Effects of electrochemically incorporated bismuth on the reduction and re-oxidation of electrolytically deposited manganese dioxide.

Abstract

The inclusion of bismuth species in electrolytically produced MnO$\sb2$ deposits on porous graphite is reported together with some electrochemical effects of the bismuth species on rechargeability of electrodeposited MnO$\sb2.$ The optimum conditions for deposition found in the study are:(UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\vbox{\halign{#\hfil&&\quad#\hfil\cr&Temperature:&85--90$\sp \circ$C\cr&Bath Composition:&0.5M to 2M H$\rm\sb2SO\sb4$\cr&&0.5M MnSO$\sb4$\cr&&0.005M to 0.01M Bi$\sp{3+}$\cr&Current Density:&5 to 20 mA cm$\sp{-2}$ (apparent)\cr}}$$(TABLE/EQUATION ENDS)The proposed mechanism for the inclusion of bismuth species is by continuous precipitation caused by high local acid strength created at the electrode by the reaction of anodic deposition of MnO$\sb2$. Bismuth species are not "included" in deposits made from nitrate and perchlorate baths but bismuth species may be absorbed into the graphite substrate and then become electrochemically active when the electrode is placed in strong alkali. Bismuth species, no matter how they are introduced into the electrochemical cell, enhance the rechargeability characteristics of MnO$\sb2$ when it is subjected to cyclic voltammetry in aqueous 9M KOH. With respect to the mechanism of reduction and re-oxidation of MnO$\sb2$ with bismuth species present in 9M KOH, the role of soluble intermediates is confirmed. It is proposed that bismuth may, among other things, act to stabilise soluble Mn$\sp{3+}$ species in solution and/or aid in the nucleation and growth process of these species that must take place in order for them to react further at the electrode. The rotating-disk cyclic voltammetry of bismuth and manganese metals is also reported. Bismuth shows good cycling behaviour with the formation and reduction of a passive oxide layer several hundred monolayers in thickness. Manganese displays a unique anodic feature on the cathodic sweep. A model for the explanation of this peak is presented.