A highly active and durable iron/cobalt alloy catalyst encapsulated in N-doped graphitic carbon nanotubes for oxygen reduction reaction by a nanofibrous dicyandiamide template

Article abstract of DOI:10.1039/c8ta01247d

Exploration of competitive electrocatalysts to replace Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cells is one of the most promising strategies to confront the energy and environmental crises. Herein, we highlighted an FeCo alloy catal

Full text of DOI:10.1039/c8ta01247d

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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JPolueransael odfoMnaotetraiadljsuCshtemmairsgtriynsA
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DOI: 10.1039/C8TA01247D
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ARTICLE
Highly Active and Durable Iron-Cobalt Alloy Catalyst Encapsulated
in N-doped Graphitic Carbon Nanotubes for Oxygen Reduction
Reaction by Nanofiberous Dicyandiamine Template
Li An, a Ning Jiang, b Biao Li, b Shixin Hua, a Yutong Fu, a Jiaxi Liu, a Wei Hao, a Dingguo Xia*,b Zaicheng
Sun*a
Exploring competitive electrocatalyst to replace Pt-based catalysts for oxygen reduction reaction (ORR) of fuel cells is one of
the most hopeful strategies to confront the energy and environmental crisis. Herein, we highlight FeCo alloy catalyst
encapsulated in N-doped graphitic carbon nanotubes (FeCo@N-GCNT-FD) as a greatly efficient non-precious electrocatalyst
for ORR. The FeCo@N-GCNT-FD catalyst exhibits a positive onset (0.96 V vs RHE), half-wave potentials (0.88 V vs RHE) and
5.6 times specific activity of commercial Pt/C at 0.70 V in alkaline media. The excellent catalytic behavior of the FeCo@N-
GCNT-FD is attributed to structural properties with a large surface area and the synergistic effect of FeCo alloy and N-GCNT,
which guarantee a large number of accessible catalytic sites and rapid mass-transfer kinetics. Theoretical calculations further
confirm that the strong synergetic and electronic effect, especially the FeCo-NG sites, provides a favorable local coordination
environment and electronic structure and lower oxygen absorbance energy. The improvement of ORR activity and durability
of the catalyst by the synergetic and electronic effects between metal and carbon provides a versatile approach for tuning
the catalytic performance of non-noble electrocatalysts.
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
in the harsh environment because they can be dissolved into the acidic
or basic solution.
To solve the problem of the metal dissolution, a variety of transition
Introduction
With the demand for low emission or zero-emission, the low cost and
high energy density battery and/or fuel cells are highly desirable to be
developed to satisfy the requirement of the car. Currently, the large-
scale commercialization of the low-temperature proton-exchange
membrane fuel cells (PEMFCs) is still hindered due to the sluggish
kinetics of the oxygen reduction reaction (ORR), which requires
expensive electrocatalyst with a high mass loading noble metal Pt to
achieve adequate power and energy densities.1-3 Even the state-of-the-
art platinum or Pt-based alloys exhibit good electrochemical
performance toward ORR, their cost, scarcity, and long-time stability
dramatically limit their practical application and commercialization.4-
7 Alternatively, if Pt-free materials, so-called costless electrocatalysts,
such as nonprecious catalyst, metal-free catalyst, with comparable
ORR electrocatalytic activity to Pt-based catalysts, can be favorably
expected to take the dominant place. Comparing with chemical inert
Pt, although other transition metals such as Fe, Co, Ni also exhibit
catalytic activity to some extent, the most critical issue is their stability
metal compounds, such as chalcogenides,8-10 oxides,11-14 and
carbides/nitrides,15-21 with considerable improvements in stability for
ORR have been developed. However, it is hard to satisfy and take the
place of Pt-based ORR catalysts due to the catalysts are unstable at
the harsh environment of a PEMFCs cathode, especially in acidic
solution.22 Recently, nanocomposites of iron carbides and carbon
16, 23
(Fe3C/C, GL-Fe/Fe5C2/NG),15,
anchoring cobalt nanocrystals
the sole metal or metal
through the plane of graphene,24
alloy/nitrogen-doped carbon synthesized via pyrolysis of complex
metal–organic framework,25, 26 the encapsulation of earth-abundant
transition metals (such as Fe, Co, Cu, Ni)27-37 in N-doped carbons
materials have been successfully developed to be stable ORR catalyst
with enhanced catalytic activities due to the synergetic effects
between metal and heteroatom-doped carbon. In addition, the outlayer
carbon material effectively prevent from the metal dissolution issue
even in the acidic media. That opens a door to develop high
performance and durability electrocatalyst for ORR. Importantly, the
intermetallic compounds with ordered structure exhibit excellent
38-40
electrocatalytic activity and long-term stability.7,
Aim to this
a.Beijing Key Laboratory of Green Catalysis and Separation, College of
Environmental and Energy Engineering, Beijing University of Technology, Beijing
100124, People’s Republic of China.
target, we propose that FeCo intermetallic wrapped with heteroatom
doped carbon is expected to be a high performance and stability.
As a heterogeneous reaction, the ORR takes place on the surface of
catalyst between liquid-solid interfaces. The catalytic activity of the
catalyst strongly depends on the specific surface area of catalyst.41 To
b.Beijing Key Laboratory of Theory and Technology for Advanced Batteries
Materials, College of Engineering, Peking University, Beijing 100871, People’s
Republic of China.
*Email - sunzc@bjut.edu.cn
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
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achieve high activity, it is highly desirable to prepare the catalyst high Materials Characterizations.
surface area. Dicyandiamide, as a principal source of graphitic-C3N4
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DOI: 10.1039/C8TA01247D
The structures of electrocatalysts are characterized by X-ray
diffraction (XRD, Bruker D8 Advance X-ray diffractometer, Cu Kα).
The morphology of the samples was examined by scanning (SEM,
Hitachi-SU-8010) and transmission electron microscopy (TEM,
TECNAI-F20), respectively. The detailed chemical composition of
the electrocatalysts was carried out by XPS (AXIS-Ultra instrument
from Kratos Analytical, Al Kα radiation, hν=1486.6 eV). The micro-
Raman spectra were recorded on Raman microscope (L2BRAM
ARAMIS) with 532 nm line of an Ar ion laser as an excitation source.
The specific surface area of the catalysts was measured by nitrogen
adsorption-desorption isotherm curve at 77 K. XAFS spectra were
recorded at beamline 1W1B of the Beijing Synchrotron Radiation
Facility. The pretreatment of the dicyandiamide was used by the
Lyophilizer (LGJ-10).
(g-C3N4), can also be served as N-doped graphitic carbon nanotube
precursor because it has abundant C and N.42-44 In this report, we
developed a simple and facile synthesis route to prepare alloy
nanoparticles wrapped with in N-doped graphitic carbon nanotube (N-
GCNT) by pyrolyzing transition metal salts and dicyandiamine
mixture. Nanofibrous dicyandiamine can form by self-assembly with
freeze-drying technique, and can be further used as template for the
formation of N-GCNT in the presence of transition metal salt in the
thermal pyrolysis process. The resulting FeCo nanoparticles
encapsulated in N-GCNT (FeCo@N-GCNT-FD) catalyst features a
bamboo-like morphology with FeCo alloy inside of N-GCNT. It
exhibits high surface area of 317.9 m2/g and a suitable wall thickness
(2~2.5 nm) of N-GCNT, which contribute to more catalytic active
sites and lower oxygen absorbance energy through the synergetic and
electronic effects. In addition, the N-GCNT effectively prevent the
alloy nanoparticles from the dissolution in both acidic and basic
environment, which endow high durability for the FeCo@N-GCNT-
FD. This method provides a versatile strategy to fabricate low-cost
and high-performance nonprecious catalysts with controllable metal
species and heteroatom-doped carbon-based materials.
Electrochemical measurements.
All the electrochemical measurements were carried out with BioLogic
VMP3 electrochemical workstation in a conventional three-electrode
configuration controlled at room temperature. A three-electrode cell
was employed using a rotating ring-disk electrode (RRDE, 5mm in
diameter from Pine Instrument) as working electrode, a Hg/HgO (0.1
M KOH) electrode as the reference electrode, and a GC film as the
counter electrode. All the potentials in the present study were referred
to the reversible hydrogen electrode (RHE), as reported previously in
Experimental
Sample preparation.
conversion method (E(RHE) = E
+ 0.933 V). The
(Hg|HgO, 0.1M KOH)
1. Pretreatment of dicyandiamide.
Hg/HgO (0.1 M KOH) reference electrode is 0.164 V versus NHE. To
prepare catalyst inks, the working electrodes were prepared as
following: 4.7 mg of the electrocatalysts and one drop 2% Nafion
solution were dispersed in 1 mL isopropyl alcohol by sonication for
30 min to form a homogeneous ink. Then 20 µL of ink was loaded
onto the polished RRDE, and the electrode was dried at room
temperature.
For the cyclic voltammetry (CV) experiments and polarization
tests, the working electrodes were conducted in a N2 and O2-saturate
0.1 mol L-1 KOH in the potential range of 0.2~1.2 V (vs RHE), with a
scan rate of 50 mV s-1, 10 mV s-1 with varying rotating speed from
400 rpm to 2500 rpm, respectively. The kinetics of the ORR for
FeCo@N-GCNT-FD, FeCo@N-GCNT and commercial Pt/C
catalysts were studied by the RDE method. The electron transfer
number of the catalyzed ORR was calculated by the Koutecky−Levich
equation at various potentials as follows：
Dicyandiamide was dissolved into a certain amount of deionized
water and the mixture was continually stirred for 30 min to form a
homogeneous 2 wt% solution. The solution was frozen in liquid
nitrogen to induce the freezing assistant assembly. Then the freezing
dicyandiamide was placed into the lyophilizer to keep above 48 h
under vacuum. The freeze-drying treated and commercial
dicyandiamide was denoted as DCDA-FD, DCDA, respectively.
2. Preparation of the FeCo@N-GCNT-FD and FeCo@N-GCNT.
In a typical synthesis, 8 mmol FeAc2, 8 mmol CoAc2·4H2O, and 32
mmol nanofibrous DCDA-FD were fully ground. Then the mixture
was heated to 800 °C at 10 °C min−1 and kept for 2 h in an Ar
atmosphere. After cooling down, the residue was collected and
washed with 0.5 M H2SO4 at 85 oC for 8 h to remove metal unwrapped
with N-GCNT, which is denoted as FeCo@N-GCNT-FD. And then,
the samples were washed by deionized water till neutral and dried at
80 °C overnight, respectively. For comparison, FeCo alloy
nanoparticles wrapped in the N-doped graphite carbon nanotube was
also synthesized by a similar process except that the commercial
dicyandiamide takes the place of nanofibrous dicyandiamide and
denoted as FeCo@N-GCNT. In order to check the effect of
temperature on the phase transformation, the different annealed
temperature (600~800 °C) samples were also synthesized. Notably,
this work refers to FeCo@N-GCNT-FD and FeCo@N-GCNT
1
1
1
1
퐵휔1
1
=
+
=
+
⁄
퐽
퐽퐿 퐽퐾
퐽퐾
2
B = 0.62nF퐶0(퐷0)2/3휈−1/6
퐽퐾 = 푛퐹푘퐶0
where J is the measured current density, JL and JK are the diffusion
limiting and kinetic current densities, respectively. ω is the angular
velocity, n is electron transfer number, F is the Faraday constant
(96485 C mol-1), C0 is the bulk concentration of O2 (1.2 × 10−3 mol
L−1),
D0
is
the
diffusion
coefficient of O2 (1.9 × 10-5 cm2 s-1), ν is the kinematic viscosity of
the electrolyte (0.01 cm2 s−1), and k is the electron-transfer rate
constant. For the Tafel plot, the kinetic current was calculated from
the mass-transport correction of RDE by:
o
annealed at 800 C except for special instructions. In addition, the
graphitic C3N4 (g-C3N4) is synthesized by the thermal treatment of
DCDA in a crucible with a cover and heated at 550 oC for 2 h in static
air at a ramp rate of 2.3 oC min-1.
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퐽 × 퐽퐿
dicyandiamide (DCDA-FD) exhibits a fluffy condition with floccule
DOI: 10.1039/C8TA01247D
morphology, which is significantly different from the pristine
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퐽퐾
=
퐽퐿 − 퐽
All RRDE measurements were performed on a Pine Instrument
device in O2 saturated 0.1 M KOH solution at a scan rate of 5 mV s-1
at 1600 rpm and the ring potential was kept at 1.4 V vs. RHE.
Hydrogen peroxide yields and the electron transfer number (n) could
dicyandiamide (Figure S1a, b). Figure S1a, b showed that pristine
DCDA shows massive and bulk morphology in millimeter or
micrometer size. However, DCDA-FD displays nanofibrous
morphology due to the rapid self-assembly of DCDA molecules in the
aqueous solution during the frozen process.42 The evolution of such a
well-ordered architecture is attributed to the fast nucleation and the
oriented growth of dicyandiamide within liquid nitrogen at the
undercooling temperature that is far below the freezing point of the
solution,42, 43 illustrating a vital influence on the morphology and
performance of the nanostructural precursor. Importantly, this fibrous
morphology is also beneficial to the formation of N-GCNT in
thermolysis process. Although both bulk and fibrous DCDA can form
N-GCNT in the presence of transition metal, fibrous DCDA-FD could
form large-scale N-GCNT with a small diameter and uniform and thin
wall thickness.
As depicted in the Scheme 1, a facile solid-state reaction method
was demonstrated to synthesize the FeCo alloy catalyst wrapped in N-
doped graphitic carbon nanotubes (denoted as FeCo@N-GCNT-FD)
simply utilizing nanofibrous dicyandiamide and metal salt
(Co(CH3COO)2 and Fe(CH3COO)2) as the precursors. Firstly, DCDA-
FD mechanically mixed with metal salts to obtained mixture, followed
by heating treatment in Ar atmosphere at high temperature 600-800
oC for 3 hours. In the pyrolysis, graphitic carbon nitride forms at low
temperature stage and further converted to graphitic carbon nanotube
with the presence of the FeCo alloy particles produced from the
decomposition of metal salts.
be determined by following the equations45, 46
IR/N
(
)
H2O2 % = 200 ×
I
R + ID
N
ID
n=4×
ID
ID+
N
Herein, ID and IR are the disk and ring currents, respectively. N is
the ring collection efficiency and the measured N value was 0.37.
Accelerated durability tests for the working electrode was performed
by running 30000 cycles between 0.6 and 1.2 V (vs. RHE) in O2-
saturated 0.1 M KOH, with a scan rate of 200 mV s-1. The
measurements of the chronoamperometric response were performed
in the O2-saturated electrolyte for 12 h by constantly bubbling O2. In
the methanol crossover tests, 1 M methanol was introduced into O2-
saturate 0.1 mol L-1 KOH. Commercial 20 wt% Pt/C (Pt/C’s loading
20 µg cm-2) was also used to compare their catalytic activities.
The electrochemically active surface area (ECSA) was estimated
from
according to a previously published report.47 The electrochemical
capacitance was determined from cyclic
the
electrochemical
double-layer
capacitance
voltammograms measured in a non-Faradaic region at different scan
rates. Electrochemical impedance spectroscopy (EIS) measurement
was carried out at 0.75 V vs RHE of 5 mV in the frequency range from
0.01 Hz to 100 kHz using an Autolab electrochemical workstation
(Autolab PGSTAT302N).
Density function theory (DFT) calculation.
All spin-polarized DFT calculations were performed by using the
Vienna Ab-initio Simulation Package (VASP)48 49with the
generalized gradient approximation PBE (Perdew-Burk-Ernzerhof)
functional.50 The plane wave basis (kinetic energy cutoff values was
520 eV) and PAW (projector-augmented wave) pseudopotential were
employed.51 The atomic positions were fully optimized until the
Hellmann-Feynman force less than 0.05 eV/Å and total energy
convergence criteria was set to 1×10-4 eV.
Our calculation employed a (6, 6) metallic carbon nanotube (CNT),
and about 10 Å vacuum layer were set to the direction of
perpendicular to the CNT axis. The metal@N-doped CNT supercell
has a lattice with x, y = 18.0 Å and z=9.73 Å. The Monkhorst-Pack
k-point grid 1×1×3 was set in all calculations. The calculations of
Gibbs free energy of ORR intermediates was followed the Nørskov’s
method52 and the work potential Uwork was defined as Uwork = min{-
ΔGi}.
Scheme 1. Schematic illustration of the formation of FeCo@N-GCNT-FD
electrocatalyst.
Figure 1a shows the XRD patterns of FeCo@N-GCNT-FD and
FeCo@N-GCNT. The peaks at around 45°, 65.5°, and 83° can be
assigned to the FeCo alloy body-centered cubic crystal structure
(JCPDS, No. 65-4131), corresponding to its (110), (200), and (211)
planes, respectively, indicating that the product contains FeCo alloy
nanoparticles. Another diffraction peak located at 26.4° is the
characteristic (002) diffraction peaks of graphitic carbon. Comparing
with FeCo@N-GCNT, the (002) diffraction peak is weaker and
broader, indicating that its domain size of (002) direction turns small.
To understand the formation of FeCo@N-GCNT-FD, the FeCo@N-
GCNT-FD samples were also synthesized at various temperatures and
denoted as FeCo@N-GCNT-FD-x, where x represents carbonization
temperature of 400, 500, 600, 700 and 800 °C. Figure S3 illustrates
X-ray diffraction (XRD) pattern of FeCo@N-GCNT-400 and 500, and
g-C3N4, indicating the DCDA converts into layered graphitic carbon
nitride at 400 °C. The sample displays yellowish brown due to the
presence of the metal salt. Raising up the temperature to 500 oC, the
Results and discussion
Dicyandiamide, which is considered to be a principal source of
graphitic-C3N4, also can be served as N-doped carbon nanotube
precursor.42, 43 As shown in Figure S1 and S2, compared with the
pristine dicyandiamide (DCDA), the freeze-drying self-assembled
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sample turns into carbon black, the XRD diffraction peak broadens investigation reveals that the lattice fringe spacing of FeCo alloy
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owing to carbonization. However, there is no metal signal is observed. nanoparticles is 0.202 nm, corresponding to tDheO(I:11100.1)0c3r9y/sCta8lTpA0la1n2e47oDf
Graphite carbon and FeCo alloy characteristic peaks formed in the FeCo phase. This result agrees with XRD results. The similar
high temperature (600-800 oC) treated samples (Figure S4), indicating morphology and structure are also observed in the case of FeCo@N-
FeCo alloy nanoparticles form in the high temperature stage. Figure GCNT sample, as shown in Figure 1d and Figure S6d-f. However, the
S5 shows SEM images of FeCo@N-GCNT-FD-600~800 °C. As N-GCNT display a larger diameter (200~300 nm) in FeCo@N-GCNT.
known, Fe and Co nanoparticles can be served as catalyst for the And the HRTEM illustrate that FeCo alloy nanoparticles are enclosed
formation of carbon tube.53, 54 Only carbon particles are observed in with a ~5.5 nm thickness graphene layers, which is thicker than that
the FeCo@N-GCNT-600 sample. That indicates that carbon nanotube in FeCo@N-GCNT-FD. Additionally, Figure S6 and S7 clearly
does not form at this stage although FeCo alloy nanoparticles have illustrate more FeCo alloy nanoparticles encapsulated in the tube in
been formed. When the thermal treatment temperature reaches 700 oC the case of FeCo@N-GCNT-FD due to nanofibrous produced in the
and above, the nanotube structures are presented in the samples. Thus, DCDA-FD.
we propose that the metal salts convert into FeCo alloy nanoparticles
To further analyze the local atomic structure of Co, Fe species in
o
at 600 C, which further work as a catalyst to promote the carbon the catalysts, extended X-ray absorption fine structure (EXAFS)
nitride into the carbon nanotube. And the FeCo alloy nanoparticles are absorption spectra were recorded for both FeCo@N-GCNT-FD and
wrapped by in situ formed carbon nanotube. Figure 1b shows N2 FeCo@N-GCNT, respectively. As seen in Figure 2a, the overall
adsorption-desorption isothermal of FeCo@N-GCNT-FD and spectrum of Co K-edge EXAFS of both samples can be distinguished
FeCo@N-GCNT. Both of them belong to type IV nitrogen adsorption- from the reference Co foil. Fourier transform (FT) is a fundamental
desorption isotherm. The loops are observed and range from the step in the data extraction and interpretation of EXAFS spectra.55 The
relative pressure of 0.4 to 1.0 indicating that mesopores and coordination environments of the Co atoms in both catalysts were
macropores coexist in the samples. The Brunauer–Emmett–Teller further corroborated using FT k3-weighted EXAFS spectrum at the Co
(BET) result reveals that the unique structure endows FeCo@N- K-edge (Figure 2b, c). We found that the two samples exhibited one
GCNT-FD catalyst with a large specific surface area of 317.9 m2 g−1, obvious FT peak located at 2.2 Å, which were mainly attributed to the
which is 2.6 times higher than the FeCo@N-GCNT (122.8 m2 g−1). scattering of metal-metal bonding (eg, Co-Co, Co-Fe, Fe-Fe), in
With such high BET specific surface area, more active sites and more comparison with the reference Co foil. Besides, the Fe K-edge
favorable mass transport of ORR relevant species can be expected EXAFS and FT-EXAFS spectra of the two samples show the similar
during the electrocatalytic ORR.
curves to that of the reference Fe foil (Figure 2d-f), also indicating
that the metal-metal bonding could be the main contributor in the as-
obtained FeCo@N-GCNT-FD and FeCo@N-GCNT. Specifically,
FeCo alloy remains after 8 h of harsh acid washing with 0.5 M H2SO4
at 85 oC, revealing the high chemical durability of the FeCo alloy
encapsulated with doped carbon nanotube, and in turn, preventing the
FeCo alloy from oxidation. The result also agrees with the XRD
measurement owing to the standard FeCo alloy (JCPDS, No. 65-4131).
Fig. 1 (a) Powder XRD patterns of FeCo@N-GCNT–FD and FeCo@N-GCNT
annealed at 800 °C under Ar atmosphere after acid etching. (b) N2
adsorption/desorption isotherms of FeCo@NGCNT-FD and FeCo@N-GCNT. (c,
d) STEM images of FeCo@N-GCNT-FD (inset HRTEM images) and FeCo@N-
GCNT (inset HRTEM images), respectively.
The morphology and structure of FeCo@N-GCNT-FD are
investigated by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). Figure 1 and Figure S6
show the typical TEM and SEM images of FeCo@N-GCNT-FD and
FeCo@N-GCNT. Both of them exhibit bamboo-like tube loaded with
alloy nanoparticles inside. In the case of FeCo@N-GCNT-FD,
average tube diameter ranges from 50 to 100 nm and the wall
thickness is 2.0±0.5 nm. The high-resolution TEM (HRTEM)
Fig. 2 (a, d) Co, Fe K-edges from FeCo@N-GCT-FD and FeCo@N-GNT as well
as Co, Fe foils, respectively. (b, c and e, f) Fourier transformed magnitudes of
the k3-weighted Co, Fe K-edge EXAFS data and first-shell fit for FeCo@N-
GCNT-FD and FeCo@N-GCNT, respectively.
The surface chemical composition and binding energy of the
FeCo@NGCT-FD were performed by X-ray photoelectron
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spectroscopy (XPS) as shown in Figure 3. The high-resolution Co and 0.5 M H2SO4 media. As shown in Figure S9a and Figure S10a,
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DOI: 10.1039/C8TA01247D
2p3/2 spectra shown in Figure 3a is deconvoluted into three peaks there was no obvious characteristic peak for FeCo@N-GCNT-FD and
centered at 779.3 eV, 782.2 and 787.3 eV, which can be attributed to FeCo@N-GCNT in the N2-saturated 0.1 M KOH media, while a well-
the existence of metallic Co, CoCxNy, and Co-Ny, Co-O, defined cathodic peak (FeCo@N-GCNT-FD at around 0.8 V and
respectively.32, 56, 57 Similarly, the deconvoluted Fe 2p spectrum shows FeCo@N-GCNT at about 0.78 V) toward ORR was obviously
the existence of three different peaks as shown in Figure 3b. The peak enhanced in O2-saturated KOH solution, indicating a pronounced
located at 707.9 eV is corresponding to the iron with zero state (Fe0), ORR electrocatalytic activity. Figure 4a shows the linear sweep
which emphasizes the existence of metallic Fe. The broad peaks at voltammetry (LSV) of FeCo@N-GCNT-FD, FeCo@N-GCNT, and
710.5 and 714.2 eV can be assigned to the 2p3/2 orbitals of Fe2+ and commercial Pt/C catalysts. Obviously, a great advantage in terms of
Fe3+ species, respectively. Interestingly, the peak at 714.2 eV ORR limiting current (6.8 mA cm-2) is observed on FeCo@N-GCNT-
represents an indication of the Fe2+-N bonding.25, 35, 46, 58, 59 According FD electrocatalyst. Moreover, FeCo@N-GCNT-FD catalyst displays
to the XPS results of the Co and Fe, it can further confirm the presence a positive onset potential (0.96 V vs reversible hydrogen electrode,
of the chemical bonding between metal-metal and metal-N in the RHE) than that of FeCo@N-GCNT (0.92 V vs RHE) and Pt/C (0.91
FeCo@NGCT-FD, respectively. Furthermore, the high-resolution N V vs RHE), and the value of the half-wave potential of the FeCo@N-
1s spectrum (Figure 2c) was divided into two dominant peaks at 398.6 GCNT-FD (0.88 V vs RHE) was 70 mV and 50 mV more positive than
and 401.0 eV, which can be assigned to pyridinic-N (20%) and that of FeCo@N-GCNT (0.83 V vs RHE) and Pt/C (0.81 V vs RHE),
quaternary-N (53%), and a minor peak at 404.4 eV, which can be respectively. Importantly, the kinetic current (JK), normalized against
attributed to pyridine N-oxides (27%).46, 60, 61 The graphitic N was the electrode geometric area, was calculated from the LSV curve
62
reported to play a crucial role in oxygen reduction reaction.46,
(Figure 4b). The FeCo@N-GCNT-FD calculated electrochemical JK
Besides, pyridinic N, due to its electron-donating properties, can serve value of 284.7 mA cm−2 at 0.7 V, which is a typical half-cell cathode
as metal anchoring sites for Fe, Co atoms.46 Moreover, in this work, potential in an operating fuel cell, is 3.4 times that of FeCo@N-GCNT
the in situ synthesized N-GCNT can not only provide strong support (84.5 mA cm-2) and 5.6 times that of Pt/C (50.6 mA cm-2), and
for the N-coordinated Co, Fe atoms but also regulate the surface exhibited a superior or comparable ORR performance to other non-
electron structure between metal and N-GCNT, which are favorable noble reference catalysts (Table S1), confirming
for ORR catalytic activity (DFT calculation will be discussed in electrocatalytic activity of FeCo@N-GCNT-FD for ORR. In addition,
detail). the electrochemically active surface
a satisfied
Furthermore, structural insights were verified by Raman spectra area (ESCA) of the FeCo@N-GCNT-FD and FeCo@N-GCNT were
(Figure 3d). Note that well-defined D band (1350 cm−1) and G band also evaluated by the double-layer capacitances (Cdl) (Figure S9).
(1577 cm−1) can provide information on the disorder and crystallinity Obviously, the slope is much higher for the FeCo@N-GCNT-FD than
of sp2 carbon materials, respectively.19, 63 On the other hand, the extent the
FeCo@N-GCNT,
indicating
that
the
of defects in catalysts can be quantified by estimating the relative FeCo@N-GCNT-FD have larger active surface areas and thus
intensities of ID/IG ratio. The higher ID/IG ratio of 0.84 was obtained confirming its high intrinsic activity. Moreover, the electrochemical
when the annealed temperature arrived at 800 o C, implying that the impedance spectra (EIS) characterization presented in Figure S10
degree of graphitization become higher with annealed temperature confirmed the overall resistance of FeCo@N-GCNT-FD is much
increased. The same results of XPS and Raman were obtained in the lower than that of FeCo@N-GCNT, benefitting for the charge transfer
sample of FeCo@N-GCT as shown in Figure S8.
and mass transfer in the process of ORR.
To further investigate the ORR reaction kinetics and pathways of
FeCo@N-GCNT-FD and FeCo@N-GCNT catalysts in 0.1 M KOH
solution, rotating disk electrode (RDE) and rotating ring-disk
electrode (RRDE) measurement were performed. The FeCo@N-
GCNT-FD catalyst percentage of the formed peroxide species during
the ORR process and the electron transfer number (n) were calculated
from RRDE data (Figure 4c, d). The peroxide species yield (%) of
FeCo@N-GCNT-FD is below 15% in the potential range from 0.1 to
0.8 V vs RHE and the average electronic transfer number n is ∼3.96.
Meanwhile, analysis of K–L plots at potentials of 0.48-0.63V vs. RHE
for FeCo@N-GCNT-FD and FeCo@N-GCNT catalysts suggest that
a four-electron pathway is the dominant catalytic mechanism (Figure
S11, Figure S12). To compare the influence of annealed temperature,
the ORR activity of FeCo@N-GCNT-FD-x in 0.1 M KOH was
measured. As shown in Figures S11, the values of n were calculated
o
about 3.8 and 3.85 for FeCo@N-GCNT-FD-600 C and FeCo@N-
Fig. 3 High-resolution XPS spectra of a) Co 2p, b) Fe 2p and c) N 1s core
levels in FeCo@N-GCNT-FD; (d) Raman spectrum of FeCo@N-GCNT-FD
annealed at 600~800 oC.
GCNT-FD-700 oC over the potential range of 0.1 to 0.8 V, respectively,
suggesting a near-four-electron oxygen reduction process. However,
FeCo@N-GCNT-FD-800 oC displays an excellent ORR activity with
more positive onset potential, half-wave potential and higher limiting
The electrocatalytic activities of the obtained FeCo@N-GCNT-FD
and FeCo@N-GCNT toward ORR were assessed in both 0.1 M KOH
o
current density (Figure S14) relative to FeCo@N-GCNT-FD-600 C
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Journal Name
and FeCo@N-GCNT-FD-700oC. It shows that improvement catalytic
activity of FeCo@N-GCNT-FD catalyst not only depends on the
morphology but also the degree of graphitization on the sample.
The methanol crossover effect and long-term ORR durability of the
electrocatalysts were evaluated through LSV, chronoamperometric
measurement at 0.75 V and accelerated durability tests (ADT). As
shown in Figure 4e, upon the injection of 1 M methanol into the
electrolyte, the FeCo@N-GCNT-FD electrocatalyst exhibits a strong
tolerance to methanol, whereas the commercial Pt/C catalyst
instantaneously led to a sharp drop of the current density due to the
introduction of methanol oxidation reaction on the surface.64
Furthermore, the results shown in Figure 4f reveal that FeCo@N-
GCNT-FD electrocatalyst exhibit excellent stability, retaining ca.
93% of the original current density after 12 h continuously running,
while the commercial Pt/C retains only ca. 60% of the initial current.
Meanwhile, the LSV curves of FeCo@N-GCNT-FD and FeCo@N-
GCNT before and after ADT measurement also exhibited a bit
degradation in the onset potential and half-wave potential with 30,000
continuous cycles between 0.6 and 1.2 V vs RHE at 200 mV s-1
(Figure S11d, Figure S12d). More importantly, the TEM results
(Figure S15) indicate that the morphology of FeCo@N-GCNT-FD,
especially the stable thickness of N-GCNT-FD, remains intact after
ADT, further confirming the remarkable durability of the FeCo@N-
GCNT-FD electrocatalyst, while the N-GCNT thickness of the
FeCo@N-GCNT catalyst has slightly changed, which may be
attributed to the more suitable thickness between N-GCNT-FD and
FeCo than the N-GCNT and FeCo, facilitating the charge transfer
between them and further intact strongly28.
In addition, FeCo@N-GCNT-FD catalyst exhibits high activity and
excellent stability for ORR not only in 0.1M KOH but also in 0.5 M
H2SO4 solution. As shown in Figure S16 and Figure S17, similarly, no
obvious featured peak for FeCo@N-GCNT-FD in the N2-saturated 0.5
M H2SO4 solution, whereas a well-defined cathodic peak (FeCo@N-
GCNT-FD at around 0.6 V and FeCo@N-GCNT at about 0.53 V)
toward ORR was enhanced in O2-saturated 0.5 M H2SO4 solution,
indicating a pronounced ORR electrocatalytic activity in acidic media.
The ORR polarization curve of FeCo@N-GCNT-FD exhibits onset
potential (ca. 0.82 V), but lower than that of the Pt/C catalyst.
However, the H2O2 yield on the FeCo@N-GCNT-FD catalyst is below
15% in the potential range of 0.1-0.6 V vs RHE investigated (Figure
S17c), also revealing a direct 4-electron-pathway dominated ORR
process in acid medium. Moreover, the LSV curves of FeCo@N-
GCNT-FD before and after ADT measurement were evaluated. After
10000 CV cycles between 0.6 and 1.2 V vs RHE at 200 mV s-1 (Figure
S17d), the onset potential and half-wave potential for FeCo@N-
GCNT-FD exhibited only a small negative shift, demonstrating that
the FeCo@N-GCNT-FD electrocatalyst also possesses high durability
for ORR in 0.5 M H2SO4 medium. However, it is a great challenge to
enhance the acidic catalytic activity compared with current references,
especially the single atomic catalyst (Table S2).
View Article Online
DOI: 10.1039/C8TA01247D
Fig. 4 The electrocatalytic activities of FeCo@N-GCNT-FD and FeCo@N-GCNT.
(a) RDE polarization curves of the FeCo@N-GCNT-FD, FeCo@N-GCNT and
commercial Pt/C electrocatalysts in O2-saturated 0.1 M KOH at 1600 rpm. (b)
Histogram of the specific activity of FeCo@N-GCNT-FD, FeCo@N-GCNT and
commercial Pt/C at different potentials. (c) RRDE curves of FeCo@N-GCT-FD
at 1600 rpm. (d) Peroxide percentage and electron transfer number (n) of
FeCo@N-GCNT-FD within the potential range of 0.1 V to 0.8 V. (e) RDE
voltammetric responses to injection of 1 M methanol into O2-saturated 0.1 M
KOH solution at a scan rate of 10 mV s-1 for FeCo@N-GCNT-FD catalyst. (f)
Chronoamperometric responses of FeCo@N-GCNT-FD and commercial Pt/C
electrocatalysts at 0.75 V in O2-saturated 0.1 M KOH, which are normalized to
the initial current responses.
To further clarify the mechanism of the enhanced activity for ORR,
DFT theoretical calculations were performed. According to the results
of N 1s XPS spectra only containing pyridinic-N (Nprid) and graphic
N (NG), the structures of FeCo@NG-GCNT and FeCo@Nprid-GCNT
were modeled and optimized, respectively, by a Fe2Co2 cluster and a
Fe2Co2 cluster encapsulated in NG-or Nprid-doped (6, 6) single-walled
GCNT (Figure 5a-d), which have been utilized as validated
geometries in the previous study.27, 35 Figure 5c and 5d illustrate the
isosurface plots of charge density of the Fe2Co2 cluster interfacing
with NG-GCNT and Nprid-GCNT, respectively. It is evident that the
significant charge redistribution occurs between the FeCo alloy
cluster and N-GCNT interface, which induces amounts of electron
transfers from the FeCo alloy clusters to N-GCNT and thus further
enhances the catalysts’ activity. However, the FeCo@NG-GCNT
structure possesses an obvious more suitable electrostatic charge than
the compound of FeCo@Nprid-GCNT, which is more beneficial for the
adsorption of oxygen species.
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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strategies for low-cost and high-performance VinewonArpticrleecOinoliunse
catalysts with controllable metal speciesDoOrI: h10e.t1e0r3o9a/Cto8mTA-0d1o2p47eDd
carbon-based materials.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21671011), Beijing High Talent
Program, and Beijing Natural Science Foundation
(KZ201710005002), China Postdoctoral Science Foundation and
Beijing Postdoctoral Research Foundation, Beijing Municipal
Fig. 5 (a, b) The optimized geometry of bare NG-GCNT and NPyrid-GCNT,
respectively. (c, d) The isosurface of charge density variation in FeCo@NG-
GCNT and FeCo@NPyrid-GCNT, respectively. Green area denotes charge
density decrease and yellow area denotes charge density increase. e) DFT-
calculated free-energy diagram for ORR on bare NG-GCNT, FeCo@NG-GCNT
and FeCo@NPyrid-GCNT catalyst surfaces. The above diagrams show the
adsorption geometry of O2, *OOH, *O, and *OH on the electrocatalysts.
The mechanistic details are shown in Figure 5e, which exhibits
free-energy paths obtained using DFT calculations for the relevant
adsorbed reaction intermediates involved in ORR on bare NG-GCNT,
FeCo@NG-GCNT, and FeCo@Nprid-GCNT. Obviously, the free
energy diagram illustrates that the three structures occur decline
tendency, confirming all electron transfer steps to be exothermic in
ORR.35, 65 For NG-GCNT, the smallest Gibbs free energy change ΔG1
of four reaction steps indicates that the most sluggish reaction step is
the hydrogenation of adsorbed O2 that represents the rate-limiting step
for ORR, which is agreement with the previously reported
heteroatom-doped carbon-based materials.66 The smallest ΔG4, as a
significant benchmark to estimate the kinetics of the ORR,
demonstrates that the fourth electron transfer reaction is the rate-
High
Level
Innovative
Team
Building
Program
(IDHT20180504). Fe, Co K-edge XAFS spectra were recorded
at beamline 1W1B of the Beijing Synchrotron Radiation Facility
(BSRF). XRD and electrochemical measurements were
performed at Large-scale Instrument and Equipment Platform of
Beijing University of Technology. The computational resources
for this work is supported by High-performance Computing
Platform of Peking University.
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DOI: 10.1039/C8TA01247D
The catalytic performance of FeCo@N-GCNT-FD catalyst exhibits enhanced intrinsic
activities and excellent durability for oxygen reduction reaction.