Cu3n nanocubes for selective electrochemical reduction of co2 to ethylene

Article abstract of DOI:10.1021/acs.nanolett.9b03324

Understanding the Cu-catalyzed electrochemical CO₂ reduction reaction (CO₂RR) under ambient conditions is both fundamentally interesting and technologically important for selective CO₂RR to hydrocarbons. Current Cu catalysts studied for the CO₂RR can show high activity but tend to yield a mixture of different hydrocarbons, posing a serious challenge on using any of these catalysts for selective CO₂RR. Here, we report a new perovskite-Type copper(I) nitride (Cu₃N) nanocube (NC) catalyst for selective CO₂RR. The 25 nm Cu₃N NCs show high CO₂RR selectivity and stability to ethylene (C₂H₄) at-1.6 V (vs reversible hydrogen electrode (RHE)) with the Faradaic efficiency of 60%, mass activity of 34 A/g, and C₂H₄/CH₄ molar ratio of >2000. More detailed electrochemical characterization, X-ray photon spectroscopy, and density functional theory calculations suggest that the high CO₂RR selectivity is likely a result of (100) Cu(I) stabilization by the Cu₃N structure, which favors CO-CHO coupling on the (100) Cu₃N surface, leading to selective formation of C₂H₄. Our study presents a good example of utilizing metal nitrides as highly efficient nanocatalysts for selective CO₂RR to hydrocarbons that will be important for sustainable chemistry/energy applications.

Full text of DOI:10.1021/acs.nanolett.9b03324

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Communication
Cu3N Nanocubes for Selective Electrochemical Reduction of CO2 to Ethylene
Zhouyang Yin, Chao Yu, Zhonglong Zhao, Xuefeng Guo, Mengqi Shen, Na Li, Michelle
Muzzio, Junrui Li, Hu Liu, Honghong Lin, Jie Yin, Gang Lu, Dong Su, and Shouheng Sun
Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b03324 • Publication Date (Web): 04 Nov 2019
Downloaded from pubs.acs.org on November 4, 2019
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Cu3N Nanocubes for Selective Electrochemical
Reduction of CO2 to Ethylene
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Zhouyang Yin,†,# Chao Yu,†,# Zhonglong Zhao,‡ Xuefeng Guo,† Mengqi Shen,† Na Li,§
Michelle Muzzio,† Junrui Li,† Hu Liu,† Honghong Lin,† Jie Yin,† Gang Lu,‡ Dong Su§ and
Shouheng Sun*,†
†Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States.
‡Department of Physics and Astronomy, California State University Northridge, Northridge,
California 91330, United States.
§Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York
11973, United States.
*Corresponding Author. Email: ssun@brown.edu.
Keywords: CO2 reduction • copper(I) nitride • perovskite structure • nanocubes •
ethylene formation
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Abstract: Understanding Cu-catalyzed electrochemical CO2 reduction reaction (CO2RR)
under ambient conditions is both fundamentally interesting and technologically important
for selective CO2RR to hydrocarbons. Current Cu-catalysts studied for the CO2RR can
show high activity, but tend to yield a mixture of different hydrocarbons, posing a serious
challenge on using any of these catalysts for selective CO2RR. Here, we report a new
perovskite-type copper(I) nitride (Cu3N) nanocube (NC) catalyst for selective CO2RR. The
25 nm Cu3N NCs show high CO2RR selectivity and stability to ethylene (C2H4) at -1.6 V
(vs reversible hydrogen electrode (RHE)) with the Faradaic efficiency of 60%, mass
activity of 34 A/g and C2H4/CH4 molar ratio > 2000. More detailed electrochemical, X-ray
photon spectroscopy, and density functional theory calculation suggest that the high
CO2RR selectivity is likely a result of (100) Cu(I) stabilization by the Cu3N structure, which
favors CO-CHO coupling on the (100) Cu3N surface, leading to selective formation of
C2H4. Our study presents a good example of utilizing metal nitrides as highly efficient
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nanocatalysts for selective CO2RR to hydrocarbons that will be important for sustainable
chemistry/energy applications.
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Active and selective CO2 reduction reaction (CO2RR) is an essential step to recycle the
over-produced CO2 back to reusable forms of carbon and to build an energy-sustainable
society.1-3 Electrochemical CO2RR can proceed under ambient conditions and has been
studied extensively to understand catalysis fundamentals and to develop efficient
catalysts for practical applications.4-8 Among all nanostructured catalysts tested thus far,
nanostructured Cu catalysts are unique to catalyze CO2RR to hydrocarbons.9-14 However,
past studies also show that it is difficult to control Cu surface catalysis under ambient
condition and the reaction often leads to the formation of a mixture of hydrocarbons, which
makes the separation of a specific hydrocarbon product extremely challenging. Recently,
a flow-cell design was used to convert CO2 more selectively to hydrocarbons, especially
to ethylene (C2H4) with the Faradaic efficiency (FE) reaching up to 70%.15,16 However,
this cell device does require corrosive alkaline electrolyte, such as 10 M KOH, to realize
the desired conversion.
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Here we introduce perovskite structured copper(I) nitride (Cu3N) nanocubes (NCs) as a
new catalyst for selective CO2RR to ethylene (C2H4) under ambient condition.
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Nanostructured transition metal nitrides have been studied for electrocatalytic
reactions17,18 but very limited number of metal nitrides were tested for CO2RR. Ni3N was
studied for CO2RR to CO.19 Cu3N nanowires prepared from nitridation of Cu(OH)2
nanowires were converted to multi-grained Cu nanowires to show high CO2RR selectivity
to C2 products20 due likely to the semiconductivity of Cu3N.21 Different from these nitride
catalysts, our Cu3N NCs have a phase-pure anti-ReO3 perovskite-type structure,22 and in
0.1 M KHCO3 electrolyte solution, they catalyze the CO2RR to C2H4 with 60% Faradaic
efficiency (FE) and 34 A/g Cu mass activity at -1.6 V vs RHE. More importantly, the NC
catalyst suppresses the formation of CH4 in the gas phase product with the C2H4/CH4
molar ratio > 2000, the highest selectivity ever reported on Cu-based CO2RR catalysis.
Our Cu3N NC structure is also stable in the CO2RR condition, showing only 7% FE drop
(from 60% to 53%) under continuous electrolysis for 20 h. The combined ease of
synthesis and high CO2RR catalytic selectivity and durability make the Cu3N NCs a
promising new catalyst for practical CO2RR to C2H4, an important industry feedstock used
to produce ethylene oxide, ethylene dichloride, ethylbenzene and polyethylene.
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Cu3N NCs were synthesized by modifying a published method.23 25 nm Cu3N NCs were
prepared by partially reducing Cu(NO3)2·3H2O with octadecylamine/oleylamine (2.5g/2.5
mL, ~1/0.8 mass ratio, see Methods for details) at 240°C, while 20 nm, and 10 nm NCs
were made at 250°C and 260℃, respectively. These synthetic results indicate that higher
reaction temperature initiates faster nucleation, consuming more Cu-precursor in the
nucleation stage and leading to the formation of smaller NCs. Another observation is that
the formation of cubic Cu3N requires the reaction temperature to be at 240°C or above.
Low reaction temperatures took longer times for the Cu precursor to nucleate and the
Cu3N seeds obtained from this slow nucleation event may be defected, and it is difficult
for the defected seeds to grow into NCs.24 As a result, at 230°C, 20 nm sphere-like Cu3N
nanoparticles (NPs) were separated. Figure 1a-c show transmission electron microscopy
(TEM) images of the as-synthesized Cu3N NCs with the lateral dimension at 9.7 ± 0.4
nm, 20.0 ± 0.7 nm, and 25.0 ± 1.5 nm, and Figure S1 shows the TEM image of the 20
nm Cu3N NPs. High resolution TEM (HRTEM) of a representative 25 nm Cu3N NC (Figure
1d) shows clear lattice fringes with their inter-fringe distance at 0.38 nm, which is close to
inter-planar distance of the (100) planes of the cubic Cu3N Pm3m structure. High-angle
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annular dark-field scanning TEM (HAADF-STEM) and STEM-electron energy loss
spectroscopy (STEM-EELS) (Figure 1e) show that the Cu and N are uniformly distributed
across the cubic structure. X-ray diffraction (XRD) of the 10, 20 and 25 nm Cu3N NC
powder further confirms that the NCs have the cubic Pm3m structure (Figure 1f), which
is an anti-ReO3 perovskite structure.22 These Cu3N NCs are colloidally and chemically
stable under ambient conditions as they showed neither aggregation nor obvious sign of
surface oxidation two weeks after their hexane dispersion was exposed to air (Figure S2).
Annealed at 200°C under Ar for 2 h, the Cu3N NCs showed no structure change (Figure
S3), but at 300℃ for 2 h, they were converted to Cu NPs (Figure S4), which was further
confirmed by thermal gravimetric analysis (TGA) (Figure S5). As these Cu3N NCs are
stable under ambient conditions, they are a viable catalyst for studying CO2RR.
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Figure 1. Characterizations of Cu3N NCs. a-c, low resolution TEM images of the as-synthesized
10 nm (a), 20 nm (b) and 25 nm (c) Cu3N NCs. d, high resolution TEM image of a representative
25 nm Cu3N NC, showing lattice fringe along <100> direction. e, HAADF-STEM image of Cu3N
NC and the corresponding STEM-EELS elemental mapping of the NC. f, XRD patterns of 10 nm,
20 nm and 25 nm Cu3N NCs, showing typical diffraction patterns that correspond to the
perovskite-type Pm3m structure.
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The Cu3N NCs were deposited on Ketjen carbon (Figure S6) and painted on carbon paper
(Toray TGP-H-060) for electrochemistry and electrocatalytic CO2RR studies in 0.1 M
KHCO3 (see Methods for details). The cyclic voltammetry (CV) curve of the 25 nm Cu3N
NCs under Ar atmosphere shows no Cu redox peaks (Figure 2a), which is different from
what is commonly observed from the Cu NP catalyst,25 indicating that there is no
measureable metallic Cu on the Cu3N NC surface. Other Cu3N NCs and Cu3N NPs show
similar CV behaviors (Figure S7). The increased current after -0.5 V is attributed to proton
reduction (hydrogen evolution reaction (HER)). CV under CO2 atmosphere is similar to
that under Ar condition except the drop of current after -0.5 V (Figure 2a) due to the
competition between CO2RR and HER.26
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With continuous CO2 flow, electrochemical CO2RR was tested under different potentials
and the gas/liquid products were characterized by gas chromatography and 1H NMR (see
Methods for details). The reduction potential dependent FEs of C2H4 formed in the
presence of different Cu3N catalysts were listed in Figure 2b. The general trend is that
the larger the NCs, the better the CO2RR selectivity. Other smaller NCs are less effective
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and their peak FEs reach 45% (20 nm NCs) and 35% (10 nm NCs) at -1.4 V. The sphere
20 nm Cu3N NP catalyst is least active with the FE of C2H4 being less than 10% in the
reduction potential range tested. The 25 nm Cu3N-catalyzed CO2RR generates C2H4
(14% FE) at -0.8 V. At -1.6 V, the FE of C2H4 reaches the maximum 60% with the mass
activity of 34 A/g and current density of 30 mA/cm2 (Figure S8), and their overall CO2RR
results are summarized in Figure 2c. In addition to C2H4, the gas products obtained from
various reduction potentials also contain H2 (< 40% FE) from HER, and CO (<9% FE)
(CO was undetectable at -1.6 V or beyond). The liquid phase products separated from
the reaction contain only formate (<33% FE) and ethanol (< 5.7% FE) over the potential
range we studied. The 25 nm Cu3N-catalyzed CO2RR is most selective at -1.6 V for
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generating C2H4 (60%) with remarkable current density (the H NMR spectrum of the
corresponding minor liquid products HCOOH and ethanol is shown in Figure S9.). We
measured the capacitive behaviors of the Cu3N NCs in a potential range from -0.05 to 0.05 V
at different scanning rates, and obtained their double-layer capacitance of 4.34 mF/cm2 (Figure
S10), which is 150 times larger than the standard capacitance of smooth Cu electrode
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(0.029 mF)27, suggesting that high current density observed from the CO2RR originates
from the large electrochemical surface area of the NC-electrode.
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We should note that compared with the common Cu metal catalyst, our Cu3N catalyst
show higher CO2RR overpotentials due most likely to the decrease in electron
conductivity of the Cu3N structure. However, since 25 nm Cu3N NCs are more active and
selective than smaller Cu3N NCs or Cu3N NPs for the CO2RR to C2H4, it is evident that
Cu3N NCs with larger fractions of (100) facets are especially selective to catalyze CO2RR
to C2H4. Another important feature of the Cu3N NC catalysis is that the catalyst
suppresses the formation of C2H6 and CH4 (only <1% FE of C2H6 + CH4 was detected),
and show the highest C2H4/CH4 ratio (> 2000) ever reported for the Cu-catalyzed
CO2RR.28
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Figure 2. Cu3N-catalyzed CO2RR in 0.1 M KHCO3 solution. a, CV curves of Cu3N NCs under Ar
(black) and CO2 (red) atmosphere. b, Reduction potential dependent ethylene formation FE of
CO2RR catalyzed by different Cu3N catalysts. c, Product distribution of CO2RR catalyzed by 25
nm Cu3N NC catalyst at different reduction potentials. d, Reduction current change (black) and
ethylene formation FE (red) over time during the continuous CO2RR catalyzed by 25 nm NCs at
-1.6 V.
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The stability of the Cu3N-catalyzed CO2RR was studied at -1.6 V (Figure 2d). The
reduction current dropped from 135 mA in the first hour to 101 mA 20 h after the continued
electrolysis. The C2H4 formation FE was at 60% in the first hour, and was stabilized at
around 53%. TEM image of the Cu3N NCs after the 20 h of electrolysis (Figure S11)
shows that there is only small degree of NC aggregation, which supports why there is the
7% FE drop from 60% to 53% during the 20 h electrolysis at -1.6 V. Compared to Cu-
based nanocatalysts, especially Cu NCs,29-31 reported previously with very limited stability
in the selected CO2RR condition, our Cu3N NCs show much enhanced stability,
suggesting that N3- stabilizes Cu+ more efficiently in the perovskite-type Cu3N structure.
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To confirm that the high CO2RR selectivity to C2H4 is from the Cu3N (100), not from Cu
(100), we prepared 29.3 ± 3.2 nm Cu NCs, denoted as 29 nm Cu NCs (Figure 3a)32 and
studied/compared their electrochemical CO2RR with 25 nm Cu3N NCs. As seen in Figure
3b, on Cu NCs, C2H4 is produced starting from -0.8 V, and the highest FE of C2H4 is about
15.4% at -1.4 V. As a comparison, the Cu3N NCs show much higher CO2RR selectivity
to C2H4 in all reduction potentials from -0.8 V to -1.8 V. As both 29 nm Cu NCs and 25
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nm Cu3N NCs have similar sizes and cube morphology, the superior CO2RR performance
of Cu3N NCs over Cu NCs suggests that the Cu-N units in the Cu3N NC structure is
essential for the NCs to achieve high CO2RR selectivity towards C2H4.
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Figure 3. Catalytic performance and surface properties of Cu and Cu3N catalysts. a, TEM image
of as-synthesized 29 nm Cu NCs. b, Reduction potential-dependent FE of ethylene obtained from
CO2RR catalyzed by 25 nm Cu3N NCs (red) and 29 nm Cu NCs (black). c, XPS of Cu and Cu3N
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NCs showing Cu 2p and Cu(II) peaks. Cu/Cu(I) peaks are too close to separate from the spectra.
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d, XPS of Cu and Cu3N NCs showing N 1s binding energy.
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To identify which Cu oxidation state, Cu(0) or Cu(I), on the Cu3N surface dominates the
CO2RR selectivity, we studied surface Cu oxidation and reduction properties of both Cu
and Cu3N NCs using X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry
(CV). The XPS of Cu NCs shows typical Cu 2p peaks at 932 and 952 eV, as well as two
Cu(II) peaks at 934 and 944 eV (Figure 3c), but they are free from N 1s peak (Figure 3d),
indicating that Cu NC surface has Cu, Cu(I) and Cu(II) components. Cu3N NCs also show
two Cu 2p peaks that slightly shift to higher energies, and a nearly invisible Cu(II) peak.
This indicates that unlike Cu NCs, the Cu3N NCs have Cu(I) enriched on their surfaces.
N 1s peak is at around 398 eV (Figure 3d), close to the common value (~397 eV) observed
from metal nitrides. CV of the Cu NCs in Ar-saturated 0.1 M KHCO3 solution exhibit two
reduction peaks at 0.35 V (Cu(II)/(Cu(I)) and -0.05 V (Cu(I)/Cu) (Figure S12), indicating
that Cu NC surface after the oxidation scan is surrounded with Cu(II) and Cu(I). These
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two reduction peaks shift to more positive potentials under CO2-saturated condition due
likely to the CO2 interaction with Cu(I)/Cu(II). Under the same CV scanning condition,
Cu3N NCs show no obvious reduction peaks (Figure 2a), indicating that on the Cu3N
surface, Cu+ is better stabilized by N3-, and Cu-N must show the desired synergistic effect
on selective CO2RR to C2H4.
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Figure 4. Relative free energy diagram and electronic density of states. a, Relative free energy
diagram for the coupling between CHOads and COads on Cu (100) and Cu3N (100) surface,
respectively. The optimized atomic geometries of the initial, transition, and final states are shown
in the insets. Blue, blue grey, grey, red, and white spheres represent Cu, N, C, O, and H atoms,
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respectively. b, The calculated density of states of the d-band on Cu (100) and Cu3N (100) surface.
The horizontal dashed lines indicate the d-band centers.
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We performed density functional theory (DFT) calculations to elucidate why Cu3N NCs
are so selective for the CO2RR to C2H4. As C-C coupling is essential for the formation of
C2 products, such as C2H4, we focus on the relevant C-C coupling reactions on the Cu3N
(100) surface. Previous work33 has established that C-C bond can be formed on a Cu
(100) surface via the coupling between two CO* molecules (to form OCCOads) or between
CHO* and CO* (to form OCCHOads). Our calculations show that the CO-CO coupling is
not favored on the Cu3N (100) surface, while the CO-CHO coupling has the energy
benefits, as shown in Figure 4a. The free energy barrier for the formation of OCCHOads
on the Cu3N (100) surface is 0.36 eV, less than half of that on the Cu (100) surface (0.74
eV). From the atomic structures of the initial, transition, and final states of the CHO-CO
coupling reaction (inset of Figure 4a), we can see that on the Cu (100) surface, CHOads,
COads, and OCCHOads occupy the Cu bridge sites, while on the Cu3N (100) surface, they
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are stabilized on the Cu(I) top sites due likely to the upshift of the Cu d-band center on
the Cu3N surface relative to the Cu (100) surface (Figure 4b), as well as the N3- induced
protonation of CO to CHO. The high energy barrier for the CO-CO coupling on the Cu3N
surface suggests that the CO-CO pathway leading to the formation of C2 products is
suppressed, leaving the OCCHO coupling a dominant pathway to C2H4.34-36
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In this work, we report a new catalyst based on Cu3N NCs for electrochemical CO2RR.
Among 10, 20, and 25 nm NCs studied, 25 nm Cu3N NCs show highest activity and
selectivity to convert CO2 to C2H4 with FE reaching 60%. More importantly, at the C2H4
formation potential (-1.6 V), C2H4 is the dominant gas product with C2H4/CH4 ratio being
>2000, and liquid products contain mostly formate (<15% FE), which makes the
separation of C2H4 from the CO2RR practical. Our studies confirm that Cu(I) is stabilized
by N in the Cu3N structure and the stabilized Cu(I) is key for the (100) Cu(I) to show
enhanced CO2RR selectivity to C2H4. DFT calculations suggest that the stabilized (100)
Cu(I) sites are energetically favored for OC-CHO (not CO-CO) coupling, inferring that the
formation of COCHOads is a dominant pathway in the Cu3N-catalyzed CO2RR to C2H4.
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This Cu3N-induced catalysis enhancement on selective CO2RR may offer a new direction
to develop highly efficient nanocatalysts for selective CO2RR to C2H4 or other
hydrocarbons.
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications website
Corresponding Author
*Prof. Shouheng Sun
Department of Chemistry, Brown University, Providence, Rhode Island 02912
E-mail: ssun@brown.edu
Author Contributions
#These authors contributed equally to this work.
Notes
The authors declare no competing financial interest
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Funding Sources
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The work was supported by the American Chemical Society Petroleum Research Fund
(57114-ND5) and in part by the Center for the Capture and Conversion of CO2, a Center for
Chemical Innovation funded by the National Science Foundation, CHE-1240020. M.M. is
supported by the National Science Foundation Graduate Research Fellowship, under Grant No.
1644760. The electron microscopy work used resources of the Center for Functional
Nanomaterials, which is a US DOE Office of Science Facility, at Brookhaven National Laboratory
under Contract No. DE-SC0012704. The work at California State University Northridge was
supported by the National Science Foundation, DMR-1828019.
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Notes
Any additional relevant notes should be placed here.
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SYNOPSIS (Word Style “SN_Synopsis_TOC”).
The perovskite-type copper(I) nitride nanocube catalyst has stable (100) Cu(I)-N facets,
favoring OC-CHO coupling for selective electrochemical reduction of CO2 to ethylene.
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