Manganese Dioxides: Chemical-Structural Disorder, Electronic Properties, Electrochemical Activity and Proton Diffusivity

ABSTRACTSeveral synthetic and natural manganese dioxides (MD) are characterized in terms of chemical composition, structural disorder and electrochemical activity; semi conducting properties are examined on pressed powdered samples. The reactivity scale established between structural disorder and electrochemical reactivity is paralleled by semi conducting properties and proton diffusivity. Conductivity values of 0.15 S.cm−1for stoechiometric β-MnO2, 1.3 10−2, 2.5 10−2 and 3.7 10−2 S.cm−1 for the defect dioxides, respectively Synthetic-Ramsdellite, CMD and EMD are measured. Proton diffusivity study by EIS provides, for low reduction rate, diffusion coefficient D values varying from 10–15 cm2/s value to 10−16 for the best reactive forms EMD and CMD; for NMD and β-MD, D is respectively 10−16 and 10−21 cm2/s. Redox reversibility is obtained around 1.3V (vs Hg.HgO) for EMD for a one electron reduction in [KOH] <IM; at -0.4V for a 2 electrons reduction in 9M KOH for Bi modified NMD.