Estimating Reaction Rate Constants from Impedance Spectra: Combining Microkinetic Modeling and Experiments of the Oxygen Evolution Reaction

Knowledge of the reaction rate constants can be vital in understanding electrochemical reaction mechanisms and their rate-determining processes. Although first-principles methods, such as density functional theory (DFT), provide valuable insight into reaction free energies and rate constants, they commonly use idealizing assumptions. As an alternative, this study demonstrates the estimation of the rate constants of the oxygen evolution reaction (OER) from electrochemical impedance measurements of a photo-electrochemical cell with a hematite anode, providing insight into the rate constants directly from measurements. Estimation is conducted using maximum likelihood estimation, minimizing the weighted error between a microkinetic model of the OER and measurements at multiple frequencies and potentials. In this way, a single set of rate constants is obtained that is optimized for multiple potentials simultaneously. The effectiveness of the estimation method is shown using synthetic measurements. Furthermore, this study shows that rate constants can be estimated from experimental measurements. However, accuracy is limited, potentially because additional processes, such as diffusive transport, are not included in the current model. The estimated rate constants are best suited for comparative analysis, rather than as absolute values, which may be influenced by specific samples. This enables analysis of differences between materials, validation of models, and prediction of electrochemical data of different material systems. This research demonstrates how combining potential and frequency dependent electrochemical impedance spectroscopy (EIS) experiments with microkinetic modeling enables the estimation of reaction rates and intermediate species coverages without relying on idealized atomistic configurations, aiding in identifying reaction mechanisms directly from experiments.