The Study of Double Perovskite-based Symmetrical Electrodes for Solid Oxide Fuel Cells and CO2 Electrolysis

Abstract

To address the issue of global climate change, it is important to replace fossil fuels with clean energy sources and reduce the concentration of carbon dioxide in the atmosphere. In this perspective, Solid Oxide Cells (SOCs) are one of the promising energy devices. Especially, SOCs can be used reversibly as Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysis Cells (SOECs). If a symmetrical electrode is adopted in SOC, which exhibits high performance while maintaining chemical stability in both oxidizing and reducing atmospheres, it can reduce difficulties and costs associated with manufacturing process. In most candidates for a symmetrical electrode, double perovskite-based materials have a potential to adopt it due to their high ionic conductivity and surface exchange coefficient. Among these candidates, PrBaFe2O5+δ (PBF) is well-known material which is stable in both oxidation and reduction atmospheres. In our previous report, it was confirmed that Sr and Ge doping in PBF (PrBa0.5Sr0.5Fe1.9Ge0.1O5+δ, PBSFG) can induce the formation of oxygen vacancy and enhancement of electrical conductivity. It was effective to strengthen SOFC performance when it used to anode. It is thought that PBSFG can be used not only as a symmetrical electrode for SOFC, but also for CO2 electrolysis, because the increase of oxygen vacancy and electrical conductivity can positively affect the CO2 adsorption and dissociation. Therefore, PBSFG was used as a symmetrical electrode of SOC, and it was evaluated in this study. The maximum power density in the SOFC was measured as approximately 1.05 W/cm2 at 800 ℃, it is the highest performance among similar conditions where double-perovskite materials were used to symmetrical electrode for SOFC. In the case of CO2 electrolysis, the maximum current density was measured as 825 mA/cm2 at 850 ℃, 1.5 V, which is higher than PBF. To identify the cause of performance enhancement, several analyses were carried out, including Electrochemical Impedance Spectroscopy (EIS), Distribution of Relaxation Time (DRT), Taffel slope, X-ray Photoelectron Spectroscopy (XPS), Electrical Conductivity Relaxation (ECR) method. Considering all the results of these analyses, it was verified that Sr and Ge doping in PBF induced the formation of oxygen vacancy and the enhancement of charge transfer rate related to CO2 adsorption and dissociation of carbon intermediate. Consequently, it was demonstrated that PBSFG exhibits excellent performance in both SOFC and CO2 electrolysis when it used as symmetrical electrode of SOC.