Hierarchical FeOOH‑Decorated Diatomite: A High‑Performance Supercapacitor Electrode

Abstract

FeOOH nanosheets were uniformly grown on porous diatomite through a simple two‑step hydrothermal route. Subsequent calcination under controlled atmospheres produced α‑Fe₂O₃ and γ‑Fe₂O₃ nanostructures, allowing systematic comparison of their electrochemical behavior in 1 M Na₂SO₄. The diatomite@FeOOH composite exhibited the highest specific capacitance of 157.9 F g⁻¹ at 0.5 A g⁻¹ and retained 98.95 % of its capacity after 1,000 cycles, underscoring its potential as a next‑generation supercapacitor electrode. The strategy is also applicable to other transition‑metal oxides for energy storage and conversion.

Background

Supercapacitors face two main bottlenecks: limited energy density and high manufacturing costs. Transition‑metal oxides, notably MnO₂, NiO, CuO, and FeOOH, have emerged as promising electrode materials due to their high theoretical capacities and environmental friendliness. Ferric oxides and hydroxides, however, are hampered by low surface area and poor conductivity. Nanostructuring can alleviate these drawbacks by boosting surface area, shortening ion diffusion pathways, and accelerating redox reactions. Yet, ferric oxide nanoparticles tend to aggregate, diminishing their electrochemical advantage. A viable solution is to anchor these nanostructures onto porous templates, thereby preserving dispersion and enhancing interfacial contact.

Diatomite, a naturally abundant, highly porous silica material, offers a low‑density, chemically stable scaffold with a large specific surface area. By decorating diatomite with ferric oxides/hydroxides, one can achieve a hierarchically porous architecture that mitigates aggregation and enhances ion transport—an approach that has been scarcely explored in supercapacitor research.

Experimental Section

Materials Synthesis

All reagents were analytical grade. The synthesis proceeded in two stages, illustrated in Figure 1. First, natural diatomite was purified by an oil‑bath protocol. MnO₂‑decorated diatomite was then prepared hydrothermally: 30 mL of 0.05 M KMnO₄ was mixed with 30 mg of purified diatomite and heated at 160 °C for 24 h in a 50 mL Teflon‑lined autoclave. The product was washed, centrifuged, and dried at 60 °C.

Next, 30 mg of the MnO₂‑decorated diatomite was treated with 30 mL of 0.01 M FeSO₄·7H₂O at 120 °C for 2 h, replacing MnO₂ with FeOOH. Calcination yielded two distinct ferric oxides: α‑Fe₂O₃ at 350 °C (2 h, O₂ atmosphere) and γ‑Fe₂O₃ at 500 °C (2 h, N₂ atmosphere).

Characterization

Morphology was examined by Zeiss Auriga FIB/SEM. Phase identification employed powder X‑ray diffraction (XRD, Cu Kα, D/max 2500). Electrochemical measurements were performed in a standard three‑electrode cell using 1 M Na₂SO₄. Working electrodes consisted of the active material (2 mg) mixed with acetylene black and PVDF (7:2:1 by weight) in NMP, cast onto 1 × 1 cm² nickel foam, and dried at 120 °C for 12 h. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) assessed performance.

Electrochemical Calculations

The specific capacitance (C_m) was calculated from GCD curves: C_m = (I Δt)/(m ΔV), where I is discharge current, Δt is discharge time, ΔV is potential window, and m is mass of active material.

Results and Discussion

SEM images (Figure 2) reveal uniform MnO₂ nanosheets on diatomite, which are fully replaced by FeOOH after the ion‑exchange step. The α‑Fe₂O₃ nanosheets are larger and more spaced than the γ‑Fe₂O₃, as shown in the lower‑magnification images. EDS mappings confirm the absence of Mn in FeOOH, corroborating the complete transformation. XRD patterns (Figure 2e) confirm the crystallographic phases: MnO₂ (JCPDS 44‑0141), FeOOH (JCPDS 29‑0713), α‑Fe₂O₃ (JCPDS 33‑0664), and γ‑Fe₂O₃ (JCPDS 33‑0664). The disappearance of MnO₂ peaks in FeOOH, α‑Fe₂O₃, and γ‑Fe₂O₃ samples confirms successful substitution.

Electrochemical tests (Figure 3) demonstrate that diatomite@FeOOH outperforms its MnO₂ and ferric oxide counterparts. At 0.5 A g⁻¹, it delivers 157.9 F g⁻¹, surpassing the other composites by a significant margin. The superior performance stems from the well‑dispersed nanosheets, high porosity, and efficient ion pathways provided by the diatomite scaffold. Cycling stability is remarkable: after 1,000 GCD cycles at 1 A g⁻¹, the capacitance retention is 98.95 %. EIS shows a low series resistance (~3 Ω) and minimal increase in charge‑transfer resistance after cycling, indicating robust interfacial kinetics.

Conclusions

We have developed a facile hydrothermal method to synthesize hierarchical FeOOH‑decorated diatomite with finely tuned nanosheet morphology. The resulting diatomite@FeOOH electrode exhibits a high specific capacitance of 157.9 F g⁻¹ at 0.5 A g⁻¹ and excellent cycle life (98.95 % after 1,000 cycles), outperforming analogous ferric oxide composites. This hierarchical architecture, combining high surface area and efficient electron/ion transport, positions diatomite@FeOOH as a promising active material for high‑performance supercapacitors. The synthesis strategy is versatile and can be extended to other transition‑metal oxides for diverse energy‑storage applications.

Abbreviations

CC

Galvanostatic charging/discharging

CV

Cyclic voltammetry

EIS

Electrochemical impedance spectroscopy

FIB/SEM

Focused ion beam scanning electron microscopy

NMP

N‑methyl‑2‑pyrrolidone

PVDF

Polyvinylidene fluoride

XRD

Powder X‑ray diffraction