Separation of Metal-N4Units in Metal-Organic Framework for Preparation of M-Nx/C Catalyst with Dense Metal Sites

Preparation of M ‑N /C Catalyst with Dense Metal Sites

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Separation of Metal‑N Units in Metal−Organic Framework for

†

Bing Hu, Xuebing Zhu, Xuhui An, Chenxiao Wang, Xiaobing Wang,* Jinling He, and Yong Zhao*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c02420

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ABSTRACT: Metal and nitrogen codoped carbon (M-N /C) materials with good

metal dispersion in a carbon matrix have attracted great attention because they can

eﬃciently catalyze various kinds of chemical/electrochemical reactions. However,

the existing strategies for preparing M-N /C materials still face the challenge of

metal site aggregation when the carbon skeleton in the precursor is substantially

lost during pyrolysis. Herein, we propose a general strategy that the increase of

metal site density and inhibition of metal aggregation could be realized by

separating M-N units in metal−organic frameworks (MOFs). The metal sites can

be well separated and distributed in carbon materials during the pyrolysis of a

metal-coordinated block copolymer, one of the speciﬁc MOFs. The strategy can be

widely applicable for the synthesis of M-N /C materials with dense metal sites,

such as Fe-, Mn-, Ni-, and Co-N /C materials, which are potentially used as the

eﬃcient catalysts for various kinds of reactions. Taking Fe-N /C as a model oxygen

reduction reaction (ORR) electrocatalyst, it shows the ORR half-wave potentials of

.90 and 0.81 V vs RHE in alkaline and acidic electrolytes, respectively.

INTRODUCTION

Metal and nitrogen codoped carbon (M-N /C) materials have

during pyrolysis to some extent, but the retention eﬀect is still

■

17,18

limited.

In order to overcome the low thermal stability of

small molecules at high temperature, the second method was

taken by using the hybrid of polymers and metal salts as

been widely used in various chemical/electrochemical

−5

6−8

reactions,

such as oxygen reduction reaction (ORR)

9,20

precursors.

For instance, pyrolysis of polyaniline and metal

and carbon dioxide reduction reaction, due to their eﬃcient

activity and high selectivity.

10,11

salt hybrids was used to prepare M-N /C materials.

The catalytic activity of M-N_x/

polymer precursor has the good thermal stability and then

provides the possibility of enriching the M-N sites to a certain

C is controlled by the quantity of metal sites, while the active

sites with good distribution and high density on the carbon

surface are no doubt attractive for their excellent perform-

extent. However, when the precursors of polymer and metal

ance. Generally, pyrolysis at high temperature is required to

salt are physically blended, the M-N units are randomly

convert the precursor with metal species to M-N /C materials

obtained in M-N /C material during the pyrolysis process,

,11

for the formation of M-N active sites with good stability.

resulting in the low possibility for obtaining the high density of

However, the carbon skeleton in the starting precursor is easily

decomposed at high temperatures (>700 °C), resulting in the

aggregation of metal atoms and low number of metal sites in

M-N units. One of the interesting and attractive strategies is

using porphyrinic metal organic frameworks (MOFs) as

23−26

polymer precursor to synthesize Fe-N /C materials.

The

13,14

the existing M-N /C materials.

It is therefore highly

periodic structure of a MOF gives rise to enrich Fe-N units

important to develop a strategy that increases the carbon

retention rate and inhibits the aggregation of metal atoms

during pyrolysis, which will be helpful to enrich the metal site

density on carbon materials.

and has the good thermal stability during pyrolysis. However,

the metal porphyrins in MOFs are densely packed, and the

problem of metal agglomeration still occurred when most of

the nonmetal elements is lost in the pyrolysis process.

Moreover, the preparation of a porphyrinic MOF is also

Many strategies have been reported to synthesize M-N /C

catalysts. The ﬁrst one is using the organic molecules

5,16

containing M-N₄units,

similar as those of metal-

Received: August 12, 2020

porphyrins, which are used directly as precursors to construct

M-N /C catalysts. But small organic molecules based

precursors are easily volatilized and decomposed at high

temperatures due to their low thermal stability. Lots of

advanced carbon materials were used to support such

precursors for inhibiting the decomposition of Fe-N units

XXXX American Chemical Society

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Scheme 1. Schematic Images for Synthesizing M-N /C Materials with (a) Metal-Coordinated Polymer and (b) Metal-

Coordinated Copolymer as Precursors

The metal site aggregation often occurs during carbon matrix pyrolysis with metal-coordinated polymer as the starting material, while the metal

aggregation in M-N /C is inhibited with metal-coordinated copolymer as precursor.

expensive and complex. It is therefore highly expected to

was controlled at 200 °C for 120 min and at 600 °C for 60 min under

a temperature program in the argon gas. The pyrolyzed samples were

treated with concentrated HCl and washed with water. The wet

powder was further dispersed in water and freeze-dried. The dry

powder was treated at the second stage at 800 °C for 2 h in the NH₃

atmosphere. The collected powder was then treated with concen-

develop an eﬀective strategy where the M-N units can be

controllably dispersed in a MOF polymer precursor, which

provides the great chance for inhibiting the metal agglomer-

ation at high temperatures, and then preparing M-N /C

catalysts with dense metal sites.

In this study, we report a facile protocol that the increase of

metal site density and inhibition of metal aggregation could be

trated HCl again. Finally, the Fe-N /C-A powder was obtained. The

preparation method for Fe-N /C-B was similar to that of Fe-N /C-A,

while no MPD was provided during Fe-POPD polymerization.

POPD-PMPD was used to prepare N/C, which was pyrolyzed at 800

realized by separating M-N units in metal−organic frame-

works (MOFs). The metal sites can be well separated and

distributed in carbon materials during the pyrolysis of metal-

coordinated block copolymers, one of the speciﬁc MOFs,

°C for 2 h in the NH

atmosphere. POPD-PMPD was mixed with

Fe(NO ) solution to form the Fe/POPD-PMPD mixture, and the

calcination process of Fe-N /C-C was similar to that for Fe-N /C-A.

Characterization. UV−visible absorption was measured by a

spectrophotometer (PC2000-ISA). X-ray diﬀraction (XRD) patterns

of samples were recorded on a powder XRD (D8-ADVANCE, Bruker,

Germany) system. Aberration-corrected high-angle annular dark-ﬁeld

scanning transmission electron microscopy (HAADF-STEM) images

were performed on a JEOL ARM200F. The energy dispersive

spectroscopy (EDS) mappings were carried out by four symmetrically

designed EDS detectors equipped on an FEI Talos F200X. Scanning

electron microscopy (SEM) was performed on a Nova NanoSEM 450

resulting in the preparation of M-N /C material with enriched

metal sites (Scheme 1). Compared to the metal-coordinated

polymer with densely packed M-N sites (Scheme 1a), the

addition of block units in the metal-coordinated copolymer not

only increases the length of the polymer chain but also

separates the M-N units, thus eﬀectively inhibiting the metal

agglomeration and increasing the density of metal sites

(Scheme 1b). Furthermore, this strategy is demonstrated to

(FEI, USA). Transmission electron microscopy (TEM) was carried

be widely applicable for the preparation of multiple M-N /C

out with a JEM 2100F TEM operated at 200 kV. Brunauer−Emmett−

Teller (BET) surface area and pore size were obtained on a

Micromeritics ASAP 2020. X-ray photoelectron spectroscopy (XPS)

was collected on a scanning X-ray microprobe (Thermo Scientiﬁc

ESCALAB 250X). ICP-MS was used to analyze the 7700x equipment

of the Agilent company. XAFS spectra at the Fe K-edge was collected

at the beamline 1W1B station of the Beijing Synchrotron Radiation

materials including of Fe-, Mn-, Ni-, and Co-N /C materials

with single-metal sites. Taking the Fe-N /C materials as model

oxygen reduction reaction (ORR) electrocatalysts, they exhibit

the excellent activity in the acidic and alkaline electrolytes.

EXPERIMENTAL SECTION

Reagents. o-Phenylenediamine (OPD), sodium bicarbonate

■

(

Facility, China. The Fe Mossbauer spectrum was recorded at 295 K

NaHCO ), and ammonium persulfate ((NH ) S O ) were pur-

using a conventional spectrometer (German Wissel MS - 500) in

constant speed mode of transmission using geometric active for the 25

chased from Macklin (Shanghai, China) Chemical Technology. m-

Phenylenediamine (MPD) and Fe(NO₃)₃were purchased from

Sigma-Aldrich (Shanghai, China). The commercial Pt/C catalyst,

which is 20 wt % with ∼3 nm platinum nanoparticles, was obtained

from Alfa-Aesar.

m Ci source of Co/Rh. The 15 μm Fe foil was used for velocity

correction of the instrument, and the data were automatically

collected by the computer. The least-square method was used to ﬁt

the spectral line. Qualitative analysis of each phase in the catalyst was

conducted by ﬁtting the parameter homogeneous power shift (IS) and

hyperﬁne ﬁeld (Hhf).

Preparation of Fe-N /C Materials. OPD (2.5 mM) was

dissolved in 25 mL of deionized water, and Fe(NO₃)₃solution

(

1.25 mM in 100 mL of deionized water) was poured into OPD

Electrochemical Characterization. Electrochemical experiments

were measured in three-electrode cells (∼25 °C) with an Autolab

electrochemical workstation (Metrohm). A saturated calomel

electrode (SCE, saturated KCl) and carbon electrode were used as

reference electrode and counter electrode, respectively. A glassy

solution and stirred for 2 min to obtain Fe-OPD coordination

compound. MPD solution (7.5 mM in 75 mL of deionized water) was

added into the above medium. The mixed solution of (NH ) S O

and NaHCO (20 mM (NH ) S O and 25 mM NaHCO in 100 mL

of deionized water) was poured into the above solution, and Fe-OPD

and MPD were copolymerized to form an Fe-POPD-MPD block

polymer suspension. The above solution was centrifuged and washed

with deionized water. Fe-POPD-PMPD wet powder was redispersed

in water and freeze-dried to obtain Fe-POPD-PMPD. It was pyrolyzed

in a tube furnace by a two-stage pyrolyzing method. The ﬁrst stage

carbon (GC) electrode (0.196 cm ) was used as working electrode.

Catalyst ink was prepared by ultrasonically dispersing 2 mg of catalyst

in a suspension with 10 μL of Naﬁon solution (5 wt %), 500 μL of

deionized water, and 500 μL of ethanol. Pt/C (20 wt %) ink was

prepared in the same way. Catalyst electrodes were obtained by

coating the catalyst ink on a glassy carbon rotating ring-disk electrode

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−

(

RRDE), and the loading amounts were 0.4 mg cm for Fe-N /C

−

and 80 μg Pt cm for Pt/C. Cyclic voltammetry (CV) tests were

carried out in N and O saturated HClO (pH = 1) and KOH (pH =

−

3) solutions. The scan rate was 5 mV s . ORR polarization curves

were obtained by sweeping the potential from 1.1 to 0.1 V in an O₂-

saturated electrolyte at a rotation speed of 1500 rpm. ORR stability

was evaluated with a rotation speed of 900 rpm. In addition, the ORR

durability was tested at a constant potential of 0.6 V for 100 h with a

rotation speed of 900 rpm. All electrode potentials were converted to

standard hydrogen electrode potentials (RHE) based on the next

equation without iR compensation. The calibration values were

E(RHE) = E(SCE) + 0.30 V and E(RHE) = E(Hg/HgO) + 0.87 V in

pH = 1 HClO and pH = 13 KOH, respectively.

RESULTS AND DISCUSSION

Synthesis and Characterization of Fe-N C Materials.

■

Synthesis of Fe-N /C as a model M-N /C material was used to

verify the advantages of the strategy that the increase of metal

site density and inhibition of metal aggregation could be

realized by separating M-N units in a MOF, that is, a metal-

coordinated block copolymer. Figure 1 illustrates the

Figure 2. (a) UV−visible absorption spectra of Fe-OPD, OPD, and

Fe(NO ) . (b) XPS survey spectra of Fe-POPD-MPD with diﬀerent

ratios of OPD and MPD. (c) FT-IR spectra of OPD, POPD2-MPD2,

and Fe-POPD2-MPD2 polymer. (d) Thermal gravimetric analyzer

(TGA) curves of Fe-POPD2-MPD2 and Fe-POPD.

can be seen from the Fe 2p spectra that Fe-POPD-MPD is

Fe (Figure S1a). The N 1s spectra show one type of N

assigned to amino groups of OPD (Figure S1b). The above

result conﬁrms the coordination of Fe and OPD in the Fe-

OPD. XPS survey spectra show that all Fe-POPD-MPD

polymers contain Fe and N elements (Figure 2b). The

elemental contents analyzed from XPS and ICP-MS (Table

S1 ) show that Fe contents gradually decrease (from 13.94 wt

in Fe-POPD2-MPD1 to 3.83 wt % in Fe-POPD2-MPD8)

with the increase of MPD ratio in Fe-POPD-MPD. We also

synthesized the metal-free copolymer POPD2-MPD2 as

reference. TEM images show that the morphology of

POPD2-MPD2 is similar to that of Fe-POPD-MPD, indicating

the coordination of Fe does not change the nanospherical

structure (Figure S2 ). Further, infrared spectra (IR) showed

that the absorption peak at 757 cm corresponds to the OPD

characteristic absorption, the absorption peaks at 1150−1134

and 848 cm correspond to the ﬂexural vibration character-

Figure 1. Preparation route for the Fe-N /C materials by using metal-

coordinated block copolymer (Fe-POPD-MPD) as the precursor.

Step I: The coordination of o-phenylenediamine (OPD) and Fe to

form Fe-OPD complex. Step II: Mixing and polymerizing Fe-OPD

and m-phenylenediamine (MPD) to form Fe-POPD-MPD copoly-

mer. Step III: Pyrolysis of Fe-POPD-MPD at 800 °C to obtain Fe-N_x/

−

C material precursor and acid leaching of Fe-N /C material to obtain

−1

the ﬁnal Fe-N /C material.

istic absorption in the benzene torus of 1, 2, 4, 5, and the

absorption peak at 3192 cm corresponds to the N-H

−1

preparation scheme for Fe-N /C. In Step I, ferric ions (Fe )

were mixed with o-phenylenediamine (OPD) to form the Fe-

OPD coordinated compounds in aqueous solution. In Step II,

Fe-OPD was mixed with diﬀerent molar ratios of m-

phenylenediamine (MPD), and then, they were oxidatively

polymerized by ammonium persulfate to obtain the block

copolymer (Fe-POPD-MPD). In this work, ﬁve types of Fe-

POPD-MPD copolymers with the Fe-OPD/MPD molar ratios

of 2:1, 2:2, 2:4, 2:6, and 2:8 were prepared, denoted as Fe-

POPD2-MPD1, Fe-POPD2-MPD2, Fe-POPD2-MPD4, Fe-

POPD2-MPD6, and Fe-POPD2-MPD8, respectively. The

UV−visible absorption spectra in Figure 2a show that the

characteristic absorption peak of OPD negatively shifts after

the formation of the Fe-OPD compound. In addition, the Fe-O

absorption peak of Fe(NO ) ranked at 300 nm disappears in

stretching vibration absorption in −NH , and the OPD peak is

wider than that of the monomer, indicating the occurrence of

−

the polymerization reaction. The absorption peak at 1380 cm

3−

corresponds to the characteristic absorption of NO . The

infrared spectra also show that Fe does not aﬀect the

characteristic absorption peaks of POPD2-MPD2 polymer

(Figure 2c). With the increase of MPD ratio in Fe-POPD-

MPD, the ratio of small-sized polymer particles decreases

gradually, and the proportion of polymer nanospheres

increases.

In step III, Fe-POPD-MPD copolymers were pyrolyzed

twice (600 and 800 °C) in a tubular furnace under an Ar

atmosphere. The iron-based aggregates were acid-leached to

form Fe-N /C-A. The Fe-N /C-B with Fe-POPD and Fe-N /

the spectra of the Fe-OPD compound, but the absorption peak

of the Fe-N absorption peak ranked at 450 nm appears. And, it

C-C with iron salt and a block polymer hybrid as the starting

precursors were also prepared, respectively, which were used to

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Figure 3. (a, d) SEM, (b, e) TEM, and (c, f) HR-TEM images of Fe-N /C-A (a, b, c) before acid pickling and (d, e, f) after acid pickling. (g) XRD

patterns for Fe-N /C-A.

illustrate the advancement of the strategy that the inhibition of

during acid etching. It indicates that aggregated metal species

metal aggregation could be realized by separating M-N units

in carbon materials, such as Fe N aggregates, gradually

in MOFs. The thermal gravimetric analysis (TGA) curves

disappear. TGA curves show that the thermal stability of Fe-

POPD is much lower than that of Fe-POPD-MPD (Figure

(Figure 2d and Table S2 ) show that, comparing with that of

Fe-POPD (the precursor of Fe-N /C-B), the carbon retention

2d). XRD patterns show that there is still an Fe C (PDF# 72-

rate of Fe-POPD2-MPD2 polymer at 800 °C substantially

increases from 32.7% to 49.0%. The carbon retention rate of

Fe-POPD2-MPD4 is further increased to 54.0%, and the

carbon retention rate of Fe-POPD-MPD polymers tends to be

stable with the increase ratio of MPD. It revealed that the

introduction of MPD units has the positive eﬀect for thermal

2125) crystal peak at 45° in the Fe-N /C-B before and after

acid pickling, and the lattice spacing of metal aggregates in

Fe-N /C-B is 0.196 nm, assigned to the (200) lattice of Fe C

(Figure S7 ). It further proves that the addition of the block can

eﬀectively inhibit the metal aggregation and improve the metal

dispersion during polymer pyrolysis.

stability of Fe-N /C materials during pyrolysis, consistent with

Structure of Iron Sites in Fe-N /C-A. The ﬁne structure

the morphology change trend of Fe-POPD-MPD with diﬀerent

MPD ratios (Figures S3, S4 and Table S3).

of the Fe sites was identiﬁed by atomic-resolution high-angle

annular dark-ﬁeld scanning TEM (HAADF-STEM), XAS, Fe

SEM and TEM images show that the morphology of Fe-N_x/

C-A is nanoparticles with the diameter of about 400 nm, and

the morphology shows no obvious change after picking (Figure

Mossbauer spectroscopy, and XPS. The HAADF-STEM image

of Fe-N /C-A shows that the white spots inside the red circle

are iron atoms. EDX mapping shows that Fe, N, and C are

). The HR-TEM image (Figure 3c) shows that the lattice

evenly distributed (Figure 4a). In order to conﬁrm the Fe

spacing of metal aggregates in Fe-N /C-A before picking is

structure of Fe-N /C-A at the atomic level, XANES and

.209 nm, corresponding to the (110) of Fe N (PDF# 72-

EXAFS were performed with iron foil, phthalocyanine (FePc),

125). The HR-TEM image (Figure 3f) shows that no Fe N

and Fe O as standard samples. As shown in the XANES

is found in Fe-N /C-A after picking. For the Fe-N /C-A before

spectra of Figure 4b, the absorption edge of Fe-N /C-A is

picking, the diﬀraction peaks at 73.53°, 40.89°, 42.94°, 56.70°,

higher than that of Fe foil but lower than that of Fe O ,

7.72°, and 75.88° correspond to crystal Fe N (Figure 3g),

revealing that the valence state of Fe in the Fe-N /C-A is

consistent with the result in the HR-TEM observation. The

located between 0 and +3. An inconspicuous pre-edge peak

XRD patterns of Fe-N /C-A show that two broad peaks at

ranked at 7113 eV in the Fe-N /C-A originates from the 1s →

about 25.7° and 44.0° belong to (002) and (100) peaks of

graphite carbon, respectively. From Figure S5 , we found that

the big porous structure appears, which is due to the acid

leaching of Fe N aggregates in Fe-N /C-A. With the increase

4pz transition along with simultaneous charge transfer of

ligand-to-metal. It indicates that Fe are conﬁned in Fe-N

units. Meanwhile, the Fe spectra in the Fe-N plane of Fe-

N /C-A is similar to that of FePc. As presented in Figure 4c,

of MPD block ratios, the big porous structure decreases

gradually, and then, it cannot be observed in Fe-POPD2-

the peak at ∼1.50 Å in the R space of Fe-N /C-A is close to

the Fe-N peak of FePc, which ascertains the presence of Fe-N

MPD6 derived Fe-N /C-A material. It conﬁrms that the

units in the Fe-N /C-A. Furthermore, compared with that of

introduction of block units signiﬁcantly improves the thermal

stability of the copolymer and metal atoms are not easy to

aggregate. Figure S6 shows the typical type of isothermal in the

Fe foil, there is no obvious Fe-Fe peak (∼2.20 Å) in Fe-N /C-

A. It further indicates that there is no Fe−Fe bond, and the

Fe site exists in the form of atomic dispersion. The results are

consistent with the observation in the HAADF-STEM. To

N adsorption/desorption experiment. As the block contents in

the copolymer increase, the speciﬁc surface areas of Fe-N /C-A

materials gradually decrease to 640 m g . The pore size of

further characterize the structure of Fe-related species in Fe-

−1

N /C-A, Fe Mo

ssbauer spectra (Figure 4d) were carried out.

Fe-N /C-A is about 3.76 nm. As the block content in the

The Mo

ssbauer spectra of Fe-N /C-A show ﬁve doublet peaks

polymer increases, some of mesopores tend to disappear,

consistent with TEM. The formation of such large pores can

(D1, D2, D3, D4, and D5) which are corresponding to square-

planar Fe-N . The corresponding ﬁtting parameters of the Fe-

be attributed to the removal of the amount of Fe N aggregates

site are summarized in Table S5. The α-Fe and γ-Fe (typical

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sextet and singlet) are not detected, and it can be concluded

that the Fe atoms in the catalyst are mainly composed of Fe-N

sites.

Further, we analyzed the element compositions and content

in Fe-N /C-A and Fe-N /C-B by XPS and ICP-MS. It shows

that, when the ratio of OPD:MPD in Fe-POPD-MPD

precursors is 1:2, the maximum content of Fe in the Fe-N _x /

C-A is 0.51 at. % (1.36 wt %) (Table S5). Figures 4e and S9a

display the Fe 2p spectra. The peaks at 709.1 and 713.5 eV are

assigned to 2p orbitals of Fe and Fe ions, respectively.

Meanwhile, the peaks at 722.6 and 725.8 eV correspond to

orbitals of Fe and Fe ions. The peak at the binding

energy of 711.3 eV indicates the existence of Fe-N species in

the Fe-N /C-A catalyst. In addition, compared with those of

Fe-N /C-B from the Fe-POPD precursor, the content of Fe-N

units is higher. The Fe-N moieties are crucial active sites for

Fe-N /C-based catalysts. As one of the eﬃcient ORR

electrocatalysts, Fe-N units initiated oxygen adsorption and

ORR catalysis, resulting in the enhancement of ORR eﬃciency.

Additionally, we characterize N species in Fe-N /C-A

materials. The N 1s spectra showed that the ﬁve peaks at

397.9, 398.4, 399.4, 400.9, and 403.9 eV correspond to

pyridinic-N, N-Fe, pyrrolic-N, graphitic-N, and oxidized-N

(

Figures S8 and S9 ), respectively. Pyridinc-N plays an

important role in the formation of Fe-N /C active sites with

local electronic structure changes. The existence of N−Fe

bonds is due to the coordination of pyridinic and pyrrolic N

with Fe, leading to the formation of catalytically active

nitrogen-iron groups. These substances act as eﬀective active

sites and directly reduce oxygen to water through the four-

electron transfer pathway. In addition, because the binding

energy of pyridinic-N and N-Fe changes little, the peak related

to pyridinc-N may also be related to N-Fe. Therefore, the

Figure 4. (a) Aberration-corrected HAADF-STEM image of Fe-N_x/

C-A, and corresponding element mapping (C: red, Fe: green, N:

blue). (b) Fe K-edge XANES spectra and (c) Fe K-edge k -weighted

FT spectra of Fe-N /C-A, Fe O , FePc, and Fe foil samples,

higher amount of pyridinc-N in the Fe-N /C-A materials than

_r_e_s_p_e_c_t_i_v_e_l_y_.₍_d₎5 Fe Mo

ssbauer spectroscopy for Fe-N /C-A. (e)

that of Fe-N /C-B indicates that the Fe-N /C-A electro-

XPS Fe 2p spectra of Fe-N /C-A.

catalysts may contain a higher amount of N-Fe species. They

Figure 5. (a) ORR polarization curves for Fe-N /C-A, Fe-N /C-B, Fe-N /C-C, and Pt/C in HClO electrolyte (pH = 1). (b) Tafel plots for Fe-

N /C-A and Pt/C. (c) Electron transfer number (n) (right) and H O yield (left) versus potential. (d) ORR polarization curves of Fe-N /C-A and

Pt/C before (solid line) and after (dashed line) 2000 potential cycles. (e) Chronoamperometric test of Fe-N /C-A and Pt/C at 0.60 V. (f) ORR

polarization curves of Fe-N /C-A before and after the addition of 0.01 M KSCN in pH = 1 HClO .

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greatly improve their ORR performance compared to that of

Fe-N /C-B. Additionally, the free pyridinic-N can capture H

and O , and then transferred to the metal sites. Therefore, the

high contents of pyridinc-N in Fe-N /C-A materials not only

provide abundant sites for ﬁxing iron atoms but also help to

improve ORR activity.

ORR Activity of Fe-N /C-A in Acidic Medium. The

above results show that separating M-N units in metal−

organic frameworks (MOFs) can eﬀectively improve the

dispersion and concentration of metal sites in the Fe-N /C.

In order to further prove the advantage of the strategy, we used

the acidic ORR as a model pathway to check the catalytic

activity of Fe-N /C by using RRDE in HClO electrolyte (pH

1) with the LSV technique. As shown in the ORR

polarization curves (Figure S10a ), the block ratio of 2:6

derived Fe-N /C-A has the initial ORR (E_o_n_s_e_t) and half-wave

(E₁_/₂) potentials of 0.90 and 0.81 V, respectively, which are the

best among the Fe-N /C materials. In addition, the ORR

selectivity of Fe-N /C-A was studied by RRDE. Fe-N /C-A

Figure 6. (a) ORR polarization curves for Fe-N /C-A and Pt/C in

(

²^:⁶⁾^s^h^o^w^e^d^t^h^e^l^o^w^H^O2 yield (Figure S10c). The

calculated electron transfer number of Fe-N /C-A (2:6) was

pH = 13 KOH. (b) Electron transfer number (n) (right) and H O

yield (left) versus potential. (c) ORR polarization curves of Fe-N /C-

.99. This result is consistent with the ORR performance for

A and Pt/C before (solid line) and after (dashed line) 2000 cycles.

the highest iron content in Fe-N /C-A (2:6) materials.

(d) CA test of Fe-N /C-A and Pt/C at 0.60 V.

Figure 5a displays the ORR performance recorded in O₂-

saturated HClO electrolyte (pH = 1). Impressively, Fe-N /C-

A electrocatalyst displays the catalytic performance with the

most positive E_o_n_s_e_tand E₁_/₂among the three electrocatalysts.

activity of Fe-N /C-A catalysts in alkaline medium was

compared with that of the previously reported Fe-N /C

The E₁_/₂of Fe-N /C-A were about 40 mV lower than those of

Pt/C catalyst (20 wt % Pt). The ORR activity of obtained Fe-

catalysts, of which the Fe-N

/C-A catalyst is at the forefront

N /C-A in acidic electrolyte ranks in the top level of Fe-based

level (Table S7 ). A H yield is lower than 1.0%, and electron

2 2

materials (Table S6). Further calculation indicates that the

Tafel slope of Fe-N /C-A materials is 66 mV dec , a little

better than that of Pt/C (70 mV dec ). It indicates the good

transfer number is calculated to be 3.99 for Fe-N /C-A,

−

indicating a 4 e transfer process in the alkaline electrolyte

−

(Figure 6b). Moreover, Fe-N /C-A exhibits the excellent

kinetic activity of Fe-N /C-A (Figure 5b). In addition, as

durability after 2000 cycles and only 7% drop during the i-t

test. In contrast, the commercial Pt/C declines by 70 mV after

2000 cycles and displays only 80% retention under the same

illustrated in Figure 5c, the H O yield for Fe-N /C-A is low,

suggesting its excellent ORR selectivity. Additionally, the ORR

stability is also very important for their application. As

displayed in Figure 5d, we tested the electrochemical cycling

conditions (Figure 6c,d). It reveals that Fe-N /C-A has the

better ORR activity and stability than those of commercial Pt/

C at an alkaline environment. These results indicate that Fe-

stability of Fe-N /C-A in O -saturated solution with Pt/C as a

^r^e^f^e^r^eⁿ^c^e^.^A^f^t^e^r²⁰⁰⁰^C^V^c^y^c^l^e^s^,^t^h^e^E¹/2 of Fe-N /C-A shows

N /C-A is the durable ORR catalyst with good selectivity.

^oⁿ^l^y^a²⁰^m^Vⁿ^e^g^a^tⁱ^v^e^s^hⁱ^f^t^,^b^u^t^t^h^e^E¹/2 of Pt/C displays a 50

mV negative shift. Chronoamperometric (CA) tests performed

General Synthesis Strategy of M-N /C Materials with

Diﬀerent Transition Metal. The above-mentioned synthesis

strategy shows the great advantage for preparing the single-

iron-atom catalysts with eﬃcient ORR catalytic activity. As the

at 0.6 V show that Fe-N /C-A exhibits a 70% retention of the

initial current density after running for 100 h, while only 40%

retention was observed for Pt/C catalyst (Figure 5e). It further

demonstrates the excellent ORR stability and durability of the

M-N /C materials are widely used as a potential catalyst for

3,9,40,41

various chemical/electrochemical reactions,

we tried to

Fe-N /C-A catalyst.

Previous reports have shown that thiocyanide (SCN ) ions

extend the synthesis strategy for other M-N /C materials. The

−

synthesis steps of M-N /C materials with Co, Ni, or Mn were

interact strongly with Fe ions, leading to the poisoning of the

similar as those of Fe-N /C-A. The appearance of the M−N

Fe-N structure and loss of catalytic activity. To clarify the

−

bond of the coordinated compounds in the UV−visible

absorption spectra proves that the M-OPD complex was

successfully obtained (Figure S11). In the SEM and TEM

ORR active sites for Fe-N /C-A, 0.01 M SCN was supported

into the electrolyte. As shown in Figure 5f, the ORR E₁_/₂

negatively shifts 80 mV and diﬀusion-limiting current density

images of M-N /C materials, the spherical morphology and no

largely decreased compared with those for original Fe-N /C-A.

metal agglomerates are observed (Figure 7). EDS mapping also

displays that metal, nitrogen, and carbon elements are evenly

These results strongly elucidate that the active sites of the

catalyst contain the Fe-N structure, as conﬁrmed by the above

distributed. Furthermore, XRD patterns of M-N /C materials

XANES and EXAFS analyses.

show two broad peaks at 25.7° and 44.0°, corresponding to

ORR Activity of Fe-N C-A in Alkaline Medium. The

002) and (100) planes of graphite carbon, respectively

(Figure S12 ). It is conﬁrmed that no metal aggregate appears

in M-N /C materials. Therefore, the strategy is a general

excellent ORR performance of Fe-N /C-A in acid medium

prompted our further exploration under alkaline condition.

The ORR activity was evaluated in KOH electrolyte (pH = 13)

using the RRDE. Strikingly, an E₁_/₂of 0.90 V can be achieved

protocol for developing the M-N /C materials with single-

for Fe-N /C-A, similar as that of Pt/C (Figure 6a). The ORR

metal sites.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

Figure 7. (a, d, g) SEM, (b, e, h) TEM, and (c, f, i) HAADF-STEM images and corresponding EDS element mapping of (a, b, c) Co-N /C, (d, e, f)

Mn-N /C, and (g, h, i) Ni-N /C.

CONCLUSION

AUTHOR INFORMATION

■

Corresponding Authors

A facile synthesis strategy is proposed to prepare the various

kinds of M-N /C materials with the metal-coordinated

Xiaobing Wang − Key Lab for Special Functional Materials of

Ministry of Education; National & Local Joint Engineering

Research Center for High-eﬃciency Display and Lighting

Technology; School of Materials Science and Engineering;

Collaborative Innovation Center of Nano Functional

Materials and Applications, Henan University, Kaifeng

copolymers as starting materials, in which M-N units are

well dispersed and separated with a blocker in MOFs. The

block unit is highly important for separating the metal sites in

the copolymer, and the carbon retention rate of the copolymer

is substantially improved, which are beneﬁcial for inhibiting the

metal agglomeration in the pyrolysis process. With Fe-N /C

475004, People ’s Republic of China; Email: 10330083@

materials as model samples, it is conﬁrmed that the existence of

single-iron atoms is enriched by several advanced techniques

vip.henu.edu.cn

Yong Zhao − Key Lab for Special Functional Materials of

Ministry of Education; National & Local Joint Engineering

Research Center for High-eﬃciency Display and Lighting

Technology; School of Materials Science and Engineering;

Collaborative Innovation Center of Nano Functional

Materials and Applications, Henan University, Kaifeng

such as XANES and Mossbauer spectra. Furthermore, Fe-N_x/

C materials show eﬃcient ORR activity and stability in acidic

and alkaline electrolytes. We believe that the above synthesis

strategy will provide a general route for the preparation of M-

N /C catalysts, which can be widely used as the eﬃcient

catalysts for diﬀerent chemical/electrochemical reactions.

475004, People ’s Republic of China; orcid.org/0000-

002-5039-4576 ; Email: zhaoyong@henu.edu.cn

ASSOCIATED CONTENT

■

Authors

Bing Hu − Key Lab for Special Functional Materials of

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02420.

Ministry of Education; National & Local Joint Engineering

Research Center for High-eﬃciency Display and Lighting

Technology; School of Materials Science and Engineering;

Collaborative Innovation Center of Nano Functional

Materials and Applications, Henan University, Kaifeng

XPS, TEM, TGA, BET, XRD, UV−visible absorption

spectra, Mossbauer ﬁtting parameters, ICP-MS data,

elemental content, and other associated electrochemical

CV analyses (PDF)

75004, People’s Republic of China

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

for Efficient Oxygen Reduction Reaction and Rechargeable Zn-Air

Article

Xuebing Zhu − Key Lab for Special Functional Materials of

Ministry of Education; National & Local Joint Engineering

Research Center for High-eﬃciency Display and Lighting

Technology; School of Materials Science and Engineering;

Collaborative Innovation Center of Nano Functional

Materials and Applications, Henan University, Kaifeng

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75004, People’s Republic of China

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Chenxiao Wang − Key Lab for Special Functional Materials of

Ministry of Education; National & Local Joint Engineering

Research Center for High-eﬃciency Display and Lighting

Technology; School of Materials Science and Engineering;

Collaborative Innovation Center of Nano Functional

Materials and Applications, Henan University, Kaifeng

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Jinling He − Key Lab for Special Functional Materials of

Ministry of Education; National & Local Joint Engineering

Research Center for High-eﬃciency Display and Lighting

Technology; School of Materials Science and Engineering;

Collaborative Innovation Center of Nano Functional

Materials and Applications, Henan University, Kaifeng

(10) Li, J.; Jiao, L.; Wegener, E.; Richard, L. L.; Liu, E.; Zitolo, A.;

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Wang, Z.; Wu, G. Thermally Driven Structure and Performance

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c02420

Evolution of Atomically Dispersed FeN Sites for Oxygen Reduction.

Angew. Chem., Int. Ed. 2019, 58, 18971−18980.

Author Contributions

B.H. and X.Z. contributed equally to the paper. The

manuscript was written through contributions of all authors.

All authors have given approval to the ﬁnal version of the

manuscript.

(

12) Zhao, L.; Zhang, Y.; Huang, L. B.; Liu, X. Z.; Zhang, Q. H.; He,

†

C.; Wu, Z. Y.; Zhang, L. J.; Wu, J.; Yang, W.; Gu, L.; Hu, J. S.; Wan, L.

J. Cascade Anchoring Strategy for General Mass Production of High-

Loading Single-Atomic Metal-Nitrogen Catalysts. Nat. Commun.

(

019, 10, 1278.

13) Sa, Y. J.; Seo, D. J.; Woo, J.; Lim, J. T.; Cheon, J. Y.; Yang, S. Y.;

Notes

Lee, J. M.; Kang, D.; Shin, T. J.; Shin, H. S.; Jeong, H. Y.; Kim, C. S.;

Kim, M. G.; Kim, T. Y.; Joo, S. H. A General Approach to Preferential

Formation of Active Fe-N _X Sites in Fe-N/C Electrocatalysts for

Efficient Oxygen Reduction Reaction. J. Am. Chem. Soc. 2016, 138,

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

(

5046−15056.

This work is supported by the National Natural Science

Foundation of China (21773055, 51702086, U1604122,

14) Hu, B. C.; Wu, Z. Y.; Chu, S. Q.; Zhu, H. W.; Liang, H. W.;

Zhang, J.; Yu, S. H. SiO -Protected Shell Mediated Templating

1805070) and the Program for Science & Technology

Synthesis of Fe-N-Doped Carbon Nanofibers znd Their Enhanced

Oxygen Reduction Reaction Performance. Energy Environ. Sci. 2018,

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