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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: epastor@ull.es, 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 922318033.astor_tejera@yahoo.es (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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