IMPROVED ELECTROCHEMICAL MEMBRANE

Abstract

This disclosure relates to polymer electrolyte membranes, and in particular, to a composite membrane having at least two reinforcing layers comprising a microporous polymer structure and a surprisingly high resistance to piercing. This disclosure also relates to composite membrane-assemblies and electrochemical devices comprising the composite membranes of the disclosure, and to methods of manufacture of the composite membranes.

Claims

1-28. (canceled)

29. A composite membrane for an electrochemical device, comprising: a) at least two reinforcing layers, each of said at least two reinforcing layers comprising a microporous polymer structure; and b) an ion exchange material (IEM) at least partially imbibed within the microporous polymer structure of the at least two reinforcing layers and rendering the microporous polymer structure occlusive; wherein the composite membrane has a thickness at 0% RH of at least 10 .Math.m; and wherein the microporous polymer structure is present in a total content of at least 15 vol % based on the total volume of the composite membrane.

30. The composite membrane according to claim 29, wherein a composition of the at least two reinforcing layers is the same or wherein a composition of the at least two reinforcing layers is different.

31. A composite membrane according to claim 29, wherein the microporous polymer structure comprises at least one fluorinated polymer, or wherein the microporous polymer structure comprises at least one fluorinated polymer and the fluorinated polymer is polytetrafluoroethylene (PTFE), poly(ethylene-co-tetrafluoroethylene) (EPTFE), expanded polytetrafluoroethylene (ePTFE), polyvinylidene fluoride (PVDF), expanded polyvinylidene fluoride (ePVDF), expanded poly(ethylene-co-tetrafluoroethylene) (eEPTFE) or mixtures thereof.

32. A composite membrane according to claim 31, wherein the fluorinated polymer is perfluorinated expanded polytetrafluoroethylene (ePTFE), or wherein the fluorinated polymer is perfluorinated expanded polytetrafluoroethylene (ePTFE) and at least one of: the composite membrane has a total content of microporous polymer structure of at least 5.5 g.Math.m.sup.-2 based on the sum of the mass per area of the all reinforcing layers present in the composite membrane; the composite membrane has a total content of microporous polymer structure content from 5.5 g.Math.m.sup.-2 to 80 g.Math.m.sup.-2 based on the sum of the mass per area of the all reinforcing layers present in the composite membrane.

33. A composite membrane according to claim 29, wherein the microporous polymer structure comprises a hydrocarbon polymer, or wherein the microporous polymer structure comprises a hydrocarbon polymer and the hydrocarbon polymer comprises polyethylene, polypropylene, polycarbonate, polystyrene, or mixtures thereof.

34. A composite membrane according to claim 29, wherein one of: the at least two reinforcing layers are in direct contact; or the composite membrane comprises at least one internal layer of ion exchange material between the at least two reinforcing layers.

35. A composite membrane according to claim 29, wherein the at least two reinforcing layers are separated by a distance d.

36. A composite membrane according to claim 35, wherein at least one of: the distance d is from 0.1 .Math.m to 20 .Math.m at 0% RH; the distance d is from 0.5 .Math.m to 5 .Math.m at 0% RH.

37. A composite membrane according to claim 29, wherein one of: the ion exchange material comprises more than one layer of ion exchange material, wherein the layers of ion exchange material are formed of the same ion exchange material; or the ion exchange material comprises more than one layer of ion exchange material, wherein a first layer of ion exchange material is formed of different ion exchange materials than ion exchange materials of a second layer of ion exchange material.

38. A composite membrane according to claim 29, wherein at least one of: the microporous polymer structure is fully imbibed with the ion exchange material; the microporous polymer structure of each of the at least two reinforcing layers has a first surface and a second surface, and wherein the ion exchange material forms a layer on at least one of the first surface or the second surface of each of the at least two reinforcing layers.

39. The composite membrane as in claim 29, wherein the microporous polymer structure of each of the at least two reinforcing layers has a first surface and a second surface, and wherein the microporous polymer structure is mostly imbibed with the ion exchange material, but comprises a region of un-imbibed or non-occlusive region of the microporous polymer structure closest to the first surface of at least one of the at least two reinforcing layers, or closest to the second surface of at least one of the at least two reinforcing layer, or both.

40. The composite membrane as in claim 39, wherein the microporous polymer structure of the at least two reinforcing layers is 90% occluded with the ion exchange material.

41. A composite membrane according to claim 29, wherein the average equivalent volume of the ion exchange material is from 240 cc/mole eq to 870 cc/mole eq, or wherein the average equivalent volume of the ion exchange material is from 350 cc/mole eq to 475 cc/mole eq, or wherein the average equivalent volume of the ion exchange material is from 240 cc/mole eq to 650 cc/mole eq.

42. A composite membrane according to claim 29, wherein the ion exchange material comprises at least one ionomer, or wherein the ion exchange material comprises at least one ionomer and at least one of: the at least one ionomer comprises a proton conducting polymer; the at least one ionomer has a density not lower than 1.9 g/cc at 0% relative humidity.

43. A composite membrane according to claim 29, wherein one of: the composite membrane has a thickness at 0 % RH of from 10 .Math.m to 115 .Math.m, or from 10 .Math.m to 90 .Math.m, or from 10 .Math.m to 40 .Math.m, or from 10 .Math.m to 30 .Math.m; or from 10 .Math.m to 20 .Math.m; the composite membrane has a thickness at 0 % RH of 15 .Math.m; the composite membrane has a thickness at 0 % RH of 25 .Math.m.

44. A composite membrane according to claim 29, wherein the composite membrane has an average failure pressure from 150 psi to 500 psi, when measured in accordance with the Average Puncture Pressure Failure Test Average Puncture Pressure Failure Test described herein.

45. A composite membrane according to claim 29, further comprising at least one backer layer removably attached to one or more external surfaces of the composite membrane.

46. A membrane electrode assembly for an electrochemical device, comprising: at least one electrode; and the composite membrane according to claim 29 in contact with the at least one electrode.

47. A membrane electrode assembly according to claim 46, wherein at least one of: the composite membrane is attached to the at least one electrode; the at least one electrode comprises a porous layer; the at least one electrode comprises carbon fibers.

48. A membrane electrode assembly according to claim 46, wherein the membrane electrode assembly is a redox flow battery membrane-electrode assembly comprising: a first electrode with a first surface and a second surface; a second electrode with a first surface and a second surface; and a composite membrane with a first surface and a second surface according to claim 29, wherein the second surface of the first electrode is in contact with the first surface of the composite membrane and the first surface of the second electrode is in contact with the second surface of the composite membrane.

49. The membrane electrode assembly according to claim 48 wherein at least one of: the redox flow battery membrane electrode assembly comprises a first electrode layer attached to the first surface of the composite membrane and a second electrode layer attached to a second surface of the composite membrane; the first electrode layer and/or the second electrode layer is a porous layer having a pore size from 1 to 200 .Math.m.

50. A membrane electrode assembly according to claim 46, wherein the at least one electrode is selected from a felt, a paper or a woven material.

51. A membrane electrode assembly according to claim 48, wherein at least one of: the first and/or second electrode is a carbon/platinum electrode with ionomer or wherein the electrode comprises an alloy with ionomer; the first and/or second electrode comprises doped carbon fibers.

52. A membrane electrode assembly according to claim 46, wherein the membrane electrode assembly is a fuel cell membrane electrode assembly comprising: a composite membrane as described according to claim 29, wherein the composite membrane has a first surface and a second surface; a first layer of electrode catalyst adhered to the first surface of the composite membrane; and a second layer of electrode catalyst adhered to the second surface of the composite membrane.

53. A membrane electrode assembly according to claim 52, wherein at least one of: the first and second layers of electrode catalyst are nanoporous layers having a pore size of up to 100 nm; the first and second layers of electrode catalyst comprise: one or more ionomer; a catalyst support such as carbon black; and platinum.

54. A membrane electrode assembly according to claim 46, wherein the membrane electrode assembly is an electrolyzer electrode assembly comprising: the composite membrane of claim 29, wherein the composite membrane has a first surface and a second surface, a first layer of electrode catalyst adhered to the first surface of the composite membrane; and a second layer of electrode catalyst adhered to the second surface of the composite membrane.

55. A membrane electrode assembly according to claim 52, wherein a layer of platinum or ruthenium catalyst is adhered to the composite membrane.

56. A fuel cell comprising the composite membrane according to claim 29, or the membrane electrode assembly of claim 46.

57. A redox flow battery comprising the composite membrane according to claim 29, or a membrane electrode assembly according to claim 46.

58. An electrolyzer comprising the composite membrane according to claim 29, or a membrane electrode assembly according to claim 46.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0099] FIG. 1A shows a schematic representation of the cross-section of a composite membrane according to an embodiment of the disclosure. The composite membrane has two reinforcing layers in direct contact with each other. Each of the reinforcing layers comprises a microporous polymer structure. The composite membrane also comprises two external layers of unreinforced ion exchange material. In this particular example, both reinforcing membranes are impregnated with the same ion exchange material (although in other examples the reinforcing layers may be impregnated with different ion exchange materials).

[0100] FIG. 1B shows the composite membrane of FIG. 1A with a backer layer.

[0101] FIG. 2 shows a schematic representation of the cross-section of a composite membrane according to another embodiment. The composite membrane has a similar construction to the composite membrane of FIGS. 1A and 1B, but the first reinforcing layer is impregnated with a first ion exchange material and the second reinforcing layer is impregnated with a second (different) ion exchange material.

[0102] FIG. 3A shows a schematic representation of a cross-section of a composite membrane according to an embodiment of the disclosure. The composite membrane has two reinforcing layers, each reinforcing layer comprising a microporous polymer structure. The two reinforcing layers are separated by an internal unreinforced layer of ion exchange material. The composite membrane also comprises two external layers of unreinforced ion exchange material on the outer surfaces of the reinforcing layers.

[0103] FIG. 3B shows a schematic representation of a cross-section of a redox flow battery membrane-electrode assembly comprising the composite membrane of FIG. 3A and two electrode layers.

[0104] FIG. 4 shows a schematic representation of a cross-section of a composite membrane according to another embodiment of the disclosure. The construction is similar to that of the membrane of FIG. 3 (having two reinforcing layers impregnated with ion exchange material, the reinforcing layers being separated by an internal layer of unreinforced ion exchange material arranged between the two reinforcing layers). This composite membrane has a single layer of unreinforced ion exchange material on the outer surface of one of the reinforcing layers, but no layer of unreinforced ion exchange material on the opposite side (i.e. on the outer surface of the other reinforcing layer).

[0105] FIG. 5 shows a schematic representation of a cross-section of a composite membrane according to another embodiment of the disclosure. The construction is similar to that of the membrane of FIG. 1, with two reinforcing layers in direct contact with each other without an intervening unreinforced ion exchange material layer. In this example, the composite membrane has a single layer of unreinforced ion exchange material on one outer side but no layer of unreinforced ion exchange material on the opposite side (i.e. the outer surface of one of the reinforcing layers is coated with a layer of unreinforced ion exchange material, but the outer surface of the other reinforcing layer is not coated with a layer of unreinforced ion exchange material).

[0106] FIG. 6 shows a schematic representation of a cross-section of a composite membrane according to another embodiment of the disclosure. The construction is similar to that of the membrane of FIG. 1, with two reinforcing layers in direct contact with each other without any intervening unreinforced ion exchange material layers and no outer layers of unreinforced ion exchange material.

[0107] FIG. 7 shows a schematic representation of a cross-section of a composite membrane according to another embodiment of the disclosure. In this embodiment the composite membrane has three reinforcing layers comprising a microporous polymer structure impregnated with an ion exchange material. All three reinforcing layers are in direct contact with each other and the composite membrane has two external layers of unreinforced ion exchange material disposed on opposite external surfaces of the composite membrane.

[0108] FIG. 8 shows a schematic representation of a cross-section of a composite membrane according to another embodiment of the disclosure. In this embodiment the composite membrane has three reinforcing layers, each reinforcing layer comprising a microporous polymer structure impregnated with an ion exchange material. The reinforcing layers separated from each other by internal layers of unreinforced ion exchange material. The composite membrane has two external layers of unreinforced ion exchange material on opposite external surfaces of the composite membrane.

[0109] FIG. 9 shows a schematic representation of a composite membrane similar to that of FIG. 6, having two reinforcing layers in direct contact with each other, with one of the reinforcing layers being fully imbibed with ion exchange material and the other reinforcing layer being mostly imbibed with the ion exchange material, but comprising a region of un-imbibed or non-occlusive region of the microporous polymer structure.

[0110] FIG. 10 shows a graph representing the average failure pressure (psi) of the composite membranes of the examples against the total ePTFE content of the composite membranes (mpa). The graph shows four series of data, each series having a comparable composite membrane thickness.

[0111] FIG. 11 shows Table 1, presenting the properties of the composite membranes of the examples.

[0112] FIG. 12 shows Table 2, presenting the properties of microporous polymer structures used in the composite membranes of the examples.

[0113] FIG. 13 shows a schematic representation of a method of manufacturing composite membranes according to embodiments of the present disclosure having reinforcing layers separated by internal layers of ion exchange material (ionomer).

DETAILED DESCRIPTION

[0114] This application discloses composite membranes for electrochemical devices with improved average failure pressure compared to state of the art composite membranes, which leads to an improved puncture resistance of the composite membrane by other components of the electrochemical device upon device assembly. Without wishing to be bound by theory, providing composite membranes with at least two reinforcing layers, each of said at least two reinforcing layers comprising a microporous polymer structure contributes significantly to the improvement in puncture resistance of the composite membrane compared to composite membranes of similar thickness and content of microporous polymer structure provided in a single reinforcing layer. In addition, for any given composite membrane thickness, increasing the total content of microporous polymer structure distributed between two or more reinforcing layers further improves the piercing resistance of the composite membrane. Without wishing to be bound by theory, providing a separation between at least two reinforcing layers within the composite membrane for any given microporous polymer content and thickness of composite membrane may further improve the piercing resistance of the composite membrane.

[0115] In some embodiments there is provided a composite membrane for an electrochemical device, comprising: [0116] a) at least two reinforcing layers, each of said at least two reinforcing layers comprising a microporous polymer structure; and [0117] b) an ion exchange material (IEM) at least partially imbibed within the microporous polymer structure of the at least two reinforcing layers and rendering the microporous polymer structure occlusive; wherein the composite membrane has a thickness at 0 % RH of at least about 10 .Math.m.

[0118] Embodiments have been described using volume-based values in order to provide a way for meaningful comparison between the composition of composite membranes comprising ionomers and microporous polymer structures of different densities.

[0119] In order to provide meaningful values of the content of microporous polymer structure within a composite membrane, but providing these values independently from the intrinsic molecular weight/matrix skeletal density of the microporous polymer structures, embodiments have been described using normalized total mass per area values. This takes into account that some embodiments may comprise different microporous polymer structures within the reinforced layers. The content of the of microporous polymer structure within a composite membrane has also been presented in mass per area values, which is a suitable measurement in embodiments comprising a single type of microporous polymer structure.

[0120] The microporous polymer structure may be present in a total content of at least about 20 vol % based on the total volume of the composite membrane. The composite membrane may have a total microporous polymer structure content of at least about 3.Math.10.sup.-6 m (i.e. at least about 3 .Math.m) based on the total area of the composite membrane divided by the matrix skeletal density of the microporous polymer structure.

[0121] Various definitions used in the present disclosure are provided below.

[0122] As used herein, the terms “ionomer” and “ion exchange material” refer to a cation exchange material, an anion exchange material, or an ion exchange material containing both cation and anion exchange capabilities. Mixtures of ion exchange materials may also be employed. Ion exchange material may be perfluorinated or hydrocarbon-based. Suitable ion exchange materials include, for example, perfluorosulfonic acid polymers, perfluorocarboxylic acid polymers, perfluorophosphonic acid polymers, styrenic ion exchange polymers, fluorostyrenic ion exchange polymers, polyarylether ketone ion exchange polymers, polysulfone ion exchange polymers, bis(fluoroalkylsulfonyl)imides, (fluoroalkylsulfonyl)(fluorosulfonyl)imides, polyvinyl alcohol, polyethylene oxides, divinyl benzene, metal salts with or without a polymer, and mixtures thereof. In exemplary embodiments, the ion exchange material comprises perfluorosulfonic acid (PFSA) polymers made by copolymerization of tetrafluoroethylene and perfluorosulfonyl vinyl ester with conversion into proton form.

[0123] As used herein, the “equivalent weight” (EW) of an ionomer or ion exchange material refers to the weight of polymer (in molecular mass) in the ionomer per sulfonic acid group. Thus, a lower equivalent weight indicates a greater acid content. The equivalent weight of the ionomer refers to the EW if that ionomer were in its proton form at 0% RH with negligible impurities. The term “ion exchange capacity” refers to the inverse of equivalent weight (1/EW).

[0124] As used herein, the “equivalent volume” of an ionomer or ion exchange material refers to the volume of the ionomer per sulfonic acid group. The equivalent volume (EV) of the ionomer refers to the EV if that ionomer were pure and in its proton form at 0% RH, with negligible impurities.

[0125] As used herein, the term “microporous polymer structure” refers to a polymeric matrix that supports the ion exchange material, adding structural integrity and durability to the resulting composite membrane. In some exemplary embodiments, the microporous polymer structure comprises expanded polytetrafluoroethylene (ePTFE) having a node and fibril structure. In other exemplary embodiments, the microporous polymer structure comprises track etched polycarbonate membranes having smooth flat surfaces, high apparent density, and well defined pore sizes.

[0126] As used herein, an interior volume of a microporous polymer structure is referred to as occlusive or “substantially occlusive when said interior volume has structures that is characterized by low volume of voids, less than 10% by volume, and being highly impermeable to gases, Gurley numbers larger than 10000 s. Conversely, interior volume of microporous polymer structure is referred to as “non-occluded” when said interior volume has structures that is characterized by large volume of voids, more than 10% by volume, and being permeable to gases, Gurley numbers less than 10000 s.

Composite Membranes

[0127] As illustrated in FIGS. 1-9, the composite membrane may include a plurality, e.g., two or more, imbibed reinforcing layers.

[0128] As shown in FIGS. 1 -4, a composite membrane 100, 200, 300, 400 is provided that includes a two reinforcing layers 105a,b, 205a,b, 305a,b, 405a,b, each reinforcing layer comprising a microporous polymer structure and an ion exchange material (e.g. ionomer) 110, 210a,b, 310, 410 impregnated in the microporous polymer structures of the reinforcing layers 105a,b, 205a,b, 305a,b, 405a,b. That is, each of the microporous polymer structures of the reinforcing layers 105a,b, 205a,b, 305a,b, 405a,b is imbibed with the ion exchange material 110, 210a,b, 310, 410. The ion exchange material 110, 210a,b, 310, 410 may substantially impregnate or occlude the microporous polymer structures of the reinforcing layers 105a,b, 205a,b, 305a,b, 405a,b so as to render the interior volume substantially occlusive (i.e. the interior volume having structures that is characterized by low volume of voids and being highly impermeable to gases). For example, by filling greater than 90% of the interior volume of the microporous polymer structure of each of the reinforcing layers 105a,b, 205a,b, 305a,b, 405a,b with the ion exchange material 110, 210a,b, 310, 410, substantial occlusion will occur, and the composite membrane will be characterized by Gurley numbers larger than 10000 s. As shown in FIGS. 1-4, the ion exchange material 110, 210a,b, 310, 410 is securely adhered to the internal and external surfaces of the microporous polymer structure of the reinforcing layers 105a,b, 205a,b, 305a,b, 405a,b forming imbibed reinforcing layers 104a,b, 204a,b, 304a,b, and 404a,b.

[0129] In some embodiments, the ion exchange material 110, 210a, 210b, 310, 410 in addition to being impregnated in the microporous polymer structures of the two reinforcing layers 105a,b, 205a,b, 305a,b forming imbibed reinforcing layers 104a,b, 204a,b, 304a,b, 404a,b, is provided as one or more additional layers 115a,b, 215a,b, 315a,b, 415a,b on one or more external surfaces of the imbibed reinforcing layer 104a,b, 204a,b, 304a,b, 404a,b. In other embodiments, the ion exchange material 410, 510 is provided only on one of the external surfaces of the imbibed reinforcing layer 404b, 504b, but not the other external surface of the composite membrane (i.e. not on the external surface of the opposite imbibed reinforcing layer 404a, 504a) (FIGS. 4 and 5). In other embodiments, the ion exchange material 610 is only provided impregnated in the microporous polymer structure 605 within the imbibed reinforcing layer 604a,b, i.e., without any additional layers of unreinforced ion exchange material, (FIG. 6). Nonetheless, the composite membrane 100, 200, 300, 400, 500, 600 may be characterized by the microporous polymer structures 105a,b, 205a,b, 305a,b, or 405a,b, 500a,b, 600a,b occupying greater than 20 % of the total volume of the composite membrane 100, 200 300, 400, 500, 600, 700, 800, 900 which total volume includes the volume of the imbibed reinforcing layers and the volume of any additional layers of unreinforced ion exchange material 115a,b, 215a,b, 315a,b,c, 415b,c, 515b, 715a,b and 815a,b,c,d if present.

[0130] In embodiments according to FIGS. 1A and 1B, a first imbibed reinforcing layer 104a may be formed by imbibing the microporous polymer structure of a first reinforcing layer 105a with the ion exchange material 110, and a second imbibed reinforcing layer 104b may be formed by imbibing the microporous polymer structure of a second reinforcing layer 105b with the same ion exchange material 110. For example, ion exchange material 110 may be imbibed into the microporous polymer structure of the first reinforcing layer 105a to form the first imbibed reinforcing layer 104a, and the same ion exchange material may be imbibed into the microporous polymer structure of the second reinforcing layer 105b to form the second imbibed reinforcing layer 104b. In this embodiment, the reinforcing layers 105a and 105b are in direct contact. In this embodiment, the composite layer has two external layers of ion exchange material 115a and 115b formed on the external surfaces 114a and 114b of the imbibed reinforcing layers 104a and 104b. The layers of ion exchange material 115a and 115b may comprise the same ion exchange material as the imbibed reinforcing layers 104a and 104b. Alternatively, the ion exchange material of one or both layers 115a and/or 115b may be different to that of the imbibed reinforcing layers 104a and 104b. The ion exchange material of both layers 115a and 115b may be the same or different.

[0131] Although only shown in FIG. 1B, in embodiments according to any one of the constructions shown in the figures, the composite membrane 100 may be provided on a backer layer 120 (FIG. 1B). The backer layer 120 may include a release film such as, for example, cycloolefin copolymer (COC) layer. In some embodiments, the composite membrane 100 may be released (or otherwise uncoupled) from the backer layer 120 prior to being incorporated in a membrane electrode assembly (MEA).

[0132] In embodiments according to FIG. 2, the composite membrane 200 may have a similar construction to that of embodiments according to FIG. 1, with two reinforcing layers 205a, 205b in direct contact with each other. In embodiments according to FIG. 2, the first imbibed reinforcing layer 204a may be formed by imbibing the microporous polymer structure of the first reinforcing layer 205a with a first ion exchange material 210a, and the second imbibed reinforcing layer 204b may be formed by imbibing the microporous polymer structure of the second reinforcing layer 205b with a second ion exchange material 210b that is different from the first ion exchange material 210a. In these embodiments, a first ion exchange material 210a may be imbibed into the microporous polymer structure of the first reinforcing layer 205a to form the first imbibed reinforcing layer 204a, and a second ion exchange material 210b may be imbibed into the microporous polymer structure of the second reinforcing layer 205b to form the second imbibed reinforcing layer 204b. In this embodiment, the composite membrane 200has two external layers of (unreinforced) ion exchange material 215a and 215b formed on external surfaces 214a and 214b of the imbibed reinforcing layers 204a and 204b. The external layers of ion exchange material 215a and 215b may comprise the same ion exchange material as at least one of the imbibed reinforcing layers 204a or 204b. The ion exchange material of both external layers of ion exchange material 215a and 215b may be the same or different. Alternatively, the ion exchange material of one or both external layers 215a and/or 215b may be different to that of the imbibed reinforcing layers 204a and 204b. Although not shown in the figure, the composite membrane may have a backer or release layer. The backer layer may be peeled or removed prior to assembly of the composite membrane in an electrochemical device.

[0133] In embodiments according to FIG. 3A, composite membrane 300 may comprise two reinforcing layers 305a and 305b imbibed with ion exchange material 310 to form imbibed reinforcing layers 304a,b. Composite membrane 300 may comprise three layers of unreinforced ion exchange material, a first layer 315a formed on the external surface 314a of the first imbibed reinforcing layer 304a, a second layer of unreinforced ion exchange material 315b formed on the external surface 314b of the second imbibedreinforcing layer 304b, and a third (internal) layer of unreinforced ion exchange material 315b disposed between imbibed reinforcing layers 304a and 304b. The ion exchange material of the unreinforced ion exchange material layers 315a,b,c may be the same or different, and it may be the same or different to the ion exchange material 310 of the imbibed reinforcing layers 304a and/or 304b.

[0134] FIG. 3B shows a redox flow battery membrane electrode assembly 350 comprising a composite membrane 300 as shown in FIG. 3A sandwiched between two electrode layers 320a and 320b disposed on the outermost surfaces of the composite membrane 300. In this particular embodiment, the electrode layer 320a is disposed on or attached to the outer surface of unreinforced ion exchange material layer 315a and electrode layer 320b is disposed on or attached to the outer surface of unreinforced ion exchange material layer 315b. The electrode layers 320a,b may be porous layers having a pore size from about 1 to about 200 .Math.m. The electrode layers 320b may be selected from a felt, a paper or a woven material. The electrode layers 320a,b may comprise doped carbon fibers.

[0135] FIG. 4 shows a schematic representation of a cross-section of a composite membrane 400 according to another embodiment of the disclosure. The construction is similar to that of the membrane 300 of FIG. 3A (having two reinforcing layers 405a,b impregnated with ion exchange material and thus forming imbibed reinforcing layers 304a and 304b). The imbibed reinforcing layers 304a,b are separated by an internal layer of unreinforced ion exchange material 415c disposed between the two imbibed reinforcing layers 404a and 404b. This composite membrane 400 has a single layer of unreinforced ion exchange material 415b on the outer surface of imbibed reinforcing layer 404b, but no layer of unreinforced ion exchange material on the opposite side (i.e. on the outer surface of the other imbibed reinforcing layer 404a).

[0136] FIG. 5 shows a schematic representation of a cross-section of a composite membrane 500 according to another embodiment of the disclosure. The construction is similar to that of the membrane of FIG. 1, with two reinforcing layers 505a, 505b in direct contact with each other without an intervening unreinforced ion exchange material layer. The two reinforcing layers 505a,bare imbibed with an ion exchange material 510 to form imbibed reinforcing layers 504a, and 504b. In this example, the composite membrane 500 has a single layer of unreinforced ion exchange material 515b on one outer side of the composite membrane (disposed on outer surface 514b of imbibed reinforcing layer 504b) but no layer of unreinforced ion exchange material disposed on the opposite side (i.e. the outer surface 514b of imbibed reinforcing layer 504b is coated with a layer of unreinforced ion exchange material 515b, but the outer surface 514a of the other reinforcing layer 504a is not coated with a layer of unreinforced ion exchange material).

[0137] In embodiments according to FIG. 6, the composite membrane 600 may include a plurality, e.g., two or more, imbibed reinforcing layers 604a and 604b (or more, but not shown) formed by two (or more) reinforcing layers 605a and 605b comprising microporous polymer structures which may be imbibed with one or more ion exchange materials 610a and 610b, which may be the same or different. In embodiments according to FIG. 6, the imbibed reinforcing layers may not have external or internal layers of unreinforced ion exchange material. That is, the composite membrane 600 may not comprise any additional layer of unreinforced ion exchange material.

[0138] In embodiments according to FIG. 7, the composite membrane 700 may comprise three imbibed reinforcing layers 704a, 704b and 704c. In these embodiments, the imbibed reinforcing layers are in contact with each other (i.e. there are no internal layers of unreinforced ion exchange material between the imbibed reinforcing layers 704a, 704c, 704b). Each of the imbibed reinforcing layers may comprise a reinforcing layer 705a,b,c comprising a microporous polymer structure. The microporous polymer structure of all of the reinforcing layers 705a,b,c may be the same. The microporous polymer structure of some of the reinforcing layers 705a,b,c may be the same while the microporous polymer structure of at least one of the reinforcing layers 705a, 705b or 705c may be different. The microporous polymer structure of the reinforcing layers 705a, 705b and 705c may be fully or partially imbibed with ion exchange material 710a, 710b and 710c respectively to render the microporous polymer structure occlusive and therefore forming imbibed reinforcing layers 704a, 705b, and 704c respectively. The ion exchange materials may comprise one or more ionomer. Each of the ion exchange materials 710a, 710b and 710c may be the same or may be different to all or some of the other ion exchange materials of the composite membrane. The composite membrane 700 shown in FIG. 7 has a layer of unreinforced ion exchange material 715a on the external surface 714a of the first imbibed reinforcing layer 704a. The composite membrane has another layer of unreinforced ion exchange material 715b on the external surface 714b of the third imbibed reinforcing layer 704b (i.e. the outermost imbibed reinforcing layer opposite reinforcing layer 704a).

[0139] In embodiments according to FIG. 8, the composite membrane 800 may comprise three imbibed reinforcing layers 804a, 804b and 804c. Each of the imbibed reinforcing layers may comprise a reinforcing layer 805a, 805b, 805c comprising a microporous polymer structure. The microporous polymer structure of all of the reinforcing layers 805a,b,c may be the same. The microporous polymer structure of some of the reinforcing layers 805a,b,c may be the same while the microporous polymer structure of at least one of the reinforcing layers 805a, 805b or 805c may be different. The microporous polymer structure of the reinforcing layers 805a, 805b and 805c may be fully or partially imbibed with ion exchange material 810a, 810b and 810c respectively to form imbibed reinforcing layers 804a, 804b, and 804c respectively. The ion exchange materials 810a, 810b and 810c may comprise one or more ionomer. Each of the ion exchange materials 810a, 810b and 810c may be the same or may be different to all or some of the other ion exchange materials of the composite membrane. The composite membrane 800 shown in FIG. 8 has a layer of unreinforced ion exchange material 815a on the external surface 814a of the first imbibed reinforcing layer 804a. The composite membrane has another layer of unreinforced ion exchange material 815b on the external surface 814b of the third imbibed reinforcing layer 804b. In addition, in these embodiments there is an internal layer of unreinforced ion exchange material 815c between imbibed reinforcing layers 804a and 804c. There is another internal layer of unreinforced ion exchange material 815d between imbibed reinforcing layers 804c and 804b.The internal layers 815c,d form a distance d between imbibed reinforcing layers 804a and 804c and between imbibed reinforcing layers 804c and 804b.

[0140] Although not specifically shown, other embodiments of composite membranes as described herein may comprise three or more imbibed reinforcing layers each comprising a reinforcing layer comprising a microporous polymer structure and an ion exchange material imbibed or partially imbibed within the microporous polymer material. In some embodiments, the composite membrane may have only one external layer of unreinforced ion exchange material on one of the external surfaces of the composite membrane. In some embodiments, the composite membrane may have external layers of unreinforced ion exchange material on both external surfaces of the imbibed layers. In some embodiments, the composite membrane may have one or more internal layers of unreinforced ion exchange material between at least two of the imbibed reinforcing layers. In some embodiments, the composite membrane may have internal layers of unreinforced ion exchange material between each of the imbibed reinforcing layers. In some embodiments, the composite membrane may have internal layers of unreinforced ion exchange material between each of the imbibed reinforcing layers and a single external layer of unreinforced ion exchange material on one of the external surfaces of the composite membrane. In some embodiments, the composite membrane may have internal layers of unreinforced ion exchange material between each of the imbibed reinforcing layers and external layers of unreinforced ion exchange material on both of the external surfaces of the composite membrane.

[0141] The imbibed reinforcing layers of the composite membrane 100, 200, 300, 400, 500, 600, 700, 800, 900 may be constructed with reinforcing layers comprising two (or more) different microporous polymer structures. For example, with reference to FIG. 1, the first imbibed reinforcing layer 104a may be formed by imbibing a first reinforcing layer 105a comprising a first microporous polymer structure with the ion exchange material 110, and the second imbibed reinforcing layer 104b may be formed by imbibing a second reinforcing layer 105b comprising a second microporous polymer structure with the same ion exchange material 110. In these embodiments, the first reinforcing layer 105a and the second reinforcing layer 105b are different. The principle of employing different types of reinforcing layers in the composite membrane architecture may be applied to embodiments according to any of the Figures. For example, in embodiments according to FIG. 2, the first imbibed reinforcing layer 204a may be formed by imbibing a first reinforcing layer 205a comprising a first microporous polymer structure with a first ion exchange material 210a, and the second imbibed reinforcing layer 204b may be formed by imbibing a second reinforcing layer 205b comprising a second microporous polymer structure 205b with a second ion exchange material 210b. In these embodiments, the first reinforcing layer 205a and the second reinforcing layer 205b are different. Therefore, in the composite membranes described herein and shown in the Figures, the first microporous polymer structure may be the same as or different from the second microporous polymer structure. The first ion exchange material may be the same as or different from the second ion exchange material.

[0142] In additional embodiments, part of the microporous polymer structure of the reinforcing layers 105a,b, 205a,b, 305a,b, 405a,b, 505a,b, 605a,b, 705a,b,c, 805a,b,c (e.g. top surface area or bottom surface area) may include a non-occlusive (i.e. the interior volume having structures that is characterized by high volume of voids and being highly permeable to gases) area (not shown in the Figures) that is free or substantially free of the ion exchange material. The location of the non-occlusive area is not limited to the top surface area of the microporous polymer structure. As provided above, the non-occlusive area may be provided on a top surface area of the microporous polymer structure of any or all of the reinforcing layers.

[0143] Yet in other embodiments, the non-occlusive area may include a small amount of the ion exchange material present in an internal surface of the microporous polymer structure as a thin node and fibril coating. However, the amount of the ion exchange material may be not large enough to render the microporous polymer structure occlusive, thereby forming the non-occlusive area.

[0144] FIG. 9 shows a schematic representation of a composite membrane 900 similar to that of FIG. 6, having two reinforcing layers 905a, 905b in direct contact with each other. Each reinforcing layer 905a, 905b comprises a microporous polymer structure. Reinforcing layer 905a has a first surface 911a, and a second surface 912a. Similarly, reinforcing layer 905b has a first surface 911b, and a second surface 912b. The microporous polymer structure 905b is fully imbibed with an ion exchange material 910b forming an occlusive imbibed reinforcing layer 904b. However, the microporous polymer structure 905a is mostly imbibed with ion exchange material 910a, but comprises a region 915a of un-imbibed or a non-occlusive area of the microporous polymer structure 905a closest to the first surface 911a of reinforcing layer 905a. Within the context of this disclosure, mostly or substantially imbibed may mean that the microporous polymer structure is about 90 % occluded with ion exchange material. Therefore, reinforcing layer 905a forms a partially imbibed reinforcing layer 904a, which is not fully occlusive. In other similar embodiments (not shown), the first reinforcing layer 905a comprises a region 915a of un-imbibed or non-occlusive area of the microporous polymer structure 905a closest to the second surface 912b of reinforcing layer 905b.

[0145] Yet in other embodiments similar to that of FIG. 9, the first reinforcing layer 905a is fully occlusive, but the second reinforcing layer 910b comprises a region of un-imbibed or non-occlusive region of the microporous polymer structure 905a closest to the first surface 911b or the second surface 912b of reinforcing layer 910b. Yet in other embodiments (not shown), both of the reinforcing layers 910a and 910b may comprise a region of un-imbibed or a non-occlusive region near one of the surfaces 911a, 912a, 911b or 912b of each reinforcing layer 910a and 910b. The microporous polymer structure of the partially imbibed reinforcing layers 904a and/or 904b may be about 90% occluded with the ion exchange material.

[0146] In embodiments in which there is no internal layer of unreinforced ion exchange material between the at least two reinforcing layers (FIGS. 1A, 1B, 2, 5, 6, 7 and 9) the at least two reinforcing layers may be in direct contact (i.e. the at least two reinforcing layers may be separated by a distance d of about 0 .Math.m.

[0147] In embodiments in which the composite membrane comprises internal layers of unreinforced ion exchange material between at least two of the reinforcing layers (FIGS. 2, 3 and 8), the at least two reinforcing layers may be separated by a distance d. The distance d may be from about 1 .Math.m to about 10 .Math.m, or from about 2 .Math.m to about 8 .Math.m, or from about 4 .Math.m to about 6 .Math.m, or from about 1 .Math.m to about 5 .Math.m, or from about 5 .Math.m to about 10 .Math.m, or from about 6 .Math.m to about 8 .Math.m. The distance d may be about 1 .Math.m, or about 2 .Math.m, or about 3 .Math.m, or about 4 .Math.m, or about 5 .Math.m, or about 6 .Math.m, or about 7 .Math.m, or about 8 .Math.m, or about 9 .Math.m, or about 10 .Math.m. The distance d may be the thickness of the internal layer of unreinforced ion exchange material disposed between two contiguous reinforcing layers.

Microporous Polymer Structure

[0148] The composite membrane may have at least two reinforcing layers comprising a microporous polymer structure. The composite membrane may have two or more reinforcing layers comprising a microporous polymer structure. For example, the composite membrane may have 2, 3, 4,5,6 7, 8, 9 or 10 reinforcing layers, each reinforcing layer comprising a microporous polymer structure.

[0149] A suitable microporous polymer structure depends largely on the application in which the composite membrane is to be used. The microporous polymer structure preferably has good mechanical properties, is chemically and thermally stable in the environment in which the composite membrane is to be used, and is tolerant of any additives used with the ion exchange material for impregnation.

[0150] As used herein, the term “microporous” refers to a structure having pores. According to various optional embodiments, the pores may have an average pore size from 0.01 to 100 microns, e.g., from 0.05 to 20 microns or from 0.1 to 1 microns.

[0151] A suitable microporous polymer structure is intended to refer to a layer having a thickness of at least about 0.1 .Math.m, optionally from about 0.5 .Math.m to about 230 .Math.m, or from about 1 .Math.m to about 100 .Math.m, or from about 1 .Math.m to about 50 .Math.m, and having an average micropore size from about 0.05 .Math.m to about 20 .Math.m, e.g., from 0.1 .Math.m to 1 .Math.m.

[0152] A suitable microporous polymer structure of the reinforcing layers 105a,b, 205a,b, 305a,b, 405a,b, 505a,b, 605a,b, 705a,b,c, 805a,b,c, 905a,b for electrochemical applications may include porous polymeric materials. The porous polymeric materials may include fluoropolymers, chlorinated polymers, hydrocarbons, polyamides, polycarbonates, polyacrylates, polysulfones, copolyether esters, polyethylene, polypropylene, polyvinylidene fluoride, polyaryl ether ketones, polybenzimidazoles, poly(ethylene-co-tetrafluoroethylene), poly(tetrafluoroethylene-co-hexafluoropropylene). In some embodiments, the microporous polymer structure of the reinforcing layers includes a perfluorinated porous polymeric material. The perfluorinated porous polymeric material may include polytetrafluoroethylene (PTFE), expanded polytetrafluoroethylene (ePTFE), polyvinylidene fluoride (PVDF), expanded polyvinylidene fluoride (ePVDF), expanded poly(ethylene-co-tetrafluoroethylene) (eEPTFE) or mixtures thereof.

[0153] In some embodiments, the microporous polymer structure includes a hydrocarbon material. The hydrocarbon material may include polyethylene, expanded polyethylene, polypropylene, expanded polypropylene, polystyrene, polycarbonate, track etched polycarbonate or mixtures thereof. Examples of suitable perfluorinated porous polymeric materials for use in fuel cell applications include ePTFE made in accordance with the teachings of U.S. Pat. No. 8,757,395, which is incorporated herein by reference in its entirety, and commercially available in a variety of forms from W. L. Gore & Associates, Inc., of Elkton, Md.

[0154] In embodiments in which the microporous polymer structure comprises ePTFE, the total mass per area of the microporous polymer structure may be from about 5.5 g/m.sup.2 to about 20 g/m.sup.2 based on the sum of the mass per area of all microporous layers present in the composite membrane. For example, In embodiments in which the microporous polymer structure comprises ePTFE, the total mass per area of the microporous polymer structure may be about 5.5 g/m.sup.2, or about 5.8 g/m.sup.2, or about 6 g/m.sup.2, or about 7 g/m.sup.2, or about 8 g/m.sup.2, or about 9 g/m.sup.2, or about 10 g/m.sup.2, or about 11 g/m.sup.2, or about 12 g/m.sup.2, or about 13 g/m.sup.2, or about 14 g/m.sup.2, or about 15 g/m.sup.2, or about 16 g/m.sup.2, or about 17 g/m.sup.2, or about 18 g/m.sup.2, or about 19 g/m.sup.2, or about 20 g/m.sup.2, based on the sum of the mass per area of all microporous layers present in the composite membrane.

Ion Exchange Material

[0155] A suitable ion exchange material may be dependent on the application in which the composite membrane is to be used. The ion exchange material preferably has an average equivalent volume from about 240 cc/mole eq to about 870 cc/mole eq, optionally from about 240 cc/mole eq to about 650 cc/mole eq, optionally from about 350 cc/mole eq to about 475 cc/mole eq, and is chemically and thermally stable in the environment in which the composite membrane is to be used. A suitable ionomer may include an ion exchange material such as a cation exchange material, an anion exchange material, or an ion exchange material containing both cation and anion exchange capabilities. In some embodiments, the ion exchange material comprises a proton conducting polymer or cation exchange material. The ion exchange material may perfluorocarboxylic acid polymers, perfluorophosphonic acid polymers, styrenic ion exchange polymers, fluorostyrenic ion exchange polymers, polyarylether ketone ion exchange polymers, polysulfone ion exchange polymers, bis(fluoroalkylsulfonyl)imides, (fluoroalkylsulfonyl)(fluorosulfonyl)imides, polyvinyl alcohol, polyethylene oxides, divinyl benzene, metal salts with or without a polymer and mixtures thereof. Examples of suitable perfluorosulfonic acid polymers include Nafion® (E.I. DuPont de Nemours, Inc., Wilmington, Del., US), Flemion® (Asahi Glass Co. Ltd., Tokyo, JP), Aciplex® (Asahi Chemical Co. Ltd., Tokyo, JP), Aquivion® (SolvaySolexis S.P.A, Italy), and 3MTM (3 M Innovative Properties Company, USA) which are commercially available perfluorosulfonic acid copolymers. Other examples of suitable perfluorosulfonic acid polymers include perfluorinated sulfonyl (co)polymers such as those described in U.S. Pat. No. 5,463,005.

Properties of the Composite Membrane

[0156] As discussed above, the composite membrane comprises microporous polymer structure and ion exchange material imbibed into the microporous polymer structure thereby forming two distinct materials that achieve improved piercing resistance of the composite membrane. Without wishing to be bound by theory, the piercing resistance of the composite membranes may be influenced by the distribution of the total content of the microporous polymer structure in multiple (i.e. at least two) reinforcing layers compared to the same content of microporous polymer structure provided in a single reinforcing layer within the architecture of the composite membrane. Furthermore, the piercing resistance of the composite membranes may be influenced by the total content of microporous polymer structure within the composite membrane.

[0157] The composite membrane may have a thickness at 0 % RH of at least about 10 .Math.m, for example from about 10 .Math.m to about 115 .Math.m, or from about 10 .Math.m to about 100 .Math.m, or from about 10 .Math.m to about 90 .Math.m, or from about 10 .Math.m to about 80 .Math.m, or from about 10 .Math.m to about 70 .Math.m, or from about 10 .Math.m to about 60 .Math.m, or from about 10 .Math.m to about 50 .Math.m, or from about 10 .Math.m to about 40 .Math.m, or from about 10 .Math.m to about 30 .Math.m, or from about 10 .Math.m to about 20 .Math.m, or from about 10 .Math.m to about 15 .Math.m, or from about 10 .Math.m to about 12 .Math.m, or from about 20 .Math.m to about 60 .Math.m, or from about 30 .Math.m to about 60 .Math.m, or from about 40 .Math.m to about 60 .Math.m, or from about 12 .Math.m to about 30 .Math.m, or from about 12 .Math.m to about 20 .Math.m, or from about 15 .Math.m to about 30 .Math.m, or from about 15 .Math.m to about 20 .Math.m, or from about 20 .Math.m to about 30 .Math.m. The composite membrane may have a thickness at 0 % RH of about 10 .Math.m, or about 11 .Math.m, or about 12, or about 13 .Math.m, or about 14 .Math.m, or about 15 .Math.m, or about 16 .Math.m, or about 17 .Math.m, or about 18 .Math.m, or about 19 .Math.m, or about 20 .Math.m, or about 21 .Math.m, or about 22 .Math.m, or about 23 .Math.m, or about 24 .Math.m, or about 25 .Math.m, or about 30 .Math.m, or about 35 .Math.m, or about 40 .Math.m, or about 45 .Math.m, or about 50 .Math.m, or about 55 .Math.m, or about 60 .Math.m, or about 65 .Math.m, or about 70 .Math.m, or about 75 .Math.m. The composite membrane may not have a thickness at 0% RH below about 10 .Math.m.

[0158] In some embodiments, the microporous polymer structure of the reinforcing layers occupies from about 15 vol % to about 70 % based on the total volume of the composite membrane, or from about 20 vol % to about 70 %,or from about 30 vol % to about 70 %, or from about 40 vol % to about 70 %, or from about 50 vol % to about 70 %, or from about 65 vol % to about 70 %, or from about 25 vol % to about 60 % or from about 20 vol % to about 50 %, or from about 20 vol % to about 40 %, or from about 20 vol % to about 30 %, or from about 40 vol % to about 60 %, or from about 40 vol % to about 50 % based on the total volume of the composite membrane. The microporous polymer structure of the reinforcing layers may be present in an amount of about 15 vol %, or about 20 vol %, or about 25 vol %, or about 30 vol %, or about 35 vol %, or about 40 vol %, or about 45 vol %, or about 50 vol %, or about 55 vol %, or about 60 vol %, or about 65 vol %, or about 70 vol %, based on the total volume of the composite membrane.

[0159] In some embodiments, the equivalent volume of the ion exchange material 110 is from about 240 cc/mole eq to about 870 cc/mol eq. The ion exchange material may have a total equivalent weight (EW) from about 400 g/eq to about 2000 g/eq SO.sub.3—. In various embodiments, the acid content of the composite membrane 100, 200, 300, 400, 500, 600, 700, 800, 900 is greater than 1.2 meq/cc, for example from 1.2 meq/cc to 3.5 meq/cc at 0% relative humidity. In various embodiments, the thickness of the composite membrane is from about 10 .Math.m to about 115 .Math.m. Specifically, according to embodiments, the thickness of the composite membrane is from about 10 .Math.m to about 115 .Math.m while the acid content of the composite membrane is kept between 1.2 meq/cc to 3.5 meq/cc.

[0160] The volume % of the microporous polymer structure of the reinforcing layers in the composite material refers to the space occupied by the microporous polymer structure, which is free of the ionomer. Accordingly, the volume % of the microporous polymer structure in the composite material is different than the imbibed layer which contains ionomer. The volume % of the microporous polymer structure in the composite material is affected by the humidity. Therefore, the experiments discussed below regarding volume % are conducted at dry conditions (e.g. 0 % relative humidity (RH)).

[0161] In some embodiments, the normalized total content of the microporous polymer structure within the composite membrane may be at least about 3.Math.10.sup.-6 m, or about 3.5.Math.10.sup.-6 m, or about 4.Math.10.sup.-6 m, or about 4.5.Math.10.sup.-6 m, or about 5.Math.10.sup.-6 m, or about 5.5.Math.10.sup.-6 m, or about 6.Math.10.sup.-6 m, or about 6.5.Math.10.sup.-6 m, or about 7.Math.10.sup.-6 m, or about 8.Math.10.sup.-6 m, or about 8.5.Math.10.sup.-6 m, or about 9.Math.10.sup.-6 m based on the total area of the composite membrane.

[0162] The equivalent weight of the ion exchange material is also affected by the humidity. Therefore, the experiments discussed below regarding equivalent weight are conducted at dry conditions (e.g. 0 % relative humidity (RH)) at an ideal state were presence of water does not affect the value of equivalent volume and meaningful comparison between different ionomers can be drawn.

[0163] As provided above, it is surprising and unexpected that the puncture resistance of the composite membrane is dramatically improved by distributing the microporous polymer structure content within two or more reinforcing layers for any given content of microporous polymer structure and composite membrane thickness.

[0164] The composite membrane may have an average failure pressure of at least about 150 psi, when measured by the Average Puncture Pressure Failure Test described hereinbelow. For example, the composite membrane may have an average failure pressure of at least about 150 psi, or at least about 160 psi, or at least about 170 psi, or at least about 180 psi, or at least about 190 psi, or at least about 200 psi, when measured by the Average Puncture Pressure Failure Test described hereinbelow.

[0165] The composite membrane may have an average failure pressure of from about 150 psi to about 500 psi, when measured by the Average Puncture Pressure Failure Test described hereinbelow, or from about 150 psi to about 450 psi, or from about 150 psi to about 400 psi, or from about 150 psi to about 350 psi, or from about 150 psi to about 300 psi, or from about 200 psi to about 400 psi, or from about 200 psi to about 350 psi, when measured by the Average Puncture Pressure Failure Test described hereinbelow.

[0166] The composite membrane may have an average failure pressure of from about 150 psi, or about 200 psi, or about 250 psi, or about 300 psi, or about 350 psi, or about 400 psi, or about 450 psi, or about 500 psi, when measured by the Average Puncture Pressure Failure Test described hereinbelow.

[0167] The membranes were prepared following a sequential coating process, such as that shown in FIG. 13. For membranes that have internal layers of ionomer between reinforcing layers, the method 1500 (FIG. 13) comprises the following steps: [0168] 1510) coating a backer with a first ionomer solution by providing a backer layer and depositing a liquid layer of the first ionomer solution; [0169] 1520) laminating (depositing) a first reinforcing layer comprising a microporous polymer structure over the liquid layer of the first ionomer solution and allowing the microporous polymer structure of the first reinforcing layer to become imbibed or at least partially imbibed with the first ionomer solution; [0170] 1530) optionally drying the laminate; [0171] 1540) coating the imbibed first reinforcing layer with a liquid layer of a second ionomer solution; [0172] 1550) lamination (depositing) a second reinforcing layer comprising a microporous polymer structure over the liquid layer of the second ionomer solution and allowing the microporous polymer structure of the second reinforcing layer to become imbibed or at least partially imbibed with the second ionomer solution; [0173] 1560) optionally drying the laminate; [0174] 1570) coating the outermost surface of the laminate which is furthest away from the backer with a final liquid layer of a third ionomer solution; and [0175] 1580) drying the laminate.

[0176] Optionally, the manufacturing method includes repeating steps 1560), 1570) and 1580) with further reinforcing layers and liquid layers of ionomer solution and drying the laminate. For example, for composite membranes comprising three reinforcing layers, a third liquid layer of a third ionomer solution may be deposited over the imbibed second reinforcing layer and applying a third reinforcing layer over the third layer of ionomer solution, and then the laminate may be dried. In some embodiments, the process comprises adding even further ionomer and reinforcing layers, and drying the laminate.

[0177] Membrane electrode assemblies may be prepared by depositing electrodes (I.e. anode and cathode) on the composite membranes by any suitable techniques known in the art. For example, solid electrode layers be pressed against the composite membrane by any suitable techniques. Alternatively, (liquid) electrode inks may be applied on the composite membrane. Upon drying the composite, the solvent of the electrode ink may dry to form a solid electrode layer. For the avoidance of doubt, the backer must be removed from the composite membrane before applying a cathode or cathode fluid diffusion layer. The ionomers in the ionomer solutions employed in each of the ionomer layers may be the same or different. The reinforcing layers employed in the electrolyte composite membrane may be all the same, or at least one of the reinforcing layers may be different.

EXAMPLES

Test Procedures and Measurement Protocols Used in Examples

Bubble Point

[0178] The Bubble Point was measured according to the procedures of ASTM F316-86. Isopropyl alcohol was used as the wetting fluid to fill the pores of the test specimen. The Bubble Point is the pressure of air required to create the first continuous stream of bubbles detectable by their rise through the layer of isopropyl alcohol covering the microporous polymer matrix. This measurement provides an estimation of maximum pore size.

Non-Contact Thickness

[0179] A sample of microporous polymer structure was placed over a flat smooth metal anvil and tensioned to remove wrinkles. Height of microporous polymer structure on anvil was measured and recorded using a non-contact Keyence LS-7010M digital micrometer. Next, height of the anvil without microporous polymer matrix was recorded. Thickness of the microporous polymer structure was taken as a difference between micrometer readings with and without microporous structure being present on the anvil.

Mass-Per-Area

[0180] Each Microporous polymer structure was strained sufficient to eliminate wrinkles, and then a 10 cm.sup.2 piece was cut out using a die. The 10 cm.sup.2 piece was weighed on a conventional laboratory scale. The mass-per-area (M/A) was then calculated as the ratio of the measured mass to the known area. This procedure was repeated 2 times and the average value of the M/A was calculated.

Apparent Density of Microporous Polymer Structure

[0181] The apparent density of the microporous polymer structure was calculated using the non-contact thickness and mass-per-area data using the following formula:

[00003]$\begin{array}{l}Apparentdensit{y}_{microporouspolymerstructure}=\\ \frac{\left\{M/A_{microporouspolymerstructure}\right\}}{\left\{non-contactthickness\right\}}=\left[g/cc\right]\end{array}$

Porosity of Microporous Polymer Structure

[0182] The porosity of the microporous polymer structure was calculated using the apparent density and skeletal density data using the following formula:

[00004]$\begin{array}{l}Porosit{y}_{microporouspolymerstructure}=\\ \frac{\left\{Apparentdensit{y}_{microporouspolymerstructure}\right\}}{\left\{Skeletaldensit{y}_{microporouspolymerstructure}\right\}}\end{array}$

Solids Concentration of Solutions of Ion Exchange Material (IEM)

[0183] Herein, the terms “solution” and “dispersion” are used interchangeably when referring to ion exchange materials (IEMs). This test procedure is appropriate for solutions in which the IEM is in proton form, and in which there are negligible quantities of other solids. A volume of 2 cubic centimeters of IEM solution was drawn into a syringe and the mass of the syringe with solution was measured via a balance in a solids analyzer (obtained from CEM Corporation, USA). The mass of two pieces of glass fiber paper (obtained from CEM Corporation, USA) was also measured and recorded. The IEM solution was then deposited from the syringe into the two layers of glass fiber paper. The glass fiber paper with the ionomer solution was placed into the solids analyzer and heated up to 160° C. to remove the solvent liquids. Once the mass of the glass fiber paper and residual solids stopped changing with respect to increasing temperature and time, it was recorded. It is assumed that the residual IEM contained no water (i.e., it is the ionomer mass corresponding to 0% RH). After that, the mass of the emptied syringe was measured and recorded using the same balance as before. The ionomer solids in solution was calculated according to the following formula:

[00005]$\begin{array}{l}\left\{\begin{array}{c}\text{wt\% solids of}\\ \text{IEM solution}\end{array}\right\}=\\ \frac{\left\{\begin{array}{c}\text{Mass of glass fiber paper}\\ \text{with residual solids}\end{array}\right\}-\left\{\text{Mass of glass fiber paper}\right\}}{\left\{\text{Mass of full syringe}\right\}-\left\{\text{Mass of emptied syringe}\right\}}=\\ \left[wt\%\right]\end{array}$

Equivalent Weight (EW) of an IEM

[0184] The following test procedure is appropriate for IEM comprised of a single ionomer resin or a mixture of ionomer resins that is in the proton form (i.e., that contains negligible amounts of other cations), and that is in a solution that contains negligible other ionic species, including protic acids and dissociating salts. If these conditions are not met, then prior to testing the solution must be purified from ionic impurities according to a suitable procedure as would be known to one of ordinary skill in the art, or the impurities must be characterized and their influence on the result of the EW test must be corrected for.

[0185] As used herein, the EW of an IEM refers to the case when the IEM is in its proton form at 0% RH with negligible impurities. The IEM may comprise a single ionomer or a mixture of ionomers in the proton form. An amount of IEM solution with solids concentration determined as described above containing 0.2 grams of solids was poured into a plastic cup. The mass of the ionomer solution was measured via a conventional laboratory scale (obtained from Mettler Toledo, LLC, USA). Then, 5 ml of deionized water and 5 ml of 200 proof denatured ethanol (SDA 3C, Sigma Aldrich, USA) is added to ionomer solution in the cup. Then, 55 ml of 2N sodium chloride solution in water was added to the IEM solution. The sample was then allowed to equilibrate under constant stirring for 15 minutes. After the equilibration step, the sample was titrated with 1N sodium hydroxide solution. The volume of 1N sodium hydroxide solution that was needed to neutralize the sample solution to a pH value of 7 was recorded. The EW of the IEM (EW.sub.IEM) was calculated as:

[00006]$E{W}_{\text{IEM}}=\frac{\left\{\begin{array}{c}\text{Mass of}\\ \text{IEM solution}\end{array}\right\}\times \left\{\begin{array}{c}\text{wt\% solids of}\\ \text{IEM solution}\end{array}\right\}}{\left\{\begin{array}{c}\text{Volume of}\\ \text{NaOH solution}\end{array}\right\}\times \left\{\begin{array}{c}\text{Normality of}\\ \text{NaOH solution}\end{array}\right\}}=\left[\frac{g}{moleeq.}\right]$

[0186] When multiple IEMs were combined to make a composite membrane, the average EW of the IEMs in the composite membrane was calculated using the following formula:

[00007]$\begin{array}{l}E{W}_{\text{IEM\_average}}=\left[\frac{\left\{\begin{array}{c}\text{Mass fraction}\\ \text{of IEM 1}\end{array}\right\}}{\left\{E{W}_{\text{IEM,1}}\right\}}+\frac{\left\{\begin{array}{c}\text{Mass fraction}\\ \text{of IEM 2}\end{array}\right\}}{\left\{E{W}_{\text{IEM,2}}\right\}}+.Math..\right)\\ {\left(\frac{\left\{\begin{array}{c}\text{Mass fraction}\\ \text{of IEM N}\end{array}\right\}}{\left\{E{W}_{\text{IEM,N}}\right\}}\right]}^{-1}=\left[\frac{g}{moleeq.}\right],\end{array}$

where the mass fraction of each IEM is with respect to the total content of all IEMs. This formula was used both for composite membranes containing ionomer blends and for composite membranes containing ionomer layers.

Equivalent Volume (EV) of Ion Exchange Material

[0187] As used herein, the Equivalent Volume of the IEM refers to the EV if that IEM were pure and in its proton form at 0% RH, with negligible impurities. The EV was calculated according to the following formula:

[00008]$E{V}_{IEM}=\frac{\left\{\begin{array}{c}\text{Equivalent Weight}\\ \text{of IEM}\end{array}\right\}}{\left\{\begin{array}{c}\text{Volumetric density}\\ \text{of IEM at 0\% RH}\end{array}\right\}}=\left[\frac{cc}{moleeq.}\right]$

[0188] The Equivalent Weight of each IEM was determined in accordance with the procedure described above. The IEMs used in these application were perfluorosulfonic acid ionomer resins the volumetric density of perfluorosulfonic acid ionomer resin was taken to be 1.9 g/cc at 0% RH.

Thickness of Composite Membrane

[0189] The composite membranes were equilibrated in the room in which the thickness was measured for at least 1 hour prior to measurement. Composite membranes were left attached to the substrates on which the composite membranes were coated. For each sample, the composite membrane on its coating substrate was placed on a smooth, flat, level marble slab. A thickness gauge (obtained from Heidenhain Corporation, USA) was brought into contact with the composite membrane and the height reading of the gauge was recorded in six different spots arranged in grid pattern on the membrane. Then, the sample was removed from the substrate, the gauge was brought into contact with the substrate, and the height reading was recorded again in the same six spots. The thickness of the composite membrane at a given relative humidity (RH) in the room was calculated as a difference between height readings of the gauge with and without the composite membrane being present. The local RH was measured using an RH probe (obtained from Fluke Corporation). The thickness at 0% RH was calculated using the following general formula:

[00009]$\begin{array}{l}Compositemembranethicknessat0\%RH=\\ =\left(\frac{CompositemembranethicknessatroomRH-\frac{M/A_{microporouspolymerstructure}}{Densit{y}_{microporouspolymerstructure}}}{1+\frac{{\lambda }_{roomRH}}{E{W}_{ionomer\text{}\_\text{}average}}\ast \frac{Molecularweigh{t}_{water}}{Densit{y}_{water}}\ast Densit{y}_{ionomer}}\right)\ast \\ \ast \left(1+\frac{{\lambda }_{RH=0\%}}{E{W}_{ionome{r}_{average}}}\ast \frac{Molecularweigh{t}_{water}}{Densit{y}_{water}}\ast Densit{y}_{ionomer}\right)\\ +\frac{M/A_{microporouspolymerstructure}}{Densit{y}_{microporouspolymerstructure}}==\left[micron\right]\end{array}$

where the parameter λ corresponds to the water uptake of the Ion Exchange Material in terms of moles of water per mole of acid group at a specified RH. For PFSA ionomer, the values for λ at any RH in the range from 0 to 100% in gas phase were calculated according the following formula:

[00010]$\begin{array}{l}\lambda =80.239\times R{H}^6-38.717\times R{H}^5-164.451\times R{H}^4+208.509\times \\ R{H}^3-91.052\times R{H}^2+21.740\times R{H}^1+0.084\end{array}$

Microporous Polymer Matrix (MPM) Volume Content of Composite Membrane

[0190] The volume % of the Microporous Polymer Matrix in each Composite Membrane was calculated according to the following formula:

[00011]$\begin{array}{l}\%Vo{l}_{MPM}=\frac{\left(\frac{M/A_{microporouspolymerstructure}}{Matrixskeletaldensit{y}_{Pmicroporouspolymerstructure}}\right)}{Compositemembranethicknessat0\%RH}=\\ \left[\%\right]\end{array}$

The Microporous Polymer Matrices used in these examples were ePTFE and track etched porous polycarbonate. The matrix skeletal density of ePTFE was taken to be 2.25 g/cc and of track etched porous polycarbonate was taken to be 1.20 g/cc.

Acid Content of Composite Membrane

[0191] Acid content of composite membranes was calculated according to the following formula:

[00012]$\begin{array}{l}AcidContent\\ =\frac{\left(CompositemembranethicknessatroomRH-\frac{M/A_{microporouspolymerstructure}}{MatrixDensit{y}_{microporouspolymerstructure}}\right)\times Densit{y}_{ionomer}}{E{W}_{ionomer}}\\ \times \frac{1}{CompositeMembranethicknessat0\%RH}=\left[\frac{moleeq}{cc}\right]\end{array}$

Ball Burst Test of Composite Microporous Layer

[0192] The mechanical strength of a composite membrane prepared in accordance with the present invention was measured by subjecting a sample to a load pressure.

[0193] A sample was fixed taut in a frame with a 45 mm diameter opening. The sample in the frame was placed into an universal testing machine AG-I of Shimadzu Corporation, Japan with an environmentally controlled chamber with the temperature and relative humidity inside of the chamber being 23° C. and 80%, respectively. A steel ball with a diameter of 6.35 mm, supported on a post, was pressed into the suspended membrane at a constant rate of 100 mm/min. The maximum load generated by the system at the sample’s break was recorded and that value is called the ball burst strength.

Average Puncture Pressure Failure Test

[0194] A sample was placed between two porous carbon electrodes (Sigracet 39AA Carbon Paper) and loaded on an Instron model 5542, with electrically isolated 14 mm diameter gold-plated cylindrical platens. The sample and electrodes area were oversized compared to the platens and extended beyond the platen to eliminate edge effects on puncture. The sample area was oversized compared to the electrodes area to prevent electrodes from touching and creating an electronic short that does not path through the sample. Electrical resistance across the membrane is measured by a Keithley 580 Micro-Ohmmeter connected to the top and bottom platens. The top platen was lowered at ambient conditions at a rate of 1 mm/min while compressive mechanical load is applied to the samples and electrical resistance measured across the sample were constantly recorded until 444.8 N (100 lbf) was applied; where a higher compression pressure may be accessed with alternative instrumentation or smaller platen active area. Membrane puncture is defined as the pressure when electrical resistance drops below 18,000 ohms, representing physical contact of the electrodes or electrode fibers through the sample. Five replicates were tested for each sample and the average of the five runs is reported as the average puncture pressure. Puncture pressure is dependent on electrode material and may significantly increase or decrease if alternative electrode materials are used.

[00013]$PuncturePressure=\frac{\left\{ForceatFailure\right\}}{\left\{PlatenSurfaceArea\right\}}=\left[psi\right]$

EXAMPLES

[0195] The composite membranes of the present disclosure may be better understood by referring to the following non-limiting examples.

[0196] To determine characteristics such as acid content, volume, and puncture resistance of the composite membrane and properties of the test procedures and measurement protocols were performed as described above. Table 1 illustrates the properties of composite membranes according to embodiments of the invention as well as comparative examples. Table 2 illustrates properties of the microporous polymer structure used in various test procedures in five series of examples in accordance with some aspects of the invention as well as comparative examples.

Ion Exchange Materials Manufactured in Accordance With Aspects of the Present Disclosure for All Examples

[0197] All ion exchange materials used in the following examples are perfluorosulfonic acid (PFSA) based ionomers with the specified equivalent weight (EW) in Table 1. All ionomers prior to manufacturing of composite membranes were in the form of solutions based on water and ethanol mixtures as solvent with water content in solvent phase being less than 50%.

[0198] A commonly known ion exchange material was used to produce a composite membrane of the present disclosure. A preferable example is a solution obtained by dispersing or dissolving a solid PFSA ionomer represented by the following general formula (wherein a:b=1:1 to 9:1 and n=0, 1, or 2) in a solvent.

##STR00001##

[0199] In some aspects, the solvent is selected from the group consisting of: water; alcohols such as methanol, ethanol, propanol, n-butylalcohol, isobutylalcohol, secbutylalcohol, and tert-butylalcohol; pentanol and its isomers; hexanol and its isomers; hydrocarbon solvents such as n-hexane; ether-based solvents such as tetrahydrofuran and dioxane; sulfoxide-based solvents such as dimethylsulfoxide and diethylsulfoxide; formamide-based solvents such as N,N-dimethylformamide and N,N-diethylformamide; acetamide-based solvents such as N,N-dimethylacetamide and N,N-diethylacetamide; pyrrolidone-based solvents such as N-methyl-2-pyrrolidone and N-vinyl-2-pyrrolidone; 1,1,2,2-tetrachloroethane; 1,1,1,2-tetrachloroethane; 1,1,1-trichloroethane; 1,2-dichloroethane; trichloroethylene; tetrachloroethylene; dichloromethane; and chloroform. In the present disclosure, the solvent is optionally selected from the group consisting of water, methanol, ethanol, propanol. Water and the above solvents may be used alone or in combinations of two or more.

Series 1

Prior Art Example 1

[0200] Prior art example 1 was prepared according to the following procedure: A first ePTFE membrane 1 with mass per area of 2.8 g/m.sup.2, a thickness of 9.6 .Math.m, an apparent density of 0.29 g/cc and a bubble point of 34.4 psi was used as microporous polymer structure of reinforcing layers. A PSFA solution as IEM with EW=810 g/mole eq SO.sub.3— (obtained from Shanghai Gore 3F Fluoromaterials Co., LTD., China), solution composition of 17.3% water, 71.5% ethanol, 11.2% solids, was coated onto the top side of a backer layer as first laydown using a drawdown bar with theoretical wet coating thickness of 3 mil (76.2 .Math.m). While the coating was still wet, a first reinforcing layer of ePTFE membrane 1 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. On the second laydown, same solution of IEM was coated onto the top surface of the first ePTFE membrane 1 using a drawdown bar with theoretical wet coating thickness of 4 mil (101.6 .Math.m). While the coating was still wet, a second reinforcing layer of ePTFE membrane 1 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. The composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. On the third laydown, a PSFA solution as IEM with EW=810 g/mole eq SO.sub.3— (obtained from Shanghai Gore 3F Fluoromaterials Co., LTD., China), solution composition of 6.2% water, 89.8% ethanol, 4.0% solids, was coated onto the top surface of the second reinforcing layerusing a drawdown bar with theoretical wet coating thickness of 3 mil (76.2 .Math.m). The composite membrane was then dried again at 165° C. The multilayer composite membrane was fully occlusive and had a layer of IEM on each side and in between of the two fully occluded microporous polymer layers. The resulting composite membrane had thickness at 0% RH of 8.66 micron.

Comparative Example 1.1

[0201] Comparative example 1.1 was made according to the following procedure: An ePTFE membrane 2 with mass per area of 6 g/m.sup.2, a thickness of 19.7 .Math.m, an apparent density of 0.29 g/cc and a bubble point of 34.8 psi was hand strained to eliminate wrinkles and restrained in this state by a metal frame. Next, a first laydown of IEM of PSFA solution with EW=810 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 32.2% water, 49.6% ethanol, 18.2% solids, was coated onto the top side of a polymer substrate (backer material). The polymer substrate (obtained from DAICEL VALUE COATING LTD., Japan) comprises PET and a protective layer of cyclic olefin copolymer (COC), and was oriented with the COC side on top. The IEM (PFSA solution) coating was accomplished using a meyer bar with theoretical wet coating thickness of 3 mils (76.2 .Math.m). While the coating was still wet, the ePTFE membrane 2 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer structure (ePTFE membrane) became fully imbibed with the IEM. The IEM also formed a layer between the bottom surface of the microporous polymer substrate and the polymer substrate. On the second laydown, a solution of IEM with the same EW and a composition of 18.4% water, 73.3% ethanol, 8.3% solids was coated onto the top surface of the ePTFE membrane 2 (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 2.5 mil (63.5 .Math.m) . The composite membrane was then dried again at 165° C., at which point it was largely transparent, indicating a full impregnation of the microporous polymer structure. The composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer structure. The resulting composite membrane had thickness at 0% RH of 8.6 .Math.m.

Comparative Example 1.2

[0202] Comparative example 1.2 was prepared according to the same procedure as described for comparative example 1.1 except that different materials were used. An ePTFE membrane 3 with mass per area of 3.9 g/m.sup.2, a thickness of 11.7 .Math.m, an apparent density of 0.34 g/cc and a bubble point of 97.5 psi was used as microporous polymer structure. A PSFA solution as IEM with EW=710 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 26.4% water, 61.6% ethanol, 12% solids, was coated onto the top side of a polymer substrate (backer material) as first laydown using a drawdown bar with theoretical wet coating thickness of 4 mil (101.6 .Math.m). While the coating was still wet, the ePTFE membrane 3 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer structure (ePTFE membrane) became fully imbibed with the IEM. The IEM also formed a layer between the bottom surface of the microporous polymer substrate and the polymer substrate. On the second laydown, solution of the same IEM with composition of 41% water, 53% ethanol, 6% solids was coated onto the top surface of the ePTFE membrane 3 (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 2 mil (50.8 .Math.m). The composite material was then dried again at 165° C., at which point it was largely transparent, indicating a full impregnation of the microporous polymer structure. The composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer substrate. The resulting composite membrane had thickness at 0% RH of 7.8 .Math.m.

Series 2

Prior Art Example 2

[0203] Prior art example 2 was prepared according to the following procedure: First, two microporous polymer structures, a first ePTFE membrane 4 with mass per area of 3.1 g/m.sup.2, a thickness of 13.3 .Math.m, an apparent density of 0.33 g/cc and a bubble point of 55.5 psi and a second ePTFE membrane 6 with mass per area of 3.0 g/m.sup.2, a thickness of 15.2 .Math.m, an apparent density of 0.20 g/cc and a bubble point of 36.6 psi were strained to eliminate wrinkles and restrained one on top of another touching on a metal frame. Next, a first laydown of solution of IEM with EW=810 g/mole eq SO.sub.3— (obtained from Shanghai Gore 3F Fluoromatenals Co., LTD., China), solution composition of 17.3% water, 71.5% ethanol, 11.2% solids, was coated onto the top side of a polymer substrate (backer layer). The polymer substrate (obtained from DAICEL VALUE COATING LTD., Japan) comprised PET and a protective layer of cyclic olephin copolymer (COC), and was oriented with the COC side on top. The coating the first laydown was accomplished using a drawdown bar with theoretical wet coating thickness of 5 mils (127 .Math.m). While the coating was still wet, the ePTFE membranes 4 and 6 previously restrained on metal frame were both laminated to the coating, whereupon the IEM solution imbibed into the pores of the microporous polymer structures. This multilayer composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer matrices became fully imbibed with the IEM. The IEM also formed a layer between the bottom surface of the microporous polymer matrix and the polymer substrate. On the second laydown, a solution of IEM with EW=810 g/mole eq SO.sub.3— (obtained from Shanghai Gore 3F Fluoromaterials Co., LTD., China), solution composition of 4% water, 95.0% ethanol, 1% solids, was coated onto the top surface of the composite material (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 0.5 mil (12.7 .Math.m). The multilayer composite material was then dried again at 165° C., at which point it was largely transparent, indicating a full impregnation of the microporous polymer matrix. The multilayer composite material was comprised of a multilayer composite membrane bonded to a substrate. The multilayer composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer structures that were touching. The resulting multilayer composite membrane had thickness at 0% RH of 7.1 micron.

Comparative Example 2.1

[0204] Comparative example 2.1 was prepared according to the same procedure as described for comparative example 1.1 except that different materials were used. An ePTFE membrane 5 with mass per area of 4.5 g/m.sup.2, a thickness of 23 .Math.m, an apparent density of 0.2 g/cc and a bubble point of 55.8 psi was used as microporous polymer structure. A PSFA solution as IEM with EW=810 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 33% water, 48.8% ethanol, 18.2% solids, was coated onto the top side of a polymer substrate (backer layer) as first laydown using a drawdown bar with theoretical wet coating thickness of 3 mil (76.2 .Math.m). While the coating was still wet, the ePTFE membrane 5 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer structure (ePTFE membrane 5) became fully imbibed with the IEM. The IEM also formed a layer between the bottom surface of the microporous polymer substrate and the polymer substrate. On the second laydown, solution of the same IEM with composition of 35% water, 56.7% ethanol, 8.3% solids was coated onto the top surface of the membrane 5 (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 3 mil (76.2 .Math.m). The composite material was then dried again at 165° C., at which point it was largely transparent, indicating a full impregnation of the microporous polymer structure. The composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer substrate. The resulting composite membrane had thickness at 0% RH of 8.8 .Math.m.

Series 3

Comparative Example 3.1

[0205] Comparative example 3.1 was prepared according to the same procedure as described for comparative example 1.1 except that different materials were used. An ePTFE membrane 7 with mass per area of 10.4 g/m.sup.2, a thickness of 62.2 .Math.m, an apparent density of 0.16 g/cc and a bubble point of 56.2 psi was used as microporous polymer structure. A PSFA solution as IEM with EW=710 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 32% water, 49.8% ethanol, 18.2% solids, was coated onto the top side of a polymer substrate (backer layer) on first laydown using a drawdown bar with theoretical wet coating thickness of 9 mil (228.6 .Math.m). While the coating was still wet, the ePTFE membrane 7 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer structure (ePTFE membrane 7) became fully imbibed with the IEM. The IEM also formed a layer between the bottom surface of the microporous polymer substrate and the polymer substrate. On the second laydown, solution of the same IEM with composition of 41% water, 53% ethanol, 6% solids was coated onto the top surface of the ePTFE membrane 7 (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 3 mil (76.2 .Math.m). The composite material was then dried again at 165° C., at which point it was largely transparent, indicating a full impregnation of the microporous polymer structure. The composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer substrate. The resulting composite membrane had thickness at 0% RH of 15.3 .Math.m.

Inventive Example 3.2

[0206] Inventive example 3.2 was prepared according to the following procedure: a first ePTFE membrane 3 with mass per area of 3.9 g/m.sup.2, a thickness of 11.7 .Math.m, an apparent density of 0.34 g/cc and a bubble point of 97.5 psi was used as microporous polymer structure of a reinforcing layer. A PSFA solution as IEM with EW=710 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 35% water, 55.1 % ethanol, 9.9% solids, was coated onto the top side of a polymer substrate (backer layer) as first laydown using a drawdown bar with theoretical wet coating thickness of 5 mil (127 .Math.m). While the coating was still wet, the first ePTFE membrane 3 restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer structure (ePTFE membrane 3) became fully imbibed with the IEM. A second laydown of the same solution of IEM was coated onto the top surface of the first membrane 3 (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 5 mil (127 .Math.m) . While the coating was still wet, a second ePTFE membrane 3 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. A third laydown of a PSFA solution with same IEM EW, solution composition of 38.0% water, 57.7% ethanol, 4.3% solids, was coated onto the top surface of the second membrane 3 using a drawdown bar with theoretical wet coating thickness of 3 mil (76.2 .Math.m). This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. The multilayer composite membrane was fully occlusive and had a layer of IEM on each side and in between of the two fully occluded microporous polymer layers of the membrane 3 with a separation distance d of about 2 .Math.m. The resulting composite membrane had thickness at 0% RH of 14.5 micron.

Inventive Example 3.3

[0207] Inventive example 3.3 was prepared according to the following procedure: a ePTFE membrane 5 with mass per area of 4.5 g/m.sup.2, a thickness of 23 .Math.m, an apparent density of 0.20 g/cc and a bubble point of 55.8 psi was used as microporous polymer structure of the reinforcing layers. A PSFA solution as IEM with EW=710 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 33% water, 52.2% ethanol, 14.8% solids, was coated onto the top side of a polymer substrate (backer layer) as first laydown using a drawdown bar with theoretical wet coating thickness of 3 mil (76.2 .Math.m). While the coating was still wet, a first ePTFE membrane 5 restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer structure (ePTFE membrane) became fully imbibed with the IEM. A second laydown of the same solution of IEM was coated onto the top surface of the first membrane 5 composite material (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 6 mil (152.4 .Math.m). While the coating was still wet, a second ePTFE membrane 5 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. A third laydown of a PSFA solution with same IEM and EW, solution composition of 10.0% water, 89.0% ethanol, 1.0% solids, was coated onto the top surface of the second membrane 5 using a drawdown bar with theoretical wet coating thickness of 1.5 mil (38.1 .Math.m) . This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. The multilayer composite membrane was fully occlusive and had a layer of IEM on each side and in between of the two fully occluded microporous polymer layers of membrane 5 with a separation distance d of about 2 .Math.m. The resulting composite membrane had thickness at 0% RH of 14.4 .Math.m .

Series 4

Comparative Example 4.1

[0208] Comparative example 4.1 was prepared according to the same procedure as described for comparative example 1.1 except that different materials were used. An ePTFE membrane 7 with mass per area of 10.4 g/m.sup.2, a thickness of 62.2 .Math.m, an apparent density of 0.16 g/cc and a bubble point of 56.2 psi was used as microporous polymer structure. A PSFA solution as IEM with EW=710 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 32% water, 49.8% ethanol, 18.2% solids, was coated onto the top side of a polymer substrate (backer layer) on first laydown using a drawdown bar with theoretical wet coating thickness of 9 mil (228.6 .Math.m). The membrane was subsequently dried in a convection oven with air inside at a temperature of 165° C. On the second laydown, solution of the same IEM with composition of 41% water, 53% ethanol, 6% solids was coated onto the top surface of membrane 7 using a drawdown bar with theoretical wet coating thickness of 3 mil (76.2 .Math.m). The membrane was again dried in a convection oven with air inside at a temperature of 165° C.The composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer substrate. The resulting composite membrane had thickness at 0% RH of 15.3 .Math.m.

Inventive Example 4.2

[0209] Inventive example 4.2 was prepared according to the following procedure: First, two microporous polymer structures of ePTFE membrane 5 as reinforcing layers with mass per area of 4.5 g/m.sup.2, a thickness of 23 .Math.m, an apparent density of 0.20 g/cc and a bubble point of 55.8 psi were strained to eliminate wrinkles and restrained one on top of another touching on a metal frame. Next, a first laydown of solution of IEM with EW=710 g/mole eq SO.sub.3-(obtained from Asahi Glass Co Ltd.), solution composition of 33% water, 52.2% ethanol, 14.8% solids, was coated onto the top side of a polymer substrate (backer layer). The polymer substrate (obtained from DAICEL VALUE COATING LTD., Japan) comprised PET and a protective layer of cyclic olephin copolymer (COC), and was oriented with the COC side on top. The coating was accomplished using a drawdown bar with theoretical wet coating thickness of 7 mils (177.8 .Math.m). While the coating was still wet, the two ePTFE membranes 5 previously restrained on metal frame were laminated to the coating, whereupon the IEM solution imbibed into the pores. This multilayer composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer matrices became fully imbibed with the IEM. The IEM also formed a layer between the bottom surface of the microporous polymer structures and the polymer substrate. On the second laydown, a solution of IEM with EW=710 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 33% water, 52.2% ethanol, 14.8% solids, was coated onto the top surface of the ePTFE membranes 5 (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 1.5 mil (38.1 .Math.m). The multilayer composite material was then dried again at 165° C., at which point it was largely transparent, indicating a full impregnation of the microporous polymer matrix.. The multilayer composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer matrices that were touching. The resulting multilayer composite membrane had thickness at 0% RH of 14.4 .Math.m.

Inventive Example 4.3

[0210] Inventive example 4.3 was prepared according to the same procedure as inventive example 3.3, described previously herein.

Inventive Example 4.4

[0211] Inventive example 4.4 was prepared according to the same procedure as described for inventive example 3.3 except that different materials were used. A ePTFE membrane 5 with mass per area of 4.5 g/m.sup.2, a thickness of 23 .Math.m, an apparent density of 0.20 g/cc and a bubble point of 55.8 psi was used as microporous polymer structure of the reinforcing layers. A PSFA solution as IEM with EW=710 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 29.8% water, 56.8% ethanol, 13.4% solids, was coated onto the top side of a polymer sheet substrate (backer layer) with a first laydown using a drawdown bar with theoretical wet coating thickness of 4 mil (101.6 .Math.m). While the coating was still wet, a first ePTFE membrane 5 previously restrained on metal frame were laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer structures became fully imbibed with the IEM. The IEM also formed a layer between the bottom surface of the microporous polymer structure and the polymer substrate. On the second laydown, solution of the same IEM with composition of 29.8% water, 56.8% ethanol, 13.4% solids was coated onto the top surface of first ePTFE membrane 5 using a drawdown bar with theoretical wet coating thickness of 5 mil (127 .Math.m). A second ePTFE membrane 5 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This multilayer composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. On the third laydown, a solution of the same IEM with composition of 35% water, 59% ethanol, 6% solids, was coated using a drawdown bar with theoretical wet coating thickness of 2 mil (50.8 .Math.m) . This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. The multilayer composite membrane was fully occlusive and had a layer of IEM on each side and in between of the two fully occluded microporous polymer layers that have a separation distance d of about 4 .Math.m. The resulting composite membrane had thickness at 0% RH of 14.4 .Math.m.

Series 5

Comparative Example 5.1

[0212] Comparative example 5.1 was prepared according to the same procedure as described for comparative example 1.1 except that different materials were used. An ePTFE membrane 8 with mass per area of 18 g/m.sup.2, a thickness of 33.4 .Math.m, an apparent density of 0.53 g/cc and a bubble point of 23.7 psi was used as microporous polymer structure. A PSFA solution as IEM with EW=810 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 32.2% water, 49.6% ethanol, 18.2% solids, was coated as first laydown using a drawdown bar with theoretical wet coating thickness of 9 mil (228.6 .Math.m). While the coating was still wet, a first ePTFE membrane 8 previously restrained on metal frame were laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. On the second laydown, solution of the same IEM with composition of 15.1% water, 18.4% ethanol, 4.2% solids was coated onto the top surface of ePTFE membrane 8 using a drawdown bar with theoretical wet coating thickness of 1.5 mil (38.1 .Math.m). This composite material was again dried in a convection oven with air inside at a temperature of 165° C. The composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer substrate. The resulting composite membrane had thickness at 0% RH of 23.8 .Math.m.

Inventive Example 5.2

[0213] Inventive example 5.2 was prepared according to the same procedure as described for inventive example 4.2 except that different materials were used. A first ePTFE membrane 5 with mass per area of 4.5 g/m.sup.2, a thickness of 23 .Math.m, an apparent density of 0.20 g/cc and a bubble point of 55.8 psi and a second ePTFE membrane 7 with mass per area of 10.4 g/m.sup.2, a thickness of 62.2 .Math.m, an apparent density of 0.16 g/cc and a bubble point of 56.2 psi were used as the microporous polymer structures for two reinforcing layers. A PSFA solution as IEM with EW=810 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 32.2% water, 49.6% ethanol, 18.2% solids, was coated as first laydown using a drawdown bar with theoretical wet coating thickness of 9 mil (228.6 .Math.m). While the coating was still wet, a first ePTFE membrane 5 and second ePTFE membrane 7 previously restrained on metal frame were laminated to the coating, whereupon the IEM solution imbibed into the pores of both ePTFE membranes. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. On the second laydown, IEM solution of the same EW with composition of 18.4% water, 73.3% ethanol, 8.3% solids was coated onto the top surface of the two ePTFE membranes 5 and 7 using a drawdown bar with theoretical wet coating thickness of 4 mil (101.6 .Math.m). This composite material was again dried in a convection oven with air inside at a temperature of 165° C. The multilayer composite membrane was fully occlusive and had a layer of IEM on each side with the two fully occluded microporous polymer layers were in contact with each other. The resulting composite membrane had thickness at 0% RH of 24.1 .Math.m.

Inventive Example 5.3

[0214] Inventive example 5.3 was prepared according to the following procedure: First, three microporous polymer structures of ePTFE membrane 5 with mass per area of 4.5 g/m.sup.2, a thickness of 23 .Math.m, an apparent density of 0.20 g/cc and a bubble point of 55.8 psi and ePTFE were strained to eliminate wrinkles and restrained one on top of another touching on a metal frame. Next, a first laydown of solution of IEM with EW=810 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 32.2% water, 49.6% ethanol, 18.2% solids, was coated onto the top side of a polymer substrate (backer layer). The substrate (obtained from DAICEL VALUE COATING LTD., Japan) comprised PET and a protective layer of cyclic olephin copolymer (COC), and was oriented with the COC side on top. The coating was accomplished using a drawdown bar with theoretical wet coating thickness of 9 mils (228.6 .Math.m) . While the coating was still wet, the three ePTFE membranes 5 previously restrained on a metal frame were laminated to the coating, whereupon the IEM solution imbibed into the pores. This multilayer composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. Upon drying, the microporous polymer matrices became fully imbibed with the IEM. The IEM also formed a layer between the bottom surface of the microporous polymer matrix and the polymer substrate. On the second laydown, a solution of IEM with EW=710 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 18.4% water, 73.3% ethanol, 8.3% solids, was coated onto the top surface of thecomposite material (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 2 mil (50.8 .Math.m). The multilayer composite material was then dried again at 165° C., at which point it was largely transparent, indicating a full impregnation of the microporous polymer matrix. The multilayer composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer matrices that were touching. The resulting multilayer composite membrane had thickness at 0% RH of 24.2 .Math.m.

Inventive Example 5.4

[0215] Inventive example 5.4 was prepared according to the same procedure as described for inventive example 4.2 except that different materials were used. A first ePTFE membrane 9 with mass per area of 9.9 g/m.sup.2, a thickness of 23 .Math.m, an apparent density of 0.43 g/cc and a bubble point of 130 psi and a second ePTFE membrane 9 of the same material were used as the microporous polymer structures. A PSFA solution as IEM with EW=810 g/mole eq SO.sub.3— (obtained from Asahi Glass Co Ltd.), solution composition of 26.0% water, 55.0% ethanol, 19% solids, was coated onto the top side of a polymer substrate (backer layer) as first laydown using a drawdown bar with theoretical wet coating thickness of 9 mil (228.6 .Math.m). While the coating was still wet, a first ePTFE membrane 9 and second ePTFE membrane 9 previously restrained on metal frame were laminated to the coating, whereupon the IEM solution imbibed into the pores of both ePTFE membranes. This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. On the second laydown, IEM solution of the same EW with composition of 18.4% water, 73.3% ethanol, 8.3% solids was coated onto the top surface of the composite material using a drawdown bar with theoretical wet coating thickness of 2 mil (50.8 .Math.m). This multilayer composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. The resulting multilayer composite membrane was fully occlusive and had a layer of IEM on each side with the two fully occluded microporous polymer layers were in contact with each other. The resulting composite membrane had thickness at 0% RH of 24.0 .Math.m.

Series 6

Comparative Example 6.1

[0216] Comparative example 6.1 was prepared according to the same procedure as described for comparative example 1.1 except that different materials were used. An ePTFE membrane 10 with mass per area of 29.5 g/m.sup.2, a thickness of 137 .Math.m, an apparent density of 0.22 g/cc and a bubble point of 43.5 psi was used as microporous polymer structure. A PSFA solution as IEM with EW=1100 g/mole eq SO.sub.3- (obtained from E. I. du Pont de Nemours and Company), solution composition of 38% water, 41.8% ethanol, 20.2% solids, was coated onto the top side of a polymer substrate (backer layer) as first laydown using a drawdown bar with theoretical wet coating thickness of 10 mil (254 .Math.m). While the coating was still wet, the ePTFE membrane 10 previously restrained on a metal frame were laminated to the coating, whereupon the IEM solution imbibed into the pores. The composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. On the second laydown, solution of the same IEM with composition of 26% water, 64.2% ethanol, 9.8% solids was coated onto the top surface of the composite material using a drawdown bar with theoretical wet coating thickness of 3 mil (76.2 .Math.m). This composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. The resulting composite membrane was fully occlusive and had a layer of IEM on each side of the microporous polymer substrate. The resulting composite membrane had thickness at 0% RH of 38.8 .Math.m.

Inventive Example 6.2

[0217] Inventive example 6.2 was prepared according to the following procedure: a ePTFE membrane 7 with mass per area of 10.4 g/m.sup.2, a thickness of 62.2 .Math.m, an apparent density of 0.16 g/cc and a bubble point of 56.2 psi was used as microporous polymer structure of the reinforcing layers. A PSFA solution as IEM with EW=1100 g/mole eq SO.sub.3— (obtained from E. I. du Pont de Nemours and Company), solution composition of 24.4% water, 56.6% ethanol, 19.0% solids, was coated onto the top side of a polymer substrate (backer layer). The polymer substrate (obtained from DAICEL VALUE COATING LTD., Japan) comprised PET and a protective layer of cyclic olephin copolymer (COC), and was oriented with the COC side on top. The coating was accomplished using a drawdown bar with theoretical wet coating thickness of 5 mils (127 .Math.m) . While the coating was still wet, a first ePTFE membrane 7 restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 125° C. Upon drying, the microporous polymer structure (ePTFE membrane 7) became fully imbibed with the IEM. A second laydown of the same solution of IEM was coated onto the top surface of the composite material (the surface opposite the polymer substrate) using a drawdown bar with theoretical wet coating thickness of 5 mil (127 .Math.m). While the coating was still wet, a second ePTFE membrane 7 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 125° C. A third laydown of the same solution of IEM was coated onto the top surface of the composite material using a drawdown bar with theoretical wet coating thickness of 5 mil (127 .Math.m) . While the coating was still wet, a third ePTFE membrane 7 previously restrained on metal frame was laminated to the coating, whereupon the IEM solution imbibed into the pores. This composite material was subsequently dried in a convection oven with air inside at a temperature of 125° C. A fourth laydown of a PSFA solution with same IEM and EW, solution composition of 20.0% water, 76.0% ethanol, 4.0% solids, was coated onto the top surface of the composite material using a drawdown bar with theoretical wet coating thickness of 5 mil (127 .Math.m). This multilayer composite material was subsequently dried in a convection oven with air inside at a temperature of 165° C. The multilayer composite membrane was fully occlusive and had a layer of IEM on each side and in between each of the three fully occluded microporous polymer layers that have a separation distance d of about 1 .Math.m. The resulting composite membrane had thickness at 0% RH of 38.6 .Math.m.

[0218] The properties of the composite membranes of the examples are presented in Table 1 (FIG. 11). The properties of the microporous polymer structures employed in the composite membranes are presented in Table 2 (FIG. 12). The improvement of the puncture resistance is illustrated in FIG. 10, which shows a graph comparing the average failure pressure of comparable composite membranes with the average failure pressure of inventive composite membranes, plotted against the normalized content of the microporous polymer structure in each composite membrane (.Math.m). Referring to FIG. 10, each datapoint is associated with average failure pressure data (psi) against the normalized content of microporous polymer structure for each series of examples, discussed below in greater detail.

Discussion of Results

[0219] Series 1 (plotted as 8 .Math.m A in FIG. 10) comprises a prior art example 1, that corresponds to Example 11.2 of WO2018/232254, which is incorporated herein in its entirety. Series 1 further comprises two comparative examples. The composite membranes have a similar thickness of about 8 .Math.m at 0% RH. Prior art example 1 has two reinforcing layers comprising ePTFE as microporous polymer structure disposed in contact with each other, while comparative examples 1.1 and 1.2 have a single reinforcing ePTFE layer. The content of ePTFE (expressed as mass of ePTFE per area of the composite membrane) is comparable in prior art example 1 and comparative example 1.1 (as shown in Table 1, the total mass per area of ePTFE in the composite membrane is about 6 g/m.sup.2). The normalized total content of microporous polymer structure as shown in Table 1, it is about 2.5 .Math.m. Comparative example 1.2 has a lower content of ePTFE in a single reinforcing layer (3.9 g/m.sup.2 or a normalized total ePTFE content of 1.7 .Math.m). All three examples provide a poor average failure pressure well under 150 psi. These composite membranes are therefore susceptible to piercing by the components of an electrochemical device, such as a redox flow battery, upon device assembly.

[0220] Series 2 (plotted as 8 .Math.m B in FIG. 10) comprises a prior art example 2 (which corresponds to example 7.3 of WO2018/232254) having two ePTFE reinforcing layers separated by an internal layer of unreinforced ion exchange material. This series also comprises comparative example 2.1 having a single ePTFE reinforcing layer. In this series, the composite membranes also have a thickness at 0% RH of about 8 .Math.m. The total ePTFE content is greater in prior art example 2 (6.1 g/m.sup.2 or a normalized total ePTFE content of 2.7 .Math.m) than in comparative example 2.1 (4.5 g/m.sup.2 or a normalized total ePTFE content of 2 .Math.m). Both composite membranes present provide a poor average failure pressure under 150 psi. These composite membranes are therefore susceptible to piercing by the components of an electrochemical device, such as a redox flow battery, upon device assembly.

[0221] Series 3 (plotted as 15 .Math.m A in FIG. 10) comprises one comparative example 3.1 having a single ePTFE reinforcing layer and two inventive examples having two ePTFE reinforcing layers. The membranes have a comparative thickness of about 15 .Math.m at 0 % RH. Although comparative example 3.1 has a greater total ePTFE content (10.4 g/m.sup.2 or a normalized total ePTFE content of 4.6 .Math.m) than inventive examples 3.2 (7.8 g/m.sup.2 or a normalized total ePTFE content of 3.6 .Math.m) and 3.3 (9 g/m.sup.2 or a normalized total ePTFE content of 4 .Math.m), the average failure pressure of comparative example 1 is unacceptable and below 150 psi, while both inventive examples have significantly improved average failure pressures above 150 psi.

[0222] Series 4 (plotted as 15 .Math.m B in FIG. 10) comprises three inventive examples having two ePTFE reinforcing layers. In FIG. 10 this series has also been plotted against comparative example 4.1 (which is the same sample as comparative example 3.1), because all the composite membranes have a comparative thickness of about 15 .Math.m at 0 % RH. These three inventive samples were prepared in a similar manner and they had the same content of ePTFE (9 g/m.sup.2 or a normalized total ePTFE content of 4 .Math.m) distributed over two reinforcing layers. The three inventive examples as well as inventive example 3.2 from series 3, which also had the same total content of ePTFE (9 g/m.sup.2), all had a comparable average failure pressures above 150 psi, unlike comparative example 3.1/4.1, which had an unacceptable average failure pressure below 150 psi, even though it had a greater total content of ePTFE (10.4 g/m.sup.2). In this series it can be seen that the separation between the reinforcing layers does not result in a significant difference in the average failure pressure.

[0223] Series 5 (plotted as 25 .Math.m in FIG. 10) had one comparative example 5.1 having a single reinforcing ePTFE layer having a total ePTFE content of 18 g/m.sup.2 (normalized total ePTFE content of 8 .Math.m), inventive examples 5.1 and 5.3 having two reinforcing ePTFE layers in direct contact and inventive example 5.2 having three reinforcing ePTFE layers in direct contact. All of these composite membranes had a comparable thickness of about 25 .Math.m and a total content of ePTFE from about 13.5 g/m.sup.2 to about 20 g/m.sup.2 (normalized total ePTFE content from 6 to 8.8 .Math.m). As shown in FIG. 10, these thicker membranes have improved average failure pressures compared to the inventive examples of series 3 and 4. Without wishing to be bound by theory, the improved average failure pressure may be due to an increased total content of ePTFE in the membrane. In addition, comparative example 5.2, which had three ePTFE layers had a significantly improved average failure pressure compared to examples 5.1 and 5.3, which had a comparable total content of ePTFE distributed over two reinforcing layers rather than three. All of the inventive examples in this series presented a superior average failure pressure compared with comparative example 5.1, which had a total content of ePTFE distributed in a single reinforcing layer (and having a comparable total content to Inventive Example 5.4).

[0224] Finally, series 6 (plotted as 41 .Math.m in FIG. 10) had one comparative example 6.1 having a single reinforcing ePTFE layer having a total mass of ePTFE per area of composite membrane of about 29.5 g/m.sup.2, and an inventive example 6.2 having three reinforcing ePTFE layers and having a total mass of ePTFE per area of composite membrane of about 31.2 g/m.sup.2. The membranes have a comparative thickness of about 40 .Math.m at 0 % RH and similar density-normalized total ePTFE content of about 13 .Math.m. Each sample showed average failure pressure well above 150 psi due to high thickness and ePTFE content. However, inventive sample 6.2 showed a marked improvement over comparative example 6.1 as none of the five samples of 6.2 tested failed when tested up to 419 psi, reported as the maximum observable average failure pressure of >419 psi.

[0225] Surprisingly, these data show that, for a given content of microporous polymer structure in a composite membrane, distributing the microporous polymer structure over at least two reinforcing layers results in a significantly improved average failure pressure compared with distributing the same content of microporous polymer structure in a single reinforcing layer. However, this observation only occurs in membranes of a minimum thickness of about 10 .Math.m at 0 % RH. As shown in series 1 and 2, composite membranes of about 8 .Math.m at 0% RH had unacceptable average failure pressures, irrespective of the number of reinforcing layers. Furthermore, increasing the number of reinforcing layers comprising a microporous polymer structure (e.g. from one to two or two to three) for a comparable total content of microporous polymer structure significantly improves the average failure pressure, as shown in series 5 and series 6. Finally, for a similar composite membrane construction (in terms of the number of reinforcing layers comprising a microporous polymer structure), increasing the total content of microporous polymer structure in the composite membrane improves the average failure pressure. Composite membranes according to this disclosure therefore are highly desirable because they have superior resistance to piercing by elements of electrochemical devices upon device fabrication, without compromising the performance of the membranes.

[0226] While the invention has been described in detail, modifications within the spirit and scope of the invention will be readily apparent to the skilled artisan. It may be understood that aspects of the invention and portions of various embodiments and various features recited above and/or in the appended claims may be combined or interchanged either in whole or in part. In the foregoing descriptions of the various embodiments, those embodiments which refer to another embodiment may be appropriately combined with other embodiments as will be appreciated by the skilled artisan. Furthermore, the skilled artisan will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.