Membrane substrates, methods of preparing membrane substrates, and methods of preparing a component of a membrane electrode assembly

Abstract

A membrane substrate in the form of a sheet is provided, comprising a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and finely divided, substantially water-insoluble silica distributed throughout the polyolefin matrix. The membrane substrate comprises sulfonic acid-functional groups bonded thereto. Alternatively, the membrane substrate comprises carboxylic acid- and / or phosphonic acid-functional groups bonded thereto. Also provided are membranes comprising the membrane substrate described above and a proton exchange ionomer impregnated into the membrane substrate. Also provided are methods for preparing a membrane substrate and methods for preparing a component of a membrane electrode assembly.

Description

MEMBRANE SUBSTRATES, METHODS OF PREPARING MEMBRANE SUBSTRATES, AND METHODS OF PREPARING A COMPONENT OF A MEMBRANE ELECTRODE ASSEMBLYFIELD OF THE DISCLOSURE

[0001] The present disclosure relates to membrane substrates suitable for use in membrane electrode assemblies, methods for preparing membrane substrates, and methods for preparing a component of a membrane electrode assembly.BACKGROUND

[0002] A membrane electrode assembly (MEA), used in fuel cells and electrolyzers, is an assembled stack of a proton-exchange membrane (PEM) or alkali anion exchange membrane (AAEM), catalyst, and flat plate electrodes. The PEM is sandwiched between two electrodes, an anode and a cathode, which have the catalyst embedded in them. The electrodes are electrically insulated from each other by the PEM. The PEM is typically a fluoropolymer such as perfluorinated sulfonic acid (PFSA), a proton-permeable, electrical insulator. PFSA has a hydrophobic polytetrafluoroethylene (PTFE) backbone and a hydrophilic, perfluorinated vinyl ether pendant side chain that is covalently bonded to a sulfonic acid group (-SO3H). This hydrophilic group also takes up water to keep the membrane hydrated, which assists in proton migration.

[0003] Factors affecting membrane performance include the level of hydration and thickness of the membrane, both of which play an important role in membrane durability and performance. Conductivity of the PEM is sensitive to relative humidity (RH); proton conductivity at low RH is too low to provide acceptable fuel cell performance. The cost of maintaining membrane hydration is high due to complications in the available water management system designs.

[0004] Reducing membrane thickness can reduce material costs and increase performance by preventing water drag or gas crossover, lowering membrane resistance and enhancing membrane conductivity. However, reducing thickness increases the risk of mechanical failure and reduces the service life (durability) of the membrane.

[0005] It would be desirable to provide membrane substrates that demonstrate lower sensitivity to relative humidity and reduced mechanical failure and gas crossover.SUMMARY OF THE DISCLOSURE

[0006] The present disclosure is directed to a membrane substrate comprising a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating substantially throughout the polyolefin matrix; and finely divided, substantially water-insoluble particulate filler comprising silica distributed throughout the polyolefin matrix. The membrane substrate comprises 10 to 90 percent by weight silica and 10 to 90 percent by weight polyolefin, based on the total weight of the polyolefin matrix and the particulate filler. The membrane substrate is in the form of a sheet and demonstrates a Gurley porosity of 1 to 1000 seconds. In certain examples, the membrane substrate comprises sulfonic acid-functional groups bonded thereto. In other examples, the membrane substrate comprises carboxylic acid- and / or phosphonic acid-functional groups bonded thereto.

[0007] The present disclosure is further directed to a membrane comprising a membrane substrate as above, further comprising a proton exchange ionomer impregnated into the membrane substrate; usually by impregnation into the network of interconnecting pores, such that the membrane demonstrates a Gurley porosity of greater than 40,000 seconds and exhibits a proton exchange capacity of greater than or equal to 0.4 mmol / g.

[0008] Also provided is a method of preparing the membrane substrate described above, the method comprising: (1 ) mixing a filler comprising silica, a polyolefin polymer, a processing plasticizer, a lubricant, optionally a free radical scavenger, and optionally an antioxidant until a substantially uniform mixture is obtained; (2) introducing the mixture to a heated barrel of a screw extruder, to which is attached a sheeting die to form a sheet; (3) forwarding the sheet to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the sheet exiting from the die; (4) stretching the sheet in at least one stretching direction to form a stretched sheet; (5) extracting the processing plasticizer from the sheet; and (6) surface-treating the silica. The silica may be treated with a sulfonic acid-functional silane to form a sulfonic acid-functional membrane substrate, or with a carboxylic and / or phosphonic acid-functional silane to form a carboxylic acid-functional and / or phosphonic acid-functional membrane substrate. Surface-treating the silica may beperformed either (i) prior to forming the mixture in step (1); or (ii) after extracting the processing plasticizer from the stretched sheet in step (5).

[0009] The present disclosure is further directed to a method for preparing a component of a membrane electrode assembly, comprising: (1 ) providing a membrane substrate prepared as described above; (2) applying a proton exchange ionomer to the membrane substrate to form an impregnated membrane; (3) annealing the impregnated membrane to form an annealed membrane; (4) soaking the annealed membrane in an aqueous solution of a mineral acid; and (5) rinsing the annealed membrane after step (4) with deionized water to form a proton exchange membrane.BRIEF DESCRIPTION OF THE FIGURES

[0010] Fig. 1 illustrates FTIR spectra of a control membrane substrate and a membrane substrate prepared in accordance with the present disclosure.

[0011] Fig. 2 is a graph of a polarization curve (voltage vs. current), comparing a commercially available proton exchange membrane with a membrane prepared in accordance with the present disclosure.DETAILED DESCRIPTION

[0012] Other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the disclosed membranes. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0013] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

[0014] Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

[0015] In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. Further, in this application, the use of “a” or “an” means “at least one” unless specifically stated otherwise. For example, “an” additive, “a” silica, and the like refer to one or more of these items. Also, as used herein, the term “polymer” is meant to refer to prepolymers, oligomers, and both homopolymers and copolymers. The term “resin” is used interchangeably with “polymer.”

[0016] The present disclosure is directed to a membrane substrate, which may be microporous. As used herein, “microporous material” or “microporous membrane” or “microporous sheet” means a material having a network of interconnecting pores, wherein, on a treatment-free, coating-free, printing ink-free, impregnant-free, and prebonding basis, the pores typically have a volume average diameter ranging from 0.001 to 1 .0 micrometer, and may constitute at least 5 percent by volume of the microporous material as discussed herein below. In other words, the volume average diameter above is reported for virgin microporous material that has not been coated, impregnated, or otherwise treated.

[0017] The polyolefin matrix may comprise ultrahigh molecular weight (UHMW) polyethylene. Non-limiting examples of ultrahigh molecular weight (UHMW) polyethylene can include essentially linear UHMW polyethylene (PE). Inasmuch as UHMW polyolefins are not thermoset polymers having an infinite molecular weight, they are technically classified as thermoplastic materials.

[0018] While there is no particular restriction on the upper limit of the intrinsic viscosity of the UHMW polyethylene, in one non-limiting example, the intrinsic viscosity can range from at least 6 deciliters / gram, or at least 7 deciliters / gram, or at least 18 deciliters / gram, to at most 50 deciliters / gram, or at most 45 deciliters / gram, or at most 18 deciliters / gram, or at most 16 deciliters / gram. Thus, the intrinsic viscosity of the UHMW may be, for example, 6 to 50 deciliters / gram, or 6 to 45 deciliters / gram, or 6 to 18 deciliters / gram, or 6 to 16 deciliters / gram, or 7 to 50 deciliters / gram, or 7 to 45deciliters / gram, or 7 to 18 deciliters / gram, or 7 to 16 deciliters / gram, or 18 to 50 deciliters / gram, or 18 to 45 deciliters / gram.

[0019] For purposes of the present disclosure, intrinsic viscosity may be determined by extrapolating to zero concentration the reduced viscosities or the inherent viscosities of several dilute solutions of the UHMW polyolefin where the solvent is freshly distilled decahydronaphthalene to which 0.2 percent by weight, 3,5-di-tert- butyl-4-hydroxyhydrocinnamic acid, neopentanetetrayl ester [CAS Registry No. 6683- 19-8] has been added. The reduced viscosities or the inherent viscosities of the UHMW polyolefin are ascertained from relative viscosities obtained at 135°C using an Ubbelohde No. 1 viscometer in accordance with the general procedures of ASTM D 4020-81 , except that several dilute solutions of differing concentration are employed.

[0020] The nominal molecular weight of UHMW polyethylene is empirically related to the intrinsic viscosity of the polymer in accordance with the following equation:M=5.37x104[n]1 37wherein M is the nominal molecular weight and [q] is the intrinsic viscosity of the UHMW polyethylene expressed in deciliters / gram. Similarly, the nominal molecular weight of UHMW polypropylene is empirically related to the intrinsic viscosity of the polymer according to the following equation:M=8.88x104[n]1 25wherein M is the nominal molecular weight and [q] is the intrinsic viscosity of the UHMW polypropylene expressed in deciliters / gram.

[0021] Often, the polyolefin matrix further comprises high density polyethylene (HDPE), which usually has a density of at least 900 kg / m3, such as 930 to 940 kg / m3. In certain examples, other thermoplastic organic polymers also may be present in the polyolefin matrix, provided that their presence does not materially affect the properties of the membrane substrate in an adverse manner. The amount of the other thermoplastic polymer which may be present depends upon the nature of such polymer. Non-limiting examples of thermoplastic organic polymers that optionally may be present in the matrix include low density polyethylene, poly(tetrafluoroethylene), polypropylene, copolymers of ethylene and propylene, copolymers of ethylene and butylene, copolymers of ethylene and (meth)acrylic (i. e., acrylic and / or methacrylic) acid, polyetherketone, polyvinylidene fluoride (PVDF), polysulfone, and / or polyethersulfone. Note that the phrase “and / or” when used in a list is meant to encompass alternative examples including each individual component in the list aswell as any combination of components. For example, the list “A, B, and / or C” is meant to encompass seven separate examples that include A, or B, or C, or A + B, or A +C, or B + C, or A + B + C.

[0022] The polyolefin may be present in the membrane substrate in an amount of at least 10 percent by weight, or at least 20 percent by weight, and at most 90 percent by weight, or at most 50 percent by weight, based on the total weight of the polyolefin matrix and particulate filler, described below. For example, the polyolefin may be present in the membrane substrate in an amount of 10 to 90 percent by weight, or 10 to 50 percent by weight, or 20 to 90 percent by weight, or 20 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler. Often the polyolefin matrix comprises a mixture of high density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE). When this mixture is used, the polyolefin matrix is typically present in the membrane substrate in an amount of 20 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.

[0023] The membrane substrates of the present disclosure further comprise finely divided, particulate, substantially water-insoluble filler comprising silica, distributed throughout the polyolefin matrix.

[0024] The filler can include any of a number of additional fillers known in the art. The filler should be finely divided and substantially water insoluble to permit uniform distribution throughout the polyolefin matrix during manufacture of the membrane substrate. Generally, the filler comprises silica, such as precipitated silica, talc, carbon black, charcoal, graphite, titanium oxide, iron oxide, copper oxide, zinc oxide, antimony oxide, zirconia, magnesia, alumina, molybdenum disulfide, zinc sulfide, barium sulfate, strontium sulfate, calcium carbonate, and / or magnesium carbonate. Often the filler comprises silica, and may further comprise an antioxidant and / or a peroxide scavenger. Anti-oxidants such as those sold by BASF under the names IRGANOX® and IRGAFOS®, and those sold by Solvay under the name CYANOX®, are suitable for use.

[0025] The finely divided, substantially water-insoluble filler may be in the form of ultimate particles, aggregates of ultimate particles, or a combination of both. With respect to silica particles, by “finely divided” is meant that the silica used in preparing the membrane substrate has an average particle size D50 of from 5 to 100 microns, or 5 to 75 microns, or 5 to 50 microns, as determined by the use of a laser diffractionparticle size instrument, such as LS230 available from Beckman Coulton, capable of measuring particle diameters as small as 0.04 micron. The D50 is selected such that average particle size of the silica is smaller than the intended thickness of the membrane substrate, discussed below. Usually at least 90 percent by weight of the silica used in preparing the membrane substrate has gross particle sizes in the range of from 5 to 40 microns; i. e., usually no larger than the thickness of the membrane. The sizes of the filler agglomerates may be reduced during processing of the ingredients used to prepare the membrane substrate. Accordingly, the distribution of particle sizes in the membrane substrate may be smaller than in the raw filler itself. By “substantially insoluble” is meant that less than 3 percent by weight, or less than 1 percent by weight of the filler particles, based on the total weight of the filler particles, dissolves into the liquid phase at 25°C when dispersed therein. This can facilitate retention of the filler in the microporous material.

[0026] The filler typically has a high surface area, which may be influenced by particle size and / or porosity, allowing the filler to carry much of the processing plasticizer used to form the membrane substrate. The surface area of the filler particles can range from at least 20 square meters per gram, or at least 25 square meters per gram, to at most 900 square meters per gram, or at most 850 square meters per gram; e. g., from 20 to 900 square meters per gram, or from 20 to 850 square meters per gram, or from 25 to 900 square meters per gram, or from 25 to 850 square meters per gram, as determined by the Brunauer, Emmett, Teller (BET) method according to ASTM C 819-77 using nitrogen as the adsorbate but modified by outgassing the system and the sample for one hour at 130°C. Prior to nitrogen sorption, filler samples are dried by heating to 160°C in flowing nitrogen (PS) for 1 hour.

[0027] In a particular example, the inorganic filler comprises precipitated silica, silica gel, or fumed silica. The silica may demonstrate a BET of 125 to 700 m2 / g, determined as noted above.

[0028] Silica gel is generally produced commercially by acidifying an aqueous solution of a soluble metal silicate, e.g., sodium silicate at low pH with acid. The acid employed is generally a strong mineral acid, such as sulfuric acid or hydrochloric acid, although carbon dioxide can be used. Inasmuch as there is essentially no difference in density between the gel phase and the surrounding liquid phase while the viscosity is low, the gel phase does not settle out, that is to say, it does not precipitate. Consequently, silica gel may be described as a non-precipitated, coherent, rigid, three-dimensional network of contiguous particles of colloidal amorphous silica. The state of subdivision ranges from large, solid masses to submicroscopic particles, and the degree of hydration from almost anhydrous silica to soft gelatinous masses containing on the order of 100 parts of water per part of silica by weight.

[0029] Precipitated silica generally is produced commercially by combining an aqueous solution of a metal silicate, ordinarily alkali metal silicate such as sodium silicate, and an acid so that colloidal particles of silica will grow in a weakly alkaline solution and be coagulated by the alkali metal ions of the resulting soluble alkali metal salt. Various acids may be used, including but not limited to mineral acids. Non-limiting examples of acids that can be used include hydrochloric acid and sulfuric acid, but carbon dioxide can also be used to produce precipitated silica. In the absence of a coagulant, silica is not precipitated from solution at any pH. In a non-limiting example, the coagulant used to effect precipitation of silica may be the alkali metal salt produced during formation of the colloidal silica particles, or it may be an added electrolyte, such as an inorganic or organic salt, or it may be a combination of both.

[0030] Precipitated silica can be described as precipitated aggregates of ultimate particles of colloidal amorphous silica that have not at any point existed as macroscopic gel during the preparation. The sizes of the aggregates and the degree of hydration may vary widely. Precipitated silica powders differ from silica gels that have been pulverized in that the precipitated silica powders generally have a more open structure, that is, a higher specific pore volume. However, the specific surface area of precipitated silica, as measured by the Brunauer, Emmet, Teller (BET) method using nitrogen as the adsorbate, is often lower than that of silica gel.

[0031] Many different precipitated silicas can be employed as the filler used to prepare the membrane substrate. Precipitated silicas are well-known commercial materials, and processes for producing them are described in detail in many United States patents, including United States Patent Numbers 2,940,830 and 4,681 ,750. The average ultimate particle size (irrespective of whether or not the ultimate particles are agglomerated) of precipitated silicas used is generally less than 0.1 micrometer, e.g., less than 0.05 micrometer or less than 0.03 micrometer, as determined by transmission electron microscopy. Non-limiting examples of suitable precipitated silicas include those sold under the HI-SIL tradename by PPG (Pittsburgh, PA).

[0032] The membrane substrate may comprise sulfonic acid-functional groups bonded thereto. Alternatively, the membrane substrate may comprise carboxylic acid-and / or phosphonic acid-functional groups bonded thereto. Such functional groups may be introduced onto the silica by condensation reaction of an acid-functional silane compound with reactive groups on the silica surface. The term “acid functional-silane compound” is meant to include silanes having one or more of the acid-functional groups mentioned above, and silane compounds having groups that may be converted to such acid-functional groups. Examples of such silane compounds include 3- (trihydroxysilyl)-l -propanesulfonic acid, (4-(2-(trihydroxysilyl)-ethyl) benzenesulfonic acid, and (2-diethylphosphatoethyl) trimethoxysilane. A mercapto-functional silane may be reacted with functional groups on the silica, followed by oxidation of the mercapto group to yield a sulfonic acid group.

[0033] The membrane substrate typically comprises at least 10 percent by weight, or at least 30 percent by weight, or at least 50 percent by weight, and at most 90 percent by weight, or at most 80 percent by weight, or at most 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler. For example, the membrane substrate may comprise 10 to 90 percent by weight, or 10 to 80 percent by weight, or 10 to 75 percent by weight, or 30 percent to 90 percent by weight, or 30 to 80 percent by weight, or 30 to 75 percent by weight, or 50 percent to 90 percent by weight, or 50 to 80 percent by weight, or 50 to 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.

[0034] The membrane substrate may further comprise a network of interconnecting pores communicating throughout the polyolefin matrix of the membrane substrate.

[0035] On a treatment-free, coating free, or impregnant-free basis, such pores can make up at least 5 percent by volume, or at least 15 percent by volume, or at least 20 percent by volume, or at least 25 percent by volume, or at least 35 percent by volume, or at least 45 percent by volume, and at most 95 percent by volume, or at most 75 percent by volume. Thus, the pores can make up 5 to 95 percent by volume, or 15 to 95 percent by volume, or 20 to 95 percent by volume, or 25 to 95 percent by volume, or 35 to 95 percent by volume, or 45 to 95 percent by volume, or 5 to 70 percent by volume, or 15 to 70 percent by volume, or 20 to 70 percent by volume, or 25 to 70 percent by volume, or 35 to 70 percent by volume, or 45 to 70 percent by volume of the membrane substrate. Often, the pores comprise at least 35 percent by volume, or even at least 45 percent by volume of the membrane substrate.

[0036] Porosity can be measured using a Gurley Densometer, model 4340, manufactured by GPI Gurley Precision Instruments (Troy, NY). The porosity valuesreported are a measure of the rate of air flow through a sample or it's resistance to an air flow through the sample. The unit of measure for this method is a “Gurley second” and represents the time in seconds to pass 100 cc of air through a 1 -inch square area using a pressure differential of 4.88 inches of water. Lower values equate to less air flow resistance (more air is allowed to pass freely). For purposes of the present disclosure, the measurements are completed using the procedure listed in the manual for MODEL 4340 Automatic Densometer. The membrane substrate may demonstrate a Gurley porosity of at least 1 second, or at least 10 seconds, and at most 1000 seconds, or at most 750 seconds. Typically, the membrane substrate demonstrates a Gurley porosity of 1 to 1000 seconds, or 10 to 1000 seconds, or 1 to 750 seconds, or 10 to 750 seconds.

[0037] The membrane substrates described above may be prepared by a method comprising:(1 ) mixing a filler comprising silica, a polyolefin polymer, a processing plasticizer, a lubricant, optionally a free radical scavenger, and optionally an antioxidant until a substantially uniform mixture is obtained;(2) introducing the mixture to a heated barrel of a screw extruder, to which is attached a sheeting die to form a sheet;(3) forwarding the sheet to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the sheet exiting from the die;(4) stretching the sheet in at least one stretching direction to form a stretched sheet;(5) extracting the processing plasticizer from the stretched sheet; and(6) surface-treating the silica with a protic acid- a sulfonic acid-functional silane to form a sulfonic acid-functional membrane substrate. In the preparation of a carboxylic acid-functional and / or phosphonic acid-functional membrane substrate, step (6) comprises surface-treating the silica with a carboxylic and / or phosphonic acidfunctional silane to form a carboxylic acid-functional and / or phosphonic acid-functional membrane substrate.

[0038] In an exemplary process, the components of the polyolefin matrix (typically in solid form such as powder or pellets), filler, processing plasticizer, and minor amounts of lubricant, antioxidant, and free radical scavenger are mixed until a substantially uniform mixture is obtained. The weight ratio of filler to polymer employed in forming the mixture is essentially the same as that of the membrane substrate to be produced.The mixture, together with additional processing plasticizer as needed, is introduced to the heated barrel of a screw extruder. Attached to the extruder is a die, such as a sheeting die, to form the desired end shape.

[0039] In an exemplary manufacturing process, when the material is formed into a sheet or film, a continuous sheet or film formed by a die is forwarded to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the continuous sheet exiting from the die. The final thickness may depend on the desired end-use application. The membrane substrate may have a thickness ranging from 5 to 300 microns, or 5 to 125 microns, or 5 to 75 microns, or 5 to 70 microns.

[0040] Optionally, the sheet may be stretched in at least one stretching direction. Stretching may take place before extraction, after extraction, or both. Stretched microporous material substrate may be produced by stretching the intermediate product in at least one stretching direction, often above the elastic limit. Usually, the stretch ratio is at least 1 .2. In many cases, the stretch ratio is at least 1 .5. Often it is at least 2. Frequently, the stretch ratio is in the range of from 1 .2 to 15. Often, the stretch ratio is in the range of from 1 .5 to 10. Usually, the stretch ratio is in the range of from 2 to 6.

[0041] The temperatures at which stretching is accomplished may vary widely. Stretching may be accomplished at ambient temperature, such as room temperature, but usually elevated temperatures are employed. Ambient temperature typically ranges from 60 to 90 °F (15.6 to 32.2 °C), such as a typical room temperature, 72°F (22.2°C). The intermediate product may be heated by any of a wide variety of techniques prior to, during, and / or after stretching. Examples of these techniques include radiative heating, such as that provided by electrically heated or gas fired infrared heaters; convective heating, such as that provided by recirculating hot air; and conductive heating, such as that provided by contact with heated rolls. The temperatures which are measured for temperature control purposes may vary according to the apparatus used and personal preference. For example, temperaturemeasuring devices may be placed to ascertain the temperatures of the surfaces of infrared heaters, the interiors of infrared heaters, the air temperatures of points between the infrared heaters and the intermediate product, the temperatures of circulating hot air at points within the apparatus, the temperature of hot air entering or leaving the apparatus, the temperatures of the surfaces of rolls used in the stretching process, the temperature of heat transfer fluid entering or leaving such rolls, or filmsurface temperatures. In general, the temperature or temperatures are controlled such that the intermediate product is stretched about evenly so that the variations, if any, in film thickness of the stretched microporous material are within acceptable limits and so that the amount of stretched microporous material outside of those limits is acceptably low. It will be apparent that the temperatures used for control purposes may or may not be close to those of the intermediate product itself since they depend upon the nature of the apparatus used, the locations of the temperature-measuring devices, and the identities of the substances or objects whose temperatures are being measured.

[0042] In view of the locations of the heating devices and the line speeds usually employed during stretching, gradients of varying temperatures may or may not be present through the thickness of the intermediate product. Also, because of such line speeds, it is impracticable to measure these temperature gradients. The presence of gradients of varying temperatures, when they occur, makes it unreasonable to refer to a singular film temperature. Accordingly, film surface temperatures, which can be measured, are best used for characterizing the thermal condition of the intermediate product.

[0043] The film surface temperatures at which stretching is accomplished may vary widely, but in general they are such that the intermediate product is stretched about evenly, as explained above. In most cases, the film surface temperatures during stretching are in the range of from 20°C to 220°C. Often, such temperatures are in the range of from 50°C to 200°C, such as from 75°C to 180°C.

[0044] Stretching may be accomplished in a single step or a plurality of steps as desired. For example, when the intermediate product is to be stretched in a single direction (uniaxial stretching), the stretching may be accomplished by a single stretching step or a sequence of stretching steps until the desired final stretch ratio is attained. Similarly, when the intermediate product is to be stretched in two directions (biaxial stretching), the stretching can be conducted by a single biaxial stretching step or a sequence of biaxial stretching steps until the desired final stretch ratios are attained. Biaxial stretching may also be accomplished by a sequence of one of more uniaxial stretching steps in one direction and one or more uniaxial stretching steps in another direction. Biaxial stretching steps where the intermediate product is stretched simultaneously in two directions and uniaxial stretching steps may be conducted in sequence in any order. Stretching in more than two directions is within contemplation.It may be seen that the various permutations of steps are quite numerous. Other steps, such as cooling, heating, sintering, annealing, reeling, unreeling, and the like, may optionally be included in the overall process as desired.

[0045] Various types of stretching apparatus are well known and may be used to accomplish stretching of the intermediate product. Uniaxial stretching is usually accomplished by stretching between two rollers, wherein the second or downstream roller rotates at a greater peripheral speed than the first or upstream roller. Uniaxial stretching can also be accomplished on a standard tentering machine. Biaxial stretching may be accomplished by simultaneously stretching in two different directions on a tentering machine. More commonly, however, biaxial stretching is accomplished by first uniaxially stretching between two differentially rotating rollers as described above, followed by either uniaxially stretching in a different direction using a tenter machine or by biaxially stretching using a tenter machine. The most common type of biaxial stretching is where the two stretching directions are approximately at right angles to each other. In most cases where the continuous sheet is being stretched, one stretching direction is at least approximately parallel to the long axis of the sheet (machine direction) and the other stretching direction is at least approximately perpendicular to the machine direction and is in the plane of the sheet (transverse direction).

[0046] Stretching the sheets prior to extraction of the processing plasticizer allows for thinner films with larger pore sizes than in microporous materials conventionally processed. It is also believed that stretching of the sheets prior to extraction of the processing plasticizer minimizes thermal shrinkage after processing.

[0047] The product passes to a first extraction zone where the processing plasticizer is substantially removed by extraction with an organic liquid, which is a better solvent for the processing plasticizer than for the organic polymer, and more volatile than the processing plasticizer. The product then passes to a second extraction zone where the residual organic extraction liquid is substantially removed by steam and / or water. The product is then passed through a forced air dryer for substantial removal of residual water and remaining residual organic extraction liquid. From the dryer, the microporous material may be passed to a take-up roll, when it is in the form of a sheet.

[0048] The processing plasticizer is a liquid at room temperature and usually it is a processing oil, such as paraffinic oil, naphthenic oil, or aromatic oil. Suitable processing oils include those meeting the requirements of ASTM D 2226-82, Types103 and 104. Those oils which have a pour point of less than 22°C, or less than 10°C, according to ASTM D 97-66 (reapproved 1978) are used most often. Examples of suitable oils include SHELLFLEX 412 and SHELLFLEX 371 oil (Shell Oil Co. (Houston, TX)), which are solvent refined and hydrotreated oils derived from naphthenic crude. It is expected that other materials, including the phthalate ester plasticizers such as dibutyl phthalate, bis(2-ethylhexyl) phthalate, diisodecyl phthalate, dicyclohexyl phthalate, butyl benzyl phthalate, and ditridecyl phthalate will function satisfactorily as processing plasticizers.

[0049] There are many organic extraction liquids that can be used in the process of manufacturing the membrane substrate. Examples of suitable organic extraction liquids include, but are not limited to, 1 ,1 ,2-trichloroethylene; perchloroethylene; 1 ,2- dichloroethane; 1 ,1 ,1 -trichloroethane; 1 ,1 ,2-trichloroethane; methylene chloride; chloroform; 1 ,1 ,2-trichloro-1 ,2,2-trifluoroethane; isopropyl alcohol; diethyl ether; acetone; hexane; heptane and toluene. One or more azeotropes of halogenated hydrocarbons selected from trans-1 ,2-dichloroethylene, 1 ,1 ,1 ,2,2,3, 4,5, 5,5- decafluoropentane, and / or 1 ,1 ,1 ,3,3-pentafluorobutane also can be employed. Such materials are available commercially as VERTREL MCA (a binary azeotrope of 1 ,1 ,1 ,2,2,3,4,5,5,5-dihydrodecafluoropentane and trans-1 ,2-dichloroethylene: 62% / 38%) and VERTREL CCA (a ternary azeotrope of 1 ,1 ,1 ,2, 2, 3, 4, 5,5,5- dihydrodecafluorpentane, 1 ,1 ,1 ,3,3-pentafluorbutane, and trans-1 ,2-dichloroethylene: 33% / 28% / 39%); VERTREL SDG (80-83% trans-1 ,2-dichloroethylene, 17-20% hydrofluorocarbon mixture), all available from MicroCare Corporation (New Britain, CT).

[0050] In the above-described process for producing a membrane substrate, extrusion and calendering are facilitated when the filler carries much of the processing plasticizer. The capacity of the filler particles to absorb and hold the processing plasticizer is a function of the surface area of the filler. Therefore, the filler typically has a high surface area as discussed above. Inasmuch as it is desirable to essentially retain the filler in the membrane substrate, the filler should be substantially insoluble in the processing plasticizer and substantially insoluble in the organic extraction liquid when microporous material substrate is produced by the above process. The residual processing plasticizer content is usually less than 15 percent by weight of the resulting microporous material and this may be reduced even further to levels, such as lessthan 5 percent by weight, by additional extractions using the same or a different organic extraction liquid.

[0051] As noted above, the membrane substrate may comprise sulfonic acidfunctional groups bonded thereto. This may be accomplished by surface-treating the silica with a sulfonic acid-functional silane to form a sulfonic acid-functional membrane substrate. Alternatively, the membrane substrate may comprise carboxylic acid- and / or phosphonic acid-functional groups bonded thereto, by surface-treating the silica with a carboxylic and / or phosphonic acid-functional silane to form a carboxylic acid-functional and / or phosphonic acid-functional membrane substrate. Surface treatment may be performed either (i) prior to forming the mixture in step (1 ); or (ii) after extracting the processing plasticizer from the stretched sheet in step (5).

[0052] In certain examples of the present disclosure, a membrane is provided, comprising: 1 ) any of the membrane substrates described above; and 2) a proton exchange ionomer impregnated into the membrane substrate. The impregnation of the proton exchange ionomer, usually into the network of interconnecting pores of the membrane substrate, renders the membrane nonporous. By “nonporous” is meant that the membrane demonstrates a Gurley porosity of greater than 40,000 seconds.

[0053] The proton exchange ionomer may comprise a sulfonate and / or sulfonic acid functional polymer, such as a sulfonate and / or sulfonic acid functional fluoropolymer. Examples of suitable proton exchange ionomers include NAFION, available from the Chemours Company, and DYNEON, available from 3M Advanced Materials Division.

[0054] The membranes described above that include the proton exchange ionomer impregnated into the membrane substrate typically exhibit a proton exchange capacity of greater than or equal to 0.4 mmol / g, and a hydrogen crossover rate of less than 20 mA / cm2, or less than 10 mA / cm2, as measured by linear sweep voltammetry (LSV). These properties render such membranes particularly suitable for use as a component of a membrane electrode assembly (MEA).

[0055] A method for preparing a component of a membrane electrode assembly is provided, comprising:(1 ) providing any of the membrane substrates prepared as described above as a starting material;(2) applying a proton exchange ionomer to the membrane substrate to form an impregnated membrane;(3) annealing the impregnated membrane to form an annealed membrane;(4) soaking the annealed membrane in an aqueous solution of a mineral acid; and(5) rinsing the annealed membrane after step (4) with deionized water to form a proton exchange membrane. The resultant proton exchange membrane is suitable for use as a component of a membrane electrode assembly in a fuel cell.

[0056] The proton exchange ionomer may be applied to the membrane substrate in step (2) by any application method known on the art, such as immersion, vacuum coating, inkjet printing, or screen printing. Immersion or vacuum coating is used most often, to maximize filling of the network of interconnecting pores.

[0057] Annealing of the impregnated membrane in step (3) may be performed at a temperature of 100 to 145°C for 30 to 60 minutes, or alternatively 100-145°C under pressure for 30 seconds to 15 minutes.

[0058] Suitable mineral acids for use in step (4) include phosphoric acid, sulfuric acid, and the like.

[0059] The present disclosure is further drawn the following aspects:1 . A membrane substrate comprising: a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating substantially throughout the polyolefin matrix; and finely divided, substantially water-insoluble particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the membrane substrate comprises 10 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin is present in the membrane substrate in an amount of 10 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; wherein the membrane substrate is in the form of a sheet and demonstrates a Gurley porosity of 1 to 1000 seconds; and wherein the membrane substrate comprises sulfonic acid-functional groups bonded thereto or wherein the membrane substrate comprises carboxylic acid- and / or phosphonic acid-functional groups bonded thereto.2. The membrane substrate of aspect 1 , wherein the membrane substrate comprises 10 to 80 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.3. The membrane substrate of any preceding aspect, wherein the membrane substrate comprises 10 to 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.4. The membrane substrate of any preceding aspect, wherein the membrane substrate comprises 30 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.5. The membrane substrate of any preceding aspect, wherein the membrane substrate comprises 30 to 80 percent by silica, based on the total weight of the polyolefin matrix and the particulate filler.6. The membrane substrate of any preceding aspect, wherein the membrane substrate comprises 30 to 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.7. The membrane substrate of any preceding aspect, wherein the membrane substrate comprises 50 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.8. The membrane substrate of any preceding aspect, wherein the membrane substrate comprises 50 to 80 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.9. The membrane substrate of any preceding aspect, wherein the membrane substrate comprises 50 to 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.10. The membrane substrate of any preceding aspect, wherein the polyolefin is present in the membrane substrate in an amount of 10 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.1 1 . The membrane substrate of any preceding aspect, wherein the polyolefin is present in the membrane substrate in an amount of 20 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.12. The membrane substrate of any preceding aspect, wherein the polyolefin is present in the membrane substrate in an amount of 20 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.13. The membrane substrate of any preceding aspect, wherein the membrane substrate demonstrates a Gurley porosity of 10 to 1000 seconds.14. The membrane substrate of any preceding aspect, wherein the membrane substrate demonstrates a Gurley porosity of 1 to 750 seconds.15. The membrane substrate of any preceding aspect, wherein the membrane substrate demonstrates a Gurley porosity of 10 to 750 seconds.16. The membrane substrate of any preceding aspect, wherein the polyolefin matrix comprises high density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE), and wherein the polyolefin matrix is present in the membrane substrate in an amount of 20 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.17. The membrane substrate of any preceding aspect, wherein the filler further comprises an antioxidant and / or a peroxide scavenger.18. The membrane substrate of any preceding aspect, wherein the silica demonstrates an average particle size D50 of from 5 to 100 microns, as determined by use of a laser diffraction particle size instrument.19. The membrane substrate of any preceding aspect, wherein the sheet has a thickness of 5 to 300 microns, or 5 to 125 microns, or 5 to 75 microns, or 5 to 70 microns.20. The membrane substrate of any preceding aspect, wherein the sheet has a thickness of 5 to 125 microns.21 . The membrane substrate of any preceding aspect, wherein the sheet has a thickness of 5 to 75 microns.22. The membrane substrate of any preceding aspect, wherein the sheet has a thickness of 5 to 70 microns.23. A membrane comprising:1 ) The membrane substrate of any preceding aspect; and2) a proton exchange ionomer impregnated into the membrane substrate; wherein the membrane demonstrates a Gurley porosity of greater than 40,000 seconds and exhibits a proton exchange capacity of greater than or equal to 0.4 mmol / g.24. The membrane of aspect 23, wherein the proton exchange ionomer comprises a polymer having phosphonic acid- and / or sulfonic acid-functional groups.25. The membrane of aspect 23 or 24, wherein the proton exchange ionomer comprises a fluoropolymer having phosphonic acid- and / or sulfonic acidfunctional groups.26. The membrane of any of aspects 23 to 25, wherein the sheet has a thickness of 5 to 300 microns, or 5 to 125 microns, or 5 to 75 microns, or 5 to 70 microns.27. The membrane of any of aspects 23 to 26, wherein the sheet has a thickness of 5 to 125 microns.28. The membrane of any of aspects 23 to 27, wherein the sheet has a thickness of 5 to 75 microns.29. The membrane of any of aspects 23 to 28, wherein the sheet has a thickness of 5 to 70 microns.30. The membrane of any of aspects 23 to 29, wherein the membrane is a component of a membrane electrode assembly (MEA).31 . The use of the membrane of any of aspects 23 to 30 in a membrane electrode assembly.32. A method for preparing a sulfonic acid-functional membrane substrate, comprising:(1 ) mixing a filler comprising silica, a polyolefin polymer, a processing plasticizer, a lubricant, optionally a free radical scavenger, and optionally an antioxidant until a substantially uniform mixture is obtained;(2) introducing the mixture to a heated barrel of a screw extruder, to which is attached a sheeting die to form a sheet;(3) forwarding the sheet to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the sheet exiting from the die;(4) stretching the sheet in at least one stretching direction to form a stretched sheet;(5) extracting the processing plasticizer from the sheet; and(6) surface-treating the silica with a sulfonic acid-functional silane to form a sulfonic acid-functional membrane substrate or surface-treating the silica with a carboxylic and / or phosphonic acid-functional silane to form a carboxylic acid-functional and / or phosphonic acid-functional membrane substrate; wherein surface-treating the silica is performed either (i) prior to forming the mixture in step (1 ); or (ii) after extracting the processing plasticizer from the stretched sheet in step (5).33. A method for preparing a component of a membrane electrode assembly, comprising:(1 ) providing the sulfonic acid-functional membrane substrate prepared according to the method of aspect 32;(2) applying a proton exchange ionomer to the sulfonic acid-functional membrane substrate to form an impregnated membrane;(3) annealing the impregnated membrane to form an annealed membrane;(4) soaking the annealed membrane in an aqueous solution of a mineral acid; and(5) rinsing the annealed membrane after step (4) with deionized water to form a proton exchange membrane.34. The method of aspect 33, wherein the proton exchange ionomer is applied to the sulfonic acid-functional membrane substrate by immersion, vacuum coating, inkjet printing, or screen printing.35. A method for preparing a carboxylic acid-functional and / or phosphonic acid-functional membrane substrate, comprising:(1 ) mixing a filler comprising silica, a polyolefin polymer, a processing plasticizer, a lubricant, optionally a free radical scavenger, and optionally an antioxidant until a substantially uniform mixture is obtained;(2) introducing the mixture to a heated barrel of a screw extruder, to which is attached a sheeting die to form a sheet;(3) forwarding the sheet to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the sheet exiting from the die;(4) stretching the sheet in at least one stretching direction to form a stretched sheet;(5) extracting the processing plasticizer from the stretched sheet; and(6) surface-treating the silica with a carboxylic and / or phosphonic acidfunctional silane to form a carboxylic acid-functional and / or phosphonic acid-functional membrane substrate; wherein surface-treating the silica is performed either (i) prior to forming the mixture in step (1 ); or (ii) after extracting the processing plasticizer from the stretched sheet in step (5).36. A method for preparing a component of a membrane electrode assembly, comprising:(1 ) providing the carboxylic acid-functional and / or phosphonic acid-functional membrane substrate prepared according to the method of aspect 35;(2) applying a proton exchange ionomer to the carboxylic acid-functional and / or phosphonic acid-functional membrane substrate to form an impregnated membrane;(3) annealing the impregnated membrane to form an annealed membrane;(4) soaking the annealed membrane in an aqueous solution of a mineral acid; and(5) rinsing the annealed membrane after step (4) with deionized water to form a proton exchange membrane.37. The method of aspect 36, wherein the proton exchange ionomer is applied to the carboxylic acid-functional and / or phosphonic acid-functional membrane substrate by immersion, vacuum coating, inkjet printing, or screen printing.

[0060] The following examples are intended to illustrate various aspects of the disclosure, and should not be construed as limiting the disclosure in any way. Components that are mentioned elsewhere in the specification as suitable alternative materials for use, but which are not demonstrated in the working Examples below, are expected to provide results comparable to their demonstrated counterparts. Unless otherwise indicated, all parts are by weight.EXAMPLESMembrane substrates preparation:Table 1. Components of Membrane Substrates1 . A precipitated silica from PPG Industries with a measured D50 of 15p.2. An Ultra High Molecular Weight Polyethylene (UHMWPE), available from Celanese Corp.3. An Ultra High Molecular Weight Polyethylene (UHMWPE), available from Celanese Corp.4. A nigh molecular weight high density polyethylene (HOPE) homopolymer with a broad bimodal molecular weight distribution, from LyondellBasell5. An antioxidant available from Cytec Industries, Inc.6. A lubricant available from HB Chemical7. A titanium dioxide available from E. I. du Pont de Nemours and Company8. A processing oil from PPC LubricantsMixture Preparation

[0061] The dry ingredients specified in Table 1 were weighed into a FM-130D Littleford plough blade mixer with one high intensity chopper style mixing blade. The dry ingredients were premixed for 15 seconds using the plough blades only. The process oil was then pumped in via a hand pump through a spray nozzle at the top of the mixer, with only the plough blades running. The pumping time for the examples varied from 45 to 60 seconds. The high intensity chopper blade was turned on, along with the plough blades, and the mixture was mixed for 30 seconds, the mixer was shut off and the internal sides of the mixer were scraped to insure all ingredients were evenly mixed. The mixer was turned back on with both high intensity chopper and plough blades turned on, and the mixture was mixed for an additional 30 seconds. The mixer was turned off and the mixture placed in a storage container.Extrusion, calendaring, and extraction

[0062] The mixtures of Examples 1 and 2 were extruded and calendered into a sheet form using an extrusion system including a feeding, extrusion and calendering system described as follows. A gravimetric loss in weight feed system (K-iron model #K2MLT35D5) was used to feed each of the respective mixtures into a 27 mm twin screw extruder (model Leistritz Micro-27gg). The extruder barrel included eight temperature zones and a heated adaptor to the sheet die. The extrusion mixture feed port was located just prior to the first temperature zone. An atmospheric vent was located in the third temperature zone. A vacuum vent was located in the seventh temperature zone. The mixture was fed into the extruder at a rate of 90 g / minute. Additional processing oil also was injected at the first temperature zone, as needed, to achieve the desired total oil content in the extruded sheet. The oil contained in the extruded sheet (extrudate) being discharged from the extruder is referenced herein as the “extrudate oil weight percent”. Extrudate from the barrel was discharged into a 15- centimeter wide sheet Masterflex® die having a 1 .5 millimeter discharge opening. The extrusion melt temperature was 203 to 210°C and the throughput was 7.5 kilograms per hour. The calendering process was accomplished using a three-roll vertical calender stack with one nip point and one cooling roll. Each of the rolls had a chrome surface. Roll dimensions were approximately 41 cm in length and 14 cm in diameter. The top roll temperature was maintained between 135°C and 140°C. The middle roll temperature was maintained between 140° C and 145°C. The bottom roll was a cooling roll wherein the temperature was maintained between 10°C and 21 °C. Theextrudate was calendered into sheet form, passed over the bottom water-cooled roll, and wound up. A sample of sheet cut to a width up to 25.4 cm and length of 305 cm was rolled up and placed in a canister and exposed to hot liquid 1 ,1 ,2-trichloroethylene for approximately 7-8 hours to extract oil from the sheet sample. Afterwards, the extracted sheet was air dried and subjected to test methods described hereinafter.Stretching

[0063] Each membrane, after extrusion, then underwent biaxial orientation stretching at Biax Laboratory, Inc. NC. MDO is machine-direction orientation, and TDO is trans-direction orientation. Example 1 was stretched after extracting the plasticizer. MDO and TDO was carried out at 132°C for Example 1 , and 110°C for the rest of samples with plasticizer. The stretching ratio is listing in Table 2.Table 2: Biaxial Orientation Stretching ConditionsSulfonic acid functionalization:

[0064] The membranes of Examples 1 and 2 were treated with 10% 3- (trihydroxysilyl)-l -propanesulfonic acid aqueous solution (see Table 3) by submerging the membrane substrates into the solution for 10 minutes to fully saturate the membrane substrates. Once fully saturated, the membrane substrate was attached to 5” x 5” metal frames with TEFLON® strips to dehydrate the coating at 105°C for ten minutes, forming membrane substrates of Examples 1 A and 2A. The formulation is listed in Table 3.

[0065] An alternative method of sulfonic acid functionalization is treatment of the silica prior to the mixture preparation (1 ). In this method, a precipitated silica slurry is mixed with, for example, 3-(trihydroxysilyl)-1 -propanesulfonic acid. The mixture may be heated in a beaker at 100°C with stirring for an hour; then the modified silica dried at 1 10 °C for 14 hours. To confirm the sulfonation, a total carbon analysis may be conducted; a control and sulfonated sample are submitted for total carbon analysis. Samples may be analyzed using a Flash 2000 Elemental Analyzer. The samples are combusted at 950 °C in a vertical reactor. The generated gasses are separated usinga GC column and quantified by a thermal conductive detector. In a particular sample of sulfonated silica prepared by this method, the control sample had <0.01 % of carbon detected while the sulfonated membrane had 0.56% of carbon, confirming the bonding of the 3-(trihydroxysilyl)-1 -propanesulfonic acid and demonstrating the efficacy of this approach.Table 3. 10% 3-(trihydroxysilyl)-1 -propanesulfonic acid aqueous solution1A nonionic surfactant available from BASFTable 4. Membrane substrate properties.

[0066] Figure 1 demonstrates FTIR spectra of a control membrane (i. e., the membrane of Example 1 ) and the sulfonic acid-functionalized membrane substrate of Example 1 A. The FTIR spectrum for the sulfonic acid-functionalized membrane sample shows new peaks compared to the control sample at 2972 cm-1and 1378 car1. These peaks correspond to C-H stretching of the S-CH2 group and SO2 asymmetric stretches, respectively, which are found in 3-(trihydroxysilyl)-1 -propanesulfonic acid, used to functionalize the membrane substrate of Example 1A. Note that the absorbance peaks for each between 1000 and 1300 cm-1extend beyond the scale of the y-axis, which shows a maximum of 0.45.Membrane pore impregnation with ionomer:

[0067] Membrane substrates 1A and 2A in Table 4 above were impregnated with ionomer to prepare proton exchange membranes. Two different pore-impregnationtechniques were examined. All membrane substrates were annealed at 130-140°C for one hour after impregnation.

[0068] A first method involved soaking a membrane substrate in a solution of 30 wt.% ionomer, 35 wt.% methanol, and 35 wt.% dimethylacetamide (DMAc) for 10 minutes, removing the membrane, wiping excess solution from the surfaces, allowing the membrane to air dry, and then brushing the solution onto both opposing membrane surfaces multiple times until the membrane became transparent. The ionomers included NAFION 2021 , a sulfonated tetrafluoroethylene-based fluoropolymer copolymer having a sulfonate equivalent weight of 1 100, available from Ion Power, Inc., and 825 EW DYNEON, a copolymer of tetrafluoroethylene (TFE) and perfluorobutanesulfonylfluoride vinyl ether having a sulfonate equivalent weight of 825, available from 3M Advanced Materials Division.

[0069] The second method involved placing a circular sample of membrane substrate in a vacuum filter funnel, covering the substrate with an ionomer solution diluted with a 50 / 50 isopropyl alcohol / water mixture, and then slowly drawing the solution through the membrane substrate via an applied vacuum for 10 minutes. The impregnated substrate was then removed from the filter funnel and the surfaces were wiped to remove excess ionomer solution. For the vacuum filling method, two different ionomer solutions were examined: 30 wt.% and 40 wt.% 1100 EW NAFION.Table 5. Proton Exchange membrane preparation*Comparative Examples D and F demonstrate a membrane prepared using the membrane substrate of Example 2, which was not sulfonated.Proton-exchange capacity (IEC) Testing:

[0070] A 2-square inch sample of each membrane was soaked in 0.5M H2SO4 for at least 48 hours, rinsed three times in 200ml DI water and the membranes were stored in distilled water for 24 hours.

[0071] The membranes were then soaked in 50ml of 0.1 M NaCI solution hours at ambient temperature (295±2 K) to convert the membrane from the H+ to the Na+ form.

[0072] The membrane samples were removed and dried at 100°C then stored in a closed descendant container at ambient temperature for 48 hours.

[0073] The NaCI solution was titrated against 0.02 M NaOH to a pH 7.0 end point using phenol red indicator. The volume of NaOH consumed was used to calculate the amount of H+ in solution. Assuming complete conversion of the membrane to the Nan- form, the ion-exchange capacity (IEC, mmol H+ I g polymer) was calculated using Equation 1 :IEC = (VNaOH x CNaOH) / W (Equation 1) wherein CNHOH is the concentration of the sodium hydroxide solution (mol / cm3), VNaOH is the volume of sodium hydroxide used (cm3) and W is the mass of dried membrane sample (g). The equivalent weight, EW (g membrane sample I SO3H group) was obtained as the reciprocal of the IEC value.Membrane Proton Conductivity:

[0074] Proton conductivity of water-equilibrated films was determined by an AC impedance method. In-plane conductivity was measured after loading a membrane sample into a BekkTech 4-electrode test cell and then submerging the cell in DI water at room temperature. A Nyquist plot was generated for the membrane sample, from which the membrane impedance was extracted. Proton conductivity was determined using Equation 2: o=L / wtR (Equation 2) where o (Siemens / cm) is the proton conductivity, L (cm) is the distance between the electrodes, w (cm) and t (cm) are the width and thickness of the dry membrane sample, and R (Q) is the measured membrane resistance (impedance) taken as the real axis value at the high-frequency intercept on the Nyquist plot.Table 6. Membrane properties2Measurements over 40,000 exceed the measurement capability of the MODEL 4340 Automatic Densometer.Membrane Electrode Assembly (MEAs) Preparation:

[0075] The membrane samples were incorporated into fuel cell membrane electrode assemblies (MEAs) by spraying-coating anode and cathode layers onto the membranes and then attaching SIGRACET 22 BB carbon paper gas diffusion layers (available from SGL Carbon).

[0076] Catalyst inks used for spraying were made from: 46.2 wt.% Pt / C (TKK Corp.) with 5 wt% NAFION 2021 ionomer and 48.8% isopropanol in an ultrasonic bath. The anode and cathode platinum loading were each 0.12 mg / cm2.MEAs Testing:

[0077] Fuel cell tests were carried out at 80°C using H2 (125 standard cubic centimeters per minute (seem)) and air (500 seem) at an absolute pressure of 100 kPa (atmospheric conditions), 150 kPa, or 200 kPa and 100% relative humidity.

[0078] MEA current-voltage data were collected using a Scribner Inc. 850e test station and a Fuel Cell Technologies 5 cm2test fixture with single serpentine anode and cathode gas channels.

[0079] Data for polarization curves (plots of voltage vs. current) were collected at 80°C under back pressures of 100, 150, and 200 kPa absolute, with hydrogen and air flowing at rates of 125 and 500 standard cubic centimeters per minute (seem), respectively.

[0080] During the polarization curve measurements, voltage was swept from 0.2 V to the open circuit voltage (OCV) at intervals of 0.05 V. At each voltage point, the current was allowed to stabilize for one minute before data collection.

[0081] An Industrial Personal Computer (IPC) was employed to monitor and control various parameters, including gas flow rate, temperature, relative humidity, and pressure. This control was achieved through the utilization of mass flow controllers for gas flow rate, T-type thermocouples for temperature measurement, membrane-type humidifiers for managing humidity levels, and pressure transducers for monitoring and adjusting pressure.Hydrogen Crossover Current:

[0082] Linear sweep voltammetry (LSV) was used to measure hydrogen crossover. Humidified hydrogen (at 125 seem) was passed through the anode flow channel and humidified nitrogen (500 seem) was fed to the cathode. Using a Gamry potentiostat (Gamry Reference), voltage was swept from 0.01 V to 0.9 V (vs SHE) at 80°C and 100 kPa abs. The scan rate was 10 mV / s. The current measured from a LSV scan is due to oxidation of hydrogen gas at the anode that leaked across the membrane. From a plot of current density vs voltage, the hydrogen crossover limiting current density was determined.Table 7. Membrane electrode assembly (MEA) propertiesHigh frequency resistance (HFR):

[0083] The current interrupt method was used to measure high frequency resistance. An AC signal was applied to the electronic load to modulate the DC load current, and the voltage and current AC response was measured by a frequency response analyzer. Testing was performed with a computer-controlled Scribner Inc. 850e test station with a 50 A / 125 W Model 890CL electronic load (Scribner Associates,Inc.) and Model 880 Frequency Response Analyzer (Scribner Associates, Inc.). EIS spectra were modeled with ZView® (Scribner Associates, Inc.). HFR and EIS data acquired with AC signal = 5% of de current.Table 8. Comparison of membrane electrical performance*NAFION 211 , NAFION 212 and NAFION 115 are cast film membranes at defined thicknesses, purchased from Ion Power, Inc. The data in Table 8 demonstrate that High-Frequency Resistance per unit thickness is lower on the experimental vs. the commercial cast films, indicating higher conductivity per unit thickness in the membrane of Example E.

[0084] Fig. 2 is a graph of a polarization curve (voltage vs. current), comparing a commercially available proton exchange membrane (NAFION 115) with the membrane of Example E. Example E demonstrates a similar performance to that of NAFION 115, at a lower thickness.Membrane puncture resistance:

[0085] A Chatillon Digital Force Gauge puncture was used to test 3 specimen sheets for puncture resistance, in at least 3 different positions. The numbers reported are the average of 3 readings.Membrane Tensile properties:

[0086] Membrane Tensile properties were measured according to ASTM D-882 (2018).

[0087] Water uptake reflects the percentage increase in weight of a membrane after soaking in water and removing the free water at the surface. High water uptake indicates high water retention capability of a membrane inside fuel cell.

[0088] Dimensional change of a membrane reflects the percentage increase in area after soaking in water and removing the free water at the surface, compared to a dry membrane.Table 9. Comparison of Membrane Mechanical Properties

[0089] The membranes of the present disclosure (Examples B and E) demonstrate higher tensile strength, while exhibiting higher water uptake, compared to a commercial ion exchange membrane.

[0090] Whereas particular examples of this disclosure have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present disclosure may be made without departing from the disclosure as defined in the appended claims.

Claims

What is claimed is:1 . A membrane substrate comprising: a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and silica distributed throughout the polyolefin matrix, wherein the membrane substrate comprises 10 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the silica; wherein the polyolefin is present in the membrane substrate in an amount of 10 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; wherein the membrane substrate is in the form of a sheet and demonstrates a Gurley porosity of 1 to 1000 seconds; and wherein the membrane substrate comprises sulfonic acid-functional groups bonded thereto.

2. The membrane substrate of claim 1 , wherein the polyolefin matrix comprises high density polyethylene (HOPE) and ultrahigh molecular weight polyethylene (UHMWPE), and wherein the polyolefin matrix is present in the membrane substrate in an amount of 20 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.

3. The membrane substrate of claim 1 or 2, wherein the filler further comprises an antioxidant and / or a peroxide scavenger.

4. The membrane substrate of any of claims 1 to 3, wherein the silica demonstrates an average particle size D50 of from 5 to 100 microns, as determined by use of a laser diffraction particle size instrument.

5. The membrane substrate of any of claims 1 to 4, wherein the sheet has a thickness of 5 to 300 microns.

6. A membrane comprising:1 ) The membrane substrate of any of claims 1 to 5; and2) a proton exchange ionomer impregnated into the membrane substrate; wherein the membrane demonstrates a Gurley porosity of greater than 40,000 seconds and exhibits a proton exchange capacity of greater than or equal to 0.4 mmol / g.

7. The membrane of claim 6, wherein the proton exchange ionomer comprises a polymer having phosphonic acid- and / or sulfonic acid-functional groups.

8. The membrane of claim 7, wherein the proton exchange ionomer comprises a fluoropolymer having phosphonic acid- and / or sulfonic acid-functional groups.

9. The membrane of any of claims 6 to 8, wherein the sheet has a thickness of 5 to 300 microns.

10. The membrane of any of claims 6 to 9, wherein the membrane is a component of a membrane electrode assembly (MEA).

11. The use of the membrane of any of claims 6 to 10 in a membrane electrode assembly.

12. A method for preparing a sulfonic acid-functional membrane substrate, comprising:(1) mixing a filler comprising silica, a polyolefin polymer, a processing plasticizer, a lubricant, optionally a free radical scavenger, and optionally an antioxidant until a uniform mixture is obtained;(2) introducing the mixture to a heated barrel of a screw extruder, to which is attached a sheeting die to form a sheet;(3) forwarding the sheet to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the sheet exiting from the die;(4) stretching the sheet in at least one stretching direction to form a stretched sheet;(5) extracting the processing plasticizer from the sheet; and(6) surface-treating the silica with a sulfonic acid-functional silane to form a sulfonic acid-functional membrane substrate; wherein surface-treating the silica is performed either (i) prior to forming the mixture in step (1); or (ii) after extracting the processing plasticizer from the stretched sheet in step (5).

13. A method for preparing a component of a membrane electrode assembly, comprising:(1 ) providing the sulfonic acid-functional membrane substrate prepared according to the method of claim 12;(2) applying a proton exchange ionomer to the sulfonic acid-functional membrane substrate to form an impregnated membrane;(3) annealing the impregnated membrane to form an annealed membrane;(4) soaking the annealed membrane in an aqueous solution of a mineral acid; and(5) rinsing the annealed membrane after step (4) with deionized water to form a proton exchange membrane.

14. The method of claim 13, wherein the proton exchange ionomer is applied to the sulfonic acid-functional membrane substrate by immersion, vacuum coating, inkjet printing, or screen printing.

15. A membrane substrate comprising: a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and silica distributed throughout the polyolefin matrix, wherein the membrane substrate comprises 10 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin is present in the membrane substrate in an amount of 10 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; wherein the membrane substrate is in the form of a sheet and demonstrates a Gurley porosity of 1 to 1000 seconds; and wherein the membrane substrate comprises carboxylic acid- and / or phosphonic acid-functional groups bonded thereto.

16. The membrane substrate of claim 15, wherein the polyolefin matrix comprises high density polyethylene (HOPE) and ultrahigh molecular weight polyethylene (UHMWPE), and wherein the polyolefin matrix is present in the membrane substrate in an amount of 20 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.

17. The membrane substrate of claim 15 or 16, wherein the filler further comprises an antioxidant and / or a peroxide scavenger.

18. The membrane substrate of any of claims 15 to 17, wherein the silica demonstrates an average particle size D50 of from 5 to 100 microns, as determined by use of a laser diffraction particle size instrument.

19. The membrane substrate of any of claims 15 to 18, wherein the sheet has a thickness of 5 to 300 microns.

20. A membrane comprising:1 ) The membrane substrate of any of claims 15 to 19; and2) a proton exchange ionomer impregnated into the membrane substrate; wherein the membrane demonstrates a Gurley porosity of greater than 40,000 seconds and exhibits a proton exchange capacity of greater than or equal to 0.4 mmol / g.21 . The membrane of claim 20, wherein the proton exchange ionomer comprises a polymer having phosphonic acid- and / or sulfonic acid-functional groups.

22. The membrane of claim 21 , wherein the proton exchange ionomer comprises a fluoropolymer having phosphonic acid- and / or sulfonic acid-functional groups.

23. The membrane of any of claims 20 to 22, wherein the sheet has a thickness of 5 to 300 microns.

24. The membrane of any of claims 20 to 23, wherein the membrane is a component of a membrane electrode assembly (MEA).

25. The use of the membrane of any of claims 20 to 24 in a membrane electrode assembly.

26. A method for preparing a carboxylic acid-functional and / or phosphonic acid-functional membrane substrate, comprising:(1 ) mixing a filler comprising silica, a polyolefin polymer, a processing plasticizer, a lubricant, optionally a free radical scavenger, and optionally an antioxidant until a uniform mixture is obtained;(2) introducing the mixture to a heated barrel of a screw extruder, to which is attached a sheeting die to form a sheet;(3) forwarding the sheet to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the sheet exiting from the die;(4) stretching the sheet in at least one stretching direction to form a stretched sheet;(5) extracting the processing plasticizer from the stretched sheet; and(6) surface-treating the silica with a carboxylic and / or phosphonic acidfunctional silane to form a carboxylic acid-functional and / or phosphonic acid-functional membrane substrate; wherein surface-treating the silica is performed either (i) prior to forming the mixture in step (1 ); or (ii) after extracting the processing plasticizer from the stretched sheet in step (5).

27. A method for preparing a component of a membrane electrode assembly, comprising:(1 ) providing the carboxylic acid-functional and / or phosphonic acid-functional membrane substrate prepared according to the method of claim 26;(2) applying a proton exchange ionomer to the carboxylic acid-functional and / or phosphonic acid-functional membrane substrate to form an impregnated membrane;(3) annealing the impregnated membrane to form an annealed membrane;(4) soaking the annealed membrane in an aqueous solution of a mineral acid; and(5) rinsing the annealed membrane after step (4) with deionized water to form a proton exchange membrane.

28. The method of claim 27, wherein the proton exchange ionomer is applied to the carboxylic acid-functional and / or phosphonic acid-functional membrane substrate by immersion, vacuum coating, inkjet printing, or screen printing.

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