High temperature proton conductors such as barium zirconate/cerate (BaCe_xZr_0.9-xY_0.1O_{3- δ} referred to as BCZY) have become increasingly more popular, with a large number of potential applications including protonic ceramic fuel/electrolysis cells (PCFC/PCEC) and membrane reactors for steam methane reforming, methane dehydroaromatization or ammonia synthesis. It was also demonstrated that PCFCs can operate with high efficiency while remaining cost competitive in the temperature range 500-600 °C compared to traditional solid oxide fuel cells at 800 °C [1].

Contrary to traditional oxygen ion conductors (such as yttria stabilized zirconia) that display only thermal expansion, chemical expansion is also present in high temperature proton conductors upon hydration (equation 1) where protonic defects are incorporated into the oxygen sublattice [2-4].

(1) H₂O (g) + O_O^x + V_O°° ↔ 2 OH_O°

First, techniques for thermal and chemical expansion measurements will be presented together with the analysis/interpretation challenges. Then, an overview of the literature data on the expansion in high temperature proton conductors will be provided. While all these measurements are performed in a single atmosphere, it is important to be able to predict the expansion with different gas compositions on both sides of the membrane to mimic a real device application. To do so, we developed a computational model based on a Nernst-Planck-Poisson formulation and included a chemo-thermo-mechanical component. Defect concentrations and mobilities were taken from previous conductivity measurement modelling [5], whereas mechanical properties (Young modulus, Poisson ratios) were determined experimentally.

References:

[1] Dubois A., Ricote S., Braun R.J., Benchmarking the expected stack manufacturing cost of next generation, intermediate-temperature protonic ceramic fuel cells with solid oxide fuel cell technology. J. Power Sources 369 (2017) 65-77.

[2] Andersson A.K.E., Selbach S.M., Knee C.S., Grande T., Chemical expansion due to hydration of proton-conducting perovskite oxide ceramics. J. Am. Cream. Soc. 97 (2014) 2654-2661.

[3] Hiraiwa C., Han D., Kuramitsu A., Kuwabara A., Takeuchi H., Majima M., Uda T., Chemical expansion and change in lattice constant of Y-Doped BaZrO₃ by hydration/dehydration reaction and final heat-treating temperature. J. Am. Cream. Soc. 96 (2013) 879-884.

[4] Hudish G., Manerbino A., Coors W.G., Ricote S., Chemical expansion in BaZr_{0.9− x}Ce_xY_0.1O_3−δ ( x = 0 and 0.2) upon hydration determined by high-temperature X-ray diffraction. J. Am. Ceram. Soc. 101 (2018) 1298-1309.

[5] Zhu H., Ricote S., Coors W.G., Kee R.J., Interpreting equilibrium-conductivity and conductivity-relaxation measurements to establish thermodynamic and transport properties for multiple charged defect conducting ceramics. Farad. Discuss. 182 (2015) 49-74.