Ceramic-ceramic (cer-cer) composites constituted by perovskite proton conductor BaCe0.65Zr0.20Y0.15O3-? (BCZ20Y15) and doped ceria (Ce0.85M0.15O2-? M = Y or Gd, YDC15 or GDC15) as electronic conductor had demonstrated notable performance as dense H2-separation membranes, with H2 flux values among the highest reported in literature for this kind of systems [1]. Long-term stability and degradation mechanisms are critical issues for these dual-phase membranes. The working conditions are really challenging: H2 purification is carried out at high temperatures in harsh reducing environments containing H2O, CO, CO2, and sulphides. In such conditions, many undesired phenomena such as structural changes, cation diffusion, mechanical alterations or chemical reactions could occur damaging the membranes properties. The purpose of the present study is to evaluate the chemical and mechanical behaviour of BCZ20Y15-GDC15 and BCZ20Y15-YDC15 composite membranes in conditions of practical interest. Materials and methods Experiments were performed on high density samples of BCZ20Y15-GDC15 and BCZ20Y15-YDC15 (50:50 or 60:40 volume ratio) prepared as described elsewhere [1]. BCZ20Y15, GDC15 and YDC15 single-phase specimens were prepared following a corresponding procedure for comparison purposes. All samples were exposed to syngas, H2 and H2S at different temperatures. The crystal structure before and after the treatments was studied by means of X-ray diffraction (XRD) while the evolution on the microstructure of the cross-sections and the surfaces of the samples was evaluated by scanning electron microscopy- Energy-dispersive X-ray spectroscopy (SEM-EDS). The characterization of the mechanical properties was performed through nano-indentation and the surface roughness was evaluated by a contact stylus profiler. Results and discussion The nano-indentation measurements allow to determine the variations occurred on the Young's modulus (E) and hardness (H) of the composite membranes and of the single-phase precursors induced by the different treatments. With the aim to detect the evolution and/or the stability of the mechanical properties, caused by harsh environments at high temperatures for a long period, nano-mechanical analyses were carried out also on samples in the "as prepared" state, having the same compositions of those thermally treated. A statistical approach was used to process the collected nanoindentation data, to achieve high reliability of the measurements. Moreover, the presence of alteration products such as carbonates in the samples treated in CO2 or of any defect induced by the reduction of ceria-based materials was evaluated by XRD and SEM-EDS investigations. The collected data indicate that the composite membranes show better overall performance in terms of both chemical and mechanical properties, compared to the single-phase precursors. Conclusions The possible degradation mechanisms together with thermally and mechanically induced stresses could compromise the long-term stability during operation of BCZ20Y15-GDC15 and BCZ20Y15-YDC15 dense membranes for H2 separation. For this purpose, these cer-cer composites were exposed to conditions of practical interest and characterized in terms of mechanical properties (Young's modulus (E) and hardness (H)), chemical composition and crystal structure (SEM-EDS and XRD) in order to obtain a better understanding of their behaviour under harsh conditions and validate their potential practical applications. 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". References 1. E. Rebollo, C. Mortalò, S. Escolástico, S.Boldrini, S. Barison, J. M. Serra, and M. Fabrizio, Energy Environ. Sci., 8, pp.3675-3686, 2015.
AN INSIGTH INTO THE CHEMICAL AND MECHANICAL STABILITY OF BaCe0.65Zr0.20Y0.15O3-d-Ce0.85M0.15O2-d (M: Y, Gd) COMPOSITES FOR HYDROGEN SEPARATION
Zin V;Deambrosis S M;Fabrizio M
2017
Abstract
Ceramic-ceramic (cer-cer) composites constituted by perovskite proton conductor BaCe0.65Zr0.20Y0.15O3-? (BCZ20Y15) and doped ceria (Ce0.85M0.15O2-? M = Y or Gd, YDC15 or GDC15) as electronic conductor had demonstrated notable performance as dense H2-separation membranes, with H2 flux values among the highest reported in literature for this kind of systems [1]. Long-term stability and degradation mechanisms are critical issues for these dual-phase membranes. The working conditions are really challenging: H2 purification is carried out at high temperatures in harsh reducing environments containing H2O, CO, CO2, and sulphides. In such conditions, many undesired phenomena such as structural changes, cation diffusion, mechanical alterations or chemical reactions could occur damaging the membranes properties. The purpose of the present study is to evaluate the chemical and mechanical behaviour of BCZ20Y15-GDC15 and BCZ20Y15-YDC15 composite membranes in conditions of practical interest. Materials and methods Experiments were performed on high density samples of BCZ20Y15-GDC15 and BCZ20Y15-YDC15 (50:50 or 60:40 volume ratio) prepared as described elsewhere [1]. BCZ20Y15, GDC15 and YDC15 single-phase specimens were prepared following a corresponding procedure for comparison purposes. All samples were exposed to syngas, H2 and H2S at different temperatures. The crystal structure before and after the treatments was studied by means of X-ray diffraction (XRD) while the evolution on the microstructure of the cross-sections and the surfaces of the samples was evaluated by scanning electron microscopy- Energy-dispersive X-ray spectroscopy (SEM-EDS). The characterization of the mechanical properties was performed through nano-indentation and the surface roughness was evaluated by a contact stylus profiler. Results and discussion The nano-indentation measurements allow to determine the variations occurred on the Young's modulus (E) and hardness (H) of the composite membranes and of the single-phase precursors induced by the different treatments. With the aim to detect the evolution and/or the stability of the mechanical properties, caused by harsh environments at high temperatures for a long period, nano-mechanical analyses were carried out also on samples in the "as prepared" state, having the same compositions of those thermally treated. A statistical approach was used to process the collected nanoindentation data, to achieve high reliability of the measurements. Moreover, the presence of alteration products such as carbonates in the samples treated in CO2 or of any defect induced by the reduction of ceria-based materials was evaluated by XRD and SEM-EDS investigations. The collected data indicate that the composite membranes show better overall performance in terms of both chemical and mechanical properties, compared to the single-phase precursors. Conclusions The possible degradation mechanisms together with thermally and mechanically induced stresses could compromise the long-term stability during operation of BCZ20Y15-GDC15 and BCZ20Y15-YDC15 dense membranes for H2 separation. For this purpose, these cer-cer composites were exposed to conditions of practical interest and characterized in terms of mechanical properties (Young's modulus (E) and hardness (H)), chemical composition and crystal structure (SEM-EDS and XRD) in order to obtain a better understanding of their behaviour under harsh conditions and validate their potential practical applications. 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". References 1. E. Rebollo, C. Mortalò, S. Escolástico, S.Boldrini, S. Barison, J. M. Serra, and M. Fabrizio, Energy Environ. Sci., 8, pp.3675-3686, 2015.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
