Single-Atom catalysts templated by metal-organic frameworks for electrochemical nitrogen reduction

Article abstract of DOI:10.1039/c9ta10206j

The electrocatalytic nitrogen reduction reaction (NRR) has much prospect for substituting the energy-consuming Haber-Bosch process. Nevertheless, its sluggish reaction kinetics and the competing hydrogen evolution reaction always result in limited ammonia yield and low faradaic efficiency (FE). In this work, an Fe-decorated porphyrinic metal-organic framework (MOF) is employed as a precursor to construct single-Atom Fe implanted nitrogen-doped carbon catalysts (Fe₁-N-C) through a mixed ligand strategy. Benefiting from the highly dispersed single-Atom Fe sites, hierarchically porous structure and good conductivity, Fe₁-N-C shows a FE of 4.51% and an ammonia yield rate of 1.56 × 10^-11 mol cm^-2 s^-1 at-0.05 V versus the reversible hydrogen electrode, superior to those of Co₁-N-C and Ni₁-N-C. Theoretical calculations reveal that Fe₁-N-C shows the lowest energy barrier of the rate-determining step during the NRR process, consistent with its highest activity obtained in experiments. This work reveals the unique potential of single-Atom catalysts for the electrochemical NRR and provides in-depth insights into the catalytic mechanism of the NRR.

Full text of DOI:10.1039/c9ta10206j

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Materials Chemistry A
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PAPER
Single-atom catalysts templated by metal–organic
frameworks for electrochemical nitrogen
reduction†
Cite this: J. Mater. Chem. A, 2019, 7,
26371
ab
ac
Rui Zhang,‡a Long Jiao,‡a Weijie Yang,
‡
Gang Wand and Hai-Long Jiang
*
The electrocatalytic nitrogen reduction reaction (NRR) has much prospect for substituting the energy-
consuming Haber–Bosch process. Nevertheless, its sluggish reaction kinetics and the competing
hydrogen evolution reaction always result in limited ammonia yield and low faradaic efficiency (FE). In
this work, an Fe-decorated porphyrinic metal–organic framework (MOF) is employed as a precursor to
construct single-atom Fe implanted nitrogen-doped carbon catalysts (Fe1-N-C) through a mixed ligand
strategy. Benefiting from the highly dispersed single-atom Fe sites, hierarchically porous structure and
good conductivity, Fe1-N-C shows a FE of 4.51% and an ammonia yield rate of 1.56 ꢀ 10ꢁ11 mol cmꢁ2
sꢁ1 at ꢁ0.05 V versus the reversible hydrogen electrode, superior to those of Co1-N-C and Ni1-N-C.
Theoretical calculations reveal that Fe1-N-C shows the lowest energy barrier of the rate-determining
step during the NRR process, consistent with its highest activity obtained in experiments. This work
reveals the unique potential of single-atom catalysts for the electrochemical NRR and provides in-depth
insights into the catalytic mechanism of the NRR.
Received 16th September 2019
Accepted 22nd October 2019
DOI: 10.1039/c9ta10206j
rsc.li/materials-a
adsorption of protons, further preventing the competitive
hydrogen evolution reaction.17 Thus, compared with other
Introduction
heterogeneous catalysts, some SACs might possess particular
advantages to suppress the HER process.
Single-atom catalysts (SACs), presenting the utmost utilisation
of metal atoms at the atomic scale, possess coordinatively
unsaturated metal atoms acting as active sites and provide
a great variety of opportunities for application in chemical
reactions.1–5 Due to their remarkable activity, high selectivity,
considerable stability and good recyclability, SACs are capable
of bridging the gap between homogeneous and heterogeneous
catalysts.6–13 Recently, SACs have been found to be promising in
electrocatalysis, such as the efficient CO2 reduction reaction
(CO2RR).14–16 Due to the atomic dispersion of metal atoms in
SACs, only top site adsorption is permitted, resulting in slug-
gish kinetics of hydrogen evolution.17 Moreover, the positively
charged metal atoms in SACs are effective to impede the
Ammonia synthesis via electrocatalytic nitrogen reduction
similar to the CO2RR is also performed at a potential range
overlapped with that of HER, where SACs might be also prom-
ising but are rarely reported so far.18–27 Reports have illustrated
homogeneous Fe-dinitrogen complexes as efficient NRR cata-
lysts.28,29 Besides, the Fe site has also been found to be essential
for the biological xation in nitrogenase.30–35 Based on the facts
above, single-atom Fe catalysts, with a well-dened structure
similar to Fe-dinitrogen complexes, might be promising elec-
trocatalysts for the NRR.36,37 Relevant theoretical predictions
have also been reported to support the promising NRR appli-
cation of Fe-N-C materials.38,39 Thus, the accurate construction
of SACs, especially single-atom Fe catalysts, is much needed in
the exploration of the efficient NRR.
aHefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory
of So Matter Chemistry, Collaborative Innovation Center of Suzhou Nano Science and
Technology, Department of Chemistry, University of Science and Technology of China,
Hefei, Anhui 230026, P. R. China. E-mail: jianglab@ustc.edu.cn
bSchool of Energy and Power Engineering, North China Electric Power University,
Baoding, Hebei 071003, P.R. China
Metal–organic frameworks (MOFs), a kind of porous crys-
talline structure assembled with metal clusters/ions and
organic ligands, have aroused widespread concern in the
research frontier.40–65 Featuring
a well-dened crystalline
cState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Fujian Institute of Innovation, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, P.R. China
dSSRL Materials Science Division, SLAC National Accelerator Laboratory and Stanford
University, 2575 Sand Hill Road, Menlo Park, CA 94025, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ta10206j
structure, a large surface area and excellent tailorability, MOFs
have been extensively applied for the rational construction of
efficient catalysts.40–65 On account of the structural regularity
and synthetic tunability, MOFs have also been proven to be
ideal precursors/templates to produce SACs with improved
metal loading via pyrolysis.66–70 In addition, the porous char-
acteristic of the MOF precursor can also be largely inherited to
‡ These authors contributed equally to this work.
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its derivative, which can greatly accelerate the catalytic process H2O three times. The sample was dissolved in chloroform (150
of MOF-derived SACs.
mL) and extracted with 1 M HCl three times and water twice.
Taking the above into consideration, single-atom Fe
embedded nitrogen-doped carbon was constructed with a por-
phyrinic MOF (PCN-222) for the investigation of the electro-
catalytic NRR. Based on ligand modulation, PCN-222(Fe), with
Synthesis of Co-TPPCOOMe and Ni-TPPCOOMe
The procedures were identical to those of Fe-TPPCOOMeCl,
except for the amount of CoCl2$6H2O (6.59 g) and NiCl2$6H2O
(6.59 g).
a
suitable molar ratio of Fe-TCPP (iron(III)meso-tetra(4-
carboxyphenyl)porphine chloride) and H2-TCPP (tetra(4-
carboxyphenyl)porphine), was synthesized. With the assis-
tance of the high-content of H2-TCPP in PCN-222(Fe), the
distance of the neighboring Fe atoms in Fe-TCPP can be effec-
tively expanded. Upon pyrolysis, the agglomeration of Fe atoms
in PCN-222(Fe) can be effectively inhibited and single Fe atom
decorated nitrogen-doped porous carbon (designated as Fe1-N-
C) can be nally obtained (Schemes 1 and S1†). In virtue of the
single-atom Fe sites and hierarchically porous structure, Fe1-N-
C shows efficient ammonia synthesis with a yield rate of 1.56 ꢀ
10ꢁ11 mol cmꢁ2 sꢁ1 and a high FE of 4.51% at ꢁ0.05 V vs. the
reversible hydrogen electrode (RHE), much better than that of
Co1-N-C and Ni1-N-C synthesized using the same procedure.
Density functional theory (DFT) calculations demonstrate that
the energy barrier for the rate-determining step (RDS) of Fe1-N-C
is lower than that of Co1-N-C and Ni1-N-C, conrming the
excellent activity of Fe1-N-C toward the NRR.
Synthesis of Fe-TCPP
A mixture of Fe-TPPCOOMeCl (0.75 g), THF (25 mL), MeOH (25
mL) and 25 mL KOH (2.63 g) solution was heated under reux at
85 ꢂC for 6 h. The organic solvent was evaporated and H2O was
replenished to the mixture. The resultant solution underwent
acidication with 1 M HCl until the pH reached 3. Then the
resultant precipitate was acquired by suction-ltration and
subsequentꢂwashing with H2O twice. The solid was eventually
dried at 60 C under vacuum.
Synthesis of Co-TCPP, Ni-TCPP and H2-TCPP
The procedures were identical to those of Fe-TCPP, except for
utilizing Co-TPPCOOMe (0.75 g), Ni-TPPCOOMe (0.75 g) and
TPPCOOMe (0.75 g).
Synthesis of PCN-222(Fe)
The catalyst preparation is similar to a reported procedure.66
Typically, ZrOCl2$8H2O (108.6 mg), Fe-TCPP (6.8 mg), H2-TCPP
(24 mg), DMF (10 mL) and CF3COOH (0.45 mL) were placed in
a Pyrex vial (20 mL) and were subsequently ultrasonically dis-
solved and heated at 120 ꢂC for 18 h. Then the resultant sample
was acquired by centrifugation and subsequent washing with
DMF three times and acetone twice. The solid was eventually
Experimental
Synthesis of meso-tetra(4-carboxyphenyl)porphine
tetramethyl ester (TPPCOOMe)
The ligands were prepared on the basis of a reported proce-
dure.71 Pyrrole (6.0 g), methyl 4-formylbenzoate (13.8 g) and
propionic acid (200 mL) were homogeneously mixed and heated
ꢂ
ꢂ
dried at 60 C under vacuum.
at 140 C under reux for 12 h. The sample was acquired via
suction-ltration and subsequent washing with ethanol, ethyl
acetate and THF. The solid was eventually dried at 60 ꢂC under Synthesis of PCN-222(Co) and PCN-222(Ni)
vacuum.
The procedures were identical to those of PCN-222(Fe), except
for utilizing Co-TCPP (7 mg) and Ni-TCPP (7 mg).
Synthesis of iron(III) meso-tetra(4-methoxycarbonylphenyl)
porphine chloride (Fe-TPPCOOMeCl)
Synthesis of Fe1-N-C
Typicalꢂly, PCN-222(Fe) was transferred to a tube furnace, heated
to 800 C in 3 h and kept for 2 h under a N2 atmosphere. The
metal oxide was further etched by soaking the sample in a HF
Generally, TPPCOOMe (1.8820 g) and FeCl2$4H2O (5.51 g) were
dispersed in DMF (100 mL) and heated at 160 ꢂC for 6 h under
reux. Then H2O (150 mL) was replenished and the precipitate
was acquired by suction-ltration and subsequent washing with
ꢂ
(20 wt%) solution at 80 C. Then the resultant precipitate was
acquired by centrifugation and subsequent washing with H2O
three times and EtOH twice. The solid was eventually dried at
ꢂ
60 C under vacuum.
Synthesis of FeNP-N-C and N-C
The procedures were identical to those of Fe1-N-C, except for
respectively utilizing Fe-TCPP (32 mg) and H2-TCPP (30 mg) as
the ligand in the synthesis of the MOF precursor.
Synthesis of Co1-N-C and Ni1-N-C
The procedures were identical to those of Fe1-N-C, except for
utilizing PCN-222(Co) and PCN-222(Ni).
Scheme 1 The fabrication of Fe1-N-C from PCN-222(Fe).
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Electrochemical NRR tests
Nuclear magnetic resonance (NMR) measurement
Electrochemical tests were performed with a Zahner Zennium Aer electrolysis, 0.9 mL electrolyte was taken out and mixed
system by utilizing a Ag/AgCl reference electrode and a Pt with 0.1 mL deuterium oxide containing dimethyl sulphoxide as
counter electrode. Before measurement, N2 was bubbled for the internal standard. The 1H NMR measurement was per-
30 min. In the electrochemical NRR, a chronoamperometry test formed with water suppression (Bruker Avance III 400 MHz). To
+
was performed with continuous bubbling of N2 at 40 sccm. For quantify the amount of 15NH4 , a series of standard 15NH4Cl
electrode preparation, the catalyst (5 mg) and Naon solution solutions (0.05 mg mLꢁ1, 0.075 mg mLꢁ1, 0.1 mg mLꢁ1 and 0.15
(30 mL) were added to 500 mL absolute ethanol. Aer sonication, mg mLꢁ1) were prepared with 0.1 M HCl. The calibration curve
the ink (120 mL) was coated on both sides of carbon paper (1 ꢀ 1 was obtained by integrating the 15N doublet of the NMR signal.
2
ꢂ
cm ) and dried at 85 C.
Results and discussion
Determination of NH3
PCN-222(Fe), with a uniform rod shaped morphology, was
Typically, 2 mL NaOH solution (1 M) including sodium citrate
(5 wt%) and o-hydroxybenzoic acid (5 wt%) was transferred to
the electrolyte (2 mL). Next, 1 mL NaClO (0.05 M) and 200 mL
C5FeN6Na2O (sodium nitroprusside) solution (1 wt%) were
replenished. When the system was maintained at 25 ꢂC for
2 h, the concentration of the obtained solution was quantied
on the basis of UV-vis absorption at 655 nm. The calibration
curve was acquired with standard NH4Cl solutions, as
depicted in Fig. S1 and S2.† The limit of detection of the
method is calculated to be 0.0095 mg mLꢁ1. The standard
NH4Cl solution was prepared with 0.1 M HCl, which is
consistent with the electrolyte utilized in the electrocatalytic
experiments.
successfully synthesized (Fig. 1a and S4†). The type-IV isotherm
features a high Brunauer–Emmett–Teller (BET) surface area of
2011 m2 gꢁ1 and the corresponding pore size distribution shows
the existence of micropores (1.2 nm) and mesopores (3.2 nm)
(Fig. S5†). The transmission electron microscopy (TEM) image
of PCN-222(Fe) can further illustrate the existence of mesopores
with high orientation (Fig. 1b). Aer pyrolysis of PCN-222(Fe)
followed by the removal of ZrO2, Fe1-N-C with the retained
rod-shape was nally obtained (Fig. 1c). N2 sorption measure-
ment of Fe1-N-C indicates a large BET surface area (440 m2 gꢁ1);
the hysteresis loop reveals the mesoporous structure (Fig. S6†).
In the powder X-ray diffraction result, the broad peaks at
around 25ꢂ and 44ꢂ correspond to the (002) and (101) facets of
the graphitized carbons (Fig. S7†). Any exceptional peaks cor-
responding to the diffraction of Fe species cannot be observed,
which is consistent with the absence of Fe particles in the TEM
image (Fig. 1c). Furthermore, energy-dispersive spectroscopy
(EDS) mapping reveals the homogeneous distribution of Fe
and N in the carbon matrix (Fig. S8†).
Determination of N2H4
As for the preparation of the coloring reagent, generally 4 g p-
dimethylaminobenzaldehyde was added to a mixture of 20 mL
concentrated HCl and 200 mL ethanol. Typically, 2 mL coloring
reagent was added to 2 mL electrolyte. When the solution was
maintained at 25 ꢂC for 15 min, the concentration of the ob-
tained solution was quantied on the basis of the UV-vis
absorption wavelength at 455 nm. The calibration curve was
acquired with standard N2H4 solutions, as depicted in Fig. S3.†
The standard N2H4 solution was prepared with 0.1 M HCl,
which is consistent with the electrolyte utilized in the electro-
catalytic experiments.
In the Raman scattering spectra of Fe1-N-C, two peaks
around 1352 cmꢁ1 (D band) and 1591 cmꢁ1 (G band) corre-
spond to disordered and graphitic carbon structures (Fig. S9†).
The relatively low intensity ratio (0.93) ID/IG illustrates the high
Calculations of the NH3 yield rate and FE
Typically, the NH3 yield rate (r) and FE are quantied as below,
½NH3ꢃ ꢀ V
r ¼
s ꢀ t
QNH3
3 ꢀ F ꢀ ½NH3ꢃ ꢀ V
Idt
ð
FE ¼
¼
Q
where [NH3] is the NH3 concentration measured from UV-vis
absorption spectra, V represents the volume of the electrolyte
(V ¼ 25 mL), s is the practical surface area, t is the time applied
for the chronoamperometry test (t ¼ 2 h) and F is the Faraday
constant.
Fig. 1 (a) Scanning electron microscopy and (b) TEM images of PCN-
222(Fe). (c) TEM and (d) aberration-corrected HAADF-STEM images of
Fe1-N-C.
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graphitization degree of Fe1-N-C. With respect to the high-
resolution N 1s X-ray photoelectron spectroscopy (XPS) spec-
trum, the binding energies at 398.5 eV, 400.2 eV, 401.1 eV and
402.8 eV correspond to pyridinic N, pyrrolic N, graphitic N and
oxidized N. The peak at 399.2 eV reveals the existence of Fe-Nx
species in Fe1-N-C (Fig. 2a). Besides, the Fe 2p spectrum of Fe1-
N-C sheds light on the oxidized state of Fe, reecting the pres-
ence of Fe-Nx series (Fig. S10†). Inductively coupled plasma
atomic emission spectroscopy and elemental analysis were also
performed to further quantify the content of Fe and N, indi-
cating an Fe loading of 1.71 wt% and a N content of 4.81 wt% in
Fe1-N-C (Tables S1 and S2†).
To further investigate the existing forms of the Fe atom in
Fe1-N-C, aberration-corrected high-angle annular dark-eld
scanning transmission electron microscopy (HAADF-STEM)
was utilized.
The isolated bright spots exhibit the atomic distribution of
Fe atoms (Fig. 1d). Apart from this, X-ray absorption spectros-
copy (XAS) was performed for understanding the coordination
environment and electronic state of Fe metal atoms. In the Fe K-
edge X-ray absorption near-edge structure (XANES) spectra, Fe1-
N-C exhibits an energy absorption prole located between that
of Fe foil and Fe2O3, demonstrating a positive valence state of Fe
atoms in Fe1-N-C (Fig. 2b). The Fourier transform-extended X-
ray absorption ne structure (FT-EXAFS) curve of Fe1-N-C
Fig. 3 (a) LSV curves of Fe1-N-C in N2 and Ar-saturated electrolyte. (b)
UV-vis spectra of the electrolyte at various potentials on Fe1-N-C. (c)
NH3 yield rate and FE of Fe1-N-C at various potentials. (d) Recycling
test of Fe1-N-C at ꢁ0.05 V.
Fe1-N-C. Proved by the fact above, potentiostatic tests were
systematically performed at a sequence of potentials under
atmospheric pressure so that the optimal conditions for NH3
formation can be achieved. As exhibited in the UV-vis spectra,
the generated ammonia reacts with phenol and hypochlorite in
the coloring reagent. The electrolyte obtained at ꢁ0.05 V has the
maximum absorption at 655 nm, corresponding to the highest
concentration of ammonia (Fig. 3b). This result manifests that
the best NRR performance of Fe1-N-C can be realized at ꢁ0.05 V
vs. RHE. To further quantify the accurate ammonia yield, a set
of control experiments were performed.72–74 The Ar control
experiments and the N2 control experiments were subsequently
conducted and all of the potential distractions (including
ammonia and NOx) to the NRR activity are well ruled out
(Fig. S11†). Under these circumstances, the highest NH3 yield
rate is calculated to be 1.56 ꢀ 10ꢁ11 mol cmꢁ2 sꢁ1, with a FE of
4.51% (Fig. 3c). During ve successive recycling tests at ꢁ0.05 V,
no obvious decay of the NH3 yield rate or FE can be observed,
demonstrating excellent stability of Fe1-N-C (Fig. 3d). Further-
more, during 60 h of continuous electrolysis, the NH3 yield rate
increases linearly and reaches a turnover number (TON) of 11,
further illustrating the excellent stability of Fe1-N-C (Fig. S12†).
Apart from ammonia, hydrazine (N2H4) might be another
potential product obtained in the NRR. In the electrocatalytic
NRR of Fe1-N-C, no N2H4 can be obtained, demonstrating the
specic selectivity of Fe1-N-C toward NH3 in nitrogen reduction
(Fig. S13†).
˚
presents a main peak at ca. 1.4 A, corresponding to Fe–N
˚
coordination. The Fe–Fe peak at 2.2 A cannot be observed,
exhibiting the exclusive existence of single-atom Fe (Fig. 2c). To
recognise the coordination structure of Fe1-N-C, EXAFS tting
was also applied. According to the best tting result, the single
Fe atom is coordinated by ꢄ4 N atoms, suggesting the existence
of Fe-N4 sites in Fe1-N-C (Fig. 2d and Table S3†).
With regard to the NRR performance of the prepared SACs,
a linear sweep voltammetry (LSV) test was rst performed. As for
Fe1-N-C, the current density in the N2 saturated electrolyte is
higher than that in the Ar saturated counterpart (Fig. 3a),
demonstrating that the NRR can be successfully catalyzed by
Accurately, the quantitative isotopic labelling experiment
using 15N2 as the feed gas was also performed to strictly deter-
mine the origin of ammonia. When Fe1-N-C underwent the
chronoamperometry test at ꢁ0.05 V for 2 h under 15N2, the
sample was analyzed by 1H nuclear magnetic resonance (NMR).
+
From the spectra, only the doublet coupling for 15NH4 can
be identied (Fig. 4a). Furthermore, the integral of the 15N
+
doublet was calculated to be 0.151, corresponding to an 15NH4
Fig. 2 (a) XPS result of N 1s of Fe1-N-C. (b) Fe K-edge XANES and (c)
FT-EXAFS curves of Fe foil, Fe1-N-C and Fe2O3. (d) EXAFS fitting result
of Fe1-N-C.
concentration of 0.076 mg mLꢁ1 (Fig. 4b). A similar result was
also obtained from the indophenol blue method, with an
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Fig. 4 (a) 1H NMR result of standard 15NH4Cl solutions. (b) The 1H NMR
calibration curve of 15NH4Cl. The blue asterisk stands for the NRR
sample of Fe1-N-C at ꢁ0.05 V.
amount of 0.077 mg mLꢁ1. Therefore, the quantitative isotopic
labelling experiments provide solid evidence to support that
NH3 is produced from the NRR.75–78 To rule out the activity of
nitrogen-doped carbon, N-C, from pyrolysed PCN-222, without
Fe was tested for the NRR. At ꢁ0.15 V, N-C exhibited poor NRR
performance, with a FE of 0.1% (Fig. S14†). To have a better
comparison, a catalyst featuring Fe nanoparticles (denoted as
FeNP-N-C) was also synthesized (Fig. S15†), which exhibited poor
NRR performance, with a faradaic efficiency of 0.21% and an
ammonia yield rate of 0.79 ꢀ 10ꢁ11 mol cmꢁ2 sꢁ1 at ꢁ0.05 V
(Fig. S16†). Furthermore, SCNꢁ, which can block the single-
atom Fe sites, has been added and leads to the decrease of
the ammonia yield rate (Fig. S17†).70 The results manifest that
the single-atom Fe sites are the real source of the high NRR
activity of Fe1-N-C. In order to make a further comparison, PCN-
222(Co) and PCN-222(Ni) have also been synthesized and used
to construct Co1-N-C and Ni1-N-C, both of which show similar
characteristics (including morphology, pore characteristics,
metal loading, N congurations and content, as well as graph-
itization degree) to Fe1-N-C (Fig. S18–S20, Tables S4 and S5†).
Besides, the state of Co and Ni in Co1-N-C and Ni1-N-C has also
been conrmed with aberration-corrected HAADF-STEM and
XAS characterization studies (Fig. S21–S24, Tables S6 and S7†).
When used as NRR catalysts, both Co1-N-C and Ni1-N-C show
a much inferior ammonia yield rate and FE to those of Fe1-N-C,
following the trend Fe1-N-C > Co1-N-C > Ni1-N-C, which further
manifests the advantage of the single-atom Fe catalyst for effi-
cient NRR (Fig. 5 and S25–S27†). Moreover, no N2H4 can be
detected in the NRR of Co1-N-C or Ni1-N-C, presenting their
excellent selectivity toward NH3 in nitrogen reduction (Fig. S28
and S29†).
Fig. 6 The reaction diagram of the NRR on the surface of Fe1-N-C.
To better understand the capability of Fe1-N-C for efficient
NRR, the variation of Gibbs free energy (DG) along the NRR
reaction pathway at the zero electrode potential (U ¼ 0) was
studied by DFT.79 The result exhibits that the NRR on Fe1-N-C
proceeds through the distal pathway, where N2 adsorption is
the RDS with a DG of 1.03 eV (Fig. 6, 7a and 7b). In comparison,
the RDS for both Co1-N-C and Ni1-N-C is the conversion from
N2* to N2H*, with a DG of 1.30 eV and 2.29 eV respectively,
which is consistent with experimental catalytic performance
and supports the high catalytic activity of Fe1-N-C theoretically
(Fig. 7a and S30†). To further investigate the difference of FE
among these catalysts, Bader charge analysis was performed.
The charge values of Fe, Co and Ni were +1.08 e, +0.87 e and
+0.84 e, respectively (Fig. 7c). Considering the electrostatic
repulsion between protons and positively charged metal atoms,
the most positive charge on the Fe atom can prevent the
approach of protons, signicantly suppressing the HER
process.17 With the lowest energy barrier of the RDS toward the
NRR and the most sluggish HER kinetics, the best NRR
performance of Fe1-N-C can be well understood.
Fig. 7 (a) Free energy plot of the NRR on Fe1-N-C, Co1-N-C and Ni1-
Fig. 5 NH3 yield rate and FE of (a) Co1-N-C and (b) Ni1-N-C at various N-C. (b) The intermediates in the distal path of the NRR on Fe1-N-C. (c)
potentials.
The Bader charge analysis of Fe1-N-C, Co1-N-C and Ni1-N-C.
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13 H. Zhang, P. An, W. Zhou, B. Y. Guan, P. Zhang, J. Dong and
X. W. Lou, Sci. Adv., 2018, 4, eaao6657.
Conclusions
14 S. Back and Y. Jung, ACS Energy Lett., 2017, 2, 969–975.
15 H. B. Yang, S.-F. Hung, S. Liu, K. Yuan, S. Miao, L. Zhang,
X. Huang, H.-Y. Wang, W. Cai, R. Chen, J. Gao, X. Yang,
W. Chen, Y. Huang, H. M. Chen, C. M. Li, T. Zhang and
B. Liu, Nat. Energy, 2018, 3, 140–147.
16 Y. Cheng, S. Zhao, B. Johannessen, J.-P. Veder, M. Saunders,
M. R. Rowles, M. Cheng, C. Liu, M. F. Chisholm, R. D. Marco,
H.-M. Cheng, S.-Z. Yang and S. P. Jiang, Adv. Mater., 2018, 30,
1706287.
17 C. Choi, S. Back, N.-Y. Kim, J. Lim, Y.-H. Kim and Y. Jung,
ACS Catal., 2018, 8, 7517–7525.
18 G.-F. Chen, X. Cao, S. Wu, X. Zeng, L.-X. Ding, M. Zhu and
H. Wang, J. Am. Chem. Soc., 2017, 139, 9771–9774.
19 M.-M. Shi, D. Bao, B.-R. Wu, Y.-H. Li, Y.-F. Zhang, J.-M. Yan
and Q. Jiang, Adv. Mater., 2017, 29, 1606550.
In summary, Fe1-N-C with atomically dispersed Fe has been
successfully constructed by rational control over the distance of
adjacent Fe atoms in a porphyrinic MOF. Thanks to the single-
atom Fe sites (excellent performance for the NRR) and hierar-
chically porous structure (accessibility of active sites), Fe1-N-C
has a superior NRR performance with an NH3 yield rate of
1.56 ꢀ 10ꢁ11 mol cmꢁ2 sꢁ1 and a FE of 4.51% at ꢁ0.05 V vs.
RHE, which surpass those of Co1-N-C and Ni1-N-C. DFT calcu-
lations reveal that the RDS of Fe1-N-C toward the NRR shows
a much lower energy barrier than that of Co and Ni counter-
parts, explaining the much better performance of Fe1-N-C
theoretically. This work offers a guideline for the construction
of outstanding SACs for the NRR and provides insightful
understanding of the related catalytic mechanism.
20 L. Zhang, X. Ji, X. Ren, Y. Ma, X. Shi, Z. Tian, A. M. Asiri,
L. Chen, B. Tang and X. Sun, Adv. Mater., 2018, 30, 1800191.
21 C. Lv, C. Yan, G. Chen, Y. Ding, J. Sun, Y. Zhou and G. Yu,
Angew. Chem., Int. Ed., 2018, 57, 6073–6076.
Conflicts of interest
There are no conicts to declare.
22 Y. Yao, S. Zhu, H. Wang, H. Li and M. Shao, J. Am. Chem. Soc.,
2018, 140, 1496–1501.
23 Y. Liu, Y. Su, X. Quan, X. Fan, S. Chen, H. Yu, H. Zhao,
Y. Zhang and J. Zhao, ACS Catal., 2018, 8, 1186–1191.
24 X. Yu, P. Han, Z. Wei, L. Huang, Z. Gu, S. Peng, J. Ma and
G. Zheng, Joule, 2018, 2, 1610–1622.
25 Y. Wang, X. Cui, J. Zhao, G. Jia, L. Gu, Q. Zhang, L. Meng,
Z. Shi, L. Zheng, C. Wang, Z. Zhang and W. Zheng, ACS
Catal., 2019, 9, 336–344.
Acknowledgements
This work is supported by the NSFC (21725101, 21871244 and
21521001), the Fundamental Research Funds for the Central
Universities (WK2060030029) and the China Postdoctoral
Science Foundation (2019TQ0298). We appreciate the support
of the 1W1B XAFS beamline of BSRF and the Supercomputing
Center of USTC.
26 L. Han, X. Liu, J. Chen, R. Lin, H. Liu, F. Lv, S. Bak, Z. Liang,
S. Zhao, E. Stavitski, J. Luo, R. R. Adzic and H. L. Xin, Angew.
Chem., Int. Ed., 2019, 58, 2321–2325.
Notes and references
1 A. Wang, J. Li and T. Zhang, Nat. Rev. Chem., 2018, 2, 65–81. 27 M. Wang, S. Liu, T. Qian, J. Liu, J. Zhou, H. Ji, J. Xiong,
2 L. Liu and A. Corma, Chem. Rev., 2018, 118, 4981–5079.
3 J. Liu, ACS Catal., 2017, 7, 34–59.
J. Zhong and C. Yan, Nat. Commun., 2019, 10, 341.
28 J. S. Anderson, J. Rittle and J. C. Peters, Nature, 2013, 501, 84–
4 P. Liu, Y. Zhao, R. Qin, S. Mo, G. Chen, L. Gu, D. M. Chevrier,
87.
P. Zhang, Q. Guo, D. Zang, B. Wu, G. Fu and N. Zheng, 29 J.-C. Liu, X.-L. Ma, Y. Li, Y.-G. Wang, H. Xiao and J. Li, Nat.
Science, 2016, 352, 797–800. Commun., 2018, 9, 1610.
5 X. Cui, H. Li, Y. Wang, Y. Hu, L. Hua, H. Li, X. Han, Q. Liu, 30 X. Cui, C. Tang and Q. Zhang, Adv. Energy Mater., 2018, 8,
F. Yang, L. He, X. Chen, Q. Li, J. Xiao, D. Deng and X. Bao,
Chem, 2018, 4, 1902–1910.
1800369.
´
´
´
31 A. J. Martın, T. Shinagawa and J. Perez-Ramırez, Chem, 2019,
6 L. Fan, P. F. Liu, X. Yan, L. Gu, Z. Z. Yang, H. G. Yang, S. Qiu
and X. Yao, Nat. Commun., 2016, 7, 10667.
7 N. Cheng, S. Stambula, D. Wang, M. N. Banis, J. Liu, A. Riese,
5, 263–283.
32 N. Lehnert, H. T. Dong, J. B. Harland, A. P. Hunt and
C. J. White, Nat. Rev. Chem., 2018, 2, 278–289.
B. Xiao, R. Li, T.-K. Sham, L.-M. Liu, G. A. Botton and X. Sun, 33 S. L. Foster, S. I. P. Bakovic, R. D. Duda, S. Maheshwari,
Nat. Commun., 2016, 7, 13638.
8 C. Zhu, S. Fu, Q. Shi, D. Du and Y. Lin, Angew. Chem., Int. Ed.,
2017, 56, 13944–13960.
R. D. Milton, S. D. Minteer, M. J. Janik, J. N. Renner and
L. F. Greenlee, Nat. Catal., 2018, 1, 490–500.
34 F. Jiao and B. Xu, Adv. Mater., 2019, 31, 1805173.
9 H. Xu, D. Cheng, D. Cao and X. C. Zeng, Nat. Catal., 2018, 1, 35 C. Guo, J. Ran, A. Vasileff and S.-Z. Qiao, Energy Environ. Sci.,
339–348.
2018, 11, 45–56.
10 H. Zhang, G. Liu, L. Shi and J. Ye, Adv. Energy Mater., 2018, 8, 36 F. Lu, S. Zhao, R. Guo, J. He, X. Peng, H. Bao, J. Fu, L. Han,
¨
1701343.
G. Qi, J. Luo, X. Tang and X. Liu, Nano Energy, 2019, 61, 420–
11 L. Zhang, Y. Jia, G. Gao, X. Yan, N. Chen, J. Chen, M. T. Soo,
427.
¨
B. Wood, D. Yang, A. Du and X. Yao, Chem, 2018, 4, 285–297. 37 Y. Qiu, X. Peng, F. Lu, Y. Mi, L. Zhuo, J. Ren, X. Liu and
12 G. Wan, X.-M. Lin, J. Wen, W. Zhao, L. Pan, J. Tian, T. Li,
J. Luo, Chem.–Asian J., 2019, 14, 2770–2779.
H. Chen and J. Shi, Chem. Mater., 2018, 30, 7494–7502.
26376 | J. Mater. Chem. A, 2019, 7, 26371–26377
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
38 X.-F. Li, Q.-K. Li, J. Cheng, L. Liu, Q. Yan, Y. Wu, X.-H. Zhang, 58 K. J. Lee, J. H. Lee, S. Jeoung and H. R. Moon, Acc. Chem. Res.,
Z.-Y. Wang, Q. Qiu and Y. Luo, J. Am. Chem. Soc., 2016, 138,
8706–8709.
39 X. Liu, Y. Jiao, Y. Zheng, M. Jaroniec and S.-Z. Qiao, J. Am.
Chem. Soc., 2019, 141, 9664–9672.
2017, 50, 2684–2692.
59 S. Ma, G. A. Goenaga, A. V. Call and D.-J. Liu, Chem.–Eur. J.,
2011, 17, 2063–2067.
60 L. Jiao, J. Y. R. Seow, W. S. Skinner, Z. U. Wang and
H.-L. Jiang, Mater. Today, 2019, 27, 43–68.
40 Z. Niu, W. D. C. B. Gunatilleke, Q. Sun, P. C. Lan, J. Perman,
J.-G. Ma, Y. Cheng, B. Aguila and S. Ma, Chem, 2018, 4, 2587– 61 D. Li, H.-Q. Xu, L. Jiao and H.-L. Jiang, EnergyChem, 2019, 1,
2599. 100005.
41 S. Zhao, Y. Wang, J. Dong, C.-T. He, H. Yin, P. An, K. Zhao, 62 L. Jiao, Y. Wang, H.-L. Jiang and Q. Xu, Adv. Mater., 2018, 30,
X. Zhang, C. Gao, L. Zhang, J. Lv, J. Wang, J. Zhang, 1703663.
A. M. Khattak, N. A. Khan, Z. Wei, J. Zhang, S. Liu, 63 Y.-Z. Chen, R. Zhang, L. Jiao and H.-L. Jiang, Coord. Chem.
H. Zhao and Z. Tang, Nat. Energy, 2016, 1, 16184. Rev., 2018, 362, 1–23.
42 B. Li, H.-M. Wen, Y. Cui, W. Zhou, G. Qian and B. Chen, Adv. 64 F.-Y. Yi, R. Zhang, H. Wang, L.-F. Chen, L. Han, H.-L. Jiang
Mater., 2016, 28, 8819–8860. and Q. Xu, Small Methods, 2017, 1, 1700187.
43 H. Wang, Q.-L. Zhu, R. Zou and Q. Xu, Chem, 2017, 2, 52–80. 65 G. Cai, W. Zhang, L. Jiao, S.-H. Yu and H.-L. Jiang, Chem,
44 M. Zhao, K. Yuan, Y. Wang, G. Li, J. Guo, L. Gu, W. Hu,
H. Zhao and Z. Tang, Nature, 2016, 539, 76–80.
45 Y.-Z. Chen, B. Gu, T. Uchida, J. Liu, X. Liu, B.-J. Ye, Q. Xu and
H.-L. Jiang, Nat. Commun., 2019, 10, 3462.
46 S.-S. Wang, L. Jiao, Y. Qian, W.-C. Hu, G.-Y. Xu, C. Wang and
H.-L. Jiang, Angew. Chem., Int. Ed., 2019, 58, 10713–10717.
2017, 2, 791–802.
66 L. Jiao, G. Wan, R. Zhang, H. Zhou, S.-H. Yu and H.-L. Jiang,
Angew. Chem., Int. Ed., 2018, 57, 8525–8529.
67 Q. Yang, C.-C. Yang, C.-H. Lin and H.-L. Jiang, Angew. Chem.,
Int. Ed., 2019, 58, 3511–3515.
68 L. Jiao and H.-L. Jiang, Chem, 2019, 5, 786–804.
47 Y.-Z. Chen, Z. U. Wang, H. Wang, J. Lu, S.-H. Yu and 69 H. Zhang, S. Hwang, M. Wang, Z. Feng, S. Karakalos, L. Luo,
H.-L. Jiang, J. Am. Chem. Soc., 2017, 139, 2035–2044.
48 L. Jiao, Y.-X. Zhou and H.-L. Jiang, Chem. Sci., 2016, 7, 1690–
1695.
49 Z. Niu, X. Cui, T. Pham, P. C. Lan, H. Xing, K. A. Forrest,
L. Wojtas, B. Space and S. Ma, Angew. Chem., Int. Ed., 2019,
58, 10138–10141.
Z. Qiao, X. Xie, C. Wang, D. Su, Y. Shao and G. Wu, J. Am.
Chem. Soc., 2017, 139, 14143–14149.
70 Y. Chen, S. Ji, Y. Wang, J. Dong, W. Chen, Z. Li, R. Shen,
L. Zheng, Z. Zhuang, D. Wang and Y. Li, Angew. Chem., Int.
Ed., 2017, 56, 6937–6941.
71 D. Feng, Z.-Y. Gu, J.-R. Li, H.-L. Jiang, Z. Wei and H.-C. Zhou,
Angew. Chem., Int. Ed., 2012, 51, 10307–10310.
50 J.-D. Xiao and H.-L. Jiang, Acc. Chem. Res., 2019, 52, 356–366.
51 X. Fang, Q. Shang, Y. Wang, L. Jiao, T. Yao, Y. Li, Q. Zhang, 72 L. F. Greenlee, J. N. Renner and S. L. Foster, ACS Catal., 2018,
Y. Luo and H.-L. Jiang, Adv. Mater., 2018, 30, 1705112. 8, 7820–7827.
52 Q. Yang, Q. Xu and H.-L. Jiang, Chem. Soc. Rev., 2017, 46, 73 L. Li, C. Tang, D. Yao, Y. Zheng and S.-Z. Qiao, ACS Energy
4774–4808. Lett., 2019, 4, 2111–2116.
53 C. A. Trickett, A. Helal, B. A. Al-Maythalony, Z. H. Yamani, 74 C. Tang and S.-Z. Qiao, Chem. Soc. Rev., 2019, 48, 3166–3180.
K. E. Cordova and O. M. Yaghi, Nat. Rev. Mater., 2017, 2, 75 L. Zhang, L.-X. Ding, G.-F. Chen, X. Yang and H. Wang,
17045.
54 N. Li, J. Liu, J.-J. Liu, L.-Z. Dong, Z.-F. Xin, Y.-L. Teng and 76 Y. Luo, G.-F. Chen, L. Ding, X. Chen, L.-X. Ding and H. Wang,
Y.-Q. Lan, Angew. Chem., Int. Ed., 2019, 58, 5226–5231. Joule, 2019, 3, 279–289.
55 D. Micheroni, G. Lan and W. Lin, J. Am. Chem. Soc., 2018, 77 H. Cheng, L.-X. Ding, G.-F. Chen, L. Zhang, J. Xue and
140, 15591–15595. H. Wang, Adv. Mater., 2018, 30, 1803694.
56 X.-K. Wang, J. Liu, L. Zhang, L.-Z. Dong, S.-L. Li, Y.-H. Kan, 78 H. Cheng, P. Cui, F. Wang, L.-X. Ding and H. Wang, Angew.
D.-S. Li and Y.-Q. Lan, ACS Catal., 2019, 9, 1726–1732. Chem., Int. Ed., 2019, 58, 15541–15547.
57 R.-B. Lin, S. Xiang, B. Li, Y. Cui, G. Qian, W. Zhou and 79 E. Skulason, T. Bligaard, S. Gudmundsdottir, F. Studt,
Angew. Chem., Int. Ed., 2019, 58, 2612–2616.
´
´
´
B. Chen, Coord. Chem. Rev., 2019, 384, 21–36.
J. Rossmeisl, F. Abild-Pedersen, T. Vegge, H. Jonsson and
J. K. Nørskov, Phys. Chem. Chem. Phys., 2012, 14, 1235–1245.
This journal is © The Royal Society of Chemistry 2019
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