1. Introduction
2. Experimental Procedure
2.1. Synthesis of NiO, GO, and NiO/GO
2.2. Characterizations
3. Results and Discussion
3.1. Physicochemical characterizations
3.2. Electrochemical analyses
4. Electrochemical Study of Asymmetric Supercapacitor
5. Conclusion
1. Introduction
Meeting the increasing energy demand requires innovative solutions to improve energy storage performance and address challenges such as cost, efficiency, and environmental impact. Electrochemical energy storage systems, including fuel cells, batteries, and supercapacitors, are essential in achieving this goal.1,2,3) Supercapacitors have garnered attention in the past decade due to their ability to rapidly store and discharge energy. These devices operate through electrostatic and redox reactions, with electrode materials falling into two categories: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs store energy by adsorbing charges on the electrode surface when bias potential is applied, offering high power density and great cycling stability.4,5) On the other hand, pseudocapacitors, typically made from transition metal compounds or conducting polymers, undergo reversible faradaic redox reactions, resulting in higher energy density but lower cycling stability compared to EDLCs.6,7,8) The development of advanced electrode materials has intensified, focusing on enhancing specific energy, specific power, and life span while remaining cost-effective.9,10,11)
The current research is focused on developing electrode materials with superior electrical conductivity, substantial specific surface area, high specific capacitance, and stability under ambient conditions. The choice of electrodes plays a critical role in determining supercapacitor performance.12,13) Recent investigations have concentrated on various transition metal oxides such as MnO2,14) Fe2O3,15) Co3O4,16) nickel oxide (NiO),17) and CuO18) and their composite materials with conductive polymers and EDLC materials.19,20,21) These studies aim to enhance the energy storage capacity and stability of pseudocapacitive materials.22) Among the number of materials, NiO has attracted significant attention due to its high specific capacitance, chemical stability, good conductivity, and favorable battery-type redox properties. Researchers are actively exploring the potential of NiO and its composites to advance supercapacitor technology by offering improved performance and durability. In this context, Muduli et al.23) coated NiO nanorods with carbon to enhance specific capacitance as well as stability. The NiO petals composited with vertically aligned reduced graphene oxide (rGO) showed areal capacitance of 175 mF cm-2 at a scan rate of 100 mV s-1.24)
In this study, porous nickel oxide/graphene oxide (NiO/GO) composite is synthesized as electrode material using a one-step hydrothermal method. The study extensively examined the structural, morphological, and electrochemical characteristics of NiO/GO through various characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. Additionally, electrochemical analyses were conducted, encompassing cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) tests, and electrochemical impedance spectroscopy (EIS). These comprehensive investigations provide valuable insights into the potential of NiO/GO composite as high performing supercapacitor electrode material, highlighting their promising electrochemical properties and paving the way for future advancements in energy storage technologies.
2. Experimental Procedure
2.1. Synthesis of NiO, GO, and NiO/GO
All chemical reagents utilized in this study were of analytical grade with a purity of 99.9 %. Nickel chloride hexahydrate (NiCl2 ‧ 6H2O) was employed as the source of Ni, while an aqueous ammonia solution served as the complexing agent. Graphene oxide (GO) was synthesized using the modified Hummers’ method and subsequently employed for the synthesis of the composite material.25,26) Double distilled water (DDW) was utilized as the solvent for material synthesis and electrolyte solution preparation.
To synthesize NiO, 50 mL of 0.015 M NiCl2 ‧ 6H2O was mixed with NH4OH to maintain a pH of 11 ± 0.1. This solution was vigorously stirred for 2 h and then subjected to a hydrothermal reaction at 120 °C for 8 h. After the reaction was completed, the prepared material was washed several times with DDW and dried at 60 °C under vacuum for 12 h. Finally, the material was annealed at a temperature of 450 °C under vacuum conditions for 3 h.
To synthesize NiO/GO, a solution of 100 mL containing 0.015 M NiCl2 ‧ 6H2O was prepared and complexed with NH4OH, maintaining a pH of 11 ± 0.1. Subsequently, 0.5 g of GO suspension with a density of 1 mg mL-1 was added to this solution. The resulting mixture was stirred for 2 h and subjected to a hydrothermal reaction in a Teflon-lined container at 120 °C for 8 h. The synthesized material was then washed with water multiple times and dried at 60 °C under vacuum for 12 h. Finally, the material underwent annealing at 450 °C under vacuum conditions for 3 h.
To fabricate the electrodes of NiO, GO, and NiO/GO drop casting method is used. The prepared material, polyvinylidene difluoride (PVDF), and activated carbon (AC) were mixed in a ratio of 8 : 1 : 1 and N-methyl-2-pyrrolidone (NMP) was used as a solvent. The prepared slurry is coated on cleansed nickel foam (NF) using a painting brush. The mass loading of each electrode was measured to calculate the specific capacitance values.
2.2. Characterizations
The XRD analysis of the synthesized NiO, GO, and NiO/GO was carried out using a X’Pert PRO Multi Purpose X-Ray Diffractometer (Malvern Panalytical, UK) equipped with Cu target (λ = 1.5406 Å) operating at 60 kV and 55 mA, with the 2θ range of 5~90°. Raman spectroscopy measurements were conducted using a Raman spectrometer by JASCO, Japan (model: NRS-5100) with an excitation laser wavelength of 532 nm. Surface morphology of prepared NiO, GO, and NiO/GO was observed using SEM instrument of JEOL, Japan (model: JSM-7900F) attached with OXFORD instruments’ energy dispersive spectroscopy (EDS) measurement system. Electrochemical measurements, including CV, GCD, and EIS, were carried out using a Zive MP6 multichannel potentiostat (WonaTech, Korea) at room temperature in a 2 M KOH electrolyte solution. A three-electrode configuration was employed, with prepared electrodes of NiO, GO, and NiO/GO serving as the working electrode, platinum wire as the counter electrode, and Hg/HgO as the reference electrode. EIS measurements were conducted at an alternating current voltage amplitude of 10 mV across a frequency range spanning from 100 mHz to 100 MHz. To evaluate the stability of the material, 5,000 cycles of GCD measurements were repeated at the current density of 5 A g-1. The specific capacitance of materials was calculated using the following relations:25,27)
where ‘i’ represent the charging current density in A g-1, ‘m’ is the mass of the active material in g, ‘Δt’ is the discharge time in seconds, and ‘ΔV’ is the potential window in V utilized during the charging or discharging process. is half of the area under the CV curve at a articular potential scan rate ‘υ’ measured in V s-1.
3. Results and Discussion
3.1. Physicochemical characterizations
To identify the crystal structure of NiO and the effect of GO on the crystal structure and crystallinity of NiO the XRD measurements were performed. The XRD patterns of NiO, GO, and NiO/GO are presented in Fig. 1(a). The XRD pattern of NiO shows distinct peaks at 37.24, 43.28, 62.85, 62.91, 75.40, 79.37, and 79.50 values of (101), (012), (110), (104), (113), (202), and (006) crystallographic planes, indicative of a rhombohedral crystal structure. The derived lattice parameters were found to be a = b = 2.95 Å, c = 7.23 Å, α = β = 90°, and γ = 120°. Remarkably, the observed XRD pattern closely matched the standard diffraction pattern (PDF no. 00-044-1159). The XRD pattern of GO shows a peak at the 2θ value of 10.3 and 21.35° corresponding to the (001) and (002) planes of GO. The peak at the lower angle indicates the large interplanar distance between the hexagonal GO sheets. The XRD pattern of NiO/GO shows a slight reduction of the intensity compared to the pristine NiO, indicating the effect of GO introduction on the crystallinity of the material. However, the crystal structure of the material remains the same. This confirms the formation of a composite of NiO/GO. Furthermore, the crystallite size (D) of NiO and NiO/GO was computed using the Scherrer equation provided below:27)
where ‘K’ is a constant with a value of 0.94, ‘λ’ is the wavelength of the X-ray radiation (λ = 0.15406 nm), and ‘β’ is the line width at the half-maximum height. The crystallite size of NiO and NiO/GO is 14.9 and 17.87 nm, respectively. The increased crystalline size will be helpful for charge storage as it will facilitate good electronic conductivity.
The Raman spectra of NiO, GO, and NiO/GO composite, spanning the spectral range of 200~1,800 cm-1, are presented in Fig. 1(b). The NiO Raman spectra exhibit vibrational modes corresponding to Ni-O bonds at 369, 520, 689, 1,047, and 1,107 cm-1. The peak at 369 cm-1 is ascribed to the first order TO1 phonon mode. The peak at 520 cm-1 is assigned to LO2 phonon mode. The peak at 689 cm-1 is associated with 2TO of the phonon vibrations.28,29) The peaks at 1,047 cm-1 and 1,107 cm-1 are ascribed to the phonon vibrations of TO + LO and 2LO modes, respectively.30,31) In the Raman spectra of GO, peaks at 1,355 cm-1 and 1,590 cm-1 are observed, corresponding to the D and G bands of the GO, respectively. The D band is associated with sp3 hybridization, while the G band is associated with sp2 hybridization of carbon atoms. The intensity ratio of the D to G band provides insights into the extent of oxidation of the graphene sheets. In the Raman spectra of NiO/GO, the peaks corresponding to NiO and GO are observed. Additionally, in NiO/GO Raman spectra, the D’ band at 1,630 cm-1 is attributed to sp3 hybridized carbon atoms restricted by the oxygen containing groups.32,33) These distinctive peaks observed in Raman spectra of NiO/GO confirm the formation of a composite material comprising both NiO and GO.
The SEM was employed to investigate the microstructural characteristics of the prepared samples. The surface morphologies of NiO, GO, and NiO/GO at various magnifications are presented in Fig. 2(a-f). At lower magnifications, NiO particles were observed to be spherical, with sizes ranging from 50 to 100 nm [Fig. 2(a, b)]. GO exhibited a sheet-like structure with a random arrangement [Fig. 2(c, d)]. The NiO/GO sample displayed a porous morphology with spherical particles of NiO and sheets of GO, with diameters averaging between 50 and 100 nm [Fig. 2(e, f)]. The porous material with GO and NiO composition is anticipated to enhance charge storage capacity. The EDS spectra of the NiO sample [Fig. 2(g)] indicated an atomic ratio of Ni to O of 1 : 1, confirming the formation of stoichiometric NiO. For the NiO/GO sample, the atomic percentage of Ni, O, and C is 24.35 %, 37.31 %, and 38.34 %, respectively [Fig. 2(h)], confirming the formation of the NiO/GO composite.
3.2. Electrochemical analyses
The supercapacitive characteristics of NiO, GO, and NiO/GO electrodes were systematically examined via CV and GCD measurements in 2 M KOH electrolyte. Fig. 3(a) shows comparative CV curves of NiO, GO, and NiO/GO at a scan rate of 2 mV s-1. The CV curves of NiO, GO, and NiO/GO show reduction and oxidation peaks. The small redox peaks observed in the GO electrode are due to the reduction and oxidation of NF in 2 M KOH during electrochemical activation before electrochemical measurements. The CV curves of NiO, GO, and NiO/GO at various scan rates of 2, 5, 10, 20, 50, and 100 mV s-1 are shown in Fig. 3(b-d), respectively. The oxidation peaks consistently shifted towards more positive potentials, while the reduction peaks showed a shift towards more negative potentials as the scan rate increased. This behavior in the CV curves of NiO/GO electrodes suggests the presence of battery-type pseudocapacitive charge storage. The specific capacitance values of NiO, GO, and NiO/GO are 389, 93, and 916 F g-1, respectively, at a scan rate of 2 mV s-1. Significantly, the specific capacitance of NiO/GO is higher than that of NiO and GO.

Fig. 3.
(a) Comparative CV curves of NiO, GO, and NiO/GO at 2 mV s-1 scan rates, CV curves of (b) NiO, (c) GO, and (d) NiO/GO at different scan rates of 2, 5, 10, 20, 50, and 100 mV s-1. (e) plot of Log i against Log scan rate, (f) i/υ1/2 against υ1/2 graphs for NiO, GO and NiO/GO. Contribution of capacitive and diffusion-controlled currents for (g) NiO, (h) GO, and (i) NiO/GO.
To evaluate the effect of GO addition on the charge storage mechanism of the NiO, we have analyzed CV curves of NiO, GO, and NiO/GO using power law, as provided below:34,35)
The peak current ip obtained for CV depends on the potential scan rate (υ) and the exponent b, where the value of b indicates the dominant charge storage mechanism. b value of 0.5 signifies diffusion-controlled charge storage, while a value of 1 suggests surface-controlled behavior. From the plot of Log (i) versus Log (υ) [Fig. 3(e)], the calculated b value is 0.54, 0.68, and 0.59 for NiO, GO, and NiO/GO electrodes, indicating that the charge storage process for NiO is primarily diffusion-controlled and for GO it is mixed due to NF support. The value of 0.59 for NiO/GO indicates that the charge storage mechanism is modified for the composite electrode. The actual contribution of each charge storage mechanism is then quantified using the following equation:34,35)
The terms isurface and ibulk represent the contributions from the surface-controlled (capacitive) process and diffusion-controlled process, respectively. These values are derived from the plot of i/υ1/2 verses υ1/2 shown in Fig. 3(f). Fig. 3(g-i) illustrate the percentage contribution of each process to the overall charge storage for the NiO, GO, and NiO/GO electrodes. The results indicate that for NiO and NiO/GO, the majority of charge storage occurs via diffusion-controlled mechanisms, while GO exhibits a higher contribution from surface-controlled charge storage. Notably, at lower scan rates, the proportion of diffusion-controlled charge storage increases across all electrodes. For the NiO/GO electrode, approximately 98 % of the charge is stored via a diffusion-controlled mechanism at a scan rate of 2 mV s-1, confirming that the dominant charge storage mechanism in NiO/GO is diffusion-controlled.
Fig. 4(a) illustrates the GCD profiles of NiO, GO, and NiO/GO electrodes at a current density of 1 A g-1. The larger charge-discharge time for the NiO/GO electrode indicates that charge storage in NiO/GO is higher than in NiO and GO. The specific capacitance values for NiO, GO, and NiO/GO, calculated from the GCD profiles measured at a current density of 1 A g-1 using Eq. (2), are 326, 81, and 893 F g-1, respectively. The GCD profiles of NiO, GO, and NiO/GO at different current densities are shown in Fig. 4(b-d), respectively. The introduction of GO significantly affects the capacitive properties of the synthesized NiO by altering the ion diffusion pathways, conductivity, and charge transport properties. The specific capacitance values decrease as the charge-discharge current density increases from 1 A g-1 to 6 A g-1. For NiO, it decreased from 326 to 116 F g-1, for GO from 81 to 28 F g-1, and for NiO/GO from 893 to 471 F g-1, indicating that the rate capacity for NiO/GO (53 %) is higher compared to NiO (36 %).
EIS serves as a valuable technique for evaluating the resistive parameters of electrode materials within an electrolyte, offering insights into crucial parameters such as the diffusion coefficient, series resistance (Rs), and charge transfer resistance (Rct) at the electrode/electrolyte interfaces. Fig. 5(a) presents the Nyquist plots of NiO, GO, and NiO/GO electrodes, where the starting point of the semicircle at high frequencies, positioned along the real axis, corresponds to Rs, representing the resistance offered by the solution in contact with the electrode. For NiO, GO, and NiO/GO electrodes, Rs was determined to be 0.95, 0.55, and 0.98 Ω cm-2, respectively. Additionally, the diameter of the semicircle observed at high frequencies was utilized to estimate the Rct value associated with charge transfer at the electrode/electrolyte interface. The Rct values of NiO, GO, and NiO/GO electrodes were determined to be 80, 58, and 51.3 Ω cm2, respectively. The relatively small Rct value of the NiO/GO sample suggests an easy charge transfer process, potentially contributing to the enhanced supercapacitive performance of the NiO/GO electrode. The stability of the NiO/GO electrode over 5,000 cycles was measured at a current density of 5 A g-1. The capacitance retention and Coulombic efficiency of the NiO/GO electrode over 5,000 cycles are presented in Fig. 5(b). The NiO/GO electrode retained 85 % of its capacitance over 5,000 cycles and showed 98.5 % Coulombic efficiency. These values indicate good electrochemical performance of the NiO/GO electrode. Table 1 presents a comparison of various transition metal oxides based on synthesis methods, capacitance, morphology, and electrochemical cycling stability. The NiO/GO composite exhibited competitive capacitance and stability, underscoring its effectiveness and suitability for supercapacitor applications.
Table 1.
Electrochemical performance of different transition metal oxides for supercapacitor application.
Material | Synthesis method | Morphology | Electrolyte | Specific capacitance (F g-1) | Stability (%) |
α-MnO236) | Anode glow discharge electrolysis | Flower-like nanoparticles | 1 M Na2SO4 | 365 at 1 A g-1 | 79.8 after 10,000 cycles at 5 A g-1 |
MnO214) | Hydrothermal | Flower-like nanoparticles | 1 M Na2SO4 |
1.5 F cm-2 at 0.11 A g-1 | - |
δ-MnO237) | Chemical deposition method | Agglomerated particles | 1 M Na2SO4 | 148.44 at 0.5 A g-1 | 82.80 after 10,000 cycles at 10 A g-1 |
MnO2-GO38) | Self-assembly redox |
Aggregated spherical nanoparticles | 0.5 M K2SO4 | 150 at 10 mV s-1 | 75 after 36,000 cycles at 1 A g-1 |
Graphene templated NiO24) |
Electron cyclotron resonance microwave plasma chemical vapour deposition | Nanosheets and petals | 1 M KOH |
175 mF cm-2 at 0.1 V s-1 | 51 after 2,000 cycles |
NiO/rGO39) |
Refluxing and microwave heating | Nanosheets and nanorods | 1 M H2SO4 |
1,400.90 at 5 mV s-1 | - |
MnO2/NiO/rGO40) | Hydrothermal | Nanorod, flower, and nanosheets | 1 M KOH | 391.6 at 0.5 A g-1 | 91 after 5,000 cycles |
Fe2O3/G41) |
Hydrothermal and thermal reduction | Nanorods and nanosheets | 1 M KOH | 387 at 1.5 A g-1 | 88.76 after 3,000 cycles |
PPy/rGO/Fe2O342) | Electrodeposition |
Nanoparticles, Nanosheets and nanospheres | 1 M KCl | 442 at 1 A g-1 | 88 after 8,000 cycles at 1 A g-1 |
Fe2O3 NTs@PPy/CC43) | Electrodeposition | Nanotubes | 1 M Na2SO4 |
237 mF cm-2 at 1 mA cm-2 | 80 after 10,000 cycles at 10 mA cm-2 |
α-Fe2O344) | Electrodeposition | Nanoparticles | 3 M KOH | 603 at 0.1 A g-1 | 85.5 after 10,000 cycles at 10 A g-1 |
Fe2O3@NiCo2O445) | Hydrothermal | Spikey core-shell | 6 M KOH |
364 C g-1 at 5 mV s-1 | 81 after 10,000 cycles at 15 A g-1 |
Melamine-assisted Co3O446) | Solution combustion | Foam-like |
4 M KOH + 4 M NaOH | 340 at 1 A g-1 | 99 after 5,000 cycles at 10 A g-1 |
Co3O416) | Hydrothermal | Cube-like | 3 M KOH | 637 at 5 A g-1 | 92.3 after 1,300 cycles at 20 A g-1 |
Mn-doped Co3O447) | Co-precipitation |
Granular and hexagonal layered structures | 1 M KOH | 537 at 1.5 A g-1 | 94 after 1,500 cycles at 100 mV s-1 |
Co3O448) | Hydrothermal | Rods | 1 M KOH | 261 at 0.25 A g-1 | 100 after 1,000 cycles at 5 A g-1 |
NiO/GO (this work) | Hydrothermal | Microsheets covered by nanoparticles | 2 M KOH | 893 at 1 A g-1 | 85 after 5,000 cycles at 5 A g-1 |
4. Electrochemical Study of Asymmetric Supercapacitor
To assess the practical applicability of the synthesized NiO/GO electrodes, an asymmetric supercapacitor (ASC) was assembled, utilizing the NiO/GO composite as the cathode and AC as the anode. Electrochemical evaluations of the ASC were conducted in a 2 M KOH electrolyte within a potential window of 1 V. The CV profiles were recorded at scan rates of 5, 10, 20, 40, 60, 80, and 100 mV s-1, as illustrated in Fig. 6(a). The maximum specific capacitance achieved was 84 F g-1 at a scan rate of 5 mV s-1. Additionally, the GCD profiles, shown in Fig. 6(b), indicate a decrease in charge-discharge time with increasing current density, with the highest specific capacitance of 77.8 F g-1 obtained at a current density of 0.5 A g-1.
The EIS measurements were performed to analyze the resistive characteristics of the ASC, with the Nyquist plot presented in Fig. 6(c). The ASC exhibits Rs of 1.91 Ω cm2 and Rct of 412.3 Ω cm2. The relatively low charge transfer resistance signifies efficient charge transport between the electrode and electrolyte. Furthermore, the cycling stability of ASC was evaluated over 3,000 CV cycles at a scan rate of 100 mV s-1 [Fig. 6(d)], demonstrating 86.87 % capacitance retention, which underscores the robust stability of the electrode materials.
5. Conclusion
The NiO/GO composite material was successfully synthesized using a hydrothermal method. The structural analysis of NiO using XRD confirmed the presence of a rhombohedral crystal structure of NiO, and the peak at 10.3° in the XRD pattern of GO indicates the successful synthesis of both materials. Raman analyses further validated the formation of the composite NiO/GO, providing additional evidence of the composition and structure of material. Furthermore, the surface morphology analysis revealed that NiO particles exhibited a spherical-type morphology and GO showed wrinkled sheet-like structure. The remarkable charge storage capacity of the NiO/GO composite electrode was demonstrated through its excellent specific capacitance of 893 F g-1 at a current density of 1 A g-1, indicating its potential for high performance supercapacitor applications. In addition, the NiO/GO composite material also exhibited a low Rct of 51.3 Ω cm-2, suggesting efficient charge transfer properties. The ASC assembled using this electrode with configuration NiO/GO//AC showed a specific capacitance of 77.8 F g-1 obtained at a current density of 0.5 A g-1. Overall, these findings highlight the potential of the NiO/GO composite material for use in supercapacitive energy storage, making it a promising candidate for future technological advancements.