Effects of electrochemically incorporated bismuth on the discharge and recharge of electrodeposited manganese dioxide films in 9m aqueous KOH

Abstract

Previous reported work has demonstrated that MnO2 can be made multiple rechargeable over the two-electron capacity by chemical modification through incorporation of a small concentration of Bi(iii) species. In the present work, conditions required for inclusion of bismuth species in electrolytically produced MnO2 deposits on porous graphite are reported together with resulting electrochemical effects of the bismuth species on rechargeability of the electrodeposited MnO2. The optimum conditions for deposition were found to be: temperature 85–90°C; bath composition 0.5 to 2m H2SO4, 0.5m MnSO4, 0.005 to 0.01m Bi3+ and current density 5 to 20 mA cm−2 (apparent). The mechanism proposed for the inclusion of bismuth species is by continuous precipitation caused by high local acidity generated at the electrode by the reaction of anodic deposition of MnO2. With respect to the mechanism of reduction and reoxidation of MnO2 in 9m KOH with bismuth species present, a previously suggested role of soluble intermediates is confirmed. It is proposed that bismuth may aid in the nucleation and growth process associated with formation of Mn(OH)2 or MnO2 from a soluble Mn(iii) intermediate. Such a process must take place in order for completion of either discharge or recharge to take place at the electrode. It seems that the role of the included Bi species is to promote a discharge and recharge mechanism of the so-called ‘heterogeneous’ kind involving a soluble Mn(iii) intermediate over an alternative, solid-state, ‘homogeneous’ pathway.