Trimetallic Oxides/GO Composites Optimized with Carbon Ions Radiations for Supercapacitive Electrodes

Abstract

Hydrothermally synthesized electrodes of Co₃O₄@MnO₂@NiO/GO were produced for use in supercapacitors. Graphene oxide (GO) was incorporated into the nanocomposites used for electrode synthesis due to its great surface area and electrical conductivity. The synergistic alliance among these composites and GO enhances electrode performance, life span, and stability. The structural properties obtained from the X-ray diffraction (XRD) results suggest that nanocomposites are crystalline in nature. The synergistic alliance among these composites and GO enhances electrode performance, life span, and stability. Performance assessment of these electrodes indicates that their characteristic performance was enhanced by C²⁺ radiation, with the uttermost performance witnessed for electrodes radiated with 5.0 × 10¹⁵ ions/cm².

1. Introduction

Due to the ever-increasing global population, the need for a sustainable environment, and global warming problems, the hunt for an alternative and more economical source of energy has been of deep interest [1,2]. Energy storage systems have consequently been the key target, as renewable energy generation and adoption rise. Specific applications involving electronic gadgets, electric cars, and the Internet are also increasing. These require an energy storage system with high energy density, high power density, long maintenance-free usage and stability, swift electrical response, and a wide range of operating temperatures. Thus, strategic research in supercapacitors has been hugely witnessed over the past years [3,4].

Supercapacitors possess excellent properties but lack remarkable energy density. However, it cuts the gap between conventional capacitors and batteries, which have low energy density and low power density, respectively [5]. There have been various studies aimed at improving the capacitance and energy density of supercapacitors, with a major focus on electrode materials [6,7,8,9]. Two observable phenomena have been observed as the intrinsic mechanism of energy storage in supercapacitors; these involve electronic double layer capacitance (EDLC) and/or pseudocapacitance [10,11]. Carbon materials (activated carbon, carbon aerogels, carbon nanotubes (CNTs) or fibers (CNFs), and graphene) possess a large surface area, and store charges through the EDLC mechanism, which involves the adsorption and desorption of electrolyte ions at the electrode/electrolyte interfaces [12,13,14]. Transition metal oxides, sulfides, nitrides, and conducting polymers, which store charges through a fast reversible reduction–oxidation process, exhibit the pseudocapacitance mechanism [15,16,17,18]. However, carbon materials generally possess greater physical and chemical stability, a higher specific surface area, and are more conductive than pseudocapacitive materials, but they have a lower theoretical capacitance. In contrast, pseudocapacitive materials maintain higher capacitance and energy density through the faradaic process but lack good electrical conductivity [19]. Supercapacitor electrodes made with a mixture of these materials exhibit a hybrid mechanism and have been reported by several authors to show a significant improvement in the electrochemical properties [20,21,22,23].

Transition metal oxides have been deeply studied as electrode materials; they possess excellent pseudo-capacitance behavior because of their weakly oriented surface ions. Redox-active metal oxides, such as manganese oxides (MnO₂, Mn₃O₄) [24], nickel oxide (NiO) [25], nickel hydroxide (Ni(OH)₂) [26], cobalt oxide (Co₃O₄) [27,28], have been confirmed to offer a good specific capacitance and remarkable reversibility. Apart from these single metal oxide electrode materials, mixed metal oxides, and metal oxide composites are also excellent for electrochemical applications. These materials generally contain a high theoretical capacitance arising from their multiple oxidation states, which in turn generate a huge pseudocapacitance effect and enhance charge storage [29,30]. In contrast, the values of their practical capacitance are quite lower than the theoretical values; this is due to the reduced activity and poor electroconductivity of a single oxide. Among the techniques for modifying the electrochemical properties of metal oxides, the formation of mixed oxides, incorporation of a conductive matrix, and irradiation of the active materials have effectively optimized the energy storage capabilities of transition metal oxides. Jiani et al. [31], synthesized Co₃O₄@MnO₂ cubic nanomaterials by ZIF-67@Mn-ZIF precursor applying a facile thermal treatment. The Co₃O₄@MnO₂ electrode offered high electrochemical properties with a specific capacitance of 413 F/g at a current density of 0.5 A/g. This can be attributed to the hollow/porous configuration and high surface area of the material, which accommodate sufficient charge and discharge processes. A mixed metal oxide (Co₃O₄-MnO₂-NiO) ternary hybrid 1D nanotube electrodeposited on a gold (Au) current collector for a supercapacitor electrode was reported by Singh et al. [32]. The material recorded good electrochemical properties with a specific capacitance as high as 2525 F/g under a potential range of 0.8 V. The combined effort of these high redox metal oxides as a distinct nanostructure improved the charge retention effect of the electrode. However, a single metal oxide-based electrode, recorded by Arunpandiyan et al. [33] using MnO₂ nanorods synthesized through a hydrothermal technique, achieved a specific capacitance of 89 F/g in 1 M KOH, but improved to 634 F/g by introducing 0.1 M K₄[Fe(CN)₆] redox additives into the electrolyte. Similarly, NiO film electrodeposited on an FTO glass substrate through pulse chronoamperometry yielded a high specific capacitance of up to 1000 F/g in 0.5 A/g current density [34]. While a Co₃O₄ film on a fluorine-doped tin oxide (FTO) glass substrate deposited through a potentiodynamic technique, as a supercapacitor electrode, also offered a high specific capacitance of about 397 F/g in a potential window of 1 V to −0.2 V [35]. These excellent electrochemical properties are thereby ascribed to the morphology, complex redox states, and the even thin film of the electrode material, which enhanced the electrode–electrolyte interfaces.

Compositing the metal oxides involves the use of high surface area materials, such as a carbon derivative, to act as a conductive medium for an even distribution of the metal oxide particles. This produces more active sites, with a resultant increase in the electrochemical properties. Graphene oxide is among the deeply researched components for compositing metal oxides [36,37,38], with an improved capacitance and cyclic stability. Ramesh et al. [39] modified the electrochemical properties of a pure NiO/MnO₂ with that of NiO/MnO₂@nitrogen-doped graphene oxide hybrid with a specific capacitance of (266.7 to 1490) F/g at a current density of 0.5 A/g). The obtained core/shell nanocomposites displayed excellent stability with 81.7% retention capacity and proved to be a promising material for supercapacitor applications [40]. Moreso, Mary et al. [41] synthesized rGO/NiCo₂O₄@ZnCo₂O₄ (RNZC) ternary composite material for supercapacity application. The nanocomposites exhibited the best electrochemical performance because of their relatively better conductivity, high reversible charging-discharging properties, and large specific capacitance of 1197 Fg⁻¹ with a maximum energy density of 62 WhKg⁻¹ at a current density of 1 Ag⁻¹. Similarly, the effect of N-doped activated carbon on synthesized NiCo₂O₄@ZnCo₂O₄ nanomaterial for supercapacitor devices was in [42]. The obtained hybrid composite of NiCo₂O₄@ZnCo₂O₄ with N-doped activated carbon exhibited a specific capaitance of 1029 Fg⁻¹ with 78.55% capacity retention at 1 Ag⁻¹. The highest energy density and power density were 101.6 WhKg⁻¹ and 1.62 W Kg⁻¹, respectively.

The above synthesized composites assisted in improving the specific capacitance of the material through: (i) offering a large surface area that helped to improve the solid–electrolyte interface, which reduced the ion diffusion pathway; (ii) the composites created a channel that improved ion and electron migration, promoting easier diffusion of electrolytes, thereby utilizing the active materials in the electrode; (iii) the interconnected nanostructures yielded an improved rate stability by minimizing structural damage due to volume expansion during cycling; (iv) the obtained improved capacitance is as a result of the small diameter which resulted to a high surface area and conductivity. The crystalline nature of the nanocomposites increased the electron transfer kinetics, resulting in a high cyclability rate of the electrode.

Several researchers have proven that electron beams of sufficient energy could enhance the nanocrystalinity of activated carbon [43,44,45]. Electron radiation deploys a remarkable effect on the activities of the supercapacitor devices, leading to improved capacitance and conductance values by influencing the lattice structure of the electrode material irrespective of the duration of exposure to the irradiation source [46,47]. Notably, the device regained its original capacitance after removing the source of irradiation. The displayed electrochemical behavior could be attributed to the enhanced induced charges and conductance of the current collectors from the irradiation effect, which resulted in total electrode capacitance. The electron radiation affects the sp² states in the carbon materials and increases the electrical conductivity as the electron dose increases due to tunneling of charge carriers through nearby conductive chains [48]. The recovery of the materials’ original capacitance after being removed from the radiation source is due to the inherent strength of the carbon nanostructured material to regain itself back to its original form [49,50]. The obtained results show that the electrochemical performance could be enhanced by being close to the dose of the irradiation source, and the material can regain its original capacitance value after being removed from the radiation sourcece [50]. The improved charge storage capability rate, as observed, is due to the re-modeling of the defects within the nanostructured carbon electrode material [51]. This structural distortion of carbon atoms occurred because the defect formation attained its threshold energy level and yielded a well-disordered graphite material [52]. Upon time, this defect reordered to the original formation, which resulted in a sharp drop in the discharge time while retaining its original value. To improve charge trapping and storage performance of an electrode, electron irradiation enhances carbon material charge storage through defect restructuring [53].

In this work, we studied the electrochemical properties of a conjugated Co₃O₄/MnO₂/NiO@GO electrode through a moderate dose of carbon ion (C⁺⁺) radiation. Results from the study show that induced defects within the electrodes enhanced their electrochemical performance, especially for low radiation doses.

2. Experimental Section

Chemicals used for synthesis of Co₃O₄@MnO₂@NiO/GO electrodes include cobalt chloride hexahydrate ((CoCl₂.6H₂O), Sigma Aldrich, St. Louis, MO, USA, 99%) as a source of Co⁺ ions, manganese (II) nitrate hexahydrate (((Mn(NO₃)₂.6H₂O), Sigma Aldrich, 99%) as a source of Mn²⁺ ions for MnO₂ and nickel nitrate hexahydrate (((Ni(NO₃)₂.6H₂O), Sigma Aldrich, 99%) as a source of Ni⁺ ions. Sodium hydroxide solution (NaOH, Sigma Aldrich, 98%) was used as the reducing agent. FTO glass (Sigma Aldrich, 99%) is the substrate used for the fabrication of various electrodes. Carbon flakes (Sigma Aldrich, 99%) were used for the synthesis of GO. All chemicals were obtained from Shanghai, China. All reagents used in this experiment were of analytical grade and used without further purification.

2.1. Synthesis of Graphene Oxide (GO)

We synthesized graphene oxide (GO) using the modified hummers method. We used a carbon flake (Sigma Aldrich, 99%) to produce GO, a product of carbon, hydrogen, and oxygen. We first used hydrofluoric acid (HF) (Sigma Aldrich, 99%) to purify the carbon flake by adding 46 mL of HF acid to 2.0 g of carbon flake and stirring for 1 h to remove impurities present in the carbon flake. Double distilled water was used to wash the solution until the pH became neutral. Then, hydrogen tetraoxosulfate (IV) acid (Sigma Aldrich, 99%) was used to exfoliate the carbon flake, while using potassium permanganate (KMnO4) (Sigma Aldrich, 99%) as a catalyst and hydrogen peroxide (Sigma Aldrich, 99%) to inhibit the exfoliation. Meanwhile, after ending exfoliation with hydrogen peroxide, hydrochloric acid (Sigma Aldrich, 99%) was added, while double distilled water was later used to wash severally until the pH was neutral, before drying in an oven. The GO produced was used in the synthesis of nanostructured composites to modify its properties for various applications

2.2. Synthesis of Co₃O₄@MnO₂@NiO/GO Electrodes

Fifty milliliters of 0.1 M each of these three salt solutions were prepared and sonicated for 30 min separately at the ratio of 1:1:1 for Co:Mn:Ni, respectively. These three salt baths were mixed in one beaker, and 1.0 M NaOH was used to bring the mixed solution to a pH of 7. At this point, 0.05 g of GO was introduced into the mixed solution, which was sonicated for one hour to obtain homogeneity. Five pieces of fluorine tin-doped (FTO) glasses with dimensions of 1 × 2 cm² each were attached to a Teflon rod with the aid of Teflon tape, inserted inside a 300 mL autoclave, and the mixed bath was transferred inside the autoclave. The covered surface area coated with the composite is 1 × 1 cm². The autoclave was well tightened and placed inside the oven, maintaining a temperature of 180 °C for seven hours. After the end of the stipulated time, the autoclave was removed from the oven, allowed to cool down to room temperature, and dark golden electrodes were formed, as shown in Figure 1a. These electrodes were rinsed with distilled water and dried in an oven at 50 °C for one hour. Figure 1a is a sketch of the steps taken during the synthesis of Co₃O₄@MnO₂@NiO/GO electrodes.

2.3. Irradiation of Co₃O₄@MnO₂@NiO/GO Electrodes

Ion implantation is an alternative technique for introducing impurities into semiconductors. It is a more flexible approach than diffusion. It is electrically controlled by regulating voltages at slow temperature changes. Ions are channeled into a well-designed beam before accelerating to very high energies in vacuo using electric and magnetic fields. The beam in a raster is scanned on top of the target, thereby enhancing the even distribution of ions over the target surface. The ions acquire sufficient energy to diffuse into the target and collide with the lattice of the electrons due to the high accelerating voltage applied. After a while, the ion comes to rest and reunite with the electron in the lattice of the target material to form a dopant atom without any change on the path of the ion and the energy of the ion nor any damage to the crystal lattice of the atom despite the lengthy penetration duration. This process is popularly known as electronic stopping.

Electrodes of synthesized Co₃O₄@MnO₂@NiO/GO composites were placed in the sample holder of a loader inside the implantation chamber [54] and radiated using 8.0 MeV carbon ions at doses of 2.25 × 10¹⁵, 5.0 × 10¹⁵, 7.25 × 10¹⁵ and 1.0 × 10¹⁶ ions/cm² while an unirradiated electrode was kept pristine. As shown in Figure 1b, the ion current was regulated throughout the irradiation process while the chamber was maintained at a pressure of 10⁻⁷ Pa. The radiation of synthesized electrodes was conducted using a Pelletron Tandem accelerator installed at the Experimental Physics Department (EPD), National Center for Physics (NCP), Islamabad, Pakistan.

2.4. Electrodes Characterizations

The structural phases of Co₃O₄@MnO₂@NiO/GO electrodes were studied by X-ray diffraction using a Rigaku Miniflex-600 X-ray diffractometer with copper radiation (Kα of λ = 0.154 nm) at a scanning speed of 1° per minute within the 10−70° 2θ range. Fourier transform infrared (FTIR) spectroscopy was used to study functional groups and bonding features of Co₃O₄@MnO₂@NiO/GO, measured within the 500−4000 cm⁻¹ wavelength range on an Alpha (II) Bruker instrument using KBr pellets at room temperature. The surface morphology features of Co₃O₄@MnO₂@NiO/GO was measured using field emission-scanning electron microscopy (FE-SEM) (model JSM-7001F, JEOL) equipped with energy-dispersive X-ray spectroscopy (EDS). GUPIXWIN computer software was used to study the PIXE spectrum, while RUMP computer software was used to study the RBS. Electrochemical characterization was performed using Gamry 3000 electrochemical workstation equipment in a 1.0 M NaOH electrolytic solution.

3. Results and Discussions

3.1. Structural Analysis

Figure 2 shows the crystal structure of hydrothermally synthesized GO with the peak at 2ϴ = 11° with (001) plane and 2ϴ = 26.62° with (002) plane, which confirms the graphene oxide formation. While Figure 3 shows the crystal structure of Co₃O₄@MnO₂@NiO/GO composite displayed distinguishing diffraction crests of CoMn₂O₄ (Co₃O₄/MnO₂) composite with JCPDS card numbers of 01-077-0471 possess peaks at 31.74°, 32.80°, 34.40°, 36.08°, 39.44°, 41.49°, 47.49°, 56.60°, and 62.79° with reflection planes of (200), (103), (013), (210), (120), (212), (023), (303), and (321) respectively. Similarly, it also exhibited diffraction peaks of NiMn₂O₄ (MnO₂/NiO) composite with a JCPDS card number of 00-001-1110 with recorded peaks angles at 31.74°, 35.01°, 36.16°, 56.52°, and 62.79° with reflection planes of (220), (311), (222), and (440), respectively. However, it also demonstrated distinctive diffraction peaks of NiCo₂O₄ (Co₃O₄/NiO) composite with a JCPDS card number of 01-073-1704, recording peaks at 31.74°, 32.89°, 41.47°, 47.58°, 58.55°, and 62.79° with reflection planes of (311), (222), (331), (422), (531), and (533), respectively. Various peaks of Co₃O₄@MnO₂@NiO/GO were sharp, strong, and fine, indicating the good crystallinity nature of the composite nanostructure, prompting possible long cycle life and stability of the fabricated sample electrodes during electrochemical processes [55]. The perceived peaks and crystal planes showed the creation of tetragonal CoMn₂O₄ with a = b = 4.9330 Å, c = 13.7100 Å, α = β = 90°, γ = 120°_, Cubic NiMn₂O₄ with a = b = c = 8.3820 Å, α = β = γ = 90° and Cubic NiCo₂O₄ with a = b = c = 9.3872Å, α = β = γ = 90° phases.

There was a graphene oxide peak at 2θ = 10.85° in the pristine sample irradiated with 2.25 × 10¹⁵ ions/cm² carbon ions, confirming the presence of GO. There was a small increase in the intensities of various peaks of the nanostructured electrodes after irradiating with a carbon ion dose of 2.25 × 10¹⁵ ions/cm², and the GO peak moved from 10.85° to 25.10°, showing movement from GO to rGO (reduced graphene oxide). However, various peak intensities also increased promptly and sharply after an increase in carbon ion radiation to 5.0 × 10¹⁵ ions/cm². The increase in the intensity of peaks at the 2.25 × 10¹⁵ and 5.0 × 10¹⁵ ions/cm² dosages indicates recrystallization and improved the crystalline nature because carbon ion radiation transported energy to the electrode’s lattices and stimulated recrystallization. This was also an increase in radiation doses to 7.25 × 10¹⁵ and 1.0 × 10¹⁶ ions/cm². Various peak intensities decreased as radiation dose increased, while rGO peak remained at its 2θ peak position but also decreased as radiation doses increased. These observed features indicate disintegration, dislocation, and defect creation at higher carbon ion radiation quantities [56,57]. We used the Debye Scherrer equation to estimate electrode crystallite size (D) using the most intensified peak. The acquired crystal sizes were 12.25, 8.86, 7.44, and 3.55 nm for pristine, doses of 2.25 × 10¹⁵, 5.0 × 10¹⁵, 7.25 × 10¹⁵ and 1.0 × 10¹⁶ ions/cm² respectively. We also estimated dislocation density (δ), number of crystals per unit area (N), and microstrain (ε) using XRD data employing Equations (1)–(3).

$$\mathsf{\delta }=\frac{1}{{\mathrm{D}}^2}$$

(1)

$$\mathrm{N}=\frac{\mathrm{d}}{{\mathrm{D}}^2}$$

(2)

$$\mathsf{\epsilon }=\frac{\mathsf{\beta }\mathrm{Cos}\mathsf{\theta }}{4}$$

(3)

where D is the crystallite size, d stands for film thickness, β represents FWHM and θ stands for value 2θ angle. Variation in structural parameters was estimated and is shown in

Table 1. Calculated structural parameters of Co₃O₄@MnO₂@NiO/GO electrodes.

S/N	Electrode/ Radiation Dose (ions/cm2)	Dislocation Density (δ) (1017m)	Number of Crystallites in a Unit Surface Area (N) (10−12m)	Microstrain (ε) (10−2)
1	Pristine	0.56	1.79	4.20
2	2.25 × 1015	1.25	4.74	6.27
3	5.0 × 1015	2.07	7.29	8.18
4	7.25 × 1015	8.90	32.04	11.26
5	1.0 × 1016	9.10	33.66	13.77

. The d-spacing of 0.24 nm corresponds to the (311) lattice plane of the Co₃O₄ crystal, and the d-spacing of 0.22 nm relates to the (200) lattice plane of the MnO₂ crystal.

3.2. Surface/MorphologicalAnalysis

The morphology and microstructure of Co₃O₄@MnO₂@NiO/GO electrodes were studied using SEM. Figure 4a is the SEM image of the pristine electrode, which demonstrates that the pristine electrode possesses a mixture of flake and granular particles evenly distributed on the surface of the substrate. The shapes were randomly distributed on the surface of the substrates. These flake- and granular-shaped particles are small in nature, which can be credited to the condensed crystallite feature of these particles. On the other hand, larger and uniformly dispersed spherical-shaped particles were observed in Co₃O₄@MnO₂@NiO/GO electrodes irradiated with 2.25 × 10¹⁵ ions/cm² as shown in Figure 4b, displaying an improved feature after reception of radiation. The SEM micrograph in Figure 4c reveals the surface morphology of Co₃O₄@MnO₂@NiO/GO electrode radiated with a 5.0 × 10¹⁵ ions/cm² dose, this electrode showed uniformly, enhanced, well-defined, and evenly distributed spherical particles. This awesome crystalline feature indicates that this radiation dose is the ideal radiation dose for optimal crystalline nature. The SEM feature presented in Figure 4d reveals the surface morphology of Co₃O₄@MnO₂@NiO/GO electrode radiated with a 7.25 × 10¹⁵ ions/cm² dose, there is shrinkage of various particles after radiation. The highly conglomerated nanocomposite has a high crystallite structure with pores on the surfaces of substrates [58]. Simply treating a nanoporous carbon with gamma irradiation alters its lattice morphology with ionizing radiations that generate free radicals within the material.

The elemental distribution of the deposited MnO₂@NiO@GO nanocomposite on the glass substrate surface is shown in Figure 4f. The figure reveals different percentage distributions of elements, which are 56 and 44% corresponding to manganese (Mn), and nickel (Ni) in that order.

Figure 5a presents the EDS outline of the Co₃O₄@MnO₂@NiO/GO pristine electrode. The existence of Co, Mn, and Ni crests showed that the electrodes were effectively fabricated with their corresponding metal nanoparticles, while Cl confirms the presence of chlorine obtained from CoCl₂·6H₂O. The manifestation of oxygen peaks with very prominent intensities indicates that the electrodes were efficaciously converted to their respective metal oxide forms. The FTIR spectrum of the Co₃O₄@MnO₂@NiO/GO pristine electrode is presented in Figure 5b. The peak located at 3445.69 cm⁻¹ indicates a stretching vibration of the O-H (hydroxide) functional group. The peak that appeared at 1789.81 cm⁻¹ in the electrode signifies a symmetric stretching of the C-O-H group. Various characteristic peaks observed between 300–1000 cm⁻¹ for the composite’s electrode denote stretching and vibration of metal oxides (Co₃O₄, MnO₂, and NiO), while peaks observed between 500–700 cm⁻¹ indicate a bending and vibration of metal oxides (Co₃O₄, MnO₂, and NiO) bonds.

Figure 6a and b shows the proton-induced X-ray emission (PIXE) and Rutherford backscattering (RBS) spectra of Pristine Co₃O₄@MnO₂@NiO/GO Electrode. GUPIXWIN computer software was used to study the PIXE spectrum, which reveals the peaks and percentage weight distribution of the sample elements, as can be seen in Table 1, while RUMP computer software was used to study the RBS, which reveals the energy and channel distribution of the sample elements, as observed in

Table 2. PIXE spectra parameters for Pristine Co₃O₄@MnO₂@NiO/GO Electrode.

Elements	Mass	% Weights by PIXE
Co	54.94	22
Mn	65.37	48
Ni	63.50	30

. Elemental distribution of Co, Mn, Ni, and minor traces elements of Cl was observed, with Mn having the highest percentage weight, energy, and channel distribution. This result confirms the supremacy of Mn, as observed in the EDS and XRD outcomes. Table 2 and

Table 3. RBS spectra parameters for Pristine Co₃O₄@MnO₂@NiO/GO Electrode.

Elements	Mass	K (ion)	Energy (MeV)	Channel
Co	54.94	0.7885	1.618	1618.49
Mn	65.37	0.7485	1.556	1556.06
Ni	63.50	0.7540	1.630	1629.90

present a review of PIXE and RBS elemental configurations. This displayed the absolute thickness and composition of the thin films.

3.3. Electrochemistry Analysis

We operated three electrode systems having Co₃O₄@MnO₂@NiO/GO fabricated working electrodes, graphite counter electrodes, and an Ag/AgCl reference electrode. Many researchers have reported the electrochemical performances of several carbon-based nanostructured materials, such as Co₃O₄, MnO₂, and Co₃O₄@MnO₂ core/shell [59,60,61]. During the electrode–electrolyte reaction, there is an obvious display of metal oxide and various oxidation states (Co³⁺/Co⁴⁺) by the cobalt (Co₃O₄) and MnO₂ materials. From previous results, the electrochemical properties of nanostructured Co₃O₄@MnO₂@NiO/GO electrodes were studied in a three-electrode configuration for CV, GCD, and EIS analysis. The commonly known three electrodes set up consisted of Pt electrode, Co₃O₄@MnO₂@NiO/GO composite, and Ag/AgCl as counter, working, and reference electrodes, respectively. To narrate the growth mechanism of Co₃O₄@MnO₂@NiO/GO composite on FTO, several cyclic voltammetry reports on bare FTO have shown that a rapid decrease in the current density from the maximum potential window has been observed [62,63]. This is evident that the CV of the bare FTO does not contribute to the resultant CV of the composite electrodes.

The measurement was performed in a 1.0 M NaOH aqueous electrolyte at room temperature in the potential windows 0–0.8 V vs. SCE with an electrode with an active mass of 0.45 mg. A NaOH solution of 1.0 molar concentration was employed as an operational electrolytic solution at various scan rates values of 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0 mVs⁻¹ and potential window of 0 to 0.7 V, as shown in Figure 7a–d, the obtained CV curves looks alike which is an evident of the irreversible nature of the Co₃O₄@MnO₂@NiO/GO composite except in Figure 7d where there is an observed distinct redox activity during the cathodic and anodic sweepings for the 7.5 × 10¹⁵ radiation dose sample. This was a result of less polarization or oxygen evolution effect after irradiation, which led to the shrinkage of various particles, as confirmed by the SEM result. The CV curves obey the charge/discharge process of Co₃O₄@MnO₂@NiO/GO composite through the redox reaction (faradaic reaction) at the electrode–electrolyte interface. From the electrochemical behavior of Co₃O₄, MnO₂, Co₃O₄@MnO₂, Co₃O₄, MnO₂, and Co₃O₄@MnO₂@NiO, the non-faradaic activity is due to the double layer capacitance, while the faradaic reaction involves the ion diffusion from the electrode to the electrolyte due to the surface bound mechanism. Therefore, CV is a product of the accumulated charge during the faradaic and non-faradaic processes [64,65,66]. The invented electrodes have shown judicious features such as their specific capacitance, energy density, and power densities [9,44,67], as shown in Figure 7a–e and using Equations (6)–(8) [6,7].

The specific capacitance values of various electrodes fabricated were deducted from CV curves using Equation (4).

$${\mathrm{C}}_{\mathrm{sp}}=\frac{1}{\mathrm{m}Vs}{{\displaystyle \int }}^{ }I\left(V\right)dV \left(\mathrm{F}/\mathrm{g}\right)$$

(4)

where m is the active mass in mg, V is the potential window used in volts, s is the sweep rate in mVs⁻¹ and I is the current applied in ampere (A).

Co₃O₄@MnO₂@NiO/GO electrodes with 1.0 mVs⁻¹ rate of the scan revealed the utmost specific capacitance of 563, 641, 732, 467, and 215 Fg⁻¹ for pristine, electrodes radiated with 2.5 × 10¹⁵, 5.0 × 10¹⁵, 7.5 × 10¹⁵, 1.0 × 10¹⁶ ions/cm² dosages of radiation, respectively. A distinct redox peak noticed during the cathodic and anodic sweepings for the 7.5 × 10¹⁵ ions cm⁻² radiation dose sample can be attributed to recrystallization due to an increased dose of carbon ions. This agrees with the XRD results in Figure 3. It is evident from the result that the 5.0 × 10¹⁵ ions/cm² radiated electrodes have the best specific capacitance. The results also indicate that at high-energy doses, specific capacitance decreased. This also confirms the high level of defects formed at high-energy dosages. The resistance and reaction kinetics of the electrodes were obtained from the EIS study. The EIS was carried out from the frequency boundary between 1.0 and 100.0 kHz, as shown in Figure 7e. A CPE diffusion model was used to study the resistances of the fabricated electrodes. The CPE model consented greatly with the Nyquist plot of the EIS. The resistances of electrolytes (Re) are 2.15, 2.55, 1.06, 3.02, and 4.80 ohms for pristine sample and 2.5 × 10¹⁵, 5.0 × 10¹⁵, 7.5 × 10¹⁵ and 1.0 × 10¹⁶ ions/cm² doses respectively. However, resistances of working electrodes (Rw) are 0.09, 0.06, 0.035, 0.17, and 1.05 ohms for the pristine electrodes radiated with 2.5 × 10¹⁵, 5.0 × 10¹⁵, 7.5 × 10¹⁵, and 1.0 × 10¹⁶ ions/cm² dosages of energy, respectively. Various acquired electrode resistances were low, an attribute of unification of GO in the course of fabricating these electrodes. The addition of GO improved electrode conductivities, hence reducing resistance. The Nyquist plots of Co₃O₄@MnO₂@NiO/GO electrodes were void of a semi-circle, indicating that electrodes possess low charge transfer resistance [2,9,68,69].

The GCD plots of Co₃O₄@MnO₂@NiO/GO electrodes shown in Figure 8a,b for current densities of 1.0 and 2.0 A/g disclose non-linear inclined voltage vs time variations [70]. Specific capacitance values were calculated using GCD plots and with the help of Equation (5).

$${\mathrm{C}}_{\mathrm{sp}}=\frac{I{{\displaystyle \int }}^{ }Vdt}{\mathrm{ms}\Delta \mathrm{V}} \left(\mathrm{F}/\mathrm{g}\right)$$

(5)

where m stands for active mass in mg, ΔV denotes potential window in volts, dt is the discharge time in seconds, I is the current in ampere (A), and s is a constant. The estimated specific capacitance values from the GCD plots were 503, 697, 811, 842, and 472 Fg⁻¹ for the pristine and electrodes irradiated with 2.5 × 10¹⁵, 5.0 × 10¹⁵, 7.5 × 10¹⁵, and 1.0 × 10¹⁶ ions/cm² dosages, respectively, using a 1.0 Ag⁻¹ current density. While at 2.0 Ag⁻¹ current density, the specific capacitance values for the pristine and electrodes irradiated with 2.5 × 10¹⁵, 5.0 × 10¹⁵, 7.5 × 10¹⁵, and 1.0 × 10¹⁵ ions cm⁻² dosages were 425, 572, 640, 658, and 399 Fg⁻¹, respectively. These results seamlessly agree with Pawar et all [71] and Li et al. [72]. The specific capacitance style attained in CV examinations was also persistent in GCD investigation. There was a voltage drop observed in the GCD plots, especially for 2.0 A/g current density. A voltage drop is caused by losses because of resistances accessible by the electrolytic solution and commonly holds an important value after numerous charge and discharge successions, symbolized by aging incidences [73]. Table 4 shows the assessment of electrochemical performance of synthesized MnO₂-based electrocatalysts as investigated in comparison with this current research.

We also carried out more evaluation of the properties of Co₃O₄@MnO₂@NiO/GO electrodes by estimating energy (E_d) and power (Pd) densities using Equations (8) and (9) [5,6,8].

$${\mathrm{E}}_{\mathrm{d}}=0.5\times {\mathrm{C}}_{\mathrm{s}} {\left(\Delta \mathrm{V}\right)}^2$$

(6)

$${\mathrm{P}}_{\mathrm{d}}=3.6\frac{{\mathrm{t}}_{\mathrm{D}}}{{\mathrm{t}}_{\mathrm{C}}}$$

(7)

where t_D denotes the time to discharge, ΔV is the potential window and t_D represent time to charge. The energy densities estimated were 296.70, 368.48, 451.78, 233.98, and 199.92 Whkg⁻¹ for pristine electrodes and electrodes radiated with 2.5 × 10¹⁵, 5.0 × 10¹⁵, 7.5 × 10¹⁵, and 1.0 × 10¹⁶ ions/cm² dosages, respectively. The power densities achieved were 30.40, 32.44, 40.55, 28.52, and 16.11 Wkg⁻¹ for the pristine electrode and electrodes radiated with 2.5 × 10¹⁵, 5.0 × 10¹⁵, 7.5 × 10¹⁵, and 1.0 × 10¹⁶ ions/cm² dosages of energy, respectively.

The cycle stability and efficiency of Co₃O₄@MnO₂@NiO/GO electrode radiated with a dosage of 5.0 × 10¹⁵ ions/cm² delivering a superior feature was analyzed using a current density of 1.0 A/g for continuous 10,000 cycles, as shown in Figure 9. Co₃O₄@MnO₂@NiO/GO-enhanced electrodes showed good cyclic stability and possessed a capacitive retention of 93.9% after 10,000 continuous cycles. This was obtained using Equation (9) [74]. However, this electrode displayed enhanced stability after receiving energy from the carbon ions. The coulombic efficiency of ions/cm² radiated Co₃O₄@MnO₂@NiO/GO electrode was estimated using the formula shown in Equation (8). The calculated coulombic efficiency (η), which describes the efficiency and rate of charge transfer in a system. This parameter quantifies electrochemical reactions, as well as energy storage devices [39].

$$\mathsf{\eta }=\frac{{\mathrm{T}}_{\mathrm{d}}}{{\mathrm{T}}_{\mathrm{c}}}\times 100\%$$

(8)

where T_d and T_c represents time to discharge and charge, respectively.

$$\mathrm{Capacitance} \mathrm{Retention} (\%)=\frac{stable capacitive value or last cycle capacitive value}{Real or first capacitive value}\times 100$$

(9)

Table 4. Comparison of the supercapacitor values of various nanostructured cobalt oxides (Co₃O₄), Manganese oxides (MnO₂) and Co₃O₄@MnO₂/GO electrodes reported in the literature.

S/NO	Composites	Morphology	Specific Capacitance (Fg−1)	Current Density (Ag−1)	Potential Window (V)	Ref.
1.	Ni(OH)2-MnO2	nanoflake core–shell	355	0.5	−0.1–0.9	[75]
2.	MnO2/NiO	nanoparticle	681	1	−1–0.9	[76]
3.	Ni–Mn	hedgehog-like hollow	1016	0.5	−0.2–1.0	[77]
4.	MnOOH/NiO	nanosheets	1625	4	0–0.6	[78]
5.	Co3O4/NiO/MnO2	nanospheres	549	0.5	−0.4–0.6	[32]
6.	Co3O4@MnO2/NGO	nanosheet	347	0.5	−0.34–0.34	[36]
7.	Co3O4@Pt@MnO2	nanoflakes	593	1	0–1.0	[79]
8.	NiO@Co3O4@MnO2	nanoflakes	792.5	2	0.35–0.5	[80]
9.	Co3O4@MnO2@PPy	nanoflakes	977	1	0–0.8	[81]
10.	Co-Ni/Co3O4-NiO	Nano heterostructure	2013	2.5	0.1–0.5	[82]
11.	Co3O4@MnO2	nanowire	413	0.5	0–0.6	[31]
12.	α-MnO2 and Co3O4	Microspheres	214.6	3		[83]
13.	rGO/NiCo2O4@ZnCo2O4	nanosheets	1197	1	0–0.55	[41]
14.	Co3O4-MnO2-NiO	nanotubes	2525	15	−0.2–0.6	[32]
15.	MnCO3 QDs/NiH–Mn–CO3	shell–nanoneedle	2641.3	3	−0.2–1.0	[84]
16.	NiO-Co3O4	mesoporous	324	2	−0.5–0.5	[85]
17.	Co3O4@MnO2@NiO/GO	nanosphere	215–732	1	0–0.8	This work

Table 4. Comparison of the supercapacitor values of various nanostructured cobalt oxides (Co₃O₄), Manganese oxides (MnO₂) and Co₃O₄@MnO₂/GO electrodes reported in the literature.

S/NO	Composites	Morphology	Specific Capacitance (Fg−1)	Current Density (Ag−1)	Potential Window (V)	Ref.
1.	Ni(OH)2-MnO2	nanoflake core–shell	355	0.5	−0.1–0.9	[75]
2.	MnO2/NiO	nanoparticle	681	1	−1–0.9	[76]
3.	Ni–Mn	hedgehog-like hollow	1016	0.5	−0.2–1.0	[77]
4.	MnOOH/NiO	nanosheets	1625	4	0–0.6	[78]
5.	Co3O4/NiO/MnO2	nanospheres	549	0.5	−0.4–0.6	[32]
6.	Co3O4@MnO2/NGO	nanosheet	347	0.5	−0.34–0.34	[36]
7.	Co3O4@Pt@MnO2	nanoflakes	593	1	0–1.0	[79]
8.	NiO@Co3O4@MnO2	nanoflakes	792.5	2	0.35–0.5	[80]
9.	Co3O4@MnO2@PPy	nanoflakes	977	1	0–0.8	[81]
10.	Co-Ni/Co3O4-NiO	Nano heterostructure	2013	2.5	0.1–0.5	[82]
11.	Co3O4@MnO2	nanowire	413	0.5	0–0.6	[31]
12.	α-MnO2 and Co3O4	Microspheres	214.6	3		[83]
13.	rGO/NiCo2O4@ZnCo2O4	nanosheets	1197	1	0–0.55	[41]
14.	Co3O4-MnO2-NiO	nanotubes	2525	15	−0.2–0.6	[32]
15.	MnCO3 QDs/NiH–Mn–CO3	shell–nanoneedle	2641.3	3	−0.2–1.0	[84]
16.	NiO-Co3O4	mesoporous	324	2	−0.5–0.5	[85]
17.	Co3O4@MnO2@NiO/GO	nanosphere	215–732	1	0–0.8	This work

4. Conclusions

Electrodes of Co₃O₄@MnO₂@NiO/GO were marvelously invented using the hydrothermal technique for applications in supercapacitors. GO was integrated into the nanostructured electrodes synthesized due to its prominent surface area and electrical conductivity. The synergistic cooperation among these transition metals and GO improved electrode properties, life span, and stability). The estimated specific capacitance values from the GCD plots were 503, 697, 811, 242, and 215 Fg⁻¹ for the pristine and electrodes irradiated with 2.5 × 10¹⁵, 5.0 × 10¹⁵, 7.5 × 10¹⁵, and 1.0 × 10¹⁶ ions/cm² dosages respectively. Electrodes radiated with a carbon ion dosage of 5.0 × 10¹⁵ ions/cm² delivered the best specific capacitance, energy density, and power density, indicating that moderate carbon ion radiation improves the electrochemical properties of nanostructured Co₃O₄@MnO₂@NiO/GO electrode.