Alkalinity Initiated Decomposition of Mediating Imidazolium Ions in High Current Density CO₂ Electrolysis

Abstract

Abstract Aqueous ionic liquid electrolytes featuring the promising 1,3‐dialkyl‐ and 1,2,3‐trialkyl‐imidazolium cations are reported in CO 2 electrolysis up to 200 mA cm −2 . A close‐to‐application flow cell setup equipped with silver gas diffusion electrode to overcome CO 2 mass transport limitations was used. Faraday efficiencies for CO as high as 68 % were achieved, while inhibiting the hydrogen evolution reaction. Particular attention is paid to the stability of the ionic liquids, which is determined by 1 H NMR spectroscopic measurements. The root cause of decomposition is the formation of hydroxide ions through reduction reactions. A high local pH arises that depends on the current density. The water content is also found to play a key role. An overall mechanism is proposed from analysis of the decomposition products including methylamine, ethylamine, formate, acetate and N 1 ‐ethyl‐ N 2 ‐methylethane‐1,2‐diamine. Methylation of the susceptible C2 position of the imidazolium ring fails to stabilize the system under high conversion rate electrolysis conditions.