Upcycling of electric arc furnace dust into ZnO–Fe₃O₄ nanocomposites for high-performance supercapacitor applications

Abstract

The growing global demand for sustainable energy has driven extensive research into efficient energy storage systems. In this study, a novel approach is proposed for the upcycling of electric arc furnace dust (EAFD), a hazardous industrial waste into high-performance ZnO–Fe₃O₄ nanocomposites for use as supercapacitor electrodes. EAFD, rich in zinc and iron oxides, was processed through sulfuric acid leaching, co-precipitation, and thermal treatment, yielding nanostructured ZnO–Fe₃O₄. Under optimized conditions leaching efficiencies reached 98.3% for Zn and 90.1% for Fe. Subsequent co-precipitation at pH 8 successfully recovered both metals into a precursor with an efficiency greater than 99%. Following the thermal treatment, the synthesized nanocomposites were employed as electrode materials in asymmetric coin-cell supercapacitors. Electrochemical characterization demonstrated a specific capacitance of 35.2 mF cm⁻², an energy density of 25.03 μWh cm⁻², and a power density of 430.81 μW cm⁻² at a current density of 0.25 mA cm⁻². Moreover, the device exhibited excellent cycling stability, retaining 81% of its initial capacitance after 7000 charge–discharge cycles. These findings demonstrate a scalable, eco-friendly approach for converting industrial waste into high-performance energy storage materials, aligning with circular economy principles and supporting environmental remediation.

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Introduction

Energy is a fundamental component of modern society, driving sectors from industrial production to communication technologies and transportation.^1,2 As the need for dependable and sustainable energy sources escalates, the role of efficient energy storage systems becomes increasingly critical. Energy storage enhances the integration of renewable sources like solar and wind, which are naturally intermittent.² Among the various storage technologies, supercapacitors have gained attention as a promising solution due to their high power density, rapid charge–discharge capability, and long cycle life.^3,4 Unlike traditional batteries, supercapacitors can provide quick bursts of energy and endure millions of cycles without significant degradation, making them ideal for applications that require fast energy delivery and durability, such as electric vehicles, grid stabilization, and portable electronics.⁵ Their unique attributes make supercapacitors a pivotal element in the transition to a cleaner and more resilient energy future.⁶

Transition metal oxides have emerged as promising electrode materials for supercapacitor applications owing to their intrinsically high specific capacitance and broad electrochemical potential windows, which enable significant enhancement of energy density.^7,8 Unlike traditional electrostatic double-layer capacitors (EDLCs), metal oxides operate via pseudocapacitive mechanisms, involving rapid and reversible redox reactions that take place both at the surface and throughout the bulk of the material.^9,10 These faradaic processes contribute to superior charge storage capabilities. Furthermore, advanced surface engineering strategies including nanoscale structuring and the incorporation of conductive carbonaceous matrices can substantially enhance the electrical conductivity, ion transport kinetics, and long-term cycling stability of metal oxide-based electrodes, thereby improving their overall electrochemical performance.^11–13 However, the synthesis of such materials often requires high-temperature treatments, controlled atmospheric conditions, or multi-step processing routes, all of which contribute to increased energy consumption and overall production costs.¹⁴ On the other hand, the utilization of active materials derived from recycling of the various waste as electrode components in supercapacitors significantly reduces production costs while simultaneously preventing the release of hazardous waste into the environment.^15–21

Electric arc furnace dust (EAFD) is generated as a byproduct of melting scrap metal during steel production, wherein metals with higher vapor pressures than iron volatilize and subsequently condense in exhaust filtration systems of the furnace. As the gases cool, oxidation reactions convert metal vapors to oxides, which then become part of the dust. Also, due to the harsh environment of the EAF and CO-bubble bursts, liquid metal and slag are carried by the high-velocity exhaust gases.²² The high zinc and iron oxide content of EAFD makes it an attractive alternative feedstock for metal recovery, offering lower cost and improved energy efficiency compared to conventional raw materials.²³ However, the presence of toxic heavy metals such as chromium, lead, and cadmium within EAFD poses significant environmental risks, as these elements can easily disperse through wind or leaching upon contact with solvents. This potential for environmental contamination underscores the critical need for the safe handling and recycling of EAFD to protect both human health and ecosystems.²⁴ Due to its hazardous composition, EAFD is classified as a dangerous industrial solid waste—coded as K061 by the U.S. Environmental Protection Agency (EPA) and as “10-02-07” by the European Waste Catalogue (EWC).²⁵

Globally, the annual generation of EAFD is estimated at approximately 7.5 million tons, yet only about 45% of this waste is currently recycled.^26,27 In many regions, limited economic incentives and inadequate regulatory enforcement lead industries to either dispose of EAFD through landfilling or disregard it entirely, resulting in its continuous accumulation.²⁸ The landfilling of EAFD not only represents a missed opportunity for resource recovery but also poses a significant environmental threat due to the potential leaching of hazardous components into soil and groundwater systems.²⁹

In the current study, a hydrometallurgical strategy was employed to recover ZnO–Fe₃O₄ nanocomposites from EAFD and to explore their application as electrode materials in asymmetric-type coin cell supercapacitors. Unlike conventional recovery routes that primarily focus on the extraction of single metals, our approach yields a functional nanocomposite with synergistic electrochemical properties. The integration of ZnO and Fe₃O₄ within a single framework enables a broadened potential window and enhanced pseudocapacitive behavior, while simultaneously offering structural stability during repeated cycling. Moreover, the direct utilization of hazardous industrial waste for the design of advanced electrode materials not only addresses a critical environmental challenge but also establishes a cost-effective pathway for sustainable energy storage technologies. The fabricated coin cell device demonstrates competitive electrochemical performance compared to previously reported waste-derived electrodes, underscoring the novelty of coupling waste upcycling with practical supercapacitor applications. This work therefore contributes to the dual advancement of circular economy strategies and high-performance electrochemical energy storage systems.

Experimental

Pre-treatment and characterization of EAFD

Initially, EAFD obtained from an EAF steel mill underwent a mixing procedure for 3 hours to ensure a homogenized raw material. The dust that had been made uniform in composition was cleansed using deionized water at a concentration of 10 g L⁻¹ and then subjected to a drying process at a temperature of 90 °C for a period of 12 hours. Subsequently, EAFD was chemically and morphologically characterized using XRD (PANalytical X'pert Pro), XRF (Panalytical Epsilon 4) and FE-SEM-EDS (Thermo Scientifıc Apreo 2 S LoVac) analyzes.

Recovery of ZnO–Fe₃O₄ from EAFD

H₂SO₄ solutions of varying concentrations (2 to 4 M) at different times (60–240 minutes), solid/liquid ratios (50–200 g L⁻¹) and temperature values (60–100 °C) were utilized to leach EAFD. Vacuum filtration was employed to eliminate the undissolved particulate from solutions. Chemical characterization of the leach solutions was conducted by ICP-OES (PerkinElmer Optima 2100 DV) analyses. Eqn (1) was utilized to calculate the leaching efficiencies (LEs) of the solutions, where m_l represents the mass of dissolved metal ions subsequent to leaching and m₀ denotes the initial weight of the metal.

The pH of the optimum leach solution was increased up to 8 by introducing ammonium hydroxide solution while mixing at 800 rpm at room temperature. The pH values were continuously measured by using a pH Meter (Hanna Edge HI2020-01) with 0.01 pH precision. After the pH adjustment, the solution was kept overnight to complete the co-precipitation process. Solid–liquid separation was conducted by using a DLAB centrifuge device at 4100 rpm for 10 minutes. The obtained precipitates were dried at 90 °C for 12 hours. The chemical and morphological characterization of the precipitate was conducted by FE-SEM-EDS (Thermo Scientifıc Apreo 2 S LoVac) analysis. Subsequently, the precipitates were calcined at 500, 750 and 1000 °C for 2 hours and the obtained samples were characterized using XRD (PANalytical X'pert Pro), FE-SEM-EDS (Thermo Scientifıc Apreo 2 S LoVac) and BET (Micromeritics 3Flex) analyses.

Production of coin cell type supercapacitors and the electrochemical tests

The synthesized ZnO–Fe₃O₄ was mixed with graphite and polyvinylidene fluoride (PVDF) in a weight ratio of 8 : 1 : 1 to function as the anode of the asymmetric supercapacitor. A combination of graphite and PVDF was then synthesized in a mass ratio of 9 : 1 to serve as the cathode. The PVDF was dissolved in 1 mL of NMP (N-methyl pyrrolidone) while stirring at a rate of 400 rpm for a duration of 5 minutes. Following this, the active compounds were incorporated into the system and agitated at a speed of 800 rpm for a duration of 30 minutes. A thickness of 200 μm was accurately attained by applying the resulting mixtures onto graphite foil utilizing the doctor blade technique. The films that were produced underwent a drying process at 90 °C for a duration of 12 hours. The films were subsequently cut into circular shapes with a diameter of 16 mm for placement into the CR2032 coin cell. The electrodes were meticulously separated using a Whatman separator, with a thickness of 25 micrometers. The mass loadings of the obtained electrodes are given in Table S2. A 6 M KOH solution served as the electrolyte, and subsequently, the coin cell device was pressed and sealed. The supercapacitors were subjected to electrochemical characterization using a Gamry Interface 1010 Potentiostat/Galvanostat for Cyclic Voltammetry (CV), Galvanostatic Charge–Discharge (GCD), and Electrochemical Impedance Spectroscopy (EIS) analyses. The specific capacitance (C_s) values of the supercapacitors were determined through CV analysis utilizing eqn (2), where a denotes the area of a single electrode, ΔV indicates the potential window, and v represents the scanning rate.

Additionally, GCD tests were conducted at the current densities of 0.25, 0.5, 1, 1.5 and 2 mA cm⁻². The operational life of the supercapacitors was assessed through GCD analyses over a span of 7000 cycles at a current density of 1.5 mA cm⁻². Energy density (E) and power density (P) of the supercapacitor were calculated using eqn (3) and (4) respectively, where ΔV represents the potential window and Δt is the discharge time.

Furthermore, EIS analyses were performed across the frequency range of 10 kHz to 10 mHz both before and after the long cycle GCD tests. Analyses were conducted using AC with a 10 mV amplitude over the frequency range of 0.01 Hz to 100 kHz. The workflow of the experimental procedure is presented in Fig. 1.

Fig. 1 The workflow of the experimental procedure.

Results and discussion

Pre-treatment and characterization of EAFD

The results of the X-ray fluorescence (XRF) analysis shown in Table 1 reveal that EAFD predominantly consists of zinc and iron, along with calcium, silicon, lead, magnesium, manganese, aluminum and phosphorus, which are the elements commonly associated with byproducts generated during iron and steel production.^30–32

Table 1 Chemical composition of EAFD obtained by XRF

Element	Fe	Zn	Ca	Pb	Mn	Si	Mg	Al	P
wt%	36.94	43.40	6.10	3.41	3.15	2.35	1.65	0.55	0.28

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Additionally, FE-SEM-EDS was employed to perform a detailed morphological and elemental characterization of electric arc furnace dust (EAFD). The FE-SEM images reveal the presence of agglomerated fine spherical or nearly circular particles, which are typical of EAFD and consistent with morphologies reported in previous studies (Fig. 2).^31,33,34 These agglomerates result from the rapid cooling and condensation of metal vapors during the furnace operation, leading to the formation of submicron-sized particles that tend to cluster together via electrostatic attraction between the particles.^22,35 Moreover, the elemental distribution observed through EDS mapping, as illustrated in Fig. 2(b), corroborates the elemental composition previously identified via XRF analysis. Key elements such as Zn, Fe, O, and trace amounts of other constituents were uniformly distributed across the sample, further validating the homogeneity of the EAFD sample.

Fig. 2 FE-SEM image of the (a) EDS mapping area of EAFD at 10 000× magnification, (b) EDS mapping results of EAFD, (c) FE-SEM image of EAFD at 50 000× magnification, and (d) FE-SEM image of EAFD at 100 000× magnification.

The phase distribution within the sample was determined by examining the XRD pattern presented in Fig. 3, and the predominant phases in the sample were identified as ZnO (98-008-1689), ZnFe₂O₄ (98-006-0554), and Fe₃O₄ (98-007-8202). These phases are known to be characteristic of EAFD and align well with the findings reported in previous studies.^26,36 The presence of ZnFe₂O₄ and Fe₃O₄ suggests a significant incorporation of iron into the spinel structure, which is commonly observed due to the high-temperature oxidative environment within the furnace. Additionally, the presence of ZnO further indicates that a substantial portion of the zinc remains in an oxidized form, typically associated with the volatilization-condensation mechanism of zinc during EAF operations.²² This phase composition provides valuable insight into the thermal history and chemical behavior of EAFD, which is essential for evaluating its potential for subsequent hydrometallurgical recovery or other recycling strategies.

Fig. 3 XRD pattern of EAFD.

Recovery of ZnO–Fe₃O₄ from EAFD

EAFD is predominantly composed of the highly stable spinel-structured franklinite phase, which poses significant challenges for recovery through hydrometallurgical routes.²⁶ Due to its thermodynamic stability, efficient dissolution of this phase typically necessitates harsh processing conditions, including elevated temperatures, extended leaching durations, and high acid concentrations.²⁷ Within the scope of this study, the influence of these critical parameters on the leaching efficiencies of zinc and iron was systematically examined.

In the preliminary stage of the leaching experiments, a solid-to-liquid (S/L) ratio of 125 g L⁻¹ and a constant temperature of 80 °C were maintained to investigate the influence of acid concentration and leaching duration on the dissolution efficiencies of zinc and iron. As shown in Fig. 4(a) and (b), at an acid concentration of 2 M, zinc and iron leaching efficiencies reached approximately 90% and 50%, respectively. It was observed that variations in leaching time had a negligible impact on metal recovery, suggesting that equilibrium was rapidly achieved under these conditions. Upon increasing the acid concentration to 3 M, leaching efficiencies marginally improved to 92% for Zn and 55% for Fe. Further elevation of the acid concentration to 4 M led to a more pronounced increase, with zinc and iron recoveries increasing to 95% and 75%, respectively. These findings demonstrate that acid concentration plays a critical role in enhancing metal solubilization, particularly for iron, which tends to form more stable oxide phases.

Fig. 4 The effect of acid concentration and time on the LE of (a) Zn and (b) Fe, the effect of temperature and acid concentration on the LE of (c) Zn and (d) Fe, and the effect of the solid/liquid ratio and acid concentration on the LE of (e) Zn and (f) Fe.

In the second phase of the study, the S/L ratio was kept constant at 125 g L⁻¹ and the leaching time fixed at 150 minutes, to isolate and assess the effects of temperature and acid concentration on the leaching efficiencies (Fig. 4(c) and (d)). At a relatively lower temperature of 60 °C, zinc leaching efficiencies ranged between 80% and 90% as the acid concentration increased. However, at elevated temperatures of 80 °C and above, zinc recovery consistently ranged from 90% to 95%, regardless of acid concentration. A similar trend was observed for iron, where both higher temperatures and acid concentrations contributed to significantly enhanced dissolution. These results emphasize the synergistic effect of temperature and acid strength in breaking down the complex oxide and spinel structures present in EAFD, thereby facilitating improved metal release into solution.

In the final phase, the impact of the solid-to-liquid ratio on leaching performance was examined by maintaining the temperature at 80 °C and the leaching duration at 150 minutes (Fig. 4(e) and (f)). This parameter was found to exert a particularly strong influence on iron recovery. Specifically, when the S/L ratio was increased to 200 g L⁻¹, the Fe leaching efficiency decreased to around 50%, likely due to reduced acid availability per unit mass of solid and hindered mass transfer. Conversely, lowering the S/L ratio to 50 g L⁻¹ significantly improved Fe recovery, reaching approximately 80%. Zinc recovery remained relatively stable across varying S/L ratios, indicating that it is less sensitive to this parameter under the examined conditions.

Table S1 displays the results of all leaching experiments performed on EAFD. A comprehensive analysis of the data reveals that the leaching efficiencies of Zn²⁺ and Fe³⁺ reached 98.3% and 90.1%, respectively, suggesting that the results are quite satisfactory in comparison to similar studies in the literature.^27,37,38

In the subsequent stage of the leaching process, the simultaneous co-precipitation of Zn²⁺ and Fe³⁺ ions from the leach solution was successfully carried out in a single-step operation, aiming to recover both metal species in a combined solid phase. After the co-precipitation process, Zn²⁺ and Fe³⁺ concentrations of the remaining solution revealed that both metal ions were precipitated with an efficiency greater than 99%. Although certain impurity elements present in EAFD were partially dissolved in the leaching process, their concentrations were too low to be incorporated into the final solid phase. During the pH adjustment and precipitation steps, these trace impurities preferentially remained in the solution rather than co-precipitating with the target species. As depicted in the FE-SEM images shown in Fig. 5, the morphology of the obtained precipitate is characterized by a highly amorphous and agglomerated structure, which is indicative of rapid nucleation and limited crystalline growth during the precipitation reaction. The irregular and clustered appearance of the particles suggests the formation of a poorly ordered phase, commonly observed under non-equilibrium conditions.

Fig. 5 (a) FE-SEM image of the EDS mapping area of the precipitate and the mapping results at 10 000× magnification, (b) FE-SEM image of the precipitate at 50 000× magnification, and (c) FE-SEM image of the precipitate at 200 000× magnification.

EDS mapping further validates the elemental composition of the precipitate. The maps confirm the uniform distribution of Zn, Fe, and O throughout the material, consistent with the targeted co-precipitation of metal hydroxides or mixed oxides. Additionally, the presence of sulfur was detected in trace amounts, which is likely a residual impurity originating from the H₂SO₄ employed during the leaching stage. The incorporation of sulfur into the solid phase may occur through surface adsorption or the formation of sulfate-containing secondary phases.³⁹

Subsequently, calcination treatments were applied to the precipitate at temperatures of 500 °C, 750 °C, and 1000 °C with the aim of disrupting the agglomerate structure, eliminating sulfur impurities, and promoting crystallization. XRD patterns of the calcined samples are presented in Fig. 6. Analysis of the diffraction peaks for the sample calcined at 500 °C revealed the presence of ZnO (98-002-8922) as well as various iron oxides, FeO (98-004-1255), Fe₂O₃ (98-007-0037), and Fe₃O₄ (98-001-1960) corresponding to different oxidation states. Additionally, ZnS (00-036-1450) and FeS (01-076-0965) phases were identified, indicating the incorporation of sulfur into the solid structure. This incorporation is attributed to the presence of sulfate ions in the leach solution, originating from the use of H₂SO₄ during leaching. Upon neutralization of the solution to approximately pH 8, these sulfur species likely co-precipitated with zinc and iron, forming sulfide compounds within the solid matrix.

Fig. 6 XRD pattern of the samples calcined at (a) 500 °C, (b) 750 °C and (c) 1000 °C.

In the sample calcined at 750 °C, the observed diffraction peaks became significantly sharper, indicating an increase in crystallinity. Furthermore, the sulfur-containing phases were no longer detected, suggesting that the sulfide compounds were successfully oxidized during the thermal treatment.^40–42 The XRD results confirmed that the structure at this temperature predominantly consisted of ZnO (JCPDS no: 01-075-1526) and Fe₃O₄ (JCPDS no: 01-075-0033) phases, signifying the formation of well-crystallized oxide materials. For the sample calcined at 1000 °C, the peaks were observed to be even more intense and well-defined, further indicating enhanced crystallinity. However, no additional or new phases were detected at this temperature, implying that the phase composition remained stable, consisting exclusively of ZnO and Fe₃O₄.

Based on the N₂ adsorption–desorption curves given in Fig. 7, BET surface area of the precipitate and the calcined samples was determined to be 10.61, 14.71, 47.19 and 0.88 m² g⁻¹ respectively. Additionally, the samples demonstrated type IV isotherms with a hysteresis loop which indicate a mesoporous structure.⁴³

Fig. 7 N₂ adsorption–desorption curves of the (a) precipitate at (b) 500 °C, (c) 750 °C and (d) 1000 °C.

FE-SEM analysis of the sample calcined at 500 °C (Fig. 8(a)–(c)) revealed the deterioration of the amorphous structure, resulting in the formation of fine, spherical particles. This structural transformation led to a moderate increase in surface area (14.71 m² g⁻¹) compared to the as-precipitated sample (10.61 m² g⁻¹). Additionally, EDS data confirmed a substantial presence of sulfur within the structure, consistent with the incorporation of sulfate species during precipitation (Fig. 8(d)). Increasing the calcination temperature to 750 °C led to the formation of a well-developed mesoporous structure, as confirmed by a significant increase in surface area to 47.19 m² g⁻¹ and the disappearance of sulfur-based impurities with only trace amounts remaining (Fig. 8(e)–(h)). At 1000 °C (Fig. 8(i)–(l)), sulfur was completely removed from the structure; however, the microstructural analysis indicated pronounced grain growth and evidence of interparticle sintering, leading to a drastic reduction in the surface area (0.88 m² g⁻¹). Given the detrimental impact of particle coarsening and sintering on electrochemical performance, particularly in terms of surface area and charge transport, the sample calcined at 750 °C was identified as the optimal candidate for the supercapacitor application.

Fig. 8 (a) EDS mapping area, (b) 50 000× magnification, (c) 200 000× magnification, and (d) EDS mapping and the spectrum of the sample calcined at 500 °C. (e) EDS mapping area, (f) 50 000× magnification, (g) 200 000× magnification, and (h) EDS mapping and the spectrum of the sample calcined at 750 °C. (i) EDS mapping area, (j) 50 000× magnification, (k) 200 000× magnification, and (l) EDS mapping and the spectrum of the sample calcined at 1000 °C.

Production of coin cell type supercapacitors and the electrochemical tests

The electrochemical performance of the produced supercapacitor was evaluated by CV, GCD and EIS tests. In the first stage, cyclic voltammetry analyses were performed at a scan rate of 100 mV s⁻¹ in various potential windows to identify the suitable operational voltage range of the supercapacitor (Fig. 9(a)). The supercapacitor has been determined to operate effectively within the voltage range of 0 to 1.6 V. However, the gas evaluation was initiated at 1.8 V and became more pronounced at 2 V.^44–46 Consequently, the optimal operating voltage range was established to be from 0 to 1.6 V, and all following electrochemical characterization studies were performed within this safe potential range. Additionally, CV analyses at 50 mV s⁻¹ were conducted for both electrodes in a three electrode configuration to determine the working potential range of individual electrodes and their contributions to charge storage mechanism of the device (Fig. S1). The specific capacitance of the ZnO–Fe₃O₄ based cathode and the graphite based anode was determined to be 275 and 97 mF cm⁻², respectively. The graphite electrode exhibited nearly rectangular CV curves, confirming its electric double-layer capacitance (EDLC) behavior, whereas the ZnO–Fe₃O₄ electrode displayed broad redox features, indicative of faradaic charge storage.⁴⁷ These results demonstrate that the device operates via a hybrid storage mechanism, in which the double-layer contribution of activated carbon is complemented by the pseudocapacitive contribution of ZnO–Fe₃O₄. Moreover, the potential separation of the electrodes allows the full device to reach an ultra-high cell voltage while each electrode individually remains within the thermodynamically stable window of the aqueous electrolyte.^48,49 Subsequently, CV analyses of the coin cell type supercapacitor were conducted at various scanning rates ranging from 10 to 500 mV s⁻¹ within the specified potential range (Fig. 9(b)). The fact that the current increases with the scan rate in the cyclic voltammetry experiments suggests that diffusion-controlled processes are the primary mechanism by which the supercapacitor stores charge.⁵⁰ This result shows that ions cannot move from the electrolyte to the active sites of the electrode instantaneously. Instead, the rate at which ions may diffuse through the electrolyte and the porous structure of the electrode material limits the movement of ions.⁵¹ Reducing scan rates facilitate the movement of electrons toward the active sites in the bulk electrode and encourage ion migration throughout the electrolyte. Consequently, the slopes of the cyclic voltammograms diminished with reduced scan rates, illustrating the ideal capacitive behavior.⁵² Additionally, the specific capacitance of the supercapacitor at a scanning rate of 10 mV s⁻¹ was determined to be 48 mF cm⁻².

Fig. 9 Cyclic voltammograms of the supercapacitor in (a) different potential ranges and (b) at different scan rates.

GCD analyses were carried out at current densities of 0.25, 0.5, 1, 1.5, and 2 mA cm⁻² to further investigate the capacitive behavior of the supercapacitor (Fig. 10). The presence of a non-linear change in the potential with respect to time is indicative of a deviation from ideal double-layer capacitive behavior. Instead of exhibiting a perfectly linear potential–time profile characteristic of EDLCs, the observed curvature implies the involvement of faradaic processes, where charge storage occurs not only through electrostatic accumulation at the electrode/electrolyte interface but also via fast, reversible redox reactions.^50,53 The possible faradaic reactions taking place in the charge–discharge process with 6 M KOH electrolyte are described as follows.^54–57

ZnO + OH⁻ ⇄ ZnOOH + e⁻ (5)

ZnO + K⁺ + e⁻ ⇄ ZnOK (6)

Fe₃O₄ + OH⁻ ⇄ Fe₃O₄OH + e⁻ (7)

Fe₃O₄ + H₂O + OH⁻ ⇄ 3FeO(OH) + e⁻ (8)

Fig. 10 GCD curves at different current densities.

Additionally, the presence of electric double-layer capacitance is indicated by a consistent and direct relationship between time and potential in the remaining sections of the discharge curves.⁵⁸ Moreover, it was observed that the discharge potential experienced a negligible decrease following the charging phase, suggesting the exceptional galvanostatic behavior.^59,60 At a current density of 0.25 mA cm⁻², the specific capacitance, energy density and power density of the supercapacitor are determined to be 35.2 mF cm⁻², 25.03 μWh cm⁻² and 430.81 μW cm⁻² respectively.

Moreover, GCD tests were conducted for 7000 cycles at a current density of 1.5 mA cm⁻² to evaluate the long-term cycling stability of the fabricated supercapacitor. The results revealed that the device retained 81% of its initial specific capacitance after 7000 cycles, indicating good electrochemical durability and stable performance over extended operation (Fig. 11(a)). The nearly equal mass loadings of the two electrodes were found to be sufficient to establish a balanced charge distribution within the device which enabled the device to operate stably over an extended potential window, which is reflected in the long-term cycling stability and the enhanced energy storage performance. Moreover, the coulombic efficiency remained around 100% throughout the entire cycling test, indicating ideal supercapacitive behavior with excellent charge–discharge reversibility (Fig. 11(b)).

Fig. 11 (a) Capacitance retention and (b) coulombic efficiency of the supercapacitor.

Furthermore, EIS analysis was performed before and after the long-cycle GCD testing, and the resulting data were fitted to an equivalent circuit. Fig. 12 presents the Nyquist plots obtained for the supercapacitor. The intersection point on the real axis in the high-frequency region of the Nyquist plot corresponds to the solution resistance (R_s). This resistance consists of three main components: the ionic resistance of the electrolyte, the intrinsic resistance of the active material, and the contact resistance at the interface between the active material and the current collector.^61,62 The semicircular zone in the Nyquist plot represents the electron transfer kinetics of redox processes, with its diameter proportional to the charge transfer resistance (R_ct). R_ct is an important quantity that describes the resistance encountered during faradaic reactions at the electrode–electrolyte interface.⁶³ It represents the ease with which electrons move between the electrode surface and the electroactive species in the electrolyte. Lower R_ct values suggest faster charge transfer kinetics and increased electrochemical activity of the electrode.⁶⁴ The Warburg impedance (W), which is a measure of the electrolyte's resistance to diffusion within the porous medium, is represented by the subsequent linear segment that follows the semicircle. A more rapid ion transfer rate between the electrode and electrolyte is indicated by a steeper gradient in this region.^65–67 The fitting results were employed to derive the values of R_s, R_ct, W, and C_dl and the corresponding values are presented in Table 2. The increase in the R_s value following 7000 cycles can be attributed to the decrease in OH⁻ ion concentration in the electrolyte during the charge–discharge process of the supercapacitor. As the cycle number increased, the accumulation of discharge products on the electrode surface resulted in a decrease in the quantity of OH⁻ ions present in the solution as a consequence of the charge/discharge process. Therefore, the R_s value exhibited an upward trend. Moreover, after 7000 cycles, the R_ct value exhibited an increase. The reduction in OH⁻ ion concentration in the electrolyte during each cycle impeded the reactions (eqn (5)–(8)) that were responsible for the formation of pseudocapacitive behavior. As a result, the contribution of redox processes in enhancing capacitive behavior diminished as the number of cycles increased, resulting in a drop in the specific capacitance after 7000 cycles.⁶⁸ On the other hand, an increase in both C_dl and W values may indicate the development of a more porous or fractured surface structure on the electrode during long-term cycling, which can expose additional electrode/electrolyte interfaces.^60,69 However, the observed increase in Warburg impedance suggests that ion diffusion has become more restricted, likely due to structural degradation, pore collapse, or the accumulation of resistive surface layers. Although the higher C_dl value implies an increased electrochemically accessible surface area, the concurrent decrease in initial capacitance after 7000 cycles indicates that these newly exposed areas may be less effective for charge storage, possibly due to reduced electrical connectivity or the formation of less active surface sites.

Fig. 12 Nyquist plots of the supercapacitor before and after the long-cycle GCD test.

Table 2 Fitted EIS results for the initial condition and at the end of 7000 cycles

	R s (Ω)	R ct (Ω)	C dl × 10^−6 (F)	W × 10^−3
1st cycle	4.49	11.33	44.43	5.916
7000th cycle	40.31	33.30	180.40	7.744

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Table 3 provides a comparative analysis between the electrochemical performance achieved in the current study and previously reported values for ZnO and Fe₃O₄ based supercapacitors in the literature. Notably, despite being produced from waste-derived materials, the supercapacitor developed in this work exhibits outstanding electrochemical performance and remarkable cycling stability, highlighting its potential for sustainable and high-performance energy storage applications.

Table 3 Electrochemical comparison of the ZnO and Fe₃O₄ based supercapacitors

Electrode compound	Electrode system	Electrolyte	Working potential range (V)	Specific capacitance	Capacitance retention	Ref.
Fe3O4 nanosheets	Three electrode	3 M LiCl	0 to 2	20.8 mF cm^−2 at 10 mV s^−1	91.7% after 2500 cycles	70
PEDOT@MnO2‖C@Fe3O4	Three electrode	1 M LiCl	0 to 2	60 mF cm^−2 at 0.9 mA	80% after 800 cycles	71
Fe3O4 incorporated carbon nanotube	Two electrode – symmetric	1 M LiPF6	0 to 2	78.5 mF cm^−2 at 1 mA cm^−2	80% after 800 cycles	72
BiFeO3	Two electrode – symmetric	1 M Na2SO4	0 to 1	7.4 mF cm^−2 at 10 mV s^−1	84% after 1000 cycles	73
Hydrogenated ZnO core–shell nanocables	Three electrode	LiCl/PVA gel	0 to 0.8	26 mF cm^−2 at 0.5 mA cm^−2	87.5% after 10 000 cycles	74
ZnO/MnO	Three electrode	1 M Na2SO4	0 to 0.8	14 mF cm^−2	—	75
ZnO/carbon nanotube	Two electrode – symmetric	1 M H2SO4	0 to 1.2	14 mF cm^−2 at 50 mV s^−1	—	76
ZnO/carbon nanowalls	Three electrode	1 M KCl	0 to 0.7	4.3 mF cm^−2 at 0.2 mA cm^−2	>100% after 26 000 cycles	77
ZnO/reduced graphene oxide	Three electrode	0.1 M KCl	0 to 0.7	19 mF cm^−2 at 1 mV s^−1	100% after 10 000 cycles	78
ZnO–Fe3O4	Two electrode – asymmetric	6 M KOH	0 to 1.6	35.2 mF cm^−2 at 0.25 mA cm^−2	81% after 7000 cycles	This work

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Conclusions

In this study, ZnO–Fe₃O₄ nanocomposites were successfully synthesized from EAFD through a hydrometallurgical route involving high-efficiency leaching and co-precipitation, followed by thermal treatment. The optimized process achieved leaching efficiencies of 98.3% for Zn and 90.1% for Fe, while the co-precipitation step ensured effective recovery of both metals. The sample calcined at 750 °C exhibited high crystallinity and minimal impurities, making it ideal for supercapacitor applications. The fabricated asymmetric coin-cell supercapacitor delivered promising electrochemical performance, including a specific capacitance of 35.2 mF cm⁻² and excellent stability, retaining 81% of its capacity after 7000 cycles. These results demonstrate the successful transformation of hazardous industrial waste into a high-value functional material with competitive energy storage performance. Overall, this work offers a sustainable and economically viable approach for both mitigating the environmental impact of EAFD and enabling the production of advanced electrode materials. This dual-purpose approach highlights the synergy between waste valorization and materials innovation, serving as a model strategy for circular and green technologies in energy storage.

Author contributions

All authors contributed to the conception and design of the study. Material preparation and data collection were performed by Ozan Aydin, Metin Gencten, and Burak Birol, and analyses were performed by Koray Bahadır Donmez, Metin Gencten and Burak Birol. The first draft of the manuscript was written by Ozan Aydin, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in the current paper.

Data availability

Data for this article are available at Github at https://github.com/ozanaydin432/Upcycling-of-EAFD-into-ZnO-Fe3O4-Nanocomposites-for-High-Performance-Supercapacitor-Applications.

Supplementary information: the results of the leaching experiments, along with the mass loadings and the cyclic voltammograms of the electrodes, is available. See DOI: https://doi.org/10.1039/d5ta06054k.

Acknowledgements

The authors would like to acknowledge that this paper is submitted in partial fulfilment of the requirements for PhD degree at Yildiz Technical University. This work has been supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK) under project number 123M490. O. Aydin would like to express his gratitude for the support of the TÜBİTAK BİDEB-2211-A Program. Moreover, M. Gencten thanks the Turkish Academy of Sciences for the Outstanding Young Scientists Award (GEBİP).

Notes and references

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