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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

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 5 9 7 6e1 5 9 8 2

0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ijhydene.2012.08.037


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

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 5 9 7 6e1 5 9 8 2 15977


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).



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.



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.



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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