Diffusion of Aluminum in Gallium

Cross Reference to Related Applications [0001] The present application claims priority to U.S. Provisional Patent Application No. 61/636,100 filed April 20, 2012, the disclosure ofwhich is incorporated by reference herein in its entirety.

Technical Field

[0002] The present invention relates to electrochemical reactors, and more particularly, to reactors for the production of energy from the oxidation of aluminum.

Background Art

[0003] It is known in the art that aluminum can be used as a fuel for the production of hydrogen which can then be harvested or used in fuel cells. But, because aluminum forms a stable protective layer upon oxidation, which limits its further reaction with the oxidant, aluminum-based systems require the use of a depassivating agent to remove the oxide layer and expose new reactive surfaces to the oxidant. Despite that the oxidation of aluminum is exothermic, aluminum has mostly been used as a source for producing hydrogen, and the heat produced has been mostly used as a waste by-product.

[0004] In US Patent No. 4,358,291, a system for the oxidation of aluminum is described using gallium as the depassivating agent and water as the oxidizing agent.

Aluminum in the solid state is reacted with water in the presence of gallium in a chamber at room temperature. Once all the aluminum is consumed, the byproducts are removed from the chamber. The released energy is quantified. Optionally, gallium may be recovered for further use. The aluminum oxide by-product is recovered; and aluminum can be recovered by electrolysis and returned to the chamber. [0005] In US Patent Application Publication 2004/0205997, a system for the production of hydrogen is described using a dual chamber, portable reactor. The system uses aluminum as a fuel source, and a sodium hydroxide solution which acts both as a

depassivating and oxidizing agent. The first chamber contains the sodium hydroxide solution; the second chamber contains the aluminum source and is the chamber where the oxidation occurs. Hydrogen is released and recovered from the second chamber. Heat is collected with heat exchangers placed within the second chamber, and aluminum hydroxide by-product forms a slurry with the excess sodium hydroxide solution. In this system, all the reactants and products of the oxidation are present in one chamber.

[0006] In US Patent No. 7,938,879, the preparation of readily-oxidizable pellets of aluminum is described using gallium as the depassivating agent. The composition of the pellets is optimized for their easy manufacture and their ability to generate an easily controllable oxidation reaction for the production of hydrogen gas and heat.

[0007] Most known systems using aluminum as a fuel have focused on the collection of hydrogen to produce energy in water-based, oxygen-rich environments. The thermal energy generated in the reaction is tempered by the heat capacity of water, and thus has usually been used as waste heat. Because of the low thermodynamic properties of water, the recovery of the thermal energy produced by the oxidation of aluminum has been ineffective for conversion into another form of energy.

Summary of the Embodiments

[0008] There are now described systems and methods for their use for the production of high thermal energy which may be harvested as a substantial anaerobic energy source for powering vehicles such as unmanned-underwater vehicles (UUVs) or unmanned aerial vehicles (UAVs), other small robotics, backup or regular underwater power sources (e.g., for oil rigs), or as an emergency power source in flooded or disaster areas.

[0009] In some embodiments, the systems rely on the use of a stream of molten depassivating agent flowing through a circuit, and passing through a source of fuel, then a source of an oxidizing agent, such as water, in small amounts with respect to the

depassivating agent such that the thermodynamic properties of the depassivating agent regulate the thermal energy produced. The stream is then passed through a heat harvester and a waste purifier before being cycled back. Optionally, the hydrogen also produced in this system may be used as an additional fuel for power generation when oxygen is available.

[0010] In other embodiments, the systems rely on a stream of an oxidant passing by a surface of a liquid pool of depassivating agent/fuel alloy. The fuel is fed into a chamber containing a pool of liquid depassivating agent, in which is continuously dissolved the fuel by diffusion and convection. The fuel reacts with the oxidant at the surface exposed to the stream of oxidant, producing heat, hydrogen and an oxide by-product of the fuel, which are entrained by the flow of oxidant. The stream is then passed through a heat harvester and the heat is converted into electrical energy. The hydrogen may be recovered downstream and used in a hydrogen fuel cell. The oxide may also be recovered downstream. The fuel concentration in the liquid gallium forms a gradient between the injection source and the interface with the oxidant.

[0011] In a first embodiment of the invention, there is provided an electrochemical reactor for use in an energy storage system having a housing configured to continuously circulate fluid in a unidirectional loop, the fluid containing predominantly liquid gallium, or a gallium alloy with indium, tin or both. The housing has an aluminum port configured to introduce a fuel, such as aluminum or an aluminum alloy, in a solid phase into the flow of fluid and a water injection port configured to introduce water into the flow of fluid. The housing is coupled to a thermal energy harvester which is configured to convert heat into electrical energy. The housing is also in fluid communication with a waste separation system, which is configured to remove aluminum hydroxide from the fluid.

[0012] In certain embodiments, the water injection port may be positioned downstream from the aluminum port. In other embodiments, the water injection port may further include a water pump configured to introduce the water into the housing. In yet other embodiments, the water pump may be equipped with a gauge to control the amount of water injected in the fluid stream.

[0013] In certain embodiments, the reactor is equipped with a pump to continuously recirculate the fluid within the housing. The pump may be a diaphragm pump, an electromagnetic pump, a magnetic drive pump, a positive displacement pump, a velocity pump, a centrifugal pump, or combinations thereof.

[0014] In certain embodiments, the waste separation system includes a centrifuge. [0015] In certain embodiments, the reactor is equipped with a vent configured to allow hydrogen to be released from within the housing. In other embodiments, the vent may include a membrane configured to keep the fiuid within the housing and to allow the hydrogen to be released from the housing. In yet other embodiments, the hydrogen may be further recovered and used in a hydrogen fuel cell to produce an additional energy source or to power the reactor.

[0016] In certain embodiments, the thermal energy harvester operates using an organic Rankine cycle engine, a Rankine cycle engine, organic Brayton engine, Brayton cycle engine, or supercritical C0 ₂ Brayton engine. In other embodiments, the thermal energy harvester operates using a Stirling cycle.

[0017] In certain embodiments, there is also provided a method of using an electrochemical reactor. In a housing forming a loop and configured to continuously circulate a fluid in one direction, a fluid containing predominantly liquid gallium is provided.

Aluminum or an aluminum alloy in a solid phase is introduced into the fluid stream at a first location in the housing so that the aluminum or the aluminum alloy substantially dissolves in the fluid. At a second location in the housing, water is introduced into the fluid stream. In certain embodiments, the first location is upstream from the second location. In the fluid stream, an aluminum oxide, hydrogen and heat are formed. The heat may then be converted into electrical energy using a thermal energy harvester coupled to the housing. The aluminum oxide is removed from the fluid, preferably substantially. The fluid stream being substantially free of aluminum oxide is then recirculated into the housing at the first location.

[0018] In certain embodiments, the aluminum alloy is an aluminum-gallium alloy.

[0019] In certain embodiments, the heat is converted into electrical energy at a third location and the aluminum oxide is removed from the fluid at a fourth location. In other embodiments, the third location is upstream from the fourth location.

[0020] In certain embodiments, the aluminum oxide is removed from the fluid using a physical separation system. In certain embodiments, the physical separation system includes a centrifuge, baffles, annular piping, and/or a separator that uses coulombic forces or electric fields. In other embodiments, the aluminum oxide is removed from the fluid using a chemical separation system that may include a chemical, such as hydrochloric acid or another suitably strong acid. In yet other embodiments, the chemical may be an additive, such as flocculating agents, binding agents, water dispersants, or surface tension agents.

[0021] In certain embodiments, the aluminum or the aluminum alloy is introduced into the fluid stream as a wire, a foil, a block, pellets, or a combination thereof.

[0022] In certain embodiments, an additive may be added to the fluid stream so as to facilitate the dissolution of the aluminum or the aluminum alloy in the fluid. In certain embodiments, the additive is a component of the fluid and circulates within the housing with the liquid gallium.

[0023] In other embodiments, the method is conducted in a housing forming a loop and configured to continuously circulate a fluid in one direction, a fluid containing predominantly liquid gallium is provided. Aluminum or an aluminum alloy in a solid phase is introduced into the fluid stream at a first location in the housing so that the aluminum or the aluminum alloy substantially dissolves in the fluid so that a liquid aluminum/gallium alloy is produced at a first location within the loop. At a second location in the housing in fluid communication with, and separate from, the first location, water is introduced into the fluid stream. In certain embodiments, the first location is upstream from the second location so that an aluminum oxide, hydrogen and heat are produced at a second location.

[0024] In yet other embodiments, the method is conducted in a housing forming a loop and configured to continuously circulate a fluid in one direction. In a first sector of the housing, the fluid is liquid gallium substantially free of aluminum oxide and aluminum. Aluminum or an aluminum alloy in a solid phase is introduced into the fluid stream so that, in a second sector of the housing, the fluid stream is substantially a liquid alloy of gallium and aluminum substantially free of aluminum oxide. Water is introduced into the fluid stream so that in a third sector of the housing, the fluid substantially includes a mixture of gallium, an aluminum oxide, and hydrogen . In certain embodiments, the first sector is upstream from the second sector. In other embodiments, the second sector is upstream from the third sector. In yet other embodiments, the first sector is upstream from the second sector, and the second sector is upstream from the third sector. Brief Description of the Drawings

[0025] The foregoing features of embodiments will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which:

[0026] FIG. 1 is a schematic depiction of an embodiment of the reactor of the invention using a circulating metallic depassivating fluid.

[0027] FIG. 2 is a schematic depiction of another embodiment of the reactor of the invention using a water stream.

[0028] FIG. 3 is a schematic depiction of another embodiment of the reactor of the invention using a water stream.

[0029] FIG. 4 is a schematic depiction of another embodiment of the reactor of the invention using a water stream.

[0030] FIG. 5 is a phase diagram of mixtures of aluminum and gallium as a function of the temperature.

Detailed Description of Specific Embodiments

[0031] Definitions. As used in this description and the accompanying claims, the following terms shall have the meanings indicated, unless the context otherwise requires:

[0032] Oxide shall mean, any compound of the fuel element bound to oxygen, for example, aluminum oxide may include AI2O3, Al(OH)3, or AIO(OH), or mixtures thereof.

[0033] The term "predominantly" means at least about 60% by weight or more.

[0034] The term "substantially" means at least about 90% by weight or more.

[0035] The term "substantially free of means at least about 99% by weight.

[0036] Aluminum metal is an energy-dense (>80MJ/L) fuel. However, aluminum is a stable metal as it forms a passivating oxide layer on its surfaces exposed to water or air. Thus, aluminum can be safely stored. Because it produces an exothermic reaction when it oxidizes in water, aluminum metal is especially promising as a power source for underwater vehicles. The use of these vehicles is severely limited by the low energy density of available anaerobic energy storage media (e.g., 0.6 MJ/L for Li-ion). [0037] Aluminum can be a revolutionary fuel for small vehicles as the power density of the fuel is up to two and a half times more energy dense (by volume) than gasoline. While the aluminum is more expensive than gasoline, its high energy density, the use of water rather than air as an oxidizer, and the relative simplicity of operation makes the fuel very appealing for use in vehicles such as unmanned-underwater vehicles (UUVs) or unmanned aerial vehicles (UAVs). Extending the mission lifetime of these vehicles by using an aluminum-gallium fuel and the costs saved by reducing the frequency of their deployment / collection easily make up for the added cost of the fuel. The advantages of this fuel are particularly stark for UUVs, where it reduces the amount of time recovery ships have to be deployed as well as negating the need for the UUVs to surface to snorkel for air; it would be possible for an UUV to be in mission mode for the entire duration of its deployment, which could be up to a month given the typical current payload size of UUVs. Other suitable applications may include small robotics, backup or regular underwater power sources (e.g., for oil rigs), or as an emergency power source in flooded or disaster areas.

[0038] Previous attempts at power production from aluminum metal have been hindered by oxide-layer passivation and slow reaction kinetics, with almost no progress made towards power production in oxygen-free environments. A scalable reactor for depassivation of aluminum metal as described herein may be a viable long-duration power source for underwater systems. An aluminum- seawater reactor as described herein can deliver a ten-fold increase in the endurance of non-nuclear underwater systems. Reactors as described herein may also find applications in emergency power supply and land-based hydro gen- storage .

[0039] FIG. 1 is a depiction of an embodiment of a reactor 10 having a housing 5 for circulating in a loop a fluid containing predominantly liquid gallium 12. Alternatively, the fluid may be an alloy of gallium with other metals, such as with indium or tin or both, wherein the gallium concentration is at least about 15% by weight. At a fuel port 13, the metal fuel, such as aluminum or an aluminum alloy 14 in a solid state is introduced into the housing. The fuel source may be provided in the form of a rod, foil, a ribbon, granules, pellets, and the like. As the fuel source dissolves within the flow of gallium, it forms a liquid alloy with gallium. At a water injection port 15, water 16 in a liquid state is introduced into the fluid stream in the housing 5. Several sources of water may be used, such as sea water, or soft water such as tap water, distilled water or water taken from a pond, a lake, or a river or collected from rain. In addition, a small amount of acid may be added to the fluid stream to speed the oxidation reaction. In the sector (I) of the housing between ports 13 and 15, the concentration of the fuel in the gallium forms a gradient with the highest concentration nearer water injection port 15. The highest fuel concentration can be estimated and controlled or fine-tuned by the temperature of the gallium in the sector and the speed of feed of the fuel source.

[0040] The liquid fuel present in the alloy reacts with the water and produces hydrogen 18, an oxide form of the fuel metal 20, and heat. In a region 40 of the housing 5, the heat is harvested in a thermal energy harvester 50 combined with and converted into another energy form such as electricity or work. The type of energy harvester used is entirely dependent on the desired power output and final use. For example, when using the system for the production of low power, the use of a Stirling cycle engine may be sufficient. When higher power is needed, such as to power large vehicles, the system may be coupled with an organic Rankine engine, a Rankine Cycle engine or Bayton cycle engine. When using to power a very large vehicle with a large system, a Brayton cycle, organic Brayton or supercritical C0 ₂ Brayton may be used as the energy harvester. Optionally, excess heat may be dissipated in the environment as a heat source with a heat exchanger 60.

[0041] In a sector (II) of the housing 5 downstream from the energy harvester 50, the fluid stream containing predominantly gallium 12 with hydrogen 18 and the oxide 20 (that is substantially free of water and/or fuel) is then entrained and passed by, or through, a waste separation system 70 to substantially remove the oxide 20 and hydrogen 18 from the fluid. The waste separation system 70 may remove the oxide 20 and hydrogen 18 from the fluid by physical and/or chemical means. For example, the waste separation system 70 may include a centrifuge that physically separates and then removes the oxide 20 from the fluid.

Alternatively, or in addition, the waste separation system 70 may include a chemical system that removes the oxide 20 from the fluid through a chemical reaction. The waste separation system 70 may include a vent 75 that allows hydrogen to be released from within the housing 5. The vent 75 may include a membrane (not shown) configured to keep the fluid within the housing 5 and to allow the hydrogen to be released from the housing 5. The reactor 10 may further include a hydrogen fuel cell (not shown) coupled to the vent 75 and configured to use the hydrogen in the hydrogen fuel cell.

[0042] In the sector (III) downstream of the waste separation system 70, the purified fluid containing substantially liquid gallium 12, or a liquid alloy of gallium, is then cycled back into the system nearby fuel port 13. Alternatively, a pump 30 may be used to propel the liquid gallium 12 through the housing 5. In the embodiment displayed in FIG.l, the pump is located between the waste separator 70 and the fuel port 13. Alternatively, the pump 30 may be placed anywhere within the housing 5. The pump 30 may be any suitable pump, such as a diaphragm pump, an electromagnetic pump, and/or a magnetic drive pump. The reactor may also be operated without a pump, and rely essentially on the variation of density or the temperature gradient of the fluid throughout the housing to circulate the fluid within the housing.

[0043] FIG. 2 is a depiction of another embodiment of a reactor 100 having a housing 105, forming a chamber containing a pool of depassivating agent 112, such as liquid gallium. The housing 105 has at least three ports, a fuel port 113 for introducing the solid fuel 114 on a side of the housing 105, such as aluminum or an aluminum alloy, directly into the pool of gallium; a water injection port 115, for introducing liquid water 116, and a port 117 for evacuating the water with the products of the reaction, the oxide 118 and hydrogen 120. The water 116 flows through the chamber and contacts the pool of depassivating agent 112 at interface 180. As the solid fuel 114 dissolves in the pool of depassivating agent 112 it becomes depassivated and migrates towards the interface 180 by diffusion or convection, where it reacts readily with the water producing heat, an oxide 118 and hydrogen 120. An energy harvester 150 is coupled to the housing to extract the heat from the circulating water and convert it into electricity or work with the use of a Rankine cycle engine or a Stirling cycle engine. A radiator 160 may be coupled to the energy harvester 150, to release excess heat further in the environment.

[0044] FIG. 3 is a depiction of yet another embodiment of a reactor 200 having a housing 205, forming a chamber containing a pool of depassivating agent 212, such as liquid gallium. The housing 205 has at least three ports, a fuel port 213 for introducing the solid metallic fuel 214 on a side of the housing 105, such as aluminum or an aluminum alloy, directly into the pool of depassivating agent 212; a water injection port 215, for introducing liquid water 216, and a port 217 for evacuating the water and the products of the reaction of the oxide 218 and hydrogen 220. The water 216 circulates within the chamber and contacts the pool of gallium 212 at interface 280. As the solid fuel 214 dissolves in the pool of depassivating agent 212, it becomes depassivated and migrates to the interface 280 by diffusion or convection, where it reacts readily with the water producing heat, an oxide 218 and hydrogen 220. Optionally, the reactor 200 may be fitted with a water pump 230 at the water inlet, water outlet or both. An energy harvester 250 is coupled to the housing to extract the heat from the circulating water and convert it into electricity or work with the use of a Rankine Cycle engine or a Stirling cycle engine. A radiator 260 may be coupled to the energy harvester 250, to release excess heat further in the environment. A heat exchanger 290 may be coupled with both inlet and outlet of the water system to preheat the water before it enters the housing 205.

[0045] FIG. 4 is a depiction of yet another embodiment of a reactor 400 having a housing 405, forming a chamber containing a pool of depassivating agent (not shown), such as liquid gallium. The housing 405 has at least three ports, a fuel port for introducing the solid metallic fuel 414 on a side of the housing 405, such as aluminum or an aluminum alloy, directly into the pool of depassivating agent; a water injection port, for introducing liquid water 416a, and a port 416b for evacuating the water and the products of the reaction of the oxide and hydrogen. The water circulates within the chamber and contacts the pool of depassivating agent at interface. As the solid fuel 414 dissolves in the pool of depassivating agent, it becomes depassivated and migrates to the interface by diffusion or convection, where it reacts readily with the water producing heat, an oxide and hydrogen. In this embodiment, the reactor 400 is fitted with two energy harvesters 450 coupled to two opposing sides of the housing to extract the heat from the circulating water and convert it into electricity or work with the use of a Rankine Cycle engine or a Stirling cycle engine. Two radiators 460 may be coupled to the energy harvesters 450, to release excess heat in the environment.

[0046] Referring to Figs. 2 to 4, in the housings, the concentration of the fuel in the pools of depassivating agent, forms a gradient with the highest concentration being at or near the fuel sources, and the lowest at the interfaces, respectively. The fuel migrates from the source to the interfaces by diffusion and convection created by the heat of the exothermic reaction of the fuel with water at the interfaces. The highest fuel concentration can be estimated and controlled or fine-tuned by the temperature of the depassivating agent in the chamber, the speed of feed of the fuel source, and the rate of the water flow. The

depassivating agent may be substantially all liquid gallium, an alloy of gallium with other metals, such as indium or tin or both, wherein the gallium concentration is at least about 15%. The fuel source may be provided in the form of a rod, foil, a ribbon, granules, pellets, and the like. The fuel source may be aluminum or an aluminum alloy.

[0047] In operation, the rate of introduction of the components depends on the temperature at which the system is operated, flow rates and desired power output. For example, referring to FIG. 1, the system may be operated at a temperature from about 200°C to about 600°C. Referring to FIGS. 2-4, the systems may be operated from about 40°C to about 100°C at atmospheric pressure, or may be operated up to about 370 °C under pressure.

[0048] The concentration of liquid aluminum, in the system of FIGS. 2-4, or in sector (I) of the system of FIG. 1, can be determined by the Al-Ga phase diagram shown in FIG. 5. For example, the fluid mixture in sector (I) of FIG. 1 will be within the region 520, with dissolution near the solid aluminum source happening near the line 510.

[0049] The flow of water in the system of FIG. 1 is adjusted to be a stoichiometric amount as governed by the equations (1), (2) or (3) of the oxidation of aluminum with water:

Al + 3 H ₂ 0 ^→ Al(OH) ₃ + 1 ½ H ₂ (1)

Al + 2 H ₂ 0 ^→ AIO(OH) + 1 ½ H ₂ (2)

2 Al + 3 H ₂ 0 ^→ A1 ₂ 0 ₃ + 3 H ₂ (3)

[0050] The flow of water in the system of FIGS. 2-4 is adjusted to maintain the desired temperature in the system.

[0051] The embodiments of the invention described above are intended to be merely exemplary; numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.