High performance H₂-permeation ceramic membranes may allow the intensification of industrial processes, reducing production and environmental costs, hence the milestone value of H₂-permeation is 1 ml·cm^-2·min^-1. Dual-phase cer-cer materials are proposed for gas separation membranes. They may combine high ambipolar conductivity (mixed protonic and electronic), and redox catalytic properties with chemical compatibility with water and acid gases at elevated temperatures and pressures in the sweeping flow. Nevertheless, the stability and non-reactivity between the phases forming the composite should be maintained at high temperatures, since the direct contact between them at high temperature under harsh environments may cause the reactivity of the materials either with the atmosphere or between them, with the subsequent lost of stability and performance (e.g. hydrogen permeation).

As a single phase, doped BaCeO₃ exhibits one of the highest proton stability and conductivity up to 600-700 °C, although it is chemically instable due to carbonation formation by exposure to moist reducing atmospheres. [1] Contrarily, some ion conducting doped CeO₂ has demonstrated to be stable in CO₂, SO₂, H₂O containing atmospheres at high temperature. [2, 3] Several authors have shown the chemical compatibility and stability in fuel cell operation and syngas conditions of systems based on doped BaCe_1-xM_xO_3-δ and Ce_1-xM_xO_2-δ containing similar dopants in both phases. [4-6]

The present work shows the evaluation of stability of composite membranes made of nominal BaCe_0.8M_0.2O_3-δ (BCMO, M= Y, Eu, Sm, Tb, Gd, Nd ) and Ce_0.8X_0.2O_2-δ (CXO, X= Y, Eu, Sm, Tb, Gd, Nd) in operating gases for WGSR. The two crystalline phases were synthesized separately via the conventional solid-state route at 1400 °C and combined in a cer-cer composite with 50:50 volume ratio. Dense pellets were prepared by calcining uniaxially pressed pellets at 1600 °C. Although both perovskite and fluorite phases are well distinguished, Rietveld refinement on XRD patterns in addition to SEM-EDS analysis of fresh-reference pellets revealed the migration of the M to the ceria phase, in disagreement with previously cited literature. However, the electrochemical properties of these composites measured by AC-conductivity still show a substantial improvement regarding the pristine single phase materials.

The highest conductivity among the tested composites was found for BaCe_0.8Eu_0.2O_3-δ (BCEO):Ce_0.8Y_0.2O_2-δ (CYO). Thus, a more comprehensive study was made. DC conductivity was measured in dry and wet H₂, and D₂ atmospheres (where H₂ and D₂ were 5% in He, and pH₂O and pD₂O correspond to 0.025 atm). Results suggested that electrons and/or oxygen ions are the predominant charge carriers in the studied temperature range. Hydrogen permeation higher than 0.6 ml·cm-2·min-1 was achieved at 700 °C, when 50% H₂ in He was used as feed gas and humidified Ar for sweeping (pH₂O=0.042 atm). This value approaches the target of 1 ml·cm-2·min-1 for proton conducting ceramics. By adding 15 vol% CO₂ in the feeding stream, chemical stability was demonstrated at 700 °C.

The stability tests have been performed at 600-900 °C in syngas (WGSR feed) that contained 15% H₂, 34% CO and 51% H₂O (volume %); and in retentate simulating conditions that contained 0.1% H₂, 90% CO₂ and 9.9% H₂O. XRD and SEM analysis after the exposure shows the decomposition of the BCLnO phase at low temperatures and the formation of carbonates. However, the carbonate peaks disappear at 900 °C and 800 °C for some of the compounds. As evidenced in former studies [4] materials with this dual phase sustain their microstructures and phase characteristics under selected conditions (15 vol% CO₂ at 700 °C). This entails these materials as suitable for other catalytic membrane reaction applications.

References

[1] D. Medvedev, A. Murashkina, E. Pikalova, A. Demin, A. Podias, P. Tsiakaras, BaCeO3: Materials development, properties and application, Progress in Materials Science, 60 (2014) 72-129.

[2] M. Balaguer, C. Solís, J.M. Serra, Structural–Transport Properties Relationships on Ce1–xLnxO2−δ System (Ln = Gd, La, Tb, Pr, Eu, Er, Yb, Nd) and Effect of Cobalt Addition, The Journal of Physical Chemistry C, 116 (2012) 7975-7982.

[3] M. Balaguer, J. García-Fayos, C. Solís, J.M. Serra, Fast Oxygen Separation Through SO2- and CO2-Stable Dual-Phase Membrane Based on NiFe2O4–Ce0.8Tb0.2O2-δ, Chemistry of Materials, 25 (2013) 4986-4993.

[4] S. Elangovan, B.G. Nair, T.A. Small, Ceramic mixed protonic/electronic conducting membranes for hydrogen separation, in, Google Patents, 2007.

[5] S. Ricote, A. Manerbino, N.P. Sullivan, W.G. Coors, Preparation of dense mixed electron- and proton-conducting ceramic composite materials using solid-state reactive sintering: BaCe0.8Y0.1M0.1O3−δ–Ce0.8Y0.1M0.1O2−δ (M=Y, Yb, Er, Eu), J Mater Sci, 49 (2014) 4332-4340.

[6] D. Medvedev, E. Pikalova, A. Demin, A. Podias, I. Korzun, B. Antonov, P. Tsiakaras, Structural, thermomechanical and electrical properties of new (1 − x)Ce0.8Nd0.2O2−δ–xBaCe0.8Nd0.2O3−δ composites, Journal of Power Sources, 267 (2014) 269-279.