AU6885498A - Electrolysis of alkali metal halide brines using oxygen cathode systems - Google Patents

Description

WO 98/45503 PCT/US98/06769 ELECTROLYSIS OF ALKALI METAL HALIDE BRINES USING OXYGEN CATHODE SYSTEMS This invention relates to a process for the electrolysis of an alkali metal halide brine in an electrolytic cell, more particularly, to the use of oxygen cathode membrane 5 assemblies for chlor-alkali electrolytic cells and to the use of flow field structures adjacent thereto. Chlor-alkali electrolytic cells electrochemically convert alkali metal halide brine solutions into halogen gas and alkali metal hydroxides. One type of chlor-alkali electrolytic cell typified by U.S Patent No. 4,191,618 to Coker et al. ("the '618 patent") 10 employs a membrane electrode assembly ("MEA") including a membrane and an oxygen depolarized cathode attached thereto such that a unitary structure is formed. The membrane itself serves as an electrolyte. The cathode is an electroconductive catalytic material bonded to the membrane. The catalyst layer is preferably covered by a porous hydrophobic layer to deter the formation of a water film over said electrode which might prevent the penetration of 15 oxygen to the electrocatalyst. While the '618 patent was an improvement over the prior art, improvements of only 0.5 to 0.6 volts were reported, probably in part due to the buildup of caustic material at the electrode. More recently, U.S. Patent 4,919,791 to Miles et al. ("the '791 patent") disclosed the use of a hydrophilic porous electrode to permit water transported through the 20 membrane to flow through the cathode, to aid in the removal of the caustic material formed at the cathode. While the flow of water facilitates the removal of caustic material it can hinder the ability of the oxygen to reach the cathode. Accordingly, there still exists a need for oxygen cathode MEAs suitable for use in chlor-alkali applications which permit the ready removal of caustic material produced at the 25 electrode and yet also permit good contact of an oxygen-containing gas with the electrode. One aspect of the present invention is a process for the electrolysis of an alkali metal halide brine in an electrolytic cell having an anode compartment having at least one anode, a cathode compartment having at least one cathode, and a membrane in contact with the cathode (thereby forming a membrane electrode assembly) which separates the anode 30 compartment from the cathode compartment. The method comprises (a) feeding the alkali metal halide brine to the anode compartment; (b) electrolyzing the alkali halide brine to 1 WO 98/45503 PCT/US98/06769 produce a halogen gas and alkali metal ions; (c) passing the alkali metal ions and water through the membrane into the cathode compartment; and (d) feeding an oxygen-containing gas to the cathode compartment so that the oxygen is reduced at the cathode and a concentrated alkali metal hydroxide solution is produced. The process is further characterized 5 by having a flow field located adjacent to the cathode to facilitate transporting the oxygen gas to the cathode and alkali metal hydroxide away from the cathode and in that the flow field comprises an electrically conductive material having a porosity of at least 30 percent and a mean pore size of at least 10 microns. Another aspect of the present invention is a process for the electrolysis of an 10 alkali metal halide brine in an electrolytic cell wherein the membrane comprises at least one active layer on the cathode side thereof, and wherein the active layer comprises (a) catalytically active particles and (b) an ionomer having an equivalent weight in the range of from 650 to 950 and which is substantially insoluble in water at a temperature of less than 100 0 C. 15 Another aspect of the invention is a process for the electrolysis of an alkali metal halide brine in an electrolytic cell where the membrane-cathode assembly further includes a flow field adjacent to the active layer which comprises a layer of an electrically conductive porous material which has at least two portions with different mean pore sizes, wherein a first portion of the layer adjacent to the membrane electrode assembly has a porosity 20 no greater than the second portion of the layer adjacent to the opposite side from the membrane electrode assembly; and wherein the second portion has a porosity of at least 30 percent; and the second portion has mean pore size which is at least 4 microns and at least two times greater than the mean pore size of the first portion. Still another aspect of this invention is a process for the electrolysis of an alkali 25 metal halide brine in an electrolytic cell wherein the cathode comprises at least two layers of catalytically-active particle containing ink on one side of the membrane, wherein at least two layers of the catalytically-active particle containing ink comprise polytetrafluroethylene polymers having pendant sulfonic acid groups, equivalent weights which differ by more than 50. 2 WO 98/45503 PCT/US98/06769 Figure 1 illustrates one embodiment of the membrane electrode assembly and the flow field useful in the electrolytic cell of the first aspect of the invention. Figure 2 illustrates one configuration of a repeat unit which may be utilized in the preparation of an electrolytic cell stack containing a plurality of electrolytic cells arranged 5 in series, which incorporates the membrane electrode assembly and flow field illustrated in Figure 1. Figure 3 illustrates a membrane electrode assembly having two active layers positioned on the same side of the membrane. Figure 4 illustrates a membrane electrode assembly having a porous layer and a 10 flow field adjacent thereto. Figures 5 and 6 depict the performance of the cells prepared in Examples 1 and 2. Figure 7 shows the experimental setup described in Example 1. In one aspect, this invention is a process for the electrolysis of an alkali metal 15 halide brine in an electrolytic cell having a membrane electrode assembly and a flow field adjacent thereto wherein the flow field comprises an electrically conductive porous material having a porosity of at least 50 percent and a mean pore size of at least 35 microns. It has been discovered that the electrolytic cell of the first aspect of the invention is able to operate at relatively high current densities and relatively low cell voltages 20 at lower gas flow rates. Typically, in an electrolytic cell, the membrane and the layer of polymer containing metal catalytically-active particles ("active layer") must be hydrated in order to be sufficiently ionically conductive. During operation of the electrolytic cell, water passes from the anode compartment through the membrane to the cathode. Water may also be present due 25 to the humidification of the oxygen-containing feed gas. However, if too much water condenses or otherwise accumulates adjacent to the active layer or within the active layer, the efficiency of the electrolytic cell is reduced, since diffusion of gas through liquid water is slow relative to its diffusion through water vapor. The porosity and pore size characteristics of the flow field in the electrolytic 30 cell of the first aspect of the invention are believed to improve the mass transfer capabilities, 3 WO 98/455U03 PCT/US98/06769 which results in lower cell voltages at high current densities. It is believed, without intending to be bound by any particular theory, that the relatively high porosity and large pores combine to preserve effective gas transport in the presence of liquid water. Because the feed gas flow is in the plane of the flow field and substantially parallel to the active layer, the liquid is swept 5 away from the active layer and out of the flow field by the gas stream, thus keeping pores open for effective transport of reactant gas to the catalytically-active particles. However, when the flow field is relatively thick, (for example, greater than 20 mils (0.51 millimeters) for air feed stoichiometry equal to 2 at 30 psig) (207 kPa) the gas velocity becomes inadequate to keep the pores clear of water. In such cases, increasing the wettability of the flow field is 10 believed to promote a type of flow condition therein which may be referred to as annular flow, wherein the liquid spreads out onto the solid surfaces of the porous structure, leaving the centers of the large pores open and available for effective gas transport. In a second aspect, this invention is a process for the electrolysis of an alkali metal halide brine in an electrolytic cell having a membrane electrode assembly which 15 comprises a solid polymer electrolyte, and at least two active layers positioned on at least the cathode side of the membrane; wherein the active layers comprise catalytically-active particles and an ionomer; the average equivalent weights of the ionomers in the layers differ by at least 50. One or more additional active layers may be positioned between the first and second active layers. 20 In a third aspect, this invention is a process for the electrolysis of an alkali metal halide brine in an electrolytic cell having a membrane electrode assembly having a solid polymer electrolyte, and at least one active layer positioned on one side of the membrane; wherein the active layer comprises (a) catalytically-active particles, and (b) an ionomer having an equivalent weight in the range of from 650 to 950 and which is substantially insoluble in 25 water at temperatures of less than 100oC. It has been discovered that the membrane electrode assemblies ("MEAs") of the second and third aspects of the invention when utilized in an electrolytic cell, provide a relatively low cell voltage at a given current density and gas flow rate. The equivalent weight of the ionomer is believed to affect the water content of 30 the active layer. It is believed, without intending to be bound by any particular theory, that the lower equivalent weight ionomer maintains a higher water content at low current densities. 4 wu ,ai/4a: PCT/US98/06769 This higher water content improves the conductivity and the accessibility of the catalytically active particles, thereby decreasing the cell voltage. However, this increase in water content can lower the performance (increase cell voltage) at higher current densities. It has been discovered that the performance at both high and low current densities may be optimized by 5 using a multi-layer active layer with a different equivalent weight ionomer in each layer. It is believed, without intending to be bound by any particular theory, that the improved performance results from the differences in hydrophilicity between the layers. The lower equivalent weight ionomer is believed to provide an area within the MEA having a high water content, for better performance at low current densities, while the less hydrophilic higher 10 equivalent weight ionomer helps transport water away from the membrane at higher current densities. In the third aspect of the invention, the use of a relatively low equivalent weight ionomer gives better performance at lower current densities. In a fourth aspect, this invention is a process for the electrolysis of an alkali metal halide brine in an electrolytic cell having a membrane electrode assembly and a layer of 15 an electrically conductive porous material adjacent thereto which has at least two portions with different mean pore sizes, wherein a first portion of the layer adjacent to the membrane electrode assembly has a porosity no greater than a second portion of the layer adjacent to the opposite side of the layer; the second portion has a porosity of at least 82 percent; and the second portion has a mean pore size which is at least 10 microns and at least ten times greater 20 than the mean pore size of the first portion. In a fifth aspect, this invention is a process for the electrolysis of an alkali metal halide brine in an electrolytic cell having a membrane electrode assembly with a nonwoven porous layer of an electrically conductive porous material adjacent thereto which has at least two portions with different mean pore sizes, wherein a first portion of the layer 25 adjacent to the membrane electrode assembly has a porosity no greater than a second portion of the layer adjacent to the opposite side of the layer; the second portion has a porosity of at least 50 percent; and the second portion has a mean pore size which is at least 35 microns and at least ten times greater than the mean pore size of the first portion. It has been discovered that the electrolytic cells of the fourth and fifth aspects 30 of the invention are able to operate at a high current density at a relatively low cell voltage, even when operated under relatively low gas pressures. 5 WO 98/45503 PCT/US98/06769 These and other advantages of these inventions will be apparent from the description which follows. In general terms the present invention is a process for the electrolysis of an alkali metal halide brine in an electrolytic cell having an anode compartment having at least 5 one anode, a cathode compartment having at least one cathode, and a membrane in contact with the cathode (thereby forming a membrane electrode assembly or MEA) which separates the anode compartment from the cathode compartment. The method comprises (a) feeding the alkali metal halide brine to the anode compartment; (b) electrolyzing the alkali halide brine to produce a halogen gas and alkali metal ions; (c) passing the alkali metal ions and water 10 through the membrane into the cathode compartment; and (d) feeding an oxygen-containing gas to the cathode compartment so that the oxygen is reduced at the cathode and a concentrated alkali metal hydroxide solution is produced. The process preferably further includes humidifying the oxygen-containing gas prior to feeding it into the cathode compartment. This can be done by bubbling the gas through water. The cathode can operate 15 at any temperature from ambient to the boiling point of the alkali metal hydroxide solution, and in general, higher temperatures are preferred. The temperature of the water bubbler should exceed the cell temperature by at least 10 0 C. When the water bubbler temperature exceeds the cell temperature, the water vapor content of the oxidant gas is increased beyond the equilibrium content at the cell temperature. It is believed that in this situation water vapor 20 will condense on the cathode solubilizing the alkali metal hydroxide, thereby increasing the removal rate of the alkali metal hydroxide and minimizing the possibility that alkali metal hydroxide will crystallize within the porous structure of the active layer or backing layer. The operating pressure of the system can be raised to any point desired to help control water evaporation, but is preferably less than about 75 psig (517 kPa). 25 Referring now to Figure 1 and Figure 4, the term "membrane electrode assembly" (1) as used herein refers to the combination of the solid polymer electrolyte (also referred to herein as a "membrane") and catalytically-active particles in the electrolytic cell assembly, regardless of its configuration or method of preparation. The layer of membrane material containing such particles are referred to as the "active layer", regardless of whether 30 such particles are incorporated into a discrete layer of polymer (2) and applied or laminated to the surface of the membrane (3), or incorporated into the membrane itself. Referring now to Figure 1, the flow field (4) is a layer of an electrically conductive porous material having a gas 6 WO 98/45503 PCT/US98/06769 stream inlet and outlet connected thereto. The flow field may comprise porous carbon material. The electrolytic cell for use in the first aspect of the invention preferably contains no impermeable flow field components having engraved, milled, or molded flow channels configured across its entire active face. These channels deliver gases directly to the active 5 layer through a porous carbon "backing layer" which supports the active layer, as illustrated in Figures 1 and 4 of U.S. Patent 5,108,849. However, the flow field, bipolar plates, and/or end plates used as supports for the electrolytic cell and to separate cells in a multi-cell configuration may contain one or more ducts therein to increase the flow of reactant gases to the flow field in the electrolytic cell of the first aspect of the invention. An example of such 10 ducts is illustrated in Figure 2. Examples of suitable porous carbon materials which may be utilized as the flow field in the first aspect of the invention include carbon paper, graphite paper, carbon felts, or other carbon-based composites which comprise at least 20 percent by weight of carbon. The flow field may have interdigitated channels cut into it to lower the pressure drop 15 introduced into the reactant gases. When desirable, the porous carbon material may be treated with a perfluorosilane or fluorine composition to increase its hydrophobicity, or oxidized, sulfonated, or coated with a hydrophilic material to increase its hydrophilicity. The conductivity of the flow field layer in the first aspect of the invention is preferably at least 0.01 Siemens/cm (S/cm), more preferably at least 0.1 S/cm, and most 20 preferably at least 0.2 S/cm. The preferred thickness of the flow field will depend on the optimal pressure drop across the flow field, but is preferably at least 1 mil( 0.02 mm), more preferably at least 5 mils (0.13 mm), and is most preferably at least 10 mils (0.25 mm); but is preferably no greater than 250 mils (6.35 mm), more preferably no greater than 100 mils (2.54 mm), and is most preferably no greater than 50 mils (1.27 mm). The porosity of the flow field 25 is preferably at least 75 percent, more preferably at least 80 percent. The mean pore size of the flow field is preferably at least 45 microns, more preferably at least 50 microns; but is preferably no greater than 1000 microns, more preferably no greater than 250 microns. The term "mean pore size" as used herein means that half the open volume of the material is contained in pores larger in diameter than the mean pore size, and half is contained in pores 30 equal to or smaller than the mean pore size. The mean pore size may be measured by any convenient method, such as by mercury porosimetry. The device used to measure the mean 7 WO 98/45503 PCT/US98/06769 pore size distribution of the layer may be calibrated using silica/alumina calibration standards (available from Micromeritics, Norcross, GA). All of the MEAs described herein may be prepared by any suitable technique unless otherwise noted. In one technique, a layer of a catalyst "ink" is first applied to a solid 5 polymer electrolyte, a carbon fiber paper, or a release substrate. Catalyst inks typically comprise catalytically-active particles (such as platinum supported on carbon), binder, solvent or dispersing aid and, optionally, a plasticizing agent. Preferably, the ink comprises catalytically-active particles, and at least one compound which functions as an ionomer, such as a polytetrafluoroethylene polymer having sulfonic acid groups and an equivalent weight 10 (based on the acid groups) in the range of from 650 to 1400. The ink also preferably contains an organic solvent or dispersing aid which permits the application of a thin, uniform layer of the catalyst/ionomer mixture to the solid polymer electrolyte, carbon fiber paper, or release substrate. To prepare the MEA useful in the second aspect of the invention, a layer of 15 catalyst "ink" is first applied to a solid polymer electrolyte, and then a layer of a second and third ink is applied to the portion of the MEA located opposite the first active layer or to a release substrate, or on top of the first active layer. The term "active layer" as used herein refers to a layer comprising a mixture of ionomer and catalytically-active particles. The MEA useful in the electrolytic cell of the first aspect of the invention may 20 be prepared by any suitable method, but is preferably prepared by applying the catalyst ink (a suspension or dispersion of the catalytically-active particles) directly to a solid polymer electrolyte as described, for example, in U.S. Patent 5,211,984. The ink is applied to the membrane in one or more applications sufficient to give a desired loading of catalytically active particles. Preferably, the layer of catalytically-active particles is prepared by applying 25 at least two inks in separate steps to form layers of the different inks. In such cases, the membrane electrode assembly comprises a solid polymer membrane having at least two layers of catalyst ink on at least one side thereof, wherein at least two layers of the catalyst ink comprise polytetrafluoroethylene polymers having pendant sulfonic acid groups, the equivalent weights of which differ by more than 50. Once prepared, the MEA is positioned 30 next to the flow field in the electrolytic celLassembly. 8 WO 98/45503 PCT/US98/06769 The electrolytic cells described herein may be incorporated in a multi-cell assembly or "stack" comprising a number of electrolytic cells arranged in series. An example of a repeat unit is illustrated in Figure 2, which shows a macroporous anode support (5), an MEA (6), a cathode flow field (7), and a bipolar separator plate (8). The bipolar separator 5 plate has ducts (9) and (10) which transport the reactants and reaction products to and from the flow field. In this configuration, the porous cathode flow field has an inert material impregnated into the border regions (the darker areas in the figure) in order to prevent the reactants and products from escaping to the outside. The border of the macroporous anode support should be solid or filled so it can deter or prevent leaks. The holes in the border 10 regions of all of the elements together form a gas manifold when they are stacked together and placed under compression. The material used to prepare the bipolar plate separator may be selected from a variety of rigid or nonrigid materials, and the plate has gas delivery ducts molded or embossed into its surface. These ducts deliver feed gas to, and remove reaction products from, the porous flow fields. In an alternative embodiment, gases and products may 15 be introduced or removed via ducts or open spaces in the porous flow field connected to manifolds. The bipolar separator plate may also have an internal structure for circulating a cooling fluid therein. If the ink from which the second active layer is prepared, is applied on top of the first active layer, the first active layer is preferably first dried sufficiently before 20 application of the second ink to prevent too much mixing of the inks. However, a minor degree of mixing of the inks at their point of contact with each other may be desirable since it will promote electrical and ionic conductivity between the active layers. After the inks have been applied, they are preferably heated under conditions sufficient to volatilize at least 95 percent of any organic solvent or dispersing aid present in the inks. 25 The term "solid polymer electrolyte" as used herein refers to a porous layer comprised of a solid polymer which has a conductivity of at least 1 x 10 -3 Siemens/cm (S/cm) under the operating conditions of the electrolytic cell, or which may be reacted with acid or base to generate a porous layer having such conductivity. Preferably, the solid polymer electrolyte comprises a film of a sulfonated fluoropolymer, or a layered composite of films of 30 sulfonated fluoropolymers having different equivalent weights. In chlor-alkali cells sulfonated membranes are preferred when the catholyte caustic strength is less than 20 percent. Above 20 percent caustic, however, it is generally preferred to use a bi-layer 9 WO 98/45503 PCT/US98/06769 membrane: sulfonated polymer adjacent to the anolyte; carboxylated polymer adjacent to the catholyte. The carboxylated polymer has a higher electrical resistance but greater hydroxyl ion rejection. Therefore it is preferred that the carboxylated polymer layer is normally thinner than the sulfonated polymer layer in the bi-layer configuration. 5 After application of a catalyst ink to a solid polymer electrolyte, the ink is preferably heated under conditions sufficient to remove enough of the organic solvent or dispersing aid so that the active layer comprises at least 99 percent by weight, more preferably at least 99.9 percent by weight of the mixture of catalytically-active particles and the ionomer. The ink is applied in an amount sufficient to provide a layer of the mixture which has a 10 thickness of at least 1 pm, more preferably at least 5 pm, and most preferably at least 10 pm; but is preferably no greater than 30 pm. The porosity of the layer is preferably at least 30 percent, more preferably at least 50 percent; but is preferably no greater than 90 percent, more preferably no greater than 60 percent. The mean pore size of the layer is preferably at least 0.01 pm, more preferably at least 0.03 pm; but is preferably no greater than 10 pm, more 15 preferably no greater than 0.5 pm, and is most preferably 0.1 pm. The thickness, porosity, and pore size characteristics referred to above refer to measurements taken when the ionomer(s) contained in the layer are in their dry and protonated form. Thereafter, the components of the MEAs useful in the second and third aspects of the invention are assembled by positioning one of the active layers in contact with the solid 20 polymer electrolyte, and then positioning the second and third active layer so that it is between the first active layer and the porous carbon material, forming the membrane/electrode assembly thereby. The term "catalytically-active particles" as used herein refers to particles of a metal or compound which are catalytic for the electroreduction of oxygen under the pressure 25 and temperature conditions in the electrolytic cell. Examples of such particles which are useful include particles of platinum, ruthenium, gold, palladium, rhodium, iridium, electroconductive and reduced oxides thereof, and alloys of such materials, either in combination with each other or with other transition metals. Other possible catalysts include but are not limited to: silver, manganese oxide, transition metal centered macrocyclic 30 molecules such as cobalt porphryn and cobalt phthalocyanine, and transition metal macrocyclic molecules heated in nitrogen to 700oC. The particles may be supported on a 10 WO 98/45503 PCT/US98/06769 suitable material, if desired, such as carbon black. Preferably, the catalytically-active particles are platinum particles supported on carbon, which preferably contain from 10 percent to 30 percent by weight of platinum. The size of the catalytically-active particles (on an unsupported basis) is preferably at least 10 A, more preferably at least 20 A; but is preferably 5 no greater than 500 A, more preferably no greater than 200 A. Larger size particles may also be utilized, or may form during cell operation by the agglomeration of smaller particles. However, the use of such particles may result in decreased cell performance. The catalytically-active particles are preferably used in an amount sufficient to provide an optimum catalytic effect under the operating conditions of the electrochemical 10 device in which they are employed. Preferably, they are utilized in an amount sufficient to provide a loading level on the cathode side of the membrane of at least 0.01 mg/cm 2 , more preferably at least 0.1 mg/cm 2 , and is most preferably at least 0.15 mg/cm 2 ; but is preferably 2 2 no greater than 0.5 mg/cm 2 , more preferably no greater than 0.35 mg/cm 2 , and is most 2 preferably no greater than 0.25 mg/cm 2 . Catalysts such as silver, manganese oxide, transition 15 metal centered macrocyclic molecules such as cobalt porphryn and cobalt phthalocyanine, and transition metal macrocyclic molecules heated in nitrogen to 700 0 C, may optimally be used in an amount sufficient to provide a loading level of up to as much as 5 mg/cm 2 . Examples of suitable organic compounds for use in the preparation of the catalyst ink include polar solvents such as glycerin, Cl- 6 alcohols, and other compounds such 20 as ethylene carbonate, propylene carbonate, butylene carbonate, ethylene carbamate, propylene carbamate, butylene carbamate, acetone, acetonitrile, difluorobenzene, and sulfolane, but is most preferably propylene carbonate. The organic compound is preferably present in an amount, based on the weight of the composition, of at least 10 percent, more preferably at least 20 percent, and is most preferably at least 30 percent; but is preferably no 25 greater than 90 percent. Such solvents in the ink function primarily as solvents or dispersing aids. Suitable ionomers for use in the preparation of the catalyst inks described herein include any polymer or oligomer having an ionic conductivity of at least 1 x 10-3 S/cm, more preferably at least 10 - 1 S/cm (under the operating conditions of the electrolytic cell), or 30 which may be reacted with acid or base to generate an oligomer or polymer having ionic conductivity. 11 WO 98/45503 PCT/US98/06769 Examples of suitable ionomers include fluoropolymers having pendant ion exchange groups, such as sulfonic acid groups in proton or salt form. Examples of such include sulfonic fluoropolymers having fluoropolymer backbones and 1 to 5 carbon atom pendent ion exchange groups attached thereto and terminating in a sulfonyl group are suitable 5 for use in the present invention. Examples of such sulfonic ion exchange group fluoropolymers are illustrated, for example, in U.S. Patents 4,578,512; 4,554,112; 4,515,989; 4,478,695; 4,470,889; 4,462,877; 4,417,969; 4,358,545; 4,358,412; 4,337,211; 4,337,137; and 4,330,654. Preferably, the ionomer has a substantially fluorinated polymer backbone and a 10 recurring pendent group having the formula: -O-(CFR)a-(CFR')b-SO 3 M (I) wherein: a and b are independently 0 or an integer of 1 to 3; a + b is at least 1; R and R' are independently selected from halogen, perfluoroalkyl, and fluorochloroalkyl; and M is independently selected from hydrogen or an alkali metal. 15 Other ionomers useful in forming both thick and thin composite membrane layers are characterized by a substantially fluorinated polymer backbone and a recurring pendent group having the formula: O-(CFR)a-(CFR')b-O-(CF 2 )c-SO 3 M (II) wherein: a and b are independently 0 or an integer of 1 to 3; c is an integer of 1 20 to 3; a + b is at least 1; R and R' are independently selected from perfluoroalkyl, halogen, and fluorochloroalkyl; and M is independently selected from hydrogen or an alkali metal. Ionomers having the above formulas are disclosed in U.S. Patents 4,478,695; 4,417,969; 4,358,545; 4,940,525; 3,282,875; and 4,329,435. The ionomer is preferably present in an amount, based on the weight of the composition, of at least 0.5 percent but 25 preferably no more than 5 percent. The ionomer may be utilized in any ionic form, such as the proton form or salt form of the corresponding oligomer or polymer. Examples 'of salt forms include quaternary ammonium, sodium, lithium, and potassium. For most chlor-alkali applications it is preferred that the ionomer be used in the sodium salt form. In the electrolytic cells useful in second aspect of the invention, the ionomers 30 used to prepare the inks preferably have an equivalent weight, based on the number of pendant 12 WO 98/45503 PCT/US98/06769 ionic groups per molecule, of at least 600, more preferably at least 700, and preferably no greater than 1200, more preferably no greater than 950. However, the ionomer must also be substantially insoluble in water at temperatures below 100oC; therefore, the minimum equivalent weight for certain fluoropolymers may be higher. The term "substantially insoluble 5 in water" as used herein means that the pure ionomer in the ionic form is at least 75 percent insoluble in distilled water at any concentration. The difference between the equivalent weight of the ionomers in at least two of the inks used to prepare the MEA is preferably at least 50, more preferably at least 100, and is most preferably at least 300; but is preferably no greater than 800, more preferably no greater than 600, and is most preferably no greater than 10 400. In the electrolytic cells useful in the second and third aspects of the invention, the ionomer used to prepare the ink preferably has an equivalent weight of at least 650, more preferably at least 700, and is most preferably at least 770; but is preferably no greater than 950, more preferably no greater than 900, and is most preferably no greater than 840. The equivalent weight of the ionomer may be determined by any suitable technique, such as 15 titration with a base, as illustrated in U.S. Patent 4,940,525. Referring now to Figure 3, which illustrates the membrane electrode assembly useful in the second aspect of the invention, a membrane (11) is shown, having two active layers positioned on the cathode side of the membrane. The active layer closest to the membrane (13) contains ionomers having a different average equivalent weight than the active 20 layer positioned adjacent thereto (15). If desired, the ionomer with the lowest average equivalent weight can be closest to the membrane. Referring now to Figure 4, which illustrates the membrane electrode assembly useful in the fourth and fifth aspects of the invention, the porous layer (16) is a layer of an electrically conductive porous material having at least two portions with different mean pore 25 sizes and is located between the active layer and the flow field. The flow field (17) may comprise a machined graphite plate, or may be primarily comprised of a thicker layer of porous carbon material as described, for example, in U.S. Patent 5,252,410. However, the porous layer (16) does not contain any catalysts which are typically present in the active layer, such as platinum. 30 The electrolytic cells usefuLin the fourth and fifth aspects of the invention contain a layer of an electrically conductive porous material (hereafter, "intermediate layer") 13 WO 98/45503 PCT/US98/06769 which is adjacent to the membrane electrode assembly and has at least two portions with different mean pore sizes. The portion of the layer adjacent to the membrane electrode assembly (18) (hereafter, "small pore region") has a mean pore size which is at least ten times smaller than the portion of the layer adjacent to the opposite side of the layer (19) (hereafter, 5 "large pore region"). Compositions suitable for use in the preparation of the intermediate layer include any organic or inorganic composition which can be fabricated into a solid layer having the porosity and pore size characteristics referred to above, and which also has sufficient dimensional, hydrolytic and oxidative stability under the operating conditions of the electrolysis cell. One method of preparing an intermediate layer having asymmetrical pore 10 size characteristics is to prepare such a layer from two or more materials having different mean pore sizes. An example of such a method is to first obtain or prepare a material having a mean pore size suitable for the large pore region (hereafter, "large pore material"), and then infiltrate and/or coat one side of the material with a composition which will reduce the porosity of a portion of the material sufficiently to obtain the smallest desired pore size, and/or 15 form a discrete layer of the composition on the outside of the material having the desired small pore characteristics. Typically, in an electrolytic cell, the membrane and the layer of polymer containing a metal catalyst ("active layer") must be hydrated in order to be sufficiently ionically conductive. During operation of the electrolytic cell, water passes through the 20 membrane and through cathode and is removed via the adjacent flow field. Water vapor is also preferably present to humidify the oxygen-containing feed gas. However, if too much water accumulates through condensation or other means adjacent to the active layer or within the active layer, the efficiency of the electrolytic cell is reduced, since diffusion of gas through liquid water is slow relative to its diffusion through water vapor. 25 It is believed, without intending to be bound, that the small pore region of the layer reduces the accumulation of excess liquid water in or next to the active layer because it serves as a semi-permeable layer or membrane which permits the water vapor generated within the active layer or present due to the humidification of the reactant gases to pass between the active layer and the flow field, but reduces or prevents condensation of water on 30 the active layer and reduces or prevents the liquid water present in the flow field or large pore region of the intermediate layer from passing back through the small pore region to the active layer. Preferably, the wettability (determined by the pore size and water-solid contact angle) 14 WO 98/45503 PCT/US98/06769 of the small pore region is such that for a sufficiently large fraction of the pores the displacement pressure required to force liquid water into these pores is larger than the hydraulic pressure in the flow field components under the prevailing condition of pressure and temperature in the electrolytic cell. 5 Examples of suitable organic compositions which may be used to prepare or infiltrate the large pore material include thermoplastic or thermosetting polymeric and oligomeric materials, such as polytetrafluoroethylenes, including those which have sulfonic acid groups, (such as NafionTM, available from DuPont), poly(alkylene oxide)s, polyolefins, polycarbonates, benzocyclobutanes, perfluorocyclobutanes, polyvinyl alcohols, and 10 polystyrene, epoxy resins, perfluoroalkyl/acrylic copolymers, polyanilines, polypyrroles, as well as mixtures thereof. Preferably, the composition is a polytetrafluoroethylene, perfluoroalkyl/acrylic copolymer, or a perfluorocyclobutane, and is most preferably a perfluorocyclobutane. Examples of suitable inorganic compositions which may be used include silver or nickel metal, nickel oxide, and titanium-based compositions. 15 The composition used to prepare the small pore region of the intermediate layer preferably contains polymer, carbon particles, and a suitable carrier. The carrier will typically infiltrate the entire large pore material, although the majority of the polymer and carbon particles will collect on or close to the surface of the side of the material to which it is applied (depending on its porosity and the size of the particles contained in the composition), thereby 20 forming the small pore region on the side of the material to which the composition is applied. Accordingly, the regions or portions of the intermediate layer having different mean pore sizes are not necessarily discrete layers, so long as at least the first 1 micron of depth of the small pore region and at least the first 50 microns of depth of the large pore region (as measured from the surface of the layer in a direction perpendicular to the layer) has the necessary pore 25 characteristics. The intermediate layer may also be prepared by applying the composition used in the preparation of the small pore region to the membrane electrode assembly, and then positioning or laminating a layer of a large pore material adjacent thereto. Alternatively, a film of the composition used in the preparation of the small pore region may be prepared 30 separately using conventional film manufacturing techniques, and then positioned or laminated between the membrane electrode assembly and the large pore material. If the 15 WO 98/45503 PCT/US98/06769 composition is applied to the MEA, it may be applied using any suitable coating technique, such as by painting or silk-screening. The small pore region of the intermediate layer is preferably at least as hydrophobic as the active layer. The composition used to prepare the small pore region is 5 preferably a liquid-based composition which will solidify after application. If the composition which is applied is solvent-based, enough of the solvent is removed to form a solid layer of material prior to assembling the electrolytic cell. Such solvent may be removed either at ambient conditions or at elevated temperatures. If appropriate, the composition is heated to increase its stability and uniformity, such as by crosslinking, molecular weight advancement, 10 or agglomerating latex particles. If the composition used to prepare the small pore region is to be applied directly to the membrane electrode assembly, the majority of the dissolved solids contained therein (such as the polymer) are preferably hydrophilic in character, since the membrane and active layer are normally prepared from a hydrophilic composition, and application of a 15 solution of a primarily hydrophobic dissolved solids would normally be expected to adversely affect the properties of the active layer. However, the composition used to prepare the small pore region is still preferably hydrophobic after being cured. Hydrophobic fillers, such as carbon fibers and/or powders treated with hydrophobic compositions such as silane- and fluorine-based compositions, may be used in 20 the compositions that are used to prepare the small pore region to give it some hydrophobic character and affect the wettability of its pores, as well as increase the porosity and mean pore size of the solidified composition. In such cases, the weight ratio of carbon fibers or powders to the other components in the composition is preferably at least 1:1, more preferably at least 3:1; and preferably no greater than 10:1, more preferably no greater than 5:1; and is most 25 preferably 3:1. If the small pore region is prepared by applying the composition to a large pore material, such as a graphite paper, the relatively fine pore structure of the paper will help keep the majority of fillers in the composition close to the surface on the side of the paper to which it is applied. Alternatively, the composition may be one which is primarily hydrophilic as applied, but hydrophobic upon curing, such as a polytetrafluoroethylene latex. If the small 30 pore region is prepared by applying a hydrophilic composition to a large pore material, a thin coating of a highly hydrophobic material such as ZonylTM 7040, a perfluoroalkyl acrylic 16 WO 98/45503 PCT/US98/06769 copolymer available from DuPont, may be applied to the side of the small pore region facing the MEA to further increase its hydrophobicity. Other examples of highly hydrophobic materials include FluoradTM FC 722 and FC 724, available from 3M. The MEA suitable for use in the fourth and fifth aspects of the invention are 5 preferably prepared by applying the catalyst ink (a suspension or dispersion of the catalyst) directly to the membrane as described, for example, in U.S. Patent 5,211,984. If the catalyst is to be applied to a porous carbon material, the composition used to prepare the small pore region is preferably applied first, followed by the catalyst ink, so that the infiltrated porous carbon material may be utilized as an intermediate layer, as well as a support layer for the 10 catalyst. However, this method, as well as any methods which require the preparation of a separate film for the intermediate layer are less preferred since such films and catalyst containing structures must typically be laminated to the membrane portion of the membrane electrode assembly in order to assemble the electrolytic cell. Such lamination processes, wherein heat and/or excessive pressure is applied to the intermediate layer, may alter or 15 damage its pore structure. Further, the composition of the intermediate layer may be formulated to minimize the voltage at which the electrolytic cell will operate at a given current density. It is believed that lower cell voltages at higher current density require the small pore region to be more hydrophobic than at lower current densities. For example, if a lower cell voltage at a 20 lower current density is desired, compositions having a higher carbon/polymer ratio (such as 5:1) are preferred for use in the preparation of the small pore region, particularly when applied to a graphite paper having a relatively low porosity. Likewise, if lower cell voltages at higher current densities are preferred, lower carbon/polymer ratios (such as 3:1) are preferred, particularly when applied to a graphite paper having a relatively high porosity. 25 The small pore region preferably has a thickness in the range of from 1 micron to 150 microns (as measured in a direction perpendicular to the intermediate layer), and has the desired porosity and pore size characteristics. More preferably, the region has a thickness in the range of from 5 to 25 microns. Preferably, the portion of the region adjacent to the MEA is sufficiently porous to permit the transmission of water vapor through the region. The 30 porosity of this portion of the region is preferably at least 10 percent. The mean pore size of the small pore region is preferably at least 0.1 micron, more preferably at least 1 micron; but is 17 WO 98/45503 PCT/US98/06769 preferably no greater than 10 microns. The mean pore size may be measured by any convenient method, such as by mercury porosimetry. The device used to measure the mean pore size distribution of the layer may be calibrated using silica/alumina calibration standards (available from Micrometrics, Norcross, GA). 5 The porosity of the small pore region is preferably at least 10 percent. Conductive fillers and non-conductive inert or fugitive fillers may be incorporated into the composition to achieve the desired pore structure. Intrinsically conductive polymers such as doped polyaniline or polypyrrole may also be used to prepare the composition in order to increase its conductivity. The pore structure of the small pore region may also be controlled 10 to some extent by the selection of the polymer or the use of an oligomeric composition. The large pore region preferably has a thickness of at least 2 mils (0.05 mm), more preferably at least 6 mils (0.15 mm); but is preferably no greater than 50 mils (1.27 mm). The porosity of this region is preferably at least 82 percent, more preferably at least 85 percent, and most preferably at least 87.5 percent. The mean pore size of the large pore region 15 is preferably at least 30 microns. The porosity and pore size values given above represent the characteristics of the small pore region for at least the first micron of its depth from the side of the intermediate layer next to the MEA and at least the first 50 microns of its depth from the opposite side of the intermediate layer, regardless of its method of preparation. Examples of suitable porous carbon materials which may be utilized as the 20 large pore material include carbon paper, graphite paper, carbon felts, or other carbon-based composites which comprise at least 20 percent by weight of carbon. When desirable, the porous carbon material may be treated with a perfluorosilane or fluorine composition to increase its hydrophobicity, or oxidized, sulfonated, or coated with a hydrophilic material to increase its hydrophilicity. If a porous carbon material is utilized as both the flow field and 25 the large pore material may have interdigitated channels cut into it to lower the pressure drop introduced into the oxygen-containing feed gas. The conductivity of the intermediate layer is preferably at least 0.01 Siemens/cm (S/cm), more preferably at least 0.1 S/cm, and most preferably at least 10 S/cm. The conductivity of the layer may be increased by the addition of conductive fillers, such as carbon fibers or particles, or by the incorporation of conductive 30 salts or polymers. Alternatives to the above porous carbon materials which may be 18 wO 9W/455U PCT/US98/06769 advantageously used as the large pore material include fine mesh woven wire screens and expanded metal, or porous metals, especially those made from silver or nickel. It has been discovered that a particularly well-suited form of catalyst ink comprises a) catalytically-active particles; b) an organic compound having a pKa of at least 18 5 and a basicity parameter 3, of less than 0.66 and c) a polymeric binder. A layer of this composition can be applied to a solid polymer electrolyte, a carbon fiber paper, or a release substrate, then heated under conditions sufficient to volatize at least 95 percent of the organic compound. The resulting composition can then be placed in contact with the solid polymer electrolyte (if the composition was not directly applied to the solid polymerelectrolyte 10 initially). It has been discovered that this catalyst ink, when used to prepare a membrane electrode assembly (MEA) having a solid polymer electrolyte, provides an MEA which provides a relatively low cell voltage at a given current density and gas flow rate in a electrolytic cell. It is believed, without intending to be bound by any particular theory, that the improved performance results from the ability of the organic compound to be easily volatized 15 when heated, which is believed to result from a low incidence of ionic, hydrogen, or covalent bonds or partial bonds formed between the organic compound and the polymeric binder, particularly when the binder is in an ionic form. Although the propensity of an organic compound to bond with the binder is difficult to quantify, the characteristics of the organic compound set forth in the above summary are measurable characteristics which are believed 20 to be indicative of a compound with a minimal or nonexistent propensity to bond with an ionomer or polar polymer. The pKa and basicity parameter reflect the acidity and basicity of the compound, respectively. It is believed that the ease with which the organic compound may be removed from the ink significantly affects the pore characteristics of the resulting active layer. Easy 25 removal of the organic compound is believed to promote a "foaming" effect in the layer, which increases the porosity of the pores of the layer. The pore characteristics affect the transport of water though the layer, which significantly affects the performance of the MEA into which it is incorporated. In addition, if the composition of the seventh aspect of the invention (hereafter, "catalyst ink") is applied directly to the membrane, it will not cause it to 30 swell excessively, since the organic compound will not bond significantly with the ionomer in the membrane. Further, the composition of the invention permits the use of Na+ or H+ forms of ionomers as the binder without significant degradation thereof when the catalyst ink is 19 WO 98/45503 PCT/US98/06769 heated to volatilize the organic compound, and provides an active layer with good long-term stability. Suitable organic compounds include organic compounds having a pKa (the negative logarithm (to the base 10) of the equilibrium constant, K, for the reaction between 5 the compound and water) of at least 18 and a basicity parameter, 3, of less than 0.66. Preferably, the pKa is at least 25. Preferably, 13 is less than 0.48, and is more preferably less than 0.40. The basicity parameter for a number of organic compounds, as well as reference procedures for its determination, are described in Kamlet et al., "Linear Solvation Energy Relationships. 23. A Comprehensive Collection of the Solvochromatic Parameters, Tr*, X, 10 and P, and Some Methods for Simplifying the Generalized Solvatochromic Equation," J. Org. Chem., Vol. 48, pp. 2877-2887 (1983). Preferably, the compound volatilizes at temperatures in the range of from 100C to 250 0 C without significant degradation which can impair the performance of the active layer. A relatively low volatilization temperature is also preferred, since organic 15 compounds (component (b)) that are not removed from the layer can add to the electrical resistance of the layer, causing poorer MEA performance. This characteristic is particularly important when the binder is utilized in its proton form, since the binder will act as a catalyst to further promote the degradation of any residual organic compound. The use of a proton form of the binder has advantages, however, since quaternary ammonium cations present in an 20 ink composition are difficult to remove and may contribute to a long "break in" period when an electrolytic cell or electrolytic cell stack is initially started. Preferably, the boiling point of the solvent is greater than 100oC so that, upon curing of the ink, the water or low boiling solvents which may be present in the ink (typically introduced to the ink by way of a commercially available binder containing such components) are removed first. 25 Examples of suitable organic compounds for use as component (b) include ethylene carbonate, propylene carbonate, butylene carbonate, ethylene carbamate, propylene carbamate, and butylene carbamate, acetone, acetonitrile, difluorobenzene, and sulfolane, but is most preferably propylene carbonate. The organic compound is preferably present in an amount, based on the weight of the composition, of at least 10 percent, more preferably at 30 least 20 percent, and is most preferably at least 30 percent; but is preferably no greater than 90 percent. 20 WO 98/45503 PCT/US98/06769 Suitable polymeric binders for use in the preparation of this preferred catalyst ink include any polymer or oligomer having ionic conductivity of at least 1 x10 -3 S/cm, more preferably at least 10 -1 S/cm (under the operating conditions of the electrolytic cell) or electrolytic cell, or which may be reacted with acid or base to generate an oligomer or polymer 5 having ionic conductivity. If the binder has pendant ionic groups, it preferably has an equivalent weight of at least 600, more preferably at least 700, and preferably no greater than 1200, more preferably no greater than 950. The equivalent weight of the binder is based on the number of pendant ionic groups per molecule, as may be determined by any suitable technique, such as titration with a base, as illustrated in U.S. Patent 4,940,525. Examples of 10 suitable binders include perfluorinated polymers and polytetrafluoroethylene polymers, and polytetrafluoroethylene polymers having pendant sulfonic acid groups, (such as NafionTM, available from DuPont). The binder is preferably present in an amount, based on the weight of the composition, of at least 0.5 percent but preferably no more than 5 percent. One advantage of the present invention is that the ionomer may be utilized in any ionic form, such 15 as the proton form or salt form of the oligomer or polymer. Examples of salt forms include quaternary ammonium, sodium, lithium, and potassium. Another method for preparing an MEA useful in the present invention comprises applying one or more layers of the catalyst ink to a release material, such as a polytetrafluoroethylene-coated substrate, curing the ink, and then laminating the cured 20 material to the membrane. A third method comprises applying one or more layers of the catalyst ink to one side of a sheet of porous carbon material, such as a carbon or graphite paper, and then placing the side of the material to which the ink was applied adjacent to the membrane. If the ink is cured before being placed next to the membrane, it should then preferably be laminated to the membrane to ensure good contact between the two. 25 The ink may be cured using any suitable method for removing at least 95 percent of component (b), as well as any other volatile organic solvents contained in the ink, such as by heating at an elevated temperature optionally under reduced pressure. Preferably, the ink is heated to a temperature at which the component (b) is volatile, but below its boiling point. If more than one ink is used to prepare the active layer of the MEA, the inks preferably 30 contain a polytetrafluoroethylene polymer having pendant sulfonic acid groups as the binder, and the layer of ink closest to the membrane has an equivalent weight which differs from the equivalent weight of the binder in the ink layer adjacent thereto by at least 50. 21 WO 98/45503 PCT/US98/06769 Preferably, the ink is heated under conditions sufficient to remove at least 99 percent, more preferably at least 99.9 percent of component (b). The ink is applied in an amount sufficient to provide a layer of the composition which, when dry and protonated, has a thickness of at least 1 pm, more preferably at least 5 pm, and most preferably at least 10 pm; 5 but is preferably no greater than 30 pm. The porosity of the layer is preferably at least 30 percent, more preferably at least 50 percent; but is preferably no greater than 90 percent, more preferably no greater than 60 percent. The mean pore size of the layer is preferably at least 0.01 pm, more preferably at least 0.03 pm; but is preferably no greater than 10 pm, more preferably no greater than 0.5 pm, and is most preferably 0.1 pm. 10 The following examples are given to illustrate the invention and should not be interpreted as limiting it in any way. Unless stated otherwise, all parts and percentages are given by weight. Example 1 An amount of 0.3125 grams of 20 percent by weight Pt on carbon particles 15 were mixed with three grams of propylene carbonate and stirred for 5 minutes. 1.8 grams of a 50 percent solution of perfluorosulfonic acid polymer (PFSA) (800 equivalent weight) in an ethanol-water solvent were added and stirred for an additional 5 minutes. 0.38 grams of 0.5M NaOH was added to the mixture, and the mixture was allowed to stir overnight. Three coats of resulting suspension were painted onto a Teflon release substrate (transfer decal), the 20 substrate and coating being heated to 95 0 C after each coat to facilitate drying. The coating on the release substrate was then pressed into a perfluorinated cation exchange membrane (obtained from DuPont under the trade name Nafion 115) which had been converted to the sodium form by soaking in 0.5M NaOH for 30 minutes, rinsed in deionized water and vacuum dried for 30 minutes at 550C. The resulting catalyst layer was 5-10 microns thick. After 25 pressing, the Teflon release substrate was removed, leaving the membrane with the active layer coated onto it. Carbon paper was made hydrophobic by submerging it into 5-10 percent solution of perfluorocyclobutene (PFCB) in mestylene. The paper was vacuum dried at 200oC for at least an hour. The edges of the paper were then sealed with epoxy requiring a room 30 temperature cure overnight. The carbon paper was held rigidly in place between the catalyst layer side of the MEA and a Lexan support block in an electrolysis cell. 22 WO 98/45503 PCT/US98/06769 In order to simplify the experimentation, 5N NaOH was used as the anolyte in place of the brine of a normal chlor-alkali cell. In this situation the anodic product is oxygen rather than the chlorine of a brine cell. As shown in Figure 7, the membrane side of the MEA faced an aperture where 5N NaOH was introduced. A bed of plastic screens to support the 5 MEA filled the aperture. A stainless steel anode was at the far end of the aperture. The NaOH solution was circulated through the aperture by pumping from a reservoir. A silver silver chloride reference electrode in the reservoir was used to measure the cathode potential. The NaOH solution was heated to 55 0 C. Oxygen was bubbled through 80oC water at 5 psig (34.5 kPa) and introduced to the cathode through the carbon paper. The gas is delivered to 10 one edge of the carbon paper via a slot in the Lexan support block, forced to travel in the plane of the carbon paper and collected at the opposite edge exiting through a second slot in the Lexan block. Current is passed between the anode and the cathode. The cathode potential is recorded at various current densities, and was compared to when nitrogen was introduced in the same circumstances in the identical system. As seen from Figure 5, the comparison 15 showed that the potential when oxygen was fed to the electrode was 0.77 to 0.90 volts less than when nitrogen was fed to the electrode. Example 2 The cathode and cell are made as in example 1 except the carbon paper flow field was made hydrophilic by placing it in a solution consisting of 0.5 g Ag 2

SO

4 ,15g Na 2

S

2 08 20 and 300 ml of concentrated H 2

SO

4 for 1 hour at 70 0 C. The difference in cathode polarization between oxygen and nitrogen feed is shown in Figure 6. The depolarization varies between 0.27 and 0.61V between 0.25 and 2.25A/in 2 (0.039 and 0.35 A/cm 2 ). 25 23

Claims (26)

1. A process for the electrolysis of an alkali metal halide brine in an electrolytic cell having an anode compartment having at least one anode, a cathode compartment having at least one cathode, and a membrane in contact with the cathode which separates the anode compartment from the cathode compartment, the method 5 comprising: a. feeding the alkali metal halide brine to the anode compartment; b. electrolyzing the alkali halide brine to produce a halogen gas and alkali metal ions; c. passing the alkali metal ions and water through the membrane into the 10 cathode compartment; d. feeding an oxygen-containing gas to the cathode compartment so that the oxygen is reduced at the cathode and a concentrated alkali metal hydroxide solution is produced; wherein a flow field is located adjacent to the cathode to facilitate transporting 15 the oxygen gas to the cathode and alkali metal hydroxide away from the cathode and wherein the flow field comprises an electrically conductive material having a porosity of at least 30 percent and a mean pore size of at least 10 microns.

2. The process of Claim 1 wherein the electrically conductive material has a thickness of at least 10 mils (0.25 mm), and a 3 inch (7.62 cm) by 3 inch (7.62 cm) portion of 20 the material can imbibe at least 0.1g of water per gram of material in ten seconds when held vertically in 3/16 inch (0.48 cm) depth of water.

3. The process of Claim 2 wherein the porous material has a thickness of at least 20 mils (0.51 mm), and a 3 inch (7.62 cm) by 3 inch (7.62 cm) portion of the material can imbibe at least 1 g of water per g of material in ten seconds when held vertically in 3/16 25 inch (0.48 cm) depth of water.

4. The process of Claim 1 wherein the electrically conductive material has a porosity of at least 50 percent.

5. The process of Claim 1 wherein the electrically conductive material has a porosity of at least 80 percent. 30

6. The process of Claim 1 wherein the electrically conductive material has a mean pore size of at least 50 microns. 24 WO 98/45503 PCT/US98/06769

7. The process of Claim 1 wherein the cathode comprises at least two layers of catalytically-active particle containing ink on one side of the membrane, wherein at least two layers of the catalytically-active particle containing ink comprise polytetrafluoroethylene polymers having pendant sulfonic acid groups, and equivalent weights which differ by more 5 than 50.

8. The process of Claim 7 wherein the layer having the lowest equivalent weight is positioned adjacent to the membrane.

9. The process of Claim 7 wherein the polytetrafluoroethylene polymers have an equivalent weight less than 950 and which are substantially insoluble in water at a 10 temperature of less than 1000C.

10. The process of Claim 1 wherein the oxygen-containing gas is humidified prior to feeding it to the cathode compartment.

11. The process of Claim 1 wherein the oxygen-containing gas is fed at such a rate that substantially no hydrogen is produced. 15

12. The process of Claim 1 wherein the electrically conductive material has at least two portions with different mean pore sizes, wherein a first portion of the layer adjacent to the cathode has a porosity no greater than the second portion of the layer adjacent to the opposite side from the cathode; and wherein the second portion has a porosity of at least 30 percent; and the second portion has mean pore size which is at least 4 microns and at least 20 two times greater than the mean pore size of the first portion.

13. A process for the electrolysis of an alkali metal halide brine in an electrolytic cell having a membrane electrode assembly which comprises a solid polymer membrane having an anode side and a cathode side, an anode compartment in contact with the anode side, and a cathode compartment in contact with the cathode side; the process 25 comprising: a. feeding the alkali metal halide brine to the anode compartment; b. electrolyzing the alkali halide brine to produce a halogen gas and alkali metal ions; c. passing the alkali metal ions and water through the membrane into the 30 cathode compartment; 25 WO 98/45503 PCT/US98/06769 d. feeding an oxygen-containing gas to the cathode compartment so that the oxygen is reduced at the cathode and a concentrated alkali metal hydroxide solution is produced; wherein the membrane electrode assembly further comprises at least one 5 active layer on the cathode side thereof, wherein the active layer comprises (a) catalytically active particles and (b) an ionomer having an equivalent weight in the rangeless than 950 and which is substantially insoluble in water at a temperature of less than 100C.

14. The process of Claim 13 wherein the active layer comprises at least 99 percent by weight of the mixture of catalytically active particles and the ionomer. 10

15. The process of Claim 13 wherein the active layer has a thickness of at least 1 lm.

16. The process of Claim 13 wherein the active layer has a thickness of at least 10 lm.

17. The process of Claim 13 wherein the active layer has a porosity of at 15 least 30 percent.

18. The process of Claim 13 wherein the active layer has a porosity of at least 50 percent.

19. The process of Claim 13 wherein the active layer has a mean pore size in the range of from 0.01 lm to 10 lm.

20 20. The process of Claim 13 wherein the catalytically-active particles are present on the cathode side in an amount sufficient to provide a loading level of at least 0.01 mg/cm 2 to 0.5 mg/cm 2 .

21. The process of Claim 13 wherein the membrane assembly further comprises at least one additional active layer comprising (a) catalytically active particles and 25 (b) an ionomer having equivalent weight which differs from the equivalent weight of the first layer by more than 50.

22. The process of Claim 21 wherein the first active layer is located between the additional active layer and the membrane.

23. The process of Claim 13 wherein the membrane electrode assembly 30 further comprises a flow field adjacent to the active layer which comprises a layer of an electrically conductive porous material which has at least two portions with different mean 26 WO 98/45503 PCT/US98/06769 pore sizes, wherein a first portion of the layer adjacent to the membrane electrode assembly has a porosity no greater than the second portion of the layer adjacent to the opposite side from the membrane electrode assembly; and wherein the second portion has a porosity of at least 30 percent; and the second portion has mean pore size which is at least 4 microns and 5 at least two times greater than the mean pore size of the first portion.

24. The process of Claim 13 wherein oxygen-containing gas is humidified prior to feeding it to the cathode compartment.

25. The process of Claim 13 wherein the oxygen-containing gas is fed at such a rate that substantially no hydrogen is produced 10

26. A process for the electrolysis of an alkali metal halide brine in an electrolytic cell having a membrane electrode assembly which comprises a solid polymer membrane having an anode side and a cathode side, an anode compartment in contact with the anode side, and a cathode compartment in contact with the cathode side; the process comprising: 15 a. feeding the alkali metal halide brine to the anode compartment; b. electrolyzing the alkali halide brine to produce a halogen gas and alkali metal ions; c. passing the alkali metal ions and water through the membrane into the cathode compartment; 20 d. feeding an oxygen-containing gas to the cathode compartment so that the oxygen is reduced at the cathode and a concentrated alkali metal hydroxide solution is produced; wherein the membrane assembly further comprises a layer of an electrically conductive porous material adjacent thereto which has at least two portions with different mean pore 25 sizes, wherein a first portion of the layer adjacent to the membrane electrode assembly has a porosity no greater than the second portion of the layer adjacent to the opposite side from the membrane electrode assembly; and wherein the second portion has a porosity of at least 30 percent; and the second portion has a mean pore size which is at least 4 microns and at least 2 times greater than the mean pore size of the first portion. 27