Chemical composition and electrochemical activity of some chemically synthesized γ-MnO2.

A series of γ-MnO₂ with Mn⁴⁺ cation vacancy fraction 1.5 < x < 2.3 was prepared via chemical synthesis. X-ray diffraction (XRD) revealed high degree of microtwinning ( T_w > 70%) and de Wolff disorder (0.32 < P_r < 0.35) in the prepared samples. Thermal analysis showed four-stage decomposition corresponding to the removal of physical water, chemisorbed water, formation of Mn₂O₃ and Mn₃O₄, respectively. The chemical composition and physical properties of the prepared samples were expressed in terms of Ruetschi’s cation vacancy model for MnO₂. The discharge behavior in 9 M KOH was governed by homogeneous solid-state reduction wherein protons and electrons were simultaneously inserted into MnO₂ lattice. Their electrochemical behavior was also studied by performing cyclic voltammetry in 9 M KOH. An attempt was made to correlate obtained energy density as a function of proton transfer rate during the discharge process. [ABSTRACT FROM AUTHOR]