Hydrangea-like NiCo-based Bimetal-organic Frameworks, and their Pros and Cons as Supercapacitor Electrode Materials in Aqueous Electrolytes

Article abstract of DOI:10.1002/zaac.201900035

Hydrangea-like NiCo-based bimetal-organic frameworks (NiCo-MOF) are synthesized in DMF-EtOH solution via a solvothermal method, using 4,4′-biphenyldicarboxylic acid as a ligand. NiCo-MOF having a highest capacity of 1056.6 F·g^–1 at 0.5 A·g^–1 and 457.7 F·g^–1 even at 10 A·g^–1 is achieved at a Ni/Co/BPDC molar ratio of 1:1:1, a temperature of 170 °C and a reaction time of 12 hours. It exhibits secondary 3D microsphere structures assembled by primary 2D nanosheet structures, good crystalline structure and good thermal stability below 350 °C in air. All the electrochemical data show that NiCo-MOF has the pros and cons as supercapacitor electrode materials in aqueous electrolytes. On the one hand, NiCo-MOF has a high capacity even at a high current density, low internal resistance, charge-transfer resistance and ion diffusion impendence, owing to the ordered coordination structure, 2D nanosheet structure and 3D assembled microsphere structure of NiCo-MOF. On the other hand, the cycling stability and rate capability are not ideal enough due to the hydrolysis of coordination bonds in aqueous electrolytes, especially, in alkaline solution. The good dispersion and high electrochemical activity of metal ions bring a high capacity for NiCo-MOF, but they result in the poor stability of NiCo-MOF. In the future work, finding a suitable organic electrolyte is an effective way to enhance the cycling stability of NiCo-MOF as well as deriving more stable skeleton materials from NiCo-MOF.

Full text of DOI:10.1002/zaac.201900035

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201900035
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Hydrangea-like NiCo-based Bimetal-organic Frameworks, and
their Pros and Cons as Supercapacitor Electrode Materials in
Aqueous Electrolytes
Qiaoqin Li,[a] Xiaoqin Wang,*[a,b] Nana Yang,[a] Fan He,[a] Yufei Yang,[a] Bohua Wu,[a] Jia Chu,[a]
Anning Zhou,[a,b] and Shanxin Xiong*[a,b]
Abstract. Hydrangea-like NiCo-based bimetal-organic frameworks
transfer resistance and ion diffusion impendence, owing to the ordered
(NiCo-MOF) are synthesized in DMF-EtOH solution via a solvo- coordination structure, 2D nanosheet structure and 3D assembled
thermal method, using 4,4Ј-biphenyldicarboxylic acid as a ligand. microsphere structure of NiCo-MOF. On the other hand, the cycling
NiCo-MOF having a highest capacity of 1056.6 F·g–1 at 0.5 A·g–1 and stability and rate capability are not ideal enough due to the hydrolysis
457.7 F·g–1 even at 10 A·g–1 is achieved at a Ni/Co/BPDC molar ratio
of 1:1:1, a temperature of 170 °C and a reaction time of 12 hours. It
of coordination bonds in aqueous electrolytes, especially, in alkaline
solution. The good dispersion and high electrochemical activity of
exhibits secondary 3D microsphere structures assembled by primary metal ions bring a high capacity for NiCo-MOF, but they result in the
2D nanosheet structures, good crystalline structure and good thermal
stability below 350 °C in air. All the electrochemical data show that
poor stability of NiCo-MOF. In the future work, finding a suitable
organic electrolyte is an effective way to enhance the cycling stability
NiCo-MOF has the pros and cons as supercapacitor electrode materials of NiCo-MOF as well as deriving more stable skeleton materials from
in aqueous electrolytes. On the one hand, NiCo-MOF has a high ca- NiCo-MOF.
pacity even at a high current density, low internal resistance, charge-
poor conductivity and narrow potential window. Conductive
polymers possess a large specific capacitance and low cost, but
they are easily detached due to the expansion in the charge and
discharge process.[2]
Introduction
Supercapacitors are
a
high-capacity energy storage
device bridging the gap between electrolytic capacitors and
rechargeable batteries. They can store 10 to 100 times more
energy per unit volume or mass than electrolytic capacitors,
can reserve and deliver charge much faster than batteries, and
can tolerate many more charge and discharge cycles than re-
chargeable batteries. They are greatly applied in the fields re-
quiring many rapid charge/discharge cycles rather than long
term compact energy storage. At present, they have been used
for regenerative braking, short-term energy storage or burst-
mode power delivery within cars, buses, trains, cranes and ele-
vators.[1] Supercapacitor performances are mainly determined
by electrode materials. The general electrode materials are car-
bon materials, metal oxides/hydroxides and conductive poly-
mers. Among them, carbon materials have a large surface area,
good conductivity, strong chemical stability, but a low specific
capacitance and low energy density. Metal oxides/hydroxides
own a large specific capacitance, high energy density, but a
Metal organic frameworks (MOFs) are a new member of the
porous material family and have been greatly attracted due to
its adjustable pore size, large specific surface area and abun-
dant organic functional groups, having potential applications
in supercapacitors,[3–6] lithium-ion batteries, catalysts, sensors,
medicine and so on. Many studies have demonstrated that me-
dium strength MOFs can be readily obtained using poly-
carboxylic acids as monodentate, bidentate or multidentate li-
gands. Some reports[7–13] confirm that BPDC can coordinate
with monometallic Co, Ni, Cd, Zn, Ba, Zr, or bimetallic
Zn(Co, Ni, Cd) to construct porous mononuclear, dinuclear, or
multinuclear MOFs with different dimensionalities. Further-
more, 4,4Ј-biphenyldicarboxylic acid (BPDC) owns more flex-
ible and larger conjugated structure than 1,4-benzenedicarbox-
ylic acid (BDC) due to the biphenyl group, which possibly
results in higher conductivity, larger pore size and more vari-
able structure for MOFs.
Ni-based MOFs usually possess high capacitance and ir-
regular shapes[4,7] whereas Co-based MOFs always own regu-
lar and stable morphologies and low capacitance.[14] Consid-
ered that both nickel ions and cobalt ions have a divalent state,
adjacent atomic number and very close ionic radius, it is pos-
sible that nickel ions and cobalt ions simultaneously coordinate
with a ligand to construct bimetallic MOFs crystals presenting
a little lattice distortion. Moreover, some reports have con-
firmed that two different metal ions may coordinate with a
* Dr. X. Wang
Fax: +86-29-85583183
E-Mail: wangxq@xust.edu.cn
* Dr. S. Xiong
E-Mail: xiongsx@xust.edu.cn
[a] College of Chemistry & Chemical Engineering
Xi’an University of Science & Technology
Xi’an, 710054, P. R. China
[b] Key Laboratory of Coal Resources Exploration and Comprehen-
sive Utilization, MLR.
Xi’an, 710021, P. R. China
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201900035 or from the au-
thor.
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ligand to form a homogeneous bimetallic MOFs rather than a area and more discharge time than the other two, indicating
mixture of two monometallic MOFs, and synthesizing bimetal- that it has a largest specific capacitance among the three.
lic MOFs is becoming a new strategy for enhancing framework Furthermore, a pair of redox peaks of the 1:1:1 sample in Fig-
stability and properties.[15–19] At present, only a few NiCo- ure 2a corresponds a couple of quite reversible redox reactions,
based MOFs have been reported as electrode materials pres- indicating that NiCo-MOF electrode has an excellent faradic
enting high electrochemical activity and energy storage ca- pseudocapacitance. Hence, it is better to adjust the Ni/Co/
pacity, in which terephthalic acid-pyrazine,[16] 2-methylimid- BPDC molar ratio to 1:1:1.
azole,[17] 1,3,5-benzenetricarboxylic acid,[18] and benzenedi-
carboxylic acid (BDC)[19] are used as ligands.
Herein, a series of hydrangea-like NiCo-MOFs are synthe-
sized via a simple solvothermal approach, depending coordina-
tion reactions between metal ions and BPDC in a DMF-EtOH
solution. Their morphologies, microstructures, pore structure,
thermal stability and capacitance performances are evaluated
by SEM, TEM, XRD, FTIR, Raman, physical adsorption and
desorption, TG and electrochemical measurements, respec-
tively. As expected, the hydrangea-like NiCo-MOF has a large
capacitance, low internal resistance, charge-transfer resistance,
and ion diffusion impendence. The pros and cons are described
in detail about NiCo-MOF as supercapacitor electrode materi-
als in aqueous electrolytes, and the corresponding essential
reasons are discussed, and the modified approaches in the fu-
ture work are offered.
Results and Discussion
Morphologies and Capacitance Performances of NiCo-
MOF Synthesized under Various Reaction Conditions
Figure 1. SEM images of bimetallic NiCo-MOF particles synthesized
under various reaction conditions.
In order to investigate the optimal Ni/Co/BPDC molar ratio,
reaction temperature and time, a series of NiCo-MOFs are syn-
thesized under various conditions. Their morphologies are ob-
In Figure 1d–f, NiCo-MOF microspheres synthesized in
served by SEM measurements and their electrochemical be- 12 h exhibit the best dispersion, most uniform size, best sphe-
haviors are performed by CV and GCD tests. SEM images are ricity, and integrity among the three. In Figure 2c–d, NiCo-
shown in Figure 1, and CV and GCD curves are exhibited in MOF synthesized in 12 h presents a much larger CV circle
Figure 2. It can be seen from Figure 1 that all the NiCo-MOFs area and more discharge time than 6 h and 18 h. Moreover,
exhibit secondary 3D hydrangea-like microsphere structures the yields of NiCo-MOF synthesized in diverse reaction time
assembled by primary 2D nanosheet structures. In Figure 1a– of 6 h, 12 h, and 18 h are calculated to 1.01 g·g–1 (BPDC),
c, NiCo-MOF microspheres at Ni/Co/BPDC molar ratio of 1.05 g·g–1 (BPDC), and 0.94 g·g–1 (BPDC), respectively.
1:1:1 have better dispersion, more uniform size, better spheric- Therefore, controlling the reaction time to 12 h can facilitate
ity and integrity than 1:1:2 and 1:2:2. According to the single uniform, well-dispersed and complete microspheres.
crystal analysis of Co-MOF using BPDC as a ligand via a
In Figure 1g–i, NiCo-MOF microspheres synthesized at
hydrothermal approach in literature,[13] the molar ratio is deter- 170 °C have better dispersion, more uniform size, better sphe-
mined to 1 between metal ions and BPDC in the Co-MOF ricity, and integrity than the other two. In Figure 2e–f, NiCo-
single crystals. The ratio of 1:1:2 is just right, but the corre- MOF microspheres synthesized at 170 °C possess a largest CV
sponding NiCo-MOF microspheres present the most serious circle area and most discharge time among the three. Addition-
agglomeration and non-uniform size, and some of them are ally, the yields of NiCo-MOF synthesized at different tempera-
defective. Whereas, the other two NiCo-MOF microspheres ture of 150, 170, and 190 °C are 1.06 g·g–1 (BPDC),
exhibit the better dispersion and sphere integrity due to the 1.05 g·g–1 (BPDC), and 1.05 g·g–1 (BPDC), respectively. Thus,
excess (metal ions/BPBC) molar ratio. It indicates that excess the temperature of 170 °C is regarded as an optimal condition.
metal ions in the reaction system are favorable for forming
In Figure 1, it is also found that NiCo-MOF microspheres
well-dispersed and complete NiCo-MOF microspheres. By the have looser surface at conditions of lower (metal ions/BPBC)
weighing and calculating, the yields of NiCo-MOF at various molar ratio, less reaction time, and lower temperature. NiCo-
ratios of 1:1:1, 1:1:2, and 1:2:2 are 1.05 g·g–1 (BPDC), MOF microspheres containing more slit-shaped micro-meso-
0.82 g·g–1 (BPDC), and 0.91 g·g–1 (BPDC), respectively. pores can be assembled by adjusting the reaction conditions,
Moreover, CV and GCD curves in Figure 2a and b show that which are favorable for the immigration and storage of electro-
NiCo-MOF at the ratio of 1:1:1 has a much larger CV circle lytic ions. Through the comparison of SEM tests and electro-
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Figure 2. Electrochemical behaviors of bimetallic NiCo-MOF electrodes synthesized under various reaction conditions.
chemical measurements, it can be determined that the optimal
process is a Ni/Co/BPDC molar ratio of 1:1:1, temperature of
170 °C and reaction time of 12 h for synthesizing NiCo-MOF
with a highest capacity. In this paper, unless otherwise stated,
NiCo-MOF is designated as a sample synthesized in this opti-
mal process.
Figure 3 shows the elemental mapping of C, O, Ni, and Co
in one NiCo-MOF microsphere. The distributions of four ele-
ments are similar to the morphology of NiCo-MOF micro-
sphere and they are homogeneously distributed throughout the
whole microsphere. Moreover, the C and O distribution are
denser than the Co and Ni distribution. All these demonstrate
that each NiCo-MOF microphere is a compound composed of
nickel ions, cobalt ions and BPDC ligands. The metal content
in the optimal NiCo-MOF are determined by an ICP test of
the digested and diluted NiCo-MOF solution. The nickel and
cobalt mass content are 13.53 wt% and 16.55 wt%, respec-
tively. The Ni/Co atomic ratio is about 1/1.21, which indicates
that cobalt ions have stronger coordination ability than nickel
ions.
Figure 3. SEM image (a) and the corresponding C (b), O (c), Co (d),
and Ni (e) elemental mapping of optimal NiCo-MOF particles.
Morphologies, Microstructures, and Capacitance
Performances of Monometallic Ni(Co)-MOF and Bimetallic
NiCo-MOF
For the comparison with bimetallic NiCo-MOF, monometal-
lic Ni-MOF and Co-MOF are also synthesized at 170 °C for
12 h. Through the weighing and calculating, the yields of
NiCo-MOF, Ni-MOF and Co-MOF are 1.05 g·g–1 (BPDC),
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0.86 g·g–1 (BPDC), and 0.79 g·g–1 (BPDC), respectively. The because all the peak sites are well consistent with that of the
morphologies of three MOFs are investigated by SEM tests reported NiCo-MOF[2] and Co-MOF[20] using BPDC as a li-
and shown in Figure 4. All the three MOFs also demonstrate gand via a hydrothermal method, other than the relative peak
secondary 3D microsphere structures assembled by primary strength. The results indicate that three samples have crystal-
2D nanosheet structures. Among them, the particle size and line structures of MOFs indeed, and their crystalline structures
surface porosity of NiCo-MOF microspheres are between Ni- are similar and their crystallinity is good. The chemical struc-
MOF and Co-MOF. Meanwhile, NiCo-MOF microspheres tures of three MOFs are also investigated by FTIR and Raman
have better sphericity and dispersion than the other two. As measurements. The FTIR spectra in Figure 5b show that three
shown in Figure 4a and b, Ni-MOF microspheres are as- MOFs have similar infrared absorption peaks, indicating they
sembled by the smallest 2D nanosheets and present the small- consist of the same organic units. A group of peaks at 1596,
est sphere-like shapes, but they are seriously aggregated. In 825, 761, and 679 cm–1 are assigned to the characteristic mode
Figure 4c and d, Co-MOF microspheres are assembled by the of aromatic ring.[21] The two strong bands at 1541 and
biggest 2D nanosheets and present the biggest flower-like 1381 cm–1 are ascribed to the asymmetric and symmetric
shapes. They have a good dispersion, loose surface but a poor stretching modes of the coordinated carboxyl group from
sphericity. In Figure 4e and f, NiCo-MOF microspheres exhibit BPDC.[7–22] All these confirm that BPDC ligands exist in the
hydrangea-like shapes, and demonstrate better sphericity than as-synthesized NiCo-MOF. Moreover, two bands at 441 cm–1
Co-MOF and better dispersibility than Ni-MOF. In order to and 528 cm–1 are associated with Ni–O and Co–O stretching
further study how NiCo-MOF hydrangeas are assembled, TEM vibrations, indicating that metal ions have coordinated with
tests are conducted to analyze their internal structures. TEM BPDC.
images in Figure 4g and h exhibit that there are many voids in
The Raman spectra of three MOFs are exhibited in Fig-
each NiCo-MOF hydrangea, which stem from the loose as- ure 5c. The four Raman bands at 1583, 1428, 1285, and
sembly of primary nanosheets. These voids may become the 1143 cm–1 are attributed to C–H in-plane bending mode of aro-
storage space of electrolyte ions. TEM tests further confirm matic ring, and the two bands at 855 cm–1 and 3067 cm–1
that the as-synthesized NiCo-MOF hydrangeas are assembled correspond to C–H out-of-plane bending mode of aromatic
by 2D nanosheets and many voids exist in their internal struc- ring. Additionally, a couple of bands at 413 cm–1 and
tures.
627 cm–1 are assigned to the Ni–O and Co–O stretching vi-
bration. All the Raman bands well coincide with that of the
reported NiCo-MOF synthesized via a hydrothermal ap-
proach.[23] These Raman results are also in good agreement
with the above FTIR analysis.
Meanwhile, the electrochemical behaviors of Ni-MOF, Co-
MOF, and NiCo-MOF are evaluated by CV, GCD, and EIS
tests, and the results are shown in Figure 6. Compared with
Ni-MOF and Co-MOF, NiCo-MOF demonstrates the larger
circle area in CV curves of Figure 6a and the longer discharge
time in GCD curves of Figure 6b, indicating that bimetallic
NiCo-MOF has a higher charge storage capability than mono-
metallic Ni-MOF or Co-MOF. This conclusion can be affirmed
by the calculation results from GCD curves at 0.5 A·g–1, in
which the specific capacitance of NiCo-MOF is 1056.6 F·g–1,
larger than that of Ni-MOF (708.8 F·g–1) and Co-MOF
(153.0 F·g–1). Furthermore, the rate capability of three MOFs
in Figure 6c exhibits that NiCo-MOF has a higher specific ca-
pacitance than Ni-MOF and Co-MOF at any current density
ranging from 0.5 A·g–1 to 10 A·g–1, which further confirms the
above conclusion.
EIS tests can get a further insight about the origin of three
MOFs’ electrochemical performances. The original Nyquist
plots of Ni-MOF, Co-MOF, and NiCo-MOF are drawn in Fig-
ure 6d. The magnified plots at low frequency part and the suit-
able equivalent circuit for Nyquist plots are also exhibited in
the inset. Three plots are composed of a depressed semicircle
at high frequency part, the slope of the 45° portion at medium
frequency part and a straight line at low frequency part. The
depressed semicircle is related to charge-transfer impedance
Figure 4. SEM images of Ni-MOF (a) (b), Co-MOF (c) (d), optimal
NiCo-MOF (e) (f) (i) particles at a Ni/Co/BPDC molar ratio of 1:1:1,
a temperature of 170 °C and a reaction time of 12 h and TEM images
of optimal NiCo-MOF particles (g) (h).
The crystalline structures of three MOFs are further ana-
lyzed by the XRD tests. The XRD patterns in Figure 5a show
that all the three MOFs have a group of similar sharp diffrac-
tion peaks at 2θ values of 6.1°, 12.4°, 15.4°, 19.7°, 29.4° and (Rct) at the electrode/electrolyte interface. The intercept of Z’
40.8°, assigned to NiCo-MOF crystals using BPDC as a ligand axis is associated with the internal resistance (Rs) of active
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Figure 5. XRD patterns (a), FTIR spectra (b) and Raman spectra (c) of Ni-MOF, Co-MOF and optimal NiCo-MOF particles.
Figure 6. CV curves (a), GCD curves (b), rate capability curve (c), Nyquist plots (d) of Ni-MOF, Co-MOF and optimal NiCo-MOF electrodes.
electrodes. The slope of straight line at low frequency part is impendence and a high capacity as observed in Figure 6a and
in connection with the impendence of ion diffusion inside the b. Fitted by equivalent circuit, NiCo-MOF presents the lower
electrode.[24] The Nyquist plot of NiCo-MOF shows a very charge-transfer resistance Rct (0.1 Ω) than Ni-MOF (0.4 Ω)
steeper line than Ni-MOF and Co-MOF at low frequency re- and Co-MOF (53.7 Ω), indicating that NiCo-MOF more easily
gion and its vertical angle is close to 90°, indicating the fast deliver charges at the electrode/electrolyte interface than Ni-
ion migration in electrode materials due to a low ion diffusion MOF and Co-MOF. Additionally, the internal resistance Rs of
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NiCo-MOF, Ni-MOF, and Co-MOF are 2.0 Ω, 2.2 Ω, and ing TG and derivative thermogravimetric (DTG) curves are
2.8 Ω, respectively, suggesting that three MOFs electrodes drawn in Figure 8. Five mass losses can be found. Three small
have a good conductivity. The EIS results indicate that bime- losses of ca. 9% at the temperature range of 25–350 °C are
tallic NiCo-MOF has a lower internal resistance, charge-trans- attributed to the removal of the adsorbed water and carbon
fer resistance and ion diffusion impendence than monometallic oxides, and a few coordinated DMF. Two large losses of 56%
Ni-MOF or Co-MOF, which are the essential reasons why bi- at the range of 350–800 °C derive from the collapse and de-
metallic NiCo-MOF possesses a higher capacity than mono- composition of organic skeleton, especially, the exclusion of
metallic MOFs.
CO2 from the evolution of carboxylic groups. The overall mass
loss of NiCo-MOF is about 64.16% and the final products are
metal oxides. The mass is almost unchanged at 450–800 °C,
indicating that the formation reaction of metal oxides has been
terminated at 450 °C. Based on the TG/DTG analysis, it can be
concluded that the as-synthesized NiCo-MOF is stable below
350 °C in air, which is more stable than NiCo-MOF synthe-
sized via a hydrothermal method.[2]
Pore Structure, Thermal Stability, and other Capacitance
Performances of Optimal NiCo-MOF
The pore structure of optimal NiCo-MOF is analyzed on an
adsorption analyzer, and the N2 adsorption-desorption isotherm
and pore distribution curve are exhibited in Figure 7. The iso-
therm presents an obvious hysteresis loop, which is usually
located in the multilayer range of physisorption isotherms and
generally associated with capillary condensation in mesopore
structures. This hysteresis loop meets the Type H3, whose ad-
sorption branch of the isotherm is like a combination of the
Types I and II isotherm. The front part in the P/P0 range of 0
and 0.5 is attributed to monolayer adsorption. The latter part in
the P/P0 range of 0.5 and 1 is related to multilayer adsorption,
capillary condensation and even liquid condensation. The iso-
therm does not exhibit any limiting adsorption at high P/P0,
which is often observed with aggregates of plate-like particles
giving rise to slit-shaped pores.[25] From Figure 1, Figure 3,
and Figure 4, a plenty of slit-shaped pores are obviously pro-
duced in the NiCo-MOF microspheres through the assembly
of NiCo-MOF nanosheets. The pore distribution curve in Fig-
ure 7b demonstrates that the slit-shaped mesopores with dia-
meters of 3–15 nm make the greatest contribution to the pore
volume, especially 3.5–5 nm. Calculated from the N2 adsorp-
Figure 8. TG and DTG curves of optimal NiCo-MOF particles in air
atmosphere.
CV tests at various scan rates and GCD tests at different
tion data, the BET surface area and average pore width of current densities are further evaluated the electrochemical be-
NiCo-MOF are 44.1 m2·g–1 and 15.1 nm, respectively. All haviors of NiCo-MOF. All the CV curves in Figure 9a present
these confirm that most slit-shaped pores are mesoporous in a pair of strong redox peaks at ca. 0.35 V/0.20 V correspond-
NiCo-MOF microspheres, which are favorable for the intercal- ing to M(III)↔M(IV), intimating a pseudocapacitive behavior.
ation of electrolyte ions. In addition, the low surface area is Moreover, as the scan rate increases, the oxidation peak shifts
associated with the unconnected slit-shaped pores. As ob- a more positive position and the reduction peak shifts a more
served in Figure 4g–h, a high density ring exists between po- negative position, deriving from an increasing polarization at
rous interior and loose surface, attributed to the tight assembly a higher scan rate. Some GCD curves in Figure 9b exhibit two
of NiCo-MOF nanosheets.
couples of GCD redox plateaus at ca. 0.35 V/0.25 V and ca.
The thermal stability of NiCo-MOF is evaluated by thermo- 0.25 V/0.20 V, assigned to M(III)↔M(IV) and M(II)↔M(III),
gravimetric (TG) analysis in air atmosphere. The correspond- respectively. The pseudocapacitive feature of NiCo-MOF is
Figure 7. N2 adsorption-desorption isotherm (a) and pore distribution (b) of optimal NiCo-MOF particles.
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Figure 9. CV curves at different scan rates (a), GCD curves at various current densities (b), cycling stability and coulombic efficiency at
1 A/g (c) of optimal NiCo-MOF electrodes in a three-electrode system; SEM images (d) (e) of optimal NiCo-MOF electrodes using graphite
paper as current collector before and after 1000-cycle GCD tests; XRD image (f) and SEM pattern (g) of optimal NiCo-MOF after soaked in a
2 M KOH solution for 24 h, and SEM image (h) NiCo-MOF after soaked in deionized water for 50 d.
further affirmed, well coinciding with the CV results. Another 481.1 F·g–1 from the initial value of 920.0 F·g–1 during 700
couple of CV redox peaks related with M(III)↔M(IV) do not cycles and tends to be stable over 700 cycles. It eventually
occur in CV curves, originating that this redox reaction is too reaches 449.2 F·g–1 (48.8% of initial value) after 1000 cycles.
fast to record. Calculated from GCD curves, specific The coulombic efficiency of NiCo-MOF decreases slightly
capacitances of NiCo-MOF are1056.6 F·g–1 (0.5 A·g–1), first and then gradually increases from the initial value of
936.8 F·g–1 (1 A·g–1), 821.3 F·g–1 (2 A·g–1), 620.0 F·g–1 98.2% to 114.9% after 1000 cycles. Why is the cycling ca-
(5 A·g–1), and 457.7 F·g–1 (10 A·g–1), respectively. The change pacitance of NiCo-MOF not stable enough in aqueous electro-
of specific capacitance with current density can be seen visu- lyte? The SEM images of NiCo-MOF electrodes using graphite
ally in Figure 6c, demonstrating that NiCo-MOF still maintains paper as current collector are observed before and after 1000
a high capacity of 457.7 F·g–1 even at 10 A·g–1 although the GCD cycles and showed in Figure 9d and e. The morphologies
rate capability of NiCo-MOF is not ideal enough. The cycling of NiCo-MOF microspheres have no change in the electrode
stability is one of the most important factors for electrode ma- fabrication process. However, the morphologies of micro-
terials in practical applications. The cycling stability of NiCo- sphere-like active materials after 1000 GCD cycles have some
MOF is examined by GCD measurements at 1 A·g–1 for 1000 changes, which are assembled by many regular hexagonal
cycles and the results are displayed in Figure 9c. The specific nanosheets and have a looser surface than the tightly as-
capacitance of NiCo-MOF at 1 A·g–1 drops gradually to sembled NiCo-MOF microspheres. But they remain the micro-
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Journal of Inorganic and General Chemistry
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sphere-shaped morphologies of NiCo-MOF assembled by pendence and higher capacity. It exhibits a large specific ca-
nanosheets. Furthermore, the XRD and SEM test of NiCo- pacitance of 1056.6 F·g–1 at 0.5 A·g–1 and 457.7 F·g–1 even at
MOF particles after soaked in a 2 m KOH solution for 24 h 10 A·g–1, and capacitance retention of 449.2 F·g–1 at 1 A·g–1
are conducted and the results are shown in Figure 9f and g, after 1000 cycles. All the electrochemical data show that
respectively. The SEM image demonstrates that some nano- NiCo-MOF has the pro of high capacity and the con of poor
sheets on the NiCo-MOF microsphere surface are peeled off cycling stability as supercapacitor electrode materials in aque-
or reconstructed after the soaking. The serious aggregation of ous electrolytes. The essential reasons on the pros and cons of
microspheres originates that the soaked sample is not ground. capacitance performances of NiCo-MOF in aqueous electro-
The XRD pattern exhibits that most NiCo-MOF on micro- lytes are revealed and the corresponding modified approaches
sphere surface is converted to Ni(OH)2 and Co(OH)2 because in the future work are suggested.
no NiCo-MOF peaks are found. Thus, the regular hexagonal
nanosheets on the microsphere surface in Figure 9e can be de-
Experimental Section
duced to be Ni(OH)2 and Co(OH)2. Ni(OH)2 and Co(OH)2 are
regarded as metal hydroxides possessing high pseudocapaci-
tance.[20,26] Hence, the NiCo-MOF derivatives have been pro-
Materials: 4,4Ј-Biphenyldicarboxylic acid was purchased from Aladdin
Industrial Corporation, China. Ni(NO3)2·6H2O, Co(NO3)2·6H2O,
duced in the cycling process and they present a lower capacity KOH, DMF, EtOH were purchased from local reagent suppliers and
used without further purification. Solef 5130-PVDF particles were pur-
chased from Solvay Co., France. Super-P conductive carbon black
(Super-P) particles were purchased from Timcal Graphite Co., Switzer-
land. Nickel foams with a thickness of 1 mm and a density of
300 g·m–3 were purchased from Tianjin Aiweixin Chemical Technol-
ogy Co., China.
than NiCo-MOF. It mainly originates that the electrochemical
activity of nickel ions or cobalt ions in NiCo-MOF is much
higher than Ni(OH)2 or Co(OH)2, attributed to the particular
ordered coordination structure of NiCo-MOF. The SEM image
of NiCo-MOF after soaked in deionized water for 50 days is
also observed and showed in Figure 9h, which depicts that the
morphology of NiCo-MOF have been changed and some
Synthesis of NiCo-MOF via a Solvothermal Approach: First, BPDC
NiCo-MOF microspheres are also reconstructed and gathered (0.125 mmol) was dissolved in 22 mL DMF-EtOH solution with a
DMF/EtOH volume ratio of 10/1 whilst stirring at 40 °C. Next,
Ni(NO3)2·6H2O (0.125 mmol) and equimolar Co(NO3)2·6H2O were
added into the above solution and dissolved whilst stirring at 40 °C.
Then, the mixture was promptly transferred into a 45 mL Teflon lined
stainless-steel autoclave and heated for 12 h at 170 °C. After the natu-
ral cooling, the precipitate was washed several times with DMF and
EtOH in turn. Finally, the brown NiCo-MOF particles were achieved
with a Ni/Co/BPDC molar ratio of 1:1:1 after dried at 60 °C for 12 h
in vacuo.
after the 50-day soaking. It further indicates that NiCo-MOF
is not stable in aqueous electrolyte, owing to the hydrolysis of
coordination bonds, especially, in alkaline solution.
All the electrochemical data show that NiCo-MOF has the
pros and cons as supercapacitor electrode materials in aqueous
electrolytes. The optimal NiCo-MOF possesses a high capacity
even at high current density, low internal resistance, charge-
transfer resistance and ion diffusion impendence as electrode
materials in aqueous electrolyte, owing to the ordered coordi-
nation structure, 2D nanosheet structure and 3D assembled
microsphere structure of NiCo-MOF. The particular structures
make metal ions highly dispersed in NiCo-MOF microspheres
and entitle NiCo-MOF high electrochemical activity. However,
the cycling stability and rate capability are not ideal enough
due to the hydrolysis of coordination bonds in aqueous electro-
lytes. In a word, the high electrochemical activity of NiCo-
MOF can bring a large capacitance, but it results in a poor
For comparison, the brown NiCo-MOF particles with different Ni/Co/
BPDC molar ratios of 1:1:2 and 1:2:2 were also synthesized according
to the above process. Therein, The BPDC concentration was invariable
for synthesizing three NiCo-MOF particles. Moreover, the light pink
Co-MOF and the light green Ni-MOF particles with a Ni(Co)/BPDC
molar ratio of 2:1 were also synthesized according to the above pro-
cess, in which 0.25 mmol Co(NO3)2·6H2O or Ni(NO3)2·6H2O were
individually added. Furthermore, the brown NiCo-MOF particles with
a Ni/Co/BPDC molar ratio of 1:1:1 were prepared in a similar process
stability. If NiCo-MOF is expected to be stable, its activity has to the above at various temperature (150 °C, 190 °C) and time (6 h,
18 h).
to be sacrificed. In the future work, finding a suitable organic
electrolyte may be an effective way to enhance the cycling
stability of NiCo-MOF as well as deriving more stable skel-
eton materials from NiCo-MOF.
Measurements: The morphologies of samples were observed on a
Phenom/Pro scanning electron microscope, a FEI INSPECT F50 field
emission scanning electron microscope and a JEOL JEM-2100 trans-
mission electron microscope, respectively. The contents of nickel and
cobalt element in NiCo-MOF were determined by a Varian 715-ES
inductively coupled plasma optical emission spectrometer. The crystal-
line structures were analyzed with a Rigaku Mini Flex 600 dif-
fractometer. The FTIR spectra were recorded with a Thermo Fisher
Scientific Nicolet IZ10. The Raman spectra were collected with a Re-
nishaw InVia Raman spectrometer with 532 nm laser source at ambient
temperature. The specific surface area and pore structure were ana-
lyzed on a Micromeritics ASAP 2020 automatic physical and chemical
adsorption analyzer in nitrogen at –196.201 °C as an adsorbent. The
thermogravimetric (TG) analysis was performed on a Germany DSC
Conclusions
A series of hydrangea-like NiCo-MOF are successfully syn-
thesized via a solvothermal method and the optimal process
has been discussed. NiCo-MOF presents secondary 3D micro-
sphere structures assembled by primary 2D nanosheet struc-
tures, good crystalline structure and good thermal stability be-
low 350 °C in air. Compared with monometallic Ni-MOF and
Ni-MOF, bimetallic NiCo-MOF has a lower internal resis-
tance, lower charge-transfer resistance, lower ion diffusion im- 200PC & TG209C thermogravimetric/differential thermal analyzer
Z. Anorg. Allg. Chem. 0000, 0–0
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Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
from room temperature to 800 °C at a heating rate of 10 °C·min–1 in
air atmosphere.
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electrolyte solution, using an Autolab PGSTAT 302N electrochemical
workstation in a three electrode system. An Ag/AgCl electrode with
saturated potassium chloride electrolyte was selected as the reference
electrode. A platinum plate was used as the counter electrode. The
working electrodes were fabricated according to the reported
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PVDF particles were dissolved in DMF for preparing an adhesive solu-
tion. A mixed pasty electrode material was achieved after the PVDF
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Supporting Information (see footnote on the first page of this article):
The Supporting Information details the preparation of the samples in
Figure 9.
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Acknowledgements
The authors thank for the financial support by National Natural Sci-
ence Foundation of China (21406176, 51503169), and the advices and
assistance from Pooi See Lee group in Nanyang Technological Univer-
sity.
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Keywords: Metal-organic frameworks; Bimetallic; Solvother-
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Received: February 22, 2019
Published Online: ᭿
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Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Q. Li, X. Wang,* N. Yang, F. He, Y. Yang, B. Wu, J. Chu,
A. Zhou, S. Xiong* .......................................................... 1–10
Hydrangea-like NiCo-based Bimetal-organic Frameworks, and
their Pros and Cons as Supercapacitor Electrode Materials in
Aqueous Electrolytes
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