Stefano Frangini

Stainless steel interconnect materials used in solid oxide fuel cells and electrolysers need to be coated to improve oxidation resistance and to mitigate Cr-vaporization. This work aimed to explore the optimal steel/coating combinations suitable for use in reversible solid oxide stacks and evaluated (Co,Mn)₃O₄ spinel, LaFeO₃ perovskite, Ce/Co and Y-based coatings, on AISI441 and Crofer 22 APU steels. The coatings were evaluated based on measurements of mass gain and oxide scale thickness after exposure at 700 and 800 °C to fuel side (90 vol.% H₂O/10 vol.% H₂) and air/oxygen side (pure O₂) atmospheres. In pure O₂, the most efficient coatings for limiting oxide scale formation and Cr evaporation, compared to the bare steel, were (Co,Mn)₃O₄ and CeCo on Crofer 22 APU. In 90 vol.% H₂O/10 vol.% H₂, the Y-based coating showed the largest improvement in oxidation resistance.

A study was conducted to explore the effects of Sr doping on the electrical properties of perovskite LaFeO₃ thin-film protective conversion coatings grown onto a K41 ferritic stainless steel, a typical interconnect material for intermediate temperature solid oxide cell (SOC) applications. The Sr-doped coatings were prepared in La₂O₃- and SrO-containing molten carbonate baths with minor added amounts of nitrate salt for accelerated coating formation. For comparison purposes, undoped coatings were obtained using the same carbonate bath, with the only difference being that SrO was replaced by inert MgO. SEM/EDX and XRD analyses were used for coating characterization and confirmed the effective incorporation of Sr but not of Mg into the LaFeO₃ layer. Although both the Sr-doped and undoped coatings consisted of a LaFeO₃ layer grown above an inner Fe-Cr spinel, the coating thickness of the Sr-doped coating was distinctly higher, approximately 2 µm, which is twice that of the undoped coating. Electrical measurements in terms of Area-Specific Resistance (ASR) were conducted at 700 °C in air and showed that Sr-doping significantly improved the electrical conductivity of the coated K41 steel. Due to the Sr-doping, the ASR values of the coated steel dropped from 60 to 37 mΩ cm² after 300 h of exposure, in spite of the higher Sr-doped coating thickness. The study concludes that Sr-doped thin-film perovskite coatings appear to be a promising solution for improved SOCs steel interconnect stability at intermediate temperatures.

The possibility of producing hydrogen from molten carbonate steam electrolysis using the well-established Molten Carbonate Fuel Cell (MCFC) technology was explored in this work. For this purpose, a 81 cm²MCFC single cell assembled with conventional cell materials was operated under alternated fuel cell/electrolysis conditions at 650°C in a binary eutectic Li₂CO₃-K₂CO₃electrolyte for about 400 h after an initial period of 650 h in which the cell worked only in the usual MCFC mode. A rapid cell performance loss in terms of cell internal resistance and electrode polarization was observed as soon as the cell started to work in the alternated fuel cell/electrolysis mode. After test completion, a post-mortem analysis was conducted to correlate the electrochemical response with cell materials degradation. Cell materials of the reverse cell were compared against a reference single cell that was assembled with the same materials and operated only in the fuel cell mode under comparable experimental conditions. Post-mortem analysis allowed to identify several serious stability issues of conventional MCFC materials when used in alternated operation modes. Thus, although the electrolyte matrix appeared almost unaffected, a significant amount of dissolved nickel was found in the matrix indicating that electrolysis operations promote an increasing chemical instability of the NiO oxygen electrode. A serious reduction of electrode porosity was also observed in both NiO oxygen and Ni metal fuel electrodes, which could explain the higher polarization resistance of the reversible cell in comparison to the reference MCFC cell. Furthermore, the oxygen current collector made with conventional 316L stainless steel was found to be seriously corroded under the alternated operation modes. Thus, the observed rapid increase in internal resistance in the reverse cell could be caused, at least in part, by an increased contact resistance between the oxygen electrode and the corroding current collector structure. Possible solutions for improving stability of electrodes and of the oxygen current collector in reverse MCFC cells were proposed and discussed in the final part of the work.

Interest on low‐temperature molten carbonate electrolysis processes (T < 600 °C) for advanced energy conversion and storage applications appears to be progressively growing in literature. However, the corrosion of metallic materials in molten carbonate salts in equilibrium with the acidic CO₂‐rich electrolysis gas atmospheres has been scarcely studied. In this work, the corrosion stability of a low‐activity FeAl aluminide diffusion coated 316 L stainless steel has been tested in a ternary carbonate eutectic mixture Li₂CO₃‐Na₂CO₃‐K₂CO₃ by 48 h‐immersion tests at 500 °C under CO₂ atmosphere. Before the corrosion testing, the coated steel samples were pre‐oxidized in air at 750 °C in order to generate a protective thin alumina film on the aluminide coating surface. The salt immersion results indicate that FeAl aluminide coatings are not stable in acid molten carbonate environments due to rapid transformation of the pre‐existing alumina film into a non‐protective mixed Fe‐Al oxide crystalline layer. Corrosion degradation has been found to consist of both general and localized forms of attack. Factors affecting FeAl aluminide coating corrosion have been analyzed and discussed in detail.

In our previous study copper oxide additions were used to accelerate the formation of perovskite LaFeO₃ conversion coatings on stainless steels from molten carbonate baths. Incorporation of copper particles into the growing coating was an additional effect resulting in the formation of a composite Cu-LaFeO₃ structure. In continuation to our previous study, the aim of this work is to report the effect of copper additions on long-term stability and performance of perovskite conversion coatings under IT-SOFC interconnect conditions. To this end, a Cu-LaFeO₃ coated K41 18Cr ferritic stainless steel was examined in air at 700°C up to 1000 h. In order to simulate properly the situation of a real IT-SOFC cell, Area Specific Resistance (ASR) and Cr-barrier properties of the coated steel were evaluated simultaneously with a special coating characterization setup. Studies were conducted by comparison with single-phase LaFeO₃ coatings obtained in a molten carbonate bath similar to that used for the formation of the composite Cu-LaFeO₃ coatings but without the addition of copper oxide. Copper addition did not change the general morphology of the perovskite coating, which remains a multi-layer coating, being composed of an outer LaFeO₃ crystalline layer, a middle Fe-rich oxide and two inner Fe-Cr rich oxide layers. However, copper was beneficial in promoting a thinner and more stable coating structure along with finer perovskite grain size. These structural improvements were further confirmed by the results obtained with electrical measurements that showed a better ASR behavior of the Cu-LaFeO₃ coatings. On the other hand, no relevant copper effects could be detected on the coating oxidation stability and on the Cr-barrier properties of the perovskite conversion coatings. Both LaFeO₃ and Cu-LaFeO₃ coatings showed similarly high coating stability and excellent Cr-barrier capability in experiments conducted at 700°C up to 1000 h. In definitive, dual-phase Cu-LaFeO₃ seem more promising systems for IT-SOFC interconnects than single-phase LaFeO₃ conversion coatings, although further improvements in ASR electrical properties are needed.