The potential of a graphene-supported porous-organic polymer (POP) for CO2 electrocatalytic reduction

Article abstract of DOI:10.1039/c6cc06773e

A one-pot, bottom-up assembly of a pyrimidine-containing porous-organic polymer (PyPOP) was conducted to homogenously deposit the PyPOP atop unmodified graphene sheets, affording a composite material PyPOP@G. The PyPOP demonstrated an appreciable affinity toward CO₂ capture but was found to be largely insulating, hindering its usage in potential electrochemical conversion of CO₂. However, its composite with graphene was found to be microporous, with maintained affinity toward CO₂ and furthermore demonstrated significant electrochemical activity toward CO₂ reduction (5 mA cm^-2 at -1.6 V), not observed in either of its two components separately.

Full text of DOI:10.1039/c6cc06773e

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The potential of a graphene-supported
porous-organic polymer (POP) for CO2
Cite this:DOI: 10.1039/c6cc06773e
electrocatalytic reduction†
Received 18th August 2016,
Accepted 2nd September 2016
Ahmed B. Soliman,ab Rana R. Haikal,a Youssef S. Hassana and
Mohamed H. Alkordi*a
DOI: 10.1039/c6cc06773e
www.rsc.org/chemcomm
A one-pot, bottom-up assembly of a pyrimidine-containing porous-
Porous organic polymers (POPs) are an emerging class of
organic polymer (PyPOP) was conducted to homogenously deposit the microporous solids with robust carbon backbones, accessible
PyPOP atop unmodified graphene sheets, affording a composite through various synthetic techniques. They are currently drawing
material PyPOP@G. The PyPOP demonstrated an appreciable affinity wide attention for CO2 capture, storage, and sequestration.19–22
toward CO2 capture but was found to be largely insulating, hindering Graphene (G), with its superior properties including mechanical
its usage in potential electrochemical conversion of CO2. However, its strength and thermal and electrical conductivity, has also
composite with graphene was found to be microporous, with main- attracted attention for various applications.23–25 As POPs and
tained affinity toward CO2 and furthermore demonstrated significant graphene constitute two distinct solids with highly contrasting
electrochemical activity toward CO2 reduction (5 mA cmÀ2 at À1.6 V), synthesis and chemical functionality, an approach to merge
not observed in either of its two components separately.
the realms of both into a composite material for application
in CO2 capture and/or conversion is of interest. As both solids
The current interest in technologies to capture and sequester are virtually insoluble in all common solvents, the approach
CO2 emissions has stimulated interest in materials capable of described here relies on bottom-up assembly of POPs from their
both CO2 capture and catalytic transformation.1,2 The bottom- soluble molecular precursors atop non-modified graphene. For
up approach of constructing microporous solids with high synthesis of POPs, the Sonogashira–Hagihara (SH) cross coupling
surface area enabled scientists to access tailorable platform reactions were successfully utilized to construct a large family of
materials. Fine tuning of these materials through judicious such microporous solids from judiciously selected molecular
selection of the building blocks resulted in demonstrated building blocks (MBBs).26
performance for heterogeneous catalysis.3,4 Of particular interest
Herein, we describe a synthetic strategy to construct a novel
are materials acting as non-noble metal-based heterogeneous composite material targeting CO2 capture and conversion. Two
catalysts for catalytic conversion of CO2 into feedstock reagents.5,6 essential components were incorporated in the design of this
Recently, catalytic moieties like metallo-salens were successfully solid; namely, a pyrimidine-based POP (PyPOP) that demon-
incorporated into the backbone of microporous solids, facilitating strated a microporous character and good affinity to CO2,20 and
catalytic conversion of CO2 into cyclic carbonates under thermal graphene, with its known desirable electrical conductivity.
conditions.7,8 However, the vast majority of such microporous
In a one-pot reaction, the bottom-up synthesis of the PyPOP
solids have poor electrical conductivity, hindering access to was conducted through an SH cross coupling reaction, in the
the rich arena of electrochemical processes. Several attempts presence of dispersed, non-modified graphene. This reaction
have recently been made to enhance the electrical properties of resulted in the homogenous deposition of the PyPOP on top
these materials.9–13 Such attempts afforded materials for appli- of graphene sheets, referred to here as PyPOP@G, Scheme 1.
cation in catalytic oxygen reduction,14,15 supercapacitors,16,17 The Fourier-transform infrared spectroscopy (FT-IR) conducted
as well as energy storage.18
on the PyPOP@G confirmed the presence of the PyPOP in the
composite, ESI.† The FT-IR spectrum of the PyPOP@G showed
several peaks characteristic of the PyPOP, including the internal
nCRC stretching at 2215 cmÀ1, pyrimidine nCQN stretching at
1564 cmÀ1 and absence of the terminal nC–H stretching found at
B3200 cmÀ1 for the starting triethynylbenzene. Further char-
acterization for the composition of the product composite
included elemental analysis, indicating significant changes
a Center for Materials Science, Zewail City of Science and Technology,
Sheikh Zayed Dist., 12588, Giza, Egypt. E-mail: malkordi@zewailcity.edu.eg
b Chemistry Department, Faculty of Science, Ain-Shams University, Abbasia, Cairo,
11566, Egypt
† Electronic supplementary information (ESI) available: Experimental procedure,
and further characterization studies including FTIR, TGA, elemental analysis, and
gas sorption measurements. See DOI: 10.1039/c6cc06773e
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Scheme 1 The one-pot, bottom-up, synthesis of the PyPOP@G. C (orange), N (blue), H (white), G (green hexagons).
to C and N contents for the PyPOP@G as compared to the Both were found to be microporous with comparable apparent
PyPOP. The observed trend of increased C content and decreased surface area, as calculated using the Brunauer–Emmett–
N content, when comparing the PyPOP to the PyPOP@G, is Teller (BET) model, Fig. 2 (BET surface area of 664 m2 gÀ1
expected due to incorporation of purely carbon-based G into the and 582.7 m2 À1, respectively). The surface area of the gra-
g
matrix of the PyPOP. The elemental analysis results, shown in phene used in this study was also measured (BET surface area
the ESI,† indicated the following composition for graphene C, of 445 m2 gÀ1), Fig. 2. Overlaying the three isotherms facilitated
94.3% and for PyPOP (PyPOP@G): C, 71.55% (76.4%); H, 2.66% the comparison which revealed that the isotherm of the com-
(2.95%); N, 21.14% (6.64%). Additional characterization of the posite occurred midway between the two isotherms for the
PyPOP and the PyPOP@G was conducted through thermogravi- PyPOP and G, as might be expected for coating the less porous G
metric analysis (TGA, ESI†). The TGAs revealed the thermal with the more porous PyPOP. Furthermore, the surface area of
stability of the PyPOP up to B350 1C, after which noticeable the composite (582.7 m2
g
À1) closely matched the weighted
and gradual weight loss was observed due to the thermal degrada- average of the surface areas of its two components, 586 m2 gÀ1
,
tion of the polymer. The same behavior was also observed for the as calculated using the total yield of the composite; 26 wt% was
composite. In the TGA for PyPOP@G, a noticeable weight loss of assigned to the starting amount of G and 74 wt% to the PyPOP
B5 wt% was observed below 100 1C. This change can be ascribed in the composite. We applied the non-local density functional
to loss of adsorbed moisture inside the pores of the PyPOP. The
TGA for G demonstrated a minor weight loss of B5 wt% when
heated up to 900 1C (ESI†). The uniform coverage of G by the
PyPOP was evident from the transmission electron microscopy
(TEM) and scanning electron microscopy (SEM) images for
PyPOP@G, Fig. 1. Further characterization studies including
solid-state NMR spectroscopy and energy-dispersive X-ray analysis
(EDX) are provided in the ESI,† and they confirmed the construc-
tion of the targeted material.
As it was previously demonstrated that the PyPOP, with its
abundant basic binding sites of aromatic pyrimidine rings, has
a favorable binding energy toward CO2,20 we opted to explore
its behavior toward CO2 sorption once composited with the
graphene. N2 sorption isotherms were measured for both
the PyPOP and the PyPOP@G to establish their microporosity.
Fig. 2 (top) N2 gas sorption isotherms for the PyPOP, graphene, and
PyPOP@G (closed symbols for adsorption and open symbols for desorption),
Fig. 1 TEM (left) and SEM (right) images of the PyPOP@G indicating
homogenous coating of the graphene by the PyPOP.
and (bottom) the corresponding PSD histograms (* denotes most significant
changes).
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theory (NLDFT) model of carbon finite pores to the adsorption
points in the three samples to calculate the corresponding pore
size distribution (PSD), Fig. 2. The PSD histograms demonstrated
close resemblance between the PyPOP and the PyPOP@G, which
is in further agreement with the BET surface area findings
discussed above. In the PSD histograms, a noticeable difference
between the PyPOP and PyPOP@G is the small shrinkage of
the pores at B6 Å and the absence of pores at B25 Å in the
composite. This change could be ascribed to subtle structural
changes upon deposition of the PyPOP on the G sheets. Close
inspection of the N2 sorption isotherms indicated the presence
of H2-type hysteresis in the N2 isotherm for the PyPOP, which
was not observed for the PyPOP@G; potentially indicating
rigidification of the PyPOP structure once in contact with the
G sheets. The observed hysteresis for the PyPOP can be ascribed
to a degree of structural flexibility, i.e. breathing of the material
upon N2 uptake, or alternatively can be ascribed to interparticle
Fig. 3 LSV for the PyPOP@G and G as the active material in CPE, 0.1 M
condensation.26,27 The isosteric heat of adsorption (Qst) for KHCO3, saturated with CO2, the inset is for PyPOP@G under N2 saturation
and CO2 saturation conditions.
CO2 into the PyPOP and the PyPOP@G was also measured
through variable temperature CO2 sorption isotherms, ESI.†
Both solids demonstrated nearly identical Qst for CO2 adsorp-
tion (B40 kJ molÀ1 at initial loadings), further confirming that the behavior in N2 saturated solution. The CO2 electroreduction
CO2 adsorption in the composite takes place predominantly on the PyPOP@G is remarkable in terms of the onset potential
within the pores of the POP. Considering the above findings, it (À1.18 V) and current density (5 mA cmÀ2 at À1.6 V) even
appears that graphene has imparted structural rigidity to the when compared to a recently published Cu foam catalyst
composite, while the PyPOP participated largely as the active (B5 mA cmÀ2 at À1.6 V vs. Ag|AgCl).29 Additionally, our
medium for gas sorption. Thus effective synergism of proper- observed onset potential compares well with that of a recent
ties arose from the successful conformal merge between the covalent-organic framework containing Co-porphyrin that
two components.
demonstrated superior catalytic activity (onset potential of
To investigate the potential of the above prepared composite À420 mV RHE @ pH 7.2, calculated to be À1.02 V vs. Ag|AgCl @
for catalytic electrochemical reduction of CO2, linear sweep pH 6.73 of our solution).30 Our catalyst compares well also
voltammetry (LSV) experiments were conducted under specific with a recent report that demonstrated CO2 electrocatalytic
reaction conditions utilized previously (0.1 M KHCO3 solution reduction behaviour for a metal–organic framework based on
saturated with CO2).28 The PyPOP was tested under such Co-porphyrin, with an onset potential of À500 mV vs. RHE in
conditions and was found to be largely insulating while a good 0.5 M K2CO3 solution.31 Finally, when graphene was tested
potential–current response was observed for the PyPOP@G. under the above conditions, Fig. 3, no significant catalytic
This result highlights the enhanced conductivity of the compo- behavior was recorded, further supporting the role of the POP
site due to the presence of the graphene. In the LSV experiment in the catalytic process observed. From the above, it is reason-
shown in Fig. 3, the first run was commenced in N2 saturated able to assume that the PyPOP imparted binding affinity of
solution, to help decipher proton reduction reaction from a graphene toward CO2 and further acted as the catalytic active site
potential CO2 reduction one. A large reduction peak was observed for CO2 electrochemical reduction, while graphene imparted
with an onset potential of À430 mV, and a second reduction wave some degree of electrical conductivity to the PyPOP, necessary to
at À1.38 V. The first reduction peak at À430 mV was assigned to conduct the CO2 reduction effectively. In aqueous solution, it is
an oxygen reduction process. This assignment was further possible for water molecules to compete for the CO2 binding sites
confirmed by the absence of this cathodic peak in the second in N-rich POPs.32 Nevertheless, in the composite described here,
run, commenced immediately after the first run, indicating the PyPOP maintained appreciable affinity toward dissolved CO2,
depletion of oxygen trapped within the pores of the composite facilitating its electrochemical reduction. Moreover, it also
during the electrode preparation. The reduction wave in the appears that cohesion, on the molecular level, between the two
second run appeared unchanged at its corresponding potential components of the composite alleviated the drawbacks from each
and was then assigned to a proton reduction process. When of the components considered separately, namely the poor con-
the solution was then saturated with CO2 through bubbling for ductivity of the polymer and the absence of proper binding sites
30 minutes (pH 6.73), the LSV experiment demonstrated one on the surface of the pristine graphene.
major reduction wave with a potential onset of À1.18 V, signifying
In conclusion, we described a one-pot, bottom-up, pathway
electrochemical reduction of CO2. This potential shift was also for constructing a novel composite material of porous organic
accompanied by increased cathodic current density in the polymer atop graphene sheets for application in CO2 capture
scanned potential window down to À1.6 V, as compared to and electrocatalytic reduction. The uniformity of the coverage
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Sandwich-Type, Graphene-Based Conjugated Microporous Poly-
mers, Angew. Chem., Int. Ed., 2013, 52(37), 9668–9672.
15 X. Zhuang, D. Gehrig, N. Forler, H. Liang, M. Wagner, M. R. Hansen,
F. Laquai, F. Zhang and X. Feng, Conjugated Microporous Polymers
with Dimensionality-Controlled Heterostructures for Green Energy
Devices, Adv. Mater., 2015, 27(25), 3789–3796.
16 K. Yuan, P. Guo-Wang, T. Hu, L. Shi, R. Zeng, M. Forster, T. Pichler,
Y. Chen and U. Scherf, Nanofibrous and Graphene-Templated
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of the graphene by the POP was demonstrated through its TEM
images. The gas sorption characteristics and electrochemical
activity of the composite were also investigated, demonstrating
a true mix of the properties of the two components as a result of
efficient compositing. Work is in progress to further investigate
the nature of the product(s) in the observed catalytic process.
These investigations help guide our future design for platform
materials capable of capture and conversion of CO2 into usable
feedstock reagents.
We acknowledge the funds from the Zewail City of Science
and Technology, the Center for Materials Science, and Egypt’s
Science and Technology Development Fund (STDF-grant 6125).
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