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PteRu catalysts supported on carbon xerogels for PEM fuelcells
J.C. Calderon a, N. Mahata b, M.F.R. Pereira b, J.L. Figueiredo b, V.R. Fernandes c,C.M. Rangel c, L. Calvillo d, M.J. Lazaro d, E. Pastor a,*aDepartment of Physical Chemistry, Institute of Materials and Nanotechnology, University of La Laguna, Avda. Astrofısico Francisco
Sanchez s/n, 38071 La Laguna, Tenerife, Spainb Laboratory of Catalysis and Materials, Associate Laboratory LSRE/LCM, Chemical Engineering Department, Faculty of Engineering,
University of Porto, 4200-465 Porto, Portugalc LNEG, Fuel Cells and Hydrogen Unit, Estrada do Paco do Lumiar 22, 1649-038 Lisbon, PortugaldCSIC e Carbochemistry Institute, Miguel Luesma Castan 4, 50018 Zaragoza, Spain
a r t i c l e i n f o
Article history:
Received 9 September 2011
Received in revised form
25 November 2011
Accepted 5 December 2011
Available online 31 December 2011
Keywords:
Carbon xerogels
Carbon treatments
Oxygenated surface groups
PteRu electrocatalysts
Direct methanol fuel cells
* Corresponding author. Tel.: þ34 679437939;E-mail addresses: [email protected], elena_p
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.12.029
a b s t r a c t
PteRu electrocatalysts supported on carbon xerogels were synthesized by reduction of
metal precursors with formate ions (SFM method). The carbon xerogel was chemically and
heat treated in order to evaluate the different procedures to generate oxygenated groups on
the surface. Temperature-programmed desorption (TPD) of xerogels showed that heat
treatment of previously chemically modified support gradually removes the oxygenated
groups from the carbon surface. Physical characterization of the catalyst was performed
using X-ray dispersive energy (EDX) and X-ray diffraction (XRD) techniques. Results
confirmed that PteRu catalysts with similar metal content (20%) and atomic ratios (Pt:Ru
1:1) were obtained.
The electrochemical activity was studied by cyclic voltammetry. Higher CO and
methanol oxidation current densities were found for catalyst deposited on chemically
treated carbon xerogel when compared with the untreated material, whereas the heat
treatment of carbon supports was in detriment of the catalytic activity. Gas diffusion
electrode preparation and MEA assembly allowed an in-house built direct methanol fuel
cell to evaluate the performance of synthesized catalysts and supports. Polarization curves
were measured and confirmed the data obtained from cyclic voltammetry regarding the
negative effect of heat treatment on the catalytic activity of these materials. Normalized
power density curves and maximum cell power per Pt weight are discussed in terms of the
operational temperature for the different materials, in comparison with results obtained
with a commercial catalyst. Moreover, relationship between catalytic activity and
oxygenated surface groups was established and it seems that carboxylic groups play a key
role in this respect.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
fax: þ34 [email protected] (E. Pastor).2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 1 7201
1. IntroductionPt(II)Cl4
2� þ B / Pt(II)Cl3B� þ Cl� (2)
Anodic catalysts for polymer electrolyte membrane fuel cells
(PEMFC) are conformed by PteRu nanoparticles supported on
carbon black, usually Vulcan XC-72 [1]. Recent works have
shown that other carbon materials, such as graphite nano-
fibers [2e4], carbon nanotubes [5e10], carbon microspheres
[11,12], hard carbon spherules [13], carbon aerogels and
xerogels [10,14,15] and mesoporous carbons [16,17], can
improve the efficiencies of the catalysts, when they are used
as supports. For example, it was reported that Pt-carbon
support interactions could improve the catalytic properties
[18] and the catalysts stability [19]. However, the causes for
such efficiency enhancement have not yet been fully deter-
mined, although it is thought that these supports can increase
the dispersion of the metal, decrease the formation of nano-
particle agglomerates and improve the diffusion of electro-
active species.
Other properties are also important concerning the role of
the carbon support in the catalyst performance. Carbon
support is not an inert material and interacts with the metal
particles. It has been reported the existence of electronic
effects, in most cases related to the increase of the electronic
density on the carbon by means of electron transfer from the
metal particles to the oxygen atoms at the surface of the
supportmaterial [20,21]. One of these effects is the decrease of
the Fermi level of the catalysts, changing the Galvani potential
and promoting electron transfer at the electrodeeelectrolyte
interface, thus accelerating the electrode processes [22].
On the other hand, it has to be considered that for carbon-
supported Pt PEMFC catalysts, metal atoms dispose free
orbitals to accept and transfer electrons [23]. Thus, the
difference in the electronic work functions of platinum
(5.4 eV) and carbon (4.7 eV) induces an increase of electron
density on the metal [22]. Nevertheless, this change in Pt
electron density is only significant if the particle size of the
microdeposit is comparable to the thickness of the double
layer [22]. Therefore, there is a relationship between particle
size and double layer thickness, i.e. the particle size of the
catalyst metal crystallite also influences the catalystesupport
interaction [24].
These interaction effects have been studied by different
techniques, as electron-spin resonance (ESR) and X-ray photo-
electronspectroscopy (XPS),demonstratingthat themaineffect
is the electron donation by platinum to the carbon support,
depending on the Fermi level of electrons in both [25,26].
Other important aspect is the role of oxygen surface groups
in the interaction between the carbon support and the metal
precursor. H2PtCl6 and [Pt(NH3)4]Cl2 are the usual precursors
for Pt/C catalyst formation, in acid and basic media, respec-
tively. Pt reduction during the impregnation step has been
studied by several authors [27e29]. An electrostatic mecha-
nism cannot describe the adsorption process of H2PtCl6 on the
carbon support [27] and the following model of interaction
was proposed to explain the impregnation of carbon with the
metal precursor:
H2Pt(IV)Cl6 þ AH þ H2O / Pt(II)Cl42� þ 2Cl� þ A� þ 5Hþ (1)
where A and B represent acid and basic surface oxygenated
groups, respectively [30], which ionize according to the
following equilibria:
HA 4 A� þ Hþ (3)
B þ Hþ 4 BHþ (4)
From this model, it is possible to conclude that the hexa-
chloroplatinic anion will deposit preferentially on basic
groups, which act as good anchoring sites for platinum on
carbon. These sites can be the p-electron rich regions in the
basal planes, which work as Lewis bases [31e33]:
Cp þ H3Oþ 4 [Cp � H3O]þ (5)
Cp þ 2H2O 4 [Cp � H3O]þ þ OH� (6)
where Cp represents the graphitized surface structure with
delocalized p electrons.
Carbonxerogels can beused as carbon supports, taking into
account their mesoporous and macroporous textures and
large pore volumes. These supports possess excellent charac-
teristics, such as high porosity, high surface area, controllable
pore size and can be produced in different forms (monolith,
thin film or powder), depending on the desired use [34]. This
material has been used in environmental technologies [35,36],
fuel cells [10,15,37], adsorption of ethylene [38], selective
hydrogenation [39] and selective hydrochlorination [40].
There are different methods of reduction of metal precur-
sors to obtain supported catalyst nanoparticles, such as the
reduction with sodium borohydride (BM) [41,42] or formic acid
(FAM) [43,44]. In particular, the FAM method does not
completely reduce the Ru precursor salt, due to the low
dissociation constant of formic acid (1.8 � 10�5) and the pH
dependence of the reduction potential of Ru. In the present
study we have applied a modified FAM method for the
synthesis of PteRu nanoparticles on carbon xerogels, where
the pH of the reaction medium was increased to 12, to ensure
complete dissociation of formic acid into formate ions, which
act as reducing agent (sodium formate method-SFM) [45].
Synthesized catalysts were analyzed by different techniques
to study their physicochemical properties. In addition, their
activities for the electrooxidation of CO and methanol were
investigated using cyclic voltammetry. Finally, materials were
tested in a direct methanol fuel monocell, in order to assess
catalyst performance and cell power densities.
2. Experimental
2.1. Carbon xerogels synthesis
Carbon xerogel (CX) was synthesized by the conventional sol-
gel condensation of resorcinol and formaldehyde, and
subsequent carbonization of the dried gel. Detailed procedure
was described elsewhere [46]. In brief, sol-gel processing was
i n t e rn 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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 17202
performed with a formaldehyde/resorcinol ratio of 2.0 at pH
5.6 (adjusted with dilute NaOH solution). The gelling and
curing step was performed for a period of three days (30, 50,
and 75 �C, one day each). The hard gel was ground to preferred
size, and then water was removed by sequential exchange
with acetone (initially containing 5% acetic acid) and cyclo-
hexane, followed by drying overnight at 80 �C. Finally, the
dried gel was carbonized at 800 �C under nitrogen flow
(100 cm3 min�1, 6 h). The carbon xerogel obtained was desig-
nated as CXUA. In order to introduce surface oxygenated
groups, a portion of CXUAwas treated with 7.0 M HNO3 for 3 h
in a soxhlet. The sample was washed several times with
distilled water, and then dried at 110 �C for 6 h. The HNO3
treated sample was denoted as CXNA. Additional samples
were prepared by heat treatment of CXNA for 30 min under N2
flow (60 cm3 min�1) at different temperatures: 400 �C(CXNA400), 600 �C (CXNA600), and 750 �C (CXNA750).
2.2. Carbon xerogel characterization
An AMI-200 Catalyst Characterization Instrument (Altamira
Instruments) was used to obtain the temperature-
programmed desorption (TPD) spectra. In a typical experi-
ment, a 100 mg sample was subjected to a 5 min�1 linear
temperature rise up to 1100 �Cunder heliumflowof 25 min�1.
A mass spectrometer (Dymaxion 200 amu, Ametek) was
employed tomonitor the desorbedCO (m/z 28) andCO2 (m/z 44)
signals. To determine the amount of each surface group,
deconvolution of the CO and CO2 TPD spectra was carried out
following a procedure previously developed for activated
carbons. A detailed description of this deconvolution process
was included in Section 3.1 and is reported also in references
[47,48]. N2 adsorption isothermsate 196 �Cwereobtainedwith
a Quantachrome NOVA 4200e multi-station apparatus, to
determine the textural properties ofmaterials. Specific surface
area (SBET) and micropore volume (Vmicro) were determined
from BET and t-plot analysis, based on the standard isotherm
for carbon materials [49]. Pore size distribution was obtained
from the desorption branch of the isotherm using the Barrett,
Joyner and Halenda (BJH) method. A FEI Quanta 400FEG envi-
ronmental scanningelectronmicroscope (SEM), equippedwith
a field emission gun (15 keV), was used to obtain micrographs
of the carbon xerogels.
2.3. PteRu catalysts synthesis
Formate ions reduction method (SFM method) was used to
synthesize PteRu catalysts with a nominal metal loading of
20% and an atomic ratio for Pt:Ru of 1:1 [45]. Carbon materials
were dispersed in 2.0 M HCOOH solution with a pH previously
adjusted to 12.0. Then, the mixture was heated at 80 �C and
metal salts solution was added under stirring. Temperature
was kept constant during this addition and for 1 h afterward.
Reaction medium was kept under stirring for 12 h, and finally
the mixture was filtered, washed and dried at 60 �C for 2 h.
2.4. Physical characterization
Metal content and PteRu atomic ratios of the synthesized
catalysts were determined by Energy Dispersive X-Ray
Analysis (EDX), using a scanning electron microscope (LEO
Mod. 440) at 20 kV, with a Si detector and a Be window. A
Universal Diffractometer Panalytical X’Pert X-ray diffraction
was used in order to obtain the XRD patterns of synthesized
catalysts. This equipment was operated with Cu-Ka radiation
generated at 40 kV and 30 mA. Scanswere done at 3� min�1 for
2q values between 20 y 100�. Dimensions of peak (220),
Scherrer’s equation and Vergard’s Law were used to calculate
the metal crystallite size [50] and lattice parameters, respec-
tively [51].
2.5. Electrochemical characterization
A three electrodes electrochemical cell was used for the
electrochemical characterization of the synthesized catalysts.
A glassy carbon bar was the counter electrode and a reversible
hydrogen electrode (RHE) placed inside a Luggin capillary was
the reference electrode. All potentials are referred to this
electrode. Theworking electrodewas prepared from a catalyst
ink using 2.0 mg of catalyst, 15 mL of Nafion� (5 wt. %, Aldrich)
and 500 mL of ultrapure water, and deposited onto a glassy
carbon disk, with 1.54 geometric area. The final Pt loading in
the catalysts was near to 13.5 mg cm�2. The supporting elec-
trolyte was 0.5 M H2SO4 (95e97%, Merck). Potentiodynamic
measurements were made using an Electrochemistry Instru-
ment mAUTOLAB III modular equipment. 18.2 MU cm H2O was
used for the preparation of all electrolyte and methanol
solutions. The electrocatalytic activity of synthesized mate-
rials was compared to that of commercial PteRu/C catalysts
from E-TEK. CO electrochemical characterization was per-
formed in order to determine the electrochemical surface
areas of electrodes (99.999%, Air Liquide). With this purpose,
this gas was bubbled into the electrochemical cell during
10min at 0.200 V vs. RHE as adsorption potential, forming a CO
monolayer on the deposited catalyst. Next, nitrogen (Micro-
GeN2, GasLab) was bubbled during 10min, to remove the non-
adsorbed CO. For methanol electrochemical characterization,
2.0 M alcohol solution in the base electrolyte was employed.
2.6. Direct methanol fuel monocell tests
Electrodes used to prepare membrane electrode assemblies
(MEA’s) consisted of a catalytic layer dispersed onto the
diffusion layer. The gas diffusion layers were prepared from
an ink composed of carbon black Vulcan XC-72R, isopropyl
alcohol (99.8%, Merck) and PTFE (58%, Dyneon), which was
painted on Toray carbon paper. This procedure was applied
for both anode and cathode. PteRu synthesized catalysts were
used as anodes and a 10 wt. % Pt/C catalyst was used as
cathode in all studied MEA’s. The catalytic layer was prepared
from an ink obtained by suspending the material in isopropyl
alcohol and stirring in an ultrasonic bath for 10 min. Later,
a 5% Nafion� solution (Electrochem Inc) was added to the
mixture. Catalyst inks were dispersed with a brush and dried
at 50 �C until a 4.0 cm�2 catalyst loading was achieved.
Nafion� 117 membranes were cleaned and turned into the
acid form by a chemical treatment in 3.0% H2O2 at boiling
temperature for 1 h, followed by a treatmentwith 0.5 MH2SO4,
also at boiling temperature, during 2 h. Membranes were
washed with boiling water for 30 between these treatments.
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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 1 7203
Cleanedmembranes were stored in ultrapure water and dried
before use. Assembly of MEA’s was performed by hot-pressing
the prepared anode and cathode on each side of the pretreated
membrane at 50 bar and 130 �C for 180 s. MEA’s weremounted
into an in-house built direct methanol fuel monocell hard-
ware. A single cell configuration was used with 25 of active
area and a single serpentine flow design on graphite plates.
Conditioning of the cell and reactants at temperatures near to
70 �C was achieved using a test station, which allowed the
MEA’s activation and polarization curve recording. Under
operational conditions, a 0.75 M aqueous CH3OH solution was
pumped through the anode of the monocell at 1.5 min�1.
Feeding of O2 through the cathode compartment was carried
out at 0.3 Lmin�1. Polarization curvesweremeasured after the
preconditioning procedure of the MEA described in ref. [52].
3. Results and discussion
3.1. Textural and surface chemical characterization ofcarbon xerogels
Table 1 presents the textural characteristics of the carbon
xerogels along with the amount of CO and CO2 evolved during
TPD. Nitric acid treatment does not induce any noticeable
change in the textural properties, but subsequent heat treat-
ment, particularly at 750 �C, results in considerable increase in
BET surface area and pore volume. Opening of otherwise
inaccessible micropores during heat treatment is possible,
which accounts for the increase in BET surface area and pore
volume. However, the average pore radius remains similar in
all the samples.
The surface oxygenated groups present on the carbon
samples were characterized by TPD. The oxygenated groups
decompose with the evolution of CO (from anhydrides,
phenols and carbonyls) or CO2 (from carboxylic acids, anhy-
drides and lactones) [39,47]. The total amounts of CO and CO2
released were calculated from the corresponding TPD spectra
and are presented in Table 1. Fig. 1 shows the TPD profiles of
the samples and Fig. 2 the concentration of surface oxygen
species obtained by deconvolution of the TPD spectra. A
multiple Gaussian function was used for fitting each spec-
trum. The numerical calculations were based on a nonlinear
routine, which minimized the sum of squared deviations,
using the LevenbergeMarquardt method to perform the iter-
ations. The use of Gaussian functions is justified by the shape
of the TPD peaks, which are a result of continuous random
Table 1 e Textural and surface chemical properties of the carb
Sample SBET (m2 g�1) Vmicro (cm3 g�1) VTa (cm3
CXUA 635 0.164 0.864
CXNA 613 0.156 0.840
CXNA400 671 0.177 0.893
CXNA600 678 0.181 0.861
CXNA750 754 0.205 0.959
a Total pore volume, estimated from the uptake at relative pressure ¼ 0
b Average pore radius from BJH method.
distributions of binding energies of the surface groups [53]. For
the carbon xerogel samples, some assumptions, justified in
previous works with activated carbons [47,48], were followed:
� The CO2 spectrum is decomposed into four contributions,
corresponding to carboxylic acids (CO2 peak #1 andCO2 peak
#2, respectively assigned to less and strongly acidic
carboxylic groups), carboxylic anhydrides (CO2 peak #3) and
lactones (CO2 peak #4). For the samples heat treated above
400 �C only one peak was considered for the carboxylic
groups, because they were processed at temperatures
higher than those corresponding to the decomposition of
carboxylic acids; so, only a small amount was observed,
probably due to some re-oxidation of the surface due to air
exposure when handling the materials.
� Each carboxylic anhydride group decomposes by releasing
one CO and one CO2 molecule. The component in the CO
spectrum corresponding to the carboxylic anhydrides (CO
peak #3) has the same shape and equal magnitude as CO2
peak #3. This peak is pre-defined from the deconvolution of
the CO2 spectra.
� In addition to carboxylic anhydrides (CO peak #3), the CO
spectrum includes contributions from phenols (CO peak #4)
and carbonyl/quinones (CO peak #5). This sequence of
decomposition temperatures (first phenols, then carbonyl/
quinones) is justified by the higher stability of the later
groups.
� The CO spectrum of sample CXNA presents a shoulder at
low temperatures that cannot be justified by the carboxylic
anhydride groups. This shoulder appears in the same region
of the decomposition of the carboxylic groups. It was
assumed that, for carbonmaterials oxidized in liquid phase,
there are two CO peaks at low temperature (CO peak #1 and
peak #2) with the same peak center andwidth at half-height
as that obtained for the carboxylic groups in the CO2 peak.
� It is assumed that secondary reactions at high temperatures
are negligible.
� The same width at half-height was imposed for CO2 peaks
#3 and #4.
� The same width at half-height was considered for phenol
and carbonyl groups.
Tables 2 and 3 show the results obtained, where TM is the
temperature of the component peak maximum, W is the
width of the component peak at half-height, and A is the
integrated component peak area. The TPD results clearly
indicate that the un-activated carbon sample, CXUA, hardly
on xerogels.
g�1) RPb (nm) CO (mmol g�1) CO2 (mmol g�1)
8.5 78 0
8.5 3349 1295
8.6 3156 488
8.6 2043 218
8.5 827 61
.98.
Fig. 1 e DeconvolutionofTPDspectra: (a)CO2spectrum; (b)COspectrum(-,TPDexperimentaldata;—, individualpeaks;d, sum
of the individual peaks).
i n t e rn 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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 17204
0
990
64 78 610 27 00
113
00
1428
78
1396
620
259184 165122
207
620
1459 14381384
0
200
400
600
800
1000
1200
1400
1600
CX-NA400 CX-NA600 CX-NA750
Sample
amount(μmolg-)
Carboxylic acidsAnhydridesLactonesPhenolsCarbonyl/quinones
CXUA CXNA
Fig. 2 e Amount of each oxygen-containing surface group
obtained by deconvolution of TPD spectra.
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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 1 7205
contains any surface oxygenated groups. On the other hand,
the nitric acid treated sample, CXNA, contains large amount
of oxygenated groups. For each heat treatment, the groups not
stable at the treatment temperature are removed. That is,
CXNA sample shows oxygenated groups that decompose as
CO and CO2 in all range of temperature. However, during the
treatment at 400 �C, all the groups that are not stable at this
temperature are removed and, therefore, only the groups that
start to decompose at 400 �C or higher temperatures can be
observed in the spectra. The same effect occurs for the
samples treated at 600 and 750 �C. In Fig. 2, this effect is
quantified. Heat treatment of CXNA at different temperatures
removes different extents of the surface groups. Treatment at
Table 2 e Results of the deconvolution of CO2 TPD spectra usin
Sample Peak #1 Peak #2
TM W A TM W A(�C) (�C) (mmol g�1) (�C) (�C) (mmol g�
CXUA e e e e e e
CXNA 308 165 742 443 105 247
CXNA400 294 140 64 e e e
CXNA600 312 200 78 e e e
CXNA750 312 200 61 e e e
Table 3 e Results of the deconvolution of CO TPD spectra usin
Sample Peak #1 Peak #2 P
TM W A TM W A TM W(�C) (�C) (mmol g�1) (�C) (�C) (mmol g�1) (�C) (�
CXUA e e e e e e e e
CXNA 308 165 117 443 105 237 562 15
CXNA400 e e e e e e 532 15
CXNA600 e e e e e e 534 14
CXNA750 e e e e e e e e
400 �C mainly removes carboxylic acid groups, whereas high
temperature treatment (750 �C) removes most of all types of
oxygenated groups leaving minor parts of phenolic and
carbonyl/quinone groups on the surface.
3.2. Physical characterization of PteRu catalysts
XRD patterns were acquired for all synthesized PteRu cata-
lysts supported on carbon xerogels (Fig. 3). Spectra exhibit the
characteristic peaks corresponding to the platinum fcc
structure, being consistent with the XRD pattern of PteRu/C E-
TEK commercial catalysts. Aweak signal near to 25 2q-degrees
was obtained, which corresponds to the C(002) graphite basal
planes [6,54], showing a low graphitization level of the carbon
supports used. The crystallite size of PteRu catalysts (Table 4)
falls between 3.6 and 4.6 nm, close to the crystallite size
determined for the commercial catalyst PteRu/C E-TEK
(4.4 nm). The calculated lattice parameters of the catalysts
were lower than 3.92 A, which corresponds to Pt. This result
suggests the formation of a PteRu alloy in all synthesized
catalysts. Finally, EDX analysis confirms that the synthesized
PteRu catalysts supported on carbon xerogels have metal
loadings near to 20% wt. and atomic proportions Pt:Ru also
close to 1:1.
3.3. Electrochemical characterization
CO electrochemical oxidation at room temperature on the
synthesized PteRu catalysts (Fig. 4) presents the lowest CO
oxidation energy for PteRu/CXNA (peak potential at 0.546 V
vs. RHE), whereas PteRu/CXNA750 displayed the highest CO
oxidation energy (peak potential at 0.670 V vs. RHE). In fact, CO
g a multiple Gaussian function.
Peak #3 Peak #4
TM W A TM W A1) (�C) (�C) (mmol g�1) (�C) (�C) (mmol g�1)
e e e e e e
562 159 184 712 159 122
532 150 259 706 150 165
534 148 27 742 148 113
e e e e e e
g a multiple Gaussian function.
eak #3 Peak #4 Peak #5
A TM W A TM W AC) (mmol g�1) (�C) (�C) (mmol g�1) (�C) (�C) (mmol g�1)
e e e e 862 183 78.12
9 184 690 188 1428 847 188 1384
0 259 692 174 1459 842 174 1438
8 27 728 183 620 838 183 1396
e 675 153 207 896 153 620
20 30 40 50 60 70 80
C(002) (220)
(200)
Inte
nsit
y / a
.u.
2θ
Pt-Ru/C E-TEK
Pt-Ru/CXNA400
Pt-Ru/CXNA750
Pt-Ru/CXNA600
Pt-Ru/CXNA
Pt-Ru/CXUA
(111)
Fig. 3 e XRD patterns of PteRu catalysts.
0.0 0.2 0.4 0.6 0.8 1.0
-0.2
-0.1
0.0
0.1
0.2
Cur
rent
den
sity
/ m
Acm
-2
Potential / V vs. RHE
PtRu/CXUA PtRu/CXNA PtRu/CXNA400 PtRu/CXNA600 PtRu/CXNA750 PtRu/C E-TEK
Fig. 4 e Cyclic voltammograms for CO oxidation on PteRu
synthesized catalysts. Scan rate: 20 mV sL1. Support
electrolyte: 0.5 M H2SO4. CO adsorption potential: 0.2 V vs.
RHE. Current densities are normalized by the electro-active
area of the electrode.
Table 5 e Electrochemical surface areas for thesynthesized catalysts.
Catalyst Electrochemical surfacearea/cm2 mg�1 catalyst
PteRu/CXUA 41,25
PteRu/CXNA 40,37
PteRu/CXNA400 42,88
PteRu/CXNA600 22
PteRu/CXNA750 27
PteRu/C E-TEK 121
i n t e rn 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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 17206
oxidation peak definition becameworse as the heat treatment
of the carbon support increased, which confirms that this is
a key factor for the catalytic activity of synthesized catalysts.
In all cases, electrochemical areas of catalysts were deter-
mined from the charge associated to a CO monolayer adsor-
bed onto the catalytic nanoparticles, and all the current
densities are referred to these electrochemical areas (in
Table 5 given as cm2/mgcatalyst).
It is remarkable the differences found in the double layer
charges for the synthesized catalysts (for details see Figure S1
and corresponding text in the Supplementary Information).
Heat treatment modified the magnitude of these charges.
Increase in the heat treatment temperature induced a growth
in double layer charges, attributed possibly to the rupture of
the structure of carbon support caused by heat treatment,
which generates new free spaces and cavities able to adsorb
ions, increasing the capacitance of the electrical double layer.
Thus, severity of heat treatment modifies in a major degree
the structure of carbon xerogels. BET areas (Table 1) also
confirm this fact, although the increase order of these areas
does not match with the electrical double layer charges.
Differences are still visible in the cyclic voltammograms (Fig. 4
and Fig. S1) after current normalization because this correc-
tion is only based on the electro-active area of catalysts,
which exclusively depends on Pt electrochemically available
surface area.
CO data can be correlated with that obtained for methanol
electrochemical oxidation (Fig. 5), in terms of the current
Table 4 e Composition, particle size and lattice parameters for
Catalyst Atomic ratio Pt:Ru Metal loa
PteRu/CXUA 58:42
PteRu/CXNA 55:45
PteRu/CXNA400 55.45
PteRu/CXNA600 49:51
PteRu/CXNA750 55:45
PteRu/C Vulcan XC-72R E-TEK 45:55
densities obtained. PteRu/CXNA showed the highest current
densities for this reaction, and in general, all synthesized
catalysts exhibited higher current densities than that ob-
tained with commercial PteRu/C E-TEK. Again, a consistent
behavior was found with respect to the heat treatment of
carbon xerogels. Methanol current densities diminish with
the increase of heat treatment temperature. Heat treatment
removes oxygen surface groups from the carbon xerogels, so
there must be a relationship between the catalytic activity of
the materials and the presence of some specific surface
oxygenated groups. TPD analysis shows that heat treatment
PteRu catalysts.
ding/wt. % Crystallite size/nm Lattice parameter/A
21 4.3 3.904
19 4.1 3.913
20 4.6 3.906
18 3.6 3.902
20 4.0 3.900
20 4.4 3.898
0.0 0.2 0.4 0.6 0.8 1.0
0.00
0.20
0.40
0.60
0.80
PtRu/CXUA PtRu/CXNA PtRu/CXNA400 PtRu/CXNA600 PtRu/CXNA750 PtRu/C E-TEK
Potential / V vs. RHE
Cur
rent
den
sity
/ m
Acm
-2
Fig. 5 e Cyclic voltammograms for methanol oxidation on
PteRu synthesized catalysts. Scan rate: 20 mV sL1. Support
electrolyte: 0.5 M H2SO4. Methanol concentration: 2.0 M.
Current densities are normalized by the electro-active area
of the electrode.
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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 1 7207
of CXNA at 400 �C, generating the CXNA400 support, princi-
pally eliminates carboxylic acids (see Figs. 1 and 2), making
the difference between CXNA and this treated material.
Accordingly, it can be stated that the presence of carboxylic
acid groups on carbon surface increases the catalytic activity
of the PteRu catalyst. Successive enhancement of heat
treatment temperature up to 600 and 750 �C, promotes the
0.2
0.4
0.6
0.2
0.4
0.6
0 100 200
0.2
0.4
0.6PtRu/CXNA750
PtRu/CXNA400
20 ºC 40 ºC 60 ºC 70 ºC
PtRu/CXUA
Pot
enti
al /
V 20 ºC 40 ºC 60 ºC 70 ºC
Current densit
20 ºC 40 ºC 60 ºC 70 ºC
Fig. 6 e Polarization curves in a DMFC monocell at different tem
supported on carbon xerogels and the commercial PteRu/C E-TE
rate: 0.75 M and 15minL1, respectively. Oxygen flow rate: 0.3 d
removal of additional oxygenated surface groups and only
carbonyls and a mall amount of phenols remain on the
carbon at the latter temperature. The presence of other
oxygenated groups, such as anhydrides, phenols and
lactones, may also play a role in the catalytic activity of PteRu
catalysts supported on carbon xerogels, although to a lower
extent.
3.4. Direct methanol fuel cell tests
In order to determine the efficiency of synthesized PteRu
catalyst supported on carbon xerogels, DMFCs tests were
carried out using them at the anode of a monocell. Fig. 6
presents the polarization curves for these materials. Poten-
tial drops were less pronounced with increasing of opera-
tional temperature. PteRu/CXNA displayed the lowest
potential drops and the highest current densities, in agree-
ment with those results obtained for CO and methanol elec-
trochemical oxidation (Fig. 5). Bearing inmind that the carbon
support of this catalyst contains a lot of oxygenated surface
groups, mainly carboxylic ones, it is possible to associate the
good performance of this catalyst to the presence of these
groups.
Comparison between polarization curves for carbon xero-
gel supported catalysts and that generated for PteRu/C E-TEK
at 70 �C (Fig. 7), shows that PteRu/CXNA presents similar
potential drop to the commercial catalyst, which displayed
the best result among the different materials. Also it can be
observed that heat treatment of carbon supports harms the
0 100 200 300
PtRu/CXNA
PtRu/CXNA600
20 ºC 40 ºC 60 ºC 70 ºC
20 ºC 40 ºC 60 ºC 70 ºC
y / mAmg Pt
PtRu/C E-TEK 20 ºC 40 ºC 60 ºC 70 ºC
peratures, using as anode the synthesized PteRu catalysts
K. Cathode: Pt/C. Concentration and methanol solution flow
m3 minL1.
0 50 100 150 200 250 300
0.1
0.2
0.3
0.4
0.5
0.6
Pot
enti
al /
V
Current density / mAmg-1Pt
PtRu/CXUA PtRu/CXNA PtRu/CXNA400 PtRu/CXNA600 PtRu/CXNA750 PtRu/C E-TEK
Fig. 7 e Polarization curves in a DMFC monocell at 70 �C,using as anodes the synthesized PteRu catalysts
supported on carbon xerogels. Cathode: Pt/C.
Concentration and methanol solution flow rate: 0.75 M and
15minL1, respectively. Oxygen flow rate: 0.3 dm3 minL1.
i n t e rn 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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 17208
performance of the monocell, in agreement with the results
obtained from the electrochemical characterization at room
temperature. Consequently, the efficiency order for PteRu
catalysts supported on carbon xerogels is:
PteRu/CXNA > PteRu/CXUA > PteRu/CXNA400 > PteRu/
CXNA750 > PteRu/CXNA600
Probably, this effect is caused by the oxygenated surface
groups which were generated during the chemical treatment
and play an important role in the catalysts performance. The
presence of these groups promotes the anchoring of nano-
particles to the carbon support [27], as well as the
nanoparticle-carbon support electronic transference
[31e33]. Otherwise, heat treatments decompose oxygenated
surface groups into CO and CO2, and the amount and type of
those groups drastically change with the treatment
temperature.
On the basis of the efficiency data recorded for the PteRu
catalysts supported on carbon xerogels, the order of impor-
tance of the surface groups is the following: i) carboxylic
groups, which justifies the best performance of PteRu/CXNA;
ii) delocalized p electrons on the basal planes of the carbon
surface (enhanced by the absence of oxygen-containing
surface groups), which explains why CXUA is the second
best sample; iii) carboxylic anhydrides, which in solution can
hydrolyze to carboxylic acids, supporting the relatively good
performance of PteRu/CXNA400, where the amount of those
groups remain practically unchanged; iv) phenol and lactone
groups seem to have a detrimental effect assuming that
PteRu/CXNA750 presents better performance than PteRu/
CXNA600.
These results can be rationalized in terms of two comple-
mentary effects: the presence of carboxylic acid and anhy-
dride groups, which might influence both the stability of the
metal crystallites [10] and the oxidation state of the active
metals, in particular Ru, as previously reported [10]; and the
conductivity of the support, which is higher in the absence of
oxygenated surface groups [55].
Fig. 7 also shows that all carbon xerogel supported mate-
rials present lower performance compared with that ob-
tained for commercial catalyst (only results for PteRu/CXNA
are similar), in spite of the data acquired from electro-
chemical characterization (Fig. 5), which displayed the lowest
current densities for the commercial catalysts during
potentiodynamic methanol electrooxidation. Possibly, this
difference is associated to the lower electrical conductivity of
carbon xerogels, which diminishes the efficiency of the
xerogel supported catalysts. It is well known that the
conductivity of carbon gels with mesoporous structure is
much lower than that for Vulcan XC-72R carbon black which
supports the commercial catalyst [1], so this could explain
why PteRu/C E-TEK presented better performance in a DMFC
configuration.
The influence of the conductivity can be deduced
comparing results in Figs. 5 and 7. In Fig. 5, it can be seen
that the current densities during methanol oxidation are
much higher for xerogel materials than for Vulcan sup-
ported one. These experiments were performed with a thin
layer of the catalyst deposited on a glassy carbon. Here the
textural properties are mainly influencing positively the
oxidation at xerogel supported catalysts, but the low
conductivity has no big influence on the results in this thin
layer. When compared the results in Fig. 7, the currents
associated to the catalysts supported on xerogels are
decreased with respect to the Vulcan material, because in
this case the conductivity is the property governing the final
behavior in the fuel cell. According to these results, it can be
suggested that if xerogel conductivity is increased, an
enhancement in the efficiency should be expected, even
over that for PteRu/C E-TEK.
Power densities at different working temperatures (Fig. 8)
showed that the highest values correspond to PteRu/CXNA.
Catalysts with heat treated carbon support displayed the
lowest DMFC performance, verifying again the impact related
to the use of different carbon xerogel. As in the case of
polarization curves, concentration of oxygen surface groups
seems to play a crucial role on the PteRu catalyst behavior.
Power densities obtained with PteRu/CXNA400, PteRu/
CXNA600 and PteRu/CXNA750, were even lower than those
obtained with PteRu/CXUA, indicating the distinct role of the
different surface groups discussed above.Moreover, a possible
structural change associated to the heat treatment of original
carbon xerogel occurs, which does not benefit the perfor-
mance of the synthesized catalyst in the monocell. It is also
observed that the use of the original CXUA without treatment
as support produces better results than the heat treated
carbon xerogels. This fact was corroborated with the
comparison between all catalysts at 70 �C given in Fig. 9. The
commercial catalyst presented the highest power density, but
the synthesized PteRu/CXNA exhibited nearly the same
performance.
Power density values have been compared with those re-
ported in the literature for PteRu anodes supported on similar
carbonaceous materials [56e58]. Maximum power density
values around 30 [56], 100 [57] and 60 [58] mW/mgPt have been
0
20
40
60
0
20
40
60
0 100 200
0
20
40
60
0 100 200 300
PtRu/CXNA600PtRu/CXNA400
PtRu/CXNA
20 ºC 40 ºC 60 ºC 70 ºC
PtRu/CXUA
20 ºC 40 ºC 60 ºC 70 ºC
Pow
er d
ensi
ty/m
Wm
gP
t
20 ºC 40 ºC 60 ºC 70 ºC
20 ºC 40 ºC 60 ºC 70 ºC
PtRu/C E-TEKPtRu/CXNA750
Current density / mAmg Pt
20 ºC 40 ºC 60 ºC 70 ºC
20 ºC 40 ºC 60 ºC 70 ºC
Fig. 8 e Power density curves in a DMFC monocell at different temperatures, using as anodes the synthesized PteRu
catalysts supported on carbon xerogels and the commercial PteRu/C E-TEK. Cathode: Pt/C. Concentration and methanol
solution flow rate: 0.75 M and 15 minL1, respectively. Oxygen flow rate: 0.3 dm3 minL1.
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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 1 7209
reported,which are in the sameorder of the values given in the
present paper. However, it has to be remarked that, in all these
references, PteRu catalystswithmetal loadings near 60%were
employed, and then thinner catalytic layers were used. It is
possible that optimizing the catalysts layer prepared in the
present work, higher power density values could be achieved,
but this will be the subject of a forthcoming publication.
0 50 100 150 200 250 300
0
10
20
30
40
50
Pow
er d
ensi
ty /
mW
mg-1
Pt
Current density / mAmg-1Pt
PtRu/CXUA PtRu/CXNA PtRu/CXNA400 PtRu/CXNA600 PtRu/CXNA750 PtRu/C E-TEK
Fig. 9 e Power density curves in a DMFC monocell at 70 �C,using as anodes the synthesized PteRu catalysts
supported on carbon xerogels. Cathode: Pt/C.
Concentration and methanol solution flow rate: 0.75 M and
15 minL1, respectively. Oxygen flow rate: 0.3 dm3 minL1.
4. Conclusions
EDX and XRD techniques demonstrated successful synthesis
of PteRu catalysts supported on carbon xerogels, with metal
loading near 20% wt and Pt:Ru atomic ratio close to 1:1.
Crystallite sizes varied between 3.6 and 4.6 nm and lattice
parameters were lower than 3.92 A of pure Pt, as expected for
the formation of a PteRu alloy. Electrochemical character-
ization confirms the large influence of the heat treatment
temperature of the support, on the final properties of PteRu
catalysts, with respect to the catalytic activity toward CO and
methanol oxidation. In general, the increase of the heat
treatment temperature unfavored the CO and methanol
electrochemical oxidations. This effect is explained by the
decrease of oxygenated surface groups on carbon xerogels.
Synthesized catalyst on carbon xerogels exhibited a good
performance in a DMFC monocell, in terms of potential drops
and power densities, the best behavior recorded with PteRu/
CXNA. Results were coherent with those obtained from the
electrochemical characterization. The carbon support of
PteRu/CXNA possesses the highest amount of carboxylic
groups, which are probably responsible for its enhanced
activity.
Electrochemical data showed that catalysts prepared with
xerogels are able to develop higher current densities during
methanol electrooxidation than the commercial one, but the
assembly in the DMFC monocell diminishes their perfor-
mance, presenting PteRu/C E-TEK the best curve. Changes in
i n t e rn 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 3 7 ( 2 0 1 2 ) 7 2 0 0e7 2 1 17210
conductivities and optimization of the structure of carbon
xerogels are essential to improve the final performance of
synthesized PteRu catalysts. Nevertheless, the results prove
that these materials might be suitable candidates for the
anode in direct alcohol fuel cells.
Acknowledgments
This work was carried out within the Accion Integrada
Hispano-Portuguesa (E-25/09 and HP2008-036). J.C.C. is
indebted to the Alban Program for the predoctoral fellowship
No. E07D403742CO. Authors acknowledge the Spanish
MICINN for financial support (MAT2008-06631-C03-01 and
MAT2008-06631-C03-02).
Appendix. Supplementary data
Supplementary data related to this article can be found online
at doi:10.1016/j.ijhydene.2011.12.029.
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