Boosting the Capacitive Performance of Cobalt(II) Phthalocyanine by Non-peripheral Octamethyl Substitution for Supercapacitors?

Article abstract of DOI:10.1002/cjoc.202000676

In this paper, pristine cobalt(II) phthalocyanine (CoPc) and non-peripheral octamethyl substituted CoPc (N-CoMe₂Pc) are the focus of electrochemical investigation. CoPc and N-CoMe₂Pc nanorods (NR) were synthesized by a facile precipitation process from sublimated bulk phthalocyanine powders and their electrochemical properties were explored. Due to the large specific surface area, the capacitance performance of the nanorods was significantly higher than that of the sublimated powder sample. N-CoMe₂Pc powder exhibited better pseudocapacity compared with CoPc powder and CoPc NR, which is attributed to enhanced charge transfer rate and improved redox activity after the introduction of octamethyl substituents on phthalocyanine ring. The maximum specific capacitance value was achieved by N-CoMe₂Pc NR based electrode, exhibiting 210.2 F g^–1 capacitance at 5 mV s^–1 scan rate and 156.1 F g^–1 at 0.25 A g^-1 current density, and also showing high efficiency and satisfactory retention. These results indicate that according to proper molecular design, N-CoMe₂Pc NR could be applied as the potential candidate for electrode material in supercapacitors.

Full text of DOI:10.1002/cjoc.202000676

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中
国 化 学
Cite this paper: Chin. J. Chem. 2021, 39, 1265—1272. DOI: 10.1002/cjoc.202000676
Boosting the Capacitive Performance of Cobalt(II)
Phthalocyanine by Non-peripheral Octamethyl
†
Substitution for Supercapacitors
a,b
b
b
b
,b,c
Minzhang Li, Rajendran Ramachandran, Yu Wang, Qian Chen, and Zong-Xiang Xu*
a
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China
Department of Chemistry, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China
Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices,
Southern University of Science and Technology, Shenzhen, Guangdong 518055, China
b
c
Keywords
Phthalocyanines | Methylation | Nanostructures | Supercapacitors | Electrochemistry
Main observation and conclusion
2
In this paper, pristine cobalt(II) phthalocyanine (CoPc) and non-peripheral octamethyl substituted CoPc (N-CoMe Pc) are the focus of
electrochemical investigation. CoPc and N-CoMe
bulk phthalocyanine powders and their electrochemical properties were explored. Due to the large specific surface area, the capaci-
tance performance of the nanorods was significantly higher than that of the sublimated powder sample. N-CoMe Pc powder exhibit-
2
Pc nanorods (NR) were synthesized by a facile precipitation process from sublimated
2
ed better pseudocapacity compared with CoPc powder and CoPc NR, which is attributed to enhanced charge transfer rate and im-
proved redox activity after the introduction of octamethyl substituents on phthalocyanine ring. The maximum specific capacitance
–
1
–1
–1
value was achieved by N-CoMe
2
Pc NR based electrode, exhibiting 210.2ꢀF g capacitance at 5ꢀmV s scan rate and 156.1ꢀF g at 0.25
A g current density, and also showing high efficiency and satisfactory retention. These results indicate that according to proper mo-
lecular design, N-CoMe Pc NR could be applied as the potential candidate for electrode material in supercapacitors.
-
1
2
Comprehensive Graphic Content
*
E-mail: xu.zx@sustech.edu.cn
View HTML Article
Supporting Information
†
Dedicated to Department of Chemistry, SUSTech, on the Occasion of her
1
0th Anniversary.
Chin. J. Chem. 2021, 39, 1265—1272
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
1265

Concise Report
Li et al.
[
22]
[23]
[13]
cell, organic memristor as well as supercapacitor. Though
few works reported phthalocyanine based materials for superca-
pacitors, the basic substitution of phthalocyanine may vary the
electrochemical performance. The size effect and the substituents
influence of phthalocyanine for supercapacitor application are still
unclear and need to be figured out.
Background and Originality Content
In order to cope with the shortage of fossil energy, the pro-
portion of clean energy will significantly increase in the next dec-
[1]
ade. But the production processes of the new energy sources
are unstable, such as wind and solar energy, so energy storage
devices are required to store the energy first. And coupled with
the rapid development of flexible wearable electronic devices, a
high-performance and light-weight device for energy storage has
In this work, we compared the supercapacitor performance of
pristine cobalt(II) phthalocyanine (CoPc) and non-peripheral oc-
tamethyl substituted cobalt(II) phthalocyanine (N-CoMe Pc) based
2
on their sublimated powder form and nanostructure. As far as we
can acknowledge, this is the first time to investigate the influence
of methyl substituents on the pseudocapacity of phthalocyanines
[
2-3]
aroused considerable interest of researchers.
which are also called electrochemical capacitors, can stand out
among various energy storage devices due to their ideal features
Supercapacitors,
like outstanding power density, fast charging and discharging rates, from the perspective of molecular structure design. The experi-
[
4]
long cycle life, low maintenance costs, etc. Generally, superca-
pacitors are divided into two types according to their main charge
storage mechanism, namely, electric double layer capacitors
mental results show that the pseudocapacitance activity of
N-CoMe Pc molecules is enhanced due to hyper-conjugation ef-
2
fect of methyl groups, and the conductivity of the material is also
enhanced because of the increased intermolecular force. In addi-
tion, the presence of methyl groups obstructs the one-dimen-
sional aggregation of phthalocyanine molecules in the preparation
process of nanostructures, which facilitates the exposure of more
active sites in the electrode material, and will directly promote
rapid ion transport at the electrode/electrolyte interface. There-
fore, the capacitance performance of N-CoMe Pc NR is improved
2
comparing with other three materials, suggesting that it is a good
candidate for pseudo capacitor materials.
(
EDLC), which release and store energy at the interface of elec-
trode/electrolyte by electrostatic interaction, and pseudocapaci-
[
5]
tors through the faradic redox reaction of electrode materials. It
is reported that carbon-based materials have been widely used in
EDLC, which can provide better electrochemical activity but suffer
from weaknesses such as narrow operating voltage windows and
[
6]
lower capacitance value. Alternatively, pseudocapacitor materi-
als, like transition metal oxides and conductive polymers, are able
to provide higher specific capacitance than EDLC, which received
[
7-8]
more attention from researchers.
Metal phthalocyanine is a kind of macrocyclic compounds
with 18 π-conjugated electron aromatic cloud delocalized around
the macrocycle. In general, the metal phthalocyanines are found
Results and Discussion
Structural analyses
The general synthesis procedure of CoPc and N-CoMe Pc are
outlined in Supporting Information (Scheme S1), which involve a
one-step synthesis from phthalonitrile and 3,6-dimethyl phthalo-
nitrile. The detailed synthetic information of Pc samples and Pc
nanostructure was described in the experimental section. The as
fabricated CoP and N-CoMe
denoted as CoP and N-CoMe
2
–
as a Pc , which tightly adsorbs lots of ions in the oxidation state
2
+
2+
2+ [9]
2
of +2, such as Cu , Co and Fe . It is extremely hard or even
impossible to extract metal ions from the central cavity of metal
phthalocyanines, unless the entire macrocyclic structure is de-
[
10]
stroyed.
However, the central metal ion can undergo redox
reaction, and the pyrrole N in phthalocyanine ring is prone to
proton reaction, resulting in highly electrochemical reversibility
2
Pc samples after sublimation were
Pc, respectively. And the CoP and
[
11-12]
and the ultrafast redox reactions.
The phthalocyanine ring
2
N-CoMe
N-CoMe
2
2
Pc nanorod samples were denoted as CoP NR and
Pc NR, respectively.
interacting with the metallic ion can provide excellent charge
transport capacity, which can lead to excellent electrochemical
[13]
The optical properties of as-sublimated phthalocyanines and
corresponding nanostructures were investigated by UV-vis ab-
sorption spectrometer. Figure 1a presents the UV-vis spectra with
monomeric absorption of as-sublimated powders dissolved in
performance. As a consequence of their electrochemical activi-
ty, metal phthalocyanines have been used in plenty of electro-
[
14]
chemical fields such as electrocatalysis,
electrochemical sen-
However, the application of metal
phthalocyanines as supercapacitor electrodes is less explored. In
010, Ozoemena et al. have evaluated the supercapacitive be-
havior of nickel(II) octa [(3,5-biscarboxylate)-phenoxy] phthalo-
[
15]
[16]
sors,
and batteries.
1
-chloronaphthalene. Both samples of CoPc and N-CoMe
hibit two strong absorbance bands: the Q band located at
00—750 nm which is assigned to the a1u(π)-e (π*) transition
2
Pc ex-
2
6
g
–
2
from the highest occupied molecular orbital (HOMO) to the low-
est unoccupied molecular orbital (LUMO) in the phthalocyanine
ring structure, and the Soret band (B band) at 300—400 nm de-
cyanine (NiOBCPPc), exhibiting ∼54 mF cm geometrical capaci-
–
2 [17]
tance at a current density of 138 μA cm . Mu et al. have fabri-
cated a three-dimensional flower-like nanostructure of iron tetra-
nitrophthalocyanine (TNFePc) for supercapacitors and a specific
[
24]
rived from the deeper a2u(π) to LUMO transition. The Q-band
main peak of N-CoMe Pc shows 23 nm redshifts compared with
–1
–1
[18]
capacitance value of 63 F g at 0.3 A g was obtained. Koodlur
2
et al. have synthesized palladium tetra aminephthalocyanine
that of CoPc due to stronger π-conjugation interaction, which is
beneficial for the charge transport ability of the material. Figure
1b presents the UV-vis spectra of as-precipitated nanorods dis-
persed in dichloromethane (DCM). The nanorod formed by
self-assembly in solution is an aggregated form of phthalocyanine
molecules. The absorption bands of both nanorods in the visible
region have a triple-peak structure because of Davydov splitting,
which is attributed to the interaction between transition dipole
moments of the adjacent phthalocyanine molecules with different
orientations that are close enough to emerge electronic transi-
–
1
(
TNPdPc) coating on glassy carbon electrode, showing 146 F g
–1 [19]
capacitance at a scan rate of 5 mV s . NiPc nanofibers were
prepared by Madhuri and his coworkers using an ultrasound
method, the calculated capacitance value was 52.28 F g (1 A g )
–
1
–1
[
20]
in 1 M H
non-peripheral octamethyl substituted copper(II) phthalocyanine
N-CuMe Pc) nanorods via precipitation for supercapacitor elec-
trode material performing in 1 M H SO electrolyte, the obtained
capacitance value was 84.3 F g at a current density of 0.5 A g .
After that, they built a composite of N-CuMe Pc nanorods and
Mxene nanosheets, the performance of the capacitor was further
2 4
SO solution. Recently, Xu et al. have fabricated the
(
2
2
4
–1
–1
[
25]
2
tions. The broad absorption Q-band of N-CoMe Pc NR is divid-
2
ed into three peaks at 626.7, 690.0 and 761.7 nm, giving a red
shift (615.1, 662.6 and 757.8 nm, respectively) relative to that of
the CoPc NR. The intensity of the higher energy peak of CoPc NR is
weaker than the lower energy peak compared with that of
[13]
improved.
In our previous works, various peripheral and
non-peripheral methyl substituted metal phthalocyanines have
[
21]
been used in organic field-effect transistor,
perovskite solar
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Boosting the Capacitive Performance of Cobalt(II) Phthalocyanine
Chin. J. Chem.
Figure 2 SEM images of (a) as-sublimated CoPc and (b) N-CoMe
2
Pc
Pc
powders; TEM images of (c) as-precipitated CoPc NR and (d) N-CoMe
2
2
NR; HRTEM images of (e) as-precipitated CoPc NR and (f) N-CoMe Pc NR.
more reactive sites, and shorter charge transfer distance, which is
[
28]
anticipant to elevate the performance of supercapacitor. Clear
and successive lattice fringes of CoPc NR and N-CoMe Pc NR can
2
be observed in the HRTEM images (Figures 2e and 2f), indicating
the high crystallinity of phthalocyanines nanostructure fabricated
by solution precipitation process.
Figure 1 (a) UV-vis absorption spectra of CoPc, N-CoMe
naphthalene; (b) UV-vis absorption spectra of CoPc NR, N-CoMe
DCM; (c) FTIR spectra of as-sublimated CoPc, N-CoMe Pc powder; (d) BET
isotherms of phthalocyanine powders and nanostructures.
2
Pc in 1-chloro-
2
Pc NR in
The BET surface areas of the four materials were investigated
2
by N
of the isotherms present classic type IV profiles with H3 hysteresis
loops at relative pressures between 0.5 and 1.0 p/p , indicating
the presence of mesopores in materials. Various surface areas
2
adsorption-desorption at 77.1 K. As shown in Figure 1d, all
[
25,26]
0
N-CoMe
The FTIR spectra of the synthesized CoPc and N-CoMe
powders are presented in Figure 1c. The peaks of both two
2
Pc NR, which confirms the β-phase of CoPc.
[29]
–1
2
Pc
2
of 37.63, 25.45, 18.09 and 13.43 m
g
were obtained by
–
1
N-CoMe Pc NR, CoPc NR, N-CoMe Pc and CoPc, respectively. BET
2
2
phthalocyanines observed in range from 500 to 1600 cm origi-
nate from the vibrations in phthalocyanine skeleton structures. In
the FTIR spectrum of CoPc, the peaks at 724, 1288, 1336, 1428
surface areas of phthalocyanine nanostructures are higher than
those of the powders, which is attributed to the size reduction of
phthalocyanine with nanostructure. Since the real pores were not
discerned in the phthalocyanine rods, it is assumed that the mes-
–
1
and 1535 cm are coming from the phthalocyanine ring vibra-
–
1
tions. The peaks appeared at 1151 and 1612 cm are assigned to
[30]
oporous behavior may come from the voids among the rods. It
the C—H bending vibrations of the aromatic ring and C=N-C
–
1
is known that high surface area could be favorable for more active
sites exposure in electrode material and directly promote the
rapid migration of ions at the electrode/electrolyte interfaces,
which can increase the utilization of the electrode material and
leads to a larger specific capacitance value; and nanosized materi-
al can shorten the charge transfer distance, thereby reducing re-
stretching. The band at 916 cm is relevant to Co-N band, indi-
cating the coordinated interaction between the 3d unoccupied
orbital of Co and N in the phthalocyanine ring. The characteristic
–
1
weak peaks at 3050 cm are attributed to the C—H stretching in
[
27]
2
aromatic rings. In the FTIR spectrum of N-CoMe Pc, the above
peaks are all present but with some shifts caused by the introduc-
tion of octamethyl groups. Meanwhile, some characteristic peaks
of methyl groups are emerging. The peaks exhibiting between
[
31]
2
sistance. Therefore, the larger surface area of N-CoMe Pc NR is
likely to elevate its electrochemical performance for supercapaci-
tor electrode.
Figure 3 shows XRD patterns of as-sublimated powder and
nanorods samples for both phthalocyanines. The XRD patterns of
–
1
2
800 and 3000 cm are signed to C—H stretching in alkanes. The
–1
bands at 1375 and 1242 cm are rooting in C—H bending and
wagging in alkanes, respectively, which also confirms the for-
[
21]
both as-sublimated CoPc and N-CoMe
2
Pc powders demonstrate
mation of N-CoMe Pc.
2
sharp peak at 2θ value of around 7°, which is the characteristic
The morphologies of as-sublimated phthalocyanines were in-
vestigated by SEM. As shown in Figures 2a and 2b, the as-sub-
limated CoPc and N-CoMe Pc powders demonstrate big micron-
2
[
32]
peaks of a metal phthalocyanine. It is worth to note that com-
paring with CoPc, both N-CoMe Pc samples exhibit a high intensi-
ty peak at 26.30° (2θ) (inter-planar spacing: 3.387 Å) inducing
2
sized rods like structure due to very strong aggregation between
phthalocyanine molecules during the sublimation process used for
purification of the crude products. CoPc presents a needle-like
shape with a length of more than 100 microns, and the length is
significantly reduced to around 10 microns after the introduction
of methyl groups, indicating that the methyl group can inhibit the
one-dimensional aggregation of phthalocyanine molecules during
sublimation. The morphologies of as-fabricated phthalocyanine
nanostructures were investigated by TEM. The images in Figures
[22]
from face-to-face stacking of phthalocyanine rings. This result
indicates that introduction of the octamethyl groups gives rise to
stronger intermolecular π-π stacking interactions between
N-CoMe Pc molecules, which is consistent with the conclusion
2
observed through UV-vis spectra. The XRD pattern of CoPc NR
shows sharp peaks and keeps almost same with CoPc powder. The
appearance of diffraction peaks at around 2θ value of 7.12° and
2
26.30° in the XRD pattern of the N-CoMe Pc NR suggests that the
2
(
c and 2d show that CoPc nanostructure contains nanorods
length < 1 μm, width < 100 nm), and the N-CoMe Pc nanostruc-
ture is also mainly composed of nanorods (length < 300 nm, width
30 nm) which are smaller than those of the CoPc NR, further
structure of phthalocyanine has not changed. These two peaks
become broader caused by large atomic relaxation appearing on
nanoscale material surfaces. Comparison of XRD peaks of
2
<
2 2
N-CoMe Pc powder and N-CoMe Pc NR indicates an obvious in-
confirming the presence of methyl group can also prevents the
one-dimensional aggregation of phthalocyanine molecules during
the self-assembly process in DI water. For supercapacitor elec-
trode material, small size often means larger specific surface area,
creasement of molecular arrangement ordering in self-assembled
nanorods. Through a comparison of self-assembly behaviors of
CoPc NR and N-CoMe
2
Pc NR, it reveals that the strong π-π interac-
tion can be maintained after precipitation process because of the
Chin. J. Chem. 2021, 39, 1265—1272
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
www.cjc.wiley-vch.de
1267

Concise Report
Li et al.
Figure 4 XPS spectra of the (a) survey, (c) N 1s and (e) Co 2p regions for
CoPc powder; (b) survey, (d) N 1s and (f) Co 2p regions for N-CoMe
2
Pc
powder.
Figures 5a—5d exhibit the CV curves of four different cobalt(II)
–
1
phthalocyanines at scan rate 5—100 mV s . First of all, the “Ca-
pacitive Potential Range (CPR)” of electrodes is optimized in be-
tween 0 to 1 V. After increased the upper and lower potential
range, it is noticed the current level increased with a sharp hump,
resulting from hydrogen and oxygen evolution reactions. Thus, the
voltage window was selected from 0 to 1.0 V according to the CV
test. The CV curve shape of these electrodes is distinctive with
that of carbon materials, which generally give a rectangular-like
curve denoting EDLC. Meanwhile, all of these CV curves present
some strong peaks, which implies that the capacitance of these
electrodes mainly comes from pseudocapacitive capacitance
Figure 3 XRD patterns of phthalocyanine as-sublimated powders and
nanorods.
methylation, which can improve the self-assembly capability and
[
23]
is beneficial to the charge transport in subsequent application.
The elemental composition and surface electron state of CoPc
and N-CoMe Pc were studied by XPS. The survey spectra in Fig-
ures 4a and 4b show that C, N and Co are present in both CoPc
and N-CoMe Pc. The element of oxygen is most likely to come
2
2
from oxygen-containing pollutants adsorbed on the surface of the
samples. Figure 4c shows the N 1s high resolution XPS spectrum
of CoPc, which is able to deconvoluted into peaks at 399.32 and
[36]
based on a redox mechanism. The CV curves of CoPc powder
(
Figure 5a) reveals a redox couple at around 0.85 and 0.65 V cor-
responding to the faradic redox-reactions of Co(II)/Co(III), which is
consistent with other cobalt phthalocyanines that have been re-
ported.
phthalocyanine have also been reported to undergo a similar re-
4
01.30 eV that can be identified as C-N=C and N-Co bonds, re-
[
37,38]
spectively. The N 1s high resolution XPS spectrum of N-CoMe Pc
2
Additionally, nickel phthalocyanine and copper
shows two peaks at 399.12 and 401.08 eV (Figure 4d), and the
binding energy of these peaks has a negative shift after the intro-
duction of octamethyl groups, which may result from that the
hyper-conjugation effect of the methyl groups would increase the
[
12,39]
dox process of central metal in supercapacitor application.
Another redox couple peaks appear at around 0.4 and 0.55 V,
corresponding to the redox-reactions of the pyrrole like nitrogen
groups in CoPc, which is also known as the protonation and
[
33,34]
electron cloud density on the phthalocyanine ring.
The lower
[
20]
binding energy is assigned to higher electron cloud density around
the bond, which also presents enhanced electrochemical activity
and more violent protonation reaction, which is expected to in-
crease the capacitance. The Co 2p high resolution XPS spectrum
of CoPc (Figure 4e) suggests that the two peaks located at 781.40
and 796.68 eV corresponds to Co 2p3/2 and Co 2p1/2, respectively.
The splitting between these two peaks is 15.3 eV, indicating that
deprotonation of phthalocyanine molecules.
This process is
more pronounced in acidic media and can greatly improve the
charge mobility of electrode material and elevate the electron
[
40]
transfer rate during electro-catalytic reactions. Figure 5b shows
the redox peaks current of N-CoMe Pc powder are getting higher
2
compared with CoPc powder, and the peak positions are also de-
–1
viated. The specific capacitance is calculated to be 94.2 F g at a
2
+
[35]
–1
cobalt species is in the formal Co valence state. The curve of
Co 2p XPS spectrum of N-CoMe Pc (Figure 4f) can be fitted by two
scan rate of 5 mV s , which is about 4.6 times higher than that of
–
1
2
CoPc (20.4 F g ). It is probably ascribed to three reasons: (1) The
size of N-CoMe Pc powder after sublimation is smaller, which is
main peaks at 781.22 and 796.56 eV that are also showing slight
shift due to the fluctuation of electron cloud density in the
phthalocyanine ring.
2
conducive to the reaction between more active sites of the elec-
trode material and electrolyte ions; (2) The introduction of methyl
groups improves the electrochemical activity of electrode material
due to the increased electron cloud density in molecules; (3) The
introduction of methyl groups enhances the intermolecular ac-
cumulation and reduces the charge transfer resistance inside the
material.
Electrochemical characterization
The electrochemical responses of powders and nanorods of
2
CoPc and N-CoMe Pc were assessed as electroactive materials for
supercapacitor electrodes, in terms of cyclic voltammetry (CV),
galvanostatic charge-discharge (GCD), and electrochemical im-
–
1
With the scan rates increasing from 5 to 100 mV s , the redox
peak current increases correspondingly, and the potential of the
oxidation peak and the reduction peak shift in the positive and
pedance spectroscopy (EIS) in 1 M H
three-electrode configuration.
2 4
SO solution applying a
1268
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Chin. J. Chem. 2021, 39, 1265—1272

Boosting the Capacitive Performance of Cobalt(II) Phthalocyanine
Chin. J. Chem.
provement of the capacitance performance of electrode material.
As expected, it is shown from Figure 5d that the area enclosed by
the CV curve of N-CoMe
2
Pc NR is further enlarged, and the spe-
–1
cific capacitance value reaches 210.2 F g at lower scan rate (5
–
1
–1
–1
mV s ), and it remains 93.8 F g at higher scan rate (100 mV s ),
demonstrating good capacitance performance, which indicates
2
that N-CoMe Pc has great potential for supercapacitor electrode.
Figure 5e shows the comparison of the CV curves of four elec-
trode materials at same scan rate. It can be seen intuitively that
their capacitance performance is ranked as follows: N-CoMe
NR > N-CoMe Pc > CoPc NR > CoPc.
2
Pc
2
Figures 6a―6d present the galvanostatic discharging curves of
bulk and nanostructured phthalocyanines within a potential win-
dow from 0.1 to 1.0 V at various current densities. The discharge
curves of four phthalocyanine samples are non-linear and show
two plateaus originating from the pseudocapacitance behavior on
[
42]
electrodes, which keeps in line with the CV results. The specific
capacitance calculation formula given in the experimental part
demonstrates that the longer discharge time corresponds to the
better the capacitance performance of the material. It can be
seen from Figures 6a―6d that the specific capacitance is a mon-
otonically decreasing function of current density, which is similar
to the CV results. This is because only part of the charging and
discharging process can be performed on the electrode surface
under high current density, which reduces the utilization rate of
the active material, resulting in a decrease in the specific capaci-
[
43]
tance obtained.
It is clearly shown in Figure 6e that the discharge time of octa-
methyl substituted cobalt(II) phthalocyanines are significantly
longer than that of pristine cobalt(II) phthalocyanines no matter
with powder form or nanostructure at the current density of 0.5
–
1
2
A g , and the N-CoMe Pc NR exhibited a prolonged discharge
time, which is due to the introduction of methyl groups and the
Figure
N-CoMe
5
Cyclic voltammetry response of (a) CoPc powder; (b)
reduction of material size. As shown in Figure 6f, a maximum spe-
2
Pc powder; (c) CoPc NR; (d) N-CoMe Pc NR; and (e) the four
2
–1
cific capacitance of 156.1 F g is achieved by N-CoMe
2
Pc NR at a
–1
electrodes at 20 mV s scan rate (f) Specific capacitance of the four
electrodes at different scan rates.
–1
current density of 0.25 A g which is much higher than the
–1
–1
N-CoMe Pc powder (80.6 F g ), CoPc NR (29.8 F g ) and CoPc
2
–1
powder (11.1 F g ), respectively. The specific capacitance of
negative directions, respectively, indicating a rapidly reversible
faradic reaction occurs between the electroactive electrode mate-
rial and the electrolyte ions and the shift is attributed to the po-
larization of electrode material beside the rapid electronic/ionic
N-CoMe Pc NR electrode is 156.1, 136.2, 120.5, 107.3, 94.2, 90.6
F g at current densities of 0.25, 0.5, 1, 2, 4, and 5 A g , respec-
tively, showing a satisfactory rate capability.
The kinetics of the four electrodes was further investigated by
EIS analysis. As shown in Figure 7a, all the Nyquist plots demon-
strate a similar shape, which is constituted by a straight line at the
lower frequency region and a semicircle at the higher frequency
region. Figure 7b shows the enlarged spectra of Nyquist plots at
the high-frequency region, the intercept on the real axis denotes
2
–
1
–1
[7]
interactions at high scan rate. It is noted that the specific capac-
itance is inversely proportional to the scan rate. This is mainly
caused by the faster reaction of the electrode at high scan rate,
and the diffusion rate of the electrolyte from the electrode/elec-
trolyte interfaces into the electrode material is insufficient to
[
41]
meet the redox reactions.
Meanwhile, this rapid reaction in
the electrolyte resistance (R ), and the diameter of the semicircle
s
electrodes cause the weakening of the oxidation peak, which is
more obvious at lower scan rate. As the evidence of Figure 4f, the
specific capacitance values of the four electrodes are reduced by
at least half at higher scan rate. Conversely, the electrolyte ions
can efficiently disseminate into the electrode materials at lower
scan rate, which results in adequate redox reactions at the sol-
id/liquid interfaces and provides higher specific capacitance val-
refers to the charge transfer resistance of the electrode/electro-
[
44]
lyte interfaces (Rct). The electrodes of octamethyl substituted
CoPc exhibited a smaller semicircle at the high-frequency region
than pristine CoPc, suggesting the enhanced charge transfer rate
while adding methyl groups, which is consistent with the previous
conclusion. Similar result was obtained in previous research re-
port. The addition of peripheral octamethyl groups on CuPc en-
hances the strength of molecular conjugate stacking and increases
the carriers transfer rate of hole transport layer, resulting a higher
[40]
ues.
Figures 5c and 5d show the CV curves of CoPc NR and
N-CoMe Pc NR at various scan rates, respectively. It is observed
[
45]
2
photo-electric conversion efficiency of pervoskite solar cells.
that the positions of the redox peaks remain unchanged com-
pared with their powders, but the intensity of all peaks increase
significantly, which corresponds to the improvement in capaci-
tance. This is attributed to the increasement of specific surface
area of the phthalocyanine nanocrystals. It is worth noting that
although the size of CoPc nanocrystals is much smaller than that
of sublimated N-CoMe Pc powder, its capacitance performance is
2
still weaker than the latter, indicating the introduction of methyl
groups on the phthalocyanine ring is more critical to the im-
All of the Nyquist curves have been fitted by the equivalent
circuit (inset diagram of Figure 7a), and the simulated R and R
ct
s
values of CoPc, CoPc NR, N-CoMe Pc, N-CoMe Pc NR can be de-
2
2
termined as 1.249, 1.186, 1.126, 0.939 Ω and 2.836, 1.942, 1.857,
0.861 Ω, respectively. The lowest R and R values of N-CoMe Pc
[
42]
s
ct
2
NR proves the better ion diffusion pathways and charge transfer
rate between the electrode/electrolyte interfaces. In addition, the
straight line of Nyquist plot in the low-frequency region reveals
the Warburg impedance (R ), which represents ideal capacitive
w
Chin. J. Chem. 2021, 39, 1265—1272
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
www.cjc.wiley-vch.de
1269

Concise Report
Li et al.
Figure 6 Galvanostatic discharging curves at different current densities
(
a) CoPc powder; (b) N-CoMe Pc powder; (c) CoPc NR; (d) N-CoMe Pc NR;
2
2
–1
Figure 7 (a) Nyquist plots, inset: equivalent circuit diagram; (b) partial
enlarged Nyquist plots and (c) Bode plots of phthalocyanine as-sublimated
powders and nanorods.
and (e) the four electrodes at 0.5 A g current density (f) Specific
capacitance of the four electrodes at various current densities.
[
46]
characteristics. The straight line of the N-CoMe
closer to the imaginary axis, which demonstrated the better
charge storage performance. The electrode of N-CoMe Pc NR with
2
Pc NR is much
–1
–1
–1
at 5ꢀmV s scan rate and 156.1ꢀF g at 0.25 A g current density,
and also showed high efficiency and satisfactory retention. So it
can be used as a promising material for application in future su-
percapacitors. Furthermore, the specific capacitance will be im-
proved by incorporating some EDLC materials, which helps to
enhance the specific surface area and leads to synergistic effect.
2
low contact resistance and charge transfer resistance is favorable
for the rapid formation of reversible faraday reaction, thus im-
proving the electrochemical performance. Furthermore, the
phase angles change with the applied frequencies as shown in the
bode plots (Figure 7c). It can be observed that the phase angles of
the four prepared electrodes are close to 45°—65° when the fre- Experimental
quency is less than 100 Hz, which involves the good pseudocapac-
[
47]
itance behavior.
The N-CoMe
2
Pc NR electrode shows higher
2
Synthesis of CoPc and N-CoMe Pc
phase angles at the lower frequency, manifesting the faster
charge transfer rate, which keeps in line with cyclic-voltammetry
and discharging profiles.
For the synthesis of CoPc powder, phthalonitrile (2.50 g, 19.5
mmol), cobalt(II) chloride (0.633 g, 4.88 mmol), and quinoline (5
mL) was mixed in a 15 mL flask. It was stirred and heated at 170 °C
surrounding by argon for 60 min, and then it was put in the air to
cool down. Then the obtained mixture was washed by deionized
water (DI water), ethanol, DCM and tetrahydrofuran (THF) suc-
cessively in the filter paper to collect solid powder and dried at
80 °C. After that, the purity of phthalocyanine was further im-
Conclusions
In summary, we have investigated the pseudo capacitive be-
havior of CoPc NR, N-CoMe
2
Pc NR, as-sublimated CoPc and
−
4
N-CoMe Pc powder in acid medium. The capacitance perfor-
2
proved via vacuum sublimation (475 °C, ∼5 × 10 Pa).
mance of the nanorods were significantly higher than that of the
as-sublimated powder sample due to the larger specific surface
N-CoMe Pc powder was fabricated referring to previous liter-
2
[
48]
ature.
A mixture of 3,6-dimethylphthalonitrile (1.0 g, 5.12
area after precipitation process. The as-sublimated N-CoMe
2
Pc
mmol), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 0.2 mL), CoCl2
(0.277g, 1.71 mmol) and 1-pentanol (5 mL) was put in a 15 mL
flask. A dark green mixture was obtained by stirring at 140 °C un-
der argon. The subsequent purification steps are consistent with
the aforementioned.
powder showed higher pseudocapacity compared with CoPc
powder and nanorods, which is attributed to enhanced charge
transfer rate and improved redox activity after methylation.
Meanwhile, it reveals that the introduction of octamethyl sub-
stituents on the phthalocyanine ring is more critical to the im-
provement of the capacitance performance of phthalocyanines.
Synthesis of N-CoMe
2
Pc NR and CoPc NR
–1
2
The N-CoMe Pc NR showed maximum capacitance of 210.2ꢀF g
N-CoMe Pc NR and CoPc NR were prepared according to a
2
1270
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© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
Chin. J. Chem. 2021, 39, 1265—1272

Boosting the Capacitive Performance of Cobalt(II) Phthalocyanine
Chin. J. Chem.
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(
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area of the active substance on the electrode was 1 × 1 cm , and
–
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the total mass of it was around 1 mg cm . All electrochemical
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(1),
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C
s
= (ʃIdV)/(vmV)
(1)
–
1
where C is the specific capacitance (F g ), I denotes the current
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–
1
–1
(
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[
C
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[
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Acknowledgement
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Manuscript received: December 13, 2020
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