High-performance solid state asymmetric supercapacitor based on electrochemically decorated 3D network-like Co3O4 architecture on NiO nanoworms

Abstract The unique 3D network-like cobalt oxide architecture was electrochemically deposited on nickel oxide nanoworms (NiONWs) with nickel foil (NF) substrate. After the anodic polarization of NF and its activation, a very thin film of NiO nanoworms with the average diameter of 30 nm and the length of 100 nm was successfully grown on anodized Ni foil (a-NF) by using a facile hydrothermal method under the mild conditions including a saturated alkaline solution of Ni2+ at 65 °C to prepare a-NF/NiONWs. Then, 3D network-like Co3O4 nanosheets with the average thickness of 25 nm were directly electrodeposited on a-NF/NiONWs by using cyclic voltammetry under the optimized experimental conditions. The morphologies and detailed geometrical structures of the electrodes were conducted by FE-SEM. The fabricated a-NF/NiONWs/Co3O4 electrode provides a high areal specific capacitance of 1320 mF cm-2 at a current density of 4 mA cm−2 and high mass specific capacitance of 2000 F g−1 at the current density of 6 A g−1. By using a-NF/NiONWs/Co3O4 as a positive electrode and reduced graphene oxide-deposited NF (RGO/NF) as a negative electrode, a solid-state asymmetric a-NF/NiONWs/Co3O4//RGO/NF supercapacitor was assembled and it exhibited a high specific capacitance of 471.4 mF cm-2 at a current density of 3 mA cm−2. The fabricated solid-state supercapacitor also showed good rate capacity and long-term cycling performance of 2000 cycles.


Introduction
To address the rapid depletion of fossil fuels and the environmental pollution issues, there is a clear urgent need to develop green and renewable energy storage technologies to achieve a secure and reliable energy supply. Among the energy storage devices such as batteries, supercapacitors (SCs) have drawn intensive research attention mainly due to their high specific power densities, fast recharge capability and excellent cycling life. Besides, SCs can be used in hybrid energy storage systems (HESS) in combination with batteries and solar panels to minimize the battery stress.
Electrode materials are considered to play an important role in the performance of SCs. So far, there are mainly three kinds of electrode materials for their SC applications including carbon-based high surface area materials, conductive polymers and transition metal oxides.
Although carbon-based SCs are the most widely used materials for commercially available SCs due to their long cycle life in the order of 10 5 , transition metal oxides can provide higher specific capacitances than carbonaceous materials and conducting polymers because of their multiple oxidation states for reversible Faradaic reactions and unique redox properties.
Among metal oxides available for SCs, cobalt oxides/hydroxides are favorable candidates due to their low cost, natural abundance, environmental compatibility, high theoretical specific capacitance of 3560 F g -1 , and high cycle life of about 20,000 cycles. Well-defined electrochemical redox activity and facilitated Co(OH)2/CoOOH redox processes are all the benefits of Co-based SCs. As reported very recently, the lattice structural similarity between Co(OH)2 and CoOOH redox couple enables a battery-mimic mechanism, resulting in high specific pseudocapacitance and long cycling life of Co-based SCs. Similar to pseudocapacitive mechanism of Co-based SCs, Ni(OH)2 and NiOOH as a redox couple of Ni-based SCs have similar lattice structures which can result in the facilitated redox processes in Ni-based SCs.
Surface structure and size of these nano-architectures for charge storage in few nanometers of the electrode surface seriously affects the specific capacitance of SCs. Actually, the limited ion diffusion within the dense nanostructure and the slow electron transport between the electrolyte and electroactive species at the electrode surface will result in low specific capacitances for Co and Ni-based SCs far from their theoretical capacities.
In this work, regarding the lattice structural similarity between Co(OH)2/CoOOH and Ni(OH)2/NiOOH redox couples and the serious effect of film thickness of supercapacitive materials on charge storage, a very thin layer of NiO nanoworms (NiONWs) were grown on ٣ anodized Ni foil (a-NF) by using a facile and scalable hydrothermal method. To the best of our knowledge, this is the first report on the growth of wormlike NiO nanostructure under the mild conditions for SC applications. Moreover, 3D network-like Co3O4 nanosheets were electrodeposited on a-NF/NiONWs by using a simple sequential-electrodeposition method.
Finally, a solid state asymmetric supercapacitor was fabricated by assembling a-NF/NiONWs/Co3O4 as a negative electrode and NF/graphene as the positive electrode.
Remarkably, the fabricated asymmetric supercapacitor (ASC) shows preeminent cycling stability with a slight decay after 2000 charge/discharge cycles in the potential window of 0 to 1.4 V with a high specific capacitance.

Fabrication of a-NF/NiONWs/Co3O4 electrode
For the fabrication of a-NF/NiONWs/Co3O4 electrode, at first, Ni foil (NF) was anodized under the applied anodic potential. For the preparation of anodized Ni foil (a-NF), Ni foil (2.0 0.5 cm 2 ) was polished with a fine emery paper (1200 SiC paper) and then was cleaned by consecutive sonication in deionized (DI) water, ethanol, and acetone for 10 min.
Anodization of NF was performed in a 100-ml solution containing 94.5 ml glycerol (C3H8O3), 5.0 ml DI water and 0.50 g KOH. Pre-cleaned NF was anodized in a twoelectrode electrochemical cell with NF (2.0 0.5 cm 2 ) as anode and Pt foil (1.0 1.0 cm 2 ) as cathode by applying +60 V for 45 min. Finally, anodized nickel foil (a-NF) was rinsed with DI water, sonicated for 10 min to remove residual electrolyte and dried in ambient air.
NiO nanoworms (NiONWs) were grown on a-NF in a supersaturated alkaline solution of Ni 2+ . The solution was prepared by the addition of appropriate amounts of 0.20 M nickel acetate solution into 20.0 mL of 1.0 M NaOH solution at 65 ˚C until a slight Ni(OH)2 precipitate appeared in solution. Then, prepared a-NF was vertically immersed in a stirred saturated alkaline Ni 2+ solution at 65 ˚C for 120 min. Finally, the fabricated a-NF/NiONWs electrode was washed with DI water and dried in air.
Network-like Co3O4 nanosheets were directly electrodeposited on a-NF/NiONWs by cycling the potential using cyclic voltammetric technique. The deposition was performed in a three-electrode electrochemical system including Ag/AgCl, Pt foil and a-NF/NiONWs as reference, counter and working electrodes, respectively. The electrodes were immersed in an electrochemical deposition bath including 15.0 ml of 0.1 M Co(NO3)2.6H2O and 5.0 ml of 2.0 M NaNO3. The pH of the solution was adjusted to 7.40 by adding an appropriate amount ۴ of 0.1 M NaOH to the solution. The potential was cycled 8 times between -1.2 V to +1.0 V at the scan rate of 50 mV s -1 (Scheme 1).
A solid-state asymmetric supercapacitor (ASC) device based on a-NF/NiONWs/Co3O4 as positive electrode and graphene-deposited NF (RGO/NF) as negative electrode was fabricated. For the fabrication of negative electrode, reduced graphene oxide (RGO) was deep-coated on NF by sequential immersing of the substrate into an aqueous GO solution (1.0 mg L -1 ) and drying in the oven at 60 ˚C for 30 min (5 times) and annealed at 300 ˚C for 3 h. Then, ASC device was simply assembled by using saturated filter paper with 1.0 M KOH as a separator.

Morphology, surface analysis, and characterizations
The anodic polarization of NF was performed at a two-electrode system by immersing NF as anode and a Pt foil as cathode in an organic-aqueous solution under the applied potential of +60 V. The preliminary experiments based on FE-SEM image analysis showed that the anodization at the applied potentials lower than +60 V did not proceed as well. So, the anodic polarization was carried out at a constant voltage of +60 V at different anodization times of 5, 30, 45 and 60 min in a solution containing 94.5 %v glycerol, 5.0 %v DI water and 0.50 %w KOH. The surface morphology of the resulted anodized Ni foil (a-NF) samples was investigated by FE-SEM ( Fig. 1).  During the anodization process, Ni species at the electrode surface is oxidized to Ni 2+ ions.
On one hand, Ni(OH)2 and NiOOH species can be formed at the electrode surface because of the basic nature of the solution. On the other hand, the resulted Ni 2+ from the anodic oxidation of Ni at the electrode surface can be expelled into the electrolyte because of the applied electric field.
Wormlike NiO nanostructures were grown on a-NF by immersing a-NF into the saturated alkaline solution of Ni 2+ without adding any additional reagent at different times of 1, 2 and 3 h. The FE-SEM images of a-NF/NiONWs samples are shown in Fig. 3. As it is clear, NiO nanoworms (NiONWs) with few tens of nanometers in diameter have been grown on a-NF substrate under the mentioned experimental conditions. The growth mechanism can be explained as follows. In a saturated alkaline solution of Ni 2+ , the concentration of dissolved Ni 2+ species are at sub-nanomolar (~nM) level even at the elevated temperatures. Also, Ni(OH)3species would not be formed in such a solution and there would not be any equilibria including Ni(OH)3or Ni(OH)4 2species. So, it seems that three distinct equilibria proceed in a saturated alkaline solution of Ni 2+ and a-NF//solution interface: Ni(OH)2 0 (aq) ⇄ Ni(OH)2 (ads) Ni(OH)2 (ads) ⇄ NiO (ads) + H2O (8) where, Ni(OH)2 0 (aq) and Ni(OH)2 (ads) are dissolved and adsorbed nickel hydroxide, respectively, and NiO (ads) is the adsorbed NiO at the electrode surface. So, for the growth of hydrated Ni(OH)2 at the electrode surface, among the well-known solubility equilibrium (Eq. 6), the adsorption/desorption equilibrium (Eq. 7) should also be considered.  S2a).

Fig. 3
After the growth of NiONWs on a-NF, followed by the activation of electrode surface by cycling the potential between 0 to +0.5 V (50 cycles) in 1.0 M KOH solution, Co3O4 nanosheets were directly electrodeposited on a-NF/NiONWs by cycling the potential (8 cycles) between -1.2 V to +1.0 V at the scan rate of 50 mV s -1 by using cyclic voltammetry.
The surface morphology of the resulted a-NF/NiONWs/Co3O4 electrode was shown in Fig.   4a-c. As it is clear, porous Co3O4 nanosheets are deposited on NiONWs forming uniform network-like structure. Such 3D interconnected nanostructures offer a large specific surface area and therefore, provide efficient contact between the electrode surface and the electrolyte, leading to fast faradic reactions.

Fig. 4
The average thickness of Co3O4 nanosheets and the interspaces between them are about 30 nm and 200 nm, respectively. This architecture offers a facilitated ion transport during fast charge/discharge processes and provides appropriate V-type channels for ion diffusion through the porous layer with greatly reduced diffusion length over which ions can transport.
Energy dispersive x-ray spectroscopy (EDS) of a-NF/NiONWs/Co3O4 confirms the presence of Ni and Co elements at the surface of sample (Fig. S2b). XRD patterns of the asdeposited Co3O4 (a-NF/NiONWs/Co3O4) and annealed samples were examined (Fig. 5)  FTIR-ATR spectra of as-prepared and annealed a-NF/NiONWs/Co3O4 samples are shown in annealing. The appeared peaks at around 1400 cm -1 , 1230 cm -1 and 1057 cm -1 are seems to be attributed to the carbonate group originating from the CO2 during ATR analysis.

Electrochemical properties of a-NF/NiONWs/Co3O4 electrode
CVs and galvanostatic charge-discharge curves were obtained in a three-electrode cell to evaluate the electrochemical properties of a-NF/NiONWs/Co3O4 and a-NF/Co3O4 electrodes. Fig. 6a shows the CVs of a-NF/NiONWs/Co3O4 at different scan rates within the potential window of 0 V to +0.5 V. The oxidation-reduction peaks of Co3O4/CoOOH species are observed in voltammograms, indicating the pseudocapacitive behavior of the fabricated electrode. The electrochemical oxidation-reduction reactions of Co3O4 are shown in the following equations: Co3O4 + H2O + OH -⇄ 3CoOOH + e - CoOOH + OH -⇄ CoO2 + H2O + e - A couple of anodic peaks exhibited at +0.12 V and +0.37 V at the scan rate of 0.02 V s -1 are attributed to the oxidation processes of cobalt species (equations 6 and 7), and a couple of cathodic peaks at about +0.10 V and +0.25 V are due to their corresponding reduction processes.
Notably, the charge transport of annealed a-NF/NiONWs/Co3O4 electrode was examined by using cyclic voltammetry (Fig. 6c). Compared to as-prepared electrode, the annealed samples showed one order of magnitude lower current densities and did not show welldefined oxidation-reduction peaks corresponding to the oxidation process of cobalt species.

٨
Finally, as discussed above, before the electrodeposition of Co3O4, a-NF/NiONWs electrode was electrochemically activated in alkaline solution by consecutive cycling the potential between 0 to +0.5 V (50 cycles) in 1.0 M KOH solution.

Fig. 6
Galvanostatic charge-discharge (GCD) curves of a-NF/Co3O4 and a-NF/NiONWs/Co3O4 electrodes in the potential range of 0 to +0.50 V at a constant current density of 6.0 mA cm -2 are shown in Fig. 7a. Two visible and separated plateaus in the charge-discharge process of a-NF/NiONWs/Co3O4 electrode resulting from the redox reactions of Co3O4 species indicate the pseudocapacitive behavior, which is consistent with the peaks observed in their corresponding CVs (Fig. 6). Also, the GCD plots exhibit asymmetric charge-discharge profiles for both a-NF/NiONWs/Co3O4 and a-NF/Co3O4 electrodes. (2), the discharge capacitance of a-NF/NiONWs/Co3O4 at 8 mA cm -2 is 1164 mF cm -2 , which is about 10 times higher than that of a-NF/Co3O4 electrode (110 mF cm -2 ), demonstrating the advantage of NiONWs thin film layer in improving the pseudocapacitance performance of electrode.
The areal discharge capacitances (CD), which are more important in practical applications, were calculated to be 859.2, 1164, 1195.2 and 1320 mF cm -2 at the current densities of 12, 8, 6 and 4 mA cm -2 , respectively. The coulombic efficiency (η), energy density (E) and power density (P) of the fabricated electrode was calculated and obtained to be 94.28%, 69.4 Wh kg -1 , and 1666.6 W kg -1 , respectively. The decrease in specific capacitance by increasing the current densities is attributed to the limited ion migration inside the active materials in shorter times. A maximum areal specific capacitance of 1320 mF cm -2 was reached at 4 mA cm -2 .

Fig. 7 A solid state asymmetric a-NF/NiONWs/Co3O4//RGO/NF supercapacitor studies
Cyclic voltammetry was performed in a three electrode system to find the electrochemical potential windows of the positive and negative electrodes (Fig. 8a). As it is clear from CVs, ٩ the potential windows are in the ranges of -0.8 to 0.0 V for negative electrode and 0.0 to +0.5 V for positive electrodes and the capacitances of two electrodes are almost equal. The working potential window of 0 to 1.4 V was selected, where the oxidation/reduction of water at the end of potential window was not occurred and the transport of charge density was higher. The voltammetric responses of ASC were also investigated at different scan rates in the capacitive potential range of 0.0 to 1.4 V (Fig. 8c). Furthermore, GCD curve at the current density of 3.0 mA cm -2 was investigated to study the capacitive performance of the fabricated ASC device (Fig. 8d). A maximum areal specific capacitance of 471.4 mF cm -2 was achieved for ASC at the current density of 3 mA cm -2 .

Fig. 8
The cycle life of the proposed ASC is a critical parameter which plays an important role in the performance of device. So, the cycle life of ASC was investigated by GCD technique at the current density of 4.0 mA cm -2 . The capacitive retention of ASC as a function of cycle numbers was shown in Fig. 9a. The proposed ASC preserves more than 75% of its initial specific capacitance after 2000 successive cycles, indicating the excellent long cycle life and electrochemical stability of fabricated ASC supercapacitor.
Electrochemical impedance spectroscopy (EIS) of assembled ASC was investigated to measure the conductivity and the charge transfer resistance of NiONWs/Co3O4 thin film (Fig. 9b). The fitted data and the equivalent circuit model are reported in Table 1 and Fig.   9c, respectively. The Rs value, i.e. the combination of ionic and electronic resistance, intrinsic resistance of the electrode materials, and diffusive and contact resistance at electrode/current collector interface, is 0.0977 Ω cm -2 that shows the high conductivity of fabricated supercapacitor.