Nitrogen/sulfur co-doped non-noble metal material as an efficient electrocatalyst for the oxygen reduction reaction in alkaline media

Article abstract of DOI:10.1039/c4ra01405g

This work demonstrates the feasibility of nitrogen/sulfur co-doped non-noble metal materials (Fe-N/C-TsOH) as platinum-free catalysts for the oxygen reduction reaction (ORR) in alkaline media. Electrochemical techniques such as cyclic voltammetry (CV), rotating disk electrodes (RDEs) and rotating ring-disk electrodes (RRDEs) are employed with the Koutecky-Levich theory to investigate the ORR kinetic constants and the reaction mechanism. It is found that the catalysts doped with TsOH (p-toluenesulfonic acid) show significantly improved ORR activity relative to a TsOH-free catalyst. The overall electron transfer numbers for the catalyzed ORR are determined to be 3.899 and 3.098, respectively, for the catalysts with and without TsOH-doping. Catalysts heat treated at 600 °C exhibit relatively higher activity. In addition, the catalyst doped with TsOH (Fe-N/C-TsOH-600) not only exhibits exceptional stability in 0.1 mol L^-1 KOH solution but also has higher methanol tolerance compared to commercial Pt/C catalyst in 0.1 mol L^-1 KOH. To some extent, increasing the Fe-N/C-TsOH-600 loading on the electrode favors a faster reduction of H₂O₂ to intermediate to H ₂O. X-ray photoelectron spectroscopy analysis indicates that pyrrolic N groups are the most active sites, and that sulfur species are structurally bound to carbon in the forms of C-S_(n)-C and oxidized -SO _(n)- bonds, an additional beneficial factor for the ORR.

Full text of DOI:10.1039/c4ra01405g

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Nitrogen/sulfur co-doped non-noble metal
material as an efficient electrocatalyst for the
Cite this: RSC Adv., 2014, 4, 19756
oxygen reduction reaction in alkaline media
Li Xu,*ab Guoshun Pan*ab and Xiaolu Liangab
This work demonstrates the feasibility of nitrogen/sulfur co-doped non-noble metal materials (Fe–N/C–
TsOH) as platinum-free catalysts for the oxygen reduction reaction (ORR) in alkaline media.
Electrochemical techniques such as cyclic voltammetry (CV), rotating disk electrodes (RDEs) and rotating
ring-disk electrodes (RRDEs) are employed with the Koutecky–Levich theory to investigate the ORR
kinetic constants and the reaction mechanism. It is found that the catalysts doped with TsOH (p-
toluenesulfonic acid) show significantly improved ORR activity relative to a TsOH-free catalyst. The
overall electron transfer numbers for the catalyzed ORR are determined to be 3.899 and 3.098,
ꢀ
respectively, for the catalysts with and without TsOH-doping. Catalysts heat treated at 600 C exhibit
relatively higher activity. In addition, the catalyst doped with TsOH (Fe–N/C–TsOH-600) not only
ꢁ1
exhibits exceptional stability in 0.1 mol L
compared to commercial Pt/C catalyst in 0.1 mol L KOH. To some extent, increasing the Fe–N/C–
TsOH-600 loading on the electrode favors a faster reduction of H to intermediate to H O. X-ray
KOH solution but also has higher methanol tolerance
ꢁ1
Received 18th February 2014
Accepted 11th April 2014
2
O
2
2
photoelectron spectroscopy analysis indicates that pyrrolic N groups are the most active sites, and that
sulfur species are structurally bound to carbon in the forms of C–S(n)–C and oxidized –SO(n)– bonds, an
additional beneficial factor for the ORR.
DOI: 10.1039/c4ra01405g
www.rsc.org/advances
catalysts, they still face multiple problems, such as unstable
catalytic activity, poor stability, limited supply, and high cost.
1
. Introduction
5
–7
Polymer electrolyte membrane (PEM) fuel cells are considered a As well, due to anode methanol penetration, oxygen reduction
promising technology to replace internal combustion engines for and methanol oxidation occur simultaneously on the Pt surface,
automotive propulsion, due to their high energy yield and low forming “cathodic mixed potential” and decreasing the cell
8–10
environmental impact. Notable among these are direct methanol performance as well as poisoning the cathode catalyst's Pt.
alkaline fuel cells (DMAFCs), which have attracted considerable Thus, developing cost-effective catalysts with high ORR catalytic
interest as alternative power sources for automobiles and activity, long-term stability, and methanol tolerance is crucial
1
portable consumer electronics. Since alcohols have a similar for advancing DMAFCs.
distribution infrastructure to petroleum, they can be easily
Owing to the inherently faster electrode kinetics and less
handled, stored, and transported, and they promise major corrosive conditions in alkaline media, a large number of cata-
improvements over gasoline combustion, including better overall lysts have been investigated for the ORR in DMAFCs. Catalysts
2
fuel efficiency and lower emissions. Nevertheless, the sluggish synthesized by pyrolyzing precursors comprising nitrogen,
kinetics of the cathodic ORR is a major factor hampering large- carbon, and transition metals have drawn signicant attention
scale implementation of DMAFCs, so most research in this area because of their reasonable activity levels, remarkable selectivity
3
currently focuses on developing efficient catalysts for the ORR.
in the catalysis of the ORR, and high tolerance to methanol, even
The ORR process can occur along a 4-electron pathway to though the mechanisms of the catalyst reaction during the heat-
directly produce H
O or a less efficient 2-electron pathway treatment process and the resulting improvement in activity are
involving the formation of H O as an intermediate. Although complicated and not fully understood.
2
4,5
2,11–13
14,15
Chu and Jiang
platinum-based nanomaterials are the most efficient ORR prepared a series of single and binary heat-treated metal-
loporphyrins (TPP as a ligand), and tested the ORR in O -satu-
with 1 mol L methanol. The best
2
2
2
ꢁ
1
ꢁ1
rated 0.5 mol L
2 4
H SO
a
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China.
E-mail: xl0522@126.com; pangs@tsinghua.edu.cn; Fax: +86 0755 2695 7008; Tel:
catalyst for oxygen reduction, which was also inert to methanol
oxidation, was heat-treated Co–/Fe–TPP. Chu and Jiang proposed
that the macrocycles were only partially decomposed by the heat
+
b
86-755-2671 1824
Shenzhen Key Laboratory of Micro/nano Manufacturing, Research Institute of
Tsinghua University in Shenzhen, Shenzhen 518057, China
ꢀ
treatment (600 C in Ar), the carbonized organic groups could
19756 | RSC Adv., 2014, 4, 19756–19765
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
ꢀ
4
form a nanostructure and the catalytic sites were MN core. Holze temperatures of 200, 400, 600, and 700 C, respectively, for 2
et al. investigated carbon-supported Fe–TMPP (TMPP stands for hours to optimize the pyrolysis temperature with respect to ORR
ꢀ
tetramethoxyphenylporphyrin) heated treated at 800 C as an electrocatalytic activity. To elucidate the effect of TsOH alone, a
active catalyst for the ORR in fuel cells using methanol as the fuel baseline sample of carbon loaded with TsOH-free Fe–N was also
16
and an alkaline carbonate electrolyte. The pyrolyzed Fe–TMPP prepared under the same conditions described above and is
catalyst was as active as Pt/C and less sensitive toward the denoted as Fe–N/C. Furthermore, the Fe–N/C–TsOH-600 catalyst
ꢁ
1
ꢀ
establishment of a mixed potential. Indeed, upon the addition of was leached in 0.5 mol L
1
H SO
2 4
at 80 to 90 C for some hours to
ꢁ
1
mol L methanol, the polarization curve of the chelate-load remove excess metals on the surface of the catalyst (denoted as
electrode remained unchanged, whereas the Pt/C electrode Fe–N/C–TsOH-600L). Then the resulting samples were ltered,
ꢀ
showed much poorer performance.
washed, and subjected to a second heat treatment at 600
C
Very recently, carbons doped or co-doped with other under N for 2 h (denoted as Fe–N/C–TsOH-600LH). In the rele-
2
1
7
18,19
20,21
heteroatoms, such as B, P,
and S,
have been developed vant gure's legend, “RT” means untreated or unpyrolyzed cata-
ꢀ
and have shown good catalytic activity for the ORR by taking lyst and “600” indicates the sample pyrolyzed at 600 C.
advantage of the different heteroatoms in the conjugated
Structural and crystal-phase analyses of the catalysts were
carbon backbone that can create new non-electroneutral sites. performed using XRD with Cu Ka radiation (l ¼ 0.15406 nm),
For instance, S and N co-doped carbons have shown better operating at 40 kV and 40 mA. Surface analysis of the catalysts
22
tolerance to methanol than commercial Pt/C electrode.
was conducted by XPS on a PHI Quantera Scanning X-ray
Nevertheless, some parameters, such as onset potential, half- Microprobe™ 5300 system (ULVAC-PHI. INC) with an Al K X-ray
wave potential, and current density, still need to be improved. anode source (hn ¼ 1486.6 eV) at 300 W and 15.0 kV.
In this work, TsOH and polypyrrole (PPy) were used as dual
dopants to synthesize a novel non-precious metal catalyst, Fe–N/
2.2. Electrochemical measurements
C–TsOH. TsOH, an organic compound, is known as a tosyl group
and was used not only as the S precursor but also as an “organic-
soluble” acid catalyst to promote the oxidation of pyrrole. ORR
activity and methanol tolerance were assessed using CV, RDE and
RRDE techniques. Powder X-ray diffraction (XRD) as well as X-ray
photoelectron spectroscopy (XPS) were employed to conrm the
material properties in terms of morphology and chemical speci-
ation. We demonstrated that the catalyst doped with TsOH
showed much better electrocatalytic activity for ORR perfor-
mance than a catalyst without TsOH doping. We then focused on
Electrochemical measurements were performed in a conven-
tional three-electrode cell using the RRDE technique in KOH
solution, ranging in concentration from 0.1 to 5.0 mol L , at
room temperature. A rotating glassy carbon disk electrode
ꢁ1
(
0
glassy carbon electrode with a geometric surface area of
.19625 cm , purchased from Gamry Instruments) was coated
2
with catalyst to form the catalyst layer and functioned as the
working electrode. According to the electrode preparation
2
method described by Qiao et al., aliquots of 5, 10, 20, 30, and 40
ꢀ
mL of homogeneous catalyst ink, consisting of 2.0 mg catalyst
evaluating Fe–N/C–TsOH catalyst heat treated at 600 C. In
ꢁ1
mL , were pipetted onto the glassy carbon (GC) disk electrode.
particular, the effect of KOH concentration on its ORR catalytic
properties was examined using the RRDE technique in 0.1–5.0
The catalyst loadings were 50.9, 101, 203.6, 305.4, and 407.2 mm
ꢁ
2
ꢁ1
g cm . A saturated calomel electrode (SCE) and Pt wire were
used as the reference and counter electrodes, respectively. All
measured potentials were converted to the standard hydrogen
electrode (SHE).
mol L KOH electrolyte. The effect of heat treatment on the
catalyst's methanol tolerance was studied, and we compared this
tolerance with that of commercially available Pt/C to evaluate the
treated catalyst's applicability in DMAFCs.
During the measurement, CVs were rst carried out on the
freshly prepared working electrode by repeatedly cycling the
potential between 0.80 and 0.30 V for more than 30 cycles at a
2. Experimental
ꢁ
1
2.1. Preparation and physical characterization of the
scan rate of 50 mV s in the presence of N bubbling. In this
2
catalysts
case, the electrode surface was determined to be clean when a
stable, reproducible voltammogram was recorded. The ORR
activity of the catalyst was tested in O -saturated KOH solutions.
2
To prepare the nitrogen and sulfur dual-doped non-precious
metal electrocatalyst (Fe–N/C–TsOH), we mixed 50 mg pyrrole
For more quantitative measurements of the ORR activity, linear
sweep voltammetry (LSV) was conducted on the catalyst-coated
RDE in the potential range between 0.76 and 0.15 V at a scan
(chemically pure) and 10 mL methanol solvent (analytically
pure; both purchased from Guoyao, Shanghai), then combined
this mixture with 120 mg carbon black (Vulcan XC 72R with a
ꢁ
1
2
ꢁ1
rate of 5 mV s in O -saturated KOH solution at various rota-
2
BET of 235 m g , purchased from Cabot), followed by 10 min
of ultrasonication. 0.25 mL 30% H and 50 mg TsOH
analytically pure, purchased from Guoyao) were added to this
tion rates, from 300 to 2100 rpm.
2 2
O
(
ultrasonicated suspension while being ground in a mortar for
some time to obtain a slurry. 10 mL methanol with 149 mg
FeSO $7H O (analytically pure) was then added to the mortar,
3
. Results and discussion
3.1. Electrochemical activity of Fe–N/C–TsOH catalysts
4
2
towards the ORR
followed by constant milling for another 45 min, then drying in
ꢀ
a vacuum at 60 C for 1 h to obtain a powder. This powder was LSV measurements of the catalysts prepared without and with
ꢁ1
further processed by thermal treatment under N
2
atmosphere at TsOH in O
2
-saturated 0.1 mol L KOH solutions were taken at 5
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 19756–19765 | 19757

View Article Online
RSC Advances
Paper
ꢁ1
mV s , and the results are presented in Fig. 1. Both Fe–N/C–
TsOH-600 and Fe–N/C-600 are catalytically active for the ORR.
However, Fe–N/C–TsOH-600 gives much higher ORR activity
than Fe–N/C-600 if both their onset potentials (Eonset) and their
half-wave potentials (E1/2) are compared. In addition, the
maximum current density for Fe–N/C-TsOH-600 is 1.7 times
higher than that of Fe–N/C-600. In other words, the addition of
TsOH considerably enhances the former catalyst's activity
relative to the latter's. According to the literature, the dopant
breaks the electroneutrality of the carbon material, since the
difference in electroneutrality between carbon and dopant
creates favorable positively charged sites for side-on O surface
2
17
adsorption. As well, pyrolysis of the catalyst in the presence of
sulfur leads to mainly amorphous carbon, resulting in
increased catalyst porosity and, in turn, enhanced catalyst
23,24
performance.
Furthermore, sulfur (or a sulfo group), whose
electronegativity is close to that of carbon, has been employed
as a dopant in the preparation of M–N catalysts, and the sulfur
x
group doped to M–N –C might be helpful for entrapping M ions
x
in an environment rich in pyrrole-type or pyridine-type
20
nitrogen.
The difference between the diffusion-limiting currents of Fe–
N/C–TsOH-600 and Fe–N/C-600 may suggest that the ORR
mechanisms they catalyze are different, particularly in terms of
the overall electron transfer number. The inset graph in Fig. 1
shows CVs of the two catalysts, which reinforce the LSV results.
A more positive peak potential value and a higher peak current
density were obtained than with Fe–N/C-600, indicating better
ORR activity aer TsOH-doping. We therefore believe that the
TsOH or the sulfur plays a key role in improving the electro-
catalytic performance of these carbon materials for the ORR.
For a more quantitative evaluation of the ORR catalytic
activity of the dual-doped catalysts developed in this work,
further RDE voltammetry measurements were carried out at
different electrode rotation rates, from 300 to 2100 rpm, as
Fig. 2 (a) and (b) Polarization curves for the ORR on Fe–N/C-600 and
ꢁ1
Fe–N/C–TsOH-600 catalysts, measured in O
KOH electrolytes at various rotation rates. Potential scan rate: 5 mV
2
-saturated 0.1 mol L
ꢁ
1
ꢁ2
s
. Catalyst loading: 101 mg cm . (c) and (d) Koutecky–Levich plots
ꢁ1
for Fe–N/C-600 and Fe–N/C–TsOH-600 in O
KOH solution at potentials of ꢁ0.6, ꢁ0.4, and ꢁ0.2 V, respectively.
2
-saturated 0.1 mol L
shown in Fig. 2(a) and (b). The overall electron transfer number
n) during the ORR process can be evaluated from the Koutecky–
(
2
Levich equation:
dꢁ
1
¼ ikꢁ1 + (0.201nFCO2
2/3 ꢁ1/6 1/2 ꢁ1
O2
i
D
n
u
)
(1)
where i is the disk ORR current density, i is the kinetic current
density, n is the overall number of electrons transferred per
d
k
Fig. 1 Polarization curves of catalysts doped with and without TsOH in
ꢁ
1
ꢁ
1
O -saturated 0.1 mol L KOH solution. Scan rate: 5 mV s . Catalyst
2
ꢁ2
molecule of O
2
reduced, F is Faraday's constant (F ¼ 96485 C
loading: 101 mg cm . The insert shows the CVs of the two catalysts in
O -saturated 0.1 mol L KOH solution. Scan rate: 5 mV s .
2
ꢁ
1
ꢁ6
ꢁ1
ꢁ1
mol ), C 2
is the concentration of oxygen dissolved (1.1 ꢂ 10
O
19758 | RSC Adv., 2014, 4, 19756–19765
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
ꢁ
3
2
mol cm ), DO2 is the diffusion coefficient of O in the bulk
ꢁ
5
2
solution (1.9 ꢂ 10 cm s), and n is the kinetic viscosity of the
ꢁ
2
2
solution (1.0 ꢂ 10 cm s). The Koutecky–Levich plots for the
ORR on both Fe–N/C–TsOH-600 and Fe–N/C-600 at three
different electrode potentials (ꢁ0.20, ꢁ0.40, and ꢁ0.60 V) are
shown in Fig. 2(c) and (d). Based on these plots, we can see that:
The average n values were determined to be 3.098 for Fe–N/C-
600 and 3.899 for Fe–N/C–TsOH-600, the latter being very close
to 4. Clearly, the catalysts doped with TsOH show a much higher
overall ORR electron number than those without TsOH doping,
indicating a signicant difference in the mechanisms they
catalyze. In other words, the ORR catalyzed by Fe–N/C–TsOH-
Fig. 3 Polarization curves for Fe–N/C–TsOH catalysts pyrolyzed at
2
different heat-treatment temperatures. Measured in O -saturated 0.1
6
00 is a 4-electron transfer process from O
N/C-600 generally tends to catalyze the ORR through a (2 + 2)-
electron pathway, producing H , which is capable of
oxidizing and splitting active sites.
2
to H
2
O, whereas Fe–
ꢁ1
ꢁ1
mol L KOH solution at a scan rate of 5 mV s . Catalyst loading: 101
ꢁ2
mg cm
.
2 2
O
The intercepts for all the Koutecky–Levich plots are slightly
higher than 0. A non-zero intercept for a Koutecky–Levich plot
demonstrates that the reaction can occur within the pores of the
catalyst rather than just on the top surface exposed to the bulk
solution, thus increasing the active surface area at slower
3
.2. Electrode catalyst loading optimization with respect to
the ORR activity
It is expected that increasing the non-noble metal catalyst
loading inside the catalyst layer should be an effective way to
improve ORR activity and performance. Therefore, we used the
RDE technique to further investigate the loading effect on ORR
activity in an O -saturated 0.1 mol L KOH solution. According
2
to our current work, the Fe–N/C–TsOH catalyst obtained by
pyrolysis at 600 C would yield the highest ORR catalytic activity.
So, to study the catalyst loading effect on ORR activity, Fe–N/C–
TsOH-600 catalyst was used to prepare catalyst layers with
different loadings (50.9, 101, 203.6, 305.4, and 407.2 mg cm ).
The resulting current–potential curves are shown in Fig. 4(a).
Increasing the catalyst loading at the disk from 50.9 mg cm to
29
25,26
kinetics and thereby affecting the slope.
Therefore, it can be
concluded that the ORR catalyzed by Fe–N/C–TsOH-600 is much
faster than that catalyzed by Fe–N/C-600.
As the potential increases, the Koutecky–Levich slope
increases for both catalysts, suggesting a decrease in the
number of electrons transferred during the ORR and, thus, a
ꢁ1
ꢀ
reduction in H O selectivity. In addition, the intercepts of the
2
ꢁ2
Koutecky–Levich plots become much greater than 0, suggesting
a decrease in the kinetics of the catalyst for the ORR. However,
the Koutecky–Levich slopes at different electrode potentials for
Fe–N/C–TsOH-600 all are much quicker than those for Fe–N/C-
ꢁ
2
ꢁ2
407.2 mg cm yields a much-improved half-potential (E1/2) and
current density (i ). In the catalyst loading range of 50 to 400 mg
d
600. This further demonstrates that the Fe–N/C has a large
ꢁ
2
cm , the monotonic increase in ORR activity with increasing
catalyst loading suggests that the electrochemically active Fe
site density (or concentration) plays an effective role in the
catalyst layer in terms of ORR activity enhancement. It is
understandable that when the electrochemically active Fe site
density rises, there are more neighboring catalyst active centers
active surface area and uniform dispersion of the catalyst on
the electrode surface aer TsOH doping, hence the faster
kinetics for the ORR catalyzed by Fe–N/C–TsOH-600 compared
to Fe–N/C-600.
30
Furthermore, we measured the polarization curves using
ꢁ
1
the RDE technique in O
2
-saturated 0.1 mol L KOH solution
available for H
2 2 2
O to be further reduced to H O, leading to a
ꢀ
for catalysts pyrolyzed from 200 to 700 C, with the unpyr-
olyzed catalyst sample (Fe–N/C–TsOH) for comparison, to
clarify the effect of thermal treatment on the ORR activity. The
results are presented in Fig. 3. Evidently, the pyrolyzed catalyst
samples all show better ORR activities than the unpyrolyzed
sample, which is consistent with the common belief that
thermal treatment can effectively improve catalysts' ORR
With increasing pyrolysis temperature, the ORR
activity keeps rising up to 600 C, then starts to decrease at
higher temperatures. This suggests that 600 C may be the
larger proportion of 4-electron pathway reactions in the whole
29
ORR process through a (2 + 2)-electron series pathway. On the
other hand, the ORR rate will be boosted as the loading
increases, because the sites participate in the ORR process
through an inner sphere mechanism to form O -metal adducts
2
such as O
layer thickness. The catalyst layer is thicker at a high catalyst
loading than at a low one; thus, the H generated in the
former case may have more time to stay within the catalyst layer
to nd another active site for further reduction before its release
to the electrolyte, as shown in Fig. 4(b).
2 x
–Fe–N . Another factor may be related to the catalyst
2
2,27,28
activity.
2 2
O
ꢀ
ꢀ
optimal heat-treatment temperature for obtaining the most
ꢀ
electroactive catalyst. Below 600 C, more Fe–N active sites
x
can be produced by increasing the pyrolysis temperature.
However, above 600 C, for example, at 700 C, the structure of
ꢀ
ꢀ
3.3. Effect of KOH concentration
the catalyst will collapse. At the same time, undesirable Electrocatalysis in alkaline media is a subject of growing
secondary species will form, possibly leading to a reduced industrial importance for fuel cells, since the ORR occurs more
concentration of Fe–N moieties on the catalyst surfaces, as readily in alkaline electrolytes than in acidic or neutral elec-
x
indicated by the XRD results below.
trolytes. For the purpose of practical applications, we studied
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 19756–19765 | 19759

View Article Online
RSC Advances
Paper
Fig. 4 (a) Current–voltage curves recorded on a GC electrode coated
by Fe–N/C–TsOH-600 catalyst with several loadings in O
2
-saturated Fig. 5 (a) Polarization curves of Fe–N/C–TsOH-600 catalyst in O
.1 mol L KOH. Potential scan rate: 5 mV s . Electrode rotation rate: saturated alkaline solutions with four different KOH concentrations, as
2
-
ꢁ1
ꢁ1
0
ꢁ2
ꢁ1
1
500 rpm. Catalyst loading: 101 mg cm . (b) ORR current density (i
0.76 and 0.0 V vs. SHE, respectively, as a function of catalyst loading rate: 1500 rpm. Catalyst loading: 101 mg cm . (b) Koutecky–Levich
data taken from (a)).
plots at different KOH concentrations.
) at marked. Electrode potential scan rate: 5 mV s . Electrode rotation
d
2
ꢁ
ꢁ
(
our catalyst's activity toward the ORR in electrolytes containing 4-electron transfer process from O to H O in all three KOH
2
2
various concentrations of KOH, to see how the catalyst activity solution concentrations.
changed with increasing KOH concentration. Fig. 5(a) presents
the polarization curves for the ORR on an electrode in O
2
-
3
.4. Electrocatalytic stability
The electrocatalytic stability of catalysts is an important
1) The catalyst has a relatively well-dened diffusion- parameter for their practical application. Fig. 6(a) shows the
limiting current plateau in all tested KOH electrolytes in the linear potential scan curves of Fe–N/C–TsOH-600 in O -satu-
potential range of ꢁ0.76 to ꢁ0.3 V, which means that the rated 0.1 mol L KOH at an electrode rotation speed of 1500
current density is limited by O diffusion from the solution to rpm aer CV with different numbers of scans, as indicated. A
the electrode surface. Furthermore, the distribution of active potential cycling test was carried out between ꢁ0.8 and 0.3 V in
saturated KOH electrolyte of various concentrations, from 0.1 to
ꢁ
1
5
mol L . We can see that:
(
2
ꢁ
1
2
ꢁ
1
sites on the electrode surface is uniform.
2
N -saturated 0.1 mol L KOH with a potential scan rate of 50
ꢁ1
(2) Increased KOH concentration resulted in a large shi of mV s . The polarization curves were measured before and aer
ꢁ
1
the onset potential to a more negative value, with a large 1500, 4800, and 6000 cycles in O -saturated 0.1 mol L KOH
2
ꢁ
1
decrease in current densities in the kinetic, mixed, and diffu- using a potential scan rate of 5 mV s and an electrode rotation
sion-limiting regions. Clearly, the diffusivity and solubility of rate of 1500 rpm. A negative shi in both half-wave potential
oxygen in KOH electrolytes can signicantly contribute to the and current density during the rst 1500 cycles of potential
mass transfer effect of oxygen diffusion through the solution, cycling is shown in Fig. 6(a). But the catalyst performance
thus affecting the current densities.
stabilizes during the last 4800 cycles, without further ORR
Fig. 5(b) shows the Koutecky–Levich plots obtained at ꢁ0.60 activity change, indicating that the surface properties of the
V for three different KOH concentrations (0.1, 1.0, and 3.0 mol catalyst are maintained during cycling. Generally, there are two
ꢁ
1
L
). From the slopes of the three lines, the overall electron distinct performance degradation stages for a non-precious
numbers can be obtained, which are, respectively, 3.9910, metal catalyst: an initial higher decay rate and a subsequent
.9738, and 3.9612. These values are fairly close to 4.0, indi- lower one. Therefore, the potential cycling method may be a
cating that the ORR catalyzed by Fe–N/C–TsOH catalyst is a promising protocol to quickly evaluate the stability of a non-
3
19760 | RSC Adv., 2014, 4, 19756–19765
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
all samples are catalytically active toward oxygen reduction. The
re-pyrolyzed catalyst shows better catalytic activity than the
others. Additionally, the onset potential of the ORR on Fe–N/C–
ꢁ
1
TsOH-600 is as high as 50 mV. Leaching in 0.5 mol L H SO4
2
for a short while causes a positive shi in the onset potential of
Fe–N/C–TsOH-600L and Fe–N/C–TsOH-600LH to 54 mV and 121
mV, respectively. It maybe that species less active toward the
ꢁ1
ORR, such as Fe
3
O
4
, dissolved in the 0.5 mol L
2 4
H SO and at
the same time increased the surface area of the catalyst. As well,
re-pyrolysis is apt to form sites that are much more active and to
allow uniform dispersion and stable distribution of the metal
on Vulcan XC 72R, thereby improving the electrocatalytic
activity of Fe–N/C–TsOH-600LH.
The complete set of oxygen reduction currents on Fe–N/C–
ꢁ
1
TsOH in 0.1 mol L KOH, detected at the disk electrode, is
summarized in Fig. 7(a). Hydrogen peroxide produced in the
course of the ORR was detected by utilizing the ring electrode in
the collection mode. The potential of the ring electrode was set
at 1.2 V, i.e., the potential at which H O oxidation proceeds as a
2
2
diffusion-limited process. Peroxide production (in %) is calcu-
lated from the ring-disk measurements, according to the
31
following equation:
c
H O2 (%) ¼ 100 [(2iring/N)/(idisk + iring/N)]
(3)
2
where the collection efficiency is N ¼ 0.22 ꢃ 5%. The amount of
peroxide produced is displayed in Fig. 7(a). In the potential
region between ꢁ0.8 and 0.2 V, the amount of peroxide is very
small (less than 5%), indicating that the ORR proceeds almost
entirely through the 4-electron pathway. This 4-electron reduc-
2
Fig. 6 (a) Polarization curves for oxygen reduction in O -saturated 0.1
ꢁ1
mol L KOH on Fe–N/C–TsOH-600 before and after a potential
ꢁ1
cycling stability test. Scan rate: 5 mV s . Electrode rotation rate: 1500
ꢁ1
2
rpm. Potential cycling was performed in N -saturated 0.1 mol L KOH
ꢁ1
with a scan rate of 50 mV s between ꢁ0.8 and 0.4 V vs. SHE for 1500, tion is conrmed by analysis of the Levich plots, i.e., from the
ꢁ
1
ꢁ1/2
4
800, and 6000 cycles. (b) Percentage of current density loss, iloss%, at linear dependence of the slopes of i versus u
, shown in
ꢁ1
different potentials for the ORR in O
2
-saturated 0.1 mol L KOH on
Fig. 7(c).
Fe–N/C–TsOH-600, as a function of the number of potential cycles.
3.5. Electrocatalytic selectivity in the presence of methanol
In a direct methanol fuel cell system, particularly with Pt-
based fuel cell catalysts, methanol crossover to the cathode is
one of the most signicant problems because of the resulting
mixed potential, which directly causes losses in efficiency and
power density. Thus, it is desirable to develop electrocatalysts
with a high selectivity towards oxygen reduction in a system
containing methanol. We therefore also investigated ORR
activity in the presence of methanol on Fe–N/C–TsOH-600 and
precious metal catalyst. To clearly assess the catalyst's stability
properties, we calculated the percentage of current density loss,
iloss%, at different potentials, as summarized in Fig. 6(b). The
iloss% is expressed as follows:
i
loss% ¼ (iini ꢁ i)/iini ꢂ 100%
(2)
The iloss% of the ORR on the catalyst rises exponentially as Pt/C, and the effect of different methanol concentrations on
the amount of potential cycling increases. This may indicate the ORR performance of both catalysts. Fig. 8 presents the
ꢁ
1
some intrinsic relationship between the active sites in the ORR curves of these two catalysts in O -saturated 0.1 mol L
2
ꢁ1
catalyst and the kinetics of the ORR. However, further experi- KOH containing 0, 0.5, 1.0, and 5.0 mol L methanol, at an
mental and modeling research is needed to clarify this electrode rotation speed of 1500 rpm and room temperature.
relationship.
Clearly, the Pt/C catalyst favors the methanol oxidation reac-
Furthermore, the Fe–N/C–TsOH-600 catalyst was leached in tion over the ORR, compared with Fe–N/C–TsOH-600. In
ꢁ
1
0
.5 mol L H SO4 to remove excess metals on the catalyst methanol-free electrolyte, no methanol-oxidation current is
2
ꢀ
surface, then put it through a second heat treatment at 600 C found for either of the catalysts. However, when the methanol
ꢁ
1
under N
oxygen reduction on Fe–N/C–TsOH catalysts subjected to potential shis from 250 to ꢁ220 mV (Fig. 8(a)). The
different conditions. The measurements were conducted in O
competitive adsorption of oxygen and methanol molecules on
saturated 0.1 mol L KOH using a potential scan rate of 5 mV the surface of Pt/C results in a mixed potential. The electrode
2
for 2 hours. Fig. 7(b) shows the polarization curves for concentration is changed from 0 to 5 mol L , the onset
2
-
ꢁ
1
ꢁ
1
s
and an electrode rotation rate of 1500 rpm. It is evident that reaction was dominated by oxidation at high molecular
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 19756–19765 | 19761

View Article Online
RSC Advances
Paper
Fig. 7 (a) Percentage of peroxide arising from Fe–N/C–TsOH treated under different conditions: Fe–N/C–TsOH-600, Fe–N/C–TsOH-600L,
and Fe–N/C–TsOH-600LH as a function of electrode potential, on the basis of the corresponding RRDE data. (b) Current–potential curves at
ꢁ1
298 K for the ORR in O
2
-saturated 0.1 mol L KOH solution, on the three kinds of catalysts treated under different conditions. (c) Levich plot at
the potential of ꢁ0.3 V vs. SHE.
concentrations, which limited the dissolved oxygen because
Pt/C is active for both oxygen and methanol. In the case of Fe–
N/C–TsOH-600, the ORR onset potential and current density
change very little with or without the presence of methanol,
ꢁ
1
ꢁ2
up to 5.0 mol L (Fig. 8(b)). At ꢁ2 mA cm , however, the
potential shis from ꢁ131 to ꢁ220 mV, indicating that
species in a methanol solution within an alkaline medium
disrupt the ORR. Nevertheless, our results show that in
ꢁ
1
0
.1 mol L KOH, this new material has a relatively higher
selectivity towards the ORR in the presence of methanol
than does Pt/C, although Fe–N/C–TsOH-600 does not have
complete methanol tolerance.
3.6. Morphology and structure of the prepared catalysts
Fig. 9 shows X-ray diffraction spectra of non-pyrolyzed and
ꢀ
pyrolyzed Fe–N/C–TsOH catalysts (at 600 and 700 C, respec-
tively). The spectra exhibit the diffraction characteristics of the
ꢀ
ꢀ
carbon support, with a broad shoulder in the range of 20 to 30
for 2q. From Fig. 9 it can be seen that Fe–N/C–TsOH-RT shows
quite strong diffraction peaks due to the crystalline nature of
FeSO $7H O. These peaks disappear aer high-temperature
4
2
treatment. In their place, additional diffraction signals form at
ꢀ
6
00 and 700 C, which may be due to the generation of metallic
iron (a-Fe), iron carbide (Fe
3
C), and iron oxide-like magnetite
32
(
Fe ), as well as FeS, respectively. These results indicate that
3 4
O
the structure of the Fe(II)–PPy precursor complex may have
decomposed during the pyrolysis process, and those species
could be types of ORR active sites, although with much less
Fig. 8 Polarization curves for O
b) Fe–N/C–TsOH-600 in O -saturated 0.1 mol L KOH electrolyte
without and with different concentrations (0.5, 1.0, and 5.0 mol L ) of
2
reduction on (a) Pt/C electrode and
ꢁ1
(
2
33
ꢁ1
activity than is expected for Fe–N sites. It is notable that Fe–N/
x
ꢁ
1
ꢀ
methanol solution. Potential scan rate: 5 mV s . Electrode rotation C–TsOH pyrolyzed at 700 C exhibits very similar peaks to the
ꢁ2
rate: 1500 rpm. Catalyst loading: 101 mg cm
.
ꢀ
catalyst pyrolyzed at 600 C, except that: (1) the shape of these
peaks of a-Fe, Fe C, and Fe for Fe–N/C–TsOH-700 become
3
3 4
O
19762 | RSC Adv., 2014, 4, 19756–19765
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Fig. 9 X-ray diffraction spectra of non-pyrolyzed and pyrolyzed Fe–
ꢀ
N/C–TsOH catalysts at 600 and 700 C, respectively. (1) Fe
O
3 4
(022);
(
(
3
2) Fe O
044).
4
(113); (3) Fe (011), Fe
C (013); (4) Fe O
3 3 4
(115); and (5) Fe
3 4
O
much stronger and sharper than for Fe–N/C-600. Evidently, due
to the formation of these less active species, the quantity of ORR
active sites is reduced, thus resulting in less ORR activity for the
30
Fe–N/C–TsOH-700; (2) when the samples further heat-treated
ꢀ
at 700 C, the diffraction signals of FeS were weakened, indi-
cated that these FeS species maybe contribute to promote the
ORR, somehow. So, to avoid the formation of additional species
Fig. 10 XPS spectra of deconvoluted N 1s (a, c, and e) and S 2p (b, d,
and f) for Fe–N/C-TsOH catalyst. (a) and (b) unpyrolyzed; (c) and (d)
pyrolyzed at 600 C; (e) and (f) pyrolyzed at 700 C.
ꢀ
ꢀ
with less ORR activity, the thermal temperature should be below
ꢀ
7
00 C for a Fe–N/C–TsOH precursor.
To obtain information about the surface atomic composition
of the pyrolyzed samples, XPS analysis was performed for N as responsible for the unique activity of Fe-containing
ꢀ
38
unpyrolyzed samples and samples pyrolyzed at 600 and 700 C. catalysts.
The broad scan of all three catalysts showed the peaks associ- Fig. 10(d) presents the S 2p spectra measured for the
ated with C, N, O, S, and Fe. This result agrees with the EDX unpyrolyzed catalyst sample (Fe–N/C–TsOH). It can be seen that
analysis. the catalyst without thermal treatment shows a large band at
The N 1s and S 2p core levels of the catalysts were recorded 167.0–171.0 eV, which can be assigned to the sulfate in the
22,39
ꢀ
for both unpyrolyzed and pyrolyzed Fe–N/C–TsOH samples. catalyst precursor.
Aer pyrolysis at 600 C (Fe–N/C–TsOH-
Fig. 10(a)–(c) show the N 1s binding energy region, where the 600), the deconvolution of the S signals gave two bands with
peaks of N 1s at 398.9, 400.5, and 401.3 eV can be attributed to binding energies of 163.8 eV and 168.8 eV, respectively, as
3
4–36
pyridinic N, pyrrolic N, and graphitic N, respectively.
From shown in Fig. 10(e). These peaks can be attributed to the
Fig. 10(a), the deconvolution of N signals for unpyrolyzed Fe– binding sulfurs in C–S(n)–C (n ¼ 1 or 2) bonds (60%) and
N/C–TsOH can be divided into two peaks, corresponding to oxidized –SO(n)– bonds (40%), which are expected to occur at the
40
pyridinic and pyrrolic N. When the thermal treatment edges of carbon. Guo et al. found that the –C–S–C– structure is
ꢀ
20
temperature is increased to 600 C, more of the pyridinic N an important factor for optimizing ORR performance. This
ꢀ
ꢀ
transforms into pyrrolic N, such that at 600 C, the fraction of fact may explain why our catalyst pyrolyzed at 600 C shows
pyrrolic N groups is the larger of the two; this catalyst exhibits better catalytic ORR activity. It is believed that pyrolysis of the
higher activity than the unpyrolyzed catalyst, as was shown by catalyst in the presence of sulfur could lead to mainly amor-
the CV and RDE results in Fig. 1. This indicates that the phous carbon, resulting in increased catalyst porosity and, in
22,24
pyrrolic N group is more active for oxygen reduction, and the turn, enhanced catalyst performance.
catalyst pyrolyzed at 600 C has more active sites (pyrrolic N) to group doped into M–N
As well, the sulfur
–C might be helpful for entrapping M
ꢀ
x
facilitate oxygen adsorption. In addition, the preferred ions in an environment rich in pyrrole-type nitrogen. Therefore,
protonation of pyridinic sites may explain why pyrolysis at it can be concluded that both sulfur doping and nitrogen
high temperatures leads to more pyrrolic N and less pyridinic doping of carbon play key roles in improving electrocatalytic
3
7
N, resulting in more stable catalysts. Furthermore, when the activity for the ORR. For the samples pyrolyzed at higher
ꢀ
Fe–N/C–TsOH samples were heat treated at 700 C, deconvo- temperatures, the S 2p peak can be tted with four different
41
lution of the N signals gave three bands that could be assigned sulfur moieties. The major contributions at binding energies
to pyridinic, pyrrolic, and graphitic N, respectively. The around 163.8 eV (35%, lower 60%), 165.0 eV (30%), and 168.8 eV
pyrrolic N peak is weaker than the one presented in Fig. 10(b), (30%) can be attributed to the binding sulfurs in C–S(n)–C (n ¼ 1
n
which agrees very well with earlier reports identifying pyrrolic or 2) bonds, conjugated –C]S– bonds, and –SO –. The
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 19756–19765 | 19763

View Article Online
RSC Advances
Paper
remaining contribution at around 162.1 eV (5%) can be
assigned to the reduced (–SH) sulfur moieties. The appearance
of –C]S– and –SH reduced the amount of C–S–C, which is why
Fe–N/C–TsOH-700 shows lower activity than the catalyst pyro-
8 C.-H. Wang, H.-C. Hsu, S.-T. Chang, H.-Y. Du, C.-P. Chen,
J.-C.-S. Wu, H.-C. Shih, L.-C. Chen and K.-H. Chen, J.
Mater. Chem., 2010, 20, 7551.
9 J.-S. Yu, M.-S. Kim and J.-H. Kim, Phys. Chem. Chem. Phys.,
2010, 12, 15274.
ꢀ
lyzed at 600 C. As mentioned above, both N and S play an
important role in improving ORR activity, with pyrrolic N and 10 D. Xia, S. Liu, Z. Wang, G. Chen, L. Zhang, L. Zhang, S. Hui
C–S–C possibly serving as the ORR catalytic sites.
and J. Zhang, J. Power Sources, 2008, 177, 296.
1 C.-H. Wang, S.-T. Chang, H.-C. Hsu, H.-Y. Du, J.-C.-S. Wu,
L.-C. Chen and K.-H. Chen, Diamond Relat. Mater., 2011,
20, 322.
1
4
. Conclusions
12 B. Merzouguia, A. Hachimia, A. Akinpelu, S. Bukola and
M. Shao, Electrochim. Acta, 2013, 107, 126.
Nitrogen/sulfur co-doped carbon materials were explored as
non-precious metal catalysts for the ORR in alkaline media.
They showed promising catalytic activity towards the ORR along
a 4-electron transfer pathway in the overall reaction, and higher
tolerance to methanol in comparison to commercial Pt/C cata-
1
3 F. Jaouen, V. Goellne, M. Lef `e vre, J. Herranz, E. Proietti and
J.-P. Dodelet, Electrochim. Acta, 2013, 87, 619.
14 R. Jiang and D. Chu, J. Electrochem. Soc., 2000, 147, 4605.
15 D. Chu and R. Jiang, Solid State Ionics, 2002, 148, 591.
16 R. Holze, I. Vogel and W. Vielstich, J. Electroanal. Chem.,
ꢁ
1
lyst in 0.1 mol L KOH.
The N and S dual-doped catalyst exhibited higher catalytic
activity for oxygen reduction in alkaline media than one doped
only with N, in terms of onset potential, half-wave potential, and
diffusion-limited current density values. In addition, the cata-
lytic activities were strongly dependent on the pyrolysis
temperature used in catalyst synthesis, with the best ORR
1
986, 210, 277.
1
1
1
2
2
2
2
7 L. Yang, S. Jiang, Y. Zhao, L. Zhu, S. Chen, X. Wang, Q. Wu,
J. Ma, Y. Ma and Z. Hu, Angew. Chem., Int. Ed., 2011, 50, 7132.
8 Z. Liu, F. Peng, H. Wang, H. Yu, J. Tan and L. Zhu, Catal.
Commun., 2011, 16, 35.
9 D. Deak, E.-J. Biddinger, K.-A. Luthman and U.-S. Ozkan,
Carbon, 2010, 48, 3635.
ꢀ
performance obtained at 600 C. Instrumental analysis and
ꢀ
XRD and XPS results all showed that 600 C may be the optimal
0 H. Wang, X. Bo, Y. Zhang and L. Guo, Electrochim. Acta, 2013,
temperature for gaining more Fe–Nx active sites, where the
pyrrolic N groups are the most active. Sulfur species structurally
bound to carbon in the forms of C–S–C and oxidized –SO –
n
bonds play the key roles in improving ORR activity.
1
08, 404.
1 R. Ahmadi, M. K. Amini and J.-C. Bennett, J. Catal., 2012,
92, 81.
2
2 J. Qiao, L. Xu, L. Ding, L. Zhang, R. Baker, X. Dai and
J. Zhang, Appl. Catal., B, 2012, 125, 197.
3 U. I. Kramm, I. Herrmann, S. Fiechter, G. Zehl, I. Zizak,
I. Abs-Wurmbach, J. Radnik, I. Dorbandt and P. Bogdanoff,
ECS Trans., 2009, 25, 659.
4 I. Herrmann, U. I. Kramm, J. Radnik, S. Fiechter and
P. Bogdanoff, J. Electrochem. Soc., 2009, 156, 1283.
5 P.-H. Matter, L. Zhang and U.-S. Ozkan, J. Catal., 2006, 239,
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 91223202), the International Science
2
2
2
&
Technology Cooperation Program of China (no.
2011DFA73410), the Tsinghua University Initiative Scientic
8
3.
Research Program (no. 20101081907), and the National Key
Basic Research Program of the China-973 Program (no.
6 H.-J. Zhang, Q. Z. Jiang, L. Sun, X. Yuan and Z.-F. Ma,
Electrochim. Acta, 2010, 55, 1107.
7 G. Hinds, NPL Report DEPC-MPE, 2005, vol. 19, p. 10.
8 C.-W.-B. Bezerra, L. Zhang, K. Lee, H. Liu, J. Zhang, Z. Shi,
A.-L.-B. Marques, E.-P. Marques, S. Wu, S. Wu and
J. Zhang, Electrochim. Acta, 2008, 53, 7703.
2011CB013102).
2
2
Notes and references
1
2
3
4
R. Dillon, S. Srinivasan, A. S. Arico and V. Antonucci, J. Power 29 F. Charreteur, S. Ruggeri, F. Jaouen and J.-P. Dodelet,
Sources, 2004, 127, 112. Electrochim. Acta, 2008, 53, 6881.
J.-L. Qiao, L. Xu, L. Ding, P.-H. Shi, L. Zhang, R. Baker and 30 L. Wang, L. Zhang and J. Zhang, Electrochem. Commun.,
J.-J. Zhang, Int. J. Electrochem. Sci., 2013, 8, 1189. 2011, 13, 447.
S. Liu, H. Zhang, Z. Xu, H. Zhong and H. Jin, Int. J. Hydrogen 31 B.-B. Blizanac, P.-N. Ross and N.-M. Markovic, Electrochim.
Energy, 2012, 37, 19065. Acta, 2007, 52, 2264.
K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science, 2009, 32 A. Velazquez-Palenzuela, L. Zhang, L. Wang, P.-L. Cabot,
323, 760.
E. Brillas, K. Tsay and J. Zhang, Electrochim. Acta, 2011, 56,
5
6
S. Wang, D. Yu and L. Dai, J. Am. Chem. Soc., 2011, 133, 5182.
4744.
F. Charreteur, F. Jaouen and J. P. Dodelet, Electrochim. Acta, 33 F. Jaouen and J.-P. Dodelet, Electrochim. Acta, 2007, 52, 5975.
2009, 54, 6622.
34 H. Wang, T. Maiyalagan and X. Wang, ACS Catal., 2012, 2,
7
C.-W.-B. Bezerra, L. Zhang, H. Liu, K. Lee, A.-L.-B. Marques,
781.
E.-P. Marques, H. Wang and J. Zhang, J. Power Sources, 2007, 35 G. Liu, X. Li, P. Ganesan and B.-N. Popov, Electrochim. Acta,
73, 891. 2010, 55, 2853.
1
19764 | RSC Adv., 2014, 4, 19756–19765
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
3
3
3
6 G. Liu, X. Li, P. Ganesan and B.-N. Popov, Appl. Catal., B, 39 L. Zhu, D. Susac, M. Teo, K.-C. Wong, P.-C. Wong,
2
009, 93, 156.
R.-R. Parsons, D. Bizzotto, K. A.-R. Mitchell and
S.-A. Campbell, J. Catal., 2008, 258, 235.
40 Y. Su, Y. Zhang, X. Zhuang, S. Li, D. Wu, F. Zhang and
X. Feng, Carbon, 2013, 62, 296.
7 G. Wu, K. Artyushkova, M. Ferrandon, J. Kropf, D. Myers and
P. Zelenay, ECS Trans., 2009, 25, 1299.
8 G. Wu, Z. Chen, K. Artyushkova, F.-H. Garzon and P. Zelenay,
ECS Trans., 2008, 16, 159.
41 J.-P. Paraknowitsch, B. Wienert, Y. Zhang and A. Thomas,
Chem. –Eur. J., 2012, 18, 15416.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 19756–19765 | 19765