Hierarchical Metal–Polymer Hybrids for Enhanced CO2 Electroreduction

Article abstract of DOI:10.1002/anie.202102193

The design of catalysts with high activity, selectivity, and stability is key to the electroreduction of CO₂. Herein, we report the synthesis of 3D hierarchical metal/polymer–carbon paper (M/polymer-CP) electrodes by in situ electrosynthesis. The 3D polymer layer on CP (polymer-CP) was first prepared by in situ electropolymerization, then a 3D metal layer was decorated on the polymer-CP to produce the M/polymer-CP electrode. Electrodes with different metals (e.g. Cu, Pd, Zn, Sn) and various polymers could be prepared by this method. The electrodes could efficiently reduce CO₂ to desired products, such as C₂H₄, CO, and HCOOH, depending on the metal used. For example, C₂H₄ could be formed with a Faradaic efficiency of 59.4 % and a current density of 30.2 mA cm^?2 by using a very stable Cu/PANI-CP electrode in an H-type cell. Control experiments and theoretical calculations showed that the 3D hierarchical structure of the metals and in situ formation of the electrodes are critical for the excellent performance.

Full text of DOI:10.1002/anie.202102193

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 10977–10982
International Edition: doi.org/10.1002/anie.202102193
German Edition: doi.org/10.1002/ange.202102193
Electrode Materials Very Important Paper
Hierarchical Metal–Polymer Hybrids for Enhanced CO2
Electroreduction
Shuaiqiang Jia, Qinggong Zhu,* Mengen Chu, Shitao Han, Ruting Feng, Jianxin Zhai, Wei Xia,
Mingyuan He, Haihong Wu,* and Buxing Han*
Abstract: The design of catalysts with high activity, selectivity,
Hybrid has emerged as an effective method for solving the
above problem. Among them, metal–polymer hybrids offer
the advantage of controlling the microenvironment of the
and stability is key to the electroreduction of CO . Herein, we
2
report the synthesis of 3D hierarchical metal/polymer–carbon
paper (M/polymer-CP) electrodes by in situ electrosynthesis.
The 3D polymer layer on CP (polymer-CP) was first prepared
by in situ electropolymerization, then a 3D metal layer was
decorated on the polymer-CP to produce the M/polymer-CP
electrode. Electrodes with different metals (e.g. Cu, Pd, Zn, Sn)
and various polymers could be prepared by this method. The
[2a,3]
active site motifs.
So far, the metal/polymer hybrids have
been successfully applied as electrocatalysts for CO RR by
2
a straightforward modification method. Recent work has
demonstrated that modification of polymer on the metal
surface was an effective strategy to improve the CO RR
2
[
4]
performance. Examples include modification of Cu surface
utilizing polydopamine, fluoropolymer, and polyaniline
(PANI), which attained 29%, and 88% Faradaic efficiency
(FE) for CH and 40% FE for C H , respectively. There is
[
5]
[6]
electrodes could efficiently reduce CO to desired products,
2
[3a]
such as C H , CO, and HCOOH, depending on the metal used.
2
4
For example, C H could be formed with a Faradaic efficiency
2
4
4
2
4
À2
of 59.4% and a current density of 30.2 mAcm by using a very
stable Cu/PANI-CP electrode in an H-type cell. Control
experiments and theoretical calculations showed that the 3D
hierarchical structure of the metals and in situ formation of the
electrodes are critical for the excellent performance.
general agreement that the incorporation of polymer can tune
the stability of intermediates in the reaction, rendering
enhanced CO RR activity. However, effects of the micro-
2
environment on the catalytic properties are still inconclusive.
Based on these considerations, some questions arise: 1) How
to manipulate the metal/polymer structure to create a desir-
able reaction microenvironment? 2) How they critically
Introduction
influence the catalytic performance of CO RR? Study on
2
Electrochemical conversion of CO2 into value-added
products has become one of the most promising routes to
these issues are helpful to design the efficient catalysts. But
conventional synthetic strategies have limitation due to their
difficulty in delicate control of the structure.
[1]
utilize abundant CO feedstocks under ambient conditions.
2
While metal-based heterogeneous catalysts commonly exhibit
high activity for CO reduction reaction (CO RR), achieving
At present, there are mainly two approaches to prepare
the metal–polymer hybrids. One is mixing metal NPs into the
polymer matrix. The other is to create polymerization and
NPs formation simultaneously via solvothermal method.
However, these procedures typically involve binders or
additives to prepare electrodes. As a consequence, the pore
volume, porosity, and accessibility of the potential active sites
would be significantly covered or impacted by these binders.
The limited numbers of active sites, and the inert basal plane
with low mass and electron transfers are still the main
2
2
high activity, selectivity and stability simultaneously remains
[2]
a challenge. Specifically, the individual metal nanoparticles
NPs) have a tend to agglomerate to reduce metal surface
energy, which often leads to change of catalytic properties.
(
[
*] S. Jia, M. Chu, S. Han, R. Feng, J. Zhai, Dr. W. Xia, Prof. Dr. M. He,
Prof. Dr. H. Wu, Prof. Dr. B. Han
[7]
drawbacks for most metal–polymer catalysts. Therefore,
new strategy to design of metal/polymer hybrids with the
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, School of Chemistry and Molecular Engineering
East China Normal University
tunable structure for boosting the CO RR performance is
2
highly desirable.
Shanghai 200062 (China)
E-mail: hhwu@chem.ecnu.edu.cn
Herein, we report an approach to design of metal/polymer
hybrids, which allows control of effective structure capable of
hanbx@iccas.ac.cn
Dr. Q. Zhu, Prof. Dr. B. Han
hosting active sites stably for CO RR. A series of three-
2
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Colloid and Interface and Thermodynamics
CAS Research/Education Center for Excellence in Molecular Sciences
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (China)
dimensional (3D) hierarchically structured metal/polymer
hybrids ((Cu, Pd, Zn, Sn)/polymer) were synthesized. It was
shown that these hybrids were efficient electrocatalysts for
CO RR and C H , CO or HCOOH could be produced,
2
2
4
respectively, depending on the kind of metal in the catalysts.
E-mail: qgzhu@iccas.ac.cn
Among them, Cu/PANI hybrid catalysts could reach a C H
FE of 59.4% with a current density of 30.2 mAcm .
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202102193.
2
4
À2
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Figure 1. Preparation and structural characterization of electrodes. A) Process used to prepare various electrodes. B) SEM image of CP. C) SEM
image of PANI-CP obtained after electropolymerization for 1 min. D) SEM image of Cu/PANI-CP (I) electrodes obtained after metal electro-
deposition for 10 min (inset: high-magnification image). E) Cross-section image of the Cu/PANI-CP (II) electrode. F) Cross-section image of the
Cu/PANI-CP (III) electrode (inset: high-magnification image). G) SEM image of the Cu-CP (IV) electrode (inset: high-magnification image).
Results and Discussion
induce formation of Cu nanosheets with 3D morphology. The
high-resolution transmission electron microscopy (HR-TEM)
images showed that the interplanar spacing was 2.10 Š,
corresponding to the d spacing of (111) planes of Cu
(Figure S4). Moreover, Cu in Cu/PANI-CP (I) show defects
(vacancies) located in the band gap on the surface (Fig-
ure 2A). These defects come mainly from two aspects. On
one hand, adding of 3,5-diamino-1,2,4-triazole (DAT) can
change the nucleation kinetics in the electrodeposition
process and promote the formation of defect structure in
We prepared a group of metal/polymer hybrids with
different structures using three synthetic routes (Figure 1A).
The metal/polymer hybrid with 3D hierarchical structure was
synthesized by coupling a continuous electrodeposition
process as shown in route I (Figure S1). The polymer layer
was first formed on the carbon paper (CP) network by in situ
[8]
electropolymerization. Subsequently, 3D metal layer is
decorated on the polymer-CP by electrodeposition to obtain
M/polymer-CP (I). Conventional strategies, including drop-
coating of metal NPs on the polymer/CP obtained by electro-
deposition (M/polymer-CP (II)), drop-coating of the solvo-
thermal synthesized metal/polymer hybrid on CP (M/poly-
mer-CP (III)), and direct electrodeposition of metal NPs on
CP by electrodeposition (M-CP (IV)) were also carried out
for comparison (Figure 1A). By route I (this work), catalysts
with different metals (Cu, Pd, Zn, Sn) and conducting
polymers (PANI), polypyrrole (PPy), polythiophene (PTh),
and their copolymers) were prepared (Figure S2).
[
9]
the catalyst. On the other hand, metal atoms can easily
nucleate and grow with defect site at the polymer surface in
[10]
the electrodeposition process. The resulting defects may
serve as active sites and the electrons on the defect states are
[11]
easily excited, leading to the enhanced conductivity.
Atomic force microscope (AFM) image indicated the thick-
ness of PANI nanofibers layer was ꢀ 3.25 mm and the most
obtain Cu layer had a thickness of ꢀ 3.46 mm (Figure S5).
Energy dispersive X-ray spectroscopy (EDS) mapping anal-
ysis confirmed the uniform dispersion of Cu, N, and C species
in Cu/PANI-CP (I) (Figure 2B), demonstrating that Cu has
been successfully grown on PANI film. Typical X-ray powder
diffraction (XRD) patterns of PANI-CP revealed represen-
tative PANI peaks at 2q = 9.58, 19.68 and 20.88. The broad
peaks at 43.28, 50.48, 42.18, 74.18, and 89.98 can be well
indexed to the (111), (200), (220), and (311) planes of Cu in
Cu/PANI-CP (I). The small peak at 36.38 was belonging to the
We then used Cu/PANI-CP (I) electrode as a prototypical
example to demonstrate the impact of the structure on
catalytic performance. Scanning electron microscopy (SEM)
showed that the original CP had a 3D network structure
(
Figure 1B). After electropolymerization, a thin layer of 3D
PANI network was covered on the surface of nanofibers of
the CP (Figure 1C). The PANI networks serve as a matrix for
hosting 3D metal layer obtained with 10min of electro-
deposition (Figure 1D and Figure S3). Very different from
the 3D hierarchically structured Cu/PANI-CP (I), Cu/PANI-
CP (II, III) shows obvious layers on substrate with noticeable
crack (Figure 1E,F). This is understandable because the Cu
ions or PANI were not in situ grown on the substrate. For
comparison, Cu-CP (IV) exhibits a large agglomeration of Cu
NPs without PANI (Figure 1G), indicating that PANI can
(111) plane of Cu O (Figure S6A). The chemical states of the
2
hybrid were also investigated by X-ray photoelectron spec-
troscopy (XPS). The curve fittings of Cu 2p and Cu LMM
Auger peaks were conducted according to the features of Cu
and Cu O (Figure S6B). These results are in good agreement
2
with the XRD analysis.
To further investigate the chemical states and electronic
structure of Cu in the hybrid, we perform X-ray adsorption
1
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tron-rich, which is beneficial to the
[16]
formation of defects.
The catalysts were used direct-
ly as working electrodes for pro-
duction of C H in KCl aqueous
2
4
electrolyte. Recent studies have
found that the low buffering capa-
bility of KCl electrolytes allowed
the electrode surface pH to in-
crease to a weakly basic range,
which facilitated the formation of
[15a,17]
C2 products.
In this study,
linear sweep voltammetry (LSV)
over various electrodes in CO2-
saturated 0.1 M KCl were deter-
mined, which shows reduction of
Figure 2. A) HR-TEM image of the Cu/PANI-CP (I) catalyst. B) Aberration-corrected TEM image and
elemental mapping of the Cu/PANI-CP (I) catalyst. C) Normalized Cu K edge XANES spectra.
D) Corresponding k3-weighted FT-EXAFS spectra. E) Morlet WT of the k3-weighted EXAFS data for
different catalysts.
CO clearly. The onset potential of
2
CO reduction over Cu/PANI-CP
2
(
I) measured was À0.40 V versus
RHE, which is less than that of all
the other catalysts (Figure S8). To
spectroscopy (XAS) analysis. The Cu K-edge spectra indi-
cates that both Cu/PANI-CP (I) and Cu-CP (IV) catalysts
retained the characteristic feature of Cu , as can be further
verify the importance of the Cu/PANI-CP (I) structure, we
determined the double-layer capacitance (C ), which was
dl
0
obtained by the charging current density at À0.05 V versus
confirmed by the Cu–Cu coordination at 2.24 Š (Fig-
RHE against the slope of scan rate (Figure 3A, Figure S9).
[
12]
ure 2C,D).
Notably, the Cu–O scattering path from the
According to the C values, we can know that the Cu/PANI-
dl
Cu O phase was also found in the X-ray absorption near edge
CP (I) catalyst had the largest electrochemically active
surface area (ECSA), which is favorable to the enhancement
2
structure (XANES) spectra of Cu/PANI-CP (I) with the
reference materials (Figure S7), indicating that Cu/PANI-CP
of the CO RR activity.
2
0
I
0
(
I) were mainly composed of Cu and Cu , and Cu was
The electrocatalytic performance of various catalysts was
[
13]
predominant. Morlet wavelet transform (WT) of the k3-
weighted extended X-ray absorption fine structure (EXAFS)
data was also performed to provide
further investigated by electrolysis of CO at different applied
2
potentials in 0.1 M KCl aqueous solution using a typical H-
further support for the existence of
CuÀCu and CuÀO bonding in the
Cu/PANI-CP (I) catalyst. As
shown in Figure 2E, Cu foil and
Cu-CP (IV) exhibit a distinct fea-
À1
ture at 5–8 Š , which represents
the Cu–Cu shell, whereas a slightly
À1
wider WT maximum at 4–10 Š ,
corresponding to Cu–O and Cu–
Cu shells, is visible in Cu/PANI-CP
[
14]
(
I) catalyst. Previous study has
shown that the selectivity in CO2
catalysis can be affected by the
oxidized phase concentration in
the surface region, which indicates
0
I
that the coexistence of Cu and Cu
is necessary to increase the pro-
[
15]
ductivity of C products.
From
2
the peaks of Cu–Cu, it is obvious
that Cu/PANI-CP (I) has a lower
Cu coordination number, demon-
strating that a large amount defects
exist in Cu of the hybrid (Fig-
ure 2C–E). It indicates that the
electron donation from PANI
made the Cu surface highly elec-
Figure 3. A) Charging current density against scan rate over different electrodes in CO -saturated
2
0
.1 M KCl aqueous electrolyte. B) Product distribution and average current density over different
electrodes. C) Long-term stability over different electrodes. Electrolysis experiments were carried out
at À1.2 V versus RHE.
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[19]
type cell, in which H O acted as a proton source. Under the
frequency data (Figure S24). At OCP, it showed that the
2
reaction conditions, no obvious liquid products were detected
by NMR spectroscopy, and H , CO, CH , C H were the
film resistance (R ) between the catalyst and substrate in the
Cu/PANI-CP (I) electrode was much smaller than that in the
Cu/PANI-CP (II, III). Moreover, the charge transfer resist-
f
2
4
2
4
gaseous products determined using gas chromatography (GC;
Figures S10 and S11). Clearly, Cu/PANI-CP (I) had much
higher FE of C H than other catalysts at all applied potentials
ance (R ) between the electrolyte and Cu/PANI-CP (I)
ct
surface was also much smaller than that of Cu/PANI-CP (II,
2
4
(
Figures S12–S15). The applied potential over the Cu/PANI-
III; Figure S23, Table S2). The smaller R and R result in the
f
ct
CP (I) was much lower, and the partial current density was
much higher. Figure S16 shows the total current density and
partial current density j(C H ) of different catalysts. A partial
lower onset potential and applied potential. Analogously, the
same trends appear in R and R at CO equilibrium potential
f
ct
2
(Figure 4A, Table S3). A reasonable interpretation of the
result is that the introduction of defects in Cu(vacancy)/PANI can
enhance electron mobility and accelerate the charge transfer
rate on Cu/PANI-CP (I) interface (Figure 4C–E, Figur-
2
4
À2
current density j(C H ) of 18.0 mAcm was achieved for Cu/
2
4
PANI-CP (I), which is about 2.2 times that of the Cu-CP (IV)
catalyst. At the applied potential of À1.2 V versus RHE, the
À2
[20]
current density over Cu/PANI-CP (I) can reach 30.2 mAcm
es S25–S28). Therefore, from the bulk to the surface, Cu/
with a C H FE of 59.4% (Figure 3B). For Cu/PANI-CP (II
PANI-CP (I) shows easier electron transfer than other Cu/
PANI interfaces (Cu/PANI-CP (II, III)) (Figure 4F), which is
conducive to enhance the activity of CO RR (Figure 3B).
2
2
4
and III), the FEs for C H were only 24.4% and 31.3%,
2
4
[21]
respectively. The performance of Cu/PANI-CP (I) is further
compared with other state-of-the-art electrocatalysts in
aqueous electrolyte, indicating that its performance is among
the best ones (Supplementary Table 1). Long-term electrol-
ysis was also performed to verify the stability of the catalysts.
Third, Cdl study reveals that the 3D hierarchical structure of
[
22]
Cu/PANI-CP (I) facilities CO diffusion (Figure 3A). From
2
Bode plots, the decrease of modulus of the impedance (log Z)
and moving of the phase angle (f) to higher-frequency region
of Cu/PANI-CP (I) indicates that more electrolyte penetrates
into the pores, which is conducive to CO2 diffusion and
As shown in Figure 3C, the CO RR activity over Cu/PANI-
2
CP (II and III) and Cu-CP (IV) decayed gradually with time
[23]
of CO electrolysis. In contrast, the FE and current density of
provides more opportunity for the reaction (Figure 4B,F),
which favors both the activity and selectivity.
2
Cu/PANI-CP (I) did not change obviously during the
reduction process (Figure 3C, Figure S17). After the reaction,
the morphology of Cu/PANI-CP (I) was well preserved, and
the composition did not change noticeably (Figures S18–S22).
It indicates that the remarkable stability of Cu/PANI-CP (I)
results mainly from the 3D hierarchical structure of the
catalyst. The highly dispersed Cu on PANI prevents agglom-
eration, which have better stability than other catalysts. All
these results illustrated that Cu/PANI-CP (I) have higher
ECSA, catalytic activity, selectivity for C H and stability than
2. The 3D hierarchical structure enhances selectivity. To
further verify the importance of the 3D hierarchical structure,
we performed the CO reduction using the Cu/PANI-CP (I)
2
electrodes prepared at different electrodeposition times. It
shows that the main product over PANI-CP was H , no C H
2
2
4
product was detected (Figure S30A), indicating that PANI is
not active for the reaction. As the amount of Cu increases
(Figure S29), the FE increased with electrodeposition time at
beginning, and then decreased slightly (Figure S30). The main
reason is that the thickness of the Cu 3D layer increases with
increasing electrodeposition time, and thus intermediates
stayed longer time in the Cu layer, which is favorable to CÀC
2
4
other catalysts.
Considering all of observations above, we conclude that
the 3D hierarchical structure obtained by in situ synthetic
strategy is critical to enhance activity, selectivity and stability.
These are further discussed below:
coupling. However, as the Cu layer was too thick, agglomer-
ation of Cu nanosheets occurred and the pores were blocked,
1
. The in situ synthetic strategy creates a favorable micro-
inhibiting diffusion of CO and electrolyte into the layer, and
2
environment to promote activity. First, in situ synthetic
strategy is conducive to the generation of 3D hierarchical
structure, which results in large ECSA (Figure 3A) and
abundant defects (Figure 2) that enhance the activity. Second,
Cu/PANI and PANI/CP contacted very well in the Cu/PANI-
CP (I) electrode because it was fabricated by in situ electro-
deposition. This structure can reduce or eliminated the
interfacial contacting resistance between the components in
the electrode, which is favorable to decrease the onset
potential and applied potential (Figure S8, Figures S12–
S15). In contrast, for Cu/PANI-CP (II, III), the synthesized
Cu NPs are embedded in the polymer. The physical contact
between Cu NPs and the polymer surface result in a tunneling
barrier for electron transfer to the active sites. To elucidate
this point, we use electrochemical impedance spectroscopy
thus the FE of C H reduced with the electrodeposition time.
2
4
3. High dispersion of metal nanosheets improves stability.
In the 3D hierarchical structure, Cu nanosheets are highly
dispersed on 3D polymer, which prevents agglomeration, and
thus Cu/PANI-CP (I) exhibited remarkable stability in
CO RR. This can be known from the fact that the FE and
2
current density of Cu/PANI-CP (I) did not change obviously
with time (Figure 3C, Figure S17).
The above findings are also applicable to the preparation
of other metal based M/PANI-CP (I) (M = Pd, Zn, Sn)
catalysts. The morphology and size of the electrodeposited
samples can be viewed from SEM images (Figure S31). It is
shown that the metals of 3D morphology in the M/PANI-CP
(I) were uniformly deposited on the 3D skeleton of PANI on
the CP. Moreover, the morphologies of these M/PANI hybrids
were significantly different from that of metals NPs deposited
directly onto CP (M-CP (IV)). The XRD and XPS results also
confirmed the successful formation of M/PANI-CP (I) (M =
Pd, Zn, Sn) catalysts (Figure S32). The electrocatalytic
[
18]
(
EIS) to study the interfacial properties of the Cu/PANI-CP
electrodes at an open-circuit potential (OCP; Figure S23) and
CO2 equilibrium potential (Figure 4A,B). Two types of
equivalent circuit was applied to fit the high and medium
1
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Figure 4. A) Nyquist and B) Bode plots of Cu/PANI-CP (I), Cu/PANI-CP (II), and Cu/PANI-CP (III) electrodes in CO -saturated 0.1 M KCl solution
2
at equilibrium potential for CO reduction. Charge difference density from top and side views of C) Cu-CP (for Cu-CP (IV)), D) Cu/PANI (for Cu/
2
PANI-CP (II) and Cu/PANI-CP (III)), and E) Cu(vacancy)/PANI interfaces (for Cu/PANI-CP (I)). The atoms in orange, silver, blue, and white represent
Cu, C, N, and H, respectively. Yellow and blue regions correspond to electron excess and electron deficiency sections. The isosurface value was
À3
set to 0.0001, 0.002, and 0.002 eŠ , respectively. F) CO diffusion and electron transfer on various Cu/PANI-CP electrodes with different
2
structures.
performance of the catalysts were also studied. Over the M/
PANI-CP (I) electrodes, the selectivity of specific products
was 84.1% (CO), 90.2% (CO), and 82.1% (HCOOH), when
Pd, Zn or Sn was used, respectively. In comparison, under the
same conditions, the non-hybrid metal M/CP (IV) catalysts
gave the selectivity of only 52.8% (CO), 73.9% (CO), and
time of intermediates in the catalyst layer, providing more
opportunity for their coupling to form a desired product.
Moreover, the 3D hierarchical structure results in high
dispersion of the metals, which prevents agglomeration of
metals and thus improves stability. We believe that the in situ
synthetic strategy has wide application in design and fabrica-
6
7.9% (HCOOH), respectively (Figure S33). Our study
tion of advanced electrocatalysts for CO electroreduction.
2
shows that other polymers can also control the morphologies
of Cu to modulate the catalytic activity (Figures S34 and
[
24]
S35).
Acknowledgements
This research was supported by the National Key Research
and Development Program of China (2017YFA0403102,
Conclusion
2
020YFA0710200) and the National Natural Science Foun-
In summary, we have presented an approach to fabricate
D hierarchical M/polymer-CP electrodes for CO electro-
reduction via in situ electrodeposition. Various electrodes,
consisting of different metals and polymers can be prepared
by this method, which have high efficiency and stability.
Detailed study indicates that the 3D hierarchical structure of
dation of China (22022307, 21733011, 21773267, 21573073).
The X-ray absorption spectroscopy (XAS) measurements
were carried out at the 1W2B beamline at Beijing Synchro-
tron Radiation Facility (BSRF), China.
3
2
the metals and in situ formation the electrodes are key points Conflict of interest
for the excellent performance. The 3D hierarchical structure
results in large surface area and abundant defects that
enhance the activity and selectivity. The interfacial charge
transfer resistances between M/polymer and polymer/CP
interfaces are very small due to the in situ electrodeposition,
leading to low onset potential and applied potential. The
enhancement of selectivity is also due to the longer staying
The authors declare no conflict of interest.
Keywords: CO reduction reaction · electrochemistry ·
2
hierarchical structures · materials science · metal–
polymer hybrids
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