WO2006008319A2 - Catalysts for fuel cell electrodes based on cobalt and its alloys, the preparation and use thereof, as well as fuel cells containing them - Google Patents

Abstract

Fuel cells catalysts comprising a polymer and metal particles of cobalt, as such or in combination with other metals, are described. Described are also methods for the preparation of the above said catalysts, their use in fuel cells and fuel cells comprising them.

Description

Catalysts based on cobalt and its alloys, their preparation and use and fuel cells containing them

Field of the invention This invention concerns catalysts for fuel cells cathodes. State of the art:

A fuel cell is a device capable of converting directly the chemical energy of a fuel into electrical power. A fuel cell works roughly as a battery, but it never dies, provided the fuel is continuously added. The process of production of electrical power in a fuel cell is silent and without mobile parts, and it occurs with the evolution of heat, water, and in certain cases of CO₂, depending on the fuel, which can be either gaseous hydrogen or a compound containing atomic hydrogen. No matter what the fuel is, every cell employs oxygen, pure or atmospheric, as a co- reagent which is transformed into water. A modern fuel cell with a polymeric electrolyte working with pure or combined hydrogen is made up of two electrodes of porous and conductive materials, separated by a polymeric membrane permeable to ions contained in the electrolyte (Figure 1). There are fuel cells that work with solid electrolytes which are in general ceramic materials and are known as Solid Electrolyte Fuel Cells (SOFC).

Hydrogen-fed fuel cells containing a polymeric membrane as solid electrolyte are known with the acronym PEMFC {Polymer Electrolyte Membrane Fuel Cell), whereas fuel cells fed with aqueous solutions of compounds that carry combined hydrogen, generally alcohols, are known with the acronym DFC, which stands for Direct Fuel Cell. In case of PEMFCs with membranes permeable to cations only, hydrogen is oxidized at the anode (negative electrode) yielding protons (H⁺) and electrons (e^"). The protons pass through the membrane towards the cathode (positive electrode), where they are consumed in the reduction of atmospheric oxygen to water that uses the electrons that arrive from the anode (Equation 1). O₂ + 4H⁺ + Ae → 2H₂O (1)

The use of an anionic-exchange polymeric membrane as electrolyte, i.e. a membrane which allows negative charges only to pass, furthers the production of negative ions, in this case OH^", in the process of oxygen reduction at the cathode, while the overall electrochemical process is left unvaried, as well as the reversible voltage of the cell.

O₂ + 2H₂O + 4e^" → 4OH^" (2) In the PEMFCs, the polymeric electrolyte is generally Nation^®, a proton-exchange fluorinated membrane, about 50-200 micrometers thick. This contains negatively charged ions (usually sulfonate groups -SO₃ ^") covalently bonded to the polymeric backbone and therefore allows the passage of protons towards the cathode but not electrons. The theoretical voltage provided by one PEMFC is about 1.23 V at 25 ⁰C, however real voltages tend to decrease to 0.7-0.8 V, with currents from 300 to 800 mA/cm². Production of heat makes up for the loss in the electrical power. Higher powers and voltages can be achieved by connecting in series more cells with bipolar plates. Such a device is called a stack, and more stacks can be assembled to yield even higher powers, up to 250 kW. Such systems have several applications, from the co-generation of power for civil and industrial uses, to mechanical traction.

By DFC (Direct Fuel Cell) we mean all the cells in which a fuel different from hydrogen is directly put in contact with the anode with no preventive treatment to extract hydrogen. The most common DFC makes use of methanol (CH₃OH), and is known as the DMFC {Direct Methanol Fuel Cell). A common DMFC of the state of art resembles a PEMFC in its configuration and working. In the DMFCs the electrolyte consists of a polymeric membrane with either proton (e.g. Nafion) or anion exchange membrane (e.g. Selemion), and the electrocatalysts contain platinum or platinum alloys with other metals. These cells work best within the range of temperature 70-100 ⁰C. The methanol is oxidized at the anode to yield protons, electrons and CO₂, while the cathode process is wholly similar to the one that takes place in the PEMFCs.

DFCs have a remarkable advantage over hydrogen fuel cells: they can use a vast range of fuels, both liquid (alcohols in general) and solid soluble in water (acids, aldehydes, sugars). These fuels are ultimately transformed into CO₂, water and energy. As a matter of fact, the electrochemical performance changes in function of the fuel and the anodic catalyst employed. Direct ethanol fuel cells are exciting much interest because this alcohol, differently from methanol is much less toxic, and moreover is a renewable resource, since one can easily get ethanol out of fermentation of a huge variety of biomasses. A DFC differs mostly from a PEMFC in that the former releases CO₂ into the environment. On the other hand, if ethanol is used as a fuel in the DEFCs (Direct Ethanol Fuel Cell), the output of carbon dioxide into the environment is offset by the chlorophylian photosynthesis process, which fixes CO₂ in the form of vegetal mass, thus closing a cycle in which energy is achieved without increasing the greenhouse effect. The electrolyte in low temperature fuel cells can be a strong acid or basic solution, like a concentrated solution of KOH in the so-called AFC (Alkaline fuel cells).

In fuel cells, both the anodic and cathode reactions occur on catalysts (or electrocatalysts) which consist of either metallic sheets, or of highly dispersed metallic nano-particles (usually 2-50 nanometers, 10^"9 m, large), supported on a porous and conductive material (for instance carbon black). Catalysts for fuel cells are generally made up of platinum or platinum-ruthenium alloys, and their purpose consists in the speeding up of the anodic and cathode reactions, which otherwise would occur too slowly to produce useful currents. The catalysts and the electrolyte are therefore two essential components for the existence and the working of fuel cells. It is known in the state of art that the more dispersed metal particles are, the better the performances catalysts provide in terms of current density (commonly expressed in mA/cm²) (see Xin et al. Chem. Commun., 2003, 394-395 and references therein). Reports about the activity of electrocatalysts in platinum & its alloys-based fuel cells with particles one or less nanometre large are not known yet in literature. The platinum loading on the electrodes for DMFC of known art can vary from 5 to 10 mg/cm², while the platinum loading on the electrodes for PEMFCs of known art can vary from 0.12 to 2 mg/cm². Even higher loadings are required to obtain appreciable current densities in DEFCs. Data and general information about fuel cells of the types PEMFC, DFC (included DMFC and DEFC) and AFC, about their working and building technology are available in: celle a combustibile, M. Ronchetti, A. lacobazzi, ENEA, February 2002 (Italy); Handbook for fuel cells, W. Vielstick and A. Lamm, Wiley, Vol. Mil; Wiley, New York, 2003; C. Lamy et al. J. Power Sources 2002, 105, 283-296; C. Lamy et al. J. Appl. Electrochem. 2001 , 31, 799-809; M. P. Hogarth and T. R. Ralph Platinum Metal Rev. 2002, 46, 146-164; M. P. Hogarth and T. R. Ralph Platinum Metal Rev. 2002, 46, 3-14. The diffusion of PEMFCs, DMFCs and DFCs, as well as any other fuel cell working with platinum is dramatically limited by the scarce natural abundance of this metal, and consequently, by its high price (natural reserves amount to just 5000 tonnes, source Johnson Matthey in Platinum Metals Rev. 2004, 48, 34). The world production of mobile phones on its own, in case all the phones were fed with fuel cells, would absorb one third of world platinum extraction (just 165 tons in 2002), while the replacement of car engines with fuel cells stacks would require more than forty times the actual production of platinum (a Mercedes class A fed with reformed hydrogen contains 180 g of platinum in its stack). Because of platinum scarce availability one can correctly suppose that a high increase in the request for platinum would cause prices to go up, until fuel cells would lose their competitiveness over other technologies for the production of power. A second constraint to the employment of platinum-based catalysts of known art involves PEMFCs too, but mostly direct alcohol fuel cells, called DAFCs (Direct Alcohol Fuel CeIf). Platinum-based cathodes are sensitive to cross-over alcohols, causing significant cathode polarizations. This phenomenon leads to a strong decrease in the cell efficiency and performance as reported by Chu et al., J Electrochem. Soc, 1994, 41 , 1770; Kuver et al., Electrochimica Acta 1998, 43, 2527; Kuver et al., J. Power Sources 1998, 74, 211. Alcohols that arrive at the cathode can be oxidised (EQUATIONS 3 and 4) consequently reducing both the concentration of oxygen and the access of oxygen to the electrocatalyst cathode. CH₃OH + 3/2 O₂ → CO₂ + 2 H₂O (3)

CH₃CH₂OH + 3 O₂ → 2 CO₂ + 3 H₂O (4)

In addition, as the oxidation of the alcohols is not always complete formation of CO is possible which can deactivate the cathode catalysts, which are generally made of Pt. Experimental evidence for the problems associated with the difficulty of oxidising CO absorbed on platinum catalysts and the passivation of the active Pt sites are reported by M. Watanabe et al. J. Phys. Chern. B 2000, 104, 1762-1768; M. Watanabe et al. Phys. Chem. Chem. Phys. 2001, 3, 306-314; M. Watanabe et al. Langmuir 1999, 15, 8757-8764; M. Watanabe et al. Chem. Commun. 2003, 828-829; H. A. Gasteiger et al. J. Chem. Phys. 1994, 98, 619-625.; S.-M. Park et al. J. Electrochem. Soc. 1995, 142, 40-45. These works showed that the combination of platinum and other metals in alloys or aggregates can reduce appreciably the anodic overpotential in DAFC cells, rendering the electrodes more tolerant to CO. Only with anodes based upon Pt/Sn ((2 mg/cm²) and cathodes based upon Pt (4 mg/cm²), at temperatures higher than 90 ⁰C and at a 3 bar oxygen pressures, potential densities of a few tens of mW/cm²have been obtained (C. Lamy et al. J. Power Sources 2002, 105, 283-296; C. Lamy et al. J. Appl. Electrochem. 2001 , 31 , 799-809). With state of the art cathodes and anodes enormous difficulties are encountered in the realisation of DFCs using ethylene glycol (C. Lamy et al. J. Appl. Electrochem. 2001 , 31 , 799-809; W. Hauffe and J. Hetbaum Electrochimica Acta 1978, 23, 299).

Many attempts have been made at reducing the cross-over of alcohols, for example by modifying the ion exchange membrane or the anode. US Pat 5, 672,438; 5,672,439, 5, 874,182; 5, 945,231. It has been observed that a reduction of the permeability of the membrane brings as a consequence less efficient ionic transport and a consequent reduction in cell efficiency.

Some of the drawbacks previously mentioned can be overcome with a few expedients, which ultimately turn out to be long and expensive. For example, cathodes can be realized which make use of platinum alloys with other transition metals, like Ru, Co, Ni, Fe, Mo e Sn (see for example D. Chu and S. Gilmann J. Electrochem. Soc. 1996, 143, 1685), or else the platinum loading can be increased up to 10 mg/cm² in both DMFC electrodes. Platinum replacement with other metals stands all the same the most interesting alternative, both for its economical implications and for cell performances stability, especially if the cell works with alcohol. Moreover, an increased capability to reduce oxygen has been reported in those catalysts where more metals have been combined (M. Watanabe et al. J. Electrochem. Soc. 1999, 146, 3750-3756).

Several procedures are known to synthesize combined metal-based electrocatalysts. In one of the commonest, a metal salt is deposited onto a conductive support, generally a carbonaceous one, like Vulcan-XC-72, and then the metal is reduced in an aqueous dispersion with an appropriate reducing agent, or with hydrogen in solid-gas conditions. A similar process is employed to add a further metal salt. The resultant material often undergoes annealing in a reductive environment or in inert gas. Such a procedure is described in the US patent US 6,379,834 B1 , Apr. 30, 2003 for anodic Pt/Mo-based electrocatalysts. Fuel cells cathode catalysts are prepared by electrodeposition of one metal at the time, generally platinum, which may be followed by the electrodeposition of other metals (see US 6,498,121 B1 (Dec. 24, 2002); Pt/Ru/Ni in US 6,517,965 B1 (Feb. 11 , 2003); Pt-Ru-Pd in US 6,682,837 B2 (Jan. 27, 2004); Pt/Ru/Ni in US 6,723,678 B2 (Apr. 20, 2004)). Other methods for the preparation of one or more metal-based cathode elecrocatalysts are even more complicated and restricted to laboratory research only; one of these methods is magnetron sputtering deposition (M. Watanabe et al. Chem. Commun. 2003, 828-829; Masahiro Watanabe, Japan patent application No.H6-225840, Aug. 27, 1994). Cathode electrocatalysts which do not make use of platinum are known in the state of the art.

US-A-482981 describes fuel cells fuelled with methanol/air, where the cathode contains a salen-cobalt complex alone or covalently bound to a copolymer obtained by vinylpyridine and polytetrafluoroethylene or by polystyrene and divinylbenzene or amine-modified copolymers, as catalytic component. The cathode realized with such compounds is interfaced with a platinum catalyzed anode. US-A-6245707 reports platinum-free catalysts for the preparation of cathodes, which can tolerate cross-over methanol.

Metals like Co, Fe, Cu, Ni, Mn, Zn, V and Ru are complexed by chelating ligands containing four nitrogen donor atoms, such as the tetraphenylporphines, and they are treated at high temperature with a flow of an inert gas, alone or in binary mixtures.

DE-A-2549083 reports the preparation of platinum-free cathodes, using linear or reticulated polymers based on iron phthalocyanines. In US-A-5240893, a method for the preparation of cathodes, which have been obtained from pyrolysis of materials containing a polymer, metals and carbon particles, is described. These metals can be cobalt, nickel, vanadium, chromium, manganese, or their mixtures. The polymer is prepared by the reaction of amine with formaldehyde or by polymerization of formaldehyde in the presence of a base. The polymer reacts with both carbon particles and the metal salts, and then it is calcined at 800 ⁰C. Some tens of mW/cm² of specific power are generated at low current density.

In US-A-5358803 it has been reported the synthesis of cathodes with catalysts made of cobalt acetate and polyacrylonitrile. WO-A-0196264 describes the synthesis of a Fischer-Tropsch catalyst. The catalyst is made of a polymer (polyacrylate or polymetacrylate) and at least two metals, which are respectively selected in the group of iron, cobalt, nickel, chromium and iron, silver, zinc, copper, platinum, zirconium, or their combinations. A comparative study of different cobalt complexes used as catalysts for the reduction of oxygen in PEMFC has shown that the immobilization of the catalysts on graphite deeply affects this reaction (T. Okada et al., J. of Inorg. and Organometallic Polymer, 1999, 9, 199). A treatment at high temperature (600 ⁰C) of cobalt complexes stabilized by chelating N₄ o N₂O₂ ligands (salen, anten, and others) supported on graphite, increases the catalytic activity without modifying the original molecular structure of the cobalt complexes. As an alternative to the treatment at high temperature of the metal complexes supported on conductive carbon materials, the electropolymerization method is known in the state of the art: F, Beduou et al., J. Mat. Chem., 1997, 7, 923 and B. Ortiz, Bull. Korean Chem. Soc, 2000, 21 , 405. Nickel and cobalt complexes, stabilized by azamacrocyclic ligands, Schiff bases and porphirins, improve their catalytic activity upon electropolymerization directly on the cathode in basic environment. In a patent application (Platinum-free electrocatalysts materials, WO 2004/036674) it has been reported that a templating polymer obtained by the condensation of a 1 ,3-diol, containing coordinating nitrogen atoms, with a 3,5-disubstituted phenol and formaldehyde or paraformaldehyde is capable to coordinate platinum-free metal salts, preferably containing iron, cobalt and/or nickel, to produce adducts; these adducts, once reduced with gaseous hydrogen or with other reducing agents, or pyrolized in an inert atmosphere at temperatures over 500 °C, produce catalytic materials for anode and cathode electrodes of PEMFC, AFC, DFC, DMFC, DEFC and in general of DAFC fuel cells. In the same patent it has been claimed that the most suitable metals for the preparation of cathode catalysts are nickel and cobalt, and the former is preferred.

Whatever the synthesis method, all known catalysts for fuel cells cathodes, and especially for PEMFC, DFC and DAFC fuel cells, contain metal particles larger than 1 nanometer (usually between 2 and 50 nm), no matter what method has been followed for their preparation (Q. Xin et al. Chem. Commun. 2003, 394-395). Undoubtedly, the activity of a catalyst, especially of a bimetallic or trimetallic one, depends upon both electronic and structural factors (M. Watanabe et al. J. Electrochem. Soc. 1999, 146, 3750-3756). The latter depend in their turn on both the synthetic procedure and the nature of the combined metals. A monograph useful for a more accurate understanding of such structure/reactivity relations has been written by P. N. Ross "The Science of Electrocatalysis on Bimetallic Surfaces", Vol. 4, J. Lipowski and P. N. Ross Jr., Wiley-lnterscience, New York, N. Y. 1997. Summary of the invention This invention describes cathode catalysts for fuel cells, containing a low metal loading, made up of metal complexes formed by cobalt salts or alloys of cobalt with other metals and polymers (already described in the said WO 2004/036674), obtained from the condensation of an 4-{1-[(phenyl-2,4-disubstituted)-hydrazine]- alkyl}-benzene-1 ,3-diol with a 3-5-disubstituted phenol and formaldehyde or paraformaldehyde in the presence of an acid or basic catalyst in water/alcohol mixtures, at temperatures between 20 and 150 ⁰C and with a molecular weight between 1000 and 50000. Description of the figures

Fig. 1 - Simplified scheme of a fuel cell working with the catalysts of the invention. Fig. 2 - Histogram showing the particle size distribution in a catalyst containing 1.0 wt. % Co loading with respect to the catalytic system metal-support (Vulcan XC- 72), realized as in process A.

Fig. 3 - Histogram showing the particle size distribution in a Co₅₀Ni₅₀ catalyst, 1.2 wt. % overall metal loading with respect to the catalytic system metal/support (Vulcan XC-72), realized as in process B. Fig. 4 - Histogram showing the particle size distribution in a Co₉₀-Fe_I0 catalyst, 2.5 wt. % overall metal loading with respect to the catalytic system metal/support (Vulcan XC-72), realized as in process C.

Fig. 5 - Histogram showing the particles size distribution in a COs₀-Ni₄₀-Vi₀ catalyst, containing 2,0 wt. % metal loading with respect to the catalytic system metal/support (Vulcan XC-72), realized as in process A.

Fig. 6 - Polarization curve of a PEMFC cell (Λ/a/7bn^®-112, H₂SO₄ 1 N), with the anode containing a commercial platinum catalyst (platinum 10% on carbon black, <1 mg Pt/cm²' Johnson Matthey), and with the cathode catalyzed with 0.10 mg Co/cm² (1.0% metal/C) at about 60 ⁰C, with pure H₂ (1 bar). Fig. 7 - Polarization curve of a DMFC cell (Λ/af/oπ^®-112, H₂SO₄ 1 N), with the anode containing a commercial Pt/Ru catalyst (platinum 20%, ruthenium 10% on carbon black, > 1mg Pt/cm²' Johnson Matthey), and with the cathode catalyzed with 0.10 mg Co₅₀-Ru₄₀-N I₁₀/cm² (1.0% metal/C) at about 65 ⁰C, fuelled with an aqueous solution of MeOH (15% v:v). Fig. 8 - Polarization curve of a DEFC cell (Selemion AMW, K₂CO₃ 1 N), with the anode containing a commercial Pt/Ru catalyst (platinum 20 wt%, ruthenium 10 wt% on carbon black, >1 mg Pt/cm², Johnson Matthey), and the cathode is catalyzed with 0.10 mg Co₆₀Ni₄₀/cm² (1.2% metal/C) at about 25 ⁰C, fuelled with an aqueous solution of EtOH (10%, v:v). Detailed description of the invention

This invention allows one to overcome the obstacles deriving from the use of known cathodes for fuel cells, thanks to their low cobalt loading (expressed in mg/cm²), where cobalt can be either alone or in combination with other metals. In the patent application WO 2004/036674 it has been reported that efficient PEMFC, DAFC and AFC fuel cells cathodes can be realized by pyrolysis of nickel or cobalt salts, coordinated by a polymer obtained from the condensation of an 4-{1- [(phenyl-2,4-disubstituted)-hydrazine]-alkyl}-benzene-1 ,3-diol with a 3-5- disubstituted phenol and formaldehyde or paraformaldehyde in the presence of an acid or basic catalyst in water/alcohol mixtures, at temperatures between 20 and 150 ⁰C and with a molecular weight between 1000 and 50000. We have now surprisingly found out that at comparable metal loading, cobalt- based catalysts are better than nickel-based ones, whatever the kind of fuel cell. Besides, cathodes containing catalysts based on cobalt alloys with other metals, like Fe, Ru, Ni, Mo, Rh, Ir, Sn, Mn, V and La, are even more efficient than catalysts containing exclusively cobalt.

The catalysts of the invention are made of metal particles, which have been obtained by pyrolysis of complexes metal salts. These metal complexes are formed of cobalt salts, or its alloys, and polymers (already described in said WO 2004/036674), which shall be denoted, from now on POLIMERO for sake of brevity, obtained from from the condensation of an 4-{1-[(phenyl-2,4-disubstituted)- hydrazine]-alkyl}-benzene-1 ,3-diol with a 3-5-disubstituted phenol and formaldehyde or paraformaldehyde in the presence of an acid or basic catalyst in water/alcohol mixtures, at temperatures between 20 and 150 ⁰C and with a molecular weight between 1000 and 50000.

Preferentially, the 4-{1-[(phenyl-2,4-disubstituted)-hydrazine]-alkyl}-benzene-1 ,3- diol is a compound with formula (A)

Where Ri is selected out of the group comprising: H and a hydrocarbon group with a number of carbon atoms from 1 to 10, possibly halogenated;

R₂ e R₃, independently, represent an electron-withdrawing group selected in the series constituted by hydrogen, halogen, acyl, ester, carboxylic acid, formyl, nitrile, sulfonic acid, aryls, linear or branched alkyls containing from 1 to 15 carbon atom, eventually functionalised with halogen atoms, or linked to each other is such a way to form one or more rings condensed with the phenyl ring, or nitro groups; and the 3,5-disubstituted phenol is a compound with the formula (B):

(B) where R₄ and R₅ independently represent an electron-donating group selected in the series constituted by H, OH, ether, amine, aryls and linear or branched alkyls, containing from 1 to 15 carbon atoms. The said polymers of the invention can be represented by the formula (C):

(C)

Where y varies from 2 to 120, x from 1 to 2, n from 1 to 3 and R₁, R₂, R₃, R₄ e R₅ are defined as above.

According to the invention, by cobalt metal salts or its combinations with other metal salts, we mean salts to be selected among the group constituted by carboxylates, halides, alcoholates, acetylacetonates, formates, oxalates, malonates, and analogous organic salts and their mixtures or carbonates and bicarbonates or their mixtures. According to the invention, metals to be used in conjunction with cobalt are preferably selected in the group Fe, Ru, Ni, Mo, Rh, Ir, Sn₁ Mn, V and La. According to this invention, by fuel cells, PEMFC, DAFC, DFC and AFC are meant.

Preferably, the catalysts of the invention, made of highly dispersed metal particles, with diameters from 2 to 50 A (1C)^"10 m), are deposited onto conductive inorganic support materials, typically amorphous carbon blacks or highly porous graphites, but even on non-conductive materials, such as porous metal oxides, like silica, alumina and ceria, for other purposes than the use in fuel cells. Before metals deposition, the support materials are purified and activated as reported in the state of the art. PURPOSES OF THE PRESENT INVENTION One of the purposes of this invention is the simple and economical realization of platinum-free cathodes based on cobalt and its alloys with other metals. This invention refers to fuel cells electrodes working at low temperature and to the process necessary for their production from a known polymer, which is denoted POLIMERO from now on, as defined before. The catalysts of the invention contain cobalt particles with a diameter from 2 to 50 A (10^"1° m), where cobalt can be either the only element, or arranged in binary and ternary combination with other metals like Fe, Ru, Ni, Mo, Rh, Ir, Sn, Mn, V and La just to say but a few. Cathodes realized with the catalysts of the invention, display properties and activities superior to those known in the state of the art for both platinum- and cobalt-based DFC even in combination with other metals, both in basic and acid ambient, with anionic and cationic exchange membranes, especially in direct alcohol fuel cells.

DESCRIPTION OF THE PRESENT INVENTION In accord with the purposes listed above, the present invention claims a new and improved method to prepare cathode catalysts for fuel cells, based on cobalt or its combinations with other metals, preferably Fe, Ru, Ni, Mo, Rh, Ir, Sn, Mn, V and La. The catalysts of the invention are made of highly dispersed metal particles, with a diameter from 3 to 50 A (10^"1° m), deposited on conductive inorganic support materials, typically amorphous carbon blacks or highly porous graphites, but even on non-conductive materials, such as porous metal oxides, like silica and alumina, for other purposes than their use in fuel cells. Before the deposition of the metals, the support materials are purified and activated as described in the state of the art. To prepare cathode catalysts, the methods A, B and C can be indiscriminately used.

Method A:

A cobalt salt, preferably cobalt acetate tetrahydrate, Co(CH₃Cθ₂)₂4H₂O, is dissolved in water and the resulting solution is added to an aqueous suspension of the POLIMERO (see WO 2004/036674, PCT/EP2003/006592). Some hours later, the resultant solid product is filtered off, washed with water and dried.

To this solid, dissolved/suspended in acetone or dimethylformamide, is added a support material such as Vulcan XC-72 or an active carbon RDBA, and the mixture is stirred for some hours. The solvent is removed by evaporation at reduced pressure, and the solid residue is heated up to 500-900 ⁰C in an inert gas atmosphere (N₂, Ar).

Method B

A cobalt salt, preferably cobalt acetate tetrahydrate, Co(CH₃CO₂)₂4H₂O, and a nickel salt, preferably nickel acetate tetrahydrate, Ni(CH₃CO₂^H₂O, are dissolved in water and added to an aqueous suspension of the POLIMERO. Some hours later, the resultant solid product is filtered off, washed with water and dried.

To this solid, dissolved/suspended in acetone or dimethylformamide is added to a support material such as Vulcan XC-72 or an active carbon RDBA, and the mixture is stirred for some hours. The solvent is removed by evaporation at reduced pressure, and the solid residue is heated up to 500-900 ⁰C in an inert gas atmosphere (N₂, Ar).

Method C

A cobalt salt, preferably cobalt acetate tetrahydrate, Co(CH₃CO₂)₂4H₂O, dissolved in water, a nickel salt, preferably nickel acetate tetrahydrate, Ni(CH₃CO₂)₂4H₂O, dissolved in water, and a vanadium salt, preferably vanadyl(IV) acetylacetonate,

(CH₃COCHCOCH₃)₂VO, dissolved in acetone, are added to an aqueous suspension of the POLIMERO. Some hours later, the resultant solid product is filtered, washed with water and dried.

To this solid, dissolved/suspended in acetone or dimethylformamide is added a support material such as Vulcan XC-72 or an active carbon RDBA, and the mixture is stirred for some hours. The solvent is removed by evaporation at reduced pressure, and the solid residue is heated up to 500-900 °C in an inert gas atmosphere (N₂, Ar).

Fuel cells containing the cathodes described in the present invention can make use of all known art anode catalysts, chosen on the basis of the fuels employed. Preparation of a cathode of the invention

The catalytic material that has been obtained with methods A, B and C is suspended in a water/ethanol mixture. PTFE (polytetrafluoroethylene, Aldrich) is added to this mixture and a flocculous compound separates, which is spread and pressed at ambient temperature over an appropriate conductive support, such as carbon paper, steel nets or conductive ceramics, just to mention some. At this time, the support is heated up to 250-350 ⁰C in an inert gas atmosphere. The metal composition in the catalysts has been determined by means of Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and verified with Energy Dispersive X-ray Spectrometry (EDXS). Histograms reported in Figures 2-5, which have been obtained from high resolution Transmission Electron Microscopy (TEM) show the distribution of the particles of a catalyst of the invention, based on cobalt, or cobalt/metal(s) combinations, as reported in detail in the description of the Figures. Histograms of cobalt particles or polymetallic particles containing cobalt in alloy with other metals, with distribution frequencies below 1 nanometer, and centred on 3-4 A, are not known for cobalt or its alloys catalyses used in fuel cells of the state of the art.

Metal particles of the catalysts of the invention are featured by a few atoms, at most a dozen, thus creating structures capable of an extraordinary cathode reactivity in fuel cells containing solid electrolytes made of both proton exchange (like Nation^®) and anionic exchange (like Asahi glass' Flemion^®) polymeric membranes. Experiments carried out with Diffuse Reflectance Infrared Spectroscopy (DRIFT) technique have shown that the catalysts of the invention, no matter what metal or metal combination is used, do not form strong bonds with CO.

Cathodes containing the catalysts of the invention convert pure oxygen, or atmospheric oxygen into water (when the electrolyte is a proton exchange membrane) or into hydroxide ions OH^" (when the electrolyte is an anionic exchange membrane).

A cathode of the invention, combined with an anode of the state of the art for fuel cells, can be used for assembling a fuel cell as the one shown in Figure 1. The performances of monoplanar fuel cells, working with electrodes of the invention, have been measured with a potenziostat in different experimental conditions.

Figures 6-8 show some examples of polarization curves for different combinations of cathodes of the invention with anodes constituted by commercial Pt or Pt/Ru- based catalysts.

In one of the possible realizations, the present invention refers to cathode electrodes catalyzed with cobalt or with cobalt combinations with other metals, like Fe, Ru, Rh, Ir₁ Ni, Pd, Mo, Sn, V, Mn and La, for PEMFC fuel cells fuelled with hydrogen, which, at the same functional characteristics of known catalyzed electrodes, make use of a lower amount of cobalt, not superior to 1 mg/cm², preferably minor or equal to 0.30 mg/cm².

In another possible realization, the present invention refers to cathodes for DAFC fuel cells containing cobalt or its combinations with other metals like Fe, Ru, Rh, Ir, Ni, Pd, Mo, Sn, V, Mn, La. Such cathodes allow for the use of alcoholic fuels such as methanol, ethanol, ethylene glycol, or sugars like glucose and sorbitol, in aqueous concentrations up to 50% in weight; in addition, they make use of an amount of cobalt or cobalt alloys with other metals not higher than 1 mg/cm², preferably lower or equal to 0.30 mg/cm². In another possible realization, the invention refers to DFC fuel cells cathodes containing cobalt or its combinations with other metals, like, Ru, Co, Rh, Ir, Ni, Pd, Mo, Sn, Mn, V and La. Such cathodes allow for the use of fuels containing combined hydrogen, like aldehydes, acids, hydrazine, metal borohydrides, in aqueous or alcoholic concentrations up to 50 wt. %, and make use of amounts of cobalt or its alloys with other metals lower tha 1 mg/cm², preferably lower than or equal to 0.30 mg/cm².

In another possible realization, the invention refers to DAFC and AFC fuel cells cathodes containing cobalt or its combinations with other metals, like, Fe, Ru, Co, Rh, Ir, Ni, Pd, Mo, Sn, Mn, V and La. Such cathodes allow for the use of alcohol fuels like methanol, ethanol, ethylene glycol, or sugars like glucose and sorbitol in aqueous concentrations up to 50% in weight, and make use of amounts of cobalt or its alloys with other metals lower than 1 mg/cm², preferably lower than or equal to 0.30 mg/cm².

For a better understanding of the invention, it is herein reported a collection of examples of the preparation of cathodes.

PREPARATION OF A COBALT-BASED CATHODE CATALYSTS

Example 1 3.6 g of cobalt(ll) acetate tetrahydrate are added to a suspension containing 7 g of the POLIMERO in 200 mL of water. The mixture is brought to pH 9 by adding 100 mL of NaOH 1 M, and energetically stirred for 15 hours at ambient temperature. A brown red precipitate forms, which is filtered off, washed several times with distilled water (3 x 40 mL) and dried under vacuum at 70 ⁰C until constant weight. Yield 9 g. Cobalt content = 3.74 wt. % (ICP-AES analysis).

36 g of Vulcan XC-72R, previously activated by reflux in 100 mL of HNO₃ 1 N, filtered, washed several times in water and heated up to 800 ⁰C for 2 h in an inert gas stream, are added to a suspension of 3.6 g of the product obtained before in 600 mL of acetone (finely dispersed with an ultrasound probe for 20 minutes). The resultant mixture is vigorously stirred at ambient temperature for 3 h; then, the solvent is removed by distillation at reduced pressure. The solid residue is put into a quartz reactor and heated up to 600 ⁰C in a nitrogen stream for 2 h. Cobalt content = 0.37 wt. % (ICP-AES analysis). PREPARATION OF A COBALT AND NICKEL-BASED CATHODE CATALYST Example 2.

3.6 g of cobalt(ll) acetate tetrahydrate dissolved in 20 mL of water and 1.8 g of nickel(ll) acetate tetrahydrate dissolved in 20 mL of water are added to a suspension containing 7 g of the POLIMERO in 200 ml_ of water. The mixture is brought to pH 9 by adding 100 mL of NaOH 1 M and vigorously stirred for 15 h at ambient temperature.

A brown red precipitate forms, which is filtered, washed many times with distilled water (3 x 40 mL) and dried in vacuum-sealed stove at 70 ⁰C until constant weight. Yield 7.8 g. Cobalt content = 2.40 wt. %, nickel content = 2.20 wt. % (ICP-AES analysis).

20 g of activated Vulcan XC-72R are added to a suspension of 2 g of the compound obtained before in 600 mL of acetone (finely dispersed with an ultrasound probe for 20 minutes). The resultant mixture is vigorously stirred at ambient temperature for 3 h; then, the solvent is removed by distillation at reduced pressure. The solid residue is introduced into a quartz reactor and heated up to 600 ⁰C in a nitrogen stream for 2 h. Cobalt content = 0.20 wt. %, nickel content = 0.21 wt %. (ICP-AES analysis). Atomic ratio Co₅₉Ni_4I

PREPARATION OF A COBALT AND IRON-BASED CATHODE CATALYST Example 3.

3.6 g of cobalt(ll) acetate tetrahydrate dissolved in 20 mL of water and 1.6 g of iron(ll) acetate 30 mL of water are added to a suspension containing 7 g of the POLIMERO in 200 mL of water. The mixture is brought to pH 9 by adding 100 mL of NaOH 1 M and vigorously stirred for 15 h.

A brown red precipitate forms, which is filtered off, washed several times with distilled water (3 x 40 mL) and dried under vacuum at 70 ⁰C until constant weight. Yield 8 g. Cobalt content = 2.20 wt. %, iron content = 2.23 wt. % (ICP-AES analysis).

20 g of activated Vulcan XC-72R are added to a suspension of 2 g of the products obtained before in 600 mL of acetone (finely dispersed with an ultrasound probe for 20 minutes at ambient temperature). The resultant mixture is vigorously stirred at ambient temperature for 3 h; then, the solvent is removed by distillation at reduced pressure. The solid residue is introduced into a quartz reactor and heated up to 600 ⁰C in a nitrogen stream for 2 h. Cobalt content = 0.20 wt. %, iron content = 0.22 wt %. (ICP-AES analysis). Atomic ratio Co₄₈Fe₅₂. PREPARATION OF A COBALT AND VANADIUM-BASED CATHODE CATALYST Example 4.

3.6 g of cobalt(ll) acetate tetrahydrate dissolved in 20 mL of water and 2.5 g of vanadyl(IV) acetylacetonate in 50 mL of acetone are added to a suspension containing 7 g of the POLIMERO in 200 mL of water. The mixture is brought to pH 9 by adding 100 mL of NaOH 1 M and vigorously stirred for 15 h at ambient temperature.

A dark brown precipitate forms, which is filtered off, washed several times with distilled water (3 x 40 mL) and dried under vacuum at 70 ⁰C until constant weight. Cobalt content = 2.40 wt. %, vanadium content = 2.60 wt. % (ICP-AES analysis). 20 g of activated Vulcan XC-72R are added to a suspension of 2 g of the products obtained before in 600 mL of acetone (finely dispersed with an ultrasound probe for 20 minutes at ambient temperature). The resultant mixture is vigorously stirred at ambient temperature for 3 h; then, the solvent is removed by distillation at reduced pressure. The solid residue is introduced into a quartz reactor and heated up to 600 ⁰C in a nitrogen stream for 2 h. Cobalt content = 0.24 wt. %, vanadium content = 0.26 wt %. (ICP-AES analysis). Atomic ratio Co₄₈V₅₂. PREPARATION OF A COBALT, NICKEL AND VANADIUM-BASED CATHODE CATALYST Example 5

3.6 g of cobalt(ll) acetate tetrahydrate dissolved in 20 mL of water, 1.8 g of nickel(ll) acetate tetrahydrate dissolved in 20 mL of water and 2.5 g of vanadyl(IV) acetylacetonate dissolved in 50 mL of acetone are added to a suspension containing 7 g of POLIMERO in 200 mL of water. The mixture is brought to pH 9 by adding 100 mL of NaOH 1 M and vigorously stirred for 15 h at ambient temperature.

A dark brown precipitate forms, which is filtered off, washed several times with distilled water (3 x 40 mL) and dried under vacuum at 70 ⁰C until constant weight. Yield 7.3 g. Cobalt content = 1.5 wt. %, nickel content = 1.7 wt. % and vanadium content = 4 wt. % (ICP-AES analysis).

20 g of activated Vulcan XC-72R are added to a suspension of 2 g of the product obtained before in 600 mL of acetone (finely dispersed with an ultrasound probe for 20 minutes at ambient temperature). The resultant mixture is vigorously stirred at ambient temperature for 3 h; then, the solvent is removed by distillation at reduced pressure. The solid residue is introduced into a quartz reactor and heated up to 600 ⁰C in a nitrogen stream for 2 h. Cobalt content = 0.15 wt. %, nickel content = 0.17 wt. % and vanadium content = 0.4 wt %. Atomic ratio = Co_2INi₂₄V₅₅. PREPARATION OF A CATHODE FOR FUEL CELL

10 g of a compound obtained as in examples 1 , 2 and 5 are suspended in 100 mL of a water/ethanol mixture 1 :1 (v/v). 3,5 g of PTFE (polytetrafluoroethylene) dispersed in water (60 wt.%) are added to the suspension, which is vigorously stirred.

20 min. later, a flocculous compound forms, which is separated by decantation. As an alternative to Vulcan, active carbon RDBA, R-5000, NSN-III or Keiten black or Raven can be used as carbonaceous support material.

Method (a): 100 mg of the product obtained before are uniformly spread onto a steel net, which is pressed at 100 Kg/cm² at ambient temperature. The electrode is heated up to 350 °C, in oven under an inert gas stream for some minutes (N₂ o Ar). Method (b): 0.5 mL of a 200 mg suspension of the POLIMERO, containing metals described in examples 1 , 2 and 5 in 50 mL of acetone, are deposited on a conductive substrate, such as silver or nickel powder, which is heated up to 500 ⁰C in an inert gas stream. Conductive substrates can be powdered ceramics (Wc, MOc, and more) as well. Method (c): this method makes use of all water-proof supports described in the state of the art. 0.5 mL of a suspension of 200 mg of the POLIMERO, containing metals as described in examples 4, 5 and 6, in 50 mL of acetone, is mixed with an electroconductive powder (3 g), suspended in water (50 mL) in the presence of 2 g of PTFE or polyethylene. After having removed the solvent by evaporation at reduced pressure, the solid residue is pressed at 100 Kg at ambient temperature, to provide all-sized thin layers or small discs, which are treated at 150 °C under inert gas.

Claims

1. Cathode catalysts for low metal-loading fuel cells, made up of metal complexes formed of cobalt salts, alone or in combination with other metals, and polymers obtained from the condensation of an 4-{1-[(phenyl-2,4- disubstituted)-hydrazine]-alkyl}-benzene-1 ,3-diol with a 3-5-disubstituted phenol and formaldehyde or paraformaldehyde in the presence of an acid or basic catalyst in water/alcohol mixtures, at temperatures between 20 and 150 ⁰C and with a molecular weight between 1000 and 50000.

2. Catalysts as in claim 1 , where the 4-{1-[(phenyl-2,4-disubstituted)- hydrazine]-alkyl}-benzene-1 ,3-diol is a compound with the following formula (A)

where Ri has been selected among the group made of: H and a hydrocarbon fragment with a number of carbon atoms from 1 to 10, possibly halogenated;

R₂ e R₃, independently, represent an electron-withdrawing group selected in the series constituted by hydrogen, halogen, acyl, ester, carboxylic acid, formyl, nitrile, sulfonic acid, aryls, linear or branched alkyls containing from 1 to 15 carbon atom, eventually functionalised with halogen atoms, or linked to each other is such a way to form one or more rings condensed with the phenyl ring, or nitro groups; and the 3,5-disubstituetd phenol is a compound with the formula (B):

(B) where R₄ and R₅, independently, represent an electron-donating group, selected in the group made of H, OH, ether, amine, aryls and linear and branched alkyls, with a number of carbon atoms from 1 to 15.

3. Catalysts as in claims 1 and 2, where the said polymers are represented by the formula (C)

(C) where y varies from 2 to 120, x from 1 to 2, n can vary from 1 to 3 and R₁, R₂, Rβ, R₄ e R₅ are defined as above.

4. Catalysts as in claims 1 -3, where the metals in combination with cobalt are chosen among the group made of: Fe, Ru, Rh, Ir, Ni, Pd, Mo, Sn, Mn, V and La.

5. Catalysts as in claims 1 - 4, where the said salts are selected in the group made of carboxylates, halides, alcoholates, acetylacetonates formats, oxalates, malonates and similar organic salts and their mixtures, or carbonates and bicarbonates or their mixtures.

6. Process for the preparation of a catalyst as in claims 1 -5 where: a cobalt salt is first complexed by the polymer as defined in the claims above.

- the resultant metallized polymer is supported on conductive and porous carbon materials; - the resultant product is:

- heated in an inert gas atmosphere at temperatures up to 1000 ⁰C

7. Process for the preparation of a catalyst as in claims 1 - 5 where:

- a mixture made of a salt or a cobalt compound and one or more metal salts is complexed by the polymer; the resultant metallized polymer is supported on porous and conductive carbon materials of the state of the art; finally, the resultant product is heated up to 1000 ⁰C in an inert gas atmosphere.

- Process for the preparation of a catalysts as in claims 1 -5 where a salt or a cobalt compound is first complexed by the polymer; the resultant complex is then treated with one or more salts; the resultant metallized polymer is supported on porous and conductive carbon materials of the state of the art finally, the resultant product is heated up to 1000 ⁰C in an inert gas atmosphere.

9. Process for the preparation of catalysts as in claims 5 -8, where the complex, or the polymer metallized with cobalt alone or with cobalt and one or more metals, is supported on porous metal oxides of the state of the art, and the product thus obtained is heated up to 1000 ⁰C in an inert gas atmosphere.

10. Cobalt-based catalyst as in claims 1-4, prepared as described in claims 5-8, where the cobalt loading varies from 0.1 to 50 wt. % with respect to the support.

11. Cobalt and iron-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and iron ranges from 99:1 to 1 :99.

12. Cobalt and ruthenium-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and ruthenium ranges from 99:1 to 1 :99.

13. Cobalt and rhodium-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and rhodium ranges from 99:1 to 1 :99.

14. Cobalt and iridium-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and iridium ranges from 99:1 to 1 :99.

15. Cobalt and nickel-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and nickel ranges from 99:1 to 1 :99.

16. Cobalt and palladium-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and palladium ranges from 99:1 to 1 :99.

17. Cobalt and tin-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and tin ranges from 99:1 to 1 :99.

18. Cobalt and molybdenum-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and molybdenum ranges from 99:1 to 1 :99.

19. Cobalt and vanadium-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and vanadium ranges from 99:1 to 1 :99.

20. Cobalt and manganese-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and manganese ranges from 99:1 to 1 :99.

21. Cobalt and lanthanum-based catalyst as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and lanthanum ranges from 99:1 to 1 :99.

22. Catalyst based on cobalt and on any transition or not-transition metal as in claims 1-4, prepared as described in claims 6-8, where the overall metal loading varies from 0.1 to 50 wt. % with respect to the support, and the atomic ratio between cobalt and the metal ranges from 99:1 to 1 :99.

23. Cobalt based catalyst as in claims 1-4, combined with two or more metals and prepared as described in claims 6-8, where the overall metal loading varies from 0.1 and 50 wt. % and the atomic ratio between metals can be any possible combination.

24. Fuel cell electrode formed of a catalyst as in claims 9-22.

25. Electrochemical fuel cell for the production of electric power, made of catalyzed cathodes as in claims 9-22, and of a proton-exchange membrane.

26. Electrochemical fuel cell for the production of electric power, made of catalyzed cathodes as in claims 9-22, and of an anion-exchange membrane.

27. Electrochemical fuel cell for the production of electric power, made of catalyzed cathodes as in claims 9-22, and of a liquid electrolyte, consisting of an aqueous solution of a strong base.

28. Electrochemical fuel cell for the production of electric power, made of catalyzed cathodes as in claims 9-22, and of a liquid electrolyte, consisting of an aqueous solution of a strong protic acid.

29. Electrochemical fuel cell for the production of electric power, made of catalyzed cathodes as in claims 9-22 and of an anode known in the state of the art.

30. Electrochemical cell defined in claims 24 and 25, containing a proton exchange membrane.

31. Electrochemical cell defined in claims 24 and 25, containing a anions exchange membrane.

32. Electrochemical cell defined in claim 26 containing a liquid alkaline electrolyte.

33. Electrochemical cell defined in claim 27 containing a liquid acid electrolyte.

34. Electrochemical cell defined in claims 24-31 , where fuel is gaseous hydrogen, reformed hydrogen, alcohols, hydrazine and hydroxylamine; borohydrides with various metal and non metal cations; aldehydes; acids; hydrocarbons, fossil fuels.