Generation of Nanoparticle, Atomic-Cluster, and Single-Atom Cobalt Catalysts from Zeolitic Imidazole Frameworks by Spatial Isolation and Their Use in Zinc–Air Batteries

Article abstract of DOI:10.1002/anie.201901109

The size effect of transition-metal nanoparticles on electrocatalytic performance remains ambiguous especially when decreasing the size to the atomic level. Herein, we report the spatial isolation of cobalt species on the atomic scale, which was achieved by tuning the zinc dopant content in predesigned bimetallic Zn/Co zeolitic imidazole frameworks (ZnCo-ZIFs), and led to the synthesis of nanoparticles, atomic clusters, and single atoms of Co catalysts on N-doped porous carbon. This synthetic strategy allowed an investigation of the size effect on electrochemical behavior from nanometer to ?ngstr?m dimensions. Single-atom Co catalysts showed superior bifunctional ORR/OER activity, durability, and reversibility in Zn–air batteries compared with the other derivatives and noble-metal Pt/C+RuO₂, which was attributed to the high reactivity and stability of isolated single Co atoms. Our findings open up a new avenue to regulate the metal particle size and catalytic performance of MOF derivatives.

Full text of DOI:10.1002/anie.201901109

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Accepted Article
Title: Spatial Isolation of Zeolitic Imidazole Frameworks-Derived Cobalt
Catalysts: From Nanoparticle, Atomic Cluster to Single Atom
Authors: Xiaopeng Han, Xiaofei Ling, Ying Wang, Tianyi Ma, Cheng
Zhong, Wenbin Hu, and Yida Deng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901109
Angew. Chem. 10.1002/ange.201901109
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10.1002/anie.201901109
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Spatial Isolation of Zeolitic Imidazole Frameworks-Derived Cobalt
Catalysts: From Nanoparticle, Atomic Cluster to Single Atom
Xiaopeng Han, Xiaofei Ling, Ying Wang, Tianyi Ma, Cheng Zhong, Wenbin Hu, Yida Deng*
chemical reduction route,[8] and impregnation method,[1b, 9] which
Abstract: The size effect of transition metal nanoparticles on
results in a great challenge to clarify the influence of crystal size
electrocatalytic performance remains ambiguous especially when
on electrocatalytic properties over a wide size ranging from
decreasing the size to atomic level. Herein, we report the spatial
nanoparticles to nanoclusters and even single atoms.
isolation of cobalt species at atomic scale through tuning zinc dopants
Recently, zeolitic imidazole frameworks (ZIFs), a subclass of
content in predesigned bimetallic Zn/Co zeolitic imidazole frameworks
metal-organic frameworks (MOFs), have been considered as
(ZnCo-ZIFs), affording the successful synthesis of nanoparticles,
atomic clusters and single atoms of Co catalysts on N-doped porous
carbon. This novel synthetic strategy allows the investigation of size
effect on electrochemical behaviours from nanometer to angstrom.
Electrochemical results demonstrate that the single Co atomic catalyst
manifests superior bifunctional ORR/OER activity, durability and
reversibility for Zn-air batteries in comparison with other counterparts
and noble metal Pt/C+RuO2, which can be attributed to the isolated
single Co atoms with high reactivity and stability. Our findings open up
a new avenue to regulate the metal particle size and catalytic
performance of metal-organic framework derivatives at multiscale level.
suitable precursors to synthesize transition metal and N-doped
porous carbon (TM-N-C) composite materials via a pyrolysis
process.[2a, 10] The increased number of coupled TM-Nx active sites
and simultaneously enhanced reactivity of active moieties are
responsible for the remarkable catalytic performance of resultant
TM-N-C electrocatalysts.[2b, 5a] Notably, bimetallic active sites can
be introduced to further enhance the ORR activity by adding
bimetallic cations during the growth of ZIFs.[5b] Meanwhile, the
homogeneous dopants of secondary metal atoms in the MOFs
can facilitate the tuning of coordination environment of metallic
centers and also provide steric hindrance for potential
agglomeration during pyrolysis.[5a] Notwithstanding, on the one
hand, the specific isolation mechanism has not been
systematically understood so far; on the other hand, seldom work
has achieved the tuning and preparing of multiscale TM particles
from nanometer to angstrom, which is essential to identify the size
influence of MOFs-derived materials on electrocatalytic
properties.
Understanding the size effect of nanoparticle catalysts on their
activities is one of the main research topics in establishing
structure-reactivity correlations in the field of heterogeneous
catalysis.[1] Particularly, single-atom catalysts are emerging as a
new research frontier,[2] featuring minimum particle size and
maximum utilization of coordinately unsaturated metal sites. On
the other hand, electrocatalytic oxygen reduction/evolution
reactions (ORR/OER) are important coupled redox reactions that
are related to various electrochemical energy conversion systems,
such as fuel cells, metal-air batteries and water-spiltting devices,[3]
etc. Regarding the investigation of size effect on ORR activity,
much effort has been devoted to Pt-based catalysts since
reducing the particle size and increasing the mass/specific
activities are essential tasks for the efficient uilization of this
precious resource.[4] To completely subtitute precious metals,
transition metal (TM, e.g., Co, Ni, Fe) catalysts have aroused
increasing interests owing to their natural advantages of low cost,
high abundance, remarkable structural stability and potential
5]
bifunctionality.[3a,
However, to the best of our knowledge,
although various studies of TM particle size effects are displayed
in traditional catalysis (e.g., cobalt particle size effects for Fischer-
Tropsch (FT) reaction),[6] little work has been done to
systematically demonstrate the size influence on ORR/OER
performance of well-defined TM-based catalysts. Moreover, the
particle size in aforementioned metal-based nanomaterials is
generally restricated in the range of approximately 1 to 20 nm by
the conventional preparation methods like thermal treatment,[5c, 7]
Figure 1. Schematic illustration of the preparation procedures of Co-NPs@NC,
Co-ACs@NC and Co-SAs@NC catalysts.
Dr. X. Han, X. Ling, Prof. C. Zhong, Prof. W. Hu, Prof. Y. Deng
School of Materials Science and Engineering, Tianjin Key Laboratory of
Composite and Functional Materials, Key Laboratory of Advanced Ceramics
and Machining Technology (Ministry of Education), Tianjin University, Tianjin
300350, P.R. China
Corresponding E-mail: yida.deng@tju.edu.cn
Dr. Y. Wang
School of Chemistry & Materials Science, Jiangsu Key Laboratory of Green
Synthetic Chemistry for Functional Materials, Jiangsu Normal University,
Xuzhou, China
Herein, we realized the spatial isolation of cobalt species at
atomic level through tuning zinc dopants content in bimetallic
ZnCo-ZIFs precursors, giving Co nanoparticles, atomic Co clusters
and Co single atoms on N-doped carbon, designated as Co-
NPs@NC, Co-ACs@NC and Co-SAs@NC, respectively. The
developed novel strategy allows the investigation of size influence
on electrochemical activity of TM catalysts with the size down to
single atomic scale. Specifically, by precisely regulating the Zn/Co
molar ratios in ZIFs structures, Co atoms can be geometrically
separated to different extents by the introduced Zn atoms upon
pyrolysis with lowest isolation degree for nanoparticles
(Zn:Co=0:1), moderate degree for atomic clusters (Zn:Co=2:1)
and highest degree for single atoms (Zn:Co=8:1), as schematically
Dr. T. Ma
Discipline of Chemistry, University of Newcastle, Callaghan, Newcastle, NSW
2308, Australia
X. Han, X. Ling and Y. Wang contributed equally to this work.
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10.1002/anie.201901109
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illustrated in Figure 1. Among them, Co-SAs@NC exhibits the (SEM) images showed the dodecagon shape of prepared ZnCo-
best ORR activity in terms of highest onset potential (0.96 V),
highest half-wave potential (0.82 V) and largest kinetic current
density (15.2 mA cm−2), closely approaching that of commercial
Pt/C (0.98 V, 0.82 V, 15.7 mA cm-2). Furthermore, Co-SAs@NC
also displays outstanding long-term durability and reversibility for
Zn-air batteries, superior to noble metal Pt/C+RuO2.
ZnCo-ZIFs were synthesized via the sodalite coordination of
Co2+ and Zn2+ nodes with 2-methylimidazole in methanol. Three
samples with Zn/Co molar ratios of 0, 2 and 8 were obtained,
designated as Co-ZIF, ZnCo-ZIF-2 and ZnCo-ZIF-8, respectively.
Compared to pure Co-ZIFs, the introduced Zn2+ dopants partially
replace the original Co2+ ions in ZnCo-ZIFs, thus the spatial
isolation of cobalt species is effectively realized at atomic level.[2a]
The X-ray powder diffraction (XRD) patterns of as-prepared ZnCo-
ZIFs were well matched with simulated ZIFs (Figure S1),
confirming the Zn-doping nature. Scanning electron microscope
ZIFs (Figure S2). After pyrolysis at 1173 K under Ar atmosphere,
the organic linkers were carbonized into N-doped porous carbon
and Zn species with low boiling point was evaporated at such a
high temperature while Co particles were remained and anchored
on N-doped carbon support.[2a] In the XRD curves of resultant
composites (Figure S3), only two broad peaks were observed in
the range of 20°-50°, which can be assigned to the (002) and (100)
planes
of
graphitic
carbon.
Further
morphological
characterizations revealed the highly dispersed and dodecagonal
nanostructures that were inherited from the initial ZIFs precursors
(Figure S4). The products were subsequently refluxed in 0.5 M
H2SO4 solution to remove the possible inactive and unstable metal
species.
Co-NPs@NC,
Co-ACs@NC
and
Co-SAs@NC
nanocomposites were finally received with no XRD signals of Co
detected owning to the low content (Figure S5).
Figure 2. (a) SEM, (b) TEM, (c) elemental mapping of Co-SAs@NC. (d) TEM image of Co-NPs@NC. Inset shows the HRTEM of an individual particle. (e, f)
HAADF-STEM images of (e) Co-ACs@NC and (f) Co-SAs@NC. (g) N2 adsorption-desorption isotherm curves. (h) Co XANES spectra of Co-SAs@NC, Co foil
and CoO. (i) Fourier-transforms of Co K-edge spectra of Co-SAs@NC and Co foil.
the spatial isolation effect of doped zinc atoms. As a result, the
The size and morphology of ZIFs were maintained after acid
mixed Zn acts as a “fence” to expand the distance of adjacent
leaching in the resultant three hybrids while their surface
Co atoms in space, rendering the multiscale controllable
became rougher with the presence of obvious porous structures
synthesis of different aggregation degree of atomic Co. The N2
(Figures 2a, b and S6, S7). Elemental mapping of Co-SAs@NC
adsorption-desorption isotherms resemble type IV (Figure 2g),
reveals the homogeneous dispersion of C, N and Co species
indicating the appearance of mesopores after acid etching,
over the entire nanoarchitecture (Figure 2c). Transmission
which is further supported by the pore size distribution curves
electron microscopy (TEM) indicates the formation of Co
(Figure S8). Brunauer-Emmett-Teller (BET) values are
characterized to be 180.1, 98.8, and 130.7 m2 g-1 for Co-
nanoparticles by thermal treatment of pure Co-ZIFs without the
isolation of Zn (Figure 2d), which is confirmed by the observed
NPs@NC, Co-ACs@NC, and Co-SAs@NC products. The large
lattice distance of 0.26 nm in the high resolution TEM (HRTEM,
specific surface area and rich porous structure are expected to
inset of Figure 2d), consistent with the spacing between the (101)
facilitate the gas diffusion and ion transportation, contributing to
plane of metallic Co. Aberration-corrected high-angle annular
the enhanced catalytic activity. Raman spectra reveal that all the
dark-field scanning transmission electron microscope (HAADF-
ID/IG ratios (ID and IG represent the intensity of D- and G-bands of
STEM) images show the generation of Co atomic clusters and
carbon materials) are 0.98 (Figure S9), suggesting the same
single atoms from pyrolyzed bimetallic ZnCo-ZIFs with increased
graphitic structures in three hybrids. This allows the exclusion of
Zn/Co molar ratios of 2:1 and 8:1 (Figure 2e, f), clearly verifying
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influence of carbon supports on the catalytic performance,
focusing on the difference from Co size and configuration.
Synchrotron X-ray absorption spectroscopy analysis was
performed to further identify the atomic structures in the
synthesized hybrids. Figure 1h presents the Co K-edge X-ray
absorption near-edge structure curves. The comparison with
reference CoO and Co foil implies that the Co single atoms in
equation, the calculated kinetic current densities (Jk) of Co-
NPs@NC, Co-ACs@NC, Co-SAs@NC and Pt/C were 7.1, 10.6,
15.2 and 15.7 mA cm−2 at 0.6 V, indicating the fast reaction
kinetics on Co-SAs@NC. The ORR catalytic performance was
also gleaned by the determined electron transfer number (n) and
yield of peroxide generation (y). As illustrated in Figure 3c, the
monitored n of Co-SAs@NC is above 3.9 and peroxide
percentage is below 3% over the potential range of 0.2-0.85 V,
demonstrating that Co-SAs@NC catalyzed ORR via an apparent
4e- mechanism, similar to Pt/C, which is recognized to be a
highly efficient reaction pathway. This is also certified by the
results from K-L determination (Figures 3d and S12). The lower
n and higher y values on Co-ACs@NC and Co-NPs@NC
suggest the ORR activities following the trend of Co-SAs@NC >
Co-ACs@NC > Co-NPs@NC. Electrochemical active surface
areas (ECSA) of 19.1, 39.2, 51.4 mF cm-2 for Co-NPs@NC, Co-
ACs@NC and Co-SAs@NC (Figure 3e) further reveals the
advantage of Co single atoms in providing more catalytically
active sites and high atomic utilization for oxygen adsorption and
Co-SAs@NC are positively charged, in good agreement with
11]
previous observations.[2a,
Fourier transform profile of Co-
SAs@NC in Figure 1i presents a primary peak at 1.47 Å,
corresponding to Co-N scattering path, which is also
corroborated by the X-ray photoelectron spectroscopy (XPS)
results (Figure S10). Compared with the spectra of Co-ACs@NC
and Co foil (Figure S11), no Co-Co path at around 2.1 Å is
observed in Co-SAs@NC, an indication of the atomically
dispersed Co single atoms.[4a,
According to Inductively
11]
coupled plasma-mass spectrometry (ICP-MS) analysis, the
mass percentages of anchored metallic Co species are 4.91,
3.95, and 1.70 wt% for Co-NPs@NC, Co-ACs@NC, and Co-
SAs@NC catalysts, respectively. These aforementioned results
collectively demonstrate the successful synthesis of metallic Co
catalysts with different particle size, highlighting the pivotal role
of Zn dopants in regulating the spatial isolation of Co atoms.
subsequent reactions, acting as
a critical factor for the
substantially enhanced ORR activity. This is also confirmed by
the highest mass activity of Co-SA@NC catalyst (Figure S13).
Moreover, electrochemical impedance spectroscopy (EIS)
discloses the smallest charge transfer resistance on Co-
SAs@NC during the catalytic process (Figure S14), resulting in
the fast reaction kinetics. Notably, the acid etching can also
benefit the ORR improvement by increasing the surface
roughness and the electrochemical active area (Figures S15,
S16 and Table S1). The remarkable ORR performance of Co-
SAs@NC catalyst can also be observed in either acidic or
neutral electrolytes (Figure S17), demonstrating its universal
catalytic capability for different practical environments.
Furthermore, preliminary results show that Co-SAs@NC also
exhibits remarkable OER activity (Figure S18), making it as a
promising bifunctional catalyst that plays an important role in
rechargeable metal-air batteries. Apart from high catalytic
activity, Co-SAs@NC hybrid also affords excellent long-term
operation durability. After a continuous chronoamperometric
period of 22.5 h, the cathodic current retention of Co-SAs@NC
is much higher than that of Pt/C (94.4 % vs 79.2 %, Figure 3f).
The remarkable catalytic stability is further corroborated by
almost overlapped CV curves after extended 5,000 cycles (the
inset of Figure 3f). Additionally, in a sharp contrast to Pt/C, the
Co-SAs@NC hybird shows considerable methanol tolerance
(Figure S19), implying its potential application for methanol fuel
cells.
A homemade Zn−air battery was further assembled to assess
the electrochemical performance of synthesized materials under
real operation conditions with zinc plate as the anode, Co-based
catalysts as the air cathode, and 6.0 M KOH and 0.2 M ZnCl2 as
the electrolyte (Figure 4a). The Co-SAs@NC based battery can
work with an open-circuit voltage (OCV) of 1.46 V (Figures 4b
and S20), higher than that involving Co-ACs@NC (1.38 V), Co-
NPs@NC (1.37 V) and Pt/C (1.41 V). The voltage-current
polarization curves indicate the best performance of Co-
SAs@NC cathode among three hybrid catalysts (Figure 4c),
which even surpasses that of the Pt/C-based battery at current
density below 100 mA cm-2. The highest power density is
achieved on battery using the Co-SAs@NC electrode (105.3
mW cm−2 at 158 mA cm−2), approaching to that of the Pt/C
(110.4 mW cm−2 at 184 mA cm−2). For a primary battery setup,
the battery catalyzed by Co-SAs@NC delivers a discharge
specific capacity of 897.1 mA h g−1 at 20 mA cm-2 based on the
mass of consumed Zn (Figure 4d), much larger than that of Pt/C
(783.5 mA h g−1). The rechargeable performance of the Co-
SAs@NC-based Zn−air battery was also evaluated (Figure S21),
which gives a superior reversibility over Pt/C+RuO2 counterpart
and rivals the electrocatalysts reported previously (Table S3).[12]
To explore the potential application in portable and wearable
devices, a flexible solid-state Zn-air battery was constructed by
zinc foil anode, KOH-poly(vinyl alcohol) (PVA) electrolyte and
carbon cloth supported Co-SAs@NC cathode (Figure 4e). A
Figure 3. (a) ORR polarization curves measured using RRDEs at 1600 rpm
and (b) Jk at 0.6 V and E1/2 for synthesized Co-NPs@NC, Co-ACs@NC, Co-
SAs@NC and Pt/C catalysts. (c) Corresponding percentage of peroxide and
electron transfer number. (d) Polarization curves of Co-SAs@NC at different
rotation rates. Inset shows the K-L plots. (e) Dependence of current densities
as a function of scan rates at 0.40 V. (f) Chronoamperometric curves of Co-
SAs@NC at 0.6 V vs RHE. Inset shows the prolonged CVs of Co-SAs@NC at
400 rpm at a scan rate of 250 mV s–1
.
The formation of series catalysts inspires us to investigate the
multiscale size effect on their electrocatalytic properties.
Rotational ring-disk electrodes (RRDEs) were employed to
assess the ORR activity of synthesized composites in O2-
saturated 0.1 M KOH. As shown in Figures 3a, b and Table S1,
Co-SAs@NC exhibits superior catalytic activity in terms of
higher onset potential (Eonset, 0.96 V), higher half-wave potential
(E1/2, 0.82 V) and larger limiting diffusion current density (Jd, 4.96
mA cm-2), which significantly outperforms those of Co-ACs@NC
(0.91 V, 0.81 V, 4.44 mA cm-2) and Co-NPs@NC (0.90 V, 0.80 V,
3.86 mA cm-2), competes with that of commercial Pt/C (0.98 V,
0.82 V, 4.98 mA cm-2) and also surpasses that of most precious
metal-free electrocatalysts reported to date (Table S2). After
correcting the diffusion-limited current by Koutecky-Levich (K-L)
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Figure 4. (a) Schematic illustration of aqueous Zn–air battery. (b) Open-circuit voltages, (c) polarization curves and corresponding power densities of Co-
NPs@NC, Co-ACs@NC, Co-SAs@NC and Pt/C-based Zn-air batteries. (d) Discharge curves of primary Zn-air batteries catalyzed by Co-SAs@NC and Pt/C at
20 mA cm-2. (e) Structural illustration of solid-state Zn-air battery. (f) Discharge–charge performance of flexible Zn-air batteries. (g) Photographs of LED screen
lighted by two batteries connected in series using Co-SAs@NC catalyst. (h) Cycling performance of the battery with Co-SAs@NC cathode at different bending
angles at 2 mA cm−2
.
high OCV of 1.40 V is obtained (Figure S22), comparable to the
value in aqueous system. The galvanostatic discharge-charge
profiles in Figure 4f suggest that the stable voltage plateau can
be maintained well for 1000 min at a current density of 1 mA cm-
2, 700 min at 2 mA cm-2 (20 min per cycle). As a demo, two
flexible batteries connected in series can power a red light-
emitting diode (LED) screen (Figure 4g) and also are capable of
maintaining extremely stable discharge-charge platforms at
different bending angles (Figure 4h), demonstrating the
considerable flexibility.
The outstanding electrocatalytic performance of the Co-
SAs@NC catalyst is attributed to the following factors. First, the
isolated single Co atoms provide maximized atomic utilization
and more catalytic sites with superior reactivity.[11] Meanwhile,
the coordination with N species in N-doped carbon support
ensures the stabilization of Co-SAs.[2a] The second reason is
that the large surface area and mesoporous structure of the
carbon matrix affords the exposure of accessible atomic sites
and the fast diffusion of O2 and electrolyte. Finally, high
conductivity of the carbon framework also favors the catalytic
process by enhancing the efficient electron transport.
In summary, this work demonstrates the controlled synthesis
of multiscale Co metallic catalysts through regulating the spatial
isolation of Co species by heteroatom Zn doping and
investigates their size-dependent electrocatalytic properties. The
Co single atoms anchored on N-doped carbon hybrid delivers
outstanding bifunctional activity, catalytic durability and
reversibility for Zn-air batteries, that is superior to most of the
precious and transition metal-based catalysts. Further
investigations highlight the isolated single atomic sites with high
reactivity and stability in facilitating the remarkable catalytic
activities. These findings should shed light on the regulation of
particle size through spatial separation and improves our
understanding on the size-performance effect of atomically
dispersed materials for chemical and electrochemical reactions.
Acknowledgements
The work was supported by Young Elite Scientists Sponsorship
Program by CAST (2018QNRC001), National Natural Science
Foundation of China (U1601216, 51602216, 51571151,
51671143 and 21805117), Tianjin Natural Science Foundation
(17JCQNJC02100) and Jiangsu Province Science Foundation
for Youths (BK20181014).
Keywords: Size effect, Single atoms, Electrocatalysis, ORR,
Metal-air battery
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10.1002/anie.201901109
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
We report the regulation of Co atoms
through spatial isolation by heteroatom Zn
doping in bimetallic ZnCo-ZIFs, enabling
the tunable particle size from nanoparticles,
atomic clusters to single atoms supported
on N-doped carbon, among which the
single atoms catalyst exhibits superior
electrocatalytic activity, catalytic durability
and reversibility for Zn-air battery owing to
the isolated Co single atomic sites with high
reactivity and durability.
Xiaopeng Han, Xiaofei Ling, Ying Wang,
Tianyi Ma, Cheng Zhong, Wenbin Hu, Yida
Deng*
Page No.1 – Page No.5
Spatial Isolation of Zeolitic Imidazole
Frameworks-Derived Cobalt Catalysts:
From Nanoparticle, Atomic Cluster to
Single Atom
This article is protected by copyright. All rights reserved.