Applicability of four recent models to the discharge behaviour of different phases of MnO₂ in alkaline and acidic/neutral electrolytes

Abstract

Manganese dioxides belonging to different crystalline phases were discharged in 9 M KOH and 5 M NH₄Cl+2 M ZnCl₂ at a constant current of 1 mA 0.1 g. These phases were characterized by X-ray diffractometry and chemical analysis. Four different models were tried for their applicability to the discharge behaviour of these phases. Almost all the four models successfully predict the values of the oxyhydroxides in the first half of the reduction range 0<r<0.5, but fail to do so in the secon half i.e. 0.5<r<1. The only exception to this is the model (Model 2) of Maskell, Shaw and Tye. This model yields, by far, the best match with the experimental data in acidic/neutral electrolyte. In alkaline electrolyte, however, even this model fails to account satisfactorily for the observed potentials in the lower half of the reduction. This failure could perhaps be explained as due to the formation of a new phase at about MnO_1.6 (r≃0.8). The cation vacancy model of Ruetschi predicts most of the physical properties rather well but the match of the theoretical potentials with the experimental values is poor, specially in the lower half.