Room temperature ionic liquids as electrolyte additives for the her in alkaline media

Abstract

Three 1-ethyl-3-methylimidazolium ([Emim])-based room temperature ionic liquids (RTILs), namely [Emim][Ac] (acetate), [Emim][EtSO4] (ethylsulfate) and [Emim][MeSO3] (methanesulfonate), are tested as additives to alkaline solutions for hydrogen evolution reaction (HER). Electrochemical measurements are performed with a platinum foil working electrode in 8 M KOH solution and after adding 1-2 vol.% of each ionic liquid. Tafel plots are constructed from the polarization curves recorded in different electrolyte compositions and the main HER kinetic parameters are determined. The highest cathodic currents are found upon [Emim][MeSO3] addition. The Volmer reaction is evidenced as the rate determining step by both Tafel analysis and electrochemical impedance spectroscopy (EIS) measurements. EIS data reveal a significant decrease of the overall impedance in the RTILs added solutions. This effect can be due to surface pre-Adsorption of the RTIL additives, which stabilize the intermediate hydrogen atoms formed in the Volmer step, thus modifying adsorption and charge transfer processes at the metal-electrolyte interface. This may lead to increased HER currents, as in the case of [Emim][MeSO3] addition. The results support the benefits of using small amounts of selected RTILs as electrolyte additives for the HER in alkaline media.