Design of Mixed Ionic-Electronic Materials for Permselective Membranes and Solid Oxide Fuel Cells Based on Their Oxygen and Hydrogen Mobility.

Oxygen and hydrogen mobility are among the important characteristics for the operation of solid oxide fuel cells, permselective membranes and many other electrochemical devices. This, along with other characteristics, enables a high-power density in solid oxide fuel cells due to reducing the electrolyte resistance and enabling the electrode processes to not be limited by the electrode-electrolyte-gas phase triple-phase boundary, as well as providing high oxygen or hydrogen permeation fluxes for membranes due to a high ambipolar conductivity. This work focuses on the oxygen and hydrogen diffusion of mixed ionic (oxide ionic or/and protonic)-electronic conducting materials for these devices, and its role in their performance. The main laws of bulk diffusion and surface exchange are highlighted. Isotope exchange techniques allow us to study these processes in detail. Ionic transport properties of conventional and state-of-the-art materials including perovskites, Ruddlesden-Popper phases, fluorites, pyrochlores, composites, etc., are reviewed.

Approaches to the design of efficient and stable catalysts for biofuel reforming into syngas: doping the mesoporous MgAl2O4 support with transition metal cations.

The mesoporous MgAl2O4 support is promising for the design of efficient and stable to coking catalysts for natural gas and biofuel reforming into syngas. This work aims at doping this support with transition metal cations (Fe, Cr, Ti) to prevent the incorporation of Ni and rare-earth cations (Pr, Ce, Zr), loaded by impregnation, into its lattice along with providing additional sites for CO2 activation required to prevent coking. Doped MgAl1.9Me0.1O4 (Me = Fe, Ti, Cr) mesoporous supports prepared by the one-pot evaporation-induced self-assembly method with Pluronic P123 triblock copolymers were single-phase spinels. Their specific surface area varies in the range of 115-200 m2 g-1, decreasing to 90-110 m2 g-1 after successive addition of the supporting nanocomposite active component 10 wt% Pr0.3Ce0.35Zr0.35O2 + (5 wt% Ni + 1% Ru) by impregnation. Mössbauer spectroscopy for iron-doped spinels confirmed the spatially uniform distribution of Fe3+ cations in the lattice without clustering being mainly located at the octahedral positions. Fourier-transform infrared spectroscopy of the adsorbed CO molecules was performed to estimate the surface density of metal sites. In methane dry reforming, the positive effect of MgAl2O4 support doping was observed from both a higher turn-over frequency as compared with the catalyst on the undoped support as well as the highest efficient first-order rate constant for the Cr-doped catalyst as compared with published data for a variety of Ni-containing catalysts based on the alumina support. In the reaction of ethanol steam reforming, the efficiency of catalysts on the doped supports is comparable, while exceeding that of Ni-containing supported catalysts reported in the literature. Coking stability was provided by a high oxygen mobility in the surface layers estimated by the oxygen isotope heteroexchange with C18O2. A high efficiency and coking stability were demonstrated in the reactions of methane dry reforming and ethanol dry and steam reforming in concentrated feeds for the honeycomb catalyst with a nanocomposite active component on the Fe-doped MgAl2O4 support loaded on the FeCrAl-alloy foil substrate.

Dry Reforming of Methane over 5%Ni/Ce1-xTixO2 Catalysts Obtained via Synthesis in Supercritical Isopropanol.

A series of 5%Ni/Ce1-xTixO2 catalysts was prepared with nickel impregnation of mixed Ce-Ti oxides obtained via synthesis in supercritical isopropanol. All oxides have a cubic fluorite phase structure. Ti is incorporated into the fluorite structure. Small amounts of impurities of TiO2 or mixed Ce-Ti oxides appear with Ti introduction. Supported Ni is presented as the NiO or NiTiO3 perovskite phase. Ti introduction increases total samples reducibility and results in stronger interaction of supported Ni with the oxide support. The fraction of rapidly replaced oxygen and the average tracer diffusion coefficient also increase. The number of metallic nickel sites decreased with increasing Ti content. All catalysts except Ni-CeTi0.45 demonstrate close activity in tests of dry reforming of methane. The lower activity of Ni-CeTi0.45 can be connected to Ni decoration with species of the oxide support. The incorporation of Ti prevents detachment of Ni particles from the surface and their sintering during dry reforming of methane.

Synthesis and Oxygen Mobility of Bismuth Cerates and Titanates with Pyrochlore Structure.

Synthesis and study of materials based on bismuth cerates and titanates were carried out. Complex oxides Bi1.6Y0.4Ti2O7 were synthesized by the citrate route; Bi2Ce2O7 and Bi1.6Y0.4Ce2O7-by the Pechini method. The structural characteristics of materials after conventional sintering at 500-1300 °C were studied. It is demonstrated that the formation of a pure pyrochlore phase, Bi1.6Y0.4Ti2O7, occurs after high-temperature calcination. Complex oxides Bi2Ce2O7 and Bi1.6Y0.4Ce2O7 have a pyrochlore structure formed at low temperatures. Yttrium doping of bismuth cerate lowers the formation temperature of the pyrochlore phase. As a result of calcination at high temperatures, the pyrochlore phase transforms into the CeO2-like fluorite phase enriched by bismuth oxide. The influence of radiation-thermal sintering (RTS) conditions using e-beams was studied as well. In this case, dense ceramics are formed even at sufficiently low temperatures and short processing times. The transport characteristics of the obtained materials were studied. It has been shown that bismuth cerates have high oxygen conductivity. Conclusions are drawn about the oxygen diffusion mechanism for these systems. The materials studied are promising for use as oxygen-conducting layers in composite membranes.

Structural and Transport Properties of E-Beam Sintered Lanthanide Tungstates and Tungstates-Molybdates.

Lanthanide tungstates and molybdates are promising materials for hydrogen separation membranes due to their high protonic conductivity. A promising approach to fabricating ceramics based on these materials is radiation thermal sintering. The current work aims at studying the effect of radiation thermal sintering on the structural morphological and transport properties of (Nd,Ln)5.5(W,Mo)O11.25-δ as promising materials for hydrogen separation membranes. The defect fluorite structure was shown to be preserved during radiation thermal sintering at 1100 °C. The presence of protons in hydrated samples was confirmed by TGA. According to four-electrode studies and the isotope exchange of oxygen with C18O2, the samples demonstrate a high proton conductivity and oxygen mobility. Residual porosity (up to 29%) observed for these samples can be dealt with during membrane preparation by adding sintering aids and/or metal alloys nanoparticles. Hence, sintering by e-beams can be applied to the manufacturing of hydrogen separation membranes based on these materials.

Carbon Formation during Methane Dry Reforming over Ni-Containing Ceria-Zirconia Catalysts.

Two series of Ni/Ce(Ti/Nb)ZrO2 catalysts were prepared using citrate route and original solvothermal continuous flow synthesis in supercritical isopropanol and studied in dry reforming of methane (DRM). TEM, XPS and FTIRS of adsorbed CO confirm influence of support composition and preparation method on the catalysts' morphology and surface features. The oxygen mobility was studied by isotope heteroexchange with C18O2. After testing in DRM, carbon deposits after catalysts' testing in DRM were investigated by temperature-programmed oxidation with thermo-gravimetric analysis. The lowest amounts of carbon deposits were obtained for unmodified Ni-CeZr and Ni-CeNbZr compositions. Ti addition lead to an increased amount of carbon, which was removed at higher temperatures. The use of supercritical supports also resulted in the formation of a higher amount of coke. Catalysts prepared by the supercritical synthesis were tested in DRM for 25 h. The highest activity drop was observed in the first three hours. For all compositions, close values of carbon deposits were revealed.

Model-Based Performance Analysis of Membrane Reactor with Ethanol Steam Reforming over a Monolith.

Membrane reactors (MR) with an appropriate catalyst are considered to be an innovative and intensified technology for converting a fuel into the hydrogen-rich gas with the simultaneous recovery of high-quality hydrogen. Characteristics of an asymmetric membrane disk module consisting of a gas-tight nanocomposite functional coating (Ni + Cu/Nd5.5WO11.25-δ mixed proton-electron conducting nanocomposite) deposited on a gas-permeable functionally graded substrate has previously been extensively studied at lab-scale using MRs, containing the catalyst in a packed bed and in the form of a monolith. The catalytic monolith consisted of a FeCrAl substrate with a washcoat and an Ni + Ru/Pr0.35Ce0.35Zr0.35O2 active component. It has been shown that the driving potential for hydrogen permeation across the same membrane in a monolithic catalyst -assisted MR is greater compared to the packed bed catalyst. This paper presents results of the study where a one-dimensional isothermal model was used to interrelate catalytic and permeation phenomena in a MR with ethanol steam reforming over the monolith, operating at atmospheric pressure and in the temperature range of 700-900 °C. The developed mathematical reaction-transport model for the constituent layers of the catalyst-asymmetric membrane assembly together with a Sieverts' equation for the functional dense layer, taking also into account the effect of boundary layers, was implemented in a COMSOL Multiphysics environment. Good agreement with the experimental data of the lab-scale MR with reasonable parameters values is provided. In numerical experiments, concentration profiles along the reactor axis were obtained, showing the effect of the emerging concentration gradient in the boundary layer adjacent to the membrane. Studies have shown that a MR with a catalytic monolith along with appropriate organization of a stagnant feed flow between the monolith and the membrane surface may enhance production and flux of hydrogen, as well as the efficiency characteristics of the reactor compared to a reactor with packed beds.

Structural and transport properties of Nd tungstates and their composites with Ni0.5Cu0.5O obtained by mechanical activation.

Nd tungstates and molybdates are promising materials for hydrogen separation membranes due to their high protonic conductivity. This work aims at elucidating the structural, textural and oxygen transport features of Nd5.5WO11.25-δ, Nd5.5W0.5Mo0.5O11.25-δ and (Nd5/6La1/6)5.5WO11.25-δ and their composites with Ni0.5Cu0.5O synthesized by mechanical activation. The oxide materials obtained were distorted double fluorites but their composites with Ni0.5Cu0.5O possess a complex phase composition. Extended defects such as grain boundaries, stacking faults and surface steps/terraces were observed in TEM images, which allow fast diffusion transport along grain boundaries (D* ∼ 10-6 cm2 s-1 at 700 °C) and slower diffusion within grains' bulk (D* ∼ 10-11, 10-12 and 10-13 cm2 s-1 at 700 °C for the rather fast, "middle" and slow channels of bulk diffusion) (2D diffusion). The model gives the best description of experimental data obtained by the isotope exchange of oxygen with C18O2 in a flow reactor. For composites with Ni0.5Cu0.5O, a significant decrease in oxygen diffusivity was shown. The reduction and subsequent reoxidation of composites resulted in an increase in oxygen mobility probably due to the partial unblocking of oxygen diffusion corresponding to the Ln tungstates/molybdates. Fine oxygen transport features allow us to increase the hydrogen yield of hydrogen separation membranes due to the proton transport mechanisms involving oxide anions and the water splitting reaction. Hence, the features of Nd tungstates and their composites with nickel(II)-copper(II) oxide studied demonstrated their high potential for use in catalytic reactors based on hydrogen separation membranes.

An Experimental Performance Study of a Catalytic Membrane Reactor for Ethanol Steam Reforming over a Metal Honeycomb Catalyst.

The present study deals with the combination of ethanol steam reforming over a monolithic catalyst and hydrogen separation by membrane in a lab-scale catalytic membrane reactor (CMR). The catalyst was comprised of honeycomb thin-walled Fechralloy substrate loaded with Ni + Ru/Pr0.35Ce0.35Zr0.35O2 active component. The asymmetric supported membrane consisted of a thin Ni-Cu alloy-Nd tungstate nanocomposite dense permselective layer deposited on a hierarchically structured asymmetric support. It has been shown that the monolithic catalyst-assisted CMR is capable of increasing the driving potential for hydrogen permeation through the same membrane as compared with that of the packed bed catalyst by increasing the retentate hydrogen concentration. Important operating parameters responsible for the low carbon deposition rate as well as the amount of hydrogen produced from 1 mol of ethanol, such as the temperature range of 700-900 °C, the water/ethanol molar ratio of 4 in the feed, have been determined. Regarding the choice of the reagent concentration (ethanol and steam in Ar), its magnitude may directly interfere with the effectiveness of the reaction-separation process in the CMR.

Effect of Asymmetric Membrane Structure on Hydrogen Transport Resistance and Performance of a Catalytic Membrane Reactor for Ethanol Steam Reforming.

The performance of catalytic membrane reactors (CMRs) depends on the specific details of interactions at different levels between catalytic and separation parts. A clear understanding of decisive factors affecting their operational parameters can be provided via mathematical simulations. In the present paper, main results of numerical studies of ethanol steam reforming, followed by downstream hydrogen permeation through an asymmetric supported membrane, are reported. The membrane module consists of a thin selective layer supported on a substrate with graded porous structure. One-dimensional isothermal reaction-transport model for the CMR has been developed, and its validation has been carried out by using performance data from a lab-scale reactor with a disk-shaped membrane. Simulations demonstrate the model's capabilities to analyze local concentrations gradients, as required to provide accurate estimates of the relationship between structure-property-performance. It was shown that transport properties of multilayer asymmetric membranes are highly related to the structural properties of each single layer.

Comparative Study of Electrical Conduction and Oxygen Diffusion in the Rhombohedral and Bixbyite Ln6MoO12 (Ln = Er, Tm, Yb) Polymorphs.

Electrical conduction and oxygen diffusion mobility in the bixbyite ( Ia3̅) and rhombohedral ( R3̅) polymorphs of the Ln6MoO12-Δ (Ln = Er, Tm, Yb; Δ = δ, δ1, δ2; δ1 > δ2) heavy lanthanide molybdates, belonging to new, previously unexplored classes of potential mixed (ionic-electronic) conductors, have been studied in the range of 200-900 °C. The oxygen self-diffusion coefficient in bixbyite ( Ia3̅) Yb6MoO12-δ phase estimated by the temperature-programmed heteroexchange with C18O2 was shown to be much higher than that for rhombohedral ( R3̅) RI (with large oxygen deficiency) and ( R3̅) RII (with small oxygen deficiency) Ln6MoO12-Δ (Ln = Tm, Yb; Δ = δ1; δ1 > δ2) oxides. According to the activation energy for total conduction in ambient air, 0.99, 0.93, and 1.01 eV in Er6MoO12-δ, Tm6MoO12-δ, and Yb6MoO12-δ bixbyites, respectively, oxygen ion conductivity prevails in the range ∼200-500 °C. Oxygen mobility data for the rhombohedral Ln6MoO12-Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) phases RI and RII indicate that the oxygen in these phases exhibits mobility at much higher temperatures, such as those above 600-700 °C. Accordingly, below 600-700 °C they have predominantly electronic conductivity. As shown by total conductivity study of Ln6MoO12-δ (Ln = Er, Tm, Yb) bixbyites ( Ia3̅) and rhombohedral phases Ln6MoO12-Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) ( R3̅) in dry and wet air, the proton conductivity contribution exists only in Ln6MoO12-δ (Ln = Er, Tm, Yb) bixbyites up to 450-600 °C and decreases with a decreasing of the lanthanide ionic radius. The obtained data on the mobility of oxygen and the presence of proton contribution in bixbyites in the 300-600 °C temperature range make it possible to confirm unequivocally that Ln6MoO12-δ (Ln = Er, Tm, Yb) bixbyites are mixed electron-proton conductors at these temperatures.