Electrochemical reduction of dioxygen in Bis (trifluoromethylsulfonyl) imide based ionic liquids

Electrochemical generation of superoxide ion (O2-) has been investigated in three room temperature ionic liquids (ILs), based on bis (trifluoromethylsulfonyl) imide anion, [N(Tf)2]−, comprising the following cations N-(3-Hydroxypropyl)pyridinium, [HPPy]+, 1-(3-methoxypropyl)-1-methylpiperidinium, [MOPMPip]+, and 1-hexyl-1-methyl-pyrrolidinium, [HMPyrr]+. Cyclic voltammetry (CV) and chronoamperometry (CA) techniques were used for the analysis of the the electrochemical process. It was found that the generated O2- was not stable in the IL based on pyridinium cation, [HPPy]+. While a stable O2- was electrochemically generated in [MOPMPip]+ and [HMPyrr]+ based ILs. CV and CA techniques were used to determine the diffusion coefficients of O2 and solubility of oxygen in the studied ILs as a function of temperature. The diffusional activation energies were then determined. It was found that [HMPyrr]+ based IL, in general, has higher diffusion coefficient and solubility of O2 and less diffusional activation energy than [MOPMPip]+ based IL. For our best knowledge, this is the first time piperidinium based ILs has been used for the electrochemical generation of O2-.

► O2- was stable in [MOPMPip]+ and [HMPyrr]+ based ILs.
► [HMPyrr]+ based IL in general has higher diffusion coefficient, electrical conductivity and O2 solubility than [MOPMPip]+ based IL.
► The density and viscosity for [MOPMPip]+ and [HMPyrr]+ based ILs decreased with temperature.