Isolated single iron atoms anchored on a N, S-codoped hierarchically ordered porous carbon framework for highly efficient oxygen reduction

Article abstract of DOI:10.1039/d1ta00384d

Atomically dispersed metal catalysts are promising candidates for the oxygen reduction reaction (ORR) and for achieving efficient energy conversion. However, rational design of single atom catalysts (SACs) with high-efficiency ORR catalytic activity and superior stability is still crucial yet challenging. Herein, anin situgas-foaming methodology is presented for constructing single Fe atoms dispersed on a N, S-codoped (Fe_SA/NSC) hierarchically ordered porous carbon (HOPC) frameworkviaone-step pyrolysis of dopamine (DA)/Fe³⁺complexes and thiourea in SBA-15 channels. HOPC structures (facilitating active site access and mass transfer) and optimized FeN₄S₂catalytic centers make Fe_SA/NSC exhibit high ORR activity with a half-wave potential (E_1/2) of 0.91 V, fuel selectivity and long-term stability (3 mV negative shift after 5000 potential cycles) in 0.1 M KOH. It even shows comparable ORR catalytic activity (E_1/2= 0.78 V) to the Pt/C catalyst in acidic electrolytes. As the air electrode in zinc-air batteries, Fe_SA/NSC demonstrates superior power density, long-term discharge stability and specific capacity to the commercial Pt/C catalyst. Thus, Fe_SA/NSC is a promising non-platinum-group metal ORR catalyst for the ORR and application in zinc-air batteries.

Full text of DOI:10.1039/d1ta00384d

Volume 9
Number 16
28 April 2021
Pages 9963–10510
Journal of
Materials Chemistry A
Materials for energy and sustainability
rsc.li/materials-a
ISSN 2050-7488
PAPER
Xin Jia et al.
Isolated single iron atoms anchored on a N, S-codoped
hierarchically ordered porous carbon framework for highly
efficient oxygen reduction

Journal of
Materials Chemistry A
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PAPER
Isolated single iron atoms anchored on a N, S-
codoped hierarchically ordered porous carbon
framework for highly efficient oxygen reduction†
Cite this: J. Mater. Chem. A, 2021, 9,
10110
a
Xinghuan Liu,‡a Xingwu Zhai,‡a Wenbo Sheng,a Juan Tu,a Zeyu Zhao,a Yulin Shi,
a
b
a
*
Caixia Xu,
Guixian Ge
and Xin Jia
Atomically dispersed metal catalysts are promising candidates for the oxygen reduction reaction (ORR) and
for achieving efficient energy conversion. However, rational design of single atom catalysts (SACs) with
high-efficiency ORR catalytic activity and superior stability is still crucial yet challenging. Herein, an in situ
gas-foaming methodology is presented for constructing single Fe atoms dispersed on a N, S-codoped
(FeSA/NSC) hierarchically ordered porous carbon (HOPC) framework via one-step pyrolysis of dopamine
(DA)/Fe3+ complexes and thiourea in SBA-15 channels. HOPC structures (facilitating active site access
and mass transfer) and optimized FeN4S2 catalytic centers make FeSA/NSC exhibit high ORR activity with
a half-wave potential (E1/2) of 0.91 V, fuel selectivity and long-term stability (3 mV negative shift after
5000 potential cycles) in 0.1 M KOH. It even shows comparable ORR catalytic activity (E1/2 ¼ 0.78 V) to
the Pt/C catalyst in acidic electrolytes. As the air electrode in zinc–air batteries, FeSA/NSC demonstrates
superior power density, long-term discharge stability and specific capacity to the commercial Pt/C
catalyst. Thus, FeSA/NSC is a promising non-platinum-group metal ORR catalyst for the ORR and
application in zinc–air batteries.
Received 14th January 2021
Accepted 24th March 2021
DOI: 10.1039/d1ta00384d
rsc.li/materials-a
signicantly benet the ORR activity and stability.11–15 Among
these catalysts, Fe-based SACs have been widely considered as
a promising candidate to replace Pt-based ORR catalysts.16–20
1. Introduction
The oxygen reduction reaction (ORR) is a crucial process
occurring in fuel cells, metal–air batteries and other energy
conversion and storage devices.1–4 However, the high thermo-
dynamic potential energy and slow dynamics of four electron
and proton transfer steps in the ORR limit its widespread
applications.5,6 Traditionally, platinum-group noble metals are
believed to have prominent electrocatalytic activities for the
ORR process, but the high cost, low abundance and poor
stability limit their commercial applications.7,8 In this regard,
development of highly active, cost-effective and non-platinum
metal catalysts for the ORR is of great signicance for large
scale applications.9,10
However, Fe-based electrocatalysts still suffer from poor elec-
trochemical stability and undesirable catalytic activity in acidic
electrolytes.1,21,22 Current reported strategies involve improving
the metal loading,23,24 doping heteroatoms25 and designing
dual-metal sites.26,27 However, the most reported Fe content in
Fe-based SACs is below 1.0 wt%, and a large amount of active Fe
species are encapsulated in the carbon framework, which
cannot participate in the reaction process of the ORR due to the
inaccessibility of SACs.24,28 Therefore, rational fabrication of Fe
based SACs with superior ORR catalytic activity and stability is
crucial yet challenging.
To cope with this challenge, optimizing the pore structures
for the construction of SACs with large surface areas and
outstanding conductivity would facilitate mass/electron trans-
fer and signicantly enhance the ORR activity.29,30 Ordered
mesoporous carbon (OMC) architectures,31 a family of porous
materials, have attracted widespread attention due to their large
specic surface areas, excellent conductivity and strong resis-
tance to corrosion.32 However, OMC structures only have
a single pore sized distribution, and a large number of active
sites in the interior of OMC cannot participate in the ORR
process. In addition, doping of metal/N with heteroatoms has
been discovered to signicantly inuence the electronic struc-
ture center of the SACs and further boost the ORR
Single atom catalysts (SACs), especially non-platinum metal
and nitrogen codoped carbon matrices (M ¼ Fe, Co, Ni, Cu, Zn,
Mn), would realize the utmost utilization of metal sites and
aSchool of Chemistry and Chemical Engineering, Key Laboratory for Green Processing
of Chemical Engineering of Xinjiang Bingtuan, Key Laboratory of Materials-Oriented
Chemical Engineering of Xinjiang Uygur Autonomous Region, Engineering Research
Center of Materials-Oriented Chemical Engineering of Xinjiang Bingtuan, Shihezi
University, Shihezi 832003, China. E-mail: jiaxin@shzu.edu.cn
bKey Laboratory of Ecophysics and Department of Physics, College of Science, Shihezi
University, Shihezi, 832003, P. R. China. E-mail: geguixian@126.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ta00384d
‡ These authors contributed equally to this work.
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performance.33–35 Thus, the design and synthesis of single O2 and reaction intermediates into or/and out of the channels;
atoms anchored on OMC with hierarchical structures and (2) the large BET specic surface areas and interpenetrating
optimizing the coordination environment of SACs are efficient mesopore architecture can provide more active sites for the ORR
ways to enhance ORR catalytic activity in both acidic and alka- and tremendously enhance the transport efficiency.39 HOPC
line media.36,37
structures (facilitating active site access and mass transfer) and
Herein, inspired by our previous research, direct calcination optimized FeN4S2 catalytic centers (modifying the electronic
of dopamine conned by graphitic carbon nitride (g-C3N4) can structures of Fe–N4 active centers) make FeSA/NSC exhibit
form 2D N-doped graphene-like nanosheets.38 We report superb oxygen reduction activity, stability and methanol toler-
a robust in situ gas-foaming approach for construction of ance in both acidic and alkaline media. In addition, FeSA/NSC
accessible iron SACs on a hierarchically ordered porous N, S- also exhibits potential prospects in Zn–air batteries.
codoped catalyst (FeSA/NSC) for boosting the ORR activity via
one-step pyrolysis of a dopamine (DA)/Fe3+ complex and thio-
2. Experimental section
2.1 Materials
urea in SBA-15 channels (Fig. 1a). The introduction of thiourea
can provide nitrogen and sulfur source for anchoring more iron
atoms and tuning the coordination environment of SACs. More
Dopamine (DA) hydrochloride and Naon (5 wt%) were ob-
importantly, thiourea plays the role of a sacricial template for
tained from Aldrich. Thiourea (99.5%) and urea (99.5%) were
constructing HOPC structures in the process of carbonization.
purchased from Tianjin Fu Yu Chemical Co. SBA-15 (no XFF01,
Such HOPC structures are especially essential in the ORR
pore diameter: 6–10 nm, specic surface area: 550–600 m2 gꢀ1
)
process owing to the following notable advantages: (1)
numerous mesopore or micropore openings on the outer
surfaces can signicantly benet movement of the electrolyte,
was bought from Jiangsu XFNANO Materials Technology Co.,
Ltd (Jiangsu, China). Iron chloride hexahydrate (99.9%) and
ethanol (99.7%) were purchased from Adamas-beta® and
Fig. 1 (a) Schematic of the synthesis process of the FeSA/NSC catalyst. Gray, white, red, yellow, blue and orange balls represent C, H, O, S, N and
Fe atoms, respectively. (b) and (c) SEM images of Fe/NC and FeSA/NSC, respectively. (d) and (e) HRTEM images of FeSA/NSC. The saffron yellow
circles represent the HOPC structures. (f) HAADF-STEM image of FeSA/NSC, with single Fe atoms highlighted by yellow circles. (g) HRTEM images
and corresponding elemental maps showing the elemental distribution of C (blue), O (green), N (red), S (purple) and Fe (orange).
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Tianjin Fu Yu Chemical Co., respectively. All the materials were spectroscopy (XPS) was performed using an Axis Ultra spec-
used without further purication.
trometer with a mono-chromatized Al Ka X-ray as the excitation
source (225 W). Brunauer–Emmett–Teller (BET) specic surface
areas were measured by nitrogen adsorption–desorption anal-
yses using a Micromeritics ASAP 2020. Raman spectra were
measured on a LabRAM HR Evolution Raman microscope with
514.5 nm laser excitation in the range of 500–2000 cmꢀ1. X-ray
absorption data at the Fe K-edge of the samples were recorded
at room temperature in the transmission mode using ion
chambers (reference samples) and uorescence excitation
mode using a Lytle detector (control samples) at beamline
BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF).
The station was operated with a Si(111) double crystal mono-
chromator. During the measurements, the synchrotron was
operated at 3.5 GeV and the current was between 150 and 210
mA. The data for each sample were calibrated with standard Fe
metal foil. Data processing was performed using the program
ATHENA. Extended X-ray absorption ne structure (EXAFS)
spectra were tted using the FEFF 6.0 code.
2.2 Preparation of the catalysts
50 mg SBA-15, 100 mg dopamine hydrochloride and 200 mg
thiourea were added in 15 mL ethanol and 15 mL deionized
water. The resulting mixture was stirred for 30 min. Further-
more, 1 mL of FeCl3$6H2O aqueous solution (47.5 mg mLꢀ1
)
was added in the mixture and the solution quickly turned dark
blue, indicating the formation of an Fe3+/dopamine complex.
The resulting mixture was continuously stirred at 60 ꢁC for
evaporating the solvent until the mixture became a solid.
Subsequently, the obtained sample was carbonized with a two-
step process under a ow of Ar in a temperature-programmable
tube furnace. It was heated at 600 ꢁC for 60 min with a heating
rate of 2.5 ꢁC minꢀ1, which was followed by further treatment at
900 ꢁC for 120 min with a heating rate of 2 ꢁC minꢀ1. The
pyrolyzed product was treated with 4 wt% HF aqueous solution
for 24 h to remove the SBA-15 template. Furthermore, the
mixture was centrifuged (10 000 rpm, 5 min) and washed with
deionized water several times. The obtained precipitates were
leached in 2 M HCl for 24 h to remove inactive iron species. The
catalyst prepared by replacing thiourea with urea in the channel
of SBA-15 (called Fe/NC-UI) was prepared via the same proce-
dure as above only replacing thiourea (200 mg) with urea (158
mg) with the same mole number. NSC and Fe/NC were
synthesized via the same procedure as above without iron or
thiourea, respectively. NC was synthesized via the same proce-
dure as above without iron and thiourea.
Fe/NSC-TO (the catalyst prepared with thiourea outside
SBA-15 channels) was prepared via the following procedure:
50 mg SBA-15 and 100 mg dopamine hydrochloride were added
in 15 mL ethanol and 15 mL deionized water. The resulting
mixture was stirred for 30 min. Furthermore, 1 mL of FeCl3-
$6H2O aqueous solution (47.5 mg mLꢀ1) was added in the
mixture. The resulting mixture was continuously stirred at
60 ꢁC for 12 h and then the solvent was slowly evaporated until
the mixture became a solid. The mixture was ground and then
put in the le margin of a porcelain boat. Subsequently,
200 mg thiourea was put in the right margin of the porcelain
2.4 DFT calculations
The all spin-polarized density functional theory (DFT) method
was employed to obtain geometric optimizations using the
Vienna ab initio simulation package (VASP).40,41 Electron
exchange–correlation is represented by the Perdew–Burke–
Ernzerhof (PBE) functional of generalized gradient approxima-
tion (GGA).42 Projector augmented wave (PAW) pseudo-
potentials were used to describe the ionic cores.43 The FeSA
/
NC and FeSA/NSC monolayers were modeled by using a 6 ꢂ 6 ꢂ
1 supercell, and Brillouin zones were sampled with a 2 ꢂ 2 ꢂ 1
grid centered at the gamma (G) point in the reciprocal space for
ꢀ
geometric optimization. A vacuum region more than 20 A along
the z-axis was set to prevent the interaction between two adja-
cent periodic images. A cut-off energy of 450 eV was adopted for
the plane-wave basis. Van der Waals (vdW) interactions have
been considered with Grimme's semiempirical DFT-D3
scheme.44 Convergence tolerances for energy and force were
ꢀꢀ1
set to 10ꢀ5 eV and 0.02 eV A , respectively. The Gibbs free
energy change (DG) for each of the oxygen reduction reaction
(ORR) steps was estimated by applying the computational
hydrogen electrode (CHE) model of Nørskov et al.45 The CHE
model uses one half of the chemical potential of gaseous
hydrogen (m(H2)) as the chemical potential of the proton–elec-
tron pair (m(H+/eꢀ)). The reaction scheme of the ORR in acidic
media at the zero electrode potential can be written as:
boat. The porcelain boat was put in
a temperature-
programmable tube furnace under a ow of Ar from right to
le. Other experimental procedures followed the same steps as
mentioned above.
2.3 Characterization
* + O2(g) + (H+ + eꢀ) / OOH*
OOH* + (H+ + eꢀ) / H2O(l) + O*
O* + (H+ + eꢀ) / OH*
(1)
(2)
(3)
(4)
Scanning electron microscopy images were acquired on a SEM
(Hitachi SU8010 Japan). Transmission electron microscopy
(Hitachi HT7700 Japan) was used to observe the morphologies
of the catalysts. The morphologies of the catalysts were char-
acterized by aberration-corrected scanning transmission elec-
tron microscopy (AC-STEM) using an FEI Titan Cubed Themis
G2 300 equipped with an energy dispersive X-ray spectrometer
(EDS). XRD patterns at wide angles (10–90ꢁ) were collected
using a Bruker D8 advanced X-ray diffractometer with Cu Ka
irradiation at 40 kV and 40 mA. X-ray photoelectron
OH* + (H+ + eꢀ) / H2O(l) + *
In alkaline media, the elementary steps are:
O2(g) + H2O(l) + eꢀ + * / OOH* + OHꢀ
(5)
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OOH* + eꢀ / O* + OHꢀ
O* + H2O(l) + eꢀ / OH* + OHꢀ
OH* + eꢀ / * + OHꢀ
(6)
(7)
1/J ¼ 1/JK + 1/JL ¼ 1/Bu1/2 + 1/JK
(11)
(12)
B ¼ 0.2nFC0(D0)2/3nꢀ1/6
(8) where J is the measured current density, JK and JL are the kinetic
and diffusion-limiting current densities, u is the rotating speed
of the disk, F is the Faraday constant (96 485 C molꢀ1); C0 is the
bulk concentration of O2 (1.2 ꢂ 10ꢀ6 mol cmꢀ3 in 0.1 mol Lꢀ1
KOH and 1.26 ꢂ 10ꢀ6 mol cmꢀ3 in 0.1 mol Lꢀ1 HClO4), D0 is the
(9)
According to the CHE model, the DG values for each ORR
step were dened as
DG ¼ DEDFT + DEZPE ꢀ TDS + DGU + DGpH
diffusion coefficient of O2 (1.9 ꢂ 10ꢀ5 cm2 sꢀ1 in 0.1 mol Lꢀ1
KOH and 1.93 ꢂ 10ꢀ5 cm2 sꢀ1 in 0.1 mol Lꢀ1 HClO4); and y is
the kinematic viscosity of the electrolyte (0.01 cm2 sꢀ1 in
0.1 mol Lꢀ1 KOH and 1.009 ꢂ 10ꢀ2 cm2 sꢀ1 in 0.1 mol Lꢀ1
HClO4).
The electron transfer number and hydrogen peroxide yield
(%H2O2) during the ORR can be determined by the RRDE
technique:
In this equation, DEDFT is the reaction energy of reactant and
product molecules adsorbed on the catalyst surface, obtained
from DFT calculations. DEZPE and DS are the change in the zero-
point energy (ZPE) and entropy at room temperature (T ¼ 298.15
K), which are obtained aer frequency calculations. DGU is the
free energy contribution related to the applied electrode
potential (U). DGpH is the Gibbs free energy change caused by
the pH value, which was calculated using
H2O2 (%) ¼ (200 ꢂ Ir/N)/(Id + Ir/N)
n ¼ (4 ꢂ Id)/(Id + Ir/N)
(13)
(14)
DGpH ¼ ln 10 ꢂ kBT ꢂ pH
(10)
where kB is the Boltzmann constant. Hence, the equilibrium
potential (U0) for the four electron transfer ORR at pH ¼ 0 and
pH ¼ 14 was determined to be 1.229 and 0.401 V versus NHE,
respectively.
where Id is the disk current, Ir is the ring current, and N ¼ 0.37 is
the current collection efficiency of the Pt ring.
2.6 Zn–air battery tests
Single-cell testing of the catalyst was performed in a laboratory-
constructed Zn–air battery. A polished Zn plate (purity >99.9%,
thickness: 0.5 mm) was used as the anode. Homogeneous ink
was loaded on 2.25 cmꢀ2 hydrophobic carbon paper and used as
the air cathode with a loading density of 1.3 mg cmꢀ2, which
was dried at 60 ꢁC for 12 h. In the Zn–air battery system,
a microporous membrane (38 mm polypropylene membrane,
Celgard 2340, American) was used as the separator, and a nickel
foam net was used as the current collector. Pictures of the
detailed assembly process of the Zn–air battery are shown in
Fig. S17.† For comparison, a Pt/C electrode with the same
catalyst loading was also prepared.
2.5 Electrochemical measurements
All of the electrochemical measurements were performed with
a CHI 760E electrochemical analyzer (Cheng Hua Instruments,
Inc., Shanghai, China) at room temperature. Electrochemical
measurements were performed in a conventional three elec-
trode cell by using Ag/AgCl (saturated KCl) as the reference
electrode, a platinum wire as the counter electrode and catalyst
coated glassy carbon (GC, 3.0 mm diameter) electrodes as the
working electrodes. To prepare the working electrode, 3.0 mg of
the catalyst was dispersed ultrasonically for 30 min in a solution
of ethanol (600 mL) and Naon (5 wt%, 30 mL). 10 mL of the well-
dispersed mixture was dropped onto a glassy carbon electrode
with a catalyst loading of 0.674 mg cmꢀ2, which was then fully
dried at room temperature. Cyclic voltammetry (CV) and linear
sweep voltammetry (LSV) were conducted with a sweep rate of
3. Results and discussion
3.1 Synthesis and structural characterization of the catalysts
50 mV sꢀ1 and 5 mV sꢀ1, respectively. All electrochemical tests It is essential to choose appropriate precursors to anchor metal
were measured in N2 or O2-saturated 0.1 M KOH aqueous ions for constructing porous structures and nally forming
solution at room temperature.
single atom catalysts.11,46 Polydopamine (PDA), a commonly
To assess the electrochemical performance of catalysts for used nitrogen-rich precursor in energy storage and conver-
the ORR in acidic media, suspensions of different catalysts (10 sion,47–49 was rst selected as a precursor complexing with metal
mL) were then dropped onto the surface of a prepolished glassy ions to prepare porous structures by the solvent evaporation
carbon electrode (GCE) followed by drying naturally. CV curves method. As shown in Fig. S1a and b,† almost no OMC structures
were measured in 0.1 M HClO4 with a scan rate of 50 mV sꢀ1
.
are observed and the ferric nanoparticles appeared in the
The LSV curves for the ORR were recorded using the rotating carbon framework, indicating that the insoluble and large size
disk electrode (RDE) technique with a scan rate of 5 mV sꢀ1. The of the PDA/Fe3+ complex could restrict their inltration into the
electrolyte was N2- or O2-saturated 0.1 M HClO4 aqueous voids of SAB-15. By contrast, OMC structures are formed in
solution.
a carbon matrix by using DA as the precursor (Fig. S1c†). The as-
The electron transfer number (n) and kinetic current density prepared DA/Fe3+ complex is soluble in ethanol/water solvents,
(Jk) were investigated based on the Koutecky–Levich (K–L) which guarantees sufficient contact with the SBA-15 template
equation:
and complete lling of the channels of SBA-15. Aer that,
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pyrocondensation polymerization of DA occurs in the channels
of SBA-15 during the carbonization process. This is conducive to
the formation of OMC structures and avoiding the agglomera-
tion of Fe species during thermal pyrolysis. As shown in Fig. 1a,
the mixture of the DA/Fe3+ complex and thiourea inltrated into
the voids of SAB-15 by the solvent evaporation process (DA/Fe/
T@SBA-15, step I). Then, decomposition of thiourea in the
channels of SBA-15 occurred during the pyrolysis process in an
argon atmosphere and eventually formed porous Fe based
carbon materials and SBA-15 compounds (Fe/NSC@SBA-15,
step II); nally, FeSA/NSC was obtained aer HF etching and
acid washing (step III).
The detailed morphologies of the catalysts were observed by
using scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) images, which show that FeSA/NSC
has HOPC structures with abundant macropores, mesopores
and micropores (Fig. 1c–e) compared to Fe/NC without addition
of thiourea (Fig. 1b). The different synthesis schematics of our
HOPC structures (Scheme S1a†) and traditional OMC structures
(Scheme S1b†) are shown in Scheme S1.† Such HOPC structures
provide sufficient exposure of active sites and facilitate rapid
electron transport. The catalysts prepared by replacing thiourea
with urea in the channels of SBA-15 (called Fe/NC-UI) also
exhibited similar HOPC structures (Fig. S2b, c and S3b†), which
conrmed that the in situ generated gases from thiourea and
urea have similar abilities to construct pore structures. Ther-
mogravimetry (TG) technology was used to analyse the thermo-
decomposition process of thiourea and urea (Fig. S4†), and the
results exhibited sharp decline between 200 and 400 ꢁC. The
decomposition of thiourea or urea could generate a huge
amount of gases and eventually form HOPC structures via an in
situ gas-foaming process. In contrast, only a small amount of
HOPC structures are observed on catalyst prepared using thio-
urea outside SBA-15 channels (Fe/NSC-TO), Fe/NC and NC
without the addition of thiourea (Fig. S2 and S3†). N2 adsorp-
tion–desorption analysis was applied to further evaluate the
porous structures and properties of the FeSA/NSC, Fe/NSC-TO,
Fe/NC-UI and Fe/NC catalysts (Fig. 2a). All of them display
a well-dened hysteresis loop at a higher N2 pressure (P/P0 ¼
0.45–0.95), suggesting the existence of mesopores.24 The Bru-
nauer–Emmett–Teller (BET) surface area and the total pore
volume of FeSA/NSC are measured to be 1224.03 m2 gꢀ1 and 1.7
cm3 gꢀ1, which are much larger than those of Fe/NSC-TO
(704.64 m2 gꢀ1 and 0.8 cm3 gꢀ1), Fe/NC-UI (1091.34 m2 gꢀ1
and 1.37 cm3 gꢀ1), and Fe/NC (858.92 m2 gꢀ1 and 0.7 cm3 gꢀ1).
Moreover, FeSA/NSC and Fe/NC-UI catalysts exhibit a wider pore
size distribution (PSD) than Fe/NSC-TO and Fe/NC (Fig. 2b), and
could be classied into three levels: (1) porous carbon skeletons
(50–200 nm) with massive macropores; (2) abundant mesopores
mainly concentrated at about 6.5 nm; and (3) a large number of
micropores (<2 nm) throughout the entire structure. By
contrast, the PSDs of Fe/NSC-TO and Fe/NC were mainly
centered at about 6.5 nm, revealing the existence of a single PSD
without addition of thiourea or urea, further strengthening the
role of thiourea or urea in constructing HOPC structures.
To further demonstrate the existential state of ferric species,
FeSA/NSC was characterized by high-angle annular dark-eld
Fig. 2 (a) Nitrogen adsorption–desorption isotherm spectra and (b)
pore size distribution (PSD) curves of FeSA/NSC, Fe/NSC-TO, Fe/NC-
UI, and Fe/NC. (c) Fe K-edge X-ray absorption near-edge structure
(XANES) and (d) Fourier transform-extended X-ray absorption fine
structure (FT-EXAFS) spectra of FeSA/NSC, Fe foil, FeS2, FeO, and
Fe2O3. Inset in (c): model of FeN4S2; Fe orange sphere, N blue spheres,
S yellow spheres and C gray spheres. High-resolution XPS spectra of N
1s (e) and (f) S 2p of FeSA/NSC.
scanning transmission electron microscopy (HAADF-STEM),
which shows that Fe species are atomically dispersed on the
carbon support (marked with yellow circles) (Fig. 1f). No
aggregated metallic species are observed in high-resolution
transmission electron microscopy (HRTEM) (Fig. 1d and e).
Energy dispersive X-ray spectroscopy (EDS) mapping images
reveal that Fe, N, S, O, and C are uniformly distributed
throughout the entire nanostructure of the FeSA/NSC catalyst.
The X-ray absorption near edge structure (XANES) spectra of
FeSA/NSC in Fig. 2c reveal that the absorption edge position is
located between those of FeO and Fe2O3, reecting that the
valence state of Fe species in FeSA/NSC is between Fe2+ and
3+
Fe .50 The main peak around 1.5 A is observed in the Fourier
ꢀ
transform (FT) k3-weighted extended X-ray absorption ne
structure (EXAFS) spectrum of FeSA/NSC, attributable to the Fe–
N coordination in the structure.23,51 No characteristic peaks of
ꢀ
ꢀ
Fe–Fe at about 2.2 A and Fe–S at about 1.8 A for FeSA/NSC are
observed, further indicating that Fe atoms are dispersed on the
HOPC framework and stabilized by nitrogen. To further identify
the local structural parameters of Fe atoms in FeSA/NSC,
quantitative EXAFS tting was conducted (Fig. S6c†). EXAFS
tting parameters listed in Table S2† evidence that the Fe atoms
in FeSA/NSC are coordinated by four N atoms with two sets of
ꢀ
ꢀ
Fe–N bond lengths (1.98 A and 1.87 A). This is attributed to the
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doping of S that plays a role in inuencing the Fe–N bond Moreover, the linear sweep voltammetry (LSV) curves were
length.33 An accurate model of FeN4S2 active sites on the HOPC measured on a rotating ring electrode (RDE) in O2-saturated
carbon framework was deduced using density functional theory 0.1 M KOH. As revealed in Fig. 3a and b, the FeSA/NSC catalyst
(DFT). Among the four feasible structures of FeN4S2 active sites shows the most excellent ORR performance with a positive half-
(Fig. S7†), the most possible structure was found (Fig. S7b†), wave potential (E1/2) of 0.91 V and the highest diffusion kinetic
which reveals very close structural parameters with those ob- current density (Jk) at 19.83 mA cmꢀ2, among all the catalysts
tained from experimental measurements (Tables S2 and S3†). investigated in this work (Fe/NSC-TO, E1/2 ¼ 0.87 V, Jk ¼ 15.48
Therefore, the centre site of FeN4S2 (inset in Fig. 2c) dominates mA cmꢀ2; Fe/NC-UI, E1/2 ¼ 0.86 V, Jk ¼ 13.28 mA cmꢀ2; Fe/NC,
the FeSA/NSC catalyst.
E1/2 ¼ 0.83 V, Jk ¼ 6.07 mA cmꢀ2; NSC, E1/2 ¼ 0.80 V, Jk ¼ 1.36
To gain deep insight into the surface chemical composition mA cmꢀ2; NC, E1/2 ¼ 0.77 V, Jk ¼ 0.40 mA cmꢀ2). The results
and elemental bonding conguration of the catalysts, X-ray demonstrate that HOPC structures and the introduction of S
photoelectron spectroscopy (XPS) analyses were conducted, into FeSA/NSC catalysts contribute to the improvement of ORR
which reveal that C, N, O, S and Fe elements are included in activity. The E1/2 and Jk of FeSA/NSC are even much better than
FeSA/NSC (Fig. S8a†), and no Si 2p signal is observed in the XPS those of commercial Pt/C (E1/2 ¼ 0.87 V, Jk ¼ 17.41 mA cmꢀ2
)
spectra of the catalysts, indicating the complete removal of SBA- (Fig. 3b). The ORR catalytic activity of FeSA/NSC is higher than
15 templates by HF. As shown in Fig. 2e, high-resolution N 1s those of most recently reported Fe-based electrocatalysts or
spectra of FeSA/NSC can be tted to ve peaks, pyridinic N at other non-precious M–N/C catalysts (Table S6†). To quantify the
397.8 eV, Fe–Nx at 398.6 eV, pyrrolic N at 399.9 eV, graphitic N at ORR pathway, the rotating ring-disk electrode (RRDE) method
400.7 eV and oxidized N at 402.4 eV. Along with S doping, the was used to monitor the yield of H2O2 during the ORR process.
high resolution S 2p spectra in Fig. 2f highlight two main peaks The H2O2 yield of FeSA/NSC is below 5% in the potential range of
corresponding to the binding energies of 163.9 eV, 164.9 eV 0.2 to 0.7 V, indicating the superior selectivity toward H2O
assigned to S 2p3/2 and S 2p1/2 of the thiophene-like structure
(C–S–C) between carbon and sulfur atoms, respectively.52
small peak at around 167.6 eV is also observed, corresponding
to the sulfate species. No metal–S bonds are detected for FeSA
A
/
NSC in either Fe 2p or S 2p spectra, further reecting that Fe
species are only anchored with N. The high-resolution Fe 2p XPS
spectra show that no signals of metallic Fe are detected in FeSA
/
NSC (Fig. S8c†). However, metallic Fe appeared in the Fe 2p XPS
spectrum of Fe/NC without the addition of thiourea or urea
(Fig. S8c†), suggesting that thiourea or urea plays an important
role in providing a N source and anchoring Fe atoms. The
contents of sulfur for FeSA/NSC, Fe/NSC-TO and NSC are 1.42,
1.28 and 1.36 at% based on XPS analysis, respectively (Table
S4†). The amount of Fe coordinated to N on FeSA/NSC samples
obtained from inductively coupled plasma atomic emission
spectroscopic (ICP-AES) measurements is 1.22 wt% (Table S5†).
X-ray diffraction (XRD) was conducted to analyse the iron and
carbon crystalline structures of the catalysts. From Fig. S9a,†
two characteristic peaks at 25ꢁ and 44ꢁ are observed in the XRD
patterns of FeSA/NSC, Fe/NSC-TE, Fe/NC-UI, Fe/NC, NSC and
NC, indicating successful carbonization aer pyrolysis. Notably,
no diffraction peaks of crystalline Fe species are observed from
the XRD patterns of FeSA/NSC, Fe/NSC-TO, and Fe/NC-UI.
3.2 ORR performances of the catalysts
Fig. 3 (a) LSV curves of FeSA/NSC, Fe/NSC-TO, Fe/NC-UI, Fe/NC,
NSC, NC and Pt/C catalysts in O2-saturated 0.1 M KOH with a rotation
rate of 1600 rpm. (b) Comparison of E1/2 and the kinetic current density
(Jk) at 0.85 V (vs. RHE) for various catalysts. (c) H2O2 yields and electron
transfer numbers (n) of different catalysts in O2-saturated 0.1 M KOH
solution. (d) ORR polarization curves of FeSA/NSC at different rotating
Aer structural and compositional characterization, the elec-
trocatalytic property measurements of as-obtained catalysts
were further explored. Cyclic voltammetry (CV) was rst con-
ducted for catalysts in N2- and O2-saturated 0.1 M KOH elec-
trolytes (Fig. S10b†). No distinctly featured peaks emerge in the rates (inset: K–L plots and electron transfer numbers). (e) LSV vol-
tammograms of various catalysts in O2-saturated 0.1 M HClO4 with
a rotation rate of 1600 rpm. (f) ORR polarization curves of FeSA/NSC
N2-saturated solution. By contrast, all the catalysts exhibit
a well-dened ORR peak between 0.75 and 0.90 V, indicating
their ORR catalytic capability in 0.1 M KOH. Among them, the
FeSA/NSC catalyst shows the most positive ORR peak potential,
and Pt/C before and after 5000 potential cycles over 0.8–1.1 V with
a sweep rate of 100 mV sꢀ1 in O2-saturated 0.1 M KOH. Inset: chro-
noamperometric responses of different catalysts in 0.1 M KOH before
which is 25 mV larger than that of the Pt/C catalyst (Fig. S10†). and after adding 0.5 M methanol.
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during the ORR process. The electron transfer number of FeSA
/
C catalyst. Meanwhile, it also exhibits better stability in 0.1 M
NSC is between 3.92 and 3.99 V from 0.2–0.7 V, evidencing that HClO4, and the E1/2 of the FeSA/NSC catalyst decays by only
FeSA/NSC undergoes a four-electron process during oxygen 8 mV aer 5000 CV cycles between 0.6 and 1.0 V, which is
reduction. It is much higher than those of other catalysts: Fe/ lower than that of the Pt/C catalyst (16 mV) (Fig. S16†). The
NSC-TO (3.84–3.98), Fe/NC-UI (3.81–3.98), and Fe/NC (3.67– resistance to methanol was investigated by adding 0.5 M
3.97) (Fig. 3c). RDE measurements with rotating speeds ranging methanol into the electrolyte for about 1000 s during chro-
from 400 to 2500 rpm were used to analyze ORR kinetics and noamperometric measurements (inset picture in Fig. 3f and
selectivity (Fig. 3d). The limited current density of FeSA/NSC S16†). It shows only a slight oscillation in the current,
increases uniformly with the increase of the rotating speed; K–L implying a great methanol tolerance in both alkaline and
plots exhibit a good linear relationship and the average electron acidic electrolytes. In contrast, the current of the Pt/C catalyst
transfer number (n) is 3.99, indicating a direct four-electron exhibits drastic uctuation aer 0.5 M methanol injection.
pathway for the ORR. Among the catalysts, FeSA/NSC shows The results demonstrate that FeSA/NSC has better stability and
the smallest Nyquist circle in electrochemical impedance methanol resistance than the Pt/C catalyst in both alkaline
spectroscopy (EIS) compared with other catalysts (Fig. S12a†), and acidic media, making it a promising candidate for
which demonstrated the quickest charge transfer ability. application in practical energy devices.
Furthermore, FeSA/NSC shows a lower Tafel slope (55.9 mV
decꢀ1) than the Pt/C electrode (67.1 mV decꢀ1) in the low over-
potential region (Fig. S12b†). It is further found that the ORR
performances of the catalysts in acidic solutions follow the
same trend as those in alkaline electrolytes: FeSA/NSC > Fe/NSC-
TO > Fe/NC-UI > Fe/NC. LSV curves in O2-saturated 0.1 M HClO4
are shown in Fig. 3e. FeSA/NSC still exhibits a higher ORR
3.3 Electrochemical performance of Zn–air batteries with
FeSA/NSC
Zinc–air batteries are promising energy devices in energy
conversion and storage.55,56 To estimate the potential applica-
tion of the FeSA/NSC electrocatalyst in energy devices, a Zn–air
battery was further tested using FeSA/NSC as the oxygen elec-
activity (E1/2 ¼ 0.78 V) than Fe/NSC-TO (E1/2 ¼ 0.75 V) and Fe/
trocatalyst (Fig. 4a). It shows an open-circuit voltage of 1.501 V
NC-UI (E1/2 ¼ 0.69 V), and is even comparable to the commer-
cial Pt/C catalyst (E1/2 ¼ 0.84 V). Moreover, the H2O2 yield of
FeSA/NSC is below 6.2% in the potential range of 0 to 0.8 V,
indicating the excellent 4-electron selectivity of the ORR in
0.1 M HClO4. The electron transfer number of FeSA/NSC in 0.1 M
HClO4 is between 3.88 and 3.95 from 0 to 0.8 V (inset in Fig. 3e),
suggesting a direct 4-electron transfer pathway. The excellent
catalytic activity of FeSA/NSC could result from the synergetic
effect of the high exposure of active sites and efficient mass
transfer through HOPC structures. In addition, the introduction
of S can also improve the overall ORR efficiency. To understand
the origin of the ORR activity of FeSA/NSC, 10 mM SCNꢀ was
used as a probe to poison Fe–N4 sites for the ORR in 0.1 M KOH.
As revealed in Fig. S13,† the E1/2 of FeSA/NSC decreased
dramatically by about 50 mV. The result suggests that single Fe
atoms in FeSA/NSC catalysts play a leading role in improving
ORR activity. Moreover, aer increasing the concentration of
SCNꢀ to 20 mM and 40 mM, the LSV curves don't show any
obvious change (Fig. S13†), indicating that nearly all Fe sites
have been blocked. However, the E1/2 value is still much larger
than that of pure SCN catalysts without Fe. The studies
demonstrate that Fe sites in the FeSA/NSC catalyst are not the
sole reason for the better ORR activity, the Fe sites may also
activate the surrounding coordinated atoms and render them
active.53
Fig. 4 (a) Schematic diagram of the Zn–air battery. (b) Photograph of
the Zn–air battery with an open-circuit voltage of 1.501 V. (c)
Discharge polarization curves and corresponding power density plots
of Zn–air batteries using FeSA/NSC and Pt/C as air electrodes,
For practical applications of single atom catalysts, ORR
catalytic stability and selectivity for the ORR are two major
factors that should be considered.54 In this work, the dura-
bility of catalysts was evaluated by chronoamperometric respectively. (d) Discharge curves of the batteries at various current
densities. (e) Specific capacities of Zn–air batteries using FeSA/NSC and
measurements. As shown in Fig. 3f, the E1/2 of the FeSA/NSC
exhibits a 3 mV negative shi aer 5000 potential cycles in
Pt/C as ORR catalysts. Long time galvanostatic discharge curves of the
Zn–air batteries at 50 mA cmꢀ2. Inset: photograph of two Zn–air
0.1 M KOH. By contrast, the E1/2 of the Pt/C shows a 12 mV
batteries lighting up an LED. (f) Stability of the Zn–air battery with FeSA
/
negative shi aer 5000 potential cycles in 0.1 M KOH, indi-
cating the better stability of FeSA/NSC than the commercial Pt/ anode at 10 mA cmꢀ2
NSC as the cathode catalyst by replenishing the electrolyte and Zn
.
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(Fig. 4b), which is close to the theoretical value of 1.65 V. The (Fig. 5a under acidic conditions and Fig. S19† under alka-
FeSA/NSC-assembled Zn–air battery exhibits a maximum power line conditions). As shown in Fig. 5a and b, the last step of the
density of 159.2 mW cmꢀ2 at a current density of 235.6 mA cmꢀ2 ORR is the most endothermic and thus is the rate deter-
(Fig. 4c), which are much higher than those of the Pt/C- mining step (RDS) for both catalysts. The DG of the RDS for
assembled Zn–air battery (76.8 mW cmꢀ2 at 151.7 mA cmꢀ2
)
FeSA/NSC (0.58 eV) is smaller than that for FeSA/NC (0.69 eV) at
and those of other previously reported oxygen electrocatalysts, equilibrium potential. It indicates that FeSA/NSC is more
e.g., N, P-doped carbon foam (55 mW cmꢀ2),57 Fe–N/P–C-700 effective for the ORR than FeSA/NC, indicating a better ORR
(133.2 mW cmꢀ2),50 and 2D mesoporous FeCo–N-x-carbon performance aer S modication. The differential charge
(150 mW cmꢀ2).58 The galvanostatic discharge curves for the density maps (Fig. 5d and e) show that more electrons would
FeSA/NSC based Zn–air battery show superior discharge stabil- accumulate on O2 (0.53 e) when adsorbed on the FeSA/NSC
ities and higher voltage values at various current densities structure compared with the FeSA/NC structure (0.50 e), which
ranging from 1 to 50 mA cmꢀ2 (Fig. 4d). A blue light-emitting is benecial for the reduction of O2.59 Furthermore, the charge
diode (LED) screen with the letters of SHZU can be easily pow- density differences and Bader charge analysis (Fig. S20†)
ered by two FeSA/NSC-assembled batteries connected in series reveal that S doping can tune the charge of Fe through the
(inset in Fig. 4e). The galvanostatic discharge curve of an FeSA
/
atomic interface effectively, which makes Fe in FeSA/NSC less
NSC based battery exhibits a stable discharge voltage at about positive than in FeSA/NC (1.07 for FeSA/NC and 1.01 for FeSA
/
1.1 V with no obvious degradation at 50 mA cmꢀ2 (Fig. 4e), NSC). The relatively low electronegativity of S could enrich the
which is much higher than that of a Pt/C-assembled battery charge around the N atom. It promotes the rate-limiting
(about 0.92 V). The specic capacity for the FeSA/NSC-equipped reductive release of OH*, reduces the reaction barrier and
battery at 50 mA cmꢀ2 is 748.4 mA h gꢀ1, which is larger than facilitates the overall ORR process.60 Therefore, the excellent
that of a commercial Pt/C-assembled battery (688.6 mA h gꢀ1). ORR activity in both alkaline and acid solutions resulting
Notably, no obvious degradation is observed aer 6 cycles for from S doping enables the engineering of charges on the Fe
about 150 hours by replenishing the electrolyte and Zn anode reactive center and HOPC structures (facilitating active site
(Fig. 4f), indicating the outstanding durability of the FeSA/NSC access and mass transfer).
catalyst in Zn–air batteries. Such an excellent performance
demonstrates again the outstanding catalytic activity and
stability of FeSA/NSC for the ORR.
4. Conclusions
In summary, a facile in situ gas-foaming strategy was developed
3.4 Theoretical evaluation of ORR activities
to synthesize single Fe atoms dispersed on a N, S-codoped
hierarchically ordered porous carbon framework by one-step
pyrolysis of dopamine/ferric chloride complexes and thiourea
conned within the channels of SBA-15. HOPC structures can
facilitate active site access and mass transfer in the ORR
process. The introduction of S can induce charge enrichment
in N and Fe for optimal O2 binding and fast electron transfer,
which make them exhibit superb oxygen reduction activity,
stability and methanol tolerance in both acidic and alkaline
media. As the air electrode in Zn–air batteries, FeSA/NSC
demonstrates superior power density, long-term discharge
stability and specic capacity to the commercial Pt/C catalyst.
Therefore, the FeSA/NSC catalyst is a promising non-platinum-
group metal ORR catalyst for practical application in Zn–air
batteries.
To gain theoretical insights into S-doping on FeSA/NSC for
high ORR reactivity and excellent ORR kinetics, DFT calcula-
tions were employed to illustrate the changes in reaction
energy and electronic structure before and aer S-doping
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Fig. 5 The Gibbs free energy diagram of the ORR on (a) FeSA/NC and This study was supported by the National Natural Science
FeSA/NSC obtained from DFT calculations. The optimized structures of
the intermediates of the ORR on (b) FeSA/NC and (c) FeSA/NSC. Gray,
white, red, yellow, blue and gold balls represent C, H, O, S, N and Fe
atoms, respectively. (d) Differential charge density and Bader charge
Foundation of China (U1703351, 52073179 and 51663021),
Bingtuan Excellent Young Scholars (CZ027205), Bingtuan
Science & Technology Nova Program and Postgraduate Educa-
tion Innovation Program of Xinjiang Uygur Autonomous Region
in 2021. We wish to thank the Analysis and Testing Center of
analysis of O2 adsorbed on (d) FeSA/NC and (e) FeSA/NSC sites. Electron
accumulation is shown in yellow and depletion in blue.
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Shihezi University for the microscopy experiments and micro- 21 R. Cao, R. Thapa, H. Kim, X. Xu, M. Gyu Kim, Q. Li, N. Park,
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