Unravelling the role of additives in the structure of non-aqueous media at the electrode surface under potential control

Abstract

Efficiency and selectivity of electrochemical reactions is controlled by the micro-environments within the electric double layer (EDL) forming the electrode-electrolyte interface. In electrocatalysis, additives can direct interfacial structure, enhancing activities. Our current level of understanding of the fundamental interactions between the solvent, the electrolyte, and additives at the electrode surface under potential control are limited. This makes a priori predictions of the EDL structure challenging. Vibrational Sum Frequency Generation (VSFG) spectroscopy allows for observation of interface-specific vibrational signatures from which interfacial species may be identified and their orientation determined, providing a way to study the fundamental behaviour of electrified interfaces. We exploit this by studying the structure of CH₃CN, in the presence of H₂O and N-methyl-2-pyrrolidone (NMP),at a gold electrode under potential control. At low concentrations of H₂O (300 ppm), VSFG signatures of CH₃CN are weak and become increasingly apparent as the concentration of H₂O increases. We conclude that this is a result of the formation of an interfacial layer with increased net ordering of CH₃CN molecules due to hydrogen bonding with H₂O disrupting the microstructured CH₃CN environment. At low concentrations of H₂O, NMP accumulates at the negatively charged electrode surface, disrupting the CH₃CN structure; however, addition of H₂O perturbs the NMP structure leading to an ordered CH₃CN interfacial layer being formed.