Study on CO₂ reduction reaction on the surface of Sr₂Fe_1.5Mo_0.5O_6−δ-Sm_0.2Ce_0.8O_1.9 porous composite materials

A method for measuring and characterizing the surface exchange reaction rate of CO₂ reduction reaction (CO₂RR) with porous two-phase cathode in solid oxide electrolysis cell (SOECs) was established. This method is based on electrical conductivity relaxation (ECR) and applied to mixed electron-ion conducting Sr₂Fe_1.5Mo_0.5O_6−δ-Sm_0.2Ce_0.8O_1.9 (SFM-SDC) composites, whose microstructure characteristics such as permeable probability, three-phase boundary (TPB) length, and surface area are determined through numerical simulation using scanning electron microscopy images. Analysis the ECR data with characteristic time distribution (DCT) model shows that the CO₂RR process involves a combination of three steps: gas diffusion, surface exchange, and their interactions. The gas diffusion contributes 18–29 % of the total resistance CO₂RR, demonstrating its significant in SOEC cathode reactions that take place in porous structures. Adding SDC generates additional active reaction sites of TPBs and thus effectively enhance the CO₂RR rate with a strengthening factor up to 10. Meanwhile, the chemical surface exchange coefficient of CO₂RR increases with the increase of TPB length, indicating that the presence of TPB significantly enhances the CO₂RR surface reaction of porous composite cathode.