Halilu Ahmed, Hayyan Maan, Aroua Mohamed Kheireddine, Yusoff Rozita, Hizaddin Hanee F
Department of Chemical Engineering, University of Malaya, Kuala Lumpur, 50603, Malaysia.
University of Malaya Centre for Ionic Liquids (UMCiL), University of Malaya, Kuala Lumpur 50603, Malaysia.
Phys Chem Chem Phys. 2021 Jan 21;23(2):1114-1126. doi: 10.1039/d0cp04903d.
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.
了解控制氧单电子电化学还原的反应机制对于在二氧化碳转化过程中可持续利用超氧离子（O2˙-）至关重要。在此，首次使用循环伏安法（CV）在相对于Ag/AgCl为-0.823 V的条件下，检测到在丁基三甲基铵双（三氟甲基磺酰）亚胺[BMAmm+][TFSI-]离子液体（IL）中稳定生成O2˙-。与该过程相关的电荷转移系数约为0.503。已确定[BMAmm+][TFSI-]是一种特定任务型离子液体，其具有由带负电荷的C(sp2)和C(sp3)的[BMAmm+]阳离子积累的大的负等价值表面密度。因此，[BMAmm+][TFSI-]对O2˙-的亲核作用不太敏感，因为从3小时的长期稳定性分析中仅记录到8.4%的O2˙-衰减。CV分析还检测到在相对于Ag/AgCl为-0.806 V的[BMAmm+][TFSI-]中O2˙-介导的二氧化碳转化，这可通过O2˙-氧化法拉第电流的消失看出。电化学阻抗谱（EIS）首次检测到[BMAmm+][TFSI-]中O2˙-的生成和二氧化碳转化机制。在O2饱和的[BMAmm+][TFSI-]中的EIS参数与在O2/CO2饱和的[BMAmm+][TFSI-]或CO2饱和的[BMAmm+][TFSI-]中检测到的参数不同。这被合理地解释为是由于在GC电极上形成了[BMAmm+][TFSI-]膜，产生了2.031×10-9 μF cm-2的双层电容（CDL）。因此，在[BMAmm+][TFSI-]中O2˙-的生成和二氧化碳利用过程中，CDL分别增加到5.897 μF cm-2和7.763 μF cm-2。由于6.86×1018 kΩ的高电荷转移电阻，发现[BMAmm+][TFSI-]中的二氧化碳极不可能被电化学转化。随后，O2˙-通过亲核加成反应途径直接介导二氧化碳转化。这些结果为在离子液体介质中利用活性氧物种利用二氧化碳提供了新的可持续机会。
