Fuel cell apparatus and associated method

An apparatus including a housing having walls is provided. The walls of the housing each have inner surfaces and outer surfaces. The walls may include apertures extending from the inner surface to the outer surface. The inner surfaces of the walls define a volume. The volume includes an electrode. The volume further includes a water-controlling separator disposed between the inner surface of the housing and the electrode. The water-controlling separator can block a flow of liquid from the electrode through the apertures to the ambient environment while allowing oxidant to flow from the ambient environment through the apertures to the electrode.

TECHNICAL FIELD

The invention includes embodiments that relate to an apparatus including a fuel cell. The invention includes embodiments that relate to a method of using the fuel cell.

DISCUSSION OF RELATED ART

An electrochemical cell may convert the chemical energy of a fuel directly into electricity without any intermediate thermal or mechanical processes. Energy may be released when a fuel reacts chemically with oxygen in the air. A fuel cell may convert hydrogen and oxygen into water. The conversion reaction occurs electrochemically and the energy may be released as a combination of electrical energy and heat. The electrical energy can do useful work directly, while the heat may be dispersed.

Fuel cell vehicles may operate on hydrogen stored onboard the vehicles and may produce little or no conventional undesirable by-products. Neither conventional pollutants nor green house gases may be emitted. The byproducts may include water and heat. Systems that rely on a reformer on board to convert a liquid fuel to hydrogen produce small amounts of emissions depending on the choice of fuel.

Other energy storage devices may include metal/air cell. Metal/air cells include a cathode that uses oxygen as an oxidant and a solid fuel anode. The metal/air cells differ from fuel cells in that the anode may be consumed during operation. Metal/air batteries may be anode-limited cells having a relatively high energy density.

A rechargeable fuel cell may be a kind of fuel cell using a hydrogen storage material as an anode and an air electrode as a cathode. The hydrogen storage material functions as both a hydrogen source for fuel and as a hydrogen oxidization catalyst. Water may be employed as an energy transformation media. When electricity is charged in the rechargeable fuel cell, water may be electrolyzed into hydrogen and oxygen. The produced hydrogen may be stored in the anode. In reverse when the electricity is exported to the loads, the hydrogen from the anode and oxygen from air constitute a fuel cell to deliver electricity. The energy stored in the rechargeable fuel cell depends on the capacity of the anode. This functionality may avoid the need for a high-pressure hydrogen container and allows for higher energy density.

As recyclable power sources, both rechargeable fuel cells and electrically rechargeable metal/air batteries may need to have a long service life. However in such a dual functional electrochemical system, where electrochemical system behaves as fuel cell in discharging process and as electrolysis cell in charging process, the air electrode may degrade quickly during charging process. The system efficiency may suffer by the compromise of the air electrode materials for both oxygen reduction and water oxidization. Further the cells of this type may include a third electrode used as cathode during charge process to reduce or prevent the degradation of air electrode. The cells may have limited specific power and energy efficiency for some applications, such as in the automotive industry. In addition conventional cells may suffer from water loss and oxygen evolution during charge process.

It may be desirable to have a fuel cell having differing characteristics or properties than those fuel cells that are currently available. It may be desirable to have a method of using a fuel cell having that differs from those methods that are currently available.

BRIEF DESCRIPTION

In accordance with an embodiment of the invention an apparatus is provided. The apparatus includes a housing having walls. The walls of the housing each have an inner surface and an outer surface. Each wall may include apertures extending from the inner surface to the outer surface. The inner surfaces of the walls define a volume. The volume includes an electrode. The volume further includes a water-controlling separator disposed between the inner surface of the housing and the electrode. The water-controlling separator can block a flow of liquid from the electrode through the apertures to the ambient environment while allowing oxidant to flow from the ambient environment through the apertures to the electrode.

An apparatus is provided in one embodiment. The apparatus includes a housing having walls. The walls of the housing each have an inner surface and an outer surface. Each wall may include apertures extending from the inner surface to the outer surface. The inner surfaces of the walls define a volume. The volume includes an electrode. The electrode is a supercapacitor electrode. The volume further includes a water-controlling separator disposed between the inner surface of the housing and the electrode. The water-controlling separator can block a flow of liquid from the electrode through the apertures to the ambient environment while allowing oxidant to flow from the ambient environment through the apertures to the electrode.

In one embodiment a method of using a fuel cell includes drawing discharge current or working voltage from an electrode is provided.

DETAILED DESCRIPTION

The invention includes embodiments that relate to a fuel cell. The invention includes embodiments that relate to a method of using the fuel cell.

According to one embodiment an apparatus is provided. The apparatus includes a housing having one or more walls. The housing may be made of a metal, an alloy, a polymer or a refractory material. The housing wall can be of different shapes and sizes. In one embodiment, the housing may be polygonal in shape. As used herein, polygonal housing includes square or rectangular housing. In one embodiment, the housing may be cylindrical in shape. The moldable housing may provide structural support and electrical support.

In one embodiment, the housing wall has an inner surface and an outer surface. The inner and outer surfaces of the wall define the apertures that are capable of passing fluid therethrough. As used herein, apertures include holes, pores, mesh and the like. The shape and size of the apertures may be selected with reference to such factors as desired flow rate of the oxidant and end use application. The inner surface of the wall defines a volume.

The electrode is disposed in the volume, and the electrode may be one of a plurality of electrodes. In one embodiment, the plurality of electrodes may be configured as a plate. In one embodiment, the plurality of the electrodes may be configured as a cylinder. The plurality of electrodes includes a first electrode, a second electrode and a third electrode. The second electrode is in ionic communication with each of the first electrode and third electrode.

The first electrode may be an air electrode. The air electrode consumes oxygen from outside ambient air during discharge, and generates oxygen during charge operation of the fuel cell. The air electrode is made of carbon matrix and a catalyst. The catalyst is capable of accelerating dissociation of molecular oxygen into atomic oxygen. In other words, oxygen from air is reduced at the air electrode and, generated free electrons conduct through the external circuit. The catalyst may include metals or metal oxide selected from platinum, palladium, ruthenium, silver, manganese dioxide, nickel oxide, cobalt oxide, perovskite oxide, or a combination of two or more thereof.

The second electrode may be an anode. The anode or negative electrode may act as both a hydrogen oxidization catalyst or as a hydrogen storage media. The anode includes a hydrogen storage material capable of receiving, storing and releasing hydrogen. The anode embodiments may include an active material supported on a current collector grid. The active material for the anode may include a hydrogen storage material, a binder material, and graphite or graphitized carbon. Other suitable active materials may include metals such as nickel, and metal oxides such as nickel oxide. Suitable nickel metal may be the commercially available trademark RANEY nickel. Suitable hydrogen storage material may be selected from hydride complexes, aluminides, borides, carbides, germanides, and silicides, or a combination of two or more thereof.

Suitable hydride complexes may include a H-M complex, where M is a metal and H is hydrogen. Such hydrides may have ionic, covalent, metallic bonding or bonding including a combination of at least one of the foregoing types of bonding. These hydrides have a hydrogen to metal ratio of greater than or equal to 1. The reaction between a metal and hydrogen to form a hydride may be a reversible reaction and takes place according to the following equation (VI):
M+(x/2)H2MHx  (VI)

Hydride complexes can store up to 18 weight percent (weight percent) of hydrogen, and have high volumetric storage densities. The volumetric storage density of hydrides may be greater than either liquid or solid hydrogen, which makes them very useful in energy storage applications. The process of hydrogen adsorption, absorption or chemisorption results in hydrogen storage and may be hereinafter referred to as absorption, while the process of desorption results in the release of hydrogen.

Suitable metal hydrides include but are not limited to Ni, Co, Al, Mn, Mo, Ti, Zn, Rh, Ru, Ir, La, Ni, Fe, Ti, Zr, W, V, B and alloys of these materials. The alloys may be selected from Rare-earth metal alloys, Misch metal alloys, zirconium alloys, titanium alloys, magnesium/nickel alloys, and mixtures or alloys thereof which may be AB, AB2, A2B, AB3or AB5type alloys. Such alloys may include modifier elements to increase their hydrogen storage capability.

The anode is disposed on an imaginary line defined by the air electrode and the third electrode. That is the anode is located between the other electrodes. In one embodiment the anode may be separated from the other electrodes by a porous matrix. The porous matrix may be a zeolite, membrane or gel placed in between the anode and each of the air electrode and the third electrode. The porous matrix may be a membrane saturated with an aqueous alkaline solution, such as potassium hydroxide (KOH). Other electrolytes suitable for use in the fuel cell may include alkaline hydroxides or salt solutions. The membrane helps to physically segregate the hydrogen and oxidant to avoid direct combustion as well as provides ionic communication.

The third electrode may be a charging electrode capable of moving a locus of oxidation during charging away from the first electrode. The third electrode may be used as positive electrode, and charging of the fuel cell takes place between the anode and the third electrode. The third electrode may be similar to a positive electrode as used in a NiMH cell. The third electrode may be made of ferro-based alloys. Suitable ferro-based alloys may include stainless steel. Suitable materials may further include one or more of nickel, cadmium, palladium, lead, gold, or platinum. The third electrode may be configured as sintered type, foamed type, fiber type or the like. Such configuration may provide an increased surface area for reaction, may enhance an ability of storing electrolyte solution within the volume of its pores and may provide diffusion control. A sintered-type nickel electrode, when used as the charging electrode, is suitable in life span. On the other hand, a foamed-type nickel electrode as well as a fiber-type nickel electrode, when used as the charging electrode, is suitable for relatively high capacity.

In one embodiment, the third electrode may include nickel and nickel hydroxide. Nickel hydroxide provides high catalytic activity and large reactive area, which helps to charge the fuel cell at lower charging voltage to reduce the loss of energy. Chemical activity may be defined as the ability of a substance to accelerate a chemical reaction in presence of the substance.

Suitable amount of nickel hydroxide in third electrode may be greater than about 10 wt percent. In another embodiment, the amount may be in a range of from about 10 wt percent to about 20 wt percent, from about 20 wt percent to about 30 wt percent, from about 30 wt percent to about 40 wt percent, from about 40 wt percent to about 50 wt percent, from about 50 wt percent to about 60 wt percent, or from about 60 wt percent to about 100 wt percent.

In one embodiment, the third electrode may include a supercapacitor electrode. The supercapacitor has a large capacitance and stores a large amount of energy in a small volume. Capacitance is proportional to the surface area of the electrodes divided by their separation distance. Simple capacitors consisting of two parallel plates reach small capacitances of the order of pico-Farad (1pF=10−12F). When such a capacitor is loaded to 1000 V, the energy content is on the order of micro-watt-second (Ws). Increasing the surface area of electrodes and minimizing the separation distance to a molecular range provides large capacitance. Capacitance of a supercapacitor is in a range of from about 10−6farad to about 103Farad and stores energy in a range of from about 10−3Ws to Watt-hour (Wh).

The supercapacitor electrode may be a large surface area porous electrode. The porous electrode may include a porous portion and a substrate. The substrate may be formed as a plate, a mesh, a foil, a sheet or the like. The substrate may be made of a conductive material or a non-conductive material. Suitable conductive material may include a metal such as ferro-based metal (e.g. stainless steel), titanium, platinum, iridium, or rhodium. Other suitable conductive material may be organic, such as a conductive plastic or graphite. The substrate may be non-conductive, if it is further coated with a conductive material. The conductive coating may be a one of the foregoing conductive materials.

Suitable material for the porous portion may be selected from one or more of carbon, carbon nanotubes, graphite, carbon fiber, carbon cloth, carbon aerogel, or conductive polymer. Other suitable material for porous portion may be selected from metallic powder or metal oxide.

In one embodiment, the porosity of the porous portion may be greater than about 10%. In another embodiment, the porosity of the porous portion may be in a range of from about 10% to about 20%, from about 20% to about 30%, from about 30% to about 40%, from about 40% to about 50%, or from about 50% to about 60%. In one embodiment, the pore size of the porous portion may be greater than about 1 nanometer. In one embodiment, the pore size of the porous portion may be in a range of from 1 nanometer to about 10 nanometers, from about 10 nanometers to about 20 nanometers, from about 20 nanometers to about 500 nanometers, or about 500 nanometers to about 1000 nanometers.

As noted, the third electrode may be formed from a supercapacitor electrode material. The amount of supercapacitor electrode material in the third electrode may be greater than about 10 wt percent. In one embodiment, the amount may be in a range of from about 10 wt percent to about 20 wt percent, from about 20 wt percent to about 30 wt percent, from about 30 wt percent to about 40 wt percent, from about 40wt percent to about 50 wt percent, from about 50 wt percent to about 60 wt percent, or from about 60 wt percent to 100 wt percent.

There are several regions within the volume. A region is defined as the space that exists between the inner surface of the wall and a surface of an adjacent electrode. A first region is bounded by the inner surface of one wall and the adjacent air electrode. A second region is bounded by the inner surface of the other wall and the adjacent third electrode.

A water-controlling separator is disposed in at least one of the regions. The separator may be a membrane that is hydrophobic or superhydrophobic. A super hydrophobic surface is defined as a surface having repellency for liquid/water or a surface that does not get wet when dipped into or placed in contact of water/liquid or that has a contact angle with water drop greater than 150 degrees. The water-controlling separator prevents liquid from passing through while allowing air/oxygen to pass there through.

In one embodiment, the water-controlling separator disposed in first region at air electrode side, prevents the flooding issue when humid air flows into the air electrode. In one embodiment, the water-controlling separator disposed in the second region at third electrode side, releases the generated oxygen during charging of fuel cell into the environment.

Suitable material for water-controlling separator may include one or more of polytetrafluoroethylene, polysulphone, polyphenylene oxide, or polyetherimide. These materials may be expanded, porous, perforated, or drawn as fibers to form a mesh, weave or mat. In one embodiment, the material may be drawn as a hollow fiber. Also, the material may be surface treated to affect such properties and characteristics as hydrophobicity/hydrophilicity, anti-fouling, and water repellency.

Suitable thickness of the water-controlling separator may be greater than about 10 nanometers. In one embodiment, the thickness of the water-controlling separator may be in a range of from about 10 nanometers to about 100 nanometers, from 100 nanometers to about 1 micrometer, from about 1 micrometer to about 10 micrometers, from about 10 micrometers to about 100 micrometers, from about 100micrometer to about 1 millimeter, or greater than 1 millimeter. In one embodiment, the thickness of water-controlling separator is in a range of from about 20 micrometers to about 200 micrometers. The water-controlling separator may be characterized by one or more properties. The properties may include pore size. In one embodiment, the pore size may be in range of from about 1 nanometer to about 10 nanometers, from about 10 nanometers to about 100 nanometers, from about 100 nanometers to about 1 micrometer, from about 1 micrometer to about 10 micrometers. Naturally, the thickness, pores size, pore configuration, and any surface treatments may cooperate to control such properties as flow rate, flow selectivity, and performance.

The working of the apparatus and the function of the fuel cell are described below with reference to illustrated embodiments. Referring to the drawings, the illustrations describe embodiments of the invention and do not limit the invention thereto.

An apparatus100in accordance with an embodiment or the invention is shown inFIG. 1. The apparatus100is a fuel cell for storing energy and producing energy. The apparatus100includes a housing102having walls104and106. The housing walls104and106have inner surfaces108and112, and outer surfaces110and114respectively. The inner surfaces108and112define a volume116. The housing walls104and106have apertures118, through which oxidant can flow into, or out of, the volume116. A first electrode120, a second electrode122and a third electrode124are disposed in the volume116. The second electrode122is separated by each of the first electrode120and the third electrode124by a membrane126saturated with potassium hydroxide (KOH). A first region128is bound by the inner surface108of the first wall and the first electrode. A second region130is bounded by the inner surface112of the second wall and the third electrode. A water-controlling separator132and134may be disposed in the region128and the region130, respectively.

During discharge, process water is consumed and air/oxidant is supplied to the air electrode to generate hydroxyl ions. In one embodiment, before supplying a flow of air/oxidant to the air electrode from the ambient environment, carbon dioxide may be removed from the flow of air/oxidant to avoid interaction between the carbon dioxide and the alkaline electrolyte.

During use of the apparatus as a fuel cell, a voltage potential can be applied between the anode and the third electrode of the fuel cell, and the electrochemical reaction can be reversed to charge the fuel cell or metal/air battery. During charging, hydrogen is stored in the anode and oxygen is produced at the air electrode, the third electrode can spatially remove the locus for the generation of oxidation away from the second electrode/anode. Generated oxygen may be released to the atmosphere through the air electrode. The stored hydrogen can react with air/oxidant to generate electricity and water during discharge. The mechanism of a fuel cell or metal/air battery may be as follows:In charging process:negative electrode: 4M+4H2O+4e→4MH+4OH−frame third electrode: 4OH−→O2+2H2O+4etotal electrolysis reaction: 4M+2H2O→4MH+O2In discharging process:negative electrode: 4MH+4OH−+4e→4M+4H2Opositive electrode: O2+2H2O+4e→4OH−total cell reaction: 4MH+O2→4M+2H2O

The air electrode may be used during the charge cycle, but may not be sufficient in some instances. For example, the air electrode may deteriorate if used to charge the fuel cell. Thus, the third electrode may be utilized as a separate oxygen generation electrode. The charge process takes place between the anode and the third electrode and the discharge process takes place between the anode and the air electrode. According to embodiments of the invention, the third electrode may be utilized to extend the cycle life over traditional structures by chemically and mechanically protecting the air electrode from degradation during recharge. Therefore, the air electrode can be free from damage during the oxygen evolution reaction.

Operation of the fuel cell at high temperature may be problematic if the temperature is high enough for water in the fuel cell to vaporize. High temperature may cause the membrane between the two electrodes to dry and lose conductivity. The fuel cell may need water in the electrolyte as well as water at the anode. Water may be generated at the air electrode. The more power a fuel cell makes, the faster the air electrode produces water and the warmer the fuel cell becomes. Because the fuel cell embodiments described herein are not necessarily closed containers, the heat generated at the air electrode may lead to evaporation of some water from the cell.

The outside temperature and humidity may influence the water management inside the fuel cell. If, under humid conditions, a fuel cell has too much water at the air electrode, oxygen cannot get to the air electrode, and the fuel cell may shut down as a result of flooding. In a dry climate, the heat from the fuel cell operation may parch the air electrode, starving it of water, and may stop the device from operating. In other words, too much water in the fuel cell may flood the air electrode, stopping the reaction and insufficient water may result losing the membrane ability to conduct OH−across the fuel cell.

The water-controlling separator may reduce water loss and oxygen evolution during the charge process. The third electrode including nickel hydroxide or supercapacitor electrode also may reduce or prevent water loss and oxygen evolution during charge process.

The water-controlling separator disposed in the first region allows air/oxidant to flow into the air electrode and reduces or prevents flooding phenomena. At the third electrode side in the second region, the water-controlling separator allows the generated oxygen during charge process to release into the environment while reducing or preventing water/electrolyte to release through and thus reducing water loss.

During the charge reaction, when the third electrode includes nickel hydroxide or supercapacitor electrode is not fully charged, the charge reaction performs as follows:
Ni(OH)+OH−→NiOOH+H2O+e−
No oxygen releases from the third electrode and water loss may be reduced. After the third electrode is fully charged, the third electrode performs as a metal electrode of a conventional rechargeable fuel cell, releases the generated oxygen.
4OH−→2 H2O+O2+4e−
In this way, water loss may be reduced by using nickel hydroxide or supercapacitor electrode, while the electrode is not in a fully charged state or condition. Additionally, the fuel cell has a desirable energy efficiency, as charge voltage is relatively lower.

Naturally, the discharge of the cell may be carried out using the anode and the air electrode. In one embodiment, the discharge of the cell may be carried out using the anode and the third electrode. In one embodiment, the discharge of the fuel cell may be carried out using the anode, the air electrode and the third electrode. So, current can be drawn from the air electrode, the third electrode or both. If current is to be drawn off of both the third electrode and the air electrode, the draw can be simultaneous or can be alternating between electrodes.

The discharge characteristics of the fuel cell according to one embodiment are shown inFIG. 2.FIG. 2includes a graph200that shows discharge voltage and discharge current over time. Time in seconds is represented on the x-axis while discharge voltage and discharge current is represented on the y-axis. A pair of curves indicated by202represents the discharge voltage204and discharge current206when discharge of the fuel cell is carried out using the anode and the third electrode. Another pair of curves indicated by208represents the discharge voltage210and discharge current212when discharge of cell is carried out using the anode and the air electrode.

The pair of curves202shows higher working voltage and discharge current than as shown by the pair of curves208. That means the cell can supply higher power density and higher energy efficiency when discharged between anode and third electrode than when discharged between the anode and the air electrode. In addition, as the Ni(OH)2or supercapacitor third electrode has the same weight as a nickel-based third electrode, and the cell maintains a high energy density.

The embodiments described herein may be examples of compositions, structures, systems, and methods having elements corresponding to the elements of the invention recited in the claims. This written description may enable those of ordinary skill in the art to make and use embodiments having alternative elements that likewise correspond to the elements of the invention recited in the claims. The scope of the invention thus includes compositions, structures, systems and methods that do not differ from the literal language of the claims, and further includes other structures, systems and methods with insubstantial differences from the literal language of the claims. While only certain features and embodiments have been illustrated and described herein, many modifications and changes may occur to one of ordinary skill in the relevant art. The appended claims cover all such modifications and changes.