Self-Assembled Fe–N-Doped Carbon Nanotube Aerogels with Single-Atom Catalyst Feature as High-Efficiency Oxygen Reduction Electrocatalysts

Article abstract of DOI:10.1002/smll.201603407

Researchers developed an efficient and universal approach for synthesizing the Fe–N-doped carbon nanotube aerogels (Fe-N-CNTAs) using tellurium nanowires as hard templates in the presence of nitrogen-containing small molecule and inorganic salt. For the f

Full text of DOI:10.1002/smll.201603407

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Aerogels
Self-Assembled Fe–N-Doped Carbon Nanotube Aerogels
with Single-Atom Catalyst Feature as High-Efficiency
Oxygen Reduction Electrocatalysts
Chengzhou Zhu, Shaofang Fu, Junhua Song, Qiurong Shi, Dong Su,
Mark H. Engelhard, Xiaolin Li, Dongdong Xiao, Dongsheng Li, Luis Estevez,
Dan Du, and Yuehe Lin*
The development of robust electrocatalysts for oxygen reduc- typically involve careful optimization of nitrogen/metal pre-
tion reaction (ORR) is the highest priority for commercializa- cursors and carbon supports, followed by a tedious acidic
[
1–3]
[8]
tion of fuel cells and other electrochemical energy devices.
leaching and further thermal treatment.
It should be
The inherently sluggish reaction kinetics of the ORR makes pointed out that most of the reported M–N–C nanostructures
Pt-based nanomaterials the most efficient catalysts currently are usually characterized by heterogeneous morphologies
[
4–6]
due to their superior electrochemical performances.
Nev- without accurate control of homogeneity of active sites. Spe-
ertheless, these noble metal-based nanocatalysts still suffer cifically, nanoscale engineering of these NPMCs with single-
from the prohibitive cost and scarcity of Pt in nature, low atom catalysis nature showed great promise in ORR due
stability, and also the issue of methanol crossover. To this to the lowest size limit and full atom utility.[18–20] Therefore,
end, rational design and synthesis of highly efficient non- the development of a facile approach to accurately design
precious metal catalysts (NPMCs) are thereby desperately this kind of single-atom catalysts is greatly needed and still
[
7–10]
needed and become a very important topic in this field.
Among them, transition metal–nitrogen–carbon (M–N–C,
remain challenging.
Aside from the composition control, rational tuning of
M = Fe, Co) based nanomaterials have been considered as the 3D porous structured carbon-based materials is another
the most promising class of NPMCs for ORR due to their effective way to enhance the ORR performance by affording
[
11–15]
high ORR activity in both alkaline and acidic media.
a high surface area and abundant exposed active sites, faster
[
21–24]
Although the nature of the active sites in M–N–C catalysts mass transport/diffusion, and electron-transfer path.
remains controversial in the scientific community, there is Two different synthetic approaches regarding this kind of
a consensus that the transition metal dopants enable them porous M–N–C nanostructures were involved. As for the first
robust catalysts and both M–N moieties and nitrogen doping method, porous M–N–C structures were obtained through
[
11,16,17]
are critical for the enhanced ORR performance.
So annealing treatment of nitrogen and metal precursors in
far the existing approaches for synthesizing M–N–C catalysts the presence of 3D templates such as mesoporous silica and
[
25–27]
silica nanoparticles.
For the other method, metal atoms
are first well embedded within the porous templates. Both
the M–N–C active sites and porous nanostructures can be
easily produced through direct carbonization at high tem-
Dr. C. Zhu, S. Fu, J. Song, Q. Shi,
Prof. D. Du, Prof. Y. Lin
School of Mechanical and Materials Engineering
Washington State University
Pullman, WA 99164, USA
[
28,29]
peratures.
issues should be addressed regarding the rational design of
D porous precursors and complicated post-treatments. Due
Despite these contributions, some critical
3
E-mail: yuehe.lin@wsu.edu
to the extremely low density, large surface area, and high
porosity, carbon-based aerogels have been an appealing class
of nanomaterials and opens up fascinating options for the
Dr. D. Su
Center for Functional Nanomaterials
Brookhaven National Laboratory
Upton, NY 11973, USA
[
30–32]
preparation of new functional electrode materials.
On
this basis, creation of novel M–N–C aerogels with special and
enhanced functions, especially the porous characteristic along
with advantageous building blocks and uniform active site
distribution at an atomic level, can offer enormous opportu-
nities to develop more state-of-the-art ORR NPMCs that can
potentially promote the development of fuel cells.
Herein, we developed an efficient and universal approach
for synthesizing the Fe–N-doped carbon nanotube aerogels
(Fe-N-CNTAs) using tellurium nanowires as hard templates
M. H. Engelhard, Dr. D. Xiao, Dr. D. Li, Prof. Y. Lin
Environmental Molecular Science Laboratory
Pacific Northwest National Laboratory
Richland, WA 99354, USA
Dr. X. Li, Dr. L. Estevez
Energy and Environmental Directory
Pacific Northwest National Laboratory
Richland, WA 99354, USA
DOI: 10.1002/smll.201603407
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Scheme 1. Schematic illustration of the synthesis procedure of Fe–N-CNTAs.
in the presence of nitrogen-containing small molecule and observed, leading to the low density of about 1.5 mg cm−3
inorganic salt (Scheme 1). For the first time, the evolution of (Figure S1, Supporting Information). TEM analysis was fur-
homogeneity of active sites with single-atom catalyst feature ther conducted and the result was shown in Figure 1B,C. It
and the creation of the CNTAs were simultaneously realized is found that interconnected CNTs with a diameter of 45 nm
through one-step hydrothermal method and subsequent heat constitute the obtained aerogel. Moreover, it was found that
treatment. Capitalizing on the advanced compositional and hollow structures with a diameter of 10 nm can be efficiently
structural features, the as-prepared catalysts possessed excel- yielded as tellurium nanowires are volatile at high tempera-
lent electrocatalytic performance for ORR and even outper- ture and can be eliminated during pyrolysis (Figure S2, Sup-
forms commercial Pt/C catalyst. Because of the convenient porting Information). Different from the previously M–N–C
and universal synthetic strategy, exceptionally improved structures derived from the direct annealing treatment of
ORR activity and durability, this kind of Fe–N-CNTAs may nitrogen and metal containing precursors, homogeneity of
find potential application in fuel cells.
composition with slightly smooth surface instead of distinct
The one-step hydrothermal approach was adopted to aggregated nanoparticles of iron species (e.g., metallic iron,
synthesize CNTAs precursor in aqueous solution at 180 °C oxides or carbides) embedded within carbon nanostructures
for 15 h in the presence of well-defined tellurium nanowires, was presented, which can be confirmed by high-resolution
glucosamine hydrochloride, and ammonium iron (II) sulfate TEM (HRTEM) (Figure 1E) and corresponding high-angle
with a mole ratio of 5. More detailed synthetic procedure annular dark-field scanning TEM (HAADF-STEM) images
is presented in the Supporting Information. The resultant (Figure 1 F). The ring-like selected area electron diffraction
hydrogel was soaked in distilled water to remove impuri- pattern also demonstrated its poor crystallinity (Figure 1D).
ties for several times and then freeze dried. Finally, after Moreover, elemental mapping images verify the existence
thermal treatment at 900 °C for 2h in nitrogen atmosphere, and highly homogeneous distribution of C, N, and Fe atoms
Fe–N-CNTAs-5-900 was obtained. Using tellurium nano- in the obtained Fe–N-CNTAs-5-900 (Figure 1G). To illus-
wires as template and glucose as carbon precursor to syn- trate the atomic structure of the Fe–N–C, high-resolution
[
33]
thesize carbon-based hydrogel was reported previously.
HAADF-STEM images were provided (Figure 1H,I). Several
Similarly, it is proposed that the successive dehydration and isolated bright dots, corresponding to heavier Fe atoms, could
polymerization of glucosamine triggered the formation of be well discerned in the carbon nanotube due to the different
the nitrogen-containing carbonaceous matrix coated on the Z-contrast between Fe and C. The small size of these dots
surface of tellurium nanowires. Meanwhile, the intermediate in the range of 0.2–0.3 nm further reveals the uniform dis-
nanowires join together and finally self-assemble into 3D persion of the individual Fe atom rather than Fe cluster and
nanowire hydrogel at the specific concentration of tellurium small crystals. As expected, the similar morphologies were
nanowires and carbon precursor. In this hydrothermal pro- also observed when the mole ratio between glucosamine
cess, metal ions interacted with hydroxyl and amino groups hydrochloride and ammonium iron (II) sulfate varied. The
were well embedded within carbonaceous matrix, leading to introduction of Fe has no effect on the resultant products in
the homogeneous distribution of metal ions within N-doped comparison to N-CNTAs-900 (Figure S3, Supporting Infor-
carbon nanowires.[
34–36]
It should be noted that the metal ions mation). As such, taking advantage of the great versatility
(Fe, Co, and Ni) homogenous doping and the creation of 3D for the synthesis of this kind of materials, Co–N-CNTAs and
nitrogen containing nanowires can be realized simultane- Ni–N-CNTAs were also obtained with the similar structural
ously through one-step hydrothermal approach, which sig- characteristics (Figure S4, Supporting Information).
nificantly simplifies the catalyst preparation and enables the
homogeneity of active sites within catalysts.
X-ray diffraction (XRD) was conducted to further ana-
lyze their crystal structure (Figure 2A). No other diffraction
The morphology of the resultant Fe–N-CNTAs-5-900 was peak was found except two distinct characteristic peaks at
[
37]
first investigated by scanning electron microscope (SEM) 2θ = 26.5° and 43° ascribing to (002) and (101), regardless
and transmission electron microscopy (TEM). As shown of the pyrolysis temperature. This result is consistent with the
in Figure 1A, a highly porous nanowire network with fairly TEM analysis mentioned above. Furthermore, X-ray photo-
uniform in size and shape without other byproducts was electron spectroscopy (XPS) was used to reveal the detailed
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Figure 1. A) SEM, B,C) TEM, E) HRTEM, and F,H,I) HAADF-STEM images of the obtained Fe–N-CNTAs-5-900. The selected area electron diffraction
pattern D) and elemental distribution G) of the C, N, and Fe atoms in the obtained Fe–N-CNTAs-5-900.
chemical composition of the resultant Fe–N-CNTAs-5 Brunauer–Emmett–Teller surface area and pore volume
Figure 2B–D). Survey scans of Fe–N-CNTAs-5 at different of Fe–N-CNTAs-5-900 were 638.5 m2
g
−1
and 1.6 cm g ,
3
−1
(
annealing temperatures confirm the existence of C, N, O, and respectively. The pores size distribution curve further indi-
Fe. Interestingly, we did not find the existence of Te in these cated their hierarchically porous nanostructures with abun-
final Fe–N-CNTAs, further verifying the fact that tellurium dant mesopores (Figure 2F). This novel nanomaterial
nanowires were completely removed at high temperatures characterized by large surface area and pore volume can not
and contributed to the evolution of the hollow structures, only make more catalytic active sites accessible but also guar-
which was much simpler than previous reports on the antee faster electron transfer and mass transport, enabling
[
38]
removal of Te nanowires. The C, N, O, and Fe contents in them very promising in electrocatalysis.
Fe–N-CNTAs-5-900 were determined to be 92.49, 2.06, 5.37,
Due to the unique nanostructured and compositional
and 0.09 at%, respectively. The nitrogen content decreased features (large surface area, 1D nanotube building block
from 4.51% to 2.06% with increasing heating temperature and homogeneity of active sites with single-atom catalyst
from 700 to 900 °C (Table S1, Supporting Information). The feature) of resultant 3D porous Fe–N-CNTAs, they are
high-resolution N1s spectrum of all Fe–N-CNTAs-5-900 can expected to be excellent ORR catalysts. To evaluate the
be deconvoluted into three different peaks at 398.3, 399.9, and electrocatalytic activities for ORR, we first carried out cyclic
4
00.9 eV, corresponding to pyridinic-N/Fe–N, pyrrolic-N, and voltammetry (CV) measurements on these newly designed
[
39–41]
graphitic-N, respectively (Figure 2D).
It is worth noting Fe–N-CNTAs-5-900 in 0.1 m KOH solution in comparison
that the peak located at 398.3 eV is also ascribed to nitrogen with N-CNTAs-900 and commercial Pt/C (Figure 3A). In
bound to the metal (Fe–N) because of the small difference in contrast to the virtually featureless CV curves within the
[
18,25]
binding energy between N−Fe and pyridinic-N.
Among entire potential range in N -saturated solution, the well-
2
them, 27% and 67.6% N atoms are in the form of pyridinic- defined cathodic peaks appeared in all the CV curves of
N and graphitic-N, respectively, and some of the pyridinic-N the three samples in the O -saturated solution. Compared
2
atoms are bonded with Fe. These dominated nitrogen-con- with N-CNTAs-900 and Pt/C catalysts, Fe–N-CNTAs-5-900
taining compositions are believed to be served as the ORR showed much more positive ORR peak (0.897 V) than
[
42]
active sites.
N-CNTAs-900 (0.777 V) and Pt/C (0.858 V). Comparison
To obtain the information on the surface area and of the ORR activity of the prepared catalysts and gaining
porosity properties of the final product, N2 physisorption further insight into the ORR process was studied using
measurement was carried out. As shown in Figure 2E, the rotating disk electrode (RDE) measurement at a scan rate
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Figure 2. A) XRD patterns and B) XPS spectra of Fe–N-CNTAs-5 annealed at different temperatures. C) XPS Fe 2p and D) N 1s spectra of the Fe–N-
CNTAs-5-900. E) Nitrogen adsorption and desorption isotherms and F) pore size distribution curve of Fe–N-CNTAs-5-900.
of 10 mV s− and was demonstrated in Figure 3B. It is clear crystallinity. To shed light on the enhanced ORR perfor-
that the Fe–N-CNTAs-5-900 exhibited a satisfactory onset mance, EASA was investigated, which is closely associated
potential and half-wave potential (0.97 and 0.88 V), which is with ORR activity. Figure S6C (Supporting Information)
more positive than that of N-CNTAs-900 (0.91 and 0.84 V) shows the typical CV curves of different Fe–N-CNTAs-5
1
and comparable to that of Pt/C (0.96 and 0.86 V). Further- curves in N -saturated 0.1 m KOH solution. Among them,
2
more, the Fe–N-CNTAs-5-900 afforded a large diffusion- Fe–N-CNTAs-5-900 possesses the highest capacitance
limiting current, which was superior to the N-CNTAs-900 associated with current density, indicative of the largest
and Pt/C. The effect of annealing temperature on the catalyst EASA and more accessibility of the surface of the carbon
[
43]
activity for ORR was studied. Although all of them present nanostructures.
Raman spectra were also provided in
−
1
the same morphology (Figure S5, Supporting Information), Figure S6D (Supporting Information). The D (≈1356 cm )
we found that annealing temperature plays a critical role in and G (≈1580 cm ) bands are ascribed to the disordered
the ORR activity and the obtained Fe–N-CNTAs-5-900 pos- carbon (sp ) and ordered graphitic carbon (sp ), respec-
sessed the best ORR activity in term of the high onset poten- tively. The nature of carbon regarding the intensity ratio of
−
1
3
2
[
44]
tial and limited current density (Figure S6A,B, Supporting D- and G-bands (I /I ) suggested that Fe–N-CNTAs-5-900
D
G
Information). The ORR performance at Fe–N-CNTAs-5-900 afford a lower I /IG value and therefore a more ordered gra-
D
was proposed to be closely associated with the improved phitic carbon structure and higher conductivity. The other
electrochemically active surface area (EASA) and carbon synthesis parameter that affects the ORR performance is the
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Figure 3. A) CV curves of Fe–N-CNTAs-5-900, N-CHNAs-900, and commercial Pt/C in N - and O -saturated 0.1 m KOH solution. The scan rate was
2
2
−
1
−1
5
1
0 mV s . B) RDE polarization curves of various catalysts in O -saturated 0.1 m KOH solution at a scan rate of 10 mV s and a rotation rate of
600 rpm. C) Tafel plots derived from Figure 2B. D) RDE polarization curves on Fe–N-CNTAs-5-900 at different rotation rates, inset: K–L plot of J
versus ω . E) Kinetic current density (j ) and the electron-transfer number (n) of the Fe–N-CNTAs-5-900 and Pt/C at different potentials. F) Current
2
−
1
−
1
k
versus time (i–t) chronoamperometric response of the Fe–N-CNTAs-5-900 and Pt/C at 0.7 V in O -saturated 0.1 m KOH at 200 rpm, respectively.
2
mass ratio between glucosamine hydrochloride and ammo- within the N-doped catalysts and provides the powerful
nium iron sulfate. As shown in Figure S7 (Supporting Infor- guidance for synthesizing high-rate ORR catalysts at single
mation), the optimized ORR catalyst was obtained when the atom level. As shown in Figure 3D, the RDE polarization
mole ratio between glucosamine hydrochloride and ammo- curves at various rotating speeds and corresponding Kout-
nium iron sulfate was 5. Besides, metal doping-dependent ecky–Levich (K–L) plots of Fe–N-CNTAs-5-900 at different
ORR behavior indicated that Fe–N-CNTAs-5-900 possessed applied potentials were provided. Accordingly, the electron
high ORR activity in comparison to the Co and Ni-based transfer number (n) of Fe–N-CNTAs-5-900 was calculated
samples (Figure S8, Supporting Information).
to be ≈4.0 at 0.4–0.7 V (Figure 3E), suggesting a mainly 4e
The Tafel curves of the catalysts were also displayed to oxygen reduction process. The significantly enhanced kinetic
assess the unique structural and compositional advantages current densities (j ) of the Fe–N-CNTAs-5-900 were also
k
of Fe–N-CNTAs-5-900 (Figure 3C). In the potential region observed at the potential range investigated. Remarkably, the
−
2
studied, the Fe–N-CNTAs-5-900 showed a Tafel slope of calculated j is 131.4 mA cm at 0.4 V, which was about 2.85-
8
k
−
1
−2
7.8 mV dec , almost the same with that of Pt/C catalysts, fold of Pt/C (46.1 mA cm ). The methanol crossover effects
illustrating a good kinetic process for ORR. Only 0.09 at% and durability were also investigated. As expected, the Fe–N-
Fe doping make the resultant catalyst more active, further CNTAs-5-900 showed better long-term stability (Figure 3F)
demonstrating the highly efficiency of the Fe–N–C active and tolerance to methanol crossover effect (Figure S9, Sup-
site compared with N-doped counterpart. This finding fur- porting Information) than Pt/C. Significantly, TEM images in
ther confirms the extreme efficiency of single atomic iron Figure S10 (Supporting Information) reveal that the structure
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