Ammonia Synthesis Under Ambient Conditions: Selective Electroreduction of Dinitrogen to Ammonia on Black Phosphorus Nanosheets

Article abstract of DOI:10.1002/anie.201813174

Constructing efficient catalysts for the N₂ reduction reaction (NRR) is a major challenge for artificial nitrogen fixation under ambient conditions. Herein, inspired by the principle of “like dissolves like”, it is demonstrated that a member of the nitrogen family, well-exfoliated few-layer black phosphorus nanosheets (FL-BP NSs), can be used as an efficient nonmetallic catalyst for electrochemical nitrogen reduction. The catalyst can achieve a high ammonia yield of 31.37 μg h^?1 mg^?1_cat. under ambient conditions. Density functional theory calculations reveal that the active orbital and electrons of zigzag and diff-zigzag type edges of FL-BP NSs enable selective electrocatalysis of N₂ to NH₃ via an alternating hydrogenation pathway. This work proves the feasibility of using a nonmetallic simple substance as a nitrogen-fixing catalyst and thus opening a new avenue towards the development of more efficient metal-free catalysts.

Full text of DOI:10.1002/anie.201813174

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Accepted Article
Title: Ammonia Synthesis Under Ambient Conditions: Selective
Electroreduction of Dinitrogen to Ammonia on Black Phosphorus
Nanosheets
Authors: Lili Zhang, Liang-Xin Ding, Gao-Feng Chen, Xianfeng Yang,
and Haihui Wang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201813174
Angew. Chem. 10.1002/ange.201813174
Link to VoR: http://dx.doi.org/10.1002/anie.201813174
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10.1002/anie.201813174
COMMUNICATION
Ammonia Synthesis Under Ambient Conditions: Selective
Electroreduction of Dinitrogen to Ammonia on Black Phosphorus
Nanosheets
[
a]
[a]
[a]
[b]
[a]
Lili Zhang, Liang-Xin Ding,* Gao-Feng Chen, Xianfeng Yang, and Haihui Wang*
Abstract: Constructing efficient catalysts for the
N
2
reduction
the π* orbital of N≡N, strengthening
weakening the N≡N bond. It is worthwhile to note that although
N
2
adsorption and
[
7]
reaction (NRR) is a major challenge for artificial nitrogen fixation
under ambient conditions. Herein, inspired by the principle of “like
an unoccupied nonbonding orbital and electron donor site with
dissolves like”, we demonstrate that, a member of the nitrogen family, abundant electron cloud density are prerequisites for a NRR
well-exfoliated few-layer black phosphorus nanosheets (FL-BP NSs)
can be used as an efficient nonmetallic catalyst for electrochemical
nitrogen reduction. The catalyst can achieve a high ammonia yield of
catalyst, the d orbital electrons in transition metals also benefit
the formation of metal-H bonds, which will exacerbate the
competitive hydrogen evolution reaction and limit the nitrogen
-
1
-1
[24]
31.37 μg h mg cat. under ambient conditions. Density functional
reduction selectivity and catalytic efficiency.
theory calculations reveal that the active orbital and electrons of
zigzag and diff-zigzag type edges of FL-BP NSs enable selective
Compared to transition metals, the weak hydrogen adsorption
of nonmetallic elements and their abundant valence electrons
should provide a more ideal nitrogen activation center.
[
25,26]
electrocatalysis of N
2
to NH
3
via an alternating hydrogenation
pathway. This work proves the feasibility of using a nonmetallic
simple substance as a nitrogen-fixing catalyst and thus opening a
new avenue towards the development of more efficient metal-free
catalysts.
Some recent studies have shown that the use of nonmetallic
components as active centers may be an effective way to obtain
[
27-31]
high selectivity.
production rate and faradaic efficiency by using boron-doped
graphene as metal-free NRR catalyst under ambient
Similar nitrogen reduction activity was also
For example, Yu et al. reported a high NH
3
a
[
29]
conditions.
Ammonia (NH
industry and agriculture.
3
) is a basic raw chemical material used in modern
[30]
observed on metal-free boron carbide nanosheets.
Nevertheless, restricted by the number of effective active sites
or doping concentration or poor conductivity, many reported
nonmetallic electrocatalysts have not shown their deserved
activity for nitrogen activation. In addition, all of the recent
studies on metal-free catalysts for the NRR have mainly focused
on nonmetallic compounds or complexes, which create complex
catalyst active centers and are difficult to purposefully optimize.
Therefore, the development of nonmetallic electrocatalysts with
high-concentration active centers and more efficient strategies to
utilize their intrinsic catalytic activity are highly desired.
[1]
At present, the energy-intensive
Haber-Bosch process is the main artificial synthesis route for
ammonia, and this process uses more than 1 % of global annual
[2,3]
energy consumption and produces carbon dioxide emissions.
In contrast, electrochemical reduction of nitrogen into ammonia
under ambient conditions is a potential strategy for sustainable
[
4-6]
ammonia production.
However, due to the strong dipole
moment of the N≡N triple bond and the vigorous hydrogen
[
7-11]
evolution reaction (HER) competing,
the development of
highly effective catalysts with sufficient activity and selectivity is
essential.
Here, for the first time, we focus our attention on a nonmetallic
simple substance and demonstrate that orthorhombic black
phosphorus in the form of few-layer nanosheets is a superior
metal-free single element electrocatalyst for nitrogen reduction
under ambient conditions. More specifically, in addition to the
characteristics of single elements, black phosphorus is attractive
as a catalyst for nitrogen reduction because its valence electron
According to available experimental and theoretical NRR data,
an active center that can easily adsorb nitrogen molecules and
[
12-15]
sufficiently activate the N≡N triple bond is very desirable.
To
this end, the search for an electrocatalytic NRR center in recent
years has mainly focused on transition-metal-based
[8,14-23]
materials.
occupied d orbitals of transition metals, the electron density from
is synergically accepted with appropriate energy and
symmetry, and then, the transition metal donates electrons to
Specifically, benefiting from the unoccupied and
2
3
2
3
structure is similar to that of nitrogen (3s 3p and 2s 2p ,
N
2
[32]
respectively) and it has an anisotropic lattice structure.
In
particular, black phosphorus is a type of layered crystal (Figure
1
a,b, Figure S1) with monolayers stacked together via weak,
[33,34]
interlayer van der Waals interactions;
these interactions
[
[
a]
b]
L. Zhang, Prof. Dr. L.-X. Ding, G.-F. Chen and Prof. Dr. H. Wang
School of Chemistry and Chemical Engineering, South China
University of Technology
Guangzhou 510640, China
E-mail: lxding@scut.edu.cn
hhwang@scut.edu.cn
Dr. X. Yang
Analytical and Testing Center, South China University of
Technology
allow black phosphorus to be exfoliated into monolayer
phosphorene or few-layer black phosphorus, releasing the
[
35-40]
maximum number of intrinsic active sites.
inherent features should facilitate chemisorption of N
Notably, such
molecules
2
and provide sufficient electrons for activation of the inert N≡N
triple bond. As a proof of concept, few-layer black phosphorus
nanosheets have a faradaic efficiency and NH
3
production rate
Guangzhou 510640, China
for the NRR in an acidic aqueous solution (0.01 M HCl) as high
as 5.07 % and 31.37 μg h mg cat., respectively.
-1
-1
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
Figure 1. (a) Schematic of black phosphorus (BP). (b) SEM image of bulk BP. (c) TEM images, (d) HR-TEM image and SAED pattern, (e) Raman spectra, (f)
Height-mode AFM image and (g) XPS spectra of FL-BP NSs.
1
/2
Few-layer black phosphorus nanosheets (FL-BP NSs) were
obtained from bulk black phosphorus (BP) by a facile liquid
exfoliation method (experiment details are given in the section of
supplementary methods in Supporting Information). The colossal
difference between the photographs of BP slurry and FL-BP NSs
suspension (Figure S2) foreshadows a successful exfoliation of
BP. The scanning electron microscopy (SEM) images in Figure
S3 confirm the sheet structure of the exfoliated FL-BP NSs. A
representative transmission electron microscopy (TEM) image of
a FL-BP NS (Figure 1c) and the higher magnification image in
the inset of Figure 1c further reveal the few-layer property of the
exfoliated BP. Additionally, clear lattice fringes in the high-
resolution TEM (HR-TEM) image along with the corresponding
selective area electron diffraction (SAED) patterns (Figure 1d)
indicate that the FL-BP NSs inherit the intact layer structure and
crystal merits of BP. The X-ray diffraction (XRD) pattern of FL-
(130.5 eV) and P 2p (131.3 eV) doublet, which represents the
characteristic binding energies of crystalline BP. The weak
oxidized phosphorus (PO
with previous reports on electronic-grade BP.
characterization fully confirmed the successful preparation of FL-
BP NSs.
The NRR electrocatalytic activity of FL-BP NSs (supported on
carbon fiber, and denoted as FL-BP NSs/CF) was performed in
x
) peak at 134.5 eV is in agreement
[
35]
The above
2
a high purity N -saturated 0.01 M HCl solution using a gas-tight
two-compartment cell (Figure S6). And the potential ammonia
product was detected and quantified by using indophenol blue
method and 1H nuclear magnetic resonance (1H NMR). Figure
S7 shows the chronoamperometry curves of electrolysis for 2 h
at different applied potentials (from -0.4 to -0.8 V) under ambient
conditions. The corresponding UV-vis absorption spectra of the
product solutions stained with indophenol indicator are shown in
Figure 2a. According to the standard curve at λ=660 nm (Figure
S8), the maximum faradaic efficiency of 5.07 % for ammonia
synthesis on the FL-BP NSs/CF catalyst electrode was achieved
o
BP NSs in Figure S4 shows two typical diffraction peaks at 16.9
o
and 34.2 , which are consistent with diffraction of the (002) and
(
004) crystal planes, respectively, confirming that the FL-BP NSs
[41]
produced by liquid exfoliation retain the initial crystalline state.
3
at -0.6 V (Figure 2b) and the highest NH yield rate was 31.37
-1
-1
The comparative Raman spectra of FL-BP NSs and BP
precipitants (without successful exfoliation) are also provided
and shown in Figure 1e. Based on previous reports, the three
μg h mg cat. at -0.7 V (Figure 2c), which is significantly higher
than that on bulk BP and powdery BP (grated bulk BP) catalyst
electrodes (Figure S9,10), as well as most nonmetallic and
metal-based catalysts reported at present (Table S1).
Furthermore, no apparent byproduct (N
product solutions (Figure S11,12), which indicates that the FL-
BP NSs/CF has a good selectivity for N electroreduction into
NH . Based on the nitrogen adsorption-desorption isotherms
-
1
-1
-1
Raman peaks at 361 cm , 441 cm and 468 cm from the BP
1
2
precipitants correspond to A
g
, B2g and A
g
modes, respectively.
Raman peaks of the FL-
2 4
H ) was detected in the
1
2
And, a blueshift in the A
g
, B2g and A
g
BP NSs is clear compared to those of the BP precipitants,
2
[34]
suggesting a reduction in the number of layers.
Moreover,
3
according to the atomic force microscopy (AFM) analysis in
Figure 1f, the exfoliated BP NSs have a thickness of ~4.1 nm (5-
(Figure S13a) and TPD spectra (Figure S13b) of bulk BP and
exfoliated sample, we speculate that the superior nitrogen
reduction properties of the FL-BP NSs are mainly due to the rich
nitrogen-adsorption active sites brought about by the lamellar
structure formed by the full exfoliation of the bulk BP.
7
layers), which is consistent with that of the FL-BP NSs (Figure
S5). The further chemical composition analysis of the FL-BP
NSs was performed using X-ray photoelectron spectroscopy
3
/2
(
XPS). As shown in Figure 1g, the FL-BP NSs exhibit a P 2p
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Figure 2. NRR behavior of FL-BP NSs/CF. (a) Corresponding UV-vis absorption spectra of the electrolyte (electrolysis at different potentials) stained with
14
+
15
+
indophenol indicator. (b) Faradaic efficiency and (c) NH
3
yield rate at various potentials. (d) 1H NMR spectra (600 MHz) of both NH
as the N source. (e) Corresponding UV-vis absorption spectra of the electrolyte (electrolysis on recycled
catalyst) stained with indophenol indicator. (f) Cycling stability results at -0.6 V (vs. RHE).
4 4
and NH produced from
14
15
the NRR reaction (at -0.6 V vs. RHE) using
N
2
or
N
2
2
To verify the source of ammonia, some necessary
comparative experiments were carried out at the optimal
potential of -0.6 V, including using Ar instead of nitrogen as the
feed gas, CF without FL-BP NSs as the working electrode, and
verify this result, the linear dependence between NH
3
production
and reaction time was also quantitatively determined by 1H
15
+
NMR spectra of NH
4
when the electrolysis was performed for
as feed gas (Figure S22d, Table S3). Here the
good linearity also means that NH can be produced sustainably
1
5
6 h using
N
2
1
5
+
N
2
as the feed gas. Specifically, no apparent NH
4
was
3
detected in the electrolyte after two hours of electrolysis when N
was replaced by Ar, and when a carbon fiber without the FL-BP
NSs was used as a catalyst electrode, respectively (Figure S14).
2
by consuming the dissolved nitrogen. Moreover, considered
about the inherent property of this catalyst, that is, the FL-BP
NSs may be not stable in the presence of water and oxygen
molecules (although the above experiments were performed
under oxygen-free condition), we also characterize the spent
catalyst to determine whether the structure and composition
have changed during NRR. The typical sheet structure
characteristic in AFM image shows that the FL-BP NSs have
good structural stability (Figure S23). However, the slightly
blurred lattice fringe in the TEM image of the spent catalyst
1
5
+
By contrast, typical NH
4
signal was detected in the presence
bubbling (based on the 1H NMR spectra,
Figure 2d). Moreover, the corresponding 1H NMR spectra had
15
of continuous
N
2
1
5
+
only a double peak of NH
of NH was completely derived from the electrochemical
reduction of N in the presence of FL-BP NSs catalyst (Figure
4
, which confirms that the formation
3
2
S15-17). Considering that the peak area of nuclear magnetic
resonance is directly related to the content of ammonia, the
1
2
(Figure S24), and the blueshift of the A
peaks (Figure S25a), as well as the enhanced oxidation peak
(PO ) intensity located at the binding energy of 134.5 eV (Figure
g g
, B2g and A Raman
1
5
+
concentration of NH
4
was further quantitatively determined by
1H NMR with external standards (Figure S18,19). As expected,
x
the calculated peak area and corresponding ammonia yield were
very close to the quantitative results from the indophenol blue
S25b), indicating that the surface of the FL-BP NSs does suffer
partial oxidation decay during the electrolysis process.
14
+
method (Table S2) and 1H NMR spectra of NH
4
when the
Accordingly, to explore the effect of the PO
ammonia synthesis, control experiment using Oxidized-FL-BP
NSs/CF as the working electrode in N -saturated solution was
x
on electrochemical
1
4
electrolysis was performed using
2
N as the feed gas (Figure
S20). This result further confirmed the high efficiency catalytic
activity of FL-BP NSs for NRR.
The durability of the FL-BP NSs catalyst for NRR was first
assessed by 5 consecutive electrolysis cycles. As shown in
Figure 2e,f and Figure S21, the faradaic efficiency and yield rate
2
conducted. As shown in Figure S26, the electrochemical
performance of ammonia synthesis on the Oxidized-FL-BP
NSs/CF is significantly lower than that on the FL-BP NSs/CF,
x
indicating that the increased PO in the FL-BP NSs catalyst does
for NH
chronoamperometric runs (about 90% performance retention at -
.6 V), which is indicative of a good stability of the FL-BP NSs in
NRR electrocatalysis during the continuous electrolytic reaction.
Time-dependent electrolytic test further confirmed the good
stability of the FL-BP NSs for NRR. From Figure S22, it can be
3
production only decrease slightly after five cycles of
not benefit for ammonia synthesis from electrochemical nitrogen
reduction (Table S4). The conclusion was also confirmed by
theoretical calculation (Figure S27). Thus, further improvements
in preventing BP degradation in the electrolyte will be beneficial
for the long-term durability of the electrocatalyst for NRR.
0
clearly seen that the time-dependent yield of NH
3
during nitrogen
reduction increases linearly with increased electrolysis time. To
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Angewandte Chemie International Edition
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COMMUNICATION
Figure 4. DFT-calculated reaction pathways and the corresponding energy
changes for the NRR on zigzag-edge (A) and diff-zigzag-edge (B) active sites
of FL-BP NSs/CF.
In summary, we have demonstrated that BP with a few-layer
nanosheet structure is a superior nonmetallic electrocatalyst for
nitrogen reduction under ambient conditions. Isotopic tracer
experiments confirmed that the synthetic ammonia was
completely derived from the supplied nitrogen. DFT calculations
support that the zigzag and diff-zigzag edges of the FL-BP NSs
are active centers for nitrogen adsorption and N≡N triple bond
activation, which enable electroreduction of N2 to NH3 via an
alternating hydrogenation pathway. This work not only proves
the feasibility of using a nonmetallic simple substance as a
nitrogen-fixing catalyst, but also provides a model for more
accurate studies on the reaction mechanism of nitrogen
reduction on nonmetallic materials. Overall, we expect this work
to play a role in the development of more efficient metal-free
nitrogen-fixing catalysts.
Figure 3. (a) Structure of ideal FL-BP NSs (top view). The green balls
represent five active sites (surface, zigzag, armchair-1, diff-zigzag and
armchair-2 edge). (b) LUMO (yellow) and HOMO (blue) of FL-BP NSs. (c)
Schematic diagram of the possible reaction mechanisms (solid line: low-
energy route, dotted line: unfavorable route) for electrochemical reduction of
N
2
to NH
3 2
at the zigzag edge (zz, top view) of FL-BP NSs under N
atmosphere.
In addition, to identify the intrinsic nitrogen adsorption active
sites and understand the possibility mechanism of nitrogen
reduction, we performed density functional theory (DFT)
calculations for the molecular orbitals of FL-BP NSs (Figure
3a,b), which due to the electron distribution in the molecular
orbital of the catalyst is the key factor to determine the active
[
7,13]
Acknowledgements
sites for nitrogen fixation.
As shown in Figure 3b, the lowest
unoccupied molecular orbital (LUMO, yellow) and the highest
occupied molecular orbital (HOMO, blue) present a nonlocalized,
asymmetric electron distribution. The electron densities of the
HOMO and LUMO are obviously concentrated on the zigzag and
diff-zigzag edges (Figure S28), and at the edges, the active
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (21776099), the Pearl
River and S&T Nova Program of Guangzhou (201610010076),
National Key R&D Program (2016YFA0202601) and
Fundamental Research Funds for the Central Universities.
2
orbitals are beneficial for adsorbing N and boosting the NRR
performance. For the zigzag and diff-zigzag edges (denoted as
zz and diff-zz, respectively), an alternating hydrogenation
pathway in an association mechanism is the plausible low-
energy NRR pathway (Figure 3c, Figure S29), which can be
described as follows (using zigzag active sites as an example):
Keywords: ammonia synthesis • nitrogen reduction reaction •
electrocatalysis • metal-free • black phosphorus
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→zz*N
N+NH
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to *NH and NH
eV) (Figure 4), the desorption of *H
energetically unfavorable (0.56 eV and 1.61 eV, respectively).
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2
→ꢁ zz*NNH →ꢁ zz*HNNH →ꢁ zz*HNNH
ꢀ
2
→ꢁ zz*H
2
NNH
2
→ꢁ zz*H
2
3 3
(l)ꢂ →ꢁ zz+NH (l)
2
NNH
2
[
3]
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2
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201813174
COMMUNICATION
COMMUNICATION
[
a]
[a]
Well-exfoliated few-layer black
phosphorus nanosheets (FL-BP NSs)
are developed as an efficient
Lili Zhang, Liang-Xin Ding,* Gao-
[
a]
[b]
Feng Chen, Xianfeng Yang, and
Haihui Wang*
[
a]
nonmetallic catalyst for electro-
chemical nitrogen reduction. The
catalyst can achieve a high ammonia
yield of 31.37 μg h mg cat. under
ambient conditions. DFT calculations
support that the zigzag and diff-zigzag
edges of the FL-BP NSs are the active
centers, which enable selective
Page No. – Page No.
Ammonia Synthesis Under Ambient
Conditions: Selective
Electroreduction of Dinitrogen to
Ammonia on Black Phosphorus
Nanosheets
-1
-1
electroreduction of N
2 3
to NH via an
alternating hydrogenation pathway.
This article is protected by copyright. All rights reserved.