Effect of Trace Water and Oxygen on the Imidazolium Cation–Copper Electrode Interface Electrochemistry

The electrochemical reduction of the [Bmim]⁺ cation in the 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]) ionic liquid (IL) at a copper electrode was studied under conditions mimicking the real-world conditions. We systematically studied the effect of water and oxygen present in the IL on the imidazolium cation-copper electrode interface electrochemistry under inert and oxygenated environments using cyclic voltammetry. Our results show that the copper valence state (Cu⁺/Cu²⁺) on the surface of the copper electrode is controlled by the potential of the electrochemical system. In addition, water is found to play a significant role in the surface electrochemistry of [Bmim]⁺ and the copper electrode. The concentration of water within the IL electrolyte is shown to influence the magnitude of the redox processes of [Bmim]⁺ and copper. Since under ambient conditions, besides water, oxygen is also present in the ILs, water can react with the superoxide anion generated via the reduction of the oxygen dissolved in the IL. OH^- from water reduction and/or trace water acts as a nucleophile for deprotonation of the imidazolium cation to form N-heterocyclic carbene (NHC). It also contributes to the enhanced oxidation of copper. Our results suggest the Bmim-carbene complex is electrocatalytically formed from [Bmim⁺] on copper/copper oxide surfaces. Initial understanding of the redox mechanisms of [Bmim]⁺, Cu, and Cu ions in nonaqueous IL electrolytes is obtained. These new understandings of redox chemistry on a copper electrode in the presence of trace water and/or oxygen in the IL are important for the control of the desired electrochemical pathways to be employed in electrochemical sensing and energy storage applications.