Auto-oxidation of redox electrodes for the selective recovery of platinum group metals

Abstract

The recovery and purification of platinum group metals (PGMs) from multicomponent solutions are essential for attaining a sustainable circular economy. Herein, we designed redox-active electrosorbents for the separation of PGM chloroanions by leveraging the auto-oxidation of redox electrodes. We synthesized a range of redox metallopolymers with tunable redox potentials and demonstrate their molecular selectivity in multicomponent PGM mixtures. Iridium and platinum chloroanions were shown to be capable of simultaneous auto-oxidation and binding to the redox polymers spontaneously. Thus, owing to the intrinsically high oxidation potential of the chloro-PGM complexes in leachate solutions, spontaneous electrochemical PGM recovery was possible without electrical or chemical input. As opposed to standard electrosorption, the energy consumption for iridium recovery is decreased by 75%. A combination of X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible (UV-vis) spectroscopy was used to track the auto-oxidation process of the redox center and iridium chloroanion. The redox potential of ferrocene polymers was found to affect the selectivity towards PGM ions, with a high molecular selectivity of over 100 achieved between Pt and Rh. Using a porous-coated redox-polymer electrosorbent, over 186 mmol Pd uptake per mole of ferrocene was achieved in the recovery of palladium from a catalytic converter leach solution. This work demonstrates an energy-efficient, process-intensified electrochemical platform for the multicomponent recovery of PGMs from waste feedstocks.