A highly selective copper-indium bimetallic electrocatalyst for the electrochemical reduction of aqueous CO2to CO

Article abstract of DOI:10.1002/anie.201410233

The challenge in the electrochemical reduction of aqueous carbon dioxide is in designing a highly selective, energy-efficient, and non-precious-metal electrocatalyst that minimizes the competitive reduction of proton to form hydrogen during aqueous CO₂ conversion. A non-noble metal electrocatalyst based on a copper-indium (Cu-In) alloy that selectively converts CO₂ to CO with a low overpotential is reported. The electrochemical deposition of In on rough Cu surfaces led to Cu-In alloy surfaces. DFT calculations showed that the In preferentially located on the edge sites rather than on the corner or flat sites and that the d-electron nature of Cu remained almost intact, but adsorption properties of neighboring Cu was perturbed by the presence of In. This preparation of non-noble metal alloy electrodes for the reduction of CO₂ provides guidelines for further improving electrocatalysis.

Full text of DOI:10.1002/anie.201410233

Angewandte
Chemie
DOI: 10.1002/anie.201410233
Electrocatalysis
A Highly Selective Copper–Indium Bimetallic Electrocatalyst for the
Electrochemical Reduction of Aqueous CO2 to CO**
Shahid Rasul, Dalaver H. Anjum, Abdesslem Jedidi, Yury Minenkov, Luigi Cavallo, and
Kazuhiro Takanabe*
Abstract: The challenge in the electrochemical reduction of
aqueous carbon dioxide is in designing a highly selective,
energy-efficient, and non-precious-metal electrocatalyst that
minimizes the competitive reduction of proton to form hydro-
gen during aqueous CO2 conversion. A non-noble metal
electrocatalyst based on a copper-indium (Cu-In) alloy that
selectively converts CO2 to CO with a low overpotential is
reported. The electrochemical deposition of In on rough Cu
surfaces led to Cu-In alloy surfaces. DFT calculations showed
that the In preferentially located on the edge sites rather than on
the corner or flat sites and that the d-electron nature of Cu
remained almost intact, but adsorption properties of neighbor-
ing Cu was perturbed by the presence of In. This preparation of
non-noble metal alloy electrodes for the reduction of CO2
provides guidelines for further improving electrocatalysis.
special attentions because it is a metal that produces hydro-
carbons during the CO2 reduction reaction.[3–13]
CO is one of the desired products from the reduction of
CO2 with high selectivity because CO can be further reduced
to oxygenates and hydrocarbons electrochemically.[14–16]
Recently, Li and Kanan reported the superior performance
of oxide-derived Cu (OD-Cu) in aqueous CO2 reduction
compared to polycrystalline Cu under the identical condi-
tions.[7] The OD-Cu electrocatalyst, however, suffers from low
product selectivity, co-generating H2.[7,13] Additionally,
although the morphology changes by the oxidation treatment
are prominent, the effective active sites on Cu-based surfaces
are still under debate, despite a large number of previous
studies.[3–16] On the other hand, noble metals, including Au
and Ag, are electrocatalysts that can efficiently reduce
aqueous CO2 to CO, with selectivities approaching
100%,[17,18] but their high cost remains a concern. Non-
noble metal bismuth-based electrode necessitates the pres-
ence of ionic liquids and non-aqueous phase to demonstrate
high performance to generate CO from CO2, and the effects
of the active components and of the electrolyte remain
unclear.[19]
Designing a CO2-reducing electrocatalyst should thus be
focused on the use of non-noble-metals that are selective and
energy-efficient. Recently, various strategies have been
reported for the conversion of CO2, including alloying of
copper with other metals,[20–23] and for the electrochemical
reduction reaction using density functional theory (DFT)
calculations to obtain higher selectivity and energy effi-
ciency.[24–27] The general trend is that the binding strengths of
the intermediates on the catalyst surface need to be adequate
and thus the importance of the surface coordinately unsatu-
rated sites has been addressed.[6,9,11,14,15] Bimetallic
approaches have also been explored by DFT calculations,
which may change the conventional electronic structure
associated with d-center theory,[24,25] and/or geometric and
ensemble effects of the metal active sites.[27] The competitive
interaction of H+/water with two metal centers may control
the reaction pathway during the electrochemical reduction of
CO2, promoting the reduction of CO2 on one or both of the
metal centers, thereby resulting in better selectivity. This
study focused on an indium electrodeposited Cu electro-
catalyst, which resulted in the formation of a Cu-In alloy that
works at moderate overpotentials with exclusive selectivity
for CO and excellent stability.
T
he development of an artificial photosynthesis process that
converts CO2 and stores the energy in the form of chemical
bonds is one of the grand challenges in modern chemistry.[1,2]
However, the limited choice of electrocatalysts that are
energy-efficient, selective, and stable increases the complex-
ity of this process.[3] Conventionally, metal electrodes have
been utilized as electrocatalysts for the aqueous CO2 reduc-
tion reaction, and the product distribution strongly depends
on the nature of the electrode surface and the electrolyte.[3]
Hori and co-workers observed that the CO2 reduction
reaction reproducibly yields CO, CH4, HCOOH and other
hydrocarbon products and that the selectivity of the products
is determined by the nature of the metallic electrode.[3,4]
Among the various metallic electrodes, copper has attracted
[*] Dr. S. Rasul, Dr. A. Jedidi, Dr. Y. Minenkov, Prof. L. Cavallo,
Prof. K. Takanabe
Division of Physical Sciences and Engineering
KAUST Catalysis Center (KCC)
King Abdullah University of Science and Technology (KAUST)
4700 KAUST, 23955-6900 Thuwal (Saudi Arabia)
E-mail: kazuhiro.takanabe@kaust.edu.sa
Dr. D. H. Anjum
Advanced Nanofabrication Imaging and Characterization Core Lab
King Abdullah University of Science and Technology (KAUST)
4700 KAUST, 23955-6900 Thuwal (Saudi Arabia)
[**] The research reported herein was supported by the King Abdullah
University of Science and Technology. L.C. and A.J. are grateful to
the KAUST Supercomputing Laboratory for the resources provided
under the project k199.
OD-Cu substrate was obtained by thermally oxidizing
a Cu metal sheet at 773 K for 2 h in static air.[13] This
treatment led to the formation of a hairy CuO nanowire
structure on Cu2O-Cu layers,[13] resulting in a surface rough-
Supporting information for this article (including experimental and
theoretical details) is available on the WWW under http://dx.doi.
org/10.1002/anie.201410233.
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ness factor that was increased 140-fold compared to that of
the pristine Cu sheet as measured by cyclic voltammetry
(Supporting Information, Table S1). The Cu-In electrode was
then prepared through electrochemical reduction of the OD-
Cu in two-electrode system with a solution containing 0.05m
and electrochemical conditions. These results indicate that
the electron transfer rates are essentially identical in these
electrodes; however, they exhibited a distinct difference in
selectivity. The effects of the applied potentials on the FEs for
OD-Cu and Cu-In are shown in Figure 1b and c, respectively.
OD-Cu began to convert CO2 at a potential of À0.3 V vs.
RHE, primarily generating H2 as the reaction product. When
the electrode was more negatively polarized, the conversion
of CO2 to CO and HCOOH improved, reaching maximum FE
of 40 and 30%, respectively, at À0.6 V vs. RHE, consistent
with previous results.[7,13] In contrast, the Cu-In electrode
catalyzed the reduction of CO2 at 0.3 V vs. RHE, to CO
selectively (FECO ꢀ 23%) while suppressing the formation of
H2 (FEH2 ꢀ 3%). We were unable to capture the remaining
products, probably because of additional Cu and/or In
reduction as the Cu/Cux+ or In/Iny+ standard redox potentials
reside in this range.[28] Moreover, at applied potentials from
À0.3 to À0.7 V vs. RHE, CO was produced as almost the sole
product of CO2 reduction, approaching an FE of 90% at
À0.5 V vs. RHE. It is clear that the presence of In along with
Cu drastically altered the nature of the electroactive species.
The partial current density for CO production (jCO) as
a function of overpotential (Tafel plot) is plotted in the
Supporting Information, Figure S3. The Tafel plot exhibits
a slope of greater than 120 mVdecÀ1, similar value to that of
OD-Cu,[7] which corresponds to a mechanism in which an
initial single-electron transfer or adsorption of reactant is the
rate-determining step.
InSO4 and 0.4m citric acid at
a current density of
À3.3 mAcmÀ2 for 90 min (ca. 18CcmÀ2). This deposition of
In underwent a rather complex reduction process in which
both reduction of the Cu oxide and deposition of In occurred
(Supporting Information, Figure S1). The surface roughness
was further improved to double that of OD-Cu (Supporting
Information, Table S1).
The as-prepared Cu-In electrode was then tested for the
reduction of CO2 in a two-compartment, three-electrode
electrochemical cell in 0.1m aqueous KHCO3 saturated with
CO2 (KHCO3/CO2). The results for the reduction of CO2
using the OD-Cu and Cu-In electrodes under chronopoten-
tiometric conditions when a geometric current density in the
electrode of À1.67 mAcmÀ2 for 1 h are shown in the
Supporting Information, Figure S2. An initial transient
potential change without the formation of any products was
observed, which most likely originates from the reduction of
oxide surfaces that are not active for CO2 reduction.[13] At
steady-state electrolysis, the potential was shifted to ca.
À0.7 V vs. RHE. Under these conditions, the Cu-In electrode
selectively converted CO2 to CO with a Faradaic efficiency of
about 95% and with almost negligible levels of H2 levels
compared to the CO selectivity of 30% obtained using OD-
Cu.
To evaluate the stability of the Cu-In catalyst, electrolysis
with long controlled potentials in 0.1m KHCO3/CO2 at À0.6 V
vs. RHE was performed, as shown in Figure 1d. The reaction
was intentionally stopped after 3.5 h and allowed to stand
overnight to observe the degradation of the electrode under
open-circuit aqueous conditions. The reaction was then
restarted for an additional 3.5 h. The results indicated that
the Cu-In catalyst is extremely stable under the conditions for
aqueous CO2 reduction, with an 85% FE for CO for 7 h.
A SEM image of the Cu-In structure is presented in the
Supporting Information, Figure S4. The microstructure con-
sists of large irregularly shaped grains ranging from 100 to
500 nm in size. The large grains are formed as a result of the
agglomeration of small nanoparticles (ca. 50 nm), which are
capped by a shell-like structure. High-resolution transmission
electron micrographs (HR-TEM) and the corresponding
calculated fast Fourier transform (FFT) patterns of the Cu-
In samples after the CO2 reduction experiments are shown in
Figure 2a. The nanostructure could be divided into two
distinct regions: the bulk and the surface. The FFT pattern of
the core clearly shows a highly crystalline structure, whereas
the FFT pattern of the shell shows a deformed crystal
structure, which may arise from the diffusion of In, with
a large atomic radius (0.155 nm), into the smaller Cu
(0.135 nm) lattice. Superimposed elemental maps of In
(green) and Cu (red) are shown in Figure 2b. The Figure
clearly shows that the surface is enriched with In with
a thickness of about 3 nm. The XRD pattern of the Cu-In
sample (Figure 3a) confirms the formation of the Cu-In
bimetallic (Cu11In9) alloy (PDF 00-065-4963), with intense
peaks for the Cu metal substrate (PDF 00-004-0836). The X-
The Cu-In electrode was subsequently tested at different
applied potentials and compared with OD-Cu. Figure 1 shows
the total current density (jtot) and Faradaic efficiency (FE) at
À0.3 to À0.7 V vs. RHE in 0.1m KHCO3/CO2. As shown in
Figure 1a, similar values for total current density were
obtained for OD-Cu and Cu-In in the same potential range
Figure 1. a) Comparison of the current density profiles for OD-Cu and
Cu-In, chronoamperometric analyses of b) OD-Cu and c) Cu-In, and
d) the long-term stability test for the Cu-In catalyst at À0.6 V vs. RHE
in 0.1m KHCO3/CO2.
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a high hydrogen overpotential metal such as In is present
around the Cu active site, effectively forming a Cu-In alloy, it
presumably inhibits the formation of H2 on Cu without
deactivating the reduction of CO2.
DFT calculations were used to shed light on the effect of
In on the bulk and surface properties of pure Cu. Comparison
of the DOS of pure Cu and of Cu/In alloys with compositions
of up to 50% In indicated that the presence of In does not
significantly affect the electronic structure of pure Cu (Fig-
ure 4a; Supporting Information, Figures S10 and S11). Fur-
Figure 2. a) HR-TEM images of Cu-In with FFT images from the bulk
and the surface (inset); b) EDS element mapping of the selected area:
In (green), Cu (red).
Figure 4. a) Density of states of Cu and of Cu11In9. The values for the
d-band center are designated. b) Site preference of In replacing one Cu
in Cu55 (Ih) cluster. The energies relative to Cu55-Ih are presented. In
the case of Cu55-Oh, the same site preference was obtained for In.
c) Side views of the three possible geometries of the (211) facet and
top views of (100) and (111) facet of Cu with one In atom replacing
a Cu atom. The energies relative to the most stable structure (left) is
presented.
Figure 3. a) XRD profiles and b) In3d and Cu2p XPS spectra of the
Cu-In sample.
ray photoelectron spectra for Cu 2p3/2 and Cu 2p1/2 and for In
3d5/2 and 3d3/2 (Figure 3b) indicate the reduction to the Cu0
and In0 metallic states. Overall, the characterization suggests
that the thin layer of bimetallic Cu-In alloy was uniformly
formed on the rough surface of OD-Cu.
The experimental data presented here raise a very
important question regarding the influence of the environ-
ment on the nature of the active species when a second metal
center is present along with Cu. A separate CO2 reduction
experiment with an In-deposited Cu sheet without an initial
oxidation treatment (Supporting Information, Figure S5)
showed only slight improvement in CO selectivity (H2
predominant), indicating that having OD-Cu as the starting
substrate is effective. This result may indicate that the high
surface area with grain boundaries,[7] and the specific surface
facets created by the oxidation/re-reduction treatment are
essential for achieving high selectivity toward CO at low
overpotentials. Another experiment with an In-deposited In
sheet (Supporting Information, Figure S6) resulted in formic
acid as the major product with trace amounts of H2 at elevated
overpotentials (typically < À0.9 V vs. RHE), indicating that
In alone will not lead to the formation of CO and Cu is
essential. Furthermore, a similar increase in selectivity
towards CO was observed using Sn and Zn as the second
metals (Supporting Information, Figures S7 and S8), suggest-
ing that the universal effects of the second metals that have
high overpotentials toward the evolution of H2 prevail. When
ther details of the calculated bulk electronic structure are
available in Supporting Information. Consistent with the
recent reports,[27] this result suggests that the role of In is to
locally modify the properties of the surfaces. For this reason,
we investigated models of possible geometrical situations
occurring at the surface of Cu/In alloys. We thus modeled the
effect of replacing one Cu atom by one In atom, as well as the
effect of adsorbing an In atom on a perfect Cu surface.
Although high In concentrations in the real alloys will result
in surfaces that cannot be represented by this model consist-
ing of a single In atom on a pure Cu system, this type of model
can offer valuable insights.[27]
The first model in the form of the Cu54In nanoparticles
indicate that the In atom prefers to specifically replace a Cu
atom on the edge of the icosahedron, and not by a Cu atom at
the corner (Figure 4b). The second model for the relative
energy of an In atom replacing one Cu atom on the three
facets, (100), (111), and (211), clearly show that the In atom
preferentially replaces a Cu atom on top of the step of the
(211) facet (Figure 4c; Supporting Information, Figure S12),
followed by replacement of a Cu atom on the (100) facet,
+ 0.10 eV, and by the (111) facet, + 0.17 eV. The different
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stabilities of the different situations, both in the nanoparticle
and the surfaces, can be clearly related to the larger size of In
compared to Cu. Indeed, the relative stability of Cu replace-
ment on the three considered Cu facets is inversely correlated
with the atom density on the surfaces. Moving to In
adsorption on a regular Cu facet (Supporting Information,
Figure S13), our results clearly indicate that the additional In
atom prefers to sit on the groove between steps on the (211)
facet, followed by adsorption on the four-fold site of the (100)
facet, + 0.18 eV, and by adsorption on the three-fold site of
the (111) facet, + 0.27 eV.
To gather information on the possible impact of a nearby
In on the adsorption energy of key intermediates, as an
example, we investigated H, CO and COOH adsorption on
the (100) facet presenting one In atom (Supporting Informa-
tion, Figures S14–S18). Our results indicate that the favored
H adsorption on the four-fold site is disfavored by 0.12 eV if
one of the four Cu atoms is replaced by an In atom, while the
CO adsorption energy on top of one of the Cu atoms is
substantially unchanged. This suggests that In atoms on the
various surfaces of the alloy could reduce the number of
multi-fold sites with strong adsorption ability. For COOH*,
the presence of In improves the stability of COOH* by
roughly 0.1 eV, possibly reducing the trend to release
HCOOH. Further details for DFT calculation are described
in Supporting Information.
rally, this material can also be applied for CO electrochemical
conversion.[16] Thus, we believe that Cu-In catalyst is the first
step toward obtaining efficient, selective and low-cost electro-
catalysts in the future.
Received: October 18, 2014
Revised: November 17, 2014
Published online: && &&, &&&&
Keywords: carbon dioxide fixation · Cu-In alloy ·
.
electrocatalysis · electrochemistry · reaction mechanisms
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Communications
Electrocatalysis
A non-noble metal electrocatalyst based
on a copper-indium (Cu-In) alloy has
been developed for the selective conver-
sion of CO2 to CO with a low overpoten-
tial. Combined experimental and theo-
retical approaches address the electronic
and geometric arrangement of the cata-
lyst surface, thus guiding the design of
the electrochemical surfaces for
S. Rasul, D. H. Anjum, A. Jedidi,
Y. Minenkov, L. Cavallo,
K. Takanabe*
&&&& — &&&&
A Highly Selective Copper–Indium
Bimetallic Electrocatalyst for the
Electrochemical Reduction of Aqueous
CO2 to CO
improved energy conversion efficiency.
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