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Accessing the Two‐Electron Charge Storage Capacity of MnO 2 in Mild Aqueous Electrolytes
Author(s) -
Mateos Mickaël,
Makivic Nikolina,
Kim YeeSeul,
Limoges Benoît,
Balland Véronique
Publication year - 2020
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.202000332
Subject(s) - electrolyte , gravimetric analysis , faraday efficiency , aqueous solution , materials science , energy storage , chemical engineering , ion , electrochemistry , inorganic chemistry , electrode , chemistry , thermodynamics , physics , organic chemistry , power (physics) , engineering
Rechargeable batteries based on MnO 2 cathodes, able to operate in mild aqueous electrolytes, have attracted attention due to their appealing features for the design of low‐cost stationary energy storage devices. However, the charge/discharge mechanism of MnO 2 in such media is still a matter of debate. Here, an in‐depth quantitative spectroelectrochemical analysis of MnO 2 thin‐films provides a set of unrivaled mechanistic insights. A major finding is that charge storage occurs through the reversible two‐electron faradaic conversion of MnO 2 into Mn 2+ in the presence of a wide range of weak Brønsted acids, including the [Zn(H 2 O) 6 ] 2+ or [Mn(H 2 O) 6 ] 2+ complexes present in aqueous Zn/MnO 2 batteries. Furthermore, it is shown that buffered electrolytes loaded with Mn 2+ are ideal to achieve highly reversible conversion of MnO 2 with both high gravimetric capacity and remarkably stable charging/discharging potentials. In the most favorable case, a record gravimetric capacity of 450 mA·h·g −1 is obtained at a high rate of 1.6 A·g −1 , with a Coulombic efficiency close to 100% and a MnO 2 utilization of 84%. Overall, the present results challenge the common view on MnO 2 the charge storage mechanism in mild aqueous electrolytes and underline the benefit of buffered electrolytes for high‐performance rechargeable aqueous batteries.