Electrochemical Deposition of Multicomponent Mixed Metal Oxides on rGO/Ni Foam for All-Solid-State Asymmetric Supercapacitor Device: Mn, Co, and Ni Oxides with Ag Doping

: In this study, an asymmetric supercapacitor (ASSC) device is assembled by the deposition and annealing of silver-doped mixed metal oxides on reduced graphene oxide (rGO)/Ni foam and activated carbon (AC) on Ni foam as positive and negative electrodes, respectively. The best performing Ag:MnCoNiO active material is synthesized on rGO/Ni foam using chronopotentiom-etry combined with heat treatment. The XRD study clearly conﬁrms the crystalline nature of the electrode with MnCo 2 O 4 and MnNi 2 O 4 phases. FT-IR and XPS studies revealed the formation of Ag:MnCoNiO/rGO on Ni foam. SEM images show a thin-ﬁlm layer of fabricated material on the surface of rGO/Ni foam. The supercapacitor properties were tested in two-and three-electrode conﬁgurations, with cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electro-chemical impedance spectroscopy (EIS) experiments in a 6 M KOH aqueous electrolyte. In the three-electrode conﬁguration, reversible faradic reactions can be observed in a potential range of 0.0 and +0.6 V vs. Hg/HgSO 4 . In the two-electrode device conﬁguration, the system exhibits a maximum energy density of 45.5 Wh kg − 1 and provides a maximum power density of 4.5 kW kg − 1 . The results showed that the doping of Ag in a MnCoNiO electrode shows promising properties, achieved by a very simple fabrication process. The results showcase the synergistic effects achieved by mixed multiple-component metal oxides, leading to improved supercapacitive properties.


Introduction
The growth in the industry of portable electronic devices and electric vehicles has motivated extensive research aiming to develop practical energy storage systems [1][2][3][4]. In this regard, fuel cells and lithium-ion batteries are considered to be prominent candidates for energy storage in the foreseeable future. Supercapacitors (SCs) have emerged as an alternative for such devices [5][6][7][8] due to certain advantages, such as their low-cost fabrication, power density, rapid charge/discharge rate, and durability. Even so, their limited energy density is a major barrier for their further commercialization [9,10]. In this regard, the fabrication of innovative materials with suitable electrochemical properties to increase the energy density in these devices is a key challenge to be overcome [11,12].
The selection of electrode material plays a key role in SC performance, as it determines the capacitance of the device, which is grouped into two main categories (i) pseudocapacitance and (ii) electrical double-layer capacitance (EDLC) [2,13]. In general, conducting polymers and various metal oxides are employed as the active electrode materials in pseudocapacitors, whereas EDLCs are based on nanostructured carbon materials, such as activated carbon (AC), carbon nanotubes, and graphene and its derivatives [14]. Alternatively, the construction of hybrid materials consisting of carbon and metal oxides has been studied as efficient materials to enhance the specific capacitance of the SCs [15].
Mixed transition metal oxides (MTMOs), due to their variable oxidation states, enable various redox processes and satisfactory structural properties are potential next-generation materials for high-performance SCs [15][16][17]. Further, carbon-based materials, such as rGO, carbon nanotubes, and activated carbon, have proven to be promising for hybrid materials, owing to their high surface area and good electrical conductivity. In recent years, much attention has been devoted to combining metal oxides with carbon-based materials that could further boost the electrochemical performance of SCs [8,11,18]. As electrodes in SCs, hybrid materials, especially with MTMOs, sometimes suffer from poor electrical conductivity and instability in contact with the electrolyte, which deteriorates the charge storage performance of the electrode. To overcome this drawback, noble metals, such as platinum [19], ruthenium [20], gold [21], or silver [22], in combination with nanomaterials, have gained attention as a way to improve the electrical conductivity on metal oxides. As a consequence of noble metal scarcity, the price of fabrication for these materials increases with increased loading. Therefore, the loading should be kept as low as possible.
Additionally, the fabrication technique is also important for the applicability of SC devices. State-of-the-art techniques for the synthesis of transition metal oxides and compositions include, but are not limited to, hydrothermal [7], chemical vapor deposition [23], spray pyrolysis [24], sol-gel [25], and electrodeposition [26]. Of all the techniques, the applicability of electrodeposition in the coating of the active material on the substrate has gained great interest due to its cost effectiveness, ease of scale-up, and facile operation [27,28].
Motivated by the advantages mentioned above, the present study aims to investigate the facile electrodeposition approach for the preparation of multiple-component mixed metal oxide, namely CoNi, MnCoNi, and Ag-doped MnCoNiO (Ag:MnCoNiO) deposited on rGO, and implementation as hybrid materials. These materials were tested as positive electrodes combined with activated carbon as a negative electrode in an asymmetric supercapacitor. All the electrochemical analysis is conducted in a 6 M KOH aqueous electrolyte solution. Structural and morphological properties were determined on Ag:MnCoNiO/rGO due to the highest displayed performance among all electrodes based on electrochemical analysis. Furthermore, the asymmetric supercapacitor (ASSC) performance of Ag:MnCoNiO/rGO as the positive electrode is further tested by assembling a device with AC as the negative electrode, exhibiting specific capacitance of 478.9 F/g at a scan rate of 5 mV s −1 .

Synthesis of Graphene Oxide (GO)
GO powder was synthesized using the improved Hummer's method [29]. The procedure was applied as follows, briefly: 1 g of pure graphite powders was firstly mixed with 30 mL concentrated H 2 SO 4 and 3.3 mL H 3 PO 4 in an ice bath and stirred for 30 min with a magnetic stirrer. Thereafter, 6 g KMnO 4 was slowly added to the above solution with magnetic stirring. The mixture was then heated at 35 • C and stirred for 2 h. Then, 100 mL of deionized (DI) water and 10 mL of H 2 O 2 (30%) were dropped gradually into the solution with continuous stirring, which turned the color of the solution yellowish. The resulting mixture was separated by centrifugation. Ethanol and 5% HCl solution was used to wash the slurry followed by deionized water up to pH = 7. Lastly, the slurry was dried in a drying oven at 60 • C for 12h and ground into a brown powder to be used in characterization.

Synthesis of MTMO Electrodes
The synthesis procedure of Ag:MnCoNiO/rGO film on Ni foam was conducted in two parts: (i) rGO was fabricated on Ni foam by electrochemical reduction of GO. In this process, 1 g/L of GO was dissolved in 0.1 M KCl aqueous solution, stirred magnetically for 1 h, and then continued to shake ultrasonically for 1 h to homogenize the electrolyte. Before the fabrication, Ar gas was used for 10 min to purge dissolved oxygen from the electrolyte. Electrochemical reduction of GO on Ni foam was carried out by cyclic voltammetry in a potential range from −1.0 V to −1.4 V at a scan rate of 10 mV/s for 10 cycles. (ii) Ag-doped MnCoNiO film was synthesized by a facile electrodeposition method on rGO/Ni foam. In a typical preparation, 0.01 M nickel (II) acetate tetrahydrate, 0.01 M manganese (II) acetate tetrahydrate, 0.02 M cobalt (II) acetate tetrahydrate, 0.01 M sodium sulfate, and 0.5 mM silver nitrate were dissolved in DI water under vigorous stirring for 10 min to form a homogenous electrolyte before the fabrication. Regarding CoNiO synthesis, the electrolyte consisted of 0.02 M cobalt (II) acetate tetrahydrate, 0.01 M sodium sulfate, and 0.01 M nickel (II) acetate tetrahydrate. For the MnCoNiO preparation, 0.01 M manganese (II) acetate tetrahydrate was added to the above mixture. All the film depositions were performed by chronopotentiometry at a constant current density of −5 mA cm −2 for 30 min on rGO/Ni foam. At the end, the fabricated samples were washed with DI water, dried in an oven (Heraeus instruments, Hanau, Germany, Trockenschrank UT 612) at 60 • C, and annealed at 500 • C with 10 • C/min for 2 h in a tube furnace. The mass loading of Ag:MnCoNiO/rGO was measured by high-precision scale (Shimadzu, Kyoto, Japan, Advanced Performance Uniblocbalance) and was about 1.2 mg in all experiments.

Characterization
The structural properties of the samples were taken by X-ray diffraction (XRD) in a range between 10 • and 90 • using Bruker D8 advanced diffractometer apparatus equipped with CuKα radiation source (λ = 1.5418 Å). Scanning electron microscopy (SEM; Hitachi TM3030 PLUS, Tokyo, Japan) equipped with energy dispersive X-ray analysis (EDX) was used to analyze the morphology and elemental composition of the fabricated samples. The elemental composition was carried out by X-ray photoelectron spectroscopy (XPS, VersaProbe II, PHI, Japan). FT-IR spectra were taken by a Bruker Vertex 80 spectrometer in the 400-4000 cm −1 region.

Preparation of an All-Solid-State Supercapacitor and Electrochemical Analyses
The activated carbon electrode was fabricated by mixing activated carbon, binder (PVDF), and conductive carbon (80:10:10) in a mortar to make a slurry with N-methyl-2pyrrolidone (NMP) solvent. The slurry was pasted on the Ni foam (1 cm × 1 cm) and dried at 60 • C in an oven for 12 h to be used as negative electrode. A gel electrolyte was prepared by dissolving 3 g of PVA and 3 g of KOH in 20 mL of DI water at 85 • C. After drying, in order to construct the ASSC device, the Ag:MnCoNiO on rGO/Ni foam was dipped in the gel electrolyte for 10 min.
The electrochemical measurements, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) were conducted by Bio-Logic VSP-3e (France) electrochemical workstation in a cell equipped with 99.99% platinum electrode as the counter electrode, Ag:MnCoNiO/rGO as a working electrode, and a mercury/mercurous sulphate (MMS) (Hg/HgSO 4 ) as the reference electrodes, respectively. A 6.0 M KOH solution was used for all tests. The CV with various scan rates and GCD under different current densities with a potential window range from 0.0 to +0.6 V was applied at room temperature. Electrochemical impedance spectroscopy (EIS) was carried out within a frequency range from 1 Hz to 100 kHz using an AC voltage with 10 mV amplitude at open-circuit potential. For the ASSC device, we developed an all-solid-state asymmetric supercapacitor (ASSC). In this configuration, Ag:MnCoNiO/rGO, as the best performing material, was used as the positive electrode and activated carbon was used as the corresponding negative electrode. Based on CV and GCD curves, the specific capacitance (C S ), energy density (E c ), and power density (P c ) can be evaluated by the equations given below [30,31]: where I dV is the integrated area under CV curve, v is the scan rate, m is the mass of the active material in three-electrode system (total mass of both electrodes in the case of two-electrode system), ∆t is the discharging time, and ∆V is the potential window.

Structural and Morphological Properties
In order to investigate the nature of the material, a structural analysis of Ag:MnCoNiO on rGO/Ni foam electrode was performed by XRD and the resulting diffraction pattern is depicted in Figure 1.  [32]. However, there is a small shift observed in the XRD diffraction peaks according to these JCPDS cards due to the partial substitution of the Ag + ions by Mn 2+ ions. A possible reason for this substitution could because Ag + ions (1.28 Å) have as larger ionic radius than Mn 2+ (0.80 Å). The intense diffraction peaks obtained for the electrode, imply the poly-crystalline metal oxide nature. The peaks correspond to cubic MnNi 2 O 4 spinel phase at 2θ = 18.9 • (hkl = 111), 30.7 • (hkl = 220), 58.8 • (hkl = 511), and 63.1 • (hkl = 440) with a lattice constant of a = b = c = 0.813 nm. The diffraction peaks at 2θ = 37.1 • , 43.1 • , and 62.3 • for MnCo 2 O 4 can be assigned to the (222), (400), and (440) lattice planes of cubic spinel structure, respectively. Also, the lattice parameters a, b, and c for MnCo 2 O 4 is 0.831 nm. There were no reflection peaks related to any impurities. In addition, no peaks associated with Ag were detected due to its low loading (while it appears in XPS), proving that Ag was successfully doped into the mixed metal oxide. The XRD result confirms the crystalline nature of the material.
The surface elemental compositions and oxidation state of the Ag:MnCoNiO/rGO electrode were analyzed by X-ray photon spectroscopy (XPS), using a Gaussian fitting model. Figure 2a- C-OH, C=O, and O=C-OH functional groups [35]. The deconvoluted O 1s XPS spectrum ( Figure 2f) displays three peaks located at 529.8, 530.2, and 531.5 eV, which are representative of Co/Mn/Ni-oxygen bonding, C=O, and O-H, respectively [13,32]. Figure 2g shows the XPS spectrum of Ag 3d doublets with binding energies located at 374.8 and 368.8 eV, which are ascribed to the Ag 3d3/2 and Ag 3d5/2 transitions [36]. This is in good agreement with the work conducted by Potlog et al., who reported that the appearance of these peaks is related to the metallic silver (Ag 0 ) [37]. The XPS analysis further indicates the successful fabrication of Ag:MnCoNiO/rGO, which is supported by XRD and EDX. The FT-IR technique was used to find the characteristic functional bonds in an electrode. Figure 3a depicts the FT-IR spectra of the GO, rGO, and Ag:MnCoNiO materials in a range between 400 and 4000 cm −1 . Regarding GO powder, the presence of the wide band nearly at 3200 cm −1 comes from O-H stretching vibration and physically adsorbed water molecules [38,39]. The other three characteristic absorption bands are indicative features  Figure 2a shows the survey spectrum for the fabricated electrode, which reveals the presence of Ni 2p, Mn 2p, Co 2p, C 1s, O 1s, and Ag 3d, consistent with EDX analysis (Figure 4). The XPS spectrum of Ni (Figure 2b) shows doublet peaks positioned at 872.5 eV and 854.7 eV, which are assigned to Ni 2p 1/2 and Ni 2p 3//2 , respectively, and it also displays multiple splitting with two satellite peaks of Ni 2p 880.2 and 861.6 eV. The corresponding satellite peaks of Ni 2p indicate the presence of a spin-orbital region of NiO [33]. Figure 2c depicts the binding energies of 642and 654 eV, which are attributed to Mn 2p 3/2 and Mn 2p 1/2 split orbital, respectively [26]. The XPS spectra of Co 2p (Figure 2d) demonstrates two pairs of peaks positioned at 779.3 and 794.5 eV, related to the characteristic spin-orbit doublets of Co 2p 1/2 and Co 2p 3/2 , respectively. In addition, there are two shakeup satellite (sat.) peaks at 801.5 and 786.8 eV, which are characteristic lines of high-spin Co 2+ [34].
The peak for C 1s XPS spectra (Figure 2e), located at around 285.5 eV, is assigned to C-C, C=C and C-H bonds. The binding energy of 289.3 eV can be assigned to C-OH, C=O, and O=C-OH functional groups [35]. The deconvoluted O 1s XPS spectrum (Figure 2f) displays three peaks located at 529.8, 530.2, and 531.5 eV, which are representative of Co/Mn/Nioxygen bonding, C=O, and O-H, respectively [13,32]. Figure 2g shows the XPS spectrum of Ag 3d doublets with binding energies located at 374.8 and 368.8 eV, which are ascribed to the Ag 3d 3/2 and Ag 3d 5/2 transitions [36]. This is in good agreement with the work conducted by Potlog et al., who reported that the appearance of these peaks is related to the metallic silver (Ag 0 ) [37]. The XPS analysis further indicates the successful fabrication of Ag:MnCoNiO/rGO, which is supported by XRD and EDX.
The FT-IR technique was used to find the characteristic functional bonds in an electrode. Figure 3a depicts the FT-IR spectra of the GO, rGO, and Ag:MnCoNiO materials in a range between 400 and 4000 cm −1 . Regarding GO powder, the presence of the wide band nearly at 3200 cm −1 comes from O-H stretching vibration and physically adsorbed water molecules [38,39]. The other three characteristic absorption bands are indicative features of the GO, which are located at 1047 cm −1 (C-O), 1634 cm −1 (C=C), and 1733 cm −1 (C=O) [40,41]. After electrochemical reduction of the GO, most of the oxygen-containing functional bands, such as hydroxyl groups (O-H), are not visible in the FT-IR spectra of the rGO. For the Ag:MnCoNiO electrode, the strong and sharp signal around 600 cm −1 might be related to the stretching vibration of the metal-oxygen bond (Mn,Co,Ni-O) [42,43].
A scanning electron microscope (SEM) image of Ag:MnCoNiO on rGO/Ni foam electrode is shown in Figure 3b. From the high-magnification image, it shows that the sample has densely packed clusters with some cracks throughout the whole substrate. These cracks could be beneficial, since they can provide easier accessibility of K + ions for enhancing specific capacitance [44]. The EDX spectrum of Ag:MnCoNiO on rGO/Ni is shown in Figure 4a

Electrochemical Characterization
In order to explore the significance of the components in the hybrid electrodes and the effects of the mixed metal oxides and compare their electrochemical behavior, a series of samples were tested: (1) bare Ni foam, (2) rGO on Ni foam, as well as (3) CoNiO, (4) MnCoNiO, and (5) Ag:MnCoNiO on rGO/Ni foam electrodes. All samples were tested with electrochemical techniques, such as cyclic voltammetry (CV), galvanostatic chargedischarge (GCD), and electrochemical impedance spectroscopy (EIS), in a three-electrode cell configuration in 6 M KOH aqueous electrolyte.
A CV comparison of the produced samples is depicted in Figure 5a, which was obtained at a scan rate of 100 mV s −1 in a potential range between 0.0 and +0.6 V.  A scanning electron microscope (SEM) image of Ag:MnCoNiO on rGO/Ni foam electrode is shown in Figure 3b. From the high-magnification image, it shows that the sample has densely packed clusters with some cracks throughout the whole substrate. These cracks could be beneficial, since they can provide easier accessibility of K + ions for enhancing specific capacitance [44]. The EDX spectrum of Ag:MnCoNiO on rGO/Ni is shown in Figure 4a, which confirms the presence of Mn, Co, Ni, O, and Ag, revealing the successful formation of Ag:MnCoNiO on rGO/Ni foam electrode and successful Ag doping. The atomic percentage of the electrode comprises 44.11% O, 2.31% S, 17.35% Co, 31.17% Ni, 1.67% Mn, 2.46% Na, 0.72% Ag, and 0.19% Cl. The elemental EDX mapping of Ag:MnCo-NiO is shown in Figure 4b and it confirms the homogenous distribution of Mn, Co, Ni, O, and Ag elements on the rGO/Ni foam surface. As can be clearly seen from Figure 5a, all the electrodes have broad oxidation waves in the anodic and a reduction wave in the cathodic scan. The presence of these peaks indicates the pseudocapacitive nature of the electrode material, which is dominated by Faradic redox reactions [2,45]. Compared to other electrodes, bare Ni foam has a very small integral area in the CV curve; hence, the specific capacitance contribution from the substrate can be neglected. It can be clearly seen from Figure 5a that from the CV profiles of five samples, the maximum integrated area under the CV curve is observed for the Ag:MnCoNiO on rGO/Ni foam, suggesting that it has the most pronounced (pseudo)capacitive behavior, owing to the improved conductivity by Ag doping. Additionally, Ag:MnCoNiO on rGO/Ni foam has the highest current density compared to all electrodes tested, indicating a fast redox reaction process. Moreover, the redox peaks of MnCoNiO are significantly larger compared with the CoNiO, indicating that the contribution of the Mn and each individual element is significant. Figure 5b illustrates the corresponding comparison of GCD curves for fabricated electrodes measured in the same potential range at a current density of 1 A/g. As can be seen clearly from Figure 5b, the GCD curves at a current density of 1 A/g for all the fabricated materials display typical charge-discharge curve shapes, suggesting that the capacitive contribution of these samples is generally due to the faradic reactions occurring at the electrode surface [46]. Apparently, all the samples show GCD curves approximately symmetrical and nonlinear in shape, revealing the good reversibility and high coulombic efficiency of the material. The Ag:MnCoNiO on the rGO/Ni foam electrode has the longest discharge time compared to the other electrodes, which indicates that this electrode has a better specific capacitance out of all the electrodes tested in this work. This result is also in agreement with the CV results. The discharging times are decreasing with current density, signifying that a high current density shortens the necessary time required for ions to migrate into the electrode [47]. Figure 5c shows a typical CV curve for Ag:MnCoNiO on the rGO/Ni foam electrode with various scan rates from 5 to 100 mV s −1 . There is no change in the shape of the CV curves for each scan rate, but a shift in the peak locations in the CV curves can be seen with the increase in scan rates. This shift can be explained by the polarization of the electrolyte at higher scan rates [13,48]. As shown in CVs in Figure 5c, there is a broad reduction peak positioned at +0.3 V, which is ascribed to the reduction of mixed metal oxides (M +4 /M +3 to M +2 and M +2 to M 0 ). In the reverse scan, there are two peaks located at +0.2 and +0.1 V, signifying oxidation of mixed metal oxides (M 0 to M +2 and further M +2 to M +4 /M +3 ) [49]. Similar CV behavior was also observed in the work conducted by Singh et al. [50]. The specific capacitance values for the Ag:MnCoNiO on the rGO/Ni foam electrode are calculated by Eq. 1 and found to be 280.3, 329.8, 378.7, 413.3, and 441.7 F/g at scan rates of 100, 50, 20, 10, and 5 mV/s, respectively. Compared to previous studies, the calculated specific capacitances for the fabricated electrode are comparable. Talluri et al. [6] synthesized (CoCrFeMnNi) 3 O 4 via a reverse co-precipitation process and they reported specific capacitance 239 F g −1 at 0.5 A g −1 from GCD curves. Further, as the scan rate increases, the specific capacitance values decrease. Such a significant decrease in capacitance with increasing scan rate is likely to be caused by the drop in accessibility of the K + ions in the solution, which cannot diffuse deep into the active sites of the electrode at higher scan rates [6].

Electrochemical Characterization
In order to explore the significance of the components in the hybrid electrodes and the effects of the mixed metal oxides and compare their electrochemical behavior, a series of samples were tested: (1) bare Ni foam, (2) rGO on Ni foam, as well as (3)    A CV comparison of the produced samples is depicted in Figure 5a, which was obtained at a scan rate of 100 mV s −1 in a potential range between 0.0 and +0.6 V. As can be clearly seen from Figure 5a, all the electrodes have broad oxidation waves in the anodic and a reduction wave in the cathodic scan. The presence of these peaks indicates the pseudocapacitive nature of the electrode material, which is dominated by Faradic redox reactions [2,45]. Compared to other electrodes, bare Ni foam has a very small integral area in the CV curve; hence, the specific capacitance contribution from the substrate can be neglected. It can be clearly seen from Figure 5a that from the CV profiles of five samples, the maximum integrated area under the CV curve is observed for the Ag:MnCoNiO on rGO/Ni foam, suggesting that it has the most pronounced (pseudo)capacitive behavior, owing to the improved conductivity by Ag doping. Additionally, Ag:MnCoNiO on rGO/Ni foam has the highest current density compared to all electrodes tested, indicating a fast redox reaction process. Moreover, the redox peaks of MnCoNiO are significantly larger compared with the CoNiO, indicating that the contribution of the Mn and each individual element is significant. Figure 5b illustrates the corresponding comparison of GCD curves for fabricated electrodes measured in the same potential range at a current density of 1 A/g. As can be seen clearly from Figure 5b, the GCD curves at a current density of 1 A/g for all the fabricated materials display typical charge-discharge

Electrochemical Performance of All-Solid-State ASSC Device
To test further practical application, we attempted to construct an ASSC device with Ag:MnCoNiO on the rGO/Ni foam as the positive electrode and AC on Ni foam as the negative electrode ( Figure 6).
Before device construction, a comparison of the CV results for both the positive and negative electrode in a three-electrode system at a scan rate of 100 mV s −1 is depicted in Figure 7a. Figure 7b depicts the CV curve of the constructed ASSC device at 100 mV s −1 , with a different potential window, verifying that the device could function suitably up to 1.8 V. The specific capacitance variations with respect to the scan rate and the inset show a series of CV curves for the ASSC device under various scan rates 5-100 mV s −1 at a potential range of 0.0 V and 1.8 V, shown in Figure 7c. It can be seen that no obvious deformation of the CV curves is visible and the reduction peak progressively shifted to a lower potential when the scan rate increased. This shift can be attributed to the polarization of the electrolyte [13]. From the CV curves, the specific capacitance values of the device are found to be 161.5, 210, 286.9, 389.3, and 478.9 F/g at scan rates of 100, 50, 20, 10, and 5 mV/s, respectively. Figure 7d displays the GCD curves of the ASSC device under different current densities within a potential range of 0-1.8 V. The observed quasi-symmetrical curves have long discharging times, revealing a good capacitive be-havior and good electrochemical reversibility. The specific capacitance decreases with increasing current density, which may be due to the decrease in mobility of the ions in the electrolyte into the inner surface of the electrode material [51]. The energy density and power density were calculated with Equations (2) and (3) for two-electrode systems. The device provided a maximum energy density of 45.5 Wh/kg and possessed maximum power density of 4500 W/kg. It can be seen from Figure 7e that the cyclic stability of the assembled ASSC device retains 86% of the coulombic efficiency at 5 A/g over 5000 cycles, which further confirms high charge-discharge reversibility of the ASSC device. A literature review is listed in Table 1, showing an overview of device performance of mixed metal oxide supercapacitors. For example, Zhang et al. [52] fabricated Co 0.5 Mn 0.4 Ni 0.1 C 2 O 4 * H 2 O electrodes via a room temperature chemical co-precipitation method and they found 0.46 mWh cm −3 and capacitance retention of 98.6% after 6000 cycles. Talluri et al. [6] prepared (CoCrFeMnNi) 3 O 4 electrode using a reverse co-precipitation approach and, according to their findings, the electrode provides 24.1 Wh kg −1 at a current density of 0.5 A g −1 . Biswal et al. [53] synthesized a cobalt-nickel-copper ternary metal oxide via electrochemical deposition followed by calcination and the device exhibited 188 F g −1 at a current density of 0.1 A g −1 and 95.1% capacitance retention after 1000 cycles. This short literature review shows that the MTMO-based supercapacitor shows comparable properties, while being fabricated in a simple, electrochemical deposition method, without any additional surface structuring. but a shift in the peak locations in the CV curves can be seen with the inc rates. This shift can be explained by the polarization of the electrolyte at hig [13,48]. As shown in CVs in Figure 5c, there is a broad reduction peak posi V, which is ascribed to the reduction of mixed metal oxides (M +4 /M +3 to M +2 a In the reverse scan, there are two peaks located at +0.2 and +0.1 V, signifying mixed metal oxides (M 0 to M +2 and further M +2 to M +4 /M +3 ) [49]. Similar CV also observed in the work conducted by Singh et al. [50]. The specific capac for the Ag:MnCoNiO on the rGO/Ni foam electrode are calculated by Eq. 1 be 280.3, 329.8, 378.7, 413.3, and 441.7 F/g at scan rates of 100, 50, 20, 10, an spectively. Compared to previous studies, the calculated specific capacitanc ricated electrode are comparable. Talluri et al. [6] synthesized (CoCrFeMnN verse co-precipitation process and they reported specific capacitance 239 F g from GCD curves. Further, as the scan rate increases, the specific capacitan crease. Such a significant decrease in capacitance with increasing scan rate caused by the drop in accessibility of the K + ions in the solution, which canno into the active sites of the electrode at higher scan rates [6].

Electrochemical Performance of All-Solid-State ASSC Device
To test further practical application, we attempted to construct an ASSC Ag:MnCoNiO on the rGO/Ni foam as the positive electrode and AC on N negative electrode ( Figure 6).   Before device construction, a comparison of the CV results for both the positive and negative electrode in a three-electrode system at a scan rate of 100 mV s −1 is depicted in Figure 7a. (f) Nyquist diagram. Additionally, the internal resistances of the device were also investigated by performing EIS tests (Figure 7f). The data were fitted using EC-lab software to the equivalent electric circuit shown at the inset. It can be observed that the spectra have two different components: (a) a semi-circle in the high-frequency region and (b) a straight line in the low-frequency region. The circumference of the semi-circle at high frequency is indicative of charge-transfer resistance (R CT ), correlated with the Faradic reactions of the device involved in the insertion/extraction of K + ions [62]. The straight line at low frequency is attributed to Warburg impedance, which refers to the diffusion resistance of K + ions into the material [63]. The evaluated value of the solution resistance (R S ), the charge-transfer resistance (R CT ), Warburg impedance (W 2 ), and capacitance (Q 1 ) of the device were found to be 2.1 Ω, 10.9 Ω, 7.243 ohm.s −1/2 , and 1.434.10 −3 F, respectively.

Conclusions
In conclusion, CoNi, MnCoNi, and Ag-doped MnCoNiO MTMOs were grown on rGO/Ni foam and characterized by electrochemical and surface science methods. An allsolid-state ASSC device was further assembled by using the best performing Ag:MnCoNiO on rGO/Ni foam as the positive electrode and AC as the negative electrode. The SEM investigations reveal the uniform distribution of MnCoNiO on rGO/Ni foam. XRD, XPS, FT-IR, and EDX characterizations support the formation of the Ag:MnCoNiO on rGO/Ni foam. In a three-electrode setup, the fabricated electrode showed a specific capacitance of 441.5 F/g at scan rate of 5 mV s −1 . In the two-electrode setup, the device exhibited an energy density of 45.5 Wh kg −1 and retained 86%. These values are also comparable with other non-structured metal-oxide-based supercapacitors. Furthermore, the experiments showed that the synergistic contribution of all the constituents gives rise to the behavior of the mixed metal oxide, not directly predictable from the individual constituents, and that leaving out one element changes the properties significantly.
Author Contributions: Field data collection, analysis, and original draft preparation: Y.E.F.; writingreview, proof reading, and editing: V.Č. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Data Availability Statement:
The data presented in the study are available on request from the corresponding author.