Marie-Laure Fontaine

Oxygen and Hydrogen Separation Membranes Based on Dense Ceramic Conductors

Publisher Summary This chapter discusses oxygen and hydrogen separation membranes based on dense ceramic conductors. The theoretical basis for understanding the transport properties in dense oxides is introduced. This treatment starts from basic defect chemistry and the equations for flux of oxygen ions and protons. The most important families of oxides and the specific compositions that demonstrate high ionic diffusion rates are introduced. The important issue of materials stability under operation is then discussed, taking into consideration, the typical membrane working condition characterized by high temperature, the presence of significant chemical, mechanical, and thermal gradients, as well as aggressive chemical components. The chapter illustrates several examples of membrane applications. The possibility of mass and heat integration in novel process designs and the potentials of dense ceramic membranes in power generation with CO2 capture are demonstrated. The chapter also discusses the latest developments in solid oxide fuel cell (SOFC) and prospects toward high-temperature water electrolysis.

Investigation of La1−xSrxCrO3−∂ (x ~ 0.1) as Membrane for Hydrogen Production

Various inorganic membranes have demonstrated good capability to separate hydrogen from other gases at elevated temperatures. Hydrogen-permeable, dense, mixed proton-electron conducting ceramic oxides offer superior selectivity and thermal stability, but chemically robust candidates with higher ambipolar protonic and electronic conductivity are needed. In this work, we present for the first time the results of various investigations of La1−xSrxCrO3−∂ membranes for hydrogen production. We aim in particular to elucidate the material’s complex transport properties, involving co-ionic transport of oxide ions and protons, in addition to electron holes. This opens some new possibilities for efficient heat and mass transfer management in the production of hydrogen. Conductivity measurements as a function of pH2 at constant pO2 exhibit changes that reveal a significant hydration and presence of protons. The flux and production of hydrogen have been measured under different chemical gradients. In particular, the effect of water vapor in the feed and permeate gas stream sides was investigated with the aim of quantifying the ratio of hydrogen production by hydrogen flux from feed to permeate and oxygen flux the opposite way (“water splitting”). Deuterium labeling was used to unambiguously prove flux of hydrogen species.

Solubility of transition metal interstitials in proton conducting BaZrO3 and similar perovskite oxides

The defect chemistry of foreign transition metals in perovskite oxides was investigated by first-principles calculations with focus on Ni and Zn in Y-doped BaZrO3. Additional transition metals (Cu, Fe, Pd, Pt, and Ag) and perovskites (SrZrO3 and SrTiO3) were considered for comparison. The octahedral interstice coordinated with square-planar oxygen could accommodate smaller cations and Ni2+ was found to be the most stable, particularly in the presence of barium vacancies. Significant solubility of Ni was substantiated only for nominally A-site deficient materials under oxidizing conditions. The computational results were corroborated by experimental studies on BaZr0.85Y0.15O3−δ with 4 mol% NiO or ZnO sintering additives. While synchrotron radiation X-ray powder diffraction of the Ni containing sample showed the presence of a BaY2NiO5 secondary phase, it could not account for the nominal amount of Ni in the sample. STEM and EDS analyses of both the Zn and Ni containing samples showed that Zn accumulated in the grain boundaries while Ni was evenly distributed within the grains and grain boundaries indicating that Ni was dissolved in the BaZrO3 structure. Furthermore, metallic Ni particles appeared on the sample surface after treatment under reducing conditions in accordance with computational predictions. The influence of interstitially dissolved Ni on proton conductivity was evaluated based on trapping of protons. Barium vacancies were found to be strong proton traps, with a binding energy of −0.80 eV, while the binding energy of protons associated with adjacent Ni interstitials was reduced to −0.20 eV.

Critical Issues of Metal-Supported Fuel Cell

Metal-Supported SOFCs (MS-SOFCs), wherein the supporting component of the cell is made of a porous alloy, are referred to as the third generation SOFC operating at temperature down to 500–650 °C. This technology is expected to decrease significantly capital and operational costs, while increasing the lifetime of cells due to lower operating temperature and higher redox stability. The chapter reviews MS-SOFC development with a focus given to main issues affecting the performance and longevity of single cells. It addresses critical issues for selection of alloy materials based on material cost, thermal expansion coefficient, corrosion rate, particle size, and Cr evaporation issues. Protective coatings, cell architecture, and advanced fabrication processes are then presented to illustrate the level of technical refinement currently achieved. Performance of produced MS-SOFCs is finally discussed to pinpoint factors contributing to major electrochemical losses and possible routes for improvement are reported.

Dual phase high-temperature membranes for CO2 separation – performance assessment in post- and pre-combustion processes

Dual phase membranes are highly CO2-selective membranes with an operating temperature above 400 °C. The focus of this work is to quantify the potential of dual phase membranes in pre- and post-combustion CO2 capture processes. The process evaluations show that the dual phase membranes integrated with an NGCC power plant for CO2 capture are not competitive with the MEA process for post-combustion capture. However, dual phase membrane concepts outperform the reference Selexol technology for pre-combustion CO2 capture in an IGCC process. The two processes evaluated in this work, post-combustion NGCC and pre-combustion IGCC, represent extremes in CO2 partial pressure fed to the separation unit. Based on the evaluations it is expected that dual phase membranes could be competitive for post-combustion capture from a pulverized coal fired power plant (PCC) and pre-combustion capture from an Integrated Reforming Cycle (IRCC).