Method for coating the surface of an organic or metallic material with particular organic compounds by means of a pulsed-current electrochemical reduction of the diazonium ions of said organic compounds

Abstract

The invention relates to a method for coating an organic or metallic material by covalent grafting of at least one organic compound A having at least one aromatic group substituted with a diazonium function, on a surface of said material, characterized in that the material is porous or fibrillar having a geometric surface area of at least 10 cm.sup.2 of material, and in that said method includes a step of continuous imposition of a non-zero pulsed current in an intensiostatic mode on the surface of the material in order to electrochemically reduce the diazonium ion or ions. The invention further relates to the resulting composite materials and to the use of such materials for manufacturing electrodes.

Claims

1. A method for coating an organic or metallic material by covalent grafting of at least one organic compound A, possessing at least one aromatic group substituted by a diazonium function, on a surface of said material, characterized in that said material is porous or fibrillar and has a geometrical surface area of at least 10 cm.sup.2 of material; and the method of coating comprises a step of continuous imposition of a non-zero pulsed current in an intentiostatic mode on the surface of the material in order to electrochemically reduce the diazonium ion or ions, wherein the step of imposition of the non-zero pulsed current is carried out in successive cycles, each cycle comprising: a grafting phase with a duration Δt.sub.1 during which a first intensity of the non-zero pulsed current (i.sub.1) applied is chosen so as to polarize the surface of the material to a potential E.sub.1 enabling the reduction of the diazonium salt and the grafting of the aromatic group, substituted by said diazonium salt, on to the surface of said material for a non-zero duration of imposition Δt.sub.1, and an idle phase of a duration Δt.sub.2 during which a second intensity of the non-zero pulsed current applied (i.sub.2) is smaller than the first intensity of the non-zero pulsed current (i.sub.1), for a non-zero idle duration Δt.sub.2, wherein the second intensity of the non-zero pulsed current (i.sub.2) is non-zero.

2. The method according to claim 1, characterized in that a value of the first intensity i.sub.1 is given in amperes by the following relationship:
i.sub.1=k×m, where: m=mass of organic or metallic material in grams; and k=2 amperes per gram of metallic material and k=5 amperes per gram of organic material.

3. The method according to claim 1, characterized in that a value of the second intensity i.sub.2 is lower than or equal to 0.05 times a value of the first intensity of the non-zero pulsed current (i.sub.1).

4. The method according to claim 1, characterized in that the duration of imposition Δt.sub.1 is given by the following relationship:
Δt.sub.1=k.sub.t×t.sub.b, in which: k.sub.t=(A)/C.sub.min, where (A) represents a concentration of the organic compound A in moles per liter and C.sub.min represents the minimum concentration in diazonium atoms, and t.sub.b represents a value constant in time.

5. The method according to claim 1, characterized in that the duration of imposition Δt.sub.1 ranges from 100 microseconds to 30 seconds, the duration of the idle time Δt.sub.2 ranges from 1 second to 5 minutes, or a combination of both.

6. The method according to claim 1, characterized in that the material is an organic material chosen from foams, felts, and superimposition of fabrics.

7. The method according to claim 6, characterized in that it comprises a step of metallization of the organic material by electrodeposition of at least one metal prior to the step of continuous imposition of the non-zero pulsed current.

8. The method according to claim 1, characterized in the material is a metallic material chosen from among the metals having a standard potential measured by a standard hydrogen electrode at 25° C. lower than zero.

9. The method according to claim 1, characterized in that the organic compound A is chosen from molecules that are insoluble or nearly soluble in water and comprise at least one arylamine function.

10. The method according to claim 9, wherein organic compound A is chosen from among the macrocyclic catalysts possessing a metal-centre at the centre of the molecule and belonging to the group consisting of: phtalocyanine, porphyrine, calixarene, crown ether and cyclopeptide families.

11. The method according to claim 1, characterized in that it is implemented in an appropriate medium comprising a protic solvent, an aprotic solvent and a supporting electrolyte.

12. The method according to claim 11, characterized in that the aprotic solvent is an organic solvent, in that the protic solvent is water and in that the appropriate medium is a mixture of organic solvent and water in a volume ratio of at least 90/10.

13. The method according to claim 5, characterized in that the duration of imposition Δt.sub.1 ranges from 0.5 to 10 seconds.

14. The method according to claim 6, characterized in that the felt is selected from the group consisting of carbon fiber felts and graphite fiber felts.

15. The method according to claim 7, further comprising that the step of metallization of the organic material by electrodeposition of at least one metal is performed in situ.

16. The method according to claim 8, characterized in that the metallic material is selected from the group consisting of tin, indium, molybdenum, gallium, vanadium, nickel, cobalt, thallium, cadmium, iron, bismuth, chromium, zinc and copper.

17. The method according to claim 16, characterized in that the metallic material is selected from the group consisting of nickel, cobalt and copper.

Description

5. LIST OF FIGURES

(1) Other features and advantages of the invention shall appear more clearly from the following description of a preferred embodiment, given by way of a simple illustratory and non-exhaustive example, and from the appended drawings, of which:

(2) FIG. 1 is a graph illustrating the results of the oxidation of hydrazine on graphite and on nickel in the absence and in the presence of catalyst;

(3) FIG. 2 is a graph presenting the results of the oxidation of hydrazine on nickel in the absence and in the presence of catalysts.

(4) FIG. 3 is a graph illustrating the results of the reduction of dioxygen on glassy carbon.

(5) FIG. 4 is a graph illustrating the results of the reduction of dioxygen on a felt metalized with nickel.

(6) FIG. 5 is a graph presenting the results of the voltammetric analysis of the reduction of dioxygen into hydrogen peroxide.

(7) FIG. 6 is a graph illustrating the results of the voltammetric analysis of a solution of hydrogen peroxide by a nickel electrode modified by 2-aminoanthraquinone.

6. EXAMPLES OF EMBODIMENTS OF THE INVENTION

(8) The general principle of the invention relies on the use of a non-zero pulsed current, with imposed current, enabling the grafting of diazonium salts in an acid hydro-organic medium on organic or metallic materials, porous or fibrillar, the specific surface area of which is at least equal to 10 cm.sup.2. The following examples serve to illustrate the invention without however being exhaustive in character.

(9) 6.1 Demonstrating the Efficiency of the Grafting of Catalysts on Organic and Metallic Materials

(10) A series of experiments was conducted to graft phtalocyanines derivatives on organic material such as graphite or metallic materials such as nickel, and to test the efficiency of the grafting by measuring the catalytic activity of the compounds once grafted relative to oxidation of a hydrazine.

(11) Hydrazine is a reducing agent that oxidizes according to the following diagram:
NH.sub.2—NH.sub.2.fwdarw.N.sub.2+4H.sup.++4e.sup.−

(12) The tested derivatives of phtalocyanines are synthesized in the laboratory and represented here below. The core Mn.sup.+ represents a metallic ion, preferably Co.sup.2+ or Fe.sup.2+.

(13) ##STR00001##

(14) The choice of using these compounds can be explained by the fact that it was observed that the catalytic activity of the phtalocyanine derivatives relative to the oxidation of hydrazine was optimum when the metal inserted into the macrocycle is Co.sup.2+ or Fe.sup.2+.

(15) In addition, these organic compounds have four chains each terminated by an aniline type amine function. For each compound therefore, there are four possibilities of anchoring by electrochemical reduction of the corresponding diazonium ions on the surface of a material.

(16) The method of coating according to the invention was implemented according to the following operational protocol. The protocol uses a felt metalized with nickel. The felt presents a spherical with a diameter of 10 cm, a thickness of 3 mm and a mass of 5.5 g. This felt is immersed in one liter of a phtalocyanine solution with a concentration equal to 10.sup.−3 mol/L.

(17) It must be noted that a phtalocyanine concentration of 10.sup.−3 mol/L is equivalent to an aniline concentration of 4.10.sup.−3 mol/L. The parameters implemented are the following: i.sub.1=11 A k.sub.t=8 Δt.sub.1=4 s i.sub.2=0.55 A Δt.sub.2=40 s
Composition of the Grafting Solution:

(18) The solutions A and B are prepared separately and then mixed in the following percentages by volume: 90% of solution A and 10% of solution B. The composition of each solution is indicated here below:

(19) a) solution A: 900 ml of DMF in which 1.2 g of phtalocyanine (molar mass=1206 g.Math.mol.sup.−1) is dissolved to obtain a final concentration of 10.sup.−3 mol.Math.L.sup.−1, and 27.5 g of NaBF.sub.4 (molar mass=110 g.Math.mol.sup.−1) is dissolved to obtain a final concentration of 0.25 mol.Math.L.sup.−1.

(20) b) solution B: 100 ml of an aqueous solution mixed with 900 ml of DMF two times:

(21) first addition of 80 ml in a solution of sodium nitrite at 0.125 mol.Math.L.sup.−1 for a final concentration, after dilution in DMF, that is equal to 10.sup.−2 mol.Math.L.sup.−1; second addition of 20 ml of a solution of a strong acid at 1 mol.Math.L.sup.−1 for a final concentration, after dilution in DMF, that is equal to 0.2 mol.Math.L.sup.−1.

(22) When the second addition is made, a waiting time of 10 min is necessary before starting the electrolysis. This waiting time is necessary because the synthesis in situ of diazonium salts was not instantaneous.

(23) FIG. 1 resembles a series of experiments firstly confirming that the catalysts once grafted do not lose their activity and secondly showing the efficiency of the catalysts and consequently the success of the covalent grafting of the phthalocyanines on the two materials (graphite and nickel). The analyses are made on samples of electrodes with a geometrical surface area of about 0.1 cm.sup.2. The intensity I is expressed in milliamperes (mA) and the potential E is expressed in volts (V) against a saturated calomel electrode (SCE).

(24) The curves of the graph of FIG. 1 represent the direct oxidation of the hydrazine on graphite and on nickel as well as the oxidation of the hydrazine after the catalyst has been grafted on to the surface of the materials. It must be noted that a better result is obtained with blocked catalysts on nickel. This better result is expressed by a shift in the curve, towards the negative potentials, of approximately 150 mV relative to the curve obtained with graphite modified by the catalyst.

(25) Thus, it has been shown that the method according to the invention enables the efficient grafting of the phtalocyanine catalysts on to graphite and nickel.

(26) 6.2 Demonstrating the Homogeneity of the Grafting of Catalysts on Organic and Metallic Material.

(27) The homogeneity of the grafting on nickel was demonstrated by measuring the anti-corrosive properties obtained after grafting. It is important to obtain the most homogeneous coating possible because the phenomenon of corrosion occurs in zones not coated by the grafting. Hydrazine is a very strong complexing agent that prompts the corrosion of a metal, which is the case for nickel, FIG. 2. In the absence of a catalyst, nickel shows a domain of corrosion situated between −1.1 V and −0.6 V. This phenomenon is well known and appears because of the complexing capacity of hydrazine on nickel which considerably lowers the potential of corrosion. In the presence of the catalyst, the corrosion disappears. A surface modification also protects the metal.

(28) Thus, it has been demonstrated that the method according to the invention enables a metallic material to be passivated homogeneously.

(29) 6.3 Reduction of Dioxygen O.sub.2 in a Solution of Sodium Hydroxide at 1 mol/L

(30) The reduction of dioxygen is implemented in fuel cells generally in a concentrated base medium. The reduction reaction corresponding to the positive pole of the cell is the following:
O.sub.2+4H.sup.++4e.sup.−.fwdarw.2H.sub.2O E°=1.23 V/SHE

(31) Depending on the pH, the release of oxygen is related to the formula: E.sub.O2/H2O=1.23−0.06 pH. As a consequence, in a solution with a pH=14, the release of dioxygen starts theoretically at 0.39 V/SHE. In practice, this value is never obtained and the best result is obtained on platinum with a reduction potential of the order of 0.1 V/SHE at pH=14.

(32) Two series of experiments were conducted in order to compare the influence of the grafting on the reduction of dioxygen. For each experiment, the reduction of dioxygen in a highly basic medium was carried out by using either a non-modified electrode or the same electrode modified by the grafting of diazonium salts. Besides, two types of different materials were used: a glassy carbon electrode and a nickel electrode.

(33) For these two electrodes, when they were modified, the phthalocyanine grafting solution was the same and had the following composition: cobalt phthalocyanine in a concentration 5.10.sup.−4 mol.Math.L.sup.−1 in a solution of DMF containing NaBF.sub.4 in a concentration of 0.5 mol.Math.L.sup.−1. 1% by volume of a solution of sodium nitrite at 1 mol.Math.L.sup.−1. 1% by volume of a solution of sulfuric acid at 0.5 mol.Math.L.sup.−1.

(34) As can be seen from FIG. 3, the grafting of phthalocyanines on the surface of the electrode improves the reduction of dioxygen as compared with the non-modified electrode. This improvement is characterized by a potential for starting reduction towards 0V/SHE and by a verticality of the signal which expresses a fast speed of electron transfer between O.sub.2 and the cobalt phthalocyanine.

(35) By contrast, unlike in glassy carbon, the dioxygen is reduced with great difficulty on nickel. Indeed, as can be seen in FIG. 4, the reduction of O.sub.2 on pure nickel does not appear in the potential domain represented. The reduction of the dioxygen becomes effective well below −0.6 V/SHE. The grafting of phthalocyanines on the surface of the nickel shows all its efficiency since the signal obtained is almost identical to the one obtained on glassy carbon (see FIG. 3). The signal has the same verticality with an increase of about 50 volts at the start of the signal.

(36) Thus, the grafting of the catalysts is advantageous on carbon and very efficient on a metallic material such as nickel. This technique gives a metallic type electrode, of which the properties relative to the reduction of dioxygen are very close to platinum. Now platinum is particularly costly, the method according to the invention enables the production of low-cost catalysts capable of reducing dioxygen at a satisfactory potential. The catalytic material created can therefore be used as an electrode in fuel cells and batteries.

(37) 6.3. Supported Synthesis of Hydrogen Peroxide H.sub.2O.sub.2 by Electrolysis by Percolation.

(38) The industrial synthesis of hydrogen peroxide is conventionally achieved by oxidation of 2-alkyl anthrahydroquinone (A) by dioxygen under heavy bubbling. This oxidation leads to the formation of 2-alkylanthraquinone (B) accompanied by a release of hydrogen peroxide. The 2-alkyl anthrahydroquinone (A) is regenerated by a reduction of 2-alkylanthraquinone (B) under a dihydrogen atmosphere. The step of reduction is usually catalyzed by a metal. This cycle is repeated successively for the industrial production of hydrogen peroxide and is represented here below.

(39) ##STR00002##

(40) Through the method of the invention, it is now possible to propose an alternative method for the synthesis of hydrogen peroxide. In other words, the present invention is allows to carry out the industrial synthesis of hydrogen peroxide through electrolysis by percolation in which the compound (A) is blocked on a porous electrode. This method regenerates the molecule (A) electrochemically without having recourse to a dihydrogen atmosphere. More specifically, the 2-alkyl anthrahydroquinone is grafted on to an electrode. A second electrode has 2-aminoanthraquinone grafted on it, according to the method of the invention, and herein plays the role of a catalyst. The oxidation-reduction reaction between the two electrodes is represented here below:

(41) ##STR00003##

(42) The diazonium salts are formed starting from the amine function in position 2 of the 2-aminoanthraquinone. The grafting composition is the following: 98% of DMF containing the 2-aminoanthraquinone at 10.sup.−3 mol.Math.L.sup.−1 and NaBF.sub.4 at 0.25 mol.Math.L−1. 1% of an aqueous solution of sodium nitrite with a concentration of 1 mol.Math.L.sup.−1. 1% of a solution of sulfuric acid with a concentration of 0.5 mol.Math.L−1.

(43) The electro-grafting process according to the invention was implemented on a nickel electrode at pH=7.

(44) The electrochemical reduction of dioxygen (O.sub.2) leads to the formation of hydrogen peroxide (H.sub.2O.sub.2) according to the reversible reaction (1):
O.sub.2+2H.sup.++2e.sup.−.fwdarw.H.sub.2O.sub.2  (1) E°=0.69 V/SHE.

(45) On the transition metals, this reaction is kinetically slow, resulting in a small quantity of H.sub.2O.sub.2 formed. On noble metals and especially for platinum, the hydrogen peroxide once formed is in reduced majority in water according to the reaction (2):
H.sub.2O.sub.2+2H.sup.++2e.sup.−.fwdarw.2H.sub.2O  (2) E°=1.73 V/SHE.

(46) The use of anthraquinone as a redox catalyst enables the quantitative and unique performance of the reaction (1) of electrochemical synthesis of hydrogen peroxide, and this can be done on all the conductive materials able of fixing the catalyst.

(47) FIG. 5 gives an account of the working of the anthraquinone grafted on a nickel electrode. On non-modified nickel, the reduction of dioxygen is not effective. By contrast, in the presence of the grafted catalyst, the reduction of dioxygen appears. As soon as the second cycle starts, when the potential varies towards the anodic potentials, the oxidation of hydrogen peroxide formed on the electrode is resumed to be oxidized in dioxygen. The formation of hydrogen peroxide under bubbling of O.sub.2 is confirmed by the analysis of a solution of hydrogen peroxide in which the modified nickel electrode is tested (see FIG. 6).

(48) A variation of the potential is performed from an initial value (Ei=−0.15 V/SHE) towards a higher anodic value (Ea) followed by a return to a cathode potential Ec with a value −0.65 V/ESH. The anodic terminal (Ea) is in the domain of oxidation of the hydrogen peroxide. Thus, the higher the value of the anodic terminal is, the greater the quantity of dioxygen formed at the electrode is consequently, during the return cycle, the reduction of dioxygen formed at the electrode occurs and the intensity of the reduction current increases in parallel to that of the anodic terminal.

(49) The method for synthesizing hydrogen peroxide developed from the method according to the invention has many advantages: there is no use of dihydrogen (H.sub.2); there is no separation of the catalyst or catalysts, the solution of hydrogen peroxide being obtained at the output of the electrochemical cell; the degree of the hydrogen peroxide solution is directly a function of the control of the intensity of the current applied to the negative terminal; according to the support material used, the peroxide solution can be synthesized in an acid solution or else in a basic solution: the method is then easily adaptable; a part of the needed dioxygen for the working of the method is provided by the electrolysis system of and more specifically from the oxidation of water which takes place at the counter electrode (positive electrode); and anthraquinone is a stable molecule through its highly robust molecular structure and therefore, as a redox catalyst, possesses a very high life time.