Amino-metalloporphyrin polymers derived Fe single atom catalysts for highly efficient oxygen reduction reaction

Article abstract of DOI:10.1007/s11426-019-9703-7

Recently, nitrogen-doped porous carbon supported single atom catalysts (SACs) have become one of the most promising alternatives to precious metal catalysts in oxygen reduction reaction (ORR) due to their outstanding performance, especially those derived from porphyrin-based materials. However, most of them involve other metal residuals, which would cause the tedious pre- and/or post-treatment, even mislead the mechanistic investigations and active-site identification. Herein, we report a precursor-dilution strategy to synthesize Fe SACs through the Schiff-based reaction via co-polycondensation of amino-metalloporphyrin, followed by pyrolysis at high temperature. Systematic characterization results provide the compelling evidence of the dominant presence of atomically dispersed Fe-N_x species. Our catalyst shows superior ORR performance with positive half-wave potential (E_1/2=0.85 V vs. RHE) in alkaline condition and moderate activity (E_1/2=0.68 V vs. RHE) under the acidic condition, excellent methanol tolerance and good long-term stability. All the results indicate Fe SACs would be a promising candidate for replacing the precious Pt in metal-air batteries and fuel cells.

Full text of DOI:10.1007/s11426-019-9703-7

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SCIENCE CHINA
Chemistry
•ARTICLES•
June 2020 Vol.63 No.6: 810–817
https://doi.org/10.1007/s11426-019-9703-7
Amino-metalloporphyrin polymers derived Fe single atom catalysts
for highly efficient oxygen reduction reaction
Qian He1†, Yuying Meng2†, Hao Zhang3, Ying Zhang3, Qingdi Sun1, Tao Gan1, Huajian Xiao3,
Xiaohui He1* & Hongbing Ji1,4*
1Fine Chemical Industry Research Institute, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China;
2Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan University, Guangzhou 510632, China;
3Fine Chemical Industry Research Institute, School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China;
4School of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, China
Received November 6, 2019; accepted February 18, 2020; published online March 27, 2020
Recently, nitrogen-doped porous carbon supported single atom catalysts (SACs) have become one of the most promising
alternatives to precious metal catalysts in oxygen reduction reaction (ORR) due to their outstanding performance, especially
those derived from porphyrin-based materials. However, most of them involve other metal residuals, which would cause the
tedious pre- and/or post-treatment, even mislead the mechanistic investigations and active-site identification. Herein, we report a
precursor-dilution strategy to synthesize Fe SACs through the Schiff-based reaction via co-polycondensation of amino-me-
talloporphyrin, followed by pyrolysis at high temperature. Systematic characterization results provide the compelling evidence
of the dominant presence of atomically dispersed Fe-Nx species. Our catalyst shows superior ORR performance with positive
half-wave potential (E1/2=0.85 V vs. RHE) in alkaline condition and moderate activity (E1/2=0.68 V vs. RHE) under the acidic
condition, excellent methanol tolerance and good long-term stability. All the results indicate Fe SACs would be a promising
candidate for replacing the precious Pt in metal-air batteries and fuel cells.
single atom catalysts, porphyrin, precursor-dilution, oxygen reduction reaction, active site
Citation:
He Q, Meng Y, Zhang H, Zhang Y, Sun Q, Gan T, Xiao H, He X, Ji H. Amino-metalloporphyrin polymers derived Fe single atom catalysts for highly
efficient oxygen reduction reaction. Sci China Chem, 2020, 63: 810–817, https://doi.org/10.1007/s11426-019-9703-7
1 Introduction
ricate SACs, e.g., carbon nanotubes [12], glucose [13], his-
tidine [14], porphyrin [15] and so on. Among of them,
porphyrin have attracted great attention because of the out-
standing multifunctional coordination compound and ideal
square-planar anchoring sites for metal atoms [16]. For ex-
ample, Wang and co-workers [17] prepared Pt single sites
through the carboxy-porphyrin-based 2D topological metal
organic frameworks with zirconium clusters (Zr6) as the
metal nodes. Cao et al. [18] reported iron-nitrogen active
sites derived from zinc chloride (ZnCl2) catalyzed cyano-
porphyrin-triazine-based frameworks. Jiang’s team [19]
created FeN4-type catalysts via a cuprous iodide (CuI) cat-
alyzed Sonogashira-Hagihara coupling reaction between
iodo-phthalocyanine and ethynyl-porphyrin. Seo’s group
Single atom catalysts (SACs) are of great interest in various
electrochemical processes due to their excellent catalytic
performance and well-defined structure for active-site in-
vestigations [1–6]. Especially in oxygen reduction reaction
(ORR), nitrogen-doped porous carbon supported single atom
catalysts (M-N-C, e.g., Fe-N-C [7], Co-N-C [8]) would be
promising candidates for replacing the precious Pt-based
materials as cathode catalysts [9–11].
Extensive precursor materials have been devoted to fab-
†These authors contributed equally to this work.
*Corresponding authors (email: hexiaohui@mail.sysu.edu.cn; jihb@mail.sysu.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
chem.scichina.com link.springer.com

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He et al. Sci China Chem June (2020) Vol.63 No.6
[20] used tetrakis(triphenylphosphine)palladium (Pd(PPh3)4)
catalysed Suzuki-Miyaura reaction to obtain the bromo-
porphyrin-based Fe-N-C catalyst. However, besides the tar-
get atomically dispersed metal species, most of them in-
volves other metal species as additives, which would cause
the tedious pre- and/or post-treatment to remove the ad-
ditives [21]. Moreover, the trace residue may mislead the
mechanistic investigations and active-site identification
[22,23]. Therefore, it is a great challenge to rationally design
and facilely synthesize the porphyrin-based SACs with un-
ambiguous identification the active sites at the atomic/mo-
lecular level.
Herein, we propose a facile precursor-dilution strategy to
prepare Fe-N-C-type SACs through the Schiff-based reac-
tion via co-polycondensation of amino-porphyrin materials,
followed by pyrolysis at high temperature. The obtained Fe
SACs show much higher ORR performance than those of Fe
nanoclusters (NCs) or Fe nanoparticles (NPs). Besides, it is
superior to commercial 20 wt% Pt/C in terms of ORR ac-
tivities, stability, and methanol resistance in alkaline condi-
tion. Additionally, it gives moderate ORR activities under the
acidic condition. This work provides an ideal model for
understanding the structure-activity relationship of the cat-
alyst and promising candidate for ORR research.
freshly distilled pyrrole (50 mmol). After refluxing for 3 h,
the solution was cooled to room temperature naturally and
250 mL absolute ethanol was added. Subsequently, the as-
obtained precipitate was filtered and washed with DMF,
CH3OH, respectively. The product was dried at 80 °C in
vacuum for 12 h, giving the atropurpureus powder with the
yield of 22% [24].
Synthesis of tetra(4-aminophenyl)porphyrin (TAPP). Un-
der the N2 atmosphere, the as-synthesized TNPP (2.0 mmol),
tin chloride anhydrous (SnCl2, 20 mmol) were dissolved in
the concentrated hydrochloric acid (100 mL) in a three-
necked round-bottomed flask (250 mL) under magnetic
stirring and refluxed at 80 °C for 3 h. The solution was
cooled to room temperature naturally and 500 mL deionized
water was added. After the system was cooled to 0 °C, 5 wt%
sodium hydroxide solution was used to adjust the pH value to
8. The as-obtained precipitate was filtered and dried at 80 °C
in vacuum for 12 h and purified by Soxhlet extraction with
chloroform. Finally, the solvent was removed under vacuum
1
and gave the nut-brown powder with the yield of 66%. H
NMR (500 MHz, CDCl3): δ 8.90 (s, 8H, pyrrole β–H), 8.00–
7.99 (d, J=7.99 Hz, 8H, Ar–H), 7.08–7.06 (d, J=7.07 Hz, 8H,
Ar–H), 4.04 (s, 8H, –NH2). ESI-MS (m/z): [M+H]+ calcd.,
675.30; found, 675.15 [24].
Synthesis of tetra(4-aminophenyl)porphyrin iron (Fe-
TAPP). Under the N2 atmosphere, the TAPP (2.0 mmol), iron
(II) chloride (FeCl2, 10.0 mmol), methanol (30 mL), N,N-
dimethylformamide (30 mL) and chloroform (90 mL) were
charged in a three-necked round-bottomed flask (250 mL)
under magnetic stirring, followed by refluxing for 3 h at
150 °C. Subsequently, system was cooled to room tempera-
ture, transferred into a separatory funnel and washed with
water three times. The organic layer was collected and dried
over Na2SO4. Finally, the solvent was removed under va-
cuum, producing the khaki powder with the yield of 68%.
MALDI-TOF MS ([M-Cl]+): m/z calcd., 728.21; found,
727.73.
Synthesis of poly-FeTAPP(20). Under the N2 atmosphere,
the FeTAPP (0.075 mmol), TAPP (1.5 mmol; FeTAPP:
TAPP=1:20, mmol:mmol), p-phthalaldehyde (3.15 mmol),
1,2-dichlorobenzeneand (40 mL), n-butanol (40 mL) and
6 M acetic acid solution were charged in a three-necked
round-bottomed flask (250 mL), the mixture was stirred and
refluxed for 24 h at 100 °C. Subsequently, system was cooled
to room temperature, the as-obtained precipitate was filtered
and purified by Soxhlet extraction with dioxane and acetone,
respectively. And the product was dried at 80 °C in vacuum
for 12 h and gave the khaki powder with the yield of 81%.
Synthesis of Fe-N-C-T (T represent the pyrolysis tem-
perature). The above resulting material poly-FeTAPP(20)
was placed in a programmable tube furnace and subjected to
the following thermal treatments in a flow of N2 at a rate of
100 mL/min. The furnace temperature was firstly increased
2 Experimental
2.1 Chemicals and materials
Pyrrole (99%), 4-nitrobenzaldehyde (97%), nitrobenzene
(99.5%), lactic acid (85%), ethanol (99.5%), N,N-
dimethylformamide (DMF, 99.9%), 1,2-dichlorobenzeneand
(99.5%), n-butanol (99.5%), acetic acid (99.5%), methanol
(CH3OH, 99.9%), p-phthalaldehyde (99.5%), and tin chloride
anhydrous (SnCl2, 99%) were purchased from Aladdin.
Chloroform (99.5%) and hydrochloric acid (AR) were ob-
tained from Guangzhou Chemical Reagent Factory (China).
Iron oxide (Fe2O3, 99.9%), sodium hydroxide (98%), iron(II)
chloride (FeCl2, 99.9%) and tetraphenylporphyrin iron(III)
chloride (FeTPPCl, 98%) were obtained from Huaweiruike
Chemical. Platinum on activated carbon (20 wt% Pt/C) and
Nafion solution (5 wt%) were bought from Johnson Matthey
and DuPont company, respectively. Pyrrole was distilled un-
der the N2 atmosphere before used. Other chemical or mate-
rials were used as received without any further purification.
2.2 Preparation procedures
Synthesis of tetra(4-nitrophenyl)porphyrin (TNPP). Typi-
cally, 4-nitrobenzaldehyde (50 mmol), nitrobenzene
(200 mL) and lactic acid (50 mL) were added into a two-
necked round-bottomed flask (500 mL) under magnetic
stirring and heated to 120 °C, followed by the addition of

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He et al. Sci China Chem June (2020) Vol.63 No.6
from room temperature to a final pyrolysis temperature (700,
800, 900, 1000, 1100 °C, respectively) at a heating rate of
5 °C min−1, and kept at this temperature for 3 h. Then the
furnace was cooled to room temperature naturally and gave
the black-colored powder product, respectively.
Synthesis of poly-TAPP (or poly-FeTAPP). Similar to poly-
FeTAPP(20), the poly-TAPP (or poly-FeTAPP) was obtained
without FeTAPP (or TAPP).
the surface of the glassy carbon rotating disk electrode
(RDE, diameter: 5.0 mm) and dried naturally, yielding an
approximate catalyst loading of 0.255 mg cm−2.
The ORR test of all the catalysts were performed on a Pine
Bipotentiostat (Pine Research Instrumentation) with a three-
electrode system in O2- or N2-saturated 0.1 M KOH with the
scan rate 10 mV s−1 at room temperature, where RDE was
used as a working electrode, graphitic carbon rods with a
diameter of 3.0 mm and KCl-saturated Ag/AgCl were used
as counter and reference electrodes, respectively. All the
potentials were conversed to the reversible hydrogen elec-
trode (RHE) by Nernst equation, Eq. (1):
Synthesis of N-C-900. Similar to Fe-N-C-900, the N-C-900
was obtained by pyrolysis poly-TAPP at 900 °C.
Synthesis of Fe-NPs/N-C-900. 5 mL FeCl2 solutions
(10 mM, H2O:CH3OH=1:1, V:V) was added to 1.0 g poly-
TAPP and stirred for 30 min at room temperature. After re-
moval of solvents by rotary evaporation, the material was
pyrolysis at 900 °C to obtain Fe-NPs/N-C-900.
E(V vs. RHE)=E(V vs. Ag/AgCl)+0.197+0.059 pH
(1)
Based on the RDE data, the electron transfer number (n)
per oxygen molecule for ORR was determined by Koutecky-
Levich (K-L) equations, Eqs. (2–4) [25]:
2.3 Characterizations
1
1
1
1
1
=
+
=
+
(2)
1/2
J JK JL JK
B
The X-ray diffraction (XRD) was operated on Bruker D8
Advanced diffractometer (Germany) with Cu Kα radiation
(λ=1.5418 Å). Elemental analysis (EA) and inductively cou-
pled plasma optical emission spectroscopy (ICP-OES) were
performed on Vario EL cube instrument and PerkinElmer
OPTIMA 8000DV (USA), respectively. Raman spectrum was
collected by Laser Micro-Raman Spectrometer. And X-ray
photoelectron spectroscopy (XPS) analysis was obtained on
the Thermo ScientificTM NexsaTM (USA). The nitrogen ad-
sorption and desorption isotherms of the materials were ob-
tained on ASAP2020M. The surface area data were then
obtained from the isotherms using the Brunauer-Emmett-
Teller (BET) method. Transmission electron microscopy
(TEM) images and scanning transmission electron micro-
scopy (STEM) images were obtained on FEI Tecnai G2 F30
and operated at 300 kV. Aberration corrected high-angle an-
nular dark-field scanning transmission electron microscopy
(AC HAADF-STEM) images and element mapping images
were obtained on a JEM-ARM200F transmission electron
microscopy (Japan) and operated at 200 kV. The extended X-
ray absorption fine structure spectra (EXAFS) and the X-ray
absorption near-edge structure (XANES) of Fe k3-edge were
performed in fluorescence mode on BL14W1 beamline at the
Shanghai Synchrotron Radiation Facility, Shanghai Institute
of Applied Physics, China, operated at 3.5 GeV with injection
currents of 140–210 mA and monochromatized with Si(111)
double-crystal.
1/6
B=0.62nFC0(D0)2/3
(3)
JK=nFkC0
(4)
where, J is the measured current density, JL and JK are the
diffusion- and kinetic-limiting current densities, ω is the
angular speed of the rotating electrode, n is the involved
electron transfer number, F is the Faraday constant (F=9.65×
104 C mol−1), C0 is the saturated O2 concentration in the
electrolyte (C0=1.2×10−6 mol cm−3), D0 is the diffusion
coefficient of O2 in the electrolyte (D0=1.9×10−5 cm2 s−1), v
is the kinetic viscosity of the solution (v=0.01 cm2 s−1), and k
is the electron-transfer rate constant. The values of n and JK
were obtained from the slope and y-intercept of the K-L plots
(or J−1 vs. ω−1/2), respectively.
For the rotating ring-disk electrode (RRDE, 5.61 mm)
technique, the n and hydrogen peroxide (H2O2) yield were
determined by the followed equations, Eqs. (5, 6) [26].
4JD
n=
(5)
JR
N
JD+
JR
N
JD+
%H2O2=200 ×
(6)
JR
N
where, JD and JR are the disk and ring current densities,
respectively. And N is the collection efficiency of the Pt ring
electrode and found to be 0.37 in our system.
Similarly, the activity under an acidic condition was tested
in 0.1 M HClO4 solution.
2.4 Electrocatalytic performance tests
Preparation of the working electrode: 2.00 mg of the elec-
trocatalyst was mixed with 20 μL of a 5 wt% Nafion solution
and 380 μL of isopropanol, and the mixture was then soni-
cated for 30 min to form the ink suspension. Subsequently,
4 μL of the homogeneous ink suspension was dropped onto
3 Results and discussion
Scheme 1 illustrates the precursor-dilution strategy [27,28]
for preparing the Fe-N-C-type catalysts. Briefly, FeTAPP

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He et al. Sci China Chem June (2020) Vol.63 No.6
(precursor, Figure S1, Supporting Information online) and
excess TAPP (diluent, FeTAPP:TAPP=1:20, mmol:mmol)
were co-polycondensation with p-phthalaldehyde to obtain
the corresponding polymer precursor (poly-FeTAPP(20)).
After pyrolysis at 900 °C for 3 h under the nitrogen atmo-
sphere, N-doped porous carbon supported Fe SACs (Fe-N-
C-900) were obtained. The corresponding ICP-OES reveals
that the actual iron loading of Fe-N-C-900 was 0.52 wt%
(Table S1, Supporting Information online). For comparison,
Fe-N-C-T catalysts (T represent the pyrolysis temperature,
from 700 to 1100 °C), metal-free N-doped porous carbon
(N-C-900), and catalyst without diluent (poly-FeTAPP-900)
were also prepared with the similar methods. Besides,
N-doped porous carbon supported iron nanoparticles (Fe-
NPs/N-C-900) was also obtained by impregnation method.
All the details were included in Table S1.
TEM and STEM images of Fe-N-C-900 show the absence
of metal nanoparticles (Figure 1(a, b)) in Fe-N-C-900. In
contrast, there were obvious nanoparticles for poly-FeTAPP-
900 (Figure S2), which were directly obtained from pyr-
olyzing the poly-FeTAPP (without diluent). On the other
hand, compare with the N-C-900 metal-free catalyst (Figure
S3), the Fe-N-C-900 catalyst displayed the clear white dots,
highlighted by yellow circles, in the AC HAADF-STEM
image (Figure 1(c)), suggesting that the Fe species highly
dispersed throughout the carbon matrix. Moreover, element
mapping image illustrated all the elements homogeneously
distributed (Figure 1(d)), indicating the TAPP diluent plays a
key role in enlarging the average distance of Fe metal atoms,
providing more free N-rich vacancies and preventing the
metal species from aggregation to nanoparticles [29].
In order to understand the status and the coordination en-
vironment of the Fe species in Fe-N-C-900 catalyst, XPS and
XAS were performed to analyze the centre atoms. Firstly,
high-resolution N 1s region (Figure 2(a)) could be well de-
convoluted into four peaks, corresponding to pyridinic-N
(398.6 eV), Fe-N (399.7 eV), pyrrolic-N (400.8 eV), and
graphitic-N (401.5 eV) [30]. It is well known that the Fe-N-C
structure and graphitic-N account for high activity and sta-
bility in electrochemical process. Secondly, the high-re-
solution of Fe 2p spectrum (Figure 2(b)) showed two major
peaks at 711.3 and 724.2 eV along with the corresponding
satellites peaks (714.5 and 730.3 eV), indicating the presence
Figure 1 The morphology of Fe-N-C-900: (a) TEM image, (b) STEM
image, (c) AC HAADF-STEM image (Fe single atoms were highlighted by
yellow circles), and (d) corresponding element mappings (color online).
of positively charged Feδ+ and Fe-N/O. Figure 2(c) showed
the absorption edge of X-ray absorption near-edge structure
(XANES) spectroscopy at the Fe K-edge of Fe-N-C-900 and
the corresponding references (FeTPPCl, Fe foil, and Fe2O3),
and the intensity of Fe-N-C-900 was situated between those
of Fe foil and Fe2O3, once again, implying that the isolated
Fe atoms carried partially positive charges [31].
As shown in Figure 2(d), the signal of Fe-N-C-900 at 1.67
Å should be attributed to Fe–N or Fe–O bonds compared
with the FeTPPCl (1.58 Å, Fe–N) and Fe2O3 (1.54 Å, Fe–O)
[31], while the absence of peak at 2.18 (Fe–Fe) and 2.52 Å
(Fe–O–Fe) clearly suggested the dominant presence of iso-
lated Fe atoms [32], which was in good agreement with the
result of AC HAADF-STEM. Secondly, the wavelet trans-
form (WT, Figure 2(e)) analysis of Fe-N-C-900 exhibited
only one intensity maximum at 4.6 Å−1, assigned to Fe–N or
Fe–O [33], while a peak at 7.9 Å−1 in the WT of Fe foil
corresponds to Fe−Fe bonds. Further information on the
local structures of Fe-N-C-900 was obtained by EXAFS
fitting curves (Figure 2(f), Table S2), suggesting the Fe–N
shell with a coordination number of 4.4 at the distance of
2.07 Å and the Fe–O shell with a coordination number of 0.8
at the distance of 2.18 Å, respectively. This indicated that the
Fe atoms connected with four N atoms and one O atom,
forming a FeN4O structure (Figure 2(f)) [34]. Therefore, a
precursor-dilution strategy was successfully explored to
synthesize the Fe SACs without using any template or ad-
ditional metals.
The structure and properties of Fe-N-C-T catalysts were
further characterized by XRD and Raman spectroscopy.
There were two pronounced broad peaks around the 20°–30°
and 40°–45° (Figure 3(a)), which could be attributed to the
Scheme 1 Schematic illustration of the preparation of Fe-N-C-type cat-
alysts. The molar ratio of FeTAPP:TAPP is 1:20 (color online).