The first step of ion-intercalation into porous electrodes is breaking of the solvent structure around them, which accounts for part of the overpotential for these process. Hence, it is necessary to have a detailed understanding of the desolvation of ions from solvents. By modifying the concentrations of metal ions in the bulk, it is possible to monitor the population of ions specifically adsorbed to the surface using surface sensitive vibrational spectroscopy at the electrode-electrolyte interface. In this work, we have used surface enhanced IR spectroscopy (SEIRAS) to study adsorption of Zn²⁺on a monolayer of 4-mercaptobenzonitrile (4-MBN) on chemically deposited gold on ZnSe crystal. -CN functional group of 4-MBN acts as Lewis base for Zn²⁺ and shows a distinct vibrational signature for -CN-Zn²⁺ bond at ~2271 cm^-1. We have used this Lewis interaction peak intensity as an indicator of adsorbed Zn²⁺ on the surface. We have studied four solvents of different dielectric constants - propylene carbonate (PC), dimethyl carbonate (DMC), dimethyl sulfoxide (DMSO), and water. We have observed that half of the population of 4-MBN at the interface coordinates with Zn²⁺ when the bulk concentration is 400 mM in PC, whereas it takes much smaller bulk concentration (~40 mM) to achieve the same population of surface-coordinated ions when DMC is used as a solvent. For DMSO, at saturated concentration of ZnCl₂, we did not observe any evidence of Zn²⁺ ions leaving the solution and coordinating with the surface-bound 4-MBN. From these observations, we show that dielectric constant of the solvent alone cannot determine the desolvation of Zn²⁺ from the solution. Specific interactions of the ion with the solvent plays a crucial role in determining the surface coverage. This work will be useful in multivalent battery research, where ion-delivery to the interface often gets complicated as it needs to have an intricate balance between solvation, mobility, and conductivity of the ions.

Anwesha Maitra

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