Exfoliation of amorphous phthalocyanine conjugated polymers into ultrathin nanosheets for highly efficient oxygen reduction

Article abstract of DOI:10.1039/C8TA11044A

It is a significant challenge to develop a high-efficiency synthetic methodology to access fully conjugated 2D conjugated polymer (CP)/covalent organic framework (COF) nanosheets (NSs) that have great application potential for electronics and energy. Herein, we report the exfoliation of a series of amorphous ethynyl-linked phthalocyanine (Pc) CPs (MPc-CPs, M = Fe, Co, Fe_0.5Co_0.5) into ultrathin MPc-CP NSs. Random coupling between the four regioisomers (with D_4h, D_2h, C_2v and C_s symmetry) of the two tetra-β-substituted phthalocyanine precursors endows the resulting phthalocyanine conjugated polymers MPc-CPs with intrinsic structural defects and a disordered framework on individual layers. This in turn induces a diminished interlayer overlapping and a weakened interlayer π-π stacking interaction, facilitating the possible exfoliation of MPc-CPs into ultrathin 2D NSs with a yield of over 50%. The direction observation by transmission electron microscopy (TEM) and atomic force microscopy (AFM) demonstrates that the ultrathin MPc-CP NSs possess a smooth surface with a uniform thickness of 1-3 nm and a lateral size of hundreds of nanometers. The as-prepared bimetallic Fe_0.5Co_0.5Pc-CP NSs were further used to fabricate a heterostructure Fe_0.5Co_0.5Pc-CP NS@G with graphene NSs as an oxygen reduction reaction (ORR) catalyst, which exhibits an onset potential of 1006 mV and a half-wave potential of 927 mV in 0.1 M KOH, representing one of the best values in an alkaline medium. Moreover, the excellent ORR activity of the exfoliated tetrapyrrole-based conjugated NSs hybridized with graphene has also been demonstrated by a Zn-air battery device, showing an open circuit voltage of 1.34 V and a peak power density of ca. 180 mW cm^?2

Full text of DOI:10.1039/C8TA11044A

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DOI: 10.1039/C8TA11044A.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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Exfoliation of Amorphous Phthalocyanine Conjugated Polym
D
e
O
r
I: 1
s
0.1
i
0
n
39/C8TA11044A
t
o
Ultrathin Nanosheets for Highly Efficient Oxygen Reduction
a
a
d
a
a
c
Wenping Liu, Chiming Wang, Lijie Zhang, Houhe Pan, Wenbo Liu, Jun Chen,
d
e
a,b
a
Dongjiang Yang, Yanjuan Xiang, Kang Wang,* Jianzhuang Jiang* and Xiangdong
Yao*b
Defects and disorders in the individual layers of ethynyl-linked phthalocyanine conjugated
polymers (CPs) diminish interlayer π−π stacking interaction, which favors the exfoliation of
these CPs into ultrathin 2D nanosheets (NSs) with high yield. In particular, the as-prepared
bimetallic NSs could be further used to fabricate heterostructure with graphene NSs to boost
the oxygen reduction in an alkaline medium.

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DOI: 10.1039/C8TA11044A
Journal Name
ARTICLE
Exfoliation of Amorphous Phthalocyanine Conjugated Polymers
into Ultrathin Nanosheets for Highly Efficient Oxygen Reduction †
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
d
a
a
c
d
Wenping Liu, Chiming Wang, Lijie Zhang, Houhe Pan, Wenbo Liu, Jun Chen, Dongjiang Yang,
e
a,b
a
b
Yanjuan Xiang, Kang Wang,* Jianzhuang Jiang* and Xiangdong Yao*
DOI: 10.1039/x0xx00000x
It is a significant challenge to develop high-efficiency synthetic methodology to access fully conjugated 2D conjugated
polymers (CPs)/covalent organic frameworks (COFs) nanosheets (NSs) that are of great application potential for electronics
and energy. Herein, we report the exfoliation of a series of amorphous ethynyl-linked phthalocyanine (Pc) CPs (MPc-CPs,
www.rsc.org/
M = Fe, Co, Fe0.5Co0.5) into ultrathin MPc-CP NSs. Random coupling between the four regioisomers (with D4h, D2h, C2v and C
s
symmetry) of the two tetra-β-substituted phthalocyanine precursors endows the resulting phthalocyanine conjugated
polymers MPc-CPs the intrinsic structural defects and disordered framework on individual layer. This in turn induces a
diminished interlayer overlapping and a weakened interlayer π−π stacking interaction, rendering the possible exfoliation of
MPc-CPs into ultrathin 2D NSs in a yield of over 50%. The direction observation by transmission electron microscopy (TEM)
and atomic force microscopy (AFM) demonstrates that the ultrathin MPc-CP NSs possess a smooth surface with a uniform
thickness of 1-3 nm and a lateral size of hundreds of nanometers. The as-prepared bimetallic Fe0.5Co0.5Pc-CP NSs was
further used to fabricate heterostructure Fe0.5Co0.5Pc-CP NS@G with graphene NSs as an oxygen reduction reaction (ORR)
catalyst, which exhibits an onset potential of 1006 mV and a half-wave potential of 927 mV in 0.1 M KOH, representing
one of the best values in an alkaline medium. Moreover, the excellent ORR activity of the exfoliated tetrapyrrole-based
conjugated NSs hybridized with graphene has also been demonstrated by the Zn-air battery device, showing an open
-2
circuit voltage of 1.34 V and a peak power density of ca. 180 mW cm .
atomic layers have emerged as one new member in the 2D
2
2-26
Introduction
nanomaterials family and attracted increasing attention.
Compared to the bulk CP/COF materials, 2D CP/COF nanosheets
have unique superiorities. For instance, their larger surface area
and more accessible active sites on the surfaces facilitate the
contact with substrate molecules under lower diffusion barrier, and
thus enhance the performance of 2D CP/COF nanosheets in
Inspired by the discovery and prosperity of graphene, artificial two-
dimensional (2D) organic conjugated layered materials with an
extended π system, such as graphdiyne, 2D organic conjugated
1
-10
polymers (CPs), and 2D covalent organic frameworks (COFs),
have received great research interest. Owing to the bottom-up
approaches, the structures and components of such 2D polymers
could be rationally designed and modulated at the atomic or
molecular level, which will offer the desirable properties and
functions with a wide range of applications in gas storage and
2
7-30
catalysis and sensing applications.
structure would enable the formation of stable dispersions of the
D CP/COF nanosheets, thus would enable large-area and low-cost
liquid-deposition techniques, such as Quasi-Langmuir-Shäfe
Additionally, the ultrathin
2
3
1-34
method, spin coating, or inkjet printing.
Generally, 2D CP/COF NSs are prepared by exfoliating their bulk
1
1,12
13-16
17,18
19-21
separation,
catalysis,
sensors,
and electronic devices.
Recently, 2D conjugated CP/COF nanosheets (NSs) with a few
3
5,36
counterparts by solvent-assisted liquid sonication,
mechanical
3
7
38
delamination,
self-exfoliation,
or sequential post-synthetic
3
9
aBeijing Key Laboratory for Science and Application of Functional Molecular and
modification. However, because of the strong interlayer π−π
interactions, the exfoliation yield of the most obtained 2D CP/COF
NSs is usually quite low. Recently, it has been reported that
introduction of a cycloaddition reaction, twist, or flexible building
units within the COF backbone is able to destroy/weaken the
Crystalline Materials, Department of Chemistry, University of Science and
Technology Beijing, Beijing 100083, China. E-mail: jianzhuang@ustb.edu.cn,
kangwang@ustb.edu.cn.
b
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan
Campus QLD 4111, Australia. E-mail: x.yao@griffith.edu.au.
c
Department of Chemistry and Chemical Engineering, Henan Institute of Science
interlayer π−π interaction, leading to easier exfoliation of the bulk
and Technology, Xinxiang 453003, China.
d
30,40,41
Collaborative Innovation Center for Marine Biomass Fibers, Materials and
COF precursors into ultrathin NSs.
extended
However, the desired
Textiles of Shandong Province, College of Chemical and Environmental
Engineering, Qingdao University, Qingdao 266071, China.
π
conjugated structure was simultaneously
e
State Key Laboratory of Catalytic Material and Reaction Engineering, Research
destroyed/weakened by the aforementioned strategies, which may
result in low electron transfer capability and thus limit their
applications in electronics and energy-related fields. Therefore, new
Institute of Petroleum Processing, SINOPEC, Beijing 100083, China.
†
Electronic Supplementary Information (ESI) available: See DOI:
1
0.1039/x0xx00000x
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methodology still need to be established to develop high-efficiency
synthetic methodology to access 2D CP/COF NSs.
View Article Online
DOI: 10.1039/C8TA11044A
Herein, we report a general approach to fabricate a series of
ultrathin 2D CP NSs (MPc-CP NSs, M = Fe0.5Co0.5, Fe, and Co) with
fully conjugated electronic structure, which was exfoliated from the
ethynyl-linked phthalocyanine (Pc) CPs prepared from the
phthalocyanine monomers, M[Pc(I)
Sonagashira–Hagihara coupling reaction. Owing to that both the
two tetra-β-substituted phthalocyanine precursors, M[Pc(I) ] and
M[Pc(ethynyl) ], are composed of four regioisomers (with D4h, D2h
symmetry), the as-synthesized CPs are of layered
4 4
] and M[Pc(ethynyl) ], through
4
2
4
4
,
4
3
C2v and C
s
structure in which the layers are abundant with intrinsic structural
defects and highly disordered framework nature. Such amorphous
structure of the individual layer with defects enables the diminished
interlayer overlapping area and weakened the interlayer π−π
4
4,45
stacking interactions,
rendering the easier and successful
exfoliation of the bulk CP materials into ultrathin NSs with a high
yield of over 50%. Meanwhile, hetero-assemblies of 2D nanolayers
4
6
such as transition metal dichalcogenides/diseleniums, layered
4
7
48
double hydroxides, and metal−organic frameworks with various
graphene NSs (including graphene, nitrogen doped graphene, and
defective graphene) have been recently received increasing interest
in fabricating versatile high performance electrocatalysts including
oxygen reduction reaction (ORR), oxygen evolution reaction (OER),
and hydrogen evolution reaction (HER) catalysts. The high activity is
due to the combination of the highly exposed active centers
Scheme 1. (a) Synthesis of the phthalocyanine based CP
NSs MPc-CP NSs (M = Fe, Co, and Fe0.5Co0.5). (b) The four
kinds of isomers (with D4h, D2h, C2v and C symmetry) for
s
tetra-β-substituted phthalocyanines. (c) Nine quadrilateral
structure models fabricated by four phthalocyanine
molecules
with
D
4h
or
D
2h
symmetry.
2
7-30
presented on 2D nanolayers
and high electron transfer
together with strong interaction
4
9-52
capability of graphene NSs
between these components to reform the electronic distribution.
Thanks to the ultrathin nature, the as-prepared bimetallic
Fe0.5Co0.5Pc-CP NSs was used to fabricate heterostructure
Fe0.5Co0.5Pc-CP NS@G with graphene NSs, which exhibits high ORR
catalysis activity in an alkaline medium.
is 27-81% lower than that between two 4D4h models, owing to the
reduced overlapping area. These results suggest that the MPc-CP
individual layers fabricated by more phthalocyanine molecules with
four kinds of symmetry are completely disordered and amorphous.
As a result, the interlayer overlapping area would be further
diminished, which weakens the interlayer π−π stacking in the CPs,
and in turn renders the possible exfoliation of the bulk MPc-CP
materials into ultrathin 2D nanosheets by the subsequent solvent-
assisted liquid sonication. With the bimetal counterparts
Fe0.5Co0.5Pc-CP NSs as representative, the yield of Fe0.5Co0.5Pc-CP
NSs could achieve ca. 36% after ultrasonic exfoliation for only 2 h in
ethanol, with the maximum yield of ca. 51% achieved after 8 h (Fig.
S2 in ESI†). The liquid sonication in other organic solvents such as
acetone, tetrahydrofuran, and N,N-dimethylformamide also gave
quite high yield of Fe0.5Co0.5Pc-CP NSs (Fig. S2 in ESI†), suggesting
the generality of the fabrication strategy.
The successful exfoliation of the bulk Fe0.5Co0.5Pc-CP material
into ultrathin Fe0.5Co0.5Pc-CP NSs is firstly evidenced by the
observation of a clear Tyndall effect when a green laser went
through the suspension obtained after the ultrasonic exfoliation
process (inset in Fig. 1a). Transmission electron microscopy (TEM)
image reveals a graphene-like structure of transparent Fe0.5Co0.5Pc-
CP NSs with a lateral size of several hundred nanometers (Fig. 1a),
further confirming the formation of ultrathin Fe0.5Co0.5Pc-CP NSs.
The thickness of the Fe0.5Co0.5Pc-CP NSs was measured by atomic
force microscopy (AFM). It is can be seen in Fig. 1b and 1c that the
Fe0.5Co0.5Pc-CP NSs possess a smooth surface with a uniform
thickness of 1.05±0.05 nm, corresponding to three layers.
Results and Discussion
Synthesis and characterization of MPc-CP NSs.
Scheme 1a schematically illustrates the approach to fabricate
ultrathin MPc-CP NSs. The bulk MPc-CP materials were prepared
4 4
from corresponding M[Pc(I) ] and M[Pc(ethynyl) ] via Sonagashira–
Hagihara coupling reaction according to previously reported
4
2
procedure.
precursors, M[Pc(I)
four constitutional isomers (with D4h, D2h, C2v and C
Since both tetra-β-substituted phthalocyanine
] and M[Pc(ethynyl) ], are actually a mixture of
symmetry)
Scheme 1b). Random coupling between the four M[Pc(I) ] and
] isomers leads to the formation of MPc-CPs with
4
4
s
(
4
M[Pc(ethynyl)
4
highly disordered individual layer. Scheme 1c shows a quadrilateral
structure fabricated by four phthalocyanine molecules with D4h or
D2h symmetry as an example to illustrate the disordered individual
layer of MPc-CPs. As can be seen, nine structural models could be
formed by random combination of four phthalocyanine molecules
with D4h or D2h symmetry. The π-π interactions between the 4D4h
model with the nine structural models were also investigated by
DFT calculations. As can be seen in Fig. S1 in ESI†, the binding
energy per phthalocyanine molecule between two different models
2
| J. Name., 2012, 00, 1-3
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Fig. 1 (a) TEM image of Fe0.5Co0.5Pc-CP NSs. Inset: Photograph of the Tyndall effect of the Fe0.5Co0.5Pc-CP NSs suspension. (b)
AFM image and (c) the corresponding height profiles of Fe0.5Co0.5Pc-CP NSs. (d) AC STEM image and (e) enlarged image of the
Fe0.5Co0.5Pc-CP NSs. (single Fe/Co atoms are highlighted by red circles). (f) HAADF-STEM image and elemental mapping of C,
N,
Co,
and
Fe
of
the
Fe0.5Co0.5Pc-CP
NSs.
Nevertheless, no obvious lattice fringes are observed in aberration- were also prepared under the same conditions with a yield of 42
corrected scanning transmission electron microscopy (AC STEM) and 60%, respectively (Fig. S6-S9 in ESI†), which show similar shape,
images (Fig. 1d), which indicates the amorphous nature of the CP thickness (1.0 nm for CoPc-CP NSs and 3.2 nm for FePc-CP NSs), and
individual layers originated from the less symmetrical isomers in surface area to the Fe0.5Co0.5Pc-CP NSs. Additionally, the diyne
Fe[Pc(I)
4
] and Co[Pc(ethynyl)
4
]. This is also supported by the fact linked Pc conjugated polymer CoPc-CP-2 fabricated from the
] could be
that the selected area electron diffraction (SAED) pattern of the alkyne–alkyne homocoupling reaction of Co[Pc(ethynyl)
4
Fe0.5Co0.5Pc-CP NSs shows broad halo rings (Fig. S3 in ESI†). also exfoliated into ultrathin NSs in the yield ca. 40% (Fig. S10 in
Moreover, from aberration-corrected high-angle annular dark-field ESI†). All above also demonstrate the generality of the present
STEM (AC HAADF-STEM) presented in Fig. 1e, bright dots (high- fabrication strategy to exfoliate CPs to ultrathin conjugated polymer
lighted by red circles) are observed, which are corresponding to NSs.
single Fe/Co atoms located in
N
4
-coordination sites of
It is worth noting again that the successful exfoliation of these
phthalocyanine ligands. In particular, the random dispersion of Pc-based CP materials to 2D ultrathin NSs by liquid sonication origin
these Fe/Co atoms also suggests the disordered framework of the the weak interlayer interactions in bulk Pc-based CP materials, due
Fe0.5Co0.5Pc-CP NSs. Energy dispersive spectroscopy (EDS) and X-ray to their intrinsic defected and highly disordered 2D networks. On
photoelectron spectra (XPS) results indicate that the Fe0.5Co0.5Pc-CP the other hand, if the CPs are with order stacking structure and less
NSs are composed of C, N, O, I, Fe and Co, in accordance with the defects, it is very hard to exfoliate them to ultrathin NSs. For
4
9
composition of bulk Fe0.5Co0.5Pc-CP material (Fig. S4 in ESI†). The example, a 2D layered Pc-based CP, denoted as FePc-CP-3, is
element mapping verifies the homogenous surface distribution of synthesized from the reaction between 1,2,4,5-benzenetetranitrile
5
3,54
Co, Fe, C, and N throughout the whole Fe0.5Co0.5Pc-CP NSs (Fig. 1f). and FeCl
2
(see the ESI† for details).
The ordered AA stacking
These analysis results, together with almost the same Fourier crystal structure of FePc-CP-3 was confirmed by Powder X-ray
transform infrared, XPS, and solid-state UV-vis diffuse reflectance diffraction (PXRD) and HRTEM (Fig. S11b and S11c in ESI†). In
spectra between bulk Fe0.5Co0.5Pc-CP and Fe0.5Co0.5Pc-CP NSs, contrast to MPc-CPs and CoPc-CP-2, only few bulk FePc-CP-3
demonstrate that the chemical composition and bonding modes material (<5%) was exfoliated into NSs with the thickness in wide
remain intact after the ultrasonic exfoliation (Fig. S5 in ESI†). In range after ultrasonic exfoliation for 8 h in ethanol, (Fig. S11d in
addition, the Brunauer-Emmett-Teller (BET) surface area of ESI†), because of the intense interlayer π−π interaction.
2
-1
2
Fe0.5Co0.5Pc-CP NSs is 80 m g calculated from the N sorption
isotherm at 77K, attributed to the mesopores formed by
aggregation of nanosheets (Fig. S5 in ESI†). This is significantly
larger than that of the bulk Fe0.5Co0.5Pc-CP (17 m g ), suggesting It is previously reported that the bulk Fe0.5Co0.5Pc-CP material
more active metal centers are exposed on the ultrathin nanosheets. exhibit good ORR activity owing to the synergetic effect between
Preparation of Fe0.5Co0.5Pc-CP NS@G heterostructure.
2
-1
4
2
The individual metal counterparts, FePc-CP NSs and CoPc-CP NSs, the proximate Fe and Co ions in the conjugated polymer.
This journal is © The Royal Society of Chemistry 20xx
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Fig. 2 (a) Schematic illustration of synthetic Fe0.5Co0.5Pc-CP NS@G. (b) TEM image of Fe0.5Co0.5Pc-CP NS@G. (c) HAADF-STEM
image and corresponding elemental mapping of C, N, Co, and Fe. (d, e) XPS spectra of Fe 2p and Co 2p of Fe0.5Co0.5Pc-CP NSs
and Fe0.5Co0.5Pc-CP NS@G. (f) The top and side views of charge density difference plot for the interfaces between
Fe0.5Co0.5Pc-CP NS and graphene. Yellow and red isosurfaces indicate charge accumulation and depletion, respectively, with
−3
an isosurface value of 0.002 e Å . For clarity, the graphene and Fe0.5Co0.5Pc-CP NS are shown in stick and ball-and-stick
models, respectively. Gray, blue, ice blue, and light green balls represent C, N, Fe, and Co atoms in Fe0.5Co0.5Pc-CP NS,
respectively.
Normally, the ultrathin NSs are more active than the bulk metal sites in Fe0.5Co0.5Pc-CP NSs. To further demonstrate this
counterpart attributed to the more exposed catalytic sites and point, density functional theory (DFT) calculations were carried out
easier mass transportation. Accordingly, it is reasonable to at the level of PBE/Lanl2DZ/3-21G (for C, H, N atoms) using
5
5-58
hypothesize that the ORR activity can be significantly improved if Gaussian 09 D.01.
Fig. 2f shows the geometry of the fully
coupling the ultrathin Fe0.5Co0.5Pc-CP NSs and graphene to fabricate relaxed Fe0.5Co0.5Pc-CP NS@G composite as well as the charge
heterostructure Fe0.5Co0.5Pc-CP NS@G. The synthesis strategy of density differences. As shown, the electrons tended to redistribute
Fe0.5Co0.5Pc-CP NS@G is shown in Fig. 2a. Owing to the conjugated around the metal sites after assembling the Fe0.5Co0.5Pc-CP NSs and
2
structure of both components, Fe0.5Co0.5Pc-CP NSs and graphene graphene NSs together, which favors to bind O molecules. This, in
NSs were easily hybridized together depending on effective combination with the highly exposed active centers, suggests the
interlayer π-π interactions. It is directly observed by TEM (Fig. 2b) optimized ORR performance of Fe0.5Co0.5Pc-CP NS@G.
that the Fe0.5Co0.5Pc-CP NSs were assembled on the surface of
graphene NSs. EDS mapping analysis indicates a coincided
distribution of Fe, Co, and N throughout the exfoliated Fe0.5Co0.5Pc-
ORR electrocatalytic activity.
CP NSs on graphene NSs (Fig. 2b). Furthermore, XPS was also The ORR property of Fe0.5Co0.5Pc-CP NS@G was evaluated by
conducted to investigate the as-prepared heterostructure. As can steady-state linear sweep voltammetry (LSV) on a rotating disk
be seen in Fig. S12, the XPS full survey of Fe0.5Co0.5Pc-CP NS@G electrode (RDE) with the rotation speed of 1600 rpm in O -
2
reveals the presence of C, N, O, I, Fe, and Co elements, well saturated 0.1 M KOH solution. For purpose of illustrating the key
consistent with the EDS results (Fig. S13 in ESI†). In the Fe 2p factors that contributed to the ORR performance of
spectrum, two peaks of the Fe 2p3/2 and Fe 2p1/2 were observed at Fe0.5Co0.5Pc-CP NS@G
,
the electrocatalytic properties of
7
2
7
09.0 and 722.9 eV of Fe0.5Co0.5Pc-CP NS@G (Fig. 2d), while the Co heterostructures with different contents were measured (Fig.
p spectrum displayed the two peaks of Co 2p3/2 and Co 2p1/2 at S14 in ESI†). As can be seen, Fe0.5Co0.5Pc-CP@G with the ratio
79.7 and 794.5 eV (Fig. 2e), respectively. Interestingly, compared between Fe0.5Co0.5Pc-CP NS and G of 2:1 exhibits the best
to the Fe0.5Co0.5Pc-CP NSs, the peaks of Fe 2p and Co 2p of catalytic activity. For comparison, the LSV curves of the composite
Fe0.5Co0.5Pc-CP NS@G shift to the lower energy, indicating a strong of bulk Fe0.5Co0.5Pc-CP and graphene (2:1) (denoted as Fe0.5Co0.5Pc-
interaction between Fe0.5Co0.5Pc-CP NSs and graphene NSs in the CP&G) as well as the commercial Pt/C (20 wt%) were also recorded.
as-prepared heterostructure. Generally, the decrease in binding As shown in Fig. 3a, Fe0.5Co0.5Pc-CP NS@G (the ratio between
energy reveals an enhanced electron screening effect because of Fe0.5Co0.5Pc-CP NS and G is 2:1) exhibited an Eonset of 1006 mV vs
the increase in electron density. These results suggest the charge RHE and an E1/2 of 927 mV vs RHE, obviously superior to
transfer between the two components in the interface of Fe0.5Co0.5Pc-CP&G (Eonset = 954 mV vs RHE and E1/2 = 855 mV vs
Fe0.5Co0.5Pc-CP NS@G, resulting in the electron accumulation at the RHE). Remarkably, with similar onset potential, the E1/2 of
4
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Fe0.5Co0.5Pc-CP NS@G is 83 mV more positive than that of the at 0.4–0.7 V vs RHE (Fig. 3c), indicating the ORR at the Fe0.5Co0.5Pc-
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-
DOI: 10.1039/C8TA11044A
commercial Pt/C (20 wt%) (E1/2 = 844 mV vs RHE), outperforming CP NS@G electrode proceeds via a 4e reduction pathway. The
most of the state-of-the-art ORR catalysts reported to date in an rotation ring-disk electrode (RRDE) measurements were further
alkaline medium (see Table S1 in ESI†). The electrochemical active carried out to determine the electron transfer number n and
surface areas (ECSAs) and mass activity of Fe0.5Co0.5Pc-CP NS@G monitor the generation of peroxide. It is can be seen in Fig. 3d, n
and Fe0.5Co0.5Pc-CP@G were also determined to further evaluate was calculated to be over 3.9 at 0.3–0.9 V vs RHE with peroxide
-
their performance. Fe0.5Co0.5Pc-CP NS@G shows an ECSA of 159 yield below 5%, confirming the 4e reduction pathway over
2
2
cm , significantly higher than that of Fe0.5Co0.5Pc-CP&G, 90 cm (Fig. Fe0.5Co0.5Pc-CP NS@G. Besides the high ORR activity, the
S15 in ESI†). The mass activity and turnover frequency of amperometric i–t test revealed that the stability of Fe0.5Co0.5Pc-CP
-1
Fe0.5Co0.5Pc-CP NS@G are determined to be 18.9 A/g and 42.5 s at NS@G is superior to Pt/C (20 wt%) (Fig. 3e). Furthermore, the XPS
.9 V vs RHE (Fig. 3b), respectively, which are significantly higher spectra of Fe0.5Co0.5Pc-CP NS@G were recorded after the i-t test,
than those of Fe0.5Co0.5Pc-CP&G (11.1 A/g and 28.7 s ) and Pt/C (20 which was almost unchanged, confirming its high stability (see Fig.
wt%) (7.5 A/g and 1.2 s ), and comparable with the best values for S16 in ESI†). Moreover, Fe0.5Co0.5Pc-CP NS@G exhibit more
Pt-free catalysts reported so far.
0
-1
-1
4
2,59-66
These results coincide well excellent tolerance for methanol compared to Pt/C (20 wt%) (Fig.
with our expectation that Fe0.5Co0.5Pc-CP NS@G would show an 3f). These results endow Fe0.5Co0.5Pc-CP NS@G an alternative
enhanced ORR performance. The high ORR catalytic activity of cathode catalyst in methanol fuel cells and metal-air batteries.
Fe0.5Co0.5Pc-CP NS@G was further revealed by the catalytic
selectivity. The electron transfer number n of Fe0.5Co0.5Pc-CP NS@G,
analysed based on the RDE measurements at different rotating
Zn–air batteries.
speeds according to the Koutecky–Levich (K–L) equation, was ca. 4 Inspired by the notable half-cell performance of Fe0.5Co0.5Pc-CP
NS@G in the ORR, this catalyst was further utilized as a cathode to
evaluate the full-cell application in the Zn–air batteries under
practical conditions (Fig. 4a). For the purpose of comparison, Pt/C
(
4
20 wt%) was also integrated into a Zn–air battery. As shown in Fig.
b, the discharge polarization and power density plots of the
Fe0.5Co0.5Pc-CP NS@G Zn–air battery show an open circuit voltage
-2
of 1.34 V and a maximum power density of ca. 180 mW cm at a
current density of 283 mA cm , which are better than those of the
Pt/C (20 wt%) (with the open circuit voltage of 1.33 V and the peak
-2
-2
power density of ca. 152 mW cm ). In addition, Fe0.5Co0.5Pc-CP
NS@G Zn–air battery exhibit stable discharge voltage at a current
-2
density of 10 mA cm , comparable to that of the Pt/C (20 wt%) (Fig.
c). To further broaden the application of Fe0.5Co0.5Pc-CP NS@G, a
4
rechargeable Zn–air battery was fabricated with the mixture of
Fe0.5Co0.5Pc-CP NS@G and RuO (a benchmark electrocatalyst for the
oxygen evolution reaction) (1:1) as the cathode. This rechargeable
Zn–air battery exhibits the very similar discharge polarization and
power density plots as the Fe0.5Co0.5Pc-CP NS@G Zn–air battery (Fig.
4
b), clearly indicating the dominant contribution of Fe0.5Co0.5Pc-CP
NS@G to the ORR activity in the cathode of the rechargeable Zn–air
battery. Moreover, long-term cycling tests at current densities of 5
-1
and 10 mA cm reveal the lack of obvious change for discharge
voltage over 200 cycles (Fig. 4d). Additionally, two rechargeable Zn–
air batteries in series could provide an enough high open circuit
voltage to power a light-emitting diode (LED) light with a rated
voltage of ≈2 V (Fig. 4e). These results disclose the great potential
of the Fe0.5Co0.5Pc-CP NS@G composite in the Zn–air batteries
applications.
Fig. 3 (a) LSV curves of Fe0.5Co0.5Pc-CP NS@G, Fe0.5Co0.5Pc-
CP&G, and Pt/C (20%) measured in O -saturated 0.1 M KOH
solution. (b) Onset potential, half-wave potential and mass
activity comparisons of Fe0.5Co0.5Pc-CP NS@G, Fe0.5Co0.5Pc-
CP&G and Pt/C (20%). (c) LSV curves at different rotation
speeds of Fe0.5Co0.5Pc-CP NS@G. The inset shows K–L plots. (d)
2
2 2
H O yield and electron transfer number of Fe0.5Co0.5Pc-CP&G
and Pt/C (20%). (e) Amperometric i–t curves of Fe0.5Co0.5Pc-CP
NS@G and Pt/C (20%) under the rotation speed of 1600 rpm.
(
(
f) Methanol tolerance test for Fe0.5Co0.5Pc-CP NS@G and Pt/C
20%).
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DOI: 10.1039/C8TA11044A
Fig. 4 (a) Schematic of the Zn–air battery. (b) The discharge polarization plots and power density curves of the Zn–air
batteries. (c) Discharge plots of the Zn–air batteries with Fe0.5Co0.5Pc-CP NS@G as well as commercial Pt/C (20%) as cathode
-2
catalysts at a current density of 10 mA cm . (d) The discharge and charge voltage profiles of the Zn-air batteries with the
−1
mixture of Fe0.5Co0.5Pc-CP NS@G and RuO (1:1) as the cathode at current densities of 5 and 10 mA mg . (e) Photographs of a
LED light (≈2 V) powered by two zinc-air batteries with the mixture of Fe0.5Co0.5Pc-CP NS@G and RuO as the cathode in series.
Conclusions
Preparation of Fe0.5Co0.5Pc-CP NSs, FePc-CP NSs, and CoPc-CP NSs
The NSs were obtained by exfoliation of bulk MPc-CP materials via a
simple sonication procedure. In a typical experiment, Fe0.5Co0.5Pc-
We report the efficient exfoliation of a series of conjugated
ultrathin MPc-CP NSs (M = Fe, Co, Fe0.5Co0.5) from their bulk
counterparts, owing to the introduction of the defects and
CP powder (20 mg) was dispersed in ethanol (60 mL). The
disorders into the individual layers, which enable diminished
suspension was then sonicated in an ultrasonication bath (KQ-
interlayer overlapping and weakened interlayer π−π stacking.
5
00DE, 40 kHz, 500W) for 8 h under room temperature. The
The ultrathin MPc-CP NSs possess a smooth surface with a
uniform thickness and a lateral size of several hundred
nanometers. The as-prepared bimetallic Fe0.5Co0.5Pc-CP NSs
can be used to fabricate heterostructure Fe0.5Co0.5Pc-CP NS@G
with graphene NSs as a high-performance ORR catalysts with
an onset potential of 1006 mV and a half-wave potential of
resulting suspension was centrifuged at 2000 rpm for 5 min to
remove the unexfoliated bulk Fe0.5Co0.5Pc-CP. The yield is calculated
by measuring the remaining weight of collected unexfoliated bulk
Fe0.5Co0.5Pc-CP and compared to the weight of original crystals
before sonication. As a result, the measured yield is 51%. FePc-CP
NSs, CoPc-CP NSs, and CoPc-CP-2 NSs were prepared by the same
procedure with the yield of 42%, 60%, and 40%, respectively.
9
27 mV in 0.1 M KOH, outperforming most of the state-of-the-
art ORR catalysts reported so far in an alkaline medium. Its
excellent ORR activity has also been demonstrated by the good
performance of the Zn-air battery device, with an open circuit
voltage of 1.34 V and a peak power density of ≈180 mW cm .
This work is surely helpful for further design and synthesis of
other ultrathin 2D CP/COF NSs with tunable conjugated
electronic and geometric structure, which might have various
promising applications in electronics and energy-related fields.
Preparation of Fe0.5Co0.5Pc-CP NS@G
Typically, a designed volume (2.0 mg mL ) of the exfoliated
Fe0.5Co0.5Pc-CP NSs was added drop by drop into graphene NSs
suspension with concentration of 1.0 mg mL under continuous
stirring for 8 h at 70ºC. The flocculated product was separated by
centrifugation. The weight ratios of Fe0.5Co0.5Pc-CP NS and G were
controlled as 1:1, 2:1, 4:1, and 8:1.
-
1
-2
−
1
Experimental Section
General Remarks
Preparation of Fe0.5Co0.5Pc-CP&G
Fe0.5Co0.5Pc-CP&G was prepared by the same procedure of
Fe0.5Co0.5Pc-CP NS@G with the Fe0.5Co0.5Pc-CP NSs replaced by bulk
Fe0.5Co0.5Pc-CP.
All reagents and solvents were reagent grade and used as received.
Fe0.5Co0.5Pc-CP, CoPc-CP, FePc-CP, and CoPc-CP-2 were synthesized
by using reported procedures.42
Acknowledgements
6
| J. Name., 2012, 00, 1-3
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Financial support from the Natural Science Foundation of 24 M. Osada and T. Sasaki, Adv. Mater., 2012, 24, 210-228.
View Article Online
China (Nos. 21631003, 21671017, and 21871024), the 25 X. Huang, C. Tan, Z. Yin and H. Zhang, DAOdIv:.10M.1a0t3e9r/.C, 82T0A11410, 4246A,
Fundamental Research Funds for the Central Universities (No. 2185-2204.
FRF-BD-17-016A), and University of Science and Technology 26 H. Sahabudeen, H. Y. Qi, B. A. Glatz, D. Tranca, R. H. Dong, Y.
Beijing is gratefully acknowledged.
Hou, T. Zhang, C. Kuttner, T. Lehnert,; G. Seifert, U. Kaiser, A.
Fery, Z. K. Zheng and X. L. Feng, Nat. Commun., 2016, 7, 13461.
7 Q. Lu, Y. Yu, Q. Ma, B. Chen and H. Zhang, Adv. Mater., 2016,
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