Design of an active and durable catalyst for oxygen reduction reactions using encapsulated Cu with N-doped carbon shells (Cu@N-C) activated by CO2 treatment

Article abstract of DOI:10.1039/c5ta06108c

Cu@N-C with the Cu particles encapsulated in N-doped carbon shells, which was activated by CO₂ treatment, is an excellent electrocatalyst for the oxygen reduction reaction (ORR) with a high activity and durability.

Full text of DOI:10.1039/c5ta06108c

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Design of an Active and Durable Catalyst for Oxygen Reduction
Reactions Using Encapsulated Cu with N-Doped Carbon Shells
Cu@N-C) Activated by CO Treatment
Received 00th January 20xx,
Accepted 00th January 20xx
(
2
a
b
a
c
a
DOI: 10.1039/x0xx00000x
Seung Hyo Noh, Min Ho Seo, Xiao Ye, Yuki Makinose, Takeyoshi Okajima, Nobuhiro
c
*d
*a
Matsushita, Byungchan Han, and Takeo Ohsaka
www.rsc.org/
Cu@N-C, where the Cu particle was encapsulated in N-doped manifestation of improved ORR activity and long-term
carbon shells and activated by CO treatment, is an excellent electrochemical stability against dissolution to acidic or
electrocatalyst for oxygen reduction reaction (ORR) with a high alkaline media and oxidation by air if they were entirely
2
1
4
activity and durability.
encapsulated in carbon shells. These works introduced new
methods to overcome the conventional limits of catalysts (i.e.,
1
activity and stability) for metal-air battery , PEMFCs, and
5
13
Oxygen reduction is one of the key chemical reactions that
power various renewable energy devices such as polymer
electrolyte membrane fuel cells (PEMFCs) and metal-air
batteries. In practice, the oxygen reduction reactions (ORRs) in
these applications are sluggish, limiting the overall system
performance. Accordingly, conventionally nano-scale Pt-based
1
6
17
biological and magnetic materials. State-of-the-art ORR
catalysts encapsulating non-precious transition metals with
with N-doped carbon shells were studied by several groups
1
8
using transition metals such as (Fe@N-C or Fe
1
FeCo
3
C@N-C ) and
9, 20
21
or NiCo alloys. In the literatures, it was observed
1
, 2
catalysts are used to reduce the activation energy for ORRs.
However, the high material cost and electrochemical
3, 4
that the noble structures dramatically improved ORR activity
compared with typical non-precious catalysts and certain
interactions between inner metals and carbon shells were
conjectured as the underlying reason.
instability
have resulted in delays for a wide range of
commercial applications of renewable energy systems.
2
2
To date, numerous non-precious metallic or metal-free
catalysts have been demonstrated to be alternatives to the
traditional Pt catalysts. For example, the ORR activities of
Previous work showed that N-doped graphene on Cu support
N-Gr/Cu) could be a promising ORR catalyst that performs as
(
2
well as the Pt(111) surface but only when the Cu is protected
against surface segregation or oxidation by oxygen or water. In
this study, we dramatically improved the thermodynamic and
electrochemical stability of Cu particles by completely
surrounding them with spherically shaped N-doped carbons
5
-8
9
macrocyclic metal complexes and metal oxides such as iron
1
0
phthalocyanine (FePc), polyaniline-iron-carbon (PANI-Fe-C),
12
11
Co O ,
3
and MnCo O
2 4
were often reported to be
4
comparable to Pt. However, the performances of these
catalysts were only approved in laboratory-scale tests and not
successfully used in actual devices for long-term operations
largely because of physical or electrochemical instabilities.
(Cu@N-C). Furthermore, we rationally designed the thickness
of the carbon shells using a CO treatment at high temperature
2
to control the atomistic interactions between the
encapsulated Cu and the N-C shell while protecting Cu from
agglomeration and electrochemical dissolution.
1
3
Recently, Fe-based materials were reported to enable the
Using first-principle density functional theory (DFT)
calculations and experimental materialization, we accurately
characterized and validated the ORR activity and long-term
durability of the Cu@N-C catalyst.
a.
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science
and Engineering, Tokyo Institute of Technology, 4259-G1-5 Nagatsuta, Midori-ku,
Yokohama, 226-8502, Japan. E-mail: ohsaka.t.aa@m.titech.ac.jp; Fax:+81-45-924-
489; Tel: +81-45-924-5404
5
b.
Department of Chemical Engineering, Waterloo Institute for Nanotechnology,
Waterloo Institute of Sustainable Energy, University of Waterloo, 200 University
Ave. W, Waterloo, ON, N2L 3G1, Canada
c.
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259
Nagatsuta, Midori, Yokohama, Kanagawa 226-8503, Japan
Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul,
d.
1
+
20-749, Republic of Korea. E-mail: bchan@yonsei.ac.kr; Fax:+82-2-321-6401; Tel:
82-2-2123-5759
Electronic Supplementary Information (ESI) available: Experimental detail,
computational detail, and additional figures in mentioned in this article
See DOI: 10.1039/x0xx00000x
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on a carbon microsphere (average size: 3.4 μm) as shown in Fig.
a, and the Cu particles were entirely encapsulated in carbon
1
shells as shown in Fig. 1b. Using the element mapping method
in Fig. 1d-g, we identified C, N, Cu, and O in Cu@N-C particles.
Particularly, the Cu particles are clearly distinguished from the
carbon microsphere supports, and N and O atoms are well
dispersed over the carbon. Defects in the carbon microspheres
2
of Cu@N-C(CO ) were found in Fig. 1h and i.
Fig. 2 (a) Overall survey spectrum, (b) Cu 2p spectrum and (c)
N 1 s spectrum of the Cu@N-C(CO ) catalyst using an X-ray
photoelectron spectroscope (XPS). (d) X-ray diffraction (XRD)
patterns of Cu@N-C(hydro), Cu@N-C(heat) and Cu@N-C(CO2).
2
2
Fig. 1 (a) FE-SEM image of the Cu@N-C(CO ) catalyst. (b) TEM
images of Cu@N-C(CO ) with three insets, which are colored
2
with red, orange, and navy boxes and have scale bars of 100,
1
00 and 20 nm, respectively. (c) HAADF-STEM image of Cu@N-
C(CO ). The elemental mapping of the HADDF-STEM image
indicates the existence of (d) Cu, (e) N, (f) C and (g) O. SEM
images of Cu@N-C(CO ) (h, i).
There were bare Cu particles without carbon shell covers in
2
2
the Cu@N-C(hydro), Cu@N-C(heat) and Cu@N-C(CO ) catalysts,
as shown in Fig. S2a-c. The XPS spectra in Fig. 2a and b (or Fig.
S5 for details) also clearly show the surface-exposed Cu
particles. The X-ray diffraction patterns in Fig. 2d indicate that
there was bulk fcc Cu (the sharp peaks), but no Cu oxide and
Cu carbide were detected in all samples. As shown in Fig. 2c
2
Cu@N-C catalyst was synthesized via sustainable precursors
such as sucrose and ammonium sulfate, which are utilized as
table sugar and fertilizer, respectively. Moreover, Cu metal
encapsulated in carbon is relatively abundant material
compared to Pt catalyst. The Cu@N-C catalyst was fabricated
according to the described procedures in the Experimental
section and Fig. S1 in the Supporting Information. We
synthesized three different types of catalysts with various
applied processes: i) hydrothermally treated “Cu@N-C(hydro)”,
2
and S6, the doping level of N in the Cu@N-C(CO ) catalyst was
approximately 1.8%, and pyrrolic N (50.5%) was dominant over
the graphitic (19.8%) and pyridinic (29.7%) N structures.
To characterize the ORR activity of the Cu@N-C materials,
rotating ring disk electrode (RRDE) voltammetry was applied,
as shown in Fig. 3a and 3b. It is clearly demonstrated that
Cu@N-C(hydro) and Cu@N-C(heat) are far less active than the
Pt/C catalyst. We found that the Cu@N-C(heat) powder lost
approximately 44 wt.% of its original mass after the CO2
treatment (Fig. S2d, e), which implies that the carbon shells
ii) “Cu@N-C(heat)”, which was heat-treated at T = 1000
℃
for
for 15
2
and O (or air) were previously used
2
2 2
h, and iii) “Cu@N-C(CO )”, which was oxidized by CO
2
3
24
min at T = 1000
℃
. CO
2
to oxidize carbon shells at high temperature. For example,
Chen et al. used CO gas to eliminate the carbon shells in
2
3
became substantially thin because of the treatment.
Surprisingly, Cu@N-C(CO ) dramatically enhanced the ORR
2
2
Fe@C nanoparticles and obtained pure carbon nanotubes
after removing Fe through an acidic treatment. We applied the
activity to a similar level to Pt/C. Figure 3b (Table S1 in
Supporting Information) shows that the onset (Eonset) and half-
2
CO treatment to regulate the carbon shell thickness via
wave (E1/2) potentials of the ORR in Cu@N-C(CO
2
) are 0.94 and
oxidation reactions at high temperature. This step is a key
process in controlling the electronic interactions between
inner Cu particles and adsorbates from the intermediates of
the ORR on the carbon shell of the Cu@N-C catalyst.
0.83 V vs. RHE (reversible hydrogen electrode), respectively.
Indeed, they were similar to those of Pt/C (1.05 and 0.85 V vs.
RHE, respectively). In addition, the measured number of
2
electrons (n) that were involved in the ORR with Cu@N-C(CO )
Figure 1 shows successfully synthesized Cu@N-C(CO
2
) particles.
2
was 3.95 (Fig. 3a). Thus, Cu@N-C(CO ) enables the reduction
Cu@N-C particles (an average size of 450 nm) were supported
2
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of O
Koutecky-Levich plots in Fig. 3c, where the slopes are constant From the beginning, the current of Pt/C steeply decreased,
regardless of the electrode potential, imply that the ORR is whereas the decrease was much milder in Cu@N-C(CO ).
dominated by the first-order kinetics between Cu@N-C(CO Cu@N-C(CO ) lost only 8 % of the initial current value even
2 2 2 2 2
to H O with less than 3 % H O produced. The linear electrochemical stability of Pt/C and Cu@N-C(CO ) (Fig. S9).
2
2
)
2
2
5
and dissolved oxygen.
after 30,000 s, but lost 33 % in Pt/C for the identical cycling
time. The excellent durability can be originated from the
encapsulation of Cu particles by N-doped carbon shells, which
may endow Cu@N-C catalyst with enough electrochemical
stability against oxidation of the core Cu in alkaline solution.
Fig. 3 ORR activity and durability of the Cu@N-C catalysts. (a)
Number of electrons and H O formation percentage. (b)
2
2
Steady-state voltammograms of the ORR with Cu@N-C(hydro),
Cu@N-C(heat), Cu@N-C(CO ), N-C(CO ) and Pt/C in O -
2
2
2
saturated 0.1 M KOH solution. The scan rate and rotating
speed were 5 mV/s and 900 rpm, respectively. (c) Steady–state
2
voltammograms of the ORR with Cu@N-C(CO ) at various
Fig. 4 Theoretical modeling of the Cu@N-C structure. (a)
rotating speeds and Koutecky−Levich plots at various electrode
potentials (inset). (d) Steady-state voltammograms of the ORR
during the potential cycling durability test (potential range:
Model system of Cu encapsulated in nitrogen-doped carbon.
(b) Charge density of Cu@N-C. Oxygen binding energies and
structures of (c) Pt(111), (d) Cu@N-C, (e) Empty@N-C and (f)
Cu nanoparticle.
0.6-1.0 V vs. RHE, scan rate: 50 mV/s) in O
KOH.
2
-saturated 0.1 M
Nevertheless, Cu@N-C(hydro) and Cu@N-C(heat) showed To identify the ORR active sites in the Cu@N-C catalysts, we
much lower ORR activities than Cu@N-C(CO ), which indicates calculated the adsorption energies of oxygen (Equation S5)
2
that the bare Cu particles around Cu@N-C(hydro) and Cu@N- using density functional theory calculations (Fig. S10 and Table
C(heat) cannot catalyze the ORR at all. In other words, the ORR S2). Figure 4a is the model system for the Cu@N-C catalysts,
activities are mainly determined by the Cu@N-C catalysts and and the strucutre is assumed as the copper particle covered by
not by the bare Cu around them. As shown in Fig. 1h and i, one layer N-doped carbon. We assumed the N doping level of
Cu@N-C(CO ) includes the defects of the carbon microsphere, 2.1 at% as proposed by XPS analysis (1.8 %), including previous
2
2
7
28
whereas Cu@N-C(heat) does not, as shown in Fig. S3g. To experimental and theoretical reports. The TC1 site, which is
understand the effect of defects on the ORR, N-doped carbon the nearest neighboring C from the doped N, strongly adsorbs
microspheres that were treated with CO (N-C(CO )) were O (3.82 eV), whereas the T
N
site (atop of N) is the weakest
2
2
2
6
fabricated, and the synthesis processes were identical to that adsorbing site (1.87 eV), which is consistent with previous
2
2
of Cu@N-C(CO ) except for the absence of the Cu precursor. results on the bulk N-Gr/Cu system. The calculated binding
2
The measured ORR activity (Eonset = 0.87 V and E1/2 = 0.71 V) of energies of O on Cu@N-C and bulk Pt(111) surfaces were 3.82
N-C(CO ) was much worse than that of the Cu@N-C(CO ) and 4.44 eV, respectively, as shown in Fig. 4. According to the
2
2
catalyst as shown in Fig. 3b, which implies that the proposed electronic structure theory for ORR catalysts of
2
9
encapsulated Cu particles play a crucial role in enhancing the Nørskov and his associates, the Cu@N-C and Pt(111) surfaces
ORR activity.
are expected to have similar ORR activities. The obtained
In addition to the superior ORR activity, Cu@N-C(CO ) exhibits oxygen adsorption energies of Empty@N-C and pure Cu were
2
outstanding electrochemical durability, as shown in Fig. 3d. In 3.43 and 5.30 eV, respectively, which implies that both
potential cycling tests between 0.6 and 1.0 V (vs. RHE), E1/2 structures are not suitable for the ORR catalyst because they
decreased by only 20 mV after 3,000 cycles. The have too weak (Empty@N-C) and too strong (pure Cu)
3
0
chronoamperometric current-time (i vs. t) responses were chemical bondings with O.
measured at a fixed potential of 0.7 vs. RHE to compare the
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enhances the ORR activity to a comparable level to the Pt(111)
surface. The electronic interactions between the inner Cu
particle and N-C shells can be described using an electronic
charge distribution as shown in Fig. 4b. The encapsulated Cu
particle clearly donates electronic charges to the N-C layers on
1
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1
1
2
Conclusions
We synthesized Cu@N-C (CO
completely encapsulated in an N-doped carbon shell, and the
shell thickness was optimized by oxidizing carbons with CO
2
), where the Cu particle was 12 Y. Liang, H. Wang, J. Zhou, Y. Li, J. Wang, T. Regier and H. Dai,
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2
.
Using experimental measurements and DFT calculations, we
found that the catalysts have outstanding properties in both
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induced electronic interaction between the inner Cu and N-C
shells. Furthermore, the N-C shells strongly protect the Cu
particle against dissolution into aqueous solutions or chemical
3
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1
1
1
4,
1
4
.
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2
) has significantly better ORR activity
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2301-2306.
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,
a
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1
Acknowledgments
2
This study was supported by the Ministry of Education, Culture, 22 S. H. Noh, D. H. Kwak, M. H. Seo, T. Ohsaka and B. Han,
Sport, Science and Technology (MEXT), Japan and the Global
Electrochim. Acta, 2014, 140, 225-231.
Frontier Program through the Global Frontier Hybrid Interface 23 T. C. Chen, M. Q. Zhao, Q. Zhang, G. L. Tian, J. Q. Huang and F.
Materials (GFHIM) of the National Research Foundation of
Wei, Adv. Funct. Mater., 2013, 23, 5066-5073.
Korea (NRF), which is funded by the Ministry of Science, ICT & 24 D. Deng, L. Yu, X. Chen, G. Wang, L. Jin, X. Pan, J. Deng, G.
Future Planning (grant number 2013M3A6B1078882). The
Sun and X. Bao, Angew. Chem., Int. Ed., 2013, 52, 371-375.
New and Renewable Energy R&D Program (20113020030020) 25 J. Xiao, Q. Kuang, S. Yang, F. Xiao, S. Wang and L. Guo, Sci.
under the Ministry of Knowledge Economy, Republic of Korea
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partially supported this work. This study was performed using 26 K. Latham, G. Jambu, S. Joseph and S. Donne, ACS
Tsubame 2.5 at the Global Scientific Information and
Sustainable Chem. Eng., 2014, , 755-764.
Computing Center of Tokyo Institute of Technology. The 27 L. Qu, Y. Liu, J.-B. Baek and L. Dai, ACS Nano, 2010,
authors gratefully acknowledge Dr. Y. Nabae for BET surface
1326.
area measurements. Seunghyo Noh also thanks the 28 D. Kwak, A. Khetan, S. Noh, H. Pitsch and B. Han,
3.
2
4, 1321-
Government of Japan for the MEXT scholarship.
ChemCatChem, 2014, 6, 2662-2670.
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