Enhanced Stability and CO/Formate Selectivity of Plasma-Treated SnOx/AgOx Catalysts during CO2 Electroreduction

Article abstract of DOI:10.1021/jacs.8b12766

CO₂ electroreduction into useful chemicals and fuels is a promising technology that might be used to minimize the impact that the increasing industrial CO₂ emissions are having on the environment. Although plasma-oxidized silver surfaces were found to display a considerably decreased overpotential for the production of CO, the hydrogen evolution reaction (HER), a competing reaction against CO₂ reduction, was found to increase over time. More stable and C1-product-selective SnO_x/AgO_x catalysts were obtained by electrodepositing Sn on O₂-plasma-pretreated Ag surfaces. In particular, a strong suppression of HER (below 5% Faradaic efficiency (FE) at a0.8 V vs the reversible hydrogen electrode, RHE) during 20 h was observed. Ex situ scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), quasi in situ X-ray photoelectron spectroscopy (XPS), and operando X-ray absorption near-edge structure spectroscopy (XANES) measurements showed that our synthesis led to a highly roughened surface containing stable Sn^?+/Sn species that were found to be key in the enhanced activity and stable CO/formate (HCOO^-) selectivity. Our study highlights the importance of roughness, composition, and chemical state effects in CO₂ electrocatalysis.

Full text of DOI:10.1021/jacs.8b12766

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Article
Enhanced stability and CO/Formate selectivity of plasma-
treated SnOx/AgOx catalysts during CO2 electroreduction
Yong-Wook Choi, Fabian Scholten, Ilya Sinev, and Beatriz Roldan Cuenya
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12766 • Publication Date (Web): 03 Mar 2019
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Enhanced stability and CO/Formate selectivity of plasma-treated
SnOx/AgOx catalysts during CO2 electroreduction
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Yong-Wook Choi1, Fabian Scholten1,2, Ilya Sinev1, Beatriz Roldan Cuenya1,2*
1Department of Physics, Ruhr University Bochum, 44780 Bochum, Germany
2Department of Interface Science, Fritz-Haber Institute of the Max Planck Society, 14195 Berlin, Germany
ABSTRACT: CO2 electroreduction into useful chemicals and fuels is a promising technology that might be used to
minimize the impact that the increasing industrial CO2 emissions are having on the environment. Although plasma-
oxidized silver surfaces were found to display a considerably decreased overpotential for the production of CO, the
hydrogen evolution reaction (HER), a competing reaction against CO2 reduction, was found to increase over time. More
stable and C1-product selective SnOx/AgOx catalysts were obtained by electrodepositing Sn on O2-plasma pre-treated Ag
surfaces. In particular, a strong suppression of HER (below 5 % Faradaic efficiency (FE) at -0.8 V vs the reversible
hydrogen electrode, RHE) during 20 h was observed. Ex situ scanning electron microscopy (SEM) combined with energy-
dispersive X-ray spectroscopy (EDS), quasi in situ X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption
near-edge structure spectroscopy (XANES) measurements showed that our synthesis lead to a highly roughened surface
containing stable Snδ+/Sn species that were found to be key in the enhanced activity and stable CO/formate (HCOO-)
selectivity. Our study highlights the importance of roughness, composition, and chemical state effects in CO2
electrocatalysis.
1. INTRODUCTION
0.97 V vs RHE)3 and lower price than Au. Moreover,
numerous studies reported that nanostructuring Ag
More efficient processes for the electrochemical CO2
reduction (CO2RR) are currently actively sought in order
to mitigate the negative effects associated with the
increase of CO2 in our atmosphere such as global
warming, while producing valuable chemicals and fuels
(CH4, C2H4, C2H5OH, etc). Additional products from the
CO2RR such as CO and formate, for which higher
selectivity at lower overpotentials can be achieved, have
been discussed to be economically more feasible than
some hydrocarbon and alcohol products considering the
price of grid electricity and the cost of CO2 capture for
CO2RR if it is not already available in a concentrated form
at an industrial site1.
surfaces
(nanoporous
materials8–10
,
nanocorals11,
nanoplates12, nanosheets13 and nanoparticles14) can lead to
a decrease in the overpotential of up to a few hundred
mV, with efficiencies for CO reaching over 90 %.
In order to further improve catalytic activity, recent
investigations have focused on oxide-derived metal
catalysts15 including Cu16, Au17, Sn18 and Pb19. In these
systems, the total/partial reduction process of the pre-
oxidized catalysts is thought to give rise to rougher
surfaces with new active sites at grain boundaries as well
as to residual cationic metal species or subsurface oxygen
which might play a role in the catalytic selectivity. In the
case of oxide-derived Ag catalysts, fundamental
understanding on the mechanisms leading to the
decreased overpotential and increased CO selectivity is
still needed20–23. For example, pre-oxidized Ag surfaces
have been reported to display stronger binding of reaction
intermediates such as COOH* and CO*20, which was
assigned to a high local pH. On the other hand, the
superior catalytic activity and selectivity of oxygen-
plasma pre-treated Ag catalysts (90% CO FE at -0.6 V vs
RHE) was assigned to locally-enhanced negative electric
fields at defects created in the Ag surfaces by the plasma
oxidation23. Unfortunately, the selectivity for thehydrogen
evolution reaction (HER), a competing reaction against
It is well known that p-block metal catalysts such as tin
(Sn), lead (Pb), indium (In), and thallium (Tl) can mainly
produce HCOO- during CO2RR, while CO can be
primarily obtained over gold (Au), silver (Ag), and zinc
(Zn) catalysts2. Although Au has displayed a remarkable
catalytic activity and high Faraday efficiency (FE) for CO
at low overpotential (87 % at -0.74 V vs the reversible
hydrogen electrode, RHE3), recent studies have tried to
find alternative catalysts owing to the high price of Au4–6
.
Non-precious metals such as Zn have also been identified
as possible candidates, although lower CO FE efficiencies
are observed3–7. Ag catalysts are considered promising
alternatives because of their high FE for CO (81.5 % at -
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CO2RR, was found to increase over time as the original
three-electrode system was used with an Ag/AgCl (3M
KCl) reference electrode and Pt-mesh as counter
electrode. Electrochemical sequences with two steps
consisting of linear sweep voltammetry (-0.75 V to -2.0 vs
AgOx films were reduced and the surface roughness
decreased. Therefore, additional work is still required in
order to develop Ag-based CO2RR catalysts with not only
improved activity and C1-product selectivity but more
importantly, also enhanced stability.
Ag/AgCl) with
a sweep speed of 100 mV/s, and
chronoamperometry at -2.0 V vs Ag/AgCl for 1 min were
employed in order to avoid the parallel deposition of
impurities such as metal salts in the aqueous solution. A
solution of 0.05 M SnCl2 (98-103 % and Alfa-Aesar) mixed
with 2 M NaOH (99.3 % and VRW Chemicals) was used
Bimetallic systems have recently attracted attention as
promising catalysts in order to achieve active but also
stable CO2RR. For instance, considerably enhanced FE
toward C1 products24,25 was obtained for systems
consisting of either Au, Ag or Cu, and Sn, In by tuning the
adsorption energy of intermediates6,26–28. Because Ag-Sn
core-shell nanoparticle catalysts lead to ~80 % FE towards
formate during 24-h CO2RR at -0.8 V vs RHE in 0.5 M
NaHCO36, Ag-based bimetallic systems have been
proposed as viable alternatives for maintaining stable C1-
product selectivity29. In the case of Ag films, Larrazábal
reported that Indium deposit on a bulk Ag electrode
(In@Ag) showed an enhanced CO current efficiency at
moderate overpotential in comparison to bulk Ag24.
However, the current efficiency for HER also increased up
to 60 % within 2 h of CO2RR coinciding with the change
of chemical state of the In species. Despite the promising
results obtained thus far on Ag films for CO2RR, further
effort to suppress HER for long-term operation is still
needed based on the fundamental understanding of the
parameters responsible for the distinct activity and
selectivity of Ag-based CO2RR catalysts.
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for
the
electrochemical
Sn
deposition.
The
electrodeposited catalysts were washed with deionized
water and subsequently dried with Ar gas.
2.2 Characterization. The morphology and chemical
state of the catalysts were investigated by SEM with a FEI
Quanta 200 FEG microscope and a liquid-N2 EDS
detector. The quasi in situ XPS measurements were
performed in an ultrahigh-vacuum setup equipped with a
Phoibos100 (SPECS GmbH) analyzer (pass energy, Epass=13
eV), and an Al anode of a XR50 (SPECS GmbH) X-ray
source (Psource
=
300 W). The transfer from the
electrochemical cell towards the UHV system is done
under pressurized argon atmosphere (99.999% Ar). All
XPS measurements were acquired in high resolution (0.05
eV) and correspond to an average of at least 5 scans. The
Casa XPS software was used for data analysis. All spectra
where aligned to the C-1s region (Eb=284.8 eV). The
presented fits of the XPS spectra where done by first
measuring the corresponding bulk references. SnO2,
SnO4, Ag2O and AgO powders (99.999% Sigma Aldrich)
were measured as reference for the different oxides and
Ar-sputtered (UHV-treated) Sn and Ag foils as reference
for the metallic spectra. All reference spectra used for
fitting where additionally constrained with respect to
their energy position (dE=0.05 eV), using the
Herein, we present Ag-based catalyst for CO/formate
production from CO2RR displaying strongly suppressed
HER (below 5 % FE at -0.8 V vs RHE) during 20 h. The
catalysts were synthesized by electrodepositing Sn on O2-
plasma pre-oxidized Ag films. Quasi in situ X-ray
photoelectron spectroscopy (XPS), operando X-ray
absorption near-edge structure spectroscopy (XANES),
and ex situ scanning electron microscopy equipped with
energy-dispersive X-ray spectroscopy (SEM-EDS) were
employed to investigate the chemical state and surface
morphology of the catalysts at different stages of the
reaction in order to extract correlations between the
structure, chemical state and composition of the
electrocatalysts and their activity and selectivity.
corresponding values reported in the literature30,31
.
To evaluate the catalytic performance, a home-built H-
type electrochemical cell separated by a Selemion anion
exchange membrane was used for CO2 electrolysis at
room temperature with an AutoLab PGSTAT128N
potentiostat. A Pt-mesh was used as counter electrode
and a leak-free Ag/AgCl electrode (LF-1, Innovative
Instruments, Inc.) as reference. CO2RR was carried out in
a purified 0.1 M KHCO3 solution by applying a potential
between -0.6 and -0.8 V vs RHE under constant 20
mL/min CO2 flow rate. Prior to the electrochemical
reaction, the electrolyte was purified with Chelex 100
Resin (Bio-Rad)32,33 in order to minimize the effect of
metal impurities in the electrolyte, and subsequently
purged with CO2 gas (99.95%) for 30 min to reach a pH
value of 6.8. The gas products were injected into an
online gas chromatograph (Agilent 7890A) every 17 min
during CO2RR. The liquid products such as formate and
to a lesser extent also alcohols were detected by high-
performance liquid chromatography (HPLC, Shimadzu)
after CO2RR. The value of the solution resistance was
2. EXPERIMENTAL DETALS
2.1 Catalyst synthesis. For the synthesis of Ag-based
catalysts, Ag foils (99.97% purity and 125 µm thickness)
were mechanically polished with silicon carbide paper
(400 grit) and subsequently rinsed in deionized water and
dried with argon (Ar). Oxygen plasma treatments (Plasma
Prep III, SPI) were employed to modify the roughness and
oxidation state of the Ag foils (denoted as AgOx). The
plasma conditions were: 20 W, 350 mTorr O2 for 2 min. In
order to electrodeposit Sn on a metallic Ag surface
(denoted as SnOx/Ag sample after air exposure) and on
the plasma-oxidized AgOx surfaces (denoted as
SnOx/AgOx sample after air exposure), a conventional
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measured by the iR interrupt method34 and
after CO2RR at -0.8 V vs. RHE for 20 h (d-f). The average
values of the composition extracted from SEM-EDS
measurements are also shown. The scale bars indicate 20
µm. The insets show higher magnification images (scale
bars: 2 µm).
electrochemical impedance spectroscopy. In order to
determine the roughness factor of the catalysts, the
double layer capacitance was measured between 0 V and
0.25 V vs RHE after 1 hour and 20 hours of reaction in 0.1
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M KHCO3 (Fig. S1 and Table S1 in supporting
information).
3.2
Chemical
state/surface
composition
characterization. To gain further information on the
chemical state of the Ag-Sn bimetallic catalysts as
prepared and after reaction, quasi in situ XPS
measurements were carried out on the SnOx/AgOx and
SnOx/Ag samples before and after CO2RR at -0.8V versus
RHE in an electrochemical cell directly attached to the
XPS system.
Operando XANES measurements were conducted with
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a
home-built electrochemical cell using Pt-mesh
(MaTeck) and a leak-free Ag/AgCl as counter and
reference electrodes, correspondingly. The measurements
were done in fluorescence mode using a planar implanted
silicon (PIPS) detector at the P64 undulator beamline of
PETRA III synchrotron radiation facility.
As expected, after O2-plasma treatment the Ag
substrate is oxidized (see Ag MNN Auger spectra in Fig.
S3). Nevertheless, after the in situ Sn electrodeposition
only metallic Ag is seen. This is directly linked to the
strong reductive alkaline conditions employed during the
Sn electrodeposition35. Moreover, it should be noted that
the deposition process does not leave behind any residual
Cl, which might influence the CO2RR mechanism (Fig. S4
and S6) by decreasing the overpotential to form the CO2
radical11.
3. RESULTS AND DISCUSSION
3.1 Morphological characterization. To synthesize
stable Ag-Sn CO2RR catalysts, O2-plasma treatments on
Ag foils and subsequent Sn electrodeposition were
employed. Figure 1 presents SEM images of SnOx/Ag and
SnOx/AgOx bimetallic catalysts as prepared (top row) and
after 20 h CO2RR at -0.8 V vs RHE (bottom row). A
comparison with a monometallic AgOx catalyst is also
included. An enhanced surface roughness is observed for
all samples due to either the O2-plasma pre-treatment or
the Sn electrodeposition. Based on the roughness factors
extracted from electrochemical double layer capacitance
measurements, the roughness was found to increase in
the order of AgOx > SnOx/AgOx > SnOx/Ag > metallic Ag
foil (see Fig. S1 and Table S1).
By comparing the integrated area of the Ag 3d and Sn
3d peaks (Fig S5), a ratio of Sn/Ag of roughly 1:1 and 3:1
was found on the surface of our SnOx/AgOx and SnOx/Ag
samples (Table S2), respectively. This ratio is higher than
that extracted by EDX since Sn is deposited on top of the
Ag substrates and XPS is a more surface-sensitive
technique than EDX. Overall, XPS data reveal that the
catalyst surface composition is stable and there is no
evidence of sintering or interdiffusion of the two metals
even after 20h of CO2RR.
A strong change of the morphology of the AgOx (Fig.
1d) and SnOx/AgOx (Fig. 1e) samples occurs after 20 h
CO2RR, while the surface of the SnOx/Ag sample (Fig. 1f)
is only mildly altered under reduction reaction
conditions. The SnOx/AgOx sample already shows a slight
change in surface roughness after 1 h reaction (Fig. S2).
Our EDS analysis shows that the entire surface of the Ag
and AgOx substrates was covered by Sn.
Figure 2. Quasi in situ Sn MNN Auger spectra of Sn
electrodeposited on (a) O2-plasma treated Ag substrate
(AgOx), (b) pristine Ag substrate. Colored spectra are Sn
MNN reference spectra (metallic Sn, SnO and SnO2) used
to fit the envelope of Auger spectra. The electrochemical
CO2RR has been performed at a working potential of -0.81
V vs RHE in 0.1 M KHCO3.
Figure 2 displays Sn MNN Auger spectra from the
SnOx/AgOx and SnOx/Ag samples acquired after 1 h and
20 h CO2RR. In both samples, features corresponding to
Figure 1. SEM images of AgOx (a, d), SnOx/AgOx (b, e),
and SnOx/Ag (c, f) catalysts acquired before (a-c), and
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metallic Sn31 are seen after the reaction together with
solid line) and the weighted spectral components
corresponding to Sn and SnO2 species.
SnOx species34. The progressive reduction of the SnOx
species is observed with increasing reaction time, but
there are still Sn oxides present on the surface of both
samples even after CO2RR at -0.8V vs RHE for 20 h (see
Fig. 2 and Table S3).
The Sn spectrum of the SnOx/Ag sample, Fig. 3(a)
resembles that of the Sn foil, showing a low-intensity
feature above the edge (so-called “white line”) at 29207.8
eV that barely changes under CO2RR. This indicates that
even though the sample surface might be oxidized, the
signal of the metallic Sn subsurface layers dominates the
XAFS spectrum. On the other hand, the sample deposited
on plasma-treated Ag (SnOx/AgOx) is severely oxidized
prior to the reaction as indicated by a more intense white
line shifted to 29209.6 eV. Under reaction conditions, the
white line gets broader but less intense and shifts slightly
to lower photon energy. To get a quantitative information
on the Sn species observed, XANES spectra were fitted
using a linear combination (LC) of the reference spectra
of a Sn foil, SnO, and SnO2, Fig. 3(b). Since XANES
spectra of both oxides show similar features and differ
only in the intensity and position of the white line, an
energy shift, fitted during linear combination analysis
served as an extra figure of merit along with the R-factor.
Thus, if two fits, using different oxides as principle
components resulted in similar R-factor values, the one
with a smaller energy shift was accepted. Hence, with
these bulk-sensitive technique, the as-prepared SnOx/Ag
sample was found to consist of 90% metallic Sn and a 10
% mixture of SnO and SnO2 with a 2:1 ratio (see Table S4
and Fig S7 for details). The LC analysis proves full
reduction of Sn in this sample under reaction conditions.
The SnOx/AgOx sample shows more severe oxidation in
its as-prepared state, with Sn0 and Sn4+ species present in
a ratio close to 1:1. In contrast to the sample supported on
pristine Ag, the SnOx film deposited on AgOx was not
fully reduced after 5 h under CO2RR conditions with 23%
of Sn remaining as SnO2.
8
9
Nonetheless, due to the quasi in situ nature of the
former measurements, and although the samples were
never exposed to air during the transfer to the XPS
chamber, they were still in contact with the electrolyte
after reaction during the transfer (5-15 s) in the absence of
an applied potential, which could result in partial re-
oxidation of the surface Sn species. In order to confirm
that SnOx species indeed remain stable under reaction
conditions at the potentials of interest, operando X-ray
absorption spectroscopy measurements were also
conducted. XANES and EXAFS were used to gain insight
into changes in the catalysts chemical state and
morphology during CO2RR. It should be noted that these
techniques are bulk-sensitive, providing information from
surface and subsurface sample regions. Furthermore, the
fluorescence signal detected here is highly dominated by
the radiation emitted by the silver support. Unfortunately
the, Ag-Kβ emission line (24941 eV) overlaps with the Sn-
Kα series (24735-25272 eV), making the measurements of
these samples at the Sn-K edge very challenging.
Nevertheless, insight into the chemical state of Sn before
and under CO2RR conditions can still be extracted by
comparing to reference spectra, Figure 3.
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b
a
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Sn foil
raw data
LC fit
CO2RR
air
SnOx/Ag
CO2RR
air
Sn foil
SnOx/AgOx
The combined results from XPS (surface sensitive) and
XANES (bulk-dominated) analysis indicate that although
the surface of the SnOx/Ag and SnOx/AgOx samples is
oxidized in their as-prepared state, the oxide film
thickness is not very thick in the case of the SnOx/Ag
sample. This is concluded based on the XAFS data
revealing a mainly reduced sample before and during
CO2RR, while only via the more sensitive XPS technique
one can detect the partial reduction of the SnOx species
during the first 20 h of CO2RR. On the other hand, SnOx
species were found to remain in the SnOx/AgOx sample
not only at the surface (XPS) but also subsurface (XAFS)
at least within the first 5 h of the reaction.
SnO
SnO2
SnO2
29180
29200
29220
29240 29180
29200
29220
29240
Photon energy (eV)
Photon energy (eV)
Figure 3. (a) Sn K-edge XANES spectra of
a
Sn
electrodeposited on O2-plasma treated Ag (SnOx/AgOx)
and on a pristine Ag foil (SnOx/Ag) measured as-prepared
and under CO2RR conditions after 5 h of activation at -0.8
V vs. RHE in 0.1 M KHCO3. Reference spectra of the
metallic Sn foil, SnO and SnO2 are also shown. Vertical
lines provide a guide for an eye and represent the white
line position of the reference Sn foil, of bulk SnO (dashed
line) and of bulk SnO2 (dotted line). (b) Sn K-edge XANES
spectrum of SnOx/AgOx (open circles) under CO2RR
conditions together with the linear combination fit (red
3.3 CO2RR performance. Figure 4 (a) shows the
selectivity of electrocatalysts toward the main products
such as CO, H2 and formate at -0.8 V vs RHE in 0.1 M
KHCO3 after 1 h CO2RR. By utilizing a plasma-oxidized Ag
foil as substrate, AgOx and SnOx/AgOx catalysts show an
outstanding selectivity toward C1 products (~95 % FE)
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Figure 4. (a) Faradaic efficiencies and (b) partial current
versus the parasitic HER (<5 % FE), whereas the SnOx/Ag
catalyst employing a metallic Ag foil as support shows
higher FE for HER (30 % after 1h CO2RR) than on the
AgOx substrates.
densities of AgOx, SnOx/AgOx, SnOx/Ag, metallic Ag and Sn
catalysts acquired after 1 h CO2RR at -0.8 V vs RHE in 0.1 M
KHCO3: HCOO- (dotted blue), CO (straight line in red) and
H2 (gray).
A correlation is also found between the roughness and
the catalytic activity, with enhanced activity for the
rougher surfaces. The O2-plasma treatment, resulting in
rough and defect-rich AgOx and SnOx/AgOx surfaces23
lead to a two-fold enhanced catalytic activity as compared
to analogous but flatter surfaces such as metallic Ag and
SnOx/Ag, as shown in Fig. 4(b). Nevertheless, for samples
with similar roughness factors (AgOx and SnOx/AgOx, see
Table S1), the product selectivity is strongly affected by
the presence of Sn, with the SnOx/AgOx sample producing
more formate than the AgOx sample, but also generating
less H2 when the performance is evaluated over a longer
period of time (20 h), Fig. 5.
In contrast, the SnOx/AgOx sample is found to be
significantly more stable in terms of total current density
and the C1 product conversion while maintaining a
constantly low (5 %) FE for H2, Fig. 5(b). A possible
explanation for the stability of the SnOx/AgOx sample is
that Sn electrodeposition on the rough AgOx surface is
able to preserve the surface roughness under reaction
conditions better than when the AgOx surface is
uncoated. In fact, according to the roughness factors
measured after different reaction times (Table S1), the
roughness of the SnOx/AgOx surface is slightly increased
after 20 h reaction, while a decrease in the roughness is
found for the pure AgOx, which we assume leads to the
loss of highly active defect sites needed for the conversion
to C1 products. The increase in the surface roughness
reported to occur for SnOx surfaces with decreasing oxide
layer thickness was previously associated to a shift in
surface orientation during electrolytic reduction36.
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Although CO2RR catalysts based on plasma-oxidized Ag
substrates display a remarkable catalytic activity and
selectivity for CO during the first hour of reaction, it is
still essential to design stable catalysts that can suppress
HER while maintaining the selectivity for C1 products. As
can be seen in Fig. 5(a), the monometallic AgOx catalyst
shows a distinct deactivation over time, with an increase
in the HER pathway likely due to the decrease in the
population of defect sites and overall decrease surface
roughness with time23.
Figure 5. Temporal evolution of the activity and selectivity
of different CO2RR catalysts: (a) AgOx, (b) SnOx/AgOx and
(c) SnOx/Ag at -0.8 V vs RHE for 20 hours during CO2RR.
Green lines (−) indicate total current densities (left y-
axis). Faradaic efficiencies (right y-axis) of CO (●) and H2
(■) are also shown. The FE value of HCOO- was obtained
after 20 hour CO2RR.
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Experimental procedures, SEM, Quasi in situ XPS, operando
XANES, and determination of surface roughness factor (PDF)
Interestingly, the ratio of the selectivity of formate and
CO on the SnOx/AgOx catalyst changed from 1:1 after the
first hour reaction (Fig. 4) to 1:3 after 20 h reaction (Fig. 5
b) in spite of sustaining ~95 % selectivity for C1 products.
This phenomenon can be explained by considering the
expected changes in the SnOx thickness25 during CO2RR36.
For instance, in Cu/Sn core/shell structures, thick (1.8
nm) tin oxide shells are selective to formate, whereas
thinner (0.8 nm) oxide shells favor CO formation. The
later was explained based on SnOx surface strain leading
to the decrease of the overpotential forming
intermediates25. Another plausible explanation for the
temporal evolution of the CO and formate selectivity
observed in Fig. 1 (b), (e) and Fig. S2 are morphological
alterations on the sample surface taking place during
electrocatalysis.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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* roldan@fhi-berlin.mpg.de.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was funded by the Cluster of Excellence RESOLV
at RUB (EXC 1069) supported by the Deutsche
Forschungsgemeinschaft and the German Federal Ministry of
Education and Research (Bundesministerium für Bildung
und Forschung BMBF) under grant #03SF0523C-‘CO2EKAT’
and by the European Research Council (ERC-725915,
OPERANDOCAT).
Furthermore, the chemical state of the Sn species
deposited on the surface appears to be a particularly
important factor determining the reaction selectivity. An
electrode consisting of both, Sn0 and Snδ+ (δ = 2,4) species
has shown a remarkable catalytic activity and FE toward
formate compared to a native SnOx or a fresh Sn0
surface18. By employing in situ spectroscopic analysis (XPS
and XANES), it is confirmed that these oxides at least
partially survive under reduction conditions37,38, or are
present as surface hydroxides in the divalent oxidation
state, that has been suggested to play a key role in
formate production over SnOx thin film electrodes39. In
summary, the enhanced catalytic selectivity for C1
products is assigned to the stabilization of cationic Sn
species on the SnOx/AgOx samples, while maintaining
adequate roughness during long-term CO2RR.
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