Performance Of Iron-and Niobium-containing Electrode Materials For Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are highly efficient and environmentally friendly electrochemical energy conversion devices, which have attracted great attention as power sources for mobile and stationary applications. The solid oxide fuel cell research trend is to the development of IT-temperature. However, with the decrease of temperature, cathodic polarization resistance increases, resulting in the decrease of the cell performance. So the development of cold cathode materials with excellent properties has become an important research direction of SOFC. In addition, the traditional anode carbon deposition, sulfur poisoning and other problems, limited to hydrocarbon fuel SOFC.Perovskite-type oxide SrCoO3-δ has three different polymorphs: the oxygen vacancy-ordered orthorhombic brownmillerite phase,2H-BaNiO3-typ hexagonal phase and3C-type cubic perovskite phase, The cubic perovskite phase of SrCoO3-δ has an outstanding catalytic activity, high oxygen permeability and desirable mixed ionic-electronic conductivities, which makes it a very vital parent compound. However, there are also several challenges in development of the cubic perovskite SrCoO3-δ prior to its practical applications. One of the major challenge issues is to stabilize the high temperature cubic structure of SrCoO3-δ at room temperature in order to ensure the structural stability. An effective way to do so is to partially substitute for the B-site of SrCoO3-δ with appropriate dopants to form a solid solution while maintaining its good ionic conductivity. As application examples, the B-site doped SrCoO3-δ oxides have been investigated for use as oxygen separation membranes, methane conversion reactors and IT-SOFC cathodes. One typical example is the case of SrCo0.8Fe0.2O3-δ (SCF) with the highest oxygen permeability in the series of La1-xSrxCo1-yFeyO3-δ perovskites, which was first investigated by Teraoka et al. for application in oxygen separation membranes. Unfortunately, the structural instability of SCF perovskite hinders its practical applications. Further studies have shown that partially substitution of higher valence cation for the B-site of SrCoO3-δ significantly improved the structural stability of the perovskite. The most effective cation was found to be Nb5+in stabilizing the structure and improving the oxygen permeability of the SrCoO3-δ-based perovskites. Recent studies have also shown that the Nb doping for Co site could obviously enhance the structural stability and electrochemical performance of La0.1Sr0.9Co1-xNbx03-δ materials compared to undoped La0.1Sr0.9CoO3-δ. Iron and Niobium co-doped perovskites SrCo0.8-xFe0.1+xNb0.1O3-δ (x=0.0and0.1, SCFNO and SCFN1) are synthesized by solid-state reaction and their structures and properties are investigated as potential cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs) based on the LaSr0.1Ga0.8Mg0.2O3-δ (LSGM) electrolyte. Fe and Nb co-doping stabilizes the cubic phase of the SCFN possessing high thermal stability at room temperature. The SCFN cathodes are chemically compatible with the LSGM electrolyte at950℃for10h. Fe and Nb co-doping significantly improves the electrical conductivity of the SCFN materials compared to the undoped SrCoO3-δ. The maximum electrical conductivities of the SCFNO and SCFN1samples are357and218S cm-1at about500℃. The polarization resistances of the SCFNO and SCFN1cathodes on the LSGM are0.091and0.115Ω cm2at700℃, respectively. The maximum power densities of the single cells with SCFNO and SCFN1cathodes on a0.3mm thick LSGM electrolyte reach756and615mW cm-2at800℃, respectively. These results indicate that the SCFN perovskites are promising cathode materials for use in IT-SOFCs.In addition, A2BB’O6type double perovskite oxides, with its large amounts of oxygen and excellent tunneling magnetoresistance effect caused the wide attention of scholars, and as the anode material of SOFC. The Mg, Mn, Mo and Cr doping are studied particularly wide. In addition, A2BB’O6double perovskite oxides, B can be different ions of different oxidation states, B’can be doped with different elements, provides more possibilities for the study of anode materials. Double perovskite oxide Sr2FeMoO6-δ Delta in CH4oxidation exhibited good catalytic activity, and hypoxia in1200℃partial pressure atmosphere can exist stably, which aroused our great interest. The SFNM1material was synthesized by solid state method, the SFNM1materials have a cubic perovskite structure stability; electrical conductivity shows a semiconducting, the electrical conductivity increases with increasing temperature; at800℃, the maximum power density of SFNMl was329mW cm-2; in the range of30-1000℃, the average thermal expansion coefficients of SFNM114.6*10-6K-1. Results showed that the SFNM1material can be used in solid oxide fuel cell as the SOFC anode.