Effects of Atomic Layer Deposition of ZrO2 on Infiltrated Mixed Ionic and Electronic Conducting Solid Oxide Cell Electrodes

Abstract

Infiltration of mixed ionic and electronic conducting (MIEC) nanoparticles into ionic or MIEC scaffolds has been employed to improve the performance of solid oxide cells (SOCs) designed to operate at relatively low temperatures (<600°C). However, the high MIEC specific surface area also hastens sintering and coarsening processes that can lead to a high rate of performance degradation for the electrodes over time. Atomic layer deposited (ALD) coatings have been investigated as a means of curbing the degradation of nano-scale MIEC electrodes. ALD is desirable for coating porous electrodes because of its self-limiting layer-growth characteristic — fully conformal coating can ideally be achieved throughout the depth of a porous electrode, including the most electrochemically active region near the electrolyte. However, results reported to date have yielded mixed results: in some cases, the ALD coating suppresses degradation, whereas in others the coating actually degrades electrode performance. Successful performance improvement seems to be dependent on a combination of factors from choice of electrode substrate and its architecture, to the coating species, its thickness throughout an electrode, and the experimental procedures for coating. This presentation seeks to better elucidate the nature of these dependencies via a study of Sr0.5Sm0.5CoO3-δ (SSC) infiltrated Gd-doped ceria (GDC) electrodes with ALD-ZrO2 coating. Life tests were carried out with degradation accelerated by using a relatively high ageing temperature of 750°C, with impedance spectroscopy measured periodically at 600°C. Rises in polarization resistance with time were substantially mitigated by 30 ALD deposition cycles, and to a lesser extent by 60 cycles, relative to uncoated samples. The overall 0-hour performance is virtually the same across the coated and uncoated samples after a short break-in period, with the benefit from ALD being entirely in the increased sustained performance. The mechanistic benefit of ALD-ZrO2 on these electrodes is parsed with the aid of microstructural and chemical characterization. Scanning electron microscopy with energy dispersive spectroscopy of cross-sections of the coated samples reveals a substantial gradient in the ZrO2 coating thickness, with several nm thickness near the free surface decreasing to sub-monolayer thickness near the electrode/electrolyte interface. This is sensible because thicker ZrO2 coatings on SSC powders were found by powder x-ray diffraction to react extensively, forming Sr- and Sm-zirconates and cobalt oxide species. The results suggest that sub-monolayer ALD-ZrO2 coatings form a very thin layer of reactants that do not cover electrochemically-active surface sites but can help suppress particle coarsening. Based on this mechanism, a coarsening model previously developed for infiltrated SSC-GDC electrodes is shown to fit the polarization resistance versus time data by assuming that the ALD layer decreases the cation diffusion. The ideal thickness is that which balances the negative effect of the reaction of active SSC material with the positive effect of a reduced effective diffusion coefficient. This thickness is sensitive to the number of ALD cycles deposited as well as the aspect ratio of the pores the ALD precursors must traverse within the electrode. The results will be contrasted to those obtained from ALD-ZrO2 on conventional La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) electrodes, which do not degrade by coarsening due to the larger feature sizes. In this case, there is no benefit from monolayer-thickness ALD-ZrO2, and thicker layers substantially degrade the performance. Figure 1