Operando Electrochemical Kinetics in Particulate Porous Electrodes by Quantifying the Mesoscale Spatiotemporal Heterogeneities

TL;DR

This study introduces a precise operando method to analyze mesoscale electrochemical kinetics in porous electrodes, revealing significant local heterogeneities and challenging previous assumptions about diffusion control.

Contribution

A novel operando technique for directly measuring local current densities in porous electrodes, resolving discrepancies between experimental and theoretical kinetic parameters.

Findings

Local current densities are two orders of magnitude higher than global averages.

Spatiotemporal heterogeneities dominate electrochemical dynamics in porous electrodes.

Diffusion is not the primary control once heterogeneities emerge.

Abstract

Electrochemical energy systems rely on particulate porous electrodes to store or convert energies. While the three-dimensional porous structures were introduced to maximize the interfacial area for better overall performance of the system, spatiotemporal heterogeneities arose from materials thermodynamics localize the charge transfer processes onto a limited portion of the available interfaces. Here, we demonstrate a simple but precision method that can directly track and analyze the operando (i.e. local and reacting) interfaces at the mesoscale in a practical graphite porous electrode to obtain the true local current density, which turned out to be two orders of magnitude higher than the globally averaged current density adopted by existing studies. Our results resolve the long-standing discrepancies between kinetics parameters derived from electroanalytical measurements and from first principles predictions. Contradictory to prevailing beliefs, the electrochemical dynamics is not controlled by the solid-state diffusion process once the spatiotemporal reaction heterogeneities emerge in porous electrodes.