Copolymer-Induced Intermolecular Charge Transfer: Enhancing the Activity of Metal-Free Catalysts for Oxygen Reduction

Article abstract of DOI:10.1002/chem.201806226

Breaking the electroneutrality of sp² carbon lattice is a viable way for nanocarbon material to modulate the charge delocalization and to further alter the electrocatalytic activity. Positive charge spreadsheeting is preferable for catalyzing the oxygen reduction reaction (ORR) and other electrochemical reactions. Analogously to the case of intramolecular charge transfer by heteroatom doping, electrons in the conjugated carbon lattice can be redistributed by the intermolecular charge transfer from the nanocarbon material to the polyelectrolyte. A copolymeric electrolyte, epichlorohydrin-dimethylamine copolymer (EDC) was synthesized. The EDC-modified carbon nanotube (CNT) hybrid was subsequently fabricated by sonication treatment and served as a metal-free carbonaceous electrocatalyst with remarkable catalytic activity and stability. The resultant hybrid presents positive charge spreadsheeting on CNT as a result of the interfacial electron transfer from CNT to EDC. DFT calculations were further carried out to reveal that the enhancement of the wrapped EDC polyelectrolyte originates from the synergetic effect of the quaternary ammonium-hydroxyl covalently bonded structure. The CNT-EDC hybrid not only provides an atomically precise regulation to modulate nanocarbon materials from inactive carbonaceous materials into efficient metal-free catalysts, but it also opens new avenues to develop metal-free catalysts with well-defined and highly active sites.

Full text of DOI:10.1002/chem.201806226

A Journal of
Accepted Article
Title: Copolymer-induced intermolecular charge transfer: Enhancing
activity of metal-free catalysts for oxygen reduction
Authors: Yihuan Yu, Zhengping Zhang, Liming Dai, and Feng Wang
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To be cited as: Chem. Eur. J. 10.1002/chem.201806226
Link to VoR: http://dx.doi.org/10.1002/chem.201806226
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Copolymer-induced intermolecular charge transfer: Enhancing
activity of metal-free catalysts for oxygen reduction
Yihuan Yu[a,b], Zhengping Zhang*[a,b], Liming Dai*[b,c], and Feng Wang*[a,b]
Abstract: Breaking the electroneutrality of sp2 carbon lattice is a
viable way for nanocarbon to modulate the charge delocalization and
to further alter the electrocatalytic activity. The positive charge
spreadsheeting is preferable for catalyzing the oxygen reduction
reaction and other electrochemical reaction. Analogously to the case
of intramolecular charge transfer via heteroatoms-doping, electrons
in conjugated carbon lattice can be redistributed by the
intermolecular charge transfer from nanocarbon to polyelectrolyte. In
strategy to engineer the interfacial charge redistribution on the
sp2 carbon lattice, thereby inducing the charge delocalization or
altering the spin density.[5,6] As results, the altered charge
redistribution could further facilitate the adsorption of oxygen
molecules, which is generally recognized as the rate-
determining step of ORR process for carbon-based catalysts.[7,8]
In most cases, the carbon networks in the incorporation with
higher electronegative nitrogen dopants (i.e., planar pyridinic
nitrogen) will present an increase of positive charge density,
which is claimed as the active configuration for ORR.[7,9-11]
However, the other forms of pyrrolic and graphitic nitrogen are
also intensely deliberated to be the active origins for ORR
instead of pyridinic nitrogen.[12-14] The huge controversy
originates from that their gross doping generated through
pyrolysis treatment is essential, which leads the uncontrollable
structure and the unavoidable coexistence of valid/ invalid
doping.[15,16] Herein, it is urgent to request for pyrolysis-free
synthetic strategies to control active sites in well-define
structures.
this work, we synthesize
a
novel copolymeric electrolyte,
epichlorohydrin-dimethylamine copolymer (EDC); the EDC modified
carbon nanotube (CNT) hybrid is subsequently fabricated by the
sonication treatment, and serves as the metal-free carbonaceous
electrocatalyst with the remarkable catalytic activity and stability. The
resultant hybrid presents the positive charge spreadsheeting on CNT
due to the interfacial electron transfer from CNT to EDC. The density
functional theory calculations are further carried out to reveal that the
enhancement of the wrapped EDC polyelectrolyte originate from the
synergetic effect of the quaternary ammonium-hydroxyl covalently
bonding structure. The novel CNT-EDC hybrid not only provides an
atomically precise regulation to modulate nanocarbon from inactive
carbonaceous materials into efficient metal-free catalysts, but also
opens new avenues to develop metal-free catalysts with well-defined
and highly active site.
On the other hand, the electron delocalization and enhanced
ORR performance can also be yielded by intermolecular charge
transfer from carbonaceous materials (electron donor) to
polyelectrolyte (acceptor).[17,18] By introducing the certain
electron-drawing polyelectrolyte, the adjacent carbon atoms can
possess the obviously positive charge density via the
intermolecular charge transfer, thereby increasing the ORR
activity.[19,20] Compared with the heteroatom-doping methods,
the noncovalent-induced charge delocalization without pyrolysis
treatment serves as a desirable and controllable strategy, which
can avoid the damage of carbon lattices, ensure electron
conductivities, and keep atomically precise regulation. In this
regard, several advances in choosing pristine carbonaceous
materials (e.g., carbon nanotube and graphene) and designing
the competitive polyelectrolyte or other electron acceptors (e.g.,
poly(diallyldimethylammonium chloride), nitrobenzene and
graphene quantum dots) have been demonstrated to tune the
electron delocalization on carbonaceous materials for the good
ORR activity.[21-23] Nevertheless, the task of comparable or
boosted activity is not trivial as the inferior intrinsic activity of
pristine carbonaceous materials.
Since the nitrogen-doped carbon nanotube (CNT) arrays
were firstly discovered as metal-free catalysts with the
remarkable electrocatalytic performance for oxygen reduction
reaction (ORR),[1] the stimulated extensive researches of metal-
free carbonaceous electrocatalysts are developed to completely
or partially replace platinum (Pt)-based catalysts in fuel cell and
metal-air battery technologies.[2-4] Among numerous research
works, heteroatom-doping has been proved as an effective
[a]
Y. Yu, Prof. Z. Zhang, Prof. F. Wang
State Key Laboratory of Chemical Resource Engineering Laboratory
of Electrochemical Process and Technology for Materials
Beijing University of Chemical Technology
Beijing 100029 (China)
E-mail: wangf@mail.buct.edu.cn (F. Wang)
zhangzhengping@mail.buct.edu.cn (Z. Zhang)
Y. Yu, Prof. Z. Zhang, Prof. L. Dai, Prof. F. Wang
Beijing Advanced Innovation Center for Soft Matter Science and
Engineering, College of Energy
Beijing University of Chemical Technology
Beijing 100029 (China)
Prof. L. Dai
[b]
[c]
As one of the important directions for this pyrolysis-free
strategy, the efficient electron acceptor can enhance the charge
delocalization for further increasing the catalytic activity of
carbonaceous materials. In the concept of intermolecular charge
transfer, rather than the conventional heteroatom-doping method,
we synthesized one kind of polyelectrolyte, epichlorohydrin-
dimethylamine copolymer (EDC), as the novel electron acceptor
with strong electronic affinity, and further prepared the EDC-
modified CNT (CNT-EDC) hybrids to serve as metal-free
carbonaceous electrocatalysts for oxygen reduction. As the
result of electron modulation, the CNT-EDC sample with positive
Center of Advanced Science and Engineering for Carbon
(Case4carbon)
Department of Macromolecular Science and Engineering Case
Western Reserve University
Cleveland, OH 44106 (USA)
E-mail: liming.dai@case.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.2018xxxxx.
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charge distribution not only presented a remarkable ORR activity
with a more positive half-wave potential than those of the
pristine CNT and classic poly(diallyldimethylammonium chloride)
polyelectrolyte modified CNT (CNT-PDDA), but also a superior
catalytic stability and selectivity.
by amorphous substance with the thickness of ca. 2 nm, which
mostly probably recognized as the EDC chains.
Figure 2. High-resolution XPS spectra of a) N 1s, b) C 1s and c) O 1s for
CNT-EDC, EDC, and CNT (without the XPS spectra of N 1s). d) Raman
spectra of the CNT-EDC and pristine CNT.
Figure 1. a) Illustration for the preparation of the EDC polyelectrolyte and
EDC-modified CNTs as metal-free carbonaceous electrocatalysts. HR-TEM
(inset: TEM) images of the b) pristine CNT and c) CNT-EDC samples.
Furthermore, we conducted the X-ray photoelectron
spectroscopy (XPS) measurement to investigate the surface
chemical states of CNT, resultant EDC and CNT-EDC. As
shown in the Figure 2a and Table S1, the high-resolution XPS
spectrum of N 1s for EDC was deconvoluted into two peaks at
403.5 and 400.7 eV, corresponding to the quaternary N (from
the quaternary-ammonium group in the chains of copolymer)
and tertiary N (from the tertiary-ammonium group on the end of
copolymer), respectively. In view of repetitive structure unit of
EDC was -CH2-CH(OH)-CH2-N(CH3)2-, the atom percentage of
N and O should be 5.26 at.%. The actual atom percentage of N
for EDC was 5.29 at.%, which was very close to the theoretical
value. Compared with the pure EDC electrolyte, the XPS
spectrum of N 1s for CNT-EDC exhibited the disapperance of
tertiary N and the low-binding-energy shift of quaternary N (at
403.2 eV), indicating that the electron on the CNT had been
transferred to the wrapped EDC chains.[19] As for the CNT-EDC
sample, the XPS of C 1s curve deconvolution (Figure 2b)
yielded into four peaks for the C=C, C–C, C–N, and O=C–O
species (at 285.0, 285.5, 286.2, and 287.1 eV, respectively),
demonstrating that the EDC electrolyte was successfully
wrapped on CNT.[21,27] Additionally, it was worth noting that the
content of O species for EDC (29.73 at.%) was higher than
theoretical value due to its strong tendency to absorb water in air.
The CNT-EDC sample showed much higher content of element
O (14.11 at.%) than CNT (3.84 at.%). Therefore, the high-
resolution XPS spectra for O 1s of EDC, CNT and CNT-EDC
(Figure 2c) were further deconvoluted into four peaks
corresponded as O–H (531.2 eV), O=C–O (532.5 eV), C–O
(533.4 eV) and absorbed O (534.4 eV).[11] The C-O peak in O 1s
spectra might really reflect the hydroxyl functional group in EDC
and the percentage of O-C (7.17 at.%) was relatively close to
theoretical value. Contrary to the EDC (2.30 at.%) and CNT
(0.76 at.%) sample, an largely increased content of absorbed O
To synthesize the quaternary-ammonium/ hydroxyl-
containing copolymeric electrolyte, the EDC polyelectrolyte was
synthesized through the ring opening polymerization method at
65 °C (Scheme S1, see the Supporting Information for details).
The chemical structure of homemade EDC polyelectrolyte was
confirmed by 13C nuclear magnetic resonance (NMR) and
Fourier transform infrared (FT-IR) spectroscopies. As shown in
Figure S1, the strong quadruple peak at around 54.0 ppm of the
EDC polyelectrolyte was ascribed to the methyl carbons of
quaternary ammonium in 13C NMR spectrum.[24] The methylene
carbons adjacent to quaternary ammonium was proved by
peaks at 70.6 and 74.1 ppm. Besides, three sharp peaks at 57.7,
61.7, and 67.7 ppm were attributed to the methylene carbons
connected to hydroxyl at different chemical environment,
respectively. In addition, the FT-IR spectra in Figure S2 showed
the absorption peaks at 1480 and 1050 and 3300 cm-1, which
were attributable to the C–N bond (in quaternary ammonium
groups), C–O bond and O–H bond (in hydroxyl groups),
respectively.[25,26] Subsequently, a homogeneous dispersion of
EDC with the hydrochloric-acid-purified multi-wall carbon
nanotube in an aqueous solution was subjected to sonication
treatment (Figure 1a). The CNT-EDC sample also exhibited the
corresponding characteristic peaks of quaternary ammonium
and hydroxyl groups, along with the absorption peaks for the
vibration of aromatic C=C (1620 cm-1) corresponding to carbon
nanotubes. To observe the detailed morphologic structure of the
CNT-EDC and CNT samples, the characterization of high-
resolution transmission electron microscopy (HR-TEM) was
conducted. The pristine CNT (Figure 1b) presented high
carbonization degree with the highly oriented lattice fringe and
the smooth surface; however, it was obviously recognized that
the nanotube wall surface in CNT-EDC (Figure 1c) was wrapped
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species (4.65 at.%) was observed for the CNT-EDC hybrid,
suggesting that the CNT-EDC hybrid could facilitate to adsorb
oxygen molecules when the CNT were combined with the EDC
electrolyte.[28] Moreover, the variations for the charge state on
carbon between the pristine CNT and CNT-EDC hybrid were
also consistent with the Raman spectra. As shown in Figure 2d,
the G-band representing the high graphization structure were
both observed in the CNT-EDC and CNT samples; however, the
G-band in the CNT-EDC sample shifted from 1576 to 1580 cm-1
in comparison with CNT, attributed to the electron transfer from
the CNT to EDC polyelectrolyte.[29]
that the CNT-EDC electrode presented the enhanced catalytic
activity with a positive onset potential (Eonset) of 0.87 V and half-
wave potential (E1/2) of 0.70 V, along with a large diffusion
limited current density (Jd, 3.52 mA cm-2, 2- and 1.2-fold
enhancements compared with CNT and CNT-PDDA,
respectively). Despite the inferior catalytic activity compared with
the commercial Pt/C electrodes, the positive Eonset of CNT-EDC
was close to that of Pt/C (0.93 V); meanwhile, with respect to
CNT and CNT-PDDA, a relatively wide current plateau and good
ORR activity of the CNT-EDC electrode indicated that the EDC-
induced intermolecular charge transfer could obviously promote
the ORR activity, and better than that caused by PDDA.
To investigate the electrocatalytic performance of the
resultant CNT-EDC sample, the ORR activities of the CNT and
CNT-EDC electrodes were first evaluated by the cyclic
voltammograms (CV) measurement, in comparison with the
homemade CNT-PDDA sample, which underwent the same
preparation procedure by using PDDA instead of EDC. As
shown in Figure 3a, all of the three electrodes showed the
Considering that the electron transfer pathway is the major
factor to evaluate the catalytic activity of ORR electrocatalysts,
we further measured the electron transfer number per O2 (n) by
-
both of the percentage of peroxide (% HO2 ) and the Koutecky–
Levich (K-L) equation. Figure S6 showed that the CNT-EDC
presented an enhanced capability to decrease the yield of the
peroxide intermediates in comparison with CNT and CNT-PDDA
(typical 2-electron transfer pathway), exhibiting between 4-
electron and 2-electron transfer pathway. Additionally, the K-L
plots of three CNT-derived electrocatalysts reconfirmed the
electron transfer number increased with introducing the EDC
polyelectrolyte (Figure S7 and S8). As one of the weakly O2-
binding materials, a rapid first-electron transfer implied the
enhanced ORR activity. To evaluate the kinetics of these metal-
free electrocatalysts, the Tafel slope was also calculated at the
low over-potential region (Figure 3c) to evaluate the rate for the
first-electron transfer in ORR process. The CNT-EDC exhibited
much smaller Tafel slope (67 mV decade-1) after being wrapped
with EDC, which also exhibited a higher transfer coefficient for
ORR kinetics with an accelerated first-electron transfer than
pristine CNT (110 mV decade-1) and CNT-PDDA (78 mV
decade-1). Furthermore, the whole kinetics for ORR process
were also evaluated by the electrochemical impedance
measurement (EIS). As shown in Figure 3d, the Nyquist plots at
open circuit potential, in which the charge-transfer resistance
(Rct) was quantified by fitting for the Randle’s equivalent circuit
(the inset in Figure 3d).[30] The CNT-EDC exhibited the smaller
Rct of 49.3 Ω than CNT (283.1 Ω) and CNT-PDDA (239.8 Ω),
indicating that the CNT-EDC presented the higher activity than
CNT and CNT-PDDA. The high catalytic stability and selectivity
of CNT-EDC were also observed in this work with 87.8% relative
retained current density after 10,000 s operation, and being free
from the methanol-crossover effect (Figure S9).
cathodic peaks in O2-saturated 0.1
M KOH electrolyte;
meanwhile, the CNT-EDC electrode exhibited the most positive
ORR peak potential (at 0.68 V vs. RHE, the same below if not
notice) and the largest peak current density (ca. 2.06 mA cm-2)
in the three electrodes (CNT: 0.63 V, 1.08 mA cm-2; CNT-PDDA:
0.66 V, 1.62 mA cm-2). As for CNT-EDC, the positively shifted
potential and enlarged current density of cathodic ORR peaks
indicated the enhanced ORR activity compared with the pristine
CNT and CNT-PDDA electrode.
Figure 3. a) CV curves of the CNT-EDC, CNT-PDDA and CNT electrodes in
O2-saturated (solid line) or N2-saturated (dashed line) 0.1 M KOH at a sweep
rate of 50 mV s-1. b) RRDE polarization curves and c) the corresponding Tafel
plots of the CNT-EDC, CNT-PDDA, CNT and commercial Pt/C electrodes in
O2-saturated 0.1 M KOH at a sweep rate of 5 mV s-1 with 1600 rpm. The ring
potential was constant at 1.5 V (top) and the disk potential was scanned at 5
mV s-1 (bottom) in 3b. d) Nyquist plots of the CNT-EDC, CNT-PDDA and CNT
electrodes at open circuit potential. Inset is the equivalent circuit, where Rs is
uncompensated resistance, Rct is charge transfer resistance, and CPE is
constant-phase element (equivalent to the double-layer capacitance).
To fundamentally analyze the enhancement of ORR
performance and catalytic mechanism of CNT-EDC, the Zeta
potential was performed to characterize the charge states of
CNT-EDC, CNT-PDDA and CNT in aqueous solution. As shown
in Figure 4a, the Zeta potential of CNT was -11 mV owing to the
negative charge of oxygen functional groups. After being
wrapped with the EDC and PDDA polyelectrolyte, the Zeta
potential of CNT-EDC positively shifted to 61 and 48 mV,
respectively, due to the cationic quaternary ammonium. The
more positive potential of CNT-EDC than CNT-PDDA indicated
that more positive charge distribution were induced by EDC than
PDDA. The positively charged property was also investigated by
The improved ORR activities were further confirmed by the
linear sweep voltammetry (LSV) measurements on a rotating
ring-disk electrode (RRDE). Figure 3b and Figure S5 showed
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using the CV measurements in Fe(CN)63-/4- solution. As shown in
Figure 4b, the CV curves of all the above three electrodes
exhibited a couple of typical redox peaks of Fe(CN)63−/4−, while
the peak current density significantly increased when the
working electrode reserved as CNT-EDC (anodic current
responses of 1.56 mA cm-2) compared with CNT and CNT-
PDDA (0.67 and 0.86 mA cm-2, respectively). The enhanced
interfacial electron transfer from CNT to EDC (Mulliken charge of
-0.465 for the nitrogen atom in PDDA).[32] Significantly, the
covalently bonded hydroxyl in EDC played an essential role in
modulating the electronic structure of quaternary ammonium. As
shown in the LUMO (lowest unoccupied molecular orbital, which
is the charge state when the polyelectrolyte accepts one
electron from CNT) spatial distributions, the electronegativity
obviously spread on the polymer skeleton from the nitrogen
atom to the oxygen atom, indicating that the obtained electron
(by quaternary ammonium) could transfer to the hydroxyl group.
As a result, the quaternary ammonium-hydroxyl covalently
bonding structure would perform as the synergetic effect to
increase the electronic affinity of EDC, which further enhanced
the intermolecular charge transfer and the ORR activity of CNT-
EDC. As an important frontier, the well-defined quaternary
ammonium-hydroxyl structure could construct more efficient
current response should be attributed to the strong electrostatic
3-/4-
interaction between Fe(CN)6
(negatively charged) and CNT-
EDC (positively charged), which could facilitate to adsorb more
negative charged species (e.g., the oxygen-containing
molecules) on the electrode surface.[31]
electron-acceptor
for
the
noncovalent-induced
charge
delocalization, and might also serve as a significant reference to
design the rational nitrogen and oxygen-codoping structure for
heteroatoms-doped carbonaceous materials.
In summary, the novel EDC polyelectrolyte with the
quaternary ammonium-hydroxyl covalently bonding structure
was designed as electron acceptor to enhance the
intermolecular charge transfer, rather than the conventional
heteroatom-doping methods to construct the efficient metal-free
carbonaceous electrocatalysts. The structural characterization
and electrochemical evaluation indicated that the EDC
polyelectrolyte had successfully synthesized and wrapped onto
the CNT, which further induced the corresponding positive
charge distribution on the adjacent carbon lattice. The CNT-EDC
sample with positive charge distribution presented a remarkable
ORR activity, along with a superior stability and selectivity in
alkaline electrolyte. The DFT theoretical calculation indicated
that the covalently bonded the quaternary ammonium-hydroxyl
performed as the synergetic effect to increase the electronic
affinity of EDC, and to further enhance intermolecular charge
transfer CNT. The novel CNT-EDC hybrid and the pre-designed
strategy in this work opens a window to develop well-defined
metal-free carbonaceous electrocatalysts in an atomically
precise regulation in clean energy conversion systems.
Figure 4. a) Zeta potential of the CNT-EDC, CNT-PDDA and CNT. b) CV
3-/4-
curves of above samples in 1 M KCl solution containing 10 mM Fe(CN)6
at
a sweep rate of 50 mV s-1. c) Calculated charge density distribution for a
single-walled carbon nanotube (left), a single-walled CNT attached with one
constitutional repeating unit of EDC (medium) or PDDA (right). d) DFT-
optimized molecular structure and LUMO spatial distributions of α electron for
the single unit of EDC+ and PDDA+ (without Cl- ion). The green and red
regions represent the positive and negative charge distributions, respectively.
To confirm the experimental results and figure out the
intrinsic enhancement of the EDC electrolyte, we developed the
density functional theory (DFT) calculations to elucidate the
change of charge state and electron structure. As demonstrated
in Figure 4c, the CNT presented the electroneutrality of sp2
carbon lattice; however, the notable positively charge
redistribution was observed for the CNT with the single-unit
molecular EDC and PDDA. Notably, the EDC induced the
stronger interfacial electron transfer than PDDA, exhibiting larger
positive charge state on the EDC-modified CNT. Figure 4d
showed the electron structure of single-unit molecular EDC and
PDDA (without Cl- ions). Owing to the existence of quaternary
ammonium, both EDC+ and PDDA+ presented the
electronegativity on the nitrogen atoms and electropositivity on
the methyl groups, which would accept electrons from CNT and
cause the positive charge distribution on adjacent carbon atoms.
The more negative Mulliken charge (-0.674) on nitrogen atom in
EDC indicated the stronger electronic affinity and faster
Experimental Section
Preparation of epichlorohydrin-dimethylamine
copolymer (EDC)
34.0 mL dimethylamine aqueous solution (33 wt.%) was decanted into a
100.0 mL round-bottom flask, and added 20.4 mL epichlorohydrin into
the round-bottom flask at 40 °C below. Then the mixture was heated and
stirred at 65 °C for 5 h. Finally, the buff liquid, as the crude product, was
purified by ethanol for five times. The synthetic mechanism of EDC via
the ring opening polymerization was shown in Scheme S1.
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Preparation of EDC functionalized carbon nanotubes
(CNT-EDC) and poly (diallyldimethylammonium chloride,
PDDA) functionalized carbon nanotubes (CNT-PDDA)
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Acknowledgements
This work was supported by the National Natural Science Funds
of China (51432003, 51802011, and 51125007); National Key
R&D Program of China (2018YFB0105500); the Start-Up Fund
for Talent Introduction of Beijing University of Chemical
Technology (buctrc201806); the Fund of High-Performance
Carbon-Based Electrodes for Energy Storage; and the
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Keywords: metal-free electrocatalysts • polyelectrolyte • carbon
nanotube • oxygen reduction reaction
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10.1002/chem.201806226
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Epichlorohydrin-dimethylamine
copolymer (EDC), as a novel kind of
polyelectrolyte, was synthesized and
carried to modify carbon nanotube to
modulate the charge delocalization on
the adjacent carbon lattice. The
covalently bonded quaternary
ammonium—hydroxyl synergistically
enhanced intermolecular charge
transfer and further improved the
catalytic activity of the resultant hybrid
for oxygen reduction.
Yihuan Yu, Zhengping Zhang*, Liming
Dai*, Feng Wang*
Page No. – Page No.
Copolymer-induced intermolecular
charge transfer: Enhancing activity of
metal-free catalysts for oxygen
reduction
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