Brass and Bronze as Effective CO2 Reduction Electrocatalysts

Article abstract of DOI:10.1002/anie.201709932

Electrochemically reducing CO₂ into fuels using renewable electricity is a contemporary global challenge that requires significant advances in catalyst design. Photodeposition techniques were used to screen ternary alloys of Cu-Zn-Sn, which includes brass and bronze, for the electrocatalytic reduction of CO₂ to CO and formate. This analysis identified Cu_0.2Zn_0.4Sn_0.4 and Cu_0.2Sn_0.8 to be capable of reaching Faradaic efficiencies of >80 % for CO and formate formation, respectively, and capable of achieving partial current densities of 3 mA cm^?2 at an overpotential of merely 200 mV.

Full text of DOI:10.1002/anie.201709932

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201709932
German Edition: DOI: 10.1002/ange.201709932
Electrocatalysis
Brass and Bronze as Effective CO Reduction Electrocatalysts
2
Jingfu He, Kevan E. Dettelbach, Aoxue Huang, and Curtis P. Berlinguette*
[
17]
Abstract: Electrochemically reducing CO2 into fuels using
renewable electricity is a contemporary global challenge that
requires significant advances in catalyst design. Photodeposi-
tion techniques were used to screen ternary alloys of Cu-Zn-Sn,
which includes brass and bronze, for the electrocatalytic
Pb) or by high rates of competitive hydrogen formation (Ni,
Fe) in the absence of expensive ionic liquids.
Alloys offer the opportunity to tune the surface properties
of electrocatalysts to optimize catalytic activity.
been shown, for example, that the activity of AuCu nano-
particles is three-fold higher than that of gold catalysts, and
that CuIn alloys can form CO with high selectivity. A Pd-Sn
alloy was reported to mediate the exclusive formation of
formate at an overpotential of 0.26 V that cannot be achieved
[18–27]
It has
[23]
reduction of CO to CO and formate. This analysis identified
2
[
22]
Cu Zn Sn and Cu Sn to be capable of reaching Faradaic
0
.2
0.4
0.4
0.2
0.8
efficiencies of > 80% for CO and formate formation, respec-
tively, and capable of achieving partial current densities of
À2
[18]
3
mAcm at an overpotential of merely 200 mV.
by any single metal.
Despite these promising results, very few alloys of
exclusively earth-abundant elements have been tested for
T
he electrolytic reduction of CO is a potential means of
2
utilizing cheap renewable electricity to produce fuels, chem-
CO reduction electrocatalysis. We disclose here the discovery
2
[
1–3]
icals, and plastics while mitigating CO emissions.
The
that the Cu-Zn-Sn compositional phase space is capable of
2
electrolytic production of CO and formate are among the
mediating efficient electrocatalytic reduction of CO to CO
2
easier products to produce from CO , with CO being
and formate. We tested these three earth abundant metals
2
particularly appealing because it can potentially be used as
because: 1) the MÀCO bonding strengths for pure phases of
[
4]
a fuel or as an intermediate for further synthesis. The
challenge is to develop an electrocatalyst capable of convert-
ing CO2 into CO and formate with high efficiency and
each of Cu, Zn, and Sn are compatible with CO and HCOOH
evolution (Supporting Information, Figure S1); and 2) Cu-
Zn, Cu-Sn, and Cu-Zn-Sn comprise the widely known alloys
brass, bronze, and white bronze, respectively. We therefore
prepared 21 films where the relative Cu, Zn, and Sn contents
were changed in 20% increments (Figure 1a) by electro-
chemically reducing the corresponding mixed-metal oxide
[
5,6]
selectivity for prolonged time periods.
State-of-the-art CO -to-CO heterogeneous electrocata-
2
[
7]
[8]
lysts include oxide-derived gold, nanostructured gold, and
nanostructured silver electrodes
[
9,10]
that achieve partial
[22,28–30]
current densities for CO formation (jCO) of 7, 40, and
films prepared by photodeposition techniques.
An
À2
3
mAcm , respectively, at overpotentials of 300 mV. We
analysis of each film revealed that the compositions
Zn Sn and Cu Sn produce HCOOH with high selectiv-
note that homogeneous electrocatalysts are also capable of
0
.8
0.2
0.2
0.8
[11,12]
achieving selective CO evolution.
The formation of
ity and efficiency, and that Cu Zn Sn is an effective CO2
0.2 0.4 0.4
formate from CO2 is most effective at carbon-supported
to CO electrocatalyst. These three catalysts were capable of
achieving partial current densities of about 3 mAcm at
a modest overpotential of 200 mV, which are among the
highest values reported to date for earth abundant metal CO2
electrocatalysts.
À2
palladium: A current density for formate production (jHCOOH)
À2
of 1 mAcm can be achieved at an overpotential of merely
[
13]
5
0 mV.
A growing number of non-noble metal CO2
reduction electrocatalysts have been unearthed (for example,
[
14]
[15]
Co nanolayers,
WSe nanoflakes,
Bi nanoflower cata-
A series of 21 electrocatalysts were synthesized by drop-
casting onto a titanium substrate a 0.6-mL aliquot drawn from
a 4.8-mL methanolic solution containing 0.1ÀaÀb mmol
CuCl , a mmol of ZnCl , and b mmol of SnCl . Each film
2
[
16]
lyst ), but the performance of earth-abundant electrocata-
lysts is confined to low current densities (for example, Zn, Sn,
2
2
2
was then exposed to near-infrared (NIR) radiation for 1 h.
This succession of deposition and irradiation steps were
repeated 7 times prior to a final 12-h exposure to NIR
[
*] Dr. J. He, K. E. Dettelbach, A. Huang, Prof. Dr. C. P. Berlinguette
Department of Chemistry, The University of British Columbia
radiation. Each film was then immersed in a 0.5m NaHCO3
2036 Main Mall, Vancouver, BC V6T1Z1 (Canada)
E-mail: cberling@chem.ubc.ca
electrolyte solution in a three-electrode cell configuration and
Prof. Dr. C. P. Berlinguette
Department of Chemical & Biological Engineering
The University of British Columbia
reduced at steady negative bias (À0.5 V vs. RHE) for 1 h prior
to film characterization and electrocatalyst testing.
The absence of chloride ligands in the as-prepared films
was confirmed by X-ray fluorescence (XRF) spectroscopy
2360 East Mall, Vancouver, BC V6T1Z3 (Canada)
and
(Supporting Information, Figure S2). An XRF analysis of the
Stewart Blusson Quantum Matter Institute
The University of British Columbia
films showed a relative metal content that did not change
substantially before or after the electrochemical reduction
step. The scanning electron microscope (SEM) image of
a prototypical as-prepared sample, Cu0.33Zn0.33Sn0.33, shows
a full coverage of metal oxide nanoparticles (Figure 2a).
2355 East Mall, Vancouver, BC V6T1Z4 (Canada)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201709932.
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1
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Cu Sn , Cu Sn , Cu Zn Sn , Cu Zn Sn , Cu Zn ,
0
.6
0.4
0.4
0.6
0.4
0.2
0.4
0.2
0.4
0.4
0.2
0.6
Cu Zn , Zn Sn , and Sn presented in Figure 1b were
0
.4
0.6
0.6
0.4
selected because they represent the different alloy phases
indicated in Figure 1a. The main crystalline phase of samples
with > 70% Cu (for example, Cu Zn Sn ) was similar to
0
.8
0.1
0.1
metallic copper. This observation is presumably because zinc
and tin atoms can both be incorporated into the lattice of
copper, which exists in the same crystalline form as brass and
bronze. When the relative copper content is less than 70%
with a tin content higher than 10%, the base-centered
monoclinic crystalline phase of h-bronze (pdf number: 045-
1
488) is present, for example, the Cu Sn sample is
0.6 0.4
characterized by a prominent signal at 30.18 and 43.08
corresponding to h-bronze and an attenuated copper signal.
For the binary Cu-Zn alloys, where the Cu ratio is below 50%,
peaks characteristic of g-brass (pdf number: 045-1488) are
observed at 35.18, 38.08, and 43.38. Mixtures of h-bronze and
metallic tin were detected for films where the copper content
was 10–30% and a tin content below 10%. Samples with
a copper content of < 10% did not alloy and only mixtures of
metallic zinc and metallic tin were detected. Alloys of zinc
and tin at room temperature are not known.
The efficacy of CO reduction was evaluated for each of
2
the films by measuring the Faradaic efficiencies and partial
current densities of all products in the three-electrode cell.
Figure 1. a) Compositional diagram for the Cu Zn Sn alloys eval-
1ÀxÀy
uated in this study. 21 different compositions were evaluated where
the metal content was changed in 20% increments. b) Powder X-ray
x
y
The Faradaic efficiency of one single product (H , CO, or
2
HCOOH) is defined as FE , where X = H , CO, or HCOOH.
X
2
diffractrograms for copper, Cu0.8Zn0.1Sn0.1, Cu0.6Sn0.4, Cu0.4Sn0.6
,
The electrolyte for these experiments, 0.5m KHCO , was pre-
3
Cu0.4Zn0.2Sn0.4, Cu0.2Zn0.4Sn0.4, Cu0.4Zn0.6, Zn0.6Sn0.4, and tin samples.
These compositions, which are indicated by the colored circles in (a),
represent the different solid-state phases of these alloys.
purged with CO for 1 h prior to catalysis. Each sample was
2
subjected to 4 h of electrolysis. Gas-phase products H and
2
CO were detected in real-time by gas chromatography, while
liquid products such as formic acid were sampled and
1
measured every hour by H nuclear magnetic resonance
(NMR) spectroscopy and calibrated against the total charge
passed during the experiment. The catalytic activities were
found to be stable over this 4 h testing period (Supporting
Information, Figure S3). The data reported in Figure 3 shows
the GC measurement 1 h into the reaction, while the NMR
data was recorded during the first two hours of electrolysis.
Titanium is comparably inert towards CO reduction (cata-
2
lytic onset ca. À0.8 V) and was used as the support for all
studies. All of the mixed-metal films tested in this study
displayed an onset of catalysis at ca. À0.4 V, and thus the
catalytic activities of the alloys were benchmarked at
a potential of À0.7 V. The contributions from the substrate
[22]
are nominal and CO reduction is favored at this potential.
2
Electrocatalysis experiments at À0.7 V on the pure
metallic copper coatings produced hydrogen almost exclu-
sively (> 95%), while zinc produced a small fraction of CO
Figure 2. a) SEM image and b)–d) energy-dispersive X-ray (EDX) spec-
troscopic elemental mapping of a film Cu0.33Zn0.33Sn0.33 on titanium.
(ca. 20%) at a modest current density. Tin yields high
selectivity of HCOOH at this potential but at a low j
of
HCOOH
À2
Elemental mapping of the electrochemically reduced films
were measured by energy-dispersive X-ray (EDX) spectros-
copy to confirm a uniform distribution of copper, zinc and tin
across the films (Figures 2b–d).
0.5 mAcm . The screening of the ternary films, summarized
in Figure 3, clearly suggest a synergistic effect between the
metals because the activities of the alloys are distinctively
higher than those of the pure metal electrodes. We gleaned
the following structure–property relationships from this
catalyst screening:
The alloying for each of the ternary films was confirmed
by benchmarking the powder X-ray diffraction (XRD)
diffractograms against the pure metallic films and known
alloys. The diffractograms for films of Cu, Cu Zn Sn ,
1) Tin appears to suppress hydrogen production in favor of
CO or HCOOH production (the jtotal and FEH2 of ternary
0
.8
0.1
0.1
2
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firmed that the metal ratios and crystalline structures of the
alloys were similar to those measured for the samples
deposited on the titanium supports (Supporting Information,
Figure S5).
The Cu Zn Sn samples were found to exhibit much
x
y
1ÀxÀy
larger current densities on the carbon cloth (Figure 4). The
Cu Zn Sn sample, for example, produced a current den-
0
.2
0.4
0.4
À2
À2
sity at À0.7 V vs. RHE of 13 mAcm (compare 3.4 mAcm
on Ti). The FE also increased to 86% from the 45%
CO
measured on the planar support at À0.4 V vs. RHE. This
observation cannot be explained exclusively by the enhanced
surface area given that the jH2 was nearly constant and only
the jCO increased. The current density measured for the
Cu Sn sample also increased 4-fold when deposited on the
0
.2
0.8
carbon cloth support, with a FEHCOOH evolution of about 85%
at À0.35 V vs. RHE.
Figure 3. a) Hydrogen evolution Faradaic efficiency (FEH2 ); b) total
current density (jtotal), c) CO evolution Faradaic efficiency (FECO),
d) HCOOH evolution faradaic efficiency (FEHCOOH), e) CO evolution
partial current density (jCO), and f) HCOOH evolution partial current
density (jHCOOH) for films of Cu Zn Sn (where x=0, 0.2, 0.4, 0.6,
x
y
1ÀxÀy
À1
0
.8, and 1, and y=0, 0.2, 0.4, 0.6, 0.8, and 1; where 0ꢀx+yꢀ1).
Figure 4. a) Cyclic voltammograms (scan rate=100 mVs ) recorded
on Cu0.2Zn0.4Sn0.4 deposited on titanium and carbon cloth support.
b) The FECO measured for the Cu0.2Zn0.4Sn0.4 samples on the two
supports. c) Cyclic voltammograms recorded on Cu0.2Sn0.8 deposited
on Ti and carbon cloth. d) The jtotal and FEHCOOH value for Cu0.2Sn0.8
subjected to electrolysis at À0.35 V for 4 h.
alloys decrease monotonously with higher tin concentra-
tions);
) Ternary alloys containing < 20% tin did not produce
2
formate, and the H :CO ratio of products ranged from 5–
2
7
0% (a range that meets the requirements of most
In conclusion, we have reported the systematic screening
of Cu-Zn-Sn ternary alloys widely known as bronze and brass
for CO2 electrocatalysis. The electrochemical testing of
a range of compositions for these samples provides clear
regions of higher CO and HCOOH evolution activities, and
the Cu Zn Sn and Cu Sn samples stand out as partic-
[
31]
industrially relevant uses for syngas );
3
) Ternary alloys with at least 20% of each metal favors CO
evolution (Figure 3c), and a higher relative concentration
of Zn to Cu is favorable. The champion CO2 to CO
catalyst measured here was Cu Zn Sn0.4;
0
.2
0.4
0.2
0.4
0.4
0.2
0.8
4
5
) Binary alloys Sn-M (where M = Cu or Zn) showed inferior
ularly efficient CO to CO and HCOOH electrocatalysts. The
2
CO -to-CO activities relative to ternary samples contain-
partial current densities for these complex mixtures reach
current densities higher than those of the constituent metals.
Combining the catalysts with high-surface-area supports
2
ing the same amounts of tin;
) Binary alloys with higher quantities of tin favor HCOOH
formation
reduced the CO reduction overpotential by about 400 mV
2
compared with that measured on planar supports. This
method revealed a means of identifying new candidate
materials to be considered for study in electrolyzer flow
cells.
The catalysts were then tested on a carbon cloth support
to facilitate higher current densities. Approximately
À2
0
.02 mmolcm of catalyst was coated on the carbon cloth
by drop-casting. Electrochemical surface area testing showed
that the capacitance on carbon cloth was 50–100 times larger
than that measured on the titanium substrate (Supporting
Information, Figure S4). XRF and XRD measurements con-
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Version of record online: && &&, &&&&
4
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Angewandte
Communications
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Communications
Electrocatalysis
Is bronze better than gold? Alloys of Cu-
Zn-Sn are found to be effective for
J. He, K. E. Dettelbach, A. Huang,
mediating the reduction of CO to CO
2
C. P. Berlinguette*
&&&& — &&&&
and formate.
Brass and Bronze as Effective CO2
Reduction Electrocatalysts
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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