Spinel LiMn2O4 nanofiber: An efficient electrocatalyst for N2 reduction to NH3 under ambient conditions

Article abstract of DOI:10.1021/acs.inorgchem.9b01707

The production of ammonia (NH₃) in the industrial scale basically relies on the traditional technology of Haber-Bosch, which is operated under harsh conditions with high energy consumption and a huge number of greenhouse gas emissions. Electrochemical N₂ reduction reaction (NRR) is a promising route for artificial N₂-to-NH₃ fixation with less energy consumption. However, an effective electrocatalyst, as a prerequisite of the NRR, is of significance. Here, we report that a spinel LiMn₂O₄ nanofiber acts as a noble-metal-free electrocatalyst for NH₃ synthesis with excellent performance under ambient conditions. The electrocatalyst, which was tested in 0.1 M HCl, has an excellent Faradaic efficiency of 7.44% and a NH₃ yield of 15.83 μg h^-1 mg_cat.^-1 at -0.50 V versus reversible hydrogen electrode. Moreover, it also possesses excellent electrochemical and structure stability.

Full text of DOI:10.1021/acs.inorgchem.9b01707

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Spinel LiMn O Nanofiber: An Efficient Electrocatalyst for N
2
4
2
Reduction to NH under Ambient Conditions
3
†
†
‡
‡
‡
‡
§
Chengbo Li, Jiali Yu, Li Yang, Jinxiu Zhao, Wenhan Kong, Ting Wang, Abdullah M. Asiri,
,
†
,‡
Quan Li,* and Xuping Sun*
†
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, China
‡
Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054,
China
§
Chemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University,
P.O. Box 80203, Jeddah 21589, Saudi Arabia
*
S Supporting Information
strongly encourages the development of noble-metal-free NRR
15−38
ABSTRACT: The production of ammonia (NH ) in the
3
electrocatalysts.3
Spinel Mn O has wide applications for
3
4
9,40
41−44
industrial scale basically relies on the traditional
technology of Haber−Bosch, which is operated under
harsh conditions with high energy consumption and a
huge number of greenhouse gas emissions. Electro-
supercapacitors
and metal-ion batteries,
but recently
it has been verified as an efficient catalyst for electrocatalytic
45,46
N reduction.
2
However, in the aqueous electrolyte, the
Faradaic efficiency (FE) of electrochemical NRR suffers from
the fierce competition of hydrogen evolution reaction
chemical N reduction reaction (NRR) is a promising
2
47
+
route for artificial N -to-NH fixation with less energy
2
3
(HER). A recent study suggests that Li in solution is able
consumption. However, an effective electrocatalyst, as a
prerequisite of the NRR, is of significance. Here, we report
to suppress the HER and thus enhance the current efficiency of
48
+
the NRR. We anticipate that the incorporation of Li into
Mn O could be a feasible way to enhance the NRR ability,
that a spinel LiMn O nanofiber acts as a noble-metal-free
2
4
3
4
electrocatalyst for NH synthesis with excellent perform-
3
which, however, has not been reported before.
ance under ambient conditions. The electrocatalyst, which
was tested in 0.1 M HCl, has an excellent Faradaic
Here, we demonstrate our recent experimental finding that
the spinel LiMn O nanofiber (LiMn O NF) behaves as a
2
4
2
4
−
1
efficiency of 7.44% and a NH yield of 15.83 μg h
3
superior NRR electrocatalyst for N reduction to NH at
2 3
−
1
mgcat. at −0.50 V versus reversible hydrogen electrode.
Moreover, it also possesses excellent electrochemical and
structure stability.
ambient temperature with excellent performance. When tested
in 0.1 M HCl, at −0.50 V versus reversible hydrogen electrode
(
RHE), this electrocatalyst affords an excellent FE of 7.44%, far
4
4
5
6
higher than those for the previous Mn O nanocube (3.0%)
3
4
and Mn O nanoparticles@reduced graphene oxide (3.52%).
3
4
This catalyst offers a NH yield (V ) of 15.83 μg h− mgcat.−1.
1
mmonia (NH ) as an important chemical is widely used
3
3
NH
3
1
,2
A
in medicaments, fertilizers, dyes, explosives, fuel, etc.
However, in the field of artificial N reduction to NH , the
It shows high electrochemical and structure stability.
2
3
The LiMn O NF was prepared by electrospinning followed
2
4
traditional technology of the Haber−Bosch process occupies a
dominant position. It is operated under rigorous conditions
with high energy consumption and a huge number of
by annealing in an air atmosphere. Figure 1a presents the X-ray
diffraction (XRD) pattern of the LiMn O NF. The character-
2
4
istic diffraction peaks at 18.61°, 36.08°, 37.75°, 43.87°, 48.05°,
3
greenhouse gas emissions. Consequently, it is urgent to
58.06°, 63.78°, 67.08°, 75.53°, and 76.55° can be assigned to
explore an energy-efficient and environmentally friendly
the (111), (311), (222), (400), (331), (511), (440), (531),
(533), and (622) planes of the cubic LiMn O structure,
process for sustainable NH synthesis under mild conditions.
3
2
4
In nature, biological N2 fixation, occurring in micro-
respectively (JCPDS 35-0728). Figures S1 and 1b show the
scanning electron microscopy (SEM) images of an as-spun
precursor and the LiMn O NF after annealing treatment,
organisms, is catalyzed by nitrogenases at room temperature
4
,5
and pressure. Similarly, electrochemical N reduction offers
2
2
4
an energy-efficient and environmentally friendly route for
respectively. Compared with the precursor, the LiMn O NF
2 4
artificial NH synthesis under ambient conditions, but this
exhibits a rough surface with a shrunken diameter of 200−300
nm (Figure 1c), which can be due to decomposition of the
polymer backbone and organic moieties in the precursor
during the annealing process. As shown in Figure 1d, after
3
process needs effective electrocatalysts to break inert NN
6
−9
with a significant activation barrier.
Many noble-metal
10
11
12
13
14
electrocatalysts (Au, Ru, Rh, Ag, Pd, etc.) for the N2
reduction reaction (NRR) have been developed. However,
because of their high cost and scarcity, their application on an
industrial scale has been limited. An immediate outlook for
large-scale industrial applications points toward the use of
systems not relying on expensive precious metals, which
annealing, the nanosized LiMn O particulates grow over the
2
4
surface of the spun fibers to form the LiMn O NF, which is
2
4
Received: June 9, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01707
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
oxygen (M−O) and surface hydroxyl groups (H−O),
5
0
respectively. For the Mn 2p region (Figure 2d), two
characteristic peaks at 641.9 and 653.5 eV can be assigned to
the BEs of Mn 2p3 and Mn 2p5 11 /, 52 2, respectively, indicating the
/2
3
+
4+
coexistence of Mn and Mn .
The electrocatalytic NRR capability of the LiMn O NF was
2
4
tested with a typical three-electrode system in 0.1 M HCl
under different potentials reported on a RHE scale. The
working electrode is the LiMn O NF/CP, the reference
2
4
electrode is Ag/AgCl (filled with a saturated KCl solution),
and the counter electrode is a graphite rod. All NRR
experiments were performed under ambient conditions in a
two-compartment cell, which is separated by a piece of Nafion
(
117) membrane. Before electrochemical measurement, all
labwares were strictly cleaned to prevent external contami-
nation. In the process of electrochemical measurement, N2
(
99.999%) was first injected into a saturator filled with 0.05 M
H SO to remove any possible nitrogen oxides, and then it
2
4
bubbled up at the bottom of cathodic compartment to saturate
the electrolyte, which reacts with protons in the electrolyte to
form NH3 on the surface of the catalyst. We use it
53
spectrophotometrically via the indophenol blue method to
determine the product of the NH concentration. We also use
the Watt and Chrisp method to determine the possible
3
Figure 1. (a) XRD pattern for LiMn O . (b and c) SEM images of
54
2
4
LiMn O . (d) TEM and (e) HRTEM images of LiMn O . (f) SAED
2
4
2
4
byproduct N H . Figures S2 and S3 show the detecting
2
4
pattern of LiMn O .
2
4
calibration curves of NH and N H , respectively. Figure 3a
3
2
4
further supported by transmission electron microscopy (TEM)
analysis. Figure 1e shows the high-resolution TEM (HRTEM)
image. It shows lattice fringes with an interplanar spacing of
0
.476 nm, consistent with the (111) plane of the LiMn O
2 4
phase. It further supported the presence of the crystalline
nature of LiMn O particulates by the selected area electron
2
4
diffraction (SAED) pattern (Figure 1f).
Figure 2a presents the X-ray photoelectron spectroscopy
(
XPS) survey spectrum of the LiMn O NF, confirming the
2 4
existence of the Mn, O, and Li elements. In the Li 1s spectrum
Figure 2b), the small atomic mass of Li leads to a lower-
(
49
intensity peak with a binding energy (BE) of 54.2 eV. Figure
2
5
c shows the O 1s spectrum. The BEs at around 529.7 and
31.2 eV are the characteristic signature of the crystal lattice
Figure 3. (a) Curves of the current density versus time for the
LiMn O NF/CP under different potentials. (b) UV−vis absorption
2
4
spectra of the electrolytes. (c) VNH3 and FE values of LiMn O NF/
2
4
CP under different potentials. (d) Amount of produced NH for the
3
LiMn O NF/CP and CP after charging at −0.50 V for 2 h in 0.1 M
2
4
HCl.
shows the time-dependent current density curves of the
LiMn O NF/CP in a N -saturated electrolyte with 0.1 M HCl
2
4
2
for 2 h, which are at diverse potentials ranging from −0.45 to
−
0.65 V and exhibit good stability. Figure S4 shows the
corresponding time-dependent current curves. After an
electrocatalytic reaction for 2 h at diverse potentials, the
electrolytes were colored with an indophenol indicator
shielded from light for 2 h. Figure 3b shows the corresponding
UV−vis absorption spectra. As shown in Figure 3c, the
−
1
−1
maximum value of V is 15.83 μg h mg
cat.
at −0.50 V. At
Figure 2. (a) XPS survey spectrum of the LiMn O NF. XPS spectra
NH
3
2
4
of the LiMn O NF in the (b) Li 1s, (c) O 1s, and (d) Mn 2p regions.
the same potential, the maximum value of the FE is 7.44%,
2
4
B
DOI: 10.1021/acs.inorgchem.9b01707
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
outperforming the previously reported Li -free Mn O nano-
Communication
+
CP is used as the catalyst to perform electrochemical reduction
of N to NH at −0.50 V, consecutive recycling tests are run six
times for a total of 12 h. As observed in Figure 4c, both V
NH
3
4
4
5
46
cube and Mn O nanoparticles@reduced graphene oxide
3
4
2
3
as well most other NRR catalysts at ambient conditions
Tables S1 and S2). We used ion chromatography to
3
(
and FE values for the LiMn O NF/CP show negligible
2
4
determine the concentration of NH (Figure S5 and Table
3
changes. The corresponding time-dependent current curves,
current density curves, and UV−vis absorption spectra (Figure
S12) confirm the high stability of the LiMn O NF/CP for
S3), suggesting that both techniques give comparable results.
Of note, bare CP has low electrocatalytic NRR activity
2
4
(
Figures 3d and S6) and no N H was detected after NRR
2 4
electrocatalytic NRR. Moreover, the LiMn O NF/CP also
2 4
(Figure S7), implying that the LiMn O NF is efficient to
2
4
shows strong durability (Figures 4d and S13). XRD (Figure
S14), HRTEM (Figure S15), and SEM (Figure S16) data
indicate that the LiMn O NF has no obvious changes in the
crystalline phase and morphology after the stability test.
In summary, the LiMn O NF has been testified as an
catalyze N reduction to NH with excellent selectivity.
2
3
In order to get better proof that NH originates from
3
2
4
electrochemical N2 reduction by the LiMn O4 NF, we
2
experiments in an electrolyte that was saturated by Ar at
2
4
−
0.50 V and in electrolyte that was saturated by N at an open-
2
efficient catalyst for electrochemical N fixation to NH at
2 3
circuit potential for 2 h. Considering the extremely small
amount of NH (m 3) in the atmosphere, we set up a blank
ambient temperature and pressure by experiments. When
tested in 0.1 M HCl, the LiMn O NF attains a excellent FE of
3
NH
2
4
−
1
−1
control experiment. As shown in Figures 4a and S8, almost no
7.44% and a large NH yield of 15.83 μg h mgcat. , with
3
strong electrochemical stability and durability. Our present
discovery not only provides people with an environmentally
friendly catalyst material for electrocatalytic N reduction but
2
+
hopefully opens up methods to design and exploit Li -based
spinel nanocatalysts for artificial N fixation applications.
2
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01707.
Experimental section, SEM and HRTEM images, UV−
vis absorption spectra, time-dependent current and
1
calibration curves, ion chromatograms, H NMR spectra,
XRD patterns, and Tables S1−S3 (PDF)
Figure 4. (a) Amount of NH produced for the LiMn O NF/CP in
3
2
4
AUTHOR INFORMATION
Corresponding Authors
E-mail: liquan6688@163.com (Q.L.).
*E-mail: xpsun@uestc.edu.cn (X.S.).
different electrolytes that were saturated by N and Ar after 2 h of
■
*
2
electrolysis at −0.50 V and an open circuit in N . (b) V
and FE
2
NH
3
values of the LiMn O NF/CP at −0.50 V for alternating 2 h cycles in
2
4
different electrolytes that were saturated by N and Ar, respectively.
2
(
c) Cycling stability test of the LiMn O NF/CP. (d) Current density
2 4
ORCID
versus time curves for the LiMn O NF/CP at −0.50 V in an N -
saturated electrolyte for 24 h.
2
4
2
Abdullah M. Asiri: 0000-0001-7905-3209
Xuping Sun: 0000-0002-5326-3838
Notes
NH occurs for all cases. Ion chromatography data also suggest
The authors declare no competing financial interest.
3
the absence of NH , as shown in Figure S9. Furthermore, we
3
also performed NRR experiments in which the LiMn O NF/
2
4
ACKNOWLEDGMENTS
■
CP was electrolyzed at −0.50 V for alternating 2 h cycles in
different electrolytes that were saturated by N2 and Ar,
respectively. The corresponding VNH3 and FE values were
This work was supported by the National Natural Science
Foundation of China (Grant 21575137).
calculated and plotted as shown in Figure 4b, and NH was
3
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DOI: 10.1021/acs.inorgchem.9b01707
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