Fabrication and performance evaluation of a novel membrane electrode assembly for DMFCs

Article abstract of DOI:10.1039/c5ra21389d

In the current study, a nano-scale perovskite LaNi_0.5Fe_0.5O₃ (LaNi_0.5Fe_0.5O₃NPs) was synthesized by a rapid ultrasonic-assisted co-precipitation method and characterized by various techniques. A modified glassy carbon electrode with Pd nanoparticles (PdNPs) and LaNi_0.5Fe_0.5O₃NPs dispersed into chitosan (CH) polymer was prepared and its catalytic activity toward methanol oxidation was investigated. Based on the electrochemical studies, the PdNPs-LaNi_0.5Fe_0.5O₃NPs-CH nanocomposite showed considerable activity for methanol oxidation in comparison to PdNPs-CH. Then a gas diffusion layer (GDL) was prepared with the ratio of 40% wt graphene carbon and 60% wt polymer mixture (consisting of 35% wt polyvinyl chloride and 65% wt polyaniline) for the micro-porous layer and a teflonated carbon cloth as the macro-porous substrate. A direct methanol fuel cell was designed, assembled and tested with the suggested PdNPs-LaNi_0.5Fe_0.5O₃NPs-CH nanocomposite as anodic catalyst and the synthetic GDL under several different conditions.

Full text of DOI:10.1039/c5ra21389d

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Fabrication and performance evaluation of a novel
membrane electrode assembly for DMFCs†
Cite this: RSC Adv., 2016, 6, 563
*
Meissam Noroozifar, Zahra Yavari, Mozhgan Khorasani-Motlagh, Tahere Ghasemi,
Seyed-Hamzeh Rohani-Yazdi and Majid Mohammadi
In the current study, a nano-scale perovskite LaNi0.5Fe0.5O3 (LaNi0.5Fe0.5O3NPs) was synthesized by a rapid
ultrasonic-assisted co-precipitation method and characterized by various techniques. A modified glassy
carbon electrode with Pd nanoparticles (PdNPs) and LaNi0.5Fe0.5O3NPs dispersed into chitosan (CH)
polymer was prepared and its catalytic activity toward methanol oxidation was investigated. Based on the
electrochemical studies, the PdNPs-LaNi0.5Fe0.5O3NPs-CH nanocomposite showed considerable activity
for methanol oxidation in comparison to PdNPs-CH. Then a gas diffusion layer (GDL) was prepared with
the ratio of 40% wt graphene carbon and 60% wt polymer mixture (consisting of 35% wt polyvinyl
chloride and 65% wt polyaniline) for the micro-porous layer and a teflonated carbon cloth as the macro-
porous substrate. A direct methanol fuel cell was designed, assembled and tested with the suggested
PdNPs-LaNi0.5Fe0.5O3NPs-CH nanocomposite as anodic catalyst and the synthetic GDL under several
different conditions.
Received 14th October 2015
Accepted 15th December 2015
DOI: 10.1039/c5ra21389d
www.rsc.org/advances
comprehensive chemical kinetics and electrochemistry as well
as in important processes such as: the oxidation of alcohols,
1. Introduction
electrochemical evolution/reduction of oxygen, reduction of
nitrogen oxides, the chemical oxidation of CO and hydrocar-
bons.6 The catalytic activity toward alcohol oxidation was
affected by both the absorption of alcohol as well as the elec-
trical and oxygen ion conductivities of perovskites. The pres-
ence of transition metal ions improves the oxidation of alcohol.7
Different polymers like chitosan are very convenient supports
for dispersing catalytic materials at the molecular level, in such
a way that each catalytic centre will be accessible to the reactive
molecules. Chitosan offers great advantages due to its non-
toxicity and suitable adhesion to the electrode substrate.8 The
addition of chitosan into noble metal containing catalysts can
signicantly improve the electrode performance for alcohol
oxidation. In this study, diverse approaches for the improve-
ment of an effective, low-cost and multifunctional anodic
catalyst for direct methanol-based fuel cells followed. In a direct
methanol fuel cell, the polymer membrane is sandwiched
between two sheets of porous backing media referred to as gas
diffusion layers or current collectors. The gas diffusion layer
(GDL) is a porous material that facilitates contact between the
methanol solution and the anodic catalyst (or the air and the
cathodic catalyst).9 Typically, a GDL consists of a macro-porous
substrate and a micro-porous layer of carbon. Carbon paper and
carbon cloth are usually used as the macro-porous substrate
due to its low ohmic resistance and high gas permeability. The
studies demonstrated that carbon cloth led higher performance
of fuel cell due to less water saturation and higher porosity.10
The micro-porous layer consists of carbon powder and increases
Fuel cells are planned to be reliable candidates for competing
against other energy generation sources. During decades of
research, different types of fuel cells have been developed.
Among these types, the direct methanol fuel cell (DMFC) seems
to be promising as a battery replacement for portable electronic
devices such as mobile phones and laptops. It has attracted
more and more attention due to its long operation time, high
energy density, no need for charging and simple system design.1
Methanol as a liquid fuel has many benets in comparison to
hydrogen, including easier production, simpler structure,
higher energy content, safer storage/transportation and lower
operating temperatures in fuel cells.2 One of the most serious
weaknesses associated with the development of DMFCs is poor
catalytic activity of the anode catalysts.3 To overcome this
problem, many electrocatalysts have been investigated. Among
them, Pt is known as the best anode material, but it is expensive
and its surface is poisoned by adsorption of methanol oxidation
by-products such as carbon monoxide (COads).4 Palladium has
a similar lattice constant and valence shell electronic congu-
ration to platinum.5 One promising approach to overcome the
high cost and easily accumulation of Pd-based materials is
to use nanoscale complex oxides, e.g., perovskites. These
compounds have been used as catalytic materials in
Department of Chemistry, University of Sistan and Baluchestan, P.O. Box 98135-674,
Zahedan, Iran. E-mail: mnoroozifar@chem.usb.ac.ir; Fax: +98-54-3344-6888; Tel:
+98-54-3344-6464
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra21389d
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electronic conductivity between the catalyst layer and the were measured with respect to this reference electrode. The
macro-porous substrate and provides non-permeable support electrolyte was a mixture of potassium hydroxide (KOH) and the
during catalyst deposition.11 There is oen hydrophobic agents corresponding methanol with a known concentration. A soni-
like polytetrauoroethylene at the GDL to allow an effective cator Euronda-Eurosonic 4D was employed for ultrasound-
management of water ow (methanol solution in at the anode assisted co-precipitation. X-ray powder diffraction (XRD) anal-
and water produced at the cathode sides).12 Extensive experi- ysis was conducted on a Philips analytical PC-APD X-ray
mental have done on the micro-porous layer of GDL to inves- diffractometer with graphite monochromatic CuKa radiation
tigate the effects of carbon powder types and loadings.13 (l ¼ 1.54 A) to verify the formation of products. The Fourier
˚
Graphene is pure carbon and also a suitable candidate for the transform infrared (FT-IR) spectra of the samples were taken
usage at the micro-porous layer. It is remarkably strong for its using a JASCO-460 FT-IR spectrometer. The surface morphology
very low weight and it conducts heat14 and electricity with great and the status of the sample were observed with scanning
efficiency.
electron microscopy (SEM) and energy dispersive X-ray (EDX)
In this study, in order to provide more effective reaction sites analysis, which was equipped with an SEM instrument (SEM-
for the catalysis; nickel-substituted lanthanides orthoferrite Philips XL30). TEM image of nanocomposite was taken using
˚
perovskite, LaNi0.5Fe0.5O3 catalysts were synthesized as nano- a Philips CM120 transmission electron microscopy with 2.50 A
meter particles (LaNi0.5Fe0.5O3NPs). The activity of Pd nano- resolution. A Zeiss EM900 transmission electron microscopy
particles (PdNPs) in the presence of LaNi0.5Fe0.5O3NPs into the with 80 keV energy was used for TEM image of graphene sheet.
appropriate amount of chitosan (CH) polymer as multifunc- SEM image of synthetic GDL was taken using a TESCAN VEGA
tional catalyst for methanol oxidation was evaluated by the LMU scanning electron microscope. An air brush kit model
electrochemical techniques. The effect of some experimental MBD-116C was employed for spray the micro-porous layer in
factors on the anodic current density and the potential of carbon cloth and nanocatalyst ink on carbon cloth-diffusion
methanol oxidation were studied. Aerwards, the graphene was layer. The resistance of the samples was measured from the
prepared through
a mild and one-step electrochemical lms by the four-point equipment with 1 mm xed distance
approach. Also, the polyaniline was synthesized with the assis- between points. The distance between the points was xed at
tance of ammonium persulfate and phenylamine. For GDL 1.0 mm. For graphene synthesis, a LEYBOLD power supply
preparation, aer a number of experiments, we found an model 667827 was employed.
appropriate ratio of graphene, polyaniline and polyvinyl chlo-
ride (PVC) as the micro-porous layer. The resulting carbon ink
was sprayed and deposited onto one side of the carbon cloth
pretreated with polytetrauoroethylene (hydrophobic agent) as
the macro-porous substrate. Finally, the functions of PdNPs-
LaNi0.5Fe0.5O3NPs-CH and PtNPs-CH nanocomposites as
anodic and cathodic catalysts were investigated in single cell
and assembled direct methanol fuel cell. The fuel cell polari-
zation curves were plotted for mentioned DMFCs.
2.3. Preparation of LaNi0.5Fe0.5O3 nanocrystalline
Aqueous solutions of 0.5 M ferric chloride (FeCl3$6H2O), 0.5 M
nickel chloride (NiCl2$7H2O) and 1 M lanthanum chloride
(LaCl3$7H2O) were prepared and mixed together in 1 : 1 molar
ratio. Deionized distilled water was employed as the solvent in
order to avoid the production of impurities in the resulted
product. 2 ml octanoic acid was added to the mixture as
a surfactant. The solution was stirred followed by adding NaOH
(1.5 M) drop by drop to keep the pH at 7–8. Aer complete
precipitation, the liquid precipitate was irradiated with ultra-
sonic waves. The sonication time was found effective in the
formation of the crystalline phase of nanoparticle. In our
experimental condition were 30 min/60 ꢀC (300 W and 40 kHz).
Aer cooling at room temperature, the resulting products were
centrifuged for 15 min at 403 RCF and washed with distilled
water and ethanol several times to remove the excess surfactant
2. Experimental
2.1. Materials
All reagents were purchased from Merck Company and used
without further purication. A natural polymer, Chitosan, ([2-
amino-2-deoxy-(1-4)-b-D-glucopyranose]), with medium molec-
ular weight of 400 000 Da was purchased from Fluka. All solu-
tions were prepared by doubly distilled water. The rods of
graphite were purchased from Gioconda Company. The teo-
nated carbon cloth, commercial GDL and Naon117 membrane
were purchased from Asian Hydrogen New Science Company
(H2 Engine Company), manufacturing & development Isfahan
science and technology town, Isfahan, Iran.
ꢀ
from the solution and then, were dried at 100 C for 8 h. The
product was calcinated at 800 ꢀC for 4 h to let the product self-
ignite and burn off the organic compound in the material. The
product was cooled to room temperature and the LaNi0.5Fe0.5
O3NPs was obtained.
-
2.4. Preparation of nanocomposites
2.2. Instrumentation
A chitosan solution (2 mg mLꢁ1) in 1% acetic acid solution was
All electrochemical studies were carried out in a conventional prepared, due to the poor solubility of the polymer, the mixture
three-electrode cell at room temperature. The working electrode was stirred to dissolve entirely and kept for 24 h, and the
substrate was a GC electrode with 0.03 cm2 surface area. A solution was ltrated to remove any impurity before use. 25 mL
platinum electrode and an Hg/HgO electrode were used as the 37% HCl solution was added to 5 mg of PdCl2 salt. During this
counter and reference electrodes, respectively. All potentials mixture, Pd2+ ions were transformed to PdCl4 that it had
2ꢁ
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better solubility. The result was mixed with a known amount of adsorbed at a constant potential (ꢂ50 mV) during 15 min. In the
CH solution using a rotary aperture (100 rpm) for 60 min. Then, following, N2 gas is purged to remove dissolved CO during 30
50 mL of freshly prepared aqueous solution of 2.5 M NaBH4 was min. Three potentials scans at 50 mV sꢁ1 between ꢁ0.6 and 0.4
added to the blend. It was kept stirring for 90 min to achieve V vs. Hg/HgO were recorded for each of the electrodes. The rst,
complete reduction (eqn (1)) of Pd nanoparticles.
second and third scans were recorded before the CO purging,
aer the N2 injection and aer second scan recording,
respectively.
4NaBH4 + H2PdCl4 / Pd + 4NaCl + 3B2H6 + 3H2
(1)
The resulting suspension (PdNPs-CH) was kept at room
conditions. 2 mg LaNi0.5Fe0.5O3NPs was mixed with 0.5 mL CH
2.8. GDL preparation
solution using an ultrasonic bath for 15 min (LaNi0.5Fe0.5O3- We carried out a number of experiments to nd the proper ratio
NPs-CH).
between graphene (as a base material), polyaniline (as an
adhesive and conductive agent) and PVC (as a exible, hydro-
phobic agent). They were compared in terms of adhesion, ex-
ibility, hydrophobicity and electrical conductivity. In order to
synthesis of carbon ink as micro-porous layer, 0.21 g polyaniline
were mixed with a known amount of methylisobutylketone as
solvent using ultrasonic waves for 20 min. Then, graphene (for
example 0.20 g) and PVC (for example 0.08 g) were added to the
suspension. The mixture was sonicated followed by adding
solvent drop by drop insofar as the suspension is suitable for
spraying. The resulting carbon ink was layer by layer sprayed
onto one side of teonated carbon cloth and followed by drying
at 40 ꢀC, consecutively. The synthetic GDL was hot-pressed
under the conditions of 85 ꢀC, 200 psi for 1 min. The sheet
resistance of three different ratio of graphene : polymer, 40 : 60,
60 : 40 and 70 : 30 was 35.4, 24.4 and 19.7 U sqꢁ1, respectively. A
proper ratio 60 : 40 of the graphene : polymer was chosen as
optimum because of low adhesive for 70 : 30 and high resis-
tance of 40 : 60 ratio.
2.5. Electrode preparation
The GC electrode with 0.03 cm2 surface area was polished with
0.05 mm alumina slurry to a mirror nish surface and was then
rinsed with distilled water. The GC electrode was subsequently
sonicated in a mixture of water : ethanol (9 : 1 v/v) for 3 min. The
GC electrode was cleaned and activated in an electrochemical cell
containing 1 M H2SO4 solution by using cyclic voltammetry (CV)
technique between ꢁ1.5 and +1.5 V at a scan rate of 100 mV sꢁ1
until a stable cyclic voltammetric prole (z15 times) was ob-
tained (only here, due to acidic media, standard calomel elec-
trode was used as reference electrode). The clean GC electrode
was modied with ‘layer by layer’ method. For electrode modi-
cation, 10 mL PdNPs-CH was spread by a pipette onto the
prepared GC electrode and the electrode was put under infrared
radiation for 10 min (GC/PdNPs-CH). Then, 10 mL LaNi0.5Fe0.5
-
O3NPs-CH was spread onto the prepared electrode. The prepared
electrode was kept under infrared radiation for 10 min. During
this period, the solvent evaporation led to the formation of
a deposited catalyst layer on the GC electrode surface (GC/PdNPs-
LaNi0.5Fe0.5O3NPs-CH).
2.9. Fuel cell operation
Initially, the prepared of suspension nanocatalysts were sprayed
uniformly onto synthetic and commercial GDLs by using the air
brush kit. The anodic catalyst loading was (2 mg cmꢁ2) PdNPs–
(1 mg cmꢁ2) LaNi0.5Fe0.5O3NPs onto the carbon cloth-diffusion
layer (PdNPs-LaNi0.5Fe0.5O3NPs-CH/CC-DL); whiles, the active
surface area of the single cells were 5 and 13 cm2. The PtNPs-CH
was rst prepared as indicated in 2.4 Section from the chloride
precursor (PtCl2) (eqn (2)) then sprayed onto GDLs.
2.6. Polyaniline and graphene synthesis
The employed method for synthesizing of the poly aniline
polymer was based on described procedures.15 An aqueous
solution of (NH4)2S2O8 was added to a solution of aniline
dissolved in aqueous HCl (1 M) and was mixed for 1 h at 5 ꢀC.
The formed precipitate was removed by ltration, washed with
1 M HCl and dried at room condition. In a typical preparation
by Liu et al.,16 the graphene nanosheets were prepared by the
electrochemical functionalization of graphite. 40 g poly-
phosphoric acid and 200 mL two-distillated water were
mixed as the electrolyte. A static potential of 3 V was applied
between the two rods of graphite by power supply in prepared
electrolyte at room temperature for 10 h. Aer corrosion, the
graphene sheets were obtained at the reactors bottom. The
black precipitate was centrifuged for 20 min at 806 RCF and
washed with distilled water several times to remove the poly-
phosphate from the solution. The resulting precipitate was
6NaBH4 + H2PtCl6 / Pt + 6NaCl + 3B2H6 + 4H2
(2)
The CC-DL with prepared PtNPs-CH with 4 mg cmꢁ2 Pt-
loading was used as cathode. A Naon117 membrane was pre-
treated by successive dipping in 5% wt H2O2 solution, doubly
distilled water, 8% wt H2SO4 solution and nally in doubly
distilled water again, at 80 ꢀC, 30 min each step. The membrane
and electrode assemblies (MEAs) were made by hot-pressing the
anode and the cathode electrodes and membrane in the middle
under the conditions of 90 ꢀC, 200 psi for 2 min. The MEAs were
assembled in the single cells for performance evaluation. Two
different single cells with 5 and 12 cm2 surface area were
employed in this study. The ow rate of methanol as fuel was
controlled using a peristaltic pump. I–V curves were obtained
ꢀ
dried at 80 C for 12 h.
2.7. CO striping
The reaction of CO oxidation on the modied electrodes was galvanostatically with an electronic load, EL200P, Daegil, and
evaluated with using the CO stripping technique. CO is controlled via a personal computer.
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of plane bending of the O–H group, respectively. For precursor,
all infrared bands of octanoic acid are observed but shi to
a lower wave number as seen in Fig. 1A (middle spectrum). In
the FT-IR spectrum, the nal product (bottom spectrum), two
strong absorptive bands at nearby 594 and 473 cmꢁ1 were
correspond to Fe–O stretching vibration and O–Fe–O bending
vibration of LaNi0.5Fe0.5O3NPs, respectively.
3. Result and discussion
3.1. LaNi0.5Fe0.5O3NPs characterization
Fig. 1A shows the FT-IR spectra of octanoic acid (up), the
product before calcination (medial), and LaNi0.5Fe0.5O3NPs
perovskite aer calcination (bottom), in the frequency range
from 4000 to 400 cmꢁ1. In Fig. 1A (up spectrum), the 2600–3000
cmꢁ1 broad band is attributed to the O–H stretching vibration
of octanoic acid. While the peak at 1712 cmꢁ1 is the typical
band of the carbonyl group. The bands at 1459 and 1379 cmꢁ1
are associated with the bending of CH2 and CH3 bonds
respectively. The absorption peaks detected at 1210–1320 cmꢁ1
and 937 cmꢁ1 were attributed to the stretching of C–O and out
The size, structure and morphology of the LaNi0.5Fe0.5O3NPs
were considered using SEM (see Fig. 1B). The SEM micrograph
was approved that the grains were composed of tremendously
ne particles and were basically secondary agglomerates of
primary particles. It was observed that nanoparticles started to
become evident within the grains and agglomerates and sphere-
like LaNi0.5Fe0.5O3NPs obtained were uniform in both
morphology and size (ꢂ45 nm).
In Fig. 1C shows the XRD pattern of LaNi0.5Fe0.5O3 perov-
skite. The analysis of XRD pattern showed crystallizes in the
˚
orthorhombic system with main diffraction peak at d ¼ 2.75 A
((1 2 1) plane). No peaks attributable to Fe2O3 weren't observed
and the initial compound was decomposed to single-phase
LaNi0.5Fe0.5O3, entirely. The sharpening of the peaks is due to
the high crystallinity of the LaNi0.5Fe0.5O3NPs. The peak
broadening conrmations in XRD patterns were shown that the
LaNi0.5Fe0.5O3 particles are nano-size. The mean size of the
LaNi0.5Fe0.5O3NPs (D) was estimated by XRD line broadening
using the Debye–Scherer equation:17
0:89 ꢃ l
D ¼
(3)
B ꢃ cos q
˚
which l is the X-ray radiation wavelength (1.54 A), q is the Bragg
angle and B is the breadth of the observed diffraction line in its
half intensity maximum. The mean crystallite size of the
LaNi0.5Fe0.5O3NPs was obtained 36.05 nm.
EDX was performed on LaNi0.5Fe0.5O3NPs for more
demonstrations. The EDX spectrum given in Fig. 1D shows the
presence of lanthanum, nickel and ferric as the component
elements.
3.2. The images investigation of PdNPs-LaNi0.5Fe0.5O3Ps-
CH, graphene and GDL
Fig. 2A displays the TEM micrograph of the PdNPs-LaNi0.5
-
Fe0.5O3NPs-CH nanocomposite on a scale of 20 nm. It was
realized from Fig. 2A that PdNPs were reduced with sizes of 3–
6 nm. Due to small particle size, the surface to volume ration
may be high. This property is an important parameter for
heterogeneous catalysts. The TEM micrograph of a graphene
nanosheet has shown in Fig. 2B. It is clear that the length and
width of graphene nanosheet were 1.76 and 1.32 mm, respec-
tively. Fig. 2C shows the SEM micrograph of the synthetic
GDL.
The two main layers of GDL (macro-porous substrate
and the micro-porous layer) are clearly observed in Fig. 2C.
The porosity of GDL is typically attributed to carbon
cloth structure. For synthetic GDL, as presented in Fig. 2C,
micro-porous layer is densely coated on the macro-porous
substrate.
Fig. 1 (A) FT-IR spectra of octanoic acid (up), product before (middle)
and after (bottom) calcination, (B) SEM, (C) XRD and (D) EDX of
LaNi0.5Fe0.5O3NPs.
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3.3. Electrochemical behaviour of GC/PdNPs-
LaNi0.5Fe0.5O3NPs-CH electrode
The CV curve of the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrode
has obtained (Fig. S1† solid line) in 1 M KOH aqueous solution
in the potential range of ꢁ1 to 0.4 V vs. Hg/HgO with a scan rate
50 mV sꢁ1. The character of the cyclic voltammograms and the
equivalent potentials were in excellent accordance to those re-
ported for noble metal-based catalysts.18 An electron was
produced on the surface of PdNPs due to be more positive
electrode potential. The anodic peak at E ¼ ꢁ0.63 V vs. Hg/HgO
is corresponded to desorption of adsorbed hydrogen.19
PdꢁHabs/ads + OHꢁ / Pd + H2O + eꢁ
(4)
The PdNPs oxide such as PdO, PdOH, Pd(OH)2, PdO2, and
PdO3 were formed in the positive potential region.20 The peak
centering at ꢁ0.30 V vs. Hg/HgO was corresponded to the
reduction of the palladium(II) oxide during the cathodic sweep.
Pd–O + H2O + 2eꢁ / Pd + 2OHꢁ
(5)
3.4. Electrochemical active surface area and accelerated
durability test of the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH
electrode
One of the functional factors for the determination of cata-
lytic activity is the electrochemical active surface area (EAS) of
palladium particles, especially for surface reactions like the
oxidation of heterogenic catalysis. The EAS of PdNPs can
be calculated by CV techniques in alkaline solution (1 M KOH
at room temperature). The voltammogram of H2 adsorption/
desorption was used to determine the EAS of PdNPs for the
GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrode (Fig. S1† solid
line). In actuality, the CV curve in the absence of methanol
was attributed to the double layer region of the modied
electrode. The columbic charge (QH) for hydrogen desorption
was employed for calculation the palladium active surface of
the electrode. The value of QH was considered as the
mean value between the amounts of charge exchanged
during the electrochemically adsorption and desorption of
H2 on Pd sites. The columbic charge (QH) for hydrogen
adsorption/desorption was employed for Pd EAS calculation
of the modied electrodes. The QH value is considered as
the mean value between the amounts of charge exchanged
during the electrochemically adsorption (Q00H) from ꢁ0.6 to
ꢁ1 V vs. Hg/HgO and desorption (Q0H) from ꢁ1 to ꢁ0.4 V vs.
Table
1 The calculated results from CV curves of GC/PdNPs-
LaNi0.5Fe0.5O3NPs-CH electrode after 1 and 100 cycles sweep with the
palladium loading of 0.31 mg cmꢁ2 in 1 M KOH aqueous solution at 50
mV sꢁ1
No. cycle QH (mC cmꢁ2
)
EAS (m2 gꢁ1
)
DPd ꢃ 10ꢁ3 (m2 gꢁ1
)
% loss
27.90
Fig. 2 The images (A) TEM of PdNPs-LaNi0.5Fe0.5O3NPs-CH, (B) TEM
of Graphene and (C) SEM of synthetic GDL.
1
8.69
6.26
6.63
4.78
4.99
3.59
100
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Hg/HgO of H2 on PdNPs sites according to following
equation;
À
Á
Q0H þ Q00
H
QH
¼
(6)
2
It was calculated by measuring the area under the peak at
potential rang of hydrogen adsorption/desorption on modied
electrodes.21
The EAS was calculated from eqn (7):
QH
EAS ¼
(7)
S ꢃ L
In this equation, L is the palladium loading (mg cmꢁ2) on the
electrode and S is a parameter relating the charge to area; this is
equal to 4.2 C mꢁ2 for palladium. This represents the charge
required for oxidation of a monolayer of H2 on PdNPs. The
values of QH and calculated EAS has reported for GC/PdNPs-
LaNi0.5Fe0.5O3NPs-CH electrode with the palladium loading of
0.31 mg cmꢁ2 in 1 M KOH aqueous solution with a scan rate
50 mV sꢁ1 in Table 1. In the previous work,19 the QH was re-
ported 3.40 mC cmꢁ2 for PdNPs lm electrode with a palladium
loading 1.10 mg cmꢁ2. It is shown that the addition of
LaNi0.5Fe0.5O3NPs to PdNPs-CH nanocomposite increased the
EAS amount. As a result, the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH
electrode has an improved dispersion and less size of PdNPs.
Tejuca expressed that on oxides with more oxidized metal
centres (e.g. Fe and Ni) could be involved in hydrogen adsorp-
tion.22 The dispersion of PdNPs was termed as the fraction of
surface-active Pd atoms in all of Pd atoms which can be
achieved:23
Fig. 3 CO oxidation (solid line) on modified electrodes: (A) GC/
PdNPs-CH and (B) GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH with the palla-
dium loading of 0.31 mg cmꢁ2 at 50 mV sꢁ1 in 1 M KOH solution. The
cycles recorded before and after the CO oxidation process are
depicted in
and
line, respectively.
EAS
It was calculated 27.90% for GC/PdNPs-LaNi0.5Fe0.5O3NPs-
CH aer 100 potential cycle. It can be seen in Table 1 that the
DPd value for the mentioned electrode decreases from 4.99 ꢃ
10ꢁ3 to 3.59 ꢃ 10ꢁ3 m2 gꢁ1 aer ADT, indicating a notable
decrease of the PdNPs active surface area due to Pd sintering
and the dissolution of Pd metal.
The recorded cyclic voltammograms of both modied elec-
trodes during the reaction of CO oxidation are represented in
Fig. 3A and B. As it can be seen from Fig. 3, there are three
considerable differences for CO oxidation on the GC/PdNPs-CH
and GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH modied electrode;
(I) The onset potential of the CO stripping peak at the GC/
PdNPs-LaNi0.5Fe0.5O3NPs-CH (ꢁ0.49 V) shied by approxi-
mately 20 mV to the positive direction compared with that GC/
PdNPs-CH (ꢁ0.47 V) electrode.
DPd
¼
(8)
À
Á
1
2
ꢃ NA ꢃ 4p ꢃ rPd
MPd
where MPd is the relative molecular weight of Pd (106.42 g
molꢁ1), NA is Avogadro number (6.02 ꢃ 1023) and rPd is Pd
atomic radius (0.14 nm). The DPd value was obtained 4.99 ꢃ
10ꢁ3 m2 gꢁ1. Furthermore, accelerated durability test (ADT) of
the nanocatalyst was carried out by continuously applying
potentials between ꢁ1 to 0.4 V with a scan rate 50 mV sꢁ1 in
1 M KOH aer 100 potential cycles. In general, the durability of
the modied electrode was tested by performing the potential
cycling as ADT in alkaline solution. The voltammogram of H2
adsorption/desorption was used to determine the change of
the PdNPs electrochemical surface area for GC/PdNPs-
LaNi0.5Fe0.5O3NPs-CH electrode by comparing H2 desorption
regions before and aer 100 potential cycles (Fig. S1† dash
line). The EAS value for the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH
electrode was decreased from 6.63 to 4.78 m2 gꢁ1 aer the
ADT. The obtained %ADT parameter in Table 1 is calculated
from eqn (9):
(II) The current density of the CO stripping at GC/PdNPs-
LaNi0.5Fe0.5O3NPs-CH (2.00 mA cmꢁ2) increased three times
compared with that GC/PdNPs-CH (0.64 mA cmꢁ2) electrode.
(III) The peak potential of the CO stripping at GC/PdNPs-CH
(¼+0.22 V) shied by 330 mV to the negative direction
compared with that GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH (¼ꢁ0.11)
electrode.
ðEASrun1 ꢁ EASrun100
Þ
%ADT ¼
ꢃ 100
(9)
EASrun1
These results can be comparable with the literature.24 Based
on Fig. 3A and B, the QCO,ads obtained 0.37 and 0.86 mC cmꢁ2
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for GC/PdNPs-CH and GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH elec- methanol and 1 M KOH aqueous solution. For comparison of
trodes, respectively. This variation is attributed to the removal the addition effect of the LaNi0.5Fe0.5O3NPs on the current
of CO poisoning on the PdNPs surface by the surface oxygen of density and the oxidation potential, palladium loading (¼0.31
the LaNi0.5Fe0.5O3NPs that is in adjacency of metal particles.
mg cmꢁ2) is similar on the GC/PdNPs-CH and GC/PdNPs-
The ECSA of the catalysts has been determined by calcu- LaNi0.5Fe0.5O3NPs-CH electrodes.
lating the area under the curve of Pd(II) oxide reduction (Qp)
3.5.1. Cyclic voltammetry. Fig. 4 represents the cyclic vol-
tammograms for the GC/PdNPs-CH and GC/PdNPs-LaNi0.5
Fe0.5O3NPs-CH electrodes in 0.80 M methanol and 1 M KOH
aqueous solution at a scan rate of 50 mV sꢁ1
using the following equation:25
-
QP
ECSA ¼
(10)
.
0:405 ꢃ l
As seen from Fig. 3, two oxidation peaks on the two modied
electrodes, which were associated to the oxidation of methanol
(Jf) and the corresponding produced intermediates (Jb). Table 2
shows the electrochemical data of the cyclic voltammograms on
GC/PdNPs-CH and GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrodes
for methanol oxidation in 0.80 M methanol and 1 M KOH with
a scan rate 50 mV sꢁ1. The onset potential is oen invoked to
evaluate the efficacy of the electrocatalysts. It is know that in the
oxidation of methanol, more negative onset potential is pref-
erable to increase the overall power of the fuel cell.26 As shown
in the obtained results for the modied electrodes (Table 2), the
corresponding onset potential of the GC/PdNPs-CH electrode
decreases according to the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH
electrode with adding the perovskite to the nanocatalyst. This
point shows that the LaNi0.5Fe0.5O3FNPs incorporation
improves the kinetics of reaction and effects on the start-up of
direct methanol fuel cell. The current density of methanol
oxidation on GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrode (Jf ¼
19.10 mA cmꢁ2) is comparable with GC/PdNPs-CH electrode (Jf
¼ 4.77 mA cmꢁ2) and the others containing noble metal at
similar concentration of methanol as electro-active material.27
This result explains that PdNPs-LaNi0.5Fe0.5O3NPs-CH nano-
composite has superior activity for methanol oxidation in
alkaline media. It is clear that both the synergistic effect
between PdNPs and structure effects of the LaNi0.5Fe0.5O3NPs
enhanced the catalytic activity of the PdNPs-LaNi0.5Fe0.5O3NPs-
CH nanocomposite for methanol oxidation. The current density
increasing of the methanol oxidation may be attribute to the
removal of CO poisoning on the PdNPs surface by the surface
oxygen of the LaNi0.5Fe0.5O3NPs perovskite that were in adja-
cency of metal particles (see ESI† for mechanism of methanol
oxidation on PdNPs-LaNi0.5Fe0.5O3NPs-CH nanocomposite).
Likely, the catalytic activity of the nanoparticles toward meth-
anol oxidation was affected by both the absorption of methanol
as well as the electrical and oxygen ion conductivities of
perovskite. The presence of transition metal ions improved the
oxidation of methanol. The surface oxygen of the LaNi0.5Fe0.5O3
nanoparticles in adjacency of metal particles removed the CO
poisoning on the surface of the palladium nanoparticles. In
where Qp is the columbic charge in mC and determined by
calculating the area under the curve of the Pd(II) oxide reduc-
tion. a proportionality constant, 0.405 mC cmꢁ2 is assumed for
the charge required for reduction of Pd(II)oxide monolayer and l
is the loading of Pd on the surface of the electrode in mg cmꢁ2
.
ECSA values were found to be 1.15 and 8.44 m2 gꢁ1 for the rst
cycle of GC/PdNPs-CH and GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH
electrodes, respectively. Also, the ECSA was calculated 6.33 m2
gꢁ1 for GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrode aer 100
cycles. The results show that the ECSA of PdNPs-LaNi0.5Fe0.5
-
O3NPs-CH is higher than PdNPs-CH, indicating higher elec-
trocatalytic activities and more effective utilization of Pd
nanoparticles over the surface.
3.5. The investigation of the methanol oxidation on GC/
PdNPs-LaNi0.5Fe0.5O3NPs-CH electrode
Electrochemical properties of modied electrode have been
investigated by cyclic voltammetry, controlled potential coul-
ometry and anodic Tafel polarization techniques in 0.80 M
Fig. 4 The cyclic voltammograms of methanol oxidation on the GC/
PdNPs-CH (dash line) and GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH (solid
line) electrodes in 0.80 M methanol and 1 M KOH aqueous solution
with a scan rate 50 mV sꢁ1
.
Table 2 The electrochemical data for methanol oxidation as estimated from CV curves of the modified electrodes in 0.80 M methanol and 1 M
KOH aqueous solution with a scan rate 50 mV sꢁ1
Electrodes
Eonset (V vs. Hg/HgO)
Ef (V vs. Hg/HgO)
Jf (mA cmꢁ2
)
Eb (V vs. Hg/HgO)
Jb (mA cmꢁ2
)
GC/PdNPs-CH
GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH
ꢁ0.67
ꢁ0.88
ꢁ0.14
ꢁ0.13
4.77
19.10
ꢁ0.30
ꢁ0.29
1.71
10.23
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3.5.3. Anodic Tafel polarization. The intrinsic activity of
PdNPs-LaNi0.5Fe0.5O3NPs-CH nanocomposite towards the
methanol oxidation was measured at 50 mV sꢁ1 in room
temperature and alkaline media. Fig. 5B shows Tafel curve of
methanol oxidation on GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH elec-
trode in 0.80 M methanol and 1 M KOH aqueous solution at the
potential range of ꢁ2 to 0 V vs. Hg/HgO with a scan rate 50 mV
sꢁ1. The values of the exchange current density (J0) and polari-
zation resistance (Rp) were obtained by extrapolating the Tafel
line to the equilibrium potential (Eeq ¼ ꢁ0.98 V vs. Hg/HgO) for
the methanol oxidation reaction. The amount of the Tafel slope
(b ¼ 0.10 V decꢁ1) was calculated from the linear portions of
a log current density vs. potential plot. It was dependent on the
coverage of pre-adsorbed CO on the methanol oxidation as
anodic reaction. The value of the Tafel slope suggests that the
adsorption of methanol on the rare earth-including catalyst was
more difficult compared to that on the noble metals.28 Tripkovic
´
et al.29 proposed that several factors such as electrode surface
structure, surface absorbents, pH values and their interactions
affect the measured Tafel slope. The Rp and J0 were calculated
3193 U and 0.22 mA cmꢁ2 for the oxidation of the 0.80 M
methanol on the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrode.
The Rp could be a criterion for charge exchange on the surface
anodic catalyst. The J0 value is corrected to the real electro-
chemical surface area of the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH
electrode.
Fig. 5 (A) The chronoamperograms of methanol oxidation on the (a)
GC/PdNPs-CH and (b) GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrodes
at 1.4 V as. Hg/HgO potential and (B) Tafel curve of GC/PdNPs-
LaNi0.5Fe0.5O3NPs-CH electrode potential in 0.80 M methanol and 1 M
KOH aqueous solution.
3.6. The effect investigation of different parameters towards
the methanol oxidation
We investigated the methanol oxidation on the GC/PdNPs-
LaNi0.5Fe0.5O3NPs-CH electrode at the different conditions such
as: the methanol concentration, scan rate and the potential
addition, metal hydroxides formed from hydrolysis can be range.
reduced to metal nanoparticles.
3.6.1. The methanol concentration. The cyclic voltammo-
3.5.2. Controlled potential coulometry. The addition effect grams of methanol oxidation on the GC/PdNPs-LaNi0.5Fe0.5
-
of functionalized LaNi0.5Fe0.5O3NPs to PdNPs-CH nano- O3NPs-CH electrode at the different methanol concentrations
composite on the current density of the methanol oxidation was were obtained in 1 M KOH aqueous solution and the potential
investigated by chronoamperometry technique for a 0.80 M range ꢁ1 to 0.4 V vs. Hg/HgO with a scan rate 50 mV sꢁ1 at
methanol concentration solution. Fig. 5A displays the chro- room temperature. The changes of the peak potential and
noamperograms of methanol oxidation on the GC/PdNPs-CH current density of methanol oxidation at forward and back-
and GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrodes at a similar ward sweep were investigated (Fig. S2A†). It could be observed
potential (E ¼ 1.4 V vs. Hg/HgO) in 0.80 M methanol and 1 M that increasing the methanol concentration up to 0.80 M
KOH aqueous solution for time ¼ 50 s. As it was evident in the increases the anodic and cathodic current density and the
absence and presence of LaNi0.5Fe0.5O3NPs, the current–time current density of methanol oxidation did not have any
curve was crossed at 138.64 and 251.28 mA cmꢁ2 amounts of considerable increase in methanol concentrations higher
current density at start time (t ¼ 0 s), respectively. The amount than 0.80 M. We believe that this effect could be due to the
of exchanged charge is associated with the area under the curve saturation of active sites on the surface of the catalyst.
that it was 38.99 and 52.77 mC for methanol oxidation on the Moreover, the oxidation reaction of methanol on modied
GC/PdNPs-CH and GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrodes electrode is controlled by the diffusion process of methanol
at the time of 50 seconds, respectively. The chronoampero- towards the catalyst surface.
grams show that the exchanged charge was increased in the
While the methanol concentration increases from 0.08 to
presence of LaNi0.5Fe0.5O3NPs. Consequently, the composite 0.80 M, the Ef and Eb shi toward the positive direction from
containing of LaNi0.5Fe0.5O3NPs was showed the higher cata- ꢁ0.16 to ꢁ0.14 V vs. Hg/HgO and ꢁ0.34 to ꢁ0.29 V vs. Hg/HgO,
lytic activity catalytic for methanol oxidation. The results of respectively. This may be attributed to the poisoning of PdNPs
chronoamperometry technique were consistent with the ob- catalyst when increasing the methanol concentration and the
tained previous consequences of cyclic voltammetry technique. oxidative removal of the adsorbed intermediates which would
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shi to a more positive potential. The plot of the logarithm of potential limit to more positive potentials, Eb and Jb were
current density vs. the logarithm of methanol concentration decreased; whiles it had a negligible effect on Jf and was
(see also Fig. S2B†) gives the reaction order.30 The reaction effectless on Ef. When limit of potential scanning was increased
order, derived from the slope of the straight line were 0.37 for from 0.3 to 0.6 V vs. Hg/HgO, for methanol oxidation, Jf/Jb
the positive and 0.77 for the negative sweep for methanol increased from 1.84 to 4.74 and also, Eb decreased from ꢁ0.26
oxidation on the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrode.
to ꢁ0.36 V vs. Hg/HgO. The forward and reverse peak currents
3.6.2. The scan rate. The cyclic voltammograms of meth- ratio (Jf/Jb) has been used to determine the tolerance of the GC/
anol oxidation on the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH elec- PdNPs-LaNi0.5Fe0.5O3NPs-CH electrode to CO as an interme-
trode at the different scan rates: 50–200 mV sꢁ1 were obtained diate of methanol oxidation. It seems that the increase of the
in 0.80 M methanol and 1 M KOH aqueous solution and the limit of potential scanning decreased the poisoning rate of the
potential rang ꢁ1 to 0.4 V vs. Hg/HgO at room temperature. PdNPs catalyst. Probably, by increasing the nal positive
Based on the CV curves, the plots of the peak potential vs. ln(n) potentials, the conversion of PdNPs to PdO was accelerated and
(Fig. S3A†) and the anodic peak current density amounts of caused a decrease in Ib.33
methanol oxidation vs. the square root of the sweeping rate
(Fig. S3B†) has been displayed. The peak potential of methanol
3.7. The performance investigation of single cells
oxidation (Ef) amplies with increasing the scan rate, and
a linear relationship (R2 ¼ 0.98) has been obtained between Ef
and ln(n). This shows that the methanol oxidation is an irre-
versible charge transfer process. The plot of Ep and ln(n) was
a straight line with a slope:31
In order to investigate the improved performance of the PdNPs-
LaNi0.5Fe0.5O3NPs-CH catalyst as an oxygen storage component,
single cell tests were carried out for the MEA made as the anode,
and the PtNPs-CH catalyst as the cathode for CH3OH/O2. The
performance of the proposed single cell system depends on the
efficiency of the electrochemical reaction at the interface
between the solid phase of electrodes (anode and cathode) and
liquid and gas phases in anode and cathode sides. The different
parameters are the effective on the mentioned reaction. We
investigated the effect of different conditions such as: temper-
ature, the concentration of methanol solution as fuel, the ow
rate of carrier stream and the concentration of the NaOH in the
carrier stream. Also, we compared the performance of the single
cell with a commercial GDL and a synthetic GDL at the same
condition.
dEp
R ꢃ T
¼
(11)
dðln nÞ ð1 ꢁ aÞ ꢃ n ꢃ F
where a stands for the electron transfer coefficient, character-
izing the effect of electrochemical potential on the activation
energy of an electrochemical reaction. The slope of Ep vs. ln(n)
plot was 0.02 V. The a value was calculated as 0.80 (n ¼ 6 and T
ꢀ
¼ 20 C) which is close to 1. This indicates that the methanol
oxidation on the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH electrode has
super kinetics. By increasing the applied sweeping rate, the
anodic peak current density of methanol oxidation has been
increased. The linear relationship (R2 ¼ 0.98) between the
square root of the scan rate and the peak current density was
estimated further the control of the overall methanol oxidation
reaction via the mass transport of the methanol from the bulk
solution to the electrode surface.32
3.7.1. The temperature. In order to investigate the
temperature effect on the performance of the fuel cell, various
temperatures ranging between 60–90 ꢀC (PO ¼ 2 bar; [meth-
2
anol] ¼ 1 M; ow rate ¼ 1.6 ml minꢁ1; [NaOH] ¼ 0 M and
Naon® 117 membrane) were tested at a single cell with 5 cm2
surface area, and the synthetic GDL. The I–V characteristics
3.6.3. The potential rang. The cyclic voltammograms of
methanol oxidation on the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH
electrode were obtained in the conditions that the nal poten-
tial was varied between 0.3 and 0.6 V vs. Hg/HgO in 0.80 M
methanol and 1 M KOH aqueous solution with a scan rate 50
mV sꢁ1 at room temperature. Table 3 shows the electrochemical
data as estimated from the corresponding CV curves. As the
result for the methanol oxidation, with increasing the nal
Table 3 The effect of upper limit of potential in positive direction on
methanol oxidation on the GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH in 0.80
M methanol and 1 M KOH aqueous solution
Forward sweep
Backward sweep
Potential Ef
range
Jf
Eb
Jb
(V vs. Hg/HgO) (mA cmꢁ2) (V vs. Hg/HgO) (mA cmꢁ2) Jf/Jb
Fig. 6 Electrical performances (cell voltage (solid) and power density
(hollow) against current density) of 5 cm2 DMFC at different temper-
atures (B) 60, (D) 70, (,) 80 and (>) 90 ꢀC using the PdNPs-
LaNi0.5Fe0.5O3NPs-CH anodic, PtNPs-CH cathodic catalysts and
synthetic GDL ([methanol] ¼ 1 M, flow rate ¼ 1.6 ml minꢁ1, [NaOH] ¼
ꢁ1 to 0.3 ꢁ0.14
ꢁ1 to 0.4 ꢁ0.14
ꢁ1 to 0.5 ꢁ0.14
ꢁ1 to 0.6 ꢁ0.14
23.35
23.51
22.60
23.45
ꢁ0.26
ꢁ0.29
ꢁ0.33
ꢁ0.36
12.68
9.62
7.65
4.95
1.84
2.44
2.95
4.74
0 M, PO ¼ 2 bar and Nafion® 117 membrane).
2
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were obtained for various methanol concentrations (PO ¼ 2 bar;
2
temperature ¼ 70 C; ow rate ¼ 1.6 ml minꢁ1; [NaOH] ¼ 0 M
ꢀ
and Naon® 117 membrane) at a single cell with 5 cm2 surface
area, and the synthetic GDL. The polarization curves have been
showed at Fig. 7. The open circuit voltages of the single cells
were 0.92, 0.95 and 0.98 V for the 1, 2 and 3 M methanol solu-
tion, respectively. The effect of the methanol concentration was
the positively and negligibly on the open circuit voltage. Aer
polarization loss, the potential of single cells were stabled at
0.35 V for 1 M, 0.39 V for 2 M and 0.41 V for 3 M methanol
solution.
The observed maximum power density of single cells were
16.79, 19.20 and 20.01 mW cmꢁ2 at the 1, 2 and 3 M methanol
solution, respectively. It is clear that the increasing concentra-
tion of methanol solution as fuel increased the single cell
performance.
Fig. 7 Electrical performances (cell voltage (solid) and power density
(hollow) against current density) of 5 cm2 DMFC at different methanol
concentrations (B) 1, (D) 2 and (,) 3 M using the PdNPs-LaNi0.5
-
Fe0.5O3NPs-CH anodic, PtNPs-CH cathodic catalysts and synthetic
3.7.3. The ow rate of methanol solution. To investigate
the effect of ow rate of methanol solution as fuel on the
GDL (temperature ¼ 70 ꢀC, flow rate ¼ 1.6 ml minꢁ1, [NaOH] ¼ 0 M, PO
2
¼ 2 bar and Nafion® 117 membrane).
performance of the single cell, the different ow rates (PO ¼ 2
2
bar; [methanol] ¼ 1 M; temperature ¼ 70 ꢀC; [NaOH] ¼ 0 M and
have been shown in Fig. 6. The open circuit voltages of the Naon® 117 membrane) were tested at a single cell with 5 cm2
ꢀ
single cells at the different temperatures were 0.79 V at 60 C, surface area, and the synthetic GDL. The I–V characteristics
0.92 V at 70 ꢀC, 0.96 V at 80 ꢀC and 0.95 V at 90 ꢀC. The have been shown in Fig. 8.
increasing temperature enhanced the open circuit voltages.
The open circuit voltages of the single cells at the different ow
Aer polarization loss, the potential of single cells were stabled rates were 0.92 V at 1.6 ml minꢁ1, 0.93 V at 2.3 ml minꢁ1 and 0.95
at 0.22 V for 60 ꢀC, 0.35 V for 70 ꢀC, 0.39 V for 80 ꢀC and 0.34 V V at 3.8 ml minꢁ1. Aer polarization loss, the potential of single
for 90 ꢀC. Also, the observed maximum power density of single cells were stabled at 0.35 V with 1.6 ml minꢁ1, 0.31 V with 2.3 ml
cells were 10.10, 16.79, 19.09 and 16.74 mW cmꢁ2 at the 60, 70, minꢁ1 and 0.33 V with 3.8 ml minꢁ1 ow rate of methanol solu-
ꢀ
80 and 90 C temperature, respectively. It can be seen that the tion. The observed maximum power density of single cell was
increasing temperature improved the single cell performance. increased from 16.79 to 17.61 mW cmꢁ2 when the ow rate of
This improvement have a higher slope at the temperature methanol solution was increased from 1.6 to 2.3 ml minꢁ1. The
ꢀ
changes from 60 to 80 C. The single cell performance levelled maximum power density of single cell was decreased to 17.53 mW
cmꢁ2 at 3.8 M ow rate of methanol solution.
ꢀ
off at temperatures higher than 80 C.
3.7.2. The methanol concentration. The methanol
3.7.4. The NaOH concentration. The main advantages of
concentration as fuel has a signicant effect on electrical the DMFC include faster methanol oxidation kinetics in an
performances such as cell voltage and power density, as one alkaline media than in acidic solutions and the potential to use
would expect. The polarization curves of the single cell system inexpensive metal catalysts.34 It is known that many catalysts
reactions (such as reduction of oxygen and oxidation of meth-
anol) perform better in alkaline electrolytes. It was also
discovered that the methanol oxidation in alkaline electrolytes
is structure insensitive,35 which opens up the opportunity for
use of perovskite-type oxides,36 which are signicantly cheaper
than Pd and Pt based catalysts, in alkaline fuel cells. Further-
more, the ionic current in the alkaline fuel cell is due to
conduction of hydroxide ions. This ionic ow is in the reverse
direction to that in proton conducting systems. The hydroxide
ions are transported through the membrane electrolyte along
a direction of the electro-osmotic drag opposing methanol ow,
thus reducing methanol crossover.37 The experimental results
show that in the fuel cell, when the pH of fuel was higher,
hydrolysis become signicant and competed with the direct
metal reduction reaction. Metal hydroxides formed from
hydrolysis can be reduced to metal nanoparticles. Due to the
formation of intermediate hydroxide phase, the nanoparticles
formation rate at high pH was limited by the reduction reac-
tions of various hydroxides and their dissolution rates. It is
know that CO2 is a by-product in DMFC from methanol
Fig. 8 Electrical performances (cell voltage (solid) and power density
(hollow) against current density) of 5 cm2 DMFC at different flow rates
(B) 1.6, (D) 2.3 and (,) 3.8 ml minꢁ1 using the PdNPs-LaNi0.5Fe0.5
-
O3NPs-CH anodic, PtNPs-CH cathodic catalysts and synthetic GDL
(temperature ¼ 70 ꢀC, [methanol] ¼ 1 M, [NaOH] ¼ 0 M, PO ¼ 2 bar
2
and Nafion® 117 membrane).
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for 0 M, 0.32 V for 0.5 M, 0.32 V for 1 M and 0.34 V for 1.5 M the
NaOH concentration in fuel stream. The observed maximum
power density of single cells were 16.79, 39.79, 49.09 and
49.47 mW cmꢁ2 at the 0, 0.5, 1 and 1.5 M the NaOH concentration
in the fuel stream, respectively. The efficacy of the NaOH addition
to the fuel stream is obvious on the single cell performance
through the increasing power density.
3.7.5. The gas diffusion layer. To compare the performance
of the synthetic and commercial GDLs, the 13 cm2 single cells
with two kinds GDLs were tested (PO ¼ 2 bar; [methanol] ¼ 1 M;
2
temperature ¼ 70 ꢀC; [NaOH] ¼ 0.5 M; ow rate ¼ 1.6 ml minꢁ1
and Naon® 117 membrane). The I–V characteristics have been
shown in Fig. 10. The open circuit voltages were 1.05 V for single
cell with the prepared GDL and 1.24 V for single cell with the
commercial GDL. Aer polarization loss, the potential of single
cells were stabled at 0.30 V for single cell with the prepared GDL
and 0.43 V for single cell with the commercial GDL. The
observed maximum power density were 34.25 and 36.72 mW
cmꢁ2 for single cell with the prepared and commercial GDLs,
respectively.
Fig. 9 Electrical performances (cell voltage (solid) and power density
(hollow) against current density) of 5 cm2 DMFC at different NaOH
concentrations (B) 0, (D) 0.5, (,) 1 and (>) 1.5 M using the PdNPs-
LaNi0.5Fe0.5O3NPs-CH anodic, PtNPs-CH cathodic catalysts and
synthetic GDL (temperature ¼ 70 ꢀC, [methanol] ¼ 1 M, flow rate ¼ 1.6
ml minꢁ1, PO ¼ 2 bar and Nafion® 117 membrane).
2
oxidation on the anode electrode and can ruin this electrode.
One solution is the carbonation of the solution due to CO2
production in alkaline electrolytes in fuel oxidation route as
follow:
Although the prepared GDL had a weaker performance than
the commercial GDL; it provided 93.27% obtained power
density of the single cell with the commercial GDL. As a result,
the prepared GDL can be an appropriate candidate.
2ꢁ
2OHꢁ + CO2 / CO3 + H2O
(12)
So, the CO2 fouling has been decreased with increasing the
NaOH concentration. However, when increasing the NaOH
concentration in the fuel carrier stream, the solid precipitation of
carbonate salts on the anode electrode has been increased.38
Consequently, this leads to a reduction in reactivity for fuel
oxidation in the system.39 Fig. 9 shows the cell performance the
4. Conclusions
In this work, PdNPs-LaNi0.5Fe0.5O3NPs-CH nanocomposite was
successfully synthesized. The GC/PdNPs-LaNi0.5Fe0.5O3NPs-CH
electrode was prepared as active catalyst for methanol oxida-
tion. Our results showed that the addition of LaNi0.5Fe0.5O3NPs
into PdNPs catalyst and the use of a more porous matrix of CH
can signicantly improve the electrode performance for meth-
anol oxidation. The factor inuencing PdNPs utilization in the
methanol oxidation reaction is intrinsically related to PdNPs
formation but using LaNi0.5Fe0.5O3NPs helps in enhancing the
DMFC performance with low Pd loading. The fuel cell results
indicated that deposited PdNPs-LaNi0.5Fe0.5O3NPs-CH and
PtNPs-CH as anodic and cathodic catalyst on the prepared GDL
provided adequate electron pathway and improved the oxida-
tion reaction. So that a single cell with 5 cm2 surface area had
49.47 mW cmꢁ2 with 2 mg cmꢁ2 PdNPs and 1 mg cmꢁ2
LaNi0.5Fe0.5O3NPs as anodic catalyst and 4 mg cmꢁ2 PtNPs as
different NaOH concentration (PO ¼ 2 bar; [methanol] ¼ 1 M;
2
temperature ¼ 70 ꢀC; ow rate ¼ 1.6 ml minꢁ1 and Naon® 117
membrane) at a single cell with 5 cm2 surface area, the ow path of
parallel and the prepared GDL. The open circuit voltages of the
single cells at the different NaOH concentrations were 0.92 V for
0 M, 0.90 V for 0.5 M, 0.91 V for 1 M and 0.90 V for 1.5 M. Aer
polarization loss, the potential of single cells were stabled at 0.35 V
cathodic catalyst when PO ¼ 2 bar; [methanol] ¼ 1 M;
2
temperature ¼ 70 ꢀC; [NaOH] ¼ 1.5 M; ow rate ¼ 1.6 ml minꢁ1
and Naon® 117 membrane.
Acknowledgements
We thank University of Sistan and Baluchestan (USB) for
nancial support.
Fig. 10 Electrical performances (cell voltage (solid) and power density
(hollow) against current density) of 13 cm2 DMFC using (B)
commercial and (D) synthetic GDL, the PdNPs-LaNi0.5Fe0.5O3NPs-CH
anodic and PtNPs-CH cathodic catalysts (temperature ¼ 70 ꢀC,
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