1 Introduction

The development of portable electronic devices, electric vehicles and many highly energy consumption application create generate enormous demand for electrochemical energy storage with higher power and energy densities [1,2,3,4]. Electrochemical capacitors, also called supercapacitors, attracted much attention due to its more excellent performance of high power density and good stability than that of the most widely used batteries, are probably the most important next-generation energy storage device [5,6,7,8,9,10,11]. According to the energy storage mechanism supercapacitor can be classified into two types which are electrical double-layer capacitors [12, 13] and faradic capacitors [14]. Among these two types, Faradic capacitors have much higher specific capacitance and energy density than those of electrical double-layer capacitors.

As a kind of energy storing device which can realize quick charge and discharge, Faradic capacitors are widely researched due to their high reliability,long lifespan and high power density [15, 16]. Transition metal oxides are reported to be a class of promising active materials for supercapacitors due to their multiple oxidation states [17,18,19]. Among them, NiCo2O4 has been suggested as a viable candidate at a low price, based on its high specific capacitance and fast Faradaic reactions. Compared to the single-component oxides, NiCo2O4 offer richer redox chemistry and combine the contributions from both Co and Ni ions [20, 21]. According to electrode structure, NiCo2O4 electrode can be classified into two basic types: binder and binder-free [22,23,24,25,26]. In the former case, the working electrode is prepared by the traditional slurry-coating technique for electrochemical measurement. In the latter case, electroactive NiCo2O4 nanostructures are directly deposited on conductive substrates used as binder-free electrodes for supercapacitors. The deposition method includes electrochemical deposition method and hydrothermal method. In this paper, we report the synthesis of nanostructured NiCo2O4@carbon cloth (NC@C) electrodes only by one-step sol–gel method. To the best of our knowledge, there have been no reports about sol–gel method to fabricate binder-less electrode in the literature by others.

2 Experimental

2.1 Preparation of samples

Synthesis of NC@C Structure. all chemicals including acetone, ethanol, citric acid, Ni(NO3)2·6H2O, and Co(NO3)2· H2O from Sinopharm Chemical Reagent Co. were of analytical grade and used without any further purification. commercial carbon clothes (WOS 1002, CeTech, the thickness is 360 μm,2 × 1 cm2 in rectangle) were cleaned by sonication in acetone, deionized water, and ethanol for 15 min each and heated at 500 °C for 2 h. In details, 1.25 × 10−3M Nickel nitrate, 2.5 × 10−3M Cobalt nitrate and 3.75 × 10−3M citric acid was dissolved in 50 ml pure H2O under vigorous stirring, resulting in light pink solution. The cleaned carbon cloth was soaked in 0.2 ml of this pink solution and then placed in an electrical oven previously designated at 70 °C for drying. After 2 h, the as-obtained precursor was further calcined in a tubular furnace at 300 °C for 2 h in air. After being slowly cooled down to room temperature, sample NC@C appeared. NiCo2O4@carbon cloth without citric acid (NC1@C) was prepared to compare with NC@C

2.2 Characterization

The structures and morphologies of as-synthesized samples were characterized by X-ray power diffraction (XRD, Rigaku D/MAX 2500, Cu Kα radiation), scanning electron microscope (SEM, FEI Nova Nano-SEM 230) and transmission electron microscope (TEM, JEOL JEM-2100F). The NC@C was direct used as working electrode without any binder. The NC@C was cut into sheets of 1 × 1 cm2. The amount of active materials on the carbon cloth was obtained by weighing the mass of the carbon cloth (treated via the anneal of 500 °C for 2 h) and the prepared sample in a high-precision analytical balance. Each electrode contains about 0.6 mg of electroactive materials, which is the same to the amount growing on carbon cloth using hydrothermal method.

2.3 Electrochemical measurements

The electrochemical performance was evaluated by using three-electrode system with Pt foil as counter electrode and Hg/HgO (KOH, a = 1) as reference electrode in 2 M KOH aqueous solution. The electrochemical measurements were carried out by using electrochemical workstation (CHI660E, Shanghai). To observe the pair of symmetrical and reversible redox peaks in high scan rates and ensure high Coulombic efficiency in even low current density testing, we measured cyclic voltammograms (CV) in the voltage range of 0−0.55 V (vs. Hg/HgO) at various scan rates, and galvanostatic charge-discharge (GCD) tests were measured in the voltage range of 0−0.52 V (vs. Hg/HgO) under different current densities.

3 Results and discussion

3.1 Physicochemical characterization

The crystalline structure and phase purity of the samples were characterized by XRD. All the diffraction peaks ((220), (311), (400), (511), and (440)) in Fig. 1 can be indexed to the spinel NiCo2O4 crystalline structure (JCPDS No: 20-0781) and no other peaks were found, except a broad peak arising from the CF substrate at 20–30 degree. The relatively broad peaks of NiCo2O4 reveal the small crystallite size or low crystallinity [27, 28].

Fig. 1
figure 1

XRD patterns of NC@C and carbon cloth

Full size image

The morphology of NC1@C is illustrated in Fig. 2a. According to the scanning electron microscopy (SEM) image, carbon fibers with rough surface could be observed, indicating the NiCo2O4 nanolayer not uniformly covered on the surface of carbon nanofibers. The morphology of NC@C is illustrated in Fig. 2b. According to the SEM image, carbon fibers with smooth surface could be observed, indicating the NiCo2O4 nanolayer uniformly covered on the surface of carbon nanofibers. The coating thickness is about 200 nm. The selected-area diffraction (SAED) pattern for NC@C (Fig. 2c inset)indicates high crystallinity, and each diffraction ring is also well indexed to NiCo2O4. Figure. 2d shows high-resolution TEM image of the NC@C, in which an interplanar spacing of 0.24 nm and 0.21 nm is assigned to the (311) and (400) lattice plane of NiCo2O4, respectively [29,30,31,32,33,34].

Fig. 2
figure 2

a SEM images of the NC1@C; b SEM images of the NC@C; c TEM images of NC@C and inset shows the SAED pattern; d HRTEM image of the NC@C

Full size image

3.2 Electrochemical characterization

To evaluate the potential of the as-synthesized NC@C acting as an electrode material for high rate supercapacitor, the electrochemical capacitive behavior was elucidated by the CV method. Figure 3a depicts representative CV curves measured at various scan rates ranging from 1 to 30 mV s−1 in a potential range of 0.0−0.55 V (vs. Hg/HgO) in 2.0 M KOH solution. The redox peaks are observed in anodic and cathodic sweeps in all CV curves, illustrating that the electrochemical capacitance of the NC@C is mainly based on the redox mechanism. The redox peaks are mainly attributed to the redox reactions related to Ni−O/Ni−O−OH or Co−O/Co−O−OH. there is only one couple of redox peaks of Ni2+/Ni3+ and Co2+/Co3+, due to the similar redox potential of Co3O4 and NiO, the structure of the NC@C which can induce appreciable broadening of some redox peaks. the peak of the CV curves current increase with increasing scan rates. Also, the shape of the CV curves is not obviously changed by increasing the scan rate indicating the good kinetic reversibility of the NC@C electrodes. It is evident that the D-value of redox peaks grows in size with the scan rates increasing, respectively. When the scan rate is 5 mV s−1, the redox peaks are around 0.33 and 0.44 V. When the scan rate is 100 mV s−1, the redox peaks are extended to 0.27 and 0.48 V. the cathodic peak shifts slightly from 0.33 to 0.27 V. This observation suggests a relatively low resistance of the electrode because of the good contact between the electroactive NiCo2O4 nanolayer and the conductive carbon clothe substrate.

Fig. 3
figure 3

a Electrochemical properties of the NC@C: b Cycle performance of the NC@C electrode and the NC1@C electrode for 3500 successive charge−discharge cycles at a current density of 1 A g−1. c galvanostatic charge−discharge(GCD) curves of the NC@C electrode at different current densities. d the first and the last three GCD cycles of 3500-cycling test at 1 A g−1

Full size image

The cycling stability of the NC@C and the NC1@Cwas also evaluated by the repeated charging/discharging measurement at a constant current density of 1 mA cm−2, as shown in Fig. 3d. Specific capacity (SC) can be determined by following equation:

$${\mathrm{SC = }}\frac{{{\mathrm{i\Delta t}}}}{{{\mathrm{m\Delta V}}}}$$

Where SC, i, Δt, m, andΔV are specific capacity (C/g), current (A), discharge time (s), the mass (g) of active materials, and the potential window (V), respectively. When a discharge current density of 1 mA cm−1 is applied, the specific capacity of NC@C electrode reaches a high value of 944.5 F g−1 and it gradually decreases to 859.5 F g−1 over 3500 cycles, resulting in an overall capacitance loss of only 8.9%. The capacity and stability of NC1@C is not the same as NC@C. The specific capacity of NC1@C electrode is just 748.5 F g−1 and it rapidly decreases to 353.0 F/g over 3500 cycles, resulting in an overall capacitance loss of 52.8%. Considering the morphologies of NC@C and NC1@C illustrated in Fig. 2, it is easy to illuminate why the capacity and stability of NC1@C electrode lower than NC@C electrode. On the NC1@C electrode, the NiCo2O4 active material is asymmetry and likely to fall off. On the NC@C electrode, the NiCo2O4 active material is homogeneous and closely combined with carbon cloth. Figure. 3c shows the galvanostatic discharge curves of as-synthesized NC@C testing under different current densities within the potential range from 0 to 0.55 V (vs. Hg/HgO). With the increase of discharge current densities, a larger voltage drop can be observed in Fig. 3c, leading to the decrease of capacitance in high-rate. It can be observed that there are voltage plateaus between 0.30 and 0.41 V, which is consistent with observations in previous reports in the literature. Figure. 3d shows the first three and the last three GCD cycles of the 3500-cycling test at a current density of 1 A g−1. The charge−discharge plateaus of the last three cycles turn into shorter than that of the beginning, resulting in a capacitance decrease.

The corresponding plot of discharge capacitance versus current density of NC@C electrode is shown in Fig. 4, along with the comparative data of NC1@C electrode. The NC@C electrode can deliver a high specific capacitance of 944.5 F g−1 at a current density of 1 A g−1 and a specific capacitance of 702.4 F g−1 is still retained at a very high current density of 20 A g−1, which are much higher than the NC1@C electrode at the same current densities.

Fig. 4
figure 4

The specific capacitance of the NC@C electrode and the NC1@C electrode at various current densities

Full size image

4 Conclusions

In conclusion, we have successfully synthesize an ultrathin NiCo2O4 nanolayer on flexible carbon cloth with strong adhesion for high-performance supercapacitors. NiCo2O4 nanolayer covered on surface of the carbon cloth is synthesized by a one-step route which is only involved a modified sol–gel method. The binder-free electrode only with simple layer covering on carbon cloth displays outstanding pseudocapacitive behaviors in 2 M KOH, which exhibits high specific capacitances of 944.5 F g−1 at 1 A g−1 and 702.4 F g−1 at 20 A g−1after activation, as well as excellent cycling stability. The specific capacitance can still retain 859.5 F g−1 (91% retention) at a current density of 1 A g−1 after 3500 cycles.