AUCU NANOFIBERS FOR ELECTROSYNTHESIS OF UREA FROM CARBON DIOXIDE AND NITRITE
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Article
AuCu nanofibers for electrosynthesis of urea
from carbon dioxide and nitrite
Songliang Liu, Shuli Yin, Ziqiang
Wang, You Xu, Xiaonian Li,
Liang Wang, Hongjing Wang
hjw@zjut.edu.cn
Highlights
AuCu nanofibers were prepared
via a one-step synthesis strategy
The catalysts possess Boerdijk-
Coxeter structure and abundant
structural defects
The catalysts exhibit noteworthy
performance for electrosynthesis
of urea
The strategy provides a strategy
for preparing defect-rich
nanofibers
Liu et al. report that AuCu self-assembled nanofibers with high-quality one-
dimensional nanofiber structure are prepared in 4-aminopyridine solution by a
one-pot chemical reduction method. They are tested as an efficient catalyst for
coupling CO2RR with NO2 RR to prepare urea.
Liu et al., Cell Reports Physical Science 3,
100869
May 18, 2022 ª 2022 The Author(s).
https://doi.org/10.1016/j.xcrp.2022.100869
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AuCu nanofibers for electrosynthesis
of urea from carbon dioxide and nitrite
Songliang Liu,1 Shuli Yin,1 Ziqiang Wang,1 You Xu,1 Xiaonian Li,1 Liang Wang,1 and Hongjing Wang1,2,*
SUMMARY
Carbon dioxide reduction reaction (CO2RR) is a promising technol-
ogy for mitigating greenhouse gas emission and achieving carbon
neutrality. However, coupling CO2RR with other reactions to pro-
duce high value-added chemicals remains a challenge. In this
work, we report self-assembled nanofibers composed of ultrathin
AuCu alloy nanowires possessing a Boerdijk-Coxeter structure
with (111)-dominant facets for the electrosynthesis of urea by
coupling CO2RR with nitrite reduction reaction (NO2RR). The rich
structural defects and AuCu bimetallic alloy composition provide a
large number of highly catalytically active sites. The constructed
AuCu nanofibers display excellent urea synthesis performance in
the electrolyte solution containing 0.01 M KNO2 with continuous
drumming of CO2, achieving a high urea yield rate of up to
3,889.6 mg h1 mg1cat. and a high faradic efficiency of 24.7%
at 1.55 V (versus Ag/AgCl). This work provides a feasible method
for the rational design of self-assembled bimetallic nanofibers for
electrosynthesis of urea under ambient conditions.
INTRODUCTION
Carbon dioxide (CO2) is one of the main gases that contribute to the greenhouse
effect.1–5 With the rapid consumption of fossil fuels, CO2 emissions will continue
to increase.6–9 In addition, due to the intervention of anthropogenic activities (indus-
trial production emissions, overfertilization), large amounts of nitrogen pollutants
(nitrite [NO2] and nitrate [NO3]) enter the ground to pollute water sources and
pose a potential threat to the environment and human health.10–13 Therefore, the
simultaneous reduction of CO2 and NOx to high value-added products is a renew-
able and economical promising approach.
Urea (CO(NH2)2) is an essential source of nitrogen for fertilizers and is widely applied
as raw material for pesticides and medicines.14–16 However, the traditional method
of synthesizing urea requires it to be carried out under harsh conditions of high tem-
perature and pressure and consumes large amounts of fossil fuels.17,18 Therefore,
much effort has been devoted to explore green and sustainable strategies for
urea synthesis.19–21 Recently, Wang and coauthors prepared urea by an electro-
chemical method coupling the N2 reduction reaction with the CO2 reduction
reaction (CO2RR).22 Nevertheless, the limited solubility of N2 in water and the high
1StateKey Laboratory Breeding Base of
activation energy barrier of NhN led to a low urea yield rate (rurea) and the corre-
Green-Chemical Synthesis Technology, College
sponding poor faradic efficiency (FE). Compared with N2, the high solubility of of Chemical Engineering, Zhejiang University of
NO2 in water and its low activation energy barrier make the utilization of NO2 Technology, Hangzhou 310014, P.R. China
as a nitrogen source for electrochemical reduction reaction coupled with CO2RR 2Lead contact
to produce urea more attractive.23 For instance, Zhang and coauthors synthesized *Correspondence: hjw@zjut.edu.cn
urea by coupling CO2RR with the NO2 reduction reaction (NO2RR) on oxygen https://doi.org/10.1016/j.xcrp.2022.100869
Cell Reports Physical Science 3, 100869, May 18, 2022 ª 2022 The Author(s). 1
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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vacancy-rich ZnO porous nanosheets.24 However, this strategy also involves individ-
ual CO2RR and NO2RR, which can generate complex product distribution, thus
reducing the FE of the urea and increasing the difficulty of separating urea from
the product.25,26 Therefore, the design and construction of electrocatalysts with
high catalytic activity and selectivity are essential for the electrochemical synthesis
of urea.
Cu-based catalysts have been proven to be highly efficient electrocatalysts for
driving CO2RR and NO2RR, but achieving highly selective synthesis of urea remains
challenging.27–32 The design and preparation of Cu-based catalysts with unique
morphology, structure, and composition is a feasible approach to significantly
improve catalytic activity and selectivity.33–38 Considering this from the aspect of
compositional engineering, alloying Cu with Au or other metals with high CO selec-
tivity to CO2RR could change the coordination structure and electronic structure of
Cu atoms, enhance the adsorption and activation of CO2, and increase the catalytic
selectivity to reduce the types of products.39–41 From the perspective of structural
design, one-dimensional (1D) nanoarchitectures have unique structural features
such as anisotropy, high flexibility, high aspect ratio, and low aggregation, which
have wide applications in the field of electrocatalysis.42–46 Among them, the con-
struction of self-assembled nanofibers (SANFs) by ultrafine 1D nanowires (NWs)
could provide a large surface area, a large number of exposed surface atomic active
sites, and excellent electron transport capacity. Despite the effort that has been
devoted to the self-assembly of NWs, until now there has been no simple and
effective method to self-assemble NWs into nanofibers.47,48 Based on the above
considerations, there is a great deal of interest in developing an effective strategy
to prepare AuCu alloyed nanofibrous catalysts for the electrocatalytic coupling of
urea synthesis.
In this work, we successfully fabricate a novel type of AuCu SANFs for the electrosyn-
thesis of urea by coupling CO2RR with NO2RR. With the assistance of 4-
aminopyridine (4-AP), AuCu nanofibers are self-assembled from AuCu ultrafine
NWs with (111)-dominant facets and Boerdijk-Coxeter structure. When used for
electrochemical urea production, the prepared AuCu SANFs exhibit excellent rurea
(3,889.6 mg h1 mg1cat.) and high FE (24.7%) in electrolytes with continuous drum-
ming of CO2. The excellent urea synthesis activity and stability of AuCu SANFs could
be attributed to the 1D parallel NW fibrous structure and AuCu bimetallic composi-
tion. This work provides an effective strategy for the simple preparation of nanofi-
brous structure and expands the application of CO2RR to successfully prepare
urea with high added value.
RESULTS
Materials characterization
The preparation process of the AuCu SANFs is illustrated in Scheme 1. AuCu SANFs
are self-assembled from ultrathin AuCu alloy NWs with the assistance of 4-AP in a
water bath at 20 C. The morphology and microstructure of the AuCu SANFs are
investigated by scanning electron microscopy (SEM), high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM), and TEM. The typical
SEM image of the AuCu SANFs displays a 1D curved nanofiber architecture with a
high yield (Figure 1A). HAADF-STEM and TEM images further confirm the formation
of uniform anisotropic nanofibrous structures, consistent with the SEM observation
results (Figures 1B and 1C). Focusing on a single 1D nanofiber, it can be observed
clearly that a nanofiber is self-assembled from numerous ultrathin NWs parallel to
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Scheme 1. The fabrication of the AuCu SANFs
each other (Figure 1D). As revealed by the HAADF-STEM image and the correspond-
ing energy-dispersive X-ray (EDX) elemental mapping, Au and Cu elements are uni-
formly distributed in the nanofibers, supporting the formation of AuCu alloys
(Figures 1E-1G). The EDX analysis of AuCu SANFs indicates that the atomic ratio
of Au:Cu is 1:1 (Figure S1).
The selected area electron diffraction (SAED) pattern strongly demonstrates the
polycrystalline features of the AuCu SANFs (Figure 2A). The high-resolution TEM
(HRTEM) images show an average diameter of 2.71 G 0.02 nm for the individual
NW, with the lattice spacing (0.23 nm) indexed to the (111) plane of face-
centered-cubic (fcc) AuCu alloy (Figures 2B and 2C).40 Moreover, a representative
set of HRTEM images of the individual AuCu NW further reveal the domains of nano-
crystal facets along the longitudinal direction of the NW (Figures 2D–2F). The
average value of the lattice stripe spacing from the different domains of AuCu NW
is 0.23 nm. Furthermore, AuCu NWs exhibit abundant twin boundaries (red ar-
rows), stacking faults (blue arrows), and atomic steps (circles), which could be used
as high catalysis active sites (Figures 2D–2F).49–51 More important, the individual
AuCu NWs that make up the nanofibers are all Boerdijk-Coxeter-type helices (line-
arly stacked regular tetrahedra, Figure 2G) and mostly terminated by (111)-type
planes. The obtained Boerdijk-Coxeter-type helix of AuCu NWs are similar to the
helix that appears in Au,52 AuPt,47 and AuAg53 NWs.
The X-ray diffraction (XRD) pattern is used to investigate the crystal structure of the
prepared nanofibers. The XRD pattern of AuCu SANFs exhibits typical diffraction
peaks of the metallic fcc structure (Figure S2). It is noted that the diffraction peaks
of AuCu SANFs in the XRD pattern are located between the diffraction peaks of stan-
dard fcc Au (Joint Committee on Powder Diffraction Standards [JCPDS]-65-8601)
and Cu (JCPDS-04-0836).40 In addition, compared with pure Au NWs, the AuCu
SANFs diffraction peak positions are slightly shifted to higher angles, indicating
the concomitant lattice contraction due to the introduction of Cu atoms into the
Au atomic layer, which further confirms the formation of the AuCu alloy.54 The sur-
face electronic states and the chemical composition of AuCu SANFs are further
investigated by X-ray photoelectron spectroscopy (XPS). In the XPS spectrum of
Au 4f, the typical peaks with binding energies at 87.4 eV could be indexed to Au
4f5/2, while that at 83.7 eV could be indexed to Au 4f7/2 (Figure 3A). It is worth noting
that the binding energy of Au 4f in AuCu SANFs has a positive shift compared with
that of pure Au NWs, which indicates that the introduction of Cu atoms can reduce
the electron density of Au atoms.54 In the high-resolution Cu 2p XPS spectrum, two
major peaks at 951.3 and 931.6 eV could be assigned to Cu(0) 2p1/2 and Cu(0) 2p3/2,
respectively (Figure 3B). The observed low content of Cu(II) species appearing at
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Figure 1. Structure of the AuCu SANFs
(A–D) SEM (A), (B) HAADF-STEM, and (C and D) TEM images of the AuCu SANFs.
(E–G) HAADF-STEM image (E) and (F and G) EDX mapping images of the AuCu SANFs.
953.7 and 933.8 eV is attributed to Cu(II) 2p1/2 and Cu(II) 2p3/2, probably owing to
the oxidation of AuCu SANFs in air.40,55 In addition, the peaks located at 962.1
and 942.5 eV could correspond to the satellite peaks of Cu 2p1/2 and Cu 2p3/2,
respectively.
A series of controlled experiments was carried out to explore the mechanism of the
formation of AuCu SANFs. In the absence of 4-AP, irregularly agglomerated nano-
particles are formed, while the excess of 4-AP causes the nanofibers to stack
together (Figure S3). 4-AP acts as a structure-directing agent in the synthesis system,
and an appropriate amount of 4-AP is essential for the successful preparation of
nanofibers. Replacing 4-AP with other structure-directing agents (F127, DM970,
PVP, CTAB) fails to form unique nanofibers (Figure S4). As shown in Figure S5,
when L-ascorbic acid (AA) is replaced with more reductive reagents, the faster reduc-
tion rates lead to the formation of bimetallic AuCu alloys with 1D NW structures.
NWs with uneven thickness are obtained by using N2H4 as a reducing agent (Fig-
ure S5A). HCOOH as a reducing agent turns the prepared AuCu SANFs into a
mixture of nanoparticles and NWs (Figure S5B). The worm-like NWs are obtained us-
ing the more reductive NaBH4 (Figure S5C). These results indicate that the choice of
AA with moderate reducing kinetics as a reducing agent is crucial for the formation of
AuCu SANFs. By changing the Au:Cu precursor ratio, Au NWs, Cu bulk particles, and
Au-Cu SANFs with different Au:Cu ratios were obtained (Figure S6). Interestingly, we
find that controlling the reaction temperature could achieve self-assembly and
disassembly of nanofibers (Figure S7). With the gradual increase in the reaction tem-
perature, the nanofibers are gradually disassembled. When the temperature is
increased to 100 C, the nanofibers are completely disassembled into uniform 1D ul-
trafine NWs (Figure S7F). The results reveal that lower reaction temperature could
slow down the reduction ability of AA, which is more favorable to the formation of
AuCu SANFs.
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Figure 2. Characterization of the AuCu SANFs
(A and B) SAED (A) and (B) HRTEM images of the AuCu SANFs. The insets in (B) display the lattice
fringes of the square region and corresponding fast Fourier transform (FFT) pattern.
(C–G) HRTEM images of several individual AuCu nanowires (NWs; red arrows [twin boundaries],
blue arrows [stacking faults], and circles [atomic steps]) (C–F), and (G) a model illustration of the
Boerdijk-Coxeter-type helical NW.
The potential formation mechanism of AuCu SANFs could be acquired from the re-
sults of the above research. The metal precursors (HAuCl4, CuCl2) are rapidly ad-
sorbed by 4-AP at 20 C. After the addition of AA, the precursors are slowly reduced
to Au and Cu atoms. As the reaction progress, the metal atoms form AuCu cores
through Ostwald maturation assembly, which reduces the total surface free energy
in the system.56,57 Then, with the assistance of 4-AP, ultrafine AuCu NWs are formed
by anisotropic epitaxial growth. Finally, to minimize the total surface free energy, the
ultrafine AuCu NWs with abundant structural defects undergo strong cohesive inter-
actions when in contact with one another and further self-assemble into AuCu SANFs
under the constraint of 4-AP.47
Electrocatalytic performances for CO2RR
AuCu SANFs were conducive to the adsorption and activation of the CO2, NO2
species due to their 1D self-assembled nanofiber structure and the synergistic ef-
fects between Au and Cu elements.30,39,58 Urea electrosynthesis by coupling
CO2RR with NO2RR was evaluated by the potentiostatic method using H-type elec-
trolytic cells at ambient temperature and pressure (Figure S8). First, the CO2RR and
NO2RR properties of AuCu SANFs were investigated. Figure 4A shows the linear
sweep voltammetry (LSV) curves of AuCu SANFs measured in Ar-saturated and
CO2-saturated 0.5 M KHCO3 solutions. It is obvious that the specific current den-
sities of AuCu SANFs in the CO2-saturated 0.5 M KHCO3 solution is higher than
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Figure 3. Characterization of the AuCu SANFs and Au NWs
(A) XPS spectra of the Au 4f for the Au NWs and AuCu SANFs.
(B) Cu 2p peaks of the AuCu SANFs.
that in the Ar-saturated 0.5 M KHCO3 solution, indicating that an effective electro-
catalytic CO2RR occurs on AuCu SANFs. Then, CO2 is passed into the 0.5 M
KHCO3 solution at a flow rate of 20 mL min1 and electrolyzed at the selected poten-
tial. There is no significant change for all of the curves, indicating that the AuCu
SANFs possess excellent catalytic stability for CO2RR (Figure S9). As shown in Fig-
ure 4B, CO is the main product of CO2 reduction occurring on AuCu SANFs, and
the FE of CO (89.5%) reaches its highest level at 1.25 V (versus Ag/AgCl). As the
potential continues to decrease, the FE of CO significantly decreases due to the
enhancement of the competing hydrogen evolution reaction (HER). The partial cur-
rent density corresponding to CO reaches 3.8 mA cm2 at 1.25 V (versus Ag/
AgCl) and the H2 partial current density is 0.4 mA cm2, resulting in the FE of H2
of only 10.5%, indicating the AuCu SANFs have efficient electrocatalytic reduction
activity and selectivity for the conversion of CO2 to CO (Figures 4C and S10). The sta-
bility of the CO2 reduction for AuCu SANFs is investigated by long-term chronoam-
perometric measurement at 1.25 V (versus Ag/AgCl) for 20 h. The current density
only slightly decays and the FE of CO is almost unchanged, indicating that the AuCu
SANFs still maintain high CO2RR activity after long-term electrolysis (Figure 4D).
Compared with the recently reported AuCu alloy catalysts, AuCu SANFs also exhibit
excellent FE of CO production (Table S1).
Electrocatalytic performances for NO2RR
Subsequently, the NO2RR performance of the AuCu SANFs was investigated. Fig-
ure 5A shows the LSV curves in the absence and addition of 0.01 M KNO2 in 0.5 M
KHCO3 solution. Compared to the electrolyte without NO2, the LSV curve in the
presence of NO2 shows a more positive onset potential and a larger current
density, indicating that the NO2RR occurs on the AuCu SANFs. Next, a chronoam-
perometry test is carried out at the selected potential for 2 h to evaluate the
electrocatalytic NO2 performance of the AuCu SANFs (Figure S11). The absorption
intensities of the remaining NO2 and the generated NH3 are measured using UV-
visible (vis) spectroscopy. The FE of NH3, NH3 yield rate (rNH3), and NH3 selectivity
at the corresponding potentials are calculated from the calibration curves
(Figures S12 and S13). The FE (78.7%) of NH3 for AuCu SANFs reaches the highest
value at 1.45 V (versus Ag/AgCl), and the corresponding rNH3 also reaches
5,254.1 mg h1 mg1cat. (Figures 5B and 5C). When the potential continues to
decrease, the rNH3 continues to increase, but the corresponding FE sharply
decreases due to the competitive HER. In addition, the NH3 selectivity of the
AuCu SANFs reaches 91%, and the selectivity of the by-products (e.g., N2) is only
9% (Figure 5D). The partial current density corresponding to NH3 at 1.45 V (versus
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Figure 4. Electrocatalytic performances for CO2RR of the AuCu SANFs
(A) LSV curves of the AuCu SANFs in an Ar-saturated and a CO 2 -saturated 0.5 M KHCO 3 solution.
(B) CO faradic efficiency for the AuCu SANFs examined with different applied potentials.
(C) The corresponding CO partial current density.
(D) The stability test conducted at 1.25 V (versus Ag/AgCl). for the AuCu SANFs. The error bars
represent SDs.
Ag/AgCl) reaches 9.8 mA cm2 and the H2 partial current density is only 1.7 mA
cm2, indicating that the NO2RR on AuCu SANFs can inhibit the occurrence of HER
(Figure S14). It is worth mentioning that the NH3 selectivity of AuCu SANFs is com-
parable to that of recently reported electrocatalysts, such as Ni-NSA-VNi.59
Electrochemical performances of urea production by coupling CO2RR with
NO2RR
The AuCu SANFs were further used as a catalyst for the preparation of urea by
coupling CO2RR with NO2RR. The CO2 was continuously drummed into the
0.5 M KHCO3 solution containing 0.01 M KNO2. Figure 6A shows the LSV curves
with and without CO2, which observed that the AuCu SANFs have a significant cur-
rent response to CO2RR. In addition, the LSV curve after the addition of 0.01 M KNO2
and CO2 exhibits the most positive onset potential and the largest current density
compared to those after the addition of 0.01 M KNO2 and the addition of CO2, indi-
cating that both CO2RR and NO2RR simultaneously occurred on AuCu SANFs
(Figure S15). The chronoamperometry test was carried out at selected different po-
tentials for 2 h. All of the curves did not significantly change, and the current density
maintained good stability (Figure S16). The generated urea was decomposed into
NH3 by urease.23,25 The absorption intensity of NH3 converted from urea was
measured by UV-vis spectrophotometry, and the concentration was further deter-
mined from the calibration curve (Figure S17). The FE of urea for AuCu SANFs
reached the highest level of 24.7% and the corresponding rurea was 3,889.6 mg
h1 mg1cat. at 1.55 V (versus Ag/AgCl) (Figures 6B and 6C). This is comparable
to the recently reported performance of ZnO-V porous nanosheets,24 lying between
those of Te-doped Pd nanocrystal23 and oxygen vacancy-rich anatase TiO2.25 As the
potential decreases further, the FE of urea decreases due to enhanced side reac-
tions. At the optimum potential of 1.55 V (versus Ag/AgCl), the FEs of H2, NH3,
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Figure 5. Electrocatalytic performances for NO2RR of the AuCu SANFs
(A) LSV curves of the AuCu SANFs in 0.5 M KHCO 3 solution without or with 0.01 M KNO2 .
(B and C) NH 3 faradic efficiency (B) and (C) NH 3 yield rate for the AuCu SANFs examined with
different applied potentials.
(D) Selectivity of NH 3 and N 2 of the AuCu SANFs at 1.45 V (versus Ag/AgCl) for 2 h. The error bars
represent SDs.
N2, and CO were 16.4%, 10.1%, 4.6%, and 42.7%, respectively (Figure 6D). The FE of
H2 production for the AuCu SANFs in 0.5 M KHCO3 + 0.01 M KNO2 solution with
CO2 at 1.55 V (versus Ag/AgCl) is decreased significantly, showing that the
CO2RR and NO2RR occurring on AuCu SANFs greatly inhibit the HER process (Fig-
ure S18). The urea selectivity, NH3 selectivity, and by-product (e.g., N2) selectivity of
the AuCu SANFs at 1.55 V (versus Ag/AgCl) were 62.7%, 25.5%, and 11.8%,
respectively (Figure 6E). The electrocatalytic stability of the AuCu SANFs was evalu-
ated by performing five continuous cycles at 1.55 V (versus Ag/AgCl). The FE of
urea and rurea remain almost unchanged after the cycles, indicating that the AuCu
SANFs have excellent catalytic stability for the preparation of urea coupled by
CO2RR and NO2RR (Figure 6F). The morphology and structure of AuCu SANFs
remain almost unchanged after the cycles (Figure S19). To explore the influence of
bimetallic composition on the performance of urea synthesis, samples with different
Au/Cu content were measured by chronoamperometry at 1.55 V (versus Ag/AgCl).
As shown in Figure S20, the rurea and FE of AuCu SANFs are greater than those of
other Au-Cu content samples. The enhanced performance of AuCu SANFs in prepar-
ing urea may be due to the synergistic effect between Au and Cu elements, and the
appropriate AuCu content can change the charge distribution between Au and Cu
and promote the formation of C-N.
A series of comparative experiments were performed to confirm further that the urea
generated originated from CO2 reduction and NO2 reduction and to eliminate the
influence of environmental or impurities in the reaction system on the measurement
results (Figures S21–S23). As shown in Figure S21, the FE of NH3 and the FE of CO
under the CO2 + NO2 condition are lower than those under the NO2 condition and
the CO2 condition, respectively. Moreover, there is a significant difference in FE at
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Figure 6. Electrochemical performances of urea production by coupling CO2RR with NO2RR of the AuCu SANFs
(A) LSV curves of the AuCu SANFs in 0.5 M KHCO 3 + 0.01 M KNO 2 solution without or with CO2 .
(B and C) Urea faradic efficiency (B) and (C) urea yield rate for the AuCu SANFs examined with different applied potentials.
(D) Faradic efficiency for different products over the AuCu SANFs at 1.55 V (versus Ag/AgCl).
(E) Selectivity of NH 3 , urea, and by-product (e.g., N 2 ) of the AuCu SANFs at 1.55 V (versus Ag/AgCl) for 2 h.
(F) Urea faradic efficiency and yield rate over the AuCu SANFs during 5 continuous cycles. The error bars represent SDs.
1.55 V (versus Ag/AgCl), indicating that urea production is probably attributed to
the consumption of CO and NH3. Under various conditions, urea was produced only
under CO2 + NO2 conditions, indicating that the urea was indeed derived from
both CO2 reduction and NO2 reduction (Figure S22). In addition, no urea was de-
tected after replacing CO2 with CO or NO2 with NH4+, suggesting that urea forma-
tion originated from intermediates before the formation of CO and NH3 (Figure S22).
To further confirm the source of urea, we conducted the isotopic labeling experi-
ment using 15NO2 as the source of nitrogen.19,20,22 As shown in Figures S23, 1H
NMR spectra of a post-electrolysis electrolyte is recorded. In contrast to the single
peak of 14N, a double peak of 15N can be observed, both exhibiting the same
peak shape and peak position as the standard sample. Based on the above results,
a possible mechanism of urea electrosynthesis was proposed (Figure S24). CO2
could be reduced to CO* intermediates (Equation 1) and NO2 could be reduced
to NH2* precursors (Equation 2) on the AuCu SANFs. Subsequently, the NH2* pre-
cursors were coupled to the CO* intermediates via a tandem reaction pathway,
thereby being converted into urea (Equation 3).
CO2 + 2H + + 2e / CO + H2 O (Equation 1)
NO2 + 6H + + 6e /NH2 + 2H2 O (Equation 2)
CO + 2NH2 / NH2 CONH2 (Equation 3)
AuCu SANFs exhibit enhanced electrocatalytic activity and excellent stability, which
could be attributed to the following reasons: (1) AuCu SANFs are composed of unique
1D AuCu NWs arranged in parallel with anisotropic and high surface area characteris-
tics, which enhance the utilization of Au atoms and Cu atoms, accelerate electron trans-
port, and could effectively avoid the agglomeration and dissolution of catalysts.
Furthermore, the 1D AuCu NWs constituting AuCu SANFs have a large number of
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structural defects (twin boundaries, stacking faults and atomic steps) acting as active
centers for catalysis.49–51,60,61 (2) AuCu SANFs have a Boerdijk-Coxeter structure
with parallel bundles of (111)-dominant facets in different orientations, which could
generate lattice strain and modulate the electronic structure, thus contributing to
enhanced electrocatalytic performance.47,62 (3) The bimetallic composition is also
essential for the excellent electrocatalytic activity and stability of the AuCu SANFs.
AuCu alloying could modulate the coordination state and electronic structure of Au
and Cu,39,40 which facilitates the adsorption and activation of reactants (CO2, NO2),
thus promoting the electrocatalytic coupling to produce urea.
DISCUSSION
In summary, AuCu SANFs with a high-quality 1D nanofiber structure have been success-
fully prepared in 4-AP solution by a one-pot chemical reduction method and tested as
an efficient catalyst for coupling CO2RR with NO2RR to prepare urea. AuCu SANFs are
self-assembled by Boerdijk-Coxeter structural AuCu alloy NWs with (111)-dominant fac-
ets. Integrating the advantages of 1D-rich defect structure, (111) facet dominance, and
bimetallic composition synergistic enhancement effect, the resultant AuCu SANFs
achieve a rurea of 3,889.6 mg h1 mg1cat., with a corresponding FE of 24.7% at
1.45 V (versus Ag/AgCl) and excellent stability. This work not only provides a feasible
approach for constructing nanofiber structure but also promotes the investigation of
the electrochemical synthesis of urea under environmental conditions.
EXPERIMENTAL PROCEDURES
Resource availability
Lead contact
Further information and requests for resources should be directed to and will be ful-
filled by the lead contact, Prof. Hongjing Wang (hjw@zjut.edu.cn).
Materials availability
This study did not generate new unique reagents.
Data and code availability
All of the data associated with this study are included in the article and supplemental
information. Additional information is available from the lead contact upon reason-
able request.
Materials and chemicals
HAuCl4, Pluronic F127 (PEO100PPO65PEO100), Nafion solution (5 wt %), and Nafion
117 membrane were received from Sigma-Aldrich. CuCl2, 4-aminopyridine (4-AP),
poly(vinylpyrrolidone) (PVP, MW = 58,000), N2H4$H2O, AA, hexadecyl trimethyl
ammonium bromide (CTAB), DM-970 ((C2H4O)n$C24H42O$C15H24O), NaBH4,
KHCO3, NH4HCO3, potassium nitrite (KNO2), sodium nitrite-15N (Na15NO2),
sodium hydroxide (NaOH), (CO(14NH2)2), (CO(15NH2)2), salicylic acid, trisodium
citrate dihydrate, sodium hypochlorite solution (NaClO), sodium nitroprusside,
p-aminobenzenesulfonamide, N-(1-naphthyl) ethylenediamine dihydrochloride,
and urease were ordered from Aladdin. HCl, HCOOH, phosphoric acid, and ethanol
were purchased from Beijing Chemical Works.
Synthesis of the AuCu SANFs
First, 47 mg of 4-AP was ultrasonically dissolved in 7 mL deionized water. Second,
1 mL HAuCl4 (20 mM) and 1 mL CuCl2 (20 mM) were quickly added to the above so-
lution and placed in a water bath at 20 C. Third, 1 mL AA (0.1 M) was quickly injected
into the mixed solution and left for 10 min. Finally, the product was cooled at room
10 Cell Reports Physical Science 3, 100869, May 18, 2022ll
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temperature and washed 3 times with water and ethanol alternately, and dried under
vacuum at 60 C.
Materials characterization
SEM was conducted on a Zeiss Gemini 500 scanning electron microscope operating
at 5 kV. TEM, HRTEM, and HAADF-STEM images were performed on a JEOL-2100F
instrument equipped with EDX spectroscopy. An XRD pattern was conducted on a
PANalyticalX’Pert Powder with a Cu Ka radiation X-ray source (l = 0.154056 nm).
XPS measurements were conducted on a VG ESCALAB MK II instrument.
Electrochemical investigation
All electrochemical experiments were performed on a CHI 660E electrochemical
analyzer. An H-type cell separated by a Nafion 117 membrane was used for con-
structing the three-electrode system. Catalyst, 5 mg, was dispersed in a mixture of
1 mL ethanol and 0.05 mL Nafion solution with ultrasonic stirring to form a homoge-
neous ink. Ink, 10 mL, was then dropped onto a piece of carbon paper (1.0 cm2) using
a micropipette and drying at room temperature to form a catalyst layer with a
loading content of 0.2 mg/cm2. The carbon rod and Ag/AgCl (3 M KCl) electrodes
were used as the counterelectrode and reference electrode, respectively.
Electroreduction of CO2 was conducted in CO2-saturated 0.5 M KHCO3 solution at
room temperature under atmospheric pressure. Before the electrolysis, the KHCO3
solution in the cathodic compartment was saturated by a constant CO2 flow for
30 min. The LSVs were recorded at room temperature from 0.7 to 1.9 V at
10 mV/s. The electrolysis was controlled at each potential under 500 rpm stirring
for 2 h, in which CO2 gas was delivered at an average rate of 20 mL/min. Electrore-
duction of KNO2 was conducted in 0.5 M KHCO3 + 0.01 M KNO2 solution at room
temperature under atmospheric pressure. Controlled potential electrolysis was per-
formed at each potential for 2 h.
Electrosynthesis of urea was conducted in CO2-saturated 0.5 M KHCO3 + 0.01 M
KNO2 solution at room temperature under atmospheric pressure. Before the elec-
trolysis, the KHCO3 solution in the cathodic compartment was saturated by a con-
stant CO2 flow for 30 min. Controlled potential electrolysis was then performed at
each potential for 2 h.
Product quantification
The gas products (H2 and CO) were analyzed by a gas chromatograph (SRI 8610C)
equipped with a thermal conductivity detector and a helium ionization detector. The
NH3 concentration was detected by a salicylic acid analysis method using UV-vis spec-
trophotometry.23,63 Two milliliters diluted electrolyte was mixed with 2 mL coloring
solution containing 0.4 M salicylic acid, 1.0 M NaOH and 0.2 M trisodium citrate
dihydrate, 50 mL oxidizing solution containing NaClO (rCl = 4–4.9), and 200 mL catalyst
solution (1 wt % Na2[Fe(CN)5NO]) for 1 h. The mixed solution after being left in the dark
for 1 h was measured using the UV-vis spectrophotometer by a TU-1900 spectropho-
tometer, and calibrated the produced indoxyl blue by the absorbance at 655 nm
wavelength. To estimate the ammonia concentration, a series of standard ammonium
bicarbonate (NH4HCO3) solutions were used to calibrate the concentration absor-
bance curve. NO2 was determined using the N-(1-naphthyl)-ethylenediamine
dihydrochloride spectrophotometric method.12,64 In detail, 0.4 g p-aminobenzenesul-
fonamide and 0.02 g N-(1-naphthyl) ethylenediamine dihydrochloride were distributed
in a mixed solution containing phosphoric acid (1 mL, r = 1.70 g/mL) and deionized
water (5 mL) to prepare the color reagent. Then, 100 mL color reagent was added to
Cell Reports Physical Science 3, 100869, May 18, 2022 11ll
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Article
5 mL diluted electrolyte and left standing for 20 min. The absorption intensity of NO2
is measured by using UV-vis spectrophotometry at a wavelength of 540 nm and the
content of NO2 was calculated by the calibration curve. The calibration curve was ob-
tained by using a series of standard KNO2 solutions.
The urea ((NH2)2CO) was decomposed by urease (Curease = 5 mg mL1; Vurea/Vurease =
10/1) into CO2 and two NH3 molecules at 40 C for 30 min23,25 After the decomposi-
tion, the total moles (nurease) of NH3 in the diluted electrolyte were measured by UV-
vis spectrophotometry and shown as 2nurea + nNH3, where 2nurea represents the moles
of NH3 produced by the decomposition. Therefore, the moles of urea (nurea) produced
were calculated by (nurease nNH3)/2. The calibration curve was obtained by using a se-
ries of standard urea solutions.
Calculation of the yield, selectivity, and FE
The calculation equations are as follows:
rNH3 = ðcNH3 3 V Þ=t 3 m (Equation 4)
Selectivity ðNH3 Þ = cNH3 =DcNO2 (Equation 5)
Selectivity ðureaÞ = curea =D cNO2 (Equation 6)
Faradaic efficiency = e 3 n 3 F=Q (Equation 7)
where cNH3 is the measured NH3 concentration, V is the electrolyte volume in
the cathode chamber (40 mL), t is the electrolysis time (2 h), m is the catalyst mass
(0.2 mg), DcNO2 is the concentration difference of NO2 before and after reduction,
curea is the measured urea concentration, e is the valence change in the reactant,
n (mol) is the products concentration, F is the Faraday constant (96,485 C mol1),
and Q (C) is the total electric quantity during each electricity passed.
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.
2022.100869.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science Foundation of
China (grant nos. 21601154, 21776255, 21972126, 21978264, and 21905250).
AUTHOR CONTRIBUTIONS
H.W. and L.W. supervised the project. S.L. conceived the idea and designed the ex-
periments. S.L. and S.Y. wrote the manuscript. S.Y., Z.W., Y.X., and X.L. discussed
the experiment results. All of authors have given their approval to the final version
of the paper.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: February 24, 2022
Revised: March 19, 2022
Accepted: April 6, 2022
Published: April 25, 2022
12 Cell Reports Physical Science 3, 100869, May 18, 2022ll
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Article
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14 Cell Reports Physical Science 3, 100869, May 18, 2022Cell Reports Physical Science, Volume 3 Supplemental information AuCu nanofibers for electrosynthesis of urea from carbon dioxide and nitrite Songliang Liu, Shuli Yin, Ziqiang Wang, You Xu, Xiaonian Li, Liang Wang, and Hongjing Wang
Supporting Information for
AuCu nanofibers for electrosynthesis of urea from
carbon dioxide and nitrite
Songliang Liu,1 Shuli Yin,1 Ziqiang Wang,1 You Xu,1 Xiaonian Li,1 Liang Wang,1 and Hongjing
Wang1,2,*
1
State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, College of
Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P. R. China.
2
Lead contact
*Correspondence: hjw@zjut.edu.cn
S1Figure S1. Structure of the AuCu SANFs (a) SEM image and (b) EDX spectrum of the AuCu
SANFs.
Figure S2. Characterization of the AuCu SANFs and Au NWs XRD patterns of the Au NWs
and AuCu SANFs.
S2Figure S3. Characterization of the AuCu SANFs TEM images of the samples prepared with (a) 0
mg of 4-aminopyridine and (b) 94 mg of 4-aminopyridine, respectively, under the typical synthesis.
Figure S4. Characterization of the AuCu SANFs TEM images of the samples prepared with (a)
F127, (b) DM970, (c) PVP and (d) CTAB, respectively, under the typical synthesis.
S3Figure S5. Characterization of the AuCu SANFs TEM images of the samples synthesized with
different reducing agent: (a) N2H4·H2O, (b) HCOOH and (c) NaBH4, respectively, under the typical
synthesis.
Figure S6. Characterization of the AuCu SANFs TEM images of AuCu samples prepared with
different metallic precursor amounts under identical conditions used for the typical synthesis. The
added metallic precursor amounts of HAuCl4, CuCl2 are (a) 2 mL, 0 mL (Au NWs); (b) 1.6 mL, 0.4
mL (Au4Cu1 SANFs); (c) 1.5 mL, 0.5 mL (Au3Cu1 SANFs); (d) 0.5 mL, 1.5 mL (Au1Cu3 SANFs);
(e) 0.4 mL, 1.6 mL (Au1Cu4 SANFs); (f) 0 mL, 2 mL (Cu Ps), respectively.
S4Figure S7. Characterization of the AuCu SANFs (a) Geometric models, TEM images of the
samples synthesized in the different temperature: (b) 20 °C, (c) 40 °C, (d) 60 °C, (e) 80 °C, and (f)
100 °C, respectively.
Figure S8. Schematic for electrocatalytic co-reduction reaction set-up with a typical
three-electrode system.
S5Figure S9. Electrocatalytic performances for CO2RR of the AuCu SANFs I-t curves at vaious
applied potentials of the AuCu SANFs in CO2-saturated 0.5 M KHCO3.
Figure S10. Electrocatalytic performances for CO2RR of the AuCu SANFs (a) H2 Faradaic
efficiency for the AuCu SANFs examined with different applied potentials. (b) The corresponding
H2 partial current density.
S6Figure S11. Electrocatalytic performances for NO2−RR of the AuCu SANFs I-t curves at vaious
applied potentials of the AuCu SANFs in 0.5 M KHCO3 + 0.01 M KNO2 solution.
Figure S12. Electrocatalytic performances for NO2−RR of the AuCu SANFs (a) UV-Vis
absorption spectra of various NO2− concentrations and (b) calibration curve used for estimating
NO2−.
S7Figure S13. Electrocatalytic performances for NO2−RR of the AuCu SANFs (a) UV-Vis
absorption spectra of various NH4+ concentrations and (b) calibration curve used for estimating
NH4+.
Figure S14. Electrochemical performances of the urea production by coupling CO2RR with
NO2−RR of the AuCu SANFs (a) The corresponding NH3 partial current density for the AuCu
SANFs examined with different applied potentials in 0.5 M KHCO3 + 0.01 M KNO2 solution. (b)
The corresponding H2 partial current density for the AuCu SANFs examined with different applied
potentials in 0.5 M KHCO3 + 0.01 M KNO2 solution.
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