Exploration of Novel Oxide Cathode Materials for Proton-Conducting Solid Oxide Fuel Cell: Phase Relation in the Ba(Zr, Y)O3-Ba(Co, Y)O3-Ba(Fe, Y)O3 System

Abstract

1. Introduction Proton-conducting solid oxide fuel cell (pc-SOFC) is expected to be one of the most efficient, highly stable, and cost-effective SOFC. As a cathode material for pc-SOFC, a composite cathode system, which comprises of both electron- and proton-conducting materials, has been examined and developed. In such cases, reaction area is limited to the triple phase boundary (TPB), and resultant cathode performance is poor in most cases. On the other hand, upon successful development of mixed proton-hole conductor (MphC) for the oxide cathode, the reaction area will be expanded to the whole surface of cathode grains, because the reaction area turned into two-dimensional boundary from one-dimensional TPB, and the efficiency of the cell will be significantly enhanced[1]. Our primary strategy to develop protonic/electronic mixed conductor for a pc-SOFC cathode is to realize a double percolation system in a single phase of solid solution, that is, Y-doped BaZrO3 (BZO) as a proton conducting host matrix, doped with Co and Fe for the formation of percolation network of electronic carriers, since BaZrO3 is reported as chemically stable perovskite with the best proton conductivity at the target temperature between 400 to 600 °C[2]. In addition, the electronic conductivity is reported in Co-[3] or Fe-doped[4] BaZrO3, while a possible low solubility of Co and its poor chemical stability has to be resolved. Fe is known to dissolve to form solid solution over the entire composition range in the BZO-BaFeO3 quasi binary system at elevated temperatures, providing both chemical stability and the hole conductivity to BZO. As the first step of the development of the MphC cathode materials, phase relation in the Ba(Zr, Y)O3-Ba(Co, Y)O3-Ba(Fe, Y)O3 system at elevated temperatures is examined to understand the feasibility of the present system to the double percolation scheme. In this study, the composition is expressed as BCFZYxyz1 for the formula, BaCox/10Fey/10Zrz/10Y0.1O3-δ, and BCFZY4411, corresponding to the composition of novel cathode for pc-SOFC reported by Duan et al.[5], is included in this compositional contour of the present study. Also further analyses including electrical conductivity and so on will be reported. 2. Experimental Powders of BaCox/10Fey/10Zr0.9-x/10-y/10Y0.1O3-δ with x and y ranging from zero to nine were synthesized via the Pechini method from nitrate aqueous solutions. Concentrations of cations in the nitrate aqueous solutions were determined by both chemical titration and an inductively coupled plasma atomic emission spectroscopy (ICP-AES). The powders were calcined at 1000 °C for 2 h, pressed into pellets using a uniaxial pressing at 1000 kgf/cm2 with an 11.3 mm diameter mold, and subsequently sintered for 10 h at either 1200, 1300, or 1400 °C. The pellets were ground into powders and X-ray diffraction (XRD) measurements were performed with the obtained powders. A field emission scanning electron microscopy and (FESEM) and an energy dispersive X-ray spectroscopy (EDX) were performed on the polished fresh surfaces of the pellets and the annealed surfaces of the pellets after thermal etching. 3. Results and Discussion From the results of XRD measurements, the most of the peaks observed in the XRD patterns were attributed to the primary cubic perovskite structure in reference to the ICDD data for cubic BaZrO3. In BCFZY3061, 3151, 3241, 3331, 2251, and 1351, peaks from the secondary perovskite phase were observed, while those from secondary and tertiary perovskite phases were observed in BCFZY4411 and 4141. The secondary and tertiary phases, however, were not clearly identified by EDX. In most of the samples, the peaks indexed with BaCO3 were observed because of the BaO-rich composition of the precursors identified by ICP-AES analyses. The other peaks were attributed to the impurity phases such as BaCoO3 and YBaCo4O7. In BCFZY3061, 3151, 3241, 3331, 4411, and 4141, partially melted area and thin plate-/needle-like crystals were observed with FESEM. These crystals were identified as YBaCo4O7 or its analog from the EDX results. In addition to the EDX analyses, from the relation between lattice constants of cubic BaZrO3-like phases and Fe/(Co+Fe+Zr) ratio, the phase relation in the BaZr0.9Y0.1O3-δ-BaFe0.9Y0.1O3-δ-BaCo0.9Y0.1O3-δ quasi-ternary system is estimated and proposed that there exist four separate solid solutions of BaZrO3-, BaFeO3-, BaCoO3- based phases and an intermediate phase between Ba(Zr, Co)Y0.1O3 and Ba(Zr, Fe)Y0.1O3.