Egg‑Albumin Assisted Hydrothermal Synthesis of Mesoporous Co3O4 Quasi‑Cubes: High‑Performance Supercapacitor Electrode

Abstract

We report a facile two‑step route to synthesize mesoporous Co₃O₄ quasi‑cubes by employing egg albumin as a bio‑surfactant during hydrothermal treatment followed by calcination at 300 °C. The resulting cubes exhibit a layered architecture, an average pore diameter of 5.58 nm and a Brunauer–Emmett–Teller surface area of 80.3 m²/g. When assembled into a working electrode and tested in 2 M KOH, the material delivers a specific capacitance of 754 F g⁻¹ at 1 A g⁻¹, retains 77 % of this value at 10 A g⁻¹, and maintains 86.7 % capacitance after 4,000 continuous charge–discharge cycles at 5 A g⁻¹. These results demonstrate that the egg‑albumin assisted Co₃O₄ quasi‑cubes are promising electrodes for next‑generation supercapacitors.

Introduction

Rapid electrification and the push for clean energy demand advanced energy storage solutions that combine high power, long life, and low cost. Supercapacitors, or electrochemical capacitors, meet these criteria by delivering rapid charge–discharge cycles and robust cycling stability. Their performance, however, is limited by the intrinsic properties of electrode materials—specifically, surface area, porosity, and electronic conductivity.

Transition‑metal oxides (TMOs) such as Co₃O₄ are attractive due to their multiple oxidation states, which enable fast pseudocapacitive redox reactions. Despite a theoretical capacitance of ~3,560 F g⁻¹, practical Co₃O₄ electrodes often fall below 1,000 F g⁻¹ because of poor electronic conductivity and volume changes during cycling.

Nanostructuring Co₃O₄ into high‑surface‑area, mesoporous morphologies can enhance electrolyte access and shorten ion diffusion pathways, thereby improving utilization of active sites. Conventional surfactants can control morphology, but bio‑surfactants like egg albumin offer environmental friendliness, low cost, and strong metal‑binding capacity, enabling precise tuning of nanostructures.

Materials and Methods

Materials

Analytical‑grade urea, cobalt(II) acetate tetrahydrate, and egg albumin (fresh eggs) were used without further purification.

Preparation of Porous Co₃O₄ Quasi‑Cubes

A mixture of 3 mL egg albumin, 2.4 g urea, and 0.3 g cobalt(II) acetate was dissolved in 37 mL de‑ionized water under vigorous stirring. The solution was sealed in a 50 mL autoclave and heated at 140 °C for 5 h. The resulting precipitate was washed, dried at 60 °C overnight, and then calcined at 300 °C for 5 h in air to yield Co₃O₄ powders. Variations in hydrothermal time (1–24 h) and albumin volume (0–5 mL) were explored to optimize morphology.

Electrode Fabrication and Electrochemical Testing

The active material (80 wt %), acetylene black (15 wt %), and PVDF binder (5 wt %) were mixed and dispersed in N‑methyl‑2‑pyrrolidone under ultrasound. The slurry was coated onto 1 × 1 cm² nickel foam, dried at 85 °C, and pressed at 10 MPa to form the working electrode. A three‑electrode cell (Pt counter, SCE reference) with 2 M KOH electrolyte was used for cyclic voltammetry (−0.1 V to 0.65 V, 2–50 mV s⁻¹), chronopotentiometry (1–10 A g⁻¹), and electrochemical impedance spectroscopy (0.01–100 kHz, 5 mV). Specific capacitance was calculated from discharge curves via C_s = I·Δt / (m·ΔV).

Characterization

X‑ray diffraction confirmed the cubic spinel phase of Co₃O₄ (JCPDS No. 43‑1003) with no detectable impurities. SEM images revealed 5–6 µm cubes composed of stacked nanolayers, while TEM showed nanoparticle‑assembled layers forming a porous framework. Raman spectra displayed characteristic modes at 468, 509, 611, and 675 cm⁻¹, consistent with Co₃O₄. XPS analysis indicated Co³⁺ and Co²⁺ states, confirming the mixed‑valence nature essential for pseudocapacitance.

BET analysis yielded a specific surface area of 80.3 m² g⁻¹ and an average pore diameter of 5.58 nm (predominantly 4.0 nm), indicative of a well‑developed mesoporous network that facilitates ion transport.

Results and Discussion

Effect of Egg Albumin and Hydrothermal Time

Without albumin, the product formed Co₃O₄ nanosheets. Adding 1 mL albumin produced well‑defined quasi‑cubes (~3–10 µm). Increasing albumin to 3–5 mL preserved the cubic morphology but reduced size to 3–4 µm, highlighting the role of albumin in directing growth through metal–protein coordination and hydrogen bonding.

Short hydrothermal times (<2 h) yielded irregular nanoparticles; 5 h was sufficient to form complete cubes, with little change observed at 15–24 h, indicating that 5 h is the optimal duration for uniform growth.

Electrochemical Performance

CV curves of the quasi‑cubes displayed well‑defined redox peaks, confirming pseudocapacitance. Specific capacitance values were 754, 712, 683, 641, 614, and 581 F g⁻¹ at 1–10 A g⁻¹, respectively. The retention of 77 % at 10 A g⁻¹ reflects excellent rate capability, while the 86.7 % capacitance retention after 4,000 cycles at 5 A g⁻¹ demonstrates outstanding cycling stability.

Compared with Co₃O₄ nanosheets (which delivered 559–452 F g⁻¹ under the same conditions), the quasi‑cubes exhibit superior performance due to their larger surface area, mesoporosity, and layered structure that facilitate rapid electron and ion transport.

Impedance Analysis

Nyquist plots show a small semicircle (<7 Ω) and a near‑vertical line at low frequency, indicating low charge‑transfer resistance and good diffusion characteristics. The quasi‑cubes have slightly higher charge‑transfer resistance than nanosheets, but the overall impedance remains low, corroborating the high rate performance.

Conclusions

We have demonstrated a green, scalable synthesis of mesoporous Co₃O₄ quasi‑cubes using egg albumin as a natural surfactant. The resulting electrodes deliver a specific capacitance of 754 F g⁻¹ at 1 A g⁻¹, retain 77 % at 10 A g⁻¹, and maintain 86.7 % capacitance after 4,000 cycles. The combination of high surface area, mesoporosity, and robust structural integrity makes these materials promising candidates for advanced supercapacitors. The egg‑albumin assisted strategy can be extended to other metal oxides to further expand the toolbox of high‑performance electrode materials.