Self-Cleaning Catalyst Electrodes for Stabilized CO2 Reduction to Hydrocarbons

Article abstract of DOI:10.1002/anie.201707478

A surface-restructuring strategy is presented that involves self-cleaning Cu catalyst electrodes with unprecedented catalytic stability toward CO₂ reduction. Under the working conditions, the Pd atoms pre-deposited on Cu surface induce continuous morphological and compositional restructuring of the Cu surface, which constantly refreshes the catalyst surface and thus maintains the catalytic properties for CO₂ reduction to hydrocarbons. The Pd-decorated Cu electrode can catalyze CO₂ reduction with relatively stable selectivity and current density for up to 16 h, which is one of the best catalytic durability performances among all Cu electrocatalysts for effective CO₂ conversion to hydrocarbons. The generality of this approach of utilizing foreign metal atoms to induce surface restructuring toward stabilizing Cu catalyst electrodes against deactivation by carbonaceous species accumulation in CO₂ reduction is further demonstrated by replacing Pd with Rh.

Full text of DOI:10.1002/anie.201707478

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A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Self-Cleaning Catalyst Electrodes for Stabilized CO2 Reduction
to Hydrocarbons
Authors: Zhe Weng, Xing Zhang, Yueshen Wu, Shengjuan Huo,
Jianbing Jiang, Wen Liu, Guanjie He, Yongye Liang, and
Hailiang Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707478
Angew. Chem. 10.1002/ange.201707478
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http://dx.doi.org/10.1002/ange.201707478

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10.1002/anie.201707478
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DOI: 10.1002/anie.200((will be filled in by the editorial staff))
CO2 Reduction
Self-Cleaning Catalyst Electrodes for Stabilized CO2 Reduction to
Hydrocarbons
Zhe Weng, Xing Zhang, Yuesheng Wu, Shengjuan Huo, Jianbing Jiang, Wen Liu, Guanjie He, Yongye
Liang,* Hailiang Wang*
Abstract: Utilizing clean and renewable energy for electrochemical
conversion of CO2 to fuels and chemical products is a promising
pathway towards zero emissions. While Cu metal catalysts have
received substantial attention due to their distinct capabilities to
catalyze CO2 electroreduction to hydrocarbons, they still suffer from
fast deactivation. Developing a Cu electrocatalyst with long-term
catalytic durability for CO2 reduction remains a big challenge. In this
work, we report a surface-restructuring strategy for realizing self-
cleaning Cu catalyst electrodes with unprecedented catalytic stability
toward CO2 reduction. Under the working conditions, the Pd atoms
pre-deposited on Cu surface induce continuous morphological and
compositional restructuring of the Cu surface, which constantly
refreshes the catalyst surface and thus maintains the catalytic
properties for CO2 reduction to hydrocarbons. Our Pd-decorated Cu
electrode can catalyze CO2 reduction with relatively stable selectivity
and current density for up to 16 h, representing one of the best
catalytic durability performances among all Cu electrocatalysts for
effective CO2 conversion to hydrocarbons. The generality of this
approach of utilizing foreign metal atoms to induce surface
restructuring toward stabilizing Cu catalyst electrodes for CO2
reduction is further demonstrated by replacing Pd with Rh.
reactions.[2a, 3] Among all the materials studied, Cu metal has received
substantial attention due to its distinct ability to generate reasonable
amounts of hydrocarbon products,[4] which requires addition of
multiple electrons and protons to a CO2 molecule and C-C bond
formation. In the past several decades, a number of strategies have
been developed to increase the activity and selectivity of Cu catalysts,
including shape control,[5] alloying,[6] surface modification[7] and
oxidative treatment.[8] However, fast deactivation remains as a major
problem for metallic Cu catalysts.[5f, 9]
While there have been numerous reports on observation of fast
decay in CO2 reduction to hydrocarbons and simultaneous rapid
dominance by H2 evolution for Cu electrodes,[9-10] the deactivation
mechanism is still under debate. One perspective is that the
deactivation is caused by cathodic deposition of metal ion impurities
in the electrolyte.[10a] It has been suggested that the metal ion
impurities can be removed by purifying the electrolyte solution via
pre-electrolysis[10a] or metal ion complexation,[9b] which is able to
maintain the product selectivity for up to 2 h. Nevertheless, other
studies find that pre-treatment of electrolyte has little improvement in
[10b, 10d, 11]
suppressing Cu catalyst deactivation.
Instead, surface
analysis reveals that accumulation of carbonaceous species may be
the main reason for catalyst poisoning.[9a, 9c, 10b-d] Possible approaches
to tackle this problem include controlling reaction pathways of
electrochemical CO2 reduction to avoid decomposition of reaction
intermediates and thus carbon accumulation on Cu surface, or
applying a periodic carbon-removing method to clean the catalyst
surface. The former has been demonstrated by altering local pH to
lower selectivity for methane,[9a, 12] although challenges still remain
because the mechanisms of CO2 electroreduction on Cu are not yet
well understood. The latter has been practically enabled by applying
The rise in CO2 emissions caused by burning fossil fuels poses
considerable risks to the environment and climate as well as all living
creatures on the earth.[1] Utilizing clean and renewable energy for
electrochemical conversion of CO2 to fuels and chemical products is
considered a promising pathway toward zero emissions.[1a, 2] Many
electrocatalysts including solid-state materials and molecular
complexes have been extensively investigated for CO2 reduction
oxidative pulses but requires considerable modification and
10d]
intervention on the reactor level.[10c,
Developing a Cu
electrocatalyst with long-term catalytic durability for CO2 reduction
remains a big challenge.
[]
Dr. Z. Weng, Y. Wu, Prof. S. Huo, Dr. J. Jiang, Dr W. Liu,
Prof. H. Wang
Here we report the discovery of a general strategy for creating
self-cleaning Cu electrocatalysts with substantially improved stability
for CO2 reduction. We first verify that plain Cu surface deactivates
quickly under the working conditions. The Faradaic efficiency (FE)
for CO2 reduction to hydrocarbons (methane and ethylene) decreases
from 56% to 13% within 2 h, while that for H2 evolution increases
from 30% to 80%. The deactivation process is accompanied with
increasing coverage of carbonaceous species on the catalyst surface.
We then find that the selectivity for CO2 reduction can be stabilized
by pre-decorating the Cu surface with Pd atoms. Our Pd-decorated
Cu electrode retains FE (H2) below 40% and FE (CH4+C2H4) above
50% during 4 h of continuous electrolysis, achieving one of the most
durable Cu-metal-based electrocatalysts with high efficiency for CO2
conversion to hydrocarbons. Under the working conditions, the Pd
decoration induces morphological and compositional restructuring of
the catalyst surface to form nanoparticles with Cu-rich surface, which
refreshes the Cu surface and thus maintains stable electrocatalytic
CO2 reduction. We further find that other noble metals such as Rh can
Department of Chemistry, Yale University, P.O. Box 208107,
New Haven, Connecticut 06520, USA
Energy Sciences Institute, Yale University, 810 West
Campus Drive, West Haven, Connecticut 06511, USA
E-mail: hailiang.wang@yale.edu
X. Zhang, Prof Y. Liang
Department of Materials Science and Engineering, South
University of Science and Technology of China, 1088
Xueyuan Road, Shenzhen 518055, China
E-mail: liangyy@sustc.edu.cn
Prof. S. Huo
Department of Chemistry, Science Colleges, Shanghai
University, 99 Shangda Road, Shanghai 200444, China
G. He
Christopher Ingold Laboratory, Department of Chemistry,
University College London, 20 Gordon Street, London
WC1H 0AJ, UK
Supporting information for this article is available on the
WWW under http://www.angewandte.org or from the
authors.
1
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perform similar functionalities as Pd. This work presents foreign-
metal-induced surface restructuring as an effective approach to
stabilizing Cu for electrocatalytic CO2 reduction.
for 15 min, the surface exhibits larger particle sizes together with
cavities (Figures 2B, S4B), which is caused by restructuring under the
electrochemical conditions. The surface coalescing continues as the
electrolysis proceeds (Figure 2C, D, and S4C, D). X-ray
photoelectron spectroscopy (XPS) analysis detects no signal of metal
elements other than Cu on the surface of the used Cu electrodes
(Figure S5), indicating that the observed deactivation is not a direct
result of metal impurity deposition. The corresponding Cu 2p and C
1s core-level XPS spectra are plotted in Figure S6. The Cu 2p spectra
all exhibit the 2p1/2 and 2p3/2 components at 952.5 and 932.7 eV,
respectively, characteristic of metallic Cu surfaces. It is noted that the
surface C/Cu ratio of the electrode increases as the electrolysis
proceeds (Figure 2E). Deconvolution of the C 1s spectra (Figure S7)
reveals that C–C is the major component of the spectrum and
responsible for the observed increase in C/Cu ratio over time (Figure
Figure 1. Distributions of gas products as the CO2 reduction electrocatalysis
proceeds on (A) Cu electrode and (B) Pd-decorated Cu electrode, and (C)
their time-dependent total current densities at –0.96 V vs RHE in CO2-
saturated 0.5 M KHCO3.
Pre-cleaned Cu mesh electrodes were first examined for
electrocatalytic CO2 reduction in pre-purified 0.5 M aqueous KHCO3
electrolyte under a constant electrode potential of –0.96 V vs the
reversible hydrogen electrode (RHE). While the gas products were
measured by an on-line gas chromatograph (GC) with an interval of
9 min during electrolysis, the products in the liquid phase were
–
2F). This excludes possible contributions from adsorbed HCO3 or
CO2 species, and alludes to deposited carbonaceous species
containing C–C bonds as a major cause of the drastic decay of CO2
reduction and rise of H2 evolution, agreeing well with the previous
studies.[9c, 10b-d, 11] Since the carbonaceous species is formed on the Cu
surface during the CO2 electroreduction process and particularly in
the pathway of CO2 conversion to CH4,[9a, 9c] the deposition rate
should be roughly proportional to the catalytic current density and
thus the deactivation cannot be mitigated by simply increasing the
electrochemically active surface area of the electrode.
In pursuit of modifying the catalytic properties of Cu, we
deposited Pd on Cu mesh electrodes via a galvanic replacement
reaction (see SI for experimental details). While the Pd decoration
does not alter the initial product distribution (Figure 1B), it turns out
to be highly effective in suppressing growth of H2 evolution. The FE
(H2) for the Pd-decorated Cu electrode remains below 40% during the
first 4 h of electrolysis (Figure 1B). FE (CH4) slightly decreases from
46% to 40%, while FE (C2H4) slightly increases from 7% to 11%.
Even after performing electrocatalysis for 16 h, the Pd-decorated Cu
electrode can keep more than 50 % of the current density devoted to
CO2 reduction (Figure S8). These results reflect that catalytic
deactivation of Cu for electrochemical CO2 reduction can be
effectively mitigated by surface Pd decoration. We note that the
distribution of the CO2 reduction products slightly changes as the
1
analyzed by H nuclear magnetic resonance (NMR) spectroscopy
after the entire electrolysis (Figure S1). Since the liquid products can
only be quantified in a time-averaged format and the electrodes
exhibit highest hydrocarbon selectivity at –0.96 V vs RHE (Figure
S2), our discussion of catalyst deactivation will be based on gas
products generated at –0.96 V. The initial gas products generated on
the Cu electrode are CH4, CO, C2H4 and H2 (Figure 1A). All the
gaseous CO2 reduction products account for a total FE of ~50%,
among which CH4 is the dominant product. The initial FE for H2
evolution is ~30%. Similar to the previous studies,[9c, 10b-d, 11] we
observe fast deactivation for the Cu electrode. The CH4 yield starts to
drop rapidly while the FE for H2 drastically increases after 24 min.
After 2 h, FE (CH4) and FE (C2H4) have decreased to 11% and 2%
respectively, while FE (H2) has increased to 80%. The Cu catalyst
loses almost all of its catalytic activity for CO2 reduction after 4 h,
with the H2 evolution reaction consuming ~100% of the current
density. Figure 1C shows the evolution of current density as the
electrolysis proceeds. The decrease in current density during the first
hour is ascribed to loss of electrochemical active surface area (Figure
S3 and Table S1). After that, the current density starts to increase
depite that the surface area continues to decrease, likely due to
increased catalytic activity for H2 evolution.
Figure 2. SEM images of (A) a fresh Cu electrode and Cu electrodes having
performed electrocatalysis for (B) 15 min, (C) 1 h and (D) 4 h. (E) Surface
C/Cu atomic ratios and (F) further breakdown of the C content for the fresh
and used Cu electrodes.
Figure 3. SEM images of (A) a fresh Pd-decorated Cu electrode and Pd-
decorated Cu electrodes having performed electrocatalysis for (B) 15 min, (C)
1 h, (D) 4 h and (E) 16 h. (F) Size distributions of the nanoparticles formed on
the used Pd-decorated Cu electrodes. (G) Surface Pd/(Cu+Pd) atomic ratios,
(H) surface C/(Cu+Pd) atomic ratios and (I) further breakdown of the C
content for the fresh and used Pd-decorated Cu electrodes.
We employ scanning electron microscopy (SEM) to image the
surface morphology of the Cu electrodes after they experience
different amounts of time of electrolysis (Figure 2A-D, S4). The fresh
Cu electrode has a relatively smooth surface with an average particle
size of ~10 nm (Figures 2A, S4A). After performing electrocatalysis
2
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electrolysis proceeds (Figure 1B), although the total FE for CO2
reduction keeps stable. Compared with the plain Cu electrode, the Pd-
decorated Cu electrode shows a much higher initial current density of
57 mA cm-2, which decreases by only 18% during the 4 h of
electrolysis (Figure 1C). Taken together, these represent one of the
most stable catalytic performances among all reported Cu metal
electrocatalysts with high efficiency for CO2 conversion to
hydrocarbons (Table S2). Note that similar electrocatalytic results can
also be obtained using polycrystalline Cu foil in place of Cu mesh
(Figure S9).
SEM images of the fresh and used Pd-decorated Cu electrodes
are shown in Figures 3A-E and S10. Compared with the fresh plain
Cu electrode, the fresh Pd-decorated Cu electrode manifests a rougher
surface with voids (Figure 3A, S10A), which is a result of the
galvanic etching and deposition processes and agrees with the
observed higher initial current density (Figure 1C) and larger surface
roughness factor (Table S1). Energy dispersive X-ray spectroscopy
(EDS) mapping shows that Pd is uniformly distributed as a thin layer
on the Cu surface (Figure S11). No X-ray diffraction (XRD) signals
other than those associated with Cu metal are detected (Figure S12),
indicating that the deposited Pd is likely in the form of small clusters
or even single atoms. Interestingly, the Pd-decorated Cu electrode
exhibits obviously different surface morphological evolution than the
plain Cu electrode during electrolysis. After 15 min of electrolysis,
nanoparticles with an average size of ~15 nm have appeared on the
electrode surface (Figure 3B, F). As the electrolysis proceeds, the size
of the nanoparticles continues to grow and their shape becomes more
and more spherical (Figure 3C-F). Only the Cu metal phase is
detected by XRD for the Pd-decorated Cu electrode after 16 h of
electrolysis (Figure S12), indicating that the formed nanoparticles are
metallic Cu.
Surface composition and chemical states of the fresh and used
Pd-decorated Cu electrodes were analyzed by XPS (Figure S13). The
fresh Pd-decorated Cu electrode contains ~12% of Pd on its surface
and the percentage decreases as the electrolysis proceeds (Figure 3G),
implying a restructuring process resulting in Cu enrichment. After 4
h of electrolysis, the Pd-decorated Cu electrode contains only ~3% of
Pd atoms on its surface. The XPS depth profile reveals that a
maximum Pd concentration exists several nanometers underneath the
surface (Figure S14B), in contrast to the fresh Pd-decorated Cu
electrode for which the maximum Pd concentration is found on the
surface (Figure S14A). The C 1s core-level XPS spectra show that the
C concentration on the electrode surface remains almost unchanged
during the first 4h of electrolysis (Figure 3H, I). Only a minor increase
in surface C concentration is observed after the electrode performing
electrocatalysis for 16 h. The evolution of surface C concentration
correlates very well with the progression of the electrode’s catalytic
selectivity for CO2 reduction, suggesting that the Pd decoration on Cu
and the consequent surface restructuring limit accumulation of
carbonaceous species on the electrode surface and thus stabilize
electrocatalytic CO2 reduction.
Taking the results together, we can conclude that deactivation of
Cu electrode for electrochemical CO2 reduction is strongly correlated
with and likely due to accumulation of carbonaceous species on the
catalyst surface (Figure 4). By decorating Cu surface with Pd, we can
induce morphological and compositional restructuring and thus
impart the catalyst electrode self-cleaning capability against carbon
accumulation, which effectively stabilizes the catalytic selectivity for
CO2 reduction. We speculate that the surface restructuring starts from
the Pd atoms acting as nucleating agents for Cu deposition. Under the
working conditions, there is a dynamic equilibrium between Cu ions
in the solution and the solid Cu electrode. In the presence of surface
Pd species, Cu ions will preferentially deposit on the Pd sites due to
their stronger affinity with Pd.[13] After the initial nucleation, Cu ions
will continue to preferentially deposit on smaller or irregularly shaped
nanoparticles as a result of their high surface energy, leading to
nanoparticle growth and re-shaping toward a more spherical
morphology. During this process, the electrode keeps refreshing its
surface and thus prevents deactivation by incorporating or desorbing
the surface carbonaceous species (Figure 4). Since the Pd-decorated
Cu eletrode possesses almost the same catalytic selectivity as the Cu
electrode and the surface is nearly free of Pd after electrocatalysis, it
can be confirmed that Pd does not directly participate in the CO2
reduction catalysis but only triggers the reconstruction and refreshing
of the Cu surface. This restructuring-induced self-cleaning
mechanism fundamentally distinguishes from those reported for Cu-
Pd bimetallic catalysts where Pd directly participates in catalysis and
often alters product selectivity.[6c-f, 14]
It is worth noting that this Pd decoration approach can
effectively suppress deactivation of Cu electrodes even in electrolyte
without pre-purification (Figure S15). Further, we demonstrate that
this approach of utilizing foreign metal atoms to induce surface
restructuring is a general strategy for stabilizing the electrocatalytic
properties of Cu electrodes for CO2 reduction. For example, similar
surface restructuring and catalytic selectivity stabilization can be
observed for Rh-decorated Cu electrodes (Figure S16).
In summary, we have developed a general strategy for achieving
stable electrocatalysts for CO2 reduction to hydrocarbons by
decorating Cu electrode surface with foreign noble atoms. Under the
working conditions, the foreign atoms induce continuous
restructuring of the Cu surface and thus render a self-cleaning catalyst
electrode showing unprecedented long-term catalytic stability. This
strategy will open up new opportunities for design and development
of robust catalysts for a wide range of electrochemical applications.
Acknowledgement
The work is supported by the Doctoral New Investigator grant from
the ACS Petroleum Research Fund and the Global Innovation
Initiative from Institute of International Education. X.Z. and Y.L.
acknowledge financial supports from "The Recruitment Program of
Global Youth Experts of China", Shenzhen fundamental research
funding (JCYJ20160608140827794), Shenzhen Key Lab funding
(ZDSYS201505291525382)
(KQTD20140630160825828).
and
Peacock
Plan
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Keywords: carbon dioxide reduction· electrocatalysis · surface
restructuring · copper · palladium
Figure 4. Schematic illustration of Pd-induced surface restructuring that can
avoids the accumulation of carbonaceous species on Cu surface.
3
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