Abstract
Atomic layer deposition (ALD) has been used to modify the surfaces of La0.6Sr0.4Co0.2Fe0.2O3- d (LSCF) cathode powders with gadolinium doped ceria (GDC) and cobalt doped zirconia (CDZ). Cyclopentadienyl based Gd and Ce sources were used for GDC, while amidinate and amide sources were used for Co and Zr, respectively. ALD was carried out between 240-260°C. Transmission electron microscopy indicated uniform coating on selected samples, depending on process recipe. Coated powders were pressed into pellets and sintered at 1000°C in air. Microstructural analysis by scanning electron microscopy (SEM) revealed good sintering behavior. Similar sintering was observed for both GDC and CDZ treated powders. Pycnometry showed porosities in the range of approximately 41% for uncoated LSCF and in the range of 37-39% for surface modified powders and mercury probe analysis showed similar pore size distributions. Symmetrical electrochemical cells were prepared from pastes of uncoated and surface modified powders using a scandium stabilized zirconia electrolyte. Electrical impedance spectroscopy showed marked improvements in polarization for the surface modified powders (Fig. 1). ZrO2 and GDC coatings decreased polarization by a factor of 2. CDZ coatings decreased polarization by a factor of up to 4. Equivalent circuit modeling suggested little effect on gas diffusion through the porous structure, consistent with similar porosity values. Intermediate and high frequency contributions were significantly affected, suggesting a strong positive impact of the surface modification on both non-charge-transfer properties and charge-transfer properties, respectively. A pre-fabricated symmetrical cell treated with GDC via ALD showed very similar behavior to a symmetrical cell fabricated from surface modified GDC. This suggests that ALD may be used in alternate ways to improve cathode performance: on a fully sintered cathode or on the powder prior to sintering. Figure 1
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