Combustion synthesis and electrochemical characterisation of Pt–Ru–Ni anode electrocatalyst for PEMFC

Applied Catalysis B: Environmental 76 (2007) 368–374 www.elsevier.com/locate/apcatb

Combustion synthesis and electrochemical characterisation of Pt–Ru–Ni anode electrocatalyst for PEMFC B. Moreno a,*, E. Chinarro a, J.C. Pe´rez b, J.R. Jurado a a

Instituto de Cera´mica Vidrio, CSIC, Campus de Cantoblanco, C/Kelsen s/n, 28049 Madrid, Espan˜a b Departamento de Quı´mica, Universidad San Pablo-CEU, E-28668 Madrid, Spain Received 21 March 2007; received in revised form 7 June 2007; accepted 16 June 2007 Available online 23 June 2007

Abstract The present work studies the synthesis by the combustion method of an anode catalyst for protonic exchange membrane fuel cell (PEMFC) employing two different fuels, that is, urea and sucrose. The unsupported pure solid solution Pt0.6Ru0.3Ni0.1 was selected from a calculated and empirical ternary phase diagram, which was previously studied. Theoretically, this particular composition exhibited single-phase features without the presence of secondary phases as RuO3 and NiO, regarding the oxygen partial pressure conditions generated during the combustion synthesis. In the X-ray diffraction (XRD) analysis of the nanoparticles synthesized by using two different fuels, a single-phase Pt0.6Ru0.3Ni0.1 alloy was detected. However, the X-ray photoelectron spectroscopy (XPS) studies showed that the nanoparticles prepared could present an onion-shell structure, in the case of the sample synthesized with sucrose as fuel, the external layers are partially constituted by Ni hydroxides, which can exhibit an active role in the hydrogen oxidation reaction. The electrochemical behaviour of this unsupported catalyst was performed by preparing MEAs, which were evaluated using a I–V polarisation curve test. The results obtained indicated that the nanoparticles prepared by sucrose have better performance, 260 mW/cm2, than those prepared using urea, 170 mW/cm2. These results are discussed in relation with the hydrogen oxidation mechanism. The results obtained reveal combustion synthesis as an appropriate method for preparing PEMFC electrocatalysts, due to its versatility, simplicity and fastness. # 2007 Elsevier B.V. All rights reserved. Keywords: Combustion synthesis; Electrocatalysts; PEMFC; Anode; Pt–Ru–Ni

1. Introduction The best and standard catalyst routinely used in proton exchange fuel cells (PEMFC) is Pt/C [1], this material catalyses the hydrogen oxidation reaction (HOR), but exhibits a fast poisoning in presence of ppm of CO. Several Pt based alloys have been proposed to inhibit this deactivation of the catalyst in the anode part, among them, Pt–Ru is the most widely studied [2–5]. Two possible explanations for the fact that alloying Pt with a transition metal can increase its CO tolerance, have been suggested. One of them is the ligand effect [6,7], in which the alloying transition metals can change the chemical properties of the first layers of Pt atoms in the surface of the catalyst, * Corresponding author. Present address: Hospital de Paraplejicos de Toledo, SESCAM, Finca la Peraleda s/n, Toledo, Espan˜a. Tel.: +34 925 24 77 00; fax: +34 925 24 77 43. E-mail addresses: [email protected], [email protected] (B. Moreno). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.06.012

lowering the density of states (DOS) on the Fermi level, reducing the CO–Pt bond energy and thus, increasing the number of Pt free sites available for H2 adsorption. Another is the bifunctional effect, the alloy metal may be better than Pt dissociating water, thus providing OH groups to react with the CO anchored at Pt sites [8,9]. Watanabe and Motoo [10] have proposed a combinated mechanism, in which ligand and bifuntional effect may coexist. Liu et al. [11] using the density functional calculations (DFT) provide the theoretical model to design the kinetic of CO oxidation, concluding that the ligand effect is the one that dominates the overall reaction in a fuel cell. Using the DFT calculations they have suggested that alloying Pt–Ru catalysts with a third metal would improve the CO tolerance of the material and would supply a significant power density. On the basis of these theories they proposed Pt– Ru–Ni alloy as an alternative to Pt due to its theoretical good performance in a H2 stream with significative amounts of CO [12,13].

B. Moreno et al. / Applied Catalysis B: Environmental 76 (2007) 368–374

There are several routes to synthesize Pt electrocatalysts for PEMFC and DMFC; among them, low temperature methods are the more commonly used. Supported Pt catalysts are prepared by colloidal method [14–16], and through its variations [17–19], i.e. by impregnation [20] and with the formation of microemulsions [21]. Trimetallic catalysts have been also synthesized employing this well-known method (colloidal) [22–24]. All these techniques have been satisfactory proved as anode catalysts preparation methods. They have shown excellent performance for metal nanoparticles fabrication, however they also presented some disadvantages, as the long time involved in the powder processing, the impurification due to the different stages of preparation and the relatively low homogeneity metal distributions, particularly when dealing with multicomponent metal formulations. In this way, combustion synthesis appears as an innovating route for multicomponent catalysts preparation, and due to its features, as a rapid and versatile technique for obtaining very fine (nanosized), alloy crystalline powders. This method has the ability to produce metal multicomponent powders due to the thermodynamical processes involved: ignition, reduction and high exothermic energy release, at the same time, in a rapid and single step which provides, usually, single phase nanoparticle powders. The use of this technique as a valid method to prepare powders with catalytical activity has been considered previously, although it has been studied mainly in ceramic materials, also, the production of metal nanoparticles by this method has been reported before [25,26]. This work studied an experimental approach to use combustion synthesis for the preparation of a novel catalyst, Pt–Ru–Ni alloy. The combustion process was followed employing a UV–Vis pyrometer, which allows to study the maximum temperature reached during the synthesis and also the propagation nature of the flame. The morphological characterisation of the powder was carried out, as well as the evaluation on a PEMFC single cell by using a membrane electrode assembly (MEA) prepared with the nanoparticles of single-phased Pt–Ru–Ni (60:30:10) as the anode catalyst. 2. Experimental 2.1. Synthesis of catalysts The selected nominal composition Pt–Ru–Ni (60:30:10) was prepared by combustion synthesis from a stoichometric mixture of (CH3–CO–CH C(O)CH3)2Pt (97%, Aldrich), (CH3–CO– CH C(O)CH3)2Ni (95%, Aldrich), (CH3–CO–CH C(O))CH3)3Ru (97%, Aldrich) and CO(NH2)2 (Aldrich, 98%) as fuel. An oxidant aid was added to achieve the stoichiometric ratio of the fuel–oxidizer mixture. Changes in the combustion conditions has been carried out, to achieve catalysts with different powder morphology, in this concern, sucrose (C12H22O11) was considered as an alternative fuel. During the synthesis, the reagents were homogenizated and heated up to 100–150 8C using a heating mantle, under continuous stirring. Once the solution began frothing, the temperature was then raised to 300 8C, in a few seconds the

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Table 1 Description of the anode loadings of MEAs assembled using as anode catalysts Pt0.6Ru0.4xNix, (x = 0.1) and the fuel used to synthesized the catalysts MEA

Anode loading (mg Pt/cm2)

Active area (cm2)

PtRuNi urea PtRuNi sucrose Pt/C commercial

0.58 0.60 0.70

25 25 25

ignition of the reactant mixture took place, as observed by the presence of an intense flame. Finally, the as-prepared powders were sieved through a 63 mm mesh. 2.2. Characterisation of the powders The phase analysis of the combustion powders was carried out employing X-ray diffraction (XRD) with a Siemens D5000 difractometer, Cu Ka radiation. The lattice parameters were calculated using Al2O3 (99.99% Fluka) as a standard. The powders morphology and microstructure was studied by both scanning electron microscopy (SEM) with a Jeol Superprobe (JXA-8900H-WD/ED combined microanalyzer) equipment and with transmission electron microscopy (TEM) using a Hitachi H-7000 instrument operating at 125 kV. X-ray photoelectron spectroscopy (XPS) analyses were performed using a Fisons ESCALAB mag 200R spectrometer. 2.3. Single cell test Membrane-electrode assemblies (MEAs) were prepared using the procedure described and modified by Chinarro et al. [27]. The catalytic layer was aerographied onto ELAT GDL microporous layer on woven web, first it was required to prepare an ink with fluidificant behaviour by mixing the appropriate amount of solvents, carbonaceous support (60 wt%), nafion solution and Pt–Ru–Ni (60:30:10) catalyst (40 wt% Pt). Nafion 1121 was employed as solid electrolyte. The assembly was prepared directly by pressing all the components together in the single cell. The electrochemical characterisation of the MEA was carried out in an I–V test station designed and developed by CSIC. The conditions used during the measurements were: P(O2) = 1 bar, P(H2) = 1.25 bar, cathode load = 0.70 mg Pt/cm2, the electrocatalyst used was Pt/ C (40 wt%/60 wt%) supplied by DeNora. The anode loadings used are depicted in Table 1, all the measurements were performed at 60 8C. 3. Results and discussion The combustion reaction was followed employing an UV– Vis pyrometer that measured the temperature of the mixture during the reaction. By this method it was possible to distinguish the differences between the fuels used in both reactions. As is described in Fig. 1(a) and (b) the synthesis with urea produces higher temperatures than sucrose and less reaction times. This fact can explain the surface composition of both samples, which will be described later, because while

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Fig. 1. Temperature dependence on time of the combustion synthesis performed, using as fuel (a) urea (b) sucrose.

employing sucrose as fuel the reaction has more time to reach the equilibrium than in the case of urea. During the synthesis of Pt0.6Ru0.3Ni0.1, the reaction promotes a reducing atmosphere; this assumption has been done on the basis of the use of raw materials, as acetil acetonates, which decompose generating reducing gases and consuming the atmospheric oxygen present in the reaction basin. It was possible to establish an equivalent oxygen partial pressure less than 1014 atm, this determination has been done on the basis of thermodinamical calculations, using the Ellingham [28] and the ternary phase equilibrium diagrams [29]. Fig. 2 shows the X-ray pattern of the Pt0.6Ru0.3Ni0.1 nanopowders prepared; a single-phased material was synthesized by combustion despite the fuel used in the synthesis method. The reflections found were shifted to higher 2u values, i.e. in the case of (1 1 1) peak from 39.83 (pure Pt reflection) to 41.59 and 40.13, for those samples synthesized with urea and sucrose, respectively. This indicates, apparently, the formation of a solid solution with the inclusion of Ni and Ru in the Pt lattice. This phase was indexed as face-centred-cubic (fcc) with ˚ in case of the a lattice parameter calculated of 3.88 A nanoparticles synthesized by urea, (namely U-sample) and ˚ for the nanoparticles prepared by sucrose (namely 3.89 A S-sample), both lattice parameters were lower than pure ˚ . Neither traces of (fcc) Ni or (hcp) Ru were metallic Pt, 3.92 A

Fig. 2. XRD patterns of the nominal composition Pt0.6Ru0.4xNix, x = 0.1, synthesized using (a) urea and (b) sucrose as fuel (—) indicates the location of the reflections associated with metallic Pt.

found; in addition the presence of NiO or RuO2 in the XRD was not noticed. The average crystallite alloy size of both samples was calculated by fitting the (1 1 1) peak and using the Debye– Scherrer equation. The values obtained were 11.3 nm in case of U-sample and 8.5 nm for S-sample. Sucrose produces powders with smaller particle size than urea, due to the differences in the temperature reached and the volume of gases released in both samples, around 800 8C and 5 mol of gases for urea and 500 8C and 23 mol of gases for sucrose. SEM micrograph in Fig. 3(a) shows the morphology of the powders of the U-sample, which are constituted by soft foam agglomerates formed by smaller particles, as is confirmed by TEM in Fig. 4(a). As it was mentioned before, the nanoparticles were agglomerated during the synthesis. Metal dispersion was evaluated by means of mapping (Fig. 3(b)–(d)), and was found to be highly homogeneous in case of Pt or Ru and with a random distribution in case of Ni. This particular metal dispersion will be discussed below. Particle size measured from TEM micrograph in Fig. 4 (a), 5–10 nm, was in the range of that calculated from XRD pattern (11.3 nm). The electron diffraction, Fig. 4(c), kept the cubic (fcc) Platinum nanocrystal structure. XPS analyses of both surfaces were performed with the aim of evaluating differences between them. The Pt 4f spectra are shown in Fig. 5(a) and (b), two peaks were found for Pt 4f7/2 at 71.0 and 72.8 eV in case of urea and 71.1 and 72.9 eV in case of sucrose sample. These binding energies indicate the presence of Pt in two different states, as Pt0 and Pt+2, with a peak ratio of (2:1) in both samples. Significantly, Ni spectra obtained are complex with one peak placed at 853.8 eV in case of U-sample and two different bands for sucrose at the binding energies of 853.8 and 855.6 eV. According to Park et al. [30] these values have been ascribed to the presence of NiO and Ni(OH)2, while in the U-sample the Ni in surface is found totally as NiO. Ru spectra, depicted in Fig. 6(d) and (e) are characterized by the presence of peaks at 280.4 and 282.8 eV in both catalysts, these bands are associated with the chemical states Ru0 and Ru6+. The Ru 3d region is often interfered by the C 1s signal that came from surface contamination. From Table 2 it can be observed that the atomic ratios between the elements are different for each sample, being the surface of the S-sample significantly enriched in Ni. The segregation of Ni to the surface has been studied and observed by others authors [30] being explained as a consequence of the difference in the vaporization heat, 509.6, 370.3 y 589.9 kJ/mol of the subsequent metals, Pt, Ni and Ru, respectively [31]. Furthermore, the atomic ratio Ru/Pt indicated that the U-sample depicted a higher Ru surface content than the S-sample. This remarkable Ru concentration in the U-sample could be one of the reasons of the lower anodic reaction efficiency. The polarization curves obtained for these catalysts are shown in Fig. 7, the maximum power density obtained in these measurements are 171 and 260 mW/cm2 for U and S-sample, respectively. Both values are lower than commercial Pt/C catalysts as it was expected but, the performance of Pt0.6Ru0.4xNix, (x = 0.1) S-sample was higher than Pt–Ru (60:40) prepared by combustion synthesis. The difference

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Fig. 3. (a) SEM micrograph of the Pt0.6Ru0.4xNix, (x = 0.1) nanopowder synthesized with urea. (b) Mapping of Ni, (c) mapping of Ru and (d) mapping of Pt.

between the activity registered for the U-sample and the Ssample is also significant, and has been ascribed to the differences noted in the XPS analysis of both catalysts. The presence of Ni hydroxides in the S-sample, detected by XPS, may have a potential activity in the HOR. Other authors had found Ni and Ni hydroxides acting not just as a promoter but also as catalyst in methanol oxidation. In particular, Park et al. [30] have proposed a mechanism for these reactions, in case of DMFC catalysts, but this mechanism could be applied in the case of PEMFC, and could explain the

differences found in the electrochemical behaviour of both catalysts. The catalytic activity found in these powders (U and Ssample) is considered an important goal in the development of fuel cell technology, it can lead to a reduction of the Pt load in the catalytic layer, involving a significant decrease in the cost associated with the catalysts with a competitive performance. On the other hand, when confirmed, the HOR catalysis of transition metal hydroxides could be a breakthrough in the total substitution of Pt.

Fig. 4. (a) TEM micrograph of the Pt0.6Ru0.4xNix, x = 0.1. (b) Electron diffraction pattern of the same powder.

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Electrochemical features of the catalysts prepared and tested were evaluated from the I–V curve obtained. Employing Tafel equation, activation overpotential of the anodic reaction was studied, in order to evaluate the influence over the catalysts response, of oxidized species in the outer catalysts surfaces. Tafel slopes were calculated obtaining a similar value in both cases, 36 mV/dec for U-sample and 37.8 mV/dec in S-sample, being these values in agreement with those determined by others authors for the same reaction using commercial catalysts [32]. These values indicate that the activation stage follows a Tafel–Volmer mechanism in which the limiting step is the dissociation reaction of the adsorbed hydrogen atoms onto the metal atoms in surface [32], reaction that is described by the following mechanism. Fig. 5. XPS spectra of Pt 4f synthesized with (a) urea and (b) sucrose.

H2 ! Had þ Had

(R.1)

H2 ! Hþ þ Had þ e

(R.2)

Had ! Hþ þ e

(R.3)

Fig. 6. XPS spectra of Ru 3d synthesized with (a) urea and (b) sucrose XPS spectra of Ni 2p (c) U-sample and (d) S-sample.

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Table 2 XPS binding energies measured for different samples synthesized in the Pt0.6Ru0.4xNix, x = 0.1 composition, and the surface atomic rations calculated from the measurements Sample

Pt 4f7/2 (eV)

Ru 3d5/2 (eV)

Ni 2p3/2 (eV)

O 1s (eV)

Ru/Pt (atm.)

Ni/Pt (atm.)

PtRuNi (60:30:10) urea

71.0 72.8 71.1 72.9

280.4 282.9 280.6 282.8

– 853.8 853.8 (18) 855.6 (82)

– 529.8 529.7

1.096

1.381

0.688

1.880

PtRuNi (60:30:10) sucrose

(66) (34) (67) (33)

(34) (66) (63) (37)

Therefore our sucrose trimetallic catalyst is considered promising due to its improved CO tolerance. 4. Conclusions

Fig. 7. I–V polarization curve of the nominal composition Pt0.6Ru0.4xNix, x = 0.1 as anode catalyst prepared by combustion, (*) U-sample, (&) S-sample and (~) Pt/C commercial.

In Fig. 8, a cross-section of the S-sample MEA studied in a SEM–EDX post-mortem analysis shows that both anode and cathode catalytic layers were homogeneously deposited onto carbon papers with an uniform thickness of 8–10 mm. Nafion 112 membrane is stable and no degradation was observed during the single cell operation, keeping its mechanical properties, it was also observed that the carbon cloth does not undergo any alteration. Employing EDX analysis, the MEA was analyzed proving that no trace of catalyst diffused through the membrane. Preliminary voltammetry stripping test indicates that the Sucrose Pt–Ru–Ni catalyst shows a peak potential for CO oxidation at +0.42 V, whereas commercial Pt–Ru/C exhibit a peak potential for CO oxidation shifted at +0.45 V.

Fig. 8. SEM micrograph of the MEA prepared with the S-sample after 10 h working.

Combustion synthesis was used as a preparation method to obtain trimetallic alloys based in the Pt–Ru–Ni system, as PEMFC anode catalysts. A single phased material with nominal composition Pt0.60Ru0.30Ni0.10 was synthesized with a particle size between 8 and 12 nm. The electrochemical characterisation of the MEA fabricated with the S-sample shows a power density of 259 mW/cm2, this behaviour has been ascribed to the higher catalytic activity of these catalysts as a consequence of its surface composition. Comparing with other synthesis methods, combustion offers a simply and rapid way to prepare complex alloys with a nanoparticle size and an onion-shell layered structure, as confirmed by XPS. Acknowledgements The authors are grateful to the financial support of APOLLON project, No. ENK5-CT-2001-00572, I3P CSIC contract, Red de Pilas de Combustible, MCYT project No. MAT02-10560E. Thanks to Prof. J.L.G. Fierro for the XPS measurements, to Dr. Mather for his useful discussion and to Dr. Rojas from the ICP for his help with the stripping experiments. References [1] R. Mohtadi, W.-K. Lee, J.W. Van Zee, Appl. Catal. B: Environ. 56 (2005) 37. [2] M. Watanabe, S. Motoo, J. Electroanal. Interfacial. Electrochem. 60 (1975) 267. [3] H.A. Gasteiger, N.M. Markovic, P.N. Ross, J. Phys. Chem. 97 (1993) 12020. [4] H. Hoster, T. Iwasita, H. Baumgastuer, W. Vielstich, Phys. Chem. Chem. Phys. 3 (2000) 337. [5] W. Zhou, Z. Zhou, S. Song, W. Li, G. Sun, P. Tsiakaras, Q. Xin, Appl. Catal. B-Environ. 46 (2003) 273. [6] H. Igarashi, T. Fujino, Y.M. Zhu, H. Uchida, M. Watanabe, Phys. Chem. Chem. Phys. 3 (2003) 306. [7] J.B. Goodenough, R. Manoharan, A.K. Shukla, K.V. Ramesh, Chem. Mater. 1 (1989) 391. [8] H.A. Gastegeir, N.M. Markovic, P.N. Ross Jr., J. Phys. Chem. 99 (1995) 8290. [9] M. Watanabe, S. Motoo, J. Electroanal. Chem. 60 (1975) 275. [10] M. Watanabe, M. Shibata, S. Motoo, J. Electroanal. Chem. 206 (1986) 197. [11] P. Liu, J.K. Norskov, Fuel Cells 1 (2001) 192–201.

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