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

Abstract

Adaptable redox-active materials hold great potential for electrochemically mediated separation processes via targeted molecular recognition and reduced energy requirements. This work presents molecularly tunable vinylferrocene metallopolymers (P-(VFc-co-X)) with modifiable operating potentials, charge storage capacities, capacity retentions, and analyte affinities in various electrolyte environments based on the hydrophobicity of X. The styrene (St) co-monomer impedes hydrophobic anions from ferrocene access, providing P(VFc-co-St) with specific response capabilities for and greatly improved cyclabilities in hydrophilic anions. This adjustable electrochemical stability enables preferential chromium and rhenium oxyanion separation from both hydrophobic and hydrophilic electrolytes that significantly surpasses capacitive removal by an order of magnitude, with a robust perrhenate uptake capacity of 329 mg/g VFc competitive with established metal−organic framework physisorbents and 17-fold selectivity over 20-fold excess nitrate. Pairing P(VFc-co-X) with other solution-specific electroactive macromolecules such as the pH-dependent poly(hydroquinone) (PHQ) and the cesium-selective nickel hexacyanoferrate (NiHCF) generates dual-functionalized electrosorption cells. P(VFc-co-X)//PHQ offers optimizable energetics based on X and pH for a substantial 4.6-fold reduction from 0.21 to 0.04 kWh/mol rhenium in acidic versus near-neutral media, and P(VFc-co-St)//NiHCF facilitates simultaneous extraction of rhenium, chromium, and cesium ions. Proof-of-concept reversible perrhenate separation in flow further highlights such frameworks as promising approaches for next-generation water purification technologies.

Electroactive Behavior of Adjustable Vinylferrocene Copolymers in Electrolyte Media

“…The effects of different comonomers and their quantities on the pseudocapacitances of the ferrocene copolymers suggest that the hydrophobicity of the comonomer impacts the accessibility of the VFc units to aqueous media and their subsequent oxidation. Correspondingly, it has previously been reported that the presence of a hydrophobic comonomer in vinylferrocene copolymers reduces their overall pseudocapacitance . This hypothesis was also supported by the roughly equivalent size properties of the three comonomers (Table ), as well as the comparable homogeneous cyclic voltammogram responses (Figure a) and calculated diffusion coefficients (Figure b and Table S1) of the resulting copolymers in organic media, all of which imply that the aforementioned dissimilarities in heterogeneous electrochemical behavior between the polymers are not a result of molecular size or electronic properties.…”

“…The P­(VFc n - co -X 1– n ) ferrocene polymers were prepared through free-radical polymerization of vinylferrocene (VFc), either with or without a comonomer using a previously reported synthesis procedure, developed based on previously reported methods. ,,,− The selected comonomer, X, was either styrene (St), methyl methacrylate (MMA), or 2-hydroxyethyl methacrylate (HEMA). Briefly, the polymers were synthesized in anhydrous 1,4-dioxane at 70 °C using 2,2′-azobis­(2-methylpropionitrile) as the radical initiator at ca.…”

“…Ferrocene polymer–carbon nanotube composites (P­(VFc n - co -X 1– n )–CNT) were consistently prepared using a previously reported noncovalent functionalization technique. ,,,, P­(VFc n - co -X 1– n ) and CNT were added to either chloroform or N , N -dimethylformamide depending on the polymer and sonicated for at least 3 h (under ice if chloroform was used), with the mixture composition held constant at 4 mg of vinylferrocene units and 4 mg of CNT per 1 mL solvent. 50 μL of the resulting dispersion was drop-cast onto Teflon-coated conductive carbon paper and spread to form an area of ca.…”

“…Repeated runs were conducted using electrodes produced from separately prepared dispersions in order to account for sample variability. The VFc/CNT mass ratio of 1:1 used in this work was selected based on its successful application in other studies for electrochemical anion separation, ,,,,,, as well as the results of a previous investigation which revealed that a PVFc/CNT mass ratio that is neither too high nor too low results in an optimized PVFc–CNT composite morphology comprising a porous and conductive three-dimensional CNT network coated conformally with PVFc …”

“…With its facile and reversible one-electron oxidation to ferrocenium, the ferrocene redox moiety has found great utility in electrochemical studies and processes as both a standalone electroactive probe and the active component in stimuli-responsive materials . The wide use of ferrocenyl compounds in areas such as medicinal chemistry, cancer research, luminescent systems, sensors, catalysis, energy storage, , and separations, − has naturally engendered a plethora of molecular derivatives and polymeric forms. In terms of the latter, ferrocene macromolecules have been formulated to contain the redox-active groups within their main chain or as pendant units, , with examples for each being the ring-opening polymerization of strained ferrocenophanes to yield poly­(ferrocenylsilanes) and the homopolymerization of vinylferrocene to obtain poly­(vinylferrocene), respectively …”