Atomic Layer Deposited Zirconia Overcoats as On-Board Strontium Getters for Improved Solid Oxide Fuel Cell Nano-Composite Cathode Durability

Abstract

In order to improve the electrochemical performance of Solid Oxide Fuel Cells (SOFCs), nano-composite cathodes (NCCs), which are fabricated by adding nano-sized mixed ionic and electronic conducting (MIEC) catalysts into backbones of partially-sintered micro-sized ionic conducting particles via precursor solution infiltration, have been intensively studied.1,2 Despite the reduced operating temperatures, poor long-term stability has still been observed for these high-performance NCCs.3 As a low-temperature, nano-sized thin-film deposition method, atomic layer deposition (ALD) has been shown to help stabilize SOFC cathodes.4 Therefore, in this work ZrO2 overcoats of various thicknesses were deposited onto 12 vol% La0.6Sr0.4Co0.8Fe0.2O3-x (LSCF) - Gd0.1Ce0.9O2 (GDC) NCCs using ALD. The initial 400oC-700oC electrochemical performance long-term 650oC degradation behavior were studied with electrochemical impedance spectroscopy (EIS). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the microstructure of NCCs with different ZrO2 thicknesses, while X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were conducted to investigate aging-induced compositional changes.Figure 1 shows that no significant change in initial polarization resistance (RP ) was observed for LSCF-GDC NCCs with 1-10 nm ZrO2 ALD overcoats between 400oC and 700oC. However, as shown in Figure 2, while no significant ohmic resistance (R0) degradation happened for any of the LSCF-GDC NCCs, different RP aging behavior was observed for cells with different ZrO2 thicknesses. Specifically, the 650oC RP degradation rate dropped from ~45%/khrs for uncoated LSCF-GDC NCCs, to ~28%/khrs, ~18%/khrs, and ~12%/khrs for NCCs with 1, 2, and 5 nm of ZrO2 overcoat, respectively, indicating improved durability. With 10 nm ZrO2 overcoat, however, the RP degradation rate increased to ~87%/khrs. SEM analyses showed no evidence of LSCF particle coarsening for any of the cells, while XPS showed less inactive Sr species on the LSCF surface for ZrO2-coated cells after aging, compared with uncoated ones. As discussed in our recent paper on the subject,5 this reduced inactive Sr species, together with the SrZrO3 phase observed from detailed XRD analyses for ZrO2 coated LSCF pellets after aging, suggested that the ZrO2 overcoats act as Sr getters and react with inactive Sr species on the LSCF surface during aging. This reaction cleans up the LSCF surface and leads to better RP stability. For 10 nm overcoats, too much SrZrO3 starts to accumulate on the LSCF surface during aging, causing the observed increase in the RP degradation rate. References T. E. Burye and J. D. Nicholas, J. Power Sources, 300, 402–412 (2015).T. E. Burye and J. D. Nicholas, J. Power Sources, 276, 54–61 (2014).M. Shah, P. W. Voorhees, and S. A. Barnett, Solid State Ionics, 187, 64–67 (2011).Y. Gong et al., Nano Lett., 13, 4340–4345 (2013).Y. Zhang, Y. Wen, K. Huang and J. D. Nicholas, ACS Applied Energy Materials, 3, 4057 (2020). Figure 1