An official website of the United States government
Per- and polyfluorinated alkyl substances (PFAS) are persistent contaminants that have been continuously detected in groundwater and drinking water around the globe. Hexafluoropropylene oxide dimer acid (tradename GenX) has been used to substitute traditional PFAS, such as PFOA, but its intense use has caused widespread occurrence in water streams and often in high levels. Here, we evaluate a redox-copolymer, poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl- co -4-methacryloyloxy-2,2,6,6-tetramethylpiperidine) (PTMA- co -PTMPMA), for the selective electrochemical removal of GenX. The amine functional groups promote affinity towards the anionic PFAS, and the redox-active nitroxide radicals provide electrochemical control for adsorption and desorption. Faster kinetics and higher uptake (>475 mg g −1 adsorbent) were obtained with the redox-copolymer when applying 0.8 V vs. Ag/AgCl potential compared to open circuit. The copolymer electrosorbents were evaluated over a wide pH range and diverse water matrices, with electrostatic-based mechanisms dependent on the state of protonation of the PFAS. Moreover, we translated the redox-electrodes from a batch to flow-by cell configuration, showing successful adsorption and release of GenX under flow and electrochemical control. Finally, prolonged exposure of GenX at reduction potentials generated smaller PFAS fragments at the redox-electrodes. To fully defluorinate GenX, the copolymer-functionalized electrodes were coupled with a boron-doped diamond (BDD) counter electrode for integrating separation and defluorination within the same device. The combined system demonstrated close to 100% defluorination efficiency. Thus, we highlight the potential of electroactive redox platforms for the reactive separation of fluorotelomers, and point to future directions for their practical implementation for water treatment.
more »
« less
Molecular Tuning of Redox‐Copolymers for Selective Electrochemical Remediation
Abstract Molecular design of redox‐materials provides a promising technique for tuning physicochemical properties which are critical for selective separations and environmental remediation. Here, the structural tuning of redox‐copolymers, 4‐methacryloyloxy‐2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TMA) and 4‐methacryloyloxy‐2,2,6,6‐tetramethylpiperidine (TMPMA), denoted as P(TMAx‐co‐TMPMA1−x), is investigated for the selective separation of anion contaminants ranging from perfluorinated substances to halogenated aromatic compounds. The amine functional groups provide high affinity toward anionic functionalities, while the redox‐active nitroxyl radical groups promote electrochemically‐controlled capture and release. Controlling the ratio of amines to nitroxyl radicals provides a pathway for tuning the redox‐activity, hydrophobicity, and binding affinity of the copolymer, to synergistically enhance adsorption and regeneration. P(TMAx‐co‐TMPMA1−x) removes a model perfluorinated compound (perfluorooctanoic acid (PFOA)) with a high uptake capacity (>1000 mg g−1) and separation factors (500 vs chloride), and demonstrates exceptional removal efficiencies in diverse per‐ and polyfluoroalkyl substances (PFAS) and halogenated aromatic compounds, in various water matrices. Integration with a boron‐doped diamond electrode allows for tandem separation and destruction of pollutants within the same electrochemical cell, enabling the energy integration of the separation step with the catalytic degradation step. The study demonstrates for the first time the tuning of redox‐copolymers for selective remediation of organic anions, and integration with an advanced electrochemical oxidation process for energy‐efficient water purification.
more »
« less
- Award ID(s):
- 1931941
- PAR ID:
- 10452990
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Functional Materials
- Volume:
- 30
- Issue:
- 52
- ISSN:
- 1616-301X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Styrene is an important commodity chemical that is highly energy and CO2intensive to produce. We report a redox oxidative dehydrogenation (redox-ODH) strategy to efficiently produce styrene. Facilitated by a multifunctional (Ca/Mn)1−xO@KFeO2core-shell redox catalyst which acts as (i) a heterogeneous catalyst, (ii) an oxygen separation agent, and (iii) a selective hydrogen combustion material, redox-ODH auto-thermally converts ethylbenzene to styrene with up to 97% single-pass conversion and >94% selectivity. This represents a 72% yield increase compared to commercial dehydrogenation on a relative basis, leading to 82% energy savings and 79% CO2emission reduction. The redox catalyst is composed of a catalytically active KFeO2shell and a (Ca/Mn)1−xO core for reversible lattice oxygen storage and donation. The lattice oxygen donation from (Ca/Mn)1−xO sacrificially stabilizes Fe3+in the shell to maintain high catalytic activity and coke resistance. From a practical standpoint, the redox catalyst exhibits excellent long-term performance under industrially compatible conditions.more » « less
-
Abstract Aqueous organic redox flow batteries (AORFBs) are highly attractive for large‐scale energy storage because of their nonflammability, low cost, and sustainability. (2,2,6,6‐Tetramethylpiperidin‐1‐yl)oxyl (TEMPO) derivatives, a class of redox active molecules bearing air‐stable free nitroxyl radicals and high redox potential (>0.8 V vs NHE), has been identified as promising catholytes for AORFBs. However, reported TEMPO based molecules are either permeable through ion exchange membranes or not chemically stable enough for long‐term energy storage. Herein, a new TEMPO derivative functionalized with a dual‐ammonium dicationic group,N1, N1, N1, N3, N3, 2, 2, 6, 6‐nonamethyl‐N3‐(piperidinyloxy)propane‐1,3‐bis(ammonium) dichloride (N2‐TEMPO) as a stable, low permeable catholyte for AORFBs is reported. Ultraviolet–visible (UV–vis) and proton nuclear magnetic resonance (1H‐NMR) spectroscopic studies reveal its exceptional stability and ultra‐low permeability (1.49 × 10−12 cm2 s−1). Coupled with 1,1′‐bis[3‐(trimethylammonio)propyl]‐4,4′‐bipyridinium tetrachloride ((NPr)2V) as an anolyte, a 1.35 VN2‐TEMPO/(NPr)2V AORFB with 0.5 melectrolytes (9.05 Wh L−1) delivers a high power density of 114 mW cm−2and 100% capacity retention for 400 cycles at 60 mA cm−2. At 1.0 melectrolyte concentrations, theN2‐TEMPO/(NPr)2V AORFB achieves an energy density of 18.1 Wh L−1and capacity retention of 90% for 400 cycles at 60 mA cm−2.more » « less
-
Abstract Electro‐responsive functional materials can play a critical role in selective metal recovery and recycling due to the need for molecular differentiation between transition metals in complex mixtures. Redox‐active metallopolymers are a promising platform for electrochemical separations, offering versatile structural tuning and fast electron transfer. First, through a judicious selection of polymer structure between a main‐chain metallopolymer (polyferrocenylsilane) and a pendant‐group metallopolymer (polyvinylferrocene), charge‐transfer interactions and binding strength toward competing metal ions are tuned, which as a result, dictate selectivity. For example, almost an order of magnitude increase in separation factor between chromate and meta‐vanadate can be achieved, depending on polymer structure. Second, these metallopolymer electrodes exhibit potential‐dependent selectivity that can even flip ion preference, based solely on electrical means—indicating a control parameter that is orthogonal to structural modifications. Finally, this work presents a framework for evaluating electrochemical separations in multicomponent ion mixtures and elucidates the underlying charge‐transfer mechanisms resulting in molecular selectivity through a combination of spectroscopy and electronic structure calculations. The findings demonstrate the applicability of redox‐metallopolymers in tailored electrochemical separations for environmental remediation, value‐added metal recovery, waste recycling, and even mining processing.more » « less
-
Abstract Exceeding the energy density of lithium−carbon monofluoride (Li−CFx), today's leading Li primary battery, requires an increase in fluorine content (x) that determines the theoretical capacity available from C−F bond reduction. However, high F‐content carbon materials face challenges such as poor electronic conductivity, low reduction potentials (<1.3 V versus Li/Li+), and/or low C−F bond utilization. This study investigates molecular structural design principles for a new class of high F‐content fluoroalkyl‐aromatic catholytes that address these challenges. A polarizable conjugated system—an aromatic ring with an alkene linker—functions as electron acceptor and redox initiator, enabling a cascade defluorination of an adjacent perfluoroalkyl chain (RF= −CnF2n+1). The synthesized molecules successfully overcome premature deactivation observed in previously studied catholytes and achieve close‐to‐full defluorination (up to 15/17 available F), yielding high gravimetric capacities of 748 mAh g−1fluoroalkyl‐aromaticand energies of 1785 Wh kg−1fluoroalkyl‐aromatic. The voltage compatibility between fluoroalkyl‐aromatics and CFxenables design of hybrid cells containing C−F redox activity in both solid and liquid phases, with a projected enhancement of Li–CFxgravimetric energy by 35% based on weight of electrodes+electrolyte. With further improvement of cathode architecture, these “liquid CFx” analogues are strong candidates for exceeding the energy limitations of today's primary chemistries.more » « less
