Cathodic Reduction Mechanisms of Manganese Dioxide from Derivative Discharge Curves

Abstract

Derivative discharge curves of ~-, 8-, or ~-MnO2 can be resolved into a set of six normal distribution functions, designated as peaks O, A, B, C, D, and E, which can each be attributed to a specific reaction. Peak O situated at the highest cell voltages is attributed to the reduction of adsorbed oxygen. The following peak A which is linked to the formation of dissolved Mn + +-ions ends when the solubility product of Mn(OH)2 is attained in the electrolyte. For peak B the over-all reaction MnO2 % Mn + + + 2H20 + 2e--> 2Mn(OH)2 is proposed. In 9M KOH the electrochemical reduction of ~-MnO2 proceeds in one step to Mn(OH)2, ~-MnO2 is at first (peak C) reduced to Mn304 and then (peak D) to Mn(OH)2, and 7-MNO2 reduces in three successive steps to an amorphous substance, Mn.~O4 and Mn(OH)2. As in practically all cases the presence of Mn(OH)2 has been observed, it is proposed that bivalent manganese is the primary reduction product which reacts in alkaline electrolyte with unreduced MnO2 to give secondary reduction products the kind of which depends on the type of MnO2 involved, the electrolyte, and time of contact. Mixing MnO2 with appropriate amounts of Mn(OH)2 gives, under comparable conditions, the same products as obtained in electrochemical reduction.Following the proposition of Balewski and Brenet (1) the first derivatives of discharge curves obtained during cathodic reduction of manganese dioxides under Leclanch6-type dry cell conditions have been examined with a pulse galvanostatic analyzer (2, 3). The derivative discharge curves obtained can be resolved into a set of up to six normal distribution functions: peaks O, A, B, C, D, and E. Their relative importance is mainly governed by the mass ratios of electrolyte to manganese dioxide in the depolarizer mix pills (3). This observation and especially the changes of the peak areas with increasing mass raUos suggest that each peak may be attributable to a specific chemical reaction taking place in the depolarizer mixes during the cathodic reduction.It is the purpose of the present communication to make proposals for this attribution. In order to obtain further evidence, discharge reactions of ~-, ~-, and ~-MnO2 in 9M KOH and in modified Leclanch6-type dry cell electrolytes have been investigated and the various reduction products formed in alkaline electrolyte have been examined by x-ray diffraction. ExperimentalSamples and discharge procedure.--The properties of the manganese dioxides used in the experiments are compiled in Table I and the compositions of the depolarizer mixes are presented in Table II. The pulse galvanostatic analyzer, preparation of the depolarizer mixes, and discharge tests together with their evaluation are described in foregoing publications (2, 3).

The Influence of Potassium Ion on the Electrodeposition and Electrochemistry of Electrolytic Manganese Dioxide

“…8) reveal evidence for a-MnO~ in these samples (14). At least four peaks are visible in the dQ/dE vs. E plot for the EMD containing no added K § similar to that shown for ~-MnOz under like conditions (14). The plots for 54 and 590 ppm K EMDs are comparable.…”

“…The differential chronopotentiograms of selected EMDs (Fig. 8) reveal evidence for a-MnO~ in these samples (14). At least four peaks are visible in the dQ/dE vs. E plot for the EMD containing no added K § similar to that shown for ~-MnOz under like conditions (14).…”

“…Manuscript submitted May 5, 1991; revised manuscript received Dec. 9, 1991. This was in part Paper 63 presented at the Seattle, WA, Meeting of the Society, Oct. [14][15][16][17][18][19] 1990.…”

λ-MnO2Positive Electrode for Fuel Cell/Battery Systems

“…z E-mail: a-tsu2mi@iis.u-tokyo.ac.jp reduced to MnOOH (discharge) and re-oxidized to MnO 2 (charge) as follows: 3,6 Mn…”