Accessing the Two‐Electron Charge Storage Capacity of MnO₂ in Mild Aqueous Electrolytes

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.

Nanostructured Electrode Enabling Fast and Fully Reversible MnO2-to-Mn2+ Conversion in Mild Buffered Aqueous Electrolytes

“…8 In our previous work, the maximal gravimetric capacity achieved in a buffered electrolyte of pH 5 was 450 mA•h•g -1 (obtained from MnO 2 thin films electrodeposited onto planar electrodes). 7 Although much better than the capacities previously reported for a range of MnO 2 -cathodes in mild unbuffered aqueous electrolytes, this gravimetric capacity remains far from the 570 mA•h•g -1 value recently achieved in a strongly acidic electrolyte (i.e., 0.1 M H 2 SO 4 ). 9 In this case, a similar conversion reaction is at work, but with H 3 O + /H 2 O acting as the proton donor/acceptor couple.…”

“…We previously attributed this second plateau to the formation of a more resistive fraction of MnO 2 at the planar electrode, a fraction which is probably much less electrically connected to the underlying current collector. 7 The lack of a second plateau with the 3D MnO 2 -GLAD-ITO electrode tends to confirm this assumption. It also supports the idea that the 3D nanostructured substrate facilitates the electrical wiring of MnO 2 by shortening the electron transport distances across the semi-conductive MnO 2 , and possibly also by strengthening the interactions between Figure 3E.…”

“…Brønsted acidity with a pK a value of 9.0 and 10.6, respectively 31. As we have previously demonstrated,7 both complexes can act as proton donors to assist the electrodissolution of MnO 2…”

“…Recently, we have revealed that the charge storage mechanism at MnO 2 electrodes is not based on an insertion process but instead on a reversible conversion principle, wherein the weak Brønsted acid AH and conjugated base Apresent in an aqueous buffered electrolyte assist the following proton-coupled electron transfer reaction at the electrode interface: 7 MnO 2(s) + 4 AH + 2 e - Mn 2+ (aq) + 4 A -+ 2 H 2 O…”

Evidence of Bulk Proton Insertion in Nanostructured Anatase and Amorphous TiO2 Electrodes

“…6 The use of buffered aqueous electrolytes was indeed proven to be a relevant strategy to stabilize the pH at metal oxide interfaces, which is key to yielding aqueous batteries with highly stable charge/discharge voltages. 7 This concept of proton insertion via a weak acid also applies to multivalent aquo-metal complexes (e.g., [Zn(H2O)6] 2+ , [Al(H2O)6] 3+ , …), which generally possess an intrinsic weak acidity through their coordinated water molecules. 5,[8][9][10] To date, most of the knowledge on the bulk insertion of protons in metal oxides comes from studies on nickel hydroxide materials.…”