Selective C-C Coupling in Carbon Dioxide Electroreduction via Efficient Spillover of Intermediates As Supported by Operando Raman Spectroscopy

Article abstract of DOI:10.1021/jacs.9b07415

Developing efficient systems for the conversion of carbon dioxide to valuable chemicals using solar power is critical for mitigating climate change and ascertaining the world's future supply of clean fuels. Here, we introduce a mesoscopic cathode consisting of Cu nanowires decorated with Ag islands, by the reduction of Ag-covered Cu₂O nanowires prepared via galvanic replacement reaction. This catalyst enables CO₂ reduction to ethylene and other C₂₊ products with a faradaic efficiency of 76%. Operando Raman spectroscopy reveals intermediate formation of CO at Ag sites which undergo subsequent spillover and hydrogenation on the Cu nanowires. Our Cu-Ag bimetallic design enables a 95% efficient spillover of intermediates from Ag to Cu, delivering an improved activity toward the formation of ethylene and other C₂₊ products. We also demonstrate a solar to ethylene conversion efficiency of 4.2% for the photoelectrochemical CO₂ reduction using water as electron and proton donor, and solar power together with perovskite photovoltaics to drive the uphill reaction.

Full text of DOI:10.1021/jacs.9b07415

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Article
Selective C-C Coupling in Carbon Dioxide Electroreduction via Efficient
Spillover of Intermediates as Supported by Operando Raman Spectroscopy
Jing Gao, Hong Zhang, Xueyi Guo, Jingshan Luo, Shaik M. Zakeeruddin, Dan Ren, and Michael Grätzel
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07415 • Publication Date (Web): 27 Oct 2019
Downloaded from pubs.acs.org on October 28, 2019
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Selective C-C Coupling in Carbon Dioxide Electroreduction via
Efficient Spillover of Intermediates as Supported by Operando
Raman Spectroscopy
a,b
b
a
b,c
b
Jing Gao , Hong Zhang , Xueyi Guo , Jingshan Luo , Shaik M. Zakeeruddin , Dan
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b,*
b,*
Ren , Michael Grätzel
a. School of Metallurgy and Environment, Central South University, Changsha, 410083
Hunan, China
b. Laboratory of Photonics and Interfaces, École Polytechnique Fédérale de Lausanne,
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015 Lausanne, Switzerland
c. Institute of Photoelectronic Thin Film Devices and Technology, College of Electronic
Information and Optical Engineering, Nankai University, Tianjin, China
*Correspondence should be addressed to D.R. and M.G.: dan.ren@epfl.ch,
michael.graetzel@epfl.ch
Abstract
Developing efficient systems for the conversion of carbon dioxide to valuable
chemicals using solar power is critical for mitigating climate change and ascertaining
the world’s future supply of clean fuels. Here, we introduce a mesoscopic cathode
consisting of Cu nanowires decorated with Ag islands, by the reduction of Ag-covered
Cu O nanowires prepared by galvanic replacement reaction. This catalyst enables CO
2
2
reduction to ethylene and other C2+ products with a faradaic efficiency of 76%.
Operando Raman spectroscopy reveals intermediate formation of CO at Ag sites which
undergo subsequent spillover and hydrogenation on the Cu nanowires. Our Cu-Ag
bimetallic design enables a ~95% effective spillover of intermediates from Ag to Cu,
delivering an improved activity towards the formation of ethylene and other C2+
products. We demonstrate a solar to chemical conversion efficiency of 4.2% for the
photoelectrochemical CO reduction to ethylene using water as electron and proton
2
donor, and solar power together with perovskite photovoltaics to drive the uphill
reaction.
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. INTRODUCTION
Various approaches have been proposed to alleviate the adverse effect caused by
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high concentration of atmospheric carbon dioxide (CO ) . One of the promising
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technologies is electrochemical reduction of CO using renewable energy, with the
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production of key chemical feedstocks such as ethylene (C H ) and fuels such as
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ethanol (C H OH) and n-propanol (n-C H OH) . Exploration of efficient and robust
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electrocatalysts for these challenging reactions is critical for developing future
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renewable energy sources and implementation of CO mitigation systems .
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Among a wide variety of the metals, copper (Cu) has the unique capability of
catalyzing the electrochemical reduction of CO to hydrocarbons and oxygenates, as
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discovered by Hori and co-workers in the 1980s . On a polycrystalline Cu electrode,
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formation of C species, e.g. carbon monoxide (CO), formate (HCOO ) and methane
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(CH ) dominates over that of C products including C H , C H OH and n-C H OH .
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Importantly, Cu electrocatalysts derived from Cu O or CuO exhibit enhanced
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selectivity towards C H and C H OH, while the formation of CH is suppressed on
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these catalysts . The improved C-C coupling on oxide-derived Cu was rationalized in
terms of an increased surface coverage by *CO (* denotes the adsorbed species)8-10
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The surface coverage of *CO can be further enhanced by introducing a second
metal (X), such as Au, Ag and Zn that selectively catalyzes the CO reduction to CO11-17.
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For example, Cu-Ag nanocoral-structured catalysts show high activity with respect to
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the formation of C hydrocarbons and oxygenates . However, previously reported
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bimetallic catalysts show unsatisfied selectivity, leading to the undesirable formation
of CH or CO. How to achieve optimal synergy between the Cu and X, enabling
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efficient C-C coupling, is yet to be investigated. Moreover, there is no spectroscopic
evidence on the synergistic effect in these bimetallic systems.
Here we uniformly disperse Ag nanoparticles on the surface of a Cu O nanowire
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(NWs) scaffold via a galvanic replacement reaction (GRR) between Ag and Cu O.
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This specific mesoscopic architecture affords an optimal corporation between Cu and
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Ag, enabling large improvements of the faradaic efficiency and current density for
ethylene formation. Ethylene is one of the most attractive CO reduction products. It
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plays a key role as a feedstock for the production of polymers and high value-added
chemicals and has the advantage over alcohols to spontaneously emerge as a gas from
the electrolyte, thus avoiding costly separation procedures.
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The improved performance of our mesoscopic catalyst is believed to be caused
by the efficient CO spillover from Ag to the Cu nanowire support. Operando Raman
spectroscopy reveals the nature of the key intermediates and their binding modes to the
two catalysts, providing detailed mechanistic insight into the mode of operation of our
bimetallic catalyst. Using perovskite solar cells together with sunlight as power source
to drive the CO reduction to ethylene on our selective CuAg cathode and water
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oxidation on the anode, we achieve a high solar-to-ethylene efficiency under air mass
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.5G illumination.
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2
. EXPERIMENTAL SECTION
.1 General
Deionized water (18.2 MΩ cm, Purelab Ultra, ELGA) was used for washing and
the preparation of solutions. All the chemicals were used without purification unless
otherwise stated. The reference electrode used in this work was Ag/AgCl (saturated
KCl, Pine), which was calibrated regularly against a reversible hydrogen electrode
(RHE, Gasketal). All the potentials cited in this work were re-scaled against RHE unless
otherwise stated.
2
.2 Preparation of catalysts and solar cells
Preparation of Cu O FTO glass substrates (TEC-15 Ω) were etched with zinc
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powder (99.9%, Sigma Aldrich) and 10% HCl solution (37%, ACS reagent, Merck) to
remove the conductive coating, followed by ultrasonic cleaning in deionized water for
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5 min and drying under compressed air. 1.5 μm-thick Cu (99.995%) was coated by a
DP650 sputter (Alliance-Concept). The sputtered Cu film was then anodized at a
-2
constant geometric current density of 8 mA cm in 3 M KOH using a two-electrode
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configuration until reaching a voltage threshold of 2.1 V (Interface 1000, Gamry). The
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resulting substrates were annealed at 600 C for 4 h in Ar atmosphere (99.9999%,
Carbagas) in a tube furnace (Lenton) to form Cu O. The temperature ramp for heating
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and cooling down was 10 C min . Cu O samples were then cut into slices with size of
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Preparation of Ag-Cu O NWs and Au-Cu O NWs HNO (65%, Sigma Aldrich)
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and AgNO (99.995% mental basis, ABCR) were dissolved sequentially in DI water,
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resulting in concentrations of 20 mM and 10 mM respectively. The prepared glass slices
with Cu O NWs were soaked into above solutions for 1 min, 3 min, 5 min and 8 min to
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form Ag-Cu O. The preparation of Au-Cu O is the same as the one of Ag-Cu O, except
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that aqueous 0.2 mM HAuCl (99.99% metal basis, ABCR) solution was used instead.
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Fabrication of perovskite solar cells Firstly, FTO substrates were cleaned via
ultraviolet ozone treatment for 20 min. 40 nm TiO compact layer was then deposited
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on FTO via spray pyrolysis at 450 °C from a precursor solution of titanium
diisopropoxide bis(acetylacetonate) in anhydrous ethanol. A mesoporous TiO layer of
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50-200 nm thick was deposited on compact TiO layer via spin coating of a 30 nm
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particle paste (Greatcell, 30 NR-D, 1:6 v/v in ethanol) at 4000 rpm for 20 s, followed
by drying at 80 °C for 10 min and sintering at 450 °C for 30 min. The resulting film
was treated with bis(trifluoromethane) sulfonamide lithium (LiTFSI) salt and sintered
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at 450 C again for 30 min . After cooling down to 150 °C, the substrates were
immediately transferred in a nitrogen atmosphere glove box for the deposition of
perovskite films. The perovskite films were prepared by anti-solvent method as
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described in our previous work . Briefly, a 1.4 M triple cation perovskite
Cs0.05(MA0.10FA Pb(I0.90Br ) precursor solution in the mixed solvent of DMF :
)
0.90 0.95
0.10 3
DMSO (4 : 1, v/v) was deposited by two consecutive spin-coating steps at 1000 rpm
and 6000 rpm for 10 s and 30 s, respectively. In the last 15 s of the second spin coating
treatment, 200 µL of chlorobenzene was dropped onto the substrate. The films were
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then annealed at 100 C for 60 min in a nitrogen-atmosphere glove box, followed by
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passivation with n-butylammonium bromide. After the perovskite deposition, a spiro-
OMeTAD solution (70 mM in chlorobenzene) doped with 20.68 µl LiTFSI (520 mg in
acetonitrile) and 35.5 µl 4-tert-butylpyridine was spin coated at 4000 rpm for 20 s.
Finally, gold top electrode with thickness of 70-80 nm was thermally evaporated under
high vacuum through a shadow mask as the top anode, which defined the device area
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as 0.25 cm . The devices were encapsulated before characterization.
2.3 Material characterization
X-ray diffraction and photoelectron spectroscopy X-ray diffraction (XRD) was
performed via Bragg-Brentano Geometry with a Lynexeye detector and a
monochromated Cu Kα radiation source. X-ray photoelectron spectroscopy was
performed on PHI VersaProbe, with an Mg Kα source. The spectra were calibrated
using C 1s peak at 284.8 eV and fitted using mixed Gaussian-Lorentzian function with
XPSPEAK software.
Electron microscopy The surface and cross-section of the catalysts were
characterized using a scanning electron microscope (SEM, Zeiss Merlin) with an
accelerating voltage of 5 kV and a probing current of 70 pA. Transmission electron
microscopy (TEM) was performed on Talos (FEI), equipped with a high-angle angular
dark field (HAADF) detector. The samples for TEM were prepared as follows. The
films were scrapped off with a sharp blade and suspended in ethanol (Fisher Chemical).
The resulting suspension was drop casted onto Au grid coated with lacey carbon (300
mesh, Ted Pella, Inc) and dried naturally.
Operando Raman spectroscopy Raman spectroscopy was carried out in a custom-
built Teflon cell during CO reduction in 0.1 M KHCO using a Raman spectrometer
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(Reinshaw). A near infrared laser (785 nm) was used as the excitation source. A water
immersion objective lens (Leica, 63×) was used for focusing and collecting the incident
and scattered laser light. The electrochemical CO reduction was performed at different
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potentials from -0.5 to -1.0 V on Cu and CuAg catalysts. Multiple spectra were
collected after the reduction of oxides and one representative spectrum was shown for
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each potential.
Double layer capacitance and under potential deposition The electrochemically
active surface areas (ECSA) of the catalysts were estimated by double layer capacitance
measurements. Cyclic voltammetry measurements were carried out at a non-faradaic
potential range from -0.114 V to -0.014 V vs. RHE at scan rates of 20, 40, 60, 80, 100,
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120 and 150 mV s . The non-faradaic current densities were plotted against scan rate
and the slope was determined as the capacitance value. The roughness factors were
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, with 30 uF cm was used as the general
dl specific
Cspecific20 Lead underpotential deposition (PbUPD) was also used to determine the number
.
of Cu sites. Cyclic voltammetry measurements were carried out from 0 V to -0.5 V vs.
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Ag/AgCl at a scan rate of 10 mV s . A Pt mesh was used as the counter electrode. The
electrolyte was prepared by dissolving NaOH and Pb(OAc) in water with the
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concentration of 100 mM and 1 mM, respectively, and was saturated with Ar before
usage.
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.4 Electrochemical reduction of CO2
The electrochemical CO reduction was performed in a custom-built two-
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compartment PEEK cell. An anion-exchange membrane (Fumasep FKS-50, Fumatech)
was used to separate the cathodic and anodic compartments. Electrodeposited IrO221 on
Ti foil was used as the counter electrode. CO (99.999%, Carbagas) saturated aqueous
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.1 M KHCO (99.99% metal basis, Sigma Aldrich) was used as the electrolyte. The
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volume of the catholyte and anolyte were 8 and 6 mL, respectively. CO was
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continuously infused into both compartments at a rate of 10 mL min (Bronkhorst
High-Tech, F201CV) during the chronoamperometric measurements. The gas products
of CO reduction were monitored by an online gas chromatography (GC, Trace ULRTA,
2
Thermo) periodically. A micropacked shincarbon column (Restek) was used to separate
the products, which were analyzed by a pulse discharge detector (PDD, Vici). Certified
gas standards with all relevant gases (H , CO, CH , C H and C H ) in CO matrix
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products dissolved, was collected after electrolysis and analyzed on a H nuclear
magnetic resonance spectrometer (NMR, Avance 500, Bruker). A pre-saturation
technique was used to suppress the signal of water. The area ratios of the products to
DMSO or phenol internal standard were compared to the standard calibration curves to
quantify the concentration of different products.
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2.5 Solar-driven electrochemical reduction of CO2
A certified silicon solar cell (Newport) was used to calibrate the light intensity of
Oriel LCS-100 Class ABB solar simulator (Newport). Three perovskite solar cells were
connected in series and the total effective illuminated area of unmasked solar cell was
-2
0
.75 cm . J-V curves of the solar cell was scan from 0 to 3.5 V at a scan rate of 5 mV
-1
s . Solar driven CO reduction was carried out in the same H-cell described above.
2
Electrodeposited IrO anode and CuAg cathode were placed in the same compartment
2
and the distance between two electrodes was kept at ca. 4 mm. Anion exchange
membrane was excluded in this system to reduce the overall resistance. 0.2 M KHCO3
was used as the electrolyte to minimize the iR drop and the volume of the electrolyte
2
was 9 mL. The surface area of CuAg was adjusted to ~0.085 cm to match the current
supplied by the photovoltaics. CuAg was pre-reduced until reaching a steady potential
before testing. J-V characteristic of the electrolyzer was measured via linear sweep
-
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voltammetry from 2 to 3.5 V at a scan rate of 5 mV s .
3
. RESULTS AND DISCUSSIONS
3.1 Preparation and characterization of Cu O and Cu O-Ag
2
2
Our method for preparing Cu O NWs and their decoration with Ag-islands, noted
2
as Cu O-Ag, is illustrated schematically in Figure 1a. Cu O NWs were synthesized
2
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from a 1.5-µm-thick Cu film, which was firstly anodized in 3 M KOH to form Cu(OH)2
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NWs (Figure S1), followed by annealing at 600 C for 4 h in Ar atmosphere . The as-
prepared Cu O showed µm-long nanowires consisting of smaller smooth particles joint
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by grain boundaries and Cu O (111) facets were identified with a lattice spacing of
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.241 nm (Figure 1b-c). Cu O-Ag was prepared by soaking Cu O into aqueous AgNO
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+
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solution for 1-8 min. Since the Cu /Cu redox couple has a lower standard reduction
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potential (0.15 V vs. standard hydrogen electrode, SHE) as compared with Ag /Ag
(0.80 V vs. SHE), the following galvanic replacement reaction (GRR) will take place
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spontaneously in acidic conditions :
+
+
2+
Cu O + 2Ag + 2H → 2Cu + 2Ag + H O
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Figure 1. Structural and chemical characterization of Cu
2
2 2
O and Ag-decorated Cu O (Cu O-Ag). (a) The
schematic route for the preparation of Cu
Cu O (insert shows low magnification TEM image); (d) SEM image, (e) HR-TEM images, (f) TEM-
HAADF image and (g) respective STEM-EDX mapping of Cu O-Ag; (h) XRD patterns of Cu O and
Cu O-Ag; (i) high-resolution Cu 2p and Ag 3d XPS spectra of Cu O and Cu O-Ag. GRR in (a) refers to
2 2
O and Cu O-Ag NWs; (b) SEM and (c) HR-TEM images of
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2
2
2
2
2
galvanic replacement reaction. Scale bars: 1 μm for (b) and (d), 5 nm for (c) and (e), 500 nm for the
insert in (c), 200 nm for (f) and (g). The indexes of standard XRD in (h) are taken from ICDD with the
PDF number: 04-016-6875 for Cu O, 04-003-5318 for Cu and 04-016-1389 for Ag.
2
After 5-min GRR in 10 mM AgNO , Ag nanoparticles (NPs) with a size of 100 -
3
2
00 nm were evenly covered on the surface of Cu O NWs (Figure 1d). Cu O (111) and
2 2
Ag (200) atomic layers were identified in the HRTEM image with lattice spacing of
.242 and 0.205 nm, respectively (Figure 1e). EDS (STEM mode) mapping of Cu O-
0
2
Ag clearly showed the dispersion of Ag NPs on the surface of Cu O NW (Figure 1f-g,
2
Figure S2).
The X-ray diffractograms of Cu O and Cu O-Ag showed peaks belonging to the
2
2
diffractions from Cu O and the Cu substrate (Figure 1h). The peaks at 38.3° and 44.4°
2
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on Cu O-Ag could be assigned to Ag (111) and Ag (200), respectively. XPS peaks at
2
2
4
~
374.5 and ~368.5 eV also confirmed the presence of Ag on Cu O-Ag (Figure 1i).
2
The occurrence of weak satellite features at ~942.9 and ~962.5 eV indicated the
2
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presence of small levels of CuO . There is no observable shift in 2p3/2 and 2p1/2
spectra of Cu in CuAg catalyst.
Cu O NWs with and without Ag loading were electrochemically pre-reduced to
2
form metallic Cu and CuAg, as demonstrated by the operando Raman spectra (Figure
S3). Interestingly, both metallic surfaces were gradually re-oxidized upon the removal
of cathodic bias, indicating the quick oxidation of the electrode surface inside the
electrolyte. This is consistent with the previous reports on other oxide-derived Cu
26
catalysts . After pre-reduction, Cu was ~1.2× rougher than CuAg as measured by
double layer capacitance and the total number of Cu sites were reduced on CuAg
(Figure S4-S5 and Table S1).
Cu and CuAg were also characterized after CO reduction reaction (Figure S6-
2
S7). The surface of Cu grains appeared rough with the coverage of small nanoparticles,
which could be attributed to the reduction of Cu O to Cu, as further confirmed by the
2
disappearance of Cu O peaks in XRD. Moreover, CuAg is likely to be phase-separated
2
after reduction, as proved by SEM and XRD (Figure S6). XPS measurement showed
that the atomic ratio of Cu/Ag increased from 0.83 to 2.18 after 60-min electrolysis,
similar observation was also reported in other bimetallic systems15, 17
.
3
.2 Improved activity and selectivity for CO reduction to C products on CuAg
2 2+
catalyst
The activity of Cu and CuAg towards CO reduction was assessed by
2
potentiostatic measurements for 60 minutes at the selected potentials from -0.60 to -
2
2
1
.20 V in a custom-built electrochemical peek cell (Figure S8-S9) . Aqueous CO -
2
saturated 0.1 M KHCO was used as the electrolyte. Gaseous products were quantified
3
during electrolysis using gas chromatography (GC) and liquid products were analyzed
1
1
27
after electrolysis using H nuclear magnetic resonance ( H-NMR, Table S2) .
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The geometric current densities of Cu and CuAg remained stable over the duration
of the 60-min electrolysis. At potentials between -1.00 and -1.20 V, the jtotal of CuAg
was 12 to 37% higher than that of Cu. Interestingly, the partial current densities for CO2
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reduction (jCO2R) on our CuAg (Figure 2a, Table S3-S4) exceeded by up to 70 % of the
jCO R on Cu at potentials < -1.00 V. The decrease of jCO R at -1.20 V is attributed to
2
2
the increased local pH at high cathodic current, which leads to the decline of the amount
of dissolved CO near the surface of electrode 10, 28.
2
Figure 2. Electrocatalytic carbon dioxide reduction performance on Cu and CuAg catalysts. (a) Total
current densities (jtotal) and partial current densities of CO
2
reduction (jCO R); (b) Partial current densities
2
of ethylene (jC H ); (c) Faradaic efficiencies of ethylene (FEC H ), ethanol (FEC H OH) and other C2+
2
4
2
4
2
5
products, (d) Faradaic efficiencies of methane (FECH4) and ethylene, and the faradaic efficiency ratio of
ethylene to methane (FEC H /FECH4); (e) Faradaic efficiency of ethylene and the total current densities
2
4
over 12 h electrolysis as a function of applied potential on Cu and CuAg catalysts. Each data point in (a)-
(d) corresponds to the average of three independent measurements and the error bars in (c) and (d)
represent the standard deviations of these measurements.
A striking improvement of activity towards ethylene was observed on CuAg, as
demonstrated by the enhancement of partial current density (jC2H4) at potentials between
-
1.00 and -1.20 V (Figure 2b). The jC2H4 reached a maximum value of -18.07 mA cm-
2
-2
at -1.05 V on our CuAg catalyst, while on Cu it was -8.53 mA cm at the same
potential, ranking our CuAg catalyst as one of the best catalysts for the conversion of
CO to ethylene in the reported literatures (Table S7).
2
The intrinsic activity reflected by the specific current density of ethylene
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(
normalized against the electrochemical active surface area) is also improved on CuAg
at potentials < -1.00 V (Figure S10). The larger specific current density of hydrogen
observed on CuAg indicates that hydrogen evolution is not suppressed at potentials <-
1.00 V, which is likely due to the additional HER activity on Ag sites. This result
demonstrates that our CuAg is distinct from previously reported CuAg alloy 15, 29-30.
The augmentation of ethylene formation on CuAg may be mainly attributed to the
action of Ag as a co-catalyst for the production of CO, as demonstrated by our control
experiment on pure Ag nanocorals (Figure S11). This local free CO near the Cu is
believed to be the key intermediate for the formation of hydrocarbons31-33. Moreover,
the mesoscopic structure of Cu also plays a critical role in the improvement of C-C
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coupling selectivity by elongating the residence time of CO, which presents an
advantage over a planar CuAg structure (Figure S12-13)34.
Our Ag decorated Cu nanowires also exhibit a much higher faradaic efficiency
for the formation of ethylene (FEC2H4) compared to the Ag-free reference (Figure 2c).
With the bare Cu nanowire cathode, a maximum FEC2H4 of 33% was achieved on Cu
at -1.00 V and the total FE of C products approached 58%, which is consistent with
2
+
previous reports using different types of oxide-derived Cu catalysts7, 26, 35-36. The
maximum FEC2H4 was greatly improved to 52% on CuAg at -1.05 V, with an
impressive total FE of ~76% for all C products. Interestingly, the faradaic efficiencies
2
+
3
7
of ethanol on CuAg and Cu catalysts do not show much difference . This is likely due
to the lack of Cu-Ag phase boundaries, which are believed to be critical in improving
1
2
the ethanol formation .
It is noted that the onset potential of methane production shifted from -0.95 V on
Cu to -1.10 V on CuAg and the faradaic efficiency of methane (FECH4) on CuAg was
much lower compared with that on Cu (Figure 2d). For example, FECH4 decreased
from 5.47% on Cu to 1.67% on CuAg at -1.10 V. The combined effects of boosting
C H while suppressing CH formation resulted in a large enhancement of the
2
4
4
selectivity for ethylene expressed by FEC2H4/FECH4 on our CuAg catalyst.
1
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Both Cu and CuAg showed good stability with continuous production of ethylene
over 12 h electrolysis (Figure 2e). The FEC2H4 on CuAg decreased from ~50% to ~44%
after 12 h. The robustness of CuAg was confirmed by SEM after stability test (Figure
S7), which showed similar morphology with the CuAg catalyst after 1 h electrolysis.
0
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3
.3 Operando Raman spectroscopy of Cu and CuAg catalysts in CO reduction
2
Operando Raman spectroscopy was carried out at different potentials after the
reduction of oxides to reveal the reaction intermediates during CO reduction, thus
2
gaining mechanistic insight on the catalytic differences between Cu and CuAg. A
custom-built Raman setup was used to probe the surface of Cu and CuAg during CO2
reduction at different potentials in 0.1 M KHCO3.
Figure 3. Operando Raman spectroscopy of Cu and CuAg catalysts. Operando Raman spectra of (a) Cu
and (b) CuAg during electrochemical reduction of CO
2 3
at different potentials in 0.1 M KHCO . The
-1
spectrum was collected at three different regions: low-frequency 200-700 cm , medium-frequency 1700-
-1
-1
2
300 cm and 2500-3500 cm . CO
2
was continuously flowed to the electrolyte. A near infrared laser
(785 nm) was used as the excitation source. All the spectra were collected after sufficient time of
electrolysis to ensure the reduction of oxides. Each spectrum is the representative of multiple
measurements at different time scale of electrolysis.
The presence of adsorbed CO on Cu was demonstrated by the appearance of
-1
Raman peaks located at 282, 355-366, and 2057-2092 cm , which correspond to the
restricted rotation of adsorbed *CO on Cu, Cu-CO stretching and C≡O stretching,
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respectively (Figure 3a) . These peaks were also observable on CuAg catalyst,
showing the vibration of adsorbed CO on Cu sites (Figure 3b). Besides, more peaks at
-1
frequencies of 460, 491-494 and 2011 cm appeared. The new peaks are likely to
originate from adsorbed CO on Ag surface, as supported by our control experiment on
rough Ag surface (Figure S14)38-39. Our observation is significantly different from
previous-reported CuZn bimetallic catalyst which showed no detectable adsorbed CO
1
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1
on Zn site . The reason could be the slightly stronger binding of CO onto Ag as
4
0
compared with Zn . It is also noted that the multiple binding sites of CO (peaks at
-
1
2011, 2061 and 2084 cm ) are not solely from Cu but also from Ag sites, which is
4
1
different in nature as compared with recently reported CuAg catalyst .
-1
The vibration of C-H was also observed at region from 2700 to 3000 cm (Figure
-1
3
a-b). Two peaks at ~2850 and ~2920 cm were detected on both Cu and CuAg
catalysts. The peaks were more obvious on CuAg than that on Cu, indicating a larger
surface population of reaction intermediates containing C-H bond (such as *CHO,
-1
*
C H O, etc) on CuAg catalyst. Remarkably, two new peaks at 2713 and 2816 cm
2 2
appeared on CuAg, which also indicates the enhanced coverage of CH-containing
intermediates. It is believed that these intermediates are critical for the formation of
4
2
hydrocarbons . However, a more precise assignment of these peaks is extremely
challenging due to the complexity of reaction intermediates that contains C-H bond.
All the signals in our Raman spectra largely benefit from the surface-enhanced
Raman scattering (SERS) effect of our nanoparticulate Cu substrate. It is noted that Ag
4
3
nanoparticles could also introduce extra SERS on CuAg . However, the differences of
signal to noise ratio of Cu-C vibrational signals on CuAg and Cu cast doubt on this
proposition (Figure 3a-b). It is unlikely that the appearance of new peaks on Cu-Ag is
due to the SERS caused by Ag nanoparticles, which is further corroborated by the
-1
observation of only traces of peaks at 2713 and 2816 cm on a thin Cu O that is
2
deposited onto an electrochemically-roughened Ag substrate (Figure S15-16).
It is also noted that the potentials where these new peaks appeared (-0.5 to -0.9
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V) on CuAg are more positive than the potential where the formation of ethylene greatly
enhanced (-1.05 V, Figure 2). This is not surprising since these intermediates undergo
very fast kinetics of being reduced at potentials <-1.0 V. Similar phenomenon has also
been reported for intermediates involved in hydrogen evolution, oxygen evolution and
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CO reduction44-47.
2
Thus, our Raman spectra demonstrated that the formation of key intermediates
(C H O ) towards hydrocarbons and oxygenates are significantly improved on the
x
y
z
surface of CuAg catalysts, due to that the desorbed CO from Ag spillover to Cu sites
and the adsorbed CO intermediates on Cu undergo further reduction. However, little
kinetic information on the process of CO spillover can be inferred from the Raman
spectrum.
3
.4 Proposed mechanism of CO reduction to C H on CuAg catalyst
2 2 4
2
Figure 4 (a) Proposed mechanism for the electroreduction of CO to ethylene on CuAg catalyst: Step 1,
the mobile gas CO produced from Ag sites readsorbs on Cu sites; step 2, CO dimerize through C-C bond
formation with electron transfer, forming adsorbed *CO-COH after proton transfer; step 3, protonation
2
of *CO-COH, accompanied by a H O desorption; step 4-7, continuous protonation of the intermediates,
resulting in C
2
H
4
formation and leaving an adsorbed *O; step 8, protonation of *O adsorbate, forming
*
OH and then H
2
O. The number of proton and electron in each step is not balanced. (b) Quantitative
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analysis of in situ CO generated from Ag sites in CuAg: production rates of CO (including *CO
intermediates and gaseous CO) and gaseous CO as a function of the potential on Cu and CuAg catalysts,
the shaded region highlights the improvement of ‘CO production’ after Ag was incorporated into oxide-
derived Cu. (c) Comparison with state-of-the-art copper and bimetallic copper-X catalysts: Faradaic
1
1-14, 17
efficiency of C2+ products and CO
.
Based on the electrocatalytic performance and operando Raman spectroscopy
results, we propose a sequential catalytic mechanism for the reduction of CO to C H
4
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for our CuAg bimetallic system (Figure 4a). Incoming CO is firstly reduced to CO on
2
either Cu O derived Cu or Ag sites by transfer of 2 electrons and 2 protons 48-49. CO
2
4
0
produced on Ag is likely to desorb due to the weak binding between CO and Ag ,
resulting in an increased local CO concentration near the surface of electrode. The local
5
0
CO may re-adsorb on Cu sites (step 1), resulting in a high surface coverage of *CO .
At a sufficiently high overpotential, i.e. < -1.0 V, CO dimerizes with the formation of
C-C bonds (step 2). This is supported by the observation of protonated CO dimer during
4
6
CO reduction by in situ FTIR spectroscopy, reported by Koper and co-workers . The
2
appearance of large amounts of CH-containing intermediates in the Raman spectra
indicates the further reduction of CO or CO dimers, leading to the formation of ethylene
step 3-8) 48.
(
3
.5 Efficient CO spillover on CuAg catalyst
Based on the preceding results, we believe that an ideal Cu-X bimetallic system
should enable the reduction of the entire CO that is generated from X. In other words,
the best synergy should exhibit 100% efficiency of CO spillover from X to Cu. To
uncover the fundamental insights into the synergy between X and Cu, we quantitatively
analyze the efficiency of CO spillover from Ag to Cu. The total production rate of CO
(
TOF ), including *CO intermediate (TOF*CO) and gaseous CO (TOFCO(g)), on Cu and
CO
CuAg catalysts was calculated against electrochemical active surface areas. For reasons
of mass balance:
TOF*CO = TOFCO - TOFCO(g)
.
The TOF difference between Cu and CuAg is defined as ΔTOF . As shown in
CO
Figure 4b, TOFCO on CuAg was much higher than TOFCO on Cu alone when the applied
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potential was more negative than -1.0 V, showing a striking enhancement from 24.2
nmol s cm on Cu to 52.6 nmol s cm on CuAg at -1.05 V (ΔTOF = 28.4 nmol s-1
-
1
-2
-1
-2
CO
-2
cm , shown in Table S8). On the other hand, the production rate of gaseous CO only
-1
-2
-1
-2
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increased from 0.7 nmol s cm on Cu to 2.3 nmol s cm on CuAg. Therefore, out of
-1
-2
-1
-2
the enhancement of TOFCO (28.4 nmol s cm ), only 1.6 nmol s cm is released as
CO gas, demonstrating that ~95% of the CO produced on Ag sites is further reduced by
Cu. We attribute this efficient CO spillover to be the major reason for the enhanced
activity towards the formation of ethylene on CuAg.
It is noted that spillover of CO occurs only at a narrow potential range from -1.0
V to -1.2 V (Figure 2a-b, Table S8). At potentials > -1.0 V, the specific current densities
for C H formation are almost identical on two catalysts. This demonstrates the small
2
4
effect of Ag on the production of C H , which is instead mainly limited by the reaction
2
4
rate of further CO reduction on Cu though Ag catalyzes CO reduction to CO in the
2
vicinity of the Cu sites at potentials > -1.0 V.
As for the detailed pathway for CO spillover from Ag to Cu, here we propose two
synergistic routes: 1) CO produced on Ag nanoparticles first desorbs into the electrolyte,
followed by diffusing and re-adsorbing on the vicinal Cu sites; 2) *CO that adsorbed
on Ag directly diffuses through the boundaries of Cu and Ag, and transfers to Cu sites.
The latter spillover pathway is more commonly studied for *H spillover in
electrochemical hydrogen evolution51-52. Here, in our system, the number of Ag sites
adjacent to Cu is much less compared to the sum of active Ag sites. Considering that
CuAg delivers a remarkable CO spillover efficiency, the majority of CO is likely to
spillover via the former pathway.
Similar CO spillover efficiency could also be achieved on CuAu bimetallic
5
3
catalysts that were prepared in similar ways under the identical test conditions . After
optimizing the galvanic replacement reaction between Cu O and HAuCl , small Au
2
4
nanoparticles with size around 30~50 nm were uniformly decorated on the surface of
Cu O NWs (Figure S17). Formation of ethylene and ethanol was maximized on CuAu
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electrode at -1.05 V with faradaic efficiencies of 38.70% and 22.60%, respectively
(Figure S18). The efficiency of CO spillover based on specific TOF was calculated to
be >90% at -1.05 V (Figure S19).
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3.6 Comparison with the state-of-the-art catalysts
Our CuAg is also compared with other Cu-based catalysts reported in literatures
in terms of faradaic efficiency11-14, 17. Figure 4c depicts the faradaic efficiencies of C2+
products and CO at the optimal potential for C formation on other Cu and CuX (X =
2
+
Au, Ag and Zn) catalysts. Our catalyst ranks as one of the best catalysts for ethylene
production in terms of faradaic efficiency and current density. Moreover, the intrinsic
activity as reflected as the specific current density for ethylene formation on CuAg also
increased by > 2 times compared to oxide-derived Cu electrodes (Figure S10).
The faradaic efficiency of CO is extremely low (~1.0%) on our CuAg catalyst as
compared with the oxide-derived Cu-Zn, which showed 10.4% of CO even at the
11
optimal potential for ethanol production . It is also noted that catalysts such as Ag@Cu
and Au/Cu exhibited only ~28.6% and ~18.3% of C products, though the faradaic
2
efficiency of CO is relatively low (3.2% and 7.0%), respectively14, 17. We postulate that
the reason could be that the active sites of metallic Cu is not as efficient as that of oxide-
derived Cu, in terms of favoring C-C coupling to form C products13, 15. Moreover, loss
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of faradaic current due to the competing hydrogen evolution reaction remains a major
challenge for CO reduction to multi-carbon products. For example, around 35% ~ 44%
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H were recorded on the reported Cu-Ag and Cu-Au catalysts, indicating the poor
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suppression of H in these bimetallic system12-14, 17.
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Another insight can be gained by comparing our CuAg and CuAu catalysts. We
observed that the selectivity towards C products on CuAu was higher at more positive
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+
potentials than that on CuAg catalyst (Figure S20). A possible reason is that the favored
potential for producing CO on Au is more positive than that of CuAg (Figure S21). The
lower overpotential for CO production at Au sites is beneficial for the accumulation of
*
CO adsorbates at less negative potential range. Since Au nanoparticles favor hydrogen
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evolution instead of CO production when the potential is negative than -1.0 V, the
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faradaic efficiency of ethylene is less on CuAu compared with CuAg . The comparison
between CuAu and CuAg highlights the importance of how well the optimal potential
of CO production and best potential for C-C coupling on Cu sites are matched.
Based on the above discussions, we highlight critical factors for designing
efficient and selective bimetallic catalysts for the formation of C products. The first
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factor is the choice of Cu and X. The use of oxide-derived Cu is more favored over
metallic Cu and X should not be too effective in terms of catalyzing the formation of
CO. The structure of the two counterparts, i.e. particle size, surface roughness and
defects, is also critical. Here, Cu O nanowires that consisted of small grains are chosen
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since this meso-structure facilitates the mobility and adsorption of CO intermediates8.
The last important factor is that optimal potential for CO production on metallic co-
catalyst should lie closely to the optimal potential window for C products formation
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on copper sites.
.7 Photosynthesis of C H from CO using water as electron and proton source
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The ultimate research goal for discovering selective electrocatalysts for CO2
reduction is to accomplish CO fixation using renewable electricity. Recently, some
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encouraging reports achieved high energy efficiencies for solar-driven CO reduction
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to carbon monoxide and formate, whose formation requires relatively low
overpotentials (Table S9)22, 54-57. Compared with carbon monoxide and formate,
ethylene is much more attractive due to its high commercial value and widespread
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industrial use . However, achieving solar-driven CO reduction to ethylene using water
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as electron and proton source with appreciable conversion efficiency remains a
challenge13, 21, 59. The solar to ethylene (STE) conversion is mainly limited by high
overvoltage losses at the cathode material and the solar to electric power conversion
efficiency (PCE) of the photovoltaic. Additional losses may arise from a mismatch
between the operating voltage and the voltage at the maximum power point (Vmpp) of
the solar cell, even though these can be largely avoided by electronic management of
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the photovoltaic power output. Here we achieved a benchmark solar to ethylene
conversion efficiency by employing our efficient CuAg catalysts and addressing the
discrepancies in the previous reports. Electrodeposited IrO was used as the anode to
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catalyze water oxidation. The overall reaction and the corresponding equilibrium cell
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potential corresponding to the standard free energy of the CO conversion reaction are :
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2CO + 2H O = C H + 3O (E = 1.15 V, G* = + 1323 kJ mol )
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Figure 5. Characterization of solar driven CO
perovskite solar cells under AM 1.5G 1 Sun solar illumination, overlaid with J-V curve of CuAg cathode
and IrO anode in the two-electrode configuration, the surface area of CuAg was normalized to the total
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reduction system. (a) J-V curve of three connected
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illuminated area of three solar cells (0.75 cm ). The maximum power point of the photovoltaics and the
operating point of the complete system are designated with dots. (b) Operating current density and
faradaic efficiency of ethylene of the complete system during the initial 1 h test.
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.2 M KHCO was used as the electrolyte to improve the conductivity and the
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catalytic activity of CuAg catalyst is slightly different from that observed in 0.1 M
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KHCO (Table S10), which is consistent with many previous works . To further
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minimize the iR drop between two electrodes, ion exchange membrane was excluded.
An overall cell voltage of about 2.8 V is required with the operating current density
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varying from -40 to -60 mA cm in order to maintain a ~50% faradaic efficiency for
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ethylene generation (Table S10). This cell voltage value (V) can be divided into four
parts:
V = E + ηethylene + ηoxygen + iR
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Where E represents the standard thermodynamic cell potential for the conversion
of CO and H O to O and C H , η and ηoxygen represent the overpotentials for
ethylene
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ethylene formation and oxygen evolution, and iR represents the ohmic loss between
two electrodes.
Based on the voltage requirement of ~2.8 V, we carefully designed perovskite
solar cells with Vmpp of ~950 mV to ensure that three of them connected in series provide
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the suitable voltage for the combined photolytic CO reduction system (Figure 5a) .
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The connected solar cells show an open-circuit voltage of 3.30 V and a short-circuit
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current of 7.66 mA cm under AM 1.5 G illumination (Figure 5b), yielding a solar-to-
electricity conversion efficiency of 18.56% from the forward curve. It should be noted
that the surface area of the cathode is also adjusted to match the current generated by
the solar cells.
The predicted operating point of the solar driven CO reduction is defined by the
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intersection of the J-V curve of the electrolyzer and the perovskite solar cells (Figure
-2
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b). The operating point (6.67 mA cm at 2.76 V) matches well the maximum power
-2
output metrics of the perovskite solar cells (6.63 mA cm at 2.80 V), indicating the
optimum solar-to-electricity conversion process in our integrated device. The
performance was further characterized without applying any external bias under AM
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.5G light illumination (Figure 5c). An average faradaic efficiency of 54% for ethylene
-2
was achieved at a stable current density of 6.68 mA cm during 60 min test,
demonstrating that the electricity-to-fuel is also under the optimal condition. This
combined system delivers a benchmark solar-to-ethylene conversion efficiency of
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.17%. Totally, a solar-to-fuel conversion efficiency of 7.78% was achieved accounting
for all reduction products (Table S11). This performance ranks our unassisted
photolytic system the most efficient solar-to-ethylene system so far reported (Table S9),
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with the added advantage that our light harvesting materials are earth-abundant in
contrast to photo-electrolysis employing tandem III-V semiconductors as absorber
material.
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4. CONCLUSIONS
In this work, we developed a Cu O derived CuAg bimetallic catalyst produced by
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galvanically replacing Cu(I) ions in the Cu O nanowires by Ag(I) from a AgNO
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solution. Synergistic interaction between oxide-derived Cu nanowires and metallic Ag
islands deposited on their surface led to a striking enhancement in the selectivity and
activity for CO reduction to ethylene, giving a faradaic efficiency of ~52% and a
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current density of -18.07 mA cm at -1.05 V. Overall, a faradaic efficiency of ~76%
for C2+ products was obtained with on CuAg. We assign the enhancement of
performance of our CuAg catalyst to the efficient CO spillover from Ag to Cu, with
~95% of the CO formed on Ag being transferred and reduced on Cu. Moreover, the
mesoscopic structure of our catalyst also engenders a higher density of active sites
compared to planar structure.
We applied operando Raman spectroscopy to confirm that CO was produced
initially at Ag sites. The weak binding of CO to Ag favors desorption of CO and
spillover to Cu sites40, 61. Remarkably, enhanced coverage of CH-containing
-1
intermediates on Cu was supported by new peaks (at 2713 and 2816 cm ) on our CuAg
catalyst. These new findings strongly support our proposed CO spillover mechanism,
which guides us towards the design of more efficient catalysts.
Based on the impressive performance of our cathode we realized a complete
photosynthetic CO reduction system using water as an electron and proton source,
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which is driven solely by solar light harvested by perovskite solar cells. The latter have
recently emerged as an attractive contender for low-cost photovoltaics and provided the
well-matched operating point for selective ethylene formation in our electrochemical
device. This artificial photosynthetic system delivers a solar-to-ethylene free energy
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conversion efficiency of 4.17% and a total solar-to-fuel conversion efficiency of 7.78%
by accounting for all reduction products. The solar-to-ethylene conversion efficiency
represents a benchmark in solar-driven CO conversion systems. This study will not
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only enable fundamental understanding of how to optimize the interplay of activity and
selectivity in CO reduction, but also facilitate future progress on catalyst development.
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Our observations will assist the transformation from an observation-driven to a design-
driven approach for photo-electrochemical CO reduction.
2
ACKNOWLEDGEMENTS
This work was financially funded by Sino-Swiss Science and Technology
Cooperation (SSSTC) 2016 under the project title “All perovskite tandems for solar to
CO fixation” from Swiss National Science Foundation (IZLCZ2–170294). J.G. is
2
financially supported by an overseas exchange scholarship from China Scholarship
Council (No. CSC201706370233). M.G. and H.Z. acknowledge the funding support
from the European’s Horizon 2020 research and innovation programme under grant
agreement No. 764047. The authors gratefully thank Aurélien Bornet (NMR, ISIC,
1
EPFL) for the technical support on H NMR, Pierre Mettraux (SCI, EPFL) for the XPS
characterization, Arnaud Magrez (IPHYS, EPFL) for assist in Raman spectroscopy and
Zaiwei Wang (LSPM, EPFL) for the XRD measurement.
AUTHOR INFORMATION
Corresponding authors:
D.R.: dan.ren@epfl.ch,
M.G.: michael.graetzel@epfl.ch
Notes
The authors declare no competing financial interest.
ASSOCIATED CONETS
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The Supporting Information is available free of charge on the ACS Publication Web
site.
Additional characterizations of Cu(OH) , Cu O and Cu O-Ag; Characterizations
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of oxide-derived Cu and CuAg catalysts; Electrochemical reduction of carbon dioxide
on Cu and CuAg; Faradaic efficiency and partial current density for products; CO2
reduction on Ag nanocorals; CO reduction on planar Cu O-Ag; Raman spectroscopic
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studies on Ag; SERS effect of Ag nanoparticles on Cu O; Calculation of CO spillover
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efficiency; CO reduction on CuAu; Solar-driven CO reduction
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