Dynamic Changes in the Structure, Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects

Article abstract of DOI:10.1002/anie.201802083

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size- and shape-controlled ligand-free Cu nanocubes during CO₂ electroreduction (

Full text of DOI:10.1002/anie.201802083

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Accepted Article
Title: Dynamic Changes in the Structure, Chemical State and Catalytic
Selectivity of Cu Nanocubes during CO2 electroreduction: Size
and Support Effects
Authors: Philipp Grosse, Dunfeng Gao, Fabian Scholten, Ilya Sinev,
Hemma Mistry, and Beatriz Roldán Cuenya
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802083
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10.1002/anie.201802083
Angewandte Chemie International Edition
Dynamic Changes in the Structure, Chemical State and Catalytic Selectivity of Cu
Nanocubes during CO2 electroreduction: Size and Support Effects
Philipp Grosse1┴, Dunfeng Gao1┴, Fabian Scholten1, Ilya Sinev1, Hemma Mistry1,2, Beatriz
Roldan Cuenya1,2,3*
1 Department of Physics, Ruhr-University Bochum, 44780 Bochum, Germany.
2 Department of Physics, University of Central Florida, Orlando, Florida 32816, USA.
3
Department of Interface Science, Fritz-Haber Institute of the Max Planck Society, 14195
Berlin, Germany
*e-mail: Roldan@fhi-berlin.mpg.de
┴ equally contributing 1st authors.
Abstract
In situ and operando spectroscopic and microscopic methods were used to gain insight into the
correlation between the structure, chemical state, and reactivity of size- and shape-controlled
ligand-free Cu nanocubes (Cu-cubes) during CO2 electroreduction (CO2RR). Dynamic changes
in the morphology and composition of Cu-cubes supported on carbon were monitored under
potential control via electrochemical atomic force microscopy, X-ray absorption fine-structure
spectroscopy and X-ray photoelectron spectroscopy. Under reaction conditions, the roughening
of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and
reduction of CuOx species observed were found to lead to a suppression of the selectivity for
multi-carbon products (i.e. C2H4 and ethanol) versus CH4. A comparison with Cu-cubes
supported on Cu foils revealed an enhanced morphological stability and persistence of Cu(I)
species under CO2RR. Both factors are held responsible for the higher C2/C1 product ratio
observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance
of the structure of the active nanocatalyst but also its interaction with the underlying substrate in
CO2RR selectivity.
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Introduction
Since a number of years there is an increasing interest in trying to understand the parameters
that govern the electrochemical reduction of CO2 (CO2RR) to design catalysts with high
selectivity for valuable chemicals and fuels.[1] Among these parameters, the catalyst structure and
chemical state are of particular importance.[2–4] Compared to polycrystalline Cu electrodes,
nanostructured Cu catalysts have shown a significantly improved CO2RR performance,
attributed to grain boundaries[5,6], Cu(100) facets[7–12], increased roughness[13], defects[14–16], low-
coordinated sites[17–19], and the presence of subsurface oxygen and Cu(I) species[20–29]
.
Previous studies[9–11] on Cu single crystals have shown the improved C-C coupling
performance of (100) facets, which was further confirmed by the high selectivity towards
ethylene observed on cubic-shaped Cu catalysts[7,8,16,21,30–36]. However, the presence of (100)
facets is not the only factor responsible for the superior activity and selectivity of cubic-shaped
Cu catalysts, with surface roughness, subsurface oxygen and Cu(I) species or Cu/Cu(I) interfaces
formed and/or stabilized under reaction conditions also playing a very important role.[21] The
function of oxygen in such structures is particularly intriguing, since on Cu(100) surfaces it was
discussed to contribute to the formation of oxygenated hydrocarbons[37]. The complexity arising
from the multiple factors affecting the catalytic performance of cubic-shaped Cu catalysts
requires a systematic study of the evolution of their structure and oxidation state under operando
CO2RR conditions.
This work focuses on the understanding of the relative importance of the different factors
responsible for specific selectivity trends observed for Cu catalysts during CO2RR, namely, the
presence and stability of Cu(100) facets, defect sites, and the content of Cu(I) species and/or
subsurface oxygen. By electrochemically growing ligand-free Cu-cubes on C supports and
comparing with analogous samples supported on Cu foils, we were able to reveal the intrinsic
behavior of the cubic-shaped Cu NPs catalysts and their dynamic evolution under CO2RR
conditions.
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Results and Discussion
Size-dependent changes in the morphology and composition of Cu-cube samples
electrodeposited on carbon paper were investigated ex situ via scanning electron microscopy
(SEM) and energy dispersive X-ray spectroscopy (EDX) before and after CO2RR, Figures 1, S1
and S2 (Suppl. docs). In all samples the Cu-cubes were found to decrease in size in the course of
the first 1 h of the reaction. Furthermore, in this process the originally flat facets and sharp edges
of the Cu-cubes were found to roughen, resulting in a porous nanocube surface, Fig. 1.
Figure 1: SEM images from size-controlled Cu-cube samples electrodeposited on carbon paper
acquired before and after CO2RR at -1.05 V for 1 h. The scale bars in the main panels are 2 µm,
those in the insets and in the images of the last column are 200 nm.
Operando electrochemical atomic force microscopy (EC-AFM) measurements carried out on
a sample with ~100 nm large cubes also revealed significant changes in their morphology
already prior to the actual CO2RR. Sample immersion in the aqueous 0.1 M KHCO3 electrolyte
at open circuit potential was observed to lead to the formation of “cracks” on the cube facets.
These cracks are likely the result of mechanical stress taking place during the solvation of Cl-
ions while being transferred from the cubes to the electrolyte. SEM-EDX data revealed that Cl
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was present throughout the entire Cu volume in the as prepared samples and therefore, the loss of
Cl leads to drastic structural changes in the entire cube volume.
Figure 2: (a) Schematic representation of the morphological evolution of Cu-cube catalysts. (b-
e) AFM images of Cu-cubes electrodeposited on highly oriented pyrolytic graphite (HOPG)
acquired in air (b), and operando EC-AFM measurements in a CO2-saturated 0.1 M KHCO3
aqueous solution at open circuit potential (c), at -1.1 V vs RHE in the same electrolyte for 1 min
(d), and after 3 h under the same conditions as in (d) and subsequent air exposure (e).
A 1 min potential pulse at -1.1 V vs RHE resulted in the further roughening of the nanocube
surface, loss of sharp corners and edges, and a decrease in the edge length of approximately
10 %. The latter is assigned to the initial reduction of Cu2O to Cu as well as some loss of Cu.
After 3h CO2RR at -1.1 V vs RHE, an additional 10% decrease of the NP size was observed,
with rough spherical-like NP shapes.
X-ray induced Cu LMM Auger electron spectra (XAES) were acquired from pristine C-
supported Cu-cubes (220 nm) before and after CO2RR for 1 h at -1.1 V vs RHE in an ultrahigh
vacuum (UHV) XPS system directly interfaced to an electrochemical cell. Thus, the sample
could be transferred in vacuum to avoid re-oxidation.[21] Additional Cu-2p XPS data are also
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included in Figure S3. A surface composition of the pristine C-supported Cu-cubes of 52%
Cu2O, 22% CuCl2, 23% CuCl, and 3% metallic Cu was obtained before reaction, Fig. 3. After
the electrochemical treatment, the cubes are mostly reduced, with only 4.6% Cu2O detected.
Interestingly, a higher content of Cu(I) species (13%) was detected after CO2RR on similarly
synthesized Cu-cubes supported on a Cu-foil, and no Cu(I) at all in the pristine Cu foil support
after CO2RR (Fig. S4). The latter finding highlights the key role of having a Cu cube/Cu foil
interaction for the stabilization of Cu(I) species.
Figure 3: Quasi in situ Cu Auger LMM XAES spectra of 220 nm Cu-cubes electrodeposited on
C paper and 250 nm cubes deposited on a Cu-foil acquired before (left) and after 1 hour of
CO2RR at -1.1 V vs RHE (right).
To gain further insight into the stability of Cl, Cu(I) species, and subsurface oxygen in our
Cu-cubes under CO2RR, operando X-ray absorption fine-structure (XAFS) measurements were
conducted on 120 nm Cu-cubes, Figures 4 and 5 (see also Figs. S5, S6, and S7 in the SI for more
details). As compared to XPS and XAES, probing about 10 nm below the surface, XAFS is a
bulk sensitive technique that reveals the overall change in the structure and composition of the
Cu-cubes. The as prepared state of the Cu-cubes shows a mixture of Cu2O and CuClx, which
almost immediately changes fully into Cu2O upon becoming in contact with the electrolyte.
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Figure 4: Normalized XANES region (a) and Fourier-transform magnitudes of k2-weighted
EXAFS data of pristine Cu-cubes (b) measured as prepared, in 0.1 M KHCO3 at open circuit
potential and in the same electrolyte after 1 h of CO2RR at -1.1V vs RHE.
In CO2-saturated 0.1 M KHCO3, the spectrum resembles greatly the Cu2O reference, with
only a very small difference where the Cl-feature was previously observed and a small decrease
of the Cu-O feature. After 1 h at -1.1 V vs RHE, the spectrum obtained for the Cu-cubes
resembles that of metallic Cu. The Fourier Transform (FT) data obtained for the Cu-cubes in the
electrolyte after 1 h CO2RR at -1.1 V vs RHE are less intense in comparison to a bulk Cu-foil,
indicating a lower atomic coordination and/or enhanced disorder. From the first shell Cu-Cu fit
after 1 h of applied potential, a coordination number (CN) of 10.8 was obtained for the Cu-cubes
(vs 12 expected for bulk fcc-Cu). The lower CN values might be indicative of the formation and
growth of pores on the Cu-cubes under reaction conditions or the partial disruption of the
nanocubes, leading to an increased surface roughness. The operando EC-AFM data also revealed
the formation of cracks (defects) on the Cu-cubes even before applying the potential once they
were exposed to the electrolyte. The porous structure seen by SEM can also be a result of the
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progressive CuOx reduction during the reaction. Under reaction conditions, these pores as well as
the cube corner and edges appear more prone to dissolution than the higher coordinated flat
facets.
Since Cu(I) species[37] have been suggested to play an important role in CO2RR selectivity, in
order to better detect small amounts of Cu-O species an additional analysis of the XAFS data by
wavelet-transform[38] (WT) was applied (Fig. 5). Details on this analysis are given in the Suppl.
documents. After processing the measured spectra and doing the Morlet WT, the spectra were
normalized to the highest signal, allowing subtraction of measured reference materials. The Cu-
cubes on C in their as prepared state display a large feature at R ~ 1.4 Å and k ~ 3.9 Å-1 which
represents the Cu-O shell, whereas at a slightly higher distance (R ~ 2 Å, k ~ 7 Å-1) the
interaction of Cu with the heavier Cl atoms is visible. For the as prepared sample, only a faint
Cu-Cu 1st and 2nd neighbor interaction is observed after the subtraction of a Cu2O reference from
the pristine Cu-cubes, indicating that the sample consists almost exclusively of Cu2O and CuClx.
In the electrolyte, the CuClx feature immediately disappears and a Cu-O-Cu feature arises at R ~
3 Å and k ~ 4 Å-1. In addition, the magnitude of the Cu-O component decreases slightly. After 1
hour of CO2RR, the most prominent feature is that of the Cu-Cu first shell (R ~ 2.3 Å, k ~ 7 Å-1).
The pristine Cu-cubes resemble almost exactly the metallic Cu reference after 1 h of CO2RR. A
deconvolution of the Cu XANES region of pristine Cu-cubes in 0.1 M KHCO3 with Cu, Cu2O,
and CuO standards, Fig. S7, yields a composition of 99% Cu and 1% (within the error margin)
Cu2O after 1 h of CO2RR. CuO could not be reasonable fitted. These findings are in good
accordance with the XPS surface analysis indicating that for the Cu cube/C system, no Cu2O
species remain after 1 h of reaction either at the surface (XPS) or in sub-surface regions (XAFS),
which is in clear contrast with previous findings for Cu-cubes/Cu-foil (Fig. 3). [21,22]
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Figure 5: Wavelet transform of Cu-cubes as prepared, in 0.1 M KHCO3, and after 1 h of CO2RR
as well as the bulk references used. The height is normalized to 1 and the subtracted images
scaled by a factor of two for better visibility.
After observing drastic changes in the morphology and chemical composition of the samples
under operando reaction conditions within the first 1-3 hours, the electrochemical performance
was studied in order to gain insight into structure/chemical-state/reactivity correlations. The
background signal from the C-paper support was subtracted from all data presented as shown in
Fig. S8. A comparison between HER Faradaic efficiency (parasitic reaction) and the sum of the
Faradaic efficiency of all products from CO2RR is displayed in Fig. 6. Detailed information on
the different products detected can be found in Figure 7 for the 220 nm Cu-cubes and in Figs. S9,
S10 and S11. Similar trends for small Cu-Au cubic NPs were reported in Ref.[39]
.
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Figure 6: Faradaic efficiency for CO2RR and HER at approx. -1.05 V vs RHE recorded during
5 h for samples with different Cu-cube sizes (580 nm, 320 nm and 220 nm). The insets display
SEM images of typical Cu-cubes measured after different reaction times. The size of the scale
bars is: 100 nm.
For the stability tests, Cu-cubes of three different initial sizes were investigated at -1.05 V vs
RHE, Fig. 6. This potential was chosen since we expect to see the highest selectivity towards C2-
C3 products.[40] The total CO2RR FE was found to increase with increasing Cu-cube size, and
achieves an initial value of ~75% on the 580 nm cubes. This is assigned to the enhanced
morphological stability of the larger cubes. A second general trend, independent of size, is a
decrease in Faradaic efficiency for CO2RR and increase in HER over time. An exception is the
220 nm Cu-cubes sample, which starts already with higher Faradaic efficiency for HER over
CO2RR. The smaller cube size on the weakly binding C support leads to a lower stability of the
Cu-cubes, leading to the loss of material into the electrolyte and possible subsequent re-
deposition as small clusters on the support and Cu-cubes. Such low-coordinated structures were
found to result in an increase in the H2 and CO production and decrease in the formation of
hydrocarbons, i.e., they favor HER over CO2RR.[41,42] This correlates with the observed
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morphological changes shown in Figs. 1 and 2, where a rounding of corners and edges and
roughening of the facets is observed.
Figure 7(a) displays the FE data for the pristine 220 nm Cu-cubes on C measured as a
function of the applied potential after 1 h of CO2RR. The production of C2 and C3 products peaks
around -1.0 V vs RHE, whilst hydrogen evolution is the lowest at this potential. At potentials
more negative than -1.0 V vs RHE, C2/C3 products decline, whilst methane production keeps
increasing.
Figure 7: (a) Faradaic efficiency of pristine 220 nm Cu-cubes on C as a function of applied
potential obtained after 1 h of CO2RR. Faradaic efficiencies for C2H4 (b), CH4 (c), and CO (d)
are also shown for the pristine 220 nm Cu-cubes on C, 250 nm Cu-cubes on Cu foil and two
reference Cu foils electropolished and O2-plasma treated (20, 20 W, 400 mTorr[21,22]).
The FE for C2H4, CH4 and CO as a function of the applied potential for the pristine 220 nm
Cu-cubes on C together with similarly synthesized 250 nm Cu-cubes supported on a Cu foil as
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well as electropolished and O2-plasma-treated Cu foils are shown in Figure 7(b-d). A very strong
support-effect is evident, with the Cu-cubes grown in the weakly-interacting C support
displaying a higher overpotential and much lower FE for C2-C3 products than the similarly-sized
Cu-cubes on the Cu foil. In fact, as can be seen in Fig.7(c), the production of CH4 of the Cu-
cubes on C resembles the metallic electropolished Cu foil, although the CO production of the
Cu-cubes on C is much higher, Fig. 7(d). It is remarkable that the pristine Cu-cubes on the Cu-
foil, which were found to better stabilize Cu(I) species under reaction conditions (Fig. 3), behave
similarly to the O2-plasma treated Cu foils. Therefore, we can confirm a direct correlation
between the stabilization of Cu(I)/Cu interfaces (Cu-cubes on Cu-foil) and the selectivity for C2-
C3 products, since metallic Cu-cubes supported on C are more selective for C1 products. This is
however not the only reason for the change in the selectivity. Even more important are the
drastic structural changes observed for the Cu-cubes on C. The roughening of the Cu(100) facets,
partial loss of the cubic shape and formation of pores as well as detachment from the surface
play a critical role in the selectivity switch reported here.
2. Conclusion
A simple electrochemical method for the synthesis of cubic-shaped nanoparticles of tunable
size supported on carbon substrates is presented here. Dynamic morphological and chemical
transformations of Cu-cubes during CO2RR were monitored using operando EC-AFM and X-ray
absorption fine-structure spectroscopy (XAFS). Drastic changes in the cube morphology were
found to take place for the Cu-cubes on C under CO2RR conditions, including the roughening
and loss of (100) facets, loss of Cu atoms from edge and corner sites, and the reduction of CuOx
species.
The selectivity for CO2RR versus HER was found to decrease with decreasing cube size,
which was assigned to more drastic changes in the cube morphology taking place under CO2RR
conditions over smaller cubes. In contrast with the findings previously reported for Cu-cubes
grown on Cu foils, a surprisingly high selectivity for CH4 as compared to C2H4 was observed
when deposited on C. The observed morphological instability of the Cu-cubes on C versus those
deposited on Cu, together with the absence of stable Cu(I) species in the former samples are
considered responsible for the preferred selectivity for C1 products.
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Experimental
A mixture of 5 mM copper sulfate-pentahydrate (CuSO4·5H2O) and 5 mM potassium chloride
(KCl) was used as starting material. Electrochemical cycling between an oxidizing (+0.55 V vs.
RHE) and a reducing potential (+0.22 V vs. RHE) with varying number of cycles (1-100),
depending on the desired cube size and coverage, lead to the electrodeposition of size- and
shape-controlled Cu-cubes with a narrow size distribution. Initially, a potential of -0.2 V vs
Ag/AgCl was held for 8 s with a subsequent ramp to +0.4 V vs Ag/AgCl for 4 s at a ramp rate of
700 mV/s. Returning to the initial potential completes the cyclic voltammetry. Cu-cube sizes
ranging from 80 nm to 1,2 µm were obtained depending on the 1 mM-100 mM KCl and
CuSO4·5H2O concentration, number of cycles, and applied potential. High surface area carbon
paper was used as substrate (Toray Carbon Paper TGP-H-060). The Cu-cubes on Cu-foil were
prepared as described in Ref. [21]
.
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). Partial
financial support from the U.S. National Science Foundation (NSF-Chemistry 1213182) is also
greatly appreciated. The XAFS experiments were performed on beamline BM25A at the
European Synchrotron Radiation Facility (ESRF), Grenoble, France. We are grateful to Aida
Serrano Rubio and Germán Castro at the ESRF for providing assistance in using beamline
BM25A.
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10.1002/anie.201802083
Angewandte Chemie International Edition
COMMUNICATION
A variety of In situ and operando
Philipp Grosse, Dunfeng Gao, Fabian
Scholten, Ilya Sinev, Hemma Mistry,
Beatriz Roldan Cuenya
spectroscopic
and
microscopic
methods were used to gain insight into
the correlation between the structure,
chemical state, and reactivity of size-
and shape-controlled ligand-free Cu
Page No. – Page No.
Dynamic Changes in the Structure,
Chemical State and Catalytic
Selectivity of Cu Nanocubes during
CO2 electroreduction: Size and
Support Effects
nanocubes
during
CO2
electroreduction. Dynamic changes in
the morphology and composition of
Cu nanocubes supported on carbon
were monitored under potential control
and correlated to product selectivity.
16
This article is protected by copyright. All rights reserved.