Microwave-irradiated novel mesoporous nickel oxide carbon nanocomposite electrodes for supercapacitor application

The present work accentuates the aspects of electrochemical analysis determined by cyclic voltammeter (CV), especially enhancement in supercapacitor's specific capacitance and energy density. In this work, nickel oxide (NiO) and nickel oxide @ reduced graphene oxide (NiO@rGO) nanocomposite materials used as electrodes were synthesized by the microwave irradiation method. Performance of the synthesized material was further characterized using X-ray diffraction, Fourier transform infrared spectroscopy, field-emission scanning electron microscope, Brunauer–Emmett–Teller specific surface-area, thermo gravimetric analysis, and CV. Furthermore, the electrochemical performance of active material at three different molarities (2 M, 4 M and 6 M) of potassium hydroxide as an electrolyte is analyzed, and observed decline in specific capacitance for synthesized nanocomposite materials in a lower state of electrolyte concentration. Accordingly, specific capacitances at 1 A/g are 270 F/g, 395 F/g at 1 A/g current density, and energy densities of 10.2 Wh/kg, 17.55 Wh/kg are observed for NiO and NiO@rGO, respectively, at 6 M KOH.


Introduction
In the present time, sustainable energy storage devices play a vital role in the modern developing society. The improvement in clean and sustainable energy is leading to growing needs for electrical storage devices. Due to these batteries and supercapacitors (SCs) are widely investigated by the research community. SCs are in the front-line of electrochemical energy storage systems designed with anode, cathode, and electrolyte sandwiched between them, separated by an insulator, and current collectors connect them externally [1].
The present research focuses on eco-friendly low toxic metal oxide electrode materials for SCs. Nanosized NiO has caught considerable attention recently due to its low cost and pseudo-capacitive behavior, but the high resistivity of NiO is a drawback for practical applications in SCs [2]. To address the challenges, carbon materials with high conductivity have been introduced to prepare nano-NiO carbon composites. Reduced graphene oxide (rGO) is an excellent candidate for manufacture new complexes containing sp2-hybridized carbon atoms with a twodimensional (2D) structure. rGO generates keen interest from researchers because of its specific characteristics such as high conductivity, large surface area, and stability in different chemical environments [3]. Furthermore, the metal oxide rGObased nanocomposite electrodes can quickly transfer electrons across the energy bands due to graphene's high work function, which encourages materials' reactivity.
Versatile routes such as solution combustion, chemical precipitation, and hydrothermal techniques regularly produce nano-metal oxide materials [4]. The microwave irradiation technique has advantages such as increased reaction kinetics, evenly distributed particles, agglomeration dissemination, and this technique reduces the reaction time and suppresses side reaction, thus enhancing the reproducibility [5]. Well understood that the working potential of the SCs is dependent on the electrochemical behavior of the electrolytes; aqueous electrolytes can achieve both high capacitance and ionic conductivity. However, KOH electrolytes are widely using for SCs applications, in the literature, commonly 1 M [6]

Materials
Nickel nitrate hexahydrate, sodium hydroxide, chemical grade from Finar make and reduced graphene oxide with research-grade were brought from Platonic Nanotech Pvt Ltd India. All the chemicals are used directly without further purification and distilled (DI) water as an aqueous medium.

Synthesis of nanocomposites
Synthesis of NiO@rGO nanocomposites: 0.1 M nickel nitrate hexahydrate were dissolved in 100 ml of DI water under constant stirring for 15 min at room temp. A further 0.5 wt% of reduced graphene oxide (rGO) was added to the above solution and continued stirring for 30 min until the homogeneous mixer formed. However, to maintain the solution pH = 7, 0.5 M sodium hydroxide was added dropwise to the above mixer. This mixer was then subjected to microwave irradiation using LG MC2846SL with an operating frequency of 2.45 Ghz, power of 900 W, and 10 min of exposure time. The final material obtained was dried at 60°C in the hot-air oven for 12 h, the resulting powder NiO@rGO nanocomposite calcinating at 350°C in a muffle furnace for 3 h.

Electrode preparation
Working electrode was prepared using synthesized nanocomposites, 80% of active material (NiO and NiO@rGO), 10% of polyvinylidene fluoride (PVDF), and 10% of activated carbon material. PVDF is resistant to adverse weather conditions over a wide range of temperatures, outstanding chemical resistance to electrodes, and an organic binder. Activated carbon is used as a solvent to develop the surface area chemically active and limit the adsorbent in the electrode and electrolyte interface. The mixed active material, PVDF, and Activated carbon material compound made as a thick slurry using motor and pestle [5] with few drops of NMP to coated on 1 9 1 cm 2 Ni foam substrate (purity 99.9%, thickness 2 mm, PPI (60-90)%, pore size 0.05-0.5 mm.) uniformly and heat to 80°C for 12 h. The electrochemical analysis performs by a three-electrode system (Ag/AgCl as reference, Pt as counter electrodes) using a Metrohm Autolab PGSTA302N (Potentiostat/ Galvanostat) equipment with NOVA 2.0.2 software.

Characterization
Thermogravimetric (TG) analysis evaluated at a temperature ranging from 25 to 600°C heating rate 10°C/min using EXSTAR TG/DTA6300. The structural properties of synthesized nanomaterials J Mater Sci: Mater Electron (2021) 32:20374-20383 characterized by X-ray diffraction (XRD) Bruker D8 advance using Cu K a (k = 1.5418 Å ) as source with 2h ranging from 20°to 80°at a scan rate of 0.5 s. Fourier transform infrared (FTIR) spectra recorded using Bruker Alpha II, Raman spectroscopic analysis carried out using CRM 215 VIR. Morphology studies performed by field-emission scanning electron microscope (FE-SEM) Zeiss instrument. Brunauer-Emmett-Teller (BET) Surface area and pore size distribution analyzed by BELSORP mini II.
3 Results and discussion 3.1 TG analysis TG analysis is used to obtain weight loss of material and verify the successful preparation of nanomaterials performed shown in Fig. 1a. The mass changes observed in three steps. Two samples indicated slight weight loss below 150°C due to the evaporation of adsorbed water molecules. While the temperature rises to 290°C, there is a decomposition of Ni(OH) 2 into NiO. Thus, weight loss absorbed is due to lattice water molecule evaporation, removal of residual sodium content, and organic matter decomposition. It speculated that nanoparticles calcined above 350°C are due to the NiO@rGO, and particles calcined above 400 oC may be the mixture of Ni, NiO, and NiO@rGO nanocomposite [12].

XRD
XRD patterns of rGO, NiO, and NiO@rGO nanocomposite are shown in Fig. 1b. The broad peak absorbed at 26.1°indicates (002) plane of bare rGO [13] shown in Fig. 1b. Five prominent peaks of NiO at 2h 37.2°, 43.2°, 62.9°,75.3°, and 79.4°related to the crystal planes of (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) matched with JC-PDS card no.78-0429 [5] having a cubic face-centered structure with lattice parameter a = 4.177 and space group: Fm3 0 m (225). The representation of the rGO peak in NiO @rGO nanocomposite prepared by the microwave irradiation method confirms the binary nanocomposite formation. No extra peaks observed in the XRD spectra of the prepared material clearly resemble the absence of any secondary peaks in the XRD patterns showing its purity. Average crystallite size calculates using the Debye-Scherrer formula. The crystallite size is predicting by peak width, which is perpendicular to planes. Different theta positions the separation size and strain broadening analysis done by Williamson and Hall equation [14]. Rapid and homogeneous microwave heating enhances the dispersion and surface area, leading to prominent intensity peaks and broader peak widths observe for NiO@rGO over NiO. More extensive crystallite size materials have a smaller surface area, and peak width is inversely proportional to crystallite size; as the crystallite size gets small, the peak gets broader. These factors induce more crystalline NiO@rGO and make a smaller crystal size after adding rGO. The average crystallite size decreases the microstrain increase; this might be due to the mechanical surface free energy of the metastable nanomaterials.

FTIR
Spectra perform to investigate metal behavior with carbon functional groups. Figure 1c shows the FTIR spectrum of irradiated microwave samples, i.e., NiO and NiO@rGO nanocomposite. The bare NiO nanomaterial shows distinct peaks at 520, 617, 1226, 1383.9, 1632, 3456 and similar peaks observed in NiO@rGO nanocomposite. The peak at 520 cm -1 indicates metal-oxygen bonding between Ni and O as vibration absorption bonding, the peak at 1383.9 cm -1 is due to the presence of hydroxyl group attained from chemically adsorbed water molecule during the synthesis of nanostructures [13]. The peak around 1628 cm -1 is due to d-H 2 O bending vibration of the water molecule absorbed by material [15]. In NiO@rGO nanocomposite, the transmission peak observed at 1645 cm-1 corresponds to the stretching vibration of the C=C aromatic link. The peak around 1070 cm -1 observed due to alkoxy and epoxy carbon-oxygen stretching vibration indicates binary nanocomposite formation of NiO@rGO [13].

Raman spectroscopy
Raman spectroscopy of NiO and NiO@rGO nanocomposites is shown in Fig. 1d. Two peaks are located at 495 cm -1 and 1074 cm -1 observed in both spectra attributed to the one phonon (1P) longitudinal optical (1P LO ) and two phonons (2P) longitudinal optical (2P LO ) of NiO vibrational modes, respectively [2,16]. It provides the information of disorder and defects in carbon-based materials by the changes in the relative intensity of two main peaks as G and D [17].
The peaks located at approximately * 1350 cm -1 and * 1575 cm -1 correspond to the D and G bonds of rGO symbolized as disorder and tangential bonds, respectively [18]. The peaks of NiO and rGO confirm the formation of NiO@rGO nanocomposite.

Morphology studies
Morphology of the synthesized nanomaterial characterized by FE-SEM is shown in Fig. 2. It is observed that the material obtained exhibits nano-circular plate-like structures [19]. The nano-circular plate assembled, intercalated with their basal surfaces, and had a mean edge length of * 60 nm and an average  thickness of * nm10, also known as twin structural materials. In twin structural nanocrystalline materials, both grain size and twin thickness decreased to the nanometer scale. Twin thickness significantly affects the deformation model of face-centered cubic structured materials, and it is unclear due to direct atomic-scale observations rarely acquired in experiments [20]. The concentration of elements analyzed by energy dispersive X-ray analysis (EDS or EDAX). Concentrations accompanied by a maximum of Ni * 60% and O along with * 40% of NiO nanostructure distributions. Ni along * 60% and O along * 39% and C along \ 1% of distributions in the case of NiO@rGO nanocomposite structures. Finally, nanostructures were nano-circular plates with uniform distribution, without agglomeration, and no impurity evidence in the composition [21].

BET specific surface area
Analysis and porous study of NiO nanostructured and NiO@rGO nanocomposite materials tested by nitrogen adsorption-desorption isotherms. As shown in Fig. 3, the result notified that both NiO and NiO@rGO nanostructured materials are Type IV isotherm. An apparent hysteresis loop at a relative pressure (1-0.6) for NiO and (1-0.45) for NiO@rGO shows the adsorption behavior of mesoporous materials. The specific surface area of NiO 0.94 m 2 g -1 , NiO@rGO 4.53 m 2 g -1 , total pore volume 0.0085226 cm 3 g -1 for NiO and 0.018861 cm 3 g -1 for NiO@rGO and mean pore diameter of NiO is 36.2 nm, NiO@rGO is 16.7 nm. An increase in the specific surface area leads to an enhancement in electrochemical specific capacitance due to its direct effect on NiO@rGO nanocomposite electrode material's reaction sites. The Barrett-Joyner-Halenda (BJH) analysis shows a narrow pore size distribution centered at 46 nm for NiO and 22 nm for NiO@rGO. An increase in specific surface area and decrement in 4 Electrochemical study 4

.1 Cyclic voltammetry (CV)
Cyclic voltammetry (CV) studies observed at 2, 5, 10 and 20 mV/s scan rates for NiO and NiO@rGO nanocomposites from -0.2 to 0.4 V as applied potential range with 2, 4, and 6 M KOH as an electrolyte are shown in Fig. 4. A scan rate of 2 mV/s was observed from the analysis to have higher specific capacitance than other scan rates. Therefore, it proposes that the scan rate is inversely proportional to specific capacitance for both NiO and NiO@rGO nanocomposite.
The reduction peaks obtained at reverse/cathodic scanning from * 0.1 to * 0.2 V, oxidation peaks obtained at forward/anodic scanning from * 0.3 V to * 0.33 V for 2 mV/s scan rate, an increase in scan rate shows oxidation-reduction peaks slightly shifted their potential values. In addition, electrochemical behavior observed at 2, 4, and 6 M KOH as electrolyte solutions notified that decreasing the electrolyte concentration decreases in specific capacitance.
The oxidation and reduction peaks of their respective anodic and cathodic scan are not symmetric due to the redox process's kinetic irreversibility [5].
The specific capacitance calculated from CV studies following formula [22]: where Cs is specific capacitance (F/g), m is mass of material coated on the electrode (mg), DV is potential applied (V)and s is scan rate (mV/s).

Electrochemical impedance spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) analysis observe for NiO and NiO@rGO nanocomposite at a frequency range of 0.1 -100 kHz with bias potential. Impedance measurements performed at 2, 4, 6 M KOH as an electrolyte shown in Nyquist plots Fig. 5a-c, respectively. The EIS data analyzed using a Nyquist Plot was a straight line in the low-frequency region and an arc in the high-frequency. The semicircle at high frequency suggests that interfacial charge-transfer resistance is significantly low because of the high conductivity. The linear shape indicates the purity of capacitive behavior, which implicates the ideal Supercapacitor. Figure 5 shows NiO and NiO@rGO nanocomposite's equivalent circuits, Rp indicates the charge transfer resistance, Rs indicates the solution resistance, and CPE is the Constant phase element. Rp relates to the electroactive surface area of the prepared electrode due to the Faradic redox process of the prepared electrodes involving the exchange of OHions. Rp of NiO@rGO is 5.25 X, and NiO is 8.91 X at 6 M KOH as an electrolyte. The diffusivity of the electrolyte-enhanced due to the

Galvanostatic charge-discharge (GCD)
Galvanostatic charge-discharge (GCD) curves of NiO and NiO@rGO nanocomposite observe at 1 A/g current density are shown in Fig. 6. Among 2, 4, 6 M KOH as an electrolyte, 6 M KOH gave superior results than other concentrations. NiO and NiO@rGO nanocomposite's specific capacitance is found 270 and 395 F/g, respectively, at 1 A/g current density.
NiO@rGO nanocomposite shows a much higher specific capacitance than NiO of 6 M KOH as an electrolyte, which is also due to the nanocomposite structures' high specific surface area. The Specific capacitances decrease gradually with an increase in discharge current density that indicates variation in the number of ions accumulated within the double layers.
The working electrodes NiO and NiO@rGO present an excellent electrochemical performance at 6 M KOH electrolyte concentration. This observation is due to the activation of electrode performance related to the kinetic process. In this process, the electrical conductivity of electrolyte concentration plays a vital role. The fact that conductivity generally increases with its concentration in aqueous solutions and this conductivity maximum at 6 M KOH electrolyte concentration [23,24] and more enough OHion concentrations than 2 M and 4 M KOH concentration, 6 M KOH facilitates penetration of electrolyte ion into an electrode, which improves the charge transfer in the bulk electrolyte and electrode [11].
The specific capacitance calculated from GCD studies used the formula [12]: where Cs g is specific capacitance (F/g), m is the mass of material coated on an electrode (mg), DV is potential difference (V), Dt is discharge time (s). The energy density is the amount of energy stored in supercapacitor, the energy density of 10.2 Wh/kg and 17.6 Wh/kg for NiO and NiO@rGO nanocomposites, respectively, were observed at 1 A/g current density from charge-discharge curves of 6 M KOH as an electrolyte.
Energy density (E) obtained from the galvanostatic test derived from the following equation [12]: where Cs g is specific capacitance (F/g), E is energy density (Wh/kg), DV is the potential difference (V). Specific capacitance and energy densities of NiO and NiO@rGO nanocomposites are shown in Table 2.

Conclusion
In this study, nano-circular plate structured nickel oxide (NiO) and nickel oxide @ reduced graphene oxide (NiO@rGO) nanocomposite materials are prepared by microwave irradiation method. TG analysis is performed to verify the successful preparation of nanomaterials and calcined temperature. Characteristic analysis for structural and morphological studies carried by XRD, and FE-SEM micrographs at different magnifications, has been noted as sphere-shaped with an average diameter of * 72 nm and thickness * 10 nm. An increase in the specific surface area from BET for NiO@rGO nanocomposite leads to an enhancement in electrochemical specific capacitance due to its direct effect on the reaction sites. The electrochemical analysis was performed by CV, GCD, and EIS at three different molarities (2 M, 4 M and 6 M) with potassium hydroxide (KOH) as an electrolyte. From CV analysis by an increase in scan rate, there is a decrement in specific capacitance and an increase in the concentration of electrolytes; there is an increment of Specific capacitance for both NiO and NiO@rGO nanocomposite. The GCD study observed that specific capacitances at 1 A/g are 270 F/g and 395 F/g, and Energy densities are 10.2 Wh/Kg and 17.5 Wh/Kg for NiO and NiO@rGO nanocomposite, respectively, at 6 M KOH as an electrolyte.
Electrochemical behavior increased due to the nano-size and high specific surface area of the nanocomposite structures. Therefore, the current study concludes that carbon composite to metal oxides improves the surface area with increases in specific capacitance and energy density for better energy storage supercapacitors.