Double-layer structure and cation-dependent solvent decomposition in acetonitrile-based electrolytes

We present an analysis of the microscopic structure of the interface between a gold electrode and acetonitrile-based electrolytes, utilising surface-enhanced infrared absorption spectroscopy in attenuated total reflection mode (ATR-SEIRAS) combined with voltammetric data. The investigation focuses on the potential-induced changes in the interactions between interfacial acetonitrile molecules and on the onset of reductive acetonitrile decomposition in Li+- and Na+-containing electrolytes. The acetonitrile molecules exhibit a potential-dependent reorientation, leading to an increase in the concentration of antiparallel dimers at the interface at negative potentials, as the nitrogen end of the molecule is pushed away from the surface. The initial stages of reductive decomposition of acetonitrile are different in the Li+- and Na+-based electrolytes. Spectral signatures characteristic of amines are seen in LiClO4 acetonitrile solutions, while amide bands are also observed in NaClO4. Because traces of water in acetonitrile must be the proton source for the reduction of interfacial acetonitrile to amines and amides, OH− must also be generated during those processes. In fact, ATR-SEIRA spectra reveal the formation and subsequent precipitation of LiOH. Precipitation of NaOH in NaClO4 seems to be absent, though. With increasingly negative potential, the reductive cleavage of acetonitrile results in the formation of several cyanide species. The corresponding cyanide-characteristic bands show a potential-dependent stretching frequency that suggests they correspond to adsorbed species. These findings highlight the effect of potential-induced solvent reorientation on solvent–solvent interactions at the interface as well as the impact of the electrolyte cation on the products of the reductive decomposition of acetonitrile.