ARTICLE
Ultralight covalent organic framework/graphene
aerogels with hierarchical porosity
Changxia Li 1 , Jin Yang 1 , Pradip Pachfule 1 , Shuang Li 1 , Meng-Yang Ye 1 , Johannes Schmidt 1 &
Arne Thomas 1 ✉
The fabrication of macroscopic objects from covalent organic frameworks (COFs) is chal-
lenging but of great signi fi cance to fully exploit their chemical functionality and porosity.
Herein, COF/reduced graphene oxide (rGO ) aerogels synthesized by a hydrothermal
approach are presented. The COFs grow in situ along the surface of the 2D graphene sheets,
which are stacked in a 3D fashion, forming an ultralight aerogel with a hierarchical porous
structure after freeze-drying, which can be compressed and expanded several times without
breaking. The COF/rGO aerogels show excellent absorption capacity (uptake of >200 g
organic solvent /g aerogel), which can be used for removal of various organic liquids from
water. Moreover, as active material of supercapacitor devices, the aerogel delivers a high
capacitance of 269 F g − 1 at 0.5 A g − 1 and cycling stability over 5000 cycles.
https://doi.org /10.1038/s41467-020-18427-3 OPEN
1 Department of Chemistry, Functional Materials, Technische Universita  t Berlin, Hardenbergstraße 40, 10623 Berlin, Germany. ✉ email: arne.thomas@tu-
berlin.de
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1234567890():,;

C ovalent organic frameworks (COFs) are highly porous
crystalline polymers constructed from lightweight ele-
ments, such as C, N, O, B, Si, a nd H, by strong covalent
bonds among the organic linkers 1 – 4 . Because of their structural
diversity, permanent porosity, long-range order, and the ver-
satile functionalities, which can be introduced into the organic
backbone, COFs are promising materials for a range of appli-
cations, such as organocatalysis 5 , gas storage 6 ,m o l e c u l a r
separations 7 , 8 , energy storage 9 , photocatalytic water splitting 10 ,
light-emitting diodes 11 , etc. However, the traditional synthetic
methods for COFs usually demand vacuum conditions, high
boiling point organic solvents such as mesitylene or 1,4-dioxane
and long reaction times (usually 48 – 72 h) 12 – 14 . More impor-
tantly, the resulting COFs are usually formed as powders, which
are hardly process able as they are insoluble a nd infusible. The
powder form is also detrimental to electric conductivity,
observed in conjugated COFs. Finally, their stacked 2D struc-
ture with micro- or small mesopores can impede mass transfer
and also the full utilization of the ir actually lar ge surface area.
Indeed, it is commonly observed that the theoretical surface
area of COFs is much higher than the measured one, which
points to dead ends and inaccessible regions within the COF
structure, even for small gas molecules. Recently, crystalline
and hierarchical porous COFs with macropores and inherent
micropores have been synthesized by employing poly styrene
spheres as a template 15 . However, these COFs are also
obtained as powders, which can be applied e.g., in electro-
chemical applications just by gluing them to an existing elec-
trode. The direct fabrication of COFs into stable 3D
architectures with control over several length scales is
thus desirable for many practical applications but still a sig-
ni fi cant challenge.
Graphene oxide (GO) is considered as an ideal precursor for
the assembly of extended architectures due to its hydrophilic
surface and large surface area, enabling versatile composite
structures with a variety of emerging material classes 16 – 18 .A s
example, GO was used to prepare graphene/MXene hydro-
gels 19 , 20 , graphene-supported metal organic framework (MOF) 21 ,
graphitic carbon nitride (g-C
3
N
4
) nanoribbon/graphene compo-
sites 22 , and boron nitride nanotubes/reduced graphene oxide
(rGO) aerogels 23 . In these composites not only the bene fi cial
properties of the single compounds are retained, but due to the
presence of graphene, they often show enhanced electrical con-
ductivity and mechanical properties. COFs exhibit low density,
good chemical stability, large surface area, and their backbone
functionality can be tailored by using appropriate monomers. 2D
COFs, which furthermore possess a π -conjugated structure,
should be perfectly suited to form composites with 2D graphene.
In this study, COF/rGO composites are prepared by a hydro-
thermal approach, which yield 3D, hierarchically porous, ultra-
light, and monolithic structures. These composite materials are
further applied for removal of oil and various organic liquids
from water and as an active material of supercapacitor-based
energy storage device.
First, a COF/rGO hydrogel is obt ained by the in si tu reactio n
of the organic linkers 1,3,5-Tri formylphloroglucinol (Tp) and
Diaminoanthraqui none (Dq) in pre sence of GO. The hydro -
thermal reac tion conditi ons lead to the reduc tion of GO to
rGO and the uniform growth of TpDq-COF along the surface
of rGO nanosheets, yielding an in timate mixing of bo th phases.
After freeze-drying of the obtained hydrogel, a COF/rGO
aerogel is fi nally formed, exhibiting a hierarchical porous
structure. Furthermore, the COF/rGO aerogel shows a low
density, go od conducti vity, redox ac tivity, and good mechan-
ical strength, yielding e xcellent absorptio n and electrochemical
properties.
Results
Materials synthesis and characterization . TpDq-COF herein was
synthesized by a hydrothermal method, which is scalable, envir-
onmentally friendly and time effective. The TpDq-COF forms via
a Schiff-base condensation between the aldehyde and amine-
groups of the respective monomers (Tp and Dq) using p-
Toluenesulfonic acid (PTSA) as catalyst (Supplementary Figs. 1
and 2a). Herein, TpDq-COF was chosen because of its redox
activity due to the presence of anthraquinone moieties within its
backbone, high chemical stability and large surface area 24 , 25 .
When GO is added to the reaction solution, a COF/rGO
aerogel is formed during the hydrothermal treatment via self-
assembly and subsequent freeze-drying, as illustrated in Fig. 1 a
and Supplementary Fig. 2b. Brie fl y, Dq and PTSA were fi rst
mixed in water to form an organic salt 26 , which was then added
to GO solution and stirred at room temperature to form a
homogeneous dispersion. Tp was added to the dispersion and
shaken thoroughly using a vortex shaker to form an extremely
dense and viscous mixture. This mixture was then transferred
into an autoclave and heated at 120 °C in an oven for 24 h to
obtain a black hydrogel. The hydrogel was thoroughly washed
with distilled water, acetone, and water to remove the PTSA and
unreacted reagents. After freeze-drying, an ultralight COF/rGO
aerogel was obtained. This approach can be easily scaled up when
a larger autoclave is used (Supplementary Fig. 5). The formation
of COF/rGO composites is possibly enabled by the presence of
oxygen containing functional groups on graphene oxide, which
promote attractive interaction or even the grafting of Dq
molecules by various organic reactions (Supplementary Fig. 6),
which might represent the initial step of COF formation on the
graphene sheets followed by COF growth after Tp is added.
The feasibility of the hydrothermal method for the synthesis of
TpDq-COF and COF/rGO hybrid is demonstrated by powder X-
ray diffraction (PXRD), Fourier transform infrared spectroscopy
(IR) spectra and X-ray photoelectron spectroscopy (XPS). The
complete disappearance of representative peak of GO (2 θ = ca.
12°) and the appearance of a new broad peak of graphene at 24°
demonstrate that GO was reduced effectively under the hydro-
thermal condition (Supplementary Fig. 7a). IR and XPS spectra
further suggest that the oxygen functional groups of GO have
been removed largely during the process (Supplementary Figs. 7b
and 8). Both COF and COF/rGO display a peak at 3.4° (2 θ )
corresponding to the re fl ection from the (100) plane of TpDq-
COF, in good agreement with the corresponding simulated XRD
pattern from the modeled structure, con fi rming the formation of
the crystalline structure of TpDq-COF (Fig. 1 c). The presence of
graphene weakens the intensity of COF peaks to some extent. For
the pure COF, the broad peak at 26° (2 θ ) can be assigned to the
π − π stacking between the COF layers, which corresponds to the
(002) plane. This peak is more pronounced for the COF/rGO. It
should be however noted, that this peak is shifted to slightly
higher angles than observed for pure rGO (Supplementary Fig. 9),
giving a fi rst hint that individual graphene layers are covered by
the COF, as in a physical 1:1 mixture of both layered materials a
broader peak combined from both (002) contributions would be
expected. The IR spectra show similar peaks for TpDq-COF and
the COF/rGO hybrid (Supplementary Fig. 10). The strong
characteristic peaks at 1240 cm − 1 (C − N), 1560 cm − 1 (C = C),
and 1615 cm − 1 (C = O) can be attributed to the formation of the
β -ketoenamine linked framework structures 25 . The vibration
frequency corresponding to the ketone (C = O) of the anthraqui-
none moiety could be assigned at 1670 cm − 1 . In contrast to rGO,
the XPS survey spectra of COF and COF/rGO clearly displays
three visible peaks of C 1 s ,N1 s and O 1 s (Supplementary
Fig. 11a). The high-resolution XPS spectra of COF/rGO with
the same types of carbon and nitrogen species as seen for pure
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TpDq-COF, further ensure the formation of the COF in presence
of graphene (Supplementary Fig. 11b ‒ d). The amount of COF
phase within the composite can be derived from the amount of
nitrogen detected by elemental analysis (Supplementary Table 1).
If not otherwise stated, the material COF/rGO discussed in the
following refers to the mass ratio of monomers: GO of 1:1 whose
COF loading is 58.5%.
N
2
sorption measurements were conducted to examine the
surface areas and porosity of COF, COF/rGO, and rGO (Fig. 1 d,
e). N
2
adsorption of the pure COF shows characteristics of a type-
I isotherm, with a steep increase at low relative pressure,
corroborating the microporosity of TpDq-COF. In addition, the
obvious hysteresis of the desorption curves indicates the presence
of mesopores. The coexistence of micro- and mesopores were
further ensured by the pore size distributions (PSD) following
nonlocal density functional theory (NLDFT). TpDq-COF has a
Brunauer – Emmett – Teller (BET) surface area of 498 m 2 g − 1 . This
value is lower than that achieved by the conventional solvother-
mal approach (1124 m 2 g − 1 ± 422) 12 , 24 , but comparable
to reported values for this COF prepared in water with acetic
acid as catalyst (489 m 2 g − 1 ) 27 . On the other hand, the speci fi c
surface area of rGO is only 37 m 2 g − 1 due to the strong π – π
stacking among graphene sheets. The mea su re me nt fo r COF/ rG O
sh ow s a simi lar is ot he rm as fo r the pu re COF bu t wit h a high
ni trog en up ta ke at high re lati ve pres sure s, indi cati ng the fo rmati on
of an add itio nal ma crop orosi ty (Fig . 1 d) . The sp eci fi c surf ace area
of COF/ rGO (246 m 2 g − 1 ) is in betw een th e v alue s obs er ved fo r the
pu re COF a nd rGO , resp ectiv ely , show ing th at bo th ma teri als are
mi xed in th e expe cted a ppro xim ate 1:1 ma ss rati o.
The COF/rGO aerogel have a low density of ca. 7.0 mg cm − 3
thus can be easily hold by a leaf (Fig. 2 a). To gain more insight
into the origin of the low density, the morphology and structure
of COF, rGO, and COF/rGO aerogel were further examined by
scanning (SEM) and transmission electron microscopy (TEM).
The TpDq-COF possesses a hollow tubular structure (Supple-
mentary Fig. 12). As shown in Fig. 2 b and Supplementary Fig. 13,
this morphology has completely changed for the COF/rGO
composites, as extended and interlinked nanosheets are observed
forming a 3D sponge-like structure. The pore sizes of these
networks are in the range of several micrometers, which is much
smaller than observed for a pure rGO aerogel showing pores of
hundreds of micrometers. Notably, no isolated COFs particles
were detected on the graphene nanosheets, indicating that the
COF grow uniformly along the surface of graphene. TEM images
of the COF/rGO sheets con fi rm that they are very thin and
partially wrinkled, pointing to a good fl exibility (Fig. 2 d and
Supplementary Fig. 14). Elemental mapping on these sheets
(Supplementary Fig. 14c) show a uniform distributions of C, N,
and O, further demonstrating that the graphene nanosheets were
fully and evenly covered by TpDq-COFs. The structure of the
COF/rGO and graphene was further investigated by atomic force
microscopy (AFM) (Fig. 2 e, f and Supplementary Figs. 15 – 16) to
elucidate the COF growth on the graphene nanosheets. The COF/
rGO nanosheets show a minimum thickness of 2.9 – 6.0 nm; while
for rGO sheets with a thickness of 1.5 – 2.0 nm are found. This
increase of average thickness might originate from the uniform
loading of a few layers of the COF on the surface of rGO.
The mechanical properties of the COF/rGO aerogel were
evaluated by measuring stress – strain curves. As shown in the
inset of Fig. 2 g and Supplementary Video 1, the as-prepared
aerogel can completely spring back to its original shape after the
stress is released. This performance originates from the complete
recovery of their 3D porous network after deformation. The
compressive stress – strain curves of COF/rGO aerogel with strains
up to 10, 20, 30, 40, and 50% are shown in Fig. 2 g. During the
unloading process, the stress always remains above zero proving
no irreversible deformation of the aerogel. The outstanding
elasticity originates from the 3D framework structure formed
during the hydrothermal process. It should be noted, that the
production of most ultralight carbon-based materials usually
requires high temperature annealing 28 – 33 . In contrast to most of
the literature reports, the synthesis temperature of the ultralight
COF/rGO aerogel presented herein is very low (120 °C). In
addition, the density of 7.0 ± 0.5 mg cm − 3 is lower than that of
TpDq-COF
PTSA Dq
GO
+
Stirring
30 min
Shaking,
20 min
Tp Transfer
1. 120 °C, 24 h
3D Ultralight aerogel
0
100
200
300
400
500
N
2
adsorption (cc g
-1
, STP)
Relative pressure ( P / P o)
COF adsorption
COF desorption
COF/rGO adsorption
COF/rGO desorption
rGO adsorption
rGO desorption
COF/rGO
Simulated TpDq
Intensity (a. u.)
2 Theta (degree)
0.0 0.2 0.4 0.6 0.8 1.0 10 20 30 40 50 2 4 6 8 10 12
0.00
0.04
0.08
0.12
0.16
d V ( d ) (cm
3
nm
–1
g
–1
)
Pore size (nm)
rGO
COF/rGO
COF
a
e d c b
0.36 nm
2.4 nm
2. Freeze drying
Fig. 1 Schematic representation of COF/rGO aerogel synthesis. a Scheme of the synthetic procedure for the preparation the COF/rGO aerogel. b Space-
fi lled model of TpDq-COF from top and side views. c PXRD patterns of the pure COF and COF/rGO compared with a simulated XRD pattern from the
modelled structure with eclipsed stacking. d N
2
adsorption-desorption isotherms of rGO, COF/rGO and COF. e , Pore size distribution for rGO, COF/rGO
and COF obtained using the NLDFT method.
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pure graphene aerogel and graphene-based composites such as G-
ZIF8, g-C
3
N
4
-G, MXene/G, and BN nanotubes/rGO (Fig. 2 h) 20 –
23 , 31 , 34 – 37 .
To test the versatility of the formation of composite hydrogels,
the GO was mixed with different amounts of the monomers and
the resulting mixtures were treated following the same protocol
(Supplementary Figs. 17 ‒ 21). With an increase of the amount of
monomers, more expanded hydrogels can be obtained. In other
words, the COF acts as an expansion agent by inhibiting the
stacking of graphene nanosheets, thereby reducing its volume
shrinkage. When the amount of monomers was raised to 2:1
related to GO, a highly expanded hydrogel is formed, which,
however, partially loses its shape after freeze-drying probably
because of the further weakened interaction between the
nanosheets. On the other hand, a COF aerogel cannot be formed
without the assistance of GO during the synthesis. Therefore, GO
plays a pivotal role in constructing the 3D COF/rGO macro-
structures. The aerogels with COF/rGO 0.5:1 and 2:1 show
similar IR peaks to COF/rGO 1:1, proving again the formation of
the COF structure on graphene (Supplementary Fig. 17c).
Moreover, in the PXRD patterns, the peak belonging to the
(100) plane appearing at 3.4° (2 θ ) becomes more pronounced
(Supplementary Fig. 17d) and the nanosheets become thicker
(Supplementary Figs. 20, 21) when the amount of COF is
increasing. The aerogel with COF/rGO 1:1 possesses a high COF
loading while maintaining intact 3D structure after freeze-drying,
yielding a relatively low density (Supplementary Table 1).
Absorption performance . Due to its highly porous structure,
high surface area, low density, and good mechanical stability, the
COF-based aerogel should be a promising absorbent for oils and
other organic pollutants. To analyze the absorption selectivity, the
COF/rGO aerogel was placed on the surface of a water and silicon
oil mixture, yielding selective absorption of the fl oating silicone
oil (dyed with Oil Red) within a few seconds (Fig. 3 a). Similarly,
when aerogel was brought in contact with underwater chloroform
(again dyed with Oil Red), fast absorption of chloroform was
observed within one second (Fig. 3 b; Supplementary Movie 2).
After this process, the oil or organic liquid could be separated
entirely, thus leaving clean water. The absorption capacity of the
pure rGO and COF/rGO aerogels was measured for various
other organic solvents. The pure rGO aerogel without COF shows
an absorption capacity of 66 – 93 times its own weight depending
on the organic solvents (Supplementary Fig. 22a). The absorption
capacities of the hybrid aerogels are much higher for all solvents
tested, showing the in fl uence of the overall lower density and
higher surface area and porosity. The COF/rGO aerogel prepared
from the monomer:GO ratio of 1:1 shows the highest absorption
capacity for a variety of organic solvents. This is in-line with the
lowest density of this composite compared to pure rGO and the
composites with other COF/rGO ratios. In addition, the micro-
porous COF provides a large speci fi c surface area, thus itself
possessing a good absorption capacity for organic solvents. The
1:1 COF/rGO aerogel possesses absorption capacity for different
solvents ranging from 98 to 240 times its own weight, which is
higher than that of many reported sorbents (Fig. 3 c, Supple-
mentary Table 2 and Supplementary Fig. 22b) 20 , 32 – 39 . The
recyclability of COF/rGO aerogel was measured by repeated
ethanol absorption and then drying in the oven. The absorption
capacity was found to be maintained above 87% after 20 cycles
(Fig. 3 d). These results demonstrate the potential of COF/rGO
aerogel for ef fi cient and recyclable oil clean-up.
Electrochemical performance . The development of ef fi cient
energy storage devices is an effective way to solve the global
energy crisis. Apart from the good conductivity and mechanical
strength of graphene, the quinone moities in the COFs backbone
can act as a redox-active unit to provide reversible Faradaic
reactions in electrochemical energy storage. The self-supporting
COF/rGO aerogels can be directly used as electrodes of a super-
capacitor without conducting additives or binders (Supplemen-
tary Fig. 23). Electrochemical measurements were carried out by
both cyclic voltammetry (CV) and galvanostatic charge/discharge
(GCD) experiments for all samples using two electrode cells in
0.5 M H
2
SO
4
aqueous electrolyte. Figure 4 a compares the CV
curves of 3D rGO/COF, rGO, and COF at the sweep rate of 50
mV s − 1 with a wide potential range of 1.5 V. The electrochemical
capacitance of pure COF is very poor without any charge and
discharge capacity due to its insulating property (Supplementary
Fig. 25). To elucidate if the formation of a composite, i.e., of COF
layers grown on rGO, is indeed necessary and bene fi cial for the
capacitive performance, TpDq-COF was also just physically
10 m 2 m 1 m
–15
–10
–5
0
5
–15
–10
–5
–16
–14
–12
5.8 nm
3.1 nm
Height (nm)
2.9 nm
Distance ( μ m)
0
2
4
6
8
10
12
Compressive stress (KPa)
Compressive strain (%)

10%
20%
30%
40%
50%
Original Compressed Released
0.0 0.5 1.0 1.5 2.0
01 0 2 0 3 0 4 0 5 0 0 5 10 15 20 25 30 35
0
400
800
1200
1600
g-C
3
N
4
nanoribbon-G
RGO/CNF
G-ZIF8
MOF-derived
carbon aerogel
This work
G/CNT
Fe
2
O
3
/C
N-CNT/CF
Ultralight G
Spongy graphene Mxene/G
Graphene
BN
Synthesis temperature (°C)
Density (mg cm –3 )
MnO
2
/CF
d c b a
h g f e
500nm
Fig. 2 Structural characterization of COF/rGO aerogels. a A photograph of an ultralight COF/rGO aerogel standing on a leaf. b , c SEM images, and d TEM
image of COF/rGO. e , f AFM image and the corresponding height pro fi les for COF/rGO. g The stress – strain curves of COF/rGO aerogel at different
maximum strains. The inset images show the snapshots of COF/rGO aerogel under compression and recovering process. h Comparison of density and
synthesis temperature of common lightweight materials. The yellow area show ultralight materials (density < 10 mg cm − 3 ).
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mixed with a conductive additive namely carbon black (super P).
However, the speci fi c capacitance of the COF-carbon black
mixture (COF-C-mix) is only 22.4 F g − 1 (32 C g − 1 ) at 0.5 A g − 1
although the conductive carbon was introduced (Supplementary
Fig. 26). I n c ont ra st, th e 3D COF /rG O ele ct ro de sh owe d obv io us
redox peaks with a dr amatic increase in speci fi c cap acity compared
t ot h ep u r eC O F ,C O F / Ca n dp u r er G Oe l e c t r o d e s( d e t a i l e d
descrip tion in Supplementary Figs. 25 – 28 ). Fi gu re 4 ba n d
Cyclohexane
Methanol
Hexane
THF
Ethyl acetate
Acetone
DMF
DMA
Ethanol
Dioxane
Toluene
Ethylene glycol
Chloroform
DMSO
Phenoxin
Silicone oil
Absorption capacity (wt.%)
0
2
4
6
8
10
12
14
16
18
20
0 5000 10,000 15,000 20,000 25,000
0 2000 4000 6000 8000 10,000 12,000
Absorption capacity (wt.%)
Cycle number
ab
cd
Fig. 3 Absorption performance of COF/rGO aerogels. Absorption of dyed silicone oil ( a ), and chloroform ( b ), from water by COF/rGO aerogels.
c Absorption ef fi ciency, and d cycling stability of COF/rGO in terms of weight gain. The error bars exhibit standard deviations based on three independent
measurements. Dimethyl sulfoxide, dimethylfor mamide, dimethylacetamide, and tetrahydrofur an are abbreviated as DMSO, DMF, DMA, and THF,
respectively.
01 0 2 0 3 0 4 0
0
10
20
– Z ″ (Ohm)
–15
–10
–5
0
5
10
Current density (A g
–1
)
Potential (V)
COF
rGO
COF/rGO
0.0
0.5
1.0
1.5
Potential (V)
Time (s)
0.5 A g –1
1 A g –1
2 A g –1
3 A g –1
4 A g –1
0
100
200
300
Cycle number
0
100
200
300
– Z ″ (Ohm)
Z ′ (Ohm)
COF/rGO
rGO
0 50 100 150 200
0.0
0.5
1.0
1.5
Potential (V)
Time (s)
1–5 cycles 4996–5000 cycles
0
50
100
150
200
250
300
Specific capacitance (F g –1 )
Specific capacitance (F g –1 )
PDC-MA
TpPa-(OH) 2
(Tempo) 100% -NiP
Dq-Tp
TaPa-Py
v-CNS-RGO
AC//PG-BBT
MWCNT@COF
COF/rGO
Name of material
c
b
a
f e d
0.0 0.5 1.0 1.5 0 200 400 600 800 1000
0 1000 2000 3000 4000 5000 0 100 200 300
0 2 468 1 0
0
100
200
300
400
Specific capacitance (F g -1 )
COF 0
100
200
300
400
Specific capacity (C g –1 )
rGO
COF/rGO
COF/rGO
Current density (A g
-1
)
Z' (Ohm)
Fig. 4 Performance of COF/rGO electrodes in a symmetrical supercapacitor device. a CV curves for rGO, COF/rGO, and COF at 50 mV s − 1 . b The
galvanostatic charge-discha rge curves of COF/rGO at a current density of 0.5, 1, 2, 3, and 4 A g − 1 . c The speci fi c capacitances and capacities calculated
from the discharge curves under different current density. d Comparative bar chart expressing the high performance of COF/rGO among all COF-based
supercapacitors in two-electrode system. e The cyclic stability of COF/rGO at a current density of 8 A g − 1 . f Impedance spectra of the rGO and COF/rGO
capacitor.
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Supplementar y Fig. 28 show the GCD curves for the COF/
rGO aerogels, COF and rGO-based supercap a citors at different
current d ensity of 0.5 – 10 A g − 1 . The C OF/ rG O hy bri ds di sp la y
triangular shap e with partial deformation, whose additional capa-
city resul t s f rom the pseudocap a city induced by redox-active
anthraquinone and extra electric double-layer capacity generated
by th e enhanced sp eci fi c surface area. In order to obtain a direct
comparison of the capacitive performance, the speci fi c capacitance
and capacity is calculated at different current densities from the
di sch arg i ng c urv e (F ig . 4 c, Su ppl em ent ary Fi g. 29 an d Su pp le -
mentary Tabl e 3). Among them, the COF/r GO aer ogel yields the-
highest speci fi c capacita nce of 269 F g − 1 in a potential window
of 1.5 V (equivalent to a sp eci fi cc a p a c i t yo f4 0 4 C g
− 1 )a tt h e
current d ensity of 0.5 A g − 1 . With the cu rrent density increasing to
10 A g − 1 , the COF/rGO can still d eliver a speci fi c capa citance of
22 2 F g -1 (292 C g − 1 ) wit h the re ten ti on of 83% c ap ac ita nc e
(Fig. 4 c). T he high speci fi c capacity and rate c apabil ity of th e COF/
rGO electrode is attr ibuted to a synergistic effect of rGO p roviding
conductivity and the COF p roviding a high surface area and red ox -
active sites, thus increasing the double layer and pseud ocapacity,
respectively. F urthermore, the formed 3D ne twork is bene fi cial for
rapid charge transfer and ion d iffusion to red o x-active sites. As a
re su lt , t he ae rog el s pre pa red u si ng a mo no me r: GO r at io of 1: 1 sho w
the highest speci fi c capacita nce. Again an optimum COF/rGO ratio
is found, as with higher COF a mounts the coating layer of the
in su lat ing C OF s pr oba bly be com es to o thi ck, re sul t in g in lo we r rat e
ca pa bil iti es (S up ple me nt ary F ig . 29) . Spe ci fi call y, the maximum
speci fi c c apa cit an ce of C OF/ rG O is t he h ig he st v alue re po rt ed f or
CO F- ba sed m ater ia ls in a two -e lec tro de sy st em (F ig. 4 d , Su pp le -
mentary Table 4) 40 – 47 . M oreover, it is als o comparable with other
potential electrode material s (Supplementary Table 5) 48 – 55 .T h e
cycling performance test of the COF/rGO device reveals a superior
retention of 96 % after 5 000 cycles, su ggesting excellent cyclic sta-
bility (Fig. 4 e). The electrochemical impedance spectroscop y (EIS)
was conducted to d isplay the charge and ion transport dynamics
(Fig. 4 f) . In t he N yq ui st pl o ts , the stra i gh t li ne at t he l ow -
frequencies reveals convenient ion transport path, while the small
semicircle at the high-f requencies indicates very low internal
electrode – electrolyte resistance and ef fi cient charge mobility in the
capacitor device 9 . Therefore, it can be concluded that the 3 D
structure of COF/ rGO material is bene fi cial for the rapid charge
transfer and ion diffusion to redox-active sites.
Discussion
In summary, COF/rGO aerogels have been fabricated by self-
assembly at low temperature following a green synthesis pathway.
As a combined result of their hierarchically porous structure,
ultralow density, good mechanical strength, and enhanced con-
ductivity, the herein developed 3D aerogels display an improved
absorption ability for organic solvents and outstanding capacitive
performance. Considering the facile preparation and excellent
performance, the 3D COF/rGO aerogel is a promising material
for environmental and energy applications.
Methods
Synthesis of Tp . 108 mmol hexamethylenetetramine (15.1 g), 49 mmol phlor-
oglucinol (6.0 g), and 90 mL tri fl uoroacetic acid were re fl uxed at 100 °C for 2.5 h
under N
2
. 150 mL HCl (3 M) was added slowly and the solution was heated at
100 °C for another 1 h. After cooling down, the solution was fi ltered through Celite
and extracted with 350 mL dichloromethan e. After that, the so lution was evapo-
rated under reduced pressure to afford 2.4 g of an off-white powder. Puri fi cation
was carried out by sublimation.
Synthesis of rGO aerogel . GO was prepared from graphite powder using a
modi fi ed Hummers ’ me thod as reported 16 . The 3D rGO aerogel was prepared by
hydrothermal reduction of GO aqueous dispersion. Brie fl y, 4.3 mL of 5 mg mL − 1
GO solution and 5 mL water were stirred for 2 h. Then, the GO aqueous dispersion
(2.3 mg mL − 1 ) was sealed in a 20 mL Te fl on-lined autoclave. Aft er heating at
120 °C in an oven for 24 h, the autoclave was cooled down to room temperature
and 3D graphene monolith was obtained by freeze-dry ing.
Synthesis of TpDq-COF . Well-ground PTSA (59.4 mg, 0.31 mmol), Dq powder
(13.4 mg, 0. 056 mmol), and 5 mL of water were mixed thoroughly and shaken well
in a vortex shaker for 5 min. Then, 7.8 mg of Tp (0.037 mmol) was add ed into the
yellow solution and shaken for another 20 min. The orange-red solution was
transferred into an autoclave and heated at 120 °C for 24 h. Then, the obtained
solid was sequent ially washed with water, acetone and water to remove unreacted
reagents and monomer fragments. Finally, the material was fi ltered, collected, and
freeze-dried or oven-dried at 80 °C (both drying methods yield similar materials).
Synthesis of COF/rGO aerogel . Well -groun d PTSA (5 9.4 mg, 0.3 1 mmol ), Dq
(13.4 mg, 0.0 56 mmol) , an d 5 mL of wa ter were mixe d thor oughly and sh aken well in
a vorte x shaker for 5 min. The yell ow solut ion was added into 4.3 mL of 5 mg mL − 1
GO di spers ion dr opwise an d stirr ed for 30 min to ob tain th e homog eneous dis per-
sion . Then, 7.8 mg of Tp (0 .037 mmol) wa s added an d the mixt ure was sh aken for
20 min. The vi scous li quid was tr ans ferred in to an auto clav e and heat ed at 120 °C fo r
24 h. Then, th e obta ined hy drog el was seque ntial ly washed wit h water , aceton e and
wat er. Af ter fr eeze -dry ing, th e COF/rG O aerog el was ob tain ed. Fo r compari son ,
anot her two CO F/rG O aerog els wi th diffe rent rat ios of monomer s and GO we re also
fabr icated un der the sa me condit ions , whil e only chan ging the mas s of PTSA , Dq
and Tp to 29.7 mg, 6.7 mg , 3.9 mg or 118. 8 mg 26.8 mg, 15 .6 mg, respe ctive ly. The
mas s rati os of monom ers: GO were 0.5 :1 and 2:1, resp ect ively . The obtain ed COF/
rGO aer ogel s are deno ted as 0.5 :1 and 2: 1, resp ect ively .
Electrochemical measurement . All samples tested as electrode materials were
measured in a symmetric two-electrode supercapacitor device with 0.5 M H
2
SO
4
aqueous solution as electrolyte. Each electrode with a thickn ess of about 2.0 mm
was prepared by cutting down the samples with a blade. A Ver nier caliper was used
to measure its length ( L ), width ( W ), and height ( H ) accurately. A fi lter paper and
two Au plates were used as separator and electron collectors, respective ly. The mass
of each electrode was calculated according to m = ρ LWH , where ρ is the bulk
density of the aerogel. Two identical electrodes were used as cathode and anode for
the device con fi guration (Suppl ementary Fig. 21). The CO F can be directly used as
a free-standing electrode by tableting the TpDq-COF powder under a pressure of 5
kN for 2 min (Supplementary Fig. 24). The thickness of the COF electrode is about
75 μ m. The device con fi guration for the COF electrode is the same as for the COF/
rGO electrode (Suppl ementary Fig. 23). A mixt ure of COF and conductiv e material
was obtained by mixing and grinding 58.5 wt.% of COF and 41.5 wt.% of carbon
super P. The resulting powder was tableted under a pressure of 5 kN for 2 min to
obtain the CO F-C-mix electrode. The mass of both electrode materials was ~1.0
mg. The speci fi c capacit ance and capacity of COF-C-mix is calculated based on the
mass of the COF.
CV curves and electrochemical impedance spectroscopy of the COF/rGO
electrodes were investigated on using a Gamry Reference 600 Potentiostat. GCD
behaviors were measured on CT3001A LAND battery testing system. The potential
range for CV and GCD tests was 0 – 1.5 V. Before the measurements, the capacitor
cell was soaked in the electrolyte for 3 h. The speci fi c gravimetric capacitance ( C
g
)
was calculated from galvanostatic discharge curves using the equation: C
g
= 4I Δ t/
m Δ V , where C
g
(F g − 1 ) is the gravimetric capacitance, I = constant discharge
current, Δ t = discharge time, m is the total mass of both electrodes, Δ V = discharge
voltage excluding the voltage drop (that is, Δ V = 1.5 V − V
drop
). The speci fi c
capacity at each electrode was calculated using the equation: Q = C
g
× Δ V = 4I Δ t/
m , where Q (C g − 1 ) is the speci fi c capacity stored, I = constant discharge current,
Δ t = discharge time, m is the total mass of both electrodes.
Absorption measurement . Before the measurements, the samples were degassed
in a vacuum oven at 70 °C for 6 h. The samples were immersed in the various
organic solvent for 5 min at the room temperature. The weight was recorded before
( W
initial
) and after ( W
adsorption
) absorption to calculate the weight gain. The
absorption capacity of the samples was calculated according to ( W
adsorption
−
W
initial
)/ W
initial
× 100%. The recyclability test was performed for ethanol by
repeating the absorption process after heating treatment at 100 °C for 2 h.
Characterization . Powder X-ray diffraction (PXRD) patterns were carried out on a
Bruker D8 Advan ce instrument with Cu K α radiation ( λ = 1.54 Å) operating at 40
kV and 40 mA. PXRD patterns were collected at a scanning speed of 2° min − 1 in
the range of 2° ‒ 60°. N
2
sorption measurements was measured on a Quantachrome
Quadrasorb SI instrument with degassing temperature of 120 °C for 12 h before the
measurement. The speci fi c surface areas were calculated by using
Brunauer – Emmett – Teller (BET) calculations and the pore size distributions were
obtained from the adsorption branch of isotherms by the non-localized dens ity
functional theory (NLDFT) model. XPS spectra were conducted on Thermo Fisher
Scienti fi c ESCALAB 250Xi. The scanning electron microscop e (SEM) measurement
were conducted on Gemini SEM 500 low vacuum high-resolution SEM. Thermo-
gravimetric analyses (TGA) were carried out on a Mettler Toledo TGA/DSC1 Star
System analyzer at a heating rate of 10 °C min − 1 under N
2
atmosphere. The
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Fourier transform infrared spectroscopy (IR) analyses of the samples were per-
formed on Varian 640IR spectrometer equipped with an ATR cell. Transmission
electron microscope (TEM) images were performed on FEI Tecnai G 2 20 S-TWIN
electron microscope with an operating voltage of 200 kV. STEM measurement was
performed with an additional upgrade using a DISS5 scan generator, attached with
a BF/ADF/HAADF-STEM detector. Elemental analyses were perfo rmed on a
Perkin-Elmer 240 element al analyzer. Atomic Force Microscopy (AFM) was
measured on Cypher AFM Microscope Asylum Research c/o Oxford Instruments
with AC Mode/Tappi ng Mode. The AFM data were analyz ed by Asylum Research
Software. The samples for AFM analyses were prepared by dispersing the samples
in a mixed solution of ethanol and water, followed by spin coating onto a Si wafer
(100). Mechanical property of the aerogels were carried out on Shimadzu AGS-X.
Data availability
The data that sup port the fi ndings of this study are available from the corresponding
authors upon request. The source data underlying Fig. 1 c – e, 2 f – h, 3 c – d and 4 a – c, e, f are
provided as a Source Data fi le. Source data are provi ded with this paper.
Received: 30 March 2020; Accepted: 18 August 2020;
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Acknowledgements
We would like to thank Jana Lutzki and Prof. Michael Gradzielski in the Stranski lab/
work group Physical Chemistry/Molecular Material Science for help in the measurement
and analysis of AFM. We thank Christi na Eichenauer for assisting in N
2
sorption and
TGA measurements, Maria Unterweger for conduc ting XRD and XPS measurements,
and Jun Wang for assisting in freeze-drying. This work was fi nancially supported by the
China Scholarship Coun cil (CSC) and the Deutsche Forschungsgemeinsch aft (DFG,
German Research Foundatio n) under Germany ’ s Excellence Strategy — EXC 2008/1
(UniSysCat) — 390540038.
Author contributions
C.L. conceived and designed the exp eriments. J.Y. performed the synthesis of Tp. C.L.
performed all performance measurem ents. J. Y., C.L., and P.P. contributed to the dis-
cussion of the synthesis of COF. C.L. and S.L. contributed to the discussi on of the
electrochemical data. M.Y. conducted the SEM analysis. J.S. analysed the N
2
adsorption
data. C.L. and A.T. wrote the manuscript. A.T. supervised the project. All authors dis-
cussed the results and commented on the manuscript.
Funding
Open Access funding provided by Projekt DEAL.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https:// doi.org/10.1038/s41467-
020-18427-3 .
Correspondence and requests for materials should be addressed to A.T.
Peer review information Nature Communic ations thanks Zhiyong Tang and the other,
anonymous, reviewer(s) for their contrib ution to the peer review of this work. Peer
reviewer reports are available.
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