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

Abstract

Herein, we report the electrochemical energy storage performance of delta-MnO2 (K-birnessite MnO2) as super-capacitor 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 similar to 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 delta-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 delta-MnO2 considerably affect the electrochemical properties of the supercapacitor electrode. (C) 2021 Elsevier B.V. All rights reserved.