Toward Tailoring of Electrolyte Additives for Efficient Alkaline Water Electrolysis: Salicylate-Based Ionic Liquids

The effect on the hydrogen evolution reaction (HER) of adding small amounts of three homemade salicylate ([Sal])-based ionic liquids (ILs), specifically 1-butyl-3-methylimidazolium salicylate ([Bmim][Sal]), 1-(3-hydroxylpropyl)-3-methylimidazolium salicylate ([COHmim][Sal]) and imidazolium salicylate ([Im][Sal]), to KOH electrolyte solutions is investigated. Density, viscosity and electrical conductivity of these nontoxic ILs are determined, further enabling determination of their ionicity. Electrochemical characterization is performed using platinum electrodes in 8 M KOH solution and after addition of 1 vol % of each ionic liquid. The measurements in the IL-added electrolytes showed higher currents and a small effect on the activation energy. Tafel analysis and electrochemical impedance spectroscopy measurements show the Volmer reaction as the rate-determining step of the HER, as well as a strong decrease of the overall impedance in the ILs-added electrolytes, which allow stable operation. The charge transfer and the polarization resistances are substantially reduced in the IL-added solutions, particularly after [Bmim][Sal] addition, with the effect of the additives becoming less important as the temperature increases. A Langmuir-type monolayer adsorption of H to the electrode surface is observed. This work shows that adding small amounts of selected ILs to the KOH electrolyte may enhance the HER in alkaline media, supporting the goal of designing efficient ionic liquid additives for water electrolysis.