In situ synthesis of Ni_xCo_yO_z–C composites with rod-like Ni@C as support for potential application in supercapacitors

Abstract

A novel Ni_xCo_yO_z–C composite has been synthesized by a one pot in situ hydrothermal method with rod-like Ni@C (Ni nanoparticles wrapped by C) as support. The structure and morphology of the composites are characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. As supercapacitor electrodes, the as-prepared composites present good electrochemical performance with a high specific capacitance (975 F g⁻¹ at the current density of 1.0 A g⁻¹) and good cycling stability (79% of capacitance still maintained after 1000 cycles). The electrochemical performance is mainly due to the active redox reaction provided by the polytype metal oxide and the faster electron transfer supplied by the conductive carbon support. These results indicate that Ni_xCo_yO_z–C composites have promising applications as supercapacitor electrode materials.

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Introduction

With the development of industrialization, both energy resource consumption and environmental pollution stress are evidently increasing. The increased demand for environmentally friendly resources has given rise to concern. Supercapacitors, advanced energy storage devices, are considered as promising systems for higher power energy storage.^1,2 In addition, supercapacitors possess stable cycling performance, short charging time and fast recharging capabilities. Usually, supercapacitors can be divided into two categories: one is electrical double-layer supercapacitors^3,4 and the other is Faradaic redox reaction pseudocapacitors.⁵ Carbon materials are often used in electrical double-layer supercapacitors because of their large specific surface area, long cycling lives, and high power density. However, carbon based supercapacitors generally exhibit low capacitances. Pseudocapacitors possess relatively high theoretical capacitance, while their shortcomings are inferior rate performance, poor cycle stability and low utilization rate.⁶ Therefore, combining carbonaceous materials (such as carbon nanotubes,^7–10 graphene,^11,12 carbon nanofibers,^13,14 porous carbon materials^15–18) with pseudocapacitive materials to enhance the performance of supercapacitors has attracted wide research interest.

Nickel- and cobalt-based metal oxides and composites have received tremendous attention over the past decades due to their potential application in the field of supercapacitors. For example, 3D graphene–Co₃O₄ composites,¹⁹ mesoporous NiO network-like hierarchical microspheres,²⁰ spinel nickel cobaltite aerogels²¹ and nickel cobaltite binary metal oxides²² have been widely investigated. Recently, several nickel based hydroxide materials, exhibiting high electrochemical performances and controllable morphology, have been fabricated.^23–25 NiO nanosheet nanotubes have also been synthesized by a simple and economical route using polymeric nanotubes (PNT) as template. However, the capacitor performance of single nickel or cobalt metal oxides cannot meet the requirement of practical applications. The hybridization of different kinds of transition metal oxides with carbonaceous materials has gradually been considered as a promising and effective method because of their low cost, environmental friendliness and high supercapacitor performance.^26–28 Improving the uniform distribution of metal oxide in carbon materials, controlling the morphology of metal oxide at the nanometer scale, and simplifying the complex process is also emphasized in research.

In this study, we developed an efficient and simple one pot hydrothermal approach to synthesize Ni_xCo_yO_z and carbon composites using Ni@C nanorods as support, which is different from the general NiCo₂O₄ synthesis method using Ni⁺ as the nickel source.^21,22 With different amounts of reaction reagents, NiO–C and NiO/NiCo₂O₄–Co₃O₄/C composites were synthesized. Electrochemical measurements indicate that the C–NiO/NiCo₂O₄–Co₃O₄ composite exhibits an improved specific capacitance and good cycle performance. Good electrochemical properties may be attributed to the active redox reaction provided by the polytype metal oxide and the conductive network of the carbon support.

Experimental methods

Preparation of the precursor, rod-like Ni@C

All the chemical reagents in the experiment were of analytical grade and used without further purification. Scheme 1 shows the synthesis process of Ni_xCo_yO_z–C composites. In a typical process, dimethylglyoxime (0.278 g) was dispersed in 24 ml ethanol and the pH was adjusted to 13.0 with 0.5 M NaOH. Then, dimethylglyoxime solution was added dropwise to 800 ml DI water containing 0.521 g NiCl₂·6H₂O under vigorous agitation. The obtained red solution was filtrated and a red sediment was obtained.²⁹ The red sediment was carbonized in argon atmosphere at 350 °C for 0.5 h and the Ni@C complex was obtained. The weight percentage of Ni in the Ni@C complex is calculated to be 53%, according to the molecular weight of dimethylglyoxime.

Scheme 1 Preparation process of the Ni@C and Ni_xCo_yO_z–C composites.

Preparation of the Ni_xCo_yO_z–C composites

For the synthesis of Ni_xCo_yO_z–C composites, Ni@C complex, urea and 30 ml ethanol were mixed and transferred to a 50 ml Teflon-lined steel autoclave. The autoclave was sealed and kept at 120 °C for 6 h. After filtration and drying, the precursors were calcined at 300 °C in atmosphere for 2.5 h to obtain the NiO–C composites (named sample 1). Furthermore, the different kinds of Ni_xCo_yO_z–C composites (sample 2 and 3) were synthesized using the same synthetic conditions as sample 1 but with different CoCl₂·6H₂O volumes. Specifically, the concentration of CoCl₂·6H₂O was added according to the amount of Ni@C, and the ratios of Ni and Co were Ni–Co = 1 : 2 (sample 2) and Ni–Co = 1 : 4 (sample 3).

Characterization

The structure of Ni@C and Ni_xCo_yO_z–C composites were examined by X-ray diffraction (XRD; Bruker D8 Advance) with Cu-Kα radiation (λ = 1.5418 Å) operating at 40 kV. The morphologies of Ni@C and Ni_xCo_yO_z–C composites were characterized by field emission scanning electron microscopy (FESEM; Hitachi, S-4800, Japan) and transmission electron microscopy (TEM, JEOL JEM-2010), respectively. The nitrogen adsorption–desorption isotherm was characterized on an ASAP (Micromeritics, USA). Specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method.

Electrochemical measurements

Electrochemical measurements were carried out with an electrochemical workstation (CHI 760D, CH instrument Inc, China) at room temperature. A conventional three-electrode system was employed with a saturated calomel electrode (SCE), a Pt plate counter electrode and a working electrode of active material in 6 M KOH aqueous electrolyte solution. The working electrode was prepared by coating Ni foam with the Ni_xCo_yO_z–C composite at appropriate weight ratios of the conductive agent (acetylene black) and polyvinylidene fluoride (PVDF). Here, the electrochemical property of Ni foam can be ignored because it is negligible compared with that of the samples.³⁰ The mass ratio of the Ni_xCo_yO_z–C composite, acetylene black and PVDF was 8 : 1 : 1, and the mass loading of active materials on Ni foam were 0.64 mg, 0.85 mg and 0.56 mg, respectively.

Results and discussion

Characterization of Ni_xCo_yO_z–C

Fig. 1(a) shows the XRD pattern of Ni@C complex. Identical diffraction peaks at 44.4, 51.8 and 76.3 can be attributed to the phase Ni. Fig. 1(b) shows the XRD pattern of sample 1. It can be seen that there present several peaks, which are in good accordance with the standard NiO phase (JCPDS 47-1049). The lattice parameters are a = 4.177 Å, b = 4.177 Å, c = 4.177 Å. When the reagent containing Co (the ratio of Ni–Co = 1 : 2) was added to the system, the composite of sample 2 was fabricated. From the XRD pattern of Fig. 1(c), it can be concluded that the as-fabricated sample 2 can be identified to the composites of NiO (JCPDS 47-1049), NiCo₂O₄ (JCPDS 20-0781)³¹ and Co₃O₄ (JCPDS 42-1467). The peaks at 31.3°, 36.7°, 38.5°, 44.9°, 55.5°, 59°, 65.2°, 68.3° (2θ) can be indexed to the (220), (311), (222), (400), (422), (511), (440), (531) faces of the NiCo₂O₄ phase, respectively. The lattice parameters of NiCo₂O₄ are a = 8.110 Å, b = 8.110 Å, c = 8.110 Å, while weak peaks at 74°, 77.2° and 78.3° (2θ) correspond to Co₃O₄. The lattice parameters are a = 8.084 Å, b = 8.084 Å, c = 8.084 Å. It can be seen that the main peaks correspond well to NiCo₂O₄ while the subsidiary peaks correspond to Co₃O₄. With a twofold increase in Co reagent (the ratio of Ni–Co = 1 : 4, sample 3), the XRD pattern of sample 3 is similar to sample 2. However, the peak intensity of the NiCo₂O₄ phase increases significantly, indicating the increase of NiCo₂O₄ content in the composite (Fig. 2(d)). It can be concluded from XRD patterns that samples 2 and 3 are composites of C–NiO/NiCo₂O₄–Co₃O₄ with different contents of Co and Ni oxide.

Fig. 1 XRD patterns of (a) Ni@C, (b) sample 1, (c) sample 2, and (d) sample 3.

Fig. 2 (a) SEM image of Ni@C complex. (b) SEM image of sample 1. (c) SEM image of sample 2 and (d) SEM image of sample 3.

Fig. 2(a) shows the SEM image of Ni@C. It can be observed that the Ni@C complex presents a nanorod structure with a smooth surface and a diameter of around 100–200 nm. After the formation of Ni_xCo_yO_z–C composite, SEM images indicate that the surface of the nanorods become rough, and metal oxide particles can be clearly observed, as shown in Fig. 2(b–d). In addition, the diameter of the nanorods increases to about 200–300 nm. SEM images indicate that Ni in the Ni@C composite can directly react with CoCl₂·6H₂O to obtain the Ni_xCo_yO_z–C composite, which can uniformly disperse the active material in the carbon support and prevent it from dropping off during the electrochemical measurement. In addition, the nanorod-like carbon support can prevent the aggregation of active material and significantly increase the specific surface area of the composite.

To provide further insights into the morphology and crystal structure of the Ni_xCo_yO_z–C composites, TEM investigations were carried out. Fig. 3(a) shows the typical TEM image of sample 3. It indicates that the sample still consists of nanorod-like structures with metal oxide particles mixed together. The diameter of the nanorod composite is around 200 nm. Fig. 3(b) is the HR-TEM image of a portion of the composite, demonstrating its crystal structure. The clear crystalline lattice with different interplane spacing can be observed. As reported, the lattice distances of Co₃O₄ and NiCo₂O₄ are nearly the same, which are difficult to recognize.^32,33 Two different directions of the lattice distance of 0.240 nm may correspond to the (311) faces of Co₃O₄ and spinel NiCo₂O₄. At the same time, two similar lattice distances of 0.207 nm and 0.212 nm corresponding to the (200) face of NiO (ref. 34) can also be observed, suggesting that the composite contains Co₃O₄, NiCo₂O₄ and NiO crystalline structures.

Fig. 3 (a) TEM and (b) HR-TEM images of Ni_xCo_yO_z–C composite (sample 3).

The energy dispersive X-ray (EDX) mappings of Ni_xCo_yO_z–C composites (sample 3) are shown in Fig. 4. It can be seen that the strong signals of carbon, nickel, cobalt, and oxygen are similar to spatial distribution in the backbone region, suggesting that the metal oxide distributed uniformly in the Ni_xCo_yO_z–C composite.

Fig. 4 (a) EDX spectrum and elemental mapping images of (b) carbon, (c) cobalt, (d) nickel, and (e) oxygen.

In general, specific surface area plays an important role in the electrochemical properties of the electrode materials. Nitrogen adsorption–desorption isotherms of Ni_xCo_yO_z–C composites are shown in Fig. 5(a–c). These isotherm profiles exhibit Langmuir type IV characteristics with an obvious hysteresis loop, indicating the characteristics of mesoporous materials. The hysteresis loop for the samples belongs to Type H3 with a slit-shaped pore. The BET specific surface areas of these three samples are measured to be 26.5 m² g⁻¹, 64.6 m² g⁻¹ and 42.8 m² g⁻¹ for samples 1, 2 and 3, respectively. It indicates that C–NiO/NiCo₂O₄–Co₃O₄ possesses a relatively higher specific surface area than that of C–NiO. It reveals a better specific surface area contrasting with others reported for NiCo₂O₄.³¹ It can be concluded that the existence of carbon support is useful for the formation of metal oxide composites with high specific surface area. The Barrett–Joyner–Halenda (BJH) pore size distribution obtained from the desorption branch of the isotherm is shown in Fig. 5(d) and (f).

Fig. 5 Nitrogen adsorption–desorption isotherms and pore-size distribution calculated by the BJH method of (a and c) sample 1, (b and e) sample 2, and (c and f) sample 3, respectively.

Electrochemical properties

To determine the electrochemical performance of the three samples, electrochemical capacitance was investigated. Fig. 6(a) shows the cyclic voltammetry (CV) curves of the three samples tested at a rate of 50 mV s⁻¹ in a 6 M KOH electrolyte. Because the area surrounded by the CV curve represents its capacity ability, it is noted that the electrochemical performance of C–NiO/NiCo₂O₄–Co₃O₄ (sample 2 and 3) is far higher than that of C–NiO (sample 1), indicating the high electrochemical performance of C–NiO/NiCo₂O₄–Co₃O₄. Fig. 6(b) shows the discharge curves of the three samples at a current density of 1 A g⁻¹. The corresponding specific capacitance C_s (F g⁻¹) was calculated from the following equation:³⁵

C_s = IΔt/mΔV

where I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the mass of the active material and ΔV (V) is the voltage change for the charge–discharge process. According to the equation, the specific capacitances of samples 1, 2, and 3 are calculated to be 101, 757, and 975 F g⁻¹, respectively, at a current density of 1 A g⁻¹. Therefore, it can be concluded that the electrochemical performance of sample 3 is better than the other two composites because of an increase in the amount of NiCo₂O₄, as shown by the XRD pattern. The discharge results are in accordance with the CV curves, which further indicate the excellent electrochemical performance of sample 3. This phenomenon may be attributed to the fact that the one-dimensional rod-like carbon network can enhance the kinetics of ions as well as electron transport both inside the electrodes and at the electrode/electrolyte interface.²⁹ In addition, multiple oxidation states provided by the binary oxide and synergistic effects offered by the carbonous material and polytype metal oxides are other key factors.³⁶

Fig. 6 (a) CV curves of sample 1, 2 and 3 at a scan rate of 50 mV s⁻¹. (b) The charge–discharge curves of sample 1, 2, and 3 at a current density of 1 A g⁻¹.

Fig. 7(a) shows the CV curves of sample 3 at different scan rates. A distinct pair of redox peaks can be clearly observed during the anodic and cathodic sweeps, suggesting that the capacitive characteristics are controlled by the Faradaic reaction. The relevant redox reactions of Ni and Co can be described as follows:^37–39

Ni_xCo_yO_z + (3x + 3y − 2z)OH⁻ + (z − x − y)H₂O ↔ xNiOOH + yCoOOH + (3x + 3y − 2z)e⁻.

Fig. 7 Electrochemical performance of sample 3. (a) CV curves at different scan rate. (b) Charge–discharge curves at different current densities. (c) Cycling stability at a current density of 8 A g⁻¹. The inset shows the Coulombic efficiency and the galvanostatic charge–discharge curves at a current density of 8 A g⁻¹ (d) EIS curves before and after 1000 charge–discharge cycles.

Furthermore, it can be observed that the current of redox peaks increases with the increasing scan rate because the rates of electronic and ionic transport are rapid enough in the applied scan rates.⁴⁰ Meanwhile, the anodic and cathodic peaks shift toward higher and lower potential, respectively. Fig. 7(b) presents the galvanostatic discharge curves. The specific capacitance values of sample 3 can be calculated from the discharge curves to be 975, 795, 671 and 427 F g⁻¹ at current densities of 1, 2, 4 and 8 A g⁻¹, respectively. The cycling performance of sample 3 was systematically investigated at a current density of 8 A g⁻¹, and is shown in Fig. 7(c). The inset of Fig. 7 indicates that sample 3 has good efficiency for each charge and discharge cycle. After several charge–discharge cycles, the galvanostatic charge–discharge curves still remain symmetric. After 1000 cycles, as much as 79% of the initial capacitance can be maintained. The measured Coulombic efficiency is around 98% (Fig. 7(c)). High Coulombic efficiency is because of the reversibility of the material, which is consistent with the CV curves. Compared with the previous studies of Co₃O₄ (ref. 33) and NiO,³⁴ the capacitance of sample 3 is better than that of pure NiO or Co₃O₄. Good cycling performance may be attributed to the synergistic effect of carbonous material and multiple kinds of oxides. In addition, carbonous material also provides electronically conductive channels for electrons to enlarge the interface between the composites and electrolyte.⁴¹ The special combination between the carbonous material and metal oxides can also tightly attach oxides to the electrode, significantly improving the stability of the electrodes.

To demonstrate the evolution of the electrochemical performance of the Ni_xCo_yO_z–C composite (sample 3) electrode, electrochemical impedance spectroscopy (EIS) was measured before and after 1000 charge–discharge cycles, as shown in Fig. 7(d). Vertical curve in the low-frequency range indicates the better capacitive behavior of the supercapacitor assembly.⁴² Simultaneously, similar EIS curves before and after discharge cycling suggest the good stability of the Ni_xCo_yO_z–C composite electrode. The high stability of the electrode may be attributed to active reversible redox reactions with metal oxides along with better electrical conductivity provided by the carbon network support.⁴³

The theoretical proportion of Ni_xCo_yO_z (82%) was calculated by 53% of Ni in Ni@C. The thermogravimetric analysis (TGA) curve is shown in Fig. 8. The result indicates that about 18.3% of weight is lost up to 800 °C in air. It suggests that the Ni_xCo_yO_z–C composite contains 18.3% carbon, which corresponds to our theoretical data.

Fig. 8 Thermogravimetric Analysis (TGA) curve of sample 3.

Conclusions

In summary, a carbon based polytype metal oxides composite (Ni_xCo_yO_z–C) was successfully synthesized with Ni@C as a support using a simple hydrothermal method. The as-prepared composites possess a rod-like morphology, large specific surface area, high conductivity and a uniform dispersion of the metal oxides. Electrochemical characterizations indicate that the Ni_xCo_yO_z–C composite is a good electroactive material for high performance supercapacitors. The specific capacitances of the Ni_xCo_yO_z–C composite can reach 975 F g⁻¹ at a current density of 1 A g⁻¹, and there is 79% of the original capacitance is retained after 1000 cycles at 8 A g⁻¹. Good electrochemical performance may result from the active redox reaction provided by the polytype metal oxide and the conductive carbon network coming from the Ni@C support. This work suggests promising applications for supercapacitors and inspires extensive research into the combination of carbonous material and multimetal oxides.

Acknowledgements

The project was supported by the Jiangsu Provincial Founds for Distinguished Young Scholars (BK20130046), the National Basic Research Program of China (2012CB933300), the NNSF of China (21275076, 61328401), the Key Project of Chinese Ministry of Education (2012058), Program for New Century Excellent Talents in University (NCET-13-0853), Research Fund for the Doctoral Program of Higher Education of China (20123223110008), Synergetic Innovation Center for Organic Electronics and Information Displays Qing Lan Project, Ministry of Education of China (IRT1148) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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