Single-atom metal-N4site molecular electrocatalysts for ambient nitrogen reduction

Article abstract of DOI:10.1039/d1cy00022e

Electrochemical N₂reduction to NH₃is an emerging energy technology, attracting much attention due to its features of mild reaction conditions and being non-polluting. In this work, we demonstrate that a well-defined cobalt tetraphenylporphyrin (CoTPP) molecule as a model catalyst exhibits good electrocatalytic nitrogen reduction activity in 0.1 M HCl electrolyte with an ammonia yield of 15.18 ± 0.78 μg h^-1mg^-1_cat.calculated by the indophenol blue method and a Faraday efficiency (FE) of 11.43 ± 0.74%. The catalyst also has satisfactory electrolytic stability and recycling test reusability. The activity displayed by the porphyrin molecular catalysts is attributed to the full exposure of the metal-N₄sites. To trace the source of ammonia, an isotope labeling experiment (¹⁵N₂as the feed gas) is used to calculate the ammonia yieldvia¹H nuclear magnetic resonance (NMR), which is close to that of the indophenol blue method. In addition, we replace the central metal to prepare CuTPP and MnTPP, and they also show electrocatalytic nitrogen reduction reaction (NRR) ability. This work proves the feasibility and versatility of using metalloporphyrin molecules as model electrocatalysts for NRR and offers a new strategy for the further development of molecular NRR catalysts.

Full text of DOI:10.1039/d1cy00022e

Catalysis
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Single-atom metal–N4 site molecular
electrocatalysts for ambient nitrogen reduction†
Cite this: Catal. Sci. Technol., 2021,
11, 2589
Sai Sun,‡ Xiaoxuan Yang,‡ Siqi Li,‡ Xinyu Chen, Ke Li, Jiaqi Lv, Wenwen Wang,
*
Dongming Cheng, Yong-Hui Wang and Hong-Ying Zang
Electrochemical N2 reduction to NH3 is an emerging energy technology, attracting much attention due to its
features of mild reaction conditions and being non-polluting. In this work, we demonstrate that a well-
defined cobalt tetraphenylporphyrin (CoTPP) molecule as a model catalyst exhibits good electrocatalytic
nitrogen reduction activity in 0.1 M HCl electrolyte with an ammonia yield of 15.18 0.78 μg h−1 mg−1
cat.
calculated by the indophenol blue method and a Faraday efficiency (FE) of 11.43 0.74%. The catalyst also
has satisfactory electrolytic stability and recycling test reusability. The activity displayed by the porphyrin
molecular catalysts is attributed to the full exposure of the metal–N4 sites. To trace the source of ammonia,
1
an isotope labeling experiment (15N2 as the feed gas) is used to calculate the ammonia yield via H nuclear
Received 4th January 2021,
Accepted 27th January 2021
magnetic resonance (NMR), which is close to that of the indophenol blue method. In addition, we replace the
central metal to prepare CuTPP and MnTPP, and they also show electrocatalytic nitrogen reduction reaction
(NRR) ability. This work proves the feasibility and versatility of using metalloporphyrin molecules as model
electrocatalysts for NRR and offers a new strategy for the further development of molecular NRR catalysts.
DOI: 10.1039/d1cy00022e
rsc.li/catalysis
renewable energy under ambient conditions. Due to its mild
reaction conditions and safe operation, the electrocatalytic
Introduction
The non-renewability of fossil fuels and global warming are
enormous challenges for the sustainable development of
human society. Developing clean energy, such as hydrogen
energy, is a promising method to solve energy problems.1,2
Compared with other carriers, ammonia has excellent
hydrogen storage capacity. The mass content of hydrogen is
17.6% in liquid ammonia and ammonia is the only carbon-
free proton carrier that will not release CO2 when it is finally
decomposed.3–6 As the second most important substance,
ammonia (NH3) is commonly used in the production of
fertilizers, pharmaceuticals, and synthetic fibers. In industry,
ammonia is mainly produced by the Haber–Bosch process
which requires high temperature and high pressure (about
400–450 °C, 10–30 MPa), but this process will lead to energy
consumption and greenhouse gas emissions.7–11 Therefore,
exploring an efficient, clean and sustainable ammonia
synthesis method is required.
NRR is currently a popular research method for ammonia
synthesis.12,13 However, due to the low solubility of N2 in
water and the high bond energy of the NN triple bond, N2
activation is still a great challenge.14–18 Both the NRR and
hydrogen evolution reaction (HER) involve a proton coupled
electron transfer process, but the HER has inherently more
favorable kinetics which hinders the rapid progress of the
NRR. As a result, the electrocatalytic NRR usually suffers
from relatively low ammonia yield and Faraday efficiency
(FE).19–21 Excitingly, various NRR electrocatalysts have been
reported to effectively address the above problems, including
noble-metal catalysts (Ru, Au, Pt, Ir, etc.),22–24 transition-
metal compound catalysts (V2O3/C, Nb2O5, Mo2C/C, N–NiO,
CaCoOx, etc.)25–29 and metal-free catalysts (boron-doped
graphene, nitrogen-doped porous carbon, Cl-doped
graphdiyne, etc.).30–33 Although the commonly used noble
metal and noble metal oxide electrocatalysts have excellent
performance, their scarcity and hign cost have limited their
application to a certain extent. Metal-based electrocatalysis
also suffers from low selectivity, and the Faraday efficiency is
often less than 10%. Moreover, the mechanism and reaction
intermediates in the NRR process cannot be accurately
studied because of non-defined structure in these catalysts.
Therefore, there is an urgent need to find defined structure,
high selectivity and cost-effective alternatives.
The electrocatalytic nitrogen reduction reaction (NRR)
achieves pollution-free conversion of N2 to NH3 with
Key Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of
Chemistry, Northeast Normal University, Changchun 130024, China.
E-mail: zanghy100@nenu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cy00022e
‡ These authors contributed equally to this work.
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Molecular catalysts have been widely used in
Catalysis Science & Technology
electrochemical energy conversion, such as the oxygen
reduction reaction, CO2 reduction reaction, and NRR,
because of the tunability of their electronic structure to
clarify the structure–activity relationship.34–37 Porphyrin
molecules with highly delocalized electrons and uniform
electron density are widely distributed in nature. The stable
coordination environment and the diversity of central metals
in metal-porphyrin achieve efficient electron and energy
transfer, so they have been used in chemical sensing,
catalysis, and photosensitive materials.38–43 Metal-porphyrin
is an atom-dispersed single-atom molecular electrocatalyst
with maximum atom utilization efficiency, which can greatly
reduce metal consumption. Most transition metals have
incompletely filled orbitals that can provide or accept
electrons, which makes them more suitable for N2 activation.
Meanwhile, the fully exposed metal–N4 sites in
metalloporphyrins are beneficial to N2 adsorption. However,
there are few studies on the application of metal-porphyrin
molecules in the electrocatalysis NRR.
Fig. 1 (a and b) SEM images of CoTPP. (c and d) TEM images of
CoTPP. (e and f) TEM images and the corresponding elemental
mapping image of CoTPP.
characterization was performed, and the results confirmed
the successful synthesis of CoTPP (Fig. S2†).
Electrochemical NRR experiments were performed in 0.1 M
HCl electrolyte using an H-shape electrochemical cell. Acidic
electrolytes can provide more protons and promote the
hydrogenation process of nitrogen. A Nafion-115 membrane
was used to separate the anode and cathode compartments,
each compartment containing 50 mL of electrolyte (Fig. S3†).
CoTPP was coated on carbon paper (mass load: 1 mg cm−2) as
the working electrode, while the Ag/AgCl electrode and graphite
rod worked as the reference electrode and the counter
electrode, respectively. The N2 gas or Ar gas flowed into 0.1 M
KOH solution, 0.1 M KMnO4 solution and 0.1 M HCl solution
in advance for purification to ensure that no N-containing
impurities in the gas enter the electrolyte. After 2 h of
electrolysis, the produced ammonia was detected and
quantified by the indophenol blue method46 and isotope
labeling experiment method (15N2 as the feeding gas),
respectively. Possible by-product N2H4 was detected and
quantified by the method of Watt and Chrisp.47 Corresponding
calibration curves are presented in Fig. S4–S6.†
Here, CoTPP molecules with well-defined Co–N4 sites
exhibit good NRR electrocatalytic activity, the ammonia yield
reaches 15.18 0.78 μg h−1 mg−1
and FE is 11.43 0.74%
cat.
at −0.3 V vs. RHE in 0.1 M HCl electrolyte. The comparison
with H2TPP molecules confirms the importance of the metal–
N4 site for the NRR. The long-term electrolytic test and cycle
test prove the stability and durability of the catalyst. We also
used an isotope labeling experiment (15N2 as the feeding gas)
and 1H NMR method to detect and calculate the ammonia
yield, the result matches the data calculation by the
indophenol blue method. In order to further illustrate the
feasibility of metalloporphyrins as NRR model molecular
electrocatalysts, the central metal of porphyrin was further
extended to other transition metals, thereby synthesizing
CuTPP and MnTPP. Experimental results show that they also
show good electrocatalytic activity.
Results and discussion
During the electrocatalytic NRR process, high-purity feed
gas (N2 gas or Ar gas) was continuously passed into the
electrolyte to ensure gas saturation. The linear scanning
voltammetry (LSV) curves of CoTPP in Ar and N2 saturated
environments are shown in Fig. 2a. Obviously, the current
density obtained in the N2-saturation state is higher, which
confirms the NRR activity of the CoTPP. After 2 h of
electrolysis, the chronoamperometry curves at potentials
from −0.1 V to −0.5 V vs. RHE are shown in Fig. 2b. The
constant current density over time indicates that the CoTPP
catalyst has good durability during the NRR test. Fig. 2c
shows UV-vis absorption spectra for the determination of
ammonia content using the indophenol blue method after 2
h of potentiostatic electrolysis, and the maximum absorption
intensity at a wavelength of 655 nm at −0.3 V vs. RHE,
indicating the highest ammonia yield at this potential. The
average ammonia yield and Faraday efficiency for three tests
were calculated and are plotted in Fig. 2d, the ammonia yield
Metal-tetraphenylporphyrin (MTPP) was synthesized by
a
simple method. The successful coordination of the metal with
tetraphenylporphyrin was confirmed by FT-IR (Fig. S1†). The
peaks at 3321 cm−1 and 962 cm−1 belong to N–H stretching
vibration and bending vibration of the porphyrin ligand,
respectively. When the metal replaces the H in the ring and
coordinates with the ligand, these two peaks weaken or
disappear. A new peak at about 1000 cm−1 was detected in the
MTPP, which was due to the strengthening of the ring vibration
(caused by the metal ion entering the porphyrin ring).44,45
The scanning electron microscopy (SEM) images and the
transmission electron microscopy (TEM) images in Fig. 1
show the nanostripe morphology of CoTPP molecules, the
width of the nanostripe is about 100 nm. The
nanomorphology and uniform size distribution of CoTPP
molecules increase the active surface area and facilitate the
adsorption of N2. X-ray photoelectron spectroscopy (XPS)
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Fig. 2 (a) Linear-sweep voltammetric curves of the CoTPP in N2-
saturated (red line) and Ar-saturated (black line) 0.1 M HCl aqueous
electrolyte with a scan rate of 5 mV s−1. (b) Chronoamperometric
curves of CoTPP for the NRR at different applied potentials. (c) UV-vis
absorption spectra of electrolytes stained with an indophenol indicator
under different applied potentials for the NRR. (d) Average ammonia
yield (left y-axis) and Faraday efficiency (right y-axis) for three tests of
CoTPP at different potentials.
Fig. 3 (a) Average ammonia yield and Faraday efficiency of the catalysts
(CoTPP and H2TPP) at −0.3 V vs. RHE in 0.1 M HCl electrolyte. (b) NH3
and N2H4 yield rates of CoTPP at −0.3 V vs. RHE in 0.1 M HCl electrolyte.
(c) UV-vis absorption spectra of electrolytes stained with an indophenol
indicator under different control experimental conditions.
and Faraday efficiency reach their maximum values, 15.18
0.78 μg h−1 mg−1
and 11.43 0.74% at −0.3 V vs. RHE,
cat.
respectively (the values of the three experiments are shown in
Fig. S7†). Under the same conditions, CoTPP exhibits
superior electrocatalytic NRR performance to H2TPP (Fig. 3a),
which is attributed to the Co–N4 sites formed by the
introduction of Co atoms, and the sites facilitate the
adsorption and activation of N2. The performance of CoTPP
molecular NRR electrocatalysts is also comparable to most
molecular and non-noble metal NRR electrocatalysts (Table
S1†). Furthermore, compared to ammonia production, the
production of hydrazine is negligible (Fig. 3b).
curve does not change significantly (Fig. 4a), which confirms
that the CoTPP electrocatalyst has long-term stability. During
the 10 h continuous electrolysis process, we sampled every 2
h, the ammonia yield and Faraday efficiency did not change
significantly,
confirming
that
CoTPP
has
stable
electrocatalytic activity (Fig. S8 and S9†). Fig. 4b shows five
cycles of testing at −0.3 V vs. RHE. The changes in ammonia
yield and Faraday efficiency are negligible. It further
illustrates that CoTPP is a stable electrocatalyst for the NRR.
XPS reveals that the composition of the catalyst has not
changed after long-term stability testing (Fig. S10†). Fig. S11†
A series of control experiments at −0.3 V vs. RHE were
conducted to verify that the source of NH3 was generated via
the electrocatalytic reduction of nitrogen by CoTPP. First, we
used the indophenol blue method to detect the 0.1 M HCl
+
electrolyte and found a small amount of NH4 , and measured
the corresponding absorbance by UV-vis absorption
spectroscopy. This explains the inevitable NH3 background in
the electrolyte, which is consistent with previously reported
studies.48,49 Then we conducted the following experiments: (a)
the catalyst was electrolyzed in Ar-saturated electrolyte for 2 h;
(b) the catalyst was electrolyzed in N2-saturated electrolyte at
open circuit potential (OCP) for 2 h; (c) bare carbon paper
without a catalyst was electrolyzed in N2-saturated electrolyte
for 2 h. The UV-vis absorption spectra (Fig. 3c) of the control
experiments show only weak signals, which is consistent with
the measured data of pure electrolytes. In other words, the NH3
generated in the system was mainly derived from the NRR
reaction on the CoTPP catalyst.
Fig. 4 (a) Time-dependent current density curve of CoTPP for 10 h of
electrolysis at −0.3 V vs. RHE. (b) Ammonia yield and Faraday efficiency
after cyclic stability test for 5 cycles. (c) 1H NMR spectra of commercial
15/14NH4Cl samples and the product obtained by using 15/14N2 as the feed
gas. (d) Ammonia yield calculated by the indophenol blue method using
14N2 as feed gas and by 1H NMR using 15N2 as feed gas, respectively.
The stability of the catalyst is an important indicator for
evaluating the catalytic performance. After 10 h of continuous
electrolysis in N2-saturated electrolyte at −0.3 V vs. RHE
potential, the current density of the chronoamperometry
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shows SEM images of CoTPP after 2 h of electrolysis. The
nanostripe morphology is not significantly different from
that before catalysis, indicating the stability of CoTPP as an
NRR molecular electrocatalyst.
To further prove the ability of the CoTPP electrocatalyst
in reducing N2 to NH3 and ensure the source of ammonia,
we conducted an isotope labeling experiment. After
electrolysis using 14N2 as the feed gas, the 1H NMR spectra
+
showed 14NH4 triplet coupling, which was well coupled to
the peak of the standard 14NH4Cl sample. Under the same
condition, we also used 15N2 as the feed gas for electrolysis,
+
and the 1H NMR spectra showed only 15NH4 doublet
coupling (Fig. 4c). The results further suggested that the
appearance of 15NH4 was produced by the electrochemical
+
nitrogen reduction of the CoTPP electrocatalyst. Because the
integrated area of the peak in 1H NMR can directly reflect
the ammonia content, we used 1H NMR and a series of
known concentrations of standard 15NH4Cl solution to
quantitatively determine the ammonia yield in the NRR. As
shown in Fig. 4d, the ammonia yield calculated from the
1
integrated area of the peaks in H NMR is close to the value
Fig. 5 (a) Average ammonia yield (left y-axis) and Faraday efficiency
(right y-axis) for three tests of CuTPP at different potentials. (b)
Average ammonia yield (left y-axis) and Faraday efficiency (right y-axis)
for three tests of MnTPP at different potentials. (c) NH3 yield rate over
MTPP (M = Co, Cu, Mn) in 0.1 M HCl electrolyte.
previously calculated by the indophenol blue method. The
coincidence of the two detection methods reveals that
CoTPP is an efficient and stable nitrogen reduction
molecular electrocatalyst.
In order to explore the versatility of molecular
porphyrins as NRR electrocatalysts, CuTPP and MnTPP
were synthesized with
a
similar reflux method. Their Experimental
molecular structure was characterized by FT-IR, as shown
in Fig. S1,† indicating that they have the same
coordination environment. The average ammonia yield of
Materials and measurements
Tetraphenylporphyrin and all other chemicals were
commercially purchased and used without further
purification. UV-vis spectra were recorded using a Shimadzu
UV-3100PC spectrophotometer. FT-IR was performed using a
Nicolet Magna 560 IR spectrometer with a wavelength range
of 4000–400 cm−1, with KBr pellets. 1H NMR spectra were
recorded on a Varian 600 MHz spectrometer. A scanning
electron microscope (Hitachi, SU-70) was used with an
CuTPP reached a maximum of 10.53
6.5 μg h−1 mg−1
cat.
at −0.3 V vs. RHE (the values of the three experiments are
shown in Fig. S12†) and the maximum Faraday efficiency
was 14.97% at −0.1 V vs. RHE (Fig. 5a). The highest
ammonia yield of MnTPP at −0.4 V vs. RHE was 7.98
0.8 μg h−1 mg−1
(the values of the three experiments
cat.
are shown in Fig. S13†), while the highest Faraday
efficiency was 8.21% at −0.5 V vs. RHE (Fig. 5b). The
time-dependent current density curves of CuTPP and
MnTPP for 10 h of electrolysis (Fig. S14 and S15†) show
that the current density does not change significantly.
These results indicate the feasibility of metalloporphyrin
molecules as model NRR electrocatalysts, which provides a
broader idea for the design of NRR electrocatalysts.
Electrochemical impedance is related to electrocatalytic
kinetics, so we conducted an electrochemical impedance
test of H2TPP and MTPP. Fig. S16† shows that MTPP
exhibits lower impedance, thus higher conductivity and
faster NRR kinetics than H2TPP. Importantly, the superior
activity of CoTPP is attributed to the proper adsorption of
N2 and N* intermediates by the Co–N4 active site, which
is consistent with the theoretical calculation results in the
known literature.50,51 Beyond, the nanostructured CoTPP
increases the number of surface active sites, which further
enhances the electrocatalytic NRR activity.
operating voltage of
3 kV and transmission electron
microscopy was carried out on a JEM-2100F (JEOL) electron
microscope. XPS measurement was carried out on a VG
ESCALABMKII, Al-Kα radiation.
Synthesis of metal-tetraphenylporphyrin (MTPP)
The MTPP (Co, Cu and Mn) molecules were synthesized
according to the previous literature52 with appropriate
modification.
1.5 g H2TPP was dissolved in 100 mL DMF to form
solution A, and 0.8 g of the corresponding metal acetate was
dissolved in 30 mL CH3OH to form solution B. Solution B
was slowly added to solution A and heated at 160 °C for 3 h.
Finally, the mixed solution was poured into 300 mL
deionized water, and then the precipitate was collected by
filtration, washed with deionized water and dried overnight
at 80 °C.
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Electrochemical measurements
series of concentrations of 0, 0.2, 0.4, 0.8 and 1.0 μg mL−1 in
0.1 M HCl solution. After three times of independent
calibrations, the calibration curve of N2H4 is y = 0.637x +
0.008, R2 = 0.999.
All electrochemical data were collected using a Princeton
Electrochemical Workstation (PMC CHS08A) under ambient
conditions. The standard three-electrode system in an
H-shape electrochemical cell equipped with a Nafion 115
membrane was used in electrochemical measurements.
Catalyst coated carbon paper was used as the working
electrode, while the Ag/AgCl electrode and graphite rod
worked as the reference and the counter electrode,
respectively. Before the NRR measurements, the feed gas
flowed into 0.1 M KOH solution, 0.1 M KMnO4 solution and
Calculations of ammonia yield rate and faradaic efficiency (FE)
The rate of NH3 yield was calculated using the following
equation:
NH3 yield = (c × V)/(17 × t × mcat.
)
0.1
M
HCl solution for purification to ensure no
where c (μg mL−1) is the measured NH3 concentration, V (mL)
is the volume of the electrolyte, t (h) is the reduction reaction
time and mcat. is the mass of catalyst loading on carbon paper.
The faradaic efficiency for the NRR was defined as the
percentage of the amount of electric charge used for
synthesizing NH3 in the total charge passed through the
electrodes during electrolysis. Assuming three electrons were
needed to produce one NH3 molecule, the FE in 0.1 M HCl
solution could be calculated as:
N-containing impurities in the gas and then high-purity feed
gas was purged to 50 mL 0.1 M HCl electrolyte for at least 30
min. The gas was maintained throughout the electrochemical
measurements. The potentials for this work were converted
to the reversible hydrogen electrode (RHE) to calibrate the
saturated Ag/AgCl electrode using the following formula:
E(vs. RHE) = E(vs. Ag/AgCl) + 0.059 × pH + 0.197 V
FE = 3F × c × V/(17 × Q)
Determination of ammonia
where F (96 500 C mol−1) is the Faraday constant, c (μg mL−1)
is the measured NH3 concentration, V (mL) is the volume of
the electrolyte, and Q (C) is the quantity of applied electricity.
The concentration of produced NH3 was determined via the
indophenol blue method using UV-vis spectrophotometry. In
detail, 2 mL electrolyte after reaction was taken from the
electrochemical cathodic chamber. This was added into 2 mL 1
M NaOH solution containing 5 wt% salicylic acid and 5 wt%
sodium citrate, and then 1 mL 0.05 M NaClO and 0.2 mL 1 wt%
sodium nitroferricyanide was added to the electrolyte. After
keeping for 2 h at room temperature, the absorption spectra
were measured using an UV-visible spectrophotometer.
Indophenol blue was formed at a wavelength of 655 nm and
absorbance at this wavelength was used to estimate the yield of
ammonia according to the standard curve. The concentration–
absorbance standard curve was calibrated using NH4Cl standard
solution with a series of concentrations of 0, 0.2, 0.4, 0.8 and 1.0
μg mL−1 in 0.1 M HCl solution. The fitting curve (y = 0.382x +
15N2 isotope labeling experiment
Isotope labeling experiments use labeled 15N2 as the feed gas
to obtain the accurate catalytic performance of CoTPP. The
method for preparing the calibration curve is as follows: (i)
prepare standard 15NH4 solutions in 0.1 M HCl at 2.5, 5.0,
+
+
10, 25 and 50 μg mL−1, respectively. (ii) Take 540 μL 15NH4
solution of each concentration and mix with 60 μL D2O in
the nuclear magnetic tube, and perform 1H NMR
+
measurement. (iii) Integrate the peak area, and 15NH4 has a
positive correlation with the integrated area to obtain a
calibration curve. The reaction solution was mixed with D2O
in proportion to test 1H NMR, and the 1H NMR peak area
was obtained by integration, then the ammonia yield was
calculated according to the calibration curve.
0.030, R2
= 0.999) shows a good linear relationship of
absorbance value with NH3 concentration by three times
independent calibrations to confirm the accuracy of the system.
Determination of hydrazine
Conclusions
N2H4 production was spectrophotometrically determined
using the Watt and Chrisp method. A mixed solution of 5.99
In summary, CoTPP with uniform nanostructure is an
efficient and stable electrocatalyst for reducing N2 to NH3
under ambient conditions. In addition, we further explored
CuTPP and MnTPP, both of which exhibited good NRR
g
para-(dimethylamino)benzaldehyde (C9H11NO), 30 mL
concentrated HCl, and 300 mL ethanol was used as the color
reagent. 5 mL electrolyte after reaction was taken from the
electrochemical cathodic chamber and the above 5 mL color
reagent was added. The mixture was kept stirring at room
temperature for 20 min. The absorbance of the solution was
measured at a wavelength of 455 nm, and the yield of N2H4
was evaluated according to the standard curve. The
concentration–absorbance curve was calibrated by the known
of concentrations of hydrated hydrazine solution with a
catalytic
activity.
This
proves
the
reliability
of
metalloporphyrin molecules as NRR model electrocatalysts,
because molecular catalysts have a well-defined structure and
clear mechanism, which is convenient for further
optimization of the catalytic process. The activity of the
metalloporphyrin electrocatalyst is attributable to the full
exposure of the metal–N4 sites, which fully adsorbs N2 and
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accelerates the N2 activation. This work provides more
choices and new ideas for the design of NRR electrocatalysts.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the financial support
from the National Natural Science Foundation of China (No.
21871042, 21471028 and 21671036), the Changbai Mountain
Scholarship, the Natural Science Foundation of Jilin Province
(No. 20200201083JC, JJKH20201169KJ), and the Natural
Science Foundation of Department of Education of Jilin
Province (JJKH20201169KJ).
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