Boosting the acidic electrocatalytic nitrogen reduction performance of MoS2 by strain engineering

Article abstract of DOI:10.1039/d0ta03622f

It has been widely confirmed that expanding the layer spacing of layer-structured MoS₂ can boost the hydrogen evolution reaction (HER) activity of MoS₂. Inspired by this, a strain engineering strategy was applied to defect-rich MoS₂ nanosheets by facilely substituting F to compress the interlayer space of MoS₂. Because of the smaller size and higher electronegativity of F as compared to S, the catalytic HER was remarkably suppressed. By considering the strongly reduced uphill energy for the hydrogenation of adsorbed N₂ on MoS₂ due to the introduction of F ions, as revealed by first-principles calculations, electrochemical nitrogen reduction reaction (NRR) activity and selectivity on the F-doped MoS₂ (F-MoS₂) catalyst under acidic conditions can be significantly boosted. Faradaic efficiency toward the NRR on F-MoS₂ was therefore enhanced to 20.6% with a maximum NH₃ yield of 35.7 μg h^-1 mg_cat^-1 at -0.2 V vs. RHE during long-term operation.

Full text of DOI:10.1039/d0ta03622f

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DOI: 10.1039/D0TA03622F
ARTICLE
Boosting the acidic electrocatalytic nitrogen reduction
performance of MoS2 by strain engineering
Received 00th January 20xx,
Accepted 00th January 20xx
Jiawei Liang,a† Shuangxiu Ma,a† Jing Li,a Yangang Wang,c* Junli Wu,a Quan Zhang,a Zhao Liu,a Zehui
Yang,a* Konggang Qub and Weiwei Caia*
DOI: 10.1039/x0xx00000x
It is widely confirmed that expanding the layer spacing of layered-structural MoS2 can boost the hydrogen evolution
reaction (HER) activity of MoS2. Inspired by this, strain engineering strategy is applied on defect-rich MoS2 nanosheets by
facilely F substitution to compress the interlayer space of MoS2 owing to the smaller size and higher electronegativity of F
than S and catalytic HER was remarkably surpressed. By considering the strongly reduced uphill energy for the
hydrogenation of adsorbed N2 on MoS2 due to the introduction of F ions revealed by the first-principle calculation,
electrochemical nitrogen reduction reaction (NRR) activity and selectivity on the F doped MoS2 (F-MoS2) catalyst in acidic
condition can be significantly boosted. Faradic efficiency toward NRR on F-MoS2 is therefore enhanced to 20.6% with the
NH3 yield high to 35.7 μg h-1 mgcat-1 at the -0.2 V vs. RHE during the long-term operation.
electrolyte is much lower than expected due to the existence
12
of competitive HER.
Actually, numerous noble-metal free
Introduction
catalysts have been developed for HER catalysis and highly
efficient noble-metal-free hydrogen generation has been
achieved.13-15 Among which, MoS2 has been considered as one
of the most promising noble-metal-free HER catalysts due to
the Pt-like property and proper Mo(III)/Mo(IV) redox potential
(0.04 V)16 and was also widely researched for NRR due to that
requirements of NRR and HER on active sites are similar.17-20
Density functional theory (DFT) calculations revealed that the
edge of MoS2 is the electrocatalytically active site for both NRR
NH3 is one of the most essential chemicals for the industrial
producing of fertilizers, dyes and resins.1, Although both
2
elements (N and H) in NH3 are abundant on earth, the intrinsic
inert property of N2, which is the most abundant molecule in
air, makes the producing of NH3 from N2 very difficult. Haber–
Bosch process, which is the primary artificial NH3 producing
strategy nowadays, undergoes with metal catalyst under high
3,
4
temperatures (400–500 °C) and pressures (15–25 MPa).
Around 1–2% of the world's energy supply is therefore
consumed by the Haber–Bosch process. Energy-efficient
strategies, represented by electrochemical nitrogen reduction
reaction (NRR), were therefore widely studied.5-10
Encouragingly, NRR using protons as hydrogen source can be
efficiently powered by electricity under ambient condition.
However, the most severe challenge for aqueous NRR is the
poor efficiency ascribed to the competition between hydrogen
evolution reaction (HER) and NRR.11 HER is supposed to be one
of the kinetically fastest electrochemical reactions due to that
only two electrons are required to accomplish the reduction
19
and HER while the basal plane is inert for NRR. Great NRR
activity in neutral condition was detected for MoS2 with
Faradic efficiency (FE) of 8.34% and NH3 yield of 29.28 µg h-1
mg-1cat in 0.1 M Na2SO4. Unfortunately, NRR catalytic activity of
MoS2 in acidic condition, which is preferred for NRR due to
that 6 H atoms are required for the fixation of every N2
molecule 20-24, is much poorer than the recently reported
cost-effective NRR catalysts
reduction activity of MoS2. Strategies were applied to enhance
the selectivity as well as NH3 yield of MoS2 based catalysts in
acidic condition.30-33 Ru/2H-MoS2 interfaces were hence
5,
25-29
ascribed to the great H+
퐸0(H+ /
+
reaction and the standard redox potentials of H /H2 (
31
engineered with the acidic NRR FE enhanced to 17.6%.
Other than the noble-metal Ru, employing cost-effective active
H )
퐸0(N /NH )
) couples are very close. As a
2 3
) and N2/NH3 (
2
32
result, even using noble metals as catalyst, the activity and
selectivity of NRR under ambient condition in aqueous
centers, including reduced graphene oxide
and single
transition metal atom33, can also boost the NRR activity and
selectivity in acidic condition.
Here in this work, a facile strain engineering strategy was
proposed to boost the acidic NRR activity and selectivity of
defect-rich MoS2 nano-sheets since strain engineering has
been widely applied for electrocatalytic activity adjustment of
a. Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China
University of Geosciences Wuhan, 430074, China. willcai1985@gmail.com (W.
Cai); yeungzehui@gmail.com (Z. Yang).
b. Shandong Provincial Key Laboratory/Collaborative Innovation Center of Chemical
Energy Storage & Novel Cell Technology, Liaocheng University, Liaocheng,
252059, China.
34-39
40-43
both metallic
and transition metal complex catalysts
.
c. College of Biological Chemical Science and Engineering, Jiaxing University, Jiaxing,
Zhejiang, 314001, China. ygwang8136@mail.zjxu.edu.cn (Y. Wang)
† These authors contributed equally to this work.
Especially for the layered-structural MoS2, interlayer space was
expanded via different methods for HER catalytic activity
improving.44-46 Inspired by this, we proposed a facile strategy
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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to compress the interlayer space of MoS2 via F doping as electron microscope (TEM) image (Figure S4) aVniedw AthrtieclehOinglihne-
DOI: 10.1039/D0TA03622F
demonstrated in Scheme 1 to reduce the HER activity and angle annular dark field scanning transmission electron
therefore enhance the NRR selectivity by considering the small microscope (HAADF-STEM) image of F-MoS2 (Figure 1c)
size of F ion and great electronegativity of F. Moreover, first- illustrates the ultrathin feature of the
F doped MoS2
principle calculation reveals that the introduction of F ions nanoflakes of F-MoS2. The corresponding EDS elemental
would also significantly lower the uphill energy for the mappings displayed in Figure 1d-f reveal that the F-MoS2
hydrogenation of adsorbed N2, leading to the formation of catalyst is predominantly composed of Mo and S with F atoms
highly active NRR catalytic centers. Due to the simultaneously doped uniformly.
enhanced NRR selectivity and activity, FE toward NRR of the F
doped Mo2S (F-Mo2S) catalyst can be therefore strongly
boosted 20.6% with the NH3 yield high to 35.7 μg h-1 mgcat-1 at
-0.2 V vs. RHE.
Scheme 1. Schematic diagram of strain engineering via F doping on the edge site of
MoS2 nanosheet.
Figure 1. (a) XRD patterns of the pristine MoS2 and F-MoS2 catalysts; (b) SEM image, (c)
HAADF-STEM image and (d-f) corresponding EDS elemental mapping images of the F-
MoS2 catalyst.
Results and discussion
High-resolution transmission electron microscopy (HR-TEM)
images are displayed in Figure 2 to in-detail reveal the micro-
morphology and strain effect of the F-MoS2 flake. It can be
detected from the HR-TEM images on edge of the F-MoS2
flakes (Figure 2a-b) that the F-MoS2 flakes mainly consist of 3-6
S-Mo-S layers with numerous defects engineered. By
measuring the thickness of a typical 5-layer nanosheet in
Figure 2b, distance of a single S-Mo-S layer can be calculated
to be 0.58 nm, which is decreased by 8% compared with the
pristine defect-rich MoS2 nanosheet prepared from the same
strategy 48. For the purpose of further confirming the effect of
F doping on compression of the MoS2 interlayer distance,
Raman analysis was carried out on both F-MoS2 and pristine
In order to confirm the successful synthesis of MoS2 and the
successful doping of F ions in F-MoS2, X-ray diffraction (XRD)
analysis was carried out (Figure 1a). All the diffraction peaks of
F-MoS2 agree well with the standard pattern of hexagonal
MoS2 (JCPDS card No. 73-1508), revealing the high purity of
the product, while all the characteristic peaks of F-MoS2 are
negatively shifted as desired due to the smaller size and higher
electronegativity of F than S. Especially, this negatively shifted
(002) peak indicates that the layer spacing of the F-MoS2
catalyst was slightly compressed since (002) facet is related to
48
the interlayer of hexagonal MoS2.47,
Scanning electron
microscope (SEM) image of the F-MoS2 catalyst in Figure 1b
reveals that the layered structure of the pristine MoS2 (SEM
image in Figure S1) was remained in F-Mo2S. Meanwhile, the
corresponding energy dispersive spectrum (EDS) reveals that
atomic ratio of F in the F-MoS2 catalyst is 1.12%, slightly higher
than designed due to the better retention ability of F during
the hyrothermal synthesis. By increasing the atomic ratio of S
to F to 2:100 during the synthesis process, the F-MoS2-2
sample was prepared. It can be found that the morphology of
F-MoS2-2 is remarkably changed compared with F-MoS2
according to the SEM image of F-MoS2-2 (Figure S2). The MoS2
layers are accumulated for F-MoS2-2 and the specific surface
area would be strongly decreased due to the higher F content.
Specific surface areas were therefore characterized using the
Brumauer-Emmett-Teller (BET) method with N2 adsorption-
desorption isotherms displayed in Figure S3. The BET surface
area of F-MoS2-2 was decreased from 18 m2/g of F-MoS2 to 12
m2/g as expected although more F ions would lead to extra
defects. The observed wrinkled structure with crumpled edge
and high transparency shown in both the transmission
MoS2 (Figure S5). Note that both the E1 and A1g bands of the
2g
F-MoS2 Raman spectrum shift to a lower wavenumber
compared with the pristine MoS2, indicating that layered-
structural MoS2 sheets were compressed by the doped F ions
49, 50. HR-TEM image on F-MoS2 surface (Figure 2c) shows that
defects are exsited. Note that numerous crystal fringes are
discontinuous in Figure 2c, indicating the existence of 1T/2H
phase interfaces, which is further manifested by the selected
area electron diffraction (SAED) pattern (insert of Figure 2c) in
the form of six independent diffraction arcs, a representative
quasiperiodic structure with rich defects.
2 | J. Name., 2012, 00, 1-3
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With the saturated gas in electrolyte changed to N2, acidic LSV
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of both MoS2 and F-MoS2 are positively sDhOifIt:e1d0.1c0o3m9/pDa0rTeAd03w62it2hF
those collected in Ar-saturated electrolyte due to the widely
approved NRR activity of MoS2 (Figure S8). Especially at 1
mA/cm2, overpotential of MoS2 was decreased by 11 mV,
attributed to reduction of N2, while this value is increased to
23 mV for the F-MoS2 catalyst. This significant difference
between the LSV curves of F-MoS2 in Ar and N2 condition
indicates the great selectivity of F-MoS2 on NRR. Similary, LSV
current density difference of F-MoS2-2 with higher F content
measured in Ar and N2 condition is also remarkable (Figure S9)
although the activity in both Ar and N2 conditions was
decayed due to the reduction of defects. By directly comparing
the LSV curves of the two catalysts collected in N2 saturated
0.05 M H2SO4 solution (Figure 3d), current density decrement
at -0.3 V vs. RHE ascribed to F doping is significantly reduced to
28.4%, ca. half of that in Ar saturated electrolyte. More
importantly, current density of F-MoS2 is high to 2.36 mA/cm2,
which is even 22.6% higher than that of the pristine MoS2
Figure 2. HR-TEM images on (a-b) edge and (c) surface of the F-MoS2 catalyst.
Sub-angstrom resolution HAADF-STEM image on surface of the
F–MoS2 were taken to clearly identify the defects and phase
interfaces of the F-MoS2 catalyst (Figure 3a). A trigonal lattice
structure corresponding to the 1T–MoS2 and the common
honeycomb lattice area of the 2H–MoS2 are both clearly visible
with interphase lined. At the same time, the uniformly dark
regions as marked in red circles confirm the existence of
defects on the F-MoS2 surface. As a supplementary evidence
for the extra surface defects, which are not as effective on
electrochemical catalysis as the edge defects, formed by F
doping. The F-MoS2 catalyst possesses higher BET surface area
(18 m2/g) compared with the pristine MoS2 (10 m2/g) due to
the remained edge defects and additionally generated surface
defects. X-ray photoelectron spectroscopy (XPS) analysis was
subsequently executed to in detail study the elemental
composition and surface chemical state of the F-MoS2 catalyst.
The survey XPS spectrum of the F-MoS2 catalyst (Figure S6)
confirms the existence of F in the F-MoS2 catalyst and the F
content in the F-MoS2 is 1.04 atom.%, very close to the SEM-
EDS result. The Mo 3d high resolution XPS spectrum of the F-
MoS2 catalyst (Figure 3b) reveals a doublet of predominantly
characteristic 1T phase peaks of Mo 3d3/2 and Mo 3d5/2 located
at 231.4 eV and 228.2 eV, which are much lower than that of
their 2H phase counterparts (232.1 eV and 229.0 eV).
Negatively shifted S 2p peak (Figure S7) can confirm the
transfer of MoS2 from 2H to 1T phase in the F-MoS2 catalyst.
Interestingly, further quantitative analysis based on the
deconvoluted Mo 3d resonances exhibited that content of 1T
phase in the F-MoS2 catalyst is increased compared with the
pristine MoS2 although the interlayer distance was
compressed.
catalyst at -0.25
V vs. RHE. The strongly changed
electrochemical catalytic property caused by the ambient
adjustment can confirm the great selectivity of active sites in
F-MoS2 toward NRR.
Figure 3. (a) Sub-angstrom resolution HAADF-STEM image on surface of the F–MoS2
flake. (b) Mo 3d high resolution XPS spectra and the pristine MoS2 and F-MoS2 catalysts.
LSV curves of MoS2 and F-MoS2 measured in (c) Ar and (d) N2 saturated 0.05 M H2SO4
solutions.
Although the additional defects and the enhanced 1T phase
content in F-MoS2 should be benificial for HER catalysis in
common sence, HER activity of MoS2 was strongly suppressed
by the doped F ions due to the strain effect according to the
linear sweep voltammogramm (LSV) curves (Figure 3c). The
HER current density at -0.3 V vs. RHE was strongly decreased
by 43.5% for the F-MoS2 catalyst compared with the pristine
MoS2 atalyst. It can be therefore indicated that HER activity of
surface defects as well as phase interfaces of MoS2 is much
lower than that of the edge-defects. In the case of F-MoS2,
activity of edge-defects was strongly reduced due to the
compression effect of F ions. As a result, it is reasonable to
imply that F-MoS2 may possess greater acidic NRR selectivity.
Since great NRR selectivity of F-Mo2S in acidic condition was
experimentally confirmed, first-principle calculation was
subsquently deployed to theoretically evaluate the effect of F
doping on the catalytic NRR activity of MoS2. The distal
pathway in the association mechanism is more feasible than
other pathways since it allows stabilizing the intermediates:
*+N2(g)→*N2→*NNH→*NNH2→*N+NH3(g)→*NH→*NH2→*N
H3→*+ NH3(g)
Mo sites on the edge of MoS2 layers were chosen as the
primary active sites of the NRR process since the Mo atom has
both empty d orbitals which can accept the long-pair electrons
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from N2 molecules and the occupied d orbitals which are NH3 yield calculated from the partial current densities for NH3
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capable of reducing the N2 triple bond significantly. The free production at various applied potential DoOnI:M10o.1S0239a/Dnd0TAF0-M36o22SF2
energy changes and the structures of each step of both MoS2 are compared in Figure 5a. Although overall current densities
and F-MoS2 in NRR process are displayed in Figure 4. It can be on these two catalysts are increased with the applied potential
noted that free energy change of nitrogen adsorption on F- during the CA test (Figure S10), NH3 yield is undependent on
MoS2 edge (-0.85 eV) is significantly lower than MoS2 (-0.38 eV), the applied potential. By considering the catalyst loading on
which suggested that the introduction of F atom can enhance the electrode, average NH3 yield at -0.20 V vs. RHE of the F-
the N2 absorption ability on F-MoS2. The Gibbs free energy MoS2 catalyst is high to 35.7 μg h-1 mgcat-1, which is among the
diagram for MoS2 (Figure 4a) showed that the first best reported cost-effective NRR catalysts (Table S1). At the
hydrogenation step of the adsorbed N2 molecule occurs same time, this value is also higher than those measured at -
difficultly with an uphill free energy change of 0.64 eV. For the 0.15 V and -0.25 V vs. RHE. In contrast, NH3 yield catalyzed by
subsequent elementary reaction steps of the distal mechanism, MoS2 at -0.2 V vs. RHE is ca. 50% lower compared with that
the hydrogenation of *N-NH2 was followed by the release of catalyzed by F-MoS2 as desired although NH3 yield of MoS2 at -
first NH3 and then the *N intermediate was formed. It can be 0.15 V vs. RHE is much higher than that of F-MoS2. As another
therefore considered that the formation of *N-NH, which important descriptor of NRR performance, FE of the catalysts
bears the highest uphill free energy of 0.64 eV, is the rate- was calculated (detail in Supporting Information) to evaluate
determining step of NRR on MoS2. For F-MoS2, the Gibbs free the selectivity of NRR catalysts (Figure 5b). As expected, NRR
energy change of the same rate-determining step is only 0.36 FE of the F-MoS2 catalyst is much greater than that of the
eV, which is 44% lower than that of MoS2 (Figure 4b), pristine MoS2 catalyst at applied potentials ranging from -0.20
aggreeing well with the significantly reduced NRR onset V to -0.30 V vs. RHE. Especially at -0.2 V vs. RHE, NRR FE of the
potential of F-MoS2 comapred with the pristine MoS2.
F-MoS2 catalyst can reach 20.6%, which is among those of the
best reported noble-metal-free catalysts (Table S1) and is also
much greater than that of the pristine MoS2. In order to
confirm the superior NRR catalytic activity of F-MoS2, we also
carried out the Nuclear Magnetic Resonance (NMR) tests to
quantitatively analyze the ammonia concentration from the
+
established standard curve (Figure S13).52 An intensive 14NH4
NMR signal was detected for F-Mo2S and the calculated
Faradaic efficiency and NH3 yield rate was similar to the UV-vis
+
method shown in Figure 5c. Also, only 15NH4 signal was
recorded with 15N2 as feeding gas depicting that and the N2 gas,
other than the residual contaminants from the catalyst
synthesis, was the nitrogen source for the formation of NH3.
Durability of NRR on F-MoS2 was subsequently investigated by
repeatedly performing the CA tests with NH3 yield (Figure 5d)
and NRR FE (Figure S14) collected. Both great activity and
Figure 4. Gibbs free energy diagrams of distal pathways for NRR on (a) F-MoS2 and (b) selectivity can be retained during the durability test. NH3 yield
is higher than 30 μg h-1 mgcat-1 and FE is higher than 20% for all
the five repeated tests. After 10 h continuously testing, NRR FE
F-MoS2 at different applied potentials. (White, yellow, red, cyan and blue balls
represent the H, S, F, Mo and N atoms, respectively.)
of NH3 production on the F-MoS2 catalyst is calculated to be
Since the effect of F doping on promotion of NRR selectivity
26%, which is even higher than that of the first two hours.
and activity of F-MoS2 are confirmed, chronoamperometry (CA)
measurements were therefore carried out in N2 saturated 0.05
M H2SO4 electrolyte (Figure S10) to quantitatively analyze the
NRR performance of F-MoS2 by utilizing the traditional
indophenol blue method 51. Typical double-cell three-electrode
electrochemical setup with catalyst coated glassy carbon
electrode as working electrode were used for the CA
measurements and the calibration curves are displayed in
Figure S11. UV-vis absorption spectra of the electrolyte stained
with indophenol indicator after 2 h electrolysis at a series of
potentials for MoS2 and F-MoS2 are displayed in Figure S12. It
is suggested that both MoS2 and F-MoS2 can catalyze NRR at -
0.15 V vs. RHE and potentials ranging from -0.30 V to -0.15 V vs.
RHE are selected to study the NRR electrocatalysis
performance.
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where ΔE is obtained directly from DFT calculatVioienwsA,rtiΔclZePOEnlinies
the change in zero-point energies (ZPE),DTOIi:s10th.1e03t9e/Dm0pTeAr0a3t6u2r2eF
(298.15 K), and ΔS is the change in entropy of products and
reactants.
Details on material characterizations, electrochemical
measurements and quantification of ammonia are provided in
Supporting Information.
Conclusions
In summary, F ions were employed into the lattice of layered-
structural MoS2 to compress the interlayer space to
simultaneously improve the NRR selectivity and activity in
acidic condition. HER activity of the F-MoS2 catalyst is strongly
reduced due to the strain effect and the NRR selectivity of the
F-MoS2 catalyst is therefore strongly enhanced. Since NRR
activity of Mo2S can also be significantly boosted by the doped
F ions according to the first-principle calculation, F-Mo2S was
therefore considered as a superior NRR catalyst with relatively
low cost. At relatively low overpotential (-0.2 V vs. RHE), NH3
Figure 5. (a) Average yield rate of NH3 production and (b) NRR FE at different applied
potentials on MoS2 and F-MoS2; (c) NMR spectra of electrolyte after NRR catalysis by F-
MoS2 (insert: comparison of Faradaic efficiency and NH3 yield rate of F-MoS2 between
UV-vis and NMR methods); (d) 10
h durability test for F-MoS2 toward N2
electrochemical reduction at -0.2 V vs. RHE.
-1
yield resulted from NRR is high to 35.7 μg h-1 mgcat with a
great NRR FE of 20.6%. By considering the great durability of F-
MoS2, this work provides a facile strategy for the destination of
efficient N2 fixation catalysts.
Methods
Catalyst synthesis. The F-MoS2 catalyst was prepared by a
45,
48
conventional hydrothermal method
.
2.2836
g
Conflicts of interest
There are no conflicts to declare.
sulfocarbamide as S source and 11.1 mg NH4F as F source
(atomic ratio of S to F is set to be 100:1) were dissolved in 40
mL deionized water under stirring for 30 min. Subsequently,
0.2358 g ammonium molybdate was added to form a
homogeneous solution. The mixed solution was then
transferred into a 50 mL Teflon-lined stainless steel autoclave
and kept at 220 oC for 18 h. After cooled to room temperature,
the product was washed with deionized water and ethanol for
three times to remove any residual reactants. Same
procedures were used for the pristine MoS2 preparation
without NH4F.
Acknowledgements
We are grateful for financial support from the National Natural
Science Foundation of China (Nos. 21875224, 21703211 and
21703212).
Notes and references
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First-principle calculation. The calculations were performed
53-55
with the Vienna Ab-initio Simulation Package (VASP).
Perdew-Burke-Ernzerhof (PBE) approximation
The
56
of density
functional theory (DFT) was used to describe the exchange-
correlation interactions and the projector augmented wave
(PAW) method 57 to account for core-valence interactions. The
energy cutoff for plane wave expansions was set to 400 eV,
and the reciprocal space was sampled by a 2×2×1 grid which is
generated by VASPKIT 58. At least 15 Å vacuum layer was
applied in the z-direction of the slab models to prevent the
vertical interactions between slabs. All structures were fully
relaxed to the ground state and spin-polarization was
considered in all calculations. The convergence thresholds for
structural optimization was set at 0.02 eV/Å in force and the
convergence criterion for energy is 1×10−5 eV.
The reaction Gibbs free energy changes (ΔG) for each step
were based on the computational hydrogen electrode model
proposed by Nørskov et al 59, 60, and can be calculated using
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ΔG = ΔE + ΔZPE - TΔS + ΔGU + ΔGpH
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Journal of Materials Chemistry A
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DOI: 10.1039/D0TA03622F
Interlayer space of MoS2 was compressed by the substituted F ions for the purpose of
rationally boosting the NRR activity and selectivity.