Transport Matters: Boosting CO2 Electroreduction in Mixtures of [BMIm][BF4]/Water by Enhanced Diffusion

Room-temperature ionic liquids (RTILs) are promising new electrolytes for efficient carbon dioxide reduction. However, due to their high viscosity, the mass transport of CO2 in RTILs is typically slow, at least one order of magnitude slower than in aqueous systems. One possibility to improve mass transport in RTILs is to decrease their viscosity through dilution with water. Herein, defined amounts of water are added to 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), which is a hydrophilic RTIL. Electrochemical measurements on quiescent and hydrodynamic systems both indicate enhanced CO2 electroreduction. This enhancement has its origin in thermodynamic/kinetic effects (the addition of water increases the availability of H+, which is a reaction partner of CO2 electroreduction) and in an increased rate of transport due to lower viscosity. Electrochemically determined diffusion coefficients for CO2 in [BMIm][BF4]/water systems agree well with values determined by NMR spectroscopy.