The two major proton-conducting solid oxide fuel cell (PCFC) performance limitations are the long-term stability as well as the concentration and mobility of proton defects in the electrolyte. The main electrolyte candidates are perovskite type, in particular, yttrium doped barium Ba(Zr/Ce/Y)O_3-s materials. However the poor grain boundary conductivity of BaZr_0.9Y_0.1O_2.95 (BZY) and the lack of chemical stability of BaCe_0.9Y_0.1O_2.95 (BCY) impede their application. The first issue can be addressed by the development of grain boundary-free thin films which exhibit conductivity level up to three orders of magnitude higher than standard sintered pellets.^[1] A solid solution of BZY and BCY, BaCe_0.2Zr_0.7Y_0.1O_2.95 (BCZY)_, demonstrates further optimized conductivity levels and chemical stability.^{[2, 3]} However, due to the low concentration of protons and surface gas sensitivity of these materials, detailed knowledge on the chemical interaction between elements from oxide host lattice and fuel cell environment is still lacking, hindering PCFC development.

In this work, we developed BaCe_xZr_0.9-xY_0.1O_2.95 epitaxial thin films and characterized their performance with electrochemical impedance spectroscopy. We investigated the doping effect on surface chemistry by using Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) in fuel cell environments (100 mtorr of H₂O, H₂, O₂ or CO₂) at operating temperature (>400°C). Upon applying water partial pressure, changes in the O 1s and Y 3d core level spectra were observed with a clear dopant dependency. These results indicate the formation of degradation products at the surface. The correlation between surface activity, composition, chemical stability, and conductivity level as well as associated new insights into the dopant effect on hydration and degradation mechanisms will be discussed.

References

[1] D. Pergolesi, E. Fabbri, A. D’Epifanio, E. Di Bartolomeo, A. Tebano, S. Sanna, S. Licoccia, G. Balestrino, E. Traversa, Nat Mater 9 (2010) (10) 846.

[2] S. Ricote, N. Bonanos, A. Manerbino, N.P. Sullivan, W.G. Coors, Journal of Materials Chemistry A 2 (2014) (38) 16107.t

[3] S. Ricote, N. Bonanos, H.J. Wang, R. Haugsrud, Solid State Ionics 185 (2011) (1) 11.

Acknowledgement

This work was supported by the Office of Naval Research (ONR) and The Advanced Light Source (ALS) funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under award DESC0001160 and Contract No. DE-AC02-05CH11231 repectively.