Three-dimensional hierarchical meso/macroporous Fe/Co-nitrogen-doped carbon encapsulated FeCo alloy nanoparticles prepared without any template or surfactant: High-performance bifunctional oxygen electrodes

Article abstract of DOI:10.1016/j.jallcom.2016.06.060

Exploring highly efficient bi-functional electro-catalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical for developing energy conversion and storage systems. Herein, for the first time, without any template or surfactant, a family of Fe/Co-N-doped three-dimensional (3D) hierarchical meso/macroporous carbon encapsulated FeCo nanoparticles (denoted as FexCo(1-x)-N/PC) have been successfully synthesized through pyrolyzing the low-cost biomass-based precursors in the NH₃ atmosphere. After optimizing the molar ratio of Fe/Co, benefiting from the collaborative advantages of the Fe/Co-N-C species, cobalt-based electro-active phases, and unique 3D electrode configuration, the resultant Fe0.5Co0.5-N/PC amazingly shows considerable ORR activity comparable with commercial Pt/C catalyst via a 4e^- pathway in alkaline electrolyte. When used for OER catalysis in an alkaline medium, Fe0.5Co0.5-N/PC achieves an extremely negative potential of 1.625 V vs. RHE at a current density of 10 mA cm^-2 and a low Tafel slope of 68.3 mV dec^-1. Furthermore, the overpotential (ΔE) between ORR and OER is just 0.813 V, which is nearly the minimum value for the non-precious metal catalysts. We believe that our present synthetic strategy can be further extended to develop other 3D hierarchical meso/macroporous carbonaceous catalysts for various applications, such as sensors, batteries, and supercapacitors.

Full text of DOI:10.1016/j.jallcom.2016.06.060

Journal of Alloys and Compounds 686 (2016) 467e478
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Three-dimensional hierarchical meso/macroporous Fe/Co-nitrogen-
doped carbon encapsulated FeCo alloy nanoparticles prepared
without any template or surfactant: High-performance bifunctional
oxygen electrodes
a
a
b
a
a, *
Mian Li , Tingting Liu , Liquan Fan , Xiangjie Bo , Liping Guo
a
Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province and Department of Chemistry, Northeast Normal University,
Changchun, Jilin Province 130024, China
College of Materials Science and Engineering, Qiqihar University, No. 42, Wenhua Street, Qiqihar 161006, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 December 2015
Received in revised form
Exploring highly efficient bi-functional electro-catalysts toward oxygen reduction reaction (ORR) and
oxygen evolution reaction (OER) are critical for developing energy conversion and storage systems.
Herein, for the first time, without any template or surfactant, a family of Fe/CoeN-doped three-
dimensional (3D) hierarchical meso/macroporous carbon encapsulated FeCo nanoparticles (denoted as
FexCo(1ꢀx)-N/PC) have been successfully synthesized through pyrolyzing the low-cost biomass-based
5
June 2016
Accepted 7 June 2016
Available online 9 June 2016
3
precursors in the NH atmosphere. After optimizing the molar ratio of Fe/Co, benefiting from the
collaborative advantages of the Fe/CoeNeC species, cobalt-based electro-active phases, and unique 3D
electrode configuration, the resultant Fe0.5Co0.5-N/PC amazingly shows considerable ORR activity
comparable with commercial Pt/C catalyst via a 4e pathway in alkaline electrolyte. When used for OER
Keywords:
Oxygen reduction reaction
Oxygen evolution reaction
FeCo alloy
Nitrogen doped carbon
Hierarchical meso/macropores
ꢀ
catalysis in an alkaline medium, Fe0.5Co0.5-N/PC achieves an extremely negative potential of 1.625 V vs.
ꢀ
2
ꢀ1
RHE at a current density of 10 mA cm and a low Tafel slope of 68.3 mV dec . Furthermore, the
overpotential ( E) between ORR and OER is just 0.813 V, which is nearly the minimum value for the non-
D
precious metal catalysts. We believe that our present synthetic strategy can be further extended to
develop other 3D hierarchical meso/macroporous carbonaceous catalysts for various applications, such
as sensors, batteries, and supercapacitors.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
efficient bi-functional electro-catalysts for both oxygen reduction
reaction (ORR) during the discharge process and oxygen evolution
reaction (OER) during the recharge process [2]. To facilitate low
over-potentials and fast kinetics in ORR/OER catalytic processes,
electro-catalysts based on the precious metals have been widely
explored and reported [3e8]. However, finding stable and efficient
electro-catalysts which drive both reactions is also difficult,
because good catalysts for ORR often tend to be poor for OER and
vice versa [9,10]. For example, those platinum based catalysts are
very excellent catalysts for ORR but which are poor for OER. Simi-
larly, catalysts based on ruthenium or iridium are the state-of-art
OER catalysts but are poor for ORR catalysis [11,12]. Recently,
most bi-functional electro-catalysts are prepared by combining Pt
The theoretically high energy density of regenerative fuel cells
especially the dual functionality regenerative fuel cell typically
(
comprises a fuel cell function (2H
is produced and an electrolyzer function (2H
is stored) and metal-air batteries makes them very desirable for
2
þ O
2
/ 2H
2
O) where electricity
2
O / 2H
2
þ O ) where
2
H
2
future energy conversion and storage applications. Significant ef-
forts have been devoted to the development of practical regener-
ative fuel cell and metal-air systems for various applications [1].
Even so, unlike conventional secondary batteries, many scientific
and technical challenges still remain to realize the potential for
practical applications, one of which being the development of
(good catalysts for ORR) and Ru/Ir (good catalysts for OER) to form a
composite [13,14]. Unfortunately, those materials used to synthe-
size these catalysts are extremely scarce and in turn demand a high
*
Corresponding author.
E-mail address: guolp078@nenu.edu.cn (L. Guo).
http://dx.doi.org/10.1016/j.jallcom.2016.06.060
925-8388/© 2016 Elsevier B.V. All rights reserved.
0

468
M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
price, which limit the technology from becoming a ubiquitous form
of renewable energy. Therefore, commercial viability of electro-
chemical devices about metal-air batteries can only be achieved
through the use of low-cost and abundant catalysts for ORR/OER.
Developing efficient and cost-effective electro-catalysts using
earth-abundant elements for ORR/OER is still highly desirable.
Some efforts have been directed towards the incorporation of
excellent noble metal-free ORR and OER catalysts into a composite,
in which the constituent ORR and OER catalysts retain their indi-
vidual properties. This strategy has been used in the past with
platinum group metals for obtaining bi-functional materials for
acid media [9], and more recently by combining OER active low-
cost transition metal oxides with carbon supports that provide
ORR functionalities for alkaline media [15]. Fe based nanostructures
have been widely explored for various kinds of electro-catalysts. To
date, in most reports on iron-based catalysts they always deliver
considerable electro-catalytic activities being similar to Pt-based
catalytic active sites and contain enough transport channels for
electrons/reactants.
With these in mind, in this work, for the first time, we attempt
to prepare novel material of three-dimensional (3D) hierarchical
meso/macroporous Fe/Co-nitrogen-doped carbon encapsulated Fe/
Co nanoparticles (NPs) from low-cost common paper towels (as the
carbon precursors) without any template or surfactant. The con-
stant dosage of paper towels (2 g) were first impregnated with
30 mL of 66.7 mM M(AC)
then dried at room temperature. Finally, the catalysts were pre-
pared by temperature-programming in a tube furnace under NH at
2
aqueous solution (M ¼ Fe and/or Co), and
3
ꢁ
ꢀ1
ꢁ
a heating rate of 5 C min and kept at 800 C for 2 h. Those
resultant catalysts are denoted as FexCo(1ꢀx)-N/PC, where x values
2 2
represent the molar ratios of Fe(AC) used in the total M(AC) and
have been controlled as 1, 0.75, 0.5, 0.25 and 0 respectively. For
example, the Fe0.75Co0.25-N/PC represents the usages of
Fe(AC) $4H O and Co(AC) $4H O are 369 mg (50 mM) and
2 2 2 2
catalysts in ORR, such as: Fe/Fe
3
CeC based catalysts; FeN
x
C/C
124.5 mg (16.7 mM) in 30 mL impregnated solution. Those 3D hi-
erarchical meso/macropores of resultant samples will provide
plenty of low-resistance channels for both electron and mass
transfer. Especially, after optimizing the x values of FexCo(1ꢀx)-N/
PC in the impregnated solutions, compared with other samples, the
resultant Fe0.5Co0.5-N/PC catalyst could get a better balance in
both OER and ORR catalyses, accompanying the minimum over-
potential between ORR and OER.
based catalysts, FeeNeC composite, etc. [16e20] However, Fe
based catalysts cannot give consideration to the highly efficient
OER catalytic activity [21,22]. Cobalt, on the other hand, is abundant
and large amounts of Co based composites have been reported to
have good OER activities, which are considerable activities even
2 2
compared with RuO and IrO catalysts in alkaline media [23e27].
Summarizing those facts mentioned above, to design and synthe-
size bi-functional catalysts for both ORR and OER with merits of
high activity, low-cost, and excellent stability; the incorporation of
good ORR catalyst (based on Fe) and OER catalyst (based on Co) into
a composite is one of the effective ways, in which the constituent Fe
and Co based catalytic active sites retain their individual properties.
Up to now, a material of FeCoN/rGO has been tentatively prepared
as ORR/OER bi-functional catalyst. Even so, may be limited by the
lack of porous structures and nonuniformity of active site disper-
sion, its ORR/OER activities is not so excellent, in order to further
enhance the total catalytic efficiency, Ru elements have been
inevitably used [28].
From a rational design point of view, an ideal electro-catalyst
should have highly active catalytic centers to facilitate a complete
reversibility of the electrochemical process; meanwhile, those
active catalytic sites should also be easily accessible to the electrons
and reactants necessary to complete the electrochemical reactions
2. Experimental
2.1. Reagents and apparatus
The paper towels were afforded by ‘mind act upon mind’ and
obtained from the local supermarket in Changchun. Fe(AC)
2 2
$4H O,
Co(AC) $4H O, KOH, and Nafion solution (5 wt%) were purchased
2
2
from Sigma-Aldrich. The commercial 20 wt% Pt/C catalyst was ob-
tained from Johnson Matthey Company (Shanghai, China). All these
chemicals were used as delivered without further treatment. Cyclic
voltammetry (CV) and rotating disk electrode (RDE, d ¼ 5 mm)
voltammetry experiments were performed with a PARSTAT 2273.
Meanwhile, all electrochemical measurements were performed in a
standard three-compartment cell. The modified RDE, Pt wire, and
Ag/AgCl electrodes acted as working, counter, and reference elec-
trodes, respectively.
[29]. Generally, the exposed edges are affected by the morphologies
and structures of catalysts. Hence, designing ORR/OER electro-
catalysts with more Fe and Co based catalytic active edge sites
and better electron/proton transport efficiency is one effective
strategy to enhance their catalytic activity. Since they were syn-
thesized for the first time, porous carbonaceous materials (such as
ordered mesoporous carbon, onion-like mesoporous carbon vesi-
cles, macroporous carbon, hierarchical porous carbon foams, etc)
have been paid intensive attention on their electrochemical appli-
cations [30e37]. However, for large-scale development and
industrialization of these catalysts, their primary obstacles are the
complex preparation processes. In fact, in recent years, for pre-
paring porous carbonaceous catalysts, carbon materials derived
from renewable biomass resources have received considerable
attention because biomass is extensively available, accessible and
recyclable [38e41]. Porous carbon composite membranes have
been prepared using low-cost common filter papers as precursors
2.2. Synthesis of catalysts and modified electrodes
The intact preparation procedures of the FexCo(1ꢀx)-N/PC cat-
alysts have been displayed in Supplementary Scheme S1. The
detailed descriptions are placed in the supporting information
(Synthesis of catalysts in the supporting information). Prior to
modification, the RDE was polished carefully with alumina pow-
ders and then cleaned with HNO
water. The catalyst ink was prepared by mixing 3 mg catalyst
powders into 1 mL Nafion solution (0.5 wt%). Then 30 L of the
3
(1: 1), ethanol, and deionized
m
catalyst ink was dropped onto the cleanly washed RDE and dried
before electrochemical experiments.
2.3. Physical characterization and electrochemical measurements of
the resultant catalysts
[42]. At the same time, A4 printing papers have also been carbon-
ized into graphene-tethered carbon fiber composite papers [43]. As
shown in Fig. S1, as the different processing methods compared
with those for filter papers and ordinary papers, the light trans-
mission tests prove that paper towels show the best transmittancy,
displaying a higher porosity. We conjecture this structure is very
suitable for preparing porous carbons dispersed by abundant Fe/Co
The detailed descriptions for the physical characterizations and
ORR/OER electro-chemical measurements of those resultant cata-
lysts were also located in the supporting information. It’s important
to note that in the electrochemical testing processes for ORR and
OER, the polarization curves were plotted as potential (E vs. RHE)
ꢀ2
versus log |j(mA cm )| to get the Tafel plots for assessing the ORR

M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
469
and OER kinetics of those resultant catalysts. By fitting the linear
portion of the Tafel plots to the Tafel equation (
Fe0.25Co0.75-N/PC, and CoeN/PC samples. It is clear that, as the
synergistic effect of Fe(AC) and Co(AC) in the nucleation process
h
¼ b log (j) þ a), the
2
2
Tafel slope (b values) can be obtained. All potentials appeared in
this paper are referred to reversible hydrogen electrode (the po-
tentials recorded and referred to Ag/AgCl in each experiment were
calculated using the formula ERHE ¼ EAg/AgCl þ 0.059 pH þ 0.197 V,
where ERHE is a potential vs. reversible hydrogen electrode (RHE),
of calcinations, Fe0.75Co0.75-N/PC, Fe0.5Co0.5-N/PC and
Fe0.25Co0.75-N/PC show smaller particle sizes than FeeN/PC and
CoeN/PC. Specially, Fe0.5Co0.5-N/PC displays the minimum parti-
cle sizes. In fact, such a geometric confinement of metal NPs within
graphitized carbon layers has been reported to enhance their
interface contact and to suppress the dissolution and agglomera-
tion of NPs, thereby promoting the electrochemical activity and
stability of the resultant hybrids [44].
E
Ag/AgCl is a potential vs. Ag/AgCl electrode, and pH is the pH value
of electrolyte. All data were reported with the typical iR compen-
sation. All current densities are the ratios of those recorded cur-
rents to the geometric area of electrode.
The morphologies and structures of as-prepared samples were
also observed by TEM image (Fig. 2). Fig. 2a shows that N/PC is
composed of hollow graphitic nanoshells with a pore size range of
3
. Results and discussion
2e200 nm; meanwhile, as shown in Fig. S4, abundant onion rings
exhibit the open structures of N/PC, demonstrating hierarchical
meso/macroporous structures have been successfully synthesized
The detailed synthesis procedures and the macrostructures of
FexCo(1ꢀx)-N/PC electro-catalysts and corresponding precursors
are presented in Scheme S1. The morphologies of resultant PC, N/
PC, and FexCo(1ꢀx)-N/PC samples are first observed by SEM im-
3
in the pyrolysis process of paper towels in NH atmosphere.
Fig. 2bed reveal that those 3D hierarchical meso/macropores have
been successfully reserved from the precursor even if the using of
metal acetates in the impregnating solution; meanwhile, most
metal NPs are encapsulated within carbon nanoshells. For FeeN/PC
2
ages. As shown in Fig. S2a, for PC calcined under N atmosphere,
lots of tubers and ravines can be obviously observed on the sur-
faces, increasing its specific surface area. Furthermore, for the N/PC
calcined under NH atmosphere, large amounts of extra meso/
3
(Fig. 2b) and Fe0.5Co0.5-N/PC (Fig. 2c), the main difference is that
less aggregation of metal NPs is observed on the surface of
Fe0.5Co0.5-N/PC. However, CoeN/PC has started to display the
characteristic bamboo-like morphology of CNTs (Fig. 2d), demon-
strating large amounts of embossments appeared in the SEM image
of CoeN/PC is really incomplete CNTs. Nevertheless, the growth
process is just remained in the first step of CNT growth. As shown in
Fig. 2e, continuing to enlarge the TEM image of Fe0.5Co0.5-N/PC
into a smaller scope. One can see most metal NPs are retained and
encapsulated in carbon nanoshells for Fe0.5Co0.5-N/PC and abun-
dant mesopores can also be observed. At last, a d-spacing of 2.02 Å
corresponding to the (110) plane of the FeCo nanocrystal have also
been found in Fig. 2f.
macropores can be clearly observed on the surfaces (Fig. S2b). As
shown in Fig. 1aec and Fig. S2c-d), it is noteworthy that large
amounts of metal NPs have encapsulated within or located on
surfaces of graphitized carbons for FexCo(1ꢀx)-N/PC samples,
suggesting the efficient assembly between metal NPs and graphi-
tized PC. Specially, the surfaces of CoeN/PC seems to be more rough
than other samples, lots of embossments can be clearly observed on
their surfaces, which could be the predecessors of carbon nano-
tubes (CNTs). As shown in Fig. S3, the low-magnification SEM of
Fe0.5Co0.5-N/PC provides a visual evidence for the homodisperse
of abundant metal NPs. Furthermore, Fig. 1a -c and Fig. S2e-f
illustrate the corresponding histograms of the particle size distri-
bution for metal NPs by counting 200 particles. One can see that the
average diameters are about 37.1, 27.0, 26.7, 30.5, and 36.4 nm for
the resultant FeeN/PC, Fe0.75Co0.75-N/PC, Fe0.5Co0.5-N/PC,
0
0
XRD images are carried out to display the compositions of PC, N/
PC, and FexCo(1ꢀx)-N/PC samples. As shown in Fig. 3a, two broad
ꢁ ꢁ
peaks at 2q values of about 26 and 43 , corresponding to the (002)
0
0
0
Fig. 1. SEM images of (a) FeeN/PC, (b) Fe0.5Co0.5-N/PC, and (c) CoeN/PC. The corresponding histograms of particle size distribution for (a ) FeeN/PC, (b ) Fe0.5Co0.5-N/PC, and (c )
CoeN/PC.

470
M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
Fig. 2. TEM images of (a) N/PC, (b) FeeN/PC, (c) Fe0.5Co0.5-N/PC, and (d) CoeN/PC. (e) Megascopic TEM image of Fe0.5Co0.5-N/PC. (f) HRTEM image of an individual CoFe alloy NP
for Fe0.5Co0.5-N/PC.
and (100) planes of carbon, respectively. However, for the
further increased intensities of D-bands for all FexCo(1ꢀx)-N/PC
samples compared with those for PC and N/PC prove that the
geometric confinement of metal NPs within graphitized carbon
layers has enhanced their interface contact and then afford more
ꢁ
FexCo(1ꢀx)-N/PC catalysts, those peaks located at 43 for (100)
planes all degrade. At the same time, five new peaks being assigned
2
to the Fe N phase (JCPDS No. 89-3939) appeared in XRD curve of
FeeN/PC. XRD curve of Fe0.75Co0.25-N/PC shows three character-
defect sites. The maximum I
that abundant content of disordered carbon increases after Fe and
N doping. However, once a part of Fe(AC) are replaced by Co(AC)
in the impregnating solutions, resultant Fe0.75Co0.25-N/PC,
Fe0.5Co0.5-N/PC, and Fe0.25Co0.75-N/PC samples show smaller I
ratios compared with FeeN/PC, demonstrating the decrease of
defect sites. Specially, for CoeN/PC, the minimum I /I ratio has
been calculated compared with other FexCo(1ꢀx)-N/PC catalysts,
but which is still larger than those for PC and N/PC. Combining the
SEM and TEM images, we preliminary analyse that large amounts of
meso/macroporous are filled by some fragmentary CNTs grown on
the surfaces, which will hugely decrease the accessible meso/
macroporous structures of CoeN/PC and then decrease the defect
sites.
D G
/I ratio of FeeN/PC (1.66) indicates
ꢁ
ꢁ
ꢁ
istic peaks located at 44.76 , 82.45 , and 65.12 , corresponding to
the Co Fe alloy (JCPDS No. 48-1618); furthermore, other diffraction
peaks are assigned to the corresponding reflections of Fe C phases
JCPDS No. 37-0999), confirming the co-existence of Co Fe alloy
and Fe phases in Fe0.75Co0.25-N/PC. Compared with
Fe0.75Co0.25-N/PC, tiny diffraction peaks for Fe C phases could still
be observed in XRD curve of Fe0.5Co0.5-N/PC accompanying a
transformation of Co Fe alloy to CoFe alloy (JCPDS No. 49-1568).
The cubic CoC phase (JCPDS No. 44-0962) can be observed for
Fe0.5Co0.5-N/PC. For Fe0.25Co0.75-N/PC, CoFe alloy continually
change to the Co Fe alloy ((JCPDS No. 48-1818); meanwhile, other
3
7
2
2
2
(
3
7
D
/
2
C
I
G
2
D G
3
7
x
7
3
strong diffraction peaks are assigned to the face centered cubic (fcc)
metallic Co (JCPDS No. 15-806). Finally, only strong and sharp
diffraction peaks associated with fcc metallic Co (JCPDS No. 15-806)
are observed in XRD curve of CoeN/PC. Hereto, we have clearly
displayed the chemical compositions of those black NPs shown in
both SEM and TEM images of those FexCo(1ꢀx)-N/PC catalysts.
Raman spectroscopy is a powerful and widely used tool for
characterizing carbonaceous materials, the typical Raman spectra
of PC and N/PC are first tested and shown in Fig. S5. They all display
two prominent peaks, which can be ascribed to the well-
2
Fig. 4aeb shows the N adsorption-desorption isotherms and
pore size distributions of FexCo(1ꢀx)-N/PC. All adsorption-
desorption curves belong to the type IV isotherms (based on
IUPAC classification) with a hysteresis loop, indicating the existence
of abundant mesopores. Meanwhile, the pore size distributions
calculated by using the BJH method from desorption branches
(Fig. 4b) show that all resultant samples have wide pore size dis-
tributions from 2 to 200 nm. At the same time, BET surface area of
ꢀ1
2
ꢀ1
documented D-band (at approximately 1359 cm , associated
with structure defects and disorders) and G-band (observed at ca.
N/PC is 634.3 m
g
with the largest total pore volume of
ꢀ1
(Fig. S6). However, BET surface areas and total pore
3
0.29 m
g
ꢀ
1
2
ꢀ1
3
ꢀ1
1
590 cm , is commonly found with all graphitic structures usually
volumes have decreased down to 242.6 m g and 0.23 m g (for
2
2
ꢀ1
3
ꢀ1
assigned to the E2g phonon of sp C atoms). The intensity ratio of D-
band to G-band, namely I /I ratio, provides the gauge for the
FeeN/PC), 254.0 m g and 0.23 m g (for Fe0.75Co0.25-N/PC),
2
ꢀ1
3
ꢀ1
2
ꢀ1
D
G
236.8 m g and 0.22 m g (for Fe0.5Co0.5-N/PC), 228.9 m g
3
ꢀ1
2
ꢀ1
amount of structural defects and reflects the extent of graphitic
crystalline in graphitic carbons. It thus evident that more defect
sites have been introduced into the carbon skeletons of N/PC
and 0.2 m
g
(for Fe0.25Co0.75-N/PC), and 175.6 m
(for CoeN/PC) (Fig. 4ced). One can see that all
g and
3
ꢀ1
0.19 m
g
FexCo(1ꢀx)-N/PC samples except for CoeN/PC show similar BET
area and pore volumes. The slight decrease in BET area and total
pore volume for CoeN/PC is mainly due to the coverage of meso/
compared with PC, a larger I
D G
/I ratio of N/PC (1.20) compared with
PC (1.09) is powerful evidence (Fig. S5). As can be seen in Fig. 3b, the

M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
471
Fig. 5bed, except for the four peaks of pyridinic-N, pyrrolic-N,
graphitic-N, and oxidized N species, an added peak of FeeN/CoeN
species located at about 399.4 ± 0.2 eV also appear in the high
resolution N 1s XPS spectra of FeeN/PC, Fe0.5Co0.5-N/PC, and
CoeN/PC, but there are large differences in contents of each N
species. On the other hand, XPS characterization results have
proved that the total metal contents are all close to 1.5 at.% for
FeeN/PC, Fe0.5Co0.5-N/PC and CoeN/PC. However, no character-
istic peaks of Fe or Co elements have been observed in the binding
energy range of Fe 2p and Co 2p for N/PC (Fig. S8a,b). The high
resolution Fe 2p3/2 and Fe 2p1/2 spectra of FeeN/PC (Fig. S8c) and
Fe0.5Co0.5-N/PC (Fig. 5e) are fitted into three peaks severally,
corresponding to the Fe(0), Fe(III)/Fe(II) ions, and satellite peaks,
respectively. As shown in Fig. 5e, the Co 2p3/2 XPS spectrum of
Fe0.5Co0.5-N/PC was fitted into four peaks located around 776.8,
780.5, 783.3, and 788.4 eV assigned to the Co(0), CoeN, Co(II) ni-
trate, and satellite peak, respectively. However, Co 2p3/2 XPS
spectrum of CoeN/PC was fitted into five peaks located around
7
76.8, 779.8, 780.8, 783.5, and 788.3 eV assigned to the Co(0), co-
balt oxide, CoeN, Co(II) nitrate, and satellite peak, respectively
Fig. S8d). Finally, the contents of pyridinic-N, metal-N, pyrrolic-N,
(
graphitic-N, oxidized N, and pyridinic-N þ metal-N species for the
N/PC, FeeN/PC, Fe0.5Co0.5-N/PC, and CoeN/PC are all calculated
and recorded in Fig. 5f. It is clear that the resultant Fe0.5Co0.5-N/PC
contains the maximum pyridinic-N, Fe/CoeN, and graphitic-N
species.
The evolution of ORR performances for PC, N/PC, and series of
FexCo(1ꢀx)-N/PC catalysts are shown in Fig. 6. All the RDE curves
2
recorded at a rotation rate of 1600 rpm in O -saturated 0.1 M KOH
solution are self-consistent, and display an obvious ORR activity
with different catalytic performances (Fig. 6a). The pure PC in 0.1 M
KOH is sluggish for ORR, evident by the low onset potential (Eonset
)
and half wave potential (E1/2); after doping N atoms into the skel-
etons of PC, the resultant N/PC will show more positive Eonset
(0.77 V vs. RHE) and E1/2 (0.67 V vs. RHE) accompanying a obvious
increase of limited diffusion current densities (j ). But these per-
L
formances are still not on the same level as commercial Pt/C. Spe-
cifically, with constantly changing the composition of precursors,
both the Eonset and E1/2 (Fig. 6b) will decrease from FeeN/PC (0.939
and 0.822 V vs. RHE) to Fe0.75Co0.25-N/PC (0.933 and 0.820 V vs.
RHE) to Fe0.5Co0.5-N/PC (0.925 and 0.813
V vs. RHE) to
Fe0.25Co0.75-N/PC (0.911 and 0.805 V vs. RHE) and then to Co0.75-
N/PC (0.839 and 0.779 V vs. RHE). One can see that, except for the
CoeN/PC, ORR catalytic activities (viewing from Eonset and E1/2) of
samples contain Fe atoms are comparable to those for the com-
mercial 20%Pt/C (0.957 and 0.817 V vs. RHE). As shown in Fig. 6c, the
high ORR electro-catalytic activities of these Fe based catalysts are
also gleaned from their much larger current densities (at 0.6 V vs.
RHE).
Fig. 3. (a) XRD patterns of resultant PC, N/PC, and FexCo(1ꢀx)-N/PC samples. (b)
Raman spectra of different FexCo(1ꢀx)-N/PC samples.
macropores by fragmentary CNTs.
X-ray photoelectron spectroscopy (XPS) was further used to
analyze the composition of N/PC, FeeN/PC, Fe0.5Co0.5-N/PC, and
CoeN/PC. Survey spectra reveal the presence of C, N, and O in four
samples, demonstrating that N atoms can be successfully doped
The ORR catalytic performances of FexCo(1ꢀx)-N/PC were
further evaluated for mechanistic and kinetic performances using
Tafel plots (Fig. 6d). Tafel slopes in low current density region are
ꢀ1 ꢀ1
3.5 mV decade on FeeN/PC, 65.7 mV decade on Fe0.75Co0.25-
6
into carbon skeletons after being calcined in NH
3
. After the precise
ꢀ1
ꢀ1
N/PC, 67.3 mV decade on Fe0.5Co0.5-N/PC, 70.6 mV decade on
quantitative analysis by XPS, the total N contents are 1.2, 2.3, 2.8,
and 2.5 at.% for resultant N/PC, FeeN/PC, Fe0.5Co0.5-N/PC, and
CoeN/PC samples. As shown in Fig. S7a-d, all C 1s XPS spectra are
ꢀ1
Fe0.25Co0.75-N/PC, and 57.2 mV decade on CoeN/PC. All these
data are close to and even surpass that recorded on a Pt/C surface
ꢀ1
(
67.6 mV decade ). From a mechanistic point of view, these results
decomposed into five components, two main peaks located at
imply the ORR mechanisms on FexCo(1ꢀx)-N/PC and Pt-based
catalysts are similar in alkaline medium and reveal the transfer of
the first electron on these FexCo(1ꢀx)-N/PC catalysts are all the rate
determining step under Temkin conditions for the adsorption of
intermediates. Overall, the ORR catalytic activities briefly decrease
by an order of FeeN/PC > Fe0.75Co0.25-N/PC > Fe0.5Co0.5-N/
PC > Fe0.25Co0.75-N/PC > CoeN/PC. Thus, we can draw a conclu-
sion that the doping of Co atoms into the catalytic systems cannot
2
~
284.5 and ~285.4 eV are assigned to sp -hybridized graphite-like
2
3
carbon (CeC sp ) and sp -hybridized diamond-like carbon (CeC
3
2
2
sp overlapping with sp carbon bound to nitrogen (NeC sp )). By
fitting the high resolution N 1s XPS spectra of N/PC (Fig. 5a), we can
assign four peaks at 398.3 ± 0.2, 400.4 ± 0.2, 401.7 ± 0.2, and
3
99.4 ± 0.2 eV to pyridinic-N, pyrrolic-N, graphitic-N, and oxidized
N species, respectively. Different from the N/PC, as shown in

472
M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
Fig. 4. (a) N
2
adsorption/desorption isotherms for FexCo(1ꢀx)-N/PC samples. (b) The corresponding pore size distributions calculated by using the BJH method from the curves
displayed in Fig. 4a. (c) BET surface area distribution histograms of the N/PC and other FexCo(1ꢀx)-N/PC samples. (d) The corresponding pore volume distribution histograms of
different samples.
Fig. 5. High resolution XPS spectra of N 1s for N/PC (a), FeeN/PC (b), Fe0.5Co0.5-N/PC (c), and CoeN/PC (d) samples. (e) High resolution XPS spectra of Fe 2p and Co 2p for resultant
Fe0.5Co0.5-N/PC sample. (f) The contents of N-bonding configurations of N/PC, FeeN/PC, Fe0.5Co0.5-N/PC, and CoeN/PC.
further promote the ORR activity, superabundant Co atoms doped
into the FeeN/PC even result in inferior ORR catalysis. By viewing
the N adsorption results shown in Fig. 4ced, surface area and pore
2
volume are the main drawbacks accounting for the inferior ORR
performance.
Finally, the ORR mechanisms occurred on the electrodes were
determined by RDE measurements at various rotation speed
(Fig. 7aeb and Fig. S9a-d), based on which the Koutecky-Levich (K-
L) plots at different potentials were also obtained for each catalyst
0
0
(Fig. 7ced and Fig. S9a -d ). One can see that the KeL plots at

M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
473
ꢀ
1
Fig. 6. (a) RDE polarization curves of ORR for pure PC, N/PC, FexCo(1ꢀx)-N/PC, and Pt/C in O
2
-saturated 0.1 M KOH with a scan rate of 5 mV s at a rotation speed of 1600 rpm. (b)
Coresponding Eonset and E1/2 values of different catalysts tested in Fig. 6a. (c) Coresponding current densities recorded at 0.6 V vs. RHE from the LSV curves shown in Fig. 6a. (d) Tafel
plots of the FexCo(1ꢀx)-N/PC and Pt/C catalysts derived and calculated from the corresponding RDE data shown in Fig. 6a.
Fig. 7. RDE voltammograms of Fe0.5Co0.5-N/PC (a) and Pt/C (b) catalysts in O
plots of the Fe0.5Co0.5-N/PC (c) and Pt/C (d) catalysts at different potentials derived from RDE measurements shown in Fig. 7a,b. (e) RRDE test of ORR on Fe0.5Co0.5-N/PC and 20 wt
Pt/C in an O -saturated 0.1 M KOH electrolyte at a rotation rate of 1600 rpm. (f) H yields and electron transfer numbers tested in 0.1 M KOH (calculated from the corresponding
and I values displayed in Fig. 7f).
-saturated 0.1 M KOH at various rotation speeds with a scan rate of 5 mV sꢀ1. Koutecky-Levich (K-L)
2
%
2
2 2
O
I
D
R

474
M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
various potentials exhibit good linearity and parallelism for FeeN/
PC, Fe0.75Co0.25-N/PC, Fe0.5Co0.5-N/PC, Fe0.25Co0.75-N/PC,
CoeN/PC, and Pt/C catalysts, indicating the first-order reaction ki-
netics for ORR with respect to the concentration of dissolved oxy-
gen. Particularly, the KeL plots of various catalysts at 0.6 V vs. RHE
are shown in Fig. S10a. The KeL plots at 0.6 V vs. RHE exhibit good
linearity and parallelism for FeeN/PC, Fe0.75Co0.25-N/PC,
Fe0.5Co0.5-N/PC, and Pt/C. After assessing the apparent number (n)
of electrons transferred during ORR according to the equation
expressed as Eq. S(1) and Eq. S(2), the four samples all display the
3.95e4.02 for Fe0.5Co0.5-N/PC and commercial Pt/C catalysts be-
tween 0.2 and 0.6 V vs. RHE respectively, revealing the intrinsic
four-electron-transfer process.
Prior to assessing the OER catalytic activities of FexCo(1ꢀx)-N/
PC, cyclic voltammograms (CVs) of N/PC and FexCo(1ꢀx)-N/PC are
2
recorded in N -saturated 0.1 M KOH solution (Fig. S11). As the
largest BET surface area, the N/PC displays the largest background
current. FeeN/PC shows a bland CV curve with the minimum
background currents. However, with adding Co atoms into the
catalytic systems, all Co-based samples show a pair of redox char-
acteristic peaks for Co(II)/Co(III) between 0.9 and 1.1 V vs. RHE [45].
Specially, the peak intensities will increase with increasing the Co
percentage in catalysts. OER catalytic activities of Pt/C and N/PC
were firstly investigated in 0.1 M KOH, their polarization curves for
ꢀ
predominantly 4e transfer pathways at this potential. However,
the slopes of K-L plots of Fe0.25Co0.75-N/PC and CoeN/PC are
much larger than other samples, demonstrating the smaller n
values. Fig. S10b displays the calculated average n values between
ꢀ1
0
.2 and 0.6 V vs. RHE got from K-L plots, which are ~4.05, ~4.01,
OER are recorded at a scan rate of 5 mV s (Fig. 8a). It is clear the
commercial Pt/C and N/PC catalysts present very poor OER electro-
catalytic activities. In contrast, under identical test conditions, the
resultant FexCo(1ꢀx)-N/PC catalysts all show acceptable perfor-
mances for OER. Furthermore, the OER catalytic kinetics of
FexCo(1ꢀx)-N/PC were explored by using corresponding Tafel
~3.96, ~3.92, ~3.79, and ~3.72 for Pt/C, FeeN/PC, Fe0.75Co0.25-N/
PC, Fe0.5Co0.5-N/PC, Fe0.25Co0.75-N/PC, and CoeN/PC catalysts. In
conclusion, The RDE technique demonstrates the predominantly
ꢀ
4
e
transfer pathways for FeeN/PC, Fe0.75Co0.25-N/PC, and
Fe0.5Co0.5-N/PC; similar to ORR catalyzed by the high-quality
commercial Pt/C catalyst. Meanwhile, the selectivity of four-
electron reduction of oxygen of Fe0.5Co0.5-N/PC and commercial
Pt/C catalysts were further tested by RRDE technique as shown in
ꢀ1
plots. Tafel slopes are calculated to be 126.8 mV decade on FeeN/
ꢀ
1
ꢀ1
PC,114.9 mV decade on Fe0.75Co0.25-N/PC, 68.3 mV decade on
ꢀ
1
Fe0.5Co0.5-N/PC, 74.5 mV decade
111.5 mV decade on CoeN/PC, indicating the most favorable ki-
on Fe0.25Co0.75-N/PC, and
ꢀ
1
Fig. 7e. Both the H
2
O
2
yields (%) and electron transfer number were
determined by the equations shown in Eq. S(3) and Eq. S(4). Sur-
prisingly, the peroxide yields of Fe0.5Co0.5-N/PC and commercial
Pt/C catalysts remain below 8.0% and 6.0% at potentials from 0.8 to
netics of Fe0.5Co0.5-N/PC catalyst during OER in alkaline medium.
ꢀ2
The potential required for the current density of 10 mA cm
(E10) is an essential parameter relevant to solar fuel cells, which is
widely used to evaluate the OER activity of catalysts. The OER Eonset
and E10 values for Fe0.5Co0.5-N/PC are as low as 1.564 and 1.625 V
0
.2 V vs. RHE; meantime, the electron transfer numbers derived
from the RRDE test (Fig. 7f) were calculated to be 3.80e3.95 and
Fig. 8. (a) IR-corrected RDE polarization curves of OER for N/PC, FexCo(1ꢀx)-N/PC, and Pt/C in 0.1 M KOH with a scan rate of 5 mV sꢀ at a rotation speed of 1600 rpm. (b) Tafel plots
1
of OER for FexCo(1ꢀx)-N/PC and Pt/C catalysts derived by the mass-transport correction of the corresponding RDE data in Fig. 8a. (c) Coresponding OER Eonset values of different
ꢀ
2
catalysts. (d) Coresponding OER catalytic potentials recorded at 10 mA cm
.

M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
475
vs. RHE, being much more negative than those for the Pt/C (1.698
and 1.926 V vs. RHE), N/PC (1.742 and 1.895 V vs. RHE), FeeN/PC
as-prepared FexCo(1ꢀx)-N/PC catalysts are normally decreasing
with the increasing of over-potentials according to the expected
evolution regarding Butler Volmer kinetics. Furthermore, the
slightly depressed semi circles observed on five materials originate
from non-ideal capacitive behaviors related to the catalyst struc-
tures themselves (i.e. abundant hierarchical meso/macroporous
structures have induced the large porosities and the obvious
capacitance distributions along the materials). On the other hand,
we can also compare OER catalytic activities of different catalysts
from EIS characterization results. As shown in Fig. 9a, Rct values of
FeeN/PC decrease significantly with increasing the over-potentials
(
1.629 and 1.743 V vs. RHE), Fe0.75Co0.25-N/PC (1.588 and 1.693 V
vs. RHE), Fe0.25Co0.75-N/PC (1.574 and 1.637 V vs. RHE), and CoeN/
PC (1.589 and 1.662 V vs. RHE) (Fig. 8ced). All these results display
that there is a large change in the intrinsic OER catalytic activity of
Fe0.5Co0.5-N/PC.
In order to get more information concerning the OER mecha-
nisms involved on FexCo(1ꢀx)-N/PC catalysts, electrochemical
impedance spectroscopy (EIS) measurements have also been car-
ried out. Fig. 9aee shows the typical Nyquist plots for five catalysts
at different potentials from 1.54 to 1.64 V vs. RHE. It can be firstly
observed that the charge transfer resistances (Rct) for OER on the
from 773.7
U
at 1.54 V vs. RHE to 418.1
U
at 1.56 V vs. RHE, to 237.4
U
at 1.58 V vs. RHE, to 137.2
U
at 1.60 V vs. RHE, to 78.4
U
at 1.62 V vs.
Fig. 9. Nyquist plots of the FeeN/PC (a), Fe0.75Co0.25-N/PC (b), Fe0.5Co0.5-N/PC (c), Fe0.25Co0.75-N/PC (d), and CoeN/PC (e) catalysts modified electrodes recorded at various OER
potentials from 1.54 to 1.64 V vs. RHE. (f) Nyquist plots of different catalysts examined at 1.6 V vs. RHE.

476
M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
RHE, and then to 44.2
faster OER process of FeeN/PC when potentials are above 1.62 V vs.
RHE. However, Fe0.75Co0.25-N/PC (Fig. 9b), Fe0.25Co0.75-N/PC
U
at 1.64 V vs. RHE. These results suggest the
enough M-N-C species for maintaining considerable ORR catalytic
activity. On the contrary, the CoeN/PC possesses more inactive
oxidized-N species with a relatively low specific surface area,
directly leading to a huge decrease in ORR catalytic activity of
CoeN/PC. For OER catalysis, as the OER catalytic efficiency of FeeN/
PC is far better than those for the N/PC and Pt/C, naturally
demonstrating the function of M-N-C in enhancing OER catalytic
activity. As introducing Co elements into the FeeN/PC, OER catalytic
efficiencies of 0.75Co0.25-N/PC and 0.5Co0.5-N/PC show great
improvement, that is because when the presence of numerous Co
(
Fig. 9d), and CoeN/PC (Fig. 9e) have displayed faster OER catalytic
processes once the potentials are above 1.60 V vs. RHE, which are all
0 mV less than 1.62 V vs. RHE of FeeN/PC, demonstrating the
2
much more efficient OER catalysis. Furthermore, for Fe0.5Co0.5-N/
PC (Fig. 9c), when applied potentials are larger than 1.58 V vs. RHE,
the trends of EIS plots and recorded Rct values from 1.58 to 1.64 V vs.
RHE are nearly the same. In Nyquist plots recorded at 1.60 V vs. RHE
ꢀ
(Fig. 9f), Fe0.5Co0.5-N/PC gives the smallest semicircle, which still
phases, Co(II) is formed with extra OH species in alkaline solution,
ꢀ
indicates that there is a smaller charge transfer resistance for
Fe0.5Co0.5-N/PC. In a word, the Fe0.5Co0.5-N/PC catalyst is a
promising candidate as a cathode OER catalyst.
resulting in the formation of Co(III). The Co(III) reacts with OH
species producing Co(IV)eO
OER occurs, in which the absorbed O
2
and H
2
O subsequently. Finally the
to Co(IV) species is desorbed
2
For evaluating the bi-functional electro-catalytic activity of
these given catalysts, the overpotential (DE) between ORR and OER
is a very important parameter, which means the loss in efficiency
for a cathodic catalyst. In general, the figure of merit used to
quantify ORR activity is the E1/2 value and OER activity is judged by
from Co(IV) [46]. It is worth to note that immoderately using Co in
the catalysts has led to an obvious decline in OER catalytic activity.
Because the coverage of meso/macroporous structures by those
fragmentary CNTs (Fig. 2d) leads to a decrease in BET area and total
pore volume of CoeN/PC, simultaneously decreasing the exposed
active sites such as M-N, pyridinic-N, etc. As the Fe0.5Co0.5-N/PC
catalyst get a balance in both structural and elemental composi-
tions, it affords the best total cathodic activities for ORR/OER
catalysis.
In addition to the catalytic activities, the durability of electro-
catalytic activity for ORR/OER catalysis is another major concern
in constructing bi-functional cathodic catalysts for metal-air bat-
teries. Satisfactorily, the Fe0.5Co0.5-N/PC catalyst is highly sus-
tainable in the test as shown in Fig. 11. One can see that the E1/2
value of the Fe0.5Co0.5-N/PC catalyst just negatively shifts 5 mV
after 3000 CV scanning cycles. At the same time, the E10 value only
shows an 8 mV of negative shift. From the above comparative an-
alyses, we then draw the conclusion that the ORR/OER catalytic
activity of our Fe0.5Co0.5-N/PC catalyst is quite stable. Meanwhile,
the characteristics of the hierarchical meso/macroporous structures
can still be clearly observed by TEM visualization even after sta-
bility test (Fig. S12).
E
10 value, thus,
D
E ¼ E10 ꢀ E1/2. In general, the smaller
DE for the
same cathode between ORR and OER, the better the catalyst serves
as a reversible oxygen electrode. Measured by this method, as
shown in Fig. 10,
PC, Fe0.25Co0.75-N/PC, and CoeN/PC catalysts are found to be
.922, 0.873, 0.812, 0.833, and 0.883 V, respectively. Compared with
the E data of Pt/C, Ir/C, Ru/C, and other catalysts reported previ-
ously (as shown in Table S1), a E value of 0.812 V for the 0.5Co0.5-
DE of FeeN/PC, Fe0.75Co0.25-N/PC, Fe0.5Co0.5-N/
0
D
D
N/PC is still the minimum, once again displaying the high efficiency
of 0.5Co0.5-N/PC for ORR/OER catalyses. Combining with the
respective ORR and OER catalytic activities of these resultant
samples, we can see that the ORR catalytic activities briefly
decrease by an order of FeeN/PC
>
Fe0.75Co0.25-N/
PC > Fe0.5Co0.5-N/PC > Fe0.25Co0.75-N/PC > CoeN/PC. However,
the OER catalytic activities will firstly increase: FeeN/
PC < Fe0.75Co0.25-N/PC < Fe0.5Co0.5-N/PC and then decrease:
Fe0.5Co0.5-N/PC > Fe0.25Co0.75-N/PC > CoeN/PC. It is clear that
0
.5Co0.5-N/PC shows the best ORR/OER total catalytic activity. As
Above all, as a noble metal-free ORR/OER catalyst, the unique
elemental and structural characteristics of 3D hierarchical porous
Fe0.5Co0.5-N/PC are favorable for boosting its best ORR/OER ac-
tivity in alkaline solution. (1) The numerous 3D hierarchical meso/
macropores (~2e200 nm) and defects on Fe0.5Co0.5-N/PC make
proton efficiently transferred. For instance, for ORR catalysis,
unique large pores (~10e200 nm) on the surface can facilitate the
shown in Fig. 5f, compared with FeeN/PC, the 0.5Co0.5-N/PC has
O
2
diffusion and the migration of superoxide and hydroxyl;
meanwhile, those small mesopores located on the surfaces (~4 nm)
are favorable for the adsorption of O molecules, which are rate-
2
determining for the whole reaction. Furthermore, in an aspectant
OER catalytic process, the 3D hierarchical meso/macropores can
Fig. 10. (a) Oxygen electrode activities of the FeeN/PC, Fe0.75Co0.25-N/PC,
Fe0.5Co0.5-N/PC, Fe0.25Co0.75-N/PC, and CoeN/PC catalysts in 0.1 M KOH with a
ꢀ
1
sweep rate of 5 mV s at 1600 rpm, (b) the oxygen electrode activities (DE) of the
FeeN/PC, Fe0.75Co0.25-N/PC, Fe0.5Co0.5-N/PC, Fe0.25Co0.75-N/PC, and CoeN/PC
Fig. 11. IR-corrected polarization curves of Fe0.5Co0.5-N/PC for both ORR and OER
catalysts.
before and after 3000 CV cycles in 0.1 M KOH solution.

M. Li et al. / Journal of Alloys and Compounds 686 (2016) 467e478
477
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[
The advantages in structure, elemental content, and ORR/OER cat-
alytic activity potentially make Fe0.5Co0.5-N/PCs a promising non-
noble metal based cathode catalyst for metal-air batteries. We
believe that our present synthetic strategy can be further extended
to develop other 3D hierarchical meso/macroporous materials for
various applications, such as sensors, batteries, and
supercapacitors.
(2015) 9658e9667.
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(
The authors gratefully acknowledge the National Natural Sci-
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