Ag doped Co₃O₄ nanoparticles for high-performance supercapacitor application

Abstract

Ag doped Co₃O₄ nanoparticles (NPs) were synthesized via a co-precipitation method changing the concentration of Ag. The crystal structure, morphology, surface area, functional group, optical band gap, and thermal property were investigated by XRD, SEM, BET, FTIR, UV–Vis, and TGA/DTA techniques. The XRD results showed the formation of single-cubic Co₃O₄ nanostructured materials with an average crystal size of 19.37 nm and 12.98 nm for pristine Co₃O₄ and 0.25 M Ag-doped Co₃O₄ NPs. Morphological studies showed that pristine Co₃O₄ and 0.25 M Ag-doped Co₃O₄ NPs having a porous structure with small spherical grains, porous structures with sponge-like structures, and loosely packed porous structures, respectively. The pristine and 0.25 M Ag-doped Co₃O₄ NPs showed BET surface areas of 53.06 m²/g, and 407.33 m²/g, respectively. The band gap energy of Co₃O₄ NPs were 2.96 eV, with additional sub-bandgap energy of 1.95 eV. Additionally, it was discovered that the band gap energies of 0.25 M Ag-doped Co₃O₄ NPs ranged from 2.2 to 2.75 eV, with an extra sub-band with energies ranging from 1.43 to 1.94 eV for all as-prepared samples. The Ag-doped Co₃O₄ as prepared samples show improved thermal properties due to the doping effect of silver. The CV test confirmed that the 0.25 M Ag-doped Co₃O₄ NPs exhibited the highest specific capacitance value of 992.7 F/g at 5 mV/s in a 0.1 M KOH electrolyte solution. The energy density and power density of 0.25 M Ag-doped Co₃O₄ NPs were 27.9 W h/kg and 3816.1 W/kg, respectively.

1. Introduction

The demand for renewable energy sources has increased in recent years with population growth, and thus researchers have a great responsibility to find advanced energy storage devices to solve the shortage of renewable energy sources [1,2]. Recently, more attention has been paid to renewable energy sources, and work with energy storage such as electric batteries, solar cells and supercapacitors has accelerated [3,4]. Among storage devices, supercapacitors have gained wide attention in recent years due to their advantages of fast charge/discharge rates, affordability, environmental-friendly, high specific capacitance, high power density, and long life cycle [[5], [6], [7], [8], [9], [10], [11], [12]]. These exceptional characteristics and supercapacitors make them suitable candidates used in many electronic devices, memory backups, mobile devices, and in the industrial field for energy storage applications [13]. Based on the nature of electrodes and charge storage mechanisms supercapacitors were categorized into electric double-layer capacitors (EDLC) and pseudocapacitors [[14], [15], [16]]. In EDLCs energy is stored and charge builds up at the interface between the electrode and the electrolytes [[17], [18], [19], [20]], but, in the case of pseudocapacitors, energy is stored by a rapid and reversible faradaic redox reaction [[21], [22], [23], [24]]. Therefore, the overall performance of a supercapacitor is determined based on the choice of its electrode material [25]. For the best results, researchers are currently working on selecting suitable electrode materials for pseudocapacitors applications [26,27]. Transition metal oxides as redox-based electrode materials are the best choice to use for this type of supercapacitor due to their excellent properties such as multiple oxidation states, efficient discharge cyclic ability, and other excellent properties for electrochemical performance [28]. As supercapacitors electrode material, transition metal oxides mainly include CeO₂ [29], MnO₂ [30], NiO [31], Fe₂O₃ [32] and Co₃O₄ [33]. Among these transition metal oxides,Co₃O₄ is cost-effective, environmentally friendly, and offers a high theoretical specific capacitance of 3560 F/g [[34], [35], [36]]. Unfortunately, the capacitance of Co₃O₄ in practical applications is quite different from the theoretical value. One reason is that electron transfer is hindered by poor conductivity, limited surface area, large volume expansion and contraction, and strong particle aggregation, resulting in the capacitance and cyclic efficiency of Co₃O₄ are limited [37,38]. The aforementioned disadvantages of Co₃O₄ result in slow kinetics, lower capacity and poor cycling stability during electrochemical experiments. To date, researchers have developed various strategies to overcome the inherent disadvantages of transition metal oxides [39]. To cope with those problems and enhance the properties of Co₃O₄, researchers can undertake multi-step techniques [40]. Among those techniques is to construct nanostructures, consisting of nanospheres, hollow structures, yolk-shell structures, and porous materials, which assist to make amends for the quantity enlargement all through cycling [41,42]. Another approach is to introduce carbonaceous composites, which now no longer handiest to relieve the volume change, however also are conducive to enhance the electronic conductivity [42,43]. Alternatively, metal doping into Co₃O₄ is likewise one a powerful approach to enhance the electrochemical performance, taking gain of the complementary and synergy among ions [41]. Previous research have confirmed that metal doping is a viable manner to modulate the electronic shape without converting the crystal shape of metal oxides and thereby attaining more desirable conductivity and advanced electroactive sites, leading to incredible electrochemical performance [[44], [45], [46]]. Thus, Co₃O₄ nanostructure materials are synthesized by various methods such as solvothermal [47], Spray pyrolysis the sol-gel method [48], thermal decomposition [49], co-precipitation [50] and hydrothermal [51]. However, these approaches are expensive, require complex instruments, and relatively longer preparation times [52,53]. Among them, the co-precipitation method is advantageous due to its fast, cost-effectiveness, and easy control during production [47,54]. Therefore, the properties and electrochemical performance of nanostructured materials can vary depending on experimental parameters such as synthesized method reaction time, concentration, solvent, raw materials and temperature [55]. Therefore, various researchers tried to optimize its electrochemical properties by alloying with various transition metal ions and it was found to be effective in improving supercapacitor performance [56,57]. For example, G. Li et al. examined that Mn-doped Co₃O₄ nanoneedles on nickel foam as a binder-free electrode for supercapacitor applications via a one-step hydrothermal method and demonstrated that the doping of Mn atoms can enhance the electrochemical activity of Co₃O₄ as well as improve the conductivity and outstanding cycling stability of Co₃O₄ with a high specific capacitance of 668.4 F/g at a current density of 1 A/g [58]. Similarly, Uma Sudharshini et al. investigated that 3% Cu-doped Co₃O₄ nanostructure materials were successfully synthesized at a lower temperature of 140 °C using different dopant concentrations via a solvothermal method and exhibited a higher specific capacity value of 812 F/g at a scan rate of 5 mV/s [59]. Similarly, Ali and Khalid. Reported that 5% Sn-doped Co₃O₄ nanorods were successfully synthesized by a solvothermal technique followed by calcination at different temperature ranges (250–400 °C) and showed that the highest specific capacitance was obtained at 350 °C calcination, which was 913.10 F/g [60]. Ali, Khalid, Tahir et al. Studied that the 4% Sb doped Co₃O₄-based electrode material prepared by hydrothermal method achieved a higher specific capacitance of 89.15 F/g at 5 mV/s investigated that 4%Sb doped Co₃O₄ based electrode material produced by a hydrothermal method has achieved a higher specific capacitance of 894.15 F/g at 5 mV/s [61]. Moreover, Khalid et al. reported that 5% Mo-doped Co₃O₄ porous nanostructure materials with different molybdenum concentrations were successfully synthesized through a simple sol–gel method and showed good conductivity and excellent specific capacitance of 858.09 F/g at a scan rate of 5 mV/s [62]. Hence, Ag-doped Co₃O₄ nanostructure materials were found to be mainly used as photocatalytic activities/electrolytes [[63], [64], [65]], Non-Enzymatic Glucose Sensor [66], supercapacitors [67], gas sensors [68], and some other applications. Different studies have been done on the fabrication of Co₃O₄ nanomaterials with alloys such as Cr, Mo, Cu, Fe, Ni, Ag, Sn and Mn, etc. Silver doping improves the electrochemical properties of some other materials has been reported in the literature, but no one has so far reported Ag-doped Co₃O₄ nanostructured materials synthesized by co-precipitation method for supercapacitor applications. Hence, Ag-doped Co₃O₄ NPs were prepared for the first time for a supercapacitor application with a simple method.

2. Experimental methods

2.1. Chemical reagents

All chemicals and reagents were used without any further purification and of analytical grade. Ammonia solution (NH₃, 99.2%), Cobalt chloride Hexahydrate (CoCl_2. 6H₂O, 99.9%), and silver nitrate (AgNO_3, 99.9%) were obtained from chemical markets. The solutions were prepared by using distilled water.

2.2. Synthesis of Ag-doped Co₃O₄ NPs

Silver doped-Co₃O₄ NPs were synthesized using a facile co-precipitation technique. In a typical synthesis, 0.3 M of Cobalt chloride hexahydrate (CoCl₂·6H₂O) and the preferred mole of (0.05, 0.1, 0.15, 0.2, and 0.25 M) silver nitrate (AgNO_3, 99.9%) were dissolved in 100 mL distilled water. Then, the resulting mixture was stirred using magnetic stirrer at 80 °C for 3 h. The pH of the mixture was adjusted to 9 by adding 0.2 M NH₃ solution dropwise to the combination of AgNO₃. Then, both products was filtered and washed with distilled water and ethanol several times. And the samples were dried in oven at temperature of 100 °C for 6 h to evaporate water and organic materials. Finally, the products were calcined in a muffle furnace at 500 °C for 4 h. A similar method was used to prepare un-doped Co₃O₄. The detailed schematic illustration of Ag-doped Co₃O₄ nanostructured materials prepared by the co-precipitation method is shown in Fig. 1.

Fig. 1.

Synthesis of Ag-doped Co₃O₄ nanostructured materials by Co-precipitation method.

2.3. Electrode preparation

Glassy carbon electrode surface has to be polished first with polishing paper and then polished thoroughly with alumina (Al₂O₃) powder slurry on a polishing cloth mounted on glass plate and then rinse with distilled water to remove alumina particles and other possible contaminants and further with deionized water. Un-doped cobalt and Silver doped cobalt oxide (Ag–Co₃O₄) oxide (Co₃O₄) with concentration of 0.25 M in 0.1 M KOH in 50 mL of volumetric flask has to be prepared by serial dilution from 5 M stock solution of Silver doped cobalt oxide and Un-doped cobalt oxide respectively. Three electrodes (Glassy carbon working electrode, reference Ag/AgCl and Pt auxiliary/counter electrode) are inserted into an electrochemical cell filled with 0.1 M KOH. Then background run need to be scanned. Stirring bar is added on the magnetic stirrer to stir the solution so that there is homogeneity of concentration from the surface of the electrode to the bulk solution. Then CV is scanned from an initial potential of +245 mV–0.0 mV and then from 0.0 mV to +245 mV at a scan rate of 5 mV/s to 100 mV/s. Finally, voltammogram of each CV scan have been labeled and saved.

2.4. Characterization

The functional groups of NPs were identified using Fourier transform infrared spectrometer (FT-IR, FT-IR 6660 (JASCO MODEL)) in the wavenumber range of 4000–500 cm⁻¹. The crystal structure and the phase purity of as prepared NPs were carried out by powder X-ray diffraction (MAXima-X XRD-7000, SHIMADZU). The morphology of the prepared NPs were analyzed using SEM, inspect™ SEM, SEM (INSPECT F50) at different magnifications. Thermal properties were Examine using TGA/DTA analysis. The optical properties were determined by Ultraviolet–Visible spectrophotometer (UV–Vis, Lambda 35 (PerkinElmer)) in the wavelength range of 200–800 nm. The Surface area were determined using Brunner-Emmet-Teller (BET) model Quanta chrome Nova Win (Quanta chrome Instruments version 11.0).

3. Results and discussion

3.1. Structural analysis

The XRD analysis was investigated to determine the structure and crystalline phase of pristine Co₃O₄ and Ag-doped Co₃O₄ NPs. All the prepared samples were examined by XRD technique, and the diffraction peaks as displayed in Fig. 2. The observed XRD peaks around at 2θ are 19.68°, 31.89°, 37.7°, 39.29°, 45.5°, 55.89° and 59.11°, corresponding to (111), (220), (311), (222), (400), (422) and (511) are crystal planes of cubic Co₃O₄, respectively. The XRD peaks are in good agreement with the JCPDS data (JCPDS 09–0 18) [69,70]. In addition, it was observed that the intensity of the peaks corresponding to the (111), (222), (422), and (511) planes were decreased, while the peaks corresponding to the (220) and (311) increased again. Doping does not affects the bulk structure, but it affects the crystallinity of the materials, the difference in the intensity of the diffraction peaks and the change of peak positions of the samples can be seen. As a result, silver doping increases the crystallinity, due to the variation of the lattice constant. The crystallite size (D) of the nanostructured materials has been estimated using the Debye-Scherer formula (equation (1)) [71].

$$\mathrm{D}=\frac{\mathbf{k}\mathbf{\lambda }}{\mathbf{\beta }\mathbf{cos}\mathbf{\theta }}$$ (1)

Fig. 2.

XRD pattern of (a) Pristine Co₃O₄ nanostructured materials, and (b) 0.25 M Ag-doped Co₃O₄ NPs.

Where, ‘‘D’’ is crystal size, ‘‘k” is the Debye–Scherer constant (0.9), ‘‘ʎ’’ represents the X-ray wavelength used (0.15406 nm), "β” is the width of the peak at half maximum intensity, and ‘‘θ" is the diffraction angle, respectively [60]. The average crystallite size was found to be equal to 19.37 and 12.98 nm for pristine Co₃O₄ and 0.25 M Ag-doped Co₃O₄, respectively. The result showed that particle size decreased with silver doping (Table 1). This small crystal value is indicative of the high surface-to-volume ratio that makes them capable of high charge storage.

Table 1.

XRD results of physical parameters of pristine Co₃O₄, and 0.25 M Ag-doped Co₃O₄ NPs.

Nanostructured materials	2θ	FWHM	D (nm)
Co3O4	31.63799	0.42607	19.37945
Ag-doped Co3O4	31.63567	0.63583	12.9861

3.2. SEM analysis

The morphology of pristine, and 0.25 M Ag-doped Co₃O₄ NPs characterized by SEM are shown in Fig. 3 (a-d) with different magnification scales. In Fig. 3(a) SEM morphologies showed that the particles are exhibit porous structure with small spherical grains at magnification scales of 10 μm [72]. Whereas, Fig. 3(b) exhibited a porous structure with sponge like structures at magnification scales of 20 μm. Fig. 3(c) showed that SEM morphologies, a loosely packed porous structure with small cracks/holes at magnification scale of 10 μm. Furthermore, Fig. 3(d) showed flower-like porous morphology at magnification scales of 20 μm. This suggested that, the addition of silver affected the surface morphology of Co₃O₄, as shown in Fig. 3(a–d). It can be seen from the SEM images that there is a clear distinction in morphology between pristine and Ag-doped Co₃O₄ nanostructured materials indicating that the doping concentration of silver has an important influence on the morphology of Co₃O₄ nanostructured materials. Thus, 0.25 M Ag-doped Co₃O₄ NPs show high porosity and better particle dispersion than that of Co₃O₄ nanostructured materials, which is a promising property to enhance the catalytic performance of the as-prepared nanostructure materials. In supercapacitor, this porous structure can offer wide surface area and great specific capacitance [73].

Fig. 3.

SEM images of (a) Pristine nanostructured materials at 10 μm, (b) Pristine nanostructured materials at 20 μm, (c) 0.25 M Ag-doped Co₃O₄ nanostructured materials at 10 μm, and (d) 0.25 M Ag-doped Co₃O₄ nanostructured materials at 20 μm.

3.3. BET surface area analysis

The specific surface areas, pore volume and pore radius of pristine Co₃O₄ and 0.25 M of Ag-doped Co₃O₄ NPs were determined via the BET analysis [74]. The BET, specific surface area, pore radius, and pore volume of synthesized nanostructure materials are shown in Table 2. The BET specific surface area and corresponding pore volume of the NPs were computed to be (53.066 m²/g, 0.07425 cc/g) and (407. 33 m²/g, 0.1153 cc/g) for Co₃O₄ and 0.25 M Ag-doped Co₃O₄ NPs, respectively. Accordingly, 0.25 M Ag-doped Co₃O₄ NPs exhibit the largest BET surface area, which is attributed to the porous structure. Furthermore, the pore radii of Co₃O₄ and 0.25 M Ag–doped Co₃O₄ NPs were to be found 13.85 and 11.56 Å, respectively. Porous structure promotes diffusion and transfer of electrolyte ions during charge and discharge process [75]. Hence, it was found that the surface area, pore volume and pore size of the Co₃O₄ sample were changed due to the presence of silver ions. In addition, from the SEM images, it can be seen that there is a clear distinction in morphology between pure and Ag-doped Co₃O₄ nanoparticles, indicating that the doping concentration of Ag has an important influence on the morphology of Co₃O₄ nanoparticles. Thus, 0.25 M Ag-doped Co₃O₄ nanoparticles show higher porosity and better particle dispersion than that of Co₃O₄ nanoparticles, which is a promising property to enhance the catalytic performance of the nanoparticles. A good catalytic reaction depends on the morphological characteristics of the material, which means a large surface area, good porosity. Thus, the BET surface area determines the number of active sites available for catalytic activity. As a result, a larger BET surface area is advantageous to the storage and shuttle electrons and ions in the electrode, resulting in a more active site participating for oxidation-reduction reaction leading to improved electrochemical potentials [76].

Table 2.

BET specific surface area, pore-volume, and pore radius of Co₃O₄ and Ag-doped Co₃O₄ NPs.

Samples	BET Surface area (m2/g)	Pore volume (cc/g)	Pore radius (Å)
Co3O4	53.066	0.07425	13.85
0.25 M Ag–Co3O4	407.338	0.1153	11.56

3.4. FT-IR analysis

FT-IR spectra of un-doped Co₃O₄ and Ag-doped Co₃O₄ NPs were recorded in the range 4000–500 cm⁻¹. Fig. 4 shows the FTIR spectra of Co₃ O₄ and Ag doped Co₃O₄ NPs at various doping levels. As shown Fig. 4, the bands at 3443 cm⁻¹ and 1626 cm⁻¹ correspond to the O–H stretching and O–H vibration of the adsorbed water molecules, respectively. Moreover, a weak band at 1386 cm⁻¹ is due to the presence of residual nitrogen groups that occurred during the combustion technique. Moreover, the band approximately at 1117 cm⁻¹ match to the coordination of Co–OH. Finally the characteristic peak at 620 cm⁻¹ can be related with the stretching and vibrations of Metal-oxygen bond, which confirms the spinel structure of Co₃O₄. Galini et al. stated that the two characteristic bands obtained for Co₃O₄ nanostructure materials are due to Metal-Oxygen vibrations, confirming the complete decomposition of precursors [77]. UmaSudharshini et al. Stated that the presence of two bands assigned to the vibrations of Co³⁺and Co²⁺ in an octahedral and tetrahedral site, respectively, thereby confirming the formation of the spinel structure Co₃O₄ [59]. The FTIR spectra observed in this study are in a good agreement with previous reported results [59,78,79].

Fig. 4.

FTIR spectra of Co₃O₄, Ag-doped Co₃O₄ (0.05 M), Ag-doped Co₃O₄ (0.1 M), Ag- doped Co₃O₄ (0.15 M), Ag-doped Co₃O₄ (0.2 M), Ag-doped Co₃O₄ (0.25 M) NPs.

3.5. UV–Vis analysis

UV–Vis measurements were made between 250 and 500 nm to examine the optical properties of the nanostructured materials as they were initially manufactured. The UV–visible spectra of un-doped Co₃O₄ and Ag-doped Co₃O₄ (0.05–0.25 M) NPs is shown in Fig. 5. It was found that when the amount of silver doping increased, the absorbance values in the UV–Visible spectra increased. It may be deduced that silver doping made the Co₃O₄ NPs more optically dense because the level of absorption is a sign of optical density [80]. The existence of two prominent absorption edges in the visible region of all measured spectra, which are attributed to the ligand-to-metal charge transfer event of (O²⁻ → Co²⁺) and (O²⁻ →Co³⁺) in Co₃O₄, is another intriguing aspect of the absorption spectrum. According to earlier reports in the literature [81]. This suggests the existence of two energy band gaps. With a variation in silver concentration, and Ag-doped Co₃O₄ exhibit different absorption bands. Using the Tauc relation from equation (2) the optical band gap of un-doped Co₃O₄ and Ag-doped Co₃O₄ samples (0.05–0.25 M) was computed [82].

$${\left(\mathbf{\alpha }\mathbf{һ}\mathbf{\nu }\right)}^{\mathit{n}}=\mathbf{A}\left(\mathbf{һ}\mathbf{\nu }-E_{\mathbf{g}}\right)$$ (2)

where, A is constant, α is absorption coefficient, hv is photon energy and E_g, is the bandgap energy and n is the constant that is equal to 2, 1/2, 2/3 and 1/3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively [83]. Fig. 6(a-f) shows Tauc plot energy bandgap of un-doped Co₃O₄, and Ag-doped Co₃O₄ (0.05, 0.1, 0.15, 0.2 and 0.25 M) nanostructured materials computed by extrapolating the linear part of these plots of (αhν) ² axis to (hν) axis. The Optical bandgap energy results showed below in Table 3.

Fig. 5.

UV–Vis spectrum of Co₃O₄, Ag-doped Co₃O₄ (0.05 M), Ag-doped Co₃O₄ (0.1 M), Ag-doped Co₃O₄ (0.15 M), Ag-doped Co₃O₄ (0.2 M) and Ag-doped Co₃O₄ (0.25 M) NPs.

Fig. 6.

Tauc plot Bandgap energy of (a) pristine, (b) Ag-doped Co₃O₄ (0.05 M), (c) Ag- doped Co₃O₄ (0.1 M), (d) Ag- doped Co₃O₄ (0.15 M), (e) Ag-doped Co₃O₄ (0.2 M), (f) Ag -doped Co₃O₄ (0.25 M) NPs.

Table 3.

Optical band gap values of Co₃O₄ and Ag-doped Co₃O₄ NPs.

Samples	Eg1 (eV)	Eg2 (eV)
Co3O4 NPs	1.95	2.96
Ag-doped Co3O4 (0.05 M)	1.94	2.75
Ag- doped Co3O4 (0.1 M)	1.92	2.60
Ag- doped Co3O4 (0.15 M)	1.85	2.57
Ag- doped Co3O4 (0.2 M)	1.69	2.53
Ag -doped Co3O4 (0.25 M)	1.43	2.22

3.6. Thermal (TGA/DTA) analysis

The thermal properties of the pristine and Ag-doped Co₃O₄ NPs were analyzed through TGA and DTA. The sample mass was about 10 mg in a corundum crucible, at a temperature between 25 °C and 900 °C at a heating rate of 20 °C per minute in the air atmosphere. Fig. 7, illustrates the TGA and DTA curves of the Co₃O₄ NPs and Ag- doped Co₃O₄ NPs. The TGA profiles of Co₃O₄ and Ag-doped Co₃O₄ NPs illustrate two stages of weight loss. The first weight loss of 0.37 mg for Co₃O₄ NPs between 25 °C and 262 °C and the corresponding endothermic peak observed at 135 °C may be because of the loss of absorbed water in the sample [67,84,85].

Fig. 7.

TGA and DTA curve of (a) Co₃O₄ nanostructured materials, (b) Ag-doped Co₃O₄ (0.05 M) nanostructured materials, and (c) Ag-doped Co₃O₄ (0.25 M) NPs.

With similar DTA curves at 342 and 506 °C, the second range (1.54 mg) weight loss in the range of 262–599C was connected to the breakdown of the precursor materials or residual organic ligands. The TGA/DTA thermal analysis patterns of Co₃O₄ nanostructured materials, however, show no change after 599C. When heated to 900 °C, the final mass loss of 8.09 mg from the starting weight of 10 mg equals a mass loss of 19.1% (Fig. 7(a)). Similar to this, the first mass loss of 0.4 mg is visible on the 0.05 M Ag– Co₃O₄ TGA curve between 25 and 254 °C, and the endothermic peak that corresponds to this temperature may be caused by the loss of physically adsorbed water (Fig. 7(b)) [67,86]. With matching DTA curves at 398 and 460 °C, the second range (0.13 mg) weight loss in the range of 254–444 °C was connected to the breakdown of residual organic ligands. The TGA/DTA thermal analysis patterns of 0.05 M Ag– Co₃O₄ NPs, however, show no change after 444 °C. When heated to 900 °C, the final mass loss of 9.47 mg from the starting weight of 10 mg is 5.3% mass loss (Fig. 7(b)). Additionally, the first mass loss of 0.36 mg between 25 and 260 °C is visible on the 0.25 M Ag– Co₃O₄ TGA curve, and the endothermic peak at 105 °C that corresponds to it may be caused by the loss of physically adsorbed water (Fig. 7(c)).

With matching DTA curves at 338 and 445 °C, the second range (0.12 mg) weight loss in the 260–415 °C range was connected to the breakdown or residual organic ligands. The TGA/DTA thermal analysis patterns of 0.25 M Ag– Co₃O₄ NPs, however, show no change after 415 °C. When heated to 900 °C, the ultimate mass loss of 9.52 mg from the starting weight of 10 mg is a mass loss of 4.8% (Fig. 7(c)). Due to the synergetic effects of Ag and Co₃O₄, the material exhibits quite varied thermal properties, according to thermal study. Hence, compared to virgin Co₃O₄, Ag-doped Co₃O₄ NPs exhibit substantially greater thermal stability.

3.7. Capacitive performance analysis

3.7.1. Cyclic voltammetry (CV) study

Electrochemical investigation were performed using cyclic voltammetry (CV). The electrochemical device consists of three electrodes with an electrolyte of 0.1 M KOH. Thus, analysis were applied for the investigation of capacitive characteristics of as-prepared pristine Co₃O₄ and 0.25 M Ag-doped Co₃O₄ nanostructured materials. Moreover, the CV curve were recorded in the potential range of 0.0 to +0.8V versus Ag/AgCl at a scan rate of 5, 10, 20, 50 and 100 mV/s and results are shown in Fig. 8 (a-c). Figure 8(a) shows the comparative CV curve of Co₃O₄, and 0.25 M Ag-doped Co₃O₄ nanostructured materials at a scan rate of 50 mV/s. Thus, the shape of the CV curves for Co₃O₄ and 0.25 M Ag-doped Co₃O₄ nanostructured materials shows a different shape and potential widow. Figure 8(b) shows the detailed CV curves of Co₃O₄ at different scan rates. As the scan rate was increased from 5 to 100 mV/s, the oxidation peak shifted from 0.41 to 0.49 V and the reduction peaks shifted from 0.25V to 0.2 V towards a higher and lower potential. Moreover, Figure 8(c) illustrates the CV curve of 0.25 M Ag-doped Co₃O₄ nanostructured materials. As the scan rate increased from 5 to 100 mV/s, the oxidation peak shifts from 0.28 to 0.34 V and the reduction peaks shifted from 0.134 V to 0.07 V towards a higher and lower potential, respectively, indicating an increase in internal resistance and a polarization effect at a higher scan rate [87]. Furthermore, the wide and sharp redox peaks have been seen from the nonlinear CV curves revealed that the pseudo-capacitance properties of the as-prepared Ag-doped Co₃O₄ nanostructured materials. The anode and cathode peaks are clearly visible. The sharp redox peaks were observed uniform during scanning rate increases from 5 to 100 mV/s, indicating good reversibility of redox reactions [88].

Fig. 8.

CV Curve of (a) Co₃O₄ and 0.25 M Ag-doped Co₃O₄ nanostructured materials at 50 mV/s, (b) Co₃O₄ nanostructured materials at different scan rates, and (c) 0.25 M Ag-doped Co₃O₄ nanostructured materials at different scan rates.

The specific capacities offered by Co₃O₄ and 0.25 M Ag-doped Co₃O₄ NPs were estimated by using the integral charge during the anodic/cathodic scan. The specific capacitance of pristine Co₃O₄ and 0.25 M Ag-doped Co₃O₄ nanostructured materials obtained from CV curves was estimated by employing equation (3) [89].

$${\mathrm{C}}_{\mathrm{s}}=\frac{\underset{\mathit{v}\mathbf{1}}{\overset{\mathit{V}\mathbf{2}}{\int }}\mathit{I}\mathit{d}\mathit{V}}{\mathit{m}\mathit{v}\Delta \mathit{V}}$$ (3)

where Cs is the specific capacitance (F/g), I represents the oxidation/reduction current for a given voltage V (v), V₁ is the lower potential limit, V₂ is the upper potential limit, v is the scan rate (v/s), and m is the mass of the electrode. According to the estimation, Co₃O₄ nanostructured materials electrode shows high specific capacitance values of 393.6, 328.7, 230.6, 143.1 and 109.8 F/g at scan rates of 5, 10, 20, 50 and 100 mV/s, respectively. Furthermore, 0.25 M Ag-doped Co₃O₄ nanostructured materials shows high specific capacitance values of 992.7, 757.8, 523.5, 289.3, and 249.7 F/g at the a scan rates of 5, 10, 50 and 100 mV/s,respectively. Hence, 0.25 M Ag Co₃O₄ NPs exhibits only high a specific capacitance value of 992.7 F/g at the scan rates of 5 mV/s (Fig. 9). It can be seen that specific capacities of 0.25 M Ag Co₃O₄ NPs were higher than from recently reported results (Table 4 [35,[58], [59], [60], [61],81,[90], [91], [92], [93], [94], [95], [96], [97], [98], [99]]) [59]. As a result at slow scanning rate, the movement of ions around the inner and outer surfaces of the prepared nanostructured materials, which makes it possible to obtain a high specific capacity [100].

Fig. 9.

The values of specific capacitance versus scan rate for pristine Co₃O₄, and 0.25 M Ag-doped Co₃O₄ NPs.

Table 4.

Comparison of specific capacities of various metal-doped Co₃O₄ NPs reported in literatures.

Materials	Specific capacitance	Scan rate	Electrolyte	Synthesis method	Reference
0.4 Sn doped Co3O4 nanowires	151.8 F/g	5 mV/s	6 M KOH	Hydrothermal method	[90]
Co3O4nanopowdes	291 F/g	10 mV/s	2 M KOH	Chemical reduction method	[91]
Hydrophilic Co3O4	315 F/g	5 mV/s	0.5 MNa2SO4	Galvanostatic electrodeposition	[92]
1% Mn-doped Co3O4 thin film	675 F/g	10 mV/s	0.1 M KOH	Sol–gel spin coat deposition	[93]
3% B-doped Co3O4 thin films	482.35 F/g	5 mV/s	6 M KOH	Spray deposition method	[94]
Ag-doped Co3O4 nanorods	584 F/g	5 mV/s	1 M KOH	Hydrothermal route method	[95]
Mn-doped Co3O4 nanoneedles	668.4 F/g	1 A/g	2 M KOH	one-step hydrothermal reaction	[58]
Ru:Co3O4 thin electrodes	628.33 F/g	1 mV/s	1 M KOH	Spray pyrolytic deposition	[96]
5% Cd doped porous Co3O4 nanosheet	737 F/g	A/g	6 M KOH	Co-precipitation method	[97]
2.5% Fe:Co3O4 thin films	429 F/g	2 mV/s	1 M KOH	Nebulizer spray pyrolysis	[81]
3% Cu-doped Co3O4 nanostructure materials	812 F/g	5 mV/s	6 M KOH	solvothermal method	[59]
Mn-doped Co3O4 oblique prisms	909 F/g	1 A/g	2 M KOH	Solvothermal reaction	[35]
Au - doped Co3O4	763 F/g	1 A/g	2 M KOH	Calcining Au@ZIF-67.	[98]
5%Sn-doped Co3O4 nanorods	842.44 F/g	5 mV/s	3 M KOH	Solvothermal method	[60]
5% Mo-doped Co3O4 Nanostructure materials	858.09 F/g	5 mV/s	3 M KOH	Sol-gel method	[42]
4%Sb-doped Co3O4 electrode	894.15 F/g	5 mV/s	3 M KOH	Hydrothermal method	[61]
Urchin-like Fe-doped Co3O4 microstructures	315.8C/g	1 A/g	2 MKOH	Hydrothermal method	(S [99].
0.25 M Ag-doped Co3O4 NPs	992.7 F/g	5 mV/s	0.1 M KOH	Co-precipitation method	This work

3.8. Energy and power density analysis

The energy and power density are two key parameters to evaluate the electrochemical performance of supercapacitors. We cannot find the energy density and power density directly from the 3-electrode system because the specific capacitance of the 3-electrode system is four times that of the 2-electrode system. Therefore, to calculate the energy density of a three-electrode system, divide the three-electrode capacitance by four. The energy density and power density of the pristine Co₃O₄ and 0.25 M Ag-doped Co₃O₄ nanostructured materials were obtained according to equations (4), (5) [[101], [102], [103]].

$$\mathbf{E}={\frac{\mathbf{1}}{\mathbf{2}}\mathit{C}\left(\Delta \mathit{V}\right)}^{\mathbf{2}}/\mathbf{3.6}$$ (4)

$$\mathbf{P}=\mathbf{3600}\mathbf{E}/\Delta \mathbf{t}$$ (5)

where, E, C, ΔV, P and Δt is the energy density (Wh/kg), capacitance of the electrode (F/g), potential window of device, power density (W/kg), and discharge time, respectively. The maximum energy density of the Co₃O₄ nanostructured materials was 6.69 W h/kg at a power density of 160.4 W/kg. Whereas, 0.25 M Ag doped Co₃O₄ nanostructured materials was 9.17 W h/kg at a power density of 472.1 W/kg. The power density of pristine Co₃O₄ and 0.25 M Ag-doped Co₃O₄ NPs in this study was higher than previously stated values in the literature [104].

4. Conclusion

In conclusion, pristine and Ag-doped Co₃O₄ NPs were successfully prepared via the Co-precipitation method with various doping concentration. XRD pattern confirmed the unchanged crystal structures of Co₃O₄ and the decrease in crystallite size with the addition of dopant Ag. SEM confirmed the formation of structures having porous structure with small spherical grains, porous structures with sponge-like structures, loosely packed porous structures with small cracks/holes, and flower-like porous morphology, respectively. Pristine Co₃O₄ and 0.25 M Ag-doped Co₃O₄ nanostructured materials have BET surface areas of 53.06 m²/g, and 407.338 m²/g, respectively. The bandgap energy values of Co₃O₄ nanostructured materials were 2.96 eV, with an additional sub-band gap energy of 1.95 eV. Furthermore, the band gap energies of Ag-doped Co₃O₄ nanostructured materials were found to be between 2.22 and 2.75 eV for all samples, with an additional sub-band corresponding to energies varying between 1.43 and 1.94 eV. The bandgap energy decreased with increasing Ag doping concentration and the obtained bandgap energy range confirmed the semiconducting nature of the prepared Ag-doped Co₃O₄ NPs. The presence of M−O bonds (M = Co, Ag) was analyzed by FTIR spectroscopy. The Ag-doped Co₃O₄ NPs show improved thermal properties owing to the doping effect of silver ions. The electrochemical study were done with the help of CV analysis in a 0.1 M KOH electrolyte solution. The influence of Ag concentration on the capacitance characteristics of Co₃O₄ nanostructured material was tested by conducting CV. In comparison to other pristine and Ag-doped Co₃O₄ samples, the 0.25 M Ag-doped Co₃O₄ sample showed the highest specific capacitance values of 992.7 F/g at a scan rate of 5 mV/s, due to its larger surface area from the reduced particle size. The TGA result confirmed that Ag-doped Co₃O₄ nanostructured materials show much stronger thermal stability than Co₃O₄ nanostructured materials. Furthermore, the energy density and power density of the Co₃O₄ and 0.25 M Ag-doped Co₃O₄ nanostructured materials were 6.69 W h/kg, 160.4 W/kg, and 27.9 W h/kg, 3816.1 W/kg, respectively.

Author contribution statement

Asab Fetene Alem, Ababay Ketema Worku: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Temesgen Atnafu Yemata, Delele Worku Ayele, Nigus Gabbiye Habtu: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Tessera Alemneh Wubieneh, Alebel abebaw, Tadele mihret kndie, Bimrew Tamrat Admasu, Minbale Admas Teshager, Addisu Alemayehu Asege, Mehary Dagnew Ambaw, Misganaw Alemu Zeleke, Alemayehu Kifle Shibesh: Conceived and designed the experiments; Wrote the paper.

Funding statement

This research did not receive any specific grant from Funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data will be made available on request.

Declaration of interest's statement

The authors declare no conflict of interest.