comparative study of aliphatic alcohols electrooxidation on zero-valent palladium complex for direct...
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Comparative study of aliphatic alcoholselectrooxidation on zero-valent palladium complexfor direct alcohol fuel cells
Mohammad Zhiani*, Somayeh Majidi, Hussein Rostami,Mohammad Mohammadi Taghiabadi
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e i n f o
Article history:
Received 27 July 2014
Received in revised form
27 October 2014
Accepted 30 October 2014
Available online xxx
Keywords:
Pd (DBA)2Alcohol electrooxidation
Direct alcohol fuel cell
Alkaline media
* Corresponding author. Tel./fax: þ98 31 339E-mail address: [email protected] (M.
Please cite this article in press as: Zhianpalladium complex for direct alcohol fuj.ijhydene.2014.10.144
http://dx.doi.org/10.1016/j.ijhydene.2014.10.10360-3199/Copyright © 2014, Hydrogen Ener
a b s t r a c t
In this paper, bis (dibenzylidene acetone) palladium (0), Pd (DBA)2, was used as an effective
catalyst for the electrooxidation of different aliphatic alcohols such as ethylene glycol (EG),
ethanol (EtOH), glycerol (Gly), methanol (MeOH)) in the alkaline media. The activity and
stability of Pd (DBA)2 were assessed for the electrooxidation of the mentioned alcohols
using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chro-
noamperometry (CA). Pd (DBA)2 exhibited significantly high anodic current density and
lower onset potential in EtOH oxidation compared to EG, Gly and MeOH. CV and CA results
demonstrated that Pd (DBA)2 is still active even after 200 CV cycles. The tolerance of Pd
(DBA)2 against poisoning intermediate products in case EtOH was higher than other
mentioned alcohols. Finally, Pd (DBA)2 successfully employed as an anode catalyst in a
passive air breathing direct alcohol fuel cell (DAFC). The maximum power density (MPD) of
30, 31, 25 and 18 mW cm�2 were achieved for EG, EtOH, Gly and MeOH, respectively. These
results indicated that Pd (DBA)2 can be a promising anode catalyst for DAFCs.
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Introduction
Compared to H2-fueled fuel cells, direct alcohol fuel cells
(DAFCs) have attracted enormous attention due to the simple
production, storage of liquid fuels and fuel purification [1,2].
Liquid fuels such as; ethylene glycol (EG), ethanol (EtOH),
glycerol (Gly) and methanol (MeOH) not only are more easily
stored and transported but also, have a higher volumetric
energy density and more energy efficiency than gaseous fuels
[3]. The volumetric energy density of EG, EtOH, Gly and MeOH
at 20 MPa is 5.79, 6.31, 6.26 and 4.82 (kWh L�1), respectively.
They are much higher than hydrogen (0.53 kWh L�1).
13263.Zhiani).
i M, et al., Comparativeel cells, International J
44gy Publications, LLC. Publ
Nevertheless, DAFCs have suffered slow kinetics of the
alcohols electrooxidation reaction on the surface of synthe-
sized catalysts in spite of great efforts, which have beenmade
for development of catalyst materials.
Pt and Pt-based catalysts have been extensively investi-
gated and recognized as the traditional catalysts with suitable
catalytic efficiency for the alcohols electrooxidation reaction
[4e6]. However, the toxic intermediate species of the alcohols
electrooxidation reaction would reduce catalytic performance
of the mentioned catalysts. In addition, the high cost and
limited supply of Pt would restrict its application in DAFCs.
As compared to the electrooxidation of MeOH, the oxida-
tion of EG, EtOH and Gly not only depend on the ability of the
study of aliphatic alcohols electrooxidation on zero-valentournal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
ished by Elsevier Ltd. All rights reserved.
Fig. 1 e Structure of Pd (DBA)2 catalyst.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e92
catalyst in oxidative removal of poisoning intermediates, but
also depend on its activity in CeC and CeH bond breakage [7].
Obviously, it is necessary to find high active and stable cata-
lysts for electrooxidation of alcohols in DAFCs.
Notableworks have been done for replacing of the Pt-based
catalysts with Pd-based catalysts toward alcohols oxidation in
DAFCs because, it is more abundant and less expensive than
Pt. Also, Pd-based catalysts have been used successfully for
the alcohols electrooxidation reaction in the alkaline envi-
ronment. For example, many researchers have devoted to
investigating of large variety of conductive materials for the
support of the Pd nanoparticles in alcohols oxidation,
including Vulcan XC-72R carbon black [8], tungsten carbides/
carbon nanotubes [9], ultrahigh-surface hollow carbon
spheres [10], carbonized TiO2 nanotube [11] and carbon mi-
crospheres [12]. Pd nanoparticles supported on multi-walled
carbon nanotubes were scrutinized for the oxidation of
EtOH, Gly or MeOH in the 2 M KOH solution in half cells [13].
The catalyst was very active for the electrooxidation of all
alcohols, with Gly providing the best performance in terms of
the specific current density and EtOH showing the lowest
onset potential.
Compositing or alloying Pd with other elements such as Sn
[14,15], Au [14,16], Ag [17], Ni [18,19], Pt [20e22] and Bi [23]
could potentially enhanced the catalysts activity, lower
degradation of the active surface and overcome the poisoning
effects by reducing surface coverage by adsorbed CO. Chen
et al. [24] found that the catalytic activity of PdeRu is
considerably higher than that of Pd toward the electro-
oxidation of EG, EtOH, and MeOH. The activity sequence of
PdeRu toward the alcohol electrooxidation was
EtOH > EG >MeOH, and PdeRu with 1:1 atomic ratio exhibited
the highest activity. They also compared the activities of
PdeRu and PteRu catalysts for alcohol electrooxidations in
alkaline media. For the electrooxidation of MeOH and EG, the
activity of PdeRu was lower than that of PteRu. For the EtOH
electrooxidation reaction, instead, the activity of PdeRu was
higher than that of PteRu.
To diminishing the CO poisoning effect on the catalyst
surface, metal oxides (CeO2, NiO, Co3O4 and Mn3O4) were
added into Pd-based catalysts in electrooxidation of alcohols
[25e27]. The results indicated that addition of the metal oxide
remarkably improves the activity and CO tolerance of the Pd-
based catalysts in alcohols electrooxidation. All investigations
suggested that Pd-based catalysts could make up for the
deficiency of Pt-based catalysts in DAFCs.
All zero-valent complexes with dl0 electronic configura-
tions have considerable catalytic interest toward alcohols
oxidation. Specially, zero-valent Pd complexes are efficient
catalysts in the field of organic synthesis and in all reactions
involving aryl, vinyl and allelic derivatives because of their
good nucleophilic properties [28]. Among factors influencing
the reactivity, electronic effects are very important, namely
oxidizing of the metal center and tuning the donor/acceptor
properties of the p-coordinated unsaturated hydrocarbon li-
gands and other co-ligands to vary the distribution of the
electron density in the complexes [29].
In our previous work [30], we have successfully employed
on first time zero-valent Pd complex, Pd (DBA)2, as an catalyst
toward the Gly electrooxidation reaction. Results showed that
Please cite this article in press as: Zhiani M, et al., Comparativepalladium complex for direct alcohol fuel cells, International Jj.ijhydene.2014.10.144
Pd (DBA)2 exhibits a superior catalytic performance with high
long-term stability for the direct Gly fuel cell in alkalinemedia.
Pd (DBA)2 was synthesized in 1970 [31]. Different substitu-
tion and addition reactions have been reported along with an
investigation of the potential utility of the Pd (DBA)2 as a
catalyst [32,33]. The complex structure of Pd (DBA)2 is shown
in Fig. 1.
In the present work, the performance of Pd (DBA)2 in
electrooxidation of different aliphatic alcohols (EG, EtOH, Gly
and MeOH) have been investigated in the half cell by cyclic
voltammetry (CV), electrochemical impedance spectroscopy
(EIS) and chronoamperometry (CA) techniques in the alkaline
medium. To determine the performance of Pd (DBA)2 in real
alkaline DAFCs, membrane electrode assembly (MEA) was
fabricated by using the Pd (DBA)2 catalyst in the anode elec-
trode. All the obtained results from the half and whole cell
indicate that Pd (DBA)2 represents an acceptable activity and
performance for DAFCs in the alkaline medium.
Experimental
Half-cell electrochemical investigation
Deposition of Pd (DBA)2 on the glassy carbonA thin film of the catalyst layer on the glassy carbon (GC)
electrode was prepared as follows: a mixture containing
2.0 mg of Pd (DBA)2 (Aldrich, molecular weight of 575 g mol�1,
melting point of 150 �C), 1 mL of 2-propanol, 1 mL of the ultra
pure water (MilliQ, Millipore) and 0.01 mL of 5 wt.% Nafion
solution (Aldrich) were sonicated for 5 min. The well-
dispersed catalyst ink was then quantitatively transferred
onto the surface of the GC electrode by using a micropipette,
and finally was dried in the oven at 60 �C for 15 min. The
catalyst loading on the electrode surface was 0.033 mg cm�2.
All chemical materials (Merck) were analytical grade.
CV, EIS and CA measurementsThe electrochemical activity and stability of Pd (DBA)2 in
alcohol electrooxidation reaction were investigated by CV, EIS
and CA techniques. Electrochemical measurements were
carried out with a conventional three-electrodic cell and an
study of aliphatic alcohols electrooxidation on zero-valentournal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Fig. 2 e Cyclic voltammograms of Pd (DBA)2 in 10 wt.% KOH
in the absence (a) and present (b) of 5 wt.% alcohol at scan
rate of 50 mV s¡1 and 25 �C.
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Auto-lab PGSTAT30 potentiostat/galvanostat at 25 �C. The GC
coated with the catalyst layer was used as a working elec-
trode. A commercial Ag/AgCl electrode and a Pt wire were also
chosen as reference and counter electrodes, respectively. All
electrochemical measurements were carried out in an alka-
line solution containing 10 wt.% KOH. The concentration of
alcohols in the solution was 5 wt.%. The alcohols examined in
the present study were EG, EtOH, Gly and MeOH.
The CV technique was carried out by scanning of the po-
tential between�0.7 and 0.4 V with the scan rate of 50mV s�1.
EIS results were obtained by scanning of the frequencies be-
tween 100 kHz and 0.005 Hz at zero volt potential. The
amplitude of the sinusoidal potential signal was 10 mV. CA
measurements were also carried out at potential of zero volt.
All potentials in this article have been reported versus
normal hydrogen electrode (NHE).
Table 1 e Comparison of electrochemical performance ofalcohol electrooxidation on Pd (DBA)2 in 10 wt.% KOH and5 wt.% alcohols with a scan rate of 50 mV s¡1 at 25 �C.
Alcohol Eonset (V) Ep (V) Jp (mA cm�2)
EG �0.32 0.14 28.4
EtOH �0.5 0.07 20.7
Gly �0.35 0.08 15.8
MeOH �0.26 0.06 5.2
DAFC preparation and evaluation
A mixture of 95 wt.% Pd (DBA)2 and 5 wt.% PTFE (Aldrich) was
ultrasonically dispersed in isopropyl alcohol and double dis-
tillated water for 20 min, to obtain the anode catalyst ink. The
ink was precisely coated on the nickel foam. The cathode
electrode was made as follows: the cathode catalyst ink was
prepared by mixing Hypermec™ K14 (Acta SpA) with 10 wt.%
Please cite this article in press as: Zhiani M, et al., Comparativepalladium complex for direct alcohol fuel cells, International Jj.ijhydene.2014.10.144
PTFE (on dry weight basis) and water, and coated on the car-
bon cloth (E-TEK) as a diffusion medium. The both anode and
cathode electrodes were dried at 60 �C for 30 min in the oven.
The anode and cathode catalyst loading were 5 and
3.5 mg cm�2, respectively. An anion-exchange membrane
(Tokuyama) was placed between two electrodes to obtain an
MEA. The MEA was then assembled into an air-breathing
DAFC for performance measurements according to V. Bam-
bagioni et al. [4].
The DAFC was tested using alkaline fuels containing al-
cohols (5 wt.% EG, 5 wt.% EtOH, 5 wt.% Gly and 5 wt.% MeOH)
and 5 wt.% KOH. All tests were performed by the fuel cell test
station (Scribner model 850e) at room temperature and
ambient pressure. The DAFC was activated according to the
procedure mentioned in Ref. [34]. Prior to recording the po-
larization curves for each fuel, the DAFC was conditioned at
the open circuit voltage (OCV) for 30 min for the stabilization.
Polarization curves were obtained by scanning the cell voltage
from the OCV to 250 mV with the scan rate of 5 mV s�1 after
reaching the steady state.
Results and discussion
Electrocatalytic activity and stability of Pd (DBA)2
The electrocatalytic activity of the Pd (DBA)2 was examined by
the CV technique in the electrooxidation of aliphatic alcohols;
EG, EtOH, Gly and MeOH. Firstly, the Pd (DBA)2 leaching test
was carried out by CV in the KOH solution. Fig. 2a shows 5th
and 80th cycles of the Pd (DBA)2 in 1 M KOH. As it can be seen,
the position and magnitude of the forward and backward
peaks did not change during 80 cycles. Therefore, it could be
concluded that Pd (DBA)2 did not leach to the KOH solution.
Fig. 2 represents CV of Pd (DBA)2 in the solution containing
10wt.% KOH in the absence (Fig. 2a) and present (Fig. 2b) of the
mentioned alcohols.
By comparing the CVs of Pd (DBA)2 in the absence and
present of the alcohols (Fig. 2a and b), a number of anodic
peaks were observed which could be clearly assigned to the
alcohols electrooxidation peaks on Pd (DBA)2. Generally, the
oxidation peak in the forward scan is corresponding to the
oxidation of freshly chemisorbed species coming from the
alcohol adsorption. In the reverse scan, the anodic peak is
correlated with the removal of carbonaceous species not
completely oxidized in the forward scan [35]. These results
demonstrate that Pd (DBA)2 is an active catalyst for electro-
oxidation of all mentioned alcohols which could be used in
alkalineDAFCs. For better comparison the activity of Pd (DBA)2
study of aliphatic alcohols electrooxidation on zero-valentournal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Fig. 3 e Cyclic voltammograms (1st, 30th, 70th, 100th, 130th, 170th and 200th cycle) of Pd (DBA)2 in the solution containing
10 wt.% KOH and 5 wt.% a) EtOH, b) EG, c) Gly and d) MeOH at room temperature with the scan rate of 50 mV s¡1.
Fig. 4 e IR spectra of Pd (DBA)2 before and after 50 CV cycles of Gly oxidation.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e94
Please cite this article in press as: Zhiani M, et al., Comparative study of aliphatic alcohols electrooxidation on zero-valentpalladium complex for direct alcohol fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.144
i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e9 5
in electrooxidation of the mentioned alcohols, all CVs were
analyzed and their results were listed in Table 1. As it can be
seen in Table 1, Pd (DBA)2 exhibits higher activity for EtOH
electrooxidation compared to other alcohols. Although the
anodic peak current density of EtOH electrooxidation on Pd
(DBA)2 is less than EG but, it occurred at more negative onset
potential compared to EG. The worst result was obtained by
the MeOH electrooxidation reaction in terms of the peak
current density and onset potential. Based on the anodic
oxidation peak current density and onset oxidation potential,
the activity sequence of Pd (DBA)2 in alcohols electrooxidation
is as follow; EtOH > EG > Gly > MeOH.
Electrocatalytic activity of the Pd (DBA)2 was also investi-
gated by CV during 200 cycles. Fig. 3aed shows consecutive
CVs for electrooxidation of EG, EtOH, Gly and MeOH,
respectively.
In all cases, the anodic peak current densities increased by
increasing of the CV cycle number. However, this trend in the
growth of the peak currents declines with further increasing
Fig. 5 e Products obtained by electro-oxidation of (a) M
Please cite this article in press as: Zhiani M, et al., Comparativepalladium complex for direct alcohol fuel cells, International Jj.ijhydene.2014.10.144
of the CV cycle number. The enhancement of the peak current
density is related to increasing the activity of Pd (DBA)2 during
potential cycling [36], and the subsequent decline in the peak
current density is attributed to two factors; (i) consumption of
the alcohol during the long time scanning, and (ii) poisoning
the catalyst by the intermediates produced during the alcohol
oxidation reaction [37]. Fig. 3 also indicates that Pd (DBA)2 still
exhibits well defined forward and reverse peaks for the alco-
hols oxidation reaction after 200 cycles.
After 200th CV cycles, the oxidation current densities of EG,
EtOH, Gly and MeOH declined approximately 19%, 14%, 52%
and 33%, respectively (Fig. 3). Higher reduction in the peak
current density of the Gly electrooxidation reaction could be
related to higher Gly oxidation intermediates. Fig. 4 indicates
FTIR spectra of Pd (DBA)2 before and after 50 CV cycles of Gly
oxidation. It shows that the catalyst structure does not change
during Gly electrooxidation. Thus, the subsequent decline in
the peak current density of Gly electrooxidation is attributed
to the catalyst poisoning.
eOH; (b) EtOH; (c) EG and (d) Gly in alkaline media.
study of aliphatic alcohols electrooxidation on zero-valentournal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Fig. 7 e Equivalent circuit of Nyquist plots analysis in
DAFCs made by Pd (DBA)2 anode catalyst with fuel
containing 5 wt.% KOH and aliphatic alcohols.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e96
Fig. 5aed shows products obtained for electrooxidation of
MeOH, EtOH, EG, and Gly, in alkaline media, respectively. The
electrooxidation of alcohols, especially polyalcohols, can be
mechanistically complicated by the occurrence of parallel
steps. In Fig. 5, it can be seen the oxidation of Gly is more
complex than other alcohols [4,35]. As it described in Fig. 3 and
Fig. 5, most of the Gly oxidation intermediates can be adsor-
bed on the catalyst active sites and therefore reduces the
catalyst active surface area. It causes a reduction in the peak
current of Gly oxidation during CV cycling. These results show
that Pd (DBA)2 catalyst has highest stability in electro-
oxidation of EtOH and lowest stability toward Gly
electrooxidation.
EIS spectra of alcohols electrooxidation on Pd (DBA)2
The EIS technique was employed to compare the impedance
characteristics of EG, EtOH, Gly and MeOH electrooxidation
reaction on Pd (DBA)2. Fig. 6 indicates the Nyquist plots of the
alcohols electrooxidation reaction on Pd (DBA)2 in the solution
containing 10 wt.% KOH and 5 wt.%mentioned alcohols at the
constant potential of zero volt, after 1st (Fig. 6a) and 200th
cycle (Fig. 6).
Fig. 6 e Nyquist plots of Pd (DBA)2 catalyst after the cyclic
voltammograms 1st (a) and 200th cycle (b) in the solution
containing 10 wt.% KOH and 5 wt.% the alcohol at potential
of 0 V.
Please cite this article in press as: Zhiani M, et al., Comparativepalladium complex for direct alcohol fuel cells, International Jj.ijhydene.2014.10.144
Although, general shapes of all Nyquist plots shown in
Fig. 6 are similar, however, the arcs diameter of the Nyquist
plots are different depend on the alcohols electrooxidation
kinetic and their structures. The Nyquist diagram comprises a
depressed semicircle at high frequencies, which is related to
the combination of the charge transfer resistance (Rct) of al-
cohols electrooxidation and the constant phase element (CPE),
followed by a straight line with a slope of nearly 45�. The latter
is due to the mass transport as modeled by the infinite War-
burg impedance [38]. The intersection of the imaginary
impedance with the real impedance reflects the solution
resistance (Rs). Based on the impedance spectra, Rct and Rs can
be extracted using the equivalent circuit set up in Fig. 7 [38].
The Nyquist plots were fitted by Zview software. The extrac-
ted resistances after 1st and 200th cycles for all alcohols were
summarized in Table 2. As it can be seen in Table 2, EtOH
electrooxidation on Pd (DBA)2 has the lowest Rct value and
MeOH has the highest value after 1st cycle. The small charge
transfer resistance of EtOH electrooxidation has a good cor-
relation with the obtained CVs results in
section Electrocatalytic activity and stability of Pd (DBA)2.
Table 2 also indicates that after 200th cycle, the values of Rs
and Rct increased for all alcohols, especially for Gly which is
related to the acceleration rate of the catalyst poisoning by
many intermediate products of Gly electrooxidation during
200 cycles [39,40]. However, Rs of the MeOH solution did not
change too much compared to other alcohols during 200 cy-
cles, because its intermediate products is lower than others
(Fig. 5), and also Pd (DBA)2 activity for MeOH electrooxidation
is lower than other alcohols, as it demonstrated in section 3.1.
Chronoamperograms of alcohols electrooxidation on Pd(DBA)2
The chronoamperometry of Pd (DBA)2 was performed in the
solution containing 10 wt.% KOH and 5 wt.% alcohols at zero
volt of potential. Fig. 8aeb shows chronoamperograms of Pd
Table 2 e The obtained resistances values from Nyquistplots of the alcohols electrooxidation reaction on Pd(DBA)2 catalyst after 1st and 200th cycles.
Alcohol After 1 cycle After 200 cycle
Rct (U cm2) Rs (U cm2) Rct (U cm2) Rs (U cm2)
EG 117 0.25 433 4.8
EtOH 95 0.05 294 1.2
Gly 937 1.1 7121 7.8
MeOH 974 5.9 4383 6.6
study of aliphatic alcohols electrooxidation on zero-valentournal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Fig. 8 e Chronoamperograms of alcohols electrooxidation
on the Pd (DBA)2 catalyst after the CV cycles (a) 1st and (b)
200th in the solution containing 10 wt.% KOH and 5 wt.%
alcohol at potential of 0 V.
Fig. 9 e IeV and power density curves of alkaline passive
air-breathing DAFC made by Pd (DBA)2 anode catalyst with
fuel containing 5 wt.% KOH and aliphatic alcohols at room
temperature.
i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e9 7
(DBA)2 in the alcohols oxidation reaction after 1st and 200th
cycles, respectively.
CA curves of all alcohols display the decay of the current
density along the time, which is related to the remaining the
adsorbed produced intermediates on the catalyst surface [36].
As it is shown in Fig. 8, current density of Pd (DBA)2 after 200th
cycle behaved with a more gently decreasing trend, because
Pd(DBA)2 is poisoned by Gly electrooxidation byproducts and
intermediates as it described before.
According to the obtained CA results, Pd (DBA)2 has higher
activity and better poisoning tolerance toward EtOH electro-
oxidation. The long-term stability results of Pd (DBA)2 in the
alcohols electrooxidation reaction are consistent with ob-
tained CV and EIS results.
Performance study of Pd (DBA)2 in a passive alkaline DAFC
A MEA based on the Pd (DBA)2 anode catalyst, commercial
FeeCo Hypermec™ cathode catalyst and Tokuyama A-006
anion-exchange membrane was used in a passive air breath-
ing DAFC.
The polarization and power density curves were obtained
in an air breathing alkaline DAFCwith different static alkaline
fuels; 5 wt.% EG, 5 wt.% EtOH, 5 wt.% Gly and 5 wt.% MeOH
(Fig. 9).
Please cite this article in press as: Zhiani M, et al., Comparativepalladium complex for direct alcohol fuel cells, International Jj.ijhydene.2014.10.144
Among all mentioned fuels, EtOH showed the highest OCV
as well as the highest maximum power density (MPD)
(31 mW cm�2). The sequence of the produced power in the
DAFC was EtOH > EG > Gly > MeOH which is similar to that
obtained results in the half-cell. In the alkaline passive air
breathing DAFC fed by the aqueous solution of EG, EtOH, Gly
and MeOH, the value for OCV is 0.68, 0.71, 0.66 and 0.51 V,
respectively.
Table 3 indicates MPD of different DAFCs based on Pd
(DBA)2 anode catalyst compared to the MPD produced by
DAFCs reported earlier in the literature [4,35,41e48]. As it can
be seen in Table 3, MPD value of Pd (DBA)2 is comparable to
what observed by typical studies on Pt, Pd and Pd compound
in the alcohols oxidation reaction roughly under the same
conditions. Although, in DAFCs based on Pd (DBA)2 were used
air instead of O2 as an oxidant gas.
From Table 3 one can distinguish that Pd (DBA)2 exhibits
higher activity for EtOH electrooxidation in comparison with
other alcohols. These results are in line with ones reported
previously for the alcohols oxidation reaction in DAFCs.
Based on these results, Pd (DBA)2 could be a good candidate
for alkaline DAFCs specially for the alkaline direct EtOH fuel
cell.
Conclusions
In this work, Pd (DBA)2, was used as an effective catalyst for
the electrooxidation of the different aliphatic alcohols such as
EG, EtOH, Gly and MeOH in alkaline media. Electrocatalytic
activity, stability and impedance of Pd (DBA)2 towards
mentioned alcohols oxidation were evaluated by CV, CA and
EIS techniques in alkaline media. Pd (DBA)2 exhibited signifi-
cantly high anodic current density and lower onset potential
in EtOH electrooxidation compared to EG, Gly and MeOH. The
activity sequence of the alcohols oxidation on Pd (DBA)2 was
EtOH> EG >Gly >MeOH. CV and CA results demonstrated that
study of aliphatic alcohols electrooxidation on zero-valentournal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Table 3 e MPD of different alkaline DAFCs.
Fuel Oxidant Anode Loading(mg cm�2)
Cathode Membrane Solution(KOH/alcohol)
MPD(mW/cm2)
T(oC) Ref.
EG O2 Pd/MWCNT 1 Hypermec K-14 Tokuyama A-006 2M, 5 wt% 5 25 4
EG O2 Pt 2 Pt/C loading 2 ADP 1M/2M 4 20 41
EG O2 PdNi 2 Hypermec K-14 Tokuyama A-006 1M/1M 35 60 42
EG air Pd(DBA)2 5 Hypermec K-14 Tokuyama A-006 10, 5% wt 25 Current study
EtOH O2 Pt/Ru 3 Pt/C AHA 1M/1M 58 25 43
EtOH O2 Hypermec K14 2 Hypermec K-14 Tokuyama A-006 1M/1M 12 30 44
EtOH O2 Pd/MWCNT 1 Hypermec K-14 Tokuyama A-006 2M, 10 wt% 18 25 4
EtOH O2 Pde(NieZn)/C 1 Hypermec K-14 Tokuyama A-006 2M, 10 wt% 65 25 45
EtOH/ air Pd(DBA)2 5 Hypermec K-14 Tokuyama A-006 2M, 5% wt 15.7 25 Current study
Gly O2 Pd/MWCNT 1 Hypermec K-14 Tokuyama A-006 2M, 5 wt% 6 20 4
Gly O2 Pt/C or Pd/C _ Pt/C ADP 4M/2M 4.2 or 2.4 25 46
Gly O2 Pde(NieZn)/C 1 Hypermec K-14 Tokuyama A-006 2M, 10 wt% 15 25 35
Gly air Pd(DBA)2 5 Hypermec K-14 Tokuyama A-006 2M, 5 wt% 7.2 25 Current study
MeOH O2 Pd/MWCNT 1 Hypermec K-14 Tokuyama A-006 2M, 10 wt% 7 25 4
MeOH Air Pt/C 2 Pt/C Nafion 1M/2M 4.5 60 47
MeOH Air PtRu/C 4 Pt/C Tokuyama A-006 1M/7M 12.8 25 48
MeOH O2 Pde(NieZn)/C 1 Hypermec K-14 Tokuyama A-006 2M, 10 wt% 22 25 35
MeOH Air Pd(DBA)2 5 Hypermec K-14 Tokuyama A-006 2M, 5% wt 7 25 Current study
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e98
Pd (DBA)2 is still active even after 200 cycles. The tolerance of
Pd (DBA)2 against poisoning intermediate products in case
EtOH was higher than other mentioned alcohols.
The performance of the MEA made by Pd (DBA)2 in the
anode side was evaluated in an alkaline passive air breathing
DAFC fed by the aqueous solution of the mentioned alcohols.
By considering the obtained peak power density, 31mWcm�2,
by the static fuel solution (EtOH) and air breathing condition at
room temperature, one can safely conclude that Pd (DBA)2exhibits acceptable activity in the anode side of the alkaline
direct EtOH fuel cell.
Acknowledgments
This work was carried out in electrochemical laboratory of
Isfahan University of Technology (IUT). The authors would
like to thanks the research council of IUT and special thanks
to Dr B. Rezaei and Dr K. Karami for their supporting. The
authors gratefully acknowledge the financial support of Iran
National Science Foundation through the project No. 93012722
and the support of fuel cell steering committee and Iranian
nano technology initiative council.
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