Prussian Blue Analogues as multivalent cation intercalation electrodes for electrochemical applications

The scarcity and economic importance of metals such as nickel, cobalt and lithium have led the European Union to classify several elements as critical or strategic materials. Europe’s dependence on third countries and the foreseeable decline in their reserves in the coming years have reinforced this classification. These metals are essential components of lithium-ion batteries, which are used in portable devices, electric vehicles, and renewable energy systems, thus becoming a key technology for the global energy transition. However, the massive use of these batteries exerts significant pressure on the supply of critical raw materials. Given the limited availability of resources and the increasing demand, the recycling of spent batteries emerges as a strategic alternative to recover valuable metals and reduce dependence on external suppliers. Due to the environmental impact and other limitations of current recycling methods, such as high consumption of chemical reagents, water, and energy, based on pyrometallurgical and hydrometallurgical processes. The search for more sustainable and efficient technologies has intensified. A novel proposal within these emerging technologies is presented in this thesis, which introduces an alternative to conventional approaches through the use of electrochemical techniques: the electrochemical ion pumping method. This innovative process aims to drastically reduce the consumption of energy, water, and chemical reagents while simultaneously improving the selectivity and efficiency of metal recovery. The implementation of this technology requires materials capable of intercalating cations, in this case focusing on divalent cations Ni²⁺ and Co²⁺, which are present in spent batteries. The materials selected for this purpose belong to the Prussian Blue Analogue (PBA) family, characterized by an open framework structure that allows the accommodation of various metallic ions. In this work, different synthesis routes were developed, varying the parameters that influence the process, such as precursor concentrations, flow rates, and the use of chelating agents. From the different syntheses, several materials were obtained, of which two were selected: one with a low vacancy content and another with a high vacancy content. Comprehensive studies of these samples demonstrated that only the material with a high level of vacancies was capable of reversibly intercalating multivalent cations, whereas the low-vacancy material exhibited competition with protons (H⁺), which limited its effectiveness. The selection of a material capable of intercalating Ni²⁺ and Co²⁺ enabled its subsequent application in the electrochemical ion pumping method. The initial experiments were carried out using a batch system consisting of two separate electrochemical cells (intercalation and recovery), each containing different electrolytes/solutions (derived from spent batteries and recovery media). The results were encouraging: concentrations of 50 mM for Ni²⁺ and 33 mM for Co²⁺ were achieved, with high selectivity over Li⁺ and Mn²⁺, other ions commonly present in spent batteries. Finally, the process was scaled up and automated through the development of a continuous-flow reactor, with a working volume 80 times larger than that of the initial system, approaching industrial conditions. In this new setup, recovery solutions containing 282 ppm of Ni²⁺ and 212 ppm of Co²⁺ were obtained, maintaining low energy consumption (5 mWh per gram of salt) and exceptional operational stability for more than 40 cycles, equivalent to one month of continuous operation. These results confirmed the technical and environmental feasibility of electrochemical ion pumping as an efficient, sustainable, and scalable tool for lithium-ion battery recycling. Consequently, this technology represents a significant advancement toward the circular recovery of critical metals, contributing to the European Union’s sustainability goals and energy security.