The Effects Of Ion Pairing On The Behavior Of CO₂ Electroreduction In Aprotic Medium

The behavior of CO2 electroreduction in aprotic medium was studied by electrochemical methods, such as cyclic voltammetry (CV), linear sweep voltammetry (LSV) and rotating disk electrode (RDE), and density functional theory (DFT) computations. The results was described as following:(1) In DMF (N,N-dimethylformamide) solution containing quaternary ammonium salts, LSV studies show that the onset potentials of CO2 reduction move to increasingly negative potentials upon increasing the alkyl chain length of the tetraalkylammonuium cations of the supporting electrolyte. Both the most positive onset potential and the lowest peak current corresponding to CO2 eletroreduction are achieved in the case of tetramethylammonium tetrafluoroborate. CO2 disturbance experiments reveal that the peak current of CO2 eletroreduction is determined by the quantity of tetramethylammonium cation adsorbed on the Ag surface. A remarkable positive shift of onset potential of CO2 eletroreduction can be observed when supporting electrolyte of tetraethylammonium tetrafluoroborate is replaced by 1-butyl-3-methylimidazolium tetrafluoroborate, but both the peak current (lp) values of CO2 eletroreduction are almost the same.And the effects of anions (such as Cl-, Br-, NO3-CH3COO-, PF6-, BF4- and ClO4-) on CO2 electroreduction process can be ignored.(2) DFT computations indicate that the onset potential of CO2 eletroreduction is dependent on the strength of ion pairing between tetraalkylammonium cations or imidazolium cations and CO2·- species. The stronger force the ion pairing owns, the more positive onset potentials of CO2 electroreduction can be obtained. The computations also predict that the ion pair ([TAA+][CO2·-]) formation is mainly driven by the attraction between the negatively charged oxygen atoms of the CO2·- species and hydrogen atoms of the alpha-methylene groups of the tetraalkylammonium cation (TAA+). The distance between the oxygen atoms of the CO2·- species and the hydrogen atoms responsible for ion pairing increases with the elongation of the TAA+ cation alkyl chain from ethyl to butyl, and the increase of this distance results in a decrease in the strength of ion pairing. Compared to the tetrahedral conformation of quaternary ammonium cation, the planar geometry of imidazolium cation allows the CO2·- species to approach closer to the planer moiety of the imidazolium cation, and there are four nearby and positively charged hydrogen atoms which are at the Nl-positions, N3-CH3,C2-H and C2-CH3 of an imidazolium cation attract the negatively charged oxygen atoms in the CO2·- species.