Photovoltaic and electrocatalytic properties of novel ball-type phthalocyanines bridged with four dicumarol

Article abstract of DOI:10.1039/c2dt12510b

The new ball-type metallo bisphthalocyanines (Co₂Pc₂ and Zn₂Pc₂) were synthesized from the corresponding [4,4′-bis(dicoumaroylphthalonitrile)] which can be obtained from the reaction of 3,3′-methylenebis(4-hydro

Full text of DOI:10.1039/c2dt12510b

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PAPER
Photovoltaic and electrocatalytic properties of novel ball-type
phthalocyanines bridged with four dicumarol†
Ümit Salan,*a Ahmet Altındal,b Ali Rıza Özkaya,a Bekir Salihc and Özer Bekaroğlu*d
Received 30th December 2011, Accepted 17th February 2012
DOI: 10.1039/c2dt12510b
The new ball-type metallo bisphthalocyanines (Co2Pc2 and Zn2Pc2) were synthesized from the
corresponding [4,4′-bis(dicoumaroylphthalonitrile)] which can be obtained from the reaction of
3,3′-methylenebis(4-hydroxy-2H-chromen-2-one) and 4-nitrophthalonitrile. The structures of the newly
synthesized compounds have been confirmed and characterized by elemental analysis, UV/Vis, IR and
1H NMR spectroscopies and MALDI-TOF mass spectrometry. Solar cells of the configuration ITO/
Co2Pc2/C60/Al and ITO/Zn2Pc2/C60/Al were fabricated. The effect of the thickness of the active Pc layer
– the thickness of the Pc layer was varied from 15 to 80 nm – on solar cells parameters has been
investigated. A nearly thickness independent open circuit voltage was observed in both structures. The
maximum photovoltaic conversion efficiency, short circuit current and fill factor were observed in ITO/
Zn2Pc2/C60/Al cell with 80 nm Pc layer to be 0.255%, 1 mA cm−2 and 0.38, respectively. The redox
properties of the ball-type complexes were investigated by cyclic voltammetry, controlled-potential
coulometry and in situ spectroelectrochemistry in DMSO–TBAP. The electrochemical measurements
showed that the complexes form ring-based and/or metal-based mixed-valence species, due to the
remarkable intramolecular interactions between the two metal phthalocyanine units. The Vulcan XC-72
(VC)/Nafion(Nf)/Co2Pc2 modified glassy carbon electrode showed much higher catalytic performance
towards oxygen reduction, compared to the VC/Nf/Zn2Pc2 modified one. It was found that the VC/Nf/
Co2Pc2 catalyst is nearly insensitive to the presence of methanol. In the presence of 1 M methanol in the
electrolyte, the catalytic performance of the Co2Pc2-based catalyst in oxygen reduction was much better
than that of the Pt-based one. Thus, it was shown that the VC/Nf/Co2Pc2 catalyst can be a good
alternative to VC/Nf/Pt as a cathode catalyst in direct methanol fuel cells.
advantages in their applications in various fields of technology
Introduction
such as fuel cells, optoelectronics, semiconductors and sensors,
over the common mononuclear and non-interacting dinuclear
Pcs. For instance, ball-type cobalt phthalocyanines displayed
high catalytic activity towards dioxygen reduction, which has
vital importance in fuel cell applications.9,10 In most cases, their
high catalytic activities were attributed to the presence of two
interacting redox-active metal centers, coordinating ability of
these metal centers for dioxygen and the suitable distance
between the two MPc units. In this respect, the comparison of
the various physicochemical properties of ball-type Pcs invol-
ving redox-active metal centers such as Co(II) or Fe(II) with the
others including redox-inactive metal centers such as Cu(II) or
Zn(II) is useful in identifying and explaining the reason for their
extraordinary properties.
The fact that the nature of the metal centers and the bridging
substituents lead to important changes in ball-type Pc character-
istics and our continuing efforts in the design of novel macro-
cycles with potential applicability in various technological areas
prompted us to synthesize new examples of this type of com-
pounds with dicumarol bridging units and different metal
centers. For a long time, binuclear Pcs have been obtained by
Ball-type phthalocyanines (Pcs) are a new class of compounds in
which two Pcs monomers arranged cofacially are bridged with
four substituents on the peripheral positions of the benzene
rings. The first synthesis of a ball-type Pc was reported in
2002.1,2 The results of our previous studies suggested that these
compounds usually display interesting electrical,3–5 electroche-
mical,6 gas sensing7 and optical properties,8 due to strong intra-
molecular interactions between the two face to face Pc rings and/
or metal centers. Therefore, this new class of compounds have
aDepartment of Chemistry, Marmara University, 34722 Kadıköy,
İstanbul, Turkey. E-mail: usalan@marmara.edu.tr, aliozkaya@
marmara.edu.tr; Fax: +90 216 347 87 83; Tel: +90 216 347 96 41
bPhysics Department, Yıldız Technical University, 34212 Davutpaşa,
İstanbul, Turkey. E-mail: altindal@yildiz.edu.tr
cChemistry Department, Hacettepe University, 06532 Beytepe, Ankara,
Turkey. E-mail: bekir@hacettepe.edu.tr
dDepartment of Chemistry, Istanbul Technical University, 34469 Maslak,
Istanbul, Turkey. E-mail: obek@itu.edu.tr
†Electronic supplementary information (ESI) available: absorption
spectra of compounds 4 and 5 in DMSO and 1H-NMR spectrum of com-
pound 5 in DMF. See DOI: 10.1039/c2dt12510b
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Fig. 1 A positive ion reflectron mode MALDI-TOF mass spectrum of
4 in α-cyano-4-hydroxycinnamic acid MALDI matrix using a nitrogen
laser (at 337 nm wavelength) accumulating 100 laser shots. Insets show
the experimental and theoretical isotopic mass distributions of the proto-
nated molecular ion.
Scheme 1 (i) K2CO3, DMSO; (ii) 4/Co(OAc)2·4H2O, 300 °C; 5/Zn-
(OAc)2·2H2O, 300 °C.
the tetramerization of phthalonitrile (or its derivatives) with basic
catalysts in a suitable solvent such as hexanol, pentanol or
dimethylformamide (DMF) at 180–200 °C. Recently researchers
have used new techniques such as ‘microwave irradiation’ or
‘heating of the solid phase’ to synthesize binuclear Pcs.
In this paper, we have used the ‘heating of the solid phase’
method for the synthesis of novel ball-type CoII 4 and ZnII
5
Pcs. Our starting compound 4,4′-bis(dicoumaroylphthalo-nitrile)
3 was synthesized from the reaction of 3,3′-methylenebis(4-
hydroxy-2H-chromen-2-one) 1 and 4-nitrophthalonitrile 2, in dry
dimethylsulfoxide (DMSO) in the presence of dry K2CO3. Com-
plexes 4 and 5 were prepared by heating 3 with excess Co
(OAc)2·4H2O and Zn(OAc)2·4H2O in a N2 atmosphere in a
sealed tube (Scheme 1).
Fig. 2 A positive ion reflectron mode MALDI-TOF mass spectrum of
5 in α-cyano-4-hydroxycinnamic acid MALDI matrix using a nitrogen
laser (at 337 nm wavelength) accumulating 100 laser shots. Insets show
the experimental and theoretical isotopic mass distributions of the proto-
nated molecular ion.
Electronic spectra are especially fruitful in establishing the
structure of ball type metallo bisphthalocyanines 4 and 5 (ESI,
Fig. S1†). The Q band observed for the compounds was attribu-
ted to the π → π* transition from the highest occupied molecular
orbital (HOMO) to the lowest unoccupied molecular orbital
(LUMO) of the Pc ring. The B bands in the UV region were
observed due to the transitions from deeper π levels to the
LUMO. The UV/Vis spectra of 4 and 5 exhibited the intense Q
bands at 675, 685 nm and 701, 684 nm, respectively. As
expected, good splitting of the Q bands in the spectra of 4 and 5
were observed. This can be attributed to strong intramolecular
interactions between the Pc rings, probably due to the ball-type
cofacial structure. 1H NMR spectra were also in good correlation
with the structure of the synthesized compounds (ESI, Fig. S2†).
Results and discussion
In this study, a novel compound 4,4′-bis(dicoumaroylphthaloni-
trile) 3 was prepared and used as a key compound for the prep-
aration of new ball-type metallo bisphthalocyanines (Co2Pc2 4
and Zn2Pc2 5) which were achieved by the reaction of 3,3′-
methylenebis(4-hydroxy-2H-chromen-2-one) 1 and 4-nitrophtha-
lonitrile 2, in dry DMSO in the presence of K2CO3, in a 67%
yield. The resulting compounds were purified by washing with
hot acetic acid and ethanol, respectively, using Soxhlet apparatus
and column chromatography. Melting points of the Pcs were
found to be higher than 350 °C.
1
Elemental analysis, IR, 1H NMR, MALDI-TOF mass and
UV/Vis spectra confirmed the proposed structures of the com-
pounds. A diagnostic feature of Pcs formation from compound 3
was the disappearance of the sharp intense CN vibration band at
2231 cm−1 in the IR spectrum. The IR spectra taken with KBr
pellets showed Ar–O–Ar peaks at 1242–1251 cm−1, an aromatic
CvC peak at around 1517–1602 cm−1, CvO peaks at
1717–1770 cm−1, an aromatic CH peak at 3044–3054 cm−1, and
an aliphatic CH2 at 2848–2940 cm−1 for the compounds 4
and 5.
In the H NMR spectrum of compound 3, which was taken in
d-chloroform, the phenyl protons appeared at 7.36 ppm (t),
7.37 ppm (d), 7.58 ppm (br t), 8.00 ppm (dd), 8.08 ppm (d),
8.59 ppm (dd), 8.66 ppm (d), and –CH2 protons at 3.85 ppm (s).
Aromatic protons of 5 appeared as multiples at 6.80–7.80 ppm
in its 1H NMR spectrum.
Positive ion and reflectron mode MALDI-TOF mass spectra
of the complexes are shown in Figs 1 and 2. Some commercial
MALDI matrices were tried to find an intense molecular
ion signal and low fragmentation under MALDI-TOF mass
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spectrometry conditions for these compounds. Mainly com-
plexes were yielded at high intensity protonated ion signals in
α-cyano-4-hydroxycinnamic acid MALDI matrix. Protonated ion
signals of 4 were observed at reasonable high intensity in reflec-
tron mode, protonated ion signals were observed at reasonably
high intensity in reflectron mode, and followed at least three CO2
elimination fragments. For both complexes, the theoretical and
experimental isotopic mass distribution of the protonated mol-
ecular ion peaks were compared (the insets in Figs 1 and 2). It
was noticed that the experimental and theoretical isotopic peak
distributions completely matched each other for each complex.
This shows that all compounds were synthesized successfully
using the synthesis route given in the Experimental section.
The characterization of the fabricated solar cells was carried
out in ambient air under an illumination of 100 mW cm−2 with
an AM1.5G sun simulator. In order to characterize the roles of
the thickness of the Pc layer in ITO/Pc/C60/Al cells, the thick-
ness of the Pc layer was varied from 15 to 80 nm. The solar cell
parameters such as short circuit current (Isc), open circuit voltage
(Voc), fill factor (FF) and photovoltaic conversion efficiency (η)
are the key parameters and should be determined under particu-
lar illumination conditions, the so called standard test con-
ditions.11 The photovoltaic conversion efficiency of a solar cell
is defined as,
dramatically with the thickness of the Pc layer. Recently, it was
discussed in the literature12,13 that the energy difference between
the HOMO of the donor and the LUMO of the acceptor is
closely correlated to the Voc value. Therefore, a thickness inde-
pendent Voc is expected because the Voc depends mostly on the
energy levels of the donor and acceptor materials. On the other
hand, there are many reports on increasing the Voc by changing
the thickness of the Pc layer. The effect of the CuPc layer thick-
ness on the open circuit voltage and short circuit current in ITO/
PEDOT:PSS/CuPc/Al solar cell devices was investigated by
Rajaputra et al.14 They concluded that the enhancement in the
Voc with thickness is primarily due to enhanced Jsc and not due
to reduced reverse saturation current density. That means, by
considering the previous reports on increasing of Voc, there is
some factor, which limits this correlation.
It should be noted that not only Isc but also FF and η show a
strong dependence on Pc layer thickness, the photovoltaic con-
version efficiency increases with increasing Pc layer thickness
reaching a value of 0.255% for a 5 layer thickness of 80 nm. The
increase in Isc and η with Pc layer thickness can be attributed to
the following two reasons: it is well known that the Pc is a good
absorber of visible light of 550–800 nm, an increase in the thick-
ness of photoactive layer leads to greater absorption and hence
more exciton generation, which in turn leads to enhanced photo-
voltaic performance. It is well established that the structural
arrangement and the overlap of the π-electron systems between
the neighbour molecules in the thin film play a fundamental role
in the electrical and optical properties of Pc compounds. There-
fore, another possible reason for the improved photovoltaic per-
formance in the thicker active layer is the change in the degree
of crystallinity, grain size and stacking arrangement of the Pc
molecules with increasing film thickness. Senthilarasu et al.15
demonstrated how the film thickness rearranges the molecular
stacks. They observed that the degree of crystallinity and grain
size increase with the film thickness. The increase in the degree
of crystallinity and grain size may result in altering the inter-
action of the π-electron systems between the different molecules,
which may affect the photovoltaic performance of the cells. The
comparison of the performance parameters of ITO/5/C60/Al and
ITO/4/C60/Al in which the thicknesses of the Pc layers were
varied from 15 to 80 nm for both devices, indicates that ITO/5/
C60/Al structure has better photovoltaic performance than ITO/
4/C60/Al for all thicknesses of Pc layer.
η ¼ ðVoc ꢀ Isc ꢀ FFÞ=Pin
where FF is given by,
FF ¼ ðImax ꢀ VmaxÞ=ðIsc ꢀ Voc
ð1Þ
Þ
ð2Þ
where Imax and Vmax are the current and voltage at the certain
point on the I–V characteristics in which the cell delivers the
maximum power density.
Fig. 3 compares the current density versus voltage (J–V)
characteristics of various cells prepared using different thick-
nesses of the Pc layers under 100 mW cm−2 illumination and the
device performance parameters are summarized in Table 1 for
numerical comparison. Although the open circuit voltages of all
devices are about 0.67 0.04 V irrespective of the thickness of
the Pc layer, the short circuit current of the same cells increases
The redox properties of 4 and 5 were examined by cyclic vol-
tammetry and controlled-potential coulometry (CPC) in DMSO–
TBAP (tetrabutylammonium perchlorate). A typical cyclic
voltammogram for 5 at 0.050 V s−1 is shown in Fig. 4A. It dis-
played four reductions at E1/2 = −0.64 V (R1), E1/2 = −0.99 V
(R2), E1/2 = −1.31 V (R3) and E1/2 = −1.7 V (R4) and an
Table 1 The effect of the thickness of the Pc layer on the photovoltaic
performance of ITO/5/C60/Al and ITO/4/C60/Al devices
Thickness (nm) Isc (mA cm−2
)
Voc (V)
FF
η (%)
15
30
45
60
80
0.30/0.12
0.40/0.18
0.65/0.28
0.80/0.45
1.00/0.53
0.63/0.64 0.33/0.20 0.062/0.047
0.66/0.65 0.33/0.18 0.087/0.062
0.68/0.65 0.34/0.17 0.150/0.088
0.70/0.71 0.33/0.19 0.185/0.107
0.67/0.68 0.38/0.23 0.255/0.104
Fig. 3 J–V characteristics of the ITO/5/C60/Al photovoltaic devices at
different thicknesses of the film of 5.
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Fig. 4 A cyclic voltammogram of 5.00 × 10−4 M 5 at 0.100 V s−1 (A) and in situ UV/Vis spectral changes monitored during its first reduction at
−0.85 V (B), second reduction at −1.15 V (C), third reduction at −1.50 V (D), fourth reduction at −1.95 V (E) and first oxidation at 0.70 V vs. SCE
(F) on Pt in DMSO–TBAP.
oxidation at E1/2 = 0.60 V vs. saturated calomel electrode (SCE)
(O1) in DMSO–TBAP. The CPC studies showed that each
broadly separated redox process involves the transfer of one elec-
tron. The transfer of one electron in each redox process suggests
that the reduction or oxidation of each ZnPc unit in dinuclear
ball-type 5 occurs at a different potential, as a result of the inter-
action between the two cofacial ZnPc units and thus, the splitting
of the molecular orbitals. It is clear from the previous studies
that the ZnII metal center is not redox-active in metallo Pcs and
thus, all redox processes of 5 are ligand-based.16,17 As illustrated
schematically in Chart 1, these processes are associated with the
presence of mixed-valence species, [ZnIIPc(−2)·ZnIIPc(−1)]+,
[ZnIIPc (−2)·ZnIIPc(−3)]− and [ZnIIPc(−3)·ZnIIPc(−4)]3−
.
The splitting of a redox process i.e., Pc(−2)/Pc(−3), for 5,
due to formation of a stable mixed-valence intermediate,
[ZnIIPc(−2)·ZnIIPc(−3)]−, is a measure of the equilibrium
5180 | Dalton Trans., 2012, 41, 5177–5187
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Chart 1 Electrode reactions for 5.
(comproportionation) constant, Kc, for a reaction such as:16
½ZnIIPcðꢁ3Þꢂ22ꢁ þ ½ZnIIPcðꢁ2Þꢂ2 O 2½ZnIIPcðꢁ2Þꢂ
ꢃ ½ZnIIPcðꢁ3Þꢂꢁ
where the mixed-valence splitting, ΔEs is related to Kc, via:
ΔEs ¼ ðRT=nFÞln Kc
ð3Þ
Fig. 5 A cyclic voltammogram of 4 at 0.050 V s−1 on Pt in DMSO–
The mixed-valence splitting energy, ΔEs, values of Pc(−2)/Pc
(−3) and Pc(−3)/Pc(−4) redox processes, i.e., the difference
between the half-peak potentials of R1 and R2, and R3 and R4,
respectively, in 5 are 0.35 and 0.39 V, respectively, indicating
remarkable stability of the relevant mixed-valence species. The
values of Kc obtained for the mixed-valence species of 5,
[ZnIIPc(−2)·ZnIIPc(−3)]− and [ZnIIPc(−3)·ZnIIPc(−4)]3−, are
8.55 × 105 and 4.08 × 106, respectively. The high Kc and ΔEs
values of 5 give evidence of the delocalization of charge among
the cofacial Pc units, and thus the formation of electrochemically
reduced mixed-valence species.
TBAP.
reversible or quasi-reversible reductions at E1/2 = −0.39 V (R1),
E1/2 = −0.99 V (R2) and E1/2 = −1.33 V vs. SCE (R3). CPC
measurements and anodic to cathodic peak separations in the
cyclic voltammogram suggested that all redox couples are one-
electron reversible or quasi-reversible processes except the
second reduction couple, R2 which involves the transfer of two
electrons per molecule. The transfer of one electron in each of
the broadly separated O1 and O2 steps implies that the oxidation
of each MPc ring of 4 occurs at different potentials as stepwise
one-electron redox couples of the two cofacial Pc units, as a
result of the interaction between them and thus, the splitting of
the molecular orbitals. The metal centre in a CoII phthalocyanine
is redox-active due to the presence of d orbital levels within
the HOMO–LUMO gap.4,16,17 Depending on its environment in
solution, the electrochemistry of cobalt Pcs is split into two sec-
tions, that referring to donor solvents and that referring to non-
donor solvents. The main difference lies in whether metal or
the ring is oxidized first. Donor solvents strongly favour the
oxidation of CoII to CoIII by coordinating along the axis to form
six coordinate species. If such donor solvents are absent, then
oxidation to CoIII is inhibited and ring oxidation occurs first.
On the other hand, the first reduction process is metal-based
both in donor and nondonor solvents. Thus, the first and the
second oxidation processes of 4 are probably metal-based
In situ spectroelectrochemical measurements enabled us to
monitor the UV/Vis spectra of electrochemically reduced species
of 5 (Fig. 4B–E). The decrease in the split Q band with the red
shift of the one at 680 nm to 697 nm during the first reduction
process, R1, and to 715 nm during the second reduction process,
R2, and the one at 651 nm to 625 nm during the second
reduction process R2 is accompanied by the increase in the
absorption between 500–600 nm and the formation of a new
band around 900 nm (Fig. 4B and C). These spectral changes,
especially the increase in the absorption between 500–600 nm,
are characteristic for ring reduction. However, in general, the
shift in the Q band is not observed during ring reductions of
mono metallo Pcs including a redox inactive metal center. The
shifts observed here should be due to the changes in the intra-
molecular interactions between the two Pc rings in ball-type 4,
upon reduction. As shown in Fig. 4D and E, the formation of
new intense bands at 561 and 579 nm during the third and fourth
reduction processes confirm the ring-based nature of these pro-
and correspond to [CoIIPc(−2)]2/[CoIIIPc(−2)·CoIIPc(−2)]+ and
2+
[CoIIPc(−2)·CoIIIPc(−2)]+/[CoIIIPc(−2)]2
redox processes,
cesses and thus, the formation of [ZnIIPc(−3)ZnPc(−4)]3− and
respectively, since each molecule of ball-type 4 involves two
CoII centers and the voltammetric measurements were carried
out in DMSO–TBAP. In situ UV/Vis spectroelectrochemical
measurements in DMSO–TBAP were also carried out to confirm
the assignment of the redox processes.
Fig. 6A shows the spectral changes recorded during the first
oxidation process of 4. The Q band in the spectrum monitored at
the beginning of the electrolysis is considerably broad, probably
due to the intra molecular interactions between the two MPc
units in 4. An alternative or additional reason for the broad Q
band may be the presence of aggregated species at high concen-
trations used in the optically transparent thin layer electrode
(OTTLE) cell. The red shift in the Q band from 670 nm to
4−
[ZnIIPc(−4)]2
species, respectively. Unfortunately, singly
oxidized [ZnIIPc(−2)·ZnIIPc(−1)]+ species could not be obtained
during the constant-potential electrolysis at 0.70 V vs. SCE
(Fig. 4F) although a well-defined redox couple was obtained
during cyclic voltammetry measurements (Fig. 4A). As shown in
Fig. 4F, the absorption of the all bands decreases during the elec-
trolysis and isosbestic points are not observed which implies that
under the conditions of constant-potential electrolysis, decompo-
sition of the complex occurs.
A typical cyclic voltammogram of 4 in TBAP–DMSO is
shown in Fig. 5. It displays two quasi-reversible oxidations at
E1/2 = 0.07 V (O1) and E1/2 = 0.60 V vs. SCE (O2) and three
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Fig. 6 In situ UV/Vis spectral changes monitored during the redox processes of 4 in TBAP–DMSO.
680 nm confirms the assignment of the first oxidation couple to
[CoIIPc(−2)]2/[CoIIPc(−2)·CoIIIPc(−2)]+. However, during the
electrolysis at 0.80 V vs. SCE, the absorptions of the all bands
decreased suggesting decomposition of the complex although a
well-defined second oxidation couple, O2 was observed in its
cyclic voltammogram. The one-electron quasi-reversible first
reduction of 4, R1, can be attributed to [CoIIPc(−2)]2/[CoIIPc-
(−2)·CoIPc(−2)]−, as evidenced by the spectral changes
observed during the electrolysis at −0.65 V vs. SCE. As shown
in Fig. 6B, the absorption of the Q band decreases with blue
shift to 613 nm which is associated with the formation of a new
band at 442 nm. These spectral changes confirm the formation
of CoII·CoI mixed-valence species.18,19 It is known from the lit-
erature20,21 that both CoII·CoI and CoI·CoI species show Q band
and metal-to-ligand charge transfer (MLCT). The new band at
442 nm can be assigned to MLCT from CoI to the Pc ring.
During the reduction at −1.25 V vs. SCE, two groups of spectral
changes are observed with different isosbestic points (Fig. 6C
and D). At the beginning, the Q band at 613 nm decreases with
red shift and nearly disappears while a new band at 734 nm
forms and the band at 442 nm increases with red shift to 464 nm
(Fig. 6C). These spectral changes indicate the formation of
CoI·CoI species. The MLCT from CoI to the Pc ring is observed
around 473 nm in the case of mono cobalt Pcs. The former spec-
tral changes are followed by an increase without shift in the band
at 734 nm and a decrease in the absorption around 600 nm
(Fig. 6D). These latter spectral changes are characteristic for ring
reduction and should correspond to the reduction of one of the
Pc rings in 4. The occurrence of two groups of spectral changes
with different isosbestic points at −1.25 V vs. SCE and the trans-
fer of two electrons per molecule during the R2 process show
that the second metal center reduction and the first ring reduction
of 4 are merge into one redox couple. Thus, R2 and R3 couples
of 4 can be assigned to [CoIIPc(−2)·CoIPc(−2)]−/[CoIPc-
(−2)·CoIPc(−3)]3− and [CoIPc(−2)·CoIPc(−3)]3−/[CoIPc(−4)]4−
.
The voltammetric electrode reactions and mixed-valence species
produced electrochemically as a result of the redox processes of
dimeric ball-type 4 are schematically illustrated in Chart 2.
The splitting of the CoII/CoIII redox process in 4, due to
formation of the stable mixed-valence intermediate,
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Chart 2 Electrode reactions for 4.
[CoIIIPc(−2)·CoIIPc(−2)]+, is a measure of the equilibrium (com-
proportionation) constant, Kc, for the reaction:
½CoIIIPcðꢁ2Þꢂ22þ þ ½CoIIIPcðꢁ2Þꢂ2 O ½CoIIIPcðꢁ2Þ
ꢃ CoIIPcðꢁ2Þꢂþ
The mixed-valence splitting energy, ΔEs, of CoII/CoIII redox
process for 4, i.e., the difference between the half-peak potentials
of O1 and O2 couples is 0.53 V which leads to Kc value of 9.61
× 108, suggesting high stability of the mixed-valence species of
[CoIIIPc(−2)·CoIIPc(−2)]+. The difference between the stepwise
reduction potentials of 4 is remarkably high, which indicates that
the mixed-valence species of [CoIIPc(−2)CoIPc(−2)]− and
[CoIPc(−2)·CoIPc(−3)]3− are considerably stable. However, it is
not suitable to determine the relevant comproportionation con-
stants due to the coalescence of second and third reduction redox
couples.
Some recent reports of our group suggested that electrocataly-
tic performances of ball-type dinuclear metallo Pcs with
cyclopentyldisilanoxy-polyhedral oligomeric silsesquioxanes,9
perfluorodecyl,22 pentaerythritol,10 1,1′-methylenedinaphthalen-
2-ol23 and dithioerythritol24 bridging units towards the oxygen
reduction reaction (ORR) are encouraging for applicability in
fuel cells. For this reason, the electrocatalytic performance of the
ball-type Pc complexes 4 and 5 towards ORR was tested by the
ring and disk polarization curves of a rotating ring disk electrode
(RRDE), modified by a mixture of the relevant Pc complex with
Vulcan XC-72 (VC) and Nafion (Nf), in O2-saturated 0.5 M
H2SO4 aqueous electrolyte solution (Fig. 7A). The onset poten-
tial (Eo) where the oxygen reduction current begins to increase,
the limiting diffusion disk current density (JL) and the half-wave
potential (E1/2) for the ORR were taken as the measures of cata-
lytic performance. The disk potential at which the current
density reaches 0.100 mA cm−2 was taken as Eo. Table 2 sum-
marizes the electrocatalytic performances of 4 and 5, in compari-
son with previously reported ball-type cyclopentyldisilanoxy-
polyhedral oligomeric silsesquioxanes-,9 pentaerythritol-10 and
dithioerythritol-bridged24 binuclear metallo Pcs. Contrary to 5,
complex 4 displays two reduction waves, well-separated from
each other. The limiting current for the first wave implies that the
ORR probably occurs via peroxo species, producing water as the
main product and some hydrogen peroxide as the side product.
The second wave, appearing at considerably negative potentials
presumably corresponds to the reduction of hydrogen peroxide
to water. The absolute ring current decreases simultaneously
while the disk current increases during the appearance of the
second wave (Fig. 7A), which confirms that hydrogen peroxide
produced at the first step as the side product is further reduced to
water during the second wave. The ORR occurs at much more
positive potentials at the VC/Nf/4 modified GCE than that at the
Fig. 7 (A) RRDE polarization curves for the electrocatalytic ORR at
0.005 V s−1 on a VC/Nf/M2Pc2 modified rotating (2500 rpm) glassy
carbon disk electrode and platinum ring electrode in O2-saturated 0.5 M
H2SO4. (B) The number of total electrons exchanged for VC/Nf/M2Pc2
modified electrodes (inset shows %H2O2 produced in the ORR as a
function of disk potential (Ering = 0.95 V vs. SCE)). (C) RDE polariz-
ation curves for the electrocatalytic ORR at 0.005 V s−1 on VC/Nf/Pt
and VC/Nf/4 modified rotating (2500 rpm) glassy carbon electrodes in
the presence and absence of 1 M methanol in O2-saturated 0.5 M
H2SO4.
VC/Nf/5 modified one. In addition, the limit current density for
the VC/Nf/4 modified GCE is much higher than that of VC/Nf/5
modified one. These findings clearly indicate the higher catalytic
activity of the former relatively.
The ring currents in RRDE voltammetry measurements,
carried out with a VC/Nf/M2Pc2 modified glassy carbon disk
This journal is © The Royal Society of Chemistry 2012
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Table 2 Electrocatalytic properties of 4 and 5, and the comparison
a direct path, i.e., oxygen reduction to water via a four-electron
process without the formation of hydrogen peroxide as an inter-
mediate is the main reaction to occur. The RRDE voltammetry
results showed that contrary to the 4-based catalyst, the reduction
wave of the VC/Nf/5 modified electrode produces approximately
equal amounts of water and hydrogen peroxide. At the potentials
between −0.050 and −0.35 V vs. SCE, the nt takes values within
the range of 2.90–3.10 (45% H2O–55% H2O2). Thus, water
selectivity of 4 in the ORR is higher than that of 5. Moreover,
for the ORR, the overpotential of the VC/Nf/4 modified elec-
trode is considerably low, in comparison with that of the VC/Nf/
5 modified electrode. According to some authors,28,29 electro-
catalytic reduction of oxygen on metallo Pcs takes place through
a redox-catalysis type of process where oxygen binding ability
and the redox potential of the central metal ions play a critical
role. The metal ion in the center of Pc ring is oxidized by
oxygen molecule during its adsorption and thus reduces it. The
metal center in 5 is redox-inactive, and thus neither oxidized nor
reduced under the conditions of electrocatalytic measurement, as
judged from their well described electrochemical behaviour.16
On the other hand, in the case of 4, the presence of two redox-
active face-to-face metal centers promotes its interaction with O2
molecules, the O–O bond breakage, the formation of suitable
intermediates and thus reduction to water, as a result of
the partial electron transfer from the filled d orbitals of the metal
to the empty or partially filled π* orbitals of oxygen.30–34 As
shown in Table 2, the catalytic performance of 4 is better than
those of previously reported ball-type pentaerythritol-,10 and
dithioerythritol-bridged24 cobalt phthalocyanines whereas it is
relatively low in comparison with the cyclopentyldisilanoxy-
polyhedral oligomeric silsesquioxanes-bridged one,9 especially
in terms of onset and half-wave potentials for the ORR.
In a direct methanol fuel cell (DMFC), methanol can diffuse
from the anode to the cathode and the performance of the fuel
cell becomes impaired when the cathode contains Pt. However,
metal N4 chelates are known to be selective catalysts and, more
particularly, to be tolerant to the presence of methanol during the
ORR.35–39 The methanol tolerance of a 4-based catalyst was also
determined and compared with that of platinum. Fig. 7C shows
rotating disk electrode (RDE) polarization curves obtained in O2-
saturated electrolyte in the presence and absence of methanol
(1.0 M) for Pt- and 4-based catalysts. Indeed, the 4-based elec-
trode, especially its first reduction wave is completely insensitive
towards methanol. The limiting current of the first wave is
also completely independent of methanol presence. Contrary to
4-based electrode, the polarization curve of the ORR on the
Pt-based electrode is strongly affected by the presence of 1.0 M
methanol in the electrolyte. The presence of 1.0 M methanol
leads to a shift of the platinum catalyzed oxygen reduction
potentials of 260 mV towards lower (more negative) potentials.
A significant methanol oxidation current is observed over a wide
range of potentials, and the catalytic activity of the Pt-based cata-
lyst for the ORR is completely blockaded by methanol over the
potential range of 0.38–0.70 V vs. SCE. Thus, until the potential
reaches 0.40 V, the ORR on the 4-based catalyst occurs at more
positive potentials in the presence of methanol, as compared to
the Pt-based one. On the basis of this result, a 4-based catalyst
can function as an alternative to Pt-based catalysts for the ORR
in DMFC applications.
with previously reported9,10,24 ball-type cobalt phthalocyanine dimers
Eo for ORRa
(V)
JL for ORRb
E1/2 for
ORR (V)
Complex
(mA cm−2
)
Ref.
Zn2Pc2 5
Co2Pc2 4
−0.01
2.07
2.63
0.92
0.86
3.30
4.27
2.17
4.50
4.84
−0.21
0.35
−0.20
−0.21
−0.14
0.41
−0.19
−0.09
0.05
twc
twc
0.45
d
Zn2Pc2d
−0.18
0.10
0.68
24
24
9
Co2Pc2e
Co2Pc2
f
Co2Pc2g
0.25
0.61
10
10
Co2Pc2
a The potential at which the current density reaches 0.100 mA cm−2 was
taken as the onset potential. b The limiting diffusion current density at
2500 rpm. c This work. d Dithioerythritol-bridged ball-type zinc and
cobalt phthalocyanine. e Ball-type cyclopentyldisilanoxy-polyhedral
oligomeric
silsesquioxanes-bridged
cobalt
cobalt
phthalocyanine.
phthalocyanine.
f Pentaerythritol-bridged
ball-type
g Pentaerythritol-bridged ball-type cobalt phthalocyanine with
heptadecafluorodecyl substituents.
electrode and a platinum ring electrode polarized at 0.95 V vs.
SCE were used to determine the number of electrons transferred,
nt, and estimate the amount of generated hydrogen peroxide and
thus water selectivity since the ORR in an acidic medium can be
either a four-electron process to form water or a two-electron
reduction to form hydrogen peroxide (Fig. 7A). It is desirable
that the ORR goes to completion and forms water via a four-
electron transfer mechanism. However, in the case of low cataly-
tic activity, oxygen gets reduced to hydrogen peroxide via a
two-electron process, but the peroxide can still get reduced
further via another two-electron process to form water. It can be
observed that the increase in the absolute ring current initiates at
the same potential as the onset potential for the ORR in both
VC/Nf/4 and VC/Nf/5 modified disk electrodes, indicating that
the generation of hydrogen peroxide is taking place (Fig. 7A).
The nt and the percentage of hydrogen peroxide produced at a
given potential was determined by the following equations:25–27
nt ¼ 4ID½ID þ ðIR=NÞꢂ
%H2O2 ¼ 100ð4 ꢁ ntÞ=2
ð4Þ
ð5Þ
where N, ID, and IR are the collection efficiency, disk current and
ring current, respectively. Fig. 7B shows the nt and percentage of
hydrogen peroxide and water generated at different potentials.
The obtained values confirm that the ORR on the 4-based elec-
trode produces a higher amount of water than that of hydrogen
peroxide, i.e., the numbers of exchanged electrons at different
potentials on the ORR wave are higher than 3 which corresponds
to the formation of equal amounts of water and hydrogen
peroxide. The nt value increases with increasing overvoltage and
becomes 3.58 (79% H2O and 21% H2O2) first, and then
decreases slightly to 3.38 (69% H2O and 31% H2O2) at the limit-
ing diffusion current plateau, leading to the production of water
as the main product. The detection of hydrogen peroxide
suggests that the first step of the series path, i.e., oxygen
reduction to hydrogen peroxide is certainly operative. However,
5184 | Dalton Trans., 2012, 41, 5177–5187
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were performed with a glassy carbon disk (5 mm dia) in O2
saturated 0.5 M H2SO4 aqueous solution under quasi-stationary
conditions (0.005 V s−1 sweep rate) at 25 °C. For RRDE ex-
periments, the working electrode was a glassy carbon disk
(5.61 mm dia) and a platinum ring leading to a collection
efficiency, N = 37%. These experiments were carried out at 2500
rpm in oxygen saturated 0.5 M H2SO4 aqueous electrolyte sol-
ution at 25 °C. The disk potential was swept at 0.005 V s−1
whereas the ring potential was held at 0.95 V vs. SCE. In order
to disperse the catalysts on a carbon support for RDE and RRDE
experiments, a mixture of the Pc compound, VC and 5% wt Nf
solution in absolute ethanol was prepared and ultrasonically
homogenized for half an hour. A given volume of this ink was
deposited using a micropipette onto a freshly polished glassy
Experimental section
All reagents and solvents obtained from commercial suppliers
were of reagent grade quality. The solvents were stored over mol-
ecular sieves (4 Å). 3,3′-Methylenebis(4-hydroxy-2H-chromen-
2-one) 1 was used as commercially supplied. IR spectra were
recorded on a Schimadzu FT-IR-8300 spectrophotometer as KBr
pellets. Electronic spectra were recorded on a Schimadzu
UV-1601 spectrophotometer. Elemental analyses were performed
1
by the Instrumental Analysis laboratory of Tübitak-Ankara. H
NMR spectra were recorded with a Mercury-Vx 400 MHz instru-
ment. Mass spectra were acquired on a Voyager-DETM PRO
MALDI-TOF mass spectrometer (Applied Biosystems, USA)
equipped with a nitrogen UV-Laser operating at 337 nm. Spectra
were recorded in reflectron mode with an average of 100 shots.
The MALDI matrix α-cyano-4-hydroxycinnamic acid was
prepared in acetonitrile–water (1 : 1, v/v) containing 0.1% tri-
fluoroacetic acid at a concentration of 10 mg mL−1. MALDI
samples were prepared by mixing sample solutions (1.0 mg
mL−1 in methanol) with the matrix solution (1 : 10, v/v) in a
0.5 mL Eppendorf® micro tube. Finally, 0.5 μL of this mixture
was deposited on the sample plate, dried at room temperature
and then analyzed.
carbon electrode leading to a catalyst loading of 106 μg cm−2
.
The amount of Nf in the ink was adjusted so that the catalyst
film was sufficiently thin, and thus its diffusion resistance was
negligible. In order to obtain a total coverage of the glassy
carbon surface, the deposited drop of ink was dried with pulsed
air at ambient temperature to evaporate the solvent quickly. Each
voltammogram was recorded with a freshly prepared electrode
due to the possible catalyst degradation in acidic medium.
The counter electrode was a Pt spiral and the reference electrode
was a SCE.
The electrochemical measurements were carried out with
a Princeton Applied Research Model VersoStat II potentiostat/
galvanostat controlled by an external PC and utilizing a three
electrode configuration at 25 °C. A platinum disk, a platinum
spiral wire and a SCE served as the working, the counter and the
reference electrodes, for cyclic voltammetry measurements. The
reference electrode was separated from the bulk of the solution
by a double bridge. Electrochemical grade TBAP in extra pure
DMSO was employed as the supporting electrolyte at a concen-
tration of 0.10 M. High purity N2 was used for deoxygenating
the solution at least 20 min prior to each run and to maintain a
nitrogen blanket during the measurements. For CPC studies, Pt
gauze working electrode (10.5 cm2 surface area), Pt wire counter
electrode separated by a glass bridge, and SCE as a reference
electrode were used. In situ spectroelectrochemical measure-
ments were carried out by an Agilent Model 8453 diode array
spectrophotometer equipped with the potentiostat/galvanostat
and utilizing an OTTLE cell with three-electrode configuration at
25 °C. The working electrode was transparent Pt gauze. Pt wire
counter electrode and a SCE reference electrode separated from
the bulk of the solution by a double bridge were used.
Ultra pure water and sulfuric acid were used to prepare the
electrolyte solution in electrocatalytic measurements. VC (Cabot
Co.), 5% Nf solution (Aldrich), extra pure ethyl alcohol (Merck),
VC supported platinum particles (ElectroChem, Inc.) and Pc
compounds were used in catalyst preparation. Glassy carbon disk
electrode, platinum ring-glassy carbon disk electrode and a pol-
ishing kid for these electrodes were purchased from Pine Instru-
ments. RDE voltammetry measurements were carried out using a
Gamry Reference 600 potentiostat/galvanostat controlled by an
external PC and utilizing a three-electrode configuration at
25 °C. Two Gamry Reference 600 potentiostats were connected
as a bipotentiostat to control the ring and disk potentials and to
collect the respective currents during RRDE experiments. A Pine
Instrument Company AFMSRCE modulator speed rotator was
employed in RDE and RRDE experiments. RDE measurements
[4,4′-Bis(dicoumaroylphthalonitrile)] (3)
A mixture of 3,3′-methylenebis(4-hydroxy-2H-chromen-2-one)
1 (0.97 g, 2.89 × 10−3 mol) and 4-nitrophthalonitrile 2 (1 g, 5.78
× 10−3 mol) in 30 mL dry DMSO was stirred at room tempera-
ture under N2. K2CO3 (1.2 g, 8.70 × 10−3 mol) was added into
the mixture over a period of 2 h. After stirring the reaction
mixture for 5 days, the undissolved salt was removed by fil-
tration. The reaction mixture was poured into water (100 mL)
and stirred. The precipitate was filtered and washed with water.
The residue was chromatographed by silica gel eluted with
CHCl3 and a gradient of CHCl3–CH3OH up to 100% CH3OH.
Yield: 1.12 g (66%). The compound is soluble in CHCl3,
1
CH2Cl2, THF, DMF. m.p. 235 °C; H NMR (400 MHz, CDCl3,
4
TMS): δ 8.66 (2H, d, J = 2.29 Hz, CH), 8.59 (2H, dd, 3,4J =
3
8.56 and 2.25 Hz, CH), 8.08 (2H, d, J = 8.56 Hz, CH), 8.00
3
(2H, dd, J = 7.96 and 1.00 Hz, CH), 7.58 (2H, br t, J = 7.93
3
3
Hz, CH), 7.37 (2H, d, J = 8.56 Hz, CH), 7.36 (2H, t, J = 7.97
Hz, CH), 3.85 (2H, br s, CH2); IR (KBr pellet) νmax/cm−1; 3044,
2923, 2231, 1667, 1606, 1532, 1407, 1277, 1253, 1160, 1110,
1055, 908, 852, 796, 745, 678, 638, 524 cm−1; elemental analy-
sis calcd (%) for C35H16O6N4; C 71.43, H 2.74, N 9.52; found:
C 71.22, H 2.87, N 9.75.
[2′,10′,16′,24′-Tetrakis(dicoumaroyl)phthalocyaninato-
dicobalt(II)] (4)
A mixture of compound 3 (0.200 g, 0.34 × 10−3 mol) and Co
(OAc)2·4H2O (0.212 g, 0.85 × 10−3 mol) was powdered in a
quartz crucible and heated in a sealed glass tube for 5 min under
dry N2 atmosphere at 300 °C. After cooling to room temperature,
5 mL of DMF was added to the residue to dissolve the product.
The precipitate was filtered, multiply washed at first with hot
This journal is © The Royal Society of Chemistry 2012
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water and then with hot ethanol in order to eliminate the
unreacted starting materials, and dried in vacuo. Then, the crude
product was washed with acetic acid, ethanol for 12 h, respect-
ively, in a Soxhlet apparatus. The residue was coarsely fractio-
nated on a silica gel column eluting with chloroform and a
gradient of chloroform–THF up to 100% THF. This blue-green
compound is soluble in THF, DMF, DMSO. Yield: 110 g (13%).
m.p. >350 °C; IR (KBr pellet) νmax/cm−1; 3044, 2940, 2850,
1770, 1717, 1589, 1520, 1463, 1404, 1242, 1088, 957, 877,
827, 748, 637, 520 cm−1; UV/Vis (DMSO): λmax(ε) = 675
(4.410), 685 (4.394), 323.5 (4.337 mol−1 dm3 cm−1); elemental
analysis calcd (%) for C140H64N16O24Co2; C, 68.02; H, 2.61;
Co, 4.77; N, 9.07; found: C, 68.25; H, 2.41; Co, 4.81; N, 8.87;
MS (MALDI-TOF): m/z 2471 (M + 1+).
Fig. 8 A schematic diagram of the device configuration.
[2′,10′,16′,24′-Tetrakis(dicoumaroyl)phthalocyaninato-
dizinc(II)] (5)
A mixture of compound 3 (0.200 g, 0.34 × 10−3 mol) and Zn-
(OAc)2·2H2O (0.186 g, 0.85 × 10−3 mol) was powdered in a
quartz crucible and heated in a sealed glass tube for 5 min under
dry N2 atmosphere at 300 °C. After cooling to room temperature,
the green reaction product was obtained, isolated and purified
using the same procedure explained above for 4. This compound
is soluble in THF, DMF, DMSO. Yield: 90 mg (10.7%). m.p.
Fig. 9 The structure of the phthalocyanine compounds.
1
>350 °C; H NMR (400 MHz, DMF, TMS): δ 6.80–7.80 (m,
56H); 3.96 ppm (s, 8H); IR (KBr pellet) νmax/cm−1; 3054, 2915,
2848, 1770, 1718, 1602, 1517, 1461, 1325, 1251, 1135, 1022,
898, 841, 721, 666, 475 cm−1; UV/Vis (DMSO): λmax(ε) = 701
(4.584), 684 (4.588), 513 (3.663), 285 (4.493 mol−1 dm3 cm−1);
elemental analysis calcd (%) for C140H64N16O24Zn2: C, 67.67;
H, 2.60; N, 9.02; Zn, 5.26; found: C, 67.92; H, 2.45; N, 9.22;
Zn, 5.15. MS (MALDI-TOF): m/z 2481 (M + 1+).
The cells were fabricated on indium tin oxide (ITO) coated
glass substrates with a sheet resistance of 20 Ω cm−2. After
masking, patterning of the bare ITO was obtained by chemical
etching in diluted HCl solution. Then the ITO coated substrates
were cleaned by ultrasonic treatment in acetone, isopropyl
alcohol, and deionized water. After being dried by N2 gas, ITO
surfaces were treated by UV-ozone for 15 min. The organic
materials used in the fabrication of solar cells were 4, 5 and C60,
they were used without any purification, as donor and acceptor,
respectively. The thin films of the Pcs with different thickness
between 15 and 80 nm were deposited on the patterned ITO
substrate by a spin coating method using DMF as solvent.
Subsequently, the films were thermally annealed at 150 °C for
15 min to evaporate the solvent. Then, on the top of the Pc layer,
function of Pc layer thickness under AM1.5 illumination
(100 mW cm−2).
Conclusions
New ball-type ZnII and CoII Pcs substituted with 3,3′-methylene-
bis(4-hydroxy-2H-chromen-2-one), 4 and 5, were synthesized
from [4,4′-bis(dicoumaroylphthalonitrile)] 3. The complexes
1
were characterized by elemental analysis, UV/Vis, IR, H NMR
and MALDI-TOFF mass spectroscopies. The structure of the
resulting compounds was refined by performing on optimized
geometry calculation in MOPAC using first AM1 then PM3
parameters (Fig. 9).40
A set of organic photovoltaic cells based on 4, 5 donor mol-
ecules and C60 acceptor molecules has been prepared. Investi-
gation of the effects of active layer thicknesses on the
performance of the photovoltaic cells indicates that the perform-
ance parameters of the solar cells such as photovoltaic conver-
sion efficiency, short circuit current and fill factor strongly
depend on the Pc layer thickness. It was observed that the device
with an 80 nm 5 layer exhibits the highest photovoltaic conver-
sion efficiency, short circuit current and fill factor. All these
findings suggest that the performance of the ITO/5/C60/Al photo-
voltaic devices can be improved by the optimization of the thick-
ness of the Pc layer. It can be said that compound 5 has a good
potential for photovoltaic applications.
C
60 with thicknesses of 20 nm and 400 nm Al layers were evap-
orated sequentially by thermal evaporation at a pressure below
10−5 mbar (Edwards Auto 500). During evaporation, there was
no vacuum break between organic (C60) and metal (Al) depo-
sition. An elipsometric technique was used to measure the thick-
ness of the films. The active area of the devices was about
5 mm2. A schematic of the device structure is shown in Fig. 8.
We have investigated the change in solar cell performance,
including short circuit current (Isc), open circuit voltage (Voc),
photovoltaic conversion efficiency (η), and fill factor (FF), as a
The electrochemical measurements of binuclear ball-type
complexes 4 and 5 showed formations of electrochemically
stable metal- and ligand-based mixed-valence species, due to the
5186 | Dalton Trans., 2012, 41, 5177–5187
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11 M. Skompska, Synth. Met., 2010, 160, 1–15.
intramolecular interactions between the two MPc units. The high
mixed-valence splitting values for these species suggested that
the interactions between the two MPc units in the complexes are
remarkable. Complex 4 showed remarkable catalytic activity
towards the ORR in an acidic medium, as compared to 5. The
distinctive catalytic performance of 4 is attributed to the redox-
active behavior of the metal centers, enhancing the interaction
with O2 molecules. The ORR on a 4-based catalyst occurs at re-
latively more positive potentials with lower overpotentials, com-
pared to some previously reported ball-type pentaerythritol-,10
and dithioerythritol-bridged24 CoPc-based catalysts. In addition,
12 F. B. Kooistra, J. Knol, F. Kastenberg, L. M. Popescu, W. J. H. Verhees,
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it was found that the catalytic activity of
a 4-based
catalyst towards the ORR in the presence of methanol is better
than that of a Pt-based one, suggesting an alternative for DMFC
applications.
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Acknowledgements
We would like to thank the Commission of Scientific Research
Project (BAPKO) of Marmara University [FEN-E-130711-0259]
and Turkish Academy of Sciences (TUBA). We also thank Prof.
Dr A. Şengül and Dr Z. Odabas for their valuable contribution
in drawing some figures.
22 M. Özer, A. Altındal, A. R. Özkaya and Ö. Bekaroğlu, Dalton Trans.,
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23 Z. Odabaş, A. Altındal, A. R. Özkaya, B. Salih and Ö. Bekaroğlu, Sens.
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24 T. Ceyhan, A. Altındal, A. R. Özkaya, B. Salih and Ö. Bekaroğlu, Dalton
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