Rune Bredesen

High temperature corrosion in SOFC environments

Abstract The corrosive environment in solid oxide fuels cells (SOFCs) using natural gas (mainly methane) as fuel is considered. In the anode chamber the gas is highly carburizing at the gas inlet and after gradual oxidation it becomes highly humid, but weakly oxidizing (high CO 2 :CO ratio) at the gas outlet; in the cathode chamber the gas is highly oxidizing (air). In addition to noble metals, nickel and chromia-forming alloys are considered as potential metallic interconnects and their corrosion behavior in the SOFC environment is discussed.

Comparison of a contactor catalytic membrane reactor with a conventional reactor: example of wet air oxidation

Abstract A wet air oxidation reaction was carried out in a gas/liquid catalytic membrane reactor of the contactor type. The oxidation of formic acid was used as a model reaction. The mesoporous top-layer of a ceramic tubular membrane was used as catalyst (Pt) support, and was placed at the interface of the gas (air) and liquid (HCOOH solution) phases. A similar reaction was carried out in a conventional batch reactor, using a steering rate high enough to avoid gas-diffusion limitations, and exactly identical conditions than for the CMR (amount of catalyst, pressure, etc.). At room temperature, the CMR showed an initial activity three to six times higher than the conventional reactor. This activity increase was attributed to an easier oxygen access to the catalytic sites. Nevertheless, the catalytic membrane gradually deactivated after a few hours of operation. Different deactivation mechanisms are presented.

Crystal structure of the mixed conductor Sr4Fe4Co2O13

The crystal structure of Sr4Fe4Co2O13 has been determined on the basis of high-resolution powder neutron diffraction data. The space group is Iba2, a=1103.19(16), b=1898.63(26), c=554.92(8) pm; R(F2)=0.117, Rwp=0.088, Rp=0.067. In the investigated multi-phase sample, the Sr4Fe4Co2O13 phase exists as a major phase together with oxygen deficient SrFeO3–x, in about equal mass fractions. Sr4Fe4Co2O13 is isostructural with Sr4Fe6O13 and adopts a variant of the perovskite type-structure where layers of corner-sharing FeO6 octahedra are separated by double layers of (Fe,Co)O4 and (Fe,Co)O5 coordination polyhedra. Condensed chains of (Fe,Co)O4 and (Fe,Co)O5 polyhedra run along [001]. Of the three non-equivalent Fe sites, the octahedral site is entirely occupied by Fe, the square pyramidal site by 61% Co and the trigonal pyramidal site by 52% Co. The refined chemical composition of the unit cell is Sr16Fe15.0Co9.0O51.84 . Possible structural reasons for the reported very high ion conductivity of Sr4Fe4Co2O13 are discussed.

Phase relations, chemical diffusion and electrical conductivity in pure and doped Sr4Fe6O13 mixed conductor materials

Abstract Phase relations and transport properties of undoped and La and/or Ti-doped Sr 4 Fe 6 O 13 have been studied. In the doped materials significant amounts of perovskite SrFeO 3− δ were formed together with SrFe 12 O 19 . An intergrowth on nanoscale of SrFeO 3− δ and Sr 4 Fe 6 O 13 was observed in the TEM for the La-doped material. The content of La was higher in the SrFeO 3− δ than in the Sr 4 Fe 6 O 13 phase. The electrical conductivity and chemical diffusion coefficient of undoped and La/Ti-doped Sr 4 Fe 6 O 13 have been measured and a defect model for undoped Sr 4 Fe 6 O 13 is presented. The transport properties of doped Sr 4 Fe 6 O 13 reflect multiphase materials mainly dominated by the perovskite phase.

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.

A wet air oxidation process using a catalytic membrane contactor

A new process for oxidation of toxic compounds in liquids has been demonstrated. The concept is based on the same principles as catalytic wet air oxidation (CWAO), but the metal catalyst is fixed to a ceramic porous membrane in a catalytic membrane reactor of the contactor type (CMR-C). Air is flowing along the surface of the contactor, and the waste liquid is supplied from the other side of the contactor through the porous contactor wall. In this way, the gas and liquid phases are driven to contact in the porous network of the catalytic contactor separating them. Fifty percent of conversion of formic acid model solution (5 g/l) was obtained in initial reactor experiments at 150 8C and 10 bar pressure, but the observed oxidation rate was low: about 0.13 mmol/s per gPt. TEM and EDS investigations of the contactor showed that 5 � /10 nm Pt particles were evenly distributed close to the surface of the mesoporous TiO2 top layer. After the experiments, a 10 � /50 nm thick aluminium-rich amorphous deposit was observed in the porous structure. The low conversion rate has been attributed to this deposit causing deactivation by encapsulation of the catalyst and plugging of the mesoporous layer of the contactor. The deposits are believed to be caused by chemical instability of a-Al2O3 in acidic aqueous environment at elevated temperature. a-Al2O3 is present in the coarse-grained membrane support. # 2003 Elsevier Science B.V. All rights reserved.

On phase relations, transport properties and defect structure in mixed conducting SrFe1.5−xCoxOz

Abstract The stability of the Sr4Fe6O13 phase, and the phase changes taking place in the presence of Co, are discussed. Reported transport parameters are analysed and discussed in relation to defects and phase changes in SrFe1.5−xCoxOz. A defect model is suggested that is in qualitative agreement with the observed transport parameters. It is suggested that defect ordering due to Co2+ ions is decisive for the transport properties.

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.

SOFC and gas turbine power systems—Evaluation of configurations for CO2 capture

Publisher Summary This chapter highlights that pressurized solid oxide fuel cells (SOFC) integrated in a gas turbine cycle is a promising power generation concept. The benefit of such combined systems is the potential for high electrical efficiency at small scale. By including an afterburner for the fuel cell, the remaining fuel in the anode exit gas is fully converted to water and CO2 while the anode and cathode streams from the fuel cell are kept separated. This enables the CO2 capture from an exhaust stream consisting of only CO2 and water. This chapter evaluates, three afterburner technologies based on different membrane conductors from the perspective of thermodynamic cycle analysis and materials technology. The total SOFC and gas turbine system with the different afterburners has been modeled in a general purpose flow sheet simulator, and mass and energy balances have been calculated. The electrical efficiency has been determined and compared for each of the three afterburners. The potential of the three technologies for future use as afterburners is evaluated.

Surface defect chemistry of Y-substituted and hydrated BaZrO3 with subsurface space-charge regions

First-principles calculations were utilized to elucidate the complete defect equilibria of surfaces of proton conducting BaZrO3, encompassing charged species adsorbed to the surface, defects in the surface layer as well as in the subsurface space-charge region and bulk. Defect calculations were performed for the BaZrO3 (0 0 1) surface with focus on protons, oxygen vacancies and Y-acceptor dopants as well as adsorbed hydroxide and oxide adions. Protons were found to exhibit a particularly strong tendency to segregate to the surface with a segregation energy of −1.3 eV. While the concentration of negatively charged Y-acceptors and hydroxide species on the outer surface can be quite high, they do not fully charge compensate the protons, yielding a net positive charge of the surface. The resulting surface potential can exceed 1 V, resulting in significant depletion of charge carriers in the subsurface space-charge region. Moreover, the results are discussed in relation to surface adsorption of water, and computational approaches for treating charged point defects in periodic slab cells are evaluated with respect to symmetry and charge compensation.