Atomic-Scale CoOx Species in Metal-Organic Frameworks for Oxygen Evolution Reaction

TLDR: In this article, a targeted on-site formation of atomic-scale CoOx species is realized in ZIF-67 by O-2 plasma, where abundant pores in Zif-67 provide channels for O2 plasma to activate the Co ions in metal-organic frameworks (MOFs), which act as active sites to catalyze the oxygen evolution reaction with an even better activity than RuO2.

Abstract: The activity of electrocatalysts strongly depends on the number of active sites, which can be increased by downsizing electrocatalysts. Single-atom catalysts have attracted special attention due to atomic-scale active sites. However, it is a huge challenge to obtain atomic-scale CoOx catalysts. The Co-based metal-organic frameworks (MOFs) own atomically dispersed Co ions, which motivates to design a possible pathway to partially on-site transform these Co ions to active atomic-scale CoOx species, while reserving the highly porous features of MOFs. In this work, for the first time, the targeted on-site formation of atomic-scale CoOx species is realized in ZIF-67 by O-2 plasma. The abundant pores in ZIF-67 provide channels for O-2 plasma to activate the Co ions in MOFs to on-site produce atomic-scale CoOx species, which act as the active sites to catalyze the oxygen evolution reaction with an even better activity than RuO2.

Figures

Figure 2. (a) TEM and (b) HAADF-STEM images of CoOx-ZIF and (c) the corresponding SAED pattern; (d) element mapping of Co, O, C, N in CoOx-ZIF;(e), (f) enlarged HAADFSTEM images of CoOx-ZIF (the atom of Co are marked by red circles).

Figure 4. (a) LSV curves for OER on ZIF-67, CoOx-ZIF, CoOx-ZIF/C, and RuO2; (b) normalized LSV curves on CoOx-ZIF and ZIF-67 by BET surface area; (c) the corresponding Tafel plots from LSV curves; (d) EIS of CoOx-ZIF and ZIF-67 recorded at a constant potential of 1.55 V vs. RHE; (e) Ring current of CoOx-ZIF on an RRDE with a ring potential of 1.5 V in O2-saturated 1 M KOH; (f) Ring current of CoOx-ZIF on an RRDE with a ring potential of 0.40 V in N2-saturated 1 M KOH.

Figure 1. (a) Preparation of CoOx-ZIF; SEM images of (b) pure ZIF-67 and (c) CoOx-ZIF (the inset shows the enlarged images and the scale bar is 200 nm), (d) XRD patterns and (e) Co 2p 3/2 XPS peaks of ZIF-67 and CoOx-ZIF.

Figure 3. (a) The XANES spectra at Co K-edge of ZIF-67, CoOx-ZIF, and references: Co metal, CoO, and Co3O4; (b) The overlaid XANES spectra of ZIF-67 and CoOx-ZIF; (c) The Fourier transfer spectra of at Co K-edge and (inset) k3 EXAFS oscillations for ZIF-67 and CoOx-ZIF.

Full text

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1
Targeted On-site Formation of Atomic-scale CoO
x
Species in Metal-
Organic-Frameworks for Oxygen Evolution Reaction
Shuo Dou,
[+],a
Chung-Li Dong,
[+],b
Zhe Hu,
[+] ,c
Yu-Cheng Huang,
b
Jeng-lung Chen,
b
Li Tao,
a
Dafeng Yan,
a
Dawei Chen,
a
Jia Huo,
a
Shaohua Shen,
d,
* Shulei Chou,
c,
* Bo Wang,
e
and
Shuangyin Wang
a,
*
a
State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and
Chemical Engineering, Hunan University, Changsha, 410082, P. R.China.
E-mail: shuangyinwang@hnu.edu.cn
b
Department of Physics, Tamkang University, Tamsui, Taiwan.
c
Institute for Superconducting and Electronic Materials, University of Wollongong,
Wollongong, New South Wales 2522, Australia.
d
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University,
Shaanxi 710049, P. R. China.
e
School of Chemistry, Beijing Institute of Technology, South Zhongguancun Street, Beijing,
10081, P. R. China.Address line 1, Address line 2, postcode, Country
[+]
These authors contributed equally to this work
Abstract:
The activity of electrocatalysts strongly depends on the number of active sites, which can be
increased by down-sizing electrocatalysts. Single-atom catalysts have attracted special
attentions due to the atomic-scale active sites. However, it is a huge challenge to obtain
atomic-scale CoO
x
catalysts. While, the Co-based metal-organic-frameworks (MOFs) own
atomically-dispersed Co ions, which motivates us to design a possible pathway to partially
on-site transform these Co ions to active atomic-scale CoO
x
species while reserving the
highly-porous features of MOFs. In this work, we, for the first time, realized the targeted on-
site formation of atomic-scale CoO
x
species in ZIF-67 by O
2
plasma. The abundant pores in
ZIF-67 provide channels for O
2
plasma to activate the Co ions in MOFs to on-site produce
atomic-scale CoO
x
species, which act as the active sites to catalyze the oxygen evolution
reaction with an even better activity than RuO
2
.
Keywords: electrocatalyst, oxygen evolution, metal-organic frameworks, atomic-scale CoO
x

2
1. Introduction
The electrochemical oxygen evolution reaction (OER) has been regarded as the core process
in metal-air batteries and water splitting devices.
[1-2]
Due to the sluggish reaction kinetics,
exploring highly efficient OER electrocatalysts is of significant demand. Noble metal oxides
of Ru/Ir are the most active OER electrocatalysts for the low overpotential and large current
density.
[3]
However, they are suffered from high cost and poor durability, which hinders the
application of these materials.
To solve this problem, many studies have been carried out to develop highly efficient and
low-cost OER electrocatalysts, such as developing transition metal compound and even
metal-free materials
[4]
. Cobalt-based materials
[5-7]
are promising alternatives to replace noble
metal oxides for OER. Especially, Co oxides have been extensively developed due to their
high performance.
[8-12]
Since the electrocatalytic process only occurs on the surface of
catalysts, it is essential to downsize Co-based species with more catalytically active sites
exposed. To this end, pushing the size limit of catalyts to the atom level is a promising
strategy.
[13]
For this purpose, we turn our attention onto metal organic frameworks (MOFs), in
which metal centers are atomically distributed. Specifically, in ZIF-67, a Co-based MOF,
Co
2+
are uniformly distributed at the atomic scale. The question is how to make use of these
Co species as active sites for OER. Previously, studies on MOFs directively used as OER
electrocatlaysts have been roported.
[14-15]
But intrinsically, ZIF-67 shows poor catalytic
activity for OER. The challenge is how to on-site transform the atomically distributed Co
2+
in
MOFs into atomic-scale active sites for OER. Co-based electrocatalysts derived from MOFs
by direct carbonization have been widely reported.
[16-18]
The sever structural shrinkage during
carbonization usually leads to a huge decrease of the surface area of MOFs precursors. In
addition, most of these Co-based electrocatalysts exist in the form of nanoparticles with
limited active sites exposed. Therefore, it is essential to develop a strategy to obtain atomic-

3
scale CoO
x
species with every active species exposed to catalyze OER while reserving the
abundant pores in MOFs to facilitate the mass transport of incoming reactants and outgoing
products.
O
2
plasma is a powerful tool to modify materials for its etching effect, and the metal atom
exposed in the O
2
atmosphere would be inevitable oxidized. Fortunately, the formation of Co
oxide species are beneficial for electrocatalyzing OER. Thus, in this work, we have applied
O
2
plasma to treat ZIF-67 to on-site produce atomic-scale CoO
x
species in ZIFs (CoO
x
-ZIF)
as an efficient OER electrocatalyst. The porous structure of MOFs provides pathways for O
2
plasma to activate the atomically dispersed Co species. In addition, plasma is highly efficient
for a rapid treatment, which would not severely destroy the bulk structure of MOFs during the
treatment. The O
2
plasma treatment on ZIF-67 leads to the on-site formation of atomic-scale
CoO
x
species in MOFs with high surface area. The as-obtained CoO
x
species in ZIF-67 show
advanced electrocatalytic performance for OER. Coupling the CoO
x
-ZIF with conductive
supports led to even better activity than RuO
2
.
2. Experimental Section
2.1 Materials preparation
The ZIF-67 was synthesized according to a literature.
[19]
In brief, 1.455 g cobalt nitrate
hexahydrate (Co(NO
3
)
2
•6H
2
O) was dissolved in 80 mL methanol. Another solution with 80
mL methanol and 1.642 g 2-methylimidazole (MeIM) was slowly added to the above
Co(NO
3
)
2
solution under stirring for 30 s. The whole mixture was kept at room temperature
for 24 h silently. ZIF-67 was obtained by centrifugation and washing with methanol for 5
times and dried at 60°C in a vacuum oven. For the O
2
plasma treatment, we applied the RF
power of 200 W, and the pressure was controlled at 120 Pa, and the treating time was 1 h.
Different treating times were also conducted to optimize the OER performance. For the
reference sample, pure ZIF-67 was placed in a tube furnace and annealed at 800 °C under N
2

Frequently asked questions

Q1. What are the contributions in "Atomic-scale coox species in metal-organic frameworks for oxygen evolution reaction" ?

In this work, the authors, for the first time, realized the targeted onsite formation of atomic-scale CoOx species in ZIF-67 by O2 plasma. The abundant pores in ZIF-67 provide channels for O2 plasma to activate the Co ions in MOFs to on-site produce atomic-scale CoOx species, which act as the active sites to catalyze the oxygen evolution reaction with an even better activity than RuO2. 

Q2. What is the role of O2 plasma in the electrocatalytic process?

O2 plasma is a powerful tool to modify materials for its etching effect, and the metal atom exposed in the O2 atmosphere would be inevitable oxidized. 

Q3. What is the role of the pores in ZIF-67?

The abundant pores in ZIF-67 provide channels for O2 plasma to activate the Co ions in MOFs to on-site produce atomic-scale CoOx species, which act as the active sites to catalyze the oxygen evolution reaction with an even better activity than RuO2. 

Q4. What was the effect of the plasma treatment on the CoOx species?

During the plasma treatment, the Co-N coordination bonds in the ZIFs were partially broken and the suspended Co species could beeasily reacted with O2 present in the system to obtain CoOx species locally. 

Q5. What is the reason why the ZIF-67 was overtreated?

While the RF power reaches up to 300 W, poor OER performance obtained probably due to that the ZIF-67 wasovertreated and the porous structure of ZIF-67 was broken down. 

Q6. What is the effect of plasma etching on the Co-N bond?

The destructive effect of plasma etching leads to the break of the Co-N coordination bond and the suspended Co species was rapidly oxidized in the presence of O2 to produce CoOx. 

Q7. Why did the surface area of ZIF-67 decrease after plasma treatment?

Thedecrease of surface area after plasma treatment is because the plasma treatment partiallydestroyed the porous structure of ZIF-67 by the etching effect. 

Q8. What was the method used for the electrochemical measurement of CoOx-ZIF?

Electrochemical measurement4 mg of CoOx-ZIF was dispersed in 2 mL ethanol followed by ultrasonication for 30 min, 100 μL 5 % Nafion solution was added to the dispersion and ultrasonication for another 30min to obtain the catalytic ink. 

Q9. How did the authors fix the ring current?

To study the reaction mechanism of OER, the authors used rotating ring-disk electrode (RRDE) andcollected the ring current by fixing the ring potential at 1.5 V vs. RHE in 1 M KOH solutionat 1600 rpm. 

Q10. What is the main absorption peak of CoOx-ZIF?

The main absorption peak (indicated by thevertical bar) of CoOx-ZIF in Figure 3b is enhanced, indicating CoOx-ZIF loses some charges at Co site and thus increases the oxidation state, shifting the absorption peak as well as theabsorption edge to higher energy. 

Q11. What is the effect of the O2 plasma treatment on CoOx-ZIF?

After O2 plasma treatment, the Co 2p 3/2 peak of CoOx-ZIF shows a slight broadening and shifting to lower binding energy (Figure S4b). 

Q12. What is the effect of oxygen on the co-Ox-ZIF?

Co sites more significantly than nitrogen and therefore giverise to higher unoccupied orbitals at Co sites in CoOx-ZIF (Figure 3b). 

Q13. What is the ohm resistance of CoOx-ZIF?

The fitted electrochemical impedance spectroscopy (EIS) indicates that the ohm resistanceof CoOx-ZIF (6.31 Ω) is smaller than that of ZIF-67 (7.60 Ω), which is consistent with the EPR results, indicating the improved conductivity of CoOx-ZIF. 

Q14. What are the main problems of OER electrocatalysts?

To solve this problem, many studies have been carried out to develop highly efficient andlow-cost OER electrocatalysts, such as developing transition metal compound and even metal-free materials[4]. 

Q15. What is the effect of O2 plasma on CoOx-ZIF?

These results confirm that Co-N coordination bonds in ZIF-67 were partially broken and the Co species was oxidized to form CoOx species by O2 plasma.