Hierarchical core-shell NiCo2O4@NiMoO4 nanowires grown on carbon cloth as integrated electrode for high-performance supercapacitors

Hierarchical core-shell NiCo2O4@NiMoO4 nanowires were grown on carbon cloth (CC@NiCo2O4@NiMoO4) by a two-step hydrothermal route to fabricate a flexible binder-free electrode. The prepared CC@NiCo2O4@NiMoO4 integrated electrode was directly used as an electrode for faradaic supercapacitor. It shows a high areal capacitance of 2.917 F cm−2 at 2 mA cm−2 and excellent cycling stability with 90.6% retention over 2000 cycles at a high current density of 20 mA cm−2. The superior specific capacitance, rate and cycling performance can be ascribed to the fast transferring path for electrons and ions, synergic effect and the stability of the hierarchical core-shell structure.

In recent years, renewable and clean energy storage technologies have attracted considerable attention due to the serious environmental pollution and fossil-fuel energy crisis [1][2][3][4][5][6] . Furthermore, the extensive demand of electric vehicles and portable electronic devices promote humans to develop more new energy storage devices [7][8][9] . Lithium-ion batteries 10 , sodium-ion batteries 11,12 and supercapacitors (SCs) 13 are potential energy storage devices currently. Among them, SCs have competitive characteristics such as high power density, long cycle life, fast charging/discharging rate, good safety and low maintenance cost 14 . Although the carbonaceous materials such as active carbon 15 , porous carbon 16 , graphene 17,18 , and carbon nanotubes 19 can deliver a high power density, the electrical double layer supercapacitive (EDLS) mechanism gives rise to their low specific capacitance and energy density. So the researchers mainly focus on developing faradaic electrode materials that have higher capacity and larger energy density. In the past few years, various faradaic electrode materials, including transition metal oxides 20 , hydroxides 21,22 , sulfides 23,24 and conductive polymers 25,26 were intensively developed. For example, MnO 2 27,28 , NiO 29 and Co 3 O 4 30 are some of the most notable faradaic electrode materials, but their intrinsic electrical conductivities are poor. Conductive polymers such as polyaniline 26 , polypyrrole 31,32 and Poly (3, 4-ethylenedioxythiophene) 33 can offer a high specific capacity, but the volume expansion leads to the poor cycling performance.
Currently, binary transition metal oxides such as NiCo 2 O 4 34-38 , NiMoO 4 39-43 and CoMoO 4 44,45 have been attracted much attention as electrode materials for supercapacitors because of their high electrical conductivity, remarkable specific capacity and environmental compatibility. For example, Lou et al. 46 fabricated the NiCo 2 O 4 nanorods and nanosheets on the carbon fiber through a facile solution method, and they delivered high capacitances of 1023.6 and 1002 F g −1 , respectively. Huang et al. 42 reported a Ni foam supported NiMoO 4 nanoplate arrays integrated electrode, showing a high specific areal capacitance of 3.4 F cm −2 at 2 mA cm −2 . However, in order to further improve the electrochemical performance, combining two types of binary metal oxides to form a unique hierarchical nanostructure is an efficient way. For example, Mai et al. 45 reported hierarchical MnMoO 4 / CoMoO 4 heterostructured nanowires, which showed an enhanced supercapacitive performance.
In addition, core-shell nanostructure electrode materials can effectively improve electrochemical performance 47 . For example, Cai et al. 48 reported a manganese oxide/carbon york-shell anode for lithium-ion batteries, presenting a high capacity and excellent cycling performance at a high current density. Rational structure design

Results and Discussion
The schematic preparation process of CC@NiCo 2 O 4 @NiMoO 4 is presented in Fig. 1. In this work, the fabrication of integrated CC@NiCo 2 O 4 @NiMoO 4 electrode contains two steps. Firstly, the aligned pristine NiCo 2 O 4 nanowires were grown on a piece of carbon cloth to fabricate the CC@NiCo 2 O 4 backbone by a facile hydrothermal reaction and post-annealing process. Afterwards, the interconnected tiny NiMoO 4 nanosheets were uniformly deposited on the CC@NiCo 2 O 4 backbone by a secondary hydrothermal process. Finally, the precursor was transformed to hierarchical core-shell CC@NiCo 2 O 4 @NiMoO 4 integrated electrode through an annealing process in the pure N 2 atmosphere. X-ray diffraction (XRD) was used to check the phase of as-prepared samples, as displayed in Fig. S1. We directly used the binder-free samples for XRD test. The patterns indicate that the pure carbon cloth has two obvious unique characteristic peak of carbon 49 . Additionally, except for the diffraction peak of carbon, all the identified peaks can be well indexed to cubic spinel NiCo 2 O 4 (JCPDS no. 20-0781) and monoclinic NiMoO 4 phase (JCPDS no. 86-0361).
The morphologies of CC@NiCo 2 O 4 , and CC@NiCo 2 O 4 @NiMoO 4 are displayed in Fig. 2. Figure 2a shows the scanning electron microscope (SEM) image of bare carbon fibers before loading active materials. The diameter of the carbon fiber is around 10-15 μ m, and the surface is smooth, which facilitates to deposit active materials on its surface. From Fig. 2b, we can see that the NiCo 2 O 4 nanowires are uniformly distributed on the carbon fiber in the low-magnification microscopy. Figure 2c,d show that the aligned NiCo 2 O 4 nanowires with average diameter of about 200 nm are vertically grown on the carbon fiber substrate. The NiCo 2 O 4 nanowires backbone structure on the carbon cloth and the space between the nanowires provide space for the growth of NiMoO 4 nanosheets. Figure 2e depicts that the surface of NiCo 2 O 4 nanowires becomes rougher and larger after the secondary hydrothermal route. In the high-magnification image (Fig. 2f), we can see that the NiCo 2 O 4 nanowires are decorated with many tiny NiMoO 4 nanosheets. The diameter of the hierarchical core-shell NiCo 2 O 4 @NiMoO 4 nanowires is about 600 nm, bigger than initial diameter of the NiCo 2 O 4 nanowires. Besides, the NiMoO 4 nanosheets can be also uniformly grown on the carbon cloth via the same hydrothermal route to form NiMoO 4 @CC integrated electrode (Fig. S2). The hierarchical core-shell NiCo 2 O 4 @NiMoO 4 can obviously improve the mass loading of unit area on the carbon cloth substrate, increasing the specific areal capacitance of the binder-free integrated electrode. More importantly, the unique nanostructure decreases the resistance of electrons from the active materials to the conductive substrate, providing higher rate performance of the electrode. Finally, the core-shell hybrid structure has better cycling stability.
We used transmission electron microscope (TEM) and High-resolution TEM (HR-TEM) to further analyze the microstructure of the hierarchical core-shell NiCo 2 O 4 @NiMoO 4 nanowires.  Figure 3d reveals a typical TEM image of the NiCo 2 O 4 @NiMoO 4 nanowire. The image shows obvious core-shell hybrid nanostructure. The tiny NiMoO 4 nanosheets are uniformly grown along the NiCo 2 O 4 wire, forming a hierarchical structure. In addition, the HR-TEM image in Fig. 3e indicates the interplanar spacing of 0.7 nm corresponding to the (001) planes of NiMoO 4 nanosheets outside, confirming the core-shell structure. NiCo 2 O 4 nanowires not only serve as active material in the integrated electrode, but also provide a fast electronic transfer path because of their higher conductivity. The backbone of NiCo 2 O 4 wires is beneficial to the rate performance of the hierarchical core-shell NiCo 2 O 4 @NiMoO 4 integrated electrode. We also analyzed the elements of the hybrid electrode material using EDS and mapping under the TEM mode. From Fig. 3f, we can see that the electrode material mainly contains Ni, Co, Mo, O elements, corresponding to the NiCo 2 O 4 and NiMoO 4 phase. Except for the four elements above, we can also find some Cu and C, which come from the Cu mesh substrate and carbon cloth fiber, respectively. The element mappings (Fig. 3g-k) clearly show that O and Ni are uniformly distributed within the whole hybrid electrode material, while Co and Mo are concentrated in the core and shell sites, respectively. So the element analysis further demonstrates that the hybrid electrode is a hierarchical core-shell nanostructure, in which NiCo 2 O 4 nanowires and NiMoO 4 nanosheets serve as core and shell, respectively. X-ray photoelectron spectroscopy (XPS) was employed to further confirm the elements and the valence states in the integrated electrode, as shown in Fig. S3. For the Ni 2p core level spectrum (Fig. S3a), the peaks located at 872.9 and 879.7 eV correspond to Ni 2p 1/2 , while the peaks at 855.4 and 861.2 eV to Ni 2p 3/2 . The gap between Ni 2p 3/2 and Ni 2p 1/2 is 17.5 eV, indicative of the Ni 2+ oxidation state 51 . From Fig. S3b, the peaks at 233.4 and 236.5 eV are ascribed to the Mo 3d 5/2 and Mo 3d 3/2 , respectively. The energy gap of 3.1 eV shows a Mo 6+ oxidation state. For Co 2p, the peaks at 779.5 and 794.6 eV are accounted for the Co 3+ , while the peaks at 781.2 and 796.3 eV are due to the Co 2+ oxidation state. The O 1s can be deconvoluted into O1, O2 and O3, which refer to the O-Co/Ni bonding, defect sites and physic-chemisorbed water, respectively 42,52 .
The porous characterization of NiCo 2 O 4 @NiMoO 4 was measured by nitrogen adsorption and desorption at 77 K. Figure S4 shows that the Brunauer-Emmett-Teller (BET) surface area is 91.97 m 2 g −1 , which means that the electrode material has rich active surface to react with electrolyte ions. Furthermore, the corresponding pore size The obtained CC@NiCo 2 O 4 @NiMoO 4 sample was directly used as integrated electrode. The electrochemical performance was investigated in a three-electrode system. We chose 3 mol l −1 KOH as electrolyte, and the reference electrode and counter electrode were standard Hg/HgO and Pt foil, respectively. Figure 4a compares the charge and discharge curves of three integrated electrodes (CC@NiCo 2 O 4 , CC@NiMoO 4 , and CC@NiCo 2 O 4 @ NiMoO 4 ) at 2 mA cm −2 . The result shows that the discharge time of CC@NiCo 2 O 4 @NiMoO 4 is the longest, indicating its best specific capacitance. Cyclic voltammetry (CV) measurement was also carried out at various scan rates at the same potential window (see Fig. 4b). Apparently, the well-defined redox peaks in each CV profile are ascribed to the reversible faradaic redox reactions of Ni/Co-O, Ni/Co-O-OH with OH − . Besides, even at high scan rate of 50 mV s −1 , the CV profile remains similar, implying excellent rate performance and chemical stability. Figure 4c presents the galvanostatic discharge curves at various current densities for CC@NiCo 2 O 4 @NiMoO 4 electrode. We can see a discharge plateau at the voltage of 0.35 V, according to the CV redox peaks. The ASC of CC@ NiCo 2 O 4 @NiMoO 4 electrode is calculated based on eqn. (1) (see Methods). Remarkably, the ASC of as-prepared CC@NiCo 2 O 4 @NiMoO 4 electrode is 2.917, 2.748, 2.406, 2.072 and 1.608 F cm −2 at current densities of 2, 5, 10, 20 and 40 mA cm −2 , respectively. Moreover, the ASC can still keep to 55.1% of the initial value at 40 mA cm −2 .
As comparisons, the ASCs of CC@NiCo 2 O 4 and CC @NiMoO 4 integrated electrode are 1.003 and 2.29 F cm −2 at 2 mA cm −2 (Fig. 4d), lower than those of CC@NiCo 2 O 4 @NiMoO 4 integrated electrode. Figure 4e shows the mass specific capacitance (MSC) calculated by mass loading density according to eqn. (2) (see Methods). The MSC of the CC@NiCo 2 O 4 @NiMoO 4 integrated electrode is 1325.9, 1249.1, 1093.6, 941.8 and 730.9 F g −1 , respectively. The values are also higher than CC@NiCo 2 O 4 and CC@NiMoO 4 electrodes at the same testing condition. The electrochemical performance of the CC@NiCo 2 O 4 @NiMoO 4 electrode is better than most of the reported individual binary metal oxides like NiCo 2 O 4 nanowires (1283 F g −1 at 1 A g −1 ) 36 , NiMoO 4 nanowires (1.27 F cm −2 at 5 mA cm −2 ) 41 , and NiMoO 4 nanosheets (1221.2 F g −1 at 1 A g −1 ) 43 . Figure 4f presents the cycling performance at 20 mA cm −2 . The results show that the ASC of CC@NiCo 2 O 4 @ NiMoO 4 integrated electrode can remain 90.6% of the initial value even after 2000 cycles, much higher than those of the CC@NiCo 2 O 4 (76.3%) and CC@NiMoO 4 integrated electrode (72.1%). This high cycling performance can be explained by the synergic effect of NiCo 2 O 4 and NiMoO 4 . Besides, the core-shell hierarchical nanostructure has better structure stability during the cycling process.
Such high ASC and rate performance of the CC@NiCo 2 O 4 @NiMoO 4 integrated electrode demonstrate the great advantage of the hierarchical core-shell nanostructure. The schematic transport and redox reaction process for electrons and electrolyte ions in the electrode is illustrated in Fig. 5a. Because NiCo 2 O 4 has higher electronic conductivity, so electrons can efficiently transfer from the carbon cloth to the NiMoO 4 nanosheets via the backbone of the NiCo 2 O 4 nanowires. So this integrated electrode has sufficient electrons to participate in the redox reaction. Furthermore, the ions can penetrate into the NiMoO 4 nanosheets to react with the inner NiCo 2 O 4 nanowires to improve the electrochemical performance of the whole integrated electrode. Finally, the hierarchical core-shell nanostructure has better stability, so it cannot be easily destroyed during the redox reaction process.
Electrochemical impedance spectroscopy (EIS) was used to analyze the electrochemical mechanism. Figure 5b shows the Nyquist plots of CC@NiCo 2 O 4 @NiMoO 4 in the frequency range of 0.01-100 kHz. As can be seen, the CC@NiCo 2 O 4 @NiMoO 4 electrode exhibits the lowest charge transfer resistance (R ct ), suggesting the largest efficient electro-active surface area. Furthermore, the diffusion resistance of the electrode is also lowest, indicating the fastest electrolyte ion diffusion during the redox reaction.
In conclusion, a two-step hydrothermal route has been carried out to fabricate hierarchical core-shell NiCo 2 O 4 @NiMoO 4 nanowires grown on carbon cloth as electrode for faradaic supercapacitors. The fabricated CC@NiCo 2 O 4 @NiMoO 4 integrated electrode shows an enhanced capacitance of 2.917 F cm −2 at 2 mA cm −2 and excellent cyclability with 90.6% retention over 2000 cycles. The superior faradaic capacitance can be accounted for the rational core-shell hierarchical nanostructure and the synergic of NiCo 2 O 4 nanowires and NiMoO 4  nanosheets. We believe that the design and synthesis method can open up a promising and efficient way to develop new electrode materials for high-performance supercapaitors.

Methods
Materials synthesis. CC@NiCo 2 O 4 @NiMoO 4 integrated electrode was fabricated by a two-step hydrothermal route followed by thermal annealing in inert gas. Firstly, the uniform NiCo 2 O 4 nanowires were grown on the carbon cloth through a facile hydrothermal reaction to form CC@NiCo 2 O 4 nanowires. Then NiMoO 4 nanosheets were synthesized based on the backbone of NiCo 2 O 4 nanowires to fabricate the CC@NiCo 2 O 4 @NiMoO 4 integrated electrode by a secondary hydrothermal reaction. In a typical process, 2 mmol Co(NO 3 ) 2 ·6H 2 O, 1 mmol Ni(NO 3 ) 2 ·6H 2 O and 9 mmol urea were dissolved into 50 ml distilled water to achieve a clear pink solution. A piece of carbon cloth was pretreated by ultrasonic in deionized water and absolute ethanol each for 30 min. Then the cleaned carbon cloth was placed in the solution. The hydrothermal temperature and time were to 120 °C and 6 h, respectively. The coated carbon cloth was cleaned by deionized water and ethanol, and then dried in oven at 80 °C for 12 h. Finally, the sample was annealed in pure N 2 atmosphere at 400 °C for 3 h to obtain CC@NiCo 2 O 4 integrated electrode.
A secondary hydrothermal reaction was adopted to grow the NiMoO 4 nanosheets on the backbone of NiCo 2 O 4 nanowires to fabricate the CC@NiCo 2 O 4 @NiMoO 4 integrated electrode. Briefly, 1 mmol Na 2 MoO 4 ·2H 2 O and 1 mmol Ni(NO 3 ) 2 ·6H 2 O were dissolved in 50 ml mixed solvent (the volume ratio of distilled water and absolute ethanol is 1:1) to achieve a precursor solution, and then put into a stainless steel autoclave. The as-prepared CC@NiCo 2 O 4 integrated electrode was sealed in the autoclave, and kept at 130 °C for 6 h. The resulting sample was taken out and rinsed with deionized water and ethanol for several times, followed by annealing at 400 °C for 2 h in pure N 2 to obtain CC@NiCo 2 O 4 @NiMoO 4 integrated electrode. We also fabricated CC@NiMoO 4 integrated electrode with the same condition.
Material Characterizations. XRD (Panalytical X'pert PRO, Holland, Cu K αl irradiation, λ = 1.5406 Å, scan rate: 5° min −1 ) was performed using a multi-purpose diffractometer to analyze the phase. The field-emission SEM (FE-SEM, FEI Sirion200, Holland) was used to characterize the morphology of the products. The microstructure and element analysis were characterized using TEM (JEOL JEM-2010F, Japan) coupled with an EDS X-ray spectrometer (Oxford Instrument). XPS measurement was carried out using Al K α X-ray. The BET surface area, pore size and volume were measured by N 2 absorption on a Micromeritics ASAP 2020 analyzer.
Electrochemical measurements. Electrochemical measurements were carried out in a three-electrode configuration on CHI760E electrochemical workstation (Chenhua, Shanghai). The binder-free CC@NiCo 2 O 4 @ NiMoO 4 was directly used as working electrode, and the nominal area was about 1 cm 2 . A Hg/HgO electrode and high-pure Pt foil served as reference and counter electrode, respectively. The electrolyte was 3 mol l −1 KOH. The mass-loading of CC@NiCo 2 O 4 , CC@NiMoO 4 , and CC@NiCo 2 O 4 @NiMoO 4 were 1.2, 1 and 2.1 mg cm −2 , respectively.
Electrochemical impedance spectroscopy (EIS) was measured using an alternating-current (AC) voltage with 5 mV, and the frequency was 0.01-100 kHz. The specific capacitance was calculated by equation (1) and (2).
where C s and C m are the areal and mass capacitance of the active material supported on the carbon cloth, respectively. i is the discharge current applied to the electrode (A); t is the discharge time from high to low potential (s) and Δ U is the gap of high and low potential (V).