Kinetic effect of the ionomer on the oxygen reduction in carbon-supported Pt electrocatalysts

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 7 8 2 8 e1 7 8 3 6

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Kinetic effect of the ionomer on the oxygen reduction in carbon-supported Pt electrocatalysts Amado Vela´zquez-Palenzuela, Francesc Centellas, Enric Brillas, Conchita Arias, Rosa Marı´a Rodrı´guez, Jose´ Antonio Garrido, Pere-Lluı´s Cabot* Laboratori d’Electroquı´mica dels Materials i del Medi Ambient, Departament de Quı´mica Fı´sica, Universitat de Barcelona, Martı´ i Franque`s 1-11, 08028 Barcelona, Spain

article info

abstract

Article history:

The mechanism of the oxygen reduction reaction (ORR) on nanoparticulated Pt/C-Nafion

Received 18 May 2012

electrodes prepared in one step has been studied to simulate the reaction in the cathode

Received in revised form

of a Polymer Electrolyte Fuel Cell (PEFC). The kinetic parameters have been obtained by

29 August 2012

hydrodynamic polarization in O2-saturated 0.01e1.00 M H2SO4 and temperatures in the

Accepted 14 September 2012

range 25.0e50.0
C. The ORR current density was maximum and practically independent of

Available online 12 October 2012

the ionomer fraction in the rage 10e55 wt% Nafion. The poorer proton conductivity for lower Nafion fractions and the formation of catalyst areas completely surrounded by

Keywords:

Nafion together with adsorption of Pt sites by sulfonate groups for higher Nafion fractions,

Pt electrocatalyst

explain the minor ORR activity in these conditions. The ionomer influence on the O2

Oxygen reduction reaction

diffusion at high overpotentials for Pt/C-Nafion was negligible when the Nafion content

Rotating disk electrode

was smaller than 20 wt%. The higher kinetic current density for Pt/C-Nafion (100 mA cm2)

Kinetic parameters

with respect to smooth Pt-Nafion (40 mA cm2), together with the smaller activation energy

Ionomer effect

of the former (25  4 kJ mol1) with respect to the latter (42  5 kJ mol1) highlighted the better properties attained by the nanosize effect. A remarkable novel result is that the reaction order of Hþ in HClO4 is close to unity, whereas in sulfuric acid it is significantly smaller and changes with potential, what has been related to the sulfate adsorption. The anomalous dependence of the charge transfer coefficient with temperature was then explained by the thermal change of the double layer structure and the variation of the coverage of adsorbed species on Pt. The more sensitive effect for Pt/C-Nafion than for smooth Pt-Nafion was ascribed to the stronger interaction between the components when the nanoparticles are involved. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Polymer electrolyte fuel cells (PEFCs) are environmentalfriendly power devices with high efficiency, low-temperature operational regime, minimal production of pollutant

exhausts and adaptability to portable electronic systems [1e4]. Despite these advantages, one of its problems is the sluggish kinetics of the oxygen reduction reaction (ORR) with respect to the hydrogen oxidation reaction (HOR), which limits the power generation in the PEFC [5,6]. The reaction mechanism of the

* Corresponding author. Tel.: þ34 93 4039236; fax: þ34 93 4021231. E-mail address: [email protected] (P.-L. Cabot). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.090

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 7 8 2 8 e1 7 8 3 6

ORR is for this reason of relevant interest. ORR could occur through a direct 4-electron pathway, originating water as final product, but also via a 2-electron pathway, producing H2O2 as intermediate, which could be further reduced in an overall 4electron reaction [7,8]. The release of H2O2 from the electrocatalyst surface is undesirable because a lower specific current is obtained and the hydrogen peroxide, a well-known oxidizing agent, can produce the degradation of components of the PEFCs such as the polymer membrane. Even in the case that 4electron pathway is dominant, the small hydrogen peroxide formation can be harmful for long-term operation [8]. A key constituent of the PEFC is the ionomer, mainly composed of perfluorosulfonate proton exchange membranes such as Nafion ones, which allow the transport of protons from the anode to the cathode. In spite of H2 and O2 solubilities in bulk Nafion are one order of magnitude higher than the corresponding values in the bulk electrolyte [9], the ionomer can introduce additional mass transport limitation due to a decrease in the diffusion coefficient of the reactants in the ionomer. Thus, the diffusion coefficient of the O2 molecule in the ionomer film is reduced about one order of magnitude with respect to the aqueous solution [10,11]. Several works described the use of the rotating disk electrode (RDE) to analyze the electrochemical behavior of Pt-based and non-Pt based as thin-layer electrodes prepared in two steps, i.e. Nafion over the electrocatalyst, to quantify the ionomer influence for the HOR and the ORR [12e16], showing the need to use small Nafion thickness to avoid such mass transport limitations. On the other hand, it is well known that the addition of the ionomer to the catalyst layer improves the PEFC performance due to the enhancement of the proton conductivity in the three-phase zone, where the solution, solid electrolyte, and reactants join together [17]. Therefore, preparing the thinlayer electrode in one step, i.e. depositing the mixture of the catalyst ink with Nafion, better represents the electrode configuration in a real PEFC. In previous work [18e20], carbonsupported nanoparticulated catalysts (Pt/C, Pt/Ru/C, etc.) were impregnated by Nafion ionomer using the one-step technique to assess the ionomer effect on the HOR. For the Pt/C-Nafion electrocatalyst, a higher performance for the HOR was found, with an insignificant mass transport limitation for Nafion contents <15e20 wt%, reaching a maximum electroactive surface area when they were about 30e40 wt%, in good agreement with fuel cell tests [1,17,18]. For the ORR, in which the effect of the preparation of the Pt/ C-Nafion electrodes in one step on the kinetics of the oxygen reduction has been less studied, it has been shown that the limiting current density is independent of the Pt load [21]. However, in the best of the authors knowledge, the in depth study of the effect of the ionomer composition of the Pt/CNafion electrodes on the kinetics of ORR for a wide range of Nafion compositions, prepared by means of the one-step technique, has not been already performed. Considering the relevance of the ORR in the PEFC cathodes, this is the main object of this paper, in which the influence of the temperature, pH, and nature of the electrolyte has been studied using hydrodynamic polarization in acidic media. Comparative results have also been obtained using smooth Pt covered by Nafion thin films.

2.

Experimental

2.1.

Materials and reagents

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High performance Pt nanoparticles supported on carbon Vulcan XC-72 (Pt/C electrocatalyst, 19.6 wt% Pt on carbon), were purchased from E-Tek. Smooth RDE Pt (3 mm in diameter) and glassy carbon (GC) electrodes (also 3 mm in diameter) were provided by Metrohm. The ionomer was a 5 wt% solution of Nafion perfluorinated ion-exchange resin in a mixture of aliphatic low molecular weight alcohols (isopropanol:n-propanol in weight ratio 55:45) and water (15e25 wt% in the mixture) supplied by Aldrich. Analytical grade ethanol from Panreac was used for the preparation of Nafion-ethanol solutions with concentrations between 0.1 and 5 wt%. Acidic electrolytes were prepared with analytical grade 96% H2SO4 and analytical grade 70% HClO4 purchased from Merck. All solutions were prepared with high-purity water obtained with a Millipore Milli-Q system (resistivity > 18 MU cm). Ar and O2 gases were Linde 5.0 (purity  99.999%).

2.2.

Preparation of the electrodes

Prior to the ink deposition, both Pt and GC tips were consecutively polished with aluminum oxide pastes of 0.3 and 0.05 mm (Buehler Micropolish II deagglomerated a-alumina and g-alumina, respectively) on a Buehler PSA-backed White Felt polishing cloth until achieving a mirror finish, being rinsed with Millipore Milli-Q water in ultrasonic bath between the polishing steps. Nafion-covered smooth RDE Pt (Pt-Nafion) electrodes with different ionomer amounts were prepared by depositing an aliquot of 2e15 ml Nafion-ethanol solution on the overall electrode surface. The coated electrode was then dried under the heat of a lamp for at least 10 min. The Nafion loading on the electrode surface varied between 15 and 975 mg cm2. In the case of the Pt/C-ionomer mixture (Pt/C-Nafion), the catalyst inks were prepared by sonicating for 45 min aqueous suspensions of Pt/C electrocatalyst powder, Nafion 5 wt% and water. The Pt/C proportion in the slurry was in the range of 3e8 mg ml1 and the Nafion content between 0.04 and 4.7 wt%. Pt/C-Nafion electrodes were obtained by depositing a stirred volume of the prepared ink on the surface of the GC disk electrode, carefully weighting such volumes with an accuracy of 0.01 mg. The recently prepared electrodes were dried under the heat of a lamp for at least 20 min. In the dry thin layer, the Pt load of the Pt/CNafion electrodes was always set to be 30 mg cm2, the Nafion content being in the range 8e71 wt%. Afterwards, the working electrode was ready to use in the Ecochemie Autolab RDE.

2.3.

Electrochemical experiments

The electrochemical experiments were performed using conventional thermostated double wall three-electrode glass cells from Metrohm of 200 ml capacity and an Ecochemie Autolab PGSTAT100 potentiostat-galvanostat with computerized control by an Autolab Nova 1.5 software. The

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3.

Results and discussion

3.1.

ORR in O2-saturated 0.50 M H2SO4

Fig. 1a shows the hydrodynamic I-E curves obtained for the ORR on Pt/C-Nafion electrode with Nafion content of 12 wt% and rotation speeds between 400 and 3600 rpm in O2-saturated 0.50 M H2SO4. A well-defined limiting current density ( jL) was always achieved at high overpotential from about 0.60 V. The polarization curve for a Pt/C-Nafion ink with a superior ionomer content of 71 wt% is also shown. As can be seen, an increase in Nafion content causes a fall in the ORR activity. In order to compare and interpret the ORR behavior under the different conditions studied, the kinetic current density ( jk) was determined in the potential region where the current starts to increase in the voltammograms of Fig. 1a, using Eq. (1) [13,23]: jk ¼

jjL jL  j

(1)

where the limiting current density ( jL) was measured at 0.2 V for each rotation speed.

a 0.00

-2.00 j / mA cm

-2

400 rpm 900 rpm

-4.00

1600 rpm

-6.00 2500 rpm 3600 rpm

-8.00 0.00

0.20

0.40

0.60

0.80

1.00

E/V

b

2.50

-2

2.00

k

j / mA cm

temperature was maintained in the range 25.0e50.0
C, with an accuracy of 0.1
C. The auxiliary electrode was a Pt rod (3.78 cm2) and the reference electrode was a double junction AgrAgClrKCl (saturated) electrode. The change of the equilibrium potential of the reference electrode with the temperature was obtained from the equation Eref (V) ¼ 0.199  1.01  103 (V
1 C )(t  25), where t is the Celsius temperature. For the equilibrium potential of the ORR, the applied equation was Eeq (V) ¼ 1.23  8.287  104 (V
C1)(t  25) [22]. All potentials given in this work are referred to SHE. After the electrolyte deaeration by Ar bubbling, 20 cyclic voltammograms at 100 mV s1, 15 at 50 mV s1 and 10 at 20 mV s1 from 0.00 to 1.00 V were performed under Ar atmosphere to assure the electrode cleanness. The ORR on the Pt electrodes was typically studied in O2-saturated 0.50 M H2SO4 solution. Prior to the measurements, O2 was bubbled for 30 min and kept over the electrolyte during the experiments. ORR hydrodynamic curves using linear sweep voltammetry at a scan rate of 5 mV s1 between 1.00 and 0.20 V were obtained, gradually increasing the rotation speed (u) from 400 to 3600 rpm. To analyze the influence of pH and the nature of the acidic electrolyte, similar ORR hydrodynamic curves were recorded at 5 mV s1 and 400 rpm in O2-saturated 0.01, 0.10 and 1.0 M H2SO4 and also in O2-saturated 0.01, 0.10 and 1.00 M HClO4 solutions. The measurements were carried out from less to higher acid concentration, the electrode being washed and dried before its transference to the following cell. The temperature effect was assessed by measuring ORR polarization curves in O2-saturated 0.50 M H2SO4 at 5 mV s1 and 400 rpm between 1.00 and 0.20 V at 25.0, 30.0, 35.0, 40.0, 45.0, and 50.0
C. These trials were made in the increasing order of temperature. Each temperature was maintained for 10 min together with O2 bubbling through the solution, thus allowing achieving the equilibrium before applying the following potential sweep.

1.50 1.00 0.50 0.00

0

10

20

30

40

50

60

70

80

% Nafion

Fig. 1 e (a) Hydrodynamic voltammograms in O2-saturated 0.50 M H2SO4 obtained at 5 mV sL1, different rotation speeds and 25.0
C for the ORR on Pt/C-Nafion (HP 20% Pt/C Vulcan XC-72 electrocatalyst, load of 30 mg Pt cmL2, Nafion 12 wt%) on a GC disk electrode. The dotted line corresponds to the ORR polarization curve at 400 rpm for Pt/C-Nafion with 71 wt% Nafion. (b) Kinetic current density vs Nafion percentage in the catalyst for the ORR on Pt/C-Nafion electrodes in O2-saturated 0.50 M H2SO4 at 25.0
C. Data at 0.85 V from hydrodynamic voltammograms recorded at 5 mV sL1 and 400 rpm.

The effect of the Nafion content on the ORR activity of the Pt/C-Nafion was analyzed from the jk values at low overpotential (E ¼ 0.85 V). As can be seen in Fig. 1b, the ORR current density was maximum and practically independent of the ionomer fraction in the range of 10e55 wt%. The inferior performance at lower Nafion fractions can be explained by poorer proton conductivity owing to the smaller impregnation of the electrocatalyst by the ionomer. Conversely, for Nafion percentages higher than about 55 wt%, a noticeable drop of the activity is detected, thus suggesting mass transport effects related to the O2 diffusion through the ionomer, in good agreement with the optimum ionomer composition range found by MEA analysis of fuel cell devices [1,17]. Comparative results were reported in previous work of our group dealing with the effect of the Nafion fraction on the electrochemical active surface area (ECSA), which was measured using the CO stripping technique [24], for the CO and the H2 oxidation

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reactions, on Pt/C or Pt-Ru/C electrocatalysts [18,19]. The decrease of the ORR current at Nafion contents higher than about 55 wt% could be related to the diminution of Pt available active sites because the excess of ionomer could create electrocatalyst regions completely surrounded by Nafion, thus making the electrochemical reactions more difficult. In addition, the specific adsorption of sulphonate groups from Nafion on the Pt surface must be also taken into account [25,26]. This negative effect on the electrochemical reaction would be more notorious at high ionomer fraction and could contribute to the observed activity reduction [27]. The KouteckyeLevich (KL) model was employed to analyze the hydrodynamic I-E curves in the plateau regions of Fig. 1a by means of Eq. (2) [15]: (2)

where the jL value for the ORR is split into three contributions: (i) the kinetic current density ( jk), which is a function of the applied potential, (ii) the limiting current density related to the diffusion through the boundary-layer ( jd), which depends on the mass transport properties through the electrolyte and (iii) the film-diffusion limiting current density ( jf), controlled by the diffusion of O2 through the Nafion layer. The jd term corresponds to the well-known Levich Eq. (3):

(7)

0.00

(3)

a -0.20 -0.40

-0.80

0.09 μm 1.53 μm 4.44 μm 8.93 μm

0.00

where Cf and Df are the solubility and the diffusion coefficient of O2 in the Nafion film, respectively. For the Pt-Nafion and Pt/ C-Nafion electrodes, the apparent ionomer thickness (L, in mm) was calculated by Eq. (5): L ¼ 0:1

XNf w rNf A

(5)

where XNf denotes the mass fraction of Nafion in the solution or in the aqueous catalyst ink (wt%), w is the mass of the deposited aliquot (mg), rNf is the Nafion density (1.98 g cm3 [9]) and A is the geometrical area of the Pt or the GC electrode surface (cm2). Combination of Eqs. (3) and (4) allows writing Eq. (2) as: 1 1 1 1 ¼ þ þ jL jk BC0 u1=2 nFCf Df L1

-0.10

Nafion wt% -0.02

-1

(4)

(6)

According to Eq. (6), by plotting ( jL)1 vs u1/2 at a given Nafion content, one should obtain a straight line with a slope equal to (BC0)1 and a Y-intercept (Y0) given by Eq. (7):

L

nFCf Df L

-1

L

jf ¼

0.00

2

b

(j ) / mA cm

2

-0.60

-1

where n is the number of electrons involved in the ORR, D is the diffusion coefficient of O2 (cm2 s1), n is the kinematic viscosity of the electrolyte (cm2 s1), C0 is the O2 concentration in the solution (equal to its solubility in the case of saturation, in mol cm3) and u is the electrode rotation speed (rpm). In 0.50 M H2SO4 at 25.0
C, the theoretical BC0 value for the 2- and 4-electron pathways of O2 are 6.98  102 and 14.32  102 mA cm2 rpm1/2, respectively, considering D ¼ 1.70  105 cm2 s1, n ¼ 1.01  102 cm2 s1 and C0 ¼ 1.30  106 mol cm3 [28]. The jf term is defined from Eq. (4):

-1

jd ¼ 0:62 nFD2=3 n1=6 C0 u1=2 ¼ BC0 u1=2

1 1 þ jk nFCf Df L1

Thus, the corresponding Y0 is a linear function of the ionomer thickness and its representation in front of L1 gives the O2 permeability (CfDf) in the ionomer. Fig. 2a shows the good linear ( jL)1 vs u1/2 plots with similar slope obtained for the ORR in the Pt-Nafion electrode, as predicted by Eq. (6). The lowest current density was achieved for the greatest Nafion thickness, in agreement with Eq. (4), thereby confirming that the ionomer can produce an additional mass transfer barrier for O2 diffusion. As illustrated in Fig. 2b, similar results were found for Pt/C-Nafion, indicating that Nafion can also limit the O2 diffusion in the one-step prepared catalyst inks. From the slopes of the KL plots, BC0 values of (13.7  0.1)  102 and (14.1  0.1)  102 mA cm2 rpm1/2 were determined for the Pt-Nafion and the Pt/C-Nafion electrodes, respectively, giving a similar overall number of transferred electrons of n ¼ 3.82  0.03 and n ¼ 3.94  0.03. This confirms that ORR mainly follows a 4-electron pathway giving H2O on both Pt electrodes, with a very minor production of H2O2 via a 2electron pathway. The slightly higher n value found for the nanoparticulated electrocatalyst could then be related to

(j ) / mA cm

1 1 1 1 ¼ þ þ jL jk jd jf

Y0 ¼

-0.20

-0.04 0.000 ω

-1/2

0.001 -1/2 / rpm

0.002

-0.30 -0.40 -0.50 0.00

0.12 μm (12 wt%) 0.32 μm (30 wt%) 1.01 μm (56 wt%) 2.22 μm (71 wt%)

0.01

0.02

0.03 -1/2

ω

0.04

0.05

0.06

-1/2

/ rpm

Fig. 2 e KouteckyeLevich (KL) diagrams for the ORR on: (a) Pt-Nafion electrodes with thickness between 0.09 and 8.93 mm and (b) Pt/C-Nafion electrodes with apparent ionomer thickness between 0.12 (Nafion 12 wt%) and 2.22 mm (Nafion 71 wt%), with a Pt load of 30 mg Pt cmL2. The inset panel shows the magnification of the linear plots in the vicinity of the Y-intercept for the Pt/C-Nafion measurements. KL data obtained in O2-saturated 0.50 M H2SO4 at 0.2 V, 5 mV sL1 and 25.0
C.

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a most efficient ORR. The 3D structure of Pt/C facilitates the readsorption of generated H2O2 on the electroactive surface of another nanoparticle to be further reduced to H2O, causing a greater n value than that found for the ORR in the Pt bulk electrode. This phenomenon has also been described for the rotating ring-disk electrode (RRDE) analysis of Pt/C [29], carbon-supported Se/Ru chalcogenide [30], and Fe-Nx/C electrocatalysts [14]. The inverse relationship of Eq. (7) allowed determining the thickness effect of Nafion (Fig. 3a and b), since (Y0)1 approaches jk when the ionomer thickness decreases to zero. In both catalysts, the Nafion effect can be neglected for L1 > 5 mm1 (L < 0.2 mm), indicating a similar mass transport resistance in the catalyst electrode prepared in one step. The same critical L value was determined in previous work focused on the HOR and the ORR using a two step technique for the ink preparation [12e16], corresponding to an equivalent Nafion fraction in the catalyst layer of z20 wt% [20]. This corroborates that the one-step electrode preparation is a valid

a

50

30 0.4

Y / mA-1 cm2

20

0.3 0.2

0

0

-1

(Y ) / mA cm

-2

40

10

0.1 0.0

0 0

0

2

5

4 6 L / μm

8

10

10 -1

15

-1

L / μm

(% Nafion) 0.00 150

0.05

0.10

0.15

100 Y / mA-1 cm2

0.020

0

-1

(Y ) / mA cm

-2

b

-1

0

50

0.015 0.010 0.005 0.000

0

0

0.0 0.2 0.4 0.6 0.8 1.0 L / μm

5

10 -1

15

-1

L / μm

Fig. 3 e Inverse of KL intercept, (Y0)L1, for the ORR on: (a) PtNafion electrodes as a function of the inverse of the Nafion thickness, LL1, and (b) Pt/C-Nafion electrodes (HP 20% Pt/C Vulcan XC-72 electrocatalyst, load of 30 mg Pt cmL2) vs the inverse of (B) the apparent thickness and (,) the Nafion percentage. The inset panels present the corresponding Y0 vs L plots. Data from hydrodynamic voltammograms in O2saturated 0.50 M H2SO4 at 0.2 V, 5 mV sL1 and 25.0
C.

technique for the kinetic analysis of the ORR and the quantification of the ionomer effect in the catalytic response. The inset panels in Fig. 3a and b present the excellent linear relationships found for Y0 vs the ionomer thickness plots for both electrodes, respectively, as expected from Eq. (7). From the corresponding Y-intercept, jk values of 40 and 100 mA cm2 were obtained for the Pt-Nafion and Pt/C-Nafion electrodes, respectively. Watanabe et al. [16] found a comparable jk ¼ 50 mA cm2 for Nafion-covered Pt bulk electrodes. The better intrinsic catalytic properties of the nanoparticulated Pt/C specimen are then highlighted by the superior jk in the higher overpotential region, more than the double than that of the smooth Pt electrode. This can not be explained by the higher surface area of the Pt nanoparticles with respect to smooth Pt, which is increased by a factor of about 20, considering the electroactive surface area of 60 m2 g1 for this system [18] and the Pt load of 30 mg Pt cm2 employed in this paper. In addition, the limiting current is expected to be independent of the Pt load, because only the electrode section is relevant in the hydrodynamic reduction process [21]. From the slopes of the above correlations, CfDf values of 6.6  106 and 3.5  105 mM cm2 s1 for the corresponding Pt-Nafion and Pt/C-Nafion electrodes were obtained. Since the theoretical data for the permeability of the O2molecule is C0D ¼ 2.6  105 mM cm2 s1, the lower CfDf value for the Pt-Nafion electrode indicates that the ionomer acts as mass transport barrier for the O2 diffusion, causing a reduction of 75% of its permeability. This phenomenon is much less significant for the Pt/C-Nafion, which suggests a change of the structure of the Nafion film with the carbon support that strongly enhances the O2-mass transport through the ionomer.

3.2.

Tafel analysis and reaction order for O2 and Hþ

Fig. 4a shows the Tafel plots obtained for the ORR in the Pt/CNafion electrode taking the jk values calculated from Eq. (1). Similar results were found for the bulk Pt electrode. Two different linear correlations with Tafel slopes close to b1 ¼ 2.303RT/F z 60 mV and b2 ¼ 2  2.303RT/F z 120 mV (61 and 111 mV for Pt-Nafion, and 77 and 134 mV for Pt/ C-Nafion), for lower and higher overpotentials, respectively. The Tafel slope close to 60 mV is the expected value for a one-electron charge transfer as the rate-determining step (rds) and under Temkin conditions of the adsorbed intermediate, as typically found for ORR on bulk Pt [31]. In contrast, the Tafel slope of about 120 mV at higher overpotentials is the theoretical value for a one-electron charge transfer as the rds but under Langmuir conditions of the adsorbed intermediate [13]. The reason for this change in the Tafel slope can be attributed to the pass of the ORR taking place on the PtOx species, mainly PtO and PteOH, to a free-oxide Pt surface after reduction of such Pt oxides [32]. This assumption is supported by the fact that the Tafel slope of ca 120 mV starts at about 0.76 V (see Fig. 4a), which is very close to 0.70 V, where the peak for the PtOx reduction appears [19]. The reaction order with respect to O2 concentration (m) was determined using the relationship between the current I and the corresponding IL at a selected potential and different rotation speeds given by Eq. (8) [11,31]:

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1.00

a b1

E/V

0.90 0.80

b2

0.70

0.60 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -2 log ( j / mA cm )

2.0 2.5

k

-3.30

log (I / A)

-3.40

b

-3.50 -3.60 -3.70 -3.80 -3.90 -0.90

-0.80

-0.70 -0.60 log (1 - I/I )

-0.50

-0.40

L

-1.00

c

d

-3.00

k

log (I / A)

-2.00

-4.00 -5.00 -6.00 -2.50 -2.00 -1.50 -1.00 -0.50

0.00 -2.50 -2.00 -1.50 -1.00 -0.50 0.00 + log ([H ] / M)

0.50

Fig. 4 e (a) Mass-transfer corrected Tafel plots for the ORR on Pt/C-Nafion electrode in O2-saturated 0.50 M H2SO4 at 5 mV sL1, 400 rpm and 25.0
C. The Tafel slopes at (b1) low and (b2) high overpotential regions are highlighted. (b) Logarithm of current vs log (1 - I/IL) at 0.75 V for the ORR on Pt/C-Nafion electrode in O2-saturated 0.50 M H2SO4 at 5 mV sL1, rotation speed between 400 and 3600 rpm, and 25.0
C. The double logarithmic plots for the kinetic current in front of proton concentration in O2-saturated (c) H2SO4 and (d) HClO4 solutions for the ORR on Pt/C-Nafion electrode at 5 mV sL1, 400 rpm and 25.0
C are also shown. Potential: (B) 0.83 V, (,) 0.78 V, (6) 0.73 V, (C) 0.68 V and (-) 0.63 V. Pt/C-Nafion electrode with a Pt load of 30 mg Pt cmL2 and 30 wt% Nafion in both cases.

  I logI ¼ logIk þ mlog 1  IL

(8)

where (1  I/IL) is equivalent to the CS/C0 ratio between the O2 concentration near the catalyst surface (CS), the practical species concentration in heterogeneous catalysis, and the bulk solution (C0). Fig. 4b presents the good straight line obtained by applying Eq. (8) at 0.75 V, in the region where the ORR on the Pt/C-Nafion electrode is under kinetic-diffusion control. The slope of these linear plots was m ¼ 1.02, indicating that ORR in the Pt/C electrocatalyst follows a first-order kinetics. The same result (m ¼ 0.96) was found for the ORR in the smooth Pt electrode. This behavior is in agreement with previous work proposing that the rds is the electron transfer involving the O2-adsorbed molecule [11].

RDE experiments of the ORR at different Hþ concentrations were also performed to determine the reaction order for this specie ( p) from Eq. (9):  p¼



vlogIk vlog½Hþ 

(9) T;E

The linear log Ik vs log [Hþ] plots shown in Fig. 4c for the ORR on the Pt/C electrocatalyst at different potentials in H2SO4 medium gave an average fractional p ¼ 0.6  0.1, changing from 0.63 (at 0.83 V) to 0.44 (at 0.63 V). A similar p ¼ 0.7  0.2 was obtained for the bulk Pt electrode, varying from 0.82 (at 0.83 V) to 0.39 (at 0.63 V). This behavior differs from that reported by Damjanovic et al. [23] for the ORR in oxide-covered Pt electrodes with two reaction orders of 0.5 and 1.0 at

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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 e n e r g y 3 7 ( 2 0 1 2 ) 1 7 8 2 8 e1 7 8 3 6

O2ðadsÞ þ Hþ þ e /HO2ðadsÞ

(10)

Although this step has also been proposed by other authors [11,13], the influence of the nature of the electrolyte on the reaction order for Hþ pointed out above was not reported previously, resulting a key subject that must be considered in order to carry out properly the mechanistic ORR analysis.

3.3.

a

-3.5

-4.5

k

log (I / A )

-4.0

-5.0

-5.5 3.10

3.20

3.30 3 -1

10 T / K

b

3.40

-1

1.40 1.20

Symmetry factor

higher and smaller overpotential Tafel regions, respectively. The fractional reaction order for Hþ suggests that it is involved in the reaction pathway not only in the rds but also in previous equilibrium reactions. However, this assumption can be considered tentatively since the Hþ concentration in the bulk may be different from that of electrocatalyst surface. Moreover, the anion adsorption on the electrode surface would vary the structure of the double layer, modifying the available concentration of Hþ for the ORR. To evaluate the influence of the adsorbed species on p, comparable hydrodynamic experiments with O2-saturated HClO4 solutions were performed. This acid was chosen since it is well known that perchlorate is more weakly adsorbed than sulphate and bisulphate [13], so these experiments can be considered as almost free from anionic adsorption. The results presented in Fig. 4d for HClO4 media gave p ¼ 0.90  0.04 for the ORR in the nanoparticulated Pt electrocatalyst, regardless of potential in the range 0.63e0.83 V. An analogous p ¼ 0.9  0.1 was obtained for the smooth Pt electrode between 0.78 and 0.88 V, where log Ik vs log [Hþ] relationship behaved linearly. The results found in HClO4 media then support a reaction order for Hþ close to the unity, indicating that the adsorption phenomena of sulphate ion on the electrocatalyst surface affects considerably the corresponding reaction order. The change of the anion surface coverage with the potential would then explain the variation of p shown in Fig. 4c. Taking into account the determined Tafel slopes and the reaction orders, one can consider that the rds for the ORR consistes in the formation of an adsorbed hydroperoxyl radical from Eq. (10):

1.00 Low overpotential

0.80 High overpotential

0.60 0.40 0.20 290

295

300

305

310

315

320

325

330

T/K Fig. 5 e (a) Arrhenius plots for the ORR on (B) Pt-Nafion (thickness 0.10 mm) and (,) Pt/C-Nafion with 30 mg Pt cmL2 and Nafion 30 wt% in O2-saturated 0.50 M H2SO4 at 5 mV sL1, 400 rpm and temperatures between 25.0 and 45.0
C. Overpotential: -0.40 V (0.83 V at 25
C). (b) Variation of symmetry factor calculated from Tafel slopes at low (E > 0.76 V) and high overpotential (E < 0.76 V) with temperature for the ORR on the above samples under the same experimental conditions.

Activation energy for the ORR

The activation energy (Ea) for the ORR in both Pt electrocatalysts were estimated by the Arrhenius equation from the I-E data obtained in RDE experiments using O2-saturated 0.50 M H2SO4 solution between 25.0 and 45.0
C. Since the ORR is a thermally activated reaction, an increase in the reduction current with increasing temperature was detected. Fig. 5a shows the excellent linear correlations found for the Arrhenius plots (log Ik vs T1), obtained for a given overpotential, after correction by the temperature, as explained in the Experimental section. The slopes of these straight lines were practically independent of the potential for each electrocatalyst, with average values of (2.2  0.3)  103 and (1.3  0.2)  103 K, corresponding to Ea values of 42  5 and 25  4 kJ mol1, for the bulk and the nanoparticulated Pt electrocatalysts, respectively. This indicates that the ORR is more favored on Pt/C. Other authors reported Ea values of z40 kJ mol1 [22] and even higher ones such as 55e60 kJ mol1 for the ORR in acidic media on Pt electrodes [33], which

matches with our result for the smooth Pt-Nafion electrode. In contrast, lower activation energies of 20e23 kJ mol1 have been found for both bulk and nanoparticulated Pt or Pt alloys (Pt3Ni and Pt3Co) [34], which are closer to that obtained by us for the Pt/C-Nafion electrode. This difference can be explained considering that the Ea value is an apparent parameter that depends of many factors, like the state of the surface, the presence or absence of ionomer and the concentration/nature of the acid employed as electrolyte. Besides, in a particular experiment, Ea could also be modified by the adsorbed species (oxygenated groups, anions, etc) and/or the change of the electrochemical double layer structure. In our work, the same conditions were used to analyze both Pt electrodes and then, the lower Ea obtained for the ORR on Pt/C-Nafion can be attributed to the nanosize effects that favor the electron transfer in the rds (for example, the modification of the Pt electronic structure because of the superior surface/volume atoms ratio).

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 7 8 2 8 e1 7 8 3 6

3.4.

Effect of the temperature on the Tafel slopes

The effect of temperature on the Tafel slopes was checked through the charge transfer coefficient (a) given by Eq. (11) [35]: a¼

g þ rb s

(11)

where g is the number of electrons transferred before the rds, s is the stoichiometric number, r is the number of electrons involved in the rds, and b is the symmetry factor. If Eq. (10) is the rds, one can take g ¼ 0 and r ¼ 1, and Eq. (11) is reduced to a ¼ b. Fig. 5b depicts the symmetry factors at both Tafel regions for smooth Pt-Nafion and Pt/C-Nafion. As can be seen, b presents an anomalous behavior with temperature. For the smaller overpotential region, a linear relationship between b and T can be observed with a slope of 12  103 K1 for Pt/C-Nafion, much greater than 7  103 K1 for smooth Pt-Nafion. In contrast, the b values at higher overpotentials for smooth Pt-Nafion only slightly increases with T (slope 7  104 K1), so the classical Tafel expression is better accomplished. The same analysis for Pt/C-Nafion shows a larger influence of the temperature on b (slope 5  103 K1). These dependences were tried to be justified by Conway’s theory [22,36,37], which considers the split of b into bH and bS related to the changes of the enthalpy and the entropy of activation with potential, respectively, as given by Eq. (12): b ¼ bH þ bS T

cathodes of PEFCs was evaluated. For Pt/C-Nafion, the higher ORR current density in the smaller overpotential region was obtained with Nafion fractions of 10e55 wt%. The minor ORR activity for lower ionomer contents was caused by the poorer proton conductivity and, for higher ionomer contents, by the excess of Nafion surrounding the nanoparticles together with the increased adsorption effect of sulfonate on Pt. KL plots at higher overpotentials confirmed the predominance of the 4electron reduction pathway in the ORR. In this potential region, jk values about 40 and 100 mA cm2 for smooth PtNafion and Pt/C-Nafion, respectively, were determined. The ionomer effects on the ORR current were negligible for L < 0.2 mm or Nafion content <20 wt%. In contrast, the reaction order for Hþ reached positive and fractional potentialdependent values in H2SO4 solutions, but close to the unity in HClO4 solutions, thus showing that it is affected by the anionic adsorption on the electrode surface and the consequent change in the double layer structure. The Ea values of 42  5 and 25  4 kJ mol1 determined for smooth Pt-Nafion and Pt/C-Nafion, respectively, also reflect the higher activity of the nanoparticulated electrocatalyst. An anomalous dependence of a with temperature was found in both Tafel regions, suggesting that it is due to the thermal modification of the double layer structure and the change of the adsorption regime of the intermediate species, leading to greater variations for the Pt/C-Nafion electrocatalyst.

(12)

Consequently, the slope of the plots in Fig. 5b gives bS, while the Y-intercept corresponds to the enthalpy factor bH. Parthasarathy et al. [38] found a variation of a in the high overpotential region with the temperature of 2.3  103 K1 for Pt/ Nafion electrodes, whereas a remained unaltered with T at lower overpotentials. In contrast, Wakabayashi et al. [22] obtained the same thermal variation for a of 2.3  103 K1 in both Tafel regions. Solorza-Feria and Duro´n [37] also reported a slope of 2.3  103 K1 for the ORR on Ru nanoparticles. The b (or a) increases with the temperature in the present work are then much higher than those reported in the literature, except at lower overpotentials for the smooth Pt-Nafion electrode, and they also lead to irrational negative values of bH, so that Conway’s model cannot be applied in our conditions. The anomalous behavior of our b values can then be ascribed to other key factors [38,39]. One of them would be the thermal change of the double layer structure and the variation of the coverage of adsorbed intermediate species (O2, anions, etc) on the active surface, which could affect the charge transfer step and therefore, the b values. Besides, the greater slopes shown Fig. 5b for Pt/C-Nafion compared to those of smooth Pt-Nafion suggests that the former is more sensitive to the thermal factors due to the changes of the ionomereelectrocatalyst interface, as a result of a stronger interaction between the components.

4.

17835

Conclusions

The influence of the ionomer on the ORR performance of Pt/CNafion and Pt-Nafion electrodes as electrocatalysts for

Acknowledgments The authors thank the financial support given by the Spanish MEC (Ministerio de Educacio´n y Ciencia) through the project NAN2004-09333-C05-03. The FPU fellowship from Spanish MEC received by A. Vela´zquez-Palenzuela to do this work is also acknowledged. This work is dedicated to the memory of Amado Vela´zquez Me´ndez.

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