Macro/Microporous Covalent Organic Frameworks for Efficient Electrocatalysis

Article abstract of DOI:10.1021/jacs.9b01226

Covalent organic frameworks (COFs) are of interest for many applications originating from their mechanically robust architectures, low density, and high accessible surface area. Depending on their linkers and binding patterns, COFs mainly exhibit microporosity, even though COFs with small mesopores have been reported using extended linkers. For some applications, especially when fast mass transport is desired, hierarchical pore structures are an ideal solution, e.g., with small micropores providing large surface areas and larger macropores providing unhindered transport to and from the materials surface. Herein, we have developed a facile strategy for the fabrication of crystalline COFs with inherent microporosity and template-induced, homogeneously distributed, yet tunable, macroporous structures. This method has been successfully applied to obtain various β-ketoenamine-based COFs with interconnected macro-microporous structures. The as-synthesized macroporous COFs preserve high crystallinity with high specific surface area. When bipyridine moieties are introduced into the COF backbone, metals such as Co²⁺ can be coordinated within the hierarchical pore structure (macro-TpBpy-Co). The resulting macro-TpBpy-Co exhibits a high oxygen evolution reaction (OER) activity, which is much improved compared to the purely microporous COF with a competitive overpotential of 380 mV at 10 mA/cm². This can be attributed to the improved mass diffusion properties in the hierarchically porous COF structures, together with the easily accessible active Co²⁺-bipyridine sites.

Full text of DOI:10.1021/jacs.9b01226

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Article
Macro/Microporous Covalent Organic
Frameworks for Efficient Electrocatalysis
Xiaojia Zhao, Pradip Pachfule, Shuang Li, Thomas Langenhahn, Mengyang Ye,
Christopher Schlesiger, Sebastian Praetz, Johannes Schmidt, and Arne Thomas
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019
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Macro/Microporous Covalent Organic Frameworks for Efficient
Electrocatalysis
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Xiaojia Zhao,† Pradip Pachfule,† Shuang Li,† Thomas Langenhahn,† Mengyang Ye,† Christopher
Schlesiger,‡ Sebastian Praetz,‡ Johannes Schmidt,† Arne Thomas*†
† Department of Chemistry, Division of Functional Materials, Technische Universität Berlin, Hardenbergstraße 40, 10623
Berlin, Germany.
‡ Institute of Optics and Atomic Physics, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany
ABSTRACT: Covalent organic frameworks (COFs) are of interest for many applications originating from their mechanically
robust architectures, low density and high accessible surface area. Depending on their linkers and binding patterns COFs mainly
exhibit microporosity, even though COFs with small mesopores have been reported using extended linkers. For some applications,
especially when fast mass transport is desired, hierarchical pore structures are an ideal solution, e.g. with small micropores
providing large surface areas and larger macropores providing unhindered transport to and from the materials surface. Herein, we
have developed a facile strategy for the fabrication of crystalline COFs with inherent microporosity and template-induced,
homogeneously distributed, yet tunable, macroporous structures. This method has been successfully applied to obtain various β-
ketoenamine-based COFs with interconnected macro-microporous structures. The as-synthesized macroporous COFs preserve high
crystallinity with high specific surface area. When bipyridine moieties are introduced into the COF backbone, metals such as Co2+
can be coordinated within the hierarchical pore structure (macro-TpBpy-Co). The resulting macro-TpBpy-Co exhibits a high OER
activity, which is much improved compared to the purely microporous COF with a competitive overpotential of 380 mV at 10
mA/cm2. This can be attributed to the improved mass diffusion properties in the hierarchically porous COF structures, together with
the easily accessible active Co2+-bipyridine sites.
INTRODUCTION
porous and crystalline COFs (COF-1 and COF-5) by Yaghi
and co-workers in 2005, several new COFs have been studied
for application in gas separation, heterogeneous catalysis,
optoelectronics and energy storage.3,19,22,23 However, due to
their synthesis condition, most reported COFs are synthesized
and isolated as microcrystalline powders, exhibiting porosities
restricted to the microporous or small mesopore regime, which
largely limits their applications for diffusion-limited
processes. Even though synthetic strategies to generate
secondary porosities within COFs have been explored,
including hollow spherical COF with mesoporous wall and
imine COF with hollow microtubular morphology,24-26 the
obtained hierarchical porous COFs usually exhibited
disconnected and uncontrolled porous architectures. The
principle limitation towards the synthesis of hierarchical COF
structures is the instability of templates in the COF synthesis
reaction medium, as most of the COFs have been synthesized
solvothermally under acidic or basic conditions. In these
regards, the synthesis of meso- or macroporous COF with
controllable porosity structures is very challenging, yet highly
desirable, particularly for diffusion-limited process such as
oxygen evolution reaction (OER), where a macroporous
structure can efficiently enhance the formation and transport
of O2 gas bubbles, thereby promoting the reaction.27-30
Extended crystalline microporous materials have been
intensively studied within recent years because of their
fascinating structural features, but also due to as they have a
range of possible applications from chemical process
industries to advanced uses in optics, electronics, catalysis and
drug delivery.1-11 However, the sole presence of micropores in
many crystalline porous materials often imposes diffusion
limitations, which is especially problematic for catalytic
processes requiring essential mass transfer as in this case just a
low utilization of active sites can be achieved.6 The
construction of hierarchical porous structure, that is the
introduction of meso- or macropore in crystalline microporous
materials is considered to be an effective way to minimize
these adverse effects, and a few progressive examples
including mesoporous crystalline zeolites4,6,12 and macroporous
MOFs13,14 have been achieved and it was shown that the
introduction of hierarchical porosity is advantageous by
substantially improving mass transfer and accessibility of the
high surface areas.
Covalent organic frameworks (COFs), as an emerging class
of crystalline porous materials with well-defined and
predictable structures exhibit low density and high accessible
surface areas.3,11,15-21 Just after the first successful report of
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Figure 1. Synthesis and characterization of macro-microporous COF. a) Schematic representation of macro-TpBpy fabrication in presence
of PSs. b) PXRD pattern of TpBpy, PS@TpBpy, and macro-TpBpy showing the preservation of crystallinity in macro-TpBpy with and
after removing the PSs. Inset: Space-filling packing diagram of TpBpy COF. c) N2 adsorption-desorption isotherms of the TpBpy and
macro-TpBpy. d) Pore size distribution of the TpBpy and macro-TpBpy, calculated from the adsorption branch of isotherms by the
NLDFT model.
Herein, we report a practical and efficient approach for the
preparation of crystalline and hierarchical COF structures, that
is with macropores in addition to their inherent microporosity
(macro-TpBpy), via template-assisted method. Polystyrene
spheres (PSs) have been utilized as hard template to enable the
formation of interconnected macropores within COFs
structures. In particular, the size of macropores can be tuned
through the size modulation of PSs and the composition can be
altered by changing the COF precursors. The obtained
macroporous COFs preserve a high crystallinity with high
specific surface area, hierarchical pore structures and
chemically stable frameworks, hence resulting in fast mass
transport and more accessible active sites. Fast mass and ion
transport in hierarchical COF structures have been confirmed
by utilizing cobalt coordinated bipyridine-based COFs as an
OER catalyst. The as-synthesized Co-coordinated macro-
microporous COFs (macro-TpBpy-Co) exhibit excellent
performance as OER catalysts, especially compared to the Co-
coordinated purely microporous COF and but also to most of
the reported cobalt-doped polymer-based electrocatalysts,
proving the advantage of hierarchically COF structures for
catalytic application.
mmol) to form an organic salt. Then the resulting salt and Tp
(63 mg, 0.3 mmol) were thoroughly shaken for 10 min, and
then the dispersion was poured into a petri dish to evaporate
excess water at room temperature for overnight. Further
drying at 80 °C for 24 h yield an orange-colored PS@TpBpy
composite (Figure S1). Subsequent, soxhlet extraction of
PS@TpBpy was conducted using tetrahydrofuran (THF) as
solvent to remove the PS template as well as unreacted
monomers, to obtain COF with macroporous structures
(macro-TpBpy). In a control experiment, pristine TpBpy was
prepared following the same process, but in the absence of PSs
(TpBpy).
To test the crystallinity of the macroporous COFs, powder
X-ray diffraction (PXRD) measurements were conducted for
macro-TpBpy, PS@TpBpy and pristine TpBpy. As shown in
Figure 1b, macro-TpBpy displays an intense peak at 3.6° (2θ)
corresponding to the reflection from the (100), very similar to
the pristine TpBpy, confirming the retention of the crystalline
structure of TpBpy even after removing the PS template.32 The
presence of broad peaks at 26° (2θ) were assigned to the (001)
plane that corresponds to the π-π stacking of the COF layers.
In addition to the essential peaks, PS@TpBpy shows a broad
peak at 19° (2θ) attributing to the amorphous PS, while the
disappearance of this peak in macro-TpBpy indicates the
complete removal of PS template after the soxhlet treatment.
Nitrogen sorption isotherms were collected to examine the
specific surface area and microporosity of pristine TpBpy and
macro-TpBpy. As shown in Figure 1c, the isotherm of pristine
TpBpy exhibits a typical type I isotherm with a steep increase
at low relative pressures, indicating the microporosity of the
COF, which was further corroborated using the corresponding
pore size distribution (Figure 1d). In the case of macro-TpBpy,
the isotherm also shows a type I isotherm but with an
additional significant increase in nitrogen uptake at high
relative pressure, indicating that an additional macroporosity
RESULTS AND DISCUSSION
Macroporous TpBpy (macro-TpBpy) was synthesized by
reacting the organic linkers 1,3,5-Trifomylphloroglucinol (Tp)
and 2,2'-bipyridine-5,5'-diamine (Bpy) in the presence of PSs
via Schiff base reaction, as schematically illustrated in Figure
1a. The monodispersed colloidal PSs used as hard template
has been synthesized according to the literature reported
procedure13 and p-Toluenesulfonic acid (PTSA) was used as a
catalyst for the COF formation.16,31 In the first step, the
colloidal PSs (~10 wt% in water, 5 ml) were mixed with the
Bpy linker (83.8 mg, 0.45 mmol) and PTSA (475.5 mg, 2.5
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has been successfully introduced by the PS template. The
THF, the obtained macro-TpBpy displays a replica of the PS
spheres yielding an interconnected uniform macroporous
structures (Figure 2c). In order to confirm the uniformity of
the macroporous structure within the materials, cross-section
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calculated pore volume further confirms the presence of
macroporosity in hierarchical macro-TpBpy, as the total pore
volume of macro-TpBpy (1.273 cm3/g) is much higher
compared to pristine TpBpy (0.381 cm3/g) (Table S2). On the
other hand, the calculated Brunauer-Emmett-Teller (BET)
surface areas of macro-TpBpy (723 m2/g) is higher than that of
pristine TpBpy (588 m2/g), showing that the formation of
macropores can efficiently promote the accessibility of
micropores.
SEM analyses were performed,
a
pronounced inner
macroporous structures was observed, similar to the porous
structure seen at the outer surface of macro-TpBpy particles
(Figure 2d). The corresponding statistical diameter of the
macropores was calculated to illustrate the macropore size
distribution (Figure S4). Furthermore, transmission electron
microscopy (TEM) prove the interconnected porous structures
(Figure 2e, f), which should be advantageous for mass transfer
e. g. during heterogeneous catalysis. In contrary, the pristine
TpBpy synthesized without PS template, features bulk
particles without any characteristic morphologies (Figure S5).
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As it can be expected that the nature of the template affect
the pore structure and morphology of the COF replica, we
studied the influence of particle size and template amount on
the porosity as well as the crystallinity of the resulting
structures. PS suspensions with different solid content (5 wt%
and 15 wt% in water) were applied to prepare the
corresponding macro-TpBpy (denoted as macro-TpBpy-5wt%
and macro-TpBpy-15wt%). Both samples show comparable
PXRD patterns as seen before, demonstrating the retention of
the robust COF framework structure (Figure S6). However,
macro-TpBpy-5wt% exhibits an inhomogeneous macroporous
structure with larger non-templated areas due to the low
amount of PSs used (Firgure S7), while the morphology of
macro-TpBpy-15wt% is comparable to macro-TpBpy-10wt%
(Figure S8). Subsequently, a series of macro-TpBpy with
macropore sizes ranging from ~160 nm to ~ 360 nm were
prepared by applying different sized PS templates (Figure 3a-
c). The corresponding macro-TpBpy replicas show the
expected macroporous sizes of 160, 320 and 360 nm,
respectively (Figure 3d-f), all with retained crystallinity
(Figure S9), confirming that this approach can be applied for
various PS templates. Furthermore, to identify the general
applicability of the proposed strategy, several other amine-
linkers (benzidine (BD); 2,6-dimethoxybenzidine (BD-Me2);
p-azoaniline (Azo) and 4,4''-diamino-p-terphenyl (TD)) were
used to fabricate the corresponding marcro-COFs (Figure S10-
13). The PXRD patterns of these macro-COFs show intense
reflection peaks corresponding to the (100) plane at the 2θ
value of 3.6° (macro-TpBD), 3.6° (macro-TpBD-Me2), 3.1°
(macro-TpAzo), and 2.9° (macro-TpTD), respectively. N2
sorption analyses of these macroporous COFs exhibit the
typical type I isotherm with the additional increase in nitrogen
uptake at high relative pressure, consistent with the one
observed for macro-TpBpy (Figure S14). In addition, SEM
images show the pronounced macroporous structures for each
of these macro-COFs, confirming the versatility of this
approach to other amine-linkers (Figure S10-13).
Figure 2. SEM images of a) monodisperse colloidal PSs, b)
PS@TpBpy, and c, d) macro-TpBpy. e, f) TEM images of macro-
TpBpy.
The morphological features of the as-synthesized PS-COF
composite and macroporous COF were characterized by
scanning electron microscopy (SEM) analyses. Figure 2a
shows the successful preparation of highly dispersed and
uniform sized PSs with an average diameter of ~270 nm. The
monodisperse colloidal PSs form a stable dispersion in
aqueous solution without any aggregation or sedimentation,
and thus can be easily used for the fabrication of homogeneous
PS-COF composites (Figure S2). As shown in Figure 2b, the
PS@TpBpy composites display a pile of densely packed PS
nanoparticles with a coating of TpBpy COF on the surface, as
confirmed from high magnification images, indicating the
formation of homogeneous PS-COF composites (Figure S3).
After removing the PS template using soxhlet extraction with
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Figure 3. SEM images of PSs and macro-TpBpy with macropore sizes of a, d) ~160 nm, b, e) ~320 nm and c, f) ~360 nm. g) General
synthesis of macro-COFs from different amines (BD, BD-Me2, Azo and TD) in presence of PSs. h) The corresponding PXRD patterns of
macro-TpBD, macro-TpBD-Me2, macro-TpAzo and macro-TpTD and their respective simulated structures. Inset: Space-filling packing
models of respective COFs.
Co-based catalytic systems are widely investigated as
promising electrocatalysts for OER.33-41 In this approach, the
bipyridine amine linker was therefore deliberately chosen as it
can strongly coordinate to metal ions, mimicking metal-
bipyridine molecular systems. Herein, as a proof of concept,
cobalt ions were used to form macro-TpBpy-Co by soaking a
defined amount of macro-TpBpy in cobalt acetate
(Co(OAc)2·4H2O) solutions in methanol and further stirring
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for 4 h at room temperature (Figure 4a).32, As reference,
TpBpy-Co was prepared by following the same protocol but
using pristine TpBpy. The PXRD pattern of macro-TpBpy-Co
and TpBpy-Co show that the crystallinity of the COF
frameworks is retained after cobalt coordination (Figure
S15a). N2 sorption analyses of macro-TpBpy-Co and TpBpy-
Co show a decreased BET surface area of 387 and 314 m2/g,
respectively, probably due to the increased mass by adding the
cobalt salt (Figure S15b). In addition, SEM images reveal the
preservation of the uniformly distributed macroporous
structures (Figure S15c, d), which are supposed to be
important to promote the accessibility of active sites and mass
transfer during the OER electrocatalytic process.
Subsequently, X-ray photoelectron spectroscopy (XPS) was
conducted to confirm the presence of Co species at macro-
TpBpy-Co and TpBpy-Co, yielding an amount of 2.84 and
3.01 atomic%, respectively (Figure S16a and Table S2). The
high resolution Co 2p spectra exhibit two main peaks centered
at 781 eV and 797 eV, which were assigned to the Co 2p3/2 and
Co 2p1/2 core levels, respectively, and the feature of the
satellite at 786 eV specifies the presence of Co2+ (Figure
S16b).43 The high resolution N1s spectra of macro-TpBpy-Co
showed an extra peak corresponding to Co-N bond appearing
at 399.6 eV compared with macro-TpBpy, pointing to a
Figure 4. Synthesis and characterization of macro-TpBpy-Co. a)
Schematic representation of the synthesis of macro-TpBpy-Co. b)
coordination of Co ions to the pyridine N within the COF
High resolution N1s spectra of macro-TpBpy and macro-TpBpy-
backbone (Figure 4b).44 Similar spectra confirm the existence
Co. c) Normalized Co K-edge XAS spectra for macro-TpBpy-Co
Co-N bond in the TpBpy-Co sample as well (Figure S16c). To
referenced by Co3O4 and Co powder; The inset shows the
further elucidate the nature of the Co species in macro-TpBpy-
Co, X-ray absorption spectroscopy (XAS) was performed. As
shown in the normalized Co K-edge spectra (Figure 4c), the
rising-edge position of macro-TpBpy-Co is located between
magnified image of the pre-edge XAS spectra. d) HAADF-STEM
images and the corresponding element mapping of C, N, O, and
Co.
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the Co powder and Co3O4 standard, indicating that the
electrode surface. For comparison, the electrocatalytic
activities of macro-TpBpy, TpBpy-Co and commercial RuO2
with the same mass loading of 0.25 mg/cm2(geometrical area)
were studied. As shown in Figure 5a, macro-TpBpy-Co
requires an overpotential of 380 mV versus the reversible
hydrogen electrode (RHE) to achieve an anodic current
density of 10 mA/cm2, much lower than that of macro-TpBpy,
TpBpy-Co and some other reported Co cation-based catalysts
(Figure S18 and Table S4). In particular, the macro-TpBpy-Co
catalyst shows an overpotential of only 430 mV at current
density of 50 mA/cm2, which is 170 mV lower than the
benchmark RuO2 catalyst (overpotential of 600 mV at 50
mA/cm2). In the corresponding Tafel slope, both macro-
TpBpy-Co and TpBpy-Co exhibit a lower linear slope of 54
mV/dec and 58 mV/dec, respectively, than that of macro-
TpBpy (339 mV/dec) and RuO2 (79 mV/dec), indicating
favorable reaction kinetics in the OER process (Figure 5b).
The superior OER performance of macro-TpBpy-Co with
respect to TpBpy-Co can be attributed to the homogeneous
and interconnected macroporous structures, which was further
confirmed by the normalization process from current density
(j) to the activity per active sites (jreal/site) (for details see SI,
Section 1). As shown in Figure 5c, a scatter plot containing
jreal/site versus applied potential was established in which a
much higher jreal/site for macro-TpBpy-Co indicates an
enhanced catalytic activity for active sites compared to
TpBpy-Co. In particular, under a higher potential with larger
amount of oxygen production, the prominent enhancement of
jreal/site for macro-TpBpy-Co gives a strong evidence for the
advantage of an additional macroporous structure that
efficiently promote the mass transfer and the accessibility of
active sites to reactants during electrocatalytic process. The
OER performance of macro-TpBpy-Co with different
macropore size have also been conducted under identical
conditions, yielding almost similar OER activities (Figure
S19a), showing that all tested pores sizes in the macropore
regime are enabling sufficient mass transport. On the other
hand, the negligible electrocatalytic activity of macro-TpBpy
evident the important role of Co ions in COFs which can
strongly coordinate with bipyridine structure to form the OER
active sites as well as improving the conductivity of catalytic
system (Figure S19b).33
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oxidation state of Co within macro-TpBpy-Co is +2 and +3.
The Fourier transformed extended X-ray absorption fine
structure (EXAFS) data reveals a variation of the neighboring
environment for Co2+, where macro-TpBpy-Co displays one
main peak at 1.5 Å, which is clearly distinct from Co3O4
standard, Co powder and Co(acac)2, confirming the existence
of Co-N first coordination shell (Figure S17). Moreover, high-
angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) imaging was conducted to
reveal the morphology and element distribution in the macro-
TpBpy-Co. The HAADF STEM images and corresponding
elemental mapping for C, N, O and Co prove the uniform
distribution of N and Co species within the macro-TpBpy-Co
matrix (Figure 4d).
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To verify the importance of Co ions in the catalysts, the
influence of thiocyanate ions (SCN-) on the OER activity of
macro-TpBpy-Co was investigated, due to the strong chelating
ability of SCN- with metal-centered catalytic sites.45,46 On
introducing SCN- into the electrolyte (10 mM), the
overpotential of macro-TpBpy-Co increased substantially at
current density of 10 mA/cm2, indicating the critical role of Co
ions during the OER catalytic process (Figure 5d).47 Moreover,
the cyclic voltammetry (CV) of macro-TpBpy-Co and TpBpy-
Co show the presence of redox peaks at 1.0-1.1 V (vs RHE)
Figure 5. OER performance of macro-COF-Co. a) OER
polarization curves and b) corresponding Tafel plots for macro-
TpBpy, macro-TpBpy-Co, TpBpy-Co and a commercial RuO2
catalyst. c) Scatter plot of jreal/site for macro-TpBpy-Co and
TpBpy-Co, versus applied potential. d) Chronopotentiometry
response of macro-TpBpy-Co as OER catalysts on addition of
10ꢀmM KSCN in 0.1ꢀM KOH after about 200 s. Inset: OER
polarization plots of macro-TpBpy-Co with and without 10ꢀmM
KSCN in 0.1ꢀM KOH, indicating that SCN− ions strongly poison
the macro-TpBpy-Co catalyst. e) Faradaic efficiency of macro-
TpBpy-Co for oxygen production. f) Chronopotentiometry test of
macro-TpBpy-Co and RuO2 catalysts at a constant current density
of 10 mA cm-2. Inset: OER polarization plots of macro-TpBpy-Co
before and after 40000s.
which can be attributed to the quasi-reversible Co(II)/Co(III)
48
couple (Figure S20).33,
The electrochemical surface area
(ECSA) of the COFs was estimated by measuring the
capacitance of the double layer (Cdl) at the solid-liquid
interface using CV. TpBpy and macro-TpBpy both show
moderate electrochemical surface areas, with a small increase
in ECSA for the COF with additional macropores, macro-
TpBpy (Figure S21).
In order to analyze the role of hierarchical porosity in
heterogeneous catalysis, the OER activity of macro-TpBpy-Co
in an alkaline aqueous electrolyte (0.1 M KOH) using a typical
three-electrode system was evaluated. The catalysts were
casted onto a rotating disk electrode (RDE), rotating at a rate
of 1600 rpm to counteract bubble accumulation at the
Subsequently, other metal ions such as iron (Fe2+), nickel
(Ni2+), copper (Cu2+), zinc (Zn2+) and manganese (Mn2+) were
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the required overpotential after 10 h of testing. The OER
polarization of macro-TpBpy-Co only exhibits a slight
negative shift (38 mV shift of the overpotential at 10 mA/cm2)
after 40000s, indicating its excellent stability in
electrochemical OER under alkaline condition. The
electrochemical surface area (ECSA) of macro-TpBpy-Co
before and after electrocatalytic catalysis was also estimated
by measuring the capacitance (Figure S21). The calculated Cdl
of macro-TpBpy-Co-40000s is 1.93×10-5 F cm-2 after long-
term electrocatalysis, very similar to the one observed for
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performance test.
XPS analyses were performed to show the nature and
environment of the different metals in the macroporous COFs.
The high resolution N1s spectra show an additional peak
which can be attributed to metal-N bonds, thus confirming the
coordination of metal ions to the pyridine moieties within the
COF backbone (Figure S22). Furthermore, the high resolution
spectra of each metal show their different oxidation states
without any detected metallic (0) species (Figure S23), which
is consistent with the PXRD results, showing no peaks for
metal particles (Figure S24). The Cdl of these catalysts were
also calculated based on capacitance showing the equivalent
electrochemical active sites for each material (Figure S25).
Subsequently, the OER performance and EIS of these catalysts
were conducted under the same conditions as for macro-
TpBpy-Co (Figure S26). However, all catalysts with other
metal species than Co exhibit a lower OER activity, showing
the advantage of Co in the here described COFs for enhanced
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macro-TpBpy-Co (1.83×10-5
F
cm-2). Subsequently, the
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structural features and chemical compositions of macro-
TpBpy-Co after OER stability tests was investigated using
SEM and XPS analyses. As shown in Figure 6a and 6b, the
morphology and macroporous structures for macro-TpBpy-Co
before and after OER analysis is preserved. XPS analyses of
macro-TpBpy-Co-40000s was conducted to analyze the
chemical and structural changes after long-term reaction under
alkaline condition. As shown in Figure 6c and 6d, no distinct
changes for the binding energy at N1s and Co2p levels
compared to that of macro-TpBpy-Co are observed, except for
the disappearance of the satellite peak (786 eV) at Co 2p in
macro-TpBpy-Co-40000s, which shows that both oxidation
states, Co (II) and Co (III), are present after OER. In addition,
the high resolution O1s spectra show a change of the peak at
533.1 eV after OER that can be assigned to a cobalt
oxyhydroxide formed after long-term catalytic process under
alkaline condition (Figure S28). Finally, OER analyses under
high current density of 50 mA/cm2 was conducted showing a
satisfying chemical stability even after a long-term reaction
(Figure S29).
OER activity.
The turnover frequency (TOF), indicating the intrinsic
catalytic activity of a catalyst at atomic level, has been
calculated to build a plot of TOF values against the applied
overpotential (Figure S27). It can be observed that macro-
TpBpy-Co shows the highest TOF value compared to TpBpy-
Co and RuO2. Moreover, the Faradaic efficiency of macro-
TpBpy-Co was determined by comparing the experimentally
produced gas volume with the theoretically calculated one
(Figure 5e, for details see SI). The good agreement of the
experimentally generated and theoretically calculated amount
of O2 reveal a Faradaic efficiency of ~98% for OER in 0.1 M
KOH, confirming the high activity as well as efficiency of
macro-TpBpy-Co.
CONCLUSION
In summary, we have developed a pathway to fabricate
hierarchical and crystalline COFs with interconnected and
uniform macroporous structures by employing a facile PS
mediated templating method. The obtained macroporous
COFs preserve their crystallinity and high specific surface
areas. Confirmation of enhanced mass transport in hierarchical
COF structures was given by testing Co-coordinated
bipyridine-based COFs as OER catalysts. The macro-TpBpy-
Co exhibits excellent performance as OER catalysts, with
much improved activity compared to the only microporous
TpBpy-Co and also compares favorably to other reported
cobalt-doped polymer-based electrocatalysts. The enhanced
performance of macro-TpBpy-Co can be attributed to the
improved mass diffusion properties in the hierarchically
porous structure, together with more accessible active sites.
The here presented PSs templating method was successfully
extended to obtain various macro-microporous imine-based
COFs by modulating the corresponding amine-linkers,
showing its potential as a convenient route for the preparation
of macro-microporous COFs with different functional groups
for various applications.
Figure 6. Structural features of macro-TpBpy-Co after
electrocatalytic analyses. SEM images a) before, and b) after and
high-resolution c) N1s and d) Co 2p spectra of macro-TpBpy-Co
before and after 40000s of OER testing.
ASSOCIATED CONTENT
Supporting Information.
Supplementary experimental methods, and characterization of the
materials and products. “This material is available free of charge
via the Internet at http://pubs.acs.org.”
Electrochemical stability of is an important factor for
catalysts in electrocatalytic process, especially for polymer-
based catalysts. To value the stability of macro-TpBpy-Co
after OER studies, a chronopotentiometry test was carried out
at a current density of 10 mA/cm2 in 0.1 M KOH. As shown in
Figure 5f, the catalyst shows almost negligible difference in
AUTHOR INFORMATION
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Journal of the American Chemical Society
Porous Covalent-Organic-Framework Membranes. Adv. Mater. 2017,
29, 1603945.
Corresponding Author
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* E-mail: arne.thomas@tu-berlin.de
17. Diercks, C. S.; Yaghi, O. M. The Atom, the Molecule, and the
Covalent Organic Framework. Science 2017, 355, 1585.
18. Pachfule, P.; Acharjya, A.; Roeser, J.; Langenhahn, T.;
Schwarze. M.; Schomacker, R.; Thomas, A.; Schmidt, J. Diacetylene
Functionalized Covalent Organic Framework (COF) for
Photocatalytic Hydrogen Generation. J. Am. Chem. Soc. 2018, 140,
1423-1427.
19. Segura, J. L.; Mancheno, M. J.; Zamora, F. Covalent Organic
Frameworks Based on Schiff-base Chemistry: Synthesis, Properties
and Potential Applications. Chem. Soc. Rev. 2016, 45, 5635-5671.
20. Feng, X.; Ding, X. S.; Jiang, D. L. Covalent Organic
Frameworks. Chem. Soc. Rev. 2012, 41, 6010-6022.
21. Halder, A.; Karak, S.; Addicoat, M.; Bera, S.; Chakraborty, A.;
Kunjattu, S. H.; Pachfule, P.; Heine, T.; Banerjee, R. Ultra-Stable
Imine-based Covalent Organic Frameworks for Sulfuric acid
Recovery: An Effect of Interlayer Hydrogen Bonding. Angew. Chem.
Int. Ed. 2018, 57, 5797-5802.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was financially supported by the China Scholarship
Council (CSC), the Alexander von Humboldt foundation, the
Deutsche Forschungsgemeinschaft (DFG, German Re-search
Foundation) under Germany´s Excellence Strategy – EXC 2008/1
(UniSysCat) – 390540038 and the Berlin Graduate School of
Natural Sciences and Engineering (BIG-NSE). We are grateful to
Christoph Fahrenson and Jan Ron Justin Simke for the SEM and
TEM measurements.
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