Structural and Potential-Dependent Metal Anion Selectivity of Redox-Metallopolymer Electrosorbents

Selective electrochemically-mediated technologies for metal separations can be essential for materials recycling and environmental sustainability. Metal recovery not only requires selectivity of the target metal over the competing electrolyte, but also selectivity between metal ions of similar size and structure in order to sort metal ions for reuse and recycling. Ferrocene polymers have been shown to be highly selective for heavy metal oxyanions through charge transfer interactions.¹ Using these polymers as electrosorbents provides the opportunity for highly tuned selectivity for different metal anions through modification of the polymer structure. We demonstrate a change in structure between a pendant group ferrocene polymer, polyvinylferrocene (PVF), and a main-chain ferrocene, polyferrocenylsilane (PFS), results in differential selectivity toward metal oxyanions.² For example, almost an order of magnitude increase in separation factor for meta-vanadate over chromate (α_VO3-,CrO42-) can be achieved by altering the polymer structure from PVF (α_VO3-,CrO42- = 5.2) to PFS (α_VO3-,CrO42- = 39) at 0.8 V vs Ag/AgCl. Through quantum chemical calculations, we elucidate how a trimethyl silane group attached to ferrocene affects charge transfer interactions between metal anions and ferrocene binding sites. In addition, we report that these ferrocene polymers exhibit potential dependent selectivity, and in some cases can even swap preference based on the applied potential, providing an additional process variable to tune selectivity that is orthogonal to structural modifications. The underlying cause of this potential dependent selectivity is undergoing further investigation. This work demonstrates the ability to tailor ferrocene polymer electrosorbents for the targeting of specific metal ions, and hints at potential applications in multicomponent separations, including metal valorization and environmental remediation.

1. 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. Nature Communications 2018, 9 (1), 4701.

2. Chen, R.; Feng, J.; Jeon, J.; Sheehan, T.; Rüttiger, C.; Gallei, M.; Shukla, D.; Su, X., Structure and Potential-Dependent Selectivity in Redox-Metallopolymers: Electrochemically Mediated Multicomponent Metal Separations. Advanced Functional Materials 2021, n/a (n/a), 2009307.