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Molecular Tuning of Redox-Copolymers for Selective
Electrochemical Remediation
Kwiyong Kim, Paola Baldaguez Medina, Johannes Elbert, Emmanuel Kayiwa,
Roland D. Cusick, Yujie Men, and Xiao Su*
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 perand 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.
Dr. K. Kim, P. Baldaguez Medina, Dr. J. Elbert, Prof. X. Su
Department of Chemical and Biomolecular Engineering
University of Illinois at Urbana-Champaign
Urbana, IL 61801, USA
E-mail: x2su@illinois.edu
E. Kayiwa, Prof. R. D. Cusick
Department of Civil and Environmental Engineering
University of Illinois at Urbana-Champaign
Urbana, IL 61801, USA
Prof. Y. Men
Department of Chemical and Environmental Engineering
University of California
Riverside, CA 92521, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202004635.
DOI: 10.1002/adfm.202004635
Adv. Funct. Mater. 2020, 30, 2004635
1. Introduction
Achieving molecular selectivity is a critical
challenge for separation processes, with
advanced materials being key to enabling more efficient technologies in water
purification and environmental remediation.[1] Selective separation of organic
anions from industrial wastewater is of
particular importance due to their environmental concern and dilute nature
(micro to millimolar range); the presence
of competing ions make it challenging to
separate by conventional adsorption or
ion-exchange methods.[2] There have been
intense efforts for targeting a broad range
of organic micropollutants,[3] but performance is often affected by intrinsic limitations in materials chemistry, as well as
the high energy, operational, and chemical
consumption costs.[4]
Electrochemical separations provide
modularity and scalability, with electrosorption methods allowing for the capture and release of target compounds based
solely on potential control.[2,5] Recently, the
focus of electrochemical separations has
shifted from bulk capacitive desalination to
ion-selective separation technologies.[1a,6]
Redox-active materials provide an attractive
electrosorption platform for the capture of
specific pollutants, including heavy metal
oxyanions, hydrophobic pollutants, and organic anions.[1a,2,7]
Redox-electrodes have been studied also as catalytic interfaces
for degradation of organic contaminants,[8] opening the door to
the integration of separation and reaction steps.[5,7b] In addition,
the asymmetric design of redox-electrodes can lead to dramatic
increases in energy efficiency for water purification systems, by
reducing operating voltages, and enabling complementary operations at the anode and the cathode.[5,7c,9]
Rational tuning of material properties is a critical direction in the development of next-generation sorbents for anion
micropollutants, especially the control of electrostatics, chargetransfer, and hydrophobic properties. Here, we demonstrate
for the first time how molecularly designed copolymers can
achieve highly selective separation of organic anions through
electrochemical control, and ultimately integrate separation
and remediation within a single electrochemical device, with
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enhanced energy efficiency. We explore polymer-functionalized
electrodes bearing amine (NH) functionalities designed to
provide high affinity for anionic organic compounds, and at the
same time, control the loading of redox-groups, for example,
nitroxide radicals (NO•), which can become charged to
oxoammonium cation (+NO), for electrically-controlled capture and release. Amine functionalities have served as attractive binding sites for anionic functionalities by targeting the
carboxylate and sulfonate head groups.[4b,10] We propose the
use of 2,2,6,6-tetramethylpiperidine as a novel platform system,
as it contains strong amine bases,[11] which can be protonated
in a broad pH range, making these systems advantageous for
adsorption of anionic functional groups. At the same time, controlled oxidation of NH to NO• provides a way of tuning
hydrophobicity and redox-charge storage, thus assisting in the
electrochemically-mediated capture and release.
To molecularly optimize the various physico–chemical properties at play, we synthesized copolymers of
4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl
(TMA)
and 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine (TMPMA),
namely, P(TMAx-co-TMPMA1−x), with x indicating the ratio
of TMA in the copolymer (Figure 1a). Incorporating NO•
radical moieties (increasing x) decreases the number of attractive NH binding sites (1−x), but at the same time increases
hydrophobicity (NO• is more hydrophobic than NH) and
reversible electrochemical activity (NO• is redox-active, while
NH is not), which are all critical for synergistic binding and
regeneration. Through mechanistic studies and separation
tests, we demonstrate that control of the copolymer ratio leads
to desirable tunability of redox-activity, hydrophobicity, and
degree of NH affinity sites, thus enabling molecular selectivity for the separation of organic compounds ranging from
persistent perfluorinated pollutants to halogenated aromatic
contaminants. Per- and polyfluoroalkyl substances (PFAS) and
halogenated compounds are particularly challenging environmental targets,[3] due to their strong chemical persistence and
distinct hydrophobicity and charge properties. Unlike purely
inorganic ions, these compounds possess surfactant-like properties (hydrophilic charged heads and hydrophobic tails), which
offer unique challenges for selective interfacial binding.
In our proposed copolymer system, the co-existing NH
and NO• functionalities synergistically combine the
affinity of NH sites with the electrostatic enhancement of
the redox-active group, NO• (Figure 1b), promoting the
electrochemically-controlled capture and release, without
the need for chemical regenerants or harsh operational
requirements (Figure 1c). Furthermore, we propose the first
integration of polymer-functionalized electrode with a boron-
Figure 1. a) Synthesis route of P(TMAx-co-TMPMA1−x). b) Redox reaction of redox-active TMA unit in P(TMAx-co-TMPMA1−x) (A denotes a generic
anion). c) A schematic illustration showing electrochemically-controlled capture and release of PFOA by P(TMAx-co-TMPMA1−x). During adsorption,
N-H sites of piperidine in TMPMA and redox-active oxoammonium in oxidized TMA units work in a synergistic way for enhanced electrostatic attraction
of PFOA. During regeneration, redox-mediated charge repulsion enhanced by nitroxyl/oxoammonium couple facilitates electrochemically-controlled
release. The ratio between TMA and TMPMA also tunes hydrophobicity, to achieve an optimal degree for PFOA binding and reversible release.
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doped diamond (BDD) electrode to achieve separation and
destruction of a model perfluorinated compound within an
asymmetric electrochemical device; the coupled regeneration
of P(TMAx-co-TMPMA1−x) with BDD allows for a modular,
energy efficient, and selective reactive separation concept for
organic anions.
2. Results and Discussion
2.1. Synthesis and Characterization of P(TMAx-co-TMPMA1−x)
P(TMAx-co-TMPMA1−x) (with x indicating the ratio of TMA in
the copolymer) was synthesized by partial oxidation of PTMPMA
with m-chloroperbenzoic acid, as illustrated in Figure 1a.
The detailed methodology can be found in Section S1.1,
Supporting Information.[12] Synthetic control of the radical
content (x) was confirmed through a number of analytical
techniques. The presence of stable NO• radicals was investigated using ultraviolet-visible (UV–vis) analysis, where typical
adsorption at 460 nm is associated with the n–π* transition of
NO• radicals.[13] An increase in the concentration of NO•
radicals, corresponding to an increase in degree of oxidation,
was observed in the UV–vis spectra (Figure 2a). Quantification of radical yields based on TEMPO standards (Figure S4,
Supporting Information) revealed 0%, 18%, 51%, 84% conversion of TMPMA to TMA at different degrees of oxidation. The
same trend in radical densities were also monitored via electron
paramagnetic resonance (EPR) (Figure S5, Supporting Information). EPR characterization of all P(TMAx-co-TMPMA1−x)
samples showed broad singlets, in accordance with previous
literature.[13] The signal intensity associated with the radical
spin density increases with higher degree of oxidation. The
oxidation of NH functionalities to NO• radicals was further
confirmed via Fourier-transform infrared spectroscopy (FTIR) (Figure S6, Supporting Information). The FT-IR signals
revealed that NO• in P(TMAx-co-TMPMA1−x) started to appear
upon oxidation as evidenced by the appearance of NO• peak
at 1467 cm−1.[12] At the same time, P(TMA84-co-TMPMA16), with
the highest radical content, did not exhibit discernable peak of
amine NH stretch in the range of 3400–3100 cm−1.[14] X-ray
photoelectron spectroscopy (XPS) also indicated the conversion
Figure 2. a) UV–vis spectra of P(TMAx-co-TMPMA1−x) copolymers in chloroform (concentration: 8 mg mL−1). b) Cyclic voltammograms of P(TMAx-coTMPMA1−x)-CNT electrodes in 0.1 m NaClO4 (scan rate: 10 mV s−1). c) PFOA uptake capacity and regeneration efficiency of P(TMAx-co-TMPMA1−x)-CNT
electrodes (adsorption: in 20 mm NaCl + 0.1 mm (black) or 1 mm PFOA (dark gray and gray) at open circuit (O.C., black, dark gray) and +1.0 V (gray) for 0.5 h;
and desorption (red) in 20 mm NaCl at −0.5 V for 0.5 h. Before desorption, adsorption was carried out at +1.0 V for 0.5 h in 1 mm PFOA + 20 mm
NaCl (gray bars). In (a–c), x denotes the fraction of TMA in P(TMA-co-TMPMA). d) PFOA uptake capacity of the P(TMA51-co-TMPMA49)-CNT electrodes
at various polarization conditions in 0.1 mm PFOA + 20 mm NaCl for 0.5 h. e) Images of water droplet on the surface of P(TMAx-co-TMPMA1−x)-coated
electrodes in contact angle measurement (contact angles were 70°, 94°, and 126° for 0%, 51%, and 51% after adsorption, respectively). f) Comparison
of PFOA uptake capacity of CNT and P(TMA51-co-TMPMA49)-CNT electrodes in 0.1 mm PFOA + 20 mm NaCl for 0.5 h. g) Scanning electron microscopy
(SEM) of P(TMA51-co-TMPMA49)-CNT electrodes, and EDS mapping of nitrogen and adsorbed fluoride. Scale bar for high-resolution SEM is 2 µm, and
scale bar for EDS and N/F mapping is 200 µm.
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of NH functionalities to NO• radicals (Figure S7, Supporting Information). The peak at the lower binding energy of
398.9–399.7 eV is assigned to NH groups, and the other peak
at about 401.3–401.5 eV is the characteristic peak of NO• radicals.[12,15] When the relative concentration of each component
representing NH and NO• is compared, the same trend
of increasing NO• content could be observed with higher
degree of oxidation. The NO• contents obtained from XPS
exhibited lower values compared to UV–vis-based quantification of NO•, and this can be ascribed to the distribution of
more polar NH group on the surface caused during the precipitation of polymers in methanol or water (see Section S1.1,
Supporting Information).
Finally, a higher electrochemical activity with higher x in
P(TMAx-co-TMPMA1−x) was clearly observed using cyclic voltammetry (CV), measured using heterogeneous P(TMAx-coTMPMA1−x) interfaces blended with CNT as a conductive
additive formed by dip-coating method (Figure 2b).[5] The
P(TMAx-co-TMPMA1−x)-CNT electrode showed homogeneous
and nanoporous morphology (Figure 2g), with the CNTs providing a porous network for ion accessibility. The reversible
peaks in Figure 2b could be ascribed to the one-electron redox
reaction between NO• and +NO, shown in Figure 1b.[13] The
normalized current increased proportionally to the increase in
oxidation degree, indicating the larger number of reversible
redox couples of NO• and +NO in P(TMAx-co-TMPMA1−x)
at higher x. However, the peak separation became larger with
higher oxidation degree, probably due to increased hydrophobicity from the replacement of more hydrophilic NH with less
hydrophilic NO•, thus limiting ion accessibility. The spectroscopic and electrochemical characterizations confirm the controllable composition of NO• and NH loadings through the
synthesis of P(TMAx-co-TMPMA1−x) copolymer, and tunable
electrochemical activity.
2.2. Selective (Electro)Sorption of a Model Compound PFOA
The separation performance of P(TMAx-co-TMPMA1−x)-CNT
electrodes, for each radical content x, were first evaluated by
carrying out the electrosorption of the model compound PFOA
at initial concentrations of 0.1 and 1 mm in the presence of
20 mm NaCl (Figure 2c). These initial concentrations translate to a molar ratio of PFOA to total nitrogen species (sum of
NH and NO•) in P(TMAx-co-TMPMA1−x) of 0.5:1 and 5:1,
respectively. First, the highest adsorption uptake of PFOA in
the absence of electricity input (at open circuit) was obtained
with polymers having a lower content of nitroxide radicals: the
non-oxidized polymer (x = 0%) showed the highest uptake at
0.1 mm (299.6 mg g−1) and the polymer with x = 18% had the
highest uptake at 1 mm (1199.3 mg g−1) (Figure 2c). The trend
of decreasing PFOA removal by polymers with >18% radical
content indicates that the piperidine groups are essential for
high PFOA binding when electricity is not applied. At the initial PFOA concentration of 1 mm (dark gray bars in Figure 2c),
the molar ratio of adsorbed PFOA to amine functional groups
in P(TMAx-co-TMPMA1−x) (PFOA/NH) was 0.48:1 at x = 0%.
The incomplete utilization of NH, even in the presence of
high initial PFOA concentration (1 mm), could be ascribed to
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the hydrophilic nature of the piperidine groups (Figure 2e).
Incorporating NO• radical moieties increase the hydrophobicity, and as a result, the utilization of NH functionalities
approached a stoichiometric ratio of 1:1 to PFOA at x = 18%
and 51%. The close to full utilization of the NH functionalities indicates that piperidine groups are critical to driving the
selective binding of PFOA under open circuit condition.
Interestingly, applying an increasingly positive potential
beyond open-circuit improved the adsorption of PFOA onto the
copolymer electrodes. For example, when the applied potential at P(TMA51-co-TMPMA49) was finely controlled in 0.1 mm
PFOA + 20 mm NaCl, a positive correlation between applied
potential and uptake of PFOA was observed (Figure 2d). In
the same electrolyte condition (0.1 mm PFOA + 20 mm NaCl),
the potential at open circuit was stabilized at −0.28 ± 0.04 V,
and applied potentials higher than that exhibited better uptake
compared to open circuit adsorption (Figure 2d). In Figure 2c,
when x was varied, applying a positive bias of +1.0 V (vs Ag/
AgCl) enhances PFOA uptake compared to open circuit,
showing >1000 mg PFOA/g adsorbent for all cases at 1 mm
PFOA, with higher uptake being observed at lower x. In particular, the molar ratio of adsorbed PFOA to NH (PFOA/
NH) in P(TMA84-co-TMPMA16) was 3.85:1 during the electrosorption at +1.0 V, indicating that PFOA binds not only
onto NH, but also onto redox-active +NO sites via electrostatic attraction (see the anion doping in Figure 1b). After electrosorption, the increase in PFOA content on the copolymer
film was evident through EDS mapping (Figure 2g), and both
survey and high-resolution XPS analysis (Figures S9,S10, Supporting Information) indicated an increase in fluorine content. The contact angle analysis also confirmed the increase in
hydrophobicity at the adsorbent layer after the electrosorption
of PFOA (Figure 2e). Our result indicates the synergistic interplay between NH affinity sites and redox-promoted electrostatic interaction at the charged +NO sites for selective PFOA
binding. The unfunctionalized CNT does not present any
noticeable uptake, either under applied potential (36.3 mg g−1
at +1.0 V) or at O.C. (10.9 mg g−1) (Figure 2f), indicating that
selective copolymer functionalization is responsible for significant uptake (30-fold increase with copolymer).
Thus, all these results provide ample evidence that surface
functionalization with P(TMAx-co-TMPMA1−x)-CNT enables
molecular selectivity for PFOA uptake in the presence of background anions. Here, we define the separation factor of PFOA
versus chloride as follows
Separation factorPFOA − /Cl − =
qPFOA − /qCl −
(cPFOA − )0 /(c Cl − )0
(1)
where (cPFOA−)0 and (cCl−)0 are the initial concentration in the
solution, and qPFOA- and qCl− are the solid-phase uptakes of
PFOA− and Cl−, the latter ratio which can be estimated from
XPS analysis (see Section S1.4, Supporting Information, for
calculation details). For example, at x = 51%, in the presence
of 200-fold excess of chloride (0.1 mm PFOA + 20 mm NaCl),
a molar ratio comparison revealed the PFOA− to Cl− ratio to
be 2.5 at open circuit, equivalent to a separation factor of 500
(Figure 3a). If final equilibrium concentrations were used to
estimate the separation factor,[1a] then the separation factor
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Figure 3. a) Separation factors of PFOA− versus chloride on P(TMA51-co-TMPMA49)-CNT electrodes after adsorption in 0.1 mm PFOA + 20 mm NaCl
for open circuit, and different positive potentials versus Ag/AgCl. b) Adsorption kinetics of PFOA by P(TMA51-co-TMPMA49)-CNT electrode at open
circuit and +1.0 V (in 0.1 mm PFOA + 20 mm NaCl). c) Equilibrium isotherm of P(TMA51-co-TMPMA49)-CNT electrode at open circuit for a range of
PFOA concentrations in 20 mm NaCl (blue line represents Freundlich isotherm). d) PFOA uptake capacity (blue dots) and regeneration efficiency (red
bars) of P(TMA51-co-TMPMA49)-CNT electrode over 5 cycles (adsorption: 0.1 mm PFOA + 20 mm NaCl at +1.0 V for 0.5 h; desorption: in 20 mm NaCl
at −1.0 V for 1 h).
toward PFOA over chloride would be 832 at these conditions.
With the positive potential at +1.0 V, the PFOA− to Cl− ratio
was decreased to 0.76, but the separation factor was 152 due to
the much lower concentration of PFOA compared to chloride,
indicating an exceptionally strong interaction between PFOA
and the copolymer (Figure 3a). Applying more positive potential resulted in more competitive attraction of chloride, thereby
leading to decreasing trend in Coulombic efficiency for PFOA
adsorption (Figure S11, Supporting Information) and also in
separation factor (Figure 3a, 37.4 at +2.5 V). This indicates that a
certain potential window (<1.2 V vs Ag/AgCl) is recommended
to achieve higher molecular selectivity (separation factor > 100).
A lower potential window also minimizes energy consumption—as will be discussed later, a practical operational potential range could be from +0.6 to +1.2 V versus Ag/AgCl at the
working copolymer electrode.
2.3. Redox-Mediated Regeneration
Here, we investigated the effect of redox-active NO• functional
groups on promoting electromediated release of the bound
PFOA species. Redox-active species have garnered intense
attention for energy storage,[2,16] and more recently, have been
show to efficiently couple charge storage with electrochemically-switched capture and release of charged pollutants.[5,7b]
As shown in XPS analysis (Figure S8, Supporting Information)
and as observed in literature,[17] one-electron oxidation of NO•
radicals forms +NO, and its fully reversible redox process
(Figure 2b) accompanies the uptake or expulsion of doped anionic species for charge neutrality (Figure 1b). We investigated
the effect of the oxidation degree and copolymer structure
on the regeneration efficiency. For each different copolymer
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(varying x), we first adsorbed the PFOA at +1.0 V versus Ag/
AgCl, and then applied −0.5 V versus Ag/AgCl to release the
adsorbed molecules. As depicted in Figure 2c, the regeneration
efficiency (defined as the ratio of PFOA recovered to adsorbed)
improved as the radical yield was increased from x = 0% to 51%.
This result indicated that anion release induced by redox-mediated charge repulsion (as shown in Figure 1b) is more efficient
than discharging based solely on polarization of a conductive
surface. For x = 51%, the regeneration efficiency was enhanced
by applying a more negative bias, exhibiting >80% regeneration
efficiency at −1.5 V (Figure S12, Supporting Information). It is
interesting to note that P(TMA84-co-TMPMA16), despite having
the highest oxidation degree, showed the poorest regeneration.
This could be ascribed to the background hydrophobic interaction between PFOA and neutral, hydrophobic PTMA layer,
which is supported by contact angle analysis indicating more
hydrophobic features at higher NO• content (Figure 2e). Our
results confirmed that compositional control of the copolymer,
with delicately tuned NH content, redox-activity, and hydrophobicity, enables not only significant uptake of PFOA but also
efficient electrochemically-mediated release of adsorbed PFAS
molecules. Through structural and electrochemical control of
the redox-copolymer electrodes, we enable reuse of the adsorbent without the need for chemical regenerants and/or harsh
operational environments.
2.4. Adsorption Kinetics and Stability
Based on the molecular optimization showing that the
highest regeneration efficiency occurs at x = 51%, P(TMA51co-TMPMA49) was selected for the subsequent experiments
on adsorption kinetics, isotherm, and cyclability. Adsorption
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kinetics were studied both at open circuit and +1.0 V in 0.1 mm
PFOA + 20 mm NaCl. As seen in Figure 3b, 81% of removal
was observed within 1 h by applying +1.0 V. The kinetics in
the first 3 h at +1.0 V outperformed open circuit, in accordance with our previous observations about the contribution of
the radical-based electrostatic interaction to enhancing PFOA
binding (Figure 2c,d). However, the maximum obtainable
uptake after reaching equilibrium (>10 h) was slightly higher
at open circuit compared to positive bias. It is hypothesized
that electrostatic interactions play a significant role of accelerating initial kinetics, but prolonged polarization induced competitive adsorption of chloride, as reflected in the separation
factor and Coulombic efficiency analysis (Figure 3a; Figure S11,
Supporting Information). Thus, in practice, electrosorption duration can be controlled to optimize separation factor.
Equilibrium isotherm studies show that a maximum capacity
of 970 mg PFOA/g adsorbent can be achieved at open circuit
(Figure 3c), which is comparable or higher than reported loadings for various chemical adsorbents for PFOA (Table S4, Supporting Information), except here we are highly selective and
electrochemically-mediated.
A cycling test was conducted with the P(TMA51-co-TMPMA49)CNT electrode being charged at +1.0 V, in the presence of 0.1 mm
PFOA and 20 mm NaCl for 0.5 h, and then discharged at −1.0 V
into an NaCl solution for 1 h. Figure 3d shows the regeneration
efficiency was maintained at >90% for a number of cycles per
step starting at cycle 2. The incomplete regeneration at the initial
release cycle 1 can be ascribed to initial irreversible adsorption
due to hydrophobic interactions between the discharged (neutral)
polymer and PFOA. Even so, the working capacity of the functionalized polymer electrode was preserved at close to 200 mg
PFOA/g adsorbent for a number of cycles. This proves the reusability advantages of our P(TMAx-co-TMPMA1−x)-CNT electrodes,
which are fully electrochemically-controlled and thus do not
require chemical regenerants for PFOA release.
2.5. Real Water Matrices and Separation of Contaminants with
Diverse Chemical Structures
To evaluate relevant environmental conditions, the removal of
PFOA was tested under different water matrices (Figure 4a),
including 20 mm NaCl, tap water, 5 mm Na2CO3, and municipal secondary wastewater effluent solution (obtained from
Urbana-Champaign Sanitary District). In all water matrices,
>93% removal efficiency was achieved within 3 h of electrosorption when spiked with 0.1 mm PFOA, and >82.5% at
the dilute initial PFOA concentration of 100 ppb (0.242 µm)
to simulate environmental conditions. We also tested the
performance of P(TMA51-co-TMPMA49)-functionalized electrodes for the removal of several PFAS compounds with different chemical structures, including perfluorocarboxylic
acids (CnF2n+1COO−), per- and polyfluoro dicarboxylic acids
(−OOC−CnF2n−COO−), and perfluoroalkanesulfonic acids
(CnF2n+1SO3−) (Figure 4b). For a given chain length (n = 8),
the open circuit adsorption of different PFAS compounds
seemed to be mainly affected by hydrophobic interaction;
for example, PFSA (perfluorosebacic acid), having double
COO− head groups and lacking primary CF bond, is less
hydrophobic and thus shows the poorest uptake at open circuit. For all cases, however, applying +1.0 V of positive potential led to greatly improved uptake, with PFSA having the
highest increase with the positive potential. The effect of the
chemical structure of PFAS on the electrosorption provides
further supporting evidence that electrostatic interaction plays
a dominant mechanism to balance the affinity interaction, and
that molecular control of the copolymer ratio is thus critical to
maximizing these selective interactions. The regeneration efficiencies of P(TMA51-co-TMPMA49)-CNT with different PFAS
compounds seemed to be also affected by the hydrophobicity
of PFAS compounds (Figure S15, Supporting Information);
having longer chain length showed decreased regeneration.
At the same time, the regeneration was facilitated in the
case of PFSA, which has two COO− head groups. Thus,
molecular tuning of copolymer ratio is shown to optimize
regeneration for various PFAS compounds. Furthermore, the
versatility of our redox-copolymer system was demonstrated by
the effective separation of halogenated aromatic compounds,
including 2,4-dichlorobenzoic acid, ibuprofen, and clofibric
acid (Figure S14, Supporting Information). All these results
support the wide applicability of our molecularly-tuned electroactive copolymers electrode to address real-world contamination under relevant conditions, and a diversity of chemical
structures.
Figure 4. a) Removal of PFOA in different water matrices spiked with 0.1 mm or 100 ppb (0.242 µm). +1.0 V was applied for 3 h. b) Uptake capacity
of different PFAS compounds with P(TMA51-co-TMPMA49)-CNT electrode at open circuit and +1.0 V (adsorption: 0.1 mm PFAS compounds + 20 mm
NaCl for 0.5 h).
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Figure 5. a) Schematic illustrations showing selective uptake of PFOA by P(TMAx-co-TMPMA1−x)-CNT during adsorption and degradation of released
PFOA by BDD electrode during regeneration. b) Simultaneous tracking of anode (BDD) and counter electrode potentials during chronopotentiometry
at 10 mA cm−2 in 0.1 mm PFOA + 20 mm NaCl for 2 h using different counter electrodes of Pt, CNT, and P(TMA51-co-TMPMA49)-CNT. (c) and (d) represent the operation of the asymmetric P(TMA51-co-TMPMA49)-CNT//BDD system for one cycle of adsorption and desorption (as depicted in [a]):
c) PFOA uptake capacity during the adsorption and defluorination efficiency during the regeneration in the asymmetric P(TMA51-co-TMPMA49)-CNT//
BDD configuration (adsorption: in 0.1 mm PFOA + 20 mm NaCl for 0.5 h). d) Tracking of anode (BDD) and cathode (P(TMA51-co-TMPMA49)-CNT)
potentials during the regeneration (desorption) of P(TMA51-co-TMPMA49)-CNT in 20 mm NaCl at 10 mA cm−2 for 5 h.
2.6. Integration of Reactive Separation
While the molecular control of the redox-copolymer allows for
the selective separation of organic contaminants, the fate of
the enriched pollutant waste after separation process must be
addressed.[18] Here, we propose a unique approach to achieve
tandem separation and degradation of the model compound
PFOA within the same electrochemical unit, by leveraging an
asymmetric electrochemical design that combines the P(TMAxco-TMPMA1−x)-functionalized CNT working electrode with a
BDD counter electrode. While the redox-copolymer acts as a
selective adsorbent, the BDD counter-electrode acts as a synergistic electrocatalyst during the regeneration step. As shown
in Figure 5a, PFOA can be first electrochemically captured by
P(TMAx-co-TMPMA1−x), whereas during release, BDD electrode
catalyzes defluorination of released PFOA. BDD anode has
proven its ability to degrade PFAS and various organic contaminants in wide applications for electrochemical advanced oxidation, showing desirable features of commercial availability, high
stability, low adsorption capacity, and high overpotential for
oxygen evolution (Figure S3, Supporting Information).[19] First,
chronopotentiometric tests on the BDD anode were carried out
at 10 mA cm−2 with various counter electrodes of platinum, CNT,
and P(TMA51-co-TMPMA49)-CNT in 0.1 mm PFOA + 20 mm
NaCl; simultaneous tracking of the potential at both working
(BDD) and counter electrodes during charging were carried out
against a reference Ag/AgCl (Figure 5b). The analysis showed
that conductive, non-redox counter electrodes (platinum and
Adv. Funct. Mater. 2020, 30, 2004635
CNT) reached significantly negative overpotentials; while the
redox-active P(TMA51-co-TMPMA49)-CNT on the other hand
remains at potentials no lower than −1.4 V (Figure 5b). Thus,
our P(TMA51-co-TMPMA49) provides an energy-efficient option
as a counter electrode during electrochemical degradation, enabling much lower voltage windows for BDD-based electrochemical oxidation processes. On the conductive counter electrodes,
water decomposition can be a major cathodic parasitic reaction
at very negative overpotentials.[5,7c] By coupling the P(TMA51-coTMPMA49)-CNT with BDD, we can efficiently store electrons
through NO•/+NO electrochemistry at more moderate overpotentials to avoid water splitting, and while its redox-process
can be directly connected to electrochemically-controlled PFOA
capture and release.
Next, tandem separation and degradation of PFOA within
a single device (Figure 5a) was experimentally realized using
an asymmetric configuration of P(TMA51-co-TMPMA49)-CNT//
BDD. First, we carried out electrosorption in a 0.1 mm PFOA +
20 mm NaCl for 0.5 h at +1.0 V, followed by applying constant
current density of 10 mA cm−2 with reversed polarity for 5 h
into a 20 mm NaCl solution to release and degrade PFOA. The
adsorption capacity was 291 mg PFOA/g adsorbent (Figure 5c),
and applying constant current density of 10 mA cm−2 achieved
a high regeneration of the bound PFOA, while pushing the
potential at the P(TMA51-co-TMPMA49)-CNT electrode to be
−1.2 V (Figure 5d). At the same time, the working potential
of BDD anode was maintained at about +4.5 V (Figure 5d), a
potential range capable of promoting the degradation of PFOA
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Figure 6. a) Energy consumption during the electroseparation of PFOA by CNT and by P(TMAx-co-TMPMA1−x)-CNT. b) PFOA removal (sky blue bar),
defluorination efficiency (orange bar), and energy consumption (brown bar) during chronopotentiometry at 10 mA cm−2 in 0.1 mm PFOA + 20 mm
NaCl for 2 h using BDD as a working electrode and different counter electrodes of Pt, CNT, and P(TMA51-co-TMPMA49)-CNT.
via electron transfer from the head group to the BDD.[19a] From
LC/MS analysis, no PFOA was found in the desorption electrolyte, implying complete degradation of released PFOA. The
final concentration of fluoride was quantified using ion chromatography, and the normalized fluoride recovery out of adsorbed
amount (291 mg PFOA/g adsorbent) was found to be 81% after
5 h electrolysis (Figure 5c), which is comparable or higher
than the defluorination ratio (i.e., CF− produced/CF− in PFOA,initial)
reported for various electrochemical PFOA degradation.[19d,20]
Its corresponding F index (i.e., CF− produced/CPFOA degraded) was
12.1, implying fluoride recovery of 12.1 out of 15 per PFOA
molecule degraded. To the best of our knowledge, our proofof-concept study is the first realization of the coupling of a
redox-active polymer with BDD electrode, providing an efficient
remediation pathway for separating and degrading enriched
PFAS compounds within a single unit operation.
2.7. Energy Analysis
Finally, the energy analysis for electrochemical separation and
defluorination highlighted the intrinsic performance advantages of our molecularly-engineered copolymer over regular
capacitive electrosorption systems. First, we achieved remarkable improvements in energy-efficiency in the electroseparation
process. Electrosorption using P(TMAx-co-TMPMA1−x)-CNT
enabled a significantly improved separation factor, leading to
high adsorption capacity and a lowering of the energy input
in the separation process down to <0.44 kWh mol−1 PFOA
(Figure 6a). On the other hand, control experiments of PFOA
electrosorption on CNT electrodes was estimated to consume
>2.67 kWh mol−1 PFOA (being potentially as high as 196 kWh
mol−1 PFOA), due to low selectivity and poor affinity of the
non-functionalized surfaces (Figure 6a). Furthermore, the
polymer-functionalized interfaces, when coupled with BDD
anode, provided a lower energy consumption via tuning of
the redox potentials. In chronopotentiometric operations for
2 h with three different counter electrodes of platinum, CNT,
and P(TMA51-co-TMPMA49)-CNT, all the configurations exhibited similar PFOA removal and defluorination performance
(Figure 6b). However, a much lower mineralization energy
was required with redox-active copolymer counter-electrodes
Adv. Funct. Mater. 2020, 30, 2004635
(P(TMA51-co-TMPMA49)-CNT) compared to conductive ones
(CNT and platinum) (Figure 6b).
For an adsorption and regeneration cycle of the P(TMA51-coTMPMA49)-CNT//BDD configuration (Figure 5a), the mineralization energy for PFOA estimated from the defluorination
performance (Figure 5c) and voltage/current profile (Figure 5d)
was found to be 62.3 kWh mol−1 F− for a 5 h operation. This
energy consumption is among the lowest reported in the literature (prior electrochemical [10–1500 kWh mol−1 F−], microwave
[4200 kWh mol−1 F−], UV [450–2000 kWh mol−1 F−], and ultrasonic [250–4200 kWh mol−1 F−]).[20b,21] The significantly lower
energy consumption can be attributed to the high defluorination efficiency, combined with the tuning of redox-potential
through the P(TMA51-co-TMPMA49)-CNT electrode. Our consideration in the energy aspect demonstrates the possibility of
using P(TMAx-co-TMPMA1−x)-CNT as not only a cost-effective
electrochemically-mediated adsorbent, but also a sustainable
counter electrode enabling energy-efficient, tandem degradation of PFOA, providing a unique pathway for next-generation,
energy-integrated water purification devices.
3. Conclusions
We have developed a copolymer of 4-methacryloyloxy-2,2,6,6tetramethylpiperidin-1-oxyl (TMA) and 4-methacryloyloxy2,2,6,6-tetramethylpiperidine (TMPMA), namely, P(TMAx-coTMPMA1−x), for the electrochemically-controlled capture and
release of organic contaminants. Control of the radical yields
(x) within the copolymer tunes the affinity toward organic
anions, and the optimal ratio between NH and NO• functionalities allows for a selective and reversible electrochemical
adsorption. P(TMAx-co-TMPMA1−x)-CNT is shown to be effective for selective capture of the model compound PFOA in the
presence of excess competing ions, with desirable features of
high adsorption capacity and cyclability. The performance of
P(TMAx-co-TMPMA1−x)-CNT was benchmarked using various
real-world water matrices spiked 0.1 mm and 100 ppb of PFOA,
and also using various PFAS with differing chain lengths and
chemical end-groups, and further expanded for the separation of halogenated aromatic contaminants. The P(TMAx-coTMPMA1−x)-CNT adsorbent electrode was integrated with a
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BDD counter-electrode to establish a unique asymmetric configuration. The combined system coupled selective separation by P(TMAx-co-TMPMA1−x)-CNT with the simultaneous
degradation by BDD during release of adsorbed PFOA, exhibiting a high defluorination performance with improved energy
efficiency.
Our work emphasizes the importance of judicious selection
and tuning of the copolymer chemistry for electrochemicallymediated separation of organic micropollutants—by controlling
the copolymer composition, we can dial-in desired properties
such as binding affinity, electrostatics, and redox-activity.
Despite the ultra-dilute concentration of organic anions and
coexistence of competing constituents, molecular design of ionselective redox-active interfaces enables an exceptional binding
affinity, with a high separation factor, and significantly enhanced
regeneration by electrochemical control. We also demonstrated
an energy-efficient and modular approach for asymmetrically
integrating redox-active interfaces with electrocatalytic BDD, for
the reactive separation of highly-stable fluorinated compounds.
The proposed approach can significantly impact future electrochemical applications for water purification. Finally, we expect
continued advances in the design of redox-materials to enable
molecularly-integrated systems which not only perform selective
separations, but also provide direct coupling with electrochemical energy storage and generation devices.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
K.K. and P.B.M. contributed equally to this work. The authors
acknowledge the support of the University of Illinois, UrbanaChampaign (UIUC) for startup funding, the support of the National
Science Foundation under Grant #1931941, the support of SURGE
Fellowship from the UIUC, and partial funding under the Illinois Water
Resources Center (IWRC) Annual grant (2019). SEM, XPS, FT-IR, GPC,
and contact angle analysis were carried in the Frederick Seitz Materials
Research Laboratory Central Research Facilities, University of Illinois.
LC-MS and EPR were carried out in the School of Chemical Science
(SCS) Mass Spectrometry Lab and EPR Lab, respectively. Major funding
for the Bruker EMXPlus was provided by National Science Foundation
Award 1726244 (2017) to the School of Chemical Sciences EPR lab at the
University of Illinois. The authors thank Furong Sun and Dr. Toby Woods
for their assistance with analysis.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
electrocatalysis, electrochemical separation, molecular selectivity, redoxactive polymers, tunability
Received: May 30, 2020
Revised: August 22, 2020
Published online: September 16, 2020
Adv. Funct. Mater. 2020, 30, 2004635
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