What Happens to MnO<sub>2</sub> When It Comes in Contact with Zn<sup>2+</sup>? An Electrochemical Study in Aid of Zn/MnO<sub>2</sub>‐Based Rechargeable Batteries
Francesca Rossi, Emanuele Marini, Marco Boniardi, Andrea Casaroli, Andrea Li Bassi, Andrea Macrelli, Claudio Mele, Benedetto Bozzini
Abstract
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.