La-doped Ba0.5Sr0.5Co0.8Fe0.2O3- d as Cathode for Protonic-Conducting Solid Oxide Fuel Cells with Enhanced Structure Stability

Abstract

Introduction As a highly efficient energy conversion device, proton-conducting solid oxide fuel cells (SOFCs) have attracted intensive attention due to its high performance at intermediate temperature.1 Ba0.5Sr0.5Co0.8Fe0.2O3- d (BSCF), as an excellent air electrode material for oxygen-ion conducting SOFCs,2 nevertheless, is not effective when employed in proton-conducting SOFCs. There are several factors limiting the performance of BSCF, including 1) the large amount of Ba2+ in A-site makes the material unstable in an atmosphere containing steam and carbon dioxide acid gases which might occupy active sites for oxygen reduction on the surface of cathode; 2) The large ionic radius of Ba2+ makes the distance between the B-site metal and oxygen far away, suppressing the electron conduction and resulting in low conductivity of BSCF.3 In this work, the modification of BSCF was achieved by partially replacing Ba2+ with La3+. La3+ has a smaller ionic radius and higher affinity with oxygen, which can reduce the unit cell size, increase the electronic conductivity, weaken the interaction between Ba2+ and O2-, and reduce the basicity of the material. This is expected to improve both the catalytic activity of the oxygen reduction reaction (ORR) and the stability of BSCF under SOFC operating condition. Experimental BSCF and La-doped BSCF (0.1La-BSCF) powders were prepared by citric acid-EDTA complex method, and were calcined at 1000 °C for 2 hours to remove the carbon residues and to obtain good crystallization. The button cells with a configuration of Ni-BaZr0.3Ce0.5Y0.2O3 (Ni-BZCY) anode support, thin layer BZCY electrolyte, and the 0.1La-BSCF air electrode were fabricated by the co-pressing method. The electrical conductivity measurements were conducted with the four-probe technique. Results and discussion XRD results indicate that the incorporation of La3+ effectively inhibits the formation of BaCO3 in humidified air, and thus improves the chemical stability of BSCF at 700 °C. Such improvement should be attributed to the improved interactions between La3+ and O2-, which decreases the electron density of Ba2+, leading to the lower basicity of BSCF. In addition, as the ionic radius of La3+ (1.36 Å) is much smaller than that of Ba2+ (1.61 Å), the introduction of La3+ can reduce the unit cell parameters, and lower the distance between B-site cation with oxygen. This effectively improves the electronic conductivity of BSCF at 600 °C from about 35 S cm-1 in air and 55 S cm-1 in O2 to 85 S cm-1 and 120 S cm-1, respectively. Fig. 1 Left: XRD patterns of BSCF and 0.1La-BSCF after synthesis and after heat treatment at 700 oC in humidified air for 100 hours. Right: Conductivity of BSCF and 0.1La-BSCF measured in air. Reference Duan, C.; Huang, J.; Sullivan, N.; O'Hayre, R., Proton-conducting oxides for energy conversion and storage. Applied Physics Reviews 2020, 7 (1), 011314.Shao, Z.; Haile, S. M., A high-performance cathode for the next generation of solid-oxide fuel cells. In Materials for Sustainable Energy: A Collection of Peer-Reviewed Research and Review Articles from Nature Publishing Group, World Scientific: 2011; pp 255-258.Xie, Y.; Shi, N.; Huan, D.; Tan, W.; Zhu, J.; Zheng, X.; Pan, H.; Peng, R.; Xia, C., A Stable and Efficient Cathode for Fluorine-Containing Proton-Conducting Solid Oxide Fuel Cells. ChemSusChem 2018, 11 (19), 3423-3430. Figure 1