Electrolyzer apparatus and method for hydrogen and oxygen production

An electrolyzer cell (10) for the electrolysis of water comprises a cathode (12) of generally tubular configuration within which is disposed an anode (16) separated from the cathode (12) by a separation membrane (14) of generally tubular configuration which divides the electrolyte chamber (15) into an anode sub-chamber 15a and a cathode sub-chamber (15b). An electrolyzer apparatus (36) includes an array (38) of individual cells (10)across each of which an electric potential is imposed by a DC generator (40) via electrical leads (42a, 42b). Hydrogen gas generated within cells (10) from electrolyte (18) is removed via hydrogen gas take-off lines (20) and hydrogen manifold line (21). By-product oxygen is removed from cells (10) by oxygen gas take-off lines (22) and oxygen manifold line (23). The electrolyzer apparatus (36) may be configured to operate either batchwise or in a continuous electrolyterecycle operation to produce high purity hydrogen at high pressure, e.g., up to about (10,000) psig, without need for gas compressors to compress product hydrogen.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns an electrolyzer apparatus and method to produce high-pressure hydrogen at pressures up to 10,000 psig or higher, by means of electrolysis of water and without necessity of separate compression equipment. Direct electrolytic generation of such high-pressure hydrogen (and by-product oxygen) is attainable by the practices of the present invention.

2. Related Art

Known electrolytic equipment, sometimes herein referred to as “electrolyzers”, using liquid electrolyte to generate hydrogen, operates in the following way. Two electrodes are placed in a bath of liquid electrolyte, such as an aqueous solution of potassium hydroxide (KOH). A broad range of potassium hydroxide concentration may be used, but optimally, a concentration of about 25 to 28% by weight KOH solution is used. The electrodes are separated from each other by a separation membrane that selectively allows passage of liquid but not gas through it. When a voltage is impressed across the electrodes (about 2 volts), current flows through the electrolyte between the electrodes. Hydrogen gas is produced at the cathode and oxygen gas is produced at the anode. The separation membrane keeps the hydrogen and oxygen gases separated as the generated gas bubbles rise through the liquid electrolyte. There is a disengagement space above the liquid electrolyte comprised of two separate chambers or two sections isolated from each other by being separated by a gas-tight barrier into two separate sections, one chamber or section to receive the hydrogen gas and the other to receive the oxygen gas. The two gases are separately removed from the respective sections of the disengagement pace for storage or venting.

SUMMARY OF THE INVENTION

Generally, in accordance with the present invention, there is provided an electrolytic apparatus and a method of generating pressurized hydrogen and by-product oxygen directly from the apparatus, without necessity of a separate pressurization step. The electrolytic apparatus, usually referred to as an “electrolyzer”, has a tubular cathode within which a rod-like anode is disposed to define between the anode and cathode an electrolyte chamber. A tubular separation membrane is disposed between the anode and the cathode to divide the electrolyte chamber into an anode sub-chamber and an electrolyte sub-chamber. In a specific embodiment, the anode, separation membrane and cathode have a coaxial configuration, so that the anode sub-chamber and the cathode sub-chamber are of concentric, annular configuration. The two electrolyte sub-chambers are respectively connected in gas-flow communication to respective gas/liquid separators to provide segregated hydrogen and oxygen sections from which the two generated gases are separately withdrawn.

Specifically, in accordance with the present invention there is provided an electrolyzer cell for the electrolysis of water having first and second opposite ends and comprising the following components. A cathode of tubular configuration is connectable to a source of DC electricity, and defines a cathode active inner surface and a cathode outer surface. An anode is connectable to a source of DC electricity, defines an anode active outer surface, and is disposed within the cathode to define therewith an annular electrolyte chamber disposed between the cathode inner surface and the anode outer surface. A separation membrane of tubular configuration is disposed within the electrolyte chamber between the cathode and the anode to divide the electrolyte chamber into an anode sub-chamber and a cathode sub-chamber. The separation membrane serves to seal against the passage therethrough of gases. First and second gas-tight seals are disposed at, respectively, the first and second opposite ends of the cell. A gas take-off connection is in liquid- and gas-flow communication with the electrolyte chamber for removing from the cell gases generated in the electrolyte chamber.

In accordance with another aspect of the invention, the gas take-off connection is dimensioned and configured to remove gas generated in the cathode sub-chamber separately from gas generated in the anode sub-chamber.

In another aspect of the invention, the cathode, separation membrane and anode are all disposed coaxially relative to each other, and the cathode inner surface, the anode outer surface and the separation membrane are each of circular configuration in transverse cross section.

Other aspects of the present invention provide that the electrolyzer cell may further comprise a pressure vessel separate from and surrounding and contacting the outer surface of the cathode or, alternatively, the cathode itself may comprise a pressure vessel. In either case, one aspect of the invention provides that the pressure vessel is capable of containing gas at an elevated pressure, which elevated pressure is at least about 10 psig. In some cases, the elevated pressure is not greater than about 10,000 psig, e.g., is not greater than about 5,000 psig.

Yet another aspect of the present invention provides that at least one of the gas-tight seals comprises an anode-sealing collar affixed to the anode adjacent one end thereof; an electrical isolation bushing, which may be cup-shaped to define a recess in which the anode-receiving collar is received, the bushing being affixed to the anode between the anode-sealing collar and the one end of the anode, the bushing engaging the anode-sealing collar; and an end fitting engaging the bushing and providing a gas-tight seal of the cathode at one end thereof.

Another aspect of the invention provides an electrolyzer comprising a plurality of electrolyzer cells as described above, first gas-flow conduits connected in liquid- and gas-flow communication between the respective cathode sub-chambers of the plurality of cells and a first gas collector; and second gas-flow conduits connected in liquid- and gas-flow communication between the anode sub-chambers of the plurality of cells and a second gas collector.

In accordance with a method aspect of the present invention there is provided a method of electrolyzing water to generate pressurized hydrogen and oxygen therefrom utilizing an electrolyzer comprising one or more electrolyzer cells. The cells individually comprise (i) a cathode of tubular configuration within which a rod-shaped anode is disposed to define an annular-shaped electrolyte chamber between the cathode and the anode, (ii) a separation membrane of tubular configuration disposed within the electrolyte chamber between the cathode and the anode to divide the electrolyte chamber into an anode sub-chamber and a cathode sub-chamber and seal the sub-chambers against gas flow therebetween. The method comprises the following steps: (a) introducing an aqueous solution of electrolyte, e.g., an aqueous solution of potassium hydroxide, into both sub-chambers of the electrolyte chamber; (b) applying a DC voltage drop across the respective anodes and cathodes of the cells to dissociate water into hydrogen at the cathode and into oxygen at the anode; and (c) separately withdrawing hydrogen and oxygen from the one or more electrolyzer cells.

In another method aspect of the present invention, the cell further comprises a pressure vessel and the hydrogen and oxygen are generated at an elevated pressure of at least about 10 psig, e.g., a pressure not greater than about 10,000 psig, or not greater than about 5,000 psig.

Method aspects of the present invention include one or more of the following, alone or in suitable combinations: the pressure differential between the hydrogen and oxygen withdrawn from the cells is maintained at not more than about 0.25 psig, preferably, not more than about 0.2 psig, and more preferably not more than about 0.17 psig.

Electrolyte and product hydrogen are flowed into a hydrogen separator, electrolyte and by-product oxygen are flowed into an oxygen separator, the respective electrolyte liquid levels in the hydrogen and oxygen separators are sensed and controlled to maintain a pressure differential between the hydrogen and oxygen withdrawn from the cells of not more than about 0.2 psig.

The electrolyte may be, but need not be, recirculated through the electrolyzer in a continuous operation.

DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS THEREOF

Referring toFIGS. 1,1A and1B, there is shown a gas-generation cell10comprising a cathode12which also serves as an outer containment shell, a separation membrane14(FIG. 1B) and an anode16. Cathode12has an inner surface12aand anode16has an outer surface16a. Surfaces12aand16aare active electrode surfaces which are exposed to, and in contact with, a liquid electrolyte18which is contained within electrolyte chamber15of gas-generation cell10. Electrolyte chamber15is defined by the space between surfaces12aand16a. As seen inFIG. 1B, separation membrane14divides electrolyte chamber15into an anode sub-chamber15acontaining an anode portion18aof electrolyte18, and a cathode sub-chamber15b, containing a cathode portion18bof electrolyte18. It is seen that the anode16, cathode12, and separation membrane14are configured coaxially, with the tubular separation membrane14disposed coaxially within the tubular cathode12and the rod-shaped anode16disposed coaxially within the separation membrane14. As shown inFIG. 1B, cathode12and separation membrane14are of annular shape in transverse cross section, thereby imparting the same cross-sectional annular shape to the anode and cathode sub-chambers15aand15b. Cathode12is separated from the anode and sealed at one end against high pressure by seal13(FIGS. 1 and 1A). A gas-tight seal12b(FIG. 1D) closes the other end of cell10. Gas-tight seal12bis shown in simplified schematic form for simplicity of illustration; its construction will be similar to that of gas-tight seal13except that, as shown inFIG. 1D, the anode16does not protrude through it, but stops short of it. A pair of gas take-off lines20and22protrude through gas-tight seal12bto establish liquid- and gas-flow communication with the interior of gas-generation cell10, as described below. The cathode12serves as the hydrogen-generating electrode and the anode16serves as the oxygen-generating electrode. The illustrated configuration of cell10separates the liquid electrolyte18into an anode electrolyte portion18aand a cathode electrolyte portion18b. The liquid electrolyte may be, for example, a 25% to 28% by weight KOH aqueous solution contained within electrolyte chamber15, i.e., between the electrodes12,16on both sides of the separation membrane14. A plurality of individual gas-generation cells formed in this manner may be assembled into an array for use in an electrolyzer, as described below.

Upon imposition of a direct current (“DC”) voltage drop, typically about from 1.5 to 3 volts, preferably about 2 volts, across cathode12and anode16, hydrogen gas is generated at cathode12within cathode sub-chamber15bof electrolyte chamber15, and oxygen gas is generated at anode16within anode sub-chamber15aof electrolyte chamber15.

The cathode component may, but need not necessarily, also serve as the pressure boundary of the electrolysis cell. That is, in some embodiments the cathode also serves as the containment or pressure vessel, whereas in other embodiments the co-axially disposed anode, separation membrane and cathode may all be contained within a pressure vessel, enabling thin-wall construction of the cathode as well as the anode.

For high pressure generation in cases where the cathode also serves as the pressure vessel, the wall thickness T of cathode12and consequently the outside diameter D of the cell10is dictated by the desired generation pressure, by material properties such as yield strength and electrical conductivity of the metal from which cathode12is made, and by practical considerations limiting the wall thickness of cathode12which, as noted above, also may serve as the containment vessel of cell10. For inexpensive steel or other suitable metal tube or pipe material, consistent with hydrogen embrittlement constraints, there are practical limits on the diameter D of individual cells for generation at 10,000 psig. These practical limits are imposed by practical limits on the wall thickness T of cathode12and result in a range of diameter D of from about 2 to 3½ inches (about 5.1 to 8.9 cm). Generally, the wall thickness T may vary from about ¼ to ⅝ inches (about 0.64 to 1.59 cm). The length L of the individual cell10is determined by the desired gas-generation rate, generation pressure, and annular flow gaps. Typically, the length L of the cell10is from about 2 to 6 feet (about 0.61 to 1.83 meters). The annular flow gaps are shown inFIG. 1Bby the radial dimension lines gc(cathode annular flow gap) and ga(anode annular flow gap). Typical dimensions for the cathode annular flow gap gcare from about 3/16 to ⅜ inches (about 0.48 to 0.96 cm), and for the anode annular flow gap gaare from about ⅛ to ¼ inches (about 0.32 to 0.64 cm).

A simple construction, shown inFIG. 1D, is used to maintain the balance of pressure across the separation membrane14within the individual cells10to within 2 inches of water (less than 0.1 psig). Maintaining such pressure balance enables maintaining product (hydrogen) purity because the separation membrane14cannot seal against gas leakage at pressure differentials exceeding a few inches of water. Gas-tight seal12bhas a circular flange11on the inside thereof in which is formed a groove (unnumbered) within which the end of separation membrane14is received to provide a gas-tight seal between cathode disengagement space19aand anode disengagement space19b. A similar grooved-flange construction may or may not be supplied at the inside of seal13(FIGS. 1 and 1A) to seal the opposite end of separation membrane14.

Gas off-take line20transports hydrogen gas from cathode disengagement space19a(FIG. 1D) within cell10above the level1of cathode electrolyte portion18bof liquid electrolyte18. Gas take-off line22transports oxygen gas from anode disengagement space19bwithin cell10above the level1′ of anode electrolyte portion18aof a liquid electrolyte18. The respective hydrogen and oxygen disengagement spaces are isolated from each other by a gas-tight bulkhead structure (not shown).

FIG. 1Cshows a second embodiment of the invention, wherein parts identical or similar to those of the embodiment ofFIG. 1Bare numbered 100 higher than the numbers used inFIG. 1B. With the single exception noted, the parts and their function of cell110ofFIG. 1Care identical to those of the corresponding parts of the embodiment ofFIG. 1B, and therefore a description of their structure and function is not repeated. In cell110, anode112is not designed to resist the operating pressures of cell110, and there is therefore provided a pressure vessel113which is separate from, but surrounds and contacts, the outer surface (unnumbered) of cathode112. Pressure vessel113has end portions (not shown) which encase the first and second ends of cell110to provide an effective pressure vessel for cell110.

The illustrated configuration of cell10enables optimization of the electrode areas for the cathode and anode. Because the gas-generation rate (of hydrogen) at the cathode is twice the gas-generation rate (of oxygen) at the anode, the respective surface areas of cathode inner surface12aand anode outer surface16aideally should have the same 2:1 ratio, or at least an approximation thereof, to allow the maximum gas-generation rate for a cell of given dimensions. The gas-generation rate is normally determined by the surface area12aof the cathode for a given material and surface conditions. In prior art parallel plate electrode configurations, where the anode and cathode are of equal surface area, there is a wasteful excess of anode surface area. In contrast, in the coaxial configuration of the present invention, the diameter of the anode is smaller than the diameter of the cathode as measured at its inner surface12a. The anode (outer) surface area is therefore smaller than the inner surface area of the cathode. The anode (outer) surface and the cathode inner surface are the surfaces in contact with the liquid electrolyte and therefore constitute the active electrode surfaces. The respective electrode diameters and annular flow gaps can be established to create a cathode-to-anode active surface area ratio near or at the optimum 2 to 1 value.

Usually, the separation membrane14ofFIG. 1Band the separation membrane114ofFIG. 1Cwill be dimensioned and configured so that the volume of sub-chambers15band115bare approximately twice the volume of their respective associated sub-chambers15aand115a. The individual cells10are sealed by providing a seal between the anode16and the containment vessel provided by the cathode12at each end of the latter. The seal must provide low voltage (˜2 volts) electrical isolation between the anode and cathode as well as sealing the cell10against liquid leakage with internal pressures in the cell of up to about 10,000 psig or more.FIG. 2is an illustration of a simple and effective seal design.

The seal13is comprised of four basic components. An anode-sealing collar24is made of metal and is welded to the anode16at an appropriate location to align it with the lower end of cathode12(FIG. 1). Collar24may alternately be made by machining anode16from a larger-diameter rod so that collar24and anode16are of one-piece, unitary construction. An O-ring groove24ais machined into the bottom end surface (unnumbered) of sealing collar24to receive an O-ring24b. An electrical isolation bushing26is of cup shape and is made of a dielectric material to provide an electrical isolation piece through which the anode16passes. Bushing26is made from non-conducting material and has an O-ring groove (unnumbered) formed about the periphery thereof to receive an O-ring26a. A high-pressure end fitting28is made of metal and provides an end piece through which the anode passes and which seals the lower end of the cathode12by means of either threading or welding. The outer diameter of the end fitting28may be threaded to provide exterior threads28ato mate with inner diameter threads (not shown) provided at both ends of the inner surface12a(FIG. 1B) of the containment vessel wall provided by cathode12. The end fitting maybe welded to the lower end of the cathode. Either arrangement forms a seal against the high gas pressure generated within cathode12.

An electrical insulating sleeve30has a sleeve bore33extending through it and is disposed within the end-fitting bore (unnumbered) extending through high-pressure end fitting28. Anode16is received within the sleeve bore33. Electrical insulating sleeve30thus serves to maintain electrical isolation between the anode16and cathode12outside the pressurized area within cathode12. Sleeve30also has an end flange30athat electrically isolates a nut32which is threaded onto the anode16, at threads17formed at or near the end thereof, and is used to preload and hold the entire assembly together. A washer34is interposed between nut32and end flange30a.

It will be appreciated that the various components, i.e., anode-sealing collar24, electrical isolation bushing26, and end fitting28are so dimensioned and configured as to position and maintain anode16at the center of the electrolyte chamber15(FIG. 1B) defined between cathode12and anode16. Structure is similarly provided to position and hold separation membrane14in place concentrically relative to anode16and cathode12. This may be accomplished by one or more suitable positioning members which are dimensioned and configured to position and maintain separation membrane14in place.

Referring now toFIG. 3, an electrolyzer apparatus36comprises an array38of individual cells10across each of which an electric potential is imposed by an electrical energy source provided, in the illustrated embodiment, by a DC generator40. Electrical leads from generator40to cells10are schematically illustrated by electrical leads42a,42b. A given hydrogen production capacity for electrolyzer apparatus36is attained by appropriately sizing individual cells10and selecting an appropriate number of such cells for connection to a common manifold system as described below. In use, a method for producing hydrogen (with an oxygen by-product) is carried out by utilizing an electrolytic apparatus as described above to produce hydrogen (and oxygen by-product) at an elevated pressure of up to 10,000 pounds per square inch gauge (“psig”), for example, a pressure range from about 0 to about 10,000 psig. The upper end of this pressure range (from about 5,000 to about 10,000 psig) is uniquely well suited to directly provide hydrogen fuel for storage in high-pressure storage vessels of hydrogen-based fuel cell-powered automobiles or other self-propelled vehicles, or portable or stationary devices. Any pressure ranges between about 0 to about 10,000 psig may of course be used. Typical of such intermediate ranges are pressures above about 3,000 psig, e.g., from above about 3,000 psig to about 10,000 psig; from about 3,500 psig to about 8,000 psig; and from about 3,500 psig to about 10,000 psig. Generation of hydrogen at pressures above 10,000 psig may be feasible in certain aspects of the invention, provided that it is economically practical for the contemplated use to provide pressure vessels and associated equipment capable of sustaining such high pressures.

An electrolyte reservoir44is supplied by make-up water pump48with make-up water from water treatment and storage zone46in order to replenish water which was dissociated by electrolysis to provide product hydrogen and oxygen. Electrolyte is taken from the electrolyte reservoir44and is fed by supply line45to electrolyte-replenishing pump50from which it is transported via electrolyte feed line51to an electrolyte manifold52which supplies the electrolyte liquid to individual cells10via electrolyte feed lines54.

Hydrogen gas generated within cells10and some electrolyte18(FIG. 1B) is removed via gas off-take lines20and hydrogen manifold line21to hydrogen separator56, wherein liquid electrolyte18(FIG. 1B) is separated from the hydrogen gas. Hydrogen product from hydrogen separator56is flowed via hydrogen discharge line60and is free to flow through check valve62and into hydrogen storage tank63, or to use or further treatment. Separated electrolyte provides a liquid seal within hydrogen separator56. Hydrogen pressure will continue to rise as hydrogen is supplied to the fixed volume storage tank63. Similarly, oxygen and liquid electrolyte18is removed from cells10by gas off-take lines22, which supply oxygen manifold line23. The oxygen gas and liquid electrolyte18flow via line23to oxygen separator64in which liquid electrolyte is separated from the oxygen. Separated oxygen flows via oxygen discharge line68at a rate, which is controlled by oxygen pressure regulator70, to an oxygen storage tank (not shown) or to venting or to use or further treatment. Separated electrolyte provides a liquid seal within oxygen separator64. The oxygen flow rate is controlled to maintain the liquid level in separator64to be equal to the liquid level in separator56. The same operational function could be performed by maintaining the pressure in separator64to be equal to the pressure in separator56. This allows the individual cells10to be operated in a flooded condition with the generated gas bubbles passing through the gas off-take lines20,22leading from each cell to the separators56,64and the common reservoir44. In such mode of operation, the levels1,1′ of electrolyte18shown inFIG. 1Dare maintained at a higher level within the apparatus illustrated inFIG. 3. The electrolyte18, in such case, floods the cells10, gas take-off lines20and22, hydrogen manifold line21and oxygen manifold line23, the electrolyte surface level in such case being at level1ofFIG. 4.

The separators56and64are sized in cross-section so as to act as a liquid trap preventing or greatly reducing electrolyte carry over and loss of potassium hydroxide. Make-up potassium hydroxide may be added to the system as needed, e.g., manually during shut-downs for periodic maintenance. In addition, the oxygen gas exiting the oxygen separator is connected to the gas space over the liquid in the electrolyte reservoir to maintain reservoir pressure at near cell pressure. This enables the electrolyte supply pump to operate as a low differential pressure circulator. Make-up water is only added to the electrolyte reservoir when level sensors in the reservoir (not shown) indicate the need to replenish the reservoir liquid.

Check valve62allows the hydrogen product gas to flow through line60into a storage tank63or to further processing or use when the hydrogen gas pressure in cells10exceeds that in line60, e.g., in the hydrogen storage tank63. A pressure sensor (not shown) acts to automatically shut off the electrical current to the electrolyzer apparatus36when the maximum design pressure in hydrogen storage tank63has been reached.

The liquid level in the hydrogen separator56is sensed by a simple level-sensing device, shown inFIG. 4, which is mounted on hydrogen separator56. Level-sensing device72comprises a pair (or more) of electrically isolated probes74,76that extend into the separator56at lengths that define the maximum and minimum desired level1of liquid electrolyte18in the separator56at, respectively, probe tips74aand76a. The electrically isolating seal is essentially the same design as the cathode/anode seal13(FIGS. 1 and 1A) described above. A low-voltage source78, typically, less than about 1.5 volts, is connected by electrical leads80,82to probes74,76and is grounded to separator56by electrical ground lead84. Electrical continuity is checked between the probes74,76and the shell of separator56. If the electrolyte level drops below the lower level, i.e., no continuity is found in either probe, the electrolyte supply pump50is actuated and electrolyte is sent to the cells. When electrical continuity is sensed on both probes74and76, the electrolyte has reached the maximum level and the electrolyte supply pump50is stopped, and no more electrolyte is sent to the cells. If the conductive electrolyte is between the two probe lengths, i.e., continuity is found on one probe only, the make-up water pump48status is left unchanged, whether on or off, until one of the two above mentioned conditions is met.

The flow of oxygen can be easily controlled to minimize the pressure differential between the separators (and therefore across the diaphragm) in either of two ways: differential pressure sensing, or liquid-level sensing.

In the differential pressure-sensing technique, the flow from the oxygen separator64is controlled by pneumatically actuated pressure regulator valve70. In this case the actuator diaphragm (not shown) of valve70is connected by lines (not shown) to sense the pressure differential between the gas in the oxygen separator64and hydrogen separator56, and opens to vent the gas space of oxygen separator64to maintain a set pressure differential. This pressure differential is set at near zero, e.g., a pressure differential of about from 0.17 to 0.2 psig, so that the pressure balance inherently keeps the liquid levels in the two separators56,64stable and equal to within the differential pressure setting.

In the direct liquid-level sensing technique, a liquid-level sensor identical to liquid-level sensing ofFIG. 4is installed on device72in the oxygen separator64. In this case the valve70regulating the flow of gas from the oxygen separator64cycles between high and low (or on and off) settings. This simple level-control scheme is satisfactory for operation of cells10. The setting of valve70is determined by the liquid electrolyte level in separator64as follows. When the valve70is at its high flow setting and the liquid level in the oxygen separator64rises and reaches the high level contact (analogous to probe tip74aofFIG. 4), the valve70is switched to its low flow-rate position by a suitable electronic control device (not shown). When the valve70is in the low flow setting and the liquid level drops and reaches the low level contact (analogous to probe tip76aofFIG. 4), the valve70is switched to its high flow-rate position by the control device.

In a different embodiment of the present invention, the electrolyte is circulated in a continuous recycle operation. This continuous-operation embodiment enables the production of high-pressure hydrogen with the potential to increase the length, and therefore the production rate, for a given cell. In the batch mode embodiment described thus far, the individual cell length is limited by a combination of the cell dimension (flow gap), gas volume generation rate, and bubble rise rate. Circulating the electrolyte upward through the cell at appropriate rates in a continuous recycle embodiment of the invention will increase the bubble rise rate via entrainment and allow longer cathode and electrode length for otherwise similarly dimensioned cells. To implement this recycle approach the separator reservoirs (items56and64inFIG. 3) would be altered by adding a return path for the electrolyte from separators56and64back to the electrolyte reservoir (item44inFIG. 3). The remainder of the apparatus schematically shown inFIG. 3and the basic control system as described above for the batch mode embodiment stays largely unaltered for the electrolyte-circulating continuous recycle embodiment.

The present invention provides at least the following advantages over the prior art.

1. The coaxial anode/cathode configuration allows very high-pressure hydrogen generation with practical wall thicknesses of conventional materials in the containment vessel provided by the cathode12. The value of this invention is further enhanced by the use of advanced pressure-containment materials, such as composite structures, which may make practical larger individual cell sizes at elevated pressures. The co-axial configuration also allows optimization of the surface areas of anode16and cathode12, as described above.

2. Independent gas/liquid separators (such as separators56,64) are used for each of the hydrogen and oxygen production sides. This allows multiple gas-generation cells10to be connected to common gas/liquid separation vessels (e.g.,56,64) and the utilization of a liquid electrolyte level control system.

3. A novel, low-cost pressure seal design for entry of the anode16into the gas-generation cell10enables satisfaction of high-pressure and electrical isolation requirements at reasonable cost.

4. The invention provides a simple, inexpensive control strategy for untended operation during hydrogen production, including automated control of the level of liquid electrolyte18, or the control of the differential pressure between the separators (56and64) and release of generated hydrogen and oxygen gases, such that high-purity gas products are obtained.

The ability of the apparatus and method of the present invention to enable hydrogen (and oxygen) production at pressures of up to or even exceeding 10,000 psig exceeds the highest direct generation pressure of about 3,000 psig that has been previously reported as attainable from prior known electrolyzers. The apparatus and method of the present invention can produce such high-pressure hydrogen without need for a separate compressor to pressurize the product hydrogen gas. Producing 10,000 psig hydrogen is key to supplying compressed hydrogen gas for fuel-cell-powered or internal combustion engine-powered vehicles at acceptable volume-to-weight ratios for onboard storage that yields a single-tank driving range equivalent to gasoline powered vehicles. The present invention allows high-pressure hydrogen production to be performed in a unique way that reduces the component cost and system complexity so that the equipment is easily affordable by individuals for commuter vehicle home fueling and for small fleet fueling applications. The invention is scalable to any given production capacity and is also practical for service-station type applications for dispensing of hydrogen to fuel-cell-powered vehicles and equipment.

The apparatus and method of the present invention may be utilized to generate pressurized hydrogen on site at locations such as service stations for hydrogen fuel cell-powered automobiles; service stations, hardware/home improvement stores, and local energy distributors for retail sale of hydrogen fuel via high-pressure canisters; and in residences, factories and office buildings for on-site energy storage and/or use in fuel cell or internal combustion engine-based portable power supply or home, garden or other appliance applications.

The present invention has been described in detail with reference to a particular embodiment thereof, but those skilled in the art will recognize that the invention may be utilized in other embodiments. Conventional known devices such as pressure-sensing and flow-rate sensing devices, and controls to operate valves and pumps, have been largely omitted from the description, as such devices and their use are well known in the art.