Heteroatom Doping By Atomic Layer Deposition for Cathodic Interface Treatment of Solid Oxide Fuel Cells
Authors: Li Haoyu , Lee Min Hwan 
Published: May 2020

Solid oxide fuel cells (SOFCs) are widely studied due to its high efficiency and fuel flexibility.1,2 In SOFCs, cathodic reaction is known to be the main source of voltage losses due to the sluggishness of oxygen reduction reaction (ORR), which led intense research efforts to develop high-performance cathodes. The main approaches include surface treatments for facilitated surface exchange and surface area enhancement by various wet processes and/or vacuum-based deposition scheme.

In this study, we demonstrate the significant impact of atomic layer deposition (ALD) treatment on the cathodic performance and operational durability. ALD has significant advantages in providing atomic scale surface overcoat with an accurate control over thickness and stoichiometry with excellent reproducibility. We use LaNi0.6Fe0.4O3-
δ (LNF) as the cathodic backbone and CeO2 (ceria) as a priori infiltrated species. Two different oxides (CeO2 and Y2O3) were used as the atomic-scale coating materials.

First, the cell with bare porous LNF cathode showed a high polarization resistance (Rp
) of 1.24 Ω cm2 at 700 °C, but with a single-step ceria infiltration (with 1 M cerium nitrate solution), a significantly lower Rp
of 0.27 Ω cm2 was obtained. An ALD-coated ceria right on top of the LNF backbone also resulted in a dramatic decrease in Rp
(0.54 Ω cm2) even with a nominal overcoat thickness of 6 Å (6 cycles of cerium and deionized water). A 6-cycle ALD coating of ceria on a ceria-infiltrated LNF showed a further improvement of cathodic performance (Rp
= 0.19 Ω cm2). A similar effect was demonstrated with an ALD coating of different material (Y2O3; 3 cycles; nominal thickness = ~3 Å , Rp
= 0.20 Ω cm2).

More interestingly, a significantly enhanced operational durability was demonstrated by ALD treatments. A sample with a single-step infiltration of ceria showed an increase of Rp
by 55% in 150 h at 700 ℃. However, a ceria ALD treatment on a ceria-infiltrated LNF resulted in a milder increase of Rp
by 33% in 150 h at 700 ℃. An yttria (Y2O3) ALD overcoat suppressed the degradation even further; only 22% increase in Rp
demonstrated in 150 h at 700 ℃.

These observations suggest that an atomic scale surface coating by ALD can be an effective approach to enhance not only the kinetics of cathodic reactions but also the operational durability at high temperatures. This is conjectured to be originated from the capability of ALD overcoat in suppressing the sintering of either backbone or infiltrated oxide nanoparticles, and partially from a change in surface chemistry facilitating surface exchange. Electron microscopy and X-ray photoelectron spectroscopy results will be discussed in this respect.

This work acknowledges the support from the U.S. National Science Foundation CAREER Award (DMR1753383) and NASA MIRO Program (NNX15AQ01A). SG and MHL also acknowledge NASA ASTAR fellowship (NNX15AW57H). The use of TEM at the Molecular Foundry was supported by the U.S. Department of Energy (Con- tract No. DE-AC02-05CH11231)

1 Z. Shao and S.M. Haile, Nature 431, 170 (2004).

2 H. Kang, S. Grewal, H. Li, and M.H. Lee, 166, 255 (2019).