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

Electrochemical energy systems rely on particulate porous electrodes to store or convert energies. While the three-dimensional (3D) porous structures are introduced to maximize the interfacial area for better overall performance of the system, spatiotemporal heterogeneities arising from materials thermodynamics are localizing the charge transfer processes onto a limited portion of the available interfaces. Here, a simple but precise method is demonstrated to directly track and analyze the operando (i.e., local and working) interfaces on the mesoscale in a practical graphite porous electrode to obtain the true local current density, which turns out to be two orders of magnitude higher than the globally averaged current density adopted by existing studies. The results shed light on the long-standing discrepancies in kinetics parameters derived from electroanalytical measurements and from first principle predictions. Contradictory to prevailing beliefs, the electrochemical dynamics are not controlled by the solid-state diffusion process once the spatiotemporal reaction heterogeneities emerge.