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Abstract

This paper compares the effects of Ni and Ru dopants in lanthanum chromite anodes by correlating structural characterization and electrochemical measurements in solid oxide fuel cells ͑SOFCs͒. Transmission electron microscope observations showed that nanoclusters of Ni or Ru metal precipitated onto lanthanum chromite ͑La 0.8 Sr 0.2 Cr 1−y X y O 3−␦ , X = Ni,Ru͒ surfaces, respectively, after exposure to hydrogen at 750-800°C. Ni nanoclusters were typically ϳ10 nm in diameter immediately after reduction and coarsened to ϳ50 nm over ϳ300 h at 800°C. In contrast, Ru cluster size was stable at Յ5 nm, and the cluster density was Ͼ10 times larger. SOFC tests were done with the doped lanthanum chromite anodes on La 0.9 Sr 0.1 Ga 0. 3,4 Clearly, nanoparticles may coarsen at the relatively high firing temperatures ͑ϳ1400°C͒ used to process SOFCs. Thus, nanoparticles must be introduced after high temperature firing steps. A new method for introducing nanoscale metal particles into oxide anodes was recently reported. The oxide material, La 0.8 Sr 0.2 Cr 1−y Ru y O 3−␦ ͑LSCrRu͒, was fired in air at elevated temperature, but when the anode was reduced during the initial SOFC operation, Ru nanoparticles Ͻ5 nm in diameter formed on the oxide surface. Experimental Procedures Powders of LSCrRu and LSCrNi were synthesized by solid-state reaction at 1200°C for 3 h, yielding particle sizes of ϳ1 to 2 m. In the discussion below, the different Ni or Ru contents are given, for example, as LSCrNi31 ͑La 0.8 Sr 0.2 Cr 0.69 Ni 0.31 O 3−␦ ͒. The SOFC anodes consisted of 50 wt % of one of the above chromite powders mixed with 50 wt % Gd-doped ceria ͑GDC͒. All SOFCs utilized La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3−␦ ͑LSGM͒ electrolytes, ϳ400 m thick. The LSGM powders were fabricated via solid-state reaction at 1250°C, followed by uniaxial pressing and sintering for 6 h at 1450°C to form the electrolyte pellets. The cathodes were La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−␦ ͑LSCF͒ mixed with 50 wt % GDC. The anodes and cathodes ͑0.5 cm 2 active area, ϳ25 m thick͒ were screen printed on the LSGM electrolytes and fired for 3 h at 1200 and 1000°C, respectively. Au current collector grids were screen printed over the electrodes and contacted using Ag wires. Single cell tests were performed as described previously 9 using a four-wire setup for current-voltage ͑I-V͒ and electrochemical impedance spectroscopy ͑EIS͒ measurements ͑BAS-Zahner IM-6͒. In life tests, the cells were first stabilized at temperature with Ar at the anode before starting H 2 flow. The H 2 flow to the cells was first humidified by bubbling the gas through H 2 O at room temperature, resulting in Ϸ3% H 2 O in H 2 . Times given in life test results are after the start of humidified H 2 flow. Measurements on various other SOFCs indicated that Ar was almost entirely purged from the anode compartment before the first electrical measurements ͑15 min͒. X-ray diffraction measurements of anode powders were done with a standard diffractometer ͑Rigaku 0.8 kW Dmax͒. Scanning electron microscopy ͑SEM͒ measurements were done along with energy-dispersive spectroscopy ͑EDS͒ ͑Hitachi S3400N-II, S3500, S3800, and S4800-II cFEG͒. High resolution electron microscopy ͑HREM͒ studies were carried out on the powder samples using a JEOL JEM-2100F electron microscope operated at 200 kV. The powders were annealed in either dry H 2 or 3% H 2 O/H 2 . Micrographs were digitally acquired on a 2 ϫ 2 k charge-coupled device camera using a Gatan Imaging Filter system. A small amount of powder was added to acetone, followed by ultrasonic mixing to achieve a particle dispersion. A drop of the resulting suspension was deposited on carbon-coated transmission electron microscope ͑TEM͒ grids ͑Ted Pella͒. Samples were stored in a desiccator overnight before HREM examination. Experimental Results The following sections describe first the structural observations of LSCrNi, with results for LSCrRu included for comparison. In the second section, electrochemical test results for SOFCs with these anodes are discussed. Structural characterization.-Ni-doped anodes.