In Situ Electrosynthesis of Peroxydicarbonate Anion in Ionic Liquid Media Using Carbon Dioxide/Superoxide System

Climate engineering solutions with emphasis on CO₂ removal remain a global open challenge to balancing atmospheric CO₂ equilibrium levels. As a result, warnings of impending climate disasters are growing every day in urgency. Beyond ordinary CO₂ removal through natural CO₂ sinks such as oceans and forest vegetation, direct CO₂ conversion into valuable intermediaries is necessary. Here, a direct electrosynthesis of the peroxydicarbonate anion (C₂O₆^2-) was investigated by the reaction of CO₂ with the superoxide ion (O₂^·-), electrochemically generated from O₂ reduction in bis(trifluoromethylsulfonyl)imide [TFSI^-] anion derived ionic liquid (IL) media. This is the first time that the IL media were employed successfully for CO₂ conversion into C₂O₆^2-. Moreover, the charge transfer coefficient for the O₂^·- generation process in the ILs was less than 0.5, indicating that the process was irreversible. Voltammetry experiments coupled with global electrophilicity index analysis revealed that, when CO₂/O₂ was contacted simultaneously in the IL medium, O₂^·- was generated in situ first at a potential of approximately -1.0 V. Also, CO₂ was more susceptible to attack by O₂^·- before any possible interaction with the IL except for [PMIm⁺][TFSI^-]. This was because CO₂ has a higher global electrophilicity index (ω_CO = 0.489 eV) than those for the [EDMPAmm⁺][TFSI^-] and [MOEMMor⁺][TFSI^-]. By further COSMO-RS modeling, CO₂ absorption was proven feasible at the COSMO-surface of the [TFSI^-] IL-anion where the charge densities were σ = -1.100 and 1.1097 e/nm². Therefore, the susceptible competitiveness of either IL cations or CO₂ to the nucleophilic effects of O₂^·- was a function of their positive character as estimated by their electrophilicity indices. As determined by experimental attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and DFT-FTIR computation, the reaction yielded C₂O₆^2- in the ILs. Consequently, the presence of O=O symmetric stretching FTIR vibrational mode at ∼844 cm^-1 coupled with the disappearance of the oxidative cyclic voltammetry waves when sparging CO₂ and O₂ confirmed the presence of C₂O₆^2-. Moreover, based on DFT/B3LYP/6-31G, pure C₂O₆^2- has symmetric O=O stretching at ∼805 and ∼844 cm^-1 when it is in association with the IL-cation. This was the first spectroscopic observation of C₂O₆^2- in ILs, and the O=O symmetric stretching vibration has peculiarity for identifying C₂O₆^2- in ILs. This will open new doors to utilize CO₂ in industrial applications with the aid of reactive oxygen species.