Investigation on the pseudocapacitive charge storage mechanism of MnO2 in various electrolytes by electrochemical quartz crystal microbalance (EQCM).

A manganese dioxide (MnO₂) film is electrodeposited onto a gold-coated quartz crystal electrode by a galvanostatic method. The scanning electron microscopy (SEM) image shows that the electrodeposited MnO₂ film consists of nanorods with diameters of 15–20 nm. The cyclic voltammetric (CV) and electrochemical quartz crystal microbalance (EQCM) results demonstrate that the MnO₂ electrodes exhibit similar specific capacitance (SC) and mass-to-charge ratios (MCR) in those electrolytes that contain the same cation but different anions. However, the SC and MCR are different in those electrolytes that contain the same anion but different cations. The SC of MnO₂ electrodes increases with a decrease of the cationic radius and an increase of the cationic charge. The smaller the cationic radius, the smaller the deviation of the MCR from the equivalent weight (EW) of the cation. The results illustrate that the pseudocapacitive charge storage mechanism of MnO₂ involves the cation in the electrolyte and its intercalation/deintercalation, which are affected by the charge number and the cationic radius. This work offers a theoretical basis for selecting suitable electrolytes to couple MnO₂-based electrodes in supercapacitors. [ABSTRACT FROM AUTHOR]