Unveiling Electrochemical Urea Synthesis by Co-Activation of CO2 and N2 with Mott–Schottky Heterostructure Catalysts

Article abstract of DOI:10.1002/anie.202101275

Electrocatalytic C?N bond coupling to convert CO₂ and N₂ molecules into urea under ambient conditions is a promising alternative to harsh industrial processes. However, the adsorption and activation of inert gas molecules and then the driving of the C–N coupling reaction is energetically challenging. Herein, novel Mott–Schottky Bi-BiVO₄ heterostructures are described that realize a remarkable urea yield rate of 5.91 mmol h^?1 g^?1 and a Faradaic efficiency of 12.55 % at ?0.4 V vs. RHE. Comprehensive analysis confirms the emerging space–charge region in the heterostructure interface not only facilitates the targeted adsorption and activation of CO₂ and N₂ molecules on the generated local nucleophilic and electrophilic regions, but also effectively suppresses CO poisoning and the formation of endothermic *NNH intermediates. This guarantees the desired exothermic coupling of *N=N* intermediates and generated CO to form the urea precursor, *NCON*.

Full text of DOI:10.1002/anie.202101275

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 10910–10918
International Edition:
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doi.org/10.1002/anie.202101275
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Urea Electrosynthesis Very Important Paper
Unveiling Electrochemical Urea Synthesis by Co-Activation of CO2
and N2 with Mott–Schottky Heterostructure Catalysts
Menglei Yuan+, Junwu Chen+, Yiling Bai, Zhanjun Liu, Jingxian Zhang, Tongkun Zhao,
Qin Wang, Shuwei Li, Hongyan He, and Guangjin Zhang*
À
Abstract: Electrocatalytic C N bond coupling to convert CO2
and N2 molecules into urea under ambient conditions is
a promising alternative to harsh industrial processes. However,
the adsorption and activation of inert gas molecules and then
the driving of the C–N coupling reaction is energetically
challenging. Herein, novel Mott–Schottky Bi-BiVO4 hetero-
structures are described that realize a remarkable urea yield
rate of 5.91 mmolhÀ1 gÀ1 and a Faradaic efficiency of 12.55%
at À0.4 V vs. RHE. Comprehensive analysis confirms the
emerging space–charge region in the heterostructure interface
not only facilitates the targeted adsorption and activation of
CO2 and N2 molecules on the generated local nucleophilic and
electrophilic regions, but also effectively suppresses CO
poisoning and the formation of endothermic *NNH inter-
mediates. This guarantees the desired exothermic coupling of
of N2/CO2 molecules.[5] To this end, tremendous efforts have
been devoted to converting N2 and CO2 to value-added
chemical products and fuels (NH3, N2H2, CH3OH,
CH3COOH, etc.).[6] Compared with the conventional thermal
catalytic and other hydrogenation processes, the utilization of
the electrochemical approach exhibits great advantages in
atom and energy economies.[7] It thus motivates researchers to
À
construct an electrochemical C N bond coupling system by
directly using CO2 and amine derivatives/nitrogen sources
(nitrate, nitrite, NO, and even N2) as feedstock.[8]
When feeding with N2 and CO2 as the reactant, the
electrochemical C-N coupling reactions can proceed to
produce valuable urea [CO(NH2)2] molecules, which is
regarded as one of the most efficient nitrogen fertilizer and
is of great significance to the chemical industry.[9] The
industrial urea synthesis is dominated by two consecutive
steps, including N2 + H2 ! NH3 and followed by NH3 + CO2
! CO(NH2)2, which operated at harsh reaction conditions
(350–5508C, 150–350 bar and 150–2008C, 150–250 bar, re-
spectively).[10] Compared with huge energy consumption
industrial processes, the electrochemical urea synthesis pro-
=
*N N* intermediates and generated CO to form the urea
precursor, *NCON*.
Introduction
Nitrogen is regarded as an indispensable element for all
life, which accounts for 78% of the atmosphere but canꢀt be
directly adopted by plants and animals.[1] Transforming N2
into valuable fertilizer under mild conditions is highly
anticipated for human beings.[2] On the other hand, excessive
carbon dioxide (CO2) emissions have aroused severe environ-
mental concerns.[3] Therefore, converting both N2 and CO2
into value-added urea molecules via C-N coupling reaction
under ambient conditions is a promising route to achieve both
vides an appealing route (N2 + CO2 + 6H+ + 6eÀ
!
CO(NH2)2 + H2O) under mild conditions.[4] However, the
related catalytic activity and selectivity for electrochemical
urea synthesis are still extremely low. The main challenges can
be ascribed as i) extraordinarily weak chemical adsorption of
inert CO2/N2 on the catalysts surface;[11] ii) the dissociation of
=
ꢀ
highly stable C O bond and N N bond requires high
overpotential;[5c,12] iii) the parallel CO2/N2 reduction reactions
strongly competes with the desired C-N coupling reaction for
urea synthesis and further results in the complex product
distribution.[7]
carbon balance strategy and high-value utilization of CO2.[4]
À1
ꢀ
Nonetheless, the chemical inert of N2 (N N, 940.95 kJmol )
À1
=
and CO2 (C O, 806 kJmol ) inevitably impede the activation
[*] M. Yuan,[+] J. Chen,[+] J. Zhang, T. Zhao, S. Li, Prof. H. He,
Prof. G. Zhang
Y. Bai
SynCat@Beijing, Synfuels China Technology Co. Ltd
Beijing 101407 (P. R. China)
CAS Key Laboratory of Green Process Engineering
State Key Laboratory of Multiphase Complex Systems
Institute of Process Engineering, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: zhanggj@ipe.ac.cn
M. Yuan,[+] J. Chen,[+] Prof. Z. Liu, J. Zhang, T. Zhao, S. Li, Prof. H. He,
Q. Wang
Engineering Design Department, Hebei Enco Petrochemical Engi-
neering Co. Ltd., Henan Branch
Henan 450000 (P. R. China)
Prof. G. Zhang
Prof. G. Zhang
Chemistry and Chemical Engineering Guangdong Laboratory
Center of Materials Science and Optoeletronics Engineering
University of Chinese Academy of Sciences
Beijing 100049 (P. R. China)
Shantou 515031 (P. R. China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202101275.
Y. Bai, Prof. Z. Liu
CAS Key Laboratory of Carbon Materials
Institute of Coal Chemistry, Chinese Academy of Sciences
Taiyuan 030001 (P. R. China)
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Aiming at this challenging field, Chen et al. reached the
can be attributed to the reduction by NaBH4 that inducing
urea production rate of 3.36 mmolgÀ1 hÀ1 and Faradaic
efficiency (FE) of 8.92% at À0.4 V vs. RHE on PdCu/TiO2
hybrids.[4] Despite the aforementioned noble metal Pd-based
catalysts that can realize urea electrosynthesis, the related
electrocatalytic activity still needs to be improved. Besides,
their real application in large-scale is restricted by high cost
and scarcity. Therefore, it is highly anticipated to develop
earth-abundant alternatives to further improve urea electro-
synthesis performance. The electrochemical C-N coupling
reaction in urea synthesis is strongly dependent on the surface
electronic state of electrocatalysts. Designing the electro-
catalyst to enhance the adsorption and activation of inert gas
molecules can be adopted as a promising strategy to promote
C-N coupling reaction and further urea formation.[13] Thus, we
supposed that Mott–Schottky heterostructures may provide
a new possibility to enhance the electrocatalytic capability of
urea synthesis.[14] The reason can be ascribed to the formed
interface can stimulate spontaneous charge transfer and then
generate a special space-charge region to drive the targeted
surface reaction. The band bending at the interface facilitates
the charge redistribution until the electrocatalytic system
reaches a thermal equilibrium state. Then, the oppositely
charged regions arise at the heterointerface and bring about
the alteration of electron density around the interface, which
is favorable for the adsorption of targeted small molecules
(CO2/N2) and the subsequent protonation process.[15] Accord-
abundant defect BiO10 decahedron models until the forma-
tion of metallic Bi site. As displayed in the Raman spectrum
(Figure 1c), the pronounced peaks detected at 327 and
367 cmÀ1 correspond to asymmetric and symmetric stretching
modes of VO43À tetrahedron unit in BiVO4. Besides, the short
(Bg) and long (Ag) asymmetric V-O stretching modes also
appear at 637 and 702 cmÀ1.[16] The intense Raman peak that
emerged at 826 cmÀ1 can be ascribed to the shorter symmetric
V-O stretching mode (Ag).[17] Compared with pristine BiVO4,
the disappearance of the vibration peaks in Bi-BiVO4 hybrids
at 367, 637, and 702 cmÀ1 results from damages to the inherent
crystal structure of BiVO4 by the reaction of NaBH4.
Although the BiO10 decahedron units were undermined and
thus produced Bi site, the XRD diffraction peaks of metallic
Bi canꢀt be found in Bi-BiVO4 hybrids, which can be ascribed
to the high dispersion and low concentration of metallic Bi. To
further certify the existence of metallic Bi, X-ray photo-
electron spectroscopy (XPS) of the hybrids was examined. It
is shown that the metallic Bi0 peak can be detected at
162.1 eV for Bi-BiVO4 hybrids in the Bi 4f region.[18] Besides,
the characteristic peaks located at 159.3 eV and 164.7 eV can
be assigned to Bi3+ 4f7/2 and Bi3+ 4f5/2, respectively (Fig-
ure 1d).[18b] The V 2p spectrum can be deconvoluted into V4+
2p (515.8 eV) peak (Figure 1e).[19] Notably, the Bi3+ 4f and
V4+ 2p peaks in Bi-BiVO4 hybrids show the negative shift of
0.2 eVand positive shift of 0.2 eV respectively, compared with
ingly, engineering distinct space-charge regions in Mott– pristine BiVO4, originating from the electronic interaction
Schottky heterojunction would be the feasible strategy for
remarkable urea electrosynthesis but it hasnꢀt been inves-
tigated so far.
between metallic Bi0 phase and BiVO4. Mott–Schottky test
was further utilized to reveal the electron transfer in Bi-
BiVO4 hybrids (Figure 1 f). BiVO4 samples possess a positive
slope, reflecting typical n-type semiconductor characteristics.
Thus, when it was combined with metallic Bi, the unique
Mott–Schottky heterostructure can be formed and the
electrons can transfer from BiVO4 to metallic Bi to form
a unique space-charge region (Figure 1g and Figure S2a–c).
The fabricated space-charge region ensures the effective
synthesis of urea at different active sites. Additionally, the
NaBH4 reduction strategy also results in the increase of
Brunauer-Emmett-Teller (BET) specific surface area from
14.7 m2 gÀ1 (BiVO4) to 30.4 m2 gÀ1 (Bi-BiVO4) (Figure 1h).
The field-emission scanning electron microscopy (FE-
SEM) image in Figure 2a reveals that pristine BiVO4
possesses an average width and length of 400 nm and 1 mm,
respectively. The corresponding EDS mapping confirms the
uniform distribution of Bi, V, and O elements (Figure 2b),
which is consistent with the XPS survey spectrum (Figure 1i).
The lattice spacing (0.312 nm) in high-magnification trans-
mission electron microscopy (HRTEM) corresponds to the
(130) crystal plane of BiVO4 (Figure S2d). Figure 2d,e proves
that the Bi-BiVO4 hybrids still maintain a similar morphology
and uniform element distribution with pure BiVO4, but the
unique heterostructural interfaces between metallic Bi and
BiVO4 can be observed (Figure 2c and f). The formation of
such heterojunction creates the unique space-charge region
(Figure S2c) with self-driven charge transfer from BiVO4 to
Bi. Thus local electrophilic and nucleophilic regions are
generated.
Herein, as the proof-of-concept catalyst, Mott–Schottky
heterostructural Bi-BiVO4 hybrids were prepared by the
NaBH4 reduction strategy and ensured spontaneous electron
transfer from BiVO4 to Bi. The rationally engineering space-
charge region guarantees the accurate adsorption and activa-
tion of N2 and CO2 and further promotes the desired
electrochemical C-N coupling reaction in urea synthesis.
Therefore, the as-prepared Mott–Schottky Bi-BiVO4 hetero-
structure electrocatalysts deliver a remarkable urea produc-
tion rate of 5.91 mmolhÀ1 gÀ1 with a FE of 12.55% at À0.4 V
(vs. RHE), outperforming those reported best values. This
work underlines the significance of the engineering space-
charge region for enhancing urea synthesis by electrocatalytic
C-N coupling reaction.
Results and Discussion
The pristine BiVO4 was directly prepared by reacting
bismuth nitrate with ammonium metavanadate by in situ
redox route. Figure 1a displays the X-ray diffraction (XRD)
patterns of as-prepared pristine BiVO4 match well with
monoclinic phase of BiVO4 (JCPDS: 83-1700). The corre-
sponding crystal structure contains BiO10 decahedron and
VO4 tetrahedron units (Figure S1). When the pristine BiVO4
is partially reduced by the NaBH4, the Bi-BiVO4 hybrids were
obtained which only exhibit relatively weak BiVO4 diffraction
peaks compared to pristine BiVO4 (Figure 1b). The reason
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Figure 1. XRD patterns of a) BiVO4 and b) Bi-BiVO4; c) Raman spectra, d) high-resolution Bi 4f spectra and e) high-resolution V 2p spectra of
BiVO4 and Bi-BiVO4; f) Mott–Schottky plots of BiVO4; g) schematic illustration of the charge transfer process in Bi-BiVO4; h) BET specific surface
area and i) XPS survey spectra of BiVO4 and Bi-BiVO4.
As for electrocatalytic urea synthesis, N2 and CO2 as
feeding gas were continuously bubbled into the H-shaped cell,
which is separated by the Nafion 211 membrane and equipped
with a three-electrode system (Figure S3). Linear sweep
voltammetry (LSV) was used to preliminarily evaluate the
potential performance of urea electrosynthesis with Bi-BiVO4
hybrids. As displayed in Figure 3a, the Bi-BiVO4 hybrids
exhibit an enhanced current density in mixed gas (N2 + CO2)
-saturated 0.1 M KHCO3 solution relative to sole gas (N2/
CO2)-saturated electrolyte, indicating the occurrence of
electrocatalytic urea synthesis. It has been reported that the
electrochemical urea production ability of the catalyst was
dominated by effectively coupling carbon dioxide reduction
reaction (CO2RR) with nitrogen reduction reaction (NRR).[4]
In another word, encouraging electrocatalytic NRR and
CO2RR performance of the Bi-BiVO4 hybrids guarantee the
electrocatalytic production of urea process (Figure 3b,c).
Constant potential electrolysis ranging from À0.3 V to À0.7 V
vs. RHE were selected to further quantitatively assess urea
electrosynthesis activity. The time-dependent current density
curves recorded for 2 h at various potentials illustrate the
superior stability of Bi-BiVO4 hybrids (Figure S4). The
generated urea and by-product NH3 were estimated by the
diacetyl monoxime method and indophenol blue method, the
related calibration curves are depicted in Figure S5.[20] Be-
sides, the possible gas products such as H2 and CO were
monitored by on-line gas chromatography. The calculated
urea yield rate and Faradaic efficiency (FE) in Figure 3d
demonstrate that when the potential reached À0.4 V vs.
RHE, the Bi-BiVO4 hybrids electrode delivers the maximum
urea yield rate of 5.91 mmolhÀ1 gÀ1 and FE of 12.55%. As
listed in Table S1, the electrocatalytic urea synthesis perfor-
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Figure 2. a) SEM image and b) the corresponding element mapping of BiVO4; c) high-magnification TEM image of Bi-BiVO4 hybrids and the
dotted line represents the heterointerfaces; d) SEM image and e) the corresponding element mapping of Bi-BiVO4 hybrids; f) the well-resolved
lattice fringe of Bi-BiVO4 hybrids in (c).
mance of Bi-BiVO4 hybrids is superior to the highest values
reported so far. However, as the potential shifted below
À0.4 V, the excessive release of CO occupied the adsorption
sites for CO2 and N2 result in dramatically reduced urea yield
and FE and further bring about the diversified reduction
products distribution (Figure 3e). Note that Bi-BiVO4 hy-
brids deliver the highest FE (15.33%) and NH3 yield rate
(7.07 mmolhÀ1 gÀ1) at À0.4 V vs. RHE during NRR, which are
higher than that of the electrocatalytic urea synthesis process
(Figure 3b). The reason can be attributed to the modulated
reaction pathways to promote electrochemical C-N coupling
reaction and further generate urea, which will be further
revealed by DFT calculations. Furthermore, a 10 h chro-
noamperometry test (Figure S6) and cycling experiments
(Figure S7) were utilized to evaluate the durability of Bi-
BiVO4 hybrids. Negligible degradation in current density and
electrocatalytic activity indicates the desirable stability of Bi-
BiVO4 hybrids. The related characterizations also prove that
the obtained Bi-BiVO4 hybrids still maintain their original
morphology, chemical state, and crystal phase after 10 h
continuous electrolysis (Figure S8). Besides, benefiting from
the enhanced solubility of CO2 in ionic liquid (IL, 1-butyl-3-
methylimidazolium tetrafluoroborate),[21] the IL-KHCO3 was
used as the electrolyte and the system exhibited the urea yield
rate of 6.64 mmolhÀ1 gÀ1 and FE of 15.8% at À0.4 V vs. RHE,
which are higher than that in pure KHCO3 solution (Fig-
ure S9a).
BiVO4 hybrids to synergistically activate CO2 and N2
molecules was further proved by isotope labeling experi-
ments. No distinguished CO(NH2)2 signal was observed when
utilizing 15N2 and CO2 as the feeding gas, only CO(15NH2)2
signal was detected in the reacted electrolyte, confirming that
urea was generated by simultaneous reduction of CO2 and N2
(Figure 3 f). Besides, the concentration of CO(15NH2)2 after
2 h electrolysis was also quantitatively detected based on the
peak area of 1H NMR (Figure 3g,h). As expected, the
calculated CO(15NH2)2 concentration is in good accordance
with that detected by the diacetyl monoxime method (Fig-
ure 3i). All these results convincingly confirm the reduction
of both N2 and CO2 into urea was catalyzed by Bi-BiVO4
hybrids.
To uncover the contribution of engineering space-charge
region on the enhancement of Bi-BiVO4 hybrids perfor-
mance, the electrochemical properties of pristine BiVO4 were
also investigated. As expected, it can be seen in Figure 4a that
pristine BiVO4 displays remarkably decreased urea yield and
FE in comparison with Bi-BiVO4 hybrids. In Figure 4b and
Figure S10, the Bi-BiVO4 hybrids also exhibit a larger electro-
chemical active surface area (ECSA) than that of pristine
BiVO4, illustrating that the designed space-charge region is
favorable for the exposure of more active sites.[22] Further-
more, the electrochemical impedance spectroscopy (EIS) of
the Bi-BiVO4 hybrids delivers smaller semicircle and higher
slope compared with that of pristine BiVO4, which verify that
Bi-BiVO4 hybrids coupled with space-charge region facilitate
the effective charge transfer and thus endow the Bi and
BiVO4 surface with a local nucleophilic and electrophilic
region (Figure 4c).[23] As the chemisorption of N2 and CO2 are
regarded as the initial step towards urea electrosynthesis, it is
critical to promote the chemisorption ability of N2 and CO2 on
Since adventitious NOx is inevitably present in the
environment and chemicals which may lead to false-positive
results, a series of control experiments were conducted to
further verify the generated urea comes from the simulta-
neous reduction of CO2 and N2 rather than contaminants
(Scheme 1 and Figure S9b). Additionally, the capability of Bi-
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Figure 3. a) LSV curves of Bi-BiVO4 hybrids at N2, CO2, and N2 +CO2 saturated electrolyte; b) NH3 synthesis with N2 as the feeding gas and c) CO
generation with CO2 as the feeding gas at various potentials for Bi-BiVO4 hybrids; d) the urea yield rate and Faradic efficiencies and e) the
corresponding product distribution of H2 (purple), CO (cyan-blue), NH3 (blue), and urea (red) with N2 and CO2 as the feeding gas at various
potentials for Bi-BiVO4 hybrids; f) 1H NMR spectra of electrolyte saturated with 15N2 + CO2 / 14N2 + CO2 after 2 h electrolysis and standard
15NH2CO15NH2/14NH2CO14NH2 solution; g) 1H NMR spectra of standard 15NH2CO15NH2 solution with various concentrations of 0.2–0.5 mgmLÀ1
h) integral area (15NH2CO15NH2/C4H4O4) concentration linear relation calibrated using standard 15NH2CO15NH2 solution; i) the urea yield of Bi-
BiVO4 hybrids after 2 h electrolysis detected by UV/Vis and 1H NMR spectroscopy.
;
the electrocatalysts. In this work, engineering space-charge
regions would be served as a promising strategy toward
significantly enhancing the chemisorption capability of inert
N2 and CO2 molecules. As revealed by theoretical simulations,
the electron-rich N atom in N2 and electron-deficient C atom
in CO2 (Figure 4d) will target adsorb on the generated
electrophilic and nucleophilic regions in Bi-BiVO4 hybrids
due to electronic interaction.[24] As shown in Figure 4e,f, both
the N2- and CO2-TPD spectrum of Bi-BiVO4 hybrids display
the stronger binding strength and larger adsorption peak as
compared to that of pristine BiVO4, verifying the enhance-
ment of the adsorption capacity for both N2 and CO2
molecules on Bi-BiVO4 hybrids.[25]
The density functional theory (DFT) calculations were
further carried out to corroborate the contribution of the
space-charge region to adsorb and activate inert gas mole-
cules. To this end, the adsorption behavior of reactant
molecules on the two different optimized structures was
firstly investigated. As revealed by Bader charge analysis
(Figure 5a), the presence of space-charge region in Bi-BiVO4
hybrids guarantees the spontaneous electron transfer from
BiVO4 to Bi (1.21eÀ), in agreement with the above XPS and
Raman characterizations. The corresponding charge accumu-
lation and depletion endows the Bi and BiVO4 interface with
the local nucleophilic and electrophilic region, which achieves
the targeted adsorption of gas molecules by the electrostatic
interaction. As expected, the adsorption energies of both N2
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Figure 4. a) Average urea yield rate and FE; b) DJ of electrocatalysts plotted against scan rate at À0.05 V vs. RHE; c) Nyquist plots of
electrochemical impedance spectra (EIS) of BiVO4 and Bi-BiVO4; d) electron density isosurface of CO2 molecule (left) and N2 molecule (right), the
color bar represents the electrostatic potential scale; e) nitrogen temperature-programmed desorption (N2-TPD) plots and f) carbon dioxide
temperature-programmed desorption (CO2-TPD) plots for BiVO4 and Bi-BiVO4.
and CO2 on the pristine BiVO4 model are much higher than
that of Bi-BiVO4 hybrids (Figure 5b,c), attesting to the
critical role of the space-charge region in promoting the gas
adsorption, which coincides well with the aforementioned
TPD results.
exothermic (Figure 5g). Subsequently, the multi-step proton-
coupled electron transfer process follows the two different
pathways that is, distal pathway and alternating pathway. As
shown in Figure 5g and Figure S12, for the case of distal
mechanism, the (H+ + eÀ) continues to attack the distal N
atom, while the (H+ + eÀ) alternatively reacts with N atoms
on both sides when following the alternating mechanism. The
potential-determining step during urea electrosynthesis is the
Although the rationally designed space-charge region
accelerates the adsorption of inert gas molecules, it is essential
to explore the adsorption sequence of N2 and CO2 on the
active sites to reveal the reaction mechanism. Compared with
CO2-TPD results, the peaks in the N2-TPD spectrum emerge
at a higher temperature and display the enhanced intensity,
ensuring that N2 gas preferentially adsorbs on the Bi-BiVO4
hybrids than that of CO2 molecule (Figure 4e,f).[4] Notably,
the bonded N2 on the local electrophilic BiVO4 region can be
further activated by energetically preferred side-on config-
uration rather than end-on configuration (Figure 5d and
Figure S11). It should be pointed out that the activation of the
N2 molecule needs to follow the previously reported “accept-
ance-donation” process. Herein, the empty eg orbitals of V
sites in VO4 tetrahedron unit firstly accept the electrons from
occupied s orbitals of N2 molecules, then the filled eg orbitals
of V sites donate their electrons to N2 empty p* orbitals,
achieving the effective activation of N2 molecule on the
surface of the electrocatalyst (Figure 5e).
generation of *NHCONH intermediate (*NCONH + H+
+
eÀ ! *NHCONH, DG = 0.48 eV) in the alternating pathway,
whereas the reductive protonation of *NCONH into
*NCONH2 requires the highest DG value of 0.59 eV for the
distal pathway. Thus, the hydrogenation of formed *NCON*
will subsequently undergo the thermodynamically feasible
alternating pathway until the release of the urea molecule.
The selectivity of the electrocatalytic urea production is
closely associated with the formation of *NCON* species. At
this stage, the possible N2 reduction or the excessive release of
CO would result in diverse product distribution and further
reduce the selectivity of urea electrosynthesis. On the one
hand, the competitive conversion of *N2 into *NNH is
inactive on the Bi-BiVO4 hybrids, as shown in Figure S13.
As seen, this process prefers to undergo at the pristine BiVO4
model, which corroborates that the rationally designed space-
charge region suppresses the NRR to some extent, let alone
the generation of side product NH3. On the other hand, the
amount of CO should be well controlled. Once the active sites
are strongly bonded with CO, which makes it difficult to
couple with the adsorbed N2 and thus retards the formation of
*NCON* species. As exhibited in Figure S14, the correspond-
When comparing the Gibbs free energy (DG) of CO2RR
with and without the participation of N2, one can conclude
that the adsorbed N2 molecule facilitates the reduction of CO2
=
to CO (Figure 5 f). Due to the *N N* possesses the matched
molecule orbitals with the released CO, the corresponding
DG indicates that the formation of *NCON* intermediate is
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Figure 5. a) Top: Planar average charge density difference along the z-direction for the Bi-BiVO4 heterojunction; bottom: charge density difference
À3
of the Bi-BiVO4 heterojunction, yellow and cyan indicate electron accumulation and depletion, respectively, with isosurface values of 0.002 eŠ
Free energy diagrams for b) N2 and c) CO2 adsorption on BiVO4 and Bi-BiVO4, the bottom figures are the corresponding calculation models;
d) N2 adsorbed on the Bi-BiVO4 by different configurations; e) simplified schematic of N2 bonding to V center; f) free energy diagrams for CO2
.
reduction with and without N2 adsorption on Bi-BiVO4 hybrids; g) electrolytic urea production via the alternating mechanism; h) mechanism of
the electrocatalytic urea synthesis based on synergistic effects of the Bi-BiVO4 Mott–Schottky heterostructure.
ing electrochemical CO-stripping experiment results demon-
strate that the CO-stripping peak of Bi-BiVO4 hybrids
(0.258 V vs. RHE) displays the negatively shift compared
with that of pristine BiVO4 counterparts (0.345 V vs. RHE),
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indicating the higher stability of Bi-BiVO4 hybrids against CO
Conflict of interest
poisoning, which is owing to the elimination of the part of
strong adsorption sites by the presence of space-charge
region.[26] Conversely, the excessive release of CO is also
not conducive to the urea electrosynthesis process. As shown
in Figure 3e, beyond the applied potential of À0.4 V, the
significantly reduced electrocatalytic urea production perfor-
mance of the Bi-BiVO4 hybrids can be ascribed to the
occupation of the adsorption sites for N2 and CO2 by excessive
release of CO.
By combining the aforementioned experimental results
and computational simulations, the overall urea electrosyn-
thesis process in Bi-BiVO4 hybrids can be summarized as
follows: (i) the spontaneous charge transfer at the hetero-
interfaces promotes the formation of space-charge region, (ii)
N2/CO2 molecules are firstly targeted adsorbed on the
generated electrophilic/nucleophilic regions by electrostatic
interaction, (iii) the adsorbed *N2 can promote CO2 reduction
to form CO, and then the generated CO will further react with
*N2 to produce the desirable *NCON* intermediate via
thermodynamically feasible electrochemical C-N coupling
reaction, (iv) the subsequent protonation process preferen-
tially undergoes the alternating mechanism until the forma-
tion of urea (Figure 5h).
The authors declare no conflict of interest.
Keywords: C–N coupling ·electrocatalysis ·Mott–
Schottky heterostructures ·urea
[1] a) M. D. Fryzuk, J. B. Love, S. J. Rettig, V. G. Young, Science
1997, 275, 1445 –1447; b) H.-P. Jia, E. A. Quadrelli, Chem. Soc.
Rev. 2014, 43, 547 –564; c) T. Wu, H. Zhao, X. Zhu, Z. Xing, Q.
Liu, T. Liu, S. Gao, S. Lu, G. Chen, A. M. Asiri, Y. Zhang, X. Sun,
Adv. Mater. 2020, 32, 2000299.
[2] a) S. L. Foster, S. I. P. Bakovic, R. D. Duda, S. Maheshwari, R. D.
Milton, S. D. Minteer, M. J. Janik, J. N. Renner, L. F. Greenlee,
Nat. Catal. 2018, 1, 490 –500; b) C. J. Pickett, J. Talarmin, Nature
1985, 317, 652 –653; c) T. Wang, Q. Liu, T. Li, S. Lu, G. Chen, X.
Shi, A. M. Asiri, Y. Luo, D. Ma, X. Sun, J. Mater. Chem. A 2021,
9, 884 –888; d) R. Zhao, Q. Geng, L. Chang, P. Wei, Y. Luo, X.
Shi, A. M. Asiri, S. Lu, Z. Wang, X. Sun, Chem. Commun. 2020,
56, 9328 –9331.
[3] a) B. Obama, Science 2017, 355, 126 –129; b) A. D. Handoko, F.
Wei, Jenndy, B. S. Yeo, Z. W. Seh, Nat. Catal. 2018, 1, 922 –934.
[4] C. Chen, X. Zhu, X. Wen, Y. Zhou, L. Zhou, H. Li, L. Tao, Q. Li,
S. Du, T. Liu, D. Yan, C. Xie, Y. Zou, Y. Wang, R. Chen, J. Huo,
Y. Li, J. Cheng, H. Su, X. Zhao, W. Cheng, Q. Liu, H. Lin, J. Luo,
J. Chen, M. Dong, K. Cheng, C. Li, S. Wang, Nat. Chem. 2020, 12,
717 –724.
Conclusion
[5] a) K. A. Brown, D. F. Harris, M. B. Wilker, A. Rasmussen, N.
Khadka, H. Hamby, S. Keable, G. Dukovic, J. W. Peters, L. C.
Seefeldt, P. W. King, Science 2016, 352, 448 –450; b) X. Cheng, J.
Wang, W. Xiong, T. Wang, T. Wu, S. Lu, G. Chen, S. Gao, X. Shi,
Z. Jiang, X. Niu, X. Sun, ChemNanoMat 2020, 6, 1315 –1319;
c) H. B. Yang, S.-F. Hung, S. Liu, K. Yuan, S. Miao, L. Zhang, X.
Huang, H.-Y. Wang, W. Cai, R. Chen, J. Gao, X. Yang, W. Chen,
Y. Huang, H. M. Chen, C. M. Li, T. Zhang, B. Liu, Nat. Energy
2018, 3, 140 –147.
[6] a) D. M. Weekes, D. A. Salvatore, A. Reyes, A. Huang, C. P.
Berlinguette, Acc. Chem. Res. 2018, 51, 910 –918; b) Z.-Q. Liang,
T.-T. Zhuang, A. Seifitokaldani, J. Li, C.-W. Huang, C.-S. Tan, Y.
Li, P. De Luna, C. T. Dinh, Y. Hu, Q. Xiao, P.-L. Hsieh, Y. Wang,
F. Li, R. Quintero-Bermudez, Y. Zhou, P. Chen, Y. Pang, S.-C.
Lo, L.-J. Chen, H. Tan, Z. Xu, S. Zhao, D. Sinton, E. H. Sargent,
Nat. Commun. 2018, 9, 3828; c) M. Yuan, Q. Li, J. Zhang, J. Wu,
T. Zhao, Z. Liu, L. Zhou, H. He, B. Li, G. Zhang, Adv. Funct.
Mater. 2020, 30, 2004208.
In summary, the NaBH4 reduction strategy was proposed
to elaborately integrate metallic Bi and semiconductor BiVO4
to form the Mott–Schottky heterojunction. The fabricated Bi-
BiVO4 hybrids achieve the maximum urea yield rate of
5.91 mmolhÀ1 gÀ1 and Faradaic efficiency of 12.55% in 0.1 M
KHCO3 at À0.4 V vs. RHE. The space-charge region at the
heterointerfaces induced by the self-driven charge transfer
promotes the targeted adsorption and activation of inert N2
and CO2 molecules on generated local electrophilic and
nucleophilic regions. Furthermore, the designed space-charge
region also facilitates the electrocatalytic kinetics and enhan-
ces the full exposure of active sites, which contribute to the
À
thermodynamically feasible coupling of the C N bond and
produce the desired *NCON* intermediate. Further explora-
tion of the space-charge region may promote the fabricated
electrocatalysts with the high urea electrosynthesis perfor-
mance.
[7] Y. Feng, H. Yang, Y. Zhang, X. Huang, L. Li, T. Cheng, Q. Shao,
Nano Lett. 2020, 20, 8282 –8289.
[8] J. E. Kim, S. Choi, M. Balamurugan, J. H. Jang, K. T. Nam,
Trends Chem. 2020, 2, 1004 –1019.
[9] M. Aresta, A. Dibenedetto, A. Angelini, Chem. Rev. 2014, 114,
1709 –1742.
[10] F. Barzagli, F. Mani, M. Peruzzini, Green Chem. 2011, 13, 1267 –
1274.
Acknowledgements
This work is supported by the National Key R&D Program of
China (No. 2020YFA0710200) and the Chemistry and Chem-
ical Engineering Guangdong Laboratory (No. 1922006).
We also thank the financial project that Key Program
for International S&T Cooperation Projects (No.
2018YFE0124600) funded by the Ministry of Science and
Technology of China.
[11] a) Y. Ding, L. Huang, J. Zhang, A. Guan, Q. Wang, L. Qian, L.
Zhang, G. Zheng, J. Mater. Chem. A 2020, 8, 7229 –7234; b) T.
Wang, S. Li, B. He, X. Zhu, Y. Luo, Q. Liu, T. Li, S. Lu, C. Ye, A.
Asiri, X. Sun, Chin. J. Catal. 2020, 42, 1024 –1029; c) T. Xu, B.
Ma, J. Liang, L. Yue, Q. Liu, T. Li, H. Zhao, Y. Luo, S. Lu, X. Sun,
Acta Phys. Chim. Sin. 2021, 37, 2009043.
[12] M. Yuan, H. Zhang, D. Gao, H. He, Y. Sun, P. Lu, S. Dipazir, Q.
Li, L. Zhou, S. Li, Z. Liu, J. Yang, Y. Xie, H. Zhao, G. Zhang, J.
Mater. Chem. A 2020, 8, 2691 –2700.
[13] Z.-H. Xue, H. Su, Q.-Y. Yu, B. Zhang, H.-H. Wang, X.-H. Li, J.-S.
Chen, Adv. Energy Mater. 2017, 7, 1602355.
Angew. Chem. Int. Ed. 2021, 60, 10910 –10918
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org 10917

Angewandte
Research Articles
Chemie
[14] K. He, T. Tadesse Tsega, X. Liu, J. Zai, X.-H. Li, X. Liu, W. Li, N.
Ali, X. Qian, Angew. Chem. Int. Ed. 2019, 58, 11903 –11909;
Angew. Chem. 2019, 131, 12029 –12035.
[15] a) M. Yuan, Y. Bai, J. Zhang, T. Zhao, S. Li, H. He, Z. Liu, Z.
Wang, G. Zhang, J. Mater. Chem. A 2020, 8, 26066 –26074; b) P.
Lu, Y. Yang, J. Yao, M. Wang, S. Dipazir, M. Yuan, J. Zhang, X.
Wang, Z. Xie, G. Zhang, Appl. Catal. B 2019, 241, 113 –119.
[16] Y. K. Kho, W. Y. Teoh, A. Iwase, L. Mädler, A. Kudo, R. Amal,
ACS Appl. Mater. Interfaces 2011, 3, 1997 –2004.
[17] a) L. Sandhya Kumari, P. P. Rao, A. N. P. Radhakrishnan, V.
James, S. Sameera, P. Koshy, Sol. Energy Mater. Sol. Cells 2013,
112, 134 –143; b) F. D. Hardcastle, I. E. Wachs, J. Phys. Chem.
1991, 95, 5031 –5041.
[21] M. Alvarez-Guerra, J. Albo, E. Alvarez-Guerra, A. Irabien,
Energy Environ. Sci. 2015, 8, 2574 –2599.
[22] M. Yuan, S. Dipazir, M. Wang, Y. Sun, D. Gao, Y. Bai, M. Zhang,
P. Lu, H. He, X. Zhu, S. Li, Z. Liu, Z. Luo, G. Zhang, J. Mater.
Chem. A 2019, 7, 3317 –3326.
[23] a) G. Zhu, X. Xie, X. Li, Y. Liu, X. Shen, K. Xu, S. Chen, ACS
Appl. Mater. Interfaces 2018, 10, 19258 –19270; b) M. Yuan, M.
Wang, P. Lu, Y. Sun, S. Dipazir, J. Zhang, S. Li, G. Zhang, J.
Colloid Interface Sci. 2019, 533, 503 –512; c) C. Xi, G. Zhu, Y.
Liu, X. Shen, W. Zhu, Z. Ji, L. Kong, Colloids Surf. A 2018, 538,
748 –756.
[24] D. Zang, Q. Li, G. Dai, M. Zeng, Y. Huang, Y. Wei, Appl. Catal.
B 2021, 281, 119426.
[25] Z. Geng, Y. Liu, X. Kong, P. Li, K. Li, Z. Liu, J. Du, M. Shu, R. Si,
J. Zeng, Adv. Mater. 2018, 30, 1803498.
[26] W. Zhang, Y. Yang, B. Huang, F. Lv, K. Wang, N. Li, M. Luo, Y.
Chao, Y. Li, Y. Sun, Z. Xu, Y. Qin, W. Yang, J. Zhou, Y. Du, D.
Su, S. Guo, Adv. Mater. 2019, 31, 1805833.
[18] a) K.-H. Ye, X. Yu, Z. Qiu, Y. Zhu, X. Lu, Y. Zhang, RSC Adv.
2015, 5, 34152 –34156; b) S. Xie, T. Zhai, Y. Zhu, W. Li, R. Qiu,
Y. Tong, X. Lu, Int. J. Hydrogen Energy 2014, 39, 4820 –4827.
[19] a) T. Soltani, A. Tayyebi, B.-K. Lee, Catal. Today 2020, 340, 188 –
196; b) M. Balamurugan, G. Yun, K.-S. Ahn, S. H. Kang, J. Phys.
Chem. C 2017, 121, 7625 –7634.
[20] a) D. Zhu, L. Zhang, R. E. Ruther, R. J. Hamers, Nat. Mater.
2013, 12, 836 –841; b) M. Rahmatullah, T. R. C. Boyde, Clin.
Chim. Acta 1980, 107, 3 –9.
Manuscript received: January 27, 2021
Accepted manuscript online: February 25, 2021
Version of record online: April 8, 2021
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