Effects of Bismuth(III)‐Oxide on the Structural and Electrochemical Properties of Pyrolusite (β‐MnO₂)

Abstract

Cycle‐life and charge/discharge curves of pure and physically Bi2O3‐doped pyrolusite (β‐MnO2) were studied. Crystalline intermediate products of the first three semi‐cycles give information about the effects of Bi2O3. A mechanism is postulated. In the first two semi cycles pyrolusite (β‐MnO2) is converted to the layered δ‐MnO2 with β‐Mn(OH)2 in the fully discharged state. In the course of the following discharge hausmannite Mn3O4, which is electrochemically inert, is formed. No hausmannite is found in presence of Bi2O3. Pieces of information on mechanistical aspects can be gathered from the initial discharge process. During the initial discharge of β‐MnO2, δ‐MnO2 is formed as an intermediate product. In the undoped case this species can be found around MnO1.75. In presence of Bi2O3, however, δ‐MnO2 is existent nearly down to MnO1.25 over the whole discharge region. The reaction path diverges at the [MnIII(OH)4(H2O)2]− ion, dissoluted during the cycling process: in presence of Bi2O3 the reaction is controlled kinetically by a quick disproportionation of the [MnIII(OH)4(H2O)2]− ions resulting in δ‐MnO2 and β‐Mn(OH)2. In absence of Bi2O3, thermodynamical control occurs, and [MnIII(OH)4(H2O)2]− reacts with [MnII(OH)4(H2O)2]2‐ forming the electrochemical non‐reactive hausmannite Mn3O4.

Rechargeability of MnO2 in KOH Media Produced by Decomposition of Dissolved KMnO4 and Bi ( NO 3 ) 3 Mixtures: II. A Reaction Viewpoint on the Role of Bi

“…By analogy with the case described in the previous section, we assume that addition reactions similar to reaction 7 can occur between Bi and Mn species >BiOH + OHMn< k5, >Bi-Mn< [9] Complex Mnatom in a octahedral site o 0-atom * OH groups Finally, this model predicts a large variation range for m. This result is consistent with the experimental data that gives m> 3 (see Table II, line 4, Ref. 10).…”

“…4 This was the first report of this phenomenon, although there have been many electrochemical studies on the corrosion and oxidation of Zn metal and alloys in different solutions. 5 - 9 The spindle crystals are a single, wurtzite-type crystal elongating along the c axis. The morphology of the spindle is simple compared with that of the tetrapod-like ZnO particle, whose structure and growth mechanism have still not been established, although various models' 0 ' 5 have beer.…”