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Covalent hybrid of hemin and mesoporous carbon as a highperformance electrocatalyst for oxygen reduction
J.B. Xu, T.S. Zhao*, L. Zeng
Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Special
Administrative Region
a r t i c l e i n f o
Article history:
Received 27 June 2012
Received in revised form
9 August 2012
Accepted 10 August 2012
Available online 8 September 2012
Keywords:
Oxygen reduction
Mesoporous carbon
Hemin
Hybrid catalyst
a b s t r a c t
A high performance hemin and mesoporous carbon hybrid electrocatalyst for the oxygen
reduction reaction (ORR) is developed by using hemin as the FeeN-containing precursor to
control the chemistry of the metal and the chemical composition of the carbon surface. As
a first step, Hemin is used as the FeeN-containing precursor to prepare the FeeN-doped
mesoporous carbon (H-MC) via a nano-casting process by using sucrose as a carbon source
and mesoporous silica as a hard template. Hemin is then used as the FeeN4-containing
precursor to prepare H-MC supported hybrid catalyst. The Fe-doped and N-doped meso-
porous carbons are also prepared and the catalytic properties of the prepared catalysts for
ORR in alkaline media are investigated. The results show that as compared with the much
more expensive Pt/C catalyst, the hybrid catalyst obtained in this work exhibits not only
a higher onset potential, but also a higher current density.
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction
The oxygen reduction reaction (ORR) is an important process
in fuel cells and other electrochemical technologies and its
kinetics has been widely studied [1e3]. Carbon-supported
noble platinum (Pt/C) is widely used as the catalyst for ORR,
but even on pure Pt, the overpotential for ORR is still in excess
of 250 mV [4]. In addition to the need to improve catalyst
performance, the cost of catalytic materials is also needed to
reduce. Therefore, the search for non-precious-metal as well
as metal-free catalysts has become one of themost active and
competitive endeavors in the field of fuel cells [1,5e12].
Transitionmetal macrocyclic compound, e.g. cobalt or iron
porphyrin, has been regarded as the most alternative ORR
electrocatalyst and considerable research has been devoted to
these non-precious-metal catalysts [6,13,14]. Jasinski [15] re-
ported for the first time that transition metal porphyrins,
namely cobalt phthalocyanine, could act as ORR electro-
catalysts in alkaline media. More recently, it has been found
that active ORR catalysts can be synthesized by pyrolyzing
a wide variety of carbon-supported transition metal/nitrogen
(MeNx/C) materials (M ¼ Co, Fe, Ni, Mn, etc., and normally
x ¼ 2 or 4) at high temperatures [6]. Hemin, a natural por-
phyrinatoiron complex, is also known to act as non-precious-
metal catalyst for ORR [16e19]. For instance, Antoniadou et al.
[17] reported that hemin was active to the ORR in both
aqueous and methanolic solutions. Arifuku et al. [19] studied
the effect of pH on the electrocatalytic reduction of oxygen on
a hemin modified glassy carbon electrode; they found that
oxygen was reduced via a one-step reduction through a four-
electron way at pH < 11, and via two successive reductions at
pH > 12. Therefore, it is believed that hemin, an easily avail-
able and cost-effective material, is a promising non-precious-
metal electrocatalyst for ORR.
* Corresponding author. Tel.: þ852 2358 8647.E-mail address: [email protected] (T.S. Zhao).
Available online at www.sciencedirect.com
journal homepage: www.elsevier .com/locate/he
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On the other hand, it has also been recognized that the
electrocatalytic activity of hemin based catalysts is still lower
than that of the Pt catalyst. One of the reasons that limit the
catalytic activity of hemin based catalysts is their low surface
density of catalytic sites. A potential solution to this problem
is to use high-surface-area materials, or the so-called “meso-
scopic structure control” [3,20,21]. In contrast to conventional
carbon supports, mesoporous carbon (MC) exhibits an
attractive property structure as a catalyst support in terms of
high specific surface area, electrical conductivity, and mass
transport [7,20,22]. It should be pointed out that the carbon
support plays a more significant role in non-precious-metal
catalysts than in Pt-based catalysts, the latter acting mainly
as a high surface area support. In the case of non-precious-
metal catalysts, carbon acts not only as a support but also as
a part of active sites [1,3,6,23]. This is because the metal (iron
or cobalt) is bound to nitrogen atoms, which themselves are
bound to the carbon graphitic structure [24]. The type of
nitrogenecarbon bonds and the geometry of the carbon
surrounding these bonds define the overall MeNeC (M, metal)
site and rules whether this site is active or not for ORR. So the
nature of MeNeC bond, i.e., the chemistry of the metal and
the chemical composition of the carbon surface, is the key for
a catalyst to obtain a high catalytic activity for ORR [2,24].
In this work, we developed a high performance hemin and
mesoporous carbon hybrid electrocatalyst for ORR. First,
hemin is used as the FeeN-containing precursor to prepare
the hemin-induced MC support. Subsequently, hemin is
doped on the prepared hemin-induced MC support to serve as
the catalyst for ORR. The activities of MC based electro-
catalysts are compared for ORR in an alkaline medium.
2. Experimental
2.1. Materials and samples prepararion
Hemin (from bovine, �90%) and hexachloroplatinic acid were
purchased from Aldrich. Vulcan XC-72 carbon powder
(particle size 20e40 nm) was procured from E-TEK Company.
5 wt.% A3-solution was received from Tokuyama and used as
received. All other chemicals were of analytical grade and
used as received.
MC was synthesized by typical templating method using
mesoporous silica (MS) and sucrose as a templating material
and a carbon source, respectively. The detailed procedures are
as follows. The aqueous sucrose solution containing diluted
sulfuric acid was impregnated into the pores of the pre-
prepared MS [21,25]. The mixture was placed in a drying
oven at 100 �C and subsequently dried at 160 �C for 6 h for
sucrose polymerization. When the mixture turned dark
brown, pyrolysis was carried out at 900 �C for 5 h under argon
gas. The resultant carbonesilica composite was treated with
diluted HF solution to remove the silica template. The
template-free carbon was filtered, washed with DI water and
dried at 100 �C to give MC. The Fe-doped MC (Fe-MC) and
hemin-induced MC (H-MC) were prepared with the same
procedure by using the FeSO4/sucrose and hemin/sucrose as
the precursos with the Fe loading of 2 wt.%, respectively. N-
dopedMC (N-MC)was prepared by the heat treatment ofMC in
Ar:NH3:H2 ¼ 1:1:1 at 900 �C for 1 h [1,26]. The hemin and
mesoporous carbon hybrid electrocatalysts were obtained by
loading hemin (Fe loading: 1.0, 1.5 and 2.0 wt.%, respectively)
into the H-MC and heat treated at 600 �C for 2 h with the
resulted samples denoted by H/H-MC-1.0, H/H-MC-1.5 and H/
H-MC-2.0, respectively. For comparison, the carbon supported
Pt catalyst with metal loading of 10 wt.% was prepared [27].
2.2. Sample characterizations
Transmission electron microscopy (TEM) images were ob-
tained by using a high-resolution JEOL 2010F TEM system
operating with a LaB6 filament at 200 kV. The samples were
dispersed in ethanol under sonication and dropped on the
carbon-coated grid and then imaged. The X-ray diffraction
(XRD) patterns of the prepared samples were obtained with
a Philips powder diffraction system (model PW 1830) using
a Cu Ka source operating at 40 keV at a scan rate of 0.025 s�1.
Nitrogen physisorption was performed on the prepared cata-
lysts to determine their textural properties (e.g., surface area
and pore size distribution). The mesoporous carbon based
samples were outgassed at 573 K in vacuum before
measurement in the Coulter SA 3100. The valence state of the
prepared samples were carried out by the X-ray photoelectron
spectroscopy (XPS) technique, which is equipped with
a Physical Electronics PHI 5600 multi-technique system using
Al monochromatic X-ray at a power of 350 W.
Electrochemical measurements were carried out in
a three-electrode cell. The glassy carbon electrode (GCE) with
an area of 0.125 cm2 was used as the working electrode, Pt foil
was employed for the counter electrode and a Hg/HgO/KOH
(1.0 M) (MMO, 0.098 V versus SHE) was used as the reference
electrode. The reference electrode was placed in a separate
chamber, which is located near the working electrode through
a Luggin capillary tube. The working electrode was modified
with the catalyst layer achieved by dropping the catalyst ink
on the GCE. The catalyst ink was prepared by ultrasonically
dispersing 10mg of the carbon catalysts in 1.9ml of ethanol, to
which 0.1 ml of 5 wt.% A3-solution was added, and the
dispersion was ultrasonicated for 30 min to obtain a homoge-
neous solution. A quantity of 10 ml of the dispersion was
pipetted out on the top of the GCE and dried in air. Solutions
were prepared from analytical grade reagents and DI water.
Polarization curves for the ORR were obtained in 0.1 M KOH
solution using the rotating disk electrode (RDE) with the speed
controlled by a Metrohm 628-10 unit.
3. Results and discussion
3.1. Physicochemical characterizations
TEM and XRD analyses were performed to confirm successful
preparation ofMS sample. FromFig. 1a, it can be observed that
the silica sample exhibit a spherical morphology with particle
sizes of around 100 nm with mesopores size of about 4 nm.
The MS sample was also characterized with XRD; the
diffraction pattern of the MS is shown in the insert of Fig. 1a.
The small-angle XRD patterns of the MS sample displays
a well resolved peak at 2q value of 0.81�, which corresponds to
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the (100) diffraction of hexagonal symmetry, similar to the
pattern of SBA-15 [28]. All the mesoporous carbons were
prepared with the resulted MS template. Fig. 1b shows
a typical TEM image of the H/H-MC sample. The EDS analysis
from the selected region indicated the Fe was detected in this
sample, which resulted from hemin.
The nitrogen adsorptionedesorption isotherms and pore
size distribution of the prepared carbon materials are shown
in Fig. 2. The gas sorption isotherms (Fig. 2a) exhibit type IV
curves, typical for mesoporous materials [3,29]. For all the
samples, the mesopore radius distribution is centered at
w4.0 nm according to the BarretteJoynereHalenda (BJH)
model (Fig. 2b derived by desorption tests), which is in
agreementwith the size of the templating silica nanoparticles.
As compared with the MC, the increased total pore volume
was observed in the Fe-MC and N-MC, and thus higher BET
(Brunauer, Emmett, Teller) surface area was obtained, while
lower BET surface area was observed in the H-MC and even
lower for the H/H-MC, since the incorporation of hemin
decreased the total pore volume. The samples characteristics,
such as the BET specific surface area and total pore volume of
the mesoporous carbon materials are summarized in Table 1.
The surface composition of the carbon samples were
characterized by XPS. The C 1s, N 1s, Fe 2p regions were
scanned and the corresponding C 1s spectra are shown in
Fig. 3. All the samples show the content of C and N, but the N
content was very low and can be neglected in the MC and Fe-
MC. Fe was found in the Fe-MC, H-MC and H/H-MC. In Fig. 3,
we can see that the C 1s peak of Fe-MC was shifted to a lower
binding energy and the binding energy shoulder was also
narrower when compared to MC, which may result from the
electron donation from iron [8]. For N-MC, the C 1s peak
shifted to a higher binding energy due to the incorporation of
nitrogen. As some carbon atoms are adjacent to nitrogen
atoms in the carbon matrix, these carbon atoms will inher-
ently have a higher C 1s binding energy since N 1s usually
shows higher binding energy (390e408 eV). The interaction
between carbon atoms and nitrogen atoms contributes to the
higher C 1s biding energy was also found in other N-doped
mesoporous carbons [8,30]. As for the H-MC and H/H-MC
Fig. 1 e XRD and TEM measurements of the MS sample (a), TEM image of the H/H-MC sample (b) and EDS analysis of the
H/H-MC (c).
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samples, the higher binding energy shifts were observed for
the C 1s peaks. As compared with the H-MC, the slightly lower
binding energy of the C 1s peak in the H/H-MC sample was
contributed by the FeeN4 centers (from hemin) indicating the
FeeNeC bonds formation between the H-MC support and
hemin [6,24]. The N contents of the mesoporous carbons were
shown in Table 1.
3.2. Electrochemical characterizations
Polarization curves for the ORR of the prepared catalysts
were recorded in 0.1 M KOH solution saturated with pure
oxygen. The LSV curves of the sample catalysts are with that
of the Pt/C catalyst in Fig. 4. It can be seen that the ORR on the
MC catalyst is diffusion controlled when the potential is less
than �0.4 V, and is under mixed diffusion kinetics control in
the potential region from �0.4 and �0.15 V. The potential
region from �0.15 and �0.1 V is the kinetics control region.
The increased catalytic activity is observed in the Fe-MC
catalyst as compared with the MC catalyst by giving
a lower overpotential and higher current density. Lefevre and
Matter proposed that the transition metal may catalyze the
formation of active sites through the growth of carbon
nanostructures with a specific architecture. For example, Fe
particles might catalyze the growth of carbon nanostructures
with a higher percentage of the edge plane exposure [31,32].
Fig. 2 e N2 sorption isotherms (a) and pore size distribution
(b) from BJH method of the prepared MC-based materials.
Table 1 e Physical properties of mesoporous carbonmaterials.
Sample BETsurface area(m2 g�1 �5%)
Total porevolume(mL g�1)
N content(from XPS, at.%)
MC 695.77 0.6305 0.08
Fe-MC 789.15 0.8468 0.59
N-MC 769.15 0.7639 0.03
H-MC 618.41 0.6596 1.15
H/H-MC 1.5 339.98 0.4131 4.93
Fig. 3 e XPS of C 1s spectra of MC-based samples.
Fig. 4 e Linear sweep voltammograms (5 mV sL1) recorded
in 0.1 M KOH solution saturated with oxygen at a rotating
rate of 2400 rpm.
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These edge planes are believed to exhibit a much higher
activity toward ORR. It is also believed that the remaining Fe
species encapsulated in the Fe-MC may exert some electron
effect on the active sites and facilitate the ORR, but they are
not active sites [8]. It should be noted that the incorporation
of Fe to the MC increased the total pore volume and BET
surface area (Table 1), which may contribute to the increased
ORR activity of the Fe-MC catalyst. As seen in Fig. 4, the
increased ORR activity is also seen in the N-MC catalyst,
which is even higher than the Fe-MC catalyst in both the
kinetic and diffusion control regions. It should be noted that
the N-MC was prepared without any transition metal in the
synthesis process. The major difference between the two
carbons is that there is 0.59 at.% nitrogen in N-MC, while
there is nearly no nitrogen in MC (from XPS result). There-
fore, the absence of any transition metal, nitrogen can be
doped into the carbon layers and produce active sites for ORR
[9,33e35]. It can be seen from Fig. 4, the ORR activity of H-MC
was significant increased over that of MC. The onset poten-
tial for H-MC is about 0.1 V, w0.18 V higher than that of MC,
with a higher ORR current over the whole potential range.
Since the FeeN4-type structure (from hemin) has been
demonstrated to be unstable after heat-treatment at 800 �C,the FeeN4 active sites for ORR can be ignored in the H-MC
catalyst [8]. The high catalytic activity of the H-MC catalyst
can be explained by the simultaneous incorporation of Fe
and N species. It turns out that the presence of Fe may
facilitate the incorporation of active nitrogen species into the
carbon matrix with a strong Lewis basicity, which can
enhance the electron-donor property of the nitrogen-doped
carbon. This in turn will weaken the OeO bond via the
bonding between oxygen and nitrogen and/or the adjacent
carbon atom [8]. Hence, the ORR activity of the H-MC catalyst
can be significantly increased. Although H-MC has greatly
improved ORR activity, but its activity is still lower than the
carbon supported noble Pt catalyst especially in the kinetic
control region.
To further increase the catalytic activity of the MC-based
catalysts, the hemin and H-MC hybrid catalysts (heat treated
at 600 �C with FeeN4 active sites maintained for ORR [8]) with
three different Fe contents were synthesized; the ORR activi-
ties were compared in Fig. 5. Similarly to previous reports by
others, there is an optimum Fe content in these non-precious
metal catalysts [8,16]. At low Fe contents (1.0 wt.% Fe) the ORR
activity increases with increasing the Fe content. When the Fe
content increases to 2.0 wt.%, the obtained catalyst show
a lower ORR activity than 1.5 wt.%. This result further
confirms that both the metal ion and the surface area are of
paramount importance in determining the electrocatalytic
activity. The optimized H/H-MC-1.5 catalyst shows an onset
potential of about 0.15 V, w50 mV higher than that of Pt/C,
with a higher ORR current over the whole potential range. In
subsequent studies, the H/H-MC-1.5 was selected as the target
catalyst.
The dynamics of ORR on the prepared catalysts were then
examined by rotating disk voltammetry. Fig. 6 shows a series
of rotating disk voltammograms of ORR at H/H-MC-1.5 cata-
lyst at different rotation rates in 0.1 M KOH saturated with
oxygen. The RDE data were analyzed using the Kouteckye
Levich equation:
1J¼ 1
JKþ 1JL
¼ 1JK
þ 1Bu1=2
(1)
B ¼ 0:62nFCoD2=3o y�1=6 (2)
JK ¼ nFkCo (3)
where J is the measured current density, JK and JL are the
kinetic and diffusion limiting current density, respectively, u
is the electrode rotation rate, n is the overall number of elec-
tron transfer, F is the Faraday constant (96,485 C mol�1), Co is
the bulk concentration of O2 dissolved in the electrolyte, Do is
the O2 diffusion coefficient, and n is the kinematic viscosity of
the electrolyte. Therefore, based on the KouteckyeLevich
equation, a plot of the inverse of the current density J�1 versus
Fig. 5 e Linear sweep voltammograms (5 mV sL1) recorded
in 0.1 M KOH solution saturated with oxygen at a rotating
rate of 2400 rpm.
Fig. 6 e Rotating disk voltammograms for H/H-MC-1.5
electrode.
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u�1/2 should yield a straight line with the intercept corre-
sponding to JK and the slopes reflecting the so-called B factor
(Eq. (2)). The number of electron transfer in the O2 reduction
process can then be calculated from the B factor by using the
literature data of Co ¼ 1.2 � 10�6 mol cm�3,
Do¼ 1.9� 10�5 cm2 s�1 and n¼ 0.01 cm2 s�1 [3]. Fig. 7 shows the
corresponding KouteckyeLevich plots for H/H-MC-1.5
compared with the H-MC electrode. Specifically, based on
Eqs. (1) and (2), the number of electron transfer (n) in ORR was
estimated to be 4.0 at the potential of 0 V. This result indicates
that the efficient reduction of oxygen was achieved on the H/
H-MC-1.5 electrode at the kinetic region. As for the H-MC
electrode, the number of n is estimated to be lower than 4
(n ¼ 3.2 at 0 V), which might indicate mixed processes. The
estimation of n for other electrodes was also carried out in
a similarmanner. For theMC, Fe-MC andN-MC electrodes, the
estimated n � 2.0, suggesting the incomplete reduction of
oxygen on these catalysts in this region.
The methanol crossover test on H/H-MC-1.5 was also per-
formed in the chronoamperometric measurements. Fig. 8
shows the corresponding response at a constant voltage of
�0.2 V for 1800 s in 0.1 M KOH solution saturated with O2 with
1.0 M methanol. For Pt/C, the ORR cathodic current vanishes,
and an anodic current of methanol oxidation appears. As for
the H/H-MC-1.5, it exhibits a stable amperometric response
for ORR, which does not suffer after introduction of methanol,
suggesting a remarkably good tolerance to fuel crossover
effects.
4. Conclusions
In summary, we developed a high performance hemin and
mesoporous carbon hybrid electrocatalyst by using hemin as
the FeeN-containing precursor to control the chemistry of the
metal and the chemical composition of the carbon surface.
The prepared catalyst exhibits a higher ORR performance than
the much more expensive Pt/C catalyst does. In addition, as
a non-precious-metal catalyst is typically insensitive to
alcohol oxidation, the use of the prepared catalyst for ORR
enables the mixed potential associated with fuel crossover to
be minimized. The present noble-metal-free catalyst can, in
principle, overcome the limitations of Pt-based systems and
provide suitable, sustainable, and cheap solutions for the
further technological development of fuel cells.
Acknowledgments
The work described in this paper was fully supported by
a grant from the Research Grants Council of the Hong Kong
Special Administrative Region, China (Project No. HKUST9/
CRF/11G).
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