Scalable hematite based symmetric supercapacitor with high capacitance for energy storage applications

1 Introduction

Scalable, low-cost electrode materials with high energy and power densities, as well as simple fabrication, are essential to broaden the application of supercapacitors (Awasthi et al., 2024, Li et al., 2019). Unlike batteries, supercapacitors store energy through electric double-layer capacitors (EDLC) or through pseudocapacitors (Liu et al., 2014), unlike diffusion-limited electrochemical reactions of a time-intensive nature. EDLCs provide high power density along with long cycle life. However, energy density is limited within EDLCs, which are typically based on materials such as activated carbon, graphene, and carbon nanotubes. Pseudocapacitors, which do employ transition metal oxides or conductive polymers, can deliver higher energy density because Faradaic reactions can be fast and reversible but materials may degrade, reducing long-term stability (Awasthi et al., 2023).

Transition metal oxides (TMOs), Fe₂O₃, MnO₂, and RuO₂, along with conductive polymers, are frequently used in pseudocapacitors due to their superior redox activity, high conductivity, high specific capacitance, power density, and low cost (Liu et al., 2014). α-Fe₂O₃ (hematite) has gained interest due to its unique structural properties, low toxicity, and environmental friendliness. This stable iron oxide with a rhombohedral crystal structure provides a robust framework for electrochemical processes (Liu et al., 2014). Thus, by incorporating α-Fe₂O₃, supercapacitors can achieve improved energy density while maintaining the high power density and cycle stability required for advanced energy storage applications (Lu et al., 2019).

However, most prior studies on α-Fe₂O₃ have emphasized tuning nanostructures via complex or non-scalable synthesis methods, often overlooking material repeatability and production scalability, for instance, hydrothermal synthesis methods have been reported (Singh et al., 2025), where they have been assembled as asymmetric device α-Fe₂O₃//NiO exhibited 797 F/g at 2 A/g that delivered energy density of 25.31 Wh/Kg. Although the capacitance and power density are high, the asymmetric Supercapacitor configuration, which should theoretically enhance performance, still yielded a relatively modest energy density.

In another work, Microwave assisted and high temperature annealing produced hematite delivered a specific capacitance of 454.6 F/g, corresponding to an energy density of 48 Wh/Kg (Lu et al., 2019), Even when the synthesis was adjusted to produce nanorods annealed at 500 °C in N₂, the energy density remained limited at 85.7 Wh/kg, despite a higher specific capacitance of 935 F/g. A similar hydrothermal approach was used to prepare an α-Fe₂O₃/rGO/SnO₂ composite, which exhibited a specific capacitance of 821 F/g at 1 A/g with an energy density of 25 Wh/kg in an asymmetric device configuration (Geerthana et al., 2022). Another study reported the synthesis of hematite structured α-Fe₂O₃ and CuO doped α-Fe₂O₃, by green method with Glycyrrhiza glabra plant extract, employed in a three electrode system which exhibited capacitance of 154.61 F/g (Latthe et al., 2025). Recently, (Vivekanandan et al., 2024), reported the synthesis of α-Fe₂O₃ nanoparticles by the hydrothermal method, where specific capacitance achieved from CV and GCD were 299.4 F/g and 351.3 F/g, respectively. Collectively, these studies indicate that enhanced electrochemical performance in hematite is often achieved through composite engineering or asymmetric device configurations. In contrast, achieving comparable performance using pristine hematite in a simple and scalable configuration remains underexplored, which motivates the present work.

In contrast to prior studies reliant on complex nanostructuring or reactor-dependent techniques, our work demonstrates that electrochemical performance can be achieved through a reproducible, low-cost, and scalable approach. α-Fe₂O₃ (hematite), a phase commonly found in rust (corrosion products of iron), was synthesized via a scalable hydrolysis-co-precipitation route using FeCl₃ as the precursor. which was assembled into a symmetric CR2032 coin cell using Nafion as a binder and copper foil as the current collector, departing from the conventional use of PVDF and stainless steel. Nafion, due to its high ionic conductivity and structural adhesion (Pradhan and Parida, 2011), combined with copper’s electrochemical performance, enabled electrolyte interaction and simplified device architecture without sacrificing performance. This streamlined fabrication strategy significantly reduces processing complexity and cost while ensuring consistent material performance. Notably, the demonstrated electrochemical stability and power capability make this approach promising for energy storage systems, where both reliability and scalability are critical.

2 Experimental section

2.1 Materials

Ferric chloride (FeCl₃, 98%), N-Methyl-2-pyrrolidone (NMP 99.5%), ammonium hydroxide solution (NH4OH,28%), Nafion 117 (∼5%) and ashless Whatman filter paper were obtained from Sigma-Aldrich (USA). Copper foil (0.05 mm thickness, 99.9% purity) and carbon black (commercial grade) were procured from Akshar Chem (India). Isopropanol (IPA, 99.9%) and ethanol (99.9%) were purchased from Merck (India). Type II water was used in all experiments.

2.2 Material synthesis

FeCl₃ was used as precursor for the synthesis of α-Fe₂O₃. A 0.3 M FeCl₃ solution was prepared in 100 mL of Type II water. The mixture was stirred continuously under magnetic agitation at 80 °C to ensure complete dissolution and achieve a homogeneous solution. During the stirring process, a distinct colour change was observed due to the hydrolysis of Fe³⁺ ions leading to the formation of Fe(OH)₃ (Farahmandjou and Soflaee, 2015), indicating the progression of the reaction. To facilitate the formation of iron hydroxide, NH₄OH was added dropwise as a precipitating agent until the pH reached ∼10, ensuring complete precipitation. This gradual addition led to the formation of a brown precipitate. The precipitate was collected via filtration using Ashless Whatman filter paper, and washed thoroughly for 5-6 cycles with Type II water to remove any residual impurities.

The filtered precipitate was dried at 80 °C temperature for 12 h to ensure complete moisture removal. This powder was then calcined at 300 °C at a ramp rate of 10 °C/min, then held for 3 h to induce the transformation to α-Fe₂O₃. As shown in Figure 1 during calcination, the material underwent dehydration and crystallisation, resulting in the formation of α-Fe₂O₃. This was confirmed by its characteristic reddish-brown colour and characteristics properties; the experimental yield was ∼60%. This process ensured the synthesis of well-formed α-Fe₂O₃, which is suitable for subsequent applications.

2.3 Electrode & device fabrication

The synthesised α-Fe₂O₃ was utilised as the active material. A slurry was prepared by using 80% α-Fe₂O₃, 10% carbon black as a conductive additive, and 10% Nafion as a binder. These materials were ground using a mortar and pestle to achieve fine particle dispersion, and then with NMP, a uniform and viscous slurry was prepared. The slurry was coated onto a 1 × 1 cm² area of the copper foil. The electrodes were then vacuum dried at 90 °C overnight. The mass loading of the active material was ∼3 mg/cm², and the average electrode coating thickness was approximately 20–30 µm.

The prepared electrode was then assembled into a CR2032 coin cell for electrochemical analysis. During the assembly, Whatman filter paper was used as the separator, 6M KOH as the electrolyte and α-Fe₂O₃-coated copper foil as the electrode. The components were stacked as per Figure 2 and sealed carefully using the crimping machine to avoid electrolyte leakage.

3 Results & discussion

3.1 Material characterisation

In this study, the electrochemical properties of α-Fe₂O₃ were investigated as an electrode material for supercapacitors, with the aim of enhancing key performance parameters, including specific capacitance and energy density. The hematite based electrodes were incorporated into compact coin cell devices to evaluate the feasibility in real-time configurations. The sample has been studied through characterisations like X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Elemental mapping, etc. Furthermore, the electrochemical analysis was performed using the Origaflex OGF500 instrument.

3.2 FE-SEM and EDAX analysis

The morphological and compositional characteristics of the synthesized α-Fe₂O₃ nanoparticles were investigated using field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray analysis (EDAX), as shown in Figure 3.

Figure 3a illustrates that the α-Fe₂O₃ nanoparticles displayed a spherical form when observed at a 500 nm range for electrochemical reactions such as ion intercalation, redox reactions, and charge transfer activities during charge-discharge cycles. The spherical morphology of α-Fe₂O₃ nanoparticles facilitates faster ion diffusion, which can contribute to enhanced energy and power density. This increased interfacial contact can promote efficient ion transport, redox activity, and charge transfer during cycling. Nanoparticles in the ∼100–200 nm range are commonly associated with high electrochemical reactivity in pseudocapacitive materials, thereby contributing to improved capacitance in Supercapacitor applications (Kuku, 2025). As seen in Figure 3c The average particle size of the nanoparticles was approximately 145 nm.

Compositional analysis through Energy Dispersive X-ray Analysis (EDAX) further confirmed the presence of iron (Fe) and oxygen (O), as shown in Figure 3b. Furthermore, the homogeneity of the synthesized α-Fe₂O₃ is highlighted by the elemental composition analysis shown in Figure 3d. This demonstrates the uniform dispersion of Fe and O throughout the sample to further confirm XPS analysis (Supplementary Figure S1) has also been done along with the EDAX. One important factor influencing the electrochemical characteristics of α-Fe₂O₃ is its elemental composition. According to the EDAX analysis, the homogeneity and effective charge distribution of the material are guaranteed by the uniform distribution of iron (Fe) and oxygen (O) throughout. To minimize localized resistances and guarantee consistent performance throughout the material, homogeneity is essential.

The performance of charge storage methods, such as electrical double-layer capacitance and pseudo capacitance, in Supercapacitor applications is determined by the interaction of morphology, elemental content, and particle size. The pseudo-capacitive characteristic of α-Fe₂O₃ spherical nanoparticles, which is fuelled by redox processes involving Fe ions, allows for high specific capacitance. High energy density and power density are guaranteed by the optimized particle size, while conductivity and charge homogeneity are improved by the consistent elemental composition. Nitrogen adsorption-desorption analysis was carried out to examine the surface area (Supplementary Figure S2) and pore structure of hematite material exhibits a type IV isotherm (Ma et al., 2010), confirming mesoporous characteristics with an average pore diameter of 63.3 nm.

X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the elemental composition and chemical states of the synthesized material (Supplementary Figure S1). The survey spectrum confirms the presence of Fe and O and a small amount of carbon which is commonly used as a reference for binding energy calibration. The high-resolution Fe 2p spectrum exhibits characteristic peaks at 710.5 eV (Fe 2p_3/2) and 724–725 eV (Fe 2p_1/2) along with satellite peak around 719 eV, which are typical signatures of Fe³⁺ in hematite (Pradhan and Parida, 2011). The O1s spectrum shows a dominant peak at 529 eV corresponding to lattice oxygen (O^2-) and a secondary peak near ∼532 eV attributed to surface hydroxyl. These results confirm the formation of hematite (Xu et al., 2022).

3.3 X-ray diffraction analysis

The crystal structure and crystal size of α-Fe₂O₃ nanoparticles has been studied using XRD as shown in Figure 4. The study was conducted over a 2θ range of 20°–80° at a scan rate of 4° per minute. By closely matching the reference pattern in the Joint Committee on Powder Diffraction Standards (JCPDS) file, the resulting diffraction pattern showed that the synthesized material was well aligned with the standard structural features of α-Fe₂O₃. The XRD pattern shows eight prominent peaks according to the Miller indices (104), (110), (012), (113), (024), (116), (214), and (300). The rhombohedral hematite structure of α-Fe₂O₃ was verified by the (104) and (110) peaks. According to JCPDS 00-033-0664, the presence of these peaks confirms the effective synthesis of pure α-Fe₂O₃ without secondary phases or impurities.

Ɗ = k λ β ⁡ cos ⁡ Ɵ (1)

Using the Scherrer equation (Equation 1), the average crystalline size of the α-Fe₂O₃ nanoparticles was estimated to be 43.7 nm. Increasing the surface area of the material improves its electrochemical activity and energy storage capability.

3.4 Electrochemical analysis

The electrochemical performance of the composite material was comprehensively evaluated using EIS, CV, and GCD analyses. EIS analysis is depicted in Figure 5a revealed a solution resistance (Rs) of approximately 1.26 Ω, which corresponds to the resistance arising from the electrolyte and electrode contact. The Nyquist plot exhibited nearly straight-line behaviour without a well-defined semicircle, suggesting the presence of diffusion related contribution. The initial slight curvature at low Zr values is attributed to the solution resistance (Rs), while the absence of a clear semicircle suggests that the charge transfer resistance (Rct) is overshadowed by diffusion contribution. The linear region observed at low frequencies indicates diffusion related behaviour, suggesting the contribution of ion transport process in the system.

The CV curves are shown in Figure 5b display quasi-rectangular shapes with distinct redox peaks. The presence of redox peaks is indicative of faradaic charge storage, which is consistent with the electrochemical behaviour of transition metal oxides, such as α-Fe₂O₃. The redox peaks exhibited a slight shift in position with increasing scan rates, which is typically associated with internal resistance. At lower scan rates (5–50 mV/s), the CV curves were well-defined with prominent redox peaks, indicating that the charge storage process was predominantly governed by faradaic reactions. As the scan rate increased, the peak current also increased, reflecting enhanced charge storage. The selected voltage window was further validated by the absence of significant distortion in CV curves and stable GCD profiles (Shivakumara et al., 2014). The broad redox peaks observed were attributed to reversible Fe³⁺/Fe²⁺ transitions, which contributed to the overall charge storage capacity, Furthermore the electrochemical behaviour suggests a mixed charge storage mechanism, involving both surfaces controlled and diffusion controlled process. The presence of redox peaks reflects faradaic activity, but does not alone confirm ideal pseudocapacitive behaviour (Li et al., 2019). The high current response, particularly at lower scan rates, suggests a favourable specific capacitance, highlighting the potential of the α-Fe₂O₃ electrode for energy storage applications. C s = Specific Capacitance (F/g); I = Current (A); m = Mass of the active material (g); Δ t = Discharge time (sec) E = Energy density Wh / Kg; v 2 = v 2 − v 1 2 (volts)P = Power Density (W/Kg).

C s = I m Δ t Δ v (2)

E = 1 2 C s v 2 x 1000 3600 (3)

P = E ∆ t × 3600 (4)

The GCD analysis is illustrated in Figure 5c provides further insights into the specific capacitance of the electrode, the slight deviation from an ideal triangular shape in the GCD curve arises from the faradic redox reactions associated with Fe³⁺/Fe²⁺ transitions (Li et al., 2019),which can contribute to pseudocapacitive charge storage in hematite. The specific capacitance decreased with increasing current density. At a current density of 0.5 A/g, the electrode exhibited a specific capacitance of 1452 F/g (Equation 2), and specific capacitance of 765 F/g was obtained at 2 A/g,1000 F/g at 1 A/g, as mentioned in Figure 5d, at lower current densities, electrolyte ions sufficient time to access a large number of electrochemically active sites within the electrode structure. However, at higher current densities rapid charge discharge processes restricts ion penetration into the inner active sites, leading to incomplete utilization of electrode material. This trend indicates that the electrode maintained its capacitive performance even at higher current densities, which is a desirable characteristic for Supercapacitor applications. Additionally, the energy and power density calculations revealed that at a current density of 1 A/g, average energy density of was calculated 125 ± 9 Wh/kg (Equation 3) and a power density of ∼1260 W/kg (Equation 4) and had a cyclic retention of 76% after 1,000 cycles in 6M KOH Figure 5e also exhibiting Coulombic efficiency of ∼99% at 1 A/g (Li et al., 2019; Ma et al., 2010; Jayakumar et al., 2024). The Ragone plot (Figure 5f) further illustrates the balance between energy and power densities, positioning the present work on hematite, compared with previously reported metal oxide based supercapacitors (Jayakumar et al., 2024; Kuku, 2025). Cyclic stability measurements were also carried out in 3M NaOH (Supplementary Figure S3) and 3M KOH (Supplementary Figure S4) electrolytes, and the corresponding results are presented in the Supporting Information for comparative analysis of material performance. The results indicated that the hematite electrode exhibits better electrochemical stability in 6M KOH compared to 3M KOH and 3 M NaOH, which can be attributed to the higher ionic conductivity and enhanced ion transport of ions in alkaline electrolyte,. These values underscore the electrode’s capability to deliver both high energy density, power density and reproducible (Supplementary Figure S5), making it a promising candidate for energy storage systems. However, extended cyclic stability beyond 1,000 cycles is limited,likely due to structural changes and volume variations during repeated cycles. Further optimization may help improve long term stability in future studies. Additionally, although copper foil provides good conductivity and ease of fabrication, its long-term stability in alkaline electrolytes may be limited. Alternative current collectors could be explored in future studies.

4 Conclusion

This study delved into the electrochemical analysis of the α-Fe₂O₃, revealing a pseudocapacitive charge storage mechanism at lower scan rates, where electrolyte ions have sufficient time to interact with the active sites. Although this study employs a fundamentally simple synthesis approach, the precise optimisation of the stoichiometric ratios and thermal parameters confers unique electrochemical properties to the resulting material, which has the potential to scale up. The electrode demonstrated an excellent capacitance of 1000 F/g at a current density of 1 A/g. The results suggest that further improvements in electrode architecture, such as nanoscale structuring or the incorporation of conductive composite materials, could enhance its rate capability, cyclic stability and overall performance in Supercapacitor applications.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fbael.2026.1795529/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: José Roberto Siqueira Junior, Universidade Federal do Triângulo Mineiro, Brazil

Reviewed by: Wujie Dong, Shanghai Jiao Tong University, China

Jibrin Yelwa, National Research Institute for Chemical Technology, Nigeria