Synthesis of nanoporous structured iron carbide/Fe-N-carbon composites for efficient oxygen reduction reaction in Zn-air batteries

Article abstract of DOI:10.1039/C6TA08050B

Large-scale industrial level applications of fuel cells and metal-air batteries have called for the development of highly efficient and low-cost oxygen reduction electrodes. Here we report the effective and simple preparation of iron carbide-embedded Fe-N-doped carbon (Fe₃C/Fe-N/C) composites using an iron-phenanthroline (Fe-Phen) complex and dicyandiamide (DCA) as the precursors that are subsequently heat treated. The optimal catalyst pyrolyzed at 800 °C (Fe-Phen-N-800) exhibits superior oxygen reduction activity with onset and half-wave potentials of 0.99 and 0.86 V in 0.1 M KOH, respectively, which are higher than those of Pt/C (onset and half-wave potentials of 0.98 and 0.84 V) at the same catalyst loading. Moreover, the obtained Fe-Phen-N-800 displays comparable activity to Pt/C in 0.1 M HClO₄ solution. Notably, the well-developed Fe-Phen-N-800 catalyst shows much higher long-term stability and better methanol tolerance than Pt/C. The results suggest that our catalyst is one of the most promising candidates to replace Pt catalysts toward oxygen reduction. Strikingly, a primary Zn-air battery using Fe-Phen-N-800 as the air cathode catalyst delivers higher voltages and gravimetric energy densities than those of a Pt/C-based system at the discharge current densities of 10 and 25 mA cm^-2, thus demonstrating the potential applications of our catalyst for energy conversion devices.

Full text of DOI:10.1039/C6TA08050B

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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ARTICLE
Synthesis of Nanoporous Structured Iron Carbide/ Fe−N−Carbon
Composites for Efficient Oxygen Reduction Reaction in Zn-Air
Batteries †
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Zheng Kun Yang, Zhi-Wei Zhao, Kuang Liang, Xiao Zhou, Cong-Cong Shen, Ya-Nan Liu, Xin Wang,
An-Wu Xu*
www.rsc.org/
Large-scale industrial level applications of fuel cells and metal-air batteries have called for the development of highly
efficient and low-cost oxygen reduction electrodes. Here we report the effective and simple preparation of iron carbide-
embedded Fe-N-doped carbon (Fe
dicyandiamide (DCA) as the precursors that are subsequently heat treated. The optimal catalyst pyrolyzed at 800 °C
Fe−Phen−N−800) exhibits superior oxygen reducꢀon acꢀvity with onset and half-wave potentials of 0.99 and 0.86 V in 0.1
M KOH, respectively, which are higher to those of Pt/C (onset and half-wave potentials of 0.98 and 0.84 V) at the same
catalyst loading. Moreover, the obtained Fe−Phen−N−800 displays comparable acꢀvity to Pt/C in 0.1 M HClO solution.
3
C/Fe−N/C) composites using iron–phenanthroline (Fe−Phen) complex and
(
4
Notably, the well-developed Fe−Phen−N−800 catalyst shows much higher long-term stability and better methanol
tolerance than Pt/C. The results suggest our catalyst is one of the most promising candidates to replace Pt catalysts toward
oxygen reduction. Striking, a primary Zn-air battery using Fe−Phen−N−800 as the air cathode catalyst delivers higher
voltages and gravimetric energy densities than those of Pt/C-based system at the discharge current densities of 10 and 25
–2
mA cm
,
thus demonstrating the potential applications of our catalyst for energy conversion devices.
active sites, which is determined by chemical compositions and the
strong synergistic effects between different components; 2) unique
porous nanostructure with high surface area, which facilitates the
1
. Introduction
The development of high-performance and low-cost oxygen
reduction reaction (ORR) electrocatalysts for implementation into
fuel cells (FCs), polymer electrolyte membrane (PEM) fuel cells and
metal-air batteries cathode is crucial for the practical applications of
these clean renewable energy conversion systems. Among metal-air
batteries, Zn-air battery has attracted considerable interest due to
its high energy density, low cost, safety and environmental
benignity. The big challenge associated with the commercialization
of Zn–air battery is still the sluggish kinetics of the ORR resulting in
large overpotential. Pure or alloyed platinum noble metal materials
fast charge transport and diffusion of ORR-relevant species, such as
+
−
−
5
2 2
H , OH , e , O , H O; 3) superior corrosion resistance in the oxygen-
riched environment. Thus, recent studies have mainly focused on
the aforementioned variables for designing and fabricating various
NPMCs towards ORR.
To date, among the various alternative NPMCs, transition-metal-
coordinating nitrogen-doped carbon catalysts labelled as M−Nx/C
(
M = Fe, Co, etc.) have been regarded as particularly promising Pt-
6
-10
free ORR electrocatalysts,
especially, those often displaying high
1
-3
activity in acidic medium. In these catalysts, the M-Nx unit is
chelated in a nitrogenous environment, which has proven to be an
are hitherto the most efficient class of ORR catalysts. The high
price, unsustainable supply, inferior durability and susceptibility to
methanol crossover effect and CO poisoning of Pt-based catalysts,
yet hinder widespread commercialization of renewable energy
technologies. To break through this bottleneck, enormous research
efforts have been devoted to explore highly active and stable non-
precious metal catalysts (NPMCs) as alternatives of Pt-based
1
1
essential structural motif in catalyzing ORR. Recently, a new family
of metal carbide nanoparticles encapsulated in graphitic carbon
shell (i.e. Fe
3
C/C) have been developed as an efficient ORR
catalysts, in which catalytic active sites are regarded as the graphitic
12-14
carbon layers activated by encapsulated nanoparticles.
Despite
4
extensive efforts, the ORR performance achieved with single-site
NPMCs is still inferior to that of the state-of-the-art Pt catalyst.
Therefore, the construction of functional hybrid catalysts with two
or more active sites might be more promising to significantly boost
materials. To achieve such desired NPMCs, several aspects should
be optimized simultaneously, calling for: 1) high density of catalytic
the ORR activity. Recently, Wei et al. prepared a hybrid of Fe
3
C and
Fe−N−doped carbon for ORR through a iron–tannin-framework ink
15
coating strategy. Wu et al. fabricated Fe
3
C/Fe−N−doped carbon
1
6
nanofibers showing high ORR activity. However, the synthetic
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routes appear to be tedious, which makes it expensive for mass neutral with water for several times and dried in vacuum at 60 °C
production. Apparently, the exploration of a facile, sustainable and overnight. The productive rate of the catalyst is ~20%. Additional
cost-effective approach to prepare hybrid NPMCs for efficient ORR samples by heating at 700, 750, 850 and 900 °C were also prepared.
is highly desirable for the industrial applications of metal-air 2.3 Catalyst characterization
batteries and fuel cells.
Herein, we report a high-performance ORR electrocatalyst collected on a Rigaku/Max-3A X-ray diffractometer with Cu Kα
composed of Fe C nanoparticles (NPs) embedded in nitrogen-doped radiation (the operation voltage and current was maintained at 40
carbon with Fe-N-enriched active sites (Fe C/Fe−N/C), which is kV and 200 mA, respectively), with a wavelength of 0.1542 nm and
The X-ray powder diffraction (XRD) patterns of the samples were
3
3
synthesized by a simple method using iron–phenanthroline (Fe- analyzed in the range of 10°≤ 2θ≤70°. Raman spectra were taken
Phen) complex and dicyandiamide (DCA) as the precursors that are using a LabRAM HR Evolution Raman microscope with laser
subsequently heat treated. The six-coordinated complex of Fe-Phen excitation at 514.5 nm at room temperature. Field emission scan
is formed with 1,10-phenanthroline (Phen) as bidentate ligand, electron microscopic (FE-SEM) images were recorded on JEOL-JSM-
producing dual active sites (Fe-Nx and Fe
3
C/C). DCA material is 6701F, field-emission scanning electron microscope and operated
kV. Transmission electron
selected due to its high N content (61%), which is favorable for the at an accelerating voltage of
5
enhancement of ORR activity of Fe−N/C catalysts. The advantages microscopic (TEM) images were obtained on a Hitachi-7650
of our synthetic route include: 1) it is a versatile, efficient and microscope at 100 kV. Thermal gravimetric (TG) measurements
controllable avenue since the used precursors are economical and were recorded on a ShimadzuTGA-50 thermal gravimetric analyzer
−1
commercially available; 2) this synthetic approach is readily cost- at a constant heating rate of 5.0 K·min under air atmosphere with
−1
effective and potential to achieve an industrial level production; 3) a gas flow of 25 mL·min . Time of Flight Secondary Ion-Mass
remarkably, the optimal sample pyrolyzed at 800 °C Spectrometer (TOF-SIMS) (Bruker Germany) was employed to get
(
Fe−Phen−N−800) shows a more positive onset potential, better idea about the coordination of the Fe-Nx-C sites in the doped
durability and higher half-wave potential as compared to material. The electrical transport property measurements were
commercial Pt/C catalyst (20 wt% Pt, Johnson Matthey) with the performed on pressed pellets using Keithley 4200-SCS
a
same mass loading in alkaline medium. To the best of our semiconductor characterization system and the four-point probe
knowledge, the ORR activity of our Fe−Phen−N−800 catalyst is method. The X-Ray photoelectron spectroscopy (XPS) was done on
among the best of NPMCs reported to date. Meanwhile, the a Perkin-Elmer RBD upgraded PHI-5000C ESCA system. The nitrogen
Fe−Phen−N−800 sample displays excellent ORR activity in acidic (N
) adsorption and desorption isotherms were measured at 77 K
2
medium. Furthermore, the performance of Zn-air batteries using Micromeritics ASAP-2020.
fabricated with the developed catalyst outperforms the Pt/C based
2
.4 Electrochemical measurements
system. It is anticipated that our novel Fe-based catalyst is a very
promising alternative to Pt/C for practical application in metal-air
batteries.
Electrochemical measurements for the prepared catalysts were
studied on CHI 660E electrochemical workstation using a three-
electrode system with a platinum counter electrode, an Ag/AgCl
4
reference electrode and with 0.1 M KOH or 0.1 M HClO as the
2
. Experimental
electrolyte. To prepare catalyst ink, 2.0 mg of the catalyst was
dispersed in 0.99 mL ethanol, followed by the addition of 10 μL 5%
Nafion solution (D520, Dupont Inc., USA) under ultrasonic
irradiation. Then, 10 μL of the catalyst ink was transferred onto a
glassy carbon (GC) electrode with a diameter of 5 mm (geometric
2.1 Materials
1,10-phenanthroline (Phen), ferrous chloride tetrahydrate
2 2
(FeCl ·4H O) and dicyandiamide (DCA) were purchased from
Sinopharm Chemical Reagent Co. Ltd. These were used as received
without further purification.
2
–2
area: 0.196 cm ), yielding a catalyst loading of 0.1 mg cm . For
comparison, commercial Pt/C catalyst (20 wt%, Johnson Matthey)
was also tested with the same loading. Prior to tests, electrolyte
2.2 Preparation
was saturated with oxygen by bubbling O
maintained over the electrolyte during the measurements. Cyclic
voltammetry (CV) curves of the catalyst under N -saturated alkaline
2 2
and a flow of O was
5
2 2
mmol (0.99 g) Phen and 1.67 mmol (0.33 g) FeCl ·4H O were
dissolved in mixture solutions of deionized water and ethanol under
flow, the solution rapidly became red color, indicating the
N
2
2
electrolyte were also measured. Linear sweep voltammetry (LSV)
measurements of the catalysts were determined using a rotating
disk electrode (RDE) (Pine Research Instrumentation, USA). The
formation of Fe-Phen complex. Then, 4.5 g DCA was added. After
removal of water and ethanol by rotary evaporation under vacuum
(76 Torr) below 60 °C, a red precursor was obtained and dried in
−1
scan rate was 10 mV s and the rotating speed was varied from 400
to 1600 rpm. The number of electrons transferred (n) during ORR
was calculated according to Koutecky-Levich equation, at various
electrode potentials:
vacuo at 60 ºC for 24 h.
The as-obtained precursors were placed in a tube furnace and
heated at 500 °C for 30 min with a ramp rate of 2 °C /min, then 800
°
C for 60 min with the same ramp rate in flowing N
this process, DCA was thermally transformed first to g-C
3
00 °C and then to N-doped carbon species coupled with Fe C
2
atmosphere. In
ꢀ
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
= ꢁ
+ ꢁ = ꢄꢅ
+
ꢆ/ꢇ
(1)
3
N
4
around
ꢁ
ꢃ
ꢂ
ꢃ
5
where J is the measured current density, J
K
L
and J are the kinetic
nanoparticles as the temperature increases. The produced black
powders were leached in 6 M HCl for 24 h at 45 °C to remove
inactive iron species. Finally, the leached samples were washed to
and diffusion-limiting current densities, ω is the angular velocity
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2
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ARTICLE
(
ω=2
calculated as below:
B = 0.62nFC Dꢈ Vꢋꢀ/ꢌ
πN, N is the rotation speed), B is Levich slope which is provide a high density of Fe-Nx active sites in subsequent heat
treatment. DCA was added as both nitrogen and carbon source.
After rotary evaporation at 60 °C, the red precursor of
Fe−Phen/DCA mixture was obtained (Fig. 1d). Finally, by pyrolysis at
ꢉ/ꢊ
(2)
ꢈ
where n is transferred electron number, F is the Faraday constant
8
00 °C, followed by acid leaching, the precursor was transformed to
−
1
−6
(
96485 C mol ), C
0
is the bulk concentration of O
usion coe cient of O
2
((1.2
×
10 mol
dark solid Fe−Phen−N−800 catalyst (Fig. 1e), exhibiting a loose,
crumpled and porous texture (Fig. 1f). The synthesis of this catalyst
can be easily scaled up, because the precursor is commercially
available and cheap (Fig. S1, ESI†).
−
3
−5
cm ), D
0
is the di
ff
ffi
2
in 0.1 M KOH (1.9
×
10
2
−1
2
cm s ), and ν is the kinematic viscosity of the electrolyte (0.01 cm
−
1
s ).
The specific kinetic current density can be calculated from K-L
equation:
ꢁ
ꢂ
∗ꢁ
Jꢍ = ꢁ
(3)
ꢂ
ꢋꢁ
Where J
density; J
K
is the kinetic current density; J is the measured current
is the diffusion-limited current density, respectively.
L
The measured potential against Ag/AgCl was converted vs. the
reversible hydrogen electrode (RHE) using Nernst equation. The
conversion was given as follows:
Eꢎꢏꢐ ꢑ=ꢑEꢒꢓ/ꢒꢓꢔꢕ + 0.059ꢖꢗ + Eꢘ
(4)
ꢒꢓ/ꢒꢓꢔꢕ
The standard potential of Ag/AgCl at 25 °C is 0.197 V, therefore
in 0.1 M KOH solution:
Eꢎꢏꢐ ꢑ=ꢑEꢒꢓ/ꢒꢓꢔꢕ + 0.966ꢑV
while in 0.1 M HClO solution:
Eꢎꢏꢐ ꢑ=ꢑEꢒꢓ/ꢒꢓꢔꢕ + 0.256ꢑV
(5)
4
(6)
Rotating ring-disk electrode (RRDE) tests were measured on CHI Fig. 1 Schematic illustration of the synthetic procedures for catalyst.
60E potentiostat. The electrolytes were 0.1 M KOH or 0.1 M HClO
(a) Preparation of Fe−Phen ink by adding FeCl into Phen solution.
7
4
.
2
−1
The disk electrode was scanned at 10 mV s , and the ring electrode (b) Addition of DCA into the ink solution. (c) Structure of the
potential was held at 1.2 V vs. RHE. The ring current collection Fe−Phen. (d) Photograph of the obtained precursor of
efficiency (N) was determined to be 37%. The hydrogen peroxide
yield (% H ) and number of electrons transferred were calculated
by the followed equations:
ꢑH = 200 ꢙ
Fe−Phen/DCA through rotary evaporation at 60 °C. (e) Photograph
and f) corresponding SEM image of the as-prepared
Fe−Phen−N−800 catalyst.
2
O
2
ꢙ
ꢚ
⁄ꢛ
%
O
(7)
(8)
ꢉ
ꢉ
ꢝꢙ
ꢚ
⁄ꢛ
ꢜ
Fig. 2a shows X-ray diffraction (XRD) pattern of as made
ꢙ
ꢜ
⁄ꢛ
n = 4ꢑ ꢙ
Fe−Phen−N−800 sample. The diffraction peaks at 37.8, 43.7, 45.0,
ꢜ
ꢝꢙ
ꢚ
⁄ꢛ
4
3
8.7, and 51.9° can be indexed to the phase of Fe C (JCPDS No 89-
D R
where I and I are the disk and ring currents, respectively.
Primary Zn-air batteries were performed on a home-built
electrochemical cell. For catalyst ink preparation, 2.0 mg of the
catalyst was ultrasonically dispersed in 1.0 mL of mixture of ethanol
(
(
0.98 mL) and 5% Nafion solution (20 μL). The homogeneous ink
2
0.25 mL) was loaded on carbon fibre paper (1.0 cm ), catalyst with
2
catalyst loadings of 0.5 mg cm , as the air cathode, and Zn foil was
used as the anode in 6 M KOH solution. All data were collected from
the as-fabricated cell with a CHI 660E electrochemical workstation
at room temperature.
3
. Results and discussion
The synthetic process of the Fe−Phen−N samples is illustrated in Fig.
. Ferrous chloride (FeCl ) was added into a mixture solution of
1
2
water and ethanol containing Phen molecules (Fig. 1a). The
coordination complex ink of Fe−Phen was rapidly formed at room
temperature due to the strong chelation between Fe (II) and Phen
ligand (Fig. 1b). The structure of the obtained Fe-phen complex is Fig. 2 (a) XRD pattern, (b) XPS survey spectrum, high-resolution (c)
displayed in Fig. 1c, in which the Fe (II) ion is confined in the center. N1s and (d) Fe2p XPS spectra of the obtained Fe−Phen−N−800
It is noted that Fe-N sextuple bonds linking three Phen moieties catalyst.
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1
7
2
867). A typical strong peak at around 26° is observed, which can efficiently protect Fe C NPs from gradually etching under harsh
3
1
8
22
corresponds to the (002) planes of graphitic carbon. No peaks ORR conditions. Moreover, the coated Fe C NPs could activate the
3
attributable to metallic iron were detected, suggesting acid washing graphitic carbon layer and trigger surrounding carbon active for
2
3
removes inactive and unstable iron species, while XRD pattern of ORR. These activated graphitic layers could further increase the
Fe−Phen−N−800 sample prior to acid washing exhibits the catalytic activity when combined with Fe-N species. HRTEM image
(
inset in Fig. 3c
to the (211) plane of Fe
scanning TEM (HAADF-STEM) image further reveals Fe
) shows a lattice spacing of 0.208 nm, corresponding
existence of metallic iron (Fig. S2, ESI
spectroscopy (XPS) measurement and thermal gravimetric (TG)
analyse (Fig. S3, ESI ) were carried out to probe the chemical
compositions and bonding configurations of and Fe in
†). The X-ray photoelectron
2
4
3
C crystal. High-angle annular dark-field
C NPs
3
†
decorated structure (Fig. 3d). The corresponding elemental
mapping shows the homogeneous distribution of C, N and O atoms
in the Fe−Phen−N−800 catalyst. The very strong Fe signal is
observed in the regions of NPs, suggesting that bright dots consist
N
Fe−Phen−N−800 sample. The XPS survey spectrum shown in Fig. 2b
clearly demonstrates the presence of four kinds of elements, C
(
89.32%), N (6.25%), O (3.65%) and Fe (0.78%). The occurrence of
of Fe C. Simultaneously, the relative weak Fe signal is found in the
3
oxygen peak presumably originates from physically adsorbed
oxygen. The N 1s XPS spectrum (Fig. 2c) reveals that the peaks at
N-doped carbon matrix, and the Fe and N signals highly overlap
with each other, indicating that some portion of Fe species complex
with doped N, which is believed to be the Fe-Nx active sites. A
further investigation of Fe-Nx active sites was carried out using
3
2
98.4, 399.6 and 401.1 eV can be assigned to pyridinic N (N1,
5.72%), pyrrolic N (N2, 30.05%) and graphitic N (N3, 44.23%),
time-of-flight secondary ion mass spectrometry (TOF-SIMS). From
+
TOF-SIMS spectrum (Fig. S4, ESI
†
), a series of FeNxCy species was
+
+
+
observed, including FeN (m/z = 59.9), FeNC (m/z = 81.9), FeN
2
+
+
(
m/z = 83.9), FeN3C2 (m/z = 121.9), FeN4C3 (m/z = 147.9),
suggesting the existence of Fe-Nx sites. The surface areas and pore
structure of the Fe−Phen−N−800 sample were characterized by N2
adsorption–desorption measurement. A type IV isotherm profile
with a hysteresis is clearly observed at a relatively higher N2
pressure (P/P = 0.5-1.0), typical of mesoporous materials (Fig. S5,
0
2
−1
ESI†
). The specific surface area of Fe−Phen−N−800 is 269 m g and
pore size distribution centers at 5 nm. High surface area and
nanoporous structure endows the Fe−Phen−N−800 as an ideal ORR
catalyst, because high surface area can allow active sites better
accessible, and abundant porosity is beneficial for fast mass
2
5
transport of reactants to active centers, thus enhancing the ORR
activity.
Fig. 3 (a, b) TEM images of the Fe−Phen−N−800 catalyst with
different magnifications. (c) HRTEM image of Fe C nanoparticles in
3
the Fe−Phen−N−800 catalyst; the down-left inset is an enlarged
HRTEM image. (d) HAADF-STEM image of the Fe−Phen−N−800
catalyst and corresponding elemental mappings of the area
indicated by the white dashed frame in (d).
1
9
respectively. The N1 and N2 atoms can coordinate with Fe species
2
0
due to their lone-pair electrons. The high contents of these two
types of N1 and N2 are favorable for the formation of Fe-Nx active
sites. The Fe2p spectrum could be deconvoluted into four peaks (Fig.
2
d). The peaks at 710.3 and 714.7 eV are attributed to the binding
2
+
3+
energies of the 2p3/2 bands of Fe and Fe species, respectively.
2
+
3+
The peaks for 2p1/2 orbitals of Fe and Fe are observed at 723.3
2
1
and 727.1 eV.
The detailed structure and morphology of as-prepared
Fe−Phen−N−800 catalyst was investigated by transmission electron
microscopy (TEM) and high-resolution TEM (HRTEM). As shown in
3
Fig. 3a, the active catalyst is composed of many solid Fe C NPs with
the size of 5-20 nm, which are randomly distributed in the carbon
matrix. The enlarged TEM image reveals that the sample has a
unique nanoporous structure (Fig. 3b). Some of the irregular pores
might be the vacancies that formed after the removal of unstable
metallic Fe species by acid treatment. Fig. 3c exhibits that Fe C NPs
3
are encapsulated by graphitic carbon layers. These graphite shells
4
| J. Name., 2012, 00, 1-3
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−
2
The limiting current density (j) of Fe−Phen−N−800 is 5.7 mA cm .
These results clearly prove Fe−Phen−N−800 is a highly efficient ORR
electrocatalyst in 0.1 M KOH. The kinetic current density normalized
to geometrical electrode surface area (j
k s
, ) of the catalyst at 0.85 V
vs. RHE was also examined. The inset in Fig. 4b shows that
−2
Fe−Phen−N−800 affords a j
, value of 7.5 mA cm , which is 2.5
k s
−2
times higher than that of Pt/C (3 mA cm ). This can be attributed to
the unique nanoporous structure that favors the quick diffusion of
oxygen species and electrolyte during the ORR process. The kinetic
reaction pathway for electrocatalytic reduction of oxygen in alkaline
was assessed by using the Koutecky–Levich (K–L) equation.
According to previous studies, the ORR process proceeds either by
two-step two-electron (2e) pathway, in which peroxide species is
produced as an intermediate species, or by a more efficient direct
four-electron (4e) pathway, in which O is converted into H O as
2
2
2
7
−
only byproduct. Generally, the 4e pathway is believed to be more
−
efficient than the 2e pathway, additionally, the peroxide species
−
produced via the 2e pathway may cause the degradation of the
catalyst. The K-L plots, derived from the RDE curves of
Fe−Phen−N−800 catalyst at various rotation speeds from 400 to
1
600 rpm (Fig. 4c), display good linearity (inset in Fig. 4c),
suggesting first-order reaction kinetics with respect to the dissolved
O concentration. The average value of electron transfer number (n)
2
was calculated to be 4.05 at the potential ranging from 0.40 to 0.60
V, indicating that the ORR over Fe−Phen−N−800 undergoes direct
2
Fig. 4 (a) CV curves of Fe−Phen−N−800 and Pt/C catalysts in N -
saturated and
O
2
-saturated 0.1
M
KOH solutions. (b) RDE four-electron (4e) reduction process. To further validate this
conclusion, rotating ring-disk electrode (RRDE) measurements were
polarization curves of Fe−Phen−N−800 and Pt/C catalysts in O
2
2
-
−
−
saturated 0.1 M KOH solution (the catalyst loading is 0.1 mg cm ). performed to determine the yield of peroxide species and 4e
Inset: comparison of the kinetic current density normalized to selectivity of Fe−Phen−N−800 catalyst. As shown in Fig. 4d, the
electrode surface area of the catalysts measured at 0.85 V (vs. RHE). peroxide yield (%) for Fe−Phen−N−800 catalyst is less than 5% over
(
c) RDE polarization curves for Fe−Phen−N−800 at various rotation the whole measured potential range and even lower than that of
rates. The inset is the corresponding K-L plots at different Pt/C in the potential range from 0.6 V to 0.85 V vs. RHE. The
potentials. (d) Peroxide yield and the electron-transfer number (n) average n is 3.97, which is in line with the results based on the K-L
of Fe−Phen−N−800 and Pt/C catalysts in O
2
-saturated 0.1 M KOH. plots, this proves that Fe−Phen−N−800 promotes the four-electron
e) Durability test of Fe−Phen−N−800 sample for 10000 cycles in O
reduction process of the oxygen reduction. The long-term stability
KOH solution. (f) Comparison of the of Fe−Phen−N−800 catalyst was evaluated in O -saturated 0.1 M
(
2
-
saturated 0.1
M
2
chronoamperometric responses of Fe−Phen−N−800 and Pt/C KOH by cycling potential between 1.1 V and 0.1 V vs. RHE at a scan
−
1
catalysts upon the addition of 3 M methanol in O
KOH solutions at 0.6 V vs. RHE.
2
-saturated 0.1 M rate of 100 mV s . The RDE curves recorded before and after 10000
cycles exhibit that the E1/2 drop for Fe−Phen−N−800 is only 13 mV
(
Fig. 4e), whereas Pt/C suffers a decrease of 75 mV for the E1/2 (Fig.
To evaluate the ORR activity of the catalysts, cyclic voltammetry
CV) and rotating disk electrode (RDE) measurements were
S6, ESI ), indicating that its durability is superior to Pt/C. The
†
(
obvious negative shift of E1/2 for Pt/C catalyst could result from the
performed for the Fe−Phen−N−800 catalyst and commercial Pt/C as
dissolution, migration and aggregation of Pt NPs during the
reference at the same mass loading. As shown in Fig. 4a, there is no
28
potential cycling. Unlike Pt/C, a small decay of performance for
cathodic peak observed for Fe−Phen−N−800 in N
KOH solution, only quasi-rectangular double-layer charging is
-saturated 0.1 M
2
Fe−Phen−N−800 is mainly ascribed to degradation of supporting
29
carbon and oxidation of active sites. The less prominent
2
6
2
recorded. In sharp contrast, under O -saturated condition, a well-
performance decay of the Fe−Phen−N−800 proves the highly stable
coordination of Fe-N with carbon moiety, which was already
suggested by its resistance to acid leaching. Chronoamperometric
defined peak at potential of around 0.87 V vs. reversible hydrogen
electrode (RHE) is clearly found for Fe−Phen−N−800 sample, which
is ∼10 mV larger than that of Pt/C catalyst (0.86 V vs. RHE), implying
its superior ORR electrocatalytic activity. To study the mass-transfer
(
CA) test was performed to probe the possible crossover effect
caused by small organic molecules (e.g. methanol). As shown in Fig.
f, there is no noticeable current density change for
kinetics of the ORR activity of the electrode, the RDE measurements
4
−
1
at 1600 rpm and 10 mV s were conducted. As shown in Fig. 4b,
the RDE polarization curve of Fe−Phen−N−800 displays a high onset
Fe−Phen−N−800, while a sharp decrease in current density is
observed for the Pt/C catalyst after the addition of 3 M methanol,
indicating that our Fe−Phen−N−800 catalyst exhibits high selectivity
for the ORR with excellent resistance to the methanol crossover.
(
Eonset: 0.99 V vs. RHE) and half-wave potentials (E1/2: 0.86 V vs.
RHE), which is 10 mV and 20 mV higher than that of Pt/C, and also
higher than of most NPMCs reported in literature (Table S1,ESI
†).
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J. Name., 2013, 00, 1-3 | 5
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ARTICLE
Journal Name
results indicate that the activity of our catalyst is higher than or
comparable to recently reported catalysts (Table S2, ESI ). The
value of j
for Fe−Phen−N−800 at potential of 0.65 V is 11.8 mA
†
k s
,
−2
−2
cm (inset in Fig. 5b), larger than that of Pt/C (10.6 mA cm ). RDE
polarization curves of Fe−Phen−N−800 at rotation speeds from 400
to 1600 rpm are presented in Fig. 5c. The corresponding K-L plots
−
1
(
inset in Fig. 5c) exhibit a good linear relationship between j and
−1/2
ω
. The average value of n is 3.98. As a result, the ORR over
Fe−Phen−N−800 catalyst also follows a dominant four-electron
pathway in 0.1 M HClO solution. Fig. 5d provides a comparison of
4
the peroxide yield and n value for the Fe−Phen−N−800 and Pt/C
catalysts in acidic condition, it can be seen that the peroxide yield is
less than 2% for our catalyst in the whole range of potentials. The
value of n for Fe−Phen−N−800 is nearly unchanged (4.0) in the
measured potential range from 0.06 V to 0.6 V, and the average n
obtained from the curve is about 3.99. These results further verify
the exclusive involvement of the preferred 4e transfer pathway
followed by Fe−Phen−N−800 material. As shown in Fig. 5e, the
durability test result reveals that the negative shift of E1/2 for
Fe−Phen−N−800 is 19 mV after 10000 cycles under acidic condition,
much less than 61 mV negative shift of E1/2 for Pt/C catalyst (Fig. S7,
ESI†
). The CA measurements exhibit that Fe−Phen−N−800 has
outstanding immunity towards methanol crossover effect (Fig. 5f).
Fig. 5 (a) CV curves of Fe−Phen−N−800 and Pt/C catalysts in O2-
To investigate the influence of the pyrolysis temperature on the
activity of Fe−Phen−N catalysts, the precursors were annealed at
various temperatures (700- 900 °C), The Raman spectra (Fig. S8,
saturated and N -saturated 0.1 M HClO media. (b) RDE polarization
2
4
curves of Fe−Phen−N−800 and Pt/C catalysts in O -saturated 0.1 M
2
−2
HClO4 solution (the catalyst loading is 0.1 mg cm ), Inset:
comparison of the kinetic current density normalized to electrode
surface area of the catalysts measured at 0.65 V vs. RHE. (c) RDE
polarization curves for Fe−Phen−N−800 at various rotation rates.
The inset is the corresponding K-L plots at different potentials. (d)
Peroxide yield and the electron-transfer number of Fe−Phen−N−800
ESI
†
) of Fe−Phen−N catalysts reveal that the ratio of integrated
intensity (I /I ) for the D band to G band was determined to be 1.96
D
G
(
700 °C), 1.58 (750 °C), 1.45 (800 °C), 1.39 (850 °C), and 1.36 (900
C), respectively. Since the value of I /I reflects the ratio of
°
D
G
3
2
disordered carbon and the graphitic carbon, the gradually
decreasing values of I /I imply that the degree of graphitization is
D
G
and Pt/C catalyst in O -saturated 0.1 M HClO . (e) Durability test of
2
4
enhanced with increasing pyrolysis temperature, which is further
proved by XRD patterns, as shown in Fig. S9, the peak centered at
around 26° becomes sharper and narrower when the pyrolysis
temperature increases from 700 to 900 °C, indicating the formation
Fe−Phen−N−800 catalyst for 10000 cycles in O -saturated 0.1 M
2
HClO4 solution. (f) Comparison of the CA responses of
Fe−Phen−N−800 and Pt/C catalysts upon the addition of 3 M
33
methanol in O -saturated 0.1 M HClO solutions at 0.8 V vs. RHE.
2
4
of more ordered graphitic structure domains, which is beneficial
for improving the electrical conductivity of the carbon support. The
electronic conductivity for the five powder samples using four point
probe method further reveals that the electrical conductivity
In addition, the obtained Fe−Phen−N−800 catalyst exhibits high
ORR activity in acidic condition. The CV curve in Fig. 5a presents a
pair of symmetric peaks at potential of about 750 mV in N
2
-
increases with the elevating pyrolysis temperature (Fig. S10, ESI†).
saturated 0.1 M HClO electrolyte for Fe−Phen−N−800. These peaks
4
The elemental compositions of Fe−Phen−N catalysts based on XPS
analysis are provided in Fig. S11. Both the contents of N and Fe are
reduced, while the content of C increases from 75.53 to 93.58 at%
are expected for a reversible redox reaction process involving Fe(II)
and Fe(III). Besides, a clearly oxidation peak appears at 790 mV in
O
2
-saturated condition for Fe−Phen−N−800, which implying iron
with increasing pyrolysis temperature (Table S3, ESI†). Fig. S12
reveals the normalized ratios of three different types of N species,
namely N1, N2 and N3, obtained from the deconvolution of high-
incorporated into the carbon matrix may serve as a mediator in the
3
0,31
electron-transfer process during the ORR.
peak potential for Fe−Phen−N−800 is 0.71 V vs. RHE, which is only
0 mV negative as compared to that of Pt/C catalyst (0.72 V vs.
RHE) in 0.1 M HClO , suggesting the excellent electrochemical
The oxygen reduction
resolution N1s spectra of Fe−Phen−N samples (Fig. S13, ESI†). It is
1
found that the content of N2 is declined, while that of N3
significantly increases with the elevated pyrolysis temperature. For
the N1 species, the decrease of N1 content is also observed, but
this tendency is not obvious at higher temperature. According to
4
activity of Fe−Phen−N−800 catalyst towards ORR. The RDE
polarization curves shown in Fig. 5b display that the values of the
E
onset and E1/2 over Fe−Phen−N−800 active material are only 38 mV
previous report, both N1 and N3 species play an important role in
and 59 mV lower than those of Pt/C catalyst in acidic medium. The
34,35
electroreduction of oxygen.
Besides, N1 can act as metal-
value of j measured for Fe−Phen−N−800 significantly increases
coordination sites because of the lone-pair electrons. RDE
polarization curves shown in Fig. S14 and S15 confirm that
(
20%) in comparison with Pt/C under the same mass loading. These
6
| J. Name., 2012, 00, 1-3
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Jou Pr nl e aa ls oe fd Mo an to et r ai ad lj su s Ct hm ea mr g ii sn ts ry A
View Article Online
DOI: 10.1039/C6TA08050B
Journal Name
ARTICLE
−1
–
Fe−Phen−N−800 sample has the best catalytic performance towards specific capacity of 708 mAh g at the current density of 10 mA cm
2
−1
ORR under both alkaline and acidic conditions. These results imply
, corresponding to the gravimetric energy density of ~892 Wh kg ,
that the optimal pyrolysis temperature is 800 °C. At this is achieved from Fe−Phen−N−800-based battery (Fig. 6c). When the
–2
temperature, the factors affecting the ORR activity, including the current density is increased to 25 mA cm , the specific capacity of
−
1
contents of N and Fe, electrical conductivity of carbon matrix and the battery is 662 mAh g , corresponding to the gravimetric energy
−1
numbers of active sites, can reach a near ideal compromise. On the density of ∼801 Wh kg . Notably, this energy density is higher than
−1
–2
−1
–2
other hand, to check whether acid treatment has any influence on 851 Wh kg at 10 mA cm and 752 Wh kg at 25 mA cm
the catalytic activity of Fe−Phen−N−800. A control catalyst was generated from Zn–air batteries with Pt/C catalyst. Such
prepared under the same conditions as Fe−Phen−N−800, but exceptional performance of Zn-air batteries is ascribed to the
without the final acid etching. The RDE polarization curves shown in superior activity of our Fe−Phen−N−800 catalyst. With respect to
Fig. S16 display that this sample exhibits an inferior ORR activity as the previously reported NPMCs, the Zn-air battery made by
compared to Fe−Phen−N−800 sample by acid etching. Fe−Phen−N−800-based catalyst presents significantly improved
Consequently, the metallic iron might not be active species for ORR. performance (see comparison in Table S4, ESI†). Most importantly,
On the contrary, it might block the accessible active sites. Before it is found that, after complete discharging, the battery could be
acid etching, the BET surface area of Fe−Phen−N−800 catalyst recovered by periodically refueling Zn foil and electrolyte without
2
−1
before acidic etching is 153 m g (Fig. S17, ESI†), smaller than that obvious decay of voltage for 5 cycles at a current density of 10 mA
2
−1
–2
of the catalyst Fe−Phen−N−800 after acid etching (269 m g ). The cm (Fig. 6d), thus confirming again the remarkable stability of the
higher surface area might be a reason that the acid-treated sample Fe−Phen−N−800 catalyst.
exhibits a much higher activity in ORR.
Finally, the practical applicability of the Fe−Phen−N−800 catalyst
Conclusions
was assessed by using Fe−Phen−N−800 as cathode catalyst in
primary Zn-air battery, commercial 20% Pt/C was also employed for
fabricating the air electrode for comparison. As shown in Fig. S18,
the as-constructed Zn-air cells display an open circuit voltage (OCV)
In summary, a cost-effective and simple way for the fabrication of
3
Fe C/Fe−N−carbon composites towards excellent ORR is
demonstrated using iron–phenanthroline coordination compound
and dicyandiamide as the precursors. It is found that the pyrolysis
temperature has a significant influence on the ORR activity of the
as-prepared catalysts. The optimal catalyst pyrolyzed at 800 ºC
(
Fe−Phen−N−800) exhibits a remarkable (superior to commercial
Pt/C) performance under alkaline condition and an excellent ORR
activity comparable to that of Pt/C under acidic solution. The ORR
over the Fe−Phen−N−800 catalyst follows the effective four-
electron-transfer route and the peroxide yield is lower than 5% in
4
both KOH and HClO media. Moreover, the Fe−Phen−N−800 catalyst
exhibits a better stability and tolerance to methanol crossover than
Pt/C catalyst. The practical applicability of our catalyst in energy
conversion devices has been demonstrated by a primary Zn-air
battery with a performance outperforming Pt/C based Zn-air
battery. We anticipate our catalyst could serve as a potential
candidate for renewable energy conversion systems, especially for
metal–air batteries and proton exchange membrane fuel cells.
Fig. 6 The galvanostatic discharge curves of Zn–air batteries using
Fe−Phen−N−800 and Pt/C as air cathode catalysts at a current
–2
–2
Acknowledgements
density of (a) 10 mA cm and (b) 25 mA cm . (c) Specific capacities
of primary Zn–air batteries with Fe−Phen−N−800 and Pt/C as The special funding support from the National Basic Research
cathode catalysts, which are normalized to the mass of completely Program of China (2011CB933700), the National Natural
consumed Zn. (d) Endurance test of the Zn–air battery with Science Foundation of China (51572253, 21271165), Scientific
–
2
Fe−Phen−N−800 catalyst at a current density of 10 mA cm . The Research Grant of Hefei Science Center of CAS (2015SRG-
battery was recharged by replenishing the Zn anode and the HSC048), and cooperation between NSFC and Netherlands
electrolyte for 5 cycles. The arrows indicate the recharge process.
Organisation for Scientific Research (51561135011) is
acknowledged.
−2
of 1.55 V by using Fe−Phen−N−800 (0.5 mg cm ) loaded carbon
paper as air cathode, which is higher than 1.52 V attained by Pt/C
catalyst. The galvanostatic discharge at current densities of 10 and
Notes and references
−
2
2
5 mA cm reveals that the voltage delivered by the Zn-air
1
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batteries from Fe−Phen−N−800 catalyst is 1.26 V and 1.21 V,
Chem. Rev., 2015, 115, 3433; (b) D. Wang, H. L. Xin, R.
respectively (Fig. 6a and 6b), which is higher than the voltage of
1
.24 V and 1.18 V derived from Pt/C-based Zn-air battery. The
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA08050B
ARTICLE
Journal Name
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA08050B
The iron carbide-embedded Fe-N-doped carbon catalyst with high-performance for oxygen reduction in Zn-air
battery was prepared through a facile and effective strategy.