Redox Polyelectrolytes with pH-Sensitive Electroactive Functionality in Aqueous Media

Abstract

A framework of ferrocene-containing polymers bearing adjustable pH-and redox-active properties in aqueous electrolyte environments was developed. The electroactive metallopolymers were designed to possess enhanced hydrophilicity compared to the vinylferrocene (VFc) homopolymer, poly-(vinylferrocene) (PVFc), by virtue of the comonomer incorporated into the macromolecule, and could also be prepared as conductive nanoporous carbon nanotube (CNT) composites that offered a variety of different redox potentials spanning a ca. 300 mV range. The presence of charged non-redox-active moieties such as methacrylate (MA) in the polymeric structure endowed it with acid dissociation properties that interacted synergistically with the redox activity of the ferrocene moieties to impart pH-dependent electrochemical behavior to the overall polymer, which was subsequently studied and compared to several Nernstian relationships in both homogeneous and heterogeneous configurations. This zwitterionic characteristic was leveraged for the enhanced electrochemical separation of several transition metal oxyanions using a P(VFc 0.63 -co-MA 0.37 )-CNT polyelectrolyte electrode, which yielded an almost twofold preference for chromium as hydrogen chromate versus its chromate form, and also exemplified the electrochemically mediated and innately reversible nature of the separation process through the capture and release of vanadium oxyanions. These investigations into pH-sensitive redox-active materials provide insight for future developments in stimuliresponsive molecular recognition, with extendibility to areas such as electrochemical sensing and selective separation for water purification.

Hydrophobicity Tuned Polymeric Redox Materials with Solution-Specific Electroactive Properties for Selective Electrochemical Metal Ion Recovery in Aqueous Environments

“…The P­(VFc-co-X) ferrocene polymers used in this work were prepared using free-radical polymerization, with synthesis procedures developed loosely based on previously reported methods. ,,− The poly­(hydroquinone) (PHQ) and nickel hexacyanoferrate (NiHCF) materials were prepared via previously reported methods.…”

“…Electrodes containing the redox-active polymers were fabricated via a solvent-based film deposition technique. Conductive electrodes were created by sonicating the redox-active species together with carbon nanotubes (CNTs) , or carbon black (CB), , and drop-casting the dispersion onto a carbon substrate. The amount of redox-active material present on the electrodes was controlled via the volume of dispersion added during the drop-casting step.…”

“…Chromium(VI) was selected due to its wide presence as a toxic contaminant, 54 and its redox-active affinity with ferrocenium. 9,10,16,49,55 In the case of dichromate, the chromium adsorption capacities were relatively unaffected by the change from perchlorate (65 mg Cr/g VFc) to nitrate (54 mg Cr/g VFc) media (Figure 5a). Even in the presence of a 20-fold excess of competing anions, these values are still ca.…”

“…Ferrocene is an example of a compound whose redox couple can be utilized to electrosorb charged species via electrochemically mediated binding. The ability of ferrocene to form ion pairs with anions has led to its application in functional materials, 9,10 and areas such as electrochemical sensing and catalysis. 11,12 Incorporating ferrocene into a conductive framework enhances its redox activity, thus enabling its use in pseudocapacitive processes such as charge storage, 13 as well as anion electrosorption.…”

“…Ferrocene is an example of a compound whose redox couple can be utilized to electrosorb charged species via electrochemically mediated binding. The ability of ferrocene to form ion pairs with anions has led to its application in functional materials, , and areas such as electrochemical sensing and catalysis. , Incorporating ferrocene into a conductive framework enhances its redox activity, thus enabling its use in pseudocapacitive processes such as charge storage, as well as anion electrosorption. − Heterogeneously secured ferrocene materials for electrochemical processes have been produced by tethering ferrocene units at the end of hydrophobic chains to form self-assembled monolayers (SAMs), − as well as by preparing them in polymeric forms for non-covalent attachment to electrode surfaces. − Macromolecules provide a convenient framework for including multiple ferrocenes within a single structure to engender the facile fabrication of electrodes with high active material loading, and can be generated with main chain or side chain moieties …”