Atomic Ni Anchored Covalent Triazine Framework as High Efficient Electrocatalyst for Carbon Dioxide Conversion

Article abstract of DOI:10.1002/adfm.201806884

Electrochemically driven carbon dioxide (CO₂) conversion is an emerging research field due to the global warming and energy crisis. Carbon monoxide (CO) is one key product during electroreduction of CO₂; however, this reduction proce

Full text of DOI:10.1002/adfm.201806884

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Carbon Dioxide Ronversion
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Atomic Ni Anchored Covalent Triazine Framework as High
Efficient Electrocatalyst for Carbon Dioxide Conversion
Chenbao Lu, Jian Yang, Shice Wei, Shuai Bi, Ying Xia, Mingxi Chen, Yang Hou,*
Ming Qiu,* Chris Yuan, Yuezeng Su, Fan Zhang, Haiwei Liang,* and Xiaodong Zhuang*
chemical, electrochemical, and photo-elec-
Electrochemically driven carbon dioxide (CO ) conversion is an emerging
research field due to the global warming and energy crisis. Carbon monoxide
2
trochemical reduction of CO into versatile
2
[
1]
species. Among them, electrochemical
reduction of CO into valuable chemicals
(
CO) is one key product during electroreduction of CO ; however, this
2
2
as fuels is a clean and sustainable strategy,
which is regarded as a promising way for
management of global carbon balance and
reduction process suffers from tardy kinetics due to low local concentra-
tion of CO on a catalyst’s surface and low density of active sites. Herein,
2
presented is a combination of experimental and theoretical validation of a
Ni porphyrin-based covalent triazine framework (NiPor-CTF) with atomically
dispersed NiN centers as an efficient electrocatalyst for CO reduction reaction
[2]
providing carbon-neutral energy. How-
ever, the conversion efficiency of CO RR
2
is critically limited by the extremely stable
chemical bonds in CO2 and competitive
4
2
(
CO RR). The high density and atomically distributed NiN centers are con-
2
4
[
3]
reduction water to hydrogen. It is imper-
ative to develop efficient electrocatalysts to
improve the product selectivity and accel-
erate sluggish kinetics.
firmed by aberration-corrected high-angle annular dark field scanning trans-
mission electron microscopy and extended X-ray absorption fine structure.
As a result, NiPor-CTF exhibits high selectivity toward CO RR with a Faradaic
2
efficiency of >90% over the range from −0.6 to −0.9 V for CO conversion and
achieves a maximum Faradaic efficiency of 97% at −0.9 V with a high current
To date, certain materials have been
widely explored to improve the perfor-
−2
mance of CO RR, including metals (e.g.,
2
density of 52.9 mA cm , as well as good long-term stability. Further calcula-
tion by the density functional theory method reveals that the kinetic energy bar-
riers decreasing for *CO transition to *COOH on NiN active sites boosts
[
4]
Au, Ag, and Cu), carbon based single-
metal-atom catalysts (e.g., Fe, Co, and
[5]
2
4
Ni), porous organic frameworks (POFs)
the performance.
and metal organic frameworks (MOFs),
[
6]
etc. Among them, POFs are a kind of
interesting materials which can be synthe-
sized by structure-defined building blocks. Most importantly,
POFs possess adjustable pore size, high specific surface area,
and excellent chemical stability, which offer promising pos-
sibilities for design of high performance catalysts. Typically,
covalent triazine frameworks (CTFs) is one of POFs which have
fully conjugated skeleton, ultrahigh specific surface area, and
abundant and flexible heteroatoms, e.g., pyridinic nitrogen spe-
cies. Such properties lead CTFs to exhibit strong affinity to CO2
1
. Introduction
Most of modern environmental and energy issues can be
tracking to excessive CO2 emissions due to explosive global
population growth and corresponding unrestrained energy
consumption. To appease the concerns about energy demands
and global warming crisis, considerable efforts have been com-
[
7]
mitted to exploring technologies for converting CO , such as
2
C. Lu, S. Wei, S. Bi, Prof. Y. Su, Prof. F. Zhang, Prof. X. Zhuang
State Key Laboratory of Metal Matrix Composites & Shanghai
Key Laboratory of Electrical Insulation and Thermal Ageing
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
Y. Xia, Prof. M. Qiu
Institute of Nanoscience and Nanotechnology
College of Physical Science and Technology
Central China Normal University
Wuhan 430079, China
Dongchuan Road 800, Shanghai 200240, China
E-mail: zhuang@sjtu.edu.cn
E-mail: qium@mail.ccnu.edu.cn
M. Chen, Prof. H. Liang
J. Yang, Prof. Y. Hou
Hefei National Research Laboratory for
Physical Sciences at the Microscale
Department of Chemistry
University of Science and Technology of China
Hefei 230026, China
Key Laboratory of Biomass Chemical Engineering of Ministry of
Education and College of Chemical and Biological Engineering
Zhejiang University
Hangzhou 310027, China
E-mail: yhou@zju.edu.cn
E-mail: hwliang@ustc.edu.cn
Prof. C. Yuan
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201806884.
Department of Mechanical and Aerospace Engineering
Case Western Reserve University
DOI: 10.1002/adfm.201806884
10900 Euclid Ave, Cleveland, OH 44106, USA
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and are considered as promising candidates
[8]
for CO RR. Unfortunately, most of reported
2
CTFs exhibit poor electrocatalytic CO RR
2
so far due to lacking of high efficient active
sites, e.g., atomically distributed M–N center
x
M = Fe, Co, and Ni).[ Rational design and
9]
(
synthesis of CTFs with rich atomically dis-
tributed active sites for CO RR remain great
2
challenge.
Herein, we report
and atomically dispersed NiN4 centers
NiPor-CTF) for electrocatalysis CO RR.
a CTF with rich
(
2
As-developed NiPor-CTF was synthesized by
conventional ionothermal approach which
contains high density and atomically distrib-
Figure 1. a) HAADF-STEM image of NiPor-CTF. Scale bar: 2 nm. b) Bright-field image and
elemental mapping for NiPor-CTF.
uted NiN sites. As an electrocatalyst, NiPor-
4
CTF exhibits high CO RR selectivity toward
2
CO production with a low onset potential
of ≈443 mV and high Faradaic efficiency (FE) of >90% at the
range of −0.6 to −0.9 V, as well as good long-term stability. Such
not only with rich NiN active site, but also with fully conju-
4
gated backbone and rich triazine unit, which are normally
beneficial for charge transfer and ion adsorption. For compar-
ison, Por based covalent triazine frameworks (Por-CTF) were
also prepared and NiPor-CTF was further pyrolyzed under
outstanding CO RR performance can be attributed to the syn-
2
ergistic effect of high density exposed NiN active sites and
4
−
1
high CO capture capability of triazine nodes (2.4 mmol g at
2
2
98 K). The density functional theory (DFT) calculation further
N atmosphere at 900 °C to produce Ni/N-rich porous carbon
2
demonstrates that the NiN centers embedded in CTF skeleton
(denoted as Ni/N-PC).
4
decreases the energy barrier for *CO transition to *COOH
The morphology of as-prepared materials was revealed by
scanning electron microscopy (SEM) and transmission elec-
tron microscopy (TEM). Typical aggregated nanoparticles were
found for the CTFs (Figures S1 and S2, Supporting Informa-
tion), which is consistent to the reported CTFs. The high-
resolution TEM images of NiPor-CTF reveal clear light spots
2
on NiN sites and boosts CO RR performance. This work pro-
4
2
vides new strategy for fabricating efficient electrocatalysts with
high density active site by rational design of porous organic
materials.
(
≈0.2 nm, Figure 1a), suggesting the atomically dispersed Ni
centers of the obtained materials. The elemental mapping
Figure 1b) indicates the uniform distribution of C, N, and Ni
2
. Result and Discussion
(
The starting and key building blocks, 5,10,15,20-tetrakis
4-cyanophenyl) porphyrin (Por) and porphyrin nickel (NiPor),
elements among NiPor-CTF. Then, the chemical structure of
as-fabricated CTFs was confirmed by Fourier transform infrared
(FT-IR) spectroscopy. The strong transmission bands located at
(
were synthesized by reported methods (for details, see the
Experimental Section).[10] Then, NiPor based covalent triazine
frameworks (NiPor-CTF) were synthesized through an iono-
thermal strategy in molten ZnCl2 (Scheme 1). Such simple
method can be easily extended to develop versatile porous
organic electrocatalysts by using rational designed building
blocks. Furthermore, this method provides a porous material
−1
1622 and 1382 cm (Figure S3, Supporting Information) indi-
[
11]
cate the successful formation of triazine rings for Por-CTF
and NiPor-CTF, as well as good thermal stability (Figure S4,
Supporting Information). Powder X-ray diffraction (PXRD)
patterns (Figure 2a) reveal that both Por-CTF and NiPor-CTF
exhibit very limited crystallinity, which can be attributed to the
harsh ionothermal reaction condition and nonplanar poly-
[
12]
merization path (Scheme 1).
Besides, no sharp Ni diffrac-
tion peaks appear in NiPor-CTF and Ni/N-PC, indicating no Ni
and Ni-based nanoparticles. Thus, NiPor-CTF with uniformly
distributed Ni can be easily prepared by simple ionothermal
approach.
Considering that the CTFs are one kind of typical conjugated
porous polymers, nitrogen physisorption was performed to
evaluate the porosity of as-prepared materials, (Figure S5, Sup-
porting Information). As a result, the Brunauer-Emmett-Teller
(
BET) specific surface area of Por-CTF, NiPor-CTF, and Ni/N-
2
−1
PC are as high as 1099, 815, and 959 m g (Table 1), respec-
tively. The specific surface area of NiPor-CTF is slight lower
than those for Por-CTF and Ni/N-PC due to the introduction of
Ni element and backbone rearrangement during the high tem-
Scheme 1. Synthesis of NiPor-CTF through ionothermal strategy. Balls
in grey, blue, and pink represent C, N, and Ni atoms, respectively. All H
atoms have been omitted. i) ZnCl , 400 °C for 20 h, and 600 °C for 20 h,
2
[
13]
then HCl (aq.) washing.
perature. Pore size distribution analyzed by nonlocal density
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Figure 2. a) PXRD patterns of Por-CTF, NiPor-CTF, and Ni/N-PC, b) high-resolution N 1s XPS spectra, c) nitrogen species configurations for Por-CTF,
NiPor-CTF, and Ni/N-PC, d) high-resolution Ni 2p XPS spectra for NiPor-CTF and Ni/N-PC, e) C K-edge XANES spectra, and f) N K-edge XANES
spectra of Por-CTF, NiPor-CTF, and Ni/N-PC.
functional theory (NLDFT) method (Figure S5b, Supporting
Information) verified the microporous and mesoporous distri-
pyrrolic-N, graphitic-N, and oxidized-N, respectively.[15] Typi-
cally, pyridinic-N and pyrrolic-N are two main species in Por-
CTF and NiPor-CTF (Figure 2c), which originate from triazine
rings and porphyrin macrocycles. The abundant pyridinic-N in
bution. Besides, CO absorption isotherms were also performed
2
to verify the capability for CO capture of as-prepared materials
2
(
Figure S6, Supporting Information). As shown in Table 1,
the materials can exhibit affinity to CO , which is crucial for
2
[
16]
Por-CTF and NiPor-CTF exhibit higher CO uptake than that
CO activation. In contrast, Ni/N-PC contains lower N per-
2
2
of Ni/N-PC. The higher CO capture capability for CTFs offers
centage (2.77 wt%, Table S1 in the Supporting Information)
and the graphitic-N is the main species, which can be presum-
ably attributed to some rearrangement and decomposition reac-
tions of the aromatic frameworks at high temperature. The
high-resolution C 1s XPS peaks located at ≈284.6, ≈285.6, and
≈288.8 eV are corresponding to CC/CC, CN, and CN/
CO species, respectively (Figures S8–S10, Supporting Infor-
2
more possibilities for CO access and transport in the porous
2
structure, which is the key for following CO conversion at pore
2
surface and active sites.[
14]
The element's composition and valence states for NiPor, Por-
CTF, NiPor-CTF, and Ni/N-PC were inspected by X-ray photo-
electron spectroscopy (XPS). The N 1s spectrum for NiPor only
shows two peaks at ≈398.8 and ≈399.5 eV, which are assigned to
carbonitrile-N and Ni–N (Figure S7, Supporting Information).
For Por-CTF, NiPor-CTF, and Ni/N-PC, the N 1s core level
XPS spectra (Figure 2b) can be fitted into the peaks at ≈398.5,
[
17]
mation). For Ni 2p spectra, two peaks presented at ≈872.7
and ≈855.5 eV for NiPor, NiPor-CTF, and Ni/N-PC can be
assigned to Ni 2p and Ni 2p (Figure 2d), indicating that
1/2
3/2
2+
[18]
Ni is the main state. Typically, two satellite peaks located
at 862.4 and 879.8 eV in NiPor monomer are likely due to the
≈
400.5, ≈401.3, and ≈402.8 eV, attributing to pyridinic-N,
Table 1. N and CO physisorption results of as-prepared materials.
2
2
2
−1 a)
2
−1 b)
3
−1 c)
3
−1 d)
Dav [nm]e)
−1 f)
Sample
SBET [m g
]
Smicro [m g
]
VTot [cm g
]
Vmicro [cm g
]
2
CO [mmol g ]
2
73 K
298 K
Por-CTF
1099
815
113
89
1.77
0.75
0.95
0.13
0.08
0.11
6.5
3.7
4.0
6.7
5.0
3.9
2.5
2.4
1.6
NiPor-CTF
Ni/N-PC
959
94
a)Specific surface area measured by BET model; b)The microporous surface area analyzed by t-plot method; c)The total pore volume measured at P/P0 = 0.99; d)The
e)
f)
microporous volume analyzed by t-plot method; Average pore size; CO2 physisorption data.
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relaxation effect. The Ni contents of NiPor, NiPor-CTF, and
Ni/N-PC are determined as 13.3 wt%, 2.40 wt%, and 1.69 wt%
by XPS analysis, respectively. The relatively low Ni content for
NiPor-CTF can be attributed to the partial decomposition of
Ni porphyrin.
observed comparing to the spectra of Ni-foil and Ni/N-PC
(Figure S12, Supporting Information). For Ni/N-PC, two peaks
located at ≈1.45 and ≈2.15 Å corresponding to Ni–N and Ni–Ni
interactions are found (Figure 3b), suggesting that part of Ni–N
sites have been destroyed (or reduced) and Ni atoms aggregated
under high temperature. Moreover, according to the first shell
fitting of EXAFS results, the NiPor-CTF spectrum fits very well
In order to further study the elemental environment of
as-prepared materials, XANES was examined. As shown in
Figure 2e, Por-CTF, NiPor-CTF, and Ni/N-PC show the similar
C K-edge XANES spectra. The peaks A1 and A2 at ≈284.3 and
with the NiN structure (Figure 3c, details in Table S2 in the
4
Supporting Information). All above results strongly prove that
≈
285.4 eV can be attributed to π* resonance. The weak peak B
the NiPor-CTF with atomically dispersed NiN centers was pre-
4
and the broad peak C located at ≈287.8 and ≈292.2 eV can be
assigned to C–N–C and σ* resonance, respectively.[ In N-edge
XANES spectra, two peaks at ≈401.2 and 407.2 eV attributed
cisely designed and synthesized.
19]
Taking the advantages of high surface area, high content
of NiN sites, fully conjugated backbone, and high CO cap-
4
2
[
20]
to graphitic-N and C–N–C were observed (Figure 2f).
An
ture capability, NiPor-CTF was considered to be one candidate
[8c,9a]
obvious peak at ≈398.5 eV, corresponding to pyridinic-N for
Por-CTF and NiPor-CTF was detected, however, not in Ni/N-PC.
This result indicates again that triazine rings in CTFs would
be degraded and reorganized under high temperature treat-
ment and result in decrease of the pyridinic-N content. Besides,
an additional peak at ≈399.2 eV assigned to NiN bond was
detected in NiPor-CTF and Ni/N-PC, proves the coordination
to electrocatalyze CO RR.
Therefore, the electrocatalytic
2
CO RR performance of obtained materials was evaluated by
2
liner sweep voltammetry (LSV). The potential at the intersection
point of the tangent in nonFaradaic region and Faradic region in
LSV curves is considered as onset potential. The LSV curves in
Figure 4a and Figure S13 in the Supporting Information reveal
that the onset potentials for NiPor-CTF, Ni/N-PC, and Por-CTF
are ≈443, ≈471, and ≈502 mV, respectively, with a current density
[21]
bond of NiN. All of these results are highly congruent with
−2
the XPS results.
of ≈0.5 mA cm . Besides, NiPor-CTF generates a high current
−2
Finally, the XANES and EXAFAS were performed to inves-
tigate the electronic structure of Ni atom in materials. The
XANES spectra of NiPor-CTF and Ni/N-PC in Figure 3a show
comparable D4h symmetry characteristic features (labeled A–D,
where peak A represents 1s to 3d transition, peak B repre-
sents 1s to 4p transition, C and D represent 1s to 4p transi-
density of ≈52.9 mA cm at −0.9 V versus reversible hydrogen
electrode (vs RHE), which is higher than that of Ni/N-PC
−
2
−2
(≈37.1 mA cm ), Por-CTF (≈6.2 mA cm ) and most reported Ni
based catalysts (Table S5, Supporting Information). Such lower
onset potential and higher current density can be attributed to
[2a]
the higher NiN sites (0.51 at%) in NiPor-CTF. Then the elec-
z
x,y
4
[9a]
tions and multiple scatter processes). And a peak appeared
at ≈8334 eV, indicating the pre-edge features of materials;
stronger intensity of pre-edge in NiPor-CTF can be attributed
to the raised dipole-allowed transitions due to the triazine rings
along the backbone.[ This result points out that the electronic
properties of CTFs can be adjusted due to conjugated skeleton
and rich electron-withdrawing triazine rings. In EXAFS, NiPor-
CTF shows a very slight shift of Ni–N first shell for NiPor in
R space (Figure S11, Supporting Information). Such shift is due
to the shortening of NiN bond under high temperature during
polymerization, matching well with the fitting results (Table S2,
Supporting Information). The main peak appeared at ≈1.45 Å
can be assigned to Ni–N first shell for NiPor-CTF (Figure 3b).
No additional Ni–Ni interaction (at ≈2.15 Å, star marked) was
trocatalysis was further performed at specific potentials to trace
the products during CO RR. As the catalytic reaction proceeds,
2
only CO and H gas can be detected, and no liquid products was
2
1
formed by H nuclear magnetic resonance (NMR) spectroscopy
6a]
(Figure S14, Supporting Information). The FE for CO and H2
production of obtained materials was measured (Figure 4b and
Figure S15, Supporting Information). Remarkably, the NiPor-
CTF exhibits high FE for CO production, exceeding 90% at the
range of −0.6 to −0.9 V and reaching to maximum FE of 97% at
−0.9 V (vs RHE), which is much better than most of reported
Ni-based materials (Table S5, Supporting Information). In com-
parison, the Ni/N-PC shows a lower FE for CO production than
NiPor-CTF, but higher than that of CTF (Figure 4b). And the CO
partial current densities for NiPor-CTF and Ni/N-PC were much
Figure 3. a) Ni K-edge EXAFS spectra of Ni-foil, NiPor-CTF, and Ni/N-PC, (inset: magnified images); b) Fourier transformation of the EXAFS spectra
of Ni-foil, NiPor-CTF, and Ni/N-PC; c) first shell fitting of Fourier transformation of EXAFS spectra for NiPor-CTF (inset: the model NiN structure in
4
NiPor-CTF. Balls in grey, blue, and pink represent C, N, and Ni atoms, respectively).
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Figure 4. CO RR performance of as-prepared materials in 0.5 m KHCO . a) LSV curves for Por-CTF, NiPor-CTF, and Ni/N-PC in CO saturated 0.5 m
2
3
2
KHCO solution; inset: enlarged LSV curves. b) CO Faradaic efficiency at various specific potentials, c) partial current density values, d) Tafel slopes
3
for Por-CTF, NiPor-CTF, and Ni/N-PC.
18.5 mF cm− for Ni/N-PC) than that of Por-CTF (6.3 mF cm−2),
indicating the higher active surface area. Typically, Ni/N-PC
exhibits higher turnover frequency (TOF) than that of NiPor-
CTF (Figure S18, Supporting Information), which can be attrib-
2
higher than that of CTF in whole potential (Figure 4c). Thus,
NiPor-CTF shows the best performance, the selectivity trend
toward CO RR is NiPor-CTF > Ni/N-PC >> Por-CTF, indicating
2
that the NiN4 plays an important role for boosting CO RR.
2
Besides, Tafel slopes for catalysts were measured to understand
uted to the higher conductivity and C . Based on the above
dl
the kinetics of CO RR. As shown in Figure 4d, the Tafel slopes
results, NiPor-CTF contains higher Ni and N content, but lower
conductivity, Cdl and TOF than those of Ni/N-PC, suggesting
2
−1
−1
for NiPor-CTF (120 mV dec ) and Ni/N-PC (130 mV dec ) are
−1
smaller than that of Por-CTF (316 mV dec ), suggesting the
that NiN centers play a more important role in electrochemical
4
higher kinetic activities.[
9b,22]
Typically, the Tafel slope values for
CO RR (Table S4, Supporting Information), and NiN centers
2
4
−1
NiPor-CTF and Ni/N-PC are close to 116 mV dec , suggesting
that the rate-determining step is initial electron transfer to
CO to form absorbed CO * intermediate, which is commonly
would be the active site toward CO RR. The specific relation-
2
ship between structure and activity should be further evaluated.
Considering that long-term stability is another significant
parameter for evaluation of electrochemical performance, the
stability of NiPor-CTF and Ni/N-PC were performed continu-
ously at 0.9 V for 20 h (Figure 5c,d). Encouragingly, NiPor-CTF
exhibited approaching stability as Ni/N-PC, with current of a
mild decrease and FE for CO maintains ≈90% after 20 h con-
tinuous reaction (Figure 5c). And LSV curves for NiPor-CTF
and Ni/N-PC only show a slight decrease after 20 h continuous
testing (Figure S19, Supporting Information). In addition, the
elemental contents and species configurations of N 1s and Ni
2p for NiPor-CTF and Ni/N-PC scarcely changed after long-time
test (Figure S20, Supporting Information), indicating the good
stability of catalysts. Such good stability of NiPor-CTF can be
2
2
[23]
appealed for metal based electrodes.
The Nyquist plots and corresponding equivalent circuit of
electrochemical impedance spectroscopy (EIS) are shown in
Figure 5a and Figure S16 in the Supporting Information. The
charge transfers resistant (R ) value for Ni/N-PC is 32.3 Ω,
ct
which is lower than that of Por-CTF (110.2 Ω) and NiPor-CTF
(
93.8 Ω) (Table S3, Supporting Information). The reduced
ct
R value for Ni/N-PC can be attributed to the improved conduc-
tivity after carbonization. In this work, Ni/N-PC shows better
conductivity but lower activity than NiPor-CTF, which may be
due to the higer content of NiN sites in NiPor-CTF. Cyclic
4
voltammetry measurements were then performed at different
−1
[24]
scan rates (varying from 5 to 100 mV s ) to assess the effective
surface area (Figure 5b and Figure S17, Supporting Informa-
tion). As measured, the NiPor-CTF and Ni/N-PC show higher
ascribed to the high chemical and thermal stability of CTFs.
For better identifying active sites in catalysts, the DFT
was also performed. As shown in Figure 6a, Figure S21 and
Figure S22 in the Supporting Information, the possible models
−2
double-layer capacitance (C , 15.2 mF cm for NiPor-CTF and
dl
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Figure 5. a) Nyquist plots for Por-CTF, NiPor-CTF, and Ni/N-PC, b) capacitive current at 0.4 V as a function of scan rate for Por-CTF, NiPor-CTF, and
Ni/N-PC, c) current–time curve for NiPor-CTF, and d) current-time curve for Ni/N-PC.
for NiPor-CTF, Por-CTF, and Ni/N-PC were built to measure
the kinetic energy barriers during the catalysis. According to
the reaction dynamics, CO reduction pathway including CO *
absorption, COOH*, and CO* formation, and CO desorption,
corresponding free energy evolution were calculated (Figure 6b).
COOH* at the potential of 0 V (Figure 6b, solid line), and it
is the rate-limiting step during reaction in which free energies
are determined for 2.003 (Por-CTF), 1.486 (NiPor-CTF), and
1.551 eV (Ni/N-PC), respectively (Figure 6b). Meanwhile, from
the transition states calculation, the intermediate barriers of
2
2
It requires an increasing free energy from CO * transition to
CO * transition to COOH* are 2.334, 0.382, and 0.638 eV for
2
2
Figure 6. a) The DFT-calculated representative models: NiPor-CTF with CO , COOH, and CO adsorbed, b) schematic Gibbs free energy profile for
2
the CO RR pathway on the three models with a pH of 7.2, and c) schematic Gibbs free energy profile for the HER pathway on the three models with
2
a pH of 7.2.
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Por-CTF, NiPor-CTF, and Ni/N-PC, respectively (Figure S23,
Supporting Information). The values for NiPor-CTF and Ni/N-
PC are smaller than that of Por-CTF, indicating that NiN4
centers in NiPor-CTF and Ni/N-PC structure can be effective
and refluxed at 155 °C for 2 h. After removing solvent and washing with
water and ethanol for three times, the solid was recrystallized for three
times to produce NiPor.
Synthesis of Por-CTF and NiPor-CTF: CTFs were prepared by
ionothermal method. Typically, Por (200 mg) and ZnCl2 (2 g) were
ground together in glove box and then transferred into a quartz ampule.
The ampule was sealed and heated to the desired temperature for certain
to decrease the energy barrier of CO * transition to COOH*,
2
which finally boosts CO RR. Furthermore, when the voltage
2
−
1
increases to 0.79 V, reaction steps for NiPor-CTF become ther-
modynamically favorable, which are lower than those of Ni/N-
period (400 °C, 20 h, then 600 °C, 20 h; 5 °C min ). After the ampule
cooled to room temperature, the obtained solid was washed with 1.0 m
HCl three times and dried in vacuum. Por-CTF was then obtained. For
NiPor-CTF, the same process was used by using NiPor as precursor.
PC (0.84 V) and Por-CTF (1.06 V), indicating the best CO RR
2
performance for NiPor-CTF, well consistent with the trend of
NiPor-CTF-400 was synthesized in molten ZnCl under 400 °C for 20 h.
2
experimental results. During CO RR process, hydrogen evolu-
2
Synthesis of Ni and N co-doped porous carbon (Ni/N-PC): NiPor-CTF
was directly pyrolyzed under 900 °C at N2 atmosphere for 2 h. Then,
after washing with 1.0 m HCl three times to remove the unstable Ni
species, Ni/N-PC was obtained.
tion reaction (HER) is the major compete reaction. According
to calculation, the rate-limiting step in HER is H O* decom-
2
position to H* and corresponding free energies for Por-CTF,
NiPor-CTF, and Ni/N-PC are 2.115, 2.550, and 2.429 eV, respec-
tively (Figure 6c). Accordingly, Por-CTF require lower potential
(
0.95 V) to overcome the energy barrier than those of NiPor-
Supporting Information
CTF (1.19 V) and Ni/N-PC (1.09 V). Thus the weaker HER per-
formance for NiPor-CTF and Ni/N-PC should be expected.
Supporting Information is available from the Wiley Online Library or
from the author.
3
. Conclusion
Acknowledgements
In summary, we report a highly porous organic material with
rich and structure-defined atomically dispersed Ni-atom through
C.L. and J.Y. contributed equally to this work. This work was financially
supported by National Natural Science Foundation of China for Excellent
Young Scholars (51722304), National Natural Science Foundation of
China (21720102002 and 5171101862), Shanghai Committee of Science
and Technology (16JC1400703), Shanghai Pujiang Talent Program
an ionothermal strategy. The strong affinity to CO and high den-
2
sity of NiN centers for catalysts ensures the facilitating CO RR.
4
2
Both experimental and computation results demonstrate that
the NiPor-CTF shows the outstanding selective conversion effi-
(18PJ1406100), and Open Project Program of the State Key Laboratory of
Photocatalysis on Energy and Environment (SKLPEE-KF201702, Fuzhou
University). Y.H. thanks the support of NSFC 51702284 and 21878270.
M.Q. thanks the support of Hubei Natural Science Foundation of China
(2018CFB531) and Self-Determined Research Funds of CCNU from
Colleges’ Basic Research and Operation of MOE (CCNU18TS048).
ciency of CO to CO. The particular molecular scaffold coordi-
2
nated with active centers can highly improve the selectivity and
efficiency toward CO RR. Such modularity strategy provides
2
new opportunities for precise design and development of versa-
tile kinds of electrocatalysts for matter conversion.
Conflict of Interest
4
. Experimental Section
The authors declare no conflict of interest.
Chemicals: Pyrrole was purchased from Aladdin Reagent.
p-cyanobenzaldehyde, boron trifluoride diethyletherate (BF ·(OC H ) ),
3
2
5 2
HCl, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and ZnCl were
purchased from Sinopharm Chemical Reagent. Dimethylformamide
2
Keywords
(
DMF), dichloromethane, and Ni(CH COO) ·4H O were purchased
carbon dioxide reduction, covalent triazine framework, energy barrier,
Faradaic efficiency, nickel–nitrogen coordination
3
2
2
from Sigma-Aldrich.
Synthesis of 5,10,15,20-tetrakis(4-cyanophenyl) porphyrin (Por):
Dichloromethane (1 L) was added to three necked round bottom flasks
Received: September 30, 2018
Revised: December 27, 2018
Published online:
(
(
2 L) and purged with argon for 30 min. Then, p-cyanobenzaldehyde
1.3 g) and pyrrole (0.67 g) were added into flask under inert atmosphere.
After stirring for 10 min, 450 mg BF ·(OC H ) was added by syringe.
3
2
5 2
The reaction was light block and stirred for 2 h and the color of mixture
changed from yellow to wine red. Then, DDQ (1.84 g) was added and
the reaction was refluxed (50 °C) for 2 h. The solvent was then removed
under reduced pressure. The obtained powder was washed by methanol
several times and dried in vacuum. After purification by column
chromatography on silica gel with dichloromethane as eluent, the purple
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