An ethynyl-linked Fe/Co heterometallic phthalocyanine conjugated polymer for the oxygen reduction reaction

Article abstract of DOI:10.1039/c8ta00516h

The Sonogashira-Hagihara coupling reaction is utilized to fabricate ethynyl-linked phthalocyanine (Pc) 2D conjugated polymers (CPs), affording the unprecedented heterometallic compound Fe_0.5Co_0.5Pc-CP and homometallic analogues MPc-CPs (M = Fe, Co) for comparison. It is confirmed that Fe and Co ions are well-defined/distributed with one metal ion encircled by four neighboring ions of the second metal in close proximity in the heterometallic conjugated polymer. Compared to the homometallic analogues, the bimetallic polymer Fe_0.5Co_0.5Pc-CP is more highly efficient for the oxygen reduction reaction (ORR), indicating that the interaction of Fe and Co ions may provide a synergistic effect on the activity. Fe_0.5Co_0.5Pc-CP exhibits similar ORR catalytic activity and superior stability compared to commercial Pt/C in alkaline media. Density functional theory calculations reveal that a fast 2 × 2e^- synergistic catalytic reaction pathway is essential for the excellent ORR in Fe_0.5Co_0.5Pc-CP.

Full text of DOI:10.1039/c8ta00516h

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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
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DOI: 10.1039/C8TA00516H
Journal Name
ARTICLE
Ethynyl-linked Fe/Co Heterometallic Phthalocyanine Conjugated
Polymer for Oxygen Reduction Reaction†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,c
a
a
a
a,b
Wenping Liu, Yuxia Hou, Houhe Pan, Wenbo Liu, Dongdong Qi,* Kang Wang,* Jianzhuang
a
b
Jiang* and Xiangdong Yao
DOI: 10.1039/x0xx00000x
Sonagashira-Hagihara coupling reaction is utilized to fabricate the ethynyl-linked phthalocyanine (Pc) 2D conjugated
polymers (CPs), affording the unprecedented heterometallic compound Fe0.5Co0.5Pc-CP and homometallic analogues MPc-
CPs (M = Fe, Co) as comparisons. It is confirmed that the Fe and Co ions are well-defined/distributed with one metal ion
encircled by four neighboring ions of the second metal in close proximity in the heterometallic conjugated polymer.
Compared to homometallic analogues, the bimetallic polymer Fe0.5Co0.5Pc-CP is highly more efficient for oxygen reduction
reaction (ORR), indicating that the interaction of Fe and Co ions may provide a synergistic effect on the activity. The ORR
activity of Fe0.5Co0.5Pc-CP is with a similar ORR catalytic activity and superior stability to the commercial Pt/C in alkaline.
www.rsc.org/
-
Density functional theory calculation reveals that a fast 2 × 2e synergetic catalytic reaction pathway is essential for the
excellent ORR in Fe0.5Co0.5Pc-CP.
separation and ordered distribution of Fe and Co atoms. However,
Introduction
so far all the state-of-the-art FeCoN
x
-based catalysts are fabricated
8
through thermal treatment (typically 800-1000°C), which is energy
intensive and not environmentally friendly. Moreover, the high
temperature thermal treatment makes it hard to approach the
atomic-level ordering of the active components in the catalysts. This
limits in-depth understanding the intrinsic reaction mechanism and
further rational design of new FeCoN
improved catalytic performance.
Two dimensional (2D) organic conjugated polymers (CPs)
are a novel class of polymeric materials with amorphous
extended π-conjugated networks. The high flexibility in the
molecular design of components and the control of network
structures for CPs makes them possessing well-defined
structure and chemical composition. It is anticipated that
simultaneous introduction of Fe- and Co-N
monomers with high ORR catalytic activity into the fabrication
of the 2D CPs could achieve Fe&Co bimetallic MN -based
catalysts with alternate distribution of Fe and Co fragments,
which would provide an effective method towards creating
high-performance ORR catalysts.
Oxygen reduction reaction (ORR) plays an important role in energy
conversion and storage devices such as fuel cells and metal-air
1
batteries. Currently, noble metal platinum (Pt) is still the best
2
catalyst for the ORR. However, the high cost and scarcity, serious
anode crossover, and methanol poisoning of Pt-based catalysts limit
their large-scale commercial applications. Non-precious high-
performance alternatives towards ORR then become highly desired
in this field, leading to the development of a wide range of non-Pt-
based electrochemical catalysts including transition metal oxides,
carbon-based metal free materials, and transition-metal-decorated
carbon materials. Among them, MN
x
-based catalysts with
3
4
9
5
6
x
-based (M = Fe, Co)
electrocatalysts have been extensively studied, which are among
the most promising systems to replace expensive Pt based
4
-macrocyclic
7,8
catalysts. Notably, recent studies suggest that Fe&Co bimetallic
MN -based materials have better catalytic activity compared to
their single-element counterparts, as shown by PCN-FeCo/C,
x
8a
x
8b
8c
8d
PANI-FeCo-C,
(Fe,Co)/N-C, and FeCo-MFR/C.
The higher
catalytic activity of the bimetallic catalysts with both Fe and Co
originates from the synergistic effect between the two metal
centers, which should be closely related to the small spatial
Herein, we developed
electrocatalyst termed
phthalocyanine (Pc) conjugated polymer by means of
Sonagashira-Hagihara coupling reaction between Fe[Pc(I) ] and
Co[Pc(ethynyl) ] {[Pc(I) ] = dianion of 2(3),9(10),16(17),23(24)-
tetraiodophthalocyanine; [Pc(ethynyl) dianion of
(3),9(10),16(17),23(24)-tetra(ethynyl)phthalocyanine}. The
a
new FeCoN
x
-based ORR
ethynyl-linked heterometallic
4
a Beijing Key Laboratory for Science and Application of Functional Molecular and
Crystalline Materials, Department of Chemistry, University of Science and
Technology Beijing, Beijing 100083, China. *E-mail: jianzhuang@ustb.edu.cn,
kangwang@ustb.edu.cn,qdd@ustb.edu.cn.
b Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan
Campus QLD 4111, Australia.
4
4
4
]
=
2
corresponding merits are summarized as follows: (1) Since
Jasinski first reported a CoPc based electrocatalyst for a fuel
c Department of Chemistry and Chemical Engineering, Henan Institute of Science
and Technology, Xinxiang 453003, China.
10
†
1
Electronic Supplementary Information (ESI) available: See DOI:
0.1039/x0xx00000x
cell cathode in 1964, the ability of metallophthalocyanines to
11
reduce molecular oxygen has been extensively studied. It has
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DOI: 10.1039/C8TA00516H
Scheme 1. Schematic diagram for the synthesis of the phthalocyanine conjugated polymers MPc-CPs (M = Fe, Co, and
Fe0.5Co0.5).
been found that metallophthalocyanines, in particular iron and 70°C for 48 h provided the heteometallic Pc-based 2D conjugated
cobalt phthalocyanines, show good ORR activity but poor network Fe0.5Co0.5Pc-CP as seen in Scheme 1. For comparative
stability. Polymerization of metallophthalocyanines could study, homometallic analogues FePc-CP (from the coupling of
significantly improve their physical and chemical robustness, Fe[Pc(I)
possibly resulting in high stability. Moreover, the planar Co[Pc(I)
conjugated molecular structure makes the phthalocyanines as conjugated polymers are insoluble in water and organic solvents
promising building blocks in the construction of 2D CP such as CHCl , CH OH, C OH, N,N-dimethylformamide (DMF),
4
] with Fe[Pc(ethynyl)
4
]) and CoPc-CP (from the coupling of
4
] with Co[Pc(ethynyl)
4
]) were also fabricated. All these
3
3
2 5
H
materials. (2) Sonagashira-Hagihara coupling reaction was tetrahydrofuran (THF), and acetone. Thermogravimetric analysis
employed to form carbon-carbon bonds between the terminal (TGA) indicated their good thermal stability with decomposition
4
alkynes in Co[Pc(ethynyl) ] and the aryl iodines in Fe[Pc(I)
4
], occurring over 350°C as shown in Fig. S1 (ESI†).
which makes the well-arranged Pc units in the network with
Fourier transform infrared (FT-IR) analysis confirms the
one chromophore originated from one starting Pc species successful fabrication of CPs from the two species of Pc
encircled by four chromophores of the other Pc species. In monomers (Fig. 1a and S2 in ESI†). The absorptions
addition, the coupling reaction was carried out under mild contributed from the central aromatic Pc macrocycle including
reaction conditions, circumventing high-temperature heat the wagging and torsion vibrations of C-H groups at 672-753
-
1
-1
treatment and retaining the structures of Pc precursors in the cm , isoindole ring stretching vibrations at 1070-1420 cm ,
catalyst materials. (3) The ethynyl-linked heterometallic Pc- and the C=N aza group stretching vibrations at 1584-1596 cm-1
based 2D conjugated polymer possesses well-defined chemical in the IR spectra of the three MPc-CPs unambiguously
1
3
composition and alternate distribution of Fe and Co fragments, demonstrate their Pc composition and structure. The C-H
providing an ideal model for understanding the origin of the stretching band at ca. 3289 cm− due to ethynyl groups of
catalytic activity. Consequently, the derived heterometallic M[Pc(ethynyl)
compound exhibits excellent ORR catalytic activity in alkaline corresponding to the C-I groups of M[Pc(I)
media, which is comparable to commercial Pt/C, due to the attenuated in the IR spectra of the three MPc-CPs.1
1
] and C-I vibration band at ca. 1033 cm−1
4
4
] get significantly
4,15
These
synergetic effect between Fe and Co in this network.
results give a good evidence for the integration of these MPc-
CPs from the coupling of M[Pc(I) ] with M[Pc(ethynyl) ] and in
4
4
particular suggesting the well-arranged Pc units in the ethynyl-
linked network with one chromophore originated from one
starting Pc species encircled by four chromophores of the
other Pc species as illustrated in Scheme 1. The solid-state UV-
vis diffuse reflectance spectrum of Fe0.5Co0.5Pc-CP shows the
phthalocyanine Soret band at 350-450 nm and Q-band at 600-
Results and Discussion
Synthesis and characterization
For the purpose of fabricating the C≡C linked phthalocyanine 2D CPs
via Sonagashira-Hagihara coupling reaction, corresponding metal
free phthalocyanine monomers, namely 2(3),9(10),16(17),23(24)-
8
00 nm, which are both slightly red-shifted compared to the
tetraiodophthalocyanine {H
tetra(ethynyl)phthalocyanine {H
following the published procedures. Metallization with Fe and Co
afforded the phthalocyaninato metal complexes Fe[Pc(I) ],
]. Sonagashira-
2
[Pc(I)
4
]} and 2(3),9(10),16(17),23(24)-
corresponding phthalocyanine monomers, suggesting a highly
conjugated nature of the polymer, Fig. 1b. This is also true for
CoPc-CP and FePc-CP (Fig. S3 in ESI†). In addition, the newly
2
[Pc(ethynyl) ]}, were prepared
4
12
4
prepared MPc-CPs were also characterized by solid-state 13
CP/MAS NMR spectroscopy, showing one broad peak at 120-
40 ppm assigned to the Pc macrocycle, (Fig. S4 in ESI†). Along
C
Fe[Pc(ethynyl)
4
], Co[Pc(I)
4
], and Co[Pc(ethynyl)
4
Hagihara coupling between Fe[Pc(I)
4 4
] and Co[Pc(ethynyl) ] with a
1
stoichiometric ratio of 1:1 in the mixed solvent of anhydrous
THF:Et (1:2) under the presence of
bis(triphenylphosphine)palladium(II) chloride and CuI as catalysts at
with the change in the metal center from Co, to Fe0.5Co0.5, and
Fe for the conjugated polymers, the signals due to the Pc
3
N
2
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respectively, corresponding to the metal coordinated pyrrole
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1 6I :,17
10.1039/C8TA00516H
and aza nitrogen species (Fig. S8 in ESI†D)O.
In addition, the
peaks assigned to iodine and oxygen were also observed in the
XPS overall spectra of the three CPs due to the residual iodine
-
groups of M[Pc(I)
4
] and axial coordinated OH or H
2
O to the
metal ions, respectively.
ORR electrocatalytic activity.
The electrocatalytic activity of MPc-CPs for ORR was initially
evaluated by CV in O -saturated and O -free electrolyte in 0.1
2
2
M KOH solution on MPc-CP-loaded glassy carbon electrodes
with 50 wt% carbon black (Vulcan XC-72) to increase the
electrical conductivity. As seen in Fig. 2a, for O -free
2
electrolyte, the current in the CV curves showed featureless
slopes for the cathodic current within the entire potential
range. In contrast, the well-defined cathodic peaks at 0.7-0.9 V
appeared in the CV curves of all the three MPc-CPs in O -
2
saturated electrolyte, indicating their ORR catalytic activity. In
particular, the peak potential positively shifted from 0.72 V for
CoPc-CP and 0.80 V for FePc-CP to 0.86 V for Fe0.5Co0.5Pc-CP,
revealing the significantly enhanced ORR catalytic activity of
the heterometallic material. Linear sweep voltammetry (LSV)
Fig.
1
(a) FT-IR spectra of Fe0.5Co0.5Pc-CP and
corresponding phthalocyanine monomers in the region of
-1
4
00-4000 cm . (b) Solid-state UV-vis diffuse reflectance
spectra of and corresponding
phthalocyanine monomers. XPS high resolution (c) Fe 2p
and (d) Co 2p spectra of CoPc-CP FePc-CP, and
Fe0.5Co0.5Pc-CP
,
Fe0.5Co0.5Pc-CP
.
curves were recorded in O
further investigate their ORR activity. In order to clarify the
factors contributing to the ORR performance of the MPc-CP
glassy carbon electrode, the electrocatalysis properties of pure
XC-72 and MPc-CP&XC-72 mixtures with different contents
were measured as the reference (Fig. S9-S11 in ESI†). For the
purpose of comparison, the ORR activity of the commercial
Pt/C (20 wt%) as well as corresponding Pc-monomers were
2
-saturated 0.1 M KOH solution to
macrocycle gradually downfield shift from 124 ppm for CoPc-
CP, to 131 ppm for Fe0.5Co0.5Pc-CP, and 135 ppm for FePc-CP
.
s
As shown in Fig. S5 (ESI†), the powder X-ray diffraction
(PXRD) patterns gives no definite evident peaks, revealing the
amorphous nature of these MPc-CPs, owing to the irreversible
nature of the Sonagashira-Hagihara coupling reaction as well
as the existence of less symmetrical isomers in
4 4
M[Pc(I) ]/M[Pc(ethynyl) ]. Scanning electron microscopy (SEM)
also studied, Fig. 2b and S12 (ESI†). As can be seen in Fig. S9
images show the morphology of the irregular agglomerated
nanoparticles of these MPc-CPs, which consist of many pieces
of nanosheets according to the transmission electron
microscopy (TEM) images (Fig. S6 in ESI†), revealing the
fabrication of the ethynyl-linked 2D network. The elemental-
mapping images of the heterometallic Fe0.5Co0.5Pc-CP (Fig. S7
in ESI†), disclose the homogeneous distribution of the iron,
cobalt, and nitrogen species throughout the whole network,
suggesting the alternate distribution of Fe and Co fragments.
This is also the same case for their homometallic analogues
(
ESI ), the ORR activity of Fe0.5Co0.5Pc-CP&XC-72 mixtures is
†
significantly superior to that of pure XC-72 and Fe0.5Co0.5Pc-CP
electrodes. The mixture with 50 wt% Fe0.5Co0.5Pc-CP exhibits
the best catalytic behavior. The same cases for the FePc-CP
loaded and CoPc-CP-loaded electrodes as shown in Fig. S10
and S11 (ESI ). These results demonstrate that the ORR
activity of the MPc-CP-loaded electrodes mainly originates
from the synergistic effect between the highly active catalytic
centers of MPc-CPs materials and high electrical conductivity
of XC-72. In addition, it is worth noting that highly conjugated
structures of these phthalocyanine-based CPs are beneficial to
improve the interaction between phthalocyanine-based CPs
and XC-72, which increases the electron-transfer efficiency
during the electrocatalytic process and in turn improves the
ORR performance.
-
†
FePc-CP and CoPc-CP
.
X-ray photoelectron spectroscopy (XPS) measurements
were performed to analyze the surface composition and
chemical valence state. As can be seen in the high-resolution
Fe 2p XPS spectra in Fig. 1c, two peaks of the Fe 2p3/2 and Fe
2
7
p
1/2 were observed at 710.0 and 723.0 eV for FePc-CP and
Fig. 2b compares the LSV curves of MPc-CPs mixed with 50
10.1 and 723.0 eV for Fe0.5Co0.5Pc-CP, indicating the existence
2
wt% XC-72 and the commercial Pt/C (20 wt%) in O -saturated
0
rotation speed of 1600 rpm. In good agreement with the CV
study result, Fe0.5Co0.5Pc-CP displays the highest activity
among the three Pc polymers in terms of onset potential
3+
16
2+
of Fe in these two polymers. The existence of Co in
Fe0.5Co0.5Pc-CP and CoPc-CP is confirmed by the two peaks of
Co 2p3/2 and Co 2p1/2 observed at 781.0 and 796.3 eV for
Fe0.5Co0.5Pc-CP and 780.9 and 796.4 eV for CoPc-CP in their
high-resolution of Co 2p XPS spectra in Fig. 1d. In the high-
resolution N 1s spectra of CoPc-CP Fe0.5Co0.5Pc-CP, and FePc-
−1
.1 M KOH solution at a scan rate of 10 mV·s with the
1
7
(E
onset), the half-wave potential (E1/2), and limiting current
,
density (J ). As shown in Fig. 2c and Table S1 (ESI ),
Fe0.5Co0.5Pc-CP exhibits an Eonset of 937 mV and an E1/2 of 848
mV with a J
L
†
CP, the N 1s peak is deconvoluted into two sub-peaks at 399.3
and 398.7 eV, 399.3 and 398.7 eV, and 399.3 and 398.5 eV,
-2
L
of -5.98 mA cm . These results are significantly
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CP shows mass activity of 11.5 A/gMPc-CP at 0.9 V vs RHE, which
View Article Online
DOI: 10.1039/C8TA00516H
is about 5 and 7 times higher than those of FePc-CP (2.42
A/gMPc-CP) and CoPc-CP (1.76 A/gMPc-CP), respectively. The
activity is also comparable to the best values reported for non-
16c,19
precious metal based electrocatalysts.
These results
suggest the synergetic effect between Fe and Co ions in
Fe0.5Co0.5Pc-CP on promoting the ORR. In order to further
verify this assumption, polarization curves of the
corresponding Pc monomers for the three Pc polymers MPc-
CPs were also recorded under the same testing conditions. As
can be seen in Fig. S12 (ESI†), the compared results clearly
indicate the only slightly increased ORR catalytic activity of
FePc-CP and CoPc-CP over their Pc monomers. However,
Fe0.5Co0.5Pc-CP shows significantly more positive Eonset and E1/2
L
as well as the greater J than those of corresponding Pc-
monomer electrode under the same loading amount (Fig. 2f).
Moreover, Fe0.5Co0.5Pc-CP displays better catalytic activity over
the mixture of FePc-CP and CoPc-CP (1:1) as seen in Fig. 2f,
indicating that the improved ORR catalytic activity of
Fe0.5Co0.5Pc-CP is due to the ordered distribution of Fe and Co
center in the close proximity.
In order to gain further insight into the ORR catalytic
activity, LSV curves on the MPc-CP-loaded electrodes with 50
wt% black carbon were recorded at the rotation speed range
of 400-2500 rpm as seen in Fig. 3a, S14, and S15 (ESI†). The
Koutecky-Levich (K-L) plots of all the MPc-CP-loaded
2
Fig. 2 (a) CVs of MPc-CPs in O -saturated (solid line) and
2
O -free (dotted line) 0.1 M KOH solution. (b) LSV curves of
MPc-CPs as well as the commercial Pt/C (20%) measured at
−
1
the scan rate of 10 mV s with the rotation speed of 1600
rpm in O -saturated 0.1 M KOH solution. (c) Half-wave
2
potential and onset potential comparisons of the
synthesized MPc-CPs as well as the commercial Pt/C (20%).
(d) Tafel plots derived from the corresponding 1600 rpm
LSV curves of the synthesized MPc-CPs as well as the
commercial Pt/C (20%). (e) Comparison of the mass activity
for MPc-CPs at 0.9 V (vs RHE). (f) LSV curves of
Co[Pc(ethynyl)
4 4
] & Fe[Pc(I) ] (1:1), CoPc-CP & FePc-CP (1:1),
2
and Fe0.5Co0.5Pc-CP in O -saturated 0.1 M KOH solution. All
the data for the synthesized MPc-CPs were measured by
doping with 50 wt% XC-72.
superior to the homometallic counterparts FePc-CP (Eonset
=
-2
9
10 mV, E1/2 = 800 mV, and J
onset = 907 mV, E1/2 = 716 mV, and J
comparable to the commercially available Pt/C catalyst (Eonset
L
= -5.76 mA cm ) and CoPc-CP
-2
(E
L
= -4.40 mA cm ), and are
Fig. 3 (a) LSV curves of Fe0.5Co0.5Pc-CP at different rotation
speeds. Insert: Koutecky–Levich (K–L) plots at different
potentials. (b) Percentage of peroxide species and the electron-
transfer number (n) of Fe0.5Co0.5Pc-CP and the commercial Pt/C
=
-
2
9
L
51 mV, E1/2 = 823 mV, and J = -6.14 mA cm ). Moreover, the
superior ORR kinetics of the heterometallic compound to the
homometallic counterparts was also clearly revealed by the
(
20%) in the potential range of 0.20-0.70 V (calculated from the
-1
significantly smaller Tafel slope of 42.3 mV dec for
Fe0.5Co0.5Pc-CP at low overpotentials than 56.6 and 135.2 mV
dec-1 for FePc-CP and CoPc-CP, respectively (Fig. 2d). The
electrochemical active surface areas (ECSAs) of the three MPc-
CPs were also calculated in 0.1 M KOH solution by the
corresponding RRDE data). (c) Amperometric i–t curves of
Fe0.5Co0.5Pc-CP and the commercial Pt/C-modified electrode
tested with the rotation speed of 1600 rpm in O
M KOH solution. (d) Methanol tolerance test with 5% methanol
in volume) in O -saturated 0.1 M KOH solution for Fe0.5Co0.5Pc-
2
-saturated 0.1
(
2
18
reported method. The ECSAs of the three MPc-CPs are
−
1
CP and Pt/C (20%) (the CV scan rate is 50 mV s ). All the data
for the synthesized MPc-CPs were measured by doping with 50
wt% XC-72.
similar under the same catalyst loading owing to their similar
polymer network structures (Fig. S13 in ESI†). The mass
activities of the three MPc-CPs were calculated to further
assess their performance. As shown in Fig. 2e, the Fe0.5Co0.5Pc-
4
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electrodes under different potentials exhibit good linearity and almost free from the methanol crossover effect, FViigew. 3Ardtic.leTOhnelisnee
parallelism, indicating the first-order reaction kinetics toward results render Fe0.5Co0.5Pc-CP a promisingDOcaI:n10d.i1d0a3t9e/Ct8oTAre00p5la16cHe
the concentration of dissolved O . The n for both Fe0.5Co0.5Pc- the cathodic Pt/C catalyst in methanol fuel cells.
2
CP and FePc-CP was calculated to be ca. 4 at 0.3-0.6 V,
-
indicating the ORR at these electrodes proceeds by a 4e
DFT calculations.
reduction pathway, similar to the commercial Pt/C electrode.
In contrast, the n derived from the K-L plots for CoPc-CP is 3.6
Based on experimental results aforementioned, the higher
-
-
±
0.1 at 0.3-0.6 V, suggesting the co-existence of 2e and 4e catalytic activity of the heterometallic conjugated network
oxygen reduction processes in this electrode. This is further Fe0.5Co0.5Pc-CP originates from the synergistic effect between
supported by the rotation ring-disk electrode (RRDE) test with the proximate Fe and Co ions. In an attempt to understand the
n calculated to be over 3.9 and low peroxide yield for the two synergetic effect, density functional theory (DFT) calculations
/
Fe-containing electrodes, Fe0.5Co0.5Pc-CP and FePc-CP, in were carried out at the level of PBE1PBE-GD3//SDD 6-
2
0
comparison with the values of ca. 3.5 and 20% for CoPc-CP at 311++G(2d,p)/6-311G(d).
Three computational models
0
.2-0.7 V (Fig. 3b and S16 in ESI
†
). These results indicate including FePc≡FePc, CoPc≡CoPc, and FePc≡CoPc, were derived
-
preferential 2e -reduction in CoPc-CP with respect to FePc- from the basic structural units of the three MPc-CPs. As
containing catalysts, in line with previous findings on catalytic expected, the metal ions act as the active sites for ORR in all
selectivity in MPc-based catalysts. Besides, the near four- the three models. For the homometallic models, the 4e-
11
electron pathway toward oxygen reduction on Fe0.5Co0.5Pc-CP
-
oxygen reduction reaction proceeds along the following
loaded electrode further reflects the high ORR efficiency of the pathways (* = surface):
-
OOH* + OH-
heterometallic catalyst. Besides the high ORR activity, the
long-term stability and possible fuel crossover effect are also
quite important. Thus, the durability of the Fe0.5Co0.5Pc-CP
with 50 wt% XC-72 and commercial Pt/C is evaluated through
chronoamperometric measurements. As shown in Fig. 3c,
O
2
+ * + H
2
O + e
→
(1)
(2)
(3)
(4)
-
→
O* + OH-
OOH* + e
O* + H
OH* + e
-
OH* + OH-
2
O + e
→
-
-
→
OH + *
When the reaction proceeds along the following path 2’
-
Fe0.5Co0.5Pc-CP exhibits a superior long-term stability to the rather than 2 after step 1, H
2
O
2
is generated by a 2e oxygen
commercial Pt/C catalyst. The almost same XPS spectra of reduction:
Fe0.5Co0.5Pc-CP before and after i-t test further confirm its high
stability (see Fig. S17 in ESI ). We believe that the
polymerization of Fe[Pc(I) and Co[Pc(ethynyl) into ORR intermediates involved in steps 1-4 and 2’ bound on the
Fe0.5Co0.5Pc-CP, which could significantly improve the physical active site at 1.23 V vs. RHE. In the FePc≡FePc model, the
OOH* + H
2
O + e-
→
2
H O
2
* + OH-
(2’)
†
Fig. 4 shows the calculated free-energy diagrams of all the
4
]
4
]
16a,c
and chemical robustness, contributes to such high stability.
2 2
activation barrier of step 2 is lower than that for 2’ (H O
In addition, after the durability test for 20000 s, the FePc-CP generation reaction). As a consequence, the ORR reaction
-
and CoPc-CP could retain around 70.1% and 77.2% of the prefers to proceed following the 4e route in the FePc≡FePc
initial current, respectively, compared to 80.6% for network of FePc-CP. It is suggested that uphill process of step
Fe0.5Co0.5Pc-CP, (see Fig. S18 in ESI†), suggesting the synergetic 2 with activation barrier of 0.56 eV is the overall rate-limiting
effect between Fe and Co ions may also be responsible for the step. For the CoPc≡CoPc model, step 2 is also the rate-limiting
-
high stability of Fe0.5Co0.5Pc-CP. Moreover, Fe0.5Co0.5Pc-CP was step for the 4e route with activation barrier of 0.58 eV, similar
to the FePc≡FePc model. However, the activation barrier of
step 2 is higher than that of step 2’ in the CoPc≡CoPc model,
-
then the 2e reduction pathway is more favorable with the
generation of a significant amount of H
2
O
2
in this model. It is
worth noting that after step 2’, H
2
O
2
generated then could be
-
reduced into OH following steps 3’ and 4.
-
-
2
H O
2
+ e + *
→
OH* + OH
(3’)
However, owing to the higher activation barrier of 0.61 eV for
step 3’ in the CoPc≡CoPc model as shown in Fig. 4a, the further
reduction of H
addition, the significant amount of H
Co centers in turn limit the further adsorption and reduction of
on these Co centers, resulting in the slow ORR kinetics as a
2
O
2
could proceed only at low potential. In
2
O
2
generated around the
O
2
whole in the CoPc≡CoPc network of CoPc-CP. This corresponds
well with the experimental findings that CoPc-CP displays
similar Eonset but significantly larger Tafel slope and smaller E1/2
in comparison with FePc-CP (Fig. 2b and 2d). In the case of the
heterometallic model FePc≡CoPc, the reaction free energy for
Fig. 4 Free-energy diagram of the ORR on (a) CoPc≡CoPc,
b) FePc≡FePc, and (c) FePc≡CoPc models at 1.23 V vs. RHE
in alkaline media. d) Proposed ORR reaction mechanism at
(
the Fe0.5Co0.5Pc-CP electrode.
the reduction of O
2 2 2
to H O at the Co center via steps 1 and 2’,
0
.49 eV, is obviously smaller than that of the reactions through
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Journal of Materials Chemistry A
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ARTICLE
Journal Name
steps 1-2, 0.56 eV, at the Fe center, inducing O
be adsorbed at the Co center with H O as the resulting heterometallic Pc-based 2D conjugated polymer Fe0.5Co0.5Pc-
2 2
2
is preferred to We have successfully prepared an ethynyl-linked Fe/Co
View Article Online
DOI: 10.1039/C8TA00516H
intermediate at this stage. Nevertheless, Fe center possesses CP with alternate distribution of Fe and Co fragments by
higher peroxide reduction reaction catalytic activity through rational design and artful synthesis. The derived heterometallic
the steps 3’ and 4 than Co center as revealed by the obviously polymer exhibits superior ORR catalytic activity to the
lower activation barrier for the former process, 0.52 eV, than homometallic counterparts, which is comparable to the
for the latter, 0.61 eV. Continuous reaction then moves from commercially available Pt/C catalyst. Based on the
the Co center to the Fe to complete the whole ORR process. As experimental analysis and DFT calculations, the excellent ORR
a total result, oxygen reduction in the heterometallic catalytic activity of Fe0.5Co0.5Pc-CP is attributed to the
FePc≡CoPc model actually proceeds through steps 1, 2’ at the synergetic effect between the proximate Fe and Co ions in the
-
-
Co center by a 2e reduction pathway via peroxide generation, conjugated network, which results in a fast 2 × 2e synergetic
-
and then through steps 3’ and 4 at Fe center by another 2e catalytic reaction pathway. This work is not only helpful for
reduction pathway via peroxide reduction with the activation gaining insight into the origin of the ORR catalytic activity for
barrier of 0.52 eV, smaller than those of both homometallic FeCoN
models (Fig. 4). Such a fast 2 × 2e synergetic catalytic reaction develop novel electrocatalysts with well-defined chemical
pathway results in the superior ORR electrochemical catalytic composition and structure for various electrochemical
x
-based catalysts, but also offers a practical way to
-
activity for Fe0.5Co0.5Pc-CP to both homometallic analogues.
To support the calculation result, the electrochemical
reactions.
activity of the MPc-CPs for H
means of the chronoamperometry method. Fig. S19 in ESI
exhibits the response curve of steady-state i−t for the MPc-CP
modified electrode to the successive addition of 1 mM H
into N -saturated 0.1 M KOH solution in the potential range of
70 and 930 mV (vs. RHE). As can be seen, the sharp step of
current reduction following the addition of H could be
observed for Fe0.5Co0.5Pc-CP FePc-CP, and CoPc-CP under the
2 2
O reduction was investigated by
Experimental Section
General Remarks
†
s
O
2 2
THF and Et
3 2
N were distilled from Na and CaH , respectively. All
2
other reagents and solvents were reagent grade and used as
5
1
2
12
received. The compounds Fe[Pc(I)
4
],
4
Fe[Pc(ethynyl) ],
2 2
O
],12 and Co[Pc(ethynyl)
12
were prepared according to
Co[Pc(I)
4
4
]
,
methods in literatures. All coupling reactions were carried out
under a nitrogen atmosphere.
applied potential below 930, 930, and 870 mV, respectively.
The polymer containing Fe shows higher activity, revealing the
The 13C CP/TOSS NMR spectra were recorded with a 4-mm
MAS probe and with a sample spinning rate of 3.0 kHz. IR
spectra were recorded as KBr pellets using a Bruker Tensor 37
effect of the Fe center towards H
support the calculation data in which the polymers with Fe
centers are promoting H reduction (step 3’ and 4).
2 2
O reduction. These results
2
O
2
-1
spectrometer with 2 cm resolution. Powder X-ray diffraction
Combined with H
it is supported that Fe0.5Co0.5Pc-CP exhibits the 2 × 2e
2
O
2
regeneration promoted by the Co centers,
(PXRD) data were collected on a Shimadzu XRD-6000
-
diffractometer using Cu-Ka radiation (I = 1.54056 Å) at room
temperature. SEM images were obtained using a JEOL JEM-
synergetic catalytic reaction pathway, as indicated by the DFT
-
calculations. In addition, the 2 × 2e synergetic catalytic
6
510A scanning electron microscopy. The morphology and
reaction pathway in Fe0.5Co0.5Pc-CP involves the transfer of
peroxide from Co centers to Fe centers. As a consequence, the
Fe and Co ions synergistic effect might get weakened along
with increasing the distance between the Fe and Co ions and
therefore result in lower ORR performance. In order to verify
mapping of the catalysts were investigated by transmission
electron microscopy (TEM, JEM-2100F, JEOL, Japan) at an
operation voltage of 200 kV. The Fe and Co contents of the
MPc-CPs were determined by inductively coupled plasma
atomic emission spectroscopy (ICP-AES) analysis with an IRIS
Intrepid II XRP instrument. XPS was carried out on PHI 5300
ESCA System (PerkineElmer, USA). Solid-state UV-Vis diffuse
reflectance spectra were recorded on an SHIMADZU UV-2600
spectrophotometer. The excitation source is Al Kα radiation.
TGA was performed on a PerkinElmer TG-7 analyzer with a
this assumption, a conjugated diyne (−C
≡C−C≡C−) linked
heterometallic Pc conjugated polymer Fe0.5Co0.5Pc-CP-2 was
synthesized by the alkyne–alkyne homocoupling reaction
between Fe[Pc(ethynyl)
distance between the Fe and Co than that in the −C
Fe0.5Co0.5Pc-CP-2 exhibits an Eonset of 912 mV
and an E1/2 of 817 mV, Fig. S20 in ESI . Both are similar to
those of FePc-CP (Eonset = 910 mV and E1/2 = 800 mV) but
smaller than those of Fe0.5Co0.5Pc-CP (Eonset = 937 mV and E1/2
48 mV). These results suggest that Fe0.5Co0.5Pc-CP-2 is very
4
] and Co[Pc(ethynyl)
4
] with larger
≡
C− linked
Fe0.5Co0.5Pc-CP
.
-1
heating rate of 10 °C min in the range of 25-900 °C under N
2
†
atmosphere. CV, LSV, and RRDE measurements were
conducted on the CHI 760E workstation (CH Instruments, Inc.)
with a RRDE-3A rotator (ALS Co., Ltd). The typical three-
electrode system was employed to evaluate the
electrochemical properties of the prepared catalysts.
Specifically, glassy carbon was the working electrode, a Pt wire
was the counter electrode, and the Ag/AgCl (in saturated KCl
solution) was the reference electrode. All potentials were
referred to the reversible hydrogen electrode by adding a
value of (0.197 + 0.059×pH) V. All the electrochemical tests in
=
8
likely to be an Fe conjugated polymer itself, without the
synergistic effect of Fe and Co, revealing the effect of the
distance between the Fe and Co ions on the their synergistic
effect.
Conclusions
6
| J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
Journal Name
ARTICLE
this study were conducted at least three times to ensure the Prior to the test, the electrolyte (0.1 M KOH solution) was
View Article Online
DOI: 10.1039/C8TA00516H
accuracy of the measurement. Besides, the iR correction was bubbled with O
applied to get rid of the influence of the Ohmic resistance, and
the effect of the doublelayer capacitance on the ORR measurement. The data was recorded at the scan rate of 50
2
for at least 30 min to make it saturated with
2
O , and a constant oxygen flow was kept during the
-1
performance of the resulting catalysts was eliminated (the mV s under static conditions when the system became stable.
corresponding methods were specified below).
For the methanol tolerance test, after injecting 5 % (volume)
methanol into the cell, the electrode was rotated for 5 min to
ensure the added methanol dispersed homogeneously in the
Synthesis of Fe0.5Co0.5Pc-CP
A
mixture of Fe[Pc(I)
Co[Pc(ethynyl) ] (80 mg, 0.12 mmol) in a mixture solvent of under static conditions again. The resulting data were
Et N (20 ml) and THF (10 ml) with the presence of catalysts corrected to remove the iR drop.
Pd(PPh Cl (2.8 mg, 14.7 μmol) and CuI (0.8 mg, 4.2 μmol)
was heated to 70°C for 48 h under N atmosphere. After the Linear sweep voltammetry (LSV) measurement
mixture was cooled to room temperature, the precipitate was The rotating speed of the working electrode is increased from
4
]
(128 mg, 0.12 mmol) and electrolyte, and then the CV measurement was carried out
4
3
3
)
2
2
2
-1
filtered and washed with THF, toluene, and methanol to 400 to 2500 rpm at the scan rate of 10 mV s . The resulting
remove any phthalocyanine monomers and catalysts. The data were corrected to remove the iR drop and the double-
product was dried at 80°C under vacuum for 12 h to yield layer effect.
Fe0.5Co0.5Pc-CP as a dark blue powder (190 mg, yield 92%). The
Fe and Co content were 3.24% and 3.82% as determined by Rotating ring-disk electrode (RRDE) measurement
ICP-AES, respectively.
The rotating speed of the working electrode was fixed at 1600
rpm with the scan rate of 10 mV s for the RRDE test. The
-
1
Synthesis of CoPc-CP
electron transfer number (n) is calculated via the following
By employing the above-mentioned synthesis procedure of equation.4b
Fe0.5Co0.5Pc-CP with Co[Pc(I)
material, CoPc-CP was isolated in the yield of 93%. The Co
content was 7.14% as determined by ICP-AES.
4
] instead of Fe[Pc(I)
4
] as starting
n = 4I
d
/ (I
% HO = 200(I / N) / (I
stands for the disk current, I
d
+ I
r
/ N)
2
-
r
d
+ I
r
r
/ N)
Where I
d
represents the ring
current, and N is the current collection efficiency of the Pt ring,
-1
Synthesis of FePc-CP
3 6
which was identified to be 0.43 in 2 mmol L K Fe[CN] and 0.1
By employing the above-mentioned synthesis procedure of M KCl solution.
Fe0.5Co0.5Pc-CP with Fe[Pc(ethynyl) ] instead of Co[Pc(ethynyl)
as starting material, FePc-CP was isolated in the yield of 95%. Koutecky-Levich (K-L) plots
4
4
]
The Fe content was 6.89% as determined by ICP-AES.
The working electrode was scanned cathodically at the rate of
-1
1
0 mV s with the rotation speed from 400 to 2500 rpm.
-1
-1/2
Synthesis of Fe0.5Co0.5Pc-CP-2
Koutecky-Levich (K-L) plots (J vs ω ) were analyzed at 0.3-
] (80 mg, 0.12 mmol) and 0.6 V. Koutecky-Levich equation: 21
4
A mixture of Fe[Pc(ethynyl)
Co[Pc(ethynyl) ] (80 mg, 0.12 mmol) in a mixed solvent of Et
20 ml) and THF (10 ml) in the presence of catalysts
Pd(PPh Cl
was heated to 70°C for 48 h under N
mixture was cooled to room temperature, the precipitate was n is transferred electron number, F (96485 C mol ) is the
1/
퐽
= 1/
퐽
퐿
+1/
퐽
퐾
= 1/(퐵휔1/2) +1/
퐽
퐾
4
3
N
2
/3
퐾
푣 퐽 = 푛퐹푘퐶
−1/6;
0
(
퐵
= 0.2푛퐹퐶
0
퐷
0
3
)
2
2
(2.8 mg, 14.7 μmol) and CuI (0.8 mg, 4.2 μmol) Where J is the measured current density, J
k L
and J are the
2
atmosphere. After the kinetic and limiting current densities, ω is the angular velocity,
-1
filtered and washed with THF, toluene, and methanol to Faraday constant, D
0
is the diffusion coefficient of O
2
in 0.1 M
(1.2
-5
2
-1
remove any phthalocyanine monomers and catalysts. The KOH (1.9 × 10 cm s ), C
0
is the bulk concentration of O
2
-6
-3
product was dried at 80°C under vacuum for 12 h to yield × 10 mol cm ), v is the kinetic viscosity of the electrolyte
2
-1
Fe0.5Co0.5Pc-CP-2 as a dark blue powder (156 mg, yield 98%).
(0.01 cm s ), and k is the electron-transfer rate constant. The
constant 0.2 is adopted when the rotation speed is expressed
in rpm.
Preparation of the working electrode
The catalyst ink was prepared by dispersing 4 mg of sample
into 1.5 mL ethanol and 0.5 mL deionized water solvent iR-Correction
containing 0.5 μL 5wt% Nafion and sonicated for 60 min. Then, The iR correction has been adopted to remove the influence of
1
0 μL of the mixture was dropped onto a polished glassy Ohmic resistance on the ORR measurements. Specifically, the
carbon electrode (4 mm in diameter). The loaded electrode electrochemical alternating current impedance spectroscopy
was placed in a 60 °C oven for 10 min to dry and then was (EIS) was utilized to measure to Ohmic resistance under the
taken out to cool down before all the tests. The corresponding ORR conditions. The potentials were calculated via the
-
2
22
catalyst loading is 0.08 mg cm
following equation:
EiR-corrected = E - iR
Cyclic voltammetry (CV).
Where i is the current, R is the uncompensated ohmic
electrolyte resistance measured via high frequency A.C.
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J. Name., 2013, 00, 1-3 | 7
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Journal of Materials Chemistry A
Page 8 of 10
ARTICLE
Journal Name
impedance in O
around 50 Ω for all the tested samples.
2
-saturated 0.1 M KOH solution, which is
5
6
(a) D. Guo, T. Kondo, J. Nakamura, R. Shibuya, C. Akiba and S.
Saji, Science, 2016, 351, 361-365; (b) L. Dai, Y. Xue, L. Qu, H.-J.
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Choi and J.-B. Baek, Chem. Rev., 2015, 115, 4823-4892; (c) J.
Zhang, L. Qu, G. Shi, J. Liu, J. Chen and L. Dai, Angew. Chem.,
Int. Ed., 2016, 55, 2230-2234; (d) Y. Jia, L. Zhang, A. Du, G. Gao,
J. Chen, X. Yan, C. L. Brown and X. Yao, Adv. Mater., 2016, 28,
9532-9538.
(a) X. Yan, Y. Jia, J. Chen, Z. Zhu and X. Yao, Adv. Mater., 2016,
28, 8771-8778; (b) P. Yin, T. Yao, Y. Wu, L. Zheng, Y. Lin, W. Liu,
H. Ju, J. Zhu, X. Hong, Z. Deng, G. Zhou, S. Wei and Y. Li, Angew.
Chem., Int. Ed., 2016, 55, 10800-10805; (c) P. Chen, T. Zhou, L.
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Y. Xie, Angew. Chem., Int. Ed., 2017, 56, 610-614; (d) H. Zhang,
S. Hwang, M. Wang, Z. Feng, S. Karakalos, L. Luo, Z. Qiao, X. Xie,
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139, 14143-14149.
Double-Layer Capacitance Correction
The ORR test was conducted in ultra-high purity nitrogen
saturated and oxygen saturated 0.1 M KOH solution,
respectively, and the final LSV data was obtained by
subtracting the LSV data measured in N
solution from the LSV results measured in O
2
-saturated 0.1 M KOH
-saturated 0.1 M
2
KOH solution. All the LSV curves in this work have been
corrected by this method.
Mass Activity
The mass activity was obtained by normalizing the kinetic
current (I
k k
) to the electrode mass. I is obtained by multiplying
J
k
(derived from the Koutecky-Levich equation at 0.9 V vs RHE)
with the geometric area of the glassy carbon disk.
7
(a) A. Zitolo, N. Ranjbar-Sahraie, T. Mineva, J. Li, Q. Jia, S.
Stamatin, G. F. Harrington, S. M. Lyth, P. Krtil, S. Mukerjee, E.
Fonda and F. Jaouen, Nature Commun., 2017, 8, 957; (b) Q. Jia,
N. Ramaswamy, U. Tylus, K. Strickland, J. Li, A. Serov, K.
Artyushkova, P. Atanassov, J. Anibal, C. Gumeci, S. C. Barton,
M.-T. Sougrati, F. Jaouen, B. Halevi and S. Mukerjee, Nano
Energy, 2016, 29, 65-82; (c) J. Li, S. Ghoshal, W. Liang, M.-T.
Sougrati, F. Jaouen, B. Halevi, S. McKinney, G. McCool, C. Ma,
X. Yuan, Z.-F. Ma, S. Mukerjee and Q. Jia, Energy Environ. Sci.,
Theoretical Calculation
Density functional theory (DFT) calculations were carried out
20
23
using PBE1PBE-GD3. A mixed basis set, including SDD for
24
Fe/Co and 6-311++G(2d,p)/6-311G(d) for C\H\N\O, was
employed. All the Gibbs free energy values were calculated
2
5
using the Nørskov model.
2
016, 9, 2418-2432; (d) U. I. Kramm, I. Herrmann-Geppert, S.
A
cknowledgements
Fiechter, G. Zehl, I. Zizak, I. Dorbandt, D. Schmeißer and P.
Bogdanoff, J. Mater. Chem. A, 2014, 2, 2663-2670.
Financial support from the Natural Science Foundation of
China (Nos. 21631003, 21671017, 21290174, and 21401009),
the National Key Basic Research Program of China (Grant No.
8
9
(a) Q. Lin, X. Bu, A. Kong, C. Mao, F. Bu and P. Feng, Adv.
Mater., 2015, 27, 3431-3436; (b) G. Wu, K. L. More, C. M.
Johnston and P. Zelenay, Science, 2011, 332, 443-447; (c) J.
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T. Yao, S. Wei, Y. Wu and Y. Li, J. Am. Chem. Soc., 2017, 139,
2
013CB933402), and China Scholarship Council. University of
Science and Technology Beijing is gratefully acknowledged.
17281-17284; (d) Y. Zhao, K. Watanabe and K. Hashimoto, J.
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Journal of Materials Chemistry A
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View Article Online
Ethynyl-linked Fe/Co Heterometallic Phthalocyanine ConjuDO
g
I:
a
10
t
.1
e
039/C8TA00516H
d
Polymer for Oxygen Reduction Reaction
a
a,c
a
a
a
a,b
Wenping Liu, Yuxia Hou, Houhe Pan, Wenbo Liu, Dongdong Qi,* Kang Wang,*
Jianzhuang Jiang* and Xiangdong Yaob
a
Heterometallic phthalocyanine 2D conjugated polymer with alternate distribution of Fe and Co
fragments has been fabricated, exhibiting excellent oxygen reduction reaction catalytic activity
comparable to Pt/C.