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NiCo2O4 nanosheets in-situ grown on three dimensional porous Ni film current collectors as integrated electrodes for high-performance supercapacitors
Journal of Power Sources 286 (2015) 371e379
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
NiCo2O4 nanosheets in-situ grown on three dimensional porous Ni film current collectors as integrated electrodes for high-performance supercapacitors Tao Wang a, b, Ying Guo a, Bo Zhao a, Shuhui Yu a, Hai-Peng Yang b, Daniel Lu a, Xian-Zhu Fu a, *, Rong Sun a, *, Ching-Ping Wong c, d a
Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, PR China College of Materials Science and Engineering, Shenzhen University, Shenzhen 518055, PR China Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong, China d School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States b c
h i g h l i g h t s 3D hierarchical porous Ni films are fabricated as effective current collectors. NiCo2O4 nanosheets are in-situ grown on the 3D hierarchical porous Ni films. The porous NiCo2O4/Ni electrodes demonstrate excellent capacitive performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2014 Received in revised form 28 March 2015 Accepted 30 March 2015 Available online 31 March 2015
Three dimensional interconnected hierarchical porous Ni films are easily fabricated as effective current collectors through hydrogen bubble template electrochemical deposition. The binder-free integrated electrodes of spinel NiCo2O4 nanosheets directly coated the three dimensional porous Ni films are facilely obtained through successively electrochemical co-deposition of Ni/Co alloy layer then followed by subsequent annealing at 350 C in air. Compared with NiCo2O4 nanosheets on smooth Ni foil or porous NiO/Ni film electrodes, the porous NiCo2O4/Ni integrated film electrodes for supercapacitors demonstrate remarkably higher area specific capacitance. The porous NiCo2O4/Ni film electrodes also exhibit excellent rate capability and cycling stability. The super electrochemical capacitive performances are attributed to the unique integrated architecture of NiCo2O4 nanosheets in-situ grown on three dimensional continuous hierarchical porous Ni collector collectors, which could provide large electrode-electrolyte interface area, high active sites, low contact resistance between current collector and active materials, fast electron conduction and ion/electrolyte diffusion. © 2015 Published by Elsevier B.V.
Keywords: NiCo2O4 nanosheets Three dimensional porous nickel film Binder free Integrated electrode Electrochemical capacitor
1. Introduction Electrochemical capacitors, also called supercapacitors, are considered as one of the most promising energy storage devices due to their advantages including much higher power density and longer cycle life than traditional batteries and larger energy density than dielectric capacitors [1e5]. Ternary nickel cobaltite (NiCo2O4) spinel has been extensively investigated as a high-performance
* Corresponding authors. E-mail addresses: [email protected] (X.-Z. Fu), [email protected] (R. Sun). http://dx.doi.org/10.1016/j.jpowsour.2015.03.180 0378-7753/© 2015 Published by Elsevier B.V.
capacitive electrode material in recent years for its excellent electrochemical performance [6]. There are a lot of reports about synthesis of NiCo2O4 materials with different structures and morphologies [7] including nanoneedles [8], nanowires [9], and nanosheets [10,11], 3D flower-shaped microsphere composed of nanopetals [7,12,13]. These nanostructured NiCo2O4 electrode materials could exhibit excellent capacitive performance owing to the large surface area, short pathways of electron and ion transport [14,15]. However, the poor intrinsic electrical conductivity of NiCo2O4 still restrict its performance although it is higher than many other metal oxides [16]. Furthermore, the traditional electrode fabrication method would be hard to provide excellent
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electron and ion transport pathways for nanostructured NiCo2O4 due to the introduction of polymer binders and electrical conductive materials, which easily resulting in “dead active materials” [10]. Only the part of these active materials which exposed to the electrolyte and connected to the current collector can effectively participate in the electrochemical charge storage process, while the “dead materials” could hardly contribute to the total capacitance, leading to un-satisfactory performance [17,18]. The electrochemical performance of metal oxides could be improved by direct growth of the low conductivity active nanomaterials on the carbon or metal conductive current collectors to form binder- and conductive agent-free integrated electrodes [8,11,19e24]. Except the excellent electrical conductivity, the current collectors of carbon materials and metal foams possess also good flexibility, high mechanical strength and low cost. The binderand conductive agent-free porous electrodes with seamless heterostructures between active materials and current collectors would enlarge the surface area and greatly shorten the diffusion path for electrons and ions, provide more efficient contact interface between electrolyte ion and active materials for high-performance supercapacitors. For example, NiCo2O4 nanosheets, nanoneedle, wires arrays grown on Ni foam exhibit greatly improved electrochemical performance with very high capacitance and excellent cycling stability [23,25e27]. NiCo2O4 nanosheets also demonstrate very good capacitive performance by coating on carbon nanotubes or carbon nanofibers [21,22]. Herein, we report NiCo2O4 nano-sheets in-situ grown on the three dimensional (3D) porous Ni film current collectors as binderand conductive agent-free integrated capacitive electrodes. The 3D porous Ni film current collectors are prepared by hydrogen bubble template electrochemical deposition [28]. Then the NiCo2O4 nanosheets are in-situ formed on the 3D porous Ni films after electrochemical co-deposition of Ni/Co alloy layer followed by heat treatment. The 3D porous NiCo2O4/Ni electrodes demonstrate far superior higher area specific capacitance than that of porous NiO/Ni electrodes or smooth NiCo2O4/Ni electrodes. The 3D porous NiCo2O4/Ni electrodes also exhibit excellent rate capability and electrochemical stability.
for several times. 2.3. Fabrication of NiCo2O4 nanosheets on 3D porous Ni film Porous NiCo2O4/Ni films were synthesized via a simple electrochemical deposition method followed by a heat treatment. Electro-deposition was performed using an electrochemical working station (CHI660E, Shanghai, China) in a three-electrode system at room temperature. The porous Ni film, platinum-plate electrode and saturated calomel electrode were used as work electrode, counter electrode and reference electrode, respectively. In a typical synthesis, 6.67 g NiSO4$7H2O and 2.5 g CoSO4$7H2O were dissolved in 100 ml of deionized water under magnetic stirring to form electrolyte. The electrochemical co-deposition of Co and Ni alloy layer on the porous nickel films or smooth nickel foils was conducted with a current density of 18 mAcm2 for 20 min. Then the Co and Ni co-deposited films were calcined at 350 C in air for 24 h. The calcined samples with porous nickel films were named porous NiCo2O4/Ni electrode, and the calcined samples with smooth nickel foils were named smooth NiCo2O4/Ni electrode. Porous nickel films were also calcined at 350 C in air for 24 h, and named porous NiO/ Ni electrode. 2.4. Fabrication of asymmetric supercapacitors Asymmetric capacitors were fabricated with porous NiCo2O4/Ni, active carbon (AC), filter paper and 6 M KOH solution as positive electrode, negative electrode, separator and electrolyte, respectively. The negative electrode was prepared as following: firstly, the active carbon, acetylene black and polytetrafluoroethylene (PTFE) in the mass proportions of 85: 10: 5 were dispersed in the ethanol to produce a homogeneous paste by magnetic stirring. Then, the resulting mixture was coated onto Ni foam substrate (1.0 1.0 cm2) using a spatula. Finally, the fabricated electrode was pressed and then dried at 80 C for 10 h. Then the porous NiCo2O4/Ni integrated film electrode, filter paper separator and AC electrode were fabricated layer by layer and soaked in 6 M KOH solution to obtain asymmetric capacitors.
2. Experimental
2.5. Characterization
2.1. Materials
The crystal phases of samples were characterized by a X-Ray powder diffractometer (XRD, Rigaku D/Max 2500, Japan) with radiation from a Cu target. The morphology was observed using a field emission scanning electron microscope (FE-SEM, FEI Nova Nano SEM 450). XPS analysis was recorded on a PHI 5800 XPS system, where Al Ka excitation source was used.
All the reagents were analytical grade and used without further purification in the experiment. Nickel sulfate (NiSO4$7H2O), cobalt sulfate (CoSO4$7H2O), nickel chloride (NiCl2) and ammonium chloride (NH4Cl), and potassium hydroxide (KOH, 90%) were purchased from Sinopharm chemical reagent Co., Ltd (Shanghai, China). Nickel foil (99.99%, 30 mm thickness) was purchased from Shenzhen Kejing Star Technology Co., Ltd (Shenzhen, China). Deionized Mini-Q water was used as the solvent. 2.2. Preparation of 3D porous Ni films Porous Ni films as current collector were prepared by electrodepositing Ni on nickel foils using hydrogen bubble dynamic template method [28]. Prior to electro-deposition, a nickel foil (1 cm 1 cm) was cleaned by ultrasonic treatment in acetone to remove oil on the surface, etched with dilute hydrochloric acid, and then washed thoroughly with deionized water. The electrodeposition was conducted in aqueous solution containing 0.2 M NiCl2 and 2 M NH4Cl with 5 Acm2 current density at room temperature. A 1 cm2 nickel foil was used as the cathode and a large area nickel foil (4 cm 4 cm) was served as anode. After 1 min deposition, the obtained samples were washed with ethanol and deionized water
2.6. Electrochemical measurement The electrochemical tests were conducted with electrochemical workstation (Zennium Zahner, Germany) using cyclic voltammetry (CV) and galvanostatic chargeedischarge (CD) with a conventional three-electrode cell in 6.0 M aqueous KOH electrolyte or asymmetric supercapacitor at room temperature. The platinum-plate electrode (1.5 cm 1.5 cm) and mercuric and mercuric oxide electrode were served as counter electrode and reference electrode, respectively. Prior to tests, the working electrode was immersed in the electrolyte for 5 h. 3. Results and discussion Fig. 1 shows the XRD patterns of porous Ni films, Ni/Co codeposited porous Ni films before and after heat treatment at 350 C for 24 h in air. It could be seen that there are strong
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Fig. 1. XRD patterns of (a) porous Ni films, (b) Ni/Co co-deposited porous Ni films and (c) porous NiCo2O4/Ni films.
diffraction peaks at 44.4 and 51.7 for the porous Ni film substrate, indexing to the (111) and (200) planes of the face-centered cubic (fcc) phase of metal Ni (JCPDS Card no.04e0850). After Ni/Co codeposition, the XRD diffraction peak positions have no obviously change comparing to the porous Ni film substrate since the Ni atoms and Co atoms formed alloy through the electrochemical codeposition. It should be noted that the diffraction peaks are wide, suggesting the small size of the electrochemical deposited metal/ alloy particles. After heat treatment at 350 C in air, the strong diffraction peaks of metallic Ni still remain in the patterns but the intensities are some weaker relative to the original co-deposited Ni films. Some small diffraction peaks are observed at 31.1, 36.6 , 44.6 , and 59.1, respectively. These peaks are attributed to the (220), (311), (400), and (511) planes of spinel NiCo2O4 (JCPDS Card no. 73e170), revealing the NiCo2O4 is successfully formed after oxidation of the co-deposited NiCo alloy layer on the porous Ni film. To study the surface chemical state of the 3D hierarchical porous NiCo2O4/Ni film, the XPS analysis is further performed. And the XPS results are presented in Fig. 2. The typical signals of Ni 2p, Co 2p and O 1s are detected based on the survey spectra (Fig. 2a). In the Ni 2p spectrum (Fig. 2b), two kinds of nickel species containing Ni2þ and Ni3þ with two shake-up satellites (denoted as “sat.”) are detected. The fitting peaks at binding energies of 837.3 eV and 856.2 eV are assigned to Ni3þ, and the peaks at 871.9 eV and 854.2 eV are ascribed to Ni2þ [12]. Besides, the typical Co 2p1/2 and 2p3/2 peaks of 3D hierarchical porous NiCo2O4/Ni film are recorded at 795.5 eV and 780.1 eV, respectively, which consist of two spineorbit doublets characteristic of Co2þ and Co3þ(Fig. 2c) [25]. The highresolution spectrum for the O 1s spectrum (Fig. 2d) shows three oxygen contributions (denoted as O1, O2 and O3). The fitting peak of O1 at 533.4 eV can be attributed to the physi- and chemiabsorbed molecular water [12]. The peak at 531.1 eV is associated with oxygen in OH groups, indicating the surface of the NiCo2O4 is hydroxylated to some extent, because of either surface oxyhydroxide or the substitution of oxygen atoms at the surface by hydroxyl groups [12]. And the peak at 529.5 eV is typical of metaloxygen bonds [26]. This result is in line with the above XRD analysis and the previous XPS result of NiCo2O4 [12,26], confirming the formation of NiCo2O4 on the 3D porous Ni skeleton after annealing at 350 C. Fig. 3 shows the typical SEM images of Ni foil substrates before
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and after porous Ni film deposition. The results obviously indicate that 3D hierarchical porous feature is successfully formed on the smooth Ni foil substrate using hydrogen bubble dynamic template method. The diameter of big pores is even larger than 20 mm and the narrow pores or gaps between particles are smaller than 100 nm. There are lots of pores with different sizes from meso to macro on the surface and in the walls of big pores. Furthermore, there are many small gaps to connect the pores in the 3D hierarchical porous films. Hydrogen bubbles would evolve at a large cathodic current density of 5 Acm2 due to water electrolysis reaction companying with metallic Ni deposition. Ni atoms could not deposit on the sites where hydrogen bubbles generate but could deposit on the other Ni substrate sites which directly expose to electrolyte solution. And the hydrogen bubbles would accumulate and become larger as farther from the generated sites of Ni substrate. Thus the hydrogen bubbles are serves as a dynamic template to form porous Ni films. As the simultaneous reactions of nickel electro-deposition and water electrolysis continue, nickel atoms would grow on the active sites of nickel foil substrate and fresh nickel deposition. Then 3D hierarchically and highly porous Ni films could be formed by the numerous electro-deposited Ni particles interconnecting each other as the pore walls, which would be as much effective current collectors relative to the traditional Ni foam current collectors with larger pores. Ni/Co co-deposited porous Ni films display almost same 3D porous feature to the porous Ni films (Fig. 4aec), indicating a thin Ni/Co layer deposition on the porous Ni films. After annealing at 350 C for 24 h in air, both of porous Ni films and Ni/Co codeposited porous Ni films still remain the porous feature in overall morphologies of low magnification SEM images (Fig. 4d, g). There is only slight change of somewhat roughness on the particles for the porous nickel films after heat treatment at the same conditions in the very high magnification SEM images (Fig. 4i), resulting from the formation of NiO layer. But greatly different microstructure is observed for the surface of Ni/Co co-deposited porous Ni films after heat treatment in the high magnification SEM images (Fig. 4e, f). There are a lot of vertical NiCo2O4 nanosheets with about 200 nm in width and about 10 nm in thickness on the surface of 3D porous films and even on the walls of deep pores. The NiCo2O4 nanosheets also interconnect each other to form mesoporous and have very rough surface. This hierarchically and highly 3D porous structure would remarkably enlarge the surface area of active materials and facilitate the mass diffusion of electrolyte in the electrodes. And the NiCo2O4 nanosheets seamless supported on the hierarchically and highly 3D porous conductive Ni film current collectors would drastically reduce the contact resistance between the Ni current collectors and NiCo2O4 active materials. In addition, the NiCo2O4 nanosheets in-situ grown and attached on the 3D porous Ni film substrate, which could prevent them aggregation thus lead to good stability. Furthermore, this is a facile, scalable and low-cost method to fabricate efficient electrodes integrated of active materials and current collectors using simple electrochemical deposition and low temperature annealing, which is much different from the traditional complicated synthesis routes and expensive fabrications. To study the electrochemical properties of the as-prepared Ni based integrated film electrodes, the cyclic voltammetry (CV) and galvanostatic chargeedischarge cycling are performed in 6.0 M KOH solution. Fig. 5 shows the CV curves of the porous NiCo2O4/Ni, smooth NiCo2O4/Ni and porous NiO/Ni film electrodes at a scan rate of 10 mV s1 in the potential window of 0e0.55 V. A couple of redox peaks are clearly observed for all the electrodes as a result of the Faradaic capacitive behavior. A pair of redox peaks at 0.23 and 0.37 V (vs. Hg/HgO) are clearly observed for the porous NiCo2O4/Ni electrode, and these peaks correspond to the reversible reaction of
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Fig. 2. XPS spectra of the porous NiCo2O4/Ni films.
Co3þ/Co4þ and Ni2þ/Ni3þ transitions associated with anions OH. The redox reactions of NiCo2O4 in the alkaline electrolyte can be expressed as the following equations of (1) and (2) [29e32]. Although two kinds of redox reactions as shown in the equations, there is no two couples of redox peaks in the CV curves. The peaks at 0.23 and 0.37 V mainly correspond respectively to the oxidation of Co3þ and the reduction of Ni3þ, according to the previous report [6]. Compared with the smooth NiCo2O4/Ni electrode, larger redox potentials are observed for the 3D porous NiCo2O4/Ni electrode. There are more pores, active sites, large surface and interface area for the 3D porous NiCo2O4/Ni electrode. This architecture could enhance the redox currents, and then might enlarge the redox potentials.
NiCo2 O4 þ OH þ H2 O4NiOOH þ 2CoOOH þ e
(1)
CoOOH þ OH 4CoO2 þ H2 O þ e
(2)
The CV integrated area of porous NiCo2O4/Ni electrode is apparently much larger compared with the smooth NiCo2O4/Ni electrode, suggesting that the porous NiCo2O4/Ni electrodes have a significantly larger specific capacitance than that of the smooth NiCo2O4/Ni electrodes. It could be attributed to the 3D hierarchical porous structure of nickel current collector substrate, creating large open and electroactive surface sites as well as effective conductance. The porous NiO/Ni electrodes display obviously shifted redox peaks in the CV curve due to the different redox mechanism described as equation of (3) [13,33]:
NiO þ OH 4NiOOH þ e
(3)
The porous NiCo2O4/Ni electrodes illustrate much higher capacitance than that of porous NiO/Ni electrodes. The results suggest that NiCo2O4 has much better electrochemically capacitive
activity than NiO, benefiting from the richer redox reaction from the combination of nickel and cobalt ions than single nickel or cobalt ion [9,20]. Thus the capacitances of porous Ni substrate and NiO layer contributed slightly to the total areal specific capacitance of the porous NiCo2O4/Ni electrodes. The high-power characteristic of the porous NiCo2O4/Ni electrodes can be identified from their voltammetric response at various scan rates. Fig. 6a shows the corresponding CV curves of porous NiCo2O4/Ni electrodes at scan rates ranging from 10 to 100 mV s1. All curves at different scan rates exhibit a similar shape. At the sweep rate of 10 mV s1, a pair of redox peaks is located at ca. 0.23 V and 0.37 V, respectively. With the scan rate increasing from 10 to 100 mV s1, the anodic peaks shift towards positive potential and the cathodic peaks shift towards negative potential. At the sweep rate of 100 mV s1, a pair of redox peaks is located at ca. 0.14 V and 0.47 V, respectively. Even at a high scan rate of 100 mV s1, the CV curve still clearly shows a pair of redox peaks, indicating that the 3D porous integrated structure is beneficial to fast chargeedischarge due to the high interface area, easy ion diffusion, low resistance and super-fast electronic transport rate between the NiCo2O4 nanosheets and Ni current collector. Fig. 6b illustrates the dependences of anodic and cathodic peak currents on different scan rates. Both of anodic and cathodic peak current values increase highly linearly with the scan rate increasing. The correlation coefficients R2 were 0.987 and 0.986 for anodic and cathodic peaks, respectively. The linear relationship between them reveals that the fast redox reaction of NiCo2O4 is controlled by surface absorption while not diffusion limitation [34,35]. Remarkably, the peak potential shifts only ca. 94 mV for a 10-time increase in the scan rate, suggesting that the 3D porous electrode possesses low polarization, resulting from the efficient electron and ion transportation in the porous integrated electrodes of NiCo2O4 nanosheets seamless grown on the high conductive and porous
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Fig. 3. SEM images of (aeb) Ni foil and (cef) porous Ni films at different magnifications.
nickel films. Galvanostatic charging-discharging tests are further conducted in a stable potential window between 0 and 0.5 V at various current densities ranging from 1 to 10 mA cm2. As expected, the curves in Fig. 7a are not triangular in shape and present charge and discharge plateaus, resulting from the redox reactions that occur in this potential range. It confirms the pseudocapacitance behavior of NiCo2O4, which is consistent with the cyclic voltammetry results. Notably, all these curves are symmetrical, suggesting a superior reversibility of redox reactions in the 3D porous NiCo2O4/Ni electrodes with a stable coulombic efficiency and a low polarization [10]. The chargeedischarge performances of the three electrodes are also evaluated at a current density of 1 mA cm2 (Fig. 7b). There are voltage plateaus in the ranging from 0.15 to 0.25 V, which is consistent with the previous reports [8,29,36]. Evidently, the porous NiCo2O4/Ni electrodes exhibit much longer chargeedischarge times and dramatically higher areal specific capacitance than the other two electrodes. The areal specific capacitance is calculated from chargeedischarge data using the equation (4):
C¼
i$Dt S$DV
(4)
where i is the applied current density used for the charge/discharge measurements, Dt is the time elapsed for the discharge cycle, S is the area of the active material and DV is the potential window. The capacitance of porous NiCo2O4/Ni electrodes is 1139 mF cm2, which is about 10 times higher than that of smooth NiCo2O4/Ni electrode (107 mF cm2) and porous NiO/Ni electrode (124 mF cm2). The specific capacitances of porous NiCo2O4/Ni electrodes at various current densities are depicted in Fig. 7c. They are as high as 1139, 1073, 1029, 953, 917 mF cm2 at the discharge current densities of 1, 3, 5, 8, 10 mA cm2, respectively. The areal specific capacitance still retains 81% when the current density increased 10 times, indicting good rate capability and superior electrochemical capacitance for porous NiCo2O4/Ni electrodes. The as-prepared electrodes of NiCo2O4 nanosheets grown on 3D porous Ni films also illustrate much higher areal specific capacitance
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Fig. 4. SEM images of (aec) Ni/Co co-deposited porous Ni films, (def) porous NiCo2O4/Ni films and (gei) porous NiO/Ni films.
Fig. 5. Comparative CV curves recorded at a scan rate of 10 mV s1 for the three different electrodes.
relative to the previously reported electrodes of NiCo2O4 nanosheets grown on commercial Ni foam (650 mF cm2 at 2 mA cm2) [37] or carbon fiber paper (289 mF cm2 at 5 mA cm2) [38]. The high performance of the 3D porous NiCo2O4/Ni electrodes are originated from the high surface area for large utilization of electroactive NiCo2O4, 3D hierarchical meso-macro porous structure for
fast penetrationediffusion of electrolyte ions and 3D porous interconnected Ni metal conductive scaffold for efficient electron transfer. Cycling performance is one of the most important factors in determining the supercapacitors for practical applications. The cycling stability of the porous NiCo2O4/Ni electrode materials is evaluated by repeated charging-discharging measurements at a constant current density of 10 mA cm2 for 1000 cycles, as shown in Fig. 7d, before the initial 100 cycles, a small decrease of capacitance is observed, which might be ascribed to incompletely activation of the interior loose porous nanostructures with open space of the electrode materials. After completely activation, the capacitance displays a slight increase and tends to a good stability from the 100th cycle onwards. During the 100th to 1000th cycles, after completely activation, the areal specific capacitance decay is only 0.9% after 1000 cycles, and the retention from the initial capacitance is 99.1%, revealing the excellent long-term cycling stability of the 3D porous NiCo2O4/Ni electrodes. To further evaluate the 3D porous NiCo2O4/Ni electrodes for practical applications, complete capacitors with 3D porous NiCo2O4/Ni electrodes and active carbon electrodes are assembled as shown in Fig. 8a. Fig. 8b gives the CV curves of the asymmetric complete supercapacitors at various scan rates ranging from 10 to 100 mV s1. The CV shape of the completed capacitors is different from that of the single NiCo2O4/Ni electrode due to the asymmetric electrode of active carbon. And the completed capacitors have larger potential window relative to the single 3D porous NiCo2O4/
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Fig. 6. Electrochemical properties of the porous NiCo2O4/Ni electrode materials measured using a three-electrode system in 6.0 M KOH aqueous electrolyte: (a) CV curves at various scan rates ranging from 10 to 100 mV s1; (b) the dependences of cathodic and anodic peak currents on scan rate.
Fig. 7. (a) Chargeedischarge curves of the porous NiCo2O4/Ni electrode at various current densities; (b) chargeedischarge curves of the three different electrodes at a current density of 1 mA cm2; (c) area specific capacitance of the three electrodes as a function of current density; (d) cycling performance for 1000 cycles of the three electrodes at a current density of 10 mA cm2.
Ni electrode. The oxidation peaks and reduction peaks on the CV curves can be observed and the peak current becomes larger with the scan rate increasing from 10 to 100 mV s1. Galvanostatic chargeedischarge testing is also measured at various current densities. It can be seen the asymmetric complete supercapacitor demonstrates a good capacitive behavior based on its symmetric and linear chargeedischarge curves (Fig. 8c). Fig. 8d shows calculations of the as-fabricated asymmetric supercapacitor based on the corresponding chargeedischarge curves in Fig. 8c. The areal specific capacitance of the asymmetric supercapacitor is calculated to be 434.4, 411.3, 393.7, 388.6 and 303.6 mF cm2 at discharge current densities of 2, 5, 8, 10 and 20 mA cm2, respectively. It was observed that 69.9% of the capacitance could be retained when the current density was increased 10 times, showing good rate capability of the asymmetric supercapacitor. It should be noted that the
areal specific capacitance and rate performance of complete supercapacitor are not as excellent as the single 3D porous NiCo2O4/Ni electrode tested in three-electrode cell, which is similar to the previous reports [39,40]. It might be attributed to the separator and the another electrode as well as the complete supercapacitor fabrication process. Thus the optimization of complete supercapacitor comprising of the 3D porous NiCo2O4/Ni electrode is on going. 4. Conclusions NiCo2O4 nanosheets supported on 3D porous Ni films are in-situ synthesized through a facile electrochemical deposition method combined with a thermal treatment in air. The 3D porous NiCo2O4/ Ni integrated film electrodes exhibit much higher areal specific
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Fig. 8. (a) Schematic illustration of asymmetric complete supercapacitors with 3D porous NiCo2O4/Ni electrodes and active carbon electrodes (b) CV curves of the asymmetric supercapacitors at different scan rates; (c) chargeedischarge curves of the asymmetric supercapacitors at different current densities; (d) area specific capacitance of the asymmetric supercapacitors as a function of current density.
capacitance of 1139 mF cm2 compared to the smooth NiCo2O4/Ni (107 mF cm2) and the porous NiO/Ni electrodes (124 mF cm2). The 3D porous NiCo2O4/Ni electrodes also demonstrate good rate capability and excellent cycling stability with only 0.9% capacitance decay after 1000 cycles at a current density of 10 mA cm2. The excellent performance of 3D porous NiCo2O4/Ni integrated film electrodes might result from the large electrochemical interface area and active sites, fast transfer of electron/electrolyte ion, low contact resistance between NiCo2O4 nanosheets and 3D porous Ni metallic current collectors. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No.21203236), Guangdong and Shenzhen Innovative Research Team Program (No.2011D052, KYPT20121228160843692), Shenzhen Electronic Packaging Materials Engineering Laboratory (2012-372), Shenzhen High Density Electronic Packaging and Device Assembly Key Laboratory (ZDSYS20140509174237196). References [1] [2] [3] [4] [5]
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NiCo2O4 nanosheets in-situ grown on three dimensional porous Ni film current collectors as integrated electrodes for high-performance supercapacitors Tao Wang a, b, Ying Guo a, Bo Zhao a, Shuhui Yu a, Hai-Peng Yang b, Daniel Lu a, Xian-Zhu Fu a, *, Rong Sun a, *, Ching-Ping Wong c, d a
Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, PR China College of Materials Science and Engineering, Shenzhen University, Shenzhen 518055, PR China Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong, China d School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States b c
h i g h l i g h t s 3D hierarchical porous Ni films are fabricated as effective current collectors. NiCo2O4 nanosheets are in-situ grown on the 3D hierarchical porous Ni films. The porous NiCo2O4/Ni electrodes demonstrate excellent capacitive performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2014 Received in revised form 28 March 2015 Accepted 30 March 2015 Available online 31 March 2015
Three dimensional interconnected hierarchical porous Ni films are easily fabricated as effective current collectors through hydrogen bubble template electrochemical deposition. The binder-free integrated electrodes of spinel NiCo2O4 nanosheets directly coated the three dimensional porous Ni films are facilely obtained through successively electrochemical co-deposition of Ni/Co alloy layer then followed by subsequent annealing at 350 C in air. Compared with NiCo2O4 nanosheets on smooth Ni foil or porous NiO/Ni film electrodes, the porous NiCo2O4/Ni integrated film electrodes for supercapacitors demonstrate remarkably higher area specific capacitance. The porous NiCo2O4/Ni film electrodes also exhibit excellent rate capability and cycling stability. The super electrochemical capacitive performances are attributed to the unique integrated architecture of NiCo2O4 nanosheets in-situ grown on three dimensional continuous hierarchical porous Ni collector collectors, which could provide large electrode-electrolyte interface area, high active sites, low contact resistance between current collector and active materials, fast electron conduction and ion/electrolyte diffusion. © 2015 Published by Elsevier B.V.
Keywords: NiCo2O4 nanosheets Three dimensional porous nickel film Binder free Integrated electrode Electrochemical capacitor
1. Introduction Electrochemical capacitors, also called supercapacitors, are considered as one of the most promising energy storage devices due to their advantages including much higher power density and longer cycle life than traditional batteries and larger energy density than dielectric capacitors [1e5]. Ternary nickel cobaltite (NiCo2O4) spinel has been extensively investigated as a high-performance
* Corresponding authors. E-mail addresses: [email protected] (X.-Z. Fu), [email protected] (R. Sun). http://dx.doi.org/10.1016/j.jpowsour.2015.03.180 0378-7753/© 2015 Published by Elsevier B.V.
capacitive electrode material in recent years for its excellent electrochemical performance [6]. There are a lot of reports about synthesis of NiCo2O4 materials with different structures and morphologies [7] including nanoneedles [8], nanowires [9], and nanosheets [10,11], 3D flower-shaped microsphere composed of nanopetals [7,12,13]. These nanostructured NiCo2O4 electrode materials could exhibit excellent capacitive performance owing to the large surface area, short pathways of electron and ion transport [14,15]. However, the poor intrinsic electrical conductivity of NiCo2O4 still restrict its performance although it is higher than many other metal oxides [16]. Furthermore, the traditional electrode fabrication method would be hard to provide excellent
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electron and ion transport pathways for nanostructured NiCo2O4 due to the introduction of polymer binders and electrical conductive materials, which easily resulting in “dead active materials” [10]. Only the part of these active materials which exposed to the electrolyte and connected to the current collector can effectively participate in the electrochemical charge storage process, while the “dead materials” could hardly contribute to the total capacitance, leading to un-satisfactory performance [17,18]. The electrochemical performance of metal oxides could be improved by direct growth of the low conductivity active nanomaterials on the carbon or metal conductive current collectors to form binder- and conductive agent-free integrated electrodes [8,11,19e24]. Except the excellent electrical conductivity, the current collectors of carbon materials and metal foams possess also good flexibility, high mechanical strength and low cost. The binderand conductive agent-free porous electrodes with seamless heterostructures between active materials and current collectors would enlarge the surface area and greatly shorten the diffusion path for electrons and ions, provide more efficient contact interface between electrolyte ion and active materials for high-performance supercapacitors. For example, NiCo2O4 nanosheets, nanoneedle, wires arrays grown on Ni foam exhibit greatly improved electrochemical performance with very high capacitance and excellent cycling stability [23,25e27]. NiCo2O4 nanosheets also demonstrate very good capacitive performance by coating on carbon nanotubes or carbon nanofibers [21,22]. Herein, we report NiCo2O4 nano-sheets in-situ grown on the three dimensional (3D) porous Ni film current collectors as binderand conductive agent-free integrated capacitive electrodes. The 3D porous Ni film current collectors are prepared by hydrogen bubble template electrochemical deposition [28]. Then the NiCo2O4 nanosheets are in-situ formed on the 3D porous Ni films after electrochemical co-deposition of Ni/Co alloy layer followed by heat treatment. The 3D porous NiCo2O4/Ni electrodes demonstrate far superior higher area specific capacitance than that of porous NiO/Ni electrodes or smooth NiCo2O4/Ni electrodes. The 3D porous NiCo2O4/Ni electrodes also exhibit excellent rate capability and electrochemical stability.
for several times. 2.3. Fabrication of NiCo2O4 nanosheets on 3D porous Ni film Porous NiCo2O4/Ni films were synthesized via a simple electrochemical deposition method followed by a heat treatment. Electro-deposition was performed using an electrochemical working station (CHI660E, Shanghai, China) in a three-electrode system at room temperature. The porous Ni film, platinum-plate electrode and saturated calomel electrode were used as work electrode, counter electrode and reference electrode, respectively. In a typical synthesis, 6.67 g NiSO4$7H2O and 2.5 g CoSO4$7H2O were dissolved in 100 ml of deionized water under magnetic stirring to form electrolyte. The electrochemical co-deposition of Co and Ni alloy layer on the porous nickel films or smooth nickel foils was conducted with a current density of 18 mAcm2 for 20 min. Then the Co and Ni co-deposited films were calcined at 350 C in air for 24 h. The calcined samples with porous nickel films were named porous NiCo2O4/Ni electrode, and the calcined samples with smooth nickel foils were named smooth NiCo2O4/Ni electrode. Porous nickel films were also calcined at 350 C in air for 24 h, and named porous NiO/ Ni electrode. 2.4. Fabrication of asymmetric supercapacitors Asymmetric capacitors were fabricated with porous NiCo2O4/Ni, active carbon (AC), filter paper and 6 M KOH solution as positive electrode, negative electrode, separator and electrolyte, respectively. The negative electrode was prepared as following: firstly, the active carbon, acetylene black and polytetrafluoroethylene (PTFE) in the mass proportions of 85: 10: 5 were dispersed in the ethanol to produce a homogeneous paste by magnetic stirring. Then, the resulting mixture was coated onto Ni foam substrate (1.0 1.0 cm2) using a spatula. Finally, the fabricated electrode was pressed and then dried at 80 C for 10 h. Then the porous NiCo2O4/Ni integrated film electrode, filter paper separator and AC electrode were fabricated layer by layer and soaked in 6 M KOH solution to obtain asymmetric capacitors.
2. Experimental
2.5. Characterization
2.1. Materials
The crystal phases of samples were characterized by a X-Ray powder diffractometer (XRD, Rigaku D/Max 2500, Japan) with radiation from a Cu target. The morphology was observed using a field emission scanning electron microscope (FE-SEM, FEI Nova Nano SEM 450). XPS analysis was recorded on a PHI 5800 XPS system, where Al Ka excitation source was used.
All the reagents were analytical grade and used without further purification in the experiment. Nickel sulfate (NiSO4$7H2O), cobalt sulfate (CoSO4$7H2O), nickel chloride (NiCl2) and ammonium chloride (NH4Cl), and potassium hydroxide (KOH, 90%) were purchased from Sinopharm chemical reagent Co., Ltd (Shanghai, China). Nickel foil (99.99%, 30 mm thickness) was purchased from Shenzhen Kejing Star Technology Co., Ltd (Shenzhen, China). Deionized Mini-Q water was used as the solvent. 2.2. Preparation of 3D porous Ni films Porous Ni films as current collector were prepared by electrodepositing Ni on nickel foils using hydrogen bubble dynamic template method [28]. Prior to electro-deposition, a nickel foil (1 cm 1 cm) was cleaned by ultrasonic treatment in acetone to remove oil on the surface, etched with dilute hydrochloric acid, and then washed thoroughly with deionized water. The electrodeposition was conducted in aqueous solution containing 0.2 M NiCl2 and 2 M NH4Cl with 5 Acm2 current density at room temperature. A 1 cm2 nickel foil was used as the cathode and a large area nickel foil (4 cm 4 cm) was served as anode. After 1 min deposition, the obtained samples were washed with ethanol and deionized water
2.6. Electrochemical measurement The electrochemical tests were conducted with electrochemical workstation (Zennium Zahner, Germany) using cyclic voltammetry (CV) and galvanostatic chargeedischarge (CD) with a conventional three-electrode cell in 6.0 M aqueous KOH electrolyte or asymmetric supercapacitor at room temperature. The platinum-plate electrode (1.5 cm 1.5 cm) and mercuric and mercuric oxide electrode were served as counter electrode and reference electrode, respectively. Prior to tests, the working electrode was immersed in the electrolyte for 5 h. 3. Results and discussion Fig. 1 shows the XRD patterns of porous Ni films, Ni/Co codeposited porous Ni films before and after heat treatment at 350 C for 24 h in air. It could be seen that there are strong
T. Wang et al. / Journal of Power Sources 286 (2015) 371e379
Fig. 1. XRD patterns of (a) porous Ni films, (b) Ni/Co co-deposited porous Ni films and (c) porous NiCo2O4/Ni films.
diffraction peaks at 44.4 and 51.7 for the porous Ni film substrate, indexing to the (111) and (200) planes of the face-centered cubic (fcc) phase of metal Ni (JCPDS Card no.04e0850). After Ni/Co codeposition, the XRD diffraction peak positions have no obviously change comparing to the porous Ni film substrate since the Ni atoms and Co atoms formed alloy through the electrochemical codeposition. It should be noted that the diffraction peaks are wide, suggesting the small size of the electrochemical deposited metal/ alloy particles. After heat treatment at 350 C in air, the strong diffraction peaks of metallic Ni still remain in the patterns but the intensities are some weaker relative to the original co-deposited Ni films. Some small diffraction peaks are observed at 31.1, 36.6 , 44.6 , and 59.1, respectively. These peaks are attributed to the (220), (311), (400), and (511) planes of spinel NiCo2O4 (JCPDS Card no. 73e170), revealing the NiCo2O4 is successfully formed after oxidation of the co-deposited NiCo alloy layer on the porous Ni film. To study the surface chemical state of the 3D hierarchical porous NiCo2O4/Ni film, the XPS analysis is further performed. And the XPS results are presented in Fig. 2. The typical signals of Ni 2p, Co 2p and O 1s are detected based on the survey spectra (Fig. 2a). In the Ni 2p spectrum (Fig. 2b), two kinds of nickel species containing Ni2þ and Ni3þ with two shake-up satellites (denoted as “sat.”) are detected. The fitting peaks at binding energies of 837.3 eV and 856.2 eV are assigned to Ni3þ, and the peaks at 871.9 eV and 854.2 eV are ascribed to Ni2þ [12]. Besides, the typical Co 2p1/2 and 2p3/2 peaks of 3D hierarchical porous NiCo2O4/Ni film are recorded at 795.5 eV and 780.1 eV, respectively, which consist of two spineorbit doublets characteristic of Co2þ and Co3þ(Fig. 2c) [25]. The highresolution spectrum for the O 1s spectrum (Fig. 2d) shows three oxygen contributions (denoted as O1, O2 and O3). The fitting peak of O1 at 533.4 eV can be attributed to the physi- and chemiabsorbed molecular water [12]. The peak at 531.1 eV is associated with oxygen in OH groups, indicating the surface of the NiCo2O4 is hydroxylated to some extent, because of either surface oxyhydroxide or the substitution of oxygen atoms at the surface by hydroxyl groups [12]. And the peak at 529.5 eV is typical of metaloxygen bonds [26]. This result is in line with the above XRD analysis and the previous XPS result of NiCo2O4 [12,26], confirming the formation of NiCo2O4 on the 3D porous Ni skeleton after annealing at 350 C. Fig. 3 shows the typical SEM images of Ni foil substrates before
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and after porous Ni film deposition. The results obviously indicate that 3D hierarchical porous feature is successfully formed on the smooth Ni foil substrate using hydrogen bubble dynamic template method. The diameter of big pores is even larger than 20 mm and the narrow pores or gaps between particles are smaller than 100 nm. There are lots of pores with different sizes from meso to macro on the surface and in the walls of big pores. Furthermore, there are many small gaps to connect the pores in the 3D hierarchical porous films. Hydrogen bubbles would evolve at a large cathodic current density of 5 Acm2 due to water electrolysis reaction companying with metallic Ni deposition. Ni atoms could not deposit on the sites where hydrogen bubbles generate but could deposit on the other Ni substrate sites which directly expose to electrolyte solution. And the hydrogen bubbles would accumulate and become larger as farther from the generated sites of Ni substrate. Thus the hydrogen bubbles are serves as a dynamic template to form porous Ni films. As the simultaneous reactions of nickel electro-deposition and water electrolysis continue, nickel atoms would grow on the active sites of nickel foil substrate and fresh nickel deposition. Then 3D hierarchically and highly porous Ni films could be formed by the numerous electro-deposited Ni particles interconnecting each other as the pore walls, which would be as much effective current collectors relative to the traditional Ni foam current collectors with larger pores. Ni/Co co-deposited porous Ni films display almost same 3D porous feature to the porous Ni films (Fig. 4aec), indicating a thin Ni/Co layer deposition on the porous Ni films. After annealing at 350 C for 24 h in air, both of porous Ni films and Ni/Co codeposited porous Ni films still remain the porous feature in overall morphologies of low magnification SEM images (Fig. 4d, g). There is only slight change of somewhat roughness on the particles for the porous nickel films after heat treatment at the same conditions in the very high magnification SEM images (Fig. 4i), resulting from the formation of NiO layer. But greatly different microstructure is observed for the surface of Ni/Co co-deposited porous Ni films after heat treatment in the high magnification SEM images (Fig. 4e, f). There are a lot of vertical NiCo2O4 nanosheets with about 200 nm in width and about 10 nm in thickness on the surface of 3D porous films and even on the walls of deep pores. The NiCo2O4 nanosheets also interconnect each other to form mesoporous and have very rough surface. This hierarchically and highly 3D porous structure would remarkably enlarge the surface area of active materials and facilitate the mass diffusion of electrolyte in the electrodes. And the NiCo2O4 nanosheets seamless supported on the hierarchically and highly 3D porous conductive Ni film current collectors would drastically reduce the contact resistance between the Ni current collectors and NiCo2O4 active materials. In addition, the NiCo2O4 nanosheets in-situ grown and attached on the 3D porous Ni film substrate, which could prevent them aggregation thus lead to good stability. Furthermore, this is a facile, scalable and low-cost method to fabricate efficient electrodes integrated of active materials and current collectors using simple electrochemical deposition and low temperature annealing, which is much different from the traditional complicated synthesis routes and expensive fabrications. To study the electrochemical properties of the as-prepared Ni based integrated film electrodes, the cyclic voltammetry (CV) and galvanostatic chargeedischarge cycling are performed in 6.0 M KOH solution. Fig. 5 shows the CV curves of the porous NiCo2O4/Ni, smooth NiCo2O4/Ni and porous NiO/Ni film electrodes at a scan rate of 10 mV s1 in the potential window of 0e0.55 V. A couple of redox peaks are clearly observed for all the electrodes as a result of the Faradaic capacitive behavior. A pair of redox peaks at 0.23 and 0.37 V (vs. Hg/HgO) are clearly observed for the porous NiCo2O4/Ni electrode, and these peaks correspond to the reversible reaction of
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Fig. 2. XPS spectra of the porous NiCo2O4/Ni films.
Co3þ/Co4þ and Ni2þ/Ni3þ transitions associated with anions OH. The redox reactions of NiCo2O4 in the alkaline electrolyte can be expressed as the following equations of (1) and (2) [29e32]. Although two kinds of redox reactions as shown in the equations, there is no two couples of redox peaks in the CV curves. The peaks at 0.23 and 0.37 V mainly correspond respectively to the oxidation of Co3þ and the reduction of Ni3þ, according to the previous report [6]. Compared with the smooth NiCo2O4/Ni electrode, larger redox potentials are observed for the 3D porous NiCo2O4/Ni electrode. There are more pores, active sites, large surface and interface area for the 3D porous NiCo2O4/Ni electrode. This architecture could enhance the redox currents, and then might enlarge the redox potentials.
NiCo2 O4 þ OH þ H2 O4NiOOH þ 2CoOOH þ e
(1)
CoOOH þ OH 4CoO2 þ H2 O þ e
(2)
The CV integrated area of porous NiCo2O4/Ni electrode is apparently much larger compared with the smooth NiCo2O4/Ni electrode, suggesting that the porous NiCo2O4/Ni electrodes have a significantly larger specific capacitance than that of the smooth NiCo2O4/Ni electrodes. It could be attributed to the 3D hierarchical porous structure of nickel current collector substrate, creating large open and electroactive surface sites as well as effective conductance. The porous NiO/Ni electrodes display obviously shifted redox peaks in the CV curve due to the different redox mechanism described as equation of (3) [13,33]:
NiO þ OH 4NiOOH þ e
(3)
The porous NiCo2O4/Ni electrodes illustrate much higher capacitance than that of porous NiO/Ni electrodes. The results suggest that NiCo2O4 has much better electrochemically capacitive
activity than NiO, benefiting from the richer redox reaction from the combination of nickel and cobalt ions than single nickel or cobalt ion [9,20]. Thus the capacitances of porous Ni substrate and NiO layer contributed slightly to the total areal specific capacitance of the porous NiCo2O4/Ni electrodes. The high-power characteristic of the porous NiCo2O4/Ni electrodes can be identified from their voltammetric response at various scan rates. Fig. 6a shows the corresponding CV curves of porous NiCo2O4/Ni electrodes at scan rates ranging from 10 to 100 mV s1. All curves at different scan rates exhibit a similar shape. At the sweep rate of 10 mV s1, a pair of redox peaks is located at ca. 0.23 V and 0.37 V, respectively. With the scan rate increasing from 10 to 100 mV s1, the anodic peaks shift towards positive potential and the cathodic peaks shift towards negative potential. At the sweep rate of 100 mV s1, a pair of redox peaks is located at ca. 0.14 V and 0.47 V, respectively. Even at a high scan rate of 100 mV s1, the CV curve still clearly shows a pair of redox peaks, indicating that the 3D porous integrated structure is beneficial to fast chargeedischarge due to the high interface area, easy ion diffusion, low resistance and super-fast electronic transport rate between the NiCo2O4 nanosheets and Ni current collector. Fig. 6b illustrates the dependences of anodic and cathodic peak currents on different scan rates. Both of anodic and cathodic peak current values increase highly linearly with the scan rate increasing. The correlation coefficients R2 were 0.987 and 0.986 for anodic and cathodic peaks, respectively. The linear relationship between them reveals that the fast redox reaction of NiCo2O4 is controlled by surface absorption while not diffusion limitation [34,35]. Remarkably, the peak potential shifts only ca. 94 mV for a 10-time increase in the scan rate, suggesting that the 3D porous electrode possesses low polarization, resulting from the efficient electron and ion transportation in the porous integrated electrodes of NiCo2O4 nanosheets seamless grown on the high conductive and porous
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Fig. 3. SEM images of (aeb) Ni foil and (cef) porous Ni films at different magnifications.
nickel films. Galvanostatic charging-discharging tests are further conducted in a stable potential window between 0 and 0.5 V at various current densities ranging from 1 to 10 mA cm2. As expected, the curves in Fig. 7a are not triangular in shape and present charge and discharge plateaus, resulting from the redox reactions that occur in this potential range. It confirms the pseudocapacitance behavior of NiCo2O4, which is consistent with the cyclic voltammetry results. Notably, all these curves are symmetrical, suggesting a superior reversibility of redox reactions in the 3D porous NiCo2O4/Ni electrodes with a stable coulombic efficiency and a low polarization [10]. The chargeedischarge performances of the three electrodes are also evaluated at a current density of 1 mA cm2 (Fig. 7b). There are voltage plateaus in the ranging from 0.15 to 0.25 V, which is consistent with the previous reports [8,29,36]. Evidently, the porous NiCo2O4/Ni electrodes exhibit much longer chargeedischarge times and dramatically higher areal specific capacitance than the other two electrodes. The areal specific capacitance is calculated from chargeedischarge data using the equation (4):
C¼
i$Dt S$DV
(4)
where i is the applied current density used for the charge/discharge measurements, Dt is the time elapsed for the discharge cycle, S is the area of the active material and DV is the potential window. The capacitance of porous NiCo2O4/Ni electrodes is 1139 mF cm2, which is about 10 times higher than that of smooth NiCo2O4/Ni electrode (107 mF cm2) and porous NiO/Ni electrode (124 mF cm2). The specific capacitances of porous NiCo2O4/Ni electrodes at various current densities are depicted in Fig. 7c. They are as high as 1139, 1073, 1029, 953, 917 mF cm2 at the discharge current densities of 1, 3, 5, 8, 10 mA cm2, respectively. The areal specific capacitance still retains 81% when the current density increased 10 times, indicting good rate capability and superior electrochemical capacitance for porous NiCo2O4/Ni electrodes. The as-prepared electrodes of NiCo2O4 nanosheets grown on 3D porous Ni films also illustrate much higher areal specific capacitance
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Fig. 4. SEM images of (aec) Ni/Co co-deposited porous Ni films, (def) porous NiCo2O4/Ni films and (gei) porous NiO/Ni films.
Fig. 5. Comparative CV curves recorded at a scan rate of 10 mV s1 for the three different electrodes.
relative to the previously reported electrodes of NiCo2O4 nanosheets grown on commercial Ni foam (650 mF cm2 at 2 mA cm2) [37] or carbon fiber paper (289 mF cm2 at 5 mA cm2) [38]. The high performance of the 3D porous NiCo2O4/Ni electrodes are originated from the high surface area for large utilization of electroactive NiCo2O4, 3D hierarchical meso-macro porous structure for
fast penetrationediffusion of electrolyte ions and 3D porous interconnected Ni metal conductive scaffold for efficient electron transfer. Cycling performance is one of the most important factors in determining the supercapacitors for practical applications. The cycling stability of the porous NiCo2O4/Ni electrode materials is evaluated by repeated charging-discharging measurements at a constant current density of 10 mA cm2 for 1000 cycles, as shown in Fig. 7d, before the initial 100 cycles, a small decrease of capacitance is observed, which might be ascribed to incompletely activation of the interior loose porous nanostructures with open space of the electrode materials. After completely activation, the capacitance displays a slight increase and tends to a good stability from the 100th cycle onwards. During the 100th to 1000th cycles, after completely activation, the areal specific capacitance decay is only 0.9% after 1000 cycles, and the retention from the initial capacitance is 99.1%, revealing the excellent long-term cycling stability of the 3D porous NiCo2O4/Ni electrodes. To further evaluate the 3D porous NiCo2O4/Ni electrodes for practical applications, complete capacitors with 3D porous NiCo2O4/Ni electrodes and active carbon electrodes are assembled as shown in Fig. 8a. Fig. 8b gives the CV curves of the asymmetric complete supercapacitors at various scan rates ranging from 10 to 100 mV s1. The CV shape of the completed capacitors is different from that of the single NiCo2O4/Ni electrode due to the asymmetric electrode of active carbon. And the completed capacitors have larger potential window relative to the single 3D porous NiCo2O4/
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Fig. 6. Electrochemical properties of the porous NiCo2O4/Ni electrode materials measured using a three-electrode system in 6.0 M KOH aqueous electrolyte: (a) CV curves at various scan rates ranging from 10 to 100 mV s1; (b) the dependences of cathodic and anodic peak currents on scan rate.
Fig. 7. (a) Chargeedischarge curves of the porous NiCo2O4/Ni electrode at various current densities; (b) chargeedischarge curves of the three different electrodes at a current density of 1 mA cm2; (c) area specific capacitance of the three electrodes as a function of current density; (d) cycling performance for 1000 cycles of the three electrodes at a current density of 10 mA cm2.
Ni electrode. The oxidation peaks and reduction peaks on the CV curves can be observed and the peak current becomes larger with the scan rate increasing from 10 to 100 mV s1. Galvanostatic chargeedischarge testing is also measured at various current densities. It can be seen the asymmetric complete supercapacitor demonstrates a good capacitive behavior based on its symmetric and linear chargeedischarge curves (Fig. 8c). Fig. 8d shows calculations of the as-fabricated asymmetric supercapacitor based on the corresponding chargeedischarge curves in Fig. 8c. The areal specific capacitance of the asymmetric supercapacitor is calculated to be 434.4, 411.3, 393.7, 388.6 and 303.6 mF cm2 at discharge current densities of 2, 5, 8, 10 and 20 mA cm2, respectively. It was observed that 69.9% of the capacitance could be retained when the current density was increased 10 times, showing good rate capability of the asymmetric supercapacitor. It should be noted that the
areal specific capacitance and rate performance of complete supercapacitor are not as excellent as the single 3D porous NiCo2O4/Ni electrode tested in three-electrode cell, which is similar to the previous reports [39,40]. It might be attributed to the separator and the another electrode as well as the complete supercapacitor fabrication process. Thus the optimization of complete supercapacitor comprising of the 3D porous NiCo2O4/Ni electrode is on going. 4. Conclusions NiCo2O4 nanosheets supported on 3D porous Ni films are in-situ synthesized through a facile electrochemical deposition method combined with a thermal treatment in air. The 3D porous NiCo2O4/ Ni integrated film electrodes exhibit much higher areal specific
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Fig. 8. (a) Schematic illustration of asymmetric complete supercapacitors with 3D porous NiCo2O4/Ni electrodes and active carbon electrodes (b) CV curves of the asymmetric supercapacitors at different scan rates; (c) chargeedischarge curves of the asymmetric supercapacitors at different current densities; (d) area specific capacitance of the asymmetric supercapacitors as a function of current density.
capacitance of 1139 mF cm2 compared to the smooth NiCo2O4/Ni (107 mF cm2) and the porous NiO/Ni electrodes (124 mF cm2). The 3D porous NiCo2O4/Ni electrodes also demonstrate good rate capability and excellent cycling stability with only 0.9% capacitance decay after 1000 cycles at a current density of 10 mA cm2. The excellent performance of 3D porous NiCo2O4/Ni integrated film electrodes might result from the large electrochemical interface area and active sites, fast transfer of electron/electrolyte ion, low contact resistance between NiCo2O4 nanosheets and 3D porous Ni metallic current collectors. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No.21203236), Guangdong and Shenzhen Innovative Research Team Program (No.2011D052, KYPT20121228160843692), Shenzhen Electronic Packaging Materials Engineering Laboratory (2012-372), Shenzhen High Density Electronic Packaging and Device Assembly Key Laboratory (ZDSYS20140509174237196). References [1] [2] [3] [4] [5]
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