To reduce Solid Oxide Fuel Cell (SOFC) cathode operating temperatures, solution infiltration has been used to fabricate Nano-Composite Cathodes (NCCs) comprised of nano-sized mixed ionic electronic conducting oxygen exchange catalysts on sintered, micro-porous, ion-conducting scaffolds.^1,2 In these cathodes, degradation due to nano-particle coarsening and/or Sr segregation are a major concern.³ Recent studies have suggested that conformal zirconia overcoats several nm thick produced via Atomic Laser Deposition (ALD) can improve the long-term stability of SOFC cathodes. ⁴

Since, the low ~200^oC ALD deposition temperatures do not coarsen the nanoparticle networks in infiltrated cathodes,⁵ here ZrO₂ ALD overcoats of various thickness were deposited onto high=performance 12 vol% La_0.6Sr_0.4Co_0.8Fe_0.2O_3-x (LSCF) - Gd_0.2Ce_0.8O₂ (GDC) NCCs. Symmetrical cathodes were produced on both sides of dense, ~500 micron thick (Y₂O₃)_0.08(ZrO₂)_0.92 electrolytes using previously described procedures.¹ Then, porous La_0.6Sr_0.4MnO₃ and gold grids were printed on both sides as current collecting layers, and 400^oC-700^oC Electrochemical Impedance Spectroscopy (EIS) measurements were performed with a push-contact setup to determine the NCC polarization resistance (Rp) from the EIS Nyquist plot x-intercepts. Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS) were also conducted on NCCs as function of overcoat thickness and degradation time at 650^oC.

Figure 1 shows that ZrO₂ ALD overcoats 1-10 nm in thickness did not significantly alter the 400-700^oC LSCF-GDC NCC Rp. However, as shown in the 1000-hour, 650^oC, open circuit Rp degradation tests of Figure 2, increased zirconia overcoat thickness resulted in increased performance stability (at least up to 5 nm of ZrO₂). Specifically, the degradation dropped from ~31.6 %/khrs for uncoated LSCF-GDC NCCs, to ~28 %/khrs, ~18 %/khrs, and ~11.7 %/khrs with 1, 2, and 5 nm of ZrO₂ overcoat, respectively. TEM analysis showed that the ZrO₂ overcoat maintained a constant, conformal, target thickness after 1000 hours at 650^oC. XPS analyses on 1000-hour, 650^oC aged cells showed that zirconia overcoats decreased the Sr/(La+Sr+Co+Fe) ratio at the surface, providing a possible explanation for the reduced degradation rates of LSCF-GDC NCCs containing ZrO₂ ALD overcoats.