Fe/Fe3C nanoparticle-decorated N-doped carbon nanofibers for improving the nitrogen selectivity of electrocatalytic nitrate reduction

Article abstract of DOI:10.1039/d0ta02317e

Nanoscale zero-valent iron has been considered to be the most promising electrocatalyst for denitrification due to its abundant resources, low price and non-toxicity. Nevertheless, the low utilization of active ingredients, inferior removal capacities (mg N g-1 Fe), and poor nitrogen selectivity are still major challenges during practical nitrate reduction. Herein, we have synthesized a one-dimensional architecture with homogeneously distributed Fe/Fe3C nanoparticles immobilized in a monodispersed carbonaceous matrix, embedded in ultra-high N-doped carbon nanofibers (Fe/Fe3C-NCNF) via a simple electrospinning strategy followed by confined reduction under a H2 atmosphere. The as-prepared Fe/Fe3C-NCNF nanostructure features well-dispersed Fe/Fe3C nanoparticles, confined reactive spaces, an interconnected nanofiber framework, and rich nitrogen doping sites, which are beneficial for integrating the synergistic catalytic effect of Fe and Fe3C, providing high-reactivity sites, boosting electron transfer, enlarging the catalyst-electrolyte interface and enhancing the surface adsorption capacity. The results reveal ultra-high nitrogen selectivity of 95% within 6 h and nearly 100% in 12 h, which are superior to other reported catalysts, and a maximum removal capacity of 2928.42 mg N g-1 Fe can be achieved, greatly improving the utilization of active ingredients. In addition, the catalysis material also shows good catalysis stability due to its structural features. The results of this study provide broad potential for the further development of iron-based functional nanostructures for electrocatalytic denitrification. This journal is

Full text of DOI:10.1039/d0ta02317e

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Fe/Fe3C nanoparticles decorated N-doped carbon nanofibers
for improving nitrogen selectivity of electrocatalytic nitrate
reduction
Yue Lan,a Junliang Chen,a Hui Zhang,a Wei-xian Zhangb and Jianping Yang*a
a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
College of Materials Science and Engineering, Donghua University, Shanghai 201620,
China
b
College of Environmental Science and Engineering, State Key Laboratory of Pollution
Control and Resources Reuse, Tongji University, Shanghai 200092, China.
E-mail for corresponding author: jianpingyang@dhu.edu.cn

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Abstract
Nanoscale zero-valent iron has been considered to be the most promising electrocatalyst
for denitrification due to abundant resources, low price and non-toxicity. Nevertheless, the
low active ingredients utilization, inferior removal capacity (mg N/g Fe) and poor nitrogen
selectivity are still the major challenges for practical nitrate reduction. Herein, we
synthesized one-dimensional architecture with homogeneously distributed Fe/Fe3C
nanoparticles immobilized in monodisperse carbonaceous matrix, which embedded in
ultra-high N-doped carbon nanofibers (Fe/Fe3C-NCNF) via simple electrospinning
strategy followed by confined reduction in H2 atmosphere. The as-prepared Fe/Fe3C-
NCNF nanostructure features well-dispersed Fe/Fe3C nanoparticles, confined reactive
spaces, interconnected nanofiber framework, and rich nitrogen doping sites, which is
beneficial to integrate the synergistic catalytic effect of Fe and Fe3C, provide high
reactivity sites, boost electron transfer, enlarge catalyst-electrolyte interface and enhance
surface adsorption capacity, respectively. The results reveal ultra-high nitrogen selectivity
of 95 % within 6 h and nearly 100 % in 12 h which is superior than other reported catalysts
and can achieve maximum removal capacity of 2928.42 mg N/g Fe, greatly improving the
active ingredients utilization. In addition, the catalysis material also shows good catalysis
stability due to structural features. The results of this study provide broad prospects for the
further development of iron-based functional nanostructrues for electrocatalytic
denitrification.
Keywords: iron nanoparticles, confined space, nitrogen-doping, carbon nanofiber,
electrocatalytic denitration

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1. Introduction
Nitrate contamination in water resources derived from overfertilization, industrial sewage
and animal manure seriously give rise to eutrophication and algae proliferation.1-2 Besides,
nitrite converted from nitrate is a major inducement of some terrible disease such as
stomach cancer, methemoglobinemiain.3-5 Thus, numerous strategy have been explored for
denitrification are ion-exchange, reverse osmosis, electrodialysis, biological denitrification
and electrocatalytic reduction. Among these approaches, electrocatalytic reduction has
been considered to be the state-of-the-art technologies for nitrate reduction because of high
treatment efficiency, low operating cost, no secondary pollution and low energy
consumption.6-8 During the process of electrocatalytic denitrification, nitrate is mainly
converted into several nitrogen-containing products including nitrite, ammonia, nitrogen
and a small amount of nitrogen oxides. Owing to its characteristics of environmentally
friendly and toilless emissions, nitrogen has become the main target product. 9-11
In the past decade, various catalysis materials including Ru, Pd, Pt, Cu, Fe, Mn have
been developed for electrocatalytic denitration.12-19 However, scarcity and high cost of
precious materials and poor stability limit their large-scale practical applications.20
Nanoscale zero-valent iron stands out from transition metals and become potential
alternative as its abundance, low cost, and well nitrogen selectivity.21-22 In this respect,
Teng et al. synthesized mesoporous carbon as carrier to support nanoscale zero-valent iron
for nitrate reduction exhibiting maximum removal capacity of 315 mg N/g Fe and 74 %

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nitrogen selectivity, which manifests that iron particles can effectively remove nitrate.23 In
order to improve nitrate removal capacity and nitrogen selectivity, Su et al. sealed iron
particles in carbon microspheres (Fe@C) with ultrahigh iron content of 74 %, which not
only developed favorable catalyst support, but also increase the iron content for
electrocatalytic denitration. This strategy realized maximum removal capacity of 1816 mg
N/g Fe, 42% nitrogen selectivity in sodium chloride system and 927 mg N/g Fe, 98%
nitrogen selectivity in sodium sulfate system within 48 h.8 Nevertheless, inefficient
catalytic performance, low active ingredients utilization are still critical challenges need to
be overcome. Therefore, there is in urgent need of developing more advanced catalyst
supports and more potent active ingredients for simultaneous achieving excellent nitrate
removal capacity and high nitrogen selectivity.
In recent years, iron/iron carbide (Fe/Fe3C) component has become a promising
alternative with efficient active substance in electrocatalysis such as ORR, OER,
photocatalysis, pollutants adsorption and Li-ion battery. The combined advantages of Fe
and Fe3C endow the hybrid superior characteristics of enhanced conductivity, strong
corrosion resistance and improved adsorption capacity, which delivers better performance
than pure metallic iron or iron carbide.24-32 Moreover, the unique electronic configuration
and catalytic properties similar to noble metal and highly activated iron atoms of Fe3C
endow it with excellent catalytic performance.33-35 As one of the typical subclass of iron-
based metal–organic frameworks (MOFs), Prussian blue (PB) not only inherits the unique

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advantages of MOFs but also can be used as a precursor to synthesize Fe/Fe3C more facile
than others because of its unique molecular structure.36-38 Owing to its special structure and
properties, Fe/Fe3C composites derived from Prussian blue have been successfully used in
various fields of zinc-air batteries, heavy metal ion removal and lithium-oxygen
batteries.39-41 PB is constructed by hexahedral Fe4[Fe(CN)6]3, where Fe3+ is connected to
the nitrogen atom of cyanide and Fe2+ is connected to the carbon atom. When PB calcined
in inert gas, reduced metal iron atoms react with carbon derived from fractured cyanide to
form Fe/Fe3C.42-45 In particular, the pyrolysis temperature is an important factor to fabricate
stable catalyst with outstanding performance. Undesirable skeleton collapsing and serious
catalyst performances deterioration are prone to occur at excessive calcination
temperature.46-50 Therefore, to avoid the destruction of internal structure, Fe/Fe3C can be
formed at appropriate temperatures in H2 atmosphere with strong reducibility. Among
various catalyst supports, one-dimensional carbon nanofiber carrier has been considered to
be the promising carbon substrate to support Fe/Fe3C nanoparticles due to many merits
such as enhanced electronic transfer capabilities, large active surface and strong stress
tolerance.51-52 In addition, the doped nitrogen atoms can also improve catalytic performance.
According to previous reported literatures and DFT calculations, the nitrogen doping sites
adjacent to carbon atom can act as an electron donor to accept lone pair electrons from
carbon atom to enhance the charge density around the carbon atom, which is conducive to
improving the covalence binding ability between carbon atoms and oxygen atom.53-54
Therefore, the adsorption capacity towards nitrate can be improved by the as-doped

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nitrogen atoms in the electrocatalyst through the enhanced binding energy of carbon atoms
and oxygen atoms of nitrate.55-56 Moreover, the modified carbon matrix by nitrogen
heteroatoms increase the defects of graphite which is elevate electrical conductivity of
continuous carbon framework.57
In this work, we designed well-dispersed Fe/Fe3C nanoparticles embedded in one-
dimensional rich N-doped carbon nanofibers (Fe/Fe3C-NCNF) via facile electrospinning
and then confined reduction in H2 atmosphere. During the pyrolysis process, the PB
microcubes embedded in PBA serve as templates to generate cavities with nearly equal
amount of Fe/Fe3C nanoparticles. In particular, this ‘confined by group’ strategy is an
effective solution to alleviate exaggerated growth and severe aggregation of Fe/Fe3C
nanoparticles. Furthermore, the continuous conductive framework acts as highway can
enlarge catalyst-electrolyte interface thus to enhance the catalyst ingredient utilization and
promote the charge transfer and mass diffusion during the electrocatalytic denitrification.
It is worth noting that the rich nitrogen doping sites and active iron species of Fe and Fe3C
play an important roles in greatly improving the electrocatalytic nitrate removal capacity
and nitrogen selectivity. As a result, we found that the prepared catalyst significantly
increases nitrate removal capacity with maximum value of 2928.42 mg N/g Fe, which
notably boost the catalytic active ingredients utilization. Moreover, the outstanding
nitrogen selectivity of 95 % within 6 h and nearly 100 % within 12 h is achieved. Besides,
the nitrogen selectivity can still exceed 95 % and removal capacity retain 93.3% after seven

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cycles showing good stability. In addition, we also explored the influences of different test
conditions and explained the unique mechanism of electrocatalytic denitrification process.
2. Experimental Section
2.1 Synthesize of Prussian blue
As previous reported,58-59 38 g polyvineypirrolydone (PVP) and 1.1 g K3Fe[CN]6·3H2O
were added to 500 mL hydrochloric acid (0.1 M) followed by stirring at room temperature
for 1 h. Then the light-yellow solution was transferred to a bottle and heated in an electric
oven at 80 °C for 24 h. The blue precipitate was collected through filtration and
centrifugation with deionized water and ethanol for several times. Finally, PB powder was
obtained by drying in a vacuum oven at 60 °C for 12 h.
2.2 Synthesize of PB/PAN-x
A certain amount of obtained PB powder (0.05 g, 0.2 g, 0.45 g) was evenly dispersed in
2.5 mL DMF solvent by sonication for 1.5 h. Then 0.36 g polyacrylonitrile (PAN) was
added into the above solution with vigorously stirring at 50 °C for 24 h. Whereafter, the
above solution was transferred to a 5 mL syringe with an 18-gauge blunt tip needle. For
the electrospinning process, the voltage of electrospinning was set to 24.5 kV and the liquid
flow rate was 1.2 mL/h. Silicone oil paper was used as receiving device to receive fiber
with the distance of 14.5 cm. Then the as-collected electrospun fibers were dried in a
vacuum oven to at 60 °C for 12 h, which named PB/PAN-1, PB/PAN-2, PB/PAN-3 (1, 2,

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3 represent that the PB mass contained in the PB/PAN fibers are 0.05 g, 0.2 g, 0.45 g,
respectively).
2.3 Synthesize of Fe/Fe3C-NCNF
The as-obtained fiber was firstly preoxidized at 280 °C for 2 h in with heating rate of
5 °C/min and then carbonized in H2 atmosphere at 500 °C for 4 h with the ramp rate of
2 °C/min. PB/PAN fibers with different PB mass (0.05 g, 0.2 g, 0.45 g) calcined at 500 °C
were named Fe/Fe3C-NCNF-1, Fe/Fe3C-NCNF-2, Fe/Fe3C-NCNF-3, respectively. In
addition, fibers with PB mass of 0.2 g were calcined at 400 °C, 600 °C named Fe/Fe3C-
NCNF-2-400 and Fe/Fe3C-NCNF-2-600, respectively.
2.4 Characterization
The microscopic morphology of electrocatalysts were characterized by field emission
scanning electron-microscopy (FESEM, Hitachi S4800), transmission electron microscopy
(TEM, JEOL JEM-2100F) operated at 200 kV. The crystalline structure and surface
chemical composition were analyzed by X-ray diffraction (XRD, Rigaku D/Max-2550 PC)
with diffractometer equipped with Cu Kα radiation and powder X-ray photoelectron
spectroscopy (XPS, ESCALAB 250Xi). Thermogravimetric was carried out using
Q5000IR TG-DTA under air atmosphere at a heating rate of 10 °C/min. Raman spectrum
was conducted using a spectrometer (HORIBA JOBIN YVON XploRA) with a He-Nelaser
excitation wavelength of 632.8 nm.

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2.5 Electrochemical measurements and data analysis
The electrocatalytic performance of the as-synthesized materials for nitrate reduction was
conducted using a three-electrode system on CHI 660E (Shanghai CHI Instruments Co.) at
given voltage -1.3 V. Platinum sheet (1 cm2) was used as a counter electrode and a saturated
calomel electrode (SCE, 0.2415 V vs NHE) was adopted as reference electrode. The
working electrode was prepared as our previous work. Briefly, the mixture of 0.5 mg
acetylene black, 4 mg prepared catalysts and 50 μl PVDF (10 g/L) were fully milled to
form catalyst ink which was further coated on 1 cm2 nickel foam and dried in a vacuum
oven at 80 °C for 12 h. Furthermore, all of the electrocatalyst performance tests were
carried out in 100 ml undivided electrolytic cell filled with 50 ml untreated solution, which
consist of different initial concentrations of sodium nitrate (25 mg/L, 50 mg/L, 100 mg/L,
150 mg/L, 200 mg/L) and required electrolyte. Depending on test conditions, different
concentrations of sodium chloride (0.01 M, 0.02 M, 0.04 M, 0.08 M) and sodium sulfate
(0.01 M, 0.02 M, 0.03 M, 0.04 M) were prepared as electrolytes. The initial pH value of
the solution was adjusted by 0.1 M HCl. At each fixed reaction time, 3 ml solution was
taken out from the reaction solution for measuring the residual concentrations of nitrate,
nitrite and ammonia by an Ultraviolet-visible (UV–vis) spectrophotometer (Evolution TM
200, Thermo Fischer Scientific, USA). As previous reported, Fe can strongly adsorb NO
and further convert into absorbed N (abs) and O (abs). The associated N (abs) can
eventually form nitrogen.60 Therefore, the products of nitrogen oxides can be ignored in

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our data analysis. The nitrate removal capacity, conversion percentage of nitrate and the
selectivity of products were calculated according to the following equations:
-
The removal capacity (RC) of NO3 :
[퐶0(푁푂3 ― ) ― 퐶푡(푁푂3 ― )] × 푉
3 ―
푅퐶푁푂
=
푚퐹푒
-
The conversation percentage (CP) of NO3 :
―
퐶0(푁푂3 ― ) ― 퐶푡(푁푂3
) × 100%
) × 100%
) × 100%
―
퐶푃푁푂 (%) =
―
3
퐶0(푁푂3
)
―
퐶푡(푁푂2
)
―
푆푁푂 (%) = 퐶 (푁푂
― ) ― 퐶푡(푁푂3
―
―
2
0
3
+
퐶푡(푁퐻4
)
+
푆푁퐻 (%) = 퐶 (푁푂
― ) ― 퐶푡(푁푂3
4
0
3
+
퐶0(푁푂3 ― ) ― 퐶푡(푁푂3 ― ) ― 퐶푡(푁푂2 ― ) ― 퐶푡(푁퐻4
)
푆푁 (%) =
× 100%
2
―
퐶0(푁푂3 ― ) ― 퐶푡(푁푂3
)
-
-
-
-
The C0 (NO3 ) represents the initial NO3 -N concentration. The Ct (NO3 ), Ct (NO2 ),
+
-
-
+
C (NH4 ) refer to NO3 -N concentration, NO2 concentration, NH4 concentration at the
fixed reaction time t (h). V is the total volume of untreated solution and mFe is the total iron
mass coated on the nickel foam.
3. Results and Discussion
The fabrication process of Fe/Fe3C-NCNF is displayed in Fig. 1a. Firstly,
K3Fe[CN]6·3H2O was selected as precursor to synthesize PB by hydrothermal approach.
Then, a series of PB/PAN nanofibers with different PB loading were constructed through

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a facile electrospinning technique via adjusting the PB mass containing in electrospinning
solution. Finally, the obtained PB/PAN nanofiber was preoxidized in air and pyrolysis in
H2 atmosphere to synthesize Fe/Fe3C-NCNF. The SEM and TEM images (Fig. S1a-b, ESI†)
show that PB microcubes are uniform in size (~ 500 nm) and well dispersed. According to
TGA curves (Fig. S2, ESI†), the result indicates the iron content of Fe/Fe3C-NCNF-1,
Fe/Fe3C-NCNF-2 and Fe/Fe3C-NCNF-3 were 11 %, 32 %, and 41 %, respectively. We
calculated the iron content of 32% through the mass of the last remaining ferric oxide
(m(Fe2O3)) and the initial mass of sample (m(S)) to be measured by following formula:
푚(퐹푒 푂 ) × 112 × 100%
2
3
Iron content (% ) =
160 × 푚(푆)
As illustrated in Fig. 1b-d, the homogeneously distributed PB microcubes embedded in
one-dimensional PAN nanofibers was constructed by a facile electrospinning. Furthermore,
the enlarged SEM images (top right inset images) of Fig. 1b-d show that, embedded
microcubes in individual fiber became more compact with the amount of PB increasing.
Fig. 1e-g indicate that the obtained products Fe/Fe3C-NCNF-x (x=1, 2, 3) after pyrolysis
remain the original one-dimensional structure. Impressively, the microcubes morphology
of PB was maintained, which can be ascribed to the uniformly coated PAN layer serving
as a buffer to prevent the structure collapse. In addition, the confined capsule can prevent
the leakage of Fe/Fe3C to eliminate the secondary pollution during subsequent
electrocatalytic process. TEM images (Fig. S3a-c, ESI † ) further reveal that Fe/Fe3C
nanoparticles are uniformly confined within carbon nanofibers. Compared with Fe/Fe3C-

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NCNF-1 and Fe/Fe3C-NCNF-3, Fe/Fe3C-NCNF-2 has better dispersibility and appropriate
iron content, which is beneficial to give full play to eletrocatalysis.
Besides, the pyrolysis temperature is also a significant factor influencing the crystal
structure and further electrocatalyst performance. X-ray diffraction (XRD) patterns shown
in Fig. 2a indicate that the characteristic diffraction peaks of Fe3C (JCPDF Card No.35-
0772) and small amount of Fe (JCPDF Card No.06-0696) can be observed in all samples.
Besides, pattern of Fe/Fe3C-NCNF-2-400 also exits weak peaks of Fe3O4, which can be
ascribed to the insufficient pyrolysis temperature to completely transfer PB into Fe/Fe3C.
The HRTEM image (Fig. S4, ESI†) of Fe/Fe3C-NCNF-2-400 further prove the presence of
Fe3O4. As the calcined temperature increases, the peak of carbon (2 θ=26.5 °) and iron
become sharper, indicating higher crystallinity and graphitization. Unfortunately, the larger
crystalline particle size and the inevitably inherent skeleton structure destruction of PB at
600 °C can be observed in Fig. S5, ESI†. Raman spectra (Fig. 2b) illustrate two peaks at
about 1340 and 1598 cm-1, which corresponds to the D and G bands of carbon materials.
Particularly, D band represents the defects of the C atom lattice and G band represents the
in-plane stretching vibration of the C atom sp2 hybrid. The intensity ratio of D peak and G
peak (ID/IG) is an important indicator to evaluate the graphitization degree. The result of
Fig. 2b manifests higher temperature is in favor of improving graphitization degree and
enhancing electric conductivity. However, as it shown in Fig. S6, ESI†, it is worth noting
that the ID/IG value of Fe/Fe3C-NCNF-3 (41% iron loading) is higher than Fe/Fe3C-NCNF-

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2 (32% iron loading), which go against with the opinion of ‘the presence of iron promotes
graphitization’.61 This phenomenon can be attributed to the aggregation of excessive
Fe/Fe3C particles hampers it catalytic capability. The surface chemical composition and
electronic structure of various samples analyzed by XPS in Fig. 2c indicates that four
elements C, N, O, Fe exist in the samples. The ultra-high N doping amount derives from
precursor PAN and cyano group in PB. According to Table S1, the content of doped N
gradually decreases from 13.57 % to 5 % as the calcination temperature increases from 400
°C to 600 °C, which means higher temperature is adverse to boosting N doping in the
carbon matrix. Broad peaks located at 723.9eV, 725.8 eV, 719.4 eV, 710.5eV, 712.3 eV
and 707.4 eV in the high-resolution Fe 2p spectrum (Fig. 2d, Fig. S7a, ESI†, Fig. S8a, ESI†)
are assigned to Fe2+ (2p1/2), Fe3+ (2p1/2), satellite peak, Fe2+ (2p3/2), Fe3+ (2p3/2) and Fe0. The
peaks of Fe0 are weaker than these of Fe2+ and Fe3+ indicating the main iron containing
species is Fe3C, which is consistent with XRD spectra. However, no obvious peak of Fe0
can be observed in Fe/Fe3C-NCNF-2-400, indicating that most iron species still exist as
iron oxides due to the low pyrolysis temperature. The result is consistent with the XRD
pattern and HRTEM image. The high-resolution N 1s spectras (Fig. 2e, Fig. S7b, ESI†
Fig. S8b, ESI†) can be divided into three different peaks: pyridinic N (398.3 eV), pyrrolic
N (400.1 eV) and graphitic N (401 eV). Among these species, graphite nitrogen is
beneficial to accelerate electron transfer, pyridine nitrogen is able to promote the
adsorption capability of nitrate during the process of electrocatalytic denitrification.
Compared with Fe/Fe3C-NCNF-2-600, Fe/Fe3C-NCNF-2 with more N doped sites can

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benefit electrocatalytic denitrification from absorption aspect. Furthermore, C-C (284.7
eV), C-N (285.6 eV), -COO (289 eV), C=O/C=N (286.8 eV) can be observed in the high-
resolution C 1s spectra (Fig. 2f, Fig. S7c, ESI†, Fig. S8c, ESI†).
The internal structure of Fe/Fe3C-NCNF-2 is further ascertained by TEM (Fig. 3a)
and HRTEM (Fig. 3b-c) images. As shown in Fig. 3a, Fe/Fe3C nanoparticles are
homogenously dispersed in carbonaceous fibers. As shown in HRTEM images (Fig. 3b-c),
Fe/Fe3C nanoparticles are wrapped by a thin carbon layer, which is ascribed to the pyrolysis
of cyano group in PB. Owing to the catalysis of metal Fe, a lattice fringe of 0.34 nm can
be observed in the outermost layer of Fe/Fe3C nanoparticle corresponding to the 002 plane
of graphite carbon. Noticeably, the carbon layer can serve as a protective layer to prevent
inner nanoparticles from severely aggregating during the pyrolysis process and effectively
alleviate Fe/Fe3C corrosion in electrolyte. The distinct lattice fringe with d-spacing of 0.20
nm, 0.24 nm correspond with the (110) plane of Fe0 and (210) plane of Fe3C, which
indicates that the decomposed cyanide groups partially reduce metal Fe to form Fe3C
during pyrolysis. The selected area electron diffraction (SAED) pattern (inset image in Fig.
3b) exhibits that the obvious diffraction ring represents the diffraction plane of graphitic
carbon and the small bright spots can be assigned to single crystalline Fe3C.19 The line scan
and EDS elemental mapping images (Fig. 3d-j) reveals that C, N, O elements are
homogeneously distributed through the whole nanofibers and Fe species are confined in
certain spaces. The EDS spectrum (Fig. S9, ESI † ) also confirm the presence of four

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elements and high N atom content.
3.2 Electrocatalytic performance
A series of catalysis materials synthesized under different experimental conditions were
applied to electrocatalytic denitrification under the same testing conditions to evaluate
denitrification efficiency and nitrogen selectivity, as shown in Fig. 4. It is worth noting that
the iron species in Fe/Fe3C-NCNF-1, Fe/Fe3C-NCNF-2, Fe/Fe3C-NCNF-3 and Fe/Fe3C-
NCNF-2-600 samples with the presence of iron and iron carbide iron show notable
improvement in nitrate removal capacity (mg N/g Fe) and nitrogen selectivity compared
with previous work, where the unmodified pure metallic Fe-based materials were served
as electrocatalyst.8,23,55 This phenomenon indicates that the introduced Fe3C shows positive
effect on nitrate removal when combined with Fe0. Since most of iron species exist in the
form of Fe3C, it is reasonable to believe that Fe3C plays the role of active ingredients during
electrocatalytic denitrification. The synergistic effects of Fe3C and Fe0 enhance the
catalytic activity and the noble-metal-like properties of Fe3C may be beneficial to improve
the nitrogen selectivity. The electrocatalytic test results show that Fe/Fe3C-NCNF-2 has
the best performance with outstanding nitrate removal of 1787.41 mg N/g Fe and ultra-
high nitrogen selectivity of 96 %. On the one hand, the abundant doped nitrogen atoms not
only enhance the adsorption capacity of nitrate, but also improve conductivity of carbon
framework for facilitating the transfer of electrons from the Fe/Fe3C core to nitrate and
intermediate products. On the other hand, the homogeneously distributed Fe/Fe3C nano-

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active ingredients confined in carbon nanofibers contribute to exposing more active sites
to improve the nitrate reduction activity and nitrogen selectivity. The nitrate removal
capacity and nitrogen selectivity of Fe/Fe3C-NCNF-1, Fe/Fe3C-NCNF-3 are respectively
1761.36 mg N/g Fe, 94% and 1369.57 mg N/g Fe, 92%, which are inferior to the
performance delivered by Fe/Fe3C-NCNF-2. Although Fe/Fe3C-NCNF-1 has well
dispersion of Fe/Fe3C nanoparticles, insufficient active sites and poor graphitization degree
lead to the inferior activity. Moreover, Fe/Fe3C-NCNF-2-600 has lower nitrate removal
capacity of 1403.81 mg N/g Fe and worse nitrogen selectivity of 93% compare to Fe/Fe3C-
NCNF-2, which can be ascribed to low nitrogen content, larger Fe/Fe3C crystalline
particles size and destroyed skeleton structure of carbon matrix.62 In addition to Fe and
Fe3C, iron oxides can also be found in Fe/Fe3C-NCNF-2-400 according to XRD spectra
due to the low reduction temperature, where the iron oxides has been confirmed as a
unfavorable electrocatalytic denitrification catalyst by previous studies.8,23 Although the
proportion of Fe and Fe3C contained in Fe/Fe3C-NCNF-2-400 make great contribution to
nitrate removal, the presence of low-reactive iron oxides, insufficient Fe/Fe3C active sites
and the poor graphitization degree significantly degrade nitrate removal activity showing
the worst removal ability (570.75 mg N/g Fe) and the lowest nitrogen selectivity (78%).
Fig. S10, ESI† shows the corresponding conversion percentage of nitrate. In addition, the
effect of various parameter conditions including initial pH value, initial Cl- concentration,
2-
-
the molarity ratio of Cl- and SO4 , initial NO3 -N concentration, reaction time and cycling
stability were discussed in detail.

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3.3 Effect of initial pH
The initial pH of the solution is an important factor that cannot be ignored in the practical
nitrate reduction. As previous reported, acidic system is favored of promoting nitrate
reduction.63-64 Therefore, different initial pH values (pH=3.0, 5.0, 7.0) were carried out to
analyze and evaluate electrocatalytic performance of Fe/Fe3C-NCNF-2 (as shown in Fig.
5a). Obviously, the removal capacity and nitrogen selectivity achieve optimal value at
initial pH 5.0. However, when the initial pH values are adjusted to 3.0 and 7.0, the removal
capability and nitrogen selectivity reduce to 1674.14 mg N/g Fe, 82 % and 1540.22 mg N/g
Fe, 88 %, respectively. The potentiodynamic polarization curves at different initial pH
values (Fig. S11a, ESI†) show that the corrosion current density of Fe/Fe3C increases from
215.5 μA to 188.4 μA as the initial pH decreases from 7.0 to 3.0. The resulting data
indicates that lower pH is conducive to Fe/Fe3C corrosion which accelerate electron
transfer from the electrocatalyst to nitrate.65 Nevertheless, severe hydrogen evolution is
accompanied by Fe/Fe3C corrosion in harsh acidic system resulting in the generated
hydrogen wrapping on the surface of the active material to form a passivation film to
prevent further contact between the active sites and the electrolyte, which deteriorates
electrocatalytic performance.66 In addition, we measured the pH value in treated solution
and found that it was higher than the initial value, which indicates that hydrogen evolution
reaction can consume hydrogen ions to produce more hydroxide. The result shows that

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faintly acidity is more favorable for electrocatalytic nitrate reduction than neutral or strong
acidity.
3.4 Effect of initial Cl- concentration
The effect of initial Cl- concentration on nitrate removal is shown in Fig. 5b. The nitrate
removal capacities are 1320.21 mg N/g Fe, 1787.41 mg N/g Fe, 1880.18 mg N/g Fe and
the corresponding nitrogen selectivity are 45 %, 96 %, 98 % at the initial Cl- concentration
0.01 M, 0.02 M, 0.04 M. The improved nitrate removal capacity can be explained by
several reasons: (1) Increased Cl- concentration can enhance the conductivity of electrolyte
facilitating ion transmission. (2) Ammonia dissolved in solution further react with
hypochlorous acid to re-expose more available active sites.55 The dramatic increase of
nitrogen selectivity can be ascribed to the breakpoint chlorination reaction. Chloride can
be oxidized to produce hypochlorite which oxidize ammonia to generate nitrogen, as shown
in the following equations (1-4).67-68 Fig. S12a, ESI† indicates that the improving Cl-
concentration decrease the content of nitrite and ammonia, promoting the conversion of
nitrite into nitrogen. However, when the Cl- concentration increases to 0.08 M, the catalytic
performance diminished. The phenomenon may be attributed to the following two reasons.
On the one hand, superfluous chloride ions occupy the catalytic active sites equivalent to
forming a passivation film wrapped on the surface of the active sites to prevent the catalytic
reaction from being initiated. On the other hand, hypochlorite ion can oxidizes nitrite into
additional nitrate (5).55,67

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2퐶푙 ― ― 2푒 ― →퐶푙2
(1)
퐶푙2 + 퐻2푂→퐻퐶푙푂 + 퐻 + +퐶푙 ―
(2)
퐻퐶푙푂→퐻 + +퐶푙푂 ―
(3)
+
3퐻퐶푙푂 + 2푁퐻4 →푁2 +5퐻 + +3퐶푙 ― +3퐻2푂
(4)
―
퐶푙푂 ― +푁푂2 ― →푁푂3 +퐶푙 ―
(5)
2-
3.5 Effect of the molarity ratio of Cl- and SO4
There are various anions in sewage which inevitably affect the practical nitrate treatment.
2-
In addition to Cl-, SO4 widely present in sewage. Therefore, it is indispensable to
2-
investigate the effect of different molarity ratio of Cl- and SO4 on denitrification using
dual electrolyte system (NaCl and Na2SO4) (Fig. 5c). Based on the above discussion, we

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fixed the total electrolyte concentration to 0.04 M. When the molarity ratio of Cl- and SO4
changes from 1:0 to 0:1, the removal capacity are 1880.18 mg N/g Fe, 2417.74 mg N/g Fe,
2472.71 mg N/Fe, 2606.67 mg N/g Fe, 2235.68 mg N/g Fe and the corresponding nitrogen
selectivity are 98 %, 98 %, 94 %, 75 %, and 32 %. The results suggest that higher Cl-
concentration is beneficial to generate nitrogen while higher SO42- concentration is favor
of conversion of nitrate into ammonia in dual electrolyte system. It can be seen from Fig.
S11b, ESI† that the corrosion current density of Fe/Fe3C in Na2SO4 system (239.1 μA) is
higher than that in NaCl system (219.5 μA), which renders nitrate to be more prone to get
electrons to be reduced to ammonia matched with the Fig. S12b, ESI†. Based on the
comprehensive consideration for nitrate removal ability and nitrogen selectivity, the
2-
2-
optimal dual electrolyte system consist of 0.01 M Cl- and 0.03 M SO4 .
-
3.6 Effect of NO3 concentration
-
The denitrification performance of Fe/Fe3C-NCNF-2 at different initial NO3 -N
concentrations is shown in Fig. 5d. It's worth noting that the result in our work is difference
from the previous reported. The nitrate removal capacity increases from 362.10 mg N/g Fe
-
to 3600.58 mg N/g Fe with the initial NO3 -N concentration increasing from 25 mg/L to
150 mg/L. As previously reported, nitrate reduction conforms to the first-order kinetic
equation. Fig. S13, ESI† indicates that the rate constant gradually drop when the initial
-
NO3 -N concentration decreases from 150 mg/L to 25 mg/L which can be attributed to
weak binding strength with nitrate, reduced effective collision frequency of activated

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molecules and competitive adsorption with hydrogen under lower nitrate concentration.69
-
However, excessively high NO3 -N concentrations (200 mg/L) results in degrading
catalytic performance. For all this, the Fe/Fe3C-NCNF-2 still exhibits high removal
capacity even at 25 mg/L due to the enhanced electronic transmission capability, sufficient
catalytic sites and excellent adsorption ability which make the electrocatalyst effectively
adsorb nitrate as much as possible through the covalent bond of carbon atoms and oxygen
-
atoms of nitrate. Notably, the nitrogen selectivity can reach above 95 % at the initial NO3
-N concentration range from 25 mg/L to 150 mg/L, and it is still over 90 % at 200 mg/L,
indicating Fe/Fe3C-NCNF-2 with outstanding nitrogen selectivity can broaden a wide
-
range of initial NO3 -N concentrations, which is much greater than other reported catalysts
shown in Table S2.
3.7 Effect of reaction time and long-term stability
The data of nitrate removal capacity and nitrogen selectivity over time were also measured
under the optimal testing conditions as follows: an initial pH 5.0, a dual electrolyte system
2-
consists of 0.03 M NaCl and 0.01 M Na2SO4 . As presented in Fig. 6a, the concentration
of nitrate gradually diminishes with the time increasing from 6 h to 36 h. However, no
obvious cumulative increase process and gradual consumption can be discovered for by-
products nitrite and ammonia which continuously keep negligible amounts in the treated
solution with the prolonging of time, suggesting that nitrate can be quickly converted to
nitrite which further be reduced to ammonia or nitrogen and the dissolved ammonia can be

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effectively oxidized to nitrogen by the active species hypochlorite. Owing to the synergistic
effect between Fe0 and highly activated iron atoms of Fe3C, Fe/Fe3C-NCNF-2 delivers
superior electrocatalytic reduction activity of nitrate and facilitates the conversion of by-
products. Moreover, the nitrogen selectivity has reached 95 % within 6 h and is almost
close to 100 % within 12 h. This result demonstrates that the Fe/Fe3C-NCNF-2 possesses
excellent activity and ultra-high nitrogen selectivity for electrocatalytic denitrification.
Long-cycle stability testing is also one of the important factors to evaluate the
electrocatalyst performance of nitrate removal. As shown in Fig. 6b, the conversation of
nitrate has hardly attenuated and the nitrate removal capacity still maintain at a high value
2256.02 mg N/g Fe retaining 93.3% of the initial value and and the nitrogen selectivity can
still exceed 95 % after seven cycles showing good catalysis stability, ascribed to the
protective carbon layer wrapped around Fe/Fe3C which can alleviate the severe corrosion
of the catalyst by the acid electrolyte. Furthermore, the confined reactive spaces can
effectively suppress the escape of Fe/Fe3C from the carbon matrix. However, the slight
decrease of nitrate removal capacity and slow-growing of by-products nitrite and ammonia
(Fig. S14, ESI†) may be ascribed to the more favorable formation of iron oxides in the
alkaline electrolyte, which inevitably reduce the activity and nitrogen selectivity of the
electrocatalyst. The corresponding conversion percentage of nitrate under different test
conditions are shown in Fig. S15, ESI† and Fig. S16, ESI†.
3.8 Catalytic mechanism for nitrate reduction reaction

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The specific mechanism of electrocatalytic nitrate reduction using Fe/Fe3C-NCNF-2 as the
catalysis material is shown in Fig. 6c. In fact, nitrate reduction is a multi-step reaction, in
which the nitrate adsorption is the first and critical step which is an important prerequisite
for triggering subsequent reactions followed by a serious of reduction process and is one
of the key factors affecting the activity of electrocatalytic denitrification.70-71 Fe/Fe3C-
NCNF-2 doping ultra-high percentage of nitrogen atoms can improve the ability of nitrate
adsorption by covalent bonding between oxygen atoms of nitrate with carbon atoms on the
surface of catalyst which simultaneously suppress the competitive adsorption of hydrogen
under acidic system at the first step of electrocatalytic denitrification. Then, the adsorbed
nitrate is reduced to nitrite by getting electrons from the catalyst which is the rate-
determining step of electrocatalytic denitrification.69 One-dimensional fiber structure can
provide a large number convenient channels for electronic transmission. In addition, the
large amount of nitrogen atoms doped in the catalyst can also improve the charge transfer.
At the same time, inevitable slightly Fe/Fe3C corrosion in weakly acidic systems
accelerating the desirable electrons transport from the catalyst to the nitrate also promotes
the conversion of nitrate to nitrite. The dissolved nitrite can be further reduced to ammonia.
More importantly, Fe0 and highly activated iron atoms in Fe3C of Fe/Fe3C greatly enhance
the electrocatalytic activity of Fe/Fe3C-NCNF-2 through synergistic effect for nitrate
removal. There are three possible ways for converting nitrite to nitrogen.60 The first way
to generate nitrogen is the association of adsorbed nitrogen by dissociating the adsorbed
-
nitrate step by step under hydrogen attack. The second way is that the unstable NO2 (abs)

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-
generating from NO3 (abs) dissociation partial disproportionate to produce N2O (abs)
which can further form nitrogen. The primary way of nitrogen production comes from the
oxidation of ammonia. Based on breakpoint chlorination theory, ammonia chloride can be
oxidized to nitrogen with the addition of chloride ion. The involved equations are shown
in (6)-(12). Noble-metal-like characteristics of Fe3C may make a contribution to improving
nitrogen selectivity. In addition, the synergistic catalytic effect of Fe and Fe3C endows the
catalyst with superior ability for almost completely converting dissolved nitrite and
ammonium to nitrogen, reflecting by the expected catalytic effectiveness of detectable
residual by-product amount and ultra-high nitrogen selectivity.
―
푁푂3 (푎푏푠) + 2푒 ― +2퐻 + →푁푂2 ― (푎푏푠) + 퐻2푂
(6)
―
푁푂2 (푎푏푠) + 푒 ― +2퐻 + →푁푂 + 퐻2푂
(7)
―
+
푁푂2 (푠표푙) + 푒 ― + 퐻 + →푁퐻4 + 퐻2푂
(8)
푁푂3 ― (푎푏푠) → 푁푂2 ― (푎푏푠) → 푁푂(푎푏푠) → 푁(푎푏푠) →푁2
(9)
3퐻퐶푙푂 + 2푁퐻4 →푁2 +5퐻 + +3퐶푙 ― +5퐻2푂
+

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(10)
푁2푂(푎푏푠) + 2푒 ― +2퐻 + →푁2 + 퐻2푂
(11)
푁푂(푎푏푠) + 푁푂(푎푏푠) + 2푒 ― + 2퐻 + →푁2푂 + 퐻2푂
(12)
The internal structure and component of catalyst after long-cycle stability testing was
characterized by SEM, TEM and XRD. It can be seen from the SEM and TEM images
(Fig. 7a-c) that there is no obvious morphological change after denitrification compared
with the freshly prepared catalyst (Fig. S17, ESI†). The catalyst coated on the electrode
slice was collected by ultrasonication in ethanol followed by centrifugation and drying to
measure XRD patterns. As shown in Fig. 7d, the peak intensity of the iron and iron carbide
has no obvious difference from the unreacted catalyst. However, the characteristic peak of
iron oxide inevitably appeared after the long-term nitrate removal, which can be explained
by the fact that alkaline electrolytic system produced by the consumption of hydrogen ions
and the generation of hydroxides is more prone to form iron oxides.
The excellent electrocatalytic performance of Fe/Fe3C-NCNF-2 shown in nitrate
reduction can be attributed to the following reasons: (1) The confined reduction strategy
can effective alleviate exaggerated growth and aggregation of Fe/Fe3C nanoparticles
during the pyrolysis process. In addition, the confined space and the thin carbon layer

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wrapped around the Fe/Fe3C nanoparticles can prevent the active ingredients from
corrosion and leak which ensure the stability of the electrocatalyst and avoid secondary
pollution. (2) The one-dimensional carbon nanofiber matrix can provide a fast electron
transfer pathway and enlarge the contact interface between solid catalyst and liquid
electrolyte thus to allows the active Fe/Fe3C nanoparticles more accessible to the target
molecules and enhance the catalyst ingredient utilization. (3) The ultra-high amount of
doped nitrogen atoms (up to 11.27 %) in the electrocatalyst is conducive to enhancing
electrical conductivity of carbon matrix and improving the adsorption of nitrate which
contribute to improve the nitrate removal capacity. (4) The Fe0 and highly activated iron
atoms in Fe3C of the well-dispersed Fe/Fe3C nanoparticles exert synergistic catalytic effect
to enhance the catalytic activity and the noble-metal-like properties of Fe3C may be
beneficial to improve the nitrogen selectivity.
4. Conclusion
In summary, we synthesized ultra-high N-doped carbon nanofibers embedded with
Fe/Fe3C (Fe/Fe3C-NCNF) immobilized in monodisperse carbonaceous matrix showing
one-dimensional architecture through simple electrospinning technique followed by
confined reduction in hydrogen atmosphere. The synthetic catalyst exhibits remarkable
electrocatalytic performance with the maximum removal capacity 2928.42 mg N/g Fe,
ultra-high nitrogen selectivity nearly 100 % and good cycling stability. The outstanding
results can be attributed to the synergistic catalytic effect of Fe and Fe3C, ultra-high N

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content, enhanced electron transfer capacity, enlarged catalyst-electrolyte interface and
improved surface adsorption capacity. The thin protector carbon layer wrapped around the
Fe/Fe3C nanoparticles and confined reactive spaces can effectively alleviate Fe/Fe3C
corrosion and suppress the escape of Fe/Fe3C from the carbon matrix, endow the Fe/Fe3C-
NCNF with good cycling stability. It is believed that this work can provide meaningful
strategy for fabricating distinguished electrocatalyst for nitrate removal.
Conflicts of interest
There are no conflicts to declare
Acknowledgements
The authors are grateful for financial support from the Fok Ying-Tong Education
Foundation of China (No. 171041), National Natural Science Foundation of China (No.
51702046), State Key Laboratory of Pollution Control and Resource Reuse Foundation
(NO. PCRRF17005), the Program for Professor of Special Appointment (Eastern Scholar)
at Shanghai Institutions of Higher Learning, State Key Laboratory for Modification of
Chemical Fibers and Polymer Materials, Donghua University.
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