Developing high-performance electrochemical energy storage devices has been one of the important issues in the energy strategic projects established all over the world (Li et al., 2017a,b,c; Wei et al., 2017; Xia et al., 2017). Among various energy storage devices, electrochemical capacitors (called as supercapacitors), with high power density, fast charge–discharge rate, and long lifespan, are considered typically as one of the most appropriate choice energy storage and conversion devices (Li et al., 2017a,b,c; Sun et al., 2017; Zheng et al., 2017a,b). According to the mechanism of charge storage, it is significantly that they not only have a large surface area for improving double-layer capacitance, but also offer a short diffusion length for abundant redox reactions, which is important for pseudocapacitance (Deng et al., 2017; Kim et al., 2017; Zheng et al., 2017a,b). Among metal compounds, binary nickel–cobalt oxides attract the extensive attention for their relatively high electrochemical performance (Li et al., 2015). For instance, the three dimensional (3D) hierarchical flower-shaped NiCo₂O₄ microsphere exhibited 1006 F g⁻¹ at 1 A g⁻¹, enhanced rate capability and excellent electrochemical stability (Lei et al., 2014). Lou et al. reported that the NiCo₂O₄ hollow spheres show 1141 F g⁻¹ at 1 A g⁻¹ and enhanced cycling stability (Shen et al., 2015). Furthermore, to maximize pseudocapacitor, one needs to construct a porous structure with a large number of active sites, and other one can design a conducting channel with high transport rates of electrons and electrolyte ions (Yuan et al., 2012).

To further improve the charge transport aiming at the intrinsic poor electrical conductivity of metal compounds, the development of electrodes with ordered nanoarrays grown directly on a collector [such as Ni foam (Li et al., 2018), Cu foil (Zhang et al., 2012; Cheng et al., 2015), Ti foil (Lu et al., 2011), and carbon cloth (Li et al., 2017a,b,c)] without any binders is particularly significant for charge storage of electrochemically inactive (Chen et al., 2017; Liu et al., 2017; Shi et al., 2017). For example, MnO₂ nanowire/CNT paper electrode display 167.5 F g⁻¹ at 77 mA g⁻¹ (Chou et al., 2008). Ni(OH)₂ nanoflakes/Ni foam delivers 1228 F g⁻¹ and 918 F g⁻¹ at 5 A g⁻¹ and 30 A g⁻¹, respectively, with excellent cycling stability (Hu et al., 2013). NiCo₂S₄ nanotube/nickel foam shows 738 F g⁻¹ at 4 A g⁻¹ (Pu et al., 2014). Nickel–cobalt hydroxide nanoarrays/carbon nanofibers reveal 1378.2 F g⁻¹ for nanorod arrays and 1195.4 F g⁻¹ for nanosheet arrays at 1 A g⁻¹ (Lai et al., 2015). NiCo₂O₄ nanoneedles on Ni foam and Ti foil binder-free electrode exhibit greatly improved electrochemical performance (Zhang et al., 2012). This electrode design renders other auxiliary components such as conductive agent and binder completely unnecessary to allow for more efficient charge and mass exchange.

Three dimensional hierarchical hybrid nanostructures composed of high conductive core materials and high-performance shell materials are promising electrode architectures for superior supercapacitors (Xiao et al., 2012; Tang et al., 2013). Herein, we first successfully fabricated a novel 3D hierarchical hybrid electrode composed of nickel nanowires@porous NiCo₂O₄ nanorods arrays core–shell structure aligned on nickel foam (NiNW@NiCo₂O₄/NF) by a facile route. In this structure, the nickel nanowires are used as high conductive core materials to support high-pseudocapacitance porous NiCo₂O₄ nanorods shell materials, and the integrated nickel nanowire/Ni foam (Ni NW/NF) composite substrate work as an excellent binder-free current collector. Benefited from the design of 3D hierarchical core–shell nanostructure, the resulting NiNW@NiCo₂O₄/NF electrode exhibits excellent electrochemical properties for supercapacitors.

Figure 1 and Figure S1 in Supplementary Material show the XRD patterns of NF, Ni NW/NF, NiCo₂O₄/NF, and Ni NW@NiCo₂O₄/NF. Thereinto the strong diffraction, two peaks at 2θ = 44.5 and 51.8 correspond to the diffraction peaks of the Ni crystal from (111) and (200) (JCPDS No. 04-0850). The well-defined diffraction peaks observed at 2θ values of 19.2°, 31.3°, 36.9°, 44.7°, 54.1°, 59.3°, and 64.9° can be successfully indexed to (111), (220), (311), (400), (422), (511), and (440) planes of the cubic NiCo₂O₄ (JCPDS No. 73-1702). No other crystallized phases are detected, indicating the high purity.

The morphology of the samples was investigated by SEM, as shown in Figure 2. Figure 2A shows that the nickel nanowires are uniformly adhered on the surface of the nickel foam. All direction extending network structure composed of the uniform prickly nickel nanowires with the diameter of about 800 nm (Figure 2D). The growth of prickly nickel nanowires with lots of bumps on the surface of nickel foam can significantly increase the surface area which is beneficial to the deposition of active materials. The morphology of NiNW@NiCo-precursor/NF is presented in Figures 2B,E, the Ni nanowire is completely covered by the NiCo-precursor nanorods, which have diameters of 100 nm and lengths about 4 µm. The NiCo-precursor nanorods grow directly from the Ni nanowire surface forming an array structure. The morphology of NiNW@NiCo₂O₄/NF after annealing is presented in Figures 2C,F, it indicates the NiNW@NiCo₂O₄/NF with high-density NiCo₂O₄ nanorods are uniformly distributed on the Ni nanowire. Typical nanorods have the lengths of about 4 μm with diameters around 100 nm. Figure S2 in Supplementary Material shows the SEM images of NiCo₂O₄/NF. The TEM image shows that lots of pores appear after annealing and form the porous NiCo₂O₄ nanorods, as shown in Figure 2G. The porous nanostructure can further enrich the active site and, hence, to improve electrochemical properties. The formation schematic illustration of Ni NW@NiCo₂O₄/NF was presented in Figure 2G. The preparation process mainly involves three steps. In the first step, nickel nanowires are generated on nickel foam surface by chemical bath deposition. In the second step, Ni–Co precursor nanorods are grown on Ni nanowires by hydrothermal process. In the third step, Ni–Co precursor nanorods are thermally transformed to NiCo₂O₄ nanorods supported on the Ni nanowires.

X-ray photoelectron spectroscopy was further employed to characterize the valence state, as shown in Figure 3. The survey spectrum in Figure 3A indicates the presence of Ni, Co, and O. The fitted fine spectra for the O 1 s, Ni 2p and Co 2p are obtained in Figures 3B–D, respectively. The O 1 s core level spectrum contains three peaks: O1, O2, and O3. The peaks of O1 at 529.3 eV and O2 at 530.8 eV are ascribed to the metal–oxygen and some defect sites with low oxygen coordination. The peak of O3 at 533.2 eV can be due to the physic-chemisorbed water. Figure 3C confirms the presence of Ni²⁺ and Ni³⁺, thereinto the peaks at 854.2 and 871.9 eV are indexed to Ni²⁺, while other peaks at 85.6 and 873.6 eV are from Ni³⁺. As presented in Figure 3D, two types of Co species can be detected from Co 2p XPS spectrum. The peaks at 780.5 and 796.0 eV are belonged to Co²⁺, while those at 779.3 and 794.0 eV are assigned to Co³⁺. These results are well consistent with the reported NiCo₂O₄.

In order to probe into the above discussion of the Ni nanowire-based electrodes on the electrochemical properties, the specific surface areas of the NiCo₂O₄/NF and NiNW@NiCo₂O₄/NF were measured by BET. Figure 4 shows the nitrogen adsorption and desorption isotherms of NiCo₂O₄/NF and NiNW@NiCo₂O₄/NF. The BET surface areas (SBET) of NiCo₂O₄/NF and NiNW@NiCo₂O₄/NF reveal 6.3 and 13.9 m² g⁻¹. In addition, the average pore diameter obtained from Barrett–Joyner–Halenda (BJH) desorption isotherm is about 18.5 and 11 nm. Thereinto, the distribution of mesoporous of NiCo₂O₄/NF and NiNW@NiCo₂O₄/NF is similar from porous NiCo₂O₄ nanorods. The results show that the high specific surface area of the NiNW@NiCo₂O₄/NF is due to the addition of nickel nanowires.

Figure 5A shows the CV curves of the NiCo₂O₄/NF and NiNW@NiCo₂O₄/NF recorded at a scan rate of 5 mV s⁻¹. Distinct redox peaks which can be observed reveal the typical faradaic capacitance features of the electrodes. The relevant redox reactions related to M–O/M–O–OH (M refers to Ni or Co) which took place on the surface of electrodes in the alkaline electrolyte could be expressed as follows (Liu et al., 2013; Shang et al., 2017).

It can be observed that the NiNW@NiCo₂O₄/NF have the larger CV curve area than NiCo₂O₄/NF demonstrating the higher specific capacitance. And the galvanostatic charge–discharge curves of the NiCo₂O₄/NF and Ni NW@NiCo₂O₄/NF at the current density of 5 mA cm⁻² are shown in Figure 5B. The comparisons of Figures 5A,B indicate a higher capacitance of NiNW@NiCo₂O₄/NF, which can be attributed to the less aggregation of active materials and more active sites provided by the novel 3D hierarchical core–shell nanostructure due to the introduction of nickel nanowires. The areal specific capacitance (F cm⁻²) of electrodes can be calculated according to the following equation (Wei et al., 2016): (3)C=Jt/ΔV

where J is the current density (A cm⁻²), t represents the discharge time (s), and ΔV designate the voltage window (V) for the galvanostatic charge–discharge measurements. By the calculation, the areal specific capacitance of NiCo₂O₄/NF is 3.5 F cm⁻² at the current density of 5 mA cm⁻², however, those of NiNW@NiCo₂O₄/NF is reach up to 7.4 F cm⁻² at the same current density. Figure 5C shows the typical CV curves of NiNW@NiCo₂O₄/NF under different sweep rates. As shown in Figure 5D, almost symmetric curves are observed during the charge/discharge processes under deferent current densities, indicating a good electrochemical capacitive characteristic and excellent reversible redox property.

Figure 6A presents the relationship between areal specific capacitance and current density of NiNW@NiCo₂O₄/NF and NiCo₂O₄/NF electrodes. The areal specific capacitances of NiNW@NiCo-NR/NF electrode are 7.44, 7.07, 6.88, 6.80, 6.73, 6.70, 6.65, 6.64, 6.55, and 6.55 F cm⁻² at different current densities of 5, 10, 15, 20, 25, 30, 40, 50, 75, and 100 mA cm⁻², respectively. This manifests that almost 88.04% of the areal specific capacitance is still retained after the current density increases from 5 to 100 mA cm⁻², which is evidently higher than that of NiCo-NR/NF electrode (47.46%), suggesting a better rate capability of NiNW@NiCo₂O₄/NF electrode which is attributed to the shorter ion channels provided by the growth of NiCo₂O₄ nanorods nanoarrays on the surface of prickly nickel nanowires than smooth nickel foam. The cycling stability of NiNW@NiCo₂O₄/NF and NiCo₂O₄/NF electrode were evaluated by the galvanostaic charge/discharge measurements for 1,500 cycles at a constant current density of 25 mA cm⁻², as shown in Figure 6B. It can be seen that both plots are gradually decreasing with the increase of cycle times. However, the downward trend of NiNW@NiCo₂O₄/NF electrode is more moderate than that of NiCo₂O₄/NF electrode, which can be owing to the buffer action in adsorption/desorption process due to the presence of nickel nanowires.

In summary, a novel 3D hierarchical core–shell structured NiNW@NiCo₂O₄/NF electrode was successfully fabricated by a facile method. The results indicate that the nickel nanowire@NiCo₂O₄/Ni foam electrode shows a high areal specific capacitance (7.4 F cm⁻² at 5 mA cm⁻²), excellent rate capability and good cycling stability. The superior electrochemical properties made it promising as electrode for supercapacitors. And this work provides a feasible way to improve rate performance.

HW and LL participated in the experiment and wrote the article. XL participated in the experiment and drew the scheme and figures. HW conceived and supervised the project and revised the manuscript. All authors contributed to the general discussion.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.