Study on Pseudocapacitance Mechanism of Aqueous MnFe[sub 2]O[sub 4] Supercapacitor

Abstract

The electrochemical mechanism leading to the pseudocapacitance of MnFe2O4MnFe2O4 in aqueous electrolyte solutions has been investigated by, in addition to electrochemical characterization, synchrotron X-ray absorption near-edge spectroscopy (XANES) and X-ray diffraction (XRD) analyses using KClKCl solution as the model electrolyte. The ferrite crystallites, prepared by a solution method, contain Mn and Fe ions randomly distributed over the tetrahedral and octahedral sites of the spinel structure and exhibit an average capacitance exceeding 100F∕g100F∕g -ferrite. Based on the data from the in situ XANES and XRD measurements and open-circuit-potential dependence on solution pH, it is concluded that the pseudocapacitance of MnFe2O4MnFe2O4 involves charge transfer at both the Mn- and Fe-ion sites, balanced by insertion/extraction of proton into/from the lattice. In addition, compared with MnO2∙nH2OMnO2∙nH2O electrode, the ferrite exhibits a far reduced, by ∼90%∼90% , lattice expansion upon cycling.