Synthesis of NiMoO₄/3D-rGO Nanocomposite in Alkaline Environments for Supercapacitor Electrodes

Abstract

Although Graphene oxide (GO)-based materials is known as a favorable candidate for supercapacitors, its conductivity needs to be increased. Therefore, this study aimed to investigate the performance of GO-based supercapicitor with new methods. In this work, an ammonia solution has been used to remove the oxygen functional groups of GO. In addition, a facile precipitation method was performed to synthesis a NiMoO₄/3D-rGO electrode with purpose of using synergistic effects of rGO conductivity properties as well as NiMoO₄ pseudocapacitive behavior. The phase structure, chemical bands and morphology of the synthesized powders were investigated by X-ray diffraction (XRD), Raman spectroscopy, and field emission secondary electron microscopy (FE-SEM). The electrochemical results showed that the NiMoO₄/3D-rGO(II) electrode, where ammonia has been used during the synthesis, has a capacitive performance of 932 Fg⁻¹. This is higher capacitance than NiMoO₄/3D-rGO(I) without using ammonia. Furthermore, the NiMoO₄/3D-rGO(II) electrode exhibited a power density of up to 17.5 kW kg⁻¹ and an energy density of 32.36 Wh kg⁻¹. These results showed that ammonia addition has increased the conductivity of rGO sheets, and thus it can be suggested as a new technique to improve the capacitance.

1. Introduction

Growing concerns about the serious environmental pollution and fossil-fuel energy crisis have attracted considerable attention to renewable and clean energy storage technologies [1]. Lithium-ion batteries, sodium-ion batteries, and supercapacitors (SCs) are currently potential energy storage devices [2]. Among them, SCs have competitive characteristics, such as high power density, long cycle life, fast charging/discharging rate [3], high safety, simple operating principles, and low maintenance costs [4,5]. Various kinds of materials have been selected to improve electrode properties. Although carbonaceous materials such as graphene and reduced GO can deliver high power density and possess high specific-area, good electrical conductivity, high chemical stability, and biocompatibility [6], faradaic electrode materials like metal oxides can provide higher capacitance and larger energy densities due to their redox reactions [7]. Recently, binary nickel-based oxides have attracted tremendous research interest because of their high theoretical capacitance and excellent electrochemical activity [8], arising from their improved electronic conductivity and surface redox reactions. Among various pseudo-active materials, NiMoO₄ is an especially attractive candidate for pseudocapacitors, offering good stability in alkaline electrolytes, high specific capacitance and accessibility [9]. Therefore, the motivation to design and fabricate an NiMoO₄ and rGO electrode with a novel hybrid nanostructure, which might combine the merits of both high specific capacitance originating from NiMoO₄ and excellent rate capability provided by rGO, is well founded.

Intensive studies have been carried out on the synthesis of NiMoO₄ with different morphologies such as nanorods [10], nanowires [11,12,13], nano-spheres [14], and nanosheets [15,16,17] for various application. Most of these studies used the hydrothermal method [8,10,11,12,13,14,15,18,19,20,21,22,23,24,25] that requires autoclave equipped with high-pressure and high-temperature, which finally resulted to the particles with large sizes and some of the papers used sonochemical [26], microwave [27], Sol-Gel [28], electrodeposition [17], and co-precipitation method [29,30]. Obviously, NiMoO₄ synthesis is much less achieved by chemical precipitation than the hydrothermal method. We also synthesized NiMoO₄ and graphene simultaneously for supercapacitor application, while those papers on chemical coprecipitation were exclusively for the synthesis of just NiMoO₄ and its photocatalyst properties. In the current work, a simple and low-cost chemical precipitation method was used to synthesize NiMoO₄/3D-rGO nanocomposites with no need of any specific facilities. In addition, this work studies the mechanism of formation of NiMoO₄ nanoparticles on the surface of the graphene oxide and its reduction, which was not discussed in previous papers. Furthermore, there are several ways to reduce GO to rGO, one of which is the chemical reduction method. In previous works, various reduction agents such as hydrazine monohydrate [31], sodium borohydrate [32], ascorbic acid [33], ammonia borane [34], and heavy metal ions [35] have been used, while in the present research an ammonia solution has been applied.

In this work, we aim to synthesis the NiMoO₄/3D-rGO electrode through a facile precipitation method for supercapacitors with particular emphasis on reduction of GO to rGO. Therefore, synergistic effects of ammonia, metal ions and low temperature have been used to eliminate large amount of GO oxygenated groups.

2. Experimental

2.1. Materials

Graphene oxide (GO), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), sodium molybdate dihydrate (Na₂MoO₄·2H₂O), and potassium hydroxide (KOH) were of reagent quality and were obtained from VWR, Sweden. Ammonia (28 wt. %-NH₃) was purchased from VWR, Sweden. Polyvinylidene fluoride (PVDF, –(C₂H₂F₂)_n–), its solvent dimethylformamide (DMF, C₃H₇NO), and starch ((C₆H₁₀O₅)_n) were obtained from Sigma, Iran.

2.2. Preparation of the NiMoO₄/3D-rGO(I) and NiMoO₄/3D-rGO(II) Nanocomposites

Figure 1 briefly illustrates the fabrication of the NiMoO₄/3D-rGO(II) nanocomposite through a simple precipitation process. The NiMoO₄/3D-rGO composite was synthesized via a precipitation method in a starch environment as follows: First, 1 g of starch was added to 20 mL of distilled water at 80 °C to form a gelation suspension. Then, 1 mg of GO was dispersed in 20 mL distilled water under 5 min ultrasonication before being added to the starch solution to form a brown suspension. To prepare an alkaline environment, 3 mL ammonia (28 wt. %) was quickly added to the above suspension to adjust the pH about 10. Then, 5 mM Ni(NO₃)₂.6H₂O and 5 mM Na₂MoO₄·2H₂O were added to the above mixture in a dropwise manner and heated at 80 °C for 1 h, followed by 10 min of stirring. Subsequently, the powder was washed with distilled water to remove the impurities, and then dried at 60 °C for 24 h in a vacuum oven. Eventually, the final powder was calcinated in a tube furnace under Ar atmosphere at 350 °C for 2 h and then cooled to room temperature naturally to obtain the NiMoO₄/3D-rGO(II) nanocomposite. The reduction of GO and the in situ formation of NiMoO₄ were achieved simultaneously, causing the generation of NiMoO₄/rGO. The same process has been done without the addition of ammonia (NH₃) and the sample is called NiMoO₄/3D-rGO(I).

2.3. Electrode Fabrication of NiMoO₄, NiMoO₄/3D-rGO(I), and NiMoO₄/3D-rGO(II) Nanocomposites

The electrodes were prepared by mixing active material, carbon black, and polyvinylidene fluoride (PVDF) with a mass ratio of 80:10:10 in DMF, followed by ultrasonication of the slurry for 10 min to form a uniform solution. The Ni foam was cleaned with HCl (10 wt. %) and ethanol during ultrasonication. Then, 2 mg·cm⁻² of the mixture was pasted onto the Ni foam (1 × 1 cm²) as the working electrode before being dried in an oven at 70 °C for 24 h.

2.4. Material Characterization

In this research, the following analytical systems were used to characterize the prepared powder. Structural characterization of the samples were characterized using an X-ray powder diffraction (XRD, Bruker with CuKα radiation (λ = 0.1542 nm), Sundsvall, Sweden). Field-emission scanning electron microscopy (FE-SEM, MAIA3, TESCAN, Sundsvall, Sweden) was used to investigate the morphology and microstructure of the powder. Raman spectroscopy (Raman microscope, Horiba Xplora plus, laser excitation at 532 nm, Sundsvall, Sweden) in the frequency range of 50–3000 cm⁻¹ was used to study the molecular bonds. A potentiostat system (VersaStat 4 with VersaStudio, Sundsvall, Sweden) was used to record the supercapacitor measurement.

3. Results and Discussion

X-ray diffraction analysis was used for studying the phase and structure of the samples. Figure 2 shows the XRD patterns of GO, rGO, NiMoO₄ NPs, NiMoO₄/3D-rGO(I), and NiMoO₄/3D-rGO(II) before and after calcination at 350 °C. X-ray diffraction spectra of the samples were carried out at 2θ = 10°–80° with a scanning rate of 0.02°/min. As can be seen, GO has a sharp peak at 2θ = 10.6° corresponded with the (002), which indicates the presence of the oxygen functional groups between the layers. This peak is disappeared in rGO pattern, implying the reduction of oxygen groups after heating at 350 °C. The XRD pattern of the NiMoO₄ NPs shows high intensity peaks, which implies the crystallinity of NiMoO₄ nanoparticles. This pattern is in accordance with Card No 00-045-0142 of JCPDS (JCPDS or ICDD -International Centre for Diffraction Data) data and related to the monoclinic phase of NiMoO₄. The peaks at 2θ = 27.21°, 30.36°, and 43.9° which are the main characteristic peaks of the formation of the α-NiMoO₄ phase, show that the sample after calcination is well crystallized and NiMoO₄·nH₂O has transformed to α-NiMoO₄ [36]. Although the results indicate that most of the phases formed are related to the α-NiMoO₄, a number of peaks at 2θ = 21.12°, 22.67°, and 32.07° signifies the presence of the orthorhombic phase of Mo₁₇O₄₇ (01-071-0566). These peaks are respectively attributed to the (510), (001), and (521) planes of Mo₁₇O₄₇ phase. In fact, Mo₁₇O₄₇ phase consists of seventeen MoO₃ phases, which has lost some of its oxygen and formed a number of vacancies, and eventually produced Mo₁₇O₄₇ instead of Mo₁₇O₅₁. XRD pattern of the NiMoO₄/3D-rGO(I) shows the exact pattern of α-NiMoO₄, with no diffraction peak of GO at 2θ = 10.6°. The peak at 2θ = 22.67° has been broaden rather than α-NiMoO₄ pattern, which is the same as rGO pattern. It can be concluded that in this case, the oxygenated groups of GO has been eliminated. The ammonia molecule is consisted of one nitrogen and three hydrogen atoms and due to the presence of lone pair on the nitrogen atom, this molecule is a strong source of electrons which is expected that it can eliminate oxygen groups of GO sheets. In order to investigate the role of the ammonia solution in the reduction process of GO, ammonia solution was added to GO suspension to adjust the pH about 10. After drying the sample at 60 °C without further heat treatment, its color changed from brown to black. According to some papers, changing in color can be the evidence of converting GO into rGO [37]. This sample is characterized before and after calcination at 350 °C. According to the XRD pattern of the NiMoO₄/3D-rGO(II) nanocomposite before calcination, there is no peak of GO at 2θ = 10.6°, which indicates that before heat treatment, ammonia caused reduction of GO sheets. Furthermore, the XRD pattern of the NiMoO₄/3D-rGO(II) nanocomposite after calcination reveals a peak at 43.3°, which corresponds to N-rGO, based on data from [38]. It reveals that nitrogen of ammonia has been doped into the rGO network at 350 °C. In addition, the peak at 2θ = 23.1° (d ~ 3.4 Å), while the GO has a peak at 2θ = 10.6° (d ~ 8.3 Å). The large space between the layers in GO is attributed to the presence of hydrocarbons, the oxygen-containing groups, and also the H₂O molecules at the edges and the distance between the sheets. In other words, decrease in the space between layers of graphene sheets in the NiMoO₄/3D-rGO(II) sample is a reason for the successful reduction of the GO sheets [39]. Results show that oxygen-containing groups can easily accept free electrons of ammonia to separate the carbon-oxygen bond, repaired π bands and form carbon radicals on GO sheets, and lead to the formation of rGO [37].

To compare the reduction rate of GO in the NiMoO₄/3D-rGO(II) nanocomposite with that of the NiMoO₄/3D-rGO(I) nanocomposite, the Raman spectra were measured for GO, NiMoO₄/3D-rGO(I), and NiMoO₄/3D-rGO(II) nanocomposites (shown in Figure 2). The Raman spectra of the pure GO (Figure 3a) shows that the D and G peaks are located at 1347.64 cm⁻¹ and 1574.98 cm⁻¹, respectively. The relative intensity ratio of the peaks (I_D/I_G) is a measure of the disorder degree, and its value is approximately 0.98 for GO [23]. In both spectra of the NiMoO₄/3D-rGO(I) and NiMoO₄/3D-rGO(II) nanocomposites, the characteristic peaks at 969 cm⁻¹ (Mo=O), 928 cm⁻¹ (Mo=O), and 697 cm⁻¹ (Ni–O–Mo) and some medium-intensity peaks at 394 cm⁻¹ (Mo–O) and 325 cm⁻¹ (Mo–O–Mo) belong to the α-phase NiMoO₄ [10,40,41] according to NiMoO₄ spectra (Figure 3a). The Raman spectra of the NiMoO₄/3D-rGO(I) nanocomposite (Figure 3b) shows that after chemical reduction of GO to rGO, the I_D/I_G ratio slightly increased from 0.98 (GO) to 1.02 (rGO), due to the increase in the amount of defects in rGO. In comparison to the NiMoO₄/3D-rGO(I), the I_D/I_G ratio of NiMoO₄/3D-rGO(II) nanocomposite (Figure 3c) was calculated to be approximately 1.049, which is higher than that of the NiMoO₄/3D-rGO(I) nanocomposite (1.02). This finding shows that ammonia reduced more oxygenated groups in the NiMoO₄/3D-rGO(II) sample. In addition, the Raman spectra of the NiMoO₄/3D-rGO(II) nanocomposite (Figure 3c) shows that the D and G peaks redshifted to the wavenumbers at approximately 1344.89 cm⁻¹ and 1574.98 cm⁻¹, respectively, compared to 1339.39 cm⁻¹ and 1577.65 cm⁻¹ in NiMoO₄/3D-rGO(I). The Raman results confirm that although the GO has been reduced in the presence of NiMoO₄ NPs, using an ammonia solution during synthesis results in more reduction of GO. In addition, the higher intensity of I_D/I_G in the NiMoO₄/3D-rGO(II) nanocomposite shows more defects and disorders in the rGO structure due to the removal of more functional groups on the surface of GO, which results in higher conductivity and provides more active sites for electron storage [23,42].

The morphologies and microstructure of the prepared NiMoO₄/3D-rGO(I) and NiMoO₄/rGO(II) nanocomposites were investigated by SEM (Figure 4). The SEM images of the both nanocomposites show a 3D interconnected network consisting of NiMoO₄ nanorods and rGO sheets. According to the SEM images of NiMoO₄/3D-rGO(I) nanocomposite, it seems that graphene oxide sheets were the nucleation centers for growing NiMoO₄ nanorods and they have been anchored the surface of reduced graphene sheets. This probably happened due to the strong electrostatic interaction between the nanorods and the reduced graphene oxide. Whereas the SEM images of NiMoO₄/3D-rGO(II) nanocomposite consisted of two different phases. It shows thick rGO sheets which can be interpret as graphite sheets. This phenomena can be attributed to the ammonia effect on reduction of the sheets. Another phase is NiMoO₄ nanorods which due to magnetic interaction has gathered together and made a separate porous structure. The results showed that starch as an environmentally friendly surfactant prevented the agglomeration of particles and eventually turned into gases (CO-CO₂) at 350 °C and resulted in a porous structure nanocomposite. Such a nanostructured morphology can provide more voids and a short diffusion path between the electrode material and the electrolyte which increases electrolyte diffusion and accessibility to reaction sites for more energy storage [23].

A possible formation mechanism for the synthesis of the NiMoO₄/3D-rGO(I) nanocomposite is schematically shown in Figure 5. It is well known that, –COOH or –OH oxygen-containing functional groups of GO sheets are the nucleation centers for combining Ni²⁺ and MoO₄²⁻ ions to form NiMoO₄. It has been reported that during the adsorption process, the release of the proton (H⁺) of –COOH or –OH functional groups to the solution results in a decrease of the equilibrium pH of the solution [43]. It can be concluded that the exchange of Ni²⁺ and H⁺ on –COOH or –OH oxygenated groups can be considered to be a mechanism for the adsorption of Ni²⁺ onto GO, and the formation of a bridge between different GO sheets. The relevant reactions in the current study can be expressed as follows:

—COOH + Ni²⁺ → —COONi²⁺ + H⁺

(1)

—OH + Ni²⁺ → —ONi²⁺ + H⁺

(2)

—2COOH + Ni²⁺ → —COONiOOC— + 2H⁺

(3)

—2OH + Ni²⁺ → —ONiO— + 2H⁺

(4)

—COOH + Ni²⁺ + —OH → —COONiO— + 2H⁺.

(5)

Electrochemical Studies

The electrochemical measurements of the samples were performed in a three-electrode cell configuration at room temperature with 3 M KOH electrolyte. In this system, a platinum sheet and a saturated calomel Hg/Hg₂Cl₂ electrode were used as the counter and the reference electrodes, respectively. Cyclic voltammetry (CV) analysis was evaluated within a potential range of 0–0.7 V at different scan rates (10–100 mVs⁻¹). Figure 6a shows the CV curves of the Ni foam, NiMoO₄ NPs, NiMoO₄/3D-rGO(I), and NiMoO₄/3D-rGO(II) nanocomposites. The redox peaks confirm that the charge-storage mechanism is mainly Faradaic (battery-type) and related to reversible chemical reactions of Ni(II) ↔ Ni(III) + e⁻ [44] which can be expressed as follows [3,45]:

Ni(OH)₂ + OH⁻ ↔ NiOOH + H₂O + e⁻

(6)

NiOOH + OH⁻ ↔ NiO₂ + H₂O + e⁻.

(7)

Mo atoms do not take part in the redox reaction. The main function of Mo element was to improve the conductivity of Nickel molybdate, thereby contributing in increasing specific capacitance and it does not show any redox peak in CV curves [46,47]. As seen, the area surrounded by the CV curve of the NiMoO₄/3D-rGO(II) hybrid electrode is larger and the redox current is higher than that of the NiMoO₄/3D-rGO(I) electrode, indicating much more specific capacitance of the NiMoO₄/3D-rGO(II) electrode. More capacitance of NiMoO₄/3D-rGO(II) supports the fact that removing more oxygenated surface functional groups of GO can provide more active sites for storing energy and increasing conductivity. Furthermore, the potential difference (ΔE) between the reduction and oxidation peaks of the NiMoO₄/3D-rGO(II) composite is 160 mV, which is approximately 81 mV smaller than that of the NiMoO₄/3D-rGO(I) electrode (241 mV, Figure 6a), indicating better electrochemical reversibility of NiMoO₄/3D-rGO(II) due to more reduction of GO sheets in the presence of ammonia [48].

Figure 6b illustrates the CV curves of the NiMoO₄/3D-rGO(II) electrode at the scan rates of 10–100 mVs⁻¹ in the potential window of 0–0.7 V. With increased scan rates, the anodic and cathodic currents increase, suggesting fast redox reactions at the electrode/electrolyte contact surface at high scan rates, which results in lower specific capacitance [22]. Due to the presence of graphene oxide sheets, in addition to the Faradaic and electrochemical reactions of NiMoO₄, formation of the electrical double-layer can lead to more charge storage in the system [49]. According to the resulting curves, it is noteworthy that by increasing the scan rate, even at the scan rate of 100 mVs⁻¹, redox peaks are still observed, indicating good chemical stability and conductivity in the hybrid structure of the electroactive materials [50].

Galvanostatic charge–discharge (GCD) tests were further performed to evaluate the capacitance of the electrodes at current densities of 1–50 Ag⁻¹. Figure 6c illustrates the GCD curves of the NiMoO₄/3D-rGO(II) electrode between 0 V and 0.5 V. A pair of voltage plateaus appear at ~0.29 and ~0.23 V and nonlinear behavior of the GCD curves, revealing the pseudocapacitive features of Ni²⁺ ions of the NiMoO₄/3D-rGO(II) electrode [18]. According to the three-electrode system, the specific capacitance of the single electrode can be estimated from the cyclic voltammetry (CV) curves and discharge curves with the Equations (8) and (9), respectively [51,52].

$$C_s=\frac{1}{\mathrm{m}\mathsf{\nu }\Delta \mathrm{V}}\int \mathrm{IdV}$$

(8)

$$C_s=\frac{\mathrm{I}\Delta \mathrm{t}}{\mathrm{m}\Delta \mathrm{V}}$$

(9)

where ν is the scan rate, I is the discharge current, Δt the discharge time, m is the mass of the active material loaded on the electrode surface, and ΔV refers to the potential range [51,52].

Stability behavior of the electrode was performed over repeated charge and discharge cycling. Therefore, GCD measurement for the NiMoO₄/3D-rGO(II) electrode were carried out at a current density of 2 Ag⁻¹ for approximately 500 cycles in 3 M KOH electrolyte (Figure 6d). Initially, the NiMoO₄/3D-rGO(II) electrode showed a significant decrease in the capacitance up to first 200 cycles. However, after 200 cycles the capacitance decreased slowly and at the end of 500 cycles, the capacitance dropped to 76% of its initial value. Good cycle stability of the NiMoO₄/3D-rGO(II) is due to the porous structure with NiMoO₄ rod morphology and low amount of oxygenated groups of GO after reduction in an alkaline environment.

Figure 6e illustrates the electrochemical impedance spectra of the NiMoO₄/3D-rGO(II) electrode before and after 500 cycles over the frequency range of 0.1 Hz to 100 kHz by applying a 10 mV-AC voltage. The Nyquist plots are usually consisted of two segments; the small semicircle in the high frequency region and the approximately straight line in the low frequency region. The diameter of the semicircle corresponds to the charge transfer resistance (R_ct) during Faradaic redox reaction. As can be seen, the Nyquist plots of both spectra show a negligible semi-circle in the high frequency region because of fast faradaic reactions, implying low R_ct and good charge transfer [53]. The intercept on the real axis at the beginning of the semicircle part determine the ESR (R_s) which is combination of electrode material resistance, ionic resistance of electrolyte and contact resistance between current collector and electrode material [54]. ESR for NiMoO₄/3D-rGO(II) electrode before cycling is 0.82 and after cycling the ESR value has increased to 0.9. Moreover, the linear line in low frequency region implies Warburg impedance (Z_W) attributed to OH⁻ ions diffusion of electrolyte. The more vertical the line, the closer to ideal capacitive behavior of the nanocomposite [53]. In the EIS plot of NiMoO₄/3D-rGO(II) electrode before 500 cycles, the straight line leans more towards the vertical Z″ axis, indicating a better capacitive performance and lower ion diffusion resistance of this electrode in contrast to NiMoO₄/3D-rGO(II) electrode after 500 cycles.

4. Conclusions

A simple precipitation approach is demonstrated to synthesize the NiMoO₄ NPs, NiMoO₄/3D-rGO(I) and NiMoO₄/3D-rGO(II) nanocomposite electrodes. The results showed that GO sheets were successfully reduced due to the disappearance of 2θ = 10.6° peak in XRD patterns and increase of I_D/I_G ratio in Raman spectrum. It can be concluded that the synergistic effect of the ammonia, metal ions and temperature benefited the deoxygenation of GO sheets. In general, addition of ammonia provided the possibility of GO deoxygenating and subsequent thermal treatment will restore the C=C bond to convert GO to reduced graphene oxide. The NiMoO₄/3D-rGO(I) and NiMoO₄/3D-rGO(II) electrodes demonstrated specific capacitances of 790 Fg⁻¹ and 932 Fg⁻¹, at a current density of 1 Ag⁻¹. An improved capacitive performance of the NiMoO₄/3D-rGO(II) electrode is attributed to the pseudocapacitance behavior of NiMoO₄ and the elimination of oxygen-containing groups which will be beneficial to the increase in the electronic conductivity of resulting rGO. In addition, the NiMoO₄/3D-rGO(II) electrode showed a good cycle life with 76% retention of the initial specific capacitance after 500 cycles.