The present invention generally relates to micro-fuel cells and more particularly to a storage apparatus containing a fuel source and water for supplying hydrogen fuel to a micro-fuel cell.
Rechargeable batteries are the primary power source for cell phones and various other portable electronic devices. The energy stored in the batteries is limited. It is determined by the energy density (Wh/L) of the storage material, its chemistry, and the volume of the battery. For example, for a typical Li ion cell phone battery with a 250 Wh/L energy density, a 10 cc battery would store 2.5 Wh of energy. It could last for a few hours to a few days depending on the usage. Recharging always requires an electrical outlet. The limited amount of stored energy and the frequent recharging are major inconveniences with the batteries. There is a need for a longer lasting, easily recharging solution for cell phone power sources. One approach to fulfill this need is to have a hybrid power source with a rechargeable battery and a method to trickle charge the battery. Important considerations for an energy conversion device to recharge the battery include power density, energy density, size and the efficiency of energy conversion.
Energy harvesting methods such as solar cells, thermoelectric generators using ambient temperature fluctuations, and piezoelectric generators using natural vibrations are very attractive power sources to trickle charge a battery. However, the energy generated by these methods is small, usually only a few milliwatts, and it requires a large volume to generate sufficient power in the hundred of milliwatts needed, making it unattractive for cell phone type applications.
An alternative approach is to carry a high energy density fuel and convert this fuel energy into electrical energy with high efficiency to recharge the battery. Radioactive isotope fuels with high energy density are being investigated for portable power sources. However, the power densities are low with this approach, and also there are safety concerns with the radioactive materials. This is an attractive power source for remote sensor type applications, but not for cell phone power sources. Among the various other energy conversion technologies, the most attractive one is the fuel cell technology because of its high efficiency of energy conversion and the demonstrated feasibility to miniaturize with high efficiency.
Fuel cells with active control systems and high operating temperature fuel cells such as active control direct methanol or formic acid fuel cells (DMFC or DFAFC), reformed hydrogen fuel cells (RHFC) and solid oxide fuel cells (SOFC) are complex systems and very difficult to miniaturize to the 2-5 cc volume needed for cell phone application. Passive air breathing hydrogen fuel cells, passive DMFC or DFAFC, and biofuel cells are attractive systems for this application. However, in addition to the miniaturization issues, other concerns include supply of hydrogen for hydrogen fuel cells, life time and energy density for passive DMFC and DFAFC, and life time, energy density and power density with biofuel cells.
Conventional DMFC and DFAFC designs comprise planar, stacked layers for each cell. Individual cells may then be stacked for higher power, redundancy, and reliability. The layers typically comprise graphite, carbon or carbon composites, polymeric materials, metal such as titanium and stainless steel, and ceramic. The functional area of the stacked layers is restricted, usually on the perimeter, by vias for bolting the structure together and passage of fuel and an oxidant along and between cells. Additionally, the planar, stacked cells derive power only from a fuel/oxidant interchange in a cross sectional area (x and y coordinates).
To design a fuel cell/battery hybrid power source in the same volume as the current cell phone battery (10 cc-2.5 Wh), a smaller battery and a fuel cell with high power density and high efficiency, and a high energy density fuel supply would be required to achieve an overall energy density higher than that of the battery alone. For example, for a 4-5 cc (1-1.25 Wh) battery to meet the peak demands of the phone, the fuel cell would need to fit in 1-2 cc, with the fuel taking up the rest of the volume. The power output of the fuel cell needs to be 0.5 W or higher to be able to recharge the battery in a reasonable time. Most development activities on small fuel cells are attempts to miniaturize the traditional fuel cell designs into a small scale, and the resultant systems are still too big for cell phone application. A few micro fuel cell development activities have been disclosed using traditional silicon processing methods in planar fuel cell configurations, and in few cases using porous silicon (to increase the surface area and power densities). See for example, U.S. Patent/Application Numbers 2004/0185323, 2004/0058226, 6,541,149, and 2003/0003347. However, the power densities of the air breathing planar hydrogen fuel cells are typically in the range of 50-100 mW/cm2. To produce 500 mW, it would require 5 cm2 or more active area. The operating voltage of a single fuel cell is in the range of 0.5-0.7V. At least four to five cells need to be connected in series to bring the fuel cell operating voltage to 2-3V for efficient DC-DC conversion to 4V in order to charge the Li ion battery. Therefore, the traditional planar fuel cell approach will not be able to meet the requirements in 1-2 cc volume for a fuel cell in the fuel cell/battery hybrid power source for cell phone use. The 3D micro fuel cell architecture described in the above mentioned patent applications attempt to solve this problem by providing more surface area to increase the power density as well as provide a modular approach to power output by adding the fuel cell modules as required. However, to achieve over all high energy density for the power source, a high energy density fuel supply needs to fit in a small volume.
A high energy density fuel source and controlled delivery of the fuel (typically hydrogen) are two important issues in the development of micro fuel cells with high energy density for portable power applications, e.g., cell phones. Among known options for the supply of hydrogen such as the H2 storage in compressed cylinders, carbon nanotubes, metal hydrides, or in metal organic frame works, the amount of hydrogen storage is limited and the energy density is typically low and they are not competitive for the specified application. Storage of hydrogen in chemical hydrides is attractive, but it requires a controlled method of releasing hydrogen gas from the chemical hydride. Once released, the storage of hydrogen gas is also difficult. Leakage caused by over production (in addition to environmental concerns) reduces energy density, and under production reduces fuel cell output. Therefore, a controlled production/flow rate is desired. Additionally, overproduction (quick chemical consumption of materials) results in a high temperature which is undesirable for material longevity and user comfort. It is further desired to maintain a small volume while avoiding consumption of power.
Accordingly, it is desirable to provide a compact and efficient storage apparatus containing a fuel source and water for supplying fuel in a controlled manner to a micro-fuel cell. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
A compact and efficient storage apparatus contains a fuel source and water for supplying hydrogen fuel to a micro-fuel cell. The storage apparatus comprises a housing defining a fuel source chamber and a plurality of water chambers, and one or more polymer crystals containing water positioned within each of the water chambers. The fuel source, such as a chemical hydride mixed with a catalyst, is positioned within the fuel source chamber, wherein the water in each of the water chambers is selectively allowed to migrate to the fuel source chamber to contact the solid fuel, thereby producing the hydrogen fuel at a desired flow rate and temperature. A conduit supplies the hydrogen fuel produced within the housing to the fuel cell.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
A high energy density fuel source and controlled delivery of the fuel for a micro-fuel cell is described herein. Water is stored in super adsorbent polymer crystals, or a hydro gel material, within a plurality of chambers. Each of the chambers are selectively “opened” so the water may migrate and mix with a solid fuel to provide hydrogen at a low temperature and at a low rate to a micro-fuel cell. The fuel is dense and compact, thereby conserving space, and the water is conveniently packaged for long term storage. The choice of the fuel (solid fuel source), the other reactant (which is water) in a convenient form (adsorbed in a polymer) and packaged into small reaction chambers separated by a controlled valve which can be opened as desired for the reaction to proceed in a safe, slow rate to produce hydrogen gas at the desired 1-3 sccm rate needed for the micro fuel cells are described in this application.
The most promising approaches are the hydrogen storage in chemical hydrides such as sodium borohydride or lithium borohydride and such or the reaction of activated sodium silicate or other metals with water. The reaction of activated metals with water is very vigorous, it is an exothermic reaction releasing lot of heat quickly and it is very difficult to control the reaction rate. Reaction of sodium borohydride and water for the generation of hydrogen is well known in the literature. For example, the article titled “Sodium Borohydride, Its hydrolysis and its use as a reducing agent and in the generation of hydrogen” by H. I Schlesinger, Herbert C. Brown, A. E. Finholt, James R. Gilbreath, Henry R. Hoekstra and Earl K. Hyde, J. Am. Chem. Soc; 1955; 75(1) 215-219 describes the reaction of sodium borohydride with water and the influence of various catalysts, and pH of water (addition of acid to the water) for this reaction. Generation of hydrogen by the reaction of sodium borohydride solution with a catalyst is also known. However the energy density of the fuel in this case is low due to the use of a dilute fuel solution. For the design of a high energy density fuel source, ideally the reaction of solid sodium borohydride (optionally mixed with a catalyst) with water is preferred. Once hydrogen is produced it will be supplied to the fuel cell to generate power. Control of the sodium borohydride with water reaction to supply only sufficient hydrogen for the fuel cell reaction is needed. To achieve high energy density for the power source, complete reaction of the fuel source (ex: sodium borohydride mixed with a catalyst) with water is also desired to maximize the fuel utilization and increase the overall system efficiency.
Referring to
It/zF=(0.125 A×60 sec)/(2*96,487)=3.89×10−5 moles,
Each of the chambers 16 of row 12 have one or more polymer crystals 22, e.g., polyacrylamide crystals, having water stored therein. Each of the chambers 18 of row 14 have a fuel source 24 stored therein. The fuel source preferably comprises solid fuel pellets comprising a mixture of sodium borohydride (NaBH4) powder and a catalyst boron oxide (B2O3) powder, but may comprise any combination of a fuel and catalyst that combines with water to produce hydrogen, e.g., sodium borohydride (NaBH4) and cobalt chloride (CoCl2). Although the solid fuel pellets are a convenient method of storing the fuel source 24, the fuel may be stored in any form, including a powder, gel, or liquid. Furthermore, though the fuel source preferably is stored in a plurality of chambers 18, an alternative embodiment comprises a single chamber containing the fuel source coupled to the plurality of chambers 16.
When a membrane 20 is opened (in a manner described hereinafter) in one of the chambers 16, water stored in the polymer crystals 22 positioned therein will migrate and mix with the fuel source 24 in the adjacent chamber 18. The mixing of NaBH4 and B2O3 results in a powder sodium boron oxide (NaBO2) and hydrogen (H2) being produced.
An example of one method of activating the storage apparatus 10 within a fuel cell apparatus is shown in
One previously known method of applying water droplets to a solid fuel results in a hydrogen flow 50 and temperature 52 shown in
Referring to
A multi-metal layer 122 comprising an alloy of two metals, e.g., silver/gold, copper/silver, nickel/copper, copper/cobalt, nickel/zinc and nickel/iron, and having a thickness in the range of 100-500 um, but preferably 200 um, is deposited on the layer 116. The multi-metal layer 118 is then wet etched to remove one of the metals, leaving behind a porous material. The porous metal layer could also be formed by other methods such as templated self assembled growth or sol-gel methods. A dielectric layer 120 is deposited on the layer 118 and a resist layer 122 is patterned in a manner well known to those in the industry on the dielectric layer 120.
Referring to
Referring to
The side walls 132 are then coated with an electrocatalyst for anode and cathodic fuel cell reactions by wash coat or some other deposition methods such as CVD, PVD or electrochemical methods (
A via, or cavity, 138 is formed (
After filling the cavity 134 with the electrolyte material, it will form a physical barrier between the anode (hydrogen feed) and cathode (air breathing) regions. Gas manifolds are built into the bottom packaging substrate to feed hydrogen gas to all the anode regions. Since it is capped on the top 136, it will be like a dead end anode feed configuration fuel cell. The fuel source described in
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.