Enhanced electrochemical performance of nanoplate nickel cobaltite (NiCo2O4) supercapacitor applications

Well-ordered, unique interconnected nanostructured binary metal oxides with lightweight, free-standing, and highly flexible nickel foam substrate electrodes have attracted tremendous research attention for high performance supercapacitor applications owing to the combination of the improved electrical conductivity and highly efficient electron and ion transport channels. In this study, a unique interconnected nanoplate-like nickel cobaltite (NiCo2O4) nanostructure was synthesized on highly conductive nickel foam and its use as a binder-free material in energy storage applications was assessed. The nanoplate-like NiCo2O4 nanostructure electrode was prepared by a simple chemical bath deposition method under optimized conditions. The NiCo2O4 electrode delivered an outstanding specific capacitance of 2791 F g−1 at a current density of 5 A g−1 in a KOH electrolyte in a three-electrode system as well as outstanding cycling stability with 99.1% retention after 3000 cycles at a current density of 7 A g−1. The as-synthesized NiCo2O4 electrode had a maximum energy density of 63.8 W h kg−1 and exhibited an outstanding high power density of approximately 654 W h kg−1. This paper reports a simple and cost-effective process for the synthesis of flexible high performance devices that may inspire new ideas for energy storage applications.


Introduction
][6][7][8][9] Electrode materials can be classied into two main groups based on the charge storage mechanism: electric double-layer capacitors (EDLCs) and pseudocapacitors (PCs). 10,113][14][15][16] On the other hand, the practical applications of EDLCs have been limited by their low specic capacitance and low energy density.1][32] To overcome this problem, further efforts have been made to prepare nanocomposites that combine NiO with other electroactive materials.Among the various PCs components, cobalt oxides (Co 3 O 4 and CoO) have become promising candidates for supercapacitor electrode applications owing to their low toxicity, low parity cost, facile preparation, and good corrosion stability. 33,34Moreover, binary metal oxides exhibit better electrochemical performance compared to single-component metal oxides owing to their outstanding electrical conductivity and multiple oxide states.
In particular, ternary nickel cobaltite (NiCo 2 O 4 ) has attracted considerable attention for its ultrahigh specic capacitance because ternary NiCo 2 O 4 combines the characteristics of simple transition metal oxides, has high specic capacitance and electronic conductivity, and better electron transport between the electrolyte and electrode surface area than binary metal oxides of nickel oxide (NiO) and cobalt oxide (Co 3 O 4 ). 35,36In particular, NiCo 2 O 4 has attracted considerable attention, owing to its superior conductivity to other binary metal oxides, such as ZnFe 2 O 4 , CoMoO 4 , MnMoO 4 , Zn 3 V 2 O 8 , and NiMnO 3 .Nickel cobaltite (NiCo 2 O 4 ) has excellent redox activity, natural abundance, low cost, environmentally benign characteristics, and nontoxicity. 37,38Thus, the fabrication of nanoplate-structured NiCo 2 O 4 using a simple and novel method to enhance its electrochemical performance is the focus of this research.Until now, there are only a few reports on the use of NiCo 2 O 4 as an electrode material for supercapacitor applications.The chemical bath deposition approach is a more common method for the preparation of functional materials than hydrothermal methods, microwave irradiation method, thermal annealing treatment approach, and mechano-chemical synthesis.Despite this, little attention has been paid to the synthesis of NiCo 2 O 4 electrode materials.The production of high-performance supercapacitors for nanostructured NiCo 2 O 4 materials by a simple method remains a challenge.
In this study, a nanostructure of NiCo 2 O 4 nanoplate-like structure was grown uniformly on highly conductive nickel foam for high-performance supercapacitor applications through a chemical bath deposition method.By rationally controlling the growth kinetics of NiCo 2 O 4 , the electrode exhibited a honeycomb nanostructure on a nickel foam surface, which provided rapid channels for ion and electron transfer and formed abundant electrochemically active sites exposed to the electrolyte surface area.The NiCo 2 O 4 nanoplate's electrode exhibited a high specic capacitance of 2791 F g À1 at a current density of 5 A g À1 and delivered an excellent energy density 63.8 W h kg À1 and high power density of 654 W kg À1 in a KOH electrolyte.This might be because the unique interconnected nanoplate's structure exhibits good reversibility and lower charge-transfer resistance of 0.2 U during the faradaic process.These supercapacitors also exhibited excellent long-life cycling stability of approximately 99.1% retention aer 3000 cycles, which might be because the nanoplate nanostructure allows convenient ion transport within and between the combs.

Experimental section
Preparation of the NiCo 2 O 4 nanoplate material on nickel foam All chemicals applied, such as nickel nitrate hexahydrate, cobalt nitrate hexahydrate, urea, and ammonium uoride, were of analytical grade and used as received.The nanoplate-like NiCo 2 O 4 nanostructure was synthesized by a simple chemical bath deposition method.The process involved a solvothermal reaction of nickel and cobalt ions as inorganic components with urea and ammonium uoride to form a precursor/self-sacricing template, which was followed by heat treatment in air to generate nanoplate-like structures.Prior to synthesis, a piece of nickel (1 cm Â 1 cm) foam was cleaned by being immersed in 2 M HCl under ultrasonication in sequence for 15 min, and rinsed with absolute ethanol and deionized (DI) water, respectively, which results in the removal of the surface layer and greasing of Ni oxide.In particular, an aqueous homogeneous solution of all chemicals applied, such as 0.42 g of Ni (NO 3 ) 2 $6H 2 O and 0.86 g of Co(NO 2 ) 3 $6H 2 O, 0.22 g of CH 4 N 2 O and 1.80 g of urea were dissolved in 100 mL of deionized water with constant magnetic stirring for 50 min.Aer stirring, the homogeneous solution formed a clear pink color solution, which indicates the active electrode of nickel cobaltite (NiCo 2 O 4 ) material.The resulting clear solution containing a piece of nickel foam was transferred into a 100 mL capped bottle and kept at 120 C for 12 h.Aer cooling to room temperature, the as-prepared nickel foam was separated by centrifugation and rinsed with absolute ethanol and distilled water.The as-prepared product was then dried with a dryer for 30 min.Finally, for the heat treatment step, the resulting prepared product was annealed under 400 C for 3 h at a heating rate of 20 C min À1 in air.The as-prepared samples supported on nickel foam were obtained.The total average NiCo 2 O 4 loading was 3 mg cm À2 .

Material characterization
The morphology, microstructure, and internal structural properties of the samples were studied by eld emission scanning electron microscopy (FESEM, S-4800, Hitachi) at an acceleration voltage of 15 kV and eld emission transmission electron microscopy (FETEM, JEOL, JEM-2100F) operated at 200 kV.The crystalline phase structure of the as-prepared sample was investigated by powder X-ray diffraction (XRD, D8 ADVANCE Bruker) at a voltage of 40 kV and a current of 40 mA using Cu Ka radiation (k ¼ 0.1542 nm).The elemental composition of the material was determined by X-ray photoelectron spectroscopy (XPS, VG Scientic ESCALAB 250) using Al Ka radiation (Scheme 1).

Electrochemical measurements
Electrochemical characterization of the NiCo 2 O 4 were carried out in a 3 M aqueous KOH solution on an electrochemical workstation (BioLogic-SP150 Korea) using a conventional three electrode system.A three-electrode system containing Ag/AgCl and platinum foil (Pt) as the reference and counter electrode, respectively, was used.The as-prepared NiCo 2 O 4 electrode were directly used as the working electrode.Cyclic voltammetry (CV) of the resulting product was performed over the potential range from À0.2 to 0.5 V vs. Ag/AgCl at different scan rates.The galvanostatic charge-discharge curves test of the prepared electrode was performed over the potential range, 0 to 0.4 V vs. Ag/ AgCl, at various current densities (A).Electrochemical impedance spectroscopy (EIS) was conducted over the frequency range, 0.1 Hz to 100 kHz with an AC amplitude of 5 mV.The specic capacitance (C sc , F g À1 ) of the electrode from GCD plots, energy density (E, W h kg À1 ) and power density (P, W kg À1 ) for the three electrode system were calculated using the following equations (eqn (1), (2), and (3)) given below: 39 where I (A) is the discharge current, Dt (s) is the discharge time, m (g) is the mass of the electroactive material of the active electrode, and DV (V) is the voltage window drop, respectively.

Results and discussion
The morphology, interior structure, and crystalline properties of the as-prepared NiCo 2 O 4 nanomaterial was examined by SEM, TEM and high-resolution TEM (HR-TEM), as shown in Fig. 1.Fig. 1a-c shows the morphology of the nanoplate-like NiCo 2 O 4 at various magnications.The nickel foam is covered uniformly by NiCo 2 O 4 nanoplate structures, indicating that the nanoplates are composed of many nanosheets.The ne nanoplates had a mean diameter of 50-80 nm.As shown in Fig. 1c, the internal structure of the as-prepared material has larger pores because of the empty spaces between the plates.In particular, this nanostructure has high specic surface areas with a bimodal pore size distributions, which play a key role in the high energy density and power density suitable for energy  2b).EDX analysis of the as-prepared product showed nickel, cobalt and oxygen (Fig. 2b), indicating a pure phase without impurities.The atomic percentage of Ni, Co, and O were 41.87%, 35.39%, and 22.73% (5 : 4 : 3), respectively.A Gaussian t of Ni 2p deconvoluted spectrum at 855.8, 861.8, 873.5, and 879.9 eV revealed Ni 2+ 2p 3/2 , Ni 3+ 2p 3/2 , Ni 2+ 2p 1/2 and Ni 3+ 2p 1/2 , respectively (Fig. 2d), indicating the presence of two spin-orbit doublets and two shakeup satellites denoted as ("Sat.")as well as +2 and +3 oxidation states in the prepared NiCo 2 O 4 material. 41In the case of the Co 2p (Fig. 2e) deconvoluted spectrum, multiple peaks at 780.5, 789.6, 795.1 and 804.2 eV were assigned to Co 2+ 2p 3/2 , Co 2+ 2p 3/2 , Co 3+ 2p 1/2 , and Co 2+ 2p 1/2 , respectively. 42The high resolution O 1s spectrum (Fig. 2f) was deconvoluted into two main peaks, 530.2 and 530.9 eV, which conrmed the formation of metal-oxygen bonding in the presence of O 1s. 43 The data clearly show that the presence of valence and mixed valence of metal ions, Ni 2+ /Ni 3+ , Co 2+ /Co 3+ , and O 1s, on the surface of the as-prepared NiCo 2 O 4 sample, which is expected to provide sufficient active sites for the redox reaction in the electroactive activity.
The electrochemical performance of as-prepared NiCo 2 O 4 as an electrode material for supercapacitors was investigated using a three-electrode system in a 3 M KOH aqueous solution as the electrolyte.Fig. 3a presents the CV curves of the active electrode.All CV curves showed a pair of redox peaks at various scan rates from 10-100 mV s À1 over the voltage range of À0.2 to 0.5 V, indicating similar pseudocapacitive behavior and good rate capability.In addition, with increasing scan rates, the redox peaks current intensity areas become larger as the scan sweep increased.Furthermore, due to polarization of the electroactive sample, the anodic peaks shied enormously towards a positive potential, whereas the cathodic peaks moved towards a negative potential with increasing scan rate because of the low resistance and fast ion and electron transfer rates.The strong peaks can be closely related to the chemical composition and morphology of the electrode material for the reversible faradaic redox reactions and redox reaction corresponding to an alkaline electrolyte according to the equations during the electrochemical measurements.
In addition, galvanic charge-discharge (GCD) measurements were taken to evaluate the specic capacitance of the electroactive material in a 3 M KOH aqueous electrolyte solution between 0-0.5 V (vs.Ag/AgCl) at various current densities of 5-10 A g À1 .The resulting curves were attributed to the faradaic redox process and were close to battery type charge storage, as shown in Fig. 3b.The voltage plateaus in the charge-discharge plots were retained at high scan rates, which indicates the high rate capability of the electrode material.According to eqn (1), the NiCo 2 O 4 electrode exhibited a specic capacitance of 2791, 2650, 2570, 2530, 2510, and 2490 F g À1 at current densities of 5, 6, 7, 8, 8, 9, and 10 A g À1 , respectively (as shown in Fig. 3c).The specic capacitance was also superior to those of recently reported exible electrodes with NiCo 2 O 4 agglomerated particles (351 F g À1 at a current density of 1 A g À1 ), 44 NiCo 2 O 4 nanoatlike particles (764 F g À1 at the scan rate of 2 mV s À1 ), 45 NiCo 2 O 4 ower-like nanostructure (658 F g À1 at a current The long-term cycling stability and coulombic efficiency of the as-prepared product as an electrode material was examined by repeating the charge-discharge plots.As shown in Fig. 4c, the NiCo 2 O 4 nanoplate-like structure exhibited excellent longterm electrochemical stability and high rate stability with a very slight decrease to 99.1%, even aer 3000 cycles (only 0.9% loss aer 3000 cycles) in a three-electrode system at a current density of 7 A g À1 .This is superior to those of previously reported 3D hierarchical ower-shaped NiCo 2 O 4 microspheres (93.2% retention aer 1000 cycles), 56 hierarchical spinal NiCo 2 O 4 nanowires (84.7% retention aer 500 cycles), 57 NiCo 2 -O 4 @MnO 2 nanowire arrays (88% retention aer 2000 cycles), 58 NiCo 2 O 4 @NiO nanowire arrays (93.1% retention aer 3000 cycles), 59 NiCo 2 O 4 @CoMoO 4 nanowire/nanoplates arrays (74.1% retention aer 1000 cycles), 60 ultrathin porous NiCo 2 O 4 nanosheet arrays (20% reduced aer 3000 cycles), 61 and NiCo 2 O 4 @NiMoO 4 nanowires (90.6% retention aer 2000 cycles). 62TEM showed that the nanohoneycombs still existed in the NiCo 2 O 4 electrode (inset in Fig. 4c) aer long term cycling, suggesting that there was no noticeable change in the morphology of the sample.Such excellent cycling stability of the NiCo 2 O 4 electrode could be attributed to the following: the unique honeycomb structure, which can alleviate the volume changes during the charge-discharge process to guarantee good stability; gradual penetration of the electrolyte ions into the electroactive material; and reduced volume expansion resulting from the rapid and long-term faradaic reaction.

Conclusion
NiCo 2 O 4 with a unique interconnected nanoplate-like structure was fabricated on the surface of the highly conductive nickel foam through a simple chemical bath deposition reaction.The interconnected honeycomb nanostructure electrode can enhance the pathway for electron transport originating from the good electronic conductivity of the nickel foam.In addition, it can reduce the transport distance of ions and enhance electrode-electrolyte contact, which leads to higher material activation, and endow it with a high specic capacitance and excellent long life cycling stability.The NiCo 2 O 4 nanoplate electrode delivered a high specic capacitance of 2791 F g À1 at a current density of 5 A g À1 and exhibited a signicantly high energy density of 63.8 W h kg À1 and high power density of 654 W kg À1 , highlighting its potential for energy storage applications.The nanoplate structure electrode material for electrochemical capacitors displayed excellent cycling stability of 99.1% retention aer 3000 cycles, even at a high current density of 7 A g À1 .As a result, the NiCo 2 O 4 electrode provides a path for high-performance supercapacitors because of its lowcost simple chemical bath deposition approach and environmental friendliness.

Fig. 1 Scheme 1
Fig.1 (a-c) Low-and high-magnification FE-SEM images and (d and e) TEM and HR-TEM images of the NiCo 2 O 4 nanoplate material.

Fig. 3
Fig. 3 (a) CV curves of the NiCo 2 O 4 nanoplate's obtained at various scan rates of 10-100 mV s À1 ; (b) galvanostatic charge-discharge curves of the NiCo 2 O 4 at different current densities of 5-10 A g À1 ; (c) specific capacitance of the three electrode material at various current densities.

Fig. 4
Fig. 4 (a) Nyquist plots of the NiCo 2 O 4 electrode, (b) Ragone plots of the device and (c) cyclic performance of NiCo 2 O 4 nanoplates during 3000 cycles at a scan rate of 7 A g À1 .