Different Reactions Define the Electrochemical Window in Imidazolium Ionic Liquids on Gold and Platinum Electrodes

Ionic liquids (IL) make excellent candidates for many energy storage devices due to unique and tuneable properties such as a high electrochemical window (ECW). We investigated water as an impurity in common 1-butyl-3-methylimidazolium (BMIM) and 1-hexyl-3-methylimidazolium (HMIM) triflate on platinum and gold electrodes in a stagnant three-electrode cell and in a flow cell coupled to a differential electrochemical mass spectrometer (DEMS). We found, that the ECW closes with increasing water content on both gold and platinum electrodes on both setups. Platinum has a smaller ECW than gold, where the difference mainly stems from the limiting reduction reaction. While both materials showed evolution of oxygen, hydrogen evolution could only be detected on platinum, indicating limitation of the ECW by different reductive reactions on gold and platinum. Our study highlights the interplay between electrode material and electrolyte for tailoring the ECW for specific applications.