of that the 3D feature is favorable for accommodating the structural change and efficient ion and electron diffusion.^26,28,29 Besides, the morphologies and structures of nanomaterials can be controlled by adjusting some reaction parameters, such as solvents, concentration of the reactant, reaction duration, temperature and capping agent, etc.⁴ However, there have been few systematic studies on the morphology control of NiCo₂O₄ electrode materials by adjusting synthesis parameters based on facile hydrothermal synthetic method, as well as the relationship between the morphologies and the corresponding supercapacitor performances.

In this work, 3D hierarchical NiCo₂O₄ with tremella-like, flower-like, urchin-like and pine-needle-like structures have been synthesized, which can be easily controlled by just changing the kinds of alkali source (or capping agent) and the addition of NH₄F. Then, the different morphologies leading to different electrochemical properties have been investigated. The urchin-like NiCo₂O₄ sample exhibits the best electrochemical performance among all the four samples. This investigation can provide a reference for future studies about morphological control and provides strategies to enhance the performance of supercapacitor electrodes.

2. Experimental

2.1. Materials synthesis

3D NiCo₂O₄ with a diversity of nanostructures including tremella-like, flower-like, urchin-like and pine-needle-like structures were prepared through a simple hydrothermal synthetic method. A typical preparation process of urchin-like NiCo₂O₄ was conducted as follows: 1 mmol Ni(NO₃)₂·6H₂O and 2 mmol Co(NO₃)₂·6H₂O (Ni and Co, molar ratio 1 : 2) was dissolved 30 mL of deionized water, followed by the addition 12 mmol of urea under stirring. The mixture stirred for 30 min and then was transferred into a 50 mL Teflon-lined stainless autoclave. The autoclave was sealed and heated at 120 °C for 6 h. After cooling to room temperature, the resulting precipitate was collected, washed with deionized water and ethanol for several times, and dried at 60 °C for 24 h. Finally, the dried powder was further annealed at 400 °C for 3 h. A similar method of preparation was used to synthesize other three NiCo₂O₄ samples. The corresponding experimental conditions of four different samples, including tremella-like NiCo₂O₄ (T-NiCo₂O₄), flower-like NiCo₂O₄ (F-NiCo₂O₄), urchin-like NiCo₂O₄ (U-NiCo₂O₄) and pine-needle-like NiCo₂O₄ (P-NiCo₂O₄), have been listed in Table 1.

Table 1 Summary of various NiCo₂O₄ morphologies and corresponding experimental conditions

Sample	Ni(NO3)2·6H2O (mmol)	Co(NO3)2·6H2O (mmol)	Urea (mmol)	Hexamethylene-tetramine (mmol)	NH4F (mmol)
T-NiCo2O4	1	2	0	12	0
F-NiCo2O4	1	2	0	12	6
U-NiCo2O4	1	2	12	0	0
P-NiCo2O4	1	2	12	0	6

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2.2. Materials characterization

The crystal phases of as-prepared products were examined by X-ray diffraction (XRD, D/max-1200X with Cu Kα radiation). The morphologies of the samples were characterized by field emission scanning electron microscopy (SEM, JSM-7800F) and transmission electron microscope (TEM, Zeiss, Libra200). The nitrogen adsorption–desorption isotherms and pore-size distributions of the as-synthesized materials were measured by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods (Micromeritics, ASAP2020).

2.3. Electrochemical measurements

A three electrode electrochemical workstation (RST 5100F, Zheng Zhou) was used for electrochemical measurements with 2 M KOH aqueous solution as the electrolyte. Hg/HgO was used as the reference electrode and platinum plate was used as the counter electrode. The working electrodes containing different active materials were prepared by mixing the as-prepared NiCo₂O₄ powders (70 wt%), acetylene black (20 wt%), and polytetrafluoroethylene (10 wt%) in a required amount of N-methyl-pyrrolidone (NMP) to form the slurries. Then, the mixed slurries were pressed onto the surface of nickel foam current collectors (1.0 cm × 1.0 cm) and dried thoroughly at 60 °C. The mass loading of the T-NiCo₂O₄, F-NiCo₂O₄, U-NiCo₂O₄ and P-NiCo₂O₄ on nickel foam were ∼1.4 mg cm⁻², ∼1.8 mg cm⁻², ∼2.1 mg cm⁻² and ∼2.0 mg cm⁻², respectively. Cyclic voltammetry (CV) curves were measured in a potential range between 0 and 0.6 V at different scan rates (10, 20, 30, 50 and 100 mV s⁻¹), and the galvanostatic charge–discharge (GCD) processes were conducted in the potential window of 0–0.55 V at different current densities (1, 2, 3, 5, 10 and 20 A g⁻¹). The electrochemical impedance spectroscopy (EIS) measurements were evaluated with potential amplitude of 5 mV in the frequency range from 0.01 Hz to 100 kHz.

3. Results and discussion

3.1. Synthesis and structural analysis

The fabrication processes of F-NiCo₂O₄, T-NiCo₂O₄, U-NiCo₂O₄ and P-NiCo₂O₄ are described in Fig. 1. Four types of NiCo₂O₄ with different morphologies have been synthesised through adjusting the kinds of alkali source and the addition of NH₄F and their detailed SEM images under different magnification are shown in Fig. 2. When the alkali source is hexamethylenetetramine (HMTA) and there is no assistance from NH₄F, T-NiCo₂O₄ constructed by cross linked ultrathin nanosheets with diameters range from 2 to 8 μm could be observed (Fig. 2a–c). After introducing NH₄F, F-NiCo₂O₄ (Fig. 2d–f) assembled from nanosheets which are thicker and more flat than that of T-NiCo₂O₄ was synthesized. Changing the alkali source from HMTA to urea without addition of NH₄F resulted in U-NiCo₂O₄ (Fig. 2g–i) with diameters range from 2 to 8 μm constructed by numerous superfine nanoneedles. P-NiCo₂O₄ (Fig. 2j–l) with the bigger diameters than that from U-NiCo₂O₄ are obtained in the presences of urea as alkali source and NH₄F as additive.

Fig. 1 Schematic diagram illustrating the fabrication processes of F-NiCo₂O₄, T-NiCo₂O₄, U-NiCo₂O₄ and P-NiCo₂O₄.

Fig. 2 SEM images of the NiCo₂O₄ with different morphologies: T-NiCo₂O₄ (a–c), F-NiCo₂O₄ (d–f), U-NiCo₂O₄ (g–i), and P-NiCo₂O₄ (j–l).

We should take note that the alkali source (HMTA and urea) and NH₄F play the important roles in formation of different NiCo₂O₄ morphologies. In the presences of HMTA as alkali source, nanosheet-assembled NiCo₂O₄ structures (T-NiCo₂O₄, F-NiCo₂O₄) can be synthesized and the whole reaction process probably proceeds through the following reactions:³⁰

(CH₂)₆N₄ + 6H₂O → 4NH₃ + 6HCHO (1)

NH₃ + H₂O → NH₄⁺ + OH⁻ (2)

Ni²⁺ + 2Co²⁺ + 6OH⁻ → NiCo₂(OH)₆ (3)

NiCO₂(OH)₆ + 1/2O₂ → NiCo₂O₄ + 3H₂O (4)

However, when HMTA is replaced by urea, well-defined nanoneedle-assembled NiCo₂O₄ structures (U-NiCo₂O₄, P-NiCo₂O₄) can be obtained, as described by the following equations:^31,32

CO(NH₂)₂ + H₂O → 2NH₃ + CO₂ (5)

CO₂ + H₂O →CO₃²⁻ + 2H⁺ (6)

NH₃ + H₂O → NH₄⁺ + OH⁻ (7)

Ni²⁺ + 2Co²⁺ + 3xOH⁻ + 1.5(2 − x)CO₃²⁻ + nH₂O → NiCo₂(OH)_3x(CO₃)_1.5(2−x)·nH₂O (8)

2NiCo₂(OH)_3x(CO₃)_1.5(2−x)·nH₂O + O₂ → 2NiCo₂O₄ + 3(2 − x)CO₂ + (3x + 2n)H₂O (9)

Comparing the two different decomposition equations above, it can be observed that CO₃²⁻ play an key role on the formation of nanoneedle-assembled NiCo₂O₄ structures. When the urea is used as the alkali hydrolyzing agent, Ni²⁺, Co²⁺ ions reacted with CO₃²⁻ and OH⁻ to form nanoneedle-assembled bimetallic (Ni, Co) carbonate hydroxide structures.^32,33 However, with the HMTA as the alkali hydrolyzing agent, Ni²⁺ and Co²⁺ ions coordinate with OH⁻ to form nanosheet-assembled bimetallic (Ni, Co) hydroxide structures.³⁴ Then, nanoneedle-assembled and nanosheet-assembled NiCo₂O₄ without obvious morphology change can be obtained after annealing.

Moreover, compared with NiCo₂O₄ structures (T-NiCo₂O₄ and U-NiCo₂O₄) without NH₄F, F-NiCo₂O₄ and P-NiCo₂O₄ with NH₄F have thicker thickness and bigger diameters demonstrating that introducing of NH₄F can promote the nanosheets and nanoneedles growing up to a bigger size. This may be attributed to the presence of F⁻, which can stimulate the nanosheets and nanoneedles formed in the initial stage to produce more active sites for further nucleation and growth.^18,35,36

The as-prepared NiCo₂O₄ samples were characterized by XRD (Fig. 3) in order to obtain the crystallographic structure. All the diffraction peaks located at 18.9°, 31.1°, 36.7°, 38.4°, 44.6°, 55.4°, 59.1° and 65.0° correspond to the (111), (220), (311), (222), (400), (422), (511) and (440) planes of cubic structured NiCo₂O₄ (JCPDS card no. 20-0781). There are no additional diffraction peaks among all the four samples, revealing that the samples are of high purity NiCo₂O₄ phase.

Fig. 3 XRD patterns of as-synthesized NiCo₂O₄ samples with different morphologies.

Further information about the U-NiCo₂O₄ structure is obtained from TEM (Fig. 4). Fig. 4a shows the low magnification TEM image of an individuate NiCo₂O₄ microsphere, indicating that the urchin-like microsphere with a diameter of ∼2 μm is constructed by a lot of ultrafine nanoneedles. As seen from the magnified TEM image (Fig. 4b), the diameter of nanoneedles is estimated to be ∼20 nm on average. The HRTEM image (Fig. 4c) displays a porous nature and shows an interplanar spacing of 0.287 nm, corresponding to the (220) plane of the NiCo₂O₄. The corresponding selected area electron diffraction (SAED) pattern (Fig. 4d) shows diffuse rings which could be well indexed to the (220), (311), (222), (400) and (511) planes of the NiCo₂O₄ crystal structure (JCPDS no. 20-0781), indicating the polycrystalline nature of the urchin-like NiCo₂O₄.

Fig. 4 (a) Low magnification and (b) medium magnification TEM images of the U-NiCo₂O₄; (c) HRTEM image of the U-NiCo₂O₄ and (d) corresponding SAED pattern.

The specific surface area and porosity of all the four NiCo₂O₄ samples were examined by BET at 77.4 K. Nitrogen adsorption/desorption isotherms of the samples (T-NiCo₂O₄, F-NiCo₂O₄, U-NiCo₂O₄ and P-NiCo₂O₄) and the corresponding pore size distributions calculated by BJH method from the desorption branch are shown in Fig. 5. These type IV isotherms with a small hysteresis loops observed at a relative pressure of 0.5–1.0 for four samples indicate the mesoporous structure.³⁷ The specific BET surface areas of T-NiCo₂O₄, F-NiCo₂O₄, U-NiCo₂O₄ and P-NiCo₂O₄ samples are 80.18, 18.93, 86.86, 61.18 m² g⁻¹, and their average pore sizes are 10.72, 12.45, 7.87, and 10.06 nm, respectively. U-NiCo₂O₄ and T-NiCo₂O₄ have the larger specific surface area than other two samples, which can provide more active sites for redox reaction, more channels and shorter distance for electrolyte ion diffusion, and improve the capacitive performance.³⁸ Therefore, the U-NiCo₂O₄ and T-NiCo₂O₄ are expected to achieve better electrochemical performance.

Fig. 5 BET results of T-NiCo₂O₄ (a), F-NiCo₂O₄ (b), U-NiCo₂O₄ (c) and P-NiCo₂O₄ (d). The insets show the corresponding pore size distributions, respectively.

3.2. Electrochemical analysis

The electrochemical measurements were conducted in a three-electrode configuration with 2 M KOH aqueous solution as the electrolyte. Fig. 6a illustrates the comparison of CV curves of four samples at a scan rate of 30 mV s⁻¹ in a potential window of 0–0.6 V. A pair of distinct redox peaks around 0.25–0.4 V and 0.4–0.55 V can be observed clearly for all the electrodes, which is mainly attributed to the faradaic redox reactions related to M–O/M–O–OH (M refers to Ni or Co) with the aid of OH⁻ anions.^39,40 The relevant redox reactions occurred on the surface of electrodes in the alkaline electrolyte could be expressed as follows:³⁵

NiCo₂O₄ + OH⁻ + H₂O ↔ NiOOH + 2CoOOH + e⁻ (10)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (11)

Fig. 6 (a) CV curves of the T-NiCo₂O₄, F-NiCo₂O₄, U-NiCo₂O₄ and P-NiCo₂O₄ at a scan rate of 20 mV s⁻¹; (b) CV curves of the urchin-like NiCo₂O₄ electrode at different scan rates; (c) GCD curves of different NiCo₂O₄ at a current density of 2 A g⁻¹; (d) GCD curves of the urchin-like NiCo₂O₄ electrode at different current densities; (e) specific capacitances versus current densities of different NiCo₂O₄; (f) cycling performance of the different NiCo₂O₄ electrodes at high current density of 10 A g⁻¹.

Apparently, the U-NiCo₂O₄ shows a larger CV curve area and current density than the other three the electrodes, demonstrating that the urchin-like architecture has a higher capacitance. Fig. 6b shows the typical CVs of the as-prepared urchin-like NiCo₂O₄ at the scan rates ranging from 10 to 100 mV s⁻¹. Obviously, with the increasing of scan rates, the position of the anodic and cathodic peaks shift toward the positive and negative potential respectively, which are mainly related to the internal resistance of the electrode.¹⁸

Fig. 6c shows the typical galvanostatic charge–discharge curves of the as-prepared four electrodes at the discharge current density of 2 A g⁻¹. The U-NiCo₂O₄ shows much longer discharge time than other three NiCo₂O₄ electrodes, this result is in good agreement with the CV measurement. The GCD curves of the U-NiCo₂O₄ electrode at different current densities of 1, 2, 3, 5, 10 and 20 A g⁻¹ are shown in Fig. 6d. The specific capacitances (C) of electrodes were calculated according to the following equation.²⁴

where I (A) is the discharge current, m (g) is the mass of electrode material, Δt (s) is the discharge time, and Δv (V) is the potential drop during discharge. The specific capacitances at various current densities are calculated based on the GCD curves of as-prepared four NiCo₂O₄ samples, and the results are shown in Fig. 6e. It is obvious to see that the U-NiCo₂O₄ shows highest specific capacitances of 436.1, 428.8, 420.2, 403.5, 366.8 and 306.6 F g⁻¹ at the current densities of 1, 2, 3, 5, 10 and 20 A g⁻¹, respectively. The second-highest capacitances of 372.9, 372, 370.5, 365.2, 353.4 and 340.7 F g⁻¹ at the same current densities are observed from T-NiCo₂O₄, while the capacitances of the other two samples (F-NiCo₂O₄ and P-NiCo₂O₄) are lower than that of U-NiCo₂O₄ and T-NiCo₂O₄. The good electrochemical performance of the U-NiCo₂O₄ with a large specific capacitance 436.1 F g⁻¹ (1 A g⁻¹) and an excellent capacitance retention of 70.3% (when the current density is increased from 1 to 20 A g⁻¹) should be credited to its unique structure with large specific surface area and porous texture.

The four NiCo₂O₄ electrode materials are conducted to prolonged GCD tests at a high current density of 10 A g⁻¹ for 1000 cycles, as shown in Fig. 6f. No distinct capacitance decay is observed for all the four samples, indicating their good cycling ability. Especially for the U-NiCo₂O₄, the specific capacitance increases gradually in the process of 1000 cycles, which can be attributed to the activation of the electrode. During this process, the electrode will be activated through the gradual infiltration of the electrolytic solution into the bulk structure and the intercalation/de-intercalation of ions through some circulations, resulting in the increase of active points inside the electrode materials, hence enhancing the specific capacitance.^19,41,42 Fig. 7a–d show the SEM images of T-NiCo₂O₄, F-NiCo₂O₄, U-NiCo₂O₄ and P-NiCo₂O₄ electrode materials after charging–discharging for 1000 cycles at a current density of 10 A g⁻¹, respectively. As can be seen, the morphologies and structures of the NiCo₂O₄ electrode materials are overall preserved with little structure deformation, except for the increased roughness on the nanosheet/nanoneedle surface, caused by the high-rate redox reactions during the repeated charge–discharge processes.^26,43

Fig. 7 SEM images of the NiCo₂O₄ electrodes after 1000 cycles: T-NiCo₂O₄ (a), F-NiCo₂O₄ (b), U-NiCo₂O₄ (c), and P-NiCo₂O₄ (d).

To further investigate the electrochemical behavior of the four NiCo₂O₄ electrodes, the corresponding EIS were carried out and shown in Fig. 8. The inset of Fig. 8 shows the enlarged view of high-frequency region and equivalent circuit diagram, where C_dl is a constant phase element accounting for a double-layer capacitor and C_ps is a pseudocapacitive element. The impedance spectra are composed of a semicircle in the high-frequency region and a nearly straight line in the low-frequency region. The intercept of plot on the real axis in the high frequency range represents the solution resistance (R_s) which is composed of the intrinsic resistance of the active material, ionic resistance of electrolyte, and the contact resistance between the electrode and current collector interface.⁴⁴ Faradaic charge transfer resistance (R_ct) which is related to the electrolyte accessible area of the electrode can be calculated from the diameter of semicircle in the high frequency range. The slope of the impedance plot in the low frequency range corresponds to the Warburg impedance (W), which represents the diffusive resistance. It can be seen that the phase angle for the impedance plot of the four samples are higher than 45° in the low frequencies, demonstrating that the electrochemical capacitive behavior of the four electrode is not controlled by diffusion process.²¹ In addition, the solution resistance, charge transfer resistance and Warburg impedance of U-NiCo₂O₄ electrode were observed to be lowest, explaining the optimal performance of the U-NiCo₂O₄ sample among the four NiCo₂O₄ electrode materials. These EIS results indicate the easy penetration of the electrolyte within the electrode and the efficient transport pathway for both electrons and ions which may be attributed the unique 1D nanoneedle-like structure of U-NiCo₂O₄.²⁹

Fig. 8 Impedance Nyquist plots of the different NiCo₂O₄ electrodes (T-NiCo₂O₄, F-NiCo₂O₄, U-NiCo₂O₄ and P-NiCo₂O₄). The inset is the enlarged view of high-frequency region and the corresponding equivalent circuit diagram.

Among the four NiCo₂O₄ electrode materials, U-NiCo₂O₄ exhibits more outstanding electrochemical performance which may be attributed to multiple contributing factors. Firstly, the U-NiCo₂O₄ with large specific surface area can facilitate the complete contact and faradaic reaction between the active material and electrolyte. Secondly, the porous texture of U-NiCo₂O₄ can accelerate the penetration and the intercalation/deintercalation of electrolyte ions within the electrode. Thirdly, the hierarchical urchin-like architecture constructed by 1D nanoneedles is advantageous in shortening transport path of ion and electron. Because of these structural features, the U-NiCo₂O₄ with the superior electrochemical performance over the other three NiCo₂O₄ structure electrodes has been achieved.

4. Conclusions

Just by simply adjusting the kinds of alkali source and the addition of NH₄F, four types of NiCo₂O₄ electrode materials with different morphologies were synthesized by a simple hydrothermal route. Among them, the U-NiCo₂O₄ exhibits the optimal electrochemical performance with a high capacitance value 436.1 F g⁻¹ at the current density of 1 A g⁻¹ as well as excellent capacitance retention of 115% (1000 cycles). The results show that the morphologies which can be controlled by adjusting synthesis conditions could lead to different electrochemical properties. This investigation also provides a reference for the rational design and synthesis of electrode materials with desirable morphology for the next-generation energy storage devices.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51302328), the Fundamental Research Projects for Central Universities (No. 106112016CDJCR131203), and supported in part by the Fundamental Research Funds for the Central Universities (No. 106112015CDJXY130006).

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