Role of imidazolium cations on the interfacial structure of room-temperature ionic liquids in contact with Pt(111) electrodes

Abstract

Room-temperature ionic liquids (ILs) have gained considerable attention as an important addition to conventional electrolytes because they exhibit large electrochemical windows and can reduce existing overpotentials in electrocatalysis. For the interfacial electrochemistry of ILs, a comprehensive understanding of molecular ions and the resulting electric double-layer structures as a function of electrode potential is mandatory, but the structures are largely different from conventional electrolytes. For that reason, we have studied the interfaces of Pt(111) in contact with ILs using 1-butyl-3-methylimidazolium [BMIM] and 1-butyl-2,3-dimethylimidazolium [BMMIM] cations as well as bis(trifluoromethylsulfonyl)imide [NTf₂] anions. We applied vibrational sum-frequency generation (SFG), where we interrogate vibrational bands from interfacial cations, anions, as well as interfacial water in situ and under potential control. Structuring of [NTf₂] anions and H₂O with electrode potential show hysteresis while a strong Stark tuning was absent. This indicates that the IL ions are oriented in the vicinity of the interface, without being directly adsorbed to the Pt(111) surface. Using the C-H stretching band from CH groups at the imidazolium ring, the ring reorientation with electrode potential was qualitatively determined. The imidazolium ring reorients as a function of potential from a more parallel orientation to an upright orientation with respect to the interfacial plane. This leads to the formation of voids in the layered structure of ions at the interface, which can be then filled with H₂O as evidenced by an increased SFG intensity from O-H stretching modes that are attributable to hydrogen-bonded interfacial water. Comparing the responses of the ILs, particularly of [BMMIM][NTf₂], shows a compact structure and a significantly pronounced rearrangement of the imidazolium ring that can also facilitates better incorporation of H₂O and significantly affects the reorientation of [NTf₂] anions and, thus, causes a pronounced hysteresis with electrode potential.