The synthesis of NiO and NiCo2O4 nanosheets by a new method and their excellent capacitive performance for asymmetric supercapacitor

Electrochimica Acta 215 (2016) 212–222

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

The synthesis ofNiO and NiCo2O4 nanosheets by a new method and their excellent capacitive performance for asymmetric supercapacitor Lixin Zhanga,b,* , Wenhui Zhenga , Hongfang Jiuc , Changhui Nia , Jianxia Changc, Guisheng Qib a b c

Chemical Engineering and Environment Institute, North University of China, Taiyuan 030051, People’s Republic of China Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan 030051, People’s Republic of China College of Science, North University of China, Taiyuan 030051, People’s Republic of China

A R T I C L E I N F O

Article history: Received 5 March 2016 Received in revised form 18 August 2016 Accepted 20 August 2016 Available online 21 August 2016 Keywords: Self-assembly Bottom-up strategy Nanosheets Asymmetric supercapacitor

A B S T R A C T

NiO and NiCo2O4 nanosheets (NSs) were controlly prepared by one method called a bottom-up strategy employing surfactant molecular self-assembly, and followed by a thermal annealing process. The products were characterized by X-ray diffraction (XRD), a field-emission scanning electron microscope (FESEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), N2 adsorption-desorption (BET) and electrochemical analysis. The results reveal that the morphology of both products are all two-dimensional sheet-like and the average length and thickness of NiO and NiCo2O4 NSs are about (220–260 nm, 9 nm) and (1
6 mm, 28 nm), respectively. Both materials have large specific surface area and porous properties, which can increase the amount of electroactive sites and facilitate the electrolyte penetration. Hence, the NiO and NiCo2O4 NSs exhibited excellent electrochemical performance with high specific capacitance of 407 and 876 F g1 (1 A g1), respectively, good rate performance and cycling life. Additionally, two asymmetric supercapacitors were fabricated by using the 2D porous materials NiO and NiCo2O4 as the positive electrode, respectively, the active carbon (AC) as the negative electrode, and 6 M aqueous KOH as the electrolyte. Two asymmetric supercapacitors showed high specific capacitance and energy density of 89 F g1 &25.99 Wh Kg1 and 119 F g1 &34.75Wh Kg1, respectively. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction The development of new energy storage techniques is a vital link in the applications of renewable energy sources [1,2]. Among the current protocols, supercapacitor with fast charging and discharging capability, high power density, long lifespan and operation safety is a new type devices between traditional capacitor and battery, which can store electrical energy and meet the increasing demands of energy storage and conversion [3,4]. Among numerous materials used in the supercapacitor, metal oxides with multiple oxidation states can achieve reversible faradaic reactions and have greater capacitance than that of carbon-active materials [5–7], which have been extensively studied. NiO as a kind of transition metal oxides has attracted increasing attention due to its higher theoretical specific

* Corresponding author at: Chemical Engineering and Environment Institute, North University of China, Taiyuan 030051, People’s Republic of China. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.electacta.2016.08.099 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

capacitance (2573 F g1 within 0.5 V) and commercial advantages such as low cost, practical availability and easy to prepare [8,9]. In the prior reports, NiO exhibited a good capacitive property. For example, zhu et al. reported a template-free approach for the synthesis of NiO porous nano-plates with a hexagonal symmetry structure, which exhibited a beneficial specific capacitance of 286.7 F g1 at a current rates of 1 A g1 [10]. Subbukalai et al. reported the preparation of NiO nanoflakes via a microwave method, which delivered a maximum specific capacitance of 401 F g1 at 0.5 mA cm2 [11]. Besides, ternary compound NiCo2O4 is another one of more promising electrode materials for supercapacitors due to its environmental benignity and relatively higher theoretical specific capacitance. Importantly, ternary compound NiCo2O4 is expected to have better electrical conductivity and offer richer redox reactions including contributions from both nickel and cobalt ions, than the two corresponding single-component oxides and cost effective [12–15]. In addition to the advantages and the potential application of these two materials in supercapacitor, much efforts are needed to improve the electrochemical properties of the two materials.

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

The ideal electrode for supercapacitors should possess the characteristics of better electrical conductivity for charge transfer and larger specific surface area for ion transport. In most potential applications, the structure of the supercapacitor electrode materials will undoubtedly play a pivotal role in their electrodechemical performance. In contrast to other types of nanostructures, two-dimensional (2D) nanostructures can provide electrode materials high specific surface areas which is allowed to storing and releasing of “particles” (such as ions, hydrogen atoms or molecules, or electric charges) and many electrochemically reactive sites [16,17]. Moreover, a golden way to enhance the redox kinetics is to create porous nanostructures with large surface area and short diffusion path of ions and electrons. Much effort have be done for obtaining the 2D materials such like Mechanical exfoliation, Liquid phase exfoliation, Unzipping nanotubes, Thermal decomposition, Chemical vapor deposition (CVD) and so all [18]. However, the preparation of homogeneous 2D nanosheets on a large scale is still a big challenge and the methods mentioned above are complex, expensive and unable to control. So how to find a facile approach for the preparation on a large scale of 2D materials is extremely desirable to meet the growing demand for such 2D materials in electronic devices. Moreover, to the best of our knowledge, there are few reports on the preparation of NiO and NiCo2O4 with a homogeneous 2D nanostructure on a large scale for supercapacitors by one method, though the capacitive property of these two kinds of materials with the 2D nanostructure has been extensively investigated. Sun’s group has successfully synthesized a few of 2D transition metal oxide nanosheet materials by a bottom-up strategy employing surfactant molecular self-assembly combined with a simple post annealing process [19]. This method has a general applicability for the synthesis of transition metal oxide material with a twodimensional structure. such as TiO2, ZnO, Co3O4, WO3, Fe3O4 and MnO2. However, to the best of my knowledge, 2D NiO and NiCo2O4 NSs have not been synthesized using this method. In this work, we use this method for successfully synthesis of 2D porous NiO and NiCo2O4 NSs. The results show that both NiO and NiCo2O4 NSs electrode materials have uniform 2D morphology with porous properties and exhibits noticeable electrochemical performance with good specific capacitance as well as better cycling stability when tested as an electrode for electrochemical evaluation in a three-electrode system. But the device can really exhibit the electrochemical performance of supercapacitor in the practical application is two-electrode system instead of three-electrode system. Therefore, two asymmetric supercapacitors (ASC) have been fabricated by using the as-prepared 2D NiO and NiCo2O4 NSs as the positive electrode, the activated carbon (AC) as the negative electrode, respectively. Both ASCs (NiO ASC and NiCo2O4 ASC) deliver a significant specific capacitance, an energy density, and good cycling stability (86.6% and 80.2% retention after 2000 cycles, respectively), which showed that both materials can be used as electrode material for ASC. 2. Experimental 2.1. Reagents The chemical reagents used for the synthesis of NiO and NiCo2O4 nanosheets were commercially available reagents. Cobalt acetate tetrahydrate (Co(Ac)24H2O), nickel acetate tetrahydrate (Ni(Ac)24H2O), hexamethylenetetramine (HMTA), ethylene glycol (EG), ethyl alcohol(EtOH) were purchased from Tianjin Guangfu Technology Development Co.LTD, China. Polyethylene oxidepolypropylene oxide-polyethylene oxide (PEO20- PPO70-PEO20, Pluronic P123) was bought from Sigma-Aldrich. All the reagents

213

Fig 1. XRD patterns of the obtained precursor and calcined products of NiO(a) and NiCo2O4(b) NSs.

used for experiments were of analytical grade and used directly without further purification. 2.2. Synthesis of NiO NSs NiO NSs were prepared according to a bottom-up strategy employing surfactant molecular self-assembly combined with a simple post annealing process proposed by Sun [19]. The experimental details were as follows. Pluronic P123 was dissolved in a mixture solution consisting of 32.9 mL methanol and 2 mL DI to form a transparent solution. After stirring for a few minutes, 0.25 g Ni(Ac)24H2O and 0.14 g HMTA were added into the transparent solution and keeping stirring until the reactants was all to be dissolved, 26 mL EG was added into the mixture and a light green solution was obtained. Then, the obtained precursor solution was statically aged for 2 days. After aging, the transparent precursor solution was transferred into a 100 mL autoclave and heated at 170  C for 5 h. The products of the solvothermal reaction were washed with ethanol and distilled water three times, respectively and dried for 10 h at 60  C in a vacuum oven. At last, the black powders were collected after calcination at 350  C for 4 h in air.

214

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

Fig. 2. FESEM images of the NiO and NiCo2O4 NSs at low magnification (a and c, respectively) and high magnification (b and d, respectively).

2.3. Synthesis of NiCo2O4 NSs

2.5. Electrochemical measurements

This preparation process is similar to that process of NiO NSs. In a typical synthesis protocol, Pluronic P123 was dissolved in a mixture solution consisting of 32.9 mL methanol and 2 mL DI to form a transparent solution. After stirring for a few minutes, 0.167 g Co(Ac)24H2O, 0.083 g Ni(Ac)24H2O and 0.282 g HMTA were added into the transparent solution and keeping stirring until the reactants was all to be dissolved, 26 mL EG was added into the mixture and a light purple solution was obtained. Then, the obtained precursor solution was statically aged for 2 days. After aging, the transparent precursor solution was transferred into a 100 mL autoclave and heated at 170  C for 24 h. The products of the solvothermal reaction were washed with ethanol and distilled water three times, respectively and dried for 10 h at 60  C in a vacuum oven. At last, the black powders were collected after calcination at 350  C for 4 h in air.

The electrochemical properties of the prepared two products were measured by a standard three-electrode system and a CHI 660 D electrochemical workstation, and the three-electrode cell configuration with NiO (or NiCo2O4) as the working electrode, platinum as the counter electrode, saturated calomel electrode (SCE) as the reference electrode and 6 M KOH as the electrolyte. The working electrode was prepared as follows:80 wt% of active materials (NiO or NiCo2O4) powder, 10 wt% of acetylene black and 10 wt% of poly(tetrafluoroethylene) were mixed together until a homogeneous black powder was obtained [20–22]. Then, a few drops of ethanol and 5 wt% of poly(tetrafluoroethylene) was added into the mixed powder. After the solvent evaporated, the sticky mixture was smeared on a nickel foam with an area of 1 cm2 and then the nickel foam was dried for 12 h at 80  C in an oven. Finally, the electrode assembly was pressed at 10 MPa. In order to evaluate the electrochemical performance of these two samples, the cyclic voltammetry measurements of the NiO (or NiCo2O4) electrode was performed at different scan rates. The charge-discharge characterization was performed at different current densities within a potential window of 0-0.45 V. Electrochemical impedance measurements were carried out between 0.01 Hz and 100 kHz. The specific capacity of the NiO and NiCo2O4 NSs is calculated via the following equation:

2.4. Instruments and characterization X-ray diffraction (XRD) patterns were obtained using a D/ MaxRB X-ray diffractometer (Japan) with Cu K irradiation source through the 2u range from 20 to 80 . The morphologies and the crystal structure of the products were done by a field-emission scanning electron microscopy (FESEM, Hitachi S-4800,) and a Japan transmission electron microscopy (TEM, JEM-1400), a highresolution TEM(HRTEM, TF20) with selected-area electron diffraction (SAED). The measurements of N2 adsorption-desorption were carried out by using a USA Micromeritic ASAP 2020 nitrogen adsorption apparatus.

C¼

c I  Dt ¼ m DV  m

ð1Þ

where C (F g1) represents specific capacity, I (mA) is the discharge current, m(mg) is the mass of the active materials, and designates

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

215

Fig. 3. Electron microscope images of 2D transition metal oxide NSs. TEM images (a, b), lattice-resolved HRTEM image (c, d) and SAED pattern (e, f) of as-prepared 2D NSs of NiO (a, c, e) and NiCo2O4 (b, d, f). The insets in (b) is an enlarged view of corresponding TEM images of NiCo2O4 NSs.

the potential drop during discharging and the total discharge time, respectively. The electrochemical performance of two as-prepared 2D porous materials was further studied by constructing two asymmetric supercapacitors. Each asymmetric supercapacitor was fabricated by using one of as-prepared materials (NiO or NiCo2O4 NSs) as the positive electrode, AC as the negative electrode, polypropylene material as the separator, and 6 M KOH solution as the electrolyte. The negative electrode is prepared as that of the positive electrode. The cyclic voltammetry and galvanostatic current charge-discharge of the asymmetric

supercapacitors were carried out on a Modulab electrochemical workstation. The cycling stability was conducted by using the Arbin battery test system (BT2000). 3. Results and discussion The crystallinity and crystal structures of the precursor and calcined products of NiO andNiCo2O4 NSs were examined by X-ray diffraction (XRD) as shown in Fig. 1. In Fig. 1a, the XRD peak positions for two samples are matched well with the hexagonal aNi(OH)20.75H2O phase (JCPDS NO. card 38-0715) with the lattice

216

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

Fig. 4. Nitrogen adsorption-desorption isotherm and the corresponding pore size distribution (insets) of 2D-shaped (a) NiO and (b)NiCo2O4 NSs.

parameters a = b = 3.08 Å and c = 23.41 Å and the cubic-structured crystalline NiO (JCPDS NO. card 040835) with the lattice parameters a = b = c = 4.171 Å, respectively. For the a-Ni(OH)2 0.75H2O, because of the weakly alkaline medium created by stepwise hydrolysis of HMTA, b-Ni(OH)2 phase, which is generally synthesized from a high pH condition, is not observed. After calcination, a-Ni(OH)2 was converted into NiO and the peak positions (red lie in Fig. 1a) at 2u = 37, 43.4, 62.8, 75.5, and 79.8 of NiO correspond to the (111), (200), (220), (311), and (222) planes of the NiO. Moreover, Fig. 1b shows the XRD patterns of the precursor of NiCo2O4 and the final NiCo2O4 sample after calcination process. All peaks of the precursor can be well indexed to nickel cobalt hydroxide (Co, Ni)O(OH) (JCPDS 29-0491). After calcining treatment, all the diffraction peaks at 2u values of 18.9, 31.2, 36.7, 44.6, 59.1 and 65.0 in the XRD patterns of NiCo2O4 can be easily indexed to the (111), (200), (311), (400), (511) and (440) reflections of cubic NiCo2O4 (JPCDS No.20-0781), demonstrating the successful fabrication of NiCo2O4. The morphologies of the NiO and NiCo2O4NSs were observed by field-emission scanning electron microscopy (FESEM). Fig. 2a and c shows the low-magnification FESEM images of the NiO and the NiCo2O4 NSs, respectively. It can be clearly seen that the morphologies of the NiO and NiCo2O4 are uniform and they are

both two-dimensional structure of nanosheets. The average diameter of the NiO and NiCo2O4NSs are in the range of 220– 260 nm and 1–6 mm, respectively. FESEM images with the highmagnification clearly reveal that the thickness of the NiO NSs (Fig. 2b) and NiCo2O4 NSs (Fig. 2d), is about 9 and 28 nm, respectively, which show clearly the ultrathin nature of NiO and NiCo2O4 NSs. The ultrathin nanosheet morphology is advantageous for efficient ion and electron transport, which will undoubtedly contribute to the enhancement of capacitance. To provide a detailed crystal structure of both materials, we further analyzed 2D NiO and NiCo2O4 NSs materials by TEM, HRTEM and SAED. Fig. 3a is the TEM image of NiO sample, and Fig. 3b and the inset of Fig. 3b are the typical TEM images of NiCo2O4 sample at different magnifications, respectively. From these three pictures we can conclude two points. Firstly, the microstructure of the NiOand the NiCo2O4 is round slice and irregular nanosheet, respectively, which is consistent with the results from the FESEM images of these two materials. Secondly, both round NiO slices and irregular NiCo2O4 NSs with a transparent feature have a porous structure on their surface, which is caused by the hydrolysis process of the weak alkaline reagent of HMTA. In this process, alkaline reagent will slowly release the gas, and these pores are caused by the outward escaped process of the gas (HCHO, NH3). It can infer that this porous feature will endow both materials a high specific surface area. Fig. 3c and d are the HRTEM images of NiO and NiCo2O4 NSs, respectively. In the two HRTEM images, the randomly orientated lattice phase reveals that the polycrystalline characteristic of both materials, and the clearly resolved lattice fringes can be discerned. The lattice spacing in Fig. 3c is calculated to be about 0.21 and 0.24 nm corresponding to the (200) and (111) planes of cubic NiO, respectively. The lattice spacing in Fig. 3d is calculated to be about 0.47 nm and 0.24 nm corresponding to the (111) and (311) planes of cubic NiCo2O4, respectively. Fig. 3e and f are the SAED patterns of NiO and NiCo2O4 NSs, respectively. Both of them display a well-defined ring, which further confirm that these two samples are of polycrystalline nature. Moreover, the SAED patterns of NiO and NiCo2O4 NSs are well indexed to the NiO and NiCo2O4 phase, respectively, which is identical with the aforementioned results of XRD and HRTEM. The specific surface area and porous nature of the NiO and NiCo2O4 NSs were examined by nitrogen-adsorption-desorption measurements, which are shown in Fig. 4. Both Fig. 4a and b exhibit a typical Langmuir type IV isotherm with an apparent hysteresis loop, indicative of a mesoporous nature of both NiO and NiCo2O4 NSs, and this information is consistent with that of Fig. 3a and d. Fig. 4a and the inset of Fig. 4a show a high Brunauer– Emmett–Teller(BET) specific surface area (139 m2 g1) and a narrow pore size distribution (2.62 nm)of NiO NSs, respectively. The specific surface area of NiCo2O4 NSs in Fig. 4b is 89 m2 g1, and a narrow pore-size distribution (2.44 nm) can be clearly observed in the inset of Fig. 4b. The relatively high specific surface areas of NiO and NiCo2O4 NSs originate from their porous structure. It has been reported such a structure of NiO NSs and NiCo2O4 NSs synthesized via the bottom-up synthetic method is favorable to supercapacitors because the porous structure with a large specific surface area can create many more convenient channels for electrolyte ions to transport, electrochemically access even more electroactive sites for energy storage as fast as possible at large current densities and therefore enhanced kinetics of the reversible redox process for charge storage [14]. In this work, we have successfully synthesized NiO and NiCo2O4 NSs by a bottom-up strategy which is proposed by Sun’ group [19]. The quintessence of this method is the surfactant molecular selfassembly process, in which amphiphilic block copolymers will form inverse lamellar micelles in a mixed solvent composed of short chain alcohol and water, as the growth templates of inorganic

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

217

Fig. 5. The formation mechanism of the two dimensional nanostructure materials.

salt precursors to grow along a selected direction. The concrete reaction mechanism is shown in Fig. 5 and this method contains four steps: Firstly, the amphiphilic block copolymers of P123 mixes with ethanol solvent and forms inverse lamellar micelles. Secondly, putting the inorganic salts and a certain amount of water into the system, the hydrated inorganic precursor salts are then limited inside the inverse layered micelle and hydrolyzed into cambium-like inorganic precursor oligomers. In this step, the content of water used to form the hydrated precursor oligomers and controlled in the phase area to form inverse lamellar micelles based on the phase behavior of the surfactant-water-oil equilibrium system [23–26]. The following step is hydrothermal treatment which can induce the micelles organizing into regular patterns and allow polycondensation between inorganic particles to improve its crystallinity. In the end, 2D materials with uniform morphology are collected after the removal of surfactants agents. In addition, as reports goes that EG was adjusted to control the hydrolysis rate of precursors and the aggregation structure of the surfactant, and it also work in the role of both co-surfactant and co-solvent in the surfactant–water system [27,28]. The reaction equations involved in the process of formation of NiO((2), (3), (4), (6)) and NiCo2O4((2), (3), (5), (7)) NSs are shown as below [29,30]: C6H12N4+ 10 H2O!6CH2O + 4 NH3H2O

(2)

NH3H2O ! NH4+ +OH

(3)

Ni2+ + 2 OH! Ni(OH)2

(4)

Ni2+ + Co2+ + 3 OH!Ni-CoOOH + H2O

(5)

Ni(OH)2! NiO + H2O

(6)

Ni-CoOOH !2 NiCo2O4 + H2O

(7)

As described above, these two kinds of 2D nanostructures of NiO and NiCo2O4 NSs can be controllably prepared by one method and it will be interesting to study the electrochemical performance of these two kinds of 2D nanostructures of material. The electrochemical properties of these two kinds of materials were comparatively studied in a standard three-electrode configuration using 6.0 M KOH as electrolyte. Fig. 6a and b show the CV curves of the NiO and NiCo2O4 NSs electrodes at different scan rates ranging from 5 to 60 mV s1 within the voltage window of 0.007 to 0.45 V, at room temperature, respectively. The CV shapes clearly reveal the pseudocapacitive characteristics of these two kinds of as-prepared electrodes, which is distinct from that of electric double-layer capacitance in which the shape is normally close to a nearly symmetrical rectangular shape. A pair of well-defined redox peaks can be observed from the CV curves of both NiO (Fig. 6a) and NiCo2O4 (Fig. 6b) NSs electrodes, which is mainly attributed to the Faradic redox reactions related to M-O/M-O-OH (M refersto Ni or Co) [31–33] and they are associated with OH anions in the electrolyte. In addition, because of the high surface area and the fastionic/electronic diffusion rate during the Faradic redox reactions at the surface, both the NiO and NiCo2O4 NSs electrodes show very prominent electrochemical properties [34,35]. In Fig. 6a and b, when the scan rate increases from 5 to 60 mV s1, the anodic peaks shift toward high potential and the

218

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

Fig. 6. (a) and (b) The CV curves of NiO round NSs and NiCo2O4 irregular NSs electrodes, respectively, at various scan rates. (c) CV curves of the as-prepared NiO round NSs and NiCo2O4 irregular NSs electrodes at a scan rate of 5 mV s1.

cathodic peaks move toward negative potential simultaneously, which is could be attributed to the polarization caused by high scan rate. The CV curves maintain an unchanged shape even at a high scan rate, implying that both electrodes enable excellent electrochemical reversibility and high-rate performance. Furthermore, the oxidation and reduction peaks of the NiO and NiCo2O4 NSs electrodes are nearly symmetrical throughout the scan range 5–60 mV s1, indicating good reversibility of redox reaction at the

electrode surface. Fig. 6c represents the CV curves of NiO and NiCo2O4 NSs electrodes at a same scan rate of 5 mV s1. Obviously, at the same scan rate, the NiCo2O4 electrode has an larger CV integrated area than that of NiO NSs electrode, suggesting that the NiCo2O4 NSs electrode has a bigger capacitance as compared with NiO NSs electrode, which may largely because that the theoretical specific capacitance of NiCo2O4 is higher than that of NiO [14,36]. The galvanostatic charge-discharge (GCD) profiles of NiO and NiCo2O4NSs were also compared in a potential window between 0 and 0.45 V at a scan rate of 1.0 A g1 as shown in Fig. 7a. Obviously, the nonlinearity in the discharge curves further verifies the pseudocapacitance behavior of NiCo2O4 coming from Faraday reaction and both 2D materials exhibited asymmetrical slope during charging and discharging processes, suggesting a good supercapacitor behavior. The NiCo2O4 NSs electrode (876 F g1) shows obviously higher specific capacitance over that of NiO NSs electrode (407 F g1) at this current. According to the specific capacity formula 1 mentioned before and combined with CD profiles, the specific capacity of NiO NSs is calculated to be 407, 400, 294, 241 and 235 F g1 at current densities of 1, 2, 3, 5 and 10 A g1, respectively. The specific capacitance of NiCo2O4 NSs electrode can reach as high as 876 F g1, 869 F g1, 636 F g1, 496F g1 and 456F g1 at 1.0 A g1, 2.0 A g1, 3A g1, 5A g1 and 10.0 A g1, respectively. When the current density was increased to 10.0A g1, the specific capacitance of the NiCo2O4 and NiO NSs electrodes decreased to 456 F g1 and 235 F g1, respectively, which was about 52% and 58% of that measured at 1.0 A g1, respectively. The cyclic performance of the NiO and NiCo2O4 NSs electrodes, which includes cycling life and specific capacitance retention of the supercapacitors are shown in Fig. 7c, tested over 1000 cycles carried out at a current density of 1 A g1. After 1000 cycles, the capacitance losses of NiCo2O4 and NiO NSs electrodes are about 12.0% and 13.3%, respectively. This suggests that the two materials have potential applications in supercapacitors. After a long period test, these two electrode materials exhibit stable electrochemical performance and high capacitance retention rate, which are very necessary and important for supercapacitors. The electrical conductivity and ion transfer of the obtained 2D supercapacitor electrodes have been investigated further by electrochemical impedance spectroscopy (EIS), which has been carried out in 6.0 M KOH solution within a frequency range of 0.01– 105 Hz at amplitude of 5 mV versus the open circuit potential. Fig. 8 shows the Nyquist impedance plots of both NiO and NiCo2O4 NSs and the insert is enlargement at high frequency region. The EIS spectra are divided into three distinct regions based on the order of decreasing frequencies to analyze. The slope of the EIS curve in the low frequency range reflects the Warburg resistance, which describes the diffusion rate of redox material in the electrolyte. The higher value of slope in case of NiCo2O4 NSs electrode over NiO NSs electrode indicates higher electrolytic ion diffusion within the NiCo2O4 NSs structure. The phase angle of the impedance plot of the NiCo2O4 and NiO NSs electrodes found to be higher than 45 in the low frequency range suggesting that the electrochemical capacitive behavior of both the electrodes is controlled by diffusion process. The shorter line at lower frequency correlates to the smaller changes in ion diffusion path and easier movement of the ions within the pores. The diameter of semicircle in the high frequency range represents the charge transfer resistance (Rct) of the electrode material resulting from the diffusion of electrons. Obviously, the diameter of semicircle for NiCo2O4 electrode is smaller than that of the NiO electrode, which indicates that NiCo2O4 NSs are ideal for fast ion and electron transport, because the larger diameter semicircle reflects higher charge-transfer resistance value. The intercept on the real axis in the high frequency range provides the equivalent series resistance (ESR), which comprises of the bulk resistance of the electrolyte, the

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

219

Fig. 7. (a) Galvanostatic discharge curves of NiO and NiCo2O4NSs electrodes at a current density of 1A g1. (b) and (c) Galvanostatic discharge curves of NiO and NiCo2O4 NSs electrodes at different current density, respectively. (d) The capacitance cycling performance of NiCo2O4 and NiO NSs electrodes at a constant current density of 1 A g1 in 1000 cycles, respectively.

Fig. 8. EIS plots of the as-synthesized 2D NiO and NiCo2O4 NSs electrodes in 6.0 M KOH solution before the cycle test (Inset: the enlarged view of both electrodes at high frequency region).

inherent resistances of the electro active material and the contact resistance at the interface between electrolyte and electrode [11,35]. The values of ESR of NiCo2O4 and NiO NSs electrodes are calculated for 0.64 and 0.76 V, respectively, which again suggests higher electrical conductivity of NiCo2O4 NSs over NiO NSs

electrode. These studies indicate that the NiCo2O4 NSs electrode has low ion diffusion resistance, which can be attributed to the incorporation of hydroxyl groups on the surface of NiCo2O4 NSs after hydrogenation. Considering the practical application, two aqueous asymmetric supercapacitors were assembled using the NiO NSs and NiCo2O4 NSs as the positive electrode and AC as the negative electrode, respectively. To build a supercapacitor with high operating voltage and high energy density, it is important to balance the charges stored at the positive electrode (Q+) and the negative electrode (Q). The charge (Q) stored by each electrode depends on the specific capacitance (C), the potential range for the charge-discharge process (DV), and the mass of the electrode (m), following the equation: Q=CDVm. To realize Q +=Q-, the mass ratio of positive and negative electrodes can be calculated by the equation: m+/m [37]. The CVs of NiO NSs and NiCo2O4 NSs (red curve) and AC electrode (black curve) were measured in a 6 M KOH solution, respectively, as shown in Fig. 9a and b, respectively. From Fig. 9a and b, it can be found that the potential windows of NiO NSs and NiCo2O4 NSs are 0.07-0.45 V, the potential window of AC is 1–0 V. The results of CVs shown in Fig. 9a and b imply that the potential window of two ASC can be as large as 1.45 V. The mass ratios of positive electrode to negative electrode of the NiO or NiCo2O4 NSs asymmetric supercapacitors, are 0.45 and 0.21, respectively. Fig. 9c and d shows the CVs of the NiO NSs and NiCo2O4 NSs ASC at various scan rates in a potential range of 0-1.45 V, respectively. All the NiO

220

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

Fig. 9. Electrochemical performances of NiO NSs//AC and NiCo2O4 NSs//AC two assembled asymmetric supercapacitors: (a) CV plots of negative electrode prepared using activated carbon and positive electrode prepared using NiO NSs at a scan rate of 5 mV s1 in 6 M KOH electrolyte; (b) CV plots of negative electrode prepared using activated carbon and positive electrode prepared using NiCo2O4 NSs at a scan rate of 5 mV s1 ; CV plots of NiO NSs//AC(c) and NiCo2O4 NSs//AC ASC (d) recorded at different scan rates, respectively; GCD plots of NiO NSs//AC(e) and NiCo2O4 NSs//AC(f) ASC recorded at different current densities, respectively.

NSs ASC CV plots are similar in shape, maintaining a pair of cathodic and anodic peaks. These redox peaks in the CV plots get shifted away from each other as the scan rates are increased. The redox peaks are indicative of the pseudocapacitive nature of an ASC and dominate enormously over the double layer contribution of AC, which is very clear from the large area of corresponding CV plots. In CVs of the NiCo2O4 NSs ASC (Fig. 9d), the oxidation peaks and reduction peaks on the CVs can be observed and the peak current becomes larger with the scan rate increasing from 5 to 60 mV s 1. However, there is no obvious distortion in the CVs even at a high scan rate of 60 mV s 1, indicating the good fast charge– discharge properties of the device [38,39]. In order to further evaluate the performance of ASC, we also performed the GCD test at various current densities (Fig. 9e and f). The shapes of the GCD curves for the NiO NSs//AC and NiCo2O4 NSs//AC ASC at different current densities are nonlinear, which further supports the fast and reversible redox peaks recorded in the CV plots. Fig. 10a manifests the plots of the specific capacitance and current density of the two asymmetric supercapacitors. The specific capacitance of the NiO NSs//AC ASC is calculated to be 89, 78, 57, 32 and 12 F g1 at current densities of 0.5, 1, 2, 3 and 5A g1, respectively. Whereas, the NiCo2O4 NSs//AC ASC manifests specific capacitance of 119, 101.5, 83, 68 and 52 F g1 at current densities of 0.5, 1, 2, 3 and 5A g1, respectively. The cycling stabilities of two ASCs were conducted at a constant current density of 0.5 A g1 for 2000 cycles, as shown in Fig. 10b. The original specific capacitance of the NiO NSs//AC ASC is 89 F g1. After 2000 cycles, 80.2% of the original capacitance is retained. The specific capacitance of the NiCo2O4 NSs//AC NSs ASC is 119 F g1 at 0.5 A g1 and about 86.6% initial specific capacitance retention can be obtained after 2000 cycles. The 86.6% capacitance retention indicates that NiCo2O4 NSs//AC ASC possess relatively good cycling performance compared to the NiO NSs//AC ASC. Further electrochemical analysis for determining the internal resistance and charge transport of the ASC was performed using

electrochemical impedance spectroscopy. Fig. 10c shows the Nyquist plots of two ASCs in the frequency range of 100 kHz0.1 Hz. The EIS plots can be easily distinguished into a distorted semicircle at the high frequency region and a straight line with a slope towards the low frequency region. The semicircle represents the charge transfer process and the sloped line represents the capacitive nature of the device. The equivalent series resistances (ESR) of NiO NSs//AC and NiCo2O4 NSs//AC ASC could be estimated from the intercept at real impedance axis and were found to be 0.79 and 0.58 V, respectively. The relatively low resistance values represent the higher diffusion and migration pathways for electrolyte ions during charge/discharge processes which are responsible for the good electrochemical performance of asymmetric device. The power and energy densities of the ASC could be evaluated from the GCD curves. The energy density and power density for the asymmetric supercapacitor can be calculated according to the following two formulas (formula 3 and 4), respectively [40]: E¼

C DV 2 7:2

ð3Þ

P¼

3:6E Dt

ð4Þ

where C (F g1) is the specific capacitance of the asymmetric supercapacitor, E (Wh kg1) is the energy density, P (KW kg1) is the power density and DV (V) is the potential drop during discharge. With the current density increasing from 0.5 to 5A g1, the Ragone plots of the NiO NSs//AC and NiCo2O4 NSs//AC ASC are shown in Fig. 10d. The NiO NSs//AC ASC asymmetric supercapacitor shows the high energy density is 25.99 Wh kg1 at a power density of 2.5KW kg1 and still remains 9.3 Wh kg1 at a high power density of 14.7KW kg1. This outperforms many previous reported ASCs, such

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

221

Fig. 10. (a) Specific capacitances of two ASCs as a function of current density; (b) capacitance retention for 2000 GCD cycles carried out at a current density of 0.5 A g1; (c) EIS plots of two ASCs in 6.0 M KOH solution before the cycle test (Inset: the enlarged view of both ASCs at high frequency region). (d) Ragone plots of two ASCs (energy density vs. power density).

as NiO//Carbon (11.6 Wh kg1 at 28W kg1) [41], MnO2 nanowire/ Graphene//Graphene (7.0 Wh kg1 at 5000 W kg1) [42], CNT@NiO//PCPs(25.4 Wh kg1 at 400 W kg1) [37]. The NiCo2O4 NSs//AC ASC delivers a high energy density of 34.75 Wh kg1 at a power density of 2.5KW kg1, and the energy density still retains a reasonable value of 15.18 Wh kg1 even at a power density as high as 24.6 KW kg1. The high energy density of our capacitor is also higher than those Co-based asymmetric supercapacitor of Ni-Co based of Graphene-NiCo2O4//Activated carbon (7.6Wh kg1 at 5600 W kg1) [43], and Ni-Co oxide//Activated carbon (7.4Wh kg1 at 1903 W kg1) [44]. And NiCo2O4 NSs@HMRAs//AC ASC(15.42Wh kg1 at 500W kg1) [45]. The excellent electrochemical performance of the NiO and NiCo2O4 NSs electrodes should be attributed to the benefit from the unique ultra-thin and mesoporous feature, which hold great promise in offering more active sites to facilitate electrochemical reactions and more efficient penetration. The above results suggested that NiO NSs and NiCo2O4 NSs synthesized via the bottom-up synthetic method are very promising electrode materials for the charge/discharge operations in supercapacitors.

round NiO NSs (about 220–260 nm in width, 9 nm in thickness) and irregular NiCo2O4 NSs (about 1–6 mm in width, 28 nm in thickness) with big specific surface area (139 m2 g1 and 89 m2 g1, respectively) have been tested for supercapacitors. The superior electrochemical performance including high specific capacitance and good cycle stability has been proved: the highest specific capacitance of NiO and NiCo2O4 NSs is 407 and 876 Fg1 (1A g1), respectively and the specific capacitance retention ratio is about 86.7% and 88.0%, respectively, after 1000 cycles at 1 A g1. Moreover, NiO NSs//AC and NiCo2O4 NSs//AC ASC have been fabricated. NiO NSs//AC ASC delivers high energy density of 25.99 Wh kg1and remarkable cycling stability with 80.2% of the initial capacity retention after 2000 cycles. Additionally, the NiCo2O4 NSs//AC ASC exhibit high specific capacitance of 119 F g1, high energy density of 34.75 Wh kg1, and excellent cycle stability. Therefore, this simple synthetic approach may provide a route for the large scale preparation of 2D porous NiO and NiCo2O4 NSs electrode materials for high energy electrochemical capacitors in future.

5.1. Conclusions

Acknowledgements

In summary, the 2D uniform nanostructure of NiO and NiCo2O4 electrodes materials synthesized in a same reaction system via a bottom-up synthetic method for the first time. The products of

The authors thank the ShanXi Provincial International Technological Cooperation of China(2012081020), ShanXi Provincial Science and Technology Foundation of China(20110321037-02),

222

L. Zhang et al. / Electrochimica Acta 215 (2016) 212–222

the ShanXi Provincial Natural Science Foundation of China (2015011016), Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering(No: CZL201505) for the financial support.

[24] [25]

References [1] J. Du, G. Zhou, H.M. Zhang, C. Cheng, J.M. Ma, W.F. Wei, L.B. Chen, T.H. Wang, Ultrathin porous NiCo2O4 nanosheet arrays on flexible carbon fabric for highperformance supercapacitors, ACS Appl Mater Interfaces 5 (2013) 7405–7409. [2] L. Yu, H.B. Wu, T. Wu, C.Z. Yuan, Morphology-controlled fabrication of hierarchical mesoporous NiCo2O4 micro-/nanostructures and their intriguing application in electrochemical capacitors, RSC Adv. 3 (2013) 23709–23714. [3] Y. Hou, L.Y. Chen, P. Liu, J.L. Kang, T.K. Fujita, M.W. Chen, Nanoporous metal based flexible asymmetric pseudocapacitors, J Mater Chem A. 2 (2014) 10910– 10916. [4] J.W. Xiao, L. Wan, S.H. Yang, F. Xiao, S. Wang, Design hierarchical electrodes with highly conductive NiCo2S4 nanotube arrays grown on carbon fiber paper for high performance pseudocapacitors, Nano Lett. 14 (2014) 831–838. [5] Y.T. Weng, H.A. Pan, N.L. Wu, G.Z. Chen, Titanium carbide nanotube core induced interface facial growth of crystalline polypyrrole/polyvinyl alcohol lamellar shell for wide-temperature range supercapacitors, J Power Sources. 274 (2015) 1118–1125. [6] J. Liang, Z.Y. Fan, S. Chen, S.J. Ding, G. Yang, Hierarchical NiCo2O4 nanosheets@halloysite nanotubes with ultrahigh capacitance and long cycle stability as electrochemical pseudocapacitor materials, Chem Mater. 26 (2014) 4354–4360. [7] Y.X. Xu, G.Q. Shi, X.F. Duan, Self-assembled three-dimensional graphene macrostructures: synthesis and applications in supercapacitors, Acs Chem Res 48 (2015) 1666–1675. [8] M.Y. Li, B.T. Dong, G.X. Gao, S.J. Ding, Synthesis of nickel oxide/reduced graphene oxide composite with nanosheet-on-sheet nanostructure for lithium-ion batteries, Mater Lett. 155 (2015) 30–33. [9] H. Zhang, X.Y. Tian, C.P. Wang, H.L. Luo, J. Hu, Y.H. Shen, A.J. Xie, Facile synthesis of RGO/NiO composites and their excellent electromagnetic wave absorption properties, Appl Surf Sci. 314 (2014) 228–232. [10] Z. Zhu, J. Ping, X. Huang, J.G. Hu, Q.Y. Chen, X.B. Ji, C.E. Banks, Hexagonal nickel oxide nanoplate-based electrochemical supercapacitor, J Mater Sci. 47 (2012) 503–507. [11] S. Vijayakumar, S. Nagamuthu, G. Muralidharan, Supercapacitor studies on NiO nanoflakes synthesized through a microwave route, ACS Appl Mater Interfaces 5 (2013) 2188–2196. [12] G.X. Gao, H.B. Wu, S.J. Ding, L.M. Liu, X.W. Lou, Hierarchical NiCo2O4 nanosheets grown on Ni nanofoam as high-performance electrodes for supercapacitors, Small 11 (2015) 804–808. [13] Q.X. Chu, W. Wang, X.F. Wang, B. Yang, X.Y. Liu, J.H. Chen, Hierarchical NiCo2O4@nickel-sulfide nanoplate arrays for high performance supercapacitors, J Power Sources. 276 (2015) 19–25. [14] L.B. Ma, X.P. Shen, H. Zhou, Z.Y. Ji, K.M. Chen, G.X. Zhu, High performance supercapacitor electrode materials based on porous NiCo2O4 hexagonal nanoplates/reduced graphene oxide composites, Chem Engin J. 262 (2015) 980–988. [15] L.B. Ma, X.P. Shen, Z.Y. Ji, X.Q. Cai, G.X. Zhu, K.M. Chen, Porous NiCo2O4 nanosheets/reduced graphene oxide composite: Facile synthesis and excellent capacitive performance for supercapacitors, J Coll Inter Sci. 440 (2015) 211– 218. [16] F. Bonaccorso, L.-G. Colombo, G.H. Yu, M. Stoller, V. Tozzini, A.C. Ferrari, R.S. Ruoff, V. Pellegrini, Graphene, related two—dimensional crystals, and hybrid systems for energy conversion and storage, Science 347 (2015) 12465011– 12465019. [17] X.F. Song, J.L. Hu, H.B. Zeng, Two-dimensional semiconductors: recent progress and future perspectives, J Mater Chem C 1 (2013) 2952–2969. [18] M.S. Xu, T. Liang, M.M. Shi, H.Z. Chen, Graphene-like two-dimensional materials, Chem Rev 113 (2013) 3766–3798. [19] Z.Q. Sun, T. Liao, Y.H. Dou, S.M. Hwang, M.-S. Park, L. Jiang, J.H. Kim, S.X. Dou, Generalized self-assembly of scalable two-dimensional transition metal oxide nanosheets, Nat Commun 5 (2014) 1–9. [20] N. Padmanathan, S. Selladurai, Solvothermal synthesis of mesoporous NiCo2O4 spinel oxide nanostructure for high-performance electrochemical capacitor electrode, Ionics 19 (2013) 1535–1544. [21] H. Wang, Q. Gao, L. Jiang, Facile approach to prepare nickel cobaltite nanowire materials for supercapacitors, Small 7 (2011) 2454–2459. [22] K. Xia, Q. Gao, J. Jiang, J. Hu, Hierarchical porous carbons with controlled micropores and mesopores for electrode materials, Carbon 46 (2008) 1718– 1726. [23] P. Alexandridis, T.A. Hatton, Poly(ethylene oxide)-poly(propylene oxide)-poly (ethylene oxide) block copolymer surfactants in aqueous solutions and at

[26]

[27]

[28] [29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

interfaces: thermodynamics, structure, dynamics, and modeling, Colloids Surfaces A: physicochem Eng Aspects 96 (1995) 1–46. R.H. Dong, J.C. Hao, Complex fluids of poly(oxyethylene) monoalkyl ether nonionic surfactants, Chem Rev 110 (2010) 4978–5022. S. Dey, A. Adhikari, U. Mandal, S. Ghosh, K. Bhattacharyya, A femtosecond study of excitation wavelength dependence of a triblockcopolymer-surfactant supramolecular assembly: (PEO)20-(PPO)70-(PEO)20 and CTAC, J Phys Chem B. 112 (2008) 5020–5026. J. Liu, A.Y. Kim, L.Q. Wang, B.J. Palmer, Y.L. Chen, P. Bruinsma, B.C. Bunker, G.J. Exarhos, G.L. Graff, P.C. Rieke, G.E. Fryxell, J.W. Virden, B.J. Tarasevich, L.A. Chick, Self-assembly in the synthesis of ceramic materials and composites, Adv Colloid Interface Sci 69 (1996) 131–180. Z.Q. Sun, J.H. Kim, Y. Zhao, F. Bijarbooneh, V. Malgras, Y. Lee, Y.-M. Kang, S.X. Dou, Rational design of 3D dendritic TiO2 nanostructures with favorable architectures, J Am Chem Soc. 133 (2011) 19314–19317. R. Zana, Aqueous surfactant-alcohol systems: a review, Adv. Colloid Interface Sci. 57 (1995) 1–64. J.H. Pan, Q.Z. Huang, Z.Y. Koh, D. Neo, X.Z. Wang, Q. Wang, Scalable synthesis of urchin- and flower like hierarchical NiO microspheres and their electrochemical property for lithium storage, ACS Appl Mater Interfaces. 5 (2013) 6292–6299. G.Q. Zhang, X.W. Lou, Controlled growth of NiCo2O4 nanorods and ultrathin nanosheets on carbon nanofibers for high-performance supercapacitors, Sci Rep. 3 (2015) 1–8. T. Peng, Z.Y. Qian, J. Wang, L.T. Qu, P. Wang, Binary cooperative NiCo2O4 on the nickel foams with quasi-two-dimensional precursors: a bridge between ‘supercapacitor’ and ‘battery’ in electrochemical energy storage, Phys Chem Chem Phys. 17 (2015) 5606–5612. A.K. Singh, D. Sarkar, G.G. Khan, K. Mandal, Hydrogenated NiO nanoblock architecture for high performance pseudocapacitor, Appl Mater Interfaces. 6 (2014) 4684–4692. X.Y. Liu, Y.Q. Zhang, X.H. Xia, S.J. Shi, Y. Lu, X.L. Wang, C.D. Gu, J.P. Tu, Selfassembled porous NiCo2O4 hetero-structure array for electrochemical capacitor, J Power Sources. 239 (2013) 157–163. X.Y. Liu, S.J. Shi, Q.Q. Xiong, L. Li, Y.J. Zhang, H. Tang, C.D. Gu, X.L. Wang, J.P. Tu, Hierarchical NiCo2O4@ NiCo2O4 core/shell nanoflake arrays as high performance supercapacitor materials, Appl. Mater. Interfaces 5 (2013) 8790– 8795. G.Y. Huang, S.M. Xu, Y.B. Cheng, W.J. Zhang, J. Li, X.H. Kang, NiO nanosheets with large specific surface area for lithium ion batteries and supercapacitors, Int J electrochem Sci. 10 (2015) 2594–2601. Q.Y. Zheng, X.Y. Zhang, Y.M. Shen, Fabrication of free-standing NiCo2O4 nanoarrays via a facile modified hydrothermal synthesis method and their applications for lithium ion batteries and high-rate alkaline batteries, Mater Res Bull. 63 (2015) 211–215. Y. Huan, H.W. Wang, Y.T. Jing, T.Q. Peng, X.F. Wang, Asymmetric supercapacitors based on carbon nanotubes@NiO ultrathin nanosheets core-shell composites and MOF-derived porous carbon polyhedrons with super-long cycle life, J Power Sources. 285 (2015) 281–290. X. Wang, A. Sumboja, M. Lin, J. Yan, P.S. Lee, Enhancing electrochemical reaction sites in nickel cobalt layered double hydroxides on zinc oxide nanowires: a hybrid material for an asymmetric supercapacitor device, Nanoscale. 4 (2012) 7266–7272. Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Asymmetric supercapacitors based on graphene/MnO2 and activated carbon nanofiber electrodes with high power and energy density, Adv Funct Mater. 21 (2011) 2366–2375. M.S. Kolathodi, M. Palei, T.S. Natarajan, Electrospun NiO nanofibers as cathode materials for high performance asymmetric supercapacitor, J Mater Chem A. 3 (2015) 7513–7522. D.W. Wang, F. Li, H.M. Cheng, Hierarchical porous nickel oxide and carbon as electrode materials for asymmetric supercapacitor, J Power Sources. 185 (2008) 1563–1568. Z.S. Wu, W. Ren, D.W. Wang, F. Li, B. Liu, H.M. Cheng, High-energy MnO2 nanowire/graphene and graphene asymmetric electrochemical capacitors, ACS Nano. 4 (2010) 5835–5842. H. Wang, C.M.B. Holt, Z. Li, X. Tan, B.S. Amirkhiz, Z. Xu, B.C. Olsen, T. Stephenson, D. Mitlin, Graphene—nickel cobaltite nanocomposite asymmetrical supercapacitor with commercial Level Mass Loading, Nano Res. 5 (2012) 605–617. C. Tang, Z. Tang, H. Gong, Hierarchically porous Ni-Co oxide for high reversibility asymmetric full-cell supercapacitor, J Electrochem Soc. 159 (2012) A651–A656. X.F. Lu, D.J. Wu, R.Z. Li, Q. Li, S.H. Ye, Y.X. Tong, G.R. Li, Hierarchical NiCo2O4 nanosheets@hollow microrod arrays for high-performance asymmetric supercapacitors, J Mater Chem A 2 (2014) 4706–4713.