Temperature-dependent structures of proton-conducting Ba(Zr(0.8-x)Ce(x)Y(0.2))O(2.9) ceramics by Raman scattering and x-ray diffraction

Unprecedented CO2-promoted hydrogen permeation in Ni-BaZr0.1Ce0.7Y0.1Yb0.1O(3-δ) membrane

Conventional Ni-BaCeO3-based membranes possess high hydrogen permeation flux but suffer serious flux degradation in CO2-containing atmosphere because of the formation of BaCO3 insulating layer. In this work, we report a novel Ni-BaZr0.1Ce0.7Y0.1Yb0.1O(3-δ) (Ni-BZCYYb) membrane, capable of both high hydrogen permeation flux and stable performance in CO2-containing atmosphere at 900 °C. Most importantly, the flux is found to be promoted rather than being diminished by CO2 normally observed for other high temperature proton conductors. The flux enhancement in Ni-BZCYYb membrane is attributed to the increase of moisture content in feed gas. When CO2 is introduced, the reverse water-gas shift reaction takes place generating H2O and CO. This work demonstrates that CO2 can be beneficial rather than detrimental for hydrogen permeation membranes that possess high chemical stability.

Testing the Chemical/Structural Stability of Proton Conducting Perovskite Ceramic Membranes by in Situ/ex Situ Autoclave Raman Microscopy

Ceramics, which exhibit high proton conductivity at moderate temperatures, are studied as electrolyte membranes or electrode components of fuel cells, electrolysers or CO2 converters. In severe operating conditions (high gas pressure/high temperature), the chemical activity towards potentially reactive atmospheres (water, CO2, etc.) is enhanced. This can lead to mechanical, chemical, and structural instability of the membranes and premature efficiency loss. Since the lifetime duration of a device determines its economical interest, stability/aging tests are essential. Consequently, we have developed autoclaves equipped with a sapphire window, allowing in situ Raman study in the 25-620 °C temperature region under 1-50 bar of water vapor/gas pressure, both with and without the application of an electric field. Taking examples of four widely investigated perovskites (BaZr0.9Yb0.1O3-δ, SrZr0.9Yb0.1O3-δ, BaZr0.25In0.75O3-δ, BaCe0.5Zr0.3Y0.16Zn0.04O3-δ), we demonstrate the high potential of our unique set-up to discriminate between good/stable and instable electrolytes as well as the ability to detect and monitor in situ: (i) the sample surface reaction with surrounding atmospheres and the formation of crystalline or amorphous secondary phases (carbonates, hydroxides, hydrates, etc.); and (ii) the structural modifications as a function of operating conditions. The results of these studies allow us to compare quantitatively the chemical stability versus water (corrosion rate from ~150 µm/day to less than 0.25 µm/day under 200-500 °C/15-80 bar PH2O) and to go further in comprehension of the aging mechanism of the membrane.