Nitrogen and Fluorine-Codoped Carbon Nanowire Aerogels as Metal-Free Electrocatalysts for Oxygen Reduction Reaction

Article abstract of DOI:10.1002/chem.201701969

The development of active, durable, and low-cost catalysts to replace noble metal-based materials is highly desirable to promote the sluggish oxygen reduction reaction in fuel cells. Herein, nitrogen and fluorine-codoped three-dimensional carbon nanowire aerogels, composed of interconnected carbon nanowires, were synthesized for the first time by a hydrothermal carbonization process. Owing to their porous nanostructures and heteroatom-doping, the as-prepared carbon nanowire aerogels, with optimized composition, present excellent electrocatalytic activity that is comparable to commercial Pt/C. Remarkably, the aerogels also exhibit superior stability and methanol tolerance. This synthesis procedure paves a new way to design novel heteroatom-doped catalysts.

Full text of DOI:10.1002/chem.201701969

A Journal of
Accepted Article
Title: Nitrogen and fluorine-codoped carbon nanowire aerogels as
metal-free electrocatalysts for oxygen reduction reaction
Authors: Shaofang Fu, Chengzhou Zhu, Junhua Song, Mark
Engelhard, Biwei Xiao, Dan Du, and Yuehe Lin
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201701969
Link to VoR: http://dx.doi.org/10.1002/chem.201701969
Supported by

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Nitrogen and fluorine-codoped carbon nanowire aerogels as
metal-free electrocatalysts for oxygen reduction reaction
Shaofang Fu,[a] Chengzhou Zhu,*[a] Junhua Song,[a] Mark H. Engelhard,[b] Biwei Xiao, Dan Du,
[c]
[a]
Yuehe Lin*[a]
Abstract: The development of active, durable and low-cost
catalysts to replace noble metal-based materials is highly
desirable to promote the sluggish oxygen reduction reaction in
fuel cells. Herein, nitrogen and fluorine-codoped three-
dimensional carbon nanowire aerogels, composed of
interconnected carbon nanowires were for the first time
synthesized by hydrothermal carbonization process. Owing to
their porous nanostructures and heteroatom-doping, the as-
prepared carbon nanowire aerogels with optimized composition
present excellent electrocatalytic activity that is comparable to
commercial Pt/C. Remarkably, the aerogels also exhibit superior
stability and methanol tolerance. This synthesis procedure
paves a new way to design novel heteroatom-doped catalysts.
under harsh environment, these carbon aerogels are promising
candidates for ORR electrocatalysts.
Aside from the morphological control, introduction of
heteroatoms (B, N, P, S and F) into the carbon structure is
another efficient strategy to improve the ORR activity of carbon-
based electrocatallysts.18-24 Theoretically, the electronegativity
difference between heteroatoms and carbon breaks the
electroneutrality of adjacent carbon atoms, thus generating
charged sites, which have been demonstrated to be favorable
for O
2
adsorption and charge transfer in ORR.4, 19
Herein, we for the first time successfully fabricated N and F-
codoped carbon nanowire aerogels (NFCNAs) through template-
directed hydrothermal carbonization. By adjusting the doping
level and carbonization condition, the electrocatalytic activity of
the NFCNAs was tuned in a wide range. Due to the synergistic
effect of dopants, good conductivity, large surface area and high
porosity, the as-synthesized NFCNAs catalysts obtained under
optimized condition present good ORR catalytic performance in
alkaline solution that is comparable to Pt/C.
Introduction
Pt-based nanomaterials have been widely used as catalysts to
facilitate the sluggish oxygen reduction reaction (ORR) due to
their good catalytic activity and 4-electron transfer process.1
However, the precious metals are expensive and scarce, which
hinders their practical applications in industry.4-6 In order to
reduce or even replace the precious metals, many efforts have
been made in developing non-precious metal and metal-free
-3
Results and Discussion
7-13
catalysts.
Among these noble metal-free catalysts,
catalytically active carbon materials are one class of the most
promising candidates. They are expected to prevent corrosion or
poisoning of metal-based active sites, thus rendering superior
stability. Carbon aerogels, composed of interconnected one-
dimensional (1D) or two-dimensional (2D) building blocks such
as carbon nanofibers/nanowires or graphene nanosheets, have
attracted intensive attentions, because they not only inherit the
properties of the unique building blocks, but also possess
structural and electrical advantages.14-17 Owing to their low cost,
good conductivity, large surface area, and sufficient durability
[a]
S. Fu, Dr. C. Zhu, J. Song, Dr. D. Du, Prof. Y. Lin
School of Mechanical and Materials Engineering
Washington State University, Pullman, WA 99164, USA
E-mail: chengzhou.zhu@wsu.edu; yuehe.lin@wsu.edu
[
b]
c]
M. Engelhard
Environmental Molecular Sciences Laboratory
Pacific Northwest National Laboratory, Richland, WA 99352, USA
[
Energy and Environmental Directory, Pacific Northwest National
Laboratory, Richland, WA 99352, USA
Figure 1. (A) TEM image of Te NWs. (B) SEM image of NFCNAs-18-1000
(inset: the corresponding digital picture). (C, D) TEM images of NFCNAs-18-
1
000.
Supporting information for this article is given via a link at the end of
the document.
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So far, several strategies have been reported to synthesize F-
doped nanomaterials, including chemical vapor deposition, arc
25, 26, 27
discharge and plasma process.
However, these methods
are hard to get extensive applications because of their high cost,
rigorous condition, high voltage and specific equipment
28
requirement. In this work, Te nanowires (NWs) were utilized as
hard templates to fabricate NFCNAs because of their ultra-thin
diameter, good dispersion and large-scale availability (Figure
29
1
A). Typically, Te NWs were first synthesized and then
dispersed in glucose solution to form a uniform mixture. Further
hydrothermal treatment under 180 °C for 15 h resulted in carbon
hydrogels. After removal of residuals, the hydrogels were
soaked in a certain amount of NH
4
F solution for 12 h. The final
products were obtained by freeze-drying and carbonization at
1
000 °C for 1 h with a heating/cooling rate of 5 °C /min (Figure
inset), leading to the graphitization of NFCNAs and
1B
decomposition of Te templates. By varying the mass ratio
between NH F and carbon materials, different aerogels were
obtained and donated as NFCNAs-R-1000 (R = NH F/carbon
materials mass ratio). The typical scanning electron microscopy
SEM) image of NFCNAs-18-1000 in Figure 1B shows that
NFCNAs exhibit highly porous 3D networks, composed of
interconnected carbon NWs with uniform size. It has been
proposed that the formation of carbonaceous matrices was
induced by dehydration and polymerization of glucose during the
4
4
(
Figure 2. (A) XRD patterns of NFCNAs-18 at different annealing temperatures.
XPS spectrum (B) and high resolution XPS spectra of N (C) and F (D) in
NFCNAs-18-1000.
hydrothermal process. In the meantime, the NWs interconnected
To gain insight into the crystal structure of the aerogels, X-ray
diffraction (XRD) was performed as revealed in Figure 2A. The
XRD patterns show the existence of two characteristic peaks at
with each other and self-assembled into 3D hydrogels.1
5, 24
The
detailed structure of the NWs in NFCNAs was further
characterized by transmission electron microscope (TEM). The
uniformly distributed NWs are clearly shown in Figure 1B with an
average diameter of ~51 nm. Similar structures can also be
observed in NFCNAs-9-1000 and NFCNAs-25-1000 as revealed
in Figure S1. Furthermore, the effect of the pyrolysis condition
on the morphology of NFCNAs was also studied. As shown in
Figure S2, the typical hollow structure can be seen in NFCNAs-
~
24.7 and 44.1°, corresponding to (002) and (100) plane of
30, 31
graphitic carbon, regardless of the pyrolysis temperature.
In
order to investigate the surface composition of NFCNAs, X-ray
photoelectron spectroscopy (XPS) analysis was conducted, as
shown in Figure 2B-D. The XPS quantitative results show that
the atomic ratio of C, O, N and F in NFCNAs-18-1000 is 93.96%,
4.2%, 1.8% and 0.04%, respectively. The N 1s spectrum of
18-800 and NFCNAs-18-900 with an inner diameter of ~ 10 nm,
NFCNAs-18-1000 was further deconvoluted into three functional
groups (Figure 2C), which include pyridic-N (397.1 eV), pyrrolic-
N (398.1 eV), and graphitic-N (400.0 eV). The deconvolution
results reveal that more atoms are in the form of pyridic N and
graphitic-N, which favors the ORR catalytic activity.32, 33 Two
which matches well with the thickness of Te NWs. The hollow
structure was also observed in CAN-1000 (Figure S3). It is worth
noting that the disappearance of a hollow structure in NFCNAs-
9-1000, NFCNAs-18-1000 and NFCNAs-25-1000 might be
attributed to the high pyrolysis temperature as well as the
introduction of F and N.
peaks can be observed in F 1s XPS spectrum in Figure 2D, that
are corresponding to ionic C-F bond (683.3 eV) and semi-ionic
C-F bond (686.9 eV). It has been demonstrated in the reported
works that the Fδ- in the ionic and semi-ionic C-F bonds is
different from free F ions in the solution.34 In NFCNAs, F still
bonds tightly with Cδ+ and shows some ionic characters. The
higher ratio of ionic C-F bond (69%) can also contribute to the
δ-
good ORR catalytic activity of the NFCNAs. In addition, N
2
adsorption/desorption measurement was carried to analyze the
surface area and pore size distribution of NFCNAs-18-1000. The
Brunauer-Emmett-Teller surface area and pore volume are
2
3
7
68.4 m /g and 0.5 cm /g (Figure S4), respectively. The pore
size distribution profile indicates that mesopores are dominant in
NFCNAs-18-1000. This novel porous structure with large
surface area contributes to faster mass transport and electron
transfer process, leading to enhanced catalytic performance.
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and NFCNAs-25-1000, which mainly attributes to the larger
electrochemical surface area (Figure S5A) and rational N and F
content. Figure S6 displays the effect of the pyrolysis condition
on the ORR activity of the aerogels. We can see much better
catalytic performance on NFCNAs-18-1000 electrode due to the
high electrochemical surface area (Figure S6A) as well as the
synergistic effect of dopants with proper composition of
heteroatoms.
2
Figure 3. (A) CV curves of Pt/C, CNAs-1000, and NFCNAs-18-1000 in N and
O -saturated 0.1 M KOH solution with a scan rate of 50 mV/s. (B) LSV curves
2
of catalysts with a scan rate of 10 mV/s at a rotating rate of 1600 rpm. (C)
Tafel plots of commercial Pt/C and NFCNAs-18-1000 catalysts.
The electrocatalytic activities of the as-synthesized aerogels
were evaluated and compared with commercial Pt/C catalyst by
cyclic voltammetry (CV) measurements in N
.1 M KOH solution with a scan rate of 50 mV/s. Figure 3A
reveals that all the catalysts display well-defined cathodic peaks
in O -saturated electrolyte while no peak is found in N -saturated
solution. Remarkably, NFCNAs-18-1000 presents more
2 2
and O -saturated
0
2
2
a
positive peak potential compared with that of the undoped
aerogels (CNAs-1000). This value is even comparable with that
observed on Pt/C catalyst, indicating the enhanced ORR
electrocatalytic activity of NFCNAs-18-1000. To further
demonstrate the good catalytic activity of the NFCNAs, we
performed linear sweep voltammetry (LSV) using rotating disk
2
Figure 4. (A) LSV curves of NFCNAs-18-1000 in O -saturated 0.1 M KOH
2
electrode (RDE) in O -saturated 0.1 M KOH solution. Figure 3B
solution under different rotating rate. (B) The corresponding K-L plots at
different voltages. (C) LSV curves of catalysts recorded using RRDE with a
scan rate of 10 mV/s at a rotating rate of 1600 rpm. (D) Number of transferred
reveals the LSV curves for CNAs-1000, NFCNAs-18-1000, and
commercial Pt/C catalysts with a rotating rate of 1600 rpm and a
scan rate of 10 mV/s. Compared with CNAs-1000 (0.841 and
electrons on Pt/C, CNAs-1000, and NFCNAs-18-1000. (E) H
different catalysts based on RRDE data.
2 2
O yield on
0.697 V), NFCNAs-18-1000 shows a more positive onset
potential and half-wave potential of 0.912 and 0.825 V, which
are comparable to that of Pt/C (0.934 and 0.844 V). To
investigate the intrinsic advantages of the unique structure, the
Tafel plots are derived from the LSV curves of these three
samples and shown in Figure 3C. The Tafel slope for NFCNAs-
To study the ORR process on the catalysts, the Koutecky-Levich
K-L) plots were obtained at different potential based on the LSV
curves with various rotating speeds (Figure 4A). The good
linearity and parallelism in Figure 4B suggests the first-order
reaction kinetics regarding the dissolved oxygen in the
solution.35, 36 It is generally accepted that ORR is a multi-step
(
18-1000 is about 70 mV/dec, which is smaller than that of
commercial Pt/C (99 mV/dec), demonstrating a faster kinetic
process of ORR on NFCNAs-18-1000 electrode. It is worth
noting that the catalytic activity of NFCNAs-18-1000 is
comparable with or even outperforms some reported catalysts
process via either a direct four-electron pathway or a two-
-
electron pathway with HO
2
as the intermediate species. A four-
electron pathway is highly desired for the catalyst since this can
obtain better ORR efficiency.5 The number of electron
transferred (n) on different electrodes were calculated from the
ring and disk currents obtained from the rotation ring-disk
electrode (RRDE) measurements (Figure 4C) based on equation
(Table S1). Moreover, the ORR catalytic activity of the aerogels
with different composition was also investigated. Figure S5B
presents that NFCNAs-18-1000 has a more positive onset
potential and half-wave potential than that of NFCNAs-9-1000
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1
.37 The average n value for NFCNAs-18-1000 is 3.7, which is
stability of NFCNAs-18-1000 is mainly attributed to the following
factors: 1) the unique porous 3D structure composed of
interconnected NWs is expected to provide large surface area,
fast electron-transfer as well as remarkable structural
robustness in alkaline condition; 2) The more electronegativity of
N and F induces positive charges on nearby C atoms, which can
similar as Pt/C (n=3.97), as shown in Figure 4D. According to
-
the RRDE results, the HO
2
yield were also obtained from
% produced on
different catalysts. In comparison with CNAs-1000, which
equation 2.37, 38 Figure 4E shows the HO -
2
presents a high HO
lower HO
2
-% value, NFCNAs-18-1000 yields much
over a large potential range, further confirming its
-
2
2
not only provide faster charge-transfer but promote O to adsorb
higher ORR efficiency.
on the surface of catalysts. By virtue of the synergistic effect
between N and F as well as the proper doping level, the as-
prepared NFCNAs exhibit enhanced catalytic activity for ORR. 3)
In contrast to the free F ions in the solution, F in the ionic and
semi-ionic C-F tightly bonds with Cδ+, making the catalysts less
vulnerable to corrosion, resulting in better stability.
ꢂꢃ
n ꢀ 4ꢁ
ꢇ1ꢈ
ꢂ ꢄ ꢂ ⁄ꢆ
ꢃ
ꢅ
δ-
ꢂꢅ⁄ꢆ
ꢉꢊꢋꢌ% ꢀ 200
ꢇ2ꢈ
ꢂ ꢄ ꢂ ⁄ꢆ
ꢃ
ꢅ
where ꢂꢃ and ꢂꢅ is the disk and ring current, respectively, which
can be obtained according to Figure 4C. ꢆ=0.37 is the collection
efficient.
Conclusions
In this work, we demonstrated a facile strategy to synthesize the
N, F-codoped metal-free ORR catalysts by combining the hard-
templating method and hydrothermal carbonization. By tuning
the concentration of dopant and the pyrolysis condition, we
successfully fabricated a class of NFCNAs, which are composed
of interconnected carbon nanowires with the diameter ~ 50 nm.
This synthesis procedure achieves simultaneous optimization of
both surface functionality and 3D porous structure, leading to the
comparable ORR activity to that of commercial Pt/C in alkaline
solution. Importantly, the NFCNAs synthesized under optimized
condition exhibit excellent long-term stability and good tolerance
to methanol crossover effect. This superior electrocatalytic
performance should be attributed to the synergistic effect of
dopants with optimal composition, highly porous structure, and
the unique 3D interconnected morphology, which could not only
facilitate the mass-transport and electron-transfer during ORR
process but eliminate the aggregation and dissolution effects
existing in metal nanoparticles-based catalysts.39 This synthetic
protocol for the development of N, F-codoped carbon nanowire
aerogels might show potential applications in the development of
other metal-free electrocatalysts with enhanced ORR
performance.
Figure 5. CV curves of NFCNAs-18-1000 (A) and Pt/C (B) with and without the
addition of 1 M methanol. (C) Current vs time curves obtained under 0.7 V for
Pt/C and NFCNAs-18-1000 in O
00 rpm.
2
-saturated 0.1 M KOH at a rotating rate of
2
To further demonstrate the superior catalytic performance of
NFCNAs-18-1000, its tolerance to methanol and durability were
also studied. Small molecules, such as methanol, can transport
through the membrane easily, thus reducing the cathodic
reactant. Moreover, the intermediate product (e.g. carbon
monoxide) might adsorb onto the surface of the catalysts,
leading to the poisoning of the cathode materials. All these
effects can lower the efficiency of the fuel cells. Both NFCNAs-
18-1000 and commercial Pt/C catalysts were subjected to the
methanol crossover tests to evaluate their tolerance to methanol.
NFCNAs-18-1000 presents a stable CV curve without any
electroactivity specific to methanol when it was added into the
solution (Figure 5A), demonstrating the remarkable tolerance to
methanol crossover effect of the NFCNAs-18-1000 electrode. In
Experimental Section
Experimental section is available in supporting information.
contrast,
oxidation/reduction peaks with the addition of 1 M methanol Acknowledgements
Figure 5B). Figure 5C displays the relative current vs time
curves obtained under 0.3 V for Pt/C and NFCNAs-18-1000 in
-saturated 0.1 M KOH at a rotating rate of 200 rpm, where
Pt/C
shows
a
characteristic
methanol
(
This work was supported by a start-up fund of Washington State
University, USA. The XPS analysis was performed using EMSL,
a national scientific user facility sponsored by the Department of
Energy's Office of Biological and Environmental Research and
located at Pacific Northwest National Laboratory (PNNL). We
O
2
NFCNAs-18-1000 only lost 5% of its original activity over 20000
s of the continuous reaction. However, a substantial loss of
current (44%) is observed on the commercial Pt/C. The
superiority of the stability on NFCNAs-18-1000 electrode was
also demonstrated by its morphology change. No obvious
changes can be observed in the TEM images of NFCNAs-18-
acknowledge Franceschi Microscopy
& Image Center at
Washington State University for TEM measurements. PNNL is a
multi-program national laboratory operated for DOE by Battelle
under Contract DE-AC05-76RL01830.
1000 after the stability test, while the Pt/C catalyst shows a
severe aggregation of the Pt particles (Figure S7). The superior
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N, F-codoped carbon nanowire
Shaofang Fu, Chengzhou Zhu,* Junhua
aerogels
hydrothermal
procedure.
were
synthesized
by
Song, Mark H. Engelhard, Biwei Xiao,
Dan Du, Yuehe Lin*
carbonization
functionalized
The
aerogels present high electrocatalytic
activity, superior stability and
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methanol-tolerance for ORR in
alkaline condition.
Nitrogen
and
fluorine-codoped
carbon nanowire aerogels as metal-
free electrocatalysts for oxygen
reduction reaction
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