Structure and Dynamics of the Electrical Double Layer during the Rapid Alternating Polarity Electro-Synthesis

Abstract

Conventional electrochemical organic synthesis uses direct current (DC) condition, where the electrode polarity is not changed during the operation. Unlike DC, alternating current (AC) introduces two more tunable parameters into the potential or current profile: frequency and waveform, allowing new possibilities for modulating reaction efficiency and selectivity. Several very recent AC electrosynthesis examples have shown that the AC can lead to enhanced chemoselectivity that cannot be reproduced by their DC counterparts1–3. For instance, Hayashi et al. presented a highly selective and easily scalable Birch-type reduction of heteroarenes by rapid alternating polarity (rAP) waveform3. AC voltage transforms the reaction kinetics presumably by affecting the mass transfer of reactive species both in the bulk solution and the electrical double layers (EDL). However, the mechanistic origin of the unique reactivity in AC electrosynthesis is underexplored. Molecular-level details are still in lack to possibly guide the rational design of AC reaction parameters.In this study, we have chosen the rAP heteroarene reduction as the example system and employed classical molecular dynamics (MD) simulations to reveal the liquid structure and dynamics in bulk and interfacial electrolyte. To capture the electrode-electrolyte interfaces, a slab-geometry simulation cell was used, where a 10 nm thick liquid electrolyte is sandwiched between two oppositely charged graphene surfaces. The multicomponent electrolyte was composed of ethanol and tetrahydrofuran (THF) as the co-solvent, [(CH3)4N]+[(BF4)]- as the salt, and a heteroarene substrate. Based on the charge distribution function statistics, the EDL layer was about 1 nm thick, so if any of the oxygen atom in the ethanol or THF is within 6 Å to the electrode surface, they are considered to be within the EDL. Under both AC and DC, the ethanol to THF ratio was higher than that in the bulk electrolyte due to stronger ion-ethanol attraction. The EDL structure responded to electric field polarity change at different time scales. First, the molecule orientation would flip also within the picosecond time scale after the polarity switch. By tracking the number of molecules in the EDL, we have found that the compositional fluctuation in the EDL converges in about 40 ps. Although it is the ion migration that gets directly affected by the alternating electric field, diffusion of charge-neutral molecules was also found to be accelerated under AC, according to the higher mean squared displacement calculated from the movement of all molecules of each species in the simulation box. This accelerated diffusion spans a larger length and longer time scales. A multi-scale model is proposed to describe both reaction kinetics and liquid structure dynamics simultaneously.