Quantification of Butler–Volmer Electron Transfer Kinetics from Scanning Electrochemical Microscopy Images

Direct-current scanning electrochemical microscopy (SECM) is an experimental technique that measures electrochemical reactions at an interface by scanning a microelectrode (ME) over the interface. A redox-active molecule (called a redox mediator) present in solution can undergo electrochemical reactions at both the ME and reactive features (RFs) at the interface. Previously, irreversible reactions at an RF have been fit from SECM images, but many reactions of interest for SECM follow reversible or quasi-reversible kinetics modeled by the Butler–Volmer equation. This work showed that pairs of SECM images (one image where the ME was reducing the redox mediator, and one where it was oxidizing the redox mediator) are necessary to fit both the exchange rate constant, k0, and overpotential, η, of the RF. SECM image pairs were fit by optimizing parameters of a finite element model to minimize the difference between the simulated and the target SECM images. Fits of synthetic image pairs achieved accurate estimates for k0 and η. Experimental SECM image pairs of a ⊘250 μm Au disk electrode in a mixed solution of FcMeOH/FcMeOH+ were also fit using this method. Overpotentials of the electrode were mostly fit to within 10 mV of their actual values. Fouling of the ME likely caused the kinetics of the FcMeOH+ + e–⇌ FcMeOH reaction (which should have been too fast for k0 to be measured by our ⊘10 μm ME) to appear to be activation-limited in some cases.