What Happens to MnO ₂ When It Comes in Contact with Zn ²⁺ ? An Electrochemical Study in Aid of Zn/MnO ₂ ‐Based Rechargeable Batteries

In the science and technology of electrochemical energy storage, different allotropes of MnO 2 , fabricated with a variety of methods, are assembled into electrodes, playing the role of cathode or oxygen reduction reaction (ORR) electrocatalyst. Often, MnO 2 ‐based cathodes are combined with Zn anodes into different types of batteries, resulting in contact between MnO 2 and its electrochemical reaction products, and Zn 2+ . Awareness is growing that this interaction adversely affects the functional performance of MnO 2 , but no definitive understanding has been reached for this issue. This study contributes, through electrochemical measurements accompanied by microscopy and Raman spectroscopy, to a better understanding of the way the electrochemical behavior of two technologically representative types of manganese dioxide ‐ hydrothermally grown α‐MnO 2 and electrodeposited γ‐MnO 2 (EDM) ‐ is degraded when these materials are exposed to neutral and alkaline aqueous solutions, containing Zn 2+ . Specifically, we highlighted different types of irreversible changes in electrochemical response, which can be interpreted with phase‐formation processes. Such changes result in the deactivation of α‐MnO 2 as ORR electrocatalyst, and of both α‐MnO 2 and EDM as zinc‐ion battery (ZIB) cathodes. The electroactivity of EDM for ZIB operation can be restored if Mn 2+ is added to the neutral electrolyte, because a phase, active in discharge, is electrodeposited during charging.