Role of Synthetic Route on the Conductivity of BaCe1-xYxO3 Proton Conductor

ROLE OF SYNTHETIC ROUTE ON THE CONDUCTIVITY OF BaCe1-xYxO3 PROTON CONDUCTOR S. Barisona, M. Battagliarina, G. Chiodellib, S. Daolioa, L. Doubovaa, M. Fabrizioa, R. Gerbasic, L. Malavasib, C. Mortalòa aIENI-CNR, Corso Stati Uniti 4 - 35127 Padova bIENI-CNR, Viale Taramelli 16 - 27100 Pavia cICIS-CNR, Corso Stati Uniti 4 - 35127 Padova Solid oxide fuel cells (SOFCs) have gained many attentions due to their high thermodynamic efficiency and low impact to the environment, as promising energy conversion system. [1] In particular, SOFCs are studied for stationary applications such as residential and industrial or power plants. The oxygen ion conductor yttria-stabilized zirconia (YSZ), the usual SOFC electrolytes, operates at high temperatures (around 1000°C), causing serious problems of stability for the cell components. Therefore, the lowering of SOFCs operating temperatures by means of new electrolyte materials is fundamental for their commercialization. In the view of the development of IT-SOFCs (Intermediate temperature SOFCs), proton conducting materials with perovskite structure (ABO3) are promising candidates as electrolytes, because of their significant proton conductivity under humidified H2 atmosphere, over the wide 300°C÷1000°C temperature range. Among these, various doped barium cerate compounds, such as BaCe1-xAxO3-? (with A = Y, Gd, Yb, Nd, Sm and ?=x/2), have been intensively investigated. In particular, BaCeO3 has been reported to have a high total conductivity of about 5.3×10-2 ?.cm when doped with 20 at.% Y (BaCe0.80Y0.20O2.9). [2] Solid-state process preparation of ceramic oxides requires high temperature treatment at 1400÷1600°C. At such temperature the well known effect of component evaporation can occur modifying the stoichiometry. Recently, a few wet chemical methodologies, as oxalate co-precipation [3], citrate [4] and Pechini process [5] were set up to prepare BCY compounds. In particular, Pechini sol-gel route has allowed synthesizing very homogeneous powders with optimal control of cation stoichiometry. [6] The main purpose of the work presented in this communication is to evaluate and compare the electrical properties of the sintered BaCe1-xYxO3-? (x=0, 0.1, 0.15 and 0.2 named BCY00, BCY10, BCY15 and BCY20 respectively) prepared by either solid state reaction or modified sol-gel Pechini method. In order to underline the proton role in the total conductivity, the electrical measurements were carried out in wet and dry atmosphere by means of Electrochemical Impedance Spectroscopy (EIS). An extensive structural and morphological investigation was performed by means of X-Ray Powder Diffraction (XRPD) and Scanning Electron Microscopy (SEM). Moreover, simultaneous differential thermal analysis and thermogravimetry (DTA/TGA) were done in dry and wet air atmosphere both on the gel precursors and on the final products to detect the formation of single phase and to evaluate the water uptake of the Y-doped cerates. The TGA analyses of gels during sol-gel synthesis showed no weight change above 1000°C, suggesting that the single phase perovskite was already formed. To evaluate the hydration of the cerate phase, thermal analyses were performed on final powders in dry air atmosphere. A 0.48% weight drop between 500°C and 700°C was detected, mainly ascribed to hydration water loss. Single phase powders were obtained in the whole range of stoichoimetry by both synthetic procedures, but for powders synthesized by sol-gel the thermal treatments were carried out at lower temperatures. In fact, pure, single phase perovskite powders with orthorhombic structure (space group Pmcn) were produced heating at 1100 °C for 2 hours. Moreover, in diffractograms of sol-gel synthesised samples it is worth highlighting the broadening of the peaks, an indication of the presence of crystallites having average sizes around 30 ± 3 nm. SEM observations confirmed that the modified Pechini process, using EDTA and ethylene glycol as complexing and polymerizing agent respectively, favoured the formation of nanostructured powders (particles sizes around 100 nm aggregated in open, sponge-like structures). In this case, full-density pellets were achieved by sintering at 1250 °C, as confirmed by SEM micrographs showing good coarsening and average grain size in the 1.2 ± 0.5 ?m range. On the contrary, lower density materials were obtained by solid state reaction microsized powders, even if sintered at 1380°C. In the wet hydrogen atmosphere (H2/H2O), the BaCe1-xYxO3-? (x=0, 0.1, 0.15 and 0.2) pellets prepared by both sol-gel and solid state syntheses, have shown significantly higher conductivities respect to dry conditions. The oxide compositions BCY15 and BCY20 show conductivities close to 10-2 ?-1cm-1 in atmosphere H2/H2O, just at the 600°C temperature. Moreover, samples prepared from nano-sized powders (sol-gel method) have higher conductivities in both dry and wet conditions in comparison with samples from solid state reaction powders. The electrical conductivity measured in isothermal conditions indicates BCY as promising electrolytes for IT-SOFCs. In particular, between 500 and 700°C (the worthwhile working temperature range for IT-SOFC) BCY15 exhibits the highest value among the considered stoichiometries. Comparing the processing conditions and the electrical conductivity values, sol-gel synthesis procedure of BCY15 revealed to be more suitable than solid state reaction. In view of the application of this material in a fuel cell, the optimization of screen printing conditions is in progress in order to deposit the electrolyte film on the supporting anode. Acknowledgment: This research has been funded by the "Celle a combustibile ad elettroliti polimerici e ceramici: dimostrazione di sistemi e sviluppo di nuovi materiali" FISR Project of MIUR. References: [1] B.C.H. Steele, A. Heinzel, Nature, 2001, 414, 345. [2] H. Iwahara, Solid State Ionics, 1995, 77, 289. [3] F. Chen, P. Wang, O.T. Sørensen, G. Meng, D. Peng, J. Mater. Chem., 1997, 7, 1533. [4] D.W. Lee, J.H. Won, K.B. Shim, Materials Letters, 2003, 57, 3346. [5] Agarwal, V.; Liu, M. J. Mater. Sci. 1997, 32, 619. [6] S. Barison, M. Battagliarin, S. Daolio, M. Fabrizio, E. Miorin, P.L. Antonucci, S. Candamano, V. Modafferi, E.M. Bauer, C. Bellitto, G. Righini, In press on Solid State Ionics.