Coupling Interfacial Redox-Reactions with In Situ Proton Generation for the Photoelectrochemical Separation of Rare-Earth Elements

Abstract

A dual-function photoelectrochemical (PEC) separation system is demonstrated for rare-earth element (REE) recovery. The sustainable release of the captured REEs is promoted through the synergistic integration of a redox-reaction for electrostatic repulsion, and in situ proton generation for ion-exchange, all driven by photoelectrochemistry. The platform consists of a redox-copolymer, poly(ferrocenylpropyl methacrylamide-co-methacrylic acid) (P(FPMAm-co-MAA)) (PFM), conjugated with carbon nanotubes (CNTs) and coated onto titanium dioxide nanorods (TNRs). The (PFM-CNT)/TNR spontaneously adsorbs up to 214.2 mg of Yttrium/g PFM by ion-exchange, and demonstrates broad applicability for other REEs. The adsorbed REEs are released through the PEC oxidation of ferrocene (Fc) to ferrocenium (Fc+), and the simultaneous PEC water splitting reactions at the TNRs that protonate the carboxylate binding groups. This dual photoelectrochemically-driven mechanism for REE release is investigated by in situ pH measurements, as well as vibrational and X-ray photoelectron spectroscopy. Through PEC approaches, a 68.8% reduction in energy consumption during REE recovery has been achieved compared to purely electrochemical systems, with a regeneration efficiency close to 100%. For NdFeB magnets from waste hard disk drives, Nd and Dy recovery efficiencies of 59.2 and 61.1% are achieved. The dual-functionality of these copolymer PEC systems offers a sustainable platform for modulating critical element recovery.