Redox-Active Magnetic Composites for Anionic Contaminant Removal from Water

Geometric Transformation of Modified Multiwalled Carbon Nanotubes-Based Heterometallic Nanostructured Material: A Model for the Electrochemical Discrimination of Insecticides

Multifunctional carbon-based materials exhibit a large number of unprecedented active sites via an electron transfer process and act as a desired platform for exploring high-performance electroactive material. Herein, we exemplify the holistic design of a heterometallic nanostructured material (MWCNTs@KR-6/Mn/Sn/Pb) formed by the integration of metals (Mn2+, Sn2+, and Pb2+) and a dipodal ligand (KR-6) at the surface of multiwalled carbon nanotubes (MWCNTs). First, MWCNTs@KR-6 was readily synthesized via a noncovalent approach, which was further sequentially doped by Mn2+, Sn2+, and Pb2+ to give MWCNTs@KR-6/Mn/Sn/Pb. The designed material showed excellent electrochemical activity for the discrimination of insecticides belonging to structurally different classes. In contrast to that of the individual building components, both the stability and electrochemical activity of heterometallic nanostructured material were remarkably enhanced, resulting in a magnificent electrochemical performance of the developed material. Hence, the current work reports a comprehensive synthetic approach for MWCNTs@KR-6/Mn/Sn/Pb synthesis by synergizing unique properties of the heterometallic complex with MWCNTs. This work also offers a new insight into the design of multifunctional carbon-based materials for discrimination of different analytes on the basis of their redox potential.

Electroactive Behavior of Adjustable Vinylferrocene Copolymers in Electrolyte Media

The redox-active properties of a series of ferrocene-containing vinyl polymers were investigated in aqueous and organic media. Each metallopolymer contained vinylferrocene (VFc) and a non-redox-active species (X), and was combined with carbon nanotubes (CNT) to generate P(VFcn-co-X1-n)-CNT composites for heterogeneous electrochemical analysis. Tunable pseudocapacitances spanning ca. 0.03-280 F/g VFc in aqueous solution were achieved by varying the copolymer composition, with P(VFc0.11-co-HEMA0.89) producing standardized values at ca. 160-180 F/g VFc even for differently hydrated anions. Additionally, the polymer-bound ferrocene/ferrocenium redox potential was seen to depend prominently on its electrolyte anion's Gibbs free energy of hydration. Although the hydrophilic chloride anion negatively influenced the electrochemical stability of the VFc units when in their PVFc homopolymer, copolymerizing them with 2-hydroxyethyl methacrylate (HEMA) and introducing perchlorate anions ameliorated their overall capacity retention by 64% and 38%, respectively. Lastly, the electrodes' responses in aprotic and protic solvents were examined for correlations with numerous solvent polarity metrics and solubility measures, with a notable observation being the stability and pseudocapacitive increase of the styrene (St)-containing P(VFc0.27-co-St0.73)-CNT from 5 to ca. 190 F/g VFc when in methanol instead of water. This study can help provide insight regarding material design considerations for redox moiety implementation in electrochemical applications.

Molecularly Selective Polymer Interfaces for Electrochemical Separations

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.

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

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.

Redox Polyelectrolytes with pH-Sensitive Electroactive Functionality in Aqueous Media

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 stimuli-responsive molecular recognition, with extendibility to areas such as electrochemical sensing and selective separation for water purification.

Recyclable structured toxic industrial nickel-containing sludge for efficient anionic contaminant adsorption

Safe, efficient, and simultaneous treatment of toxic industrial sludge and anionic contaminant crisis in one route still remains a persistent global challenge. Herein, we proposed a facile waste-control-waste conceptual design strategy to develop low-cost and high-performance sludge-based adsorbent for not only recycling of toxic waste nickel-containing sludge (NCS) but for the efficient removal of anionic contaminants in wastewater. The as-designed Ni-Al layered double oxides/calcined NCS (Ni-Al LDOs/CNCS) (216.96 m²/g, 0.44 cm³/g) with hierarchical porous structure possessed a larger specific surface area and well-developed porosity compared with raw NCS (60.52 m²/g, 0.26 cm³/g). It was proved that a higher hydrothermal temperature (180 °C) and a longer hydrothermal time (24 h) both promote the in situ assembly of LDHs nanosheets on CNCS surface. Significantly, the sludge-based adsorbent displayed high adsorption capacity towards five representative anions including F^- (~ 31.1 mg/g), SO₄^2- (~ 37.7 mg/g), NO₃^- (~ 21.8 mg/g), Cl^- (~ 28.0 mg/g), and H₂PO₄^- (~ 35.8 mg/g). Furthermore, the adsorbent maintained desirable adsorption capacity even after 6 adsorption/desorption cycles. Therefore, this study could be potentially extended toward design of other industrial waste sludge-derived high value-added advanced materials and for wastewater treatment applications.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.