Characteristic time integration to characterize gas diffusion and surface reaction steps of the oxygen reduction reaction via porous electrocatalysts for solid oxide fuel cells

Abstract

An analytical method, characteristic time integration (CTI), is proposed to characterize the gas transportation and surface reaction steps of the high-temperature oxygen reduction reaction (ORR) in porous composite electrocatalysts for solid oxide fuel cells. The new method is developed from the previously reported characteristic time distribution (DCT) theory that is proposed to analyze ORR kinetics in porous electrode materials based on electrical conductivity relaxation (ECR) experiments. This method considers all the characteristic times in the DCT spectrum, gives the weighted average characteristic time, defines the minimum characteristic time, and thus calculates the intrinsic surface exchange coefficient as well as the hindrance factor of gas diffusion. This method is demonstrated with porous composites consisting of the mixed electron–ion conductor Sr₂Fe_1.5Mo_0.5O_6−δ (SFM) as the electrocatalyst and the oxygen-ion conductor Sm_0.2Ce_0.8O_1.9 (SDC) as the additive. The composites have close porosities in the range of 0.345–0.38 and similar pore size distributions but different SDC contents and thus various densities in three-phase boundaries (TPBs), which are obtained through three-dimensional reconstruction using back-scattered electron microscopy images. The effective surface exchange coefficients of the composites are determined using the CTI method based on ECR profiles, and the surface exchange coefficient at TPBs and the activity per unit length of the three-phase line are calculated. At an SDC content of 9.06 vol%, the composite exhibits the highest ORR rate and TPB activity. In addition, the gas diffusion effect is calculated with the minimum characteristic time, which proves that the attenuation effect of gas diffusion in pores on the ORR process can reach 15.5% in the porous composite.