Oxygen Reduction Reaction in 1-Ethyl-3-Methylimidazolium Triflate with Addition of Triflic Acid As a Proton Source
The electrochemical experiments adopted a 3-electrode cell configuration. A Pt/GC (glassy carbon) electrode (~1.6 mm in diameter, MF-2013 BASi) was used as the working electrode and a Pt wire (~0.5 mm in diameter, MW-1032 BASi) was used as the counter electrode. The reference electrode was assembled with a Pt wire inserted in a glass tube filled with the studied ionic liquid solution containing 50:50 mol% Fc/Fc+as the internal potential standard. During testing, the solution was continuously purged with nitrogen or oxygen.
Fig. 1 Compares cyclic voltammetry (CV) curves at the Pt electrode in HTF added EMI-TF in N2 and O2 atmospheres, respectively. Under a N2 atmosphere, the reduction peak occurring at around -1 V vs. Fc/Fc+ at the Pt electrode can be ascribed to the hydrogen evolution reaction (HER), which increases with increasing HTF concentration. In an O2atmosphere, both HER and ORR occur and the two peaks overlap for the Pt electrode. However, on the negative scan ORR occurs earlier than HER. In the presence of oxygen, HTF contributes to the HER current increase and to the positive shift of the ORR onset potential. At a concentration of 0.1 M, the ORR onset potential shifts positively by almost 1 V. On Pt, adding HTF is beneficial for ORR improvement, however, it is accompanied by the HER.
In an attempt to separate the ORR and HER, CV curves were obtained for the GC electrode, and are compared in Fig. 2. As can be seen, HER occurs at a much more negative potential. In this case, ORR itself can be compared in the O2atmosphere without interference from the HER peak. On the GC electrode, with addition of HTF, ORR current slightly increases while the onset potential does not move in the positive direction as it does on Pt. This indicates that the effect of the proton species differs depending on the electrode. For both electrodes, ORR becomes irreversible when HTF exceeds a certain level, indicating that proton readily reacts with the superoxide.
Fig. 1 Comparison of CV curves at the Pt electrode in HTF added EMI-TF at a scan rate of 50 mV s-1 in N2 and O2.
Fig. 2 Comparison of CV curves on GC electrode in HTF added EMI-TF at a scan rate of 50 mV s-1 in N2 and O2.
Using Gaussian09, Mulliken charges of carbon and nitrogen atoms in various cations of ionic liquids containing triflate anion were calculated. The results, shown in Fig. 3, demonstrate that EMI+is the least stable cation among all the selected cations in the presence of superoxide.
Fig. 3 Mulliken charges of carbon and nitrogen atoms in various cations calculated using Gaussian09.
In summary, unlike BDMI-TF, in which the addition of proton species significantly increased ORR activity on both Pt and GC electrodes, ORR onset potential shifts positively on Pt and ORR current increases slightly on GC in EMI-Tf. This is likely related to the instability of the cation in the presence of superoxide.
 M. C. Buzzeo, R. G. Evans, and R. G. Compton, ChemPhysChem, 5, 1106 (2004).