JP2024542127A - Oxygen-supplied fuel cell system and method of use - Google Patents

Abstract

【summary】
SOLUTION
The fuel cell system includes a hydrogen storage container and an oxygen storage container, a fuel cell stack, an air supply, an oxygen supply generating an internal oxygen pressure, at least one cathode, and at least one cathode exhaust. The system can switch between the air supply and/or the oxygen supply to supply the cathode of the fuel cell, and can perform air operation at low power consumption conditions and oxygen operation at high power consumption conditions. Oxygen can be recirculated from the cathode exhaust at a rate higher than the stoichiometric rate, and the internal oxygen pressure can rise to 1.1 bar or more. After the air is exhausted from the fuel cell stack, an automatic control system can recirculate oxygen, and a cathode exhaust recirculation can be performed, in particular an oxygen exhaust recirculation. The fuel cell can be configured to have ports with removable connections for refilling the hydrogen storage container and the oxygen storage container.
[Selected Figure] Figure 1

Description

This application claims the benefit of U.S. Provisional Patent Application No. 63/181,994, filed April 30, 2021, U.S. Provisional Patent Application No. 63/194,413, filed May 28, 2021, and U.S. Provisional Patent Application No. 17/727,138, filed April 22, 2022.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT This invention was made with Government support under Department of Energy Contract No. DESC0017144. The Government may have certain rights in this invention.

This disclosure relates generally to fuel cell systems, and more particularly to oxygen-supply fuel cell systems.

Fuel cells are electrochemical devices that convert energy from hydrogen and oxygen into water and electricity. Fuel cells are particularly useful because they can store large amounts of energy in the form of hydrogen. Storing more energy requires larger hydrogen tanks, but this is a relatively low-cost form of energy storage compared to other forms of energy storage such as batteries. The oxygen required for the electrochemical reaction is easily obtained from air. As only water is produced, hydrogen fuel cells can be a zero-emission source of power if the hydrogen is produced from renewable sources, such as by electrolysis of water. Fuel cells are therefore useful for many applications, including stationary energy storage and transportation.

Those skilled in the art will appreciate that the primary limitation of fuel cells is power density, which directly impacts the cost and size of the fuel cell system. Fuel cell power output is typically limited by the cathode where oxygen is converted to water. The oxygen must be transported from the air channels to the cathode catalyst layer. As higher current is drawn from the cell, the air becomes nitrogen-rich, water-rich and oxygen-depleted until a "mass transport limited current" or "maximum power output" is reached. The power density limitation necessitates the use of larger and more expensive fuel cell stacks to provide sufficient power. Often fuel cell systems are hybridized with batteries to provide maximum power output. The drawback is that the hybrid approach often increases the cost and complexity of the system.

There are several approaches that fuel cell developers have explored to improve mass transport limited current. Fuel cells can be designed to improve mass transport, such as a mix of hydrophobic and hydrophilic pores. Optimizing the fuel cell configuration can help improve mass transport. However, even with these approaches, the maximum current density is limited by oxygen transport. At the system level, the air flow rate can be increased to improve mass transfer. In some cases, during maximum power operation, the air is compressed to improve oxygen partial pressure and mass transfer. The limitation of these approaches is the introduction of additional components and parasitic losses. Even with high compressed air flow rates, the fuel cell current density is still limited by oxygen transport.

Those skilled in the art will appreciate that fuel cells can be operated with pure oxygen instead of air. The first fuel cells developed for aerospace applications used pure oxygen [Fuel Cell Handbook - available at https://www.osti.gov/servlets/purl/769283]. Pure oxygen is known to increase the voltage of a fuel cell, which can result in increased power density. The main drawback of using pure oxygen is that the oxygen must be sourced and stored on-board, which significantly increases the cost, weight, and size of the system. As a result, on-board storage of oxygen reduces power density compared to supplying oxygen from air.

In some cases, the inventors have envisioned a system that operates on both air and, in certain cases, oxygen. U.S. Pat. No. 4,657,829 discloses a system in which an on-board electrolyzer produces hydrogen and oxygen to power a fuel cell system while a hydrogen generating reformer is in operation. Patent application WO 2021/118660 A2 discloses an aerospace system in which a fuel cell is primarily powered by air and, in emergency situations, by oxygen produced from an integrated electrolyzer when air is not available. However, this prior art does not disclose a method for rapidly increasing the power density of a fuel cell system on demand. Relying on an on-board electrolyzer to generate oxygen increases the complexity, weight, and size of the system, thus reducing the power density of the system. Furthermore, the prior art does not disclose a method for minimizing the release of unused pure oxygen from the system. Allowing unused oxygen to escape through the cathode outlet, as is done in air-based fuel cells, increases the need for on-board oxygen, thus further reducing the power density of the system.

The present invention relates to a means of energy storage and power production. Energy storage is important in a variety of applications, including storing renewable energy to meet fluctuations in generation and demand. Energy storage is also important in transportation, where energy must be stored on-board the vehicle and converted to power to meet load requirements. Hydrogen fuel cells provide a zero-emission means of storing energy and producing power when needed. However, power production is often the limiting factor in air-fed hydrogen fuel cell systems. The present invention reveals a means of increasing the power production of fuel cell systems using oxygen while maintaining a high energy density.

The fuel cell system may include a hydrogen storage container in fluid communication with a hydrogen supply and an oxygen storage container in fluid communication with an oxygen supply. In a preferred embodiment, there is a fuel cell configured as a fuel cell stack and in fluid communication with the hydrogen and oxygen supplies, and other features of the system may include an air supply, an oxygen supply that generates internal oxygen pressure, at least one cathode, and at least one cathode exhaust.

There may be means for switching between the air and/or oxygen supplies to supply the cathode of the fuel cell, and automatic control means for supplying oxygen from the oxygen supply to the fuel cell to produce a predetermined power level at the fuel cell. As part of the above, in a preferred embodiment there may be means for recirculating oxygen from the cathode exhaust at a rate higher than the stoichiometric rate, and means for increasing the internal oxygen pressure to above 1.1 bar to at least 30 bar. In another preferred embodiment there may be an automatic control system for recirculating oxygen after air is exhausted from the fuel cell stack, specifically this may be controlled by a timer, an electrochemical oxygen sensor, or other means known to those skilled in the art.

In a preferred embodiment, there may be a recirculation of the cathode exhaust, in particular the oxygen exhaust. This may include a means for initiating the recirculation of the oxygen exhaust when at least 50% of the air is expelled from the fuel cell by oxygen. The fuel cell system may be configured with ports with removable connections for refilling the hydrogen storage vessel and the oxygen storage vessel. In some embodiments, the fuel cell system may be combined with a refueling system having a dual hydrogen and oxygen filling nozzle.

In another set of embodiments, there may be an automatic control system for a fuel cell system including a fuel cell that is switchable between air and oxygen operation, with the air operation being possible under low power consumption conditions and the oxygen operation being possible under high power consumption conditions.

Without limiting the scope of the fuel cell disclosed herein, reference will now be made to the drawings and figures.
FIG. 1 shows current-potential and current-power curves for a proton exchange membrane (PEM) fuel cell operated with air at 150 kPa and 250 kPa compared with the same PEM fuel cell operated with pure oxygen at 250 kPa. FIG. 2 shows a diagram of an embodiment of a backup power fuel cell system. FIG. 3 shows a diagram of an embodiment of a fuel cell system design integrated with an electrolyzer with removable ports to allow movement of fuel cell system parts. FIG. 4 shows a diagram of an embodiment of a fuel cell system including examples of some possible control communication lines. These diagrams are provided to aid in understanding exemplary embodiments of the fuel cell system and its use, as described in more detail below, and should not be construed as unduly limiting the specification. In particular, the relative spacing, position, size, and dimensions of the various elements shown in the figures may not be drawn to scale and may be exaggerated, reduced, or otherwise altered for clarity. Additionally, those skilled in the art will appreciate that various alternative configurations have been omitted simply to improve clarity and reduce the number of figures.

The present invention includes a fuel cell system that is powered by using hydrogen at the anode and air at the cathode during normal operation, and pressurized pure oxygen at the cathode during periods of high power demand. In a preferred embodiment, when high power output is required from the fuel cell, a valve or series of valves is actuated by a control system. The valves switch the cathode gas supply from air to pure pressurized oxygen. A first valve can switch the input gas supply to oxygen. When oxygen has expelled most of the air from the fuel cell, a second valve begins recirculating and pressurizing the oxygen. As used herein, "purge" means that at least 50% of the air at the fuel cell cathode has been replaced with oxygen. Activation of the first valve may be initiated manually by a user of the system. In a preferred embodiment, activation of the valves may be initiated automatically by a fuel cell control system. The fuel cell control system can initiate the switching of the valves based on an increase in power demand, which may be done by measuring the voltage drop across the fuel cell stack caused by the increase in power demand, or by other signals known to those skilled in the art. In a preferred embodiment, actuation of the second valve may be initiated by a timer based on a known drain time, by measurement of the fuel cell stack voltage at a known operating current, by an oxygen sensor, or by other means known to those skilled in the art.

Those skilled in the art will appreciate that fuel cells have a higher voltage at similar current density in oxygen compared to air. In a preferred embodiment, actuation of the first valve is performed within 5 seconds, preferably within 50 milliseconds, of an increase in power demand, allowing for a rapid response to increased power demand. In a further preferred embodiment, the pure oxygen exhausted from the fuel cell stack is passed through a dehumidifier to remove excess moisture, after which the oxygen is recirculated using an oxygen safe blower. In a preferred embodiment, the oxygen is pressurized to an absolute pressure of greater than 1.1 bar to improve fuel cell performance. In some cases, the oxygen is pressurized to an absolute pressure of 30 bar or more to enhance fuel cell performance. In other preferred embodiments, the incoming air can be filtered and purified to remove particles and contaminants that could potentially compromise the safe use of pure pressurized oxygen at the cathode.

In some embodiments, the control system can also include a capacitor. The capacitor can provide increased power for a short period of time while air replaces oxygen at the fuel cell cathode. In this case, the system can respond to power demands more quickly. When the output level drops, the capacitor can receive power from the fuel cell before the oxygen is completely displaced by air and the fuel cell current drops. In this case, the fuel cell does not experience high voltage at low current. One skilled in the art will appreciate that high voltages associated with pure oxygen at low current can degrade the fuel cell cathode more quickly.

In a preferred embodiment, the fuel cell may be a proton exchange membrane (PEM) fuel cell. The PEM fuel cell may provide hydrogen in a dead-end configuration. In a further preferred embodiment, the fuel cell may be a liquid-cooled PEM fuel cell with internal manifold oxidant flow paths. Liquid cooling has many advantages, including the ability to maintain stack temperatures at low or high power levels without the inefficiencies of using a high stoichiometric flow of oxidant to provide cooling. Internal manifold oxidant has many advantages, including the ability to recirculate unused oxygen back into the stack rather than releasing it to the atmosphere.

In a preferred embodiment, the fuel cell stack is in fluid communication with stored hydrogen on the anode side and with air or stored oxygen on the cathode side, as best shown in FIGS. 1-4. The hydrogen and oxygen storage can be sized appropriately depending on the application of the system. Specifically, the oxygen storage can be sized to have sufficient oxygen volume to allow for the use of pure oxygen during the high power duty cycle of the system, but less than the stoichiometric amount of oxygen for the hydrogen. For example, if the system operates at high power 50% of the time, the maximum number of moles of oxygen storage is 25% of the hydrogen storage, since 2 moles of hydrogen are consumed per mole of oxygen. In another example, if the system operates at high power 10% of the time, the maximum number of moles of oxygen storage is 5% of the hydrogen storage, since 2 moles of hydrogen are consumed per mole of oxygen. As a result, the relatively small oxygen tank can eliminate the need for large fuel cells and hybridization with batteries, capacitors, turbines, and/or other high power energy conversion means, which are complex, expensive, and large and space consuming. Additionally, those skilled in the art will appreciate that in the unlikely event that the oxygen reservoir is completely depleted of oxygen, the system can be designed to continue to operate at a reduced power level using atmospheric air and hydrogen.

In addition, as best seen in FIG. 3, the hydrogen and oxygen storage units may be fitted with ports to allow for rapid refilling of the gas. In other cases, liquid hydrogen and/or liquid oxygen may be used. In yet another example, the hydrogen and oxygen storage units may be connected to a water electrolysis device. In a preferred embodiment, the hydrogen and oxygen ports are fed from a dual hydrogen and oxygen filling station, allowing both hydrogen and oxygen to be refilled simultaneously. In a further preferred embodiment, the hydrogen and oxygen ports are connected to hydrogen and oxygen nozzles on a single handle, allowing the user to easily refill both tanks at the same time. In another preferred configuration, the flow of oxygen may be restricted, resulting in a refill time similar to when a larger volume of hydrogen is refilled at the same time, thus minimizing the rate of oxygen. In a preferred embodiment, the oxygen port and nozzle are covered when not in use and kept clean and free of particles and debris.

EXAMPLES Example 1 PEM Fuel Cell Operation A ^{25 cm2} fuel cell system was constructed with a sulfonated tetrafluoroethylene-based fluoropolymer copolymer (NAFION®, The Chemours Company FC, LLC, Delaware, USA) 211 membrane, 20 weight percent platinum/carbon anode catalyst, and 40 weight percent platinum/carbon cathode catalyst. The catalyst was first mixed with NAFION® ionomer, water, and alcohol solvent to form an anode ink and a cathode ink, respectively. The inks were applied to a polytetrafluoroethylene (PTFE) (TEFLON®, The Chemours Company FC, LLC, Delaware, USA) backing layer and transferred to the membrane by hot pressing. The loading of platinum on the anode and cathode was 0.2 mg/ ^cm2 , respectively. The fuel cell was mounted in a Fuel Cell Engineering Test Station with a 25 ^cm2 manifold.

The fuel cell was supplied with pure hydrogen on the anode side and air or oxygen on the cathode side. A three-way valve and mass flow controller were used to switch the gas supply to the cathode. A back pressure regulator controlled the operating pressure of the cell. A humidifier was used to humidify the incoming gas to near 100% relative humidity at 80°C. The manifold was heated to 80°C. The current-voltage characteristics of the fuel cell were measured under 150 kPa and 250 kPa air and 250 kPa oxygen, as shown in Figure 1. As shown, very high power and efficiency can be obtained when the cell is operated under pressurized oxygen.

Those skilled in the art will appreciate that even higher power output can be achieved using thinner membranes. NAFION® 211 is 25 microns thick, but it is possible to produce membranes thinner than 8 microns. With thinner membranes, the resistance drops proportionately and power densities can exceed 5 watts per square centimeter.

Additionally, those skilled in the art will appreciate that PEM fuel cells can be operated at pressures up to about 30 bar if the fuel cell and seals are properly designed. Higher hydrogen and/or oxygen pressures dramatically increase the NEMST potential of the fuel cell at a given current. Furthermore, it is known that the rate of oxygen reduction increases with oxygen partial pressure. Finally, higher pressures also improve mass transfer. As a result, the higher pressure operation possible with the present invention can dramatically increase the power density and efficiency of the fuel cell.

Example 2 Stationary Backup Power System In one embodiment of the present invention, a fuel cell system with oxygen power boost capability can be used to provide backup power. The system can use a PEM fuel cell stack made from multiple cells similar to Example 1. A preferred process flow diagram for this example is shown in FIG. 2. Valves 1 and 2 are three-way valves. Valve 1 is arranged to supply pure oxygen or air as the oxidant to the fuel cell by a blower (not shown for simplicity). In normal operation, air is used as the oxidant for the fuel cell. Valve 1 is arranged to supply oxygen based on increased power demand, low fuel cell voltage measurement, high current measurement, and/or low oxygen concentration sensor measurement. The oxidant passes through a water exchange subassembly which humidifies the incoming gas and removes moisture from the cathode outlet gas.

Excess water from the exhausted oxidant can be removed in this subassembly. The humidified oxidant is fed to the cathode inlet of the fuel cell. The exhausted oxidant (pure oxygen, oxygen-depleted air, or a mixture thereof) is fed from the cathode outlet to the water exchange subassembly and then to valve 2. In air operation, valve 2 exhausts oxygen-depleted air to the atmosphere. In oxygen operation, valve 2 recirculates pure oxygen to the oxygen inlet side. To switch between air and oxygen operation, valve 1 is arranged to first supply oxygen. An oxygen sensor, stack voltage, or timing mechanism is then monitored to determine when nitrogen is exhausted from the cathode of the fuel cell. At that point, valve 2 is arranged to recirculate unused oxygen to prevent large amounts of unused oxygen from being exhausted. If the oxygen storage is depleted, the valve switching is disabled and the system operates only with air as the oxidant. A control system, not shown for simplicity, monitors stack voltage, stack current, and oxygen concentration while also controlling the valve arrangement. In a further preferred embodiment, the fuel cell operates with pressurized oxygen. Increasing the oxygen pressure increases the cell potential and improves performance. The fuel cell may also be operated with pressurized hydrogen. When both valve 1 and valve 2 are configured for closed-loop oxygen operation, the oxygen gas pressure is increased, increasing the inlet oxygen pressure from the oxygen reservoir.

In a preferred embodiment, the fuel cell stack can be liquid cooled to prevent overheating of the fuel cells. During periods of high power demand, the circulation rate of the liquid coolant through the fuel cell stack can be increased by the control system. Heat can be removed from the liquid coolant using a heat exchanger, radiator, or other methods known to those skilled in the art of temperature control.

In a further preferred embodiment, during high power operation, product water may be removed from the fuel cell stack via an oxygen recycle device that removes excess water and/or cools the oxygen. Examples of such devices include, but are not limited to, condensers, membrane separators, or other types of dehumidifiers. In this manner, dry oxygen may be returned to the fuel cell stack.

In a preferred embodiment, hydrogen and oxygen are replenished by a water electrolyser that supplies the respective gas storage tanks. In this embodiment, the gas tanks are replenished when power is available, and the fuel cell may be operated with oxygen when power demand is high, and with air at low power when oxygen is unavailable or low power is required for extended periods of time. In a preferred embodiment, the electrolyser uses an anion exchange membrane (AEM) to keep the oxygen and hydrogen separated. In a preferred embodiment, the electrolyser operates at high pressure (10-1,000 bar), which allows for the production of hydrogen and oxygen gases that can be economically stored without further mechanical compression. Compressing water with a pump is less energy consuming and less inefficient than compressing gas with a mechanical compressor. Thus, such an embodiment has significant advantages over conventional electrolysis systems that use compressors.

Typically, in some embodiments of the invention, the electrolyzer operates at 30 bar or more. In some cases, it may be advantageous to operate the hydrogen side of the electrolyzer at a higher pressure than the oxygen. Many applications, including hydrogen fuel cells for transportation, may require high pressure hydrogen. On a molar basis, oxygen storage tanks are less expensive because only half the amount of oxygen is produced compared to hydrogen from water electrolysis. In some cases, higher oxygen pressures may raise safety concerns. In some cases, the system may not require large amounts of oxygen storage, as oxygen is readily available from the ambient air. In the described systems, at least a portion of the energy is stored in the form of both oxygen and hydrogen.

Those skilled in the art will appreciate that there are many ways to produce hydrogen, including methane reforming, photoelectrochemical water splitting, ammonia cracking, hydrocarbon cracking, and the like. The hydrogen tank may be refilled with hydrogen produced by any method, not necessarily by an electrolyzer. The hydrogen may be provided from a hydrogen pipeline, not necessarily from a tank. Additionally, those skilled in the art will appreciate that pure oxygen may be produced from other methods, including pressure swing adsorption from air, membrane separation from air, fractional distillation of air, or other sources. The oxygen storage tank may be refilled with oxygen produced by any method.

Example 3 Mobile Power System In another preferred embodiment of the present invention, a fuel cell system with oxygen power boost function can be used to provide a mobile power source. A preferred process flow diagram for this example is shown in Figures 3 and 4. This particular system embodiment operates similarly to Example 2, except that in this example, the hydrogen and oxygen storage is configured in two parts. One set of tanks is connected to a water electrolysis device that refills the tanks when power is available. A second set of tanks is mounted on a mobile vehicle. The tanks mounted on the vehicle can be refilled at any time using a removable connector attached to the electrolysis device tank. In this approach, the electrolysis system is effectively a dual hydrogen and oxygen filling station for the fuel cell vehicle. In another embodiment, the electrolysis device can be located away from the vehicle's fueling station, and gas can be transported from the electrolysis device to the fueling station and then from the fueling station to the vehicle.

In one embodiment, oxygen can be produced by separating oxygen from air using any number of available techniques known to those skilled in the art, including pressure swing adsorption or membrane separation. In a preferred embodiment, the air separator can be part of a stationary unit that produces oxygen for an oxygen filling station. In another preferred embodiment, the air separator can be onboard a vehicle. The air separator can be powered by regenerative braking or auxiliary power while onboard the vehicle when the vehicle is in use. Alternatively, the air separator can be powered through an electrical connection, such as a common outlet, when the vehicle is idling.

In a preferred embodiment, the fuel cell stack can be liquid cooled to prevent overheating of the fuel cells. During periods of increased power demand, the circulation rate of the liquid coolant through the fuel cell stack can be increased by the control system. Heat can be removed from the liquid coolant using a heat exchanger, radiator, or other methods known to those skilled in the art of temperature control.

In one embodiment, the control system may also include a capacitor. The capacitor may provide increased power for a short period of time while air replaces oxygen at the fuel cell cathode. In this case, the system may respond to power demands more quickly. When the power level drops, the capacitor may receive power from the fuel cell before the oxygen is sufficiently removed by air to cause the fuel cell current to drop. In this case, the fuel cell does not experience high voltages at low currents. One skilled in the art will appreciate that high voltages associated with pure oxygen at low currents may cause the fuel cell cathode to deteriorate more quickly.

In a further preferred embodiment, during high power operation, product water may be removed from the fuel cell stack via an oxygen recycle device that removes excess water and/or cools the oxygen. Examples of such devices include, but are not limited to, condensers, membrane separators, or other types of dehumidifiers. In this manner, dry oxygen may be returned to the fuel cell stack.

Those skilled in the art will appreciate that there are many ways to produce hydrogen, including methane reforming, photoelectrochemical water splitting, ammonia cracking, hydrocarbon cracking, and the like. The mobile hydrogen tank may be refilled with hydrogen produced by any method, including from a hydrogen pipeline and not necessarily from a tank. Additionally, those skilled in the art will appreciate that pure oxygen may be produced from other methods, including pressure swing adsorption from air, membrane separation from air, fractional distillation of air, and the like. The mobile oxygen tank may be refilled with oxygen produced by any method.

Example 4 Dual Refueling Nozzle In another preferred embodiment, a mobile fuel cell system may be refueled at a dual hydrogen and oxygen filling station. The filling station may include a system for simultaneously refueling a vehicle's hydrogen and oxygen tanks. In a preferred embodiment, the vehicle storage tanks may be refueled using a set of nozzles where the flow rate of gas through both nozzles may be controlled by the same system. In a further preferred embodiment, both nozzles may be located on the same handle for user convenience, and thus the vehicle's refueling port is designed to mate with the dual nozzles.

Example 5 - Delivery Truck In a preferred embodiment, the fuel cell system described in Example 3 can be placed in a truck. The advantage of this embodiment is that the truck can carry a load with zero emissions. During acceleration periods, the fuel cell operates in oxygen mode. When the truck is not accelerating, the fuel cell operates in air mode. The present invention minimizes the size of the fuel cell and eliminates the need for heavy and expensive batteries while achieving zero emissions and power requirements. Refueling connections can be conveniently installed at the home base of the truck fleet or along a frequently traveled corridor.

Example 6 Vertical Take-Off and Landing In another preferred embodiment, the fuel cell system described in Example 3 can be deployed in a zero-emission vertical take-off and landing aircraft. The advantage of this embodiment is that the vehicle can carry a heavier load with zero emissions. The fuel cell can operate in oxygen mode during periods of ascent, acceleration, and/or maneuvering. The fuel cell can operate in air mode when the vehicle is descending or traveling at average speeds. The present invention minimizes the size of the fuel cell and eliminates the need for heavy and expensive batteries while achieving zero emissions and power requirements. Refueling connections can be conveniently installed at the vehicle's home base or along a frequently traveled corridor.

Example 7 High Power Fuel Cell System The high power fuel cell system described in the above examples may be of benefit in many other embodiments. Those skilled in the art will appreciate that the technology is useful in applications such as ships, cargo ships, passenger ships, trains, rail cars, mining equipment, construction equipment, fleet vehicles, material handling equipment, portable electronics, stationary equipment, and other such applications known to those skilled in the art. Those skilled in the art will also envision systems incorporating mixtures or combinations of the elements described in the above examples.

The claim relates to a fuel cell system including a hydrogen storage container in fluid communication with a hydrogen supply and an oxygen storage container in fluid communication with an oxygen supply. In a preferred embodiment, there are fuel cells configured as a fuel cell stack and in fluid communication with the hydrogen and oxygen supplies. Other features of the system may include an air supply, an oxygen supply that generates internal oxygen pressure, at least one cathode, and at least one cathode exhaust.

There may be means for switching between an air supply and/or an oxygen supply to supply the cathode of the fuel cell, and automatic control means for supplying oxygen from the oxygen supply to the fuel cell to produce a predetermined power level at the fuel cell. There may be cooling system means having a liquid coolant for removing waste heat from the fuel cell or fuel cell stack.

As part of the above, in a preferred embodiment there may be means for recirculating oxygen from the cathode exhaust at a rate greater than the stoichiometric rate and means for increasing the internal oxygen pressure to 1.1 bar or greater.

In another preferred embodiment, there may be an automatic control system for recirculating oxygen after air is exhausted from the fuel cell stack, and in particular, this means may be controlled by a timer.

In yet another embodiment, there may be a means for dehumidifying the oxygen exhaust from the cathode exhaust before recycling it to the fuel cell, and an oxygen sensor for detecting when air is exhausted from the fuel cell stack.

In a preferred embodiment, there may be recirculation of the cathode exhaust, particularly the oxygen exhaust. This may include means for initiating recirculation of the oxygen exhaust when at least 50% of the air is purged from the fuel cell by oxygen. The cooling system may also have a variable speed pump for recirculating the liquid coolant at an increased rate during high power mode operation.

In two specific embodiments, they are in the same range, but the fuel cell can produce more than 1 watt per square centimeter when operating in high power mode and/or double its maximum power output when operating in oxygen mode compared to air mode.

The fuel cell may be configured with ports with removable connections for refilling the hydrogen and oxygen storage containers, and the nozzles associated with the ports may be fixed to the single handle.

In another set of embodiments, there may be an automatic control system for the fuel cell system including the fuel cell that is switchable between air operation and oxygen operation, with air operation in low power consumption conditions and oxygen operation in high power consumption conditions. The terms "lower" and "higher" as applied to power consumption are not subject to precise parameters as known to those skilled in the art, but will be established by the capacity of the fuel cell and the needs of the user in a particular case. Oxygen operation uses substantially pure pressurized oxygen. "Pure" oxygen is defined herein as oxygen that is at least 50% pure _O2 , preferably at least 90% pure _O2 , and most preferably at least 95% pure _O2 . This allows the fuel cell to be operated with pure pressurized oxygen at current densities of greater than about 500 mW/ ^cm2 in one embodiment, greater than about 1,000 mW/ ^cm2 in another embodiment, and greater than about 2,000 mW/ ^cm2 in yet another embodiment. In yet another set of embodiments, the current density may reach 5,000 mW/ ^cm2 or more.

Various means for controlling the switch from air to oxygen operation will be understood by those skilled in the art, and may include, for example, events such as an increase in power demand to the control system, a predetermined fuel cell voltage measurement, a predetermined oxygen sensor measurement, a predetermined current measurement, and other means and events. By way of example only and not limitation, such an event may control a mixing valve that is switchable between oxygen and air input.

In yet another set of embodiments, there may be a fuel cell system including a hydrogen storage container and an oxygen storage container in addition to a fuel cell in fluid communication with the hydrogen storage container and the oxygen storage container, at least one cathode, and a cathode exhaust. There may be means for providing an air supply and an oxygen supply to the cathode of the fuel cell, thereby allowing oxygen to at least partially vent the air from the fuel cell, and such gas switching may be at least partially controlled by an automatic control scheme that provides oxygen to the fuel cell when higher power levels are required.

The system may further include cooling means for removing waste heat from the fuel cell with a liquid coolant, and a fill port with a removable connection for refilling the hydrogen and oxygen storage containers. There may also be a means for initiating recirculation of the oxygen exhaust when at least 50% of the air has been displaced from the fuel cell by the oxygen.

Numerous changes, modifications, and variations of the preferred embodiments disclosed herein will be apparent to those skilled in the art, all of which are anticipated and contemplated to be within the spirit and scope of the present disclosure. For example, while certain embodiments have been described in detail, those skilled in the art will appreciate that the foregoing embodiments and variations can be modified to incorporate various types of substitutions, additional or alternative materials, relative arrangements of elements, sequence of steps and additional steps, and dimensional configurations. Thus, although only some variations of the products and methods are described herein, it will be understood that the implementation of such additional modifications and variations, and their equivalents, are within the spirit and scope of the methods and products defined in the following claims. The corresponding structures, materials, acts, and equivalents of all means or step-plus-function elements in the following claims are intended to include any structures, materials, or acts for performing a function in combination with other elements specifically recited in the claims.

Claims (20)

1. A fuel cell system comprising:
a. a hydrogen storage container in fluid communication with a hydrogen supply;
b. an oxygen storage container in fluid communication with the oxygen supply;
c. a fuel cell stack in fluid communication with the hydrogen supply and the oxygen supply, an air supply, an oxygen supply generating an internal oxygen pressure, at least one cathode, and at least one cathode exhaust;
d. means for switching between said air supply and/or said oxygen supply to supply said cathode of said fuel cell;
e. an automatic control means for supplying oxygen from said oxygen supply to said fuel cell to produce a predetermined power level in said fuel cell;
f. a cooling system having a liquid coolant to remove waste heat from said fuel cell system. The system of claim 1, further comprising means for recirculating oxygen from the cathode exhaust at a rate greater than the stoichiometric rate. The system of claim 1, further comprising means for increasing the internal oxygen pressure to 1.1 bar or more. The system of claim 1, further comprising an automatic control system for recirculating oxygen after air is exhausted from the fuel cell stack. The system of claim 1 further comprising a means for dehumidifying the oxygen exhaust from the cathode exhaust before recirculating it to the fuel cell. The system of claim 1 further includes an oxygen sensor that detects when air is exhausted from the fuel cell stack. The system of claim 1, further comprising means for initiating recirculation of oxygen exhaust when at least 50% of the air is exhausted from the fuel cell by oxygen. The system of claim 1, wherein the cooling system further comprises a variable speed pump that recirculates the liquid coolant at an increased rate during high power mode operation. The system of claim 1, comprising a fuel cell that produces more than 1 watt per square centimeter of power when operated in a high power mode. The system of claim 1 further includes a fuel cell that has twice the maximum output when operated in oxygen mode compared to air mode. The system of claim 1, further comprising ports having removable connections for refilling the hydrogen storage container and the oxygen storage container. The system of claim 11, wherein the ports include removable nozzle connections for refilling the hydrogen storage container and the oxygen storage container and are secured to a single handle. An automatic control system for a fuel cell system that is switchable between air operation and oxygen operation, the control system switching the fuel cell to air operation under low power consumption conditions and to oxygen operation under high power consumption conditions. The system of claim 13, wherein the oxygen operation uses pure pressurized oxygen. 15. The system of claim 14, wherein said fuel cell operates at a current density of greater than about 500 mW/ ^cm2 with said pure pressurized oxygen. 15. The system of claim 14, wherein said fuel cell operates at a current density of greater than about 1,000 mW/ ^cm2 with said pure pressurized oxygen. 15. The system of claim 14, wherein said fuel cell operates at a current density of greater than about 2,000 mW/ ^cm2 on said pure pressurized oxygen. The system of claim 13, wherein the control system switches from air operation to oxygen operation based on an event selected from a group of events consisting of an increase in power demand on the control system, a predetermined fuel cell voltage measurement, a predetermined oxygen sensor measurement, and a predetermined current measurement. 1. A fuel cell system comprising:
a. a hydrogen storage container;
b. an oxygen storage container;
c. a fuel cell in fluid communication with the hydrogen storage vessel and the oxygen storage vessel, at least one cathode, and a cathode exhaust;
d. means for providing an air supply and an oxygen supply to the cathode of the fuel cell, thereby allowing oxygen to at least partially displace air from the fuel cell;
e. an automatic control scheme that supplies oxygen to the fuel cell when higher power levels are required;
f. cooling means for removing waste heat from said fuel cell with a liquid coolant;
g. a port with a removable connection for refilling said hydrogen storage container and said oxygen storage container. 21. The system of claim 20, further comprising means for initiating recirculation of oxygen exhaust when oxygen exhausts air from the fuel cell.

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