B-Cu-Zn Gas Diffusion Electrodes for CO2 Electroreduction to C2+ Products at High Current Densities

Article abstract of DOI:10.1002/anie.202016898

Electroreduction of CO₂ to multi-carbon products has attracted considerable attention as it provides an avenue to high-density renewable energy storage. However, the selectivity and stability under high current densities are rarely reported. Herein, B-doped Cu (B-Cu) and B-Cu-Zn gas diffusion electrodes (GDE) were developed for highly selective and stable CO₂ conversion to C₂₊ products at industrially relevant current densities. The B-Cu GDE exhibited a high Faradaic efficiency of 79 % for C₂₊ products formation at a current density of ?200 mA cm^?2 and a potential of ?0.45 V vs. RHE. The long-term stability for C₂₊ formation was substantially improved by incorporating an optimal amount of Zn. Operando Raman spectra confirm the retained Cu⁺ species under CO₂ reduction conditions and the lower overpotential for *OCO formation upon incorporation of Zn, which lead to the excellent conversion of CO₂ to C₂₊ products on B-Cu-Zn GDEs.

Full text of DOI:10.1002/anie.202016898

Angewandte
Research Articles
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 9135–9141
International Edition: doi.org/10.1002/anie.202016898
German Edition: doi.org/10.1002/ange.202016898
CO2 Electroreduction
Hot Paper
B-Cu-Zn Gas Diffusion Electrodes for CO2 Electroreduction to
C2+ Products at High Current Densities
Yanfang Song, Jo¼o R. C. Junqueira, Nivedita Sikdar, Denis Öhl, Stefan Dieckhçfer,
Thomas Quast, Sabine Seisel, Justus Masa, Corina Andronescu, and Wolfgang Schuhmann*
Abstract: Electroreduction of CO2 to multi-carbon products
has attracted considerable attention as it provides an avenue to
high-density renewable energy storage. However, the selectivity
and stability under high current densities are rarely reported.
Herein, B-doped Cu (B-Cu) and B-Cu-Zn gas diffusion
electrodes (GDE) were developed for highly selective and
stable CO2 conversion to C2+ products at industrially relevant
current densities. The B-Cu GDE exhibited a high Faradaic
efficiency of 79% for C2+ products formation at a current
density of À200 mAcmÀ2 and a potential of À0.45 V vs. RHE.
The long-term stability for C2+ formation was substantially
improved by incorporating an optimal amount of Zn. Oper-
ando Raman spectra confirm the retained Cu+ species under
CO2 reduction conditions and the lower overpotential for
*OCO formation upon incorporation of Zn, which lead to the
excellent conversion of CO2 to C2+ products on B-Cu-Zn
GDEs.
overcoming the low solubility of CO2 in aqueous electrolytes
(approx. 35 mm) and shortening the diffusion path for the
reaction, GDE-based systems can potentially reach industry-
relevant current densities as opposed to the bathed electrodes
utilized in a conventional H-cell.[7–9]
As catalyst for eCO2RR, Cu has been intensively studied
due to its unique ability to produce hydrocarbons and
alcohols, which is attributed to a moderate CO* binding
energy.[10,11] The pre-oxidation of Cu has previously been
reported to greatly improve the intrinsic catalytic properties
towards C2+ products formation.[12] The catalytic role played
by Cu+ and subsurface oxygen was later revealed, where Cu+
interacts synergistically with Cu0 to promote C2+ products
À
formation owing to the lower energy barrier for C C
coupling.[13–18] Despite some recent efforts on plasma-activat-
ed Cu,[16] electro-redeposited Cu,[17] and multi-hollow
Cu2O,[18] the stabilisation issue of Cu+ under eCO2RR
condition remains challenging.
Introduction
Introducing modifier elements has been considered as an
efficient strategy to reduce the tendency of Cu+ reduction at
negative potentials.[19–22] Boron has an electron configuration
of 2s22p1 and thus exhibits Lewis acidity. The empty electronic
orbital of B tends to receive electrons from Cu, tuning the
local electronic structure of Cu with positive valence sites and
making it more Lewis acidic, both of which are favourable for
boosting the formation of C2+ products in eCO2RR. A B-
doped oxide-derived-Cu catalyst exhibited a higher Faradaic
efficiency (FE) of 48.2% toward the formation of C2+
products than that of its boron-free oxide-derived-Cu coun-
terpart (30.5%), owing to stabilisation of the Cu+ species by
the introduction of B.[21] By doping different amounts of B, the
valence state of Cu could be tuned to achieve optimum C2+
Worldwide climate change has become a regular topic in
the news, as anthropogenic CO2 levels have continued to
steadily increase.[1–3] The direct conversion of CO2 into
chemical feedstocks or high energy-density fuels driven by
renewable electricity is a potentially attractive technology to
negate CO2 emissions. However, despite many efforts being
devoted to improving electrochemical CO2 valorisation,
development is still in the early-stages due to sub-optimal
activity and product selectivity.[4–6] Employing gas diffusion
electrode (GDE) systems has emerged as a promising ap-
proach to promote the electrochemical CO2 reduction
reaction (eCO2RR) towards possible applications.[3,7] By
[*] Dr. Y. Song, J. R. C. Junqueira, Dr. N. Sikdar, Dr. D. Öhl, S. Dieckhçfer,
T. Quast, Dr. S. Seisel, Prof. W. Schuhmann
Analytical Chemistry-Center for Electrochemical Sciences (CES)
Faculty of Chemistry and Biochemistry
Prof. C. Andronescu
Chemical Technology III, Faculty of Chemistry and CENIDE
Center for Nanointegration University Duisburg Essen
Carl-Benz-Strasse 199, 47057 Duisburg (Germany)
Ruhr University Bochum
Universitätsstrasse 150, 44780 Bochum (Germany)
E-mail: wolfgang.schuhmann@rub.de
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.202016898.
Dr. Y. Song
ꢀ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution Non-Commercial
NoDerivs License, which permits use and distribution in any
medium, provided the original work is properly cited, the use is non-
commercial and no modifications or adaptations are made.
CAS Key Laboratory of Low-Carbon Conversion Science and Engi-
neering
Shanghai Advanced Research Institute
Chinese Academy of Sciences
99 Haike Road, Shanghai 201203 (P. R. China)
Dr. J. Masa
Max Planck Institute for Chemical Energy Conversion
Stiftstrasse 34–36, 45470 Mülheim an der Ruhr (Germany)
Angew. Chem. Int. Ed. 2021, 60, 9135 –9141
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
9135

Angewandte
Research Articles
Chemie
products formation with a high FE of 79% at À1.1 V vs. RHE
in 0.1m KCl.[22] However, the current densities in these
reports (ꢀ 70 mAcmÀ2) are far below any possible applica-
tion.
In addition to a high FE towards particular desirable
products and high current density, stable long-term operation
is also highly important for viable application of a given
catalyst for eCO2RR.[23,24] In addition to the challenge of
inherent chemical instability of Cu-based catalysts, catalyst
degradation and cathodic corrosion under high cathodic
potentials are similarly severe drawbacks.[7,23–27] Inhibition of
these processes to stabilize Cu-based catalysts is important
yet still challenging for the development of efficient eCO2RR
systems.
Herein, we report a B-doped Cu (B-Cu) catalyst for
efficient eCO2RR at industry-relevant current densities using
a GDE electrolysis system. During operation especially at
high current densities, GDEs are suffering from flooding due
to electrowetting. Notably, this flooding problem, which
characteristically leads to a low concentration of CO2 at the
triple-phase boundary due to low CO2 solubility, was miti-
gated by controlling the amount of the hydrophobic binder
PTFE (polytetrafluoroethylene) during catalyst ink prepara-
tion and the gas-liquid-catalyst interphase was further opti-
mized by tuning the catalyst loading. As a result, a highest FE
of 78% for C2+ products formation (C2H4 49%, C2H5OH
22%, C3H7OH 7%) was achieved on B-Cu GDE with 10% of
PTFE and 0.5 mgcmÀ2 catalyst loading at À200 mAcmÀ2 and
À0.45 V vs. RHE. Furthermore, a sacrificial anode of Zn,
a less noble metal, was employed to stabilize B-Cu catalyst
during eCO2RR. With an optimum amount of incorporated
Zn nanosheets (0.025 mgcmÀ2), the long-term stability for C2+
product formation at À200 mAcmÀ2 was significantly im-
proved with an overall C2+ product current density of
À194 mAcmÀ2. Operando electrochemical Raman spectros-
copy (OERS) results show that the active Cu+ species is stable
even at high reduction potentials, the overpotential for *OCO
formation is positively shifted by incorporation of Zn nano-
sheets, both of which contribute to the excellent eCO2RR
performance of B-Cu-Zn GDE.
Figure 1. XRD patterns of A) B-Cu nanoparticles and B) Zn nano-
sheets. TEM images of C) B-Cu nanoparticles and D) Zn nanosheets.
HRTEM images of E) B-Cu nanoparticles and F) Zn nanosheets.
9008877) are both observed in the XRD pattern of the as-
synthesized Zn sample (Figure 1B), with the intensity of the
oxide peaks being notably higher compared to B-Cu due to
faster oxidation of metallic Zn.
The transmission electron microscopy (TEM) image
(Figure 1C) reveals the nanoparticle morphology of the as-
synthesized B-Cu sample with some degree of agglomeration.
The corresponding size distribution histogram (Figure S1)
shows that the B-Cu nanoparticle sizes were in the 23–31 nm
range. The as-synthesized Zn sample (Figure 1D) exhibits
nanosheet morphology. High-resolution transmission elec-
tron microscopy (HRTEM) images of the B-Cu sample
(Figure 1E) show lattice fringes with three different d-
spacings of 3.01, 2.46, and 2.13 Š, which correspond to
Cu2O (110), (111), and (200), respectively, while no metallic
Cu-related lattice fringes were observed. This is most likely
due to the surface oxide-covered metallic Cu phase making it
difficult to be observed by HRTEM. Similarly, only ZnO
(100) and (101) lattice fringes with d-spacings of 2.81 and
2.47 Š, respectively, were identified in Zn nanosheets (Fig-
ure 1F).
Results and Discussion
The B-Cu catalyst was synthesized by a facile and scalable
method[22,28,29] using concentrated sodium borohydride
(NaBH4) as both the reducing agent and boron source under
continuous Ar purging. Zn nanosheets were synthesized by
electrochemical deposition of Zn on a Cu foil followed by
scratching them off. Figure 1A displays the X-ray diffraction
(XRD) pattern of the as-synthesized B-Cu sample. The main
peaks at 2q values of 43.38, 50.48, and 74.18 are corresponding
to the (111), (200) and (220) planes, respectively, of cubic
¯
metallic Cu with a space group of Fm3m (COD: 9013014). In
addition, weak peaks of cubic Cu2O (111), (200), and (220)
¯
planes with a space group of Pn3m (COD: 1000063) are
X-ray photoelectron spectroscopy (XPS) was employed in
order to study the chemical states and composition of the
catalysts. The XP spectrum of B-Cu nanoparticles presents
a sharp Cu2p3/2 peak at 932.4 eV (Figure S2), which can be
observed due to oxidation in air or Cu-B interaction with the
solvent during synthesis. Similarly, peaks assigned to hexa-
gonal Zn (P63/mmc, COD: 9012435) and ZnO (P63mc, COD:
9136
www.angewandte.org
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 9135 –9141

Angewandte
Research Articles
Chemie
assigned to Cu0 and/or Cu+ species.[30] To distinguish Cu0 and
At a current density of À400 mAcmÀ2, H2 is the only
Cu+, the Cu LMM Auger transition (Figure 2A) was addi-
tionally recorded, which indicates that the surface of B-Cu
nanoparticles mainly consisted of a Cu+ (Cu2O) oxidation
state at a kinetic energy of 916.7 eV. This result is consistent
with the HRTEM analysis. The presence of B in B-Cu was
confirmed by B1s XPS. Only B sub-oxide signals at about
191.0 eV were observed in the B1s spectrum (Figure 2B),
since the solubility of B in Cu is extremely low and elemental
B is typically oxidized at the surface. The B:Cu atom ratio in
the B-doped Cu sample is 0.0144 as determined by XPS.
Figure 2C presents the Zn2p XP spectrum of Zn nanosheets
with a sharp Zn2p3/2 peak at 1022 eV. Zn0 and Zn2+ (ZnO) can
be distinguished using the Zn LMM spectrum (Figure 2D), in
which the well-resolved peak at around 987.8 eV is assigned
to ZnO. ZnO as the main component on the surface of Zn
nanosheets matches well with the HRTEM analysis in
Figure 1F.
detectable product, and the electrolyte penetrated the whole
GDE as leakage was observed.
To mitigate the flooding problem, we added varying
amounts of PTFE to the catalyst ink to enhance the hydro-
phobicity of the B-Cu catalyst layer on top of the GDE. By
increasing the content of PTFE relative to the total mass
loading of the catalyst from 5 to 10 and 20%, the hydro-
phobicity of the B-Cu GDE gradually increased as shown in
the corresponding contact angle measurements (Figure S5).
As expected, the hydrogen evolution reaction (HER) was
substantially decreased and the formation of C2+ products,
mainly ethylene and ethanol, was enhanced accordingly
(Figure S4). For B-Cu-modified GDEs with 5% PTFE, the
FE for H2 decreased to 69% at a current density of
À200 mAcmÀ2 and a potential of À0.59 V vs. RHE, the FE
for C1 products increased to 8.4% (CO 3.1%, formate 4.4%),
and the FE for C2+ products increased to 17% (C2H4 5%,
C2H5OH 12%; Figure 3A, Figure S4B). When the content of
PTFE was further increased to 10%, the FE for H2 was
further reduced to 28% at a current density of À200 mAcmÀ2
and a potential of À0.54 V vs. RHE, at which the FE for C1
products decreased to 5% (CO 4%, formate 1.0%), and the
FE for C2+ products increased to 70% (C2H4 43%, C2H5OH
23%, C3H7OH 4%; Figure 3A, Figure S4C). However,
further increase in the PTFE content to 20% did not lead
to a decrease in the HER (32% at À200 mAcmÀ2, À0.54 V vs.
RHE). Concomitantly, the FE for C1 products increased to
8% (CO 6%, formate 2%), and the FE for C2+ products
decreased to 53% (C2H4 27%, C2H5OH 20%, C3H7OH 5%;
Figure 3A, Figure S4D). This is most likely due to a decrease
in conductivity of the B-Cu catalyst/PTFE layer and possible
blocking of the active sites of the catalyst. B-Cu GDEs with
10% PTFE exhibited the maximum FE for the formation of
C2+ products and the minimum FE for H2 and C1 products
formation.
The electrocatalytic activity and selectivity of the catalysts
towards eCO2RR were evaluated in a H-type GDE cell
(Figure S3) employing 1m aqueous KOH as electrolyte. In
our experiments, the catalyst ink was directly drop-coated on
carbon paper with a hydrophobic microporous gas diffusion
layer (GDL) to obtain the GDE (see details in Supporting
Information). GDEs having only B-Cu in the catalyst layer
show that the predominant product during eCO2RR is H2
over the whole applied current range (Figure S4A). The
Faradaic efficiency (FE) for H2 was 78%, the FE for C1
products (CO and formate) was 4.8%, and the FE for C2
products (C2H4 and C2H5OH) was 5.8%, at a current density
of À200 mAcmÀ2 and a potential of À0.6 V vs. RHE (Fig-
ure 3A, Figure S4A). The comparatively high FE for H2
evolution can be ascribed to the high hydrophilicity of the
B-Cu catalyst as demonstrated by contact angle measure-
ments (Figure S5). Moreover, the hydrophilic property of B-
Cu accelerates flooding of the GDE, thus limiting eCO2RR.
The selectivity of eCO2RR to C2+ products on B-Cu
GDEs with different contents of PTFE can be further
evaluated by the FE for the C2+/C1 products ratio. As shown
in Figure 3B, B-Cu-10%PTFE is highly selective for the
formation of C2+ products at potentials more negative than
À0.43 V vs. RHE. Particularly, at À0.59 V vs. RHE and
À300 mAcmÀ2, the C2+/C1 ratio of B-Cu-10%PTFE reaches
14.4, which is superior to electrodes with 0, 5, and 20% PTFE.
The selectivity of eCO2RR to C2+ products on B-Cu-
10%PTFE GDEs reached up to 93.5% (Figure S8).
The overpotential for eCO2RR at a total current density
of À10 mAcmÀ2 decreases with increasing PTFE added to the
B-Cu GDE (Figure S9). The total current density, as well as
the partial current densities for C2+ product formation were
greatly enhanced upon the addition of PTFE on the B-Cu
GDE (Figure 3C). B-Cu-10%PTFE shows the maximum C2+
geometric current density of À184 mAcmÀ2 at À0.59 V vs.
RHE, which is around 16 times higher than that obtained
using B-Cu without PTFE (À11.6 mAcmÀ2). We conclude
that the intermediate amount of PTFE (10%) can balance
hydrophobicity and conductivity of B-Cu and mitigate the
flooding problem of the GDE, thereby boosting the con-
version of CO2 towards C2+ compounds.
Figure 2. A) Cu LMM and B) B1s XPS spectra for B-Cu nanoparticles.
C) Zn2p and D) Zn LMM spectra for Zn nanosheets.
Angew. Chem. Int. Ed. 2021, 60, 9135 –9141
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
9137

Angewandte
Research Articles
Chemie
Figure 3. A) FE of H2, C1, and C2+ products on B-Cu electrodes with 0, 5, 10, and 20% PTFE at a current density of À200 mAcmÀ2 and a potential
of À0.60, À0.59, À0.54, and À0.54 V vs. RHE, respectively. B) C2+/C1 ratio and C) partial current density of C2+ products at different potentials on
B-Cu electrodes with different contents of PTFE. D) FE for the formation of H2, C1, and C2+ products on B-Cu electrodes with 0.1, 0.2, 0.5, and
1.0 mgcmÀ2 at a current density of À200 mAcmÀ2 and a potential of À0.64, À0.53, À0.45, and À0.54 V vs. RHE, respectively. E) FE for the
formation of various C2+ products at a catalyst loading of 0.5 mgcmÀ2 B-Cu at different potentials. F) Partial current density of C2+ products at
different potentials on B-Cu electrodes with different loadings. Product quantification was performed by means of gas chromatography (Figure S6)
and 1H-NMR spectroscopy (Figure S7).
The three-phase interphase within the GDE plays a key
role for the eCO2RR, which can be affected by the thickness
of the catalyst layer. Hence, we further changed the mass
loading of the catalyst to investigate the effect of catalyst
thickness on the eCO2RR at the three-phase interphase. The
thickness of the catalyst layer on the top of GDL gradually
decreases from 26.7 mm to 13.4, 5.5 and 2.5 mm when
decreasing the mass loading of the catalyst from 1 mgcmÀ2
to 0.5, 0.2 and 0.1 mgcmÀ2, respectively (Figure S10). With
decreasing mass loading of B-Cu, the FE for H2 formation
was further decreased to about 20% with a catalyst loading of
0.5 and 0.2 mgcmÀ2, respectively, at À200 mAcmÀ2, while it
increased to 56% when only 0.1 mgcmÀ2 of the catalyst was
used (Figure S11). In the case of a too thin catalyst layer, the
electrolyte will penetrate into the GDL causing flooding
which is ultimately promoting the HER and inhibiting the
eCO2RR. For example, on B-Cu GDEs with a mass loading of
only 0.1 mgcmÀ2, H2 formation accounted for 85% of the
current at À300 mAcmÀ2. Further increase of the current
density to À400 mAcmÀ2 led to increasing electrowetting and
hence penetration of the electrolyte through the GDE,
ultimately causing the GDE to lose its function.
increased to 0.5 and 8% at loadings of 0.2 and 0.1 mgcmÀ2,
respectively (Figure S11, S12), suggesting that the formation
of C1 products increases with decreasing catalyst loading
(Figure 3D). Typically, at À200 mAcmÀ2, the FE for C2H4
first increased to 49% for loadings of 0.5 and 0.2 mgcmÀ2, and
then decreased to 18% for a loading of 0.1 mgcmÀ2, whereas
the FE for C2H5OH gradually decreased to 22 and 18% for
loadings of 0.5 and 0.2 mgcmÀ2, and further decreased at 14%
for a loading of 0.1 mgcmÀ2 (Figure S11). In summary, the FE
for C2+ compounds including C2H4, C2H5OH, C3H7OH, and
acetate was 78% for a loading of 0.5 mgcmÀ2 (Figure 3D),
consisting of 49% C2H4, 22% C2H5OH, 7% C3H7OH at
a current density of À200 mAcmÀ2 and a potential of À0.45 V
vs. RHE (Figure 3E).
The current density at potentials more negative than
À0.40 V vs. RHE first increases by decreasing the catalyst
loading from 1 to 0.5 mgcmÀ2 and then decreases with further
lowering the mass loadings to 0.2 and 0.1 mgcmÀ2 (Fig-
ure S13), which coincides with the partial current for the
eCO2RR. This can be further confirmed by a comparison of
the partial current densities for C2+ formation as shown in
Figure 3F. The highest C2+ current density of À198 mAcmÀ2
(at À0.49 V vs. RHE) was achieved at a loading of 0.5 mgcmÀ2
of B-Cu GDE. In contrast, the maximum C2+ current density
for catalyst loadings of 1.0, 0.2, and 0.1 mgcmÀ2, was 184, 152,
and 69 mAcmÀ2 at more negative potentials of À0.59, À0.53,
The formation of CH4 was completely suppressed at
a loading of 1.0 mgcmÀ2, however, decreasing the loading to
0.5 mgcmÀ2 led to CH4 formation with a FE of around 0.02%
at À200 mAcmÀ2. The FE for CH4 formation gradually
9138
www.angewandte.org
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 9135 –9141

Angewandte
Research Articles
Chemie
and À0.56 V vs. RHE, respectively. This implies that the mass
The current density at potentials more positive than
loading of the B-Cu catalyst, and thus the thickness of the
catalyst layer, considerably influences the location of the
three-phase interphase and the contribution of eCO2RR to
the overall current. A suitable penetration depth of the
electrolyte on the one hand together with a sufficient
diffusion depth of gaseous CO2 on the other hand are
necessary to maximize the activity and efficiency of eCO2RR.
To address the cathodic corrosion issue of Cu catalysts at
high cathodic potentials, which limits the long-term stability
of such catalysts, the less noble metal Zn was employed as
sacrificial anode to protect the B-Cu catalyst. For this, a B-Cu-
Zn catalyst was prepared by physical mixing of B-Cu nano-
particles and Zn nanosheets on a carbon paper as support.
The B-Cu loading was fixed at 0.5 mgcmÀ2 while the Zn
loading was increased gradually from 0.01 to 0.075 mgcmÀ2.
The maximum FEs for the formed C2H4, C2H5OH and
C3H7OH and their corresponding potentials can both be
adjusted by varying the Zn loading in the B-Cu-Zn catalyst
(Figure S14). The catalyst with a composition of 0.5B-
Cu:0.025Zn showed the highest FE for alcohols (34%), with
C2H5OH (31%) and C3H7OH (3%) at À0.45 V vs. RHE and
a total current density of À200 mAcmÀ2 (Figure 4A,B).
Further increasing the Zn loading results in a decline of the
maximum FE for the formation of C2H5OH, which can be
attributed to the excess CO generated on Zn (Figure S15) and
the competing HER. The maximum FE for the formation of
C2H4 monotonically decreases from 49% on B-Cu to 27% on
0.5B-Cu:0.075Zn. Hence, the selectivity for alcohols versus
C2H4 (FE alcohols / FE C2H4) can be tuned from 0.57 on
a bare B-Cu electrode to 1.04 on a 0.5B-Cu:0.075Zn
electrode by the amount of added Zn nanosheets (Fig-
ure 4A).
À0.45 V vs. RHE was highest using a 0.5B-Cu:0.025Zn GDE,
while at potentials more negative than À0.45 V vs. RHE, the
current density was highest at a higher Zn loading of 0.5B-
Cu:0.05Zn GDE (Figure S16). Further increasing the Zn
loading to 0.075 mgcmÀ2 resulted in a drop of current density
due to the excess of Zn and fewer Cu active sites for eCO2RR.
The partial current densities for the formation of C2+ products
are further compared in Figure 4C. Using a 0.5B-Cu:0.025Zn
GDE, the highest current density for the formation of C2+
products is obtained over almost the whole potential range,
with an observed value of À194 mAcmÀ2 at À0.49 V vs. RHE
being similar to that of the B-Cu electrode. In contrast, using
GDEs with catalyst loadings of 0.5 mgcmÀ2 B-Cu GDEs
mixed with 0.01, 0.05, and 0.075 mgcmÀ2 Zn nanosheets,
maximum C2+ product formation current densities of À158,
À156, and À120 mAcmÀ2 were obtained at potentials of
À0.52, À0.47, and À0.56 V vs. RHE, respectively. Hence, an
optimum amount of Zn (0.025 mgcmÀ2) is required for
minimizing cathodic Cu corrosion while retaining reasonable
activity of C2+ product formation.
The long-term eCO2RR stability of the catalyst is one of
the key factors for potential applications, notwithstanding the
fact that maintaining the catalyst integrity still remains
challenging for GDE-based systems. The stability of the
0.5B-Cu:0.025Zn electrode in comparison with a B-Cu
electrode was evaluated by performing eCO2RR electrolysis
at a high current density of À200 mAcmÀ2. The experimental
results shown in Figure 4D and Figure S17 reveal excellent
stability of the 0.5B-Cu:0.025Zn electrode with a negligible
decay of the FE for the formation of C2+ products. In contrast,
the FE for the formation of C2+ products on B-Cu GDEs
starts to attenuate after 1 h of eCO2RR electrolysis at the
expense of increasing H2 formation. Thus, the incorporation
of Zn nanosheets remarkably improved the long-term
eCO2RR stability of the catalyst, however with sacrificing
a little bit of the FE for the formation of C2+ products at the
beginning of the electrolysis. To better understand the
difference in the observed long-term stability of these two
kinds of GDEs, the change in composition and chemical states
after electrocatalysis was further investigated by recording Cu
and Zn LMM Auger spectra before and after the stability test.
Qualitative assessment of the Cu LMM Auger spectra
(Figure S18) reveals Cu+ with a peak maximum at around
916.3 eV kinetic energy to be the main surface component in
both the 0.5B-Cu:0.025Zn GDE and the B-Cu GDE even
after long-term eCO2RR. Besides Cu+, a contribution of
metallic Cu manifests as a shoulder at a kinetic energy of
around 918.5 eV. Interestingly, that contribution diminishes
nearly completely after 4 h of electrolysis for the B-Cu GDE,
whereas the 0.5B-Cu:0.025Zn GDE retains a visible shoulder
corresponding to Cu0 after the same duration of electrolysis.
These findings demonstrate the stabilization of the metallic
Cu state on the GDE surface by Zn in 0.5B-Cu:0.025Zn.
Further evidence for the sacrificial anodic protection effect of
Zn is obtained from the Zn LMM Auger spectra on 0.5B-
Cu:0.025Zn GDE before and after 15 min of eCO2RR
(Figure S19). While the as-prepared GDE shows similar
contributions of metallic Zn and ZnO, the GDE after 15 min
Figure 4. A) FE for the formation of C2H4 and alcohols and the ratios
of alcohols/C2H4 on B-Cu electrodes with different amounts of Zn at
a current density of À200 mAcmÀ2. B) FE for the formation of various
C2+ products on 0.5B-Cu:0.025Zn electrodes at different potentials.
C) Partial current density for the formation of C2+ products at different
potentials on B-Cu electrodes with different amounts of Zn.
D) FE for the formation of various C2+ products on 0.5B-Cu and 0.5B-
Cu:0.025Zn GDEs at a current density of À200 mAcmÀ2 and 4 h of
electrolysis.
Angew. Chem. Int. Ed. 2021, 60, 9135 –9141
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
9139

Angewandte
Research Articles
Chemie
of eCO2RR exhibits a stronger decay of the Zn0 peak intensity
compared to the one of ZnO. Notably, no traces of surface Zn
are detectable after 4 h of electrolysis (spectrum not shown)
confirming the anodic Zn corrosion process concomitantly
stabilizing Cu+ and Cu0. This finding does not only explain the
higher stability of the 0.5B-Cu:0.025Zn GDE, but also
confirms the synergistic function of the presence of Cu+ and
Cu0 to promote C2+ product formation. The obtained
Cu LMM Auger spectra (Figure S18) confirmed that both
the 0.5B-Cu:0.025Zn GDE and the B-Cu GDE mainly
exhibit Cu+ on the surface even after long-term eCO2RR.
Besides Cu+, a fraction of Cu0 was also observed which
marginally increased during electrolysis. The 0.5B-
Cu:0.025Zn GDE shows a more stable Cu+/Cu0 ratio after
4 h of electrolysis with a lower Cu+/Cu0 ratio due to the Zn
protection as compared with the 0.5B-Cu GDE. This finding
does not only explain the higher stability of the 0.5B-
Cu:0.025Zn GDE, but also confirms the synergistic function
of the presence of Cu+ and Cu0 to promote C2+ product
formation. The obtained Zn LMM Auger spectra on 0.5B-
Cu:0.025Zn GDE (Figure S19) shows a significant decrease
of both Zn content and metallic Zn after long-term eCO2RR,
which confirms the sacrificial protection effect of Zn on the B-
Cu catalyst.
To further investigate the origin of the eCO2RR perfor-
mance of both the B-Cu and 0.5B-Cu:0.025Zn GDEs,
operando Raman spectroscopy measurements were carried
out by sequential application of potentials from 0.27 to
À1.43 V vs. RHE. First, the structural evolution of the catalyst
was monitored (Figure 5). Both 0.5B-Cu and 0.5B-
Cu:0.025Zn GDEs exhibit Raman peaks at 218, 522, and
640 cmÀ1, which were ascribed to the Cu2O phase.[18,31,32]
These Raman signals remain essentially unchanged under
operando conditions demonstrating the stability of the active
Cu+ species even at high reduction potentials. This is expected
to contribute to the efficient conversion of CO2 to C2+
products at the comparatively high applied current densities.
Operando electrochemistry Raman measurements were addi-
tionally used to detect possible intermediates of the eCO2RR
on 0.5B-Cu and 0.5B-Cu:0.025Zn electrodes. The peak at
1072 cmÀ1 is assigned to the anti-symmetrical stretching of
*OCO,[32,33] which can be observed from a potential of below
À0.23 V vs. RHE and increases in intensity at more cathodic
potentials for the 0.5B-Cu electrode (Figure 5A). In contrast,
the 0.5B-Cu:0.025Zn electrode already shows a sharp *OCO
vibrational peak at a potential of 0.27 V vs. RHE, which
gradually weakens at more cathodic potentials (Figure 5B).
This implies that the activation of CO2 can be achieved at
more positive potentials for the 0.5B-Cu:0.025Zn electrode
additionally supporting the observed higher eCO2RR activity
for the 0.5B-Cu:0.025Zn electrode as compared with the
0.5B-Cu electrode (Figure 4C). Raman peaks at 283, 344, and
2095 cmÀ1 for both 0.5B-Cu and 0.5B-Cu:0.025Zn electrodes
correspond to adsorbed CO on Cu,[31–34] which represents the
À
key intermediate for C C coupling and C2+ products for-
mation. The peak at about 2713 cmÀ1 is associated to the C H
À
vibration[35] and further confirms the formation of hydro-
carbons.
Conclusion
A B-Cu GDE system was developed for selective CO2
conversion to C2+ products at seldom reported high current
densities. The flooding problem in this GDE system, that is,
liquid electrolyte domination at the triple-phase boundary,
was mitigated through controlling the amount of added PTFE
and by optimizing the catalyst loading leading to optimisation
of the gas-liquid-catalyst interphase. The FE for the formation
of C2+ products was as high as 78% on a 0.5 mgcmÀ2 B-Cu
GDE with 10% PTFE at a current density of À200 mAcmÀ2
and a potential of À0.45 V vs. RHE. By incorporation of an
optimum amount of Zn nanosheets (0.025 mgcmÀ2), the
activity and long-term stability of the B-Cu catalyst were
substantially improved reaching a maximum current density
for the formation of C2+ products of À194 mAcmÀ2. Oper-
ando electrochemical Raman results demonstrate the stable
presence of Cu+ species during eCO2RR and the positive shift
of the overpotential for *OCO formation upon incorporation
of Zn, which concertedly result in the excellent eCO2RR
Figure 5. Operando electrochemistry Raman spectra for a A) 0.5B-Cu
GDE and a B) 0.5B-Cu:0.025Zn GDE during eCO2RR at different
applied potentials.
9140 www.angewandte.org
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 9135 –9141

Angewandte
Research Articles
Chemie
[13] D. Ren, Y. Deng, A. D. Handoko, C. S. Chen, S. Malkhandi, B. S.
performance of the B-Cu-Zn GDE. These findings suggest
a strategy for optimizing GDE structures to ultimately
improve the eCO2RR activity, selectivity, and stability for
the formation of value-added chemicals/fuels at high current
densities which may become suitable for potential applica-
tions.
Yeo, ACS Catal. 2015, 5, 2814 –2821.
[14] Q. Zhu, X. Sun, D. Yang, J. Ma, X. Kang, L. Zheng, J. Zhang, Z.
Wu, B. Han, Nat. Commun. 2019, 10, 3851.
[15] A. Eilert, F. Cavalca, F. S. Roberts, J. Osterwalder, C. Liu, M.
Favaro, E. J. Crumlin, H. Ogasawara, D. Friebel, L. G. M.
Pettersson, A. Nilsson, J. Phys. Chem. Lett. 2017, 8, 285 –290.
[16] H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev,
Y. W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser, B.
Roldan Cuenya, Nat. Commun. 2016, 7, 12123.
Acknowledgements
[17] P. De Luna, R. Quintero-Bermudez, C. T. Dinh, M. B. Ross, O. S.
´
Bushuyev, P. Todorovic, T. Regier, S. O. Kelley, E. H. Sargent,
The project has received funding from the ERC (CasCat
[833408]) and by the Deutsche Forschungsgemeinschaft in the
framework of the research unit FOR 2397e2 (276655237) as
well as under Germanyꢀs Excellence Strategy— EXC 2033–
390677874— RESOLV. CA acknowledges funding by the
BMBF in the framework of the NanomatFutur project
“MatGasDif” (03XP0263). NS acknowledges the Alexander
von Humboldt foundation for a Postdoc fellowship. Y.S. is
grateful to the International Clean Energy Talent Program
(iCET) hosted by the Future Energy Profile at Mälardalen
University in cooperation with the China Scholarship Council
(CSC) and the Applied Energy Journal. The authors acknowl-
edge the support of Dr Ulrich Hagemann from the DFG core
facility ICAN (DFG RI 00313) for a part of the XPS
measurements. Sandra Schmidt is acknowledged for help
with the SEM measurements. Open access funding enabled
and organized by Projekt DEAL.
Nat. Catal. 2018, 1, 103 –110.
[18] P. P. Yang, X. L. Zhang, F. Y. Gao, Y. R. Zheng, Z. Z. Niu, X. Yu,
R. Liu, Z. Z. Wu, S. Qin, L. P. Chi, Y. Duan, T. Ma, X. S. Zheng,
J. F. Zhu, H. J. Wang, M. R. Gao, S. H. Yu, J. Am. Chem. Soc.
2020, 142, 6400 –6408.
[19] Z. Yin, C. Yu, Z. Zhao, X. Guo, M. Shen, N. Li, M. Muzzio, J. Li,
H. Liu, H. Lin, J. Yin, G. Lu, D. Su, S. Sun, Nano Lett. 2019, 19,
8658 –8663.
[20] Z. Q. Liang, T. T. Zhuang, A. Seifitokaldani, J. Li, C. W. Huang,
C. S. Tan, Y. Li, P. De Luna, C. T. Dinh, Y. Hu, Q. Xiao, P. L.
Hsieh, Y. Wang, F. Li, R. Quintero-Bermudez, Y. Zhou, P. Chen,
Y. Pang, S. C. Lo, L. J. Chen, H. Tan, Z. Xu, S. Zhao, D. Sinton,
E. H. Sargent, Nat. Commun. 2018, 9, 3828.
[21] C. Chen, X. Sun, L. Lu, D. Yang, J. Ma, Q. Zhu, Q. Qian, B. Han,
Green Chem. 2018, 20, 4579 –4583.
[22] Y. Zhou, F. Che, M. Liu, C. Zou, Z. Liang, P. De Luna, H. Yuan,
J. Li, Z. Wang, H. Xie, H. Li, P. Chen, E. Bladt, R. Quintero-
Bermudez, T. K. Sham, S. Bals, J. Hofkens, D. Sinton, G. Chen,
E. H. Sargent, Nat. Chem. 2018, 10, 974 –980.
´
´
[23] S. Popovic, M. Smiljanic, P. Jovanovic, J. Vavrac, R. Buonsanti, N.
Hodnik, Angew. Chem. Int. Ed. 2020, 59, 14736 –14746; Angew.
Chem. 2020, 132, 14844 –14854.
Conflict of interest
[24] L. Fan, C. Xia, F. Yang, J. Wang, H. Wang, Y. Lu, Sci. Adv. 2020,
6, eaay3111.
[25] C. T. Dinh, T. Burdyny, M. G. Kibria, A. Seifitokaldani, C. M.
Gabardo, F. P. G. de Arquer, A. Kiani, J. P. Edwards, P. De Luna,
O. S. Bushuyev, C. Zou, R. Quintero-Bermudez, Y. Pang, D.
Sinton, E. H. Sargent, Science 2018, 360, 783 –787.
[26] A. I. Yanson, P. Rodriguez, N. Garcia-Araez, R. V. Mom, F. D.
Tichelaar, M. T. M. Koper, Angew. Chem. Int. Ed. 2011, 50,
6346 –6350; Angew. Chem. 2011, 123, 6470 –6474.
[27] B. Vanrenterghem, M. Bele, F. R. Zepeda, M. Sala, N. Hodnik, T.
Breugelmans, Appl. Catal. B 2018, 226, 396 –402.
[28] S. Ma, M. Sadakiyo, M. Heima, R. Luo, R. T. Haasch, J. I. Gold,
M. Yamauchi, P. J. Kenis, J. Am. Chem. Soc. 2017, 139, 47 –50.
[29] Y. X. Duan, F. L. Meng, K. H. Liu, S. S. Yi, S. J. Li, J. M. Yan, Q.
Jiang, Adv. Mater. 2018, 30, 7873 –7889.
[30] H. Jung, S. Y. Lee, C. W. Lee, M. K. Cho, D. H. Won, C. Kim,
H. S. Oh, B. K. Min, Y. J. Hwang, J. Am. Chem. Soc. 2019, 141,
4624 –4633.
[31] Y. Deng, B. S. Yeo, ACS Catal. 2017, 7, 7873 –7889.
[32] Y. Yang, S. Ajmal, Y. Feng, K. Li, X. Zheng, L. Zhang, Chem.
Eur. J. 2020, 26, 4080 –4089.
[33] W. Shan, R. Liu, H. Zhao, Z. He, Y. Lai, S. Li, G. He, J. Liu, ACS
Nano 2020, 14, 11363 –11372.
The authors declare no conflict of interest.
Keywords: B-doped Cu ·CO2 electroreduction ·
gas diffusion electrode ·multi-carbon products ·Zn nanosheet
[1] P. Tans, R. Keeling, NORR/ESRL. http://www.esrl.noaa.gov/
gmd/ccgg/trends/.
[2] W. D. Jones, J. Am. Chem. Soc. 2020, 142, 4955 –4957.
[3] C. Chen, J. F. K. Kotyk, S. W. Sheehan, Chem 2018, 4, 2571 –
2586.
[4] Y. Y. Birdja, E. Perez-Gallent, M. C. Figueiredo, A. J. Gçttle, F.
Calle-Vallejo, M. T. M. Koper, Nat. Energy 2019, 4, 732 –745.
[5] M. B. Ross, P. D. Luna, Y. Li, C. T. Dinh, D. Kim, P. Yang, E. H.
Sargent, Nat. Catal. 2019, 2, 648 –658.
[6] S. Zhang, Q. F. Rong, T. J. Meyer, Chem 2018, 4, 2571 –2586.
[7] T. Burdyny, W. A. Smith, Energy Environ. Sci. 2019, 12, 1442 –
1453.
[8] M. Ma, E. L. Clark, K. T. Therkildsen, S. Dalsgaard, I. Chor-
kendorff, B. Seger, Energy Environ. Sci. 2020, 13, 977 –985.
[9] F. P. García de Arquer, C. T. Dinh, A. Ozden, J. Wicks, C.
McCallum, A. R. Kirmani, D. H. Nam, C. Gabardo, A. Seifito-
kaldani, X. Wang, Y. C. Li, F. Li, J. Edwards, L. J. Richter, S. J.
Thorpe, D. Sinton, E. H. Sargent, Science 2020, 367, 661 –666.
[10] A. Vasileff, C. Xu, Y. Jiao, Y. Zheng, S. Z. Qiao, Chem 2018, 4,
1809 –1831.
[34] D. Ren, B. S.-H. Ang, B. S. Yeo, ACS Catal. 2016, 6, 8239 –8247.
[35] D. Öhl, Y. U. Kayran, J. R. C. Junqueira, V. Eßmann, T.
Bobrowski, W. Schuhmann, Langmuir 2018, 34, 12293 –12301.
Manuscript received: December 20, 2020
Revised manuscript received: February 6, 2021
Accepted manuscript online: February 9, 2021
Version of record online: March 10, 2021
[11] C. W. Lee, K. D. Yang, D. H. Nam, J. H. Jang, N. H. Cho, S. W.
Im, K. T. Nam, Adv. Mater. 2018, 30, 1704717.
[12] C. W. Li, M. W. Kanan, J. Am. Chem. Soc. 2012, 134, 7231 –7234.
Angew. Chem. Int. Ed. 2021, 60, 9135 –9141
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
9141