Electrochemical C-N coupling with perovskite hybrids toward efficient urea synthesis

Article abstract of DOI:10.1039/d1sc01467f

Electrocatalytic C-N coupling reaction by co-activation of both N2 and CO2 molecules under ambient conditions to synthesize valuable urea opens a new avenue for sustainable development, while the actual catalytic activity is limited by poor adsorption and coupling capability of gas molecules on the catalyst surface. Herein, theoretical calculation predicts that the well-developed built-in electric field in perovskite hetero-structured BiFeO3/BiVO4 hybrids can accelerate the local charge redistribution and thus promote the targeted adsorption and activation of inert N2 and CO2 molecules on the generated local electrophilic and nucleophilic regions. Thus, a BiFeO3/BiVO4 heterojunction is designed and synthesized, which delivers a urea yield rate of 4.94 mmol h-1 g-1 with a faradaic efficiency of 17.18% at -0.4 V vs. RHE in 0.1 M KHCO3, outperforming the highest values reported as far. The comprehensive analysis further confirms that the local charge redistribution in the heterojunction effectively suppresses CO poisoning and the formation of the endothermic ?NNH intermediate, which thus guarantees the exothermic coupling of ?NN? intermediates with the generated CO via C-N coupling reactions to form the urea precursor ?NCON? intermediate. This work opens a new avenue for effective electrocatalytic C-N coupling under ambient conditions.

Full text of DOI:10.1039/d1sc01467f

Showcasing research from Professor Guangjin Zhang’s
laboratory, Institute of Process Engineering, Chinese
Academy of Sciences, Beijing, China.
As featured in:
Electrochemical C–N coupling with perovskite hybrids
toward efficient urea synthesis
In this work, Zhang et al. proposed a unique ultrasonic bath
method to fabricate perovskite structural BiFeO /BiVO
3
4
hybrids. The local charge redistribution in BiFeO /BiVO
3
4
hybrids promotes the targeted adsorption and activation
of inert N and CO molecules and further electrochemical
2
2
C-N coupling reactions to form urea with high efficiency.
The authors appreciate the help from Chinese artist, Miss
Lina Zhao, for the designing of this cover image.
See Guangjin Zhang et al.,
Chem. Sci., 2021, 12, 6048.
rsc.li/chemical-science
Registered charity number: 207890

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Electrochemical C–N coupling with perovskite
hybrids toward efficient urea synthesis†
Cite this: Chem. Sci., 2021, 12, 6048
ab
ab
cd
c
ab
All publication charges for this article Menglei Yuan, Junwu Chene, Yiling Bai, Zhanjun Liu, Jingxian Zhang,
have been paid for by the Royal Society
of Chemistry
ab
a
fg
*abg
Tongkun Zhao, Qiaona Shi, Shuwei Li, Xi Wang
and Guangjin Zhang
2 2
Electrocatalytic C–N coupling reaction by co-activation of both N and CO molecules under ambient
conditions to synthesize valuable urea opens a new avenue for sustainable development, while the
actual catalytic activity is limited by poor adsorption and coupling capability of gas molecules on the
catalyst surface. Herein, theoretical calculation predicts that the well-developed built-in electric field in
perovskite hetero-structured BiFeO
thus promote the targeted adsorption and activation of inert N
local electrophilic and nucleophilic regions. Thus, a BiFeO /BiVO
3
/BiVO
4
hybrids can accelerate the local charge redistribution and
and CO molecules on the generated
heterojunction is designed and
with a faradaic efficiency of 17.18% at
, outperforming the highest values reported as far. The comprehensive
2
2
3
4
À1 À1
synthesized, which delivers a urea yield rate of 4.94 mmol h
À0.4 V vs. RHE in 0.1 M KHCO
g
3
analysis further confirms that the local charge redistribution in the heterojunction effectively suppresses
CO poisoning and the formation of the endothermic *NNH intermediate, which thus guarantees the
exothermic coupling of *N]N* intermediates with the generated CO via C–N coupling reactions to
Received 13th March 2021
Accepted 11th April 2021
DOI: 10.1039/d1sc01467f
form the urea precursor *NCON* intermediate. This work opens
electrocatalytic C–N coupling under ambient conditions.
a new avenue for effective
rsc.li/chemical-science
À1
À1
However, the highly stable double-bond (C]O, 806 kJ mol ) in
1
Introduction
CO
2
molecules and the triple-bond (N^N, 940.95 kJ mol ) in
Nitrogen (N ), accounting for 78% of the atmosphere, exists in
2
N molecules make the inert gas molecules difficult to activate
2
13–17
the gaseous form that can't be directly utilized in biology and mildly.
Although conventional industrial processes such as
the Haber–Bosch method and carbon capture and sequestra-
generated from industries and transportation is the principal tion achieve the activation of inert gas molecules, their further
1–3
2
chemistry elds. On the other hand, carbon dioxide (CO )
4
–8
greenhouse gas causing serious environmental concerns.
Thus, converting N and CO
into value-added fuels and complex synthetic processes.
utilization is impeded by the large energy consumption and
18–22
In comparison, energy-saving
2
2
chemical products via the C–N coupling reaction is a promising and environmentally benign electrocatalysis technologies such
approach for not only mitigating environmental issues and as the nitrogen reduction reaction (NRR) and the carbon
9–12
energy crisis but also high-value utilization of N and CO .
dioxide reduction reaction (CO
2
RR) are drawing growing
2
2
23–30
attention.
Besides, the emerged electrochemical C–N
coupling reaction may provide a new possibility of enhancing
the spectrum of products of CO2 by using CO2 and amine
a
CAS Key Laboratory of Green Process Engineering, Beijing Key Laboratory of Ionic
Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems,
Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190,
China. E-mail: zhanggj@ipe.ac.cn
2
derivatives/nitrogen sources (nitrate, nitrite, NO, and even N )
9,31,32
as feedstock.
The desired product urea [CO(NH
the electrochemical C–N bond formation occurs by employing
b
2 2
) ] will be produced when
Center of Materials Science and Optoelectronics Engineering, University of Chinese
Academy of Sciences, Beijing, 100049, China
c
9
State Key Laboratory of Coal Conversion, CAS Key Laboratory of Carbon Materials, both N
2
and CO as the feeding gas. Urea is commonly utilized
2
Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China
d
as the general feedstock in industry and the primary fertilizer
SynCat@Beijing, Synfuels China Technology Co. Ltd., Huairou District, Beijing,
33,34
for agriculture.
The industrial urea synthesis proceeds by
1
01407, China
two consecutive processes, including N + H / NH followed
e
2
2
3
Sino Shaanxi Nuclear Molybdenum Industry Co. Ltd., Tongguan, 714300, China
Key Laboratory of Luminescence of Optical Information, Ministry of Education, Beijing
f
by NH
3
+ CO
2
/ CO(NH
2
)
2
, which operate under harsh reaction
ꢀ
ꢀ
conditions (350–550 C, 150–350 bar and 150–200 C, 150–250
Jiaotong University, Beijing, 100044, China
g
35
Chemistry and Chemical Engineering Guangdong Laboratory, Shantou, 515031, bar, respectively). Compared with the complex industrial
China
synthetic process, simultaneous electrocatalytic xation of N
and CO driven by a renewable energy source under ambient
2
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1sc01467f
2
1
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9
conditions provides a clean route for urea production. The
main issues in electrochemical synthesis of urea lie in three
2
Results and discussion
aspects: (1) extraordinarily weak chemical adsorption of inert Conspicuously, the charged surface can give rise to a signicant
29,30,36,37
30
CO /N on the catalysts surface;
(2) the dissociation of effect on the targeted adsorption of reactant molecules. In this
2
2
the highly stable C]O bond and N^N bond requires high regard, constructing built-in elds at the heterointerface to
13,15
overpotential;
2 2
(3) the parallel reaction of CO /N reduction promote a spontaneous electron transfer process will supply
suppresses the efficiency of C–N coupling and strongly more possibilities for generating desirable two opposite
41
competes with the desired urea formation reaction, further charged regions. This design concept has been conrmed by
9
resulting in a large distribution of complex products.
the investigation of various perovskite-based semiconductor
Although a few noble metal Pd-based catalysts can achieve catalysts, which can constitute distinct p–n junctions and thus
4
0
urea electrosynthesis, the related catalytic activity showed accelerate local charge redistribution at the heterointerface.
À1 À1
a maximum urea yield rate of 3.36 mmol h
8
g
with a FE of The band theory of solids has conrmed that the behavior of
.92%. Besides, the high price and scarcity of the used noble electron transfer is strongly correlated to the work function of
metals impede their real application on the large scale. Thus, semiconductors. As presented in Fig. 1a and b, the theoretical
further improvement of the catalytic activity with earth- simulation results showed that the work function values of
abundant materials remains a challenge. Inspired by the BiVO and BiFeO were 3.23 eV and 6.30 eV, respectively, which
9
42
4
3
2 2
4 3
different electronic structures of N and CO molecules, tuning allow self-driven electron transfer from BiVO to BiFeO . The
the electronic state of the electrocatalyst's surface can be prole of the planar averaged electrostatic potential along the z-
a feasible strategy to optimize the adsorption of reactant gas direction for BiFeO /BiVO hybrids is depicted in Fig. 1c.
3
4
molecules and suppress the competing electro-reduction reac- Compared with pristine BiVO and BiFeO that possess periodic
4
3
tions to promote urea generation. In this regard, fabricating lattice potential, BiFeO /BiVO hybrids showed a big build-in
3
4
a built-in electronic eld to accelerate the local charge redis- potential at the interfaces. Additionally, electron density
tribution in perovskite heterostructures displays fascinating difference calculation was further performed to reveal the
potential to deliver impressive urea electroproduction activity. charge distribution at the interface of BiFeO /BiVO hybrids. As
3
4
On the one hand, perovskite structured transition-metal oxide described in Fig. 1d, the charge depletion and accumulation are
ABO ) semiconductors possess a distinctive electronic struc- represented by the cyan region and yellow region, respectively.
(
3
ture, which can lead to the alteration of electron density when An increase of the charge density was observed at the BiFeO3
38,39
combining with other domains.
On the other hand, the work surface and a decrease at the BiVO surface, which conrmed
4
function difference in perovskite heterostructures drives the that the spontaneous electron transfer induced the local charge
local charge redistribution by the band bending at the hetero- redistribution around the interface and endowed the surfaces of
interface, which is preferred for targeted adsorption and acti- BiFeO3 and BiVO4 with local nucleophilic and electrophilic
2 2 2
vation of inert small molecules such as N , CO , H O, and so regions. Bader analysis results further demonstrate that BiVO4
40
on. Despite the encouraging merits, inducing the local charge can transfer 2.33 electrons to BiFeO . Previous research has
3
redistribution in perovskite hybrids toward the specic urea proven that the chemisorption of inert CO and N is the initial
2
2
electrosynthesis hasn't been explored.
In this work, with the aid of theoretical simulation, we the N atom in the N2 molecule and the C atom in the CO2
exploited perovskite structured p-type BiFeO and an n-type molecule are electron-rich and electron-decient, respectively.
semiconductor BiVO to fabricate innovative p–n hetero- Therefore, it can be deduced that the rationally designed
junction electrocatalysts, aiming at enabling spontaneous BiFeO /BiVO hybrids with local nucleophilic and electrophilic
step for electrocatalytic urea production. As shown in Fig. S1,†
3
4
3
4
electron transfer at the heterointerfaces by the desirable built-in regions would adsorb targeted reactant molecules by electro-
electric eld. The obtained perovskite structured BiFeO /BiVO
static interaction. To conrm the above hypothesis, density
3
4
showed high electrocatalytic activity toward the C–N coupling functional theory (DFT) calculations were performed to reveal
reaction to synthesize urea, and exhibits a FE of 17.18% in 0.1 M the gas adsorption behavior on the surface of the electro-
KHCO3 at À0.4 V vs. RHE. The urea yield rate can reach catalyst. As expected, the calculated adsorption energies of
À1 À1
4.94 mmol h
g
which is much higher than the recently re- BiFeO /BiVO hybrids for N2 and CO2 are À0.17 eV and
3
4
ported best values with Pd-based electrocatalysts. The activity À0.06 eV, much lower than those of individual architectures of
can further be improved by adding an ionic liquid to the elec- BiFeO and BiVO (Fig. 1e and f), proving that N and CO show
3
4
2
2
trolyte. The generated local electrophilic and nucleophilic a stronger tendency to adsorb on the surface of BiFeO /BiVO4
3
regions enhanced the targeted adsorption and activation of hybrids than on pristine BiFeO and BiVO due to the interfacial
3
4
inert N2 and CO2 molecules and balanced the competing interaction triggered by the well-dened space-charge region,
electro-reduction reactions to further promote the formation of which is favorable for much readily chemisorbing inert gas
the *NCON* intermediate via the C–N bond coupling reaction. molecules and thus facilitating the electrocatalytic urea
Thus, engineering a built-in electric eld to facilitate local production process.
charge redistribution has been proposed as an appealing
As a proof-of-concept experiment, perovskite structured
strategy to enhance electrocatalytic C–N bond coupling and BiFeO /BiVO hybrids were designed and synthesized by the
3
4
further urea synthesis.
facile ultrasonic bath method. The eld-emission scanning
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Fig. 1 The calculated electrostatic potentials and work functions for (a) BiVO
4
, (b) BiFeO
/BiVO
4
heterojunction, the yellow and cyan color indicate electron accumulation and depletion, respectively, with an
3
and (c) BiFeO
3
/BiVO
4
heterojunction surfaces; (d)
planar average charge density difference along the z-direction for the BiFeO
difference of the BiFeO /BiVO
2 3 4 3 4
isosurface value of 0.013 e A˚ ; free energy diagrams for (e) N and (f) CO adsorption on BiFeO , BiVO and the BiFeO /BiVO heterojunction.
3
4
heterojunction, the bottom image shows the charge density
3
À3
2
electron microscopy (FE-SEM) image in Fig. 2a reveals that which indicates the possible interaction between the two
43,44
BiFeO /BiVO heterostructures possess a rice-like morphology distinct domains.
As further revealed by Mott–Schottky (M–
3
4
with an average length of 1 mm and a diameter of approximately S) curves, BiFeO with a negative slope (Fig. 3d) and BiVO with
3
4
4
00 nm, respectively. The relevant elemental mapping demon- a positive slope (Fig. 3e) matched well with the apparent char-
strates the evenly distributed Bi, Fe, V, and O elements. In acteristics of a p-type and an n-type semiconductor, respec-
comparison, pristine BiVO still maintains the same tively. In comparison, the as-prepared BiFeO /BiVO hybrids
morphology as heterostructured hybrids (Fig. 2c), whereas the exhibited evident p–n heterojunction features (Fig. 3f), which
BiFeO displays an irregular nanoparticle structure (Fig. 2b). As furnish the rational architecture to achieve the desired local
surveyed by high-resolution transmission electron microscopy charge redistribution by the above theoretical prediction. In
4
3
4
3
(
0
HR-TEM), the well-resolved lattice fringes of 0.282 nm and Raman spectra (Fig. 4a), the typical vibrational bands at 137 and
À1
.312 nm corresponded to the (104) plane and (130) plane of 171 cm
represent the Fe–O–Fe bonds of the BiFeO3
whereas they exhibit a slightly negative shi
45,46
BiFeO3 and BiVO4 crystals (Fig. 2e). Meanwhile, the distinct sample,
interface generated by the intimate contact of BiFeO and BiVO accompanied by an intensity decrease when coupled with BiVO
4
3
4
conrms the representative establishment of nanoscale heter- domains. Likewise, the detected V–O stretching modes (637,
À1
3À
ostructures (Fig. 2d).
702, and 826 cm ) and VO
4
tetrahedron bending modes (327
À1
In Fig. 3a and b, the XRD spectra show that the hexagonal and 367 cm ) also negatively shied aer the establishment of
47–49
BiFeO
3
structure (JCPDS: 86-1518) and monoclinic BiVO
4
phase the heterostructure with the space-charge region.
broadening of vibrational modes and position shiing in
samples. Concerning BiFeO /BiVO hybrids, both sets of BiFeO /BiVO hybrids manifest the intense coupling interaction
This
(JCPDS: 83-1700) can be obtained in pristine BiFeO and BiVO
3
4
3
4
3
4
50,51
diffraction peaks are consistent with those of pristine samples, between BiFeO and BiVO .
Besides, compared to the UV-Vis
spectrum of pristine samples, the dominant peak in BiFeO
to higher diffraction angles (Fig. 3c), BiVO hybrids exhibited an obvious blue shi, evidencing the
3
4
except for the slight shi of the characteristic peaks of BiFeO
and partial peaks of BiVO
3
3
/
4
4
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3 4 3 4
Fig. 2 (a) SEM image and the corresponding elemental mapping of BiFeO /BiVO hybrids; SEM images of (b) BiFeO and (c) BiVO ; (d) high-
3 4 3
resolution TEM image of BiFeO /BiVO hybrids and the dotted line represents the heterointerfaces; (e) the well-resolved lattice fringe of BiFeO /
BiVO hybrids in Fig. 2d.
4
52
existence of the charge transfer effect (Fig. 4b). To further illustrate the presence of Bi, Fe, V, and O elements in the ob-
reveal the electronic effects between BiFeO and BiVO , the tained electrocatalysts, which is consistent with the above SEM
chemical components and the alteration of valence states in the characterization. The high-resolution Bi 4f spectrum displayed
3
4
3
+
3+
formed catalysts were further examined by X-ray photoelectron two predominant peaks of Bi 4f7/2 and Bi 4f5/2 at the binding
53,54
spectroscopy (XPS). The survey spectra, displayed in Fig. 4c, energies of 159.3 eV and 164.7 eV (Fig. 4d).
3 4 3 4 3 4 3 4
Fig. 3 XRD patterns of (a) BiFeO , (b) BiVO and (c) BiFeO /BiVO ; Mott–Schottky plots of (d) BiFeO , (e) BiVO and (f) BiFeO /BiVO .
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Fig. 4 (a) Raman spectra, (b) UV-Vis spectra, (c) XPS survey spectrum and (d) high-resolution Bi 4f spectrum of BiFeO
3 4 3 4
, BiVO and BiFeO /BiVO ;
(
e) high-resolution Fe 2p spectrum of BiFeO
3
3 4
and BiFeO /BiVO ; (f) high-resolution V 2p spectrum of BiVO
4
and BiFeO /BiVO .
3
4
Besides, two distinct peaks centered at 713.4 eV and 726.9 eV online gas chromatography monitored the possible gas prod-
in the Fe 2p region can be attributed to the binding energies of ucts such as CO and H . It has been reported that the electro-
And the XPS chemical catalytic activity of the C–N coupling reaction for urea
peak positioned at 515.8 eV in the V 2p spectrum illustrates the production was dominant as a result of effectively coupling the
2
3
+
3+
55,56
Fe 2p3/2 and Fe 2p1/2, respectively (Fig. 4e).
4
+
presence of V (Fig. 4f). It is noteworthy that when BiFeO
coupled with BiVO , the emerged new peaks in BiFeO /BiVO
4
3
was carbon dioxide reduction reaction (CO
reduction reaction (NRR). As displayed in Fig. 5a and b, the
hybrids are assigned to Fe 2p and V 2p peaks in contrast to BiFeO /BiVO hybrids possess a high NH faradaic efficiency
2
RR) with the nitrogen
4
3
2
+
5+
3
4
3
pristine samples, elucidating that the apparent electronic (FE) (12.81%) for the NRR and CO FE (20.21%) for the CO RR,
2
interactions between BiFeO
formation of p–n heterojunctions.
related changes in the valence state further demonstrate that examined in 0.1 M KHCO
the charge transferred from BiVO to BiFeO . All these results CO , N , or CO + N ). As depicted in Fig. 5c, the distinctly
convincingly suggest the successful establishment of unique p– enhanced current density in CO + N saturated electrolyte
3
and BiVO
4
7,58
domains are due to the which establish the baseline of N
2
and CO
More importantly, the The linear sweep voltammetry curves (LSV) were initially
saturated with different gas feeds (Ar,
2
reduction activity.
5
3
4
3
2
2
2
2
2
2
n heterojunctions and the transfer of electrons between BiVO4 relative to that in CO2 and N , respectively, indicates the
2
and BiFeO . The induced local charge redistribution contrib- occurrence of the electrocatalytic C–N coupling reaction for
3
utes to the targeted adsorption of reactant molecules and thus urea production. A potentiostatic experiment was further per-
enhances the electrocatalytic urea production ability.
The electrocatalytic activity of BiFeO /BiVO hybrids for the catalytic urea production of BiFeO
C–N coupling reaction toward urea production was tested in working potentials. As shown in Fig. S4,† the corresponding
a 0.1 M KHCO
formed to quantitatively assess the performance of electro-
3
4
3
/BiVO hybrids at different
4
3
solution utilizing an H-type two-compartment chronoamperometry curves within the potential range of À0.3 V
cell separated by a Naon 211 membrane, which is equipped to À0.7 V exhibit stable current density aer electrolysis for 2 h.
with a three-electrode conguration (Fig. S2†). Ultrahigh purity As demonstrated in Fig. 5d, the urea yield rate and the corre-
CO2 gas (99.999%) and N2 gas (99.999%) were continuously sponding FE increase with the increase of applied potential.
À1 À1
purged into the cathodic chamber with the same ow rate of 5 The highest urea yield rate is 4.94 mmol h
g
with a FE of
À1
mL min during the electrolysis process. The possibly gener- 17.18% at À0.4 vs. the reversible hydrogen electrode (RHE),
ated liquid products (urea and NH
3
) in the electrolytes were outperforming all the reported values for Pd based catalysts
spectrophotometrically measured by the diacetyl monoxime (Table S1†). However, at more negative potentials, the electro-
2
2,59
method and indophenol blue method.
Meanwhile, the catalytic urea production performance decreases, which may
and CO
related calibration curves are displayed in Fig. S3.† Besides, result from the occupation of the active sites for N
2
2
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Fig. 5 (a) NH
LSV curves of BiFeO
corresponding product distribution of H
and CO as the feed gas at various potentials for BiFeO
3
synthesis with N
/BiVO hybrids in Ar, N
(purple color), CO (cyan-blue color), NH
2
as the feed gas and (b) CO generation with CO
2 3 4
as the feed gas at various potentials for BiFeO /BiVO hybrids; (c)
3
4
2
, CO and N + CO saturated electrolyte; (d) the urea yield rate and faradaic efficiencies and (e) the
2
2
2
2
3
(red color) and urea (blue color) for urea production with N
2
2
1
15
14
2
3
/BiVO
4
hybrids; (f) H NMR spectra of electrolyte saturated with
2 2 2
N + CO / N + CO
15
15
14
14
1
15
15
after 2 h of electrolysis and standard NH
2
CO NH
2
/
NH
2
CO NH
2
solution; (g) H NMR spectra of standard NH
2 2
CO NH solution at
À1
15
15
15
concentrations of 0.2–0.5 mg mL ; (h) integral area ( NH
2
CO NH
2
/C
4
4
H O
4
)–concentration linear relation calibrated using standard NH -
2
1
5
1
CO NH
2
solution; (i) the urea yield of BiFeO
3
/BiVO
4
hybrids after 2 h of electrolysis determined using the UV-Vis spectrum and H NMR spectra.
À1 À1
reduction by the excessively released CO (Fig. 5e). Additionally,
the as-obtained BiFeO /BiVO
current density aer a 10 h long-term chronopotentiometry test
Fig. S5†). Likewise, when conducting ve cycling tests, negli- the C–N coupling reaction with CO and N , a rigorous protocol
h
3
g ) than that obtained in pristine KHCO solution at the
3
4
hybrids retain 96% of the initial same potential (Fig. S8†).
To gain solid proof that the produced urea originated from
(
2
2
gible change is observed in the electrocatalytic urea production was employed to avoid false-positive results caused by the
performance (Fig. S6†), attesting to the superior electrocatalytic contamination of environmental NO (Fig. S9†). Under all the
X
stability of BiFeO
terization studies further reveal that the BiFeO
were well-matched with their original morphology, crystal urea electrosynthesis (Fig. S10†).
phase, and chemical states aer 10 h of continuous electrolysis ment was further utilized to corroborate the N source of the
3
/BiVO
4
hybrids. The corresponding charac- control experiment conditions, negligible urea was detected,
/BiVO hybrids which excludes the possible effect of environmental NO on the
isotopic labeling experi-
3
4
X
1
5
N
2
1
5
(Fig. S7†), corroborating their robust structural stability. generated urea [CO(NH
2 2 2 2
) ]. The standard CO( NH ) sample
Impressively, with the aid of enhanced CO adsorption capacity displays two dominant peaks at approximately 5.35 ppm and
2
60
1
of an ionic liquid, when the 1-butyl-3-methylimidazolium 5.47 ppm in H Nuclear Magnetic Resonance (NMR) (Fig. 5f),
tetrauoroborate–KHCO3 ([Bmim]BF –KHCO ) electrolyte was while the standard CO( NH ) sample possesses one distinct
used in the system, a higher electrocatalytic activity for urea peak at about 5.42 ppm. When utilizing
production is achieved (FE: 20.75%, urea yield rate: 5.42 mmol feed gas, the detected H NMR signals of the produced urea
1
4
4
3
2 2
1
5
N
2
and CO
2
as the
1
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Fig. 6 (a) Average urea yield; (b) carbon dioxide temperature-programmed desorption (CO
2
-TPD) spectra; (c) DJ of electrocatalysts plotted
3 4 3 4
against scan rate at À0.05 V vs. RHE; (d) Nyquist plots of electrochemical impedance spectra (EIS) of BiFeO , BiVO and BiFeO /BiVO .
1
5
matched well with the standard CO( NH
2
)
2
signals. Addition- BiVO
was also quantitatively local nucleophilic and electrophilic regions and thus promotes
detected by H NMR, and the related signal integration– the targeted adsorption of CO and N molecules, which is in
4 3 4
hybrids endows the surfaces of BiFeO and BiVO with
1
5
2 2
ally, the concentration of CO( NH )
1
2
2
concentration linear relation is exhibited in Fig. 5g and h. As good agreement with the aforementioned theoretical predic-
expected, the calculated urea concentration is consistent with tion. The electrochemically active surface area (ECSA) results
À2
the quantitative results of the diacetyl monoxime method suggest that the BiFeO
3
/BiVO
4
hybrids (76.6 mF cm ) display
(
Fig. 5i). All these results are convincingly indicative of urea a higher electrochemical double-layer capacitance (C ) than
dl
À2
À2
originating from the C–N coupling reaction from N and CO
pristine BiFeO3 (33.2 mF cm ) and BiVO4 (49.6 mF cm ),
2
2
catalyzed by BiFeO
For comparison, the pristine BiFeO
evaluated under identical conditions. At the optimal potential molecule adsorption and activation (Fig. 6c and S12†). Elec-
3
/BiVO
4
hybrids.
which signies that local charge redistribution promotes the
3
4 3 4
and BiVO were also exposure of more active sites in BiFeO /BiVO hybrids for gas
of À0.4 V, the electrocatalytic activities of BiFeO
3
(urea yield trochemical impedance spectroscopy (EIS) analysis further
/BiVO hybrids possess a smaller semi-
À1 À1
rate: 1.41 mmol h
g
, FE: 4.35%) and BiVO
4
(urea yield rate: reveals that the BiFeO
3
4
À1 À1
2.50 mmol h
g , FE: 7.59%) were much inferior to those of circle and higher slope than pristine samples, evincing that the
BiFeO /BiVO hybrids for urea production (Fig. 6a). This indi- presence of local charge redistribution in BiFeO /BiVO hybrids
3
4
3
4
cates that the local charge redistribution in BiFeO /BiVO
hybrids plays a critical role in enhancing electrocatalytic urea electrocatalytic process (Fig. 6d).
signicantly promotes electron/ion transfer kinetics during the
3
4
29,43
production. As revealed by temperature-programmed desorp-
tion (TPD), compared with pristine BiFeO and BiVO , BiFeO
BiVO
desorption peak in the CO
S11†), elucidating that the local charge redistribution in BiFeO
To obtain deeper insight into the C–N coupling reaction
mechanism toward the electrocatalytic urea production, DFT
3
4
3
/
4
hybrids exhibited stronger binding strength and larger calculation was further employed to assess the intermediate
- and N -TPD spectra (Fig. 6b and variation and energy barrier during the reaction process. The
corresponding BiFeO /BiVO heterostructured architecture is
2
2
3
/
3
4
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3 4 2 2
Fig. 7 (a) The calculation model of the BiFeO /BiVO heterostructure; free energy diagrams for (b) N adsorption and (c) CO reduction with and
2 3 4
without N adsorption on the BiFeO /BiVO heterostructure; the electrolytic urea production via (d) alternating and (e) distal mechanisms.
displayed in Fig. 7a. Since the chemisorption of inert CO and appeared at a higher temperature and exhibited enhanced
2
N
2
molecules is the initial step for electrocatalytic urea intensity, demonstrating that the BiFeO
production, it is critical to reveal the competitive adsorption possessed stronger N chemisorption ability than that for CO
between these gas molecules. In comparison with CO -TPD, the molecules (Fig. 6c and S11†). In other words, the C–N coupling
peaks in the N -TPD spectrum of the heterostructured hybrids reaction was initiated with chemisorption of N molecules on
3
/BiVO
4
hybrids
2
2
9
2
2
2
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2021 The Author(s). Published by the Royal Society of Chemistry
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3 4
Fig. 8 The schematic electrocatalytic urea production mechanism based on BiFeO /BiVO p–n heterostructure synergistic effects.
the BiFeO
3
/BiVO
4
hybrids. Besides, the free energy of the N
2
were prone to pursue the distal mechanism until the release of
adsorption with a side-on conguration was more negative than urea molecules rather than the alternating pathway from the
31
that of an end-on conguration, suggesting that N
adsorbed on the local electrophilic BiVO regions via the more
stable side-on conguration (Fig. 7b). Impressively, as identi- closely associated with the formation of *NCON* intermediates.
2
preferably thermodynamic perspective.
4
The selectivity of the electrocatalytic urea production is

ed in Fig. 7c, the BiFeO /BiVO hybrids required the highest The possible N reduction or the excessive release of CO would
3
4
2
Gibbs free energy (DG) of 0.59 eV to reduce CO
decreased to 0.44 eV when the N
neighboring BiVO regions. Therefore, it can be deduced that hand, the DG for the reductive protonation of *N
the adsorbed/activated N
reduction of adsorbed CO
2
, while the DG result in a decrease in the efficiency of the electrocatalytic C–N
2
molecules emerged in coupling reaction and further reduce the selectivity. On the one
into *N H (N
molecules would facilitate the + H + e + * / *NNH), which is regarded as the potential-
to CO on local nucleophilic BiFeO determining step (PDS) toward the NRR, was calculated to be
4
2
2
2
+
À
2
2
3
61
regions. Then the C–N coupling reaction spontaneously 0.14 eV (Fig. S13†), which was much higher than that for the
occurred between the activated N molecules (*N]N*) and in C–N coupling reaction to form *NCON*. Such a signicantly
2
situ generated CO to form the *NCON* intermediate, due to the larger energy barrier makes BiFeO /BiVO hybrids inactive for
3
4
3
matching molecular orbitals. The corresponding DG further the electrocatalytic NRR, let alone generating side product NH .
corroborates that the formation of the *NCON* intermediate is On the other hand, the release of the generated *CO interme-
an exothermic process. Once the *NCON* intermediate is diate was strongly associated with the selectivity of the elec-
generated, the subsequent hydrogenation of *NCON* would trocatalytic urea production. Thus, the stability of the as-
produce two possible reaction pathways involving distal and prepared electrocatalysts against CO poisoning was evaluated
alternating mechanisms. Concerning the alternating pathway, through an electrochemical CO-stripping experiment. As shown
+
À
the (H + e ) alternately reacted with the two N atoms, while the in Fig. S14,† the CO-stripping peak of BiFeO /BiVO4 hybrids
3
protonation continuously attacked the distal N atoms for the (0.251 V vs. RHE) exhibits a more negative shi than that of
distal mechanism. As displayed in Fig. 7d and e, when the pristine BiFeO
reduction of *NCON* follows the distal pathway, the required (0.265 V vs. RHE), indicating that the BiFeO
DG decreased to 0.54 eV compared to that of the alternating possessed higher stability against CO poisoning, which is due to
3
(0.261 V vs. RHE) and BiVO
4
counterparts
3
/BiVO
4
hybrids
pathway (0.72 eV), suggesting that the BiFeO
3
/BiVO
4
hybrids the elimination of some of the strong adsorption sites by the
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62
local charge redistribution. Notably, the amount of CO should thank the nancial project of the Key Program for International
be well controlled. As conrmed by Fig. 5e, when the applied S&T Cooperation Projects (No. 2018YFE0124600) funded by the
potential exceeded À0.4 V, the excessively released CO occupied Ministry of Science and Technology of China.
the adsorption sites for N and CO and resulted in a remark-
2
2
able decrease of the FE for urea production.
By combining the aforementioned experimental results and
computational simulations, the overall urea electrosynthesis
process can be summarized in the following steps: (i) the built-
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