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A high-throughput search for direct methanol fuel cell anode electrocatalysts of type PtxBiyPbz
Applied Surface Science 254 (2007) 653–661 www.elsevier.com/locate/apsusc
A high-throughput search for direct methanol fuel cell anode electrocatalysts of type PtxBiyPbz Jing Jin a,d,3, Mark Prochaska b,d,3, Dominic Rochefort a,d,1, David K. Kim c,d, Lin Zhuang a,d,2, Francis J. DiSalvo a,d, R.B. van Dover c,d, He´ctor D. Abrun˜a a,d,* a
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, United States Department of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, United States c Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, United States d Cornell Fuel Cell Institute, Cornell University, Ithaca, NY 14853, United States b
Received 27 February 2007; accepted 4 June 2007 Available online 20 July 2007
Abstract We used a high-throughput method to screen for direct methanol fuel cell anode electrocatalysts in the Pt–Bi–Pb system. Previous studies showed that PtBi and PtPb (both NiAs structure type) were active electrocatalysts for the oxidation of formic acid, but only PtPb was active in oxidizing methanol. We synthesized thin films with continuous composition spreads of the three elements by magnetron sputtering at deposition temperatures from ambient to 510 8C. A fluorescence method was then used to identify compositions that were active toward methanol oxidation. Only films deposited between temperatures of 160 and 400 8C showed electrocatalytic activity. The areas that were active for methanol oxidation showed predominantly the NiAs structure type according to XRD, with optimal activity for compositions near PtBi0.01Pb0.53. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05.Zx; 82.47. a; 82.47.Gh Keywords: Combinatorial chemistry; Fuel cell anode catalysts
1. Introduction Many companies are developing direct methanol fuel cells for power sources in portable electronics due to the low cost, ease of storage and distribution of methanol [1]. However, problems with anode electrocatalysts, typically containing Pt, challenge their entry into the electronics market. Carbon monoxide, a common reaction intermediate in methanol oxidation [2], tends to poison the surface of pure Pt electrocatalysts and prevent further oxidation. Methanol
* Corresponding author. Tel.: +1 607 255 4720. E-mail address: [email protected] (H.D. Abrun˜a). 1 Present address: De´partement de Chimie, Universite´ de Montre´al, CP6128 Centre-Ville, Montre´al, Que., Canada H3C 3J7. 2 Present address: Department of Chemistry, Wuhan University, Wuhan 430072, China. 3 These authors contributed equally to this work. 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.06.077
oxidation on Pt surfaces is also very slow, resulting in lower power output from the fuel cell. Numerous studies on PtRu alloys as anode electrocatalysts have demonstrated their superiority over Pt alone in terms of resistance to CO poisoning and faster kinetics [3–5]. However, PtRu still oxidizes methanol at an insufficient rate, and the Ru tends to migrate away from the catalyst surface into the bulk as well as leach from the surface under some conditions [6]. There is also evidence that PtRu alloy is not the active electrocatalyst; instead, bulk quantities of hydrous Ru-oxides mixed with Pt metal are responsible for the increased activity [7,8]. Recent studies showed that the ordered intermetallic compounds PtBi and PtPb oxidized formic acid with a lower onset potential and much higher steady-state current than Pt or PtRu [9,10]. PtBi also showed remarkable resistance to poisoning upon exposure to CO, as revealed by cyclic voltammetry and DEMS measurements, while PtPb was only mildly affected by CO [11]. PtBi, while active for formic acid
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oxidation, showed no activity toward methanol oxidation. PtPb oxidizes methanol at potentials slightly positive of that found for PtRu but at higher current densities [12]. Ordered intermetallic compounds offer new clues in the vast search for new fuel cell electrocatalysts. However, even with this narrowed search, traditional methods of synthesis of one composition at a time would be time-intensive for screening binary and ternary ordered intermetallic compounds alone. We recently introduced a high-throughput method for synthesizing thin film continuous composition spreads of two and three elements [13]. We geometrically arranged three sputter guns and controlled sputter rates to produce composition spread thin films of Pt, Bi, and Pb on a Si wafer. The films contain approximately 60% of the interior compositions of the ternary phase diagram. We then screened the films for electrochemical activity using a fluorescence method [14,15]. In a film with a composition spread of Pt, Bi, and Pb, we found active compositions having the same structure as PtPb (NiAs structure type). According to semi-quantitative (standardless) microprobe measurements, the active electrocatalyst area had compositions near Pt0.38(Bi/Pb)0.62, which is within experimental error of the 1:1 Pt:M ratio expected for a NiAs structured compound. In this paper, we completed a more thorough analysis of several Pt–Bi–Pb composition spread films. Our objective was to screen most of the Pt–Bi–Pb ternary phase diagram for compositions that are electrochemically active to methanol oxidation and determine the effect of deposition temperature on activity. The binary phase diagrams of Pt–Bi and Pt–Pb show the existence of several intermetallic compounds (line compounds) at room temperature: Bi2Pt, BiPt, PtPb4, PtPb, and Pt3Pb. The binary phase diagram of Bi–Pb shows the existence of the compound Bi3Pb7, but this phase has a stoichiometric width dependent on the temperature. A ternary phase diagram of Pt, Bi, and Pb [16] is shown in Fig. 1. The
crystallographic structures of BiPb3Pt, BiPb7Pt2, Bi3PbPt2, Bi7PbPt4 and Bi13Pb7Pt10 have been reported [17–21]. Although each of these compositions is reported to have a unique crystallographic structure, Bi and Pb could not be distinguished by X-ray diffraction. The first two are related to the PtPb4 structure, while the last three are related to the PtBi2 structure. This is expected since the ternary compositions have similar ratios of Pt to Bi/Pb, as shown by lines (a) and (b) in Fig. 1. Additional compositions along line (b) in Fig. 1 (Bi2Pb4Pt3, Bi3Pb5Pt4, and BiPbPt) have also been reported, although their detailed structure is unknown and assumed to be related to that of PtBi2. 2. Experimental 2.1. Synthesis of composition spread thin films The details of our synthesis, testing, and characterization methods have been described in a previous paper [13]. We deposited six films with composition spreads of Pt, Bi, and Pb at different substrate temperatures (ambient, 100, 160, 265, 400 and 510 8C), one film of just Pt and Pb at 265 8C, and one film of just Pt and Bi at 260 8C. All substrates were 3 in. diameter ndoped (As-doped) Si wafers. Before depositing the composition spread layer, we sputtered a uniform Ta underlayer as a ˚. diffusion barrier with a thickness of approximately 1200 A Table 1 lists the composition spread films (labeled #1 through #8 throughout the paper) with the elements present and deposition temperature for each one. We chose the currents supplied to each sputter gun so as to simultaneously deposit all elements with approximately the same molar deposition rate at the center of the composition spread. In film #1 through #6, ˚ /s at sputter gun 1 deposited Pt at a current of 100 mA (1.5 A substrate center), sputter gun 2 deposited Bi at a current of ˚ /s at substrate center), and sputter gun 3 deposited 58 mA (3.4 A ˚ /s at substrate center). We also Pb at a current of 57 mA (2.9 A used these deposition rates for film #7, except that gun 2 was turned off so as to obtain a Pt–Pb binary composition spread. The time of deposition for all the films was 320 2 s. This produced a composition spread layer thickness of approxi˚ at the center of film #1 through #6 and 1410 A ˚ at mately 2500 A the center of film #7. The deposition of film #8 used different rate calibrations: Pt ˚ /s at substrate center) and Bi from from gun 1 at 112 mA (1.7 A Table 1 Information on the synthesis of the seven composition spread thin films
Fig. 1. A ternary phase diagram of Pt, Bi, and Pb showing compositions that have been investigated. The compositions along line (a) all contain 20 at.% Pt, while the compositions along line (b) all contain 33 at.% Pt.
Film
Elements present
Deposition temperature (8C)
Fluorescence onset potential (mV vs. Ag/ AgCl, 25 mV)
#1 #2 #3 #4 #5 #6 #7 #8
Pt, Pt, Pt, Pt, Pt, Pt, Pt, Pt,
Ambient 100 160 265 400 510 265 260
Inactive Inactive 255 175 175 Inactive 210 Inactive
Bi, Bi, Bi, Bi, Bi, Bi, Pb Bi
Pb Pb Pb Pb Pb Pb
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˚ /s at substrate center). The time of gun 2 at 57 mA (4.2 A deposition was 240 2 s, for an approximate composition ˚ at the center. spread layer thickness of approximately 1400 A 2.2. Fluorescence screening Our method of screening the films for efficient electrocatalysts, initially described by Mallouk and Smotkin [14], was based on the detection of fluorescence arising from quinine, a molecule that exhibits intense blue fluorescence at pH values below 5. Our testing protocol consisted of using the entire film as a working electrode in a solution containing 5 M methanol (Burdick & Jackson), 0.1 M CF3SO3Na (Aldrich), and 0.5 mM quinine (Aldrich). Solutions were prepared using Milli-Q reagent water (Mill-Q, Millipore, 18.2 MV cm resistivity). Fluorescence images were taken using a Nikon D100 camera (Nikon, Japan) and Nikon capture 4 camera control software. A CHI900 SECM (CH Instruments, Austin, TX) was used in the voltammetric studies. All potentials are expressed relative to a Ag/AgCl reference electrode without regard for the liquid junction. We scanned the applied potential toward positive values at a rate of 5 mV/s while monitoring the fluorescence intensity over the entire film. As the oxidation of methanol in an active area of the film generates protons, the local pH decreases below 5 and we observe fluorescence of the quinine. The most active areas will fluoresce at the lowest potentials and before areas of lower activity.
Fig. 2. The diagram shows the location of the nine regions of the film that were characterized before and after electrochemical testing. Also indicated the approximate locations of the Pt, Bi, and Pb sputter guns relative to the regions.
the fluorescence method described above. We then used the same characterization techniques (XRD and EDS) in the same nine regions to see if the composition at any point changed as a result of electrochemical testing. If any portions of the film showed fluorescence activity, we cut out a 1 cm 1 cm section of that part of the film and performed XRD, WDS, and XPS measurements on it. For those films deposited at 160 8C and lower or at 510 8C (films that were entirely inactive), we cut a 1 cm 1 cm section from the same region that showed activity in films deposited at intermediate temperatures and performed the same measurements. 3. Results and discussion
2.3. Characterization methods 3.1. Electrochemical tests of films We characterized the composition spread films using a JEOL 8900 EPMA electron microprobe to estimate the bulk compositions with EDS and WDS. In both types of measurements, we used electron beam energies of 10 keV. EDS measurements were semi-quantitative, with an error of approximately 10%, and performed in beam scanning mode over an approximate area of 1150 mm 800 mm. WDS measurements were done in spot mode, with a beam width in the order of 1 mm. WDS composition determinations were completed using Pt, Bi2Se3, and PbS standards for Pt, Bi, and Pb content, respectively, and TiO2 for oxygen content. The computer program GMRFILM produced thin film corrections of the WDS results. To investigate the composition of the surface, we used two different XPS machines: a Surface Science SSX-100 and a VG Escalab 220i-XL. A Bruker AXS general area diffraction detector system (GADDS) allowed us to observe the X-ray diffraction (XRD) spectra of small regions of the film (approximately 2 mm in diameter or less), while DiffracPlus Basic 4.0 matched the spectra to known structure types. For each film, we performed two sets of characterizations. In the first set, we studied nine different regions, shown in Fig. 2. At a single point in each of the regions, we measured the XRD spectrum with GADDS and composition with EDS. Next, we performed electrochemical measurements on the sample using
As an example of our electrochemical test results, we provide a detailed description of film #5, shown in Fig. 3. A sequence of fluorescence images taken at different applied potentials is given in panels (a)–(d). The relative positions of the Pt, Bi, and Pb sputtering sources are labeled in panel (a). At the initial potential (0.0 V) there was no fluorescence. There was some contrast in the image due to the surface topography of the film; this contrast was also observed under normal white light illumination. As the film potential was increased to 125 mV at 5 mV/s, we saw no indication of blue fluorescence. At an applied potential of +175 mV, a faint fluorescent spot appeared near region 8 (see Fig. 2). At 300 mV, the fluorescence intensity from this region greatly increased. Using an analysis method we have proposed in another paper (reference: a paper in preparation), the fluorescence onset potential was used to quantify and compare the performance of the active areas of the films. The color intensity of the active region, measured in arbitrary units with a linear scale from 0 to 255, was plotted as a function of the applied potential (shown in panel (e) of Fig. 3). Before the onset of fluorescence, the background intensity was typically between 0 and 50. We therefore, defined the fluorescence onset potential as the potential at which the color intensity reaches 50. For the active area of film #5, the onset potential was 175 25 mV.
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Fig. 3. Images of the fluorescence test on film #5 (Pt–Bi–Pb composition spread deposited at 400 8C). Images (a)–(d) show the raw image of the film in a potential scan from 0 to 300 mV vs. Ag/AgCl. Panel (a) also shows the relative locations of the Pt, Bi, and Pb sources. A plot of the color intensity vs. applied potential is shown in (e) for the area with the highest intensity fluorescence.
Table 1 provides a listing of the fluorescence onset potentials for the most active area of each film. For the Pt–Bi–Pb series at different deposition temperatures, there was a change in the electrocatalytic activity for methanol oxidation. In the films deposited at ambient temperature, 100 and 510 8C, there was no activity observed anywhere on the film up to 300 mV. At intermediate deposition temperatures, the onset potential was lowest for films deposited at approximately 265 and 400 8C. The locations of the active areas were similar in all the films that showed activity. We infer that temperature plays an important role in affecting the electrocatalytic activity, possibly by determining the composition and/or crystallinity of the material exposed at the surface.
We also tested the activity of two binary composition spread films (Pt–Bi and Pt–Pb) to further investigate the effect of mixing the two main group metals (Bi and Pb). For Pt–Bi, there was no fluorescence even when the potential ramped up to 400 mV. For Pt–Pb, we observed an active area, again near region 8 on the film. The fluorescence onset potential was 210 25 mV, close to that of the ternary composition spread prepared under the same conditions (film #4). The addition of Bi appears to slightly lower the onset potential (increase the activity) by about 35 mV (although the error bars are large enough that this enhancement could be zero), even though PtBi is completely inactive. Certainly the addition of some Bi does not eliminate electrocatalytic activity.
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Fig. 4. Images of film #5 (a) before and (b) after electrochemical testing. The approximate locations of the Pt, Bi, and Pb sputter guns are also shown in the images.
3.2. Characterization of films before and after electrochemical testing As an example of characterization of the nine regions of a composition spread before and after electrochemical testing, we provide detailed results of film #5 (see Table 1) and give a brief summary of the others. Fig. 4 shows the physical appearance of film #5 before and after electrochemical testing. Regions 1 and 4, close to the Pt gun, had reflective surfaces that remained so after electrochemical testing. The other regions in the sample had dull textures and were darker the closer they were to the Bi and Pb guns. These regions became somewhat lighter in color after testing. As will be shown, we believe this change in coloration is due to Pb leaching from the film at a relatively positive potential versus Ag/AgCl (above 300 mV). Fig. 5 shows the results of semi-quantitative composition measurements, obtained with EDS, of each of the nine regions of film #5. The chart shows the composition of the regions before and after electrochemical testing. Most of the regions maintained their composition, within experimental error, during testing. The exceptions were in regions 6 and 8. These regions showed a significant drop in the atomic percent of Pb: 11% in region 6 and 36% in region 8. The locations of the drops in Pb content corresponded to areas of the films that showed changes in physical appearance as well, such as a change in coloration. Fig. 6 shows the resulting XRD spectra of all nine regions of film #5 before and after electrochemical testing. Also super-
Fig. 5. Semi-quantitative composition measurements, determined with EDS, of all nine regions of film #5 before and after electrochemical testing. The regions with an asterisk indicate that the measurement was completed after the fluorescence test.
imposed on the XRD spectra in Fig. 6 are literature peak positions for the following elements and compounds: PtPb (ICDD PDF file #6-374), Pt (ICDD PDF file #4-802), PtxPb (ICDD PDF file #6574), PtPb4 (ICDD PDF file #6-463), Pb5Bi8O17 (ICDD PDF file #41-405), Bi2O3 (ICDD PDF file #45-1344), Pb (ICDD PDF file #4-686). Regions 1, 2, 5, and 7 contained the least significant changes in XRD spectra before and after electrochemical testing, matching the location of regions that showed few changes in physical appearance and composition (that is, regions that were in close proximity to the Pt gun). These regions also showed common structure types: all four had peaks of varying intensity that correspond to XRD spectra of PbPt (pdf #6-374), although the intensity of the PbPt peaks in region 1 were weak and only appeared after electrochemical testing. It is possible that this appearance of PbPt is due to a preferred orientation of the phase that was not detected in the initial spectrum. The fact that the relative intensities of the PbPt peaks are different from those of the powder diffraction file suggests a preferred orientation. Most of the peaks of region 1 matched both the Pt (pdf #4-802) and PbPtx (pdf #6-574) structure types, but the peaks were slightly shifted from Pt by approximately 0.28 and from PbPt3 by approximately 0.58. This shift is too large to be solely the result of strain in the film. The GADDS instrument allows one to position the sample to the correct height using a laser beam positioning device, but this is difficult to confirm using the Bradley–Jay function [22] since there are only the three peaks for each phase in the spectrum. Another possible cause of the shifts is that some Pb or Bi may have substituted into sites in the lattice. The other regions of film #5 showed significant changes in XRD spectra. Regions 3 and 6 generally showed peaks at the same values of 2u before and after electrochemical testing, but the intensity of those peaks increased by approximately 10% in most cases. One exception to this observation is in region 3 at 278, where the peak increased by 133%. A few peaks disappeared entirely, notably those at 2u = 298 and 408 in region 6. Region 4 showed peaks that matched PbPtx and smaller peaks that matched Pt. The Pt peaks increased in intensity after electrochemical testing, with the tallest peaks increasing by 150–200%. This may suggest that Pb leached from the region, although EDS measurements showed no significant drop in the atomic percentage of Pb. The most drastic changes occurred in regions 8 and 9. In region 8, peaks at 2u = 308, 33.58, 38.58, 55.58, 59.58, and 628 have either decreased in intensity or disappeared altogether. Each of these peaks matched the Pb4Pt (pdf #6-463)
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Fig. 6. XRD spectra of all nine regions of film #5. The best matches to the peaks of known elements and compounds from the set of powder diffraction files are shown by the vertical lines in each spectrum.
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structure type. Region 9 also showed significant decreases in the intensity or disappearance of peaks that match the Pb (pdf #4686) or Pb4Pt structure types. It is interesting to note that the regions that showed the most significant changes in our measurements (regions 6, 8 and 9) had some of the highest Pb content. This suggests that during electrochemical testing, Pb had a tendency to leach from regions of the films where there was a large amount of Pb. This is probably due to the relatively low standard reduction potential of the Pb2+/Pb couple ( 0.3474 V versus Ag/AgCl). A scan of the potential of the film in the positive direction will oxidize Pb to Pb2+ into solution before Bi or Pt, which have higher standard reduction potentials. An exception to this rule is in region 7, where there was 56–57 at.% Pb. However, the XRD spectrum of this region strongly matched the PbPt structure type, and the ratio of Pt:Pb was close to 1:1. If the intermetallic compound PbPt had indeed formed in region 7, the Pb could only be oxidized if the free energy of bond formation between Pb and Pt were overcome in addition to the free energy required to remove two electrons from Pb. This would increase the reduction potential of Pb into solution and promote the stability of the Pb against oxidation. Using a method described by Miedema et al. [23,24], we estimated the enthalpy of formation of PtPb to be 26.6 kJ/mol of atoms. If we neglect the entropy contribution to the free energy of formation, this shifts the standard reduction potential of Pb oxidation by 0.138 V in the positive direction. The other films showed similar behavior. Significant changes in composition and XRD spectrum were more likely to occur in regions 6, 8, or 9, usually with a decrease in the atomic percentage of Pb. Notable exceptions to this observation occurred in film #6 (an inactive film deposited at 510 8C), where the composition of almost every region was stable. This is probably due to the fact that the Pb content is lower with increasing deposition temperature, so there is less of it in elemental form to leach. A higher deposition temperature, especially if it is above the melting point of Pb (327 8C), would increase the vapor pressure of the Pb and make it less likely to stick to the thin film. XRD spectra of regions 6, 8, and 9 of film #6 also showed less significant changes than film #5. In these regions, each of the peaks decreased in intensity but their positions in 2u remained the same. In region 6 of film #2 (an inactive film deposited at 100 8C), the atomic percentage of Pb actually increased by 13%, while Bi decreased by 11 at.%. This could have been due to the oxidation of Pb from parts of the film and resettlement at other locations. There may also have been some Bi leaching that is significant in this region but not detectable in others. 3.3. Characterization of active areas after electrochemical testing Fig. 7 shows the composition, measured by WDS, of the active areas for film #3, #4, #5, and #7, along with inactive areas on film #1, #2, and #6 that had the same location as the active areas on other samples. As can be seen from the chart and as expected from the position on the films, the Bi content in the
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Fig. 7. Bulk composition, measured with WDS, of the active areas of film #3, #4, #5, and #7. Also shown the bulk compositions of inactive areas of film #1, #2, and #6 that have similar positions on the film as active areas on other samples.
active areas was low, never more than 10 at.%. The oxygen content in active areas in part arises from surface oxidation of the film on exposure to air and perhaps from oxygen/water contamination in the sputtering system. The sputtering vacuum chamber is typically pumped down to base pressures on the order of several microTorr, and may still have enough oxygen present to be incorporated into the films at some level. Fig. 8 shows the XRD spectra of the same active and inactive areas characterized with WDS. Each of these areas had peaks that are similar in location in 2u, but the peak intensities increased and line widths decreased with increasing deposition temperature. This is expected, since at higher temperatures closer to the melting point the deposited atoms have higher surface and bulk mobility, resulting in grain growth. Note that the incongruent melting point of PtPb is 790 8C and the two highest deposition temperatures are 63 and 74% of this temperature on an absolute scale. The majority of the peaks in each spectrum matched the position of the PtPb/PtBi structure type (pdf #6-374). However, the relative intensities of the peaks differ from those of a powder pattern. This suggests that the crystallites of the active areas may have a preferred orientation. An additional peak at 348 appears frequently in all samples, regardless of composition, and is due to the Ta underlayer. The XRD spectra of the same area on different active and inactive films suggest that given the location of activity on a Pt– Bi–Pb thin film composition spread, we can predict the structure type of the active area. However, structure type is not sufficient to predict activity. Similarly, the composition data show that composition is also not sufficient for the prediction of activity in some area of the film. These data suggest that the bulk composition of the film is not the controlling factor in determining electrochemical activity. This is perhaps not a surprise, since the activity occurs at the film surface. It is possible that the surface composition, structure, and morphology is different than the bulk and is likely to be a function of the deposition temperature. Fig. 9 shows the composition of metallic elements of the same areas in each film (whether it is active or inactive) as
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Fig. 8. XRD spectra of the same active and inactive areas as those measured in Fig. 7. The best matches to the location of peaks of known elements and compounds from the set of powder diffraction files are shown by the vertical lines in each spectrum.
measured by XPS. This measurement describes the composition at the surface instead of the bulk, as is the case with EDS and WDS measurements. In comparing the surface compositions in Fig. 9 to bulk compositions in Fig. 7, a notable
difference was the greater atomic percentage of Pb and Bi at the surface. The rise of these elements to the surface could have been due to their lower melting points, which would give them greater mobility during deposition. Fig. 9 shows that a higher
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the active areas had similar structure types, we found that neither structure type nor composition (in the bulk or at the surface) were determinants of active electrocatalysts. The reasons for this result are still under investigation. Acknowledgements
Fig. 9. Surface composition of the metallic species, measured by XPS, of the same active and inactive areas described in Fig. 7.
deposition temperature appeared to decrease the atomic percentage of Pb at the surface, as evidenced for film #3 through #7. The data show that surface composition does not determine the appearance of activity in the fluorescence tests. Film #1 and #2 could lead us to believe that the high Pb content prevents electrochemical activity, but film #6 was also inactive and showed a low fraction of Pb. The results point out the need for more surface characterization techniques to better understand the relationship between deposition temperature and electrochemical activity. 4. Conclusions We have used a combinatorial method to screen a large number of candidates for fuel cell anode electrocatalysts of type PtxBiyPbz. We deposited six composition spread thin films of Pt, Bi, and Pb at different deposition temperatures from ambient to 510 8C. We also deposited binary composition spreads of Pt and Bi at 260 8C and Pt and Pb at 265 8C. Our results show that the optimal electrocatalyst in this system was deposited at 400 8C (determined by fluorescence screening), has an average composition of Pt0.46Bi0.09Pb0.20O0.25 (determined by WDS), and has the same structure type as the ordered intermetallic compound PtPb (determined by XRD). A composition spread deposited with only Pt and Pb confirms that low fractions of Bi do not deactivate the electrocatalyst, but may in fact decrease the onset potential by up to 50 mV. We sampled nine points on each of the films with EDS and XRD measurements before and after electrochemical testing by fluorescence. We found that areas that are close to the Pb source tend to change composition and structure significantly. This is probably due to the low standard reduction potential of Pb leading to the dissolution of the elemental form, or leaching of Pb from Pb-rich phases such as PtPb4. The deposition temperature plays a crucial role in synthesizing active electrocatalysts in the Pt–Bi–Pb system. We found that the fluorescence activity is optimal when the deposition temperature is between 260 and 400 8C. Although
We would like to thank John Hunt for his assistance in obtaining data with WDS and EDS, Maura Weathers for her help in obtaining XRD data, and John Shu for acquiring data with XPS. We also acknowledge the Cornell Center for Materials Research in maintaining GADDS and microprobe facilities at Cornell University. This work is supported through the Department of Energy, grant DE-FG02-03ER46072, and the National Science Foundation. References [1] M. Cropper, Fuel Cells 4 (2004) 236–240. [2] H.A. Gasteiger, N. Markovic, P.N. Ross Jr., E.J. Cairns, J. Phys. Chem. 97 (1993) 12020–12029. [3] H.F. Oetjen, V.M. Schmidt, U. Stimming, F. Trila, J. Electrochem. Soc. 143 (1996) 3838–3842. [4] J.B. Goodenough, A. Hamnett, B.J. Kennedy, R. Manoharan, S.A. Weeks, J. Electroanal. Chem. 240 (1988) 133–145. [5] M. Watanabe, M. Uchida, S. Motoo, J. Electroanal. Chem. 229 (1987) 395–406. [6] P. Piela, C. Eickes, E. Brosha, F. Garzon, P. Zelenay, J. Electrochem. Soc. 151 (2004) A2053–A2059. [7] J. Long, R.M. Stroud, K.E. Swider-Lyons, D.R. Rolison, J. Phys. Chem. B 104 (2000) 9772–9776. [8] D.R. Rolison, P.L. Hagans, K.E. Swider, J.W. Long, Langmuir 15 (1999) 774–779. [9] E. Casado-Rivera, Z. Ga´l, A.C.D. Angelo, C. Lind, F.J. DiSalvo, H.D. Abrun˜a, Chem. Phys. Chem. 4 (2003) 193–199. [10] E. Casado-Rivera, D.J. Volpe, L. Alden, C. Lind, C. Downie, T. Va´zquezAlvarez, A.C.D. Angelo, F.J. DiSalvo, H.D. Abrun˜a, J. Am. Chem. Soc. 126 (2004) 4043–4049. [11] D. Volpe, E. Casado-Rivera, L. Alden, C. Lind, K. Hagerdon, C. Downie, C. Korzeniewski, F.J. DiSalvo, H.D. Abruna, J. Electrochem. Soc. 151 (2004) A971–A977. [12] L.R. Alden, C. Roychowdhury, F. Matsumoto, D.K. Han, V.B. Zeldovich, H.D. Abruna, F.J. DiSalvo, Langmuir 22 (2006) 10465–10471. [13] M. Prochaska, J. Jin, D. Rochefort, L. Zhuang, F.J. DiSalvo, H.D. Abruna, R.B. van Dover, Rev. Sci. Instrum. 77 (2006) 054104. [14] E. Reddington, A. Sapienza, B. Gurau, R. Viswanathan, S. Sarangapani, E.S. Smotkin, T.E. Mallouk, Science 280 (1998) 1735–1737. [15] J.E. Vitt, R.C. Engstrom, Anal. Chem. 69 (1997) 1070–1076. [16] P. Villars, A. Prince, H. Okamoto, Handbook of Ternary Alloy Phase Diagrams, ASM International, Materials Park, OH, 1995. [17] T. Matkovic, K. Schubert, J. Less-Common Met. 59 (1978) P35–P40. [18] K. Schubert, S. Bhan, T.K. Biswas, K. Frank, P.K. Panday, Naturwissenschaften 55 (1968) 542–543. [19] T. Biswas, K. Schubert, J. Less-Common Met. 19 (1969) 223–243. [20] Y.C. Bhatt, K. Schubert, Z. Metallkd 71 (1980) 550–553. [21] Y.C. Bhatt, K. Schubert, J. Less-Common Met. 70 (1980) P39–P45. [22] B.D. Cullity, S.R. Stock, Anonymous Elements of X-ray Diffraction, Prentice Hall, Inc., Upper Saddle River, NJ, 2001, pp. 363–384. [23] A.R. Miedema, P.F. de Chatel, F.R. de Boer, Physica B+C 100 (1980) 1– 28. [24] A.K. Niessen, F.R. de Boer, R. Boom, P.F. Chatel, W.C.M. Mattens, A.R. Miedema, CALPHAD 7 (1983) 51–70.
A high-throughput search for direct methanol fuel cell anode electrocatalysts of type PtxBiyPbz Jing Jin a,d,3, Mark Prochaska b,d,3, Dominic Rochefort a,d,1, David K. Kim c,d, Lin Zhuang a,d,2, Francis J. DiSalvo a,d, R.B. van Dover c,d, He´ctor D. Abrun˜a a,d,* a
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, United States Department of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, United States c Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, United States d Cornell Fuel Cell Institute, Cornell University, Ithaca, NY 14853, United States b
Received 27 February 2007; accepted 4 June 2007 Available online 20 July 2007
Abstract We used a high-throughput method to screen for direct methanol fuel cell anode electrocatalysts in the Pt–Bi–Pb system. Previous studies showed that PtBi and PtPb (both NiAs structure type) were active electrocatalysts for the oxidation of formic acid, but only PtPb was active in oxidizing methanol. We synthesized thin films with continuous composition spreads of the three elements by magnetron sputtering at deposition temperatures from ambient to 510 8C. A fluorescence method was then used to identify compositions that were active toward methanol oxidation. Only films deposited between temperatures of 160 and 400 8C showed electrocatalytic activity. The areas that were active for methanol oxidation showed predominantly the NiAs structure type according to XRD, with optimal activity for compositions near PtBi0.01Pb0.53. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05.Zx; 82.47. a; 82.47.Gh Keywords: Combinatorial chemistry; Fuel cell anode catalysts
1. Introduction Many companies are developing direct methanol fuel cells for power sources in portable electronics due to the low cost, ease of storage and distribution of methanol [1]. However, problems with anode electrocatalysts, typically containing Pt, challenge their entry into the electronics market. Carbon monoxide, a common reaction intermediate in methanol oxidation [2], tends to poison the surface of pure Pt electrocatalysts and prevent further oxidation. Methanol
* Corresponding author. Tel.: +1 607 255 4720. E-mail address: [email protected] (H.D. Abrun˜a). 1 Present address: De´partement de Chimie, Universite´ de Montre´al, CP6128 Centre-Ville, Montre´al, Que., Canada H3C 3J7. 2 Present address: Department of Chemistry, Wuhan University, Wuhan 430072, China. 3 These authors contributed equally to this work. 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.06.077
oxidation on Pt surfaces is also very slow, resulting in lower power output from the fuel cell. Numerous studies on PtRu alloys as anode electrocatalysts have demonstrated their superiority over Pt alone in terms of resistance to CO poisoning and faster kinetics [3–5]. However, PtRu still oxidizes methanol at an insufficient rate, and the Ru tends to migrate away from the catalyst surface into the bulk as well as leach from the surface under some conditions [6]. There is also evidence that PtRu alloy is not the active electrocatalyst; instead, bulk quantities of hydrous Ru-oxides mixed with Pt metal are responsible for the increased activity [7,8]. Recent studies showed that the ordered intermetallic compounds PtBi and PtPb oxidized formic acid with a lower onset potential and much higher steady-state current than Pt or PtRu [9,10]. PtBi also showed remarkable resistance to poisoning upon exposure to CO, as revealed by cyclic voltammetry and DEMS measurements, while PtPb was only mildly affected by CO [11]. PtBi, while active for formic acid
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oxidation, showed no activity toward methanol oxidation. PtPb oxidizes methanol at potentials slightly positive of that found for PtRu but at higher current densities [12]. Ordered intermetallic compounds offer new clues in the vast search for new fuel cell electrocatalysts. However, even with this narrowed search, traditional methods of synthesis of one composition at a time would be time-intensive for screening binary and ternary ordered intermetallic compounds alone. We recently introduced a high-throughput method for synthesizing thin film continuous composition spreads of two and three elements [13]. We geometrically arranged three sputter guns and controlled sputter rates to produce composition spread thin films of Pt, Bi, and Pb on a Si wafer. The films contain approximately 60% of the interior compositions of the ternary phase diagram. We then screened the films for electrochemical activity using a fluorescence method [14,15]. In a film with a composition spread of Pt, Bi, and Pb, we found active compositions having the same structure as PtPb (NiAs structure type). According to semi-quantitative (standardless) microprobe measurements, the active electrocatalyst area had compositions near Pt0.38(Bi/Pb)0.62, which is within experimental error of the 1:1 Pt:M ratio expected for a NiAs structured compound. In this paper, we completed a more thorough analysis of several Pt–Bi–Pb composition spread films. Our objective was to screen most of the Pt–Bi–Pb ternary phase diagram for compositions that are electrochemically active to methanol oxidation and determine the effect of deposition temperature on activity. The binary phase diagrams of Pt–Bi and Pt–Pb show the existence of several intermetallic compounds (line compounds) at room temperature: Bi2Pt, BiPt, PtPb4, PtPb, and Pt3Pb. The binary phase diagram of Bi–Pb shows the existence of the compound Bi3Pb7, but this phase has a stoichiometric width dependent on the temperature. A ternary phase diagram of Pt, Bi, and Pb [16] is shown in Fig. 1. The
crystallographic structures of BiPb3Pt, BiPb7Pt2, Bi3PbPt2, Bi7PbPt4 and Bi13Pb7Pt10 have been reported [17–21]. Although each of these compositions is reported to have a unique crystallographic structure, Bi and Pb could not be distinguished by X-ray diffraction. The first two are related to the PtPb4 structure, while the last three are related to the PtBi2 structure. This is expected since the ternary compositions have similar ratios of Pt to Bi/Pb, as shown by lines (a) and (b) in Fig. 1. Additional compositions along line (b) in Fig. 1 (Bi2Pb4Pt3, Bi3Pb5Pt4, and BiPbPt) have also been reported, although their detailed structure is unknown and assumed to be related to that of PtBi2. 2. Experimental 2.1. Synthesis of composition spread thin films The details of our synthesis, testing, and characterization methods have been described in a previous paper [13]. We deposited six films with composition spreads of Pt, Bi, and Pb at different substrate temperatures (ambient, 100, 160, 265, 400 and 510 8C), one film of just Pt and Pb at 265 8C, and one film of just Pt and Bi at 260 8C. All substrates were 3 in. diameter ndoped (As-doped) Si wafers. Before depositing the composition spread layer, we sputtered a uniform Ta underlayer as a ˚. diffusion barrier with a thickness of approximately 1200 A Table 1 lists the composition spread films (labeled #1 through #8 throughout the paper) with the elements present and deposition temperature for each one. We chose the currents supplied to each sputter gun so as to simultaneously deposit all elements with approximately the same molar deposition rate at the center of the composition spread. In film #1 through #6, ˚ /s at sputter gun 1 deposited Pt at a current of 100 mA (1.5 A substrate center), sputter gun 2 deposited Bi at a current of ˚ /s at substrate center), and sputter gun 3 deposited 58 mA (3.4 A ˚ /s at substrate center). We also Pb at a current of 57 mA (2.9 A used these deposition rates for film #7, except that gun 2 was turned off so as to obtain a Pt–Pb binary composition spread. The time of deposition for all the films was 320 2 s. This produced a composition spread layer thickness of approxi˚ at the center of film #1 through #6 and 1410 A ˚ at mately 2500 A the center of film #7. The deposition of film #8 used different rate calibrations: Pt ˚ /s at substrate center) and Bi from from gun 1 at 112 mA (1.7 A Table 1 Information on the synthesis of the seven composition spread thin films
Fig. 1. A ternary phase diagram of Pt, Bi, and Pb showing compositions that have been investigated. The compositions along line (a) all contain 20 at.% Pt, while the compositions along line (b) all contain 33 at.% Pt.
Film
Elements present
Deposition temperature (8C)
Fluorescence onset potential (mV vs. Ag/ AgCl, 25 mV)
#1 #2 #3 #4 #5 #6 #7 #8
Pt, Pt, Pt, Pt, Pt, Pt, Pt, Pt,
Ambient 100 160 265 400 510 265 260
Inactive Inactive 255 175 175 Inactive 210 Inactive
Bi, Bi, Bi, Bi, Bi, Bi, Pb Bi
Pb Pb Pb Pb Pb Pb
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˚ /s at substrate center). The time of gun 2 at 57 mA (4.2 A deposition was 240 2 s, for an approximate composition ˚ at the center. spread layer thickness of approximately 1400 A 2.2. Fluorescence screening Our method of screening the films for efficient electrocatalysts, initially described by Mallouk and Smotkin [14], was based on the detection of fluorescence arising from quinine, a molecule that exhibits intense blue fluorescence at pH values below 5. Our testing protocol consisted of using the entire film as a working electrode in a solution containing 5 M methanol (Burdick & Jackson), 0.1 M CF3SO3Na (Aldrich), and 0.5 mM quinine (Aldrich). Solutions were prepared using Milli-Q reagent water (Mill-Q, Millipore, 18.2 MV cm resistivity). Fluorescence images were taken using a Nikon D100 camera (Nikon, Japan) and Nikon capture 4 camera control software. A CHI900 SECM (CH Instruments, Austin, TX) was used in the voltammetric studies. All potentials are expressed relative to a Ag/AgCl reference electrode without regard for the liquid junction. We scanned the applied potential toward positive values at a rate of 5 mV/s while monitoring the fluorescence intensity over the entire film. As the oxidation of methanol in an active area of the film generates protons, the local pH decreases below 5 and we observe fluorescence of the quinine. The most active areas will fluoresce at the lowest potentials and before areas of lower activity.
Fig. 2. The diagram shows the location of the nine regions of the film that were characterized before and after electrochemical testing. Also indicated the approximate locations of the Pt, Bi, and Pb sputter guns relative to the regions.
the fluorescence method described above. We then used the same characterization techniques (XRD and EDS) in the same nine regions to see if the composition at any point changed as a result of electrochemical testing. If any portions of the film showed fluorescence activity, we cut out a 1 cm 1 cm section of that part of the film and performed XRD, WDS, and XPS measurements on it. For those films deposited at 160 8C and lower or at 510 8C (films that were entirely inactive), we cut a 1 cm 1 cm section from the same region that showed activity in films deposited at intermediate temperatures and performed the same measurements. 3. Results and discussion
2.3. Characterization methods 3.1. Electrochemical tests of films We characterized the composition spread films using a JEOL 8900 EPMA electron microprobe to estimate the bulk compositions with EDS and WDS. In both types of measurements, we used electron beam energies of 10 keV. EDS measurements were semi-quantitative, with an error of approximately 10%, and performed in beam scanning mode over an approximate area of 1150 mm 800 mm. WDS measurements were done in spot mode, with a beam width in the order of 1 mm. WDS composition determinations were completed using Pt, Bi2Se3, and PbS standards for Pt, Bi, and Pb content, respectively, and TiO2 for oxygen content. The computer program GMRFILM produced thin film corrections of the WDS results. To investigate the composition of the surface, we used two different XPS machines: a Surface Science SSX-100 and a VG Escalab 220i-XL. A Bruker AXS general area diffraction detector system (GADDS) allowed us to observe the X-ray diffraction (XRD) spectra of small regions of the film (approximately 2 mm in diameter or less), while DiffracPlus Basic 4.0 matched the spectra to known structure types. For each film, we performed two sets of characterizations. In the first set, we studied nine different regions, shown in Fig. 2. At a single point in each of the regions, we measured the XRD spectrum with GADDS and composition with EDS. Next, we performed electrochemical measurements on the sample using
As an example of our electrochemical test results, we provide a detailed description of film #5, shown in Fig. 3. A sequence of fluorescence images taken at different applied potentials is given in panels (a)–(d). The relative positions of the Pt, Bi, and Pb sputtering sources are labeled in panel (a). At the initial potential (0.0 V) there was no fluorescence. There was some contrast in the image due to the surface topography of the film; this contrast was also observed under normal white light illumination. As the film potential was increased to 125 mV at 5 mV/s, we saw no indication of blue fluorescence. At an applied potential of +175 mV, a faint fluorescent spot appeared near region 8 (see Fig. 2). At 300 mV, the fluorescence intensity from this region greatly increased. Using an analysis method we have proposed in another paper (reference: a paper in preparation), the fluorescence onset potential was used to quantify and compare the performance of the active areas of the films. The color intensity of the active region, measured in arbitrary units with a linear scale from 0 to 255, was plotted as a function of the applied potential (shown in panel (e) of Fig. 3). Before the onset of fluorescence, the background intensity was typically between 0 and 50. We therefore, defined the fluorescence onset potential as the potential at which the color intensity reaches 50. For the active area of film #5, the onset potential was 175 25 mV.
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Fig. 3. Images of the fluorescence test on film #5 (Pt–Bi–Pb composition spread deposited at 400 8C). Images (a)–(d) show the raw image of the film in a potential scan from 0 to 300 mV vs. Ag/AgCl. Panel (a) also shows the relative locations of the Pt, Bi, and Pb sources. A plot of the color intensity vs. applied potential is shown in (e) for the area with the highest intensity fluorescence.
Table 1 provides a listing of the fluorescence onset potentials for the most active area of each film. For the Pt–Bi–Pb series at different deposition temperatures, there was a change in the electrocatalytic activity for methanol oxidation. In the films deposited at ambient temperature, 100 and 510 8C, there was no activity observed anywhere on the film up to 300 mV. At intermediate deposition temperatures, the onset potential was lowest for films deposited at approximately 265 and 400 8C. The locations of the active areas were similar in all the films that showed activity. We infer that temperature plays an important role in affecting the electrocatalytic activity, possibly by determining the composition and/or crystallinity of the material exposed at the surface.
We also tested the activity of two binary composition spread films (Pt–Bi and Pt–Pb) to further investigate the effect of mixing the two main group metals (Bi and Pb). For Pt–Bi, there was no fluorescence even when the potential ramped up to 400 mV. For Pt–Pb, we observed an active area, again near region 8 on the film. The fluorescence onset potential was 210 25 mV, close to that of the ternary composition spread prepared under the same conditions (film #4). The addition of Bi appears to slightly lower the onset potential (increase the activity) by about 35 mV (although the error bars are large enough that this enhancement could be zero), even though PtBi is completely inactive. Certainly the addition of some Bi does not eliminate electrocatalytic activity.
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Fig. 4. Images of film #5 (a) before and (b) after electrochemical testing. The approximate locations of the Pt, Bi, and Pb sputter guns are also shown in the images.
3.2. Characterization of films before and after electrochemical testing As an example of characterization of the nine regions of a composition spread before and after electrochemical testing, we provide detailed results of film #5 (see Table 1) and give a brief summary of the others. Fig. 4 shows the physical appearance of film #5 before and after electrochemical testing. Regions 1 and 4, close to the Pt gun, had reflective surfaces that remained so after electrochemical testing. The other regions in the sample had dull textures and were darker the closer they were to the Bi and Pb guns. These regions became somewhat lighter in color after testing. As will be shown, we believe this change in coloration is due to Pb leaching from the film at a relatively positive potential versus Ag/AgCl (above 300 mV). Fig. 5 shows the results of semi-quantitative composition measurements, obtained with EDS, of each of the nine regions of film #5. The chart shows the composition of the regions before and after electrochemical testing. Most of the regions maintained their composition, within experimental error, during testing. The exceptions were in regions 6 and 8. These regions showed a significant drop in the atomic percent of Pb: 11% in region 6 and 36% in region 8. The locations of the drops in Pb content corresponded to areas of the films that showed changes in physical appearance as well, such as a change in coloration. Fig. 6 shows the resulting XRD spectra of all nine regions of film #5 before and after electrochemical testing. Also super-
Fig. 5. Semi-quantitative composition measurements, determined with EDS, of all nine regions of film #5 before and after electrochemical testing. The regions with an asterisk indicate that the measurement was completed after the fluorescence test.
imposed on the XRD spectra in Fig. 6 are literature peak positions for the following elements and compounds: PtPb (ICDD PDF file #6-374), Pt (ICDD PDF file #4-802), PtxPb (ICDD PDF file #6574), PtPb4 (ICDD PDF file #6-463), Pb5Bi8O17 (ICDD PDF file #41-405), Bi2O3 (ICDD PDF file #45-1344), Pb (ICDD PDF file #4-686). Regions 1, 2, 5, and 7 contained the least significant changes in XRD spectra before and after electrochemical testing, matching the location of regions that showed few changes in physical appearance and composition (that is, regions that were in close proximity to the Pt gun). These regions also showed common structure types: all four had peaks of varying intensity that correspond to XRD spectra of PbPt (pdf #6-374), although the intensity of the PbPt peaks in region 1 were weak and only appeared after electrochemical testing. It is possible that this appearance of PbPt is due to a preferred orientation of the phase that was not detected in the initial spectrum. The fact that the relative intensities of the PbPt peaks are different from those of the powder diffraction file suggests a preferred orientation. Most of the peaks of region 1 matched both the Pt (pdf #4-802) and PbPtx (pdf #6-574) structure types, but the peaks were slightly shifted from Pt by approximately 0.28 and from PbPt3 by approximately 0.58. This shift is too large to be solely the result of strain in the film. The GADDS instrument allows one to position the sample to the correct height using a laser beam positioning device, but this is difficult to confirm using the Bradley–Jay function [22] since there are only the three peaks for each phase in the spectrum. Another possible cause of the shifts is that some Pb or Bi may have substituted into sites in the lattice. The other regions of film #5 showed significant changes in XRD spectra. Regions 3 and 6 generally showed peaks at the same values of 2u before and after electrochemical testing, but the intensity of those peaks increased by approximately 10% in most cases. One exception to this observation is in region 3 at 278, where the peak increased by 133%. A few peaks disappeared entirely, notably those at 2u = 298 and 408 in region 6. Region 4 showed peaks that matched PbPtx and smaller peaks that matched Pt. The Pt peaks increased in intensity after electrochemical testing, with the tallest peaks increasing by 150–200%. This may suggest that Pb leached from the region, although EDS measurements showed no significant drop in the atomic percentage of Pb. The most drastic changes occurred in regions 8 and 9. In region 8, peaks at 2u = 308, 33.58, 38.58, 55.58, 59.58, and 628 have either decreased in intensity or disappeared altogether. Each of these peaks matched the Pb4Pt (pdf #6-463)
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Fig. 6. XRD spectra of all nine regions of film #5. The best matches to the peaks of known elements and compounds from the set of powder diffraction files are shown by the vertical lines in each spectrum.
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structure type. Region 9 also showed significant decreases in the intensity or disappearance of peaks that match the Pb (pdf #4686) or Pb4Pt structure types. It is interesting to note that the regions that showed the most significant changes in our measurements (regions 6, 8 and 9) had some of the highest Pb content. This suggests that during electrochemical testing, Pb had a tendency to leach from regions of the films where there was a large amount of Pb. This is probably due to the relatively low standard reduction potential of the Pb2+/Pb couple ( 0.3474 V versus Ag/AgCl). A scan of the potential of the film in the positive direction will oxidize Pb to Pb2+ into solution before Bi or Pt, which have higher standard reduction potentials. An exception to this rule is in region 7, where there was 56–57 at.% Pb. However, the XRD spectrum of this region strongly matched the PbPt structure type, and the ratio of Pt:Pb was close to 1:1. If the intermetallic compound PbPt had indeed formed in region 7, the Pb could only be oxidized if the free energy of bond formation between Pb and Pt were overcome in addition to the free energy required to remove two electrons from Pb. This would increase the reduction potential of Pb into solution and promote the stability of the Pb against oxidation. Using a method described by Miedema et al. [23,24], we estimated the enthalpy of formation of PtPb to be 26.6 kJ/mol of atoms. If we neglect the entropy contribution to the free energy of formation, this shifts the standard reduction potential of Pb oxidation by 0.138 V in the positive direction. The other films showed similar behavior. Significant changes in composition and XRD spectrum were more likely to occur in regions 6, 8, or 9, usually with a decrease in the atomic percentage of Pb. Notable exceptions to this observation occurred in film #6 (an inactive film deposited at 510 8C), where the composition of almost every region was stable. This is probably due to the fact that the Pb content is lower with increasing deposition temperature, so there is less of it in elemental form to leach. A higher deposition temperature, especially if it is above the melting point of Pb (327 8C), would increase the vapor pressure of the Pb and make it less likely to stick to the thin film. XRD spectra of regions 6, 8, and 9 of film #6 also showed less significant changes than film #5. In these regions, each of the peaks decreased in intensity but their positions in 2u remained the same. In region 6 of film #2 (an inactive film deposited at 100 8C), the atomic percentage of Pb actually increased by 13%, while Bi decreased by 11 at.%. This could have been due to the oxidation of Pb from parts of the film and resettlement at other locations. There may also have been some Bi leaching that is significant in this region but not detectable in others. 3.3. Characterization of active areas after electrochemical testing Fig. 7 shows the composition, measured by WDS, of the active areas for film #3, #4, #5, and #7, along with inactive areas on film #1, #2, and #6 that had the same location as the active areas on other samples. As can be seen from the chart and as expected from the position on the films, the Bi content in the
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Fig. 7. Bulk composition, measured with WDS, of the active areas of film #3, #4, #5, and #7. Also shown the bulk compositions of inactive areas of film #1, #2, and #6 that have similar positions on the film as active areas on other samples.
active areas was low, never more than 10 at.%. The oxygen content in active areas in part arises from surface oxidation of the film on exposure to air and perhaps from oxygen/water contamination in the sputtering system. The sputtering vacuum chamber is typically pumped down to base pressures on the order of several microTorr, and may still have enough oxygen present to be incorporated into the films at some level. Fig. 8 shows the XRD spectra of the same active and inactive areas characterized with WDS. Each of these areas had peaks that are similar in location in 2u, but the peak intensities increased and line widths decreased with increasing deposition temperature. This is expected, since at higher temperatures closer to the melting point the deposited atoms have higher surface and bulk mobility, resulting in grain growth. Note that the incongruent melting point of PtPb is 790 8C and the two highest deposition temperatures are 63 and 74% of this temperature on an absolute scale. The majority of the peaks in each spectrum matched the position of the PtPb/PtBi structure type (pdf #6-374). However, the relative intensities of the peaks differ from those of a powder pattern. This suggests that the crystallites of the active areas may have a preferred orientation. An additional peak at 348 appears frequently in all samples, regardless of composition, and is due to the Ta underlayer. The XRD spectra of the same area on different active and inactive films suggest that given the location of activity on a Pt– Bi–Pb thin film composition spread, we can predict the structure type of the active area. However, structure type is not sufficient to predict activity. Similarly, the composition data show that composition is also not sufficient for the prediction of activity in some area of the film. These data suggest that the bulk composition of the film is not the controlling factor in determining electrochemical activity. This is perhaps not a surprise, since the activity occurs at the film surface. It is possible that the surface composition, structure, and morphology is different than the bulk and is likely to be a function of the deposition temperature. Fig. 9 shows the composition of metallic elements of the same areas in each film (whether it is active or inactive) as
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Fig. 8. XRD spectra of the same active and inactive areas as those measured in Fig. 7. The best matches to the location of peaks of known elements and compounds from the set of powder diffraction files are shown by the vertical lines in each spectrum.
measured by XPS. This measurement describes the composition at the surface instead of the bulk, as is the case with EDS and WDS measurements. In comparing the surface compositions in Fig. 9 to bulk compositions in Fig. 7, a notable
difference was the greater atomic percentage of Pb and Bi at the surface. The rise of these elements to the surface could have been due to their lower melting points, which would give them greater mobility during deposition. Fig. 9 shows that a higher
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the active areas had similar structure types, we found that neither structure type nor composition (in the bulk or at the surface) were determinants of active electrocatalysts. The reasons for this result are still under investigation. Acknowledgements
Fig. 9. Surface composition of the metallic species, measured by XPS, of the same active and inactive areas described in Fig. 7.
deposition temperature appeared to decrease the atomic percentage of Pb at the surface, as evidenced for film #3 through #7. The data show that surface composition does not determine the appearance of activity in the fluorescence tests. Film #1 and #2 could lead us to believe that the high Pb content prevents electrochemical activity, but film #6 was also inactive and showed a low fraction of Pb. The results point out the need for more surface characterization techniques to better understand the relationship between deposition temperature and electrochemical activity. 4. Conclusions We have used a combinatorial method to screen a large number of candidates for fuel cell anode electrocatalysts of type PtxBiyPbz. We deposited six composition spread thin films of Pt, Bi, and Pb at different deposition temperatures from ambient to 510 8C. We also deposited binary composition spreads of Pt and Bi at 260 8C and Pt and Pb at 265 8C. Our results show that the optimal electrocatalyst in this system was deposited at 400 8C (determined by fluorescence screening), has an average composition of Pt0.46Bi0.09Pb0.20O0.25 (determined by WDS), and has the same structure type as the ordered intermetallic compound PtPb (determined by XRD). A composition spread deposited with only Pt and Pb confirms that low fractions of Bi do not deactivate the electrocatalyst, but may in fact decrease the onset potential by up to 50 mV. We sampled nine points on each of the films with EDS and XRD measurements before and after electrochemical testing by fluorescence. We found that areas that are close to the Pb source tend to change composition and structure significantly. This is probably due to the low standard reduction potential of Pb leading to the dissolution of the elemental form, or leaching of Pb from Pb-rich phases such as PtPb4. The deposition temperature plays a crucial role in synthesizing active electrocatalysts in the Pt–Bi–Pb system. We found that the fluorescence activity is optimal when the deposition temperature is between 260 and 400 8C. Although
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