Oxide film formation and diffusion processes in near-surface layers of current collectors in solid oxide fuel cells

A method is developed for modifying the surface of current collectors in solid-oxide fuel cells (SOFC) prepared from ferrite stainless steel (Crofer22APU). Diffusion of the protective coating material into the Crofer22APU bulk and reverse diffusion of steel components into the coating are studied. The cross-sectional microstructure and composition are studied by the electron-microscopic technique. The elemental composition of the junction between the current collector and the lanthanum-strontium manganite cathode is studied depending on the time of service-life tests in the SOFC working mode (50–6000 h). The formation of the Cr₂O₃ oxide islet structure on the current collector surface at the steel/coating interface is observed. It is shown that the mutual diffusion of coating components (Ni) and Crofer22APU steel together with the redox reaction at the interface prevent the chromium diffusion to the surface and protect the steel current collector from oxidation.     

Nickel-copper-chromium catalyst for selective methane oxidation to synthesis gas at short residence times

Data on the selective oxidation of methane to synthesis gas on a 9% NiCuCr/2% Ce/(ϑ + α)-Al₂O₃ catalyst in dilute mixtures with Ar at short residence times (2–3 ms) are presented. The composition, structure, morphology, and adsorption properties of the catalyst with respect to oxygen and hydrogen before and after reaction were studied using XRD, BET, electron microscopy with electron microdiffraction, TPR, TPO, and TPD of oxygen and hydrogen. The following optimum conditions for the preparation and pretreatment of the catalyst for selective methane reduction were found: the incipient wetness impregnation of a support with aqueous nitrate solutions; drying; and heating in air at 873 and then at 1173 K (for 1 h at either temperature) followed by reduction with an H₂-Ar mixture at 1173 K for 1 h. At a residence time of 2–3 ms (space velocity to 1.5 × 10⁶ h⁻¹) and 1073–1173 K, the resulting catalyst afforded an 80–100% CH₄ conversion in mixtures with O₂ (CH₄/O₂ = 2: 1) diluted with argon (97.2–98.0%) to synthesis gas with H₂/CO = 2: 1. The selectivity of CO and H₂ formation was 99.6–100 and 99–100%, respectively; CO₂ was almost absent from the reaction products. The catalyst activity did not decrease for 56 h; carbon deposition was not observed. A possible mechanism of the direct oxidation of CH₄ to synthesis gas is considered.     

Non Aqueous Titration and Catalytic Conversion of Cyclohexanol as a Test of Surface Acidity

Summary. The textural characteristics, including surface area, mean pore diameters, and total pore volume of Cr₂O₃–CuO/Al₂O₃ solid catalysts were determined from the low temperature adsorption of N₂ at 77 K. The structural properties were investigated using XRD. The surface acidity of calcined samples was determined using two comparable methods, including the non-aqueous titration of acidic groups with n-butylamine and dehydration/dehydrogenation activity of cyclohexanol. XRD patterns assigned a crystalline CuO and γ-Al₂O₃ for 723 K calcinations products of lower Cr₂O₃ content. The gradual increase of calcinations temperature promoted the crystallinity of Cr₂O₃ and resulted in solid–solid interaction of CuO and Cr₂O₃ forming CuCr₂O₄. The textural parameters varied with both calcinations temperature and catalyst composition. The surface acid density (DAS) increased with the increase of chromia content up to 0.132 mole% Cr₂O₃, while the rise of calcinations temperature led to a decrease of surface acidity. The dehydration/dehydrogenation of cyclohexanol as well as n-butylamine titration succeeded in characterizing of surface acidity. Present address: Chemistry Department, College of Science, King Faisal University, Al-Hofuf 31982, Saudi Arabia     

TG Measurements of the Oxidation Kinetics of Fe-Cr Alloy with Regard to its Application as a Separator in SOFC

The high-temperature oxidation behavior of a ferritic alloy (SUS 430) in a SOFC environment, corresponding to the anode (H₂/H₂O gas mixture) and cathode (air) operating conditions, was determined with regard to application of the alloy as a metallic separator material in SOFC. The oxidation kinetics of Fe-16Cr alloy (SUS 430), was studied by thermogravimetry in H₂/H₂O gas mixtures with p_H/p_HO=94/6 and 97/3 and in air, in the temperature range 1023-1223 K, for 3.6 up to 1080 ks. It was found that the protective oxide scale, composed mainly of Cr₂O₃ with uniform thickness and excellent adhesion to the metal substrate, grows in accordance with the parabolic rate law. The dependence of the parabolic rate constant, k_p, of the scale on temperature obeys the Arrhenius equation: k_p=6.8×10^-4 exp (-202.3 kJ mol^-1R^-1T^-1) for H₂/H₂O gas mixtures with p_H/p_HO=94/6. The determined k_p was independent of the oxygen partial pressure in the range from 5.2×10^-22 to 0.21 atm at 1073 K, which means that the rates of growth of the scale on Fe-16Cr alloy in the above-mentioned atmospheres are comparable. The oxidation test results on Fe-16Cr alloy in H₂/H₂O gas mixtures and air demonstrate the applicability of SUS 430 alloys as a separator for SOFC. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mn–Co spinel protective–conductive coating on AL453 ferritic stainless steel for IT-SOFC interconnect applications

This paper presents research on the synthesis and properties of the Mn_1.5Co_1.5O₄ (MC) spinel powder, as well as its application for the preparation of a MC thick film on the AL453 steel to be used for metallic interconnect material in IT-SOFCs. In order to prepare the MC micropowder with excellent homogeneity of the chemical and phase compositions, EDTA gel processes were utilized. In order to improve the contact electrical resistance between an AL453 steel interconnect and the La_0.8Sr_0.2FeO₃ (LSF) cathode and protect the cathode from Cr poisoning, the surface of the AL453 steel was coated with a protective manganese cobaltite spinel matrix using screen printing in combination with an appropriate heat treatment. The oxidation of the AL453/MC composite layer carried out in the air–H₂O gas mixture at 1,073 K for 55 h showed that the spinel coating may serve as an effective barrier against outward Cr diffusion from the AL453 steel and, therefore, significantly inhibit the formation of volatile Cr vapors from the chromia scale. The contact ASR study of the interconnect–cathode interface in the AL453/MC/LSCM/LSF/LSCM/MC/AL453 system carried out in the range of 723?1,073 K in air showed a very large drop in ASR compared to the resistance of the AL453/LSCM/LSF/LSCM/AL453 system without the spinel coating.     

In Situ Study of the Surface Oxidation of FeCr Alloys Using Grazing Incidence X‐ray Absorption Spectroscopy (GIXAS)

The surface oxidation of FeCr alloys with 18, 28, and 43 mass‐% Cr was investigated in situ using grazing‐incidence X‐ray absorption spectroscopy (GIXAS) at the chromium and iron K‐edges. Oxidation in air was monitored in situ in the temperature range from 290 K to 680 K. The standard GIXAS data analysis is extended for the treatment of a single layer model in order to estimate the chromium concentrations of the oxide layer and of the near‐interface substrate as well as the oxide layer thickness. XANES analysis shows transitions from b.c.c. Fe to corundum type Fe₂O₃ and from b.c.c. Cr to corundum type Cr₂O₃. The initial oxide layers are 1.1‐1.4 nm thick and contain 60‐90 mass‐% chromium, while the near‐interface substrate is depleted in Cr. During heating, iron oxide growth dominates up to 560‐600 K. Then the chromium oxide layer loses its passivation effect and Cr oxidation sets in.     

Synthesis and Properties of The Solid Solutions Formed in The Fe2V4O13–Cr2V4O13 System

DTA and XRD studies of the Fe₂V₄O₁₃–Cr₂V₄ O₁₃ system have shown that continuous solid solutions of a Fe_2–xCr_xV₄O₁₃ type, bearing a Fe₂ V₄ O₁₃ structure, are formed in the system. With the increasing degree of the Cr3+ ion incorporation into the Fe₂ V₄ O₁₃ structure, a contraction of the solid solution crystal lattice develops. Solid solutions of a Fe_2–x Cr_x V₄ O₁₃ type melt incongruently, their melting temperature increasing from 953 to 1003 K with increase in the degree of the Cr³⁺ ion incorporation. The solid product of melting Fe_2–x Cr_x V₄ O₁₃ solid solutions for 0.2<x >1.2 is the Fe_1–x Cr_x VO₄ solution phase, and for x ≤0.2 and x ≥1.4 – the Fe_1–x Cr_x VO₄ phase as well as FeVO₄ or CrVO₄ , respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermodynamic possibilities and constraints of pure hydrogen production by a chromium,nickel, and manganese-based chemical looping process at lower temperatures

The reduction of chromium, nickel, and manganese oxides by hydrogen, CO, CH₄, and model syngas (mixtures of CO + H₂ or H₂ + CO + CO₂) and oxidation by water vapor has been studied from the thermodynamic and chemical equilibrium point of view. Attention was concentrated not only on the convenient conditions for reduction of the relevant oxides to metals or lower oxides at temperatures in the range 400–1000 K, but also on the possible formation of soot, carbides, and carbonates as precursors for the carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of very stable Cr₂O₃ to metallic Cr by hydrogen or CO at temperatures of 400–1000 K is thermodynamically excluded. Reduction of nickel oxide (NiO) and manganese oxide (Mn₃O₄) by hydrogen or CO at such temperatures is feasible. The oxidation of MnO and Ni by steam and simultaneous production of hydrogen at temperatures between 400 and 1000 K is a difficult step from the thermodynamics viewpoint. Assuming the Ni—NiO system, the formation of nickel aluminum spinel could be used to increase the equilibrium hydrogen yield, thus, enabling the hydrogen production via looping redox process. The equilibrium hydrogen yield under the conditions of steam oxidation of the Ni—NiO system is, however, substantially lower than that for the Fe—Fe₃O₄ system. The system comprising nickel ferrite seems to be unsuitable for cyclic redox processes. Under strongly reducing conditions, at high CO concentrations/partial pressures, formation of nickel carbide (Ni₃C) is thermodynamically favored. Pressurized conditions during the reduction step with CO/CO₂ containing gases enhance the formation of soot and carbon-containing compounds such as carbides and/or carbonates.     

Thermal characterisation of alumina supported chromium and platinum–chromium catalysts

This study presented results on reduction of alumina supported chromium and platinum–chromium catalysts using temperature programmed reduction method (TPR). It has been shown that catalysts after earlier oxidation step but without calcinations one undergo reduction in lower temperature in comparison to calcined only catalysts. Moreover, addition platinum to Cr/Al₂O₃ catalysts also caused decrease of reduction temperature. It has been observed that over the examined catalysts oxidation CO to CO₂ and reduction CO to CH₄ occurs. However, on Pt–Cr catalysts both reactions proceed at lower temperature compare to Cr catalysts.     

Photocatalytic selective oxidation of CO with O2 in the presence of H2 over highly dispersed chromium oxide on silica under visible or solar light irradiation

A highly dispersed Cr⁶⁺-oxide species on silica (Cr/SiO₂) was found to act as an efficient photocatalyst for the selective oxidation of CO into CO₂ with O₂ in the presence of H₂ under visible (λ>420 nm) or solar light irradiation at 293 K. UV-Vis, photoluminescence and FT-IR investigations revealed that the selective reactivity of the photoexcited tetrahedral Cr⁶⁺-oxide species ([Cr⁵⁺−O⁻]^*) with CO, as well as the high reactivity of the photoreduced Cr⁶⁺-oxide species (Cr⁴⁺-oxide species) with O₂ both play significant roles in this reaction.     

Activities of δ-Al2O3-supported bimetallic Pt?NiO and Co?NiO catalysts for methane autoreforming: Oxidation studies

The methane oxidation activities of Pt−NiO and Co−NiO bimetallic catalysts have been investigated as part of a larger research program on the autothermal reforming of methane (combined methane oxidation and steam reforming) in a fluidized bed reactor. Experiments at atmospheric pressure and 783–1023 K for both catalysts showed that the reaction was more selective towards H₂ production at CH₄∶O₂ ratios greater than unity. Light-off temperature increased with decreasing CH₄∶O₂ ratios, but increase in gas velocity (beyond minimum fluidization) increased the light-off temperature. Co−NiO was as promising as the more expensive Pt−NiO catalyst for the oxidation.     

The effect of water vapor on the oxidation behavior of 9%Cr steels in simulated combustion gases

The effect of the O₂ and H₂O content on the oxidation behavior of the 9%Cr steel P91 was studied in the temperature range of 600–800 °C. The oxidation rates under the various experimental conditions were determined by in-situ thermogravimetry. In dry oxygen a protective scale growth occurs with an oxidation rate controlled by diffusion in the scale. In presence of water vapor, after an incubation period, the scales become non-protective, as a result of a change of the oxidation limiting process. The water vapor effect is especially apparent in the temperature range of 600–700 °C, whereas at higher temperatures hardly any effect was found. The destruction of the protective scale by water vapor does not only depend on the H₂O content but also on the H₂O/O₂-ratio. Received: 15 July 1997 / Revised: 5 February 1998 / Accepted: 10 February 1998     

Computational insights into the molecular mechanisms for chromium passivation of stainless-steel surfaces

First principles DFT calculations are used to gain insights into the molecular mechanism of Cr passivation of FeCr alloy surfaces. The systems studied represent early stages of oxidation of FeCr alloys when the oxide layers extend just a few atomic layers into the bulk. A Monte-Carlo atom-swapping algorithm was developed to efficiently explore possible atomic positions and identify the most promising structures that yield overall energy lowering. Analysis of the resulting low energy structures show that the surface oxide layer is rich in chromium while there is a reduction in chromium in the metallic phase near the alloy-oxide interface. Furthermore, there is an increased concentration of Fe near the oxide-air surface. Analysis of the molecular structure of the oxide layers found that oxidized Cr was predominantly in the Cr₂O₃ phase, while oxidized Fe was present as both FeO and Fe₂O₃. We propose that the oxidative variability of Fe facilitates O diffusion in the iron-rich phases because of the range of geometries available for accommodating the O atom. In contrast, O diffusion is less facile in Cr, which has little variability in oxidation state.     

Comparison of structure and electrochemical properties for 5 V LiNi0.5Mn1.5O4 and LiNi0.4Cr0.2Mn1.4O4 cathode materials

Spinel LiNi_0.5Mn_1.5O₄ and LiMn_1.4Cr_0.2Ni_0.4O₄ cathode materials have been successfully synthesized by the sol–gel method using citric acid as a chelating agent. The structure and electrochemical performance of these as-prepared powders have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and the galvanostatic charge–discharge test in detail. XRD results show that there is a small Li _y Ni_1-y O impurity peak placed close to the (4 0 0) line of the spinel LiNi_0.5Mn_1.5O₄, and LiMn_1.4Cr_0.2Ni_0.4O₄ has high phase purity, and the powders are well crystallized. SEM indicates that LiMn_1.4Cr_0.2Ni_0.4O₄ has a slightly smaller particle size and a more regular morphological structure with narrow size distribution than those of LiNi_0.5Mn_1.5O₄. Galvanostatic charge–discharge testing indicates that the initial discharge capacities of LiMn_1.4Cr_0.2Ni_0.4O₄ and LiNi_0.5Mn_1.5O₄ cycled at 0.15 C are 129.6 and 130.2 mAh g⁻¹, respectively, and the capacity losses compared to the initial value, after 50 cycles, are 2.09% and 5.68%, respectively. LiMn_1.4Cr_0.2Ni_0.4O₄ cathode has a higher electrode coulombic efficiency than that of the LiNi_0.5Mn_1.5O₄ cathode, implying that Ni and Cr dual substitution is beneficial to the reversible intercalation and de-intercalation of Li⁺.     

Studies on Thermal Stability of Titanium Substituted Iron Molybdenum Spinel Oxide

The thermal stability of the solid solutions of Fe₂Mo_1–xTi_xO₄ for x=0.0 to 1.0 in air, had been investigated in the temperature range 303–1173 K using differential thermal analysis and thermogravimetry (DTA and TG). The products obtained by heating the sample in air, at different temperatures, have been characterized by X-ray diffraction and IR-studies. The results show that all the ferrite samples undergo surface oxidation during initial heating. On heating to 823 K, the samples undergo oxidation of the octahedral site cations only and forma cation deficient spinel phase. On further heating in air, the ferrites undergo complete oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dehydrogenation of Isobutane to Isobutene with Carbon Dioxide over SBA‐15‐Supported Chromia‐Ceria Catalysts

A series of SBA‐15‐supported chromia‐ceria catalysts with 3% Cr and 1%–5% Ce (3Cr‐Ce/SBA) were prepared using an incipient wetness impregnation method. The catalysts were characterized by XRD, N₂ adsorption, SEM, TEM‐EDX, Raman spectroscopy, UV–vis spectroscopy, XPS and H₂‐TPR, and their catalytic performance for isobutane dehydrogenation with CO₂ was tested. The addition of ceria to SBA‐15‐supported chromia improves the dispersion of chromium species. 3Cr‐Ce/SBA catalysts are more active than SBA‐15‐supported chromia (3Cr/SBA), which is due to a higher concentration of Cr⁶⁺ species present on the former catalysts. The 3Cr‐3Ce/SBA catalyst shows the highest activity, which gives 35.4% isobutane conversion and 89.6% isobutene selectivity at 570 °C after 10 min of the reaction.     

Oxygen deficiency and phase transitions in SrCo1– x – y Fe x Cr y O3–δ ( x =0.10–0.40, y =0–0.05)

Oxygen-deficient phases based on perovskite-like strontium cobaltites-ferrites are promising mixed conductors for high-temperature electrochemical applications. The p(O₂)-T-δ diagrams for the oxide systems SrCo_{1– x – y} Fe _x Cr _y O_{3– δ} (x=0.10–0.40; y=0–0.05) were studied at 500–1000 °C in the oxygen pressure range from 10^–5 to 0.21 atm using the coulometric titration technique and thermogravimetric analysis. Stability limits of the cubic perovskite phases having a high oxygen ionic conductivity were evaluated as functions of temperature, oxygen partial pressure and oxygen nonstoichiometry. It was found that doping with chromium and increasing the iron content in SrCo(Fe,Cr)O_{3– δ} both lead to a considerable enlargement of the cubic perovskite phase existence domain towards lower temperatures and reduced oxygen pressures. Electronic Publication     

High Temperature Oxidation of Fe22Cr-alloy

The isothermal and constant heating rate oxidation behaviour of the alloy Fe₇₈Cr₂₂ was examined in air with 1% H₂O and in 7% H₂/93% Ar with 1% or 12% H₂O. The measurements were performed in the temperature range 700–1300°C. The effect of surface treatment prior to oxidation was examined. A Cr₂O₃ scale developed slowly up to900°C. At 1100°C a catastrophic oxidation was observed after heat treatment for 70 h in air with 1% H₂O and in 7% H₂/93% Ar with 12% H₂O. The scale developed in these cases consisted of iron rich oxides such as Fe₂O₃ or FeCr₂O₄, in contrast to the more protective Cr₂O₃ scale seen under other test conditions. Possible causes for the catastrophic oxidation are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of Cr(VI) compounds during the thermal decomposition of amorphous chromium hydroxide

Chromium hydroxide (CH) was prepared by the reduction of purified sodium chromate using starch. CH was then used to prepare chromium oxide (Cr₂O₃). Results of thermogravimetric, X-ray diffraction, Fourier transform infrared spectroscopy, and chemical analyses suggested that a small amount of the initial Cr(VI) content speeds up the oxidation and reduction reactions, thereby promoting the purity and crystallization of Cr₂O₃. Cr(VI) in CH induced the evolution of CH and the formation of Cr(VI) containing compounds including CrO₃, NaCr(CrO₄)₂, Cr₃O₈, and Cr₅O₁₂ at low sintering temperature. Furthermore, homogeneous Cr₂O₃ nanoparticles with 99 % purity and particle size of 50 nm were obtained.     

Influence of protective layers on SOFC operation

Corrosion kinetics of ferritic alloys/steels (Crofer22APU, ITMLC, ZMG232L) were studied at high temperature. An extent of corrosion was evaluated by measuring the oxide scale thickness and the weight gain as a function of heating time. It is shown that even porous layer applied to interconnect can significantly reduce the rate of the steel oxidation. Contribution of the “oxide component” into the total degradation of the SOFC stack performance is estimated. Different protection materials and combinations were tested to analyze their influence on the processes of high temperature oxidation and long-term degradation of Fe-Cr steels. It has been shown that “more soft” materials on the basis of spinels (Mn(Co_{1 − x} Fe _x )₂O₄, Cu_{1 − x} Ni _x Mn₂O₄) are most suitable materials for the use as protective layers in comparison to perovskites. The efficiency of different protective materials was also tested in the real SOFC stacks designed in cooperation with company Staxera GmbH. It has been shown that applied spinel materials can effectively increase the long-term stability of the SOFC stacks.