Preparation of Silver Molybdate-Decorated Reduced Graphene Oxide Nanocomposite Using Ionic Liquids for High-Performance Energy Storage Application: A Greener Approach

Abstract

Achieving high energy density while maintaining high power density and long cycle life in supercapacitors, particularly in supercapatteries (SCs), through a thermally stable, greener ionic liquid approach remains a significant challenge for an advanced energy storage application. In this work, we prepared high conductive and high charge storage capability bimetallic transition metal molybdate [Ag₂Mo₂O₇ (AgM)], synergistic with reduced graphene oxide (rGO) coated on nickel foam (AgM/rGO/NF). The physio-chemical characterization revealed a ball-like cluster morphology wrapped in rGO nanosheets and a spinel-type cubic structure using scanning electron microscopy (FE-SEM) displays and X-ray diffraction (XRD) analyses. Further, the electrochemical performance of AgM/rGO/NF electrode achieved a remarkable specific C_sp value of 573.63 F/g at a current density of 1.0 A/g in 3 M KOH electrolyte. An asymmetric SCs (ASCs) device was fabricated using AgM/rGO/NF as the positive and rGO as the negative electrodes, achieving a wide potential window of 1.3 V. The ASC demonstrated an energy density of 16.71 Wh/kg at a power density of 642.98 W/kg, highlighting AgM/rGO/NF’s potential as an advanced electrode material for energy storage applications.

1. Introduction

The increasing emissions of greenhouse gases resulting from fossil fuel combustion and industrial activities present a significant threat to the global environment [1]. As human development progresses, the continued reliance on fossil fuels raises concerns about a potential future energy crisis and environmental degradation [2]. This necessitates the need to transition toward sustainable energy sources, such as wind, solar, and tidal power. While these sources offer promising solutions, their intermittent nature highlights the urgent need for advanced energy storage systems to ensure a sustainable future [3].

In this regard, supercapatteries (SCs) are considered highly promising energy storage devices due to their unique characteristics, including long cycle life, high power density, reasonable energy density, and eco-friendly fabrication [4,5]. An SC comprises four primary components: positive and negative electrodes, a separator, an electrolyte, and current collectors. Energy is stored and released at the electrodes during the charging and discharging processes, which involve both ionic and electronic transport. An ion-conductive electrolyte facilitates ionic transport while inhibiting electron movement between the electrodes. The current collectors enable electron flow through the external circuit, and the separator prevents short circuits while ensuring efficient ion transmission [6]. Among these components, the electrode plays a crucial role in determining the performance of SCs [7]. These electrodes operate via two charge storage mechanisms: non-faradaic and faradaic processes. In non-faradaic processes, charge storage occurs through the electrochemical double-layer capacitance (EDLC) through physical separation at the electrode/electrolyte interface. On the other hand, faradaic processes involve ion movements into and out of the electrode, resulting in changes in the redox state of metal and charge transfer. Faradaic charge storage can be divided into two types: pseudocapacitive (PSCs), where charge storage occurs on the surface, and battery-type (BSCs), where it happens in the bulk. Among these mechanisms, PSCs are preferred because of their fast, reversible chemical nature which provides high surface area, high conductivity, and greater capacitance that are desirable for enhanced energy density in PSC materials [6,8,9]. Enhancing energy density while maintaining high power density and long cycle life remains a significant challenge in SC technology [2]. Therefore, improving the energy density by increasing the potential window is vital, as it is proportional to the square of the voltage window. This can be achieved by assembling two different materials for the positive electrode (cathode) and negative electrode (anode) in asymmetric SCs (ASCs) [1,3,10]. For efficient ASC fabrication, conducting polymers and transition metal oxide (TMO)-based PSC materials are preferable due to their cost-effectiveness, unique physiochemical properties, high theoretical capacitance, and fast charge transfer kinetics [10].

Among various TMOs, including ruthenium oxide, cobalt oxide, nickel oxide, iron oxide, vanadium oxide, copper oxide, tungsten oxide, iridium oxide, manganese oxide, molybdenum oxide (MoO₃), and silver oxide (Ag₂O), Ag₂O stands out as a more stable material due to its affordability, environmental friendliness, and favorable electrochemical properties compared to the environmentally toxic Ru₂O. Ag₂O’s suitability for energy storage applications stems from its ability to exist in multiple oxidation states (+1, +2) and form various oxide phases (AgO, Ag₃O₄, Ag₄O₃, Ag₂O₃, and Ag₂O), enabling enhanced charge storage capacity. However, Ag₂O suffers from low conductivity due to its insulating nature [8]. This limitation can be addressed by combining Ag₂O with molybdate oxides (MoO_x where X = 1, 2, 3, and 4), which exhibit improved conductivity due to multiple redox states (Mo³⁺ to Mo⁶⁺) and high chemical stability provided by strong covalent bonds. The bimetallic transition metal molybdate Ag₂Mo₂O₇ (AgM) synergistically combines the high charge storage capability of Ag₂O with the improved conductivity and stability of MoO_x, making it an excellent candidate for advanced energy storage applications [11,12].

Recently, several studies have demonstrated the potential of Ag₂Mo₂O₇ for advanced energy storage applications. For instance, Ana Silvia González et al. synthesized high-performance 3D nanostructured silver electrodes using an antireplica/replica template-assisted method for micro-supercapacitors, achieving 95% capacitance retention after 2600 cycles [8]. Similarly, Amirreza Safartoobi et al. developed bead-like Ag₂MoO₄ nanofibers coated on Ni foam for dual visible light-driven heterogeneous photocatalysis and high-performance supercapacitor electrodes, reporting 93% capacitance retention after 5000 cycles [11]. J. Johnson William et al. synthesized mesoporous β-Ag₂MoO₄ nano-potatoes via a rapid wet chemical route, achieving 82% capacitance retention after 5000 cycles [12]. S. Rajkumar et al. synthesized nanostructured Ag₂Mo₂O₇ via a one-step precipitation process, obtaining 85.2% capacitance retention after 5000 cycles [9]. However, these methods often involve toxic chemicals, are time-consuming, and require high-cost specialized equipment. Consequently, a multifunctional, non-flammable, greener ionic liquid solvent approach is preferable for producing high-quality materials with efficient electrochemical performance [13]. In this work, Ag₂Mo₂O₇ was synthesized using a greener approach involving 1-butylimidazolium glycolate ionic liquid solvent and subsequently composited with reduced graphene oxide (rGO) to enhance conductivity and improve the electrochemical performance of SCs. The rGO in the AgM/rGO composite plays a dual role: it removes oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl, creating a conductive network that accelerates charge transfer and it serves as a protective layer to maintain the stability of Ag₂Mo₂O₇ particles [3]. As a result, the fabricated AgM/rGO/NF electrode exhibits a high specific capacitance of 573.63 F/g, contributing to an enhanced energy density of 16.71 Wh/Kg at a power density of 642.29 W/kg for AgM/rGO||rGO/NF ASCs device. The results authenticate that the assembled device holds great promise as a potential electrode material for advanced energy storage applications.

2. Experimental Methods

2.1. Required Chemicals

Silver nitrate hexahydrate (AgNO₃.6H₂O, ≥99.0%), ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄.4H₂O, ≥98.0%), 1-butylimidazolium glycolate, ethanol (C₂H₆O, ≥99.8%), hydrochloric acid (HCl, 37%), potassium hydroxide (KOH, ≥99.0%), carbon black (C₅, ≥97.0%), polyvinylidene fluoride (PVDF: (CH₂CF₂)_n, ≥99.0%), and n-methyl pyrrolidone (NMP: C₅H₉NO, ≥99.0%) were obtained from M/s Sigma-Aldrich, Taiwan, and used without further purification. Deionized water (DI.H₂O, ≥99.0%) was used throughout the experiments.

2.2. Synthesis of AgM and AgM/rGO

AgM was synthesized by dissolving 1 mmol of (NH₄)₆Mo₇O₂₄.4H₂O (solution A) and 2 mmol of AgNO₃.6H₂O (solution B) separately in 50 mL of DI.H₂O. Solution A was then added drop-wise to solution B under vigorous stirring at 80 °C for 30 min. Subsequently, 2 mL of previously synthesized 1-butyl imidazolium glycolate ionic liquid was added and centrifuged, washed with ethanol, and DI.H₂O. The resulting product was then dried at 80 °C, yielding Ag₂Mo₂O₇ nanoparticles, which were named as AgM.

For the preparation of the AgM/rGO composite, highly conductive rGO prepared via a modified Hummer method was incorporated into the AgM [14]. The fabrication of the electrode materials in this study employed a slurry preparation method, which represents a solution-processing route. Consequently, the AgM/rGO composite was prepared using the typical grinding method in 1:1 ratio. The detailed synthesis process is illustrated in Figure 1 and used to analyze the physio-chemical nature of as-prepared AgM/rGO.

2.3. Physical Characterization

To investigate the physical properties of the as-prepared AgM, rGO, and AgM/rGO material, various analytical technique was employed. In detail, field emission scanning electron microscopy (FESEM-SM-7500F manufactured by JEOL, Japan), paired with energy dispersive X-ray spectroscopy (EDX), was used to visualize the surface morphology and elemental composition of the material. The X-ray diffraction (XRD, X’Pert Pro by PANalytical, Malvern, UK) and X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Inc., Chigasaki, Japan/PHI 5000 Versa Probe III) were utilized to assess the purity, crystal structure, and chemical states of the prepared materials.

2.4. Electrochemical Characterization

2.4.1. Electrode Preparation of Synthesized Material

To evaluate the electrochemical performance, electrodes were prepared using nickel foam (NF) cut into 1 × 1 cm² pieces. Prior to the coating process, the surface-oxide layer, and impurities on the NF were removed by washing with a 10% HCl solution, followed by washing with 10 mL of ethanol and DI.H₂O for 10 min under sonication. Subsequently, the NF was dried under N₂ gas atmosphere to prevent oxidation and degradation, which helps maintain structural integrity. The cleaned and highly conductive NF was then used for coating.

For the coating process, a slurry was initially prepared by mixing active material (AgM and AgM/rGO), carbon black, and PVDF in the ratio of 70:20:10, using NMP as the solvent. After thoroughly grinding the mixture, the slurry was homogenously coated onto the surface of NF and dried at 60 °C for 12 h. The coated NF was then used as the working electrode for electrochemical analyses.

The electrochemical behavior of the electrodes was studied using a Metrohm Auto Lab-(PGSTAT302N) electrochemical workstation. The electrochemical characteristics and reaction kinetics of prepared electrodes were analyzed using technique such as cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS). From the CV and GCD data, the areal capacitance (C_areal) from CV and specific capacitance (C_sp) from GCD were calculated using Equations (1) and (2) [15,16,17,18], as shown below:

C_areal = ∫IdV/A × Ʋ × ΔV(1)

C_sp = I × Δt/m × ΔV(2)

In the equation, “C_areal” stands for areal capacitance (F/cm²), “∫IdV” is the integrated area under the CV curve, “Ʋ” stands for scanrate (V/s), “Cₛₚ” stands for specific capacitance (F/g), “I” represents discharge current (A), “A” indicates active material area of electrode, “m” denotes the mass of the active material, “Δt” signifies discharge time (s), and “Δv” refers to the working potential window.

2.4.2. Assembling of Asymmetric Supercapattery Device

To evaluate the real-time application of the AgM/rGO/NF electrode, an asymmetric supercapattery (ASC) was assembled in a sandwich-type configuration. In this setup, the AgM/rGO/NF composite served as the positive electrode, while rGO was used as the negative electrode. The fabricated ASCs are designated as AgM/rGO||rGO/NF. A filter membrane soaked in 3 M KOH acted as both the separator and an electrolyte (charge) reservoir. The separator was made larger than the working electrode to prevent short circuit.

To validate the practicability of the assembled AgM/rGO||rGO ASCs, the energy and power densities were calculated using Equation as shown in (3) and (4) [15,18,19];

E_s = ½ C_sp ×ΔV²(3)

Ps = E_s × 3600/Δt(4)

In the equation, “Eₛ” refers to the energy density (Wh/kg), while “Cₛₚ” stands for specific capacitance (F/g) derived from GCD curves. “ΔV” indicates the potential range of the assembled device, “Pₛ” denotes the power density (W/kg), and “Δt” represents the discharge time in seconds (s).

3. Results and Discussions

3.1. Surface Analyses

The surface morphology and elemental composition of the as-prepared AgM, rGO, and AgM/rGO composite materials were analyzed using FE-SEM, and the results are depicted in Figure 2a–m. The different magnifications of AgM in Figure 2a–c show uniform, dense, ball-like clusters. These clusters were formed when (NH₄)₆Mo₇O₂₄ and Ag (NO₃)₂ were dissolved in green solvent H₂O in the presence of the templating agent, 1-butyl imidazolium glycolate ionic liquid. The ionic liquid plays a critical role in controlling the morphology of AgM. During the synthesis process, the dissolved molecules dissociated into Ag²⁺ and MoO₄²⁻ ions, which became encapsulated by the ionic liquid. This encapsulation creates a highly structured solvation shell around the ions, stabilizing them in the solution phase. The cation (1-butyl imidazolium) and anion (glycolate) can interact with the precursors and intermediate species, leading to anisotropic growth of the material. The electrostatic interactions between the ions and the ionic liquid molecules contribute to the ordered arrangement of the solvation shell [20]. When oppositely charged ions approach each other, the protective solvation shell breaks under electrostatic forces, enabling the nucleation and formation of AgM (Ag₂Mo₂O₇). Additionally, the unique physicochemical properties of the ionic liquid, such as its hydrogen bonding, high ionic conductivity, and reduced surface tension, promote anisotropic growth and self-assembly of the nanoparticles. As the Ag₂Mo₂O₇ particles collide within the ionic liquid environment, the electrostatic and van der Waals forces facilitate their adhesion and growth into larger, ball-like clusters. The ionic liquid not only stabilizes the particles but also directs their aggregation into dense, spherical morphologies by preventing excessive agglomeration. This templating action enables precise control over the morphology and uniformity of the Ag₂Mo₂O₇ nanoparticles. The detailed mechanism of AgM formation is illustrated in Figure 3, and the corresponding chemical reactions are presented in Equations (5)–(7) [21];

(NH₄)₆Mo₇O₂₄ + H₂O → 6NH₄⁺_(aq) + 7MoO4²⁻_(aq)(5)

2 AgNO₃ → 2Ag⁺ + 2NO₃^–(6)

6NH₄⁺_(aq) + 7 MoO₄^2–_(aq) + 2Ag⁺ + 2NO₃^– → 2Ag₂Mo₂O₇ + 6NH₄⁺ + 2NO₃^–(7)

In addition, the prepared rGO, as shown in Figure 2d, displays densely arranged, multilayered, corrugated sheets-like morphology. The corrugation at the edges of the nanosheets is a result of the loss of oxygen-containing functional groups during the reduction process of GO. This reduction leads to strong attractions and interactions between the surface groups on the graphene-like layers [2,22,23]. After the incorporation of the rGO sheets into the AgM material, an AgM/rGO composite is formed and wrapped over the ball-like cluster of AgM, as depicted in Figure 2e–g. AgM morphology is maintained after incorporation with rGO nanosheets, providing more active sites that facilitate greater ion adsorption [24]. Furthermore, the EDX and mapping analyses in Figure 2h–m demonstrate the homogenous distribution of Ag, Mo, O, and C in AgM/rGO composite, signifying a synergistic effect that enhances electron/ion transport, improved stability, and facilitated redox activity, contributing to efficient electrochemical performance [25,26,27].

3.2. XRD Analysis

The crystal structure and phase purity of the as-prepared AgM, rGO, and AgM/rGO material were examined using XRD analysis, as shown in Figure 4. The AgM displayed diffraction peaks at 2θ angle of 14.1°, 23.5°, 24.4°, 25.9°, 26.9°, 28.4°, 29.1°, 30.2°, 31.8°, 31.9°, 32.4°, 32.9°, 33.9°, 35.1°, 35.8°, 37.1°, 37.9°, 39.3°, 43.1°, 43.9°, 44.9°, 45.6°, 48.1°, 48.8°, 49.6°, 51.0°, 51.4°, 52.7°, 53.2°, 54.3°, 55.6°, 55.7°, 56.7°, 58.7°, 59.5°, 60.8°, 62.8°, 64.9°, 66.7°, and 67.9° corresponding to the (0 1 0), (0 −1 2), (−1 −1 1), (0 −2 1), (−1 0 2), (0 2 0), (−1 −1 2), (1 −2 2), (−1 1 2), (−1 2 1), (−2 0 1), (2 0 0), (0 1 2), (0 −1 3), (0 2 1), (−1 −2 2), (1 −3 2), (−2 −1 1), (1 1 2), (1 −3 3), (−2 0 3), (1 0 3), (0 −2 4) (−3 0 1), (−3 1 2), (−3 0 2), (−1 −2 4), (0 −3 4), (0 −4 2), (−1 3 2), (0 −4 3), (−2 −1 4), (−3 −1 2), (2 −2 4), (−1 4 1), (−4 2 0), (2 −4 4), (2 −5 3), (−4 0 1) and (−4 4 0) planes, respectively. These peaks indicate an anorthic crystal system with a P-1 space group, confirmed by JCPDS no. 75-1505, validating an Ag₂Mo₂O₇. Furthermore, no peaks corresponding to metallic Ag, AgO, Ag₂O, Ag₃O₄, or MoO₃ were observed, confirming the formation of coherent Ag₂Mo₂O₇ material [11,12,21].

For the AgM/rGO composite, peak were observed at 2θ angle of 14.1°, 23.5°, 24.4°, 25.9°, 26.9°, 28.4°, 29.1°, 30.2°, 31.8°, 31.9°, 32.4°, 32.9°, 33.9°, 35.1°, 35.8°, 37.1°, 37.9°, 39.3°, 43.1°, 43.9°, 44.9°, 45.6°, 48.1°, 48.8°, 49.6°, 51.0°, 51.4°, 52.7°, 53.2°, 54.3°, 55.6°, 55.7°, 56.7°, 58.7°, 59.5°, 60.8°, 62.8°, 64.9°, 66.7°, and 67.9° corresponding to the (0 1 0), (0 −1 2), (−1 −1 1), (0 −2 1), (−1 0 2), (0 2 0), (−1 −1 2), (1 −2 2), (−1 1 2), (−1 2 1), (−2 0 1), (2 0 0), (0 1 2), (0 −1 3), (0 2 1), (−1 −2 2), (1 −3 2), (−2 −1 1), (1 1 2), (1 −3 3), (−2 0 3), (1 0 3), (0 −2 4) (−3 0 1), (−3 1 2), (−3 0 2), (−1 −2 4), (0 −3 4), (0 −4 2), (−1 3 2), (0 −4 3), (−2 −1 4), (−3 −1 2), (2 −2 4), (−1 4 1), (−4 2 0), (2 −4 4), (2 −5 3), (−4 0 1) and (−4 4 0) planes of AgM. Additionally, a graphite peak at 2θ angle of 26.48°, and 44.22° corresponding to the (002) and (100) plane, were observed, confirming the presence of highly electrically conductive rGO synergized with AgM. These peaks match the hexagonal structure of the sp² carbon, which are in good agreement with JCPDS no. 41-1487, confirming the removal of oxygen-containing functional groups such as hydroxyl, epoxy, and carbonyl group during the reduction process of GO [28,29,30,31,32,33]. Furthermore, the diffraction peak positions validate the successful incorporation of rGO into the AgM material, indicating a strong interaction of the composite material [2].

3.3. XPS Analysis

To analyze the specific electronic states and validate the chemical oxidation state of the as-prepared AgM/rGO composite material, XPS analysis was studied and the results are shown in Figure 5a–d. The results clearly show the presence of Ag 3d, Mo 3d, O 1s, and C 1s elements in AgM/rGO material. In Figure 5a, the high-resolution spectrum of Ag 3d shows two peaks for Ag 3d_5/2, and Ag 3d_3/2 at a binding energy of 367.38 eV and 373.39 eV. Generally, the spin-orbit Ag 3d can exist in two oxidation states such as monovalent silver ions (Ag⁺) and metallic silver (Ag⁰). The energy gap between Ag 3d_5/2 and Ag 3d_3/2 is 6.01 eV indicating the presence of higher oxidation states, confirming the existence of Ag²⁺ and the absence of ionic Ag⁺ and Ag⁰ in the sample [11,21].

Further, the high-resolution Mo 3d in Figure 5b exhibits two peaks at spin-orbit of Mo 3d_5/2 and Mo 3d_3/2 at binding energy around 231.52 eV and 234.78 eV. This indicates that the material exhibits oxidation on the surface of MoO_x (x = 1, 2, 3, and 4) [34]. In addition, the high-resolution O 1s spectrum in Figure 5c indicates the existence of two deconvoluted peaks at a binding energy of around 529.94 eV and 531.77 eV, ascribed to the metal-oxide (M–O) and O–C=O, respectively [11].

Additionally, the incorporation of rGO into the AgM material is validated through a high-resolution C 1s spectrum. The results displayed in Figure 5d reveal the presence of C=C/C–C, C–OH, and O–C=O functionalities at binding energy around 283.59 eV, 285.01, and 288.81 eV, respectively [35]. This suggests that the prepared material has reduced oxygen-containing functional groups, which typically hinder electron/ion transport [9]. Concurrently, this treatment enhances the formation of sp² hybridization, which is highly conductive due to the continuous delocalization of π-electrons. These delocalized electrons facilitate efficient electron transfer across the rGO sheets, creating more pathways for electron mobility and improving the AgM/rGO electrochemical performance [36]. This further confirms the successful incorporation of rGO sheets onto the surface of AgM, contributing to enhanced conductivity and electrochemical performance [2,35,37,38,39].

3.4. Electrochemical Studies

3.4.1. Electrochemical Evaluation in Three-Electrode Systems

The electrochemical behavior of the as-prepared electrodes was evaluated using a 3-electrode setup in a 3 M KOH electrolyte and the results are presented in Figure 6a–d. The CV curves of the as-prepared AgM/NF and AgM/rGO/NF electrodes were recorded at a scan rate of 50 mV/s within a potential window of −0.3 to +0.6 V, as displayed in Figure 6a. From the results, the AgM/rGO/NF electrode exhibits an enlarged CV area due to the transfer of improved electrochemical active sites provided by composited rGO, which helps to enhance the performance of the electrode [12]. This configuration increases the distance between the graphene sheets, which allows easier penetration of electrolyte ions [40]. As a result, enhanced ion transport leads to a higher current response, indicating a quasi-conversion reaction mechanism with efficient electron movement and favorable reversibility during the faradaic process [11].

In the framework of pseudocapacitive storage, the electrochemical behavior observed for the AgM/NF and AgM/rGO/NF electrodes, as highlighted in the CV analysis involved electron transfer and charge redistribution. The three anodic current peaks (A1, A2, and A3) observed during the CV scan correspond to the sequential oxidation of silver at different stages. The first peak (A1) arises from the dissolution of silver as it reacts with hydroxide ions (OH⁻) from the electrolyte solution, forming silver hydroxide complexes ([Ag(OH)₂]⁻) on the surface of the electrode. The [Ag(OH)₂]⁻ complex can diffuse into the solution and, upon reaching a supersaturated state, adhere to the electrode surface, forming Ag₂O. This is a typical pseudocapacitive reaction, where the surface redox reactions contribute to charge storage. The electrochemical oxidation and reduction of silver ions contribute to the overall energy storage mechanism. The peak A2 represents the further oxidation of Ag₂O into AgO, indicating a continuation of the pseudocapacitive charge storage process, while the A3 peak, which appears during the reverse scan, corresponds to the further electrooxidation of silver metal to form Ag₂O. This quasi-reversible process ensures that silver ions are continuously cycled between their oxidized and reduced states, enabling efficient energy storage and release. The cathodic peaks (C1 and C2) observed during the reverse scan represent the reduction of AgO to Ag₂O and, ultimately, the reduction of Ag²⁺ ions back to Ag⁰. The peak C1 corresponds to the reduction of silver and oxygen species, reinforcing the reversible nature of the pseudocapacitive mechanism. The C2 peak, which appears at a more negative voltage, represents the final reduction of Ag²⁺ to Ag⁰, completing the cycle of energy storage and release. The pseudocapacitive mechanism is based on the electrochemical reaction of silver with hydroxide ions, where the charge storage is due to the redox processes involving silver in different oxidation states (Ag⁰, Ag²⁺, AgO, and Ag²O). These reactions lead to a high surface area for ion interaction, allowing the electrode to efficiently store and release charge. The equations for the pseudocapacitive storage mechanism can be summarized as follows [8]:

Ag + 2OH⁻_ads ↔ [Ag (OH)₂]⁻_ads + e⁻(8)

[Ag (OH)₂]⁻_ads ↔ [Ag (OH)₂]⁻_aq(9)

2Ag + 2OH⁻ ↔ Ag₂O + H₂O +2e⁻(10)

Ag₂O + 2OH⁻ ↔ 2AgO + H₂O +2e⁻(11)

2AgO + H₂O +2e⁻ ↔ Ag₂O + 2OH⁻(12)

Significantly, the presence of the Mo atom increases electrical conductivity without involving in the reaction. The synergetic nature of Ag and Mo enhances the electrical conductivity and improves the electrochemical behavior of the as-prepared AgM/rGO/NF electrode [11]. Furthermore, to understand the nature of charge storage, CV at various scan rates were recorded and presented in Figure 6b. The corresponding redox reaction of AgM/rGO/NF electrode maintained its shape while increasing its scan rate with an increase in the current response. This behavior indicates that the prepared AgM/rGO/NF electrode exhibits stable charge storage performance, validating the complete conversion of Ag to AgO (oxidation) and Ag²⁺ to Ag⁰ (reduction) processes [11].

To analyze the rate performance of the as-prepared AgM/rGO/NF electrode, the material exhibits a specific capacitance of 771.38 F/g at 50 mV/s (areal capacitance of 2700 mF/cm²) and a high capacitance value of 1055.83 F/g at 5 mV/s (areal capacitance of 3695 mF/cm²). This happens because at high scan rates, the electrolyte ions are limited to the surface of the electrode whereas at lower scan rates, ions can penetrate deeper into the electrode, allowing for better diffusion and resulting in higher capacitance [41,42]. This characteristic reveals that the electrode exhibits pseudo-capacitance behavior where electron transfer during redox reactions is highly dependent on the voltage scan rate, primarily due to polarization within the electrode material [8]. In addition, to evaluate the charge-discharge efficiency, the GCD curves were analyzed at various current densities within a potential window of −0.2 to +0.4 V, as presented in Figure 6c. The GCD measurement exhibits a non-linear curve with sudden distortion during the discharge process. This behavior is apparent that the prepared AgM/rGO/NF electrode undergoes a quasi-conversion reaction involving a faradaic process at the electrode-electrolyte interface, consistent with the redox peaks observed in the CV analysis. These results further validate the pseudocapacitive nature of the electrode, where redox reactions contribute to energy storage and release [43]. The C_sp was calculated and observed to be 573.63 F/g at a current density of 1 A/g, and the increasing current density gradually decreases the discharge time. This phenomenon occurs due to high current density which leads to a greater voltage drop that mitigates the effectiveness of the faradaic process. The sudden potential drop may be caused by a quasi-conversion reaction, where unoxidized species undergo oxidation during discharge. This behavior is due to the electrochemical nature of metallic Ag which directly oxidizes from the 0 to +2 state by losing two electrons [12].

The Nyquist plot in Figure 6d provides the kinetics of the AgM/rGO/NF electrode at its open circuit potential (OCP). The semicircle in a high-frequency region defines a charge transfer controlled process (R_ct), indicating the rate at which charge can transfer between the electrode and the electrolyte [44,45,46]. The R_ct value of the as-prepared AgM/rGO/NF was observed to be 0.76 Ω. Further, the point at which the semicircle touches the X-axis provides a solution resistance (R_s) value. The as-prepared AgM/rGO/NF exhibits 1.64 Ω of R_s which indicates high conductivity that gives efficient charge transfer at the electrode/electrolyte interface, facilitating ion permeation and enhancing overall electrochemical performance [47].

3.4.2. Electrochemical Evaluation of AgM/GO Electrode in a Two-Electrode System

From the result of the three-electrode system, the as-prepared AgM/rGO/NF electrode was chosen to investigate the practicable applicability by assembling it into ASC. In ASCs fabrication, the composite AgM/rGO/NF serves as the positive electrode and rGO as the negative electrode (AgM/rGO||rGO/NF ASCs), using 3 M KOH electrolyte and a filter paper as a separator. The rGO exhibits a large working potential window in negative potential regions (−0.9 to 0 V), and a high theoretical surface area with high capacitance [3]. The operating potential window for AgM/rGO||rGO/NF ASCs is achieved to be 1.3 V. The CV curves for the prepared AgM/rGO/NF (−0.3 to 0.6 V) and rGO (−0.9 to 0 V) were recorded at 50 mV/s and are presented in Figure 7a. The assembled AgM/rGO||rGO/NF ASCs device result reveals the combination of EDLC from the negative rGO electrode and pseudo-capacitance from the positive AgM/rGO/NF electrode [2,48,49]. Further, the CV curves at different scan rates were recorded and displayed in Figure 7b, validating the quasi-rectangular CV curves with distinct redox peaks demonstrating a non-capacitive faradaic energy storage mechanism. The high current response at a high scan rate of 50 mV/s along with a sustained CV curve, suggests that the AgM/rGO||rGO/NF ASCs device exhibits excellent reversibility and strong rate capability.

In addition, the GCD curves of AgM/rGO||rGO/NF ASC device at different current densities confirm the faradic process, characterized by a non-linear quasi-symmetric charge/discharge curve that indicates balanced charge storage between the positive and negative electrode (Figure 7c). Figure 7d displays the Nyquist plot of the assembled ASCs device which exhibits 1.85 Ω of R_s and 9.35 Ω of R_ct in high-frequency regions, highlighting efficient electron/ion transport. Furthermore, the life cycle of assembled ASCs device was assessed by performing 5000 continuous charge/discharge cycles at a constant current density of 4 A/g (Figure 7e). The high C_sp in the initial cycle is due to the continuous penetration of ions in the electrolyte and the slight decrease in C_sp at the last 100 cycles is due to the occurrence of numerous redox reactions in assembled ASCs device [8,50]. The resultant device maintains 92.1% capacitance retention indicating the stable life of the assembled ASCs device.

To investigate the energy and power densities of assembled AgM/rGO||rGO/NF ASCs device, a Ragone plot was plotted by comparing with previous pieces of literature and is presented in Figure 7f [51,52,53,54,55,56]. The result reveals that the assembled device exhibits an energy density of 16.71 Wh/Kg at a power density of 642.98 W/Kg, which aligns with the excellent super-capacitive performance of AgM/rGO||rGO/NF ASCs [48,49,50,51,52,53]. Therefore, the extended potential window and balanced electron/ion transfer between the electrodes authenticate that the assembled AgM/rGO||rGO/NF ASCs device has been a potential candidate for advanced energy storage systems [57].

4. Conclusions

In summary, an eco-friendly and highly efficient AgM/rGO composite was successfully synthesized using a simple, cost-effective, and green ionic liquid method. The resulting AgM/rGO material exhibited a distinctive ball-like cluster morphology enveloped by rGO nanosheets and displayed a spinel-type cubic structure. The presence of Ag, Mo, and O in AgM, along with C=C bonds in rGO confirmed the successful formation of AgM/rGO composite. To evaluate its electrochemical performance, electrodes comprising AgM/NF, rGO/NF, and AgM/rGO/NF were analyzed in a 3 M KOH electrolyte. Among these, AgM/rGO/NF demonstrated superior performance, achieving a higher C_sp of 573.63 F/g at a current density of 1.00 A/g. Furthermore, to estimate the practicability, an ASC device (AgM/rGO||rGO/NF) was fabricated and exhibited a wide potential window of 1.3 V with a capacitance retention of 92.1%. The fabricated device delivered an energy density of 16.71 Wh/kg at a power density of 642.98 W/kg proposing that the AgM/rGO||rGO/NF composite will be a potential and sustainable electrode material for advanced energy storage applications.

Author Contributions

C.M.B.: Conceptualization, Investigation, Methodology, Writing—original draft, Writing—review and editing. Z.A.: Methodology, Validation. M.S.: Funding acquisition, Resource, Project administration, Supervision. A.V.M.: Formal analysis. C.K.: Data curation, Investigation, Software, Supervision, Writing—review and editing. A.-Y.L.: Resource, Validation, Project administration. Y.-J.H.: Validation and Formal analysis. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors extend their appreciation to the Deanship of Research and Graduate Studies at King Khalid University for funding this work through Large Research Project under grant number RGP2/209/45.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures

Figure 1. Schematic representation of the AgM/rGO composite material.

Figure 2. (a–c) FE-SEM image of AgM, (d) rGO, (e–g) AgM/rGO composite, (h) EDX, (i–m) EDX mapping of AgM/rGO composite material.

Figure 3. Formation mechanism of self-assembly of AgM material.

Figure 4. XRD analysis of AgM, rGO, and AgM/rGO composite material.

Figure 5. XPS spectrum of (a) Ag 3d spectrum, (b) Mo 3d spectrum, (c) O 1s spectrum, (d) C 1s spectrum of prepared AgM/rGO composite material.

Figure 6. (a) CV scan rate of AgM/NF and AgM/rGO/NF electrodes at 50 mV/s, (b) different scan rates, (c) GCD at different currents, and (d) EIS of AgM/rGO/NF electrodes (Inset: Randel’s circuit).

Figure 7. (a) CV curve of prepared positive and negative AgM/rGO/NF and rGO/NF electrodes, (b) CV curve at various scan rates of assembled AgM/rGO||rGO/NF ASCs device, (c) GCD profile at different current densities, (d) Nyquist plot (Inset: Randel’s circuit), (e) capacitance retention, and (f) Ragone plot of assembled AgM/rGO||rGO/NF ASCs device.

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