CATHODE BONDED TO ION EXCHANGE MEMBRANE FOR USE IN ELECTROLYZERS FOR ELECTROCHEMICAL PROCESSES AND RELEVANT METHOD FOR CONDUCTING ELECTROLYSIS

Abstract: A cathode constituted by a gas and liquid permeable layer bonded to a ion exchange membrane or diaphragm and comprising an electrolyte resistant binder, parti­ cles of electrocatalytic material and particles of an electrically conductive material having a higher hydrogen overvoltage than the electrocatalytic parti­ cles. Effective porosity is imparted to the layer of particles by means of a sacrificial, pore-forming agent and by leaching out such agent after the particles have been bonded together and the layer formed is in its desired thickness, preferably after it has been depos­ ited upon the diaphragm or the membrane.

Description: This invention relates to a cathode bonded to an ion exchange membrane or diaphragm, for use in electrolyzers for electrochemical processes, particularly for the electrolysis of chloride to generate chlorine and alkali hydroxide or water electrolysis to generate oxygen and hydrogen. It further relates to a method for generating chlorine or hydrogen. It is known to perform such processes in such electrolyzers with other electrodes. In order to achieve maximum production with a minimum consumption of electrolytic power it has been proposed to use electrolyzers wherein at least one electrode is bonded to one side of the membrane. The other electrode may be bonded to the other side of the membrane or may be pressed against such side or even spaced a short distance therefrom. Such electrolyzers and the relevant electrolysis process are described for example in U.S patent No. 4,224,121. Said patent describes a bonded electrode which comprises a porous coating on one side of the diaphragm, the coating comprising particles of an electrocatalytic material which is capable of functioning as an inert-to electrolyte electrode material at a relatively low overvoltage the particles being bonded together by a binder or polymer capable of resisting attack during use of the coating as an electrode for example in the above mentioned electrolytic processes. The coating is made porous so as to be permeable to electrolyte with which it comes in contact. Typical electrode particles used on the cathode side include platinum group metals and their electroconductive oxides. GB-A-20 87 433 discloses a cathode for electrolyzing alkali metal chloride brine in an electrolytic cell. The cathode consists of an electrically conductive substantially non-electrocatalytic material which is in contact with the cathodic surface of the membrane. The electrically conductive material is selected from group IB metals and corrosion resistent, electrically conductive compounds thereof. The cathode may further comprise an electrocatalyst which may be a group VIII transition metal having a lower hydrogen evolution overvoltage than the electrically conductive material. US-A-4,457,815 discloses a cathode for the electrolysis of aqueous ionic media. The cathode consists of a porous coating bonded to the membrane and comprises an electrocatalyst and a surface catalyst. The electrocatalyst has a significantly lower overvoltage for the intended reaction than the surface catalyst. The electrocatalyst includes the metals of group VIII of the periodic table and preferably has an enhanced surface area. The surface catalyst materials include insoluble metals, metal oxides, metal hydroxides, metal carbides, metal nitrides, metal borides, metal sulfides and mixtures thereof. The problem underlying the present invention is to provide an electrode, particularly a cathode, which has an improved operating voltage and a longer active live time, compared with conventional electrodes. According to the present invention, an electrode and more particularly a cathode is provided which exhibits a remarkably longer active lifetime compared with conventional cathodes and further allows for a lower cell voltage and an outstanding saving in the energy consumption. The invention relates to a cathode constituted by a gas and liquid permeable porous coating bonded to a ion exchange membrane, said cathode comprising low hydrogen overvoltage electrocatalytic particles , electrically conductive, corrosion resistant particles having a higher hydrogen overvoltage than said electrocatalytic particles and a binder resistant to electrolyte attack and suitable for cementing the particles and for bonding said coating to the membrane characterized in that the low hydrogen overvoltage electrocatalytic particles are ruthenium dioxide (RuO₂) or a mixed oxide of palladium and titanium (PdOTiO₂) and the higher hydrogen overvoltage electrically conductive particles are silver. Basically the amount of electroconductor is directed to maintaining or even increasing the electrical conductivity typical of the platinum group metal compounds, while lowering the noble metal load per unit area of electrode surface at which electrolysis takes place. The upper limit for the amount of electroconductor is given by the necessity to keep the hydrogen overvoltage of the mixtures below a certain threshold value. As a matter of fact, the maximum allowed hydrogen overvoltage of the mixture should be about 0.2 Volts in a 30-35% NaOH solution, at a temperature of 90 °C and at a cathode current density of 1000 Ampères per square meter of cathode surface. Conveniently, the cathode must be highly porous and permeable to allow for the electrolyte, e.g. the catholyte, flow therethrough so that the electrolysis reaction may take place when the electrolyte comes into contact with the exposed surface of the low overvoltage particles. Further, the cathode must exhibit a good electrical conductivity so that electric current, supplied by a current distributor which may be a screen, a wire mat or other conductor, may flow through the conductive particles contained in the cathode and be distributed to the electrocatalytic particles. According to one embodiment of this invention and in order to obtain the necessary porosity, the cathode initially contains a solid leachable material such as aluminum powder or flakes, water soluble inorganic salts or organic compounds, which may be in small crystals or even in needles or strands. After the mixture is bonded to one side of the membrane, the leachable material may be leached from the mixture to produce channels through which catholyte can move to contact the conductive, electrocatalytic particles and the evolved hydrogen can escape. A suitable binder, resistant to the aggressive cell environment, is used to obtain an adequate bonding. Preferred binders include processable polymers of organic monomers which on polymerization form a carbon chain and which have fluorine attached to the chain often to the substantial exclusion of other radicals or in any event as the preponderant radical attached thereto. Such materials include polymers of tetrafluoroethylene and/or chlorotrifluoroethylene and similar polymers which may also contain cation exchange groups. The mixture of the cathode components may be heated and fused or sintered to cement the particles together. Alternatively a solution or slurry or suspension of such polymer in a liquid may be mixed with the low overvoltage particles and the conductor particles and the mixture dried and treated to produce a self sustaining sheet or a suitable coating on the diaphragm. Where a separate sheet is produced the sheet may be bonded to the diaphragm in a second manufacturing step. The particles of the conductor as well as the particles of the low overvoltage material may be in any convenient shape or size which may be distributed throughout the binder to provide substantially uniform conductivity and overvoltage over the entire surface thereof from end to end or side to side. Conveniently the conductor as well as the low overvoltage material may be in the form of a powder. Alternatively either or both of the particles may be in the form of threads, wires, strands or the like having a length substantially greater than their cross section. The structure of the electrodes of the present invention, as well as the materials and the manufacturing procedure utilized for producing the same are illustrated in detail in the following description. THE MEMBRANE The ion exchange membrane or diaphragm, whereto the cathode is bonded, is constituted by a thin sheet of a hydrated cation exchange resin characterized in that it allows passage of positively charged ions and it minimizes passage of negative charged ions, for example Na+ and Cl- respectively. Two classes of such resins are particularly known and utilized; in the first one the ion exchange groups are constituted by hydrated sulphonic acid radicals attached to the polymer backbone or carbon-carbon chain, whereas in the second one the ion exchange groups are carboxylic radicals attached to such chain or backbone. As it is well known, the best preferred resins for industrial applications, (such as the electrolysis of alkali metal halides, alkali metal hydroxide due to their higher chemical resistance to the electrolytes, are obtained by utilizing fluorinated polymers. In industrial applications, when utilizing fluorinated cationic membranes, a higher electrical conductivity has been obtained by increasing the number of sulphonic or carboxylic radicals attached to the polymer backbone : these membranes, which permit reduction of the cell voltage, are defined as "low equivalent weight membrane". However, these membranes are strongly hydrated and architectonically opened and thus a remarkable and undesirable diffusive migration of catholyte, for example alkali hydroxides, from the cathode side to the anode side, may be experienced with the consequent reduction of the electrolysis current efficiency. An efficient inhibition of the catholyte migration, e.g. alkali hydroxide, is achieved by utilizing high equivalent weight membranes, that is membranes having a relatively small number of ion exchange groups attached to the polymer backbone. These membranes, however, exhibit a low electrical conductivity and cause a remarkable increase of the cell voltage. The above drawbacks have been overcome in industrial applications by combining the two types of membranes into a single membrane wherein the surface in contact with the catholyte, e.g. alkali hydroxide, in the cathode compartment, is constituted by a thin resin layer having high equivalent weight (for example a thickness of 50 µm) bonded to a thicker layer (for example having a thickness of 200 µm) constituted by low equivalent weight resin, in contact with the anolyte (for example alkali metal halide) in the anode compartment. Said bilayer membranes, when used in conventional cells of the state of the art (e.g. the so-called zero-gap system wherein the electrode is in contact with the membrane, and the so-called finite-gap cells wherein the electrode is spaced from the membrane) must exhibit a sufficient mechanical resistance: This may be obtained by inserting inside the membrane a reinforced fabric, by dispersing fibers of a suitable length inside the polymer or by a combination of both. Further, the membrane surface may be coated by a thin layer of hydrophilic material, such as metal oxides, e.g. SiO2, TiO2, ZrO2, in order to avoid or reduce adhesion to its surface by gas bubbles, especially hydrogen gas bubbles evolved in the course of the electrolysis. Ion exchange membranes exhibiting the above mentioned characteristics are produced by Du Pont under the trade mark of Nafion(R) (e.g. Nafion 954, 961) and by Asahi Glass under the trade mark of Flemion(R) (e.g. Flemion 783). The use of at least one electrode bonded to a cation exchange membrane permits use of other types of membranes with respect to conventional membranes. The membranes which may be utilized are characterized by absence of the hydrophilic layer, whose role is efficiently played by the electrode bonded to the membrane absence of reinforcing fabric or dispersed fibers and consequently reduced overall thickness, as the electrode bonded to the membrane provides for a high mechanical resistance. The development of a reliable, industrially applicable technology for bonding at least one electrode to a cation exchange membranes allows to utilize low cost and low voltage drop membranes, which turns out in an appreciable energy saving, as it will be clearly illustrated in the following examples. Suitable membranes are produced by Du Pont, for example bilayer membranes type NX10119, having an overall thickness of 150 µm. Diaphragms of other constructions including those having coatings of other construction or composition as part of the diaphragm structure may be used in the electrolytic process of this invention. THE ELECTRODES As previously stated, the electrode advantageously comprises a porous layer of low hydrogen overvoltage particles, conductor particles, strands or the like to improve or maintain conductivity and the binder to bond together the conductor and low hydrogen overvoltage material to produce porous layer electrodes. To insure adequate porosity, a leachable pore-forming material is added and leached out after the layer has been formed or deposited. The components of the mixture utilized for producing the electrodes are characterized as follows: the binder is constituted by a resin resistant to the electrolyte attack and at least partially compatible with the material constituting the ion exchange membrane. Suitable binders are constituted by polytetrafluoroethyelene particles. The preferred formulation is an aqueous solution, or emulsion or suspension of such particles. Similar results have been obtained by utilizing Du Pont (Teflon® T-30) and Montefluos-Italia (Algoflon® D-60) products which are both constituted by very thin particles of polytetrafluoroethylene in the range of 0.1-1 µm stabilized in an aqueous medium, by adding suitable dispersing agents. It is believed that appreciable results could be obtained also with other fluorinated polymers particles, for example copolymers of tetrafluoroethylene-hexafluoropropene, polyvinyldenfluoride, polyvinylfluoride, polytetrafluoroethylene containing ionic ion exchange groups attached to the polymer backbone, such as sulphonic radicals or carboxylic radicals. the conductor particles are finely divided usually substantially spheroidal and have the following characteristics : Type Preparation Method or availability Granulometry (µm) Specific area (BET) Silver reduction by NaBH₄ 1 1 m2/g Silver commercial (Johnson & Matthey) 1 1 m2/g The conductor serves to maintain and more advantageously to improve the overall electroconductivity of the electrode. Thus the conductor particles have a surface exposed to contact with the low overvoltage particles (i.e. the electrocatalyst) which surface is highly electroconductive, For example the conductor (silver particles) has a substantially greater electroconductivity than ruthenium oxide or like platinum group oxide. Consequently silver serves to improve the overall electroconductivity of the electrode layer. According to an embodiment of the present invention, a very thin and fine conductive metal screen, for example having a mesh number higher than 50, is utilized as current conductor. For example, a nickel or preferably a silver screen may be pressed against the ion exchange membrane, whereto a coating constituted by a mixture of a fluorinated binder, low hydrogen overvoltage electrocatalytic components and leachable components (for example aluminum powder), has been previously applied. The membrane-coating-conductive screen assembly is then subjected to heating, under pressure, for carrying out the sintering treatment, as illustrated hereinafter, and then to a leaching treatment. In a further embodiment, the conductive screen may optionally be coated by a metal or a metal compound belonging to the platinum group, or by a compound such a Raney nickel or the like. The low overvoltage material may include materials such as listed in the following table : The RuO2 samples thus obtained have been subjected to X-rays diffraction. The samples obtained by the Adams method show only the typical rutile, RuO2, spectrum, while the samples obtained by thermal decomposition appear to be constituted by a mixture of RuO2 and a second component which is isomorphous with K2RuC16. The content of this second component decreases by increasing the decomposition temperature and is practically nil with a decomposition temperature of 700°C. The most suitable decomposition temperature appears to be about 600°C, as at higher temperatures the electrocatalytic activity degree is exceedingly low, while at lower temperatures the coating, when operated as cathode, tends to loose ruthenium as a consequence of both mechanical and electrochemical actions, which is clearly unacceptable. Illustrative data are reported in Example 6. In a further embodiment of the present invention, the conductor, in the form of powder, strands, wires or the like, may be coated by a thin film of electrocatalytic material having low hydrogen overvoltage. For example, the silver particles may be coated according to conventional techniques, such as electroless or galvanic deposition in a fluidized bath, by metals belonging to the platinum group or precursors alloys of Raney nickel or similar materials. The coated particles may be used alone or, according to an embodiment of the present invention, in admixture with uncoated particles of a conductive material in a suitable ratio. Samples of cathodes bonded to an ion exchange membrane have been prepared utilizing, as the low hydrogen overvoltage component, Raney nickel (produced by Carlo Erba - Italy) instead of compounds of metals belonging to the platinum group. The relevant data are reported in Example 8. The leachable component is constituted by commercial aluminum powder (e.g. produced by Merck, average diameter : 125 µm), previously subjected to surface oxidation utilizing diluted nitric acid. Different materials, other than aluminum powders, may be utilized provided that they are easily leachable. Suitable materials are for example zinc powder, tin powder, alkali metal salts (such as carbonates, sulphates, chlorides). In the specific case of alkali metal salts, it is obviously necessary to adapt the fabrication process by resorting to formulations based on dry powders. Interesting results have been obtained by utilizing said alternative materials, as illustrated in the following description. THE PREPARATION PROCESS The above described components have been utilized for producing the electrodes according to one of the following procedures, illustrated hereinafter by resorting to practical examples . PROCEDURE A The first step consists in preparing a coagulum or paste containing the various components (e.g polytetrafluoroethylene, RuO2, a metal more electroconductive than RuO2 such as silver, and a porosity promoter such as aluminum) in the desired ratio. A suspension of 0.7 g of Algoflon D60® produced by Montedison are added to the mixture containing 3 g of silver powder , 0.8 g of RuO2 powder and 0.65 gr. of aluminum powder. The aluminum powder is previously oxidized by using diluted nitric acid. The compound is then homogenized and isopropylic alcohol is added thereto, under suitable stirring. The coagulum (high viscosity phase) is separated from the liquid phase and then applied as a thin film over an aluminum sheet, previously oxidized by means of diluted nitric acid. After drying at 105°C, sintering is carried out at 325°C for ten minutes. The aluminum sheet, coated by the sintered film, is then applied onto the cathode side of a Du Pont NX 10119, 140 x 140 mm, membrane, at 175°C under a pressure comprised between 50, 65 bar and 60, 78 bar (50 and 60 kg/cm2) for 5 minutes. The membrane is then immersed in 15% sodium hydroxide for two hours at 25°C, in order to completely dissolve the aluminum sheet and the aluminum powder utilized as porosity promoter. PROCEDURE B The first step of this alternative procedure consists in preparing a paint having a lower viscosity than the above mentioned coagulum of PROCEDURE A and containing the various components (for example, polytetrafluoroethylene, RuO2, silver and aluminum) in the desired ratios. For this purpose, a suspension of 0.7 g of Algoflon D60® (Montefluos), previously diluted, is added to the mixture containing 3 g of silver, 0.8 g of RuO2, 0.65 g of aluminum powder, previously oxidated by means of diluted nitric acid. After homogenization, 5 grams of methylcellulose or other equivalent material such as cellulose derivates (acetate, ethylate etc.) glucose, lactic and pyruric acid etc. are added to the compound in order to avoid coagulation and to obtain a liquid of sufficient viscosity as to be applied like a paint. Said liquid is then applied, by brushing or by other equivalent technique, onto an aluminum sheet previously oxidated by diluted nitric acid. The operation is repeated until the desired amount of the noble metal is obtained. Then, sintering is carried out in oven at 340°C for 1 hour. The pre-formed sheet thus obtained is then bonded onto the cathodic surface of the membrane at 20,26-81,04 bar (20-80 kg/cm2), preferably 40,52-50,65 bar (40-50 kg/cm2) at 175°C. After pressing and mechanically removing the aluminum sheet, the membrane is subjected to alkali leaching treatment in a 15% sodium hydroxide solution for 12-24 hours up to complete solubilization and extraction of the pore-forming agent. PROCEDURE C In this third alternative, a suspension of polytetrafluoroethylene, previously diluted is utilized. For example, a Du Pont Teflon® T-30 suspension is diluted with distilled water in order to obtain a final content of 0.1 grams of polytetrafluoroethylene per milliliter (ml) of liquid. Then, 4 ml of this diluted suspension are added to 200 ml of distilled water and heated until boiling. An amount of 1.5 grams of a low overvoltage material is then added to the boiling diluted polytetrafluoroethylene solution. The electrocatalytic material and the polytetrafluoroethylene coagulate and are separated from the liquid phase through filtering. The filtered coagulum, after drying, is mechanically crushed, broken up and then mixed with about 500 grams of finely powdered solid carbon dioxide. The homogenized mixture is then applied in a uniform layer onto a tantalum sheet. The solid carbon dioxide is sublimated through infrared irradiation and the residue, applied in a uniform layer onto the tantalum sheet, is sintered at 300-340°C, preferably at 310-330°C, for ten minutes. The sintered film is finally applied onto the cathode side of a Du Pont Nafion NX 10119 membrane, under a pressure of 101.3 bar (100 kg/cm2), at 175°C for about 5 minutes. The samples obtained by the above procedures have been subjected to different tests; however, it has to be understood that the present invention is not intended to be limited to these specific examples, since various modifications of both the instrumentality employed and the steps of the process may be introduced and fall within the scope of the invention. EXAMPLE 1 Various samples of a coating of the present invention and consisting of silver and polytetrafluoroethylene, bonded to a Du Pont NX 10119 membrane, were prepared according to the afore described procedure A. The tests were aimed to verify the electrical resistivity variations over the coating as a function of the ratio between silver and polytetrafluoroethyelene. The following components were utilized : commercial silver powder (Johnson & Matthey) having an average diameter of the spheroidal particles of 1 µm and a specific surface (BET) of 1 m2/g, in a quantity sufficient to obtain a load of 100 g per square meter of membrane surface. polytetrafluoroethylene (Du Pont Teflon T-30) suspension in a quantity sufficient to obtain the following percentages by weight of the final coating bonded to the ion exchange membrane : 15 - 35 - 40%, which correspond to 35 - 60 - 70% by volume respectively. aluminum powder (Merck Co.) having an average diameter of 125 microns, and previously oxidized by means of diluted nitric acid, in a weight ratio of 1.5 with respect to the polytetrafluoroethylene weight. The electrical resistivity of the coating was determined by the four-heads system, the two central heads (connected to a high impedance voltmeter) having a contact surface of 1 x 10 mm and a distance of 10 mm apart. The resistivity (IR) values, reported in Table 1, are accordingly conventionally indicated in ohm/cm. A PTFE content lower than 15% produces a mechanically unstable coating. The lowest electrical resistivity values of the coating bonded to the membrane allow for improved current distribution and reduced cell voltage. Therefore, the following examples are referred to coatings which, after leaching of the porosity promoter, exhibit a content of PTFE of 10-20% by weight. EXAMPLE 2 Various samples of a coating, containing only a conductor and PTFE particles, bonded to the cathode side of a Nafion® NX 10119 membrane, were prepared. After leaching the aluminum powder, the coating exhibited an average content of 10-20% by weight of PTFE. The initial content of aluminum powder before leaching was in a ratio of 1.5 with respect to the PTFE weight. The electrical resistivity of each sample was detected following the same procedure described in Example 1 and the relevant data are reported in Table 2. The above data show that the coating resistivity is not only a function of the electrical conductivity of the conductor by it is especially a function of the contact resistivity among the various component particles, depending on the nature and thickness of the superficial oxide film formed at each particle surface. Similar results were obtained with coating prepared following the afore described procedures B and C. EXAMPLES 3 The same samples of Example 2 were subjected to various tests for establishing their resistance to chemical corrosion, which tests consisted in immersion in a sodium hydroxide solution containing hypochlorite (2 g/l as active chlorine) at ambient temperature, for two hours. These tests were aimed to verify the behaviours of the various coating samples under the same conditions which prevail during shut-down of industrial electrolyzers. The electrical resistivity (IR) of each coating sample was detected both before and after each test and after subsequent cathodic polarization in 30% sodium hydroxide. The relevant data are reported in TABLE 3. The above data clearly indicate that the coating based on silver and WC are suitable for industrial applications. In particular, silver undergoes surface corrosion with formation of a chloride or basic chloride film, as the increased electrical resistivity indicates. Under cathodic polarization (as it would occur under real conditions, during start-up operations after a shut-down) this film is re-converted into metal, the electrical resistivity thus returning to the low initial values. WC is completely inert but the observed higher electrical resistivity values clearly indicate that its use in industrial applications would involve a penalty in the cell voltage. The samples utilizing nickel or copper particles as conductors are subject to irreversible deterioration due to the action of active chlorine. Actually, also a prolonged cathodic polarization proves useless and the initial electrical resistivity values cannot be restored. EXAMPLE 4 A series of coating samples containing, besides the conductors (silver, nickel, WC), also varying quantities of RuO2 powder as a low hydrogen overvoltage compound of metal belonging to the platinum group (obtained by the Adams method), were prepared following the aforementioned procedure A. The coating was characterized by an average content of PTFE of 10-20% by weight (determined after leaching the aluminum powder, used as porosity promoter, in a ratio 1.5 times the weight of the PTFE). For comparison purposes, various samples based only on RuO2 and PdOTiO2 were prepared without adding any electrical conductor. Furthermore, two samples, based on platinum black and PTFE, were prepared according to the teachings of U.S. Patent 4,224,121 and utilized as conventional reference electrodes. More particularly these two samples were prepared by following the procedures shown in the above patent at page 10 (lines 38-68) and page 11 (lines 1-31) as summarized hereinbelow : platinum salt in the form of chloride is mixed with an excess of sodium nitrate or equivalent alkali metal salt and the final mixture is fused in a silica dish at 500-600°C for 3 hours. The residue is washed throughly to remove the nitrates and halides. The resulting aqueous suspension of oxides is reduced at room temperature by using an electrochemical technique, or, alternatively, by bubbling hydrogen through it. The product is dried thoroughly, ground, and sieved through a nylon mesh screen. Usually, after sieving the particles have an average 4 µm diameter. Finally the metal powder is blended with the graphite-Teflon(R) mixture. For all of the samples, a cation exchange membrane Du Pont NX 10119 was utilized. The 140 x 140 mm electrode samples were utilized as cathodes in laboratory cells, under the following conditions : anode : titanium expanded sheet having a thickness of 0.5 mm, diamond dimensions 2 x 4 mm and 140 x 140 mm as projected area, activated by a catalytic coating of RuO2-TiO2, obtained by conventional thermal decomposition technique. cathode : electrode bonded to membrane prepared as illustrated in Example 3, abutting against a current distributor constituted by 25 mesh nickel fabric having a wire thickness of 0.2 mm. A resilient compressible nickel wire mat was disposed between the nickel fabric and the electrode samples and exerted pressure, as illustrated in U.S. Patent 4,343,690 - 4,340,452 anolyte : brine containing 220 g/l NaCl at 90°C catholyte : 33% sodium hydroxide at 90°C current density : 3 kA/m2 The initial voltage values and those after 30 days of operation are reported in Table 4. The above results clearly show that : when silver is utilized as the conductor : a load of 10 gr/m2 of RuO2 or platinum black is sufficient to ensure an improved cell voltage, 0.2 V lower than the voltage obtained by utilizing silver alone. when utilizing nickel as the conductor : an increased cell voltage with respect to silver, 0.1 to 0.12 higher, is detected even if silver is added, confirming thus the important role played by the electrical resistivity of the coating, which has to be as low as possible. when utilizing WC as the conductor : the cell voltage is increased by about 0.15 volts with respect to silver, which constitutes a further confirmation of the importance of the coating electrical resistivity. when utilizing RuO2 alone or PdOTiO2 alone : without silver the cell voltage results increased by about 0.1 V even if higher loads of noble metals (for example 200 gr/m2) are introduced. The electrical resistivity of coatings based uniquely on RuO2 or on PdOTiO2 appeared to fall in the range of 5-10 ohm/cm. when utilizing coatings based on mixtures of conductors and platinum group metal compounds : the same cell voltages are obtained as with conventional coatings of the art but a lower load of noble metal per square meter is required. In the particular case of RuO2 - silver and of platinum black-silver mixtures, a 0.1 volt, lower cell voltage is measured utilizing a noble metal load of 10-20 g/m2 (minimum load required according to the state of the art technique : 40-80 g/m2) samples prepared according to the state of the art technique, (last two items of Table 4), for comparison purposes : soon after 30 days of operation an initial detaching of the coating from the membrane is experienced. The coating samples according to the present invention resulted unimpaired. EXAMPLE 5 Coating samples were prepared varying the aluminum powder content, the content of silver (150 g/m2), RuO2 (40 g/m2 by the Adams method) and PTFE (10% of the final weight detected after leaching the aluminum powder) being the same. These tests were aimed to ascertain the role played by the coating porosity. All of the samples were prepared following the aforementioned procedure B. The samples were tested under the same electrolysis conditions as described in Example 4. The results are reported in the following Table 5. The above data clearly show that the optimum weight ratio between aluminum and PTFE is 1.5. Below this ratio, the porosity is unsufficient to grant a complete exploitation of the RuO2 due to lower active area and lower mass transfer of both reagents and products through the catalytic layer, while higher ratios tend to provide for less mechanically stable structures and for an increased electrical resistivity (0.08 ohm cm versus 0.05 ohm/cm) EXAMPLE 6 Coating samples were prepared in order to determine the effect of different types of RuO2 on the cell voltage. All of the samples were prepared following the aforementioned procedure B and utilizing the following quantities of material : RuO2 40 g/m2 Silver 150 g/m2 PTFE 15% of the final coating weight aluminum powder 1.5 times the PTFE weight Du Pont Nafion 10119 membranes were utilized. The following RuO2 types were utilized : RuO2 obtained by the Adams method RuO2 obtained by thermal decomposition at 500°C, consisting of a mixture for 50% of rutile RuO2 and 50% of a compound which is isomorphous with K2RuCl6 (determined by X-rays diffraction) RuO2 obtained by thermal decomposition at 600°C and consisting of a mixture for 70% of rutile RuO2 and 30% of said isomorphous compound. RuO2 obtained by thermal decomposition at 700°C, consisting 100% of rutile RuO2. RuO2 obtained by chemical oxidation at 40°C, via the hydrogen peroxide route, of commercial Ru metal powder RuO2 obtained by thermal decomposition at 450°C, in presence of hydroxylamine as oxidizing controlling agent, consisting of a mixture for 35% of rutile RuO2 and 65% of a compound isomorphous with K2RuCl6 All the above RuO2 types, after preparation, were submitted to the final crushing in order to obtain the product in a desired powder form (1 µm). The coating samples were tested under the same electrolysis conditions as illustrated in Example 4. The relevant data are reported in Table 6. The above data demonstrate that RuO2 obtained by thermal decomposition is noticeably more catalytic than the types obtained by the Adams and chemical methods, notwithstanding its lower specific surface (1.5 m2/g versus 80 m2/g). The failure of the samples prepared at 500°C (thermal method) was due to a non complete oxidation of the precursor ruthenium salt (RuCL3.3H2O, type) to the desired final product (RuO2). The failure of the sample prepared by the chemical method was attributed to the surface oxidation of the metallic Ruthenium powder which is unstable in concentrated caustic solutions in the presence of active chlorine diffusing through the membrane from the anode to the cathode side during shut down conditions. The surprising better behaviour of the sample prepared at low temperature (450°C), in respect to the previous one obtained at 500°C, is ascribed to the role played by NH2OH which leads to the complete oxidation of the ruthenium salt more effectively than oxygen gas. EXAMPLE 7 Various samples, prepared following the aforementioned procedure A and containing silver (150 g/m2), RuO2 (by the Adams method - 30 g/m2), PTFE (15% of the final coating weight, after leaching the aluminum powder utilized in a ratio of 1.5 parts for each part of PTFE), were tested under the same electrolysis conditions illustrated in Example 4, but for the alkali metal concentration and current density. The most characterizing data are reported in the following Table 7. The above data clearly show that the cathodes of the present invention can undergo high current densities without any mechanical damage and further provide for an efficient performance also when in contact with remarkably concentrated sodium hydroxide solution, wich are forbidden in the conventional zero-gap, narrow gap or finite gap cells. This unexpected behaviour may be ascribed to the particular nature of the cathodes bonded to ion exchange membranes described in the present invention. These cathodes in fact are characterized in a porous, capillary internal structure wherein the evolution of hydrogen gas bubbles inside the pores and the release of said bubbles towards the aqueous sodium hydroxide solution completely eliminate the concentration polarization phenomena, which are typical of the other conventional processes. EXAMPLE 8 Various samples of cathodes bonded on an ion exchange Nafion® NX 10119 membrane, were prepared following procedure A utilizing the most advantageous ratios but substituting the electrocatalytic platinum group metals compounds with Raney nickel, produced by Carlo Erba, Italy. These samples were characterized by PTFE (Algoflon® D60 - Montefluos, Italy) : 15% by weight aluminum powder : 1.5 parts for each PTFE part After leaching the aluminum powder, the samples were tested under the same electrolysis conditions illustrated in Example 4. The relevant data are reported in the following Table 8. The above results clearly indicated that silver, which substantially reduces the coating resistivity, allows for a more efficient exploitation of the low hydrogen overvoltage electrocatalysts, not only of those based on the platinum group metals. These last ones, however, are the most preferred, compared with electrocatalyst based on Raney nickel or similar compounds, for their higher resistance to active chlorine attack (during shut down operations) and to poisoning by iron or heavy metal traces, which may be contained in the sodium hydroxide. EXAMPLE 9 Four cathodes, identified as samples A, A′ and samples B, B′, bonded to a Dupont Nafion (R) NX 10119 membrane, were prepare according to "procedure B". The final coating compositions, after leaching the aluminum powder, were as follows : The samples, 140 x 140 mm, were operated, initially for 15 days, in commercially pure catholytes and subsequently , again for the same period of time, in contaminated catholytes containing impurities such as iron or mercury compounds. The working conditions and the electrochemical performance of the above samples are reported in Table 9. From these experimental results it can be concluded that : metallic platinum and ruthenium dioxide behave quite similarly in commercially pure electrolyte ruthenium dioxide performs better than metallic platinum in contaminated catholyte. EXAMPLE 10 A series of samples having varying thicknesses of the coating, bonded to a bilayer ion exchange membrane 150 µm thick, were prepared following procedure B. The following materials were utilized : RuO2 (Adams method) in a quantity equal to 18% of the final coating weight PTFE (Algoflon D60 - Montefluos, Italy) 10% of the final coating weight commercial silver (Johnson & Matthey) 72% of the final coating weight aluminum powder ( Merck Co.) in a ratio of 1.5 parts for each PTFE part. The samples were tested under the same electrolysis conditions as in Example 4 and the relevant results are reported in the following Table 10. The above results show that the same performances or even better ones are obtained with very thin coatings and thus with lower silver loads and particularly with lower noble metal loads per square meter of membrane surface. In any case the coating composition and process for preparing said samples are to be maintained within the most preferred conditions already defined in the preceding examples. EXAMPLE 11 Various cathodes bonded to three different types of membranes were prepared according to procedure B. The final coating composition, after leaching the aluminum powder, was as follows : RuO2 : 12 g/m2 silver : 50 g/m2 PTFE : 8 g/m2 The following membrane types were utilized : Du Pont Nafion® 902 bilayer sulphocarboxylic, reinforced membrane having a thickness of 250 microns Du Pont Nafion® NX10119 bilayer sulphocarboxylic, unreinforced membrane having a thickness of 150 µm experimental, bilayer sulphocarboxylic unreinforced membrane, having a thickness of 80 µm experimental, bilayer, carboxylic, unreinforced membrane, having a thickness of 90 µm The samples, 140 x 140 mm, were tested under the same electrolysis conditions illustrated in Example 4. The relevant data are reported in the following Table 11. As expected, the reinforced membrane, whose utilization is unavoidable in conventional electrolyzer, utilizing the zero-gap, narrow gap or finite gap technology, provide for higher voltages, due to the higher thickness and to the presence of internal reinforcement (fabric or dispersed fibers). The possibility to utilize unreinforced membranes, which are characterized by remarkably lower voltages, is particularly advantageous for the technology based on bonding of the electrodes, in particularly cathodes, of the present invention. In fact, the electrode bonded to the membrane represents an efficient reinforcement which provides for mechanical stability and easy handling of the membrane otherwise bound to being ruptured under mechanical stresses during operation (pressure pulsations, pressure differentials between anode and cathode compartments). This surprising result constitutes one of the substantial innovative steps of the present invention. EXAMPLE 12 Various cathodes bonded to the bilayer sulphocarboxylic membrane Dupont, Nafion(R) NX 10119, 150 µm thickness, were prepared according to "Procedure B" but, instead of a single layer, two layers, one subsequent to the other, were applied on the membrane. The first layer, directly contacting the membrane and composed by silver, represented the barrier layer for keeping down to low values the hydrogen gas and caustic soda back diffusion through the membrane from the cathode side to the anode one ; the second layer, separated from the membrane by the previous one and composed by ruthenium dioxide and silver in a proper ratio represented the electrocatalytic coating for the hydrogen evolution. The final composition of the coatings after leaching the aluminum powder present in both layers, was as follows : As expected, the presence of a barrier layer between the membrane and the electrocatalytic coating improves the performance of the cathode bonded system. EXAMPLE 13 A cathode, prepared according to procedure A, was bonded to a 130 µm thick, anion exchange membrane (Asahi Glass, Selemoin (R), CMV/CMR type) The coating composition, after leaching the aluminum powder utilized in a ratio of 1.5 part for each PTFE part, was as follows : RuO2 : 12 g/m2 silver : 50 g/m2 PTFE (Algoflon® D60 - Montefluos, Italy) : 8 g/m2 The sample, 100 x 1000 mm, was tested for water electrolysis, under the following conditions : anode : nickel expanded sheet - 0.5 mm thick, diamond dimensions 2x4 mm membrane-cathode assembly in contact with the anode and pressed thereto by a resilient compressible nickel wire mat current distributor : 25 mesh nickel fabric (wire thickness 0.2 mm) interposed between the cathode bonded to the membrane and the nickel mat. anolyte and catholyte : 25% KOH at 80°C current density : 3 KA/m2 For comparison purposes, a similar cell was provided with an un-bonded cathode constituted by an expanded nickel sheet having a thickness of 0.5 mm and activated by galvanic coating constituted by nickel containing RuO2 particles dispersed therein. The voltage detected with the bonded cathode was 1.9 V, while the voltage detected with the un-bonded cathode was 2.05 V. EXAMPLE 14 A cathode, prepared according to Procedure A, was bonded to a sulphonic - 200 µm thickness - cation exchange membrane, Dupont Nafion (R) 120. The coating composition, after leaching the aluminum powder utilized in a ration of 1.5 parts for each PTFE part, was as follows : - RuO2 : 12 g/m2 - Ag : 50 g/m2 - PTFE : 8 g/m2 (suspension of Algoflon® D60 - Montefluos Italy) The sample, 100 x 1000 mm, was tested for water electrolysis under the conditions described above in Example 13. In addition, the electrolytic cell was equipped with a chamber for mixing the degased anolyte and the catholyte together, in order to counterbalance the polarization of concentration created by the cationic membrane and to allow for feeding the anodic and cathodic compartments with the same electrolytes. A similar cell was provided with an un-bonded cathode constituted by an expanded nickel sheet having a thickness of 0.5 mm and activated by galvanic coating constituted by nickel containing RuO2 particles dispersed therein. The voltage detected with the bonded cathode was 1.96V, whereas the one with the un-bonded cathode was 2.11V.

Claims: A cathode constituted by a gas and liquid permeable porous coating bonded to a ion exchange membrane, said cathode comprising low hydrogen overvoltage electrocatalytic particles , electrically conductive, corrosion resistant particles having a higher hydrogen overvoltage than said electrocatalytic particles and a binder resistant to electrolyte attack and suitable for cementing the particles and for bonding said coating to the membrane characterized in that the low hydrogen overvoltage electrocatalytic particles are ruthenium dioxide (RuO₂) or a mixed oxide of palladium and titanium (PdOTiO₂) and the higher hydrogen overvoltage electrically conductive particles are silver. The cathode of claim 1 characterized in that the coating has a surface resistivity lower than 0.1 ohm/cm. The cathode of claim 1 characterized in that the hydrogen overvoltage of the electrocatalytic layer is lower than 0.2 volts at 1000 Ampere/m2. The cathode of anyone of the preceeding claims characterized in that the binder is polytetrafluoroethylene contained in a ratio of 10-20% of the coating weight. The cathode of anyone of the preceeding claims characterized in that the ruthenium dioxide (RuO2) is obtained by thermal decomposition in the presence of air at 600°C for two hours. The cathode of claim 4 or 5 characterized in that the coating has the following composition by weight : RuO2 18% - silver 72% PTFE 10%. The cathode of claim 6 characterized in that the the silver load is comprised between 50 and 75 g/m2. The cathode according to any of the preceding claims characterized in that it utilizes an unreinforced ion exchange membrane. The cathode of claim 8 characterized in that the membrane thickness is 150 µm. Method of generating chlorine, which comprises electrolyzing aqueous alkali chloride in a cell having oppositely charged electrodes separated by an ion exchange membrane substantially impervious to electrolyte flow characterized in that the cathode of the cell has a structure as defined in claims 1 to 9 and is bonded to said ion exchange membrane. Method of generating hydrogen by means of a water electrolysis process, which comprises electrolyzing aqueous alkali metal hydroxide in a cell having oppositely charged electrodes separated by an ion exchange membrane substantially impervious to electrolyte flow characterized in that the cathode in said cell has a structure as defined in claims 1 to 9 and is bonded to said ion exchange membrane. A precursor of a cathode according to anyone of claims 1 to 9 which additionally comprises particles of a pore-forming leachable agent.

Inventors: MARTELLI, GIAN NICOLA, NIDOLA, ANTONIO
Applicants: DE NORA PERMELEC S.P.A.
Application year: 1987
Main classification: C25B11
IPC codes: C25B9