Abstract
Ceramics samples with the nominal composition [(ZrO2)0.95(Y2O3)0.05]1-x[PrOy]x and praseodymia contents of x = 0.05–0.15 were prepared by the direct firing of compacted 5YSZ + PrOy mixtures at 1450–1550 °C for 1–9 h and characterized for prospective applicability in reversible solid oxide cells. XRD and SEM/EDS analysis revealed that the dissolution of praseodymium oxide in 5YSZ occurs via the formation of pyrochlore-type Pr2Zr2O7 intermediate. Increasing PrOy additions results in a larger fraction of low-conducting pyrochlore phase and larger porosity, which limit the total electrical conductivity to 2.0–4.6 S/m at 900 °C and 0.28–0.68 S/m at 700 °C in air. A longer time and higher temperature of firing promotes the phase and microstructural homogenization of the ceramics but with comparatively low effect on density and conductivity. High-temperature processing leads to the prevailing 3+ oxidation state of praseodymium cations in fluorite and pyrochlore structures. The fraction of Pr4+ at 600–1000 °C in air is ≤2% and is nearly independent of temperature. 5YSZ ceramics with praseodymia additions remain predominantly oxygen ionic conductors, with p-type electronic contribution increasing with Pr content but not exceeding 2% for x = 0.15 at 700–900 °C. The average thermal expansion coefficients of prepared ceramics are in the range of 10.4–10.7 ppm/K.
Highlights
Limited fossil fuel reserves and growing environmental concerns push forward the development of alternative energy technologies based on renewable sources—wind, solar, biomass, and others
While the solid oxide electrolysis cell (SOEC) may utilize electrical energy supplied by a renewable source to split water and generate hydrogen as energy storage or fuel, the solid oxide fuel cell (SOFC) converts chemical energy stored in the form of hydrogen back to electricity on demand [1,2,3,4,5]
The emphasis in this work was on the effect of PrOy content and direct firing conditions on properties relevant for the application including phase composition, ionic and electronic transport properties, Pr oxidation state and associated redox behavior, and thermomechanical compatibility with 8YSZ in terms of thermal expansion
Summary
Limited fossil fuel reserves and growing environmental concerns push forward the development of alternative energy technologies based on renewable sources—wind, solar, biomass, and others. The common view is that high oxygen partial pressure buildup within the pores, grain boundaries and other defects in the near-interface electrolyte layer and at the electrode/electrolyte interface under anodic polarization results in intergranular fractures in the electrolyte surface layer and cracking and delamination at the electrode/electrolyte interface [7,8,9] These irreversible degradation processes are contributed or promoted by the interdiffusion of cations with the formation of undesired phases at the interface, insufficient ionic conductivity, and, the electrochemical activity of the oxygen electrode, and other factors. The emphasis in this work was on the effect of PrOy content and direct firing conditions (temperature and time) on properties relevant for the application including phase composition, ionic and electronic transport properties, Pr oxidation state and associated redox behavior, and thermomechanical compatibility with 8YSZ in terms of thermal expansion
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