Different Reactions Define the Electrochemical Window in 1-Butyl-3-Methylimidazolium Triflate on Gold and Platinum Electrodes.

Ionic liquids (IL) make excellent candidates for many energy storage devices due to unique and tunable properties such as a large electrochemical window (ECW). Water as an impurity in 1-butyl-3-methylimidazolium (BMIM) triflate is investigated on platinum and gold electrodes in a stagnant glass cell and in a flow-cell coupled to a differential electrochemical mass spectrometer (DEMS). It is found that the ECW closes with increasing water content on both gold and platinum electrodes in both setups. Platinum has a smaller ECW than gold, where the difference mainly stems from the limiting reduction reaction, as identified based on DEMS. Below 1.11 M_H2O/L_IL, the anodic reaction is predominantly IL decomposition and above the oxygen evolution reaction for both materials. The cathodic limit is given by the hydrogen evolution reaction for platinum independent of water content and gold above 1.66 M_H2O/L_IL, while it is IL decomposition below. The study highlights the interplay between electrode material and electrolyte for tailoring the ECW for applications involving intentional or unintentional mixing of water with IL.