A Comparison of Cobalt and Platinum Extraction in Hydrophobic and Hydrophilic Ionic Liquids: Implication for Proton Exchange Membrane Fuel Cell Recycling

Starting from a real Proton Exchange Membrane Fuel Cell (PEMFC) leachate, separation of the strategic metals platinum(IV) and cobalt(II) was carried out using two distinct strategies applying ionic liquids (ILs), either via solvent extraction (SX) or acidic aqueous biphasic system (AcABS). The versatility of ILs and their ability to provide original solutions to common issues in the field of used device recycling is illustrated, with both approaches critically compared. The highly hydrophobic tetradecylpyridinium bis(trifluoromethanesulfonyl)imide ([C-14 pyr][NTf2]) was synthesized and applied to the separation of Pt/Co. This solvent was able to selectively extract over 98% of Pt(IV) from the PEMFC leachate in one step, while leaving Co(II) in the aqueous phase, and exhibited the highest reported partition of Pt(IV) in a [NTf2]-based IL. In a second approach, the AcABS, based on the fully water-miscible tributyltetradecylphosphonium chloride ([P-44414]Cl), HCl, and water, is compatible with the concentrated nature of the leachate and extracts both platinum(IV) and cobalt(II) quantitatively. In a second stage, a selective precipitation step enabled the recovery of platinum(IV) in the form of an organometallic complex while leaving the coextracted cobalt(II) in solution. This work represents the first utilization of an AcABS for the recycling of a real waste technological object. These two different types of ILs in both cases resulted in an efficient separation of Pt(IV) from Co(II), using two very different pathways. The merit and transferability of each approach is critically compared, and suggestions are made as to the suitable condition range for each technique.