Nitrogen-doped carbon-enriched MOF and derived hierarchical carbons as electrode for excellent asymmetric aqueous supercapacitor

Article abstract of DOI:10.1016/j.jallcom.2021.158764

This article aims to use nitrogen-doped metal p-π conjugated-organic frameworks and derived carbon-rich electrode materials for high-performance electrode materials. Nitrogen-doped contributes to the energy storage performance, and the p-π conjugated-organic ligands of the conjugated system can not only improve the stability of the structure, but increase the conductivity of the material. Gradient carbonization can explore the maximum electrochemical potential of MOF-B, from which the previous 1271.0 increased to 2727.5 F·g^?1 at a current density of 1.0 A·g^?1. The excellent capacitance retention rate is 86.67% of the initial value at a high current density after 20,000 cycles. Besides, the MOF-B-600//AC also shows a high energy density of 63.62 Wh·kg^?1 at a power density of 400 W·kg^?1 and good coulomb efficiency of 90.7%. As a result, the excellent electrochemical performance of the nitrogen-doped and carbon-enriched MOF-B-600 has potential application prospects in energy storage devices.

Full text of DOI:10.1016/j.jallcom.2021.158764

Journal of Alloys and Compounds 867 (2021) 158764
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Nitrogen-doped carbon-enriched MOF and derived hierarchical carbons
as electrode for excellent asymmetric aqueous supercapacitor
⁎
Dongdong Huanga,c, Li Chenb, Liguo Yuea,b, Fan Yanga, Hao Guoa,d, , Wu Yanga,⁎⁎
a Key Lab of Bioelectrochemistry and Environmental Analysis of Gansu Province, Key Lab of Eco-Environments Related Polymer Materials of MOE, College of
Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, PR China
b School of Materials and Energy, Guangdong University of Technology, Guangzhou Higher Education Mega Center, No. 100 Waihuan Xi Road, Guangzhou 510006,
PR China
c CAS Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences (CAS), Lanzhou 730000, PR China
d Gansu International Scientific and Technological Cooperation Base of Water-Retention Chemical Functional Materials, Lanzhou 730070, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
This article aims to use nitrogen-doped metal p-π conjugated-organic frameworks and derived carbon-rich
electrode materials for high-performance electrode materials. Nitrogen-doped contributes to the energy
storage performance, and the p-π conjugated-organic ligands of the conjugated system can not only im-
prove the stability of the structure, but increase the conductivity of the material. Gradient carbonization can
explore the maximum electrochemical potential of MOF-B, from which the previous 1271.0 increased to
2727.5 F·g−1 at a current density of 1.0 A·g−1. The excellent capacitance retention rate is 86.67% of the initial
value at a high current density after 20,000 cycles. Besides, the MOF-B-600//AC also shows a high energy
density of 63.62 Wh·kg−1 at a power density of 400 W·kg−1 and good coulomb efficiency of 90.7%. As a result,
the excellent electrochemical performance of the nitrogen-doped and carbon-enriched MOF-B-600 has
potential application prospects in energy storage devices.
Received 11 November 2020
Received in revised form 21 December 2020
Accepted 13 January 2021
Available online 3 February 2021
Keywords:
MOF
p-π conjugation
Supercapacitor
Carbon enrichment
N-doped
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
main trend of current research. Metal organic framework (MOF)
nanomaterials combine the redox reaction of pseudocapacitance and
Energy storage materials and devices will be one of the key in-
dustries of the “5G Revolution”. The mature application and devel-
opment of supercapacitors will be the focus of attention in the fields
of future communications, energy, automobiles, and artificial in-
telligence (AI) products [1]. Traditional electrode materials for su-
percapacitors are mainly carbon materials, which have good
electrical conductivity and excellent cycle stability [2–4], but the
specific capacitance cannot meet people's needs. For transition
metal oxides [5], phosphides [6], sulfides [7], etc., they have high
specific capacitance, but the structure is unstable due to the phase
change of the electrochemical reaction [8], and the excellent cycle
stability of carbon materials cannot be achieved. The design of
electrode materials with excellent performance has become the
the stability of carbon based electric double layer capacitor (EDLC),
giving the supercapacitor a reliable and stable application value [9].
Compared with traditional carbon material supercapacitors, in-
vestigators are constantly exploring the durability characteristics of
high energy density on the basis of the application lifespan, through
the continuous modification and tame of the electrode mate-
rial [10–13].
Electrode materials with excellent electrochemical performance
are often formed through pure physical mixing, but the resulting
materials often do not have an excellent lifespan [14,15]. However,
based on the designable characteristics of MOF structure and func-
tion, the organic monomers used to synthesize MOF are usually
modified so that their structure contains target modifiers. Xu et al.
gave a detailed introduction and interpretation of the application of
new MOF materials in the field of energy storage [16]. In addition,
using typical MOF-5 as a precursor, a high-performance electrode
material by encapsulation of sulfur with hierarchically carbon was
synthesized by pyrolysis [17]. Using designed π-conjugated organic
ligands, the synthesized Ni3(HITP)2 electrode materials can exhibit
excellent cycle stability and low equivalent series resistance without
⁎
Corresponding author at: Key Lab of Bioelectrochemistry and Environmental
Analysis of Gansu Province, Key Lab of Eco-Environments Related Polymer Materials
of MOE, College of Chemistry and Chemical Engineering, Northwest Normal
University, Lanzhou 730070, PR China.
⁎⁎
Corresponding author.
E-mail addresses: haoguo12@126.com (H. Guo), xbsfda123@126.com (W. Yang).
https://doi.org/10.1016/j.jallcom.2021.158764
0925-8388/© 2021 Elsevier B.V. All rights reserved.

D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
any conductive agents(> 5000 S m−1) [18]. It can be seen that the
conductive MOF electrode material stabilizes the structure through
the conjugate system. In other words, it can make electronic devices
work for a long time. Furthermore, an electrode with accurate
carbon content can also be designed according to the length of the
organic ligand [19]. The introduction of heteroatoms into the organic
ligands can enhance the wettability of the electrode surface with the
electrolyte, which is more conducive to rapid electron transfer [20].
Good interface compatibility and fast charge transfer are un-
doubtedly creating good conditions for the kinetics of redox reac-
tions [21].
The metal ion (cluster) center of the MOF material is the active
center of the redox reaction, and the transition metal ion (cluster)
quickly gains and loses electrons during the charge and discharge
process, providing ultra-high specific capacity [22]. The backbone
(Organic ligand) in the MOF structure often acts as a conductive
medium in the electrode material, just like a wire connecting each
active site together [23]. In view of organic ligands, designing or-
ganic ligands that are liable to conduct electricity is also an im-
portant part of excellent MOF electrode material preparation. Almost
all reports of carbonization are to transform amorphous carbon to
graphite microcrystalline structure and increase its conductivity
[24]. This is one of the most effective methods to design a conductive
grid of a conjugate system plus a carbonization method to obtain a
highly conductive electrode material [25]. The combination of MOF
and carbon material provides ultra-high specific capacity and spe-
cific energy density through redox reaction, and the carbon material
consolidates the cycle stability and excellent conductivity of the
composite material.
2.2. Apparatus
Fourier transform infrared spectra (FT-IR) were achieved on a
FTS-3000 spectrometer (Finland) with an argon ion laser
(λ = 514 nm). The crystal structure of the materials was characterized
by X-ray powder diffraction (PXRD, Bruker D8 Advance), using Cu-K
(α) radiation (λ = 1.50405 Å). X-ray photoelectron spectroscopy (XPS,
Thermo Fisher Scientific, ESCALAB 250Xi, USA) was used to measure
the chemical composition and chemical state of the elements on the
surface of the material. The morphology and structure of the syn-
thesized materials were investigated using field emission scanning
electron microscopy (SEM, Zeiss Ultra Plus, Germany) and JEM-1200
EX transmission electron microscope (TEM, JEOL, Japan). Specific
surface area and pore size distribution (PSD) curves were de-
termined by the Brunauer–Emmett–Teller (BET) Micromeritics ASAP
2010 in a liquid nitrogen atmosphere at −196 °C. Electrochemical
tests such as cyclic voltammetry (CV), galvanostatic charge-dis-
charge (GCD) and electrochemical impedance spectroscopy (EIS)
were implemented on a CHI660E electrochemical workstation
(Chenhua, Shanghai, China). The cycle stability was performed on
the LAND CT2001A battery test system.
2.3. Synthesis of Ni-MOF
The p-π conjugated-organic ligands were synthesized by an ul-
trasonic-assisted method. The 0.02 mol (2.743 g) PABA and 0.01 mol
(1.341 g) TPAL are respectively dissolved in 15 mL of hot ethanol
solution to form a transparent solution, and then the two solutions
are mixed and sonicated. A large amount of yellow solid matter
appeared in the reaction liquid, which was obtained by sedimenta-
tion filtration and recrystallization to improve the purity of the li-
gand. The components in the recrystallized product were analyzed
by thin layer chromatography (TLC). When the recrystallized product
showed only a single spot on the TLC plate, it proved that the pro-
duct was effectively purified. Finally, the product was dried at 80 °C
for 24 h under the vacuum oven and called ligand A (Yield: 2.976 g).
In this way, ligand B was obtained with 3ABA and TPAL.
The MOF were synthesized by a one-step solvothermal method
according to the reported approach [26]. In a typical synthesis,
0.7428 g ligand A, 0.2908 g Ni(NO3)2·6H2O, were dissolved in 20 mL
DMF, add a small amount of hexadecyl trimethyl ammonium bro-
mide (10 mg) as a templating agent and 5 mL triethylamine as a
–COOH release agent with sonicating in an ultrasound bath for
15–20 min at room temperature. Then it was heated under con-
trolled temperature at 85 °C for 2 days in a Teflon-lined stainless
steel autoclave. The resulting colorless crystals were collected, wa-
shed with DMF and alcohol successively. Finally, the product was
dried at 60 °C for 12 h under the vacuum oven, the product MOF-A
was obtained (Yield: 0.6871 g, 66.8% based on Ni). The product
MOF-B was obtained by the above-mentioned method (Yield:
0.4378 g, 42.5%).
In this paper, we reported a simple approach to synthesize two
kinds of p-π conjugation and nitrogen-containing organic ligands by
Schiff base reaction, and then coordinate with transition metal to
form nitrogen-doped and carbon-enriched electrode materials for
high-performance electrode materials. After gradient carbonization,
the multi-hierarchical, carbon-rich, nitrogen-containing MOF mate-
rials can be realized, which not only promotes the increase of MOF
graphitization, but also constructs more active sites for redox reac-
tions. Benefit from the design of material structure and the im-
provement of conductivity, MOF-B-600 showed an excellent specific
capacitance of 2727.5 F·g−1 at a current density of 1 A·g−1 and a ca-
pacitance retention of 86.67% after 20,000 cycles. An asymmetric
aqueous supercapacitor MOF-B-600//AC manifested an energy den-
sity of 63.62 Wh·kg−1 with a maximum power density of 400 W·kg−1
.
High-performance electrochemical properties indicate that the de-
veloped nitrogen-doped and carbon-enriched MOF electrode mate-
rial is expected to become potential electrode materials for practical
applications.
2. Experimental
2.1. Preparation of materials
2.4. Carbonization of materials
4-Aminobenzoic acid (PABA, 99.3%) and 1,4-phthalaldehyde
(TPAL, 98%) were purchased from Aladdin Reagent Inc. 3-
Aminobenzoic acid(3ABA,99%) was purchased from Beijing HWRK
Co., Ltd. Ethanol (99.8%), N, N-dimethylformamide (DMF, 99.7%)
were purchased from Rionlon. Ni(NO3)2.6 H2O(98.0%) was purchased
from Tianjing Zhiyuan Chemical Reagent Co., Ltd. Activated carbon
(AC) was purchased from Shanghai Sino Tech Investment
Management Co., Ltd. (China), with a specific surface area of more
than 2100 m2·g−1 and a particle size of about 10 µm. 5 wt%
Polytetrafluoroethylene (PTFE, 60 wt% dispersion in water) was also
purchased from Aladdin Reagent Inc. The deionized water was ob-
tained by laboratory secondary distillation. All reagents were ana-
lytical grade, which were employed without further purification.
The prepared material was evacuated in a tube furnace, which
was filled with nitrogen gas. After repeating three times, it was
heated to 400 °C, 500 °C, 600 °C and 700 °C at a rate of 5 °C/min for
2 h, and then was cooled to room temperature and afterward named
MOF-B-T(MOF-B-400, MOF-B-500, MOF-B-600, MOF-B-700).
2.5. Preparation of electrode
MOF-A (5 mg, 80 wt%), acetylene black (0.93 mg, 15 wt%) and
PTFE aqueous solution (5 wt%) were mixed in ethanol. After ultra-
sonic treatment for 30 min, the mixture was placed in an infrared
lamp to dry. Weigh two nickel foam with a size of 1 × 2 cm2, the
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D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
mixture is ground to paste and smeared evenly on an area of
1 × 1 cm–2 of nickel foam surface, two electrodes were prepared with
a similar mass of active substances. Each electrode sheet is loaded
with about 2 mg·cm−2 of active material. Then they were dried at
50 °C for 24 h under the vacuum oven. The nickel foams were
pressed under 10 MPa for 20 s, to prevent the coated active material
from falling off during the test. Weigh the quality of nickel foamed
before and after coating, calculate the effective active material mass
to evaluate electrochemical performance.
three-electrode system with 6 M KOH aqueous solution as the
electrolyte. Two electrode sheets with close mass of active charge
were weighed, and an asymmetric supercapacitor was assembled
with 6 M KOH aqueous solution as the electrolyte. Estimate the
specific capacitance (Cm) of a single electrode in a three-electrode
configuration from the CV curves [27]:
V
c
1
Cm
=
I (V)dV
V
a
m (V
Va)
(1)
c
In this equation, ʋ represents the scan rate, m is the mass of
active material (g), Vc and Va refer to high and low potential limit of
the CV measurement, and I is the instant current.
2.6. Calculations
The working electrode, the counter electrode (CE, 1 mm platinum
wire) and the reference electrode (RE, Hg/Hg2Cl2) are involved in the
Besides, the specific capacity of the materials can also be calcu-
lated from discharging time according to the following Eq. (2) [28]:
Fig. 1. (a) SEM, (b) TEM, (c) HRTEM and (d) SADE pattern of MOF-A; (e) SEM, (f) TEM, (g) HRTEM and (h) SADE pattern of MOF-B.
3

D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
I
m
t
V
represent the (001) (101) and (12-1) crystal planes from the inside
to the outside.
Cm
=
(2)
The FT-IR spectra can be used to determine the structure of the
synthetic material. Fig. 3a shows the infrared spectra of PABA, ligand A
and ligand B. Strong skeleton vibrations appear at 1640–1600 cm−1 and
584 cm−1, during to the C═N [32] of synthetic organic ligands. From the
infrared spectra of ligand A and ligand B, it was also found that the band
of aldehyde C-H in raw material PABA disappeares at 2750 cm−1, in-
dicating that the ligands were synthesized successfully. There is no C═O
peak at 1720–1700 cm−1 [33] in the infrared spectra of MOF-A and MOF-
B in Fig. 3b, indicating that the O on the carboxyl group involves in
coordination. At the same time, the stretching vibrations of water mo-
lecules at around 3420–3400 cm−1 and 3700–3600 cm−1 indicate that
coordinated H2O molecules and a small number of raw materials are
present within the Ni-based material [34]. In PXRD diffraction patterns
of the MOF-A and MOF-B, the sharp and strong diffraction peaks prove
that both materials have good crystallinity [35] (Fig. 3c and d). The
diffraction peak positions of MOF-A and MOF-B are the same, and
strong diffraction peaks at 2θ = 2.55°, 4.21°, 8.81°, 38.16° and 51.61° are
indexed to the (001), (101), (12-1), (32-1) and (334) planes, respectively.
The nitrogen adsorption-desorption isotherms of MOF-A and MOF-B
(Fig. 3e and f) display a type IV curve, which exhibits a larger BET
specific surface area of 141.61 m2·g−1 and 161.50 m2·g−1. Both of them
present an obvious hysteresis loop in the relatively high pressure (P/P0
≥ 0.60) and a monolayer and multilayer adsorption at low relative
pressure (P/P0 < 0.60) [36]. The pore size distribution curves of MOF-A
and MOF-B in the inset show that the pore size distribution are 13.60
and 17.17 Å respectively, which belongs to a microporous type and more
favorable to charge transfer. The BET specific surface area of MOF-B-600
increased from 161.50 to 209.9 m2·g−1, which indicates carbonization is
beneficial for removing impurities in MOF pores (Fig. S1, Supplemen-
tary Information). Increasing specific surface area is more conducive to
the occurrence of redox reactions. However, the pore size distribution is
reduced to 12.68 Å, which is also closely related to the metal nitride
formed during the carbonization process.
The existence of C, N, O, and Ni elements are verified by the XPS
spectrum (based on Cls = 284.6 eV) of MOF-A and MOF-B, which
confirms that organic ligands and metal ions (clusters) successfully
combined through coordination bonds (Fig. 4a and c). As presented
in the high-resolution XPS spectra Fig. 4b and d, there are two major
peaks at 873.4 and 855.73 eV with a spin-orbital energy separation
of 17.67 eV, corresponding to Ni 2p1/2 and Ni 2p3/2, respectively. The
two other peaks centered at around 879.76 and 861.73 eV corre-
sponded to Ni 2p1/2 and Ni 2p3/2 satellites, respectively, and the
appearance of Ni2+ and Ni3+ also provides the possibility for redox
reactions [37]. Thermal gravimetric analysis (TGA) graph of the MOF-
A and MOF-B are shown in Fig. 4e and f curves trend can be divided
into three sections, the first loss of water and solvent, followed by
the unstable bond fracture and the final structure collapse. Differ-
ential thermal analysis (DTA) shows that the Ni atoms of MOF-A and
MOF-B are oxidized at 310–350 °C, followed by the C and N atoms of
the organic ligands are oxidized at 450–500 °C, and all are exo-
thermic reaction. Near the 936 °C is an endothermic reaction due to
the decomposition of the metal oxide.
where I refers to the discharge current (A), t represents the discharge
time (s), m (m = ma + mc in asymmetric system) is the mass of active
material (g), and ∆V is the range of GCD voltage (V).
Asymmetric supercapacitors (ASC) can measure the practical
application value of electrode materials. In order to ensure that the
anode (MOF-B-600) and cathode (AC) charges are equal, calculate
the mass of the anode and cathode loaded active material according
to Eq. (3) [29]:
m+
m
C ·
V
V
R =
=
C+·
(3)
+
Energy density (E, Wh·kg−1) and power density (P, W·kg−1) cal-
culations are based on Eqs. (4) and (5) [30,31]:
Cm V2
E =
(4)
(5)
2
E
t
P =
where Cm is the specific capacitance of the symmetric super-
capacitors calculated according to the GCD curves, ∆t (s) and ∆V (V)
represent discharge time and voltage range (∆V=V– + V+).
3. Result and discussion
3.1. Structure and morphology characterization
Fig. 1 shows the microstructure, the different morphology of
MOF-A and MOF-B is due to the difference in the structure of their
organic ligands. Fig. 1a and e show the SEM of MOF-A and MOF-B,
both of which present petal-like structures. However, there are
obvious differences in size and thickness and MOF-B shows a
larger flake shape and a thinner thickness. The SEM image of the
graded carbonized MOF-B, meticulous surface structure change
and the distribution of surface elements carbonized at 600 °C are
shown in Fig. 2. It can be seen from Fig. 2(a–e) that as the carbo-
nization temperature increases the MOF dereved materials be-
come loose and orderly, however, the structure agglomerates and
the regularity of order is broken at a high temperature of 700 °C.
From the meticulous structure of carbonization matters showed in
Fig. 2(h–j), it is also found that as the increase of carbonization
temperature the number of surface nanoparticles is also in-
creasing, which may be attributed to the formation of metal ni-
trides. The EDS mapping of MOF-B-600 shows that the types of
elements are consistent with XPS, and there is no obvious change
after carbonization (Fig. 2(k–n)). It can also be seen from the ele-
ment distribution that the elements in the selection are evenly
distributed. From its element content distribution (Table S1, Sup-
plementary Information), it is not difficult to find that MOF-B-600
is a carbon-rich electrode material, and this characteristic can be
traced back to the organic ligand for the synthesis of MOF-B. It can
be seen from the TEM image that the sheet-like structure of MOF-A
is not well presented due to agglomeration, while the loose
structure of MOF-B shows corresponding wrinkles (Fig. 1b and f).
In the high-resolution transmission electron (HRTEM) microscope
image (Fig. 1c and g), displaying a clearer crystal lattice the dis-
tance between 10 lattices can be accurately measured by the
software (Gatan Digital Micrograph) matched with the instrument,
and the average value can be obtained. It is known that the lattice
spacing of MOF-A and MOF-B on the (001) crystal plane is 0.205
and 0.232 nm, respectively. The selected area electron diffraction
(SAED) pattern in Fig. 1d and h are made up of concentric circles,
indicating that the nanoparticles MOF-A and MOF-B are poly-
crystalline crystalline systems. The clear concentric circles
3.2. Electrochemical characterization
The electrochemical performances of MOF-A and MOF-B are
presented in Fig. 5, which are measured in a three-electrode system
in 3.0 M KOH aqueous solution. CV curves exhibit a pair of redox
reaction peaks, which prove that MOF-A and MOF-B electrode ma-
terials are pseudocapacitance behavior (Fig. 5a). This feature is also
confirmed in the GCD curves (Fig. 5b), and the middle position of the
small house-shaped GCD curve is also the section where the redox
reaction proceeds. It can be seen from Eq. (1) the closed area of the
CV curve represents the specific capacitance. At a scan rate of
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D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
Fig. 2. (a–j) SEM diagrams of MOF-B at different temperatures, (k–n) EDS mapping images of MOF-B-600.
5

D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
Fig. 3. (a, b) FT-IR spectra, (c, d) PXRD patterns and (e, f) N2 adsorption-desorption isotherm (The inset is the pore size distribution curves).
80 mV·s−1, the closed area of MOF-B is significantly higher than that
of MOF-A, which proves that the specific capacitance of MOF-B is
higher than that of MOF-A. Meanwhile, the discharge time of MOF-B
Fig. S3. The charge-storage mechanism may be probably explained
by the following redox reactions:
[Ni3(OH)2(C22H14O4) 2·(H2O)4]·2H2O+OH––e– ↔ [Ni3O(OH)(C22H14O4)
·
is also longer than that of MOF-A at a current density of 1.0 A·g−1
.
2
(H2O)4]·2H2O+H2O
(6)
According to Eq. (2), the specific capacitance of MOF-B is also greater
than that of MOF-A. The CV curves at different scan rates and the
GCD curves at different current densities are shown in detail in
The specific capacitance at the various current densities is shown
in Fig. 5c, and it is found that the specific capacitance of MOF-B is
6

D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
Fig. 4. XPS survey spectrum and Ni 2p high resolution scan (a, b) MOF-A, (c, d) MOF-B, TGA and DTA curves of (e) MOF-A, (f) MOF-B.
always greater than that of MOF-A. The maximum specific capaci-
tances of MOF-A and MOF-B are 706.7 and 1271.0 F·g−1 at a current
density of 1.0 A·g−1. When the current density is increased by 20
is only 76.19%. It can be seen that MOF-B can be developed and
applied in real life as a high-performance electrode material. EIS
measurements were performed to further understand the dynamics
of charge transfer in electrode materials, and the results are pre-
sented in Fig. 5e. It is found the intercept of the arc and horizontal
axis is similar in the high-frequency region, which represents their
resistance of the electrolyte and the electrode is not much different.
The straight line close to 45° in the middle-frequency region reflects
the Warburg impedance (W). The slope value represents the ion
diffusion/transport speed in electrolyte at the low-frequency region.
The slope of MOF-B in the low-frequency region is slightly larger
than that of MOF-A, indicating that it has certain advantages in
times, the specific capacitances of the two are 409.5 and 660 F·g−1
,
respectively. The comparison of MOF-A and MOF-B with other
reported materials is shown in Table 1. It can be seen that the
synthesized two MOF materials have certain advantages in electro-
chemical performance. Especially, MOF-B can be developed and
applied in real life as a high-performance electrode material.
After 10,000 cycles at a current density of 10 A·g−1, the specific
capacitance of MOF-B remains at 93.10% of the initial capacitance
(Fig. 5d). However, the specific capacitance retention rate of MOF-A
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D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
Fig. 5. Electrochemical performance of the MOF-A and MOF-B in 3 M aqueous KOH electrolyte: (a) CV curves at a scan rate of 80 mV·s−1, (b) GCD profiles at a current density of
0.5 A·g−1, (c) the curves of specific capacitance versus current density, (d) Cycling performance, (e) Nyquist plots in the frequency range of 100 kHz–0.01 Hz (The inset is the
equivalent circuit), (f) the linear relation between the peak current and the square root of scan rates.
charge transfer. According to the equivalent circuit (Fitted by the
ZSimDemo software), Rs(0.61 Ω), Cdl, Rct (2.55 Ω), Q and W re-
present the solution impedance, double-layer capacitors, the re-
sistance between the electrode and the electrolyte, constant phase
angle components and Warburg impedance, respectively. The peak ip
of MOF-A and MOF-B has an approximately linear relationship with
the square root of the scan rate (Fig. 5f), and the fitting parameters
are displayed in Table S2. The linear parameters obtained by this
fitting indicate that the redox reaction at the electrolyte|electrode
interface conforms to the quasi-reversible and diffusion control
process [47,48].
Because MOF-B has outstanding advantages in electrochemical
performance, it is carbonized at a gradient temperature to study the
trend of its electrochemical performance with temperature change.
MOF-B material carbonized at gradient temperature still exhibits
pseudo-capacitance behavior from Fig. 6a and b. However, compared
with other temperature carbonized materials, MOF-B-600 has the
largest area of closed CV curves and the longest discharge time of
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D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
Table 1
The current (i) with the scan rate (ν) obeys the power law, as
shown in Eqs. (7) and (8) [50,51].
Comparison of electrochemical performances with other different materials.
Electrode materials
Voltage
range (V)
current
density
Specific
Ref.
b
(7)
(8)
i = a
capacitance
(A·g−1
)
(F·g−1
)
log i = log a + b log
65Ni-MDH
ZIF-ppy-2
Zn/Ni-MOF
Zn-MOF/PANI(ZMP)
Co-MOF
Co/Ni-MOF/CFP
Ni-MOF/CNT
rGO-Ni-doped MOF
Ni-based-MOF
MOF-A
0～0.5
0～0.6
0～0.35
0～0.8
0～0.6
0～0.5
-0.1～0.6
0～0.5
0～0.5
0～0.4
0～0.4
1.0
215
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
This
work
0.5
0.25
1.0
0.6
2.0
0.5
0.05
1.0
554.4
1620
477
206.76
1044
1765
758
726
706.7
1271.0
The value of b is related to the electrochemical process of the
material. b ≤ 0.5 means battery type, on the contrary, 0.5 < b ≤ 1
means mainly capacitance. After taking the logarithm of Eq. (7),
draw the relationship diagram between logi and logv (Fig. 6e), and
the relationship shows a linear function. Extracting the slope value
(b value) of the linear correlation line, it mainly shows the me-
chanism of the battery below 0.15 V, while it is the capacitance
characteristic at other potentials (Fig. 6f). It is generally believed that
the CV current is composed of diffusion current and pseudocapaci-
tance current. Eqs. (9) and (10) [52] illustrate this entire well.
1.0
1.0
MOF-B
i(V) = k1v + k2v1/2
(9)
GCD curves. That is to say, 600 °C is the optimal carbonization
temperature. Compared with MOF-B, carbonization product can
achieve bigger potential without changing the original framework.
At the same time, the ymmetry of the redox peak and charge-dis-
charge time of the carbonized material has also been improved to a
certain extent, indicating that the electrochemical reversibility of the
electrode material has also been improved [49]. After calculation,
the specific capacitances of MOF-B-400, MOF-B-500, MOF-B-600
and MOF-B-700 are 489.75, 1013, 2727.5 and 1280 F·g−1 at a current
density of 1.0 A·g−1. Nyquist plots of different carbonization tem-
peratures can find that the intercept between the arc and the hor-
izontal axis does not change significantly (Fig. 6c). However, the
overall slope of the low-frequency region is reduced, indicating that
the wettability of the high-temperature carbonized material be-
comes poor, which is not conducive to the diffusion of electric
charges.
i(V)/v1/2 = k1v1/2 + k2
(10)
After calculation, we found that 72.71% of the total current arose
from the capacitive process at 60 mV·s−1, which was shown in
Fig. 6g. The contribution rates of the two mechanisms at different
scan rates are shown in Fig. 6h. In the process from 5 to 100 mV·s−1
,
the contribution of the capacitance increased from 58.94% to 76.25%,
which indicates that most of the charge storage in the MOF-B-600
electrode is a capacitance process, which facilitates fast rechargeable
storage.
3.3. Electrochemical performance of MOF-B-600//AC asymmetric
supercapacitors
The electrochemical properties of the water-based asymmetric
supercapacitor assembled with MOF-B-600 and AC are shown in
Fig. 7. In the three-electrode system, CV curves of MOF-B-600 and AC
at a 60 mV·s−1 show that their ΔV values are 0.6 and 1.0 V, respec-
tively (Fig. 7a). Theoretically, the asymmetric supercapacitor po-
tential window is the sum of the potential windows under the three-
electrode system, and this conjecture has also been confirmed in
subsequent work. Fig. 7b displays the CV curves of the MOF-B-600//
AC ASC under different potential windows, and slight polarization at
the positive electrode appears in the potential window of 0–1.6 V, so
the maximum potential window is 0–1.6 V. This method can provide
an effective theoretical basis for selecting the best potential window.
The area of the CV curve increases with the increase of the scan rates
(Fig. 7c), and present a quasi-rectangular shape. This shows that the
asymmetric MOF-B-600//AC supercapacitor mainly exhibits the
characteristics of carbon materials after assembly, which is in-
separable from the characteristics of the long-chain organic ligands
in MOF-B. The GCD curves at different current densities are shown in
Fig. 7d, and the long discharge time is attributed to the high specific
capacitance of MOF-B-600. It can be seen that the formed MOF-B-
600//AC asymmetric supercapacitor is dominated by two electro-
chemical behaviors. The GCD curves show that the specific capaci-
tances are 207.1, 180.5, 163.9, 126.8 and 79.4 F·g−1 at current densities
of 0.5, 1.0, 2.0, 5.0, 10.0 A·g−1, respectively. Fig. 7e shows the re-
lationship between the energy density and the power density of the
MOF-B-600//AC. It can be seen that MOF-B-600//AC supercapacitor
exhibits a high energy density of 63.62 Wh·kg−1 at a power density of
400 W·kg−1. The coulomb efficiency (the ratio of the discharge time
to the charge time) shows a trend that first decays, then increases,
and finally becomes constant (finally 90.7%, Fig. 7f). The appearance
of this phenomenon is related to the long-term soaking of the
electrode sheet, which causes the active material to be inactivated or
And through the contact angle experiment, it is also confirmed
that as the carbonization temperature increases, the contact angle
between the electrolyte and the electrode material also increases
with the degree of carbonization, and the wettability of the elec-
trode material gradually deteriorates (Fig. S2). The precursors
MOF-A and MOF-B both show good wettability, with contact an-
gles of 26.26 and 27.3°, respectively. However, after gradient
temperature carbonization, the contact angle increases con-
tinuously with the increase of carbonization temperature(66.4→
112.5°). This phenomenon is attributed to the gradual conversion
of metal ions (clusters) in the precursor into oxides, resulting in
the gradual decline of the infiltration effect of the electrolyte. The
four-point probe van der Pauw technology also confirmed the ef-
fect of gradient carbonization on the conductivity of the material
(Table S3). It can be seen from the table that the conductivity of the
precursors MOF-A and MOF-B is obviously not as good as that of
the carbonized MOF-B-T. Moreover, the conductivity will continue
to increase with the increase of the carbonization temperature,
until the material is completely carbonized and its conductivity no
longer changes significantly. Therefore, gradient carbonization can
effectively increase the conductivity of the electrode material, and
its value will increase by several orders of magnitude. The stability
of the MOF-B-600 was tested at a current density of 10 A·g−1 for
20,000 cycles (Fig. 6d). After 10,000 cycles of testing, the decrease
in capacitance retention rate may be due to the inactivation or
shedding of the active material of the electrode sheet, while the
increase at ~12,000 may be due to the activation of the internal
active material, and finally specific capacitance retention rate is
stabilized at 86.67%. The CV curve of MOF-B-600 is studied in
detail, and the important mechanism that affects the behavior of
the battery or capacitor is learned.
9

D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
Fig. 6. Electrochemical performance of MOF-B with different carbonation temperature in three-electrode system: (a) CV curves at 60 mV·s−1, (b) GCD profiles at 1.0 A·g−1, (c)
Nyquist plots, (d) Cycling performance of MOF-B-600 at 10 A·g−1, (e) log (v) versus log (i) plot, (f) b-values versus potential, (g) the capacitive contribution at a scan rate of
60 mV·s−1, (h) the ratio of surface capacitive effects and diffusion controlled intercalation contributions.
10

D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
Fig. 7. Electrochemical performance of MOF-B-600//AC: (a) CV curves of AC and MOF-B-600 at a 60 mV·s−1, (b) CV curves of MOF-B-600//AC under different potential windows at
a 50 mV·s−1, (c) GCD profiles of MOF-B-600//AC at different scan rates, (d) GCD profiles of MOF-B-600 at different current densities, (e) Ragone plot of MOF-B-600//AC asymmetric
supercapacitor [51-57], (f) Coulombic efficiency.
fall off, and then the active material inside the electrode sheet is
activated, showing an upward trend.
performance and the application research of carbon-enriched
high-performance electrode materials are explored. Carbonized
MOF-B shows an excellent electrochemical performqance, with a
specific capacitance of 2727.5 F g−1 at 1.0 A g−1. In addition, MOF-B-
600 in the two-electrode system, the energy density and power
density were 63.62 Wh kg−1 and 400 W kg−1. The nitrogen-con-
taining conjugated and carbon-enriched MOF and its carbonized
derivative electrode materials have good research value and ap-
plication prospects.
4. Conclusion
In summary, nitrogen-doped organic p-π conjugated ligands are
synthesized by a simple Schiff base reaction with an ultrasound-
assisted method to synthesize carbon-enriched MOF electrode
materials. The influence of isomer structure on electrochemical
11

D. Huang, L. Chen, L. Yue et al.
Journal of Alloys and Compounds 867 (2021) 158764
CRediT authorship contribution statement
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[22] P.A. Shinde, Y. Seo, S. Lee, H. Kim, Q.N. Pham, Y. Won, S. Chan Jun, Layered
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metal-organic framework with high electrical conductivity for electrochemical
sensing, J. Chin. Chem. Soc. 66 (2019) 522–528.
Dongdong Huang: Writing - review & editing. Li Chen: Data
curation. Liguo Yue: Investigation, Writing
Fan Yang: Investigation, Resources. Hao Guo: Project administration.
Wu Yang: Writing - review & editing, Funding acquisition.
- original draft.
Declaration of Competing Interest
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
[24] K. Liu, X. Zheng, K. Wang, C. Wang, M. Chen, Sodium metal-assisted carboniza-
tion of pyrrole to prepare N-doped porous carbons for high-rate performance
supercapacitors, Carbon 153 (2019) 265–273.
Acknowledgments
[25] J. Cai, Y. Song, X. Chen, Z. Sun, Y. Yi, J. Sun, Q. Zhang, MOF-derived conductive
carbon nitrides for separator-modified Li–S batteries and flexible super-
capacitors, J. Mater. Chem. A 8 (2020) 1757–1766.
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solvothermal synthesis of BiOI microsquares as a novel electrode material for
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Authors were very grateful to National Natural Science
Foundation of China (21665024, 21765019) and Key Lab of Polymer
Materials of Gansu Province for their financial supports.
Appendix A. Supporting information
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Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jallcom.2021.158764.
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