Synthesis and enhanced electrochemical supercapacitive properties of manganese oxide nanoflake electrodes

MnO_2+δ (Manganese oxide) nanoflakes were synthesized for use as electrode material in electrochemical supercapacitors. The nanoflakes were produced via RF-magnetron sputtering with various excess oxygen contents (δ), and the electrochemical supercapacitive properties of the MnO_2+δ nanoflakes were investigated as a function of δ with the use of a Na₂SO₄ electrolyte. The excess oxygen (δ) induces the MnO_2+δ nanoflakes to form a thin open structure, and μ-Raman measurements revealed that the MnO_2+δ nanoflakes formed a birnessite phase with a layered structure. X-ray photoelectron spectroscopy was used to obtain quantitative information on both the oxidation state and the chemical composition of the nanoflake electrodes. The crystallinity of the nanoflakes improved when the oxygen partial pressure increased during sputtering. At an optimal δ~0.6, the electrochemical stability and the capacity retention significantly improved, and electrochemical impedance spectroscopy revealed that easy access of Na⁺ ions into the nanoflakes at an optimal δ value resulted in a low diffusion resistance, playing a key role in determining the improvement in the supercapacitor characteristics.