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Influence of the velocity of Pt ablated species on the structural and electrocatalytic properties of Pt thin films
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Influence of the velocity of Pt ablated species on the structural and electrocatalytic properties of Pt thin films C. Hamel c, S. Garbarino c, E´. Irissou a, F. Laplante b, M. Chaker c, D. Guay c,* a
National Research Council Canada e Industrial Materials Institute, 75 Blvd de Mortagne, Boucherville, Quebec, Canada J4B 6Y4 Centre de recherche et de de´veloppement Arvida, 1955 Blvd Mellon, C.P. 1250, Saguenay, Quebec, Canada G7S 4K8 c INRS, Energie, Materiaux et Telecommunications, 1650 boulevard Lionel Boulet, Varennes, Quebec, Canada J3X 1S2 b
article info
abstract
Article history:
Platinum was deposited by pulsed laser deposition at different kinetic energy by varying
Received 18 March 2010
the He background pressure in the deposition chamber. As a result, the porosity of the film
Received in revised form
varies from 5 to 86% as the He pressure is increased. This yields to an increase of the
22 April 2010
electrochemically active surface area and to an increased resistance to poisoning by CO, as
Accepted 26 April 2010
evidenced by a 45 mV shift of the peak potential of the CO stripping peak towards less
Available online 14 June 2010
positive values. Similarly, the electrocatalytic activity of the films for the oxygen reduction reaction, as measured by the potential at half limiting diffusion current, is enhanced.
Keywords:
However, a comparison between the intrinsic electrocatalytic activities of the films show
Oxygen reduction reaction
that they have similar values (within a factor of 1.8) and do not significantly differ from that
Pulsed laser deposition
of a polycrystalline Pt disk, indicating that the increased activity for the ORR is mainly due
Porous electrode
to a geometric (electrochemically active surface area) effect. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As Pt and Pt alloy remain the most efficient electrocatalysts to increase the reaction rates of several electrochemical reactions, notably in the field of polymer electrolyte membrane fuel cell (PEMFC) where it is found at both the anode and the cathode, extensive research has been devoted to optimize its activity, selectivity and long term stability. Platinum being also expensive and scarce, different approaches have been considered to minimize the Pt content, including the synthesis of Pt and Pt alloyed nanoparticles of controlled size [1] deposited on a high surface area support. It is beyond the scope of this paper to review all these studies and the interested reader should consult reviews on the subject [2,3]. Several deposition methods to prepare nanoparticles and thin films revealed enhanced electrocatalytic activity for reactions that are important in PEMFCs. Among them, pulsed
laser deposition (PLD) was recently studied as a clean and versatile method for the preparation of Pt nanoparticles with controlled size [4] and for Pt thin films with different morphologies [5]. Moreover, crossed-beam pulsed laser deposition (CBPLD), whereby two targets of dissimilar materials are ablated by two independent laser beams, can be applied to prepare Pt alloy thin films on a variety of substrates. Thus, PteSn [6] and PteNi [7] were obtained by CBPLD and investigated for ethanol oxidation and oxygen reduction reaction, respectively. Metastable PteAu thin films were also synthesised by CBPLD and exhibited very interesting electrocatalytic properties for the oxygen reduction reaction [8]. In PLD, the deposition parameters have a very drastic influence on the structural and morphological properties of the deposited films [9e11]. For example, deposition in low background pressure and/or small target-to-substrate distance yields to high kinetic energy deposition conditions,
* Corresponding author. Tel.: þ1 450 929 8141; fax: þ1 450 929 8102. E-mail address: [email protected] (D. Guay). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.04.148
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where the atoms and ions inside the plasma plume have a high velocity. Likewise, deposition in high background pressure and/or large target-to-substrate distance yields to low kinetic energy conditions and species inside the plasma plume have small velocity. In the case of Pt and PteRu alloy, different deposition conditions were shown to have a profound effect on the morphology of the film deposited on a porous substrate [12]. Thus, under high kinetic energy conditions, Pereira et al. recently replicated the pore structure of an anodic aluminum oxide (AAO) membrane, therefore yielding to the preparation of functionally modified Pt, PtRu and Au macroporous films whereas low kinetic energy conditions yielded to the clogging of the porous substrate. More recently, the crystalline structure of Pt films was also found to vary with the kinetic energy conditions prevailing during the deposition. At high kinetic energy, dense and highly (111) oriented thin films were obtained, while porous and polycrystalline films were formed under low kinetic energy conditions [5]. This study follows previous reports showing that this effect is not limited to Pt and that Au thin films display the same transition from highly (111) oriented to polycrystalline as the kinetic energy of the plasma plume is decreased [13e15]. Herein, we report on the effect of the deposition parameters i.e. the background gas pressure on the structural, morphological and electrochemical properties of Pt thin films deposited on carbon substrates. The deposition parameters will be shown to have a strong influence on the velocity of the Pt species that are impinging on the carbon substrate and that this plasma characteristic has a determining effect on the morphology, porosity and electrochemically active surface area of the films. Finally, the effect of all these parameters on the CO oxidation behaviour and the activity of the resulting Pt films for the oxygen reduction reaction will be assessed.
2.
Experimental
Platinum films were deposited by pulsed laser deposition (PLD) under different He gas pressures (from 105 to 10 Torr). A target of pure Pt (99.99%) was ablated with a focused laser beam (pulsed KrF laser, l ¼ 248 nm, pulse length 17 ns, repetition rate 100 Hz). The laser fluence was set at 8 J/cm2, while the target-to-substrate distance was set at 5 cm. The velocity (v) of the deposited Pt species was determined by time-offlight (TOF) emission spectroscopy, using the experimental set-up described in detail elsewhere [13e15]. Details on the measurements of the velocity of Pt are given elsewhere [5]. The deposition rates were measured at the different He gas pressures [16] and the number of laser pulses was adjusted to reach a constant Pt film thickness. The crystalline structure of the Pt films was investigated by x-ray diffraction (XRD) using a Bruker AXS D8 advance diffractometer with Cu Ka radiation. All measurements were performed at a grazing incidence angle of 2 . The morphology of the Pt films was characterized by scanning electron microscopy (SEM) using a Hitachi S-4700 Field-Emission SEM. The surface morphology and roughness of the samples were investigated by scanning tunneling microscopy (STM) using a Nanoscope III microscope from Digital Instrument.
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The porosity of the Pt films was determined by X-ray reflectometry (XRR) using a Philips diffractometer (PANAnalytical X’Pert PRO) with a Cu Ka radiation source. The XRR curve is obtained by plotting the scattered X-ray intensity against the angle of incidence (q). At low incidence angle, total external reflection occurs, whereas X-ray beam is partially absorbed by the sample above a critical angle (qc). The critical angle is related to the film density r using the relation: qc ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r$re $l2 $Z$NA =p$A
where re is the electron radius, l the X-ray wavelength, Z the atomic number, NA Avogadro’s number, and A the atomic mass. qc is defined as the angle at which the intensity is half that of the plateau value corresponding to total reflection. The porosity is given by 1 r/rbulk, where rbulk is the bulk density of platinum. For these measurements, Pt films were deposited on Si substrates. For electrochemical characterization, the Pt films were deposited on 5 mm diameter carbon disks (amorphous graphite, 99.997%, Goodfellow Corporation). The Pt/C samples were inserted into a rotating ring-disk electrode (RRDE) shaft and used as working electrodes in a standard three compartment cell. The ring-disk electrode was controlled by a bipotentiostat (Pine Instrument Company, AFCBPI), coupled with a rotation controller (Pine Instrument Company). Prior to measurement of the electrochemically active surface area (EASA), the Pt/C electrodes were conditioned in de-aerated H2SO4 solution by cycling between 0.05 V and 1.30 V at 50 mV s1 until reproducible voltammograms were obtained. For the oxygen reduction reaction (ORR) studies, the Pt ring electrode was held at 1.20 V. At that potential, the oxidation of peroxide produced at the disk is under diffusion control [17] while the Pt/C working electrode potential was swept from 1.00 to 0.05 V at 20 mV s1. Voltammetric profiles were recorded at room temperature in de-aerated (Argon N5.0, Praxair) and in O2 saturated (N4.3, Praxair) 0.5 M ultrapure sulphuric acid (A300-212, Fisher Scientific). Carbon monoxide (N2.5, Praxair) adsorption was performed by bubbling purified CO (oxygen trap, CRS) for 3 min with the electrode potential held at 0.30 V. Residual CO was removed by bubbling Ar for 15 min and two successive cyclic voltammograms (CV) were recorded at 15 mV s1. Prior to each electrode characterization, the glassware and the electrochemical cell were cleaned according to a well established method [18]. The auxiliary and the reference electrode were a platinum gauze and reversible hydrogen electrode (RHE), respectively. All potential values are referred to the RHE scale.
3.
Results and discussion
3.1.
Characterization of the as-deposited Pt thin films
XRD patterns of Pt films deposited over a large range of velocities (v), from ca. 11.3 to 0.03 km s1, are displayed in Fig. 1. The diffraction pattern of the carbon substrate (without Pt) is also shown (curve a in Fig. 1) for sake of comparison. The XRD patterns of all Pt/C samples show the five characteristic diffraction peaks of the face centered cubic (fcc) lattice of
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a
b
c
d
e
f
Fig. 1 e XRD patterns of Pt/C electrodes deposited at different v values: (a) carbon substrate, (b) 11.3 km sL1, (c) 4.6 km sL1, (d) 3.0 km sL1, (e) 10L1 km sL1 and (f) 0.03 km sL1.
polycrystalline platinum. The XRD pattern of Pt/C with v ¼ 0.03 km s1 (curve f) is dominated by the diffraction peaks of the carbon substrate, whereas the contribution from the carbon substrate decreases with increasing v. Indeed, the XRD pattern of the Pt/C sample with v ¼ 11.3 km s1 (curve b) shows only the characteristic diffraction peaks of Pt. As shown elsewhere, PLD deposition at high Ek on a Si substrate gives highly (111) oriented Au [13e15] and Pt [5] thin films, whereas deposition at low kinetic energy conditions yields to polycrystalline Au and Pt films. Obviously, the surface mobility of Au and Pt species deposited at low and high kinetic energy are different, with adatom motion being favored at higher Ek values, yielding surface reorganization and minimization of the energy of the whole system. In the case of the Pt/C samples prepared in this study, the XRD
patterns do not exhibit any evidence of preferential orientation. It is believed that the nature of the substrate (most probably its mass and its thermal properties) has an influence on the growth characteristics of the films and a more definite answer to our ability to growth (111) preferentially oriented Pt/ C samples will await a more detailed study of the influence of these parameters. SEM micrographs of Pt/C samples are shown in Fig. 2. The thickness of all samples, as determined from SEM crosssectional views (not shown) remains constant with a mean value of ca. 30 nm (10 nm). Dense and compact films are observed in Fig. 2a and b, corresponding to v ¼ 11.3 and 4.6 km s1, respectively. For v lower than 4.6 km s1 (Fig. 2c, d and 2e), the Pt films appeared increasingly more porous and samples prepared at the lowest v exhibit a cauliflower-type of structure. This increase in the porosity of the film as v is decreased might explain the increased contribution of the carbon substrate to the XRD pattern of the films prepared at the lowest v value (see Fig. 1f). The mean crystallite size, estimated from the full width at half maximum (FWHM) of the corresponding XRD patterns using the Scherrer equation, and the porosity of the films, determined from XRR measurements, are plotted as a function of v in Fig. 3a and Fig. 3b, respectively. As seen in Fig. 3a, the Pt mean crystallite size remained constant at ca. 5 nm for v 2 km s1. For films prepared at higher v values, the mean crystallite size increases steadily and reaches 30 nm at 11.3 km s1. This increase of the crystallite size is thought to occur as a result of the increased mobility of the Pt adatom at the surface of the substrate as v is increased. Crystallite size determination performed by STM measurements (open circles in Fig. 3a), gives strikingly similar values. This allows us to conclude that the Pt bulk and surface crystallites display the same mean size and that both are affected the same way by the kinetic energy of the laser ablated species. Fig. 3b depicts how the porosity varies with v. For v ¼ 11.3 km s1, the porosity of the Pt films is ca. 5%, which means that the density of the film is close to the theoretical bulk Pt density (21.4 g cm3). However, as v decreased from 11.3 to 102 km s1, the porosity of the film increased to reach 86% at the lowest v value.
3.2. Electrochemical properties of the as-deposited Pt thin films CVs of Pt/C at different v are shown in Fig. 4. CVs recorded in Ar-saturated H2SO4 electrolyte are presented in the left-hand side panel, while those recorded after the adsorption of a monolayer of CO are depicted in the right-hand side panel. For comparison, CVs for a polycrystalline Pt disk recorded in the same conditions are also shown in Fig. 4a and g. CVs of all Pt/C electrodes show the typical response of polycrystalline platinum in H2SO4 electrolyte, (i) a hydrogen adsorption/desorption region from 0.05 to 0.40 V, (ii) double layer region from 0.40 to 0.80 V, and (iii) the Pt oxidation/ reduction region at potentials above 0.80 V. A detailed analysis of the voltammetric curves was performed by determining the position of the hydrogen desorption peaks (at ca. 0.12 and 0.26 V) and of the Pt oxide reduction peak (at ca. 0.77 V). Similar electrode potential values (0.02 V) were found in each case,
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a
b
Fig. 3 e (a) Mean crystallite size and (b) porosity of Pt/C electrodes deposited at different v values.
Fig. 2 e SEM micrographs of Pt/C electrodes deposited at different v values: (a) 11.3 km sL1, (b) 4.6 km sL1, (c) 3.0 km sL1, (d) 10L1 km sL1 and (e) 0.03 km sL1.
indicating that all Pt/C films exhibit the same electrochemical behaviour, independently of the applied kinetic energy. Moreover, these values are similar to those recorded on a Pt disk (see Fig. 4a), which confirmed that the electrochemical surface properties of Pt thin films prepared by PLD do not
differ significantly from those of polycrystalline platinum. In particular, there is no indication of a (111) preferential surface orientation of the Pt/C sample with v ¼ 11.3 km s1, consistent with the corresponding XRD pattern, which displays all the characteristic diffraction peaks of polycrystalline Pt. In the right-hand side panel of Fig. 4, a full CO monolayer was pre-adsorbed at the surface of the Pt/C electrodes and two subsequent CVs in Ar-saturated electrolyte were recorded (only the forward sweeps are shown for sake of clarity). CO-blocked Pt sites for hydrogen adsorption are evidenced by the low currents observed below 0.40 V while CO oxidation started at potentials larger than ca. 0.70 V. The subsequent recovery of the hydrogen desorption behaviour of the platinum surface in the second positive sweep is indicative of the full CO monolayer stripping during the first anodic sweep. The CO oxidation peak was observed at ca. 0.820 V for the polycrystalline Pt disk electrode (Fig. 4g) whereas all Pt/C electrodes display a CO oxidation peak at lower potential values. Indeed, the main CO oxidation peak gradually decreased towards less anodic potential values from ca. 780 mV to 735 mV as v decreased from 11.3 km s1 to 0.03 km s1, indicating that it is more facile to oxidize CO at the more porous surface than at the denser surface. The electrochemically active surface area (EASA) of Pt/C sample was calculated by integrating the hydrogen desorption charge (taking into account the double layer contribution) involved in the potential region below 0.40 V, assuming the extensively used value of 210 mC per cm2 Pt [19]. In Fig. 5, EASA values were found to increase as v varies from 11.3 to 3.0 km s1, indicating that a higher number of platinum surface atoms are accessible as v is decreased in that energy range. Such enhancement of the EASA is in good agreement with the morphological changes previously described in Fig. 2 and correlates also well with the enhanced porosity observed
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a
g
b
h
c
i
d
j
e
k
f
l
Fig. 4 e Cyclic voltammetries (CV) in Ar-saturated 0.5 M H2SO4 solution of Pt/C films deposited at different v values: (a and g) polycrystalline Pt disk, (b and h) 11.3 km sL1, (c and i) 4.6 km sL1, (d and j) 3.0 km sL1, (e and k) 10L1 km sL1, (f and l) 0.03 km sL1. In a to f, the sweep rate was 50 mV sL1. In gel (CO stripping voltammetry), the sweep rate was 15 mV sL1.
when v is varied in that range (Fig. 3b). They are also in good agreement with the change observed in the surface roughness of the film. For example, the Rs and the EASA values are increased by a factor of 1.52 and 1.38, respectively, as v is decreased from 4.6 to 3.0 km s1. For v < 3.0 km s1, EASA values were found to decrease as the kinetic energy is decreased. Such a behaviour seems quite surprising as the porosity of the films is the highest (and the mean crystallite size the lowest) when Pt is deposited at low v. This decrease of the EASA value for v < 3.0 km s1 is consistent with the total CO oxidation charges calculated from the first anodic sweep shown in Fig. 4. Indeed, EASA values can also be obtained from these CO oxidation charges by using a correction factor of 420 mC cm2 Pt [19]. As seen in Fig, 5, both EASA values follow the same trend with respect to v, confirming the fact that the EASA values do not scale with the porosity of the film. It is worth mentioning that even if the EASA values estimated using the CO stripping charges, EASACO, followed the same general trend as the EASA values determined from the Hupd charges, EASAH, the former corresponds only to 54 6% of the latter. This is not the case on a polycrystalline Pt disk as EASACO and EASAH give the same value (within 6%). This means that the factor 2 difference observed on Pt films prepared by PLD is not an artefact. This difference might arise
as a consequence of (i) CO species being not linearly adsorb onto Pt surface atoms, which is implicitly assumed in taking a value of 420 mC cm2 Pt to estimate EASACO, or/and (ii) limited access of adsorbed CO to the Pt atoms at the surface of
Fig. 5 e Variation of the electrochemically active surface area (EASA) determined from the hydrogen and CO oxidation charges with respect to v.
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the film. This may indeed be the case as suggested by the presence of a residual oxidation current in the hydrogen desorption potential region in the first sweep of Fig. 4h to l. However, in that case, the EASACO/EASAH ratio variation with v and the porosity of the films would have been expected. This issue will be dealt with in more details in a forthcoming study. However, it remains to be explained why the EASA (Fig. 5) does not scale with the porosity of the films (Fig. 3b). In the remaining of this study, the intrinsic electrocatalytic activity of Pt/C electrodes does not differ by more than a factor of 1.8 when the velocity is varied over more than 3 orders of magnitude. Moreover, a factor less than 1.8 between the intrinsic electrocatalytic activity of any particular Pt/C electrode and the bulk Pt disk was observed. Assuming that Pt deposited at different kinetic energies has the same intrinsic electrocatalytic activity for the ORR, this means that the estimation of the electrochemically active surface area is valid. Therefore, it seems that the porosity determined by XRR measurements is the sum of an open (electrochemically accessible) and close (not electrochemically accessible) porosity. Therefore, the discrepancy between the porosity and the EASA measurements could indicate that the fraction between the close and the total porosity is increased as the kinetic energy of the ablated species is decreased. This issue will be the subject of a forthcoming study. In Fig. 6, a set of polarization curves in O2 saturated 0.5 M H2SO4 solution at different rotation rates is shown for Pt/C electrodes. The potential at half limiting diffusion current density, which is representative of the electrocatalytic performance of the Pt catalysts for the ORR, shifts towards more positive values as v is decreased from 11.3 km s1 to 0.03 km s1. It is also worth noting that Pt/C electrodes exhibit a distinctly different behaviour in the hydrogen sorption potential region (Edisk < 0.40 V) as compared to polycrystalline Pt disk. On Pt disk electrode, the hydrogen sorption peaks are not observed in O2 saturated H2SO4 solution and, O2 reduction current decreases are due to the formation of H2O2 at
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a hydrogen covered platinum surface. In contrast, the CV of all Pt/C samples exhibit the two characteristic hydrogen adsorption peaks that are superimposed of the O2 reduction current, indicating that both processes are occurring simultaneously. These surface processes are scarcely observed concomitantly in the case of polarization curves in O2 saturated solution under forced convection conditions (u ¼ 1600 rpm) and this behaviour is more typically observed in a quiescent acidic solution in presence of remnant O2 [20]. Hence, the superimposition of Hupd peak currents to the O2 current during the RRDE experiments at Pt/C electrodes illustrate the inability of the electroactive species to react with all the Pt atoms accessible to the electrolyte. As seen in Fig. 6, there is some small variation in the diffusion limited current of the various Pt/C electrodes, which is attributed to different porosity and surface roughness [21] and/or to the high pseudocapacity of the carbon support at 20 mV s1 [22]. Fig. 7a presents a representative set of ring-disk results in O2 saturated 0.5 M H2SO4 electrolyte and at different rotation rates obtained on a Pt/C electrode prepared at v ¼ 11.3 km s1.
a
b
c
Fig. 6 e Polarization curves (20 mV sL1) in O2 saturated 0.5 M H2SO4 solution at 1600 rpm for Pt/C electrodes deposited at different v.
Fig. 7 e (a) Polarization curves of Pt/C electrode deposited at Ek [ 11.3 km sL1 (20 mV sL1) in O2 saturated 0.5 M H2SO4 solution at different rotation rates, (b) corresponding ring current densities (Ering [ 1.20 V) and (c) resulting KouteckyeLevich plots for the current densities displayed in (a).
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Table 1 e ORR parameters as a function of the velocity (v) of Pt ablated species. v (km s1) 102 3 101 3.0 4.6 11.3 Pt RDE
n 4.30 4.00 3.80 3.85 3.35 3.70
jkgeo at E ¼ 0.85 V (mA cm2 geo) 9.39 5.27 7.81 5.95 2.16 0.59
0.25 0.20 0.20 0.30 0.05 0.05
jk cm2 Pt at E ¼ 0.85 V (mA cm2 Pt)
XH2 O2 (%) (idisk at E ¼ 500 mV)
XH2 O2 (%) (idisk at E ¼ 100 mV
0.5 0.5 0.6 0.9 1.8 1.2
1.2 1.1 1.5 2.1 5.0 1.8
317 194 206 239 184 315
The oxygen reduction currents displayed in Fig. 7a shows a potential region where the ORR is under mixed kineticdiffusion control (Edisk > 0.70 V), followed by a region where the current for the ORR is limited by diffusion (Edisk < 0.60 V). Similar data were obtained for all Pt/C electrodes investigated in this study. The analysis of such voltammetric profiles lead to the determination of parameters involved in the ORR: (a) From the purely diffusion controlled potential region, Levich linear plots (idisk vs w1/2) allowed us estimate the number of electrons, n, involved in the overall reaction (see Table 1), using data taken from the literature for the diffusion coefficient (D ¼ 1.8 105 cm2 s1) and concentration (C ¼ 1.13 106 mol cm3) of dissolved oxygen in 0.5 M H2SO4 at 20 C [23]. (b) From the mixed controlled potential region, the kinetic current, ikinetic, can be evaluated using the Kouteck1/2 ) (see Fig. 7c). yeLevich relationship (i1 disk vs w (c) Finally, from the ring current that corresponds to the oxidation of H2O2, the fraction of H2O2 formed at the cathode can be determined, which is given by XH2 O2 ¼
2iring =N idisk þ iring =N
where N, idisk and iring correspond to the collection efficiency of the ring, the disk current and the ring current, respectively. As seen in Fig. 7b, the ring current represents a negligible fraction of the disk current at high potential but increases in the Hupd potential region (Edisk < 0.40 V). Selectivity towards H2O2 formation, XH2 O2 , was calculated at 500 mV since the cathode potential remains close to that value under fuel cell operating conditions [24]. XH2 O2 at 100 mV were also calculated since O2 migration into the anodic compartment could lead to the formation of H2O2 at the anode, and degradation of the catalyst layer and membrane could occur [25]. The values of all parameters previously mentioned are summarized in Table 1 for the Pt/C electrodes prepared at different v values. For the Pt disk electrode, the number of electrons involved in the overall ORR reaction is n ¼ 3.7, close to the value of 4 expected for the ORR at Pt catalysts. For the Pt/C samples, there is a slight but consistent increase of the n value as v is decreased from 11.3 to 0.03 km s1. This is attributed to an enhanced contribution of the 4e- reduction pathway for porous electrodes synthesised at low kinetic energies. This assumption is confirmed by the XH2 O2 values (at Edisk ¼ 500 mV), which
6 29 25 24 20 27
is found to decrease at lower v value. The same trend is observed for XH2 O2 measured at Edisk ¼ 100 mV. The kinetic current densities were calculated from a KouteckyeLevich analysis and the resulting data are reported in Table 1. The kinetic current densities are expressed relative to the geometric area of the underlying carbon substrate and to the real Pt electrochemically active surface area, evaluated from the EASACO. As expected, higher geometric current densities were found for the whole range of Pt/C samples as compared to polycrystalline bulk Pt. The highest value of jkgeo is 9.39 mA cm2 at 0.85 V is obtained for Pt/C sample prepared at v ¼ 0.03 km s1. This is about 20 times higher than jkgeo recorded on a Pt disk electrode. In comparison, the variation of the current density normalized to the Pt EASACO are much less important (less than a factor of 1.8), indicating that the large fluctuation observed previously in the value of jkgeo is mainly due to a variation of the EASA of the samples. As far as we can tell, there is no systematic variation of jk cm2 Pt with respect to the kinetic energy of the Pt species used to prepare the deposits that would indicate that a preferential structure exists at the surface of the Pt films. Again, this result differs notably from what was observed for the high kinetic energy deposition conditions of Pt on Si substrate and might be related to the peculiar thermal properties of the substrate. This issue warrants further investigation.
4.
Conclusions
During pulsed laser deposition, varying the helium background pressure in the deposition chamber allows for a control over the kinetic energy of the Pt species impinging on the substrate that in turn allows for a control over the porosity and surface morphologies of the films. In the case of carbon monoxide poisoning, the more porous Pt film are the more electroactive. In the case of the oxygen reduction reaction, there is no systematic variation of the intrinsic electrocatalytic activity and all Pt/C electrodes displays an intrinsic electrocatalytic activity that does not differ by more than a factor of 1.8 from that of a polycrystalline Pt disk. Strategies to prepare highly (111) oriented Pt films on conductive substrate are being developed to take advantage of the fact that Pt adatoms deposited under high kinetic energy conditions possess an increased surface mobility that allow them to diffuse and re-arrange themselves to minimize the total values of the system.
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Acknowledgements [12]
This work was made possible due to the financial support of the National Science and Engineering Research Council (NSERC) of Canada and the Canada Research Chair program. One of us (CH) also acknowledges the financial support of the Fonds Que´be´cois de la Recherche sur la Nature et les Technologies (FQRNT).
[13]
[14]
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[21]
[22]
[23]
[24]
[25]
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and electrochemical hydriding properties. Int J Hydrogen Energy 2009;34:5778e84. Pereira A, Laplante F, Chaker M, Guay D. Functionally modified macroporous membrane prepared by using pulsed laser deposition. Adv Funct Mater 2007;17:443e50. Irissou E, LeDrogoff B, Chaker M, Guay D. Correlation between plasma expansion dynamics and gold-thin film structure during pulsed-laser deposition. Appl Phys Lett 2002;80:1716e8. Irissou E, Le Drogoff B, Chaker M, Guay D. Influence of the expansion dynamics of laser-produced gold plasmas on thin film structure grown in various atmospheres. J Appl Phys 2003;94:4796e802. Irissou E, LeDrogoff B, Chaker M, Trudeau M, Guay D. Nanostructured gold thin films prepared by pulsed laser deposition. J Mater Res 2004;19:950e8. Irissou E, Vidal F, Johnston T, Chaker M, Guay D, Ryabinin AN. Influence of an inert background gas on bimetallic cross-beam pulsed laser deposition. J Appl Phys 2006;99:034904-1e034904-12. Paulus UA, Schmidt TJ, Gasteiger HA, Behm RJ. Oxygen reduction on a high-surface area Pt/Vulcan carbon catalyst: a thin-film rotating ring-disk electrode study. J Electroanal Chem 2001;495:134e45. Angerstein-Kozlowska H. In: Bockris JOB, Conway BE, Sarangapani S, editors. Comprehensive treatise of electrochemistry, vol. 9. New York: Plenum; 1984. p. 15. Vielstich W, Lamm A, Gasteiger H. In: Vielstich W, Lamm A, Gasteiger H, editors. Handbook of fuel cells: fundamentals, technology and applications, vol. 4. New York: Wiley and Sons, Ltd.; 2003. p. 2690. Maye MM, Kariuki NN, Luo J, Njoki P, Wang L, Lin Y, et al. Electrocatalytic reducttion of oxygen: gold and goldeplatinum nanoparticle catalysts prepared by twophase protocol. Gold Bull 2004;37:217e24. Wei YC, Liu CW, Chang YW, Lai CM, Lim PY, Tsai LD, et al. The structure-activity relationship of PdeCo/C electrocatalysts for oxygen reduction reaction. Int J Hydrogen Energy 2010;35:1864e71. Mayrhofer KJJ, Strmcnik D, Blizanac BB, Stamenkovic V, Arenz M, Markovic NM. Measurement of oxygen reduction activities via the rotating disc electrode method: from Pt model surfaces to carbon-supported high surface area catalysts. Electrochim Acta 2008;53:3181e8. Kucernak A, Jiang J. Mesoporous platinum as a catalyst for oxygen electroreduction and methanol electrooxidation. Chem Eng J 2003;93:81e90. Sethuraman VA, Weidner JW, Haug AT, Pemberton M, Protsailo LV. Importance of catalyst stability vis-a`-vis hydrogen peroxide formation rates in PEM fuel cell electrodes. Electrochim Acta 2009;54:5571e82. Jusys Z, Behm RJ. Simulated “Air Bleed” oxidation of adsorbed CO on carbon supported Pt. Part 2. Electrochemical measurements of hydrogen peroxide formation during O2 reduction in a double-disk electrode dual thin-layer flow cell. J Phys Chem B 2004;108:7893e901.
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Influence of the velocity of Pt ablated species on the structural and electrocatalytic properties of Pt thin films C. Hamel c, S. Garbarino c, E´. Irissou a, F. Laplante b, M. Chaker c, D. Guay c,* a
National Research Council Canada e Industrial Materials Institute, 75 Blvd de Mortagne, Boucherville, Quebec, Canada J4B 6Y4 Centre de recherche et de de´veloppement Arvida, 1955 Blvd Mellon, C.P. 1250, Saguenay, Quebec, Canada G7S 4K8 c INRS, Energie, Materiaux et Telecommunications, 1650 boulevard Lionel Boulet, Varennes, Quebec, Canada J3X 1S2 b
article info
abstract
Article history:
Platinum was deposited by pulsed laser deposition at different kinetic energy by varying
Received 18 March 2010
the He background pressure in the deposition chamber. As a result, the porosity of the film
Received in revised form
varies from 5 to 86% as the He pressure is increased. This yields to an increase of the
22 April 2010
electrochemically active surface area and to an increased resistance to poisoning by CO, as
Accepted 26 April 2010
evidenced by a 45 mV shift of the peak potential of the CO stripping peak towards less
Available online 14 June 2010
positive values. Similarly, the electrocatalytic activity of the films for the oxygen reduction reaction, as measured by the potential at half limiting diffusion current, is enhanced.
Keywords:
However, a comparison between the intrinsic electrocatalytic activities of the films show
Oxygen reduction reaction
that they have similar values (within a factor of 1.8) and do not significantly differ from that
Pulsed laser deposition
of a polycrystalline Pt disk, indicating that the increased activity for the ORR is mainly due
Porous electrode
to a geometric (electrochemically active surface area) effect. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As Pt and Pt alloy remain the most efficient electrocatalysts to increase the reaction rates of several electrochemical reactions, notably in the field of polymer electrolyte membrane fuel cell (PEMFC) where it is found at both the anode and the cathode, extensive research has been devoted to optimize its activity, selectivity and long term stability. Platinum being also expensive and scarce, different approaches have been considered to minimize the Pt content, including the synthesis of Pt and Pt alloyed nanoparticles of controlled size [1] deposited on a high surface area support. It is beyond the scope of this paper to review all these studies and the interested reader should consult reviews on the subject [2,3]. Several deposition methods to prepare nanoparticles and thin films revealed enhanced electrocatalytic activity for reactions that are important in PEMFCs. Among them, pulsed
laser deposition (PLD) was recently studied as a clean and versatile method for the preparation of Pt nanoparticles with controlled size [4] and for Pt thin films with different morphologies [5]. Moreover, crossed-beam pulsed laser deposition (CBPLD), whereby two targets of dissimilar materials are ablated by two independent laser beams, can be applied to prepare Pt alloy thin films on a variety of substrates. Thus, PteSn [6] and PteNi [7] were obtained by CBPLD and investigated for ethanol oxidation and oxygen reduction reaction, respectively. Metastable PteAu thin films were also synthesised by CBPLD and exhibited very interesting electrocatalytic properties for the oxygen reduction reaction [8]. In PLD, the deposition parameters have a very drastic influence on the structural and morphological properties of the deposited films [9e11]. For example, deposition in low background pressure and/or small target-to-substrate distance yields to high kinetic energy deposition conditions,
* Corresponding author. Tel.: þ1 450 929 8141; fax: þ1 450 929 8102. E-mail address: [email protected] (D. Guay). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.04.148
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where the atoms and ions inside the plasma plume have a high velocity. Likewise, deposition in high background pressure and/or large target-to-substrate distance yields to low kinetic energy conditions and species inside the plasma plume have small velocity. In the case of Pt and PteRu alloy, different deposition conditions were shown to have a profound effect on the morphology of the film deposited on a porous substrate [12]. Thus, under high kinetic energy conditions, Pereira et al. recently replicated the pore structure of an anodic aluminum oxide (AAO) membrane, therefore yielding to the preparation of functionally modified Pt, PtRu and Au macroporous films whereas low kinetic energy conditions yielded to the clogging of the porous substrate. More recently, the crystalline structure of Pt films was also found to vary with the kinetic energy conditions prevailing during the deposition. At high kinetic energy, dense and highly (111) oriented thin films were obtained, while porous and polycrystalline films were formed under low kinetic energy conditions [5]. This study follows previous reports showing that this effect is not limited to Pt and that Au thin films display the same transition from highly (111) oriented to polycrystalline as the kinetic energy of the plasma plume is decreased [13e15]. Herein, we report on the effect of the deposition parameters i.e. the background gas pressure on the structural, morphological and electrochemical properties of Pt thin films deposited on carbon substrates. The deposition parameters will be shown to have a strong influence on the velocity of the Pt species that are impinging on the carbon substrate and that this plasma characteristic has a determining effect on the morphology, porosity and electrochemically active surface area of the films. Finally, the effect of all these parameters on the CO oxidation behaviour and the activity of the resulting Pt films for the oxygen reduction reaction will be assessed.
2.
Experimental
Platinum films were deposited by pulsed laser deposition (PLD) under different He gas pressures (from 105 to 10 Torr). A target of pure Pt (99.99%) was ablated with a focused laser beam (pulsed KrF laser, l ¼ 248 nm, pulse length 17 ns, repetition rate 100 Hz). The laser fluence was set at 8 J/cm2, while the target-to-substrate distance was set at 5 cm. The velocity (v) of the deposited Pt species was determined by time-offlight (TOF) emission spectroscopy, using the experimental set-up described in detail elsewhere [13e15]. Details on the measurements of the velocity of Pt are given elsewhere [5]. The deposition rates were measured at the different He gas pressures [16] and the number of laser pulses was adjusted to reach a constant Pt film thickness. The crystalline structure of the Pt films was investigated by x-ray diffraction (XRD) using a Bruker AXS D8 advance diffractometer with Cu Ka radiation. All measurements were performed at a grazing incidence angle of 2 . The morphology of the Pt films was characterized by scanning electron microscopy (SEM) using a Hitachi S-4700 Field-Emission SEM. The surface morphology and roughness of the samples were investigated by scanning tunneling microscopy (STM) using a Nanoscope III microscope from Digital Instrument.
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The porosity of the Pt films was determined by X-ray reflectometry (XRR) using a Philips diffractometer (PANAnalytical X’Pert PRO) with a Cu Ka radiation source. The XRR curve is obtained by plotting the scattered X-ray intensity against the angle of incidence (q). At low incidence angle, total external reflection occurs, whereas X-ray beam is partially absorbed by the sample above a critical angle (qc). The critical angle is related to the film density r using the relation: qc ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r$re $l2 $Z$NA =p$A
where re is the electron radius, l the X-ray wavelength, Z the atomic number, NA Avogadro’s number, and A the atomic mass. qc is defined as the angle at which the intensity is half that of the plateau value corresponding to total reflection. The porosity is given by 1 r/rbulk, where rbulk is the bulk density of platinum. For these measurements, Pt films were deposited on Si substrates. For electrochemical characterization, the Pt films were deposited on 5 mm diameter carbon disks (amorphous graphite, 99.997%, Goodfellow Corporation). The Pt/C samples were inserted into a rotating ring-disk electrode (RRDE) shaft and used as working electrodes in a standard three compartment cell. The ring-disk electrode was controlled by a bipotentiostat (Pine Instrument Company, AFCBPI), coupled with a rotation controller (Pine Instrument Company). Prior to measurement of the electrochemically active surface area (EASA), the Pt/C electrodes were conditioned in de-aerated H2SO4 solution by cycling between 0.05 V and 1.30 V at 50 mV s1 until reproducible voltammograms were obtained. For the oxygen reduction reaction (ORR) studies, the Pt ring electrode was held at 1.20 V. At that potential, the oxidation of peroxide produced at the disk is under diffusion control [17] while the Pt/C working electrode potential was swept from 1.00 to 0.05 V at 20 mV s1. Voltammetric profiles were recorded at room temperature in de-aerated (Argon N5.0, Praxair) and in O2 saturated (N4.3, Praxair) 0.5 M ultrapure sulphuric acid (A300-212, Fisher Scientific). Carbon monoxide (N2.5, Praxair) adsorption was performed by bubbling purified CO (oxygen trap, CRS) for 3 min with the electrode potential held at 0.30 V. Residual CO was removed by bubbling Ar for 15 min and two successive cyclic voltammograms (CV) were recorded at 15 mV s1. Prior to each electrode characterization, the glassware and the electrochemical cell were cleaned according to a well established method [18]. The auxiliary and the reference electrode were a platinum gauze and reversible hydrogen electrode (RHE), respectively. All potential values are referred to the RHE scale.
3.
Results and discussion
3.1.
Characterization of the as-deposited Pt thin films
XRD patterns of Pt films deposited over a large range of velocities (v), from ca. 11.3 to 0.03 km s1, are displayed in Fig. 1. The diffraction pattern of the carbon substrate (without Pt) is also shown (curve a in Fig. 1) for sake of comparison. The XRD patterns of all Pt/C samples show the five characteristic diffraction peaks of the face centered cubic (fcc) lattice of
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a
b
c
d
e
f
Fig. 1 e XRD patterns of Pt/C electrodes deposited at different v values: (a) carbon substrate, (b) 11.3 km sL1, (c) 4.6 km sL1, (d) 3.0 km sL1, (e) 10L1 km sL1 and (f) 0.03 km sL1.
polycrystalline platinum. The XRD pattern of Pt/C with v ¼ 0.03 km s1 (curve f) is dominated by the diffraction peaks of the carbon substrate, whereas the contribution from the carbon substrate decreases with increasing v. Indeed, the XRD pattern of the Pt/C sample with v ¼ 11.3 km s1 (curve b) shows only the characteristic diffraction peaks of Pt. As shown elsewhere, PLD deposition at high Ek on a Si substrate gives highly (111) oriented Au [13e15] and Pt [5] thin films, whereas deposition at low kinetic energy conditions yields to polycrystalline Au and Pt films. Obviously, the surface mobility of Au and Pt species deposited at low and high kinetic energy are different, with adatom motion being favored at higher Ek values, yielding surface reorganization and minimization of the energy of the whole system. In the case of the Pt/C samples prepared in this study, the XRD
patterns do not exhibit any evidence of preferential orientation. It is believed that the nature of the substrate (most probably its mass and its thermal properties) has an influence on the growth characteristics of the films and a more definite answer to our ability to growth (111) preferentially oriented Pt/ C samples will await a more detailed study of the influence of these parameters. SEM micrographs of Pt/C samples are shown in Fig. 2. The thickness of all samples, as determined from SEM crosssectional views (not shown) remains constant with a mean value of ca. 30 nm (10 nm). Dense and compact films are observed in Fig. 2a and b, corresponding to v ¼ 11.3 and 4.6 km s1, respectively. For v lower than 4.6 km s1 (Fig. 2c, d and 2e), the Pt films appeared increasingly more porous and samples prepared at the lowest v exhibit a cauliflower-type of structure. This increase in the porosity of the film as v is decreased might explain the increased contribution of the carbon substrate to the XRD pattern of the films prepared at the lowest v value (see Fig. 1f). The mean crystallite size, estimated from the full width at half maximum (FWHM) of the corresponding XRD patterns using the Scherrer equation, and the porosity of the films, determined from XRR measurements, are plotted as a function of v in Fig. 3a and Fig. 3b, respectively. As seen in Fig. 3a, the Pt mean crystallite size remained constant at ca. 5 nm for v 2 km s1. For films prepared at higher v values, the mean crystallite size increases steadily and reaches 30 nm at 11.3 km s1. This increase of the crystallite size is thought to occur as a result of the increased mobility of the Pt adatom at the surface of the substrate as v is increased. Crystallite size determination performed by STM measurements (open circles in Fig. 3a), gives strikingly similar values. This allows us to conclude that the Pt bulk and surface crystallites display the same mean size and that both are affected the same way by the kinetic energy of the laser ablated species. Fig. 3b depicts how the porosity varies with v. For v ¼ 11.3 km s1, the porosity of the Pt films is ca. 5%, which means that the density of the film is close to the theoretical bulk Pt density (21.4 g cm3). However, as v decreased from 11.3 to 102 km s1, the porosity of the film increased to reach 86% at the lowest v value.
3.2. Electrochemical properties of the as-deposited Pt thin films CVs of Pt/C at different v are shown in Fig. 4. CVs recorded in Ar-saturated H2SO4 electrolyte are presented in the left-hand side panel, while those recorded after the adsorption of a monolayer of CO are depicted in the right-hand side panel. For comparison, CVs for a polycrystalline Pt disk recorded in the same conditions are also shown in Fig. 4a and g. CVs of all Pt/C electrodes show the typical response of polycrystalline platinum in H2SO4 electrolyte, (i) a hydrogen adsorption/desorption region from 0.05 to 0.40 V, (ii) double layer region from 0.40 to 0.80 V, and (iii) the Pt oxidation/ reduction region at potentials above 0.80 V. A detailed analysis of the voltammetric curves was performed by determining the position of the hydrogen desorption peaks (at ca. 0.12 and 0.26 V) and of the Pt oxide reduction peak (at ca. 0.77 V). Similar electrode potential values (0.02 V) were found in each case,
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a
b
Fig. 3 e (a) Mean crystallite size and (b) porosity of Pt/C electrodes deposited at different v values.
Fig. 2 e SEM micrographs of Pt/C electrodes deposited at different v values: (a) 11.3 km sL1, (b) 4.6 km sL1, (c) 3.0 km sL1, (d) 10L1 km sL1 and (e) 0.03 km sL1.
indicating that all Pt/C films exhibit the same electrochemical behaviour, independently of the applied kinetic energy. Moreover, these values are similar to those recorded on a Pt disk (see Fig. 4a), which confirmed that the electrochemical surface properties of Pt thin films prepared by PLD do not
differ significantly from those of polycrystalline platinum. In particular, there is no indication of a (111) preferential surface orientation of the Pt/C sample with v ¼ 11.3 km s1, consistent with the corresponding XRD pattern, which displays all the characteristic diffraction peaks of polycrystalline Pt. In the right-hand side panel of Fig. 4, a full CO monolayer was pre-adsorbed at the surface of the Pt/C electrodes and two subsequent CVs in Ar-saturated electrolyte were recorded (only the forward sweeps are shown for sake of clarity). CO-blocked Pt sites for hydrogen adsorption are evidenced by the low currents observed below 0.40 V while CO oxidation started at potentials larger than ca. 0.70 V. The subsequent recovery of the hydrogen desorption behaviour of the platinum surface in the second positive sweep is indicative of the full CO monolayer stripping during the first anodic sweep. The CO oxidation peak was observed at ca. 0.820 V for the polycrystalline Pt disk electrode (Fig. 4g) whereas all Pt/C electrodes display a CO oxidation peak at lower potential values. Indeed, the main CO oxidation peak gradually decreased towards less anodic potential values from ca. 780 mV to 735 mV as v decreased from 11.3 km s1 to 0.03 km s1, indicating that it is more facile to oxidize CO at the more porous surface than at the denser surface. The electrochemically active surface area (EASA) of Pt/C sample was calculated by integrating the hydrogen desorption charge (taking into account the double layer contribution) involved in the potential region below 0.40 V, assuming the extensively used value of 210 mC per cm2 Pt [19]. In Fig. 5, EASA values were found to increase as v varies from 11.3 to 3.0 km s1, indicating that a higher number of platinum surface atoms are accessible as v is decreased in that energy range. Such enhancement of the EASA is in good agreement with the morphological changes previously described in Fig. 2 and correlates also well with the enhanced porosity observed
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a
g
b
h
c
i
d
j
e
k
f
l
Fig. 4 e Cyclic voltammetries (CV) in Ar-saturated 0.5 M H2SO4 solution of Pt/C films deposited at different v values: (a and g) polycrystalline Pt disk, (b and h) 11.3 km sL1, (c and i) 4.6 km sL1, (d and j) 3.0 km sL1, (e and k) 10L1 km sL1, (f and l) 0.03 km sL1. In a to f, the sweep rate was 50 mV sL1. In gel (CO stripping voltammetry), the sweep rate was 15 mV sL1.
when v is varied in that range (Fig. 3b). They are also in good agreement with the change observed in the surface roughness of the film. For example, the Rs and the EASA values are increased by a factor of 1.52 and 1.38, respectively, as v is decreased from 4.6 to 3.0 km s1. For v < 3.0 km s1, EASA values were found to decrease as the kinetic energy is decreased. Such a behaviour seems quite surprising as the porosity of the films is the highest (and the mean crystallite size the lowest) when Pt is deposited at low v. This decrease of the EASA value for v < 3.0 km s1 is consistent with the total CO oxidation charges calculated from the first anodic sweep shown in Fig. 4. Indeed, EASA values can also be obtained from these CO oxidation charges by using a correction factor of 420 mC cm2 Pt [19]. As seen in Fig, 5, both EASA values follow the same trend with respect to v, confirming the fact that the EASA values do not scale with the porosity of the film. It is worth mentioning that even if the EASA values estimated using the CO stripping charges, EASACO, followed the same general trend as the EASA values determined from the Hupd charges, EASAH, the former corresponds only to 54 6% of the latter. This is not the case on a polycrystalline Pt disk as EASACO and EASAH give the same value (within 6%). This means that the factor 2 difference observed on Pt films prepared by PLD is not an artefact. This difference might arise
as a consequence of (i) CO species being not linearly adsorb onto Pt surface atoms, which is implicitly assumed in taking a value of 420 mC cm2 Pt to estimate EASACO, or/and (ii) limited access of adsorbed CO to the Pt atoms at the surface of
Fig. 5 e Variation of the electrochemically active surface area (EASA) determined from the hydrogen and CO oxidation charges with respect to v.
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the film. This may indeed be the case as suggested by the presence of a residual oxidation current in the hydrogen desorption potential region in the first sweep of Fig. 4h to l. However, in that case, the EASACO/EASAH ratio variation with v and the porosity of the films would have been expected. This issue will be dealt with in more details in a forthcoming study. However, it remains to be explained why the EASA (Fig. 5) does not scale with the porosity of the films (Fig. 3b). In the remaining of this study, the intrinsic electrocatalytic activity of Pt/C electrodes does not differ by more than a factor of 1.8 when the velocity is varied over more than 3 orders of magnitude. Moreover, a factor less than 1.8 between the intrinsic electrocatalytic activity of any particular Pt/C electrode and the bulk Pt disk was observed. Assuming that Pt deposited at different kinetic energies has the same intrinsic electrocatalytic activity for the ORR, this means that the estimation of the electrochemically active surface area is valid. Therefore, it seems that the porosity determined by XRR measurements is the sum of an open (electrochemically accessible) and close (not electrochemically accessible) porosity. Therefore, the discrepancy between the porosity and the EASA measurements could indicate that the fraction between the close and the total porosity is increased as the kinetic energy of the ablated species is decreased. This issue will be the subject of a forthcoming study. In Fig. 6, a set of polarization curves in O2 saturated 0.5 M H2SO4 solution at different rotation rates is shown for Pt/C electrodes. The potential at half limiting diffusion current density, which is representative of the electrocatalytic performance of the Pt catalysts for the ORR, shifts towards more positive values as v is decreased from 11.3 km s1 to 0.03 km s1. It is also worth noting that Pt/C electrodes exhibit a distinctly different behaviour in the hydrogen sorption potential region (Edisk < 0.40 V) as compared to polycrystalline Pt disk. On Pt disk electrode, the hydrogen sorption peaks are not observed in O2 saturated H2SO4 solution and, O2 reduction current decreases are due to the formation of H2O2 at
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a hydrogen covered platinum surface. In contrast, the CV of all Pt/C samples exhibit the two characteristic hydrogen adsorption peaks that are superimposed of the O2 reduction current, indicating that both processes are occurring simultaneously. These surface processes are scarcely observed concomitantly in the case of polarization curves in O2 saturated solution under forced convection conditions (u ¼ 1600 rpm) and this behaviour is more typically observed in a quiescent acidic solution in presence of remnant O2 [20]. Hence, the superimposition of Hupd peak currents to the O2 current during the RRDE experiments at Pt/C electrodes illustrate the inability of the electroactive species to react with all the Pt atoms accessible to the electrolyte. As seen in Fig. 6, there is some small variation in the diffusion limited current of the various Pt/C electrodes, which is attributed to different porosity and surface roughness [21] and/or to the high pseudocapacity of the carbon support at 20 mV s1 [22]. Fig. 7a presents a representative set of ring-disk results in O2 saturated 0.5 M H2SO4 electrolyte and at different rotation rates obtained on a Pt/C electrode prepared at v ¼ 11.3 km s1.
a
b
c
Fig. 6 e Polarization curves (20 mV sL1) in O2 saturated 0.5 M H2SO4 solution at 1600 rpm for Pt/C electrodes deposited at different v.
Fig. 7 e (a) Polarization curves of Pt/C electrode deposited at Ek [ 11.3 km sL1 (20 mV sL1) in O2 saturated 0.5 M H2SO4 solution at different rotation rates, (b) corresponding ring current densities (Ering [ 1.20 V) and (c) resulting KouteckyeLevich plots for the current densities displayed in (a).
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Table 1 e ORR parameters as a function of the velocity (v) of Pt ablated species. v (km s1) 102 3 101 3.0 4.6 11.3 Pt RDE
n 4.30 4.00 3.80 3.85 3.35 3.70
jkgeo at E ¼ 0.85 V (mA cm2 geo) 9.39 5.27 7.81 5.95 2.16 0.59
0.25 0.20 0.20 0.30 0.05 0.05
jk cm2 Pt at E ¼ 0.85 V (mA cm2 Pt)
XH2 O2 (%) (idisk at E ¼ 500 mV)
XH2 O2 (%) (idisk at E ¼ 100 mV
0.5 0.5 0.6 0.9 1.8 1.2
1.2 1.1 1.5 2.1 5.0 1.8
317 194 206 239 184 315
The oxygen reduction currents displayed in Fig. 7a shows a potential region where the ORR is under mixed kineticdiffusion control (Edisk > 0.70 V), followed by a region where the current for the ORR is limited by diffusion (Edisk < 0.60 V). Similar data were obtained for all Pt/C electrodes investigated in this study. The analysis of such voltammetric profiles lead to the determination of parameters involved in the ORR: (a) From the purely diffusion controlled potential region, Levich linear plots (idisk vs w1/2) allowed us estimate the number of electrons, n, involved in the overall reaction (see Table 1), using data taken from the literature for the diffusion coefficient (D ¼ 1.8 105 cm2 s1) and concentration (C ¼ 1.13 106 mol cm3) of dissolved oxygen in 0.5 M H2SO4 at 20 C [23]. (b) From the mixed controlled potential region, the kinetic current, ikinetic, can be evaluated using the Kouteck1/2 ) (see Fig. 7c). yeLevich relationship (i1 disk vs w (c) Finally, from the ring current that corresponds to the oxidation of H2O2, the fraction of H2O2 formed at the cathode can be determined, which is given by XH2 O2 ¼
2iring =N idisk þ iring =N
where N, idisk and iring correspond to the collection efficiency of the ring, the disk current and the ring current, respectively. As seen in Fig. 7b, the ring current represents a negligible fraction of the disk current at high potential but increases in the Hupd potential region (Edisk < 0.40 V). Selectivity towards H2O2 formation, XH2 O2 , was calculated at 500 mV since the cathode potential remains close to that value under fuel cell operating conditions [24]. XH2 O2 at 100 mV were also calculated since O2 migration into the anodic compartment could lead to the formation of H2O2 at the anode, and degradation of the catalyst layer and membrane could occur [25]. The values of all parameters previously mentioned are summarized in Table 1 for the Pt/C electrodes prepared at different v values. For the Pt disk electrode, the number of electrons involved in the overall ORR reaction is n ¼ 3.7, close to the value of 4 expected for the ORR at Pt catalysts. For the Pt/C samples, there is a slight but consistent increase of the n value as v is decreased from 11.3 to 0.03 km s1. This is attributed to an enhanced contribution of the 4e- reduction pathway for porous electrodes synthesised at low kinetic energies. This assumption is confirmed by the XH2 O2 values (at Edisk ¼ 500 mV), which
6 29 25 24 20 27
is found to decrease at lower v value. The same trend is observed for XH2 O2 measured at Edisk ¼ 100 mV. The kinetic current densities were calculated from a KouteckyeLevich analysis and the resulting data are reported in Table 1. The kinetic current densities are expressed relative to the geometric area of the underlying carbon substrate and to the real Pt electrochemically active surface area, evaluated from the EASACO. As expected, higher geometric current densities were found for the whole range of Pt/C samples as compared to polycrystalline bulk Pt. The highest value of jkgeo is 9.39 mA cm2 at 0.85 V is obtained for Pt/C sample prepared at v ¼ 0.03 km s1. This is about 20 times higher than jkgeo recorded on a Pt disk electrode. In comparison, the variation of the current density normalized to the Pt EASACO are much less important (less than a factor of 1.8), indicating that the large fluctuation observed previously in the value of jkgeo is mainly due to a variation of the EASA of the samples. As far as we can tell, there is no systematic variation of jk cm2 Pt with respect to the kinetic energy of the Pt species used to prepare the deposits that would indicate that a preferential structure exists at the surface of the Pt films. Again, this result differs notably from what was observed for the high kinetic energy deposition conditions of Pt on Si substrate and might be related to the peculiar thermal properties of the substrate. This issue warrants further investigation.
4.
Conclusions
During pulsed laser deposition, varying the helium background pressure in the deposition chamber allows for a control over the kinetic energy of the Pt species impinging on the substrate that in turn allows for a control over the porosity and surface morphologies of the films. In the case of carbon monoxide poisoning, the more porous Pt film are the more electroactive. In the case of the oxygen reduction reaction, there is no systematic variation of the intrinsic electrocatalytic activity and all Pt/C electrodes displays an intrinsic electrocatalytic activity that does not differ by more than a factor of 1.8 from that of a polycrystalline Pt disk. Strategies to prepare highly (111) oriented Pt films on conductive substrate are being developed to take advantage of the fact that Pt adatoms deposited under high kinetic energy conditions possess an increased surface mobility that allow them to diffuse and re-arrange themselves to minimize the total values of the system.
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 5 ( 2 0 1 0 ) 8 4 8 6 e8 4 9 3
Acknowledgements [12]
This work was made possible due to the financial support of the National Science and Engineering Research Council (NSERC) of Canada and the Canada Research Chair program. One of us (CH) also acknowledges the financial support of the Fonds Que´be´cois de la Recherche sur la Nature et les Technologies (FQRNT).
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