TY - JOUR

T1 - Mechanistic insights into carbon dioxide utilization by superoxide ion generated electrochemically in ionic liquid electrolyte

AU - Halilu, A.

AU - Hayyan, M.

AU - Aroua, M.K.

AU - Yusoff, R.

AU - Hizaddin, H.F.

PY - 2021/1/14

Y1 - 2021/1/14

N2 - 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 (CDL). Therefore, during the O2- generation and CO2 utilization in [BMAmm+][TFSI-], the CDL 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.

AB - 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 (CDL). Therefore, during the O2- generation and CO2 utilization in [BMAmm+][TFSI-], the CDL 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.

KW - Addition reactions

KW - Carbon dioxide process

KW - Charge transfer

KW - Cyclic voltammetry

KW - Electrochemical electrodes

KW - Electrochemical impedance spectroscopy

KW - Electrolytes

KW - Electrolytic reduction

KW - Ionic liquids

KW - Oxygen

KW - Carbon dioxide utilization

KW - Charge transfer coefficient

KW - Double-layer capacitance

KW - Electrochemical reductions

KW - Ionic liquid electrolytes

KW - Long term stability

KW - Negatively charged

KW - Nucleophilic additions

KW - Carbon dioxide

U2 - 10.1039/d0cp04903d

DO - 10.1039/d0cp04903d

M3 - Journal article

VL - 23

SP - 1114

EP - 1126

JO - Physical Chemistry Chemical Physics

JF - Physical Chemistry Chemical Physics

SN - 1463-9076

IS - 2

ER -