Atomic Layer Deposited Zirconia Overcoats as On-Board Strontium Getters for Improved Solid Oxide Fuel Cell Nanocomposite Cathode Durability
Yubo Zhang, Yeting Wen, Kevin Huang, Jason D. Nicholas
Abstract
Here, a flow type Atomic Layer Deposition (ALD) reactor was used to deposit 1-10 nm thick porous ZrO<sub>2</sub> overcoats within the pores of conventional La<sub>0.6</sub>Sr<sub>0.4</sub>Co<sub>0.8</sub>Fe<sub>0.2</sub>O<sub>3-x</sub> (LSCF)-infiltrated Ce<sub>0.9</sub>Gd<sub>0.1</sub>O<sub>1.95</sub> (GDC) Solid Oxide Fuel Cell (SOFC) cathodes. Here, both coated and uncoated cathodes displayed initial 650 °C polarization resistance (R<sub>p</sub>) values of 0.09 ± 0.03 Ω cm<sup>2</sup>. However, improved stability was observed for cells with zirconia overcoats ≤ 5 nm thick. Specifically, 1000-hour, symmetric cell, open-circuit, 650 °C R<sub>p</sub> degradation rates decreased from 45 %/khrs for uncoated LSCF-GDC Nano-Composite Cathodes (NCCs) to 28 %/khrs, 18 %/khrs, and 12 %/khrs for identical LSCF-GDC NCCs with 1, 2, and 5 nm of zirconia overcoat, respectively. In contrast, identical LSCF-GDC NCCs with 10 nm of zirconia overcoat displayed 650 °C R<sub>p</sub> degradation rates of 87 %/khrs. Scanning Electron Microscopy and controlled atmosphere impedance tests showed no significant changes in the LSCF infiltrate particle size or microporosity gas concentration polarization resistance with 1000 hours of 650 °C aging. Instead, X-ray Photoelectron Spectroscopy indicated that zirconia overcoats decreased the amount of “surface Sr” on the LSCF, and X-ray Diffraction detected SrZrO<sub>3</sub> in samples with 5 or 10 nm thick zirconia overcoats. Hence, the lower degradation rates of LSCF-GDC NCCs with 1-5 nm thick zirconia overcoats were attributed to “clean up” of deleterious “surface Sr” from the LSCF surface via the formation of SrZrO<sub>3</sub>, while the higher degradation rates of LSCF-GDC NCCs with 10 nm thick zirconia overcoats were attributed to the accumulation of excessive amounts of SrZrO<sub>3</sub> hindering oxygen incorporation into the LSCF.