Changing the Product Selectivity for Electrocatalysis of CO2 Reduction Reaction on Plated Cu Electrodes

Article abstract of DOI:10.1002/cctc.201901748

Electrochemical reduction of carbon dioxide (CO₂RR) on various types of Cu electrodes to useful chemicals and fuels has attracted much attention. Herein, we comparatively investigate the distributions of CO₂RR products over electroplated Cu, chemically plated boron-doped Cu (Cu?B) and electroplated phosphorus-doped Cu (Cu?P) electrodes. A global Faradaic efficiency of more than 50 % can be reached for the C2+ (ethylene, ethanol and n-propanol) products on both plated Cu?B and Cu?P electrodes at ～?1.15 V vs. RHE in 0.1 M KHCO₃ electrolyte. Moreover, in situ surface enhanced infrared spectroscopy results together with quantitative analysis of the CO₂RR products reveal a more facile conversion/depletion of the *CO intermediate after B- and P-doping, for which Cu?B promotes the C2+ products while Cu?P enhances both C2+ generation and CH₄ evolution at faster *CO consumption. The present work suggests the vital role of *CO in the step of C?C bonding formation and highlights that the metalloid doping may alter the reactivity and selectivity of the intermediate.

Full text of DOI:10.1002/cctc.201901748

Accepted Article
Title: Changing the Product Selectivity for Electrocatalysis of CO2
Reduction Reaction on Plated Cu Electrodes
Authors: Hong Li, Xianxian Qin, Tian-Wen Jiang, Xian-Yin Ma, Kun
Jiang, and Wen-Bin Cai
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Changing the Product Selectivity for Electrocatalysis of CO2
Reduction Reaction on Plated Cu Electrodes
Hong Li, [a] Xianxian Qin, [a] Tianwen Jiang, [a] Xian-Yin Ma, [a] Prof. Dr. Kun Jiang,*[b] Prof. Dr.
Wen-Bin Cai*[a]
a Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative
Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University,
Shanghai 200438, China.
Email: wbcai@fudan.edu.cn.
b Institute of Fuel Cells, School of Mechanical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China.
Email: kunjiang@sjtu.edu.cn.
Invited contribution to the Special Issue:
Electrocatalysis: From Batteries to Clean Energy Conversion
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Abstract
Electrochemical reduction of carbon dioxide (CO2RR) on various types of Cu electrodes to useful chemicals
and fuels has attracted much attention. Herein, we comparatively investigate the distributions of CO2RR
products over electroplated Cu, chemically plated boron-doped Cu (Cu-B) and electroplated phosphorus-doped
Cu (Cu-P) electrodes. A global Faradaic efficiency of more than 50% can be reached for the C2+ (ethylene,
ethanol and n-propanol) products on both plated Cu-B and Cu-P electrodes at ~ -1.15 V vs. RHE in 0.1 M
KHCO3 electrolyte. Moreover, in situ surface enhanced infrared spectroscopy results together with quantitative
analysis of the CO2RR products reveal a more facile conversion/depletion of the *CO intermediate after B- and
P-doping, for which Cu-B promotes the C2+ products while Cu-P enhances both C2+ generation and CH4
evolution at faster *CO consumption. The present work suggests the vital role of *CO in the step of C-C bonding
formation and highlights that the metalloid doping may alter the reactivity and selectivity of the intermediate.
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Introduction
Electrochemical CO2 reduction to value-added chemicals using renewable energies has attracted significant
attention as a potential solution to reduce anthropogenic carbon emissions as well as to fulfill clean electricity
storage.[1] Among CO2 reduction products, C2+ hydrocarbons and oxygenates like ethylene (C2H4), ethanol
(C2H5OH) and n-propanol are highly desirable owing to their impressive energy densities as compared to C1
compounds.[2] Copper (Cu) is the only metal capable of reducing CO2 to multi-carbon products at relatively
high yields, though still challenged by the H2 evolution side reaction and the simultaneous generation of other
C1 products like CO and CH4 in aqueous electrolytes.[3]
So far, different strategies have been proposed to optimize the structure and composition of Cu-based
catalysts toward selective multi-carbons products generation.[4] Earlier single crystalline studies suggest that
Cu(100) is more selective for C2H4 generation vs. CH4 evolution as compared to Cu(111).[5] For poly-crystalline
Cu electrodes, the control over morphology,[6] nanoparticle size,[7] surface roughness[8] grain boundary density[9]
has been demonstrated to tune up the C2+ selectivity as well. As for the component optimization, the alloying
of Cu with Pd,[10]Ag[9b, 11] or Zn[12] has been proposed to improve the selectivity toward ethylene and ethanol
products. Recently, the incorporation of a metalloid component to modify the electronic structure and thus
tuning the binding strength of metal surfaces has attracted considerable interest. Our earlier study on boron-
doped Pd catalyst reveals the enhanced formate production over a wide potential window as compared to un-
doped Pd, resulting from a downshifted d-band center of surface Pd atoms with B doping.[13] Along this line,
metalloid doped Cu catalysts, such as Cu-S,[14] Cu-B[15] and Cu-N catalysts[16] have been explored as well for
the CO2RR. Varying the metalloid component may lead to different product distributions. Specifically, that the
Cu-S catalyst promotes the C1 products while the other two favor the C2+ products. As the C2+ products
promoters, the previous Cu-B and Cu-N catalysts were prepared via wet-chemical synthesis,[15a] high
temperature calcination,[15b] and hydrothermal synthesis.[16] The catalysts thus obtained are hardly applicable for
in situ spectroelectrochemical measurements.
In addition to the above efforts, a better understanding of the Cu electrode/electrolyte interface is highly
demanded for developing Cu-based catalysts as the electrochemical CO2RR is a very complex reaction
involving multiple proton and electron transfers. High sensitivity in situ electrochemical vibrational
spectroscopy is a powerful tool to tackle this challenge.[17] Yeo and co-workers employed in-situ Raman
spectroscopy revealing that the C2 products generation started after the reduction of surface CuOx into metallic
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Cu.[18] Koper et al. carried out a complementary IRAS and SERS study to clarify the role of supporting
electrolyte and water content in determining products distribution for CO2RR in an organic solvent.[19] By using
attenuated total reflection surface enhanced infrared spectroscopy (ATR-SEIRAS), Xu et al. identified a CO
adsorption band at 2058 cm–1 on oxide-derived Cu different from that on Cu-poly, and assigned it to the CO
bound to the Cu(100) facet, which is responsible for its enhanced C2+ selectivity.[20] Waegele et al. used the
same technique and clarified that atop-bound CO rather than bridge-bonded CO is an on-pathway intermediate
for C1 and C2+ products.[21] It is thus of particular interest to extend in situ spectroscopic study of CO2 reduction
from monometallic Cu to metalloid-doped Cu electrodes.
In the present work, Cu film electrodes with or without metalloid B- (Cu-B) and P-doping (Cu-P) are
fabricated by (electro) chemical plating method. By this means, we are able to comparatively study the
distributions of the CO2 reduction products over four different polycrystalline copper electrodes, i.e.,
mechanically polished Cu, electroplated Cu, chemically plated B-doped Cu and electroplated P-doped Cu. Both
the plated Cu-B and Cu-P electrodes exhibit a significantly higher portion of C2+ products as compared to the
un-doped counterparts in 0.1 M KHCO3 electrolyte. Furthermore, in situ ATR-SEIRAS is extended to probe the
potential dependent variation of the CO intermediates at the plated Cu, Cu-B and Cu-P film electrodes, to
provide a correlation of the enhanced C2+ selectivity with the CO coverage.
Experimental
Preparation of plated Cu electrodes
The Cu foil (99.95%, 20 mm  20 mm  0.2 mm, Beijing Research Institute of Nonferrous Metals) was first
mechanically polished with 1500-grit abrasive paper (STARCKE) to remove surface oxides. The freshly
polished Cu foil was ultra-sonicated sequentially and repeatedly in acetone and Milli-Q water (> 18.2 Mῼ·cm,
Millipore), and eventually dried by a high purity nitrogen stream.
The polished Cu foil with its backside being sealed served as the substrate for electroplating or chemical
plating a desired Cu film. Specifically, the un-doped Cu film was prepared by electrochemical deposition at -
1.1 V (vs Ag/AgCl) for 100 s in a stirred aqueous solution consisting of 12 mM CuSO4 and 0.11 M H2SO4 at
room temperature; The electrodeposition of the Cu-P film was carried out in a stirred aqueous solution of 12
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mM CuSO45H2O, 0.48 M NaH2PO2H2O and 0.034 M Na3C6H5O72H2O at -1.1 V (vs Ag/AgCl) for 20 min at
70℃, with the plating bath pH adjusted to ca. 8 using a NH4OH solution. The Cu-B film was chemically
deposited according to the following procedures: 60 ml of a 12 mM CuSO45H2O + 0.034 M Na3C6H5O72H2O
solution was prepared, and its pH was adjusted to ca. 10.5 and its temperature was controlled at 55 °C; 2 mL of
0.1 g mL-1 DMAB solution prepared with cold water was injected quickly into the above CuSO4 precursor
solution under stirring, followed by placement of the polished Cu foil for 5 min.
The as-plated films were rinsed with copious amount of Milli-Q water and dried by the nitrogen stream for
subsequent measurements.
Material characterizations
The surface morphologies of the plated films on Cu foil were characterized by a scanning electronic
microscopy (SEM, FEI, Quanta 450 FEG). The crystalline phases of the plated films were characterized by a
powder X-ray diffractometer using Cu Kα radiation from 35º to 80º (Bruker D2 Phaser). The surface electronic
structures of the plated films were analyzed by using an ESCALAB 250 XI X-ray photoelectron spectrometer
(Thermo Scientific) with the monochromatic Al Kα radiation (1486.6 eV), and the binding energies were
calibrated with reference to the C 1s peak at 284.6 eV. To quantify the chemical compositions of the plated Cu-
B and Cu-P electrodes, these two films were firstly deposited onto carbon paper substrates following the same
protocol above, then dissolved into freshly prepared aqua regia. Inductively coupled plasma-atomic emission
spectroscopy (ICP-AES, Thermo Elemental IRIS Intrepid) was employed to analyze the compositional
information. Ex situ X-ray absorption spectrometry on the Cu K-edge was performed at the BL14LW beamline,
Shanghai Synchrotron Radiation Facility, by recording the fluorescence signals of electroplated Cu, Cu-B and
Cu-P on carbon paper, respectively.
CO2RR Electrochemical measurements
All CO2 reduction measurements were run at room temperature in a customized H-type two compartment
glass cell separated by a Nafion 117 membrane with gas inlets and outlets. Each compartment contained 50 mL
of 0.1 M KHCO3 electrolyte. The solution in the cathodic compartment was purged with CO2 (Air liquide,
99.999%) for 40 min prior to the start of electrolysis. A constant flow rate of CO2 gas (30 sscm, monitored by
Alicat Scientific mass flow controllers) was continuously delivered into the cathodic compartment to keep CO2-
saturation during electrolysis. A plated Cu film served the working electrode with a bunch of graphite rods and
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a Ag/AgCl electrode used as the counter electrode and reference electrode, respectively. A CHI 660b
electrochemical workstation was employed for the potential control and current measurement. All potentials
(
)
reported were converted to the reversible hydrogen electrode (RHE) scale in this work using 퐸 vs. RHE =
(
)
퐸 vs. Ag/AgCl + 0.199 V + 0.0591 × pH (herein pH 6.8), with a manual iR drop compensation (70% of the
determined value). Before the start of a CO2RR measurement, the electrode was subjected to the potential
cycling from -0.54 V to 1.16 V vs RHE in an Ar-saturated 0.1 M KHCO3. The electrochemical active surface
area (ECSA) of a plated sample was estimated via its double layer capacitance by measuring and plotting the
double layer charging current as a function of scan rate.
Quantification of CO2RR products
The gas products were analyzed by an online gas chromatography (GC) with a Gas Chromatograph 2060
from Shanghai Ramiin Instruments equipped with a flame ionization detector (FID) coupled with a methanizer
and a thermal conductivity detector (TCD). The detection limits of CO on FID, and H2 on TCD, are determined
to be 1 and 5 ppm, respectively. The FID was used to quantify hydrocarbons, CO, CO2 and the TCD to quantify
H2. The gas products were sampled after a continuous electrolysis of ~15 min under each potential. The aqueous
products were analyzed by a 500 MHz nuclear magnetic resonance (NMR) spectrometer from Bruker Company
with a water suppression technique. Typically, 700 μL of the electrolyte after 2 h’s electrolysis was mixed with
35 μL of D2O solution containing phenol as the internal standard.
In-situ ATR-SEIRAS measurements
The Cu-B, Cu-P or Cu overfilm plated on a Au-coated reflecting plane of semicylindrical Si prism was used
as the working electrode for in situ ATR-IR measurement, with the Ag/AgCl electrode and the graphite rod as
the reference and counter electrodes, respectively. The infrared spectra were acquired by using a Thermoelectric
IR spectrometer (Thermo Fisher IS50) equipped with an MCT-A detector at a spectral resolution of 8 cm-1 and
with an unpolarized IR radiation at an incidence angle of ca. 65º. All the spectra are shown in the absorbance
unit as -log (I/I0), where I and I0 represent the intensity of the reflected radiation of the sample and reference
spectra, respectively. Experimental details including chemical deposition of the Au films, setup of ATR cell
etc., can be found elsewhere.[22]
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Results and Discussion
DMAB, as a reducing reagent and a boron source, initially used for wet chemical synthesis of Pd-B/C catalyst
by our group,[13, 23] is further extended in this work for chemically plating the Cu-B film on a polished Cu
substrate under modified reaction conditions. One-step electroplating of the Cu-P film was also tested here using
Na2H2PO2 as the phosphorus source. Typical SEM images of the mechanically polished Cu foil, electroplated
Cu, chemically plated Cu-B and electroplated Cu-P films are shown in Figure 1(a-d), respectively. In
comparison to the bare substrate with relatively flat surface, the Cu-B film largely consists of densely packed
nanoparticles, the Cu-P film consists of nanoflakes whereas the electroplated Cu film appears to have a densely
packed underlayer scattered with larger nanoparticles on its top.
Figure 1. SEM images of the plated (a) polished Cu, (b) electroplated Cu, (c) Cu-B and (d) Cu-P. (e) symmetric
XRD patterns of mechanically polished Cu (black), electroplated Cu (purple), Cu-B (red) and Cu-P(blue)
electrodes. (f) Further XRD pattern subtraction is performed by employing the polished copper foil as the
reference sample with the (220) peak intensity of each sample being normalized.
The symmetric XRD characterizations suggest polycrystalline features for these four Cu electrodes (Figure
1e), without any detectable copper oxide phases, although the Cu foil substrate has the (220) preferred texture.
The diffraction peaks at 2θ degrees of 43.46º, 50.62º and 74.40º are indexed to the crystal planes of Cu(111),
(200) and (220), respectively (JCPDS 04-0836). Since all the Cu electrodes show a predominant texture of
Cu(220) from the underlying Cu foil, a simple subtraction is performed by normalizing the Cu(220) peak
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intensity and then using the polished Cu foil as the reference to emphasize the crystalline structure of the plated
Cu films (Figure 1f). After subtraction, the diffraction peaks are very weak for the electroplated Cu film,
indicating approximately similar crystalline phases of the plated Cu film on the Cu foil. On the contrary,
prominent (111) and relative weak (200) peaks are observed on the plated Cu-B and Cu-P films, suggesting the
polycrystalline feature with certain (111) preference. Also, noted are the typical bipolar features at 74.4 º and
50.6 º in Figure 1f with the upward lope at a smaller angle and the downward lope at a larger angle due to a
slightly negative shift of diffraction angles for the Cu-B film, suggesting an effective B doping in the Cu
nanoparticle subsurface. In comparison, the XRD characteristic angles for the electroplated Cu-P film appear
less shifted, suggestive of a significantly lower doping concentration of P or a different form of doping. The
presence of B (or P) in the resulting Cu-B or (Cu-P) film can be further demonstrated by the XPS measurement
(vide infra). Furthermore, a similar grain size distribution for the above electrodes is revealed upon calculating
their (111) diffraction peaks by Scherer equation, i.e., 28.0, 30.8, 28.2 and 32.0 nm for polished Cu, plated Cu,
Cu-B and Cu-P, respectively.
Figure 2. (a) X-ray photoelectron spectroscopy and (b) Cu K-edge X-ray absorption spectra of electroplated
Cu, Cu-B, Cu-P electrodes, together with other reference samples.
The chemical states of Cu surface layers in the plated Cu, Cu-B and Cu-P films are first studied by ex situ
core-level X-ray photoelectron spectroscopy (XPS). As shown in Figure 2a, two sharp peaks centered at ca.
932.4 eV and 952.2 eV in the Cu 2p region are observed for the Cu-B, Cu-P and Cu films, which can be
attributed to Cu0 2p3/2 and Cu0 2p1/2, respectively.[7a] This metallic Cu feature can also be seen from the Cu Auger
spectra (Figure S1),[24] and further reinforced by synchrotron radiation Cu K-edge X-ray absorption near-edge
spectra (XANES, Figure 2b). In contrast to the Cu(I)/(II) oxide feature on Cu2O and CuO references, a same
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absorption edge at 8979 eV are observed corresponding to Cu(0) for all 3 plated samples and in accordance with
standard Cu foil reference. No metal impurities are detected based upon the XPS survey spectra (Figure S2)
and no relevant metalloid doping is found in the electroplated Cu film (Figure S3), while both the B 1s signal
for the Cu-B film and the P 2p signal for the Cu-P film (as shown in the core level XPS spectra in Figure S4
and the ICP-AES spectra in Figure S5) are clearly observed. These suggest that DMAB and NaH2PO2 serve as
the effective B and P sources, respectively. It is noted that though some oxygen and carbon signals are observed
on the XPS survey spectra, Cu 2p XPS, K-edge XANES, together with XRD and Auger analysis all suggest a
metallic surface state for the above plated Cu electrodes. Therefore, we prudently assign these O and C signals
to the surface adsorbed CO2 and O2 species due to the sample exposure to air during the transfer process.
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Figure 3. (a) Steady-state current densities averaged from 2-h CO2RR electrolysis at each applied potential,
together with the Faradaic efficiencies of (b) H2, (c) HCOO-, (d) CO, (e) CH4, (f) C2H4, (g) ethanol (C2H5OH),
and (h) n-propanol (C3H7OH) for Cu-B, Cu-P, electroplated Cu and polished Cu foil electrodes. The error bars
represent two independent samples.
Chronoamperometric CO2 reduction was performed over four different Cu electrodes at fixed potentials
between -0.8 and -1.15 V in CO2-saturated 0.1 M KHCO3 electrolyte (pH 6.8). The gas products were analyzed
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by gas chromatography (Figure S6), and the liquid products by nuclear magnetic resonance spectroscopy
(Figure S7). The averaged geometric current densities at each applied potential were plotted in order to evaluate
the electrocatalytic activities of the plated Cu- B, Cu-P, Cu films and the polished Cu foil, as shown in Figure
3a while the linear sweep voltammetry curves are plotted in Figure S8. Electrochemical surface areas of the
four Cu electrodes are evaluated based upon their double layer capacitance measurements (Figure S9).
Throughout the interested potential window, all the four Cu electrodes deliver a similar overall current density
but a very different product selectivity. FEs for H2 and other CO2 reduction products as a function of applied
potential are plotted in Figure 3b-h, for which the sum up efficiencies for all major products are ca. 82-105%
(Figure S10). As compared to the polished Cu and the plated Cu, both Cu-B and Cu-P films show a lower
selectivity toward H2 evolution side reaction. FEs for formate and CO keep decreasing with negatively going
potential on all Cu electrodes, while FE for CH4 keeps increasing especially for the mechanically polished Cu.
The maximum overall FE for C2+, i.e., 54.5% is recorded at ~ -1.08 V vs. RHE on the Cu-B electrode, consisting
of 32.1% for C2H4, 16.3% ethanol and 6.1% n-propanol. Similarly, the Cu-P electrode also demonstrates an
enhanced C2+ FE as compared to the un-doped Cu, reaching up to 50.3% at ~ -1.15 V (vs. 37.8% on the
electroplated Cu and 26.9% on the polished Cu, Figure S11) including 33.5% for C2H4, 13.7% ethanol and 3.4%
n-propanol.
Figure 4. (a) Faradaic efficiencies for different Cu electrodes at ~ -1.15 V (vs RHE). (b) Partial current densities
of C2+ products for different Cu electrodes as a function of potential.
To better compare the C2+/C1 selectivity on different Cu electrodes investigated, the distribution histograms
of CO2RR products at ~ -1.15 V with maximum C2+ FEs are plotted, together with the scatter plot showing the
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corresponding C2+/C1 values in Figure 4a. The overall CO2RR FEs (apart from H2 evolution) are ca. 64.9%
for the polished Cu, 46.1% for the electroplated Cu, 56.1% for the chemically plated Cu-B and 62.6% for the
electroplated Cu-P, respectively. The highest current faradaic efficiency ratio of C2+/C1 is achieved on the Cu-
B electrode (ca. 20.3) with an overall C2+ FE of 53.5%, which presents a 4.5-fold enhancement as compared to
the electroplated Cu film, or a 29.0-fold enhancement over the polished Cu foil. Similarly, for the Cu-P electrode,
we achieved a C2+/C1 ratio of 4.6, which is about a 6.5-fold improvement over the polished Cu. The overall
C2+ partial current densities are also plotted in Figure 4b, both the Cu-B and Cu-P electrodes surpass the un-
doped Cu ones for selective generation of the C2+ products over the whole potential window. Specifically, at ~
-1.15 V vs RHE, the Cu-B film delivers 9.0 mA cm-2, and the Cu-P film 8.0 mA cm-2 for the C2+ partial current
density, which is 2.3-fold and 2.0-fold as high as the polished Cu, respectively. Besides, throughout the
interested potential window from -0.80 V to -1.15 V vs RHE, the multi-carbon product generation activities
(partial current densities) on the Cu-B and Cu-P films surpass those on the polished Cu foil by a factor up to 5.4
and 4.3 at ~ -1.0 V vs RHE, respectively.
Since a similar geometric current density is observed for all the Cu electrodes (Figure 3a), the local pH
change due to proton consumption at cathode surface is unlikely the major contributor to this observed
selectivity change. Furthermore, the traditional viewpoint on the crystalline plane effect is that the (111) facet
is not favorable for C2+ product selectivity. In the meanwhile, SEM characterizations suggest these four
polycrystalline Cu electrodes show quite different morphologies of secondary particle packing density and grain
boundary density, which could potentially contribute to altering the CO2RR product distribution. To better
address this morphology effect, we have prepared another Cu-B film electrode with a prolonged chemical
plating time of 12 min. As shown in Figure S12, the 5-min and 12-min Cu-B samples show a different
morphology but a similar subsurface B doping content from DMAB, as a matter of fact, both of them
demonstrate an enhanced C2H4/CH4 selectivity as compared to the electroplated Cu, thus highlighting the
metalloid doping effect as a main factor for promoting the C-C bond formation in this study. In fact, according
to the DFT calculations,[15a] the B- doped Cu (111) electrode can greatly improve the C2+ products selectivity
as compared to the B-doped Cu (100) electrode by decreasing the energy barrier for the C-C coupling during
the CO2RR, in agreement with our experimental data.
To track the real-time surface intermediate information during the CO2RR over these plated Cu electrodes,
high sensitivity in situ electrochemical ATR-SEIRAS measurements are performed on Cu-B, Cu-P, and
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electroplated Cu film electrodes on Au-coated Si prism.[22] Figure 5a to 5c show the ATR-SEIRAS spectra
acquired during negative-going potential sweep in CO2-saturated 0.1 M KHCO3. For all the three Cu film
electrodes, linearly bonded CO (COL) from CO2 reduction shows up at ~ -0.38 V vs. RHE and thereafter, at
frequencies of ~ 2060-2077 cm-1.[25] The adsorbed CO bands are gradually blue-shifted with increasing band
intensities due to the dipole-dipole coupling effect as the potential moves stepwise from ~ -0.4 to ~ -0.7 V, and
then red-shifted at more negative potentials (< ~ -0.7 V, toward a more negatively charged surface) due to the
Stark effect, which is consistent with previous reports.[25a, 26]
We further integrate the COL band intensities as a function of applied potential for these plated Cu electrodes,
as depicted in Figure 5d. In the negative going potential sweep, the maximum COL band intensity locates at -
0.67 V for Cu-B (red), -0.71 V for Cu-P (blue) and -0.76 V for Cu (black), respectively. Moreover, with more
negative potential applied, the ν(COL) band on Cu-B or Cu-P decreases more rapidly than that on electroplated
Cu though at a similar overall current density. It is known that *CO on Cu surfaces can go through three different
pathways: 1) get directly released from surface as CO(g), 2) via *CHO pathway to form CH4(g), 3) via C-C
coupling step to form C2 and C3 products.[27] Herein, by on-line GC quantification analysis of CO(g) and CH4(g)
products (Figures 3d and 3e), it is found that both pathway (1) and pathway (2) are suppressed with B- doping
and pathway (1) is suppressed with P-doping. Given the superior C2+ generation performance on Cu-B and Cu-
P (Figure 3f-h) as compared to those on electroplated Cu, this rapid consumption of *CO on Cu-B and Cu-P
contributes mainly to the enhanced multi-carbon product generation from a more facile pathway (3) of C-C
coupling step. Specifically, even faster consumption of *CO on Cu-P at potentials negative of -0.95 V may
come from concurrent enhancement of pathway (2).
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Figure 5. In situ electrochemical ATR-SEIRAS spectra of CO on (a) Cu-B, (b) Cu-P, and (c) electroplated Cu
electrodes in CO2-saturated 0.1 M KHCO3 solution. (d) the integrated band intensity of linearly adsorbed CO
(COL) on Cu-B, Cu-P and electroplated Cu electrodes as the function of cathodic potential, respectively and
LSV curves obtained on Cu-B, Cu-P, and electroplated Cu electrode at a scan rate of 1 mV s-1 in CO2-saturated
0.1 M KHCO3 solution.
Conclusion
In summary, we have investigated the electrochemical CO2 reduction on four kinds of plated polycrystalline
Cu electrodes. Ex situ XPS, Auger and XANES analysis suggest a metallic Cu surface structure regardless the
plating condition. With the metalloid doping, both Cu-B and Cu-P exhibit an enhanced selectivity toward multi-
carbon products generation, At -1.15 V vs. RHE, 53.5% and 50.3% Faradaic efficiency (FE) toward C2+
products generation are observed on the Cu-B and Cu-P electrodes in 0.1 M KHCO3, demonstrating a 29.0- and
6.5-times enhancement of C2+/C1 ratio as compared to the polished Cu electrode, respectively. Besides,
throughout the interested potential window from -0.80 V to -1.15 V, the multi-carbon product generation activity
(partial current density) on Cu-B and Cu-P surpass the polished Cu foil by a factor up to 5.4 times and 4.3 times
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at ~ -1.0 V vs RHE, respectively. Comparative study on these plated Cu film electrodes by in situ
electrochemical ATR-SEIRAS reveals a more facile conversion of linear bounded *CO intermediate on Cu-B
and Cu-P surfaces as compared to un-doped Cu. Furthermore, in considering together on-line GC/NMR
products quantification and in situ ATR-SEIRAS analysis, we find that P-doping leads to a concurrent
enhancement for both CH4 and C2+ generation while B-doping leads to a significantly higher C2+/C1
selectivity as compared to electroplated Cu without metalloid doping, both are at the sacrifice of *CO surface
intermediate.
Acknowledgements
This work was supported by the NSFC (Grant No. 21733004), the 973 Program of MOST (Grant No.
2015CB932303), and the International Cooperation Program of STCSM (Grant No. 17520711200). K.J.
acknowledges the startup funding from Shanghai Jiao Tong University. We thank Mr. Kang Liu at Central
South University for the assistance in XANES data analysis.
Keywords
Carbon dioxide reduction; Copper plating; Copper electrode; Metalloid doping; ATR-SEIRAS
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Table of Contents Graphic
B- and P-doped Cu catalysts: metalloid-doping promotes C-C bond formation during electrochemical
CO2 reduction toward multi-carbon products on plated Cu-B and Cu-P film electrodes, in particular
the Cu-B electrode yields a significantly higher C2+/C1 ratio.
18
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