Abstract
Cobalt-phthalocyanine (CoPc) catalysts have shown great promise for
enhancing the performance of lithium-oxygen (Li-O2)
batteries, offering benefits such as efficient lithium storage, high
reversible capacity, improved cycle performance, and enhanced charge and
discharge capacity. In this study, we focused on the synthesis of
CoPc-carbon nanotube (CNT) composites with porous structure. The
composites were prepared by synthesizing CoPc compounds and interacting
them with CNTs using a 3D ball mill shaker. Comprehensive spectroscopic
techniques including NMR, FTIR, and UV were employed to characterize the
newly synthesized phthalonitrile and phthalocyanine compounds. The
surface morphologies of the composite materials were investigated using
SEM, EDX, mapping and TEM analysis, enabling the determination of
particle sizes and chemical compositions. XRD and XPS analyses confirmed
that the composite structures were consistent with the existing
literature. BET analysis revealed multilayer isotherm for the
composites, indicating their favorable properties. The composite
catalysts were incorporated into batteries and their performance was
evaluated through various electrochemical tests. Notably, the CoPc1-CNT
composite exhibited a remarkable discharge capacity of 3400 mAh
g-1catalyst in the prepared battery.
Keywords: Li-O2 batteries, Porous Structure,
Composite catalysts, Cobalt Phthalocyanine, Carbon nanotube
Introduction
Intermittent renewable energy generation poses significant challenges to
our energy systems[1]. Unlike traditional fossil
fuel-based power generation, renewable energy sources such as solar and
wind are subject to natural variations and are not constantly
available[2]. This intermittency creates a
mismatch between energy supply and demand, as renewable energy
production does not always align with peak energy consumption
periods[3]. As a result, excess renewable energy
may be generated during low-demand periods, while insufficient energy is
available during high-demand periods[4]. This
situation leads to inefficient energy utilization and potential grid
instabilities[5]. To address these challenges,
efficient energy storage solutions are needed[6].
Energy storage systems can capture surplus energy during times of high
production and release it during periods of high demand, ensuring a more
balanced and reliable energy supply[7]. By
effectively storing renewable energy, we can overcome the limitations of
intermittency, enhance grid stability, and promote the wider integration
of renewable energy sources into our energy systems. The storage of
energy obtained from renewable sources is a matter of great significance
in our quest for sustainable power solutions. Fuel cells and batteries,
as the forefront of energy storage systems, rely on fundamental
components such as electrodes, membranes, and electrolytes or fuels[8-19]. Among these technologies,
Li-O2 batteries have garnered immense attention due to
their high specific energy reaching approximately 11,425 Wh
kg-1, making them highly appealing for next-generation
energy storage applications [12-18, 20]. However,
despite their potential, several challenges need to be addressed.
Currently, electric vehicles powered by state-of-the-art Li-ion
batteries struggle to meet the demand, with limited ranges of less than
100 miles per charge. In contrast, Li-O2 batteries
possess the potential to achieve capacities that can rival those of
gasoline vehicles. To fully harness this potential, it is crucial to
develop cost-effective and advanced bifunctional electrocatalysts for
oxygen reduction reaction (ORR) and oxygen oxidation reaction (OER) in
Li-O2 batteries [12-18, 20].
Nonetheless, there are obstacles to overcome.
Aprotic Li-O2 batteries typically consist of a Li metal
anode, a porous air cathode, a separator, and an electrolyte that
facilitates the migration of Li+ ions between the
electrodes [21]. The aprotic electrolytes are
liquid solutions of lithium salts dissolved in organic solvents. Within
a lithium-air battery cell, a lithium-ion conductive membrane is
sandwiched between a lithium metal anode and a porous air cathode.
However, Li-O2 batteries face several technical
limitations that hinder their practical implementation. One of the
primary challenges is the stagnation of ORR and OER in the aprotic
solution. This leads to large discharge overpotentials and overcharge
potentials, which reduce the overall energy efficiency of the battery.
These overpotentials result from sluggish kinetics and limited catalytic
activity, making it difficult to achieve efficient oxygen
electrochemistry.
Furthermore, electrode and electrolyte instability under high charge
potentials poses a significant challenge [22]. The
high reactivity of lithium metal anodes with the electrolyte can lead to
the formation of unstable solid-electrolyte interphase (SEI) layers,
hindering ion transport and causing capacity fading over repeated
charge–discharge cycles [22, 23]. Electrode
degradation and dissolution of active materials can also occur, further
limiting the battery cycle life and overall performance[24, 25]. However, recent studies have shown that
the pulsed current can boost the stability of lithium metal anode and
improve the performance of lithium-oxygen batteries[26]. The choice of suitable electrolytes for
Li-O2 batteries is also crucial. The compatibility between electrolyte
and electrode materials as well as the stability of electrolyte at high
potentials is vital to prevent side reactions and maintain the battery
performance. However, finding electrolytes that exhibit high ionic
conductivity, good stability, and low reactivity with electrode
materials remains a challenge.
Another limitation is the complexity of the oxygen crossover issue[27]. Oxygen crossover from cathode to anode can
cause parasitic reactions, leading to side reactions and reduced
efficiency [28]. It can also contribute to the
formation of lithium peroxide (Li2O2)
and other unwanted byproducts, which can accumulate and affect the
battery performance and capacity [29, 30].
In addition to these challenges, the limited understanding of the
underlying electrochemical mechanisms and reaction intermediates in
Li-O2 batteries hampers the development of effective
strategies for improving their performance and stability. The complex
interplay between electrode materials, electrolyte, and catalytic
processes requires further investigation and optimization to overcome
the limitations of Li-O2 batteries and enable their
widespread practical application. [11, 31-48].
Additionally, the catalyst support material also holds significance
since ORR and OER take place on the catalyst surface. By dispersing the
catalyst particles on a support, the surface area is maximized, enabling
a more efficient utilization of the catalysts [8, 18,
45].
Transition metals, owing to their activity and good conductivity, are
widely employed as catalysts, particularly in the form of nanomaterials
for rechargeable Li-O2 batteries. Carbon, with its high
degree of dispersion, good conductivity, and affordability, has emerged
as a promising catalyst support material [49-51].
Carbon nanomaterials including nanotubes have garnered considerable
attention due to their enhanced capacity and stability, offering
distinct advantages over conventional carbon black, such as improved
stability and higher conductivity [52]. Moreover,
Co-phthalocyanines have shown promising attributes for
Li-O2 batteries, including good coulombic activity[20, 53], favorable lithium storage, high
reversible capacity, improved cycling performance[54], increased discharge energy, elevated open
circuit potential [55], slow increase in internal
pressure, and enhanced charge and discharge capacity[56]. Significant advancements have been made in
addressing the limitations of Li-O2 batteries and
improving their performance. Previous studies have focused on developing
efficient catalysts for oxygen reduction and oxidation reactions,
exploring new electrode materials, and optimizing the electrolyte
composition. These efforts have led to the discovery of promising
catalyst materials such as transition metal compounds and carbon
nanomaterials that exhibit enhanced catalytic activity and stability.
Moreover, researchers have investigated the use of new electrolytes with
improved compatibility and stability. However, despite these
advancements, several limitations persist. Challenges such as
electrode/electrolyte instability, limited cycle life, and low energy
efficiency still need to be overcome. The complex interplay between
different components of Li-O2 batteries necessitates
further research to gain comprehensive understanding of the underlying
mechanisms and to develop novel strategies for achieving higher
performance and reliability.
This study aims to investigate the potential of cobalt-phthalocyanine
(CoPc) catalysts for Li-O2 batteries and enhance their performance. The
study focuses on using CoPc-CNT (carbon nanotubes) composites with
porous structure as catalyst materials. The compounds were synthesized
and interacted with CNT in 3D ball mill shaker to form the composites.
The synthesized compounds were characterized using spectroscopic methods
and the surface morphologies of the composited compounds were examined
through SEM, EDX, Mapping and TEM analyses. XRD, BET and XPS analyses
were conducted to assess the structures of the composites and verify
their compatibility with previous researches. Batteries were prepared
using the composite catalysts and their efficiency was evaluated through
various electrochemical tests.
Parts of Experimental
Preferred Materials and methods
All solvents and reagents which are hexanol (anhydrous, ≥99%),
chloroform (CHCl3), DMF (N,N-Dimethylformamide,
anhydrous, 99.8%), isopropyl alcohol (HPLC grade, 99.9%), acetonitrile
(HPLC grade, ≥99.9%), tetrahydrofuran (THF), 4-((trifluoromethyl)thio)
phenol, 4-(Imidazol-1-yl)phenol, 4-nitrophthalonitrile, anhydrous
K2CO3 (≥99.0 purity), % 5
NaHCO3 (Reagent Plus®, ≥99.5%, powder),
1,8-diazabicyclo [5.4.0 ] undec-7-ene (DBU), CoCl2,NiCl2 were purchased from commercial suppliers and used
without. FT-IR spectra were recorded using the Perkin Elmer Spectrum 65
model FT-IR Spectrometer. Chromatography was performed using Merck grade
60 silica gel. The purity of the products was assessed at each step by
TLC (MeOH/H2O (1:1), SiO2) and ethyl
acetate/hexane (17:3) systems. TLC spots were visualized under a CAMAG
model UV lamp. Melting point determinations were conducted using a
GallenKamp model device. UV-vis absorption spectra were obtained using
Evolution 220 UV-visible Spectrophotometer with solutions of
1x10-5M concentration. Mass spectra were acquired
using Agilent 6545 QTOF MS model instrument. NMR spectra were recorded
using AVANCE III 400 MHz NaNoBay FT-NMR model device. X-ray measurements
were carried out using D8-QUEST diffractometer. SEM, EDX and mapping
spectra were acquired using FEI Quanta 650 Field Emission SEM model
device. XRD spectra were obtained using Bruker D8 DISCOVER. TEM analyses
were conducted using FEI TALOS F200S TEM 200 kV model device. All
experiments were performed under a dry N2 atmosphere.
Long-term charge–discharge cycles of the batteries were performed using
an 8-channel battery analyzer. The Li-O2 cell battery
tests were conducted using a Gamry interface 1000
potentiostat/galvanostat device within a potential range of 2.5 – 4.5 V
under O2 flow at 1 atm. The interface impedances of the
air cathodes were measured before and after the charge and discharge
cycles by applying an amplitude of 10 mV in the frequency range of
105 – 10-1 Hz.
The procedure for synthesis
4-(4-((trifluoromethyl)thio)phenoxy)phthalonitrile(P1) and
4-(4-(1H-imidazol-1-yl)phenoxy)phthalonitrile (P2)
4-(thifluoromethyl)thio)phenol( 5.5 mmol, 1.068g) or
4-(Imidazol-1-yl)phenol (5.5 mmol, 0.881g) and excess of
K2CO3 (5.78 mmol, 1.903g) were added to
dry DMF (11mL) under nitrogen atmosphere and dissolved by mixing. Then,
4-nitrothalonitrile (5.59 mmol, 0.952 g) was added dropwise to the
rapidly stirring mixture at 50-60°C (Scheme 1). The reaction mixture was
allowed to react for 3 days under nitrogen atmosphere. Afterwards, the
reaction mixture was cooled in ice-water mixture (200
cm3). The resulting creamy product was filtered and
washed with distilled water until neutralization To remove any unreacted
products, the solid was dissolved in chloroform and washed with 5%
NaHCO3 solution. The resulting creamy solutions were
then dried using anhydrous Na2SO4 and
the solvents were subsequently removed under vacuum[57]. The purity of the reactions was assessed
using TLC in a mixture of CHCl3:MeOH (100:3). Following
purification, the compounds were crystallized in chloroform.
Subsequently, the compounds were stored in a vacuum oven at 30°C for one
week.SCHEME 1Yield (P1): 59.06%. Ma (P1): 320.29 g/mol. Melting Point (P1):
81.2-82.9°C. FT-IR (cm-1); 3066 (w, Ar-CH), 2926 (w,
Alip-CH), 2229 (C≡N, st), 1675(C=C), 1578 (C=N), 1487(st), 1253
(Ar-S-Alip CH), 1078, 1013, 950, 850, 824. 1H
NMR(CDCI3, ppm): 7.79-7.76 (d, 2H, phenyl) 7.74(s, 1H,
phenyl), 7.36 (d, 2H, phenyl), 7.32 (d, 2H, phenyl) 7.14 (d, 2H,
phenyl). EI/MS m/z: 321.026 [M]+.
Yield (P2): %70.47. Ma(P2): 286.29g/mol. Melting Point (P2):
130.2-133.1°C. FT-IR (cm-1); 3026 (w, Ar-CH), 2920 (w,
Alip-CH), 2259 (C≡N, st), 1675(C=C), 1590 (C=N), 1481(st), 1081, 1013,
947, 841, 810. 1H NMR(CDCI3, ppm):
7.87(s, CH, phenyl), 7.78(d, CH, phenyl), 7.52(d,2CH, phenyl), 7.34(d,
CH, phenyl), 7.32(d, CH, phenyl), 7.27(d, CH, phenyl), 7.24(s, CH,
phenyl), 7.22(d, 2CH, d). EI/MS m/z: 287.099 [M]+.
Synthesis of (2(3), 9(10), 16(17), 23(24) –
tetrakis(4-(4-(trifluoromethyl)thio)phenoxy) phthalocyaninato- metal
free) (Pc1) and its Co Compound(CoPc1)
4-(4-((trifluoromethyl)thio)phenoxy)phthalonitrile (1.128 mmol, 0.361g)
without salt or anhydrous CoCl2 (0.282 mmol, 0.0366g)
was added to dry hexanol (4 mL) and dissolved by stirring under nitrogen
at 160°C, then degassed DBU (1.128 mmol, 0.168 mL) was added dropwise
(Scheme 2). The reactions continued at 160 °C for 5 days under nitrogen.
The greenish dark blue products were cooled to room temperature after
diluting with hexane. The products were repeatedly washed with MeOH and
i-PrOH followed by CH3CN [57]. The
purity of Pc1 and CoPc1 was checked by TLC (Hexane: Ethylacetate)
(17/3). The presence of double spots indicated that P1 remained
partially unreacted in the medium. After Pc1 and CoPc1 were dissolved in
approximately 1 mL of chloroform, they were loaded onto the
chromatograph and separated by a fractionated column. Firstly, they were
run with hexane/ethyl acetate (10:1) and the leading impurity phases
were removed, and the remaining parts were taken from the chromatotrome
with 5:1 hexane/ethyl acetate. To avoid the loss of yields, the
remaining parts of Pc1 and CoPc1 were removed with chloroform in the
chromatotrome, which was cleaned with methanol.SCHEME 2Yield(Pc1): 9.3%. Ma(Pc1): 1347.34 g/mol, Melting point (Pc1):
205.1-207.8°C. FT-IR (cm-1); 3059 (w,Ar-CH), 2925
(Alip-CH), 1580 (C=N), 1568 and 1213 (Ar-S-Alip-CH), 1066, 993, 822,
724. EI/MS m/z: 1348 [M]+.
Yield(CoPc1): %7.2. Ma(CoPc1):1404.1581 g/mol, Melting point (CoPc1):
265.1-267.8°C. FT-IR (cm-1); 3010 (w,Ar-CH), 2905
(Alip-CH), 1620 (C=N), 1500 and 1120 (Ar-S-Alip-CH), 966, 943, 822, 724.
EI/MS m/z: 1404 [M]+.
Synthesis of (2(3), 9(10), 16(17), 23(24) – tetrakis
4-(4-(1H-imidazol-1-yl)phenoxy) phthalocyaninato- metal free) (Pc2)
and its Co Compound(CoPc2)
4-(4-(1H-imidazol-1-yl)fenoksi)ftalonitril (1.128 mmol, 0.322 g) without
salt or anhydrous CoCl2 (0.282 mmol, 0.0366g) was added
to dry hexanol (4 mL) and dissolved by stirring under nitrogen at 160°C,
then degassed DBU (1.128 mmol, 0.168 mL) was added dropwise (Scheme 3).
The reactions continued at 160 °C for 5 days under nitrogen. The
greenish dark blue products were cooled to room temperature after
diluting with hexane. The products were repeatedly washed with MeOH and
i-PrOH followed by CH3CN[57]. With
the mixture of (MeOH:H2O)(1:1), Pc2 and CoPc2 were
purified from impurities and single spots were obtained.SCHEME 3Yield(Pc2):12.6%. Ma(Pc2): 1211.34g/mol. Melting point
(Pc2):>280°C. FT-IR (cm-1); 3059
(w,Ar-CH), 2925 (Alifatik-CH), 1580 (C=N), 1568, 1066, 993, 822 and 724.
EI/MS m/z: 1211[M]+.
Yield(CoPc2): 7.8%. Ma(CoPc2): 1268.4075 g/mol. Melting point(CoPc2):
>280°C FT-IR (cm-1); 2950 (w,Ar-CH), 2870
(Alip-CH), 1680 (C=N), 1510, 1120, 963, 850 and 724. EI/MS m/z:1269
[M]+.
Synthesis of CNT-MPc (Carbon Nanotube supported
Metalophthalocyanine) Composites
A homogeneous and nanosized composite was obtained by mixing
metalophthalocyanine (CoPc1 or CoPc2) and CNT in equal weight[58] for 2 h with 3D mechanical mixer at 1200 rpm.
Preparation of cathode material for Battery Tests
The MPc-CNT composite and PVDF were mixed in the ratio of 95:5,
respectively using 3D ball milling for 6 hours. Subsequently, a slurry
was prepared by adding NMP (N-Methyl-2-Pyrrolidinone). The prepared
slurry was then subjected to ultrasonic treatment in an ultrasonic bath
at 55 °C for 30 min. The cathode material was obtained by applying the
slurry onto a nickel foam (radius 16 mm) using the roll coating
technique. Before coating, the Ni foams underwent preprocessing step by
immersing them in 3.0 M HCl in an ultrasonic bath for 30 min. The Ni
foams were then washed with distilled water and further treated with
acetone in an ultrasonic bath for 30 min. After drying at 110°C and
storing in a desiccator, the Ni foams served as the current collector.
The dried Ni foams were loaded with 31.57 mg slurry for CoPc1-CNT and
18.91 mg for CoPc2-CNT, respectively. Following overnight drying at
70°C, the Ni foams were left in the desiccator for 5 min before being
transferred to a glove box for battery preparation. Batteries were
assembled using 20 mm diameter separator, 16 mm diameter Li anodes, 19
mm diameter cathode and 1.0 M LiPF6 in DMSO electroyte.
Results and discussions
Evaluation of 1H-NMR Spectra
The NMR spectra of aryl-substituted phthalonitrile compounds typically
exhibit polysubstituted peaks resembling those of monosubstituted
benzene, typically appearing at approximately δ=7.90-7.00 ppm.
The NMR spectrum of the P1 molecule, as shown in Figure 1a, reveals
peaks corresponding to the protons labeled as 1 (CH, d) and 2 (-CH, s)
in the range of 7.79–7.75 ppm. Additionally, in the provided spectrum,
the peaks for the protons numbered 3 (CH, d) and 4 (2-CH, d) were
observed in the range of 7.36-7.26 ppm, while the peak for the 5 (2-CH,
d) proton appeared at 7.14 ppm. The integral areas and observed peaks in
the spectrum indicate the successful synthesis of
P1.[57].
Upon analyzing the NMR spectrum of the P2 compound, as depicted in
Figure 1b, peaks corresponding to the protons labeled as 1-8 (from 1 to
8) were observed at chemical shifts of 7.8, 7.79, 7.52, 7.34, 7.32,
7.27, 7.244, and 7.21 ppm, respectively [57]. The
presence of these peaks provides confirmation of successful synthesis of
the product. The agreement between observed peaks and integral areas
further supports the synthesis of the product.FIGURE 1.
Evaluation of FT-IR Spectra
Phthalonitrile and Phthalocyanine compounds were also characterized by
FT-IR spectroscopy and the spectra are given in Fig. 2-3. When the
literature is reviewed, it is seen that the vibration of C≡N is
approximately 2200 cm-1, Ar-C is between 3200 and 2950
cm-1, and Alip-CH is between 2950 and 2800
cm-1[57]. In this study, the characteristic C≡N
peak for phthalonitrile compounds coded as P1 and P2 was observed at
2229 and 2259 cm-1, respectively. During the
cyclotetramerization of P1-P2 to Pc1-Pc2, the sharp CN peak at 2229 and
2259 cm-1 disappeared, respectively. When the IR
spectra of Pc1, Pc2, CoPc and CoPc2 were examined, they were obtained
very similar to the spectra of the starting materials (P1 and P2) except
for minor vibrational shifts. It was observed that the C= N peaks of Pc1
and Pc2, which were observed around 1580 in the FT-IR spectrum, shifted
to 1620 cm-1 in metalophthalocyanines called CoPc1 and
CoPc2. The weak N-H peaks observed in phthalonitriles named P1 and P2
were not observed in the phthalocyanine and metalophthalocyanine yields.
The flat peak observed around 3500 in Pc2 compound was obtained due to
humidity. The fingerprint region in metalophthalocyanine compounds
(CoPc1 and CoPc2) was shifted to the right when compared to free
phthalocyanines (Pc1 and Pc2). The observed peaks were consistent with
the literature, indicating that the compounds were synthesized[57].FIGURE 2 AND 3
UV-measurement
It is known that there are two distinct regions in the UV-vis spectra of
phthalocyanine complexes, attributed to π-π* transition from the HOMO
(highest occupied molecular orbital) to the LUMO (lowest unoccupied
molecular orbital) of the Pc ring (Q-band) and deeper π-π* transitions
(B-Band; UV region). The Q Band is observed around 650-700 nm while the
B Band is observed around 300 nm. Based on this information, only B
bands were observed around 300 and 290 nm respectively in the UV
spectrum of P1 and P2 compounds (Fig. 4-5). When the UV spectrum of P1
and Pc2 compounds is compared with that of P1 and P2 compounds, it is
seen that there are also Q bands in the spectrum at 670 and 680 nm,
respectively. When the UV spectra were examined, the Q band for CoPc1
was observed at 640(Qx) and 680(Qy) nm, while the Q band for CoPc2 was
observed at 650(Qx) and 690(Qy) nm. In the spectrum, the Q bands of
CoPc1 and CoPc2 were observed more intensely than the Q bands of Pc1 and
Pc2. These results were found to be in agreement with the literature[59, 60].FIGURE 4 AND 5
Surface Morphology
The successful binding of CoPc1 and CoPc2 on the carbon nanotube was
confirmed by scanning electron microscopy (SEM). The SEM micrographs
seen in Fig. 6(a-b) show the morphologies of CoPc1 and CoPc2 loaded onto
CNT surfaces. The bonding of MPc (CoPc1 and CoPc2) on carbon surfaces
via intermolecular interactions can be explained as a possible cause of
diverse loading of MPc [61]. The bonding between
the aromatic structure of the carbon surface and the macrocyclic ligand
of MPc is a non-covalent π−π interaction [58].
Energy dispersive X-ray (EDX) analysis (Fig. 6c-d) was used to determine
the chemical composition of the prepared CNT_MPc composites. Fig. 6(c)
EDX spectrum showed successful fixation of CoPc1 on the carbon composite
and the presence of C, N, O, F, S and Co elements. The elemental
analysis of the CoPc1-CNT and CoPc2-CNT compounds and the corresponding
weight percentages are shown in the table overlaid on Fig. 6c and 8d.
Fig. 6(d) EDX spectrum shows the successful fixation of CoPc2 on the
carbon composite similar to that of CoPc1 and the presence of the
elements C, N, O and Co. The presence of desired elements in accordance
with EDX and the homogeneous distribution in the mappings of CoPc1 and
CoPc2 seen in Fig. 6(e-f) show that the composite has been synthesized.FIGURE 6TEM micrographs of CoPc1-CNT (a) and CoPc2-CNT (b) composites are given
in Fig. 7. It is clear from the TEM micrographs that the
metalophthalocyanines are loaded onto the CNT. In general,
metalophthalocyanines interact strongly with oxygen functional groups
and oxygen to create complexes [58]. The loading
of MPc on the carbon surface also depends on the entity of functional
oxygen groups in the CNT. CoPc1-CNT and CoPc2-CNT composites exhibit
similar property with relatively smaller metalophthalocyanine particles
appearing mostly inside the nanotubes. CoPc1-CNT and CoPc2-CNT
composites have relatively smaller metalophthalocyanine particles mostly
appearing inside the nanotubes. The TEM micrograph shows the
interplanetary spacing and particle size of MPc-CNT molecules for CoPc1
and CoPc2, respectively, in the range of about 26.7–66.9 nm and
17.0–37.5 nm.FIGURE 7
XRD
Fig. 8 shows XRD plots of multiwalled CNT composites of CoPc1 and CoPc2
taken between 10 and 90°. Due to severe plastic deformation caused by
high-energy ball impacts, the compounds lost their crystallinity and
wider XRD peaks were observed [62]. In addition,
the semi-amorphous structure of the composites causes the peaks to be
flat. According to the information obtained from the literature, CNT
exhibits diffraction peaks at 2θ = 26° and 43° corresponding to the
graphite planes of (002) and (100) [63]. However,
the definitions of the peaks seen are given below with reference card
numbers. Looking at the reference PDF cards, it is understood that the
structure contains Metal-phthalocyanine and CNT composite. 13,163°,
21.622°, 25.397°, 26.505°, 34.459°, 37.917°, 40.304° and 44.066° (Carbon
Nitride) - Reference No: PDF 01-087-1526. 19.075°, 25.230°, 29.317°,
34.102°, 36.313°, 37.871°, 38.638°, 38.707°, 39.702°, 40.1711°, 40.617°,
40.740° and 42.822° ( Cobalt Oxalate,
CoC2O4)- Reference No: PDF 00-037-0719.
21.984° and 31.704° (Tetracyanoethylene, C6N4)- Reference No: PDF
00-037-1895. 33,794° (Nitrogen, N) - Reference No: PDF 01-082-2781.
25.919° and 42.959° (Carbon Nanotube)- Reference No: PDF 00-058-1638.FIGURE 8
XPS analysis
XPS provided an opportunity to further investigate the elements and
chemical states on the surface of CoPc1-CNT and CoPc2-CNT composites.
The XPS spectrum (Fig. 9) showed C, O and N peaks on the surface of
CoPc1-CNT and CoPc2-CNT composites. According to the XPS data in Table 1
for CoPc1-CNT composite, the percentages of C, O, N, F, S2p and Co2p
atoms on the surface for CoPc1-CNT composite reflect 80.4%, 6.8%,
4.4%, 5.4%, 1.8% and 0.6%, respectively. According to the XPS data
in Table 1 for CoPc2-CNT composite, the percentages of C, O, N and Co2p
atoms on the surface for CoPc1-CNT composite reflect 83.3%, 8.2%,
6.6%, and 0.3%, respectively. When the XPS spectrum (Fig.9) obtained
for the two composites is examined, the peaks for C1s, O1s and N1s were
obtained at approximately 285 eV, 395 eV and 530 eV, respectively, and
this agrees with the literature[64, 65].FIGURE 9
Electrochemical Tests
The electrochemical performances of the constructed CoPc1-CNT and
CoPc2-CNT composite were first investigated in a three-electrode system
in 0.5 M LiCIO4 (15g ethylene carbonate + 15g diethylene
carbonate) electrolyte. Fig. 10 displays the CV curves at a scan rate of
0.10 V s−1. The Ni Mesh graph shows the conversions of
1, 2 and 3 numbers. 1 is Ni/Ni2+, 2 is
Ni2+/Ni8/3+, and 3 is
Ni8/3+/Ni3+. When the CV curves
shown in fig. was examined, it was observed that different peaks were
observed in CoPc1-CNT and CoPc2-CNT composites because of the closed
surface with composite materials unlike the voltammogram of Ni Mesh, and
the current increased compared to the bare Ni Mesh electrode in the same
potential range. The peaks of the CoPc composites are shown as 1 and 2
for oxidation and 1’ and 2’ for reduction conversions. The 1, 1’ are the
Co/Co2+ conversion and 2, 2‘ are the
Co2+/Co3+ conversions. The increase
in the current and the observation of the oxidation peaks proved the
accuracy of the work done.FIGURE 10According to the EIS graph given in Fig. 11, the Ni mesh base material
was given the highest resistance. After coating the Ni mesh with
CoPc1-CNT and CoPc2-CNT composite materials, the resistance ratiois
given as the inset figures. As can be seen from the impedance
measurements, the charge transfer resistance is smaller for the
CoPc1-CNT composite than CoPc2-CNT.FIGURE 11Adsorption isotherms have specific isotherms that can be classified
between type I and type VI according to IUPAC. The distinctive
tendencies of the adsorption isotherms depend on the nature of the pore
structure, the adsorption mechanism and the adsorbent/adsorbate
interactions. Each type of adsorption isotherm appears to have unique
ability to adsorb water vapors within a certain range of relative
humidity (RH) or relative pressure (P/Po). Since the adsorption
isotherms of CoPc1-CNT and CoPc2-CNT the composites seen in Fig. 12
illustrate the Type-VI, which sign multilayer isotherm absorption[66].FIGURE 12The charge/discharge curves of CoPc1-CNT and CoPc2-CNT the composites
shown in Fig. 13, provide valuable insights into their electrochemical
performance. For the CoPc1-CNT composite, the charging process exhibited
a capacity of approximately 2000 mAh g-1 within the
potential range of 3.2 V to 4.5V. During the discharge phase, the
potential ranged from 4.0 V to 2.0 V. Notably, an abrupt drop in
capacity was observed initially reaching around 500 mAh
g-1. Subsequently, a stable discharge region spanning
from 500 mAh g-1 to 3000 mAh g-1 was
observed followed by a final decrease in capacity until reaching 2.0 V.
Overall, the CoPc1-CNT composite exhibited discharge capacity of 3400
mAh g-1. Capacities were calculated according to
catalyst weight.
In contrast, the charge/discharge curve of the CoPc2-CNT composite, as
depicted in Fig. 13, displayed a lower capacity of approximately 1100
mAh g-1 during charging within the potential range of
3.1 V to 3.85 V. The discharge phase spanned from 3.1 V to 2.0 V with a
discharge capacity of approximately 1400 mAh g-1.
Notably, an initial increase followed by a sudden decrease in charge
curve of CoPc2-CNT composite was observed. This behavior can be
attributed to a weak bond between the Co metal and the Pc2 leading to an
additional redox reaction and subsequent oxidation of the metal.
The observed differences in capacity and behavior between CoPc1-CNT and
CoPc2-CNT composites highlight the influence of the phthalocyanine
structure on the electrochemical performance of composite materials. The
higher capacity and more stable discharge behavior of the CoPc1-CNT
composite show a stronger interaction between CoPc1 and CNT, leading to
improved charge transfer kinetics and enhanced battery performance.
These findings emphasize the importance of rational design and selection
of phthalocyanine materials for composite electrode applications as they
directly impact the electrochemical performance of Lithium-air
batteries. Further investigations are needed to gain a deeper
understanding of the underlying mechanisms and optimize the composite
composition to unlock the full potential of phthalocyanine based
composites for advanced energy storage systems.FIGURE 13
Conclusion
In this study, CoPc@CNT composites were successfully synthesized by
interacting two different phthalocyanines, Pc1 and Pc2, with CNT. The
synthesized phthalocyanines were characterized using spectroscopic
methods and employed in the fabrication of Lithium-air batteries.
Various electrochemical tests were conducted to assess the performance
of the prepared batteries.
As anticipated, the CoPc1-CNT and CoPc2-CNT composites exhibit
characteristic oxidation and reduction peaks corresponding to
Co/Co2+ and
Co2+/Co3+ redox processes. Impedance
measurements revealed that CoPc1-CNT composite exhibited lower charge
transfer resistance compared to CoPc2-CNT. BET analysis indicated that
both CoPc1-CNT and CoPc2-CNT composites exhibited multilayer isotherm
absorption.
Furthermore, the battery using the CoPc1-CNT composite demonstrated a
charging capacity of 2000 mAhg-1 and a discharge
capacity of 3400 mAh g-1catalyst. On
the other hand, the battery incorporating the CoPc2-CNT composite
exhibited lower efficiency. These findings provide that the CoPc1-CNT
composite holds promise for enhancing the performance of lithium-air
batteries.
Overall, this study successfully demonstrated the synthesis of CoPc@CNT
composites, characterized their electrochemical properties, and
evaluated their performance in Lithium-air batteries. Further
investigations are warranted to optimize the composite composition and
explore its potential for advanced energy storage applications.AcknowledgmentThe Scientific and Technological Research Council of Turkey (TUBITAK)
(Project No: 118C472) supported this study. The authors thank TUBITAK,
Osmaniye Korkut Ata University, and OKUMERLAB.