Upscaled model for diffusion and serial reduction pathways in porous electrodes

Abstract

Abstract Multiscale modelling of coupled diffusion and serial reduction reactions in porous micro-electrodes is developed in this work. The governing coupled equations at the pore scale in the case of two reduction reactions, as for instance, the serial reaction pathway for oxygen reduction to hydrogen peroxide and subsequently to water, are upscaled to obtain a macroscopic model describing the process in an effective medium at the electrode scale. This new macroscopic model, obtained from the volume averaging technique, is validated through comparisons with results of 3D Direct Numerical Simulations of the pore-scale model. The excellent agreement between the two approaches proves the relevance of the macroscale model which reduces to a 1D problem in the configuration under concern, providing a drastic speedup in the computation of the solution. Numerical results obtained with the macroscopic model are successfully compared to experimental data obtained by voltammetry with porous gold electrodes of different thicknesses operating the serial pathway of oxygen reduction to water. Results highlight the ability of this new macroscopic model to predict the electrode behavior and show that the second reduction reaction of hydrogen peroxide plays an important role in the current production.