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Durability of different carbon nanomaterial supportswith PtRu catalyst in a direct methanol fuel cell

Annukka Santasalo-Aarnio a, Maryam Borghei b, Ilya V. Anoshkin b, Albert G. Nasibulin b,Esko I. Kauppinen b, Virginia Ruiz b,c, Tanja Kallio a,*aResearch Group of Fuel Cells, Department of Chemistry, Aalto University, P.O. Box 16100, 00076 Aalto, FinlandbNanomaterials Group, Department of Applied Physics, Aalto University, P.O. Box 16100, 00076 Aalto, FinlandcCIDETEC-IK4 (Centre for Electrochemical Technologies), Paseo Miramon 196, 20009 Donostia-San Sebastian, Spain

a r t i c l e i n f o

Article history:

Received 4 July 2011

Received in revised form

11 October 2011

Accepted 1 November 2011

Available online 9 December 2011

Keywords:

Carbon nanotubes

Carbon nanofibers

Methanol oxidation

Direct methanol fuel cell (DMFC)

* Corresponding author. Tel.: þ358 9470 2258E-mail address: tanja.kallio@tkk.fi (T. Kal

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.11.009

a b s t r a c t

PtRu catalysts with similar particle size and composition were deposited on three different

carbon supports: Vulcan, graphitized carbon nanofibers (GNF) and few-walled carbon

nanotubes (FWCNT) and their performance for methanol oxidation was studied in an

electrochemical cell and in a single cell DMFC. The electrochemical results indicate that

with PtRu/GNF and PtRu/FWCNT higher current densities are obtained and oxidation

intermediates deactivate the surface less compared to the same catalyst on Vulcan

support. Conversely, PtRu/Vulcan provided the highest open circuit voltage OCV and

current densities in DMFC experiments due to a well-optimized electrode layer structure.

Because stability is a key requirement for fuel cell commercialization, 6-day-long fuel cell

stability tests were carried out, showing that PtRu/Vulcan degraded significantly. This was

due to the collapse of the secondary structure of the electrode layer revealed by post

characterization of the membrane electrode assembly (MEA) with SEM and TEM. PtRu/GNF

exhibited slightly poorer initial performance but better stability because the structure of

the anode layer was maintained. PtRu/FWCNT showed the worst initial performance and

long-term stability. The good stability of non-optimized PtRu/GNF MEAs shows the

potential of these novel nanocarbon supported catalysts as stable fuel cell components

after proper MEA optimization.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction With the aim of reducing costs associated with the precious

Fuel cells running on liquid fuels such as direct methanol fuel

cells (DMFCs) are very promising energy sources for low power

electronic applications but improvements in performance and

design, as well as cost reduction are still necessary [1]. In

particular, developing more efficient and durable electro-

catalysts for methanol oxidation is crucial for achieving

enhanced performance and increasing the lifetime of DMFCs.

3; fax: þ358 9470 22580.lio).2011, Hydrogen Energy P

metal, nanosized mono and bimetallic catalysts deposited on

conducting carbons with high surface area are typically used.

Along with the nature of the catalysts, the search for better

andmore stable anodematerial has comprised evaluating the

influence of the carbon support on the catalytic activity. A

good catalyst support should have high surface area and

conductivity, exhibit good permeability to reactants and have

electrochemical stability under fuel cell operating conditions.

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 ) 3 4 1 5e3 4 2 43416

In this regard, many types of carbon nanostructures have

been investigated as supports for anode electrocatalysts in

DMFCs such as single-walled carbon nanotubes (SWCNTs),

multi-walled carbon nanotubes (MWCNTs), graphitized car-

bon nanofibers (GNFs), fullerenes and carbon nanohorns, due

to their appealing properties such as high surface area, elec-

tronic conductivity as well as chemical and mechanical

stability [2e16]. Most reports have demonstrated the superior

performance of highly porous tubular nanocarbons and

compared them to carbon black in terms of catalyst utilization

and dispersion. Thus, 2-fold enhancements of the catalytic

activity have been observed for MWCNT-supported Pt [6] or

PtRu [7,8] and for SWCNT-supported PtRu [9] and up to a 4-fold

increase for GNF-supported Pt [10,11] with respect to the

carbon black-supported catalysts. PtRu colloids on GNFs also

have 64%higher activities than the unsupported catalysts [17].

On the other hand, there are comparatively few systematic

studies where the performance of electrocatalysts deposited

in the same fashion on several carbon supports and from

various commercial sources is compared [17e20]. These

studies revealed notable differences in the activity trends as

well as in the enhancement factors (with respect to carbon

black) for the different carbon supports, with CNT-supported

catalysts either outperforming [18,21] or exhibiting poorer/

comparable activity [19,22,23] to carbon black. These con-

trasting results highlight the importance of conducting

further studies to gain more insight into the role of the cata-

lyst support. Furthermore, most of the reports on the impr-

oved activities of CNT or GNF-supported catalysts do not

address the crucial issue of the long-term stability and when

they do, conclusions on the stability of novel catalyst supports

are inferred from chronoamperometry or voltammetry tests

in electrochemical cells [8,10,19e21,23]. However, the catalyst

environment in the DMFC membrane electrode assembly

(MEA) differs considerably from that in a half-cell electro-

chemical set-up, which has motivated the current study

where activity and stability of PtRu catalysts on different

carbon supports have been compared in both scenarios.

Specifically, we have investigated both the catalytic

activity and long-term stability of PtRu catalysts deposited

following the same protocol on a series of carbon supports,

namely GNFs, few-walled CNTs (FWCNTs) and Vulcan. Metal

loadings, MEA preparation and DMFC testing were compa-

rable for the different electrocatalysts. In other reports where

the influence of the catalyst support was evaluated, catalyst

particle differed considerably for each support [17], which

makes more difficult to assess separately the effect of the

support. Here special effort has been done to achieve

comparable catalyst size and composition on the different

carbon supports to minimize differences arising from the

catalyst.Wewill show that the stability trends for catalysts on

different supports observed in chronoamperometry do not

necessarily correlate with long-term stability tests in DMFC

MEAs. Activities from chronoamperometric tests decrease in

the trend FWCNTs > GNFs > Vulcan, whereas the activities

observed in DMFC tests followed the opposite trend:

Vulcan>GNFs> FWCNTs. Therefore, caution is advisedwhen

attributing enhanced catalytic performance to novel electro-

catalysts based solely on electrochemical measurements.

Moreover, this also addresses the question of different MEA

preparation methods that need to be optimized also for the

high surface area catalyst supports.

2. Experimental methods

2.1. Catalyst material preparation

Vapour grown carbon nanofibres were kindly supplied by

Showa Denko (product reference VGNF). These are highly

graphitized nanofibers with a diameter of w150 nm and

a length of w10 mm. Prior to catalyst deposition, GNFs were

subject to oxidative surface functionalization by refluxing

them in a 1:1 (vol/vol%) mixture of HNO3 2 M and H2SO4 1 M at

120 �C for 6 h. Few-walled carbon nanotubes (FWCNT) with

2e5 walls (<6 nm diameter, w 1 mm length) were obtained by

catalytic pyrolysis of CH4 diluted with 80% H2 at 950 �Caccording to a method reported elsewhere [24]. The catalyst is

a mixture of CoeMo oxides (5 at.%) supported on MgO. After

the synthesis, residual catalyst was removed (<1% w.) from

the FWCNTs by washing in HCl, followed by rinsing with

deionized water and drying in vacuum. Prior to PtRu deposi-

tion, FWCNTs were acid treated to introduce surface func-

tional groups by refluxing them in a 1:1 mixture of HNO3 2 M

and H2SO4 1 M at 120 �C for 4 h. Vulcan XC72R powdered

carbon black with 96.11 kg m�3 bulk density (Cabot Corpora-

tion) was used as received for catalyst deposition.

2.2. Deposition of PtRu on Vulcan, FWCNTs and GNFs

Ru-Pt catalysts (w25% total metallic loading, Pt:Ru 1:1 atomic

ratio) were deposited on the different carbon supports by

reduction of the corresponding metal precursors, RuCl3 and

K2PtCl6, with NaBH4 according to a previously reported

procedure [25]. Briefly, the carbon supports were ultra-

sonicated in a solution containing ethylene glycol and water

for 20 min and then the required amount of the metal

precursors (with Pt:Ru atomic ratio of 1:1 and PtRu:C weight

ratio of 1:3) was added and ultrasonicated for 20 min. After

that, a solution of 0.04 M NaBH4 and 0.4 M NaOH was added

dropwise and the resulting suspension was kept under ultra-

sonication at 60 �C for 2 h for complete reduction. Finally the

suspension was filtered and the solid product was collected,

washed thoroughly with water and acetone and dried in

vacuum at 60 �C overnight.

2.3. Physical characterization of the materials

Scanning electron microscopy (SEM) was performed on a JEOL

JSM-7500FA field emission scanning electron microscope

equipped with an energy-dispersive X-rays spectrometer

(EDXS) for evaluation of catalyst metal loadings. For cross-

section imaging, MEAs were frozen in liquid nitrogen and

cut. Transmission electron microscopy (TEM) was done with

a Tecnai 12 Bio Twin transmission electron microscope with

LaB6 gun at 120 kV. As-prepared pristine catalyst powder and

degraded catalysts, scratched from the MEA anode, were

dispersed in isopropanol by short ultasonication and a drop

cast on TEM grids. Powder X-ray diffraction spectra were ob-

tained by a Bruker D8 Advance X-ray diffractometer using Cu

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 ) 3 4 1 5e3 4 2 4 3417

Ka radiation and a Lynx Eye fast detector with scan conditions

of 2s/0.03�. The BET surface area was determined with N2

adsorption/desorption at 77 K temperature with a FlowSorb

2300 II instrument byMicromeritics. A four-probemethodwas

used for measuring the electrical conductivities of the MEA

anodes.

2.4. Electrochemical characterization of the materials

The aim of the electrochemical characterization was to

reproduce the conditions in a fuel cell anode as closely as

possible and therefore, a similar ink as used for fuel cell MEA

was prepared with 5 mg of the studied catalyst material,

Nafion� solution (5 wt % Aldrich) and isopropanol (Merck,

p.a.). The ink was mixed carefully with a magnetic stirrer and

an ultrasonic bath. 5-ml droplet of the ink was placed onto

a glassy carbon electrode which was cleaned and polished

with alumina solution and an ultrasonic bath. The ink was

dried in air over night. An electrochemical cell was assembled

with this glassy carbon electrode as a working, a platinum coil

as a counter electrode and standard calomel electrode (SCE) as

a reference; nevertheless, all the potentials mentioned are

referred to the reversible hydrogen electrode (RHE). All the

glassware was carefully boiled in MQ water (0.04 mS cm�1,

Millipore) and rinsed various times prior to the measure-

ments. 0.1 M HClO4 (Merck, 70 wt %) was freshly prepared and

used as an electrolyte in order to ensure that no significant

anion adsorption from the electrolyte occurs on the PtRu

surface [26].

The surface potential of the working electrode was

controlled all the time when the electrode was in the elec-

trolyte that was de-aerated prior to the experiments. The

experiments were performed with a potentiostat/galvanostat

PGSTAT100 Autolab system. Firstly, a cyclic voltammogram

(20 mV s�1) was obtained to ensure that the electrode prepa-

ration had succeeded. The catalyst surface cleaning was done

with CO adsorption/oxidationmethod because cycling to high

potentials would have caused ruthenium dissolution [27]. The

electrolyte was purged with CO gas (99.99%) for 2 h while the

electrode was kept in 0.1 V vs. RHE potential and rotated at

300 rpm to ensure easy diffusion to the catalyst surface.While

the potential was still held the gas was changed to N2 and

purged for 30 min to remove the CO from the electrolyte.

Subsequently, a cyclic voltammogram with three scans was

obtained at 10 mV s�1 to show the CO oxidation peak behav-

iour. After this cleaning cycle voltammograms of the catalyst

surface were performed at 20 mV s�1. In this report, all the

currents are presented normalized by the catalyst loading on

the working electrode and all voltammograms refer to the

features in the third cycle, where steady-state response was

attained.

After the measurements in pure electrolyte, the solution

was changed to one containing the same electrolyte and 1 M

methanol. To prevent changes inmethanol concentration, the

gas inlet was first directed into a bubbler bottle containing the

same solution as the cell and methanol saturated gas was

directed to the experimental cell. Cyclic voltammogramswere

recorded with a rotating disc electrode (RDE) at 10 mV s�1 rate

with and without 1800 rpm rotation (Pine instruments). To

further study the catalyst poisoning additional

chronoamperometric measurements for 1 h were obtained at

0.7 V vs. RHE potential.

2.5. Fuel cell experiments

Prior to the fuel cell experiments a membrane electrode

assembly (MEA) with each of the studied catalyst materials

was made. The catalyst ink was prepared as a mixture of

catalyst (60 wt % of Pt on Vulcan (Alfa Aesar) for cathode and

the investigated PtRu catalyst for anode), Nafion� solution and

isopropanol solvent. The ink was first stirred with a magnetic

stirrer for several hours and the Vulcan catalyst was sonicated

with an ultrasonic bath. The slurry was painted onto the

membrane materials with an airbrush and dried in a vacuum

oven for 2 h. The metal loading of the cathode was

2� 0.2mg cm�2 and for the anode 1� 0.2mg cm�2. The reason

for using cathodes with higher catalyst loading and same

Nafion loading at the anode was to ensure that the differences

seen in the fuel cell results were purely dependent on the

anode catalyst supporting material. When both electrodes

were painted onto the membrane, the MEA was heat pressed

at 130 �C, with 50 kN pressure for 120 s. The fuel cell was

assembled with Teflon insulators, gas diffusion layers (carbon

cloths) and a MEA, closed and tightened evenly with 10 kN

force.

The fuel cell experiments were performed in a single cell

DMFC with surface area of 7.29 cm2. 1 M methanol (Merck,

p.a.) fuel was fed to the anodewith 1.5mlmin�1 rate and dried

oxygen gas (5.0 Aga) to the cathode at 300 ml min�1. The

temperature of the cell was controlled to be 70 �C � 1 �C. Thecell was stabilized over night with the studied methanol fuel

(0.2 ml min�1) and oxygen with 30 ml min�1 and normalized

2 h prior to the polarization experiments with higher flow

rates. The polarization curves were measured with a PGSTAT

20 instrument with an Ecochemie 10 A current booster

BSTR10A and a GPES software from the open circuit voltage

(OCV) to 0.05 V at the rate of 0.5 mV s�1. In addition, long-term

stability test for the MEAs at 70 � 1 �C was performed after

measuring the polarisation curves. The flow rates of the fuels

were lowered to 0.5 ml/min for the methanol and 90 ml min�1

for oxygen and stabilized for 2 h. The potential control was

applied and kept at constant 0.4 V for 137 h while the current

was observed as a function of time. After the measurement

the polarisation curve was obtained by the same method

described above. Finally, the degraded MEAs were removed

from the cell and the cross-sections were imaged with SEM

and catalyst material scratched from the MEA anode was

analysed by TEM.

3. Results and discussion

3.1. Structural characterization of the materials

As indicated above, special effort has been devoted to

obtaining catalysts with comparable metal loading, Pt:Ru

atomic ratio and particle size (Table 1) on the different carbon

supports with clearly different BET surface areas increasing in

order GNF < Vulcan < FWCNT. Metallic content and Pt:Ru

atomic ratios are determined from the pristine catalyst

Table 1 e Characterization data for PtRu-carbonnanomaterials. The fresh catalyst diameter has beendetermined from the pristine catalyst powder and thedegraded one from the MEA anode after fuel cellexperiments.

PtRu-Vulcan PtRu-GNF PtRu-FWCNT

Metal w% 22.6 23.5 26.0

Pt:Ru atomic ratio

(EDXS)

1.0 1.1 1.1

PtRu diameter

(TEM, nm)

fresh/degraded

3.3/4.5 3.2/3.5 2.8/3.3

PtRu diameter

(XRD, nm) fresh

3.01 3.47 3.34

Fig. 1 e TEM images of as-prepared PtRu catalysts on Vulcan (a

DMFC (def).

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powder as the average from multiple spot EDXS analysis of Pt

and Ru emission intensities. TEM images and histograms

(Fig. 1) indicate that all powder catalysts exhibit good particle

dispersion and narrow particle size distribution, with more

aggregates observed on GNFs and FWCNTs than on Vulcan.

Mean particle sizes, which are summarized in Table 1, do not

differ considerably for the different as-prepared catalysts.

Fig. 2 presents XRD patterns of as-prepared catalysts,

which clearly show the diffraction peaks of PtRu alloy crystal

faces (111), (200), (220) and (311) at around 40.2�, 46.8�, 68.3�,82.3�, respectively, characteristic of a face-centered cubic (fcc)

structure with a cell constant of 3.88 Ǻ for all catalyst samples.

The average particle size was estimated from the Pt (111)

diffraction peak according to the Scherrer formula for the

), GNF (b) and FWCNT (c) and after 137 h stability tests in

Fig. 2 e XRD patterns of as-prepared catalyst powders.

Fig. 3 e Cyclic voltammograms of PtRu catalyst on different

carbon supports in 0.1 M HClO4, scan rate of 20 mV sL1.

PtRu/Vulcan (dashed, black line), PtRu/FWCNT (thin, red

solid line), PtRu/GNF (thick, green solid line). (For

interpretation of the references to colour in this figure

legend, the reader is referred to the web version of this

article.)

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 ) 3 4 1 5e3 4 2 4 3419

different catalysts (Table 1), which is in good agreement with

the values calculated from the TEM images. Hexagonal carbon

peak C(002) at around 26� is dominant but the other charac-

teristic peaks C(100) around 42� and C(101) at 45� overlap with

the PtRu diffraction pattern and consequently are not well

resolved. Moreover, graphite peaks C(004) around 54� and

C(110) around 78� can be observed only for the GNF support

showing highest intensities. The comparable structure ach-

ieved for the PtRu nanoparticles in the three catalystmaterials

enables assessing separately the influence of the type of

carbon support on the observed catalytic properties of the

ensemble.

3.2. Electrochemical characterization of the materials

The prepared PtRu catalyst materials on different supports

have been studied in an electrochemical cell to determine

their activity for methanol oxidation. The catalyst materials

have been cleaned with a CO adsorption/oxidation procedure

and not by cycling to high voltages to avoid the dissolution of

ruthenium from the alloy [27]. This is also the reason why all

the experiments are performed only up to 0.7 V. Electro-

chemical surface areas calculated from the CO adsorption/

oxidation experiments result in similar values in range of

12e14 m2 g�1 for carbon supported alloy catalyst materials.

This is expected as special effort was done to synthetize

materials with similar particle size and distribution. However,

these values have not been used to normalize the currents

because details of CO adsorption on ruthenium surface are

unknown; instead the current are normalized with catalyst

loading.

The voltammograms obtained in 0.1MHClO4 are presented

in Fig. 3. All three catalysts exhibit the typical features of PtRu

nanoparticle catalysts in HClO4 electrolyte [28,29]: a clear

electrosorption of hydrogen on platinum is observed at

potentials between 0.05 and 0.1 V. Shortly after this the

ruthenium oxide formation initiates at 0.2 V [28]. This poorly

resolved hydrogen adsorption region is the reason why

hydrogen adsorption/desorption on platinum surface can not

be used for electrochemical surface area determination.

Instead measured currents have been divided by the catalyst

loading on the electrode. For FWCNT support the active

surface area is higher than for the other two supports and

therefore slightly higher currents are obtained. Moreover,

with PtRu/FWCNT an additional peak at the platinum oxide

region is observed around 0.55 V that has been previously

reported in H2SO4 electrolyte at 0.65 V [6,8,30] and might be

due to electroactive surface groups on carbon support [31]

resulting from the oxidative acid treatment of FWCNTs.

Fig. 4A shows the activity of PtRu catalyst on different

supports towards methanol oxidation. The cyclic voltammo-

grams indicate that around four times higher currents are

obtained with PtRu/FWCNT support and even eight times

higherwith PtRu/GNF compared to PtRu/Vulcan. These results

are in accordance with the results obtained by other groups

when comparing separately CNT to Vulcan [8,9,19,21e23] or

GNF to Vulcan [10]. On the other hand, there are only few

reports where the performance of MWCNT, SWCNTs from

several suppliers and GNF supports are compared by the same

group [18] observing that all PtRu on CNT supports are more

active for methanol oxidation than PtRu/GNFs.

In general, cyclic voltammetry is not a good method to

model the conditions that catalyst and support material need

to tolerate in the fuel cell, where current is drawn with

constant potential. Therefore, chronoamperometric experi-

ments (Fig. 4B) have also been performed at 0.7 V. As can be

seen from Fig. 4B with all of the supports the current obtained

at 0.7 V in the cyclic voltammogram (Fig. 4A) is notmaintained

for a long time due to the platinum catalyst sites poisoning by

methanol oxidation intermediates [29,32]. Especially, the high

current obtained with PtRu catalyst on GNF support decreases

4 times after around 500 s, approaching the value of PtRu/

Vulcan after 1 h. Even if the maximum current density

Fig. 4 e Methanol oxidation on PtRu catalyst on different

carbon supports in 0.1 M HClO4 and 1 M methanol

solutions. (A) Cyclic voltammograms with 1800 rpm and

10 mV sL1 and (B) chronoamperometric curves at 0.7 V

potential. (a) PtRu/Vulcan (black line), (b) PtRu/FWCNT (red

line), (c) PtRu/GNF (green line). (For interpretation of the

references to colour in this figure legend, the reader is

referred to the web version of this article.)

Fig. 5 e DMFC polarization curves for different PtRu catalyst

supports at 70 �C. On cathode 2 mg cmL2 Pt on Vulcan is

used in each case. PtRu catalyst loading for anode is

around 1 mg cmL2. (a) PtRu/Vulcan (black line), (b) PtRu/

FWCNT (red line), (c) PtRu/GNF (green line). (For

interpretation of the references to colour in this figure

legend, the reader is referred to the web version of this

article.)

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 ) 3 4 1 5e3 4 2 43420

obtained for PtRu/FWCNT with cyclic voltammetry is almost

half that of PtRu/GNF, the same catalyst on a FWCNT support

seems to be clearly more stable, retaining 69% of the initial

current after 1 h at 0.7 V. This value is considerably high

compared to the 10% reported by Li et al. for the same exper-

iment with PtRu catalyst on CNT [8]. Current drops of 16% [19]

and 78% [23] have been reported for other CNT-supported

catalysts during chronoamperometric measurements at

lower potential (0.4 V). The wide range of the results obtained

by different groups illustrates the relevance of catalyst and

support preparation methods.

3.3. Fuel cell performance

Direct comparison of results reported by different research

groups on the fuel cell performance of various catalyst

supports is a challenging task due to the variety of experi-

mental protocols used for MEA preparation and fuel cell

operating conditions. To the best of our knowledge no attempt

to evaluate the long-term performance of fuel cell catalysts on

several different nanocarbon supports in the same DMFC set-

up has been previously done. In Fig. 5 polarization curves of

DMFC with similar catalyst loading and ionomer content but

different anode catalyst supports are presented. All the pris-

tine MEA anodes show electronic resistivity in the range of

1e2 mU m. The open circuit voltage (OCV) depends e.g. on

operating temperature, methanol crossover through the

Nafion membrane, selected catalysts and electrode layer

structure. For the three support materials all the conditions

remain the same except the electrode layer structure. The

OCV value obtained for PtRu/Vulcan MEA (590 mV, Table 2) is

in agreement with previously published data [9,17,33] and

PtRu/GNF MEA has only 20 mV lower OCV. However, a signif-

icantly lower OCV (450 mV) has been obtained for the PtRu/

FWCNT MEA compared to the other supports. This trend

agrees with that observed by Liu et al. [9] with higher catalyst

loadings.

PtRu/VulcanMEA yields high current density (299mA cm�2,

Table 2) even with very low (1 mg cm�2) anode loadings. PtRu/

GNF MEA also produces a reasonable current density of

231 mA cm�2 whereas the value for PtRu/FWCNT is moderate

(158 mA cm�2). SEM images of the MEA cross-sections for the

different catalysts before the experiments (Fig. 6) can help to

explain the differences observed for fuel cell performance and

OCV. Anode thicknesses have also been determined (Table 2)

from the MEA cross-section SEM images. The fresh PtRu/

Vulcan anode layer is considerably thinner than the electrode

layers of the other supports, which are almost 8 times thicker

even for a same catalyst loading. This is due to the higher

surface area of the tubular nanocarbon supports and conse-

quently larger volumes for the same catalyst loading. Also from

the SEM cross-section images it can be seen that PtRu/Vulcan

anodes are optimally structured due to the years of practice in

Table 2 e The performance of the different MEAs thicknesses the anode electrodes by SEM analysis before and after the137 h chronoamperometric experiment in a fuel cell.

Support OCV (initial) Max. current density OCV (after) Current density Anode thickness

V mA cm�2 V Decreaseafter 137 h

Fresh mm Degradedmm

Thicknesschange

Vulcan 0.59 299 0.57 14% 8 � 1 27 � 3 þ2.7%

FWCNT 0.45 158 0.46 35% 60 � 9 63 � 8 �0.04%

GNF 0.55 231 0.54 1% 65 � 5 56 � 4 0%

The range of thicknesses and mean value with errors for each electrode is given.

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 ) 3 4 1 5e3 4 2 4 3421

optimizing the MEA preparation conditions. It is important to

keep in mind that MEA fabrication plays a vital role in DMFC

performance. When dealing with supports with totally

different surface area, the optimal solvent and Nafion solution

contents for Vulcan supported-catalyst are not necessary the

Fig. 6 e Cross-section SEM images of fresh (aec) and degraded

(b, e) and FWCNT (c, f).

best ones for GNF and FWCNT-supported ones. This has been

noticed by us and also by other groups for PEMFC MEAs and

a detailed study of optimizing ink composition for catalysts on

FWCNT and GNF supports will be subject of a further separate

study.

(def) MEA anodes with PtRu catalysts on Vulcan (a, d), GNF

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 ) 3 4 1 5e3 4 2 43422

Nevertheless, a basic practical requirement for the use of

these materials as cell components in commercial fuel cells is

a good long-term stability. This type of tests are time

consuming and therefore often absent in many reports

assessing the performance of novel catalyst supports. Here

potentiostatic experiments in DMFC for 137 h at 0.4 V were

performed after measuring the polarisation curves in order to

evaluate the durability of the catalyst structure (Fig. 7).

Immediately after this experiment another polarisation curve

was measured and the losses in OCV and maximum current

density are presented in Table 2. To further study the loss and

agglomeration of the catalyst particles, EDXS analysis and

TEM images of the used anode catalyst were taken and are

presented in Fig. 1def along with the catalyst particle size

distribution. EDXS characterization of the catalysts after

stability tests do not reveal significant metal loss in any of the

supports. However, TEM analysis of the degraded catalysts

(Fig. 1) show particle size growth and broader size distribu-

tions, the extent of which varies with the catalyst support

(Table 1).

As can be seen from Fig. 7, PtRu/Vulcan produces the

highest current density, as expected from the polarisation

curve (Fig. 5), nevertheless, the current density degradation is

rapid and no steady value seems to be attained even for such

a long experiment. After the 6-day experiment the OCV

decreased only 20 mV (Table 2), however, the 14% loss of the

maximum current density is problematic from the point of

view of applications. This degradation can be explained by

comparing SEM images of the fresh and degraded PtRu on

Vulcan support (Fig. 6a and d). In the fresh anode, the PtRu/

Vulcan layer exhibits a three-dimensional net structure that

allows efficient liquid and gas flow through the whole anode

layer. Conversely, several cross-section SEM images of the 6-

day used MEA (Fig. 6d) show that the secondary structure of

the anode layer is collapsed, inhibiting the transport of the

Fig. 7 e The current density obtained at 0.4 V with different

support materials in a DMFC at 70 �C. (a) PtRu/Vulcan (black

line), (b) PtRu/FWCNT (red line), (c) PtRu/GNF (green line).

(For interpretation of the references to colour in this figure

legend, the reader is referred to the web version of this

article.)

fluids through the electrode. Moreover, the anode layer has

swelled in diluted methanol solution and the anode thickness

has increased almost 3.5 times (Table 2). Moreover, the mean

particle size of PtRu catalyst on Vulcan support increases from

3.3 nm to 4.5 nm (Fig. 1d) and the size distribution becomes

clearly broader compared to fresh catalyst material.

A similar phenomenon can also explain the poor stability

of the PtRu/FWCNT MEA during the 6-day test (Fig. 7, Table 2).

As seen in the SEM images, the fresh PtRu/FWCNT anode layer

shows spongy morphology offering pathways for fluid flow

(Fig. 6c) which leads to the high anode thickness (Table 2).

Nevertheless, after the 6-day experiment a dramatic current

drop was observed, which is mainly due to the loss of path-

ways in the anode layer clearly visible in Fig. 6f. Moreover,

TEM images also reveal the presence of more bundles in the

PtRu/FWCNT anode after the stability test, which together

with the increase in average particle size from 2.8 nm to

3.3 nm and loss of cavities in the anode layer, could contribute

to the highest current drop (35%) noted for the investigated

PtRu MEAs.

Even though PtRu/GNF catalyst shows high activity

towards methanol oxidation in RDE measurements (Fig. 4) it

exhibits poorer performance than PtRu/Vulcan in DMFC

experiments (Fig. 5). These results contrast with those re-

ported by Guo et al. [34] for PtRu catalyst on carbon nanofibers

that showed modest activity for methanol oxidation in elec-

trochemical cell but still slightly higher performance in DMFC

compared to Vulcan. Unfortunately, no detailed information

on MEA preparation process was provided by the authors. In

our study the current density of 231 mA cm�2 obtained with

the PtRu/GNF anode (Fig. 5) is surprisingly high even though

the anode thickness is 8 times thicker compared to the PtRu/

Vulcan anode. Interestingly, the current density changes

during the 6-day experiment are almost negligible (Table 2).

The reason for this better stability can be inferred from SEM

characterization of the MEA cross-section (Fig. 6b and e) that

shows how the spongy three-dimensional structure of PtRu/

GNF anode has been preserved during the 6-day measure-

ment, unlike with the other supports. Moreover, the particle

size growth is comparatively less pronounced for PtRu/GNFs

from degraded MEAs, which further explains the remarkable

stability of these catalysts.

As a mechanically and chemically resistant material, GNF

is able to withstand the mechanical and electrochemical

stress induced by the fluid flow, electrochemical reactions and

dimensional changes upon starting and shutting down of the

fuel cell. Thus, we envisage that after suitable optimization of

MEA preparation process for the PtRu/GNF anodes, these

materials could provide higher current densities with better

stability than conventional PtRu/Vulcan anodes.

4. Conclusions

In thiswork PtRu catalyst has been deposited on three different

carbon support materials with special effort to obtain compa-

rable particle size and composition in order to evaluate the

effect of the support in the catalytic activity and stability of the

materials in DMFC anodes. Very similar metal loadings,

particle sizes and even distributions were attained for the

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 ) 3 4 1 5e3 4 2 4 3423

catalysts on the different carbon supports. Electrochemical

experiments show that the highest currents are reached with

PtRu catalyst on GNF support whereas PtRu/FWCNT seems to

be more tolerant to methanol oxidation intermediates. Never-

theless, catalysts on both tubular supports exhibit clearly

higher activity towards methanol oxidation and better toler-

ance for COads poisoning than the same catalyst on Vulcan

support. When the same catalysts are compared in a DMFC

anode an inverse trend is observed, with PtRu/Vulcan exhibit-

ing the highest OCV and current densities. However, when

subject to the long-term chronoamperometry experiments this

catalyst degrades fast, the three-dimensional layer structure of

the Vulcan anode is lost, resulting in a gradual decrease of

current density that is problematic from the application point

of view. PtRu/FWCNT produced moderate current densities

and suffered more pronounced current density losses and

anode layer structural changes. On the other hand, although

PtRu/GNF provided slightly lower initial current density than

PtRu/Vulcan, negligible current loss was detected during the 6-

day experiment, indicating a more stable nature of the anode

layer structure. Overall, further optimization of MEA prepara-

tion for the novel catalyst supports in view of the MEA struc-

tural characterization after the long-term stability tests shall

lead to enhanced performance as DMFC anode catalyst layers.

Acknowledgements

The Academy of Finland (T.K., V.R., M.B. and A.G.N., Academy

Research Fellowship) and the Spanish Ministry of Science and

Innovation (V.R., Prog. Ramon y Cajal) are acknowledged for

financial support. We also thank Dr. Iratxe de Meatza for

helpful discussions and Kati Vilonen for help with the BET-

area determination.

r e f e r e n c e s

[1] Borup R, Meyers J, Pivovar B, Kim YS, Mukundan R,Garland N, et al. Scientific aspects of polymer electrolyte fuelcell durability and design. Chem Rev 2007;107:3904e51.

[2] Lee K, Zhang J, Wang H, Wilkinson DP. Progress in thesynthesis of carbon nanotube- and nanofiber-supported Ptelectrocatalysts for PEM fuel cell catalysis. J ApplElectrochem 2006;36:507e22.

[3] Antolini E. Carbon supports for low-temperature fuel cellcatalysts. Appl Catal B Environ 2009;88:1e24.

[4] Kosaka M, Kuroshima S, Kobayashi K, Sekino S, Ichihashi T,Nakamura S, et al. Single-wall carbon nanohorns supportingPt catalyst in direct methanol fuel cells. J Phys Chem C 2009;113:8660e7.

[5] Vinodgopal K, Haria M, Meisel D, Kamat P. Fullerene-basedcarbon nanostructures for methanol oxidation. Nano Lett2004;4:415e8.

[6] Saha MS, Li R, Sun X. High loading and monodispersed Ptnanoparticles on multiwalled carbon nanotubes for highperformance proton exchange membrane fuel cell. J PowerSources 2008;177:314e22.

[7] Chien C-C, Jeng K- T. Effective preparation of carbonnanotube-supported Pt-Ru electrocatalysts. Mater ChemPhys 2006;99:80e7.

[8] Li L, Xing Y. Pt-Ru nanoparticles supported on carbonnanotubes as methanol fuel cell catalysts. J Phys Chem C2007;111:2803e8.

[9] Liu Z, Ling XY, Guo B, Hong L, Lee JY. Pt and PtRunanoparticles deposited on single-wall carbon nanotubes formethanol electro-oxidation. J Power Sources 2007;167:272e80.

[10] Ma Y, Jiang S, Jian G, Tao H, Yu L, Wang X, et al. CNx

nanofibers converted from polypyrrole nanowires asplatinum support for methanol oxidation. Energy Environ Sci2009;2:224e9.

[11] Bessel CA, Laubernds K, Rodriguez NM, Baker RTK. Graphitenanofibers as an electrode for fuel cell applications. J PhysChem B 2001;105:1115e8.

[12] Li W, Wang X, Chen Z, Waje M, Yan Y. Pt-Ru supported ondouble-walled carbon nanotubes as high-performance anodecatalyst for direct methanol fuel cells. J Phys Chem B 2006;110:15353e8.

[13] Wang X, Li W, Chen Z,Waje M, Yan Y. Durability investigationof carbon nanotube as catalyst support for proton exchangemembrane fuel cell. J Power Sources 2006;158:154e9.

[14] Ahmadi R, Amini MK. Synthesis and characterization of Ptnanoparticles on sulfur-modified carbon nanotubes formethanol oxidation. Int J Hydrogen Energy 2011;36:7275e83.

[15] Chem Y, Zhang G, Ma J, Zhou Y, Tang Y, Lu T. Electro-oxidation of methanol at the different carbon materialssupported Pt nano-particles. Int J Hydrogen Energy 2010;35:10109e17.

[16] Tsai M-C, Yeh T-K, Tsai C- H. Methanol oxidation on carbon-nanotube-supported platinum and platinum-rutheniumnanoparticles prepared by pulsed electrodeposition. Int JHydrogen Energy 2011;36:8261e6.

[17] Steigerwalt ES, Deluga GA, Lukehart CM. Pt-Ru/carbon fibernanocomposites: synthesis, characterization, andperformance as anode catalysts of direct methanol fuel cells.A search for exceptional performance. J Phys Chem B 2002;106:760e6.

[18] Girishkumar G, Hall TD, Vinodgopal K, Kamat PV. Single wallcarbon nanotube supports for portable direct methanol fuelcells. J Phys Chem B 2006;110:107e14.

[19] Chetty R, Kundu S, Xia W, Bron M, Schuhmann W, Chirila V,et al. PtRu nanoparticles supported on nitrogen-dopedmultiwalled carbon nanotubes as catalyst for methanolelectrooxidation. Electrochim Acta 2009;54:4208e15.

[20] Selvaraj V, Alagar M. Pt and Pt-Ru nanoparticles decoratedpolypyrrole/multiwalled carbon nanotubes and theircatalytic activity towards methanol oxidation. ElectrochemCommun 2007;9:1145e53.

[21] Jiang S, Zhu L, Ma Y, Wang X, Liu J, Zhu J, et al. Directimmobilization of Pt-Ru alloy nanoparticles on nitrogen-doped carbon nanotubes with superior electrocatalyticperformance. J Power Sources 2010;195:7578e82.

[22] Gan L, Lv R, Du H, Li B, Kang F. High loading of Pt-Runanocatalysts by pentagon defects introduced in a bamboo-shaped carbon nanotubes support for high performanceanode of direct methanol fuel cells. Electrochem Commun2009;11:355e8.

[23] Liu Z, Lee JY, Chen W, Han M, Gan LM. Physical andelectrochemical characterization of microwave-assistedpolyol preparation of carbon-supported PtRu nanoparticles.Langmuir 2004;20:181e7.

[24] Rakov EG, Grishin DA, Gavrilov YV, Rakova EV, Nasibulin AG,Jiang H, et al. The morphology of pyrolytic carbon nanotubeswith a small number of walls. Russ J Phys Chem 2004;78:2222e7.

[25] Huang T, Liu J, Li R, Cai W, Yu A. A novel route forpreparation of PtRuMe (Me ¼ Fe, Co, Ni) and their catalyticperformance for methanol electrooxidation. ElectrochemCommun 2009;11:643e6.

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 ) 3 4 1 5e3 4 2 43424

[26] Lai SCS, Koper MTM. Electro-oxidation of ethanol andacetaldehyde on platinum single-crystal electrodes. FaradayDiscuss 2008;140:399e416.

[27] Arico AS, Srinivasan S, Antonucci V. DMFCs: fromfundamental aspects to technology development. Fuel Cells2001;1:133e61.

[28] Gojkovic SL, Vidakovic TR, Durovic DR. Kinetic study ofmethanol oxidation on carbon-supported PtRuelectrocatalyst. Electrochim Acta 2003;48:3607e14.

[29] Lin WF, Iwasita T, Vielstich W. Catalysis of COelectrooxidation at Pt, Ru, and PtRu alloy. An in situ FTIRstudy. J Phys Chem B 1999;103:3250e7.

[30] Lin Y, Cui X, Yen CH, Wai CM. PtRu/carbon nanotubenanocomposite synthesized in supercritical fluid: a novel

electrocatalyst for direct methanol fuel cell. Langmuir 2005;21:11474e9.

[31] Wang J, Yin G, Shao Y, Zhang S, Wang Z, Gao Y. Effect ofcarbon black support corrosion on the durability of Pt/Ccatalyst. J Power Sources 2007;171:331e9.

[32] Liu P, Logadottir A, Norskov JK. Modeling the electro-oxidation of CO and H2/CO on Pt, Ru, PtRu and Pt3Sn.Electrochim Acta 2003;48:3731e42.

[33] Santasalo A, Kallio T, Kontturi K. Performance of liquid fuelsin a platinum-ruthenium-catalysed polymer electrolyte fuelcell. Platinum Metals Rev 2009;53:58e66.

[34] Guo J, Sun G, Wang Q, Wang G, Zhou Z, Tang S, et al. Carbonnanofibers supported Pt-Ru electrocatalysts for directmethanol fuel cells. Carbon 2006;44:152e7.