(Invited) Surface Contaminant Effects on the Properties of Pr-Doped Ceria Determined Via Wafer Curvature and X-Ray Diffraction Measurements
Authors: Ma Yuxi , Nicholas Jason D. 
Published: February 2020

Praseodymium doped ceria (PCO) is a model mixed ionic electronic conductor (MIEC) solid oxide fuel cell (SOFC) cathode material. Under SOFC operating conditions, PCO undergoes the reaction (1):
½ O2 + 2Pr’
Ce + VӦ = 2PrCe + Oo
x

The kinetics of this reaction can be represented by the chemical oxygen surface exchange coefficient (kchem). It has been reported that surface contaminants can affect the kinetics of this reaction, either boosting or deteriorating the oxygen exchange performance of MIECs (2, 3). Hence, in this work, the effect of surface contaminants on the Pr0.1Ce0.9O2-δ (10PCO) thin film properties were investigated.
First, YSZ (yttria doped zirconia) or MgO (magnesium oxide) single crystal substrates were coated with (100)-oriented >21 nm average grain sized 10PCO thin films containing various surface contaminants via Pulsed Laser Deposition. The stress-temperature behavior of these samples was then measured using a dual substrate wafer curvature measurement technique utilizing a multi-beam optical stress sensor (MOSS) to determine the 280-700oC in-plane 10PCO thermal-chemical expansion coefficient, Young’s modulus, and oxygen nonstoichiometry, as described in Ref. (4). The curvature relaxation of these samples reacting to abrupt changes in the surrounding oxygen partial pressure from synthetic air (20% O2-80% Ar) and 10 times diluted synthetic air (2% O2-98% Ar), and vice-versa, were measured to determine the high temperature 10PCO kchem. Finally, the 500-700oC films strains in the 10PCO|YSZ samples were analyzed by measuring the 10PCO (200) peak position via high temperature X-Ray diffraction (HTXRD) and compared to the film stresses measured via MOSS to determine the Young’s modulus and thermal-chemical expansion coefficient (assuming no difference between the in-plane and out-of-plane values).
The close agreement of the data in Figures 1a and 1b show that the in-plane and out-of-plane bulk properties measured here were identical to each other, within the measurement errors, and similar to some of the previous values reported for bulk 10PCO. Further, no difference in bulk properties were observed for films with various surface contaminants. However, as shown in Figure 2, Si surface impurities significantly reduced kchem while noble metal surface impurities likely increased it. These results 1) will be important for evaluating the real-world tolerance of 10PCO in real-world SOFCs, 2) highlight the benefit of using electrode-free kchem measurement techniques, and 3) help explain the large kchem variation seen in the literature for identical materials at identical conditions.

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Figure 1