Potential-Dependent Structure of Ionic Liquids and Solvated Lithium Ions on a Gold Electrode: A Surface-Enhanced Infrared Study
SEIRAS experiments have been performed on an Au thin film electrode chemically deposited on a Si prism in a vacuum-compatible glass cell. The electrode potential was controlled by a potentiostat against a Pt quasi-reference electrode and corrected by the redox potential of Fc/Fc+ couple. LiFSA and LiTFSA are added to [BMIM][FSA] and [BMIM][TFSA], respectively, to control the concentrations of Li+. SEIRAS measurements have been performed during potential scans where under potential deposition of Li is not observed.
Potential-dependent SEIRA spectra of [BMIM][TFSA]/Au electrode showed the threshold potentials for steep increasing/decreasing in the intensities of the vibrational bands, which indicated the existence of the energy barrier for anion-cation exchange at the first ionic layer on the electrode.2 Addition of the Li salt resulted in more positive threshold potential for the exchange of cations to anions at the first layer. We concluded that the anionic complex (e.g. [Li(TFSA)2]-) have higher potential barrier to replace cations at the first layer than TFSA- owing to its bulky structure. The formation of anionic Li complex is supported by the decrease of the band intensity of the solvated TFSA- at potentials negative of pztc (potential of zero total charge). Anionic Li complex moves further away from the electrode at the negative potential. The difference between [BMIM][FSA] and [BMIM][TFSA] will be reported in the presentation.