SCREEN ASSEMBLY FOR AN ELECTROCHEMICAL CELL

Abstract: A screen assembly for use in an electrochemical cell for supporting and facilitating the hydration of a solid membrane. The screen assembly is comprised of two planar screen layers having strands and openings wherein the strands of the second layer are positioned substantially across the openings of the first layer. In an embodiment the openings of the layers are diamond shaped wherein the openings of the first layer are positioned generally orthogonal to the openings in the second layer. The strands of the screen assembly support, and the openings facilitate the hydration, of the membrane of an electrochemical cell.

Description: SCREEN ASSEMBLY FOR AN ELECTROCHEMICAL CELL Background of the Invention: Field of the Invention This invention relates generally to proton exchange membrane electrochemical cells. In particular, this invention relates to a screen assembly that sustains the integrity and structure of the proton exchange membrane, facilitating operation at high pressure and high fluid flow rates with increased reliability. Brief Description of the Related Art Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells, including electrolysis cells having a hydrogen water feed. A proton exchange membrane electrolysis cell functions as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to FIGURE 1, in a typical single anode feed water electrolysis cell 101, process water 102 is reacted at oxygen electrode (anode) 103 to form oxygen gas 104, electrons, and hydrogen ions (protons) 105. The reaction is created by the positive terminal of a power source 106 electrically connected to anode 103 and the negative terminal of a power source 106 connected to hydrogen electrode (cathode) 107. The oxygen gas 104 and a portion of the process water 102' exit cell 101, while protons 105 and water 102" migrate across proton exchange membrane 108 to cathode 107 where hydrogen gas 109, is formed. The typical electrochemical cell includes a number of individual cells arranged in a stack with fluid, typically water, forced through the cells at high pressures. The cells within the stack are sequentially arranged including a cathode, a proton exchange membrane, and an anode. The cathode/membrane/anode assemblies (hereinafter "membrane assembly") are supported on either side by packs of screen or expanded metal which are in turn surrounded by cell frames and separator plates to form reaction chambers and to seal fluids therein. The screen packs establish flow fields within the reaction chambers to facilitate fluid movement and membrane hydration, and to provide mechanical support for the membrane and a means of transporting electrons to and from electrodes. As stated above, the screen packs support the membrane assembly. The membrane is typically only about 0.002 - 0.012 inches in thickness, when hydrated, with the electrodes being thin structures (less than about 0.002 inches) of high surface area noble metals pressed or bonded to either side of the membrane and electrically connected to a power source. When properly supported, the membrane serves as a rugged barrier between the hydrogen and oxygen gases. The screen packs, positioned on both sides of the membrane against the electrodes, impart structural integrity to the membrane assembly. Due to the high pressure differential that exists in an operating cell, however, the membrane and electrode on the low pressure side can be forced into the screen packs. Conventional screen packs have a number of disadvantages and drawbacks. For example, existing screen packs comprise multiple layers of screen material formed from 0.007 inches (0.178 millimeters (mm)) - 0.010 inches (0.254 mm) thick metal strands having pattern openings of 0.125 inches (3.17 mm) by 0.053 inches (1.35 mm) to 0.071 inches (1.80 mm) (commonly known as 3/0 screen). Under typical operating conditions, a pressure differential of about 390 psi forces the membrane assembly into the openings of the first layer of screen on the low pressure side of the cell. Due to the extrusion of the membrane into this screen layer, the membrane stress in the center of a screen opening increases to about 4,600 psi, while the membrane material has a maximum rating of only about 2,000 psi. Consequently, high axial stresses may force the screen strands into the membrane over time, thereby filling the screen void areas with membrane material. Alternatively, the membrane may rupture, allowing mixing of hydrogen and oxygen gases. These prior art screens having a large openings, i.e. about 3/0, failed to provide sufficient mechanical support, screens having smaller openings failed to provide adequate mass flow characteristics. As is discussed in U.S. Patent No. 5,296,109, to Carlson et al., unsuccessful attempts have been made to utilize screens having a smaller opening size, and therefore less open area. These attempts have yielded decreased fluid movement resulting in poor membrane hydration and dry spots, i.e. localized drying out of the membrane, which also ultimately leads to cell failure. Due to the problems associated with the use of small opening screens, Carlson et al. teaches the use of a porous plate to attain the desired mass flow characteristics and support of the membrane assembly. The porous plate of Carlson et al. allows water access to the electrode, solving the problem of the oxygen blockage of the openings. However, because this plate is expensive to produce, difficult to attain repeatability with respect to uniform thickness, porosity, mechanical properties, etc., it is not a viable alternative for a relatively low cost electrochemical system or for a mass production part. What is needed in the art is a readily manufactured, improved screen assembly which provides structural integrity without adversely affecting the cell's mass flow characteristics. Summary of the Invention: The above-described drawbacks and disadvantages of the prior art are alleviated by the electrochemical cell screen assembly and electrochemical cell in accordance with the present invention. The electrochemical cell screen assembly comprises: a first screen layer having openings and a second screen layer having openings larger than the first screen layer openings, with the second screen disposed parallel to and in contact with the first screen layer. The electrochemical cell of the present invention comprises: an electrolyte membrane; an electrode disposed on each side of the membrane; and a screen assembly disposed adjacent to each of the electrodes, with at least one of the screen assemblies having a first screen layer with openings and a second layer with openings larger than the first layer openings, wherein the second screen layer is disposed parallel to and in contact with the first screen layer. The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. Brief Description of the Drawings: Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: FIGURE 1 is a schematic diagram of a prior art electrochemical cell showing an electrochemical reaction; FIGURE 2 is a cross sectional view of an electrochemical cell showing the relationship of the cell components; FIGURE 3 is a plan view of a screen assembly of the present invention; and FIGURE 4 is a partial cross section of an isometric view of a screen assembly according to the present invention shown supporting a membrane. FIGURE 5 is a partial cross section of an isometric view of a screen assembly according to the present invention shown supporting a membrane. The figures are meant to further illustrate the present invention and not to limit the scope thereof. Detailed Description of the Preferred Embodiment: The screen pack of the present invention comprises multilayers of screens assembled such that the layer adjacent the electrode has a smaller opening and/or strand size to provide improved support to the membrane. Referring to FIGURE 2, cell 1 includes proton exchange membrane 8 having an anode 3 and a cathode 7 bonded to either side, with the periphery of membrane 3 installed between a pair of cell frames 21. Oxygen screen pack 43 is installed inside the active area of the frame assembly between anode 3 and oxygen separator plate 45, while hydrogen screen pack 22 is installed inside of cell frame 21 between hydrogen separator plate 24 and cathode 7. In operation, process water 2 enters inlet port 25 and a portion of the water is diverted into oxygen screen pack 43. A portion of the water 2 not diverted into screen pack 43, continues along conduit 25 formed by axially aligned holes in the components comprising the stack, and enters subsequent cells in the cell stack (not shown) positioned outside of the cell 1. The portion of process water 2 diverted through screen pack 43 contacts anode 3 where the water electrochemically converts to oxygen gas and protons. Oxygen gas, as well as excess water, exhausted from the cell through similar porting arrangements as those which directed the water to the anode 3. The generation of gases in the cell combine with external pressure regulation produces a large pressure differential between the oxygen side and the hydrogen side of the cell. This pressure differential forces membrane 8 and cathode 7 against the opposing screen pack. It should be noted that the direction of the pressure differential, i.e. greater or lower pressure on the cathode side, is dependent upon the application requirements of the electrochemical system. Referring to FIGURE 3, for example, a first screen layer 50 and a second screen layer 51 comprise the first two layers of hydrogen screen pack 22. Screen layers 50, 51 are preferably comprised of planar screen layers preferably having elongated, such as diamond shaped, openings 52, 53 formed by strands 54, 55 respectively. In a preferred embodiment, the openings are placed such that the openings and opening in subsequent screens are not in phase or oriented in the same direction, with placing openings 52 of screen layer 50 substantially orthogonal to the diamond shaped openings screen layer 51 such that diamond shaped openings 52 are partially obstructed by strand 55 especially preferred. The preferred shape of the openings is dependent upon mass flow characteristics and the structural integrity attained with a screen pack having those openings. Diamond shaped openings are shown by way of example, however, other shaped openings are advantageously contemplated by the present invention, such as ovals, circles, and hexagons, among others, with elongated shaped openings preferred. Although diamond openings in adjacent screen layers are preferably oriented orthogonally to one another, other orientations are possible. Furthermore, as is illustrated in FIGURE 5, the openings of the various screen layers 50, 51' can have a different geometry, i.e. diamond and square, respectively, with preferred size and geometry of the screen layers dependent upon mass flow characteristics. FIGURES 4 and 5, in conjunction with FIGURE 2, illustrates the ability of the membrane support device to provide structure and integrity to the membrane 8 during operation. The pressure differential between the cell halves during operation causes cathode 7 and membrane 8 to press against strands 54 of first screen layer 50, deflected out of plane into diamond shaped perforation 52, where the displacement is arrested by strands 55 of second screen layer 51 (51'). Screen layer 51 (51') supports screen layer 50, while subsequent layers (not shown) support both screen layers 50 and 51 (51'). The preferred number of screen layers is dependent upon the side of the cell in which the screen pack will be employed, the pressure differential of the cell, the size of the openings formed by the strands, and the desired mass flow characteristics. In an electrolysis cell, for example, operating with a 390 psi pressure differential, and using diamond shaped openings of about 0.046 inches (1.17 mm) by about 0.077 inches (1.96 mm), the low pressure side of the cell has screens 50, 51 (51') and subsequent screens, totaling 9 screens, while the high pressure side of the cell has screens 50, 51 (51') and subsequent screens, totaling 4 screens. The screen of subsequent layers is also preferably placed in a pattern having the openings of subsequent layers placed orthogonal to preceding layers. The screen layers can be a perforated sheet or a woven mesh, formed from a metal plate or strands or other electrically conductive material. Preferably, the screens are formed from a metal which is inert in the electrochemical cell environment, electrically conductive, and capable of providing sufficient structural integrity to the membrane assembly, such as niobium, zirconium, titanium, or tantalum, among others, and alloys thereof. It is anticipated that the screen can be comprised of expanded metal, stamped sheet, chemically milled sheet, woven screen or any commercially viable embodiment that yields a substantially planar screen member. The screens of the present invention preferably employ at least a first thin layer to prevent cutting and rupture of the membrane assembly. This thin screen has a strand thickness (t) below about 0.005 inches (0.127 mm) with below 0.004 inches (0.102 mm) preferred and about 0.002 inches (0.051 mm) to about 0.0035 inches (0.089 mm) especially preferred. This thin screen layer supports the membrane assembly, preventing extrusion into the screen pack and inhibiting cutting of the membrane assembly by the strands of the screen. Subsequent screen layers can have a similar strand thickness, with a strand thickness of about 0.005 inches or greater preferred due to the enhanced structural integrity attained with thicker strands. In addition to a reduced screen strand thickness, it is also preferable to employ a reduced opening size for at least the first screen layer, with a reduced opening size employed for subsequent screen layers based upon mass flow demands. The actual size of the openings (perforations or holes) is dependent upon the desired mass flow rate and number of screen layers to be employed. For a diamond pattern, in an electrolysis cell operating at a 390 psi pressure differential, a diamond size of less than 0.125 inches (3.17 mm), "b" (width), by less than 0.071 inches (1.80 mm), "a" (height) 3/0, can be employed, with a size of about 0.125 inches (3.17 mm) to about 0.050 inches (1.27 mm), "b", by about 0.071 inches (1.80 mm) to about 0.027 inches (0.686 mm), "a", preferred, i.e. about 3/0 to about 5/0 preferred, and a 4/0 screen, about 0.077 inches (1.96 mm) by about 0.033 inches (0.838 mm) to about 0.046 inches (1.17 mm) especially preferred for the screen layer adjacent the electrode (see "a" and "b" in FIGURE 3, respectively). Subsequent layers can also employ small opening sizes or can have an opening size larger than the opening size of the screen layer adjacent the electrode to improve mass flow characteristics. It has been discovered that mass flow problems associated with oxygen blocking the holes in a screen or other medium, preventing water access to the electrode, can be substantially eliminated by employing a combination of strand thickness and opening size. For example, a large strand thickness requires a large opening size in order to enable flow through the screen. However, a thin strand thickness, i.e., about 0.003 inches (0.076 mm), can be used with 3/0 and even 4/0 screen without adversely affecting the mass flow characteristics thereof. Consequently, the appropriate screen size to attain the desired structural integrity while maintaining acceptable mass flow characteristics can be determined by controlling the strand thickness based upon the opening size. Essentially, as the opening size decreases, the strand size must also decrease. For example, a 3/0 screen can have a strand thickness of about 0.007 inches (0.178 mm), while a 4/0 or 5/0 screen should have a strand thickness less than 0.005 inches (0.127 mm), with a thickness of about 0.003 inches (0.076 mm) or thinner preferred. With reference again to FIGURES 3, 4 and 5, it is an important aspect of the present invention that openings 52 of layer 50 are not unduly obstructed by strands 55. The present invention allows for adequate hydration of membrane 8 by facilitating fluid transport through the unobstructed portion of openings 52 by providing for large openings in layer 50, due to the preferred elongated geometry of the strand openings, and by utilizing relatively thin strands 55. The membrane support device having the above-described features enables enhanced protection from screen pack induced damage, good flow distribution with high fluid pressures and flow rates, and low cost manufacturing. This device is useful all types of electrolysis cells, and particularly beneficial for use on the side of the water feed and/or the low pressure side of the membrane. In the preferred embodiment the unsupported length and out of plane displacement of the membrane is reduced. As a result of the reduction in elongation of the membrane the stresses within the membrane assembly are reduced to within an acceptable range for the type of material used. While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation. What is claimed is:

Claims: Claims 1. A water side electrochemical cell screen assembly, comprising: a first screen layer having first openings having a size of about 0.077 inches (1.96 mm) or less by about 0.033 inches (0.838 mm) or less and a thickness of less than 0.005 inches (0.127 mm), wherein said size and said thickness are interrelated such that the combination of said size and said thickness enable the passage of water and a gas through said first openings; and a plurality of subsequent screen layers having second openings, wherein said subsequent screen layers are disposed parallel to and in contact with said first screen layer. 2. A water side electrochemical cell screen assembly as in Claim 1, wherein said thickness is about 0.004 inches (0.102 mm) or less. 3. A water side electrochemical cell screen assembly as in Claim 1, wherein said thickness is about 0.0035 inches (0.089 mm) or less. 4. A water side electrochemical cell screen assembly as in Claim 1, wherein at least a portion of said second openings having a larger size than said first openings size. 5. A water side electrochemical cell screen assembly as in Claim 1, wherein at least a portion of said subsequent screen layers have second strands having a second thickness greater than said first thickness. 6. A water side electrochemical cell screen assembly as in Claim 5, wherein said first thickness is about half of said second thickness. 7. A water side electrochemical cell screen assembly as in Claim 1, wherein said first openings and said second openings have a substantially elongated, diamond or oval shaped geometry. 8. A water side electrochemical cell screen assembly as in Claim 7, wherein said first openings are disposed generally orthogonal to said second openings. 9. A water side electrochemical cell screen assembly as in Claim 1, wherein said first openings and said second openings have different geometries. 10. A water side electrochemical cell screen assembly as in Claim 1, wherein at least a portion of said subsequent screen layers have second strands having a second thickness substantially equal to said first thickness. 11. A water side electrochemical cell screen assembly as in Claim 10, wherein said portion of subsequent screen layers have an opening size substantially equivalent to said first openings size. 12. An electrochemical cell, comprising: an electrolyte membrane having a first gas side and a second gas side; a first gas electrode disposed on said first gas side of said membrane; a second gas electrode disposed on said second gas side of said membrane; a first gas screen assembly disposed adjacent to said first gas electrode, said first gas screen assembly having a first screen layer having first openings having a first size of about 0.077 inches (1.96 mm) or less by about 0.033 inches (0.838 mm) or less and a first thickness of less than 0.005 inches (0.127 mm) and at least one subsequent screen layer having subsequent openings, wherein said subsequent screen layer is disposed parallel to and in contact with said first screen layer and said first size and said first thickness are interrelated such that the combination of said first size and said first thickness enable the passage of water and the first gas through said first openings; and a second gas screen assembly disposed adjacent to and in contact with said cathode electrode. 13. An electrochemical cell as in Claim 12, wherein the electrochemical cell has a pressure differential across said membrane such that a pressure at said first gas side of said membrane is lower than pressure at said second gas side of said membrane. 14. An electrochemical cell as in Claim 13, wherein said subsequent openings have a subsequent size and a subsequent thickness, wherein said first thickness is less than said subsequent thickness. 15. An electrochemical cell as in Claim 14, wherein said first thickness is about half of said second thickness or less. 16. An electrochemical cell as in Claim 12, wherein said first openings and said subsequent openings have a substantially elongated, diamond, or oval shaped geometry. 17. An electrochemical cell as in Claim 16, wherein said first openings are disposed generally orthogonal to said subsequent openings. 18. An electrochemical cell as in Claim 12, wherein said first openings and said second openings have different geometries. 19. An electrochemical cell as in Claim 11, wherein said first thickness is about 0.0035 inches (0.089 mm) or less. 20. An electrochemical cell, comprising: an electrolyte membrane having a first gas side and a second gas side; a first gas electrode disposed on said first gas side; a second gas electrode disposed on said second gas side; a first gas screen assembly disposed adjacent to said first gas electrode, said first gas screen assembly having a first screen layer having first openings having a first size of about is less than 0.125 inches (3.17 mm) by less than 0.071 inches (1.80 mm) and said first thickness is about 0.0035 inches (0.089 mm) or less and at least one subsequent screen layer having subsequent openings, wherein said subsequent screen layer is disposed parallel to and in contact with said first screen layer and said first size and said first thickness are interrelated such that the combination of said first size and said first thickness enable the passage of water and the first gas through said first openings; and a second gas screen assembly disposed adjacent to and in contact with said cathode electrode. 21. An electrochemical cell as in Claim 20, wherein the electrochemical cell has a pressure differential across said membrane such that a pressure at said oxygen side of said membrane is lower than pressure at said hydrogen side of said membrane. 22. An electrochemical cell as in Claim 20, wherein said subsequent openings have a subsequent size and a subsequent thickness, wherein said first thickness is less than said subsequent thickness. 23. An electrochemical cell as in Claim 22, wherein said first thickness is about half of said second thickness or less. 24. An electrochemical cell as in Claim 20, wherein said first openings and said subsequent openings have a substantially diamond or oval shaped geometry. 25. An electrochemical cell as in Claim 24, wherein said first openings are disposed generally orthogonal to said subsequent openings. 26. An electrochemical cell as in Claim 20, wherein said first size is about 0.050 inches (1.27 mm) to about 0.125 inches (3.17 mm) by about 0.027 inches (0.686 mm) to about 0.071 inches (1.80 mm). 27. An electrochemical cell as in Claim 20, wherein said first size is about 0.050 inches (1.27 mm) to about 0.007 inches (1.96 mm) by about 0.027 inches (0.686 mm) to about 0.033 inches (0.838 mm). 28. An electrochemical cell, comprising: an electrolyte membrane having a high pressure side and a low pressure side, wherein said low pressure side has a pressure lower than a said high pressure side; a high pressure electrode disposed on said high pressure side; a low pressure electrode disposed on said low pressure side; a low pressure screen assembly disposed adjacent to said low pressure electrode, said low pressure screen assembly having a first screen layer having first openings having a first size of about 0.077 inches (1.96 mm) or less by about 0.033 inches (0.838 mm) or less and said first thickness is about 0.0035 inches (0.089 mm) or less and at least one subsequent screen layer having subsequent openings, wherein said subsequent screen layer is disposed parallel to and in contact with said first screen layer and said first size and said first thickness are interrelated such that the combination of said first size and said first thickness; and a high pressure screen assembly disposed adjacent to and in contact with said cathode electrode.

Inventors: MOLTER TRENT M, MOLTER, TRENT, M.
Applicants: PROTON ENERGY SYSTEMS
Application year: 1999
Main classification: C25B9
IPC codes: C25B1, C25B9, C25B13, H01M8