Direct Extraction of Platinum Nanoparticles from Fuel Cells with Ionic Liquids: Part 1. Implementation and Optimization of the Process Parameters

Abstract

The cost of proton-exchange membrane fuel cells (PEMFCs) comes mainly from platinum nanoparticles, which are used as a catalyst; it represents 40% of the stack price for large-scale production. It is thus crucial to reduce their cost to produce cheaper devices, which could compete with fossil energy on the industrial market. One way to reach this goal would be to recover the Pt catalyst from the membrane electrode assembly (MEA) for further recycling. For now, current end-of-life (EoL) technologies are mainly based on hydrometallurgical and pyro-hydrometallurgical processes for recovering platinum, which are identified to be energy-demanding and generate high amounts of toxic liquids and gaseous effluents. To meet sustainability and circular economy criteria in the recycling of noble metals, our approach was based on the use of ionic liquids (ILs) to both extract and stabilize platinum in the form of metallic nanoparticles (Pt NPs), thus avoiding the use of strong acids and the emissions of hydrofluoric acid (HF), which make the waste management of conventional processes complicated. Thirteen different ILs were selected to investigate how their structural composition as well as their physicochemical properties may affect the extent of Pt extraction, and their ability to stabilize detached nanoparticles. This screening study showed that ionic liquids could interact with all of the elements of the active layer and allowed us to delineate the key parameters that ILs should possess to achieve the best extraction performance: hydrophilicity, hydrogen bonding ability, and the coordinating ability of the anions. The best result was obtained with trihexyltetradecylphosphonium chloride (P66614Cl, commercial Cyphos IL 101; 120 °C and 6 h), which not only led to an extraction extent up to >90% of the Pt present initially on the catalytic layer but also allowed in a single step to detach the Pt NPs from the carbon support. The metallic Pt NPs suspended in P66614Cl were found stable with diameters of around 2–3 nm, as evidenced by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analyses. Compared to conventional processes, this safer and convenient route to recover Pt catalysts from MEAs directly in their metallic form by simple immersion of the electrode in the appropriate IL opens up new perspectives in terms of rare earth metal recycling from material composites.