Platinum group metals (PGMs) are of vital importance to various industries such as manufacturing, transportation, and energy economy. To secure the supply chain of PGMs, it is important to selectively extract target species from the mining ores and recycle spent automobile catalytic converters or e-waste. However, PGMs are challenging species to separate due to very similar physical and chemical properties and low concentration and complex speciation depending on pH or type of electrolyte used.1 Electrochemical separation is environmentally benign with good modularity and scalability. The development of next-generation electrodes with selective interactions for metal-binding and recovery is critical for innovating sustainable mining processes.Redox electrodes were also shown to be efficient in controlling the operational voltage windows in aqueous-phase electrochemical systems, by increasing the Faradaic efficiency towards ion-binding and suppressing side reactions that would otherwise increase energetic costs. At the same time, polymer-functionalized electrodes have been shown to facilitate selective electrodeposition and regeneration of metals, thus providing an alternative approach for energy-efficient metal recovery. Redox-active polymers containing ferrocene have shown high selectivity towards anionic species such as metal oxyanions of arsenic and chromium.2 By modulating functional groups of redox polymers, charge-transfer behavior can be changed, and it can also provide new binding sites for target ions. We investigated a series of ferrocene-containing redox polymers to understand the kinetic and equilibrium uptake of PGM chloroanions. Different functional groups such as amide, alkyl, amine, and ester, respectively can modulate different redox potential within metallopolymers due to electron donating or electron withdrawing effects. The effect of these homopolymers was evaluated, with high uptake and regeneration being achieved for a range of PGMs. Going further than individual monomers, PGM-specific ligands were designed and incorporated into redox polymer by copolymerization. By leveraging heteroatom ligands as metal-specific functionality, the selectivity of the redox-copolymers in different pairs of PGM binary mixtures was tuned beyond the original homopolymers. The effect of ligand binding strength, ratio between redox group to ligand, and releasing solution on selectivity and uptake was investigated in detail.(1) Kim, K.; Candeago, R.; Rim, G.; Raymond, D.; Park, A. H. A.; Su, X. Electrochemical approaches for selective recovery of critical elements in hydrometallurgical processes of complex feedstocks. iScience 2021, 24 (5), 31, Review. DOI: 10.1016/j.isci.2021.102374.(2) Su, X.; Kushima, A.; Halliday, C.; Zhou, J.; Li, J.; Hatton, T. A. Electrochemically-mediated selective capture of heavy metal chromium and arsenic oxyanions from water. Nat. Commun. 2018, 9 (1), 4701. DOI: 10.1038/s41467-018-07159-0.
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