Copper-Gold Interactions Enhancing Formate Production from Electrochemical CO2 Reduction

Article abstract of DOI:10.1021/acscatal.9b03158

Strong interactions between two different types of metal nanoparticles can dramatically change their electrocatalytic properties but are underexplored. Herein we show that interactions with Au can turn Cu, which by itself is neither selective nor active for electrochemical CO₂ reduction to formate, into a much improved electrocatalyst for the same reaction. Our Cu/Au catalyst produces formate in a significant yield at -0.4 V vs the reversible hydrogen electrode in a near-neutral electrolyte and achieves a partial current density of 10.4 mA cm^-2 and a Faradaic efficiency of 81% at -0.6 V, which is 15 times more active and 4 times more selective than the control Cu catalyst derived in the same way but without Au. Electrochemical and spectroscopic studies reveal that the metal-metal interactions in the Cu/Au catalyst lead to the disappearance of Au's characteristic electrocatalytic activity for reducing CO₂ and for oxidizing CO and to the stabilization of Cu¹⁺ species on the Cu surface at CO₂ reduction potentials. Enhanced formate production from CO₂ electroreduction is now unlocked for the Cu-Au bimetallic system, implicating vast possibilities to improve electrocatalytic reactivity utilizing metal-metal interactions.

Full text of DOI:10.1021/acscatal.9b03158

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Article
Copper-Gold Interactions Enhancing Formate
Production from Electrochemical CO2 Reduction
Zixu Tao, Zishan Wu, Xiaolei Yuan, Yueshen Wu, and Hailiang Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b03158 • Publication Date (Web): 21 Oct 2019
Downloaded from pubs.acs.org on October 22, 2019
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Copper-Gold Interactions Enhancing Formate Production from Electrochemical CO2 Reduction
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Zixu Tao1,2, Zishan Wu1,2, Xiaolei Yuan1,2,3, Yueshen Wu1,2, Hailiang Wang*1,2
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1Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
2Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States
3School of Chemistry and Chemical Engineer, Nantong University, Nantong, Jiangsu, 226019, P. R.
China
Abstract
Strong interactions between two different types of metal nanoparticles can dramatically change their
electrocatalytic properties but are underexplored. Herein we show that interactions with Au can turn
Cu, which by itself is neither selective nor active for electrochemical CO2 reduction to formate, into a
much improved electrocatalyst for the same reaction. Our Cu/Au catalyst produces formate in a
significant yield at -0.4 V vs. the reversible hydrogen electrode in a near-neutral electrolyte and achieves
a partial current density of 10.4 mA cm-2 and a Faradaic efficiency of 81% at -0.6 V, which is 15 times
more active and 4 times more selective than the control Cu catalyst derived in the same way but without
Au. Electrochemical and spectroscopic studies reveal that the metal-metal interactions in the Cu/Au
catalyst lead to the disappearance of Au’s characteristic electrocatalytic activity for reducing CO2 and
for oxidizing CO and to the stabilization of Cu1+ species on the Cu surface at CO2 reduction potentials.
Enhanced formate production from CO2 electroreduction is now unlocked for the Cu-Au bimetallic
system, implicating vast possibilities to improve electrocatalytic reactivity utilizing metal-metal
interactions.
Keywords: CO2 reduction, electrocatalysis, metal-metal interactions, copper-gold, formate-
selective
*Corresponding author: Hailiang Wang. Email address: hailiang.wang@yale.edu
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Artificial carbon fixation is a sustainable way to produce organic chemicals and to control the
atmospheric CO2 level, which could help alleviate the resource shortage and global warming issues1. In
principle, the process involves reducing CO2 with high-energy electrons from various sources, for
example, from oxidation of H2O or H2S as in the natural processes of photosynthesis and
chemosynthesis, or from H2 as in the chemical industry1. Electrochemical CO2 reduction, because of its
mild and controllable working conditions and its compatibility with renewable energy sources, is a
promising CO2 fixation technique2. Formate, as a 2-electron reduction product from CO2, is easily
accessible and valuable3. Typical formate-selective CO2 reduction electrocatalysts include some Group
14 and 15 metals, Pd and their alloys4-9. Sn, Pb and Bi are known for high formate selectivity (Faradaic
efficiency, FE > 90%; all selectivity values are referenced to FE unless otherwise noted) but often times
require potentials as negative as -1 V vs. the reversible hydrogen electrode (RHE; all potentials are
referenced to the RHE unless otherwise noted)10-16; Pd can reduce CO2 to formate with nearly zero
overpotential, though the stability is seriously harmed by CO poisoning9.
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Among all the electrocatalysts for CO2 reduction, Cu metal is perhaps the only one that can reduce
CO2 to many products including CO, formate, CH4, C2H4 and alcohols across a wide potential range.
However, Cu itself is not selective to any of these products, and the overall CO2 conversion efficiency
is often compromised by the hydrogen evolution reaction (HER). For example, a polycrystalline Cu
mesh catalyzes CO2 reduction to CH4, C2H4, CO and some liquid products with a total FE < 70% at -
0.96 V17; oxide-derived Cu can generate 33% formate and 40% CO at -0.55 V 18. Various approaches
have been taken to improve the selectivity and activity toward a single product19-24. For example, S-
modified Cu nanoparticles achieved 80% formate at -0.8 V20; Cu foil treated with O2 plasma produced
60% ethylene at -0.9 V24; Oxide-derived Cu nanowires on Cu mesh were 61.8% selective for producing
CO at -0.4 V25.
Metal-metal interactions play important roles in electrocatalysis. For example, the HER activity of
Pd-based bimetallic systems was enhanced by optimizing the H adsorption energy via the interactions
between the Pd overlayer and the metal substrate26. Research on Pt3M (M=Ni, Co, Fe, Ti, V)
demonstrated the effects of alloying on surface adsorbates and electrocatalytic O2 reduction27. More
recently, bimetallic electrocatalyst systems have been studied for CO2 reduction28-31. A study of Au-
coated Cu foil revealed that CO spillover from Au to Cu surface activates CO2 reduction to ethanol and
*Corresponding author: Hailiang Wang. Email address: hailiang.wang@yale.edu
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n-propanol at lower overpotentials21. A CuSn3 material exhibited high formate selectivity (95%) at -0.5
V vs. RHE32. Our previous studies also demonstrated metal-metal interactions tuning CO2 reduction
catalysis: Pd on polycrystalline Cu induced continuous surface reconstruction and stabilized the
CH4/C2H4 production by Cu17; a single Pb-Pd electrocatalyst showed bifunctional activity for CO2-
formate interconversion due to reversible reconstruction under controlled electrochemical conditions33.
Despite these progresses, it remains interesting and challenging to discover new metal-metal
interactions leading to more desirable catalytic properties.
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Herein, we report a new phenomenon of metal-metal interactions between Cu and Au, where the
Au acts as a promoter to the Cu catalyzing electrochemical CO2 reduction to formate (Figure 1). Our
Cu/Au catalyst derived from a physical mixture of CuO and Au nanoparticles is able to produce formate
in a significant yield at -0.4 V vs. RHE. Compared to CuO-derived Cu, it shows 15 times enhanced
formate production, with the partial current density increased from 0.67 to 10.4 mA cm-2 and the FE
elevated to over 80% at -0.6 V. This represents the most active CO2-to-formate electrocatalyst with Cu
as the primary component. The chemical states and properties of the surface Au and Cu sites are
substantially modified as a result of the metal-metal interactions. The originally CO-selective Au
nanoparticles are fully deactivated from producing CO; their characteristic electrochemical CO
oxidation behavior is also absent. In addition, the interactions render more Cu1+ sites under the CO2
reduction conditions, which may be related to the higher selectivity and activity for formate production.
Figure 1. Schematic illustration of CO2 reduction on the Cu/Au catalyst. The metal-metal interactions
promote formate production on Cu and deactivate CO formation on Au.
The CuO and Au nanoparticles were synthesized separately following previously published
*Corresponding author: Hailiang Wang. Email address: hailiang.wang@yale.edu
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methods with slight modifications (see Supporting Information, SI)34-35. Their X-ray diffraction (XRD)
patterns (Figure S1a) match well with CuO (ICSD # 69758) and Au (ICSD # 44362), respectively. The
CuO and Au nanoparticles have average sizes of 8 nm and 7 nm, respectively, as confirmed by the
broadened diffraction peaks and the electron microscopy images (Figure S1b, c). Loaded on carbon
fiber paper and tested in CO2-saturated 0.5 M KHCO3 aqueous electrolyte, the Au nanoparticles show
high selectivity (> 80% FE) for CO between -0.5 V and -0.7 V (Figure S2a, b), which agrees with
previously published results on Au nanoparticle electrocatalysts36. The CuO nanoparticles, which were
electrochemically reduced to Cu metal prior to the CO2 reduction measurement18, exhibit potential-
dependent selectivity changes characteristic for Cu (Figure 2a)37. Between -0.4 V and -0.6 V, H2 and
CO account for nearly all the reduction current. As the working electrode potential is switched stepwise
to the more negative direction, C2H4 and CH4 emerge in sequence. Liquid products are generated
between -0.6 V and -1.0 V, as implied by the < 100% total FE of all the gas products. More than 40%
of the electrons go to the HER across the entire potential range.
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The Cu/Au catalyst was prepared by loading a mixture of the CuO and Au nanoparticles (8:1 mass
ratio) on a carbon fiber paper electrode followed by an electrochemical pre-reduction. Compared to the
Cu catalyst, this catalyst shows comparable partial current densities (Figure S3) but much lower FEs
(Figure 2b) for H2 and CO in the potential range between -0.4 V and -0.8 V. At -0.6 V, FE (H2) and FE
(CO) are 21.8% and 3.5%, respectively, indicating significant conversion of CO2 to liquid product(s).
To accumulate and quantify the liquid product(s), CO2 reduction electrolysis was performed at -0.6 V.
Formate was detected as the only product in the electrolyte (Figure S4). The Cu/Au catalyst shows an
81% selectivity for formate with a partial current density of 10.4 mA cm-2. Compared to Cu, which
exhibits a FE (formate) of 19% and a partial current density of 0.67 mA cm-2 at -0.6 V, the Cu/Au
catalyst is 4 times higher in selectivity and 15 times higher in activity (i.e. current density) for reducing
CO2 to formate. The onset potential for formate production is at least 200 mV more positive (Figure 1a,
b). In terms of overpotential, selectivity and reaction rate, this Cu/Au catalyst is competitive among
formate-selective CO2 reduction electrocatalysts (Table S1). Among all catalysts with Cu as the primary
component, it is arguably the most active one reported to date. This is very different from previously
reported Cu-Au bimetallic alloy nanoparticles where CO2 reduction to CO was optimized while formate
production remained at <5% FE, implying totally different metal-metal interactions in these two
*Corresponding author: Hailiang Wang. Email address: hailiang.wang@yale.edu
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systems38. As the Cu/Au ratio is varied (with CuO/Au mixing ratios of 16:1 and 4:1, denoted as Cu/Au-
A and Cu/Au-B, respectively), the FE of formate and its partial current density both change but remain
much higher compared to those for the pure Cu catalyst (Figure 2c, d), supporting the positive effects
of the Cu-Au interactions. In a 4-hour long electrolysis at -0.6 V, the Cu catalyst only maintained a total
current density of 3.6 mA cm-2 with an averaged formate FE of 19% (Figure 2e). In contrast, 12 mA
cm-2 with 79% formate selectivity was measured for the Cu/Au catalyst (Figure 2f).
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Figure 2. CO2 reduction performance of the Cu/Au catalyst in CO2-saturated 0.5 M KHCO3. (a-b): Potential-
dependent gas-phase product distribution for (a) Cu and (b) Cu/Au. (c-d): (c) FE distribution and (d) partial
*Corresponding author: Hailiang Wang. Email address: hailiang.wang@yale.edu
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current densities of products for CO2 reduction catalyzed by Cu, Cu/Au, Cu/Au-A and Cu/Au-B at -0.6 V.
(e-f): 4-hour stability test for CO2 reduction catalyzed by (e) Cu and (f) Cu/Au at -0.6 V. The average formate
FEs were quantified after CO2 reduction electrolysis.
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To understand the origins of the unprecedented enhancement in electrocatalytic activity and
selectivity of Cu by Au for CO2 reduction to formate, we performed structural characterizations for the
Cu/Au catalyst after the pre-reduction step. XRD shows that it is a mixed phase of Cu and Au without
forming any Cu-Au alloy (Figure S5a). The catalyst manifests a morphology of coalesced particles with
Au nanoparticles distributed in the Cu matrix reduced from CuO (Figure S5b, d-f), which is different
from the original CuO and Au nanoparticles. The bulk Cu:Au atomic ratio was measured to be 95:5
(Figure S5c). X-ray photoelectron spectroscopy (XPS) depth profiling indicates that the two elements
co-exist on the surface (Figure S5g). Both the microstructure and the electrochemical surface areas of
the Cu/Au catalyst are similar to the Cu catalyst (Figure S6), implying that the CuO reduction process
is not altered by the presence of the Au nanoparticles.
Au is well known for its electrocatalytic activity for selective CO2 reduction to CO. However, in
our Cu/Au catalyst, the Au component does not manifest its intrinsic activity. We thereby propose that
the surface Au sites are deactivated because of their interactions with Cu. This hypothesis is further
supported by our electrochemical CO oxidation studies. Measured in 0.5 M aqueous KHCO3, the Au
nanoparticles clearly exhibit the characteristic activity of Au metal toward electrochemical oxidation of
CO at around 1.1 V in both the forward and reverse scans (Figure 3a)39. In contrast, this CO oxidation
behavior is absent for the Cu/Au catalyst despite the presence of the redox waves associated with Cu
(Figure 3b). The other Cu/Au catalysts with different Cu:Au ratios do not show CO oxidation activity
either (Figure S7), confirming that the deactivation of Au is caused by the presence of the Cu component
in the catalyst structure. Note that the Au nanoparticles were synthesized in the presence of
cetyltrimethylammonium bromide. The surface ligand may have played a role in the metal-metal
interactions40, which warrants further investigation.
*Corresponding author: Hailiang Wang. Email address: hailiang.wang@yale.edu
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Figure 3. Cyclic voltammograms of (a) Au nanoparticles and (b) the Cu/Au catalyst in 0.5 M KHCO3
saturated with N2 or CO.
Quasi-in-situ XPS41-43 was performed to study the surface structure of the Cu/Au catalyst. The surface
Au sites are in the Au0 state, as evidenced by the binding energy of the Au 4f electrons (Figure 4a). The
binding energy of the Cu 2p electrons and the absence of their satellite peaks demonstrate that the Cu2+ from
the original CuO nanoparticles has been fully reduced (Figure 4b). Given that Cu1+ and Cu0 cannot be
distinguished in the Cu 2p XPS spectrum (Figure S8), we acquired the Cu LMM Auger spectrum to analyze
the oxidation state. While the surface of the Cu catalyst is dominated by Cu0, the Cu/Au catalyst contains a
significant number of Cu1+ sites (Figure 4c). Further experiments show that the oxidation states of the Au
and Cu sites are retained after long-term electrolysis (Figure S9). These results suggest that the Cu-Au metal-
metal interactions stabilize Cu1+ on the surface under reducing potentials, which is likely related to the
enhanced performance for CO2 reduction18, 44
.
Figure 4. (a) Au 4f XPS spectra of the Cu/Au catalyst and Au nanoparticles. (b) Cu 2p XPS spectra and (c)
Cu LMM auger spectra of the Cu/Au and Cu catalysts.
*Corresponding author: Hailiang Wang. Email address: hailiang.wang@yale.edu
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In summary, we have developed a formate-selective CO2 reduction electrocatalyst based on an
intimate mixture of Cu and Au nanoparticles. The metal-metal interactions make the Cu 15 times more
active and 4 times more selective toward CO2 conversion to formate; the Au is deactivated for
converting CO2 to CO. These findings highlight the promise of utilizing inter-nanoparticle metal-metal
interactions to improve CO2 reduction electrocatalysis.
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Supporting Information
Experimental details and additional figures.
Acknowledgement
This work was supported by the National Science Foundation (Grant CHE-1651717)
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*Corresponding author: Hailiang Wang. Email address: hailiang.wang@yale.edu
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*Corresponding author: Hailiang Wang. Email address: hailiang.wang@yale.edu
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