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Cite This: J. Phys. Chem. Lett. 2018, 9, 7153−7159

Origin of the Selective Electroreduction of Carbon Dioxide to
Formate by Chalcogen Modified Copper
Rodrigo García-Muelas,† Federico Dattila,† Tatsuya Shinagawa,‡ Antonio J. Martín,‡
Javier Pérez-Ramírez,*,‡ and Núria López*,†
†

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Institute of Chemical Research of Catalonia, The Barcelona Institute of Science and Technology, Av. Països Catalans 16, 43007
Tarragona, Spain
‡
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg
1, 8093 Zurich, Switzerland
S
* Supporting Information

ABSTRACT: The electrochemical reduction of atmospheric CO2 by renewable
electricity opens new routes to synthesize fuels and chemicals, but more selective and
efficient catalysts are needed. Herein, by combining experimental and first-principles
studies, we explain why chalcogen modified copper catalysts are selective toward formate
as the only carbon product. On the unmodified copper, adsorbed CO2 is the key
intermediate, yielding carbon monoxide and formate as carbon products. On sulfur,
selenium, or tellurium modified copper, chalcogen adatoms are present on the surface and
actively participate in the reaction, either by transferring a hydride or by tethering CO2
thus suppressing the formation of CO. These results highlight the active role of chalcogen
centers via chemical steps and point toward basicity as the key descriptor for the stability
and selectivity of these catalysts.

T

steps have also been proposed;17,24,25 see Scheme 1. The DFTCHE model predicts medium-to-high overpotentials for
eCO2RR and describes selectivity trends observed for C1 and
C 2 products on different metals and surface orientations.8,10,17,23 Carbon monoxide is the key intermediate for
most eCO2RR products, with the exception of formate. The
linear scaling relationships (LSRs) between the binding
energies of different intermediates on transition metals26
impose constraints, limiting the optimization of metallic
catalysts or alloys.8,9,27 As we show in the present letter,
surface modifiers can break LSRs by adding chemical
(potential-independent) steps, thus improving the selectivity
control.
Copper is unique in that it reduces CO2 to compounds
demanding more than two electron transfer steps with
reasonable selectivity.2,4 Recent studies have pointed out that
p-block elements5,24,29−34 can act as modifiers and that tiny
amounts of sulfur29,30 and selenium29 switch the eCO2RR
selectivity toward formate, otherwise a minor product, while
largely inhibiting the undesired hydrogen evolution reaction
(HER). The aim of our work is to unravel the mechanistic
origin of the selectivity switch reported for the sulfur-modified
copper catalyst, Cu−S, and illustrate if the effect persists for
other chalcogenides. To this end, we have performed DFT

he electrochemical CO2 reduction reaction (eCO2RR)
driven by renewable electricity can mimic the natural
photosynthetic cycle and thus is a key element to meet climate
targets.1,2 In spite of the populated catalogue of catalysts
identified, with carbon monoxide, hydrocarbons, and formate
as more frequently reported products,3−5 this technology
remains at an incipient stage as existing materials are
suboptimal regarding activity, selectivity, stability, and
scalability for practical purposes.6 These complications are
related to the lack of robust structure-performance relationships, limited by (i) the complexity of the reaction network,
(ii) the challenging application of in situ studies in electrochemical environments,7 and (iii) the simplifications in the
models representing electrochemical processes at the molecular scale.
Theoretical attempts to explain the eCO2RR over welldefined transition metal surfaces8−11 combine Density Functional Theory (DFT) and the computational hydrogen
electrode (CHE) approach.10,12,13 This strategy allows for
solvent contributions to be introduced through approximate
models,14−16 while electric potential and pH effects can be
added as linear corrections.16,17 Neither the effect of the
applied potential on adsorption nor the impact of pH on
selectivity (known to control methane and ethylene
formation18−22) are fully included in the simulations, although
significant advances have been made lately (see ref 23 and
references therein). On clean metals, the reaction starts with
the adsorption of CO2, followed by a succession of protoncoupled electron transfers (PCETs),17 although decoupled
© 2018 American Chemical Society

Received: October 21, 2018
Accepted: December 11, 2018
Published: December 11, 2018
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Scheme 1. (a) Simplified Reaction Mechanisms for the eCO2RR toward Formate/Formic Acid (Paths 1−2, in Olive and
Yellow) and CO (Path 3, Red), and for the Parasitic HER (Path 4, Gray) on Clean Cu;28, a (b) Reactions Mechanisms in
Which the Chalcogens Act as Active Centers: CO2 Tethering (Path 6, Light Green), Heyrovsky-like Hydride Shuttle (Path 7,
Dark Green), and the HER (Path 5, Black).b

The full mechanism is presented in Scheme S1. bThe charge of an adsorbed chalcogen, δ−, depends on the external potential U.

a

over the metallic copper particles when exposed to air.33 The
introduction of the chalcogen modifiers did not alter the
crystallite size. In addition, as chalcogens are present at low
concentrations they likely prevented the identification of any
related bulk chalcogen reflections. The presence of chalcogen
species on the topmost surface (approximately 1 nm) of the
synthesized catalysts was clearly indicated by the time-of-flight
secondary ion mass spectroscopy (ToF-SIMS) analysis (Figure
S5). X-ray photoelectron spectroscopy (XPS) analysis
suggested a very limited presence of surface chalcogenide
phases in the as-synthesized materials, as shown for measurements before eCO2RR testing (Figure 1b; see signal at binding
energies of ca. 162, 54, and 573 eV for CuxS, CuxSe, and
CuxTe, respectively). The chalcogen-free catalyst (Cu−⌀) was
prepared by skipping the addition of chalcogen species29,30
(see Experimental Procedures in Section S1), resulting in
unmodified Cu2O particles (see Figure S3).
Cu−Se and Cu−Te exhibit slightly better catalytic performance than Cu with a mild preference for HCOO− among the
eCO2RR products measured with chronoamperometry (CA)
at −0.6 V vs Reversible Hydrogen Electrode (RHE), Figure 1c.
In contrast, over Cu-S, HCOO− is the main carbon product,
being in equal proportion with H2 whereas only trace amounts
of CO were detected. The Cu−S becomes more selective
toward formate at higher overpotentials until the presence of
CO and more complex products, typically associated with
clean copper, becomes detectable at −0.9 V vs RHE (Figures
S6−S7).35 At −0.8 V, the incipient volcano-like behavior in
terms of selectivity observed at −0.6 V manifests now
clearly (Figure 1c),suggesting the presence of a general effect
modulated by the nature of the chalcogen. Comparison of our

simulations for the systems including O, S, Se, and Te as
dopants and compared them to the experimental systems. The
true state of O-containing samples is difficult to assess (see
below).
We synthesized three copper catalysts from its oxidic Cu2O
phase, modified by sulfur (Cu−S), selenium (Cu−Se), or
tellurium (Cu−Te) via a solvothermal route.30 The fresh
samples exhibited microsized aggregates containing nanometric particles (Figures S1−S4) with a chalcogen content of
1−3 at. % relative to copper (Table 1). The chalcogens were
uniformly distributed, as shown by energy dispersive X-ray
spectroscopy coupled to scanning electron microscopy (EDXSEM, Figure 1a). As for the crystalline structures, Cu−S, Cu−
Se, and Cu−Te exhibited X-ray diffraction patterns assigned to
Cu2O accompanied by traces of the metallic Cu phase (Figure
S2), likely due to the rapid formation of a native oxide layer
Table 1. Double-Layer Capacitance, CDL in mF cm−2, and
XPS- or EDX-Measured Chalcogen Elemental Content,
before and after Electrocatalytic Testing, in Atomic
Percentage Relative to Cua
relative content
CDL
Cu−⌀
Cu−S
Cu−Se
Cu−Te

XPSfresh

XPSused

EDXfresh

EDXused

2.9
1.4
2.3
2.8

78.2 ± 0.1
3.4 ± 1.0
2.3 ± 2.2
n.q.b

77.9 ± 0.1
1.1 ± 1.1
1.5 ± 1.4
10.0 ± 8.2

45.8
0.6
0.2
1.1

48.7
0.6
n.q.b
0.7

a
For the Cu2O-derived Cu catalyst (Cu−⌀), the percentages refer to
oxygen. bn.q. nonquantifiable.

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Figure 1. (a) EDX elemental maps of chalcogen elements for the corresponding fresh catalysts. White contours indicating the particle borders are
added as a visual aid (see Figure S4). Scale bars: 2 μm. (b) Chalcogen XPS spectra before and after the eCO2RR testing at −0.6 V vs RHE showing
their presence after reaction. The peak indicated by asterisk (*) originates from the Auger emission in the Cu LMM region. (c) Product
distribution over the copper-chalcogen catalysts, obtained by chronoamperometry at −0.6 (top) and −0.8 V (bottom) vs RHE for 1.5 h in 0.1 M
KHCO3 saturated with CO2 (pH 6.7). Polycrystalline copper10 yields around 10% of both HCOO− and CO at −0.6 V, and 20%/30%, respectively,
at −0.8 V. Further tests regarding the electric potential dependence and the role of alkaline cations are presented in Figures S6−S8 and Section S3.

as adatoms sitting on surface fcc sites (Table S2) leading to a
relative composition of 2.7 at. % (X = 1, Cu = 36) mimicking
the experimental content, Table 1. Other structural models
were tested but found less stable (Table S2). The removal of S,
Se, and Te as H2S, HSe−, and H2CO2Te occurs at more
negative potentials than U = −0.92, − 1.06, and −0.87 V vs
RHE, respectively (Table 2), explaining why all Cu−X systems

pure Cu (Cu−⌀) with polycrystalline, oxide-derived Cu reveals
close similarities in product distribution,10 suggesting the
removal of oxygen atoms from the copper surface under
reaction conditions. In situ studies on oxide-derived copper
catalysts have hinted at the presence of oxygen near the
surface.32,36,37 Nevertheless, the absence of such an oxidic
phase has also been suggested33 and the promotion of carbon
products formation was associated with grain boundaries.33,34,38 Under the eCO2RR conditions, the surface of the
electrocatalysts reconstructs (Figure S1) and the chalcogen
content seems to decrease within the limited accuracy at such
low concentrations (Table 1). Accordingly, XPS excitations
around the Cu 2p peak showed the presence of Cu2O and
Cu2+ (Figure S9), whereas peaks compatible with the copper
chalcogenide phases built up (Figure 1b; Tables 1, S1).39 The
broad peaks at higher binding energies in Figure 1b are
ascribable to oxidic phases,40 presumably formed upon
exposure of the chalcogenides samples to air during characterization.41 Further ToF-SIMS analysis confirmed the increase in
the relative abundance of chalcogen on the topmost layer upon
the reaction (Figure S5). Upon the restructuring process, the
surface remains populated by chalcogen atoms, although some
leaching occurs.30 We remark that the presence of copper
chalcogenide phases after the eCO2RR testing is not expected
from a thermodynamic point of view, because Pourbaix
diagrams predict metallic copper as the most stable phase
under operation conditions (Figure S10); however, small
domains cannot be fully discarded. In summary, the
unmodified catalyst (Cu−⌀) can be identified as polycrystalline Cu under operation conditions, since (i) the removal of
oxygen atoms or hydroxyl groups is largely favored (Tables S2
and S3) and (ii) the product distribution of Cu−⌀ follows that
of polycrystalline Cu (Figure 1c). Therefore, the selectivity
patterns observed for Cu−S, Cu−Se, and Cu−Te can be
ascribed to the chalcogen modifier.
To unravel the origin of the selectivity patterns, polycrystalline Cu was modeled by the lowest energy Cu(111) surface.
Our calculations show that the results of the Cu(211), (110),
and (100) facets yield qualitatively the same results, Figure
S11. In the model, the chalcogens (X = O, S, Se, Te) are added

Table 2. Computed Properties of the Chalcogen Modified
Cu(111) Surfacea
system

ΔGH2X

ΔGHX−

ΔGXCO2H2

qX*

εp − εF

Δ(εd − εF)

Cu−O
Cu−OH
Cu−S
Cu−Se
Cu−Te

−1.28
−0.60
+0.92
+1.22
+1.71

−0.84
−0.16
+0.94
+1.06
+1.48

+1.86
−
+2.95
+3.74
+0.87

−0.93
−0.59
−0.59
−0.43
−0.21

−2.50
−4.22
−1.61
−1.28
−0.88

−0.16
−0.11
−0.07
−0.05
−0.07

a

ΔGH2X, ΔGHX−, ΔGXCO2H2: desorption energies for the chalcogens to
produce H2X, HX−, and XCO2H2 (X = O, S, Se, Te), in eV. qX*:
Bader charges of the adsorbed chalcogen, in |e−|. εp − εF: center of the
chalcogen p-band with respect to the Fermi level of the system, in eV.
Δ(εd − εF): d-band center shift for the adjacent Cu atoms upon
anchoring of the chalcogen, in eV.

were stable at the working potentials from −0.6 to −0.8 V vs
RHE.30 At more cathodic potentials than −0.90 vs RHE, the
Cu−S performance starts resembling that of polycrystalline
copper,10 Table 2 and Figures S6. The stability of the Cu−S
system was further tested, Figures S12−S13, suggesting the
progressive loss of sulfur from the surface at deeper cathodic
potentials.
To understand the mechanistic implications of the eCO2RR
on Cu, we start by analyzing the reactions listed in Scheme 1a
where the parasitic hydrogen evolution reaction is also
presented. In the reaction network, either coupled or
sequential proton and electron transfers appear, and their
relative contributions are given by their different dependence
on the potential and the interface pH. To elucidate the origin
of the selectivity switch imposed by the chalcogen modifier, we
modeled CO2 adsorption as an electron transfer process and
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Figure 2. Gibbs free energy profiles on clean and chalcogen modified Cu(111) surfaces at U = −0.6 V vs RHE. (a) Electrochemical CO2 reduction
toward formate (path 1, olive), CO (path 3, red), and hydrogen evolution reaction (path 4, gray) on clean Cu(111). Path 2 is also downhill from
CO2*− and is omitted for simplicity. For S-modified surfaces (b) presents path 6 starting from tethered CO2*− toward formate and (c) the Smediated formate path 7 and HER. PCET: proton-coupled electron transfer. ET: electron transfer. PT: proton transfer. CS: chemical step. TS:
transition state in CS. The profiles for U = 0.0 and −0.8 V vs RHE are shown in Figure S16, and those for other chalcogens, in Figure S17.

stabilize the CO2*− intermediate, thus simultaneously
increasing the Faradaic efficiency or production of HCOO−
and CO, until mild overpotentials are reached.10 The parasitic
HER, by contrast, occurs through either a classical Volmer−
Tafel mechanism, path 4, or a Volmer−Heyrovsky mechanism,
omitted here for simplicity. As both eCO2RR and HER have
H* as a common intermediate, a mild metal−hydrogen
bonding strength promotes both reactions.47 Applied to
other transition metals, the reaction network in Scheme 1a
predicts that they are poor formate-producing catalysts.23,48,49
The energy profile for Cu shown in Figure 2a at U = −0.6 V
can be explained as follows: the markedly endergonic CO2
adsorption, Table S4, requires considerable overpotentials to
trigger its reduction. The HER largely predominates at
potentials more anodic than −0.8 V. As CO2 adsorption
becomes more exergonic, paths 1 and 2 start competing with
the HER (path 4). At higher overpotentials, the increasingly
larger CO2*− coverage enhances the production of formate
and CO,50 accounting for the concomitant HER decrease. In
our energy profiles, Figure 2a, the HER is preferable over the
CO2 reduction. The larger Faradaic efficiency toward formate
could come from mass-transport phenomena19,51,52 (local pH
at the interface, local CO2 concentration) not included in our
model.
Modification of the copper surface with chalcogen (X)
adatoms leads to stable configurations with a polarized X−
metal bond (Xδ−) according to the Bader charges of δ = −0.93,
− 0.59, − 0.43, and −0.21 |e−| for O, S, Se, and Te
respectively, Table S3. Along the series, the bond is more
covalent as the partially empty p-states of the chalcogens lie

decoupled all the other steps along the whole reaction network
when possible.17,42 The rate-determining step in the eCO2RR
process is the difficult CO2 adsorption both on pure metal
electrodes and in molecular catalysts.3,17,24 Raman spectroscopy has recently evidenced that the resulting carboxylate species
is the first intermediate in the CO2 conversion to formate on
copper.43,44 From a mechanistic perspective, successful CO2
adsorption and activation imply the endothermic bending of
the O−C−O bond, which requires 3.05 eV for the neutral
molecule but only 1.23 eV for the negatively charged species,
Figure S14. Therefore, its activation implies the electron
transfer to its high lying LUMO.45 On the surface, CO2*− can
adopt four possible conformations,17 depending on the surface
potential, Figure S15a. The most stable adsorbed structure at
U = −0.6 V is η2(C,O), where both C and O are bonded to the
surface.
Then a proton can be adsorbed onto the copper surface as
H* through a PCET step, path 1 in Scheme 1a, reacting with
the carboxylate via a chemical step yielding monodentate
HCOO*−, which desorbs as formate, Figure S15b,46 similar to
CO2 reduction on PdHx.17 A proton from solution is
transferred to a terminal oxygen of CO2*−, giving rise to a
carboxyl intermediate (path 2) that reacts with H* to produce
formic acid. Formic acid desorbs spontaneously and converts
to formate due to the solution’s pH (buffered at 6.7).
Alternatively, the COOH* intermediate decomposes to CO*
and water through either decoupled or concerted proton−
electron transfer, path 3. The desorption of CO from Cu is
endergonic,9 Figure 2a, thus allowing further reduction toward
methanol and hydrocarbons.10 More cathodic potentials can
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closer to the highest, unperturbed Cu bands (the d-band is
invariant; see Tables 2 and S3). At more negative potentials,53
the partial charge of the chalcogens, Xδ−(U), increases to δ =
−0.98, −0.76, −0.66, and −0.53 |e−|. The Xδ− surface site acts
as a nucleophile to trap CO2 concomitant with an electron
transfer, producing a chalcocarbonate Xδ−CO2*−, path 6 in
Scheme 1b. The computed free energies for S-decorated Cu
(new paths 5, 6, and 7) are shown in Figure 2. Due to its
geometry, Xδ−CO2*− cannot form CO, as it would imply
breaking the C−X and C−O bonds simultaneously. In parallel,
a proton can adsorb on the neighboring Cu from the solution
with an electron from Cu to form H*.42 The reaction proceeds
with H* attacking the carbon atom in Xδ−CO2*− to produce
formate, as indicated by the inset labeled TS in Figure 2b. This
potential-independent chemical step presents an activation
energy of 1.02 eV. The Xδ− site can also attract protons to
produce Xδ−H, paths 5 and 7 in Scheme 1b. This species can
attack either a proton or a CO2 in the solution, to produce
hydrogen (HER path 5) or formate (path 7) respectively in
Heyrovsky-like mechanisms. Paths 6 and 7 agree with the Tafel
plot in Figure S7, which indicates that the rate-determining
step for the reaction involves a single electron transfer (slope
120 mV dec−1). Finally, the presence of the chalcogen prevents
the adsorption of CO2 on its neighborhood (destabilizing
adsorption by 0.53, 0.27, and 0.12 eV for first, second, and
third nearest Cu sites, respectively), blocking unselective paths
occuring on Cu-only sites, Figure 2.
The energy profiles in Figure 2 enable the rationalization of
the experimental catalytic trends over Cu−S, Cu−Se, and Cu−
Te in Figure 1c. Since all steps in path 7 become exergonic
from U = −0.6 V vs RHE toward more cathodic potentials and
the chalcogen atom destabilizes path 2, the chalcogen centers
become highly selective for the eCO2RR toward formate. As
the formation of XH* becomes more endergonic for heavier
chalcogens, the activity of paths 5 and 7 is reduced along the
chalcogen series (Table S4). As already noted, the blocking
effect of Se and Te in neighboring Cu atoms is milder than that
of S, Table S4, in line with the experimental trends observed in
Figure 1.
The differences in activity and selectivity can be traced back
to an intrinsic property of the adsorbed chalcogenides as both
the tethering of CO2 (Figure 3a) and the formation of XH*
(Figure 3b) depend on the donor ability of X. Figure 3c−d
compile the key energy figures in paths 6 and 7 as a function of
the basicity obtained as the p-band center of the chalcogen.54
In Figure 3c (path 6) the low basicity enhances the stability of
the XCO2*− adsorbate, while higher basicity makes HCOO−
formation more exergonic. In Figure 3d (path 7), the limiting
factors are the formation of XH* and formate, where the
former becomes more endergonic at larger basicities. In turn
the HER is controlled by H adsorption on Cu (gray) and does
not depend on the nature of the adatom. The optimal crossing
point between the stability of the key intermediate and formate
appears at −2.5 (path 6) and −2.2 eV (path 7), respectively.
Thus, in both cases the chalcogen that shows the closest value
is sulfur, which is in agreement with its unique catalytic
performance among the family of chalcogen modifiers
identified in Figure 1. It is also significant that the stability
of the chalcogens on the surface correlates with the basicity,
Figure S18.
In summary, the selective production of formate upon
chalcogen modified copper is due to (i) the presence of basic
sites where CO2 is tethered blocking its dissociation toward

Figure 3. Most important intermediates for formate production (a)
via CO2 tethering on the chalcogen adatom or (b) via the Heyrovsky
mechanism (path 6 and 7 in Scheme 1, respectively). (c−d) Gibbs
energies (U = 0.0 V vs RHE) of the crucial intermediates as a function
of the basicity of the chalcogen, computed as the center of the
chalcogen p-band. Blue dots correspond to the energy of the
precursors Xδ−CO2*− and Xδ−H, respectively shown in the (a−b)
panels, while the energy of HCOO− is shown in green. The largest
activity is expected when the green and blue lines cross as the process
will be isoenergetic. Gray dots represent the adsorption energy of
hydrogen on copper atoms in the vicinity of X. Lines were added as a
guide to the eye.

CO, (ii) the ability of chalcogens to directly transfer hydrides
to CO2, and (iii) the effective blocking of unselective paths on
the surrounding Cu atoms. Our results point out the role of
chemical steps in electrochemical processes.

■

ASSOCIATED CONTENT

S
* Supporting Information

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpclett.8b03212.
Experimental procedures, computational details, cation
effect, supplementary schemes, figures, tables, and
references (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: jpr@chem.ethz.ch (J.P.-R.).
*E-mail: nlopez@iciq.es (N.L.).
ORCID

Rodrigo García-Muelas: 0000-0002-2219-5027
Javier Pérez-Ramírez: 0000-0002-5805-7355
Núria López: 0000-0001-9150-5941
Notes

The authors declare no competing financial interest.
The DFT data that support the findings of this study are
available in ioChem-BD55 at DOI 10.19061/iochem-bd-1-92.
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■

ACKNOWLEDGMENTS
We are thankful for the financial support from MINECO
(Grant Number CTQ2015-68770-R), ETH Zurich, and the
European Union under Projects A-LEAF (732840-A-LEAF)
and ELCoREL (721624-ELCOREL). The ScopeM of ETH
Zurich and the Barcelona Supercomputing Centre (BSC-RES)
are thanked for access to their facilities and generous
computational resources.

■

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