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Article

Porous NiMoO4-NrGO as a Battery-Like Electrode Material for Aqueous Hybrid Supercapacitors

by
Shahrzad Arshadi-Rastabi
1,2,*,
Rasoul Sarraf-Mamoory
1,*,
Ghadir Razaz
2,
Nicklas Blomquist
2,
Jonas Örtegren
2 and
Håkan Olin
2
1
Department of Materials Engineering, Tarbiat Modares University, Tehran 14115111, Iran
2
Department of Natural Sciences, Mid Sweden University, 85170 Sundsvall, Sweden
*
Authors to whom correspondence should be addressed.
J. Compos. Sci. 2023, 7(6), 217; https://doi.org/10.3390/jcs7060217
Submission received: 31 March 2023 / Revised: 13 May 2023 / Accepted: 22 May 2023 / Published: 26 May 2023
(This article belongs to the Special Issue Composites for Energy Storage Applications, Volume II)
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Versions Notes

Abstract

:
Recently, much research has investigated nanocomposites and their properties for the development of energy storage systems. Supercapacitor performance is usually enhanced by the use of porous electrode structures, which produce a larger surface area for reaction. In this work, a biocompatible polymer of starch medium was used to create the porous nanostructure. Two powders, i.e., Nickel molybdate/reduced graphene oxide (NiMoO4-rGO) and Nickel molybdate/nitrogen-doped reduced graphene oxide (NiMoO4-NrGO), were synthesized using the deposition method in a medium containing starch, nickel nitrate salts, sodium molybdate, and graphene oxide powder. In terms of electrochemical performance, the NiMoO4-NrGO electrode displayed a higher specific capacitance, i.e., 932 Fg−1 (466 Cg−1), than the NiMoO4-rGO electrode, i.e., 884 Fg−1 (442 Cg−1), at a current density of 1 Ag−1. In fact, graphene oxide sheets could lose more oxygen groups in the presence of ammonia, resulting in increased electrical conductivity. For the asymmetric supercapacitor of NiMoO4-NrGO//AC, the specific capacitance at 1 Ag−1, energy density, and power density were 101.2 Fg−1 (111.32 Cg−1), 17 Wh kg−1, and 174.4 kW kg−1, respectively. In addition, this supercapacitor material displayed a good cycling stability of over 82%.

1. Introduction

Energy is among the most important issues facing the world today. Decreasing fossil fuel sources and their environmental problems has compelled scientists to strive to replace them with renewable and clean energy sources [1,2]. Various energy storage systems such as batteries and supercapacitors have recently been developed, and supercapacitors have attracted particular interest. The limitations of batteries have restricted their use in systems that require high power density [3,4]. Compared with batteries, supercapacitors have about 10 times the power density and a faster charge–discharge cycle of 1~30 s. Different category of materials such as carbonic, polymeric, and metal oxide/hydroxide materials have been used in supercapacitors, with metal oxides commonly being utilized due to their unique properties [5]. In addition to single metal oxides such as MnO2 [6], Co3O4 [7], NiO [8], and RuO2 [9], binary metal oxides such as CoMoO4 [10], NiMoO4 [11], and MnMoO4 [12] are considered promising electrode materials. Compared with single metal oxides, binary metal oxides with multiple oxidation states and high electrical conductivity enhance energy storage systems. Meanwhile, metal molybdates are an important group of inorganic materials that provide high electrical conductivity. However, the high electrical conductivity, low price, abundance, and good chemical and thermal stability of NiMoO4 make it an appropriate alternative to use as an electrode in supercapacitors [13]. NiMoO4 is synthesized using different techniques and apparatuses in, for example, hydrothermal [14,15], microwave [16], sonochemical [17], and sol–gel [18] synthesis. Various forms of NiMoO4, such as nanorod [11], nanocluster [19], nanosphere [20], flower-like [21], and nanoplate [22] NiMoO4, have been synthesized and explored.
Based on literatures, using the “pseudocapacitive” or the “battery-type” for Ni-based material such as NiMoO4 is still controversial [23]. Nickel molybdate (NiMoO4) displays oxidation–reduction (redox) behavior and also has a good specific capacitance. NiMoO4 is a combination of MoO3 and NiO metal oxides. The strong redox reaction of Ni2+ ions and the good electrical conductivity of MoO42– ions make NiMoO4 highly suitable for use as an electrode material in supercapacitors [13,24]. NiMoO4 has better supercapacitor properties than its binary compounds MoO3, with a specific capacitance of 176 Fg−1 [25], and NiO, with a specific capacitance of 414.6 Fg−1 [26]. In addition, utilizing carbon materials such as activated carbon (AC), carbon nanotubes (CNT), and three-dimensional (3D) graphene as a scaffold for the metal oxide NiMoO4 results in enhanced electrical conductivity, chemical stability, and ultimately enhancement of supercapacitor performance [27]. The 3D network structure of graphene provides many electron transfer paths and active sites, and also favors mechanical stability during cycling performance.
This research aimed to create a 3D structure of reduced graphene oxide and nickel molybdate to combine the beneficial properties of both materials and eventually enhance the performance of supercapacitors. The NiMoO4-NrGO and AC electrodes were applied to construct an asymmetric supercapacitor. The electrochemical performance of the fabricated supercapacitor was investigated by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrical resistivity (EIS), and cycling performance tests using a potentiostat. The material for the synthesized electrode was characterized using different techniques. In addition, to optimize the electrolyte, the effect of pH on the electrochemical behavior of the NMO-NrGO electrode and the optimal electrolyte concentration to use in the whole supercapacitor cell were investigated.

2. Experimental Section

2.1. Materials and Methods

The primary chemical materials used in this research along with their specifications are given in Table S1.

2.2. Preparation of NiMoO4-rGO and NiMoO4/N-Doped rGO Hybrid Nanocomposite

The following steps were used to synthesize the powders of NiMoO4-rGO and NiMoO4/N-doped rGO hybrid nanocomposite through a precipitation method: (a) a solution containing 1 g starch in 20 mL distilled water was prepared at 80 °C to provide a gelation suspension; (b) 1 mg graphene oxide was dispersed in 20 mL distilled water and added to the starch suspension; (c) 3.0 mL ammonia (28 wt%) was added to the suspension of starch and graphene oxide to adjust the pH to about 10; (d) two solutions containing 20 mL of 5 mM Ni(NO3)2·6H2O and 20 mL of 5 mM Na2MoO4·2H2O, respectively, were added drop-wise to the suspension of starch, graphene oxide, and ammonia under continuous stirring for 1 h at a temperature of 80 °C; (e) the synthesized powders were washed with distilled water to remove the extra materials and impurities. The powders dried in a vacuum oven at a temperature of 60 °C for 24 h; (f) the final powders were calcinated in a tube furnace with Ar atmosphere at a temperature of 350 °C for 2 h; and (g) finally, two groups of powder were synthesized and labeled: NiMoO4/reduced graphene oxide (NMO-rGO) and NiMoO4/N-doped reduced graphene oxide (NMO-NrGO). Note that step (c) was not performed when synthesizing NMO-rGO. The process of synthesizing the electrode materials is schematically illustrated in Figure 1.

2.3. Preparation of NiMoO4-rGO and NiMoO4-NrGO Hybrid Electrodes Using Ni Foam Current Collectors

First, the nickel foam was immersed in the ultrasonic bath of 1 M HCl (37 wt%) for 5 min to remove the impurities from the surface; then, it was cleaned with water and ethanol. The synthesized powders (80 wt%), AC (10 wt% AC), and polyvinylidene fluoride binder (10 wt% PVDF) were mixed in an ultrasonic bath. The nickel foam was then uniformly coated with this mixture. Next, the electrodes were dried at the temperature of 60 °C for 24 h in an oven. All electrodes were soaked in the 3.0 M KOH electrolyte for 30 min before the electrochemical tests, all of which were performed at room temperature.

2.4. Material Characterization

To characterize and study the properties of NMO-rGO and NMO-NrGO samples, various analytical methods such as X-ray diffraction (XRD Bruker 2D phaser with Cu Kα radiation (λ = 0.1542 nm, Bremen, Germany), scanning electron microscopy (TESCAN MIRA3 SEM, Brno, Czech Republic), and Fourier-transform infrared spectroscopy (ABB Bomem FTIR system, ATR method, Västerås, Sweden) were used. Furthermore, Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) gas adsorption measurements (Micromeritics TriStar II analyzer, Norcross, GA, USA) were used to study the specific surface area and pore size distribution of the materials.

2.5. Electrochemical Investigations

The electrochemical performance of NMO-rGO and NMO-NrGO electrodes was measured using a potentiostat instrument (VersaStat 2.63.3) with different modes, such as CV, GCD, and electrochemical impedance spectroscopy (EIS). The synthesized electrodes were investigated as working electrodes in a three-electrode setup. The reference and counter electrodes were calomel and platinum, respectively, and the electrolyte was 3.0 M KOH. CV was performed in the voltage range of 0–0.7 V at scan rates of 5–100 mV s−1. GCD tests were conducted in the voltage range of 0–0.5 V at current densities of 1–50 Ag−1. Electrochemical impedance spectroscopy (EIS) measurements were carried out at 0.1–100 KHz and a voltage amplitude of 10 mV. To build an asymmetric supercapacitor, AC electrode, NMO-NrGO electrode, and cellulose paper were used as the negative electrode, positive electrode, and separator, respectively. The specific capacitance is calculated based on the equations in supplementary [28,29].

3. Results and Discussion

3.1. Material Characterization of the Electrodes

Figure 2a shows the FTIR spectra of the pure GO, NMO-rGO, and NMO-NrGO nanocomposites in the range from 400–4000 cm−1. Based on the FTIR spectrum of GO, most of the peaks are attributed to the graphene oxide functional groups. These correspond to the C=O carbonyl group (~1765 cm−1), aromatic C=C bonds (~1650 cm−1), the C–OH bond (~1397 cm−1), deformation of the C–O–C bond (~1208 cm−1), and C–O bonds (~1038 cm−1) [30,31]. Furthermore, a broad band of the hydroxyl (O–H) group between ~2800 and ~3600 cm−1 contributed to the water molecules adsorbed on the surface of the samples [32]. The FTIR spectra of the NMO-rGO and NMO-NrGO nanocomposites show that the strong absorption peak at 3250 cm−1 is reduced significantly, which confirms the transformation of GO to rGO [33] after compositing with NiMoO4. In addition, as can be seen in the FTIR spectra of the NMO-rGO and NMO-NrGO nanocomposites, the oxygenated groups of C–OH, C–O, C=O, and C–O–C either disappeared or weakened. This also supports the reduction of GO sheets to rGO during the adsorption of Ni2+ to the negatively charged oxygenated groups of graphene oxide [34]. The characteristic bonds at 444, 750, and 836 cm−1, respectively, are related to Mo–O–Mo, the Mo–O–Ni vibration mode, and the Mo=O bond of NiMoO4 [35,36].
Figure 2b illustrates the XRD patterns of the GO, NMO-rGO, and NMO-NrGO nanocomposites. It can be observed that a sharp peak for GO appearing at 2θ = 10.6° (002) is absent for the NMO-rGO and NMO-NrGO nanocomposites. This implies the reduction in oxygen groups after compositing with NiMoO4 and heating at 350 °C [37,38]. Other peaks occurring in NMO-rGO and NMO-NrGO nanocomposite patterns with high intensity are attributed to the crystalline NiMoO4 nanoparticles [38,39]. Based on the standard diffraction pattern data other papers [36,40,41], the JCPDS code number of the NiMoO4 powder is 00-045-0142.
The SEM images of the NMO-rGO and NMO-NrGO nanocomposites are presented in Figure 3a–d. These show the NiMoO4 nanorods decorating the rGO sheets, providing a 3D structure and forming many porosities through the nanorods. This porous nanostructure provides short diffusion paths that enhance the accessibility of the reaction sites to the electrolyte. This increases the charge transfer and thus the energy and power density [34]. The electrostatic attachment of metal ions to the functional groups of the GO sheets results in the reduction of GO to rGO and then the growth of NiMoO4 nanoparticles on the rGO sheets [31,36,42]. Based on the SEM images of the NMO-NrGO nanocomposite (Figure 3c,d), relatively thick rGO sheets can be seen, which could be a sign of multilayered graphite. This phenomenon can be due to the effect of ammonia on reducing the graphene oxide functional groups [38]. It can be concluded that the formation of hydrogen bonds between the functional groups and free electrons of ammonia resulted in more reduction in graphene oxide. This neutralized the surface charge of the graphene oxide sheets as well as reducing the electrostatic repulsion between the graphene oxide sheets and increasing their adhesion, thereby producing multilayered graphite.
Furthermore, the N2 adsorption–desorption isotherms using BET and the size distribution of the pores using BJH are given in Figure 3e,f. For the porous nanostructure of NMO-rGO, the specific surface area and the average volume of pores are calculated as 23.8 m2 g−1 and 0.41 cm3 g−1, respectively, whereas the specific surface area and the average volume of pores for the NMO-NrGO nanocomposite are 22.3 m2 g−1 and 0.38 cm3 g−1, respectively.

3.2. Electrochemical Properties of NMO-rGO and NMO-NrGO Electrodes

Figure 4a shows the CV curves of NMO-rGO and NMO-NrGO electrodes at a scan rate of 10 mV s−1. Comparison of the obtained specific capacitances and the areas under the curves indicates a higher specific capacitance for the NMO-NrGO than the NMO-rGO electrode. As can be seen in Figure 3e,f, the specific areas for both the NMO-rGO and NMO-NrGO electrodes are similar. Thus, the enhancement of the specific capacitance of the NMO-NrGO electrode versus the NMO-rGO electrode could not be due to surface area reaction. It can be said that the removal of a large percentage of the oxygenated functional groups attached to the GO sheets in the alkaline environment of ammonia led to an increase in the electrical conductivity of the rGO sheets and an increase in the energy storage capacitance [43]. In addition, the potential difference (∆E) between the anodic and cathodic peaks in the NMO-NrGO electrode is less than in the NMO-rGO electrode. This indicates that the greater reduction in GO also enhanced the electrochemical reversibility of the NMO-NrGO electrode [44]. Figure 4b shows the GCD curves of the NMO-rGO and NMO-NrGO electrodes in the voltage range of 0–0.5 V at a current density of 1 Ag−1. The NMO-rGO and NMO-NrGO electrodes display specific capacitances of 884 Fg−1 (442 Cg−1) and 932 Fg−1 (466 Cg−1), respectively. The nonlinear shape of the GCD curves and the presence of a pair of voltage plateaus at voltages of about 0.2 and 0.3 V show that the specific capacitances of both electrodes were affected by the Faradaic reactions of Ni2+/Ni3+ ions in the NiMoO4 nanoparticles [45,46], agreeing with the CV results. Figure 4c shows the specific capacitances of the NMO-rGO and NMO-NrGO electrodes versus different scan rates of 10–100 mV s−1. The curves show that increasing the scan rate decreases the specific capacitance for both electrodes. The highest specific capacitance value was obtained at a scan rate of 10 Ag−1, equivalent to 411.42 Fg−1 (288 Cg−1) and 495.7 Fg−1 (347 Cg−1) for NMO-rGO and NMO-NrGO electrodes, respectively. The bar chart in Figure 4d illustrates the specific capacitance–current density diagram for the NMO-rGO and NMO-NrGO electrodes. It is observed that the specific capacitances of both electrodes decreased with the increase in current density from 1 to 100 Ag−1. The decrease in the specific capacitance can be attributed to the voltage drop, insufficient active sites, and high-speed redox reactions [47]. At high current densities, ions cannot transfer as quickly as electrons, so the ions limit the redox process and the electron transfer. Thus, a limited number of electrolyte ions penetrate the electroactive materials. Furthermore, because it is mostly the outer layers of the electroactive materials that are involved in the charge storage process, small amounts of the active materials are involved in the redox reactions [48,49]. The highest specific capacitance value for the NMO-NrGO electrode was obtained at a current density of 1 Ag−1, which is equivalent to 932 Fg−1 (466 Cg−1). As can be seen, at the current density of 50 Ag−1, the capacitance retention of NMO-NrGO electrode was about 52%, whereas the capacitance retention of the NMO-rGO electrode calculated around 8%. It demonstrates the better performance of the NMO-NrGO nanocomposite to use in supercapacitors at high current densities. Figure 4e shows the cycling performance of the NMO-rGO and NMO-NrGO electrodes at a current density of 2 Ag−1 and in the voltage range of 0–0.5 V. Both electrodes started with a high specific capacitance of approximately 800 Ag−1 (400 Cg−1). The cycling performance of the NMO-rGO electrode initially decreased rapidly, but the degradation rate slowed after 50 cycles. The NMO-NrGO electrode displayed more specific capacitance and stable cycling performance over 500 cycles, finally retaining 76% of its specific capacitance in the first cycle. In addition, multiple types of electrolytes, including acidic (H2SO4), alkaline (KOH), and neutral (Na2SO4) electrolytes, were employed to evaluate the impact of pH and electrolyte concentration on the electrochemical behavior of the NMO-NrGO electrode. KOH electrolyte was used at a variety of concentrations, including 1.0, 2.0, 3.0, and 4.0 M [50,51,52]. The resultant CV curves are shown in Figure S2.

3.3. Electrochemical Properties of an NMO-NrGO//AC Asymmetric Supercapacitor

An asymmetric supercapacitor was fabricated using an NMO-NrGO electrode and AC in 3.0 M KOH. Figure 5a shows the CV diagram of both electrodes, i.e., NMO-NrGO as the cathode and AC as the anode, at a scan rate of 10 mV s−1. In the CV curve of the NMO-NrGO electrode, a pair of oxidation–reduction peaks can be observed, indicating Faradaic reactions and the battery-like behavior of NiMoO4. In contrast, the CV curve of the AC electrode is almost rectangular without oxidation–reduction peaks, which is specific to carbon materials and indicates that the electrical double layer is formed. Based on the specific capacitances of the NMO-NrGO electrode (495.7 Fg−1) and the AC electrode (138.1 Fg−1) at 10 mV s−1, and on the potential range of both electrodes, the mass ratio was calculated as 0.4 m+/m− [53,54]. According to this ratio, the amounts of active material on both positive and negative electrodes were considered to be 2 and 5 mg, respectively. Therefore, the voltage window of the NMO-NrGO//AC asymmetric cell can be increased from 0.7 V in a symmetric supercapacitor to 1.1 V in an asymmetric supercapacitor. Figure 5b shows the CV curves of the NMO-NrGO//AC asymmetric supercapacitor in the voltage range of 0–1.1 V and at scan rates of 5–50 mV s−1. All the CV loops display almost rectangular shape and weak redox peaks. The redox peaks in the CV curves for the NMO-NrGO//AC asymmetric supercapacitor confirm that the charge is stored in the form of Faradaic reactions. In addition, the quasi-rectangular shapes of the CV curves are related to the charge stored in the form of an electrical double-layer capacitive (EDLC) [55]. As can be seen in Figure 5b, up to the voltage of 0.6 V, the supercapacitor stores electrical energy via an electric double layer formed through the physical mechanism, whereas when increasing the working voltage to 0.8 V, it stores the energy in Faradaic form through redox reactions. Furthermore, the CV curves do not display significant changes at high scan rates, indicating the good capacitance of the supercapacitor and its quick response to the current [56]. The maximum specific capacitance for the NMO-NrGO//AC supercapacitor was equivalent to 72.3 Fg−1 at a scan rate of 5 mV s−1.
Figure 5c shows the GCD curves of the NMO-NrGO//AC asymmetric supercapacitor in the voltage range of 0–1.1 V and different current densities of 1–20 Ag−1. The nonlinear behavior of the GCD curves caused by redox reactions and the battery-like properties of the metal oxide of NiMoO4 confirm the cathodic and anodic peaks in the CV curves (Figure 5b). The highest calculated specific capacitance (based on the total mass of the active material of two electrodes) was at about 101.2 Fg−1 (111.32 Cg−1) at a current density of 1 Ag−1. This supercapacitor maintained its electrochemical efficiency to some extent even at a high current density. Specific capacitance values versus different current densities are given in Figure 5d, illustrating that the specific capacitance decreased with increasing current density. Figure 5d shows a good specific capacitance equivalent to 62 Fg−1 at a current density of 20 Ag−1, indicating good electron transfer and ion intercalation abilities. It can be stated that the NMO-NrGO//G supercapacitor has a good ability to store energy at high current densities. Figure 5e shows the cycling performance of the NMO-NrGO//AC asymmetric supercapacitor determined by continuous charge–discharge testing at a current density of 2 Ag−1 and in a voltage window of 0–1.1 V. As can be seen, the trend of the specific capacitance during 1000 cycles indicates slight degradation of about 18% during cycling. This implies that the NMO-NrGO//G asymmetric supercapacitor has good stability, likely due to the combination of short intercalation paths in the porous nanostructure of NiMoO4 and the high electrical conductivity of the graphite material. Figure 5f presents an EIS analysis of the NMO-NrGO//AC supercapacitor before and after 1000 cycles over a frequency range of 0.1 Hz–100 KHz. Based on the Nyquist plot, the NMO-NrGO//AC supercapacitor had an internal resistance of ESR = 3.1 Ω and a charge transfer resistance of Rct = 1.43 Ω before the cycling. The internal resistance after the cycling was ESR = 3.4 Ω, which is similar to that before the cycling, indicating good supercapacitor stability. The low internal resistance is due to the large specific surface area and high conductivity of the electroactive material. The charge transfer resistance (Rct) after cycling increased slightly to 2.1 Ω from the initial level. The increase in Rct was probably due to the electroactive material peeling off from the electrode substrate, as a result of decreased adhesion between substrate and active material during the electrochemical performance. In addition, the slope of the linear part of the NMO-NrGO//AC impedance curve before the cycling performance is almost vertical. This indicates lower Warberg resistance (ZW) and better capacitive behavior before the cycling testing [57]. ZW is related to the diffusion rate of the OH− ions intercalating to the electrode. In the Ragone plot, shown in Figure 5g, the energy density and power density [58] of the NMO-NrGO//AC asymmetric supercapacitor synthesized in this work are compared with those of other symmetric and asymmetric supercapacitors reported in the literature. The energy density and power density of the NMO-NrGO//AC supercapacitor investigated here are 17 Wh kg−1 and 174.24 kW kg−1, respectively. These are good levels, compared with those of other reported supercapacitors, for example: NiCo2S4@NiMoO4/NF//AC (21.4 Wh kg−1 and 58 kW kg−1) [59], NF@NiMoO4@C//AC (61 Wh kg−1 and 21.7 kW kg−1) [60], β-NiMoO4//AC (4.53 Wh kg−1 and 252.2 kW kg−1) [61], NiMoO4//AC (54 Wh kg−1 and 24 kW kg−1) [45], MnCo2O4@NiMoO4//AC (42 Wh kg−1 and 852.3 kW kg−1) [62], and g-C3N4/NiMoO4//rGO (90 Wh kg−1 and 300 kW kg−1) [63].
Jcs 07 00217 g005 550
Figure 5. (a) CV curves of the NMO-NrGO and AC electrodes at a scan rate of 10 mV s−1 in the three-electrode setup; (b) CV curves in the voltage range of 0–1.1 V and at various scan rates of 5–50 mV s−1; (c) GCD curves with nonlinear behavior at the potential range of 0–1.1 V and at various current densities of 1–20 Ag−1; (d) specific capacitance vs. various current densities of 1–20 Ag−1; (e) cycling performance at 2 Ag−1 after 1000 cycles; (f) comparison of the Nyquist plot, before and after 1000 cycles; and (g) Ragone plot of the NMO-NrGO//AC asymmetric supercapacitor synthesized here compared with other symmetric and asymmetric supercapacitors reported in the literature (Xu 2016 [60], Wang 2017 [57], Moosavifard 2015 [45], Zhang 2017 [56], Albohani 2019 [58] and Mehraz 2019 [59]).
Figure 5. (a) CV curves of the NMO-NrGO and AC electrodes at a scan rate of 10 mV s−1 in the three-electrode setup; (b) CV curves in the voltage range of 0–1.1 V and at various scan rates of 5–50 mV s−1; (c) GCD curves with nonlinear behavior at the potential range of 0–1.1 V and at various current densities of 1–20 Ag−1; (d) specific capacitance vs. various current densities of 1–20 Ag−1; (e) cycling performance at 2 Ag−1 after 1000 cycles; (f) comparison of the Nyquist plot, before and after 1000 cycles; and (g) Ragone plot of the NMO-NrGO//AC asymmetric supercapacitor synthesized here compared with other symmetric and asymmetric supercapacitors reported in the literature (Xu 2016 [60], Wang 2017 [57], Moosavifard 2015 [45], Zhang 2017 [56], Albohani 2019 [58] and Mehraz 2019 [59]).
Jcs 07 00217 g005

4. Conclusions

NMO-rGO and NMO-NrGO nanocomposites were prepared using a simple precipitation method in starch medium. The electrochemical performance indicates that NMO-NrGO had a higher specific capacitance of 932 Fg−1 (466 Cg−1). This higher specific capacitance is attributed to the greater reduction in negative functional groups on GO sheets, which enhanced the electrical conductivity and eventually the specific capacitance. The maximum specific capacitance of the NMO-NrGO//AC supercapacitor was about 101.2 Fg−1 (111.32 Cg−1) at a current density of 1 Ag−1. The cycling performance of the supercapacitor indicated good stability over 1000 cycles with only a slight degradation of 18%. The calculated energy density and power density of the NMO-NrGO//AC supercapacitor were about 17 Wh kg−1 and 174.24 kW kg−1, respectively.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jcs7060217/s1. Table S1: The primary chemicals used in this work; Figure S1: The Energy dispersive X-ray (EDX) mapping analysis of NiMoO4; Figure S2: CV curves of the NMO-NrGO electrode: (a) in 3.0 M H2SO4, 3.0 M KOH, and 3.0 M Na2SO4 electrolytes; (b) in KOH electrolyte with concentrations of 1.0, 2.0, 3.0, and 4.0 M.

Author Contributions

S.A.-R.: investigation, methodology, synthesis, and writing the original-draft, R.S.-M.: supervision; G.R.: formal analysis and editing the manuscript; N.B.: providing laboratory facilities and technical assistance; J.Ö.: editing and formal analysis and H.O.: supervision and editing the article. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

The authors acknowledge and thank Tarbiat Modares University and Mid Sweden University for supporting this research with laboratory facilities, supervision, and technical assistance.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic image of synthesizing the electrode materials.
Figure 1. Schematic image of synthesizing the electrode materials.
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Figure 2. (a) Fourier–transform infrared spectrometry (FTIR) spectra; (b) X-ray diffraction (XRD) patterns of the pure GO, NMO-rGO, and NMO-NrGO nanocomposites.
Figure 2. (a) Fourier–transform infrared spectrometry (FTIR) spectra; (b) X-ray diffraction (XRD) patterns of the pure GO, NMO-rGO, and NMO-NrGO nanocomposites.
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Figure 3. SEM images of: NMO-rGO nanocomposite at (a) low magnification and (b) high magnification; and NMO-NrGO nanocomposite at (c) low magnification and (d) high magnification. BET adsorption–desorption isotherm with BJH plot of pore size distribution (inset) for (e) NMO-rGO and (f) NMO-NrGO nanocomposites.
Figure 3. SEM images of: NMO-rGO nanocomposite at (a) low magnification and (b) high magnification; and NMO-NrGO nanocomposite at (c) low magnification and (d) high magnification. BET adsorption–desorption isotherm with BJH plot of pore size distribution (inset) for (e) NMO-rGO and (f) NMO-NrGO nanocomposites.
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Figure 4. (a) Cyclic voltammetry (CV) diagram at a scan rate of 10 mV s−1; (b) galvanostatic charge–discharge (GCD) diagram at a current density of 1 Ag−1; (c) specific capacitance vs. scan rate; (d) specific capacitance vs. current density, and (e) cycling performance of NMO-rGO and NMO-NrGO electrodes.
Figure 4. (a) Cyclic voltammetry (CV) diagram at a scan rate of 10 mV s−1; (b) galvanostatic charge–discharge (GCD) diagram at a current density of 1 Ag−1; (c) specific capacitance vs. scan rate; (d) specific capacitance vs. current density, and (e) cycling performance of NMO-rGO and NMO-NrGO electrodes.
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Arshadi-Rastabi, S.; Sarraf-Mamoory, R.; Razaz, G.; Blomquist, N.; Örtegren, J.; Olin, H. Porous NiMoO4-NrGO as a Battery-Like Electrode Material for Aqueous Hybrid Supercapacitors. J. Compos. Sci. 2023, 7, 217. https://doi.org/10.3390/jcs7060217

AMA Style

Arshadi-Rastabi S, Sarraf-Mamoory R, Razaz G, Blomquist N, Örtegren J, Olin H. Porous NiMoO4-NrGO as a Battery-Like Electrode Material for Aqueous Hybrid Supercapacitors. Journal of Composites Science. 2023; 7(6):217. https://doi.org/10.3390/jcs7060217

Chicago/Turabian Style

Arshadi-Rastabi, Shahrzad, Rasoul Sarraf-Mamoory, Ghadir Razaz, Nicklas Blomquist, Jonas Örtegren, and Håkan Olin. 2023. "Porous NiMoO4-NrGO as a Battery-Like Electrode Material for Aqueous Hybrid Supercapacitors" Journal of Composites Science 7, no. 6: 217. https://doi.org/10.3390/jcs7060217

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