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

•Low cyclic stability of layered δ-MnO2 supercapacitor electrode was investigated.•Na+ ions intercalation on the electrode surface was observed via EDAX and XPS analysis.•Slow charge/discharge process leads to permanent intercalation of Na+ electrolytic ion.•Prolonged cyclic test shows Na+ ions repl...

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Published in:Journal of alloys and compounds 2022-02, Vol.892, p.162199, Article 162199
Main Authors: Justin Raj, C., Manikandan, Ramu, Sivakumar, Periyasamy, Opar, David O., Dennyson Savariraj, A., Cho, Won-Je, Jung, Hyun, Kim, Byung Chul
Format: Article
Language:English
Subjects:
Aqueous electrolytes
Capacitance
Chemical reactions
Electrochemical analysis
Electrochemical impedance spectroscopy
Electrode materials
Electrodes
Energy storage
Impedance spectroscopy
Intercalation
Ion intercalation
Layered nanostructure
Manganese dioxide
Manganese oxide
Photoelectrons
Spectrum analysis
Stability tests
Supercapacitor
Supercapacitors
Surface stability
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Summary:•Low cyclic stability of layered δ-MnO2 supercapacitor electrode was investigated.•Na+ ions intercalation on the electrode surface was observed via EDAX and XPS analysis.•Slow charge/discharge process leads to permanent intercalation of Na+ electrolytic ion.•Prolonged cyclic test shows Na+ ions replaced some host K ions in layered δ-MnO2. [Display omitted] 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.
ISSN:0925-8388
1873-4669
DOI:10.1016/j.jallcom.2021.162199