Method to determine the oxygen reduction reaction kinetics via porous dual-phase composites based on electrical conductivity relaxation

The kinetics for the oxygen reduction reaction (ORR) via porous dual-phase composites are critical for high-temperature electrochemical devices such as solid oxide fuel cells. Herein, a method was proposed to determine the chemical surface exchange coefficient (k(chem)) and reveal the ORR process of porous dual-phase composites based on electrical conductivity relaxation measurements and the distribution of characteristic time (DCT) model. The method was demonstrated with porous La0.6Sr0.4Co0.2Fe0.8O3-delta-Sm0.2Ce0.8O1.9 (LSCF-SDC) composites, whose geometric properties, such as percolation probability and three-phase boundary (TPB) length, were determined from 3D structures utilizing numerical simulation. DCT analysis showed the ORR process involved a combination of three steps: gas diffusion, surface exchange, and their interaction. The gas diffusion contributed up to 22% of the ORR resistance, even though the composite porosity was as high as 50%. Also, k(chem) was greatly improved by adding SDC to form dual-phase composites and the highest improvement was achieved at similar to 10 vol% SDC. It is suggested that the improvement was related to TPB, but the reaction should go beyond the TPB lines. The present method is also useful for analyzing CO2 reduction and vapor splitting reactions in solid oxide electrolysis cells.