Exploiting spinel manganese oxide decorated with silver nanoparticles as electrodes for supercapacitor application | Scientific Reports

Scientific Reports volume 15, Article number: 21597 (2025) Cite this article

This work presents a novel approach to the development of supercapacitor technology through the integration of a gel polymer electrolyte (GPE) and Ag nanoparticle (NP) modified Mn3O4 electrodes. To the best of our knowledge, this is the first study to employ a GPE comprising poly(vinylidene fluoride)-co-hexafluoropropylene (PVdF-HFP) as the host polymer, propylene carbonate (PC) as the plasticizer, and magnesium perchlorate (Mg(ClO4)2) as the salt, in conjunction with Ag NP-modified Mn3O4 electrodes. The study also introduces a pioneering low-temperature ultrasonication method for the attachment of Ag NPs to Mn3O4, which eliminates the need for a reducing agent. This approach is characterized by its simplicity, cost-effectiveness, and scalability, offering significant advantages over conventional methods. The electrochemical performance of the resulting supercapacitor cells, featuring the modified electrodes and novel GPE, was comprehensively evaluated, yielding a single electrode specific capacitance of 9.38 F g⁻¹, with an energy density of 1.9 Wh kg⁻¹, and a power density of 30.8 W kg⁻¹. The findings demonstrate the potential of this new system to enhance energy storage capabilities, marking a substantial advancement in supercapacitor research, and this study sets the foundation for future investigations into scalable, high-performance energy storage solutions, emphasizing both innovation in material design and process optimization.

The use of supercapacitors is becoming increasingly prevalent in a variety of applications that require the delivery or storage of energy rapidly and efficiently. These include regenerative braking systems in electric vehicles, backup power systems, and a variety of consumer electronics, where the ability to rapidly charge and discharge is crucial. Furthermore, supercapacitors are being employed in renewable energy systems to mitigate fluctuations in power output1,2. Supercapacitors, also called ultracapacitors, are energy storage devices that are capable of storing and delivering energy at a considerably faster rate than conventional batteries1,2. They can be classified into two primary categories: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs utilize electrostatic charge separation at the interface between an electrode and an electrolyte to store energy, whereas pseudocapacitors employ faradaic redox reactions involving electron transfer to achieve energy storage2,3. The materials most commonly utilized as electrodes in supercapacitors include activated carbon (employed in EDLCs due to their elevated surface area), metal oxides (such as RuO2 and MnO2), and conducting polymers (including polyaniline and polypyrrole), which are frequently used in pseudocapacitors due to their ability to undergo reversible redox reactions2,3.

Transition metal oxides have been the subject of considerable research interest as electrode materials for supercapacitors, due to their high theoretical capacitance, excellent electrochemical stability, and environmentally benign nature2,3,4,5,6. Among these, manganese oxides (MnO2, Mn3O4, etc.) are of particular interest due to their low cost, natural abundance, and multiple oxidation states, which facilitate efficient redox reactions and are therefore essential for high energy and power density7,8.

Manganese oxide in the form of Mn3O4, a mixed-valence compound containing Mn2+ and Mn3+ ions, has been identified as a promising candidate for use in supercapacitor electrodes. Mn3O4 exhibits notable electrochemical properties, including a high specific capacitance, which is a consequence of its unique spinel structure (where Mn2+ ions occupy the tetrahedral site and Mn3+ ions occupy the octahedral site of the crystal structure) that provides abundant active sites for intercalation and deintercalation of ions7,8. Various synthesis techniques have been utilized with the objective of optimizing the morphology, surface area, and crystalline structure of Mn3O4, each contributing to the electrochemical performance of the material8,9,10,11,12. The most prevalent techniques employed for the synthesis of Mn3O4 encompass hydrothermal, sol-gel, and co-precipitation methodologies, among others8,9,10,11,12. Li et al.9 designed a straightforward one-step hydrothermal process to facilitate the growth of Mn3O4 nanorods on Ni foam. The resulting nanorods, with diameters of approximately 100 nm and lengths between 2 and 3 μm, demonstrated a notable specific capacitance of 263 F g⁻¹ at 1 A·g⁻¹9. The electrochemical performances were realized in a three-electrode system, with 4 M NaOH liquid electrolyte solution9. Raj and co-authors10 investigated a facile synthesis of Mn3O4 nanoparticles at room temperature via a simple chemical precipitation method. The electrochemical performance of Mn3O4 nanoparticles yielded a specific capacitance of 322 F g⁻¹ at 0.5 mA cm− 2, a good rate capability of 223 Fg− 1 at 15 mA cm− 2, as well as a notable cycling life, with capacitance retention at 77% after 1000 cycles10. Luo et al.11 proposed a simple method to fabricate Mn3O4 cubes, which, on electrochemical examinations, delivered an excellent specific capacitance of 667 F g− 1. at a scan rate of 1 mV s− 1, 583 F g− 1 at a low current density of 1 A g− 1, respectively. The EIS tests were performed in a three-electrode system, utilizing a 6 M KOH liquid electrolyte solution11. Fang et al.12 employed a colloidal method to synthesize ultrafine Mn3O4 nanowires, resulting in a specific capacitance of 433.1 F g− 1 at a current density of 0.5 A g− 1, with a voltage range from − 0.5 to 1.1 V in a 1 M Na2SO4 electrolyte, in a three-electrode system cell examination. The material was also tested in two configurations, using a cathode electrode consisting of ultrafine Mn3O4 nanowires and an active carbon (AC) anode electrode12. The constructed device exhibited a high energy density of 26.7 Wh kg⁻¹ at a power density of 442 W kg⁻¹, and revealed excellent cycling stability, retaining 75.8% of its initial capacitance value after 64,000 charge/discharge cycles12.

However, the intrinsic conductivity of Mn3O4 is relatively low, which can limit its overall electrochemical performance7,8,9,10,11,12. In this regard, substantial research has been undertaken into surface modification strategies to address this limitation. These strategies involve the incorporation of conductive additives, the creation of composite structures, or doping with other elements to enhance electrical conductivity and electrochemical activity. The modifications can considerably improve charge transfer kinetics, cycling stability, and specific capacitance. For instance, Cuéllar-Herrera et al.13 employed milling and ultrasonic processes to fabricate Mn3O4/rGO composites. A composition of 70% Mn3O4 and 30% rGO exhibited a specific capacitance value of 525 Fg− 1 at 5 mV s− 1 in a three-electrode cell13. The material was also examined in a symmetrical configuration, demonstrating a stability of 85% after 5000 charge-discharge cycles. The specific energy density was found to be 6.25 Wh kg⁻¹ at a power density of 125 Wh kg⁻¹ and a current density of 0.5 Ag⁻¹, and 5.36 Wh kg⁻¹ at a power density of 500 Wh kg⁻¹ and a current density of 2 A g⁻¹13. A facile one-step solvothermal process was proposed to obtain a flowerlike Mn3O4/rGO compound by Zhang et al.14. The electrochemical evaluation was done in a three-electrode system using 2 M KOH liquid electrolyte, in which the Mn3O4/rGO electrode displayed a high specific capacity of 118.4 mAh g− 1 at a current density of 1 A g− 1, accompanied by an excellent cycling stability of 88.1% after 4000 cycles at a current density of 10 A g− 114.

In a recent study, Chinnaiah and colleagues15 employed a green sol-gel synthesis involving an extract of Withania somnifera to synthesize an Ag/Mn3O4 nanocomposite. The achieved Ag/Mn3O4 electrode was examined in 1 M KOH liquid electrolyte and revealed a specific capacitance of 338 F g⁻¹ at a current density of 1 A g⁻¹, with a cyclic retention of 87.4%15.

The objective of this study is to examine the influence of modifying the surface of manganese oxide (Mn3O4) with a spinel structure through the incorporation of silver nanoparticles (Ag NPs) on its energy storage potential. Building on our prior works, in which we successfully demonstrated the surface modification of activated carbon16, multiwalled carbon nanotubes17, and cobalt oxide5 with Ag NPs for use as electrode materials in symmetric supercapacitor configurations, the current research offers a novel extension of this approach. Our previous studies demonstrated that the integration of Ag NPs significantly enhanced the electrochemical performance of these materials, reinforcing the effectiveness of our methodology.

This study introduces a novel application of a gel polymer electrolyte (GPE) for the evaluation of supercapacitor performance. To the best of our knowledge, this represents the first investigation utilizing a GPE composed of poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) as the host polymer, propylene carbonate (PC) as the plasticizer, and magnesium perchlorate (Mg(ClO4)2) as the conducting salt, in combination with manganese oxide (Mn3O4) electrodes modified with silver nanoparticles (Ag NPs). In addition, a new low-temperature ultrasonication technique was developed to modify the surface of Mn3O4 with Ag NPs, eliminating the need for chemical reducing agents. This approach is notable for three features: its operational simplicity, direct approach, and cost-effectiveness. Collectively, these characteristics contribute to its potential as a viable pathway for scalable synthesis. The resulting supercapacitor devices, incorporating both the modified electrodes and the novel GPE, were subjected to comprehensive electrochemical characterization, signifying a substantial contribution to the advancement of energy storage technologies.

Manganese oxide (Mn₃O₄, 97%), poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP, > 99.9%) were purchased from Sigma-Aldrich. Silver nitrate (AgNO₃) and ethanol (96%) were procured from Lachema. Activated carbon (Activated charcoal) was purchased from CHEMPUR. All chemicals used were of analytical grade and were used as received.

The manganese oxide material was surface modified with 1, 3, and 5 wt% Ag nanoparticles using the procedure described below. Silver nitrate (AgNO₃, pure p.a., Lachema) was first dissolved in 50 mL of ethanol (96%, Lachema). Subsequently, manganese oxide (Mn₃O₄, Sigma-Aldrich) was introduced to the nitrate–ethanol solution of Ag that had been prepared previously. The synthesis was conducted in an ultrasonic bath cleaner for 2 h, with 30 min of sonication separated by 30 min intervals. The dark-brown suspension was then air-dried for several hours at temperatures ranging from 60 °C to 150 °C for several hours. In the final stage of the procedure, the manganese oxide materials with 1, 3, and 5 wt% of silver were ground in an agate mortar to achieve a fine powder. Figure 1 presents the synthesis procedure of Mn3O4@Ag composites, along with the proposed mechanism for the reduction of AgNO₃ without the need for a reducing agent. The precise pathway governing the nucleation and growth of silver nanoparticles under these conditions remains to be fully elucidated. Given that no chemical reducing agents were employed and that the materials were not exposed to the elevated temperatures typically required for the complete thermal decomposition of silver nitrate, it is hypothesized that the Mn3O4@Ag composite itself may have played a direct role in facilitating the reduction of Ag⁺ ions. Furthermore, it is proposed that this reduction process occurred in parallel with the photolysis and/or partial thermal decomposition of AgNO3, potentially enhanced by the ultrasonication treatment applied during synthesis. The resulting samples were labelled M1, M3, and M5, and a control sample without Ag nanoparticles was labelled M0 (Mn3O4).

The preparation process of Mn3O4@Ag composites with the proposed mechanism of reduction of AgNO3 without the need for a reducing agent.

The X-ray diffractometry (XRD) was performed on a Panalytical X’Pert Pro MPD (Multipurpose Diffractometer). Data collection was performed over a range from 10 to 90° with a scanning rate of 1.5° (2θ)/min with CuKα radiation (45 kV, 40 mA, λ = 1.5406 nm). The crystal phases were identified by referencing diffraction patterns in a licensed library from the International Centre for Diffraction Data (ICDD). The Raman spectra were acquired on a DXR Raman microscope (Thermo Scientific) with a 32-two-second scan, laser 532 nm laser (3 mW) under a 10 × objective of an Olympus microscope. The X-ray photoelectron spectroscopy (XPS) apparatus, comprising a SPECS PHOIBOS 100 hemispherical analyzer with a 5-channel detector and a SPECS XR50 achromatic X-ray source equipped with an Al and Mg double anode, was utilized to analyze the surface composition of the samples and the chemical states of the elements. Scanning electron microscopy (SEM) was conducted using an SEM/FIB Zeiss Crossbeam 350 (Germany). Energy-dispersive X-ray spectroscopy (EDX) was performed using an Ametek EDAX Octane Elite with an accelerating voltage of 7 kV to minimize the penetrating depth. Transmission electron microscopy (TEM) investigations were conducted utilizing the FEI Talos F200X microscope, operated at an accelerating voltage of 200 kV. EDX measurements were performed in scanning transmission electron microscope (STEM) mode employing a Super-X system with four SDDs. The specimen for TEM investigations was suspended in ethanol and deposited onto amorphous carbon, which was embedded on a Cu grid (Pacific Grid 300 mesh).

The electrodes were prepared using M0, M1, M3, and M5 materials. To fabricate the working electrodes, the prepared active (M0, M1, M3, and M5) material (80 wt%), activated carbon (10 wt%), and poly (vinylidene) fluoride as binder (10 wt%) were thoroughly mixed. A homogeneous slurry of the above materials was made by using acetone as a common solvent. The prepared slurry was brush-coated over activated carbon cloth (AVCarb, USA) with a geometric area of ~ 1 cm2, and allowed the electrodes to evaporate at room temperature (RT) for ~ 7 h for the removal of solvent (acetone in the present case). The loading mass on the electrodes was between 3.3 mg and 6.3 mg. Figure 2 shows the schematic illustration of the electrode fabrication process. The electrodes were tested in a two-electrode set-up using PVdF-HFP-PC-Mg(ClO4)2 as gel polymer electrolytes18. The cells were prepared by sandwiching a gel polymer electrolyte is placed between two symmetrical electrodes. To evaluate the electrochemical performance of the cell, cyclic voltammetry (CV) (at different scan rates and in the potential range from 0 to 1.4 V), ac impedance spectroscopy (frequency range from 200 kHz to 10 kHz), galvanostatic charge-discharge testing with repetitive cyclic testing (varying current density and in the potential range from 0 to 1.4 V). All the electrochemical measurements were carried out using a BioLogic VMP3 electrochemical workstation. Below is the configuration of the cells:

Cell A: M0 | PVdF-HFP-PC-Mg (ClO4)2 | M0.

Cell B: M1 | PVdF-HFP-PC-Mg (ClO4)2 | M1.

Cell C: M3 | PVdF-HFP-PC-Mg (ClO4)2 | M3.

Cell D: M5 | PVdF-HFP-PC-Mg (ClO4)2 | M5.

All the cells were rested for 4–5 h before electrochemical measurements, which ensured the enhanced electrochemical performance of the cells because of proper soaking of electrodes and electrolytes.

Schematic illustration of the electrode fabrication process.

As illustrated in Fig. 3a, the XRD patterns of the pristine (M0), and Ag-modified Mn3O4 (M1, M3, and (M5) powders exhibit a high degree of similarity to the tetragonal phase spinel-like Mn3O4 pattern (JCPDS card no. 24–0734)19,20,21,22, thereby confirming that the modification of the surface using Ag nanoparticles does not result in phase segregation. As shown in Fig. 3a, the twelve characteristic XRD peaks of Mn3O4/Ag powders, observed at 2θ angles of 18.1°, 28.9°, 31.1°, 32.3°, 36.1°, 38.1°, 44.5°, 50.8°, 54.1°, 55.9°, 58.6°, and 59.9°, correspond to the (101), (112), (200), (103), (211), (004), (220), (105), (312), (303), (321), and (224) crystal planes of tetragonal Mn3O4, with lattice constants a = 5.76 Å, c = 9.47 Å, and space group I41/amd (Hausmannite, JCPDS card no. 24–0734)19,20,21,22. The peaks observed at 38° and 44° are attributed to the overlapping phases of Mn3O4 (004) and (220), as well as Ag (111) and (200). A simplified form of the Scherrer equation was employed to estimate the average crystallite size of Mn3O4 in pristine and in each Ag-modified material. The average crystallite size of Mn3O4 was found to be approximately 33 nm.

The XRD pattern (a) and Raman spectra (b) of M0, M1, M3, and M5 samples.

It is important to note that interesting findings were obtained from Raman spectroscopy (Fig. 3b). The Raman spectra of each Mn3O4 sample were recorded in the range of 200 to 800 cm− 1. Furthermore, close observation of the Raman spectra of Mn3O4 samples surface-modified with Ag nanoparticles (M1, M3, and M5) reveals a strong alignment with the unmodified Mn3O4 sample (M0), as demonstrated in Fig. 3b. As shown in Fig. 3b, a sharp active band located at ∼651 cm–1 was observed for all samples. This observation serves to confirm the crystalline nature of the samples analyzed, corresponding to the Mn–O stretching vibrations of divalent manganese ions (Mn2+) in the coordination of the tetrahedral (MnO6)20,22,23,24. The vibrational mode (A1g) at 651 cm− 1 is a representative characteristic of the tetragonal structure of Mn3O4, found in the mineral hausmannite. Furthermore, the two broad weak bands observed at ∼311 and ∼367 cm− 1 are attributed to the out-of-plane bending modes of Mn–O (the Eg symmetry mode) and the asymmetric stretch of the bridge oxygen species Mn–O–Mn, respectively20,22,23,24. It is important to note that all the peaks in the resulting spectrum are indicative of the tetragonal structure of Mn3O4. The results of the present study are in close alignment with those reported in the literature20,22,23,24. The surface modification of Mn3O4 powders by Ag in the 1–5%wt. the range does not result in any qualitative changes to the spectrum, thereby indicating that the tetragonal I41/amd symmetry remains unaltered. However, as the amount of silver used in the modification process increased, an increase in the intensity of the A1g peaks and a shift of ∼ 5 cm− 1 was observed in the M1, M3, and M5 samples, compared to the pristine Mn3O4 material. It is recognized that Ag particles can trigger a surface-enhanced Raman scattering effect25.

XPS survey spectra of M0 and M1 samples (a), high-resolution spectra of Mn 2p (b), O 1s (c), and Ag 3d (d) of M3 sample.

The X-ray photoelectron spectroscopy (XPS) analysis revealed the presence of Mn and O in the M0 and M3 samples, with Ag, and C detected exclusively in the M3 sample (refer to survey XPS spectra Fig. 4a). The high-resolution spectra for Mn 2p, O 1s, and Ag 3d from the M3 sample are presented in Fig. 4b-d. Specifically, Fig. 4b illustrates the XPS spectrum of Mn 2p, where two peaks at 641.1 eV and 652.8 eV correspond to the Mn 2p3/2 and Mn 2p1/2 levels, respectively. The band gap energy difference between these two peaks, 11.7 eV, further supports the presence of Mn3O426,27,28,29. Furthermore, analysis of the asymmetric O 1s peak (Fig. 4c) revealed two primary components: one at 530.0 eV, attributed to Mn–O bonding within the spinel Mn3O4 structure (with Mn2+ in MnO and Mn3+ in Mn2O3), and another at 532.0 eV, indicative of chemisorbed oxygen on the surface of the spinel crystallites26,27,28,29. Moreover, the XPS results demonstrate that the deposition of Ag is an effective process, with the metallic silver phase being identified on the Mn3O4 surface (see Fig. 4d). The binding energies of 368.1 eV and 374.1 eV correspond to the Ag 3d5/2 and Ag 3d3/2 peaks of metallic silver17, respectively.

The morphology of all samples (M0, M1, M3, and M5) was examined using scanning electron microscopy (SEM) (refer to Fig. 5). For selected sample M3, which exhibited the most favourable electrochemical performance, transmission electron microscopy (TEM) techniques were employed (refer to Fig. 6). SEM analysis of the pristine Mn3O4 (M0) sample revealed particles with irregular shapes and rough surfaces. These particles were densely aggregated with minimal visible porosity, and their sizes ranged from approximately 1 to 3 μm. The addition of Ag (samples: M1, M3. M5) led to a noticeable alteration of the Mn₃O₄ surface, characterized by increased granularity and irregularity as the Ag content increased. While the pristine Mn3O4 particles maintained their size range of 1 to 3 μm, the presence of Ag nanoparticles, less than 500 nm in size, became increasingly evident in samples with higher Ag content (M3 and M5). The higher-resolution SEM images for M1, M3, and M5 samples (see Fig. S1 in the Supplementary Material) reveal a clear visualization of the surface morphology and serve to confirm the presence of metallic Ag nanoparticles on the Mn3O4 surface, existing as small spherical structures. As the Ag content increases from 1 to 5%, there is an observable shift in the distribution of the nanoparticles, which become more densely distributed and slightly larger (a tendency towards agglomeration is also evident). Notably, sample M5 exhibits the most uniform and abundant coverage.

SEM images of M0, M1, M3, and M5 samples. Magnification: 2.5 KX. (scale bar 3 μm).

In order to verify the presence of Ag nanoparticles in each surface-modified sample, an EDS analysis was performed, and the results are provided in the Supplementary Material (refer to Fig. S2–S5). The analysis indicated that Ag was present in its metallic form (Fig. S2–S5). To gain a deeper understanding of the sample morphology, the presence of Ag, and to obtain information about the average size of Ag, TEM measurements were conducted for sample M3 with a three-weight content of Ag (see Fig. 6).

As demonstrated in Fig. 6, the TEM analysis of sample M3, which consists of Mn3O4 powder, confirmed the presence of Ag on the surface. Figure 6b clearly shows the presence of crystalline silver nanoparticles. To determine the distribution of silver particles on the analyzed sample, elemental mapping (HAADF-EDX) of Mn, O, and Ag was conducted (see Figs. 6c-h). The mapping results are presented in a series of images in Figs. 6(e, f, and h), indicating a uniform and well-distributed Ag NPs on the Mn3O4 surface. Additionally, the Mn3O4 surface exhibited excellent coverage by Ag nanoparticles as highlighted in Figs. 6(e, f and h). The generation of a histogram of the silver nanoparticle sizes was facilitated by the TEM images, as demonstrated in Fig. 6i. The application of a LogNormal distribution to the data yielded an average silver nanoparticle diameter of Xc = 2.2 nm.

(a) TEM image of the sample M3, (b) with a zoomed-in view showing a deposition of the Ag NPs, (c) HAADF image, (d-g) EDX mapping of Mn, Ag, O, (h) HAADF with EDX image of Ag NPs, and (i) histogram of the silver nanoparticle size.

Furthermore, the nitrogen adsorption-desorption isotherms have been recorded for all samples and are provided and discussed in the Supplementary Material (Fig. S6 and Table S1).

The pristine Mn3O4 and Ag-surface modified Mn3O4 electrodes (M0, M1, M3, and M5) were tested and evaluated as electrode materials for supercapacitors in a two-electrode system using magnesium ion-based gel polymer electrolyte. The cells mentioned above (Cell A-Cell D) were first tested using cyclic voltammetry (CV) at different scan rates in the potential range from 0 to 1.4 V. The CV curves of all cells at a scan rate of 5 mV s− 1 and in the potential range from 0 to 1.4 V are shown in Fig. 7a. As can be seen from the curves, cell A exhibits a symmetrical curve without any significant redox peaks, but in the case of cell B-cell D, redox peaks have been observed, it is due to the introduction of silver nanoparticles in the manganese oxide network. In order to understand the scenario, the electrochemical reaction between Mn3O4 and silver nanoparticles involves the transfer of electrons between the two, wherein Mn3O4 acts as an oxidizing agent and silver will act as a reducing agent and hence silver in the Mn3O4 network is interesting for electrochemical applications particularly to increase the stability of the cells. All the curves are reversible and hence show excellent reversibility of the redox processes. Also from the figures, it can be clearly seen that cell C exhibits the highest sweep area which clearly means that the cell has higher capacitance and lower resistance as compared to other cells. The capacitance values of cell A have been calculated using equation S1, whereas for cell B-cell D, the capacitance values were calculated using the Trasatti method (equation S2), the equations are provided in the Supplementary Material. The specific capacitance of a single electrode has been found to be of the order of ~ 8.31 F g− 1. Figure 7b shows the CV curves of cell C at different scan rates, viz. 5, 10, 20, 30, 50, 100 mV s− 1 in the potential range from 0 to 1.0 V, the rate capability of the cell is very good, and the deviation of CV curves from the ideal capacitive curves are mainly due to the contribution caused by redox reaction. Additionally, with the increase in the scan rates, the CV curves are smooth and free from redox peaks, it is mainly due to the fast switching in and out of ions, hence receiving less time for the redox reactions. Figure 7c shows the variation of specific capacitance with respect to scan rates, as can be seen from the figure, the cell follows the standard behavior and with the increase in scan rates, capacitance values gradually decreases which is clearly due to the insufficient time for the electrolyte ions (Mg2+ and ClO4−) to switch in and out of ions and hence there are incomplete electrochemical reactions leading to the reduced capacitance of the cells.

The relationship between peak current (I) and scan rate (V) provides insight into the dominant electrochemical mechanisms within the electrode30. By plotting log(I) versus log(V) (Figure S7) for low scan rates (5–100 mV s− 1), the slope of the plot can be used to predict the charge storage mechanism. A slope value of 0.5 indicates that the process is diffusion-controlled, whereas a slope of 1 signifies capacitive contributions31. When the slope lies between 0.5 and 1, it suggests a mixed mechanism involving both capacitive and diffusion-controlled processes32. In the case of cell C, the slope value was determined to be 0.57, indicating that the total charge storage is derived from a combination of capacitive and diffusion-controlled mechanisms. The individual contributions of these mechanisms can be calculated using specific equations S6, S7, and S8 given in the supporting information and the Fig. 7d illustrates the variation in capacitive (Qs​) and diffusion-controlled (Qd) charge contributions at different scan rates. At a low scan rate of 5 mV/s, the extended time available for ion movement allows ions to diffuse deeply into the bulk of the electrode material, resulting in a higher diffusion-controlled contribution (Qd=68.54%) compared to the capacitive contribution (Qs=31.46%). As the scan rate increases, the time for ion diffusion becomes limited, leading to surface-dependent capacitive processes where charge is stored electrostatically or through rapid redox reactions at the surface. The capacitive contribution significantly increases to 74.17%, while the diffusion-controlled contribution decreases to 25.83%.

(a) Cyclic voltammetry curves of cell A-cell D at a scan rate of 5 mV s− 1, (b) cyclic voltammetry curve of cell C at different scan rates, (c) variation of specific capacitance of cell C with respect to different scan rates and (d) Capacitive and diffusion-controlled charge contributions of cell C at scan rates of 5 and 100 mV/s.

Figure 8a shows the representative impedance spectroscopy curves of cell A-cell D, which also reveal the impact of adding silver in the Mn3O4 network. Following the CV results, three weight percentages of silver nanoparticles in the electrode network show the best electrochemical performance. It was also found out that charge transfer resistance is the least for cell C as compared to other cells, and hence the capacitance value is the highest (8.32 F g− 1). The capacitance values are calculated by using equation S3 (provided in supplementary information). It signifies that 3 wt% of silver is optimized, and it also slows the resistance between the electrode and electrolytes. To further confirm the electrochemical results, galvanostatic charge-discharge (GCD) studies have been carried out for all cells at a current density of 0.058 A g− 1, and in the potential window of 0 to 1.4 V. The results have been shown in Fig. 8b. As can be seen, GCD curves show the redox nature of prepared electrodes, and the results are in-line with other electrochemical results mentioned above. The specific capacitance of cells was found to be of the order of 9.38 F g− 1, with an energy density of 1.9 Wh kg− 1 and power density of 30.8 W kg− 1, respectively. Figure 8c shows the GCD curves of cell C at different current densities and in the potential window of 0 to 1.0 V. Table S2 shows the literature comparison of Mn3O4 material and its composites for supercapacitor application. It is important to highlight that this study introduces a simple, cost-effective, and environmentally friendly method for depositing Ag nanoparticles onto the Mn3O4 surface without the use of any reducing agents. Notably, this work presents, for the first time, the investigation of the Mn3O4@Ag composite in the fabrication of a symmetrical supercapacitor cell incorporating a gel polymer electrolyte. As can be seen from the graphs that at lower current density, the cells show higher capacitance, it is potentially due to the facile diffusion of protons through the manganese network, at lower current densities it is easier for the protons to infiltrate into the inner cores of the network unlike at higher current densities where the ions only interact with outer surface of the electrode, thereby decreasing the capacitance of the cells. Figure 8d shows the long-term cyclic stability performance of cell C to check the chemical stability of the prepared materials. It is worth noting that in the case of supercapacitors, the electrodes are in direct contact with electrolyte materials, and the stability of electrodes in such cases is critically important, and it can be tested by performing long-term cyclic testing. Cell C was cycled continuously for ~ 15,000 galvanostatic cycles in the potential window from 0 to 1.0 V and at a current density of 0.058 A g− 1. The cell was stable throughout cyclic testing; in fact, the capacitance was increased by almost double after cycling testing, clearly confirming the chemical stability of the M3 material. Additionally, the coulombic efficiency was also nearly 100% throughout the cycles. The SEM characterization with EDX (Fig. S8, Fig. S9) reveals the morphological stability of the materials. Also, from micro-images, the presence of electrolyte ions is confirmed, which may be attached to the electrode materials during charging and discharging measurements.

(a) Electrochemical impedance spectroscopy curves of cell A-cell D at a frequency of 10 mHz, (b) galvanostatic charge discharge curves of cell A-cell D, (c) galvanostatic charge discharge curves of cell C at different current densities and (d) cyclic testing and cyclic retention of cell C at a current density of 0.058 A g− 1 (inset shows the few cycles).

This work presents a pioneering application of a gel polymer electrolyte (GPE) for supercapacitor technology, utilizing a novel combination of PVdF-HFP as the host polymer, propylene carbonate (PC) as the plasticizer, and magnesium perchlorate (Mg(ClO4)2) as the salt, in conjunction with Ag nanoparticle (NP)-modified Mn3O4 electrodes. The study demonstrates the successful use of this GPE and electrode configuration in a two-electrode supercapacitor system, achieving a specific capacitance of 9.38 F g− 1 and excellent energy and power densities of 1.9 Wh kg− 1 and 30.8 W kg− 1, respectively. Notably, the supercapacitor cells exhibited outstanding stability, retaining high performance over approximately 10,000 charge-discharge cycles at a current density of 0.058 A g− 1. The incorporation of silver nanoparticles within the Mn3O4 electrodes significantly enhanced the capacitive performance, underscoring their potential for energy storage applications. The study also introduces a novel, low-temperature ultrasonication method for attaching Ag NPs to Mn3O4. This method is simple, cost-effective, and scalable, and eliminates the need for a reducing agent, offering a promising and sustainable method for future supercapacitor manufacturing. The research contributes significantly to energy storage technologies, providing valuable insights for the development of high-performance, scalable supercapacitor systems.

The data will be available on the request from the corresponding author (Dr. Monika Michalska, [email protected]).

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Authors are thankful to Dr Kamil Bochenek (IPPT PAN) for SEM-EDX measurements. M. M. would like to thank Dr A. Malinowska (Łukasiewicz – IMiF) for the comparative discussion regarding XRD analysis. This work was financially supported by the Ministry of Education, Youth and Sports, Czech Republic (contract no. 8F21007), National Centre for Research and Development, Poland (V4-Japan/2/17/AtomDeC/2022) through the research project cooperation between the AtomDeC Consortium by funding received from the Visegrad group(V4)-Japan 2021 2nd Joint Call on “Advanced Materials”. M.M. would like to acknowledge the financing support through the European Union under the REFRESH - Research Excellence For REgion Sustainability and High-tech Industries (project no. CZ.10.03.01/00/22_003/0000048) via the Operational Programme Just Transition. M. Michalska would like to thank for the assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LM2023066. This work was also supported by the VSB – Technical University of Ostrava (project No. SP2025/044).

Department of Chemistry and Physico-Chemical Processes, Faculty of Materials Science and Technology, VSB – Technical University of Ostrava, 17. listopadu 2172/15, Ostrava-Poruba, 708 00, Czech Republic

Monika Michalska & Martin Sarman

Institute of Fundamental Technological Research, Polish Academy of Sciences, Pawińskiego 5B, Warsaw, 02-106, Poland

Janhavi Sharma, Chandini Kumar & Amrita Jain

Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, Żwirki i Wigury 101, Warsaw, 02-089, Poland

Kamil Sobczak

Center for Solar Cells and Renewable Energy (CSRE), Department of Physics, SSBSR, Sharda University, Greater Noida, 201310, India

Pramod Kumar Singh

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M.M.: conceptualization, methodology, formal analysis, investigation, writing – original draft, writing – review & editing, resources, visualization, funding acquisition. M.S.: formal analysis, investigation, writing – original draft, visualization. J.S.: investigation. C.K.: investigation, writing – original draft. K.S.: formal analysis, investigation, writing – original draft. P.K.S.: formal analysis, writing – review and editing. A.J.: methodology, formal analysis, investigation, writing – original draft, writing – review & editing, resources, funding acquisition.

Correspondence to Monika Michalska or Amrita Jain.

The authors declare no competing interests.

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Michalska, M., Sarman, M., Sharma, J. et al. Exploiting spinel manganese oxide decorated with silver nanoparticles as electrodes for supercapacitor application. Sci Rep 15, 21597 (2025). https://doi.org/10.1038/s41598-025-04476-5

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Received: 06 February 2025

Accepted: 27 May 2025

Published: 01 July 2025

DOI: https://doi.org/10.1038/s41598-025-04476-5

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