Facile synthesis of three-dimensional structured carbon fiber-NiCo2O4-Ni(OH)2 high-performance electrode for pseudocapacitors

Two-dimensional textured carbon fiber is an excellent electrode material and/or supporting substrate for active materials in fuel cells, batteries, and pseudocapacitors owing to its large surface area, high porosity, ultra-lightness, good electric conductivity, and excellent chemical stability in various liquid electrolytes. And Nickel hydroxide is one of the most promising active materials that have been studied in practical pseudocapacitor applications. Here we report a high-capacitance, flexible and ultra-light composite electrode that combines the advantages of these two materials for pseudocapacitor applications. Electrochemical measurements demonstrate that the 3D hybrid nanostructured carbon fiber–NiCo2O4–Ni(OH)2 composite electrode shows high capacitance, excellent rate capability. To the best of our knowledge, the electrode developed in this work possesses the highest areal capacitance of 6.04 F cm−2 at the current density of 5 mA cm−2 among those employing carbon fiber as the conductor. It still remains 64.23% at 40 mA cm−2. As for the cycling stability, the initial specific capacitance decreases only from 4.56 F cm−2 to 3.35 F cm−2 after 1000 cycles under a current density of 30 mA cm−2.

T he ever increasing demand for portable electronic equipment, and electric or hybrid electric vehicles, as well as the concerns of environmental pollution have stimulated the rapid development of the electro-chemical energy storage and conversion devices (EESCs). These devices typically include lithium-ion batteries and pseudo-capacitors [1][2][3][4] . In particular, pseudo-capacitors, as a typical representative of EESCs, have been widely studied due to their numerous merits, such as fast charge and discharge properties, long service life and high power density, wide range working temperature, high safety and the environmentally friendly feature 5,6 .
In the development of pseudo-capacitors, electrode materials are well known as one of the key factors affecting their electro-chemical performance 7,8 . So far, most of the investigations have focused on utilizing metals (e.g., Ni foil, Ni foam, Ti foil, Cu foil, etc.) as the current collector and transition metal oxides (e.g., NiO, NiOH, CoO, Co 4 O 3 , NiCo 2 O 4 , MnO 2 ) as active materials to fabricate high-performance electrodes of pseudocapacitors [9][10][11][12][13][14][15] . These electrode materials were found to exhibit impressive pseudo-capacitive performance. Among all those materials, nickel hydroxide is a promising electrode material for pseudocapacitor applications due to its high theoretical capacitance (2082 F g 21 for Ni(OH) 2 within 0.5 V) and good electrochemical reversibility in alkaline electrolyte, low costs, abundant natural resources, and excellent environment compatibility [16][17][18][19] . However, how to obtain Ni(OH) 2 active material with excellent electrochemical performance and how to fabricate a high energy density and high power density electrode containing Ni(OH) 2 remain big challenges 20  nanorods and ultrathin nanosheets electrode materials based on carbon nanofibers, the electrode shows excellent electrochemical performance due to its fine 3D hybrid nanostructure 23 . Recently, Nickel Cobalt Sulfide directly deposited on carbon fiber also shows high specific capacitance of 1418 F g 21 at 5 A g 21 with excellent rate capability 24 . Although these studies have very excellent achievements, it is still necessary and of great significance to unceasingly improve the electrode performances of pseudo-capacitors to meet the ever increasing social needs.
In this work, we designed and fabricated a unique 3D hybrid nanostructured electrode using the commercial two dimensional (2D) textured carbon fiber (CF) as the conductive substrate due to its good conductivity, ultra-lightness, excellent chemical stability and outstanding flexibility [25][26][27] . Then, needle-like NiCo 2 O 4 was assembled uniformly and vertically on the surface of carbon fibers to form 3D hybrid structured CF-NiCo 2 O 4 materials, the latter of which was found to have a better electrical conductivity compared to the binary metal oxides such as NiO and Co 3 O 4 as reported in the previous studies 9, 21,28 . Within the composite, the 3D hybrid structured CF-NiCo 2 O 4 serves as a supporting material, while polycrystalline Ni(OH) 2 was deposited on the surface of the NiCo 2 O 4 needles by using an electrochemical deposition technology. In this way, a new ultra-light, flexible and porous 3D hybrid structured CF-NiCo 2 O 4 -Ni(OH) 2 electrode was synthesized. The preparation process is shown in Fig. 1. Electrochemical measurements demonstrate that the 3D hybrid nanostructured CF-NiCo 2 O 4 -Ni(OH) 2 electrode shows a high capacitance with excellent rate capability and good cycling stability.

Results
The hydrothermally synthesized supporting material was firstly characterized by X-Ray Diffraction (XRD) (see Fig. 2a). All the diffraction peaks in the XRD pattern can be readily indexed as spinel NiCo 2 O 4 , according to the standard card (JCPDS Card No. 20-0781). The morphology of the as-prepared product was studied by scanning electron microscopy (SEM). The typical low-magnification SEM micrograph ( Fig. 2b) clearly illustrates that the 3D hybrid nanostructured supporting material is composed of numerous needle-like NiCo 2 O 4 with a sharp tip uniformly arranged on the CF surface. High-magnification SEM micrograph (Fig. 2c) reveals that the length of a NiCo 2 O 4 needle is approximately 4 mm, and the diameter of the nano-needle ranges from being several nanometers at the tip of the needle to around 100 nm near the end of the needle. Bright-field transmission electron microscopy (TEM) images show that the samples are polycrystalline (Fig. 2d). High-resolution TEM micrographs (Fig. 2f) show the lattice fringes with interplane spacing of 2.34 nm, corresponding to the (222) plane of the spinel phase NiCo 2 O 4 in agreement with our XRD results. Then, selected-area electron diffraction (SAED) measurements were carried out, and the results show that the NiCo 2 O 4 needles have a polycrystalline structure, which is consistent with the observation in Fig. 2e. Therefore, we have illustrated that the well-defined 3D hybrid nanostructured CF-NiCo 2 O 4 supporting materials can be obtained by using a facile, lowcost, effective solution route.
The polycrystalline Ni(OH) 2 wrinkled nanosheets were subsequently deposited on the surface of the NiCo 2 O 4 needles using a simple electrochemical deposition method. The total mass of the CF-NiCo 2 O 4 -Ni(OH) 2 electrode is 17.29 mg, and that of the loaded Ni(OH) 2 active material is 2.44 mg cm 22 . The CF-NiCo 2 O 4 -Ni(OH) 2 electrode was then analyzed using Raman spectroscopy. As seen in Fig. 3a, the Raman peaks at 185, 467, 508, and 651 cm 21 , corresponding to F 2g , E g , F 2g , and A 1g modes of the NiCo 2 O 4 nanowires, respectively 21,29 . Two new peaks are observed at 459 cm 21 and 534 cm 21 after electro-deposition, corresponding to the symmetric Ni-OH stretching mode and to the Ni-O stretching/ vibrational mode, respectively [30][31][32] . In Fig. 3b and c, Ni(OH) 2 was deposited uniformly on the surface and in the interspace of the 3D nanostructured supporting material. TEM micrographs show that the synthesized Ni(OH) 2 is of a 3D wrinkled nanostructure consisting of densely wrinkled nanosheets (Fig. 3d, e). All the surface of the NiCo 2 O 4 needles have been completely coated, indicating the formation of a highly porous 3D hybrid nanostructure. The corresponding SAED results (the inset of Fig. 3d) exhibit a diffraction ring pattern, indicating that the 3D wrinkled Ni(OH) 2 is of a polycrystalline structure. The different diffraction rings can be readily indexed to the different crystal planes of the Ni(OH) 2 phase. Fig. 3e illustrates that Ni(OH) 2 is firmly connected to the surface of NiCo 2 O 4 needles, which eventually forms a 3D wrinkled and porous nanostructure, consistent with the aforementioned SEM images analysis. The unique 3D hybrid porous nanostructure implies good electrochemical capacitive performance of the sample.
Then, the unique 3D hybrid porous structured CF-NiCo 2 O 4 -Ni(OH) 2 product was used as the electrode material in three-electrode cell. Fig. 4a shows the typical cyclic voltammetry (CV) curves of the pseudocapacitor with the CF-NiCo 2 O 4 -Ni(OH) 2 electrode at different scanning rates in the potential range from 20.2 V to 0.55 V. The CV curves clearly demonstrate that the CF-NiCo 2 O 4 -Ni(OH) 2 electrode shows typical pseudocapacitive characteristics and excellent reversibility. In specific, a pair of representative redox peaks are clearly visible in each voltammogram at the scanning rate of 2, 4, 6, 8, and 10 mV, which is obviously distinct from the electric double-layer capacitance characterized by nearly rectangular CV curves. By comparing the CV curves with CF-NiCo 2 O 4 supporting materials in Fig. 4b and by comparing the discharging curves of the  www.nature.com/scientificreports CF-NiCo 2 O 4 -Ni(OH) 2 electrode with CF-NiCo 2 O 4 supporting materials at the current density of 5 mA cm 22 (Fig. S1), we can see that the CF-NiCo 2 O 4 -Ni(OH) 2 electrode demonstrates better reversible and charging/discharging properties. The capacitive properties of the unique 3D hybrid porous structured CF-NiCo 2 O 4 -Ni(OH) 2 electrode mainly originate from the strong faradaic redox reaction of polycrystalline Ni(OH) 2 nanosheets. The capacitance of electrode was measured and could be correlated with the reversible reactions of Ni-O/Ni-O-OH by using the following equation 33 : In Fig. 4a, the peak current increases significantly at the scanning rate from 2 mV to 10 mV, which indicates that the kinetics of the interfacial faradic redox reactions are rapid enough, which facilitates the transmission of electronic and ionic species. With increasing scan rates, the potential of the oxidation peak shifts in the positive direction and that of the reduction peak shifts in the negative direction, which is mainly related to the internal resistance of the electrode.
To further evaluate the application potential of 3D hybrid porous structured CF-NiCo 2 O 4 -Ni(OH) 2 electrode, galvanostatic charge-discharge measurements were carried out between 20.1 and 0.45 V (vs. SCE) at various current densities, and the corresponding discharge curves of electrode are shown in Fig. 4c. The shapes of five curves are very similar and show ideal pseudocapacitive behaviour with sharp responses and small internal resistance drop. Moreover, there is a potential platform in every discharge curve. It is the typical pseudo-capacitance characterization of transition metal compounds that is in agreement with the result obtained from CV curves in Fig. 4a. This phenomenon is caused by a charge transfer reaction or electrochemical absorption-desorption process at the electrode-electrolyte interface. According to the formula (2), the calculated areal capacitance is shown in Fig. 4d. Encouragingly, the capacitances are 6.04, 5.72, 5.24, 4.56, 3.88 F cm 22 (corresponding to 2475, 2344, 2147, 1869, 1590 F g 21 , respectively) when the dischare current densities are 5, 10, 20, 30 and 40 mA cm 22 , respectively. To the best of our knowledge, the areal capacitances obtained from the CF-NiCo 2 O 4 -Ni(OH) 2 electrode in this work are the highest values among those employing 2D textured carbon fiber as the conductor. The capacitance gradually decreases as the current density increases. The reason is that at a higher current density the incremental voltage drops and the active material involved in the redox reaction is insufficient. In comparison with the specific capacitance of 6.04 F cm 22 measured at the current density of 5 mA cm 22 , the specific capacitance decreases to be 3.88 F cm 22 when the current density increases to 40 mA cm 22 , but still remains 64.2% of the original value. These results illustrate that the CF-NiCo 2 O 4 -Ni(OH) 2 electrode possessed excellent rate capability.
In addition, to investigate the stability of CF-NiCo 2 O 4 -Ni(OH) 2 electrode, the cycling endurance of the electrode was evaluated by repeatedly charging-discharging measurements at a constant current density of 30 mA cm 22 and the results are shown in Fig. 4e. The areal capacitance is around 4.56 F cm 22 in the first cycle and it gradually decreases to 3.35 F cm 22 after 1000 cycles, corresponding to a 26.6% decrease in the initial specific capacitance. Although the cycling stability is not good enough, the capacitance of the CF-NiCo 2 O 4 -Ni(OH) 2 electrode after 1000 cycles is still higher than the areal capacitance of the initial charging/discharging cycle reported in some recent studies, even at a heavy current density up to 30 mA cm 22 21,22,24 .

Discussion
The aforementioned electrochemical measurements demonstrate that the 3D hybrid nanostructured CF-NiCo 2 O 4 -Ni(OH) 2 electrode material shows excellent pseudocapacitive behavior, such as higher capacitance, better rate capability and cycling stability. Note that the areal capacitance of this hybrid electrode is as high as 6.04 F cm 22 at the current density of 5 mA cm 22 . The excellent electrochemical performance of the CF-NiCo 2 O 4 -Ni(OH) 2 electrode material may be attributed to its 3D hybrid structure of multiscale, ranging from nanoscale to microscale. First, the 3D hybrid structure of the welldefined CF-NiCo 2 O 4 supporting materials provide an efficient 3D electric-conductive network, which facilitates the high-speed electron transport and serves as the foundation for electro-deposited Ni(OH) 2 . Second, the 3D CF-NiCo 2 O 4 supporting material also has more active surface area for accommodating more active material Ni(OH) 2 . Third, the 3D wrinkled framework of Ni(OH) 2 nanosheets allows the facile penetration of electrolyte into the electrode, and promotes the surface redox reactions, which offers a relatively high electro-active surface area, as described in the schematic diagram in Fig. 1.
Energy density and power density are two key factors for evaluating the power applications of pseudocapacitors. A good pseudocapacitors can provide high energy density and capacitance. The power density and energy density were estimated from discharge curves by the formulas (3) and (4). As shown in Fig. 4f, the energy density of the CF-NiCo 2 O 4 -Ni(OH) 2 electrode was able to reach 9.14 kWh m 22 at a power density of 14.83 W m 22 , and still remained at 5.86 kWh m 22 at a power density of 109.9 W m 22 . Compared with other 3D hybrid porous structures that use 2D textured carbon fiber as the current collector (e.g.: Nickel Cobalt Sulfide, Nickel-Cobalt Hydroxide Nanosheets,NiCo 2 S 4 Nanotube) and are applied as electrode of pseudocapacitors, the CF-NiCo 2 O 4 -Ni(OH) 2 electrode exhibited higher areal capacitance and energy density (See Table 1).

Conclusions
To sum up, the 3D hybrid nanostructured CF-NiCo 2 O 4 supporting materials were successfully synthesized by using a facile solution method and a simple annealing treatment. By using the 3D hybrid nanostructured CF-NiCo 2 O 4 materials as the supporting materials, we have fabricated a new electrode material for pseudocapacitors via electrochemical deposition -the 3D hybrid nanostructured CF-NiCo 2 O 4 -Ni(OH) 2 . The total mass of the composite electrode was found to be 17.29 mg only. Electrochemical measurements demonstrate that the pseudocapacitor with the synthesized CF-NiCo 2 O 4 -Ni(OH) 2 electrode exhibits high specific capacitance and excellent cycling stability. To the best of our knowledge, the CF-NiCo 2 O 4 -Ni(OH) 2 electrode in this work shows the highest areal capacitance of up to 6.04 F cm 22 at the current density of 5 mA?cm 22 , among those employing carbon fiber as the supporting substrate. The capacitance of the electrode still remains to be 3.35 F cm 22 at the current density of 30 mA?cm 22 after 1000 cycles. The excellent performance of the electrode material is attributed to the unique 3D hybrid micro/nano-structure of the CF-NiCo 2 O 4 -Ni(OH) 2 electrode. The performance we achieved suggests that the 3D hybrid nanostructured electrode prepared using simple synthesis process and developed in this work have great potential in various energy storage technologies. Therefore, we expect that pseudocapacitors with better electrochemcial performance could be fabricated by using the 3D hybrid nanostructured electrode.

Methods
All the reagents used in the experiments are of analytical grade and are used without further purification.
The synthesis of the CF-NiCo 2 O 4 supporting materials. In a typical procedure, 1 mmol Ni(NO 3 ) 2 ?6H 2 O, 2 mmol Co(NO 3 ) 2 ?6H 2 O, and 5 mmol urea were dissolved in 35 ml. deionized (DI) water. After stirring for about 10 min, a transparent pink-colored solution was obtained. The solution was then sealed in a Teflon-lined stainless autoclave. A piece of clean carbon cloth substrate of 3 3 7 cm 2 in size was immersed into the reaction solution. Afterwards, the autoclave was put in an oven, and heated to 120uC and kept at that temperature for 5 h. After cooling down naturally to room temperature, the sample was collected and then washed with DI water and absolute ethyl alcohol for several times before it was dried in a vacuum oven at 80uC for 2 h. Finally, the dried sample was calcined at 350uC in the air for 2 h with the heating rate at 4uC min 21 .
The fabrication of the 3D hybrid nanostructured CF-NiCo 2 O 4 -Ni(OH) 2 . The CF-NiCo 2 O 4 supporting materials were accurately weighed and cut into 1 3 1 cm 2 pieces, before being employed as the work electrode in the 100 ml pure and transparent 0.1 M Ni(NO 3 ) 2 solution for a standard three-electrode cell at room temperature (about 25uC), where the Pt foil serves as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Electro-deposition was performed at the potential of 20.9 V for 400 s. Then, the obtained sample was washed with DI water and ethanol for several times. The product was then dried at 100uC for 10 h in a drying oven. The mass of the CF-NiCo 2 O 4 -Ni(OH) 2 and the pure Ni(OH) 2 was 17.29 mg cm 22 and 2.44 mg cm 22 , respectively, being weighed with a high precision analytical balance of Sartorius BT25S.
Electrode materials characterizations and electrochemical measurements. X-ray diffraction (XRD) patterns were collected with a SHIMADZU XRD-7000 X-ray diffractometer in a highly intensive Cu Ka irradiation (Voltage: 40.0 Kv, Current: 30.0 mA, l: 1.5406 Å ). The morphology of the products was examined by fieldemission scanning electron microscopy (Hitachi, SU-8010). TEM and HRTEM images were recorded on a JEOL JEM-2100F microscope that operates at 200 kV. Transmission electron microscope (TEM) images were taken through a JEOL-2100F microscope. The electrodeposited CF-NiCo 2 O 4 -Ni(OH) 2 product was used as the working electrode with 2.44 mg Ni(OH) 2 active material. The electrochemical tests were conducted in a CHI660D electrochemical workstation in an aqueous KOH electrolyte (2.0 M) with a three-electrode cell where Pt foil serves as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The areal capacitance of the electrode materials can be calculated according to the following equation:

C~IDt=ADV ð2Þ
In which I is the discharge current (A), Dt is the discharge time (s), DV is the voltage window (V) and A is the area of the electrode material (cm 2 ). The energy density (E) and power density (P) are important parameters for pseudocapacitor device. In this study, we also evaluated the energy density and power density of the Ni 3 S 2 /Ni electrodes from the discharge cycles as follows: P~E=t ð4Þ Where C is the areal capacitance (F m 22 ), t is the discharge time (s), and DV is the voltage window (V).