Abstract: Solid oxide fuel cell (SOFC) cathodes are typically composite materials with porous structure to meet gas diffusion requirements. However, the relationship between electrode structure and gas diffusion impedance remains to be further elucidated. Taking the typical composite cathode lanthanum strontium cobalt ferrite - doped ceria (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-Sm_0.2Ce_0.8O_1.9, LSCF-SDC) as the research object, this study reveals the relationship between cathode structure and gas diffusion impedance by combining electrochemical impedance spectroscopy (EIS), distribution of relaxation times (DRT) analysis, and three-dimensional (3D) reconstruction methods. The results indicate that under galvanostatic discharge, the diffusion impedance increases nearly linearly with cathode thickness. Specifically, at a high current density of 1.0 A·cm⁻², a non-zero intercept of 0.113 Ω·cm² appears in the impedance-thickness fitting, attributed to the mass transfer limitations imposed by the current collector and the external stagnant layer. Microstructural analysis demonstrates that as the dominant pore size increases from ~0.3 μm to ~1 μm, a fundamental transition in the gas transport mechanism from Knudsen diffusion to molecular diffusion occurs, resulting in a substantial reduction in diffusion impedance. However, while the increased porosity facilitates mass transfer, it leads to a decrease in triple-phase boundary (TPB) density from 233.5 to 196.7 μm·μm^-3, thereby inducing an increase in surface exchange impedance. Comprehensive analysis demonstrates that addition of 15%(in mass) ammonium oxalate achieves an optimal balance between mass transport capability and reaction activity. In this work, a quantitative diffusion impedance model incorporating porosity, tortuosity, and pore size factor is established, providing verifiable experimental evidence and theoretical support for the microstructural design of SOFC cathodes.

Key words: gas diffusion impedance, diffusion impedance model, oxygen reduction reaction, LSCF-SDC composite cathode, solid oxide fuel cell