Excess potential of anion adsorption/desorption at ionic liquid/electrode interfaces
Takashi IWAHASHI1, Wei ZHOU2, Doseok KIM3, Yukio OUCHI1
1Tokyo Institute of Technology, Tokyo, Japan
2Shanghai University, Shanghai, China
3Sogang University, Seoul, Korea (Republic of)
Ion adsorption/desorption at electrolyte/electrode interfaces is of fundamental importance in electrochemistry due to its critical role in the electrochemical reactions. Room temperature ionic liquids (RTILs) provide electrolyte/electrode interfaces without neutral solvent, causing characteristic electrochemical properties and reactions, different from that at the diluted electrolyte/electrode interfaces. It has been reported that the RTIL/electrode interfaces exhibit hysteresis behavior in the ion adsorption/desorption processes; the applied potential response of ion adsorption/desorption differs with respect to the previous history of the potential application. It indicates that the ion adsorption/desorption does not occur at point of zero charge (PZC) but requires an excess potential, which is needed to overcome the activation energy of ion desorption. The activation energy of ion desorption is considered to be derived from the adsorption energy of ion and/or the activation barrier for ion desorption. However, the origin of the excess potential, the activation energy of ion desorption, is still unclear due to lack of quantitative information on the activation energy.
In this study, we examined the anion adsorption/desorption processes at the RTIL/Pt electrode interfaces by infrared-visible sum-frequency generation (IV-SFG) vibrational spectroscopy. The excess potentials for anion/cation desorption are quantitatively determined by comparing the potential response of SF signal from the anion on Pt with that estimated from electric double layer (EDL) model. The anion adsorption/desorption behavior at neat RTIL/Pt interface is compared with that at binary RTIL/Pt interface to investigate the factors determining the activation energy of anion/cation desorption.
The hysteretic ion adsorption/desorption at the interfaces of the Pt electrode with neat RTILs composed of 1-butyl-3-methylimidazolium ([C4mim]+) cation with trifluoromethanesulfonate ([OTf]−) and bis(trifluoromethanesulfonyl)amide ([TFSA]−) anions is found to be due to large activation energy of the anion desorption. In addition, the adsorption energy of anion estimated from the ion co-adsorption at binary RTIL/Pt interface is larger than that of cation, but is much smaller than the activation energy of the anion desorption deduced from the excess potential of anion desorption. Thus, pronounced hysteretic potential response of RTIL/Pt interface can be dominantly caused by large activation barrier for anion desorption, but not by the adsorption energy of anion. The origin of large activation barrier for anion desorption is also elucidated by comparing the hysteretic potential response of neat RTIL/Pt interfaces with that of binary RTIL/Pt interface. As a result, we found that the activation barrier for anion desorption strongly depends on the ion species in the bulk or diffuse layer rather than that in EDL. These results suggest that the activation barrier for anion desorption, the origin of hysteretic potential response, can be attributed to the activation energy required for restructuring of ionic layering. Details will be discussed in the presentation.