Built-in Electric Field Triggered Interfacial Accumulation Effect for Efficient Nitrate Removal at Ultra-Low Concentration and Electroreduction to Ammonia

Article abstract of DOI:10.1002/anie.202109785

A built-in electric field in electrocatalyst can significantly accumulate higher concentration of NO₃^? ions near electrocatalyst surface region, thus facilitating mass transfer for efficient nitrate removal at ultra-low concentration and electroreduction reaction (NO₃RR). A model electrocatalyst is created by stacking CuCl (111) and rutile TiO₂ (110) layers together, in which a built-in electric field induced from the electron transfer from TiO₂ to CuCl (CuCl_BEF) is successfully formed. This built-in electric field effectively triggers interfacial accumulation of NO₃^? ions around the electrocatalyst. The electric field also raises the energy of key reaction intermediate *NO to lower the energy barrier of the rate determining step. A NH₃ product selectivity of 98.6 %, a low NO₂^? production of <0.6 %, and mass-specific ammonia production rate of 64.4 h^?1 is achieved, which are all the best among studies reported at 100 mg L^?1 of nitrate concentration to date.

Full text of DOI:10.1002/anie.202109785

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Built-in Electric Field Triggered Interfacial Accumulation Effect
for Efficient Nitrate Removal at Ultra-Low Concentration and
Electroreduction to Ammonia
Authors: Wu-Ji Sun, Hao-Qin Ji, Lan-Xin Li, Hao-Yu Zhang, Zhen-Kang
Wang, Jing-Hui He, and Jian-Mei Lu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202109785
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10.1002/anie.202109785
Angewandte Chemie International Edition
RESEARCH ARTICLE
Built-in Electric Field Triggered Interfacial Accumulation Effect
for Efficient Nitrate Removal at Ultra-Low Concentration and
Electroreduction to Ammonia
Wu-Ji Sun, Hao-Qin Ji, Lan-Xin Li, Hao-Yu Zhang, Zhen-Kang Wang, and Jing-Hui He,* Jian-Mei Lu*
[*]
W.-J. Sun, L.-X. Li, H.-Y. Zhang, Prof. J.-H. He, and Prof. J.-M. Lu
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Suzhou Nano Science and Technology, National
United Engineering Laboratory of Functionalized Environmental Adsorption Materials, Soochow University, Suzhou 215123, P. R. (China)
E-mail: jinghhe@suda.edu.cn ; lujm@suda.edu.cn
Prof. H.-Q. J and Z.-K. Wang
College of energy, Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou
215006, China.
Abstract: Nitrate removal from water with low pollution concentration
and its subsequent electroreduction to higher-value ammonia is
becoming progressively essential. However, this process still suffers
from low selectivity and activity due to slow mass transfer of nitrate
and the induced limited reagent supply to reaction interface. Herein,
we demonstrate that a built-in electric field in electrocatalyst can
as the nitrogen source receive much attention, as inspired by the
low dissociation energy of the N=O bond (204 kJ mol−1) and
unlimited solubility of polar NO3– ions.[5] Therefore electrocatalytic
nitrate reduction reaction (NO3RR) to ammonia[6] are of great
interest to reduce energetic consumption as well as water
treatment.
–
significantly accumulate higher concentration of NO3 ions near
To date, various metals, metal oxides and two-dimensional
materials displays great potential in electrocatalytic nitrate
reduction reaction to ammonia (NO3RR).[7] However, the present
NO3RR electrocatalysts still suffers from low ammonia production
rate toward nitrate low than 100 mg L–1 as the mass transfer was
too weak due to small concentration gradient near the electrode
region. On the other hand, electrocatalytic reduction of nitrate to
electrocatalyst surface region, thus facilitating mass transfer for
efficient nitrate removal at ultra-low concentration and
electroreduction reaction (NO3RR). A model electrocatalyst are
created by stacking CuCl (111) and rutile TiO2 (110) layers together,
in which a built-in electric field induced from the electron transfer from
TiO2 to CuCl (CuCl_BEF) is successfully formed as demonstrated by
density functional theory (DFT) calculations and X-ray absorption near
edge structure (XANES). Molecular dynamics simulation and finite
element analysis suggest that this built-in electric field effectively
8]
ammonia involves an eight-electron, nine proton process.[7c,
–
Many intermediates such as highly toxic NO2 , hydroxylamine and
diamine may leak as byproducts if the adsorption from catalysts
is weak.[8a, 9] Therefore, the simple metal/support architectures of
catalysts will result in low selectivity, low Faradaic efficiency, and
in consequence release of toxic byproducts, which are
unsatisfactory in practical water treatment.
–
triggers interfacial accumulation of NO3 ions around the
electrocatalyst. The electric field also rises the energy of key reaction
intermediate *NO to lower down energy barrier of the rate determining
–
step. A NH3 product selectivity of 98.6%, a low NO2 production of
<0.6%, and mass-specific ammonia production rate of 64.4 h–1 is
achieved with this proof-of-concept system, which are all the best
among studies reported at 100 mg L–1 of nitrate concentration to date.
Herein, we demonstrate that a built-in electric field in
electrocatalyst can significantly accumulate higher concentration
–
of NO3 ions near electrocatalyst surface region, thus facilitating
mass transfer of reagent for efficient nitrate removal at ultra-low
concentration and subsequent electroreduction to higher-value
ammonia. A model electrocatalyst CuCl_BEF was created by
stacking CuCl (111) and rutile TiO2 (110) layers together to form
a built-in electric field induced by electrons transfer from TiO2 to
CuCl, as demonstrated by density functional theory (DFT)
calculations and X-ray absorption near edge structure (XANES).
The built-in electric field effectively triggers interfacial
Introduction
In recent decades, low concentration nitrate in surface and ground
water has been increasing as vast amounts of nitrous oxides
emitted from fossil fuels combustion, as well as fertilizers and
pesticides discharged from industry, agriculture and domestic
sewage.[1] Nitrite causes acid rain, photochemical smog,
methemoglobinemia and cancers. Therefore the Safe Drinking
Water Act (SDWA) regulations proposed by United States
Environmental Protection Agency stated that public water
systems should not have nitrate above 10 milligrams per liter (mg
L−1).[2] Therefore, Nitrate removal from water with low pollution
concentration and its subsequent electroreduction to higher-value
product is becoming progressively essential.
–
accumulation of NO3 ions around the electrocatalyst and also
rises the energy of key reaction intermediate *NO proved by
molecular dynamics simulation and finite element analysis.
Nitrate was converted to NH3 with a selectivity of 98.6%, a low
NO2– production of <0.6% and a mass-specific production rate of
64.4 h–1, which are both the best among all reported at 100 mg L–
1 of nitrate to date (Supporting Information Table S1).
Ammonia, as one of major fertilizers, is mainly produced
through Haber-Bosch process,[3] which consumes 1–2% of global
energy and releases equal amount of CO2.[4] As an alternative,
utilizing hydrogen from water to electrochemically reduce nitrate
1
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RESEARCH ARTICLE
Results and Discussion
Figure 1. Schematic illustration of CuCl_BEF electrocatalyst with a build-in electric field. a) Illustration of the CuCl_BEF in-situ formed
on the MXene surface. b) Differential charge density of CuCl_BEF. The side view of the charge transfer difference between CuCl and
TiO2 layers, where the yellow and cyan areas indicate the charge accumulation and depletion with iso-surfaces at 0.004 e/Å3. The
green, red, blue, and cyan balls refer to Cl, O, Cu, and Ti atoms, respectively. c) and d) Molecular dynamics simulation of CuCl and
–
CuCl_BEF in KNO3 (100 mg L–1) solution. Scale bar, 0.5 nm. e) Distribution of NO3 along the z-axis electrode distance based on
molecular dynamics simulation. f) and g) Finite element analysis of ions distribution difference near CuCl and CuCl_BEF in KNO3
solution.
To create a built-in electric field in electrocatalysts, we stacked
CuCl and TiO2 with different work functions as a model catalyst
(Figure 1a). Stacking any two dissimilar materials (at least one is
semiconductor), charge will cross their interface to produce a
built-in electric field until their Fermi levels align. Density
functional calculations were first employed to survey crystal lattice
match and electron transfer between the CuCl and rutile TiO2. The
CuCl is of zincblende-like structure, and its (111) plane
considerably corresponded with (110) surfaces of rutile TiO2
because of the substantial chemical bond formation between O
and Cu (Figure 1b). The theoretically calculated differential
charge distribution show that 0.57 electrons per super unit cell
transfer from the TiO2 layer, producing a space charge density of
approximately 0.5 e–/nm3 to create a CuCl-based catalyst with a
built-in electric field (noted as CuCl_BEF). Our theoretical
prediction was consistent with earlier ultraviolet photoelectrons
spectroscopy and high-resolution electron energy-loss
spectroscopy of the CuCl on rutile TiO2(110) in ultra-high-
vacuum,[10] where Cu–O bond is formed and the work functions
difference between these two materials, ∆Φ, was approximately
0.8 eV. On the condition of the built-in electric potential ∆U (=∆
Φ/e, e is the electron charge) was mainly confined to the nearest
stacking CuCl and TiO2 layers with a thickness of d =1 nm, the
built-in electric field strength was roughly estimated to be 8×108 V
m–1 using E=∆U/d, very strong compared to normal p-n junctions
with space charge layers generally of several nanometer in
thickness.
Compared to a pure CuCl catalyst without this built-in electric
field, CuCl_BEF attracts higher concentration of K+ and NO3– ions
at the side of interface from the electrolyte, as demonstrated by
molecular dynamics simulation (Figures 1c, d). We placed
CuCl_BEF and CuCl in NO3– electrolyte (100 mg L–1). After 10 ns,
the maximum concentration of NO3– in the diffusion layer as 1–2
nm away from the side of CuCl_BEF reaches 12.3 ions/nm3,
which is higher than that of on pure CuCl surface (6.3/nm3 Figure
1e).
2
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RESEARCH ARTICLE
Figure 2. a) and b) DFT calculations of reaction pathways for NO3– reduction to NH3 on the surface of CuCl_BEF and pure CuCl. The
key intermediates are marked in pink shadow. c) Schematic illustration of in-situ differential electrochemical mass spectrometry
measurement. d) The gaseous intermediate/products are collected and analyzed by mass spectroscopy.
To understand such the accumulation effect of the built-in electric
field, we further simulate the adsorption using finite element
analysis. The CuCl_BEF was modeled by two stacking layers of
same thicknesses as in the DFT calculation, and the interaction
between NO3– and heterojunction was described by electrostatics
equations solved in a two-dimensional box. After just 0.1 ns, the
with diffusion caused by the concentration gradient to drive mass
transfer, significantly improving the reaction rate.
The built-in electric field also significantly modulates the
reaction kinetics by changing the formation barriers of the key
intermediates by tuning the oxidation state of Cu(I), as revealed
by the DFT calculations. The electroreduction of nitrate to
ammonia generally involves an eight electron and nine proton
process, as previously reported, which occurs in nine elementary
steps (Figure 2a and Supporting Information Figure S1). Nitrate
ions are first adsorbed on CuCl_BEF with a formation free energy
change of –2.79 eV (using the initial reactant state as the
reference level).
–
built-in electric field had already accumulated 140 mg L–1 of NO3
near the junction, whereas the pure CuCl had no such effect. The
accumulative distance extended to approximately 3 nm away
from the electrocatalyst surface, this is comparable to the typical
thickness of the electric double layer. Under an electrochemical
reaction, this promotional migration effect works synergistically
3
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Figure 3. Morphology and structure of the CuCl_BEF electrocatalyst. a) TEM image of as-synthesized CuCl_BEF. The inset presents
the elemental mapping images of the white dotted box area. b) Representative HRTEM image of CuCl_BEF. The insets show a
representative crystal plane of CuCl (111) and (220). c) XRD pattern of CuCl_BEF. The asterisk represents the characteristic peaks of
MXene. d) Cu K-edge XANES spectra. The inset indicates average Cu oxidation state determination in CuCl_BEF, using the Cu K-
edge energy shift of the reference Cu foil, Cu2O and CuO.
–
After that, *NO3 reacts with one proton-electron pair to produce
*NO2 with a free energy of –4.1 eV. Afterward, NO2 can attack the
second proton-electron pair to produce *ON (–4.36 eV). After the
formation of *ON on the CuCl_BEF surface, the reaction could
occur in two case, the fourth proton-electron pair can attack *NO
either through O side or N side to form *NOH or *ONH species,
which represent two different mechanisms, i.e. *NOH pathway,
and *ONH pathway respectively. From *NO to *NOH, the
calculated free energy change increases sharply (ΔG=0.39 eV),
which is thus energy unfordable in our experimental condition.[7f,
energy of *ON binding configuration on CuCl_BEF than the pure.
As previously calculated for the adsorption of nitric oxide on Cu(I)
in zeolite model catalyst, NO can bonds to Cu(I) either through N–
Cu or O–Cu bond.[12] In both modes, there is an unpaired electron
donation from NO π* orbital to Cus, d and back donation from Cu
dyz to NO π* orbital, leading to an overall electron donation from
NO to Cu(I).[12a] Thus, with the built-in electric field, an electron
richer Cu(I) suppress the electron donation from NO but facilitate
the back donation to NO π*. While the donation attenuates the
binding energy and rises the free energy, the back donation
weakens the N=O bond and promotes further hydrogenation. This
considerably explains why CuCl_BEF has a much lower formation
barrier for Intermediates 1. Similarly, for Intermediates 2, pure
CuCl has a formation barrier of ΔG = 0.17 eV, while CuCl_BEF
has not barrier (–1.97 eV Figure 2a). Conversely, since the
release barrier of NO2– is rather high (ΔG =2.64 eV) and there is
an extra ion accumulation effect from the built-in electric filed, the
NO2– will be more difficult to desorb from CuCl_BEF than from
11]
In contrast, the formation of *ONH species is more favorable
with the free energy increases of only 0.18 eV. Thereafter, *ONH2
and *ONH3 species might be spontaneously formed on the
surface of catalyst with negative ΔG of –0.48 eV and –0.03 eV,
respectively. The *OH intermediate can be formed by releasing
one ammonia to the electrolyte, and the free energy change for
this step increases slightly. The eighth electrochemical step is the
formation of *H2O species on the CuCl_BEF surface with a
negligible free energy change (ΔG=0.07 eV). In short, the highest
free energy change is from *NO*ONH/*ONH (Intermediate 1),
and *ONH3/*NH2OH release NH3 or H2O (Intermediate 2),
consequently, these steps are considered as rate-determining
steps of the entire reduction pathway.
We compared the reaction barriers on CuCl_BEF with pure
CuCl and pure TiO2 without a built-in electric field. The ΔG for
Step 1 on CuCl_BEF is 0.18 eV, which is significantly reduced
compared to theses on pure CuCl (ΔG =0.42 eV, Supporting
Information Figure S2) and pure TiO2 (ΔG =0.78 eV, Supporting
Information Figure S3). This is due to the relatively higher free
–
pure CuCl. Because of the relatively fast hydrogenation of NO2 ,
–
–
there is less chance to release NO2 , explaining the ultralow NO2
yield. Subtly, the energy barrier of H2 formation on CuCl_BEF is
0.73 eV. This high barrier inhibits the hydrogen evolution reaction
(HER), and thus, ammonia production over CuCl_BEF is
efficiently promoted (Supporting Information Figure S4).[13]
To prove the above mechanism, in-situ electrochemical online
differential electrochemical mass spectrometry (DEMS)[14] was
used to sample the reaction intermediates (Figure 2c). In DEMS
experiments, the electrocatalyst slurry was pasted on the PTFE
membrane and conductively connected through Au foil to
4
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10.1002/anie.202109785
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RESEARCH ARTICLE
electrochemical work station. The PTFE membrane is porous and
its back side is exposed to the mass spectroscopy, allowing
penetration of different gaseous products with varied mass-to-
charge (m/z) ratio to mass spectrograph. When four voltage
pulses from –0.1 to –1.4 V were applied in series to the
electrocatalyst, the m/z signals of 46, 30, 33, 17 that correspond
to NO2, NO, ONH3, and NH3, respectively, were detected (Figure
2d). Particularly, the NO2 signal was considerably weak,
confirming its low production rate.
the MXene sheets, and they possess similar morphologies and
dimensions (Supporting Information Figures S10, 11). When the
Cu-to-Ti molar ratio is 10%, the highest density of nanoshuttles is
obtained, as confirmed by both TEM and XRD (Figure 3c and
Supporting Information Figure S12). In contrast, a higher ratio of
Cu-to-Ti exhaust mostly Ti (0) and collapse the MXene layers,
whereas a lower Cu:Ti ratio resulted in the relatively low density
of the nanoshuttles with relatively poor crystallinity. While the
dimension of the CuCl nanodots is as thin as one monolayer in
CuCl_BEF, the built-in electric field in CuCl_BEF is strong enough
to show its dual roles. This is further proved by that the
electrochemical active surface area (ECSA) of CuCl_BEF is up to
900 cm–2, which is considerably larger than that of pure MXene
(195 cm–2) and other reported electrocatalysts (Supporting
Information Figure S13). Therefore, CuCl_BEF catalysts render
an ideal model to survey the CuCl catalyst with built-in electric
field effect in nitrate electroreduction.
To experimentally prepare the CuCl_BEF, we employed
Ti3C2Tx MXene as both support and Ti source because of its large
surface area, rich surface group and reducibility.[15] Ti3C2Tx
MXene aqueous solution was prepared by etching Ti3AlC2 power
using LiF in concentrated HCl and it comprised Ti3C2 monolayers
terminated by OH and Cl groups.[16] The MXene surfaces were
negatively charged (Supporting Information Table S2) after
release of H+ groups, and thus, positive Cu2+ ions were adsorbed,
reduced and precipitated. A dark precipitate was produced from
Ti3C2Tx MXene dispersion after adding CuSO4 solution dropwise.
Transmission electron microscopy (TEM) and high-angle annular
dark field image (HAADF) scanning TEM imaging (Insert in Figure
3a) show that the nanoshuttles comprised the uniform distribution
of Ti, Cu, O, and Cl on the MXene layers (Figure 3a and
Supporting Information Figure S5). These nanoshuttles are
approximately 50 nm in length, 10 nm in width judging from TEM
and 13.3 nm in height measured by atomic force microscopy
(AFM) (Supporting Information Figure S6). The high-resolution
TEM (HRTEM) images reveal that these nanoshuttles are mainly
composed by rutile TiO2 as its characteristics (110) and (101)
diffraction fringes, dominating on the entire nanoshuttles.
Additionally, there are a few (1–5) of CuCl nanodots in each
nanoshuttle, showing lattice fringes of CuCl (111) and CuCl (220)
embedded on surfaces. The CuCl nanodots are in the dimension
of 1–2 nm, approximately one layer thick considering its most
exposed (111) plane from X-ray diffraction (XRD), and CuCl_BEF
is formed on MXene sheets (Figure 3b). XRD patterns detected
(111), (220) and (311) planes of CuCl (PDF no. 06–0344), as well
as the (110), (101) and (211) planes of rutile TiO2 (PDF no. 21–
1276, Figure 3c).
Inspired by the enhanced mass transfer and reaction kinetics
using the built-in electric field, we quantified the NO3RR activity of
CuCl_BEF. Linear sweep voltammetry (LSV) demonstrated that
starting from –0.4 V (vs. Reversible Hydrogen Electrode, RHE,
hereafter), the CuCl_BEF catalyst displays significantly larger
–
–
current density in 100 mg L–1 of NO3 solution than in NO3 free
solution (Figure 4a). To verify the origin of ammonia and eliminate
the possible interference from electrocatalyst or external
environment, the 1H NMR spectra of 15N–NO3 and 14N–NO3
–
–
isotopic labeling were analyzed. The 1H NMR spectra of
commercial (14NH4)2SO4 and (15NH4)2SO4 samples were collected
as standard spectra.[19] The 1H NMR spectra of 15N–NH4+ showed
double peaks at δ = 7.11 and 6.92 ppm, while 14N–NH4+ showed
triple peaks at δ = 7.15, 7.02 and 6.89 ppm (Figure 4b), confirming
that the formation of ammonia originated from the
electroreduction of nitrate.[20] The concentration of ammonium,
1
determined by the peak area of HNMR (Supporting Information
Figures S14, 15), was also consistent with the result using
Nessler’s reagent method (Supporting Information Figures S16–
18 and Table S3). Altogether, the results demonstrate the
excellent performance and reliability of electrocatalytic reduction
of nitrate to ammonia.
To explore the valence of Cu in CuCl_BEF, X-ray absorption
near edge structure (XANES) and X-ray photoelectron spectrum
(XPS) were conducted (Figure 3d). The oxidation state of Cu in
CuCl_BEF shifts to +0.68 by Cu K-edge EXAFS spectra, using
Cu foil, Cu2O and CuO as the references (Figure 3d),[17] further
confirming the electron transferring from TiO2 to CuCl and the
successful creation of the built-in electric field. The Cu 2p peak in
XPS is closely corresponded with CuCl (Supporting Information
Figure S7). The Cl 2p peak displays two chemical environments,
corresponding to formation of Cl–Cu and Cl–Ti bonds (Supporting
Information Figure S8). Because of OH– species exist on MXene
surface, the major Cu (I) is CuCl rather than Cu or Cu2O as
verified by the Cu LMM Auger spectrum (Supporting Information
Figure S9).[7e, 18]
While the Cl– source for CuCl_BEF is solely from MXene
surface, the formation of CuCl_BEF relies on Cl– diffusion on the
MXene surface and this two-dimensional confinement determine
the limited size and distribution of CuCl nanoshuttles. When the
Cu-to-Ti molar ratio in the reactants increases from 1%, 5%, 10%,
20% to 50%, the obtained nanoshuttles are evenly distributed on
By varying the applied potential, the nitrate conversion and
ammonia production rate reach maximum at –1.0 V (Supporting
Information Figure S19), which is in consequence chosen as the
operation voltage. We further test the performances of CuCl_BEF
electrocatalysts with Cu-to-Ti molar ratios varying from 1%, 5%,
10%, 20% to 50%. The CuCl_BEF with Cu/Ti=10% shows the
maximum NH3 yield rate at 2.15 mg h–1 cm–2 and NH3 selectivity
up to 95.6% within 2 hours, thus, the ratio is chosen (Supporting
Information Figure S20). By calculating specific activity based on
the unit copper mass (Supporting Information Figure S21), the
NH3 yield rate of CuCl_BEF electrocatalyst is 64.4 h–1 (1.82 mg h-
1
cm-2). To the best of our knowledge, this is the highest among
–
all reported NO3RR activities using NO3 (<100 mg L–1)
(Supporting Information Table S1). NO3RR follow a first-order
reaction kinetics towards 100 mg L–1, reflect that the CuCl_BEF
has sufficient activity and diffusion is rate-limiting factor. The
nitrate conversion is continuously improved to 92.0% and NH3
selectivity up to 98.6% by prolonging of reaction time after 6 h
(Figure 4c, Figure S22).
5
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Figure 4. NO3RR activity of the CuCl_BEF electrocatalyst. a) Linear sweep voltammetry curves of CuCl_BEF tested in 0.5 M Na2SO4
–
with and without 100 mg L–1 NO3 . The "j" is current density (j/A m-2) which is the amount of charge per unit time that flows through a
unit area of electrode. b) 1H NMR spectra of the electrolyte after electrocatalytic NO3RR using 15NO3– and 14NO3– as the nitrogen source.
–
c) Time dependent concentration change of NO3 , NO2– and NH3 tested at –1.0 V. d) Seven consecutive recycling tests at –1.0 V. e)
Selectivity of ammonia with different initial NO3– concentrations. f) Average NH3 yield rate within 6 h of using different electrocatalysts.
In addition, The nitrate conversion and ammonia production with
20 mg L-1 and 50 mg L-1 initial nitrate are continuously improved
by prolonging of reaction time after 6 h (Supporting Information
Figure S23).The steady concentrations of nitrate and nitrite then
are below 8 and 0.6 mg L–1 respectively, and both are lower than
the concentration stated in the Safe Drinking Water Act (SDWA)
regulations (10 and 1 mg L–1) proposed by United States
Environmental Protection Agency.[2] The NO3RR performance of
the CuCl_BEF electrocatalyst at different pH and temperature
were compared. The results showed that the NO3RR was
favorable under pH>7, but unfavorable under pH=3. This
attributed to the dominant hydrogen evolution reaction (HER)
under acidic atmosphere, which suppressed NO3RR. On the other
hand, the temperature was favorable for NO3RR at 25℃. The
activity of CuCl_BEF electrocatalyst reduce at lower temperatures
(0 ℃) and the ammonia is easy to escape at higher temperatures
(35 ℃). (Supporting Information Figures S24, 25).). Additionally,
the CuCl_BEF catalyst exhibits good stability, sustaining an
ammonia selectivity of >90% and nitrite selectivity of <1% after
seven cycles of running (Figure 4d) with intact nanoshuttles
CuCl + MXene (20.1%, 38.8%, 79.3%), TiO2 + MXene (60.2%,
54.8%, 73.7%) and MXene (20.5%, 27.3%, 81%, Supporting
Information Figure S27). The superiorities of CuCl_BEF over
others in NO3RR proves that the effective promotion of the
NO3RR performance is due to the built-in electric field rather than
from TiO2 or MXene.
Conclusion
In conclusion, we demonstrate that a built-in electric field in
electrocatalyst can significantly accumulate higher concentration
of reagent ions near the electrocatalyst surface region to improve
mass transfer for the efficient removal of nitrate at ultra-low
concentration and conversion to ammonia. A strong built-in
electric field in the CuCl_BEF electrocatalyst, induced by
electrons transfer from TiO2 to CuCl, effectively triggers interfacial
accumulation of NO3– ions around the electrocatalyst, as revealed
by the molecular dynamic simulation and finite element analysis.
The electric field also rises the energy of the key reaction
intermediate *NO to reduce energy barrier of the rate-determining
step. Taking advantage of the built-in electric field, nitrate at low
concentration was efficiently converted with a high ammonia
production selectivity (98.6%), low nitrite (0.6 mg L–1) and a
copper mass-specific ammonia yield rate up to 64.4 h–1, which are
all the highest among all reported results at 100 mg L–1 to date.
Our results will inspire the use of a built-in electric field as an
efficient strategy for the further development of NO3RR
electrocatalysts, and it may also be effective for other
electrocatalytic reactions.
–
morphologies (Supporting Information Figure S26). When NO3 –
N concentration varies from 20 to 200 mg L–1, the selectivity of
ammonia remains at high-levels (>80%), and it approximately
100% at 100 and 200 mg L–1 with negligible nitrite production,
displaying a wide concentration tolerance (Figure 4e).
We compare the catalytic performance of CuCl_BEF to that of
a simple blend of CuCl and MXene (denoted by CuCl + MXene),
a mixture of TiO2 nanoparticles and MXene (TiO2 + MXene), and
pure MXene under identical test condition. The NH3 yield rate of
CuCl_BEF (1.82 mg h–1 cm–2) is significantly higher than that of
the latter three (0.85, 0.57 and 0.34 mg h–1 cm–2, respectively,
Figure 4f). Additionally, the Faradaic efficiency, NO3– conversion
rate and ammonia selectivity of CuCl_BEF are 44.7%, 92.0%,
98.6%, respectively, which are significantly higher than those of
6
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[12] a) P. Kozyra, M. Radon, J. Datka, E. Broclawik, Struct. Chem. 2012, 23,
1349-1356; b) W. F. Schneider, K. C. Hass, R. Ramprasad, J. B. Adams,
J. Phys. Chem. B 1998, 102, 3692-3705.
Acknowledgements
We gratefully acknowledge the financial support provided by the
National Key R&D Program of China (2020YFC1818401,
2017YFC0210906), National Natural Science Foundation of
China (21978185, 21938006, 21776190), Basic Research Project
of Leading Technology in Jiangsu Province (BK20202012),
Suzhou Science and Technology Bureau Project (SYG201935)
and the project supported by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
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Keywords: Built-in electric field • Electroreduction of nitrate to
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10.1002/anie.202109785
Angewandte Chemie International Edition
RESEARCH ARTICLE
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A model electrocatalyst are created by stacking CuCl (111) and rutile TiO2 (110) layers together, in which a built-in electric field
induced from the electron transfer from TiO2 to CuCl (CuCl_BEF) is successfully formed as demonstrated by density functional theory
(DFT) calculations and X-ray absorption near edge structure (XANES). Molecular dynamics simulation and finite element analysis
suggest that this built-in electric field effectively triggers interfacial accumulation of NO3– ions around the electrocatalyst. The electric
field also rises the energy of key reaction intermediate *NO to lower down energy barrier of the rate determining step. A NH3 product
selectivity of 98.6%, a low NO2– production of <0.6%, and mass-specific ammonia production rate of 64.4 h–1 is achieved with this
proof-of-concept system, which are all the best among studies reported at 100 mg L–1 of nitrate concentration to date.
8
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