Degradation Mitigation of Infiltrated Solid Oxide Fuel Cell Cathodes Via Atomic Layer Deposited Zirconia Overcoats

Abstract

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 ~200oC 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% La0.6Sr0.4Co0.8Fe0.2O3-x (LSCF) - Gd0.2Ce0.8O2 (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 La0.6Sr0.4MnO3 and gold grids were printed on both sides as current collecting layers, and 400oC-700oC 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 650oC. Figure 1 shows that ZrO2 ALD overcoats 1-10 nm in thickness did not significantly alter the 400-700oC LSCF-GDC NCC Rp. However, as shown in the 1000-hour, 650oC, 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 650oC. XPS analyses on 1000-hour, 650oC 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. Figure 1