General Strategy to Fabricate Metal-Incorporated Pyrolysis-Free Covalent Organic Framework for Efficient Oxygen Evolution Reaction

Zhi Gao, Le Le Gong, Xiang Qing He, Xue Min Su, Long Hui Xiao, and Feng Luo*

Article

General Strategy to Fabricate Metal-Incorporated Pyrolysis-Free

Covalent Organic Framework for Eﬃcient Oxygen Evolution

Reaction

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c00235

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sı Supporting Information

ABSTRACT: Because of the permission of the manipulations of modular construction on

the atomic level, covalent organic frameworks (COFs) have attracted extensive attention in

the electrocatalytic ﬁeld. Owing to the lack of metal sites in pristine COFs constructed only

by metal-free organic building units, it generally exhibits extremely low electrocatalytic

activity. Thereby, linking metal sites on the backbone of pyrolysis-free COFs but not loading

them on the surface to enhance the electrocatalytic activity is highly desirable but still

remains a huge challenge. To this end, herein, we report an eﬃcient and general cation-

exchange strategy to synthesize Ni/Fe metal-ion-incorporated COFs (Ni Fe₁₋_x@COF-SO₃)

for the oxygen evolution reaction (OER) based on the fundamental structure design of

COFs. Impressively, the turnover frequency (TOF) value in Ni Fe @COF-SO reaches

.5 0.5

−

.14 s at the overpotential of 300 mV, which outperforms most recently reported OER

electrocatalysts, indicative of ultrahigh metal-atom utilization eﬃciency.

INTRODUCTION

Transition-metal-based heterogeneous catalysts are the most

widely used in a large number of important chemical reactions

of metal sites. Thus, presently they have mainly served as

precursors to fabricate various electrocatalysts via high-

■

temperature pyrolysis. Only few metal-incorporated pyrol-

ysis-free COF materials were reported as electrocatalysts for

the oxygen evolution reaction (OER). In these reports,

incorporating a catalysis-active metal center into the porphyrin

unit in the COF and simultaneous combination with excellent

owing to their low cost, high stability, and repeatability, but

their catalytic activity is often limited by the low atom

utilization eﬃciency. As is already known, homogeneous

catalysts can fully contact with reactant and thus improve the

catalytic performance. However, the poor recyclability

seriously hinders their industrial application. In recent years,

conductive support present an eﬀective strategy. However,

from the viewpoint of the COF structure, the strong π−π

interactions between adjacent porphyrin rings would to some

extent aﬀect the accessibility of the metal center during the

reaction process and consequently reduce metal-atom

utilization eﬃciency. On the other hand, pristine COFs

also can directly serve as support to load metal species for the

OER. For example, the sp N-rich ﬂexible COF and low-band-

gap benzimidazole COF have been reported to be ideal

supports to load nanoscale Co Ni (OH) and Ni N taking

fabrication of single-atom catalysts has attracted extensive

attention because they can combine the advantages of

homogeneous and heterogeneous catalysts, exhibiting unique

catalytic properties and maximized atom eﬃciency for low

cost. Nevertheless, the preparation methods for single-atom

catalysts often encounter harsh reaction conditions and

multistep synthesis processes, overshadowing the industrial

application. Thus, developing a simple and general strategy to

maximize the atom utilization eﬃciency of catalysts is

meaningful but remains a huge challenge.

Covalent organic frameworks (COFs), constructed by

strong covalent bonds using various organic molecules as

building units, possess uniform pore structure, high accessible

advantage of the nanoscopic conﬁnement of the pore structure,

,11a

respectively.

The resulting catalysts exhibited excellent

performance for the OER. However, starting from the

molecular design based on the variability of organic building

units, developing a general and simple approach to fabricate

surface area, and excellent chemical stability. In particular,

they can be facilely custom-made and precisely modiﬁed by

choosing appropriate functionalized organic units. Owing to

these merits, we believe that the COFs can be elaborately

designed to maximize the atom utilization eﬃciency. In recent

years, COFs have attracted much attention as promising

Received: January 22, 2020

electrocatalysts. Pristine COFs usually aﬀord poor con-

ductivity and low electrocatalytic activity because of the lack

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 1. Schematic Illustration of the Preparation Process of Ni Fe @COF-SO

.5 0.5

metal-incorporated pyrolysis-free COF electrocatalysts for the

OER process with high atom utilization eﬃciency is still virgin.

In this work, a general and eﬀective cation-exchange strategy

was proposed to incorporate the metal sites into the COFs’

skeleton. As shown in Scheme 1, this approach contains three

RESULTS AND DISCUSSION

■

We ﬁrst prepared COF-SO H by reacting 2,4,6-triformylphlor-

oglucinol and 2,5-diaminobenzenesulfonic under solvothermal

conditions via a Schiﬀ base reaction according to our previous

work. NH @COF-SO was subsequently obtained by

steps, one being the deliberate design of −SO H-anchored

ammoniating COF-SO H in concentrated ammonia−water.

COF, COF-SO H, one being the amination of pristine COF-

The metal-ion-incorporated Ni Fe @COF-SO (x = 0, 0.2,

1−x

SO H by NH ·H O to generate the ammoniated phase of

0.5, 0.8, 1.0) with diﬀerent components were easily prepared

by a cation-exchange process between NH₄⁺and metal-

NH @COF-SO , and the next one being the ion exchange

between NH and metal ions to obtain the metal-incorporated

including cations. As shown in Figure 1a,b, COF-SO H shows

the eclipsed stacking ABA mode with the stacking distance of

.4 Å determined using the Material Studio program, in which

phase of Ni Fe @COF-SO . Density functional theory

1−x

(

DFT) calculations demonstrate the successful formation of

−

the coordination bond between the metal and oxygen atom of

SO H units among adjacent layers hold the opposite

−

SO₃units in the COF. The OER activity indicates that the

arrangement fashion rather than the common same-side

mode. Deprotonation is achieved in NH @COF-SO , and

cation-exchange strategy based on the fundamental molecular

design can improve the atom utilization eﬃciency, enabling a

high TOF of 0.14 s⁻in optimized Ni Fe @COF-SO at the

NH₄⁺ions are located on the pore and further stabilized by

hydrogen bonds (Figure 1c).

0.5 0.5

overpotential of 300 mV, which is superior to most recently

reported outstanding OER electrocatalysts. Moreover, the

simultaneous incorporation of Ni and Fe can induce signiﬁcant

bimetallic electronic interaction, thus beneﬁting for the further

improvement of the intrinsic activity of single metal sites.

The crystalline nature and structures of metal-free COF-

SO H and NH @COF-SO , as well as Ni/Fe-incorporated

Ni Fe₁₋_x@COF-SO₃(x = 0, 0.2, 0.5, 0.8, 1.0), were

determined by powder X-ray diﬀraction (PXRD) and the

Materials Studio Forcite molecular dynamics module method.

Inorg. Chem. XXXX, XXX, XXX−XXX

structure without the presence of any undesired species such as

Article

starting metal precursor, metallic Ni/Fe, or Ni/Fe-oxide

(

Figure 2b and Figure S1). C CP-MAS solid-state NMR

spectroscopy further demonstrates the β-ketoenamine-linked

structure in COF-SO H, NH @COF-SO , and Ni Fe @

0.5 0.5

COF-SO (Figure 2c,d).

Infrared (IR) spectra were performed to further understand

the structure of these COFs. IR spectra of diﬀerent COFs

including COF-SO H, NH @COF-SO , and Ni Fe₁₋_x@COF-

SO (x = 0, 0.2, 0.5, 0.8, 1.0) all show two bands at about 1272

−

−1

cm (CN) and 1586 cm (CC) assigned to β-

ketoenamine-based framework structures (Figure 3A). The

−1

characteristic bands at 1079, 814, 705, and 533 cm conﬁrm

−

−1

the appearance of SO species. A new band at 3136 cm

(

NH with hydrogen bonds) was detected in NH @COF-SO

compared to COF-SO H, which strongly proves the successful

deprotonation of the SO H unit under ammonium

hydroxide solution. In comparison, Ni Fe @COF-SO (x =

1−x

Figure 1. Space-ﬁlling view of the structure of COF-SO H (a, b) and

, 0.2, 0.5, 0.8, 1.0) exhibits relatively weaker intensity at 3136

NH @COF-SO H (c).

−1

cm , indicating that a part of NH ions in NH @COF-SO

are exchanged by metal-contained cations, well consistent with

the low metal content in Ni Fe1−x@COF-SO (x = 0, 0.2, 0.5,

0.8, 1.0) determined by inductively coupled plasma atomic

As shown in Figure 2a, PXRD patterns of COF-SO H and

NH @COF-SO match well with the corresponding simulated

data, proving their successful synthesis. After incorporating

metal ions, the resulting Ni Fe @COF-SO still maintains

emission spectroscopy (ICP-AES) tests (Table S1).

As demonstrated by XPS survey spectra, COF-SO

NH @COF-SO are only composed of C, N, O, and S

H and

1−x

the characteristic diﬀraction of NH @COF-SO with no extra

peaks, indicating the retention of the robust COF framework

elements, and the successful incorporation of Ni and Fe in

Ni Fe @COF-SO is doubtless (Figure 3B). The N 1s

.5 0.5

Figure 2. (a, b) PXRD patterns of diﬀerent samples. (c) Schematic for diﬀerent carbon atoms in the COF. (d) Solid state ¹³C CP-MAS spectrum

of COF-SO H, NH @COF-SO , and Ni Fe @COF-SO .

0.5 0.5

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 3. (A) IR spectrum of COF-SO H (a), NH @COF-SO (b), Ni@COF-SO (c), Fe@COF-SO (d), Ni Fe @COF-SO (e), Ni Fe @

0.2 0.8

0.5 0.5

COF-SO (f), and Ni Fe @COF-SO (g). (B) XPS survey spectra of Ni Fe @COF-SO (a), COF-SO H (b), and NH @COF-SO (c). (C)

0.8 0.2

0.5 0.5

XPS spectra of the N 1s region in COF-SO H (a), NH @COF-SO (b), and Ni Fe @COF-SO (c).

0.5 0.5

spectra of COF-SO H, NH @COF-SO , and Ni Fe @COF-

0.5 0.5

SO are compared. As shown in Figure 3C, only one peak at

00.0 eV related to secondary N was detected in COF-SO H,

while a new peak at about 402.0 eV assigned to NH₄was

found in NH @COF-SO , indicating the successful deproto-

nation of the −SO H group in COF-SO H. It should be

noticed that the peak intensity related to NH₄is obviously

weaker in Ni Fe @COF-SO relative to NH @COF-SO ,

which undoubtedly proves that NH₄in NH @COF-SO is

.5 0.5

partially exchanged by metal-contained cations. From the

above results, it is evidential that COF-SO H, NH @COF-

SO , and metal-ion-incorporated Ni Fe @COF-SO (x = 0,

1−x

.2, 0.5, 0.8, 1.0) have been successfully synthesized.

The periodic density functional theory (DFT) calculations

were performed using the Vienna ab initio simulation package

VASP) code to further determine the connection mode

(

between the metal and COF backbone. Spin-polarization was

Figure 4. SEM images of COF-SO

H (a), NH @COF-SO (b), and

3 4 3

Ni Fe @COF-SO (c). (d) TEM image of Ni Fe @COF-SO .

calculated to determine the lowest energy magnetic conﬁg-

0.5 0.5

urations. Ni and Fe elements belong to the same class of VIII

and possess similar physical and chemical properties. Thereby,

determine the nanoﬁber network structure in Ni Fe @COF-

.5 0.5

the coordination modes in bimetallic NiFe@COF-SO should

SO without the detectable nanoparticles (Figure 4d and

be the same as those in monometallic Ni@COF-SO . Thus,

Figure S5). SEM energy dispersive spectrometry (EDS)

mapping reﬂects the uniform distribution of Ni, Fe, C, O, N,

and S elements in Ni Fe @COF-SO (Figure S6).

only the Ni@COF-SO was investigated in detail. As shown in

Figure S2 , the Ni site in Ni@COF-SO takes the common

0.5 0.5

octahedral geometry ﬁnished by three coordinated water

Upon acquiring the structural information, we are now in a

position to examine the OER activities of Ni Fe₁₋_x@COF-SO₃

samples (x = 0, 0.2, 0.5, 0.8, 1) containing monometallic and

bimetallic COFs, as well as metal-free NH @COF-SO and the

molecules, one chlorine atom, and two oxygen atoms of

−

SO .

In order to further disclose the structural information, N₂

adsorption−desorption, scanning electron microscopy (SEM),

and transmission electron microscopy (TEM) were carried

commercial IrO benchmark in a standard three-electrode

system. According to the linear sweep voltammetry (LSV)

curves (Figure 5a), the metal-free NH @COF-SO requires

the overpotential of as high as 543 mV to deliver 10 mA cm ,

indicative of extremely poor activity for the OER. In the case of

out. The 77 K N adsorption−desorption tests (Figure S3)

−2

indicate that COF-SO H possesses the BET surface area of

−1

06 m g and pore size of 1.2 nm. After ammoniating, the

resulting NH @COF-SO delivers smaller BET surface area

metal-incorporated Ni Fe @COF-SO electrodes, the sig-

niﬁcantly lower overpotential is required to aﬀord the same

1−x

−1

(

110 m g ). After incorporating metal ions, the BET surface

−1

area increases to 281 m g in Ni Fe @COF-SO , which

may possibly be due to the partial replacement of NH₄with a

large ion radius (1.43 Å) by metal ions with a relatively smaller

current density (Figure 5b), suggesting that metal centers are

0.5 0.5

the key active sites for the OER process. Moreover, the

bimetallic COFs require lower overpotentials to reach the

−2

ion radius (Ni for 0.69 Å and Fe for 0.61 Å). The SEM

current density of 10 mA cm as compared to monometallic

image of COF-SO H shows a well-deﬁned nanoﬁber network

COFs (Ni@COF-SO and Fe@COF-SO ). Among diﬀerent

structure (Figure 4a), while NH @COF-SO shows a similar

bimetallic COFs, Ni Fe @COF-SO with the Ni/Fe mass

0.5 0.5 3

morphology (Figure 4b). After incorporating metal ions, the

resulting Ni Fe @COF-SO (x = 0, 0.2, 0.5, 0.8, 1.0) still can

ratio of 0.5:0.5 exhibits the lowest overpotential of 308 mV at

−

10 mA cm , which is even better than the commercial IrO

1−x

maintain the initial morphology (Figure 4c and Figure S4).

TEM and high-resolution TEM (HRTEM) images further

benchmark (327 mV). The corresponding Tafel slopes derived

from LSV curves are also analyzed to gain insight into the

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 5. (a) LSV curves toward the OER of Ni Fe @COF-SO (x = 0, 0.2, 0.5, 0.8, 1), NH @COF-SO , and commercial IrO . (b)

1−x

−

Overpotential at 10 mA cm . (c) Tafel slopes. Mass activities (d) and TOFs (e) at the overpotential of 300 mV in diﬀerent samples. (f) Nyquist

plots at the overpotential of 308 mV. (g) ECSA evaluation. (h) Current density at 400 mV overpotential plotted against ECSA (2C ) of diﬀerent

samples. (i) Chronoamperometric curves of Ni Fe @COF-SO .

.5 0.5

COFs electrodes. As shown in Figure 5c, the Tafel slope of

instead of COF-SO H. ICP-AES results reveal that the metal

−

Ni Fe @COF-SO is 83 mV dec , being much smaller than

species are not detected in −SO H-free Ni Fe @COF

0.5 0.5

those of other COFs including Ni@COF-SO (118 mV

(Table S1 ), and the negligible OER activity in Ni Fe @COF

0.5 0.5

−

−1

dec ), Fe@COF-SO (123 mV dec ), Ni Fe @COF-SO

is proven (Figure S8 ), which further demonstrates that the

0.2 0.8

−

−1

(

113 mV dec ), Ni Fe @COF-SO (103 mV dec ), and

NH @COF-SO (267 mV dec ), which reﬂects the fastest

synthesis of −SO H-including COF-SO H plays a pivotal role

0.8 0.2

−

to achieve the incorporation of metal species and excellent

OER activity.

OER kinetics in Ni Fe @COF-SO .

.5 0.5

In order to demonstrate the crucial role of −SO H units in

In addition, the mass activity (MA) as an important

quantitative metric was determined to assess the OER

performance of diﬀerent electrodes. As shown in Figure 5d,

COF-SO H for OER catalysis, the p-phenylenediamine and

,4,6-triformylphloroglucinol were used as building units to

synthesize the −SO H-free COF according to the method

at the overpotential of 300 mV, the MA of Ni Fe @COF-

0.5 0.5

−1

reported by U. K. Kharul et al. As shown in Figure S7, the

SO (52.31 A g ) is much higher than those of Ni@COF-SO

−

−1

XRD pattern of the −SO H-free COF matches well with the

(2.94 A g ), Fe@COF-SO (0.51 A g ), Ni Fe @COF-

0.2 0.8

−1

−

data reported, demonstrating its successful synthesis. Then,

SO (3.28 A g ), Ni Fe @COF-SO (12.46 A g ), and

3 0.8 0.2 3

−

incorporating metal ions to the −SO H-free COF was carried

commercial IrO (29.78 A g ), well in accordance with the

out according to the same procedure for the synthesis of

lowest overpotential. To further disclose the intrinsic activity,

the TOF was calculated at an overpotential of 300 mV

Ni Fe @COF-SO in the presence of −SO H-free COFs

0.5 0.5

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 6. XPS spectra of Ni 2p (a) and Fe 2p (b) regions in diﬀerent catalysts.

assuming that the metal sites in Ni Fe @COF-SO are active

times improvement. This strongly proves that the remarkably

1−x

(

Figure 5e). Remarkably, the TOF value of Ni Fe @COF-

enhanced OER activity in bimetallic COF (Ni Fe @COF-

.5 0.5

0.5 0.5

−

SO reaches 0.1442 s , which is about 17.59-, 103-, 17.59-,

SO ) relative to monometallic COF (Fe@COF-SO ) is

.29-, and 9.74-fold higher than those in Ni@COF-SO

attributed to not only the increased ECSA, but probably also

the strong electronic eﬀect between Ni and Fe sites, which

would result in the increased intrinsic catalytic activity.

Taking into account that the durability of electrocatalysts is

−

−1

(

0.0082 s ), Fe@COF-SO (0.0014 s ), Ni Fe @COF-

0.2 0.8

−1

−

SO (0.0082 s ), Ni Fe @COF-SO (0.0336 s ), and

0.8 0.2

−

commercial IrO₂(0.0148 s ), respectively, exhibiting a

signiﬁcant bimetallic synergistic eﬀect. Based on the activity

metric mentioned above (e.g., overpotential, Tafel slope, mass

activity, and TOF), it can be reasonably concluded that

bimetallic COFs possess better OER activity relative to

monometallic COFs, and Ni Fe @COF-SO is the best

one among Ni Fe @COF-SO (x = 0, 0.2, 0.5, 0.8, 1.0)

electrodes and even superior to the commercial IrO₂

benchmark.

22a

of crucial importance for practical applications, the long-

term stability of Ni Fe @COF-SO was evaluated by a

.5 0.5

chronoamperometric (i−t) measurement at diﬀerent poten-

tials. Impressively, Ni Fe @COF-SO can maintain the

.5 0.5

−

current densities of 20, 50, and 100 mA cm at three

diﬀerent applied potentials (Figure 5i), respectively, exhibiting

excellent stability. Besides, the PXRD pattern demonstrates

that Ni Fe @COF-SO after chronoamperometric measure-

0.5 0.5

1−x

.5 0.5

To determine the reason for the enhanced OER activity in

bimetallic COFs, the possible factors that may aﬀect the

performance were carefully explored. First, electrochemical

impedance spectra (EIS) of diﬀerent samples were measured.

As revealed by Nyquist plots (Figure 5f), the semicircle

diameter of Ni Fe @COF-SO is smaller than those of other

ment still can keep its initial crystalline structure without the

formation of nanoparticles determined by HRTEM (Figures

S10 and S11). The nanoﬁber network structure of Ni Fe @

.5 0.5

COF-SO and the uniform distribution of diﬀerent elements in

it are also well retained, as demonstrated by SEM and the

corresponding EDX mapping images (Figure S12 ). Moreover,

the binding energies of Ni 2p and Fe 2p in Ni Fe @COF-

.5 0.5

samples, proving more highly eﬃcient electron transport at the

.5 0.5

electrolyte−electrode interface, which will promote the

electrocatalytic OER process. Second, the electrochemical

active surface area (ECSA) is determined based on the double-

SO after OER stability tests shift to higher values compared to

those in fresh Ni Fe @COF-SO , which should be due to

.5 0.5

the possible formation of NiFe-hydroxide during the prolonged

22b

layer capacitance (C ) estimated by the simple cyclic

water oxidation process (Figure S13).

voltammogram (CV) method.

Clearly, the C_d_lin

X-ray photoelectron spectroscopy (XPS) characterization

was carried out to disclose the electronic interaction between

−

Ni Fe @COF-SO (0.237 mF cm ) is higher than those

.5 0.5

−

in Ni Fe @COF-SO (0.163 mF cm ), Ni Fe @COF-

Ni and Fe sites on OER activity. In the case of NiFe-based

.8 0.2

0.2 0.8

−2

−

−2

SO₃(0.103 mF cm ), Ni@COF-SO (0.084 mF cm ), and

bimetallic electrocatalysts, Ni is generally regarded as the

−

−2

Fe@COF-SO₃(0.077 mF cm ) (Figure 5g). The best OER

activity in Ni Fe @COF-SO may be related to the largest

mainly active sites for the OER, while the incorporation of Fe

can modulate the electronic state of Ni, leading to the shift of

Ni 2p binding energy to a higher value and thus enhancing the

.5 0.5

ECSA. CV curves measured in the non-Faradaic potential

range 1.23−1.33 V vs the reversible hydrogen electrode (RHE)

are presented in Figure S9. Moreover, the relationship of

current density at the overpotential of 400 mV with ECSA in

diﬀerent COFs was also investigated. As shown in Figure 5h,

the samples possessing larger ECSA exhibit higher current

density, further conﬁrming that the ECSA of catalysts is

actually a vital contributor for improving the OER perform-

OER performance. From the Ni 2p spectra (Figure 6a), it

can be seen that these COFs present two typical peaks at about

856 and 874 eV, which are assigned to Ni 2p and Ni 2p₁_/₂

electronic conﬁgurations, respectively. Obviously, the bind-

ing energy of Ni 2p in bimetallic COFs is higher than that of

monometallic COF (Ni@COF-SO ) while the binding energy

of Fe 2p in bimetallic COFs transfers to a lower value

compared to that in Fe@COF-SO₃(Figure 6b), which

indicates that incorporating Fe can change the coordination

environment of Ni species and thus promote the formation of

strong Ni−Fe electronic interactions. Among the bimetallic

COFs, Ni Fe @COF-SO shows the highest Ni 2p binding

ance. However, after investigating this result carefully, we can

ﬁnd that the ECSA has more than a 3-fold increase from 0.154

−

−2

mF cm in Fe@COF-SO to 0.474 mF cm in Ni Fe @

0.5 0.5

COF-SO , while the corresponding current density increases

from 4.66 to 48.66 mA, showing signiﬁcantly an almost 11

.5 0.5

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

330013, P. R. China; orcid.org/0000-0001-6380-2754;

Article

energy, implying that the strongest Ni−Fe electronic

interaction is generated when the mass ratio of Ni/Fe is

equal to 0.5:0.5. Noticeably, the COFs possessing a higher

binding energy exhibit a more excellent OER performance.

These results are in accordance with the conclusions in the

reported literature, in which the synergetic eﬀect of mixed

metals can eﬃciently modulate the electronic property of

Email: ecitluofeng@163.com

Authors

Zhi Gao − State Key Laboratory of Nuclear Resources and

Environment, School of Chemistry, Biology and Materials

Science, East China University of Technology, Nanchang

330013, P. R. China

26,27

active sites, thereby enhancing the OER activity.

There-

fore, it can be reasonably concluded that the optimized

electronic structure of Ni active sites induced by Fe plays a

crucial role for improving the OER activity.

Le Le Gong − State Key Laboratory of Nuclear Resources and

Environment, School of Chemistry, Biology and Materials

Science, East China University of Technology, Nanchang

330013, P. R. China

Xiang Qing He − State Key Laboratory of Nuclear Resources

and Environment, School of Chemistry, Biology and Materials

Science, East China University of Technology, Nanchang

330013, P. R. China

Furthermore, in order to further demonstrate the existence

of a synergy interaction between Ni and Fe in bimetallic

Ni Fe @COF-SO for OER catalysis, the mechanical

.5 0.5

mixtures of Ni@COF-SO and Fe@COF-SO (Ni@COF-

SO /Fe@COF-SO ) were prepared, and the corresponding

OER performance was investigated. As shown in Figure S14,

the OER activity of Ni@COF-SO /Fe@COF-SO is much

Xue Min Su − State Key Laboratory of Nuclear Resources and

Environment, School of Chemistry, Biology and Materials

Science, East China University of Technology, Nanchang

330013, P. R. China

lower than that in bimetallic Ni Fe @COF-SO , which

0.5 0.5

further conﬁrms that the synergy interaction between Ni and

Fe in Ni Fe @COF-SO plays a crucial role for the

enhanced OER performance.

Long Hui Xiao − State Key Laboratory of Nuclear Resources

and Environment, School of Chemistry, Biology and Materials

Science, East China University of Technology, Nanchang

.5 0.5

30013, P. R. China

CONCLUSIONS

■

In summary, we have demonstrated that the cation-exchange

strategy based on the molecular design of COFs is a general

way to obtain a metal-incorporated pyrolysis-free COF for

OER electrocatalysis. The resulting COF electrocatalysts

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c00235

Notes

(

Ni Fe @COF-SO , x = 0, 0.2, 0.5, 0.8, 1.0) can preserve

The authors declare no competing ﬁnancial interest.

1−x

the initial structure of metal-free NH @COF-SO . The

optimized Ni Fe @COF-SO with only 5 wt % metal

.5 0.5

ACKNOWLEDGMENTS

−

■

content shows high TOF values of 0.14 s , which is superior

to the corresponding monometallic COFs and most recently

reported excellent OER electrocatalysts, presenting outstand-

ing metal-atom utilization eﬃciency. Moreover, the high

chemical stability and OER stability of Ni Fe @COF-SO

This work was ﬁnancially supported by the Natural Science

Foundation of Jiangxi Province of China (20192BAB213001),

Foundation of Jiangxi Educational Committee (GJJ180406),

Research Foundation for Advanced Talents of East China

University of Technology (DHBK2018043), and the National

Natural Science Foundation of China (21871047; 21661001).

0.5 0.5

are also determined. The excellent performance in Ni Fe @

.5 0.5

COF-SO is ascribed to the prominent electronic synergistic

eﬀect and the unique advantage of the COFs’ structure, which

aﬀords enhanced intrinsic activity of single metal sites,

favorable charge transfer, and mass transport. The general

and eﬀective strategy proposed in this work opens a new

avenue to design metal-incorporated pyrolysis-free COF

electrocatalysts, which also exhibits huge potential in other

ﬁelds beyond the OER.

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ASSOCIATED CONTENT

sı Supporting Information

■

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00235.

Additional data and ﬁgures including XRD patterns,

structures, nitrogen adsorption isotherm, SEM images,

HRTEM images, SEM-EDS mappings, LSV curves, CV

plots, PXRD image, and high-resolution Ni 2p and Fe 2p

spectra (PDF )

Ni(OH) Heterostructure for an Enhanced Oxygen Evolution

AUTHOR INFORMATION

■

Reaction. Nanoscale 2019, 11, 3599−3605. (g) Ren, H.; Sun, X.;

Du, C.; Zhao, J.; Liu, D.; Fang, W.; Kumar, S.; Chua, R.; Meng, S.;

Kidkhunthod, P.; Song, L.; Li, S.; Madhavi, S.; Yan, Q. Amorphous

Fe −Ni −P −B −O Nanocages as Efficient Electrocatalysts for Oxygen

Evolution Reaction. ACS Nano 2019, 13, 12969−12979.

Corresponding Author

Feng Luo − State Key Laboratory of Nuclear Resources and

Environment, School of Chemistry, Biology and Materials

Science, East China University of Technology, Nanchang

Inorg. Chem. XXXX, XXX, XXX−XXX

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