Abstract:
One embodiment of a method of operating an electrochemical cell system comprises: flowing supply water to an anode electrode of an electrolysis cell, applying a first current density to the electrolysis cell, electrolyzing the supply water at the anode electrode wherein hydrogen ions and a first portion water migrate to a cathode electrode of the electrolysis cell, collecting the first portion of water in a chamber in fluid communication with the cathode electrode, monitoring a first portion water level in the chamber, when the first portion water level attains a first selected level, decreasing the supply water flow to the anode electrode a sufficient amount to draw the first portion water from the chamber to the anode electrode, and electrolyzing the first portion water.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This non-provisional application claims the benefit of the filing date of Provisional Patent Application No. 60/379,448 filed May 10, 2002, and Provisional Patent Application No. 60/319,549, filed Sep. 13, 2002, both of which are hereby incorporated by reference in its entirety. 
     
    
     
       BACKGROUND  
         [0002]    This disclosure relates to electrochemical cells, and, more particularly, to an electrolysis system capable of high pressure operation.  
           [0003]    Electrochemical cells are energy conversion devices that are usually classified as either electrolysis cells or fuel cells. Proton exchange membrane electrolysis cells can function as hydrogen generators by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to FIG. 1, a section of an anode feed electrolysis cell of the related art is shown at  10  and is hereinafter referred to as “cell  10 .” Reactant water  12  is fed to cell  10  at an oxygen electrode (anode)  14  where a chemical reaction occurs to form oxygen gas  16 , electrons, and hydrogen ions (protons)  15 . The chemical reaction is facilitated by the positive terminal of a power source  18  connected to anode  14  and a negative terminal of power source  18  connected to a hydrogen electrode (cathode)  20 . Oxygen gas  16  and a first portion  22  of the water are discharged from cell  10 , while protons  15  and a second portion  24  of the water migrate across a proton exchange membrane  26  to cathode  20 . At cathode  20 , hydrogen gas  28  is formed and is removed for use as a fuel or a process gas. Second portion  24  of water, which is entrained with hydrogen gas, is also removed from cathode  20 .  
           [0004]    Another type of water electrolysis cell that utilizes the same configuration as is shown in FIG. 1 is a cathode feed cell. In the cathode feed cell, process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode. A power source connected across the anode and the cathode facilitates a chemical reaction that generates hydrogen ions and oxygen gas. Excess process water exits the cell at the cathode side without passing through the membrane.  
           [0005]    Electrochemical cell systems generally include one or more individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a membrane electrode assembly (hereinafter “MEA”) defined by the cathode, the proton exchange membrane, and the anode. Each cell typically further comprises a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA may be supported on either or both sides by flow field support members such as screen packs or bipolar plates disposed within the flow fields, and which may be configured to facilitate membrane hydration and/or fluid movement to and from the MEA. Because a differential pressure often exists across the MEA during operation of the cell, pressure pads or other compression means are employed to maintain uniform compression of the cell components, thereby maintaining intimate contact between flow fields and cell electrodes over long time periods.  
           [0006]    While existing electrolysis cells are suitable for their intended purposes, there still remains a need for an improved apparatus and method of electrolyzing water to produce hydrogen gas for use in a hydrogen-powered application such as a fuel cell.  
         SUMMARY  
         [0007]    Disclosed herein is an electrolysis system and a method of operating an electrochemical cell system. One embodiment of the method of operating the electrochemical cell system comprises: flowing supply water to an anode electrode of an electrolysis cell, applying a first current density to the electrolysis cell, electrolyzing the supply water at the anode electrode wherein hydrogen ions and a first portion water migrate to a cathode electrode of the electrolysis cell, collecting the first portion of water in a chamber in fluid communication with the cathode electrode, monitoring a first portion water level in the chamber, when the first portion water level attains a first selected level, decreasing the supply water flow to the anode electrode a sufficient amount to draw the first portion water from the chamber to the anode electrode, and electrolyzing the first portion water.  
           [0008]    One embodiment of the electrolysis system comprises: an electrolysis cell comprising a fluid accumulation chamber disposed in fluid communication with a cathode side of the electrolysis cell, a gravity feed water source disposed in fluid communication with an anode side of the electrochemical cell, a control valve disposed upstream of the electrolysis cell and downstream of the water source, and a level sensing unit disposed in the fluid accumulation chamber, wherein the level sensing unit is in operable communication with a power source and the control valve, and wherein the power source is in operable communication with the electrolysis cell. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Refer now to the drawings, which are meant to be exemplary and not limiting, and wherein like elements are numbered alike in the several Figures.  
         [0010]    [0010]FIG. 1 is a schematic representation of a prior art anode feed electrolysis cell.  
         [0011]    [0011]FIG. 2 is a schematic representation of one embodiment of a high pressure electrolysis system.  
         [0012]    [0012]FIG. 3 is a perspective view of one embodiment of a water source, a phase separator, a control valve, and a cell gravity fed by the water source.  
         [0013]    [0013]FIG. 4 is an exploded schematic representation of one embodiment of a high pressure electrolysis cell.  
         [0014]    [0014]FIG. 5 is an exploded schematic view of one embodiment of a hydrogen flow field structure.  
         [0015]    [0015]FIG. 6 is a sectional view of one embodiment of a water/hydrogen chamber of a high pressure electrolysis cell.  
         [0016]    [0016]FIG. 7 is a perspective view of one embodiment of a level sensing unit.  
         [0017]    [0017]FIG. 8 is a flow chart of one embodiment of a system for controlling anode and cathode feed operation.  
         [0018]    [0018]FIG. 9 is a schematic representation of one embodiment of an anode-cathode feed high pressure electrolysis cell. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Disclosed herein is an electrochemical cell, a cell system, and a method of operating the electrochemical cell system. The cell comprises electrodes disposed at opposing surfaces of a proton exchange membrane. This cell design is capable of high pressure (e.g., greater than or equal to about 2,400 pounds per square inch (psi)) electrolysis of water. For example, the cell can generate and withstand operating pressures of up to and exceeding about 2,400 psi. This system, which is capable of anode and cathode feed electrolysis, can attain operating pressure without the use of pumps, compressors, or the like. During operation, water is fed to the anode of the cell. Upon electrolysis of the water fed to the anode, hydrogen and some water is accumulated at the cathode of the cell. Upon filling of a fluid accumulation chamber at the cathode, the water feed to the anode is stopped or reduced, and the pressure of the hydrogen is utilized to drive the water accumulated in the fluid accumulation chamber back through the proton exchange membrane, thereby causing further electrolysis.  
         [0020]    Although the disclosure below is described in relation to a proton exchange membrane electrochemical cell employing hydrogen, oxygen, and water, other types of electrochemical cells and/or electrolytes may be used, including, but not limited to, phosphoric acid, and the like. Various reactants can also be used, including, but not limited to, hydrogen, bromine, oxygen, air, chlorine, iodine, and the like. Upon the application of different reactants and/or different electrolytes, the flows and reactions change accordingly depending upon the particular type of electrochemical cell. Furthermore, while the discussion below is directed to an electrolysis cell in which either the anode or cathode may be fed with reactant water, it should be understood by those of skill in the art that fuel cells and regenerative fuel cells (combinations of electrolysis and fuel cells) are also within the scope of the embodiments disclosed.  
         [0021]    An electrochemical cell comprises a membrane electrode assembly having a proton exchange membrane, a cathode disposed at a first side of the proton exchange membrane, and an anode disposed at a second side of the proton exchange membrane, and a water/hydrogen chamber disposed at the cathode. The water/hydrogen chamber is configured to receive water through the proton exchange membrane and to receive hydrogen gas and to utilize a pressure of the hydrogen gas to drive the water back through the proton exchange membrane. A level sensing unit may be disposed at the water/hydrogen chamber to detect a level of water in the water/hydrogen chamber. The level sensing unit may comprise at least two probes disposed in electrical communication with each other. A hydrogen flow field may be disposed intermediate the water/hydrogen chamber and the cathode. The hydrogen flow field may comprise a frame having a lip extending about a periphery of the frame and a support ring disposed at an inner peripheral surface of the lip of the frame. The support ring preferably defines a boundary of the hydrogen flow field. A fluid outlet may be disposed at the water/hydrogen chamber. The fluid outlet may be disposed in fluid communication with a hydrogen-powered application. A vent may also be disposed at the water/hydrogen chamber.  
         [0022]    Referring to FIG. 2, one embodiment of a high pressure electrolysis system that utilizes gravimetric means to allow water to flow to the cell is shown at  30  and is hereinafter referred to as “system  30 .” System  30  provides for a cell integrated with a fluid storage arrangement in which high operating pressures may be achieved without the use of pumps or compressors. In some embodiments, the power input is about 1.48 volts to about 3.0 volts, with current densities being about 50 A/ft 2  (amperes per square foot) to about 4,000 A/ft 2 .  
         [0023]    System  30  comprises a cell/fluid storage unit  32 , hereinafter referred to as “cell  32 ,” a water source (e.g., a vessel  44 ) that supplies a gravity-fed water stream to cell  32 , a phase separation unit  34  disposed in fluid communication with cell  32 , and a hydrogen gas outlet  36  from which hydrogen gas can be supplied to an application  38 . A control/power unit  40  comprising a power source in electrical communication with cell  32  and a controller in operational communication with various valves, sensors, and system  30  components supplies power to cell  32 , as well as to various components associated with system  30 , and controls the operation of system  30 . The various controls and sensors associated with system  30  include, but are not limited to, valves, transmitters and controllers (e.g., temperature, pressure, flow, level, and the like), and sensors (e.g., level, pressure, temperature, flow, and the like). Optionally, a pump  47  may be used to increase water flow through the cell to aid in the removal of oxygen gas.  
         [0024]    Cell  32  comprises at least one membrane electrode assembly (not shown) and a level sensing unit  42 . Level sensing unit  42  is disposed within the fluid storage reservoir of cell  32  and is configured to sense the collection of water at the cathode. Water is fed to cell  32  via vessel  44 , which is preferably a collapsible container that allows water to be supplied to cell  32  in batches. Optionally, to avoid the need for pumps, vessel  44  comprises a drain at a lower end thereof so as to rely upon a gravity feed to cell  32 . Fluid communication is maintained between vessel  44  and cell  32  through an optional control valve  46 . A check valve  48 , as well as various sensors and controllers, may also be disposed intermediate an outlet of control valve  46  and an inlet of cell  32  to prevent the backflow of fluids from cell  32 . Phase separation unit  34  is disposed in fluid communication with cell  32  to receive excess water not consumed in the electrolysis operation.  
         [0025]    Hydrogen gas outlet  36  is disposed at an outlet of cell  32  and comprises a supply line  50  and a vent line  52 . Supply line  50  is in fluid communication with application  38 , which may be, for example, a fuel cell. As with the other components of system  30 , various sensors, valves, and controllers can be employed in hydrogen gas outlet  36 . For example, a check valve  54  is preferably disposed within supply line  50  intermediate application  38  and cell  32  to prevent the backflow of hydrogen gas from application  38  to cell  32 . A drying apparatus (not shown; e.g., a swing pressure dryer, an adsorption column, dessicant, or the like) may also be incorporated into supply line  50 . Additionally, an auxiliary fill port  56  may be optionally disposed in fluid communication with supply line  50  intermediate application  38  and check valve  54 . Auxiliary fill port  56  may be utilized to supplement the hydrogen gas supply to application  38 , or it may allow system  30  to be interconnected with any other type of hydrogen generation system to supply single or multiple applications. Vent line  52  preferably comprises a vent device  58  through which hydrogen gas may be vented from cell  32  and a pressure sensor/transmitter  60 . Although vent device  58  may be a rupture disk, vent device  58  is preferably a vent valve controllable in response to a pressure sensed at cell  32  by the sensor portion of sensor/transmitter  60 .  
         [0026]    Referring now to FIG. 3, an embodiment of the arrangement of vessel  44 , phase separation unit  34 , control valve  46 , and cell  32  is shown. Although cell  32  may be directly and continuously supplied with water, vessel  44  is preferably a modular unit that allows water to be supplied to cell  32  in batches, thereby enabling the quality of each batch to be ascertained prior to its being fed to cell  32 . A drain line  66  receives the water from vessel  44  and directs the water through control valve  46  to cell  32 . Cell  32  and control valve  46  may be disposed within a housing (not shown). Phase separation unit  34  can be mounted so as to be at a higher elevation than cell  32  (and optionally vessel  44 ) in order to effect the gravity draining of water from phase separation unit  34  back to cell  32  (or optionally vessel  44 ).  
         [0027]    Vessel  44  preferably comprises a collapsible material  62  (e.g., a bag, bladder, membrane, or the like) disposed within a frame  64 . Frame  64  is preferably mounted at an angle with respect to drain line  66  through which water is removed from vessel  44 . Alternately, vessel  44  may simply be a rigid structure or a rigid structure with a movable divider disposed therein. Vessel  44  may further include a filter disposed at an outlet to remove contaminants or particulate matter from the water as it is fed from vessel  44 . Cell  32  may also receive a continuous water supply.  
         [0028]    Referring now to FIGS. 4 through 6 where cell  32  is shown in greater detail. Cell  32  comprises a body  68  and a head  70 . Body  68  forms the actively operational component of cell  32  and comprises a hydrogen flow field structure  84 , a membrane electrode assembly  72  (MEA  72 ) comprising a proton exchange membrane  73  and electrodes  75 ,  77  disposed at opposing sides of proton exchange membrane  73 , an oxygen flow field structure  74 , and an end plate  82 . Body  68  is connected to head  70  via any suitable means, such as, for example, bolts, clamps, and the like.  
         [0029]    Hydrogen flow field structure  84  comprises a frame  86  and a hydrogen flow field member  92  disposed within frame  86 . A pressure pad  71  may optionally be disposed at hydrogen flow field structure  84  intermediate hydrogen flow field member  92  and head  70 . Oxygen flow field structure  74  comprises a frame  76  and an oxygen flow field member  80  disposed within frame  76 . A pressure pad  79  may optionally be disposed intermediate oxygen flow field structure  74  and end plate  82 . Flow field members  80 ,  92  may be screen packs, bipolar plates, or the like. Frames  76 ,  86  can be any dielectric material that is compatible with the electrochemical cell environment. Possible frame materials include, but are not limited to, glass-filled polycarbonates, thermosets, thermoplastics, rubber materials (e.g., polyetherimide, polysulfone, polyethersulfone, polyarylether ketone, combinations of the foregoing materials, and the like), and mixtures comprising at least one of the foregoing materials.  
         [0030]    In one exemplary embodiment of hydrogen flow field structure  84 , shown with reference to FIG. 5, hydrogen flow field structure  84  comprises frame  86 , a support ring  88  disposed at frame  86  and hydrogen flow field member  92 , and a porous plate  93  disposed within the opening of frame  86 . Frame  86  preferably includes a lip  104  extending about the periphery of frame  86  at one side thereof. Lip  104  is configured and dimensioned to receive and retain support ring  88  and to structurally support frame  86  upon the exertion of forces (e.g., pressure) in the directions outward from hydrogen flow field member  92 . Support ring  88  is preferably fabricated from metal and is more preferably fabricated from steel.  
         [0031]    A buss plate  99  may be disposed adjacent to hydrogen flow field member  92 . A gasket  101  can also be disposed at the juncture of support ring  88  and frame  86  between buss plate  99  and the assembled support ring  88  and frame  86  to effectively retain hydrogen flow field member  92  between frame  86 , porous plate  93 , and buss plate  99 . An optional perforated plate  97  may be disposed adjacent to pressure pad  71  on the side of buss plate  99  opposite hydrogen flow field member  92 . A gasket  102  (which may be, for example, an o-ring) enables a seal to be maintained between the head and hydrogen flow field structure  84 . As stated above, the end plate, the oxygen flow field structure, the MEA, and hydrogen flow field structure  84  can be connected to head  70  via a bolt, a clamp, or the like.  
         [0032]    Referring to FIG. 6, head  70  is shown in greater detail. Head  70  comprises a shell  106  having a fluid accumulation chamber, e.g., a water/hydrogen chamber  108 , defined therein. Water/hydrogen chamber  108  is a pressure chamber that operates as a phase separation unit that is integral with the operative body portion of the electrochemical cell and is configured to retain water and hydrogen gas at operating pressures. An open side  91  of shell  106  defining water/hydrogen chamber  108  can be positioned against the hydrogen flow field structure and bolted to the body of the cell such that hydrogen generated during the electrolysis of water (as well as any residual water) can be received in water/hydrogen chamber  108 .  
         [0033]    Level sensing unit  42  can be mounted within shell  106  such that probes  110  of varying lengths extend into water/hydrogen chamber  108  perpendicular to the level of water in water/hydrogen chamber  108 . As the water level rises in water/hydrogen chamber  108 , an electrical short is created between probes  110 , thereby indicating the level of water in water/hydrogen chamber  108 .  
         [0034]    Level sensing unit  42  is shown in greater detail with reference to FIG. 7. Level sensing unit  42  provides feedback to the control/power unit via at least two probes extending from a fitting  112  mounted in an opening in the shell. Each probe  110 , three of which are shown, comprises an electrically conductive material that extends into the water/hydrogen chamber of the shell. Level sensing unit  42  operates by the detection of an electrical short caused when the water level in the shell contacts any two probes  110 . Varying levels of water can be detected by varying the lengths of probes  110 . For example, as a rising water level engages a long probe  110  and a medium length probe  110 , the control/power unit detects an electrical short that indicates a corresponding water level. As the water level rises to engage a shorter probe  110 , the control/power unit detects an electrical short that indicates a different corresponding water level.  
         [0035]    Fitting  112  can comprise a seal  114  through which probes  110  are inserted. The material from which seal  114  is manufactured is preferably such that the hydrogen environment of the head can be hermetically sealed from the environment adjacent to the cell and such that long-term joint stability between probes  110  and the integrity of seal  114  can be maintained. Preferably, seal  114  is fabricated from borosilicate glass, and more preferably alumino-borosilicate glass. Other materials from which seal  114  may be fabricated include ceramic, glass, epoxy, VITONÂ®(a fluoroelastomer commercially available from DuPont Dow Elastomers L. L. C.), adhesive, a combination of epoxy and VITONÂ®, and the like, as well as a combination comprising at least one of the foregoing materials.  
         [0036]    Probes  110  can comprise any electrically conductive material compatible with the operating environment of the cell. Possible materials include metals such as copper, iron, ferrous materials (e.g., steel), silver, nickel, cobalt, titanium, and the like, as well as alloys and combinations comprising at least one of the foregoing materials. Probes  110  are preferably fabricated of an iron/nickel/cobalt alloy (e.g., an alloy comprising about 45% to about 55% iron (Fe), about 43% to about 53% nickel (Ni), and less than or equal to about 7% cobalt (Co) (such as about 0.5% to about 7% Co)) brazed at an end thereof to an end of a stainless steel pin (e.g., 316 stainless steel). The braze material is preferably silver (Ag) and/or copper (Cu), and the braze is preferably disposed within seal  114  to limit or prevent the contamination of the cell with the braze material. Other materials from which probes  110  can be fabricated include, the foregoing listed materials plated with Ni and/or gold (Au), and the like, as well as combinations comprising at least one of the above listed materials.  
         [0037]    Referring now to all the Figures, operation of system  30  is initiated by the gravity feed of water from the water source  44  to cell  32  at the anode side of proton exchange membrane  73 . Once water is received into cell  32 , subjected to an electric current, and reduced to hydrogen ions and oxygen. The hydrogen ions pass through the proton exchange membrane  73  to the cathode electrode of the cell while the oxygen is removed from the cell and, for example, vented to the atmosphere. As the water at the cathode side of the cell accumulates, level sensing unit  42  detects the level of water and transmits a water level signal to control/power unit  40 . When a predetermined level of water has been detected, control/power unit  40  signals optional control valve  46  and/or pump  47  to stop or restrict the flow of water to the anode (e.g., by closing at least partially or closing fully).  
         [0038]    Once the manipulation of control valve  46  and/or pump  47  causes the flow of water to the anode side of the cell to either stop or slow down, the direction of the flow of water is reversed and the cell is fed from the cathode. In the cathode feeding of the cell, it is believed that the pressure at the cathode side of the proton exchange membrane forces the accumulated water back through the proton exchange membrane to the anode, causing further electrolysis. The amount of water electrolyzed (and hydrogen generated) is a function of the current density applied to the cell. The system can optionally use the same current density in both anode and cathode feed modes. Due to the highly complex and dynamic reactions that take place during cathode feed mode, when operated in this manner, it has been found that the membrane is subjected to an undesirable and irreversable drying condition.  
         [0039]    Under liquid anode feed operating conditions, water is carried along with hydrogen ions through the membrane during electrolysis. This extra water provides a benefit of maintaining the hydration state of the membrane and prolonging its life. Since water is being transported to the electrode from outside the membrane electrolysis will continuously occur under a constant supply water condition. However, when the electrochemical cell is switched into a cathode feed mode, the mechanism of the reaction changes. As described above, cathode feed mode takes place when water flow to the anode electrode is stopped. Once the water immediately adjacent to the anode electrode is electrolyzed, the anode electrode will start to consume water from the cathode side.  
         [0040]    As shown in FIG. 9, there are two forces acting on the cathode water that will tend to drive it to the anode electrode: 1) back diffusion, and 2) hydraulic flux. The back diffusion phenomena is due to the characteristics of the membrane material which&lt;in its natural state, wants to remain hydrated with a high water content (w c ). As will be discussed below, cathode feed electrolysis tends, to c some varying degree, to dry the membrane as water is consumed within the membrane. This consumption further enhances the back-diffusion activity. The second phenomena, hydraulic flux, occurs due to the pressure differential between the two sides of the cell. When a large pressure differential conditional exists, such as when the hydrogen gas on the cathode side is greater than or equal to about 2,000 psi, with the anode side being at ambient pressure, the pressure differential acts to force the water back through the membrane under hydraulic pressure. While the hydraulic flux will be present any time there is a pressure differential, in very high differential applications, the hydraulic flux will be the dominant mode for transporting water back across the membrane.  
         [0041]    Since current is still being applied to the cell, electrolysis will continue. However, the water now being electrolyzed is provided via the back diffusion and hydraulic flux within the membrane rather than on the surface adjacent anode flow field. Thus, as the water molecule reaches the anode electrode, the molecule is disassociated, with the oxygen continuing on into the anode flow field and the hydrogen ion reversing direction to migrate back to the cathode side of the cell. Due to electro-osmotic flux, some water molecules will also be dragged back with the hydrogen ions through the membrane. This has the effect of preventing some of the water being driven by hydraulic flux from reaching the anode electrode and being electrolyzed. At high current density operation, the combination of the required reactant water for electrolysis and electo-osmotic flux will be greater that the water being migrating from the cathode. Under these conditions, cathode feed electrolysis will continue, but will reduce the amount of water within the membrane and cause irreversible drying of the membrane. This drying of the membrane shortens the expect life of the electrolysis cell. The drying will be most noticable on the membrane surface adjacent to the anode electrode. The amount of water molecules disassociated and the amount of drying that occurs is directly proportional to the current density and the pressure differential between the two sides of the cell.  
         [0042]    To eliminate the drying and achieve a long life for the membrane, the amount of water diffused by the pressure differential should be balanced with the current density during cathode feed mode. As shown in FIG. 9, the ideal water balance at the anode electrode would be:  
         Hydraulic Flux+Back diffusion≧Electro-osmotic flux+Water electrolysis| 
         [0043]    In other words, the water driven back through the membrane would be greater than or equal to the water pulled by electro-osmotic flux plus the water consumed by the electrolysis reaction. By maintaining this condition, a longer life is attained while maximizing hydrogen production. To achieve this, the control system  40  was modified such that when the pump  47  is disabled, or control valve  46  is closed (which occurs only in cathode feed mode), the current density for electrolysis is reduced. In the exemplary embodiment, the water is electrolyzed at 500 amperes per square foot (asf) in anode feed and reduced to 200 asf in cathode feed. In addition, by operating at a reduced current density, electrolysis continues, allowing hydrogen gas to be produced while extending the life of the electrolysis cell.  
         [0044]    [0044]FIG. 8 illustrates the control logic in switching from anode to cathode feed electrolysis. Operation is initialized at box  120 , and a level sensing unit  42  (see also FIG. 2) detects a predetermined low water level (e.g., at or below P1). To enable anode feed electrolysis, the water is turned on (e.g., either the optional pump  47 , and/or optional control valve  46  are enabled) allowing water to flow to the anode electrode (box  124 ) where the water is electrolyzed in box  126 . The water continues to be electrolyzed until the predetermined water level is detected by level sensing unit  42 . To switch to cathode feed, the current density is reduced in box  130  and the water flow is ceased (or at least restricted to a sufficent amount to consume water from the cathode electrode side of the cell) in box  132 . It should be appreciated that, while the actions in boxes  130  and  132  are shown serially, they may also occur simultaniously. Water from the cathode is consumed until the the level sensing unit  42  detects a predetermined low water level (e.g., a level of less than or equal to P1), the control system  40  then loops back via  136  to box  124  to resume the flow of water to the anode electrode.  
         [0045]    The above-described electrochemical cell and its exemplary embodiments provide for the operation of an electrolysis system at higher pressures than those achievable with other electrolysis systems. Such higher pressures can be achieved without the use of pumps or compressors. Furthermore, by alternating the direction of feed water to the cell to define a combination anode/cathode feed cell, the cell can be operated more efficiently.  
         [0046]    The use of a gravity-fed water system reduces the amount of space required for the system by eliminating, or reducing the size of a feed pump. Furthermore, for system applications in which the water supply is not continuous, the modularity of the vessel provides a supply of contaminant free feed water. Moreover, the collapsibility feature of the water vessel permits gravity feed without the need to vent the vessel, thereby eliminating the need to filter the gas vented from the vessel. Additionally, longer life and higher hydrogen gas output may be achieved by applying a different current density to anode and cathode feed electrolysis.  
         [0047]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the disclosure.