Origin of Discharge Potential Decrease in Bi 2 O 3 -Modified 2-Electron Birnessite Electrodes in Alkaline Electrolytes

Abstract While Bi 2 O 3 -modified MnO 2 electrodes in alkaline electrolytes are promising for large scale applications with 2–electron discharge-charge processes, the discharge potential decrease with cycling needs to be clarified, together with the role of the bismuth redox mediator. In this study, we performed reaction analysis of Bi 2 O 3 -modified birnessite electrodes using operando X-ray diffraction and X-ray absorption fine structure measurements to detail the behavior of the reaction products and intermediates, in comparison with the behavior of the birnessite electrode without modification. The discharge potential decrease from the 1st to 9th discharge (still higher than that of the non-modified sample) was correlated with the evolution of the interlayer spacing of the birnessite host as well as the formation of the intermediate and the final products. The results suggest that the modification of a Birnessite-structured MnOOH intermediate phase ( δ -MnOOH) leads to the low redox potential of the bismuth mediator, probably owing to the local pH increase at the electrode surface, indicating the importance of controlling the local conditions of the redox mediator for better electrode potential stability.