A study on charge storage mechanism of α-MnO2 by occupying tunnels with metal cations (Ba2+, K+)

The co-existence of two mechanisms, the surface chemisorption mechanism (SCM) and the tunnel storage mechanism (TSM), for the charge storage of MnO2 is investigated. Sample α-MnO2 whose tunnels (2 × 2) are almost vacant is synthesized by chemical co-precipitation technique. To verify whether the charge storage of as-prepared α-MnO2 involves the charges based on the SCM besides the TSM, Ba2+ and K+ are intercalated into the tunnels of α-MnO2 by refluxing and samples Ba-MnO2 and K-MnO2 are obtained, respectively. By powder X-ray diffraction, ICP and the extraction test, it is confirmed that Ba2+ and K+ occupy the tunnels and are difficult to remove. By electrochemical analyses, we have found that the charge storage of Ba-MnO2 and K-MnO2 basically depends on the SCM, as Ba2+ and K+ stabilized in the tunnels in advance block the intercalation/deintercalation of cations from the electrolyte. Thus, it can be inferred that for α-MnO2 two mechanisms contribute to the charge storage. The contributions of α-MnO2's mechanisms are estimated. The charges based on the TSM predominantly account for 81.2–89.9% of the total charges. The charges on the SCM may contribute to 7.9–16.8% and are not negligible. The charges from double-layer capacitance may be negligible, since it is only 1.1–4.1%.