Visualization of local electrocatalytic activity of metalloporphyrins towards oxygen reduction by means of redox competition scanning electrochemical microscopy (RC-SECM)

Article abstract of DOI:10.1016/j.electacta.2009.02.047

The redox competition mode of scanning electrochemical microscopy (RC-SECM) has been utilized to visualize the local electrocatalytic activity of metalloporphyrin spots towards oxygen reduction in 0.1 M phosphate buffer as electrolyte solution. The metall

Full text of DOI:10.1016/j.electacta.2009.02.047

Electrochimica Acta 54 (2009) 4971–4978
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Visualization of local electrocatalytic activity of metalloporphyrins towards
oxygen reduction by means of redox competition scanning electrochemical
microscopy (RC-SECM)
Ayodele O. Okunola1, Tharamani Chikka Nagaiah1, Xingxing Chen1,
Kathrin Eckhard1, Wolfgang Schuhmann1, Michael Bron∗,1
Analytische Chemie - Elektroanalytik & Sensorik, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The redox competition mode of scanning electrochemical microscopy (RC-SECM) has been utilized to
visualize the local electrocatalytic activity of metalloporphyrin spots towards oxygen reduction in 0.1 M
phosphate buffer as electrolyte solution. The metalloporphyrin spots were obtained by electrochemically
induced deposition using a droplet cell. Tetratolyl porphyrins (TTPs) of Mn, Fe and Co have been inves-
tigated, with that containing Mn as central metal atom showing highest catalytic activity. The multiple
stable oxidation states of Mn were seen as a key factor in the influence of the metal ion on the catalytic
activity. From the RC-SECM results, it is shown that oxygen reduction at a manganese TTP (MnTTP) mod-
ified electrode surface yielded the least amount of H2O2 when compared to iron TTP (FeTTP) and cobalt
TTP (CoTTP). As further confirmed by means of rotating disc electrode (RDE) measurements this was
attributed to the high activity of MnTTP for H2O2 reduction.
Received 8 November 2008
Received in revised form 13 February 2009
Accepted 16 February 2009
Available online 25 February 2009
Keywords:
Electrodeposition
Electrocatalysis
Oxygen reduction
Scanning electrochemical microscopy
SECM
© 2009 Elsevier Ltd. All rights reserved.
Metalloporphyrins
1. Introduction
which influences the overall (electro)catalytic activity of the com-
pound under investigation. Co and Fe containing metalloporphyrins
Macrocyclic organic N4-complexes of transition metals like met-
alloporphyrins and metallophthalocyanins are known to catalyze a
variety of redox reactions. These reactions are found in porphyrin-
containing enzymes such as sulfite reductase [1], nitrate reductase,
cytochrome c oxidase [2], blue copper oxidases, pseudocatalase
[3], photosystem II [4], nitrogenase and hydrogenase [5]. In each
case, the enzyme contains one or more coordinatively bound metal
atoms in the active site, and in many cases the enzyme may also
include active peripheral metal sites. In 1964, Jasinski [6,7] reported
that Co porphyrins are able to electrocatalyze the oxygen reduction
reaction (ORR). Since then, a large number of papers appeared deal-
ing with the electrocatalytic oxygen reduction using such so-called
N4-chelates, which were summarized in several reviews [8]. The
ORR at such complexes which involves a transfer of at least two elec-
trons through the central metal ion to molecular oxygen requires
that molecular oxygen is reversibly bound to the central metal ion.
This phenomenon further reiterates the importance of the nature of
the central metal atom particularly regarding its affinity for oxygen
have been investigated in combination with carbon nanotubes
[9,10], methanol tolerant electrocatalysts [11], in polyaniline com-
plexes [12,13] or as composite catalysts along with other transition
metal oxides [14]. While metalloporphyrins containing Co and Fe
as central metal ions have been investigated in greater detail,
especially for two- and four-electron oxygen reduction reac-
tions, only a few reports on manganese-containing N4-macrocyclic
complexes with respect to their role in ORR [15,16] have been
published.
The investigation of the electrocatalytic activity of N4-
macrocyclic metal complexes towards oxygen reduction generally
involves electrochemical techniques such as voltammetry and
amperometry [11–24]. However, more and more reports appeared
that suggested the use of scanning electrochemical microscopy
(SECM) in its many modes to evaluate the activity of catalysts
[25–28]. For example, the sample generation-tip collection (SG-TC)
mode [26] and the tip generation-sample collection (TG-SC) mode
of SECM have been used to visualize the activity of catalysts for
local O2 reduction [29,30]. In the TG-SC mode, the SECM tip, which
is positioned close to the sample, locally generates O2, which is sub-
sequently reduced at the sample under investigation. The Faradaic
current flowing through the sample is used as the analytical sig-
nal to evaluate its local electrocatalytic activity. Recently, the redox
∗
Corresponding author. Tel.: +49 234 32 26200; fax: +49 234 32 14683.
E-mail address: michael.bron@rub.de (M. Bron).
1
ISE member.
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.02.047

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competition mode of SECM (RC-SECM) where the tip and the sam-
ple under investigation are made to compete for the same analyte
in solution in a bipotentiostatic measurement has been suggested
[31]. In this case, the current at the tip is used as analytical signal.
This approach helped to overcome the challenge observed in the
TG-SC mode where a significant contribution of the background
signal at larger samples limits the possibility of high resolution
imaging [29]. In the RC-SECM mode, the local concentration of O2
in the gap between SECM tip and sample is kept high by applying
repetitive water splitting potential pulses to the tip before the deter-
mination of O2 reduction activity. The RC-SECM image is generally
obtained in such a manner that the oxygen reduction at the tip will
lead to a constant current signal until the tip is scanned across an
O2 consuming site of the sample. This consumption alters the local
was purchased from VWR, Darmstadt, Germany; K2HPO4·3H2O
from Merck, Darmstadt, Germany; meso-tetratolyl porphyrin-
Co(II) (CoTTP) from Porphyrin Systems (Lübeck, Germany). TBABF4
(tetrabutylammonium tetrafluoroborate) was from Fluka (Buchs,
Switzerland); N,N-dimethylformamide (DMF) was from J.T. Baker
(Deventer, The Netherlands). For the synthesis of metallo-
porphyrins, substituted benzaldehydes, trifluoroboron etherate
(BF3·OEt2), tetraphenylchlorophosphate (Ph4PCl) were obtained
from Merck (Darmstadt, Germany). 2,3-Dichloro-5,6-dicyano-p-
benzoqui none and pyrrole were purchased from Sigma–Aldrich
(Steinheim, Germany). All chemicals were of analytical grade and
used as received. MnTTP and FeTTP were prepared from a one-
flask synthesis of porphyrins which followed a standard literature
protocol [34–38].
O2 concentration and hence the diffusional fluxes of O2 towards the
SECM tip thereby leading to a decrease in the current flow through
the tip. The electrocatalytic properties of the sample are therefore
reflected by the modulation of the current signal at the SECM tip.
SECM imaging of local O2 reduction has generally elucidated
the local catalytic activity of a sample but does not provide infor-
mation on the number of electrons involved in the ORR, which
is represented by the formed reaction products. The selectivity of
a catalytically active sample either towards a 4-electron (respec-
tively sequential 2 × 2-electron) reduction of oxygen to H2O or a
2-electron reduction to H2O2 is of high importance for the opti-
mization of potential electrocatalysts. Recently, a sequential dual
imaging mode of the SECM was introduced in which it was possible
to conduct a simultaneous visualization of both the local electrocat-
alytic activity and the selectivity of the investigated catalyst [32]. In
that approach, two images are simultaneously obtained where one
image elucidates the activity of the sample towards oxygen reduc-
tion and the other visualizes the produced (if any) H2O2 during the
ORR.
In a preceding publication [33], we investigated the properties
of electrodeposited2 metalloporphyrins with respect to their elec-
trocatalytic activity for O2 reduction and were able to demonstrate
the advantages of the electrochemically induced deposition of met-
alloporphyrins as compared with dip- or drop-coating processes for
electrode modification.
In this contribution, SECM is applied to investigate the elec-
trocatalytic behavior of electrodeposited metalloporphyrins films
on electrode surfaces towards O2 reduction. Tetratolyl porphyrins
(TTPs) which contain Mn, Fe and Co as central metal ions have
been investigated. The impact of the catalyst loading, which was
varied during the electrochemical deposition of the metallopor-
phyrin layers, that of the central metal ion on the catalytic behavior
of these metalloporphyrins towards ORR as well as their selec-
tivity towards electrocatalytic reduction of O2 was investigated.
Comparing results from SECM and rotating disc electrode (RDE)
measurements made it possible to further elucidate the mechanism
of catalytic O2 reduction.
2.2. SECM measurements
The SECM instrumentation has been described before [39]. The
main components are stepper motor driven x–y–z stages (Owis,
Staufen, Germany), a bi-potentiostat (PG 100; Jaissle, Waiblingen,
Germany) and an in-house written control software programmed
under Visual Basic 6.0 (Microsoft, Unterschleißheim. Germany). All
SECM measurements were carried out in 0.1 M phosphate buffer
which was prepared by mixing 0.1 M KH2PO4 and 0.1 M K2HPO4
(1:1) in triply distilled de-ionized water and adjusting the pH
value to 7. A buffered solution has been chosen in order to avoid
any unwanted influence of pH changes due to the ORR. Pt-micro-
electrodes (Ø = 25 ␮m) used as SECM tips were fabricated according
to an earlier established procedure [40] using Pt wire purchased
from Goodfellow (Bad Nauheim, Germany). Other components of
the electrochemical cell include a miniaturized Ag/AgCl/3 M KCl ref-
erence electrode, to which all potentials in this work refer, a Pt-foil
as counter electrode and an indium-tin-oxide (ITO)-coated glass
plate.
During the redox competition mode of SECM the sequential
pulse profile applied to the SECM tip was: +50 mV for 500 ms (no-
effect potential), +1200 mV for 200 ms (O2 production), −600 mV
for 500 ms (O2 reduction and data acquisition), +1200 mV for
200 ms (O2 production) and +600 mV for 500 ms (H2O2 oxidation
and data acquisition).
2.3. Sample preparation
The metalloporphyrin spots were prepared by
electrodeposition from 3 mM solution of each metallopor-
a pulsed
a
phyrin in 0.1 M TBABF4 in DMF. The spots were deposited
on a Sigradur® (HTW Hochtemperatur-Werkstoffe, Thierhaupten,
Germany) glassy carbon (GC) plate in a specially designed electro-
chemical droplet cell composed of a Pt wire as counter electrode
and a Ag/AgCl/3 M KCl electrode as reference electrode, adapted
from the work of Hassel et al. [41]. Each pulse applied in the droplet
cell for the electrochemical deposition of the metalloporphyrin fol-
lowed a sequential potential profile of 0 V for 2 s, 1.6 V for 3 s and
0 V for 2 s. In our previous work, the number of pulses was varied
between 10, 20, 30 and 50 pulses to control the catalyst loading on
the GC plate, and 20 pulses turned out to be the optimum [33]. Thus,
20 deposition pulses were used if not otherwise stated through-
out the study. Since the electrodeposition occurs via ring oxidation,
and the ring system is the same in all systems, we assume that
similar amounts of porphyrin are deposited in all cases. This is
further supported by the similarity of the CVs obtained on the vari-
ous metalloporphyrins in the potential region where ring oxidation
takes place [33]. The metalloporphyrin spots were left to dry briefly
and the GC plate was rinsed carefully with DMF to remove loosely
attached material. The spot was then left to air-dry. Fig. 1 shows a
schematic representation of the electrochemical droplet cell.
2. Experimental
2.1. Materials
The metalloporphyrins used (both commercially purchased
and locally synthesized) are abbreviated as follows: manganese
TTP (MnTTP), iron TTP (FeTTP) and cobalt TTP (CoTTP). KH2PO4
2
Very often the term “electropolymerization” can be found in literature for
the electrochemical deposition of metallo-porphyrins and -phthalocyanins. In this
paper, however, the more general term “electrodeposition” is used. In this electrode-
position, likely first a radical cation is formed by oxidation, which attacks a second
porphyrin to form a dimer and so on. The exact structure and mechanism, however,
is not clear.

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4973
Fig. 2. Schematic representation of the reactions taking place in the gap between the
SECM tip and the metalloporphyrin-modified sample during the redox competition
mode of SECM in which the SECM tip and the sample compete for the available O2.
Note that the tip current during oxygen reduction is also influenced by the amount
of H2O2 available.
observed small current signal at less negative Es above the spot
indicates a small but almost insignificant contribution from the
topography of the electrodeposited metalloporphyrin spot.
Fig. 1. Scheme of the electrochemical droplet cell for the electrodeposition of met-
alloporphyrins.
2.4. RDE measurements
3.2. Effects of catalyst loading on catalytic activity
For the RDE measurements, O2 reduction at the modified elec-
trode was monitored in O2 saturated (by bubbling O2 gas through
the solution for 15–20 min) 0.1 M phosphate buffer at pH 7 (see Sec-
tion 2.2). Using linear sweep voltammetry (LSV), a classical three-
electrode configuration comprising a 1 mm Pt wire counter elec-
trode, a Ag/AgCl/3 M KCl reference electrode and the metallopor-
phyrin modified 3 mm glassy carbon working electrode was used.
A potential window from +0.7 V to −0.7 V at a scan rate of 5 mV s−1
was employed. All RDE measurements were carried out using an
Autolab PGSTAT12 (Eco Chemie, Utrecht, The Netherlands) poten-
tiostat. The 3 mm glassy carbon working electrode was rotated at
rotation speeds of 100, 400 and 900 rpm by an analytical rotator,
model ASR2 (Pine Instrument Company, Grove City, PA, USA).
In the pulsed electrodeposition formation of the metallopor-
phyrins spots, the number of pulses has been varied between 20,
30 and 50 leading to increased thickness of the metalloporphyrin
deposit. Using MnTTP as a test sample, the baseline corrected RC-
SECM line scans obtained from these deposits are displayed in
Fig. 4. Obviously, the electron transfer process using a film formed
by 20 pulsed deposition cycles showed highest activity. This is
in agreement with earlier work on CV investigations on similar
electrochemically deposited metalloporphyrin films [33]. Higher
metalloporphyrin loadings thus do not necessarily lead to higher
overall catalytic activity. One reason for this behavior might be the
semi-conducting properties of the porphyrin film.
3.3. Effects of central metal ion on catalytic activity
3. Results
The electrocatalytic reduction of O2 at a metalloporphyrin
depends largely on the central metal ion because the ORR occurs by
a transfer of electrons via the central metal when bound reversibly
to O2. Other factors such as the electron withdrawing or donat-
ing nature of the ring substituents on the porphyrin macrocycle
also contribute to this step. To study the influence of the cen-
tral metal ion, tetratolyl porphyrins which contain Fe, Co and Mn
have been investigated. Fe and Co are better known metals in this
respect but the multiple stable oxidation states of Mn deserve closer
attention. Using the already established electrodeposition parame-
ters (Section 3.2) FeTTP, CoTTP and MnTTP were electrochemically
deposited using the electrochemical droplet cell. The catalyst spots
were scanned using the RC-SECM mode to elucidate their catalytic
activity towards O2 reduction (Fig. 5). The observed current dif-
ferences represent the changes in local O2 concentration which
is supposed to be indicative of the local catalytic activity of the
metalloporphyrin-modified surface. Due to the fact that the ring
substituents for all investigated metalloporphyrins are the same,
the central metal atom has obviously a severe influence on the
observed local tip current. From Fig. 5 it appears that electrode-
posited MnTPP is the most active catalyst, followed by FeTPP, while
surprisingly CoTPP exhibits only a low activity. This is unexpected
given the fact that Co porphyrins are known O2 reduction catalysts.
3.1. The effect of sample potential
A combination of the TG-SC, SG-TC and RC-SECM modes has
been employed in order to visualize the local electrocatalytic
activity of metalloporphyrin-modified glassy carbon surfaces. The
working principles of this combination have been previously
described in detail [32]. Using this combination it is possible to
visualize sequentially in a single scan on the one hand the local
O2 consumption of a catalytically active surface and on the other
hand the H2O2 produced during the ORR. The GC plate upon which
the sample is electrochemically deposited is polarized at a poten-
tial Es which is negative enough to provoke O2 reduction at the
sample. A schematic representation of the reactions taking place
in the space between the tip and the sample is shown in Fig. 2.
The tip is positioned at a z-distance of about 10 ␮m away from the
sample (using SECM approach curves in the feedback mode). Thus,
the SECM tip forms a local thin-layer cell together with the sample
surface hindering diffusion of redox species to or from bulk solution.
The sample potential, Es, has been varied (−600, −400 and
−200 mV) in order to investigate the effects of Es on the observed
catalytic ORR activity. The influence of Es on the catalytic activity
as observed in the RC-SECM image is displayed in Fig. 3. O2 reduc-
tion over the metalloporphyrin spots can already be observed at a
sample potential of −200 mV. As expected, the observed catalytic
activity for ORR increases at more negative sample potentials. At
−600 mV a good contrast between the active spot and the sig-
nificantly less active non-modified GC surface is obtained. The
3.4. Selectivity for oxygen reduction
Simultaneously with the investigation of the activity for local
O2 reduction, H2O2 production has been visualized by applying

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Fig. 3. RC-SECM images showing the effects of sample polarization potential (Es) on the electrocatalytic O2 reduction for an electrochemically deposited MnTTP spot. Sample
was polarized at −200, −400, and −600 mV, respectively. Color contrasts correspond to the measured cathodic tip current. Lighter and elevated areas correspond to lower
negative O2 reduction current which is indicative of higher electrocatalytic activity at the sample (0.1 M phosphate buffer, pH 7, 25 ␮m Pt tip, tip-to-sample distance 10 ␮m,
data acquisition after 500 ms of the detection pulse).
a sequential pulse profile to the SECM tip as described in Section
2.2. Following the O2 injection pulse the tip potential is switched
to 600 mV to invoke H2O2 oxidation. Hence, an anodic current is
expected at the SECM tip assuming that H2O2 which is formed dur-
ing the ORR will diffuse across the gap from the sample to the SECM
tip following a substrate generation-tip collection scheme. SECM
images for H2O2 production detected during the ORR are displayed
in Fig. 6 for all three metalloporphyrin spots used in this study.
Surprisingly, H2O2 detection gave rise to a negative tip current.
For a better visibility, the H2O2 signal obtained over the MnTPP spot
has been inverted. It is note worthy that the GC plate upon which the
spot is deposited also reduces O2 in a 2-electron transfer reaction
to H2O2 at the applied sample potential of Es = −600 mV. During
the scanning procedure, the H2O2 produced from the GC surround-
ing the metalloporphyrin spot diffuses to the SECM tip where it is
oxidized yielding an increased background current. Whether the
GC plate produces more H2O2 than the catalyst spot or not will
therefore determine the observed current difference which is plot-
ted in the SECM images. A small amount of H2O2 produced thus
would lead to a current lower than that observed over the unmod-
ified GC surface. If the metalloporphyrin spot however produces
more H2O2 than the bare GC, the observed current signal over this
spot will be positive. Since the scans for all investigated metallo-
porphyrins lead to a lower current in H2O2 production than the GC
surface, but a higher O2 reduction current, it can be deduced that
all metalloporphyrin spots produce less H2O2 at the used sample
potential than the bare GC surface. This can serve as a first indication
that O2 reduction over the electrochemically deposited metallo-
porphyrin spots does not occur according to a simple 2-electron
transfer mechanism under formation of H2O2. It can be assumed
that the primarily formed H2O2 is further reduced to H2O at the
metalloporphyrin-modified sample while H2O2 formed at the non-
modified GC surface is not further reduced and can hence diffuse
to the SECM tip. Alternatively, O2 may at least in part (e.g. at some
special active sites) be reduced in a 4-electron transfer reaction to
H2O, thus circumventing the formation of H2O2.
In order to verify these assumptions, the RC-SECM experiments
were repeated but this time on a conducting surface which has a
high overpotential with respect to O2 reduction. An ITO coated glass
slide was modified with 5% (v/v) dimethlydichlorosilane in CHCl3
to increase the hydrophobicity of the surface. Metalloporphyrin
spots were electrodeposited on the ITO coated glass slide using
the droplet cell. In Fig. 7, the related RC-SECM images obtained
both in the O2 reduction and the H2O2 detection potential pulse
sequence is shown. As expected, the current obtained above the
metalloporphyrin spot due to the oxidation of H2O2 at the SECM
tip is now positive. The amount of H2O2 is lowest over the MnTPP
spot and highest over the CoTPP-modified surface. The latter fact
is note worthy keeping in mind the low activity for O2 reduction
at the electrodeposited CoTPP spot observed during the RC-SECM
measurements (see Fig. 5). Even though ITO proved to be the better
Fig. 4. Baseline corrected RC-SECM x-line scans (at y = 1100 ␮m) of electrochemi-
cally deposited MnTTP spots at different deposition cycles for the formation of the
MnTPP deposit (0.1 M phosphate buffer, pH 7, Es = −600 mV, data acquisition after
500 ms of the detection pulse).

A.O. Okunola et al. / Electrochimica Acta 54 (2009) 4971–4978
4975
Fig. 5. (A) RC-SECM O2 reduction images of FeTTP, CoTTP and MnTTP spots. Color contrasts correspond to the measured cathodic tip current. (B) Baseline corrected x-line
scans (y = 1100 ␮m) at each sample (sample polarized at −600 mV, 0.1 M phosphate buffer, pH 7, 25 ␮m Pt tip, tip-to-sample distance 10 ␮m, data acquisition after 500 ms of
the detection pulse).
substrate for H2O2 detection, further experiments with ITO have not
been carried out due to low adhesion of the electrodeposited film,
causing severe experimental problems, and only rarely a sample
was stable for the duration of an SECM experiment.
distilled water and allowed to air-dry before use. Linear sweep
voltammogramms were recorded at 100, 400 and 900 rpm and the
curves subjected to the standard Koutecky–Levich analysis result-
ing in the calculated kinetic current. A plot of the logarithmic kinetic
current density against the potential (Tafel plot) for the different
metalloporphyrin films is displayed in Fig. 8. In this case and in
contrast to the results from RC-SECM measurements, the CoTPP
exhibits the highest catalytic activity for O2 reduction. As can be
seen, the Tafel plots show certain nonlinearity. This is due to the
fact, that the number of active sites involved in the ORR differs with
potential. While at less negative potentials, the oxygen is supplied to
the whole film, leading to a linear Tafel plot, at more negative poten-
tials (i.e., with a highly active catalyst) all the oxygen is reduced at
the outer layer of the thin film. Thus, at more negative potentials
less active sites are involved, leading to a virtual decrease in activity
and a nonlinearity of the Tafel plot.
3.5. RDE measurements
In order to further confirm the applicability of RC-SECM in
the investigation of the electrocatalytic activity of metallopor-
phyrins towards O2 reduction, an RDE was used for voltammetric
measurements under controlled mass-transfer conditions. The
metalloporphyrins were electrochemically deposited on 3 mm
diameter GC electrodes. The electrochemical deposition of the met-
alloporphyrins was done by applying a single pulse at 1.6 V for
90 s. Different deposition parameters compared to the SECM mea-
surements have been used since it turned out that the procedure
established for the droplet cell did not yield satisfactory results
in the case of direct electrodeposition on the 3 mm GC rods. After
deposition, the electrodes were carefully rinsed by dipping in triply
From the Koutecki–Levich plots (see supplementary
information, determined at −0.4 V), the average number of
electrons transferred for each of the electrodeposited films has

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A.O. Okunola et al. / Electrochimica Acta 54 (2009) 4971–4978
Fig. 6. (A) H2O2 oxidation above electrochemically deposited FeTTP, CoTTP and MnTTP spots during ORR. H2O2 produced at the sample is oxidized at the tip at a potential of
+600 mV. The H2O2 oxidation current above the catalyst spot is a relative value to that at the bare glassy carbon surface. These images were obtained sequentially in the same
RC-SECM scan with those shown in Fig. 5 under the same experimental conditions. For better visibility the tip current values were inverted for the MnTTP spot. (B) Baseline
corrected x-line scans at each sample (sample polarized at −600 mV, 0.1 M phosphate buffer, pH 7, 25 ␮m Pt tip, tip-to-sample distance 10 ␮m, data acquisition after 500 ms
of the detection pulse).
been calculated to be approximately 3.5 for MnTTP, 2.2 for CoTTP
and 3.7 for FeTTP. From these values, it is clear that O2 reduction
at CoTTP favors the 2-electron transfer process, hence producing
exclusively H2O2 as already observed in the SECM experiments.
When comparing RC-SECM with RDE results, there seems to
be an inconsistence. With both methods, the detected activity of
the MnTPP and FeTPP deposits was similar. However, in contrast to
RC-SECM, RDE measurements showed CoTPP to be the most active
catalyst for ORR. Both methods, however, qualitatively agree in the
observation that CoTPPproduces much more H2O2 during O2 reduc-
tion than do the other two metalloporphyrins. To explain these
differences, a microscopic picture of the processes occurring in the
gap between tip and sample has to be envisaged (see Fig. 2). In the
case of a complete reduction of O2 to H2O, which is nearly achieved
over MnTPP, the case is rather simple: the amount of O2 in the gap
between sample and tip is lowered due to consumption by MnTPP,
thus the tip current is lowered which reflects the catalytic activity
of MnTPP. In the case of an incomplete reduction of O2, however,
as in the case of CoTPP, H2O2 accumulates in the gap between tip
and sample. Here, not only the remaining O2 is reduced at the tip
but also the produced H2O2. Thus the current at the tip is higher
than it should be if only the remaining O2 was contributing, thus
indicating an apparent lower consumption of O2 by the sample and
hence a lower activity for ORR.
Furthermore, in this case the metalloporphyrin spot is contin-
uously exposed to higher H2O2 concentrations since the sample
potential is applied throughout the whole experiment. A deteriora-
tion of the metalloporphyrin film by H2O2 can be assumed leading
additionally to a decreased catalytic activity of CoTPP with respect
to ORR as seen in the RC-SECM experiment. Preliminary experi-
ments, in which a CoTPP film has been exposed to a 3.3 M H2O2
solution for 40 min, indeed showed a decrease in activity.

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4977
Fig. 7. RC-SECM images of electrocatalytic O2 reduction at an electrochemically deposited MnTTP spot on an ITO coated glass slide. The images were obtained under the same
experimental conditions as for the images displayed in Fig. 5. (A) O2 reduction and (B) H2O2 oxidation (sample polarized at −600 mV, 0.1 M phosphate buffer, pH 7, 25 ␮m Pt
tip, tip-to-sample distance 10 ␮m, data acquisition after 500 ms of the detection pulse).
3.6. H2O2 reduction
As derived from the RDE measurements (see Fig. 8), MnTPP
showed an average number of 3.5 electrons per reduced O2
molecule. Additionally, the MnTPP spot showed the lowest H2O2
oxidation current in the RC-SECM measurements (see Fig. 6). As
pointed out above, besides an intrinsically improved selectivity
of the MnTPP for ORR following a 4-electron transfer pathway, a
sequential electron transfer under intermediate formation of H2O2
can equally explain the observed results. In this case, the multiple
stable oxidation states of the Mn porphyrin may exhibit a facilitated
further reduction of H2O2, potentially even immediately after its
formation. Control experiments were run to verify this hypothesis.
CoTTP and MnTTP films were electrochemically deposited on 3 mm
GC electrodes using the established protocol and the films obtained
Fig. 9. Linear sweep voltammograms showing the electrocatalytic reduction of H2O2
in deaerated 1 M H2O2 in 0.1 M phosphate buffer at (a) CoTTP and (b) MnTTP films.
Scan rate: 50 mV s−1
.
were tested for H2O2 reduction in 1 M H2O2 in 0.1 M phosphate
buffer solution while excluding O2 by purging the solution with Ar
for about 20 min before the experiment. Fig. 9 shows the reduction
of H2O2 at MnTTP and CoTTP films in the absence of oxygen. The
results revealed that MnTTP is indeed significantly more active for
H2O2 reduction even at the same oxygen reduction potential. This
confirms that the MnTTP spot is able to further reduce H2O2 to H2O.
4. Conclusion
The redox competition mode of scanning electrochemical
microscopy is a versatile tool in investigating the local electro-
catalytic activity of surface immobilized O2 reduction catalysts.
Here, its application has been successfully extended to electrode-
posited films of Co, Fe and Mn porphyrins. Mn porphyrins show
Fig. 8. Tafel plots displaying the electrocatalytic activity towards O2 reduction on
glassy carbon electrodes which were modified with electrodeposited (a) FeTTP, (b)
CoTTP and (c) MnTTP (0.1 M phosphate buffer, pH 7).

4978
A.O. Okunola et al. / Electrochimica Acta 54 (2009) 4971–4978
a high selectivity towards water formation in oxygen reduction.
The effects of catalysts loading and central metal ion on the cat-
alytic activity of metalloporphyrins towards O2 reduction have been
demonstrated by RC-SECM measurements with comparable results
to those obtained from RDE measurements. Furthermore, SECM has
been shown to be a tool to obtain information on the selectivity
in oxygen reduction and might thus complement RRDE measure-
ments. Future work will be directed towards the investigation of
a larger library of metallo N4-macrocycles with respect to elec-
trocatalytic activity and selectivity for ORR and a more detailed
elucidation of the influence of the pH-value on the electrocatalytic
activity of electrodeposited metalloporphyrin films.
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The authors are grateful to the MIWFT-NRW for financial support
within the framework of the Nachwuchsgruppe “Mikroelek-
trochemie zur Optimierung von Heterogenkatalysatoren für
PEM-Brennstoffzellen” and to Dr. Thomas Erichsen for software
adaptations and trouble shooting. Ayodele Okunola is grateful to
the German Academic Exchange Service (DAAD) for a PhD scholar-
ship. Dr. Tharamani Chikka Nagaiah is grateful to the Alexander von
Humboldt Foundation for a postdoctoral fellowship.
Appendix A. Supplementary data
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