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

Halilu, A. and Hayyan, M. and Aroua, M.K. and Yusoff, R. and Hizaddin, H.F. (2019) In Situ Electrosynthesis of Peroxydicarbonate Anion in Ionic Liquid Media Using Carbon Dioxide/Superoxide System. ACS Applied Materials and Interfaces, 11 (29). pp. 25928-25939. ISSN 1944-8244

Abstract

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 (C2O62–) 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 C2O62–. 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 (ωCO2 = 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 C2O62– 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 C2O62–. Moreover, based on DFT/B3LYP/6-31G, pure C2O62– 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 C2O62– in ILs, and the O=O symmetric stretching vibration has peculiarity for identifying C2O62– in ILs. This will open new doors to utilize CO2 in industrial applications with the aid of reactive oxygen species.