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

Abstract

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 La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ–Sm_0.2Ce_0.8O_1.9 (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 ∼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.