Electrochemical capacitors or the so-called supercapacitors are intermediate devices between batteries which can store high amounts of energy but with a low power and conventional dielectric capacitors which can supply higher power but the energy density is low. Since Lee and Goodenough announced the first capacitive behavior of manganese dioxide in neutral media in 1999,~1 pseudocapacitive properties of manganese oxides have been extensively investigated for not only achieving the highest capacitance values but also understanding the charge-storage mechanism.~(2-4) Capacitive properties of manganese oxides is strongly dependent upon morphologies, crystalline structures, and defects. Relationship between the crystalline structures and their pseudocapacitive properties has been investigated exhaustively by Brousse et al.~5 and Devaraj et al.~6 Both research groups pointed out that birnessite of a 2D layered structure with large interlayer separation (~0.7 nm) has higher specific capacitance than 1D and 3D crystals. This can be associated with bicontinuous networks of solid and pores which enable both electrons and cations to move fast and reversibly. Besides, birnessite is a clathrate compound of Mn oxide host with cations as guests. Charge transfer taking place between the host layers and guest ions in the limited space may tune the structure. In 2004, Nakayama et al. presented an electrochemical route to fabricate birnessite-type layered Mn oxides as a thin film.~7 The deposited Mn oxide layers are lying parallel to the surface of electrode.~8 This leads to a difficulty in electron transfer along a direction perpendicular to the substrate and does not allow easy access of charge-compensating cations in solution to the interlayer surface. In the present study, the birnessite film electrodeposited was intercalated with various transition metal ions, followed by repetitive potential cycling. As a main result, the morphology of the electrodeposited birnessite changed drastically, while its pseudocapacitive performance was highly improved.
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