Effect of Ti sublayer on the ORR catalytic efficiency of dc magnetron sputtered thin Pt films

international journal of hydrogen energy 35 (2010) 4466–4473

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Effect of Ti sublayer on the ORR catalytic efficiency of dc magnetron sputtered thin Pt films Osman Ozturk a,*, Oguz Kaan Ozdemir a, Isılay Ulusoy a, Ali Sems Ahsen a, Evelina Slavcheva b a b

Nanotechnology Research Center, Gebze Institute of Technology, 41400 Kocaeli, Turkey Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Sofia, Bulgaria

article info

abstract

Article history:

A series of thin Pt films were deposited by dc magnetron sputtering directly on

Received 16 December 2009

a commercial hydrophobic carbon paper substrate having a thin microporous Vulcan-XC72

Received in revised form

layer or upon a thin Ti sublayer sputtered on the top of the microporous carbon film. The

5 February 2010

electrocatalytic properties of the sputtered Pt films toward the oxygen reduction reaction

Accepted 15 February 2010

were investigated in 0.5 M H2SO4 solution and in a hydrogen PEM fuel cell. The catalyst with

Available online 23 March 2010

˚ thick Ti sublayer is robust, mechanultralow Pt loading of 22 mg cm2 deposited on a 33 A ically stable, possesses highly developed surface area and improved catalytic efficiency. Its

Keywords:

performance as a MEA cathode in a single hydrogen PEM fuel cell (577 mA cm2 at 0.4 V cell

Electrocatalysis

voltages and a maximum power of 0.954 W) proved to be much superior compared to that

ORR

of MEA with the same cathode Pt loading but without Ti sublayer (173 mA cm2 at 0.4 V,

Sputtered Pt films

0.231 W, respectively). ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Ti PEMFC

1.

Introduction

The extensive research and development in the field of PEM fuel cells during the last decade resulted in huge improvement of their performance. The achieved power values in the range 0.5–0.7 W cm2 satisfy completely the power needs of the numerous commercial electronic devices requiring autonomous low power supply such as mobile phones, laptops, digital cameras [1,2] and are attractive for automotive and stationary applications [3]. Nevertheless the achieved substantial progress the PEM fuel cells are not broadly utilized because their cost and durability are still not satisfactory. These are considered as the most serious obstacles for the PEM FC commercialisation. In that regard, the electrocatalysis is one of the weakest points due to the large amount of

precious metal used. Much of the research on the proton exchange membrane fuel cell (PEMFC) is focused on improving the catalytic activity of cathode electrocatalysts where the oxygen reduction reaction (ORR) takes place. The slow kinetics of the ORR is responsible for about 70% of the total losses that occur in PEMFC [3–7]. The most common cathode catalyst by now is the platinum and in addition to performance improvement, decrease in the catalytic loading is highly desirable for fuel cell cost reduction. The thin film deposition method of magnetron sputtering (MS) which is widely used for integrated circuit manufacturing, recently finds application as an alternative catalyst preparation and electrode assembling technique. The method is particularly favourable for applications in hydrogen energy converting electrochemical cells with

* Corresponding author. Present address. Department of Physics, Gebze Instute of Technology, Cayirova-Gebze, Kocaeli 41400, Turkey. Tel.: þ90 262 605 1317; fax: þ90 262 605 3101. E-mail address: [email protected] (O. Ozturk). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.077

international journal of hydrogen energy 35 (2010) 4466–4473

Table 1 – Samples under study with catalyst loading of 22 mgPt cmL2, sputtered on hydrocarbon paper substrate I3/C1 with upper micro porous layer of Vulcan XC 72 and sputtered Ti film with varying thickness Sample No.

1

2

3

4

˚ Ti, A Abbreviation

0 I3/C1/Ti0

13 I3/C1/Ti13

22 I3/C1/Ti22

33 I3/C1/Ti33

polymer electrolyte membrane (PEM), including fuel cells, water electrolysers and unitized regenerative fuel cells. It allows deposition of thin compact films upon a selected substrate material and ensures simplicity of the catalysts preparation as well as improved stability, durability, and utilisation [8–19]. The sputter conditions are easily controlled in obtaining homogeneous, well-dispersed and reproducible thin films of various single and composite materials with a very low loading. Using this method an essential decrease in the fuel cell cathode Pt loadings without reduction in performance has been already achieved [8,12–15]. It has been demonstrated that the sputter method insures very efficient usage and high mass activity of the platinum [16–18]. The technique was also applied for deposition of thin iridium oxide electyrocatalytic films as anode material for PEM water electrolysis and a 10-fold decrease of the catalyst loading has been reported compared to conventional iridium anodes [19]. In the present paper the catalytic efficiency and stability of thin sputter deposited Pt films with ultra low loading of 22 mgPt cm2 are investigated as catalyst for oxygen reduction reaction. The aim is to study the role of the substrate material on the electrochemically active surface area and the catalytic activity in terms of exchange current density, jo and activation energy, DG, as well as to access the catalytic performance characteristics at real operating conditions in a hydrogen PEMFC.

and 1.5  103 mbar. The distance between Pt target and the substrate was set to 50 mm. The catalytic layer was deposited either directly on a commercial hydrophobic carbon paper covered with a thin microporous film of Vulcan-XC72 (I3/C1, Freudenberg) or upon an ultrathin Ti film with varying thick˚ ) sputtered on the top of the I3/C1 ness (13, 22, and 33 A substrate. The main role of Ti was to serve as an adhesion sublayer for the Pt. The structure and morphology of the obtained composite materials were studied by XPS analysis using Phobus 150 Specs electron analyzer with conventional X-Ray source (AlKa). The quartz crystal microbalance was used to monitor the platinum coverage. For thickness determination XPS signals of the Ag 3d5/2 attenuation was used as a function of the time of platinum exposure. The amount of Pt was calculated using the formula developed by Tanuma, Powell, and Penn (TPP formula) [20]. The catalytic efficiency of the films toward ORR in 0.5 M H2SO4 solutions was investigated by means of cyclic voltametry (CV) and linear sweep voltametry (LSV) on rotating disc electrode (RDE), using Pine Instrument Bipotentiostat, model AFCBP1, controlled by PC and commercial software. The experiments were carried out in a three electrode electrochemical cell with Pt wire counter electrode and saturated Ag/ AgCl reference electrode. All potentials in the paper were referred vs. RHE. The tests were performed in the temperature range 30–80  C. In order to obtain reproducible data, prior to ORR measurements the electrodes were electrochemically pre-treated by cycling the potential in the range 0.25 to 1.3 V for 50 cycles at scan rate of 100 mV s1. RDE polarization curves were recorded in oxygen saturated solution. The potential was swept in the range from  0.12 V to þ 1.0 V (vs. RHE) at scan rate of 10 mV s1. The RDE polarisation curves obtained at different rotating speed in the range 200–1600 rpm were used to construct the Koutecky–Levich plots (1/j vs. u1/2) according to the equations: 1=j ¼ 1=jkin þ 1=jdif

2.

Experimental

1=jdif ¼ 1= bu1=2

E (V)

A series of thin Pt films were deposited by dc magnetron sputtering in argon plasma using a BesTec 222 UHV Sputtering System. The sputtering chamber was evacuated to a base pressure of 5  109 mbar and during sputtering process (after feeding the chamber with Ar gas) it was between 1.4  103 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0

200 rpm 400 rpm 600 rpm 900 rpm 1600 rpm

0,0

1,0

2,0

3,0

4,0

5,0

6,0

-2

j (mA.cm )

Fig. 1 – Oxygen reduction current on I3/C1/Ti0 electrode, obtained at varying rotation rates in O2-saturated 0.5 M H2SO4 at 30 8C.

4467

2=3




b ¼ 0:62nFAD0 n1=6 CO

(1) (2) (3)

where j is the measured current density, jkin is the current density due to charge transfer, jdif is the diffusion current density, u is the rotation velocity expressed in revs min1, [rpm], and b is the Levich constant, n is the number of electrons transferred per O2 molecule, F, Faraday constant, Co, concentration of O2 in the bulk, Do, the diffusion coefficient, and n, the kinematic viscosity of the solution [21]. The kinetic current density extracted from Koutecky-Levich plots was applied to construct the mass transfer corrected Tafel plots and to calculate the apparent exchange current density, jap o , of the ORR. Cyclic voltammograms in deaerated 0.5 M H2SO4 at 100 mV s1 were recorded and used to assess the electrochemically active surface area, EASA, of the films via integration of the area under the hydrogen adsorption/desorption peaks. The real exchange current density, jo was calculated dividing jap o by EASA for each sample. Finally, the ORR activation energy was calculated applying Arrhenius plots, jo1/T.

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international journal of hydrogen energy 35 (2010) 4466–4473

3,0 2,0 1,0 -2

j (mA.cm )

The catalytic activity of the test samples at real operating PEMFC conditions was evaluated in a laboratory hydrogen fuel cell. The sputter deposited thin Pt film electrodes were integrated in a membrane electrode assembly (MEA, 5 cm2) by hot pressing on a Nafion 112 polymer membrane. The anode used for all PEMFC tests was 22 mgPt cm2 deposited on I3/C1. The cathode side of the MEA contained a sample from the series under study. The experiments were carried out in the temperature range 30–80  C at hydrogen partial pressure of 3 bar, oxygen gas pressure of 3 bar and 100% humidity.

-2,0 30 oC

-3,0

60 oC

-4,0 -5,0 -0,2

80 oC 0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

E (V)

Fig. 3 – Cyclic voltammograms of 22 mgPt cmL2 I3/C1/Ti0 electrode recorded in Ar-saturated 0.5 M H2SO4 at different temperatures; potential scan rate 100 mV sL1.

3.

Results and discussion

3.1.

Experimental results in acid solution

The test samples prepared by dc magnetron sputtering of Pt (22 mgPt cm2) on carbon paper substrates are presented in Table 1. They are further referred as I3/C1/TiX samples where ˚ ] of the supporting Ti sublayer. X is the thickness in [A The RDE experiments in O2-saturated acid solutions are used to obtain information about the intrinsic catalytic activity of the sputtered Pt films in terms of exchange current density, jo and activation energy, DG. The 0.5 M H2SO4 electrolyte acts as a model for a fuel cell environment, providing protons to the reaction at the cathode similar to a Nafion membrane, which transports the incoming protons from the anode to the cathode in the PEM fuel cell. Oxygen within the solution diffuses to the surface of the electrode and reacts to form water, consuming electrons. By rotating the electrode at different rates, the mass transfer limitations due to the slow diffusion of O2 can be evaluated and the pure kinetic current, giving a measure for the true catalytic performance at specific voltages can be obtained. Fig. 1 shows the linear sweep voltammograms for ORR on sample 1 (I3/C1/Ti0 – 22 mgPt cm2, no Ti sublayer) recorded at 30  C cell temperature with varying rotation rates. The welldefined mass transfer and kinetic limited regions, typical for the reduction of oxygen on Pt catalyst are clearly seen on the curves. Taking into consideration the operating temperatures typical for hydrogen PEMFC, the catalytic behaviour of the I3/C1/Ti0 sample was further tested, gradually increasing the temperature of the solution to 80  C at 10  C steps. The results showed that up to 60  C the current density

increases in the whole range of potentials, while above this temperature the electrode performance falls down (Fig. 2), particularly in the diffusion limited potential range. Similar results were obtained during the cyclovoltammetry tests carried out in Ar-saturated 0.5 M H2SO4 at different temperatures as shown in Fig. 3. The observed shift of the ORR peak potential to more positive values with the increasing temperature is a clear indication for facilitation of the ORR. The cyclic voltammograms were used to calculate the electrochemically active surface area of the electrode under study, applying the well established procedure of integration the area under the hydrogen adsorption/desorption peaks and using the value of 210 mC cm2 (the charge required for adsorption of hydrogen monolayer on 1 cm2 of smooth Pt electrode) as a correction factor [21]. The calculations showed that the EASA increases gradually with temperature reaching  a maximum of 35.98 m2 g1 Pt at 50 C. At higher test temperatures the values of EASA decrease in accordance with the results from the RDE tests. In order to clarify the reason for the registered decrease in the performance of the catalyst at elevated temperatures, XPS analysis of the film was performed before and after the CV and RDE experiments. The Pt 4f spectrum is presented in Fig. 4. To find the amount of platinum for each sample the intensity of the Pt 4f7/2 peak was calculated. The highest peak was observed for the as-prepared sample. According to the TPP formula it corresponds to 22 mg cm2 platinum loading.

1,0

120000

Count Rate (cnts.sec-1)

E (V)

0,0 -1,0

o

0,8

30 C

0,6

60 oC 80 oC

0,4 0,2 0,0 0,0

1,0

2,0

3,0

4,0

5,0

6,0

-2

j (mA.cm )

Fig. 2 – RDE polarization curves on 22 mgPt cmL2 I3/C1/Ti0 electrode obtained in O2-saturated 0.5 M H2SO4 at different temperatures at rotation rate of 1600 rpm.

4f7/2

100000 80000

4f5/2

I3/C1/Ti0_as-prepared I3/C1/Ti0_aCV I3/C1/Ti0_aRDE

60000 40000 20000 0 50

60

70 80 Binding Energy (eV)

90

100

Fig. 4 – The XPS spectra of the 22 mgPt cmL2 I3/C1/Ti0 electrode: as-prepared (AP), after cyclic voltammetry (aCV), and after rotating disk (aRDE) experiments.

international journal of hydrogen energy 35 (2010) 4466–4473

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Fig. 5 – EDX analysis of the samples22 mgPt cmL2 I3/C1/Ti0 (a) and I3/C1/Ti33 (b) before (left) and after (right) cyclovoltammetry tests.

The ratio between the intensity of Pt 4f7/2 peak before and after I3/C1/Ti0 electrochemical tests was used to calculate the change in the catalyst loading. The calculations based on the XPS data, showed that the electrochemical treatment of the film at elevated temperatures results in essential decrease of the Pt loading – from 22 mg cm2 in the asprepared film to 12.1 mg cm2 (45% loss) after CV and

11.3 mg cm2 (49%loss) after RDE experiments. In addition, it was found that the amount of carbon in the upper micro porous film of the substrate also decreases. These results are clear evidence for electrode degradation related to weak adhesion, dissolution of the Pt catalytic film, corrosion of the carbon, etc. phenomena, causing decrease in the EASA and worse electrochemical behaviour.

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3,0

1,0 0,8

I3/C1/0 I3/C1/Ti13 I3/C1/Ti22

0,6

I3/C1/Ti33

1,0 0,0

E (V)

-2

j (mA. cm )

2,0

-1,0 -2,0

I3/C1/0

-3,0

I3/C1/Ti13

-4,0

I2/C1/Ti22

-5,0 -0,2

0,4 0,2

I3/C1/Ti33

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

0,0

1,6

0,0

1,0

2,0

E (V)

3,0 -2 j (mA.cm )

4,0

5,0

6,0

Fig. 6 – Cyclic voltammograms of the test samples recorded in 0.5 M H2SO4 at 60 8C; scan rate 100 mV sL1.

Fig. 7 – RDE polarization curves in 0.5 M H2SO4 at 30 8C; rotation rate 1600 rpm.

It is known that Pt being a noble metal does not adhere very well when sputtered as a thin film on variety of surfaces. At the same time Ti is a very active metal with excellent adhesion to nearly all materials and is often used as an adhesion promoter. Therefore, a thin Ti layer was sputtered on the I3/C1 substrate before depositing the catalytic Pt film. The goal was to prevent the observed degradation of the electrode ensuring improved adhesion of the catalyst to the substrate and at the same time isolating the carbon paper from the reaction zone. To determine the optimal thickness of the adhesion sublayer, ˚ thick Ti sublayers were samples containing 13, 22, and 33 A tested. The cyclovoltametry tests carried out in Ar-saturated ˚ Ti 0.5 M H2SO4 solution showed that the introduction of 13 A in the I3/C1/Ti13 sample has no positive effect on the electrochemical active surface area, EASA, of the electrode and does not improve its stability. The EASA of the I3/C1/Ti22 sample at low temperatures is higher than that of the sample without Ti, while above 40  C the performance of the electrode declines again and the EASA decreases. In contrast, the I3/C1/ Ti33 sample shows stable performance in the whole temperature range tested. At low temperatures the EASA of I3/C1/ Ti33 is lower compared to the sample without Ti (I3/C1/Ti0). However, EASA is stable with the increasing temperature and no degradation of the I3/C1/Ti33 electrode was registered by the XPS spectra taken after both CV and RDE experiments. These findings were confirmed also by the EDX analysis carried out over the whole surface of the samples before and after electrochemical tests. The obtained qualitative results are presented in the diagrams on Fig. 5. As it is seen after electrochemical treatment the amount of Pt in the unsupported I3/C1/Ti0 film decreases nearly by a factor of two, while for I3/C1/Ti33 sample no changes in the Pt loading were registered.

In Fig. 6 are compared the cyclic voltammograms of all samples under study at temperature of 60  C. The calculated data about the EASA are summarised in Table 2. The electrocatalytic activity of Pt films toward the oxygen reduction was assessed applying the method of linear sweep voltammetry and Koutecky–Levich plots based on Eq. (1). The RDE polarization curves show characteristic behaviour reported in the literature for ORR on Pt in acid solutions with a welldistinguished region of kinetic mixed, and diffusion limited reaction rate. At very low overpotentials, the process is governed by the charge transfer, then in the range down to 0.75–0.5 V the ORR reaction proceeds under mixed diffusionkinetic control, after which the current starts to level out reaching diffusion limited values at potentials of 0.25–0.2 V. With the increase of the temperature the ORR onset electrode potential shifts to higher anodic values and an increase in the rate of the reaction is registered. It is interesting to note that the reduction of oxygen on the samples containing Ti sublayer commences much earlier (about 70–80 mV) than on the I3/C1/Ti0 sample. This effect is registered in the whole temperature range for all samples. It is most pronounced in the case of I3/C1/Ti33 at 30  C as shown in Fig. 7. The exchange current density, jo, is known to be a qualitative measure for the intrinsic activity of the catalyst. In order to calculate jo, it is necessary first to construct the mass transfer corrected Tafel slopes, which in turn, requires determination of the kinetic current density. One of the common approaches to calculate jkin, is to solve Eq. (1) regarding the kinetic current at different potentials and given rotating speed. This results in the relation (4), known as mass-transport correction for rotating disk electrodes [3,22]. 1,0

I3/C1/0 I3/C1/Ti22 I3/C1/Ti33

Table 2 – EASA (m2 gL1 Pt ), calculated from the cyclic voltammetry tests. Sample Temperature 30 40 ( C) 50 60 70 80

I3/C1/Ti0 I3/C1/Ti13 I3/C1/Ti22 I3/C1/Ti33 27.16 33.41 35.98 35.29 28.17 19.6

23.46 26.51 29.41 31.27 21.61 17.23

32.16 36.44 33.17 30.56 28.13 19.47

24.32 25.62 23.52 24.75 26.95 23.35

E (V )

0,8 0,6 0,4 0,2 0,0 -4,3

-3,8

-3,3

-2,8

-2,3

-1,8

-1,3

-2

log jk (A.cm )

Fig. 8 – Mass transfer corrected Tafel plots for the samples under study.

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international journal of hydrogen energy 35 (2010) 4466–4473

Table 3 – Kinetic parameters calculated from the mass transfer corrected Tafel plots. Samples

Temperature ( C)

b (V dec1)

2 jap o (A cm )

jo (A cm2)

DG (kJ mol1)

I3/C1/Ti0

30 40 50 60 70 80

0.158 0.166 0.156 0.146 0.149 0.138

0.005248 0.005754 0.007244 0.008913 0.009772 0.011749

1.832E08 1.722E08 2.013E08 2.525E08 3.468E08 6.031E08

20.62

I3/C1/Ti22

30 40 50 60 70 80

0.126 0.138 0.142 0.137 0.137 0.124

0.01047 0.01318 0.00513 0.01585 0.01349 0.01862

3.26E08 3.62E08 1.55E08 5.19E08 4.8E08 9.56E08

18.25

I3/C1/Ti33

30 40 50 60 70 80

0.143 0.135 0.139 0.137 0.140 0.120

0.00891 0.0128 0.0135 0.0162 0.0158 0.027

3.663E08 4.996E08 5.738E08 6.546E08 5.862E08 1.156E07

16.04

(4)

where j is the experimentally measured current density, jlim. is the measured diffusion limited current density, and jkin. is the kinetic current density. The obtained mass transfer corrected Tafel plots are presented in Fig. 8. They show a wide linear range (over 400 mV), which allows to determine the values of the Tafel slope and the apparent values of the exchange ap current density, j0 . All experimental results are summarized in Table 3. For all samples a single Tafel slope was obtained with a value close to 120 mV dec1, indicating the first electron transfer reaction according the Eq. (3) as a rate determining step of the reaction: O2 þ e /O 2ads

(5)

Usually, the presence of oxides on the Pt surface alters the kinetics of the ORR and yields a different Tafel slope [23]. Since this is obviously not the case for the sputtered deposited thin Pt catalytic films under study, the observed strong promotion of the ORR on the Pt films sputtered upon Ti sublayer must have another origin. It is well known that the combination of d-metals with dissimilar electronic character often exhibits a pronounced synergetic effect on the electrocatalysis. The main catalytic activity originates by the hyper-d-metallic phase, which prevails on the catalyst surface, while hypo-d-phase contributes to the catalyst overall synergetic effect by the so-called Strong Metal-Support Interaction (SMSI) [24]. The realisation of such an effect could explain the registered shift in the ORR starting potential to more positive potentials (Fig. 7). If this is really the case, the SMSI should manifest itself in shift of binding energy, BE, of the Pt4f XPS spectra, change in the bond strength of the intermediate species adsorbed on the catalytic surface, and/or a decrease in the activation energy of the ORR. The RDE experiments carried out at various test temperatures were used to validate this hypothesis. For this purpose

the Arrhenius plots of the calculated exchange current density as function of the reverse temperature were constructed, following the Eq. (6): dlogj0 DG ¼ jð1=TÞ 2; 3R

(6)

where DG is the activation energy, j is the measured current density, jo is the exchange current density, and R, the gas constant. Since it was found that the very thin Ti film does not improve the stability of the Pt catalyst during the electrochemical treatment, the I3/C1/Ti13 sample was not further examined. The results obtained with the other samples are presented in Fig. 9. The exchange current density increases linearly with the test temperature for all samples under study. The calculated values of the activation energy for all catalytic films are in good agreement with the values reported in the literature about polycrystalline Pt in sulfuric acid solutions (20–25 kJ mol1) [3], indicating high activity of the sputtered ultrathin Pt films.

-5 I3/C1/0 -6

I3/C1/Ti22

-2

j$jlim jlim  j

Log j o (A .cm )

jkin ¼

I3/C1/Ti33

-7 -8 -9 -10 2,80

2,90

3,00

3,10

3,20

3,30

3,40

-1

1/Temp*1000 (1.K )

Fig. 9 – Arrhenius plot for the ORR exchange current density at different test samples, containing 22 mgPt.cmL2 with and without Ti sublayer on the carbon paper substrate.

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international journal of hydrogen energy 35 (2010) 4466–4473

1

80000 60000 40000

1

0,8

0,8

0,6

0,6

0,4

20000

0

0 50

60

70

80

90

0,4

I3/C1/Ti33

0,2

0,2

I3/C1/Ti0 0

100

200

100

300

Power(W)

I3/C1/Ti33 I3/C1/Ti0

100000

E (V)

-1

Count Rate (cnts.sec )

120000

400

500

600

0

-2

J (mA.cm )

Binding Energy (eV)

Fig. 12 – Cell voltage and power density as function of current density for MEAs with 22 mgPt cmL2 on I3/C1/Ti0 (bold line) and 22 mgPt cmL2 on I3/C1/Ti33 (dashed line) cathodes and 22 mgPt cmL2 on I3/C1/Ti0 anode at 80 8C; 100% RH, 3 bar partial pressure of H2 and O2.

Fig. 10 – XPS spectra of 22 mgPt cmL2 I3/C1/Ti0 and 22 mgPt cmL2 I3/C1/Ti33 test samples.

The presence of Ti sublayer affects the activation energy which decreases further reaching the lowest value of ˚ thick Ti sublayer. The 16 kJ mol1 for the sample with 33 A lower activation energy means facilitation of the ORR. The reaction needs less energy to proceed on the active Pt sites when Pt is grafted on Ti. These results are in accordance with the already mentioned shift of the onset ORR potential to more positive values, the calculated higher exchange current density, and the observed improved stability of the Tisupported catalytic films. Thus, the assumption about the before mentioned SMSI effect caused by electronic interactions between the hyper-d Pt and the hypo-d Ti metals, most clearly manifested for the I3/C1/Ti33 sample, can be considered as well justified. The data from the XPS analysis support this assumption as well. The XPS spectra for the I3/C1/Ti0 and I3/C1/Ti33 samples are compared in Fig. 10. They show a slight shift (about 0.20 eV) of the binding energy, BE, of the Pt4f peak toward more positive values in presence of Ti.

3.2.

Experimental results in a laboratory single PEMFC

The sputtered Pt films with ultra low catalytic loading were envisaged for application as cathodes in hydrogen PEMFC. Therefore, the I3/C1/Ti33 sample which has shown the most promising catalytic activity in 0.5 M H2SO4 was integrated in MEA and tested as cathode for ORR during real operation conditions in a laboratory single PEMFC. Nafion 112

membrane was used as a proton exchange electrolyte and I3/C1/Ti0 as an anode for H2 oxidation. The tests were carried out at varying temperatures in the range 30–80  C at 100% RH. The partial pressure of the reagents (both H2 and O2) was 3 bars. The experiments were terminated when the cell voltage reached the value of 0.4 V. The results obtained are shown in Fig. 11. They were compared to the results obtained on MEA with unsupported I3/C1/Ti0 cathode, having the same Pt loading. An improved fuel cell performance with the increasing temperature was found for both test samples as the one with Ti sublayer demonstrated superior characteristics. The MEA with I3/C1/Ti33 cathode had about 100 mV more positive open circuit potential and achieved three times higher current density at given cell voltage. The favourable influence of Ti sublayer integrated in the structure of the cathode is illustrated in Fig. 12 where the cell voltage and power density of both MEAs at operating temperature of 80  C are presented as function of the applied current density. The power density of the MEA with I3/C1/Ti0 cathode reaches maximum of 0.046 W cm2 at 173 mA cm2, while in the presence of Ti the power density increases remarkably, reaching a maximum of 0.195 W cm2 at 577 mA cm2. The results obtained are in accordance with the improved catalytic activity of the samples containing Ti proven by CV and RDE experiments carried out in sulphuric acid solution.

1,00 30 oC 60 oC 80 oC

E (V)

0,80 0,60

4.

Conclusions

600

The research performed shows the possibilities offered by method of magnetron sputtering to fabricate highly active and cost efficient composite catalytic films with precisely controllable ultra low loadings and homogeneous highly developed ˚ surface. It demonstrates that the incorporation of only 33 A

Fig. 11 – H2/O2 PEMFC polarisation curve with 22 mgPt cmL2 on I3/C1/Ti33 as cathode and 22 mgPt cmL2 on I3/C1/Ti0 as anode at varying test temperature; 100% RH, 3 bar partial pressure of H2 and O2.

thick titanium sublayer as a catalyst support ensures an improved chemical and mechanical stability and results in essential increase in ORR catalytic activity of the sputtered Pt films. Based on the calculated values of the ORR activation energy, EDX and XPS analysis, the observed favourable effect of Ti is prescribed to realisation of SMSI between the hyper-d Pt

0,40 0,20 0,00 0

100

200

300

400

500

-2

j (mA.cm )

international journal of hydrogen energy 35 (2010) 4466–4473

and hypo-d Ti metals. The beneficial influence of Ti found in acid aqueous solutions sustains also during PEMFC tests. The MEA with 22 mgPt cm2 on I3/C1/Ti33 cathode catalyst shows a sixfold increase of the fuel cell performance characteristics compared to the MEA with unsupported Pt catalyst with the same loading. Thus, an essential cost reduction of the PEM electrocatalysis could be achieved.

Acknowledgements The research has been carried out in the frame of the project EVRENA-108M139 and the authors gratefully acknowledge the financial support by TUBITAK.

references

[1] Wainright JS, Savinell RF, Liu CC, Litt M. Microfabricated fuel cells. Electrochim Acta 2003;48:2869–77. [2] Gruber D, Ponath N, Mueller J, Lindstaedt F. J Power Sources 2005;150:67–72. [3] Gasteiger HA, Kocha SS, Sompalli B, Wagner FT. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl Catal B Environ 2005;56:9–35. [4] Gasteiger HA, Panels JE, Yan SG. Dependence of PEM fuel cell performance on catalyst loading. J Power Sources 2004;127: 162–71. [5] Kadirgan F, Kannan AM, Atilan T, Beyhan S, Ozenler SS, Suzer S, Yo¨ru¨r A. Carbon supported nano-sized Pt–Pd and Pt–Co electrocatalysts for proton exchange membrane fuel cells. Int J Hydrogen Energy 2009;34:4387–94. [6] Wee J-H, Lee K-Y, Kim SH. Fabrication methods for low-Ptloading electrocatalysts in proton exchange membrane fuel cell systems. J Power Sources 2007;165:667–77. [7] Wang Zhen-Bo, Zuo Peng-Jian, Chu Yuan-Yuan, Shao Yu-Yan, Yin Ge-Ping. Durability studies on performance degradation of Pt/C catalysts of proton exchange membrane fuel cell. Int J Hydrogen Energy 2009;34:4387–94. [8] Haug Andrew T, White Ralph E, Weidner John W, Huang Wayne, Shi Steven, Stoner Timothy, et al. Increasing proton exchange membrane fuel cell catalyst effectiveness through sputter deposition. J Electrochem Soc 2002;149:A280–7. [9] Alvisi M, Galtieri G, Giorgi L, Giorgi R, Serra E, Signore MA. Sputter deposition of Pt nanoclusters and thin films on PEM fuel cell electrodes. Surf Coat Technol 2005;200:1325–9. [10] Huang Kuo-Lin, Lai Yi-Chieh, Tsai Cheng-Hsien. Effects of sputtering parameters on the performance of electrodes

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

4473

fabricated for proton exchange membrane fuel cells. J Power Sources 2006;156:224–31. Makino Koji, Furukawa Kazuyoshi, Okajima Keiichi, Sudoh Masao. Performance of Sputter-deposited platinum cathodes with nafion and carbon loading for direct methanol fuel cells. J Power Sources 2007;166:30–4. Cha SY, Lee WM. Performance of proton exchange membrane fuel cell electrodes prepared by direct deposition of ultrathin platinum on the membrane surface. J Electrochem Soc 1999;146:4055–60. Mukerjee S, Srinivasan S, Appleby AJ. Effect of sputtered film of platinum on low platinum loading electrodes on electrode-kinetics of oxygen reduction in protonexchange membrane fuel-cells. Electrochim Acta 1993;38: 1661–9. Ticianelli EA, Beery JG, Srinivasan S. Dependence of performance of solid polymer electrolyte fuel-cells with low platinum loading on morphological-characteristics of the electrodes. J Appl Electrochem 1991;21:597–605. Chang Chi-Lung, Chang Tang-Chun, Ho Wei-Yu, Hwang JJ, Wang Da-Yung. Electrochemical performance of PEM fuel cell with Pt–Ru electro-catalyst layers deposited by sputtering. Surf Coat Technol 2006;201:4442–6. Hirano S, Kim J, Srinivasan S. High performance proton exchange membrane fuel cells with sputter-deposited Pt layer electrodes. Electrochim Acta 1997;42:1587–93. Radev I, Slavcheva E, Budevski E. Investigation of nanostructured platinum based membrane electrode assemblies in ‘‘EasyTest’’ cell. Int J Hydrogen Energy 2007;32: 872–7. O’Hayre R, Lee S-J, S-W Cha, Prinz FB. A sharp peak in the performance of sputtered platinum fuel cells at ultra-low platinum loading. J Power Sources 2002;109:483–93. Labou D, Slavcheva E, Schnakenberg U, Neophytides S. Performance of laboratory polymer electrolyte membrane hydrogen generator with sputtered iridium oxide anode. J Power Sources 2008;185:1073–8. Tanuma S, Powell CS, Penn DR. Proposed formula for electron inelastic mean free paths based on calculations for 31 materials. Surf Sci 1987;192:L849. Bard AJ, Faulkner L. In: Electrochemical methods: fundamentals and applications, vol. 341. New York: Wiley; 2001. p. L849–57. Macia MD, Campina JM, Herrero E, Feliu JM. On the kinetics of oxygen reduction on platinum stepped surfaces in acidic media. J Electroanal Chem. 2004;564. Sarapuu A, Kasikov A, Laaksonen T, Kontturi K, Tammeveski K. Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions. Electrochim Acta 2008;53:5873–80. Jaksic MM. Hypo-hyper-d-electronic interactive nature of interionic synergism in catalysis and electrocatalysis for hydrogen reactions. Int J Hydrogen Energy 2001;26:559–78.