Charge storage mechanism of α-MnO₂ in protic and aprotic ionic liquid electrolytes

In this work we have investigated the charge storage mechanism of MnO₂ electrodes in ionic liquid electrolytes. We show that by using an ionic liquid with a cation that has the ability to form hydrogen bonds with the active material (MnO₂) on the surface of the electrode, a clear faradaic contribution is obtained. This situation is found for ionic liquids with cations that have a low pKa, i.e. protic ionic liquids. For a protic ionic liquid, the specific capacity at low scan rate rates can be explained by a densely packed layer of cations that are in a standing geometry, with a proton directly interacting through a hydrogen bond with the surface of the active material in the electrode. In contrast, for aprotic ionic liquids there is no interaction and only a double layer contribution to the charge storage is observed. However, by adding an alkali salt to the aprotic ionic liquid, a faradaic contribution is obtained from the insertion of Li⁺ into the surface of the MnO₂ electrode. No effect can be observed when Li⁺ is added to the protic IL, suggesting that a densely packed cation layer in this case prevent Li-ions from reaching the active material surface.