Direct Molecular Evidence of Proton Transfer and Mass Dynamics at the Electrode-Electrolyte Interface.

Proton transfer has been widely regarded as a key step in many electrochemical and biological processes. However, direct molecular evidence has long been lacking. In this work, we chose the electrochemical oxidation of acetaminophen (APAP) as a model system and utilized in situ liquid time-of-flight secondary ion mass spectroscopy (ToF-SIMS) to molecularly examine proton solvation and transfer in this process. In addition, we successfully captured and identified the transient radical intermediate, providing solid molecular evidence to resolve an important debate in electron transfer-proton transfer oxidation mechanism of APAP. Moreover, the potential-dependent behaviors of both inert ions and electroactive species during the dynamic potential scanning were chemically monitored in real time and the mass diffusion mechanism regarding the electroactive and nonelectroactive species was revealed under polarized conditions. The results are consistent with our computer simulations. The observations in this work greatly improved our understanding of proton transfer and mass dynamics occurring at the electrode-electrolyte interface in complex electrochemical processes.