Exposing {001} Crystal Plane on Hexagonal Ni-MOF with Surface-Grown Cross-Linked Mesh-Structures for Electrochemical Energy Storage

Article abstract of DOI:10.1002/smll.201902463

Hexagonal nickel-organic framework (Ni-MOF) [Ni(NO₃)₂·6H₂O, 1,3,5-benzenetricarboxylic acid, 4-4′-bipyridine] is fabricated through a one-step solvothermal method. The {001} crystal plane is exposed to the largest hexagonal surface, which is an ideal structure for electron transport and ion diffusion. Compared with the surrounding rectangular crystal surface, the ion diffusion length through the {001} crystal plane is the shortest. In addition, the cross-linked porous mesh structures growing on the {001} crystal plane strengthen the mixing with conductive carbon, inducing preferable conductivity, as well as increasing the area of ion contact and the number of active sites. These advantages enable the hexagonal Ni-MOF to exhibit excellent electrochemical performance as supercapacitor electrode materials. In a three-electrode cell, specific capacitance of hexagonal Ni-MOF in the 3.0 m KOH electrolyte is 977.04 F g^?1 and remains at the initial value of 92.34% after 5,000 cycles. When the hexagonal Ni-MOF and activated carbon are assembled into aqueous devices, the electrochemical performance remains effective.

Full text of DOI:10.1002/smll.201902463

FULL PAPER
Electrochemical Energy Storage
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Exposing {001} Crystal Plane on Hexagonal Ni-MOF
with Surface-Grown Cross-Linked Mesh-Structures
for Electrochemical Energy Storage
Yan Li, Yuxia Xu, Yong Liu, and Huan Pang*
it has been found that the organic tem-
Hexagonal nickel-organic framework (Ni-MOF) [Ni(NO3)2·6H2O,
plate can guide the nucleation and growth
of inorganic nanocrystals by artificial
modification or synthesis to obtain the
ideal shape, size, and final properties.[3,4]
Metal-organic framework (MOF), a new
type of porous crystalline material, is
composed of two main components:
inorganic vertices (metal ions or clus-
ters) and organic linkers.[5,6] By varying
metal ions and organic ligands, over
20 000 varied MOFs have been reported,
and their structures are not only abun-
dant but also designable.[7,8] In addition
to their various structures and composi-
tions, MOF has high surface areas, large
pore volumes, and uniform pore disper-
sions compared to conventional micro-
porous and mesoporous materials (zeolite,
mesoporous SiO2, and activated carbon
(AC)).[9–11] These inherent characteristics
make MOF very promising in a variety
1,3,5-benzenetricarboxylic acid, 4-4′-bipyridine] is fabricated through a one-
step solvothermal method. The {001} crystal plane is exposed to the largest
hexagonal surface, which is an ideal structure for electron transport and ion
diffusion. Compared with the surrounding rectangular crystal surface, the ion
diffusion length through the {001} crystal plane is the shortest. In addition,
the cross-linked porous mesh structures growing on the {001} crystal
plane strengthen the mixing with conductive carbon, inducing preferable
conductivity, as well as increasing the area of ion contact and the number
of active sites. These advantages enable the hexagonal Ni-MOF to exhibit
excellent electrochemical performance as supercapacitor electrode materials.
In a three-electrode cell, specific capacitance of hexagonal Ni-MOF in the
−1
3.0 m KOH electrolyte is 977.04 F g and remains at the initial value of 92.34%
after 5,000 cycles. When the hexagonal Ni-MOF and activated carbon are
assembled into aqueous devices, the electrochemical performance remains
effective.
of applications including gas adsorption and separation,[12,13]
catalysis[14] Y, drug delivery,[15] proton conductivity and
sensing.[16] Furthermore, several MOFs have been investigated
for various electrochemical applications, such as supercapaci-
tors, batteries, and fuel cells.[17–19]
1. Introduction
Nanocrystals possess multifarious size and shape-related char-
acteristics for a variety of fields including electronics, catalysis,
and optics.[1,2] Owing to the characteristics of nanocrystal
growth kinetics, yet synthesis process often needs rigorous
experimental conditions, and nanocrystals are difficult to con-
trol in size and shape. In addition, additives like ligands and
metal ions are required on many occasions. In recent years,
Supercapacitors (SCs), as a key enabling electrical energy
storage technology, have drawn continuing attention because
of their superior power densities, charge–discharge rates, and
longer cycling lives than secondary batteries.[20,21] It is well
known that the electrochemical properties of electrochemical
energy storage devices mainly depend on the electrode mate-
rials. According to the working mechanism of supercapaci-
tors, electrode materials are classified into carbon materials[22]
(electric-double-layer capacitors) and pseudocapacitor electrode
materials including metal sulfide,[23] metal oxides,[24] metal
selenide,[25] metal hydroxide,[26] conductive polymers,[27] and
so on, which mainly store energy by reversible redox reac-
tions. Recently, rapidly growing MOF materials have achieved
extensive specific surface area, large pore volume, and uni-
form pore dispersions, which can provide potential electrode
candidates for SCs.[28] The supercapacitive application of MOF
can be mainly classified into two types. One is to use MOF as
a template to prepare MOF derivatives: metal oxides, metal
nanoparticles, porous carbon compounds, etc.[29,30] The other
is that pure MOFs are directly applied to electrode materials
for SCs and their capacitance are based on the reversible redox
Y. Li, Y. Xu, Prof. H. Pang
School of Chemistry and Chemical Engineering
Guangling College
Yangzhou University
Yangzhou 225002, Jiangsu, P. R. China
E-mail: panghuan@yzu.edu.cn
Dr. Y. Liu
Collaborative Innovation Center of Nonferrous
Metals of Henan Province
Henan Key Laboratory of High-Temperature Structural
and Functional Materials
School of Materials Science and Engineering
Henan University of Science and Technology
Luoyang, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201902463.
DOI: 10.1002/smll.201902463
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reactions of the metal centers.[31–34] However, most MOFs lack
adequate conductivity and chemical durability when utilized
directly as SCs electrode materials, and their capacitance is
usually low.[35] To enhance capability, one method is MOF
micronano-crystallization: reducing the particle size to the
micro/nanometer scale could lead to a significant decrease in
the path of electrolyte ion diffusion and an increasing specific
surface area of the active materials. The electrochemical prop-
erties would obviously be enhanced hence.[36,37] Yaghi’s team
reported the synthesis of diverse MOF nanocrystals and found
that zirconium MOF possessed fairly nice areal capacitance
(5.09 mF cm−2) in SCs.[38] Through great efforts, Wang and cow-
orkers have proven that PANI-ZIF-67-CC in KCl electrolyte has
a high specificity of 2,146 mF cm−2 capacitance at 10 mV s−1.[39]
Inspired by the pioneering work of the predecessors
and considering the fact that applying MOF directly to SCs
electrode material is still difficult. In this work, we report
the facile synthesis of hexagonal [Ni(HBTC)(4,4′-bipy)]
(HBTC = 1,3,5-benzenetricarboxylic acid, 4,4′-bipy = 4,4′-bipy-
ridine) materials with good hexagonal morphology (denoted
by Ni-MOF). The largest exposed face (001) crystal plane
could shorten the path of ion diffusion and electron trans-
port. Moreover, cross-linked network structures grown on the
(001) crystal plane are easily mixed with the conductive carbon
to form a shorter electron transport length. At the same time,
this structure forms a large number of pores, ensuring a large
ionic contact surface and increasing the number of electroac-
tive sites. In view of this structural advantage, we subsequently
studied the electrochemical properties of the obtained Ni-MOF
by conventional three-electrode cell. Hexagonal Ni-MOF elec-
trode showed a specific capacitance of 977.04 F g−1 at 0.5 A g−1
in a 3.0 m KOH solution. In cyclic performance testing, when
current density is 0.5 A g−1, specific capacitance remains at
92.34% after 5000 cycles. More importantly, hexagonal Ni-MOF
and AC were utilized as the positive and negative materials,
respectively, to assemble aqueous devices. It was found that the
aqueous devices revealed a capacitance of 189.23 F g−1 at a cur-
rent density of 0.5 A g−1, great cycling stability, as well as high
energy density.
images (Figure S1, Supporting Information) confirm the fact
that the resulting product structure is of a certain thickness.
After further enlargement, we found that a cross-linked mesh
structure appeared on the surface of the hexagonal Ni-MOF
(Figure 1a3–e3). With the extension of solvothermal time, the
mesh structure gradually appeared from vagueness to clarity,
then back to vagueness, and finally disappeared. The surfaces
of the six squares gradually changed from smooth to rough to
smooth. Compared with the other four reaction time samples,
the Ni-MOF morphology obtained at 18 h (Y3) is relatively
complete and uniform, and the mesh structure on the surface
of the six squares is the clearest. Based on its special surface-
grown mesh structures, its growth process was explored.
Through reducing the amount of reactants, the samples were
synthesized at 80 °C for 18 h and characterized by SEM. As
shown in Scheme S1 (Supporting Information), when the
amount of the reactant is 0.375 mmol, the obtained products
are piled up in sheet-like structures and initially have a hex-
agonal shape. By doubling the amount (0.75 mmol), the orig-
inal sheet-like structures gradually accumulate into a hexagon,
and the surface presents the closely packed mesh structures.
Doubling the amount again (1.5 mmol), the products form a
clear hexagonal shape and the dense surface mesh structures.
Therefore, it is found that whole growth process is the accu-
mulation of sheet-like structures via controlling the amount
of reactants. The elemental mapping in Figure S2 (Supporting
Information) demonstrates that C, N, O, and Ni are dispersed
homogeneously in Ni-MOF (Y3). Meanwhile, to further ascer-
tain the detailed elemental composition, X-ray photoelectron
spectroscopy (XPS) analysis was performed (Figure S3, Sup-
porting Information). The XPS survey spectrum exhibits
that hexagonal Ni-MOF consists primarily of Ni, C, N, and
O elements, unambiguously confirming the existence of
all elements. Six peaks are perceived in the Ni 2p spectrum
(Figure S4, Supporting Information). There are two main spin
orbital peaks: one at 872.8 eV and the other at 855.3 eV, with
a spin energy separation of 17.5 eV, corresponding to Ni 2p1/2
and Ni 2p3/2, respectively, which is a feature of Ni2+. The two
peaks located at 881.1 and 863.23 eV correspond to Ni 2p1/2
and Ni 2p3/2 satellites, respectively. These results verify pres-
ence of Ni2+ in Ni-MOF, consistent with the previous literature
reports.[40,41] The peaks of Ni 2p1/2 and Ni 2p3/2 spin orbitals
located at 877.63 and 859.84 eV, respectively, correspond to
Ni3+. It can be found that the peak area of Ni2+ is significantly
larger than that of Ni3+, which indicates that the Ni elements
in hexagonal Ni-MOF mainly exist in the form of Ni2+. More-
over, we attempted to determine the effect of temperature on
the morphology: the fixed reaction time was 18 h, the reaction
temperature was changed to 40, 60, 80, 100 °C, and the other
conditions were equal. The corresponding SEM images were
shown in Figure S5a–d (Supporting Information). It could be
found that of the four temperatures, 80 °C is the optimum
temperature for the synthesis of hexagonal Ni-MOF. In order
to further prove the optimal reaction condition (80 °C, 18 h) of
hexagonal Ni-MOF, the morphology of obtained samples under
other reaction times was continuously explored at these four
temperature. As shown in Figure S6 in the Supporting Infor-
mation, it is found that the hexagonal morphology of the sam-
ples is incomplete at low temperature (40, 60 °C). As time goes
2. Results and Discussion
Hexagonal Ni-MOF ([Ni(HBTC)(4,4′-bipy)]) was synthesized
via a one-pot hydrothermal method using nickel nitrate hexa-
hydrate as the metal source with H3BTC and 4,4′-bipy as the
organic ligand and the solvent of DMF. The MOF was synthe-
sized by controlling reaction temperature and time to explore
its growth process, and its electrochemical properties of super-
capacitors were studied.
The morphologies at different magnifications of as-synthe-
sized Y1-5 were examined by scanning electron micro-
scopy (SEM). In Figure 1, a1–e1 are low-magnification SEM
images at a solvothermal temperature of 80 °C and solvo-
thermal times of 6, 12, 18, 24, and 30 h, respectively. It can
be clearly seen that obtained Ni-MOF is a typical six-square
structure. Figure 1a2–e2 displays a corresponding further
enlarged single hexagonal Ni-MOF with an average thickness
of ≈1 µm. High-resolution transmission electron microscopy
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to (001), (002), and (003) have three distinct diffraction peaks
of 7.62°, 15.59°, and 23.62°, respectively. The exposed (001)
crystal plane corresponding to the hexagonal face is conducive
to strong diffraction peaks of the (001), (002), and (003) crystal
planes. Combined with morphology of the hexagonal Ni-MOF,
as shown in the schematic diagram (Figure 2c), the top and
bottom hexagonal crystal planes correspond to the (001) crystal
plane, and the rectangular crystal planes correspond to (100),
(1-10), and (010) crystal planes. The maximum exposure sur-
face of (001) is a perfect structure for electron transport and
electrolyte solution diffusion. The interlinked 4,4′-bipy-nickel
chains provide a conductive path for the electrons on a plane
perpendicular to (001). Moreover, the cross-linked porous mesh
structures growing on the (001) crystal plane facilitate the
storage and diffusion of electrolyte solutions, which may have
excellent electrochemical properties as electrodes in superca-
pacitors. In short, the XRD patterns match the SEM images
well. In addition, it can be seen that the patterns of Y1 and
Y2 are slightly rough, with low crystallinity and poor crystal
form, which may be due to the short solvothermal time. With
prolonging solvothermal time, the peaks in the Y3-5 patterns
are intense and sharp, indicating that the samples have high
crystallinity and relatively stable crystal form. The formation
of hexagonal Ni-MOF was further confirmed by performing
Fourier transform-infrared radiation (FT-IR) spectroscopy. As
shown in Figure 2d, the same FT-IR band of the five samples
confirmed that the hexagonal Ni-MOF retained the same func-
tional groups. From the strong absorbed peaks at 1610, 1549,
and 1434 cm−1, it was inferred that there is a benzene ring

skeleton vibration (C C) in the MOF. In addition, the absorp-
tion peak caused by the C–H out-of-plane bending vibration
at 764 cm−1 and overtone peak at 1717 cm−1 further bore out
the presence of the benzene ring. Signals at 815 and 717 cm−1
correspond to 1,3,5 trisubstituted peaks on the benzene
ring. The strong peak of carboxylate (COO−) was observed at
1365 cm−1. C–O stretching vibrations at 1221 and 1261 cm−1
also confirmed the carboxylic acids.
Figure 1. SEM images of hexagonal Ni-MOF obtained at varied
solvothermal time at 80 °C: (a1-3) 6 h, (b1-3) 12 h, (c1-3) 18 h, (d1-3)
24 h, (e1-3) 30 h.
In a three-electrode test system, the electrochemical perfor-
mance of bare Ni-foam electrode was evaluated via exploiting
Pt as counter electrode and Hg/HgO as reference electrode.
As shown in the Figure S7 in the Supporting Information,
the bare electrode is charged and discharged rapidly under the
voltage window of 0.49 V. At the current density of 0.5 A g−1,
the specific capacitance is only 15.23 F g−1. Next, the hexagonal
Ni-MOF (Y1-5) were fabricated into electrodes and a series
of electrochemical performance tests were carried out in the
three-electrode cell. Cyclic voltammogram (CV) curves of cor-
responding voltage window ranges of the five samples (Y1-5)
were accomplished at 20 mV s−1 to explore the suitable poten-
tial windows (Figure S8, Supporting Information). Figure 3a
and Figure S9 (Supporting Information) represent CV curves of
Y3 and the other four materials from 5 to 100 mV s−1, respec-
tively. These CV curves all clearly show a pair of redox peaks,
indicating Faraday behavior based on the hexagonal Ni-MOF
electrodes. Such faradaic features were produced by the sur-
face redox reaction of the electrode during the electrochemical
experiment. Additionally, the two redox peaks may be explained
by the redox reaction of Ni(II) to Ni(III) on the surface. A
similar process has been proposed for explaining the redox
on, the changes of hexagonal surface mesh structures are not
obviousorevennomeshstructures.Athightemperature(100°C),
the samples are partially damaged, and the size becomes large
and uneven.
The crystal structures of obtained hexagonal Ni-MOF were
further investigated by X-ray diffraction (XRD) in Figure 2a,
where Y1-5 are XRD patterns of five different reaction times
(6, 12, 18, 24, 30 h) at a solvothermal temperature of 80 °C. The
obtained results are consistent with the previously reported
(Ni(HBTC)(4,4′-bipy)),[42] indicating that the five hexagonal
Ni-MOF samples are materials with the same crystal struc-
ture. Figure 2b is a view of the structures along ab-axis. These
hexagonal Ni-MOFs have 3D porous pillared-layer frame-
work structures. The 2D layer consists of the central metal
ion Ni (II) and partially deprotonated H3BTC, which are then
connected by a 4,4′-bipy ligand as columns along the c-axis
between these layers to form a 3D highly porous framework
(Figure 2b inset). The crystallographic planes corresponding
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Figure 2. a) XRD patterns of the sample Y1-5. b) View of structures of hexagonal Ni-MOF 2D layer along ab-axis. Over the upper right corner, a view
of structure perpendicular to the 2D layer. c) Schematic view of a single crystal structure of hexagonal Ni-MOF. d) FT-IR spectra of the sample Y1-5.
reaction of Ni-based MOF ([Ni3(OH)2(C8H4O4)2(H2O)4]·2H2O
and Ni3(btc)2·12H2O) materials.[43] Thus, the charge-storage
mechanism may be probably described as the following redox
reactions given in Equations (1) and (2)
a large amount of charge, resulting in the high charge storage
capacitance. The GCD studies at different current densities
of 0.5, 0.6, 1, 2, 3, and 5 A g−1 were performed, and the cor-
responding GCD curves are shown in Figure 3c for Y1 and in
Figure S10 (Supporting Information) for Y1, Y2, Y4, and Y5.
The relationship between the calculated specific capacitance
value and the current density is shown in Figure 3d. It can be
observed that the specific capacitance values of Y3 are 977.04,
887.75, 788.26, 709.19, 673.48, and 612.25 F g−1 from 0.5 to
5 A g−1, respectively, which are much higher than the other four
samples. The cycling stabilities of Y1-5 were further evaluated
at 0.5 A g−1. As can be observed from Figure 3e, the specific
capacitance of Y3 can retain 93.55% of the initial value in the
first 2000 cycles, and remains at 92.34% of the initial value at
the end of the 5000 cycle, indicating good cycling stability. The
decay of capacitance occurs mainly before 1000 cycles, and after
5000 cycles in succession, the value of the capacitance remains
at 92.34% of the initial value. The main reason for the decrease
of capacitance may be that during the continuous cycle, OH−
repeatedly intercalates and deintercalates at the interface of
electrode/electrolyte, resulting in the structural collapse of the
hexagonal Ni-MOF and hindering the electrolyte wetting elec-
trode. The comparison of the specific capacitance of different
Ni-based MOF materials is given in Table S1 (Supporting Infor-
mation), and the value of Y3 is comparable to previous studies.
Figures S11 and S12 (Supporting Information) show the SEM
Ni
Ni
(
(
II
)
s + OH− ↔ Ni
( )
II)(OH
+ e−
(1)
(2)
ad
II)(OH
)
↔ Ni
(
III)(OH
)
s + e−
ad
With the scanning speed increasing, curve area and peak cur-
rent continue to increase, exhibiting good capacitive behavior
and charge storage characteristics.
Galvanostatic charge–discharge (GCD) curves of Y1-5 elec-
trodes at 0.5 A g−1 are shown in Figure 3b. The shapes of
curves present characteristics of pseudocapacitance, which is
consistent with CV. The corresponding specific capacitance
values are calculated as 582.93, 945.65, 977.04, 825.80, and
542.98 F g−1 for Y1-5, respectively. In combination with the
SEM images, the clearer the mesh structures on the hexagonal
surface, the higher the relative specific capacitance. Compared
with the other four samples, the mesh structures grown on the
surface of Y3 are the most clear and complete; they easily fully
mix with conductive carbon, which is more conducive to facile
electrolyte distribution through electrochemical active sites and
expanding specific surface area of the active materials. There-
fore, the high surface area of Y3 is more effectively used to hold
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Figure 3. Electrochemical properties of as-prepared hexagonal Ni-MOF in a three-electrode cell with 3.0 m KOH aqueous solution. a) CV curves of
Y3 at different scanning speeds; b) GCD curves of Y1-5 at 0.5 A g−1. c) GCD curve of Y3 various current densities. d) Specific capacitance at different
current densities of Y1-5. e) Cyclic performance of Y1-5 at 0.5 A g−1
.
and XRD patterns of Ni-MOF (Y3) after 5000 cycles using a
three-electrode cell. According to the SEM image, the hexagonal
Ni-MOF structures are collapsed. The XRD pattern shows that
the crystal of the sample after cycling is very weak, which is
quite different from the pattern before cycling. Although the
Ni-MOF material has undergone some changes in the working
process, it still has excellent specific capacity and good electro-
chemical stability. Excellent stability is important for high per-
formance supercapacitors. To further investigate its stability,
Figures S13 and S14 (Supporting Information) provide the pic-
ture, SEM and XRD patterns of Y3 after soaking in 3.0 m KOH
electrolyte for 24 h. From the SEM image, the sample still main-
tains a hexagonal shape after soaking, and the mesh structures
on the surface disappear. XRD shows that the crystal shape after
immersion becomes weak, but still has the same composition
as the sample before soaking.
The electrochemical characteristics of the five samples were
estimated via electrochemical impedance spectroscopy (EIS).
As shown in Figure S15 (Supporting Information), the slope of
Y1 is larger than the other four in the low-frequency region,
implying the lower ion diffusion resistance and superior elec-
trochemical performance of the Y3 electrode.
In order to ulteriorly study hexagonal Ni-MOF electrodes for
practical applications, aqueous devices were fabricated in 3.0 m
KOH with the hexagonal Ni-MOF and AC. Figure S16 (Sup-
porting Information) shows CV curves for several voltages for
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Figure 4. Electrochemical properties of as-prepared hexagonal Ni-MOF//AC aqueous devices. a) CV curves of Y3 at different scanning speeds; b) GCD
curves of Y1-5 at 0.5 A g−1. c) GCD curve of Y3 various current densities. d) Specific capacitance at different current densities of Y1-5. e) Cyclic
capacitance of Y1-5 at 0.5 A g−1. f) Ragone plot of the hexagonal Ni-MOF//AC aqueous device exhibiting the relationship between energy density and
power density (inset: a blue LED powered by two series-wound aqueous devices).
hexagonal Ni-MOF//AC aqueous devices at 20 mV s−1. All of
them reveal pseudocapacitive behavior. GCD curves from 0.8 to
1.5 V at 0.5 A g−1 were measured (Figure S17, Supporting Infor-
mation) to further procure opportune charge–discharge voltage
values. Figure 4a and Figure S18 (Supporting Information) pre-
sent the CV curves from 5 to 100 mV s−1. It is obvious that the
CV curve remains well-shaped at the sweep speed of 100 mV s−1,
suggesting that aqueous devices have excellent rate performance.
The GCD curves of Y1-5//AC aqueous devices with current
density of 0.5 A g−1 are presented in Figure 4b. Corresponding
specific capacitance values are 131.25, 170.36, 189.23, 119.62,
and 67.92 F g−1. Compared with the other four samples, the
specific capacitance of Y3 is 189.23 F g−1, which is the largest
and is in agreement with CV. The GCD curves across a range of
current densities (0.5, 0.6, 1, 2, 3, and 5 A g−1) are displayed in
Figure 4c and Figure S19 (Supporting Information). The shapes
of all charge and discharge curves are tightly linear and exhibit
a typical triangular symmetric distribution. Among them, the
calculated specific capacitance values of the Y3//AC aqueous
device are 189.23, 167.08, 149.14, 132.76, 124.14, and 116.38 F g−1,
respectively (Figure 4d), which are higher than the other four
groups. Moreover, the cyclic stabilities are evaluated (Figure 4e).
Under the current density of 0.5 A g−1, the capacitance of Y3//AC
changed from 189.23 to 164.20 F g−1 and remained at 86.77%
after 5000 charging and discharging cycles. At the open circuit
voltage, EIS spectra (Figure S20, Supporting Information) were
measured to ascertain their small charge transfer resistance.
Beyond dispute, Y3 is the smallest. To investigate the overall
properties of hexagonal Ni-MOF//AC aqueous devices, Ragone
plots are drawn in Figure 4f from GCD curves based on their
two-electrode system. The Ragone plots demonstrate the relation-
ship between energy density and power density for five different
aqueous devices. It can be found that the hexagonal Ni-MOF
(Y3)//AC manifests a high energy density of 55.26 Wh kg−1
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Figure 5. Schematic diagram of charge transfer process on (001) and rectangular crystal planes, respectively.
at a power density of 362.50 W kg−1 and maintains a value of
33.98 Wh kg−1 under 3624.87 W kg−1, and its splendid rate per-
formance is distinctly higher than those of the other four devices.
Meanwhile, the Ni-MOF (Y3)//AC device performs well com-
pared to the similar materials that have been reported recently
(Figure S21, Supporting Information), such as Ni3(btc)2·12H2O//
AC (16.5 Wh kg−1), Ni-MOF//AC ASCs (21.05 Wh kg−1),
and GM-LEG@Ni-MOF//AC ASC (32.7 Wh kg−1). To demon-
strate that the device can be employed in practical application,
the aqueous device was used to power a blue light-emitting diode
(LED), which is presented in the inset of Figure 4f.
In this present work, hexagonal Ni-MOF can be successfully
applied to SCs electrode materials with outstanding properties
including large capacitance, good rate performance and excellent
cycle stability. First, the materials offer a 3D porous layer frame-
work structure. Large layer spacing is beneficial for electrolyte
diffusion and OH− intercalation/deintercalation. Second, the
exposed (001) crystal plane shortens the path of electron trans-
port and ion diffusion. As shown in Figure 5, when electrons
and ions pass through the electrode materials, the length of
passing through the (001) crystal plane is noticeably shorter
than the surrounding rectangular crystal plane. The transpor-
tation path is shortened, the transportation rate is accelerated,
and the electrochemical performance is improved. Finally, the
cross-linked mesh structures growing on the (001) crystal plane
enhances mixing with the conductive carbon, causing the elec-
trode materials to become more conductive and accelerating
electron transfer at the electrode–electrolyte interface. Simul-
taneously, mesh structures could increase the ion contact area
and expose more active sites. In this instance, it would be easier
for substrate molecules to interact with the active site and
further improve the electrical properties.
method. The electrochemical performance of hexagonal
Ni-MOF was investigated via a three-electrode cell and aqueous
device. In a three-electrode cell, sample Y3 electrode displayed
a maximum specific capacitance of 977.04 F g−1 at 0.5 A g−1,
and held 92.34% of the initial value after 5000 cycles. More
importantly, the Y3//AC aqueous device possesses specific
capacitance of 189.23 F g−1 at 0.5 A g−1. The preeminent electro-
chemical performance could be ascribed to essential features of
the hexagonal Ni-MOF, such as favorable {001} exposed facets
and the surface-grown cross-linked mesh structures. Ni-based
MOF materials have extensively served as SCs electrode mate-
rials. However, as MOF materials, the hexagonal Ni-MOF has
the disadvantage of poor electrical conductivity. Combining
MOFs with highly conductive materials involving carbon nano-
tubes, graphenes, and conductive polymers represents a prom-
ising future research trend.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by the National Natural Science Foundation
of China (NSFC-21671170, 21673203, and 21201010), the Top-notch
Academic Programs Project of Jiangsu Higher Education Institutions
(TAPP), Program for New Century Excellent Talents of the University in
China (NCET-13-0645), the Six Talent Plan (2015-XCL-030), and Qinglan
Project. The authors also acknowledge the Priority Academic Program
Development of Jiangsu Higher Education Institutions and the technical
support we received at the Testing Center of Yangzhou University.
3. Conclusions
Conflict of Interest
In conclusion, we have designed and prepared the hexagonal
Ni-MOF materials using a controllable one-step solvothermal
The authors declare no conflict of interest.
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Keywords
cross-linked mesh structures, exposing crystal plane, hexagonal Ni-MoF,
supercapacitors
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©
Small 2019, 1902463
1902463 (8 of 8)
2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim