Stabilization of Ni-YSZ Nanocomposite Anodes by Deposition of a Thin YSZ Overlayer

Micro solid oxide fuel cells (μ-SOFCs) are energy conversion devices that are being considered for portable devices, such as laptop computers and mobile phones.  As the low operating temperatures of μ-SOFCs (600 °C) result in high electrode resistances, one solution to this is to increase the triple phase boundary (TPB) length.  In order to achieve this goal, ethylene glycol-based polymeric precursors of the Ni and YSZ components, which allow atomic level mixing of ions, were used to form nanocomposite Ni-YSZ thin films in this work. This, in turn, resulted in Ni-YSZ anode layers with an average grain size of ca. 25 nm, producing a very high TPB length (in comparison to conventional Ni-YSZ anodes) and hence a low electrode polarization resistance of 0.64 Ω.cm² at 550 °C in humidified H₂.

It has been reported that Ni-YSZ thin films, prepared by pulsed laser deposition (PLD), exhibit Ni grain growth at operating temperatures (400-600 °C), resulting in a deleterious effect on the long-term stability of the electrode performance [1]. Although the YSZ network within the composite films constrained Ni grain growth to some extent, significant grain growth at the outer surface of the film could not be inhibited [1]. To overcome this problem for the polymeric precursor-deposited Ni-YSZ films developed here, we deposited a thin, porous YSZ layer (ca. 100 nm in thickness) on top of the Ni-YSZ film. This led to a decrease in the polarization resistance degradation rate, from 3.7% per hour at 550 °C in Ni-YSZ thin films to only 0.39 % per hour at the same temperature.The results obtained in this study show that atomic level mixing, resulting from the use of polymeric precursors, yields very good electrode performances that can be stabilized by the application of additional layers. This will allow for significantly lower μ-SOFC cost as a result of the increase in power density and improved robustness. 

Acknowledgements:

The authors gratefully acknowledge the Eyes High PDF Program at the University of Calgary and Alberta Innovates – Technology Futures (AITF) for the support of AB, as well as the Natural Sciences and Engineering Research Council of Canada (NSERC) for the overall financial support of this work.

References:

1. U. P. Muecke, K. Akiba, A. Infortuna, T. Salkus, N. V. Stus and L. J. Gauckler, Solid State Ionics, 178 (2008) 1762.