Atomic Layer Deposition Functionalized Composite SOFC Cathode La_0.6Sr_0.4Fe_0.8Co_0.2O_3-δ -Gd_0.2Ce_0.8O_1.9: Enhanced Long-Term Stability

Abstract

We report that the long-term stability of a conventional mixed oxide-ion and electron conducting solid oxide fuel cell cathode, La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3−δ -Gd 0.2 Ce 0.8 O 1.9 (LSCF-GDC) composite, can be markedly improved by functionalizing its surfaces with a conformal layer of nanoscale ZrO 2 films through atomic layer deposition (ALD). Over a >1100 h testing at 800 °C, the overcoated LSCF-GDC cathode exhibited respective polarization and ohmic area-specific-resistances 3 and 1.5 times lower than the pristine sample, whereas the pristine LSCF-GDC cathode degraded at a rate 4 times faster than the overcoated one. The multifunctionality of porosity, mixed conductivity, and suppressed Srenrichment enabled by the nanoscaled ALD-ZrO 2 overcoats are attributed to the performance retention observed for the overcoated cathode.

Modification of LSF-YSZ Composite Cathodes by Atomic Layer Deposition

“…As pointed out in the Introduction, two previous studies reported uniform deposition of porous ZrO 2 films in porous electrodes, with growth rates of roughly 0.6 nm/cycle. 4,5 In contrast, growth rates in our work were more than 20 times lower and the large effect of ALD modification on the electrode impedances in our work implies the films must be reasonably dense.…”

“…The precursors used in the present study were tetrakis (2,2,6,6-tetramethyl-3,5-heptanedionato) zirconium (Zr(TMHD) 4 , Strem Chemical, Inc.), tris(2,2,6,6-tetramethyl-3,5-heptanedionato) lanthanum (La(TMHD) 3 Because the ligands for the La and Zr precursors could not be oxidized with O 2 or water at the deposition temperature, the samples used in this study were removed from the system and oxidized in a muffle furnace at 773 K for 5 min after each deposition cycle. Later studies showed that the ligands could be effectively oxidized at the deposition temperature using NO 2 as the oxidant and that identical growth rates for ZrO 2 on γ-Al 2 O 3 could be achieved by sequential dosing of the Zr(TMHD) 4 , precursor and NO 2 . As discussed in previous papers, 15,16 varying the exposure times and pressures did not affect deposition rates for the conditions used in these experiments.…”

“…[1][2][3][4] For example, Gong et al reported that degradation rates for cathodes based on La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) decreased significantly after depositing a 5-nm ZrO 2 film. 3,4 While they reported a slight increase in the initial electrode impedance, the performance of the ALD-modified electrode surpassed that of the unmodified electrode after less than 100 h and exhibited a much lower impedance after 900 h of operation at 1073 K. In another example of cathodes modified by ZrO 2 ALD films, the initial impedance of composite cathodes of Sr-doped LaMnO 3 (LSM) and yttria-stabilized zirconia (YSZ) actually decreased following deposition of films as thick as 60 nm. 5 However, studies in which electrodes were modified by ALD with other oxides have shown deleterious effects on performance.…”

Advances in Cathode Materials for Solid Oxide Fuel Cells: Complex Oxides without Alkaline Earth Metal Elements

“…[ 79,130,152,180 ] In particular, the development of new electrode materials free from alkaline earth metal elements has received much attention. In the past two decades, many alkaline earth element-free metal oxides of different structures have been developed, such as perovskite, Ruddlesden-Popper, and spinel structures.…”

Atomic Layer Deposition on Porous Materials: Problems with Conventional Approaches to Catalyst and Fuel Cell Electrode Preparation

“…While growth rates will depend on the precursor and surface, these are typically reported to be between 0.01 and 0.1 nm/cycle, values that correspond well to that which would be calculated based on the thickness of a precursor monolayer [28]. In apparent contradiction with this, two groups have reported growing ZrO 2 films in porous SOFC electrodes at rates between 0.5 and 0.6 nm/cycle [29,30]. Another group reported growing metallic W films at 0.4 nm/cycle, although the diameter of a W atom is only 0.25 nm [31].…”