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Hydrogen is one of the most promising energy carriers. Currently, the most used technology to produce electricity from hydrogen, particularly for green mobility, is the technology based on the use of Proton Exchange Membrane Fuel Cells (PEMFC). Platinum, pure or alloyed with another element, remains essential in the design of efficient catalysts either for anode or cathode catalytic layer in PEMFC. This metal is rare and expensive hence recycling it becomes an important issue for the hydrogen sector development. Chemical ways to recover platinum from PEMFC have been already studied 1-2 based on Pt leaching using either a mix of HCl/HNO3 or HCl/H2O2. In our project, we investigate an electrochemical process to leach platinum contained in aged Pt based catalysts to further resynthesize catalysts from the recovered Pt salts. The electrodissolution of platinum becomes significant with the oxidation of the metal (around 1.1 V vs RHE) but particularly when the oxidized platinum is reduced (around 0.7 V vs RHE) 3. Moreover, the dissolution is enhanced when a complexing agent as chloride anion is introduced 4. The idea of our process is to study the impact of different protocol of steps between high potential (to oxidize platinum) and low potential (to reduce platinum oxide). The improvement of the process is linked to the upper and lower potential values, the number of steps and the nature of the electrolyte and the complexing agent. Our main parameter to monitor the dissolution is the changes in electrochemical real surface area (ECSA) associated to transmission electron microscopy (TEM) images of the catalyst at different stages of the protocol as illustrated in Figure 1. In terms of methodology a model study on fresh commercial Pt/C catalyst has been first performed to determinate the optimal conditions of Pt dissolution followed by the application of the protocol on membrane electrode assemblies (MEA) aged in real conditions. 1 L. Duclos, L. Svecova, V. Laforest, G. Mandil, et P.-X. Thivel, « Process development and optimization for platinum recovery from PEM fuel cell catalyst », Hydrometallurgy, vol. 160, p. 79-89, mars 2016, doi: 10.1016/j.hydromet.2015.12.013. 2 L. Duclos, M. Lupsea, G. Mandil, L. Svecova, P.-X. Thivel, et V. Laforest, « Environmental assessment of proton exchange membrane fuel cell platinum catalyst recycling », J. Clean. Prod., vol. 142, p. 2618-2628, janv. 2017, doi: 10.1016/j.jclepro.2016.10.197. 3 S. Cherevko, A. R. Zeradjanin, G. P. Keeley, et K. J. J. Mayrhofer, « A Comparative Study on Gold and Platinum Dissolution in Acidic and Alkaline Media », J. Electrochem. Soc., vol. 161, n° 12, p. H822-H830, 2014, doi: 10.1149/2.0881412jes. 4 S. Geiger, S. Cherevko, et K. J. J. Mayrhofer, « Dissolution of Platinum in Presence of Chloride Traces », Electrochimica Acta, vol. 179, p. 24-31, oct. 2015, doi: 10.1016/j.electacta.2015.03.059. Figure 1

Rechargeable Li-ion batteries have been developed and marketed over the past 30 years and are used today in different fields like electric vehicles, portable electronics, etc. In a few years, the development of this technology has been exponential, and it has quickly imposed on the energy storage market. However, it is important to note that this technology is very dependent on metals (Co, Ni, Cu, Al, Li ...). These metals are rare, expensive and non-renewable. In addition, they are unevenly distributed in the earth's crust and their production is often very polluting. Thus, the massive use of this technology is creating an imbalance between the needs for metals and the existing primary resources, leading irrevocably to environmental and economic problems.Recycling this waste is therefore essential to both limit the overexploitation of primary metal resources and reduce their ecological impact. In terms of batteries recycling, two main families of processes can be distinguished. The first, known as pyrometallurgy, is based on the high-temperature treatment of waste. In contrast, hydrometallurgical processes are processes with significantly lower energy costs, as these processes are carried out at near ambient temperatures. The principle is to dissolve metals in their ionic form. Toxic acids (and their mixtures) are very often used, but other methods are possible (bases, complexing agents, oxidants). Once in solution, the metals can be recovered in the form of salts, hydroxides or in their metallic form by conventional chemical processes (precipitation, crystallization, etc.) or electrochemical processes (electro-deposition). Several separation / purification steps are sometimes necessary. Relatively few studies have looked at closed-loop recycling, i.e. the resynthesis and reuse of recycled materials in batteries. Thus, the objective of this project is to test the feasibility of closed-loop recycling of cathode materials from Li-ion batteries based on the use of innovative and less toxic recycling routes, and to verify the electrochemical performance of recycled materials at the end of the recycling process. To build this green cycle, the present study has been carried out using both model cathode materials (LCO, NMC) and real cathodes from spent drone batteries, that have been discharged, opened and dismantled in a glove box. All materials have, of course, been characterised (X-ray diffraction, SEM) and the present metals content has been quantified (wet digestion followed by AAS analyses) in order to be able to establish reliable material balances at the end of the recycling process. The developed closed-loop recycling process was composed of three subsequent steps: 1) The cathode materials recovered have been leached in a deep eutectic solvent medium (DES) composed of ethylene glycol – choline chloride mixture, replacing the concentrated acids traditionally used. DES is a new class of green solvents, that is, non-harmful to human health and the environment compared to conventional organic solvents. They consist of a mixture of products (solids) which, at a given ratio, become liquid (eutectic), and which have the combined properties of the different constituents of the mixture. 2) Synthesis of recycling active material from the DES media by precipitation and calcination. 3) Assembly of button cells, followed by electrochemical tests and batteries post-mortem analyses (X-ray diffraction, SEM) have been carried-out at the last step. Composite electrodes made of recycled active materials, polymer binder and carbon have been formulated and assembled into button cells with a Li counter-electrode and a conventional separator (e.g. Celgard impregnated with liquid electrolyte). The prepared batteries were then recycled by electrochemical methods coupled with impedance measurements. The aging of the cells was carried out through one hundred charge / discharge cycles and a specified «fast charging» test developed in our laboratory, allowing to calculate the lithium diffusion coefficient in the cathode material. The results have shown that the recycling of metals has been successfully carried out using a deep eutectic solvent (DES), in a closed-loop manner. The leaching protocol have been optimized at the optimum conditions: ChCl: EG (1:2) with 0.8 mol.L-1 of HCl, with a ratio(wt.) S/L of 1/50, heated to 87.5 °C, for 2 h. After being completely dissolved, the metals have been then recovered from the DES by precipitation, calcined and reused as active materials. Using this method, the DES solvent has also been recovered and reused in a closed-loop without suffering any damage. This work has also shown that the recovered battery materials can be reused into the battery manufacturing cycle without changing their electrochemical performance. Indeed, this process allowed to obtain a homogeneous powder with a morphology similar to the initial materials (particle size -1). Figure 1