Manganese Cobalt Spinel Oxides Research Articles

TEM Characterization of Long Term Aged Interconnect Oxide Layers from Real Stack

Interconnects from real SOFC stacks operated for 9000h and 20000h are characterized by means of SEM and TEM. This study focuses on the air side of the interconnect, which has been coated with manganese cobalt spinel oxide (MCO) doped with iron. Differences between the zones of air channel and interconnect rib are highlighted. Iron particles are found in the thermally grown oxide (TGO) exposed to the air channel. Manganese-rich crystals are observed close to the steel bulk for the rib contact zone, and in the bottom part of the TGO for the air channel zone. Cobalt is segregating at grain boundaries in the MCO after 20000h. More investigation is necessary to fully understand all mechanisms but the additional information gathered in the present study should help in better designing accelerated testing and modelling.

MnxCo3-xO4 spinel oxides as efficient oxygen evolution reaction catalysts in alkaline media

The design of efficient electrocatalysts for oxygen evolution reaction (OER) is an essential task in developing sustainable water splitting technology for the production of hydrogen. In this work, manganese cobalt spinel oxides with a general formula of MnxCo3-xO4 (x = 0, 0.5, 1, 1.5, 2) were synthesised via a soft chemistry method. Non-equilibrium mixed powder compositions were produced, resulting in high electrocatalytic activity. The oxygen evolution reaction was evaluated in an alkaline medium (1 M KOH). It was shown that the addition of Mn (up to x ≤ 1) to the cubic Co3O4 phase results in an increase of the electrocatalytic performance. The lowest overpotential was obtained for the composition designated as MnCo2O4, which exhibited a dual-phase structure (∼30% Co3O4 + 70% Mn1.4Co1.6O4): the benchmark current density of 10 mA cm−2 was achieved at the relatively low overpotential of 327 mV. The corresponding Tafel slope was determined to be ∼79 mV dec−1. Stabilities of the electrodes were tested for 25 h, showing degradation of the MnCo2O4 powder, but no degradation, or even a slight activation for other spinels.

Diffusion couple study of the interaction between Cr2O3 and MnCo2O4 doped with Fe and Cu

Manganese cobalt spinel oxides are promising coating materials for the protection of ferritic stainless steel interconnects in solid oxide fuel cell (SOFC) stacks. The interaction between such coatings and the steel is here studied using diffusion couples as a model system. The interaction between MnCo2O4, MnCo1.7Fe0.3O4 and MnCo1.7Cu0.3O4 spinels and Cr2O3 was studied in air at 900 °C. In all cases, a reaction layer rich in Co and Cr formed at the interfaces. Using Pt-particles to mark the original interface reveals that the reaction layers grow by diffusion of Co (and Mn) from the spinel oxides to the Cr2O3/reaction layer interface. The growth of the reaction layers followed parabolic kinetics with rate constants of 1.3 × 10−5 μm2 s−1 for the MnCo2O4/Cr2O3 couple, 8.6 × 10−6 μm2 s−1 for the MnCo1.7Fe0.3O4/Cr2O3 couple, and finally 1.2 × 10−4 μm2 s−1 for the MnCo1.7Cu0.3O4/Cr2O3 couple.

Metal-defected spinel MnxCo3-xO4 with octahedral Mn-enriched surface for highly efficient oxygen reduction reaction

Manganese-cobalt spinel oxides are considered as a class of promising and low-cost electrocatalysts for oxygen reduction reaction (ORR), whose performances largely depend on their electronic structures which can be effectively optimized by defect engineering. Herein, metal defects (manganese vacancies and cobalt vacancies, i.e. VMn and VCo) were in-situ introduced into spinel MnxCo3-xO4via a simple solvothermal treatment followed by thermal calcination. Mn-Co glycerolate precursors not only enable controllable synthesis of spinel oxides with variable metallic ratios, but also play a key role in constructing metal defected crystals for their lamellated structure. As a result of the formation rate difference between manganese and cobalt glycerolate, a unique Mn-enriched surface is formed, leading to the increase of highly active sites for ORR. Importantly, the presence of metal defects, confirmed by XRD (X-ray diffraction), element analysis and XAFS (X-ray absorption fine structure spectroscopy), leads to greatly increased electrical conductivity and O2 adsorption ability, thus bringing about enhanced ORR activity. Especially, metal-defected Mn1.5Co1.5O4, with the optimal Mn/Co ratio, exhibits comparable activity and superior durability to those of the benchmark Pt/C in ORR and displays excellent discharge performance in Zn-air batteries for practical application. This work provides a new way to optimize the electrocatalytic performance of mixed metal spinel oxides via rational defect engineering.

Thermal expansion and electrical conductivity of Fe and Cu doped MnCo2O4 spinel

Manganese cobalt spinel oxides are promising coating materials for corrosion protection of metallic interconnects in solid oxide fuel cell stacks. This work investigates how Fe and Cu doping affect the crystal structure, thermal expansion and electrical conductivity of the MnCo2−xMxO4 (M = Cu, Fe; x = 0.1, 0.3, 0.5) spinel oxides. Single phase cubic spinels were successfully prepared by spray pyrolysis. The electrical conductivity between room temperature and 1000 °C increased with addition of Cu and decreased with addition of Fe. The thermal expansion coefficient (TEC) between 50 and 800 °C decreased from 14.4 to 11.0 × 10−6 K−1 going from MnCo2O4 to MnCo1.5Fe0.5O4. The TEC of the Cu substituted materials did not follow any obvious trend with composition and was likely influenced by precipitation of CuO during heating. Based on their physical properties, the Fe doped materials are the most attractive for application as SOFC interconnect coatings.

Effect of coating density on oxidation resistance and Cr vaporization from solid oxide fuel cell interconnects

Manganese cobalt spinel oxides are promising materials for protective coatings for solid oxide fuel cell (SOFC) interconnects. To achieve high density such coatings are often sintered in a two-step procedure, involving heat treatment first in reducing and then in oxidizing atmospheres. Sintering the coating inside the SOFC stack during heating would reduce production costs, but may result in a lower coating density. The importance of coating density is here assessed by characterization of the oxidation kinetics and Cr evaporation of Crofer 22 APU with MnCo1.7Fe0.3O4 spinel coatings of different density. The coating density is shown to have minor influence on the long-term oxidation behavior in air at 800 °C, evaluated over 5000 h. Sintering the spinel coating in air at 900 °C, equivalent to an in-situ heat treatment, leads to an 88% reduction of the Cr evaporation rate of Crofer 22 APU in air-3% H2O at 800 °C. The air sintered spinel coating is initially highly porous, however, densifies with time in interaction with the alloy. A two-step reduction and re-oxidation heat treatment results in a denser coating, which reduces Cr evaporation by 97%.

Properties of Spinel Protective Coatings Prepared Using Wet Powder Spraying for SOFC Interconnects

A wet powder spraying (WPS) method was applied to prepare protective layers of manganese cobalt spinel oxide (MCO) on commercial stainless steels of solid oxide fuel cell (SOFC) interconnects. Densification of the MCO layer was dependent on the thermal treatment condition in which a reactive sintering increased the coating density. An applied MnO sublayer reacted with the MCO layer during sintering, and resulted in secondary phase formation and limited densification. Both MCO and MnO-MCO coatings were adhering to the steel substrates and suppressed chromium diffusion during oxidation. The area specific resistance (ASR) of a test specimen including the MCO layer was 0.03 Ωcm2 at 700 °C after 800 h, lower than with MnO-MCO, which is believed to be due to the denser protective layer of the single phase and more limited chromia scale growth. Thus, an effective protective layer of metallic interconnects could be prepared using a simple wet coating method after optimization of the sintering conditions.

Composition effects of spinel MnxCo3−xO4 nanoparticles on their electrocatalytic properties in oxygen reduction reaction in alkaline media

In this work, we synthesized a series of manganese cobalt spinel oxide (MnxCo3−xO4) nanoparticles (NPs) covering the whole composition range (x = 0.0. 0.4, 0.8, 1.4, 1.9, and 3.0) and investigated their electrocatalytic properties in relation with the oxygen reduction reaction (ORR) in alkaline media. The NP samples were synthesized by sonochemical reactions of Mn(OAc)2 and Co(OAc)2 (Ac = acetyl) in a water-dimethylformamide mixed solvent. The four samples in the Co-rich side have the cubic structure whereas the other two samples in the Mn-rich side have the tetragonal structure. The X-ray photoelectron spectroscopy and electrochemical analyses data indicate that the distribution of manganese and cobalt ions between the two metal ion sites of the spinel structure in our NP samples conforms to that of the bulk counterparts in the literature. The electrocatalytic data show that the ORR mechanism is changed when the structure is changed from cubic to tetragonal. The highest ORR activity was observed with the x = 0.4 sample. The electrochemical stability of this sample is higher than that of commercial Pt/C.

Lifetime prediction for manganese cobalt spinel oxide coatings on metallic interconnects

To prevent chromium poisoning of cathodes in solid oxide fuel cells, metallic interconnects are coated with protective oxides. One commonly used coating is manganese cobalt spinel oxide (MCO). Although MCO acts as a barrier to oxidation of interconnects, formation of native oxide scales on interconnects still occurs. As a result of native scale growth during fuel cell operations, the strength of the MCO interface will degrade. In addition, the spallation is generally driven by the temperature coefficient of expansion mismatch between the native scales and the MCO. Thus MCO spallation is likely to be dependent on the thicknesses of the different layers and is most likely to occur when cooling the fuel cell from high operating temperatures. In this study, the effects of the native scale thickness on MCO spallation are explored. To obtain interfacial fracture energy as a function of native scale thickness, room temperature, four-point bend experiments are performed on coated interconnects with various native scale thicknesses. By comparing the evolving interfacial fracture energy from experiments with the interfacial fracture energy obtained from an analytical model, coating lifetime is predicted.

Spinel coatings on metallic interconnects: Effect of reduction heat treatment on performance

Typical manganese cobalt spinel oxide (MCO) coating processes involve both a reduction and an oxidation heat treatment to produce dense and well adherent coatings on metallic interconnects for solid oxide fuel cells (SOFCs). In a high volume production, to achieve the required cycle-time and capacity throughput, it is necessary to perform the reduction heat treatment in a continuous manner rather than batch firing. However, the continuous reduction heat treatment in furnaces requires large capital investments and also incurs high operating costs. Therefore, it is the reduction heat treatment that contributes to a significant portion of overall coating costs. For reduced cost and increased throughput, it would be preferable to eliminate the reduction heat treatment and oxidize the green coatings directly. However, it is anticipated that coatings without the reduction heat treatment would have poor adherence and density which would result in reduced functionality. In the present paper, the performances of MCO coatings processed with and without the reduction heat treatments are evaluated in terms of interfacial fracture energy from mechanical bend tests and area specific resistance (ASR) from electrical tests. Quantifying these mechanical and electrical properties allows for assessment of the benefits of the reduction firing.

Transition Metal Doping of Manganese Cobalt Spinel Oxides for Coating SOFC Interconnects

Manganese cobalt spinel oxide coatings have been shown to be effective for preventing chromium volatilization from interconnect alloys and associated cathode poisoning in solid oxide fuel cells. A reaction layer consisting of chromium-containing spinel formed due to interaction of the coating with the chromia scale on the alloy surface and increases the overall area specific resistance (ASR) of the system. Reduction in ASR can be accomplished by reducing the thickness and/or increasing the electrical conductivity of the reaction layer, which includes both the high-chromium spinel phase and the chromia scale. This paper reports the effects of transition metal dopants in manganese-cobalt spinel on the reaction products and chromia scale formed on a ferritic stainless steel alloy. Dense single-phase doped spinel ceramics were prepared through process optimization. High temperature diffusion couple experiments with sintered spinel and ferritic stainless steel SS 441 were carried out to understand the spinel-alloy interactions. Doping of the spinel oxide led to a reduction in the thickness of the chromia scale formed on the alloy. The difference in the morphologies and compositions of interaction layers at 800°C and 900°C provides insight on the formation and growth mechanism of a high-chromium spinel.

Electrical Properties, Cation Distributions, and Thermal Expansion of Manganese Cobalt Chromite Spinel Oxides

As the oxidation and chromium volatilization of chromia‐forming alloy interconnects can cause Solid oxide fuel cells (SOFC) cathode poisoning and cell degradation, spinel coatings like Mn1.5Co1.5O4 have been applied as a barrier to oxygen and chromium diffusion. To evaluate their long‐term stability, the properties of the reaction layer between the Mn1.5Co1.5O4 coating and Cr2O3 scale formed on the alloy surface need to be characterized. Therefore, compositions of Mn1.5−0.5xCo1.5−0.5xCrxO4 (x = 0–2) were prepared to investigate their electrical properties, cation distributions, and thermal expansion behavior at high temperature. With increasing Cr content in manganese cobalt spinel oxides, the cubic crystal structure is stabilized and the electrical conductivity and coefficient of thermal expansion both decrease. The cation distributions determined from neutron diffraction show that Cr and Mn have stronger preference for octahedral sites in the spinel structure as compared with Co.

Crystal Structure, Chemical Stabilities and Electrical Conductivity of Fe-Doped Manganese Cobalt Spinel Oxides for SOFC Interconnect Coatings

The application of ceramic coatings has been demonstrated as an effective technique to suppress the oxidation scale growth and chromium volatilization of ferritic stainless steels used for SOFC interconnects. Manganese cobalt spinel oxides are promising materials for this application. In this work, the effects of iron doping in the MnCo2-xFexO4 (x = 0–0.7) spinel oxide on the crystal structure, reaction with chromia, cation distributions, and electrical conductivity at high temperature were investigated. With increasing the iron content, the lattice parameters increase and the electrical conductivity decreases. During reaction with chromia, iron does not appear in the chromium-rich reaction layer even for the sample with heaviest doping (e.g. MnCo1.3Fe0.7O4). Neutron diffraction studies suggest that cobalt has a stronger preference for the tetrahedral sites while manganese and iron favor the octahedral sites.

Mechanical characterization of oxide coating–interconnect interfaces for solid oxide fuel cells

This paper reports on the characterization of interfaces between oxide coatings and metallic interconnects that are used in planar solid oxide fuel cells. With the reduction of operating temperatures to 800°C, it is possible to replace ceramic interconnects with less expensive stainless steels. However, when incorporating chromia-forming metallic interconnects, steps must be taken to inhibit chromium poisoning. One approach to prevent chromium poisoning, is to deposit dense, protective coatings, such as manganese cobalt spinel oxide (MCO). The brittle nature of MCO makes it susceptible to damage under mechanical and thermal stresses during operation. A four point bend experiment is designed to assess the strength and adhesion of reduced and oxidized coatings deposited on SS441 or Crofer interconnects. Resulting tensile cracking patterns on the convex side of bend specimen are used to quantify the interfacial shear strength with a shear lag model. Using energy based fracture mechanics, interfacial fracture energy is calculated from the strain at the onset of coating spallation. Scanning electron microscopy images of the cracked coating surfaces are processed to analyze the failure mechanisms, crack spacing and spalled areas. At 3% strain, the weakest interface is found in the Crofer system with the oxidized coating.

Interactions Between SOFC Interconnect Coating Materials and Chromia

Manganese cobalt oxides are promising coating materials for reducing chromium volatilization, and thus the associated cathode poisoning, from interconnect alloys in solid oxide fuel cells (SOFCs). Interaction between this coating and the oxide scale formed on the alloy during fuel cell operation can lead to changes in the coating composition and thus its performance. In this study, the properties of manganese cobalt spinel oxides and the reaction between manganese cobalt spinel oxides and chromia were investigated. The reaction product consists of two layers: a layer in contact with chromia that grows by the diffusion of cobalt and manganese from (Mn,Co)3O4 toward the chromia and an intermediate layer that grows by the diffusion of chromium through the reaction layer. The effect of dopants on the coating performance was also investigated. With the addition of iron or titanium, the rate of reaction between the spinel coating and the chromia scale can be decreased significantly, which would reduce the risk of scale spallation and provide an increase in the lifetime of the interconnect and thus the fuel cell.