Ceramic coatings for SOCs interconnects processed by electrophoretic deposition: electrical characterization

The integration of renewable energy (RE) sources in the power system is a key strategy to reduce fossil fuel consumption and promote the transition towards a sustainable energy system. To this purpose, hydrogen as energy vector will be a key instrument for meeting the EU Green Deal objectives. In this context, solid oxide cells (SOCs) are promising electrochemical devices for the conversion of chemical energy into electrical energy and vice versa. Solid oxide cells devices are of great interest for efficient and clean power generation and for highly efficient conversion of electricity to hydrogen, fuels and chemicals using high temperature electrolysis. High-performance advanced ceramic materials are at the core of the technological innovations needed to reach a sustainable and climate-neutral economy and society. Due to working temperature between 650-850 °C, ferritic stainless steels are the most widely used for interconnects (IC) in SOC technology. At high oxygen partial pressures and in the presence of steam, volatile chromium species form, CrO₂(OH)₂ , thus poisoning the electrochemical activity with detrimental consequences on the SOC performance. Manganese-cobalt based spinels are the widest commercial solution, used as a protective coating due to their thermomechanical compatibility with IC and high electric conductivity. These ceramic-based coatings are used to protect metallic IC from oxidation and to limit chromium release and poisoning of the oxygen electrode; both effects can compromise the durability and the efficiency of the SOC stack. The doping of MnCo spinels with transition metals (Fe and Cu) has been previously evaluated as an interesting approach to obtain modification of electric and thermomechanical properties of the manganese cobaltite pure spinels. This work focuses on iron and copper doped MnCo based coatings, deposited by electrophoretic deposition method. The steel considered is CROFER 22 APU. Samples of four different compositions were produced by electrophoretic co-deposition of Mn1,5Co1,5O4, Fe2O3 and CuO in different amounts. As deposited samples were sintered in reductive and oxidizing atmosphere in a two-step heat treatment, obtaining a dense and compact coating. After each step XRD and SEM analyzes were carried out, in order to verify properties of each deposition. Oxidation kinetics were tested on each coating by thermogravimetric test, at 750°C for 1000 h. The mass gain was monitored over time to assess the oxygen uptake caused by metal oxidation. TEM-SAED characterization was carried out on 5Fe5CuMCO and 10Fe10CuMCO samples at AGH Univesrity of Science and Technology, in Krakow. In parallel, pellets of different compositions of Cu and Fe doped MCO were produced by pressing pre-sintered powders, then sintered again to obtain a dense and uniform material. Pellets were used for dilatometric measurements, to investigate the effect of doping on the thermomechanical properties; doped samples did not exhibit the cubic to tetragonal transition. High Cu concentration leads to a higher CTE, while Fe has the opposite effect. The research findings reported in the MSc thesis demonstrated the feasibility of achieving a mixed spinel structure by a fast and low cost production method. Coatings obtained were dense, stable and had shown compatible thermomechanical properties with CROFER 22 APU. Thus, they were performant in reducing oxygen partial pressure on the scale.