Perovskite-Based Catalysts for Direct Methanol Fuel Cells - The

J. Phys. Chem. C 2007, 111, 9573-9582

9573

Perovskite-Based Catalysts for Direct Methanol Fuel Cells Aidong Lan and Alexander S. Mukasyan* Department of Chemical and Biomolecular Engineering, Center for Molecularly Engineered Materials, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: NoVember 7, 2006; In Final Form: March 8, 2007

As an effort to further explore the possible complex oxide catalysts for methanol electrooxidation, a library of perovskites (ABO3; A ) Ba, Ca, Sr, La; B ) Fe, Ru) were synthesized and tested. A novel screening strategy, featuring energy-efficient and rapid solution combustion (SC) synthesis techniques in combination with a throughput catalyst activity testing method, was employed. It was demonstrated that most of these mixed-conductor complex perovskites with ruthenium on the B-site are promising candidates for the development of effective catalysts. A possible reaction pathway on the perovskite (e.g., ARuO3) surface is proposed in analogy to the well-established reaction mechanism of methanol oxidation on a Pt surface. Additional experiments by using ethanol and formic acid as fuels were conducted to give further insights on the proposed reaction pathway. Furthermore, composite perovskite-Pt compositions were also synthesized directly by the SC method for the design of multifunctional catalysts, where the optimum amount of noble metal was found to be ∼10 wt %. These novel catalysts, containing 4 times less platinum, display comparable apparent catalytic activity to standard Pt-Ru alloy. Structure and surface properties of these novel catalysts were also studied by using X-ray photoelectron spectroscopy and X-ray diffraction techniques. The above findings strongly suggest that the proposed approach for design of multifunctional catalysts is practical and effective.

1. Introduction One of the drawbacks of the direct methanol fuel cell (DMFC) is the poisoning of the anodic catalyst. During the lowtemperature oxidation of the fuel, the reaction intermediate, i.e., carbon monoxide, strongly binds to the catalyst surface and blocks the active sites. Because of this, a large quantity of noble metal material (e.g., Pt) is typically required, which leads to an increased cost. Also, it is well-known that pure Pt as a catalyst is ineffective in avoiding this poisoning effect. Among the different approaches to solving this problem, the most successful has been the use of a Pt-Ru alloy. The addition of Ru substantially improves the CO tolerance of the catalyst, and there has been a great deal of research on the optimization of the alloy composition and structure.1,2 In our studies, we have followed a different approach for the development of an effective, low-cost, multifunctional anodic catalyst for direct alcohol-based fuel cells. It is proposed that mixed conductive complex oxides, some of which are excellent catalysts for CO oxidation,3 could be suitable alternative materials for such an application. More specifically, it is known that a variety of perovskites (ABO3) are electronically conductive and also possess excellent proton transport properties.4 In addition, the surfaces of perovskite compounds are acidic, and hence, they are stable and active in a strong acid environment. Finally, the vast applications of perovskites for reformation of hydrocarbons suggest that such materials should have a better tolerance toward oxidation of reactive sites than Pt alloys.5 In our previous work,6 we demonstrated a novel methodology for finding effective perovskite-based catalysts. Instead of utilizing conventional labor-intensive and time-consuming catalyst exploration methods, a screening strategy featuring energy-efficient and rapid solution combustion (SC) synthesis * Author to whom correspondence should be addressed. Phone: (574) 631-9825. Fax: (574) 631-8366. E-mail: [email protected].

techniques in combination with a throughput catalyst activity testing method was employed. It was shown that the SrRuO3 composition possessed a sizable activity for methanol electrooxidation in the conditions similar to those for DMFCs. It was also revealed that the direct incorporation (during the combustion reaction) of a small amount of platinum on the perovskite surface leads to a significant enhancement of its catalytic activity. In this work, a library of perovskites (ABO3; A ) Ba, Ca, Sr, La; B ) Fe, Ru) were synthesized and tested for electrooxidation of methanol. It was proven that most of these mixedconductor complex perovskites with ruthenium on the B-site are promising candidates for the development of effective catalysts. However, two families, LaRuO3 and SrRuO3, showed the best performance. Composite perovskite-Pt catalysts were also synthesized directly by SC method, where the optimum amount of noble metal was found to be ∼10 wt %. These catalysts demonstrate apparent catalytic activity similar to the standard Pt-Ru alloy. However, the former involve 4 times less platinum than the standard. The above findings confirm the effectiveness of the suggested approach for design of multifunctional catalysts. 2. Experimental Procedure 2.1. Catalyst Synthesis. Solution combustion synthesis is an attractive technique for the production of different oxides, including ferrites, zirconia, and perovskites.7 It involves a selfsustained reaction between an oxidizer (e.g., metal nitrate) and a fuel (e.g., glycine, hydrazine). In this work, metal nitrates (Alfa Aesar) Me(NO3)x (where Me ) Ba, Ca, Sr, La, Fe) and glycine (C2H5NO2) were used to synthesize different catalysts. Ruthenium nitrosyl nitrate solution (Alfa Aesar) was utilized to prepare Ru-based perovskites. Although SC can be performed in different reaction modes, including volume combustion synthesis (VCS), self-propagating sol-gel combustion (SGC),

10.1021/jp067343p CCC: $37.00 © 2007 American Chemical Society Published on Web 06/14/2007

9574 J. Phys. Chem. C, Vol. 111, No. 26, 2007

Lan and Mukasyan

TABLE 1: Catalysts Characteristics before and after Calcination composition

surface area, BET (m2/g)

XRD identification as-synthesized

BET after calcinations, (m2/g)

2.2 1.5 18.5 9.1 17

SrFeO3, Sr(NO3)2 BrRuO3, Br(NO3)2 CaO SrRuO3, Sr(NO3)2 La2O3, LaRuO3

1.5 1.4 2.3 4.0 8.5

SrFeO3 BaRuO3 CaRuO3 SrRuO3 LaRuO3

and impregnated combustion (IC),9,10 we chose to use the conventional VCS approach. First, reactants are dissolved in water and the obtained solution is thoroughly mixed to essentially reach homogenization on the molecular level. After preheating to its boiling point (100 °C), water evaporates followed by solution self-ignition at some temperature (Tig), which varies for different systems (e.g., ∼130 °C for LaRuO3 and 250 °C for SrRuO3). After ignition, the temperature rises rapidly (up to 103 °C/s) to values in the range of 3001000 °C. High temperature, accompanied by intensive gasification (CO2, N2, steam) during a short time period (0.1-1 s), converts the initial solution to a fine, well-crystallized powder. In general, under equilibrium conditions, the combustion reaction in such systems can be represented as follows:

(95νφ)CH NH CO H + ν45(φ - 1)O w 25 10 5 + ( νφ)CO + φH O + ν( φ + 1)/2N 9 18 9

Meν(NO3)ν + MeOν/2(s)

2

2

2

(g)

(g)

2

2

2

(g)

2

where Meν is a metal with valence ν, φ is a fuel to oxidizer ratio. φ ) 1 means that the initial mixture does not require atmospheric oxygen for complete oxidation of the fuel, whereas φ > 1 (0.3 V).16 In analogy to the well-established reaction mechanism of methanol oxidation on a Pt surface,2,15 a possible reaction pathway on the perovskite (e.g., ARuO3) surface can be described as follows:

CH3OH(sol) + ARuO3(H2O) f ARuO3(CH3OH) + H2O (1) ARuO3(CH3OH) f ARuO3(CO) + 4H+ + 4e(several steps) (2) ARuO3(H2O) f ARuO3(OH) + H+ + e-

(3)

ARuO3(CO) + ARuO3(OH) f CO2 + H+ + e-

(4)

It is assumed that in the range of potential where methanol oxidation occurs, the surface of ARuO3 undergoes several processes, including the competition for adsorption sites between water and methanol, which plays a decisive role in the investigated reaction. To explain the existence of the first peak on the typical CV curves for ARuO3 perovskites, we assume that reactions 1-3 take place at a potential even below 0.3 V. In this region, methanol adsorption is partially inhibited by adsorbed hydrogen or water on the perovskite surface. The methanol has to displace adsorbed species in order to be dehydrogenated and oxidized on the electrode surface, and it is suggested that the perovskite structures assist the dissociative adsorption and electrooxidation of methanol at low potentials. Note that no such peaks were observed in the case when after the same conditioning procedures the anodes were fed with humidified N2 or CO (see also Figure 4). These results strongly indicate that the anodic activities observed at this potential range in the methanol and ethanol can be exclusively assigned to electrooxidation of these alcohols. It is known that when a perovskite surface is exposed to water, vapor hydroxyl [OH] groups can readily form.5 A high concentration of hydroxyl groups on the surface certainly favors the complete methanol oxidation. Under such premises, there is a likelihood that there are small scales of dissociate adsorption, and complete oxidation taking place even in the low-potential region, which partially contribute to the sizable current density of peak 1. Since catalytic behavior is primarily determined by the transition metal on the B-site of perovskite (ABO3) structures, the futility of SrFeO3 toward oxidation of methanol simply indicates that Fe does not form an electrocatalytic active site on the surface.

The further decay of current density is due to CO adsorption on the surface sites, which has higher activation energy of reaction. When the sweeping potential is above 0.3 V, the kinetics of CO oxidation (reaction 4) picks up. However, simultaneously the chemisorption of water was also accelerated. Consequently, the competition of methanol with water for adsorption sites becomes visible. Considering the relatively low current intensities of peak 2 observed for investigated perovskites, it is suggested that dehydrogenation of methanol on the surface of perovskites is an intrinsically slow process. Accordingly, at higher potentials, methanol adsorption becomes a ratelimiting step, and for this reason the reaction rate passes through a maximum near 0.4 V and then decays. To get further insights on the methanol oxidation mechanism on the perovskite surface, additional experiments were conducted with different anodic fuels. Four fuels were employed, including humidified N2, humidified CO, 1 M ethanol, and 1 M formic acid solutions. Representative results obtained for SrRuO3 and LaRuO3 catalysts are shown in Figure 4. 3.2.1. Humidified N2. The purpose to use this “fuel” is to create a scenario where no organic species are present in the vicinity of the electrodes. As shown in Figure 4, no distinguishable activities were detected for both catalysts throughout the investigated potential range. Note that, even in the low-potential region where the hydrogen desorption peak is expected to appear, the current density is fairly small compared to the peak 1 observed in the case of methanol oxidation (Figure 3). This result does support the assumption that the parallel methanol chemisorption takes place at low potential on the surface of perovskites. 3.2.2. Humidified CO. Since CO, arising from the dissociation of methanol, is known to be the main poisoning species at the electrode surface, the behavior of perovskite electrodes in a CO environment is important for evaluation of their potential for a fuel cell application. As can be seen in Figure 4, the onset of CO oxidation occurred at 0.35 V for both perovskite catalysts. Also note that current densities for the La-based catalyst are much higher than those for Sr-based perovskites. For the sake of comparison, Table 2 lists potentials for the onset of oxidation for different fuels on perovskite- and Pt-based catalysts obtained in this study. It is noteworthy that the onsets of oxidation for CO occur at much lower potentials for perovskites as compared to Pt and almost coincide with Pt-Ru electrodes. In addition, as shown in Figure 4, the oxidation curves of both positive and negative sweeping directions exhibit a clear symmetry, which indicates that the process is facile and only charge transfer is the controlling factor of the reaction. Note, that no measurable CO oxidation current density was observed during similar experiments on AFeO3-based perovskites (not shown in the graphs). It is well-known that during electrooxidation of alcohols on a Pt-Ru catalyst, ruthenium activates water and provides preferential sites for OH group adsorption at low potential (0.2 V vs RHE).17 These OH groups are essential for complete oxidation (to CO2) of the intermediate chemisorbed species. As a result, the oxidation of CO on ARuO3 perovskites can be accredited to the presence of Ru. 3.2.3. Ethanol. The oxidation of ethanol is a more complex process with respect to methanol oxidation because its full oxidation involves 12 electrons in dehydrogenation and the breakage of a C-C bond.18 Evidently in Figure 4, the major standout feature of ethanol oxidation is the absence of peak 1 and lower current densities at low potential. The monotonic ascent of current can be understood assuming that the increase of sweeping potential facilitates charge transfer, which is the

Perovskite-Based Catalysts for Methanol Fuel Cells

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9577

Figure 4. CV for SrRuO3 and LaRuO3 electrodes subjected to different anode fuels (temperature 80 °C; sweeping rate 6 mV/s).

TABLE 2: Potential for the Onset of Oxidation of Different Fuels on Perovskite and Pt-Based Catalysts (Data Were Obtained at 80 °C, 6 mV/s)

catalyst

onset potential of methanol (V vs DHE)

onset potential of ethanol (V vs DHE)

onset potential of formic acid (V vs DHE)

onset potential of CO (V vs DHE)

SrRuO3 LaRuO3 Pt Pt-Ru

0.28 0.30 0.48 0.27

0.33 0.33 0.50 0.31

0.30 0.32 0.28 0.28

0.33 0.35 0.50 0.33

controlling factor of a complex ethanol dissociation process, whereas resultant sparse CO coverage plays a secondary role.

3.2.4. Formic Acid. Although no longer being considered as a prototype for the electrooxidation of methanol,16 the reaction of formic acid offers some additional insights on our perovskite catalysts. As shown in Figure 4, in the case of formic acid oxidation, a small peak appeared in the low-potential range. Contrasting to the other alcohols fuels, the chemisorption process of each formic acid molecule involves only two electrons. Consequently, CO coverage rapidly becomes a predominant factor that leads to observed current decay. Another interesting result is the slightly higher onset potential, which was found for formic acid oxidation on perovskite-based catalysts as compared to Pt (see Table 2). It is generally accepted that on the Pt surface, formic acid electrooxidation proceeds


Lan and Mukasyan

Figure 5. Comparison of the performance for as-prepared and calcinated perovskite-based catalysts: cell temperature 90 °C, sweeping rate of 6 mV/s, 0.5 M methanol.

TABLE 3: Some Parameters of Investigated Pt-Containing Catalysts sample type

catalyst loading (mg/cm2)

actual Pt loading (mg/cm2)

BET surface area (m2/g)

LaRuO3 + 1 wt % Pt LaRuO3 + 5 wt % Pt LaRuO3 + 10 wt % Pt LaRuO3 + 15 wt % Pt SrRuO3 + 1 wt % Pt SrRuO3 + 5 wt % Pt SrRuO3 + 10 wt % Pt SrRuO3 + 15 wt % Pt Pt-black Pt-Ru

5.9 6.9 6.8 6.5 6.2 5.8 6.1 6.2 3.0 4.7

0.06 0.34 0.68 0.97 0.06 0.29 0.61 0.92 3.00 3.10

9.7 7.8 7.0 6.4 8.8 7.8 7.3 6.8 30 62

through the dual-pathways mechanism.16 This involves complete oxidation through a reactive intermediate (RI) to form CO2 in parallel with a partial-oxidation pathway to CO. The former path results in a lower potential of onset of complete oxidation of formic acid. Accordingly, considering the observation in this study, it is very likely that no RI formed on the surface of the perovskite, and thus no parallel reaction pathways take place on the surface of perovskite electrodes. 3.3. Complex Perovskite-Pt Catalysts: Synthesis and Electrochemical Characterization. One of the most important conclusions that can be drawn from the above results is that Ru-containing perovskites are catalytically active for alcohol oxidation. However, it is also clear that these perovskites are not as effective as standard Pt-Ru alloy catalysts. Furthermore, our results point toward this ineffectiveness being the result of slow kinetics of C-H bond scission, the essential step of methanol dissociation. Since platinum is universally regarded as the best catalyst to break the C-H bond, combining Pt and perovskite to form a bifunctional catalyst for alcohol oxidation is the next logical step to be pursued. Indeed, our previous results demonstrated that the catalytic activity of Pt can be significantly enhanced by addition of Pt into a SrRuO3 perovskite.6 In this study, the La- and Sr-based compositions, due to their apparently better activities among

the investigated perovskite (see Figure 2), were selected as the basic families for the design of composite perovskite-Pt catalysts. As mentioned in section 2.1, all complex powders were synthesized by adding a Pt-containing reagent (e.g., tetraammine platinum nitrate) into the initial metal nitrate + glycine solution, followed by direct SC synthesis of the catalysts. It should be pointed out that the catalytic activities of these perovskite + Pt complex catalysts on methanol oxidation appeared to be sensitive to the calcination treatment. Figure 5 illustrates the typical influence of calcination treatment on catalyst performance. Mixed results are observed on perovskites without Pt. Calcination, which leads to better perovskite structure, enhances the catalytic activity of SrRuO3, whereas it diminishes catalytic activity for LaRuO3. Meanwhile for perovskite + Pt complex catalysts, the results suggest that calcination treatment generally has a negative effect on overall catalytic activity. Taking into account the complex changes underwent on the surface of these perovskites during the heat treatment, we do not have a straightforward explanation for the above effects (further discussion of this issue can be found in section 3.5). Accordingly, detailed electrochemistry tests have been conducted on a series of as-prepared perovskite + Pt complex catalysts. The loading density, Pt concentration, and BET surface areas of the investigated systems are listed in Table 3. The most


J. Phys. Chem. C, Vol. 111, No. 26, 2007 9579 TABLE 4: Potential for the Onset of Oxidation for Methanol at Various Temperatures on Complex Perovskite + Pt and Pt-Ru Catalysts

Figure 6. I-V polarization curves of various Pt-based catalyst anode electrodes in Nuvant systems: cell temperature 80 °C, sweeping rate of 6 mV/s, 0.5 M MeOH.

important statistically proven results (I-V polarization curves) are presented in Figure 6. For the sake of comparison, the data on the Pt-Ru and Pt-black standards are also shown. Several observations can be outlined. First, for both LaRuO3 + Pt and SrRuO3 + Pt catalysts, a substantial decrease in the onset potential of methanol oxidation compared to that of pure Pt-black is observed. It is noteworthy that LaRuO3 + Pt exhibits more negative onset potential (ca. 0.26 V) than that of SrRuO3 + Pt (ca. 0.30 V). Second, the apparent activities of the complex catalyst with ∼10 wt % Pt are comparable with those for the Pt-Ru standard. Third, the saturation effect is evident, i.e., activities of the complex catalysts do not increase with increasing Pt loading above 10 wt %. Finally, the performance of the La-based perovskite is in general higher as compared to Srbased composition. The bifunctional feature of these complex catalysts is evident from CV curves shown in Figure 7 for compositions with a

catalyst

onset potential at 60 °C (V vs DHE)



SrRuO3 + 10 wt % Pt LaRuO3 + 10 wt % Pt Pt-Ru

0.35 0.32 0.32

0.32 0.29 0.30

0.29 0.27 0.27

low concentration of Pt (e.g., 1 wt %). Upon close inspection, two contributing origins of the observed catalytic activity from these complex catalysts can be distinguished, i.e., perovskite at lower potential (0.1-0.4 V) and Pt at higher potential (above 0.5 V). Comparison between perovskite + Pt catalysts and Ptblack quite clearly reveals that the presence of perovskite in the material is responsible for initiation of methanol oxidation at lower potential. At higher potential, the lack of methanol chemisorption on perovskite is complemented by the addition of Pt. As noted above, it is found that there is a saturation point (in Pt wt %) for the enhancement of catalytic activity for these complex compositions. As can be seen in Figure 6, an increase of Pt concentration in the range of 1-10 wt % leads to a significant increase of electrocatalytic activity. However, a further rise (e.g., up to 15 wt %) in metal quantity does not provide a sizable enhancement in material performance. The most spectacular results are observed on the LaRuO3 + 10 wt % Pt composite, which exhibits almost identical I-V characteristics as the Pt-Ru standard. It is a remarkable fact, considering that the former catalyst has only 0.61 mg/cm2 of actual Pt loading compared to 3.1 mg/cm2 for the Pt-Ru sample (see Table 3). Nevertheless, in spite of the striking similarity between perovskite complex catalysts and Pt-Ru alloy, it is not clear yet if they exhibit a similar bifunctional mechanism, i.e., Ru-containing perovskites render Pt atoms more susceptible to OH adsorption to overcome CO poisoning and thus promote the overall catalytic activity.2 In addition, the onset potentials

Figure 7. Comparison of the CV obtained from perovskite + Pt and Pt-based standard catalysts electrodes by the Nuvant system: cell temperature 80 °C, sweeping rate of 6 mV/s, 0.5 M methanol.


Lan and Mukasyan TABLE 5: Binding Energy, fwhm (Full Width of Half-Maximum), and Relative Intensity for Different Ru Species as Observed from Pt(4f) Spectra for a Variety of Pt-Based Catalysts

catalysta SrRuO3 + 10% Pt (AP) SrRuO3 + 10% Pt (CAL) LaRuO3 + 10% Pt (AP) LaRuO3 + 10% Pt (CAL)

a

Pt species

BE of Pt (4f7/2) (eV)

fwhm (eV)

relative intensity (%)

Pt0 Pt2+ Pt4+ Pt4+ Pt4+ Pt4+ Pt0 Pt2+ Pt4+

71.9 73.2 76.1 74.9 75.1 76.3 71.5 72.6 75.9

1.6 2.2 2.3 2.0 2.3 1.5 1.2 1.3 1.4

46 30 24 100 62 38 49 27 24

AP ) as-prepared; CAL ) calcinated.

TABLE 6: Binding Energy, fwhm, and Relative Intensity for Different Ru Species as Observed from Ru(3p) Spectra for a Variety of Ru-Based Catalysts Figure 8. Potentiostatic measurement results on a various catalysts. The potential is set at 0.4 V vs DHE. catalysta

of methanol oxidation at various temperatures on complex perovskite + Pt and Pt-Ru catalysts are listed in Table 4. It can be seen that all onset potentials shift negatively as the temperature increases. 3.4. Kinetics of Complex Perovskite-Pt catalysts. A CV gives qualitative results for the evaluation of the electrodes apparent activity but cannot provide accurate kinetic data since the potential is not kept constant. Such data can be obtained by using a potentiostatic approach, i.e., chronoamperometry, which involves a current measure after a step change in the electrode potential. In our work, such measurements were conducted at different constant potentials in the range of 0.1-0.7 V for time periods of 1-5 h in Nuvant systems. Figure 8 shows examples of such current-time transients recorded for perovskite-based and standard catalysts at a constant anode potential of 0.4 V, which is a potential of technological interest. The typical current-time transient is an exponential decay curve. The total measured current, iT, involves at least three components: Faradaic current iF, the charging current iC, both decaying with time t, and a noise current (iT ) iF + iC + noise). As iC decays exponentially, after a short period (several seconds) the current becomes predominantly of a Faradaic nature, approaching a steady-state value. As can be seen from Figure 8b, there are decent steady current densities for the perovskites SrRuO3 and LaRuO3. This can serve as decisive evidence to prove the Faradaic nature of peak 2 observed in Figure 3. It is noteworthy that the steady current density observed for LaRuO3 + 15% Pt (Pt loading 0.97 mg/cm2; Figure 8a) is higher than the Pt-Ru standard sample which has an actual Pt loading of 3.1 mg/cm2. This result implies that novel perovskite-Pt catalysts could deliver superior performance compared to the standard Pt-Ru in real fuel cell systems. 3.5. Surface Characterization of Perovskite-Based Catalysts. It was shown above that perovskite-platinum composite catalysts exhibit significant electrocatalytic activity toward oxidation of methanol under the conditions similar to those for DMFCs. However, it is still elusive which element and what surface structure contribute to this high activity. In order to gain a better insight into the surface properties of our perovskite + Pt composite materials, further investigations have been undertaken on two sets of promising perovskite/Pt composites (i.e., LaRuO3 + Pt and SrRuO3 + Pt systems) by XPS.

Ru species

BE of relative Ru 3p3/2 fwhm intensity (%) (eV) (eV)

LaRuO3 + 10% Pt (AP)

Ru4+ Ru6+ RuO2‚xH2O LaRuO3 + 10% Pt (CAL) Ru4+ Ru6+ SrRuO3 + 10% Pt (AP) Ru4+ Ru6+ SrRuO3 + 10% Pt (CAL) Ru4+ Ru6+ SrRuO3 (AP) Ru4+ Ru6+ RuO2‚xH2O SrRuO3 (CAL) Ru4+ Ru6+ LaRuO3 (AP) Ru4+ Ru6+ LaRuO3 (CAL) Ru4+ Ru6+ a

463.5 465.6 467.9 463.6 465.6 462.4 465.3 463.5 465.8 462.4 464.8 467.6 463.1 465.7 462.5 465.3 463.3 465.1

2.3 3.2 2.5 3.2 2.8 3.4 4.0 3.2 3.1 3.3 3.1 2.8 3.0 3.9 3.7 3.5 2.8 2.9

13 66 21 63 34 68 32 64 36 59 31 10 56 44 71 29 61 39

AP ) as-prepared; CAL ) calcinated.

Accordingly, detailed scans were recorded for the Pt 4f, Ru 3p signals. Tables 5 and 6 list the binding energies, full width of half-maximum (fwhm), and relative intensity for different Pt and Ru species, obtained by quantitative treatments of Pt 4f7/2 and Ru 3p3/2 peaks. The Pt 4f7/2 peak observed in most catalysts are typically deconvoluted into three components: one around 71.4 eV and two others with higher binding energies, around 72.8 and 75.1 eV, which have been assigned to Pt with valences of 0, +2, and +4, respectively.19,20 In turn, the Ru 3p3/2 peak is deconvoluted into a component at 462.0 eV (Ru0) and higher binding energy components at 463.0 and 467.1 eV, which are ascribed to RuO2, RuO3, and hydrated RuO2‚xH2O species.19,20 Inspection of Table 5 shows that calcinated LaRuO3 + 10 wt % Pt exhibits predominantly the Pt0 state, while in its assynthesized counterpart Pt is totally oxidized. Note that corresponding X-ray diffraction (XRD) results (Figure 9a) support this observation. Indeed, prominent Pt peaks are observed in calcinated samples but are not visible in as-prepared samples. However, in the case of SrRuO3 + 10 wt % Pt, a reverse trend is observed. Pt peaks show up in XRD results for as-prepared samples and disappear for calcinated ones (see Figure 9b). Also, XPS results clearly reveal the existence of the Pt0 state in asprepared samples, whereas only the fully oxidized state of Pt was probed in calcinated samples.


J. Phys. Chem. C, Vol. 111, No. 26, 2007 9581 structural measurements (EXAFS) under reaction conditions. After these experiments this issue will be discussed in detail. 4. Concluding Remarks In this work, we employ a new screen strategy to synthesize and test a variety of complex oxides materials (perovskite) for their catalytic activity toward methanol oxidation. It was shown that most of the mixed-conductor complex perovskites (ABO3) with ruthenium on the B-site are promising candidates for the development of effective anodic catalysts for DMFC. Furthermore, composite perovskite-Pt catalysts synthesized directly by SC exhibit apparent catalytic activity similar to the standard Pt-Ru alloy. The corresponding potentiostatic test result implies that these novel perovskite-Pt catalysts could deliver superior performance compared to the standard Pt-Ru in real fuel cell systems. From the synthesis standpoint, the specific surface area of the catalyst can be improved by using another recently developed combustion-based method, so-called impregnated solution combustion (ISC) synthesis.9,10 It was shown that the application of ICS for synthesis of complex oxide compositions indeed led to significant enhancement of specific surface area of produced catalysts. Although it is premature to say these perovskite-based catalysts are legitimate alternatives of standard Pt-Ru yet, the findings in this work do warrant a further study on this direction. Moreover, this work provides a practical road map toward finding competitive electrocatalysts in the jungle of the perovskite world. It is believed that this study of perovskite-based catalysts may hold a key to a low-cost solution for synthesis of effective catalysts for DMFC.

Figure 9. XRD results for (a) LaRuO3 + 10% Pt and (b) SrRuO3 + 10% Pt.

It is noteworthy that as-prepared LaRuO3 + Pt complex samples, in which only oxidized Pt states are probed, outperform their calcinated counterparts, which show a prominent Pt0 presence (evident in Figure 5). In addition, as-prepared SrRuO3 + Pt does not show a noticeable advantage over calcinated SrRuO3 + Pt in terms of catalytic activity. Note that the Pt0 state was probed only in the former catalyst. These results strongly suggest that platinum in oxidative states +2 and +4 plays an active role toward methanol electrooxidation for complex perovskite-Pt compositions. However, it is not yet clear why Pt is present in different oxidative states in LaRuO3 + Pt and SrRuO3 + Pt samples under similar treatments. A close survey of Table 6 indicates that no photoemission intensity corresponding to zero-valence Ru was detected for any investigated catalyst. In other words, in these samples, Ru exists only in oxidized forms. Since we accredited Ru for the observed electrocatalytic activity of investigated perovskites (see Figure 2), the conclusion can be drawn that Ru in oxidized form is catalytically active toward methanol oxidation. In fact, this statement is in good agreement with the results obtained by others.21-23 We understand the fundamental importance of statement regarding “active” Ru in oxidized form. Certainly, it is premature to jump to any final conclusive statement on this interesting point. Currently, we are preparing for in situ catalyst fine

Acknowledgment. This work was supported by the U.S. Army CECOM RDEC through Agreement AAB07-03-3-K414. Such support does not constitute endorsement by the U.S. Army of the views expressed in this publication. References and Notes (1) Dircks, K. Recent Advances in Fuel Cells for Transportation Application. SAE special publication; 1999, 1425, 39. (2) Lamy, C.; Lima, A.; LeRhun, V.; Delime, F.; Coutanceau, C.; Le´ger, J.-M. J. Power Sources 2002, 105, 283. (3) Tejuca, L. G.; Fierro, J. L. G.; Tascon, J. M. D. AdV. Catal. 1989, 36, 237. (4) Norby, T. Solid State Ionics 1999, 125, 1. (5) Pena, M. A.; Fierro, J. L. G. Chem. ReV. 2001, 101, 1981. (6) Deshpande, K.; Mukasyan, A. S.; Varma, A. J. Power Sources 2006, 158, 60. (7) Patil, K. C.; Aruna, S.; Mimami, T. Curr. Opin. Solid State Mater. Sci. 2002, 6, 507. (8) Goodenough, J. B. Localized to Itinerant Electronic Transition in PeroVskite Oxides; Springer-Verlag: New York, 2001; p 19. (9) Mukasyan, A.; Epstein, P.; Dinka, P. Proc. Combust. Inst. 2007, 31, 419. (10) Dinka, P.; Mukasyan, A. J. Phys. Chem. B 2005, 109, 21627. (11) Diaz-Morales, R. R.; Liu, R.; Fachini, E.; Chen, G.; Segre, C. U.; Marinez, A.; Cabrera, C.; Smotkin, E. S. J. Electrochem. Soc. 2004, 151, A1314. (12) Liu, R. X.; Smotkin, E. S. J. Electroanal. Chem. 2002, 535, 49. (13) Deshpande, K.; Mukasyan, A. S.; Varma, A. Chem. Mater. 2004, 16, 4896. (14) Iwasita, T. Electrochim. Acta 2002, 47, 3663. (15) Leger, J. M. Electrochim. Acta 2005, 50, 3123. (16) Lipkowski, J.; Ross, P. N. Electrocatalysis; Wiley-VCH: New York, 1998. (17) Hitmi, H.; Beligsir, E. M.; Le´ger, J.-M.; Lamy, C.; Lezna, R. O. Electrochim. Acta 1994, 39, 407. (18) Song, S. Q.; Zhou, W. J.; Zhou, Z. H.; Jiang, L. H.; Sun, G. Q.; Xin, Q.; Leontidis, V.; Kontou, S.; Tsiakaras, P. Int. J. Hydrogen Energy 2005, 30, 995.

9582 J. Phys. Chem. C, Vol. 111, No. 26, 2007 (19) Cattaneo, C.; Sanchez de Pinto, M. I.; Mishima, H.; Lopez de Mishima, B. A.; Lescano, D.; Cornaglia, L. J. Electroanal. Chem. 1999, 461, 32. (20) Raman, R. K.; Shukla, A. K.; Gayen, A.; Hegde, M. S.; Priolkar, K. R.; Sarode, P. R.; Emura, S. J. Power Sources 2006, 157, 45.

Lan and Mukasyan (21) Bockris, J. O’M.; Otagawa, T. J. Phys. Chem. 1983, 87, 2960. (22) Bockris, J. O’M.; Otagawa, T. J. Electrochem. Soc. 1984, 131, 290. (23) Lin, W. F.; Jin, J. M.; Christensen, P. A.; Scott, K. Electrochim. Acta 2003, 48, 3815.

Perovskite-Based Catalysts for Direct Methanol Fuel Cells - The

Recommend Documents