ESR study of spin adducts of the direct electrocatalytic decomposition of light aliphatic alcohols in a polymer electrolyte fuel cell

Article abstract of DOI:10.1007/s11172-010-0276-z

Spin adducts of methanol and ethanol electrocatalytic oxidation were detected by the spin trap method using a tiny H₂/O₂ fuel cell (FC) designed for ESR in situ with a Nafion/Pt membrane electrode assembly. Spin adducts of intermediates of the direct electrooxidation of ethanol, which have not been observed earlier, were obtained by the variation of oxidation conditions, in particular, the FC potential. The work of the FC was controlled by monitoring the diagnostic curves potential3-current density, power density3-current density, and efficiency-power density.

Full text of DOI:10.1007/s11172-010-0276-z

Russian Chemical Bulletin, International Edition, Vol. 59, No. 8, pp. 1543—1548, August, 2010
1543
ESR study of spin adducts of the direct electrocatalytic decomposition
of light aliphatic alcohols in a polymer electrolyte fuel cell
M. K. Kadirov,a M. I. Valitov,a I. R. Nizameev,a D. M. Kadirov,b and Sh. N. Mirkhanovc
aA. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 73 2253. Eꢀmail: kadirov2004@mail.ru
bKazan State Technological University,
68 ul. Karla Marksa, 420015 Kazan, Russian Federation
cKazan State University,
8 ul. Kremlevskaya, 420008 Kazan, Russian Federation
Spin adducts of methanol and ethanol electrocatalytic oxidation were detected by the spin
trap method using a tiny Н2/O2 fuel cell (FC) designed for ESR in situ with a Nafion/Pt
membrane electrode assembly. Spin adducts of intermediates of the direct electrooxidation of
ethanol, which have not been observed earlier, were obtained by the variation of oxidation
conditions, in particular, the FC potential. The work of the FC was controlled by monitoring
the diagnostic curves potential—current density, power density—current density, and effiꢀ
ciency—power density.
Key words: fuel cell, polymer polyelectrolyte, diagnostic curves, methanol, ethanol, free
radical, ESR spectroscopy, spin trap, spin adduct.
Increasing interest in the development of direct alcoꢀ
hol fuel cells (DAFC) with the protonꢀexchange memꢀ
brane (PEM), especially working on methanol to put in
action electric travel facilities, has recently been observed.1
However, methanol has substantial drawbacks, for inꢀ
stance, it is relatively toxic, its boiling point is low (65 °С),
and it is not primary fuel. Therefore, other alcohols, in
particular, those obtained from biomasses, are considered
as alternative types of fuels.
Ethanol is an attractive fuel for electric travel facilities
based on fuel cells (FC), because it is not so toxic and is
easily prepared by fermentation of sugarꢀcontaining raw
materials. However, there are several unsolved problems,
being a serious obstacle in practical use of DAFC.
1. High activation barrier of the direct electrochemiꢀ
cal oxidation of alcohol (DEOA) at the anode.
2. Contamination of the Pt catalyst with coproduct
carbon monoxide (CO) formed during the DEOA. At
present researchers try to solve these problems by using
various, not purely platinum metallic catalysts, such as
PdRu, PtRu, PtSnX (X = Ni, Co, Mn, V), which someꢀ
times show a lower degree of contamination and better
characteristics.2,3
4. Degradation of the protonꢀexchange membrane,
because it works under rather extreme conditions: high
working temperature (from 90 °С and higher), the action
of metallic catalysis, and aggressive fuels and radical prodꢀ
ucts of their decomposition, particularly, hydroxyl and
superoxide radicals, viz., НО• and НОО•.
The use of ESR technique for detection of the radicals
formed makes it possible to monitor and control reactions
that occur on the surface and and in the bulk of the PEM.
The complete oxidation of ethanol is a 12ꢀelectron
process
EtOH + 3 H2O
2 CO2 + 12 H+aq + 12 e–.
(1)
Thus, many intermediates are involved in the reacꢀ
tion. Therefore, search for stageꢀbyꢀstage investigation of
this process is an urgent task.
A spin trap of the "nitrone" type, namely, 5,5ꢀdiꢀ
methylꢀ1ꢀpyrrolineꢀNꢀoxide (DMPO), was used to
detect shortꢀlived radicals in the working FC for ESR. The
formation of the spin adduct DMPO/X from the DMPO
spin trap and radical Х• is given below (reaction (2)).
The spin adducts DMPO/X exhibit the characteristic
ESR spectra with splittings from nitrogen and the proton
3. Penetration of an alcohol molecule through the
polymer membrane.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1506—1511, August, 2010.
1066ꢀ5285/10/5908ꢀ1543 © 2010 Springer Science+Business Media, Inc.

1544
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
Kadirov et al.
serving as the catalyst and grids of a thin Pt wire. The grids are
arranged at two sides of the catalystꢀcovered membrane and
serve as both electrodes and gas dividers.
(2)
First, fluoroplastic tube 3 is placed on a fluoroplastic cylinder,
which is mounted of body 1 and cap 2 with membrane—electrode
assembly 4 in the inner hole, thus transforming the components
into a single whole. The electrodes are connected to the external
circuit using thin silver connecting wires. Tubes, through which
hydrogen and oxygen are fed to the FC under low pressure and
their excess is removed, are connected to the channels. The side
of the membrane—electrode assembly, where hydrogen comes
through channels 1 and 2, serves as an anode, and the cathode is
the side where oxygen comes. The FC is placed in the ESR
spectrometer cavity in such a manner that the electric field lines
of the cavity standing waves would be perpendicular to the surface
covered with the catalyst of the membrane. Electrochemical
reactions of hydrogen oxidation and oxygen reduction occur on
the catalyst surface between the electrodes and membrane.
Hydrogen atoms split into the component electron and proton
on the catalyst surface in the negatively charged anodic part.
Protons diffuse to the cathode through the membrane, and
electrons move through the external circuit making useful work.
The electrons reach the cathodic side and reduce oxygen atoms
to bivalent ions. These ions form water and heat upon the
reaction with diffused protons. The FC performs the direct
conversion of the fuel energy to electricity, omitting poorly
efficient combustion processes that occur with high losses.
Experiments were carried out with H2/O2, viz., FC for
ESR with the surface density 0.2—1.0 mg cm–2 Pt/Nafion 117.
A 1 М solution of DMPO in water or alcohol (10 μL) was
uniformly deposited on the anodic or cathodic side. Spectra
were recorded 5 min after the beginning of the work of the FC
under the short circuit conditions. Conventionally high potentials
(>0.3 V) were used, and low potentials (<0.1 V) were also used
in particular experiments, which is additionally mentioned during
DMPO
DMPO/X
nearest the radical center. The large database on spin
adducts are available at the site.4
Experimental
The ESR method was used to study processes and products
in the FC.
By analogy to the prototype described earlier,5 we designed a
FC for ESR more reliable and simpler in operation.6 The scheme
of the FC for ESR is shown in Fig. 1.
The FC for ESR consists of body 1, cap 2, fluoroplastic tube
3, and membrane—electrode assembly 4. Channels providing
access of hydrogen and oxygen and removal of their excess are
made in the body and cap. Body 1 and cap 2 are connected in
such a way that metallic tubes 5, which are the ends of two
channels in the body, enter the corresponding two channels in
the cap, and the channels are joined into a single system. The
internal diameter of tube 3 is equal to the external diameter of
the cylindrical FC mounted of two parts. The body and cap have
holes for mounting of membrane—electrode assembly 4 consistꢀ
ing of the Nafionꢀtype membrane with supported Pt particles
5
a
the description. The flow rate of hydrogen was 14 cm3 min–1
and that of oxygen was 7 cm3 min–1
,
.
1
4
ESR spectra were detected on 3ꢀcm EMX (Bruker),
RadioPAN SE/Xꢀ2543, and REꢀ1306 radiospectrometers with
frequencies of 9.3—9.9 GHz. The former two spectrometers
are equipped with a rectangular cavity ТЕ102, and REꢀ1306 has
a cylindrical cavity ТМ110. The polarizing magnetic field was
modulated with a frequency of 100 kHz, and the sensitivity of
the spectrometers was (1—5)•1012 spin T–1. The polarizing
magnetic field induction was measured with ERꢀ035 and Sh1ꢀ1
NMR magnetometers and a Hall sensor. Frequencies in a
microwave cavity were measured with an EiP371 frequency
meter. The standard mode of ESR spectra detection was used
with autotuning of the klystron signal frequency by the measuring
cavity. The choice of detection modes was determined by
requirements of undisturbed recording of the first derivative of
the ESR signal. The measurement inaccuracy of magnetic
parameters depends mainly on inaccuracies of the frequency
meter and magnetometer, stability of resonance conditions, and
ESR linewidth, being 3•10–2 G for HFC constants and 1•10–4
for g factors.
2
4
b
5
1
2
c
Magnetoresonance parameters and relative intensities of
spectral components were determined, as a rule, by computer
simulation of experimental ESR spectra. The SimFonia (Bruker)
and WinSim (NIEH/NIH) simulation programs were used, and
the latter makes it possible to determine the basic parameters of
the isotropic spectrum by automatic fitting.
3
Fig. 1. Scheme of the fuel cell for ESR: a, front projection;
b, sagittal projection; c, fluoroplastic tube. For clarification,
see the text.

Decomposition of aliphatic alcohols in FC
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1545
Results and Discussion
The main reason for worsening of the characteristics is
the absence of carbon support, which sharply increases
the contact surface of platinum catalyst particles with the
polyelectrolytic membrane. However, this fact does not
impede to use the FC for ESR with the purpose of modelꢀ
ing processes in real lowꢀtemperature FC.
A comparison of the polarization curve (see Fig. 2, b)
and the curve of the dependence of the power density on
the current density (see Fig. 2, a) shows that the further
increase in the current density after the power density
maximum was achieved results in a decrease in the FC
power because of the concentration polarization, which is
caused by the depletion of the nearꢀelectrode space with
reactants involved in the redox reactions.
Although the FC is not optimized to the maximum
production of electric power, the efficiency of the input
energy conversion to the useful energy is rather high,
being ~25% (Fig. 2, c), which is due to the high efficiency
of the method of electric energy generation in the FC.
The experimental spectrum (Fig. 3) of the DMPO
spin adduct obtained on the anodic side of the FC for
ESR is a triplet of triplets, where the small multiplet
1 : 1 : 1 is ascribed to one 14N nucleus (aN = 16.2 G)
and the large triplet 1 : 2 : 1 is ascribed to two equivalent
The diagnostic curves (1) of the FC for ESR for the
work with the Н2/O2 fuelꢀoxidation system are shown in
Fig. 2. The curve of the dependence of the power density
on the current density shows (see Fig. 2, a) that the maxiꢀ
mum current density of 34 mW cm–2 is achieved at a
current density of 100 mA cm–2. These values are more
than an order of magnitude lower than the same characꢀ
teristics of the known7 FC with the optimized parameters.
p/mW cm–2
a
35
30
25
20
1
15
10
2
5
0
–5
0
E/V
1.0
20
40
60
80 100 120 i/mA cm–2
b
a
1
0.8
0.6
0.4
0.2
1
2
2
0
20
40
60
80 100 120 i/mA cm–2
3440
3460
3480
3500
3520 H/G
η
b
1
0.7
0.6
0.5
0.4
0.3
0.2
0.1
c
1
2
2
–5
0
5
10
15
20
25 p/mW cm–2
3440
3460
3480
3500
3520 H/G
Fig. 2. Diagnostic curves of Н2/O2 in the absence of (1) and with
the addition (2) to the anodic side of 10 μL of an aqueousꢀ
ethanol (3 : 1) solution for the platinum—Nafion assembly
with the surface density of Pt 1.0 mg cm–2. For clarification, see
the text.
Fig. 3. Experimental ESR spectra of (1) spin adducts detected
in an aqueous DMPO solution on the anodic (а) and cathodic (b)
sides in the Н2/O2 FC for ESR; (2) the corresponding simulated
spectra.

1546
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
Kadirov et al.
1Н protons (аH = 21.8 G). The individual ESR linewidth
is ΔНpp = 2.0 G. The closest values of the constants met in
literature are as follows: aN =16.35 G, аH = 22.18 G (see
Ref. 8) and aN = 16.0 G, аH = 22.0 G (see Ref. 9).
The spin adduct is formed in reaction (2), where
Х = Н.10 Atomic hydrogen is generated from gaseous
hydrogen on the catalyst surface during the oxidation
of the fuel
a
1
2
DMPO/CH2OH
DMPO/H
45%
55%
Pt + Н2
2 Pt—(Н•)ads
2 H+ + 2 e–.
(3)
3420
3440
3460
3480
3500 3520 H/G
By definition, the potential of this reaction is
E°a = 0.000 V. In Eq. (3) the symbol of platinum is italiꢀ
cized to emphasize that the metallic surface is considered
rather than one particular atom regardless of the mechaꢀ
nism of bonding with the hydrogen atom.
b
1
Under the same conditions, but with supporting of an
aqueous DMPO solution on the cathodic side, the quartet
ESR spectrum is observed for the spin adduct DMPO/ОН
(see Fig. 3, b) due to the same splitting from the 14N
nucleus (aN = 16.0 G) and 1Н proton (аН = 16.0 G). Here
ΔНpp = 1.7 G. The most similar published data are the
following: aN = 14.7 G, аН = 14.7 G (see Ref. 11) and
aN = 14.8 G, aH = 14.8 G (see Ref. 12).
2
3420
3440
3460
3480
3500
3520 H/G
Fig. 4. Experimental ESR spectra of (1) spin adducts detected in
an aqueousꢀmethanol DMPO solution on the anodic (а) and
cathodic (b) sides in the Н2/O2 FS for ESR; (2) the total simuꢀ
lated spectra.
The reactions at the cathode are the reduction of oxyꢀ
gen atoms by electrons coming from the anode through
the electric circuit and addition of protons diffused from
the anode through the membrane to form water with the
equilibrium potential E°c = 1.229 V
with the hyperfine coupling (HFC) constants aN = 15.4 G,
aH = 21.8 G is caused, most likely, by the direct anodic
oxidation of methanol, and the •СН2ОН radical is one of
the primary radical products of this process. The literature
data13 for this adduct are the following: aN = 15.92 G,
aH = 22.56 G.
4 H+ + 4 e– + O2
2 H2O.
(4)
The DMPO/ОН adduct is formed due to another
reaction, which inevitably occurs at the cathode with the
equilibrium potential E°c = 0.682 V
The weak spectrum of the same adduct DMPO/
СН2ОН is detected when depositing an aqueousꢀmethanol
solution of DMPO on the cathodic side (see Fig. 4, b).
The free radical •СН2ОН is the result of the attack of the
methanol molecule by the hydroxyl radical. However, no
spin adduct of the hydroxyl radical itself was observed
under these experimental conditions.
The diagnostic curves (2) of the Н2/O2 FC for
ESR with the membrane—electrode assembly 1 mg cm–2
Pt/Nafion obtained by the addition of 10 μL of an aqueꢀ
ousꢀethanol (3 : 1) solution to the anodic side are also
presented in Fig. 2. As can be seen from the polarization
curve, the potential and current density values decrease
sharply and, hence, the power density decreases (more
than tenfold). The efficiency η also decreases, being 15%,
while for the pure Н2/O2 FC it is 25. This worsening
of the FC characteristics is related, first of all, to the
contamination of the Pt catalyst with coproduct carbon
2 H+ + 2 e– + O2
H2O2.
(5)
Hydrogen peroxide decomposes instantly to two hydroxyl
radicals on the surface of sponged platinum, which is
really the platinum supported on Nafion. The results preꢀ
sented agree with the conclusions7 for the FC for ESR
operated under the short circuit conditions.
Light aliphatic alcohols are actively studied as a poꢀ
tential fuel for FC. The deposition of 10 μL of a 1 М
aqueousꢀmethanol solution of DMPO on the anodic side
of the FC for ESR and the work during more than 5 min
under the short circuit conditions gave the experimental
ESR spectrum (see Fig. 4, a). The decoding and autoꢀ
matic fitting of the spectrum indicate that we deal with
the total spectrum of the adducts DMPO/СН2ОН (45%)
and DMPO/H (55%). The origins of the hydrogen adduct
was discussed above, and the DMPO/СН2ОН adduct

Decomposition of aliphatic alcohols in FC
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1547
monoxide (CO) formed during the DAFC, which is the
main obstacle in the application of alcohols as fuel for FC.
As shown by IR spectroscopy,14—17 pure Pt electrodes
are rapidly poisoned with such strongly adsorbed interꢀ
mediates as CO due to dissociative chemisorption, which
is the main reason for worsening of the FC characteristics.
Other compounds were also identified by IR spectroscopy:
reaction intermediates, such as acetaldehyde, and acetic
acid, and other byꢀproducts.16 Using αꢀ(4ꢀpyridylꢀ1ꢀoxide)ꢀ
Nꢀtertꢀbutylnitrone (POBN) as a spin trap and the hydroꢀ
gen/oxygen FC for ESR in an aqueousꢀethanol soluꢀ
tion, we detected the spin adduct POBN/СН(ОН)СН3
on the cathodic side, whereas the spin adducts POBN/
СН(ОН)СН3 and POBN/Н were observed on the anodic
side.11 The appearance of the •СН(ОН)СН3 radical on
the cathodic side was explained by the attack of ethanol
molecules by the hydroxyl radicals, whereas their formaꢀ
tion on the anodic side is due to the partial electrochemiꢀ
cal oxidative decomposition by an alcohol molecule.
The use of DMPO as a spin trap made it possible to
obtain additional information. For example, the adduct
DMPO/ОН (aN =14.1 G, aH = 14.1 G, similar literature
data20: aN = 14.3 G, aH = 14.3 G) is formed along with
the spin adduct DMPO/СН(ОН)СН3 (aN = 15.5 G,
aH = 22.2 G, literature data: aN = 15.5 G, aH = 22.2 G;18
aN = 15.7 G, aH = 22.4 G (see Ref. 19) in the ratio 1 : 4 on
the cathodic side (Fig. 5, a). The spin adducts DMPO/
СН(ОН)СН3 and DMPO/Н were observed on the anꢀ
odic side (see Fig. 5, b), indicating, as published earlier,12
the partial electrochemical oxidative of an ethanol molꢀ
ecule and splitting of a hydrogen molecule on the platiꢀ
num surface to atoms.
a
1
2
DMPO/
CH(OH)CH3
80%
20%
DMPO/OH
3440
3460
3480
3500
H/G
b
1
2
DMPO/
CH(OH)CH3
57%
43%
DMPO/H
3440
3460
3480
3500
3520 H/G
c
1
2
The work of the FC at low potentials found the spin
adducts DMPO/СН(ОН)СН3 (aN = 15.7 G, aH = 23.0 G)
and DMPO/СОСН3 (aN = 14.9 G, aH = 18.8 G, literaꢀ
ture data21: aN = 13.3 G, aH = 17.5 G) on the anodic side
of the FC (see Fig. 5, c), indicating that the following
reactions occur:
DMPO/
CH(OH)CH3
DMPO/
COCH3
50%
25%
25%
DMPO/OH
Pt + С2Н5ОН
+ H+ + e–,
Pt—[СН(ОН)CH3]ads
+
(6)
(7)
3440
3460
3480
3500
H/G
Fig. 5. Experimental ESR spectra of (1) spin adducts detected
in an aqueousꢀethanol DMPO solution on the cathodic (а),
anodic (b), and anodic at low potentials (c) sides in the Н2/O2
FC for ESR; (2) the total simulated spectra.
С2Н5ОН
СН3—СHO + 2 H+ + 2 e–,
Pt—(CO—CH3)ads
Pt + СН3—СHO
+ H+ + e–.
+
(8)
of the spin adducts. The paramagnetic intermediates of
ethanol electrooxidation were specified by the variation
of the FC potential.
In addition, the spin adduct DMPO/ОН (aN = 14.6 G,
aH = 14.5 G) was detected, which is the result, most
likely, of the dissociative adsorption of water
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 09ꢀ03ꢀ12264ꢀ
ofiꢀm), the Presidium of the Russian Academy of Sciꢀ
ences (Program for Basic Research No. 18), and the State
Contract No. 02.352.11.7070.
Pt + H2O
Pt—OHads + H+ + e–.
(9)
Thus, the ESR detection of the spin adducts of
electrocatalytic ethanol oxidation in situ in the FC
allowed us to record the polarization curves and spectra

1548
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
Kadirov et al.
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Received February 19, 2010;
in revised form June 1, 2010