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DOI: 10.1039/C5RA03774C (Paper) RSC Adv., 2015, 5, 48181-48186

Synthesis of a flower-like Co-doped Ni(OH)2 composite for high-performance supercapacitors†

Qi Wang, Suwen Liu*, Haiyan Sun and Qifang Lu
School of Material Science and Engineering, Qilu University of Technology, Jinan, 250353, P. R. China. E-mail: liusw@qlu.edu.cn; Fax: +86-531-89631227; Tel: +86-531-89631231

Received 3rd March 2015 , Accepted 18th May 2015

First published on 18th May 2015

Abstract

Flower-like architectures of a Co-doped Ni(OH)2 composite were prepared using Ni(NO3)2·6H2O, Co(NO3)2·6H2O and CO(NH2)2 as raw materials via a hydrothermal process. The as-prepared sample was characterized in detail by XRD, SEM, FT-IR and other techniques. The XRD and SEM analysis indicate the microarchitecture was accumulated by Ni(OH)2 nanorods with good crystallinity. The electrochemical properties of the Co-doped Ni(OH)2 composite was also investigated, and a three electrode system was used to test the electrochemical properties and the Co-doped Ni(OH)2 composite shown higher activity than pure Ni(OH)2 which was obtained by Ni(NO3)2·6H2O and CO(NH2)2 as raw materials via a hydrothermal process. The electrochemical properties could be seen in the CV curves, galvanostatic discharge curves and the specific capacitance based of the CV curves and discharge curves.

1. Introduction

Supercapacitors or electrochemical capacitors have attracted considerable attention in recent years because they can provide instantaneously a higher power density than batteries and higher energy density than conventional dielectric capacitors.1–4 And supercapacitors are environmentally friendly.5,6 Advanced supercapacitors must be developed with higher operating voltages and higher energy without sacrificing the power delivery and cycle life to meet the energy demands for practical applications in the future.7 So they have been regarded as one of the most promising energy storage devices in practical use. Electrode materials widely used in supercapacitors are carbon, conducting polymers and metal oxides/hydroxides. Supercapacitors using conducting polymers or metal oxides/hydroxides are pseudocapacitors (PCs). Their capacitances arise from electrochemical adsorption of ions or redox reactions. Faradaic pseudocapacitance usually occurs at or near the electrode surface and the processes are usually fast and reversible.5 However, most of the oxide materials are not suitable for industrial use because of their high cost.

Recently, Ni(OH)2 has been considered as an attractive candidate in supercapacitor because of its high theoretical specific capacitance and excellent redox behavior.8 Besides, Ni(OH)2 is environmentally friendly and inexpensive9,10 Ni(OH)2 has two phases: α- and β-Ni(OH)2.5 In α-Ni(OH)2, there are many exchangeable anions and water molecules intercalated into the interlayer. So the interlayer space of α-Ni(OH)2 is larger than the β-Ni(OH)2, which endows it better electrochemical activity. In addition, α-Ni(OH)2 transforms to γ-NiOOH after the charging process, in which the oxidation state of α-Ni(OH)2 is about 3.3 to 3.7.11 Whereas in β-NiOOH (the charged state of β-Ni(OH)2), the oxidation state of Ni is about 2.9. That is to say, in the electrochemical process, the α-Ni(OH)2/γ-NiOOH pair can deliver more electrons than β-Ni(OH)2/β-NiOOH. So α-Ni(OH)2 usually shows better electrochemical performance than β-Ni(OH)2. However, the α-Ni(OH)2 phase is very hard to prepare because it is unstable and can easily transform to β-Ni(OH)2.

Cobalt is the earliest and most be researched as the additive. It is generally believed that by codeposition method, Ni(OH)2 which has adding cobalt exists in the form of Ni1−xCox (OH)2 solid solution. And Co could replace the position of part Ni and existing as Co3+ in the process of the whole cycle, which greatly increases the number of the cationic impurity defects in the lattice defects of Ni(OH)2/NiOOH. The existence of defects can increase the protons (H+) in and out of the degrees of freedom in the process of charging and discharging, and improve the reversibility of nickel electrode reaction.

Materials with the 3D hierarchical structures can significantly shorten the diffusion lengths for both electrolyte ions and electrons. Considering that most of the reactions of supercapacitors occur at or near the surface of electrodes, materials with high surface area and suitable pore sizes (2–5 nm) usually display higher discharge specific capacitance because it is easily immersed by electrolytes.12 Besides, the proton transfer of electrolytes within the pores can be eased. As the reaction speed of charge/discharge is rapid in supercapacitors, these structures are favorable for improving the electrochemical utilization of active materials.13

In this paper, flowers-like architectures of the Co-doped Ni(OH)2 composite were synthesized through a simple hydrothermal method with Ni(NO3)2·6H2O, Co(NO3)2·6H2O and CO(NH2)2 as raw materials. The Ni(OH)2 can be obtained by Co(NO3)2·6H2O and CO(NH2)2 as raw materials via a hydrothermal process. The novel nanostructure of the product exhibits better electrochemical properties than the pure Ni(OH)2.

2. Experimental section

2.1. Synthesis

Synthesis of Co-doped Ni(OH)2. All reagents were of analytical grade and were used without further purification. In a typical synthesis, 1.745 g Ni(NO3)2·6H2O, 1.746 g Co(NO3)2·6H2O and 2.5 g CO(NH2)2 were dissolved into deionized water under magnetic stirring to form a transparent solution. Then the solution was transferred to a teflon-lined stainless steel autoclave and heated to 150 °C and maintained at that temperature for 5 h. Subsequently, the autoclave was cooled to room temperature naturally. After that, the precipitate was collected by centrifugation, washed with deionized water and ethanol for 3 times and dried at 70 °C for 12 h in air to obtain the final product. Synthesis of Ni(OH)2. With the same procedural as synthesis above, 1.745 g Ni(NO3)2·6H2O and 2.5 g CO(NH2)2 were used to prepare pure Ni(OH)2.

2.2. Characterization

The crystal structure of the product was determined on an X-ray diffractometer (XRD, Rigaku D/Max 2200 PC) with a graphite monochromator and CuKα radiation (λ = 0.15418 nm). The morphology of the product was determined using a field emission scanning electron microscope (FE-SEM, TOSHIBA S4800) and transmission electron microscope (TEM, JEM 1011). Fourier transform infrared spectroscopy (FT-IR) was carried out on an IFS66 V/S & HYPERION 3000 spectrometer (Bruker Optics, Germany) in the range 400–4000 cm−1. The electrochemical properties were measured using an electrochemical workstation (Shanghai CHI660E).

2.3. Electrochemical properties measurement

A three electrode system was used. Hg/HgO electrode was used as reference electrode. Platinum wire was used as counter electrode. 6 M aqueous solution of KOH was used as electrolyte. The working electrode was prepared as follows: the material was first uniform mixed with acetylene black and PVDF (Polyvinylidene Fluoride) in a ratio of 8[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 by weight, and then made them paste adding the NMP (N-methyl-2-pyrrolidone); the paste was applied to a 1 cm × 1 cm Ni net and dried at 70 °C for 12 h, and then the Ni net was maintained at 10 MP force for 1 minute. The electrode mass of Co-doped Ni(OH)2 composite and pure Ni(OH)2 is 6 mg and 9 mg, respectively.

3. Material characterization

Typical XRD patterns of the flowers-like Co-doped Ni(OH)2 and pure Ni(OH)2 are shown in Fig. 1a. The XRD pattern of the Co-doped Ni(OH)2 composite is similar to that of pure Ni(OH)2, indicating that Co-doped Ni(OH)2 composite has been well synthesized. All of the reflections in the XRD pattern in Fig. 1a can be indexed to rhombohedral α-Ni(OH)2 with lattice parameters of a = b = 3.08 Å and c = 23.41 Å (JCPDS, no. 38-715), which is in good agreement with the reported pattern for Ni(OH)2.14 The four characteristic peaks at 12.1°, 24.6°, 33.3° and 59.4° correspond to the (003), (006), (101) and (110) diffraction planes, respectively. According to the Bragg formula, the calculated basal spacing is 0.73 nm, which is in accordance with the reported values.14,15 In addition, there are only one peak from other phases were detected indicating that the Co-doped Ni(OH)2 composite is of purity. And the XRD pattern of Co-doped Ni(OH)2 composite depict reflection at 2θ values of 19.5°, which can be indexed as (002) crystal planes of α-Co(OH)2 phase (JCPDS, no. 74-1057). Moreover, the relative intensity of the corresponding diffraction peaks for the Co-doped Ni(OH)2 composite is significantly decreased compared to those of the pure Ni(OH)2 sample, demonstrating a change in grain sizes of Ni(OH)2 particles influenced by the doped Co. The average crystalline sizes of the synthesized samples calculated from the (003) diffraction peak using Scherrer's formula are 15.9 and 18.0 nm for the pure Ni(OH)2 sample and the Co-doped Ni(OH)2 composite, which is consistent with the above XRD analysis.
image file: c5ra03774c-f1.tif
Fig. 1 XRD patterns (a) and FTIR spectrum (b) of the Ni(OH)2 and the Co-doped Ni(OH)2 composite.

To further confirm the XRD results, the as-prepared the Co-doped Ni(OH)2 composite was examined by Fourier transform infrared (FTIR) spectroscopy in the range of 400–4000 cm−1 and the results are shown in Fig. 1b. The band at 3435 cm−1 corresponds to the O–H vibration of hydrogen-bonded hydroxyl groups and intercalated water molecules located in the interlamellar space of Ni(OH)2.14,16 The very strong absorption band at 2250 cm−1 is the typical vibration of C[triple bond, length as m-dash]N triple bonds in the OCN− anions, which are the byproducts of urea hydrolysis.16 The weak band around 1637 cm−1 can be assigned to the bending mode of the interlayer water molecule. The absorption at around 1487 cm−1 is assigned to the C[double bond, length as m-dash]O in the carbonate ions. Additionally, the absorption band located at 1167 cm−1 is probably related to the presence of carbonate ions derived from the adsorption of atmospheric CO2 or hydrolysis of urea.16 Moreover, the band at 1383 and 831 cm−1 can be attributed to the interlayer nitrate anion, whereas the two bands around 639 and 484 cm−1 are ascribed to the δOH and νNi–OH vibrations, respectively.17,18 The bands at ca. 965 and 513 cm−1 are assigned to the stretching vibration of Co–OH bond.19 The strong peak at 652 cm−1 is assigned to the δOH of hydroxyl group.

The morphology and structure of Ni(OH)2 and Co-doped Ni(OH)2 composite were observed by SEM. Fig. 2a clearly shows that Ni(OH)2 sample is composed of many well-defined flowers-like architectures with diameters of ca. 3.9 μm, but the surface is not smooth with some flakes. The Co-doped Ni(OH)2 composite shows a three-dimensional flowers-like structure with considerable sticks on its surface (Fig. 2b), and its diameter is 2.7 μm. From the image of EDS (Fig. 2c), Co element was examined. The doped Co changed the morphology of the Ni(OH)2.


image file: c5ra03774c-f2.tif
Fig. 2 SEM image of the Ni(OH)2 (a) and Co-doped Ni(OH)2 composite (b) and the EDS of the Co-doped Ni(OH)2 composite (c).

TEM images of Co-doped Ni(OH)2 composite in Fig. 3b reveal that the three-dimensional flowers-like structure are made of nanorods. Compared with Ni(OH)2 (Fig. 3a), we can see that the nanorods composing the Co-doped Ni(OH)2 composite is longer. From the Fig. 3a, we just observe the surface of the Ni(OH)2 are made of fine suede.


image file: c5ra03774c-f3.tif
Fig. 3 TEM image of the Ni(OH)2 (a) and Co-doped Ni(OH)2 composite (b), the scale bars of the inserted image is 200 nm and 500 nm, respectively.

N2 adsorption–desorption isotherms of the sample (Fig. 4) show an obvious hysteresis loop in the relative pressure range of 0.01–1.0. The adsorption isotherm belongs to a Type IV hysteresis loop and a Type H2 hysteresis loop, according to Brunauer–Deming–Deming–Teller classification.20 From the corresponding BJH curve inserted in Fig. 4, the sample did not have a relatively narrow pore size distribution. The Co-doped Ni(OH)2 composite shows a BET surface area of 94 cm2 g−1, which is the 1.3 times than the Ni(OH)2 (Fig. S1†).


image file: c5ra03774c-f4.tif
Fig. 4 Typical N2 adsorption–desorption isotherms of the Co-doped Ni(OH)2 composite and pore size distribution curve obtained from the N2 desorption isotherm branch based on BJH.

4. Electrochemical properties

Cyclic voltammetry (CV) is generally used to characterize the capacitive behavior of an electrode material. Fig. 5b shows the typical CV curves of the Co-doped Ni(OH)2 composite at different scan rates in 6 M KOH aqueous solution. All the CV curves consist of a pair of strong redox peaks, indicating that the capacitance characteristics are mainly governed by Faradaic redox reactions, which is very distinct from that of electric double layer capacitors that usually produce a CV curve close to an ideal rectangular shape.13 The symmetric characteristic of the anodic and cathodic peaks indicates the excellent reversibility of the Co-doped Ni(OH)2 composite electrode. In addition, it can be seen that the shapes of these CV curves show almost no significant change as the scan rates increase from 1 to 50 mV s−1, implying the improved mass transportation, excellent electron conduction within the nanoparticles, and small equivalent series resistance. With increasing scan rates, the potential of the oxidation peak shifts in the positive direction and that of the reduction peak shifts in the negative direction, which is mainly related to the internal resistance of the electrode. However, the CV curves of the pure Ni(OH)2 (Fig. 5a) do not have obvious anodic peak, these CV curves show almost no significant change as the scan rates increase from 2 to 50 mV s−1. Fig. 5c shows the CV curves of the Ni(OH)2 and Co-doped Ni(OH)2 composite at a scan rate of 2 mV s−1. The anodic peak (positive current density) for the Ni(OH)2 and Co-doped Ni(OH)2 composite occurred around 0.54 V and 0.38 V (vs. Hg/HgO), respectively, indicates an oxidation process related to the oxidation of α-Ni(OH)2 to γ-NiOOH, whereas the cathodic peak (negative current density) observed around 0.29 V and 0.24 V (vs. Hg/HgO) corresponds to a reduction process following the Faradaic reactions of Ni(OH)2:21
 
α-Ni(OH)2 + OH− ↔ γ-NiOOH + H2O + e− (1)
image file: c5ra03774c-f5.tif
Fig. 5 CV curves of Ni(OH)2 (a) and Co-doped Ni(OH)2 composite (b) at various scan rates in 6 M KOH. CV curves of Ni(OH)2 and Co-doped Ni(OH)2 composite (c) at 2 mV s−1 scan rate. Specific capacitance of the Ni(OH)2 and Co-doped Ni(OH)2 composite (d) as a function of the scan rates based of the CV curves.

Compared with those for the pure Ni(OH)2 electrode, the oxidation and reduction potentials for the Co-doped Ni(OH)2 composite electrode shift remarkably to the negative direction, which suggests that the Co-doped Ni(OH)2 composite electrode can be charged more easily. It is probably due to the combination of the Co with the Ni(OH)2, which makes the electron transfer and the intercalation/deintercalation of electrolyte ions more easier. The potential difference between anodic and cathodic peaks for the Co-doped Ni(OH)2 composite electrode is obviously smaller than that for the pure Ni(OH)2 electrode, implying the higher reversibility of the Co-doped Ni(OH)2 composite electrode due to the potential difference usually used as an estimate of the reversibility of the electrochemical redox reaction.22

The specific capacitance of the electrode can be calculated from the CV curves according to the following equation:13

 
image file: c5ra03774c-t1.tif(2)
where C is the specific capacitance (F g−1) based on the mass of the electroactive materials, s is the potential scan rate (mV s−1), m is the mass (g) of the electroactive materials in the electrodes (g cm−2), V is the potential (V) and i is the response current density (A cm−2). The specific capacitance of our as-prepared pure Ni(OH)2 and Co-doped Ni(OH)2 composite electrodes at different scan rates in 6 M KOH is presented in Fig. 5d. It can be seen that the specific capacitance of the Co-doped Ni(OH)2 composite electrode is always higher than that of the pure Ni(OH)2 electrode at different scan rates. When the scan rate is increased to 50 mV s−1, the specific capacitance of the Co-doped Ni(OH)2 composite electrode retain 37% of its initial value, indicating the positive synergistic effect of Co and Ni(OH)2 in the composite. Moreover, the specific capacitance decreases gradually with increasing scan rate, which can be attributed to the diffusion effect limiting the diffusion and migration of the electrolyte ions within the electrode at high scan rates, resulting in low electrochemical utilization of the Ni(OH)2 particles.23 The theoretical specific capacitance of the Ni(OH)2 is 2082 F g−1.1 A high specific capacitance of the Co-doped Ni(OH)2 composite of 1807 F g−1 can be obtained at 2 mV s−1. Therefore, the high capacitance of the Co-doped Ni(OH)2 composite electrode can be ascribed to the synergistic effect of Co and Ni(OH)2. Firstly, Co can increase the proton conductivity, and improve the proton diffusion coefficient. Secondly, a part of Co replace the position of Ni, and Co in the process of the whole cycle exist as Co3+, therefore it greatly increased the number of cationic impurities defects in the Ni(OH)2 and NiOOH lattice. The existence of defects can be increased the degrees of the protons (H+) freedom in the process of charging and discharging, improving the reversibility of nickel electrode reaction. Because of the high performances of the Co-doped Ni(OH)2 composite, it is highly desirable to develop a negative electrode material with superior electrochemical performance to assemble asymmetric supercapacitors with a wider voltage range and thus higher energy density than each of the components.

Fig. 6 shows the galvanostatic discharge curves of the Ni(OH)2 and Co-doped Ni(OH)2 composite in 6 M KOH. All the discharging profiles in Fig. 6a and b present the similar feature: in the lower potential range the potential varies linearly with time, and in the higher potential range the potential changes nonlinearly with time. For example, at a current density of 0.5 A g−1, the charging profile for the Co-doped Ni(OH)2 composite electrode in Fig. 6b displays a nonlinear potential decrease in the potential range of 0.49–0.20 V and a linear one in the potential range below 0.20 V. Apparently, for the pure Ni(OH)2 and Co-doped Ni(OH)2 composite electrode materials prepared, the pseudocapacitance contributes the main part of the total capacitance, and the double-layer capacitance does a small part.


image file: c5ra03774c-f6.tif
Fig. 6 Galvanostatic discharge curves of the Ni(OH)2 (a) and Co-doped Ni(OH)2 composite (b) at different constant current densities. Variation of SC with discharging current density of the electrodes (c).

SC values of the electrodes were calculated according to the following equation:24

 
image file: c5ra03774c-t2.tif(3)
in which, Cs is the SC (Fg−1), I the constant current (A), t the discharge time (s), ΔV the total potential deviation (V), m the mass of the active material in an electrode. Variation of SC with discharging current density for the pure Ni(OH)2, Co-doped Ni(OH)2 composite electrodes was given in Fig. 6c. With increasing current density from 0.2 to 2 A g−1, SC value of the pure Ni(OH)2 electrode decreases from 1442.3 to 465.2 F g−1, showing a capacitance decay of 67%. However, SC value of the Co-doped Ni(OH)2 composite electrode decreases from 2126.6 to 1783.3 F g−1 when increasing current density from 0.2 to 5 A g−1, exhibiting a capacitance decay of 16%. Obviously, the Co-doped Ni(OH)2 composite electrode exhibits a much larger SC and higher rate capability, which is in agreement with the results of the CV curves.

Galvanostatic charge and discharge measurements were conducted to study the cycle life and rate performance of the Co-doped Ni(OH)2 composite at a scan rate of 10 A g−1 for 1000 cycles. Fig. 7 shows the capacitance retention ratio of the Co-doped Ni(OH)2 composite charged as a function of the cycle number. It is worth noting that after the initial 500 cycles, the sample displays a maximum discharge capacity. This activation process can be assigned to the nature of α-Ni(OH)2. Usually deeds an activation process to reach the maximum discharge capacity.25 On the other hand, the Co-doped Ni(OH)2 composite microspheres we produced are composed by nanorods. The interconnection between these nanorods may produce many macropores. It takes some time for the electrolyte to an improvement in the surface wetting of the electrode during extended cycling.26 After 1000 cycles, the sample displays an excellent long cycle life with only 5.8% deterioration of its initial specific capacitance, demonstrating superior long-term electrochemical stability.


image file: c5ra03774c-f7.tif
Fig. 7 Cyclic performance of the Co-doped Ni(OH)2 composite in 6 M KOH at the current density of 10 A g−1.

5. Conclusions

Flowers-like Co-doped Ni(OH)2 composite was successfully prepared through a simple hydrothermal reaction and its electrochemical properties were studied. The samples were consisted of nanorods, and flowers-like composite formed during the synthesis. The Co-doped Ni(OH)2 composite shows a BET surface area of 94 cm2 g−1, which is the 1.3 times than the Ni(OH)2. Therefore, it owns higher electrochemical properties, for there are the as-prepared flowers-like microarchitecture and the doped of Co. Cycle stability of flowers-like Co-doped Ni(OH)2 composite is good. After 1000 cycles, the sample displays an excellent long cycle life with only 5.8% deterioration of its initial specific capacitance, demonstrating superior long-term electrochemical stability.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51172133 and 21401114), Key Project of Chinese Ministry of Education (Grant no. 211098), Project of university innovation of Jinan (Grant no. 201311034) and Ministry of Education of Shandong Province (Grant no. J13LA01 and J14LC02). The authors also thank the Analytical Center of Qilu University of Technology for technological support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03774c
This journal is © The Royal Society of Chemistry 2015
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