Highly N/O co-doped ultramicroporous carbons derived from nonporous metal-organic framework for high performance supercapacitors

Article abstract of DOI:10.1016/j.cclet.2020.09.029

A new nonporous Zn-based metal-organic framework (NPMOF) synthesized from a high nitrogen-containing rigid ligand was converted into porous carbon materials by direct carbonization without adding additional carbon sources. A series of NPMOF-derived porous carbons with very high N/O contents (24.1% for NPMOF-700, 20.2% for NPMOF-800, 15.1% for NPMOF-900) were prepared by adjusting the pyrolysis temperatures. The NPMOF-800 fabricated electrode exhibits a high capacitance of 220 F/g and extremely large surface area normalized capacitance of 57.7 μF/cm² compared to other reported MOF-derived porous carbon electrodes, which could be attributed to the abundant ultramicroporosity and high N/O co-doping. More importantly, symmetric supercapacitor assembled with the MOF-derived carbon manifests prominent stability, i.e., 99.1% capacitance retention after 10,000 cycles at 1.0 A/g. This simple preparation of MOF-derived porous carbon materials not only finds an application direction for a variety of porous or even nonporous MOFs, but also opens a way for the production of porous carbon materials for superior energy storage.

Full text of DOI:10.1016/j.cclet.2020.09.029

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Highly N/O co-doped ultramicroporous carbons derived from nonporous
metal-organic framework for high performance supercapacitors
Yangyi Gu, Ling Miao, Ying Yin, Mingxian Liu, Lihua Gan, Liangchun
Li
PII:
S1001-8417(20)30533-7
https://doi.org/10.1016/j.cclet.2020.09.029
CCLET 5858
DOI:
Reference:
To appear in:
Chinese Chemical Letters
Accepted Date:
18 September 2020
Please cite this article as: Gu Y, Miao L, Yin Y, Liu M, Gan L, Li L, Highly N/O co-doped
ultramicroporous carbons derived from nonporous metal-organic framework for high
performance supercapacitors, Chinese Chemical Letters (2020),
doi: https://doi.org/10.1016/j.cclet.2020.09.029
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Communication
Highly N/O co-doped ultramicroporous carbons derived from nonporous metal-
organic framework for high performance supercapacitors
Yangyi Gu, Ling Miao, Ying Yin, Mingxian Liu, Lihua Gan, Liangchun Li*
Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai
200092, China
*Corresponding author.
E-mail address: lilc@tongji.edu.cn
Graphical Abstract
A new nonporous Zn-based metal-organic framework readily synthesized from a high nitrogen-containing ligand can be
converted into ultramicroporous carbons with narrow pore size distribution and 20.18% N/O co-doping by direct carbonization.
The NPMOF-800 fabricated electrode exhibits very high capacitance of 220 F/g and shows excellent energy, power density, and
99.1% capacitance retention rate after 10000 charge-discharge cycles.
Article history:
Received 10 August 2020 Received in revised form 15 September 2020 Accepted 16 September 2020 Available online
ABSTRACT
A new nonporous Zn-based metal-organic framework (NPMOF) synthesized from a high nitrogen-containing rigid ligand was converted into porous
carbon materials by direct carbonization without adding additional carbon sources. A series of NPMOF-derived porous carbons with very high N/O
contents (24.1% for NPMOF-700, 20.2% for NPMOF-800, 15.1% for NPMOF-900) were prepared by adjusting the pyrolysis temperatures. The
NPMOF-800 fabricated electrode exhibits a high capacitance of 220 F/g and extremely large surface area normalized capacitance of 57.7 µF/cm2
compared to other reported MOF-derived porous carbon electrodes, which could be attributed to the abundant ultramicroporosity and high N/O co-
doping. More importantly, symmetric supercapacitor assembled with the MOF-derived carbon manifests prominent stability, i.e., 99.1% capacitance

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retention after 10000 cycles at 1.0 A/g. This simple preparation of MOF-derived porous carbon materials not only finds an application direction for a
variety of porous or even nonporous MOFs, but also opens a way for the production of porous carbon materials for superior energy storage.
Keywords:
N/O co-doped Ultramicroporous Metal-organic frameworks Porous carbons Supercapacitors
With the increasing demand for energy and power, it is urgent to find suitable energy storage equipments. As novel energy storage
devices, supercapacitors have received more and more attention due to their higher power density, faster charge-discharge rate and longer
cycle life than conventional batteries [1, 2]. Supercapacitors can be divided into two types [3]. One is capacitors that undergo a Faraday
electrochemical reaction at the electrode surface, also known as pseudocapacitors. The other is called electrochemical double-layer
capacitors (EDLCs), which store energy by accumulating static charge in the electric double layer at the interface of the electrolyte and
the electrode. EDLCs usually use porous carbons with high surface area as electrode materials.
The capacitance of supercapacitors mainly depends on the electrode materials. Carbon materials are the earliest electrode materials
used in capacitors with large surface area, good electrical conductivity, thermal and chemical stability. They are also rich in resources
and inexpensive [4, 5]. For the future applications of porous carbon materials to supercapacitors, it mainly requires that porous carbon
materials possess high surface area, reasonable pore size distribution, and high electrical conductivity and wettability [6‒8]. In the field
of energy storage, the high surface area is beneficial to increase the storage sites of carbon materials and the device energy density. But
it is not general that the larger the surface area means the better performance. The matching of pore size and electrolyte ion size is also
an important factor affecting the electrochemical properties [9, 10]. Generally, porous carbons with uniform pore size distribution exhibit
higher energy density. In addition, by introducing heteroatoms such as B, N, P and S, the wettability of materials can be improved and
the pseudo-capacitive effect can also be introduced to enlarge the electrochemical capacitance [11, 12]. Porous carbon materials are
readily prepared by pyrolysis of organic precursors (e.g., polymers) and subsequent physical or chemical activation [13‒16]. Albeit this
method can produce porous carbon materials with high surface area, the pore size distribution is generally wide and the structure is highly
disordered, impeding realization of the pore-electrolyte matching and further functionalization. In order to solve this problem,
carbonization of crystalline metal-organic frameworks (MOFs) at various temperatures to produce relatively ordered porous carbons has
been emerging as an effective strategy.
MOFs are crystalline materials constructed by self-assembly of organic ligands and metal ions (or clusters). As a new type of porous
materials, due to their large surface area, adjustable pores and active metal sites, MOFs show extraordinary potential in gas storage and
separation, catalysis, ion exchange and electrochemistry [17‒22]. MOFs have become the most attractive precursors and templates for
the preparation of porous carbons due to their rich diversity and highly ordered porous structures [23, 24]. Thus MOF-derived carbon
materials often exhibit relatively controllable and narrow pore size distribution as well as good electrical conductivity and easy
heteroatom-doping as active sites [6]. Xu et al. first introduced furyl alcohol (FA) into the pores of MOF‒5 (Zn as the metal), and then
carbonized in the argon atmosphere at 1000 °C to produce a porous carbon material with very high specific surface area (2872 m2/g) and
pore volume (2.06 m3/g) [25]. Since then many other MOFs have been utilized in the field of electrochemistry as energy storage and
conversion materials in virtue of their merits [26‒32]. Among them, ZIFs rich in N heteroatoms of their organic ligands are frequently
used to prepare electrode materials for supercapacitors [33‒35]. Chaikittisilp et al. carbonized commercially available ZIF‒8 without any
other carbon sources, and the resultant porous carbons exhibited high electrochemical performance [36]. Except for the narrow pore size
distribution and heteroatoms doping, ultramicroporosity of the porous carbons has been long-sought since it is propitious to efficient
transportation of the small electrolyte ions and extremely high cycle stability, yet challenging in synthesis [9]. Direct carbonization of
microporous or nonporous MOFs (NPMOFs) could be a promising and efficient way to prepare ultramicroporous carbons. In particular,
numerous NPMOFs have hitherto been reported but much less investigated because of the useless nonporosity, and therefore exploration
of their conversion to intriguing ultramicroporous carbons for high-performance electrodes are of great significant and worthy of
intensive study.
In this context, an organic ligand with very high nitrogen content was successfully prepared and used to synthesize NPMOF. The
NPMOF was directly carbonized under the temperature of 700 °C, 800 °C and 900 °C to obtain porous carbon materials, which were
denoted as NPMOF-700, NPMOF-800 and NPMOF-900, respectively. Among them, the specific capacitance of the supercapacitor
assembled by NPMOF-800 in 6 mol/L KOH is 220 F/g, which is very high compared with the reported MOF-derived carbon materials.
Moreover, the two-electrode test showed that high capacitance and superb cycle stability could be achieved by NPMOF-800 electrode.
The capacitance retention rate was over 99.1% after 10000 charge-discharge cycles. These prominent performances were attributed to
the intriguing structure of the NPMOF-derived porous carbons, i.e., high nitrogen/oxygen doping, generated ultramicroporosity and
narrow pore size distribution.
As demonstrated in the Fig. 1, firstly the high nitrogen-containing ligand (BITA, 37.82% N) was designed and readily synthesized in
large scale (Supporting information). And then a straightforward solvothermal reaction of BITA with Zn(NO3)2•6H2O afforded NPMOF
as octahedron crystals (Fig. 1b). The crystal structure of NPMOF was intuitively characterized by single crystal X‒ray diffraction and
‒
depicted in Figs. 1c, d and Fig. S1 (Supporting information), showing the nonporous nature with coordinated NO3 counter ions and
MeCN molecules tightly encapsulated in the very small pores. Unlike the other highly porous MOFs the pores of which usually collapse
‒
or shrink during the high temperature carbonization, the easily decomposed NO3 ions and volatilized MeCN molecules in NPMOF
should result in various gases such as H2O and CO2 released from the solid NPMOF crystals, facilitating the generation of ultramicropores.
As a result, the MOF-derived porous carbon materials were readily prepared by pyrolysis at 700 °C, 800 °C and 900 °C, denoted as

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NPMOF-700, NPMOF-800 and NPMOF-900, respectively (Supporting information).
Fig. 1. Schematic illustration for the preparation of (a) MOF-derived porous carbonsand single crystal structure of NPMOF. (b) Photos
of the NPMOF crystals. (c) Structure with encapsulated acetonitrile molecules showing in space-filling model. (d) The coordination state
of Zn metal (gray: C atom, red: O atom, blue: N atom, yellow: Zn atom, white: H atom).
Scanning electron microscope (SEM) images confirm that the as-synthesized NPMOF consists of uneven dispersed crystals in
octahedron shape, and the diameters of NPMOF are ranged from 10 μm to 100 μm (Figs. 2a and b). As revealed by the SEM images,
small-sized crystals of the obtained MOF-derived porous carbon materials largely retain the original octahedron shape from the parent
NPMOF and exhibit a smooth surface without any large pores or cracks. In comparison, the large-sized particles splinter severely (Figs.
2c, d and Fig. S2 in Supporting information). The TEM images (Figs. 2e, f and Fig. S3 in Supporting information) intuitively show the
nature of micropores, mesopores and macropores of the NPMOF-800 material.
Fig. 2. SEM images of (a, b) NPMOF crystals and (c, d) NPMOF-800. (e, f) TEM images of NPMOF-800.
The XRD patterns of the NPMOF are illustrated in Fig. 3a. The diffraction peaks of pristine NPMOF are in good agreement with the
XRD patterns simulated from single crystal structure. After carbonization, the XRD patterns of the MOF-derived porous carbons exhibit
broad diffraction peaks at 23° and 44° assignable to the typical (002) and (100) lattices of amorphous carbons (Fig. 3b) [37].
Thermogravimetric and differential scanning calorimetry (TG‒DSC) curves of NPMOF were measured under nitrogen atmosphere, and
gave a deep understanding for the carbonization process (Fig. S4 in Supporting information). The acetonitrile molecules begin to overflow
at 60 °C. When the temperature reaches around 350 °C, the coordinated nitrates initiate the decomposition and the ligands start to degrade,
giving out volatiles (e.g., CO2, H2O) and facilitating the generation of ultramicropores [38]. Compared to the initial weight of NPMOF
crystals, 60% weight loss is detected at 900 °C by TG analysis. The IR spectra of the MOF-derived porous carbons exhibit absorption

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peaks for C‒N bending vibration (1290 cm‒1), and C=N stretching vibration in triazolidine rings (1450 cm‒1) (Fig. S5 in Supporting
information).
Raman measurements were carried out to explore the detailed carbon structure. The Raman spectra for the MOF-derived porous
carbons exhibit D and G bands located at 1350 cm‒1 and 1580 cm‒1, respectively (Fig. S6 in Supporting information). These peaks are
the components of the disordered carbon structure (D band) and graphitic carbon structure (G band) [39]. The relative ratios of D band
to G band (ID/IG) are calculated based on the peak height to evaluate the crystallization degree of graphitic sp2‒type carbon (sp2‒C)
structures [40]. The ID/IG value of NPMOF-900 (0.996) is the smallest and all the ID/IG values of NPMOF-derived porous carbons are ca.
1.0, indicating the high graphitization degree of these carbon materials.
Fig. 3. XRD patterns of (a) NPMOF and (b) MOF-derived porous carbons. (c) Nitrogen sorption isotherms and (d) pore size distributions of NPMOF, NPMOF-
700, NPMOF-800 and NPMOF-900.
The porosity of these MOF-derived carbons was investigated by N2 sorption. The degassing process was conducted under vacuum
conditions at 150 °C for 5 h before measurements. As shown in Fig. 3c, the NPMOF exhibits type I isotherms, and the MOF-derived
porous carbons present the combined characteristics of type I and IV isotherms, along with a slightly upward tendency at P/P0 > 0.9 [41].
The very low adsorption of NPMOF indicates its nonporous feature. The sharp N2 uptakes at very low relative pressure (P/P0 < 0.1)
prove the presence of micropores [36]. In the range of 0.1 to 0.9 (0.1 < P/P0 < 0.9), a little hysteresis loops are observed, attributed to the
narrow slit-like pores. At high relative pressure (P/P0 > 0.9), the N2 uptakes are steep, indicating that the MOF-derived porous carbons
may have some macropores embedded in the micropores (e.g., interparticle voids) [36]. The nonlocal density functional theory (NLDFT)
equilibrium model was applied to evaluate pore size distribution. As shown in Fig. 3d, the ultramicropores of the obtained MOF-derived
porous carbons with diameters around 0.5 nm may be ascribed to the removal of Zn nanoparticles, and the micropores with diameters
about 1 nm could be attributed to the volatilization of gases generated from the nitrates and pyrolysis of the BITA ligands.
Different carbonization temperatures led to a great impact on the porosity of the derived carbon materials. The Brunauer‒Emmett‒
Teller (BET) surface areas (SBET), micropore areas (Smicro), total pore volumes (Vpore) and micropore volumes (Vmicro) of these porous
carbons are summarized in Table S2 (Supporting information). The micropore volumes were calculated by the t-plot method. The
NPMOF-800 possesses a relatively high surface area of 381 m2/g, which is much higher than that of the NPMOF-700 (82 m2/g). However,
the surface area of the NPMOF-900 sample with higher graphitization dramatically decreased to 43 m2/g. Furthermore, the ratio of the
microporous volumes (Vmicro/Vpore) in NPMOF-800 is almost three times of that in NPMOF-900, contributing to the better electrochemical
performance. Benefiting from the microporous structure, the surface areas of these MOF-derived porous carbons are very high compared
with the parent NPMOF. The chemical and electronic states of the elements that exist within these porous carbons were further
investigated via X‒ray photoelectron spectroscopy (XPS). In the wide-scan XPS spectra (Fig. S7 in Supporting information), four peaks
are observed and assigned to the typical peak of Zn 2p, C 1s, N 1s, O 1s at 1020.4 eV, 283.9 eV, 397.5 eV, 531.3 eV, respectively. As
the temperature rises from 700 °C to 900 °C, the content of N decreases from 17.77 to 8.28%, and the content of C increases accordingly
while the content of O is almost constant (Table S3 in Supporting information). The total N and O doping amount decreases from 24.08%
for NPMOF-700 to 15.14% for NPMOF-900. It is notable that based on our strategy NPMOF-800 possesses highly graphitic structure,
yet the N content still retains 13.91%.
The narrow-scan XPS spectra are presented in Fig. 4. The N 1s spectra of all the porous carbons can be mainly fitted into four peaks
centered at ~ 397.4 eV, 398.8 eV, 399.9 eV and ~ 400.7 eV. The N1 peak at 397.4 eV is assigned to the formation of Zn‒N bonds (N
atoms bonded to Zn atoms) [42]. The N2 peak at 398.8 eV is corresponded to pyridinic‒N (N‒6) [41]. The N3 peak centered at 399.9 eV
is attributed to pyrrolic‒N (N‒5) [43]. The N4 peak at the highest binding energy (400.7 eV) undoubtedly represents graphitic‒N (N‒Q)
[44]. It is revealed that various types of N atoms have been steadily doped into the carbon structure. N‒6 and N‒5 are considered to
contribute to the faraday reaction and provide pseudocapacitance, while N‒Q can increase the conductivity of the material. Moreover,
the wettability of carbon materials can be further increased due to the different electronegativity of N and C atoms. The percentages of

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doped N content estimated from the XPS spectra are 17.77% (for NPMOF-700), 13.91% (for NPMOF-800) and 8.28% (for NPMOF-
900). It is rational that the C‒N bonds are gradually destroyed with the increasing of temperature, thereby leading to the lower N content
in the NPMOF-900. The O 1s spectra are assigned to the following peaks: 530.6 eV is attributable to Zn‒O [45], 531.6 eV is assignable
to C=O and 532.7 eV is ascribed to C‒O [46, 47]. The C 1s spectra can be assigned into six peaks (Fig. S8 in Supporting information).
The C1 peak at ~283.0 eV is deconvoluted to metal carbide, while the C2 peak at 284.0 eV is related to a graphitic carbon structure (sp2‒
C) [48]. The C3 peak at 284.8 eV belongs to sp3 C‒C [49]. The C4 peak at 285.9 eV is assigned to C atoms directly bonded to oxygen
(C‒O) [48]. The C5 peak at 286.2 eV is ascribed to sp3 C‒N [50]. The small peak centered around 287.6 eV (C6) is related to C=O [48].
The binding energy values of Zn 2p3/2 and Zn 2p1/2 are found at 1022.0 eV and 1045.0 eV, respectively [42].
Fig. 4. High-resolution XPS spectra of NPMOF-700 ((a) N 1s, (b) O 1s), NPMOF-800 ((c) N 1s, (d) O 1s) and NPMOF-900 ((e) N 1s, (f) O 1s).
As energy storage and conversion devices, supercapacitors have both the characteristics of conventional capacitors and the batteries
that can charge and discharge of capacitors rapidly and store energy. Recently the MOF-derived porous carbon materials have proved to
show great potential in high-performance supercapacitors because of their unique structures and heteroatoms doping [51‒53]. To evaluate
the electrochemical properties of MOF-derived porous carbons, electrochemical experiments were carried out using a three-electrode
system in 6 mol/L KOH electrolyte under the potential window of ‒1 V to 0 V. The CV and GCD curves of MOF-derived porous carbons
electrodes conducted at different potential scan rates and current densities are shown in Fig. 5. The NPMOF-700 displays a distorted
rectangular CV shape, whereas other electrodes present quasirectangular CV shapes (Fig. 5b). The distorted rectangular CV shape
indicates the ion-diffusion limitation in micropores [54]. The CV curves of NPMOF-800 electrode at varied scan rates from 5 mV/s to
100 mV/s retain relatively rectangular shapes, indicating good charge propagation with excellent rate capability (Fig. 5e). In order to
evaluate the specific capacitance of the MOF-derived porous carbons electrodes, galvanostatic charge-discharge measurements were
conducted at various current densities in a three-electrode system. All of the electrodes display a quasi-linear appearance with a slight
bend, implying the pseudocapacitance impact derived from N/O doping. It is well known that the presence of some heteroatom (N, O, P,
S, etc.) functionalities can enhance the capacitance by the pseudo-capacitive effect [55, 56]. As shown in Fig. 5a, the specific capacitance
values obtained at a current density of 1 A/g are 66 F/g (NPMOF-700), 220 F/g (NPMOF-800) and 207 F/g (NPMOF-900), respectively.
These values are among the previously reported high capacitances of MOF-derived porous carbon electrodes (Table S4 in Supporting
information) [31, 36‒57, 58]. Fig. 5a shows the charge-discharge curves at 1 A/g. NPMOF-800 is at a premium due to its larger surface
area and pore volume. The specific capacitance of NPMOF-700 is only 66 F/g at a current density of 1 A/g, while it drastically increases
to 220 F/g and 207 F/g for NPMOF-800 and NPMOF-900. Nyquist plots are applied to gain an insightful understanding of the ion-
diffusion and charge-transfer behaviors within electrodes (Fig. 5c) [54]. For each sample, the Nyquist plot showed a small semicircle at
high frequency and a linear trait at low frequency. At low frequencies, the vertical curve is featured, indicating a nearly ideal capacitive
behaviour. The equivalent series resistance (Rs) calculated from the intercept point at the Z′-axis, are 0.50 Ω, 0.51 Ω and 0.53 Ω for
NPMOF-700, NPMOF-800 and NPMOF-900 electrodes, respectively. The semicircle diameter represents the charge transfer resistance
(Rct), the fitted Rct value of NPMOF-700 is 3.9 Ω, which was much higher than the value of NPMOF-800 (0.59 Ω) and NPMOF-900
(0.54 Ω), indicating lower speeds of electron transfer and ion transport. The charge transfer resistance is too high for NPMOF-700, which

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is the main reason leading to its poor performance. Additionally, the straight line of the Nyquist plot represents the warburg impedance
(W). Clearly, the Rct (0.59 Ω) and W (0.42 Ω) for NPMOF-800 can ensure the efficient electron transfer and fast ion diffusion.
In consideration of factors such as effective surface area, pore size distribution and heteroatom doping, NPMOF-800 delivers the most
excellent electrochemical performance. It is worth mentioning that the BET surface area normalized capacitance of NPMOF-800 is 57.7
µF/cm2 and among the highest values for all the porous carbon materials, which is ascribed to the abundant ultramicropores (Table S4 in
Supporting information).
Fig. 5. Electrochemical performances measured in a three-electrode system using 6 mol/L KOH electrolyte: (a) GCD curves at 1 A/g. (b) CV curves at 5 mV/s.
(c) Nyquist plotsof NPMOF-700, NPMOF-800 and NPMOF-900 electrodes. (d) GCD curves at different current densities. (e) CV curves at different scanning
rates. (f) The relationships between current densities and capacitancesof NPMOF-800 electrodes.
To further characterize the electrochemical performances of NPMOF-800 electrode, symmetric two-electrode devices were also
assembled using 6 mol/L KOH electrolyte. The CV curves show a typical rectangular shape from 0 to 1 V with the sweep rates ranging
from 5 mV/s to 100 mV/s, suggesting rapid electrochemical responses in aqueous electrolyte solutions (Fig. 6a) [40]. The CV shape is
mirror-symmetric, even at a high scan rate, indicating the high reversibility of the sample. The charge-discharge current densities are
varied from 0.2 A/g to 10 A/g to evaluate the specific energy and power of the device. All the GCD curves under various current densities
show typical linear discharge curves, demonstrating low internal resistances which are in consistent with EIS measurements [59]. Nyquist
plots (Fig. 6c) demonstrate the positive effect of NPMOF-800 as a beneficial conductive scaffold due to the low Rs (0.41 Ω) and Rct (1.3
Ω). The symmetric device shows a high capacitance value of 168 F/g (at 0.5 A/g). Ragone plot in relation to energy-power densities is
displayed (Fig. 6f). At the current density of 0.2 A/g, the device exhibits high specific energy of 6.25 Wh/kg at the power of 100 W/kg.
Furthermore, the specific energy maintains at 5.07 Wh/kg when the specific power increases to 1000 W/kg at 5 A/g. The energy density
of NPMOF-800 device is comparable and even higher than other carbon materials reported previously [35, 41, 60‒62]. To further
demonstrate the advantages of NPMOF-800 for supercapacitors, the life cycle test was carried out using GCD studies at a current density
of 1 A/g up to 10000 cycles (Fig. 6g). After 10000 cycles, NPMOF-800 device still remains over 99.1% of the initial specific capacitance,
which is overwhelmingly high compared with previously reported values (Table S4). These results highlight the advantages of our
symmetric capacitors meeting the requirements of high stability, excellent rate capability, and high cycle performance. These interesting
features of this symmetric supercapacitor open a new era for researchers to explore high-performance electrochemical energy storage
devices based on MOF-derived porous carbon materials.

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Fig. 6. Electrochemical performances of NPMOF-800 electrode measured in a two-electrode system using 6 mol/L KOH electrolyte: (a) GCD curves at different
current densities. (b) CV curves at different scanning rates. (c) Nyquist plot. (d) The relationship between current densities and capacitances. (e) Rate performance
measurements. (f) Ragone plot. (g) Cycling stability at 1 A/g.
In summary, we synthesized a new nonporous NPMOF using rigid nitrogen-containing ligand and made it a porous carbon material
by direct carbonization. By optimizing the pyrolysis temperatures, the material with the best performance was finally found to be
NPMOF-800. The specific capacitance of NPMOF-800 can reach 220 F/g and an extremely high surface area normalized capacitance of
57.7 µF/cm2 has been achieved, attributed to its super high heteroatom content (N: 13.91%, O: 6.27%) and ultramicroporosity features.
In addition, symmetrical two-electrode device prepared by NPMOF-800 has excellent energy and power density, and the capacitance
retention rate after 10000 charge-discharge cycles is 99.1%. This work not only provides an effective way to prepare high performance
supercapacitors, but also proposes new applications for numerous porous or even nonporous MOFs.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgment
This work was financially supported by the National Natural Science Foundation of China (Nos. 21501135, 21875165), the
Fundamental Research Funds for the Central Universities, and the Recruitment Program of Global Experts of China.
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