Bio-inspired iron/sulfur/graphene nanocomposite and its use in the catalysis of the oxygen reduction reaction at room temperature in alkaline media on a glassy carbon electrode

Article abstract of DOI:10.1002/jccs.201800326

This work demonstrates the performance of a bio-inspired iron/sulfur/graphene nanocomposite as a non-platinum electrocatalyst for the oxygen reduction reaction (ORR) in an alkaline medium. The catalyst shows the most positive ORR onset potential (1.1 V vs. RHE) according to its unique structure in the alkaline medium (KOH solution, pH = 13) at low temperature (T = 298 K). The catalyst is evaluated by the rotating-disk electrode (RDE) method under various rotating speeds (0–2,000 rpm) in the potential range ?0.02–1.18 V vs. a rechargeable hydrogen electrode (RHE). The number of transferred electrons, as one of the most important parameters, is almost constant over a wide range of potentials (0.1–0.8 V), which indicates a more efficient four-electron pathway from O₂ to H₂O on the FePc-S-Gr surface. The mean size of catalyst centers are in the nanoscale (<10 nm). The estimated Tafel slope in the appropriate range is about ?110 mV per decade at low current density, and E_1/2 of FePc-S-Gr displays a negative shift of only 7.1 mV after 10,000 cycles.

Full text of DOI:10.1002/jccs.201800326

Received: 25 August 2018
Revised: 20 October 2018
Accepted: 23 October 2018
DOI: 10.1002/jccs.201800326
A R T I C L E
Bio-inspired iron/sulfur/graphene nanocomposite and its use
in the catalysis of the oxygen reduction reaction at room
temperature in alkaline media on a glassy carbon electrode
Behnam Seyyedi | Bahar Ahmadi Variani | Esmaeil Habibi
Nanotechnology Research Centre, Urmia
This work demonstrates the performance of a bio-inspired iron/sulfur/graphene
University, Urmia, Iran
nanocomposite as a non-platinum electrocatalyst for the oxygen reduction reaction
Correspondence
Behnam Seyyedi, Nanotechnology Research
(ORR) in an alkaline medium. The catalyst shows the most positive ORR onset
potential (1.1 V vs. RHE) according to its unique structure in the alkaline medium
(KOH solution, pH = 13) at low temperature (T = 298 K). The catalyst is evalu-
Centre, Urmia University, Shahid Beheshti
Avenue, Urmia 5715944931-165, Iran.
Email: b.seyyedi@urmia.ac.ir
ated by the rotating-disk electrode (RDE) method under various rotating speeds
(0–2,000 rpm) in the potential range −0.02–1.18 V vs. a rechargeable hydrogen
electrode (RHE). The number of transferred electrons, as one of the most important
parameters, is almost constant over a wide range of potentials (0.1–0.8 V), which
indicates a more efficient four-electron pathway from O2 to H2O on the FePc-S-Gr
surface. The mean size of catalyst centers are in the nanoscale (<10 nm). The esti-
mated Tafel slope in the appropriate range is about −110 mV per decade at low
current density, and E1/2 of FePc-S-Gr displays a negative shift of only 7.1 mV
after 10,000 cycles.
KEYWORDS
electrocatalyst, fuel cell, graphene, iron phthalocyanine, nanomaterial, non-noble
metal, oxygen reduction reaction
limitations. ORR plays an important role in electrochemical
energy generation and storage systems (such as fuel cells
and Li-ion batteries), which are rapidly developing. The fuel
cell, as a next-generation energy system, supplies electricity
by the conversion of chemical energy, and its performance is
strongly dependent on the ORR kinetics.[6–11] Extensive
research has been carried out to develop various NPMCs for
ORR.[12–16] Since different electrocatalysts lead the ORR in
aqueous solutions through two pathways, many precursors
have been introduced with different performances – either
through an efficient two-step, two-electron pathway from O2
to hydrogen peroxide, or a more efficient four-electron way
from O2 to H2O.[17–19] Recently, many studies have shown
that the M-N4-C structure is the most efficient catalyst for
ORR, where M is a transition metal such as iron, copper,
1
| INTRODUCTION
Graphene is defined as an ordinary and regular two-
dimensional (2D) structure consisting of an isolated, single
layer of carbon in a honeycomb structure, which has unique
and significant features. Graphene’s noteworthy attributes
include long-term operational stability, high strength, econ-
omy, and good conductivity for electricity and heat.[1–5]
Because of its good electrical conductivity, graphene is a
suitable candidate as a substrate for non-precious-metal cata-
lysts (NPMCs). NPMCs supported by carbon, including gra-
phene, are a serious possible alternative for platinum
catalysts. Because of the slow kinetics of the oxygen reduc-
tion reaction (ORR), platinum and its derivative are widely
used as catalysts for ORR; however, these have considerable
© 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2018;1–7.
http://www.jccs.wiley-vch.de
1

2
SEYYEDI ET AL.
cobalt, or zinc.[20–24] In this context, the cytochrome enzyme
is a promising material for electrocatalysis of ORR in nature.
Cytochrome uses iron atoms in the form of Fe (N4) as cata-
lyst for oxygen reduction.[25] In this work, the Fe (N4) cen-
ters are stabilized by the iron phthalocyanine structure.
Phthalocyanines and their derivatives are a widely investi-
gated as important functional materials. Metal phthalocya-
nines, as 2D metal-organic materials, form M (N4) centers
with most transition metals; moreover, they have been inves-
tigated as catalysts for various reactions including ORR and
redox systems. Although metal phthalocyanines and their
derivatives have not been commercialized as catalysts, they
have often been investigated in the several applications
because of their chemical and thermal stability and specific
properties.[26–32]
for 10 min. The FePc powder was filtered, washed, and dried
in an air oven at 80ꢀC overnight. Elemental microanalysis
(using a Flash EA112 automatic elemental analyzer) was
used for analyzing the synthesized FePc (%), and the results
are as follows: Fe 5.38; C 36.55; N 10.62; Cl 47.31; and H
0.14. The elemental mass percentages for iron (Fe%), carbon
(C%), nitrogen (N%), chlorine (Cl%), and hydrogen (H%)
were calculated by the following equations:
ꢀ
ꢁ
MFe × 1
MFePc
Fe% ¼
× 100,
× 100,
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
ꢀ
ꢁ
MC × 32
C% ¼
MFePc
ꢀ
ꢁ
MN × 8
N% ¼
× 100,
MFePc
This study demonstrates a bio-inspired, high-perfor-
mance, iron-based catalyst for ORR in alkaline media
(KOH, 0.1 mol/L) at low temperature. The performance of
the new bio-inspired catalyst (FePc-S-Gr) is compared with
ꢀ
ꢁ
MCl × ð16−xÞ
Cl% ¼
× 100,
MFePc
ꢀ
ꢁ
MH × x
those of iron phthalocyanine (FePc) and platinum (Pt0.2/C0.8
)
H% ¼
× 100,
MFePc
electrocatalysts. The new catalyst is found to give a much
higher ORR activity than metal-free organic N-C and Pt-
based catalysts in alkaline electrolytes: especially, it is con-
siderable in terms of EOnset, E1/2, n (the number of trans-
ferred electrons), Tafel slope, and catalyst durability.
Elemental microanalysis, thermogravimetric analysis (TGA),
inductively coupled plasma (ICP) analysis, Fourier transform
infrared (FT-IR) spectroscopy, powder X-ray diffraction
(PXRD), transmission electron microscopy (TEM), and
scanning electron microscopy (SEM) were employed to
evaluate the catalyst’s structure.
where MFe, MC, MN, MCl, MH, and MFePc are the molar mass
of iron, carbon, nitrogen, chlorine, hydrogen, and FePc
(FeC32N8HxCl16–x), respectively, and n is the number of
chlorine atoms in the FePc structure. Results of the compari-
son of experimental and theoretical data for the elemental
microanalysis of the prepared FePc shows the value of
x as ~2.
The synthesis of the catalyst (FePc-S-Gr) included the
following steps. First, about 75 mmol of Na2S was added to
75 mL of dimethylformamide (DMF) to obtain a solution;
after that, the temperature was decreased to −5ꢀC in an ice
bath. Then, about 75 mmol of dried graphene was broadcast
into the solution, and the mixture was reacted at −5ꢀC for
12 hr. The obtained samples were filtered, washed, and dried
in an air oven at 80ꢀC overnight. To prepare FePc-S-Gr,
500 mg of the dried powder and 500 mg of the obtained
FePc were dispersed in DMF (75 mL) and then cooled to
−5ꢀC. The mixture was allowed to react for 24 hr. The
FePc-S-Gr powder was filtered, washed, and dried in an air
oven at 80ꢀC overnight.
2
| EXPERIMENTAL
2.1 | Reagents and materials
All chemicals were of analytical grade purchased from
Sigma-Aldrich and used without further purification. All
electrodes used were bought from Tianjin Aida Co., Ltd
(China). Deionized water was used to prepare the aqueous
solutions.
2.3 | Characterization
2.2 | Preparation of FePc and FePc-S-Gr
The FT-IR spectra for graphene, FePc-S-Gr, and FePc were
recorded using a ThermoNicolet (model NEXUS FT-IR
670) spectrometer between 4,000 and 400 cm−1 with the
standard KBr pellet method. XRD patterns of FePc,
graphene, and FePc-S-Gr were obtained using an X’Pert
Pro-Panalytical diffractometer with Cu Kα radiation (wave-
length = 1.54 Aꢀ) at room temperature. The 2θ angular data
were collected between 2ꢀ and 80ꢀ at the scan rate of 1ꢀ per
step. To determine the Fe loading weight to the final ORR
iron-based catalyst, the amounts of iron and sulfur in the
FePc was synthesized using a simple microwave oven under
atmospheric pressure. Water used in the synthesis was dis-
tilled before use. A mixture of iron(II) chloride, high-purity
urea, and phthalic anhydride in the weight ratio 1:4:4 was
dissolved in a saturated solution of NaCl (25 mL). Next,
100 mg ammonium molybdate as catalyst and 0.5 mmol lauric
acid as capping agent to suppress the flocculation of
nanoparticles were added to the solution. The chemicals
were dissolved in a beaker and reacted using a microwave
oven (Samsung-900 W) at a low temperature (100–125ꢀC)

SEYYEDI ET AL.
3
obtained FePc-S-Gr sample were analyzed by an ICP optical
emission spectrometer (ICP-OES) using an ICP-MS 7900
(Agilent) (Fe: 14,071 ppm and S: 8,046 ppm) instrument.
The catalyst particles size and morphology were studied by
SEM and TEM using an FESEM-MIRA III (TESCAN) and
EM10C-100 kV (Zeiss), microscopes, respectively.
In general, FePc has different crystal phases, and the two
peaks at 2θ = 6.9ꢀ and 9.1ꢀ at room temperature correspond
to the α-FePc phase and monoclinic crystal system in the
(200) and (102) planes, respectively.[34,35] The FePc and
FePc-S-Gr diffraction patterns represent the peaks at 6.9ꢀ,
15.6ꢀ, 24.6ꢀ, and 25.4ꢀ with different intensities identical to
those of α-phase and monoclinic FePc (JCPDS Card No
14-0926).[36,37] The average size of the FePc and FePc-S-Gr
grids, D, along the (200) direction was estimated from the
Debye–Scherer equation:
The catalytic performance of the obtained samples for
ORR was evaluated by liner sweep voltammetry (LSV) and
cyclic voltammetry (CV) using an Autolab PGSTAT302N
potentiostat/galvanostat. The measurements were done by
employing a conventional three-electrode cell. Next, a glassy
carbon rod (2 mm diameter) was used as the counter elec-
trode[33] and a Ag/AgCl electrode was employed as the ref-
erence electrode. Afterward, a catalyst-film-coated glassy
carbon rotating-disk electrode (RDE) with a surface area of
0.19625 cm2 was used as a beneficial electrode. The catalyst
ink was produced by adding 2 mg of the obtained powder to
a solution containing 50 μL Nafion (5 wt%) and 450 μL
deionized water. Moreover, the suspension was sonicated for
30 min. In the next step, 20 μL of the catalyst ink was
placed on the RDE surface and dried slowly to form the dry
catalyst film (catalyst loading ≈0.4 mg/cm2). Additionally,
CVs and LSVs were performed in an O2-saturated KOH
(0.1 mol/L) solution by varying the potential from −0.02 to
1.18 V vs. a reversible hydrogen electrode (RHE) at the scan
rate of 50 and 10 mV/s, respectively. Also, the electrolyte
temperature was maintained at 25 ꢁ 1ꢀC and the back-
ground CV and LSV measurements were performed in the
N2-saturated KOH (0.1 mol/L) solution.
0:9λ
β:cosθ
D ¼
,
ð6Þ
where θ, λ, β, and D are the angle of incidence, the wave-
length of the radiation, full width at half-maximum (FWHM
in radians), and the crystalline grid size, respectively. The
mean size of crystalline domains was calculated for α-FePc
as ~0.19 nm and for FePc-S-Gr ~0.21 nm; the difference in
the crystalline grid size could be due to the sulfur bridge
between α-FePc and graphene. The FePc-S-Gr XRD pattern
shows a diffraction peak at 2θ = 22.69ꢀ, coinciding with the
sulfur-carbon value.[38]
3.2 | FT-IR spectrum
To locate the functional groups of FePc, Figure 2 displays
the FT-IR spectra of FePc, graphene, and FePc-S-Gr with a
resolution of 2 cm−1 in the region of fundamental frequen-
cies from 400 to 4,000 cm−1
.
It is quite clear that IR spectral matching cannot be
achieved because of the strong couplings. The band appear-
ing at ~3,444 cm−1 in the infrared spectra of graphene and
FePc-S-Gr was assigned to an OH vibration of adsorbed
water.[39] The NH stretching and bending vibrations were
not observed at ~3,289 and 1,006 cm−1, respectively, in
FePc and FePc-S-Gr. The absence of these bands is due to
the replacement of hydrogen by the iron cation. The C–H
stretching vibrations of the benzene rings are observed at
~3,028 cm−1 (FePc and FePc-S-Gr). The C–C stretching
3
| RESULTS AND DISCUSSION
3.1 | Powder X-ray diffraction
PXRD was used to study the crystal phase structures of
FePc, graphene, and FePc-S-Gr, which are shown in
Figure 1.
FIGURE 1 XRD pattern of graphene, FePc-S-Gr, and FePc
FIGURE 2 FTIR spectra of FePc, graphene, and FePc-S-Gr

4
SEYYEDI ET AL.
vibrations of pyrrole and isoindole structures in the phthalo-
cyanine skeleton are observing at ~1,643, 1,458, 1,376,
1,183, 1,131, 1,079, 885, and 655 cm−1. The C–H in-plane
bending vibrations of the aryl structure appear at 1,470 and
1,536 cm−1. The peaks at 1,440 and 1,291 cm−1 determine
the =N–C bond and the stretching vibration of aromatic phenyl
ring. The complex bands appearing at 700–1,400 cm−1 were
assigned to C–N in isoindole, C–Cl, and C–S stretching
vibration, the C–H in-plane bending vibrations, and the C–H
out-of-plane bending vibrations.[38,40,41]
were used in the temperature range 25–800ꢀC. The heating
rate and fluid flow were 10ꢀC/min and 50 mL/min, respec-
tively (Figure 4).
Steps of weight change, loss of moisture, and thermal
decomposition indicate the thermal behavior of the sam-
ples. The first step of weight change for FePc, due to the
loss of moisture, occurs at 100–150ꢀC (Figure 4b).
Figure 4b–d shows the oxidation steps of FePc, FePc-S-
Gr, and graphene, respectively. At different steps of oxi-
dation reactions, nitrogen, carbon, chlorine, and iron
atoms are oxidized. The second step of changes in
Figure 4c shows the decomposition of connected FePc on
graphene sheets, which does not appear in Figure 4d for
graphene.
3.3 | SEM and TEM characterization
Figure 3e–g shows the microstructure images of the FePc-S-
Gr catalyst, which was was obtained by a field-emission
SEM. A network structure of FePc-S-Gr consisting of ran-
domly crumpled sheets was observed in the form of a multi-
layer solid, which might be attributed to the presence of
foreign sulfur atoms. The TEM images indicate the homoge-
neously amorphous texture of graphene in which the Fe (N4)
centers with diameter less than 20 nm are deposited on the
surface of the carbon particles (Figure 3a–c)
3.5 | Electrochemical evaluation for ORR
CVs of FePc and FePc-S-Gr in O2-saturated KOH solution
were measured at the scan rate of 50 mV/s (Figure 5).
FePc shows a weak peak in the O2-saturated solution,
due to ORR activity, at about 0.8 V, while for FePc-S-
Gr, a well-defined cathodic peak appears, which indicates
the excellent catalytic performance for ORR. To further
investigate the ORR performance, RDE curves were
obtained for FePc-S-Gr at various rotation rates
(200–2,000 rpm). Figure 6 shows the onset potential of
the catalyst for the ORR of ~1.1 V (vs. RHE) after cor-
recting for the background current at iORR = −0.02
mA/cm2. The number of transferred electrons (n) of the
Fe-S-Gr catalyst was calculated by using the Koutecky–
Levich (K–L) equation[42]
3.4 | Thermogravimetric analysis
TGA results obtained in air atmosphere shows the oxidation
behavior of FePc, FePc-S-Gr, and graphene. The samples
(a)
(b)
(a)
(c)
(d)
(e)
(f)
(b)
(c)
(d)
FIGURE 4 (a) Thermogravimetric analysis of the samples in air. (b–d)
Relation of ln[(W0/W∞)/(Wt/W∞)] vs temperature of chlorinated FePc, FePc-
S-Gr, and graphene in air atmosphere
FIGURE 3 SEM (d–f) and TEM (a–c) images of the synthesized FePc-
S-Gr

SEYYEDI ET AL.
5
FIGURE 5 Cyclic voltammograms of the synthesized samples
FIGURE 7 K–L plots for FePc-S-Gr
TABLE 1 Number of electrons transferred of FePc-S-Gr
E (V)
n
ðJ−1¼7,084:7x−0:307,R2¼0:9991Þ
0:1 V
3.96
3.96
0:2 V
J−1¼7,080:8x + 7:2175,R2¼0:9988
ð
Þ
ðJ−1¼7,088:1x + 9:8428,R2¼0:9984Þ
0:3 V
3.95
3.95
0:4 V
J−1¼7,088:1x + 10:843, R2¼0:9984
ð
Þ
Þ
0:5 V
0:6 V
0:7 V
3.95
3.94
J−1¼7,088:1x + 12:843, R2¼0:9984
ð
J−1¼7,115:3x + 29:09, R2¼0:9999
ð
Þ
ðJ−1¼7,080:7x + 41:766, R2¼0:9974Þ
3.96
3.96
ðJ−1¼7,072:1x + 60:63,R2¼0:9991Þ
0:8V
FIGURE 6 Polarization curves for catalyst performance at various rotating
rates
1
1
J−1 ¼ JL−1 + JK−1
¼
+
,B
Bω0:5 JK
qffiffiffiffiffiffiffiffi
qffiffi
¼ ð0:201Þ:n:F:CO : 3 DO :1
,
ð7Þ
2
6
2
2
ν
where J is the measured current density, Jk is the kinetic cur-
rent density, and ω is the rotation rate of the electrode
(F = 96,485ꢀC/mol, CO2 = 1.2 × 10−6 moL/mL, DO2 = 1.9
× 10−5 cm2/s, υ = 0.01 cm2/s).
The K–L plots were obtained for the ORR on a synthe-
sized sample (Figure 7). Using Equation (6), the slopes
remain approximately constant over the potential range
0.1–0.6 V. The number of transferred electrons (n) for FePc-
S-Gr is close to 4 (Table 1).
Figure 8 shows the calculated Tafel slopes for FePc-S-
Gr. The electrocatalytic activity of the sample for ORR is
reflected by its Tafel slope of the diffusion-corrected kinetic
current density.
FIGURE 8 Calculated Tafel slope for FePc-S-Gr
A Tafel slope of −110 mV/dec was observed at low cur-
rent density (high potential) and of −119.3 mV/dec was
obtained in the high current density range (low potential).
To be clear, the Tafel slope of commercial Pt/C is ~60 mV
and 120 dec–1,[42] which would be used as a reference. Prin-
cipally, the range of Tafel slope for ORR is between 30 and

6
SEYYEDI ET AL.
4,000 cycles are now routinely used in the literature. After
10,000 cycles, the E1/2 of FePc-S-Gr displays a negative
shift of only 7.1 mV (Figure 9), which is lower than most
reported values.[42]
Figure 10 indicates comparison between EOnset and E1/2
of the FePc-S-Gr electrocatalyst and Pt0.2/C0.8 as a standard
reference, from which the higher performance of the new
catalyst can be seen.
4
| CONCLUSIONS
We demonstrated a new non-precious metal electrocatalyst
for the ORR in this paper. The results reflect the intrinsic
activity and selectivity of the prepared catalyst. The catalyst
exhibited limiting current density and half-wave potential
comparable to those of a platinum-based electrocatalyst
because of the good mass transport on the catalyst. ORR
occurs on the surface of FePc-S-Gr catalyst in an aqueous
solution by a multistep reaction. The particle size of the
obtained powders is in the nanoscale (<10 nm). The cata-
lytic activity of prepared iron-based catalyst is comparable
to that of the Pt/C electrocatalyst used as a reference. The
Tafel slope of diffusion-corrected kinetic current density is
−110 mV/dec at low current density. The FePc-S-Gr nano-
structure exhibits superior durability of performance by
using a NPMC in the system.
FIGURE 9 ORR polarization plots of the FePc-S-gr electrocatalyst before
and after 10,000 potential cycles between 0.2 and 1.1 V (vs. RHE) in an O2-
saturated KOH solution
130 mV/dec in alkaline solution. This value allows the cata-
lyst to determine the rate-controlling reaction when the four-
electron pathway and the two-electron pathway compete. On
a metal/metal oxide surface, the rate-determining step is a
pseudo- two-electron pathway, which gives a Tafel slope of
≈60 mV/dec at high potential. However, on a pure metal
surface, the first electron transfer is the rate-determining
step, resulting in a Tafel slope of ≈120 mV/dec. Such a high
Tafel slope for FePc-S-Gr indicates that the protonation of
O
2− is smoother than that of a Pt-based electrocatalyst; how-
ACKNOWLEDGMENT
ever, the protonation of O2− is the rate-limiting step on the
active sites. The small difference in the Tafel slope of FePc-
S-Gr indicates that the formed iron oxide at the catalyst
surface is insignificant. Therefore, the ORR occurs by the
four-electron pathway at the surface of FePc-S-Gr.
We thank the staff of the Nanotechnology Research Center,
Urmia University, for their assistance with the electrochemical
experiments.
ORCID
Catalyst durability test was carried out by cycling the
potentials between 0.2 and 1.1 V (vs. RHE) at 50 mV/s in
O2-saturated KOH (0.1 mol/L) solution to evaluate the elec-
trochemical stability of the FePc-S-Gr catalysts. More than
Behnam Seyyedi
https://orcid.org/0000-0001-8084-9108
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