STABILITY OF BaCe0.65Zr0.20Y0.15O3-d AND Y- OR Gd-DOPED CERIA IN SIMULATED OPERATIONAL ENVIRONMENTS FOR H2 SEPARATION

Introduction The development of mixed ionic-electron conductors (MIEC) has made possible the potential use of these materials in different applications, including energy and environmentally related processes. In particular, the property of dense membranes based on ceramic proton conductors to selectively incorporate hydrogen into their structure as charge protonic defects make these systems of great interest for the production of high purity H2 from low-quality gas mixtures at T > 600 °C. Hydrogen molecules are split into protons and electrons that are transported from the high H2 pressure side to the low H2 pressure side of the membrane without external power supply (operating in non-galvanic conditions). Unfortunately, these systems show H2 fluxes considerably lower than those of Pd membranes (the milestone of 1 mL·min-1·cm-2 has not been reach to date). However, the working temperatures and durability exhibited by these oxides make them appealing for applications under harsh conditions, allowing the intensification of industrial processes that results on the improvement of the overall efficiency by coupling reaction and separation in a single unit (membrane reactors) [1]. Among this kind of membranes, dual-phase composites have shown enhanced H2 permeation respect to single-phase MIEC membranes. In fact, in previous studies, our group reported promising permeation flux values (ranging between 0.12 and 0.27 mL·min-1·cm-2 at ?750°C under wet 50% H2) for the ceramic-ceramic (cer-cer) membranes based on BaCe0.65Zr0.20Y0.15O3-? (BCZ20Y15)-Ce0.85M0.15O2-? (MDC15, M = Y and Gd) [2]. In this work, the stability behaviour of BCZ20Y15-MDC15 composites was thoroughly investigated. The samples were exposed to syngas, H2S and reducing atmospheres and studied by means of different analytical techniques such as SEM, XRD or impedance spectroscopy. In such conditions many detrimental effects could occur: BaCe1-xZrxO3-based systems can be attacked by CO2 and H2O, ceria-based compounds expand due to the Ce4+ reduction and H2S is a poison for many materials, even at low concentrations (a few ppm). Moreover, the phases that form the composites could react between them or suffer from other structural changes as cation migration. All these effects could result in a loss of the membranes performances. The behaviour of the cer-cer composites in those practical conditions was compared to that of the single-phase materials, in order to understand the possible degradation mechanisms and determine the potential applications of these systems in energy and environmental areas. Materials and methods BCZ20Y15-GDC15 and BCZ20Y15-YDC15 dense membranes in 50:50 and 60:40 volume ratios were prepared by ball-milling BCZ20Y15 (Marion Technologies, France) and GDC15 or YDC15 (prepared by solid state reaction) powders followed by uniaxial pressing and sintering. Dense sintered single-phase BCZ20Y15, GDC15 and YDC15 precursors were prepared in analogous conditions for comparison. The samples were exposed to different atmospheres (H2, syngas, H2S) and then characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Impedance (EIS) and X-ray photoelectron (XPS) spectroscopies investigations were also performed for the H2S treated samples. Results and discussion The crystal structure and phase stability of composite membranes and single-phase precursors were studied by XRD technique coupled with SEM investigations, comparing the results obtained after the different treatments with those recorded for the as prepared samples. BaCeO3-BaZrO3-based perovskites have critical stability issues in atmospheres containing CO2 and H2O (i.e. wet/ dry syngas) caused by the decomposition of the perovskite structure into barium carbonates and cerium oxide. Rietveld refinement on the XRD patterns and SEM analysis of the samples exposed to different syngas treatments do not show the formation of carbonates, demonstrating good chemical stability of the composites against wet/dry CO2 atmospheres. SEM investigations of the cross-sections of single-phase doped-ceria samples after the exposure to H2 and syngas environments show alterations and defects on the microstructure that can be attributed to the reduction of Ce4+ to Ce3+. These results are confirmed by Rietveld refinement. On the contrary, BCZ20Y15-GDC15 and BCZ20Y15-YDC15 composites do not present such defects on the cross-sections. These results suggest that the dense structure is preserved under the reducing and aggressive conditions investigated, avoiding any gas leakage of the membrane due to cracks or similar defects that are critical during H2 separation operation. Furthermore, Ce-rich materials can interact with H2S leading to the formation of sulphur-containing compounds (sulphides, sulphates, oxysulphides). The nature of the interaction is somehow more complicated than for the H2 and syngas atmospheres. For this reason, in addition to SEM and XRD investigations, EIS and XPS measurements were also performed. Total conductivity was measured under 2000 ppm of H2S from 400 to 700 °C by EIS, showing a drop at 600°C and 700°C. The decrease of the conductivity depends on the concentration of H2S, as suggested by the tests performed from 700 to 2000 ppm at 700°C. XRD patterns do not present peaks of by-products and SEM-EDS investigations reveals S only in some regions of the composite surfaces. On the other hand, XPS analysis has detected S. The complementary information gathered by the different techniques suggests that interaction with H2S seems limited to the surface, while the ceramic bulk appears unaltered. Conclusions Dual-phase membranes based on BaCe0.65Zr0.20Y0.15O3-? and Ce0.85M0.15O2-? (MDC15, M = Y and Gd) demonstrate very good stability in H2 and syngas atmospheres while in H2S the different tests performed suggest an alteration process involving the surface. EIS measurements show a decrease on the conductivity that depends on the concentration. The tolerance to H2S is relatively good taking into account that the concentration in operating conditions is at much lower values (for instance, mixtures produced from natural gas commonly contains level of tens ppm of H2S). Overall, the studied composite membranes appear suitable for membrane reactors and H2-separation applications on the basis of the stability results of the present study. Acknowledgments The authors are grateful to Dr. Rosalba Gerbasi, Dr. Naida El Habra and Dr. Simona Barison for their help in XRD and SEM analyses. This work has been funded by the Italian National Research Council - Italian Ministry of Economic Development Agreement "Ricerca di sistema elettrico nazionale". This work has been funded by the Italian National Research Council - Italian Ministry of Economic Development Agreement "Ricerca di sistema elettrico nazionale". References 1. C. Mortalò, S. Barison, E. Rebollo, M. Fabrizio, Mixed Ionic-Electronic Conducting Membranes for Hydrogen Separation in Membrane engineering for the treatment of gases, Volume 1, 2nd Edition, Ed. E. Drioli, G. Barbieri and A. Brunetti, RSC, in press. 2. E. Rebollo, C. Mortalò, S. Escolástico, S.Boldrini, S. Barison, J. M. Serra, and M. Fabrizio, Energy Environ. Sci., 8, pp.3675-3686, 2015.