Synthesis of a Boron–Imidazolate Framework Nanosheet with Dimer Copper Units for CO2 Electroreduction to Ethylene

Article abstract of DOI:10.1002/anie.202106004

Fundamental understanding of the dependence between the structure and composition on the electrochemical CO₂ reduction reaction (CO₂RR) would guide the rational design of highly efficient and selective electrocatalysts. A major impediment to the deep reduction CO₂ to multi-carbon products is the complexity of carbon–carbon bond coupling. The chemically well-defined catalysts with atomically dispersed dual-metal sites are required for these C?C coupling involved processes. Here, we developed a catalyst (BIF-102NSs) that features Cl^? bridged dimer copper (Cu₂) units, which delivers high catalytic activity and selectivity for C₂H₄. Mechanistic investigation verifies that neighboring Cu monomers not only perform as regulator for varying the reaction barrier, but also afford distinct reaction paths compared with isolated monomers, resulting in greatly improved electroreduction performance for CO₂.

Full text of DOI:10.1002/anie.202106004

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Synthesis of Boron-imidazolate Framework nanosheet with Dimer
Cu units for CO2 Electroreduction to Ethylene
Authors: Ping Shao, Wei Zhou, Qin-Long Hong, Luocai Yi, Lirong
Zheng, Wenjing Wang, Hai-Xia Zhang, Huabin Zhang, and
Jian Zhang
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10.1002/anie.202106004
Angewandte Chemie International Edition
RESEARCH ARTICLE
Synthesis of a Boron-Imidazolate Framework Nanosheet with
Dimer Cu Units for CO2 Electroreduction to Ethylene
Ping Shao,†[a] Wei Zhou,†[b] Qin-Long Hong,[a] Luocai Yi,[a] Lirong Zheng,[d] Wenjing Wang,[a] and Hai-Xia
Zhang,*[a] Huabin Zhang,[c] Jian Zhang*[a]
[a]
P. Shao, Q.-L. Hong, L Yi, Dr. W. Wang, and Prof. H.-X. Zhang, Prof. J. Zhang
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Fuzhou, Fujian 350002, P. R. China
E-mail: Zhanghaixia@fjirsm.ac.cn; zhj@fjirsm.ac.cn.
[b]
[c]
[d]
Dr. W. Zhou
Department of Applied Physics, Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Faculty of Science
Tianjin University
Tianjin 300072, P. R. China
Prof. H. Zhang
KAUST Catalysis Center (KCC)
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955-6900, Saudi Arabia
Prof. L. Zheng
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics
Chinese Academy of Sciences
No. 19 Yuquan Road, Beijing 100049, China
†These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Fundamental understanding of the dependence between
the structure and composition on the electrochemical CO2 reduction
reaction (CO2RR) would guide the rational design of highly efficient
and selective electrocatalysts. A major impediment to the deep
reduction CO2 to multi-carbon products is the complexity of carbon-
carbon bond coupling. The chemically well-defined catalysts with
atomically dispersed dual-metal sites are required for these C-C
coupling involved processes. Here, we developed a catalyst (BIF-
102NSs) that features Cl- bridged dimer copper (Cu2) units, which
delivers high catalytic activity and selectivity for C2H4. Mechanistic
investigation verifies that neighboring Cu monomers not only
perform as regulator for varying the reaction barrier, but also afford
distinct reaction paths compared with isolated monomers, resulting
in greatly improved electroreduction performance for CO2.
utilization and exceptional catalytic performance, especially
provide mechanistic insight into catalytic process.[5] Currently,
Cu-based materials are the most selective catalysts for
electrocatalytic CO2RR to deeply reduced C2+ products.
Although high activities of multi-carbon products have been
obtained by tailoring strategies on Cu-based catalysts, such as
morphology and facet controlling,[6] size tuning,[7] defect
engineering and dopant modification,[8] the formation mechanism
for C2+ species has not been fully resolved.[9] For the most
accepted one, the C-C formation step through *CO dimerization
or coupling pathway is regarded as the rate-determining step.[10]
To improve the formation of multi-carbon products, the precursor
of the adsorbed CO (*CO) on the surface should be present in
close proximity to one another. This requires specific dual-metal
sites in the catalyst, which may produce synergistic effect on two
adjacent sites to facilitate the C-C bond coupling. However, by
downsizing the metal nanoparticles for fabrication of dimer-
copper catalysts remains a significant challenge. Therefore, it is
meaningful to synthesize dimer-copper catalysts with well-
defined structures as a model electrocatalyst for the studies of
CO2RR.
Introduction
The electrochemical conversion of CO2 and water into fuels
and chemicals, ideally if driven by renewable energy, holds great
promise to alleviate the depletion of fossil resources and reduce
the carbon emission.[1] As CO2 is a thermodynamically stable
and kinetically inert molecule, the electrochemical CO2 reduction
reaction (CO2RR) is a process that requires high energy input.[2]
To date, numerous efficient electrocatalysts for converting CO2
into C1 products (carbon monoxide, methane and formic acid)
have been reported with high Faradaic efficiency and
selectivity.[3] Comparing with C1 products, C2 and C2+ species
(ethylene, ethanol, propane, etc.) are especially attractive
because of their higher volumetric energy densities and broader
applications. However, due to the higher kinetic barrier in the C-
C coupling process, producing multi-carbon hydrocarbons and
oxygenates is a big challenge for the CO2RR.[4]
Metal−organic frameworks (MOFs), constructed by metal
nodes and organic ligands, are a class of distinctive crystalline
materials with precise spatial arrangement of metal atoms.[11]
Owing to their structural adjustability, MOFs can offer a highly
versatile platform for catalysis.[12] However, the reaction
condition for CO2RR is somewhat harsh, and most of the
reported MOFs can hardly maintain pristine crystallinities during
the reaction process. An effective strategy to improve the
chemical stability of MOFs is to construct robust metal-ligand
coordinate bonds by using basic N-donor ligands, such as
azolate ligands.[11a] Boron imidazolate frameworks (BIFs) are
one kind of azolate-based MOFs constructed by the crosslinking
of metal ions and boron imidazolate ligands.[13] Because of the
presence of robust M-N coordination bonds, together with the
As one holy grail in the synthesis science, construction of
catalysts at molecular scale can lead to extremely high atom
1
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10.1002/anie.202106004
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RESEARCH ARTICLE
strong covalent bonds (B-N), BIFs hold promise for the
development of chemical stable crystalline catalysts for CO2RR.
Additionally, predesigned boron imidazolate ligands provide
room for adjusting the microenvironments around the active
centers. Recent studies demonstrated that introducing isolated
square-planar Cu site in BIF can stabilize the reaction
node is bridged by two μ2-bridging Cl- ions in BIF-102, while Cu2
node is bridged by one μ2-bridging O from COO- and one
bridging η2-COO- in BIF-103 (Figure 1 and S2). Unlike BIF-102
and BIF-103, BIF-104 contains mononuclear Cu ions as metal
-
nodes linked by three BH(mim)3 ligands to form a 2D layer
(Figure S2c and f). Cu ions in BIF-104 also adopt the five-
coordinated triangular bipyramid modes with three N atoms from
intermediates
and
improve
the
activity
for
CO2
photoreduction.[13c] Besides this, introducing bimetallic Cu sites
in catalysts can alter the adsorption energy barrier of reaction
intermediates over active sites and promote C–C coupling for
CO2 reduction reaction.[7a]
three BH(mim)3 ligands and two O atoms from chelating
-
carboxylate of the pendent auxiliary benzoic acid ligand (Figure
1b). The powder X-ray diffraction (PXRD) investigations showed
that both the as-synthesized bulk crystals and the exfoliated
nanosheets matched well with the simulated one from the crystal
structures, indicating that the crystal structures of BIF-102, BIF-
103 and BIF-104 were well retained (Figure S2g-i). Furthermore,
all these samples can maintain their structural integrity after
soaking in acidic/basic (1 ≤ pH ≤ 12) aqueous solutions,
indicating the extraordinary chemical stability of boron
imidazolate frameworks (Figure S22-S24).
Ultrathin nanosheets morphology of the as-synthesized
hybrids can be discerned well by transmission electron
microscopy (TEM), atomic force microscopy (AFM) (Figure 2, S3,
and S4). The nearly transparent feature of the BIF-102NSs
demonstrates their ultrathin thickness (Figure 2 a-b). Energy
dispersive spectroscopy (EDS) measurement indicates that BIF-
102NSs electrocatalysts contain Cu, Cl, C, and N, without other
impurities (Table S1). Element mapping on these nanosheets
also confirms these elements are uniformly distributed
throughout entire BIF-102NSs (Figure 2c-d). AFM offers more
detailed information of these nanosheets. As shown in Figure 2e
and f, the exfoliated BIF-102NSs display uniform thickness of
about 1.2 nm. Benefitting from the ultrathin nanosheets of the
2D layered structure; these catalysts may expose more active
sites for improving efficient catalytic activity in electrocatalytic
reduction of CO2.
Figure 1. Coordination environment of BIF-102NSs and BIF-104NSs and the
illustration showing the preferential reaction pathways of different model
systems.
With these considerations, we have successfully elaborated
dimer-copper catalysts Cu(II)2[BH(mim)3]2Cl2 (BIF-102, mim = 2-
methylimidazole) and Cu(II)2 [BH(mim)3]2(HCOO)2 (BIF-103) with
varied
bridging
ligands.
For
reference,
catalyst
Cu(II)2[BH(mim)3]2(BA)2 (BIF-104, BA = benzoate) with isolated
Cu sites has also been developed (Figure 1 and Figure S2).[17]
Electrocatalytic results clearly demonstrated that the dimer-
copper sites with the cooperative nature could deliver efficient
faradic efficiency (FE) for the generation of C2H4 product.
Furthermore, the Cl- ions bridged BIF-102NSs shows better
performance with higher selectivity for C2H4. This work suggests
that modulating the coordination environment of metal centers is
one way to tailor the product selectivity of CO2RR, providing
insights into the influence of different metal clusters on CO2RR.
Results and Discussion
As depicted in Figure S1, bulk crystals of BIF-102, BIF-103,
and BIF-104 were firstly synthesized under solvothermal
condition by reacting BH(mim)3- with different copper salts. Then,
the as-prepared bulk crystals were dispersed in ethanol and
exfoliated by ultrasonication, resulting in the formation
nanosheets (noted as BIF-102NSs, BIF-103NSs, and BIF-
104NSs). Single-crystal X-ray diffraction analysis revealed that
all of them have 2D layer frameworks. BIF-102 and BIF-103
have almost identical host frameworks which contain binuclear
Figure 2. a) TEM and b) HRTEM image of BIF-102NSs electrocatalysts; c)
High-angle annular dark-field (HAADF) STEM image and d) corresponding
mapping of Cu, Cl,
C and N in BIF-102NSs; e) AFM image and f)
corresponding height of as-synthesized BIF-102NSs.
For gain more accurate structural information about the
chemical status and elemental composition of the samples, X-
ray absorption near-edge structure (XANES) and X-ray
photoelectron spectra (XPS) are performed. As shown in
XANES (Figure 3a), the absorption edges of BIF-102NSs, BIF-
103NSs and BIF-104NSs are clearly presented with nearly
-
Cu2 as metal nodes further connected by six BH(mim)3 ligands
(Figure S2a-b). In both structures, Cu ions adopt the five-
coordinated triangular bipyramid modes in which three N atoms
from three BH(mim)3- ligands. The only difference is that the Cu2
2
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10.1002/anie.202106004
Angewandte Chemie International Edition
RESEARCH ARTICLE
identical profiles. The absorption edge positions are comparable
to that of CuO, suggesting the divalent state of Cu ions in the
three electrocatalysts.[8b,14] The XPS survey scans clearly display
the presence of B, C, N, and Cu in BIF-102NSs, BIF-103NSs
and BIF-104NSs. Moreover, the existence of Cl is also detected
in BIF-102NSs (Figure S5a-c). The Cl 2p spectrum for BIF-
102NSs shows the main peaks at 199.1 eV, which should be
assigned to Cl- anion (Figure 3b).[15] The N 1s spectra of all three
samples are quite similar and deconvoluted into two peaks
curves are measured from 0 to −1.0 V versus reversible
hydrogen electrode (vs. RHE) in both CO2 and Ar saturated 0.5
M KHCO3 electrolyte (Figure S6, S7). The greater current
density and more positive onset potential in CO2 saturated
solution for each sample demonstrated that the three BIFs show
possibly favorable activity toward CO2RR compared with
hydrogen evolution reaction (HER). As revealed by LSV in
Figure 4a, BIF-102NSs displayed the most positive onset
potential of -0.51 V and the highest total current density at
potential from -0.60 V to 0.96 V when comparing with BIF-
103NSs and BIF-104NSs. BIF-102NSs exhibited a current
density of 12.1 mA cm-2 at 0.94 V, which is about 2.08 and 3.1
times larger than that for BIF-103NSs (5.8 mA cm-2) and BIF-
104NSs (3.9 mA cm-2) at the identical operation voltage,
respectively.
centered at 398.8 and 399.7 eV, corresponding to the N-B and
12b]
N-Cu bonding, respectively (Figure 3c).[5e,
The high-
resolution XPS Cu 2p spectra of the samples display the
dominant peak of about 934.6 eV, closing to that of Cu2+
compounds with N ligands (Figure 3d).[16]
To elucidate the electrocatalytic performances, cyclic
voltammograms (CV) and linear sweep voltammograms (LSV)
Figure 3. a) XANES spectra at the Cu K-edge of the BIF-102NSs, BIF-103NSs, BIF-104NSs, Cu foil, Cu2O, and CuO; b) Cl 2p XPS spectrum of BIF-102NSs; c)
N 1s spectra and d) Cu 2p spectra of BIF-102NSs, BIF-103NSs, and BIF-104NSs.
Figure 4. a) LSV curves of BIF-102NSs, BIF-103NSs, and BIF-104NSs electrocatalysts in CO2 saturated 0.5 M KHCO3 electrolyte, respectively; b) The FE of
valuable gas and liquid products of BIF-102NSs electrocatalysts at different applied potentials in CO2 saturated electrolyte; c) Mass spectrum of gases from the
electrocatalytic reduction of 13CO2 with BIF-102NSs as electrocatalysts; d) The FEC2H4 of BIF-102NSs, BIF-103NSs, and BIF-104NSs electrocatalysts at different
applied potentials in CO2 saturated electrolyte; e) Comparison of the CO2RR gas productions activity between BIF-102NSs, BIF-103NSs, and BIF-104NSs
electrocatalysts at -1.0 V vs. RHE in CO2 saturated electrolyte; f) The Cdl of BIF-102NSs, BIF-103NSs, and BIF-104NSs electrocatalysts.
3
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Electrolysis at fixed potentials are further performed to monitor
the products of CO2 reduction by gas chromatography (GC) and
1H nuclear magnetic resonance (NMR) spectroscopy. The
detected gas-phase products involve CO, CH4, C2H4, and a trace
amount of C2H6. Besides, the liquid-phase products of HCOOH
is also monitored in all those catalytic processes. The generated
products are highly relevant to the operation electrode potentials
(Figure S8, S9, and S10). Major product of HCOOH with the
highest Faradaic efficiency (FE) of 29.58% is detected over BIF-
102NSs at lower overpotentials (Figure S8c). With the negative
shifting of electrode potential, FEHCOOH decreases gradually
while C2H4 becomes the dominant CO2 reduction product. The
FEC2H4 significantly increases as potential shifted from -0.8 V to -
1.0 V and achieved 11.30% at -1.0 V, which is much higher than
those of FE calculated from the CO, CH4, and C2H6 production
(Figure 4b). However, hydrogen evolution reaction (HER)
become more competitive at more negative potentials, the FE of
valuable products gradually decrease while FE of H2 increase. In
addition, Ar instead of CO2 is also introduced into the same
reaction system to reveal the origin of the products and only H2
was detected (Table S2). The 13CO2 isotopic investigation is
further conducted for confirming the origin of the products. As
shown in Figure 4c, the peak of m/z = 30 assigning to the 13C2H4
is clearly presented, further demonstrating that the carbon
source of C2H4 originates from the introduced CO2.
Electrocatalytic stability for the BIF-102NSs is also characterized
by using chronoamperometric and related FEC2H4 curves. At the
applied potential of -1.0 V, BIF-102NSs exhibited no obvious
degradation of current density and the faradaic efficiency of
C2H4 during after electrolyzing 5 h at -1.0 V (Figure S11).
existences of binuclear Cu2 units in BIF-102 and BIF-103
structures obviously weaken the electronic states in the vicinity
of the Fermi level, resulting in much weaken interactions
between the CO2 molecular and reactive centers. Furthermore,
comparing with BIF-103, some new enhanced hybridized
electronic states appear in BIF-102, as observed from the
calculated projected density of state (PDOS) of Cu atoms
(Figure 5b). The emergence of new electronic states could be
attributed to the existence of Cl species in BIF-102 with higher
electronegativity, which may also weaken the binding between
CO2 molecules and copper clusters and result in a moderate
adsorption energy. In another hand, as the C-C coupling is a key
step in the formation of C2H4 products, moderate adsorption is a
critical property to the further coupling between the CO2
molecules from the adjacent Cu sites, realizing the C-C coupling
in an efficient way. The adsorption free energy (E0) of CO2 is
also investigated. The binding of CO2 is energetically favored on
all the three catalysts, as evidenced by these negative values
(Figure 5c). The binding forces of BIF-102 is moderate, which is
weaker than both BIF-103 and BIF-104. Corresponding
distribution of charge densities has also been investigated,
further proving that the coupling of CO2 molecules around the
copper cluster of BIF-102 is easier than that of BIF-103 and BIF-
104 (Figure 5d).
Comparisons for the catalytic performances among BIF-
102NSs, BIF-103NSs and BIF-104NSs are also conducted
(Figure 4d and 4e). As observed, BIF-102NSs and BIF-103NSs
with binuclear Cu2 units exhibit superior activity and selectivity
for C2 hydrocarbons (C2H4) than that of BIF-104NSs with
isolated Cu sites, demonstrating that the binuclear Cu2 units
work as promoter in the evolution of C2 products. Moreover, BIF-
102NSs deliver better performance than that of BIF-103NSs in
the evolution of C2H4. This fact should be attributed to the
chlorine bridges in BIF-102NSs induced modulating effects
between
the
binuclear
Cu2
units.
Correspondingly,
electrochemical double-layer capacitances (Cdl) are calculated
from CV curves to estimate the electrochemical active surface
area (Figure 4f and S12). BIF-102NSs exhibit the largest Cdl
value (3.07 mF cm-2) than those of BIF-103NSs (1.63 mF cm-2)
and BIF-104NSs (1.08 mF cm-2), demonstrating that BIF-
102NSs provide more active sites being beneficial to CO2
electroreduction reaction. Moreover, the electrochemical
impedance spectroscopy (EIS) (Figure S13) also indicates that
BIF-102NSs exhibit the smallest curvature radius of the
semicircular Nyquist plots. The tafel plot of the logarithm values
of current density (log j) against overpotential demonstrated that
the BIF-102NSs exhibited the smallest Tafel slope (Figure S14).
These results implied that the BIF-102NSs has highest charge
transfer efficiency when comparing with those of BIF-103NSs
and BIF-104NSs.
Figure 5. DFT calculation results. Calculated TDOS a) and PDOS-Cu b) of
BIF-102, BIF-103 and BIF-104; c) the adsorption free energy of CO2 at BIF-
102, BIF-103 and BIF-104; d) Calculated distribution of charge density of BIF-
102, BIF-103 and BIF-104.
Conclusion
In this work, we constructed a dual-copper site that is
anchored on ultrathin boron imidazolate layers (BIF-102NSs) to
tailor the reactivity and selectivity of CO2 electroreduction
reaction. Electrocatalytic results clearly demonstrated that BIF-
102NSs could efficiently enhance the faradic efficiency of C2H4
products (FE of 11.3%), which is 1.6-fold enhancements of its
iso-reticular counterparts (BIF-103 FE of 7.15%) and 3.2-fold
enhancements of its single-metal counterparts (BIF-104 FE of
3.55%). Based on the X-ray absorption near-edge structure
To unveil the effects of dimer-copper units and atomic
configuration on the reaction mechanism of CO2RR, density-
functional theory (DFT) simulations are performed. Figure 5a
shows the density of states (DOS) of BIF-102, BIF-103 and BIF-
104. Comparing with the mononuclear structure of BIF-104, the
(XANES),
X-ray
photoelectron
spectra
(XPS),
and
4
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10.1002/anie.202106004
Angewandte Chemie International Edition
RESEARCH ARTICLE
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Our findings point out a new avenue to modulate the synergetic
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reaction path of CO2 electroreduction advancing further
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Acknowledgements
This work is supported by National Natural Science Foundation
of China (21935010, 21773242), National Key Research and
Development Program of China (2018YFA0208600), and the
Strategic Priority Research Program of the Chinese Academy of
Sciences (XDB20000000).
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Keywords: CO2 reduction • electrocatalysis • crystal • metal-
organic framework • catalyst
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5
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10.1002/anie.202106004
Angewandte Chemie International Edition
RESEARCH ARTICLE
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[17] The X-ray crystallographic coordinates for structures reported in this
article have been deposited at the Cambridge Crystallographic Data
Centre (CCDC) under deposition numbers CCDC BIF-102 (2065618),
BIF-103 (2065619) and BIF-104 (2065620). These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
6
This article is protected by copyright. All rights reserved.

10.1002/anie.202106004
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
The chemically well-defined catalysts with atomically dispersed dimer-copper sites are elaborated to modulate the reaction path of
CO2 electroreduction reaction (CO2RR). BIF-102NSs catalyst with Cl- bridged dimer copper (Cu2) units shows higher catalytic activity
and selectivity for C2H4, because the neighboring Cu monomers perform as regulator for varying the reaction barrier and afford
distinct reaction paths compared with isolated monomers.
7
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