CHARACTERIZATION OF MECHANICAL PROPERTIES AND DEFECTS OF SOLID-OXIDE FUEL CELL MATERIALS
Wachsman, Eric D
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Solid-oxide fuel cells (SOFCs) have the potential to help meet global energy demands by efficiently converting fuel to electricity. The technology currently requires high temperatures and has reliability limitations. A critical concern is the structural integrity of the cell after redox cycling at operating temperatures. As new materials are developed to reduce operating temperatures and improve redox stability, the effect of the environment on the mechanical properties must be studied. Ceria-based systems have allowed the operating temperature to be decreased to the 600℃ range. For this reason, a three-point bend apparatus was developed which could test materials up to 650℃ in reducing environments. Using this apparatus, it was shown how pore geometry and amount affected strength of porous gadolinium doped ceria (GDC) at 650℃ with lower aspect ratio pores, leading to higher fracture strength due to crack tip blunting. The strength of Ni-GDC/GDC half-cell coupons showed no dependence on loading orientation at elevated temperatures in air, but were 47% weaker when the electrolyte was placed in tension under H2 as compared to when the electrolyte was placed in compression. It was also determined that a reduced Ni-GDC/GDC coupon could be exposed to air for an extended period of time and reheated under H2 with no effect to the strength, allowing for more options when processing and preparing cells. A new anode material, SrFe0.2Co0.4Mo0.4O3-δ (SFCM), was investigated for chemical expansion, oxygen non-stoichiometry, and mechanical properties. SFCM maintains phase purity under reducing conditions, with little changes to lattice parameter between oxidation and reduction, but under oxidation, SFCM forms Sr2Co1.2Mo0.8O6 impurities. SFCM supports a large degree of non-stoichiometry, up to δ = 0.176 at 600℃, due to a low enthalpy of formation for oxygen vacancies of 44.3 kJ mol−1. Fracture toughness of SFCM was determined to be (0.124 ± 0.023) MPa√m in air at room temperature and (0.286 ± 0.038) MPa√m at 600℃. The strength of SFCM-GDC half-cells increased by 31% upon heating to 600℃, after which reduction decreased strength by 29%. Reduction and redox cycling were shown to only decrease the characteristic strength, not alter the structural flaw distribution, as microcracks uniformly grew.