Phase-Selective Syntheses of Cobalt Telluride Nanofleeces for Efficient Oxygen Evolution Catalysts

Article abstract of DOI:10.1002/anie.201701998

Cobalt-based nanomaterials have been intensively explored as promising noble-metal-free oxygen evolution reaction (OER) electrocatalysts. Herein, we report phase-selective syntheses of novel hierarchical CoTe₂ and CoTe nanofleeces for efficient

Full text of DOI:10.1002/anie.201701998

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Accepted Article
Title: Phase Selective Synthesis of Unique Cobalt Telluride
Nanofleeces for Highly Efficient Oxygen Evolution Catalyst
Authors: Qiang Gao, Chuan-Qi Huang, Yi-Ming Ju, Min-Rui Gao, Jian-
Wei Liu, Duo An, Chun-Hua Cui, Ya-Rong Zheng, Wei-Xue Li,
and Shu-Hong Yu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201701998
Angew. Chem. 10.1002/ange.201701998
Link to VoR: http://dx.doi.org/10.1002/anie.201701998
http://dx.doi.org/10.1002/ange.201701998

Angewandte Chemie International Edition
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COMMUNICATION
Phase Selective Synthesis of Unique Cobalt Telluride
Nanofleeces for Highly Efficient Oxygen Evolution Catalyst **
†
†
Qiang Gao , Chuan-Qi Huang , Yi-Ming Ju, Min-Rui Gao, Jian-Wei Liu, Duo An, Chun-Hua Cui, Ya-
Rong Zheng, Wei-Xue Li*, and Shu-Hong Yu*
Abstract: Co-based nanomaterials have been intensively explored
as promising noble metal free oxygen evolution reaction (OER)
electrocatalysts. Herein, we first report phase selective synthesis of
amorphous cobalt-phosphate-based material (Co-Pi),[8] Co-Pi/α-
Fe
composites,[9] Co-OEC (oxygen evolving complex),[10] and
so on. Besides advantages such as earth-abundant, low cost
and high yield, these Co-based materials can generate O under
mild conditions and modest overpotentials. Previous efforts have
demonstrated that CoSe nanobelts and CoSe /Mn
composites are promising OER catalysts in alkaline media.
Here, we show overwhelming interests in Te, as an element
among the transition metal-based chalcogenides, because
compared to O, S and Se atoms, Te atom exhibits more metallic
character, which is a favourable property for electrocatalysts due
to better electron conductivity.[12]
2
O
3
novel hierarchical CoTe
catalyst. The CoTe nanofleeces exhibited excellent electrocatalytic
activity and stablity for OER in alkaline media. It has been
discovered that the CoTe catalyst exhibited superior OER activity to
the CoTe catalyst and comparable to the state-of-the-art RuO
2
and CoTe nanofleeces for efficient OER
2
2
2
2
3 4
O
[
11]
2
2
catalyst. Density functional theory calculations showed that the
binding strength and lateral interaction of the reaction intermediates
2
on CoTe and CoTe are essential for the order of their overpotential
and the trend variation at different condition. This study provides
value insights for rational design of noble metal free OER catalysts
with high performance and low cost by use of Co-based
chalcogenides.
Although many metal tellurides have been synthesized,
researches into cobalt telluride hierarchical structures and their
applications are still rare.[ Among various fabricating methods,
Te nanowire directed synthesis is emerging as one of the most
popular strategies for the fabrication of 1D telluride
13]
Oxygen evolution reaction (OER) plays a significant role in
many electrochemical processes such as water splitting and
solar fuel synthesis.[1] Design and synthesis of active, stable,
and low-cost catalytic materials for water splitting is pivotal to the
development of sustainable energy sources for powering fuel
cells.[2] In practice, however, large-scale electrochemical
production of hydrogen from water splitting is greatly constrained
by two fundamental limitations: the high overpotentials of the
OER, and the lack of stability of electrode materials.[3] Currently,
the most widely used catalysts for OER in electrolysis cells are
ruthenium (Ru) and iridium (Ir) oxides, although these elements
are among the rarest elements on earth.[4] Therefore, there is an
urgent demand of alternative catalysts which are not only
suitable for large-scale adoption but also remain high efficiency.
In the last few decades, cobalt (Co)-based OER catalysts
have been developed for potential applications of water
nanostructures,[14] because ultrathin nanowires are desirable
[15]
templates for the fabrication of 1D nanostructures.
Herein, we
report a facile chemical transformation process for phase
selective synthesis of uniform hierarchical CoTe and CoTe
nanofleeces using ultrathin Te nanowires as templates, which
shows highly efficient OER performances. The CoTe catalyst
exhibited superior OER activity to the CoTe catalyst and
comparable to the state-of-the-art RuO catalyst in basic media.
Furthermore, such hierarchical CoTe nanofleeces also perform
very stable in 0.1 M KOH electrolyte. Density functional theory
DFT) calculations showed that the distinct binding strength and
lateral interaction of the reaction intermediates on CoTe and
2
2
2
2
(
2
CoTe is essential for the relative order of their overpotential and
trend variation at different conditions.
Firstly, ultrathin Te nanowires (Figure S1) were systhesized by
the modified method described previously.[16] The hierarchical
oxidation in alkaline environments, including nanostructured
2
CoTe nanofleeces were typically synthesized through an polyol
5]
and related hybrids,[6] Co
nanoparticles,[7]
Co
3
O [
4
2
O
3
reduction approach using the ultrathin Te nanowires as
templates. The X-ray diffraction (XRD) patterns (Figure S2) of
o
o
the samples obtained at 200 C and 220 C can be indexed to
orthorhombic CoTe (JCPDS 11-0553) and hexagonal CoTe
JCPDS 34-0420), respectively. The Co:Te atomic ratio
[
*] Dr.Q. Gao,[†] Mr. Y. M. Ju, Dr. M. R. Gao, Dr. J. W. Liu, Mr. D. An, Dr. C.
H. Cui, Dr. Y. R. Zheng, Prof. Dr. S. H. Yu
2
(
Department of Chemistry, Division of Nanomaterials and Chemistry,
Hefei National Laboratory for Physical Sciences at Microscale,
Collaborative Innovation Center of Suzhou Nano Science and
Technology, Center for Excellence in Nanoscience, Hefei Science
Centre of CAS, University of Science and Technology of China, Hefei,
Anhui 230026, P. R. China
determined by Energy-dispersive X-ray spectroscopy (EDS)
analysis (Figure S3) agrees well with the XRD result. The X-ray
photoelectron spectroscopy (XPS) data in Figure S4a reveal the
electron-binding energies of Co in CoTe
2
and CoTe both
E-mail: shyu@ustc.edu.cn
correspond to Co2+ cations.[17] Furthermore, synchrotron X-ray
[
†]
Mr. C. Q. Huang, Prof. Dr. W. X. Li
absorption spectroscopy analysis was also performed to further
determine the oxidation states of the cobalt (Figure S4b), which
was demonstrated to be +2 for both CoTe and CoTe as well.
2
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academic of Sciences, Dalian 116023, P. R. China
Department of Chemical Physics, Hefei National Laboratory for Physical
Sciences at Microscale, iChEM (Collaborative Innovation Center of
Chemistry for Energy Materials), Center for Excellence in Nanoscience,
University of Science and Technology of China, Hefei, Anhui 230026, P.
R. China
E-mail: wxli70@ustc.edu.cn
[
†] These authors contributed equally to this work.
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Angewandte Chemie International Edition
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COMMUNICATION
evolution of this reaction (Figure S6). After keeping the solution
at 110 oC for 1 h, all of the peaks could be indexed to the
hexagonal phase of Te. When the reaction temperature was
o
increased to 200 C for 15 min, trace peaks of CoTe
2
appeared
began to form.
Upon prolonging reaction time to 30 min, the product consisted
predominantly of CoTe , although the hexagonal phase of Te
in the XRD pattern, which indicated that CoTe
2
2
was also present. No hexagonal Te diffraction peaks were
observed when the reaction time was up to 1 h, which indicated
that the Te was completely transformed into CoTe .
2
Figure 1. SEM images of hierarchical CoTe
2
(a) and CoTe (d) nanostructures.
HRTEM image of one single CoTe (b) and CoTe (e) nanofleece. (c, f)
2
HAADF-STEM characterization of one single nanofleece and the
corresponding elemental mappings of Te (red) and Co (green).
The morphologies and sizes of the products were studied by
field-emission scanning electron microscopy (FESEM) and
transmission scanning electron microscopy. Figure 1a shows a
Figure 2. (a) Schematic illustration of the proposed growth mechanism for
hierarchical CoTe nanostructures. (b) SEM image of the product obtained at
2
110 oC for 60 min; SEM images of the products obtained at 200 oC for (c) 0
min; (d) 5 min; (e) 10 min; (f) 15 min; (g) 20 min; (h) 30 min; (i) 60 min; (j) 120
min.
2
typical SEM image of the hierarchical CoTe structures with
mean diameter of about 200 nm and length up to several
micrometers. The SEM image indicates that a large amount of
small nanofleeces align perpendicularly to the growth axis,
forming remarkable 1D hierarchical nanostructures. The
representative SEM image of as-prepared products clearly
reveals that the small nanofleeces have diameters of mere ~10
nm and lengths of tens to hundreds nanometers. Figure 1b
The samples taken from the synthesis mixtures at different
time intervals were then observed by SEM to capture the
morphology evolution process (Figure 2). Figure 2a shows the
schematic illustration of the proposed growth mechanism for
2
hierarchical CoTe nanostructures. In order to boil off the trace
o
amount of water, we first heated the solution at 110 C for 1 h.
shows a typical HRTEM image of a single CoTe
revealing that the CoTe nanofleeces are structurally uniform
with interlayer spacing about 2.71 Å, which corresponds to (120)
lattice plane of the orthorhombic CoTe The elemental
mappings (Figure 1c) indicate that Co and Te elements are
uniformly distributed in the product. Figure 1d shows a typical
SEM image of CoTe nanofleeces, which have the similar
2
nanofleece,
During this process, the diameter of the nanowires grew from 7
nm to ~30 nm, and the surface of the nanowire turned much
rougher (Figure S7). When the temperature rose to 200 oC,
plenty of bumps appeared on the surface of nanowires. With the
extension of time, more and more bumps came out and
gradually grew into long nanofleeces, which eventually covered
the whole surface of nanowires when the reaction lasted for 1 h.
As the reaction proceeded to 2 h, all the nanofleeces continued
to grow, and the average diameter reached to about 20 nm.
Note that we have conducted a series of experiments to
optimize the conditions for synthesis of high-quality hierarchical
CoTe2 nanostructures. In our experiments, we found that the
composition of surfactants (ratio of OAm/OA), the amount of
2
2
.
2
hierarchical structure with the CoTe nanofleeces. A typical
HRTEM image (Figure 1e) of a single CoTe nanofleece reveals
that the CoTe hierarchical structures are structurally uniform with
interlayer spacing about 2.86 Å, which corresponds to (101)
lattice plane of the hexagonal CoTe. Moreover, the
adsorption-desorption isotherms of the product (Figure S5)
reveal that the hierarchical CoTe and CoTe nanofleeces have
N
2
2
2
Co(acac) , the reaction temperature and time play important
roles in the formation of uniform hierarchical CoTe2
nanostructures (Figure S8-S11).
2
-1
BET surface areas of 97 and 75 m g , respectively, which are
fairly high for transition metal chalcogenide materials. Such a
high surface area should be attributed to the tiny branched
nanofleeces with an average diameter of about 10 nm.
To understand the formation mechanism of hierarchical CoTe
2
nanofleeces, the growth process was carefully tracked by time
dependent experiments. We used XRD to analyze the phase
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Figure S13c, d, the original hierarchical nanostructure of CoTe
2
was still kept after 2,000 potential cycles electrolysis, while the
morphology of CoTe was destroyed, which should be
responsible to the different stabilities of CoTe
catalysts. The excellent stability of the CoTe can be evidenced
further from the chronoamperometric measurements. As shown
2
and CoTe
2
-2
in Figure S15, at the current density of 10 mA cm , the
overpotential produced in the CoTe catalyst exhibited no
2
measurable increase over 24 h of continuous operation. This
extraordinary durability shows promise for practical applications
of the catalysts over the long term.
We then studied the OER activities of the products obtained at
different time intervals (Figure S16a). The low OER activities of
the products obtained at 15 min and 30 min should be attributed
to poor crystallinity and impurity of remnant Te nanowires, while
the low OER activities of the products obtained at 2 h could be
the result of low surface area. We also studied the OER
activities of the products obtained at different temperatures
Figure 3. (a) Polarization curves for OER of several catalysts as indicated. (b)
Tafel plot (overpotential versus log current) derived from (a). (c, d) OER
polarization curves of hierarchical CoTe and CoTe nanofleeces before and
2
after different cycles of accelerated stability test.
(
Figure S16b). The low OER activities of the products obtained
at 180 oC should also be attributed to poor crystallinity and
impurity of remnant Te nanowires, while the low OER activities
To appraise the catalytic properties of the new hierarchical
o
CoTe
oxygen, film of hierarchical CoTe
onto glassy carbon electrodes for cyclic voltammetry (CV) in O
saturated 0.1 M KOH. Similar measurements for hierarchical
CoTe nanostructures, Pt/C (20-wt.%) and commercial RuO
2
nanostructures for electrochemical oxidation of water to
of the products obtained at 250 C should be due to the phase of
2
nanostructures was prepared
CoTe and the even lower surface area.
2
-
2
reference were also performed for comparison. Figure 3a shows
their linear sweeps in an anodic direction, from which we can
see that the hierarchical CoTe nanostructures exhibit greater
2
current as compared to hierarchical CoTe nanostructures, RuO
2
and Pt/C references. The current density of 10 mA cm-2 can be
achieved at a small overpotential of ~0.357 V for our catalyst,
which is better than the best reported Co
3
O
4
/graphene catalyst
with the similar loading (0.25 mg cm ). Note here that the
overpotential of current density at 10 mA cm-2 of the CoTe
-
2 [6]
2
catalyst is comparable to or smaller than those of the well-
studied Co-based OER catalysts in the literature (Table S1).
Figure 3b shows that the Tafel slope of the hierarchical CoTe
nanostructures is ~32 mV/decade, which is smaller than that of
RuO hierarchical CoTe nanostructures, and previously
reported Co-based OER catalysts such as CoSe /Mn
/graphene hybrids at similar
2
Figure 4. Standard free energy diagram for OER at zero potential, equilibrium
potential and theoretical onset external potential. (a) and (b) for CoTe(101)
2
,
and CoTe
2
(120) without pre-adsorbed O*, (c) and (d) for CoTe(101) and
2
3 4
O
CoTe (120) with pre-adsorbed O* (0.5 ML).
2
composites,[11a] Co
O
4
[5]
[6]
3
3 4
and Co O
loading, and comparable to the best known OER catalysts in
basic media.[18] We calculated a high TOF of 0.20 s-1 associated
To rationalize the superior electrochemical performance of
cobalt telluride and dependence on the phase, we propose that
high surface area, electrical conductivity and activity of CoTe2
and CoTe towards the reaction intermediates are essential. First,
the hierarchical structure of CoTe2 nanofleeces offers large
surface areas, which increases contact between the electrolytes
and electrode materials, thus enhancing the electrocatalytic
activities of the electrodes. This can be justified by the larger
with CoTe
KOH, which was higher than 0.12 s of CoTe nanostructures
and comparable to 0.22 s-1 of RuO
In addition, the hierarchical CoTe
also exhibited high stability for OER. Figure 3c shows that after
,000 potential cycles, the CoTe nanostructures almost
2
nanostructures at overpotential of 350 mV in 0.1 M
-1
2
.
2
nanostructures catalyst
2
2
2
-1
afforded the same j-V curves as initial catalyst, only with
negligible drops of the anodic current, while the anodic current
showed a loss of about 80% and 70% for the hierarchical CoTe
BET area of 97 and 75 m g for CoTe and CoTe, respectively
2
(Figure S5). To be more relevant with electrochemical active
surface area of materials, the capacitance of the corresponding
nanostructures (Figure 3d) and the state-of-the-art RuO
2
(Figure
double layer of CoTe and CoTe nanostructures were measured.
2
S12) at 1.60 V versus RHE, respectively. We performed XRD
and XPS measurements after 2,000 potential cycles (Figure S13, 0.357 mF cm , while the capacitance of CoTe electrode is 0.266
S14), which showed that no obvious phase and valence
As shown in Figure S17, the capacitance of CoTe electrode is
2
-2
-2
mF cm . Therefore, the CoTe2 catalysts have
a higher
changes were observed after long-term electrolysis. As shown in
electrochemical active surface area than that of CoTe, which is
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Angewandte Chemie International Edition
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COMMUNICATION
in favor of enhancing the electrochemical activity. Second, the
21431006, 21225315, 21321002), the National Basic Research
Program of China (Grants 2014CB931800, 2013CB933900,
2013CB834603), and the Users with Excellence and Scientific
Research Grant of Hefei Science Center of CAS (2015HSC-
UE007), and the Chinese Academy of Sciences (Grants KJZD-
EW-M01-1, KGZD-EW-T05 and XDA090301001) and the
Fundamental Research Funds for the Central Universities
(WK2060190045).
kinetics of electrode reactions for CoTe
2
and CoTe
nanostructures were probed by electrochemical impedance
spectroscopy (EIS) technique (Figure S18). The smaller Rct
value of CoTe electrode than that of CoTe suggests its higher
2
electrical conductivity.
DFT investigation (see Supporting Information for details) of
the two dominated facets CoTe(101) and CoTe
2
(120) (Figure
S19 and S20) showed that CoTe
2
and CoTe bind differently with
Keywords: electrocatalyst
• cobalt telluride • hierarchical
the reaction intermediates involved in OER.[19] The standard
structure • oxygen evolution reaction
reaction free energy profile G (Figure 4), taking into account of
the electrochemical condition,[19b, 20] shows that the third step is
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In summary, we have demonstrated
transformation process for phase selective synthesis of novel
hierarchical CoTe and CoTe nanofleeces using ultrathin Te
nanowires as templates. This earth-abundant CoTe catalyst
exhibited excellent OER activity and stability in alkaline medium.
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2
2
2
CoTe nanostructures played an essential role on their superior
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3
Acknowledgements
[
X-ray
Photoelectron
Spectroscopy
Database.
The authors acknowledge the funding support from the National
Natural Science Foundation of China (Grants 21521001,
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COMMUNICATION
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Angewandte Chemie International Edition
10.1002/anie.201701998
COMMUNICATION
COMMUNICATION
Qiang Gao, Chuan-Qi Huang, Yi-Ming
Ju, Min-Rui Gao, Jian-Wei Liu, Duo An,
Chun-Hua Cui, Ya-Rong Zheng, Wei-
Xue Li*, and Shu-Hong Yu*
Page No. – Page No.
Phase Selective Synthesis of Unique
Cobalt Telluride Nanofleeces for
Highly Efficient Oxygen Evolution
Catalyst
Text for Table of Contents
Hierarchical nanofleees of noble metal free electrocatalyst: Hierarchical CoTe
2
nanofleeces were successfully synthesized by
using ultrathin Te nanowires as templates, which exhibited excellent electrocatalytic activity and stablity for oxygen evolution reaction
OER) in alkaline media. The CoTe catalyst exhibited superior OER activity to the CoTe catalyst and comparable to the state-of-the-
art RuO catalyst, which was supported by DFT calculations.
(
2
2
This article is protected by copyright. All rights reserved.