Precursor organic solutions of tetravalent metal phosphates and pyrophosphates and their use for electrode modification and for the preparation of composite membrane for fuel cells working at temperatures >90°C and/or at low relative humidity

The interest for polymeric electrolyte fuel cells (PEMFC) is considerably grown since these electrochemical generators do not produce fine parti- cles or toxic gases and, furthermore have a better performance than thermal motors. A massive replacement of the present vehicles with new electrical vehi¬ cles supplied by fuel cells is expected to have a beneficial effect not only on the air pollution of large towns but also could slow down the present fuel burning speed, thus decreasing also the danger due to sun house effects.

In spite of research efforts in all the most industrialized nations, the mass production of PEMFC electrical vehicles is hindered by various problems, especially related to the efficiency of the state of the art of electrodes, that have not yet the requested exchange currents, and to the proton conducting membranes of the state of the art, which do yet possess high proton conductivity when working at low relative humidity. Even when very expensive platinum electrodes and the best presently available perfluorosolfonic membranes are used, PEMFCs dramatically decrease their performance at temperatures greater than 900C and at relative humidity lower than 70%. In practice, the present PEMFCs for cars are obliged to operate in the temperature range 70-900C and at relative humidity greater than 75%, thus making complicate and expen¬ sive either the cooling of the cells, especially in summer, or the water management. In a previous patent it has been shown that the presence of inorganic particles in the interfacial electrodes/membrane regions considerably improves the performance of PEMFCs at temperatures greater than 1000C (G. Alberti et al. EP1205994)

This important result has been later confirmed also by American re- searchers (L. Krishnan et al. Abstracts of 201st Meeting of ECS, Phila¬ delphia May 12-17, 2002).

It has been reported in literature (see, as an example, the recent review of G. Alberti, M. Casciola, Annu. Rev. Res. 2003, 33:129 and references therein) that an improvement of PEMFCs performance at temperatures greater than 900C can be obtained by insertion of inorganic nano- particles in the polymeric matrix of the membranes used in these de¬ vices.

Thus, the facility and economy of the insertion of inorganic particles in the electrodes/membrane interfacial regions and/or inside ionomeric membranes of the state of the art assumes a relevant importance for commercial developments of PEMFCs.

Such insertion is not easy to be performed since the inorganic particles to be inserted must be preferably very insoluble in water and in common organic solvents and they have furthermore very low vapour pressures. A very promising procedure for these insertions is based on the possibil¬ ity of preparing organic solutions containing the components of the inor¬ ganic particles to be inserted.

Such solutions must preferably have the property that the insoluble par¬ ticles are formed only when the solvent is eliminated, eventually after a thermal treatment. These solutions can therefore be considered as solu¬ ble precursors of insoluble inorganic particles A large part of the inorganic particles already inserted in ionomeric membranes are based on silica or metal oxides such as titania and zir- conia usually obtained for decomposition with water of the correspond¬ ing metal alcoxides (A.S. Aricό, V. Antonucci, 1999, EP 0926754; Roziere et al., WO0205370).

Recently, the preparation of precursor organic solutions of tetravalent metal phosphate-sulfophenylenphosphonates having compositions M(IV)(O3P-G)2-X(O3P-Ar-SO3H)x, where G is a generic organic or inor¬ ganic radical, Ar is an arylenic radical, has been reported (G. Alberti et al. WO 03/081691 A2).

The lamellar tetravalent metal phosphates such as zirconium phosphate Zr(O3P-OH)2, are of interest for the acid surface of the lamellae; there¬ fore, they have been inserted, with very promising results, in membranes for medium temperature fuel cells (P. Costamagna et al., 2002, Electro- chimica Acta 47:1023; M. Yamashita et al. Abstracts of the 201st Meet¬ ing of ECS, Philadelphia May 12-17, 2002; B. Bauer et al. WO 03/077340 A2).

In this case, since the precursor organic solutions of zirconium phos- phate were yet unknown, the insertion has been performed with more complicated procedures. In the patent WO 96/29752 the "in situ" precipi¬ tation has been used. The membrane is first contacted with a solution - A -

containing a zirconyl salt in order to obtain the replacement of protons of -SO3H groups by ion exchange with zirconium. Then, by contacting the membrane with phosphoric acid the -SO3H is regenerated and "in situ" precipitation of zirconium phosphate is obtained. Thus, this method re- quires the presence of acid groups in the polymer to be modified. In the patent WO 03/077340 A2, after an exfoliation process of Zr(O3P-OH)2 with amines, gels of said compound in organic solvents can be pre¬ pared. These gels are then dispersed in organic solution of ionomers. This procedure cannot be used for the filling of pre-formed porous mem- branes since the lamellar particles cannot enter inside small pores and they therefore remain on the external surface of the porous membrane.

Recently it was surprisingly found that precursor organic solutions of la¬ mellar tetravalent metals acid phosphates can be also prepared, thus making possible an easier insertion in the matrix of ionomeric mem¬ branes, inside the pores of porous membranes and deposition on the catalytic surfaces of the electrodes.

A detailed investigation on the stability of these solutions showed that the stability can be increased: a) by increasing the basicity of the organic solvent (this property can be easily deduced from its Kb value); b) by de¬ creasing the temperature; c) by increasing the [phosphoric acid]/[M(IV)] ratio.

The said precursor solutions can be prepared with different [phosphoric acid]/[M(IV)] ratio. In the case in which this ratio is exactly two, only M(IV)(O3P-OH)2 is obtained when the solvent is eliminated.

However, it can be pointed out that in some cases the use of [phospho- ric acid]/[M(IV)] ratios greater than two could be convenient since the stability of precursor solutions is increased. Obviously, an excess of phosphoric acid remains after the solvent evaporation and it must be eliminated (e.g., by washing with a suitable solvent).

These important results convinced us to attempt the preparation of pre¬ cursor solutions also for the three-dimensional acid phosphates such as M(IV)[O2P(OH)2HO2PO(OH)].

This class of phosphates has been only recently discovered (G. Alberti et al. It Patent. PG 2003 A 000005) and it is of great interest since all the examined compounds exhibit very high proton conductivity (1-3x10"2 Scrτϊ1a 1000C) even at very low (<1%) relative humidity.

Also in this case it was possible to find the conditions in which stable precursor solutions are formed. Thus, this discovery makes not only possible an easy insertion of said compounds in the interfacial elec¬ trodes/membrane regions and inside ionomeric membrane of the state of art, but also permits their insertion inside the pores of ceramic or polymeric membranes, thus enlarging in significant manner their poten- tial applications. Of particular interest is their insertion inside polybenzo- imidazole membranes (PBI) where the three-dimensional acid phos¬ phates can partially or completely replace the phosphoric acid. Finally, an investigation on the thermal stability of said compounds showed that cubic pyrophosphates, M(IV)P2O7, are formed at temperatures greater than 120°-130°C. Due to their insolubility, high thermal and chemical stability as well as for their acid surfaces, M(IV)P2O7 particles can be used for the modification of electrodes and membranes of medium tem¬ perature PEMFCs.

Due to the thermal stability of pyrophosphates, the solvent can be elimi¬ nated also at high temperatures. Thus, even solvent with high boiling point can be used for the preparation of precursor solutions of M(IV)P2O7.

Precursor solutions of tetravalent metal pyrophosphates are particularly suitable for filling porous ceramic membranes to be used at high tem¬ perature.

It is an object of the present invention the preparation of a variety of or¬ ganic solutions containing tetravalent metal salts and phosphoric acid that, at room temperature or lower, do not give place to gelations o pre¬ cipitations for a sufficiently long time (at least one hour) in order to per¬ mit the use reported in the description and the claims and from which, for evaporation of the solvent it can be possible the direct preparation of insoluble compounds of composition Zr(O3P-OH)2, M(IV)[O2P (OH)2]2 [O2PO(OH)] e M(IV)P2O7 with cubic structure. However in other cases gels may be preferred.

It is a further object of the present invention for obtaining an easy filling of the pores of porous membranes either of polymeric or ceramic type.

It is a further object of the present invention the use of said solutions for obtaining an easy insertion of nano-particles of said compounds inside the matrix of organic or inorganic polymers provided that they are solu¬ ble in the same solvents.

The use can be extended also to polymers soluble in solvents different from those of the organic solutions object of the present invention, pro¬ vided that they are mixable with said organic solutions and do not pro¬ voke a fast gelation of the solution or the precipitation of the compound to be dispersed in the polymeric matrix. It is a further object of the present invention the use of said solutions for obtaining an easy insertion of said nano-particles in the elec¬ trodes/membrane interfaces of PEMFCs, either as pure compounds or in mixture with proton conducting ionomers such as Nafion and sulfonated PEK.

The following examples have the purpose of facilitating the understand¬ ing of the invention, and do not intend to limit in any manner its scope, which is solely defined by the appended claims.

The organic solutions and organic gels of the M(IV) compounds normally contain only one compound. However, mixtures of different compounds are possible.

EXAMPLES

EXAMPLE 1

This example illustrates the detailed preparation of a DMF solution con- taining a zirconyl salt and phosphoric acid from which zirconium phos¬ phate of α- type is obtained. Some data on the stability of these solu¬ tions are also reported.

8.7 g of anhydrous zirconyl propionate (Magnesium Elektron Limited, England) are dissolved in 40 mL of DMF. Taking into account that the composition of this compound was found to be ZrOi.27(CHsCH2COO)1.46 (MW=217.9 Dalton), the above amount corresponds to 0.04 mol.

Separately, 0.08 mol of anhydrous phosphoric acid (7.84 g) are dis- solved in 40 mL of DMF. The former solution is slowly added, under stir¬ ring at room temperature, to the last solution. A clear solution is obtained ([Zr(IV)]=0.5M). When the solution is warmed at 8O0C, the formation of a compact and transparent gel is observed (the gel is usually formed in less than 30 minutes). The solid obtained after evaporation of the sol¬ vent at 80 and 1400C does not contain, as shown by 1H NMR measure¬ ments, appreciable amount of propionates, but the presence of DMF is still evident. When the solid is washed with HCI 1M, a solid of composi¬ tion Zr(OH)o.6(θ3POH)i.7 is obtained The X-ray powder diffraction pattern shows the peaks of zirconium phosphate with a layered strutcture of α- type (compare curves a and b of figure 1). From the titration curve an amount of acid phospates of 5.8 meq/g is obtained.

EXAMPLE Ibis

This example illustrates the detailed preparation of an DMF solution con¬ taining hafnium oxide chloride propionate and phosphoric acid from which hafnium phosphate of α- type is obtained. Some data on the sta¬ bility of these solutions are also reported.

A mixed hafnium (IV) oxide chloride propionate used in this example was prepared in laboratory. A weighted amount of HfOCI2-8H2O (Strem Chemicals) and propionic acid (Aldrich) are mixed in a glass open vessel in the molar ratio 1 :3. The mixture is kept under stirring at 600C by using an oil bath in order to obtain a solid residue. Chemical analysis showed that the anhydrous solid has the composition (MW=317.2 Dalton).

12.7 g of the above compound (corresponding to 0.04 mol of Hf, previ¬ ously dehydrated at 1000C for 30 minutes) are dissolved in 40 mL of DMF. Separately, 0.08 mol of anhydrous phosphoric acid (7.84 g) are dissolved in 40 mL of DMF. The former solution is slowly added, under stirring at room temperature, to the last solution. A clear solution is ob- tained ([Hf(IV)]=0.5M). When the solution is warmed at 80°C, the forma¬ tion of a compact and transparent gel is observed after about 30 min¬ utes. The solid obtained after evaporation of the solvent at 80 and 140°C for about 2 hours does not contain, as shown by 1H NMR measure¬ ments, appreciable amount of propionates, but the presence of DMF is still evident. After washing with HCI 1M, a solid with a molar ratio [phos¬ phate mol]/[Hf mol]=1.9 is obtained

EXAMPLE 1 tris

This example illustrates the detailed preparation of a DMF solution con¬ taining a titanium salt and phosphoric acid from which titanium phos- phate of α- type is obtained. Some data of the stability of these solutions are also reported.

0.08 mol of anhydrous phosphoric acid (7.84 g) are dissolved in 68 ml_ of isobutanol. 11.36 g of titanium propoxide (98%, Aldrich), Ti(OCH2CH2CHs)4 ((MW=284 Dalton), corresponding to 0.04 mol, are added under stirring to the solution of the phosphoric acid, obtaining a clear solution ([Ti(IV)]=0.1M). When the solution is warmed at 800C, the formation of a compact and transparent gel is observed (it takes usually less than 30 minutes). The X-ray powder diffraction pattern shows the peaks of semicrystalline titanium phosphate with a layered strutcture of α- type (compare curves a and b of figure 2). Chemical analysis showed that in the solid the molar ratio [phosphate mol]/[Ti mol] is 1.7+0.1.

EXAMPLE 2

This example illustrates the detailed preparation of a 3-hexanol solution containing a zirconyl salt and phosphoric acid from which zirconium phosphate of composition Zr[O2P(OH)2]2[O2PO(OH)], ZrP3 is obtained. Some data on the stability of these solutions are also reported.

According to a procedure analogous to that described in examples 1- 1tris, 0.008 mol of anhydrous zirconyl propionate (1.74 g) are dissolved in 40 mL of 3-hexanol while 0.024 mol of anhydrous phosphoric acid (2.35 g) are dissolved in 40 mL of 3-hexanol. The solution of phospohric acid is then slowly added at 00C and under stirring to the solution of zir- conyl propionate ([Zr]=O-IM). The behaviour of the obtained solution at temperatures > 800C is very similar to that of the solution described in examples 1-1tris. Solvent evaporation at 800C leaves a residue which, as shows the X-ray powder diffraction pattern, has a layered structure of α- type (see figure 1 , curve b). If the solid is left at 80-900C, the gradual conversion into the phase Zr[O2P(OH)2HO2PO(OH)] and the disappear- ance of the α-phase is observed. The X-ray powder diffraction pattern obtained after two days of thermal treatment at 800C is reported in figure 4, curve b.

EXAMPLE 2 bis

This example illustrates the detailed preparation of a 3-hexanol solution containing hafnium oxide dichloride and phosphoric acid from which haf¬ nium phosphate of composition Hf[O2P(OH)2J2[O2PO(OH)], HfP3 is ob¬ tained. Some data of the stability of these solutions are also reported.

0.41 g of HfOCI2 (1.53x10~3 mol of Hf obtained from dehydration at 1000C for 30 minutes of Hafnium (IV) oxide dichloride octahydrate sup¬ plied by Strem Chemicals) are dissolved in 3 mL of 1-propanol. About 75% of propanol is evaporated and then 3-hexanol is added until the volume is 7.8 mL.

Separately 0.46 g of anhydrous phosphoric acid (4.68x10"3 mol) are dis¬ solved in 7.8 mL of 3-hexanol. The solution of phosphoric acid is then slowly added, at 00C and under stirring, to the solution of hafnium oxide dichloride. A clear solution is obtained. The behaviour of the obtained solution at temperatures > 800C is very similar to that of the solution de¬ scribed in example 1. Solvent evaporation at 800C leaves a residue which, as shown the X-ray powder diffraction pattern, has the structure of a hafnium phosphate of α- type (see figure 2, curve a). If the solid is left at a temperature of 80-900C, a gradual conversion into the phase Hf[O2P(OH)2MO2PO(OH)], and the disappearance of the layered α- phase, is observed. The X-ray powder diffraction pattern obtained after 12 days of thermal treatment at 800C is reported in figure 4, curve b.

EXAMPLE 3

This example illustrates the detailed preparation of a 3-hexanol solution containing an zirconyl salt and phosphoric acid from which zirconium pyrophosphate of composition ZrP2O7 is obtained. Some data on the stability of these solutions are also reported.

According to a procedure analogous to that described in example 2, a clear solution is prepared. The solvent is at first evaporated at 800C and then the residue is heated at 1800C for one day. The X-ray powder dif¬ fraction pattern (see figure 5, curve b) shows the formation of zirconium pyrophosphate with a cubic structure.

EXAMPLE 3 bis

This example illustrates the detailed preparation of a 3-hexanol solution containing a titanium salt and phosphoric acid from which titanium pyro- phosphate of composition TiP2O7 is obtained. Some data on the stability of these solutions are also reported.

0.144 mol of anhydrous phosphoric acid (14.11 g) are dissolved in 73 mL of 3-hexanol. 6.816 g di titanium propoxide (Aldrich), corresponding to 0.024 mol, are added at 00C under stirring to the solution of the phos¬ phoric acid, obtaining a clear solution ([Ti(IV)]=0.3). The solvent is at first evaporated at 800C and then the residue is heated at 1800C for 18 hours. The X-ray powder diffraction pattern (see figure 6, curve b) shows the formation of titanium pyrophosphate with a cubic structure.

EXAMPLE 4

This example gives a detailed description of the use of the organic solu¬ tion reported in example 1 to fill the pores of a porous polymeric mem¬ brane with zirconium phosphate. Case of a porous polytetrafluoroethyl- ene (PTFE, or Teflon membrane).

A clear DMF solution is prepared according to the procedure described in the example 1. A PTFE membrane (Fluoropore Membrane Filters, MiI- lipore, pore size 0.5 μm ; thickness 60 μm; porosity 85%, initial weight 0.0391 g) is completely covered with the solution for about 60 minutes in order to permit a good infiltration of the solution inside the pores. The membrane is taken out from the solution and the liquid excess on the external faces of the membrane is quickly eliminated (e.g., by contacting alternatively the two membrane faces with a paper filter), while the sol¬ vent inside the pores is eliminated by drying at 800C for about 1 hour and then at 1400C overnight. The final weight of the membrane is 0.0462 g with a weight increment of 18%. The entire filling procedure can be repeated several times depending on the wished pore filling degree.

EXAMPLE 4bis

This example gives a detailed description of the use of the organic solu¬ tion reported in example 1 to fill the pores of a porous polymeric mem¬ brane with hafnium phosphate. Case of a porous polytetrafluoroethylene membrane.

According to the procedure described in the example 2 bis a clear solu¬ tion with [Hf]=O.1 M is prepared. A PTFE membrane is completely covered with the solution at a tem¬ perature of 0-30C, according to the procedure described in the example 4. The weight increment of the membrane is 26 wt%. The X-ray powder diffraction pattern obtained after 10 days of thermal treatment is reported in figure 7, curve b. The peak diffraction at 2Θ=18°C is due to the mem¬ brane polymer.

EXAMPLE 4tris

This example gives a detailed description of the use of the organic solu¬ tion reported in example 2 to fill the pores of a porous polymeric mem¬ brane with zirconium phosphate of composition Hf[O2P(OH)2MO2PO(OH)], HfP3. Case of a porous polytetrafluoroethyl- ene membrane.

According to the procedure described in the example 2bis, a clear solu¬ tion with [Hf]=O.1 M and [phosphoric acid mol]/[Hf mol]=6 is prepared. A PTFE membrane is completely covered with the solution acoording to the procedure described in the example 4 bis. The solvent is eliminated at 50-600C overnight and then the membrane is mantained at 800C to allow the conversion of the inorganic compound into HfP3 phase. The X ray powder diffraction pattern obtained after 10 days of thermal treat¬ ment is reported in figure 4, curve c. The peak diffraction at 2Θ=18°C is due to the membrane polymer.

EXAMPLE 5

This example gives a detailed description of the use of the organic solu- tion reported in example 3 to fill the pores of a porous polymeric mem¬ brane with zirconium. Case of a porous polytetrafluoroethylene mem¬ brane. According to the procedure described in the example 2, a clear solution with [Zr]=O.1 M and [phosphoric acid mol]/[Zr mol]=6 is prepared. A PTFE membrane is completely covered with the solution according to the pro¬ cedure described in the example 4 bis. The solvent is eliminated at 800C overnight and then the membrane is left at 180°C for one day to allow the conversion of the inorganic compound. The X ray powder diffraction pattern obtained after the thermal treatment is reported in figure 5, curve c and shows the formation of zirconium pyrophosphate with a cubic structure (compare with figure 5, curve a).

EXAMPLE 5 bis

This example gives a detailed description of the use of the organic solu¬ tion reported in example 3 to fill the pores of a porous polymeric mem- brane with hafnium pyrophosphate of composition HfP2O7. Case of a porous polytetrafluoroethylene membrane.

A PTFE membrane is completely covered with the solution prepared according to the procedure described in the example 4 tris, then the membrane is treated as described in the example 5. The X-ray powder diffraction pattern obtained after the thermal treatment is reported in fig¬ ure 8, curve b and shows the formation of hafnium pyrophosphate with a cubic structure.

EXAMPLE 6

This example gives a detailed description of the use of the organic solu¬ tion reported in examples 1-3 or similar, for a partial filling of the pores of a porous inorganic membrane with zirconium phosphates or pyrophos- phates in order to prepare a membrane with catalytic properties. Case of a zirconium oxide tubular asymmetrical ceramic membrane filled with particles of Hf[O2P(OH)2HO2PO(OH)] A zirconium oxide tubular asymmetrical ceramic membrane (TAMI tri- channel, thickness of the thin layer 0.14 μm) is out gassed under vac¬ uum in a desiccator. The membrane, kept under vacuum at 0-30C, is then completely covered with the solution, prepared according to the procedure reported in example 4 tris, for about 10 minutes. The number of the filling steps is chosen in order to have a partial filling of the pores, preferably in the range 30-70 wt%

EXAMPLE 7

This example illustrates the use of the organic solutions reported in the examples 1-1 tris for preparing a composite membrane consisting of a polymeric matrix of the state of art filled with a given percentage of the wished particles. Case of the sulfonated polyetherketone (s-PEK) filled with 20 wt % particles of zirconium phosphate.

1.0 g of s-PEK (FuMA-Tech 1.4), with ion-exchange capacity 1.4x10"3 equivalent/g, previously dehydrated at 800C overnight are dissolved in 10 ml_ of DMF at 120°C. To this solution 1.65 ml_ of the solution of the example 1 are added. The resulting mixture is kept under stirring for 1 hour at room temperature and then poured on a glass plate. The solvent is evaporated at 80°C for 5 hours and at 120-130°C for 2 hours. The membrane is then detached from the glass support by immersion in wa¬ ter, washed with diluted HCI solution, washed with a mixture 1 :1 v/v of ethanol/water and stored at room temperature. The percentage of zirco- nium phosphate in the anhydrous membrane is 20 wt% and the mem¬ brane thickness is 0.008 cm.

EXAMPLE 7 bis

This example illustrates the use of the organic solutions reported in the examples 1-1 tris for preparing a composite membrane consisting of a polymeric matrix of the state of art filled with a given percentage of the wished particles. Case of the Fumion filled with 10 wt% particles of zir¬ conium phosphate.

1.0 g of Fumion (perfluorinated polysulfonic acid, FuMA-Tech), with ion- exchange capacity 0.9x10"3 equivalent/g, previously dehydrated at 800C overnight are dissolved in 10 ml_ of DMF at 800C. To this solution 0.8 mL of the solution of the example 1 are added. The resulting mixture is kept under stirring for 1 hour at room temperature and then poured on a glass plate. The solvent is evaporated at 800C for 5 hours and at 120- 1300C for 2 hours. The membrane is then detached from the glass sup¬ port by immersion in water, washed with diluted HCI solution, washed with a mixture 1 :1 v/v of ethanol/water and stored at room temperature. The percentage of zirconium phosphate in the anhydrous membrane is 10 wt% and the membrane thickness is 0.008 cm.

EXAMPLE 7 tris

This example illustrates the use of the organic solutions reported in the examples 2-2 bis for preparing a composite membrane consisting of a polymeric matrix of the state of art filled with a given percentage of the wished particles. Case of the Fumion filled with 16.5 wt% particles of hafnium phosphate.

0.217 g of anhydrous Fumion are dissolved in 8 mL of a mixture 1 :1 v/v of 3-hexanol/1-propanol at 400C for about 4 hours.

According to the procedure described in example 2 bis, a clear solution with [Hf]=O.1 M and [phosphoric acid mol]/[Hf mol]=6 is prepared. 1.16 mL of this solution are added at room temperature and under stirring to the polymer solution. The resulting mixture is kept under stirring for 1 hour at room temperature and then poured on a glass plate. The solvent is evaporated at 55°C overnight and at 800C for 2 days and then the membrane is directly detached from the glass support without using any solvent. The percentage of hafnium phosphate in the anhydrous mem¬ brane is 16.5 wt%. The X-ray powder diffraction pattern is reported in figure 9, curve b.

EXAMPLE 8

This example illustrates the use of the organic solutions reported in the examples 2-2 bis for preparing a composite membrane consisting of a polymeric matrix of the state of art filled with a given percentage of the wished particles. Case of the Fumion filled with 30 wt% particles of Hf[O2P(OH)2I2[O2PO(OH)].

0.217 g of anhydrous Fumion are dissolved in 8 ml_ of a mixture 1 :1 v/v of 3-hexanol/1-propanol at 400C for about 4 hours.

According to the procedure described in example 2 bis, a clear solution with [Hf]=O.1 M and [phosphoric acid mol]/[Hf mol]=10 is prepared. 1.98 ml_ of this solution are added at room temperature and under stirring to the polymer solution. The resulting mixture is kept under stirring for 1 hour at room temperature and then poured on a glass plate. The solvent is evaporated at 55°C overnight and at 800C for 3 days and then the membrane is directly detached from the glass support. The percentage of Hf[O2P(OH)2MO2PO(OH)] in the anhydrous membrane is 30 wt%. The X-ray powder diffraction pattern is reported in figure 10, curve b.

EXAMPLE 9

This example illustrates the use of the organic solutions reported in the examples 2-2 bis for preparing a composite membrane consisting of a polymeric matrix of the state of art filled with a given percentage of wished particles. Case of the Fumion filled with 16 wt % particles of cu¬ bic hafnium pyrophosphate.

According to the procedure described in example 7 tris a composite membrane is prepared. After thermal treatment of the membrane at 1200C for 2 hours and 18O0C overnight a composite membrane contain¬ ing 16 wt% of HfPaCv is obtained. The X-ray powder diffraction pattern is reported in figure 11 , curve b.

EXAMPLE 10

This example illustrates the use of the organic solutions reported in the examples 1-1 tris, to insert inorganic particles in the interface regions electrodes/membrane; case of Hf(OsPOH)2.

According to the procedure described in example 1 bis a clear solution of the precursor of α-HfP in DMF is prepared. The solution is directly sprayed on the gas diffusion electrode surface (e.g. an ELAT™ elec¬ trode by De Nora North America). The solvent is at first evaporated by thermal treatment at 800C for about 30 minutes and then completely eliminated by thermal treatment at 140-150°C for 5-6 hours.

EXAMPLE 10 bis

This example illustrates the use of the organic solutions reported in the examples 2-2tris, to insert inorganic particles in the interface regions electrodes/membrane; case of HfP3.

According to the procedure described in example 2 bis a clear solution of the precursor of HfP3 in 3-hexanol is prepared. The solution is directly sprayed on the gas diffusion electrode surface. The solvent is at first evaporated by thermal treatment at 800C for about 30 minutes and then completely eliminated by thermal treatment at 120-1300C for 5-6 hours. EXAMPLE 10 tris

This example illustrates the use of the organic solutions reported in the examples 3-3bis, to insert inorganic particles in the interface regions electrodes/membrane; case of cubic titanium pyrophosphate.

According to the procedure described in example 3 bis a clear solution of the precursor of TiP2O7 in 3-hexanol is prepared. The solution is di¬ rectly sprayed on the gas diffusion electrode surface. The solvent is evaporated by thermal treatment at 170-1800C for 6-7 hours. The ex¬ cess of phosphoric acid is removed by washing the electrode with etha- nol. The residues of ethanol are finally removed by evaporation.

EXAMPLE 11

This example illustrates the use of the organic solutions reported in the examples 1-3, to insert inorganic particles in the interface regions elec¬ trodes/membrane; case of α-HfP in Nafion.

According to the procedure described in example 1 bis a clear solution of the precursor of α-HfP in DMF is prepared. 0.15 ml. of this solution are added, under stirring to 10 g of Nafion solution (5 wt% in alcoholic solution by Aldrich). The solution is directly sprayed or painted on the gas diffusion electrode surface. The solvent is at first evaporated by thermal treatment at 800C for about 30 minutes and then completely eliminated by thermal treatment at 130-1400C for 5-6 hours.

EXAMPLE 11 bis This example illustrates the use of the organic solutions reported in the examples 1-3, to insert inorganic particles in the interface regions elec¬ trodes/membrane; case of cubic zirconium pyrophosphate in Nafion

According to the procedure described in example 3, a clear solution of the precursor of ZrPaO7 in 3-hexanol is prepared. 0.2 mL of this solution are added, under stirring to 10 g of Nafion solution. The solution is di¬ rectly sprayed or painted on the gas diffusion electrode surface. The sol¬ vent is at first evaporated by thermal treatment at 800C for about 30 minutes and then completely eliminated by thermal treatment at 170- 1800C for 5-6 hours. The excess of phosphoric acid is removed by washing the electrode with ethanol. The residues of ethanol are finally removed by evaporation.

As reported before, the preparation of gels of zirconium phosphate in organic solvents with a procedure based on the exfoliation of pre-formed microcrystals of this proton conductor, has been already reported in the PCT patent No. WO 03/077340 A2.

It was now found that similar gels of zirconium and hafnium phosphates in organic solvents can be directly obtained by the precursor solutions with an easier procedure.

As reported before, the quick formation of dense gels is observed when the precursor solutions are warmed at temperatures higher than 30- 400C. The exact nature of these gels is at the moment unknown. Taking into account that insoluble M(/IV) phosphates are obtained after elimina¬ tion of the solvent, it is likely that these gels are constituted by small clusters of layered M(IV) phosphates. Since the number of species pre- sent along borders of very small layered particles can be a significant fraction of the total species, edge-edge interactions among these parti¬ cles could be preferred to surface-surface interactions. In this case, the gels could be seen as house-card arrangements of very small particles, with the organic solvent trapped between them. When the solvent is eliminated, the instability of said house-card arrangement could lead to layered arrangements of the planar particles. Since the formation of these gels is obtained in times relatively short, the size of the layer parti¬ cles is expected to be even smaller than that of particles obtained by ex¬ foliation of pre-formed M(IV) phosphates. In any case, although addi¬ tional studies have to be carried out to better clarify the nature of these gels and the exact size of the particles, the easy and economical prepa- ration as well as the stability even long time after their preparation is of great practical importance for the preparation of composite proton con¬ ducting membranes.

Example 12

This example illustrates the use of precursor solutions with molar ratio (H3PO4/Zr = 2) to prepare a stable zirconium phosphate-DMF gel.

A precursor DMF solution of zirconium phosphate of α-type is first pre- pared as reported in the example 1.

The precursor solution is heated at 800C for 30 min. The formation of a compact and transparent gel of zirconium phosphate containing a large amount of trapped DMF is obtained. In the experimental conditions used in this example the wt/wt% of zirconium phosphate is 12%. This gel can be conserved in closed vessels and used even after for a very long time from its preparation.

Example 12 bis

This example illustrates the use of precursor solutions with molar ratio (H3PCVHf = 2) to prepare a hafnium phosphate-DMF gel. A precursor DMF solution of hafnium phosphate of α-type is first pre¬ pared as reported in the example 1 bis.

The precursor solution is heated at 800C for 30 min. The formation of a compact and transparent get of hafnium phosphate containing a large amount of trapped DMF is obtained. In the experimental conditions used in this example the wt/wt% of hafnium phosphate is 15%. This gel can be conserved in closed vessels and used even after for a very long time from its preparation.

Example 12 tris

This example illustrates the use of precursor solutions with molar ratio H3PO4/HF = 3 to prepare a stable hafnium phosphate-DMF gel.

A precursor DMF solution of hafnium phosphate of a α-type is first pre¬ pared as reported in the example Ibis with the molar ratio H3PO4/Hf in this solution = 3.

The precursor solution is heated at 8O0C for 30 min. The formation of a compact and transparent gel of hafnium phosphate containing a large amount of trapped DMF is obtained. Since in this case the used ratio HsPCVHf ration was 3, an excess of phosphoric acid remains in the DMF gels. This excess of phosphoric acid can be eliminated by washing the gel two or three times with DMF.

In the experimental conditions used in this example the wt/wt% of haf¬ nium phosphate is 20%.This gel can be conserved in closed vessels and used even after for a very long time from its preparation. Example 13

This example illustrates the use of gel reported in the example 12 to prepare a composite Fumion membrane filled with nano particles of zir- conium phosphate) A weighed amount of Fumion (corresponding to 1g of anhydrous iono- mer) is dissolved under vigorous stirring in 8g of DMF at 80°C. To this solution, 0,44 g of the gel of the example 12 are added. The mixture is held under stirring at room temperature for 1 hour and then poured on a glass plate. The solvent is evaporated at 800C for 5 hours and at 120- 1300C for 2 hours. The membrane is then detached from the glass sup¬ port by immersion in water, washed with diluted HCI solution, washed with a mixture 1 :1 v/v of ethanol/water and stored at room temperature. The percentage of zirconium phosphate in the anhydrous membrane is 5% and the membrane thickness is 0,006 cm.

Example 13 bis

This example illustrates the use of gel reported in the example 12bis to prepare a composite Fumion membrane filled with nano particles of haf¬ nium phosphate) A weighed amount of Fumion (corresponding to 1 g of anhydrous iono- mer) is dissolved under vigorous stirring in 8g of DMF at 800C. To this solution, 0,35 g of the gel of the example 12bis are added. The mixture is held under stirring at room temperature for 1 hour and then poured on a glass plate. The solvent is evaporated at 800C for 5 hours and at 120- 1300C for 2 hours. The membrane is then detached from the glass sup¬ port by immersion in water, washed with diluted HCI solution, washed with a mixture 1 :1 v/v of ethanol/water and stored at room temperature. The percentage of hafnium phosphate in the anhydrous membrane is 5% and the membrane thickness is 0,008 cm.