Synthesis and characterisation of the perovskite-related cuprate phases YSr2Cu2MO7+y(M = Co, Fe) for potential use as solid oxide fuel cell cathode materials

Abstract

In this paper we report the synthesis and characterisation of the perovskite cuprate phases YSr 2 Cu 2 MO 7+y (M=Co, Fe) to examine their potential for use as cathode materials in Solid Oxide Fuel Cells (SOFC). Both samples showed conductivities of ≈10 Scm -1 at 900°C and were also shown to be stable at this temperature in N 2 . For YSr 2 Cu 2 FeO 7+x , semiconducting behaviour was observed up to ≈550°C, with a decrease in conductivity at higher temperatures, attributed to oxygen loss reducing the charge carrier concentration.In the case of YSr 2 Cu 2 CoO 7+y , semiconducting behaviour was observed over the range of temperatures studied, although a small but significant steep increase in conductivity was observed above 800°C. High temperature X-ray diffraction studies of this particular phase showed that this increase in conductivity coincided with an orthorhombictetragonal structural transition, accompanied by a significant reduction in cell volume. In addition to measurements in air, conductivities were also measured with varying p(O 2 ) (0.2-10 -5 atm.) at 900°C, and this data showed significant hysteresis between measurements on reducing and re-oxidising, suggesting poor oxide ion transport, poor oxygen surface exchange kinetics, or significant structural changes on varying p(O 2 ).Chemical compatibility studies of these phases with SOFC electrolytes at temperatures between 900 and 1000°C showed reaction in all cases. In the case of CeO 2 based electrolytes, the reaction led to the formation of the "fluorite-block" phases, (Y/Ce) 2 Sr 2 Cu 3-x M x O 9+y (M=Co, Fe), and samples of these were subsequently prepared and the conductivities measured. Similar hysteresis between conductivity measurements on reducing and re-oxidising were also observed for these samples.

Electrochemically Induced Phase Changes in La2CuO4 During Cathodic Electrocatalysis

“…For example, oxygen vacancies have been shown to directly affect kinetics of both OER and ORR in these materials, and oxygen hyperstoichiometry influences catalytic activity by providing access to high valent oxidation states of transition metals within the lattice . The high ionic conductivity with respect to O 2− anions has also made RPOs a central focus in solid oxide fuel cells, resulting in the synthesis and structural characterization of a large selection of ternary and quaternary compositions . The comprehensive electrochemical behavior of RPO materials is not typically explored in detail.…”

Control of Co content and SOFC cathode performance in Y 1−y Sr 2+y Cu 3−x Co x O 7+δ

“…The orthorhombic 3a p structure of YSr 2 Cu 2 CoO 7 (illustrated in Fig. 1) was first reported by Huang et al [29], and a later study at high temperature showed that it undergoes a transformation to a tetragonal structure above 800 C [30]. This compound is a layered perovskite, with Y, Sr, Co and Cu layers ordered along the stacking axis.…”

“…The electronic properties of YSr 2 Cu 2 MO 7þd (M ¼ Fe, Co) were studied by Sansom et al [30,31], who reported a p-type semiconductor with a total conductivity of 15 S cm À1 at 800 C, whilst thermogravimetric analysis showed that no oxygen loss occurred in the temperature range 25e900 C under air and nitrogen atmospheres. The material was found to be reactive towards the common electrolytes La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 2.85 , YSZ, GDC and apatite at elevated temperatures.…”

Electrochemical characterization of PrBa2−xSrxCu3O6+δ layered oxides as innovative and efficient oxygen electrode for IT-SOFCs

“…The change of oxidation states of the mixed valence cations influences the conductivity values due to the concentration of electron carriers (holes, h • ). The work carried out by Sansom et al [47] on cuprates with the 1:2:3 structure, YSr 2 Cu 2 CoO 7 and YSr 2 Cu 2 FeO 7 , also reports a hysteresis in conductivity values between measurements performed in reducing and oxidizing conditions, which is assigned to a poor oxide ion transport, poor oxygen surface exchange kinetics, or significant structural changes on varying oxygen partial pressure. For YBCO, different resistivity values during cooling and heating were measured even with ramps of 0.1°C min − 1 .…”