Electric Potential-Driven Acid/Base Chemistry: Kinetics of Electrochemical Interfacial Proton Transfer and Transport

Proton transfer at solid/liquid interfaces is a fundamental step in many complex biological and electrocatalytic processes. Previous model studies using electrodes modified with self-assembled monolayers (SAMs) of carboxylic acid-terminated alkanethiols have demonstrated that interfacial proton transfer is controlled by the local electrochemical microenvironment. The thermodynamic driving force for electrochemically driven protonation/deprotonation of acid/base SAMs is governed by a combination of the electric potential at the SAM/solvent interface, the pK_a of the acid group, and the solution pH. Here, we develop a kinetic model to describe electric potential-driven protonation/deprotonation as a two-step process. This comprises a reversible proton transfer step at the SAM/electrolyte interface (i.e., (de)protonation) and a proton transport step describing the motion of protons as they traverse the diffuse electrical double layer to and from the solution bulk. The kinetics of the transport step are investigated using finite element simulations, providing numerical estimates for the transport rate constants under combined diffusional and migrational transport modes. Using the dependence of these rate constants on the electric potential at the SAM/electrolyte interface, we define situations where the overall rate expression is limited by either (de)protonation, proton transport, or a combination of both. From this analysis, we determine a lower limit for the acid group pK_a of ≈3, above which proton transfer at the plane of acid dissociation is generally the rate-determining step. The electric potential-driven proton transfer/transport kinetic model developed herein provides a general approach to treat electric potential-driven coupled ion transfer and transport phenomena, with potential applications including proton-coupled electron transfer processes, ion intercalation in alkali metal batteries, and ion transport across biological membranes.