ww.sciencedirect.com

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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 4

Available online at w

journal homepage: www.elsevier .com/locate/he

Electrochemical oxidation of alcohols on PteTiO2

binary electrodes

Bjorn Hasa a, Evangelos Kalamaras a, Evangelos I. Papaioannou a,Labrini Sygellou b, Alexandros Katsaounis a,*aDepartment of Chemical Engineering, University of Patras, GR26504, Greeceb Institute of Chemical Engineering Sciences (FORTH/ICE-HT), GR26504, Greece

a r t i c l e i n f o

Article history:

Received 14 June 2013

Received in revised form

28 August 2013

Accepted 17 September 2013

Available online 11 October 2013

Keywords:

Direct alcohol fuel cell

Methanol

Ethanol

PteTiO2

* Corresponding author. Tel.: þ30 2610962757E-mail addresses: alex.katsaounis@chem

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.09.1

a b s t r a c t

In this study PteTiO2 binary electrodes were prepared by means of thermal decomposition

of chloride precursors on Ti substrates, characterised by X-ray Diffraction (XRD), Scanning

Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), electrochemical

techniques and CO stripping and used as anodes for alcohol oxidation. The minimization

of the Pt loading without electrocatalytic activity losses was also explored. TiO2 was chosen

due to its chemical stability, low cost and excellent properties as substrate for Pt disper-

sion. It was found that TiO2 loading up to 50% results in Electrochemically Active Surface

(EAS) increase. The EAS of Pt(50%)-TiO2(50%) was found to be almost one order of magni-

tude higher than that of pure Pt while the EAS of samples with Pt loading lower than 30%

was negligible. The above conclusion has been confirmed both by following the charge of

the reduction peak of platinum oxide and by CO stripping experiments. All samples have

been evaluated during the electrochemical oxidation of methanol and ethanol. In both

cases the Pt(50%)-TiO2(50%) electrode had better electrocatalytic activity than the pure Pt

anode. The observed higher performance of the binary electrodes was mainly attributed to

the enhanced Pt dispersion as well as the formation of smaller Pt particles by the addition

of TiO2.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction electric vehicles [1]. Alcohols are suitable fuel because they

In various applications, fuel cells are widely recognized as

very promising devices to obtain electric power directly from

the combustion of chemical compounds. Fuel cells are also

advantageous because they release less, if any, harmful

emissions during operation and, in some cases, the fuel used

to power fuel cells can be considered renewable. Especially

low temperature fuel cells that utilize alcohols as a direct fuel

(Direct Alcohol Fuel Cells, DAFCs) are widely proposed as

possible power generators for mobile applications such as

; fax: þ30 2610997269.eng.upatras.gr, alex.katsa2013, Hydrogen Energy P10

have a high energy density (energy per unit volume) and exist

in liquid form at room temperature, which allows for easier

handling, transportation and storage. Many studies have been

aimed toward direct methanol fuel cells since methanol has

good kinetics of oxidation compared to other alcohols at low

temperatures and is known to be oxidized completely to CO2,

resulting in the maximum transfer of electrons [2]. However,

the toxicity of methanol and its high crossover rate in fuel

cells have led some researchers to investigate other, safer,

alcohols, such as ethanol [3e9]. Among several metal

ounis@enveng.tuc.gr (A. Katsaounis).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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 415396

catalysts, Pt based electrodes have shown high electro-

catalytic activity towards methanol and/or ethanol oxidation

[10,11]. One of themajor drawbacks of Pt is that the CO species

which are formed during the oxidation process occupy the

active sites resulting in catalyst deactivation [12e15].

For the case of methanol oxidation the overall reaction can

be expressed by the following (6 electron process) equation:

CH3OH þ H2O / CO2 þ 6Hþ þ 6e� (1)

Several studies have focused on the elucidation of the re-

action mechanism in terms of identification of reaction in-

termediate adsorbed species and their distribution on the

electrode surface as well as the understanding of the rate

determining step [16]. Detailed mechanistic studies using

various techniques (such as in situ infrared reflectance spec-

troscopy [17,18]) have shown that in a simplistic model, the

adsorption of methanol molecule on the Pt surface can lead

into two types of adsorbed species, i.e COads and ($CHO)adswhich were further oxidised to the final oxidation product

[16]. In acidic media, Pt followed by Ir are the most effective

metals for breaking the CeH bonds during the adsorption step

[16,19,20]. However, the strongly adsorbed CO species have

been identified as the main poisoning species which are

blocking the active sites and diminishing the dissociation rate

of methanol. Oxidation of these adsorbed species is consid-

ered to be the rate determining step of the overall reaction

[16,17,21]. It is commonly accepted that the OH adsorption

from water dissociation is necessary to oxidize the strongly

adsorbed CO species [16,22]. Thus, it is necessary to introduce

another metal with the ability to decrease the coverage of CO

adsorbed species and increase the coverage of OH species.

Ruthenium is recognised as an excellent catalyst for water

dehydrogenation and enhancement of methanol electro-

oxidation through minimisation of the CO species formation

rate [22,23]. This is the reason why PteRu catalysts are

nowadays the best commercially available anodes for direct

methanol fuel cells.

In general, there are three different effects which occur

when binary or ternary electrodes are used as anodes in

DAFCs. The first one is the cooperative (or bi-functional) effect

[24e26] which is the case for the PteRu electrodes mentioned

above. There is a bifunctional mechanism where the second

or the third alloy metal activates water at low potentials and

hence promotes the CO electro-oxidation. The second one is

the electronic (or ligand) effect [27e30] where the addition of the

second metal results in Pt work function modification. Thus,

the PteCO bond is weakening due to the shift of the d electron

from the alloy metal to Pt. The last effect considered as more

likely in this study is the geometric effect. In this case, the

second element is generally inactive having the role of the

support in order to achieve better Pt dispersion and thus

higher active surface.

During the last decades many studies have been focused

on the development of Pt-based novel electrocatalysts, such

as Pt-based bimetallic alloys, nanoparticle mixtures, and

composites [31e41]. Metals such as Co, Os, Sn, Ir, Pd,W, Ni, Au,

Fe have been proposed for Pt based binary or even ternary

electrodes with enhanced electrocatalytic performance dur-

ing alcohols oxidation [42e44]. However, the high cost of these

electrodes is one of the major drawbacks for their further

commercialisation.

On the other hand, Pt together with stable metal oxides,

such as PteMOx (M ¼ Ru, W, or Ti) and more specific PteTiO2

have been studied, mostly as potential cathodes in direct

alcohol fuel cells [45e52] or as photoanodes in photo-

electrochemical cells [53e58]. Thus, various mechanisms have

been discussed for the role of TiO2 and the electrocatalytic ac-

tivity of PteTiO2 towards oxygenereduction reaction (ORR) and

methanol or ethanol tolerance. For cathodic electrodes, incor-

porating TiO2 with Pt is supposed to help in mitigating the ag-

gregation of Pt particles and protecting the Nafion membrane

against peroxide radicals formedduring the cathodic reduction

of oxygen. PteTiO2 electrodes are very attractive due to their

cost effectiveness and acid stability [45e50]. TiO2 shows high

durability in relation to conventional carbon supports; however

its electronic conductivity is relatively lower resulting in

increased ohmic resistance for the cell [45,47]. In addition, it is

easier to disperse Pt nanoparticles on TiO2 rather than on car-

bon supports which are characterized by their hydrophobicity.

Additionally, setting good dispersion of the Pt nanoparticles

even after carbon surface modification with oxidizing acids or

surfactants has been reported to be difficult [59].

The majority of the PteTiO2 electrodes reported in the

literature were prepared by solegel method, electrodeposition

andmagnetron sputtering [45e51,60]. In our case, the PteTiO2

electrodes were prepared bymeans of thermal decomposition

of chloride precursors at high temperature following the

preparation method of DSA type electrodes [61e63]. The films

were deposited on Ti substrates instead of classical carbon

substrates. It is well known that different preparation tech-

niques result in materials with different properties. Thus,

electrodes with the same composition could behave differ-

ently if the procedure of the preparation alters. During this

study, PteTiO2 binary electrodes with different metal ratios

were prepared, characterized physicochemically and electro-

chemically and afterwards were studied as anodes for meth-

anol and ethanol electrooxidation.

2. Experimental

The PteTiO2/Ti electrodes were prepared bymeans of thermal

decomposition of H2PtCl6 (Fluka) and TiCl3 (SigmaeAldrich) at

500 �C in air taking into account that the deposition yield of Pt

and TiO2 is about 60% and 100%, respectively [63e65]. Both the

precursors were dissolved in isopropanol (Sigma Aldrich e

34863) to get solutions of 100 mM H2PtCl6 and 25 mM TiCl3,

followed by sonication for 15 min. Appropriate ratios were

used to obtain electrodes with different Pt and TiO2 loadings.

A small amount (50 mL) of the precursor solution was applied

with a micropipette on a Ti plate (15 � 15 mm) previously

treated in boiling 1 M oxalic acid (�97% Fluka) solution. The

sample was heated first at 70 �C for 10 min followed by

calcination at 500 �C for 10 min. This procedure was repeated

several times until a final specific mass of 1.9 � 0.1 mg/cm2

was reached. The samples were finally treated at 500 �C for

60 min. Seven electrodes with different Pt:TiO2 molar ratios

were prepared (Pt:TiO2 / 100:0, 85:15, 65:35, 50:50, 35:65, 15:85

and 0:100).

Fig. 1 e XRD spectra of the Ti substrate, Pt/Ti, TiO2/Ti and

Pt(50%)-TiO2(50%)/Ti.

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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 4 15397

XRD measurements were carried out by the use of Siemens

D-5000 XRD instrument. The crystallite size calculation was

based on X-ray line broadening of the diffraction peak accord-

ing to Scherrer’s equation. The morphology of the surface was

explored with SEM using a Zeiss LEO SUPRA 35VP electron

microscope. The surface analysis studies were performed in a

UHV chamber (P < 10�9 mbar) equipped with an SPECS LHS-10

hemispherical electron analyzer. The XPS measurements were

carried out at room temperature using unmonochromatized

AlKa radiation under conditions optimized for maximum

signal (constant DEmodewith pass energy of 97 eV giving a full

width at halfmaximum (FWHM) of 1.7 eV for theAu 4f7/2 peak).

Themeasurements took place at two sample tilt angles, q, 0 and

45� with respect to the sample surface normal. The analysed

area was a rectangle with dimensions 2.5 � 4.5 mm2. The XPS

core level spectra were analysed using a fitting routine, which

can decompose each spectrum into individual mixed Gaus-

sianeLorentzian peaks after a Shirley background subtraction.

ThemainC1s peak at 284.7 eV from superficial carbonwas used

in binding energy (BE) corrections for specimen charging.

Electrochemical measurements were carried out in a single

compartment three-electrode cell (50 mL) using an Autolab

potentiostat (mAutolab Type III, Eco Chemie). The counter elec-

trodewasaPtwirewhile thereferenceelectrodewasamercurous

sulphate electrode, MSE (REF 621, Radiometer Analytical) with a

potential of þ0.64 V versus standard hydrogen electrode (SHE).

Thedatawere recorded andanalysedwith the softwareGPES. All

electrodes were studied as anodes for methanol (SigmaeAldrich

e 34860) and ethanol (Riedel-de Haen e 32221) oxidation under

acidicconditions (1MHClO4,AlfaAesar, 60%e 033263).Thewater

(18.3 MU cm) used for the preparation of the solutions was puri-

fied with an Easypure RF compact ultrapure water system

(Barnsted). The geometric area of the studied anodes was

0.785 cm2. During alcohols oxidation, a continuous Ar flowrate

(80 cc/min)was applied in the cell in order to avoidmass transfer

limitations. In order tomaintain the same turbulence induced by

thegasbubblingand thussimilarmass transfermechanisms, the

Ar flowrate was adjusted and measured electronically while the

position of the jet was exactly in the same position in every

experiment. The bulk concentration of the alcohol practically

remained stable due to its high initial value (1 M) and the short

duration of the experiments (30e40 min). In addition, fresh so-

lutionswere always used for every new experiment while all the

presented voltammograms (if not stated otherwise) were the

steady state voltammograms after many scans.

For the CO stripping experiments, CO gas (5% in He) was

preadsorbed on the electrocatalyst surface by bubbling

through the electrolyte for appropriate time (20e60 min for

surface saturation by CO) at�0.6 V (vsMSE) in 1MHClO4. After

Ar (5 N) purging for 10 min to eliminate any dissolved CO in

the electrolyte the adsorbed CO was oxidized by CV with a

scan rate of 30 mV/s and the CO stripping curve was obtained.

Two subsequent CV cycles were recorded to verify the com-

plete oxidation of the adsorbed CO.

3. Results and discussion

Fig. 1 shows XRD patterns of the Ti substrate as well as the

patterns of three more samples (Pt, TiO2 and Pt(50%)-

TiO2(50%)).Well defined peaks for Pt-containing samples were

observed for loadings down to 30%. TiO2 was observed mostly

in anatase phase in consistence with both the low treatment

temperature (500 �C) and the procedure which was followed

during the preparation of the samples. The formation of solid

solutions was excluded since no remarkable shifts were

observed between the main peaks of Pt and TiO2 (Fig. 1).

Scherrer equation (for the peak of Pt(111) at 2q ¼ 40�) was

applied in order to estimate the size of the Pt crystallite size.

The diameter of the crystallites as a function of the Pt loading

is shown in Fig. 2. The more the TiO2 loading the lower the

crystallite size of Pt. However, after a critical TiO2 loading of

50% the size of the crystallite remains constant (about 18 nm),

two and a half times smaller than that in the case of pure Pt. In

the literature, it has been already reported [66e68] that TiO2

and especially TiO2 in anatase phase [59] could act as a very

good substrate for Pt dispersion. The above result is an indi-

cation that the surface area of the TiO2 containing electrodes

is higher than that of pure platinum.

The effect of TiO2 loading on the Pt metal particles can also

be observed by Scanning Electron Microscopy. Fig. 3 shows

SEM images of pure Pt and Pt(50%)-TiO2(50%) electrodes.

Although it is difficult to calculate accurately the size of the

particles from these images, it is of the same order of

magnitude like that calculated from the XRD patterns. In

addition, the films are quite porous while the Pt particles are

very well dispersed and uniformly decorated on the surface in

Fig. 2 e Effect of TiO2 on the Pt particles diameter.

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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 415398

the case of TiO2 presence. In the case of pure Pt, it seems that

there is a formation of a thin platinum film which follows the

structure of the Ti substrate.

Photoelectron spectra of the Pt4f regions for all analysed

samples at 0� analyzer exit angle are shown in Fig. 4. Corrected

BE values for the main detectable elemental states, excluding

from the ubiquitous superficial ‘carbon’ at 284.7 eV, were ob-

tained from detailed spectra (not shown) of the C1s, O1s and

Ti2p regions. The binding energy of Ti2p is at 459.0 eV for

Ti(4þ) in TiO2 [69], the O1s peak consists of two components at

530.8 eV from O associated with Ti in TiO2 [69] and near

532.5 eV in all samples is assigned to surface hydroxyl groups

or organic oxygen related to atmospheric contamination.

Fig. 3 e SEM pictures of the surface of Pt (top)

Deconvolution of the Pt4f spectra are shown in Fig. 4 where

the most intense doublet with binding energies of 71.3

(Pt 4f7/2) and 74.6 eV (Pt 4f5/2) is attributed to metallic Pt [70].

However, for the low Pt loading sample, Pt(15%)-TiO2(85%),

additional states are observed: peaks at 72.2 (Pt 4f7/2) and

75.6 eV (Pt 4f5/2) assigned to Ptdþ (2<d < 4) [71,72]. Measure-

ments recorded at 45�exit angle shown no significant differ-

ences in peak shapes, positions and changes in relative

intensities of Pt4f and Ti2p indicating that neither Pt or Ti are

closer to the surface. The only changes observed are in relative

peak intensities Ptdþ and Pt0 for sample Pt(15%)-TiO2(85%)

where the Ptdþ/Pt0 ratio increases with increasing analyser

exit angle from 0� to 45� indicating that Pt oxide tends to cover

the Pt. Using the total peak area of Pt4f, O1s, Ti2p and C1s

peaks, in each sample and the appropriate sensitivity factors

(based on Wagner’s collection and adjusted to the trans-

mission characteristics of analyser EA10) and equations, the

average relative atomic concentration in the analysed region,

can be determined. Table 1 shows the intensity ratio Pt4f/Ti2p

as well as the surface atomic concentrations (Pt%:TiO2%). The

calculated values for all samples are in agreement with the

nominal values within an experimental error of 10%. Thus, in

all Figures the Pt loading is taken from the values reported in

the experimental section.

Cyclic Voltammetry was initially used for the electro-

chemical characterization of the samples. Fig. 5 shows vol-

tammograms of pure Pt, pure TiO2 and Pt(50%)-TiO2(50%)

under acidic conditions (1 M HClO4) without alcohol at 25 �Cand u ¼ 50 mV/s. The observed current densities of pure TiO2

were negligible as expected under these conditions and

without radiation [59,73]. On the other hand, Pt gave a typical

voltammogram containing the formation of platinum oxide

at 0.2 V (vs MSE), the reduction of the oxide at 0.05 V (vs MSE)

and the hydrogen adsorptionedesoprtion area. Introduction

and Pt(50%)-TiO2(50%) (bottom) samples.

Fig. 4 e Photoelectron spectra of the Pt4f regions for all analysed samples at 0 �C analyzer exit angle.

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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 4 15399

of TiO2 resulted in a notable increase of the total charge and

more specifically in a big increase of the current densities

observed during the formation and reduction of the platinum

oxide and the hydrogen adsorptionedesorption area. Thus,

the electrochemically active surface area (EAS) is expected to

be higher in the case of TiO2 containing samples. The

enhancement of the Pt active surface was observed for TiO2

loadings up to 50% while further increase of TiO2 resulted in

the opposite effect. The EAS was estimated both from the

charge of the reduction peak (divided by 440 mCb/cm2) and by

CO stripping experiments (by dividing the charge of the

oxidation peak with 420 mCb/cm2). The latter are presented in

Table 1 e Intensity ratio Pt4f/Ti2p and surface atomicconcentrations (Pt%:TiO2%).

Sample NominalPt:TiO2

molar ratios

Pt4f/Ti2p Surface atomicconcentration

Pt:TiO2

Pt(100%)-TiO2(0%) 100:0 e 100:0

Pt(85%)-TiO2(15%) 85:15 11.8 83:17

Pt(65%)-TiO2(35%) 65:35 7.05 71:29

Pt(50%)-TiO2(50%) 50:50 2.42 49:51

Pt(35%)-TiO2(65%) 35:65 1.27 34:66

Pt(15%)-TiO2(85%) 15:85 0.94 23:77

Fig. 6 for the pure Pt and the Pt(50%)-TiO2(50%) sample. The

quantity of the adsorbed CO was much higher in the second

case while a second sharp oxidation peak was observed at

0.03 V (vs MSE). It should be mentioned that the required

adsorption time in order for the surface to be saturated by CO

was three times higher in the case of Pt(50%)-TiO2(50%)

compared with that of pure Pt. In addition, electrodes with Pt

loading less than 30% were totally inactive for CO adsorption,

in agreement with the results of CV where the voltammo-

grams of these samples (Pt loading < 30%) were similar with

those of pure TiO2. This observation could be explained by

the percolation theory and the discontinuity of the film

leading to low surface conductivity [74e76]. In addition, the

very small Pt nanoparticles may be encapsulated by TiO2

resulting in a behaviour similar to that of pure TiO2 films.

Fig. 7 shows the effect of TiO2 loading on the EAS based on

the CV and CO stripping experiments. There was almost an

one order of magnitude increase of the active surface as the

TiO2 increased up to 50%. This enhancement could be more

notable if we further normalize with Pt mass which is less in

the case of PteTiO2 electrodes. Although previous studies

[52,77] have already reported increase of the EAS, none of

them succeeded similar enhancement of the active surface.

The knowledge of the EAS helps to explain the effective

role of TiO2 on this type of binary electrodes. As already

Fig. 5 e Cyclic Voltammograms of Pt, TiO2 and Pt(50%)-

TiO2(50%) electrodes under acidic conditions (1 M HClO4) at

25 �C. The current is normalized by the geometric surface

area. u [ 50 mV/s.

Fig. 7 e Effect of TiO2 loading on the Electrochemically

Active Surface (EAS) based on the results of Cyclic

Voltammetry (open symbols) and of CO stripping (closed

symbols).

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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 415400

discussed in the introduction part, the cooperative, electronic

and/or geometric effects could take place during the operation

of a binary electrode. Taking into account that TiO2 is inactive

under the investigated conditions we believe that cooperative

effect is not playing an important role in our case. In order to

Fig. 6 e Cyclic Voltammograms (first and second scan) after

CO adsorption at constant potential, E [ L0.6 V (vs MSE) of

Pt (top) and Pt(50%)-TiO2(50%) (bottom) electrodes under

acidic conditions (1 M HClO4) at 25 �C. The current is

normalized by the geometric surface area. u [ 50 mV/s.

investigate any electronic effects resulting in Pt work function

changes we plotted the total charge of the CV (under acidic

conditions (1MHClO4) without alcohol) normalized by the EAS

for each sample as a function of the TiO2 loading. As we can

observe in Fig. 8, although the TiO2 affects the total charge,

qtotal, the normalized charge remains interestingly stable for

all samples. This supports that the effective role of TiO2 is

mostly due to a geometric effect (increase of the active sur-

face) rather than due to an electronic effect. TiO2 seems to act

as a second and more effective substrate on which Pt is better

dispersed and decorated as small metal particles resulting in

an increase of the active surface area. This conclusion seems

to be partially in disagreement with other previous studies

[58,77,78] where the enhanced electrocatalytic activity has

been explained by a possible bi-functional mechanism during

alcohol oxidation and more specifically by a possible eOH

adsorption on the TiO2 surface. It should be emphasized

however the different preparation technique which was used

in this study for the PteTiO2 anodes. Our results support

strongly the statement that different preparation techniques

result in electrodes with different properties. The case of

PteTiO2 electrodes with similar Pt:TiO2 ratio like ours, though

prepared by a totally different preparation method (applica-

tion of TiO2 coating on Pt/CNT) is characteristic [77]. Addition

of TiO2 resulted in negligible change of the EAS but at the same

time in a remarkable enhancement of the electrocatalytic

activity during methanol oxidation [77]. This is the case of bi-

functional mechanism in contrast with our geometric effect.

PteTiO2 electrodes with intermediate behaviour were re-

ported in [79] where TiO2 layer was deposited on Si substrate

using a hydrothermal method and Pt nanoparticles were

Fig. 9 e Electrochemical oxidation of 1 M Ethanol (top) and

1 M Methanol (bottom) under acidic conditions (1 M HClO4)

at 25 �C using Pt and Pt(50%)eTiO2(50%) electrodes.

u [ 30 mV/s.

0 1000 2000 3000 4000t / sec

0

20

40

60

i/mAcm

-2

0

20

40

60

i/mAcm

-2

E=0.185 V (vs MSE)

1M MeOH

50:50

65:35

85:15100:0

35:65

E=0.185 V (vs MSE)

1M EtOH

50:50

85:15100:0

35:65

65:35

Fig. 10 e Amperometric measurements under constant

potential E [ 0.185 vs MSE during electrochemical

oxidation of 1 M Ethanol (top) and 1 M Methanol (bottom)

under acidic conditions (1 M HClO4) at 25 �C.

Fig. 8 e Effect of TiO2 loading on the total charge, qtotal (left

y-axis) and the normalized charge, qtotal,N by the EAS

charge (right axis).

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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 4 15401

pulse-electrodeposited on the porous TiO2 support. In that

case TiO2 affected significantly both the EAS and the

normalized by the EAS current densities during methanol

oxidation. This could be the case of bi-functional effect in

conjunction with a geometric effect.

All samples were evaluated under methanol and ethanol

electrooxidation. In agreement with CV experiments all

samples with Pt loading lower than 30% were totally inactive

since they behave like pure TiO2 electrodes. The negligible

electrocatalytic activity of TiO2 anodes during methanol

electroeoxidation reaction has been already reported in the

past [59]. Fig. 9 shows voltammograms during electrochemical

oxidation of 1 Mmethanol (Fig. 9 down) and 1 M ethanol (Fig. 9

up) using pure Pt and Pt(50%)-TiO2(50%) electrodes under

acidic conditions (1 M HClO4). The onset of methanol elec-

trooxidation in the case of pure Pt is at 0.05 V (vsMSE) followed

by a monotonous increase of the electrocatalytic rate until

0.3 V (vs MSE). Above this potential, the current density of the

CV connected with the rate of the alcohol electrooxidation

decreases due to the PtOx formation (Fig. 5). A second oxida-

tion peak was observed during the backward scan (at 0.08 V vs

MSE) due to the reduction of the PtOx (Fig. 5) and the re-

availability of the active platinum sites. This behaviour is in

full agreement with previous reported studies on methanol

oxidation on Pt anodes [80e83]. The electrocatalytic activity of

the anode seems to increase dramatically in the case of TiO2

containing electrodes. The methanol oxidation rate is one

order of magnitude higher in the case of Pt(50%)-TiO2(50%)

anode in consistence with the geometric effect which was

discussed in previous paragraphs and believed to be the pre-

dominant phenomenon in our electrodes. It should be noted

however that we cannot exclude the case of a small electronic

effect mostly due to the shift which was observed both for the

onset of the oxidation and the oxidation peak to lower

potentials (w150e200mV andw20e60mV respectively). If the

geometric effect was the only effect, the potentials of the

oxidation peaks would have remained constant.

Although the electrocatalytic behaviour of the electrodes

were qualitatively similar in the case of ethanol oxidation

(Fig. 9 up) the total enhancement of the rates was lower (up to

300%) while a shift of the oxidation peaks to higher potentials

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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 415402

was observed in the case of TiO2 containing electrodes. The

fact that ethanol electrooxidation leads to further in-

termediates besides carbon monoxide such as acetic acid

[4,5,7,9,84,85] which are also adsorbed on the surface without

being further oxidized may be the reason for this behaviour.

The existence of other intermediates is supported by the

presence of extra oxidation peaks (which are clear in the case

of pure Pt) during the forward scan of the CV (Fig. 9 up).

Chronoamperometric measurements were performed at

constant potential (E ¼ 0.185 V) both under methanol and

ethanol electrooxidation (Fig. 10). Although the 65% of the

electrocatalytic activity decreases after one hour at constant

potential, the current densities in the case of Pt(50%)-TiO2(50%)

anodes were always higher both for methanol and ethanol

oxidation. Interestingly and in agreement with the CV experi-

ments the current density in the case of Pt(35%)-TiO2(65%)

anode was very quickly diminished and finally reached values

similar or even lower to those observed with pure platinum.

4. Conclusions

Various PteTiO2 binary electrodes were prepared and char-

acterised by XRD, SEM, XPS, electrochemical techniques and

CO stripping. It was found that TiO2 loading up to 50% resulted

in an increase of the Electrochemically Active Surface (EAS).

The EAS of Pt(50%)-TiO2(50%) was almost one order of

magnitude higher than that of pure Pt while for samples with

Pt loadings lower than 30% the EAS diminished. This conclu-

sion has been confirmed both by following the charge of the

reduction peak of platinum oxide and CO stripping experi-

ments. All samples were also evaluated during electro-

chemical oxidation of methanol and ethanol. Undermethanol

and ethanol electrooxidation, the Pt(50%)-TiO2(50%) electrode

exhibited better electrocatalytic activity than the pure Pt

anode. The observed higher performance of TiO2 loading

electrodes (for TiO2 up to 50%) was mainly attributed to the

enhanced Pt dispersion as well as to the formation of smaller

Pt particles.

Acknowledgement

Authors are thankful to Dr. V. Drakopoulos and FORTH/ICE-

HT for SEM analysis and to Laboratory of Inorganic and Ana-

lytic Chemistry of University of Patras (Professor P.G. Kout-

soukos) for XRD measurements. We also thank our reviewers

for helpful suggestions.

r e f e r e n c e s

[1] EG&G Technical Services, Inc.Fuel cell handbook.Morgantown, WV: US Department of Energy; 2006

[2] Rao V, Cremers C, Stimming U, Cao L, Sun S, Yan S, et al.Electro-oxidation of ethanol at gas diffusion electrodes aDEMS study. J Electrochem Soc 2007;154:B1138e47.

[3] Kim I, Han OH, Chae SA, Paik Y, Kwon SH, Lee KS, et al.Catalytic reaction in direct ethanol fuel cells. Angew Chemiee Int Edition 2011;50:2270e4.

[4] Lai SCS, Kleijn SEF, Ozturk FTZ, Van Rees Vellinga VC,Koning J, Rodriguez P, et al. Effects of electrolyte pH andcomposition on the ethanol electro-oxidation reaction. CatalToday 2010;154:92e104.

[5] Lamy C, Rousseau S, Belgsir EM, Coutanceau C, Leger JM.Recent progress in the direct ethanol fuel cell: developmentof new platinum-tin electrocatalysts. Electrochim Acta2004;49:3901e8.

[6] Lobato J, Canizares P, Rodrigo MA, Linares JJ. Testing avapour-fed PBI-based direct ethanol fuel cell. Fuel Cells2009;9:597e604.

[7] Rousseau S, Coutanceau C, Lamy C, Leger JM. Direct ethanolfuel cell (DEFC): electrical performances and reactionproducts distribution under operating conditions withdifferent platinum-based anodes. J Power Sources2006;158:18e24.

[8] Vigier F, Coutanceau C, Perrard A, Belgsir EM, Lamy C.Development of anode catalysts for direct ethanol fuel cells. JAppl Electrochem 2004;34:439e46.

[9] Zhou WJ, Li WZ, Song SQ, Zhou ZH, Jiang LH, Sun GQ, et al.Bi- and tri-metallic Pt-based anode catalysts for directethanol fuel cells. J Power Sources 2004;131:217e23.

[10] Lamy C, Lima A, LeRhun V, Delime F, Coutanceau C,Leger JM. Recent advances in the development of directalcohol fuel cells (DAFC). J Power Sources 2002;105:283e96.

[11] Vigier F, Rousseau S, Coutanceau C, Leger JM, Lamy C.Electrocatalysis for the direct alcohol fuel cell. Topics in Catal2006;40:111e21.

[12] McGovern MS, Waszczuk P, Wieckowski A. Stability ofcarbon monoxide adsorbed on nanoparticle Pt and Pt/Ruelectrodes in sulfuric acid media. Electrochim Acta2006;51:1194e8.

[13] Hepel M, Kumarihamy I, Zhong CJ. Nanoporous TiO2-supported bimetallic catalysts for methanol oxidation inacidic media. Electrochem Commun 2006;8:1439e44.

[14] Antolini E, Salgado JRC, Gonzalez ER. The methanoloxidation reaction on platinum alloys with the first rowtransition metals. The case of Pt–Co and –Ni alloyelectrocatalysts for DMFCs: a short review. Appl Catal BEnviron 2006;63:137e49.

[15] Aramata A, Kodera T, Masuda M. Electrooxidation ofmethanol on platinum bonded to the solid polymerelectrolyte, Nafion. J Appl Electrochem 1988;18:577e82.

[16] Leger JM. Mechanistic aspects of methanol oxidation onplatinum-based electrocatalysts. J Appl Electrochem2001;31:767e71.

[17] Hamnett A. Mechanism and electrocatalysis in the directmethanol fuel cell. Catal Today 1997;38:445e57.

[18] Bedam B, Lamy C, Leger JM. New York: Plenum Press; 1992.[19] Dau H, C L, T R, M R, S R, P S. The mechanism of water

oxidation: from electrolysis via homogeneous to biologicalcatalysis. Chem Cat Chem 2010;2:724e61.

[20] Wang J, Holt-Hindle P, MacDonald D, Thomas DF, Chen A.Synthesis and electrochemical study of Pt-based nanoporousmaterials [21] Advances in electrochemical science andengineering. Electrochim Acta 2008;53:6944e52.

[21] Iwasita-Vielstick T. Chapter 3. Progress in the study ofmethanol oxidation by in situ, ex situ and on-linemethods. In: Gerscher H, .W. TC, editors. Advances inelectrochemical science and engineering. Weinheim: VCH;1990. p. 127.

[22] Lima A, Coutanceau C, Leger JM, Lamy C. Investigation ofternary catalysts for methanol electrooxidation. J ApplElectrochem 2001;31:379e86.

[23] Wang K, Gasteiger HA, Markovic NM, Ross Jr PN. On thereaction pathway for methanol and carbon monoxideelectrooxidation on Pt–Sn alloy versus Pt–Ru alloy surfaces.Electrochim Acta 1996;41:2587e93.

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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 4 15403

[24] Watanabe M, Motoo S. Electrocatalysis by ad-atoms: part III.Enhancement of the oxidation of carbon monoxide onplatinum by ruthenium ad-atoms. J Electroanal Chem1975;60:275e83.

[25] Yajima T, Wakabayashi N, Uchida H, Watanabe M. Adsorbedwater for the electro-oxidation of methanol at Pt–Ru alloy.Chem Commun 2003;9:828e9.

[26] Watanabe M, Motoo S. Electrocatalysis by ad-atoms: partXVIII. Enhancement of carbon monoxide oxidation onrhodium and iridium electrodes by oxygen adsorbing ad-atoms. J Electroanal Chem 1986;202:125e35.

[27] Binning Jr RC, Liao MS, Cabrera CR, Ishikawa Y, Iddir H, Liu R,et al. Density functional calculations on CO attached toPtnRu(10en) (n ¼ 6e10) clusters. Int J Quantum Chem2000;77:589e98.

[28] Ishikawa Y, Liao MS, Cabrera CR. Oxidation of methanol onplatinum, ruthenium and mixed Pt–M metals (M¼Ru, Sn): atheoretical study. Surf Sci 2000;463:66e80.

[29] Liao MS, Cabrera CR, Ishikawa Y. A theoretical study of COadsorption on Pt, Ru and Pt–M (M¼Ru, Sn, Ge) clusters. SurfSci 2000;445:267e82.

[30] Neophytides SG, Murase K, Zafeiratos S,Papakonstantinou G, Paloukis FE, Krstajic NV, et al.Composite hypo-hyper-d-Intermetallic and interionic phasesas supported interactive electrocatalysts. J Phys Chem B2006;110:3030e42.

[31] Jiang L, Sun G, Sun S, Liu J, Tang S, Li H, et al. Structure andchemical composition of supported Pt–Sn electrocatalysts forethanol oxidation. Electrochim Acta 2005;50:5384e9.

[32] Yi Q, Chen A, Huang W, Zhang J, Liu X, Xu G, et al.Titanium-supported nanoporous bimetallic Pt–Irelectrocatalysts for formic acid oxidation. ElectrochemCommun 2007;9:1513e8.

[33] Shan CC, Tsai DS, Huang YS, Jian SH, Cheng CL.PteIreIrO2NT thin-wall electrocatalysts derived from IrO2

nanotubes and their catalytic activities in methanoloxidation. Chem Mater 2007;19:424e31.

[34] Yang LX, Bock C, MacDougall B, Park J. The role of the WOx

ad-component to Pt and PtRu catalysts in theelectrochemical CH3OH oxidation reaction. J ApplElectrochem 2004;34:427e38.

[35] Jayaraman S, Jaramillo TF, Baeck SH, McFarland EW.Synthesis and characterization of PteWO3 as methanoloxidation catalysts for fuel cells. J Phys Chem B2005;109:22958e66.

[36] Wu YM, Li WS, Lu J, Du JH, Lu DS, Fu JM. Electrocatalyticoxidation of small organic molecules on polyaniline-Pt-HxMoO3. J Power Sources 2005;145:286e91.

[37] 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.

[38] Bock C, Paquet C, Couillard M, Botton GA, MacDougall BR.Size-selected synthesis of PtRu nano-catalysts: reaction andsize control mechanism. J Am Chem Soc 2004;126:8028e37.

[39] Deivaraj TC, Chen W, Lee JY. Preparation of PtNinanoparticles for the electrocatalytic oxidation of methanol.J Mater Chem 2003;13:2555e60.

[40] Luo J, Maye MM, Kariuki NN, Wang L, Njoki P, Lin Y, et al.Electrocatalytic oxidation of methanol: carbon-supportedgold–platinum nanoparticle catalysts prepared by two-phaseprotocol. Catal Today 2005;99:291e7.

[41] Coutanceau C, Brimaud S, Lamy C, Leger JM, Dubau L,Rousseau S, et al. Review of different methods for developingnanoelectrocatalysts for the oxidation of organiccompounds. Electrochim Acta 2008;53:6865e80.

[42] Song S, Wang Y, Tsiakaras P, Shen PK. Direct alcohol fuelcells: a novel non-platinum and alcohol inert ORRelectrocatalyst. Appl Catal B Environ 2008;78:381e7.

[43] Zhou W, Zhou Z, Song S, Li W, Sun G, Tsiakaras P, et al. Ptbased anode catalysts for direct ethanol fuel cells. Appl CatalB Environ 2003;46:273e85.

[44] Zhao X, Yin M, Ma L, Liang L, Liu C, Liao J, et al. Recentadvances in catalysts for direct methanol fuel cells. EnergyEnviron Sci 2011;4:2736e53.

[45] Ioroi T, Siroma Z, Fujiwara N, Yamazaki SI, Yasuda K. Sub-stoichiometric titanium oxide-supported platinumelectrocatalyst for polymer electrolyte fuel cells. ElectrochemCommun 2005;7:183e8.

[46] Chen JM, Sarma LS, Chen CH, Cheng MY, Shih SC, Wang GR,et al. Multi-scale dispersion in fuel cell anode catalysts: Roleof TiO2 towards achieving nanostructured materials. J PowerSources 2006;159:29e33.

[47] Shim J, Lee CR, Lee HK, Lee JS, Cairns EJ. Electrochemicalcharacteristics of Pt–WO3/C and PteTiO2/C electrocatalystsin a polymer electrolyte fuel cell. J Power Sources2001;102:172e7.

[48] Fu Y, Wei ZD, Chen SG, Li L, Feng YC, Wang YQ, et al.Synthesis of Pd/TiO2 nanotubes/Ti for oxygen reductionreaction in acidic solution. J Power Sources 2009;189:982e7.

[49] Xiong L, Manthiram A. Synthesis and characterization ofmethanol tolerant Pt/TiOx/C nanocomposites for oxygenreduction in direct methanol fuel Cells. Electrochim Acta2004;49:4163e70.

[50] GustavssonM, Ekstrom H, Hanarp P, Eurenius L, Lindbergh G,Olsson E, et al. Thin film Pt/TiO2 catalysts for the polymerelectrolyte fuel cell. J Power Sources 2007;163:671e8.

[51] Song H, Qiu X, Li X, Li F, Zhu W, Chen L. TiO2 nanotubespromoting Pt/C catalysts for ethanol electro-oxidation inacidic media. J Power Sources 2007;170:50e4.

[52] Selvarani G, Maheswari S, Sridhar P, Pitchumani S,Shukla AK. Carbon-supported Pt–TiO2 as a methanol-tolerant oxygen-reduction catalyst for DMFCs. J ElectrochemSoc 2009;156:B1354e60.

[53] He C, Abou Asi M, Xiong Y, Shu D, Li X. Photoelectrocatalyticdegradation of organic pollutants in aqueous solution usinga PteTiO2 film. Int J Photoenergy 2009;2009.

[54] Li Q, Wang K, Zhang S, Zhang M, Yang J, Jin Z. Effect ofphotocatalytic activity of CO oxidation on Pt/TiO2 by stronginteraction between Pt and TiO2 under oxidizingatmosphere. J Mol Catal A Chem 2006;258:83e8.

[55] Sasaki T, Koshizaki N, Yoon JW, Yamada S, Koinuma M,Noguchi M, et al. Photoelectrochemical behavior of the Pt/TiO2 nanocomposite electrodes prepared by Co-sputterdeposition. Electrochemistry 2004;72:443e5.

[56] Sasaki T, Nichols WT, Yoon JW, Koshizaki N.Photoelectrochemical behaviors of Pt/TiO2 nanocompositethin films electrodes prepared by PLD/sputtering combinedsystem. In: MRS Proceedings. vol. 846, 2004. p. 223e8.Copyright ª Materials Research Society, 2005.

[57] Spǎtaru T, Marcu M, Spǎtaru N. Electrocatalytic andphotocatalytic activity of Pt–TiO2 films on boron-dopeddiamond substrate. Appl Surf Sci 2013;269:171e4.

[58] Park KW, Han SB, Lee JM. Photo(UV)-enhanced performanceof Pt–TiO2 nanostructure electrode for methanol oxidation.Electrochem Commun 2007;9:1578e81.

[59] Guo X, Guo D-J, Qiu X-P, Chen L-Q, Zhu W-T. Excellentdispersion and electrocatalytic properties of Pt nanoparticlessupported on novel porous anatase TiO2 nanorods. J PowerSources 2009;194:281e5.

[60] Yoo SJ, Jeon TY, Lee KS, Park KW, Sung YE. Effects of particlesize on surface electronic and electrocatalytic properties ofPt/TiO2 nanocatalysts. Chem Commun 2010;46:794e6.

[61] Comninellis C, Pulgarin C. Anodic oxidation of phenol forwaste water treatment. J Appl Electrochem 1991;21:703e8.

[62] Trasatti S. Electrocatalysis: understanding the success ofDSA�. Electrochim Acta 2000;45:2377e85.

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 8 ( 2 0 1 3 ) 1 5 3 9 5e1 5 4 0 415404

[63] Comninellis C, Vercesi GP. Problems in DSA� coatingdeposition by thermal decomposition. J Appl Electrochem1991;21:136e42.

[64] Panakoulias T, Kalatzis P, Kalderis D, Katsaounis A.Electrochemical degradation of Reactive Red 120 using DSAand BDD anodes. J Appl Electrochem 2010;40:1759e65.

[65] Papastefanakis N, Mantzavinos D, Katsaounis A. DSAelectrochemical treatment of olive mill waste water on Ti/RuO2 anode. J Appl Electrochem 2010;40:729e37.

[66] Banu A, Spataru N, Teodorescu V, Maraloiu A, Voiculescu I,Marcu A, et al. Structural and electrochemicalcharacterization of TiO2/Pt hybrid catalyst system for directbio-ethanol fuel cell. J Optoelectron Adv Mater2010;12:1189e93.

[67] Kim MY, Jung SB, Kim MG, You YS, Park JH, Shin CH, et al.Preparation of highly dispersive and stable platinumcatalysts supported on siliceous SBA-15 mesoporousmaterial: roles of titania layer incorporation and hydrogenperoxide treatment. Catal Lett 2009;129:194e206.

[68] Kim MY, Park JH, Shin CH, Han SW, Seo G. Dispersionimprovement of platinum catalysts supported on silica,silica-alumina and alumina by titania incorporation and pHadjustment. Catal Lett 2009;133:288e97.

[69] Stefanov P, Shipochka M, Stefchev P, Raicheva Z, Lazarova V,Spassov L. XPS characterization of TiO2 layers deposited onquartz plates. J Phys Conf Series 2008;100.

[70] Liang Y, Zhang H, Zhong H, Zhu X, Tian Z, Xu D, et al.Preparation and characterization of carbon-supported PtRuIrcatalyst with excellent CO-tolerant performance for proton-exchange membrane fuel cells. J Catal 2006;238:468e76.

[71] Bancroft GM, Adams I, Coatsworth LL, Bennewitz CD,Brown JD, Westwood WD. ESCA study of sputtered platinumfilms. Anal Chem 1975;47:586e8.

[72] Barr TL. An ESCA study of the termination of the passivationof elemental metals. J Phys Chem 1978;82:1801e10.

[73] Frontistis Z, Daskalaki VM, Katsaounis A, Poulios I,Mantzavinos D. Electrochemical enhancement of solarphotocatalysis: degradation of endocrine disruptorbisphenol-A on Ti/TiO2 films. Water Res 2011;45:2996e3004.

[74] Moya JS, Lopez-Esteban S, Pecharroman C. The challenge ofceramic/metal microcomposites and nanocomposites. ProgMater Sci 2007;52:1017e90.

[75] Fan Z. A new approach to the electrical resistivity of two-phase composites. Acta Metallurgica Et Materialia1995;43:43e9.

[76] McLachlan DS, Blaszkiewicz M, Newnham RE. Electricalresistivity of composites. J Am Ceram Soc1990;73:2187e203.

[77] Song H, Qiu X, Li F. Effect of heat treatment on theperformance of TiO2-Pt/CNT catalysts for methanol electro-oxidation. Electrochim Acta 2008;53:3708e13.

[78] Hepel M, Dela I, Hepel T, Luo J, Zhong CJ. Novel dynamiceffects in electrocatalysis of methanol oxidation onsupported nanoporous TiO2 bimetallic nanocatalysts.Electrochim Acta 2007;52:5529e47.

[79] Chen CS, Pan FM. Electrocatalytic activity of Pt nanoparticlesdeposited on porous TiO2 supports toward methanoloxidation. Applied Catal B Environ 2009;91:663e9.

[80] Nichols RJ, Bewick A. SNIFTIRS with a flow cell: theidentification of the reaction intermediates in methanoloxidation at Pt anodes. Electrochim Acta1988;33:1691e4.

[81] Bambagioni V, Bianchini C, Marchionni A, Filippi J, Vizza F,Teddy J, et al. Pd and Pt–Ru anode electrocatalysts supportedon multi-walled carbon nanotubes and their use in passiveand active direct alcohol fuel cells with an anion-exchangemembrane (alcohol ¼ methanol, ethanol, glycerol). J PowerSources 2009;190:241e51.

[82] Freitas RG, Santos MC, Oliveira RTS, Bulhoes LOS, Pereira EC.Methanol and ethanol electroxidation using Pt electrodesprepared by the polymeric precursor method. J PowerSources 2006;158:164e8.

[83] Hassan HB. Electrodeposited Pt and Pt-Sn nanoparticles on Tias anodes for direct methanol fuel cells. Ranliao HuaxueXuebao/J Fuel Chem Technol 2009;37:346e54.

[84] Bianchini C, Bambagioni V, Filippi J, Marchionni A,Vizza F, Bert P, et al. Selective oxidation of ethanol toacetic acid in highly efficient polymer electrolytemembrane-direct ethanol fuel cells. Electrochem Commun2009;11:1077e80.

[85] Sine G, Foti G, Comninellis C. Boron-doped diamond (BDD)-supported Pt/Sn nanoparticles synthesized inmicroemulsion systems as electrocatalysts of ethanoloxidation. J Electroanal Chem 2006;595:115e24.