Accessing the Two‐Electron Charge Storage Capacity of MnO2 in Mild Aqueous Electrolytes |
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Authors: | Mickaël Mateos Nikolina Makivic Yee‐Seul Kim Benoît Limoges Vronique Balland |
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Institution: | Mickaël Mateos,Nikolina Makivic,Yee‐Seul Kim,Benoît Limoges,Véronique Balland |
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Abstract: | Rechargeable batteries based on MnO2 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 MnO2 in such media is still a matter of debate. Here, an in‐depth quantitative spectroelectrochemical analysis of MnO2 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 MnO2 into Mn2+ in the presence of a wide range of weak Brønsted acids, including the Zn(H2O)6]2+ or Mn(H2O)6]2+ complexes present in aqueous Zn/MnO2 batteries. Furthermore, it is shown that buffered electrolytes loaded with Mn2+ are ideal to achieve highly reversible conversion of MnO2 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 MnO2 utilization of 84%. Overall, the present results challenge the common view on MnO2 the charge storage mechanism in mild aqueous electrolytes and underline the benefit of buffered electrolytes for high‐performance rechargeable aqueous batteries. |
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Keywords: | conversion mechanisms proton‐coupled electron transfer rechargeable aqueous batteries spectroelectrochemistry |
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