FeN: X and γ-Fe2O3 co-functionalized hollow graphitic carbon nanofibers for efficient oxygen reduction in an alkaline medium

Article abstract of DOI:10.1039/d0ta00073f

The exploration of efficient, stable, and inexpensive oxygen reduction reaction (ORR) electrocatalysts to replace Pt/C in fuel cells and metal-air batteries still remains an ongoing challenge. Herein, we report an efficient ORR electrocatalyst composed of single-atom FeN_x and γ-Fe₂O₃ nanoparticle co-functionalized hollow graphitic carbon nanofibers (FeN_x/Fe₂O₃-CNFs). It is found that the FeN_x species serve as the active sites, while the γ-Fe₂O₃ nanocrystals function as a co-catalyst to boost the ORR catalytic activity. The obtained FeN_x/Fe₂O₃-CNFs exhibit desirable ORR electrocatalytic activity with a 4-electron transfer pathway, a half-wave potential of 0.81 V approaching that of commercial Pt/C, low hydrogen peroxide yields (<6% at 0.2-0.7 V), long-term stability (87.14% after 30 000 s), and excellent methanol tolerance. The assembled Zn-air battery based on the FeN_x/Fe₂O₃-CNFs has an open circuit voltage of 1.51 V and superior energy density of 920 W h kg^-1. This work highlights the significant contribution of the co-catalyst in electrocatalysis.

Full text of DOI:10.1039/d0ta00073f

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Materials for energy and sustainability
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Li, R. Yu, S. Xi, J. Wu, D. Zhao, L. Mai and L. Zhou, J. Mater. Chem. A, 2020, DOI: 10.1039/D0TA00073F.
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Number 12
March 2018
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
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DOI: 10.1039/D0TA00073F
ARTICLE
FeN and γ-Fe O Co-Functionalized Hollow Graphitic Carbon
x
2
3
Nanofibers for Efficient Oxygen Reduction in Alkaline Medium
a
a
a
a, b
c
b
a
Qiang Yu, Sitian Lian, Jiantao Li, Ruohan Yu, Shibo Xi, Jinsong Wu, Dongyuan Zhao, Liqiang
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
Mai and Liang Zhou *
DOI: 10.1039/x0xx00000x
The exploration of efficient, stable, and inexpensive oxygen reduction reaction (ORR) electrocatalysts to replace Pt/C in fuel
cells and metal-air batteries still remains an ongoing challenge. Herein, we report an efficient ORR electrocatalyst composed
of single-atom FeN
found that the FeN
ORR catalytic activity. The obtained FeN
transfer pathway, a half-wave potential of 0.81 V approaching commercial Pt/C, low hydrogen peroxide yields (< 6% at 0.2
x
and γ-Fe
species serve as the active sites, while the γ-Fe
/Fe -CNFs exhibits desirable ORR electrocatalytic activity with a 4-electron
2
O
3
nanoparticle co-functionalized hollow graphitic carbon nanofibers (FeN
x 2 3
/Fe O -CNFs). It is
x
2 3
O
nanocrystals function as a co-catalyst to boost the
x
2 3
O
–
0.7 V), long-term stability (87.14% after 30000 s), and excellent methanol tolerance. The assembled Zn-air battery based
-1
x 2 3
on the FeN /Fe O -CNFs has an open circuit voltage of 1.51 V and superior energy density of 920 Wh kg . This work highlights
the significant contribution of co-catalyst in electrocatalysis.
recent study by Wan et al. found that the Fe/Fe
3
C in Fe-N-C
could boost the ORR performance of FeNx. That is to say, the
Fe and/or Fe C can function as a co-catalyst during the ORR
process. A question arises: is there any other material which can
act as a co-catalyst and boot the ORR activity of M-N species
likes Fe or Fe C?
Herein, we report the construction of FeN
nanoparticle co-functionalized hollow graphitic carbon
nanofibers (FeN /Fe -CNFs). It is find that the γ-Fe
nanoparticles can function as a co-catalyst and effectively
improve the ORR activity of FeN . The FeN /Fe -CNFs
3
4
Introduction
3
Oxygen reduction reaction (ORR) has become a pivotal process
1
,2
in fuel cells and metal-air batteries. Commercial Pt/C is
currently the most widely employed ORR electrocatalyst.
However, it suffers from high cost, scarcity, poor stability, and
x
3
2 3
and γ-Fe O
3
-5
x
CO/methanol poisoning.
Hence, the exploration of
alternative electrocatalysts with superior electrocatalytic
activity, high stability, and low cost has being become
x
O
2 3
2 3
O
6
-9
increasingly significant.
x
x
2 3
O
Metal-nitrogen-carbon based materials (M-N-C, M = Fe or Co
demonstrates a half-wave potential approaching commercial
Pt/C. More importantly, the limiting current density and long-
term stability of FeN /Fe O -CNFs in alkaline medium surpasses
x 2 3
those of commercial Pt/C. This work highlights the significant
contribution of co-catalyst in electrocatalysts.
etc.) have been identified as an alternative family of ORR
electrocatalysts.1
0-14
In 1964, Jasinski pioneered the
employment of metal phthalocyanines, a family of macrocyclic
1
5
complexes containing M-N
demonstrated that annealing M-N
4
moieties, for ORR. Later, it was
containing compounds in
4
inert atmosphere could lead to significantly improved ORR
activity.16 Recently, it has been found that the pyrolysis of
inexpensive C, N-containing precursors and transition metal
Experimental
Synthesis of FeN /Fe O -CNFs and FeN /Fe C-CNFs
x 2 3 x 3
1
,17-21
species can also lead to M-N-C based ORR catalysts.
Since
2
2-31
electrocatalysts:
then, M-N-C has been extensively investigated.
The composition of M-N-C catalysts is relatively complex and
they usually contain N-doped carbon, M-N moieties, and M-
rich nanoparticles. Such complexity makes the real active sites
Phenolic resin nanofibers (RNFs) were synthesized based on our
37
x
previous report. Typically, the as-made RNF aerogel (0.3g) and
Fe(NO ·9H O (2.5 mmol) were dispersed in 50 mL water and
)
3 3
2
3
2-36
and role of M-rich nanoparticles elusive.
It has been
stirred at 25 °C for 24h to allow the RNF to absorb the ferric ions.
After ferric ion adsorption, the sample was collected by
centrifugation and freeze-dried. Subsequently, the sample was
generally accepted that M-N is the active sites for ORR. A
x
carbonized in N
at 50°C for 10 h to remove most of the Fe species. The sample
was then subjected to pyrolysis at 800 °C for 2 h in N or H O/N
/Fe -CNFs,
2 2 4
at 800 °C for 1h and treated with 0.5 M H SO
a. State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, P. R. China.
Nanostructure Research Centre, Wuhan University of Technology, Wuhan
b.
2
2
2
4
30070, P. R. China.
to obtain the FeN
respectively.
x
/Fe
3
C-CNFs and FeN
x
2 3
O
c.
Institute of Chemical and Engineering Sciences, A*STAR (Agency for Science,
Technology and Research), Singapore 627833, Singapore.
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/D0TA00073F
x 2 3 x 2 3
Fig. 1 Schematic illustration for the synthesis of FeN and γ-Fe O nanoparticle co-functionalized hollow graphitic carbon nanofibers (FeN /Fe O -CNFs).
equation: E(RHE) = E(SCE) + 0.0591*pH + 0.241. Cyclic
voltammetry (CV) and Linear sweep voltammetry (LSV) curves
were recorded at 5 and 10 mV s- respectively. LSV curves with
different rotation rates were conducted at a rotating disk-
electrode (RDE). The electron transfer number (n) at different
potentials was calculated based on the Koutecky-Levich (K-L)
equation:
Characterization:
1
The morphology of the samples was studied by scanning
electron microscope (SEM, JEOL-7100F) and transmission
electron microscope (TEM, JEM-2100F and Titan Themis G2 60-
3
00). The X-ray diffraction (XRD) patterns were collected on a
Bruker D2 PHASER. The N sorption results were obtained on a
Tristar-3020 N adsorption analyzer at 77 K. The surface
2
1
1
1
=
+
0.62푛퐹퐷 02 /3
퐶0 푣1/6ω1/2
2
퐽
퐽퐾
compositions of the catalysts were analyzed by an X-ray
photoelectron spectroscopy (XPS) instrument (VG MultiLab
where J and J
k
are the measured current density and kinetic
2
000). Thermogravimetric analysis (TGA) was performed on a
current density, respectively; n stands for the transferred
electron number; F is the Faraday constant (96500 cm ); C
NETZSCH STA 449F5 under air. Raman spectra was obtained by
using HORIBA LabRAM micro-Raman spectroscopy. The
-1
0
is
-6
the saturated concentration of O
2
in 0.1 M KOH (1.2×10 mol
Mössbauer tests were conducted by
a
Wissel MS-500
-1
-6
2
-1
cm ); D
0
2
is the diffusion coefficient of O (1.87×10 cm s )
5
7
spectrometer. A Co (Rh) source with activity of 25 mCi was
employed. The velocity calibration was processed with a room
temperature γ-Fe absorber. X-ray absorption near edge
structure (XANES) and Extended X-ray absorption fine structure
2 -1
solution; v is the kinematic viscosity (0.01 cm s ); and ω is the
angular rotation rate of the electrode (rad s ).
The percentage of H and n were also performed by
rotating ring-disk electrode (RRDE) tests according to the
following equati퐼o푟/n푁:
-1
2 2
O
(EXAFS) data were performed on the XAFCA beamline of
Singapore Synchrotron Light Source.
H O % = 200
2
2
I푑 + (퐼푟/푁)
4
퐼푑
Electrochemical measurement:
푛 = 퐼푑 + (퐼푟/푁)
Electrochemical measurements were carried out with a CHI
7
60E workstation (Chenhua) in a standard three-electrode
system. The well-dispersed inks were prepared by mixing 5 mg
FeN /Fe -CNFs, 5 mg Vulcan XC72R carbon, and then added
where I
d r
and I are the disk current and ring current,
respectively. And N is the current collection efficiency (0.37).
Tolerance to methanol: LSV curves with a rotation rate of
x
2 3
O
into 800 μL isopropanol, 150 μL deionized water, and 50 μL
Nafion solution. After thorough ultrasonic treatment, 10 μL
well-dispersed ink was dropped onto a glassy carbon electrode
1
600 rpm were carried out to evaluate the methanol tolerance
of FeN /Fe -CNFs and Pt/C catalyst. 3 M methanol was
x
2 3
O
injected into alkaline medium.
KSCN toxicity tests: To investigate the role of Fe-N in the ORR
x
2
(GCE) surface with an area of 0.196 cm . 0.1 M fresh O
2
-
saturated KOH solution was used as electrolyte in all ORR tests.
The GCE, Pt sheet, and saturated calomel electrode (SCE) were
used as working electrode, counter electrode, and reference
electrode, respectively. All potentials were calibrated to
reversible hydrogen electrode (RHE) on the basis of Nernst
activity of FeN
injected into the alkaline medium before LSV tests.
x 2 3
/Fe O -CNFs, 10 mM fresh KSCN solution was
2
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8
– 14 graphitic layers with interlayer distances of 3.4 Å,
View Article Online
DOI: 10.1039/D0TA00073F
corresponding to the (002) plane of graphitic carbon. γ-Fe O
2 3
nanoparticles can be clearly observed in the CNFs (Fig. 2d). The
interplanar spacings (0.25 nm) observed in Fig. 2d agree well
with the (311) lattice spacing of γ-Fe O . Besides the γ-Fe O
2 3 2 3
nanoparticles, Fe species (both atomically dispersed Fe single
atoms and few-atom clusters) can also be observed in other
areas of the CNFs as shown in the high-angle annular dark-field
scanning transmission electron microscopy image (HAADF-
STEM, Fig. 2f). The Fe is also confirmed by energy dispersive X-
ray spectroscopy (EDS) elemental mapping (Fig. 2g). As a single
Fe-atom is unstable when exposed to air, it is speculated that
the Fe species are stabilized by N-coordination and exists as
FeN
sample FeN
carbon nanofibers (FeN
For comparison, the sample without steam activation was
also synthesized and characterized. It consists of FeN and Fe
co-functionalized CNFs (FeN /Fe C-CNFs, Fig. S4). Direct
carbonization of the RNFs leads to amorphous N-doped CNFs
(Fig. S5). Different from FeN /Fe -CNFs, neither FeN /Fe C-
CNFs nor N-doped CNFs shows the hollow tubular structure,
x
. From the above characterizations, we tentatively term the
and γ-Fe co-functionalized hollow graphitic
-CNFs).
x
2 3
O
x 2 3
/Fe O
x
3
C
x
3
Fig. 2 The SEM image of resin nanofibers (a); SEM (b), TEM (c), and HRTEM (d – e) images
of FeN /Fe -CNFs; HAADF-STEM image of FeN /Fe -CNFs (f); EDS elemental mapping
of C, N, O, and Fe (g). The inset in (a) is digital photo of the RNFs monolith.
x
2
O
3
x
2 3
O
x
O
2 3
x
3
Zn-air batteries assembly: The Zn-air batteries were demonstrating that the steam activation plays a significant role
constructed with polished Zn plate (thickness: 0.25 cm) as in forming the hollow structure. To further explore the reason
anode, hydrophobic carbon cloth (W1S1009, CeTech Co., Ltd) for the formation of hollow graphitic structure, the steam
loaded with well-dispersed ink as air cathode, and 6.0 M KOH as activation temperature is varied from 700 to 900 °C. The hollow
electrolyte. The ink is the same as above ORR test and the mass graphitic structure begins to form at 700 °C (Fig. S6) although
-2
most CNFs retain amorphous at this temperature. Most of the
CNFs can be converted into hollow graphitic CNFs at 800 °C and
the hollow graphitic CNF morphology is partially destroyed at
loading of catalyst is 1 mg cm . Zn-air battery performance was
tested under air using customized electrochemical cell.
9
00 °C (Fig. S7).
Results and discussion
The synthesis of FeN /Fe O -CNFs involves three major steps: (I)
x 2 3
XRD (Fig. 3a) pattern of the FeN /Fe O -CNFs shows two
typical diffractions for γ-Fe O (JCPDS No. 39-1346) and two
2 3
x
2 3
diffractions for graphitic carbon (JCPDS No. 26-1079). As for
FeN /Fe C-CNFs, the XRD pattern shows two diffractions for
graphitic carbon, while no diffraction for Fe C can be detected.
The existence of Fe C nanocrystals in FeN /Fe C-CNFs can only
be demonstrated by HRTEM image (Fig. S4c). The high
graphitization degree of FeN /Fe -CNFs can be confirmed by
Raman spectra (Fig. S8). The FeN /Fe -CNFs shows a relatively
low I /I ratio of 1.03, much lower than that of FeN /Fe C-CNFs
1.15).
The surface area and porosity were studied by N
(Fig. S9). The BET surface area and pore volume of FeN
ferric ion adsorption, (II) carbonization, and (III) steam
activation (Fig. 1). 3-Aminophenol-formaldehyde RNFs
prepared by a scalable sol-gel method are employed as the
x
3
3
precursor.37 The abundant hydroxyl (-OH) and amino (-NH
groups on the surface of RNFs (Fig.S1) enable the effective
3
x
3
2
)
3
+
o
x
2 3
O
adsorption of ferric ions (Fe ). During carbonization at 800 C,
the ion species are partially reduced to Fe C and the remaining
part can be stabilized by N, forming FeN species. Meanwhile,
the Fe C catalyzes the conversion of RNFs into partially graphitic
x
2 3
O
3
D
G
x
3
x
(
3
2
sorption
2 3
/Fe O -
CNFs. Thus, the product after secondary carbonization is
o
x
designated as FeN
x
/Fe
3
C-CNFs. With steam activation at 800 C,
2
-1
3 -1
CNFs reach 712 m g and 1.15 cm g (Table S1), respectively.
In addition, the t-Plot microporous surface area and pore
the Fe species help further graphitization, the amorphous
carbon is etched away by steam, and the Fe C is oxidized into γ-
Fe . As a result, FeN /Fe -CNFs is obtained.
The obtained RNFs exhibit a monolith morphology with
3
2
-1
volume of the FeN
x
/Fe
and 0.16 cm g , respectively. The (microporous) surface area
and pore volume of FeN /Fe -CNFs are slightly higher than
those of FeN /Fe C-CNFs. In electrocatalysis, the higher surface
2 3
O -CNFs are calculated to be 308 m g
O
2 3
x
2 3
O
3
-1
x
2 3
O
ultralow density, which can be sit on a flower (Fig. 2a inset). SEM
shows that the RNFs are composed of uniform nanofibers with
a diameter of ~25 nm (Fig. 2a, Fig. S2). After ferric ion
x
3
area can expose more active sites, and thus contribute to the O
adsorption and reduction effectively.
2
2
adsorption, pyrolysis in N , and steam activation, their average
x 2 3
The XPS survey spectrum of FeN /Fe O -CNFs shows an Fe
diameter is reduced to ~20 nm and the surface become rough
content of 0.35 wt.% (Fig. S10). TGA (Fig. S11) in air shows a
weight loss of ~85 wt.%, corresponding to an Fe content of 10.5
wt.%. The Fe content determined from XPS is far below that
(
(
Fig. 2b). TEM (Fig. 2c and Fig. S3) and high-resolution TEM
HRTEM, Fig. 2d – 2e) images clearly reveal the hollow graphitic
x 2 3
structure of FeN /Fe O -CNFs. The wall of CNFs is composed of
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Fig. 3 XRD patterns (a) of FeN
CNFs (d); Fe K-edge XANES (e) and FT k -weighted EXAFS spectra (f) of FeN
x
/Fe
2
O
3
-CNFs and FeN
x
/Fe
3
x 2 3 x 2 3
C-CNFs; high-resolution N1s (b) and Fe2p (c) XPS spectra of the FeN /Fe O /Fe O -
-CNFs; 57Fe Mössbauer spectrum of the FeN
3
x
/Fe
2
O
3
-CNFs, Fe foil and Fe
2
O
3
.
measured from TGA. This is because most of Fe species are appearance of a weak pre-edge peak at ~7117 eV suggests the
3
encapsulated in graphitic carbon shells, making it difficult to be existence of FeN
detected by the XPS technique. The high-resolution N 1s weighted EXAFS (Fig. 3f) spectrum of FeN
spectrum (Fig. 3b) shows three types of N species: pyridinic-N a strong peak for Fe-N/O bonds at ~1.5 Å. It should be noted
x
structure. The Fourier-transformed (FT) k -
x 2 3
/Fe O -CNFs exhibits
or Fe-N
x
species (398.2 eV), pyrrolic-N (400.6 eV), and graphitic- that the bond distance between Fe-N and Fe-O is very small, and
N (401.4 eV).7
,38-40
x 2 3
Remarkably, the content of pyridinic-N the peak of FeN /Fe O -CNFs is obviously wider than that of
2 3
reaches as high as 46.56%, which can effectively improve the Fe O
at ~1.5 Å. In general, the XANES and EXAFS results are well
activity of catalyst and endow it with excellent surface consistent with the HAADF-STEM, XPS, and Mössbauer
wettability and onset potential, while the graphitic-N (16.43%) spectroscopy characterizations. All these characterizations
will be beneficial to the improvement of diffusion-limited demonstrate the existence of FeN
performance.41 The high-resolution Fe 2p spectrum of
The obtained FeN /Fe -CNFs was confirmed to have super
FeN /Fe -CNFs (Fig. 3c) can be deconvoluted into five ORR properties due to its unique structure with one
components: the 2p3/2-2p1/2 spin-orbit doublet for Fe at 710.3 dimensional (1D) nanostructure, high graphitization degree,
x x 2 3
in FeN /Fe O -CNFs.
x
2 3
O
x
2 3
O
2
+
3
+
and 723.0 eV, the 2p3/2-2p1/2 spin-orbit doublet for Fe at 713.2 high surface areas, as well as the well-dispersed FeN
and 725.0 eV, and the satellite peak at 717.2 eV. It should be The CV (Fig. S12) curves of FeN /Fe -CNFs and FeN
noted that the peak at 710.3 eV can also be assigned to the FeN CNFs show an oxygen reduction peak at 0.83 V (vs. RHE), much
species owing to the slight difference in binding energy positive than that of N-doped CNFs (0.71 V). LSV (Fig. 4a) curve
x
species.
x
O
2 3
x
/Fe C-
3
x
4
2,43
57
between FeN
spectrum of FeN
components (Fig. 3d). The D1 and D2 doublets can be assigned V) and FeN
to FeN species attribution to a low and medium spin state, that of commercial 20 wt.% Pt/C. In addition, the FeN
respectively,
x
and pyridinic-N.
/Fe -CNFs can be fitted into four 0.81 V. Such an E1/2 is higher than those of N-doped CNFs (0.70
/Fe C-CNFs (0.79 V), and is only 17 mV lower than
2 3
/Fe O -
x 2 3
The Fe Mӧssbauer of the FeN /Fe O -CNFs shows a half-wave potential (E1/2) of
x
2 3
O
x
3
x
x
4
4,45
while the other two sextets are mainly from CNFs shows the highest limiting current density (Ilimit = ~6 mA
2
+
3+
-2
Fe and Fe species, suggesting the co-existence of FeN
Fe in hollow graphitic CNFs. The FeN
believed to be active sites for ORR, account for 54.7 % of the kinetic control region (0.9 V), the J
x
and γ- cm ) among all catalysts, which is close to its theoretical value
O
2 3
x
species, which are at 1600 rpm and suggests the rapid mass-transfer kinetics. At
of FeN /Fe -CNFs reaches
0.50 mA cm , which is close to that of Pt/C. The effects of
k
x
2 3
O
-2
total Fe (Table S2).
X-ray absorption spectroscopy was further conducted to activation temperature on ORR performance have been
analyze the atomic and electronic structure of Fe species in evaluated by varying the activation temperature from 700 – 900
FeN
exhibits an absorption edge closed to Fe
valence state of Fe species in FeN /Fe
x 2 3
/Fe O -CNFs. The Fe K-edge XANES (Fig. 3e) spectrum °C. Both the CV and LSV results demonstrate that the sample
3
+
2 3
O , suggesting the Fe
activated at 800 °C outperforms that prepared at 700 and 900
x
2 3
O -CNFs. Besides, the °C (Fig. S13). Although the sample prepared at 900 °C
4
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Fig. 4 LSV curves of FeN
.1 M KOH (1600 rpm at 10 mV s ) (a); LSV curves of FeN
from 400 to 1600 rpm (inset: the corresponding K-L plots) (b); electron transfer number
and H yields of FeN /Fe -CNFs at 0.2 – 0.7 V (c); stability tests of FeN /Fe -CNFs
and Pt/C at 0.75 V (d).
x
/Fe
2
O
3
-CNFs, FeN
x
/Fe
3
C-CNFs, Pt/C, and CNFs in O
2
-saturated
x 2 3
Fig. 5 Zn-air battery performances based on FeN /Fe O -CNFs and Pt/C catalysts. OCV
stability (a), the insets are digital photos showing the OCV; discharge polarization and
corresponding power density curves (b); discharge curves at 10 mA cm-2 (c); rate
performance at different current densities (d), the inset is a digital photo of 60 LEDs
powered by two aqueous Zn-air batteries in series.
-1
0
x
2 3
/Fe O -CNFs at rotating rates
2
O
2
x
2
O
3
x
2 3
O
shows the highest surface area, the hollow graphitic CNF
structure is partially destroyed owing to over activation, leading
to a decrease in catalytic performance.
To investigate the electron transfer number (n) during ORR,
LSV curves at various rotating rates were recorded (Fig. 4b). Due
to the decreased diffusion distance at high rotation speeds, the
converted into γ-Fe
treatment. And we found that the γ-Fe
act as a co-catalyst for ORR, which is even more effective than
Fe C in boosting the activity of Fe-N . Although the origin of
enhanced ORR activity is not clear at this stage, it is speculated
there may exist certain interaction between the Fe
nanocrystals and neighboring FeN , say alteration of the Fe's
2
O
3
nanocrystals via
a facile steam
2 3
O
nanocrystal can also
3
x
2 3
O
I
limit increases proportionally with the rotation speed. The K-L
plots show a perfect linearity at various potentials. The n is
determined to be ~4 for FeN /Fe -CNFs, demonstrating a
direct 4-electron reduction ORR pathway. The peroxide specie
yields and corresponding n of FeN /Fe -CNFs were also
determined by RRDE tests. The H yields of FeN /Fe -CNFs
x
charge density. Further studies, such as density functional
theory calculations, will be performed in the future to help us
x
2 3
O
understand the detailed role of Fe
Inspired by the excellent ORR performance, aqueous Zn-air
batteries were assembled based on the FeN /Fe -CNFs
2 3
O in Fe-N-C catalyst in ORR.
x
2 3
O
2
O
2
x
2 3
O
x
2 3
O
keep below 6 % in the potential range of 0.2 – 0.7 V (Fig. 4c),
confirming a direct 4-electron transfer pathway for ORR. When
catalyst using 6.0 M KOH as the electrolyte. As shown in Fig. 5a,
the open circuit voltage (OCV) of Zn-air battery based on
FeN /Fe O -CNFs reaches 1.51 V, which is slightly higher than
x 2 3
the battery based on commercial Pt/C catalyst (1.48 V). The
constructed devices also show good OCV stability after 1800 s.
From the discharge polarization curve (Fig. 5b), the maximum
compared to FeN
lower H yields and higher electron transfer number (Fig.
S14). Thus, the high ORR activity of FeN /Fe -CNFs is proved
by the 4-electron reduction of oxygen and the very low H
yields. The obtained FeN /Fe -CNFs catalyst also
demonstrates outstanding methanol tolerance performance
Fig. S15). With the addition of methanol, the LSV curve of
FeN /Fe -CNFs presents no obvious change, while the LSV
x 3 x 2 3
/Fe C-CNFs, the FeN /Fe O -CNFs shows
2 2
O
x
2 3
O
2 2
O
x
2 3
O
current density and power density of FeN
x
/Fe
2
O
3
-CNFs based
-2
-2
battery are calculated to be 152.87 mA cm and 69.42 mW cm ,
respectively. Both values are higher than those of the battery
based on Pt/C catalyst. In addition, the Zn-air battery also has
stable discharge curve with voltage platform of ~1.21 V. Under
(
x
2 3
O
curve of Pt/C shows a sharp deformation. The long-term
stability represent another key parameter for ORR catalysts.
-2
a current density of 10 mA cm , the discharge capacity
x 2 3
During the stability test (30000 second), the FeN /Fe O -CNFs
-1
normalized to consumed Zn is up to 761 mAh g , corresponding
shows a slight current loss of 12.86 % (Fig. 4d). Under the same
conditions, the commercial Pt/C loses 41.67 % of the activity.
Considering the complexity of the catalyst, SCN- poison
-1
to an energy density of 920 Wh kg (Fig. 5c). Although the
average discharge voltage of Pt/C based Zn-air battery reaches
-1
~
(
1.23 V, its specific capacity (744 mAh g ) and energy density
experiment was performed to block the Fe-N
injecting KSCN, the ORR activity of FeN /Fe
significantly degraded, demonstrating the active role of Fe-N
ORR (Fig. S16). This is well consistent with previous reports
which showed Fe-N species were the active sites of Fe-N-C
x
species. After
-CNFs was
in
-1
915 Wh kg ) are slightly lower. Moreover, the rate
x
2 3
O
performance of Zn-air battery was tested at current densities of
x
-2
2
– 10 mA cm (Fig. 5d). The Zn-air battery shows a very stable
discharge voltage platform, and the discharge voltage
decreases slowly with the increase of current density. When the
x
catalysts for ORR.2
9, 46-50
In fact, a recent study by Wan et al.
C could boost the ORR activity of
In the present study, the Fe C nanoparticles are
-2
current density returns to 2 mA cm , the voltage platform can
demonstrated that the Fe/Fe
3
return to the initial level. It is worth mentioning that the Zn-air
3
4
Fe-N
x
.
3
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
Page 6 of 8
ARTICLE
Journal Name
1
1
2 Y. Y. Xu, P. L. Deng, G. D. Chen, J. X. Chen, Y. YaVnie,wKA.rtQiclie,OHn.linFe.
batteries in series can successfully light up 60 LEDs, indicating
the excellent practical application potential.
Liu, and B. Y. Xia, Adv. Funct. Mater., 2019, 1906081.
DOI: 10.1039/D0TA00073F
3 G. Saianand, A.-I. Gopalan, J.-C. Lee, CI. Sathish, K.
Gopalakrishnan, G. E. Unni, D. Shanbhag, V. D. B. C. Dasireddy,
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Conclusions
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903937.
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1
1
1
4 B. Y. Guan, L, Yu and X. W. Lou, Adv. Sci., 2017, 4, 1700247
5 R. Jasinski, Nature, 1964, 201, 1212-1213.
In summary, we have constructed a highly efficient ORR catalyst
consisted of FeN
functionalized hollow graphitic carbon nanofibers (FeN
CNFs). Featuring FeN active sites, γ-Fe nanocrystals as co-
catalyst, and 1D hollow graphitic CNF structure, the resultant
FeN /Fe -CNFs shows superior ORR activity in terms of 4-
x
single-atoms and γ-Fe
O
2 3
nanocrystals co-
6 P. Neta, J. Phys. Chem., 1981, 85, 3678-3684.
7 S. Gupta, D. Tryk, I. Bae, W. Aldred and E. Yeager, J. Appl.
Electrochem., 1989, 19, 19-27.
x 2 3
/Fe O -
x
2 3
O
1
1
2
2
8 W. Gang, K. L. More, C. M. Johnston and Z. Piotr, Science,
2
011, 332, 443-447.
x
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O
9 B. Y. Guan, Y. Lu, Y. Wang, M. H. Wu and X. W. Lou, Adv. Funct.
Mater., 2018, 28, 1706738
electron transfer pathway and low hydrogen peroxide yields.
Besides, the high limiting current density, long-term stability,
0 R. Jiang, L. Li, T. Sheng, G. Hu, Y. Chen and L. Wang, J. Am.
Chem. Soc., 2018, 140, 11594-11598.
1 Z. Qiao, S. Hwang, X. Li, C. Wang, W. Samarakoon, S.
Karakalos, D. Li, M. Chen, Y. He, M. Wang, Z. Liu, G. Wang, H.
Zhou, Z. Feng, D. Su, J. S. Spendelow and G. Wu, Energy &
Environ. Sci., 2019, 12, 2830-2841.
and excellent methanol tolerance of FeN
competitive among previous reported M-N-C electrocatalysts.
The assembled Zn-air battery based on FeN /Fe -CNFs
x 2 3
/Fe O -CNFs are also
x
2 3
O
demonstrates a high open circuit voltage, power density, and
energy density. The present research highlights the important
role of the co-catalyst in electrocatalysts.
2
2
2 K. L. Ai, Y. L. Liu, C. P. Ruan, L. H. Lu and G. Q. Lu, Adv. Mater.,
2013, 25, 998-1003.
3 D. H. Deng, L. Yu, X. Q. Chen, G. X. Wang, L. Jin, X. L. Pan, J.
Deng, G. Q. Sun and X. H. Bao, Angew. Chem. Inter. Ed., 2013,
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2, 371-375.
Conflicts of interest
There are no conflicts to declare.
2
4 Q. Wang, Z. Y. Zhou, Y. J. Lai, Y. You, J. G. Liu, X. L. Wu, E.
Terefe, C. Chen, L. Song and M. Rauf, N. Tian, S. G. Sun, J. Am.
Chem. Soc., 2014, 136, 10882-10885.
2
2
2
2
5 M. Q. Wang, W. H. Yang, H. H. Wang, C. Chen, Z. Y. Zhou and
S. G. Sun, ACS Catal., 2014, 4, 3928-3936.
6 Y. Hu, J. O. Jensen, W. Zhang, N. C. Lars, W. Xing, J. B. Niels
and Q. F. Li, Angew. Chem. Inter. Ed., 2014, 126, 3749-3753.
7 W. X, Yang, X. J. Liu, X. Y. Yue, J. B. Jia and S. J. Guo, J. Am.
Chem. Soc., 2015, 137, 1436.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21673171), the Programme of Introducing
Talents of Discipline to Universities (B17034), the National
Natural Science Fund for Distinguished Young Scholars
8 F. L. Meng, Z. L. Wang, H. X. Zhong, J. Wang, J. M. Yan and X.
B. Zhang, Adv. Mater., 2016, 28, 7948-7955.
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Table of contents
FeN and γ-Fe O nanoparticle co-functionalized hollow graphitic carbon nanofibers exhibit
x
2 3
desirable ORR electrocatalytic activity.