Recycling Zinc and Graphite from Spent Zn–C Batteries to Synthesize ZnFe₂O₄@Graphene Nanosheets Composites for High-Performance Supercapacitor Electrodes

Abstract

The rapid increase of battery and carbon waste has raised an urgent demand for efficient recycling methods, along with the development of environmentally friendly energy storage materials. In this study, zinc ferrite (ZnFe₂O₄, ZFO) was synthesized from spent batteries, while graphene nanosheets (G_Ns) was obtained from recycled carbon, and then combined to form ZFO@G composites for supercapacitor applications. Characterization techniques including XRD, Raman, SEM, and electrochemical analyses (CV, GCD, EIS) were employed to elucidate the crystal structure, surface morphology, and electrochemical performance of the materials. The results reveal that the incorporation of G_Ns improves the electrochemical performance of ZFO, delivering a specific capacitance of 606.69 F g^–1 at 1 A g^–1 with a capacitance retention of ∼58.8% after prolonged cycling. This study not only demonstrates the potential of recycled materials for supercapacitor applications but also contributes to mitigating environmental issues caused by battery waste.

1. Introduction

The rapid growth in demand for batteries, fueled by the widespread adoption of consumer electronics, electric vehicles, and energy storage systems, has amplified concerns about battery waste, posing significant environmental risks. Improper disposal of spent batteries, particularly zinc–carbon (Zn–C) batteries, can release heavy metals and toxic chemicals, endangering human health and ecosystems. , In 2023, China alone exported 13.29 billion Zn–C batteries, reflecting their extensive use. The global Zn–C battery market, valued at USD 6.2 billion in 2023, is projected to grow at a compound annual growth rate of 4.5% through 2030. However, the rapid increase in discarded batteries, particularly Zn–C batteries, has created numerous environmental challenges. Traditional disposal methods such as landfilling or incineration not only cause soil and water pollution due to heavy metals but also lead to the loss of valuable resources such as zinc, manganese, and graphitematerials that are essential in modern industry and energy technology. − Within the context of the circular economy and sustainable development, the advancement of efficient, eco-friendly, and economically feasible Zn–C battery recycling technologies has become an urgent global and national demand.

In response to the pressing need for efficient, safe, and environmentally friendly energy storage solutions, supercapacitors (SCs) have emerged as a promising technology thanks to their high-power density, fast charge–discharge capability, and outstanding cycling life. , However, their major limitation lies in their low energy density (5–10 W h kg^–1), which is only a fraction of that of Li-ion batteries (100–250 W h kg^–1). This restricts the ability of SCs to fully replace batteries in long-term storage applications; instead, they are commonly used as complementary devices in systems that require instantaneous power, high durability, and fast charging, such as hybrid electric vehicles, portable electronics, and renewable energy systems. Consequently, the development of novel electrode materials capable of enhancing energy density while maintaining the intrinsic advantages of SCs has become a central research focus in the field of energy storage.

Graphene has become a research hotspot in supercapacitor studies due to its unique two-dimensional structure, which provides a large surface area, excellent electrical conductivity, and high mechanical strength, even surpassing carbon nanotubes (CNTs). , Notably, the effective surface area of graphene is minimally affected by pore size distribution, allowing electrolyte ions to access both sides simultaneously and thereby optimizing charge storage. Therefore, graphene and its derivatives such as reduced graphene oxide (rGO) are regarded as highly promising substrates for supercapacitor electrodes. Meanwhile, transition metal oxides such as MnO₂, FeO, NiO, and CuO... exhibit high electrochemical capacity but suffer from poor conductivity and limited cycling stability. , Combining graphene/rGO with transition metal oxides has been shown to yield significant benefits: for example, Jhankal et al. developed MnO₂ nanoneedles/rGO composites for sodium-ion supercapacitors, achieving a capacitance of 216 F g^–1 and an energy density of 24.25 W h kg^–1 at 1 A g^–1, with stable performance over 6000 cycles. Similarly, Askari et al. synthesized ZnFe₂O₄–rGO composites via a hydrothermal method, obtaining a specific capacitance of 101 F g^–1 at 0.5 A g^–1. Tiwari et al. reported layered ZnFe₂O₄/RGO heterostructures fabricated by a one-step hydrothermal process on nickel foam, which showed excellent potential for supercapacitor applications. These findings highlight the potential of graphene–metal oxide composites for improving energy density and cycling durability, encouraging further exploration toward practical applications.

Recent studies have explored the recycling of spent Zn–C batteries to recover valuable components for electrochemical energy storage applications. In particular, our previous works have focused on the recovery and reutilization of carbon materials derived from spent Zn–C batteries, as well as the development of novel aqueous electrolytes based on ZnSO₄–H₂SO₄ systems, demonstrating improved electrochemical stability and performance in zinc-based energy storage devices. These studies highlight the potential of battery waste valorization through targeted reuse of individual components. However, most existing reports, including our earlier contributions, primarily address either recycled carbon materials or electrolyte engineering separately. ,, In contrast, the present work advances beyond these approaches by simultaneously recycling both graphite and zinc from the same spent Zn–C batteries and integrating them into a unified ZnFe₂O₄@G composite electrode for supercapacitor applications, thereby extending recycling strategies toward functional electrode material design.

Inspired by these developments, and in contrast to previously reported ZnFe₂O₄–graphene composites synthesized from commercial precursors, this study introduces a recycling-oriented approach to address both battery waste and supercapacitor limitations. Graphite rods recovered from spent Zn–C batteries are exfoliated into graphene nanosheets (G_Ns) using a four-electrode plasma-assisted electrochemical method, which differs fundamentally from conventional chemical oxidation routes. Concurrently, zinc extracted from the same batteries is combined with iron to synthesize ZnFe₂O₄ via a hydrothermal process. The resulting ZnFe₂O₄@G composite features a well-integrated hybrid architecture that synergistically combines electric double-layer capacitance and pseudocapacitance, demonstrating the feasibility of converting spent Zn–C battery components into functional supercapacitor electrode materials as a proof-of-concept recycling strategy. A comprehensive quantitative assessment of environmental impact, scalability, and economic feasibility is beyond the scope of the present work.

2. Experiments

2.1. Materials

Zinc–carbon batteries at their end-of-life were collected from the supermarket. Sulfuric acid (H₂SO₄, 95%) was bought from Samchun Chemical Co., Ltd.; N-methyl-2-pyrrolidone (NMP) were purchased from Shanghai Zhanyun Chemical Co., Ltd. Polyvinylidene difluoride (PVDF), poly­(vinyl alcohol) (PVA) and carbon black were produced from Aladdin and Macklin, respectively. Ferric nitrate nonahydrate (Fe­(NO₃)₃·9H₂O, ≥98%), OXFORD LAB FINE CHEM LLP, India. Sodium hydroxide (NaOH, pellets, ≥98%, Merck Life Science GmbH, Germany). Absolute ethanol (CH₃CH₂OH, ≥99.7%, AR, Guangdong Guanghua Sci-Tech Co., Ltd., China) was used in the experiments.

2.1.1. Synthesis Graphene Nanosheets

Graphene nanosheets (G_Ns) were synthesized via an electrochemical plasma method using a four-electrode plasma system (Figure a). Graphite recovered from spent zinc–carbon batteries was employed as the precursor material. In the synthesis process, four graphite rods connected to the cathode and one graphite rod connected to the anode were immersed in 1 M NaOH solution and subjected to plasma discharge at 80 V and 1 A for 1 h. The plasma treatment promoted the exfoliation of graphite layers while simultaneously introducing oxygen-containing functional groups (−OH, −COOH, −CO) into the carbon framework, resulting in the formation of graphene nanosheets. The resulting suspension was repeatedly filtered and washed with deionized water and ethanol until neutral pH, followed by drying at 80 °C for 24 h to obtain graphene nanosheets.

1.

Process to prepare (a) graphene nanosheets, (b) ZnFe₂O₄ nanoparticles, and (c) ZnFe₂O₄@G composites. Photographs taken by the author Thuy Trang T. Vuong.

2.1.2. Synthesis Zinc Ferrite Nanoparticles

Zinc ferrite (ZnFe₂O₄, ZFO) nanoparticles were synthesized via a hydrothermal method (Figure b). First, spent zinc–carbon batteries were mechanically treated to separate the zinc-containing fraction, which was then dissolved in 1 M H₂SO₄ at a solid-to-liquid ratio of 1:1 to obtain a Zn²⁺ solution. After impurity removal, the Zn²⁺ solution was mixed with 1 M Fe­(NO₃)₃·9H₂O at a molar ratio of Zn²⁺/Fe³⁺ = 1:1. The mixture was stirred magnetically for 20 min, followed by the dropwise addition of 4 M KOH until the pH reached 11 to form a precursor precipitate. Hydrothermal treatment was carried out at 220 °C for 22 h to induce the formation of the characteristic spinel structure of ZnFe₂O₄. The resulting product was collected by centrifugation at 6000 rpm for 20 min, washed repeatedly with deionized water and ethanol, and then dried at 110 °C for 24 h. Finally, ZnFe₂O₄ nanoparticles with a homogeneous crystalline structure and ultrafine particle size were obtained.

2.1.3. Synthesis ZnFe₂O₄@G

The ZnFe₂O₄@G composite material (ZFO@G) was synthesized via a simple wet-chemical process (Figure c). Initially, zinc ferrite (100 mg ZnFe₂O₄) and graphene nanosheets (100 mg G_Ns)­0 powders were mixed at a mass ratio and subsequently dispersed in 100 mL of ethanol (C₂H₅OH) solvent. This mixture was then subjected to ultrasonication for 30 min to ensure homogeneous dispersion and optimize the interfacial interaction between the two components. Following ultrasonication, the composite material was recovered by filtration, followed by multiple washing cycles with ethanol to completely remove impurities and residual solvent. Finally, the resulting material was dried in an oven at 80 °C for 24 h to obtain the purified ZnFe₂O₄@G composite powder.

2.2. Characterization of Materials

The morphology and structure of the samples were investigated using various characterization techniques, including X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα radiation, λ = 1.5406 Å, 2θ range of 10–80°), Raman spectroscopy (Horiba LabRAM HR Evolution, 532 nm excitation laser), and scanning electron microscopy (SEM, Hitachi SU8000 and JEOL JSM-IT800, accelerating voltage 5–15 kV) equipped with energy-dispersive X-ray spectroscopy (EDX) for elemental analysis. The specific surface area and pore structure were analyzed by N₂ adsorption–desorption isotherms at 77 K using a Micromeritics MicroActive analyzer (Serial #377, USA) after degassing the samples at 165 °C for 8 h.

2.3. Electrochemical Studies

The electrode preparation was carried out using a slurry casting method. A mixture comprising 80 mg of the active material (80 wt %), 10 mg of carbon black (10 wt %), and 10 mg of PVDF binder (10 wt %) was dispersed in 2 mL of solvent. This mixture was thoroughly stirred on a magnetic stirrer and subsequently ultrasonicated to yield a homogeneous slurry. An aliquot of 20 μL of the slurry was then dropped-casted onto a 1 cm² carbon paper substrate and dried under vacuum at 60 °C for 24 h. The resulting electrode had a final active mass loading of approximately 1 mg.

Electrochemical analysis was performed using a standard three-electrode configuration. The electrolyte chosen was a 1 M NaOH aqueous solution, selected based on the criteria of adequate ionic conductivity, electrochemical stability, and compatibility with the Ag/AgCl reference electrode. In this setup, the prepared electrode served as the working electrode (WE), a platinum (Pt) foil as the counter electrode (CE), and a commercial Ag/AgCl electrode (in saturated solution) was utilized as the reference electrode (RE).

The symmetric solid-state supercapacitor (SSC) device was assembled according to a sandwich-like structure: active material/PVA-NaOH/active material. The two electrodes were first immersed in the NaOH gel polymer electrolyte. Subsequently, a separator (a piece of blotting paper or filter paper) measuring 1.5 cm × 1.5 cm (corresponding to an area of 2.25 cm²) was soaked in the NaOH gel polymer electrolyte and then placed between the two electrodes to form the complete supercapacitor structure.

To investigate the charge storage capability of the samples, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge (GCD) measurements were performed using a CS 310 electrochemical workstation (Corrtest, China).

The specific capacitance of the three-electrode system was calculated from the cyclic voltammetry curves (C _CV, F g^–1), and from the galvanostatic charge–discharge curves (C _GCD, F g^–1) using the following equation: ,

$$C_{\mathrm{C}\mathrm{V}}=\frac{\int I\mathrm{d}V}{\mathrm{\Delta }V\times m\times \nu }$$ 1

and

$$C_{\mathrm{GCD}}=\frac{I\times \mathrm{\Delta }t}{\mathrm{\Delta }V\times m}$$ 2

where I (A) is the current, dV is the differential potential, ∫I dV is area enclosed by the cyclic voltammetry curves, ΔV (V) is the potential window, m (g) is the mass of active material, Δt is the discharge time at a constant current, and ν (V s^–1) is the scan rate.

For the symmetric two-electrode configuration, the specific capacitance of a single electrode was calculated according to ,

$$C_{\mathrm{S}}={4\mathrm{C}}_{\mathrm{GCD}}$$ 3

The energy density (E, W h kg^–1) and power density (P, W kg^–1) of the symmetric two-electrode device were calculated using the following equations: ,

$$E=\frac{C_{\mathrm{s}}\times {(\mathrm{\Delta }V)}^2}{2}\times \frac{1000}{3600}$$ 4

$$P=\frac{E\times 3600}{\mathrm{\Delta }t}$$ 5

3. Results and Discussion

3.1. Structural and Morphological Attributes

The structure and phase composition of the prepared samples were investigated using powder XRD. Figure a illustrates the XRD pattern of the ZFO@G composite, ZFO and G_Ns materials. Specifically, the XRD spectrum of the G_Ns sample exhibits a diffraction peak at 2θ = 26.57°, corresponding to an interlayer spacing of ∼0.335 nm, which can be assigned to the (002) plane of graphitic carbon or partially reduced graphene oxide. In addition, a broad diffraction peak at approximately 2θ ≈ 40° is observed, corresponding to the (100) plane. These features indicate a multilayer graphene-based structure with a low degree of long-range ordering, suggesting the presence of stacked graphene layers rather than fully oxidized graphene oxide. The XRD pattern of the ZFO sample shows characteristic peaks at 2θ ≈ 29.9° (220), 35.2° (311), 42.8° (400), 53.2° (422), 56.6° (511), and 62.4° (440). These peaks perfectly match the standard card JCPDS no. 22-1012, confirming the formation of pure spinel cubic ZnFe₂O₄ crystal. − For the ZFO@G composite sample, the XRD pattern displays the characteristic peaks of the ZnFe₂O₄ phase, while the characteristic peaks of G_Ns are no longer obvious. This can be attributed to the excellent dispersion of the graphene layers within the ZnFe₂O₄ crystal lattice or the partial reduction of the oxygen functional groups, which results in the loss of the long-range order of G_Ns.

2.

(a) XRD pattern, and (b) Raman spectroscopy of G_Ns, ZFO, and ZFO@G composites.

The Raman spectra of G_Ns, ZFO, and ZFO@G composite samples are shown in Figure b, highlighting the structural features and interactions between the carbon matrix and the ferrite phase. The G_Ns spectrum exhibits two characteristic carbon bands: the D band at ∼1350 cm^–1, associated with structural defects, and the G band at ∼1580 cm^–1, corresponding to the E_2g mode of sp² carbon. A weak 2D band at ∼2700 cm^–1 further indicates the multilayered and highly oxidized nature of G_Ns. The presence of pronounced D and G bands suggests abundant oxygen functional groups and disordered sp³ domains. − The ZFO sample shows characteristic Raman peaks at ∼305 cm^–1 (E_g), ∼460 cm^–1 (T_2g), and ∼625 cm^–1 (A_1g), confirming the formation of the spinel ZnFe₂O₄ structure. − The intense and symmetric A_1g mode indicates good crystallinity, while the moderate peak broadening implies a nanocrystalline structure with lattice defects. For the ZFO@G composite sample, the Raman spectrum simultaneously exhibits signals from both G_Ns and ZFO, confirming the successful combination of the two material phases. In addition to the vibrational peaks of ZFO, the two characteristics D (∼1350 cm^–1), and G (∼1590 cm^–1) peaks are still clearly observed. Notably, the I _D/I _G ratio shows a slight increase compared to pure G_Ns, indicating the introduction of additional defect sites due to the anchoring of ZFO nanoparticles on the G_Ns surface, while the overall graphitic framework is largely preserved. This change is generally attributed to the partial reduction of oxygen groups on G_Ns during the synthesis process, simultaneously leading to the formation of chemical bonds between GO and ZFO, which helps improve the electrical conductivity and structural stability of the composite.

Figure illustrates the SEM images combined with the EDX spectra of the G_Ns, ZFO, and ZFO@G composite materials. As shown in Figure a,b, the electrochemical exfoliation process using graphite rods recovered from spent carbon–zinc batteries successfully produced highly uniform, few-layer G_Ns sheets with large lateral dimensions. Such morphology provides a high surface area and abundant active sites favorable for the anchoring of metal oxide nanoparticles. The EDX spectrum (Figure c) shows that G_Ns primarily consists of the element C (95.2 at. %), along with a small amount of O (4.8 at. %), which is consistent with the presence of oxygen functional groups in the graphene oxide structure. Figure d,e shows SEM images of the ZFO sample, exhibiting agglomerated nanoparticles ranging from nano- to submicron size, distributed nonuniformly and forming porous clusters. The rough surface and numerous voids may enhance electrolyte contact but can also limit electrical conductivity due to particle aggregation. The EDX spectrum (Figure f) confirms the elemental composition of O, Fe, and Zn with atomic ratios consistent with the spinel ZnFe₂O₄ structure (O: 50.9 at. %, Fe: 35.0 at. %, Zn: 14.1 at. %). In contrast, the ZFO@G composite (Figure g,h) displays a well-integrated hybrid structure where ZnFe₂O₄ nanoparticles are uniformly distributed and tightly anchored onto the surface and within the interlayer spaces of G_Ns. The carbonaceous matrix effectively prevents nanoparticle agglomeration while providing continuous electron transport pathways, which is expected to enhance the electrochemical performance compared to the individual components. The EDX spectrum (Figure i) demonstrates the coexistence of C, O, Fe, and Zn (C: 58.1 at. %, O: 23.7 at. %, Fe: 12.4 at. %, Zn: 5.8 at. %), confirming the successful immobilization of ZnFe₂O₄ on the G_Ns framework. This result validates the stable formation of the composite structure, which both maintains the high electrical conductivity from G_Ns and ensures the provision of numerous electrochemically active sites from ZnFe₂O₄, promising a significant improvement in the material’s performance in energy storage and conversion applications. Notably, the plasma-exfoliated G_Ns exhibits a porous structure with abundant defects and exposed edge sites, which significantly increases the electrolyte-accessible surface area. This porous hybrid architecture facilitates rapid ion diffusion and ensures that a large fraction of surface-accessible redox sites of ZnFe₂O₄ can participate in charge storage, thereby contributing to the high specific capacitance observed.

3.

SEM images and EDX spectra of (a–c) G_Ns, (d–f) ZFO, and (g–i) ZFO@G composites.

Figure presents the N₂ adsorption–desorption isotherms of the G_Ns, ZFO, and ZFO@G samples. All three materials exhibit the characteristic Type IV curve according to the IUPAC classification, accompanied by a hysteresis loop typical of mesoporous materials. For G_Ns, the hysteresis loop is clearly visible in the region of P/P ₀ > 0.8, confirming the presence primarily of large mesopores. Conversely, the ZFO sample shows a sharp increase in adsorption capacity even at a low P/P ₀ region, reflecting the coexistence of micropores and small mesopores. The ZFO@G sample displays an intermediate curve between G_Ns and ZFO, with a higher adsorption capacity than pure G_Ns. This suggests that the combination of GO and ZFO successfully created a hierarchical porous structure, which maintains the large mesopores while also benefiting from the contribution of smaller mesopores.

4.

N₂ adsorption–desorption isotherms and pore size distribution of (a) G_Ns, (b) ZFO, and (c) ZFO@G.

The BET analysis results (Table ) reveal a clear distinction in surface area and pore size among the three materials. The relatively low BET surface area of the G_Ns sample (∼23 m² g^–1) can be attributed to the restacking of few-layer graphene sheets during drying and degassing, which is commonly observed in graphene-based materials produced without chemical spacers. ,, In addition, nitrogen adsorption measurements may underestimate the electrochemically accessible surface area of defect-rich graphene materials, where charge storage is dominated by surface defects and edge sites rather than physical porosity. , The GO sample exhibits the lowest specific surface area (22.9 m² g^–1) and a relatively large average pore diameter (∼24–28 nm), consistent with its role as a mesoporous framework. The ZFO sample shows the highest surface area (79.1 m² g^–1) with a notable microporous contribution (19.4 m² g^–1) and a smaller average pore diameter (∼10–11 nm), indicating a typical mesoporous–microporous structure. In contrast, the ZFO@G composite presents intermediate values (45.3 m² g^–1) with an average pore diameter of ∼15–16 nm. The reduction in surface area compared to pristine ZFO may result from the partial encapsulation and dispersion of ZFO nanoparticles within the G_Ns matrix; however, this configuration contributes to a well-balanced pore distribution between micropores and mesopores. These findings suggest that ZFO@G possesses an optimized hybrid architecture that combines the excellent electrical conductivity of G_Ns with the high active surface area of ZFO.

1. BET Surface Area and Pore Structure Parameters of G_Ns, ZFO and ZFO@G Samples.

sample	BET surface area (m2 g–1)	micropore area (m2 g–1)	pore volume (cm3 g–1)	average pore diameter (nm, BET)
GNs	22.87	0.18	0.1392 (ads)/0.1620 (des)	24.35 (ads)/28.33 (des)
ZFO	79.09	19.42	0.1760 (ads)/0.1838 (des)	15.53 (ads)/16.22 (des)
ZFO@G	45.32	1.18	0.2110 (ads)/0.2135 (des)	10.67 (ads)/10.80 (des)

3.2. Electrochemical Characterizations

3.2.1. Electrochemical Studies of Three Electrode System

The cyclic voltammetry (CV) curves of G_Ns, ZFO, and ZFO@G materials were recorded using a three-electrode configuration in 1 M NaOH electrolyte, with the applied potential ranging from −1.3 to 0.5 V at various scan rates (100, 50, 20, 10, and 5 mV s^–1), as shown in Figure . For the G_Ns sample (Figure a), the CV curves exhibit nearly rectangular shapes with low current response, indicating a typical electric double-layer capacitance (EDLC) behavior that originates from the reversible adsorption/desorption of OH^– ions on the oxygen-rich surface. In contrast, the ZFO sample (Figure b) displays distinct redox peaks, characteristic of pseudocapacitive behavior associated with the reversible redox reactions of Zn and Fe species in alkaline medium. The possible electrochemical reactions can be described as follows

$${\mathrm{Z}\mathrm{n}}^{2+}+{2\mathrm{O}\mathrm{H}}^{-}\leftrightarrow {\mathrm{Z}\mathrm{n}(\mathrm{O}\mathrm{H})}_2\leftrightarrow \mathrm{Z}\mathrm{n}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}$$

$$\mathrm{F}\mathrm{e}\mathrm{O}\mathrm{O}\mathrm{H}+e^{-}\leftrightarrow {\mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H})}_2$$

$${\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4+{\mathrm{O}\mathrm{H}}^{-}\leftrightarrow \mathrm{Z}\mathrm{n}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3+e^{-}$$

5.

CV curves of (a) G_Ns, (b) ZFO, (c) ZFO@G at different scan rates (5–100 mV s^–1), and (d) comparison of CV curves for G_Ns, ZFO, and ZFO@G at 5 mV s^–1.

Notably, the ZFO@G composite (Figure c) exhibits both quasi-rectangular shapes and well-defined redox peaks, indicating the synergistic contribution of the EDLC behavior from G_Ns and the pseudo capacitance of ZFO. This combination results in a significantly higher current density compared to the individual components. When directly compared at the same scan rate of 5 mV s^–1 (Figure d), the CV curve of ZFO@G displays the largest enclosed area and superior current response, confirming that the integration of G_Ns and ZFO not only enhances electrical conductivity but also increases the number of electrochemically active sites. Therefore, the ZFO@G composite demonstrates excellent electrochemical performance, making it a promising electrode material for supercapacitors in alkaline media. Based on the CV curves, the specific capacitances of G_Ns, ZFO, and the ZFO@G composite were calculated at various scan rates according to eq , and the corresponding results are summarized in Table .

2. Capacitance Value was Extracted From the CV Curve.

	C (F g–1)
electrode	5 mV s–1	10 mV s–1	20 mV s–1	50 mV s–1	100 mV s–1
GNs	60.17	54.14	47.17	38.18	31.15
ZFO	85.79	66.57	54.75	42.18	33.22
ZFO@G	205.85	183.54	154.7	119.64	91.54

The electrochemical impedance spectroscopy (EIS) profiles of the G_Ns, ZFO, and ZFO@G electrodes are presented in Figure , and the equivalent circuit fitting parameters are summarized in Table . The Nyquist plots of the supercapacitor devices, recorded over a frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV, reveal that at high frequencies, the solution resistance (R ₀) of all three samples is small (∼1–1.5 Ω), indicating negligible influence from the electrolyte. However, significant differences appear in the semicircle region at intermediate frequencies, which corresponds to the charge transfer resistance (R _C). The G_Ns and ZFO electrodes exhibit large semicircles with R _C values of 716 Ω and 399 Ω, respectively, suggesting sluggish reaction kinetics and poor electron transport capability. In contrast, the ZFO@G composite shows the smallest semicircle with a R _C of only 331 Ω, confirming that the incorporation of ZnFe₂O₄ with graphene significantly enhances electrical conductivity and facilitates ion exchange at the electrode–electrolyte interface. , At low frequencies, all three samples display nearly vertical lines associated with Warburg impedance (W _s), which reflects ion diffusion processes. The ZFO@G curve exhibits a steeper slope, indicating more favorable ion diffusion. The equivalent circuit fitting results further support this observation, as the ZFO@G electrode shows a much lower film resistance (R _a = 40.4 Ω) and reduced Warburg parameters (σ_W = 85.3, δ_W_s = 59.7) compared to G_Ns (R _a = 86.8 Ω, σ_W = 8.40 × 10⁶, δ_W_s = 265) and ZFO (R _a = 80.4 Ω, σ_W = 732, δ_W_s = 33). These findings demonstrate that the ZFO@G composite not only preserves the intrinsic pseudocapacitive behavior of ZnFe₂O₄ but also benefits from the superior electrical conductivity and interconnected network structure of graphene. Consequently, the composite exhibits reduced internal resistance, accelerated electrochemical kinetics, and improved ion diffusion. This synergistic effect is the key mechanism underlying the outstanding electrochemical performance of ZFO@G compared with its individual counterparts. ,

6.

Electrochemical impedance spectra of G_Ns, ZFO, and ZFO@G composites with the corresponding equivalent circuit model.

3. Equivalent Circuit Parameters Obtained From EIS Fitting for G_Ns, ZFO, and ZFO@G Samples.

sample	R 0 (Ω)	R a (Ω)	p_CPEa	T_CPEa	R c (Ω)	p_CPEc	T_CPEc	σ_W	δ_Ws
GNs	1.15	86.8	0.593	0.0236	716	0.941	0.0297	8.40 × 106	265
ZFO	1.56	80.4	0.574	0.0166	399	1	0.0312	732	33
ZFO@G	1.2	40.4	0.665	0.0399	331	1	0.0690	85.3	59.7

Figure presents the galvanostatic charge–discharge (GCD) curves of G_Ns, ZFO, and the ZFO@G composite at various current densities (1–10 A g^–1). The GO sample (Figure a) exhibits nearly linear and symmetric GCD curves, characteristic of an electric double-layer capacitance (EDLC) mechanism. However, the short charge–discharge duration and low specific capacitance indicate a limited number of accessible charge storage sites. In contrast, the ZFO sample (Figure b) displays distinct nonlinear segments in the GCD curves, consistent with the pseudocapacitive behavior arising from the reversible redox reactions of Zn and Fe species, accompanied by a longer charge–discharge time compared to G_Ns. Notably, the ZFO@G composite (Figure c) shows the longest charge–discharge duration and a combination of quasi-linear and curved profiles, confirming the synergistic contribution of the EDLC mechanism from G_Ns and the pseudo capacitance of ZFO. It should be noted that the first charge–discharge cycle exhibits an apparent Coulombic efficiency exceeding 100%, which is attributed to irreversible electrochemical activation processes, including electrolyte penetration, surface reconstruction, and the activation of pseudocapacitive redox sites. Therefore, the electrochemical performance evaluation is based on subsequent stabilized cycles rather than the initial cycle. This synergy results in a significantly enhanced specific capacitance, especially at low current densities, while maintaining well-preserved GCD shapes even at high current densities-indicating excellent rate capability. When directly compared at the same current density (Figure d), ZFO@G exhibits a markedly higher specific capacitance than the individual G_Ns and ZFO samples. These results demonstrate that the integration of ZnFe₂O₄ with graphene not only improves electrical conductivity and oxide particle dispersion but also creates a hybrid architecture that increases the number of electrochemically active sites and facilitates ion transport within the electrode. ,

7.

GCD curves of (a) G_Ns, (b) ZFO, (c) ZFO@G at different current density (1–10 A g^–1), and (d) comparison of GCD curves for GO, ZFO, and ZFO@G at 1 A g^–1.

Furthermore, the specific capacitances of the G_Ns, ZFO, and ZFO@G electrodes, derived from the GCD curves using eq , are presented in Figure a. At a low current density of 1 A g^–1, the ZFO@G composite exhibits the highest specific capacitance of approximately 600 F g^–1, significantly surpassing that of ZFO (∼150 F g^–1) and G_Ns (∼70 F g^–1). As the current density increases to 10 A g^–1, although all three samples show a decrease in capacitance due to ion transport limitations, ZFO@G still retains a value above 150 F g^–1, demonstrating a markedly superior rate capability compared to G_Ns and ZFO. In addition, the comparison of the electrochemical performance of the ZFO@G composite with previously reported materials is summarized in Table . This improvement can be attributed to the synergistic interaction between the EDLC mechanism of G_Ns and the pseudo capacitance behavior of ZFO, as well as the hybrid structure that enhances electrical conductivity and reduces internal resistance.

8.

(a) Specific capacitances following current densities of G_Ns, ZFO, and ZFO@G composites.

4. Electrochemical Studies of Carbon Materials in This Report in Comparison With Previous Report.

material	method to preparation	electrolyte	specific capacitance (F g–1)	energy density	refs
ZnFe2O4@rGO	hydrothermal	2 M KOH	∼352.9 F g–1 @ 1 A g–1	∼6.7 W h kg–1
ZnFe2O4@rGO	microwave hydrothermal (44.3 wt % rGO)	2 M KOH	∼628 F g–1 @1 A g–1	-
ZnFe2O4 NPS	coprecipitation	1 M H2SO4	186.6 F g–1 @ 1 A g–1	-
graphene nanosheets	electrochemical exfoliation	1 M ZnSO4·H2SO4	149.68 F g–1 @ 0.5 A g–1	60.08 W h kg–1
graphene nanosheets	electrochemical exfoliation	1 M Na2SO4	44.25 F g–1 @ 0.5 A g–1	6.15 W h kg–1
ZFO@G	electrochemical exfoliation	1 M KOH	606.69 F g–1 @ 1 A g–1	215.7 W h kg–1	this work

3.2.2. Electrochemical Studies of Devices

Based on the electrochemical stability evaluation of the electrode materials using a three-electrode system, a symmetric all-solid-state supercapacitor employing the ZFO@G composite as the active material was further investigated. The device was fabricated in a “sandwich” configuration, consisting of electrode/gel polymer electrolyte/electrode. The gel polymer electrolyte was prepared by dissolving PVA in a 1 M NaOH solution, resulting in a homogeneous gel suitable for application in solid-state supercapacitors.

Figure a presents the cyclic voltammetry (CV) curves of the symmetric solid-state supercapacitor at various scan rates (5–100 mV s^–1), exhibiting a quasi-rectangular shape accompanied by distinct redox peaks. This behavior indicates the coexistence of two charge storage mechanisms: (i) electric double-layer capacitance (EDLC) originating from graphene, attributed to its high surface area and reversible OH^– ion adsorption, and (ii) pseudo capacitance arising from the reversible redox reactions of Zn and Fe species in ZFO. , As the scan rate increases, the current response proportionally rises while the overall curve shape remains nearly unchanged, suggesting highly reversible electrochemical reactions and efficient ion diffusion. The galvanostatic charge–discharge (GCD) curves recorded at different current densities (0.5–2.5 A g^–1) display symmetric profiles with long discharge times, indicative of high Coulombic efficiency (Figure b). Notably, at a low current density of 0.5 A g^–1, the longest discharge time corresponds to the highest capacitance. As the current density increases, the discharge time gradually decreases but the curves retain a linear shape, confirming the device’s excellent rate capability and structural stability under high current operation.

9.

Electrochemical performance of the symmetric solid-state supercapacitor based on ZFO@G (a) CV curves at different scan rates (5–100 mV s^–1), (b) GCD curves at various current densities (0.5–2.5 A g^–1), (c) Nyquist plot with equivalent circuit fitting, (d) specific capacitance at different current density, (e) Ragone plot showing energy and power density, (f) cycling stability at 2.5 A g^–1 for 2000 cycles.

Figure c shows the electrochemical impedance spectroscopy (EIS) spectrum of the ZFO@G-based solid-state device, along with the fitted curve based on the equivalent circuit model. The Nyquist plot displays an intercept at the high-frequency region, a semicircle at the midfrequency region, and a nearly vertical tail at low frequencies, which are characteristic of an electrochemical storage process involving solution resistance, charge transfer resistance, and ion diffusion within the porous structure [34]. The fitting results show that the solution resistance (R ₀) of ZFO@G is approximately 1.77 Ω, indicating good electrode–electrolyte contact. The film resistance (R _a = 66.7 Ω) is relatively low, reflecting the enhanced electrical conductivity due to the synergistic effect between graphene and ZFO [46]. The charge-transfer resistance (R _c = 715 Ω) corresponds to the redox kinetics occurring at the active surface, demonstrating pseudocapacitive behavior. The CPE parameters (p_CPEa = 0.760 and p_CPEc = 0.768 < 1) indicate a porous and heterogeneous surface, typical of oxide–carbon hybrid electrodes. Moreover, the Warburg coefficients (σ ≈ 50.3, δ ≈ 111) suggest favorable ion diffusion within the hierarchical pore network, which is crucial for maintaining capacitance at high current densities. Overall, the ZFO@G electrode exhibits low internal resistance and efficient ion transport, contributing to its superior electrochemical performance.

Based on the GCD curves, the specific capacitance of the electrode materials was calculated using eq and is presented in Figure d. The ZFO@G device exhibited a specific capacitance of 354 F g^–1 at a current density of 0.5 A g^–1, and 128 F g^–1 at 2.5 A g^–1, demonstrating good capacitance retention at high current densities. This excellent performance is attributed to the synergistic effect between the conductive graphene network and the pseudocapacitive redox-active sites of ZFO. The Ragone plot (Figure e) indicates that the device delivers an energy density of 118 W h kg^–1 at a power density of 875 W kg^–1, and still maintains 45 W h kg^–1 at a high-power density of 8000 W kg^–1, highlighting a well-balanced trade-off between energy and power densitiesa key requirement for high-performance supercapacitors. Furthermore, the cycling stability of the device was evaluated via GCD measurements at 2.5 A g^–1 over 2000 cycles (Figure f), where the electrode retained approximately 58.8% of its initial capacitance. Although a noticeable decrease is observed, this retention is comparable to or higher than many reported metal oxide-based materials. The degradation is primarily attributed to the repetitive expansion and contraction of ZFO particles during cycling, leading to structural cracking and weakened electrode–electrolyte contact, thereby reducing the number of electrochemically active sites. In addition, the pronounced capacitance decay at higher current densities is mainly associated with limited ion diffusion and increased polarization in the solid-state configuration under fast charge–discharge conditions. At high current densities, electrolyte ions may not fully access the redox-active sites of the ZFO component, leading to incomplete faradaic reactions and reduced utilization of active material. Moreover, charge imbalance between the two identical electrodes and irreversible side reactions in the solid-state electrolyte can contribute to the nonideal Coulombic efficiency observed at high rates.

4. Conclusions

In conclusion, ZFO@G material has been successfully fabricated from used zinc carbon batteries through an environmentally friendly recycling method. ZnFe₂O₄ was synthesized from recycled zinc and combined with graphene nanosheets (G_Ns) obtained from waste graphite rods, forming a hybrid composite structure. Structural and morphological characterizations (XRD, Raman, SEM/EDX, BET) confirmed the formation of pure spinel ZnFe₂O₄, the homogeneous dispersion of oxide nanoparticles on graphene sheets, and the creation of a hierarchical porous network.

Electrochemical measurements (CV, GCD, EIS) demonstrated that ZFO@G exhibits a synergistic contribution from the electric double-layer capacitance (EDLC) of G_Ns and the pseudo capacitance of ZnFe₂O₄, resulting in a high specific capacitance (∼600 F g^–1 at 1 A g^–1), good rate capability, and enhanced cycling stability. Moreover, the symmetric solid-state supercapacitor based on ZFO@G showed a remarkable energy density of ∼220 W h kg^–1 at low power density and maintained stable performance even at high current loads.

These findings highlight the great potential of ZFO@G as a high-performance electrode material for supercapacitors, while simultaneously demonstrating a sustainable strategy to recycle spent batteries into value-added energy storage materials, in line with the principles of green energy and circular economy.

All the data supporting this article has been included in the research article. If the raw data is required, it will be made available on request.

⊥.

T.T.T.V. and V.V.T. contributed equally to this work. P.L.N. and V.V.T., T.T.T.V. came up with a research idea. T.T.T.V., V.V.T. did the experiment. T.T.T.V., V.V.T., P.N.L.P. wrote the main manuscript text. P.L.N., T.T.T.V., V.V.T. edited the manuscript until it was complete.

The authors declare no competing financial interest.