Abstract

Solid oxide electrolyzer cells (SOECs) are a promising technology for production of green hydrogen because of their high efficiency and great potential for integration with industrial processes, that enables the use of waste heat and steam. However, the lifetime of the SOECs is currently only about five years, while many critical elements (e.g., rare earth elements (REEs), Ni, Co) are used in the cells. Therefore, a recycling process is mandatory for scaling up the technology. Most critical elements are located in the membrane electrode assembly (MEA), the active part of the cell. Since the investigated MEAs are electrolyte supported, meaning that the thin electrolyte (approx. 100 µm) provides the mechanical support to the MEA, they are very brittle, which prevents mechanical separation steps reported in the literature for other cell designs. Therefore, a direct hydrometallurgical approach is pursued in this work. The MEA can be crushed followed by a leaching process with 4.5 M HCl at 80 °C previously published by our research group, resulting in a leaching solution mainly composed out of Ce, Co, Fe, Gd, La, Mn, Ni, and Sr, while the solid electrolyte consisting of Sc-doped zirconium oxide remains undissolved. This work deals with an efficient separation process of the critical elements from the leaching solution to prepare pure oxides that can be reused for the fabrication of new cells. The first separation step involves the selective precipitation of Fe at pH 3; however, the poor filterability of the resulting Fe₂O₃ ∙ x H₂O requires the optimization of the precipitation parameters, such as temperature and time. Subsequently, Ce can be oxidized to Ce(IV) by hydrogen peroxide addition and the Ce(IV) is selectively precipitated at a pH 4-5, thereby avoiding the high effort of separating Ce from the other REEs by solvent extraction. This precipitation is sensitive to all experimental conditions, especially pH and temperature, which must be carefully controlled to ensure complete precipitation of Ce without co-precipitation of other elements. The remaining leaching solution can then be divided into two streams by adding Na₂SO₄ which allows the precipitation of Sr as SrSO₄, and Gd as well as La as NaREE(SO₄)₂ ∙ H₂O. To achieve a high precipitation yield of Gd, which is the only heavy REE in the studied system, application of elevated temperatures is required. After this step, Co, Mn, and Ni are the only elements remaining in solution. Both streams are further processed to prepare pure oxides. All separations steps are optimized experimentally, supported by thermodynamic calculations performed with OLI studio 12.5.