Angstrom-Thick Oxide Overcoat on Decorated Nanoclusters for Enhanced Performance and Durability of SOFC Cathodes
Authors: Li Haoyu , Kang Hung-Sen , Grewal Simranjit , Lee Min Hwan 
Published: December 2020

A solid oxide fuel cell (SOFC) is an energy conversion system with high conversion efficiency, high power density and fuel flexibility.1 Since its durability and cost issues are largely originated from the high temperature operation, intense efforts have been made to reduce the operational temperature to an intermediate range of 600 – 800 °C.2 The decrease in temperature, however, results in a significant decrease in the electrochemical kinetics in particular of the oxygen reduction reaction (ORR) occurring at the cathode.3,4

Atomic layer deposition (ALD) is an emerging low-temperature chemical vapor deposition variant that can deposit well-dispersed islands or uniform films at the atomic scale even on a highly corrugated geometry.8,9 By leveraging the capability, researchers have successfully improved SOFC durability mainly by suppressing the agglomeration of high-surface-area electrodes10–13 and/or dopant segregation to the surface of perovskite-based cathodes.14,15 Considering the capability of ALD to perform a conformal atomic-scale treatment on complex geometry, an ALD treatment can be best leveraged when applied to a structure of extreme surface area as opposed to conventional porous backbone structures. However, its application to a high-surface-area electrodes (e.g. infiltrated electrodes) and associated analysis is rarely reported.

In this presentation, we demonstrate that a uniform atomic-scale oxide overcoat (either ceria or yttria with a nominal thickness of 0.7 – 1.5 Å) by ALD is highly effective not only in the enhancement of thermal stability of underlying ceria nanoparticles (5 – 20 nm) infiltrated on LaNi0.6Fe0.4O3-
d (LNF) backbone (~100 nm) but also in the facilitation of oxygen reduction kinetics. Since LNF does not suffer from Sr segregation, a major degradation mechanism,17 the analysis of cell performance degradation is simplified. Based upon electrochemical and physical characterization of a series of infiltrated and/or ALD-treated samples, we discuss the impact of surface treatment on the nanoscale morphology, surface chemistry and electrode performance.

We provide a quantitative analysis of the thermal agglomeration of NPs with and without ALD treatment and prove the close correlation between the NP agglomeration and electrode performance degradation. To the best of our knowledge, this is the first work showing the correlation in a quantitative manner. In addition, we demonstrate that an angstrom-scale ALD overcoat dramatically enhances the electrode performance in terms of polarization resistance and its activation energy for both bare LNF and ceria-infiltrated LNF electrodes. The improved electrode activity from ALD treatment is mainly ascribed to a significant facilitation of O2 dissociative adsorption with a partial reduction, with the aid of surface-specific oxygen-deficient and catalytically active ceria.

The authors acknowledge the support from the Korea Institute of Industrial Technology (KITECH) and U.S. National Science Foundation CAREER Award (DMR 1753383). The use of TEM at the Molecular Foundry was supported by the U.S. Department of Energy (Contract No. DE-AC02-05CH11231). The XPS work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (Contract DE-AC52-07NA27344).

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Kang H., Grewal S., Li H., Lee M. H. Effect of Surface-Specific Treatment by Infiltration into LaNi6Fe4O3-δ Cathodic Backbone for Solid Oxide Fuel Cells, 166, 255, 2019