Molecularly Selective Polymer Interfaces for Electrochemical Separations

Abstract

The molecular design of polymer interfaces has been key for advancing electrochemical separation processes. Precise control of molecular interactions at electrochemical interfaces has enabled the removal or recovery of charged species with enhanced selectivity, capacity, and stability. In this Perspective, we provide an overview of recent developments in polymer interfaces applied to liquid-phase electrochemical separations, with a focus on their role as electrosorbents as well as membranes in electrodialysis systems. In particular, we delve into both the single-site and macromolecular design of redox polymers and their use in heterogeneous electrochemical separation platforms. We highlight the significance of incorporating both redox-active and non-redox-active moieties to tune binding toward ever more challenging separations, including structurally similar species and even isomers. Furthermore, we discuss recent advances in the development of selective ion-exchange membranes for electrodialysis and the critical need to control the physicochemical properties of the polymer. Finally, we share perspectives on the challenges and opportunities in electrochemical separations, ranging from the need for a comprehensive understanding of binding mechanisms to the continued innovation of electrochemical architectures for polymer electrodes.

Unraveling the Role of Solvation and Ion Valency on Redox-Mediated Electrosorption through In Situ Neutron Reflectometry and Ab Initio Molecular Dynamics

“…Selective separation of metal ions from multicomponent mixtures represents a compelling challenge for critical materials recovery and recycling, as well as for environmental remediation . Ion-electrosorption using redox metallopolymers features low energy consumption, no need for chemical regenerants, modularity, and scalability, making it an ideal candidate for both small and large scale separations; furthermore, adsorption and desorption of target anions is directly controlled by tuning the applied bias in a switch-like mechanism. − Currently, incomplete understanding of adsorption mechanisms taking place at redox-active interfaces constitutes a major bottleneck in rational design of better performing selective redox metallopolymers . In particular, it is not clear how selectivity is influenced by solvation effects, ion valency, and morphological response of the film upon adsorption, at equilibrium and during the adsorption and desorption transients (i.e., in dynamic conditions). ,, In situ characterization of redox polymer behavior becomes a necessary step to advance separations, sensing, and energy technologies .…”

“… 2 − 4 Currently, incomplete understanding of adsorption mechanisms taking place at redox-active interfaces constitutes a major bottleneck in rational design of better performing selective redox metallopolymers. 5 In particular, it is not clear how selectivity is influenced by solvation effects, ion valency, and morphological response of the film upon adsorption, at equilibrium and during the adsorption and desorption transients (i.e., in dynamic conditions). 3 , 6 , 7 In situ characterization of redox polymer behavior becomes a necessary step to advance separations, sensing, and energy technologies.…”

Redox-Responsive Halogen Bonding as a Highly Selective Interaction for Electrochemical Separations

“…Since the modulation of XB strength relies heavily on the presence of a conjugated π system between the redox moiety and XB, for either amplifying or decreasing the sigma hole, ,, we demonstrated XB electrosorption with ferrocene moiety as a redox-active group. Ferrocene exhibits notable physical and electrochemical stability, along with low E 1/2 potential (∼0.15–0.25 V Ag/Ag + ), , offering significant advantages for incorporating it as a redox moiety in our XB molecule. From a synthesis perspective, a wide range of commercially available ferrocene-based building blocks makes it more convenient to synthesize ferrocene molecules with various conjugates .…”

“…Ferrocene exhibits notable physical and electrochemical stability, along with low E 1/2 potential (∼0.15–0.25 V Ag/Ag + ), , offering significant advantages for incorporating it as a redox moiety in our XB molecule. From a synthesis perspective, a wide range of commercially available ferrocene-based building blocks makes it more convenient to synthesize ferrocene molecules with various conjugates . Among halogen atoms, iodine can promote a strong electron-depleting region (σ-hole) on the elongation of the covalent bond (C–I) due to its high polarizability. , In addition, our unique halogen bonding donor molecule features a polymerizable styrene moiety.…”

“…The design of molecularly selective interactions with strong binding and reversibility is of paramount importance for electrochemically mediated separations in a heterogeneous platform, − particularly for the sustainable recovery of value-added compounds in nonaqueous media. − Redox-active platforms have been powerful for the selective recovery of value-added species; ,− however, their applications have primarily been restricted to ion sorption applications in the aqueous phase, where selective electrosorption has mostly relied on electrostatic interactions between the redox-active motif and ions. ,− With the growing industries of pharmaceutical or chemical synthesis, ,, there is an increasing need for the exploration, development, and implementation of tunable intermolecular interactions to expand the scope of selective electrochemical separation to nonaqueous media. Here, we design a redox-responsive halogen bonding polymer with a robust and selective noncovalent interaction for electrochemical separations in nonaqueous media.…”