EQCM Investigation on Electrodepositon and Charge Storage Behavior of Birnessite-Type MnO2

We have fabricated birnessite-type layered manganese oxide by cathodic reduction of aqueous permanganate ions (MnO4-) in the presence of various alkaline metals. The electrodeposition and charge storage processes were monitored by means of electrochemical quartz crystal microbalance (EQCM). Molar masses acquired for the electrodeposition process revealed that MnO2 grows concurrently with the incorporation of the coexisting cations and water molecules. The molar mass decreases in the order of Li > Rb similar to K similar to Cs > Na, which is in good agreement with the order of crystallinity. Clearly, the degree of crystallinity in the formation of birnessite depends on the incorporation amount of the guest ions and water. The birnessite-modified electrode increases and decreases in mass during cathodic and anodic scans in Na2SO4 electrolyte. The molar masses obtained were 21.6 and 24.9 g per mole of electrons, respectively, both of which coincide with the atomic weight of Na (23.0 g).