G-C3N4 templated synthesis of the Fe3C@NSC electrocatalyst enriched with Fe-Nx active sites for efficient oxygen reduction reaction

Article abstract of DOI:10.1039/c9ta01837a

A porous, efficient, and durable nonprecious electrocatalyst for the oxygen reduction reaction (ORR) under both alkaline and acidic conditions is synthesized by a green, solvent-free, one-pot, economical, scalable, and template-free strategy. In situ formed graphitic carbon nitride (g-C₃N₄) is utilized as a sacrificial template for constructing the core-shell Fe₃C@NSC electrocatalyst with a highly porous substructure and nanosheet-like morphology. The presence of glucose facilitates the homogeneous distribution of nanoparticles and establishes the nanosheet structure, otherwise agglomeration and formation of nanotubes occur in the Fe₃C@NSC electrocatalyst. Optimizing the synthetic conditions generates a highly active ORR electrocatalyst with a unique mesoporous architecture, large surface area, high nitrogen doping level, encapsulated Fe₃C nanoparticles in NS co-doped carbon layers and multiple exposed edge active centers. The best performing electrocatalyst displays outstanding ORR performance under alkaline conditions with onset potential and half-wave potential (E_1/2) values of 1.059 and 0.938 V vs. the reversible hydrogen electrode (RHE), much higher than those of the state-of-the-art commercial Pt/C (20%), and admirable performance in acidic media. In addition, it exhibits 4e^- selectivity, tolerance toward methanol, and superb durability with zero degradation after 12000 potential cycles under both alkaline and acidic conditions. These exceptional characteristics make the as-synthesized electrocatalyst one of the top-level non-precious ORR electrocatalysts reported to date.

Full text of DOI:10.1039/c9ta01837a

View Article Online
View Journal
Journal of
Materials Chemistry A
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. A. Eissa, S. G.
Peera, N. H. Kim and J. H. Lee, J. Mater. Chem. A, 2019, DOI: 10.1039/C9TA01837A.
This is an Accepted Manuscript, which has been through the
Volume
4 Number 1 7 January 2016 Pages 1–330
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry A
Accepted Manuscripts are published online shortly after
Materials for energy and sustainability
www.rsc.org/MaterialsA
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

Please do not adjust margins
Journal of Materials Chemistry A
Page 1 of 16
View Article Online
DOI: 10.1039/C9TA01837A
ARTICLE
g-C3N4 templated synthesis of Fe3C@NSC electrocatalyst enriched with
Fe-Nx active sites for efficient oxygen reduction reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Ahmed A. Eissa,ab Shaik Gouse Peera,a Nam Hoon Kima and Joong Hee Leeac*
DOI: 10.1039/x0xx00000x
A porous, efficient, and durable nonprecious electrocatalyst for oxygen reduction reaction (ORR) under both alkaline and
acidic conditions is synthesized by a green, solvent-free, one-pot, economical, scalable, and template-free strategy. In-situ
formed graphitic carbon nitride (g-C3N4) is utilized as a sacrificial template for constructing core-shell Fe3C@NSC
electrocatalyst with highly porous substructure and nanosheets-like morphology. The presence of glucose facilitates the
homogenous distribution of nanoparticles and establishing the nanosheets structure, otherwise agglomeration and formation
of nanotubes occur in Fe3C@NSC electrocatalyst. Optimizing the synthetic conditions generates a highly active ORR
electrocatalyst with unique mesoporous architecture, large surface area, high nitrogen doping level, encapsulated Fe3C
nanoparticles in NS co-doped carbon layers and multiple exposed edge active centers. The best performing electrocatalyst
displays outstanding ORR performance under alkaline conditions with onset potential and half-wave potential (E1/2) of 1.059 and
0.938 V vs reversible hydrogen electrode (RHE), much higher than the state-of-the-art commercial Pt/C (20%), and admirable
performance in acidic media. In addition, it exhibits 4e selectivity, tolerance toward methanol, and superb durability with zero
degradation after 12000 potential cycles under both alkaline and acidic conditions. These exceptional characteristics make the
as-synthesized electrocatalyst one of the top-level non-precious ORR electrocatalysts reported to date.
transition metals, especially Fe and Co, in combination with the doped
heteroatoms, like S, N, P, and B, which are believed to form highly
active MNC bonds.13,14 In particular, Fe and nitrogen co-doped
1. Introduction
The universal requirement for clean and sustainable energy
sources has intensified rapidly and continuously in the last few
decades, due to the extensive use of limited non-renewable energy
sources such as fossil fuels. As a result, the emergence of efficient
renewable energy technologies such as supercapacitors, solar cells,
batteries and fuel cells has recently received extensive attention.1
Electrochemical ORR is the efficiency determining reaction in several
electrochemical conversion and storage devices including fuel cells
and metal-air batteries because of its sluggish kinetics.2 This
sluggishness is due to the high overpotential value of ORR at the
cathode, which results in low efficiency of fuel cell performance. So
far, platinum (Pt) and Pt-alloys catalysts supported on carbon are
considered excellent electrocatalysts for ORR.3 However, the high
cost, scarcity, non-selectivity for ORR in the presence of methanol,
sensitivity to impurities, and poor stability of Pt limit the successful
commercialization of fuel cells.4 This has tremendously motivated
global research communities to develop low-cost and stable non-
precious metal electrocatalysts (NPMCs) for ORR catalysis.5 Various
researchers have come up with various NPMCs that include transition
metal based chalcogenides,6 oxides,7 nitrides,8 oxynitrides,9
carbides,10 phosphides,11 and metal-free heteroatom-doped carbon
compounds.12 The most popular and promising NPMCs so far are the
carbon catalysts (FeNC) are emerging as the most promising
candidate noble-metal-free electrocatalysts for ORR.15-17 The uneven
charge distribution (spin and electron density) around the heteroatoms
and the formation of MNC active sites, and their synergistic
interactions, collectively accelerate the O2 adsorption and breaking of
oxygen molecules, and hence facilitate the ORR process.15,18 In
addition to the catalytic ORR activity, high surface and meso-porosity
are essential for enhancing the density of active sites and promoting
the mass diffusion of O2 molecules towards the electrode surface. The
traditional methods for fabricating unique mesoporous materials,
substantially depend on the use of rigid templates, such as mesoporous
silica templates.19 The FeNC catalysts based on silica templates
generally have stable open frameworks, compared to other
mesoporous structures.15 However, the silica templates are employed
only as hard templates, where further numerous complicated and
tedious procedures are required to remove the silica template, by using
risky hydrofluoric acid or hot alkaline liquor. These complicated and
time-consuming steps ultimately impede the scale-up production and
increase the manufacturing cost of the catalysts. Furthermore, the
electrocatalytic activity may deteriorate, because a portion of the
active sites may be inescapably wasted during the post synthesis
processes to get rid of the template through acidic or basic etching.
Consequently, a question immediately emerges. Is it conceivable to
establish a practical procedure in which a homogeneous rigid template
can be included within the precursors, where the annealing at high
temperatures drives the formation of unique mesoporous structure
with high surface area and felicitous removal of the template at the
same time through a mechanochemical one-pot solid-state reaction?
This is the essential motivation underlying the current work.
a. Advanced Materials Institute of BIN Convergence Technology (BK21 Plus Global
Program) & Department of BIN Convergence Technology, Chonbuk National
University, Jeonju, Jeonbuk 54896, Republic of Korea. E-mail:
jhl@chonbuk.ac.kr
b. Department of Chemistry, South Valley University, Qena, 83523, Egypt.
c. Carbon Composite Research Centre, Department of Polymer-Nano Science and
Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, Republic of
Korea
†Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 2 of 16
ARTICLE
Journal Name
no further processes are required. More importantly, it is a green
gC3N4 is a two-dimensional layered polymer composed of only
View Article Online
synthesis strategy, where solvents are not employed at any point of
carbon, nitrogen, and some impurity of hydrogen, therefore it is a very
promising sustainable material for many applications. It is thermally
stable up to 600 °C, and further increment of the pyrolysis temperature
and duration results in significantly enhancing both of its crystallinity
and surface area.20 gC3N4 nanosheets can act as anchoring sites for
the nanoparticles decoration to form a hybrid system. Due to its low
thermal stability at around 600 °C, it is possible to utilize gC3N4
nanosheets as in-situ, sacrificial-structural directing template to
induce porosity in the catalyst. There have been few studies reported
recently with gC3N4 derived ORR catalysts.21-25 However, most of
the synthetic protocols involve multiple steps and using large amounts
of solvents and concentrated acids in order to prepare the final
electrocatalyst, making the fabrication process more complicated and
time-consuming. For example, J. Li et al. reported a strategy to
synthesize N-doped graphene (NGA) as metal-free electrocatalyst for
DOI: 10.1039/C9TA01837A
the reaction, and consequently, no waste disposal problems related to
solvent need to be considered.
By considering all the above issues, we herein demonstrate
an effective in situ solid-state-polymerization reaction to fabricate a
highly-efficient and durable ORR electrocatalyst by utilizing gC3N4
as a sacrificial template through a fast, scalable, one-pot, and solvent-
free mechanochemical synthesis protocol. The electrocatalyst
composed of ferric carbide (Fe3C) nanoparticles homogenously
dispersed and embedded in N and S co-doped carbon nanosheets
(Fe3C@NSC) enriched with the highly active FeNx sites. It has been
synthesized by the manual grinding of the reactants; Fe(NO3)3.9H2O,
dicyandiamide (DCDA), thiourea, and glucose as renewable
feedstocks. DCDA is utilized as N source and to obtain the thermally
removable gC3N4 template at around 550 °C.20 The thermal
ORR from g-C3N4 derived from melamine, in a two-step pyrolysis decomposition of gC3N4 template results in the formation of porous,
process.21 The NGA catalyst showed low ORR activity of onset two-dimensional wrinkled carbon nanosheets, in addition to high-
potential and E1/2 values of 0.914 V and 0.828 V vs RHE in the level of N doping, and encapsulating Fe3C nanoparticles. Further, the
generated gases during the thermal decomposition of the sacrificial
alkaline medium. M.-Q. Wang et al. prepared FeNC catalyst from
FeCl3·6H2O, dicyandiamide and Ketjenblack EC 600JD in a water gC3N4 template, such as NH3 and CO2, play a fundamental role in
assisted complexation, drying, and pyrolysis process, which is a three- inducing high porosity and increasing the surface area of the obtained
step synthetic process.22 The size of Fe-based nanoparticles was electrocatalyst. Thiourea is used as a S source; meanwhile glucose is
difficult to control. As a result, the catalyst displayed very low activity applied as a carbon source. It is noteworthy that the use of glucose
for ORR with E1/2 value of 0.75 V vs RHE, in acidic medium. Very assists the homogeneous distribution of Fe3C nanoparticles
recently, S. Sultan et al. reported a strategy to synthesize Co1.08Fe3.34 throughout the carbon substructure and allows the construction of
encapsulated in few-layers nitrogen-rich graphitic tube carbon nanosheets-like structures in Fe3C@NSC catalyst, otherwise
(Co1.08Fe3.34@NGT) of low surface area (235 m2 g1), using a mixture
of melamine, Co(ClO4)2, and FeCl3.26 The catalyst displayed excellent
onset potential and E1/2 values of 1.03 and 0.94 V vs RHE. H. Yu et
al. synthesized nitrogen-doped carbon nanosheets (N-CNS) as a
metal-free electrocatalyst in a three-step synthetic process, where g-
carbon nanotubular structures of low surface area are produced, in
addition to the carbon nanosheets structures. A series of samples were
prepared at various temperatures, and indicated as Fe3C@NSC/T,
where T is the annealing temperature. The Fe3C@NSC electrocatalyst
obtained at 900 °C possessed ideal N-doping level (5.64%), large
C3N4 was firstly prepared by the thermal polymerization of urea, then specific surface area (473 m2 g1), unique mesoporous architecture,
the obtained g-C3N4 was coated with a carbon source, in a and abundant exposed highly active sites. As result, the
a
hydrothermal reaction, and finally the product was calcined at 900 electrocatalyst exhibited very high electrocatalytic activity toward
ºC.23 The resulted N-CNS catalyst showed low onset potential and E1/2 ORR in the alkaline medium, compared to the benchmark commercial
values of 0.888 and 0.754 V vs RHE, in the alkaline electrolyte. Y. Pt/C (20%), with excellent values for the onset potential and E1/2 of
Zheng et al. synthesized organometallic complex of Co-C3N4/CNT 1.059 and 0.938 V vs RHE. It also showed admirable performance
using pristine carbon nanotubes (CNTs), tri-s-triazine (C6N8), DCDA under acidic conditions with onset potential and E1/2 values of 0.924
and Co salt, in a very long-time three-step fabrication process.25 and 0.796 V vs RHE. More importantly, it presented outstanding
Taking into account the above issues regarding the construction of g- durability after 12000 potential cycles with zero degradation in the onset
potential, and negligible change in E1/2, 4e transfer selectivity, and
resistance toward methanol oxidation under both alkaline and acidic
conditions. To the best of our knowledge, the performance of the
herein as-synthesized electrocatalyst makes it one of best non-
precious metal electrocatalysts that have been reported to date for
ORR in both alkaline and acidic electrolytes. (Table S1 and S2†).
C3N4-based electrocatalysts including metal-free and/or metal-doped
electrocatalysts, it urgent to develop an efficient synthetic protocol to
construct highly active and durable ORR electrocatalyst by a fast,
single-step, scalable, and tunable route for the fuel cells and metal-air
batteries applications at the industrial level.
On the other hand, solution chemistry synthesis involves
fabricating the materials by using a certain volume of solvent like,
methanol, ethanol, N,N-dimethylformamide, or trimethylamine.
These solution-based protocols create critical environmental
problems, due to the utilization of toxic organic solvents, and the
production of large quantities of waste during the synthesis process
and subsequent washing steps.27 In addition, it requires additional
steps to purify the product and remove the solvent molecules, which
may be difficult to completely remove from the final product. On the
other hand, mechanochemical synthesis techniques include solvent-
2. Results and Discussion
2.1. Materials Characterization
Fig. 1 elucidates the synthesis procedure of Fe3C@NSC
electrocatalyst. During pyrolyzing the precursors by a temperature-
programmed reaction (Fig. S1†), DCDA can form 2D gC3N4
nanosheets at temperature around 550 °C. The formed intermediate
free routes for fabricating the materials by applying mechanical gC3N4 nanosheets can be used as an in-situ template for the confined
energy by ball milling or simple manual grinding of the reactants, to development of 2D carbon nanosheets from glucose. Furthermore, the
induce the chemical reaction between them and yield a solid
product.27 The significant features of mechanochemical synthesis are
scalability, quick-fabrication process, no loss of material occurs
during work up, ease of handling, the products do not need further
purification processes to get rid of solvent and impurities, and the
relatively high yield of products, because the solidsolid reactions
produce high quantity of products in the form of powder, and thereby,
internal hydrogen bonds in the NH2/NH groups between the
layered gC3N4 will be broken at pyrolysis temperature from 550 to
600 °C, which results in promoting the surface contact of gC3N4 with
polysaccharide.28 On increasing the temperature over 600 °C, gC3N4
begins to completely degrade, resulting in providing active sites,
plentiful edges and surface defects in the final product, therefore
leading to the simultaneous consolidation of N and S into the carbon
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 16
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
matrix, and producing Fe3C@NSC electrocatalyst enriched with the ratio of disordered-amorphous carbon in the prepared materials. There
View Article Online
is no shift of (002) peak for the different catalDysOtIs:,1w0.h10ic3h9/sCu9gTgAe0s1t8s3th7Ae
existence of the same interlayer spacing in all materials. The increase
of temperature from 800 to 1000 °C did not affect the intensity of this
peak, indicating that almost all samples have the same content of
either graphitic or amorphous carbon. The remaining peaks are
attributed to the crystalline planes of orthorhombic ferric carbide
(Fe3C) phase (PDF#96-901-2188). The PXRD pattern for the catalyst
prepared at 800 °C exhibited only a very few peaks related to the Fe3C
phase; however, by increasing the reaction temperature to 900 °C,
more peaks corresponding to Fe3C phase became much more
pronounced, indicating the existence of perfect orthorhombic Fe3C
phase, which is believed to be highly active for ORR catalysis.29
When the pyrolysis temperature was further raised to 1000 °C, the
intensities of the Fe3C phase diffraction peaks diminished, indicating
the low content of Fe3C phase. This is because of the metastable Fe3C
nanoparticles are sensitive to temperature, and tend to degrade at
elevated temperatures.33
FeNx highly active sites, without any need for further post-treatment
to remove the template. The formation of Fe3C nanoparticles can be
illustrated as follows: during the carbonization at high temperatures,
the iron precursor is reduced to the extremely active Fe nanoparticles.
Thereafter, the carbon atoms spread inside/outside the crystal lattice
of Fe, leading to the formation of Fe3C nanoparticles encapsulated in
graphitic carbon shells.29 The formation of core-shell architecture is
pivotal for keeping a stable electrocatalytic performance for long
periods under harsh conditions. Parallel to the above processes, the
generated pyridinic and pyrrolic nitrogen atoms have the chance to
react with Fe atoms to form FeNx moieties. Both Fe3C and FeNx
have been demonstrated to be tremendously active sites for catalyzing
the ORR.29, 30 By achieving the formation of nanosheets by employing
glucose, the specific surface area has been enlarged dramatically from
199 to 473 m2 g which resulted in exposure of abundant active sites
and consequently maximizing the catalytic efficiency.
Fig. 1. Schematic illustration for the fabrication procedure of Fe3C@NSC electrocatalyst.
To emphasize the formation of g-C3N4, we investigated the
Fig. 2b displays the Raman spectra for the electrocatalysts
material obtained by pyrolyzing the precursors at temperature of pyrolyzed at different temperatures, where two distinguished peaks
were observed at 1350 cm1 and 1590 cm1, which correspond to the
D and G bands, consecutively. D band is associated with the sp3-
deficiencies and disorders, while G band provides knowledge about
the crystallinity and bond stretching of sp2-bonded pairs in carbon
materials.34 This elucidates the formation of graphitized carbon with
various imperfections/dislocations in Fe3C@NSC. The ratio between
the intensities of D and G peaks (ID/IG) is an effective way to obtain
qualitative knowledge about the structural imperfections in carbon
nanomaterials, as well as providing information about the quantitative
measurement of the edge plane exposure.35 The catalysts prepared at
550 °C by means of powder X-ray diffraction (PXRD) (Fig. S2†). The
PXRD pattern exhibits a typical g-C3N4 phase, indicating that DCDA
was indeed thermally polymerized to g-C3N4 at this temperature. The
main peak at 2θ = 27.3° is attributed to the (002) plane; meanwhile,
the peak at 2θ = 13° is ascribed to the (110) plane.31 Fig. 2a shows the
PXRD patterns for the materials synthesized at different carbonization
temperatures. All samples showed a typical (002) diffraction peak at
26°, with interlayer spacing of 0.34 nm, which is close to the interlayer
spacing of pristine graphite (0.335 nm)32, indicating that all catalysts
have graphitic structure. The broadness of this peak proposes a higher
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 4 of 16
ARTICLE
Journal Name
different pyrolysis temperatures exhibited almost the same ID/IG ratio, corrosion than a thinner one. The presence of another pVeiaekw AnretixcltetOon2linDe
band at 2925 cm1 is ascribed to an extra band (D + G), which is
which is very close to unity, clarifying that they approximately
contain the same degree of graphitization and disorder. This
conclusion supports the surveillances from PXRD, where the
pyrolysis temperature had almost no influence on the carbon peak
observed in PXRD analysis (Fig. 2a). The graphitization is essential
DOI: 10.1039/C9TA01837A
related to the integration of the overtones of D and G bands.37 In
addition, there is a shoulder between G and D bands in all samples.
These characteristics are consistent with the properties of few-layered
graphene characterized by the existence of structural imperfections.38
These conclusions were confirmed by the transmission electron
microscopy (TEM) and high-resolution TEM (HRTEM) images as
will be elucidated in the next sections.
for
enhancing
the
electrical
conductivity;
meanwhile,
defects/dislocations are favorable for N and S doping, thereby
enhancing the potential active sites for the adsorption of ORR
intermediates, and hence, increasing the electrocatalytic activity
towards ORR. It observable that there is a peak at around 2693 cm1 assign the specific surface area and pore size distribution (PSD) for
in all electrocatalysts, which corresponds to 2D band. The existence the prepared materials under different conditions. Fig. 2c shows the
of this broad peak suggests the existence of multilayer graphene-like N2 adsorption/desorption isotherm linear plots for the catalysts
Brunauer–Emmett–Teller (BET) analysis was performed to
Fig. 2. a) XRD patterns, b) Raman spectra, c) N2 adsorption-desorption isotherms, and d) corresponding pore size distribution plots of
Fe3C@NSC electrocatalysts synthesized at different calcination temperatures. The dotted rectangle in d clarifies the comparison of the pore
size distribution between the various electrocatalysts in the microporous region.
structure in the prepared catalysts.36. Principally, the high intensity of prepared at different temperatures. The isotherms display a typical
2D band indicates the formation of ultra-thin graphene; meanwhile, type IV, with a sharp uptake at relative low pressure, which is
the broad 2D peak of low intensity reveals the formation of thick distinctive for mesoporous materials, suggesting that plenty of
graphene with multi-layers.34 Fig. 2b shows obviously that all the mesopores are exist in these catalysts.18 Increasing the pyrolysis
prepared catalysts offered a broad 2D band of low intensity, alluding temperature from 800 to 900 °C resulted in significant enhancement
of the BET surface area from 360 to 472 m2 g. However, elevating
the temperature to 1000 °C led to diminishment of the BET surface
area to 403 m2 g which might be due to the deterioration of the
meso-porosity during the annealing high temperatures. The existence
the efflorescence of multilayer graphene matrix. A thick graphitic
structure would provide more edge sites and defects which are
preferential geometric structures for N and S doping. Furthermore, a
thicker structure would be more durable and more resistant to
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 16
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
of mesoporous structure was confirmed by the PSD curves (Fig. 2d). its ORR performance. The Fe3C@NSC electrocatalyst prepared
View Article Online
The average PSD emphasized the presence of mesopores with without glucose displayed an irregular mDoOrpI:h1o0l.o1g03y9/wC9itThA0l1a8r3g7eAr
diameters centered at about 3.88 nm. The electrocatalysts showed a particles accompanied by the existence of short nanotubes (indicated
clear hierarchical pore size distribution, indicating the ideal porosity by yellow circles and yellow arrows, respectively in Fig. S7†).
which is useful for ORR effectiveness. In addition, Fig. 2d displays a Consequently, it showed convincing evidence that the presence of
secondary PSD at 1.72 nm, demonstrating that the high surface areas glucose precursor during the synthetic process of the electrocatalyst
of the synthesized materials are ascribed to the micro-porosity, as well facilitated the distribution of Fe source and inhibited the nanoparticles
as the meso-porosity. Intriguingly, the sample prepared at 900 °C aggregation during the carbonization process, which resulted in the
offers the highest one that contains the largest number of micropores formation of small Fe-based nanoparticles within the carbon
(dotted rectangle in Fig. 2d) and mesopores. The micropores were substructure, instead of larger aggregates.
declared to be one of the pivotal contributors for generating highly
active FeNC ORR electrocatalysts, where the FeNx active sites
are generally created between the edges of graphitic sheets.39 On the
other hand, the mesopores are significantly required for enhancing the
mass transfer of the reactants and products during the ORR catalysis
to accomplish efficient reaction kinetics.40 Table S3† shows clearly
that the microporous structure of Fe3C@NSC/900 (58 m2 g) was
greatly enhanced, compared to that of Fe3C@NSC/800 (18 m2 g)
and Fe3C@NSC/1000 (7 m2 g) during the in-situ fabrication process,
which is important for the production of FeNx active sites.
Consequently, Fe3C@NSC/900 was predicted to exhibit the best ORR
performance among all the synthesized electrocatalysts where it has
the largest surface area and highest ratio of micropores as well as
mesopores. To demonstrate the synergistic effect of the precursors on
the properties of Fe3C@NSC/900, we measured the BET surface area
for the catalysts synthesized in absence of sulfur source or glucose
(Fig. S4†). The BET surface area decreased from 473 to 335 or 199
m2 g in the absence of sulfur source or glucose, respectively
concluding that both of thiourea and glucose are essential for
promoting the surface area and hence strengthening of the
electrochemical performance towards ORR. This conclusion
demonstrates the reason for the deterioration of the ORR efficiency in
absence of glucose, where the BET surface area sharply decreased to
199 m2 g. It is conspicuous from Fig. S7 and S10† that the
generation of carbon nanotubes and agglomeration of the
nanoparticles occurred within the catalyst when glucose was absent
during the fabrication process of the electrocatalyst, which resulted in
reducing the surface area to much extent, and hence depreciation the
effectiveness of the catalyst towards ORR. Based on the BET studies
for the various electrocatalysts, it can be concluded that the
collaboration between all precursors is fundamental for promoting the
surface area, and hence the catalytic performance for the optimized
electrocatalyst.
Fig. S5† shows the FESEM images for the electrocatalysts
fabricated at different pyrolysis temperatures. The electrocatalyst
synthesized at the carbonization temperature of 800 °C consists of
carbon nanosheets without apparent nanoparticles (Fig. S5a-c†).
When the temperature was elevated to 900 °C, considerable quantities
of Fe-based nanoparticles were generated, and homogeneously
dispersed throughout the carbon matrix (Fig. S5d-f†). The uniform
distribution of the Fe3C nanoparticles is due to the protective carbon
layers surrounding these nanoparticles, which allow them to be
separated from each other. Further increase in the pyrolysis
temperature resulted in irregular distribution and enlarging the size of
the nanoparticles, as a result of nanoparticles agglomeration at higher
temperatures (Fig. S5g-i†). Furthermore, the hierarchical structure
obtained at the temperature of 900 °C was almost destroyed, which
led to decreasing the meso-porosity of the prepared catalyst, and
inhomogeneous distribution of the nanoparticles within the carbon
substructure. The elemental mapping images of the Fe3C@NSC/900
electrocatalyst shows that the N, S, Fe, O, and C elements were
uniformly disseminated throughout the carbon substructure, and no
agglomerated nanoparticles were detected (Fig. S6†). The presence of
glucose during the electrocatalyst fabrication had a remarkable
influence on its morphology as well as its substructure, and hence on
The formation of nanotubes in the absence of glucose can be
illustrated as follows: DCDA is firstly converted into g-C3N4 at
temperature around 550 °C. As the temperature is further increased,
g-C3N4 begins to decompose into small gaseous molecules, such as
nitrogen and cyano fragments beyond 700 °C.20 Parallel to this
reaction, Fe precursor is also reduced to metallic Fe and/or Fe3C
nanoparticles which intern serve as a catalyst to assist the growth of
nanotubes from the gasified molecules produced from DCDA.
Furthermore, the formation of large Fe3C nanoparticles as
consequence of agglomeration in absence of glucose facilitates the
construction of nanotubes. On the other hand, glucose assisted the
formations of nanosheets structure instead of nanotubular structure,
where the g-C3N4 produced from the thermal condensation of DCDA
attach and confine the amorphous carbon intermediate produced
from glucose in its interlayer space. By this strategy, it prevents g-
C3N4 layers from stacking together, where it condenses in a
cooperative process in the gaps between the layers of g-C3N4 at
600 °C.41 In addition, the confined Fe based nanoparticles between
the layers of g-C3N4 are prohibited from agglomeration, as a result of
the exfoliation of g-C3N4 by the action of glucose. This leads to the
homogenous distribution of Fe-based nanoparticles of small sizes
which could not able to catalyze the growth of nanotubes.
The detailed structural and morphological studies of the as-
synthesized electrocatalysts were investigated by TEM and HRTEM.
The TEM images for the Fe3C@NSC/900 electrocatalyst (Fig. 3a-c)
display a good dispersion of the encapsulated Fe3C nanoparticles
throughout the carbon matrix. The lattice dimension of the dark
nanoparticles encapsulated inside the carbon layers was estimated to
be 0.204 nm (Fig. 3e). This distance corresponds to the (031)
crystallographic planes of the orthorhombic Fe3C phase, suggesting
that these embedded nanoparticles are Fe3C nanocrystals,42 which
agrees with the XRD analysis. The lattice fringes of the encased
nanoparticles cores are conspicuous, indicating the crystalline nature
of the orthorhombic Fe3C nanoparticles. The HRTEM images (Fig. 3d,
e and S8a-b†) assure the presence of both graphitic and amorphous
carbon framework, substantiating the emergence of graphitic carbon
with a lattice spacing of 0.34 nm between the encapsulating layers,
which matches with the (002) plane. It is noteworthy that some
morphological deformations were noticed on the graphitic carbon
framework (yellow circles in Fig. S8b and c†). These defects are
probably induced by N and S doping. This suggests that the graphitic
carbon shells have a porous nature, which is anticipated to enhance the
mass transfer of O2 molecules towards the core Fe3C nanoparticles
during the ORR. The interconnected porous graphitic carbon
framework provides convenient open space and compendious spreading
channels for the reactants, as well as intermediates in the ORR, which
excessively accelerate both the charge and mass transportations.
It is clear in Fig. 3d and e that the graphitic carbon films
surrounding the Fe3C nanoparticles consist of a few layers (indicated
by green arrows, ↕). These carbon shells can effectively safeguard the
Fe3C nanoparticles from progressive etching under severe ORR
conditions, consequently leading to steady electrocatalytic
performance.43 Furthermore, the encased Fe3C nanoparticles could
stimulate the graphitic carbon layers, and induce the neighboring
carbon to become active centers for ORR.44 More essentially, such
core-shell configuration enables a corroborative interaction between
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 6 of 16
ARTICLE
Journal Name
the graphitic carbon layers and Fe3C nanoparticles, which could
The samples synthesized at temperatures of 800ViaenwdArt1ic0le0O0nl°inCe
enhance the electrochemical activity, as well as electrocatalytic showed similar morphologies to Fe3C@NSC/D90O0I:e1l0e.c10tr3o9c/aCt9aTlyAs0t1(8F3i7gA.
durability.45 It is also apparent from Fig. 3d and e that the growth of S9†). However, the sample pyrolyzed at 1000 °C exhibited
the graphitized carbon layers is well interlinked between and around aggregation of the nanoparticles (indicated by the yellow arrows) over
the nanoparticles, resulting in the establishment of multidirectional the carbon nanosheets, due to the high temperature. The TEM images
channels throughout the carbon matrix, which leads to the formation for the electrocatalyst fabricated without glucose precursor (Fig. S10†)
of more defects and enhancement of the electron transfer process. This emphasize the observations noticed from FESEM images (Fig. S7†),
type of nanostructure straightforwardly increases the electrocatalytic where the absence of glucose during synthesizing the electrocatalyst
performance of the electrocatalyst. In the scanning TEM (STEM) led to the formation of large particles as well as short carbon
image obtained using a high-angle angular dark-field (HAADF) nanotubes. This dramatically resulted in decreasing the BET surface
area from 473 to 199 m2 g. The Fe precursor ratio had a great
detector (Fig. 3f), the clear appearance of some regions at various
Fig. 3. a-c) TEM images and d, e) HRTEM images of the as-synthesized Fe3C@NSC/900 electrocatalyst. The Fe3C nanoparticles embedded
in NSC shells were labelled with yellow circles. The green arrows (↕) show the thickness of the shells surrounding Fe3C nanoparticles. f)
HAADF-STEM image with EDS linear scan spectrum, and g-k) the elemental maps corresponding to C, Fe, N, S, and O elements of the
Fe3C@NSC/900 electrocatalyst
scales correspond to the various elemental compositions of the influence on the size and distribution of the Fe3C nanoparticles within
Fe3C@NSC/900 electrocatalyst.46 In particular, the embedded and the carbon matrix as displayed in Fig. S11†. Low proportions of Fe
scattered luminous areas represent Fe atoms, because Fe has the precursor
such
as
0.4:1:1:4
highest atomic number among the scanned elements; meanwhile, the [(Fe(NO3)3.9H2O:thiourea:glucose:DCDA)] led to the formation of
surrounding region with pale luminousness indicates the remaining small nanoparticles, which were distributed well throughout the
elements (C, N, O, and S), which have lower atomic numbers than Fe. carbon nanostructure (Fig. S11a,b). On the other hand, the high
This distinctly shows that the iron species were completely coated by proportions of Fe such as 1.6:1:1:4 resulted in agglomerating of the
NSC layers. The homogenous distribution of the elements was proved nanoparticles on the surface of the carbon nanosheets with
by HAADF-STEM and elemental mapping (Fig. 3f-k), where C, Fe, inhomogeneous distribution (Fig. S11c,d). To demonstrate the
N, S, and O were uniformly distributed throughout the electrocatalyst influence of the used temperature program on the electrocatalyst
nanostructure, implying that N and S atoms were proficiently morphology and hence on its catalytic performance, we synthesized
hybridized into the carbon substructure.
the electrocatalyst at 900 °C for 2 h, using the heating rate of 5 °C/min
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 16
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
pyrrolic N.58 It has been reported that the existence of graphitic N
without fixing the temperature at 550 °C for 2 h (Fig. S12†). The
electrocatalyst showed excessive aggregation of the nanoparticles,
which led to constructing of very large Fe-based particles as shown in
Fig. S13†. We can conclude that the carbonization temperature,
reactants ratio, type of precursors, and the temperature program
applied during the synthesis process have significant influences on the
morphology, composition, and nanostructure of the synthesized
electrocatalysts, and hence on their electrocatalytic efficiency.
Therefore, optimizing all these parameters is of great importance to
prepare a highly electroactive-durable catalyst.
The wide range of X-ray photoelectron spectroscopy (XPS)
characterization of the samples prepared at various temperatures
distinctly emphasized the existence of Fe, O, N, C, and S elements
within the surface of the electrocatalysts (Fig. 4a). Table S4†
summarizes the atomic surface contents of these elements. The nitrogen
content decreased from 7.37% to 2.9% when the temperature was
View Article Online
enhances the ORR current density by preferring the four-electron
DOI: 10.1039/C9TA01837A
transfer pathway.47
Fig. 4c and S15† display the high-resolution Fe 2p spectra of the
various electrocatalysts. The peaks situated at 710.2 and 713.6 eV are
ascribed to the binding energies of the 2p3/2 orbitals of ferrous (Fe2+)
and ferric (Fe3+) species; meanwhile, the peaks at 720.5 and 724.7 eV
can be attributed to the binding energies of the 2p1/2 orbitals of Fe2+
and Fe3+ ions, consecutively. It has been reported that the metal ion
centers could enhance the oxygen reduction activity, where they can
boost the initial adsorption of oxygen molecules and the reaction
intermediates.59 The peak at 707.9 eV may be correlated to zero-
valence metallic Fe or Fe-carbide.17 The peak located at 711.3 eV
indicates to the existence of chemical bonding between Fe and N
species in the as-synthesized electrocatalyst,60,61 which further
confirms the existence of FeN moieties throughout the
elevated from 800 °C to 1000 °C. To further investigate the nature of electrocatalyst substructure. The FeN moieties are crucial active
nitrogen bonding in the electrocatalysts, the high-resolution N 1s spectra
were measured (Fig. 4b). The N 1s spectrum of the Fe3C@NSC
electrocatalysts could be deconvoluted into four kinds of bonding
centered at the binding energies of 398.3, 399.0, 400.1, and 401.2 eV,
corresponding to pyridinic nitrogen,47 Fe coordinated nitrogen
(NFe),48 hydrogenated nitrogen (NH, pyrrolic N)49, and graphitic N,50
respectively indicating that nitrogen atoms were truly incorporated into
the carbon framework. The comparative proportions of the
deconvoluted peak areas of the N 1s were calculated to determine the
contents of the miscellaneous nitrogen bonding configurations in
Fe3C@NSC electrocatalysts (Fig. S14†). The content of pyridinic N
slightly diminished from 24.8 to 24.27% when the pyrolysis
temperature was raised from 800 to 900 °C. The further increment in
the temperature (1000 °C) resulted in considerable decrease in the
content of pyridinic N (13.3%). This is due to the weak thermal stability
of the pyridinic nitrogen atoms, where they are either transformed into
pyrrolic or graphitic form.50, 51 The high quantity of pyridinic N has been
suggested to be beneficial for edge plan exposure, which positively
contributes to the ORR electrocatalytic activity, as the edge sites are
more propitious for the ORR.52 Moreover, the free pyridinic N can
capture the protons and oxygen molecules, which thereafter are
transferred to the catalytic sites.53
sites for FeNC based ORR catalysts in both acidic and alkaline
electrolytes,17,29,62 since they prefer the initial oxygen adsorption,
which lead to enhancing the ORR efficiency. Moreover, the FeN
species contribute to the high stability of Fe3C@NSC catalyst,
especially in acidic medium, where they are difficult to be leached out
by the acid.17 It is noteworthy that no satellite peaks at ≈ 719.0 eV
corresponding to iron oxide (Fe-O bonding) were observed in the
high-resolution Fe 2p spectrum, which emphasized the absence of any
iron oxide phase in the synthesized electrocatalysts.63 This discloses
that the Fe atoms are essentially bonded with nitrogen in the carbon
framework.
The high-resolution XPS spectra of S 2p revealed that the doped
S atoms are present in four discrete forms (Fig. 4d and S16†). The S-
doping can introduce defects and modify the electronic structure of
FeNC electrocatalysts, specially the FeNx active sites due to its
large atomic radius and lower electronegativity; therefore, it enhances
their electrocatalytic activity.64 The peak detected at lower binding
energy (162.7 eV) is assigned to the S-2 species attached to Fe atoms
(FeSx).65 The peaks located at 163.2 and 164.8 eV are attributed to the
S 2p3/2 and S 2p1/2 peaks of sulfide species (CSC) in the synthesized
electrocatalyst. The CSC moieties have been stated as being
efficient active centers for catalyzing the ORR.66 The peak at 168.5
eV is ascribed to the C-SOx-C interaction,60 which has been reported
to enhance the efficient adoption of oxygen molecules,67 since they
increase the spin density of the carbon material.18 The presence of
these different types of sulfur states asserted the inclusion of sulfur
atoms in the carbon matrix. The absence of the peak related to thiol
groups (~160.0 eV) in the XPS spectrum of S suggests that most of
The presence of NFe bonds is due to the coordination of the
pyridinic and pyrrolic nitrogen atoms to Fe which leads to the formation
the catalytically active NFe moieties.53,54 These species react as
efficient active sites for the direct reduction of oxygen to water through
a four-electron transfer process.52 The formation of NFe active sites
demonstrates the important synergistic effect between the doped N and
Fe in boosting the ORR process. It has been demonstrated that the NFe the doped S atoms were situated at the edges of the carbon
moieties participate in the pyridinic N signal through XPS.52,55 In other
words, due to the small variation between the binding energies of
pyridinic N and NFe, the peak related to pyridinic N may also involve
a contribution from Fe-coordinated nitrogen (NFe).56 Consequently,
the determined high quantity of pyridinic N in the Fe3C@NSC/900
electrocatalyst suggests the possibility of a high quantity of NFe
species embedded in the carbon matrix of the electrocatalyst. This result
proposes that Fe3C@NSC/900 catalyst had multitudinous NFe species,
which resulted in promoting the electrocatalytic performance of the
catalyst towards ORR.29 The content of pyrrolic N reduced from 28.82%
at 800 °C to 24.68% at 900 °C, then increased again to 28.83% when
the temperature was elevated to 1000 °C. The disadvantageous
influence of pyrrolic N atoms on the ORR is because of their remarkable
contribution to the reduction of oxygen to hydrogen peroxide.57 Herein,
the Fe3C@NSC/900 catalyst showed the lowest percentage of pyrrolic
N (24.68%). The amount of graphitic N increased continuously from
22.72% at 800 C to 39.03% at 1000 °C, revealing that more graphitic
nitrogen existed within the surface of Fe3C@NSC/1000. This signifies
the higher thermal stability of graphitic N, compared to pyridinic N and
nanostructure.68 It is noteworthy that the formation of FeSx bonds as
well as thiophene-like structures (CSC) in the herein synthesized
electrocatalyst excessively boosted the electrocatalytic activity of
FeNC catalytic system for ORR, where the FeSx moieties can
enhance the electron transfer during the catalytic process,69
meanwhile the CSC sites perform an important role in decreasing
the electron localization around Fe-based centers, and hence reduce
the ORR activation barrier on FeNC system,70 contributing to an
outstanding ORR performance. The samples prepared at 800 °C and
900 °C approximately possessed similar content of S (0.8 and 0.83%,
respectively), however the atomic percentage of S diminished to
0.73%, when the annealing temperature was elevated to 1000 °C
(Table S4†). It is interesting to observe that the Fe3C@NSC/900
electrocatalyst had the maximum proportion of carbon to oxygen (9.2)
comparing to 4.9 and 7.7 for Fe3C@NSC/800 and Fe3C@NSC/1000,
respectively which indicates that Fe3C@NSC/900 possessed the
highest enhanced graphitization among all the electrocatalysts, which
in turn led to the improving the electrocatalytic efficiency toward the
ORR. The XPS characterization concludes that the Fe3C@NSC/900
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 8 of 16
ARTICLE
Journal Name
electrocatalyst is wealthy of multiple kinds of active sites; namely which is attributed to the deficiency of the active sites constructed for
View Article Online
pyridinic N, graphitic N, coordinated Fe, and sulfur-carbon catalyzing the ORR, since the low calcinationDtOemI: 1p0e.r1a0t3u9r/eCr9eTsAu0lt1e8d37iAn
coordination, which led to the superb activity and durability of the as- low degree of graphitization and inferior conductivity of the catalyst.
synthesized electrocatalyst as will be elucidated in the following Increasing the calcination temperature to 800 °C resulted in much
sections.
enhancement of the onset potential and E1/2, as well as the diffusion-
limiting current density, due to the promotion of the graphitization
Fig. 4. a) XPS survey spectrum, and b) fitted high-resolution XPS spectra of the N 1s for the Fe3C@NSC electrocatalysts fabricated at different
annealing temperatures. c, d) Fitted high-resolution XPS spectra of Fe 2p and S 2p in the Fe3C@NSC electrocatalyst synthesized at temperature
of 900 °C.
degree and the increased content of the efficient active sites. By
raising the temperature to 900 °C, the electrocatalyst efficiency
2.2. Electrochemical Performance
The electrocatalytic characteristics of the materials synthesized at
various pyrolysis temperatures were firstly examined towards ORR
by LSV and CV techniques in O2-saturated 0.1 M KOH electrolyte
(Fig. 5a and S17†). Fig. 5b shows the influence of the pyrolysis
temperatures on the values of the onset potential, E1/2, and reduction
peak potential for the different electrocatalysts. We determined the
onset potential in this work as the potential at which the current
density exceeds a threshold value of 0.1 mA cm2. The material
prepared at 700 °C showed very poor catalytic activity towards ORR,
towards ORR became the best, as can be seen from the values of onset
potential, E1/2 and limiting current density (1.059 V, 0.938 V and 7.87
mA cm2, respectively). The high value of the limiting current density
for the Fe3C@NSC/900 electrocatalyst is because of the moderated
value of mass loading (0.4 mg cm) applied on the electrode surface
and the high density of the multitudinous efficient active sites for
catalyzing the ORR such as FeNx, FeSx, C-S-C, CSOxC, pyridinic
N, and graphitic N, which lead to the reduction of more volume of
the O2 at electrode surface. Furthermore, the electrocatalyst has large
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 16
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
surface area (473 m2 g) and high mesoporosity which boosts the
because of: (1) the high-density of different active sitesVrieewprAertsicelnetOednlinine
mass transport of the reactants and products during the ORR catalysis Fe3C, FeNx, NC, and SC, (2) the high surfaDceOaI:re10a.1(407393/mC92TgA0)18w3i7tAh
resulting in accomplishing efficient reaction kinetics. The highly optimal porosity, which is much needed to expose more active sites for
graphitic shells surrounding the Fe3C nanoparticles enhance the the reactant species and boost the oxygen diffusion to the active sites,
electrocatalyst conductivity and facilitates the electron transfer from (3) the satisfied contents of nitrogen and sulfur dopants, which were
Fe3C cores to the NSC shells. Consequently, we can attribute the high uniformly dispersed throughout the electrocatalyst nanostructure, (4)
value of limiting current density to: moderate value of mass loading, in-situ construction of graphene-like structure, which is serviceable for
high density of active sites, and efficient mass and electron increasing the number of active sites, (5) the catalyst almost having the
transportations. The further increase in the carbonization temperature same degree of graphitization and disorders that are necessary for
to 1000 or 1100 °C reduced the ORR performance, due to the promoting the ORR efficiency by increasing the electric conductance
following reasons: (1) decomposition of the ferric carbide phase, and number of active sites, respectively, and (6) the encapsulated Fe3C
which has an essential role in enhancing the ORR activity,29 (2) nanoparticles activate the surrounding graphitic shells to become
diminishment of the number of active sites due to the excessive loss active toward the ORR. More importantly, these graphitic shells
of N and S contents, as was detected by the XPS analysis, where perfectly protect Fe3C nanoparticles from corrosion, leading to
greater loss of N and S occurred at elevated temperatures, (3) maintaining high performance for long time operation.
deterioration of the porosity and decrease of the BET surface area, and
(4) agglomeration of the electrocatalyst nanoparticles at higher rotation rates of the Fe3C@NSC/900 (Fig. 5c), reveal that the onset
temperatures. potentials stayed fixed under different rotation rates, while the current
The rotating disk electrode (RDE) measurements at different
It can be concluded that 900 °C is the optimum annealing densities were increased with increasing the rotation speed, due to the
temperature to fabricate the Fe3C@NSC electrocatalyst, since it enhancement of the mass transport, demonstrating the kinetic-
showed the best electrocatalytic performance among all the prepared controlled process of ORR.71,72 The electron transfer numbers (n)
electrocatalysts, which is even better than that of commercial Pt/C were calculated from RDE measurements according to the Koutecky–
(20%), as can be seen from the values of the onset potential, E1/2, and Levich (K–L) equation. The superb linearity and near coincidence of
limiting current density (1.059 V, 0.938 V and 7.87 mA cm2 for the KL plots (Fig. 5d), indicate the first-order reaction kinetics with
Fe3C@NSC/900, vs 1.018 V, 0.895 V and 6.44 mA cm2 for Pt/C
respect to the concentration of dissolved oxygen and identical electron
Fig. 5. a) LSV curves of Pt/C (20%) and Fe3C@NSC electrocatalysts fabricated at different carbonization temperatures in O2-saturated 0.1 M
KOH electrolyte at scan rate of 5 mV s1 and electrode rotation rate of 1600 round-per-minute (rpm). b) Correlation between temperature and
onset potential/half-wave potential/reduction peak potential of the electrocatalysts. c) LSV curves of Fe3C@NSC/900 at different electrode
rotation speeds at scan rate of 5 mV s1 in O2-saturated 0.1 M KOH electrolyte. d) Corresponding KL plots of Fe3C@NSC/900 at various
potentials. e) Tafel plots of Pt/C (20%), and Fe3C@NSC catalysts synthesized at different temperatures. f) LSV curves of Fe3C@NSC/900
and the electrocatalysts prepared without DCDA, glucose, or sulfur source in O2-saturated 0.1 M KOH electrolyte at 1600 rpm and sweep rate
of 5 mV s1.
(20%), respectively). The outstanding ORR performance shown by transfer numbers per oxygen molecule at different potentials, which
Fe3C@NSC/900 electrocatalyst represents one of the best reported resulted in complete ORR kinetics.19 The values of
n for
diverse Fe-based electrocatalysts towards ORR in alkaline medium Fe3C@NSC/900 were estimated to be 3.94-3.97, at varied potentials
(Table S1†). This superior electrocatalytic performance is mainly ranging from 0.3 to 0.7 V vs RHE, which were higher than the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 10 of 16
ARTICLE
Journal Name
corresponding n values of the electrocatalysts fabricated at 800, 1000, and limiting current density (at potential of 0.4 V vs RHE) by 28 mV
View Article Online
and 1.63 mA cm, consecutively as shoDwOnI: i1n0.1F0i3g9./CS92T2A†0.18T3h7Ae
or 1100 °C (Fig. S19†). These values are similar to the theoretical
value of 4 for Pt/C (practical value of n= 3.95, Fig. S18†), suggesting
the high selectivity towards complete the four-electron transfer
pathway, and the direct reduction process of the dissolved oxygen
molecules to hydroxyl ions (OH) in alkaline medium. To ensure the
excellent ORR activity of the as-synthesized electrocatalysts, Tafel
slopes were determined by analyzing the corresponding LSV plots at
low overpotentials, wherein the ORR rate relied on the rate of the
surface reaction on the electrocatalyst.73 As seen in Fig. 5e, the Tafel
slope of Fe3C@NSC/900 is 71 mV dec which is slightly lower than
that of Pt/C (75 mV dec1) as well as that for the other catalysts
prepared at various temperatures. This emphasizes that
Fe3C@NSC/900 has a faster electron transfer rate than Pt/C and other
catalysts, which resulted in a prominent ORR efficiency. To further
assure the higher electrocatalytic performance of Fe3C@NSC/900
comparing to the commercial Pt/C (20%), the electrochemical
accessible (active) surface area (EASA) have been estimated from the
electrochemical double-layer capacitance (Cdl).26,63 for both
electrocatalysts under the same experimental conditions in N2-
saturated 0.10 M KOH (Fig. S20a,b†). In addition, we calculated these
deterioration in the electrocatalyst activity is mainly due to the
nanoparticles agglomeration, which led to the inhomogeneous
distribution of the nanoparticles within the carbon matrix and
decreasing the number of the exposed active sites. This observation
assures the importance of using slow heating rate and fixing the
temperature at 550 °C during the electrocatalyst production, in order
to allow the formation of gC3N4, which has essential role in the
uniform distribution of the Fe-based nanoparticles throughout the
carbon substructure, and hence enhancing the electrocatalyst activity.
To get more insights about the reasons for the high electrocatalytic
efficiency of Fe3C@NSC/900, four additional samples were
synthesized under the same conditions without the addition of Fe
precursor, DCDA, glucose, or sulfur source, and examined by LSV
technique (Fig. 5f). The material prepared without Fe precursor
presented the lowest values of both onset potential and E1/2, due to the
absence of the efficient active sites for ORR such as Fe3C and FeNx
when Fe was not included during the synthesis process. This assures
the importance of Fe-based active sites in enhancing the ORR activity.
The sample prepared in the absence of DCDA showed the lowest
values of both onset potential and E1/2, as well as diffusion limiting
current density, indicating the worst performance for ORR. The poor
performance in the absence of glucose (onset potential and E1/2 were
0.997 and 0.851 V, Fig. S23†) is mainly due to the large diminution
in the BET surface area (199 m2 g compared to 473 m2 g). This
diminishment in the BET surface area is due to the conversion of the
morphology from two-dimensional nanosheets architecture with a
large accessible surface area to one-dimensional nanotubes with lower
surface area (Fig. S5d-f and S7†). Moreover, the Fe-based
nanoparticles were agglomerated, and were mostly not protected by
the carbon shells (Fig. S10†). The onset potential, E1/2, and diffusion-
limiting current density were decreased to 1.023 V, 0.903 V, and 4.75
mA cm respectively in the absence of the S source. This reveals that
S doping is an important parameter for enhancing the electrocatalytic
activity for ORR, where the sulfur species, such as thiophene S, can
modulate the electron spin density in the carbon framework, which
ultimately leads to changing the internal charge distribution and
promoting the chemisorption of oxygen species.74 In addition, the
FeSx species promote the electron transfer during the catalytic
process.69 Therefore, we can attribute the much reinforcement in the
ORR activity; as a result of S doping, to the synergistic effect of the
homogenously dispersed FeSx and FeNx species at the atomic level as
well as the formation of thiophene-like structures (CSC) in the
Sdoped FeNC electrocatalyst. These conclusions are in full
agreement with results reported by Y. Qiao et al.69 where the
formation of FeS and CSC bonds as a result of the sulfuration of
FeNC catalyst have promoted the original activity of FeNx active
sites. In addition to FeSx and CSC species, the presence CSOxC
interaction, has been reported to enhance the efficient adoption of
oxygen molecules, and hence increase the ORR reactivity.
Furthermore, the BET surface area was increased from 335 to 473 m2
g because the involvement of S led to creating more defects in the
carbon architecture, which resulted in increasing the microporous
surface area from 4 to 58 m2 g (Table S3†). The much enhancement
in the micro-porosity of the electrocatalyst could provide plentiful
accessible active sites, and therefore leads to boosting the ORR
efficiency. We can conclude that the significant promotion in the
ORR activity of the FeNC catalytic system as a result of S doping,
not only because of the increase in the specific surface area, but also
because of the formation of new S-based active sites which leads to
the increment in the density of ORR active sites, contributing to the
superior ORR performance. This emphasizes the fundamental role
played by the dual doping of N and S into the carbon matrix in
generating more active sites, and hence increasing the ORR efficiency
values for the electrocatalyst synthesized without
S source
(Fe3C@NC/900) in order to demonstrate the synergistic effect of N
and S co-doping. The calculated Cdl and EASA values for Fe3C@NSC
were 1.415 mF and 47.17 cm2, much better than the Cdl and EASA
for commercial Pt/C (1.09 mF and 36.33 cm2). For Fe3C@NC/900,
the values of Cdl and EASA were 0.88 mF and 29.33 cm2. From these
values, we can conclude that the S-doping resulted in enhancing the
ORR performance not only because of the formation of new S-based
active sites for ORR, but also due to the promotion in the EASA where
the value of the EASA has been increased from 29.33 to 47.17 cm2,
when S has been co-doped with N. It is clearly seen that the S-doping
significantly facilitated the access of the internal pore structure of the
electrocatalyst to the electrolyte, which leads to enhancing the
utilization of the electrocatalyst surface area, thereby promoting the
EASA, and hence boosting the ORR performance. The increased
EASA elucidates partially the enhanced ORR catalytic activity of
Fe3C@NSC/900 over the commercial Pt/C (20%) and
Fe3C@NC/900, which emphasizes the promising electrocatalytic
activity of Fe3C@NSC/900 electrocatalyst towards the ORR, with
respect to the commercial Pt/C (20%) and Fe3C@NC/900.
The Fe loading is one of the most important fabrication
parameters for the catalysts, which can be proficiently adjusted by
altering the mass of Fe precursor during fabricating the electrocatalyst.
The electrocatalyst synthesized without Fe showed very weak
performance due to the absence of the Fe-based active sites such as
Fe3C nanoparticles and FeNx centers, which have predominant role in
promoting the electrocatalyst efficiency for ORR (Fig. S21†). The
electrocatalyst fabricated with low Fe precursor ratio (0.4:1:1:4 of
Fe(NO3)3.9H2O:thiourea:glucose:DCDA) exhibited onset potential
and E1/2 of 1.036 and 0.922 vs RHE, comparing to 1.059 and 0.938 V
for the electrocatalyst synthesized with the optimum Fe precursor
ratio (1:1:1:4). On the other hand, using higher Fe ratio led to more
degradation in the activity (onset potential and E1/2 values are 1.032
and 0.905 V) due to diminishment of the number of the exposed active
sites because of the nanoparticles agglomeration. These results
illustrate that adjusting the precursors ratios has significant role in
enhancing the ORR performance of the as-synthesized electrocatalyst.
Since the temperature program applied for fabricating the
electrocatalyst had a great influence on its morphology, it highly
affected the catalytic activity of the elctrocatalyst (Fig. S22†). The
electrocatalyst fabricated using the second temperature program (Fig.
S12†) displayed less catalytic performance towards the ORR
comparing to the electrocatalyst obtained by employing the first
temperature program (Fig. S1†), where it offered less values of E1/2
10 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 11 of 16
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
compared to the mono-doped electrocatalyst. By considering all these shells prohibit the Fe3C cores from leaching; consequently, the
View Article Online
electrochemical measurements together, our results disclose that the Fe C@NSC/900 maintained its durability, even after thousands of
DOI: 10.1039/C9TA01837A
3
coexistence of all the precursors together during the electrocatalyst potential cycles. The long-term stability of Fe3C@NSC/900 and
fabrication is necessary for achieving a well-defined mesoporous commercial Pt/C was confirmed by current-time chronoamperometric
architecture, high BET surface area, and symmetrical distribution of measurements for 30 h (Fig. 6c). The Fe3C@NSC/900 electrocatalyst
the different types of active sites, which are much required for manifested very slight alleviation in performance with 97.2%
producing a highly efficient electrocatalyst.
retention of current density. In contrast, there was a much greater
The durability of non-precious ORR electrocatalysts is a very reduction in the current density of commercial Pt/C, where it retained
important criterion to evaluate and employ them for practical only 73.4% of its initial current density, which further emphasizing
applications, instead of Pt-based electrocatalysts. The RDE curve of the outstanding long-term durability of the Fe3C@NSC/900
the Fe3C@NSC/900 electrocatalyst after the potential cycling of electrocatalyst.
working electrode for continuous 12000 potential cycles displayed a
The resistance of ORR electrocatalysts to methanol (MeOH)
negligible negative half-wave potential shift of 2 mV, while the onset crossover is important for practical implementations as cathode
potential showed approximately no change (Fig. 6a). In contrast, the catalysts in methanol-based fuel cells. Upon the injection of 2 M
RDE measurements of Pt/C disclosed that the onset potential and E1/2 MeOH into the O2-saturated 0.1 M KOH solution after 1000 s, no
showed more negative potential shifts of 32 and 34 mV, consecutively remarkable alteration in the current density was noticed for
(Fig. 6b). Moreover, the reduction in the diffusion-limiting current Fe3C@NSC/900 electrode (Fig. 6d). In contrast, there was a
density for Pt/C was higher than that of Fe3C@NSC/900. The significant variation in the current density for Pt/C electrode after
Fig. 6. Durability tests of Fe3C@NSC/900 and Pt/C (20%): a, b) LSV curves of Fe3C@NSC/900 and Pt/C (20%) before and after 12000
potential cycles in O2-saturated 0.1 M KOH electrolyte. c) Current disintegration of Fe3C@NSC/900 and Pt/C (20%) for 30 h at constant
potential of 0.6 V (vs RHE) in O2-saturated 0.1 M KOH at 1600 rpm. d) Chronoamperometric response of methanol at Fe3C@NSC/900 and
Pt/C (20%) electrodes at 1600 rpm upon the injection of methanol after 1000 s into the O2-saturated 0.1 M KOH solution. e) CV curves of
Fe3C@NSC/900 in absence and presence of 2 M methanol. f) Influences of the SCN ions on the ORR electrocatalytic activity of the
Fe3C@NSC/900 catalyst, in O2-saturated 0.1 M KOH electrolyte at scan rate of 5 mV s1 and electrode rotation speed of 1600 rpm.
Fe3C@NSC/900 electrocatalyst was scrutinized again by TEM and MeOH injection into electrolyte, because of the MeOH oxidation on
HRTEM, to inspect the morphological variations of the electrocatalyst the Pt/C electrode surface. After 5000 s, only 55.8% of the initial
after the durability test in basic medium (Fig. S24a-c†). It is obvious current density was maintained compared to 99.8% for
that there were no observable alterations in the morphology of the Fe3C@NSC/900. The resistance to MeOH was emphasized by CV in
electrocatalyst after the durability test. Moreover, the Fe3C the absence and presence of 2 M of MeOH (Fig. 6e). We observed no
nanoparticles cores were still clearly seen, and competently variation in the CV curve of Fe3C@NSC/900 before and after the
safeguarded by the encapsulating graphitic carbon shells (yellow addition of MeOH; meanwhile, the CV curve of Pt/C exhibited a very
circles in Fig. S22b and c†). This indicates that the Fe3C@NSC/900 clear oxidation peak when MeOH was injected into the electrolyte
core-shell nanostructure is reluctant to corrosion, and possesses the (Fig. S25†). These results demonstrate that Fe3C@NSC/900 is not
ability to maintain its structural perfection, even after 12000 potential sensitive to MeOH, implying the better electrocatalytic selectivity for
cycles. This observation further demonstrates the outstanding stability ORR and the strong tolerance to MeOH crossover, which means that
of Fe3C@NSC/900 electrocatalyst, where the protective graphitic it may be a promising candidate as a cathode material to substitute
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 12 of 16
ARTICLE
Journal Name
commercial Pt/C for direct methanol fuel cells. The electrocatalytic resulted in increasing the values of onset potential and E1/2, from 0.863
activity of Fe3C@NSC/900 was tested in O2-saturated 0.1 M KOH and 0.732 V to 0.924 and 0.796 V, consecutively (Fig.V7ieaw, iAnrtsicelte)O. Tnlihnee
DOI: 10.1039/C9TA01837A
solution containing 10 and 20 mM KSCN to investigate the catalytic high value of the onset potential (0.924 vs RHE) reflects the
exceptionally low overpotential of the ORR over Fe3C@NSC/900
electrocatalyst in acidic medium. Increasing the pyrolysis temperature
to 1000 °C caused the onset potential and E1/2 to negatively shift to
0.876 and 0.755 V. Consequently, 900 °C is the optimum temperature
to fabricate the electrocatalyst, where the electrocatalyst prepared at
this temperature showed the highest values of onset potential and E1/2
as well as diffusion-limiting current density, among all the as-
synthesized electrocatalysts in both alkaline and acidic electrolytes.
The onset potential and E1/2 for Pt/C catalyst were 0.954 and 0.828 V
vs RHE which are more positive than those values of Fe3C@NSC/900
by only 30 and 32 mV, respectively. Indeed, it can be observed that
the reduction of oxygen by the Fe3C@NSC/900 electrocatalyst in the
acidic medium is slower than that in the alkaline medium. This is
because of the protonation of the doped N atoms in acidic medium
hinders the charge delocalization. In addition, the intermediate H2O2
formed in the acidic medium is less stable on the active sites of
contribution from FeNx active centers (Fig. 6f). It is well known that
thiocyanate ions (SCN) can poison the FeNx centers.29,48,49 When
10 mM KSCN were introduced to the cell containing Fe3C@NSC/900
as a cathode material, the onset potential and E1/2 were negatively
shifted by 54 and 33 mV, respectively, indicating that the ORR
performance was significantly inhibited due to the blocking of FeNx
active centers by SCN ions, where these ions coordinate with Fe and
compete with O2. Upon injecting more concentration of SCN (20
mM), the electrocatalytic activity worsened, where the onset potential
and E1/2 were negatively shifted by 74 and 62 mV, consecutively. In
addition, the diminishment in the diffusion-limiting current density
was significantly more after injecting more concentration of SCN.
These results suggest that more FeNx active centers were blocked,
when more concentration of SCN ions was introduced into the
electrolyte. When the electrode was washed multiple times with water
and retested in a fresh O2-saturated 0.1 M KOH solution, the ORR
performance almost completely recovered. These results further prove
that the existence of Fe species in combination with nitrogen species
provides crucial active sites for the ORR in the co-doped mesoporous
carbon nanostructures.

Fe3C@NSC than the Lewis basic form HO2 formed in the alkaline
medium.54,75
To get accurate and deep insight into the electron-transfer kinetics
implicated in the ORR process over Fe3C@NSC/900 and Pt/C (20%)
in the acidic solution, the LSVs at different rotation speeds in O2-
Fig. 7. a) RDE voltammograms of Pt/C (20%) and Fe3C@NSC electrocatalysts, fabricated under various annealing temperatures in O2-
saturated 0.1 M HClO4 electrolyte at a sweep rate of 5 mV s1 and rotation speed of 1600 rpm. The inset shows the relation between
temperature and the onset potential/half-wave potential of the electrocatalysts. b) Tafel plots of Pt/C (20%) and Fe3C@NSC electrocatalysts
synthesized at different carbonization temperatures. c) Chronoamperometric response of methanol crossover on Fe3C@NSC/900 and Pt/C
(20%) at 1600 rpm upon methanol addition after 1000 s into O2-saturated 0.1 M HClO4 solution. d) Current-time chronoamperometric
response of Fe3C@NSC/900 and Pt/C (20%) for 20 h in O2-saturated 0.1 M HClO4 at 1600 rpm. e, f) ORR polarization curves for
Fe3C@NSC/900 and Pt/C (20%) before and after 12000 potential cycles in O2-saturated 0.1 M HClO4 electrolyte.
The extraordinary electrocatalytic performance of the saturated 0.1 M HClO4 electrolyte were measured (Fig. S26a and c).
The parallelism and linear relationship between J1 and ω0.5 at various
potentials ranging from 0.30 to 0.55 V (vs RHE) indicate that the ORR
catalyzed by the Fe3C@NSC/900 electrocatalyst follows a first order
reaction kinetics with respect to the concentration of dissolved oxygen
Fe3C@NSC catalysts toward ORR under alkaline conditions
encouraged us to study their performance in acidic medium. The ORR
activity was studied in 0.1 M HClO4 electrolyte using RDE setup (Fig.
7a). Elevating the carbonization temperature from 800 to 900 °C
12 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 13 of 16
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
in the solution. The electron transfer number for the ORR over Optimizing the various reaction parameters leads constructing a
highly electroactive-durable catalyst with onset potential, E
View Article ,Onalninde
Fe3C@NSC/900 was calculated from the linear fitting of the KL
DOI: 10.1039/C9TA01837A
1/2
plots, and found to be higher than 3.93 over the potential range from limiting current density of 1.059 V, 0.938 V, and 7.87 mA cm,
0.3 to 0.55 V. This value was very close to that of Pt/C catalyst (n= respectively, in the alkaline solution. The electrocatalyst exhibits
3.96, Fig. S26d†), indicating that the ORR proceeded through a four- selectivity of four-electron transfer, tolerance toward methanol
electron transfer pathway in the acidic medium, which involve the oxidation, and excellent durability after 12000 potential cycles, with
direct reduction of oxygen gas molecules to water molecules. As zero degradation in the onset potential, and 2 mV degradation in the
shown in Fig. 7b, the catalyst synthesized at 900 °C exhibited the E1/2. Furthermore, it presents 97.2% activity retention after continuous
lowest Tafel slope (69 mV dec), in comparison with the catalysts
synthesized at 800 or 1000 °C (78 and 74 mV dec respectively).
The almost similar slope for Fe3C@NSC/900 and benchmark Pt/C (66
mV dec) indicates that an identical reaction mechanism of ORR
occurred on the surface of both electrocatalysts, which proposes that
the rate-determining stage in the ORR is the first-electron transfer
stage.76 As displayed in Fig. 7c, upon the injection of MeOH into the
electrolyte, the commercial Pt/C catalyst exhibited a dramatic
variation in the current, and the sign of the voltammetric current was
even changed, due to the oxidation of methanol over the surface of the
catalyst. The current was gradually retained to the negative side
because of the ORR; however, only 19.9% of the initial current was
retained. On the other hand, no remarkable alteration in the current
happened for the Fe3C@NSC/900, after adding methanol to the
electrolyte solution. Moreover, the current maintained almost its same
value (99.3%) within 4000 s from the methanol injection. The
sensitivity of Fe3C@NSC/900 to MeOH was assured by the CV
technique in the absence and presence of 2 M MeOH (Fig. S27a†). It
is noticeable that the CV curve is the same as that before injecting
MeOH into the electrolyte solution, while the commercial Pt/C
showed an obvious peak due to the oxidation of MeOH on its surface
(Fig. S27b†).
The Fe3C@NSC/900 electrocatalyst offered a very high stability
with time, where it retains 98.5% of the initial current density after the
operation for 20 h (Fig. 7d). In contrast, commercial Pt/C displayed
only 75.9% retention under the same conditions. These data indicate
that the as-synthesized electrocatalyst possesses superior robustness
to Pt/C (20%) in the acidic medium. The outstanding durability of
Fe3C@NSC/900 was confirmed by the accelerated durability (ADT)
test in O2-saturated HClO4 solution in the potential range from 0.5 to
1.1 V vs RHE, to check the long-term durability of Fe3C@NSC/900
and Pt/C under the operating conditions (Fig. 7e and f). Obviously,
the LSV curve of Fe3C@NSC/900 recorded after 12000 potential
cycles exhibited only a slight negative shift of E1/2 (5 mV), while the
onset potential still possessed almost the same value as before the
ADT, indicating that no changes had occurred in the intrinsic activity
of the electrocatalyst (Fig. 7e). On the other hand, the onset potential
and E1/2 of Pt/C were degraded by 27 and 52 mV, respectively (Fig.
7f). These results reveal that the Fe3C@NSC/900 electrocatalyst has
better sustained catalytic performance than the commercial Pt/C at
low pH values during the long-time operation, in which the Pt
nanoparticles tend to agglomerate into larger particles, resulting in
disintegration of the catalyst performance. Considering all the above
results together reveals that Fe3C@NSC/900 is an efficient-durable
electrocatalyst for the ORR with a selective four-electron pathway and
is free from methanol crossover. As a result, Fe3C@NSC/900
provides a promising candidate electrocatalyst, instead of commercial
Pt/C catalyst in fuel cells and metal-air batteries.
operation for 30 h. In the acidic medium, the electrocatalyst shows
very convincing electrochemical performance with onset potential
and E1/2 of 0.924 and 0.796 V, respectively. However, it outperforms
Pt/C (20%) catalyst regarding the methanol tolerance and endurance,
where it offers no change in the onset potential and negligible change
of 5 mV in the E1/2 compared to 27 and 52 mV deterioration in the
onset potential and E1/2 for Pt/C after 12000 potential cycles. The
much enhanced electrocatalytic activity and durability of
Fe3C@NSC/900 can be attributed to its multiple marvelous
characteristics such as high specific surface area, unique mesoporous
nanostructure, and the enrichment of various active centers including
FeNx species, pyridinic N, graphitic N, C-S-C and Fe-N-C structures
as evidenced by the XPS analysis. Moreover, the N and S co-doped
graphitic shell layers surrounding the Fe3C nanoparticles prevent their
agglomeration during the accelerated durability test and hence
guarantee their long-term durability. Eventually, the existence of the
Fe3C nanoparticles inside NSC shells lowers the local work function
on the graphitic shells because of the easy electron transport from the
Fe3C core to the NSC shells. Taking into consideration the cheap
nature of the employed precursors and the superb catalytic
performance of the synthesized electrocatalyst, this work introduces a
successful strategy to rationally design and construct high-
performance noble metal-free ORR electrocatalysts by a green,
potentially scalable, low-cost, and tunable route.
Conflict of Interest
The authors declare no conflict of interest.
Acknowledgments
This research was supported by the Basic Science Research Program
(2017R1A2B3004917) through the National Research Foundation
(NRF), funded by the Ministry of Science and ICT & Future Planning
of Republic of Korea.
References
1.
2.
3.
Z. F. Huang, J. Wang, Y. Peng, C. Y. Jung, A. Fisher and
X. Wang, Adv. Energy Mater., 2017, 7, 1700544.
J. Wang, K. Wang, F.-B. Wang and X.-H. Xia, Nat.
Commun., 2014, 5, 5285.
C. Wang, H. Zhang, J. Wang, Z. Zhao, J. Wang, Y. Zhang,
M. Cheng, H. Zhao and J. Wang, Chem. Mater., 2017, 29,
99159922.
4.
5.
6.
7.
8.
L. Yang, X. Zeng, W. Wang and D. Cao, Adv. Funct. Mater.,
2018, 28, 1704537.
C. Zhu, H. Li, S. Fu, D. Du and Y. Lin, Chem. Soc. Rev.,
2016, 45, 517531.
P. Ganesan, M. Prabu, J. Sanetuntikul and S. Shanmugam,
ACS Catal., 2015, 5, 36253637.
Y. Xue, S. Sun, Q. Wang, Z. Dong and Z. Liu, J. Mater.
Chem. A, 2018, 6, 1059510626.
S. Mondal and C. R. Raj, J. Phys. Chem. C, 2018, 122,
1846818475.
3. Conclusions
In summary, we have established a sustainable one-step approach
to synthesize efficient and durable non-precious electrocatalyst for
ORR. The presence of glucose leads to the homogenous distribution
of the nanoparticles, preventing the formation of aggregates and
construction of nanosheets architecture instead of nanotubes, thereby
maximizing the surface area. The existence of sulfur source drives the
formation of more active sites and promotes the surface area.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 13
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 14 of 16
ARTICLE
Journal Name
9.
M. Chisaka, Y. Yamamoto, N. Itagaki and Y. Hattori, ACS 36.
Appl. Energy Mater., 2017, 1, 211219.
O. Frank, M. Mohr, J. Maultzsch, C. Thomsen, I. Riaz, R.
Jalil, K. S. Novoselov, G. TsoukleDriO, IJ:.10P.a10rt3Vh9iee/wnCiA9orTtsiAcla0en1O8dn3liK7nAe.
Papagelis, ACS Nano, 2011, 5, 22312239.
K.-H. Wu, D.-W. Wang, X. Zong, B. Zhang, Y. Liu, I. R.
Gentle and D.-S. Su, J. Mater. Chem. A, 2017, 5,
32393248.
B. Mecheri, V. C. Ficca, M. A. C. de Oliveira, A.
D’Epifanio, E. Placidi, F. Arciprete and S. Licoccia, Appl.
Catal. B Environ., 2018, 237, 699707.
M. Lefèvre, E. Proietti, F. Jaouen and J.-P. Dodelet, Science,
2009, 324, 7174.
J. Park, Y. Nabae, T. Hayakawa and M.-a. Kakimoto, ACS
Catal., 2014, 4, 37493754.
X. H. Li, S. Kurasch, U. Kaiser and M. Antonietti, Angew.
Chem. Int. Ed., 2012, 51, 96899692.
Y. Zhang, L.-B. Huang, W.-J. Jiang, X. Zhang, Y.-Y. Chen,
Z. Wei, L.-J. Wan and J.-S. Hu, J. Mater. Chem. A, 2016, 4,
77817787.
Y. Zhang, W.-J. Jiang, L. Guo, X. Zhang, J.-S. Hu, Z. Wei
and L.-J. Wan, ACS Appl. Mater. Interfaces, 2015, 7,
1150811515.
Y. Hu, J. O. Jensen, W. Zhang, L. N. Cleemann, W. Xing,
N. J. Bjerrum and Q. Li, Angew. Chem. Int. Ed., 2014, 53,
36753679.
M. Xiao, J. Zhu, L. Feng, C. Liu and W. Xing, Adv. Mater.,
2015, 27, 25212527.
N. Jian and R. E. Palmer, J. Phys. Chem. C, 2015, 119,
1111411119.
S. N. Bhange, S. M. Unni and S. Kurungot, J. Mater. Chem.
A, 2016, 4, 60146020.
K. Yuan, S. Sfaelou, M. Qiu, D. Lützenkirchen-Hecht, X.
Zhuang, Y. Chen, C. Yuan, X. Feng and U. Scherf, ACS
Energy Lett., 2018, 3, 252260.
E. Li, F. Yang, Z. Wu, Y. Wang, M. Ruan, P. Song, W. Xing
and W. Xu, Small, 2018, 14, 1702827.
L. Lai, J. R. Potts, D. Zhan, L. Wang, C. K. Poh, C. Tang,
H. Gong, Z. Shen, J. Lin and R. S. Ruoff, Energy Environ.
Sci., 2012, 5, 79367942.
J. Zhu, M. Xiao, C. Liu, J. Ge, J. St-Pierre and W. Xing, J.
Mater. Chem. A, 2015, 3, 2145121459.
K. Artyushkova, A. Serov, S. Rojas-Carbonell and P.
Atanassov, J. Phys. Chem. C, 2015, 119, 2591725928.
J. Wei, Y. Liang, Y. Hu, B. Kong, G. P. Simon, J. Zhang, S.
P. Jiang and H. Wang, Angew. Chem. Int. Ed., 2016, 55,
13551359.
10.
11.
12.
13.
E. Hu, X. Y. Yu, F. Chen, Y. Wu, Y. Hu and X. W. Lou,
Adv. Energy Mater., 2018, 8, 1702476.
37.
R. Wang, X. Y. Dong, J. Du, J. Y. Zhao and S. Q. Zang,
Adv. Mater., 2018, 30, 1703711.
J. Zhu, M. Xiao, P. Song, J. Fu, Z. Jin, L. Ma, J. Ge, C. Liu,
Z. Chen and W. Xing, Nano Energy, 2018, 49, 2330.
H. Tan, J. Tang, J. Kim, Y. V. Kaneti, Y.-M. Kang, Y.
Sugahara and Y. Yamauchi, J. Mater. Chem. A, 2019, 7,
13801393.
Q. Cheng, S. Han, K. Mao, C. Chen, L. Yang, Z. Zou, M.
Gu, Z. Hu and H. Yang, Nano Energy, 2018, 52, 485493.
Z. Huang, H. Pan, W. Yang, H. Zhou, N. Gao, C. Fu, S. Li,
H. Li and Y. Kuang, ACS Nano, 2018, 12, 208216.
M. Sun, D. Davenport, H. Liu, J. Qu, M. Elimelech and J.
Li, J. Mater. Chem. A, 2018, 6, 25272539.
S. H. Ahn, X. Yu and A. Manthiram, Adv. Mater., 2017, 29,
1606534.
38.
39.
40.
41.
42.
14.
15.
16.
17.
18.
19.
43.
44.
M. H. Naveen, K. Shim, M. S. A. Hossain, J. H. Kim and
Y. B. Shim, Adv. Energy Mater., 2017, 7, 1602002.
G. A. Ferrero, K. Preuss, A. Marinovic, A. B. Jorge, N.
Mansor, D. J. Brett, A. B. Fuertes, M. Sevilla and M.-M.
Titirici, ACS Nano, 2016, 10, 59225932.
20.
21.
W.-J. Ong, L.-L. Tan, Y. H. Ng, S.-T. Yong and S.-P. Chai,
Chem. Rev., 2016, 116, 71597329.
J. Li, Y. Zhang, X. Zhang, J. Han, Y. Wang, L. Gu, Z. Zhang,
X. Wang, J. Jian and P. Xu, ACS Appl. Mater. Interfaces,
2015, 7, 1962619634.
M.-Q. Wang, W.-H. Yang, H.-H. Wang, C. Chen, Z.-Y.
Zhou and S.-G. Sun, ACS Catal., 2014, 4, 3928-3936.
H. Yu, L. Shang, T. Bian, R. Shi, G. I. Waterhouse, Y. Zhao,
C. Zhou, L. Z. Wu, C. H. Tung and T. Zhang, Adv. Mater.,
2016, 28, 50805086.
L. Yang, D. Cheng, H. Xu, X. Zeng, X. Wan, J. Shui, Z.
Xiang and D. Cao, PNAS, 2018, 115, 66266631.
Y. Zheng, Y. Jiao, Y. Zhu, Q. Cai, A. Vasileff, L. H. Li, Y.
Han, Y. Chen and S.-Z. Qiao, J. Am. Chem. Soc., 2017, 139,
33363339.
45.
46.
47.
48.
22.
23.
49.
50.
24.
25.
51.
52.
53.
26.
27.
S. Sultan, J. N. Tiwari, J. H. Jang, A. M. Harzandi, F.
Salehnia, S. J. Yoo and K. S. Kim, Adv. Energy Mater.,
2018, 8, 1801002.
C. Schneidermann, N. Jäckel, S. Oswald, L. Giebeler, V.
Presser and L. Borchardt, ChemSusChem, 2017, 10,
24162424.
28.
29.
Q. Li, D. Xu, X. Ou and F. Yan, Chem. Asian J., 2017, 12,
18161823.
W.-J. Jiang, L. Gu, L. Li, Y. Zhang, X. Zhang, L.-J. Zhang,
J.-Q. Wang, J.-S. Hu, Z. Wei and L.-J. Wan, J. Am. Chem.
Soc., 2016, 138, 35703578.
54.
55.
L. Lin, Q. Zhu and A.-W. Xu, J. Am. Chem. Soc., 2014, 136,
1102711033.
J. Li, S. Ghoshal, W. Liang, M.-T. Sougrati, F. Jaouen, B.
Halevi, S. McKinney, G. McCool, C. Ma and X. Yuan,
Energy Environ. Sci., 2016, 9, 24182432.
Z. Liu, F. Sun, L. Gu, G. Chen, T. Shang, J. Liu, Z. Le, X.
Li, H. B. Wu and Y. Lu, Adv. Energy Mater., 2017, 7,
1701154.
C. Santoro, R. Gokhale, B. Mecheri, A. D'Epifanio, S.
Licoccia, A. Serov, K. Artyushkova and P. Atanassov,
ChemSusChem, 2017, 10, 32433251.
W. Niu, L. Li, X. Liu, N. Wang, J. Liu, W. Zhou, Z. Tang
and S. Chen, J. Am. Chem. Soc., 2015, 137, 55555562.
N. Ramaswamy, U. Tylus, Q. Jia and S. Mukerjee, J. Am.
Chem. Soc., 2013, 135, 1544315449.
P. Chen, T. Zhou, L. Xing, K. Xu, Y. Tong, H. Xie, L.
Zhang, W. Yan, W. Chu and C. Wu, Angew. Chem. Int. Ed.,
2017, 56, 610614.
30.
31.
32.
33.
J. Liang, R. F. Zhou, X. M. Chen, Y. H. Tang and S. Z.
Qiao, Adv. Mater., 2014, 26, 60746079.
F. Fina, S. K. Callear, G. M. Carins and J. T. Irvine, Chem.
Mater., 2015, 27, 26122618.
56.
57.
S. Gupta, S. Zhao, O. Ogoke, Y. Lin, H. Xu and G. Wu,
ChemSusChem, 2017, 10, 774785.
Z. Schnepp, A. E. Danks, M. J. Hollamby, B. R. Pauw, C.
A. Murray and C. C. Tang, Chem. Mater., 2015, 27,
50945099.
J.-B. Wu, M.-L. Lin, X. Cong, H.-N. Liu and P.-H. Tan,
Chem. Soc. Rev., 2018, 47, 18221873.
58.
59.
60.
34.
35.
D. Voiry, J. Yang, J. Kupferberg, R. Fullon, C. Lee, H. Y.
Jeong, H. S. Shin and M. Chhowalla, Science, 2016, 353,
14131416.
14 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 15 of 16
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
61.
Z. Tan, H. Li, Q. Feng, L. Jiang, H. Pan, Z. Huang, Q. Zhou,
View Article Online
H. Zhou, S. Ma and Y. Kuang, J. Mater. Chem. A, 2019, 7,
DOI: 10.1039/C9TA01837A
16071615.
62.
Y. J. Sa, D.-J. Seo, J. Woo, J. T. Lim, J. Y. Cheon, S. Y.
Yang, J. M. Lee, D. Kang, T. J. Shin and H. S. Shin, J. Am.
Chem. Soc., 2016, 138, 1504615056.
63.
64.
T. Palaniselvam, V. Kashyap, S. N. Bhange, J. B. Baek and
S. Kurungot, Adv. Funct. Mater., 2016, 26, 21502162.
Q. Li, W. Chen, H. Xiao, Y. Gong, Z. Li, L. Zheng, X.
Zheng, W. Yan, W. C. Cheong and R. Shen, Adv. Mater.,
2018, 30, 1800588.
65.
66.
67.
68.
69.
70.
B. Vinayan, T. Diemant, R. J. Behm and S. Ramaprabhu,
RSC Adv., 2015, 5, 6649466501.
X. Qing, J. Shi, C. Ma, M. Fan, Z. Bai, Z. Chen, J. Qiao and
J. Zhang, J. Power Sources, 2014, 266, 8898.
S. Bag, B. Mondal, A. K. Das and C. R. Raj, Electrochim.
Acta, 2015, 163, 1623.
Z.-S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng and K.
Müllen, J. Am. Chem. Soc., 2012, 134, 90829085.
Y. Qiao, P. Yuan, Y. Hu, J. Zhang, S. Mu, J. Zhou, H. Li,
H. Xia, J. He and Q. Xu, Adv. Mater., 2018, 30, 1804504.
H. Shen, E. Gracia‐Espino, J. Ma, K. Zang, J. Luo, L. Wang,
S. Gao, X. Mamat, G. Hu and T. Wagberg, Angew. Chem.
Int. Ed., 2017, 56, 1380013804.
71.
72.
S. Yasuda, A. Furuya, Y. Uchibori, J. Kim and K.
Murakoshi, Adv. Funct. Mater., 2016, 26, 738744.
X. Huang, Z. Zhao, L. Cao, Y. Chen, E. Zhu, Z. Lin, M. Li,
A. Yan, A. Zettl and Y. M. Wang, Science, 2015, 348,
12301234.
73.
74.
75.
76.
Z. Li, M. Shao, L. Zhou, R. Zhang, C. Zhang, M. Wei, D.
G. Evans and X. Duan, Adv. Mater., 2016, 28, 23372344.
I. Y. Jeon, S. Zhang, L. Zhang, H. J. Choi, J. M. Seo, Z. Xia,
L. Dai and J. B. Baek, Adv. Mater., 2013, 25, 61386145.
K. Wan, Z.-p. Yu, X.-h. Li, M.-y. Liu, G. Yang, J.-h. Piao
and Z.-x. Liang, ACS Catal., 2015, 5, 43254332.
T. Shinagawa, A. T. Garcia-Esparza and K. Takanabe, Sci.
Rep., 2015, 5, 13801.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 15
Please do not adjust margins

Journal of Materials Chemistry A
Page 16 of 16
Table of contents
View Article Online
DOI: 10.1039/C9TA01837A
Noble metal-free electrocatalyst for efficient oxygen reduction reaction under both alkaline and
acidic conditions has been fabricated by one-pot template-free strategy.