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Oxygen reduction on well-defined platinum nanoparticles inside recast ionomer
Electroehimica
Act& Vol. 41. No. 2, pp. 307-314. 1996 Copyright 0 1995 Elsevier Science Ltd. Printed in Great Britain. AU rightsnarvcd
Pergamon
0013-4686/96 59.50 + 0.00
0013-46%(95)00305-3
OXYGEN REDUCTION NANOPARTICLES A. IRC, 2
GAMEZ,
ON WELL-DEFINED PLATINUM INSIDE RECAST IONOMER D. RICHARD
and P.
GALLEZOT
Avenue Albert Einstein, 69626 Villeurbanne, France F. GLOAGUEN,
R. FAURF,,R. DURAND*
CREMGP, BP 75, 38402 St Martin d’HQes, France (Received 24 January 1995; in revisedform 30 May 1995) Abstract-The objective of the study was to investigate the effect of particle size on the catalytic activity for oxygen reduction reaction at platinum/recast ionomer interface. To obtain experimental evidence of this effect, porous electrodes of well-defined geometry and very well calibrated Pt particles on graphite were used. The catalytic powders were prepared by cationic exchange and character&d by TEM and H and CO electrochemical adsorptions. For oxygen reduction, a loss of catalytic activity with the decreased platinum particle size is confirmed. This activity loss is correlated to the stronger adsorption of oxygenated species under inert atmosphere and during oxygen reduction. No effect of the inter-particles distance was found even when the particles are 1.2nm in diameter and about 1Onm away; the use of graphite powder also prevents a too strong shielding effect of the catalyst support. Key words: oxygen electroreduction, particle size effect, platinum, graphite substrate.
1. INTRODUCTION Numerous studies have been reported on the kinetics of oxygen reduction reaction (ORR) in acid media on carbon supported Pt nanoparticles[l-21. Some authors[l] correlated the rate of this reaction to particle size and shape whereas others[2] found no particle size effect. Gloaguen et aI.[3] have recently described a test procedure to obtain the kinetic parameters for ORR at Pt/C-Nafion interface. By using various Pt/Vulcan powders from E-TEK Inc., a particle size effect, in a diameter range of 2 to 26 nm, have been already observed[4]. In order to validate these preliminary results, ORR kinetic measurements at various Pt/graphite powders in Nafion were performed and reported in this paper. Transmission electronic microscopy (TEM) and hydrogen and carbon monoxide electrosorption allowed to accurately characterise the catalytic powders. The influences of the inter-particle distance[2] and of the 0, partial pressure were also studied. 2. EXPERIMENTAL 2.1. Reagents and electrodes The Nafion solution is a 5 w/w 1100 EW Nafion solution from Du Pont. The 0.1 M H,SO, solution was prepared from Suprapur Merck concentrated sulphuric acid and very pure water (Millipore super Q system). The working electrode was a compact thin layer of catalytic powders and recast Nafion, deposited on a rotating disk electrode, and immersed * Author to whom correspondence should be addressed.
in the dilute sulphuric solution. The catalytic layer coating procedures and the electrochemical equipment have been described elsewhere[3]. The potentials are reported vs. the equilibrium potential taken by Pt/C inside the Nafion phase saturated with hydrogen; these Nafion rhe potentials are identical to those obtained vs. a hydrogen electrode in any acid solution in equilibrium with Nafion. The catalyst support was a graphite powder (HSAG 300 Lonza) of 3OOm’ g- ’ in specific area. The graphite HSAG 300 is a non-microporous support, which porosity is essentially an intergranular mesoporosity. This powder is made of platelets of several graphitic sheets. Most of the surface area developed consists in basal planes of graphitic structure which are very convenient for the study of the particle size or inter-particle distance. XRD patterns show thin peaks for (0 0 2n) planes (corresponding to an average d spacing of 0.37nm) at contrary with the patterns observed for carbon blacks like Vulcan which are much broader. Indeed this later support which consists of small balls (z250nm in diameter) presents irregularities or defects at its surface. Moreover the curvature of these balls could make more difficult the interpretation of any inter-particle distance effect. 2.2. Catalyst preparation The metal deposition methods used were the cationic exchange for initial loading and the refill method for controlled particle size increase. The main interest of the exchange method is that it allow for the separate control of the metal loading and of the particle size, provided that the loading is within the limit due to the exchange capacity of the support. This loading is determined by the number 307
308
A.
Table 1. Effect of the reduction conditions cle size Temperature rate /(K min - ‘)
et al.
upon the parti-
Reduction temperature /K
1 0.1
GAMEZ
Table 3. Effect of the inter-particle
d/nm 2 1.2
513 513
pt/wO/
d/nm”
X/nmb
6.2 2.3 7.1 4.0
1.2 1.2 2.1 2.1
8.2 14.5 18.1 25.2
distance SA/(pAcm-‘) 9.6 10.1 17.7 18.7
From TEM observation. b Calculated according to:
l
Catalyst after a
maintained for 2 h at the reduction temperature increase from room Hydrogen flow 151 h-‘.
temperature
temperature.
of functional groups at the surface of the support, while the particle size is essentially controlled by the reduction conditions. This is at contrary of what occurs in the case of the impregnation method when the size usually depends upon the loading. Table 2. Platinum
particle size comparison measuring techniques
using different
Technique TEM
co
H
Catalyst pt/wo/o
djnm
d/nm
d/nm
6.2 7.1’ 8.gb 11.6’
1.2 2.1 3.3 4.3
1.3 2.2 3.3 4.4
1.4 2.3 3.4 4.3
‘JJ and ‘: lst, 2nd and 3rd refill from d = 2 nm (see Table 1).
Fig. 1. TEM micrographs
of Pt/C powders
with different
X = J21.4nd3(100 - m)S,/(m x 6000) - d, where S, is the specific area (mZg-‘) of the carbon support and m the mass fraction of Pt in the catalytic powder.
The cationic exchange method has been previously described[5] but will be briefly resumed. A solution of tetramine platinous salt was added to a suspension of the carbon support in ammonia. The solid was then filtered to ensure that only the cations bounded to the support are kept, the excess being washed away. Thus an increase of the functional surface groups amount was necessary to obtain catalytic powders with a high enough platinum loading. This was achieved by liquid phase oxidation of the graphite powder by a NaOCl solution. After the ionexchange and the filtration, the catalyst was dried and reduced under H, flow. The, average size of the metallic particles were fixed by controlling the reduction conditions (temperature increase and H, flow) (Table 1).
Oxygen reduction on well-defined platinum nanoparticles
In order to obtain homogeneous distributions of particles, the refill method of Mezeno et al.[6] was used instead of thermal sintering to prepare larger particles. This method uses a surface redox reaction to coat the particles \kith a new layer of atom after each refill. An uniform growth of the particles, and a resulting homogeneous size distribution was thus obtained. The catalytic powders used for the study of the particle size effect (Table 2) were prepared by successive retills starting from the first powder on the list (6.2 wt% Pt), prepared by cationic exchange; they show homogeneous size distribution of increasing mean diameter. The catalytic powders used for the study of interparticle distance (Table 3) include two of the previous powders (6.2 and 7.1 wt% Pt) and two other powders (2.3 and 4.0wt% Pt) prepared by cationic exchange with loading below the exchange capacity limit and reduced under conditions required to obtain the desired particle size (ie 1.2 and 2.1 nm respectively).
Particle size (nm)
B
Particle size (nm)
C
2.3. Characterisation
Particle size (nm)
I
,
Fig 2. Distribution of particles sizes in Pt/C catalyst. The average particle size (dVA) is the volume-area mean diameter calculated from the distribution according to dVA = Zf~:/Xf~~. (A) dVA = 1.2nm; (B) 2.1 nm; (C) 3.3nm; (D) 4.4nm.
-6
! 0.1
309
of the catalytic powders
The particles of platinum were measured by TEM (Fig. 1). A suspension, in ethanol, of the catalytic powder was deposited on a microscope grid coated with an amorphous carbon film. The grid was observed using a JEOL 100 CX microscope. The mean particle size was obtained by measuring the diameter of a sutkient number of particles to ensure a good statistics. The surface mean diameter was calculated according to dVA = Z?jjd:jIAd:, where x is the frequency of occurrence of the particles of diameter d, in the sample, from the particle size distribution given in Fig. 2. The electrochemical active Pt surface area of the thin layer was measured by coulometries of hydrogen adsorption&sorption (0.05 to 0.4 V, 5 mV s- ‘) and oxidation of adsorbed CO (0.1 to 1.0 V; 5 mV s-i) in a N, purged 0.1 M H,SO, solution. For the smallest Pt loading, due to the electrochemical activity of the carbon substrate, the coulometry of hydrogen adsorption-desorption is difficult to accu-
I
0.5 0.9 0.7 E /V (RHE) Fig. 3. CO oxidation: voltammograms (5 mV s-l) for thin layer of Pt/C and Nafion with various particle sizes: 1.2 nm; --- 4.3 nm. Current densities related to the real area. 0.3
310
A. GAMEZ et al.
-
d=1.2
nm - - - d=4.3
nm
-12, 0
0.4
0.8
1.2
I
E /V (RHE) Fig. 4. Reduction of adsorbed oxygen species as a function of Pt particle size: volgammograms (5mV SK’) under inert atmosphere for thin layers of Pt/C and Nafion (1.2 nm; --- 4.3nm). Current densities related to the real area. rately calculate, but the CO peaks are still well defined (Fig. 3). The measurement of Pt surface area in contact with Nafion and available for electrochemical reactions allows for the evaluation of the ratio: total platinum area/geometric area. Since the Pt loading of the layers was known, it was possible to estimate the specific catalyst area S (m’ g- ‘) and the average particle size d (nm). Assuming spherical particles: d = 6ooo/(21.4 s) where 21.4 is the platinum density (gcmm3). The particle sizes calculated by electrochemical measurement are close to those obtained by TEM observations (Table 2) which means that the overall area of the catalyst dispersed in the active layer is available for the electrochemical reaction. The reduction of the adsorbed oxygenated species at the platinum particle surface under inert atmosphere was also investigated. The voltammograms (Fig. 4) were recorded in the same conditions of scan rate and potential range as the oxygen reduction studies (see 2.4.); the voltammetric curves strongly depending on the scan rate (Fig. 5).
2.4. Oxygen reduction The oxygen reduction current-potential curves were obtained from slow scan voltammograms (1.3 to 0.3 V; 5 mVs_‘) at various rotation frequencies (250 to 2000rpm) in a 0.1 M H,SO, solution under 0.2 and 1 atm of oxygen. Before starting to record each current-potential curve the electrode was held at 1.3 V during 30s. Analogous current-potential curves were obtained when the starting potential was l.OV instead of 1.3 V. Starting at rather high potentials (es l.OV) is useful for comparisons with real fuel cells cathodes. For very thin active layers (z 1pm) the electrode is mostly under kinetic control[3] (ie the ohmic and the diffusion limitations within the active layer are negligible). For every rotation frequencies , the kinetic current density i was then calculated using: i = (ini,_J(iL.n - in); in being the measured current density using the geometric area and i,+ the corresponding limiting current density in solution.
3. RESULTS
AND DISCUSSION
The voltammograms under inert atmosphere of freshly prepared thin active layer are sometimes featureless during the first cycles. With continued cycling (0.05 to 1.3 V; lOOmVs_‘) the characteristic platinum waves rapidly develop and after 10 scans reproducible voltammograms are obtained. Some authors showed[7] that alternate anodiccathodic treatments change the surface structure of dispersed Pt and at the same time modify its electrocatalytic properties. No significant differences in the catalytic activities, for ORR and for oxidation of CO adsorbed, were however found here when the active layers were previously scanned between 0.05 to 1.3 V or between 0.05 to 1.0 V. Figure 6(A) shows the mass activity (MA) (kinetic current density/catalyst loading ratio) as a function of the Pt particle size for oxygen reduction reaction in Nafion. At contrary with previous results[l], no maximum in mass activity is observed between 1.2 and 4.4nm; MA tends to decrease slowly with increasing the particle size above 5 nm in diameter. Figure 6(B) shows the specific activity SA (kinetic current density/real catalyst area ratio) as a function of the Pt particle size for oxygen reduction reaction in Nafion. SA was measured at 0.85 V, 293 K and 1 atm. SA tends to decrease with decreasing particle size. Similar results were found in Nafion for Pt/Vulcan powders from E-TEK Inc.[4], but the decrease for the smaller particles was more pronounced leading to a maximum in the MA for particle sizes around 3.5 nm in diameter. In life test of polymer electrolyte fuel cells, little apparent current density loss was observed while the oxygen cathode area decreases considerably due to the particle sintering. This was explained by an increase of SA with the particle sizer 173. The loss of catalytic activity with decreasing the particle size, also observed in sulphuric and phosphoric media, was explained by Watanabe et a/.[23 by either diffusion or mutual interaction when the particles are too close together (<20nm). In this
Oxygen reduction on well-defined platinum nanoparticles
ri iv r-1 / pAcm-2
[mV
311
5-11-l
-1
Fig. 5. Voltammetric reduction of adsorbed oxygenated species as a function of scan rate (50; --5; .-.0.5 mV s- ‘) under inert atmosphere for thin layer of Pt/C and Nafion: ratios current densities/scan rate and cd. related to the real area; particle size 1.2 nm.
work, this effect was not observed even when the distance between the particles is ten times their diameter (Table 3). The local current density is anyway much lower than the spherical limiting current density (i + i,, sphcr) so, the diffusion and/or the shielding effect of inter-particle distance should be accounted for when the particles are very close together (ie, between about one and five times their diameter). Moreover, a possible shielding effect of the catalyst support is probably more important when the particle sizes are smaller than 2 nm because these particles are probably spherical and/or can be situated in nanopores but this effect is much less pronounced for graphite platelets (eg, HSAG 300) than for carbon black (eg, Vulcan XC72) of ill-defined shape. Calculations are currently developed in our
laboratory for these various local conditions but the preliminary results show they cannot explain the main size effect. A loss of catalytic activity was also found in gas phase catalysis under oxygen atmosphere: the deactivation due to a strong chemisorption of oxygen atoms at the platinum surface is more pronounced when the particle size are smaller than 3.0nm[9]. Some authors[lO] have allocated this deactivation to the irreversible formation, under oxygen atmosphere, of PtO, species, but this assumption seems not realistic under the electrochemical conditions used for ORR at Pt/Nafion interface. A strong deactivation due to the over-oxidation of Pd was also observed for particles smaller than 2.5 nm in liquid phase oxidation of glucose[ 111.
A. GAMEZ et al.
312
.
40
20
SA @4
I
.
m-2,
. .
.
20 i
10
,
where i is the kinetic current density per real catalyst area, AG# the chemical part of the activation energy at low coverages, t-0 represents the change of adsorption energy of reaction intermediates with total coverage 8. This change of adsorption energy with the particle size might be due to the existence of specific adsorption sites at the surface of the platinum particles[l] or to electronic effects[12]. However, the differences in energy levels of the valence electrons will eventually become appreciable when nearly all atoms in the particles are surface atoms (ie, for particle sizes smaller than lOnm)[13] which is not the case here. Figure 4 shows that, under inert atmosphere, the amount of oxygenated species still adsorbed, at any potential above 0.8 V (ie, Temkin isotherm), is larger when the particle size decreases. According to equation (I), this implies a diminution of 1i 1 and then a loss of specific activity (SA = (i1at 0.85 V). Figure 7 validates this result: the change of the Tafel slope (Temkin to Langmuir isotherm) takes place at 0.8 V for very small particles (1.2 nm) and at 0.87V for bulk platinum. During oxygen reduction, the oxygenated species are then more strongly adsorbed on the nanoparticle surface than on the bulk platinum surface. According to the Damjanovic theory[S] for electroreduction of oxygen at prereduced bulk platinum surface in acid media the oxygen adsorption is fast, the following step:
.
t
n
Fig. 6. (A) Mass activity MA for oxygen reduction at 0.85 V @he), 1 atm. and 293 K, as a function of the Pt particle size; (B) Specific activity SA for oxygen reduction at 0.85 V (rhe), 1 atm. and 293 K, as a function of the Pt particle size.
In this work, the SA decrease for the smallest particles might be correlated to a stronger adsorption of the oxygenated species. For high coverage 0 of oxygenated species (ie, Temkin isotherm), with any electrochemical rate determining step, we will find a kinetic equation as : -i
cc exp -
AG’ + are + j3FE RT
H+ + e- -
1
products
(2)
1 (1)
d = 1.2
--
5 ’ z
+
is rate determining and the other steps (ie, reduction of O,d and OH,,) are under quasi-equilibrium. For example, under Temkin conditions of adsorption (in the 60mV dec- ’ region) the reaction rate is given
1
_ -3.00 NI $
r.d.s. (o,),,
nm
bulk
-4.00-
$ rl -5.00-
-6.00 !
0.6
0.7
0.8 E /V
0.9
b
I 1
(NE)
Fig. 7. Tafel plots for oxygen reduction (293 K, 1 atm) at various Pt/Nafion interfaces. Current densities related to the geometrical area. Roughness factor: 2.1 for bulk Pt and around 10 for 1.2nm particles, leading to real area current densities 3.5 times larger for the bulk Pt in 0.85 V potential region (cf: Fig. 6 and [4]).
313
Oxygen reduction on well-defined platinum nanoparticles
by: -i
a
P~~[H+]~‘*
exp z [
1
4. CONCLUSION (3)
For oxygen reduction, a loss of catalytic activity with the decreased platinum particle size is confirmed in the well-defined condition allowed by:
As expected from this r.d.s., the reaction order of oxygen at bulk Pt/Nafion interface was determined to be unity[14]. The same result was obtained for not too small Pt particles in Nafion[lS]. The adsorption energy of oxygenated species is however larger at the nanoparticle surface than at the bulk platinum surface and then the first electrochemical step could not be the rate determining step. This would also implies a different ORR mechanism for very small nanoparticles. Considering moreover non-prereduced electrodes enhances the effect of these oxygenated species. To clarify these problems, the effect of proton concentration inside recast ionomer (Nafion type) is not easy to measure, but the oxygen partial pressure effect on the reaction rate for Pt nanoparticles was investigated using two partial pressures (1 and 0.2 atm.). For the rather high potential range (eg, 0.8, 0.9V), the measured current densities at very small particles (1.2nm)/Nafion interface seems to be proportional to (po2)xwith 0.3 < x -=z0.7 (Fig. 8) and not to poI as expected from the r.d.s. (2). In our opinion, depending on the oxygenated species coverage (ie, on the pretreatment and the potential), the r.d.s. for ORR on very small nanopartitles could be the step (2), or the reduction of Oa,, or OH,, or both (ie, two r.d.s. steps, with negligible backward term, could be considered for ORR, like for the simpler HER). Anyway ORR experiments with prereduced nanoparticles, which provide higher current density than with non-prereduced particles, present the same difflculties :
(a) the kinetic measurement procedure, (b) the utilisation of homogeneous and controlled particle size and (c) utilisation of graphite powders for the catalyst support which reduces the possibility ing effect of the support.
Alternate anodic-cathodic treatments from 0.05 to 1.3 V or from 0.05 to 1.0 V led to no significant variations of the catalytic activities. No effect of the inter-particles distance was found when the particle size is 1.2nm and the particles are about lOnm apart. This activity loss is correlated to the stronger adsorption of oxygenated species on very small particles. Acknowledgements-The authors gratefully acknowledge the ADEME and ECOTECH (VPE Program) for their support.
REFERENCES 1. K. Kinoshita, J. electrochem.Sot., 137,845 (1990). 2. M. Watanabe, H. Sei. P. Stonehart. J. electroanal. Chem.. 261, 375 (1989). 3. F. Gloaguen, F. Andolfatto, R. Durand, P. Ozil, J. appl. Electrochem. 24,863 (1994). 4. A. Kabbabi, F. Gloaguen, F. Andolfatto, R. Durand, .I. electroanal. Chem. 373.251 (1994). 5. D. Richard, P. Gallezdt, in Prep6ration oj Catalysts IV, (Edited by B. Delmon et al.), Elsevier, Studies in Surface Science and Catalysis series (31), Amsterdam, p. 74 (1987). 6. J. C. Menezo, M. F. Denanot, S. Peyrovi, J. Barbier, Appl. Cat. 15,353 (1985). 7. I. Bakos and G. Honanyi, J. electroanal. Chem. 347, 383 (1993).
(a) c.d. is not proportional to po, and (b) Pt oxidation states deduced from XANES studies are slightly higher under ORR than under inert atmosphere[ 161; it is therefore difficult to accept a quasi-equilibrium for the last steps.
I -3.8 -
+
of a shield-
0.2
atm
+
1.0
atm
\
I
-5.8 0.7
0.74
0.82
0.78
E
/V
0.86
I
(RHE)
Fig. 8. Effect of 0, partial pressure (1 and 0.2 atm.) on the Tafel plots for ORR at 293 K. Current densities related to the real area. Pt particle size: 1.2 nm.
314
A. GAMEZet al.
8. D. B. Sepa, M. V. Vojnovic, L. M. Vracar, A. Darnjanovie, Electrochim. Acta, 32, 129 (1987). 9. F. V. Hanson, M. Boudart, J. Catal. 53, 56 (1978). 10. J. M. H. Dirkx, H. S. van der Baan, J. Catal. 67, 1 (1981). 11. M. Besson, P. Gallezot, F. Lahmer, G. F&he, P. Fuentes, in Catalysis of orgnic reactions, (Edited by J. R. Kosk, T. A. Johnson), Marcel Dekker, New York, p. 169 (1994). E. Passalacqua, L. Pino, A. S. Arico, V. 12. N. Giordano, Antonucci, M. Vivaldi, K. Kinoshita, Hectrochim. Acta, 36, 102 (1991).
13. M. Che, C. 0. Bennett, in Advances in Catalysys, Academic Press, New York, 36, 55 (1989). S. Srinivasan, A. J. Appleby, C. R. 14. A. Parthasarathy, Martin, J. electrochem. Sot., 139,2856 (1992). 15. S. Mukerjee, S. Srinivasan, J. MC Breen, E.C.S. Spring Meet. Abstr., San Fransisco, p. 1288 (1994). and toll., Oxygen Electrochemistry Symp., 16. R. Durand E.C.S. Fall Meet., Chicago (1995). 17. M. S. Wilson, F. H. Garzon, K. E. Sickfacus, S. Gottesfeld, J. electrochem. Sot. 140,2872 (1993).
Act& Vol. 41. No. 2, pp. 307-314. 1996 Copyright 0 1995 Elsevier Science Ltd. Printed in Great Britain. AU rightsnarvcd
Pergamon
0013-4686/96 59.50 + 0.00
0013-46%(95)00305-3
OXYGEN REDUCTION NANOPARTICLES A. IRC, 2
GAMEZ,
ON WELL-DEFINED PLATINUM INSIDE RECAST IONOMER D. RICHARD
and P.
GALLEZOT
Avenue Albert Einstein, 69626 Villeurbanne, France F. GLOAGUEN,
R. FAURF,,R. DURAND*
CREMGP, BP 75, 38402 St Martin d’HQes, France (Received 24 January 1995; in revisedform 30 May 1995) Abstract-The objective of the study was to investigate the effect of particle size on the catalytic activity for oxygen reduction reaction at platinum/recast ionomer interface. To obtain experimental evidence of this effect, porous electrodes of well-defined geometry and very well calibrated Pt particles on graphite were used. The catalytic powders were prepared by cationic exchange and character&d by TEM and H and CO electrochemical adsorptions. For oxygen reduction, a loss of catalytic activity with the decreased platinum particle size is confirmed. This activity loss is correlated to the stronger adsorption of oxygenated species under inert atmosphere and during oxygen reduction. No effect of the inter-particles distance was found even when the particles are 1.2nm in diameter and about 1Onm away; the use of graphite powder also prevents a too strong shielding effect of the catalyst support. Key words: oxygen electroreduction, particle size effect, platinum, graphite substrate.
1. INTRODUCTION Numerous studies have been reported on the kinetics of oxygen reduction reaction (ORR) in acid media on carbon supported Pt nanoparticles[l-21. Some authors[l] correlated the rate of this reaction to particle size and shape whereas others[2] found no particle size effect. Gloaguen et aI.[3] have recently described a test procedure to obtain the kinetic parameters for ORR at Pt/C-Nafion interface. By using various Pt/Vulcan powders from E-TEK Inc., a particle size effect, in a diameter range of 2 to 26 nm, have been already observed[4]. In order to validate these preliminary results, ORR kinetic measurements at various Pt/graphite powders in Nafion were performed and reported in this paper. Transmission electronic microscopy (TEM) and hydrogen and carbon monoxide electrosorption allowed to accurately characterise the catalytic powders. The influences of the inter-particle distance[2] and of the 0, partial pressure were also studied. 2. EXPERIMENTAL 2.1. Reagents and electrodes The Nafion solution is a 5 w/w 1100 EW Nafion solution from Du Pont. The 0.1 M H,SO, solution was prepared from Suprapur Merck concentrated sulphuric acid and very pure water (Millipore super Q system). The working electrode was a compact thin layer of catalytic powders and recast Nafion, deposited on a rotating disk electrode, and immersed * Author to whom correspondence should be addressed.
in the dilute sulphuric solution. The catalytic layer coating procedures and the electrochemical equipment have been described elsewhere[3]. The potentials are reported vs. the equilibrium potential taken by Pt/C inside the Nafion phase saturated with hydrogen; these Nafion rhe potentials are identical to those obtained vs. a hydrogen electrode in any acid solution in equilibrium with Nafion. The catalyst support was a graphite powder (HSAG 300 Lonza) of 3OOm’ g- ’ in specific area. The graphite HSAG 300 is a non-microporous support, which porosity is essentially an intergranular mesoporosity. This powder is made of platelets of several graphitic sheets. Most of the surface area developed consists in basal planes of graphitic structure which are very convenient for the study of the particle size or inter-particle distance. XRD patterns show thin peaks for (0 0 2n) planes (corresponding to an average d spacing of 0.37nm) at contrary with the patterns observed for carbon blacks like Vulcan which are much broader. Indeed this later support which consists of small balls (z250nm in diameter) presents irregularities or defects at its surface. Moreover the curvature of these balls could make more difficult the interpretation of any inter-particle distance effect. 2.2. Catalyst preparation The metal deposition methods used were the cationic exchange for initial loading and the refill method for controlled particle size increase. The main interest of the exchange method is that it allow for the separate control of the metal loading and of the particle size, provided that the loading is within the limit due to the exchange capacity of the support. This loading is determined by the number 307
308
A.
Table 1. Effect of the reduction conditions cle size Temperature rate /(K min - ‘)
et al.
upon the parti-
Reduction temperature /K
1 0.1
GAMEZ
Table 3. Effect of the inter-particle
d/nm 2 1.2
513 513
pt/wO/
d/nm”
X/nmb
6.2 2.3 7.1 4.0
1.2 1.2 2.1 2.1
8.2 14.5 18.1 25.2
distance SA/(pAcm-‘) 9.6 10.1 17.7 18.7
From TEM observation. b Calculated according to:
l
Catalyst after a
maintained for 2 h at the reduction temperature increase from room Hydrogen flow 151 h-‘.
temperature
temperature.
of functional groups at the surface of the support, while the particle size is essentially controlled by the reduction conditions. This is at contrary of what occurs in the case of the impregnation method when the size usually depends upon the loading. Table 2. Platinum
particle size comparison measuring techniques
using different
Technique TEM
co
H
Catalyst pt/wo/o
djnm
d/nm
d/nm
6.2 7.1’ 8.gb 11.6’
1.2 2.1 3.3 4.3
1.3 2.2 3.3 4.4
1.4 2.3 3.4 4.3
‘JJ and ‘: lst, 2nd and 3rd refill from d = 2 nm (see Table 1).
Fig. 1. TEM micrographs
of Pt/C powders
with different
X = J21.4nd3(100 - m)S,/(m x 6000) - d, where S, is the specific area (mZg-‘) of the carbon support and m the mass fraction of Pt in the catalytic powder.
The cationic exchange method has been previously described[5] but will be briefly resumed. A solution of tetramine platinous salt was added to a suspension of the carbon support in ammonia. The solid was then filtered to ensure that only the cations bounded to the support are kept, the excess being washed away. Thus an increase of the functional surface groups amount was necessary to obtain catalytic powders with a high enough platinum loading. This was achieved by liquid phase oxidation of the graphite powder by a NaOCl solution. After the ionexchange and the filtration, the catalyst was dried and reduced under H, flow. The, average size of the metallic particles were fixed by controlling the reduction conditions (temperature increase and H, flow) (Table 1).
Oxygen reduction on well-defined platinum nanoparticles
In order to obtain homogeneous distributions of particles, the refill method of Mezeno et al.[6] was used instead of thermal sintering to prepare larger particles. This method uses a surface redox reaction to coat the particles \kith a new layer of atom after each refill. An uniform growth of the particles, and a resulting homogeneous size distribution was thus obtained. The catalytic powders used for the study of the particle size effect (Table 2) were prepared by successive retills starting from the first powder on the list (6.2 wt% Pt), prepared by cationic exchange; they show homogeneous size distribution of increasing mean diameter. The catalytic powders used for the study of interparticle distance (Table 3) include two of the previous powders (6.2 and 7.1 wt% Pt) and two other powders (2.3 and 4.0wt% Pt) prepared by cationic exchange with loading below the exchange capacity limit and reduced under conditions required to obtain the desired particle size (ie 1.2 and 2.1 nm respectively).
Particle size (nm)
B
Particle size (nm)
C
2.3. Characterisation
Particle size (nm)
I
,
Fig 2. Distribution of particles sizes in Pt/C catalyst. The average particle size (dVA) is the volume-area mean diameter calculated from the distribution according to dVA = Zf~:/Xf~~. (A) dVA = 1.2nm; (B) 2.1 nm; (C) 3.3nm; (D) 4.4nm.
-6
! 0.1
309
of the catalytic powders
The particles of platinum were measured by TEM (Fig. 1). A suspension, in ethanol, of the catalytic powder was deposited on a microscope grid coated with an amorphous carbon film. The grid was observed using a JEOL 100 CX microscope. The mean particle size was obtained by measuring the diameter of a sutkient number of particles to ensure a good statistics. The surface mean diameter was calculated according to dVA = Z?jjd:jIAd:, where x is the frequency of occurrence of the particles of diameter d, in the sample, from the particle size distribution given in Fig. 2. The electrochemical active Pt surface area of the thin layer was measured by coulometries of hydrogen adsorption&sorption (0.05 to 0.4 V, 5 mV s- ‘) and oxidation of adsorbed CO (0.1 to 1.0 V; 5 mV s-i) in a N, purged 0.1 M H,SO, solution. For the smallest Pt loading, due to the electrochemical activity of the carbon substrate, the coulometry of hydrogen adsorption-desorption is difficult to accu-
I
0.5 0.9 0.7 E /V (RHE) Fig. 3. CO oxidation: voltammograms (5 mV s-l) for thin layer of Pt/C and Nafion with various particle sizes: 1.2 nm; --- 4.3 nm. Current densities related to the real area. 0.3
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-
d=1.2
nm - - - d=4.3
nm
-12, 0
0.4
0.8
1.2
I
E /V (RHE) Fig. 4. Reduction of adsorbed oxygen species as a function of Pt particle size: volgammograms (5mV SK’) under inert atmosphere for thin layers of Pt/C and Nafion (1.2 nm; --- 4.3nm). Current densities related to the real area. rately calculate, but the CO peaks are still well defined (Fig. 3). The measurement of Pt surface area in contact with Nafion and available for electrochemical reactions allows for the evaluation of the ratio: total platinum area/geometric area. Since the Pt loading of the layers was known, it was possible to estimate the specific catalyst area S (m’ g- ‘) and the average particle size d (nm). Assuming spherical particles: d = 6ooo/(21.4 s) where 21.4 is the platinum density (gcmm3). The particle sizes calculated by electrochemical measurement are close to those obtained by TEM observations (Table 2) which means that the overall area of the catalyst dispersed in the active layer is available for the electrochemical reaction. The reduction of the adsorbed oxygenated species at the platinum particle surface under inert atmosphere was also investigated. The voltammograms (Fig. 4) were recorded in the same conditions of scan rate and potential range as the oxygen reduction studies (see 2.4.); the voltammetric curves strongly depending on the scan rate (Fig. 5).
2.4. Oxygen reduction The oxygen reduction current-potential curves were obtained from slow scan voltammograms (1.3 to 0.3 V; 5 mVs_‘) at various rotation frequencies (250 to 2000rpm) in a 0.1 M H,SO, solution under 0.2 and 1 atm of oxygen. Before starting to record each current-potential curve the electrode was held at 1.3 V during 30s. Analogous current-potential curves were obtained when the starting potential was l.OV instead of 1.3 V. Starting at rather high potentials (es l.OV) is useful for comparisons with real fuel cells cathodes. For very thin active layers (z 1pm) the electrode is mostly under kinetic control[3] (ie the ohmic and the diffusion limitations within the active layer are negligible). For every rotation frequencies , the kinetic current density i was then calculated using: i = (ini,_J(iL.n - in); in being the measured current density using the geometric area and i,+ the corresponding limiting current density in solution.
3. RESULTS
AND DISCUSSION
The voltammograms under inert atmosphere of freshly prepared thin active layer are sometimes featureless during the first cycles. With continued cycling (0.05 to 1.3 V; lOOmVs_‘) the characteristic platinum waves rapidly develop and after 10 scans reproducible voltammograms are obtained. Some authors showed[7] that alternate anodiccathodic treatments change the surface structure of dispersed Pt and at the same time modify its electrocatalytic properties. No significant differences in the catalytic activities, for ORR and for oxidation of CO adsorbed, were however found here when the active layers were previously scanned between 0.05 to 1.3 V or between 0.05 to 1.0 V. Figure 6(A) shows the mass activity (MA) (kinetic current density/catalyst loading ratio) as a function of the Pt particle size for oxygen reduction reaction in Nafion. At contrary with previous results[l], no maximum in mass activity is observed between 1.2 and 4.4nm; MA tends to decrease slowly with increasing the particle size above 5 nm in diameter. Figure 6(B) shows the specific activity SA (kinetic current density/real catalyst area ratio) as a function of the Pt particle size for oxygen reduction reaction in Nafion. SA was measured at 0.85 V, 293 K and 1 atm. SA tends to decrease with decreasing particle size. Similar results were found in Nafion for Pt/Vulcan powders from E-TEK Inc.[4], but the decrease for the smaller particles was more pronounced leading to a maximum in the MA for particle sizes around 3.5 nm in diameter. In life test of polymer electrolyte fuel cells, little apparent current density loss was observed while the oxygen cathode area decreases considerably due to the particle sintering. This was explained by an increase of SA with the particle sizer 173. The loss of catalytic activity with decreasing the particle size, also observed in sulphuric and phosphoric media, was explained by Watanabe et a/.[23 by either diffusion or mutual interaction when the particles are too close together (<20nm). In this
Oxygen reduction on well-defined platinum nanoparticles
ri iv r-1 / pAcm-2
[mV
311
5-11-l
-1
Fig. 5. Voltammetric reduction of adsorbed oxygenated species as a function of scan rate (50; --5; .-.0.5 mV s- ‘) under inert atmosphere for thin layer of Pt/C and Nafion: ratios current densities/scan rate and cd. related to the real area; particle size 1.2 nm.
work, this effect was not observed even when the distance between the particles is ten times their diameter (Table 3). The local current density is anyway much lower than the spherical limiting current density (i + i,, sphcr) so, the diffusion and/or the shielding effect of inter-particle distance should be accounted for when the particles are very close together (ie, between about one and five times their diameter). Moreover, a possible shielding effect of the catalyst support is probably more important when the particle sizes are smaller than 2 nm because these particles are probably spherical and/or can be situated in nanopores but this effect is much less pronounced for graphite platelets (eg, HSAG 300) than for carbon black (eg, Vulcan XC72) of ill-defined shape. Calculations are currently developed in our
laboratory for these various local conditions but the preliminary results show they cannot explain the main size effect. A loss of catalytic activity was also found in gas phase catalysis under oxygen atmosphere: the deactivation due to a strong chemisorption of oxygen atoms at the platinum surface is more pronounced when the particle size are smaller than 3.0nm[9]. Some authors[lO] have allocated this deactivation to the irreversible formation, under oxygen atmosphere, of PtO, species, but this assumption seems not realistic under the electrochemical conditions used for ORR at Pt/Nafion interface. A strong deactivation due to the over-oxidation of Pd was also observed for particles smaller than 2.5 nm in liquid phase oxidation of glucose[ 111.
A. GAMEZ et al.
312
.
40
20
SA @4
I
.
m-2,
. .
.
20 i
10
,
where i is the kinetic current density per real catalyst area, AG# the chemical part of the activation energy at low coverages, t-0 represents the change of adsorption energy of reaction intermediates with total coverage 8. This change of adsorption energy with the particle size might be due to the existence of specific adsorption sites at the surface of the platinum particles[l] or to electronic effects[12]. However, the differences in energy levels of the valence electrons will eventually become appreciable when nearly all atoms in the particles are surface atoms (ie, for particle sizes smaller than lOnm)[13] which is not the case here. Figure 4 shows that, under inert atmosphere, the amount of oxygenated species still adsorbed, at any potential above 0.8 V (ie, Temkin isotherm), is larger when the particle size decreases. According to equation (I), this implies a diminution of 1i 1 and then a loss of specific activity (SA = (i1at 0.85 V). Figure 7 validates this result: the change of the Tafel slope (Temkin to Langmuir isotherm) takes place at 0.8 V for very small particles (1.2 nm) and at 0.87V for bulk platinum. During oxygen reduction, the oxygenated species are then more strongly adsorbed on the nanoparticle surface than on the bulk platinum surface. According to the Damjanovic theory[S] for electroreduction of oxygen at prereduced bulk platinum surface in acid media the oxygen adsorption is fast, the following step:
.
t
n
Fig. 6. (A) Mass activity MA for oxygen reduction at 0.85 V @he), 1 atm. and 293 K, as a function of the Pt particle size; (B) Specific activity SA for oxygen reduction at 0.85 V (rhe), 1 atm. and 293 K, as a function of the Pt particle size.
In this work, the SA decrease for the smallest particles might be correlated to a stronger adsorption of the oxygenated species. For high coverage 0 of oxygenated species (ie, Temkin isotherm), with any electrochemical rate determining step, we will find a kinetic equation as : -i
cc exp -
AG’ + are + j3FE RT
H+ + e- -
1
products
(2)
1 (1)
d = 1.2
--
5 ’ z
+
is rate determining and the other steps (ie, reduction of O,d and OH,,) are under quasi-equilibrium. For example, under Temkin conditions of adsorption (in the 60mV dec- ’ region) the reaction rate is given
1
_ -3.00 NI $
r.d.s. (o,),,
nm
bulk
-4.00-
$ rl -5.00-
-6.00 !
0.6
0.7
0.8 E /V
0.9
b
I 1
(NE)
Fig. 7. Tafel plots for oxygen reduction (293 K, 1 atm) at various Pt/Nafion interfaces. Current densities related to the geometrical area. Roughness factor: 2.1 for bulk Pt and around 10 for 1.2nm particles, leading to real area current densities 3.5 times larger for the bulk Pt in 0.85 V potential region (cf: Fig. 6 and [4]).
313
Oxygen reduction on well-defined platinum nanoparticles
by: -i
a
P~~[H+]~‘*
exp z [
1
4. CONCLUSION (3)
For oxygen reduction, a loss of catalytic activity with the decreased platinum particle size is confirmed in the well-defined condition allowed by:
As expected from this r.d.s., the reaction order of oxygen at bulk Pt/Nafion interface was determined to be unity[14]. The same result was obtained for not too small Pt particles in Nafion[lS]. The adsorption energy of oxygenated species is however larger at the nanoparticle surface than at the bulk platinum surface and then the first electrochemical step could not be the rate determining step. This would also implies a different ORR mechanism for very small nanoparticles. Considering moreover non-prereduced electrodes enhances the effect of these oxygenated species. To clarify these problems, the effect of proton concentration inside recast ionomer (Nafion type) is not easy to measure, but the oxygen partial pressure effect on the reaction rate for Pt nanoparticles was investigated using two partial pressures (1 and 0.2 atm.). For the rather high potential range (eg, 0.8, 0.9V), the measured current densities at very small particles (1.2nm)/Nafion interface seems to be proportional to (po2)xwith 0.3 < x -=z0.7 (Fig. 8) and not to poI as expected from the r.d.s. (2). In our opinion, depending on the oxygenated species coverage (ie, on the pretreatment and the potential), the r.d.s. for ORR on very small nanopartitles could be the step (2), or the reduction of Oa,, or OH,, or both (ie, two r.d.s. steps, with negligible backward term, could be considered for ORR, like for the simpler HER). Anyway ORR experiments with prereduced nanoparticles, which provide higher current density than with non-prereduced particles, present the same difflculties :
(a) the kinetic measurement procedure, (b) the utilisation of homogeneous and controlled particle size and (c) utilisation of graphite powders for the catalyst support which reduces the possibility ing effect of the support.
Alternate anodic-cathodic treatments from 0.05 to 1.3 V or from 0.05 to 1.0 V led to no significant variations of the catalytic activities. No effect of the inter-particles distance was found when the particle size is 1.2nm and the particles are about lOnm apart. This activity loss is correlated to the stronger adsorption of oxygenated species on very small particles. Acknowledgements-The authors gratefully acknowledge the ADEME and ECOTECH (VPE Program) for their support.
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(a) c.d. is not proportional to po, and (b) Pt oxidation states deduced from XANES studies are slightly higher under ORR than under inert atmosphere[ 161; it is therefore difficult to accept a quasi-equilibrium for the last steps.
I -3.8 -
+
of a shield-
0.2
atm
+
1.0
atm
\
I
-5.8 0.7
0.74
0.82
0.78
E
/V
0.86
I
(RHE)
Fig. 8. Effect of 0, partial pressure (1 and 0.2 atm.) on the Tafel plots for ORR at 293 K. Current densities related to the real area. Pt particle size: 1.2 nm.
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13. M. Che, C. 0. Bennett, in Advances in Catalysys, Academic Press, New York, 36, 55 (1989). S. Srinivasan, A. J. Appleby, C. R. 14. A. Parthasarathy, Martin, J. electrochem. Sot., 139,2856 (1992). 15. S. Mukerjee, S. Srinivasan, J. MC Breen, E.C.S. Spring Meet. Abstr., San Fransisco, p. 1288 (1994). and toll., Oxygen Electrochemistry Symp., 16. R. Durand E.C.S. Fall Meet., Chicago (1995). 17. M. S. Wilson, F. H. Garzon, K. E. Sickfacus, S. Gottesfeld, J. electrochem. Sot. 140,2872 (1993).