Thermal and Chemical Expansion of Proton-Conducting Ceramics BaCexZr0.9-XY0.1O3-d (x=0 and 0.2)
Authors: Hudish Grant , Ricote Sandrine , Coors Grover 
Published: February 2020

For more than three decades, perovskite materials based on doped barium/strontium cerate/zirconate have been studied as potential hydrogen separation membranes and electrolyte for proton ceramic fuel cells or electrolysis cells.  It is well known that cerates are unstable in carbon dioxide and high steam containing atmospheres. Additionally, higher conductivities are obtained with barium instead of strontium on the A-site of the perovskite. Therefore, devices are designed using solid solution of barium cerate-zirconate with low amount of cerium (BaCexZr0.9-xY0.1O3-δ, BCZY).

The protonic defects (OHo
·) are incorporated in the material by the hydration equation, also known as the Wagner equation [1] (Eq. 1), in which water dissociates in oxygen vacancies.

H2O (g) + Vo
··  + Oo
x ↔ 2 OHo
·  (1)

The incorporation and removal of protonic defects create chemical expansion and contraction respectively, which have a tremendous impact on the mechanical strength of the device: the difference of coefficient of thermal expansion (CTE) between the electrolyte and the electrodes increases in the hydration/dehydration region and can cause failure of the device, especially as the trend is to decrease the electrolyte thickness. Therefore, it is imperative that the dehydration temperature and chemical expansion is considered for sealing, startup, and operation of devices based on proton-conducting ceramics.

There is limited high temperature X-ray diffraction (HT-XRD) work on BaCexZr0.9-xY0.1O3-δ materials. Hiraiwa et al.
 [2] studied the chemical expansion/contraction of BaZr0.8Y0.2O3-δ (BZY20) in dry oxidizing atmosphere and reported the presence of chemical expansion in the temperature range of 300-450 °C, due to the dehydration of the sample (water incorporated during fabrication) and also found that the chemical expansion depends on the high temperature thermal history of the part.  Andersson et al. [3] recorded HT-XRD on hydrated BZY in flowing synthetic air (1 ppm H2O) and determined a dehydration temperature of about 300 °C.

In this work, HT-XRD was used to determine the thermal and chemical expansion of BZY10 (BaZr0.9Y0.1O3-δ) and BCZY27 (BaCe0.2Zr0.7Y0.1O3-δ). Two sets of experiments will be presented and the pros and cons of each will be detailed: in-situ hydration and pre-hydration via autoclave. Figure 1 displays the lattice parameter of BCZY27 recorded while cooling in dry 4% H2 and 4% H2 containing 1 or 3% moisture. The knee in the curve observed in the temperature range 300-500°C in moist conditions is related to hydration of the material. The lower CTE in this temperature range comes from the competition between the contraction due to cooling and the expansion due to protonic defect incorporation. Additionally, an ideal start-up procedure for BCZY based materials will be proposed.

[1] V.S. Stotz, C. Wagner, Ber Bunsenges Phys. Chem. 70 (1967) 781

[2] C. Hiraiwa, D. Han, A. Kuramitsu, A. Kuwabara, H. Takeuchi, M. Majima, T. Uda, J. Am. Ceram. Soc. 96 (2013) 879

[3] A.K.E. Andersson, S.M. Selbach, C.S. Knee, T. Grande, J. Am. Ceram. Soc. 97 (2014) 2654

Figure 1