Arrangement of flow structures for use in high differential pressure electrochemical cells

Abstract

The present disclosure is directed towards flow structures in electrochemical cells for use in high differential pressure operations. The flow structure on the low pressure-side of the cell has a larger surface area than the flow structure on the high-pressure side of the cell at the flow structureMEA interface. The boundary of the high pressure flow structure is entirely within the boundary of the low pressure flow structure. A seal around the high pressure flow structure is also contained within the boundary of the low pressure flow structure. In such an arrangement, high fluid pressures acting on the electrolyte membrane from the high-pressure side of the cell is fully and continuously balanced by the flow structure on the low pressure-side of the membrane. Use of the low pressure flow structure as a membrane support prevents the rupture or deformation of the membrane under high stresses.

Claims

1. An electrochemical cell comprising: a first electrode, a second electrode, and an electrolyte membrane disposed therebetween; a first flow structure adjacent to the first electrode, the first flow structure comprising a first planar surface along a side facing the-electrode; a second flow structure adjacent to the second electrode, the second flow structure comprising a second planar surface along a side facing the electrode; and a seal around the second flow structure, wherein the seal surrounds an outermost perimeter of the second flow structure; wherein a perimeter of the second electrode is entirely within a perimeter of the first electrode; wherein the first flow structure is configured to withstand higher structural force than the second flow structure; wherein an area of the first planar surface is larger than an area of the second planar surface; and wherein the entire seal is located outside a boundary of the second planar surface and within a boundary of the first planar surface.

2. The electrochemical cell of claim 1, wherein the boundary of the second planar surface is within the boundary of the first planar surface.

3. The electrochemical cell of claim 1, wherein the seal comprises a polymeric gasket.

4. The electrochemical cell of claim 1, wherein the first flow structure comprises a compacted porous metallic substrate.

5. The electrochemical cell of claim 4, wherein the first flow structure is compacted to a density level greater than that of the second flow structure.

6. The electrochemical cell of claim 4, wherein the first flow structure comprises at least one micro-porous material layer laminated onto the compacted porous metallic substrate.

7. The electrochemical cell of claim 1, wherein the second flow structure comprises a porous metallic substrate.

8. The electrochemical cell of claim 7, wherein the second flow structure comprises at least one micro-porous material layer laminated onto the porous metallic substrate.

9. The electrochemical cell of claim 1, further comprising a bipolar plate positioned adjacent to the second flow structure.

10. The electrochemical cell of claim 9, further comprising a reinforcement layer between the bipolar plate and the electrolyte membrane, wherein the reinforcement layer is positioned around the outside of the perimeter of the second electrode.

11. The electrochemical cell of claim 10, wherein the reinforcement layer is bonded onto the electrolyte membrane.

Description

(1) The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the various aspects of the invention.

(2) FIG. 1 illustrates an exploded schematic view showing the various components of a prior art Proton Exchange Membrane (PEM) fuel cell;

(3) FIG. 2 illustrates a cross-sectional view of an electrochemical cell for use in high differential pressure operations, in accordance with exemplary embodiments of the present disclosure;

(4) FIGS. 3A-3C illustrate plan views of the high pressure and low pressure flow structures for various electrochemical cell geometries, in accordance with exemplary embodiments of the present disclosure; and

(5) FIG. 4 illustrates an expanded view of a portion of the electrochemical cell depicted in FIG. 2, in accordance with exemplary embodiments of the present disclosure.

(6) It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

(7) Reference will now be made to certain embodiments consistent with the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like parts. It is to be understood that the although the present disclosure is described in relation to a high differential pressure electrochemical cell, the devices and methods of the present disclosure can be employed with various types of electrochemical cells, including, but not limited to, electrochemical cells operating under high differential pressures.

(8) The present disclosure is directed towards the arrangement of three-dimensional porous flow structures inside electrochemical cells for use in high differential pressure operations. In illustrative embodiments, one of the flow structures in the electrochemical cell is exposed to higher fluid pressures during operation than the flow structure on the other side of the electrolyte membrane. For instance, when an electrochemical cell is configured as a hydrogen compressor, the flow structure on the cathode side of the membrane is exposed to higher pressures than the flow structure on the anode side. Hereinafter, the flow structure that is exposed to higher fluid pressures during operation is referred to as the high pressure flow structure and the flow structure that is subjected to comparatively lower fluid pressures is referred to as the low pressure flow structure. In exemplary embodiments, the low pressure flow structure has a larger surface area than the high pressure flow structure along the sides parallel to the membrane. In such an arrangement, the high fluid pressures acting on the electrolyte membrane is fully and continuously balanced by structural support from the three-dimensional porous substrate (i.e., the flow structure) on the low pressure-side of the membrane. Use of the low pressure flow structure as a membrane support obviates the need for additional membrane reinforcement structures to support the membrane against high stresses. The continuous support provided to the membrane by the low pressure flow structure enables the use of traditional, thin electrolyte membranes (e.g., PFSA (perfluorosulfonic acid) membranes having a thickness <30 m) in high differential pressure operations without resulting in membrane deformation or failure.

(9) FIG. 2 shows a cross-sectional view of an electrochemical cell 20 for use in high differential pressure operations. As illustrated in FIG. 2, cell 20 comprises an electrolyte membrane 40 which is flanked by a high pressure flow structure 22 on one side and a low pressure flow structure 28 on the other side. High pressure flow structure 22 and low pressure flow structure 28 are surrounded by bipolar plates 30 and 31, respectively, which separate electrochemical cell 20 from the neighboring cells in the stack. Bipolar plate 30 is situated on the high pressure-side of cell 20 and bipolar plate 31 is situated on the low pressure-side of the cell. A seal 25 is provided between bipolar plate 30 and membrane 40 to prevent leakage of high pressure gas. Seal 25, also referred to herein as the high pressure-side seal, pinches the membrane against low pressure flow structure 28. In exemplary embodiments, seal 25 comprises an elastomeric or polymeric sealing material, for example, silicone, EPDM (ethylenepropylene-diene-monomer), fluoroelastomer, nitrile rubber (Buna-N), PTFE (polytetrafluoroethylene), polysulfone, polyetherimide, polychenylene sulfide, PEEK (polyether ether ketone), polyimide, PET (polyethylene terephthalate), PEN (polyethylene naphthalate), HDPE (high-density polyethylene), polyurethane, neoprene, acetal, nylon, polybutylene terephthalate, NBR (acrylonitrile-butadiene rubber), etc.

(10) As illustrated in FIG. 2, high pressure flow structure 22 has a smaller surface area than lour pressure flow structure 28 at the flow structureMEA interface, i.e., on the sides facing electrolyte membrane 40. In exemplary embodiments, the boundary of high pressure field 22 at the flow structure-MEA interface is completely encompassed by the boundary of low pressure flow structure 28. In such an arrangement where low pressure flow structure 28 has a larger surface area than high pressure flow structure 22, the high fluid pressure acting on electrolyte membrane 40 from the high pressure flow structure is continuously balanced by the structural support provided by the low pressure flow structure located on the other side of the membrane. Such an arrangement ensures that every part of the membrane 40 that is exposed to high fluid pressure is supported by the low pressure flow structure 28. The uniform and continuous support provided by the low pressure flow structure 28 protects against high stress points on membrane 40 which are known to cause membrane failure. The reinforcement provided by low pressure flow structure 28 further ensures that membrane 40 does not flex excessively under the high pressure, thereby preventing rupture. In an exemplary electrochemical cell used for hydrogen compression, the cell was able to operate at differential pressures higher than about 12,000 psi without rupturing the membrane, with differential pressure being measured as the difference between the inlet hydrogen pressure (which can range from about 10 psi to about 0 psi, or from about 0 psi to about 25 psi, about 100 psi, about 500 psi, about 1000 psi, or about 6000 psi) and the compressed hydrogen pressure (which can range from the lower bound of the inlet hydrogen pressure to higher than about 12,000 psi).

(11) FIGS. 3A-3C show plan views of the high pressure flow structure 22 and low pressure flow structure 28 for various possible electrochemical cell geometries. FIG. 3A illustrates an arrangement where the flow structures of the electrochemical cell are circular framed; FIG. 3B illustrates an arrangement where the flow structures are rectangular framed; and FIG. 3C illustrates an arrangement where the flow structures are oval framed. Various other flow structure geometries are possible depending upon the design of the electrochemical cell. As illustrated in FIGS. 3A-3C, the perimeter of the high pressure flow structure on the side facing membrane 40 is contained entirely within the perimeter of the low pressure flow structure on the side facing the membrane. FIGS. 3A-3C further demonstrate that seal 25 is contained within the perimeter of the low pressure field on the side facing the membrane, such that the high pressure-side sealing is accomplished against the contiguous low pressure flow structure. In a design where the low pressure flow structure is equal to or smaller than the high pressure field, any potential gap between the bipolar plate and the low pressure flow structure (e.g., at the extremities of the low pressure flow structure) can create a failure point for the membrane-electrode-assembly. By encompassing the high pressure-side seal within the boundary of the low pressure flow structure, any gaps between bipolar plate 31 and low pressure field 28 are not exposed to high pressure field 22. Such an arrangement further ensures that discontinuities in the low pressure side (e.g., any portion of the membrane that is not supported by the low pressure flow structure) are not exposed to high pressures. In exemplary embodiments, all of the high pressure-side seals in the entire cell stack are within the perimeters of the respective low pressure flow structures.

(12) In an illustrative embodiment, flow structures 22, 28 are fabricated using metal foams or other porous metallic substrates. In one such embodiment, an open, cellular flow structure is formed by compacting a highly porous metallic material, such as, a metal foam, sintered metal frit, or any other porous metal. The porous metallic material can comprise a metal, such as, stainless steel, titanium, aluminum, nickel, iron, etc., or a metal alloy, such as, nickel chrome alloy, nickel-tin alloy, etc. In some illustrative embodiments, the size of the pores in the metallic material can range from about 10 to about 1000 m. For example, the pore size of the metallic material can range from about 20 m to about 1000 m, such as from about 50 m to about 1000 m, from about 20 m to about 900 m, etc, from about 30 m to about 800 m, from about 40 m to about 700 m, from about 50 m to about 600 m, from about 60 m to about 500 m, from about 70 m to about 500 m, from about 100 m to about 450 m, from about 200 m to about 450 m, and from about 350 m to about 450 m. In certain embodiments, the average pore size of the metallic material is about 400 m, about 500 m, or about 800 m. In some embodiments, the void volume of the metallic material can be greater than about 75%, greater than about 80%, greater than about 85%, greater than about 90%, greater than about 95%, about 75%, about 80%, about 85%, about 90%, or about 95%. The compaction process increases the overall strength of the porous metallic material. For instance, in one embodiment, the yield strength of the porous metallic material before compaction is 30 psi and after compaction the strength increases to 14,000 psi.

(13) In certain embodiments, low pressure flow structure 28 is compacted to a density level greater than that of high pressure flow structure 22. In some embodiments, a porous metallic material intended to form low pressure flow structure 28 is compacted to an exposed axial stress level (P.sub.exposed) equal to or greater than the intended operational pressure (P.sub.operation) of the electrochemical cell. In some embodiments, the ratio of the exposure stress and the operational pressure (P.sub.exposed/P.sub.operation) ranges from a value of about 1 to about 1.5. For example, if an electrochemical cell is intended to be operated at a differential pressure of about 4,000 psi, then a porous metallic material forming low pressure flow structure 28 is compacted to a stress level equal to or greater than about 4,000 psi.

(14) In some embodiments, the compacted porous metallic matrix is laminated on one side with a micro-porous material layer (MPL) to form the flow structure. For example, the porous metallic matrix can be laminated with the MPL before the compaction process, or the porous metallic matrix can be laminated with the MPL after the compaction process. Lamination can include calendering, pressing, or coating the MPL onto the porous material. The flat, smooth laminated surface can be placed adjacent to the electrolyte membrane of an electrochemical cell. In illustrative embodiments, the average pore size of the laminated MPL is less than the average pore size of the compacted layer, which can create a porosity gradient through the metallic flow structure and facilitate the distribution of mechanical support to the electrolyte membrane. In exemplary embodiments, the MPLs have average pore size ranging from about 0.5 m to 10 m.

(15) In additional embodiments, the MPL is coated with an electrocatalyst layer if the electrocatalyst is not integral to the membrane electrode assembly. The resulting laminated structure can be arranged in the electrochemical cell with the electrocatalyst layer positioned adjacent to the membrane. In some embodiment where MPL is not used, the electrocatalyst layer can be coated directly onto the compacted porous metallic matrix substrate on the side facing the electrolyte membrane.

(16) FIG. 4 shows an expanded view of area 50 in FIG. 2 to further illustrate the various components of an exemplary embodiment of electrochemical cell 20 at the flow structureMEA interface. As illustrated in FIG. 4, high pressure flow structure 22 and low pressure flow structure 28 are laminated with MPLs 52A and 52C respectively, on the sides facing electrolyte membrane 40. In exemplary embodiments, electrodes 54A and 54C are placed adjacent to, bonded, laminated, directly cast, or coated onto the flow structures 22 and 28, respectively. In some embodiments, electrochemical cell 20 comprises a reinforcement border 56 around the periphery of high pressure field 22 between bipolar plate 30 and electrolyte membrane 40. In certain embodiments, reinforcement border 56 is located along the area between the boundaries of the high pressure and the low pressure flow structures. In such embodiments, seal 25 is positioned between reinforcement layer 56 and bipolar plate 30, as illustrated in FIG. 4, because seal 25 is located in the area between the boundaries of the high pressure and low pressure flow structures 22. In exemplary embodiments, reinforcement layer 56 comprises a polymeric material, for example, silicone, EPDM (ethylenepropylene-diene-monomer), fluoroelastomer, nitrile rubber (Buna-N), PTFE (polytetrafluoroethylene), polysulfone, polyetherimide, polychenylene sulfide, PEEK (polyether ether ketone), polyimide, PET (polyethylene terephthalate), PEN (polyethylene naphthalate), HOPE (high-density polyethylene), polyurethane, neoprene, acetal, nylon, polybutylene terephthalate, NBR (acrylonitrile-butadiene rubber), etc. In some embodiments, reinforcement border 56 is bonded to electrolyte membrane 40 to create a integrated flow structure-electrode-membrane-border assembly to reduce the number of processing steps during the cell manufacturing and assembly stage.

(17) Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.