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C catalyst from carbonyl complexes for fuel cell applications
Electrochimica Acta 47 (2002) 3733 /3739 www.elsevier.com/locate/electacta
Preparation of a Pt Ru/C catalyst from carbonyl complexes for fuel cell applications /
A.J. Dickinson a,*, L.P.L. Carrette b, J.A. Collins c,*, K.A. Friedrich d,*, U. Stimming c a
Department of Chemistry, Loughborough University, Ashby Road, Loughborough, Leicestershire LE11 3TU, UK b Advanced Materials Editorial Office, Wiley-VCH, Pappelallee 3, 69496 Weinheim, Germany c Physik Department E19, Technische Universita¨t Mu¨nchen, James-Frank-Straße, 85748 Garching, Germany d Center for Solar Energy and Hydrogen Research Baden-Wuerttemberg, Helmholtzstrasse 8, D-89081 Ulm, Germany Received 1 March 2002; received in revised form 5 May 2002
Abstract A new method for producing highly active, carbon supported, Pt /Ru catalysts for methanol oxidation is presented. The catalyst is produced from carbonyl metal complexes by deposition of the precursors on carbon in a high boiling-point solvent, and a simple method of producing the necessary Pt carbonyl complex is also reported. Well distributed PtRu catalysts with high dispersion and narrow size distribution are obtained. Comparison of the catalyst produced by this method against commercial and in-house sulphito route catalysts show that it has a promising activity as well as presenting an easier route to ternary and quaternary catalysts. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Direct methanol fuel cell; Methanol electrooxidation; Fuel cell catalysts; Platinum, ruthenium, polymer electrolyte membranes; Membrane electrode assembly
1. Introduction Although fuel cells are by no means new devices, the interest in fuel cell research has increased significantly over the last decade due to higher environmental awareness, promising results obtained by several research laboratories and the prospect of using these devices for a wide range of applications. Catalysts used in low temperature fuel cells are generally based on highly dispersed platinum on an inert support, which is electronically conducting. As anode catalysts, PtRu alloy particles on carbon support are momentarily the state-of-the-art catalyst for reformate and direct methanol polymer electrolyte fuel cells. The fabrication of these catalysts has been one of the major subjects of investigations carried out to improve performance and catalyst utilization as well as lowering the costs of fuel cells. One of the major factors limiting the practical * Corresponding authors E-mail addresses: [email protected] (A.J. Dickinson), [email protected] (J.A. Collins), [email protected] (K.A. Friedrich).
development of direct methanol fuel cell is the insufficient performance of the electrocatalysts. This is especially true for the anode catalyst where few electrode materials have been shown to be capable of oxidizing methanol in acidic media, and of these only Pt-based materials display a high enough stability and activity to be attractive as catalysts. Much work on the oxidation of organic molecules and specifically on methanol oxidation have been carried out over the last decades with respect to finding new catalyst compositions. A new interest in fuel cells has developed in an effort to produce commercially viable high energy density power sources that are portable and environmentally friendly [1 /4]. The properties of the fuel cell anode have received considerable attention, with efforts being focused on the development of an improved understanding of the reaction pathways associated with the oxidation of hydrocarbon fuels such as methanol and formic acid [5 /8], and the fabrication of poison-resistant and highly active anode materials [9 /11]. Recent advances include the discovery of an ‘optimized’ multicomponent catalyst for methanol oxidation [12,13], construction of high surface area and poison-resistant
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 3 4 3 - 2
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nanoscale anodes [14,15], and the development of a detailed understanding of the role of crystal orientation and composition on reactivity and poisoning of numerous binary catalyst compositions [9,16/18]. Since on pure platinum, methanol oxidation is strongly inhibited by poison formation, the study of bimetallic catalysts is of widespread interest because the addition of a second metal often improves the activity and selectivity of metallic catalysts for low-temperature fuel cell applications. Binary Pt alloys such as Pt /Ru or Pt /Sn have been suggested as catalyst materials for the fabrication of polymer electrolyte membrane (PEM) fuel cell anodes with increased CO tolerance since the early 1970s [19,20]. Particular interest has focused on the application of these materials as anodes in methanol fuel cells for electric vehicles, with much research into the structure, composition and mechanism of their catalytic activity, being undertaken both at a fundamental and applied level [21 /23]. Presently, binary Pt /Ru catalyst materials for methanol oxidation are prepared and tested in diverse forms: Pt /Ru bulk alloys [9,11,16,24/ 31], Ru electrodeposits on Pt [32,33], Ru adatom or Ru nanoparticle modified polycrystalline Pt [34,35] and Pt single crystal electrodes [24,25,32,33,36 /38], carbon supported Pt /Ru alloys derived from electrochemical codeposition [23,39,40] and Ru adsorbed on Pt [41]. Several studies demonstrated that the alloyed Pt/Ru material with a composition of Pt:Ru /1:1 exhibits the highest activity for CO oxidation [9,39,42]. It is worth noting that despite the diversity of methods for catalyst preparation, all these materials present an enhanced activity towards methanol oxidation. Although the enhancement effect of Ru on methanol oxidation has been well known for decades and has long been considered in the development of fuel cells, many details concerning the enhancement in catalytic activity, especially concerning the mechanisms on the atomic scale, are not yet well understood. Apart from the electronic effect of Ru on the bond strengths of the adsorbates [43], a bi-functional mechanism is considered to be responsible for the enhancement effect [26]. The latter effect involves the adsorption of some oxygencontaining species on ruthenium atoms at, compared with platinum, lower potentials. This species, in turn, is necessary for the oxidation of intermediates such as CO or COH. By this means, the onset of methanol oxidation is shifted from around 450 mV for pure platinum catalysts down to 250 mV for Pt/Ru electrodes (vs . RHE). This can easily be verified by electrochemical online mass spectroscopy (DEMS), which allows the detection of the onset of CO2 formation at the respective potentials [44]. The preparation of supported catalysts can be carried out by numerous methods. A common method is the reduction of metal chloride salts, which is a simple and straightforward chemical preparation technique
although it can lead to significant amounts of chloride poisoning. Another chemical preparation technique is based on the oxidation and subsequent reduction of metal sulphite salts, which can be prepared from the chloride metal salts. This preparation method has an advantage over the straight reduction of chloride salts in the fact that no chloride ions are present during the deposition of the metals onto the support. It has been shown that the presence of chloride ions during the deposition can have a negative effect on the later performance of the catalysts for methanol oxidation. A negative point of the sulphito method is that the preparation requires more time and is complicated, and the number of sulphito salts with known preparations is limited. It is also possible to prepare catalysts by an electrochemical reduction which can give an insight into the effect of size distribution and/or the chemical structure of the support on the performance of the catalyst, although this technique is severely limited in the amount of catalyst that can be prepared, and the particle sizes obtainable by such electrochemical methods, approximately 300 /500 nm, have been shown to be non-optimal for methanol oxidation [45]. A new way of preparing a wide range of Pt based catalysts could lie in the methodology presented in this paper. A well known method of preparing Pt carbonyl complexes has been modified and simplified to produce a carbonyl complex which can be used as a precursor to make Pt based catalysts [46,47]. The method involves the oxidation of a metal chloride salt with carbon monoxide, which easily leads to the formation of a carbonyl complex. This complex is then thermally decomposed to form the catalyst. Previously, a Pt catalyst, of high surface area has been obtained by Machida et al. by thermal decomposition of appropriate high molecular weight Pt carbonyl compounds [48,49]. But, this catalyst was unsupported, unlike the present carbon supported catalyst. Another unique aspect of the present work, is that it is a binary Pt /Ru catalyst which is prepared, the first such case from carbonyl precursors, that we are aware of. Apart from the composition of a catalyst surface, another important aspect is its structure. Many surface reactions require suitable atomic configurations at the surface, which allow the reactants to be in an appropriate binding situation and geometric orientation towards each other. Thus the influence of surface structure on catalytic activity cannot be overemphasized, for example which local Pt /Ru configuration is the best for effective complete methanol oxidation. This paper outlines initial experiments showing the effect of composition and structure of a catalyst prepared in a novel manner, on catalytic activity. Thus, although the importance of the fact that a binary Pt /Ru catalyst can produce a noticeable enhancement of current for
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methanol oxidation at low potentials was not ignored in the field of fuel cell research, it has taken more than three decades for the possibility of technically using the PtRu /CH3OH systems to be recognized. Much of the data presented here is not only significant for increasing the understanding of methanol oxidation mechanisms on mixed-metal, catalytic electrodes, but also furthers the understanding of noble metal deposition processes.
2. Experimental A Pt carbonyl complex was prepared by purging CO through an aqueous solution of Chloroplatinic acid (H2PtCl6) (10 mg cm 3) for 24 h with constant mechanical stirring. The solution initially had an orange color, which became cherry red with the formation of the Pt(CO)2 precipitate. At the end of the 24 h, a dark colored precipitate had formed. This was then filtered, followed by drying under a flowing CO atmosphere. Once dried the Pt carbonyl became a black crystalline solid, which was highly hygroscopic. Normally the filtrate was still slightly colored, in this event the solution was re-purged with CO causing more Pt carbonyl to form until a yield near to 100% was achieved, indicated by a colorless solution. A sample of the precipitate dissolved in Tetra Hydro Furan (THF) was utilized to determine the structure of the [Pt3(CO)3(m2-CO)3]2 dianion present, using Fourx ier Transform Infra Red (FTIR) spectroscopy. The IR absorption measurement was performed using a FTS6000 spectrometer from BioRad. All functions are remote-controlled by the WINIR software installed on a PC, thus supplying a user interface. The spectrometer was configured for the mid infrared range from 4000 to 400 cm 1 with a standard ceramic IR source, and a KBr beam splitter. A liquid nitrogen cooled EG&G MCT (mercury cadmium telluride) IR detector was used. The solution was used to form a thin film between two CaF2 plates and using a p-polarized input beam, the IR spectra were recorded with the spectrometer in rapid scan mode, i.e. the moving mirror of the Michelson interferometer is driven continuously and data is sampled at points of zero intensity of the HeNe laser signal. The beam traversed a polarization filter consisting of 0.12 mm wide strips of aluminium on a KRS-5 substrate. The filter can be set so as to polarize the light either parallel or perpendicular to the reflection plane (in this case perpendicular). The spectral resolution (Dn ) was set to 4 cm 1. In this configuration, the acquisition of one scan takes about 0.3 s, although it is necessary to average several scans to obtain a sufficient signal-tonoise ratio, typically 400, giving the final spectrum. The spectrometer is continuously purged with dry air, to protect the water-soluble KBr beam splitter and to
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remove moisture that would generate H2O bands in the spectra obtained. The Pt/Ru catalyst was prepared in the following manner: 216 mg of [Ru3(CO)12], 254 mg of [Pt(CO)2]x and 300 mg of Vulcan XC-72R carbon black were mixed with 300 ml of o-Xylene in a round bottom flask. This mixture was then refluxed at 143 8C for 24 h with constant mechanical stirring. The mixture was then allowed to cool to room temperature before the oXylene was removed using a rotary evaporator. Once separated the catalyst was allowed to dry under air for 7 days. Initial experiments were carried out under an Ar atmosphere, however, the catalysts thus obtained displayed very poor performances, probably due to a poor deposition of the Pt. In order to improve the deposition of Pt, all future refluxing was carried out under air in order to maximize the decomposition of the Pt carbonyl complex. Once drying had been completed, 100 mg of the catalyst powder was mixed with 353 mg Nafion solution (5% in aliphatic alcohols, Du Pont) and then diluted with a 1:1 mixture of MilliporeTM water (18 MV) and 2Propanol (Merck). The resulting ink was then placed in an ultra-sonic bath until the catalyst powder had fully dispersed, approximately 15 min. This ink was then sprayed onto a piece of TGPH-060 Toray carbon paper which was kept at 130 8C throughout the process. The spraying machine used consisted of a motorized X /Y table controlled by a CNC Automation Controller C116-4 isel† (Conrad electronics) which also regulated the flow of the catalyst suspension through a Walther Pilot Domino nozzle. The controller was run by an inhouse written computer program that ensured an even distribution of the catalyst on the substrate, this program allowed for the spraying of square electrode sheets of variable size. A 12 mm diameter disk was mechanically punched out of the electrode sheet and placed into a Kel-F† electrode holder and was contacted to the rear by a gold wire. A silicon rubber washer was used to seal the electrode into the holder and it also controlled the geometrical area of the electrode exposed to the electrolyte at 0.5 cm2. The design of the electrode holder allowed for a stream of Ar gas to be passed around the back of the electrode. In turn, the holder was placed in a water jacketed three-electrode glass cell fitted with a Luggin capillary contacting the reference electrode (a Mercury/Mercury Sulfate (MMS) from Sentek-UK Ltd.) which was positioned outside the cells thermal jacket to maintain isothermal operation at room temperature even when the rest of the cell was heated. The counter electrode was a Pt gauze positioned at the opposite side of the cell to the working electrode. The cell design facilitated the purging of the electrolyte with Argon during operation.
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The electrodes were tested for methanol oxidation in an Argon saturated electrolyte consisting of 1.5 M CH3OH (methanol) and 1 M H2SO4 at 25, 45 and 65 8C. The half cell used was unsuitable for operation above 65 8C as methanol evaporation occurred at to high a rate. Cyclic voltammetry and polarization data were collected using a HEKA PG310 Potentiostat/ Galvanostat system. The cycles were run between / 650 and 850 mV versus MMS (Mercury Mercurous Sulfate) for methanol oxidation at a scan rate of 20 mV s 1. Polarization data were collected with the cell under galvanostatic control and were carried out manually. Each value was collected after allowing the electrode to reach a steady-state value, usually this occurred within 3 /5 min. Two other catalysts were also tested in the cell to act as controls. The first was an in house produced 50% w/o Pt /Ru/C with a 1:1 atomic ratio prepared using a sulphito route. The second was a commercial catalyst obtained from E-Tek Inc., 40% w/o Pt /Ru/C with a 1:1 atomic ratio. Transmission electron microscopy (TEM) was carried out on the Pt/Ru/C catalyst, prepared via the carbonyl route, with a small quantity of the supported catalyst dusted onto a standard Cu grid (400 mesh) covered with a carbon film (Plano). Applying the catalyst in this manner, instead of in solution, avoids the formation of aggregates due to uncontrolled agglomeration, during the evaporation of the solution droplet. Thus any aggregates observed have originated from the initial catalyst preparation. The TEM micrograph was recorded in the bright-field mode on a PHILIPS CM 20 (University of Ulm), with a point resolution of 0.23 nm. The negative images with a magnification of 600 000 were magnified photographically on the print by a factor of 3.5 and digitized with a scanner. After further magnification, 258 particles from three images could be analyzed and the distribution of the particle sizes was evaluated. EDX analysis was performed with an Inca 3000 Oxford Instruments detector. Reference PtRu/C samples of known composition were used to ascertain the validity of the results.
3. Results and discussion An FTIR spectrum of the platinum carbonyl complex dissolved in THF is shown in Fig. 1. Platinum carbonyl cluster dianions have the general formula [Pt3(CO)3(m2CO)3]2 with x /2,3,4,5 [46]. Infrared solution spectra x in THF for each of these species are known from the literature [46]. The spectra show an expected lowering of terminal and bridging carbonyl frequencies upon successive reduction of the dianions II /V. From Fig. 1, the very strong band at 2056.4 cm 1, as well as the strong band at 1869.6 cm 1 and weak bands at 1889.9 and
1825.5 cm 1 are suggestive of the presence of the complex with x /5, [Pt15(CO)30]2 while the very strong band at 2040.4 cm 1, the strong band at 1844.5 cm 1 and weak band at 1830.4 cm 1, imply the presence of the complex with n /4, [Pt12(CO)24]2 . Thus it is likely that a mixture of these two dianions is present in the THF /Pt carbonyl complex solution. This is also shown by the blue /green color of the solution obtained, which is indicative of the latter complex [46,47]. In Fig. 2 a typical TEM image of the carbonyl prepared catalyst is shown with the corresponding size distribution obtained from the evaluation of 258 particles from three images (all particles in the images which could be discerned clearly were used for the analysis). As can be seen the distribution of the particles is homogenous on the support and the catalyst is well dispersed, consisting of particles averaging 2.5 nm with a standard deviation of 9/0.45 nm. The smallest particle detected has a size of about 1.6 nm whereas the largest one in the three images is about 4 nm. From XRD measurements larger particle sizes according to the Scherrer formula were derived, however, XRD is sensitive to the volumetric distribution of the particles, and therefore, a small number of large particles can lead to differences compared with TEM. A detailed analysis and comparison of the different methods is in preparation. From the EDX analysis of the sample it is derived that the Ru deposition is more effective compared with the Pt deposition since a high weight percentage of Ru was found. The ratios of Pt to Ru in the Xylene solution were chosen in order to aim for a 50:50 atomic ratio. However, values in the range of 62/67 atm.% Ru and correspondingly 38/33 atm.% Pt were obtained from different EDX measurements on the sample. This indicates that the Pt carbonyl decomposition is still inhibited with respect to the Ru carbonyl decomposition at the temperatures used. Therefore, the deposition properties and yields of the different carbonyls have to be taken into account in the catalyst synthesis of PtRu and of the supported catalyst compositions. In summary, this catalyst preparation procedure results in homogenously distributed small (2.5 nm) PtRu particles on the support with a narrow size distribution. Due to the decomposition properties the catalyst is Ru-rich. Fig. 3 shows the I /V curves collected for the carbonyl prepared catalyst as a function of temperature. Increasing the temperature increases both the maximum current density obtainable and lowers the potential obtained at a given current density, both features indicative of an effective electrocatalyst. It can be seen that there is a far greater increase in performance on going from 25 to 45 8C than there is going from 45 to 65 8C. This is to be expected, since the improvement in methanol oxidation kinetics is not linear with temperature. Since the 65 8C offers the highest performance the
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Fig. 1. IR spectrum of the Pt carbonyl complex in THF.
results taken at this temperature are discussed in more detail in this paper. In Fig. 4, the performance of the carbonyl prepared catalyst compared with a sulphito preparation and a standard catalyst obtained from E-Tek Inc, is illustrated. The results presented have been normalized for the different catalyst loadings present on the electrodes tested. These results clearly show a cross over point for all three catalysts. At low currents (i.e. low over-potentials) the E-Tek catalysts displays the
best performance while the carbonyl catalyst appears the worst of the three. An inferior performance of the carbonyl catalyst is not surprising since the PtRu atomic ratio of 33:67 is sub-optimal for methanol oxidation. Most studies on dispersed catalysts have found that a 1:1 atomic ratio exhibited best performance [52]. Furthermore, the published studies agree that a Rurich catalyst shows inferior performances [52]. However, the trend of inferior performances at low potentials for the carbonyl catalyst is reversed at values above /145
Fig. 2. TEM image of Pt/Ru catalyst prepared via carbonyl route. The dark features correspond to the Pt/Ru catalyst particles. Insert: Size histogram for the primary particles. The histogram is obtained after counting 258 particles from TEM micrographs recorded with a magnification of 600 000.
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Fig. 3. I /V curves for 50% w/o Pt /Ru/C prepared by the carbonyl route, taken at 25, 45 and 65 8C in 1 M H2SO4 and 1.5 M MeOH.
Fig. 4. Comparision of the mass activity curves for a 50% w/o Pt/Ru/ C prepared by the carbonyl route, a 50% w/o Pt/Ru/C prepared by the sulphito route and a 40% w/o Pt/Ru/C from E-Tek at 65 8C in 1 M H2SO4 and 1.5 M MeOH.
Ag 1 of metal. Recent work has suggested that there is an optimal particle size for CO oxidation, producing particles below this size will cause an increase in the potential needed for the oxidation process to occur since the adsorption of oxygen-containing species is strongly inhibited [50]. The interpretation proposed in this paper is that the particle size obtained for the in house catalysts is below this optimal size, however, at high potentials CO oxidation is not the only rate determining step, and therefore, the activity becomes dominated by a surface area effect. This would explain the crossover of the results. The electrochemically active surface area (EASA), obtained for each electrode by CO stripping supports this interpretation, see Table 1. The EASA is calculated assuming the targetted amount of metal is present. From Table 1, it can be seen that the E-Tek Table 1 EASAs measured by CO stripping for each of the catalysts tested. Catalyst
Symbol
EASA (m2g 1)
E-Tek Sulphito Carbonyl
j k
22.3 26.3 27.7
Fig. 5. Comparision of the activity of a 50% w/o Pt/Ru/C prepared by the carbonyl route, a 50% w/o Pt /Ru/C prepared by the sulphito route and a 40% w/o Pt /Ru/C from E-Tek at 65 8C in 1 M H2SO4 and 1.5 M MeOH, normalised for the electrochemical active area.
catalyst exhibits the lowest EASA, while the carbonyl prepared catalyst is the highest. Fig. 5 shows the I /V response data for all three catalysts normalized for their EASA. It is interesting to note that the performance of both in-house catalysts are very similar, with only small differences being apparent at either low or high current values. This would seem to indicate that the two catalysts are very similar in nature to each other although the PtRu ratio of the carbonyl catalyst is sub-optimal. The differences at either end of the current range could be due to particle size effects [51]. At high currents it would seem to suggest that the smaller particles behave in a superior manner toward methanol oxidation. However, this result shows that the sulphito method and the carbonyl method produce catalysts with very similar surface activities. The poorer performance of the E-Tek catalyst would seem to indicate that it has a less active surface composition than the in-house prepared catalysts, possibly due to a slight poisoning of the surface by impurities.
4. Conclusions The work presented in this paper has shown a simple method of producing a reproducible Pt carbonyl complex from chloroplatinic acid. The catalyst is prepared by decomposing the carbonyls in a high boiling-point solvent. This catalyst preparation procedure results in homogenously distributed small (2.5 nm) PtRu particles on the support with a narrow size distribution (0.5 nm). From the chemical composition of the catalyst (ratio of Pt:Ru 33:67) as derived from EDX it is concluded that the deposition of the Pt carbonyl is inhibited. It has also been shown that this carbonyl complex can be combined with a Ru carbonyl compound to produce supported Pt /Ru catalyst for methanol oxidation in a simple one step preparation. A catalyst produced by this novel method has an activity at least equivalent to that of a catalyst produced by the conventional sulphito proce-
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dure in spite of a sub-optimal PtRu ratio, but offers a much easier route to ternary and quaternary anode catalysts.
Acknowledgements Financial support by VDMA Gesellschaft for Research and Innovation (VFI GmbH) and partners is gratefully acknowledged. Part of this research has also been supported by a Marie Curie Fellowship of the European Community programme, ‘Energy, Environment and Sustainable Development’, under contract number ERK5-CT-1999-50002. The authors are grateful to Dr. Banhart, University of Ulm, for recording the TEM micrograph. Furthermore, the authors thank Professor R. Hiesgen, Fachhochschule Esslingen, for recording of EDX spectra and for helpful discussions.
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Preparation of a Pt Ru/C catalyst from carbonyl complexes for fuel cell applications /
A.J. Dickinson a,*, L.P.L. Carrette b, J.A. Collins c,*, K.A. Friedrich d,*, U. Stimming c a
Department of Chemistry, Loughborough University, Ashby Road, Loughborough, Leicestershire LE11 3TU, UK b Advanced Materials Editorial Office, Wiley-VCH, Pappelallee 3, 69496 Weinheim, Germany c Physik Department E19, Technische Universita¨t Mu¨nchen, James-Frank-Straße, 85748 Garching, Germany d Center for Solar Energy and Hydrogen Research Baden-Wuerttemberg, Helmholtzstrasse 8, D-89081 Ulm, Germany Received 1 March 2002; received in revised form 5 May 2002
Abstract A new method for producing highly active, carbon supported, Pt /Ru catalysts for methanol oxidation is presented. The catalyst is produced from carbonyl metal complexes by deposition of the precursors on carbon in a high boiling-point solvent, and a simple method of producing the necessary Pt carbonyl complex is also reported. Well distributed PtRu catalysts with high dispersion and narrow size distribution are obtained. Comparison of the catalyst produced by this method against commercial and in-house sulphito route catalysts show that it has a promising activity as well as presenting an easier route to ternary and quaternary catalysts. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Direct methanol fuel cell; Methanol electrooxidation; Fuel cell catalysts; Platinum, ruthenium, polymer electrolyte membranes; Membrane electrode assembly
1. Introduction Although fuel cells are by no means new devices, the interest in fuel cell research has increased significantly over the last decade due to higher environmental awareness, promising results obtained by several research laboratories and the prospect of using these devices for a wide range of applications. Catalysts used in low temperature fuel cells are generally based on highly dispersed platinum on an inert support, which is electronically conducting. As anode catalysts, PtRu alloy particles on carbon support are momentarily the state-of-the-art catalyst for reformate and direct methanol polymer electrolyte fuel cells. The fabrication of these catalysts has been one of the major subjects of investigations carried out to improve performance and catalyst utilization as well as lowering the costs of fuel cells. One of the major factors limiting the practical * Corresponding authors E-mail addresses: [email protected] (A.J. Dickinson), [email protected] (J.A. Collins), [email protected] (K.A. Friedrich).
development of direct methanol fuel cell is the insufficient performance of the electrocatalysts. This is especially true for the anode catalyst where few electrode materials have been shown to be capable of oxidizing methanol in acidic media, and of these only Pt-based materials display a high enough stability and activity to be attractive as catalysts. Much work on the oxidation of organic molecules and specifically on methanol oxidation have been carried out over the last decades with respect to finding new catalyst compositions. A new interest in fuel cells has developed in an effort to produce commercially viable high energy density power sources that are portable and environmentally friendly [1 /4]. The properties of the fuel cell anode have received considerable attention, with efforts being focused on the development of an improved understanding of the reaction pathways associated with the oxidation of hydrocarbon fuels such as methanol and formic acid [5 /8], and the fabrication of poison-resistant and highly active anode materials [9 /11]. Recent advances include the discovery of an ‘optimized’ multicomponent catalyst for methanol oxidation [12,13], construction of high surface area and poison-resistant
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 3 4 3 - 2
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nanoscale anodes [14,15], and the development of a detailed understanding of the role of crystal orientation and composition on reactivity and poisoning of numerous binary catalyst compositions [9,16/18]. Since on pure platinum, methanol oxidation is strongly inhibited by poison formation, the study of bimetallic catalysts is of widespread interest because the addition of a second metal often improves the activity and selectivity of metallic catalysts for low-temperature fuel cell applications. Binary Pt alloys such as Pt /Ru or Pt /Sn have been suggested as catalyst materials for the fabrication of polymer electrolyte membrane (PEM) fuel cell anodes with increased CO tolerance since the early 1970s [19,20]. Particular interest has focused on the application of these materials as anodes in methanol fuel cells for electric vehicles, with much research into the structure, composition and mechanism of their catalytic activity, being undertaken both at a fundamental and applied level [21 /23]. Presently, binary Pt /Ru catalyst materials for methanol oxidation are prepared and tested in diverse forms: Pt /Ru bulk alloys [9,11,16,24/ 31], Ru electrodeposits on Pt [32,33], Ru adatom or Ru nanoparticle modified polycrystalline Pt [34,35] and Pt single crystal electrodes [24,25,32,33,36 /38], carbon supported Pt /Ru alloys derived from electrochemical codeposition [23,39,40] and Ru adsorbed on Pt [41]. Several studies demonstrated that the alloyed Pt/Ru material with a composition of Pt:Ru /1:1 exhibits the highest activity for CO oxidation [9,39,42]. It is worth noting that despite the diversity of methods for catalyst preparation, all these materials present an enhanced activity towards methanol oxidation. Although the enhancement effect of Ru on methanol oxidation has been well known for decades and has long been considered in the development of fuel cells, many details concerning the enhancement in catalytic activity, especially concerning the mechanisms on the atomic scale, are not yet well understood. Apart from the electronic effect of Ru on the bond strengths of the adsorbates [43], a bi-functional mechanism is considered to be responsible for the enhancement effect [26]. The latter effect involves the adsorption of some oxygencontaining species on ruthenium atoms at, compared with platinum, lower potentials. This species, in turn, is necessary for the oxidation of intermediates such as CO or COH. By this means, the onset of methanol oxidation is shifted from around 450 mV for pure platinum catalysts down to 250 mV for Pt/Ru electrodes (vs . RHE). This can easily be verified by electrochemical online mass spectroscopy (DEMS), which allows the detection of the onset of CO2 formation at the respective potentials [44]. The preparation of supported catalysts can be carried out by numerous methods. A common method is the reduction of metal chloride salts, which is a simple and straightforward chemical preparation technique
although it can lead to significant amounts of chloride poisoning. Another chemical preparation technique is based on the oxidation and subsequent reduction of metal sulphite salts, which can be prepared from the chloride metal salts. This preparation method has an advantage over the straight reduction of chloride salts in the fact that no chloride ions are present during the deposition of the metals onto the support. It has been shown that the presence of chloride ions during the deposition can have a negative effect on the later performance of the catalysts for methanol oxidation. A negative point of the sulphito method is that the preparation requires more time and is complicated, and the number of sulphito salts with known preparations is limited. It is also possible to prepare catalysts by an electrochemical reduction which can give an insight into the effect of size distribution and/or the chemical structure of the support on the performance of the catalyst, although this technique is severely limited in the amount of catalyst that can be prepared, and the particle sizes obtainable by such electrochemical methods, approximately 300 /500 nm, have been shown to be non-optimal for methanol oxidation [45]. A new way of preparing a wide range of Pt based catalysts could lie in the methodology presented in this paper. A well known method of preparing Pt carbonyl complexes has been modified and simplified to produce a carbonyl complex which can be used as a precursor to make Pt based catalysts [46,47]. The method involves the oxidation of a metal chloride salt with carbon monoxide, which easily leads to the formation of a carbonyl complex. This complex is then thermally decomposed to form the catalyst. Previously, a Pt catalyst, of high surface area has been obtained by Machida et al. by thermal decomposition of appropriate high molecular weight Pt carbonyl compounds [48,49]. But, this catalyst was unsupported, unlike the present carbon supported catalyst. Another unique aspect of the present work, is that it is a binary Pt /Ru catalyst which is prepared, the first such case from carbonyl precursors, that we are aware of. Apart from the composition of a catalyst surface, another important aspect is its structure. Many surface reactions require suitable atomic configurations at the surface, which allow the reactants to be in an appropriate binding situation and geometric orientation towards each other. Thus the influence of surface structure on catalytic activity cannot be overemphasized, for example which local Pt /Ru configuration is the best for effective complete methanol oxidation. This paper outlines initial experiments showing the effect of composition and structure of a catalyst prepared in a novel manner, on catalytic activity. Thus, although the importance of the fact that a binary Pt /Ru catalyst can produce a noticeable enhancement of current for
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methanol oxidation at low potentials was not ignored in the field of fuel cell research, it has taken more than three decades for the possibility of technically using the PtRu /CH3OH systems to be recognized. Much of the data presented here is not only significant for increasing the understanding of methanol oxidation mechanisms on mixed-metal, catalytic electrodes, but also furthers the understanding of noble metal deposition processes.
2. Experimental A Pt carbonyl complex was prepared by purging CO through an aqueous solution of Chloroplatinic acid (H2PtCl6) (10 mg cm 3) for 24 h with constant mechanical stirring. The solution initially had an orange color, which became cherry red with the formation of the Pt(CO)2 precipitate. At the end of the 24 h, a dark colored precipitate had formed. This was then filtered, followed by drying under a flowing CO atmosphere. Once dried the Pt carbonyl became a black crystalline solid, which was highly hygroscopic. Normally the filtrate was still slightly colored, in this event the solution was re-purged with CO causing more Pt carbonyl to form until a yield near to 100% was achieved, indicated by a colorless solution. A sample of the precipitate dissolved in Tetra Hydro Furan (THF) was utilized to determine the structure of the [Pt3(CO)3(m2-CO)3]2 dianion present, using Fourx ier Transform Infra Red (FTIR) spectroscopy. The IR absorption measurement was performed using a FTS6000 spectrometer from BioRad. All functions are remote-controlled by the WINIR software installed on a PC, thus supplying a user interface. The spectrometer was configured for the mid infrared range from 4000 to 400 cm 1 with a standard ceramic IR source, and a KBr beam splitter. A liquid nitrogen cooled EG&G MCT (mercury cadmium telluride) IR detector was used. The solution was used to form a thin film between two CaF2 plates and using a p-polarized input beam, the IR spectra were recorded with the spectrometer in rapid scan mode, i.e. the moving mirror of the Michelson interferometer is driven continuously and data is sampled at points of zero intensity of the HeNe laser signal. The beam traversed a polarization filter consisting of 0.12 mm wide strips of aluminium on a KRS-5 substrate. The filter can be set so as to polarize the light either parallel or perpendicular to the reflection plane (in this case perpendicular). The spectral resolution (Dn ) was set to 4 cm 1. In this configuration, the acquisition of one scan takes about 0.3 s, although it is necessary to average several scans to obtain a sufficient signal-tonoise ratio, typically 400, giving the final spectrum. The spectrometer is continuously purged with dry air, to protect the water-soluble KBr beam splitter and to
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remove moisture that would generate H2O bands in the spectra obtained. The Pt/Ru catalyst was prepared in the following manner: 216 mg of [Ru3(CO)12], 254 mg of [Pt(CO)2]x and 300 mg of Vulcan XC-72R carbon black were mixed with 300 ml of o-Xylene in a round bottom flask. This mixture was then refluxed at 143 8C for 24 h with constant mechanical stirring. The mixture was then allowed to cool to room temperature before the oXylene was removed using a rotary evaporator. Once separated the catalyst was allowed to dry under air for 7 days. Initial experiments were carried out under an Ar atmosphere, however, the catalysts thus obtained displayed very poor performances, probably due to a poor deposition of the Pt. In order to improve the deposition of Pt, all future refluxing was carried out under air in order to maximize the decomposition of the Pt carbonyl complex. Once drying had been completed, 100 mg of the catalyst powder was mixed with 353 mg Nafion solution (5% in aliphatic alcohols, Du Pont) and then diluted with a 1:1 mixture of MilliporeTM water (18 MV) and 2Propanol (Merck). The resulting ink was then placed in an ultra-sonic bath until the catalyst powder had fully dispersed, approximately 15 min. This ink was then sprayed onto a piece of TGPH-060 Toray carbon paper which was kept at 130 8C throughout the process. The spraying machine used consisted of a motorized X /Y table controlled by a CNC Automation Controller C116-4 isel† (Conrad electronics) which also regulated the flow of the catalyst suspension through a Walther Pilot Domino nozzle. The controller was run by an inhouse written computer program that ensured an even distribution of the catalyst on the substrate, this program allowed for the spraying of square electrode sheets of variable size. A 12 mm diameter disk was mechanically punched out of the electrode sheet and placed into a Kel-F† electrode holder and was contacted to the rear by a gold wire. A silicon rubber washer was used to seal the electrode into the holder and it also controlled the geometrical area of the electrode exposed to the electrolyte at 0.5 cm2. The design of the electrode holder allowed for a stream of Ar gas to be passed around the back of the electrode. In turn, the holder was placed in a water jacketed three-electrode glass cell fitted with a Luggin capillary contacting the reference electrode (a Mercury/Mercury Sulfate (MMS) from Sentek-UK Ltd.) which was positioned outside the cells thermal jacket to maintain isothermal operation at room temperature even when the rest of the cell was heated. The counter electrode was a Pt gauze positioned at the opposite side of the cell to the working electrode. The cell design facilitated the purging of the electrolyte with Argon during operation.
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The electrodes were tested for methanol oxidation in an Argon saturated electrolyte consisting of 1.5 M CH3OH (methanol) and 1 M H2SO4 at 25, 45 and 65 8C. The half cell used was unsuitable for operation above 65 8C as methanol evaporation occurred at to high a rate. Cyclic voltammetry and polarization data were collected using a HEKA PG310 Potentiostat/ Galvanostat system. The cycles were run between / 650 and 850 mV versus MMS (Mercury Mercurous Sulfate) for methanol oxidation at a scan rate of 20 mV s 1. Polarization data were collected with the cell under galvanostatic control and were carried out manually. Each value was collected after allowing the electrode to reach a steady-state value, usually this occurred within 3 /5 min. Two other catalysts were also tested in the cell to act as controls. The first was an in house produced 50% w/o Pt /Ru/C with a 1:1 atomic ratio prepared using a sulphito route. The second was a commercial catalyst obtained from E-Tek Inc., 40% w/o Pt /Ru/C with a 1:1 atomic ratio. Transmission electron microscopy (TEM) was carried out on the Pt/Ru/C catalyst, prepared via the carbonyl route, with a small quantity of the supported catalyst dusted onto a standard Cu grid (400 mesh) covered with a carbon film (Plano). Applying the catalyst in this manner, instead of in solution, avoids the formation of aggregates due to uncontrolled agglomeration, during the evaporation of the solution droplet. Thus any aggregates observed have originated from the initial catalyst preparation. The TEM micrograph was recorded in the bright-field mode on a PHILIPS CM 20 (University of Ulm), with a point resolution of 0.23 nm. The negative images with a magnification of 600 000 were magnified photographically on the print by a factor of 3.5 and digitized with a scanner. After further magnification, 258 particles from three images could be analyzed and the distribution of the particle sizes was evaluated. EDX analysis was performed with an Inca 3000 Oxford Instruments detector. Reference PtRu/C samples of known composition were used to ascertain the validity of the results.
3. Results and discussion An FTIR spectrum of the platinum carbonyl complex dissolved in THF is shown in Fig. 1. Platinum carbonyl cluster dianions have the general formula [Pt3(CO)3(m2CO)3]2 with x /2,3,4,5 [46]. Infrared solution spectra x in THF for each of these species are known from the literature [46]. The spectra show an expected lowering of terminal and bridging carbonyl frequencies upon successive reduction of the dianions II /V. From Fig. 1, the very strong band at 2056.4 cm 1, as well as the strong band at 1869.6 cm 1 and weak bands at 1889.9 and
1825.5 cm 1 are suggestive of the presence of the complex with x /5, [Pt15(CO)30]2 while the very strong band at 2040.4 cm 1, the strong band at 1844.5 cm 1 and weak band at 1830.4 cm 1, imply the presence of the complex with n /4, [Pt12(CO)24]2 . Thus it is likely that a mixture of these two dianions is present in the THF /Pt carbonyl complex solution. This is also shown by the blue /green color of the solution obtained, which is indicative of the latter complex [46,47]. In Fig. 2 a typical TEM image of the carbonyl prepared catalyst is shown with the corresponding size distribution obtained from the evaluation of 258 particles from three images (all particles in the images which could be discerned clearly were used for the analysis). As can be seen the distribution of the particles is homogenous on the support and the catalyst is well dispersed, consisting of particles averaging 2.5 nm with a standard deviation of 9/0.45 nm. The smallest particle detected has a size of about 1.6 nm whereas the largest one in the three images is about 4 nm. From XRD measurements larger particle sizes according to the Scherrer formula were derived, however, XRD is sensitive to the volumetric distribution of the particles, and therefore, a small number of large particles can lead to differences compared with TEM. A detailed analysis and comparison of the different methods is in preparation. From the EDX analysis of the sample it is derived that the Ru deposition is more effective compared with the Pt deposition since a high weight percentage of Ru was found. The ratios of Pt to Ru in the Xylene solution were chosen in order to aim for a 50:50 atomic ratio. However, values in the range of 62/67 atm.% Ru and correspondingly 38/33 atm.% Pt were obtained from different EDX measurements on the sample. This indicates that the Pt carbonyl decomposition is still inhibited with respect to the Ru carbonyl decomposition at the temperatures used. Therefore, the deposition properties and yields of the different carbonyls have to be taken into account in the catalyst synthesis of PtRu and of the supported catalyst compositions. In summary, this catalyst preparation procedure results in homogenously distributed small (2.5 nm) PtRu particles on the support with a narrow size distribution. Due to the decomposition properties the catalyst is Ru-rich. Fig. 3 shows the I /V curves collected for the carbonyl prepared catalyst as a function of temperature. Increasing the temperature increases both the maximum current density obtainable and lowers the potential obtained at a given current density, both features indicative of an effective electrocatalyst. It can be seen that there is a far greater increase in performance on going from 25 to 45 8C than there is going from 45 to 65 8C. This is to be expected, since the improvement in methanol oxidation kinetics is not linear with temperature. Since the 65 8C offers the highest performance the
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Fig. 1. IR spectrum of the Pt carbonyl complex in THF.
results taken at this temperature are discussed in more detail in this paper. In Fig. 4, the performance of the carbonyl prepared catalyst compared with a sulphito preparation and a standard catalyst obtained from E-Tek Inc, is illustrated. The results presented have been normalized for the different catalyst loadings present on the electrodes tested. These results clearly show a cross over point for all three catalysts. At low currents (i.e. low over-potentials) the E-Tek catalysts displays the
best performance while the carbonyl catalyst appears the worst of the three. An inferior performance of the carbonyl catalyst is not surprising since the PtRu atomic ratio of 33:67 is sub-optimal for methanol oxidation. Most studies on dispersed catalysts have found that a 1:1 atomic ratio exhibited best performance [52]. Furthermore, the published studies agree that a Rurich catalyst shows inferior performances [52]. However, the trend of inferior performances at low potentials for the carbonyl catalyst is reversed at values above /145
Fig. 2. TEM image of Pt/Ru catalyst prepared via carbonyl route. The dark features correspond to the Pt/Ru catalyst particles. Insert: Size histogram for the primary particles. The histogram is obtained after counting 258 particles from TEM micrographs recorded with a magnification of 600 000.
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Fig. 3. I /V curves for 50% w/o Pt /Ru/C prepared by the carbonyl route, taken at 25, 45 and 65 8C in 1 M H2SO4 and 1.5 M MeOH.
Fig. 4. Comparision of the mass activity curves for a 50% w/o Pt/Ru/ C prepared by the carbonyl route, a 50% w/o Pt/Ru/C prepared by the sulphito route and a 40% w/o Pt/Ru/C from E-Tek at 65 8C in 1 M H2SO4 and 1.5 M MeOH.
Ag 1 of metal. Recent work has suggested that there is an optimal particle size for CO oxidation, producing particles below this size will cause an increase in the potential needed for the oxidation process to occur since the adsorption of oxygen-containing species is strongly inhibited [50]. The interpretation proposed in this paper is that the particle size obtained for the in house catalysts is below this optimal size, however, at high potentials CO oxidation is not the only rate determining step, and therefore, the activity becomes dominated by a surface area effect. This would explain the crossover of the results. The electrochemically active surface area (EASA), obtained for each electrode by CO stripping supports this interpretation, see Table 1. The EASA is calculated assuming the targetted amount of metal is present. From Table 1, it can be seen that the E-Tek Table 1 EASAs measured by CO stripping for each of the catalysts tested. Catalyst
Symbol
EASA (m2g 1)
E-Tek Sulphito Carbonyl
j k
22.3 26.3 27.7
Fig. 5. Comparision of the activity of a 50% w/o Pt/Ru/C prepared by the carbonyl route, a 50% w/o Pt /Ru/C prepared by the sulphito route and a 40% w/o Pt /Ru/C from E-Tek at 65 8C in 1 M H2SO4 and 1.5 M MeOH, normalised for the electrochemical active area.
catalyst exhibits the lowest EASA, while the carbonyl prepared catalyst is the highest. Fig. 5 shows the I /V response data for all three catalysts normalized for their EASA. It is interesting to note that the performance of both in-house catalysts are very similar, with only small differences being apparent at either low or high current values. This would seem to indicate that the two catalysts are very similar in nature to each other although the PtRu ratio of the carbonyl catalyst is sub-optimal. The differences at either end of the current range could be due to particle size effects [51]. At high currents it would seem to suggest that the smaller particles behave in a superior manner toward methanol oxidation. However, this result shows that the sulphito method and the carbonyl method produce catalysts with very similar surface activities. The poorer performance of the E-Tek catalyst would seem to indicate that it has a less active surface composition than the in-house prepared catalysts, possibly due to a slight poisoning of the surface by impurities.
4. Conclusions The work presented in this paper has shown a simple method of producing a reproducible Pt carbonyl complex from chloroplatinic acid. The catalyst is prepared by decomposing the carbonyls in a high boiling-point solvent. This catalyst preparation procedure results in homogenously distributed small (2.5 nm) PtRu particles on the support with a narrow size distribution (0.5 nm). From the chemical composition of the catalyst (ratio of Pt:Ru 33:67) as derived from EDX it is concluded that the deposition of the Pt carbonyl is inhibited. It has also been shown that this carbonyl complex can be combined with a Ru carbonyl compound to produce supported Pt /Ru catalyst for methanol oxidation in a simple one step preparation. A catalyst produced by this novel method has an activity at least equivalent to that of a catalyst produced by the conventional sulphito proce-
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dure in spite of a sub-optimal PtRu ratio, but offers a much easier route to ternary and quaternary anode catalysts.
Acknowledgements Financial support by VDMA Gesellschaft for Research and Innovation (VFI GmbH) and partners is gratefully acknowledged. Part of this research has also been supported by a Marie Curie Fellowship of the European Community programme, ‘Energy, Environment and Sustainable Development’, under contract number ERK5-CT-1999-50002. The authors are grateful to Dr. Banhart, University of Ulm, for recording the TEM micrograph. Furthermore, the authors thank Professor R. Hiesgen, Fachhochschule Esslingen, for recording of EDX spectra and for helpful discussions.
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