Studies On Novel Solid Oxide Fuel Cell Cathode Materials Based On Sr₂Fe_1.5Mo_0.5O_6-δ

Current research is focusing on the development of intermediate temperature solidoxide fuel cells (IT-SOFCs), which are operated within the600-800oC range so as todiminish materials attenuation, long-term instability and high cost. However, relatively lowoperation temperature decreases the catalytic activity of the oxygen reduction reaction(ORR) that occurs in cathode, and increases the polarization loss. Thus, the performance ofcathodes is becoming a limiting factor to IT-SOFCs. Therefore, recently, great efforts havebeen devoted to modifying the conventional cathode materials or developing new materialsthat can efficiently work at intermediate temperature. Sr2Fe1.5Mo0.5O6δ(SFM) as a newperovskite oxide, has recently attracted much attention because it show excellent redoxstability and catalytic activity in both oxidizing and reducing environments.In this work, we propose a novel strategy to prepare the nanoporous compositecathodes Sr2Fe1.5Mo0.5O6δ-Sm0.2Ce0.8O1.9(SFM-SDC) for IT-SOFCs by a facileself-combustion technology. This strategy can produce a nanocomposite material withhomogeneously distributed SFM and SDC phases. The SDC particles were smaller, about10-100nm in diameter, than the SFM particles (100-200nm), and the size of the pores incomposite was approximately1μm. The electrochemical impedance spectroscopy (EIS)results exhibit that SFM-SDC40(wt%60:40) cathode has best electrochemical performancewith low polarization resistance (Rp). Bi-layer cathodes SDC/SFM-SDC were fabricated,and excellent electrochemical performance of such composite cathodes were observed. TheSDC interlayer significantly decreases the Rpof cathode and accelerates the charge transferprocess. As a result, the Rpof the SDC/SFM-SDC40bi-layer cathodes is almost50%lessthan that of SFM-SDC40cathode on YSZ electrolyte at800oC, and Rpis only0.11cm2.The anode-supported single cells with SDC interlayer also demonstrate significantlyenhanced power performance in a temperature range of650-800°C compared with the cellswithout SDC interlayer. The maximum power densities of NiO-YSZ/YSZ/SFM-SDC andNiO-YSZ/YSZ/SDC/SFM-SDC were0.33and0.78W cm2at650°C,1.77and2.11Wcm2at800°C, respectively. Sr2Fe1.5-xNixMo0.5O6δ(x=0,0.05,0.1,0.2,0.4)(SFNM) materials have beensynthesized by sol-gel combustion method and studied towards application as cathodematerials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The crystalstructure, microstructure, thermal expansion, element valence, conductivity andelectrochemical properties have been characterized as a function of Ni content. Thesymmetrical structure of cubic lattice in perovskite oxides is confirmed. SFNM materialspossess the3D interconnected network microstructure composed of nanoparticles. Anincreasing Ni substitution results in the unit cell shrinkage and the increase of thermalexpansion coefficient (TEC). Furthermore, Doped Ni element exhibits low oxidation state(Ni2+). B-site Ni-doped affects the equilibrium between Fe3+/Mo5+and Fe2+/Mo6+, which isdirectly related to the conductivity. The SFNM conductivity was apparently improved,reaching60S cm-1at450oC when x=0.1, which is more than twice that of SFM sample.Sr2Fe1.4Ni0.1Mo0.5O6δ(SFN0.1M) cathodes showed excellent electrochemical performanceand lowest Rp.The Rpof SFN0.1M cathode was approximately50%of that of SFM.Moreover, the maximum power densities of single cell based on SFN0.1M cathode were0.92and1.27W cm2at700and750°C, respectively.Perovskites Sr2-xBaxFe1.5Mo0.5O6δ(x=0,0.2,0.4,0.6,0.8,1) have been prepared aspotential cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs)via combustion method. The expansion of unit cell is derived from the substitution of thelarge radius Ba, and the impurity phase structure is detected when x=0.8and1.Concurrently, TECs increase with the increasing content of doped Ba. A-site doped Ba inperovskite has an insignificant impact on the existence and the ratio of Fe3+/Mo5+andFe2+/Mo6+species in Sr2-xBaxFe1.5Mo0.5O6δ(x=0.2,0.4,0.6)(SBFM) samples. Comparedwith Sr2Fe1.5Mo0.5O6δ, SBFM cathodes exhibit better electrochemical performance, whichhas been confirmed by EIS. In particular, Sr1.8Ba0.2Fe1.5Mo0.5O6δ(SB0.2FM) shows thelowest Rp. The peak power densities of single cell based on SB0.2FM cathode are0.87and1.30W cm2at700and750°C, respectively. All these results show SBFM materials arepromising cathode materials for IT-SOFCs.The mechanism of the influence of B-sit Ni-doped and A-site Ba-doped for SFMmaterial performance is discussed in this paper，which is analyzed through the lattice change and the change of Fe and Mo element valence state in materials. It is supposed thatdoped Ni2+is conducive to the formation of oxygen vacancies, at the same time theequilibrium between Fe3+/Mo5+and Fe2+/Mo6+is affected. The performance enhanced ofSFNM material via B-site Ni-doped is attributed to the two factors. And the performance ofSBFM material is enhanced after A-site Ba substitution owing to the unite cell expansion,which accelerates the movement of free electrons in the crystal lattice. The equilibriumreaction between Fe3+/Mo5+and Fe2+/Mo6+is not obvious influence on the properties ofSBFM material. We concluded that material performance is optimal when the amount of thedouble electronic couples Fe3+/Mo5+and Fe2+/Mo6+equals.