A study on charge storage mechanism of α-MnO 2 by occupying tunnels with metal cations (Ba 2+, K +)

The co-existence of two mechanisms, the surface chemisorption mechanism (SCM) and the tunnel storage mechanism (TSM), for the charge storage of MnO ₂ is investigated. Sample α-MnO ₂ whose tunnels (2 × 2) are almost vacant is synthesized by chemical co-precipitation technique. To verify whether the charge storage of as-prepared α-MnO ₂ involves the charges based on the SCM besides the TSM, Ba ²⁺ and K ⁺ are intercalated into the tunnels of α-MnO ₂ by refluxing and samples Ba-MnO ₂ and K-MnO ₂ are obtained, respectively. By powder X-ray diffraction, ICP and the extraction test, it is confirmed that Ba ²⁺ and K ⁺ occupy the tunnels and are difficult to remove. By electrochemical analyses, we have found that the charge storage of Ba-MnO ₂ and K-MnO ₂ basically depends on the SCM, as Ba ²⁺ 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-MnO ₂ two mechanisms contribute to the charge storage. The contributions of α-MnO ₂'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%.