Origin of capacitance decay for a flower-like δ-MnO₂ aqueous supercapacitor electrode: The quantitative surface and electrochemical analysis

Herein, we report the electrochemical energy storage performance of δ-MnO₂ (K-birnessite MnO₂) as supercapacitor electrode material in Na₂SO₄ aqueous electrolyte. The electrode exhibited considerable electrochemical performances due to the fast intercalation/deintercalation reactions of Na⁺ on the pseudocapacitive MnO₂ surface. However, a long-term cyclic stability test of the electrode at a low specific current (1 A g⁻¹) demonstrated a decline in its initial capacitance value to the tune of ~ 21%. To quantify the above discrepancy, the electrochemical intercalation of Na⁺ ions on the electrode surface was quantitatively studied employing electrochemical impedance spectroscopy, EDAX analysis and X-ray photoelectron spectroscopy. Further, the surface of the electrode was analyzed by performing complete charge and charge/discharge measurements at a low specific current of 0.1 A g⁻¹. These results disclosed that, besides the surface intercalation/deintercalation reactions, some Na⁺ ions have permanently substituted into the bulk (layer) of δ-MnO₂ by replacing the host K ions from the layered nanostructure. Thus, this finding suggests that Na⁺ ions replaced in the site of K in δ-MnO₂ considerably affect the electrochemical properties of the supercapacitor electrode.