EP2160779A2 - Catalysts having low platinum content for fuel cells - Google Patents

Abstract

The present invention relates to the field of fuel cell, in particular to electrode compositions for fuel cells (PEMFC - Polymer Electrolyte Membrane Fuel Cell) with low platinum content and discloses the use of a compound of formula M1xHy-xM2M312O40, wherein M1 is selected from the group consisting of Cs+, Rb+, K+, NH4+, x represents the number salified protons from M1 in the corresponding acid, M2 is P or Si and M3 is W or Mo; y = 3, when M2 is P and y = 4 when M2 is Si. Said compound is used in a composition with a noble metal or an alloy thereof, preferably Platinum, for the preparation of an electrode composition for fuel cells.

Description

Catalysts having low platinum content for fuel cells

The present invention relates to the field of fuel cell, in particular to electrode compositions for fuel cells (PEMFC - Polymer Electrolyte Membrane Fuel Cell) with low platinum content.

Background of the invention

Fuel cells have been known since long time; however they have not yet reached a satisfactory development as far as their large scale production and use.

The problem of providing efficient and low cost electrode configurations has not yet been satisfactorily solved.

Electrode processes, namely hydrogen oxidation (HOR) and oxygen reduction (ORR), require the use of catalysts, which are still based on platinum or its combinations with other metals.

The use of platinum implies high production costs. Recent development of nanostructured carbon supports having high surface development allowed reducing platinum loading of one order of magnitude, from 4 mg/cm² to 0.4 mg/cm^2' However, these values are still too high for a large scale economy, especially for certain applications, such as vehicle traction.

Therefore, there is the felt need to dispose of alternative electrode configurations and catalysts with much lower costs with respect to the present ones.

It is well-known to use CS_2.5H_0.5PW₁₂O₄₀ as proton conducting additive in Nafion-based membranes (V. Ramani, et al; Electrochim. Acta 50(2005)118; V. Ramani, et al. J. Power Sources 152(2005)182; V. Ramani, et al., J. of Membrane Science 232(2004)31; V. Ramani, et al. J. of Membrane Science 266(2005)110). L. Wang et al. Electrochim. Acta 52(2007)5479. M. Li. et al. Electrochemical and Solid-State Letters (2006), 9(2), A92-A95).

In other papers, the use of insoluble salts of heteropolyacids as additives for modifying Nafion membranes is disclosed (Y. S., Kim, et al., J. of Membrane Science 212(2003)263).

In T. Kukino, et al., Solid State Ionics 176(2005)1845, modification of a sulfonated polymer with an acid is disclosed.

A pellet of the compound Cs₂HPWi₂O_4O was used as separator instead of Nafion in a H₂/O₂ cell with Pt catalysts at 120 ⁰C.

Phosphotungstic acid (HPA) was used as solvent in fuel cells (N. Giordano, et al., Electrochim. Acta 41(1996)397). A solution of this acid was used as immobilized electrolyte in a glass matrix (Fluka Chemie Glass Microfiber Felt) in a cell with two "gas diffusion"-type electrodes with Pt catalyst. The task of the acid is to transport protons (similar to Nafϊon). It was found that the acid itself accelerates the oxygen reduction reaction.

W. B. Kim, et al., Science 305(2004)1280, disclose a gold catalyst, used for oxidizing CO to CO₂ through the reaction with PMθi₂O₄₀ ^3~ (CO + H₂O + PMθi₂O₄₀ ^3~ → CO₂ + 2H⁺ + PMOi₂O₄O^5") in a fuel cell fed with H₂ and O₂. The reduced form of the heteropolyacid is pumped to the cathode, where it is reduced regenerating the starting compound. The overall effect is cleaning of hydrogen fuel from CO, which poisons the oxidation Pt catalysts and an increase of the cell current two to heteropolyacid regeneration.

Substituted heteropolyacids used as catalysts are disclosed by M-Chen Kuo, et al. "Electro- catalyst materials for fuel cells based on polyoxometallates - K₇ or H₇[P₂Wi₂θ₆i)Fe^iπ(H₂O)] or Hi₂[P₂ Wi₅O₅₆)Fe^iπ ₄ (H₂O)₂](EIeCt. Acta 52(2007)2051).

This reference disclosed the direct use of salts as catalysts for hydrogen oxidation and oxygen reduction with poor results and not relevant to the effects of the present invention.

B. R Limoges et al (Elect. Acta 50(2005)1169) disclose the use of polyacids of the [PMθ(i₂_ series both as cathode and anode catalysts with no Pt in PEM.

R. J. Stanis et al. (J. Electrochem. Soc. 155(2008)B155) disclose the use of substituted heteropolyacids with W, Mo and V to mitigate Pt poisoning caused by CO.

US 2002/0111267A1 discloses the use of ammonium metatungstate [(NH₄)₆H₂ Wi₂O₄₀)] as precursor for in situ formation of a catalyst for H₂ oxidation without Pt.

US 5,298,343 discloses different kinds of metals (Pd, Pt, Ru, Rh, Ir, Os) and ammonium tung- state and molybdate.

The compounds differ from the ones disclosed in the present invention in that they do not contain heteratoms (P or Si).

A number of published papers deal with surface modifications of Pt nanoparticles with heter- opolyacids. These references are not relevant to the present invention, since the do not use insoluble salts (Renata Wlodarczyk, et al., J. Power Sources 159 (2006) 802); Renata WIo- darczyk, et al., Electrochim. Acta. 52 (2007) 3958-3964); Kulesza RJ, Marassi R, Karnicka K, et al. Reviews on Advanced Material Science 15 (2007) 225.; Chojak, Malgorzata; Kolary- Zurowska, Aneta; Wlodarczyk, Renata; Miecznikowski, Rrzysztof; Karnicka, Katarzyna; Pa- lys, Barbara; Marassi, Roberto; Kulesza, Pawel J. Electrochimica Acta (2007), 52(18), 5574-5581.; Kulesza, P. J.; Marassi, R.; Karnicka, K.; Wlodarczyk, R.; Miecznikowski, K.; Chojak, M.; Kolary, A. ECS Transactions (2006), 1(6, Proton Exchange Membrane Fuel Cells V, in Honor of Supramaniam Srinivasan), 107-118).

Summary of the invention

It has now been found that the use of salts of big cations (Cs⁺, K⁺, Rb⁺, NH₄ ⁺) of heteropolya- cids of the H3PW12O40, H₄SiWi₂O₄O, H₃PMOi₂O₄O, H₄SiMOi₂O₄O kind allows preparing catalysts for fuel cells with low platinum content.

Advantageously, the present invention allows to lower platinum content to the order of tenths ooff μμgg//ccmm²² ((aabboouutt 2200--220000 μμgg//ccmm²²));; tthheessee vvaalues are far lower than the present ones, especially as far as reduction of oxygen is concerned.

Therefore, it is an object of the present invention the use of compound comprised in formula M_xH_y__xM²M_j2O₄₀ , where M¹ is selected from the group consisting of Cs⁺, Rb⁺, K⁺, NH₄ ⁺, x represents the number of protons salified by M¹ in the corresponding acid, M² is P or Si and M³ is W or Mo; y = 3, when M² is P and y = 4 when M² is Si.

Said salts cannot be directly used as catalysts, because they are poorly or no active for the in- terested reactions. However, addition of small amounts of a noble metal or an alloy thereof, such as for example Pt or an alloy thereof, such as Pt-M alloys, where M is selected from the group consisting of Co, Cr, V, Cu, Fe, V and Sn, preferably alloys such as Pt_xCθi__x and

Pt_xNii__x, prepared in different manners, allows the preparation of composite catalyst layers (in the foregoing also named electrode composition) which are very active both for the HOR re- action (Pt, PtRu) and the ORR reaction (Pt-M, wherein M is Co, Cr, V, Cu, Fe, V).

In particular, it is an object of the present invention an electrode composition comprising a compound as above disclosed doped with a noble metal or an alloy thereof, preferably Platinum or an alloy thereof. Said composition is useful for the preparation of composite electrodes for fuel cells comprising one of the above compounds doped with a noble metal or an alloy thereof, preferably Platinum or an alloy thereof. The compositions can be prepared according to the following methods, given in exemplary way for Platinum:

1. electrochemical deposition using dissolution/corrosion of a Platinum counter-electrode as Platinum source;

2. formation of composite electrodes by admixing one of the above compounds with high surface development carbon-supported Pt nanoparticles, for example Vulcan^® XC-72; this preparation is especially suitable in case of above mentioned Pt-M alloys;

3. formation of electrodes with one of the above mentioned compounds, where Pt is chemically introduced during the preparation phase of the insoluble compound. Another object of the present invention is the use of the compounds and compositions above mentioned for the preparation of electrodes, in particular for hydrogen oxidation or oxygen reduction in fuel cells.

Also objects of the present invention are the electrodes comprising the above mentioned com- pounds or compositions and the related fuel cells comprising at least one of the said electrodes.

These and other objects of the present invention will be disclosed in detail also by means of Examples and Figures, wherein:

Figure 1 shows a XANES spectrum (X-ray Absorption Near Edge Structure) of the compound Pt-Cs₂.₅PWi2 (Pt 3, 5 weight %) and a Pt leaf.

Figure 2 shows a voltammogram referred to electrode of Example 1.

Figure 3 shows a cyclic voltammogram obtained with the same electrode of Figure 2 in H₂SO₄ solution of saturated with Ar after CO adsorption CO.

Figure 4 shows progressive Pt electrochemical deposition of Pt on an electrode similar to the one mentioned above.

Figure 5 shows vo Mammograms obtained with rotating electrode of Example 2.

Figure 6 shows vo Mammograms obtained in Example 3.

Figure 7 shows vo Mammograms obtained in Example 4.

Figure 8 shows oxygen reduction curves applying the electrodes of Example 5.

Figure 9 shows a series of curves obtained in sequence with the rotating electrode according to Example 6.

Fig. 10 shows three RDE vo Mammograms for oxygen reduction obtained with three different electrodes as disclosed in Example 7.

Fig. 11 shows three RDE voltammograms and the corresponding ring currents for oxygen re- duction and H₂O₂ oxidation as disclosed in Example 8.

Fig. 12 shows the first and the 600^th RDE voltammograms for oxygen reduction as disclosed in Example 9.

Figure 13 shows two polarization curves obtained in cells according to Example 10. Figure 14 shows X- spectra of the electrode before and after 300 running hours of the cell of Example 10.

Figure 15 shows polarization curves obtained on cells assembled with MEA as in Example 11.

Figure 16 shows polarization and power curves obtained with cells assembled disclosed in Example 12.

Figure 17 shows polarization and power curves obtained with cells assembled as disclosed in Example 13.

Detailed description of the invention

The compounds comprised in the formula are well-known and can be prepared by titrating the starting acid with a salt of the metal M, for example a chloride or carbonate. See for example, Y. Izumi, M. Ogawa, K. Urabe, Applied Catalysis A: General 132(1995)127.

The preferred compounds are those of formula M_xH_{y x}M²M_j2O₄₀ , wherein M is Cs⁺, NH₄ ⁺ and Rb⁺ and x is selected between 2 and 3, M² is phosphorus (P) and M³ is tungsten (W). Par- ticularly preferred are the compounds wherein x is 2.5. For sake of simplicity, in the foregoing the salts will be named by using the notation M¹XM²M³I₂ (for ex. CS2.5PW12 means the compound Cs_{2 5}H_{0 5}PWi₂O₄O; (NH₄)₂PWi₂ is for (NH₄)₂HPWi₂O₄₀).

For the purposes of the present invention, the compounds Cs_2.5Ho_.5PWi₂0₄o; (NH₄)₂HPWi₂O₄₀; Rb_{2 5}H_{0 5}PWi₂O₄₀ are preferred.

Other than their insolubility in aqueous environment, the main feature of the salts is their high surface area and high acidity. Properties of micro-mesoporous aggregates making the final product of the preparation are of particular interest. During the precipitation phase, the micro- crystals aggregate to form tertiary structures having dimensions of the order of 1 μm (T. Oku- hara, T. Nakato, Catalysis Survey (Japan) 2(1998)31). Surface area depends on cation type, preparation conditions and number of protons which are substituted. The area increases with a very rapid trend when the number of substituted protons is about 2. In case of Cs_xPWi₂, surface area reaches values of the order of 150 for x = 3. In tertiary structures, dimensions of mesopores range from 20 to 500 A with an area equal to about 30% of the total area. The micropores (width <2 nm) cover the remaining of the measured area. Another characteristic pf tertiary structures is high acidity (Hammett acidity function H₀ < -13.6) and the hydrophobic character. When Platinum is introduced in the preparation phase (see for example: Yanyong Liu and co- workers; J. Of Molecular Catalysis A: Chemical 141 (1999) 145-153), the precipitation is carried out by titrating with a salt of the cation of interest a mixture of the acid added with a salt of Pt (H₂PtCl₆ or Pt(NHs)₄(OH)₂) in suitable proportions and such as to the desired final product (for ex. Pt-Cs₂.₅PWi₂) contains Pt from 3 to 20-30% by weight.

Il precipitate, after suitable phases of filtration and solvent elimination in oven at 50⁰C, can be used as such for the preparation of catalytic layers or previously submitted to reduction treatments with hydrogen at 100-300⁰C or with reducing agents such as NaBH₄ or formic acid. As it can be seen in the cited literature, and from our experiments Platinum particles tend to lo- calize inside micro-mesopores and they cannot be visualized neither by high-resolution TEM, probably because of small dimensions (determined by micropores dimensions) and of their being inside a matrix made by heavy metals (W, Mo). The presence of Pt was nonetheless evidenced by XANES (X-ray Absorption Near Edge Structure) measures as shown in Figure 1 at L2 threshold of Pt. As it can be seen, the spectrum of the compound Pt-Cs_2-5PW₁₂ (Pt 3,5 weight %) is very similar to the one of a Pt leaf, even though both the position and the form of the threshold in the case of the composite indicate that Pt is partially oxidised. This demonstrates that Pt is present in the tertiary structure.

Preparation of inks and of electrodes

Electrodes for studies with rotating disc or disk ring electrodes were prepared following the procedure below. Known amounts of the different compounds, either as such or doped with Pt, carbon with high surface area, such as for example marketed as Vulcan^® and a copolymer of sulfonated tetrafluoroethylene, of which cui a preferred example is the one known as Nafϊon^® (5% by weight hydroalcoholic solution), or another suitable copolymer, were mixed in isopropanol. In a typical preparation, the weight ratios Compound:Vulcan:Nafion are 2:1 :1,1. The mixture was stirred for 24 hours. The presence of Vulcan in the mixture is for maintaining the electric contact among the aggregates of the compound. The so obtained inks were used for preparing thin films on Glass Carbon electrodes(GC). Typically, 5 μl of mixture were deposited on the surface of the electrode and left to dry on air for about 1 hour. Electrochemistry of the electrodes so prepared was studied by cyclic voltammetry or voltammetry with disc rotating (Glassy Carbon (GC) or disk ring (GC disk, Pt ring) electrode in H₂SO₄ or HClO₄ solutions, using a three-electrode cell with a counter electrode made of a Pt leaf immersed in the same compartment containing the electrode under investigation (working electrode) and a reference (standard calomelane electrode (SCE)) connected with the working compartment through a Luggin capillary. In parallel and for comparison purposes, electrodes containing only Vulcan or 20 w/w % Pt Vulcan XC-72 with or without the compound were prepared. For the preparation of MEA electrodes (Membrane Electrode Assembly), the catalytic inks were prepared following the same procedures described above. The so obtained inks were layered on GDL (Gas Diffusion Layer) LT 1200W (E-Tek). The so obtained electrodes (5 cm²) were tested in single cells (Fuel Cell Technologies, Inc.) provided with graphite blocks with single serpentine for gas flow using a membrane of the Nafϊon NRE 211-212 type (Ion Power) as proton conductors. The cells were sealed by compression using suitable sealing gasket. Alternatively, MEA (Membrane Electrode Assembly) were prepared using the same type of membrane and a standard hot pressing technique. Working pressure of gases ranger from 1 to 3 bars. The gases were pre -humidified with relative humidity ranging from 30 to 100% (RH Relative Humidity). Open circuit gas flow is 50-100 ml/s- and automatically increases based on current according to stoichiometric ratios (loading based total flow).

Other embodiments of electrodes are possible according to the common knowledge in the field.

Fuel cells using the teaching of the present invention can be manufactured in a conventional manner and do not require particular explanations.

The following examples further illustrate the invention.

Example 1

Electrochemical doping

A cyclic voltammogram (50 mV/s) of film, prepared as above disclosed on a GC electrode, containing non doped CS2.5PW12 in the potential window of -0,05 - 1,05 vs. RHE (Reversible Hydrogen Electrode) is shown in Figure 2. The experiment was run in a conventional electrochemical cell with a Pt "flag" counter electrode immersed in the same solution as the working counter electrode. The current in the zone of potential near to the cathode limit is characteristic of the reduction of the compound CS2.5PW12 deposited on an inert electrode support and corresponds to the reaction

Cs₂.₅PWi2^VIθ4o^"0-⁵ + and^" ^ Cs₂.₅PW^vWii^VI θ4o^"L5

as can be deducted from the corresponding cyclic voltammograms of PW₁₂O₄₀ ^" adsorbed on glass carbon electrodes or in acid solution (Renata Wlodarczyk, Malgorzata Chojak, Krzysz- tof Miecznikowski, Aneta Kolary, Pawel J. Kulesza and Roberto Marassi, J. Power Sources 159 (2006) 802). Electrochemical activity takes place in the characteristic zone of electrochemical activity of the H₂/H⁺ pair on Pt electrodes. The voltammogram relative to the same electrode after about 12 hours of scans in the same potential interval is shown in Figure 2 (continuous line). From this figure, an increase of both anode and cathode currents is noted in the same zone of potential and the curve progressively takes the characteristic form of a wave of absorption/evolution of H₂ on a Pt electrode. The presence of Pt on the electrode results evident in Figure 3, which shows a cyclic voltammogram obtained with the same electrode in H₂SO₄ solution saturated with Ar after adsorption of CO. The peak at about 0.9 V is charac- teristic of CO stripping of adsorbed on Pt. Since the only possible source of Pt is the dissolution/corrosion of the counter electrode during cycling, from this series of experiments we deduct that the process of "activation" produces electrochemical deposition of Pt in the VuI- can/CsPWi₂/Nafion matrix. This conclusion was also confirmed by a subsequent experiment wherein the Pt counter electrode was substituted by a sheet Carbon Paper (insert in Figure 2). In this case, if a slight increase of currents is excluded, the starting voltammogram and the one obtained after 12 hours of activation are practically coincident. No CO stripping wave was observed running an experiment with the same modalities seen in Figure 3, when activation was made using a Carbon Paper electrode.

The amounts of deposited Pt varies with the experimental conditions and depends on the number of activation cycles and/or the presence in solution of Pt(II)/Pt(IV) ion complexing agents, such as for example Cl^" ions, which shift the of oxidation/corrosion potential of metallic Platinum towards more negative values with respect to those pf the Pt/Pt(II) pair in sulphuric acid. For the electrode relative to this example an estimation of the Pt loading was made on the basis of the calculation of the area electrochemically active for irreversible oxi- dation of adsorbed CO to CO₂ (Figure 3) (reaction Pt-CO + H₂O → Pt + CO₂ +2e^" +2 H⁺; charge necessary for oxidising a CO monolayer on Pt 0.484 mC/cm^"2) and of an estimation of the mean radius of the Pt particles. The loading corresponding in μg/cm² is 1.41 and 3.52 μg/cm² assuming a particle mean radius equal to 2 and 5 nm, respectively. From this, we deduce that, although we cannot exclude a priori the possibility of deposition of Pt particles di- rectly on Vulcan VC-72, the major part of the metal deposited be localized in the micro- mesopores as explained previously.

Figure 4 shows the progressive electrochemical deposition of Pt on an electrode similar to the one disclosed above using as test the progressive growth of the wave of oxygen reduction in saturated acid solutions at room temperature. Curve 4a shows the starting voltammogram in solution saturated with argon; curves 4b, 4c and 4d show the voltammograms obtained in solutions saturated with oxygen after 1250, 2500 and 3750 cycles of activation, respectively. The example shows demonstrates that the amounts of deposited Pt increases with increasing of the number of cycles of activation and that electrodes obtained are active for the reaction of oxygen reduction (ORR).

Subsequent experiments with 20% Pt Vulcan (XC-72) - CS2.5H0.5PW12 mixtures, wherein Pt was intentionally introduced using commercial catalysts showed voltammograms similar among them and similar to the ones obtained introducing Pt in the Vulcan / CS2.5H0.5PW12 mixture through electrochemistry. In this case, waves have higher definition because of the higher Platinum content.

Example 2

All the electrodes prepared as disclosed in Example 1 were tested in solutions saturated with H₂ at atmospheric pressure using a rotating electrode. As comparison element the following electrodes prepared with the only 20%Pt Vulcan XC-72 were used. The Pt loadings are the following:

Test N. 1 : electrode 20%Pt Vulcan XC-72: Pt 70 μg/cm².

Test N. 2: electrode 20%Pt Vulcan XC-72 - Cs₂.₅H₀.₅PWi₂ (1 :2 w/w %) Pt 67 μg/cm² pre- pared by mechanical mixing.

Test N. 3: electrode Vulcan XC-72 - Cs_2.5H_0.5PWi₂ (1 :2 w/w %). Electrochemical activation, calculated Pt loading 5-10 μg/cm².

The vo Mammograms obtained with rotating electrode are shown in Figure 5.

Efficacy of the method of electrochemical preparation is evident from the fact that similar performances are obtained for hydrogen oxidation with Pt loadings much lower than those obtained using electrode N. 1 or N. 2.

Example 3

This Example is relative to the use of electrodes prepared using for the preparation of catalytic inks for hydrogen oxidation.

Test N. 1 : electrode 20%Pt Vulcan XC-72: Pt 70 μg/cm².

Test N. 2: electrode 20%Pt Vulcan XC-72 - Cs_2.5PWi₂ (1 :2 w/w %) Pt 67 μg/cm²

Test N. 3: electrode Vulcan XC-72 - (NH₄)₂HPWi₂ (1 :2 w/w %). Pt electrochemically introduced (calculated Pt loading 5-10 μg/cm²).

The voltammograms obtained with above mentioned electrodes at 2,500 rpm in solution 0.5M H₂SO₄ saturated with hydrogen at 25 ⁰C are shown in Figure 6. As in the preceding case, it is evident the efficacy of the method of electrochemical preparation in obtaining catalytic layers with low Pt content of which are very active for the reaction of hydrogen oxidation even though Cs⁺ is substituted with NH₄ ⁺. Example 4

Test N. 1 : electrode 20%Pt Vulcan XC-72: Pt 70 μg/cm²

Test N. 2: electrode 20%Pt Vulcan XC-72 - (NH)4PW12 (1 :2 w/w %) Pt 67 μg/cm²

Test N. 3: electrode Vulcan XC-72 - Cs2.5PW12 (1 :2 w/w %) (loading of Pt calculated67 μg/cm²).

The voltammograms obtained with the electrodes mentioned above at 1,600 rpm in 0.5M H₂SO₄ solution saturated with oxygen at 25 ⁰C are shown in Figure 7. The curves demonstrate that the presence of the insoluble salts of heteropolyacids in the inks obtained by admixing Pt 20% Vulcan XC-72 makes the oxygen reduction kinetically more favourable as evidenced by the shift of the curve of reduction towards more positive potentials. The effect is more evident in the case of Cs⁺ salts.

Example 5

Test N. 1 : electrode Vulcan XC-72

Test N. 2: electrode Vulcan XC-72 - (NH)₄PWi₂ (1 :2 w/w %)

Test N. 3: electrode Vulcan XC-72 - Cs_2.5PWi₂ (1 :2 w/w %).

All three electrodes were electrochemically activated using the same number of cycles (about 1,200). Pt loadings, estimated as previously disclosed are of the order of 2-5 μg cm^"2. The curves of oxygen reduction (1,660 rpm) shown in Figure 8 demonstrate that the electrochemical activation for dissolution/corrosion of the counter electrode has as effect the formation of electrodes active in the reaction of oxygen reduction. Shifting towards positive potentials of the wave of reduction caused by the presence of the salts demonstrates that these increase the velocity of the reduction process. The salt containing Cs, in accordance with the results of other Examples, is the most active in promoting the reduction reaction.

Example 6

The compound Pt-Cs_2-5PW₁₂ (Pt 3,5% by weight), prepared according to the procedure disclosed above using H₂PtCl₆, was used for the preparation of thin layers on a glass carbon electrode as previously disclosed (Vulcan XC-72: Compound: Nafϊon 2:1 :0.8 by weight) without previous reduction treatment with NaBH₄ or hydrogen. Therefore, the electrode contains oxidised Pt. Said electrode (Pt loading 48,6 μg cm^"2) was directly polarized in 0.5 M H₂SO₄ solu- tion saturated with oxygen at 25°C. Figure 9 shows a series of curves obtained in sequence with the rotating electrode. The sequence shows that the activity towards the oxygen reduc- tion increases with increasing of the number of cycles till stabilize after about 30 cycles. The experiment demonstrates that Ie subsequent polarizations reduce the starting Pt(IV) to metallic Pt and therefore the compound containing the starting Pt(IV) can directly be used, for example, for the preparation of MEA directly activated in the cell. Active electrodes, either for preparation of film or MEA, for oxygen reduction without previous activation by polarization can be obtained starting from the compound after treatment with reducing agents as NaBH₄ or gaseous hydrogen.

Example 7

The composites Pt-Cs_2-5PW₁₂ (Pt 5% o 20% w%), prepared as described above by using H₂PtCl₆, were used to prepare thin layers on a GC electrode and activated as in Example 6. The layers contained Vulcan XC-72: composite: Nafϊon with a weight ratio 1.3:1 :0.07. The RDE vo Mammograms in a fresh solution 0.1 M HClO₄ saturated with oxygen at room temperature are shown in Fig. 10 together with one obtained in the same condition with an electrode prepared using the reference catalyst 20% Pt- Vulcan XC-72. The Pt loadings of the three layers were the same: i.e. 15 μgcm^"2. As it may be seen, the curves relative to the layers containing Pt-Cs_2-SPWi₂ are shifted to more positive potentials than the one containing the reference catalyst. The positive shift is greater for the composite with the higher Pt content. The half wave potentials are 0.842 V, 0.866 V and 0.891 V for the 20%Pt Vulcan, the 5% and 20% Pt-Cs_2-5PWi₂, respectively. This means that the catalytic activity of the catalyst contain- ing Cs_2-5PWi₂ is greater than that of the reference catalyst. The catalytic activity may be expressed in terms of mass specific activity at 0.9 V. The values measured from Fig. 10 are 0.114 A/mgp, for 20%Pt- Vulcan, 0.135 and 0.153 A/mg_Pt for the electrodes Pt-Cs_2-5PWi₂ with 5 e 20 % Pt, respectively. This corresponds to an increase of 18% and 34% in mass specific activity for the composites containing 5% and 20 % Pt with respect to the reference catalyst whose measured mass activity is in agreement with the value of 0.13 A/mg_Pt quoted at 60 ⁰C in H.A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner "Activity benchmark and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalyst for PEMFCs", Applied Catalysis B: Environmental 56(2005)9).

Example 8

Fig. 11a shows a comparison between the RDE voltammograms obtained with a ring disk electrode (disk GC, ring Pt) with the reference catalyst and with Pt-Cs_2-5PWi₂ 20% Pt and equal loadings (15 μgcm^"2). As it may be seen from the ring currents (Fig. l i b) recorded at 1.2 V, while the potential of the film at the GC disk was swept in the negative direction, are higher for the reference catalyst. As the ring current are proportional to the quantity of H₂O₂ produced at the disk, this means that the catalyst containing the heteropolyacid salt is more effective than the reference catalyst in favouring the 4 electron reduction of oxygen to water. Example 9.

Fig. 12 shows the first and 600^th RDE voltammograms obtained with a layer of 20%Pt- CS2.5PW12 prepared as previously described and activated by electrochemical cycling in 0.5 H2SO4 saturated with O₂ at 25°C (Pt loading 31 μgcm^"2). The experiment was conducted tak- ing one RDE curve every hour for a total of 600 hours to establish the stability of the layer electrochemical activity. Less than 2% decrease of the diffusion current measured at 0.5 V and a 4 mV negative shift of the half way potential was found. This testifies the stability of the electrochemical activity of the deposit and, indirectly, the stability of the prepared catalyst over prolonged use. Example 10

In Figure 13 are shown two polarization curves obtained in single cells (5 cm² active area) in the following conditions: pressure 3 bar, cell temperature 70⁰C, RH anode and cathode 100%.

MEA were prepared according to the previously described procedure. The anodic catalytic layer was prepared in both cases using Pt 20% Vulcan XC-72 with a Pt loading of 1 mg cm^"2. In cell (a) the cathodic catalytic layer was made of a mixture of Vulcan XC-72 + not previously reduced (Pt 3.5 % by weight) + Nafion in 1 :2:0.6 weight ratio. Pt loading was 37 μg cm^"2. In cell (b) cathodic catalytic layer was made of a mixture of Pt 10 % Vulcan XC- 72 + Vulcan XC-72 + Nafion in 1 :2,1 :1.15 weight ratio. Pt load was 57 μgcm^"2. Inner resistances of the two cells were of the same order of magnitude (150 mΩ cm²). As it could be ob- served, the cell containing the compound has much better performances than the one containing only Pt 10% Vulcan XC-72, especially in the low current density zone, where the limiting factor is the kinetic of the oxygen reduction reaction. The effect is in line with what disclosed in Example N. 4. It will be also noted that in general the salts of the present invention are hydrophobic. This is favourable in the case they are used in the cathode, since water is eliminated, avoiding flooding of the electrode. The cell (a) worked continuously for about 300 hours without appreciable degradation of its performances and of the electrode as demonstrated by X-spectra of the electrode before and after 300 working hours (Figure 14), thus confirming the validity of the stability test disclosed in Example 9.

Example 11

Figure 15 shows polarization curves obtained on cells assembled with MEA having the following compositions:

Cell (a) 10% Pt Vulcan XC-72 : Vulcan XC-72 : Nafion (1 :1 :1.5 weight %) at anode and cathode; Pt loading: 50 μg cm^"2

Cell (b) anode 10% Pt Vulcan XC-72: Vulcan XC-72, Cs₂.₅PWi₂: Nafion (1 :1:1 :1.5 % by weight) Pt loading: 43 μgcm^"2; cathode as in Cell (a). Cell (c) anode as in Cell (a); cathode 10% Pt Vulcan XC-72: Vulcan XC-72, Rb₂.₅PWi₂: Nafion (1 : 1 : 1 : 1.5 % by weight) Pt loading 40 μg cm^"2.

As it can be observed, performances of the cell containing the compounds Rb_2-5PW₁₂ CS2.5PW12 at anode or at cathode are better than those containing only 10% Pt Vulcan-XC-72. This demonstrates that addition of the salt to the mixture improves performances of catalytic layers both for hydrogen oxidation and oxygen reduction.

Example N. 12

Fig. 16 shows a typical polarization curve obtained using a cell with a cathode made up with 20%Pt-CsPWi2 (Cs: Vulcan 2:1 weight ratio). The anode was a commercial electrode with a Pt loading 0.5 mg/cm² (LT 120EW from E-TEK). The Pt loading at the cathode was increased by a factor of about 4 (0.2 mg/cm²) with respect to Examples 10-11 to demonstrate that higher loading of heteropolyacid salt, that comes with a higher Pt loading, does not alter the activity of the catalytic layer and results in high power densities. The oxygen electrode was prepared using the salt doped with Pt(IV) by painting the ink on a commercial GDL (LT 1200W from E-TEK). Pt(IV) was reduced to Pt by treating the electrode at 100 ⁰C with 5% H₂ gas in Argon. The reduced electrode was then washed with a 0.5 M H₂SO₄ solution followed by water and assembled in a MEA with a Nafion 212 membrane by hot pressing. The cell was run at 70 ⁰C and anode and cathode Relative Humidity of 30% and at 3 bar absolute pressure at both electrodes. The curves demonstrate that the cathode catalyst may work at high current densi- ties with a low Pt content also in condition of low relative humidity.

Example 13

Fig. 17 shows a typical polarization curve obtained using a cell with an anode made up with 20%Pt-CsPWi₂ (Cs:20% Pt- Vulcan 2:1 weight ratio) prepared by precipitating Cs_2-5PWi₂ in the presence of 20%Pt- Vulcan XC-72. Pt loading at the anode was 0.2 mg/cm². The cathode was a commercial electrode with a Pt loading of 0.5 mg/cm² (LT120EW from E-TEK). The MEA was assembled by hot pressing using with a Nafion 212 membrane. The cell was run at 70 ⁰C and anode and cathode Relative Humidity of 100% and at 2 bar absolute pressure at both electrodes. The curves demonstrate that the composite of the invention, prepared using carbon supported Pt works well as oxidation catalyst at high current densities.

Claims

1. Use of a compound of formula , wherein M¹ is selected from the group consisting of Cs⁺, Rb⁺, K⁺, NH₄ ⁺, x represents the number salifϊed protons from M¹ in the corresponding acid, M² is P or Si and M³ is W or Mo; y = 3, when M² is P and y = 4 when M² is Si, for the preparation of a catalyst for fuel cells.

2. Use according to claim 1, wherein M is selected fro the group consisting of Cs⁺, Rb⁺, K⁺, NH₄ ⁺, x is 2 or 3, M² is P and M³ is W.

3. Use according to claim 2, wherein x is 2.5.

4. Use according to claim 3, wherein said compound is selected from the group consisting of CS_{2 5}HO ₅PWI₂O₄₀, (NH₄)₂HPWi₂O₄₀ and Rb_{2 5}H₁₁₅PWi₂O₄₀.

5. Use according to one of claims 1-3, wherein said compound is in micro-mesoporous form.

6. Electrode composition comprising a compound of claims 1-4 doped with a noble metal or an alloy thereof.

7. Composition according to claim 5, wherein said noble metal is Platinum or an alloy thereof.

8. Composition according to claim 5 or 6, wherein said metal or said alloy is supported on carbon with high surface development.

9. Composition according to on of claims 5-7, further comprising a copolymer of sulfonated tetrafluoroethylene.

10. Process for the preparation of the electrode composition of one of claims 5-7, comprising the electrochemical deposition of the noble metal or an alloy thereof in the compound of one of claims 1-4 by corrosion/dissolution of a counter electrode of said noble metal.

11. Process according to claim 8, wherein said noble metal is Platinum or an alloy thereof.

12. Process for the preparation of the electrode composition of one of claims 5-7, comprising mixing a compound of one of claims 1-4 with a noble metal or an alloy thereof, said noble metal being supported.

13. Process according to claim 10, wherein said noble metal or an alloy thereof is supported on carbon with high surface development.

14. Process according to claim 10 or 11, wherein said noble metal is Platinum or an alloy thereof.

15. Process for the preparation of the electrode composition of one of claims 5-7, comprising chemical introduction of a noble metal during the preparation phase of the compound of one of claims 1-4.

16. Process according to claim 15, wherein the precipitation is carried out by titrating with a salt of the cation of interest M¹ a mixture of the acid HM²M³I₂O₄ added with a salt of said noble metal.

17. Process according to claim 16, wherein said noble metal is present in a concentration between 3 and 20% w/w.

18. Process according to one of claims 16 or 17, wherein the precipitate obtained from said titration is submitter to a reduction treatment.

19. Process according to one of claims 15-18, wherein said noble metal is Platinum.

20. Use of a composition of one of claims 5-8 for the preparation of catalysts for hydrogen oxidation or for oxygen reduction in fuel cells.

21. Electrode comprising the composition of one of claims 5-8.

22. Fuel cell comprising at least one electrode of claim 21.