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Journal Article•DOI•

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

Shuo Dou1, Chung-Li Dong2, Zhe Hu3, Yu-Cheng Huang2, Jeng Lung Chen2, Li Tao1, Dafeng Yan1, Dawei Chen1, Shaohua Shen4, Shulei Chou3, Shuangyin Wang1 •
Hunan University1, Tamkang University2, University of Wollongong3, Xi'an Jiaotong University4
01 Sep 2017-Advanced Functional Materials (Wiley)-Vol. 27, Iss: 36, pp 1702546
TL;DR: 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.

Summary (2 min read)

Jump to: [1. Introduction] – [HO2] – [3. Results and Discussion] – [OER.] and [4. Conclusion]

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].
  • Fortunately, the formation of Co oxide species are beneficial for electrocatalyzing OER.
  • The O2 plasma treatment on ZIF-67 leads to the on-site formation of atomic-scale CoOx species in MOFs with high surface area.
  • For the O2 plasma treatment, the authors applied the RF power of 200 W, and the pressure was controlled at 120 Pa, and the treating time was 1 h.
  • X-ray photoelectron spectroscopic (XPS) measurements were carried out on an AXIS ULTRA (Kratos Analytical).

HO2

  • The Faradaic efficiency (ε) was determined by collecting the ring current when fixing the disk current at 200 μA and ring potential at 0.4 V vs. RHE in N2-saturated 1 M KOH solution.
  • Id is the disk current, Ir is the ring current, and N is the current collection efficiency (0.21 in this study) which was determined by IrO2 catalyst thin film electrode.
  • The electrochemical surface area (ECSA) was evaluated by measuring the double layer capacitance method via CVs at different scan rate from 20 to 100 mV s-1 in the range of no Faradaic processes occurred.
  • The ECSA could be calculated from the double layer capacitance according to: ECSA= Cdl Cs Where.
  • The turnover frequency (TOF) was evaluated by the following equation [15]: TOF= J×A 4×m×F.

3. Results and Discussion

  • As illustrated in Figure 1a, O2-plasma was applied to treat ZIF-67.
  • As shown in Figure S3, the absorption band at 425 and 1580 cm-1 are assigned to the stretching vibration of Co-N and C=N, respectively.
  • Notably, the intensity of this small feature is higher in ZIF-67 than CoOx-ZIF which indicates the higher oxidized states of Co in ZIF-67 than in CoOx-ZIF.
  • Thus, the lower intensity observed in CoOx-ZIF indicates more symmetrical atomic structures around Co after plasma treatment.
  • It is well known that the poor conductivity of ZIF-67 hinders their electrocatalytic applications.

OER.

  • The Tafel plots were also collected to investigate the OER kinetics in Figure 4c.
  • The CoOx-ZIF displays a smaller Tafel slope (70.3 mV dec -1) than pristine ZIF-67 (108.8 mV dec1), which demonstrates the intrinsic reason for CoOx-ZIF owning better OER activity than pristine ZIF-67.
  • To study the reaction mechanism of OER, the authors used rotating ring-disk electrode (RRDE) and collected the ring current by fixing the ring potential at 1.5 V vs. RHE in 1 M KOH solution at 1600 rpm.
  • The presence of Co3O4 provides active sites to catalyze OER, thus shows better OER performance than the pristine.

4. Conclusion

  • In summary, the authors have successfully obtained the atomic-scale CoOx species in the MOFs through a simple but efficient plasma treatment.
  • The atomic-scale CoOx species provide rich active sites for OER, demonstrating highly efficient electrocatalytic activity, which is even better than RuO2.
  • The unique atomic-scale dispersed structure of MOFs provides excellent precursors for the on-site formation of atomic-scale catalyst species for OER.
  • The abundant pores in the ZIF-67 provide channels for O2 plasma to activate the atomic Co ions in MOFs to on-site produce atomic-scale CoOx species.
  • Furthermore, the remained large surface area and etched surface of ZIFs ensures excellent mass transport during OER.

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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
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Beijing University of Chemical Technology1
01 Oct 2018-Journal of Power Sources
TL;DR: In this article, the authors reviewed the activity, stability and durability for oxygen evolution reaction in alkaline medium of different types of recently reported electrocatalysts such as Ni, Co, NiCo, Fe, Se, Mo, Cu, Mn, Zn, V, Ti/Ta, and metal free based earth-abundant-electrocatalyst.

374 citations

Journal Article•DOI•
Creating coordinatively unsaturated metal sites in metal-organic-frameworks as efficient electrocatalysts for the oxygen evolution reaction: Insights into the active centers

[...]

Li Tao1, Chun-Yu Lin2, Shuo Dou1, Shi Feng1, Dawei Chen1, Dongdong Liu1, Jia Huo1, Zhenhai Xia2, Shuangyin Wang3, Shuangyin Wang1 •
Hunan University1, University of North Texas2, Shenzhen University3
01 Nov 2017-Nano Energy
TL;DR: In this paper, a Co-containing MOF was used to create coordinately unsaturated metal sites (CUMSs) as catalytic centers for oxygen evolution reaction (OER) in Zeolitic imidazolate frameworks.

347 citations

Journal Article•DOI•
A Cobalt-Iron Double-Atom Catalyst for the Oxygen Evolution Reaction.

[...]

Lichen Bai1, Chia-Shuo Hsu2, Duncan T. L. Alexander1, Hao Ming Chen2, Xile Hu1 •
École Polytechnique Fédérale de Lausanne1, National Taiwan University2
11 Sep 2019-Journal of the American Chemical Society
TL;DR: Electrochemical, microscopic, and spectroscopic data including those from operan-do X-ray absorption spectroscopy, reveal a dimeric Co-Fe moiety as the active site of the catalyst.
Abstract: Single-atom catalysts exhibit well-defined active sites and potentially maximum atomic efficiency. However, they are unsuitable for reactions that benefit from bimetallic promotion such as the oxygen evolution reaction (OER) in an alkaline medium. Here we show that a single-atom Co precatalyst can be in situ transformed into a Co-Fe double-atom catalyst for the OER. This catalyst exhibits one of the highest turnover frequencies among metal oxides. Electrochemical, microscopic, and spectroscopic data, including those from operando X-ray absorption spectroscopy, reveal a dimeric Co-Fe moiety as the active site of the catalyst. This work demonstrates double-atom catalysis as a promising approach for the development of defined and highly active OER catalysts.

315 citations

References
More filters
Journal Article•DOI•
Exceptional chemical and thermal stability of zeolitic imidazolate frameworks

[...]

Kyo Sung Park1, Zheng Ni1, Adrien P. Côté1, Jae Yong Choi2, Rudan Huang3, Fernando J. Uribe-Romo1, Hee K. Chae2, Michael O'Keeffe4, Omar M. Yaghi1 •
University of California, Los Angeles1, Seoul National University2, Beijing Institute of Technology3, Arizona State University4
05 Jul 2006-Proceedings of the National Academy of Sciences of the United States of America
TL;DR: Study of the gas adsorption and thermal and chemical stability of two prototypical members, ZIF-8 and -11, demonstrated their permanent porosity, high thermal stability, and remarkable chemical resistance to boiling alkaline water and organic solvents.
Abstract: Twelve zeolitic imidazolate frameworks (ZIFs; termed ZIF-1 to -12) have been synthesized as crystals by copolymerization of either Zn(II) (ZIF-1 to -4, -6 to -8, and -10 to -11) or Co(II) (ZIF-9 and -12) with imidazolate-type links. The ZIF crystal structures are based on the nets of seven distinct aluminosilicate zeolites: tetrahedral Si(Al) and the bridging O are replaced with transition metal ion and imidazolate link, respectively. In addition, one example of mixed-coordination imidazolate of Zn(II) and In(III) (ZIF-5) based on the garnet net is reported. Study of the gas adsorption and thermal and chemical stability of two prototypical members, ZIF-8 and -11, demonstrated their permanent porosity (Langmuir surface area = 1,810 m 2 /g), high thermal stability (up to 550°C), and remarkable chemical resistance to boiling alkaline water and organic solvents.

5,512 citations

Journal Article•DOI•
Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

[...]

Wee-Jun Ong1, Lling-Lling Tan1, Yun Hau Ng2, Siek Ting Yong1, Siang-Piao Chai1 •
Monash University1, University of New South Wales2
20 May 2016-Chemical Reviews
TL;DR: It is anticipated that this review can stimulate a new research doorway to facilitate the next generation of g-C3N4-based photocatalysts with ameliorated performances by harnessing the outstanding structural, electronic, and optical properties for the development of a sustainable future without environmental detriment.
Abstract: As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

5,054 citations

Journal Article•DOI•
Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction

[...]

Charles C. L. McCrory1, Suho Jung1, Jonas C. Peters1, Thomas F. Jaramillo2•
California Institute of Technology1, Stanford University2
30 Oct 2013-Journal of the American Chemical Society
TL;DR: In this paper, the authors report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts for water oxidation.
Abstract: Objective evaluation of the activity of electrocatalysts for water oxidation is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a current density of 10 mA cm–2 per geometric area, approximately the current density expected for a 10% efficient solar-to-fuels conversion device. Utilizing ...

4,808 citations

Journal Article•DOI•
Single-atom catalysis of CO oxidation using Pt1/FeOx

[...]

Botao Qiao1, Aiqin Wang1, Xiaofeng Yang2, Lawrence F. Allard3, Zheng Jiang4, Yi-Tao Cui1, Jingyue Liu5, Jingyue Liu1, Jun Li2, Tao Zhang1 •
Dalian Institute of Chemical Physics1, Tsinghua University2, Oak Ridge National Laboratory3, Chinese Academy of Sciences4, University of Missouri–St. Louis5
01 Aug 2011-Nature Chemistry
TL;DR: Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.
Abstract: Platinum-based heterogeneous catalysts are critical to many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the surface active-site atoms are used. Catalysts with single-atom dispersions are thus highly desirable to maximize atom efficiency, but making them is challenging. Here we report the synthesis of a single-atom catalyst that consists of only isolated single Pt atoms anchored to the surfaces of iron oxide nanocrystallites. This single-atom catalyst has extremely high atom efficiency and shows excellent stability and high activity for both CO oxidation and preferential oxidation of CO in H-2. Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.

4,446 citations

Journal Article•DOI•
A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles.

[...]

Jin Suntivich1, Kevin J. May1, Hubert A. Gasteiger1, John B. Goodenough2, Yang Shao-Horn1 •
Massachusetts Institute of Technology1, University of Texas at Austin2
09 Dec 2011-Science
TL;DR: The high activity of BSCF was predicted from a design principle established by systematic examination of more than 10 transition metal oxides, which showed that the intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an eg symmetry of surface transition metal cations in an oxide.
Abstract: The efficiency of many energy storage technologies, such as rechargeable metal-air batteries and hydrogen production from water splitting, is limited by the slow kinetics of the oxygen evolution reaction (OER). We found that Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3–δ (BSCF) catalyzes the OER with intrinsic activity that is at least an order of magnitude higher than that of the state-of-the-art iridium oxide catalyst in alkaline media. The high activity of BSCF was predicted from a design principle established by systematic examination of more than 10 transition metal oxides, which showed that the intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an e g symmetry of surface transition metal cations in an oxide. The peak OER activity was predicted to be at an e g occupancy close to unity, with high covalency of transition metal–oxygen bonds.

3,876 citations

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Ultrathin metal–organic framework nanosheets for electrocatalytic oxygen evolution

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Frequently Asked Questions (15)
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. 

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. 

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. 

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. 

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. 

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. 

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

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. 

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. 

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. 

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). 

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

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. 

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]. 

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.