Perfluorinated Covalent Triazine Framework Derived Hybrids for the Highly Selective Electroconversion of Carbon Dioxide into Methane

Article abstract of DOI:10.1002/anie.201807173

Developing cost-effective electrocatalysts for high-selectivity CO₂ electroreduction remains challenging. We herein report a perfluorinated covalent triazine framework (CTF) electrocatalyst that displays very high selectivity in the electroredu

Full text of DOI:10.1002/anie.201807173

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Accepted Article
Title: Perfluorinated Covalent Triazine Framework for High-selectivity
Electroconversion CO2 into CH4
Authors: Yuanshuang Wang, Junxiang Chen, Genxiang Wang, Yan Li,
and Zhenhai Wen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201807173
Angew. Chem. 10.1002/ange.201807173
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10.1002/anie.201807173
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COMMUNICATION
Perfluorinated Covalent Triazine Framework Derived Hybrids for
High-selectivity Electroconversion CO2 into CH4
Yuanshuang Wang[a] +, Junxiang Chen[a] +, Genxiang Wang[a] +, Yan Li[a], Zhenhai Wen[a]*
Abstract: Developing cost-effective electrocatalysts for high-
selectivity CO2 electroreduction remains a daunting challenge. We
here report a perfluroinated covalent triazine framework (CTF)
electrocatalyst that holds an impressive high selectivity towards CO2
electroreduction conversion into CH4 with faradaic efficiency of
99.3% in aqueous electrolyte. Systematic characterization and
electrochemical study, coupling with density functional theory
calculations, demonstrate that co-covalent of nitrogen and fluorine in
CTF provides a unique pathway that is inaccessible with the
individual components for CO2 electroreduction, contributing to the
catalytic sites with high selectivity for CO2 conversion into CH4.
tetrafluoroterephthalonitrile as source.[6h] For comparison, a
series of nitrogen and fluorine co-doped carbon (FN-CTF-X, X
represent the synthetic temperature) were prepared by further
treatment of FN-CTF-400 at different temperature, and nitrogen
and chlorine co-doped CTF (ClN-CTFs) were prepared using the
same method by using F-free precursors (Supporting
Information, Figure S1).[6g]
The synthesis process is schematically described in Figure 1a,
in which the FN-CTF-400 samples were prepared by a one-step
polymerization with the assistance of ZnCl2. The morphologies
were examined by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). One can see the FN-
CTF-400 holds
a
three-dimensional (3D) structure with
Fossil fuels have become the major energy sources to meet
the global energy demands for economic and social
development since the industrial revolution. The excessive
consumption of fossil fuels would discharge the vast amount of
CO2 that could destruct the carbon cycle among ocean, land,
and atmosphere, causing the enormous worries on the emerged
global warming issues.[1] It is thus highly desirable to develop an
effective strategy or technique to re-balance the carbon cycle by
reducing the amount of CO2 in the atmosphere. Electrochemical
reduction of CO2 has been regarded as a promising route to
recycle CO2 into value-added fuels and chemicals with
harnessing electricity generated from renewable energy sources
(e.g., windy, solar).[2] Therefore, tremendous efforts have been
put into exploring efficient electrocatalysts, including a variety of
transition metal and their complexes, and metal free carbon
materials are promising candidates.[3] Nevertheless, we still must
strive to develop robust electrocatalysts that can highly
selectively reduce CO2 in aqueous electrolyte.[2d, 4]
interconnected polymer framework according to the SEM image
(Figure 1b) and a large amount of micropores based on TEM
observation (Figure 1c). The high resolution TEM (HRTEM)
image for the FN-CTF-400 is displayed in Figure 1d, revealing
the sparse crystalline lattice with space of ~0.39 nm, as
evidenced by the corresponding intensity profile for the line scan
across the lattice fringes (inset of Figure 1d). The single
graphitic layers are seen to be bending likely due to the fluorine
and nitrogen doping.[7] And the energy dispersive X-ray
spectroscopy (EDS) elemental map indicates that N and F are
homogeneously distributed on the entire structure (Figure 1e
and Figure S2). The morphologies for the other counterpart
samples were also characterized with displaying similar
morphology to the FN-CTF-400 (Figure S3).
Recently, covalent triazine framework (CTF), due to its
ultralarge surface area, has received tremendous attentions for
a variety of potential applications in the fields of catalytic
supports, gas capture and storage, energy conversion and
storage, optoelectronics and semiconductors.[5] Moreover, it is
rather convenient to tune heteroatom doping in CTF by using
heteroatom-containing molecular building-block as resource,[6]
offering the possibility for developing CTF evolved materials as
tunable electrocatalysts for CO2 reduction. Unfortunately, the
metal-free CTF based materials as CO2 reduction electrocatalyst
is rarely reported so far although highly deserved.
We herein report the synthesis of perfluorinated covalent
triazine framework (FN-CTF-400) with the ionothermal
assistance of
molten ZnCl2
at
400°C
by using
Figure 1. (a) Reaction schemes and the structure of the FN-CTF-400; (b)
SEM image of FN-CTF-400; (c) TEM image and (d) HRTEM image with
corresponding intensity profile for the line scan across the lattice fringes of the
FN-CTF-400; (e) the EDS elemental mapping of the FN-PCTF-400.
[a]
Y. S. Wang, J. X. Chen, G. X. Wang, Y. Li, and Prof. Dr. Z. H.
Wen
The samples were also studied using a variety of techniques,
including X-ray power diffraction (XRD), Raman spectra, X-ray
photoelectron spectroscopy (XPS), and N2 adsorption/desorption.
XRD patterns for the FN-CTF-400 and the CTF derived
materials (i.e., FN-CTF-550, FN-CTF-700 FN-CTF-900) exhibit
two weak and broad peaks (Figure S4), implying its poor
crystallinity. Raman spectra of the FN-CTFs samples obtained at
different temperature were performed to further study the local
CAS Key Laboratory of Design and Assembly of Functional
Nanostructures, Fujian Provincial Key Laboratory of
Nanomaterials, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences
Fuzhou 350002, P. R. China
E-mail: wen@fjirsm.ac.cn
These authors contributed to this work equally
+
Supporting information for this article is given via a link at the end
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10.1002/anie.201807173
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disorder and crystallinity. The intensity ratio of peaks for
structure defects (ID) and crystalline graphitic carbon (IG) in the
samples enhance slightly with decreasing temperature (Figure
2a), suggesting the samples obtained at increased temperature
likely decrease the defects of samples. Additionally, the
elemental composition, chemical state and electronic state of the
elements of the FN-CTFs samples were investigated by X-ray
photoelectron spectroscopy (XPS). Figure 2b displays the
survey XPS spectra for the set of FN-CTFs samples. The FN-
CTF-400 manifests an eye-catching signal at 686.04 eV
corresponding to F1s,[6h] and the FN-CTF-550 presents a
significant decrease in F1s peak intensity, while the FN-CTF-700
and the FN-CTF-900 notably show no F1s peak. Moreover, with
the increased synthetic temperature, the FN-CTFs samples
display distinctly decrease in intensity for N1s accompanying
with the increase in intensity for C1s, which indicate that
increasing temperature would diminish the doping level of
heteroatom, being consistent with the decreased defects density
verified by Raman analysis. We made a further analysis on the
high resolution XPS spectrum of N1s and F1s for the FN-CTFs
(Figure 2c-d and Figure S5). The F1s spectra for the FN-CTF-
400 (Figure 2d) can be deconvoluted into two component peaks
at 687.3 and 690.0 eV, corresponding to covalent C-F and semi-
ionic C-F, respectively,[8] while there are no signals of F element
in XPS spectra of FN-CTF-700 and FN-CTF-900 (Figure S5d
and 5f).
isotherms curves for the FN-CTFs samples. The FN-CTF-900
exhibits a type-IV isothermal curve with hysteresis loop at high
P/P0 value that indicates complex micro-mesoporous structure.
The other three FN-CTFs samples while feature type-I isotherm
curve that imply the dominant microporous structure. The FN-
CTF-400 holds a BET surface area 817 m2 g-1, this value is
remarkably lower than those of FN-CTF-550 (1131 m2 g-1), FN-
CTF-700 (2040 m2 g-1), and FN-CTF-900 (2162 m2 g-1),
indicating more pores were produced with increasing
temperature. Pore size distributions were estimated according to
the adsorption branch of the nitrogen isotherms by density
functional theory (DFT). The pore size distributions studies show
that FN-CTF-400 and FN-CTF-550 have a major peak centered
at around 0.54 nm and some other peaks below 2 nm. FN-CTF-
700 has two pore size distributions at 1.2 nm and 2.8 nm, and
FN-CTF-900 has a major peak centered at 0.5 nm and 4.5 nm
with significantly broad distribution in the microporous and
mesoporous range (Figure S6). Figure 2f presents the CO2
adsorption/desorption isotherms curve for the set of FN-CTFs
samples. All the materials have good CO2 adsorption likely
because of their surface functionalities. The sequence of the
CO2 capturing values at 1 bar for CTFs is FN-CTF-700 (6.02
mmol g-1), FN-CTF-550 (4.61 mmol g-1), FN-CTF-900 (4.01
mmol g-1), and FN-CTF-400 (2.66 mmol g-1).
The electrocatalytic properties of the CTFs samples towards
CO2 electroreduction were investigated in a closed three-
electrode system with CO2 saturated aqueous 0.1 M KHCO3
solution as electrolyte. Based on cyclic voltammetry (CV) curves
(Figure S7), one can observe the FN-CTF-400 shows
significantly increased current density in CO2-saturated
compared with that in N2-saturated electrolyte plus the onset
potential at around -0.1 V, demonstrating the FN-CTF-400
exhibits good catalytic activity for CO2 reduction. We further
analyzed the products generated in the range of -0.4 to -1.2 V
upon electrocatalysis, to this end, the gas and liquid products
were monitored by gas chromatography (GC) and nuclear
magnetic resonance, respectively. In this way, the catalytic
selectivity of CO2 electro-reduction can be fairly evaluated.
Figures 3a-3d outline the faradaic efficiency (FE) of various
CTF-based CO2 electrocatalysts for production of CH4, CO, and
H2, which are denoted as FECH4, FECO, and FEH2, respectively.
Notably, the FN-CTF-400 behaves a highly catalytic selectivity
toward CO2 electroreduction into CH4 with
a dominant
competitive advantage over hydrogen evolution reaction, as
evidenced by a FECH4 of around 78.7% with a small FEH2 of
below 15% in potentials between -0.4 and -0.6 V (Figure 3a).
More impressively, the FECH4 at FN-CTF-400 can approach up
to 99.3% in the potential range of -0.7 ~ -0.9 V and gradually
decrease to 65% with enhancement in FEH2 at potential above
1.0 V. The FN-CTF-550 exhibits a decreased selectivity for CH4
production with a peak FECH4 of ~ 62.3 % at -0.7 V and a relative
high FEH2 of 41.6% at -0.4 V (Figure 3b). The FN-CTF-700 and
the FN-CTF-900, in contrast, tend to favor catalytic conversion
of H2O and CO2 into H2 and CO rather than CH4. Specifically,
the FN-CTF-700 shows a peak value (58.2 %) for FECO at -0.8 V
and a peak value (57.16 %) for FEH2 at -0.4 V (Figure 3c), and
the FN-CTF-900 shows the highest value for FECO (42 %) at -0.7
V and for FEH2 (80 %) at -0.4 V (Figure 3d). It should be pointed
Figure 2. (a) Raman spectra and (b) XPS survey spectra of the FN-CTFs; (c)
high resolution N 1s XPS spectra and (d) high resolution F 1s XPS spectra for
FN-CTF-400 (e) N2 adsorption/desorption isotherm curve and (f) CO2
adsorption/desorption isotherm curves for FN-CTFs
The adsorption/desorption properties of gas molecules of N2
and CO2 are investigated by Brunauer-Emmett-Teller (BET)
method. Figure 2e presents the N2 adsorption/desorption
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out there are no liquid hydrocarbon compound products for all
FN-CTF electrocatalysts based on NMR testing reports.
saturated electrolyte was from CO2 other than the catalysts or
electrolyte. The electrochemical surface areas for FN-CTF-400
and the bare carbon paper were evaluated based on the double
layer capacitive (Cdl) by cyclic voltammetry (Figure S11a and
b).[9] The Cdl of FN-CTF-400 (965 μF cm-2) is far larger than that
of the bare carbon paper (3.58 μF cm-2) (Figure S11c),
confirming the CO2RR activity stems from the FN-CTF-400
catalysts.
The above results evidence the FN-CTF-400 possess an
impressively high catalytic selectivity toward CO2 conversion into
CH4, it is thus desirable to study what peculiarity in FN-CTF-400
give rise to such high-selectivity catalysis. The FN-CTF-400,
according to characterization and previous report,[6d,10] was likely
a composite consisting of majorly polymer and small amounts of
pyrolyzed carbon, because it was prepared at a relatively low
temperature. The polymer in FN-CTF-400 provides desirable
structureres for capturing CO2 molecules and hydrophobic
environment to suppress hydrogen evolution,[6h,11] while the F/N
codped carbon offer the favorable catalytic activity sites for CH4
production. For the other FN-CTF samples, most of the polymer
structures have turned into graphite structure that tends to
catalyze CO2 reduction into CO. Besides, it is interesting to
investigate the role of F-doping due to the varied F-doped level
in the set of FN-CTF samples. The FN-CTF-400, which possess
the highest F-doping level, indeed exhibits the highest selectivity
for CO2 electroreduction into CH4 (i.e., FECH4), followed by the
FN-CTF-550 that hold the second position in F-doping level and
FECH4. For both the FN-CTF-700 and the FN-CTF-900 that do
not contain fluorine, they favorably catalyze reduction CO2 into
CO, rather than CH4. It is thus rational to infer the F-doping in
FN-CTF do play a key role in selectively reducing CO2. We
additionally prepare chloride containing TIF (ClN-CTF-400) with
performing the associated characterization (Figure S12-14). The
electrochemical measurements (Figure S15) are conducted to
study its catalytic activities and selectivity, which indicates the
ClN-CTF-400 also perform decently toward selectively
catalyzing conversion CO2 into CH4 with a maximum FECH4 of
around 74.9 % at -0.8 V. In addition, three N-containing only
CTFs (N-CTF, Figure S16-S19) were also prepared by using the
same method as FN-CTF-400 with F-free materials as
precursors, respectively. All the three N-CTF catalysts show a
preferred CO products with a relatively low FECH4 (Figure S20)
and a tiny CH4 production rate (Figure S21) over the potential
range of -0.2~-1.2 V. The set of electrochemical experiments on
controlled samples strongly evidence that the F-doping plays a
critical role in regulating the selectivity of the product in the
electrocatalysis of CO2 reduction.
Figure 3. Faradaic efficiency for CH4, CO and H2 versus potential on
electrodes of (a) FN-CTF-400; (b) FN-CTF-550; (c) FN-CTF-700; and (d) FN-
CTF-900. (e) the production rates for H2, CO and CH4 at FN-CTF-400 at
different applied potential; (f) current densities corresponding to CH4
production on the set of FN-CTFs samples.
The production rates were also calculated by GC analysis of
the gas production (Figure 3e and Figure S8). As expected, with
increasing applied voltage, the FN-CTF-400 manifests an
enhanced production rate of CH4 from 1.05 mol h-1 m-2 at -0.7 V
to 7.75 mol h-1 m-2 at -1.2 V (Figure 3e), with a much higher CH4
production rate than that of the FN-CTFs-550 (Figure S8) and
the other two FN-CTFs counterparts, which show a high
production rate for either H2 or CO (not show). Figure 3f depicts
the current density regarding to CH4 production at different
potential for the set of samples. The FN-CTF-700 and the FN-
CTF-900 show no CH4 production in the potential range of -0.4 ~
-1.2 V. By contrast, the FN-CTF-400 and the FN-CTF-550
manifests a remarkable current density corresponding to CH4
production starting at -0.4 V, and a gradual increase in current
density with enhancing the applied voltage until reach a stable
value at -1.2 V. Obviously, the FN-CTF-400 performs better with
a much higher current density of CH4 than the FN-CTF-550. The
catalytic stability for selectively CO2 reduction (into CH4) was
also evaluated through monitoring variation of the FECH4 at
intervals, the FN-CTF-400 can still maintain a FECH4 of ~91.7%
after 5-hours continuous running at -0.8 V (Figure S9). We
conducted the electrochemical tests at FN-CTF-400 electrode in
N2 saturated electrolyte and the bare carbon paper electrode in
CO2 saturated electrolyte with monitoring the products (Figure
S10), no CH4 product is produced in both cases, verifying that
the carbon in CH4 production at FN-CTF-400 electrode in CO2
Density functional theory (DFT) calculation was performed to
study why the FN-CTFs-400 electrocatalysts hold such high
selectivity in conversion of CO2 into CH4. To this end, the
computational hydrogen electrode (CHE) method proposed by
[12]
Nørskov et al
was applied, in which the limiting potential (Ul)
was calculated to estimate the activity and selectivity (Sec 2 of
supporting information). The computational models were built
based on the physical characterization, where N and F -doped
carbon are modeled with all the possible doping configurations
in references of the former work[13] as depicted in Figure S1. The
reaction pathway for CO2 elctroreduction into CO (g) and CH4 (g)
[8] is described as RS1-RS14 (Sec. 2.2 of supporting information).
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The CHO* pathway (RS4-RS8) is more likely to be the exact
pathway,[7] as indicated in Figure S22. For N doping, the edge
graphite N, including the single-N graphitic (gN) and pair-N
graphitic (2gN), are the active sites, while for N & F co-doping,
pyrrolic N site is also active for CO2 electroreduction. Figure 4
displays the top views and the associated DFT reaction free
energy diagram (FED) of the active sites for CO2
electroreduction. Figure 4a-4b and Figure 4c-4e represent the
N-CTF and NF-CTF catalysts, respectively. According to the
FEDs, it is clear that, among all the active sites, only Edge-gN is
more prone to produce CO, while the other four active local
configurations produce CH4: (1) Experiments illustrates the TB-
CTF-400, NP-CTF-400 and NB-CTF-400 with only N-doping
structures produce both CH4 and CO (Figure S20a-S20c), based
on our DFT calculations, the existence of both Edge-gN and
Edge-2gN are the catalytic sites for these samples, since these
two sites are responsible for the production of CO (Figure 4a)
and CH4 (Figure 4b), respectively; (2) FN-CTF-700 (Figure 3c)
and FN-CTF-900 (Figure 3d) are also the N-doped structures,
while the high temperature treatment impair activity of the pair-N
(Edge-2gN) due to destruction, leading to that Edge-gN become
the main contribution. CO accordingly becomes the main
product on these two catalysts; (3) when F is covalent to the N-
containing catalysts, the Edge-gN-F (Figure 4c) and Edge-2gN-F
(Figure 4d) will be formed accompanying with pyrrolic N as
active site upon F doping (Figure 4e). According to their FEDs
(Figure 4c-4e), CH4 will be the main products for FN-CTF-400.
This matches well with the Faraday efficient diagram for FN-
CTF-400, which suggests CH4 is the main product; (4) For FN-
CTF-550, some of the F would escape upon carbonization, the
originally existing site Edge-gN-F in FN-CTF-400 has recovered
to the F-free site Edge-gN, which is active site for the production
of CO. Such correspondence to the experimental selectivity
verifies the accuracy of DFT based mechanisms, revealing the
key active sites and their aiming products, which could be an
important guidance for strategies of synthesizing more advanced
materials that possess higher selectivity and activity on CO2RR
in the future.
In summary, we prepared a set of covalent triazine framework
(CTF) catalyst with tunable the surface functionality by
introducing a variety of heteroatom doping. We demonstrate the
CTF with substantial F-covalent can function as
a high-
selectivity electrocatalyst for CO2 electroreduction to CH4 with a
faraday efficiency of approaching 100%. The systematic
electrochemical study, coupling with DFT calculation,
demonstrate the ultrahigh selectivity observed for FN-CTF-400
can be attributed to the doped F which regulates the activity of N
and make it more conductive to CH4 production. This work
provides an important guidance for strategies to design and
synthesize advanced electrocatalysts that possess high
selectivity and activity on CO2 electroreduction.
Acknowledgements
We would like to thank 1000 Plan Professorship for Young
Talents, Hundred Talents Program of FuJian Province, the
Fujian Science and Technology Key Project (Item Number.
2016H0043), the National Natural Science Foundation of China
(Grant No. 21703250) for financial support.
Keywords: High-selectivity electrocatalysis • CO2 reduction•
Covalent triazine framework• Catalytic sites
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10.1002/anie.201807173
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Yuanshuang Wang [a] +, Junxiang Chen
[a] +Genxiang Wang[a] +, Yan Li[a] and
Zhenhai Wen*[a]
The F-containing COF materials
shows an unprecedented high
selectivity with faradaic efficiency of
99.3% toward electrochemical
conversion of carbon dioxide into CH4.
DFT calculation demonstrates that co-
covalent of nitrogen and fluorine
contributes the catalytic sites for CH4
production.
Page No. – Page No.
Perfluorinated Covalent Triazine-
based Framework as High-selectivity
Electrocatalysts for Conversion CO2
into CH4
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