1. Introduction

In response to the needs and ecological interests of modern society, there is a growing interest in new, low-cost, and environment-friendly energy storage systems[1]. Among the various energy storage devices, supercapacitors (also known as ultracapacitors or electrochemical capacitors) have been attracting attention as new energy storage devices with high power density, long cycle life, simple charging circuits, and no memory effect compared to batteries[2]. Potential applications of these supercapacitors include emergency power devices used in memory backup, portable electronic equipment, and electric vehicles (EVs) that require high power density. Supercapacitors can also provide high power needed during vehicle acceleration in combination with a battery or a fuel cell and recover the energy used during braking. In addition, supercapacitors are considered a breakthrough in energy storage as the next-generation energy storage devices because they can store as much energy as a battery and can be fully charged within 1~2 min.

To date, various carbon materials, metal oxides, and conductive polymers have been used as electrode materials for supercapacitors. Different carbon materials have been studied as electrode materials for supercapacitors to secure high output density and high capacitance at the same time[3]. Carbon materials such as carbon nanotubes, carbon blacks, carbon nanofibers, activated carbons, and graphenes are the examples of electrical double layer capacitor (EDLC) materials. EDLC stores and emits energy by charge separation at the Helmholtz interface between the electrolyte and the electrode. Thus, for electrodes operated on the principle of EDLC, high specific surface area is the largest factor promoting high specific capacitance, which is a very important characteristic of EDLC electrode materials.

Among carbon materials, graphenes or reduced graphene oxide (rGO) is a material with a single layer of a two-dimensional structure of graphite, with hexagonal sp²-hybridized planar carbon isotopes. These graphene materials have high flexibility, wide specific surface area, and high electrical conductivity, and are considered the next-generation electrode materials; however, they have π-π stacked sheets due to Van der Waals interactions at adjacent layers between the graphene sheets. The stacked graphene layers lose their corresponding specific surface area and fail to improve electrochemical performance[ 4,5]. Therefore, it is important to prevent the reaggregation of graphene by maximizing the theoretical properties and ensuring an effective specific surface area to use graphene as an electrode material.

In order to maximize the electrochemical properties of the electrodes, several studies have been conducted not only on carbon-based materials but also on transition metal oxides and conducting polymers. Some studies were conducted on the electrode materials by forming a complex between conductive polymers, such as polypyrrole and polyaniline, and carbon-based materials[9,10]. Various transition metal oxides such as Ni(OH)₂[6], Co₃O₄[7], Fe₂O₃[8], NiO[9], and RuO₂[10] are considered promising electrode materials due to their high capacitance values. RuO₂ has a higher capacitance value than other transition metal oxides, but it has a disadvantage of the high cost and toxicity of the metals.

Spinel nickel cobaltite (NiCo₂O₄), among various metal nanomaterials used as electrodes, has many electrically active points and its electrical conductivity is at least twice that of conventional Co₃O₄ and NiO single metal oxides. It is also a low cost, environmentally friendly, and a widely used electrode material[11]. As the two metals form a spinel structure, nanostructures present in various states of oxidation can enable richer redox reaction for spinel nickel cobaltite so that it can accumulate more charge[12-14].

Recently, many researchers have been interested in the preparation of composites that can secure both the electrical double layer capacitance and pseudo-capacitance values and at the same time exhibit synergy through the interactions between them. Such a composite can have improved electrochemical properties such as higher specific capacitance, rate capability, and cyclic stability than a single material. Therefore, inorganic materials and graphenes have been reported to have high capacitance and excellent rate capability[12-14]. However, all parameters such as high capacitance, rate capability, and cycling stability could not be easily fulfilled. So, we would like to improve and guarantee the above parameters by combining two different nanostructured materials. It was proposed that novel composites consist of 2-dimensional rGO and petal-like 3-dimensional NiCo₂O₄/rGO composites.

NiCo₂O₄/rGO composites were fabricated using a hydrothermal method by adding different amounts of graphene oxide (GO) to the nickel cobalt linear double hydroxide (NiCo LDH) precursors solution. The GO and NiCo were reduced to rGO and NiCo₂O₄ using heat treatment. NiCo₂O₄ acts as a medium for the exfoliation of reduced graphene oxide sheets by effectively preventing the aggregation and restacking of rGO. Moreover, high electrochemical performance of NiCo₂O₄/rGO composites as a capacitor was demonstrated. The petal-like NiCo₂O₄/rGO composites exhibit an excellent electrochemical performance, specifically, high specific capacitance (1,755 Fg^-1 at 2 Ag^-1), good rate capability, improved internal resistance, and excellent cycling stability (only 15% loss after 3,000 cycles). The data obtained were used to determine the optimum weight percent of graphene oxide in NiCo₂O₄/rGO composites.

2. Experimental

2.1. Synthesis of graphite oxide

Graphite oxide was synthesized from natural graphite (SP-1, Bay Carbon) using a modified Hummers’ method. Graphite powder (1 g) was added into a mixture of 98% sulfuric acid (46 mL), sodium nitrate (1 g), and potassium permanganate (5 g). The solution was maintained at 43 ℃ for 2 h. Next, 30% H₂O₂ (7 mL) and deionized (DI) water (80 mL) were slowly added into the solution. A bright yellow solution obtained after the oxidation reaction was filtered, washed with 10% HCl, and the precipitate was washed with ethanol and DI water via centrifugation (3,600 rpm, 5 min) to remove residual graphite. The graphite oxide powder was obtained after freeze-drying for 30 h.

2.2. Synthesis of NiCo₂O₄/rGO composites

NiCo₂O₄/rGO composites were synthesized with different weight ratios of GO-to-NiCo₂O₄. Various amounts of graphite oxide (0.025, 0.05, 0.075, 0.1, and 0.2 g) were dispersed in deionized water (80 mL) and sonicated for 30 min to produce graphene oxide. Then, 1 mmol of Nickel(II) nitrate hexahydrate (Ni(NO₃)₂⋅6H₂O), 2 mmol of Cobalt(II) nitrate hexahydrate (Co(NO₃)₂⋅6H₂O), and 8 mmol of urea were separately dissolved in 20 mL ethanol (EtOH). The two solutions were then mixed and 1 M sodium hydroxide solution was added into the solution to maintain the pH at 6. After the solution was vigorously stirred for 30 min, the resulting solution was sealed into a Teflon-lined stainless- steel autoclave at 100 ℃ for 6 h. We finally obtained the NiCo/GO composite after filtering, washing, and drying the solution in air at 60 ℃ for 12 h. The obtained NiCo/GO was transferred to the tube furnace and heated to a higher temperature. This step was conducted at 300 ℃ under air atmosphere using a heating rate of 1 ℃ min^-1. NiCo₂O₄/rGO composites were obtained by heat treatment and reduction. The obtained composites were called 25G_NCO, 50G_NCO, 75G_NCO, 100G_NCO, and 200G_NCO based on the amount of GO in the composites. For comparison, we also prepared a sample without GO, referred to as NoG_NCO. In addition, we synthesized pure rGO for a comparison study.

2.3. Characterization methods

The microstructure of NiCo₂O₄/rGO composites was analyzed using field emission scanning electron microscopy (FE-SEM, Carl Zeiss, Supra 25) and transmission electron microscopy (H-7600 (Hitachi)). X-ray diffraction (XRD) analysis of materials was performed on Empyrean Series 2 X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å). The Fourier transform infrared (FT-IR) spectra were recorded on Perkin Elmer FT-IR Spectrum Two in the range of 650~ 4,000 cm^-1.

2.4. Electrochemical characterization

All electrochemical measurements were performed in a three-electrode cell system using Iviumstat (Ivium Technologies, Netherlands) at room temperature. Working electrodes were prepared from 85 wt% of NiCo₂O₄/rGO composite powders (prepared as described in section 2.2), 10 wt% of carbon black (Super-P, Alfa-Aesar) as a conductive agent, and 5 wt% polyvinylidene fluoride (PVDF) as a binder. Then, the mixture was mixed with N-methyl-2-pyrrolidine (NMP) as the solvent using an agate mortar. The mixed slurry was coated on a Ni foam substrate (1 cm × 1 cm) and dried in a vacuum oven at 80 ℃ for 12 h. The average weights of active materials loaded on the current collector were 3 to 4 mg. A saturated calomel electrode and platinum wire were used as the reference and the counter electrodes, respectively. The tests were performed in 6 M aqueous KOH electrolyte. Cyclic voltammetry (CV) measurements were carried out between 0.0 and 0.4 V at scan rates of 5, 10, 20, 30, 50, and 100 mVs^-1. The specific capacitance of the composite can be calculated from the Galvanic charge/discharge results according to the following equation: C = IΔt/mΔV, where I is the response current density (Ag^-1), ΔV is the difference between the highest and the lowest potential (V), m is the mass of the active material in the electrodes (g), and Δt is the discharge time (s). Galvanostatic charge/discharge curves were plotted in the same current density range of 1, 2, 3, 5, and 10 Ag^-1. The electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 100,000 ~0.01 Hz at the open circuit potential with an alternating current perturbation of 5 mV.

3. Results and Discussion

3.1. Structure characterizations

Schematic illustration of the synthesis process is presented in Figure 1. In the dispersion of graphene oxide, Ni²⁺ and Co₂₊ cations from Ni(NO₃)₂⋅6H₂O and Co(NO₃)₂⋅6H₂O were anchored to the negatively charged surface of graphene by electrostatic attraction[15]. After coprecipitation with urea and sodium hydroxide, a nucleation occurred and NiCo LDH particles were produced on graphene oxide under hydrothermal treatment. The NiCo LDH that existed between the graphene layers likely prevented the restacking of the neighboring sheets. The agglomeration of LDH particles was avoided by the exfoliated graphene oxide. After annealing at 300 ℃, graphene and LDH were reduced to rGO and NiCo₂O₄, respectively. Consequently, the active surface area of graphene, which would be exposed to electrolyte ions, increased and lead to fast ion and electron transport.

Figure 2 shows the XRD spectra of the pristine rGO and NiCo₂O₄, and the 75G_NCO composites. The peak of 23.8° in the XRD pattern of rGO was obtained, but the GO signature diffraction peak of 11° disappeared. The reduction from GO to rGO was properly performed [16]. The XRD patterns of pure NiCo₂O₄ and 75G_NCO composites were compared with the previous report[16]. Spinel NiCo₂O₄ with a cubic structure was formed (JCPDS No. 20-0781). Since the XRD pattern has no peaks corresponding to other characteristics, the composite material produced has high purity. For the 75G_NCO composite, the d-spacing of the (311) diffraction peak of NiCo₂O₄ was 2.4 Å. These results suggest that NiCo₂O₄ effectively prevents the restacking of rGO and maintains a wider d-spacing of graphene layers for 75G_NCO composites. There have been a few reports on diffraction peaks disappearing or weakening when rGO reaggregation was effectively prevented[ 17,18]. However, the weaker peak strength of 75G_NCO composites than that of pure NiCo₂O₄ shows lower crystallinity of NiCo₂O₄ than of 75G_NCO composites. Such structural properties may lead to more structural defects that may improve the electrochemically active sites and provide more ion diffusion channels with the electrolyte[19].

Figure 3 presents the FT-IR spectra of pristine rGO, NiCo₂O₄, and rGO-NiCo₂O₄ composites. The peak at 1,625 cm^-1 is attributed to the stretching of carboxyl (-COOH)[20]. The NiCo₂O₄ spectrum indicates that strong peaks at lower frequencies (652 cm^-1) can be assigned to the stretching vibrations of the Co-O bonds in cobalt oxide[21]. In the spectrum of the NiCo₂O₄/rGO nanocomposites, bonds at 652 cm^-1 along with the other bands at 3,000~3,600 cm^-1, and 1,635 cm^-1 were observed. 3,000~3,600 cm^-1, could be assigned to the OH- group of carboxyl (-COOH) or remained moisture. 1,635 cm^-1 could be related to the oxidation state of graphene. From the IR spectra results, we could say that the NiCo₂O₄ oxide molecular structure had been sustained after the formation of composite with layer-structured graphene.

Surface morphology and microstructure of the composite materials were also investigated. The FE-SEM images of (a) pristine rGO, (b) NiCo₂O₄ (c), (d) 75G_NCO composites and TEM images of (e) NiCo₂O₄ and (f) 75G_NCO composites are presented in Figure 4. The 3-dimensional NiCo₂O₄ nanoparticles are distributed evenly over the rGO sheet and act as a material that minimizes the loss of surface area as a spacer, which prevents repositioning with the adjacent sheets as well as contributing to its own capacitance. The NiCo nanoflake/GO composite was reduced to the NiCo₂O₄/rGO composite through heat treatment. The TEM image (Figure 4 (e), (f)) shows that NiCo₂O₄ is well connected to each other through rGO, securing an open space between adjacent rGO sheets and shortening the ion diffusion path. Figure 4 (g) shows that the amount of graphene exceeding the amount of the metal oxide is injected and the reaggregation of the graphene occurs again, reducing the effective specific surface area.

3.2. Electrochemical performance

The cyclic voltammograms of the synthesized composite electrodes are shown in Figure 5 (a). To compare the electrochemical behavior of pristine rGO, NoG_NCO, and 75G_NCO, cyclic voltammetry (CV) curves of the samples in 6 M KOH electrolyte were measured at the scan rate of 10 mVs^-1 in the potential range of 0.0~0.4 V. Samples containing nickel cobaltite showed clear redox peaks attributed to the pseudocapacitive feature of nickel cobaltite. The redox peaks of the two pairs indicated that the charge adsorption/desorption of the electrode had a greater effect on the pseudo-capacitance than the electric double layer capacitance (Figure 5). As reported in previous studies [11,22,23], nickel cobaltite is active in the voltage range of 0~0.55 V and the pseudocapacitive behavior in the alkaline electrolyte can be expressed as follows:

In addition, the specific capacitance can be measured by calculating the internal area of the CV. The higher the capacitance, the wider the internal area of the graph. Figure 5 (b) shows that the 75G_NCO composites show the greatest electrochemical performance. The 75G_NCO sample can undergo more redox reactions because it has more active sites with a larger specific surface area and is more electrically conductive than the other samples.

Figure 5 (c) shows the movement of the redox peaks of the 75G_NCO composite in CV curves at different scan rates in the range of 5~100 mVs^-1. At low scan rates, the electrodes can interact with large amounts of electrolyte ions through a large specific surface area and ions can freely pass through the inside/outside of the electrode material, securing a high specific capacitance value. However, as the scan rate increases, it becomes difficult to effectively transfer the electrolyte ions into the porous electrode, and consequently, the specific capacitance is reduced because a rapid oxidation-reduction reaction does not proceed. As a result, generally all samples show a fast decay at high scan rates[15].

The galvanostatic charge/discharge behavior of the composites was measured in the potential range of 0.0~0.4 V. Figure 6 (a) illustrates the typical first charge/discharge curves of the composite electrodes at the current density of 1 Ag^-1. The results exhibited the same tendency as the CV curves in Figure 5 (a). Two voltage steps are shown in the curve of the discharge process. The voltage drop in the first step (0.4 V to 0.2 V) follows a straight-line similar to that of the electrical double layer capacitance. The second step (slow potential decay from 0.2 V to 0.13 V) indicates the performance of pseudo-capacitance due to the redox reaction of nickel cobaltite. The GCD curve in Figure 6 (b) shows a decrease in capacitance as graphene is added near 75G_NCO, a trend similar to that of CV. The main reasons for this phenomenon are (1) the synergy by the formation of a composite of graphene and nickel cobaltite and (2) the prevention of the restacking of graphene sheets. In more detail, for the first reason, it is possible to explain the increase in capacitance in the process of using both pseudo-capacitance and electric double layer capacitance of the composite material composed of graphene and nickel cobaltite. Second, owing to the 3-dimensional nickel cobaltite structure, it is possible to partially prevent the reaggregation of the adjacent rGO sheets for securing the surface area of the complex and contributing to the capacitance due to electrochemically active sites. These two reasons ensure the improved ion transfer and excellent electrochemical reversibility, leading to an increase in the specific capacitance while increasing the graphene amount of the 75G_NCO composite material. However, as the concentration of graphene increases beyond the 75G_NCO composite, the specific capacitance decreases because the amount of graphene exceeding the amount of the metal oxide is injected and the reaggregation of the graphene occurs again, reducing the effective specific surface area. Figure 6 (c) shows that the 75G_NCO composite has a stable redox reaction even at a high current density.

The specific capacitance (capacitance per unit area) is evaluated using the formula C = IΔt / mΔV, where I is the discharge current, Δt is the discharge time, m is the mass of the active material, and ΔV is the voltage change after a full charge. The specific capacitance of the 75G_NCO was calculated as 1,710, 1,755, 1,748, 1,700, and 1,575 Fg^-1 at the current densities of 1, 2, 3, 5, and 10 Ag^-1, respectively (Figure 7).

The impedance response as a function of frequency of the composite electrode was demonstrated in a Nyquist plot. Electrochemical impedance spectroscopy (EIS) is a plot in which Z′ and Z″ represent the real and imaginary parts of the impedance axis, respectively. Figure 8 displays the impedance responses of NoG_NCO and 75G_NCO composites with high frequency. EIS is divided into three major components according to the frequency domain and each element is known as[24] charge transfer resistance (Rct), bulk resistance (R_B), and Warburg resistance (Z_W). The charge transfer resistance is measurable at high frequency generated by bilayer capacitance values and is expressed in a semicircular form. Warburg impedance is represented by the slope of the curve at low frequency, which means that the slope of the curve is close to the shape of the straight line and closer to the ideal capacitor behavior. The slope of 75G_NCO composites is closer to the shape of the straight line than NoG_NCO, which can be interpreted as an improvement in electrical conductivity through composite formation (Figure 8).

The cyclic stability of the 75G_NCO composite electrode was evaluated in the potential range of 0.0~0.4 V at a scan rate of 10 Ag^-1 for more than 3,000 cycles and the results are shown in Figure 9. For the initial 300 cycles, NoG_NCO showed a retention higher than 75G_NCO, but the retention abruptly decreased at larger number of cycles. The capacitance of the 75G_NCO composites showed a gradual decrease (5%) over 2,000 cycles while maintaining the performance at > 90% in the initial 300 cycles. Finally, the cyclic stability was higher than 85% of the initial capacity after a 3% reduction in 3,000 cycles.

4. Conclusions

In summary, NiCo₂O₄/rGO composites were prepared by introducing various amounts of rGO on NiCo₂O₄. The combination of rGO with NiCo₂O₄ enhanced the electrochemical performance of rGO because rGO enhanced the charge transport by facilitating ion diffusion. In addition, the impedance plots showed that the composites had the higher rates of ion diffusion and charge transport with a low bulk resistance. The 75G_NCO composites showed the best electrochemical properties such as specific capacitance, rate capability, and cycle performance. These capacitive performances are the result of the inherent constitution and electrochemical activity of composites based on multiple oxidation states, as well as the enhanced porosity and specific surface area by the addition of 2-dimensional graphene materials and the improved electrical conductivity. The 75G_NCO composites showed the highest specific capacitance of 1,755 Fg^-1 in a 6 M KOH electrolyte at a current density of 2 Ag^-1. In addition, this composite showed a high rate capability (~92% retention at 10 Ag^-1) and excellent cycle performance (~85.7% retention of initial value after 3,000 cycles). These composites could be applied to the electrode of supercapacitors which exhibits high energy density and high power density together.