Surface Oxygen Exchange Reaction Research Articles

Mo-doped La0.4Sr0.6FeO3-δ hollow fiber membrane for air separation and methane conversion

Ceramic oxygen transport hollow fiber La0.4Sr0.6Fe0.95Mo0.05O3-δ (LSFM5) membranes were fabricated via combined phase inversion and sintering technique. High-temperature X-ray diffraction experiments indicated that LSFM5 oxide undergoes second order phase transition at temperature above 300 °C from rhombohedral to cubic structure. The inner and outer surfaces of LSFM5 membranes were modified by acid etching and by depositing a surface decoration layer. For both kinds of membranes with modified surfaces, the enhancement of oxygen fluxes was observed at operating temperature range 860–980 °C, which indicates that the surface oxygen exchange reactions play a decisive role in the control of the oxygen transport. Catalytic experiments of methane partial oxidation (POM) by Ni/LSFM5 based oxygen-permeable membrane reactor exhibited good performance both in the mode with synthetic air on the supply side, and in the mode of POM coupled with CO2 splitting on the supply side.

Enhanced oxygen reduction reaction activity and electrochemical performance of A-site deficient (La0.6Sr0.4)1-xFe0.8Ni0.2O3−δ cathode for solid oxide fuel cells

In this work, (La0.6Sr0.4)0.9Fe0.8Ni0.2O3-δ (LSFN90), a stable, highly ORR-active and cost-efficient perovskite oxide, is developed as cathode materials for solid oxide fuel cell (SOFC). The introduction of A-site deficiency results in the crystal expansion of the cubic perovskite phase and an increase in oxygen vacancy concentration at operating temperature. The LSFN90 cathode displays good oxygen reduction reaction activity and low polarization resistance values. The A-site deficiency facilitates the diffusion of oxygen ions in the electrode and accelerates the surface oxygen exchange reaction. LSFN90 is used as cathode materials for SOFC to prepare anode-supported single cells, achieving maximum power densities of 1.51, 1.27, 0.95 and 0.63 W cm−2 under wet hydrogen (3%H2O–97%H2) atmosphere at 850, 800, 750 and 700 °C, respectively. The introduction of A-site deficiency can greatly enhance the oxygen reduction reaction activity and electrochemical performance of the cathode, demonstrating that LSFN90 has significant potential as a cathode material for practical applications in solid oxide fuel cells.

Surface modification with cobalt oxide nanocluster for enhanced surface oxygen exchange reactions of GDC-LSCF membrane: fabrication, electrochemical kinetic properties, and stability

Surface modification with cobalt oxide nanocluster for enhanced surface oxygen exchange reactions of GDC-LSCF membrane: fabrication, electrochemical kinetic properties, and stability

Porous an hollow nanofibers for solid oxide fuel cell electrodes

Among the diverse approaches for improving the electrode performance of solid oxide fuel cells operating at intermediate temperatures, the use of nanofiber-based electrodes has provided large improvement owing to their large specific surface area, continuous conduction pathway, and highly porous structure. However, the low thermal stability at increased temperature often limits the process compatibility and sustainability during operation. In this study, we fabricated nanofiber-based electrodes with a high porosity and hollow shape using one-step electrospinning with a hydrogel polymer, which exhibited largely improved performance and excellent thermal stability. A porous-nanofiber-based cell exhibits a polarization resistance of 0.021 Ωcm2 and maximum power density of 1.71 W/cm2 at 650 °C, which is an improvement of 34.3% and 14.7% compared to that of a solid-nanofiber-based cell, respectively. Comprehensive analyses of the microstructures and chemistry indicate that the performance increase is mainly attributable to the enhanced surface oxygen exchange reactions owing to the extended reaction sites with lower energy barriers by the high porosity and enriched oxygen vacancies in the nanofibers.

Electrospun composite nanofibers for intermediate-temperature solid oxide fuel cell electrodes

Considering the dominant loss associated with surface oxygen exchange reactions among other complex electrochemical processes, the design of an electrode structure for the reasonable operation of solid oxide fuel cells is challenging. The surface oxygen exchange reaction can be considerably facilitated using composite nanofibers containing the electrode, electrolyte, and catalyst. The composite nanofiber electrodes containing Pd show the smallest polarization resistance of 0.031 Ωcm2 and the maximum power density of 0.7 W/cm2 at 650 °C, which are 39.2% and 12.5% improved values compare to the catalyst-free composite nanofiber electrodes, respectively. These results provide an facile fabrication strategy for developing high-performance electrodes for use in solid oxide fuel cells.

Rational design via tailoring Mo content in La2Ni1-xMoxO4+δ to improve oxygen permeation properties in CO2 atmosphere

Perovskite (ABO3) and ruddlesden-popper (A2BO4) oxides are typical mixed conducting ceramic membrane materials for oxygen separation from air. In particular, ruddlesden-popper (RP) membrane display high CO2 resistance despite their relative low oxygen permeation flux compared to perovskite oxide membranes. Element-doping is an important method to improve the oxygen permeability. In this work, the mechanism of oxygen transfer process through one RP ceramic La2Ni1-xMoxO4+δ (x = 0, 0.025, 0.05, 0.1, 0.2) membranes was investigated. An optimum doping level (x = 0.05) in La2Ni1-xMoxO4+δ was found. The optimized composition of La2Ni0.95Mo0.05O4+δ membrane could not only improve surface oxygen exchange reactions, but also promote oxygen ion bulk diffusion through the dense layer. The maximum oxygen flux of La2Ni0.95Mo0.05O4+δ membrane reached 3.27 mL min−1 cm−2 at 1000 °C. Furthermore, La2Ni0.95Mo0.05O4+δ membrane high stability in CO2 atmosphere. When sweeping gas was switched from helium to pure CO2, the oxygen fluxes were only reduced by 5% and stabilized at 2.75 mL min−1 cm−2 at 950 °C. Our results highlight the efficiency of Mo-doping strategy to simultaneously improve the oxygen permeability and stability of A2BO4+δ-type oxide membranes.

Development of Pr2-xSrxCuO4±δ mixed ion-electron conducting system as cathode for intermediate temperature solid oxide fuel cell

Mixed ionic-electronic conducting oxides Pr2-xSrxCuO4±δ (x = 0, 0.2, 0.4, 0.5 and 0.6) are synthesized by combusting mixed precursors (metal acetates) under microwave heating followed by microwave sintering at 1000 °C for 4 h. Each composition of Pr2-xSrxCuO4±δ solid solutions crystalizes into tetragonal structure within one single phase across the compositional range 0 ≤ x ≤ 0.5. The T′-phase Pr2CuO4±δ transforms to the T*-phase at x = 0.4. Crystallite size reduces from 801(9) to 728(6) nm with an increase in the Sr content. The maximum dc-conductivity (σdc = 63.1(2) S cm−1 at 700 °C) and minimum activation energy (Ea = 0.118(3) eV, below pseudo-transition temperature 620 °C) in Pr1.6Sr0.4CuO4±δ is ascribed to optimum concentration of extrinsic dissociated defects. The performance of electrochemical symmetrical cells (Pr2-xSrxCuO4±δ/ Ce0.9Gd0.1O1.95/ Pr2-xSrxCuO4±δ) prepared using inkjet printing is found to be superior than spin coated (cathode) symmetrical cells. Increase in oxygen ion conductivity and surface oxygen exchange reaction rate with increasing Sr dopant concentration improve overall electrochemical performance of cathode. Minimum electrode polarization resistances 0.30(7) Ω cm2 and 0.21(3) Ω cm2 (at 700 °C) are obtained for symmetrical cells of Pr1.6Sr0.4CuO4±δ cathode using spin-coating and inkjet printing methods, respectively. Electrochemical impedance spectroscopy studies suggest that the charge transfer step is the rate-limiting step during oxygen reduction reaction.

Effect of Porous Layer on Oxygen Semi Permeation and Surface Exchange of CaTi0.9Fe0.1O3-δ Membranes

Oxygen transport membranes are dense ceramic oxides that allow oxygen diffusion along a chemical potential gradient. CaTi0.9Fe0.1O3−δ(CTF) exhibits an interesting tradeoff between performance and stability; despite a lower oxygen flux, it exhibits a superior thermal, mechanical, and chemical stability. To optimize the performance of CTF, it is important to establish the rate-limiting step of oxygen transport mechanism, either surface oxygen exchange reactions, or bulk diffusion. We prepared CTF membranes with and without a porous layer. The limiting step was determined using the ratio between chemical potential drop of oxygen near the surface and across the bulk. We find that bare CTF membranes exhibit a mixed-control below 750 °C, changing to a bulk-limited process with increasing temperature. The porous layer mitigates surface limitation and increases the flux by 35% at lower temperature.

Enhanced High Oxygen Permeation of Mixed-Conducting Multichannel Hollow Fiber Membrane via Surface Modified Porous Layer

The oxygen permeation performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) mixed-conducting multichannel hollow fiber (MCMHF) membranes was improved by surface modification via spin-spraying of a La0.6Sr0.4CoO3−δ (LSC) porous layer. At 1173 K, the oxygen permeation flux of the modified membranes was clearly enhanced and reached 9.68 mL·cm–2·min–1, which is a remarkable high value in the field of mixed-conducting oxygen permeation membrane processes. Theoretical calculations demonstrated that the oxygen transport resistance, especially the surface exchange resistance, obviously decreased as a result of the modified LSC porous layer. Moreover, the process of oxygen permeation through the modified membrane was controlled by both bulk diffusion and the surface oxygen exchange reaction, whereas the oxygen permeation of the unmodified membrane was dominantly controlled by the surface oxygen exchange reaction. The modified MCMHF membranes showed generally stable oxygen permeation fluxes over 100 h at 1173 K.

Surface Segregation and Inter-Diffusion of Cations and Impurities in Microelectrodes for Solid Oxide Fuel Cells and Electrolyzers

The surface segregation and inter-diffusion of cations and impurities in La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) micro-patterned electrode structures on a single crystal Zr0.84Y0.16O1.92(YSZ) electrolyte was investigated by high-resolution ion beam analysis (Time-of Flight Secondary Ion Mass Spectrometry and Low-Energy Ion Scattering). The outermost and near surface of these microelectrodes showed significant changes in the distribution of the cations and impurities after micro-patterning, annealing and electrochemical testing of the cells. As occurring for bulk LSCF ceramics, strong segregation of Sr towards the electrode surface is observed during annealing of the microelectrode, as well as due to the thermal treatments required for electrochemical measurements. This segregation might have a drastic effect on the surface oxygen exchange reaction as the electrochemically active transition metals (Fe, Co) are not directly exposed to the gas phase. Impurities, such as Si and Ca, are also observed at the annealed electrolyte surface. Furthermore, the microstructured electrode showed that impurities can also migrate between the different components (i.e. Na and Si impurities migrate from the electrolyte to the LSCF electrode, showing a “cleaner” surface after the thermal treatments). This study has provided a further understanding of the migration and segregation of cations and impurities taking place in real devices, building upon previous studies performed on ceramics materials. Figure 1

Highly stable La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fibre membrane for air separation swept by steam or steam mixture

Perovskite oxide ceramic membranes have the potentials for industrial oxygen production, particularly to replace the conventional expensive air separation units, which is of significance to improve the viability of the clean energy technologies like IGCC and Oxyfuel projects. In most of these applications, the steam presence is unavoidable thus requiring these membranes to be stable in the steam-containing atmosphere under high temperatures. In this work, La0.6Sr0.4Co0.2Fe0.8O3−α (LSCF) perovskite hollow fibre membrane was prepared and tested for O2 separation in the presence of steam in the sweep gas. Three membranes were successfully tested at temperature of 900°C using the sweep containing 3%, 4–12% and 100% steam (balanced by helium) for 370, 200 and 500h, respectively. In the long run, the steam can etch the membrane surface, as evidenced by the leaching of iron and the formation of strontium carbonate. This etching process however is too slow to cause substantial damage on the membrane performance. Using pure steam sweep, the LSCF membrane was operated at 900°C for nearly 500h without any sign of O2 flux decay, compared favourably with Ba0.5Sr0.5Co0.8Fe0.2O3−α (BSCF) hollow fibre membranes, which only lasted for 90h. In contrast to the severe poisoning effects of other acid gases like SO2 or NOx on the O2 permeation through similar LSCF membrane, the detrimental effect of steam is negligible. In fact, the presence of minor steam less than 12% in the sweep is kinetically favourable for the surface oxygen exchange reactions, leading to the improvement of the O2 flux.

Effects of Water on Cathodic Performance of Ba0.6La0.4CoO0.3 on the Cell Using LaGaO3-Based Oxide for Electrolyte

Effects of water addition to oxidant on the cathodic overpotential of La-doped BaCoO 3 electrode were investigated. It was found that the overpotential of La-doped BaCoO 3 decreased by addition of H 2 O to oxygen. In particular, the decrease in overpotential by addition of H 2 O was significant with decreasing operating temperature of the cell. Therefore, it became clear that the maximum power density increased by addition of H 2 O to oxidant. The improved cathodic activity by addition of H 2 O was further studied by using 18 O tracer exchange measurement. Although the diffusion coefficient was independent of the coexisting water, the surface exchange coefficient increased greatly by addition of water. 18 O- 16 O isotopic exchange reaction under coexisting H 2 16 O demonstrated that the surface oxygen exchange reaction through H 2 O is very fast and the coexisting water catalytically accelerates the dissociation reaction of gaseous oxygen molecule into oxide ion. The positive effects of humidified oxidant were stable over 50 hours of testing.

Oxygen exchange and diffusion in the near surface of pure and modified yttria-stabilised zirconia

By studying the oxygen transport through yttria-stabilised zirconia (YSZ), a strategy could be proposed which should lead to a reduction in the operating temperature of the solid oxide fuel cell (SOFC) to the intermediate temperature range without loss in performance. The combination of isotopic exchange depth profiling with low energy ion scattering (LEIS) and elastic recoil detection analysis (ERDA) has shown a complex structure affecting the surface oxygen exchange reaction and self-diffusion in 10 mol% yttria-doped zirconia. Remarkable is the presence of a thin (about 6 nm) layer at the external surface showing resemblance with the monoclinic phase. The results suggest a significant improvement in the surface oxygen exchange with respect to the values reported in literature when impurity oxides are prevented from segregating to the external surface. A possible operating temperature of around 850 °C seems feasible. Improvements in the surface oxygen exchange by addition of a surface catalyst reported in literature are also attributed to the removal of impurities. Further decrease in operating temperature, down to at least 725 °C, should be possible by removing the impurities in the bulk, which should lead to a considerable increase in the grain boundary diffusion and by reduction of the electrolyte thickness.

Stabilized bismuth oxide-noble metal mixed conducting composites as high temperature oxygen separation membranes

Oxygen concentration cells, with dense dual phase composite membranes made from erbia-stabilized bismuth oxide and a noble metal (Au, Ag), were investigated in the temperature range 650–850°C under controlled oxygen partial pressure gradients. An electrochemical treatment was applied to interpretation of the oxygen permeation data. It is found that the composite membranes exhibit high oxygen permeability relative to the single phase bismuth oxide, since oxygen ions and electrons are allowed to transport through the oxide and metal phase, respectively. The oxygen permeability of the silver-containing composite is at least one order higher than that of the gold-containing one, which can be explained by the fact that silver has a higher catalytic activity than gold for the surface oxygen exchange reaction and thus less limitations are exerted on the overall oxygen transport.

Kinetics of UO 2 oxidation in steam atmosphere

An adsorption model for calculation of UO 2 oxidation in H 2O/H 2 gas mixture is proposed. The model is based on the assumption that the rate of fuel oxidation by steam is controlled mainly by surface oxygen exchange reactions which are described in terms of Langmuir-like theory. Corrections to the oxidation rate related to the volume diffusion of oxygen are taken into account using an adiabatic approach to the diffusion problem. Parameters of the model were optimized on the experiments with steam pressure up to 10 5 Pa at 1200 to 2000 K.