The electrochemistry of platinum phthalocyanine microcrystals - III. Electrochemical behaviour in aqueous electrolytes

Article abstract of DOI:10.1016/S0013-4686(01)00469-8

Platinum phthalocyanine (PtPc) films applied to gold and glassy carbon (GC) surfaces have been shown to exhibit complex electrochemistry in aqueous media. Upon potentiodynamic cycling three anodic (0.81, 1.10 and 1.27 V) and two cathodic peaks (1.12 and 0.59 V vs SCE) develop. These processes are ascribed to phthalocyanine ring oxidation and reduction processes. This phthalocyanine ring electrochemistry is thought to be controlled by anion doping within the film, the large separation between corresponding anodic and cathodic processes being manifestations of barrier potential to anion movement within the film. A large reduction peak at -0.6 V is attributed to the mass expulsion of anions from the film.

Full text of DOI:10.1016/S0013-4686(01)00469-8

Electrochimica Acta 46 (2001) 2573–2582
www.elsevier.nl/locate/electacta
The electrochemistry of platinum phthalocyanine
microcrystals
III. Electrochemical behaviour in aqueous electrolytes
Richard J.C. Brown, Anthony R. Kucernak *
Department of Chemistry, Imperial College of Science, Technology and Medicine, London SW7 2AZ, UK
Received 1 December 2000; received in revised form 7 February 2001
Abstract
Platinum phthalocyanine (PtPc) films applied to gold and glassy carbon (GC) surfaces have been shown to exhibit
complex electrochemistry in aqueous media. Upon potentiodynamic cycling three anodic (0.81, 1.10 and 1.27 V) and
two cathodic peaks (1.12 and 0.59 V vs SCE) develop. These processes are ascribed to phthalocyanine ring oxidation
and reduction processes. This phthalocyanine ring electrochemistry is thought to be controlled by anion doping within
the film, the large separation between corresponding anodic and cathodic processes being manifestations of barrier
potential to anion movement within the film. A large reduction peak at −0.6 V is attributed to the mass expulsion
of anions from the film. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Platinum phthalocyanine; Electrochemical behaviour; Phase tranformation; Microcrystals; Abrasion
1. Introduction
complexes may be centred on the phthalocyanine ring,
at the metal centre or at both [6]. These processes are
influenced by several factors including the nature of the
substituents on the ring, the nature and oxidation state
of the central metal, and the properties of the axial
ligands and solvent [7].
Whether oxidation occurs at the ring or at the central
metal in MPc complexes depends exclusively on the
relative energy levels of the frontier metal orbitals
(d-orbitals for transition metal MPcs) and the ring p
orbitals, in particular the HOMO and LUMO orbitals
[8]. It is usually possible to observe two successive
oxidations or reductions of the phthalocyanine ring by
removal of electrons from the HOMO or addition of
electrons to the LUMO respectively. If metal orbitals
lie at energies within the HOMO–LUMO gap, oxida-
tions or reductions or both may be observed at the
central metal. In such circumstances metal to ligand
charge transfer (MLCT) or ligand to metal charge
transfer (LMCT) may occur accompanied by colour
Phthalocyanines [1] represent a very important class
of organic macrocycle in modern chemistry. Metalloph-
thalocyanines (MPcs) have found particular uses as
electrocatalysts for a variety of reactions [1]. The most
common electrocatalytic application for these MPcs has
been in oxygen reduction with the goal being the devel-
opment of fuel cell cathode electrocatalysts [2,3].
The incorporation of different metals into the core of
the phthalocyanine ring and variations in the sub-
stituents on the periphery of the ring (particularly cou-
pling aromatic groups) results in compounds that have
varied properties. Changes in oxidation states often
result in reversible and dramatic colour changes in
MPcs [4,5]. The redox processes occurring in MPc
* Corresponding author. Tel.: +44-(0)20-7-5945831; fax:
+44-(0)20-7-5945804.
E-mail address: a.kucernak@ic.ac.uk (A.R. Kucernak).
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0013-4686(01)00469-8

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R.J.C. Brown, A.R. Kucernak / Electrochimica Acta 46 (2001) 2573–2582
changes. p-Donor or acceptor ligands may directly
change the position of the metal orbitals with respect to
the HOMO–LUMO gap and may affect the electro-
chemistry that can be observed. Variation of the solvent
may also have an effect in this respect.
Sulphuric acid and sodium hydroxide (AnalaR) were
used without further purification. Solutions were made
up with distilled, deionised water (Millipore MilliQ
system). The working electrodes were either glassy car-
bon (0.39 cm2) or gold (0.28 cm2) housed in PTFE
sheaths (Oxford Electrodes Ltd, UK).
More recently platinum phthalocyanine (PtPc) has
been considered as an alternative to first row transition
metal MPcs whose stability is poor under the high
temperature, high potentials and harsh acidic environ-
ments encountered within fuel cells. PtPc suffers less
under such conditions and thin films (200 nm) of PtPc
on gold have been shown to promote catalytic oxygen
reduction in base [9,10]. The electrochemistry of PtPc
has never been fully characterised, as such study is
hindered by the low solubility of PtPc in most solvents.
This has meant that most studies have involved com-
posite PtPc electrodes or thin layers of PtPc evaporated
onto a conducting substrate. An examination of dis-
solved PtPc in 1-chloronaphalene at 150°C by Camp-
bell et al. revealed two reduction waves at −1.23 and
−1.61 V against the ferrocene couple [11]. PtPc can be
electrocrystallised from a chloronapthalene solution at
120°C to form fine 20 nm diameter fibres which display
two anodic and two cathodic electrochemical processes
[12]. A study of the electrochemical properties of PtPc/
poly-bisphenol-A-carbonate (PBC) and PtPc/PBC/car-
bon composite electrodes in propylene carbonate by
Before use, the discs were polished down though
grades of aluminium oxide paper to 1 mm before being
thoroughly washed with deionised water and dried in a
stream of argon (purity \99.9%). PtPc layers were
applied by means of physical abrasion of the PtPc
crystals onto the electrode surface.
Electrochemical experiments were performed using
an EcoChemie AUTOLAB PGSTAT30 potentiostat. A
standard three-electrode arrangement was employed us-
ing working electrodes as described above, a large-area
platinum flag counter electrode and a saturated calomel
electrode (SCE) as a reference, against which all
voltages will be referred to and displayed henceforth. A
three-compartment cell was used with a Luggin capil-
lary and a glass sinter separating working and counter
electrode compartments. All solutions were saturated
either with argon (\99.9%) or oxygen (\99.9%) by
bubbling the appropriate gas though the solution for 30
min prior to each experiment and keeping a positive
pressure above the solution during experiments.
Kogan and Kakushi [13,14] gives
a complicated
voltammetric structure and is characterised by two
anodic and four cathodic redox transitions. Their anal-
ysis points towards a complicated regime involving
phase transitions within the composite electrode and
several anodic and cathodic redox transitions.
In a previous study, the electrochemical behaviour in
acetonitrile of PtPc microcrystals attached by a process
of dry abrasion to an electrode surface without the need
of a polymeric binder was characterised [15].
The aims of our current study are to extend the
measurements to aqueous electrolytes and to elucidate
the processes occurring within these films and if possi-
ble to relate these processes to structural changes within
the film. A recent review of voltammetry of solid mi-
croparticles immobilised on electrode surfaces provides
useful background to the techniques for immobilising
powders on electrodes [16].
3. Results and discussion
3.1. First scan discrepancy
The most striking feature of the electrochemistry of
PtPc film on glassy carbon (GC) is the evolution of the
voltammogram during cycling, as has been previously
reported for this system in non-aqueous electrolytes
[15]. Fig. 1 shows the first and twentieth scans for an
abrasively deposited film of PtPc on a GC electrode.
The main features obvious from these scans are the
disappearance of the peak IIIA at 1.19 V and its corre-
sponding reductive couple, IIIC at 0.93 V, and the
appearance of a new peak IIA, at 1.12 V. The signifi-
cant difference between the first and subsequent scans is
attributed to the irreversible intercalation of anions into
the PtPc microcrystals, as confirmed by the significant
excess of anodic charge passed compared to the charge
recovered during the cathodic scan. This result is simi-
lar to that seen for this system in acetonitrile electrolyte
[15].
Since the fresh solid phase may not be conducting
enough in its initial state for significant electrochemistry
to be observed, there may need to be an initial bulk
induction of solvent into the film to aid conduction and
act like an inert conducting binder in composite elec-
trodes. Indeed highly conductive PtPc salts have been
prepared by oxidation accompanied by anion insertion
2. Experimental
PtPc was synthesised by heating finely crushed ph-
thalonitrile and PtCl2 at 200°C in the absence of a
solvent for 1 h [17]. Two separate sublimation steps
purified the product, each repeated three times. In the
former, unreacted phthalonitrile was sublimed from the
crude product at 140°C and 0.1 mbar pressure, and in
the latter, PtPc was sublimed from the intermediate
product at 550°C and 0.1 mbar pressure.

R.J.C. Brown, A.R. Kucernak / Electrochimica Acta 46 (2001) 2573–2582
2575
into the lattice [12,18]. This process may be considered
quite separately from the anion/cation ingress and
egress that may accompany the electrochemistry of
subsequent scans. This first scan phenomenon has often
been explained in terms of a percolation or switching
potential for polymer films [19–21]. The percolation
threshold is the potential at which the conducting spe-
cies in a film generates a well connected network. An
interesting feature of the percolation threshold is the
appearance of a sharp variation of the electrical resis-
tance of the solid phase, as previously found for PtPc
films in non-aqueous electrolytes [15]. This is a critical
phenomenon determined by only a small variation in
the concentration of the conducting species or the
electrode potential [19–21]. Such a process has a barrier
potential associated with it and the result of this is that
there is a much larger potential (the onset potential)
required initially to ‘turn on’ the electrochemical solid
phase. In this way, the first scan often exhibits much
sharper redox characteristics at more extreme potential
than subsequent scans. Behaviour like this must also be
determined by the packing and arrangement of the
initial solid phase and may be more consistent with
porous phases.
maximum after about five scans, and then decay away.
The very broad peak IA shows a steady increase of
height with scan number; its growth is correlated with
the production of a very broad cathodic background,
seen on the reverse scan. Peak IIA, which is absent in
the first scan, steadily grows over the first five scans,
whereas peak IVA more quickly peaks and then broad-
ens and shifts to more anodic potentials. The corre-
sponding cathodic peak IVC shows a similar variation,
shifting to more cathodic potentials on successive scans.
Peak IIC, which starts off as a double-lobed response,
increases in height and shows a shift to more cathodic
potentials.
The assignment of couples in this diagram was deter-
mined from an analysis of the voltammetric response of
an aged film during variation of the scan limits, Fig. 3.
Both the anodic and cathodic limits were modified up
to a maximum scan range of 0–1.4 V. The anodic limits
were varied from 1.0 to 1.4 V in 0.1 V increments, and
the cathodic limits were varied between 0 and 1.2 V.
The electrochemistry between these scan limits consists
of three anodic peaks and three cathodic ones. It is
evident that the process responsible for peak IA does
not show an obvious reduction process, but instead
leads to a very broad cathodic response. Peak IIA
produces a corresponding cathodic peak, IIC, with a
very large peak separation of 0.5 V. This separation in
non-aqueous electrolytes has been attributed to a phase
change process [15]. Peak IVA leads to the production
of a cathodic peak, IVC, with a 0.15 V peak separation.
Limiting the cathodic end-point of the scans confirms
the assignment of peaks. Once a cathodic endpoint is
chosen which does not lead to the reduction process
embodied in peak IIC, the corresponding anodic pro-
cess, IIA, disappears (Fig. 3f). Similarly, choosing a
scan limit within which IVC does not occur leads to the
disappearance of IVA (Fig. 3g).
3.2. E6olution of the electrochemistry
After the highly irregular first scan, the electrochem-
istry of the PtPc microcrystalline film slowly evolves.
After about five scans the voltammogram shows no
further changes apart from some variation in peak
heights, and it reaches a steady state after about twenty
scans; such an ‘aged’ film shows very little further
variation. The evolution of this voltammogram is ac-
companied by a change in peak height and a slight shift
in the position of some of the peaks during successive
voltammetric scans, as seen in Fig. 2. The evolution of
peak height with scan number is shown inset in this
diagram. It can be seen that peak-heights go through a
It is proposed that the pairs of peaks II and IV are
due to oxidation and reduction of the phthalocyanine
Fig. 1. Cyclic voltammetry of an abrasively deposited PtPc layer applied to a glassy carbon in 0.2 mol dm−3 sulphuric acid. 1st scan
(—); 20th scan (·····). Scan rate 0.050 V s−1
.

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Fig. 2. Cyclic voltammetry of an abrasively deposited PtPc layer applied to a glassy carbon in 0.2 mol dm−3 sulphuric acid; 2nd
scan (—); 3rd scan (·····); 4th scan (– – –); 5th scan (– · – ·). Scan rate 0.050 V s−1. Inset: absolute value of peak height as a
function of scan number.
ring. Peak III is presumably also due to Pc ring oxida-
tion/reduction although it is often so small as to be
unobservable between IVC and IIC. As peak III is
predominantly seen in the first scan, it is presumed that
this process is the same as for peak II, it merely occurs
in a slightly different environment. Peak IA has previ-
ously been attributed to anion doping at the surface of
a PtPc–poly(bisphenol-A-carbonate) (PBC) composite
electrode [13,14]. A similar explanation relating to pro-
cesses at the film ꢀ electrolyte interface could also be
applied here. IA has a small cathodic counterpart hid-
den by IIC as determined in the envelope opening
analysis. The marked development of the voltam-
mogram from the initial scan and the higher overpoten-
tial for bulk Pc oxidation on the first scan is attributed
to the initial doping–de-doping process bringing about
a change in the bulk film structure, as is seen with
conducting polymers. Certainly the double layer current
is much smaller for the first scan than for subsequent
ones.
versus the scan rate in the diagram inset into Fig. 4.
The peak height to scan rate plots were fitted to a
function of the form
iP=Awx
(1)
where iP is the peak height, w is the scan rate, A is a
constant, and x is the power to which the scan rate is
raised. For solid-state processes that are not under
diffusion control, a value for the exponent of close to
one would be expected. The value calculated for this
exponent, obtained by fitting the curves to the above
function were found to be close to unity for all peaks
except for peak IVC, which was found to have a value
close to 0.85. The experimental dependence of ip on w
for several known cases of nucleation–growth have
been shown to have values of x intermediate between
1/2 and 1 [22]. The precise value of x is dependent upon
the nature of the nucleation–growth kinetics and may
even depend upon the scan rate [23,24]. The reduction
of the oxidised form of the PtPc microcrystals at peak
IVc thus appears to follow a nucleation–growth pro-
cess, entirely consistent with what has been found in
non-aqueous electrolytes at macroscopic electrodes [15]
and at microelectrodes [25].
3.3. Scan rate dependence
The dependence of the voltammetry of an aged film
with scan rate over the range 5–100 mV s−1 is shown
in Fig. 4. The peak positions within the voltam-
mograms of the aged film remain constant with scan
rate and show no shift in potential, apart from peak IA,
which shifts to more anodic potentials as the scan rate
increases. The peak current for each peak is plotted
The ratio of the total cathodic to anodic charge for
the voltammograms varies quite significantly with scan
rate. A plot of this ratio with scan rate is displayed in
Fig. 5. At high scan rates almost complete charge is
recovered whereas at lower scan rates the charge re-
versibility is poor. This contrasts to the case in acetoni-

R.J.C. Brown, A.R. Kucernak / Electrochimica Acta 46 (2001) 2573–2582
2577
trile in which aged films showed good charge reversibil-
ity independent of scan rate [15]. Inset into Fig. 5 is a
cyclic voltammogram of the film at a very low scan rate
of 0.002 V s−1. The poor charge reversibility appears to
be as a result of a significant excess of anodic charge at
high potentials which is not recovered in peak IVC. This
peak appears quite small in comparison to its anodic
counterpart, IVA. This anodic excess is not due to an
irreversible conversion within the film, as repeated cy-
cling between these potential limits results in an excess
anodic charge much larger than could be expected for
the amount of microcrystalline PtPc deposited on the
electrode. It thus seems that the excess anodic charge
must be due to reaction with the electrolyte. The large
discrepancy between the charge contained in peaks IVA
and IVC suggests that the material produced during the
oxidation process at IVA reacts with the electrolyte to
regenerate the precursor PtPc material and this leads to
a reduction in the size of IVC. If we write the reaction
occurring at IVA and IVC as
represents the same molecular stack of phthalocyanine
molecules which have been further oxidised, then the
irreversible reaction occurring is the oxidation of the
aqueous electrolyte to oxygen resulting in the regenera-
tion of the starting material
2[PtPc]2x+ +H2O“2[PtPc]x++1/2O2+2H+
(3)
The observation that the peak currents are directly
proportional to the scan rate, consistent with electron
transfer of an adsorbed species, is at odds with previous
work on PtPc–PBC electrodes [13,14] which showed a
relationship proportional to the square root of the scan
rate. This may indicate that in the well evolved voltam-
mogram the oxidation–reduction peaks are mainly due
to significant redox chemistry within the film. However
it is possible that the peaks do not represent a clear-cut
single process. Peak IIA increases in size dramatically if
the system is left to rest at its open circuit potential for
a few minutes. This observation, coupled with the
larger potential difference between peak pair II com-
pared with the redox couple IV, may indicate a con-
comitant second-order phase transition or anion
doping. The large potential differences between corre-
sponding peaks were also observed in the PtPc–PBC
composite electrode. Such a difference is incompatible
with electrochemical irreversibility since there is no
change in peak separation with scan rate. It is thought
that the overpotential may be related to the steric
barriers to migration of relatively large anions within
the film [13,14]. Although several theories [26–30] have
been offered to explain this phenomenon, two seem
most appropriate to the system under consideration.
The electrochemistry of polyaniline films [10] has
been characterised in terms of percolation threshold
potentials whereby one observes an electrochemical
switching of the film resulting in rapid and large
changes in the conduction of the polyaniline. Such a
theory is consistent with observations in this study of
higher overpotentials for film oxidation on the first
cycle compared to subsequent cycles. The second and
mathematically more rigorous theory describes such
behaviour as ‘unusual quasi-reversibility (UQR)’ [13].
UQR describes the anion uptake and removal into films
as an N-shaped free energy relationship and the film
itself as a series of domains. The exact characteristics of
the free energy relationship will determine the absolute
nature of the electrochemical response. However, the
features of a system exhibiting UQR match very well
the system under investigation in this study, namely: (1)
the potentials of peak current remain almost constant
with scan rate, (2) peak currents are directly propor-
tional to scan rate, (3) repetitively cycled voltam-
mograms are (eventually) congruent, i.e. no irreversible
chemical changes, (4) large potential separations for
corresponding anodic and cathodic peaks, (5) anodic
and cathodic peaks are of differing shape. A more
[PtPc]+x −e− ? [PtPc]2x+
(2)
where [PtPc]+x represents the partially oxidised PtPc
material of undetermined stoichiometry, and [PtPc]x2+
Fig. 3. Cyclic voltammograms with different endpoints for an
abrasively deposited PtPc layer applied to a glassy carbon
electrode in 0.2 mol dm−3 H2SO4. The electrode was aged
before the scans by cycling the potential 20 times between the
anodic and cathodic limits. Scan rate 0.050 V s−1
.

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R.J.C. Brown, A.R. Kucernak / Electrochimica Acta 46 (2001) 2573–2582
Fig. 4. Cyclic voltammetry as a function of scan rate for an aged abrasively deposited PtPc layer applied to a glassy carbon electrode
in 0.2 mol dm−3 H2SO4. Scan rate of 0.005 V s−1 (······); 0.01 V s−1 ( – –); 0.025 V s−1 ( – · – ·); 0.050 V s−1 (– ·· – ··); 0.1
V s−1 (—).
physical explanation for this response is provided by
Bond et al., who consider the large potential differ-
ences between the oxidation and reduction peaks in
TCNQ to be due to a large surface free-energy re-
quirement needed to nucleate the new phase during
the solid–solid phase transformation [31–33].
Chronoamperometry is a powerful technique for
studying nucleation–growth controlled phase transfor-
mations [34–37]. It has provided definitive evidence
for the solid–solid phase transformations controlled
by nucleation and growth in TCNQ microcrystals
[31–33]. Peter et al. [38,39] and Lacroix et al. [40,41]
have investigated the fast switching of the elec-
trochromism of polyaniline using this technique. A
previous paper has shown, using chronoamperometry
of PtPc abrasively deposited on microelectrodes in
non-aqueous solution, that a number of different
solid–solid phase transformations can be identified de-
pending on the initial and final oxidation state of the
PtPc [25].
Fig. 5. Ratio of cathodic to anodic charge as a function of scan rate for an aged abrasively deposited PtPc layer applied to a glassy
carbon electrode in 0.2 mol dm−3 H2SO4. Potential scan limits of 0 and 1.4 V. Inset: cyclic voltammogram of the same film at a
scan rate of 0.002 V s−1
.

R.J.C. Brown, A.R. Kucernak / Electrochimica Acta 46 (2001) 2573–2582
2579
Fig. 6. Current–time transients at different potential for an aged abrasively deposited PtPc film applied to a glassy carbon electrode
in 0.2 mol dm−3 H2SO4. Inset: plot of the current versus reciprocal square root of time. Potential steps from 0 to: 0.4 V (—); 0.825
V (– – –); 1.17 V (······); 1.35 V (– ·· – ··).
Chronoamperometry has been used to study the cur-
rent response after potential steps from the reduced
state of the PtPc film at 0 V to a more positive
potential, Fig. 6. The current–time transients do not
show any inductive peaks or loops. Indeed they do not
even show the cross-over response observed for these
materials in acetonitrile electrolyte [15]. However, the
the potential exceeds about 0.8 V. The broad anodic
peak which then appears would seem to be related to
the first scan discrepancy, as successive scans which do
not sweep as far cathodically as the reductive feature
produce voltammograms which closely replicate those
seen previously, i.e. Fig. 2.
The origin of the large cathodic peak (−0.6 V) is
certainly the most interesting feature of the above elec-
plots do not neatly fit the Cottrell equation (I8t−1/2
)
at short times after the double layer charging transients,
Fig. 6 (inset). This proves that the processes occurring
are not diffusion controlled. Indeed it is possible that
the growth and nucleation process is occurring so
quickly that its response is hidden within the double
layer response. This is certainly a possibility since these
are such thin layers. However, it is of interest that the
last two steps (past peaks IIA and IVA) respectively
exhibit a finite current response over quite long time
scales, indicating the presence of a slow oxidation pro-
cess over longer time periods. Such a process may be
associated with the deep intercalation of the PtPc mi-
crocrystals which is not obtained simply by potential
cycling, or it may be due to the oxidation of solvent by
the oxidised PtPc mentioned above, Eq. (3).
3.4. Anion dependence
Fig. 7 displays the effect of different anions on the
electrochemistry of the abrasively deposited PtPc mi-
crocrystals. The diagram compares the response seen in
0.2 mol dm−3 perchloric acid, Fig. 7a, with that seen in
0.2 mol dm−3 sulphuric acid, Fig. 7b–d. The response
seen in perchloric acid solution is much broader and
less well defined than that seen in sulphuric acid. In
comparison, an extra reduction peak is seen at −0.68
V in sulphuric acid. By varying the anodic scan limit, it
can be seen that this reductive feature only occurs when
Fig. 7. Comparison between cyclic voltammetry of an aged
abrasively deposited PtPc layer applied to a glassy carbon
electrode in (a) 0.2 mol dm−3 HClO4; (b)–(e) 0.2 mol dm−3
H2SO4. (b)–(d) show the evolution of the cathodic feature as
the upper scan limit is increased. (e) shows the response of the
film after it has been aged. Scan rate: 0.05 V s−1
.

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R.J.C. Brown, A.R. Kucernak / Electrochimica Acta 46 (2001) 2573–2582
Fig. 8. Comparison between cyclic voltammetry of an aged abrasively deposited PtPc layer applied to a glassy carbon electrode in
3.0 mol dm−3 sulphuric acid, (—); and 0.2 mol dm−3 sulphuric acid, ( ·····). Inset: peak potential variation with natural logarithm
of sulphuric acid concentration for peaks IIA (ꢀ); and IIC (ꢁ). Scan rate: 0.05 V s−1
.
trochemistry. This feature has, to the best of the authors’
knowledge, never been observed for any system. We
attribute this peak to the expulsion of sulphate anions
from the PtPc films. Certainly the size of the peak
currents is much larger than the PtPc redox chemistry
(Fig. 7e). The proportionality of the peak current to the
square root of the scan rate seems to confirm that this
is a process concerning non-absorbed species. Consistent
with the above observations, this peak has an anodic
counterpart (1.05 V) with a very large potential differ-
ence, presumably the mass re-inclusion of sulphate
anions within the PtPc lattice as previously speculated
for PtPc redox peaks. This corresponding anodic peak
is extremely broad with the peak current not propor-
tional to the square root of the scan rate, probably due
to the presence of other PtPc redox electrochemistry
within its limits. Clearly this stripping and redoping of
sulphate anions prevents the voltammogram of the PtPc
film from developing in the manner seen in the positive
potential range. The second peak in the stripping area
may well be due to removal of sulphate bound differ-
ently within fresh PtPc films in an environment that
disappears with cycling of the film and rearrangement of
the PtPc lattice.
ance of a new anodic peak, aA, and the corresponding
reduction peak, aC. The second is a shift in the position
of the peaks in the voltammogram.
The formation of the new peak, aA, is related to
changes in anion strength in a very complicated way. It
is not entirely obvious whether this peak is revealed in
neutral solutions of high sulphate concentration, al-
though in acidic solutions it becomes much more promi-
nent. The appearance of peaks aA and aC appears in
many ways to mirror the peak labelled III seen during
the first scan in Fig. 1. The reappearance of this peak in
highly acidic solutions is difficult to explain, but may be
related to protonation of the phthalocyanine ring which
occurs in acidic solutions:
PtPc+H+ ? HPtPc+
(4)
Thus appearance of the aA/C peaks at higher pH may
represent the electrochemical reaction of the protonated
phthalocyanine
HPtPc+−e− ? HPtPc2+
(5)
The peaks in the cyclic voltammograms vary in posi-
tion as a function of the anion concentration. The
positions of most peaks show a complex behaviour,
typically showing a maximum in their potential as a
function of anion concentration. One set of peaks which
does not show this are peaks IIA and IIC, which show
a monotonic decrease in potential as a function of
increasing electrolyte strength. Both of these peaks show
an 0.081 V decade−1 shift in peak position with sulphate
concentration. Over the pH range studied, over 90% of
the sulphuric acid exists as the bisulphate anion. Thus
we expect that the reaction occurring be of the type
3.5. Dependence on anion concentration
Fig. 8 shows the cyclic voltammogram of an aged
abrasively deposited PtPc film on a glassy carbon elec-
trode in 3.0 mol dm−3 sulphuric acid. For comparison,
the voltammogram of the same electrode in 0.2 mol
dm−3 is also displayed. Two features of this voltam-
mogram are of specific interest. The first is the appear-

R.J.C. Brown, A.R. Kucernak / Electrochimica Acta 46 (2001) 2573–2582
2581
PtPcx[HSO4]+e− ? xPtPc+HSO4−
(6)
Another shows a very large peak-potential separation
of 0.5 V. The most anodic of the couples shows a peak
separation of 0.15 V. Virtually all peaks show a linear
peak height dependence and no potential shift with
scan rate.
As has been seen previously, this peak couple shows
a very large peak potential difference which is not due
to electrochemical irreversibility, but rather the pres-
ence of a solid–solid phase transformation with a large
surface free energy requirement to nucleate the new
phase. Feldberg and Rubenstein contend that the gen-
eralised free energy expression controlling quasi-re-
versibility in these phases is given by [30]:
In comparison to non-aqueous electrolytes, at high
potentials there appears to be a chemical reaction be-
tween the oxidised PtPc and water, which regenerates
the reduced PtPc and presumably produces molecular
oxygen. This process is especially noticeable at slow
scan rates. Chronoamperometric transients show no
indication of nucleation–growth transients, and it is
assumed that these processes occur so quickly as to be
masked by the double-layer charging process. Sulphate,
but not perchlorate, may be reductively expelled from
the PtPc microcrystalline film. This produces a material
which shows an electrochemical response similar to the
nascent material before it has undergone electrochemi-
cal cycling. Most of the peaks show a complex shift in
potential as a function of anion concentration apart
from the main intercalation/expulsion peaks which
show an 0.081 V decade−1 shift. The results are inter-
preted as supporting a mechanism whereby a quantity
of cations is inserted along with the anions into the
PtPc crystals. At low pH values a new peak appears in
the voltammograms. This is presumed to be caused by
the electrochemical response of the protonated plat-
inum phthalocyanine.
DG=DG°+RTln[ fN(n/nmax)]−RTln[c/c0]
(7)
Where DG is the change in molar free energy for the
insertion process (DG=0 at equilibrium), DG° is the
standard molar free energy, fN(n/nmax) is a generalised
N-shaped function of n/nmax of the electro-active film
into which particles of concentration c (standard state
c0) are inserted. Hence it is not unexpected that an
increase in electrolyte concentration causes a decrease
in the molar free energy and a decrease in peak poten-
tial. In the case where the activities of the PtPc and its
charge transfer salt [PtPcxHSO4] are unity, we may
assume a Nernstian-like behaviour
2.303RT
E=E0−
log10aHSO
(8)
−
4
nF
The variation in peak position with potential is mea-
sured to be 0.081 V decade−1, and for a response of the
type expected from Eq. (8), this would imply that the
number of electrons transferred is only 0.7, somewhat
less than expected. EQCM studies, to be reported in a
following paper, show that not only are anions inserted
into the PtPc films, but quantities of cations are too.
These cations act to balance part of the charge associ-
ated with the insertion of the bisulphate ions, and thus
lead to a lower than expected value of n.
Acknowledgements
We thank Dr N. Long and Mr Mongay-Batalla for
providing PtPc samples. We wish to thank Johnson
Matthey for the loan of precious metals. This project
has been funded by the UK EPSRC and MOD/DERA
under grant GR/L 57920. RJCB was supported by an
EPSRC Quota studentship.
4. Conclusions
A novel deposition method has been used to study
PtPc microcrystals in aqueous electrolytes. The voltam-
metric properties of the PtPc are related to the redox
chemistry of the PtPc itself. Such electrochemistry can
no longer be related to the superior ordering of com-
posite electrodes or vacuum deposited films since the
type of film deposited here will not have comparable
ordering. The electrochemistry exhibited is consistent
with a regime of unusual quasi-reversibility for a film
exhibiting many structural domains [13].
Similar to the case in non-aqueous electrolytes, a
very distinct first-scan discrepancy is seen [15]. The
electrochemistry of the film evolves over many cycles,
requiring about twenty cycles to reach an almost invari-
ant scan. In the anodic range the electrochemistry is
composed of three couples. One is poorly defined.
References
[1] C.C. Leznoff, A.P.B. Lever (Eds.), Phthalocyanines:
Properties and Applications, VCH, London, 1990.
[2] A. Hamnett, G.L. Troughton, Chem. Ind. (1992) 480.
[3] A. Hamnett, Philos. Trans. R. Soc. London, Ser. A 354
(1996) 1653.
[4] H. Li, T.F. Guarr, J. Electroanal. Chem. 297 (1991) 169.
[5] N. Toshima, T. Tominaga, S. Kawamura, Bull. Chem.
Soc. Jpn. 69 (1996) 245.
[6] S. Mho, B. Ortiz, S. Pak, D. Ingersoll, N. Doddapaneni,
J. Electrochem. Soc. 142 (1995) 1437.
[7] N. Toshima, T. Tominga, Bull. Chem. Soc. Jpn. 69 (1996)
2111.
[8] A.B.P. Lever, S.R. Pickens, P.C. Minor, S. Licoccia, B.S.
Ramaswamy, K. Magnell, J. Am. Chem. Soc. 103 (1981)
6801.

2582
R.J.C. Brown, A.R. Kucernak / Electrochimica Acta 46 (2001) 2573–2582
[9] C. Paliteiro, A. Hamnett, J.B. Goodenough, J. Elec-
[26] N. Toshima, T. Tominaga, S. Kawamura, Bull. Chem.
Soc. Jpn. 69 (1996) 245.
troanal. Chem. 160 (1984) 359.
[10] C. Paliteiro, A. Hamnett, J.B. Goodenough, J. Elec-
troanal. Chem. 239 (1988) 273.
[27] J. Cao, K. Aoki, Electrochim. Acta 41 (1996) 1787.
[28] R.M. Torresi, S.I. Cordoba de Torresi, E.R. Gonzalez, J.
Electroanal. Chem. 461 (1999) 161.
[29] J.M. Green, L.R. Faulkner, J. Am. Chem. Soc. 105 (1983)
2950.
[11] R.H. Campbell, G.A. Heath, G.T. Hefter, R.C.S. Mc-
Queen, J. Chem. Soc., Chem. Commun. (1983) 1123.
[12] J. Jiang, A. Kucernak, Synth. Met. 114 (2000) 209.
[13] I.L. Kogan, K. Yakushi, Electrochim. Acta 43 (1998)
2053.
[30] S.W. Feldberg, I.J. Rubinstein, J. Electroanal. Chem. 240
(1988) 1.
[14] I.L. Kogan, K. Yakushi, J. Mater. Chem. 7 (1997) 2231.
[15] J. Jiang, A. Kucernak, Electrochim. Acta 45 (2000) 2227.
[16] F. Scholz, B. Meyer, in: A.J. Bard, I. Rubenstein (Eds.),
Electroanalytical Chemistry, vol. 20, Marcel Dekker, New
York, 1998, pp. 1–86.
[17] A.B.P. Lever, E.R. Milaeva, G.P. Speier, in: C.C.
Leznoff, A.B.P. Lever (Eds.), Phthalocyanines: Properties
and Applications, vol. 3, VCH, New York, 1993, p. 1.
[18] H. Yamakado, K. Yakushi, N. Kosugi, H. Kuroda, A.
Kawamoto, J. Tanaka, T. Sugano, M. Kinoshita, S.
Hino, Bull. Chem. Soc. Jpn. 62 (1989) 2267.
[19] J. Cao, K. Aoki, Electrochim. Acta 41 (1996) 1787.
[20] M. Kalaji, L. Nyholm, L.M. Peter, J. Electroanal. Chem.
313 (1991) 271.
[21] M. Kalaji, L. Nyholm, L.M. Peter, J. Electroanal. Chem.
325 (1992) 269.
[31] A.M. Bond, S. Fletcher, P.G. Symons, Analyst 123 (1998)
1891.
[32] A.M. Bond, S. Fletcher, F. Marken, S.J. Shaw, P.G.
Symons, J. Chem. Soc., Faraday Trans. 92 (1996) 3925.
[33] M.F. Suarez, A.M. Bond, R.G. Compton, J. Solid State
Electrochem. 4 (1999) 24.
[34] T. Yea, K.M. Yin, J. Carbajal, R.E. White, J. Elec-
trochem. Soc. 138 (1991) 2869.
[35] T.F. Otero, E. Angulo, Solid State Ionics 63–65 (1993)
803.
[36] K. Aoki, J. Electroanal. Chem. 373 (1994) 67.
[37] T.F. Otero, H. Grande, J. Rodriguez, J. Electroanal.
Chem. 394 (1995) 211.
[38] M. Kalaji, M. Peter, L.M. Abrantes, J.C. Mesquita, J.
Electroanal. Chem. 274 (1989) 289.
[39] M. Kalaji, L. Nyholm, L.M. Peter, J. Electroanal. Chem.
313 (1991) 271.
[40] J.C. Lacroix, K.K. Kanazawa, A. Diaz, J. Electrochem.
Soc. 136 (1989) 1308.
[22] S. Fletcher, C.S. Halliday, D. Gates, M. Westcott, T.
Lwin, G. Nelson, J. Electroanal. Chem. 159 (1983) 267.
[23] E. Bosco, S.K. Rangarajan, J. Chem. Soc., Faraday
Trans. 1 77 (1981) 483.
[41] J.C. Lacroix, A. Diaz, J. Electrochem. Soc. 135 (1988)
1457.
[24] S.K. Rangarajan, Faraday Symp. Chem. 12 (1989) 103.
[25] J. Jiang, A. Kucernak, Electrochim. Acta 46 (2001) 1223.
.
.