Hydrothermal growth of ferrous hydroxide terephthalate as a new positive electrode material for supercapacitors

Article abstract of DOI:10.1039/C8DT02377H

Ferrous hydroxide terephthalate, Fe(OH)(C₈H₅O₄), is first prepared by an in situ hydrothermal growth method on nickel foam, which can be directly used as the positive electrodes for supercapacitors with superior pseudocapacitive behaviors. It displays high specific capacitance of 5.25 F cm^?2 (656.3 F g^?1) at 10 mA cm^?2 (1.25 A g^?1).

Full text of DOI:10.1039/C8DT02377H

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Hydrothermal growth of ferrous hydroxide terephthalate as a
new positive electrode material for supercapacitors
Received 00th January 20xx,
Accepted 00th January 20xx
Xiuling Ou, Shuijin Lei,* Xinlai Zhang, Kun Wan, Yifan Wang, Wei Zhou, Yanhe Xiao and Baochang
Cheng
DOI: 10.1039/x0xx00000x
www.rsc.org/
Ferrous hydroxide terephthalate, Fe(OH)(C8H5O4), is first prepared friendliness, earth abundance and low cost for commercial
by an in-situ hydrothermal growth method on nickel foam, which applications.13–15 However, these familiar Fe-based materials
can be directly used as the positive electrodes for supercapacitors mostly exhibit capacitive behavior in negative potential
with superior pseudocapacitive behaviors. It displays high specific window. The reports on Fe-based positive electrode materials
capacitance of 5.25 F cm−2 (656.3 F g−1) at 10 mA cm−2 (1.25 A g−1).
are relatively limited. According to the standard reduction
potentials of iron element, Fe3+ can be reduced into Fe2+ in a
potential range of −0.55~0.77 V. Furthermore, its standard
reduction potential is changeable with different ligands.16
Actually, thanks to the broad potential range, the prominent
pseudocapacitive behavior also can been observed in the
positive potential region for Fe-based electrode materials.17–22
In this work, Ni-foam supported ferrous hydroxide
terephthalate (Fe(OH)(Tp)) has been successfully prepared for
the first time, which can be directly employed as the positive
electrode for pseudocapacitors with favorable electrochemical
performances. The fabricated Fe(OH)(Tp) electrode can exhibit
high specific capacitance, good rate capability, and superior
cycling stability.
Introduction
The increasing concerns about energy and environmental
issues have triggered extensive research on clean and
renewable energy as well as sustainable energy storage
systems.1–2 Supercapacitors, as a competitive electrochemical
energy storage device, can bridge the gap between the high
power density of traditional capacitors and high energy density
of batteries/fuel cells.3–4 Generally, supercapacitors can be
classified into two types: electrical double layer capacitors
(EDLCs) dependent on charge accumulation at the
electrode/electrolyte interfaces, and pseudocapacitors based
on the fast and reversible faradaic redox reactions at or near
the electrode surface.5 Accordingly, the frequently used
electrode materials of supercapacitors can be grouped into
Experimental
three main categories: carbon-based materials for EDLCs, Preparation of Fe(OH)(Tp)
transition metal compounds and conducting polymers for
Typically, a piece of Ni foam (1 cm
× 3 cm) was etched with 3
pseudocapacitors.6–7 In supercapacitors, electrode materials
show significant influences on electrochemical performances.
Therefore, seeking new electrode materials is of great
importance and still remains challenging.8
M HCl for 10 min to remove the oxide layer, and sequentially
washed with deionized water and absolute ethanol by
ultrasonication for 5 min, and then dried in vacuum. To
prepare Ni foam supported Fe(OH)(Tp), 0.199 g (1 mmol) of
FeCl2·4H2O and 0.166 g (1 mmol) of terephthalic acid were
respectively dissolved in 20 mL of deionized water and mixed
together under magnetic stirring. The resulting solution was
loaded into a 50 mL Teflon-lined autoclave, and the cleaned Ni
foam was completely immersed into the solution. After
hydrothermal reaction at 150 °C for 12 h, the reacted Ni foam
was picked out, rinsed with deionized water and absolute
ethanol, and finally dried in vacuum at 50 °C for 12 h. The mass
loading of the sample was measured by weighting the
increased mass of the Ni foam after coating using an AB265-
S/FACT microbalance with a sensitivity of 0.01 mg (Mettler
Toledo, Switzerland).
Transition metal compounds are the most notable electrode
materials for pseudocapacitors because of their multiple
oxidation states for efficient redox charge transfer.9–12 Among
them, Fe-based compounds such as iron oxides, iron
oxyhydroxides and ferrites have been recognized as the
desired electrode materials for supercapacitors due to their
significant merits including rich redox processes for high
specific capacitance, non-toxicity for environmental
School of Materials Science and Engineering, Nanchang University, Nanchang,
Jiangxi 330031, China.
* Corresponding author. E-mail: shjlei@ncu.edu.cn
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Sample characterizations
1379 cm−1 are derived from asymmetric and symmetric stretch
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DOI: 10.1039/C8DT02377H
−
−
of the carboxylate group [i.e. νas(–COO ) and νs(–COO )] of
X-ray diffraction (XRD) patterns were recorded on
a
1
terephthalate moiety. The peaks at 3055 and 1500 cm− are
PANalytical Empyrean diffractometer with Cu Kα1 radiation (λ
= 1.5406 Å, operating at 40 kV and 40 mA). Scanning electron
microscopy (SEM) images were taken from a JSM-6701F (JEOL,
Japan) field emission scanning electron microscope and a
Quanta 200F environmental scanning electron microscope (FEI,
Netherlands). The Fourier transform infrared (FTIR) spectrum
due to the aromatic C–H and C=C stretching, respectively.
Additionally, the bands at 818 and 1015 cm−1 corresponding to
para out-of-plane aromatic C–H wag and para in-plane
aromatic C–H bend also can be clearly observed. Three bands
1
at 3588, 3425 and 3345 cm− represent the OH stretching
vibrations from OH− and water. The band at 605 cm−1 then can
be assigned to Fe–O vibration. Therefore, the FTIR results
further confirm the formation of iron terephthalate.
was obtained on
a Tensor II FTIR spectrometer (Bruker,
Germany) in KBr medium at room temperature.
The morphology of the as-grown Fe(OH)(Tp) sample on Ni
foam was investigated by SEM observations. As shown in Fig.
1c, the low-magnification SEM image reveals that the Ni foam
skeleton is fully covered by a mass of Fe(OH)(Tp) sample
through the in-situ growth. The dense coating of the sample
on such a three-dimensional framework of Ni foam would
necessarily make for a high mass loading of the active
materials. It is worthwhile to point out that the high mass
loading per area is generally a critical requirement for the
practical applications of supercapacitor devices. The
corresponding high-magnification SEM image (Fig. 1d) shows
that the product consists of micro-sized polyhedral crystals
with a relatively broad size distribution. These Fe(OH)(Tp)
polyhedra are tightly anchored on the Ni foam substrate,
guaranteeing a good contact between the active materials and
current collector for supercapacitor applications.
To evaluate the electrochemical performances of the as-
fabricated electrode, cyclic voltammetry (CV), galvanostatic
charge/discharge (GCD), and electrochemical impedance
spectroscopy (EIS) tests were conducted. Fig. 2a displays the
CV curves of the Fe(OH)(Tp) electrode within the potential
range of 0~0.5 V (vs. Hg/HgO) at different scan rates. The clear
redox peaks imply that the typical capacitive characteristic of
the Fe(OH)(Tp) electrode should be mainly dominated by
Faradaic redox reaction, which may be described as Fe(OH)(Tp)
+ OH− ↔ Fe(OH)2Tp + e−. It is noted that the oxidation peaks
Electrochemical measurements
The prepared Fe(OH)(Tp) on Ni foam was directly applied as
the positive electrode with an active area of 1
×
1 cm2. The
negative electrode was prepared by pasting the slurry
consisting of activated carbon, acetylene black and
polyvinylidene fluoride binder with a mass ratio of 8:1:1 on
nickel foam. The asymmetric supercapacitor (ASC) device was
assembled as a Teflon Swagelok type cell using nonwoven
fabric membrane as the separator and 3 M KOH aqueous
solution as the electrolyte. A typical three-electrode system
was used to test the electrochemical properties of the
electrode on a CHI 760E electrochemical workstation (Chenhua
Instruments, Shanghai, China) in 3 M KOH aqueous electrolyte
using platinum sheet and Hg/HgO electrode as the counter and
reference electrodes, respectively. The electrochemical
performances of the ASC device were then tested on the
electrochemical workstation using a two-electrode mode.
Results and discussion
The phase structure of the product grown on Ni foam was
identified by XRD, as shown in Fig. 1a. Evidently, in the high
angle region (2θ > 40°), three distinctly strong diffraction peaks
located at 44.3°, 51.7° and 76.2° can be undoubtedly indexed
to the face-centred cubic Ni phase (JCPDS Card 65−0380),
corresponding to the Ni foam substrate. As for the remaining
XRD peaks, it is noteworthy that no JCPDS cards with respect
to Fe-based terephthalates can be found to match with them.
Sherif has reported three forms of iron terephthalates (JCPDS
Cards 33−1722; 33−1723; 33−1724),23 but all the three card
files are unavailable for indexing the as-grown sample.
Fortunately, it is interesting that the XRD patterns of the
sample are actually similar with nickel hydroxide terephthalate
also reported by Sherif (35−1677).23 In this context, it should
be believable that the synthesized product on Ni foam
substrate can be indexed to iron (II) hydroxide terephthalate.
Accordingly, the calculated lattice constants of a = 9.7858 Å, b
= 7.4202 Å, c = 11.0682 Å and α = 101.230°, β = 83.904°, γ =
92.791° with a triclinic system (space group P−1) can be
achieved. It can be seen that the experimental XRD patterns
can be in good agreement with the corresponding simulated
ones. Therefore, the Ni foam supported sample can be
determined as ferrous hydroxide terephthalate. FTIR
spectroscopy (Fig. 1b) was further examined to verify the
structure of the product. Two dominant bands at 1575 and
Fig.
1 (a) XRD patterns, (b) FTIR spectrum, (c) low-magnification and (d) high-
magnification SEM images of the as-synthesized Ni foam supported Fe(OH)(Tp) sample.
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are hard to be distinguished, which may be attributed to the operating potential window (0.5 V). ThereforeV, ietwhAerticrleeOsunllintes
overlap of anodic peaks with the O2 evolution potential.19, 22 As demonstrate the outstanding supercapaDcOitIi:v1e0.p10e3r9f/oCr8mDaTn02c3e77oHf
usual, a steady increase in the CV current can be observed the prepared Fe(OH)(Tp) electrode at a high mass loading level.
with the increase of scan rate, because the small sweep rate The plot of the calculated areal and gravimetric specific
will enable an enough duration for ion access in the electrode capacitances as a function of the applied areal and gravimetric
and then the charge transport can be rapid enough. current densities is illustrated in Fig. 2c. The specific
Meanwhile, the increase of scan rate also gives rise to a capacitance always decreases with the increase of current
change in hysteresis of the oxidation (anodic) and reduction density due to the diffusion effect. It can be found that the
(cathodic) peaks, which gradually shift to more positive and prepared Fe(OH)(Tp) electrode shows 64.2% retention of the
more negative potentials, respectively. It is generally thought initial capacitance when the current density is increased from
to be resulted from the increase of the charge diffusion 10 to 50 mA cm−2, indicating a good rate capability. To explore
polarization in the pseudocapacitive electrode materials.
the dependence of specific capacitance on mass loading of
GCD curves were recorded at various current densities for Fe(OH)(Tp), the comparative Fe(OH)(Tp) samples with
further evaluating the capacitive performance of the different mass loading were prepared for different reaction
Fe(OH)(Tp) electrode as charted in Fig. 2b. The obvious voltage time. The SEM images (Fig. S1) reveal that more and denser
plateaus of the GCD curves further verify the pseudocapacitive crystals are formed on Ni foam with increase of reaction time.
behavior, and the well symmetric nature indicates its ideal Fig. S2a-b gives the CV curves at 5 mV s−1 and GCD curves at 10
electrochemical capacitive characteristic with good mA cm−2 of the Fe(OH)(Tp) electrodes prepared for 4~12 h. It
reversibility and high coulombic efficiency. The areal specific can be seen that the CV loop area and discharge time increase
capacitance (Ca, F cm−2) values of the Fe(OH)(Tp) electrode can with mass loading. Their specific capacitances are outlined in
be determined from the discharge curve for each current Fig. S2c, in which the areal capacitance monotonically
density according to the following equation: Ca = (IΔt)/(SΔV), increases with mass loading while the gravimetric capacitance
where I is the discharging current (A), Δt represents the shows a maximum at the mass loading of 5.4 mg cm−2.
discharging time (s), S is the area of the electrode (1 cm2), and
EIS measurement was also carried out to investigate the
ΔV designates the potential window (0.5 V), respectively. interfacial properties of the Fe(OH)(Tp) electrode. The Nyquist
Accordingly, a highest areal capacitance of 5.25 F cm−2 at 10 plot of EIS is presented in Fig. 2d, which includes a semicircle
mA cm−2 can be obtained. Encouragingly, based on the mass part in the high frequency region followed by a linear part in
loading (~8 mg cm−2) of the active materials on Ni foam, the the low frequency region. The diameter of the semicircle is
Fe(OH)(Tp) electrode can deliver a maximum gravimetric ascribed to the interfacial charge transfer resistance, while the
capacitance (Cg) of 656.3 F g−1 at 1.25 A g−1. The theoretical intercept at x-axis in the high frequency region is associated
gravimetric specific capacitance of Fe(OH)(Tp) can be with the equivalent series resistance (ESR) generally involving
calculated to be 810.9 F g−1 based on the equation Cm = (n × the intrinsic resistance of the electrode, the ionic resistance of
F)/(M ×ΔV), where n is the number of electrons transferred in the electrolyte, and the contact resistance. The straight line
the redox reaction (n
=
1
in this case), F is corresponding to the Warburg impedance is related to the
the Faraday constant (96485 C mol−1), M is the molecular diffusion of electrolyte ions in the host materials. In our
weight of the active material (237.98 g mol−1), and ΔV is the
research, the direct growth of Fe(OH)(Tp) sample on Ni foam
as a binder-free electrode ensures the intimate contact
between the active materials and current collector with small
contact resistance and Ohmic polarization. Hence, the
prepared Fe(OH)(Tp) electrode exhibits a very small ESR of
about 0.62 Ω. Compared with the contact resistance, the
electrode shows a relatively larger charge-transfer resistance
of 2.71 Ω because of the densely-packed crystals with limited
accessible surface area. Moreover, the sharp slope of the
Warburg line reveals the fast ion diffusion in the electrode
materials and the ideal capacitive behavior. It is believable that
Fig. 2 Electrochemical behaviors of the prepared Fe(OH)(Tp) electrode in 3 M KOH
aqueous electrolyte with a three-electrode model: (a) CV curves at different scan rates;
(b) GCD curves at different applied areal current densities; (c) the areal and gravimetric
specific capacitances of the Fe(OH)(Tp) electrode as a function of the current density;
(d) EIS Nyquist plots of the Fe(OH)(Tp) electrode.
Fig. 3 (a) Cycling performance and coulombic efficiency of the Fe(OH)(Tp) electrode
over 5000 cycles at a current density of 50 mA cm−2; (b) the GCD curves of the first and
the 5000th cycles for the Fe(OH)(Tp) electrode.
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the 3D framework of the Ni foam substrate has rich void space
to facilitate the diffusion of electrolyte within the electrode
bulk reducing the diffusion resistance. The favorable electrical
conductivity of the obtained Fe(OH)(Tp) electrode would be
advantageous in charge storage.
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DOI: 10.1039/C8DT02377H
The cycling stability of the Fe(OH)(Tp) electrode was
inspected by the consecutive GCD tests at a current density of
50 mA cm−2 for 5000 cycles. As plotted in Fig. 3a, the electrode
shows a relatively quick decrease of capacitance (~16.4%)
within initial 2500 cycles, and then gradually becomes more
stable. Finally, it shows a capacitance retention of 78.2% after
5000 cycles. For the sake of comparison, the first and 5000th
GCD curves are also provided in Fig. 3b. Their closely
conforming profile also reveals the stability of the prepared
Fe(OH)(Tp) electrode. To further demonstrate its cycling
stability, Fig. S3 presents the SEM images of the electrode
after cycling test. It can be found that the Fe(OH)(Tp) crystals
are still densely coated on Ni foam skeleton. Typically, these
crystals almost maintain their polyhedral morphology, just
except that the frizzy nanosheets are formed on their surface
resulting from electrochemical reactions. What’s more, the
prepared Fe(OH)(Tp) electrode also displays a steady and high
coulombic efficiency above 94%, as witnessed by the favorable
symmetry of the GCD curves. These results prove the excellent
electrochemical stability and reversibility of the electrode.
The electrochemical performances of the activated carbon
negative electrode are presented in Fig. S4. The results reveal
that it has good electrochemical properties and can be applied
in supercapacitor applications. Fig. S5 gives the contrastive CV
curves of the positive and negative electrodes at 50 mV s−1 for
charge balance. The CV curves of the assembled ASC device at
different scan rates within a voltage window of 1.5 V are
displayed in Fig. 4a. They all exhibit a constant shape, even at a
high scan rate of 100 mV s−1, suggesting the good capacitive
behavior and excellent rate performance. The GCD curves of
the ASC device (Fig. 4b) were also collected at various current
Fig. 5 (a) Cycling stability and coulombic efficiency of the ASC device over 5000 cycles at
a current density of 20 mA cm−2; (b) the GCD curves of the first and the 5000th cycles
for the ASC device.
densities, and the calculated specific capacitances are outlined
in Fig. 4c. A maximum specific capacitance of 53.4 F g−1 can be
achieved at 0.5 A g−1. Based on EIS in Fig. S6, the ESR and
charge-transfer resistance of the ASC device are about 2.05
and 4.8 Ω, respectively. It also has a low ion diffusion
resistance according to the high slope of Warburg line.
Energy density and power density are crucial for practical
application of supercapacitors. Generally, the energy density (E)
and power density (P) can be determined by the equations of E
= CV 2/(2
×3.6) and P = 3600E/t, respectively, where C is the
gravimetric capacitance (F g−1), V is the operating potential
window (V), and t is the discharge time (s). Fig. 4d presents the
Ragone plot of the fabricated ASC device. It can deliver a
highest energy density of 16.68 Wh kg−1 at a power density of
375 W kg−1 with a voltage window of 1.5 V, and the energy
density still can remain 10.26 Wh kg−1 as the power density is
increased to 1875 W kg−1.
The cycling stability of the ASC device was investigated by
the long-term GCD tests at 20 mA cm−2 for 5000 cycles. As
shown in Fig. 5a, the ASC device exhibits a capacitance
retention of 87.3% and the coulombic efficiency above 96%.
For detail, the corresponding first and 5000th GCD curves are
further supplied in Fig. 5b. Similarly, the consistent shape and
symmetric feature of the GCD curves further verify the good
stability and reversibility of the ASC device. The superior
electrochemical performances of the prepared Fe(OH)(Tp)
electrode as well as ASC device manifest its great potential
applications in energy storage.
Conclusions
In summary, the densely coated Fe(OH)(Tp) crystals have been
directly grown on nickel foam with a high mass loading
through a simple hydrothermal route, which can be profitably
used as the binder-free electrode for pseudocapacitors. The
fabricated electrode shows high areal and gravimetric specific
capacitances of 5.25 F cm−2 and 656.3 F g−1 at the current
densities of 10 mA cm−2 and 1.25 A g−1, respectively. More
significantly, the assembled ASC device can deliver a high
energy density of 16.68 Wh kg−1 at a power density of 375 W
kg−1 with a voltage window of 1.5 V. As a new Fe-based
electrode material, the prepared Fe(OH)(Tp) reveals great
potential for future applications in supercapacitors.
Fig. 4 Electrochemical behaviors of the assembled asymmetric supercapacitor device
with a two-electrode model: (a) CV curves at different scan rates; (b) GCD curves at
different applied areal current densities; (c) the areal and gravimetric specific
capacitances of the Fe(OH)(Tp) electrode as a function of the current density; (d) the
Ragone plot of the ASC device.
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Conflicts of interest
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DOI: 10.1039/C8DT02377H
There are no conflicts to declare.
Acknowledgements
Financial supports by National Natural Science Foundation of
China (21461014), the Project for Young Scientist Training of
Jiangxi Province (20153BCB23022), and the Natural Science
Foundation of Jiangxi Province (20151BAB206016) are
gratefully acknowledged.
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