WO2010083219A1

WO2010083219A1 - Membraneless microfluidic fuel cell

Info

Publication number: WO2010083219A1
Application number: PCT/US2010/020902
Authority: WO
Inventors: Jonathan Posner; Kamil Salloum
Original assignee: Arizona Board Of Regents, For And On Behalf Of, Arizona State University
Priority date: 2009-01-13
Filing date: 2010-01-13
Publication date: 2010-07-22

Abstract

A fuel cell includes a plurality of individual fluid passageways each leading to a common outlet. Each fluid passageway includes an anode and a cathode. A plurality of fuel ports are connected to the fluid passageways and supply fuel to the passageways upstream of the anodes to flow in electrolyte towards the outlet. A plurality of oxidizer ports are connected to the fluid passageways and supply oxidizer to the passageways upstream of the cathodes to flow in electrolyte. The arrangement of the ports and the anodes and cathodes enables the fuel to oxidize at the anodes to generate electrons for conduction to a load and oxidation products, the oxidizer to reduce at the cathodes using electrons received from the load and generate reduction products, the oxidation products and reduction products to react to form a by-product, and the flow including the by- product to exit via the common outlet.

Description

MEMBRANELESS MICROFLUIDIC FUEL CELL

Field of the Invention

[0001] The present application relates to a fuel cell, and more particularly to a membraneless microfluidic fuel cell, in which liquid fuel and liquid oxidant are interfaced through a flow system.

Background of the Invention

[0002] If fuel cells are to become viable portable power sources in the future, solutions to a number of difficult, persistent technical problems are needed. Many of these problems are associated with the presence of the proton exchange membrane, which is highly sensitive to various factors, such as operating temperatures and membrane humidity. Efforts in portable applications have largely focused on reducing the size of proton exchange membrane (PEM) fuel cells. By portable power sources, this is generally referring to substitutes for batteries that power portable electronic devices. This approach carries all the cost and efficiency issues associated with larger scale PEM fuel cells. Moreover, the reduction in size exaggerates some of these problems, and introduces even further problems that require resolution for a commercially viable product. [0003] One approach has been to deliver laminar flows of oxidizer and fuel saturated electrolytes into a single channel with a cathode on one side and an anode on another. See, e.g., Membraneless Vanadium Redox Fuel Cell Using Laminar Flow, Ferrigno et al., J. Amer. Chem. Soc. 2002, 124, 12930-12931; Fabrication and Preliminary Testing of a Planar Membraneless MicroChannel Fuel Cell, Cohen et al., J. Power Sources, 139, 96- 105; and Air-Breathing Laminar Flow-Based Microfluidic Fuel Cell, Jayashree et al., J. Am. Chem. Soc, 2005, 127, 16758-16759. See also, U.S. Patent Nos. 7,252,898 and 6,713,206. Each of these is incorporated into the present application by reference in their entirety for background teachings.

[0004] This approach has various shortcomings. First, the fuel and oxidizer will mix downstream of the entry point, wasting the majority of the reactants. Second, the diffusivity of many oxidizers leads to mixed potentials at the anode due to oxidizer crossover to the anode. This takes energy away from the circuit and also leads to inefficiency of the cell. Third, a mass transport boundary layer builds up on the electrodes which generates mass transport losses in the fuel cell and decreases performance. Fourth, the architecture of the cell is restricted to the geometries, length scales, and electrolytes where laminar flow is ensured.

[0005] U.S. Patent Publication Nos. 2003/0165727 and 2004/0058203 disclose mixed reactant fuel cells where the fuel, oxidant and electrolyte are mixed together and then flow through the anode and cathode. These publications are incorporated herein by reference. According to these publications, the anode is allegedly selective for fuel oxidation and the cathode is allegedly selective for oxidizer reduction. The designs in these publications have significant shortcomings. First, the amount of some oxidizers that can be typically carried by an electrolyte is relatively low (e.g., the oxygen solubility in an electrolyte is typically quite low relative to fuel solubility). This means that a relatively high flow rate is required for the mixed reactants to ensure that an ample amount of oxidizer is flowing through the cell. That is, a relatively high flow rate is required to maximize oxidizer exposure and reaction at the cathode. But increasing the flow rate requires increased work, thus detracting from the overall power efficiency of the cell. Increasing the flow rate also advects the reactants downstream before they can fully react, resulting wasted reactants. Moreover, electrodes that are selective by virtue of their material properties tend to have lower reaction activity rates than non-selective electrodes. Because the designs in these two publications rely primarily on the use of selective electrodes for both the cathode and anode, this further detracts from the efficiency of the cell. [0006] The present application addresses the aforementioned challenges without a proton exchange membrane.

Summary of the Invention

[0007] According to an aspect of the present invention, there is provided a fuel cell that includes a plurality of individual fluid passageways each leading to a common outlet. Each fluid passageway comprises an anode and a cathode. The fuel cell also includes a plurality of fuel ports connected to the fluid passageways for supplying fuel to the passageways upstream of the anodes to flow in electrolyte towards the outlet, and a plurality of oxidizer ports connected to the fluid passageways for supplying oxidizer to the passageways upstream of the cathodes to flow in electrolyte. The arrangement of the fuel and oxidizer ports and the anodes and cathodes enables (a) the fuel to oxidize at the anodes to generate electrons for conduction to a load and oxidation products, (b) the oxidizer to reduce at the cathodes using electrons received from the load and generate reduction products, (c) the oxidation products and reduction products to react to form a byproduct, and (d) the flow comprising the by-product to exit via the common outlet. [0008] According to an aspect of the invention, there is provided a method for generating electrical current using a fuel cell that includes a plurality of individual fluid passageways each leading to a common outlet, each fluid passageway comprising an anode and a cathode. The method includes flowing a first flow comprising a fuel and an electrolyte simultaneously across or through the anodes. The fuel is oxidized at the anodes to generate electrons for conduction to a load and oxidation products in the electrolyte. The method also includes flowing a second flow comprising an oxidizer simultaneously across or through the cathodes. The oxidizer receives electrons from the load and is reduced by the cathodes to generate reduction products in the electrolyte. The method also includes merging the first flow and the second flow so that the oxidation products and the reduction product react to form by-products in the electrolyte and complete an electrochemical reaction in the fuel cell, and flowing a flow comprising the by-products and the electrolyte through the outlet.

[0009] According to an aspect of the invention, there is provided a method for generating electrical current with a fuel cell system, the fuel cell system comprising a plurality of primary fuel cells and at least one secondary fuel cell, each fuel cell comprising an anode and a cathode. The method includes flowing a first flow comprising a fuel and a first electrolyte across or through the anodes of the primary fuel cells, the fuel being partially oxidized at the anodes to generate electrons for conduction to a load and oxidation products in the electrolyte with excess fuel remaining in the electrolyte, and flowing a second flow comprising an oxidizer and a second electrolyte across or through the cathodes of the primary fuel cells, the oxidizer receiving electrons from the load and being partially reduced by the cathodes to generate reduction products in the electrolyte with excess oxidizer remaining in the electrolyte. The method also includes communicating the first flow and the second flow with each other through a third electrolyte so that the oxidation products and the reduction products react to form byproducts in the electrolyte and complete electrochemical reactions in the primary fuel cells, flowing the excess fuel and electrolyte from the primary fuel cells across or through the anode of the secondary fuel cell; and flowing the excess oxidizer and electrolyte from the primary fuel cells across or through the cathode of the secondary fuel cell to complete an additional electrochemical reaction in the secondary fuel cell.

[0010] According to an aspect of the invention, there is provided a fuel cell system that includes a plurality of fuel cells, each fuel cell comprising a fluid passageway comprising an anode and a cathode, a fuel port connected to the fluid passageway for supplying fuel to the passageway upstream of the anode to flow in electrolyte, an oxidizer port connected to the fluid passageway for supplying oxidizer to the passageway upstream of the cathode to flow in electrolyte, a fuel and electrolyte outlet connected to the fluid passageway at or near the anode, and an oxidizer and electrolyte outlet connected to the fluid passageway at or near the cathode. The arrangement of the fuel and oxidizer ports and the anode and cathode enables (a) the fuel to oxidize at the anode to generate electrons for conduction to a load and oxidation products, (b) the oxidizer to reduce at the cathode using electrons received from the load and generate reduction products, (c) the oxidation products and reduction products to react to form a by-product at an intersection in the fluid passageway, (d) the flow comprising the oxidation products to exit the fuel cell via the fuel and electrolyte outlet, and (e) the flow comprising the reduction products to exit the fuel cell via the oxidizer and electrolyte outlet. The fuel and electrolyte outlet of at least one of the fuel cells is connected to the fuel port of another fuel cell and the oxidizer and electrolyte of at least one of the fuel cells is connected to the oxidizer port of said another fuel cell. [0011] Other aspects, features, and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.

Brief Description of the Drawings

[0012] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0013] Figure 1 is a schematic view of a fuel cell in accordance with an embodiment of the present invention;

[0014] Figure 2 is a more detailed schematic view of a portion of the fuel cell of Figure l; [0015] Figure 3 is a detailed schematic view of a portion of a fuel cell in accordance with an embodiment of the present invention;

[0016] Figure 4 is an exploded view of the fuel cell of Figure 1;

[0017] Figure 5 is a schematic, exploded cross-sectional view of the fuel cell of Figure l;

[0018] Figure 6 is a schematic cross-sectional view of an embodiment of the fuel cell of Figure 1;

[0019] Figure 7 is a schematic cross-sectional view of an embodiment of the fuel cell of Figure 1;

[0020] Figure 8 is a schematic view of a fuel cell in accordance with an embodiment of the present invention;

[0021] Figure 9 is a detailed schematic view of a portion of the fuel cell of Figure 8;

[0022] Figure 10 is a schematic view of a fuel cell in accordance with an embodiment of the present invention; and

[0023] Figure 11 is a schematic view of a fuel cell system that includes a plurality of fuel cells of Figure 10.

Detailed Description of Embodiments of the Invention

[0024] The Figures illustrate embodiments of various aspects of the inventions claimed. These embodiments are in no way intended to be limiting, and are intended only as an example for facilitating an understanding of the principles of the claimed inventions. In some instances, various components are illustrated schematically, as it is understood many different structures may be used.

[0025] Figure 1 is a schematic top view of an electrochemical fuel cell 10 in accordance with an embodiment of the present invention. As illustrated, the fuel cell 10 includes a plurality of anodes 12 that are each connectable to an external load L. The anodes 12 may be made out of a catalyst material so that when the fuel comes into contact with the anodes 12, the anodes 12 oxidize fuel and generate electrons for conduction to the load L and oxidation products. An oxidation product is an ionic or molecular byproduct of the fuel's oxidation that has donated at least one electron. An oxidation product may also be referred to as a cation because the loss of an electron may result in a positive charge. However, the cations may be supported in the electrolyte by negative ions. [0026] The fuel cell 10 also includes a plurality of cathodes 14 that are each connectable to the external load L. The cathodes 14 may be made out of a catalyst material so that when the cathodes 14 are connected to the anodes 12 via the load L, the cathodes reduce the oxidizer. The cathodes 14 are configured to receive electrons from the load L to reduce an oxidizer (oxidant) when the oxidizer comes into contact with the cathodes 14 to form reduction products and complete an electrochemical circuit. A reduction product is an ionic or molecular byproduct of the oxidizer that has gained at least one electron. A reduction product may also be referred to as an anion because the gain of an electron may result in a negative charge. However, the anions may be supported in the electrolyte by positive ions.

[0027] In the embodiment of Figure 1, eight pairs of anode and cathodes are illustrated, with each anode 12 being in series with its corresponding cathode 14 along a respective fluid passageway 18 so that reduction of the oxidizer occurs downstream of oxidation of the fuel relative to the direction of fluid flow in the fuel cell 10, as will be discussed in further detail below.

[0028] As illustrated in Figure 1, the fuel cell 10 also includes a plurality of fuel inlets 16 that are configured to receive the fuel that is to be oxidized by the anodes 12. Each anode 12 has a corresponding fuel inlet 16 and is connected to its corresponding fuel inlet 16 by a fluid passageway 18, which may be in the form of a conduit or channel, as shown in greater detail in Figure 2. The fuel inlets 16 are located radially outwardly of the anodes 12. The fluid passageways 18 are configured to allow the fuel entering the fuel inlets 16 to flow towards and across and/or through the anodes 12. The fuel cell 10 also includes a plurality of electrolyte inlets 20 that are also located radially outwardly of the anodes 12. As illustrated, each electrolyte inlet 20 is located adjacent to, but separate from, a corresponding fuel inlet 16, and is also connected to a corresponding fluid passageway 18. In the illustrated embodiment, the electrolyte that enters the electrolyte inlet 20 and the fuel that enters the fuel inlet 16 combine with each other, thereby forming what may be referred to as an anolyte, prior to passing across or through the anode 12. In an embodiment, the electrolyte and the fuel may be mixed together in advance (and thus single inlets for both, instead of single inlets for each may be used). The illustrated embodiment is not intended to be limiting in any way. [0029] The fuel cell 10 also includes a plurality of oxidizer (oxidant) inlets 22 that are connected to the fluid passageway 18 at locations that are downstream (i.e., away from the fuel inlet 16) of the anodes 12. hi the embodiment illustrated in Figure 1, the oxidizer inlets 22 are connected to the fluid passageway 18 at locations that are upstream of the cathodes 14. A second plurality of electrolyte inlets 24 are provided and are connected to the fluid passageway 18 at locations downstream of the anodes 12 and upstream of the cathodes 14, but downstream of the anodes. Electrolyte, which may be the same type of electrolyte that is provided to the fuel cell 10 via the electrolyte inlets 20, may enter the second electrolyte inlets 24 and mix with the oxidizer, thereby forming what may be referred to as a catholyte, prior to the oxidizer flowing across the cathodes 14. After the catholyte flows past (by flowing across and/or through) the cathodes 14, the electrolyte and any by-products of the electrochemical reactions that take place in the fluid passageway 18 may flow out of the fuel cell 10 via a common outlet 28, which is located at the center of the fuel cell 10. The overall flow direction of the fluids in the fuel cell 10 is radially inward and towards the outlet 28.

[0030] In an embodiment, the oxidizer and electrolyte may be mixed together in advance (and thus single inlets for both, instead of separate inlets for each may be used). Also, the additional electrolyte may be omitted and the oxidizer may be delivered directly into the electrolyte already in the fluid passageway 18.

[0031] As illustrated in Figure 1, a single oxidizer inlet 22 may be connected to two adjacent fluid passageways 18 and the second electrolyte inlets 24 may also be connected to two adjacent fluid passageways in an alternating manner, which may allow for a more compact design as compared to a design that includes eight oxidizer inlets and eight second electrolyte inlets. In an embodiment (not illustrated), the fuel inlets 16 and the electrolyte inlets 20 may be connected to adjacent fluid passageways 18 in a similar manner. The illustrated embodiment is not intended to be limiting in any way. [0032] Figure 3 illustrates an embodiment in which the anode 12 and the cathode 14 are in a parallel flow configuration. As illustrated, the electrolyte inlet 20 has two connections, one to the fluid passageway 18 upstream of the anode 12 and one to a fluid passageway 26 that includes the cathode 14. The oxidizer inlet 22 is also connected to the fluid passageway 26 upstream of the cathode 14 so that the oxidizer that enters the oxidizer inlet 22 mixes with the electrolyte that enters the electrolyte inlet 20 prior to the oxidizer coming in contact with the cathode 14. The second electrolyte inlet 24 is located downstream of the anode 12, and is located downstream of the cathode 14 as well, even though it is located in the same position relative to the other ports. The electrolyte that enters the second electrolyte inlet 24 may be used to transport any neutral by-products that are generated by the electrochemical reaction that takes place at the intersection of the fluid passageways 18, 26, i.e., at the location where the anolyte and catholyte first contact each other.

[0033] Figure 4 illustrates an exploded perspective view of the fuel cell 10 of Figure 1. As illustrated, the fuel cell 10 includes a plurality of layers, including a first layer 30, a second layer 32, and a third layer 34. The layers 30, 32, 34 are desirably electrochemically inert and non-conductive so that they may contact each other without contributing to the electrochemical reaction that takes place in the fuel cell 10 or causing any short-circuits within the fuel cell 10. The layers 30, 32, 34 may be constructed separately and stacked together to form the fuel cell.

[0034] The first layer 30 includes the plurality of fuel ports 16, electrolyte ports 20, oxidizer ports 22, and second electrolyte ports 24. Each of the ports 16, 20, 22, 24 may be in the form of an aperture, which may be circular in shape as illustrated, or may have any other suitable shape. Although all of the ports 16, 20, 22, 24 are illustrated as being the same size and shape, they do not necessarily have to have the same size and shape. For example, the fuel port may be larger or smaller than the oxidizer port, depending on the desired flow rates and pressures to be realized within the fuel cell. The ports 16, 20, 22 may be created in the first layer 30 by micromacbining, etching, lithography, or any other suitable technique.

[0035] The second layer 32 includes the fluid passageway 18 and other side branch passageways 18s that are configured to connect the ports 16, 20, 22, 24 to the main branch fluid passageway 18 when the fuel cell 10 is assembled. Although all of the passageways are illustrated as having the same width, each passageway may be individually configured to have the desired width and shape so that the desired flow rates and pressures may be realized within the fuel cell. The fluid passageways 18, 18s are preferably designed so that the flow of the fuel, electrolyte(s), and oxidizer is a laminar flow. At least the fluid passageways 18 of the second layer 32 are configured to extend all the way through the second layer 32 so that they will be exposed to the third layer 34 upon assembly of the fuel cell 10, as discussed below. In an embodiment, the side branch passageways 18s also extend through the second layer 32. The passageways may be created by micromachining, etching, lithography, or any other suitable technique.

[0036] The fluid passageways 18 created in the second layer 32 may span the microfluidic to millifluidic range, i.e., the smallest dimension, such as the depth of the channel, may be in the range of about 1 μm to about 10 mm. The lengths of the passageways may be designed so that the most efficient reactant utilization may be achieved, and may depend on the concentrations of the particular reactants in the catholyte and the anolyte. In an embodiment, the length of the channels may be selected within an aspect ratio that is based upon the Peclet number (Pe), as specified by equation (1) below:

Pe = UHID (1) where U is the average velocity of the catholyte or anolyte in the channel, which may be controlled by the flow rate, H is the characteristic dimension of the channel (such as the width or height), and D is the diffusion coefficient of the catholyte or anolyte that is flowing in the channel. Preferably, the channel geometry and flow rates are selected so that a high Peclet number, such as greater than 10 is achieved, so as to substantially prevent diffusive intermixing of the catholyte and the anolyte at the intersection points, (contact zones).

[0037] As illustrated in Figure 4, the third layer 34 includes the anodes 12 and the cathodes 14. As illustrated, each anode 12 is spaced from its corresponding cathode 14 so that there is a gap 38 therebetween. The gap 38 is configured to electrically insulate each anode 12 from its corresponding cathode 14 to prevent a short circuit in the fuel cell 10. In an embodiment, an insulator (not shown) may be located within the gap 38 and may be configured to allow fluids to flow therethrough. The anodes 12 and the cathodes 14 may be deposited onto the third layer 34 by any suitable deposition technique, such as sputtering, printing, self assembly, etc.

[0038] Alternatively, the anodes and cathodes can be deposited directly onto or into the second layer 32 and the third layer 34 may then act to seal the assembly and conduct the electrodes to the load.

[0039] The third layer 34 is also configured to allow the anodes 12 and cathodes 14 to be connected with the load L. For example, in an embodiment, suitable openings 40, such as the one shown in Figure 5, may be provided in the third layer 34 so that an electrically conducting material 42 may be placed into the opening and contact the anode 12 or cathode 14 from an underside of the third layer 34 (with the anode 12 and cathode 14 being on a top side). Each opening 40 should be configured so that any fluid that flows over or through the associated electrode (anode 12 or cathode 14) does not enter the opening 40 and leak through the third layer 34 and out of the fuel cell 10. In an embodiment, the opening 40 may be filled with an electrically conducting material, i.e., an electrically conducting material may be deposited in the opening so that it is permanently affixed to the third layer 34. The load L may then be connected to the material that is in the opening 40, which would provide an electrical connection to the anode 12 or cathode 14. Alternatively, an electrically conductive material 42s (see Figure 5) can be deposited on the third layer 34 itself such that the current is conducted from the electrodes to the load in the plane of the third layer 34. The conductive material is patterned such that it does not connect the anode and cathode directly, but through the desired external load. [0040] The anode 12 may comprise any electrically conductive material that is coated with a suitable catalyst for oxidizing the fuel as the fuel passes over the anode 12. In an embodiment, the anode 12 may at least partially comprise a porous material that is the catalyst itself. Non-limiting examples of catalysts that may be used include platinum, ruthenium, palladium, nickel, gold, and carbon or alloys of the aforementioned. The porous material may be, for example, a catalyst coated carbon cloth, a porous foam, a packed bed of catalyst particles and/or colloidal crystals that allows the fuel to pass therethrough and oxidizes the fuel as it passes.

[0041] Similarly, the cathode 14 may comprise any electrically conductive material that is coated with a suitable catalyst for reducing the oxidizer as the oxidizer passes over the cathode 14. Non-limiting examples of catalysts that may be used include platinum, ruthenium, palladium, nickel, gold, and carbon or alloys of the aforementioned. In an embodiment, the cathode 14 may at least partially comprise a porous material that is the catalyst itself. The porous material may be, for example, a catalyst coated carbon cloth, a porous foam, a packed bed of catalyst particles and/or colloidal crystals that allows the oxidizer to pass therethough and reduces the oxidizer as it passes.

[0042] To assemble the layers, 30, 32, 34, the second layer 32 may be placed on the third layer 34 so that each electrode pair (one anode 12 and one cathode 14) lines up with a corresponding fluid passageway 18, as illustrated by Figure 5. In an embodiment where the anode 12 is made up of a porous catalyst material and is configured to allow the anolyte to flow therethrough, the anode 12 may have the same height as the fluid passageway 18, as illustrated by Figure 6. In an embodiment illustrated in Figure 7, the anode 12 extends into the fluid passageway 18, but has a height that is less than the height of the fluid passageway 18. In this configuration, the anolyte will pass over, i.e. across, the anode 12. Although the cathode 14 is not illustrated in Figures 5-7, it should be understood that configurations of the cathode 14 that are similar to the illustrated anode 12 may be used.

[0043] The first layer 30 may be placed on the second layer 32 so that the ports 16, 20, 22, 24 are aligned with their respective side branch fluid passageways 18s. Upon stacking of the layers, the first layer 30 provides a cover for the fluid passageways that are provided by the second layer 32. The arrangement of the layers 30, 32, 34 provides a compact, low profile fuel cell 10, which may be stacked or otherwise combined with other fuel cells having the same or substantially the same design.

[0044] To operate the fuel cell 10, the fuel ports 16 may be fluidly connected to a fuel source via a suitable fuel manifold. Similarly, the oxidizer ports 22 may be fluidly connected to an oxidizer source via a suitable oxidizer manifold. The electrolyte ports 20, 24 may be fluidly connected to a single electrolyte source when the same electrolyte is supplied to both ports 20, 24, or to separate electrolyte sources when different electrolytes are supplied to the electrolyte ports 20, 24, with suitable electrolyte manifold(s). [0045] The fuel, oxidizer, and electrolyte(s) may be fed to their respective ports 16, 22, 20, 24 by gravity, surface forces, such as surface tension or electroosmotic flow, or a mechanically driven force. In an embodiment, suitable flow generators, such as pumps or pressurized source, may be used to generate the flows of the fuel, oxidizer, and electrolyte(s) through their respective ports 16, 22, 20, 24 and into the fluid passageways 18.

[0046] Figure 8 illustrates an embodiment of a fuel cell 110 in which a single electrolyte port 120 provides a flow of electrolyte that merges with the flow of fuel provided by a fuel port 116 and the flow of oxidizer (oxidant) provided by an oxidant port 122. As illustrated, the flow of electrolyte is split when the electrolyte enters a fluid passageway 118 so that part of the flow of electrolyte combines with the flow of fuel in a first part 117 of the fluid passageway 118 to create an anolyte and part of the flow of electrolyte combines with the flow of oxidant in a second part 119 of the fluid passageway 118 to create a catholyte. As illustrated, the first part 117 of the fluid passageway includes an anode 112 and the second part 119 of the fluid passageway includes a cathode 114. As the anolyte contacts the anode 112, the anolyte gives up electrons that are conducted to the load L and forms oxidation products, and as the catholyte contacts the cathode 114, the catholyte gains electrons from the load L and forms reduction products. The flow of the anolyte merges with the flow of the catholyte at 123, after the anolyte has been oxidized by the anode 112 and the catholyte has been reduced by the cathode 114. The merged flow, which now also includes by-products of the electrochemical reaction exits a common waste outlet 128.

[0047] Figure 9 illustrates a more detailed view of the split of the flow of the electrolyte that enters the fuel cell 110. Because there is no membrane between the flow of the fuel and the flow of the oxidant, there is an intersection 121 where the flows of electrolyte, fuel, and oxidizer interface or communicate with each other. Another intersection 123 is located where the flows merge after the anolyte has passed across or through the anode 112 and the catholyte has passed across or through the cathode 114 (see Figure 8). The intersection 121 is arranged relative to the anode 112 and the cathode 114 so that the anolyte flow containing the oxidation products in the first part 117 of the fluid passageway interacts or communicates with the catholyte flow containing the reduction products in the second part 119 of the fluid passageway to enable the necessary ionic or molecular exchange between the two flows to complete the reaction. The electrolyte that flows into the intersection 121 acts as a barrier for direct reaction of the fuel and oxidizer. Thus, the overall fuel cell reaction may be characterized by (a) the oxidation of fuel to generate oxidation products and electrons for conduction to the load L, (b) the reduction of the oxidizer supported by receiving electrons from the load L, and (c) the exchange of the oxidized and reduced products at, for example, the intersection 121. [0048] The intersection 121 may also be called an exchange zone because the intersection 121 is the location at which, for example in an acidic fuel cell, the ions are exchanged between the fuel and the oxidizer, although reactions do not necessarily have to occur at the intersection. In embodiments in which ions are exchanged in the exchange zone, the exchange zone may be called an ion exchange zone, although such a term is not intended to be limiting in any way. By-products that are generated may be neutral, but they do not have to be neutral. Although the fluid will be "net-neutral," the individual species within the fluid may have charges. The intersection 123 serves as a location where the anolyte and catholyte mix and may react.

[0049] Figure 10 illustrates an embodiment of a fuel cell 210 that includes a pair of fuel ports 216 that are located near opposite ends of an anode 212, and a pair of oxidant ports 222 that are located near opposite ends of a cathode 214. An electrolyte port 220 is centrally located relative to a fluid passageway 218 in which the anode 212 and the cathode 214 are located. A fuel and electrolyte (anolyte) outlet 227 is configured to allow oxidation products that are generated when the fuel is oxidized at the anode 212, as well as unoxidized anolyte, to exit the fuel cell 210. Similarly, an oxidant and electrolyte (catholyte) outlet 229 is configured to allow reduction products that are generated when the oxidant receives electrons from the cathode 214, as well as unreduced catholyte, to exit the fuel cell 210. Intersections 221, which maybe ion exchange zones as described above, are located between respective ends of the anode 212 and the cathode 214 where the electrolyte enters the fluid passageway 218. The unreacted anolyte and unreacted catholyte that flow to the respective exits 227, 229 can still be reacted in a subsequent reaction process in, for example, a similar fuel cell. The embodiments illustrated herein do not limit themselves to further reaction in a fuel cell.

[0050] A plurality of the fuel cells 210 of Figure 10 may be connected, as shown in Figure 11, to create a fuel cell system 410. The fuel cells 210 may share a common fuel supply, a common oxidizer supply, and a common electrolyte supply with the use of suitable manifolds or the like. As illustrated, a single fuel cell 310, which includes a fuel inlet 316, an oxidizer inlet 322, an electrolyte inlet 320, a fuel and electrolyte outlet 327, and a oxidant and electrolyte outlet 329 may be connected so that fuel, electrolyte, and oxidation products that are exhausted from two of the fuel cells 210 via their respective fuel and electrolyte outlets 229 may become the supplies of fuel and electrolyte for the fuel cell 310. Similarly, the fuel cell 310 may be connected so that the oxidizer, electrolyte, and reduction products that are exhausted from two fuel cells 210 become the supplies of oxidizer and electrolyte for the fuel cell 310.

[0051] The operation of the fuel cell 310 is substantially the same as the operation of the fuel cell 210 shown in Figure 10, with the exception that the flows entering the fuel cell 310 already include oxidation products or reduction products. By allowing the exhaust flows, which still contain some amount of fuel and oxidizer, from the fuel cells 210 to enter the additional fuel cell 310, additional current may be generated, thereby resulting in increased fuel utilization and thermodynamic efficiency. Of course, more or less fuel cells 210 may be connected, and additional fuel cells 310 may be connected to achieve optimum efficiency for the fuel cell system. The illustrated embodiment is not intended to be limiting in any way. [0052] In an embodiment, the fuel may be hydrogen saturated sulfuric acid and the oxidant may be oxygen saturated sulfuric acid. For an acidic cell using such reactants, oxidation of the fuel at the anode 12 may be generally represented by the following equation:

H₂ -» 2H⁺ + 2e^" (2) and reduction of the oxidant (oxidizer) at the cathode 14 may be represented by the following equation:

0.5O₂ + 2e^" + 2H⁺^H₂O (3) and the net reaction of the system is:

H₂ + 0.5O₂ -> H₂O (4)

Thus, the byproduct of these reactions is water.

[0053] For an alkaline fuel cell using, for example, potassium hydroxide for the electrolyte rather than sulfuric acid, oxidation of the fuel at the anode 12 maybe generally represented by the following equation:

H₂ + 2OH- -» 2H₂O + 2e^" (5) and reduction of the oxidant (oxidizer) at the cathode 14 may be represented by the following equation:

O₂ + 2H₂O + 4e^" -» 4OH- (6) and the net reaction of the system is:

H₂ + 0.5O₂ -> H₂O (7) which is the same net reaction described above in reference to an acidic fuel cell in equation (4).

[0054] For either type of fuel cell (i.e., acidic or alkaline), other possible reactions may occur, including various intermediary reactions, or different reactions when different reactants are used. The system generates an open circuit voltage based on the potentials of its respective half cell reactions. When current is drawn through the load L, this voltage will generally decrease in value to zero, the point of maximum extractable current, or short circuit voltage.

[0055] The electrodes (i.e., anode and cathode) can be made up of any electrically conductive material that is coated with a suitable catalyst. In an embodiment, each electrode comprises a porous material that is the catalyst itself, including but not limited to a catalyst coated carbon cloth, a porous foam, a packed bed of catalyst particles, and/or colloidal crystals. [0056] In addition to any fuel, oxidant, electrolyte or catalyst material mentioned above, any of the following in various combinations may be used in any of the embodiments described above, as well as in any other embodiment within the scope of any aspect of the invention.

[0057] Electrodes/Catalysts: Platinum, Platinum black, Platinized metal (any), Nickel, Nickel Hydroxide, Manganese, Manganese Oxides (all states), Palladium, Platinum Ruthenium alloys, Nickel Zinc alloys, Nickel Copper alloys, Gold, Platinum black supported on metal oxides, Platinum Molybdenum alloys, Platinum Chromium alloys, Platinum Nickel alloys, Platinum Cobalt alloys, Platinum Titanium alloys, Platinum Copper alloys, Platinum Selenium alloys, Platinum Iron alloys, Platinum Manganese alloys, Platinum Tin alloys, Platinum Tantalum alloys, Platinum Vanadium alloys, Platinum Tungsten alloys, Platinum Zinc alloys, Platinum Zirconium alloys, Silver, Silver/Tungsten Carbide, Iron tetramethoxyphenyl porphorin, Carbon or Carbon Black. [0058] Fuels: Formic acid, Methanol, Ethanol, 1-proponal, 2-propoanl, Cyclobutanol, Cyclopentanol, Cyclohexanol, Benzyl alcohol, Lithium, Zinc, Aluminum, Magnesium, Iron, Cadmium, Lead, Acetaldehyde, Propionaldehyde, Benzaldehyde, Ethylene glycol, Glyoxal, Glycolic acid, Glyoxylic acid, Oxalic acid, 1,2-propanediol, 1,3-propanediol, Glycerol, Hydrogen, Vandium(π)Λ/anadium(Iir), Carbon Monoxide, Sodium Borohydride, Other Borohydrides (e.g. Potassium), and other metal redox systems e.g.: Iron/chromium, Nickel/cadmium.

[0059] Oxidants: Air, Oxygen gas, Dissolved Oxygen, Hydrogen Peroxide, Potassium Permanganate, Vanadium(IV)/Vanadium(V) and Manganese Oxide. [0060] Electrolytes: Potassium Hydroxide, Sodium Hydroxide, Sulfuric acid, Nitric acid, Formic acid, Phosphoric acid, Trifluoromethanesulfonic acid (TFMSA), Ionic liquids (all types), Acetimide, Fluoroalcohol emulsions, and Perflourocarbon emulsions (e.g. Flourinert®).

[0061] The foregoing illustrated embodiment(s) have been provided solely for illustrating the structural and functional principles of the present invention and are not intended to be limiting. To the contrary, the present invention is intended to encompass all modifications, substitutions, alterations, and equivalents within the spirit and scope of the following appended claims.

Claims

IN THE CLAIMS:

1. A fuel cell comprising: a plurality of individual fluid passageways each leading to a common outlet; each fluid passageway comprising an anode and a cathode; a plurality of fuel ports connected to the fluid passageways for supplying fuel to the passageways upstream of the anodes to flow in electrolyte towards the outlet; and a plurality of oxidizer ports connected to the fluid passageways for supplying oxidizer to the passageways upstream of the cathodes to flow in electrolyte; wherein the arrangement of the fuel and oxidizer ports and the anodes and cathodes enables (a) the fuel to oxidize at the anodes to generate electrons for conduction to a load and oxidation products, (b) the oxidizer to reduce at the cathodes using electrons received from the load and generate reduction products, (c) the oxidation products and reduction products to react to form a by-product, and (d) the flow comprising the by-product to exit via the common outlet.

2. The fuel cell according to claim 1, further comprising a plurality of electrolyte ports, each electrolyte port being configured to provide the electrolyte to at least one fluid passageway.

3. The fuel cell according to claim 1, wherein each oxidizer port is configured to provide the oxidizer to two fluid passageways.

4. The fuel cell according to claim 1, wherein the fuel cell comprises a plurality of planar layers, and wherein the fuel ports and the oxidizer ports are provided by a first layer.

5. The fuel cell according to claim 4, wherein the fluid passageways are provided by a second layer.

6. The fuel cell according to claim 5, wherein the anodes, the cathodes, and the common outlet are provided by a third layer.

7. The fuel cell according to claim 1, wherein at least part of each anode is porous and is configured to allow the fuel and electrolyte to flow therethrough.

8. The fuel cell according to claim 7, wherein at least part of each cathode is porous and is configured to allow the oxidizer and electrolyte to flow therethrough.

9. The fuel cell according to claim 1, wherein at least part of each cathode is porous and is configured to allow the oxidizer and electrolyte to flow therethrough.

10. The fuel cell according to claim 1, wherein each anode and each cathode comprises a catalyst.

11. The fuel cell according to claim 1, wherein each anode and each cathode is electrically conductive.

12. The fuel cell according to claim 1, wherein the fuel is selected from the group consisting of hydrogen, methanol, ethanol, carboxyl acid, borohydride, and vanadium.

13. The fuel cell according to claim 1, wherein the oxidizer is selected from the group consisting of nitric acid, peroxide, permanganate, and vanadium oxide.

14. The fuel cell according to claim 1, wherein the electrolyte is selected from the group consisting of sulfuric acid, organic buffer, and hydroxide.

15. The fuel cell according to claim 1, wherein the common outlet is located in the center of the fuel cell and the fluid passageways extend radially inwardly towards the common outlet.

16. The fuel cell according to claim 1, wherein the fuel cell is a microchip.

17. The fuel cell according to claim 1, wherein in each fluid passageway, the cathode is downstream of the anode.

18. The fuel cell according to claim 1, wherein each fluid passageway comprises a first branch that comprises the anode and a second branch that comprises the cathode, the second branch merging with the first branch at a location downstream of the anode.

19. A method for generating electrical current using a fuel cell comprising a plurality of individual fluid passageways each leading to a common outlet, each fluid passageway comprising an anode and a cathode, the method comprising: flowing a first flow comprising a fuel and an electrolyte simultaneously across or through the anodes, the fuel being oxidized at the anodes to generate electrons for conduction to a load and oxidation products in the electrolyte; flowing a second flow comprising an oxidizer simultaneously across or through the cathodes, the oxidizer receiving electrons from the load and being reduced by the cathodes to generate reduction products in the electrolyte; merging the first flow and the second flow so that the oxidation products and the reduction product react to form by-products in the electrolyte and complete an electrochemical reaction in the fuel cell; and flowing a flow comprising the by-products and the electrolyte through the outlet.

20. The method according to claim 19, wherein at least one anode and at least one cathode are in series and the second flow merges with the first flow between the anode and the cathode that are in series.

21. The method according to claim 20, wherein the cathode is located downstream relative to a direction of flow from the anode.

22. The method according to claim 19, wherein at least one anode and at least one cathode are in parallel and the first flow and the second flow merge after the first flow crosses or passes through the anode and the second flow crosses or passes through the cathode.

23. The method according to claim 19, wherein the fuel is selected from the group consisting of hydrogen, methanol, ethanol, carboxyl acid, borohydride, and vanadium.

24. The method according to claim 19, wherein the oxidizer is selected from the group consisting of nitric acid, peroxide, permanganate, and vanadium oxide.

25. The method according to claim 19, wherein the electrolyte is selected from the group consisting of sulfuric acid, organic buffer, and hydroxide.

26. The method according to claim 19, further comprising flowing the electrolyte in parallel with the first flow and the second flow to an intersection where the first flow and the second flow interact.

27. The method according to claim 26, wherein there is no membrane at or in the intersection.

28. A method for generating electrical current with a fuel cell system, the fuel cell system comprising a plurality of primary fuel cells and at least one secondary fuel cell, each fuel cell comprising an anode and a cathode, the method comprising: flowing a first flow comprising a fuel and a first electrolyte across or through the anodes of the primary fuel cells, the fuel being partially oxidized at the anodes to generate electrons for conduction to a load and oxidation products in the electrolyte with excess fuel remaining in the electrolyte; flowing a second flow comprising an oxidizer and a second electrolyte across or through the cathodes of the primary fuel cells, the oxidizer receiving electrons from the load and being partially reduced by the cathodes to generate reduction products in the electrolyte with excess oxidizer remaining in the electrolyte; communicating the first flow and the second flow with each other through a third electrolyte so that the oxidation products and the reduction products react to form byproducts in the electrolyte and complete electrochemical reactions in the primary fuel cells; flowing the excess fuel and electrolyte from the primary fuel cells across or through the anode of the secondary fuel cell; and flowing the excess oxidizer and electrolyte from the primary fuel cells across or through the cathode of the secondary fuel cell to complete an additional electrochemical reaction in the secondary fuel cell.

29. The method according to claim 28, wherein the first, second, and third electrolytes are the same electrolyte.

30. The method according to claim 29, wherein the first, second, and third electrolytes are provided by a common source.

31. A fuel cell system comprising: a plurality of fuel cells, each fuel cell comprising a fluid passageway comprising an anode and a cathode; a fuel port connected to the fluid passageway for supplying fuel to the passageway upstream of the anode to flow in electrolyte; an oxidizer port connected to the fluid passageway for supplying oxidizer to the passageway upstream of the cathode to flow in electrolyte; a fuel and electrolyte outlet connected to the fluid passageway at or near the anode; and an oxidizer and electrolyte outlet connected to the fluid passageway at or near the cathode, wherein the arrangement of the fuel and oxidizer ports and the anode and cathode enables (a) the fuel to oxidize at the anode to generate electrons for conduction to a load and oxidation products, (b) the oxidizer to reduce at the cathode using electrons received from the load and generate reduction products, (c) the oxidation products and reduction products to react to form a by-product at an intersection in the fluid passageway, (d) the flow comprising the oxidation products to exit the fuel cell via the fuel and electrolyte outlet, and (e) the flow comprising the reduction products to exit the fuel cell via the oxidizer and electrolyte outlet, and wherein the fuel and electrolyte outlet of at least one of the fuel cells is connected to the fuel port of another fuel cell and the oxidizer and electrolyte of at least one of the fuel cells is connected to the oxidizer port of said another fuel cell.

32. The fuel cell system according to claim 31, wherein each of the fuel cells further comprises an electrolyte port connected to the fluid passageway for flowing the electrolyte into the respective fuel cell.