Subsequently, in order to promote thorough interdiffusion of homogeneous yttria-zirconia alloys, short-period (bilayer period < 1 nm) yttria/zirconia laminates were and annealed under the same conditions as the long-period laminates. 2-4 mol% YSZ films with thicknesses 20-35 nm As-deposited YSZ is amorphous; annealed is polycrystalline Annealed YSZ with 3 mol% yttria (3YSZ) is tetragonal, as expected from bulk phase diagram 6 Annealed short-period laminate exhibits columnar grain structure (~5-25 nm grain diameter) that spans entire film thickness XRR indicates density increase from 5.78±0.29 g/cc to 6.13±0.31 g/cc (commercial bulk 3YSZ, 6.05 g/cc), likely due to removal of excess H 2 O incorporated during ALD process and film crystallization Densification was accompanied by 2.8 (±1.7) % reduction in film thickness Detailed electrical conductivity measurements were carried out on selected as-deposited and heat treated samples using the van der Pauw technique. Electrical characterization of ultrathin (25-35 nm thick) 3YSZ films indicated a 10 times enhancement of the total electrical conductivity compared to bulk 3YSZ 6 and is about 15 times higher than that of bulk polycrystals with 2% yttria 7. It is interesting to note that the films show very high total conductivity even with very low yttria content. Further investigation of yttria-doping effects and phase transformation kinetics are needed to understand the observed increase in electrical conductivity of these yttria-zirconia alloys. 6. Fevre, M., et al, Local order and thermal conductivity in yttria-stabilized zirconia. I. Microstructural investigations using neutron diffuse scattering and atomic-scale simulations.
We demonstrate that alternating the oxygen partial pressure gradient across a yttria-stabilized zirconia (YSZ) electrolyte membrane prior to solid oxide fuel cell (SOFC) testing with nanoporous Pt electrodes greatly increases (e.g. by >70-fold at 350°C) peak power density compared with devices without pretreatment. Transiently altering the oxygen activity at the cathode-YSZ interface appears to change the wetting characteristics of the nanoporous Pt, significantly affecting the stability and low-temperature performance of the SOFCs. Image analysis and impedance spectroscopy results suggest that an increase in triple-phase boundary area fraction at the cathode-YSZ interface contributes to the observed effect.Nanoporous Pt is a common electrode material for research on low-to-moderate temperature (e.g. ≤550°C) solid oxide fuel cell (SOFC) devices. However, a major difficulty associated with nanoporous Pt electrodes, even at low temperatures, is the tendency for grain coarsening and film break-up on the surface of electrolytes such as yttria-stabilized zirconia (YSZ). [1][2][3][4][5][6] These morphological changes can result in severe loss of triple-phase boundaries (TPBs -regions where gas, electrolyte, and electrode coexist), which in turn increases the activation loss and overpotential required to run the fuel cell. At low-to-moderate temperature, slow cathode kinetics tend to limit the performance of SOFCs, and high TPB area is a key structural feature for efficient O 2 reduction and oxygen ion incorporation into the electrolyte at the cathode side of the cell. [6][7][8] We fabricated SOFCs with nanoporous Pt electrodes and investigated the evolution of electrode morphology during operation. Device performance was correlated with changes in Pt morphology during SOFC testing. To the best of our knowledge, this is the first study presenting the time-evolution and performance of nanoporous Pt electrodes at various SOFC operating temperatures. In addition, a simple gas pretreatment was found to lead to immediate, significant, and repeatable enhancement of SOFC power density.YSZ SOFC devices were fabricated using commercially available polycrystalline 8-YSZ substrates (8 mol% Y 2 O 3 Ceraflex by Marketech Intl., Inc.) and DC-sputtered, nanoporous platinum electrodes. Prior to electrode deposition, each 100-μm-thick, 25 mm × 25 mm YSZ substrate was treated with oxygen plasma for 1 min on each side to remove any carbonaceous residue left over from the original substrate fabrication process. Immediately thereafter, the substrate was placed in a DC sputtering system for electrode deposition. Each cathode and anode consisted of a 115-nm-thick layer of nanoporous Pt, and was sputtered at room temperature under 10.0 Pa of argon pressure and 50 W power for 150 s. While the anode was blanket-deposited, the cathode was 11 mm × 11 mm in area, positioned at the center of the YSZ substrate, and thus defined the active area (and the reported current densities) of the SOFCs. Details of the electrochemical measurements performed on ...
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