Oxygen electroreduction on heat-treated multi-walled carbon nanotubes supported iron polyphthalocyanine in acid media

Article abstract of DOI:10.1016/j.electacta.2014.09.064

Multi-walled carbon nanotubes (MWCNTs) supported iron phthalocyanine (FePc), binuclear iron phthalocyanine (bi-FePc) and iron polyphthalocyanine (FePPc) were prepared by a solvothermal process. The resulting FePc/MWCNTs, bi-FePc/MWCNTs and FePPc/MWCNTs were heat-treated in argon (Ar) atmosphere at various temperatures ranging from 500 to 900°C to obtain optimized catalysts for the oxygen reduction reaction (ORR). The crystal structure, morphology and chemical environment of the catalysts were examined by ultraviolet-visible (UV-Vis) spectroscopy, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure spectroscopy (XAFS). The electrocatalytic activity of the obtained catalysts was measured using a rotating disk electrode (RDE) technique in 0.5 mol L^-1 H₂SO₄ solution saturated with oxygen. The ORR activity of the heat-treated FePPc/MWCNTs was found to be better than that of the heat-treated bi-FePc/MWCNTs and FePc/MWCNTs. Furthermore, the heat-treatment temperature greatly influenced the catalytic ORR ability of the catalysts. The FePPc/MWCNTs heat-treated at 800°C exhibited a four-electron transfer process and the best ORR activity (E_ORR = 0.79 V vs. RHE), methanol resistance, and stability (current loss = 13% at -0.13 V vs. Hg/Hg₂SO₄ after 55 h). XPS indicated that pyridine-type nitrogen, not graphitic-N, played a critical role in determining the electrocatalytic ORR activity of the amples. XAFS showed that the coordination geometry around Fe was close to square planar in structure, suggesting that the Fe-N₄ structure was produced by the high temperature treatment.

Full text of DOI:10.1016/j.electacta.2014.09.064

Electrochimica Acta 147 (2014) 343–351
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Oxygen Electroreduction on Heat-treated Multi-walled Carbon
Nanotubes Supported Iron Polyphthalocyanine in Acid Media
Rui Zhang a, Yingxiang Peng a, Zhipan Li a, Kai Li a, Jie Ma a, Yi Liao a, Lirong Zheng c,
b,**
Xia Zuo a, , Dingguo Xia
*
a
Department of Chemistry, Capital Normal University, Beijing 100048, PR China
College of Engineering, Peking University, Beijing 100871, PR China
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, PR China
b
c
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 20 June 2014
Received in revised form 2 September 2014
Accepted 15 September 2014
Available online 28 September 2014
Multi-walled carbon nanotubes (MWCNTs) supported iron phthalocyanine (FePc), binuclear iron
phthalocyanine (bi-FePc) and iron polyphthalocyanine (FePPc) were prepared by a solvothermal process.
The resulting FePc/MWCNTs, bi-FePc/MWCNTs and FePPc/MWCNTs were heat-treated in argon (Ar)
atmosphere at various temperatures ranging from 500 to 900 ꢀC to obtain optimized catalysts for the
oxygen reduction reaction (ORR). The crystal structure, morphology and chemical environment of the
catalysts were examined by ultraviolet-visible (UV–Vis) spectroscopy, X-ray diffraction (XRD), high-
resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-
ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure spectroscopy (XAFS). The
electrocatalytic activity of the obtained catalysts was measured using a rotating disk electrode (RDE)
technique in 0.5 mol Lꢁ1 H2SO4 solution saturated with oxygen. The ORR activity of the heat-treated
FePPc/MWCNTs was found to be better than that of the heat-treated bi-FePc/MWCNTs and FePc/
MWCNTs. Furthermore, the heat-treatment temperature greatly influenced the catalytic ORR ability of
the catalysts. The FePPc/MWCNTs heat-treated at 800 ꢀC exhibited a four-electron transfer process and
the best ORR activity (EORR = 0.79 V vs. RHE), methanol resistance, and stability (current loss = 13% at
–0.13 V vs. Hg/Hg2SO4 after 55 h). XPS indicated that pyridine-type nitrogen, not graphitic-N, played a
critical role in determining the electrocatalytic ORR activity of the amples. XAFS showed that the
coordination geometry around Fe was close to square planar in structure, suggesting that the Fe-N4
structure was produced by the high temperature treatment.
Keywords:
Iron polyphthalocyanine
Multi-walled carbon nanotubes
Oxygen reduction reaction
Heat-treated
XAFS
ã 2014 Elsevier Ltd. All rights reserved.
1. Introduction
promising non-platinum electrocatalysts for ORR, such as precious
metal Ru-based materials [5,6], non-precious metal chalcogenides
Proton exchange membrane fuel cells (PEMFCs) have found a
wide variety of commercial applications and have the potential to
radically alter the contemporary energy landscape. At the present
moment, the main factor preventing the successful commerciali-
zation of PEMFCs is their high price [1]. Because platinum and
platinum alloy electrocatalysts are used for the oxygen reduction
reaction at the cathode side of PEMFCs, the high cost of platinum
significantly increases the total price of fuel cell devices. Moreover,
platinum and platinum alloy electrocatalysts still suffer from other
serious problems, including: poor methanol tolerance and low
stability [2–4]. These issues have driven the development of some
[7,8] and transition metal macrocycle materials [9,10].
Transition metal-nitrogen catalysts containing MN4 macro-
cycles (tetraazaannulenes, porphyrins and phthalocyanines) have
gained huge interest owing to their high electrocatalytic activity
toward oxygen reduction reaction and their competitive price on
the market. Since 1964, when Jasinski observed that cobalt
phthalocyanine catalyzed the ORR [11], numerous MN4 macrocy-
clic compounds such as phthalocyanines and porphyrins have been
widely explored. However, while most metal phthalocyanines and
porphyrins have been found to exhibit good initial oxygen
reduction performance [12–14], their stability still requires further
improvement [15,16]. To improve the catalytic activity and stability
of these materials for the ORR, heat-treatment in an inert
atmosphere above 500 ꢀC has been used to stabilize MN4 macro-
cycles on a carbon based support [17–21]. The performance of a
PEMFC is essentially governed by the electrocatalyst support
* Corresponding author. Tel.: +86 10 68903086; fax: +86 10 68903040.
** Corresponding author. Tel. :+ +86 10 82529046; fax: +86 10 82529010.
E-mail addresses: zuoxia@mail.cnu.edu.cn (X. Zuo), dgxia@pku.edu.cn (D. Xia).
http://dx.doi.org/10.1016/j.electacta.2014.09.064
0013-4686/ã 2014 Elsevier Ltd. All rights reserved.

344
R. Zhang et al. / Electrochimica Acta 147 (2014) 343–351
materials, because they strongly influence the electrocatalyst
performance, durability and efficiency. Black Pearl and Vulcan XC-
72R are commonly used as catalyst support materials, but their
small surface area means that the resulting electrodes are only
weakly active toward the electroreduction of oxygen. Owing to
their unique electrical and mechanical properties, MWCNTs
exhibit promising potential to promote electron transfer when
used as catalyst supports in electrochemical reactions [22–25].
Thus, the use of MWCNTs as a support for heat-treated MN4
macrocycles may increase their electrocatalytic performance and
efficiency for the ORR beyond those achievable with Black Pearl
and Vulcan XC-72R.
In the literature, MWCNTs-supported transition metal-nitrogen
catalyst materials are always prepared by depositing metal
phthalocyanines onto MWCNTs using impregnation, ultrasonic
synthesis or solid-phase synthesis methods [26–28]. The weak
interaction between the metal phthalocyanine and the MWCNTs
mean that the amount of metal phthalocyanine coated onto the
MWCNTs is low, resulting in a limited number of active sites after
heat-treatment and consequently a low catalytic activity for the
ORR. The general aim of the present work was to prepare catalysts
using MWCNTs as the carbon support material and FePPc, bi-FePc,
and FePc, which have large conjugated systems, as precursors. The
catalysts were synthesized by a solvothermal method and then
pyrolyzed in Ar atmosphere at different temperatures. The
catalytic ability and stability of the catalysts towards the ORR in
acidic media were evaluated and the influences of the heat-
treatment temperature and the degree of conjugate of phthalocy-
anine are discussed. Finally, a variety of characterization methods
were used to explore the structure of the active site.
of 8 ꢀC minꢁ1 in a tubular furnace under a 0.1 L minꢁ1 flow of high
purity Ar for 1 h. The synthesized materials were labelled
FePc/MWCNTs-800 ꢀC and bi-FePc/MWCNTs-800 ꢀC.
2.2. Electrochemical measurements
For the electrochemical measurements, the working electrode
was a rotating disk electrode, consisting of a glassy carbon (GC)
disk with a diameter of 4 mm. A Catalyst ink was prepared by
ultrasonically dispersing 2.5 mg of the catalyst in 0.5 mL of a
0.1wt% Nafion1 ethanol solution. Then, 20 mL of the catalyst ink
was dropped onto the surface of the polished GC and dried at room
temperature. RDE and chronoamperometry measurements were
performed with a SP-200 electrochemical workstation (Bio-Logic
SA) using a conventional three-electrode system. A platinum net
and a Hg/Hg2SO4 with saturated K2SO4 solution electrode were
used as the counter electrode and reference electrode, respectively.
All measurements were carried out at room temperature with
0.5 mol Lꢁ1 H2SO4 solution as the electrolyte in the potential range
between 350 mV and -700 mV at a scan rate of 5 mV sꢁ1. The ORR
current densities given in the figures were normalized with respect
to the geometrical area (A) of the GC electrodes.
2.3. Physical characterizations
The morphology, composition and microstructure of the
synthesized catalyst materials were investigated by high-resolu-
tion transmission electron microscopy (HRTEM; FEI Tecnai F20,
Japan) and energy-dispersive X-ray spectroscopy (EDX; FEI Tecnai
F20, Japan). The structural changes in the catalysts after heat-
treatment were explored by X-ray diffraction (XRD; D8 Advance,
2. Experimental
Bruker, Gꢁer1many). A 2
u
angular region from 10ꢀ to 70ꢀ, a scan rate
of 5ꢀ min , and a step of 0.02ꢀ were used for the XRD measure-
ments. The chemical states and the amount of nitrogen contained
in the catalysts were analyzed by X-ray photoelectron spectrosco-
2.1. Electrocatalyst preparation
2.1.1. Preparation of pyrolyzed FePPc/MWCNTs
py (XPS; Thermo ESCALAB 250, UK) with Al K
the excitation source. UV ꢁ Vis spectroscopic data was collected
using Hitachi U-3900 spectrophotometer. The electronic
a X-ray radiation as
1,2,4,5-tetracyanobenzene (0.25 mmol), FeCl3 (0.125 mmol) and
MWCNTs (0.01 g, outer diameter = 6ꢂ13 nm, length = 2.5ꢂ20
m
m,
a
Sigma Aldrich) were dispersed in a quinoline solvent. The mixture
was stirred at 220 ꢀC under nitrogen atmosphere for 3 h, filtered,
washed with ethanol, acetone, methanol, 2wt% HCl, and deionized
water in sequence, and then dried under vacuum at 80 ꢀC
overnight. The prepared catalyst sample was denoted as FePPc/
MWCNTs. Then the FePPc/MWCNTs was pyrolyzed at 500, 600,
700, 800 and 900 ꢀC, respectively, for 1 hour using a heating rate of
8 ꢀC minꢁ1 in a tubular furnace under a 0.1 L minꢁ1 flow of high
purity Ar. The sample was allowed to cool down in the oven under
Ar atmosphere. The resulting catalysts were correspondingly
structure and local geometry of the Fe contained in the catalysts
were investigated by X-ray absorption fine structure spectroscopy
(XAFS; BSRF synchrotron facility). The Fe K-edge spectra were
measured in the fluorescence mode, using a double-crystal Si (111)
monochromator. XAFS consists of two parts, the X-ray absorption
near edge structure (XANES) and the extended X-ray absorption
fine structure (EXAFS). The data in the EXAFS region were analyzed
using the Athena software package.
3. Results and Discussion
labelled
FePPc/MWCNTs-500 ꢀC,
FePPc/MWCNTs-600 ꢀC,
and
FePPc/MWCNTs-700 ꢀC,
FePPc/MWCNTs-800 ꢀC
3.1. Characterization of electrocatalysts
FePPc/MWCNTs-900 ꢀC. For comparison, MWCNTs-supported iron
phthalocyanine (FePc) and binuclear iron phthalocyanine (bi-FePc)
catalysts were also synthesized in the same manner.
To confirm the successful formation of the electrocatalysts, the
UV–vis absorption spectra of dimethyl sulfoxide (DMSO) suspen-
sions of the electrocatalyst samples were obtained. Fig. 1a, b, and c
are the spectra of FePPc/MWCNTs, bi-FePc/MWCNTs, and
FePc/MWCNTs, respectively. They show typical electronic spectra
with two strong absorption regions, one in the UV region near
318 nm (B-band) and the other in the visible region in 658ꢂ693 nm
(Q-band) [29], indicating that iron phthalocyanine has been
formed. The B-bands of the three catalysts appear at the same
energy, but the Q-bands of FePPc/MWCNTs and bi-FePc/MWCNTs
are shifted to higher energies compared with that of
FePc/MWCNTs. This suggests that the energy of the highest
occupied molecular orbital (HOMO) was the same for the
materials, whereas the lowest unoccupied molecular orbitals
(LUMO) of FePPc/MWCNTs and bi-FePc/MWCNTs probably had
2.1.2. Preparation of pyrolyzed FePc/MWCNTs and bi-FePc/MWCNTs
1,2-Dicyanobenzene (0.5 mmol), FeCl3 (0.125 mmol), MWCNTs
(0.01 g) were dispersed in a quinolone solvent. The mixture was
stirred at 220 ꢀC under nitrogen atmosphere for 3 h, filtered,
washed with ethanol, acetone, methanol, 2wt% HCl, and deionized
water in sequence, and then dried in vacuum at 80 ꢀC
overnight. The obtained sample was recorded as FePc/MWCNTs.
The bi-FePc/MWCNTs were prepared by replacing 1,2-Dicyano-
benzene (0.5 mmol) with 1,2-Dicyanobenzene (0.375 mmol) and
1,2,4,5-tetracyanobenzene (0.065 mmol), but using the same
synthetic procedure. The obtained FePc/MWCNTs and
bi-FePc/MWCNTs were pyrolyzed at 800 ꢀC using a heating rate

R. Zhang et al. / Electrochimica Acta 147 (2014) 343–351
345
Fig. 2. X-ray diffraction patterns of (a) MWCNTs, (b) FePPc-700 ꢀC, (c) FePPc/
MWCNTs-500 ꢀC, (d) FePPc/MWCNTs-600 ꢀC, (e) FePPc/MWCNTs-700 ꢀC, (f) FePPc/
MWCNTs-800 ꢀC, and (g) FePPc/MWCNTs-900 ꢀC.
Fig. 1. UV–vis spectra of (a) FePPc/MWCNTs, (b) bi-FePc/MWCNTs, (c) FePc/
MWCNTs, (d) FePPc/MWCNTs-800 ꢀC, (e) bi-FePc/MWCNTs-800 ꢀC, and (f) FePc/
MWCNTs-800 ꢀC.
Fig. 3. HRTEM images of (A) FePPc/MWCNTs, (B) FePPc/MWCNTs-800 ꢀC, (D) bi-FePc/MWCNTs, (E) bi-FePc/MWCNTs-800 ꢀC, (G) FePc/MWCNTs and (H) FePc/MWCNTs-
800 ꢀC, EDX images of (C) FePPc/MWCNTs-800 ꢀC, (F) bi-FePc/MWCNTs-800 ꢀC and (I) FePc/MWCNTs-800 ꢀC.

346
R. Zhang et al. / Electrochimica Acta 147 (2014) 343–351
higher energy than that of FePc/MWCNTs, resulting in a smaller
HOMO-LUMO gap for FePPc/MWCNTs and bi-FePc/MWCNTs than
for FePc/MWCNTs. This in turn means that FePPc/MWCNTs and
bi-FePc/MWCNTs are more able to donate electrons owing to their
larger phthalocyanine rings. Fig 1d, e, and f show the UV–vis
spectra of FePPc/MWCNTs-800 ꢀC, bi-FePc/MWCNTs-800 ꢀC, and
FePc/MWCNTs-800 ꢀC, respectively. The two typical absorption
peaks (B-band and Q-band) have disappeared, which indicates that
the phthalocyanine ring decomposed at 800 ꢀC and its chemical
structure changed. In fact, the phthalocyanine ring structure was
found to be destroyed and the peaks disappeared at 500 ꢀC
(Fig. S1).
3.2. Electrocatalytic properties of FePPc/MWCNTs pyrolyzed at various
temperatures
Fig. 4A displays linear sweep voltammograms measured in
acid media to study the effect of the heat-treatment temperature
on the ORR activity of the FePPc/MWCNTs modified electrodes.
The current density greatly increased with heat treatment
temperature from 500 to 800 ꢀC and started to fall when
the temperature was further increased to 900 ꢀC. This trend
suggests that a large number of ORR active sites may have been
formed on the heat-treated FePPc/MWCNTs catalyst within the
temperature range of 600ꢂ800 ꢀC.
The above UV–vis results suggest that heat-treatment had a great
influence on the structure of the prepared electrocatalysts.
Therefore, XRD analyses were carried out to clarify the relationship
betweenthepyrolysistemperatureandthestructureofthecatalysts.
The XRD patterns of FePPc/MWCNTs heat-treated at 500, 600, 700,
800, and 900 ꢀC are shown in Fig. 2c–g, respectively, with the
diffraction patterns of FePPc-700 ꢀC (Fig. 2b) and MWCNTs (Fig. 2a)
Fig. 4B shows the dependences of the limited current density
and onset potential of the FePPc/MWCNTs modified electrodes on
the FePPc/MWCNTs heat treatment temperature. The highest
limited current density and most positive onset potential were
ꢁ7.35 mA cmꢁ2 and 150 mV, respectively, obtained when
the catalyst was pyrolyzed at 800 ꢀC. Hence, the catalyst that
displays the highest electrochemical activity for the ORR was
FePPc/MWCNTs-800 ꢀC. The XRD patterns (Fig. 2) indicated that
presented for comparison. A very broad obvious peak at 2u
= 23–26ꢀ
can be seen in Fig. 3b, which was assigned to the amorphous carbon
produced by the pyrolyzation of FePPc at 700 ꢀC. In comparison, the
other samples show a relatively sharp diffraction peak (002) at
well-crystallized Fe-Nx phase (2u
= 35.6ꢀ) formed at 800 ꢀC and
decomposed into Fe (2u
= 64ꢀ) at 900 ꢀC, which explains why
800 ꢀC was the optimum pyrolyzation temperature.
2u
= 26ꢀ which can be attributed to the graphitic plane of the
MWCNTs[30]. AllthesamplesexceptFePPc-700 ꢀC(Fig. 2b)showed
a distinct diffraction peak at 2
= 43ꢀ, which is close to the (101)
u
diffraction peak of graphite [31], another typical diffraction peak of
MWCNTs. When FePPc/MWCNTswas pyrolyzedat800and 900 ꢀC, a
new diffraction peak assigned to the (110) plane of the Fe-Nx phase
clearly appeared at 2u
= 35.6ꢀ (Fig. 2f and g, respectively). This
implies that awell-crystallized Fe-Nx phasewas formed at800 ꢀC by
the thermal decomposition of the phthalocyanine ring. However,
another obvious diffraction peak appeared at 64ꢀ after
heat-treatment at 900 ꢀC, (Fig. 2g), which indicates that part of
the Fe-Nx probably decomposed to form a new species. It has been
previously reported that a well-crystallized Fe-Nx phase formed at
800 ꢀC can partially decompose into Fe at 900 ꢀC [32,33], so the
diffraction peak at 2u
= 64ꢀ may correspond to the characteristic
diffraction peak of Fe. In view of these results, it is plausible to
assume that FePPc/MWCNTs-800 ꢀC contained more Fe–Nx phase
and a more orderly structure than the other samples.
The morphology of catalysts before and after heat-treatment at
800 ꢀC was observed by HRTEM. Fig. 3 is the HRTEM and EDX
images of the catalysts. Compared with the unmodified MWCNTs
(Fig. S2), it can be observed that FePPc forms a uniform coating on
the surface of MWCNTs with a thickness of about 10 nm (Fig. 3A).
However, the distribution of bi-FePc and FePc on the surface of
MWCNTs is not uniform, and the thickness of bi-FePc and FePc
dispersing on the surface of MWCNTs is only about 3 nm and 2 nm,
respectively (Fig. 3D and G). That is because the degree of
conjugation has a significant impact on the distribution of
phthalocyanine on the surface of MWCNTs. The greater the degree
of conjugation, the stronger the force between phthalocyanine and
MWCNTs is, which resulted the most uniform distribution and the
maximum thickness of FePPc coated on the surface of MWCNTs.
Fig. 3B, E and H are the HRTEM images of FePPc/MWCNTs-800 ꢀC,
bi-FePc/MWCNTs-800 ꢀC and FePc/MWCNTs-800 ꢀC, respectively.
It can be seen clearly that the thickness of residue produced by
phthalocyanine ring pyrolyzed at high temperature became small,
which maybe results from the destroyed phthalocyanine ring
through high temperature. From Fig. 3B, E and H, we can see that
FePPc/MWCNTs-800 ꢀC remained the most uniform distribution
and the biggest thickness, about 5 nm. The presence of Fe in the
EDX image (Fig. 3C, F and I) confirms that an iron phase was
produced by the pyrolyzation of the phthalocyanine ring and
covered the surface of the MWCNTs.
Fig. 4. (A) Linear sweep voltammograms for FePPc/MWCNTs pyrolyzed at different
temperatures, measured in O2 saturated 0.5 mol Lꢁ1 H2SO4 at 1600 rpm and a scan
rate of 5 mV sꢁ1: (a) 500 ꢀC, (b) 600 ꢀC, (c) 700 ꢀC, (d) 800 ꢀC, and (e) 900 ꢀC. (B)
Dependence of limited current density and onset potential of FePPc/MWCNTs on
pyrolyzation temperature.

R. Zhang et al. / Electrochimica Acta 147 (2014) 343–351
347
Fig. 5. (A) Linear sweep voltammograms for FePPc/MWCNTs-800 ꢀC measured at
Fig. 6. (A) RDE polarization curves of FePPc/MWCNTs-800 ꢀ
C
measured at
400–2025 rpm and a scan rate of 5 mV sꢁ1 in O2 saturated 0.5 mol Lꢁ1 H2SO4
2025 rpm and a scan rate of 5 mV sꢁ1 in O2 saturated 0.5 mol Lꢁ1 H2SO4 with
and without 1 mol Lꢁ1 of methanol. (B) Chronoamperometric responses of the
catalysts in O2 saturated 0.5 mol Lꢁ1 H2SO4 solution.
solution. (B) Koutecky-Levich plots for oxygen reduction on FePPc/MWCNTs-800 ꢀ
in O2 saturated 0.5 mol Lꢁ1 H2SO4 solution.
C
rotation rate is expressed in rpm [36]. The extrapolated K-L lines
show non-zero intercepts, indicating that the oxygen reduction
process is under mixed kinetic-diffusion control within a large
range of potentials. From the slope of the K-L plots, the n value of
the catalyst is 3.7 at ꢁ300 to ꢁ600 mV, indicating that the oxygen
reduction on the FePPc/MWCNTs-800 ꢀC catalyst proceeded
mainly by the 4eꢁ reduction process.
In direct methanol fuel cells (DMFCs), methanol may diffuse
from the anode to cathode through the Nafion1 membrane.
This crossed-over methanol poisons the Pt catalyst on the cathode
and decreases the output current density, resulting in a mixed
potential and reduction in both cell performance and fuel
utilization. However, MN4 chelates are known to be selective
catalysts and, more particularly, to be tolerant to the presence
of methanol during ORR [37,38]. Fig. 6A shows RDE polarization
curves obtained in O2 saturated electrolyte in the presence and
absence of methanol (1 mol Lꢁ1) for the FePPc/MWCNTs-800 ꢀC
catalyst. The current density and the onset potential exhibited
almost no change in the presence or absence of methanol
(1 mol Lꢁ1). Therefore, the present FePPc/MWCNTs-800 ꢀC
Fig. 5A shows linear sweep voltammograms at various rotation
rates for ORR on the FePPc/MWCNTs-800ꢀC catalyst. The ORR
limited current density for the FePPc/MWCNTs-800ꢀC catalyst
prominently increased with rotation rate. Fig. 5B shows the
Kouteckye-Levich (K-L) plots of
the ORR for the
FePPc/MWCNTs-800ꢀC catalyst. The number of electrons trans-
ferred per O2 molecule (n) is calculated from the K-L equation as
follows [34]:
1
J
1
1
¼
þ
JK JLiM
2
3
1
ꢁ6
JLim ¼ 0:20nFD
y
C
0V21 ¼ BV21
where J (mA cmꢁ2) is the measured current density; Jk (mA cmꢁ2
)
and JLim (mA cmꢁ2) are the kinetic and diffusion limited current
density, respectively; F is the Faraday constant (F = 96485C molꢁ1);
D
is the diffusion coefficient of O2 in 0.5 mol Lꢁ1 H2SO4
(D = 1.8 ꢃ 10ꢁ5 cm2 sꢁ1);
y is the kinetic viscosity (y
= 0.01 cm2
sꢁ1); C0 is the bulk concentration of O2 (C0 = 1.13 ꢃ10ꢁ6 mol cmꢁ3);
V
is the rotation rate [35]. The constant 0.2 is adopted when the
modified electrode is insensitive towards methanol at a

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R. Zhang et al. / Electrochimica Acta 147 (2014) 343–351
The current losses exhibited by FePPc/MWCNTs-800 ꢀC and
FePPc/MWCNTs were nearly 13 and 20%, respectively, after
55 h, indicating that the heat treatment was useful for improving
the stability of the catalyst.
3.3. Electrocatalytic properties of FePPc/MWCNTs-800 ꢀC, bi-FePc/
MWCNTs-800 ꢀC and FePc/MWCNTs-800 ꢀC
Fig.
7
shows linear sweep voltammograms obtained
for GC electrodes modified with FePPc/MWCNTs-800 ꢀC,
bi-FePc/MWCNTs-800 ꢀC, and FePc/MWCNTs-800 ꢀC. As demon-
strated in Fig. 7, the limited current densities and the onset
potentials of the FePPc/MWCNTs-800 ꢀC, bi-FePc/MWCNTs-800 ꢀC,
and FePc/MWCNTs-800 ꢀC catalysts were ꢁ7.35, ꢁ6.03, and
ꢁ5.60 mA cmꢁ2, and 150, 80, and 60 mV, respectively. According
to these results, the trend in the electrocatalytic activity of the
catalysts for the ORR was FePPc/MWCNTs-800 ꢀC > bi-FePc/
MWCNTs-800 ꢀC > FePc/MWCNTs-800 ꢀC. The excellent perfor-
mance of FePPc/MWCNTs-800 ꢀC can be attributed to its beneficial
morphology and structure. Obviously, the degree of conjugation of
FePPc is the biggest, so the force between FePPc and MWCNTs is the
strongest. From TEM image in Fig. 3A, it can be seen that FePPc
formed a uniform coating on the MWCNTs with a thickness of
about 10 nm, which is also the largest. After high temperature
heat-treating, the FePPc/MWCNTs-800 ꢀC remained the most
uniform distribution and the biggest thickness. FePPc/MWCNTs-
800 ꢀC may have had more active sites, resulting in its best
electrochemical performance.
Fig. 7. Linear sweep voltammograms for (a) FePc/MWCNTs-800 ꢀC, (b) bi-FePc/
MWCNTs-800 ꢀC, and (c) FePPc/MWCNTs-800 ꢀ
C
measured in O2 saturated
0.5 mol Lꢁ1 H2SO4 at 1600 rpm and a scan rate of 5 mV sꢁ1
.
concentration of 1 mol Lꢁ1, which represents a good resistance
against methanol poisoning. To examine their ORR stability,
chronoamperometry tests of the FePPc/MWCNTs catalysts
before and after pyrolyzation at 800 ꢀC were conducted at
ꢁ130 mV vs. Hg/Hg2SO4 and the results are shown in Fig. 6B.
Table 1
Kinetic parameters of the electrocatalysts during ORR.
Catalyst
Electrolyte
Eonset/mV
JLim/mA.cm-2
E1/2/mV
n
Current loss
FePPc/CNTs-800(2025rpm, 5mV/s)
FePc/C(2500rpm, 5mV/s)[39]
bi-FePc/CNTs(2500rpm, 10mV/s)[40]
FePc/CNTs(1600rpm, 10mV/s)[41]
0.5 M H2SO4
0.5 M H2SO4
0.5 M H2SO4
0.5 M H2SO4
790
800
850
715
7.76
5
3.4
4.12
615
600
650
529
3.7
3.9
3.8
–
13%(55 h)
35%(3 h)
77%(14 h)
–
Eonset and E1/2 vs. RHE
Fig. 8. XPS N1s spectra of (A) FePPc/MWCNTs-700 ꢀC, (B) FePPc/MWCNTs-800 ꢀC, (C) bi-FePc/MWCNTs-800 ꢀC, and (D) FePc/MWCNTs-800 ꢀC.

R. Zhang et al. / Electrochimica Acta 147 (2014) 343–351
349
Table 2
Relative concentration of nitrogen species in the electrocatalysts as determined by XPS.
Sample
Pyridinic (%)
Pyrrolic (%)
Graphitic (%)
FePPc/MWCNTs-700ꢀC
FePPc/MWCNTs-800ꢀC
bi-FePc/MWCNTs-800ꢀC
FePc/MWCNTs-800ꢀC
18.32(398.6eV)
27.44(398.8eV)
7.01(398.3eV)
4.21(398.7eV)
20.76(399.2eV)
28.33(399.5eV)
23.70(399.3eV)
19.05(399.4eV)
60.91(401.0eV)
44.23(401.2eV)
69.29(401.2eV)
76.74(401.4eV)
To objectively evaluate the ORR activity of this catalyst, we
compared the performance of FePPc/MWCNTs-800 ꢀC with those
reported for other materials [39–41], the results are shown in
Table 1. In this comprehensive consideration, FePPc/MWCNTs-
800 ꢀC presents superior performance to those reported in the
literature.
The structure of the active sites of MN4 catalysts for the ORR
after heat-treatment appears to be complicated and is not yet fully
understood. However, several decades of effort have led to the
proposal that the chemical states and amount of nitrogen
contained in such catalysts have a great influence on the catalytic
activity [42–45]. XPS was used to detect the chemical states and
the content of the nitrogen in the catalysts. Fig. 8 presents the N1
XPS spectra of FePPc/MWCNTs-700 ꢀC, FePPc/MWCNTs-800 ꢀC,
bi-FePc/MWCNTs-800 ꢀC, and FePc/MWCNTs-800 ꢀC. According to
the literature [46–48], the N1s peaks at 398.6 ꢄ 0.3 eV,
399.5 ꢄ 0.3 eV, 401.3 ꢄ 0.3 eV and 403.3 ꢄ 0.3 eV can be assigned
to pyridinic-N, pyrrolic-N, graphitic-N, and pyridine-N-oxide,
respectively. Among these species, pyridinic-N and graphitic-N
are believed to play critical roles in determining the ORR activity
and stability of MN4 macrocycles. Conversely, pyridine-N-oxide
seems to be useless in improving ORR activity [49]. As shown in
Fig. 8, all spectra fit well with the three peaks known to originate
from pyridinic-N (ꢂ398.6 eV), pyrrolic-N (ꢂ399.3 eV), and
graphitic-N (ꢂ401.2 eV). The relative concentrations of each of
the nitrogen species are listed in Table 2. It is clear that the
pyridinic-N concentration of the catalyst pyrolyzed at 800 ꢀC was
greatly increased in comparison to that of the sample pyrolyzed at
700 ꢀC. The dominant peak at 398.7 eV in the spectra suggested
that pyridinic-N is the major state in the N1s (Fig. 8B), and should
be responsible for the high ORR activity. This is why the
FePPc/MWCNTs-800 ꢀC catalyst had a higher activity than that
of the FePPc/MWCNTs-700 ꢀC catalyst. Furthermore, the pyridinic-
N
concentration significantly increased with the degree of
conjugation of the phthalocyanine, but the graphitic-N concentra-
tion decreased. Combined with the electrocatalytic activity results,
it is inferred that pyridinic-N, not graphitic-N, plays a critical role
in determining electrocatalytic ORR activity.
As mentioned earlier, catalyst materials prepared through
pyrolysis have been found to be more active and stable for ORR
than MN4 macrocycles, leading researchers to believe that a new
active site is formed during such heat-treatment. Some research
groups have proposed two possible active site configurations, Fe-
N2 and Fe-N4 [50,51]. To further study the catalytic mechanism,
XAFS was used to analyze the activity sites of the electrocatalyst
samples.
Fig. 9A and B depict the XANES spectra of the catalysts, and the
insets are enlargements of the pre-edge peak at 7114 eV. The
small pre-edge peak at 7114 eV is assigned to a dipole-forbidden
but quadrupole-allowed 1s!3d transition, typical for the
octahedral or square-pyramidal Fe (III) complexes [52]. There
are three kinds of phthalocyanine ring structure: square planar,
square pyramidal and octahedral [53]. If the small pre-edge peak
at 7114 eV is weak, it means that the coordination geometry
around Fe is closer to the square planar structure. Fig. 9A shows
that the pre-edge peak of FePPc/MWCNTs-800 ꢀC was weaker
than that of the FePPc/MWCNTs, which indicates that the high
temperature treatment likely changed the phthalocyanine ring
into a square planar Fe-N4 structure. From the Fig. 9B, the order of
height of the pre-edge peak at 7114 eV was FePPc/MWCNTs-
800 ꢀC < bi-FePc/MWCNTs-800 ꢀC < FePc/MWCNTs-800 ꢀC, which
matches the trend observed for the electrochemical performance
of the pyrolyzed catalysts (Fig. 7). It can thus be inferred that the
active sites of the catalysts may be the Fe-N4 square planar
structure, which can combine with O2 favorably and to improve
ORR activity. Actually, the molecular structure of highly
conjugated metal phthalocyanines tends to be planar. Therefore,
the Fe-N4 square planar structure is likely to form after heat
treatment.
In the Fe K-edge Fourier transform EXAFS spectra of the
electrocatalysts (Fig. 10A and B), the first strong peak at 1.4 Å
(before phase shift correction) is caused by backscattering from the
Fig. 9. XANES spectra of the electrocatalysts.

350
R. Zhang et al. / Electrochimica Acta 147 (2014) 343–351
were then heat-treated at 500, 600, 700, 800, and 900 ꢀC to
optimize their ORR activity. Heat-treatment at 800 ꢀC was found to
yield the highest catalytic activity for the ORR. XRD was used to
characterize the structural changes in the catalysts after heat-
treatment, and revealed that while the Fe-Nx phase formed before
800 ꢀC, at 900 ꢀC the Fe compounds in the catalysts began to
convert to a metallic state. Compared with the bi-FePc/MWCNTs-
800 ꢀC and FePc/MWCNTs-800 ꢀC catalysts, the FePPc/MWCNTs-
800 ꢀC catalyst possessed a high performance towards the ORR in
terms of both onset potential and current density. The overall
electron transfer number of the ORR catalyzed by FePPc/MWCNTs-
800 ꢀC was 3.7, suggesting that the ORR process was dominated by
a 4-electron transfer pathway that produced H2O. TEM images
revealed FePPc formed a uniform coating on the MWCNTs with a
thickness of about 10 nm, the largest among the obtained catalysts.
After heat-treatment, FePPc/MWCNTs-800 ꢀC maintained the most
uniform distribution and largest thickness, which may have
increased the amount of active sites available for the ORR. XPS
indicated that the amount of pyridine-type nitrogen increased
with the degree of conjugation of the phthalocyanine precursor,
indicating that pyridine-type nitrogen, not graphitic-N, plays a
critical role in determining the ORR activity of such catalysts.
EXAFS indicated that the structure of the phthalocyanine ring was
destroyed through heat-treatment, but that the Fe-N4 structure
was maintained during heat treatment at 800 ꢀC. It can be inferred
that the smaller the radius of the Fe-N4 structure of a catalyst, the
better its ORR activity is. Moreover, the special pre-edge peak at
7114 eV in the XANES spectrum indicated that the complexes
showed a better ORR performance if the coordination geometry
around Fe was closer to the square planar Fe-N4 structure. It is
believed that the active sites in the present catalysts may be Fe-N4.
Acknowledgements
This study was supported by the National Natural Science
Foundation (11179033) and Beijing Natural Science Foundation
(2102012 and 2132017).
Appendix A. Supplementary data
Fig. 10. Fe K-edge Fourier transform EXAFS spectra of the electrocatalysts (k2-
weighed, not phase corrected).
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.elec-
tacta.2014.09.064.
four N atoms. The second peak at 2.3 Å is caused by backscattering
from the eight carbon atoms of the pyrrole ring bonded to carbon
[54].AsshowninFig.10A, theheightandthepositionofthefirstpeak
of FePPc/MWCNTs-800 ꢀC and FePPc/MWCNTs were almost the
same, which indicates that the Fe-N4 structure was basically
unchanged after pyrolysis. However, the height of the second peak
of FePPc/MWCNTs-800 ꢀC was significantly reduced, whichsuggests
a reduction in the number of the eight carbon atoms of the pyrrole
ring bonded to other carbons. This indicates that the structure of
phthalocyanine ring was destroyed by the high temperature, which
is consistent with the UV–vis results. In Fig. 10B, the position of the
first peak of FePPc/MWCNTs-800 ꢀC is the lowest compared with
those of FePc/MWCNTs-800 ꢀC and bi-FePc/MWCNTs-800 ꢀC, which
indicates that the radius of the Fe-N4 structure of FePPc/MWCNTs-
800 ꢀC was smaller than the others. Further comparison with the
electrochemical results allows us to conclude that the radius of the
Fe-N4 structure affected the ORR activity of the catalyst. The smaller
the radius, the better the ORR activity is.
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