Abstract
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.
Highlights
There is a growing interest in the application of supercapacitors (SCs) spurred by their power and cycle performance
A similar response is observed for EtOHIM, with a slight tilt of the cyclic voltammetry (CV) curve reflecting an increased resistance in the electrolyte, related to the higher viscosity and lower conductivity compared to EMIM TFSI, see Table S2 in supplementary information
For the protic ionic liquid EIM TFSI, a clear faradaic peak centered at 0.6 V vs Ag/Agþ is observed in the CV during oxidation at a low scan rate (1 mV sÀ 1)
Summary
There is a growing interest in the application of supercapacitors (SCs) spurred by their power and cycle performance. In the case of protic ionic liquids (PILs), protons on the cation can be available for electrochemical interactions with a faradaic material [15], which could enable a similar mechanism as present with aqueous elec trolytes. There has been only a few studies using PILs in combination with e.g. RuO2 or MnO2 electrodes [16,17], showing that redox-transitions took place with protons transferred from the electro lyte to the electrode In these studies the electrolyte contained an excess amount of acid, it was not a true ionic liquid, and the proton activity could be related to the acid. The electrochemical response of MnO2 electrodes previously studied in aqueous electrolytes [11], is here investigated with these ionic liquid electrolytes and the mechanism discussed in relation to the proton availability, interaction energies, and geometry. At the same time this interaction efficiently blocks interaction between Liþ and the MnO2
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