Electrochemical recovery of platinum from spent proton exchange membrane fuel cells using ionic liquid melts

International audience; Proton Exchange Membrane Fuel Cells (PEMFC) are made of a membrane electrode assembly (MEA), constituted by two electrodes separated by a polymer membrane. The electrodes consist of a carbon substrate on which platinum nanoparticles are immobilized with a platinum loading ranging from 0.15 to 4 mg.cm-² [1]. The amount of platinum used in MEA represents 25 % of the MEA's price. With the future emergence of PEMFC, platinum used in MEA should represent a very important industrial market - approximately 20 % of platinum production- for platinum recovery. At the moment, only extractive pyro-hydro-metallurgical processes are known for the recovery of platinum contained in PEMFC [2]. Typically, in these processes, MEA are calcinated to eliminate the carbon substrate and the polymer membrane. Ashes obtained from this calcination are then leached by aqua regia, in order to recover the platinum. Calcination step leads to a high consumption of energy and induces the use of a fume treatment system. Moreover, the use of aqua regia, a highly corrosive solution, induces NOx emission and tight working conditions. It is then very important to find a new treatment pathway which can be more friendly environmental.The aim of this work is to develop an electrochemical route for platinum recovery from MEA using ionic liquids (ILs). ILs are interesting solvents for hydrometallurgical treatments purposes due to their low vapor pressure and their chemical stability leading to safer/greener processes. As the platinum of MEA's electrodes is already immobilized on carbon substrates, the leaching of platinum can be achieved by electrochemical dissolution. The leached platinum can then be recovered by electrodeposition. For this purpose, ILs are particularly promising due to their wide electrochemical window allowing the electrodissolution and the electrodeposition of numerous metals [3].Our results show that simultaneous electroleaching and electrodeposition of platinum is achievable in ionic liquids melts containing chloride ions. The leaching rates are comparable to those obtained by chemical dissolution in aqua regia without the use of any oxidant in the ionic liquid medium. The influence of applied potential or current was studied, as well as the influence of temperature and chloride content. The leaching efficiency and rate were evaluated by Atomic Absorption Spectrometry. By modulating the composition of the ionic liquid medium, we show that it is possible to recover platinum by electrodeposition at the counter electrode simultaneously to its electroleaching. This single cell process was then successfully applied to the recovery of platinum from MEA's electrodes, in ambient atmosphere.