OXYFLUORIDES, ELECTRODES CONTAINING THEM AND THEIR USE FOR HYDROGEN PRODUCTION

Abstract

Oxyfluoride derivatives and their preparation, as well as their uses as catalysts in electrochemistry, including the electrodes and electrochemical cells comprising them. These may be useful for hydrogen production.

Claims

1. A compound of formula (I): ##STR00005## wherein M1, M2, M1, M2, M1, M2 are transition metals of the d/f block or alkaline-earth elements in the periodic table; u, v, x, y and z are identical or different and are such that u+v<1; 0<x,y,z<1; w=x+u(y?x)+v(z?x).

2. The compound according to claim 1 where M1, M1 and M1 belong to d/f block or alkaline-earth elements in the periodic table and M2, M2 and M2 belong to d/f transition elements of the periodic table.

3. The compound according to claim 1, wherein: either x=0.5 and y=z=u=v=0 or x=y=0.5 and u=0.5 and z=v=0 with M2=M2 or x=0.3 and y=0.7 and u=0.5 and z=v=0 with M2=M2.

4. The compound according to claim 1, wherein M1 and M1, M1 are selected from the group consisting of Mn, Fe, Co, Ni, Cu, Zn and M2 and M2, M2 are chosen from Mn, Fe, Co, Ni, Ru and Rh.

5. The compound according to claim 1, wherein the compound (I) is selected from the group consisting of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5, Ni.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5, Zn.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5, Co.sup.2+.sub.0.25Ni.sup.2+.sub.0.25Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5, Co.sup.2+.sub.0.15Fe.sup.2+.sub.0.35Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5, Co.sup.2+.sub.0.75Fe.sup.3+.sub.0.25O.sub.0.25F.sub.1.75, Ni.sup.2+.sub.0.15Co.sup.2+.sub.0.425Fe.sup.3+.sub.0.425O.sub.0.425F.sub.1.575, Ni.sup.2+.sub.0.25Co.sup.2+.sub.0.375Fe.sup.3+.sub.0.375O.sub.0.375F.sub.1.625, and Ni.sup.2+.sub.0.5Co.sup.2+.sub.0.25Fe.sup.3+.sub.0.25O.sub.0.25F.sub.1.75, Ni.sup.2+.sub.0.75Co.sup.2+.sub.0.125Fe.sup.3+.sub.0.125O.sub.0.125F.sub.1.875.

6. A solid solution of the compound according to claim 1.

7. An electrode comprising the compound of formula (I) according to claim 1 as a catalyst.

8. The electrode according to claim 7, comprising carbon paper loaded with an ink comprising the compound of formula (I).

9. An electrochemical cell comprising the electrode according to claim 7.

10. The electrochemical cell according to claim 9 which is a water electrolyzer and/or a fuel cell.

11. A process of preparation of the solid solution according to claim 6, which comprises the following steps: synthesizing a hydrated fluorinated precursor of formula (II): ##STR00006## where x, y, z, u, v, M1, M2, M1, M2, M1, M2 are defined as in claim 1, by evaporation of a concentrated hydrofluoric acid (HF) solution of the dissolved metal salts or by precipitation of said solution; and thermally treating the compound of formula (II) at a temperature comprised between 150 and 400? C.

12. A compound of formula (II): ##STR00007## where x, y, z, u, v, M1, M2, M1, M2, M1, M2 are defined as in claim 1.

13. A process of preparation of hydrogen comprising electrolyzing water with an electrochemical cell according to claim 9.

Description

FIGURES

[0079] FIG. 1 is a schematic representation of an electrolyzer (A) and representation of overpotential applied to the anode at 10 mA.Math.cm.sup.?2 (B).

[0080] FIG. 2 is a representation of modulating the energy levels of the binding strength between the active sites and the intermediates and its impact on the overpotential.

[0081] FIG. 3 represents the cyclic voltammetry measurement of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 showing the current density as a function of the potential applied.

[0082] FIG. 4 illustrates the Tafel slope of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 on Glassy Carbon and Carbon Paper substrate indicating that for every increase of 27 mV of applied potential, the current density increases by one order of magnitude.

[0083] FIG. 5 shows the evolution of the mass activity (left) and TOF (right) as a function of the applied potential for Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5.

[0084] FIG. 6 represents the Arrhenius plot for Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 to determine the activation energy of the rate limiting step.

[0085] FIG. 7 illustrates the chronopotentiometric measurement of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 over extended periods of time shows no obvious activity decrease.

[0086] FIG. 8 represents the XRD patterns of Co.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7, Ni.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7 and Co.sup.2+.sub.0.5Ni.sup.2+.sub.0.5Fe.sup.3+F.sub.5(H.sub.2O).sub.7 matching with the PDF card number 00-037-0794 (CoFeF.sub.5(H.sub.2O).sub.7).

[0087] FIG. 9 represents the XRD patterns of Co.sup.2+.sub.0.75Fe.sup.2+.sub.0.25F.sub.2(H.sub.2O).sub.4 and Co.sup.2+.sub.0.25Ni.sup.2+.sub.0.5Fe.sup.2+.sub.0.25F.sub.2(H.sub.2O).sub.4 matching with the PDF card number 00-025-0243 (CoF.sub.2(H.sub.2O).sub.4)

[0088] FIG. 10 represents the volume of the cell as a function of the composition for Co.sup.2+.sub.xFe.sup.2+.sub.1?xF.sub.2(H.sub.2O).sub.4 with x=0-1. The evolution is in accordance to the Vegard's law which confirms the metal ratio.

[0089] FIG. 11 represents the volume of the cell as a function of the composition for Ni.sub.xCo.sup.2+.sub.(1?x)/2Fe.sup.2+.sub.(1?x)/2F.sub.2(H.sub.2O).sub.4, with x=0-1. The evolution is in accordance to the Vegard's law which confirms the metal ratio.

[0090] FIG. 12 represents the EDX spectra of Co.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7 and Ni.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7, ratios of metal cations M.sup.3+/M.sup.2+ are close to one.

[0091] FIG. 13 represents the TXRD of Co.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7 under ambient atmosphere showing the formation of the intermediate phase Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 with rutile structure type (characteristic peaks surrounded in the black circles).

[0092] FIG. 14 represents the thermogravimetric curve of Co.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7 under ambient air.

[0093] FIG. 15 represents the XRD patterns of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 and Ni.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 obtained by thermal treatment of Co.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7 and Ni.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7, respectively.

[0094] FIG. 16 illustrates the comparison of FTIR spectra of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 and Ni.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 with those of their corresponding hydrated precursors, Co.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7 and Ni.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7.

[0095] FIG. 17 represents the N.sub.2 adsorption-desorption isotherms of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 and Ni.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 and corresponding BJH pore size distributions.

[0096] FIG. 18 represents the TEM pictures showing the mesoporosity of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 and Ni.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5.

EXAMPLES

Example 1: Electrocatalytic Properties of Solid Solutions M.SUP.2+..SUB.x.M.SUP.3+..SUB.1?x.O.SUB.1?x.F.SUB.1+x

[0097] Electrocatalytic measurements are performed in a standard 3-electrode setup in a custom-built glass single compartment reaction cell. Hg/HgO reference and Pt or carbon counter electrodes are used for all measurements. For a working electrode substrate, Toray Carbon Paper is used as a model high-surface area support (catalyst loading of 5 mg.Math.cm.sup.?2). Alternatively, a rotating disk setup with a glassy carbon surface is used (catalyst loading of 0.1 mg.Math.cm.sup.?2). To generate a catalyst-coated working electrode, a catalyst ink is generated by sonicating 300 ?l ethanol, 100 ?l de-ionized water, 10 ?l of 5% Nafion? solution, 4 mg catalyst and 0.2 mg multiwalled carbon nanotubes (10-40 nm diameter). The ink is spread onto a working electrode surface and allowed to dry under ambient conditions. 1.0 M KOH is used as the electrolyte in all measurements at room temperature (approximately 21? C.) and 95% compensation of the solution resistance is used correct correction. The solution resistance is measured at open circuit at 100 KHz frequency before each measurement. Turnover frequencies (TOFs) are calculated by dividing the reaction rate (extracted from the current density, assuming 4 electrons extracted from each water molecule) by the redox-active cobalt species, calculated through integrating the area under the Co(II/III) redox peak at 0.1 V vs. RHE (Reversible Hydrogen Electrode).

Example of Co.SUP.2+..SUB.0.5.Fe.SUP.3+..SUB.0.5.O.SUB.0.5.F.SUB.1.5

[0098] The Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 catalyst-loaded carbon paper electrode exhibits 220 mV overpotential for a current density of 10 mA.Math.cm.sup.?2 (FIG. 3). The Tafel slope, measured in a current density range of 1-100 mA.Math.cm.sup.?2, is calculated to be 27 mV.Math.decade.sup.?1 (FIG. 4). At 300 mV overpotential, the mass activity of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 reaches 846 A.Math.g.sup.?1, with a TOF of 21 s.sup.?1 per electroactive site (or 0.5 s.sup.?1 using the total mass of cobalt deposited) (FIG. 5). The high activity is also evident through the low activation energy of 28.9 KJ.Math.mol.sup.?1, calculated from the Arrhenius plot (FIG. 6). The activity is stable for more than 500 h, as measured through chronopotentiometric tests at current densities of 10, 50, 200 and 1000 mA.Math.cm.sup.?2 (FIG. 7).

[0099] Overall, the electrocatalytic performance of Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 is arguably the best reported for a cobalt-based material under the above-mentioned standard testing conditions in terms of a combination of overpotential, mass activity, turnover frequency and stability. This conclusion is attained after a careful comparison of quantitative performance metrics of notable catalysts reported over the last several years (Table 3).

[0100] Table 3 gathers their electrochemical properties in comparison with precious metals oxides and with the best reported oxohydroxides. It should be noted that in order to properly compare catalytic performances of anodic materials, it is necessary to take into account several electrochemical characteristics as well as the measuring conditions: [0101] overpotential (mV), [0102] Tafel slope (mV.Math.dec.sup.?1): the lower it is, the lower the increase in potential to be applied to obtain a high current density and therefore a high hydrogen production rate, [0103] turnover frequency (TOF, s.sup.?1): reflects the efficiency of a catalytic site (frequency of reactions). The higher the TOF, the better the activity, [0104] mass activity (mA.Math.g.sup.?1) at a given overpotential, [0105] temperature of the electrolytic medium, here RT, [0106] concentration of the medium in KOH, here 1 M.

TABLE-US-00003 TABLE 3 Electrochemical characteristics of best catalysts in literature, LDH stand for Layer Double Hydroxide, G for Gelled, CP for Carbon Paper and GC for Glassy Carbon. Mass Bulk TOF Tafel activity** Catalysts Substrate ?* (mV) (s.sup.?1)** (mV .Math. dec.sup.?1) (A .Math. g.sup.?1) Reference Co.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5 CP 220 0.5 27 846 Present invention LDH-(Co,Fe) CP 331 0.01 85 32 Zhang et Science (8 2016, 3 (6283), 333 337. G-(Co,Fe)OOH GC 271 0.043 60 162 Zhang et 2016 G- CP 233 0.09 Zhang et al. N (Co,Fe,Mo)OOH Catal. 2020, 985-9924 G- CP 212 0.24 Zhang et (Co,Fe,Mo,W)OOH 2020 G- GC 217 0.46 37 1175 Zhang et (Co,Fe,W)OOH 2016 LDH-(Ni,Fe) GC 269 0.07 117 Zhang et 2016 LDH/GO-(Ni,Fe) GC 210 0.1 42 McCrory et al. Am. Chem. So 2015, 137 (13 4347-4357 (Ni,Fe,Mo)OOH CP 201 0.1 32 Zhang et 2020 (Ni,Fe,Mo,W)OOH CP 205 0.14 28 Zhang et 2020 IrO.sub.2 CP 260 0.01 45 McCrory et 2015 *evaluated at 10 mA .Math. cm.sup.?2 **evaluated at 300 mV indicates data missing or illegible when filed

Example 2: Synthesis of M.SUP.2+.M.SUP.3+.F.SUB.5.(H.SUB.2.O).SUB.7 .and M.SUP.2+..SUB.x.M.SUP.2+..SUB.1?x.F.SUB.2.(H.SUB.2.O).SUB.4

[0107] The synthesis of oxyfluorinated solid solutions M.sup.2+.sub.xM.sup.3+.sub.1?xO.sub.1?xF.sub.1+x is carried out in two steps: the first one is the synthesis of a hydrated fluorinated precursor M.sup.2+M.sup.3+F.sub.5(H.sub.2O).sub.7, frequently written MMF.sub.5.7H.sub.2O, or M.sup.2+.sub.xM.sup.2+.sub.1?xF.sub.2(H.sub.2O).sub.4 in concentrated hydrofluoric acid (HF) and the second one consists of a moderate thermal treatment in ambient atmosphere to obtain the solid solutions M.sup.2+.sub.xM.sup.3+.sub.1?xO.sub.1?xF.sub.1+x.

[0108] For the synthesis of MMF.sub.5.7H.sub.2O: The metal salts (chlorides, nitrates, carbonates, phosphates, sulfates, acetates . . . ) are dissolved in a concentrated HF solution. Then the solution is evaporated at a temperature below the boiling point of the solution until the beginning of the precipitation. The solution is cooled at ambient air and the as-synthesized solid is filtered out, washed and dried leading to MMF.sub.5(H.sub.2O).sub.7 with a yield around 70%. These synthesis methods can be extending to three (or more) metals by multi-substitution as well as on +II and/or +III metal cations such as M.sup.2+.sub.1?xM.sup.2+.sub.xM.sup.3+F.sub.5(H.sub.2O).sub.7 or M.sup.2+M.sup.3+.sub.1?xM.sup.3+.sub.xF.sub.5(H.sub.2O).sub.7. As alternative synthesis, after dissolution of the metal salts, the precipitation is triggered by an addition of an alcohol such as ethanol at room temperature. The solid is recovered by the same aforementioned protocol.

[0109] For the synthesis of M.sup.2+.sub.xM.sup.2+.sub.1?xF.sub.2(H.sub.2O).sub.4: the metal salts (chlorides, nitrates, carbonates, phosphates, sulfates, acetates ...) are dissolved in alcohol (ethanol, isopropanol, . . . ). The solvent should be degassed prior to the reaction by bubbling it with argon for 30 min. After the degassed concentrated HF (bubbling with argon for 30 min) is added allowing the hydrated fluoride to precipitate. The as-synthesized solid is filtered out, washed and dried leading to MMF.sub.2(H.sub.2O).sub.4 with a yield around 70%. These synthesis methods can be extending to three (or more) metals by multi-substitution such as M.sup.2+.sub.1?xM.sup.2+.sub.xM.sup.2+F.sub.2(H.sub.2O).sub.4.

Example 2.1: Co.SUP.2+.Fe.SUP.3+.F.SUB.5.(H.SUB.2.O).SUB.7

[0110] CoCl.sub.2.6H.sub.2O (237.9 mg) and Fe(NO.sub.3).sub.3.9H.sub.2O (404.0 mg) are dissolved into 10 ml of a concentrated hydrofluoric acid solution (28 mol.Math.L.sup.?1, HF.sub.48%). The reaction mixture is placed in a Teflon Becher and stirred for 1 h at 100? C. in an oil bath until the formation of a precipitate. After cooling, the mixture is filtered, washed with technical ethanol and dried at room temperature giving pink powder.

Example 2.2: Ni.SUP.2+.Fe.SUP.3+.F.SUB.5.(H.SUB.2.O).SUB.7

[0111] Ni(NO.sub.3).sub.2.6H.sub.2O (290.8 mg) and Fe(NO.sub.3).sub.3.9H.sub.2O (404.0 mg) are dissolved into 10 mL a concentrated hydrofluoric acid solution (28 mol.Math.L.sup.?1, HF.sub.48%). The reaction mixture is placed in a Teflon Becher and stirred for 1 h at 100? C. in an oil bath until the formation of a precipitate. After cooling, the mixture is filtered, washed with technical ethanol and dried at room temperature giving green powder.

Example 2.3: Co.SUP.2+..SUB.0.5.Ni.SUP.2+..SUB.0.5.Fe.SUP.3+.F.SUB.5.(H.SUB.2.O).SUB.7

[0112] CoCl.sub.2.6H.sub.2O (119.0 mg), Ni(NO.sub.3).sub.2.6H.sub.2O (145.4 mg) and Fe(NO.sub.3).sub.3.9H.sub.2O (404.0 mg), are dissolved into 10 mL a concentrated hydrofluoric acid solution (28 mol.Math.L.sup.?1, HF.sub.48%). The reaction mixture is placed in a Teflon Becher and stirred for 1 h at 100? C. in an oil bath until the formation of a precipitate. After cooling, the mixture is filtered, washed with technical ethanol and dried at room temperature giving brownish powder.

Example 2.4: Co.SUP.2+..SUB.0.75.Fe.SUP.2+..SUB.0.25.F.SUB.2.(H.SUB.2.O).SUB.4

[0113] CoCl.sub.2.6H.sub.2O (892.2 mg), and FeCl.sub.2.4H.sub.2O (248.5 mg), are dissolved into 50 ml of isopropanol. The latter has been, prior to the synthesis, carefully degassed by bubbling it using argon for 30 min. The reaction mixture is placed in a round bottom flask previously flushed with argon. Once the solubilisation is completed, 10 mL of degassed concentrated hydrofluoric acid solution (28 mol.Math.L.sup.?1, HF.sub.48%) (30 minute by bubbling it using argon) is added to the solution and the solution is allowed to stir for 30 min under argon atmosphere until precipitation is complete. Finally, the mixture is filtered, washed with technical ethanol and dried at room temperature giving a pink powder.

Example 2.5: Co.SUP.2+..SUB.0.25.Ni.SUP.2+..SUB.0.5.Fe.SUP.2+..SUB.0.25.F.SUB.2.(H.SUB.2.O).SUB.4

[0114] CoCl.sub.2.6H.sub.2O (119 mg), Ni(NO.sub.3).sub.3.6H.sub.2O (290.8 mg) and FeCl.sub.2.4H.sub.2O (99.8 mg), are dissolved into 15 mL of isopropanol. The latter has been, prior to the synthesis, carefully degassed by bubbling it using argon for 30 min. The reaction mixture is placed in a round bottom flask previously flushed with argon. Once the solubilisation is completed, 3 mL of degassed concentrated hydrofluoric acid solution (28 mol.Math.L.sup.?1, HF.sub.48%) (30 minute by bubbling it using argon) is added to the solution and the solution is allowed to stir for 30 min under argon atmosphere until precipitation is complete. Finally, the mixture is filtered, washed with technical ethanol and dried at room temperature giving a brownish powder.

Example 3: Synthesis of Solid Solutions M.SUP.2+..SUB.x.M.SUP.3+..SUB.1?x.O.SUB.1?x.F.SUB.1+x

[0115] A thermal treatment at moderated temperature (+/?250? C.) in atmospheric conditions of hydrated fluorides MMF.sub.5(H.sub.2O).sub.7 leads to the formation of solid solutions M.sub.0.5M.sub.0.5O.sub.0.5F.sub.1.5 along the general reaction:

##STR00004##

Example 3.1: Co.SUP.2+..SUB.0.5.Fe.SUP.3+..SUB.0.5.O.SUB.0.5.F.SUB.1.5

[0116] Co.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 is obtained by thermal treatment of Co.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7 under ambient atmosphere at 240? C. for 1 h in a furnace corresponding to an experimental weight loss of 43.4% close to the theoretical value 43.9% (m.sub.before=175 mg, m.sub.after=99 mg).

Example 3.2: Ni.SUP.2+..SUB.0.5.Fe.SUP.3+..SUB.0.5.O.SUB.0.5.F.SUB.1.5

[0117] Ni.sup.2+.sub.0.5Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 is obtained by thermal treatment of Ni.sup.2+Fe.sup.3+F.sub.5(H.sub.2O).sub.7 under ambient atmosphere at 290? C. for 1 h 30 min in a furnace corresponding to an experimental weight loss of 44.7% close to the theoretical value 44% (m.sub.before=434 mg, m.sub.after=240 mg).

Example 3.3: Co.SUP.2+..SUB.0.25.Ni.SUP.2+..SUB.0.25.Fe.SUP.3+..SUB.0.5.O.SUB.0.5.F.SUB.1.5

[0118] Co.sup.2+.sub.0.25Ni.sup.2+.sub.0.25Fe.sup.3+.sub.0.5O.sub.0.5F.sub.1.5 is obtained by thermal treatment of Co.sup.2+.sub.0.5Ni.sup.2+.sub.0.5Fe.sup.3+F.sub.5(H.sub.2O).sub.7 under ambient atmosphere at 280? C. for 30 min in a furnace corresponding to an experimental weight loss of 46% close to the theoretical value 44.0% (m.sub.before=371 mg, m.sub.after=200 mg).

Example 3.4: Co.SUP.2+..SUB.0.75.Fe.SUP.3+..SUB.0.25.O.SUB.0.25.F.SUB.1.75

[0119] Co.sup.2+.sub.0.75Fe.sup.3+.sub.0.25O.sub.0.25F.sub.1.75 is obtained by thermal treatment of Co.sup.2+.sub.0.75Fe.sup.2+.sub.0.25F.sub.2(H.sub.2O).sub.4 under dry synthetic air at 300? C. for 1 h in a furnace previously flushed for 30 min under dry synthetic air, corresponding to an experimental weight loss of 41.5% close to the theoretical value 43.7% (m.sub.before=436 mg, m.sub.after=255 mg).

Example 3.5: Co.SUP.2+..SUB.0.25.Ni.SUP.2+..SUB.0.5.Fe.SUP.3+..SUB.0.25.O.SUB.0.25.F.SUB.1.75

[0120] Co.sup.2+.sub.0.25Ni.sup.2+.sub.0.5Fe.sup.3+.sub.0.25O.sub.0.5F.sub.1.5 is obtained by thermal treatment of Co.sup.2+.sub.0.25Ni.sup.2+.sub.0.5Fe.sup.2+.sub.0.25F.sub.2(H.sub.2O).sub.4 under dry synthetic air at 220? C. for 1 h in a furnace previously flushed for 30 min under dry synthetic air, corresponding to an experimental weight loss of 39.1% close to the theoretical value 40.2% (m.sub.before=139.5 mg, m.sub.after=85 mg).

Example 4: Characterization of M.SUP.2+.M.SUP.3+.F.SUB.5.(H.SUB.2.O).SUB.7 .and M.SUP.2+..SUB.x.M.SUP.3+..SUB.1?x.O.SUB.1?x.F.SUB.1+x

[0121] XRD=X-ray diffraction [0122] SEM-EDX=Scanning Electron Microscope coupled with Energy Dispersive X-ray [0123] TXRD=Thermal X-ray Diffraction [0124] TGA=ThermoGravimetry Analysis [0125] FTIR=Fourier Transformed Infrared Spectroscopy [0126] TEM=Transmission Electron Microscopy

Example 4.1: Characterization of M.SUP.2+.M.SUP.3+.F.SUB.5.(H.SUB.2.O).SUB.7

[0127] Experimental XRD patterns match with the known XRD pattern of the CoFeF.sub.5(H.sub.2O).sub.7 (PDF card number 00-037-0794) (FIG. 8). Small shifts of diffraction peaks related to different metal ionic radii are observed. The metal ratio M.sup.3+/M.sup.2+ close to one, obtained by SEM-EDX, is in good agreement with M.sup.2+M.sup.3+F.sub.5(H.sub.2O).sub.7 formulations (FIG. 12).

Thermal Analyses of Co.SUP.2+.Fe.SUP.3+.F.SUB.5.(H.SUB.2.O).SUB.7

[0128] TXRD: The monitoring of the structural evolution of Co.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5 as function of the temperature exhibits four domains (FIG. 13): [0129] (1) RT-100? C.: stability of CoFeF.sub.5(H.sub.2O).sub.7, [0130] (2) 100-200? C.: dehydration of CoFeF.sub.5(H.sub.2O).sub.7 leading to the formation of CoFeF.sub.5(H.sub.2O).sub.n (n<7), [0131] (3) 220-280? C.: formation of Co.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5 with rutile structure type, [0132] (4) from 300? C.: formation of oxides CoO and CoFe.sub.2O.sub.4.

[0133] TGA: The formulation of the intermediate phase Co.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5 observed by TDRX is confirmed by TGA under ambient air (FIG. 14). CoFeF.sub.5(H.sub.2O).sub.7 undergoes the following decompositions upon thermal treatment, the experimental weight loss values are in good agreement with theoretical values: [0134] 100? C.<T<200? C.: CoFeF.sub.5(H.sub.2O).sub.7.fwdarw.CoFeF.sub.5+7H.sub.2O (% .sub.theo=37.6%/%.sub.exp=37.3%) [0135] 200? C.<T<280? C.: CoFeF.sub.5+H.sub.2O.fwdarw.2Co.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5+2HF (%.sub.theo=43.4%/%.sub.exp=43.9%) [0136] T>280? C.: 2Co.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5+ 3/2H.sub.2O.fwdarw.?CoO+?CoFe.sub.2O.sub.4+3HF (%.sub.theo=53.6%/%.sub.exp=52.8%)

Example 4.2: Characterization of Solid Solutions M.SUP.2+..SUB.x.M.SUP.3+..SUB.1?x.O.SUB.1?x.F.SUB.1+x

[0137] XRD: XRD patterns of Co.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5 and Ni.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5 match with the PDF card (01-070-1522) of FeOF (rutile structure type) and show line profiles which are characteristic of low crystalline compounds (FIG. 15).

[0138] FTIR: Water removal of hydrated fluorides CoFeF.sub.5(H.sub.2O).sub.7 and CoFeF.sub.5(H.sub.2O).sub.7 after thermal treatment is confirmed by FTIR (FIG. 16). The FTIR spectra of CoFeF.sub.5(H.sub.2O).sub.7 and CoFeF.sub.5(H.sub.2O).sub.7 present a broad signal between 3300 and 2800 cm.sup.?1 and a sharp peak centered at 1600 cm.sup.?1 attributed to the stretching (?.sub.OH) and bending (?.sub.HOH) modes respectively. After thermal treatment, these signals are no longer present, confirming H.sub.2O removal.

[0139] The thermal treatment induces a significant increase of the specific surface area (S.sub.BET) between the hydrated fluorides and the oxyfluorides: CoFeF.sub.5(H.sub.2O).sub.7 (3 m.sup.2.Math.g.sup.?1), NiFeF.sub.5(H.sub.2O).sub.7 (1 m.sup.2.Math.g.sup.?1), Co.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5 (24 m.sup.2.Math.g.sup.?1), and Ni.sub.0.5Fe.sub.0.5O.sub.0.5F.sub.1.5 (17 m.sup.2.Math.g.sup.?1) (FIG. 17).

[0140] TEM images show that the increase of S.sub.BET is probably related to an emerging porosity related to the removal of HF and H.sub.2O gas molecules during the thermal decomposition (FIG. 18). This nanostructuration, in agreement with XRD patterns, is also confirmed by N.sub.2 adsorption/desorption isotherms showing type IV hysteresis corresponding to mesoporous structure according to the IUPAC classification; the mesopores diameters range from 2 up to 10 nm (FIG. 17).