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

TLDR: Self-assembled M-N-doped carbon nanotube aerogels with single-atom catalyst feature are for the first time reported through one-step hydrothermal route and subsequent facile annealing treatment to exhibit excellent oxygen reduction reaction electrocatalytic performance even better than commercial Pt/C in alkaline solution.

Abstract: Self-assembled M-N-doped carbon nanotube aerogels with single-atom catalyst feature are for the first time reported through one-step hydrothermal route and subsequent facile annealing treatment. By taking advantage of the porous nanostructures, 1D nanotubes as well as single-atom catalyst feature, the resultant Fe-N-doped carbon nanotube aerogels exhibit excellent oxygen reduction reaction electrocatalytic performance even better than commercial Pt/C in alkaline solution.

Figures

Figure 1. SEM (A), TEM (B, C), HRTEM (E) and HAADF-STEM (F, H and I) images of the obtained Fe-N-CNTAs-5-900. The selected area electron diffraction pattern (D) and elemental distribution of the C, N and Fe atoms in the obtained Fe-N-CNTAs-5-900.

Figure 3. (A) CV curves of Fe-N-CNTAs-5-900, N-CHNAs-900 and commercial Pt/C in N2and O2-saturated 0.1 M KOH solution. The scan rate was 50 mV s -1 . (B) RDE polarization curves of various catalysts in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s -1 and a rotation rate of 1600 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 −1 versus ω −1 . (E)

Figure 2. XRD (A) and XPS (B) spectra of Fe-N-CNTAs-5 annealed at different temperatures. XPS Fe 2p (B) and N 1s (D) spectra of the Fe-N-CNTAs-5-900. Nitrogen adsorption and desorption isotherms (E) and pore size distribution curve (F) of Fe-N-CNTAs5-900.

Full text

BNL-113588-2017-JA
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,
Junzheng Chen, Luis Estevez, Dan Du, and Yuehe Lin
Accepted by Small
February 2017
Center for Functional Nanomaterials
Brookhaven National Laboratory
U.S. Department of Energy
USDOE Office of Science (SC),
Basic Energy Sciences (SC-22)
Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under
Contract No. DE-SC0012704 with the U.S. Department of Energy. The publisher by accepting the
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1
DOI: 10.1002/((please add manuscript number))
Article type: Communication
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, Junzheng Chen, Luis Estevez, Dan Du, and Yuehe
Lin*
[*] 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
E-mail: yuehe.lin@wsu.edu
Prof. Y. Lin, M. Engelhard, Dr. D. Xiao, Dr. D. Li,
Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory,
Richland, WA 99354, USA
Dr. X. Li, Dr. J. Chen, Dr. L. Estevez
Energy and Environmental Directory, Pacific Northwest National Laboratory, Richland, WA
99354, USA
Dr. D. Su
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York
11973, USA
C. Zhu and S. Fu contributed equally to this work.
Keywords: Aerogels. transition metal–nitrogen–carbon structures. single-atom catalysts.
nonprecious metal catalysts. oxygen reduction reaction
Finely controlled synthesis of high active and robust non-precious metal catalysts with
excellent catalytic efficiency for oxygen reduction reaction (ORR) is extremely vital for
successful implementation of fuel cells and metal batteries. Unprecedented ORR
electrocatalytic performances and the diversified synthetic procedure in term of favorable
structure/morphology characteristics make transition metals-derived M–N–C (M=Fe or Co)
structures the most promising nanocatalysts. Herein, using the nitrogen-containing small
molecular and inorganic salt as precursors and ultrathin tellurium nanowires as templates, we
for the first time successfully synthesized a series of well-defined M-N-doped carbon
nanotube aerogels with single-atom dispersion through one-step hydrothermal route and
subsequent facile annealing treatment. Taking advantage of the porous nanostructures, one-

2
dimensional building block as well as homogeneity of active sites and single-atom catalyst
feature, the resultant Fe-N-doped carbon nanotube aerogels exhibited excellent ORR
electrocatalytic performance even better than that of the commercial Pt/C in alkaline solution,
having great potential in fuel cell applications.
The development of robust electrocatalysts for oxygen reduction reaction (ORR) is the
highest priority for commercialization of fuel cells and other electrochemical energy
devices.
[1-3]
The inherently sluggish reaction kinetics of the ORR makes Pt-based
nanomaterials the most efficient catalysts currently due to their superior electrochemical
performances.
[4-6]
Nevertheless, these noble metal-based nanocatalysts still suffer from the
prohibitive cost and scarcity of Pt in nature, low stability, and also the issue of methanol
crossover. To this end, rational design and synthesis of highly efficient non-precious metal
catalysts (NPMCs) are thereby desperately needed and become a very important topic in this
field.
[7-10]
Among them, transition metal–nitrogen–carbon (M–N–C, M = Fe, Co) based
nanomaterials have been considered as the most promising class of NPMCs for ORR due to
their high ORR activity in both alkaline and acidic media.
[11-15]
Although the nature of the
active sites in M–N–C catalysts remains controversial in the scientific community, there is a
consensus that the transition metal dopants enable them robust catalysts and both M–N
moieties and nitrogen doping are critical for the enhanced ORR performance.
[11,16,17]
So far the
existing approaches for synthesizing M–N–C catalysts typically involve careful optimization
of nitrogen/metal precursors and carbon supports, followed by a tedious acidic leaching and
further thermal treatment.
[8]
It should be pointed out that most of the reported M–N–C
nanostructures are usually characterized by heterogeneous morphologies without accurate
control of homogeneity of active sites. Specifically, nanoscale engineering of these NPMCs
with single-atom catalysis nature showed great promise in ORR due to the lowest size limit

3
and full atom utility.
[18-20]
Therefore, the development of a facile approach to accurately
design this kind of single-atom catalysts is greatly needed and still remain challenging.
Aside from the composition control, rational tuning of the 3D porous structured carbon-based
materials is another effective way to enhance the ORR performance by affording a high
surface area and abundant exposed active sites, faster mass transport/diffusion and electron-
transfer path.
[21-24]
Two different synthetic approaches regarding this kind of porous M-N-C
nanostructures were involved. As for the first method, porous M-N-C structures were
obtained through annealing treatment of nitrogen and metal precursors in the presence of 3D
templates such as mesoporous silica and silica nanoparticles.
[25-27]
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
temperatures.
[28,29]
Despite these contributions, some critical issues should be addressed
regarding the rational design of 3D porous precursors and complicated post-treatments. Due
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 preparation
of new functional electrode materials.
[30-32]
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 opportunities 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 (CNTAs) using tellurium nanowires as hard templates in the
presence of nitrogen-containing small molecule and inorganic salt (Scheme 1). For the first
time, the evolution of homogeneity of active sites with single-atom catalyst feature and the
creation of the CNTAs were simultaneously realized through one-step hydrothermal method
and subsequent heat treatment. Capitalizing on the advanced compositional and structural

Frequently asked questions

Q1. Why is carbon-based aerogels an appealing class of nanomaterials?

Due 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 preparation of new functional electrode materials. [30-32] 

Q2. What is the description of the M-N-CNTAs?

Subsequent annealing treatment gives rise to the M-N-CNTAs characterized by advanced features including porous nanotube networks and homogeneity of active sites with single atom catalyst feature. 

Q3. What is the promising class of NPMCs for ORR?

Among them, transition metal–nitrogen–carbon (M–N–C, M = Fe, Co) based nanomaterials have been considered as the most promising class of NPMCs for ORR due to their high ORR activity in both alkaline and acidic media. [11-15] 

Q4. What is the common method of synthesizing carbon nanotubes?

Using tellurium nanowires as template and glucose as carbon precursor to synthesize carbon-based hydrogel was reported previously. [33] 

Q5. What is the synthesis parameter that affects the ORR performance?

The other synthesis parameter that affects the ORR performance is the mass ratio between glucosamine hydrochloride and ammonium iron sulfate. 

Q6. What is the effect of annealing on the ORR activity of Fe-N-?

The ORR performance at Fe-N-CNTAs-5-900 was proposed to be closely associated with the improved electrochemically active surface area (EASA) and carbon crystallinity. 

Q7. What is the consensus on the nature of the active sites in M–N–C catalysts?

Although the nature of the active sites in M–N–C catalysts remains controversial in the scientific community, there is a consensus that the transition metal dopants enable them robust catalysts and both M–N moieties and nitrogen doping are critical for the enhanced ORR performance. 

Q8. What is the first method used for the synthesis of M-N-C structures?

As for the first method, porous M-N-C structures were obtained through annealing treatment of nitrogen and metal precursors in the presence of 3D templates such as mesoporous silica and silica nanoparticles. [25-27] 

Q9. What is the Tafel slope of the Fe-N-CNTAs-5-900?

In the potential region studied, the Fe-N-CNTAs-5-900 showed a Tafel slope of 87.8 mV dec -1 , almost the same with that of Pt/C catalysts, illustrating a good kinetic process for ORR. 

Q10. What is the synthesis of M-N-doped carbon nanotubes?

Self-assembled M-N-doped carbon nanotube aerogels with single-atom catalyst feature are for the first time reported through one-step hydrothermal route and subsequent facile annealing treatment. 

Q11. What is the description of the Fe-N-CNTAs?

As expected, the Fe-N-CNTAs-900 showed better long-term stability (Figure 3F) and tolerance to methanol crossover effect (Figure S9) than Pt/C. Significantly, TEM images in Figure S10 reveal that the structure of nanowires was well maintained after stability test, while Pt/C catalyst showed obvious aggregations, confirming the better stability of Fe-N-CNTAs-5-900 during electrochemical process. 

Q12. What is the diffraction peak of Fe-N-CNTAs?

It is worth noting that the peak located at 398.3 eV is also ascribed to nitrogen bound to the metal (Fe−N) because of the small difference in binding energy between N−Fe and pyridinic-N. [18, 25]