Mechanistic insights into carbon dioxide utilization by superoxide ion generated electrochemically in ionic liquid electrolyte

Abstract

Understanding the reaction mechanism that controls the one-electron electrochemical reduction of oxygen is essential for sustainable use of the superoxide ion (O₂˙⁻) during CO₂ conversion. Here, stable generation of O₂˙⁻ in butyltrimethylammonium bis(trifluoromethylsulfonyl)imide [BMAmm⁺][TFSI⁻] ionic liquid (IL) was first detected at −0.823 V vs. Ag/AgCl using cyclic voltammetry (CV). The charge transfer coefficient associated with the process was ∼0.503. It was determined that [BMAmm⁺][TFSI⁻] is a task-specific IL with a large negative isovalue surface density accrued from the [BMAmm⁺] cation with negatively charged C(sp²) and C(sp³). Consequently, [BMAmm⁺][TFSI⁻] is less susceptible to the nucleophilic effect of O₂˙⁻ because only 8.4% O₂˙⁻ decay was recorded from 3 h long-term stability analysis. The CV analysis also detected that O₂˙⁻ mediated CO₂ conversion in [BMAmm⁺][TFSI⁻] at −0.806 V vs. Ag/AgCl as seen by the disappearance of the oxidative faradaic current of O₂˙⁻. Electrochemical impedance spectroscopy (EIS) detected the mechanism of O₂˙⁻ generation and CO₂ conversion in [BMAmm⁺][TFSI⁻] for the first time. The EIS parameters in O₂ saturated [BMAmm⁺][TFSI⁻] were different from those detected in O₂/CO₂ saturated [BMAmm⁺][TFSI⁻] or CO₂ saturated [BMAmm⁺][TFSI⁻]. This was rationalized to be due to the formation of a [BMAmm⁺][TFSI⁻] film on the GC electrode, creating a 2.031 × 10⁻⁹ μF cm⁻² double-layer capacitance (C_DL). Therefore, during the O₂˙⁻ generation and CO₂ utilization in [BMAmm⁺][TFSI⁻], the C_DL increased to 5.897 μF cm⁻² and 7.763 μF cm⁻², respectively. The CO₂ in [BMAmm⁺][TFSI⁻] was found to be highly unlikely to be electrochemically converted due to the high charge transfer resistance of 6.86 × 10¹⁸ kΩ. Subsequently, O₂˙⁻ directly mediated the CO₂ conversion through a nucleophilic addition reaction pathway. These results offer new and sustainable opportunities for utilizing CO₂ by reactive oxygen species in ionic liquid media.