A highly active defect engineered Cl-doped carbon catalyst for the N2reduction reaction

Article abstract of DOI:10.1039/d0ta11298d

eNRR is a promisingly environment-friendly strategy to obtain ammonia under ambient conditions. For metal-based catalysts, the competitive adsorption of H⁺over N₂remains the primary hurdle for high ammonia yield and faradaic efficiency. Carbon-based metal-free catalysts attract our attention for the potential of being promising NRR catalysts due to their natural low HER activity. In this study, we prepared a modified Zn-based metal-organic framework (Zn-BTC) decorated with Cl ions by simply adding NaCl in the synthetic process. The existence of Cl ions in the framework contributes to the generation of large pores during thermolysis. Moreover, -Cl and -COCl species were formedin situand connected with carbon atoms in the defect area. Benefitting from such a structure, this catalyst achieves a high ammonia yield rate of 103.96 μg h^?1mg_cat^?1and a faradaic efficiency of 21.71% at room temperature. Theoretical results also proved that the newly produced -COCl and -Cl functional groups could attract more electrons from adjacent carbon atoms due to their stronger electronegativity, thus improving its affinity towards N₂while lowering the HER activity as reactive sites.

Full text of DOI:10.1039/d0ta11298d

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Materials Chemistry A
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A highly active defect engineered Cl-doped carbon
catalyst for the N2 reduction reaction†
Cite this: J. Mater. Chem. A, 2021, 9,
5807
Minxue Huang, ‡a Shipeng Gong, ‡a Sini Wang,c Kang Yang,
Shi Chen,
a
a
ad
c
ab
*
*
Changlai Wang,
Xiangkai Kong
and Qianwang Chen
eNRR is a promisingly environment-friendly strategy to obtain ammonia under ambient conditions. For
metal-based catalysts, the competitive adsorption of H+ over N2 remains the primary hurdle for high
ammonia yield and faradaic efficiency. Carbon-based metal-free catalysts attract our attention for the
potential of being promising NRR catalysts due to their natural low HER activity. In this study, we
prepared a modified Zn-based metal–organic framework (Zn-BTC) decorated with Cl ions by simply
adding NaCl in the synthetic process. The existence of Cl ions in the framework contributes to the
generation of large pores during thermolysis. Moreover, –Cl and –COCl species were formed in situ and
connected with carbon atoms in the defect area. Benefitting from such a structure, this catalyst achieves
Received 19th November 2020
Accepted 23rd December 2020
a high ammonia yield rate of 103.96 mg hÀ1 mgcat and a faradaic efficiency of 21.71% at room
À1
temperature. Theoretical results also proved that the newly produced –COCl and –Cl functional groups
could attract more electrons from adjacent carbon atoms due to their stronger electronegativity, thus
improving its affinity towards N2 while lowering the HER activity as reactive sites.
DOI: 10.1039/d0ta11298d
rsc.li/materials-a
strategies to elevate the selectivity are at the expense of the
ammonia yield.10,11 As such, several transition metal-based
Introduction
catalysts were studied due to their available d-orbital and
donable p-electrons, but their unfavorable affinity towards N2,
poor conductivity, and strong agglomeration tendency impedes
their application.12
The major challenge encountered in industrial-scale ammonia
production is developing an alternative way to replace the
energy-consuming Haber–Bosch process. Electrocatalytic
nitrogen reduction reaction (eNRR) is a promising route at
room temperature and atmospheric pressure. The critical
bottleneck lies in adsorbing and activating nitrogen for the high
bond energy of nonpolar N^N triple bond.1 Highly active
electrocatalysts that create active sites for absorbing N2 inten-
sively and retarding HER as much as possible are desired to
achieve efficient nitrogen xation. A pioneering study found
that numerous noble metal-based catalysts (e.g., Ru, Au, and
Pt)2–9 favor the competitive adsorption of H+ over N2, which
causes unsatised efficiency. As reported, the faradaic efficiency
of most Au-based catalysts are below ten percent and further
Compared with the metal-based catalysts, it has been
demonstrated that carbon-based metal-free materials can be
the ideal candidates for NRR catalysts owing to their inert
reaction for HER and numerous physical properties such as low
cost and stable corrosion resistance.13,14 Previous theoretical
studies have suggested that an efficient NRR electrocatalyst may
have empty orbits to accept the lone pair electrons of nitrogen.15
In this view, a perfect graphene network with uniformly
distributed sp2 electrons is not an active electrocatalyst.
Therefore, some researchers have put effort into tuning the
electronic structure by replacing the C atoms in the network
with heteroatoms such as N, P, and B.16,17 However, the different
atomic radius between C and heteroatoms increases the diffi-
culty of this method. Little attention has been paid to the
oxygen-containing functional groups in the carbon framework
as they can also be used to change the electronic structure of the
adjacent carbon atoms due to their electron affinity. There exist
abundant O species, such as –OH and –COOH, particularly in
the defect area of the carbon matrix.18 As a result, it provides
a much feasible approach to create defects in the carbon
network and modify the O species for increasing their electro-
negativity. The halogen ions with a strong electronegativity are
a suitable choice to be used to modify the O species through the
aHefei National Laboratory for Physical Science at Microscale, Department of
Materials Science and Engineering, University of Science and Technology of China,
Hefei 230026, China. E-mail: cqw@ustc.edu.cn
bAnhui Province Key Laboratory of Condensed Matter Physics at Extreme Conditions,
High Magnetic Field Laboratory of Chinese Academy of Sciences, Hefei 230031, China
cKey Laboratory of Green and Precise Synthetic Chemistry and Application, Ministry of
Education, Anhui Province Key Laboratory of Pollutant Sensitive Materials and
Environmental Remediation, Huaibei Normal University, Huaibei, Anhui 235000,
China. E-mail: kxk@chnu.edu.cn
dDepartment of Materials Science and Engineering, Center of Super-Diamond and
Advanced Films, City University of Hong Kong, Kowloon, Hong Kong, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta11298d
‡ These authors contributed equally to this work.
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substitution reaction.19 Among all halogen ions, Cl ions are less and CR (without saline) samples were amorphous carbon with
corrosive and safer than FÀ, BrÀ, and IÀ, and NaCl is the partial graphitization. The broad peaks located at 23ꢀ and 43ꢀ
cheapest halogen precursors. Thus, it gives us a novel idea to are assigned to (002) and (100) planes of the graphitic carbon.
gra Cl atoms on the carbon to obtain a superb ammonia Two predominant peaks at approximately 1344 cmÀ1 and
yielding catalyst with a better nitrogen affinity and much higher 1596 cmÀ1 in the Raman spectra were used to study the degree
selectivity.
of graphitization. The two overlapping peaks in Fig. S2† could
Herein, we used a facile way to introduce Cl atoms into be deconvoluted into four individual peaks (D1, D3, D4, and G).
graphene catalysts by mixing the saline solution with the In general, the G band is relevant to the E2g vibration mode of
precursor solution at room temperature. During pyrolysis, the the defect-free sp2 carbon structures, and the D1 band is asso-
Cl atoms were in situ linked to carbon atoms in graphene to ciated with the A1g symmetric vibration of edge plane defects,
form C–Cl and –COCl species. By quantifying the NH3 yield, an where the ratio of ID /IG could deduce the content of defects in
1
ammonia-yielding rate of 103.96 mg hÀ1 mgcatÀ1 and a faradaic the samples. As for DC-Cl, the value of ID /IG was found to be
1
efficiency of 21.71% were observed. Further, the isotopic 1.98, which being larger than 1.47 of CR revealed a much higher
labelling of 15N2 experiments conrms catalytic performance defect degree. Besides, the high-resolution transmission elec-
towards NRR. The NEXAFS spectra in combination with theo- tron microscope (HRTEM) images (Fig. 1c and d) also reveal
retical calculations proved that the formation of new functional that DC-Cl is full of nanoholes with diameter around 50 nm,
groups (–Cl and –COCl) was benecial to the active carbon sites while CR exhibited negligible nanoholes (Fig. 1e), thereby cor-
of NRR catalysts.
responding to the value in the BET text (Fig. S3 and Table S1†).
The distinct porous structure was rationalized by the etching
effect of NaCl and the high-temperature pyrolyzation. The
decomposition of ZnBTC to ZnO occurred at around 450 ꢀC,
while CO2, CO, and Zn were evaporated through ZnO + C /
Zn(g) + CO at a high temperature. During this period, more
number of nanoholes and exposed surfaces were generated,
which contributed to a promising electrochemical perfor-
mance. The detailed pyrolysis process revealed by the ther-
mogravimetric analysis (Fig. S4†) also proved that more weight
loss occurred in the samples with saline. The images of the
Results
The facile synthesis of DC-Cl precursors was carried out by
adding a mixed solution of NaCl and Zn(CH3COO)2$2H2O into
a 1,3,5-benzenetricarboxylate solution (Fig. 1a). NaCl not only
contributes to the porous structure but also provides the source
of Cl. The products were washed and then pyrolyzed in a N2
atmosphere at 900 ꢀC for 3 h. The X-ray diffraction patterns
(Fig. S1†) directly identied that both the DC-Cl (with saline)
Fig. 1 (a) Schematic of the synthetic process of DC-Cl. (b) The FESEM image of DC-Cl. (c and d) The transmission electron microscopy (TEM)
images of DC-Cl. (e) The HAADF-STEM image of CR (inset: high-resolution transmission electron microscopy (HRTEM) image of CR). (f) The
HAADF-STEM image of DC-Cl. (g–i) Images of elemental mapping of C, O, and Cl, respectively.
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elemental mapping of DC-Cl shows that C, O, and Cl are and Cl 2p, respectively. The XPS spectrum of C 1s can be
present, while only C and O elements were present in CR deconvoluted into four peaks, which belonged to C]C (284.8
(Fig. 1f–h and S5†).
eV), C–O (285.5 eV), C–Cl (286.2 eV), and C]O (288.3 eV)
To further prove the existence of Cl and investigate its (Fig. 2c).30 Correspondingly, the Cl 2p peaks located at 198.5 eV
modulation effects, the Fourier transform infrared spectroscopy (2p3/2) and 200.1 eV (2p1/2) are attributed to the C–Cl bond
(FTIR) analysis of samples before and aer pyrolyzation was (Fig. S8†).31,32 The contact angles of DC-Cl are smaller than those
carried out, and the results are compared in Fig. 2a. Two of CR, which means that the better wettability of DC-Cl was
representative peaks at 622.4 cmÀ1 and 698.6 cmÀ1 are attrib- inuenced by the Cl ion embedment.33 Besides, N2 temperature-
uted to the C–Cl vibrations. This proves that chlorine atoms programmed desorption (N2-TPD) measurements were also
were linked to the precursors via forming new bonds with performed. As shown in Fig. 2d, DC-Cl exhibited a broad
carbon. It can be seen from Fig. S6† that a blue shi is observed temperature region for N2 desorption. This can be explained by
in the bands of both C]O and C–C stretching vibrations, the multiple contributions from both the porous structure and
conrming that the adsorbed chlorine ions were successfully Cl modulation. Moreover, it displays a desorption edge at round
introduced into the nanoholes of precursors.20,21
500 ꢀC, much higher than CR (ca. 300 ꢀC), thereby suggesting its
Fig. 2b shows the NEXAFS spectra for CR and DC-Cl. The stronger N2 binding strength than CR (Fig. 2d). This is bene-
sharp resonance around 285.4 eV results from the p* excita- cial for adsorbing more nitrogen molecules on the DC-Cl
tions of C–C bonds. A slight positive shi of 0.3 eV is observed surface, in turn enabling the free inert N2 to get activated for
for DC-Cl, which can be attributed to a higher valence of carbon further reduction.
atoms caused by the Cl modulation.22–25 Apparently, two new
The electrocatalytic nitrogen xation was carried out in an H-
peaks at 297.4 eV and 300.1 eV appeared in case of DC-Cl, with type cell at room temperature and atmospheric pressure. Before
the peaks being assigned to s*CÀCl resonance, indicating that the electrochemical tests, the solution was bubbled by owing Ar or
incorporated Cl ions existed in the form of sp3 interactions N2 for 30 min. LSV curves of DC-Cl in Ar- or N2-saturated 0.05 M
within the carbon matrix.26–29
H2SO4 are provided in Fig. S9.† A higher current density was
The presence of Cl ions and their bonding format was observed in the N2 atmosphere, and it indicates the occurrence
further investigated by X-ray photoemission spectroscopy (XPS). of N2 reduction and NH3 production under this condition.
The survey spectrum (Fig. S7†) of DC-Cl exhibits three peaks at According to the polarization prole, the NRR performance was
285 eV, 535 eV, and 200.8 eV, which are attributed to C 1s, O 1s, measured from À0.6 V to À1.0 V versus RHE for two hours. The
Fig. 2 (a) Fourier transform infrared spectroscopy (FTIR) of samples before (ZnBTC + NaCl) and after pyrolyzation (DC-Cl). (b) Near Edge X-ray
Absorption Fine Structure (NEXAFS) of CR and DC-Cl. (c) The XPS spectra of C 1s of DC-Cl and CR (inset: contact angles of DC-Cl and CR). (d) N2-
TPD profiles of DC-Cl and CR.
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Fig. 3 . (a) Chronoamperometry curves from À0.6 V to À1.0 V for 2 h. (b) UV/vis absorption spectra from À0.6 V to À1.0 V. (c) NH3 yield (red
column) and faradaic efficiency (blue line) from À0.6 V to À1.0 V. (d) Summary of the catalytic performances for some reported NRR elec-
trocatalysts. (e) NH3 yield (red column) and faradaic efficiency (blue line) of DC-Cl during 6 consecutive cycles at À0.8 V. (f) Ammonia yield and
concentration of NH4+ in the electrolyte with different catalyst loadings using ion chromatography.
corresponding chronoamperometry curves recorded in Fig. 3a catalysts (Fig. 3d). For comparison, NaF and NaBr were also
shows the good stability for DC-Cl during the nitrogen reduc- used for the catalyst preparation, while the measurements
+
tion process. The yields of NH4 were calculated by both the demonstrated that NaCl possesses higher activity towards N2
colorimetric method and cation chromatography method reduction and NH3 production (Fig. S13†).30,31,34,35 Notably, both
(Fig. S10†). As for DC-Cl, the UV/vis absorption spectra at each the ion chromatography and colorimetric measurements illus-
+
voltage are given in Fig. 3b, and the highest NH4 yield is trate that the ammonia concentration was scaled up over time
attained at À0.8 V versus RHE. Accordingly, the highest and extended time reduction leads to an equal yield rate, which
ammonia yielding rate was 103.96 mg hÀ1 mgcatÀ1, which is 7.0 proved the indeed NH4 production from N2 reduction
+
times as high as that of CR without Cl modulation (Fig. 3c and (Fig. S15†). DC-Cl was pyrolyzed in a hydrogen ow to remove Cl
S13†). The corresponding faradaic efficiency was found to be atoms and obtain DC, with the ammonia rate decreasing
16.86%. When the potential was increased to À0.6 V versus dramatically. The long-term stability test shows that DC-Cl can
RHE, the measured faradaic efficiency reached 21.71%. Both of maintain good durability for six successive cycle measurements
these two descriptors were superior to most of the reported at À0.8 V vs. RHE of 2 h (Fig. 3e). The charge transfer resistance
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(Rct) before and aer NRR was found to be 3.926 U and 4.837 U, functional theory (DFT) calculations. As displayed in Fig. S23,†
respectively, and the ignorable changes imply the structural the perfect graphene was inert to adsorb the N2 molecule. Based
robustness of the catalysts (Fig. S20†). There is nearly no vari- on the experimental results, two couple of models, involving
ation in the XRD spectra and TEM morphology of DC-Cl aer DC-OH versus DC-Cl (DC with –Cl functional group) and DC-
the cycling tests, demonstrating the robust nature of the catalyst COOH versus DC-COCl, were constructed for comparison. As
(Fig. S21 and S22†).
shown in Fig. S24,† the DG values of *2H adsorption for DC-Cl
Nørskov et al. have reported that the exact quantication of and DC-COCl were À2.08 eV and À2.23 eV, which were lower
ammonia from electrochemical nitrogen xation is facilely than those for DC-OH (À2.01 eV) and DC-COOH (À2.19 eV),
affected by the contamination from air, electrolytes, ion- respectively. As the optimal |DG| for HER should be close to 0,
conducting membranes, or nitrogen-containing compounds the smaller DG values obtained on both the DC-Cl and DC-COCl
in nitrogen gas stream, or even from the catalysts itself.36 suggest that Cl modulation can effectively suppress the
Herein, a series of control experiments were performed to hydrogen generation process on DC, thereby leading to more
demonstrate the yield of ammonia that was actually derived rapid kinetics for NRR. The energy proles for the electro-
from the electrochemical reduction of N2. First, the Ar or N2 gas catalytic NRR processes are evaluated in Fig. 4a and b, which are
was puried through a ltration container lled with 0.05 M associated with their corresponding optimized structures
H2SO4 before the gas ow into an H-type cell. The H2SO4 elec- shown in Fig. 4c, d and S25.† It is clear from the gure that DC-
trolyte and solution involved in the indophenol-blue method OH needs much higher energy to overcome the barriers of N2
were prepared freshly in order to eliminate the possible accu- molecule adsorption (N2 / *N2, 1.99 eV) and hydrogenation of
mulation of ammonia in the lab environment. The Naon *NNH (*NNH / *NHNH, 2.14 eV). Apparently, DC-Cl was
membrane was washed by mixed acid and freshwater before much facile for N2 molecule adsorption, exhibiting a signi-
each electrochemical test. Besides, we performed measure- cantly smaller energy barrier of 0.18 eV, which is in accordance
ments with Ar under the same conditions as the N2 electro- with the above N2-TPD observations, as discussed in Fig. 2d.
reduction test, and the detected yield of ammonia was Besides, the hydrogenation of *NNH on DC-Cl becomes
extremely small (Fig. 3c). Note that all yields of ammonia completely barrierless, releasing 0.17 eV free energy. Moreover,
calculated above were excluded for calculating the yield under the rate-limiting step in NRR on DC-Cl changes to the desorp-
the Ar experiment. In addition, a control experiment of DC-Cl tion of *NH2, which requires 1.13 eV free energy, much lower
with N2 blowing for 2 h at an open circuit potential (OCP) was than the rate-limiting step for DC-OH (2.14 eV). Aer adsorbing
conducted, in which negligible ammonia was detected, thereby the N2 molecule, the bond length of N^N on DC-Cl was
˚
revealing the high purity of feeding gas without nitrogenous observed to be 1.26 A, which is larger than that on DC-OH (1.25
˚
impurity (Fig. S14†). Further, the crossover gas text was carried A), demonstrating that the doping of Cl was benecial to
out in N2 or Ar saturated electrolytes for six cycles (Fig. S17†). It weaken N^N bonds. Similar results can also be obtained on
also proves that the produced ammonia was evolved from the DC-COCl (Fig. 4b), which exhibits a more energetically efficient
feed gas N2 rather than environmental pollution. An electro- rate-limiting step for N2 molecule adsorption (1.74 eV) and is
reduction experiment with a blank electrode at À0.8 V vs. RHE also lower than that of DC-COOH. To sum up, the Cl modula-
(Fig. S14†) also conrms that the ammonia originates from the tion tends to exist in the form of –Cl and –COCl functionaliza-
electroreduction performance of our catalysts. To reliably verify tion, and both of them were evidenced to decrease the energy
that the produced NH3 originates from the electroreduction of barriers of the rate-limiting step for NRR, enabling a boosted
N2, the labeling experiments using 15N2 as a feeding gas was capability for N2 reduction on DC.
conducted, and the peak of 15NH4+ was detected in the 1H NMR
To further investigate the change in the electronic structure
spectra (Fig. S12†).
of defective carbon aer doping Cl atoms and the bonding
The relationship between the ammonia yield and the loading nature of the N2 molecule on the catalysts, Bader charge
of the catalyst was also explored (Fig. 3f). It is evident that the distribution, density of states (DOS), and charge density
increase in the catalyst loading leads to a higher current differences of these models were studied. For an ideal NRR
response in LSV curves and more ammonia production in the catalyst, it is expected to simultaneously possess empty orbitals
+
electrolyte (Fig. S18†). Furthermore, the concentration of NH4
to accept the electron pair of N2 and occupied orbitals to donate
increases almost linearly, and the calculated ammonia yield electrons to the antibonding orbital of the N2 molecule.15,39 As
nearly remain unchanged with loadings from 0.07 mg cmÀ2 to shown in Fig. S26,† the introduction of –Cl and –COCl species
0.21 mg cmÀ2 in the same conditions, excluding the effects of results in the decrease of electrons around the active carbon
low catalyst loading will lead to high or false performance. atoms due to their strong electronegativity. In Fig. S27,†
Further increasing the loadings will not lead to an equal compared with DC-OH and DC-COOH, the DOS of active sites of
performance because a thicker catalyst layer may affect the DC-Cl and DC-COCl moved towards lower energy, indicating
transfer of reactants (both electrons and protons).37,38 There- that the empty p orbitals of carbon atoms become energetically
fore, a catalyst loading of 0.14 mg cmÀ2 was optimal in this favorable to accept the lone-pair electrons from N2, which agree
electrochemical measurement.
with the above results obtained in the Bader analysis. Fig. 4e
In order to identify the actual catalytic sites and understand illustrates that the interaction between the p orbitals of C and
the Cl modulation effects on accelerating the adsorption and p* of N2 can induce the partial occupation of the formed p–p*
activation of N2 molecules for NRR, we carried out density orbitals. This means the electrons of carbon atoms were back-
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Fig. 4 The side-view and top-view structure models of intermediates in NRR for (a) DC-COCl. (b) DC-COOH. Gibbs free energy diagrams of the
N2 electroreduction for (c) DC-Cl and DC-OH. (d) DC-COCl and DC-COOH. (e) Electronic density of states (DOS) and schematic illustrations of
p orbitals of DC-Cl, p orbitals of the N2 molecule, and their interaction within the DC-Cl–N2 configuration. (f) Charge density differences of DC-
Cl with the adsorption of N2.
donated to p* of N2, thus weakening the bond strength of N^N, carbon atoms to make them act as newly recognized active sites
which is in favor of the following hydrogenation.40–42 Besides, it for NRR with high efficiency.
is also shown in Fig. 4f that the charge transfer between DC-Cl
and N2 was in a two-way format, namely charge accumulation
and depletion, which can be observed for both N2 molecule and
Conclusion
C atoms. This result is in accordance with the acceptance-
In summary, we developed a defect engineered Cl-doped carbon
donation process as described in the DOS calculations. The
catalyst for nitrogen reduction. This unique porous structure
same results can also be obtained on DC-COCl via the DOS and
was full of nanoholes with diameter around 50 nm and had
charge density differences, as displayed in Fig. S28.† Therefore,
a stronger ability to select and bind with N2 while suppressing
–Cl and –COCl can effectively tune the electronic structure of
the HER process. The graed Cl atoms modied the oxygen-
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containing functional groups in the carbon network and thus 11 Z. H. Xue, S. N. Zhang, Y. X. Lin, H. Su, G. Y. Zhai, J. T. Han,
formed –Cl and –COCl, which can tune the p orbital electrons of
carbon atoms and make them highly active sites. The catalyst
Q. Y. Yu, X. H. Li, M. Antonietti and J. S. Chen, J. Am. Chem.
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shows a superior NRR performance with the ammonia yielding 12 Y. T. Liu, X. Chen, J. Yu and B. Ding, Angew. Chem., Int. Ed.,
rate of 103.96 mg hÀ1 mgcat
and a faradaic efficiency of
2019, 58, 18903–18907.
À1
21.71%. Thus, this work provides a rational thought for 13 Z. Lin, Y. Yang, M. Li, H. Huang, W. Hu, L. Cheng, W. Yan,
designing a highly active metal-free catalyst for nitrogen
xation.
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C. Wang, L. Hu and Q. Chen, Angew. Chem., Int. Ed., 2019, 58,
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14 P. Jiang, J. Chen, C. Wang, K. Yang, S. Gong, S. Liu, Z. Lin,
M. Li, G. Xia, Y. Yang, J. Su and Q. Chen, Adv. Mater.,
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15 C. Ling, X. Niu, Q. Li, A. Du and J. Wang, J. Am. Chem. Soc.,
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Conflicts of interest
The authors declare no competing nancial interest.
16 Z.-H. Sheng, H.-L. Gao, W.-J. Bao, F.-B. Wang and X.-H. Xia, J.
Mater. Chem., 2012, 22, 390–395.
Acknowledgements
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18 C. Su, M. Acik, K. Takai, J. Lu, S. J. Hao, Y. Zheng, P. Wu,
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20 Y. Xin, C. Wang, Y. Wang, J. Sun and Y. Gao, RSC Adv., 2017,
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This study was supported by the National Natural Science
Foundation (NSFC, 51772283, 21271163, 21972145), the
National Key R&D Program of China (Grant No.
2016YFA0401801), the Fundamental Research Funds for the
Central Universities (WK2060000032) and the Hong Kong
Scholars Program (XJ2019022). The calculations were
completed on the supercomputing system in the Super-
computing Center of USTC. The authors thank the so X-ray
magnetic circular dichroism end station (XMCD) of National
Synchrotron Radiation Laboratory (NSRL) for assistance with
XANES measurements.
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