Nanocasting synthesis of an iron nitride-ordered mesopore carbon composite as a novel electrode material for supercapacitors

Article abstract of DOI:10.1039/c7ra08704g

Herein, a series of novel iron nitride (Fe₂N)/cubic-ordered mesoporous carbon (OMC) composites (Fe₂N@OMC) were synthesized via a facile nanocasting route and ammonia calcination. Physical characterization results indicate that small non-aggregated Fe₂N nanoparticles were evenly distributed on the external surface and in the pores of the carbon material with a linear array of mesopores in a regular interval arrangement. An electrochemical test was carried out using Fe₂N@OMC as a negative electrode of a supercapacitor in 6 mol L^-1 KOH aqueous electrolytes. The results indicated that the obtained Fe₂N@OMC-2 exhibited a superior specific capacitance of 547 F g^-1 at 1 mV s^-1 and 520 F g^-1 at 0.5 A g^-1, an excellent rate capability (398 F g^-1 at 20 A g^-1 with capacitance retention of 76%), and an outstanding cycling stability. After 1000 cycles, the specific capacitance retention of Fe₂N@OMC-2 remained 85%, which was much higher than 28% of the bare Fe₂N.

Full text of DOI:10.1039/c7ra08704g

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PAPER
Nanocasting synthesis of an iron nitride-ordered
mesopore carbon composite as a novel electrode
material for supercapacitors†
Cite this: RSC Adv., 2017, 7, 44619
*
Li Sun, Jian Feng, Linlin Qi, Ishaq Muhammad, Jabeen Maher,
Liyang Xu,
*
Xiaoyu Cheng and Weiming Song
Herein, a series of novel iron nitride (Fe2N)/cubic-ordered mesoporous carbon (OMC) composites
(Fe2N@OMC) were synthesized via a facile nanocasting route and ammonia calcination. Physical
characterization results indicate that small non-aggregated Fe2N nanoparticles were evenly distributed
on the external surface and in the pores of the carbon material with a linear array of mesopores in
a regular interval arrangement. An electrochemical test was carried out using Fe2N@OMC as a negative
electrode of a supercapacitor in 6 mol Lꢀ1 KOH aqueous electrolytes. The results indicated that the
obtained Fe2N@OMC-2 exhibited a superior specific capacitance of 547 F gꢀ1 at 1 mV sꢀ1 and 520 F gꢀ1
at 0.5 A gꢀ1, an excellent rate capability (398 F gꢀ1 at 20 A gꢀ1 with capacitance retention of 76%), and
an outstanding cycling stability. After 1000 cycles, the specific capacitance retention of Fe2N@OMC-2
remained 85%, which was much higher than 28% of the bare Fe2N.
Received 7th August 2017
Accepted 1st September 2017
DOI: 10.1039/c7ra08704g
rsc.li/rsc-advances
under operation at high-intensity current, which is not expected
for electrode materials used in energy utilization and storage
systems.11–14 Previous reports have proved that transition metal
1. Introduction
In recent years, as there is an increasing requirement for the
storage and utilization of a mass of energy, electrochemical
devices have been developing rapidly. Among them, super-
capacitors have attracted signicant attention due to their high
energy density, great rate capacity, and cycling stability.1–3 Thus,
many researchers are engaged in the investigation and research
of electrode materials for supercapacitors. Transition metal
oxides (Fe2O3, Co3O4, and MnO2), bimetallic oxides, and
hydroxides are always used as electrode materials because of
high capacitance of transition metal compounds.4–8 For
instance, Yu et al. presented the design and development of
novel Ag/PEDOT:PSS-nanopillar/MnO2 layers by a layer struc-
ture for high-performance exible supercapacitors. This elec-
trode provided excellent specic capacitances of 862 F gꢀ1
(based on MnO2) at a current density of 2.5 A gꢀ1.9 Thomas et al.
have reported an all-solid-state, energy harvesting and storing
ribbon that integrates a solar cell and a supercapacitor. In this
device, MnO2 deposited on Cu(OH)2 nanotube arrays was used
nitrides, such as chromium nitride, cobalt nitride, vanadium
nitride, and iron nitride, used as electrode materials for
supercapacitors have more advantages than transition metal
oxides due to their superior electrical conductivity (4000–
55 500 S cmꢀ1) and outstanding pseudocapacitive behav-
iour.15–18 For example, Lu et al. demonstrated a novel core–shell
structure in which TiN nanowires were covered with ultrathin
amorphous carbon protective layers. This TiN solid-state
supercapacitor demonstrated a remarkable 91.3% retention in
aqueous electrolytes aer 15 000 cycles without any oxidation.19
Yu et al. synthesized tungsten oxynitride (WON) nanowires on
carbon cloths, and the WON asymmetric supercapacitor
anode showed a high volumetric capacitance of 4.95 F cmꢀ3 at
12.5 mA cmꢀ3
.
Metal nitrides are considered to be the most
20
promising representatives of new generation of electrode
materials for electrochemical storage device such as lithium-ion
batteries (LIBs), fuel cells, sodium-ion batteries, and super-
capacitors.21–24 The unique nature of M (metal)–N (nitride)
bonding can provide simultaneous multiple model contributions
from metallic, covalent, and ionic bonding.25 Moreover, transi-
tion metal nitrides may be able to provide similar properties to
the noble metals due to their similar electronic structures.26
Among many kinds of metal nitrides, Fe2N is recognized as
the best choice as a pseudocapacitive electrode material due to
its outstanding electrical conductivity, low cost, low resistance,
and environmental friendliness.27,28 However, in the synthesis
of transition metal nitrides, products always form a large
as an electrode, and the capacitance of the nal electrodes was
increased to 145.15 mF cmꢀ2
.
However, the high electrical
10
resistivity of transition metal compounds affects the power
density; this results in a signicant reduction in capacitance
College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar,
Heilongjiang 161006, P. R. China. E-mail: qdsongweiming@163.com;
sunli04070215@163.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra08704g
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number of bulks of transition metal nitrides; this leads to low
utilization rate of their high capacitances in a strong current
system. To overcome this problem and enhance the electro-
chemical performance, carbon material and transition metal
nitride composites have been synthesized by researchers. The
carbon materials such as ordered mesoporous carbons (OMC),
activated carbons, carbon nanobers, and carbon nano-
tubes29–32 with a high specic surface area can increase the
electric conductivity of composite materials and provide
a carbon skeleton to support the uniform deposition of metal
nitride particles, thus increasing the utilization efficiency of
transition metal nitrides.33–36
There are only a few investigations and research on the use
of iron nitride as an electrode material for supercapacitors.
Zhu et al. reported that Fe2N/graphene was synthesized via
deposition of ZnO on a graphene nanosheet surface. Then,
ZnO and Fe3+ were subjected to ion-exchange and nally
annealed under an atmosphere of ammonia. The synthetic
composite tested as an electrode in an electrolyte of 1 M LiCl
showed a specic capacitance of 170 F gꢀ1 at a scan rate of
10 mV sꢀ1 and 120 F gꢀ1 at 100 mV sꢀ1 in a three-electrode
system.37 A Fe2N/activated carbon composite was fabricated
Fig. 1 Schematic for the nanocasting synthesis of OMC and Fe2-
N@OMC composites.
under anꢀ1inert atmosphere at a temperature ramp rate of
2
ꢁC min and kept for 3 h. The calcined product was then
removed out of the silica template using 10 wt% HF solution,
and pure OMC was obtained.
2.3 Synthesis of Fe2N, Fe2N@OMC-1, Fe2N@OMC-2, and
Fe2N@OMC-3
Fe2N was prepared by a straightforward route. First, 1.00 g
FeCl3$6H2O was dried at 80 ꢁC in a vacuum oven, and then
anhydrous FeCl3 was ground to form a ne powder. Subse-
quently, the powder sample was tiled into a thin layer at the
bottom of a quartꢀz1boat and calcined at 800 ꢁC for 2 h at a heating
rate of 5ꢁC min under the ow of ammonia at a rate of
100 mL minꢀ1. The calcined product was Fe2N. Fe2N@OMC-1,
Fe2N@OMC-2, and Fe2N@OMC-3 were prepared using
a vacuum nanocasting method, as shown in Fig. 1. OMC
weighing 0.10 g was rst dispersed in 20 mL of DI water under
sonication for 2 h, and different amounts (0.25 g, 0.50 g, and
1.00 g) of FeCl3$6H2O were added to 20 mL DI water under
stirring for 8 h to obtain a homogeneous mixture. The subse-
quent steps (drying, grinding, and calcination) are the same
as those in the procedure for preparing Fe2N. The obtained
products were Fe2N@OMC-1, Fe2N@OMC-2, and Fe2N@OMC-3.
´
via hydrothermal and ammonia treatments by Agata Sliwaka
et al. The specic capacitance of this composite used as an
electrode active material for supercapacitors operating in 6 M
KOH is 498 and 371 F gꢀ1 at the scan rates of 10 and 100 mV sꢀ1
,
respectively.38 In this study, we rst report the synthesis of
a series of novel Fe2N@ordered cubic mesoporous carbon
composites (Fe2N@OMC), and the resultant best composite as
an active material of the electrode for supercapacitors operating
in 6 M KOH shows a superior capacitance values of 506 F gꢀ1 at
the scan rates of 10 mV sꢀ1 and 435 F gꢀ1 at the scan rates of
100 mV sꢀ1
.
2. Experimental
2.4 Material characterization
2.1 Materials
The sample materials were characterized by X-ray diffraction
˚
(XRD, D8 FOCUS, Bruker, Germany) with Cu Ka 1.5406 A radi-
Triblock copolymer Pluronic P123 (MW ¼ 5800, PEG–PPG–PEG)
was purchased from Macklin Biochemical Co. Ltd (Shanghai,
China). Iron(III) chloride hexahydrate (FeCl3$6H2O), HCl water
solution (AR, 36.0–38.0%), and other chemicals were supplied
by Kemiou Chemical Reagent Co. Ltd (Tianjin, China).
ation. The microstructure of the sample materials was observed
using a transmission electron microscope (TEM, H-7650, Hita-
chi, Japan) at a voltage of 100 kV, a high-resolution trans-
mission electron microscope (HRTEM, Tecnai G2 F20, FEI, USA)
at a voltage of 200 kV, and a scanning electron microscope
(SEM, S-3400, Hitachi, Japan). Thermogravimetric analysis
(TGA) was performed using a Pyris1TGA (PerkinElmer Instru-
2.2 Synthesis of the KIT-6 template and OMC
The cubic three-dimensional (3D) ordered mesoporous silica ments) under an air atmosphere at a heating rate of 5 ꢁC minꢀ1
.
template KIT-6 was fabricated using the synthesis procedures Nitrogen sorption analysis was conducted at 77 K using
reported in the literature,29 and the 3D-ordered mesoporous a physical adsorption instrument (AUTOSORB-1, Quantach-
carbon (OMC) materials were prepared using sucrose as rome, USA). The specic surface area was determined by the
a carbon source and KIT-6-ordered mesoporous silica as a hard Brunauer–Emmett–Teller (BET) method, and the pore size
template. As shown in Fig. 1, 1.00 g of sucrose precursor and distribution calculation was derived from the adsorption
1.00 g of KIT-6 were added to 20 mL DI water and stirred to branches of the isotherms by the Barrett–Joyner–Halenda (BJH)
obtain a homogeneous mixture. The sucrose precursor was theory. X-ray photoelectron spectroscopy (XPS) was performed
made to continuously ow into a mesoporous silica template by using a spectrometer (ESCALAB 250Xi, Thermo Fisher, USA)
vacuum impregnation at room temperature. The obtained with Al Ka radiation (1486.6 eV). The shis in energy (charging)
mixture was dried at 80 ꢁC, and then, the dried solid was ground of the XPS spectra were calibrated using the C 1s peak at
to form a ne powder. This powder was then calcined at 800 ꢁC 284.6 eV as a reference peak.
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results support the complete conversion of FeCl3 into Fe2N by
NH3 annealing, as shown in eqn (3). The two broad peaks at
approximately 23.8 and 43.6ꢁ in the XRD pattern of OMC arise
from the (002) and (100) planes of graphite-type carbon,
respectively.
2.5 Electrochemical characterization
The electrochemical measurements were carried out using
a standard three-electrode cell, where Pt foil and Hg/HgO elec-
trode were used as the counter and reference electrodes,
respectively, in a 6 M KOH aqueous solution. The working
electrodes were prepared as follows:
heat
FeCl3 þ NH3
Fe N þ HCl
(3)
ꢀꢀꢀꢀ!
2
The clean nickel foams (1 ꢂ 10 cm2) were prepared in
advance. Then, 1 ꢂ 1 cm2 nickel foams were coated by mixing
80 wt% active material (OMC, Fe2N, Fe2N@OMC-1, Fe2-
N@OMC-2, or Fe2N@OMC-3) with 10 wt% polytetrauoro-
ethylene (PTFE) as a binding agent and 10 wt% acetylene black
conductor. The mass of the active materials on each electrode
was approximately 4.00 mg. Cyclic voltammetry (CV) and gal-
vanostatic charge/discharge measurements were carried out by
an electrochemical work station (CHI660E). The electro-
chemical impedance spectroscopy (EIS) measurement was per-
formed using an electrochemical work station (PAR VersaSTAT
4) at an open circuit potential from 100 kHz to 0.01 Hz with an
alternating current amplitude of 5 mV. The specic capacitance
(C, F gꢀ1) of the electrode materials can be calculated from the
galvanostatic discharged curves and the cyclic voltammograms
according to the following equations:
Transmission electron microscopy (TEM) was performed to
further observe the microstructure and the distribution of Fe2N
particles for the synthesized products. The TEM image of OMC
shows a highly ordered mesoporous structure with a linear array
of mesopores in a regular interval arrangement (Fig. 3a).39 The
TEM image of Fe2N@-OMC-2 indicates a homogeneous distri-
bution of small and non-agglomerative Fe2N nanoparticles
(with a diameter of 10–30 nm) on the surface of OMC, as shown
in Fig. 3c. By contrast, the TEM image of Fe2N@OMC-1 shows
that the well-ordered structure is still retained and the presence
of Fe2N particles almost cannot be observed in Fig. 3b. On the
other hand, the image of Fe2N@OMC-3 revealed an uneven
distribution of big Fe2N particles on the surface of OMC, and
the ordered structure was severely damaged, as shown in
Fig. 3d. The high-resolution TEM (HRTEM) image (Fig. 3e)
exhibits well-resolved lattice fringes, again conrming the high-
crystalline nature of the Fe2N nanoparticles for Fe2N@OMC-2.
The lattice fringe spacings are measured to be 0.21 and
IDt
C ¼
(1)
Um
ð
Idt
nm
C ¼
(2)
where I is the current (A), U is the operating cell voltage (V), t is
the time (s), n is the scan rate (V sꢀ1), and m is the mass of the
active material in the electrode (g).
3. Results and discussion
X-ray diffraction (XRD) analysis conrmed the presence of Fe2N
in all the samples except OMC, as shown in Fig. 2. The peaks at
2q ¼ 37.4 (021), 40.9 (200), 43.0 (121), 56.9 (221), 68.0 (023), and
76.0ꢁ (321) are consistent with the orthorhombic Fe2N phase
(JC-PDS #50-0958). No iron oxide phase was observed. These
Fig. 2 The XRD patterns of OMC, Fe2N, Fe2N@OMC-1, Fe2N@OMC-2, Fig. 3 TEM images of OMC (a), Fe2N@OMC-1 (b), Fe2N@OMC-2 (c),
and Fe2N@OMC-3.
and Fe2N@OMC-3 (d). HRTEM image of Fe2N@OMC-2 (e).
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Table 1 Porous property of composite materials identified by BET and
BJH
0.44 nm, which are assigned to the d-spacing of (102) and (200)
planes of the orthorhombic Fe2N phase (JC-PDS #50-0958),
respectively, and the result is consistent with that of the analysis
of XRD.40 X-ray energy-dispersive spectrometer (EDS) elemental
mapping was performed to further conrm the element distri-
bution. The SEM image (Fig. S1a ESI†) of Fe2N@OMC-2 and its
EDS elemental mapping data (Fig. S1c and d ESI†) show that the
particles are mainly C (red), Fe (purple), and N (green), which
are uniformly distributed in Fe2N@OMC-2.
Sample name
SBET (m2 gꢀ1
)
VT (cm3 gꢀ1
)
DBJH (nm)
OMC
Fe2N
Fe2N@OMC-1
Fe2N@OMC-2
Fe2N@OMC-3
1609
62
1232
919
583
2.13
0.02
1.55
0.99
0.44
5.29
14.2
5.03
4.88
2.62
The content of Fe2N calculated from thermogravimetric
analysis (Fig. S2 and Table S1, ESI†) was 28.1, 40.3, and
57.1 wt% in Fe2N@OMC-1, Fe2N@OMC-2, and Fe2N@OMC-3,
respectively. For further characterization of the porous struc-
ture of the prepared composites, the N2 adsorption–desorption
isotherm (Fig. 4a) and pore size distribution curves (Fig. 4b)
were obtained by nitrogen adsorption–desorption analysis. The
samples OMC, Fe2N@OMC-1, and Fe2N@OMC-2 exhibit type IV
curves with an H1 hysteresis loop, which indicates the meso-
porous nature of the synthesized composite materials due to the
presence of OMC. In comparison, the H1 hysteresis loop cannot
be observed in the N2 adsorption–desorption isotherm of Fe2N
and Fe2N@OMC-3 (Fig. 4a). The textural properties and
parameters, such as the BET surface area (SBET), the total pore
volume (VT), and the average pore diameter (DBJH), are esti-
mated, and the data value are summarized in Table 1. The BET
surface area, total pore volume, and average pore diameter of
initial OMC are 1609 m2 gꢀ1, 2.13 cm3 gꢀ1, and 5.29 nm,
respectively. The porosity development in the composite mate-
rials is gradually reduced with the increasing deposition of Fe2N
on the carbon surface. Particularly, when the content of Fe2N
increases to 57 wt% of Fe2N@OMC-3, the BET surface area
signicantly decreases from 1609 to 583 m2 gꢀ1 from OMC to
Fe2N@OMC-3 due to the blockage of the pore system by the big
Fe2N particles deposited on the external carbon surface, which
can be detected in the TEM image in Fig. 3d. This nding is also
supported by a signicant decrease in the total pore volume
from 2.13 cm3 gꢀ1 for OMC to 0.44 cm3 gꢀ1 for Fe2N@OMC-3.
For Fe2N@OMC-1, the porosity of OMC is preserved to
a greater extent (SBET: 1232 m2 gꢀ1, VT: 1.55 cm3 gꢀ1), but the
content of Fe2N is only 28 wt% of Fe2N@OMC-1. The BET
surface area of Fe2N@OMC-2 is much higher than that of
Fe2N@OMC-3 (919 vs. 583 m2 gꢀ1). The total pore volume of
Fe2N@OMC-2 is 0.99 cm3 gꢀ1, which is only two-fold lower than
that of OMC, 2.13 cm3 gꢀ1. Moreover, the content of Fe2N can
reach 40.3 wt% of Fe2N@OMC-2. The pore size distribution
calculated from DBJH reveals that the widths of pores are 5.29,
14.2, 5.03, 4.88, and 2.62 nm in OMC, Fe2N, Fe2N@OMC-1,
Fe2N@OMC-2, and Fe2N@OMC-3, respectively.
X-ray photoelectron spectroscopy (XPS) measurements were
performed to analyze the surface information of the synthesized
products. The surface iron concentration of Fe2N@OMC-1,
Fe2N@OMC-2, and Fe2N@OMC-3 is 1.7, 6.2, and 14.6 at%,
respectively (Table S1, ESI†). The N 1s XPS spectra of Fe2-
N@OMC-2 (Fig. 5c) and Fe2N@OMC-3 (Fig. 5e) were divided
into ve component peaks, which corresponded to nitrogen
bonded with metal nitride (ꢃ397.6 eV), pyridinic-N (ꢃ398.4 eV),
pyrrolic-N (ꢃ399.8 eV), graphitic-N (ꢃ400.8 eV), and nitrogen
oxide (ꢃ401.9 eV) functionalities.29,38 The N 1s XPS spectrum of
Fig. 4 The nitrogen adsorption–desorption isotherms (a) and pore
diameter distribution (b) of OMC, Fe2N, Fe2N@OMC-1, Fe2N@OMC-2,
and Fe2N@OMC-3.
Fig. 5 High-resolution XPS spectra of N 1s of Fe2N@OMC-1 (a), Fe2-
N@OMC-2 (c), and Fe2N@OMC-3 (e). Fe 2p spectra of Fe2N@OMC-1
(b), Fe2N@OMC-2 (d), and Fe2N@OMC-3 (f).
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Fe2N@OMC-1 indicates no metal nitride functionality (Fig. 5a) distributed on the surface of OMC. The higher specic surface
as compared to that of Fe2N@OMC-2 and Fe2N@OMC-3, area of Fe2N@OMC-2, which implies a better distribution of
whereas Fe2N really exists in the Fe2N@OMC-1, as demon- Fe2N particles on the surface of OMC as compared to that of
strated by XRD analysis. This nding indicated that iron nitride Fe2N@OMC-3, is a reason for the stronger pseudocapacitive
also exists in the pores of OMC and this part of iron nitride behavior reected by the voltammetry curve. However, excessive
cannot be detected via XPS, which also applies to Fe2N@OMC-2 specic surface area of Fe2N@OMC-1 will sacrice the content
and Fe2N@OMC-3. The high-resolution XPS spectrum of the as- of Fe2N, thereby decreasing the electrochemical pseudoca-
made samples for the Fe 2p level exhibits main peaks at around pacity, which is not expected. For Fe2N@OMC-1, Fe2N@OMC-2,
707.5 and 721.4 eV that correspond to the Fe 2p3/2 and Fe 2p1/2 and Fe2N@OMC-3 composites, both the charge storage mech-
levels, respectively. They are assigned to Fe(II) in iron nitride. anism of the electric double-layer behavior and faradaic redox
Moreover, the peaks appear at 710.0, 715.6, 723.8, and 729.1 eV, reactions derived from OMC and Fe2N, respectively, must be
which are attributed to the presence of Fe–O bonds.41,42 The investigated and studied. In Fig. 6b, a symmetrical triangular
electrochemical performance of the obtained samples was can be observed remarkably in the galvanostatic charge–
analyzed in a standard three electrode system. The cyclic vol- discharge curves of OMC, which demonstrates that OMC has an
tammogram of OMC shows a rectangular and symmetric shape, outstanding double-layer capacitance and the platforms of the
indicating a typical electric double-layer (EDL) behaviour, and triangular shape shown by the galvanostatic charge–discharge
all cyclic voltammograms of Fe2N-containing samples exhibit curves of Fe2N, Fe2N@OMC-1, Fe2N@OMC-2, and Fe2N@OMC-3
undoubtedly the reversible faradaic redox reactions attributing correspond to the redox peaks of cyclic voltammogram at
to Fe2N, as shown in Fig. 6a. All the composites of Fe2N@OMC-1, ꢀ0.75 O ꢀ0.62 V and ꢀ1.05 O ꢀ0.94 V. Moreover, it was
Fe2N@OMC-2, and Fe2N@OMC-3 have sharper and higher obvious that the discharge time curves (Fig. S3 ESI†) of synthetic
redox peaks as compared to bare Fe2N; this indicates that the products were in the order of OMC < Fe2N < Fe2N@OMC-3 <
presence of OMC can effectively enhance the electrochemical Fe2N@OMC-1 < Fe2N@OMC-2. For Fe2N@OMC-2, excellent
performance of these composites due to better utilization of capacitance values of 547 F gꢀ1 at 1 mV sꢀ1 and 520 F gꢀ1 at
Fe2N during charging and discharging when Fe2N is evenly 0.5 A gꢀ1 can be calculated by voltammetry and galvanostatic
charge–discharge. Fe2N@OMC-2 has better specic capacitance
as compared to those reported in other studies,37,38 as shown in
Table S2.† In the application of supercapacitors, the material of
the OMC-based Fe2N composite with very high specic capaci-
tance has never been synthesized before. The capacitance of
Fe2N@OMC-2 is more than two times that of bare Fe2N at
1 mV sꢀ1 and 0.5 A gꢀ1 and dramatically decreases in the strong
current and high scan rate regions (Fig. 6c and d). The specic
capacitances of Fe2N@OMC-1, Fe2N@OMC-2, and Fe2N@OMC-3
were 310, 398, and 245 F gꢀ1 at a high current density of
20 A gꢀ1, maintaining 75, 76, and 61% of the initial capacitance
at 0.5 A gꢀ1. For comparison, bare Fe2N exhibits a specic
capacitance of only 50 F gꢀ1 at a current density of 20 A gꢀ1; this
fully illustrates the signicance of OMC of carbon-based Fe2N
composite in improving the energy density. In addition, for
supercapacitors, the cyclic stability is another indispensable
performance indicator. All the prepared products were run for
1000 charge–discharge cycles at a current density of 1 A gꢀ1, as
shown in Fig. 6e. It can be seen that all carbon-based Fe2N
composites show excellent cyclic stability aer 1000 cycles due
to a high specic surface area and the appropriate porous size.
Particularly, Fe2N@OMC-2 shows a much higher retention of
85% of the initial capacitance as compared to bare Fe2N, which
exhibits only 28% retention. Moreover, it was clearly observed
that only a very small amount of Fe2N nanoparticles of
Fe2N@OMC-2 accumulated aer 1000 cycles (Fig. S4b ESI†)
compared to those before 1000 cycles (Fig. S4a ESI†). The stable
nanostructure made the Fe2N@OMC-2 electrode have an
outstanding cyclic stability. The nanocasting route can achieve
a homogeneous dispersion of small non-aggregated Fe2N
nanoparticles on the external surface and in the pores of OMC;
this has been demonstrated by nitrogen sorption analysis, SEM,
TEM, HRTEM, and XPS results. This feature can improve the
Fig. 6 Cyclic voltammograms of the synthetic products at 1 mV sꢀ1 (a),
galvanostatic charge/discharge curves performed at 0.5 A gꢀ1 (b),
specific capacitance as a function of the scan rate (c) and current
density (d), cycle stability at a current density of 1 A gꢀ1 (e), and Nyquist
plots with an inset representing the high-frequency region (f).
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transport of ions from the bulk electrolyte to the composite densities increased from 0.5 to 20 A gꢀ1, and a superior cyclic
material. The collective effect of the two factors, OMC providing stability of 85% was retained aer 1000 cycles. This study
more active sites due to the high specic surface area and the constitutes the rst demonstration of employing the composite
high pseudocapacitance of Fe2N, accounts for the outstanding of Fe2N and OMC as an electrode material for supercapacitors,
electrochemical performance of the Fe2N@OMC-2 composite as which can effectively enhance the performance of energy
an electrode material for supercapacitors.38
storage devices.
The resistive characteristics of electrode materials were
analyzed by EIS (Fig. 6f). In the Nyquist plots of OMC, Fe2-
N@OMC-1, and Fe2N@OMC-2, a small semicircular line corre-
sponding to the interfacial charge-transfer resistance is noticed
in the high-frequency region, which indicates that all the
samples OMC, Fe2N@OMC-1, and Fe2N@OMC-2 have a small
Conflicts of interest
There are no conicts to declare.
charge-transfer resistance. The slope lines at an angle of 80–90ꢁ Acknowledgements
are observed markedly in the lower frequency region of OMC,
We gratefully acknowledge the support for this study provided
Fe2N@OMC-1, and Fe2N@OMC-2, which show an excellent
charge–discharge capacitive behavior because of the fast
charge-transfer rate from the active electrode material to the
current collector.40,43 By contrast, no obvious semicircle is
detected for bare Fe2N and Fe2N@OMC-3; this demonstrates
the presence of charge-transfer resistance, resulting in a poor
rate capability.44 In addition, the slope lines at an angle near 45ꢁ
for Fe2N and Fe2N@OMC-3 reveal the occurrence of the War-
burg impedance, standing for ion-diffusion resistance.40,45 The
ion-diffusion resistance of Fe2N and Fe2N@OMC-3 is higher
than that of OMC, Fe2N@OMC-1, and Fe2N@OMC-2; this may
be caused by the aggregated and bulky particles of Fe2N,
making ion diffusion in the electrolyte become more difficult.46
The EIS measurements can further prove that the composite of
OMC and small non-aggregated Fe2N particles is advantageous
in ion-transport, which plays a signicant role in rate perfor-
mance and specic capacitance.
Different electrolytes can better test the performance of the
electrodes.47 The Fe2N@OMC-2 electrode was tested in 1 M
LiOH, NaOH, and KOH electrolytes, as shown in Fig. S5 ESI.† It
is clear that the redox peaks of CV curves increase in turn as the
size of the cations in the electrolytes decreases (K+ > Na+ > Li+).
This observation indicates enhanced pseudocapacitive reac-
tions with smaller cations as they are easier to get intercalated
by the National Natural Science Foundation of China (No.
21501104), the Natural Science Foundation of Heilongjiang
Province (B2015014), and the University Nursing Program for
Young Scholars with Creative Talents in Heilongjiang Province
(UNPYSCT-2016088).
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44624 | RSC Adv., 2017, 7, 44619–44625
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