In this study, amorphous manganese oxide (MnO₂) nanostructured thin films were synthesized by a simple hydrothermal method. It is well known that the nanostructure plays a crucial role in energy storage applications. Herein, MnO₂ nanostructures ranging from plates to flakes were synthesized without the use of any hard or soft templates. The 4+ oxidation state of Mn was confirmed by X-ray photoelectron spectroscopy. The MnO₂ nanoflake structure has a specific surface area of 46 m²g⁻¹, which provides it with an excellent rate capability and an exactly rectangular cyclic voltammogram (CV) curve. The MnO₂ nanoflake electrode has a high specific capacitance of about 433 Fg⁻¹, an energy density of 60 Whkg⁻¹ at 0.5 mAcm⁻², and an excellent cyclic stability of 95% over 1000 CV cycles in 1 M aq. Na₂SO₄. Kinetics analysis of the charge storage in the nanoflake MnO₂ sample shows a 55.6% diffusion-controlled contribution and 44.4% capacitive-controlled contribution to the total current calculated at a scan rate of 100 mVs⁻¹ from the CV curve.

Cobalt ferrite thin films were grown by PLD at different temperatures as an electrode material for supercapacitors. The films deposited at room temperature exhibited the best power density (3277 W kg⁻¹) and energy density (17 W h kg⁻¹) values.

The hydrothermal method has been utilized to synthesize graphitic carbon nitride (g-C₃N₄) polymers and cobalt oxide composites effectively. The weight percentage of g-C₃N₄ nanoparticles influenced the electrochemical performance of the Co₃O₄-g-C₃N₄ composite. In an aqueous electrolyte, the Co₃O₄-g-C₃N₄ composite electrode, produced with 150 mg of g-C₃N₄ nanoparticles, revealed remarkable electrochemical performance. With an increase in the weight percentage of g-C₃N₄ nanoparticles, the capacitive contribution of the Co₃O₄-g-C₃N₄ composite electrode increased. The Co₃O₄-g-C₃N₄-150 mg composite electrode shows a specific capacitance of 198 F/g. The optimized electrode, activated carbon, and polyvinyl alcohol gel with potassium hydroxide were used to develop an asymmetric supercapacitor. At a current density of 5 mA/cm², the asymmetric supercapacitor demonstrated exceptional energy storage capacity with remarkable energy density and power density. The device retained great capacity over 6k galvanostatic charge–discharge (GCD) cycles, with no rise in series resistance following cyclic stability. The columbic efficiency of the asymmetric supercapacitor was likewise high.

The pursuit of efficient and economical catalysts for water splitting, a critical step in hydrogen production, has gained momentum with the increasing demand for sustainable energy. Among the various electrocatalysts developed to date, cobalt oxide (Co₃O₄) has emerged as a promising candidate owing to its availability, stability, and catalytic activity. However, intrinsic limitations, including low catalytic activity and poor electrical conductivity, often hinder its effectiveness in electrocatalytic water splitting. To overcome these challenges, substantial efforts have focused on enhancing the electrocatalytic performance of Co₃O₄ by synthesizing composites with conductive materials, transition metals, carbon-based nanomaterials, and metal–organic frameworks. This review explores the recent advancements in Co₃O₄-based composites for the oxygen evolution reaction and the hydrogen evolution reaction, emphasizing strategies such as nanostructuring, doping, hybridization, and surface modification to improve catalytic performance. Additionally, it examines the mechanisms driving the enhanced activity and stability of these composites while also discussing the future potential of Co₃O₄-based electrocatalysts for large-scale water-splitting applications.