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

• Main Authors: Halilu, Ahmed, Hayyan, Maan, Aroua, Mohamed Kheireddine, Yusoff, Rozita, Hizaddin, Hanee F.
• Format: Article
• Published: Royal Society of Chemistry 2021
• Subjects: QC Physics; QD Chemistry; TP Chemical technology
• Online Access: http://eprints.um.edu.my/28001/

Understanding the reaction mechanism that controls the one-electron electrochemical reduction of oxygen is essential for sustainable use of the superoxide ion (O-2(-)) during CO2 conversion. Here, stable generation of O-2(-) 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 similar to 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(sp(2)) and C(sp(3)). Consequently, BMAmm(+)]TFSI-] is less susceptible to the nucleophilic effect of O-2(-) because only 8.4% O-2(-) decay was recorded from 3 h long-term stability analysis. The CV analysis also detected that O-2(-) mediated CO2 conversion in BMAmm(+)]TFSI-] at -0.806 V vs. Ag/AgCl as seen by the disappearance of the oxidative faradaic current of O-2(-). Electrochemical impedance spectroscopy (EIS) detected the mechanism of O-2(-) generation and CO2 conversion in BMAmm(+)]TFSI-] for the first time. The EIS parameters in O-2 saturated BMAmm(+)]TFSI-] were different from those detected in O-2/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 x 10(-9) mu F cm(-2) double-layer capacitance (C-DL). Therefore, during the O-2(-) generation and CO2 utilization in BMAmm(+)]TFSI-], the C-DL increased to 5.897 mu F cm(-2) and 7.763 mu F 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 x 10(18) k omega. Subsequently, O-2(-) 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.