Elucidating Spatial Distribution of Electrochemical Reaction in a Porous Electrode by Electrochemical Impedance Spectra for Flow Batteries

A porous electrode is an essential component in a flow battery, and its structure determines the battery's performance. The coupling of the multi-temporal-spatial-scale processes (e.g., electrochemical reaction, mass transfer, charge transfer) makes the recognition of each process complicated. Herein, a symmetric flow cell device is developed, and the electrochemical impedance measurement (two- or three-electrode configuration) is realized to elucidate the electrochemical processes. First, the effect of flow rate and concentration on the impedance spectra is investigated to identify the electrochemical processes. Second, the distributed resistance is quantified to describe the spatial distribution of the electrochemical reaction. It is found that the electrochemical reaction occurs near the membrane side at a low polarization current, and the reaction zones spatially extend from the membrane side to the current collector with the increase of imposed polarization. Such an evolution of the spatial distribution stems from the trade-off between the mass transfer and the ion conduction in the porous electrode. This work provides an experimental method to nondestructively probe the electrochemical processes, and the result provides guidance for developing innovative electrode structures for flow batteries.