In Situ X-Ray Absorption Spectroscopic Studies on Proton Conduction Mechanism of Zr, Y Co-Doped Barium Cerate for Organic Hydride Fuel Cell Operating at Intermediate Temperature

Abstract

Introduction A new type of fuel cell utilizing organic hydrides as a fuel, such as methylcyclohexane – toluene system attracts a lot of attention because they are good hydrogen career from the point of view of safety, easy handling, and high energy density. This type of fuel cell using proton using proton conductive oxide electrolyte is required to operate at intermediate temperature (400°C) from the perspective of reaction efficiency and thermal decomposition of organic hydride. In order to develop the fuel cell system, proton conductive electrolyte with higher conductivity and chemical stability is needed, and proton conducting perovskite oxides are anticipated to achieve above requirements. Among these perovskite oxides, BaCe0.9-xZrxO3-δ has been investigated widely due to its high proton conductivity and chemical stability. [1] However, further enhancement of proton conductivity is required to apply the material for fuel cell application, to investigate the proton conducting mechanism. In this study, we analyzed the fine structure of BaCe0.9-xZrxO3-δ by in situX-ray absorption spectroscopy (XAS), and reveal its proton conducting mechanism. Experimental BaCe0.9-xZrxO3-δ was prepared by the solid-state reaction method. Stoichiometric BaCO­­­­­­­3, CeO­2, ZrO2, and Y2O3 powders were mixed by ball milling with ethanol at 400 rpm for 10 hours. After drying, they were calcined at 1400°C for 10 hours in air, and then grounded by ball milling again with ethanol for 20 hours. Dried powder was pressed at 120 kg/cm2 into cylindrical pellets. These pellets were vacuum packed and hydrostatic pressed at 200 MPa for 5 min. They were embedded in the same composition powder and sintered at 1700°C for 10 hours in air. After sintering, the pellets were polished with a diamond slurry by a grinding machine. For conductivity measurements, Pt paste was applied to both phases of the pellets. The measurement was carried out under wet 3% H2/N2, wet 21%O2/N2 (pH2O = 1.7×103 Pa), and dry 21%O2/N2. The XAS measurement was carried out at BL01B1 of SPring-8. Two kinds of samples for X-ray absorption measurements were prepared as follows. For ex situ measurements, sintered pellets were grounded by an alumina mortar and mixed with BN powder. Obtained powder was molded into cylindrical pellets by uni-axial press. For in situ sample, these pellets were polished and their thickness became 100 mm. For in situ XAS measurements, the temperature range was room temperature to 600°C, and the atmosphere was wet 3% H2/N2 (pH2O = 1.7×103 Pa) and dry 21%O2/N2. Results and Discussion The metal cation – oxide distance were calculated by XRD and EXAFS analysis. Ce-O and Y-O distance were significantly longer than average M-O distance. It implies that in BaCe0.9-xZrxO3-δ­ structure, CeO6 and YO6 octahedral are tilting due to their larger ionic radii than Zr4+. These tilting sites contribute to reduction of symmetry in the crystal structure. At high temperature, Ce-O and Zr-O distance became drastically longer. Increase of the M-O length mitigates the repulsion between proton and metal cation, promoting the proton conductivity. Ce-O distance was shortened in wet 3% H2 atmosphere. It implies that protonic defect is stabilized around Ce4+because of its low electronegativity. This work revealed the relation between fine structure of proton conducting perovskites and their proton conductivity, and showed the guide of material design with higher proton conductivity. REFERENCES [1]K. Katahira, Y. Kohchi, T. Shimura and H. Iwahara, Solid State Ionics, 138, 91 (2000) [2] A. Longo, F. Giannici, A. Balerna, C. Ingrao, F. Deganello,and A. Martorana, Chem. Mater., 18, 5782 (2006).