One hypothesis about the origin of enhancement under rAP is that exposing the electrode to the oppositely charged potential provides time to replenish reagents and eliminate products from the diffusion layer near the electrode surface. Thus, upon renewed application of the reaction potential, the interface is refilled with a high concentration of substrate, enabling high conversion rates while simultaneously avoiding competitive side reactions. Based on this premise, similar performance as observed under rAP should be achievable under an applied DC potential if the supply and removal of species to and from the electrode surface is rationally controlled.
Furthermore, in reactions that required both an oxidation and reduction of intermediates to yield the final product, rAP is hypothesized to eliminate the need to transfer relevant intermediates across the bulk solution. This then enables enhanced performance of a reaction compared to running under DC conditions. However, it can be envisioned that a reactor engineered to have extremely high rates of mass transfer could move intermediates across the bulk solution at sufficient rates to yield the same performance as running under rAP.
To verify these hypotheses, we built electrochemical flow reactors which allow for precisely controlled mass transport rates. We compared conditions of high and low mass transport, with and without rAP, to evaluate the impact of mass transport in several reactions of interest to pharmaceutical electrosynthesis. Our data confirm that high mass transport rates can mimic rAP’s ability to replenish reagents at the interface and cycle intermediates.
Based on our findings, we were then able to employ controlled mass transport to achieve comparable organic electrosynthesis selectivity and yield under DC conditions compared to rAP. Our results provide novel insight into the role of mass transfer in rAP’s ability to mediate interfacial reagent concentrations to enhance reaction performance and chemoselectivity.