Investigation of Pseudocapacitive Charge-Storage Reaction of MnO[sub 2]⋅nH[sub 2]O Supercapacitors in Aqueous Electrolytes

Pseudocapacitive charge-storage reaction of in several aqueous alkali and alkaline salts solutions, including , , , , and , has been studied on fine-grained thin films and particles which possess the -type crystal structure. In situ synchrotron X-ray diffraction analysis shows that charge transfer at Mn sites upon reduction/oxidation of is balanced by bulk insertion/extraction of the solution cations into/from the oxide structure, which causes reversible expansion and shrinkage in lattice spacing of the oxide during charge/discharge cycles. Electrochemical quartz-crystal microbalance and X-ray photoelectron spectroscopy data further indicate that plays the predominant role in all cases, while the extent of participation of alkali cations first decreases and then increases with ionic size. The charge-storage reaction can be summarized as: , where is alkali cation.