Pt catalyst with low content of Pt nano-particles

Journal of Electroanalytical Chemistry 671 (2012) 24–32

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The kinetics of the hydrogen oxidation reaction on WC/Pt catalyst with low content of Pt nano-particles M.D. Obradovic´ a, S.Lj. Gojkovic´ b, N.R. Elezovic´ c, P. Ercius d, V.R. Radmilovic´ b, Lj.D. Vracˇar b, N.V. Krstajic´ b,⇑ a

Institute of Chemistry, Technology and Metallurgy – ICTM, University of Belgrade, 11000 Belgrade, Njegoseva 12, Serbia Faculty of Technology and Metallurgy, University of Belgrade, 11000 Belgrade, Karnegieva 4, Serbia Institute for Multidisciplinary Researches, University of Belgrade, 11030 Belgrade, Kneza Vislesava 1, Serbia d National Center for Electron Microscopy, LBLN University of California, Berkeley, USA b c

a r t i c l e

i n f o

Article history: Received 14 October 2011 Received in revised form 30 December 2011 Accepted 27 January 2012 Available online 24 February 2012 Keywords: Hydrogen oxidation Mechanism Tungsten carbide Platinum catalyst

a b s t r a c t The catalytic activity of WC/Pt electrocatalysts towards hydrogen oxidation reaction (HOR) in acid solution was studied. Tungsten carbide (WC) prepared by polycondensation of resorcinol and formaldehyde in the presence of ammonium metatungstate salt and CTABr surfactant was used as the support of a Pt electrocatalyst (WC/Pt). The obtained WC/Pt electrodes were characterized by XRD, HRTEM, EDS, EELS and electrochemical measurements. HRTEM analysis showed that the WC particles possess a core–shell structure with a metallic tungsten core and a shell composed of a mixture of tungsten carbides shell (WC and W2C). The WC/Pt catalyst is composed of well-dispersed sub-nanometer Pt clusters which consist of a few to several tens of Pt atoms. EELS measurements indicate that the WC particles function as nucleation sites for Pt nanoparticles. Based on the Tafel–Heyrovsky–Volmer mechanism the corresponding kinetic equations were derived to describe the HOR current–potential behavior over the entire potential region on RDE. The fitting showed that in the lower potential region HOR on Pt proceeds most likely via the Tafel–Volmer (TV) pathway. The kinetic results also showed that the WC/Pt(1%) when compared to the standard C/Pt(1%) electrode led to a remarkable enhancement of the hydrogen oxidation in an acidic medium, which was explained by H-spill-over between platinum and tungsten carbide. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Tungsten carbide (WC) has been considered as an anode material for hydrogen [1] or methanol [2] polymer electrolyte membrane fuel cells (PEMFCs) since Leavy and Boundort [3] first showed that WC materials possess catalytic properties similar to those of platinum group metals, due to their isoelectronic structure to platinum. Unfortunately, WC is not an inert material. When exposed to water, WC undergoes continuous oxidation and dissolution [4,5]. The exact nature of the formed tungsten oxides is difficult to characterize and is strongly dependent on the applied potential, electrolyte composition and surface pretreatment [6]. The relatively undefined composition of the surface oxide layers is probably the major reason for the irreproducible hydrogen adsorption potentials at tungsten carbide [7]. The activity for hydrogen oxidation reaction (HOR) and stability of high area tungsten carbides in acidic electrolytes depend on their preparation method. The activity of WC was related to the carbon deficiency and oxygen replacement in carbon layers of the WC lattice [8,9]. ⇑ Corresponding author. Tel.: +381 113303682. E-mail address: [email protected] (N.V. Krstajic´). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2012.01.026

Most significant for the electrocatalysis at tungsten carbide is the presence of oxygen species (tungsten oxides) at the catalyst surface. Ross and Stonehart [10] discussed the effect of the surface composition of tungsten carbides on the activity for HOR. The HOR activity of the carbon deficient, oxygen containing carbides was significantly higher than that of the stoichiometric carbide. The increased activity of the oxygen substituted carbide was due to a reduced interaction of the surface with the electrolyte, resulting from the covalent tungsten-oxygen bonding. However, when used as an anodic material under PEFC conditions, WC alone exhibits poor electrocatalytic activity although it showed tolerance to CO poisoning [11,12]. The HOR activity of WC was four orders of magnitude lower than that of Pt [10]. One of the reasons is its low specific area, due to a high preparation temperature which lead to formation of rather large particle sizes. Lately, a great effort has been directed towards studies on the fundamental surface properties and new methods for preparation of WC materials that could be used as an electrocatalyst [13,14]. It has been reported that WC possesses three different crystalline phases (b-W2C, a-WC, and b-WC1x) depending on the synthetic route and reaction conditions [15]. In order to improve the catalytic activity of tungsten carbide towards HOR the effect of the addition

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of a second metal, such as Ni, Co, Fe, Mn or Mo was also investigated [16–18]. However, addition of Ni, Fe or Mn resulted in a lower activity for HOR, owing to the reduced surface area of the catalysts. At the same time, Co–WC catalyst carburized at 600 °C exhibited better catalytic activity as compared to WC catalyst [18]. It has been found that combination of Pt with both a-WC [19–21], b-W2C [22] and b-WC [23] resulted in a high catalytic activity for hydrogen oxidation. While the W2C/Pt catalyst showed similar HOR activity and kinetics as C/Pt catalyst [22], WC/Pt catalysts prepared with high surface area WC exhibited higher specific activity then C/Pt catalyst [24,25]. It has been postulated that Pt was accelerating the dissociative adsorption of H2, which was the ratedetermining step for the HOR [25] and WC could take over the rest steps in the mechanism of the hydrogen oxidation reaction. In addition, it has been found that the stability of WC was extended to higher positive potentials in the presence of a small amount of Pt [26]. The enhanced stability of the Pt–WC surface was attributed to the strong bonding between Pt and WC, most likely at defect regions of the WC surface, which prevents the surface oxidation. Some reports found that the charge for H adsorption/desorption on Pt supported on WC was higher compared with that on C supports [27,28], which was explained by H+-spill-over between Pt and WC. According to our best knowledge, in the published reports, the HOR activity of various tungsten carbides or Pt/WC catalysts was mainly treated by comparing RDE polarization curves, or the kinetic currents determined using Koutecky–Levich linear relationship, at the selected potential. The aim of the present paper is to demonstrate that the kinetics of the HOR at WC/Pt catalysts can be generally treated through the Tafel–Heyrovsky–Volmer pathway, and it is possible to determine kinetic parameters of those elemental steps by analyzing RDE polarization curves. The kinetics of the HOR was investigated at home-made WC/Pt and commercial C/Pt catalysts in order to examine the specific role of WC on enhanced activity of Pt towards the HOR. The Pt catalyst content was very low (1 mass%) in both electrodes, because that is the only way to see any noticeable difference in their RDE polarization curves, taking into account the fact that the HOR is an extremely fast reaction on Pt. 2. Experimental 2.1. Preparation of WC Mesoporous WC was prepared by polycondensation of resorcinol (99% purity E. Merck) and formaldehyde (Fluka Chemie) in the presence of cetyltrimethylammonium bromide (CTABr) surfactant, using the modified method proposed by Ganesan et al. [28]. In a typical synthesis, 6.14 g of CTABr was dissolved in 20 ml of distilled water and added to the solution containing 4.073 g of ammonium metatungstate (AMT), 1.13 g of resorcinol and 1.7 ml of formaldehyde in 10 ml H2O. Then the solution was decanted in a glass tube, sealed and placed for 3 days at 25 °C, 1 day at 50 °C and 3 days at 85 °C. During this procedure the solution was transformed to gel from which cryogel was prepared by the freeze-drying method [29]. The gel was immersed in a tenfold higher volume of t-butanol (p.a. quality, Centrohem, Beograd) for one day and rinsed twice with new t-butanol to displace the liquid contained in the gel. The sample was pre-frozen at 30 °C for 24 h. After that, it was dryed frozen for 20 h at the pressure of 4 mbar. The red colored cryogel was calcinated at 1173 K for 1 h in Ar flow and 2 h in H2 flow (2 cm3 s1) [30]. 2.2. Physicochemical characterization The BET surface area and pore size distribution of the WC sample were calculated from nitrogen adsorption/desorption

25

isotherms at 196 °C, using the gravimetric McBain method. Pore size distribution was estimated by applying the BJH method [31] to the desorption branch of isotherms, and mesoporous surface and micropore volume were estimated using the high resolution as plot method [32]. The phase structure, crystallinity and size of the synthesized WC and WC/Pt catalysts were studied with an X-ray diffractometer (XRD, JEOL 6300F microscope) with Cu Ka radiation (k = 0.154056 nm). Transmission electron microscopy (TEM), measurements were performed using the FEI (Fillips Electronic Instruments)-CM200FEG super-twin and TEAM0.5 ultra-twin transmission electron microscopes, operating at 200 kV and equipped with the Gatan 1k  1k and 2k  2k CCD cameras, respectively. Specimens were prepared for transmission electron microscopy by making suspension of the catalyst powder in ethanol, in an ultrasonic bath. The suspension was dropped onto clean holey carbon grids and then dried in air. The chemical composition of W–WC core–shell particles were characterized by energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). 2.3. Catalyst preparation The Pt catalyst was deposited on a WC support by a conventional borohydride reduction method. The preparation process can be described as follows: 40 mg of the WC powder was dispersed in 20 ml of high-purity water (Millipore, 18 MX cm) in ultrasonic bath, and then mixed with appropriate amount of H2PtCl6 aqueous solution (10 mg ml1). The mixture of metal salt and support was reduced by using an excess of sodium borohydride solution. The precipitate was washed with high-purity water and then dried at 80 °C. The Pt loading of the samples was 1 mass% and 10 mass%. 2.4. Electrode preparation Four milligrams of WC/Pt catalysts was ultrasonically suspended in 1.0 ml of 2-propanol and 50 ll of Nafion solution (5 mass%, Aldrich) to prepare catalyst inks. Then, 10.0 ll of ink was transferred with an injector to the clean gold disk electrode (5 mm diameter, with geometric surface area of 0.196 cm2). After volatilization of alcohol, the electrode was heated at 80 °C for 10 min. The Pt loading was 0.4 lg (1 mass%). In order to compare the catalytic activity of WC/Pt against a conventional C supported Pt catalyst, a commercial catalyst XC-72R/Pt (20 mass%, E-Tek) was thoroughly mixed and homogenized with appropriate amount of XC-72R powder to attain 1 mass% of Pt loading. 2.5. Electrochemical characterization A conventional three-compartment cell was used for electrochemical characterization. The working electrode compartment was separated by fritted glass discs from other two compartments. A reversible hydrogen electrode (RHE) in the same solution as that of the working electrode was used as the reference electrode. A large-area platinum sheet of 5 cm2 geometric area was used as the counter electrode. The electrochemical measurements were performed in 0.5 mol dm3 HClO4 solution (Spectrograde, Merck), prepared with high-purity water, at the temperature of 25 °C. The experiments were performed by potentiodynamic method. A PAR Model 273 Potentiostat/Galvanostat was used for all electrochemical experiments. Polarization curves for the HOR were recorded at the scan rate of 2 mV s1. The cyclic voltammetry (CV) experiments were carried out in the potential range between hydrogen and oxygen evolution in

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N2 saturated 0.5 mol dm3 HClO4 with various scan rates at a rotating speed of 2500 rpm. The electrochemically active surface area of Pt was determined by the stripping of underpotentially deposited (upd) copper [33]. This method is applicable to WC + Pt system since the anodic peak of Cu stripping is more positive than the peak for hydrogen deintercalation from the hydrous tungsten oxide. In addition, WC alone was found [34] to be inactive for Cu upd. Copper was underpotentially deposited from the unstirred solution of 0.10 mol dm3 H2SO4 and 2.0  103 mol dm3 CuSO4 at the potential of 0.330 V vs. RHE, which is about 15 mV more positive than the equilibrium potential of Cu electrodeposition in the electrolyte applied. After 2 min of deposition, which is enough to form a for full monolayer, the electrode potential was swept anodically and a stripping voltammogram was recorded.

3. Results and discussion 3.1. Characterization of WC and WC–Pt An XRD pattern of the as-prepared WC is presented in Fig. 1. The diffraction peaks at 31.77°, 35.98°, and 48.26° correspond to (0 0 1), (1 0 0), and (1 0 1) facets of WC, respectively, while the 2H of 34.52°, 38.03°, 39.57°, 52.3°, 61.8°, and 69.77° correspond to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), and (1 0 3) facets of W2C. In addition, the sharp peaks at 40.26°, 58.27°, and 73.19° correspond to (1 1 0), (2 0 0), and (2 1 1) facets of metallic W. Hence, the XRD pattern consists of well defined diffraction peaks of W, W2C, and WC phases and does not indicate the presence of tungsten oxides after preparation procedure, which means that during carburization process all of the starting amount of tungsten oxide was converted into a mixture of WC, W2C and metallic W. An XRD pattern of WC/Pt (not shown) does not show peaks arising from the Pt nanoparticles due to very low Pt content. Figs. 2a and 3a depict a typical high-angle angular dark field (HAADF) scanning transmission electron microscope (STEM) images of the as prepared WC and WC/Pt with 1 wt.% Pt, respectively. The tungsten carbides particles are nanocrystals with a typical bimodal particle size distribution: larger above 5 nm in diameter and smaller ones below 2 nm in diameter (Fig. 2a). The corresponding HRTEM image of WC particle (Fig. 2b) shows a complex core–shell structure with a core of metallic tungsten and a shell made of a mixture of tungsten carbides (WC and W2C). Elemental

Fig. 1. XRD pattern of WC support.

analysis of the shell by EDS is shown in the inset of the Fig. 2b. The O element detected in the shell is probably associated with the reaction of the highly active surface of tungsten carbides with the oxygen from the air and corresponding tungsten oxides are probably amorphous in nature. Taking into account the fact that preparation procedure was conducted at 900 °C the origin of O element in the shell is not non-converted tungsten oxides, because the corresponding diffraction peaks of oxide phases must be present in XRD pattern. It has been well documented that under normal conditions, on contact with ambient atmosphere the fresh tungsten carbide chemisorbed oxygen resulting in the formation of WOx species [35,36]. It can be concluded that after 3 h of carburization at 900 °C, carbon reduced the tungsten oxide in the precursor to a mixture of W, WC, and W2C. Also, there is carbon in excess and the supporting material consists of un-reacted amorphous carbon with embedded W particles covered with WC and W2C phases. The WC/Pt(1 wt.%) sample (Fig. 3a) is composed of well-dispersed sub-nanometer Pt clusters which mostly consist of a few to several tens of Pt atoms. Some of the individual atoms are clearly resolved in this HRSTEM image. FFT taken from the HAADF image shown in Fig. 3b shows clearly that this Pt particle is imaged close to 110 zone axis. Even subnanometer sized cluster marked in Fig. 3b shows similar, although slightly distorted, fcc pattern, typical for Pt cluster. The EELS spectrum in the inset of Fig. 3b, shows the presence of W and Pt by their M4,5 energy loss edges, and thereby indicates that WC particles are functioning as nucleation sites for Pt nanoparticles. Nitrogen adsorption isotherms of WC, as the amount of N2 adsorbed as a function of relative pressure at 196 °C, are shown in Fig. 4a. The isotherms are identified as type IV which is characteristics of mesoporous materials. The specific surface area calculated by the BET equation, SBET, was 85 m2 g1. The pore size distribution (PSD) is shown in Fig. 4b shows that the WC is mesoporous with the most of the pore radius lower than 2 nm. 3.2. Cyclic voltammetry results Cyclic voltammetry (CV) data for the WC and WC/Pt catalysts with different Pt loadings are presented in Fig. 5. CV was performed to determine the electrochemically active surface area and to elucidate the adsorption properties of the catalysts. The CV curve of the WC (Fig. 5a, curve 3) shows a very clear adsorption/desorption peak at 0.1 V vs. RHE. Its reversible nature suggests that WC nanoparticles are active for H(upd) adsorption/desorption without the presence of Pt species. The presence of WC nanoparticles in the WC/Pt catalysts results in a few changes in the CV of the Pt electrode. In the presence of very low Pt content, the H(upd) adsorption/desorption reaction (WC/Pt(1%) catalyst) is enhanced and takes place in a narrow potential range (curve 2 in Fig. 5a). The WC/Pt catalyst with higher Pt content has a CV with two H(upd) adsorption/desorption peaks (curve 1) whose shape is similar to the CV of Pt but its first H(upd) peak is suppressed until the second desorption peak is shifted to a more positive potential (0.3 V) and that indicates the H-spill-over. In order to check the possibility that these WC particles to exhibit a H-spill-over effect when they are in contact with Pt, the electrochemically active surface area of the catalysts, Seasa, was calculated from the charge associated with the anodic desorption peak of (upd) hydrogen with a reference to 210 lC cm2 for polycrystalline Pt, and from Cu stripping measurements (Fig. 5b). The corresponding results are presented in Table 1. The electrochemically active surface area of the catalysts calculated from the H(upd) region is about 30% higher than the value obtained from Cu stripping measurements. These results prove the presence of H-spill-over effect of WC particles, which could be reverse or direct.

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Fig. 2. (a) A HAADF-STEM micrograph of WC with the typical bimodal particle size distribution of WC nanocrystals; (b) TEM image of the core–shell structure of a WC particles. Black dot in the shell indicates e-beam size (1.2 nm) and its position during EDS analysis.

Hydrogen that is adsorbed and dissociated on the Pt surface, (Eq. (1a)) could spill over onto the tungsten oxide species, WOy (Eq. (1b)), which are present at WC surface. Then, desorption of spiltover H atoms could take place from HxWOy by migration back to Pt species (reverse spill-over).

Hþ þ e þ Pt $ Pt—Hads

ð1aÞ

xPt—Hads þ WOy $ Hx WOy þ xPt

ð1bÞ

H2 ! 2Hþ þ 2e

The corresponding elementary steps for the Tafel–Heyrovsky– Volmer mechanism on Pt catalysts are: kT

H2 þ 2Pt () 2H  Pt Tafel reaction kT

kH

However, having in mind the CV response of pure WC, H(upd) could be directly adsorbed on WOy species and then spill over onto the Pt surface. Due to the presence of a H-spill-over effect of WC particles, the determination of electrochemically active surface area of Pt nanoparticles from the stripping of underpotentially deposited Cu (UPD) copper is relevant. 3.3. Kinetics of the hydrogen oxidation at WC/Pt catalysts 3.3.1. Theoretical considerations The hydrogen oxidation reaction (HOR) in acid media can be written as:

ð2Þ

H2 þ Pt () H  Pt þ Hþ þ e Heyrovsky reaction kH

kV

H  Pt () Pt þ Hþ þ e Volmer reaction kV

ð3Þ

ð4Þ

ð5Þ

In the Tafel–Volmer pathway the dissociative adsorption of a hydrogen molecule is followed by two separate one-electron oxidations of adsorbed H atoms. However, in the Heyrovsky–Volmer pathway, a one-electrooxidation occurs simultaneously with chemisorption, followed by another one-electron oxidation of the adsorbed H atom. In a steady-state, the kinetics of the HOR for the simultaneous occurrence of the Tafel–Volmer and Heyrovsky–Volmer routes can be described in terms of the reaction rate of the elementary steps by the following equations:

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Fig. 3. (a) A high resolution HAADF-STEM image of WC/Pt with sub-nanometer Pt clusters attached to the WC particles. (b) EELS spectrum of Pt cluster (inset of Fig. 3b) shows the presence of W and Pt by their M4,5 energy loss edges.

v  F ¼ I ¼ Fðv H þ v V Þ ¼ 2Fðv T þ v H Þ

ð6Þ

where v is the overall reaction rate and vs with different subscripts are the reaction rates of the elementary steps (3–5). It has been recently shown [37,38] that the overall reaction rate of the HOR when a Langmuir-type adsorption is considered for the adsorbed hydrogen, in the presence of mass-transfer limitations and assuming that the electron transfer coefficient is ½, can be written as:

    2  H I ¼ I0T ð1  HH Þ2 1  IIL  ð1  H0H Þ2 H H0H h      FE   FE i H  ð1  H0H Þ H exp  2RT þI0H ð1  HH Þ 1  IIL exp 2RT H0



HH FE ¼ exp  c RT H0H

ð8Þ

where c is also a function of H0H . Previously, it was shown by simulations [39] that H0H ranges from 0.09 to 107. Incorporating Eq. (8) into Eq. (7) and letting (1  HH) and (1  H0H ) be unity, Eq. (7) can be presented in more simplified form:

   I 2F E  exp  1 IL c RT      I FE FE FE exp  exp  þ I0H 1  exp  IL 2RT 2RT c RT

I ¼ I0T

ð7Þ

H

ð9Þ

This can be rearranged as:

0 H

where HH and H are the coverages of the reaction H intermediate at the potential E, and at the reversible potential, E0, respectively; IL is the H2 diffusion limiting current at high potential where cH2 approaches zero; I0T and I0H are the exchange current densities of Tafel and Heyrovsky steps, respectively. Dependence of HH on g (or E vs. RHE) can be obtained by solving the equation dH=dt ¼ 2v T þ v H  v V ¼ 0, following the approximation that the concentration of the reaction intermediate does not change in time. Wang et al. [38] found that simple expression can be used under the assumption that the Volmer reaction rate is sufficiently larger than the Tafel and Heyrovsky reaction rates. For sufficiently small H0H and E P 0 follows:



h I¼ ¼

I0T ð1  exp



2F E c RT

h   FE F Ei E  exp F þ I0H exp 2RT  exp 2RT c RT FE 1 þ II0TL þ I0H exp 2RT IL

i

Ik 1 þ IILf ð10Þ

The numerator in the above equation corresponds to the kinetic current, Ik, when the limiting current, IL ? 1. The sum of the two positive terms in the denominator represents the kinetic current of the forwards reactions, If.

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Fig. 4. (a) Nitrogen adsorption isotherms for WC support. Solid symbols-adsorption, open symbols-desorption. (b) BJH pore size distribution.

Here, I0T, I0H and c are the three essential kinetic parameters. Wang et al. [38] showed that the HOR takes place dominantly through the Tafel–Volmer pathway on Pt for E 6 50 mV, where the current rises rapidly in an inverse exponential function. In this case, Eq. (9) can be presented in a more simplified form:

I ¼ I0T

   I 2FE  exp  1 IL c RT

ð11Þ

and rearranged as:

I¼



I0T  IL I0T þ IL



  2FE 1  exp  c RT

ð12Þ

or:

 2FE I I exp   ¼1 I0T IL c RT

ð13Þ

If the I0T  IL condition is fulfilled (this condition can be easily fulfilled even in RDE measurements), then the second term on the right side of Eq. (14) can be neglected and Eq. (14) becomes:

E¼

c RT 2F

ln

 IL  I IL

ð14Þ

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Fig. 5. (a) Cyclic voltammograms for WC (curve 3) and WC/Pt electrodes with different amount of Pt, recorded at a sweep rate of 100 mV s1 in N2 saturated 0.5 mol dm3 HClO4 solution a 25°C. (b) Cu stripping voltammetry for WC/Pt(1 wt.%) electrode in 0.1 mol dm3 H2SO4 solution at sweep rate of 20 mV s1.

Eq. (15) is the similar to the Nernstian equation (E ¼  RT lnðILII Þ) for a pure diffusion controlled reaction and which is 2F L frequently used to prove that the HOR takes place as the reversible reaction. 3.3.2. Determination of the kinetic parameters Fig. 6a presents the hydrogen oxidation polarization curves for several rotation speeds, obtained at a scan rate of 2 mV s1 for the WC/Pt(1 wt.%) catalyst. The current increases rapidly with potential on each curve and reaches a limiting value at ca. 75 mV (RHE). Fig. 6b displays the Levich–Koutecky plot for the HOR, where the near-zero intercept and the linear behavior indicate that the current at the positive potential limit is essentially diffusion limited for this catalyst. The HOR polarization curves presented in Fig. 6a were first analyzed in low potential region using Eq. (15) in order to calculate the kinetic parameter, c. The solid lines in Fig. 7 show that agreement with the data obtained for the value of c = 1.42, which does not depend on the rotation rate. Assuming also that HOR takes place dominantly through the Tafel–Volmer pathway in the low potential region, and if, for instance E 6 10 mV the overall reaction rate can be presented by Eq. (11) in which the exponential may be expanded and higher terms neglected, so that one obtains:

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Table 1 Calculated kinetic parameters for the HOR at WC/Pt(1%) and C/Pt(1%) catalysts. Electrode (0.4 lg Pt)

C/WC–Pt(1%) Vulcan/Pt(1%)

Electrochem.active surface area (cm2)

I0T

c

I0H 2

mg1Pt

HUPD desorpt.

CuUPD desorpt.

mA

mA cm

A

0.53 0.23

0.40 0.23

16 4

40 17

40 10

2

1

mA

mA cm

A mg

1 0.3

2.5 1.3

2.5 0.8

Pt

1.42 2.2

The exchange current densities are calculated using the value of electrochemically active Pt surface area estimated by Cu stripping voltammetry.

Fig. 7. Measured (symbols) and fitted (line) polarization curve for the HOR on WC/ Pt(1%) at different rotation rates of RDE in hydrogen saturated 0.5 mol dm3 HClO4 solution at 25 °C. Fitted curve was obtained using Eq. (15) in order to determine the kinetic parameter c.

Fig. 6. (a) Polarization curves obtained with RDE at 2 mV s1 for H2 oxidation in 0.5 mol dm3 HClO4 solution at C/WC–Pt(1%) catalyst, for several rotating speeds. (b) Corresponding Levich–Koutecky plot for the HOR at 0.3 V vs. RHE.

 I 2F IL  I0T ¼ E c RT I0T þ IL

Fig. 8. Experimental data (symbols) and fitted (lines) linear polarization curves for the HOR on WC/Pt(1%) RDE in hydrogen saturated 0.5 mol dm3 HClO4 solution at 25 °C. Fitted curves were obtained using Eq. (15) and the kinetic parameter c = 1.42.

ð15Þ

Fig. 8 presents the corresponding polarization data recorded at different rotation rates in the low potential region and the corresponding values of the exchange current for Tafel step, I0T were calculated from the slope of linear I–E (RHE) response (Eq. (15)), (mean value is presented in Table 1.

Now, it is possible to estimate the last third kinetic parameter, (I0H) by fitting the RDE polarizations curves in the whole potential range with Eq. (9). The solid line in Fig. 9 shows that agreement with the polarization data (symbols) is obtained by the best fits with one variable kinetic parameter. The values of the calculated

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Fig. 9. Measured (symbols) and fitted (line) polarization curves for the HOR on WC/ Pt(1%) RDE in hydrogen saturated 0.5 mol dm3HClO4 solution at 25 °C. Fitted curve was obtained using Eq. (9) and previously determined kinetic parameters c = 1.42 and I0T = 16 mA.

Fig. 10. Kinetic current for the HOR on WC/Pt(1%) electrode at 25 °C calculated using the numerator in Eq. (10) with I0T = 16 mA and I0H = 1.0 mA and c = 1.42. Dash and dotted lines represent the contributions from TV and HV pathways, respectively.

kinetic parameters of the HOR at WC/Pt(1 wt.%) electrode that fit the polarization RDE curves are presented in Table 1. It can be concluded that the Tafel–Volmer pathway is responsible for the high HOR activity on WC–Pt(1 wt.%) catalyst at E 6 50 mV, where the current rises rapidly in an inverse exponential fashion to a value close to I0T. Further increase is realized mainly through Heyrovsky–Volmer pathway, which is important at the more positive potentials, where ITV levels off. It is interesting to note that in the literature, the E vs. ln ((IL  I)/ IL) relationship is most often identified with the Tafel plot (assuming reversible kinetics of the HOR) That is wrong, because when

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Fig. 11. Polarization curves obtained with RDE at 2 mV s1 for the HOR in 0.5 mol dm3 HClO4 solution at C/Pt(1%) catalyst, for several rotation speeds.

Fig. 12. Measured (symbols) and fitted (line) polarization curve for the HOR on C/ Pt(1%) RDE at different rotating rates in hydrogen saturated 0.5 mol dm3 HClO4 solution at 25 °C. Fitted curve was obtained using Eq. (15) in order to determine the kinetic parameter c.

Tafel step controls the overall reaction rate, the current, I, depends as an inverse exponential fashion on potential, E. This is in agreement with the theoretical prediction that if a preceding chemical adsorption is followed with fast electron transfer reaction, as it is proposed in the kinetic analysis for HOR, then it is not correct to use the Butler–Volmer equation in performing the kinetic of the reaction. In addition, by comparing the dependence of the kinetic current, Ik on the potential E, for the HOR, (Ik is calculated by using the numerator in Eq. (10) and the determined kinetic parameters), and anodic polarization curve for the HOR, derived from a half-cell under actual PEMFC operating condition [40,41], one can see very good conformity (Fig. 10).

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were determined. The fitting procedure shows that the HOR on WC/Pt and C/Pt catalysts most likely proceeds most likely via the Tafel–Volmer pathway in the lower potential region, while the Heyrovsky–Volmer pathway is operative in the higher potential region. The WC/Pt(1 wt.%) catalyst exhibits four times higher mass specific activity for the HOR compared to the conventional carbon supported Pt catalyst. The enhanced catalytic activity of the WC/Pt catalyst is probably cause by the presence of a H-spill-over effect of WC nanoparticles. Acknowledgments This work is financially supported by the Ministry of Science and Technological Development, Republic of Serbia, under Contract No. 172054. All TEM characterizations have been performed at National Center of Electron Microscopy, LBLN, University of California, Berkeley. References

Fig. 13. Polarization curves obtained with rotating disk electrode at 2 mV s1 for H2 oxidation in 0.5 mol dm3 HClO4 solution at C/Pt(1%) and WC/Pt catalysts, at rotating speed of 600 rpm.

In order to find the influence of WC on the activity of Pt catalyst for the HOR, the polarization measurements were also conducted on a commercial C/Pt electrode, with the same Pt content (mass.1%). Fig. 11 presents the HOR polarization curves for several rotation speeds, recorded at a scan rate of 2 mV s1 for a C/Pt(1 wt.%) electrode. For this electrode, the linearity of E  ln((IL  I)/IL), irrespective of rotation rate is also achieved, indicating that the Tafel– Volmer (TV) pathway for the HOR is operative in the lower potential range (Fig. 12). The polarization curves for the HOR on WC/Pt(1 wt.%) and C/ Pt(1%) RDE electrodes are presented together in Fig. 13, for comparison. It is evident that WC/Pt(1 wt.%) catalyst exhibits higher catalytic activity. The kinetic parameters of the HOR at C/Pt(1 wt.%) electrode were determined in an identical way as in the case of WC/Pt(1 wt.%) catalyst and presented in Table 1, as well. In order to compare the activities of these two catalysts, specific and mass specific activities are also presented. On the basis of the determined kinetic parameters for the HOR it can be concluded that the WC/Pt(1 wt.%) electrode exhibits four times higher mass specific activity and about 2.5 times higher specific activity compared to C/Pt(1 wt.%) catalyst. The higher activity of the WC/Pt catalyst could be a result of the presence of the H-spill-over effect which is probably also operative also in the potential range where the HOR takes place. WC plays the role in accelerating the dissociative adsorption of H2 at Pt, which is the rate-determining step for the HOR. 4. Conclusions The kinetics and mechanism of the HOR on a home-made WC/ Pt(1%) electrode in a perchloric acid solution has been studied. Based on the Tafel–Heyrovsky–Volmer mechanism the corresponding kinetic equations were derived to describe the HOR current behavior on RDE over the entire potential range, Applying these equations in the analysis of the polarization curves measured for WC/Pt(1%) at RDE, the exchange currents of Tafel, (I0T), and Heyrovsky, (I0H), elemental steps and adsorption parameter, (c),

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