Protic Imidazolium Ionic Liquids for Electroreduction of CO2

Abstract

Protic ionic liquids characterized by permutable protons have important applications in electrochemistry. This work represents an unprecedented report of the use of such electrolytes for CO2 electroreduction (ECR) into CO at 10 bar and near room temperature (45 °C) on zinc electrodes. It was observed that when protic ILs are synthesized from strong acids, the competition with the hydrogen evolution reaction increases. In contrast, the use of softer acids in the synthesis significantly enhances CO production. Imidazolium-based protic ILs were chosen due to their known high CO2 solubility and consequently their potential to increase reaction productivity. The electrolytes were electrochemically characterized, their conductivities and diffusion coefficients were determined, and their performance was compared. Despite the aqueous nature of the electrolyte, remarkably, 100% Faradaic efficiencies were obtained with [HMIM][Lac] containing 50 wt % water. The performance of the aqueous protic 50 wt % [HMIM][Lac] electrolyte surpassed the performance of the non-protic 50 wt % [EMIM][OTf] electrolyte. Furthermore, among the investigated water concentrations, the maximum CO production for the protic 50 wt % [HMIM][Lac] (177 µmol/cm2) is of the same order as the magnitude of 90 wt % [EMIM][OTf] (271 µmol/cm2), an almost pure IL, more expensive, and less sustainable electrolyte. The non-fluorinated bio-based lactate anion concurs to the higher sustainability of the process. Hydrophilic protic ILs are thus promising as electrolytes for ECR.