Experimental optimization of hydrothermal treatment and nitric acid leaching process for the recovery of YSZ from end-of-life Solid Oxide Fuel Cells

Energy transition is one of the fundamental steps needed to fight climate change and to lead towards sustainable technology able to fulfill the energy requirements of the present while preserving the planet. The commitments to limit the increase of temperature on Earth below 1.5°C and to reach net-zero CO2 emissions by 2050 move the energy sector to a renovation, in which renewable energy sources will be pivotal. In order to pursue sustainable paths, the application of the concept of circular economy is fundamental also in the energy sector; efforts to recover critical materials used in RES systems will be important for a life cycle sustainability. In this context, hydrogen technology can play a key role being an efficient and flexible energy vector, thanks to energy conversion in electrochemical devices such as solid oxide cells. This MSC thesis is carried in the context of the Best4Hy project (SustainaBlE SoluTions FOR recycling of end of life Hydrogen technologies), which focuses on the development and validation of existing and novel recycling processes for two key FCH products: PEMFC and SOC. One of the main objectives is the identification of EoL SOC cell materials and components to be recycled and re-used to process new cells, including minimum 30% of recycled critical raw materials, with a reduction of the overall cost of the FC stack. This thesis is focused on the optimization of a recovery process for SOC anode materials, based on Yttria Stabilized Zirconia and Nickel; the possibility of upscaling the process in a pilot plant is also investigated. The optimization extends some previous results obtained with two separated treatments of anode material’s powders, a hydrothermal treatment performed at 200°C for 4 hours to disintegrate the zirconia and an acid leaching at 80°C for 2 hours in a 2.2M HNO_3 solution to selectively extract nickel, and aims to reduce the energy needs while maintaining a sufficient effectiveness of the recovered material. To simplify the process, by reducing time and consumptions, the possibility of merging the two treatments was tested in laboratory. Encouraging results were obtained with the two separated treatments, the nickel extraction was successfully achieved, and the hydrothermal treatment has proven to be effective, leading to a specific surface area of 16 m^2 g^(-1) in the reference range of 12 – 20 m^2 g^(-1). The selective extraction of Nickel was less significant in the joined process as compared to the separated treatments, presumably due to the use of a non-stirred reactor. Most probably, the stirring is fundamental for the leaching and its effect should be investigated deeper in the pilot plant. The YSZ recovered by the lab-scale recycling process has been fully characterized to assess the target of the acceptance criteria required for their re-use in the manufacturing process adopted at industrial level, for the EoL cells. To demonstrate the feasibility of the process in industrial scale, an upscaling from the early-stage laboratory scale is necessary. In this work the first steps to determine the critical issues of this upscaling are described, and the different levels of a possible modelling were investigated to provide a preliminary starting point for the future development of a reactor model that can be useful to support the experimental testing. Further analysis on the pilot plant will be required to reduce the energy needs, and less concentrated solutions or the use of weaker acids should be investigated.