DE69306816T2 - MEMBRANE ELECTRODE STRUCTURE FOR ELECTROCHEMICAL CELLS - Google Patents

Description

The present invention relates to an improved membrane electrode structure for use in an ion exchange membrane electrolytic cell. In particular, the invention relates to the use of an intermediate layer for the membrane electrode structure of chlor-alkali electrolytic cells to reduce the amount of hydrogen in chlorine.

It is known to effect electrolysis by so-called solid polymer electrolyte (SPE) type electrolysis of an alkali metal chloride, wherein a cation exchange membrane of a fluorinated polymer is connected to a gas-liquid permeable catalytic anode on one surface and/or a gas-liquid permeable catalytic cathode on the other surface of the membrane.

This prior art electrolytic method is extremely advantageous as electrolysis at low cell voltage because the electrolytic resistance caused by the electrolyte and the electrolytic resistance caused by bubbling of hydrogen gas and chlorine gas generated in the electrolysis can be effectively reduced. This was considered difficult to achieve in electrolysis with cells of other configurations.

A high proportion of hydrogen in chlorine causes problems in the chlorine liquefaction process. Additional steps are required to prevent the formation of dangerous gas mixtures. The hydrogen problem represents a serious disadvantage if no cost-effective method of reducing the hydrogen content can be found.

The anode and/or cathode electrolysis cells of the prior art are bonded to the surface of the ion exchange membrane in such a way that they are partially embedded therein. The gas and the electrolyte solution are easily permeated to remove from the electrode the gas formed by electrolysis on the electrode layer in contact with the membrane. This means that very few gas bubbles adhere to the membrane after they are formed. Such a porous electrode usually consists of a thin porous layer formed by uniformly mixing particles acting as an anode or a cathode with a binder. It has been found that when an electrolytic cell with an ion exchange membrane directly bonded to the electrode is used, the anode in the electrolytic cell is brought into contact with hydroxide ions migrating back from the cathode compartment, and accordingly the anode material is required to be both chlorine-resistant and alkali-resistant in this prior art process and an expensive material must be used. When the electrode layer is directly bonded to the ion exchange membrane, a gas is formed by the electrode reaction between an electrode and membrane and certain deformation phenomena of the ion exchange membrane cause deterioration of the membrane properties. In such an electrolytic cell, the current collector for the electrical supply of the electrode layer, which is bonded to the ion exchange membrane, should be in close contact with the electrode layer. If close contact is not achieved, the cell voltage can be increased. The cell structure for bringing the current collector into safe contact with the electrode layer is therefore disadvantageously complicated according to the state of the art.

In addition, in chlor-alkali electrolysis cells in which the cathode is directly connected to the membrane, hydrogen permeates through the membrane, penetrates into the anolyte space and mixes with the chlorine. High hydrogen levels are then found in the chlorine, which causes problems in the liquefaction process.

Previous means of reducing hydrogen content include 1) using a platinum black layer on the anode side of the membrane, 2) using a layer (e.g. Ag) that is less electroactive than the electrode layer itself between the membrane and the electrode, 3) using thicker membranes, and 4) using a membrane with a lower permeation rate for the permeation of hydrogen. These methods have proven to be expensive and ineffective.

Perfluoromembranes used as membranes for electrolysis reactions usually have relatively low water contents. Compared to conventional ion exchangers with the same water contents, the conductivity of the perfluoromembranes is unusually high. This is due to the phase separation present in the perfluoroion membranes. The phase separation greatly reduces the tortuosity for sodium ion diffusion. The path for hydrogen ion diffusion is the aqueous ionic region and the amorphous fluorocarbon region. Therefore, the tortuosity experienced by the hydrogen molecules for the phase-segregated fluorocarbon membranes is also low compared to conventional hydrocarbon ion membranes.

The phase segregation properties of the fluorocarbon membranes provide the high migration rates for sodium ions; thus, a relatively low ionic resistance is also the cause of the high hydrogen diffusion rates and the resulting high proportion of hydrogen in chlorine. In addition, the high hydrogen permeation rate is even further enhanced by the high solubility of hydrogen in the fluorocarbon membranes due to the hydrophobic interaction between hydrogen molecules and the fluorocarbon chains. Therefore, reducing the hydrogen permeation rates by increasing the thickness of the membranes or modifying the structure of the membranes would not be very effective, since the sodium migration rate would be reduced if one tried to reduce the hydrogen diffusion rate. and the twisting effect is difficult to introduce due to phase separation.

A retardation layer is defined as a layer for slowing down hydrogen permeation between the electrode layer and the membrane. Any type of layer can have some effect in slowing down hydrogen permeation as long as it is 1) inactive with respect to the electrolytic production of hydrogen and 2) flooded. The latter condition is also important for low resistance (i.e. low voltage and good performance). Based on these considerations, a layer made of a mixture of inert solid particles (usually inorganic) and binders (usually organic) would serve best for this purpose.

The need for a binder is obvious: the binder can (1) bind the components in the barrier layer together and also (2) provide the required adhesion between the retardation layer and the electrode layer and between the retardation layer and the membrane. The function of the solid particles is also twofold: (1) provide physical strength to the barrier layer so that very limited interpenetration between different layers occurs during manufacture and (2) form an agglomerate with the binder.

The reason why the retardation layer is more effective than the membrane itself in slowing down hydrogen permeation is that (1) it allows the migration of hydroxide and hydrogen ions at a faster rate, so that a relatively small voltage penalty is incurred. On the other hand, the Coulomb interaction exerted by the sulfonate and carboxylate groups in the membrane slows down sodium ion diffusion. The situation is even worse when the membrane is immersed in highly corrosive solutions, as in the case of the chlor-alkali membrane. This is particularly serious in the case of carboxyl membranes. It is It is assumed that ion pairing between sodium ions and carboxylate groups and hydroxide ions is the cause of the very low diffusion rate when dehydration of the membrane occurs under this condition. The solubility of hydrogen in the etching solution is much lower than in the membrane, so that the permeation rate (the product of diffusion coefficient and solubility) of hydrogen can be reduced by a higher factor compared to that of sodium and hydroxide ions. By introducing the mixture of inorganic particles and binder and with the required morphology, the resistance of the etching solution is increased. The ratio of the specific resistance of the porous medium saturated with electrolyte, Rp, to the volume resistance of the same electrolyte solution, Rb, is usually called "formation resistivity factor".

F = Rp/Rb = X/O.

This equation describes the relationship between F and the "electrical twist", X, and O, the porosity. X is different from the hydraulic twist, which takes into account the fact that the path travelled by the diffusing species is increased by the presence of impermeable barrier materials. On the other hand, besides hydraulic twist, X also takes into account the special effects due to the convergent-divergent nature of the capillaries, called constrictedness.

Since the conductivity is proportional to the diffusion rate of the clay species, the specific formation resistivity factor is also related to the diffusion rate in the porous medium, Dp, and the diffusion rate in the bulk of the electrolyte, Db, by the following equation:

F = Rp/Rb = Db/Dp

The present invention provides an improved membrane electrode structure for use in electrochemical cells, which comprises a retardation layer between a ion exchange membrane and the cathode. The retardation layer comprises a mixture of 5 to 80 weight percent inorganic solid particles with 95 to 20 weight percent of a thermoplastic polymer binder having a melting point of 230°F to 540°F (110 to 232°C).

As described above, the function of the retardation layer is to provide porosity and distortion, so as to impede hydrogen diffusion. That is, when making a retardation layer with a porosity in the range of 5 to 90 percent, it is preferable to have a porosity in the range of 20 percent to 60 percent, preferably in the range of 30 percent to 50 percent. Preferably, the ratio of distortion/porosity is in the range of 2-500, preferably in the range of 5-100, and more preferably in the range of 10-50.

The inorganic solid particles comprise one or more of the borides, carbides and nitrides of the metals of Groups IIIB, IVA, IVB, VB and VIB of the Periodic Table. Typical examples of suitable materials include SiC, YC, VC, TiC, BC, TiB, HfB, BV2, NbB2, MOB2, W2B, VN, Si3N4, ZrO2, NbN, BN and Tib. Preferably, silicon carbide is used.

The binder used in the invention comprises new ion exchange polymers which can be used alone or mixed with non-ionic thermoplastic binders. These polymers are copolymers of the following monomer I with monomer II.

Monomer I is represented by the general formula: CF₂=CZZ' (1),

wherein Z and Z' are independently selected from the group consisting of -H, -Cl, -F or -CF₃.

Monomer II consists of one or more monomers, selected from compounds represented by the general formula:

Y-(CF₂)a-(CFRf)b-(CFRf)c-O-[CF(CF₂X)-CF₂-O-]n-CF=CF₂ (II)

wherein

Y is SO₂Z,

Z is -I, -Br, -Cl, -F, -OR or -NR₁R₂; is,

R is a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical,

R₁ and R₂ are independently selected from the group consisting of -H, a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical,

a is 0-6,

b is 0-6,

c is 0 or 1,

provided that a+b+c is not equal to 0,

X is -Cl, -Br, -F or mixtures thereof when n> 1,

n is 0 to 6, and

Rf and Rf are independently selected from the group consisting of -F, -Cl, perfluoroalkyl radicals having from 1 to 10 carbon atoms and fluorochloroalkyl radicals having from 1 to 10 carbon atoms.

The ionic binders can be mixed with a non-ionic binder such as a fluorinated hydrocarbon like Teflon.

According to the present invention, at least one of the electrodes, preferably the cathode, is bonded to an ion exchange membrane by a retardation layer for use in an electrolytic cell, in particular in a chlor-alkali cell, to slow down the diffusion of hydrogen.

The composition for producing the retardation layer is preferably in the form of a suspension of agglomerates of particles with a diameter of 0.1 to 10 µm, preferably 1 to 4 µm. The suspension can be formed with an organic solvent which can be easily removed by evaporation, such as halogenated hydrocarbons, alkanols, ethers and the like. Freon is preferred.

The inorganic particles are mixed with the binder particles to make up 5 to 80% by weight of the total particles. A higher proportion of inorganic particles results in poor adhesion of the electrode layer to the retardation layer. The mixture of inorganic particles with the binder is then suspended in an organic solvent and applied to an ion exchange membrane. The suspension can be applied by spraying, brushing, screen printing and the like to distribute the inorganic particles substantially over the entire retardation layer.

After the organic solvent has been removed, the composition is hot-pressed, preferably by means of a roller or press at 80 to 220°C under a pressure of 0.01 to 150 kg/cm2, to bond the layer to the membrane. Thereafter, an electrode layer can be hot-pressed onto the barrier layer under the same conditions.

The retardation layer has a thickness of 0.3 to 3.0 mils (0.0076 to 0.0762 mm), preferably 0.4 to 1.0 mils (0.0102 to 0.0254 mm).

The cation exchange membrane on which the porous non-electrode layer is formed may be made of a polymer having cation exchange groups such as carboxylic acid groups, sulfonic acid groups, phosphoric acid groups and phenolic hydroxy groups. Suitable polymers include copolymers of a vinyl monomer such as tetrafluoroethylene and chlorotrifluoroethylene and a perfluorovinyl monomer having an ion exchange group such as a sulfonic acid group, carboxylic acid group and phosphoric acid group, or a reactive group that can be converted into the ion exchange group. It is also possible to use a membrane made of a polymer of trifluoroethylene in which ion exchange groups such as sulfonic acid groups have been introduced or a polymer of styrene-divinylbenzene in which sulfonic acid groups have been introduced.

The cation exchange membrane is preferably made of a polymer having the following units. Mole percent

wherein X represents a fluorine, chlorine or hydrogen atom or -CF₃, X' represents X or CF₃(CH₂)m, m represents an integer from 1 to 5.

The typical examples of Y have the structures that bind A to fluorocarbon groups, such as

x, y and z represent an integer of 1 to 10, respectively, Z and Rf represent -F or a C₁-C₁₀ perfluoroalkyl group, and A represents -COOM or SO₃M or a functional group such as -CN, -COF, -COOR₁, -SO₂F and -CONR₂R₃ or -SO₂NR₂R₃, which is convertible into -COOM or SO₃M by hydrolysis or neutralization, and M represents hydrogen or an alkali metal atom, and R₁ represents a C₁-C₁₀ alkyl group.

A cation exchange membrane with an ion exchange group content of 0.5 to 4.0 milliequivalents/gram is preferred dry polymer, particularly from 0.8 to 2.0 milliequivalents/gram of dry polymer made from the copolymer.

In the cation exchange membrane made of a copolymer having the units (M) and (N), the proportion of units (N) is preferably in the range of 1 to 40 mol percent, preferably 3 to 25 mol percent.

The cation exchange membrane used in this invention is not limited to one made of only one kind of the polymer. It is possible to use a laminated membrane made of two kinds of the polymers having a lower ion exchange capacity in the cathode side, for example, having weakly acidic ion exchange groups such as carboxylic acid groups in the cathode side and strongly acidic ion exchange groups such as sulfonic acid groups in the anode side.

The cation exchange membrane used in the present invention can be prepared by blending a polyolefin such as polyethylene, polypropylene, preferably a fluorinated polymer such as polytetrafluoroethylene and a copolymer of ethylene and tetrafluoroethylene.

The electrode used in the present invention has a lower overvoltage than that of the material of the porous non-electrode layer bonded to the ion exchange membrane. Thus, the anode has a lower chlorine overvoltage than that of the porous layer on the anode side and the cathode has a lower hydrogen overvoltage than that of the barrier layer on the cathode side in the case of electrolysis of alkali metal chloride. The material of the electrode used depends on the material of the retardation layer bonded to the membrane.

The anode is usually made of a platinum group metal or a platinum group alloy, a conductive Platinum group metal oxide or a conductive reduced oxide thereof.

The cathode is typically a platinum group metal or alloy, a conductive platinum group metal oxide, or an iron group metal or alloy, or silver.

The platinum group metals are Pt, Rh, Ru, Pd, Ir. The cathode consists of iron, cobalt, nickel, Raney nickel, stabilized Raney nickel, stainless steel, stainless steel treated by etching with a base.

The preferred cathode materials for use with the retardation layer of the present invention are Ag and RuO2.

The polymers comprising the binders of the present invention also preferably have an equivalent weight within a certain desired range, namely from 550 to 1200. It is possible to adjust the polymer preparation steps to produce a polymer having an equivalent weight within the desired range. The equivalent weight is a function of the relative concentration of the reactants in the polymerization reaction.

The polymers comprising the binders of the present invention preferably have a melt viscosity within a certain desired range. It is possible to adjust the polymer preparation steps to produce a polymer having a melt viscosity within the desired range. The melt viscosity is based on the concentration of the initiator and the reaction temperature.

The polymer obtained by one of the above-mentioned processes is then hydrolyzed in a suitable basic solution to convert the non-ionic thermoplastic form of the polymer into the functional ionic form which The hydrolysis step is of particular importance in the process because during the hydrolysis step the non-functional polymer film is heated and reacted as shown below, during which process the polymer is softened and swollen with steam in a controlled manner. Incomplete hydrolysis results in covalently bonded functional groups whose lack of mobile ions results in isolating regions within the binder. The density of the hydrolysis solution is preferably between 1.26 and 1.28 grams per ml at room temperature. The hydrolysis process requires 2 moles of NaOH for each mole of functional group in the polymer as shown in the equation below:

-CF₂SO₂Z + 2NaOH → -CF₂SO₃Na + NaZ + H₂O

where Z is -I, -Br, -Cl, -F, -OR or -NR₁R₂,

R is a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical,

R1 and R2 are independently selected from the group consisting of -H, a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical, preferably phenyl or lower alkyl-substituted phenyl.

For hydrolysis, the copolymers are placed in the hydrolysis bath at room temperature with inert sieve materials that keep the copolymers in the liquid, ensuring that no bubbles are trapped on the films. The bath is then heated to 60ºC to 90ºC and kept at this temperature for at least four hours to ensure complete hydrolysis and expansion to the correct value.

After the hydrolysis step under heating, the bath is allowed to cool to room temperature and the polymers are then removed from the bath and rinsed with high purity deionized water, then placed in a bath of deionized water to wash out residual ionic substances.

When forming the retardation layer, a pore former can be used, which can be leached out after production. It is advantageous that the pores created by the pore former are connected to one another and extend from the membrane-electrode interface into the electrode-catholyte interface.

The present invention is further illustrated by certain examples and references which are given for illustrative purposes only and are not intended to limit the present invention.

example 1 Production of binders

This example demonstrates the preparation of a sulfone fluoropolymer with an equivalent weight of 794 and a low shear melt viscosity of 50,000 poise (dyne sec-cm-2) at 250ºC and 4.25 sec-1 and a 100ºC water absorption of 50 percent.

To a 132 L glass-lined reactor equipped with an anchor stirrer, H-baffle, platinum resistance temperature device, and a jacket for temperature control were charged 527 grams of ammonium perfluorooctanoate, 398.4 grams of Na2HPO4.7H2O, 328.8 grams of NaH2PO4.H2O, and 210.8 grams of (NH4)2S2O8. The reactor was then evacuated to 0.0 atmospheres as determined by electronic pressure reading, and then an inert gas (nitrogen) was added to pressurize the reactor to 448 kPa. This was done four times, after which the reactor was evacuated again. 99 liters of de-oxygenated deionized water were added, the stirrer was started and heat was applied to the jacket. The stirrer was set at 250 revolutions per minute (rpm) and then 15 ml of a terminating agent such as isopropyl alcohol was added followed by 16.65 kg of 2-fluorosulfonyl perfluoroethyl vinyl ether. Once the temperature reached 50ºC, the reactor was Tetrafluoroethylene (TFE) gas was fed at a rate of 0.5 to 0.567 kg per minute until a pressure of 1060 KPa was achieved over a period of 17 minutes. The addition was continued until a total of 18.18 kg of TFE had been fed to the reactor. At this time, the addition was stopped and then nitrogen was bubbled through the headspace of the system and room temperature water was introduced into the reactor jacket. The materials react with each other to form a latex. The latex was transferred to a larger vessel for separation and stripping of residual monomer. After the contents were allowed to settle, a bottom drain valve was opened to allow monomer to drain in a separate phase. The vessel was then heated and a vacuum was applied to remove any additional monomer components. A brine system then circulates brine at 20ºC through cooling coils in the vessel to freeze the latex, causing coagulation into large polymer agglomerates. After freezing was complete, the latex was allowed to thaw with gentle heating (room temperature water) and the latex was transferred to a centrifuge where it was filtered and washed several times with deionized water. The latex polymer cake was then dried overnight in a rotary cone dryer under vacuum (969 Pa) at 110ºC. The water content of the polymer was tested using Karl Fischer reagent and found to be 140 ppm. The isolated polymer was weighed and the weight was found to be 23.18 kg. The equivalent weight of the above polymer was found to be 794.

The binder can be produced either in thermoplastic form or in ionic form. To produce it in thermoplastic form, the dried polymer was dispersed in a suitable solvent and ground into a fine dispersion.

To prepare the polymer in ionic form, the polymer was then hydrolyzed in an approximately 25 wt.% NaOH solution. The density of the hydrolysis solution was between 1.26 and 1.28 grams per ml at room temperature. The hydrolysis process required 2 moles of NaOH for each mole of functional group in the polymer, as shown in the following equation:

-CF₂SO₂F + 2NaOH → -CF₂SO₃Na + NaF + H₂O

The polymers were placed in the hydrolysis bath at room temperature with inert sieving materials that kept the polymers in the liquid, ensuring that no trapped bubbles were present. The bath was then heated to 60ºC to 90ºC and then held at this temperature for at least four hours to ensure complete hydrolysis and expansion to the correct value.

After the heating hydrolysis step, the bath was allowed to cool to room temperature and the polymers were then removed from the bath and rinsed with high purity deionized water, then placed in a bath of deionized water to wash out residual ionic substances.

Example 2

A suspension of SiC particles and the copolymer of Example 1 was formed in Freon in a ratio of 70:30. The resulting layer had a twist/porosity ratio in the range of 5-50. The composition was sprayed onto a Dow high performance sulfone-carboxyl bilayer ion exchange membrane. After stripping off the solvent, the composition was hot pressed at 475ºF (246ºC) and (0.5-100 psi) 3.5-700 kPa to form a layer 0.4 mil (0.0102 mm) thick. A 1 mil (0.0254 mm) thick electrode layer of Ag/RuO2/binder (76 percent:10 percent:8 percent) was then sprayed onto the barrier layer. The entire assembly was then heated to 400ºF (204ºC) and pressed together at (0.5-100 psi) 3.5-700 kPa.

The electrode/retardation layer/membrane was then treated to obtain the final shape for electrolysis. This could involve electrolysis in a suitable solution to hydrolyze the membrane and/or the binder if either is in the thermoplastic form.

The resulting structure can be used as a membrane for a chlor-alkali electrolysis cell.

Claims (7)

1. A membrane electrode structure for use in an electrochemical cell having an ion exchange membrane with a cathode layer, characterized in that the membrane electrode structure also has a retardation layer between the membrane and the cathode layer, the retardation layer being a mixture of 5 to 80 weight percent inorganic solid particles with 20 to 95 weight percent of a thermoplastic ion-conductive polymer binder having a melting point of 230°F to 540°F (110 to 232°C), the inorganic solid particles containing at least one of the borides, carbides and nitrides of metals of Groups IIIB, IVA, IVB, VB and VIB of the Periodic Table, the binder consisting of a copolymer of a monomer of the general formula: CF₂=CZZ' (1), wherein Z and Z' are independently selected from the group consisting of -H, -Cl, -F or -CF₃, and at least one monomer selected from compounds represented by the general formula: Y-(CF₂)a-(CFRf)b-(CFRf)c-O-[CF(CF₂X)-CF₂-O-]n-CF=CF₂ (II) wherein Y is SO₂Z, Z is -I, -Br, -Cl, -F, -OR or -NR₁R₂; is, wherein R is a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical, R₁ and R₂ are independently selected from the group consisting of -H, a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical, wherein a is 0-6, b is 0-7, c is 0 or 1, provided that a+b+c is not equal to 0, X is -Cl, -Br, -F or mixtures thereof when n> 1, n is 0 to 6, and Rf and Rf are independently selected from the group consisting of -F, -Cl, perfluoroalkyl radicals having from 1 to 10 carbon atoms and fluorochloroalkyl radicals having from 1 to 10 carbon atoms. 2. The structure of claim 1, wherein the retardation layer has a thickness of 0.3 to 3 mils (0.0076 to 0.0762 mm). 3. The structure of claim 1, wherein the retardation layer has a porosity of 5 percent to 90 percent and a twist/porosity ratio in the range of 2-500. 4. The structure of claim 1, wherein the inorganic particles are silicon carbide. 5. The structure of claim 1, wherein the binder contains nonionic thermoplastic polymer material. 6. The structure of claim 1, wherein the cathode comprises a platinum group metal oxide and silver. 7. An electrochemical cell having an electrode-membrane structure, characterized in that the membrane-electrode structure comprises the structure of claim 1.