CH658199A5 - LOW OHMIC RESISTANCE DIAPHRAGMS APPLICABLE IN ALKALINE ELECTROLYSERS. - Google Patents

Description

The present invention relates to diaphragms of low ohmic resistance applicable in alkaline electrolysers.

The production of alkaline electrolysers, for the production of hydrogen intended for the storage of electrical energy during periods of low consumption, implies, for reasons of profitability, operation at relatively high current densities (i> 400 mA / cm2) for a terminal voltage lower than 1.9 volts. This condition can be satisfied if the electrolyser is operated at temperatures between 100 and 200 ° C.

The separation of hydrogen from oxygen is obtained by the interposition between the electrodes of the electrolyzer of a diaphragm permeable to the electrolyte (for example aqueous KOH) and impermeable to the gases formed at the electrodes.

The operation of the electrolyser at temperatures above 100 ° C requires the availability of diaphragms which meet the following specifications:

- high ionic conductivity to avoid energy losses by Joule effect (ohmic resistance of the order of at most 0.1 to

0.3 Q / cm2);

- high chemical stability in strongly alkaline (KOH 6N to 10N) and oxidizing medium;

- good temperature resistance;

- low permeability to oxygen and hydrogen so as to ensure a purity of the gases greater than 99.5%;

• - mechanical strength sufficiently high to avoid tears and cracks during the mounting of the diaphragms; 5 - good wettability avoiding the accumulation of gas bubbles on the diaphragm.

Diaphragms and separators with high chemical stability and temperature resistance have been described in various patents, in particular in the case of alkaline batteries. Thus, United States Patent No. 3,749,604 reports diaphragms which can be used in such silver-zinc type cells to prevent diffusion of silver oxides to the zinc electrode. These diaphragms consist of porous substrates covered on both sides with a layer of porous polymer containing 15 inorganic oxides and hydroxides such as Zr02, Zr (OH) 4, Th02, etc. The presence of these inorganic compounds serves to increase the hydrophilicity and the electrical conductivity of the diaphragm because these compounds constitute ion exchangers, although their exchange capacity is not very high. It is specified that the particle size of the inorganic oxides must be between 74 and 700 microns and preferably between 149 and 700 microns. However, the holder noted that by using the process described in this patent and in particular that by respecting the conditions relating to the particle size of the inorganic oxides recommended by its authors, one obtained diaphragms whose pores have diameters exceeding in generally several microns, which is too high to prevent the diffusion of gases through the diaphragms. Moreover, this patent emphasizes, among other things, the pore diameters of between 5 and 75 microns. On the other hand, if we use oxides with a much smaller particle size, for example less than 50 microns, we obtain diaphragms whose pores are small enough to prevent the passage of gases, but their electrical conductivity is then too low to be able to use them profitably in alkaline chlorinators. Indeed, their electrical resistances measured in KOH 10N at 110 ° C range between 0.5 and 2 fî / cm2.

In other patents, such as in U.S. Patent No. 3,713,890, the possibility of introducing inorganic compounds such as ZrOz, Th02, Ce02, calcium, potassium salts, etc. is reported. in an organic binder consisting of a chemically stable polymer 40. In this latter patent, the implementation of the polymer used as a binder differs from that used in patent No. 3749604. Indeed, the polymer is not dissolved in an organic solvent before being mixed with the powder of the organic compound, but it is mixed, in the form of an aqueous dispersion, with an aqueous dispersion of the inorganic compound, the particles of the polymer and those of the inorganic compound having to be of diameter less than 10 microns. The final dispersion obtained is poured onto a flat support, evaporated and the resulting dry film is sintered at the sintering temperature of the polymer. Apart from the so small grain size of the inorganic compound, no other condition relating to this compound is mentioned, in particular as regards the shape of the grains and the apparent density of the powder. The preparation of the diaphragms by sintering the polymer, according to the method described in this patent, has the drawback of not inducing porosity in the binder. The electrical conductivity of the diaphragm thus prepared is therefore too low, in particular if the powder of the inorganic compound used does not satisfy the morphological conditions indicated below. In addition, if it is sought to increase the electrical conductivity of the diaphragm by increasing the content of the inorganic compound, the polymer no longer sintered and the resulting diaphragm no longer has any mechanical strength.

In addition, the wettability of the currently known diaphragms is not always satisfactory. However, good wettability prevents the accumulation of gas bubbles on the diaphragm and promotes the penetration of the electrolyte into the pores of the diaphragm.

The present invention relates to improved diaphragms which do not have the drawbacks mentioned above and which can

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658,199

be applied both in alkaline electrolysers and in alkaline batteries.

According to the present invention, there are provided diaphragms of homogeneous and regular texture, of good wettability, of low ohmic resistance and of good mechanical strength, characterized in that they consist of membranes of thickness up to 700 microns , made of a porous polymer containing oxides, hydroxides or inorganic salts, high pore volumes and low solubility in KOH, the following alkali and alkaline earth metals: sodium, potassium, calcium or magnesium, and the following metals: zirconium, thorium or cerium, these diaphragms having an ohmic resistance measured in KOH ION at 110 ° C of at most 0.3 fi / cm2, a porosity rate greater than 50% with pore dimensions of at most 1 micron and a mechanical strength defined by a tensile strength of at least 25 kg / cm2, a modulus of elasticity (Young's modulus) of at least 200 kg / cm2 and a burst strength of at least 3 kg / cm2.

The polymer from which the diaphragms are made according to the invention can be chosen from hydrophilic and hydrophobic polymers having good stability in a strongly alkaline medium and at high temperature (of the order of 110 to 160 ° C). Examples of suitable polymers according to the invention include polysulfones, polyoxyphenylenes, polyquinoxalines, pyrrones and the like (hydrophilic), polytetrafluoroethylenes and the like (hydrophobic).

Preferably, the following conditions are observed:

1. The inorganic compound introduced into the polymer has a low apparent density and / or a high pore volume. In other words, the shape of the grains constituting said inorganic compound must be irregular and tormented rather than compact and rounded. An advantageous form is that of sticks, needles and the like. These grains can also themselves have a high clean porosity, thereby lowering the apparent density of the powder. The choice of this characteristic of the inorganic compound makes it possible to give the final diaphragm a high porosity rate.

2. The particle size of the inorganic compound corresponds to very fine grains having diameters less than 50 microns and preferably less than 20 microns. In addition, a finer particle size makes it possible to obtain greater homogeneity of composition and greater regularity of the texture of the diaphragm with a lower frequency of appearance of defects (holes, cracks, etc.), which reduces the risks. passage of gases through the diaphragm.

3. The polymer serving as a binder for inorganic compounds is used in such a way that this binder is itself porous or microporous. This porosity can be obtained by methods known in the technical literature, chosen according to the nature of the polymer. Thus, the so-called phase inversion method (R.F. Kesting "Syntheticpolymeric membranes", page 117, editor MacGraw Hill 1971) can be used in the case of a polymer soluble in organic solvents. The method used in United States Patent No. 3,749,604 is derived from this. It is also possible to introduce a porophore agent into the polymer which is then eliminated, totally or partially, either by bringing the diaphragm to the decomposition temperature of the porophore agent (eg carbonates), or by extracting the agent diaphragm porophore from a solvent. However, it is important that the pores of the diaphragm are sufficiently small to prevent the diffusion of gases through these pores.

4. A high porosity makes it possible to increase the thickness of the diaphragms, therefore their mechanical strength, while retaining their conductivity and further reducing the risks of the appearance of holes and cracks which would favor the passage of gases through the diaphragm .

5. When the polymer has a pronounced hydrophobicity, it is important that the content of inorganic compound is high (> 55% by weight) so that the wettability and the conductivity of the latter are sufficient.

Patents mentioning the preparation of battery separators or diaphragms for batteries or electrolysers from inorganic oxides or hydroxides and an organic binder do not usually meet any of the conditions listed above or at least not the first.

The diaphragms prepared by respecting the conditions listed above also exhibit remarkable stability over time of their electrical properties, good mechanical strength and good wettability.

The examples which follow illustrate the invention in no way limiting.

Example 1:

Two diaphragms of equal thickness (about 500 microns) referenced A and B are prepared by the phase inversion method, using the polysulfone "P 1700" (sold by Union Carbide) as a binder and as a heat exchanger. 'ions of magnesium oxide.

The phase inversion method used consists in pouring onto a flat stainless steel support, using an applicator, a solution of polysulfone in dimethylformamide to which the powder was added beforehand, with vigorous stirring. MgO. It is important to ensure that the powder is dispersed evenly in the solution. After this operation, the support is immersed 2s in an aqueous bath for a few minutes. The diaphragm detaches itself from its support in the aqueous bath. After washing with water and drying, it is ready for use.

The two diaphragms A and B have the same composition by weight, namely 40% of polysulfone for 60% of MgO, but in the case of diaphragm A, MgO is used with an apparent density 0.17 g / cm3 and an average particle size, determined at the Coulter counter, 5 microns for the number average and 12 microns for the weight average, while in the case of diaphragm B, MgO is used with an apparent density of 0.74 g / cm 3 and an average particle size of 8 microns for the number average and 20 microns for weight average.

The ohmic resistance of the two diaphragms, measured in KOH 10N at 110 ° C, is 0.07 Q / cm2 for diaphragm A and 0.62 Q / cm2, almost 9 times higher than that of diaphragm 40 A The rate of porosity measured with the porosimeter with jnercure is from 70 to 75% for the diaphragm A and from 45 to 50% for the diaphragm B.

The volume distribution of the pore radii turns out to be as follows:

45 - for diaphragm A:

• 16% of the total pore volume consists of pores with a radius of less than 0.02 microns;

• 20% of the total pore volume consists of pores with a radius between 0.02 and 0.1 micron;

50 • 20% of the total pore volume consists of pores with a radius between 0.1 and 0.2 microns;

■ 36% of the total pore volume consists of pores with a radius between 0.2 and 0.3 microns;

55 • 8% of the total pore volume consists of pores with a radius between 0.3 and 0.5 microns,

- for diaphragm B:

• 25% of the total pore volume consists of pores with a radius of less than 0.1 microns;

60 • 25% of the total pore volume consists of pores with a radius between 0.1 and 0.2 microns;

■ 12.5% of the total pore volume consists of pores with a radius between 0.2 and 0.3 microns;

65 • 20.8% of the total pore volume consists of pores with a radius between 0.3 and 1 micron;

• 16.7% of the total pore volume consists of pores with a radius between 1 and 4 microns.

658,199

A series of diaphragms A is prepared by varying their weight content of MgO from 45 to 60% and their ohmic resistance is measured as previously. Figure 1 shows the variation of the ohmic resistance (R) in 14N KOH medium at 110 ° C of the diaphragms as a function of their MgO content of apparent density: 0.17 g / cm3.

A series of diaphragms B is likewise prepared with weight contents of MgO of between 50 and 85%. FIG. 2 shows the variation of the ohmic resistance (R) of these diaphragms in 14N KOH medium at 110 ° C. as a function of their MgO content of apparent density: 0.74 g / cm 3.

The comparison of the two figures speaks for itself.

The endurance test carried out on an electrolysis cell, in the absence of any catalyst, shows that the diaphragm A exhibits remarkable stability over time of its properties. Figure 3 indicates that after 1000 hours of operation under a current density of 0.4 A / cm2, the total cell voltage is 1.98 volts and the voltage drop (iR) in the diaphragm is 0, 06 volt. The anode potential is 0.68 volts and the cathode potential is —1.23 volts. The test was carried out with a diaphragm whose composition by weight was 50% of MgO for 50% of polysulfone.

The test conditions were as follows:

- temperature: 110 ° C

- electrolyte: KOH 30%

- pressure in the cell: 20 bars

- nickel anode and cathode

- current density: 0.4 A / cm2

The mechanical strength of the diaphragms according to the invention has been found to be good, even for the high porosity rates: for diaphragm A, there is a tensile strength of 160 to 180 kg / cm2 and a Young's modulus of several hundred kg / cm2.

Example 2:

Five MgO powders of neighboring average particle size are used, each having a different apparent density ranging from 0.17 g / cm 3 to 1.2 g / cm 3 and a diaphragm with a thickness of 400 to 500 microns is prepared with each of the powders. method indicated in Example 1. The five diaphragms have substantially the same weight composition.

Their ohmic resistance is measured in KOH 10N at 110 ° C. FIG. 4 indicates the variation of the ohmic resistance (R) of the diaphragms in KOH 14N medium at 110 ° C as a function of the apparent density of MgO (g / cm3).

Using a mercury porosimeter, the pore volume of the five MgO powders used above is measured. FIG. 5 gives the variation of the ohmic resistance (R) of the diaphragms in 14N KOH medium at 110 ° C. as a function of the pore volume of the MgO powder used to prepare the diaphragms, volume expressed in mm3 / g.

Example 3:

A diaphragm with a thickness of 400 to 500 microns is prepared according to the method described in Example 1. For this, MgO powder with an apparent density of 0.17 g / cm 3 is used, sieved so as to remove the grains whose diameter is greater than 20 microns. The sieving made it possible to lower the ohmic resistance of the diaphragm from 0.07 fi / cm2 to 0.05 Q / cm2.

The same sieved MgO powder is used and hydrolyzed by immersion in 10N KOH at 140 ° C. for a period of 72 hours. The powder is washed with water and dried, then it is verified by X-ray diffraction that it has been completely hydrolysed to Mg (OH) 2. This powder of Mg (OH) 2 is used to prepare a diaphragm under the same conditions as above. Its ohmic resistance measured under the same conditions as above is 0.06 Si / cm2.

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Example 4:

Diaphragms of thickness 400 to 500 microns of type A and B are prepared, according to a method similar to that indicated in Example 1, using polyphenylquinoxaline in place of poly-io sulfone. The polyphenylquinoxaline solvent being 1,1,2,2 t-trachloroethane which is not miscible with water, a bath containing acetone which is miscible with 1.1 is used instead of the aqueous bath , 2.2 tetrachloroethane.

The diaphragms are washed in acetone, then in water and dried.

It can be seen that the ohmic resistance of the diaphragms A is much lower than that of the diaphragms B.

Example 5:

The method described in US Pat. No. 3,713,890 is used to prepare a diaphragm 250 to 300 microns thick, from an aqueous dispersion of polytetrafluoroethylene 30N (marketed by Du Pont de Nemours) mixed with an aqueous dispersion of Mg (OH) 2. The 25 Mg (OH) 2 powder is obtained by hydrolysis of an MgO powder of low apparent density and sieved under the same conditions as in Example 3. In addition, a pore-forming agent is introduced into the dispersion. occurrence of (MgC03) 4Mg (0H) 25H20 powder. The dispersion, well homogenized by mechanical stirring, is then partially evaporated at 60 ° C. for a few hours until a thick paste is obtained which is placed on a flat support and which is spread by pressing with a roller. steel to make a sheet about 300 microns thick. The weight composition in solid material of this sheet is as follows: 35 - 30% of polytetrafluoroethylene

- 65% Mg (OH) 2

- 5% of (MgC03) 4Mg (0H) 25H20

This sheet is then placed in an oven, the temperature of which is raised in stages to 360 ° C., the sintering temperature of the polytetrafluoroethylene at which the sheet is kept for approximately one hour. This treatment gives the sheet good mechanical strength. Its ohmic resistance, measured in KOH 10N at 110 ° C, is 0.3 fi / cm2.

Instead of the powder of Mg (OH) 2, use is made of a powder composed of 50% by weight of the powder of Mg (OH) 2 above and of 50% of powder of Zr (OH) 4 sieved with the same way as the Mg (OH) 2 powder and a diaphragm with a thickness of 200 microns is prepared, as indicated above. Its ohmic resistance, determined in KOH 10N at 110 ° C, is close to 0.3 fl / cm2. The polymer used in this example being a hydrophobic polymer, it has been found that, in this case, the use of inorganic compounds at a content of less than 55% by weight leads to too low wettability and to too high resistivity, clearly greater than 0.3 fì / cm2.

55 The mechanical strength of the diaphragms obtained in this example is characterized by a tensile strength> 25 kg / cm2, a modulus of elasticity of 200 kg / cm2 and a burst strength of 4 kg / cm2.

It goes without saying that the present invention has been described for purely explanatory and in no way limitative and that any useful modification may be made without departing from its scope.

3 sheets of drawings

Claims (8)

658199 2 1. Diaphragm of homogeneous and regular texture, good wettability, low ohmic resistance and good mechanical strength, characterized by the fact that it consists of membranes of thickness up to 700 microns, made of a porous polymer containing oxides, hydroxides or inorganic salts, with high pore volumes and low solubility in KOH, chosen from the following alkali and alkaline earth metals: sodium, potassium, calcium or magnesium, and the following metals: zirconium, thorium or cerium , these diaphragms having an ohmic resistance measured in KOH ION at 110 ° C of at most 0.3 fl / cm2, a porosity rate greater than 50% with pore dimensions of at most 1 micron and mechanical strength defined by a tensile strength of at least 25 kg / cm2, a modulus of elasticity called Young's modulus of at least 200 kg / cm2 and a burst strength of at least 3 kg / cm2. 2. Diaphragm according to claim 1, characterized in that it has a thickness of between 200 and 700 microns. 3. Diaphragm according to one of claims 1 and 2, characterized in that the polymer of which it consists is chosen from hydrophilic polymers of the polysulfone, polyoxyphenylene, poly-quinoxaline, pyrron and similar type, or hydrophobic of the poly-tetrafluoroethylene type or the like having good stability in a strongly alkaline medium and at a high temperature of 110 to 160 ° C. 4. Diaphragm according to one of claims 1 to 3, characterized in that the radii of the grain pores are distributed between a value less than 0.02 microns and a value at most equal to 1 micron. 5. Diaphragm according to one of claims 1 to 4, characterized in that the powders of the inorganic compounds entering into its constitution taken alone or in mixtures have a low apparent density resulting in a high pore volume greater than 10 mm3 / g. 6. Diaphragm according to claims 3,4 and 5, characterized in that, when the polymer is of the hydrophobic type, the content of inorganic compound is greater than 55% by weight. 7. An alkaline electrolyser comprising a diaphragm according to one of claims 1 to 6. 8. An alkaline battery comprising a diaphragm according to one of claims 1 to 6.