Design and optimisation of cobalt-free protective coatings for SOCs applications

To mitigate the polluting emissions from the energy sector, a shift towards renewable energy sources is imperative. However, the fluctuating nature of renewable energy production necessitates efficient and sustainable energy storage solutions for grid management. Reversible Solid Oxide Cells (SOCs) offer a promising solution, serving as energy conversion devices with high efficiency and minimal environmental impact. During operation, typically in the range of 650°C to 850°C, ferritic stainless-steel materials (FSS) commonly used as interconnects release volatile chromium species when the protective chromia scale layer is exposed to oxygen or water, which can migrate toward the cathode, leading to its poisoning. This work focuses on developing and characterizing advanced coating techniques to enhance the performance and longevity of interconnect materials in SOCs. To limit chromium migration and enhance the oxidation resistance, utilizing the electrophoretic deposition (EPD) method, an Mn-Cu based spinel (〖Mn〗_1.7 〖Cu〗_1.3 O_4) coating has been deposited on AISI441, the FSS material used as interconnect. This spinel selection aims to provide an alternative to the conventional manganese and cobalt-based coatings, which pose health and social risks due to the cobalt extraction process. Additionally, attempts have been made to improve coating properties through iron doping. A Design of Experiment (DoE) procedure has been conducted to efficiently study the deposition parameters and determine the optimal settings to achieve the desired coating thickness. After deposition, the sintering process was explored through two distinct approaches: a conventional method and a Rapid Thermal Process (RTP). The conventional approach involved the use of a tubular oven and a chamber furnace to respectively reduce or oxidize the samples, conducting two-step and one-step sintering processes at different temperatures. To analyze the impact of conventional sintering on the spinel, hot stage microscopy (HSM) was employed to simulate the process and examine material shrinkage over varying temperatures and durations. The rapid thermal process technique involves the utilization of radiant heating sources to rapidly heat materials, providing precise control over temperature profiles and rapid heating and cooling cycles. This approach allowed the achievement of acceptable densification levels in a shorter period. Following both sintering processes, the treated samples underwent X-ray diffraction analysis to investigate the phase composition. The microstructural investigation was conducted using scanning electron microscopy to assess coating adhesion to the substrate and the level of densification achieved with the thermal treatment. Additionally,, dilatometry analysis was conducted to evaluate the coefficient of thermal expansion at different temperatures ranges. The electrical properties were tested in terms of area-specific resistance (ASR) in an oxidizing atmosphere at high temperatures. Short-term and long-term tests were conducted, with the former aimed at studying the behaviour of the samples at different temperatures, and the latter focused on examining the impact of oxide scale evolution on electrical performance over time. The long-term tests were performed at 750 °C for approximately 500 hours in a synthetic air atmosphere, revealing low resistivity and good stability over time. A post-ageing microstructural characterization