Effective recovery strategies for critical and hazardous materials from end-of-life solid oxide cells
SAFFIRIO S. 1, FIORE S. 2, SANTARELLI M. 3, SCHIAVI I. 4, PYLYPKO S. 5, STROPNIK R. 6, MORI M. 6, LOTRIC A. 6, SMEACETTO F. 1, FIORILLI S. 1
1 Politecnico di Torino - Department of Applied Science and Technology, Turin, Italy; 2 Politecnico di Torino - Department of Environment, Land, and Infrastructure Engineering, Turin, Italy; 3 Politecnico di Torino - Department of Energy, Turin, Italy; 4 Environment Park S.p.A., Turin, Italy; 5 Elcogen AS, Tallin, Estonia; 6 University of Ljubljana, Ljubljana, Slovenia
Solid oxide cells (SOCs) are a group of electrochemical devices able to operate reversibly for clean and highly efficient power generation, but also for renewable energy conversion to hydrogen, fuels and various valuable chemicals. However, the development of economically viable recycling strategies for the recovery and re-utilization of raw materials is required to achieve their full deployment.
Within this frame, the BEST4Hy project 1 aims at the development and validation of efficient and scalable recovery pathways for critical raw and hazardous materials from end-of-life (EoL) SOCs. These include La and Co from the LSC (La0.6Sr0.4CoO3) oxygen electrode, and Ni from the Ni-YSZ (Yttria-stabilized Zirconia) fuel electrode of EoL cells dismantled from stacks. In view of re-manufacturing, a considerable effort is additionally addressed to the recovery of YSZ, accounting for around 40% wt of a cell and thus owing a high related economic value.
For this purpose, the present study specifically focuses on the optimization of a simple multi-step process based on existing technologies and aimed at the recovery of YSZ and Ni from EoL Ni-YSZ composite electrodes. To achieve so, the Ni-YSZ composite material was mechanically separated from the LSC electrode, milled and subsequently hydrothermally treated for further disaggregation. HNO3-assisted leaching was then performed on the disaggregated powders at 80°C, already enabling the recovery of YSZ at this stage. The leached Ni was re-precipitated in the form of an oxalate compound and calcinated to obtain NiO, as a precursor for electrode re-manufacturing.
An extensive investigation on the optimal operating parameters was performed for each step of the process. As a result, both YSZ and NiO recovered powders targeted the reference criteria concerning particle size distribution (PSD), specific surface area (SSA) and chemical purity for re-manufacturing. Noticeably, the crystalline phase of YSZ was fully retained after its exposition to HNO3, yielding a 92% wt recovery of the initial amount contained in the Ni-YSZ electrode. Furthermore, the full recovery of the leached metallic Ni and its conversion to high-purity NiO were proved.
Each step was optimized also considering energy and mass balances, amount of used reagents, and liquid and solid waste flows. A life-cycle assessment (LCA) of the overall recovery process was carried out accordingly to determine the environmental impact of re-manufacturing in comparison with the production of virgin YSZ and NiO.
Further optimization and upscaling of the developed process are currently ongoing at a higher technological level to improve its overall efficacy and validate it on a pilot scale.
1 This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 101007216. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe research.
