Covalent Phenanthroline Framework Derived FeS@Fe3C Composite Nanoparticles Embedding in N-S-Codoped Carbons as Highly Efficient Trifunctional Electrocatalysts

Article abstract of DOI:10.1002/adfm.201803973

Efficient and earth-abundant materials with multifunctional electrocatalytic properties within a wide range of pH are the new darlings for developing green energy conversion and storage techniques. A novel porous covalent phenanthroline framework (Fe-Phen-COFs) that involved Fe-DMSO (dimethyl sulfoxide) coordination complexes is successfully synthesized using 3, 8-dibromophenanthroline and 1, 3, 5-benzenetriboronicacid trivalent alcohol ester as a rigid building block via Suzuki coupling reaction. Fe-Phen-COFs as the self-carrier enriched with Fe, S, N, and C is pyrolyzed to produce N-S-codoping carbons with embedded core–shell Fe₃C and FeS composite nanostructures (FeS/Fe₃C@N-S-C). The FeS/Fe₃C@N-S-C-800 obtained by pyrolysis at 800 °C exhibits efficient trifunctional electrocatalytic activity for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) within a wide pH range. Impressively, the ORR half-potential of FeS/Fe₃C@N-S-C-800 reaches 0.87 V in 0.1 m KOH, more positive than the previously reported Pt-free electrocatalysts. It could be utilized as the advanced air electrode materials in zinc–air batteries, which exhibit an excellent power density and cycling stability superior to those of Pt/C-based zinc–air battery. Thermal conversion of novel Fe-Phen-COFs provides an effective strategy to prepare high-performance trifunctional electrocatalytic materials for the new-generation powerful energy conversion technologies.

Full text of DOI:10.1002/adfm.201803973

FULL PAPER
Electrocatalysts
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Covalent Phenanthroline Framework Derived FeS@Fe3C
Composite Nanoparticles Embedding in N-S-Codoped
Carbons as Highly Efficient Trifunctional Electrocatalysts
Fantao Kong, Xiaohong Fan, Aiguo Kong,* Ziqian Zhou, Xiaoying Zhang, and Yongkui Shan*
efficiently catalyze these electrochemical
Efficient and earth-abundant materials with multifunctional electrocatalytic
properties within a wide range of pH are the new darlings for developing
green energy conversion and storage techniques. A novel porous covalent
phenanthroline framework (Fe-Phen-COFs) that involved Fe-DMSO (dimethyl
sulfoxide) coordination complexes is successfully synthesized using 3,
processes, for completely eliminating
Pt-based and Ir/Ru-based noble metal
catalysts with limiting resources and
[
7–11]
time-drifting catalytic efficiency.
For
minimizing the cost of the electrode cata-
lytic layers, it is undoubtedly the most
promising that a trifunctional electro-
catalyst can high-efficiently catalyze the
three electrocatalytic processes in one
electrolyte. In the past decades, some
carbon-based NPMC electrocatalysts
with competitive catalytic performance
have been reported by optimizing the
morphologies and local structures of
8-dibromophenanthroline and 1, 3, 5-benzenetriboronicacid trivalent alcohol
ester as a rigid building block via Suzuki coupling reaction. Fe-Phen-COFs
as the self-carrier enriched with Fe, S, N, and C is pyrolyzed to produce
N-S-codoping carbons with embedded core–shell Fe C and FeS composite
3
nanostructures (FeS/Fe C@N-S-C). The FeS/Fe C@N-S-C-800 obtained
3
3
by pyrolysis at 800 °C exhibits efficient trifunctional electrocatalytic activity
for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and
hydrogen evolution reaction (HER) within a wide pH range. Impressively, the
[
12–16]
catalytic materials.
of electrocatalysts were only efficient for
However, most
ORR half-potential of FeS/Fe C@N-S-C-800 reaches 0.87 V in 0.1 m KOH, more
3
[
17,18]
a single electrochemical process.
The
positive than the previously reported Pt-free electrocatalysts. It could be utilized
as the advanced air electrode materials in zinc–air batteries, which exhibit an
excellent power density and cycling stability superior to those of Pt/C-based
zinc–air battery. Thermal conversion of novel Fe-Phen-COFs provides an
effective strategy to prepare high-performance trifunctional electrocatalytic
materials for the new-generation powerful energy conversion technologies.
active ORR electrocatalyst often exhibited
poor OER or HER performance and vice
[19,20]
versa.
It is still a challenge to develop
high-performance and low-cost trifunc-
[
17,21,22]
tional NPMCs.
A vast range of carbon-supported non-
precious metal-based materials, including
metal, alloy, metal oxides, metal chalco-
genides, metal carbides, metal nitrides,
1
. Introduction
nonprecious metal-based N-doped carbon (M-N-C), and heter-
oatom-doped carbons have been investigated as advanced elec-
[
11,23–30]
Electrical splitting water into hydrogen and oxygen and recov-
ering them in fuel cells could serve as a green and renewable
energy conversion and storage technology.[1–6] The high-rate
oxygen reduction or evolution reaction (ORR or OER) and
hydrogen evolution reaction (HER) at lower overpotentials
are of great importance to the enhancement of energy utili-
zation rate and output power in these green energy systems.
Currently, tremendous attentions have been paid on exploring
low-cost nonprecious metal electrocatalysts (NPMCs) to
trocatalytic materials for HER, OER, and ORR
Among
them, carbon-coupled Fe C materials with the optimizing
3
morphologies including core–shell structures, nanofibers,
nanospheres and nanoribbons were demonstrated to be higher
[
31–40]
active for ORR, even in acidic solution.
sition metal sulfides such as CoSx
Nevertheless, tran-
[
41–46]
[47–50]
and NiSx
exhib-
ited more efficient catalytic performance for HER and OER.
It should be a promising pathway to create the enhancing
multifunctional electrocatalytic materials for these electrode
reactions by coupling different functionality of several metal
[
51–53]
compounds in one composite material.
However, it does
Dr. F. T. Kong, X. H. Fan, Prof. A. G. Kong, Z. Q. Zhou,
X. Y. Zhang, Prof. Y. K. Shan
School of Chemistry and Molecular Engineering
East China Normal University
Shanghai 200241, P. R. China
E-mail: agkong@chem.ecnu.edu.cn; ykshan@chem.ecnu.edu.cn
not mean that high-efficiency multifunctional eletrocatalysts
could be obtained by simply mixing different components on
carbon supports. Hence, the challenge in this route is how
to realize the synergistically enhancing of several catalytic
activities by controlling the formation and homogeneous
distribution of different components in carbons and synchro-
nously optimizing the local structures of composites including
porosity and conductivity.
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201803973.
DOI: 10.1002/adfm.201803973
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Figure 1. Preparation schematic and structural confirmation of Phen-COFs and Fe-Phen-COFs. A) Schematic representation for the synthesis of Fe-
1
3
Phen-COFs and the derived FeS/Fe C@N-S-C electrocatalysts. B) C CP-MAS solid-state NMR spectrum of Phen-COFs. The inset image is chemical
3
formula of Phen-COFs structural unit. C) FT-IR spectra of a) 3, 8-DBPT, b) Phen-COFs, c) Fe-Phen-COFs and d) FeS/Fe C@N-S-C-800. D) The UV–vis
3
diffuse reflectance spectra of b) Phen-COFs and c) Fe-Phen-COFs.
Covalent organic frameworks (COFs) with adjustable 2. Results and Discussion
microcosmic structure and ordered porosity in molecule level
can endow the derived carbon materials with target high-
2.1. Confirmation of Phen-COFs and Fe-Phen-COFs
[
54–57]
efficiency electrocatalytic performance.
The pyrolysis
of COFs could serve as an effective method in precisely con-
trolling the type and distribution of dopants or composites
on porous carbons.[ The thermal conversion of COFs with
incorporated porphyrin macrocycles produced more efficient
metal-contained ORR electrocatalysts with high-turnover
The new Phen-COF structure was synthesized by Suzuki cou-
pling reaction using 3, 8-DBPL and BTA as the rigid building
block (Figure 1A). The chemical structure of Phen-COFs was
confirmed by the solid state 13C cross-polarization magic angle
spinning (CP-MAS) NMR spectrum (Figure 1B). The resonance
signal near 128.7 ppm was caused by the coupling benzene
rings, indicating that the 3, 8-DBPL blocks were successfully
coupled to BTA by polymerization. In addition, the peaks
at 141.1 and 149.9 ppm were attributed to the carbon atoms in
the pyridine ring, confirming the presence of the phenanthroline
58]
[
59]
metal-N sites. However, COFs-derived metal-based carbon
4
nanocomposites with high-efficient activities for three reac-
tions are still scarce. Herein, we used 3, 8-dibromophen-
anthroline (3, 8-DBPL) and 1, 3, 5-benzenetriboronic acid
pinacol ester (BTA) as building blocks to synthesize porous
COFs with open metal-chelating sites. Fe and the solvent
molecules (DMSO) can easily absorbed into such COFs to
create FeNC and FeOS moieties as the precursors of
Fe C and FeS, which were converted into FeS@Fe C nano-
[
60,61]
structure in the prepared Phen-COFs.
Furthermore, the
Fourier transform Infrared (FT-IR) spectra (Figure 1C) of Phen-
COFs was similar to that of 3, 8-DBPL, but the CBr stretching
−
1
vibrational absorption peak at 532 cm of 3, 8-DBPL molecule
disappeared after the formation of polymer Phen-COFs, indi-
cating that the CBr bond broken and coupling polymerization
3
3
particles composite embedding in N-S-codoped carbons
FeS/Fe C@N-S-C) by heat treatment. The prepared FeS/
(
3
[
57]
Fe C@N-S-C composites exhibit high-efficient trifunctional
3
occurred.
electrocatalytic performance for HER, ORR, and OER in the
wide pH range.
The synthesized Fe-Phen-COF networks were obtained
from the reflux coordination of Phen-COFs and Fe(Ac)2 in
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Figure 2. A) The high-resolution N 1s XPS spectrum, B) survey XPS spectra, C) The deconvoluted S 2p XPS spectrum, and D) Fe 2p XPS spectra of
a) Fe-Phen-COFs and b) FeS/Fe C@N-S-C-800. E) XRD patterns and F) Raman spectra of a) FeS/Fe C@N-S-C-700, b) FeS/Fe C@N-S-C-800, c) FeS/
3
3
3
Fe C@N-S-C-900.
3
DMSO by solvothermal reaction. The IR absorption band
2.2. Preparation, Composition, and Morphology of FeS/
Fe C@N-S-C Materials
3
−
1
−1
−1
at 3030 cm (CH), 1585 cm (CN), and 725 cm (ring
deformation) of pyridinic rings with similar structures
[
62]
of Phen-COFs disappeared in the Fe-Phen-COFs.
The
Owing to the fact that developing the efficient and stable
NPMCs for ORR, OER, and HER is of great interest in the
commercialization of the high-performance sustainable energy
devices on a mass scale, a large variety of the synthesis methods
were reported. In general, the different catalysts were prepared
by the different methods due to the high dependence of the
morphology of the catalysts on the preparation conditions and
the difference between the chemical properties of different
component elements. However, the normal pyrolysis of simple
macrocycles or mixtures of nitrogen and metal precursors only
leads to uncontrolled agglomeration or inhomogeneous micro-
structure and thus insufficient exposure of the active sites
infrared characteristic peak of FeN was newly observed
−
1
at 995 cm . The solid-state UV–visible diffuse reflectance
spectra (Figure 1D) of Fe-Phen-COFs showed the absorp-
tion peak (375 nm), which was from redshift of absorption
signals of Phen-COFs (344 nm). It suggested that N-coordi-
nated Fe(II) ions affected the conjugated system of phenan-
[
63]
throline structure. The high-resolution N 1s XPS spectrum
(
3
Figure 2A-a) of Fe-Phen-COFs revealed the N1s peak around
99.8 eV, which could be attributed to FeN moieties.
[
64]
These results testified that Fe(II) ions were coordinated to
N-chelate sites in Phen-COFs.
[
66,67]
Interestingly, the IR spectra of Fe-Phen-COFs revealed the
and poor mass transport.
In addition, MOFs are optimal
IR adsorption peak at 946 cm− and 475 cm , corresponding
1
−1
precursors that contain carbon and metal and/or nitrogen pre-
cursors together into one framework for the preparation of
the nonprecious metal electrocatalysts. Nevertheless, directly
carbonized MOFs often lead to metal aggregation and structure
breakdown, thus reducing the pore size and surface area. Com-
pared with the traditional synthesis of the nonprecious metal
electrocatalysts, the catalyst prepared by us possesses three
distinctive features. First, the prepared neo-precursor with Fe-
Phen and -DMSO co-chelating sites simultaneously contain
carbon, nitrogen, iron, and sulfur sources. In the pyrolysis
process, unstable FeN and FeOS moieties undergo local
decomposition to form small-sized nanoclusters consisting of
crystalline iron carbides and iron sulfides and phenanthroline
moiety was carbonized to form the N-doped carbon as matrixes
of the catalysts. The precursors of the three key component
were included in one framework (or one molecule), which
to of S=O and FeO that should origin from the direct coor-
[
62]
dination of DMSO solvent molecules to Fe (II) ions.
The
XPS survey spectra of Fe-Phen-COFs (Figure 2B-a) indicated
the presence of C, N, O, S, Fe elements in Fe-Phen-COFs. The
deconvoluted S 2p spectrum of Fe-Phen-COFs (Figure 2C-a)
displayed a peak at 166.0 eV, ascribing to the S 2p3/2 signals
[
65]
of coordination species formed by iron ions and DMSO.
This further indicated that the coordination complexes of
DMSO and iron (II) ions have been involved into Phen-COFs,
2
+
probably in the form of octahedral [Fe(Phen)(C H SO)] or
2
6
2
+
[
Fe(C H SO) ] . The observation of TEM images for Fe-Phen-
2 6 6
COFs revealed an amorphous accumulation of compounds
Figure S1, Supporting Information). The coordination
(
compounds will be used to act as the source for generating
FeS and Fe C nanoparticles and carbon-based matrix in the
3
catalysts.
display an obvious difference from the synthesis of FeS/Fe C
3
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Figure 3. Morphology and structural characterization of as-prepared FeS/Fe C@N-S-Cs. A,B) SEM (scale bar: 1.00 um) and C,D) TEM images (scale
3
bar: 100 and 50 nm) of FeS/Fe C@N-S-C-800. G,H) HRTEM images (scale bar: 5 nm) of FeS/Fe C@N-S-C-800,revealing lattice fringes of the FeS,
3
3
Fe C phase, and wrapped graphitic carbon edges, respectively. E,F) HAADF-STEM image and I) elemental mappings for Fe of FeS/Fe C@N-S-C-800.
3
3
J) STEM-EDX images of FeS at the white circle mark in H of FeS/Fe C@N-S-C-800.
3
loaded on the nonmetal doped carbons reported in the litera-
ture.[ Next, the unstable FeN and FeOS moieties were
included in one framework as precursor, which is in favor of
creating the higher active site density and the uniform distri-
bution of the active species. Finally, the FeN moiety closely
neighbor FeOS moiety in one framework as precursor,
which facilitates the construction of the two-phase composite
structures consisting of the iron carbides and iron sulfides in
the nanoparticles (Figure S2, Supporting Information). Such
strength of these two bands (I /I ) often evaluated the content
D G
68]
2
of defects and average crystallite size of the sp domains in the
[
70–72]
derived FeS/Fe C@N-S-C samples.
The calculated I /IG
3
D
values were 1.06, 1.04, and 1.02 for the FeS/Fe C@N-S-C-700,
3
-800, and -900, respectively. It suggested that the crystallite
domains of carbon layers gradually increased with the elevation
of carbonization temperature.
The FeS/Fe C@N-S-C-800 observed in scanning electron
3
microscope (SEM) image (Figure 3A,B) showed a piece-stacked
structures are conducive to the synergistic interaction of Fe C
particle morphology. A clearer transmission electron micros-
3
and FeS.
copy (TEM) image (Figure 3C,D) of FeS/Fe C@N-S-C-800
3
The large-angle X-ray diffraction patterns (XRDs) of FeS/
exhibited that some core–shell structures were embedded in
the carbon matrix. The core particles had two different crystal-
line phases. The observation of high-resolution transmission
electron microscopy (HRTEM) (Figure 3G,H) further displayed
that these core particles with small size in diameter were encap-
sulated into the graphitic carbon layer. The lattice spacing with
a d-spacing of 2.11 and 1.72 Å agreed well with the distance
Fe C@N-S-C carbonized at different temperatures were shown
3
in Figure 2E. The diffraction humps at 2θ = 18.7° (101), 29.9°
(
(
0
110), 33.6° (112), 47.1° (211), 53.1° (300), 63.2° (008), and 71.4°
118) were assigned to hexagonal FeS crystal phase (PDF#37-
477). The rest diffraction peaks at 2θ = 37.7° (210), 44.6° (220),
and 45.8° (112) agreed well with the characteristic diffraction
[
62]
peaks of orthorhombic Fe C crystal phase (PDF#35-0772).
of (211) and (110) planes of Fe C and FeS, respectively. The
3
3
These results indicated that FeS and Fe C phases were formed
two-phase composite structures consisting of iron carbides and
iron sulfides in the nanoparticles were clearly observed on the
surface of carbon layers (Figure S2, Supporting Information).
Furthermore, high-angle annular dark field scanning transmis-
sion electron microscopy (HAADF-STEM) and elemental map-
pings images (Figure 3E,F,I) were used to verify Fe elemental
3
after pyrolysis treatment of Fe-Phen-COFs. There were no
obvious detectable diffraction signals originating from the (002)
and (101) planes of graphitic carbons,[ which may be due
69]
to higher diffraction intensity of Fe C and FeS nanoparticles.
3
However, the Raman spectroscopy (Figure 2F) of FeS/Fe C@N-
3
S-C obtained at different pyrolysis temperatures showed the
clear CC adsorption signals at 1338 and 1588 cm , assigning
to typical D and G bands of carbon materials. The relative
distribution in the FeS/Fe C@N-S-C-800. The analysis of
HRTEM combined with STEM-EDX (Figure 3J) indicated that
3
−1
the presence of FeS in FeS/Fe C@N-S-C-800 with the atomic
3
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Figure 4. A) N adsorption–desorption isotherms. B) Pore distribution curves obtained by BJH method based on the adsorption branch. a) FeS/
2
Fe C@N-S-C-700, b) FeS/Fe C@N-S-C-800, and FeS/Fe C@N-S-C-900.
3
3
3
percentage ratio of Fe (2.17 at%) and S (2.01 at%) was approxi-
mately similar to 1:1 in the crystalline particles. The coexistence
distribution of Fe, S, C, and O atoms in the surface of the pre-
pared catalyst. XPS measurements were further conducted to
investigate the chemical composition and the chemical envi-
of Fe C@C and FeS@C core–shell structures in derived
3
carbons avoided the serious agglomeration of Fe C and FeS
ronment of component elements of FeS/Fe C@N-S-C-800. The
3
3
nanoparticles and provided a stable carbon peripheral pro-
survey XPS spectrum of FeS/Fe C@N-S-C-800 (Figure 2B-b)
3
tection for them.[
25,73]
Core–shell Fe C@C and FeS@C may
also revealed the presence of C, N, O, S, and Fe elements, which
indicated that the N, Fe, and S elements were successfully
implanted in the carbon frameworks. The surface N content
3
synergistically exert stable multifunctional electrocatalytic
activity toward ORR, HER, and OER.
The nitrogen adsorption–desorption analysis indicates that
of FeS/Fe C@N-S-C-800 is about 3.55 at%, lower than that of
3
the as-synthesized FeS/Fe C@N-S-C materials possessed the
Fe-Phen-COFs precursors (4.54 at%, Table 2). The high-resolu-
3
typical type I isotherms (Figure 4A). Specifically, the adsorption
of N increased sharply at a lower relative pressure P/P , indi-
tion N 1 s spectra (Figure 2A-b) of FeS/Fe C@N-S-C-800 were
3
deconvoluted into four peaks centered at 398.4, 400.0, 401.1, and
402.3 eV, corresponding with pyridinic-N, pyrrolic-N, graphitic-
2
0
[
74]
cating the presence of micropores in the FeS/Fe C@N-S-Cs.
3
[75]
A remarkable increase in the middle and high P/P with a clear
N, and oxidized-N, respectively. It is clear that the pyridinic N
and graphitic N were dominant N species in the carbon frame-
works, which were believed as more effective N-doping type
0
hysteresis loop also confirmed the presence of mesopores and
some macropores. The listed parameters in Table 1 showed
[76]
that FeS/Fe C@N-S-C-800 possessed the highest Bruauer–
for enhancing activity of carbons. The high-resolution Fe 2p
3
2
−1
Emmett–Teller specific surface area (S ) of 288 m g and
total pore volume of 0.35 cm g . This S
survey spectra (Figure 2D-b) of FeS/Fe C@N-S-C-800 revealed
BET
3
3
−1
is not very higher
photoelectron signals in the Fe 2p1/2 and Fe 2p3/2 region, con-
BET
[
77,78]
than other carbon-based materials, probably owing to higher
firming the presence of Fe (II) and/or Fe (III) state.
more, the peak at a binding energy of 707.8 eV in Fe 2p region
Further-
mass content of Fe C and FeS particles. The micropore and
3
3/2
[79]
mesoporous pore size obtained from calculation by Horvaih–
Kawazoe (HK) (Figure S3, Supporting Information) and Bar-
rett–Joyner–Halenda (BJH) (Figure 4B) method was centered
also revealed the presence of Fe C. For the S 2p spectrum in
3
Figure 2C, four well-defined peaks were observed. Two peaks
located at 163.7 and 164.9 eV were attributed to the existence of
thiophene-like (CS, CS), while the peak at 168.4 eV
at 0.49 and 3.8 nm. These N -sorption behaviors testified the
2
hierarchical porosities of FeS/Fe C@N-S-Cs, which could facili-
was characteristic of the oxide of sulfur (SO ). The two peaks
3
x
tate the exposure of more active species and enhance the mass-
transport capacity in the catalysts.
confirmed that S species have also been doped in the carbon
[
80,81]
lattices or edges in the form of CS, CS or CSO .
The
x
[82]
S 2p peak at 161 eV was assigned to S2 structure. It proved
that S ions from the decomposition of Fe-DMSO coordination
compounds reacted with Fe(II) and formed FeS nanoparticles
−
2
.3. Surface Ingredients on FeS/Fe C@N-S-Cs
3
encased in carbon layers. FeS species in the FeS/Fe C@N-S-Cs
3
−1
The SEM elemental mapping results of FeS/Fe C@N-S-C-800
were also found by several peaks below 500 cm in Raman
3
[83]
(Figure S4, Supporting Information) showed that a reasonable
spectra, agreeing with FT-IR and XRD results.
Table 1. The textural parameters of FeS/Fe C@N-S-C-n materials.
3
SBET [m g−1]
2
Smicropore [m g−1]
2
Sexteral [m g−1]
2
Vtotal [cc g−1
0.24
]
Vmicropore [cc g−1
]
Samples
dHK [nm]
0.49
dBJH [nm]
3.8
FeS/Fe3C@N-S-C-700
FeS/Fe3C@N-S-C-800
FeS/Fe3C@N-S-C-900
166
288
266
118
200
158
48
88
0.07
0.11
0.07
0.35
0.49
3.8
108
0.41
0.49
3.8
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respectively.[85] The results of the FT EXAFS and 57Fe Möss-
bauer spectroscopy analysis clearly certified that the prepared
FeS/Fe C@N-S-C catalysts contain FeS and Fe C as the main
Table 2. The surface species analyses of Fe-Phen-COFs and FeS/
Fe C@N-S-C-800 by XPS (at%).
3
3
3
Samples
N
NPyd
—
NPyr
—
Ng/N-C Fe-N N-oxide
components.
Fe-PTCPs
4.54
3.55
32.4
10.2
61.7
—
5.9
5.6
FeS/Fe3C@N-S-C-800
31.6
52.6
2
.5. Electrochemistry Properties for the ORR
2
.4. Local Microstructures of FeS/Fe C@N-S-Cs
The electrocatalytic properties of FeS/Fe C@N-S-C toward ORR
3
3
were first investigated in alkaline medium by cyclic voltam-
metry (CV) and rotating disc electrode (RDE) techniques. An
In order to further understand and explore the catalytic behavior
of the prepared catalysts, FeS/Fe C@N-S-C catalyst was fur-
impressive peak potential of FeS/Fe C@N-S-C-800 was found
3
3
ther characterized with X-ray absorption spectroscopy (XAS)
to emerged at 0.85 V (vs RHE) in O -saturated 0.1 m KOH solu-
2
5
7
and Fe Mössbauer spectroscopy measurements to confirm
the chemical environment of the iron atoms in the prepared
catalysts. As can be seen in Figure 5A, the X-ray absorption
tion, in contrast to the nonfeature CV curves in N -saturated
2
solution (Figure 6A), suggesting that the FeS/Fe C@N-S-C
3
catalyst possessed high efficiency electrocatalytic activity for ORR
in alkaline medium. Additionally, to further explore the ORR
properties of the prepared catalytic materials, the polarization
near edge structure (XANES) edge of FeS/Fe C@N-S-C shifted
3
to higher energy relative to the absorption edge of Fe foil, indi-
cating the increase of the Fe oxidation state. The local struc-
curves for FeS/Fe C@N-S-C-n prepared at different pyrolysis
3
−1
tural information of the iron species in the FeS/Fe C@N-S-C
temperatures were obtained at a scan rate of 5 mV s and a
rotation rate of 1600 rpm (Figure 6B), displaying that FeS/
3
catalyst was probed using Fourier transformed X-ray absorption
fine structure (FT-EXAFS) spectroscopic analysis (R space). The
three FT peaks displayed in Figure 5B clearly revealed iron-
Fe C@N-S-C-800 possessed the best ORR activity in 0.1 m KOH
3
solution. The half-wave potential on FeS/Fe C@N-S-C-800
3
[
69]
based local microstructures in the FeS/Fe C@N-S-C catalyst.
(E1/2 = 0.87 V) was more positive than that on the commercial
Pt/C (0.84 V). The corresponding current density (5.2 mA cm−
at 0.2 V) was comparative to that on the Pt/C-JM catalyst
(20 wt%, Johnson Matthey). The Tafel slope of ORR on FeS/
3
2
The FT peak at ≈1.99 Å can be well defined as a FeS configu-
ration with a bond length of ca. 2.39 Å (all the radial distances
given in figure 5B are without phase correction, the true bond
length = 1.99 + 0.4 Å because of a phase shift). The FT peak at
−
1
Fe C@N-S-C-800 (90 mV decade ) (Figure S5A, Supporting
3
≈
2.45 Å can be fitted as FeFe scattering with a bond length
Information) was smaller than those on other catalysts, signi-
of ca. 2.85 Å. This bond length belonged to the Fe-Fe pair in
iron sulfide.[ The FT peak at ≈1.51 Å may arise from the
FeN / FeC / FeO scattering. However, it should be noted
that an inherent limitation of XAS is the inability to differen-
tiate N from C and O due to similar atomic numbers of them.
We cannot use FT EXAFS to conclusively identify the presence
fying the favorable ORR kinetics of FeS/Fe C@N-S-C-800 in
3
84]
0.1 m KOH medium.
The ORR catalytic behaviors of FeS/Fe C@N-S-C-n were
3
also evaluated in 0.1 m HClO electrolyte. The corresponding
4
CV curves (Figure 6D) confirmed that FeS/Fe C@N-S-
3
C-800 had an obvious cathodic peak potential (Ep = 0.71 V)
in O -saturated 0.1 m HClO solution. The RDE polarization
[
84]
or absence of iron nitrides or carbides in the final catalyst.
2
4
Therefore, we measured ex situ 57Fe Mössbauer spectroscopy
to identify the different chemical environment of iron in FeS/
curves of FeS/Fe C@N-S-C-800 (Figure 6E) showed that the
3
half-wave potential was about 160 mV negative shift compare
with a commercial Pt/C catalyst (0.80 V), indicating the better
ORR catalytic performance compared with the reported non-
precious catalysts in acidic electrolyte. The diffusion-limiting
Fe C@N-S-C sample. The results were shown in Figure 5C.
3
The spectrum was composed of four components: three sextets
and one singlet. The fitted Mössbauer parameters were shown
in Table 3. The singlet was assigned to γ-Fe, while the hyper-
fine parameters of the three sextets S1 (sextet 1), S2, S3 cor-
−2
current of ORR on FeS/Fe C@N-S-C-800 was 5.1 mA cm at
3
0.2 V, which was slightly lower than that over the commercial
−
2
responded to iron sulfide (FeS), iron carbide (Fe C), and α-Fe,
Pt/C catalyst (5.3 mA cm at 0.2 V). Compared with other
3
Figure 5. A) Fe K-edge XANES spectra of FeS/Fe C@N-S-C-800 and Fe foil. B) Fourier transform EXAFS of FeS/Fe C@N-S-C-800. C) 57Fe Mössbauer
3
3
spectrum of the FeS/Fe C@N-S-C-800 catalyst.
3
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Table 3. The fitted Mössbauer parameters for the FeS/Fe C@N-S-C-800
catalyst at room temperature.
have the ORR activity in neutral electrolyte. It can be seen in
Figure 6H that the LSV curves of the as-prepared N-S-C-800,
3
FeS/Fe C@N-S-C-n, and Pt/C catalysts were obtained at a
3
Samples
IS
QS
HF
LW
Assignment
rotating speed of 1600 rpm. Remarkably, the FeS/Fe C@N-S-
3
[
mm s−1] [mm s−1] [Tasel] [mm s−1]
C-800 catalyst exhibited a half-potential (E1/2 = 0.56 V) compa-
FeS/Fe3C @N-S-C Singlet
0.08
0.91
0.33
0.10
—
—
0.28
0.46
0.44
0.64
γ-Fe
FeS
rable to that on the Pt/C catalyst (0.575 V) for ORR and higher
−
2
Sextet 1
Sextet 2
Sextet 3
0.15
0.00
−0.03
31.35
20. 70
32.84
limiting current (5.5 mA cm at −0.1 V) than that over the
−
2
Pt/C catalyst (5.3 mA cm at −0.1 V) in the neutral electrolyte.
To gain insight into the ORR activity, Tafel plots (Figure S5C,
Supporting Information) on the as-prepared catalysts were
further studied. The results indicate that Tafel slope value of
Fe3C
α-Fe
−
1
catalytic materials, the smaller Tafel slope on FeS/Fe C@N-S-
C-800 (150 mV decade ) (Figure S5B, Supporting Information)
85 mV dec for the FeS/Fe C@N-S-C-800 is much lower than
3
3
−1
that for all the samples tested. The relatively lower Tafel slope
revealed a favorable reaction kinetics in 0.1 m HClO solution.
indicated that FeS/Fe C@N-S-C-800 has an outstanding ORR
4
3
These results imply that FeS/Fe C@N-S-C-800 has the highly
reaction kinetics.
3
efficient ORR catalytic activity in both alkaline and acidic media.
The ORR electrocatalytic activities of the prepared FeS/
The electrocatalytic durability of FeS/Fe C@N-S-C-800 was
3
investigated by chronoamperometric method in three electro-
lytes. As displayed in the Figure 6C,F,I, no obvious change in
the peak potential of the CV curves was observed before and
Fe C@N-S-C-n were also investigated in neutral 0.01 m PBS
3
buffer. As shown in Figure 6G, a well-defined cathodic peak
appeared at 0.65 V (vs RHE) in O -saturated 0.01 m PBS electro-
after running 5000 cycles in O -saturated 0.1 m KOH, 0.1 m
2
2
lyte and featureless cyclic voltammogram existed in N -saturated
HClO as well as 0.01 m PBS solutions, indicating high surviva-
2
4
buffer, which indicated that the FeS/Fe C@N-S-C-n catalysts
bility of the catalytic activity structures. These results suggested
3
Figure 6. Cyclic voltammograms (CV) obtained in O and N saturated A) 0.1 m KOH, D) 0.1 m HClO , and G) 0.01 m PBS electrolytes of FeS/Fe C@N-
2
2
4
3
S-C-800. Polarization curves of a) N-S-C-800, b) FeS/Fe C@N-S-C-700, c) FeS/Fe C@N-S-C-800, d) FeS/Fe C@N-S-C-900, and e) commercial Pt/C
3
3
3
(
20 wt%) in O saturated B) 0.1 m KOH, E) 0.1 m HClO , and H) 0.01 m PBS electrolytes. CVs run for 5000 cycles recorded in O saturated in C) 0.1 m
2
4
2
−
1
KOH, F) 0.1 m HClO , and I) 0.01 m PBS electrolytes for FeS/Fe C@N-S-C-800 (electrode rotation speed, 1600 rpm; scan rate, 5 mV s ).
4
3
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Figure 7. LSV curves of FeS/Fe C@N-S-C-800 at different rotation rates in A) 0.1 m KOH, C) 0.1 m HClO , and E) 0.01 m PBS electrolytes. K-L plot and
3
4
the corresponding electron transfer numbers for FeS/Fe C@N-S-C-800 in B) 0.1 m KOH, D) 0.1 m HClO , and F) 0.01 m PBS electrolytes.
3
4
that the FeS/Fe C@N-S-C-800 had better ORR stability and
durability than commercial Pt/C in a wide pH range. The
Figure S6A, Supporting Information) and neutral conditions
(2.32 V for 10 mA cm , Figure S7A, Supporting Information),
3
−
2
N-S-codoping carbon-encapsulated FeS@Fe C nanostructures
impressively, FeS/Fe C@N-S-C-800 afforded a current density
of 10 mA cm at an electrode potential of 1.80 V under alkaline
3
3
−
2
might restrain the dissolution and agglomeration of Fe C and
3
FeS nanoparticles, thus enhancing the electrocatalytic stability
and durability.
conditions (Figure 8A). This fact illustrates that the OER cata-
lytic activity of FeS/Fe C@N-S-C-800 is comparable to those
3
The ORR kinetics process of the FeS/Fe C@N-S-C-800
of the analogous catalysts by recently reported (see Table S1 in
the Supporting Information for detailed comparison). However,
there are still some gaps compared with the state-of-the-art IrO2
catalyst. From Tafel plots constructed by the polarization curves
(LSV curves in Figure 8A), the Tafel slopes can be obtained by
3
catalysts was investigated according to the RDE polarization
curves at different rotating rates in three media (Figure 7A–C).
The corresponding K-L plots (Figure 7D–F) obtained from the
polarization curves for FeS/Fe C@N-S-C-800 between 0 and
3
[
87]
0
0
.6 V in 0.1 m KOH, 0.1 to 0.4 V in 0.1 m HClO and 0.1 V to
fitting of the straight line segment of Tafel plots as shown in
4
−
1
.35 V in 0.01 m PBS electrolytes exhibit the parallel lines at var-
Figure 8B, which shows a Tafel slope of 81 mV dec , implying
ious potentials, indicating a first-order process with the same
a relatively fast kinetic reaction rate on FeS/Fe C@N-S-C-800
3
number of electrons transferred per O molecule in the oxygen
electroreduction at different potentials. The transferred elec-
tron numbers (n) derived from the slopes of the linear K-L plots
for OER. Such kinetic feature is very similar to those of the
reported catalysts.
2
[
86]
[42,88–90]
The catalytic performance of FeS/Fe C@N-S-C-800 for
3
at 0.3 V (vs RHE) on FeS/Fe C@N-S-C-800 in alkaline, neutral,
HER was also explored in N -saturated 0.5 m H SO , 0.01 m
3
2
2
4
acidic solutions were 3.9, 3.7 and 4.1 respectively, confirming
PBS and 1 m KOH electrolytes, respectively. The LSV curves in
Figure 8C exhibit the high-efficiency catalytic activity of FeS/
Fe C@N-S-C-800 for HER in 0.5 m H SO solution. The over-
that ORR catalyzed by FeS/Fe C@N-S-C-800 occurs mainly by
3
the direct 4-electron reduction pathway from O to H O within
2
2
3
2
4
a wide pH range with low overpotential.
potential of HER on FeS/Fe C@N-S-C-800 at a current den-
3
−
2
sity of 10 mA cm was −174 mV, more positive than that on
other FeS/Fe C@N-S-Cs catalysts. The catalytic activity of FeS/
3
2
.6. Electrochemistry Properties for the OER and HER
Fe C@N-S-C-800 for HER was better than those of the reported
3
nonprecious electrocatalysts under acidic conditions in the lit-
[
91]
To evaluate the electrocatalytic activity for OER, the OER LSV
erature, such as Fe C/Mo C@NPGC (−980 mV), Co S @C
3 2 9 8
[
92]
[93]
curves on FeS/Fe C@N-S-C-n materials were measured in
(−240 mV), and Mo C@NC-2-750 (−240 mV) (more details
were listed in Table S2 in the Supporting Information). The
3
2
O -saturated 1 m KOH, 0.01M PBS and 0.5 m H SO solutions,
2
2
4
respectively. Although the OER activity of FeS/Fe C@N-S-
C-800 was not prominent under acidic (2.41 V for 10 mA cm ,
FeS/Fe C@N-S-C electrode exhibited a catalytic activity closer to
Pt/C electrode for HER compared with the analogous catalyst.
3
3
−
2
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Figure 8. A) The LSV curves and B) corresponding Tafel slopes on different catalysts for OER in 1 m KOH solution. C) The LSV curves and D) corre-
−
1
sponding Tafel slopes on different catalysts for HER in 0.5 m H SO solution (electrode rotation speed, 1600 rpm; scan rate, 5 mV s ).
2
4
Tafel slope of HER over FeS/Fe C@N-S-C-800 (Figure 8D)
(LSV) technique. As showed in Figure S8A,D (Supporting
Information), the potential of the OER and HER polarization
curves at the current density of 10 mA cm on the catalysts
3
−
1
was 34 mV dec and the smallest among the tested materials
−
1
−1
−2
(
493 mV dec for FeS/Fe C@N-S-C-700, 54 mV dec for FeS/
3
−1
Fe C@N-S-C-900, and 40 mV dec for Pt/C), indicating an
in 1 m KOH solution slightly shifted (≈20 and ≈15 mV) after
running 2000 cycles. In the meantime, by the same durability
measurement method, the negligible potential shift of the OER
and HER polarization curves was observed at the current den-
3
intrinsically improved HER kinetics. Moreover, FeS/Fe C@N-
3
S-C-800 also exhibited proper catalytic performance for HER
with the overpotential of −446 mV in 1 m KOH electrolyte and
798 mV at a current density of 10 mA cm−2 in 0.01 m PBS
buffer (Figures S6C and S7C, Supporting Information). These
sity of 10 mA cm in 0.5 m H SO electrolyte (Figure S8B,E,
−2
−
2
4
Supporting Information) and 0.01 m PBS buffer (Figure S8C,F,
results attest that FeS/Fe C@N-S-C-800 has the effective HER
Supporting Information). These results indicated the slight deg-
3
activity in a wide pH range.
radation of FeS/Fe C@N-S-C-800 for OER and HER in a wide
3
Moreover, the variation rule of electrocatalytic activity of FeS/
pH range. The good durability of the FeS/Fe C@N-S-C-800 cat-
3
Fe C@N-S-C-800 for ORR, OER and HER with the PH of elec-
trolyte (Table 4) was consistent with the literature results.
alyst could be derived from its unique constitution with metal
species closely wrapped in the carbon matrix.
3
[
94–96]
These results sufficiently illustrated that FeS/Fe C@N-S-C-800
For exploring the possibility of FeS/Fe C@N-S-C-800 catalyst
3
3
catalyst possesses multifunctional electrocatalytic properties in
the wide range of PH.
in the practical application, the home-made zinc–air batteries
were assembled using the FeS/Fe C@N-S-C-800 catalyst as
3
The long-term durability of the FeS/Fe C@N-S-C-800 catalyst
the active air electrode and Zn foil as the anode for an energy-
3
for OER and HER was investigated by linear sweep voltammetry
related practical application (Figure 9). We also constructed
Table 4. The comparison of the electrochemical catalytic properties of the FeS/Fe C@N-S-C-800 catalyst in the wide range of PH.
3
Sample
ORR
OER/HER
0
.1 m KOH
0.1 m HClO4
0.01 m PBS
1 m KOH
0.5 m H2SO4
0.01 m PBS
FeS/Fe3C@N-S-C-800
Eonset
E1/2
Eonset
E1/2
Eonset
E1/2
ηOER
0.57
ηHER
ηOER
1.18
ηHER
ηOER
1.09
ηHER
1.02
0.87
0.88
0.64
0.92
0.54
0.45
0.17
0.80
Note: Eonset and E1/2 represent the onset potential (V vs RHE) and half-wave potential (V vs RHE) of ORR, respectively. ηOER/ηHER represent the overpotential (V vsRHE) at
the current density of 10 mA cm− of OER or HER.
2
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Figure 9. The FeS/Fe C@N-S-C-800-based zinc–air batteries. A) Polarization and power density curves of the zinc–air batteries with the FeS/Fe C@N-
3
3
−
2
S-C-800 or Pt/C as catalysts. B) Galvanostatic discharge–charge cycling curve at 20 mA cm of rechargeable zinc–air battery with different catalysts.
C) Galvanostatic discharge–charge cycling curves at 20 mA cm of rechargeable zinc–air batteries with the FeS/Fe C@N-S-C-800 catalyst. D) Photograph
−2
3
of a light-emitting diode (LED, 3 V) powered by two batteries connected in series.
reference battery using Pt/C catalyst loaded on carbon cloth
as the air electrode. Figure 9A displayed the polarization and
which exhibit an excellent operation stability. So we believe that
FeS/Fe C@N-S-C-800 catalyst is a promising catalyst toward
3
power density curves for the FeS/Fe C@N-S-C-800 and Pt/C
the new renewable energy technologies.
3
air electrodes. The measured open-circuit potentials (OCPs) of
the resultant zinc–air batteries were as high as 1.43 and 1.44 V
In order to gain insight into the catalytic origin and behavior
of FeS/Fe C@N-S-C-800 during ORR, OER, and HER, the
3
for FeS/Fe C@N-S-C-800- and Pt/C-derived air electrodes
catalysts without FeS/Fe C (N-S-C-800) was prepared (see the
3
3
respectively. The FeS/Fe C@N-S-C-800 catalyst showed a peak
Experimental Section) and FeS/Fe C@N-S-C-800 catalysts were
specially treated with 10% hydrochloric acid solution for 12 h
3
3
−2
current density of ≈65 mA cm and a peak power density of
−
2
≈
63 mW cm , which was higher than that of the Pt/C-based
to obtain the catalyst FeS/Fe C@N-S-C-800-HCl. The XRD pat-
3
−2
counterpart (peak current density: ≈65 mA cm ; peak power
terns of FeS/Fe C@N-S-C-800-HCl revealed the weaker peak
3
−2
density: ≈60 mW cm ). Moreover, durability of the batteries
intensity of FeS and Fe C nanoparticles (Figure S9, Supporting
3
has also been evaluated using a galvanostatic recurrent pulse
Information), which point out that a part of FeS@Fe C nano-
3
−2
method at a constant current density of 20 mA cm for 24 h
with a 5 min to recharge and 5 min to discharge. As shown in
particles in the catalysts were etched-off after undergoing acid-
leaching. The results of electrochemical activity investigation
for ORR, OER, and HER in Figures 6B,E, 8A,C, and 10 show
Figure 9B, the FeS/Fe C@N-S-C-800 catalyst produced an ini-
3
tial charge potential of 2.34 V and discharge potential of 0.85 V
with a voltage gap of 1.49 V, confirming a higher round-trip
efficiency of 37.6% compared to 27.1% of Pt/C electrode. After
that the catalytic activity of FeS/Fe C@N-S-C-n is obviously
3
higher than those over the catalysts without FeS/Fe C (N-S-
3
C-800) and catalyst FeS/Fe C@N-S-C-800-HCl, which clearly
3
2
4 h of circulation, the zinc–air battery with the FeS/Fe C@N-
testify that catalytic activity of FeS/Fe C@N-S-C-n for ORR,
3
3
S-C-800 catalyst showed high stability (Figure 9C), whereas the
control zinc–air battery with the Pt/C catalyst suffered a slightly
decline in the discharge potential after 24 h. Both results from
the galvanodynamic test and galvanostatic discharge–charge
test indicated that the electrocatalytic performance and stability
OER, and HER originates from the introduction of FeS/Fe C
3
in N-S-codoped carbons. In the meantime, it was found that
FeS has highly hydrophilicity and accelerates the adsorptions of
−
+ [97]
aqueous substrates (O , H O, OH , or H ), but FeS delivers
2
2
sluggish ORR kinetic process and has little catalytic activity for
[
97,98]
of the FeS/Fe C@N-S-C-800 catalyst were superior to that of
OER and HER due to the intrinsic low activity
and Fe/Fe C
3
3
the Pt/C (20 wt%) catalyst in the zinc–air battery test. As an
example for practical application (Figure 9D), multiple zinc–air
batteries with serial connection were fabricated and integrated
into circuits to power a red light-emitting diode (LED, 3 V),
could assist the stabilization of the peroxide intermediate, pro-
−
mote a 4e ORR process and exhibits the preferable catalytic
[
99–101]
activity toward OER and HER,
but is insufficient toward
[
84]
O adsorption. In the prepared catalysts, FeS/Fe C@N-S-C-n,
2
3
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Figure 10. LSV curves of FeS/Fe C@N-S-C-800 and FeS/Fe C@N-S-C-800-HCl in A) 0.1 m KOH solution and B) 0.1 m HClO solution for ORR. LSV
3
3
4
curves of FeS/Fe C@N-S-C-800 and FeS/Fe C@N-S-C-800-HCl in C) 1 m KOH solution for OER and D) 0.5 m H SO solution for HER.
3
3
2
4
FeS, and Fe C composite nanoparticles as a coupling body
nanostructures. Impressively, the optimized FeS/Fe C@N-S-
3
3
embedding in N-S-codoped carbons catalyst (FeS/Fe C@N-
C-800 not only exhibited high-efficient ORR catalytic activity in
acidic (E1/2 = 0.64 V) and basic conditions (E1/2 = 0.87 V), but
also show higher activities for HER and OER. At the same time,
3
S-C). In the procedure of ORR over FeS/Fe C@N-S-C-n, FeS
3
may improve the adsorption capacity of the catalyst for oxygen
and raise the reactive rate of the direct four-electron reduction
pathway from O to H O catalyzed by Fe C, thus endowing
the investigation results demonstrate that the FeS/Fe C@N-S-
3
C materials possess trifunctional electrocatalytic activity (ORR,
OER, HER) within a wide pH range from acidic to basic condi-
2
2
3
FeS/Fe C@N-S-C-n with excellent catalytic performance. In the
3
process of OER and HER over FeS/Fe C@N-S-C-n, FeS could
tions. The synergistic interaction between FeS and Fe C nano-
3
3
promote the adsorptions capacity of the catalyst for aqueous
particles embedded in N-S-codoped carbon surface contributed
to such effective trifunctional electrocatalytic performance. Fur-
thermore, a primary zinc–air battery by using the as-prepared
−
+
substrates (H O, OH , or H ) and facilitate reactants contact
2
with the catalytic active sites Fe C, which would be helpful to
3
accelerate the rate-determining reaction steps, thereby boosting
FeS/Fe C@N-S-C-800 composite as the air cathode exhibited
3
the catalytic performance of FeS/Fe C@N-S-C-n for OER and
a remarkable peak power density and durability superior to
those of its counterpart with the reference Pt/C air cathode. The
strategy of rational design and orientated derivative provide a
feasible route for preparing high-efficiency multifunctional elec-
trocatalytic materials with inorganic nanoparticles composite to
respond to the target electrocatalytic reactions in a wide range of
3
HER. All this being said, Fe-related species (Fe C and FeS)
3
could synergistically enhance the electrocatalytic activity of the
FeS/Fe C@N-S-C-n for ORR, OER, and HER. Among them,
3
Fe C is the initiator of catalytic activity and FeS is a synergistic
3
cocatalyst.
PH. The prepared FeS/Fe C@N-S-C-800 could be a promising
3
catalyst toward the new renewable energy technologies.
3
. Conclusion
Novel porous phenanthroline-based covalent organic frame-
works with open metal-chelate sites were constructed using
rigid building blocks. The metal implant of this Phen-COFs
was achieved in DMSO solvent, which made Fe-DMSO coor-
dination complexes involving in the Phen-COFs. The thermal
conversion of this special COFs results into the formation of
4
. Experimental Section
Reagents and Chemicals: All chemicals were purchased from
Sinopharm Group Chemical Reagent Co., Ltd. and used without further
purification.
Synthesis of Covalent Phenanthroline Frameworks: 3, 8-Dibromo-
phenanthroline (0.676 g, 2 mmol) and BTA (0.456 g, 2 mmol) were
dissolved into a mixed solvent of tetrahydrofuran (THF) and N,N-
N-S-codoping carbons with embedded core–shell Fe C and FeS
3
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dimethylformamide (DMF) (VTHF/VDMF = 2:1) in three-necked flask. The
disk electrode (5 mm in diameter, Pine) or a glassy carbon RRDE
(5.6 mm in diameter, Pine) was used as the working electrode. An Ag/
AgCl electrode (KCl, 3 m) was employed as reference electrode. The
counter electrode was a Pt wire in the ORR and OER measurement or a
graphite rod in the HER. The catalyst ink was prepared by ultrasonically
dispersing 10 mg of catalyst in a solution containing 1.28 mL of ethanol
containing 30 µL Nafion (5 wt%) solution to form a homogeneous
suspension. Then, a little of catalyst ink was pipetted onto the surface
of the polished glassy carbon electrode and dried at room temperature.
The electrocatalytic properties for ORR in alkaline electrolytes were
carried out in O2 saturated 0.1 m KOH solution and the potential range
saturated solution of Na CO (30 mL) and tetrakis(triphenylphosphine)
2
3
palladium (0) (0.08 g, 0.07 mmol) were then added, and the resulting
mixture was stirred and refluxed at 80 °C for 12 h in a flowing
nitrogen atmosphere. After the reaction system above cooled to room
temperature, an ammonia solution (15 mL, 5 wt%) was added and
stirred for 15 min. A yellowish solid was collected by filtrating and
washing with deionized water. The final Phen-COFs were obtained after
being dried in vacuum at 60 °C.
To obtain iron-coordinated phenanthroline polymer (Fe-Phen-COFs),
the above-prepared Phen-COFs (0.3 g) and Iron (II) acetate (1.2 g) were
dissolved together in 30 mL of DMSO. Then, the resulting mixture was
stirred and refluxed at 80 °C for 12 h under flowing nitrogen. A large amount
of ethanol was added to the cooled reaction mixtures. The precipitation was
collected and dried in vacuo to give a brown solid of Fe-Phen-COFs.
−
1
from −1.2 and +0.2 V (vs Ag/AgCl) at a scan rate of 5 mV s at the
ambient temperature. Acid tests were studied in O2 saturated 0.1
m
HClO4 solution and the potential range was between 1.0 and −0.2 V
(vs Ag/AgCl). Neutral tests were studied in O2 saturated 0.01 m PBS
solution and the potential range was between −1.0 and 0.4 V (vs Ag/
AgCl). The general loading of FeS/Fe3C@N-C-n samples on the working
Synthesis of FeS/Fe C@N-S-C: The Fe-Phen-COFs products were
3
pyrolyzed at 700–900 °C for 4 h in a quartz tube furnace under nitrogen
−
1
−2
atmosphere, with a heating rate of 2 °C min . The prepared samples
electrode was 0.3 mg cm in 0.1 m KOH and 0.01 m PBS electrolytes and
−
2
were marked as FeS/Fe C@N-S-C-n (n is the thermal-treatment
0.6 mg cm in 0.1 m HClO4 solution. The catalyst loading of Pt/C (JM)
3
−
2
temperature). For comparison, the FeS/Fe C@N-S-C-800 was leached in
was 0.1 mg cm on the electrode in both electrolytes for comparison.
All the potentials here were referenced to the reversible hydrogen
electrode (RHE) potentials. The kinetic parameters have been calculated
by Koutecky–Levich (K-L) equations[
3
1
0% of HCl solution for 12 h to give FeS/Fe C@N-S-C-800-HCl sample.
3
The catalysts without FeS/Fe C (N-S-C-800) were prepared by paralyzing
3
104]
the dried mixture of Phen-COFs and DMSO at 800 °C for 4 h in a quartz
tube furnace under nitrogen atmosphere.
1
J
1
JL
1
JK
1
ω
1
JK
Characterization: The XRD was performed on a D8 Advance X-ray
diffractometer (Bruker AXS, Germany) with Cu Kα radiation (40 kV,
=
+
=
+
/2
(1)
1
B
40 mA). The morphology and microstructure of samples were
characterized by SEM (Hitachi ST-4800) and TEM (JEM-2010) with an
acceleration voltage of 200 kV. The HAADF-STEM was carried out using
JEM-2100F TEM (LVEM5, Japan). The chemical bonding states of samples
were analyzed by XPS with Al Kα radiation (15 kV, 150 W) carried out on
Axis Ultra DLD, and the C 1s (BE = 284.8 eV) as a reference to perform
2/3
B = 0.62 nFC0
JK = nFkC0
(
D0
)
γ −
1/6
(2)
(3)
spectrum calibration. N adsorption/desorption measurements were
in which J, JK, and JL are the measured current density, kinetic and
diffusion-limiting current densities, respectively; ω is the angular velocity
of the disk, n is the number of electrons transferred in ORR, F is the
Faraday constant (F = 96 485 C mol L−1), C0 is the bulk solubility of
O2, D0 is diffusion coefficient of O2, γ is the kinematic viscosity of the
electrolyte, and k is the electron transfer rate constant. The number of
electrons transferred (n) can be obtained from the slope of the K-L plots.
Linear sweep voltammetry (LSV) for OER and HER were conducted
2
measured at 77 K in the Micromeritics ASAP 2020 analyzer. Measurement
of Raman spectra of the samples was performed on a GX-PT-1500 (150)
13
instrument with a 532 nm excitation laser (1 mW). Solid-state C NMR
spectra were measured at ambient temperature on a Bruker AVANCE III
400 MHz spectrometer. Room temperature UV–visible diffuse reflectance
spectra were conducted on a Shimadzu UV 2401 PC with an integrating
sphere attachment. The chemical structure of the samples was analyzed
by the Fourier transform infrared spectra (Bruker Tensor 27) in KBr pellets.
XAFS Measurement: The X-ray absorption find structure spectra
−
1
with a scan rate of 5 mV s in 1.0 m KOH, 0.01 m PBS, and 0.5 m H2SO4
solutions at room temperature, where graphite rod was employed as
the counter for HER measurements. The general loading of catalysts
(Fe K-edge) were collected at 1W1B station in Beijing Synchrotron
−
2
Radiation Facility (BSRF), China. The storage rings of BSRF were
operated at 2.5 GeV with a maximum current of 250 mA. Using Si (111)
double-crystal monochromator, the data collection was carried out in
transmission mode using ionization chamber. All XAS spectra were
recorded in room conditions in fluorescence mode.
in the OER and HER on the working electrode was 0.6 mg cm .
The iR-compensation was performed by electrochemical impedance
spectroscopy to correct the compensated potential. The LSV curves
were replotted as overpotential (η) versus log current (log j) to get Tafel
plots for quantification of the OER and HER kinetic performance of
as-prepared catalysts. By fitting the linear part of the Tafel plots to obtain
the Tafel slope (b) according to the Tafel equation ( η = a + b log(j)).
Zinc–Air Batteries: Rechargeable zinc–air batteries in a two-electrode
XAFS Analysis and Results: The acquired EXAFS data were processed
according to the standard procedures using the ATHENA module
implemented in the IFEFFIT software packages.[
102]
3
The k -weighted
[
105]
EXAFS spectra were obtained by subtracting the postedge background
from the overall absorption and then normalizing with respect to the edge-
system were assembled according to the following procedures.
The air cathode was prepared by loading the catalyst (with a loading
3
of 4 mg cm−2) onto a carbon cloth substrate. A polished Zn foil was
jump step. Subsequently, k -weighted χ(k) data of Cu K-edge were Fourier
−1
applied as the anode, and 6 m KOH filled with 0.2 m Zn(Ac) was applied
transformed to real (R) space using Hanning windows (dk = 1.0 Å )
to separate the EXAFS contributions from different coordination shells.
To obtain the quantitative structural parameters around central atoms,
least-squares curve parameter fitting was performed using the ARTEMIS
2
as the electrolyte to form zincate (Zn(OH)42 ) to ensure reversible Zn
electrochemical reactions at the anode. The discharge polarization and
power density plots were recorded using a galvanodynamic method.
Galvanostatic discharge–charge cycling curves were performed at room
temperature, in which one cycle consists of one discharge step followed
−
[
103]
module of IFEFFIT software packages.
57Fe Mössbauer spectra were recorded at room temperature using
a Wissel accelerated Mossbauer spectrometer with a proportional
by one charge step of the same current density and duration time
5
7
(20 mA cm−2 for 24 h with a 10 min cycling).
counter. Moving Co (Pd) in a constant acceleration mode was used as
the radioactive source. The spectrum was fitted with a combination of
Lorentzian lines.
Electrochemical Measurement: The electrocatalytic properties (ORR,
OER, HER) of the as-prepared catalysts were characterized in a three-
electrode system by CV, RDE techniques, and ring disk electrode (RRDE)
techniques on the CHI-760C electrochemical analyzer. A glassy carbon
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Adv. Funct. Mater. 2018, 1803973
1803973 (12 of 14)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.afm-journal.de
[
24] L. Shang, H. Yu, X. Huang, T. Bian, R. Shi, Y. Zhao,
Acknowledgements
G. I. N. Waterhouse, L. Z. Wu, C. H. Tung, T. Zhang, Adv. Mater.
The authors are grateful for financial support from the China National
Natural Science Foundation (No. 21303058), the Shanghai Municipal
Natural Science Foundation (No. 13ZR1412400), and the key project
of Shanghai Science and Technology Committee (No. 14231200300).
The authors are thankful to the beamline 1W1B-XAFS station of Beijing
Synchrotron Radiation Facility (BSRF) for XAS measurements.
2
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Conflict of Interest
The authors declare no conflict of interest.
Keywords
carbon, covalent phenanthroline frameworks, FeS@Fe C, Pt-free
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