Electrocatalytic Reduction of CO2 in Imidazolium-Based Ionic Liquids
Résumé
Different abbreviation systems for denoting imidazolium-based room temperature ionic liquids (RTILs) are reviewed. Impact of some physicochemical properties of RTILs within the mass transfer steps of electrochemical reactions and particularly electrocatalytic reduction of CO2 is addressed in detail. Most available data on conductivity, viscosity, H2O content, and CO2 solubility for different RTILs are summarized and compared with conventional aqueous and nonaqueous solvent supporting electrolyte (SSE) systems in the present article. Furthermore, this article provides a wide point of view in CO2 electrochemical conversion by comparing the most relevant achievements reported on metallic electrodes such as Ag, Bi, Pb, or Sn and metal-free electrodes such as nitrogen-doped carbon nanofibers using pure imidazolium-based RTILs, binary (RTIL/H2O and RTIL/acetonitrile(AN)) and ternary (RTIL/AN/H2O) mixtures. Those approaches are based on mixtures using RTILs either as a supporting electrolyte by diluting them in different molecular solvents (mainly AN and/or H2O) or as co-catalyst together with a conventional electrolyte salt in solution. Product selectivity for the electrochemical reduction of CO2 changes as a function of SSE composition in some of those binary and ternary mixtures, being CO the dominant product when RTIL is the main component and HCOOH the major one when AN dominates the SSE system. Moreover, the co-catalyst role of imidazolium-based cations is evidenced and possible CO2 adduct intermediates formed are discussed. Particular emphasis is devoted to the imidazolium–CO2 interaction and the catalytic properties for the electrochemical reduction of CO2 exhibited by different materials used as electrode. Those activity results are reviewed by comparing their reaction overpotential, partial current density, and Faradaic efficiency for each product as a function of the SSE system. Some selected results for the electrochemical reduction of CO2 in aqueous media are also included for the sake of comparison.