Highly porous nanocomposite of NiMoS₄ nanosheets and reduced graphene oxide for energy storage application

Supercapacitors (SCs), also known as electrochemical capacitors, have been widely studied for alternative energy storage devices because they possess high power capabilities, high charge/discharge rates, and very long cycle-life [1–6]. However, SCs still have a low energy density and are not yet practical applications. There are two main strategies to improve the supercapacitors' energy density, including increasing the specific capacitance and enlarging the working window potential. While the capacitance is mainly due to the characteristics of the active electrode materials, the operating window potential is dependent on the properties of electrolytes and the structure of supercapacitors.

In the current study, we proposed to prepare a nanomaterial with high capacitance, high conductivity, and excellent porosity for supercapacitors. The prepared composite consists of bimetallic sulfides (NiMoS₄) and reduced graphene sheets (RGO). It was reported that NiMoS₄ had an excellent theoretical specific capacitance because of its Faradaic redox reactions with multiple valence states and active sites. Besides, this material is abundant and environmentally friendly [7–9]. However, NiMoS₄ shows a low intrinsic electric conductivity [10, 11]. The combination of NiMoS₄ and high conducting materials such as graphene is studied to overcome this issue [2, 10].

Xu et al have recently reported preparing NiMoS₄/RGO composite by the hydrothermal method [10]. The optimally designed composite showed a high specific capacity of 124 mAh g⁻¹ at 1 A g⁻¹. However, this method required many steps and a long-time reaction that may need a high-cost process. This paper used a fast and effective microwave-assisted one-step approach to prepare high porous RGO/NiMoS₄ nanocomposites. Ethylene glycol was used as a green solvent to minimize the use of organic solvents. The prepared nanocomposites provide a high surface area to facilitate the fast electron transport and electrolytes loading in the electrodes. The obtained product is believed to be a promising active material for high-performance supercapacitors.

2.1. Materials

2.2. Preparation of NiMoS₄/RGO nanocomposites

2.3. Characterisation

2.4. Electrochemical measurements

The electrochemical measurements, including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) cycles, and electrochemical impedance spectroscopy (EIS), were performed using a PGSTAT100N (Netherlands) in both three-electrode and two-electrode cells. The electrolyte is 6 M KOH solution. The counter and reference electrodes were a platinum plate and a silver chloride electrode (AgCl/Ag). The working electrodes were prepared by mixing 80 wt% active material with 15 wt% acetylene black (> 99.9%), 5 wt% Nafion solution, and 1 ml ethanol (99.9%) to form a black slurry. The resulting paste was pressed on nickel foam (NF, 1 cm × 1 cm) and dried at 60°C for 10 h. Each electrode contained 1.5 mg of active material. All the NF collectors were cleaned with 3 M HCl solution in an ultrasonic bath for 10 min and washed with DI water, and then dried in a vacuum at 60°C overnight before coating the composites.

The measurements were carried out at room temperature. The specific capacitance (C_s ) was calculated according to the equation: C = It/mΔV, where I, t, m, and ΔV are the applied current (A), discharge time (s), the composite mass (g), and voltage window (V), respectively.

An asymmetric supercapacitor (ASC) was fabricated using the NG-2 as active cathode material and the RGO as active anode material. The cathode and anode electrodes are prepared by the same procedure as described above. Two electrodes were directly packed with the filter paper separator between them.

The energy density (E, Wh kg⁻¹) and power density (P, kW kg⁻¹) are calculated according to the equations:

$$\begin{eqnarray}&&E=\displaystyle \frac{{C}_{s}{\rm{\Delta }}{V}^{2}}{2},\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&P=\displaystyle \frac{E}{t}.\end{eqnarray} \tag{ 2 }$$

where C_s , ΔV and t are specific capacitance (F g⁻¹), potential range (V), and discharge time (h).

The morphology of the bare NiMoS₄ and various RGO/NiMoS₄ nanocomposites can be observed by SEM images (figure 1) and more clearly in TEM images (figure 2). The bare NiMo₂S₄ is in the structure of agglomerated particles (figure 1(a)). After the RGO loadings, the morphology of the prepared nanocomposites was significantly changed. Various forms of the NiMoS₄ covered the surface of RGO sheets. At a small amount of RGO loading (figure 1(b)), many NiMoS₄ particles were coated on RGO sheets' surface. However, the composite had a hierarchical porous structure with an even distribution of sheet-like NiMoS₄ on the surface of RGO sheets (figures 1(c) and (d)) at the GO precursor's extensive contents. The NiMoS₄ nanosheets are of several nanometers in thickness and are intercrossed together. More clear structures of the prepared composites are presented in the TEM images, as shown in figure 2. The RGO/NiMoS₄ composite's porous structures provide a large open space and many electroactive surface sites, shortening the electrolytes' movement in the electrodes during redox reactions and charge/discharge processes. It will also increase the contact interfaces between electrodes and electrolytes, which will improve electrochemical performance [13]. The NG-2 sample showed the best porous and uniform layer of NiMoS₄ nanosheets distributed on the graphene surface. Thus, it was used for other characterizations and tested for electrochemical properties.

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The surface information and the oxidation state of the RGO/NiMoS₄ nanocomposite are conducted using XPS, as shown in figures 3 & 4. Figure 3 presented the XPS survey of the RGO/NiMoS₄ composite. The main atomic compositions are Ni (9.76%), Mo (39.15%), and S (51.09%). The observed peaks of Ni 2p, Mo 2p, S 2p, C 1 s, and O 1 s confirmed nickel, molybdenum, sulfur, carbon, and oxygen elements existing on the obtained sample's surface. For their oxidation states, the high resolutions of Ni 2p, Mo 3d, S 2p, and C 1 s peaks are displayed in figure 4. The Ni 2p spectrum (figure 4(a)) can be deconvoluted into two peaks at binding energies of 856.8 (2p_3/2) and 874.0 eV (2p_1/2) according to the Ni²⁺ spin-orbit doublets. Also, there are two shake-up satellites (labelled as 'Sat.') that appeared at 861.6 and 880.5 eV due to the Ni³⁺ active species, respectively [13–15]. The Mo 3d spectrum comprised two peaks at binding energies of 232.6 (3d_3/2) and 235.6 eV (3d_5/2) with a splitting width of approximately 3.0 eV that are attributed to Mo⁶⁺ and Mo⁵⁺ ions (figure 4(b)) [15, 16]. The high-resolution S 2p peak can be deconvoluted into peaks at 163.5 (2p_1/2) and 162.1 eV (2p_3/2), indicating the presence of terminal S²⁻ ion. Figure 4(d) shows the C 1 s spectrum with a prominent peak at 284.5 eV (C–C bonds) and a weak peak at 286.7 eV (C=O bonds), which indicates the reduction of GO. The O 1 s peak is according to the multiplicity of adsorbed water at or near the surface. Thus, the XPS analysis confirms the prepared nanocomposite comprised Ni²⁺, Ni³⁺, Mo⁵⁺, Mo⁶⁺, and S²⁻ ions.

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Figure 5 presents the XRD patterns of GO, bare NiMoS₄, and RGO/NiMoS₄ (NG-2). The GO spectrum shows a sharp peak at 11.86^o (figure 5(a)). The diffraction peaks of bare NiMoS₄ were observed at 2θ of 12.78, 33.87, 36.37, and 44.74^o that are assigned to MoS₂ (JCPDS card no. 24-0513) [17]. Other several diffraction peaks appeared at 2θ of 18.04, 26.45, and 34.80^o, according to MoO₃ (JCPDS card no. 05-0508) [18]. Besides, some well-defined peaks of Ni₂S₃ (JCPDS card no. 44-1418) were found at 2θ of 22.15, 28.75, 38.51, 43.15, 49.13, and 55.73^o [19]. For the RGO/NiMoS₄ composite, the XRD pattern exhibited an excellent assignment with the bare NiMoS₄ spectrum's prominent peaks as described above. Besides, the characteristic peak of GO at 2θ of 11.86^o disappeared, indicating the reduction of GO. These results suggested the formation of the RGO/NiMoS₄ nanocomposite. Also, the chemical structure of samples was investigated by FTIR spectra (Fig. S1). Compared to the spectra of RGO and NiMoS₄, the RGO/NiMoS₄ has two peaks at 605 and 1035 cm^-1 that correspond to the Ni-S or Mo-S vibrations. Besides, the peaks at 1045 (C=O stretching) and 1747 cm⁻¹ (C–O stretching) in the GO structure disappeared, confirming the formation of RGO.

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The electrochemical properties were first determined in a three-electrode system. The as-prepared composites were coated as active materials on working electrodes. The Ag/AgCl electrode and platinum foil were used as counter and reference electrodes, respectively. Figure 6(a) shows the CV curves of bare NiMoS₄ and different composites at the same scan rate of 5 mV s⁻¹. All electrodes display a typical pseudocapacitive characteristic. At this scan rate, an exact couple of redox peaks is seen. These redox peaks were assigned to the redox conversions of Mo⁵⁺/Mo⁶⁺ and Ni²⁺/Ni³⁺ ion couples. The NG-2 electrode exhibited the highest responding current. It can be explained that conductive RGO sheets coated with the porous and uniform NiMoS₄ nanosheets accelerated electron and ion transport, resulting in improved electrochemical performance.

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EIS curves determine the electron resistances of the electrodes and the electrode/electrolyte interface. The high-frequency semicircle is attributed to the charge transfer resistance (R_ct), while the low-frequency line corresponds to capacitance behavior. Besides, the intercept of a semicircle with the real axis is an equivalent series resistance (ESR). The ESR includes the electrolyte solution's resistance, the active material's intrinsic resistance, and the active material/current collector's contact resistance. Figure 6(b) presents the EIS plots of NiMoS₄ and different RGO/NiMoS₄ electrodes measured at a frequency range of 10 kHz to 10 MHz at 5 mV in a 6 M KOH solution. The ESR of all composites is much smaller than that of bare NiMoS₄, suggesting lower diffusion resistance and R_ct between the electrolyte and the active composites. It can be explained that the RGO nanosheets enhanced the conductivity and porosity of the prepared composite. Moreover, the semicircle diameter in RGO/NiMoS₄ curves is smaller than that of bare NiMoS₄, indicating the rapid charge transport in the RGO/NiMoS₄ structure.

Figure 6(c) presents the anodic/cathodic peak currents versus the potential scan rates. All peak currents are linearly proportional to the scan rates, indicating the composite's rich redox reactions during the charging/storing process. Moreover, there are still typical redox peaks observed at a high scan rate of 100 mV s⁻¹, suggesting a fast electron and ion movement in the electrode structure, good reversibility of redox reactions, and excellent rate capability [20, 21]. The electrochemical performance of the NG-2 electrode is determined by CP measurements at five current densities, as shown in figure 6(d).

The specific capacitance (C_s) could be calculated from the discharge curves. The C_s of 1273, 1003, 970, 933, and 780 F g⁻¹ can achieve at a current density of 1, 1.5, 2.5, 3.5, and 6.5 A g⁻¹, respectively. The rough comparison of the prepared material's electrochemical performance and some published related works is presented in table S1. The C_s value of the RGO/NiMoS₄ composite is higher than those of some nickel-molybdenum sulfides. The prepared electrode showed a high capacitance rate with a 61.2% retention from 1.0 to 6.5 A g⁻¹. The enhanced capacitance rate is due to the rapid charge movement in a high porosity and conductivity of the RGO/NiMoS₄ composite.

The electrochemical properties were second measured in a two-electrode system. An ASC device was prepared using RGO/NiMoS₄ (NG-2) as the positive electrode, RGO as the negative electrode. A 6 M KOH solution was used as the electrolyte. The mass proportion of the positive and negative electrodes was determined to achieve good performance based on the charge balance of q⁺ = q⁻. The charge stored by two electrodes (q) can be calculated by the following [19]: q = IΔt = C_s .ΔV.m. Therefore, the mass proportion is calculated by the equation [22]:

$$\begin{eqnarray}&&\displaystyle \frac{{m}^{+}}{{m}^{-}}=\displaystyle \frac{{C}_{s}^{-}{\rm{\Delta }}{V}^{-}}{{C}_{s}^{+}{\rm{\Delta }}{V}^{+}}.\end{eqnarray} \tag{ 3 }$$

From a three-electrode system, at the current density of 1 A g⁻¹, the C_s values are 1146 F g⁻¹ (NG-2) and 172 F g⁻¹ (RGO). The potential windows are 0–0.5 V (NG-2) and 0–1.0 V (RGO) based on the CV results. Thus, the optimum mass ratio of the cathode and anode electrodes is approximately 0.30. The electrochemical performance of this ASC device was presented in figure 7. From the CV curves at different potential windows (figure 7(a)), the device's full voltage window is 1.5 V. Figure 7(b) presents the CV curves measured at different scan rates, ranging from 2 to 100 mV s⁻¹. Redox peaks can be seen on the CVs even at 100 mV s⁻¹, indicating the pseudocapacitive characteristic attributed to the NG-2 electrode. Besides, the CV curves' shapes were unchanged much even at a high scan rate, confirming an excellent rate capability of the device.

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Figure 7(c) shows the ASC device's CP profiles at various current densities from 1 to 4 A g⁻¹. The C_s is 80 F g⁻¹ at 1 A g⁻¹ and maintains 80.6% (325 F g⁻¹) at 4 A g⁻¹. The ASC device's cycling performance was tested at the current density of 4 A g⁻¹ (figure 7d). The capacitance remains over 62% after 3500 cycles, indicating the ASC's good stability. On the other hand, the device's EIS curves before and after 3500 cycles have a similar shape, suggesting the obtained device's good cyclic stability. The device exhibits an E of 82.5 W h kg⁻¹ at a P of 1000 W kg⁻¹, which is better than those of previous related reports (as shown in Fig. S2), such as Co₃S₄/CoMo₂S₄-rGO//AC (33.1 W h kg⁻¹ at 850 W kg⁻¹) [22], CoMoS₄//AC (27.2 W h kg⁻¹ at 400 kW kg⁻¹) [23], Co₉S₈/NSA//AC (20.0 W h kg⁻¹ at 828.5 W kg⁻¹) [24], and Co-Mo-S//AC (42.6 W h kg⁻¹ at 850.3 kW kg⁻¹) [25], suggesting a great potential of our ASC device for practical supercapacitor applications.

A high mesoporous RGO/NiMoS₄ nanomaterial was prepared successfully by an effective microwave-assisted approach. The prepared composite showed a high specific capacitance of 955 F g⁻¹ at 1.0 A g⁻¹ in a three-electrode measurement. The ASC device packed using RGO/NiMoS₄ cathode and RGO anode has an energy density of 82.5 Wh kg⁻¹ at a power density of 1000 W kg⁻¹. Besides, the device showed good cyclic stability of 62% after 3500 cycles. The prepared composite appears as a high potential nanomaterial for supercapacitor application.

This research was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.99-2018.310.