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Electrocatalysis
Supporting information
CO2 Electroreduction on Cu-Modified Platinum Single Crystal Electrodes in Aprotic
Media
Alexander V. Rudnev1,2*, Maria R. Ehrenburg1*, Elena B. Molodkina1, Inna Botriakova1, Alexey I.
Danilov1, Thomas Wandlowski2
1
A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow 119991, Russia
2
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern,
Switzerland
*Corresponding authors
[email protected] (A.V. Rudnev)
Tel: (+41)-31-631-5385
Fax: (+41)-31-631-3994
[email protected] (M.R. Ehrenburg)
Tel: (+7)-495-955-4455
Deposit Washing and Stabilization
Another serious issue is to remove the modified electrode from the solution without additional
modification of its surface when it is brought in contact with the atmosphere or another solution.
When a wet deposit is removed into air, fast corrosion processes with nonuniform copper dissolution
may occur on the deposit surface. To avoid this effect, the deposit was washed in water saturated by
Ar and/or H2 to remove the drop of acidic solution from its surface in an inert atmosphere. One
should take into account that contact with hydrogen-saturated water may cause additional copper
deposition from the solution drop remaining on the electrode after it is removed from the cell which
is undesirable. All operations must be carried out very swiftly to minimize the time of contact with
air. After washing, the electrode must be thoroughly dried by an intensive stream of Ar.
As electrode washing and transfer between the cells may cause deposit dissolution, we carried
out special experiments to estimate the amount of the remaining deposit on the basis of its
dissolution charge. The difference in the charges of copper deposition and dissolution may be due to
two effects. Firstly, the surface may be etched in air before the deposit is finally dried, especially in
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the case of the Pt(100) and Pt(110) electrodes. This effect is most pronounced in the case of deposits
obtained in perchlorate solutions, where there is no strongly adsorbing anion on the copper surface
that could inhibit copper corrosion in air. Secondly, cathodic deposition currents correspond to two
processes: formation of neutral Cu atoms and Cu+ ions. The contribution of the latter process may
reach 10%, especially at low potentials at the growing stage. In the case of Pt(111), the current yield
was at least 90% in sulfate solutions; thus, the electrode was removed from the cell without any
noticeable deposit modification in air. In the case of deposits on Pt(100) and Pt(110), the scatter is
higher, but accurate observation of the procedure of fast washing in Ar-saturated water allows
obtaining deposits with a high state of preservation.
Supplementary data
Fig. S1 shows that copper is preferentially deposited along the steps on a Pt(100) electrode at
rather low overpotentials.
Fig. S1. AFM images of Cu deposition on Pt(100). Frame size: (a) 20×20 µm2 and (b) 4×4 µm2. Cu
deposit was obtained at η1 = –25 mV for 45 s from 0.5 M H2SO4+10 mM CuSO4.
Fig. S2. Current transients of Cu deposition on Pt(hkl) electrodes in an 0.5 M H2SO4+10 mM CuSO4
solution. (a) solid line: η1 = –0.066 V (2 s), η2 = –0.036 V (900 s); dashed line: η1 = –0.096 V (2 s),
η2 = –0.031 V; (b) solid line: η1 = –0.075 V (2 s); η2 = –0.045 V (900 s); dashed line: η1 = –0.110 V
(2 s); η2 = –0.040 V; (c) solid line: η1 = –0.095 V (2 s), η2 = –0.065 V (900 s); dashed line: η1 = –
0.140 V (2 s); η2 = –0.055 V (900 s). Corresponding AFM images are shown in the main text in
Fig. 4.
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Fig. S3. First-cycle voltammograms of (a) Pt(hkl) single crystals and (b) Cu deposits on Pt(hkl) in
argon-saturated (solid) and CO2-saturated (dashed lines) 0.1 M TBAPF6 in acetonitrile. The sweep
rate is 50 mV s-1. The insets in (b) show representative AFM images of the quasi-epitaxial Cu
deposits on Pt(hkl). The frame size is 10×10 µm2. The parameters of Cu deposition are given in
Table S1.
Table S1. Average parameters of Cu/Pt(hkl) electrodes used in CO2-reduction experiments in Fig.
S3. The data were extracted from AFM analysis of at least 10 images for each electrode.
Roughness
Copper
Electrode Deposition conditions* Thickness of Cu deposits in
equivalent monolayers**
factor*** coverage****
Cu/Pt(111) η1 = –0.115 V for 2 s
η2 = –0.050 V for 900 s
136
1.06 ± 0.04
0.68 ± 0.01
315
1.13 ± 0.04
0.81 ± 0.04
346
1.09 ± 0.02
0.99 ± 0.01
Cu/Pt(100) η1 = –0.091 V for 5 s
η2 = –0.025 V for 900 s
Cu/Pt(110) η1 = –0.128 V for 5 s
η2 = –0.043 V for 900 s
* Pt(111) was exposed at 0.55 V (vs. Cu reference electrode) for 500 s to activate surface before Cu
deposition.
** Thickness of Cu deposits in equivalent monolayers was estimated on the basis of Cu deposition
charges per two-electron process and with account for densities of Pt surface atoms on different
Pt(hkl)-(1x1) faces.
*** Roughness factors were roughly estimated by roughness analysis of AFM images using the
WSxM software package [1].
**** Area of a Pt(hkl) surface covered by copper normalized to the whole geometric area of Pt(hkl);
estimated from AFM images using the WSxM software package [1].
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The strongest deactivation was observed for CO2 reduction on Cu/Pt(110) electrode in PC
solution (Fig. S4). Deactivation indicates that the electrode surfaces can be poisoned by products of
CO2 reduction. The strongest deactivation for Cu/Pt(110) can be explained by presence of many
structural defects on Cu crystallites. They have a shape of triangular prism, and the angle between
Pt(110) substrate and side faces of the crystallites is close to 45°. The latter suggests the presence of
(100) sites in terraces and/or in steps. For more details see Ref. [2].
j / mA cm
2
0.0
-0.2
-0.4
20
15
10
7
5
3
2
1
-2.0
-1.5
-1.0
E vs. Ag/AgCl / V
Fig. S4. The CVs of Cu/Pt(110) in argon-saturated (black) and CO2-saturated (red curves) 0.1 M
TBAPF6 in PC. The sweep rate is 50 mV s-1. The numbers of consecutive cycles in the CO2-saturated
solution are indicated in the figure.
Fig. S5. The CVs of Cu(111) in argon-saturated (solid curves) and CO2-saturated (dashed curves) 0.1
M TBAPF6 in (a) PC and (b) AN. Dotted curve represents the CV of Pt(111) (taken from Fig. 5a),
and dash-dotted curve represents the CV of Cu/Pt(111) (taken from Fig. 5b). The sweep rate is 50
mV s-1.
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Roughness analysis
The actual surface area was estimated by a utility in WSxM software [1] by considering the
surface as a collection of 3D triangles (each of them formed by a point and its first neighbours).
Then, we calculate the area of those singles triangles and obtain the real area of the sample by adding
them up. Obviously, this approach depends on AFM image resolution and sharpness of a cantilever,
and leads to underestimation of roughness factor. We performed analysis of 10 to 20 different images
for each Cu/Pt(hkl) electrode surfaces. Indeed, the increase of image resolution (number of points
and lines, e.g. 256x256, 512x512, 1024x1024) leads to only a minor increase of determined real area:
e.g. estimated roughness factor was increased only slightly (2-4 %) for images with low resolution
(256x256 points) and high resolution (1024x1024 points), respectively. Moreover, for small-area
images roughness factor was slightly higher as compared to large-size images due to better resolving
nanostructural defects. Thus, for estimation of actual area we used images with small and medium
frame sizes (typically 1x1 to 3x3 µm2) with resolution of at least 512x512 points (1 to 6 nm).
Sharpness of cantilever (< 10 nm tip radius in this work) and resolution of AFM images were not
enough to resolve atomic-height relief in such large-scale images. However, the quasi-epitaxy of
copper deposits suggests the presence of flat terraces on copper crystallites with respective
crystallographic orientations, which leads to such not high total roughness (Fig. S6).
Nonetheless, roughness underestimation for Cu/Pt(110) could be higher than that for Cu/Pt(111)
and Cu/Pt(100), because side faces of Cu crystallites on Pt(110) have ~45° angle respectively
substrate plane. Simply, the Cu crystallites on Pt(110) have shape of a triangular prism, while in case
of Pt(111) and (100) the Cu crystallites have rather wide and flat top side of respective
crystallographic orientation (see Fig. S6).
Fig. S6. Ex situ AFM images and respective cross sections of Cu deposits on (a) Pt(111), (b) Pt(100),
and (c) Pt(110). ). Frame sizes are indicated. Cu deposits were obtained from 0.5 M H2SO4+10 mM
CuSO4 at (a) η = –50 mV for 500 s, (b) η1 = –0.091 V for 5 s and η2 = –0.021 V for 900 s, (c) η1 = –
0.128 V for 5 s and η2 = –0.043 V for 900 s.
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References
1. I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, A.M. Baro, Rev.
Sci. Instrum. 78, 013705 (2007).
2. A.V. Rudnev, E.B. Molodkina, M.R. Ehrenburg, R.G. Fedorov, A.I. Danilov, Y.M. Polukarov,
J.M. Feliu, Russ. J. Electrochem. 45, 1052-1063 (2009).
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