Residual Chlorine Induced Cationic Active Species on a Porous Copper Electrocatalyst for Highly Stable Electrochemical CO2 Reduction to C2+

Article abstract of DOI:10.1002/anie.202102606

Electrochemical carbon dioxide (CO₂) reduction reaction (CO₂RR) is an attractive approach to deal with the emission of CO₂ and to produce valuable fuels and chemicals in a carbon-neutral way. Many efforts have been devoted to boost the activity and selectivity of high-value multicarbon products (C₂₊) on Cu-based electrocatalysts. However, Cu-based CO₂RR electrocatalysts suffer from poor catalytic stability mainly due to the structural degradation and loss of active species under CO₂RR condition. To date, most reported Cu-based electrocatalysts present stabilities over dozens of hours, which limits the advance of Cu-based electrocatalysts for CO₂RR. Herein, a porous chlorine-doped Cu electrocatalyst exhibits high C₂₊ Faradaic efficiency (FE) of 53.8 % at ?1.00 V versus reversible hydrogen electrode (V_RHE). Importantly, the catalyst exhibited an outstanding catalytic stability in long-term electrocatalysis over 240 h. Experimental results show that the chlorine-induced stable cationic Cu⁰/Cu⁺ species and the well-preserved structure with abundant active sites are critical to the high FE of C₂₊ in the long-term run of electrochemical CO₂ reduction.

Full text of DOI:10.1002/anie.202102606

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 11487–11493
International Edition: doi.org/10.1002/anie.202102606
German Edition: doi.org/10.1002/ange.202102606
CO2 Reduction
Residual Chlorine Induced Cationic Active Species on a Porous Copper
Electrocatalyst for Highly Stable Electrochemical CO2 Reduction to
C2+
Minhan Li, Yuanyuan Ma, Jun Chen, Robert Lawrence, Wei Luo, Marco Sacchi, Wan Jiang, and
Jianping Yang*
Abstract: Electrochemical carbon dioxide (CO2) reduction
reaction (CO2RR) is an attractive approach to deal with the
emission of CO2 and to produce valuable fuels and chemicals
in a carbon-neutral way. Many efforts have been devoted to
boost the activity and selectivity of high-value multicarbon
products (C2+) on Cu-based electrocatalysts. However, Cu-
based CO2RR electrocatalysts suffer from poor catalytic
stability mainly due to the structural degradation and loss of
active species under CO2RR condition. To date, most reported
Cu-based electrocatalysts present stabilities over dozens of
hours, which limits the advance of Cu-based electrocatalysts
for CO2RR. Herein, a porous chlorine-doped Cu electro-
catalyst exhibits high C2+ Faradaic efficiency (FE) of 53.8% at
À1.00 V versus reversible hydrogen electrode (VRHE). Impor-
tantly, the catalyst exhibited an outstanding catalytic stability in
long-term electrocatalysis over 240 h. Experimental results
show that the chlorine-induced stable cationic Cu0/Cu+ species
and the well-preserved structure with abundant active sites are
critical to the high FE of C2+ in the long-term run of
electrochemical CO2 reduction.
provide fuels and commodity chemicals in a carbon-neutral
way.[1] To date, a variety of electrocatalysts have been
developed for CO2RR, while the reduction products are
highly catalyst-specific.[2] Cu-based materials stand out as
a unique category of electrocatalysts for CO2RR because of
their capability to convert CO2 into valuable deep reduction
products, such as long-chain hydrocarbons and multicarbon
oxygenate (C2+ products).[3] However, to data, Cu-based
CO2RR electrocatalysts suffer from large overpotentials,
unsatisfactory selectivity toward C2+ products and poor
catalytic stability under CO2RR conditions, which impede
the application of CO2RR to valuable C2+ products. In that,
enormous studies have been carried out to improve the
activity and FE of C2+ products. Various key factors and
À
strategies that can facilitate C C coupling have been pro-
posed, such as surface roughness,[4] particular facets,[5] grain
boundaries,[6] subsurface oxygen and oxidized copper spe-
cies,[7] alloying and doping strategy,[8] tandem strategy,[9]
electrolyte design,[10] and electrolyzer engineering.[11] Apart
from activity and selectivity, stability is also a key perfor-
mance benchmark for Cu-based CO2RR catalysts.[12] How-
ever, the stability of Cu-based electrocatalysts in CO2RR,
which is particularly important for the commercialization of
this technology, remains less investigated across the research-
es on CO2RR.[13]
Introduction
Electrochemical CO2 reduction reaction (CO2RR) pow-
ered by renewable energy has emerged as a promising
technology for sustainable energy storage and atmospheric
CO2 reduction. Concurrently, CO2RR process is able to
To date, different deactivation mechanisms have been put
forward on Cu-based CO2RR catalysts, such as poisoning,
dissolution, structural degradation, loss of active phases,
accumulation of carbonaceous species and reduction of CuOx
species.[13b,14] In general, the vulnerable structure and active
sites of the catalysts may change as the reaction proceed,
which largely result in the poor stability of Cu-based electro-
catalysts in the course of CO2RR. To mitigate the disintegra-
tion of structure and the loss of active sites, graphene oxide
wrapped and thick CuOx outer layer protected Cu nanowires
showed enhanced stability for both structure and catalytic
performance.[15] Recently, a Cu nanowire catalyst was re-
ported to maintain a high selectivity toward C2H4 for over
200 hours of electrocatalysis, pointing out the importance of
stable stepped sites that formed through in situ during
electrochemical activation.[16] Therefore, it is of great impor-
tance to maintain the stability of catalyst structure and active
sites for improving the catalytic performance of Cu-based
CO2RR electrocatalysts. On the other hand, although the
conclusion is still pending, the crucial role of oxidized Cu
species for the improvement of C2+ production has been
widely investigated. Given the positive relevance between
[*] M. H. Li, Dr. Y. Y. Ma, Prof. W. Luo, Prof. W. Jiang, Prof. J. P. Yang
State Key Laboratory for Modification of Chemical Fibers and
Polymer Materials, International Joint Laboratory for Advanced Fiber
and Low-dimension Materials, College of Materials Science and
Engineering, Donghua University
Shanghai 201620 (P. R. China)
E-mail: jianpingyang@dhu.edu.cn
Prof. J. Chen
ARC Centre of Excellence for Electromaterials Science, Intelligent
Polymer Research Institute, Australian Institute of Innovative
Materials, Innovation Campus, University of Wollongong
Wollongong, NSW 2522 (Australia)
Dr. R. Lawrence, Dr. M. Sacchi
Department of Chemistry, University of Surrey (UK)
Prof. W. Jiang, Prof. J. P. Yang
Institute of Functional Materials, Donghua University
Shanghai 201620 (China)
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.202102606.
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oxidized Cu species and C2+ production, the
stability of oxidized Cu species is important for
maintaining high selectivity of C2+ products.[17]
To improve selectivity and stability of Cu-
based catalysts, the effect of halogen ions has also
been investigated, focusing on either constructing
unique Cu nanostructures or specifically adsorbed
halogen ions on the catalysts surface or specifi-
cally adsorbed halogen ions on the catalysts
surface.[18] Recent studies showed that the residual
I and modified F in the Cu matrix have been
demonstrated to boost C2+ production through
either stabilized Cu+ species or facilitated inter-
mediates adsorption.[19] In this work, we proposed
a halogen stabilization strategy that residual-
chlorine in the Cu matrix induced stable active
species for CO2RR. A halogen-containing pre-
cursor, Cu4(OH)6FCl nanosheets (CuOHFCl
NSs), was first synthesized by a facile hydro-
thermal process and then electrochemically acti-
vated to Cl-doped porous Cu catalysts under
CO2RR condition. The catalysts showed high FE
of C2+ products and suppressed hydrogen evolu-
tion reaction (HER). Importantly, the catalysts
exhibited an outstanding stability in a long-term
electrocatalysis over 240 hours. The stable Cl-
induced cationic active sites and the well-pre-
served structure of catalyst were found to be
responsible for the superior catalytic stability.
Figure 1. Characterization of as-prepared CuOHFCl NSs. a) Gram-scale CuOHFCl
NSs powder obtained in one batch of hydrothermal synthesis in a 100 mL autoclave.
b) XRD pattern and Rietveld refinement result of CuOHFCl NSs c) Structural
schematic diagrams of CuOHFCl NSs d) TEM image, e) HRTEM, f) SAED pattern,
g)–j) elemental mapping, and k)–n) XPS results of as-prepared CuOHFCl NSs. Scale
bars: d) 2000 nm. e) 10 nm. f) 5 nmÀ1. g–j) 50 nm.
(6.61 at%) in the as-prepared CuOHFCl nanosheet (Fig-
ure 1k–n and Table S1).
The as-prepared electrode was in situ activated for 10 min
at À1.00 VRHE in (denoted as e-CuOHFCl) prior to CO2RR
evaluation. The potential-dependent FEs of gas and liquid
Results and Discussion
The CuOHFCl NSs were prepared via a facile hydro-
thermal method. The sapphire powder of as-prepared
CuOHFCl NSs can be synthesized at gram-scale in a single
hydrothermal synthesis using an autoclave of 100 milliliters
(Figure 1a). The X-ray diffraction (XRD) pattern and
Rietveld refinement result of the CuOHFCl NSs were shown
in Figure 1b (Rwp = 9.63%, Rp = 11.4%). The as-
prepared CuOHFCl NSs possesses kagome lay-
ered structure with CuI atoms in the kagome
lattice and CuII atoms between kagome layers
(Figure 1c).[20] Scanning electron microscopy
(SEM) and transmission electron microscopy
(TEM) confirmed the sheet-like morphology of
the as-prepared CuOHFCl NSs with a few mi-
crons in planar scale and about tens of nano-
meters in thickness (Figure 1d and Figure S2).
Both the high-resolution transmission electron
microscopy (HR-TEM) (Figure 1e) and the se-
lective area electron diffraction (SAED) pattern
(Figure 1 f) pointed out a highly ordered hexag-
onal crystalline structure. The energy dispersive
products were shown in Figure 2a and b, respectively. The FE
of H2 was suppressed below 30% from À0.75 to À1.00 VRHE
with a lowest FE of H2 of 26.3% at À1.00 V1, indicating the
high selectivity toward CO2RR. After that, the FE of H2
gradually increased due to the limited CO2 mass transfer and
intensified HER at more negative potential than À1.00 VRHE
.
Figure 2. The CO2RR performance of electrochemical activated e-CuOHFCl nano-
sheet. a) Potential-dependent FEs of gas phase and total products. b) Potential-
dependent FEs of liquid phase products. c) Potential-dependent partial current
densities of H2, C1, and C2 and corresponding total current density. d) Current
density and quantity of charge in long-term stability test. e) FEs of all products in
long-term stability test.
spectroscopy (EDS) elemental mapping analysis
shows the uniform distribution of elemental Cu,
Cl, F, and O (Figure 1g–j). The XPS analysis also
confirmed the existence of Cl (7.23 at%) and F
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With the reduction potential decreases, CO and formate, as
to determine the long-term CO2RR stability of e-CuOHFCl
catalyst in H-cell at À1.00 VRHE
[16,22] A higher current density
the major C1 products, showed similar trends and their FE
values decreased consistently. The production of CH4 started
at À1.00 VRHE and the FE of CH4 was relatively low in the
tested range. Therefore, the FE of C1 products decreased
consistently from 51.2% to 15.7% with the decreased
potential. The FE of C2H4 increased with the decreased
potential and reached the peak of 36.3% at À1.00 VRHE. After
that, however, the FE of C2H4 decreased slightly due to the
marked rise in FE of H2. The liquid C2+ products, including
.
was observed for the first electrocatalysis cycle (Figure 2d),
which is likely due to the reduction of CuOHFCl to metallic
Cu (Figure S7).[22] The rest electrocatalysis cycles showed
relatively stable current densities. While the corresponding
quantity of charge that passed through the interface of
electrode indicated a slight decline in the activity during the
whole stability test of 240 h (Figure 2d). The damaged
catalyst layer and detached catalyst caused by the stirring
and formed bubble is likely responsible for the decreased
activity (Figure S8). The FEs of the reduction products during
the long-term CO2RR test were relatively stable without
obvious deactivation as shown in Figure 2e (Figure S9 and
S10), demonstrating the outstanding catalytic stability of e-
CuOHFCl catalyst. To the best of our knowledge, the stability
of our catalyst outperforms most reported Cu-based CO2RR
catalysts (Table S2).
1
ethanol, 1-propanol, and acetate, were detected by H NMR
method (Figure S3a,b) and achieved the maximum FEs of
12.0%, 6.1%, and 0.9%, respectively, at À0.89 to À1.00 VRHE
.
Totally, the FE of C2+ products showed a volcano shape with
the applied potentials and reached 53.8% at À1.00 VRHE. The
total current density, and partial current densities for C1 and
C2+ products were shown in Figure 2c. The partial current
density for C2+ products increased with the decreased applied
potentials and reached the maximum of 15.0 mAcmÀ2 at
À1.05 VRHE. The H2 partial current density was suppressed
below 5.0 mAcmÀ2 at potentials above À1.0 VRHE, while it
increased significantly at more negative potentials and
became the major product at À1.10 VRHE. Interestingly,
although the FE of C1 products showed a significant change
with the applied potentials, its partial current density only
varied in the range of 2.11 to 5.87 mAcmÀ2. Further, if one
considered merely the 2 electron transfer products (CO and
formate), their partial current densities were almost the same
at all applied potentials (Figure S4), which means that the
production rates of these two products was nearly constant at
all applied potentials.
To gain an in-depth understanding of the excellent
stability of e-CuOHFCl NSs, the electrodes after the stability
tests were taken out from catholyte and immediately rinsed
with large amount of DI water (the spent catalysts after 4 h
and 240 h of CO2RR are denoted as CuOHFCl-4 and
CuOHFCl-240, respectively). The TEM images of the spent
catalyst after 4 h of CO2RR test showed that the CuOHFCl
NSs maintained highly porous structure and a sheet-like
morphology with roughened surface and edge (Figure 3a).
The lattice fringe in the HR-TEM confirmed that the
nanosheets were assembled by interconnected metallic Cu
grains after reaction (Figure 3b). The HAADF-STEM image
and elemental mapping showed that the uniform distribution
of Cu, F, and Cl in the porous Cu nanosheets (Figure 3c–f).
After stability test of 240 h, the catalysts still possessed the
morphology of porous sheet, while the shape of the sheet
resembled a leaf of bamboo (Figure 3g and i). Taking a closer
look at the tips and edges of the “leaf”, the catalyst showed
a highly sawtooth edge and layered structure (Figure 3h and
Figure S11), which provided abundant stepped and low-
coordinated sites that are favorable for the production of C2
products.[5a,16,18b] Furthermore, the elemental mapping dem-
onstrated that Cu, F, and Cl elements were evenly distributed
in the spent CuOHFCl-240 sample (Figure 3j–l). Therefore, it
is believed that the well-retained porous nanosheet morphol-
ogy and abundant step sites evolved during the CO2RR test
may contribute to maintain the high FE of C2+ products
during the long-term stability test.
As shown by the elemental mapping and EDS (Fig-
ure S12), F and Cl still existed in the catalysts after CO2RR
stability test. The residual halogens and chemical state of Cu
were further examined by XPS spectra after CO2RR tests.
Obviously, a peak corresponding to Cl 2p could be detected
on the catalyst after 4 h of CO2RR test (Figure 4a). In
contrast, the absence of Cl 2p peak in Cu(OH)2-CA sample
excluded the possible exogenous Cl contamination from the
electrolyte or the reference electrode in the cathode cell
(Figure 4a). However, after 240 h of CO2RR test, only a small
hump was shown at the position of Cl 2p in the high-
resolution Cl XPS spectra (Figure 4a), which may be caused
by the detachment of catalyst and the interference of Nafion
To explore the effects of halogens and nanosheet mor-
phology on the CO2RR catalytic performance, two control
samples were prepared and tested for CO2RR under the same
condition. Cu(OH)2 nanosheets without halogen elements
were prepared by
a modified reported method (Fig-
ure S5a).[21] Hydrogen accounted for the major reduction
product over the activated Cu(OH)2 nanosheets in the range
of applied potentials (Figure S5b), while the maximum FE of
C2H4 products was only 22.7% at À1.00 VRHE (34.2% for total
C2+ products). The other control sample was obtained by
reducing the CuOHFCl nanosheets in H2/Ar atmosphere at
3508C for 4 h (denoted as h-CuOHFCl). The nanosheet
morphology was destroyed and bulky particles emerged for h-
CuOHFCl sample (Figure S5c). The h-CuOHFCl sample also
exhibited an inferior catalytic performance compared with
CuOHFCl NSs (Figure S5d). H2 accounted for the major
product with FEs ranging from 52.2% to 68.8% over h-
CuOHFCl sample, while the maximum FE of C2H4 was only
13.3% at À0.99 VRHE. In addition, to exclude the effect of
electrochemical surface area (ECSA), double layer capaci-
tance of different catalysts was measured (Figure S6). Con-
sidering the higher double layer capacitance values of Cu-
(OH)2 NSs and h-CuOHFCl than that of e-CuOHFCl, the
partial current density normalized by ECSA of e-CuOHFCl
electrocatalyst was even higher than the control samples,
indicating its higher intrinsic activity toward C2+ products.
The stability was crucial for the application of CO2RR.
Therefore, we carried out consecutive electrocatalysis cycles
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binder and accumulated adsorbates. Nafion binder is usually
used in preparing powdery catalyst inks allowing the strong
adhesion between the catalysts and the current collector.
However, it imposes difficulty to study the surface properties
of the spent electrodes after CO2RR test as surface-attached
Nafion may interfere the XPS characterization, which is
evidenced by the ultra intense F-C peak and high F content
(Figure 4b. F content: 45–56 at% according to XPS) from
Nafion binder (Figure S13). To avoid the interference of
Nafion binder, we prepared another two electrodes without
Nafion binder and studied their surface properties by XPS
after CO2RR tests at À1.00 VRHE for 4 h and 24 h (denoted as
CuOHFCl-4-BF and CuOHFCl-24-BF, BF means binder-
free). The electrodes after CO2RR tests were stored and
transferred under inert atmosphere and then in situ Ar
sputtered for 300 s before XPS measurements. As shown in
À
the high-resolution XPS spectra of F, the F C bond dis-
appeared when Nafion was removed demonstrating the
successful elimination of the interference from Nafion binder
(Figure 4c). A weak peak corresponding to the F-Cu can be
observed after 4 h of CO2RR test,[19a] while no F peaks can be
detected after 24 h of CO2RR test, indicating the gradual
leaching of F element as the reaction proceeded. However,
different from F, trace amount of Cl element seemed to be
relatively stable in the e-CuOHFCl catalyst (Figure 4d),
which confirms the Cl residual in the catalyst during the long-
term CO2RR test. The Cu:Cl atomic ratio obtained by XPS
measurements before and after 300 s Ar sputtering indicated
that Cl existed in both the surface and bulk of the catalysts
and the surface Cl content was relatively higher than the bulk
(Figure S14 and Table S3). To further investigate the possible
residue of halogens in the Cu matrix, we annealed the
CuOHFCl NSs by annealing in air and reducing in H2/Ar
atmosphere in a tube furnace, respectively. The thermal
gravimetric (TG) analysis showed that the CuOHFCl nano-
sheets could decompose completely in air at higher than
ꢀ 4458C (Figure S15a). The CuOHFCl nanosheets decom-
posed to CuO after annealing at 5008C in air for 4 h (denoted
as a-CuOHFCl, Figure S15b,c). The XPS spectra of F and Cl
confirmed the absence of F and the presence of Cl in the a-
CuOHFCl sample (Figure S15d,e). Further, the H2 reduced h-
CuOHFCl sample contained a mixed phase of Cu and CuCl
(Figure S16a) and the XPS spectra also confirmed the
coexistence of Cu0 and Cu+ in h-CuOHFCl sample (Fig-
ure S16b). Similarly, F was removed while Cl still existed in
the h-CuOHFCl sample (Figure S16c,-d). Therefore, although
the reason remains unclear, it can be concluded that Cl
remained in the Cu matrix of e-CuOHFCl catalysts during
CO2RR test, while F was less stable and leached out in the
process of CO2RR test.
Figure 3. The TEM images and elemental mappings of e-CuOHFCl
catalysts at different stages in the long-term stability test. TEM (a,b)
and elemental mapping (c–f) after 4 h of electrocatalysis. TEM (g,h)
and elemental mapping (i–l) show that the morphology of the catalysts
after 240 h of electrocatalysis resemble bamboo leaves (inset in (g)).
Scale bars: a) 200 nm. b) 5 nm. c)–f) 200 nm. g) 50 nm. h) 10 nm. i)–
l) 50 nm.
The Cu 2p spectra showed that the chemical state of the
catalyst was Cu0 or Cu+ after CO2RR (Figure 4e and g). As it
is impossible to distinguish Cu0 and Cu+ in the XPS spectra of
Cu 2p, the Cu LMM Auger spectra was measured (Figure 4 f
and h).[12] Based on the peak position of Cu LMM spectra at
ꢀ 570.1 eV, the predominating copper species on the surface
of CuOHFCl-4 and CuOHFCl-240 catalysts were Cu+ after
CO2RR test.[23] For comparison, a halogen-free Cu(OH)2
nanosheet sample was prepared and tested for CA at the
Figure 4. XPS measurements of electrodes after electrocatalysis. Elec-
trodes with Nafion binder: a) Cl 2p. b) F 1s, e) Cu 2p, f) Cu LMM; and
electrodes without Nafion binder (Ar sputtered before XPS measure-
ment) c) F 1s, d) Cl 2p, g) Cu 2p, h) Cu LMM.
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same potentials as CuOHFCl nanosheets.[21b] The Cu LMM
jointly contribute to Cu-based electrocatalysts in
CO2RR.[10,32]
Although copper halides derived Cu-based electrocata-
spectra of Cu(OH)2 nanosheets after CA test (denoted as
Cu(OH)2-CA) showed a sharp peak at ꢀ 568.6 eV, indicating
the overwhelming metallic Cu0 sites on the surface of
Cu(OH)2-CA sample. It should be noted that the ex situ
XPS has limited capability to probe the surface state of Cu as
metallic copper is prone to be oxidized once exposing to air.
Although we tried to isolate electrodes after CO2RR tests
from air, the surface oxidization seems to be inevitable.
However, the quite different predominate Cu LMM Auger
peaks in e-CuOHFCl and Cu(OH)2-CA implied the different
oxidation state in these two samples. As discussed above, to
eliminate the interference of Nafion binder, the Cu LMM
Auger spectra were measured on the electrodes without
Nafion binder after 4 h and 24 h of CO2RR tests. Further, Ar
sputtering was used prior XPS measurement to remove
surface layer on the catalysts that was possibly oxidized or
contaminated. According to the split peak, it was shown that
the mixed oxidation state of Cu0 and Cu+ existed in the
binder-free catalysts under CO2RR condition (Figure 4h).[24]
Previously, the crucial role of Cu+ sites or mixed Cu0-Cu+ sites
for the production of C2+ products has been widely demon-
strated by both theoretical and experimental studies.[12,17b,25]
Various strategies have been developed for the purpose of
construction and stabilization of Cu+ or mixed Cu0-Cu+ sites,
such as hydroxide/oxide-derived Cu-based catalysts,[7d] plas-
ma treatments,[25b] doping strategies,[8a,26] ligand stabiliza-
tion,[23b] core–shell structures,[17c,27] and spatial confinement
strategy.[17a] Therefore, the stability of cationic Cu sites (Cu+
or mixed Cu0-Cu+) is crucial to maintain the catalytic
performance of Cu-based electrocatalysts.[7c,18b,23a]
lysts have been widely studied, the effects of residual halogens
in Cu matrix remains unclear. Luna and co-workers found
that no chlorine remained on the surface of the sol-gel copper
oxychloride (Cu2(OH)3Cl) derived catalysts after CO2RR
reaction.[17b] However, Vasileff and co-workers revealed that
trace amount of iodine species on the iodide-derived copper
catalysts, which altered the oxidation state of adjacent Cu
sites and favored the C2 production.[19b] Recently, a F modified
Cu catalysts derived from Cu(OH)F precursor was fabricated
and F was found to boost the C2+ production through
[19a]
À
a hydrogen-assisted C C coupling mechanism.
In this work, the Cl ions were found to be more stable than
F ions in the e-CuOHFCl electrocatalysts under CO2RR
conditions and the residual Cl still existed after long-term
CO2RR test. The heteroatomic doping in Cu matrix can
induce the change in chemical state and electronic structure
of Cu via electron transfer between Cu atoms and the doped
atoms.[33] Non-metallic elements, including nitrogen and
boron, have also been demonstrated to modify the electronic
structure of Cu and induce positively charged Cu sites.[8a,26]
The residual I ions also promoted a high content of Cu+
species in iodine-modified Cu catalyst after CO2RR.[18b]
Given the larger electronegativity of Cl (3.16) than the other
doped heteroatoms in Cu matrix, such as Ag (1.93), B (2.04),
N (3.04), and I (2.66), it is reasonable to deduce that mixed
oxidation state of Cu0 and Cu+ in e-CuOHFCl electrocatalyst
is likely caused by the residual Cl in Cu matrix. This
conception is also supported by the lack of Cu+ species in
the halogen-free control sample of Cu(OH)2 after CO2RR,
which showed inferior FE of C2+ compared to e-CuOHFCl.
Therefore, it is believed that the synergistic effects between
Cu+ and Cu0, which were stabilized by residual Cl in e-
CuOHFCl electrocatalysts after CO2RR, favored the produc-
tion of C2+ products.[7b, 25a] Furthermore, the stable Cu0-Cu+
sites during the long-term stability test are partly responsible
for the excellent catalytic stability.
Discussion
Halogen doping and oxidation of Cu
The effects of halogen ions on the Cu-based CO2RR
electrocatalysts have been widely investigated. On one hand,
the halogen ions play an important role in constructing Cu-
based nanostructures and stabilizing cation Cu species.[18a,28]
The electropolished Cu foil evolved into quite different Cu
nanostructures when immerged or electrochemically ano-
dized in halogen-containing solutions, including KCl, KBr,
and KI.[10,18b] The unique nanostructures and advantageous
facets of cuprous halides derived Cu electrocatalysts was
believed to play a significant role for enhanced C2+ selecti-
vity.[18a] On the other hand, halogen ions in electrolyte that
strongly adsorb on the surface of Cu catalysts could influence
the surface charge property as well as the binding strength of
certain intermediates.[18e,29] The addition of IÀ ions into the
CsHCO3 electrolyte was proposed to contribute to the
stabilization of Cu+ species under CO2RR reaction by
adsorbed iodine ions.[30] Similarly, the use of KCl electrolyte
was also demonstrated to induce a biphasic Cu2O-Cu catalyst
and preserve Cu+ species under CO2RR.[31] Some works
demonstrated the chemical effect of adsorbed halogen ions
prevailed over the nanostructuring effect when both effects
Stability
The catalytic stability is as important as activity and
selectivity for catalysis. However, the studies on the catalytic
stability of Cu-based CO2RR electrocatalysts are far from
enough to understand the degradation mechanisms and
propose the stabilization strategies. In the limited stability
studies, the loss of well-defined facets and the changes in the
morphology change and structure are major reasons for the
decreased activity and selectivity.[34] Cu nanocubes with
preferential Cu(100) facet have been proved to be efficient
[5a,35]
À
for C C coupling in CO2RR.
However, the nanocubic
structure tended to deteriorate under CO2RR condition via
a nanoclustering and coalescence mechanism.[36] The evan-
escence of advantageous Cu(100) facet and increased amount
of formed small nanoclusters that favored the HER, which led
to the degradation of CO2RR.[34a,36] Similarly, the coarsen of
the dendritic Cu nanostructures in the course of CO2RR
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Angewandte
Research Articles
Chemie
resulted in the loss of high index facets, which was believed to
be responsible for the degradation of catalytic perform-
ance.[13a,14a] A recent study demonstrated that a thicker and
smoother CuxO protective layer obtained by controlled
surface oxidation could inhibit the disintegration of Cu
nanowires and improve its catalytic stability to more than
22 h.[15b] In this study, it was demonstrated that abundant
active sites that were induced under CO2RR conditions were
well-retained even after 240 h of electrocatalysis, which may
be due to the thermodynamical stability of these active sites
under CO2RR conditions.[16] The well-preserved sheet-like
morphology with porous and layered structure would also
facilitate the exposure of active sites. On the other hand, it
was found that the copper oxidation state changed with the
applied potentials as well as the time under reaction
conditions.[17b] In this context, the stability of cationic Cu
species (Cu+ or Cu0-Cu+) are also of great importance for the
catalytic stability in CO2RR.[17a,25b] Therefore, it is believed
that the mixed oxidation state of Cu0-Cu+ caused by the
residual Cl atoms in e-CuOHFCl electrocatalyst is the key to
maintain a stable FE of C2+ products in long-term CO2RR.
Based on the above analysis, it is proposed that the stable Cu0-
Cu+ active sites as well as the integrate structure of the
catalysts collectively ensure the outstanding stability of e-
CuOHFCl electrocatalysts in CO2RR.
the Program for Professor of Special Appointment (Eastern
Scholar) at Shanghai Institutions of Higher Learning, State
Key Laboratory for Modification of Chemical Fibers and
Polymer Materials, Donghua University.
Conflict of interest
The authors declare no conflict of interest.
Keywords: chlorine ·CO2 reduction reaction ·copper ·
electrocatalysis ·mixed oxidation states
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Conclusion
To summarize, we have proposed a strategy to achieve
high selective and stable Cu-based electrocatalysts for
CO2RR through residual-chlorine induced stable active
species. A novel porous Cl doped Cu electrocatalyst derived
from electrochemically activated CuOHFCl NSs was synthe-
sized. The e-CuOHFCl electrocatalysts exhibited a high FE of
C2+ products of 53.8% at À1.00 VRHE and a large partial
current for C2+ products of 15 mAcmÀ2 at À1.05 VRHE. The Cl
element was found to be more stable than F element in
derived e-CuOHFCl electrocatalysts and remained stable in
the Cu matrix under CO2RR conditions. The residual Cl
induced Cu0-Cu+ sites as well as favorable morphology of the
e-CuOHFCl electrocatalysts were found to boost the C2+
production. Importantly, the e-CuOHFCl electrocatalyst
presented an outstanding catalytic stability for CO2RR over
240 h. The residual-chlorine induced stable Cu0-Cu+ sites and
structural stability of the catalyst were found to be critical to
maintain the high FE of C2+ in the long-term run of CO2RR.
This study demonstrates new strategies to improve the
catalytic stability of Cu-based CO2RR electrocatalysts.
Acknowledgements
The authors are grateful for financial support from the Fok
Ying-Tong Education Foundation (No. 171041), Shanghai
Science and Technology Committee (19DZ2270100 and
20WZ2500200), National Natural Science Foundation of
China (91963204), International Joint Laboratory for Ad-
vanced fiber and Low-dimension Materials (18520750400),
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Manuscript received: February 20, 2021
Accepted manuscript online: March 8, 2021
Version of record online: April 8, 2021
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