Potential-Resolved Snapshot Impedance Spectroscopy for Exploring the Mechanism of a Complex Reaction Undergoing Parallel Pathways

Abstract

Here we report an electrochemical snapshot approach with rate modulation, where impedance spectra are; captured as snapshots on varied potentials. Using this method, we managed potential-resolved mechanistic analysis of a complex reaction composed of multiple and parallel steps. The snapshot impedance spectroscopy is based on transforming the E-I data in the time domain to those in the frequency domain, and the rate modulation of specific protonation steps is based on controlling the buffer capacity and pH of the solution. We applied this strategy to monitoring changes in impedances during p-benzoquinone reduction as a function of the scanned potential, which allowed us to elaborately analyze its chemical and electrochemical reaction steps in the frame of parallel electrochemical-chemical-electrochemical (ECE) and electrochemical-chemical-disproportionation (ECD) mechanisms. As a result, we found that the second electron transfer of the reaction occurs either directly from the electrode or indirectly via disproportionation, and the relative contributions of each pathway depend on pH. While well-buffered solutions have mostly been used in studies in the past, poorly buffered solutions with appropriate buffer capacities are best taken advantage of for the purpose of controlling the protonation levels.