WO2010148198A1

WO2010148198A1 - Multipass membraneless full cell

Info

Publication number: WO2010148198A1
Application number: PCT/US2010/038990
Authority: WO
Inventors: Jonathan Posner; Kamil Salloum
Original assignee: Arizona Board Of Regents For And On Behalf Of Arizona State University
Priority date: 2009-06-17
Filing date: 2010-06-17
Publication date: 2010-12-23

Abstract

A fuel cell having a first fluid passageway, a second fluid passageway, and an electrolyte passageway positioned at a first reaction zone between the first and second fluid passageways. The first fluid passageway has at least one anode and the second fluid passageway has at least one cathode. The fuel cell also has a fuel port in fluid communication with the first fluid passageway for supplying a fuel stream to the first fluid passageway, and an oxidizer port in fluid communication with the second fluid passageway for supplying an oxidizer stream to the second fluid passageway. The electrolyte passageway also has an electrolyte stream in fluid communication with the fuel stream and the oxidizer stream that creates and maintains a separated interface between the fuel stream and the oxidizer stream.

Description

MULTIPASS MEMBRANELESS FUEL CELL

Field of the Invention

[0001] The present application relates to a fuel cell, and more particularly to a multipass membraneless microfluidic fuel cell, in which liquid fuel and liquid oxidant are interfaced multiple times 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, but not exclusively, referring to substitutes for batteries that power portable electronic devices. This approach carries all the cost issues and challenges associated with larger scale PEM fuel cells. Moreover, the reduction in size aggravates some technical challenges that require resolution for a commercially viable product.

[0003] There are several low temperature membraneless fuel cells that avoid the use of a membrane. One approach has been to deliver parallel 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, 124:12930-12931 (2002); Fabrication and Preliminary Testing of a Planar Membraneless MicroChannel Fuel Cell, Cohen et al., J. Power Sources, 139:96-105 (2005); Air-Breathing Laminar Flow-Based Microfluidic Fuel Cell, Jayashree et al., J. Am. Chem. Soc, 127:16758-16759 (2005); Microfluidic Fuel Cells: A Review, Kjeang, et al., J. Power Sources, 186:353-369 (2009); and Counter Flow Membraneless Microfluidic Fuel Cell, Salloum et al., J. Power Sources, 195:6941-6944 (2010). See also, U.S. Patent Nos. 7,252,898 and 6,713,206.

[0004] However, delivering parallel laminar flows of oxidizer and fuel saturated electrolytes into a single channel with a cathode on one side and an anode on another has various shortcomings. First, the fuel and oxidizer will mix downstream of the entry point, which wastes some of the reactants. Second, the diffusivity of the reactants leads to mixed potentials at the electrodes due to cross-over to the adjacent electrode. This cross-over reduces the overall potential of the fuel cell, which results in lower thermodynamic efficiencies. Third, a mass transport boundary layer builds up on the electrodes, which generates mass transport losses in the fuel cell and decreases performance. Finally, the architecture of the cell is restricted to the respective geometries, length scales, and electrolytes that insure substantially laminar flow.

[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. As disclosed in these publications, the anode is allegedly, or otherwise purported to be, selective for fuel oxidation and the cathode is allegedly, or otherwise purported to be, selective for oxidizer reduction. The respective 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, i.e., 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 in wasted reactants. Moreover, electrodes that are selective by virtue of their material properties tend to have lower reaction activity rates than non-selective electrodes. Finally, the designs in these two publications rely primarily on the use of selective electrodes for both the cathode and anode; this further detracts their performance and applicability.

[0006] In one aspect, the present application addresses the aforementioned challenges without the use of a proton exchange membrane. SUMMARY OF THE INVENTION

[0007] Presented herein is a fuel cell, comprising a first fluid passageway, a second fluid passageway, and an electrolyte passageway positioned at a first reaction zone between the first and second fluid passageways. In one aspect, the first fluid passageway comprises at least one anode and the second fluid passageway comprises at least one cathode.

[0008] The fuel cell further comprises a port for a reducing agent (such as fuel) in fluid communication with the first fluid passageway for supplying a fuel stream to the first fluid passageway, and an oxidizing agent (or oxidizer) port in fluid communication with the second fluid passageway for supplying an oxidizer stream to the second fluid passageway. In another aspect, the electrolyte passageway comprises an electrolyte stream providing fluidic and ionic communication with the fuel stream and the oxidizer stream. The electrolyte stream can be non-reacting and can maintain a separated fluidic interface between the fuel stream and the oxidizer stream. In one aspect, the electrolyte stream maintains substantially laminar flow.

[0009] In another aspect, the fuel cell further defines a plurality of reaction zones between the fuel stream, the oxidizer stream, and the electrolyte stream. In this aspect, at each reaction zone the electrolyte stream creates and maintains a separated fluidic and ionic interface between the fuel stream and the oxidizer stream. In another aspect, the electrolyte stream of each reaction zone can be a common or a dedicated electrolyte stream. Thus, the electrolyte stream can be varied in each of the reaction zones.

[0010] These and other objects of the present application will be clear when taken in view of the detailed specification and disclosure in conjunction with the appended figures.

DETAILED DESCRIPTION OF THE DRAWINGS

[0011] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate certain aspects of the instant invention and together with the description, serve to explain, without limitation, the principles of the invention. Like reference characters used therein indicate like parts throughout the several drawings.

[0012] Fig. 1 is a schematic view of one aspect of a reaction zone in a membraneless microfluidic fuel cell.

[0013] Fig. 2 is a schematic view of one aspect of the fuel cell of Fig. 1, showing a plurality of reaction zones.

[0014] Fig. 3 is a schematic view of the fuel cell of Fig. 2, showing a scheme for interconnections between the reaction zones connected to a load, according to one aspect.

[0015] Fig. 4 is a cross-sectional view of one aspect of a membraneless microfluidic fuel cell.

[0016] Fig. 5 is a perspective view of a prototype of one aspect of a membraneless microfluidic fuel cell.

[0017] Fig. 6 is a schematic view of a multi-pass fuel cell, according to one aspect.

[0018] Figs. 7A, 7B, and 7C graphically illustrate polarization data for the multi-pass fuel cell of Fig. 6 operating at varied reactant and/or electrolyte flow rates.

[0019] Fig. 8 graphically illustrates and compares polarization and power density curves for a single cell and the multi-pass fuel cell of Fig. 6.

[0020] Fig. 9 graphically illustrates the overall fuel utilization for a first cell (Cell 1) of the multi-pass fuel cell of Fig. 6 and both cells of the multi-pass fuel cell at varying reactant flow rates.

[0021] Fig. 1OA graphically illustrates the thermodynamic losses of a conventional fuel cell.

[0022] Fig. 1OB graphically illustrates the thermodynamic losses of a multi-pass fuel cell, according to one aspect.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention can be understood more readily by reference to the following detailed description of the invention and the Examples included therein and to the Figures and their previous and following description.

[0024] Before the present systems, articles, devices, and/or methods are disclosed and described, it is to be understood that this invention is not limited to specific systems, specific devices, or to particular methodology, as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. [0025] The following description of the invention is provided as an enabling teaching of the invention in its best, currently known embodiment. To this end, those skilled in the relevant art will recognize, and appreciate that many changes can be made to the various aspects of the invention described herein, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof.

[0026] As used in the specification and the appended claims, the singular forms "a,"

"an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "an anode" includes two or more such anodes, and the like.

[0027] Ranges can be expressed herein as from "about" one particular value, and/or to

"about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as "about" that particular value in addition to the value itself. For example, if the value "10" is disclosed, then "about 10" is also disclosed. It is also understood that when a value is disclosed that "less than or equal to" the value, "greater than or equal to the value" and possible ranges between values are also disclosed, as appropriately understood by the skilled artisan. For example, if the value "10" is disclosed then "less than or equal to 10" as well as "greater than or equal to 10" is also disclosed. It is also understood that throughout the application, data is provided in a number of different formats and that this data represents endpoints and starting points, and ranges for any combination of the data points. For example, if a particular data point "10" and a particular data point "15" are disclosed, it is understood that greater than, greater than or equal to, less than, less than or equal to, and equal to 10 and 15 are considered disclosed as well as between 10 and 15. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are also disclosed.

[0028] "Optional" or "optionally" means that the subsequently described event or circumstance can or can not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0029] Presented herein is a fuel cell, comprising a first fluid passageway 100, a second fluid passageway 200, and an electrolyte passageway 300 positioned at a reaction zone 20 between the first and second fluid passageways. In one aspect, the first fluid passageway 100 comprises at least one anode 110 and the second fluid passageway 200 comprises at least one cathode 210.

[0030] The fuel cell further comprises a fuel port 120 in fluid communication with the first fluid passageway 100 for supplying a fuel stream 105 to the first fluid passageway, and an oxidizer port 220 in fluid communication with the second fluid passageway 200 for supplying an oxidizer stream 205 to the second fluid passageway 200. In another aspect, the electrolyte passageway 300 comprises an electrolyte stream 305 in fluid communication with the fuel stream 105 and the oxidizer stream 205. In one aspect, the electrolyte stream 305 creates and maintains a separated interface between the fuel stream 105 and oxidizer stream 205. In this aspect, the electrolyte that enters the electrolyte port 320 and the fuel that enters the fuel port 120 interface with each other and flow through the remainder of the fuel passageway 100, and similarly the electrolyte from electrolyte port 320 and oxidizer from oxidizer port 220 interface and flow through the remainder of the oxidizer passageway 200.

[0031] It is contemplated that the interaction between the respective reactant (fuel or oxidizer) and the electrolyte primarily serves to maintain a separated interface where the fuel and oxidizer do not mix. In one aspect, the electrolyte mixes with the respective reactant (fuel or oxidant), therefore enhancing the conductivity of the fluid passing through the first and second fluid passageways 100 and 200. In another aspect, the fuel and oxidizer already exhibit high conductivities, in which case the electrolyte stream still serves the function of maintaining sufficient conductivity for ion exchange through the reaction zone 20. In a further aspect, all of the streams maintain substantially laminar flow. [0032] In another aspect, as shown in Figure 2, the fuel cell further defines a plurality of reaction zones 20 between the fuel stream, the oxidizer stream, and the electrolyte stream. In this aspect, at each reaction zone 20 the electrolyte stream 305 creates and maintains separated interfaces between the fuel stream 105 and the oxidizer stream 205. As can be seen in Figure 2, each consecutive reaction zone is in a downstream direction with respect to the fuel stream and each consecutive reaction zone is in a downstream direction with respect to the oxidizer stream. That is to say, the second reaction zone is downstream from the first reaction zone. At each reaction zone, or substantially adjacent each reaction zone, the first fluid passageway comprises an anode and the second fluid passageway comprises a cathode.

[0033] Figure 3 is a schematic view of the fuel cell 10 showing a plurality of anodes

110 that are each connectable to an external load. In one aspect, the anodes 110 can be made out of a supported electrocatalyst material so that when the fuel comes into contact with the anodes 110, the anodes 110 oxidize fuel and generate electrons for conduction to the load and oxidation products, hi this aspect, the electrocatalyst serves as the catalyst while the support can serve as the main pathway for electron conduction. In another aspect, at least one anode 110 can be embedded in at least a portion of the first fluid passageway 100. For example and without limitation, the anode could be embedded in a porous medium and/or a packed bed in the first fluid passageway, and electron shuttling can occur through this embedded electrocatalyst to the substrate contact or to an electrical interconnect 115, described more fully below. As used herein, an "oxidation product" is an ionic or molecular byproduct of the fuel's oxidation that has donated at least one electron. An "oxidation product" can also be referred to as a cation because the loss of an electron can result in a positive charge. However, the cations can be supported in the electrolyte by negative ions.

[0034] Figure 3 also shows a plurality of cathodes 210 that are each connectable to the external load. The cathodes 210 can be made out of a catalyst material so that when the cathodes 210 are connected to the anodes 110 via the load, the cathodes reduce the oxidizer. The cathodes 210 are configured to receive electrons from the load to reduce an oxidizer (oxidant) when the oxidizer comes into contact with the cathodes 210 to form reduction products and complete an electrochemical circuit. In another aspect, at least one cathode 210 can be embedded in at least a portion of the second fluid passageway 200. For example and without limitation, the cathode could be embedded in a porous medium and/or a packed bed in the second fluid passageway, and electron shuttling can occur through this embedded electrocatalyst from the substrate contact or to an electrical interconnect 115, described more fully below. As used herein, a "reduction product" is an ionic or molecular byproduct of the oxidizer that has gained at least one electron. A "reduction product" can also be referred to as an anion because the gain of an electron can result in a negative charge. However, the anions can be supported in the electrolyte by positive ions.

[0035] In one aspect, the fuel cell 10 further comprises a plurality of electrical interconnects 115 to electrically couple adjacent reaction zones 20, as illustrated in Figure 3. In another aspect, the electrical interconnects can be conductive material configured to transfer electrons from the anode 110 of a first reaction zone to the cathode 210 of an adjacent reaction zone, minimizing resistive losses. Therefore, the electrons produced from an anode can be transferred to an adjacent cathode for consumption there without external connections. In still another aspect, the electrical interconnects 115 can be catalytic as the electrocatalyst can be embedded in the fluid passageways 100, 200, 300, and coupled to the interconnects. Alternatively, in another aspect, it is contemplated that a single common electrical "pad" can connect each anode 110 of the plurality of anodes, and a similar pad can connect each cathode 210 of the plurality of cathodes. In this aspect, neither pad would cross a reaction zone 20, and the two pads would only be coupled through the load.

[0036] In the aspect shown in Figure 2, seven reaction zones 20 are illustrated.

Reaction zone 1 represents the schematic shown in Figure 1, in which the outlets for each of the fuel stream and the oxidant stream, which each contain electrolyte and the unreacted portion of the reactant, are used in the next reaction zone. As illustrated in Figure 2, the unreacted fuel and oxidant from reaction zone 1 flow to reaction zone 2, which operates on the same principals as shown in Figure 1. It is contemplated that this sequential recycling can be repeated multiple times. For example, and without limitation, Figure 2 shows seven reaction zones, but more or fewer zones are contemplated. In this manner both high thermodynamic and coulombic efficiencies can be maintained through appropriate loading.

[0037] One skilled in the art will appreciate that the thermodynamic efficiency, Ej =

V/V_R, increases when a fuel cell is operated at a higher voltage V with respect to a maximum theoretical reversible voltage V_R. However, the current output from the fuel cell /is low compared to the maximum extractable current I_max, therefore the coulombic efficiency, Ec = I/I_max, decreases. As illustrated herein, by allowing unreacted fuel and oxidizer to pass through further downstream reaction zones 20, the total current /is increased, which in turn results in a higher coulombic efficiency and higher overall cell efficiency, E — Eτ*Ec-

[0038] Figure 4 illustrates one possible embodiment of a multipass separated flow micro fluidic fuel cell which has a radial flow pattern. As illustrated, in this aspect, the reaction zones are substantially along a single axis. The advantage to the illustrated radial design is to maximize an ion transfer zone area in a small volume without jeopardizing the laminar flow feature. In one aspect, the electrolyte stream 305 interfaces between the fuel stream 105 and oxidizer stream 205 at each reaction zone 20. In another aspect, the fuel, electrolyte, and/or the oxidant comprise a reactant with high conductivity. The illustrated embodiment is not intended to be limiting in any way.

[0039] In one aspect, the electrolyte passageway 300 comprises an insulating porous bed 310, such that the first fluid passageway 100 and the second fluid passageway 200 are electrically insulated from one another so as not to cause any short-circuits within the fuel cell and maintain a laminar flow even at higher flow rates.

[0040] As mentioned herein, the fuel cell comprises a fuel port 120, an oxidizer port

220, and an electrolyte port 320. Each of the ports 120, 220, and 320 can be in the form of an aperture, which can be circular in shape as illustrated, or can have any other suitable shape. Although all of the ports 120, 220, 320 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 can 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 120, 220, 320 can be created by micromachining, etching, lithography, or any other suitable technique.

[0041] Although all of the passageways are illustrated as having the same width, each passageway can be individually configured to have the desired width and shape so that the desired flow rates and pressures can be realized within the fuel cell. The fluid passageways, in one aspect, are designed so that the flow of the fuel, electrolyte(s), and oxidizer is a laminar flow. As shown in Figure 4, the passageways can be created by stacking a plurality of routing plates 400, each defining various bores or slots which, when stacked, create the fluid passageways, hi this aspect, the anodes and cathodes are sandwiched between each layer. In the aspect shown in Figure 4, the anodes are radially inward of the cathodes, but other configurations are contemplated.

[0042] The fluid passageways 100, 200, 300 can span the microfluidic to millifluidic range, i.e., the smallest dimension, such as the diameter of the passageway, can be in the range of about 1 μm to about 10 mm. In one exemplary aspect, the lengths of the passageways can be designed so that the most efficient reactant utilization can be achieved, and can depend on the concentrations of the particular reactants in the fuel and the oxidant.

[0043] The anode 110 can comprise any electrically conductive material that supports a suitable electrocatalyst for oxidizing the fuel as the fuel passes over the anode 110. In an aspect, the anode 110 can at least partially comprise a porous material that is the catalyst itself. Non-limiting examples of catalysts that can be used include platinum, ruthenium, palladium, nickel, gold, and carbon or alloys of the aforementioned and the like. The porous material can be, for example, a catalyst coated carbon cloth, a porous foam, a packed bed of catalyst particles, colloidal crystal, or reverse opal that allows the fuel to pass therethrough and oxidizes the fuel as it passes.

[0044] Similarly, the cathode 210 can comprise any electrically conductive material that supports a suitable electrocatalyst for reducing the oxidizer as the oxidizer passes over the cathode 210. Non-limiting examples of catalysts that can be used include platinum, ruthenium, palladium, nickel, gold, and carbon or alloys of the aforementioned and the like. In an embodiment, the cathode 210 can at least partially comprise a porous material that is the catalyst itself. The porous material can be, for example, a catalyst coated carbon cloth, a porous foam, a packed bed of catalyst particles, colloidal crystals, or reverse opal that allows the oxidizer to pass therethrough and reduces the oxidizer as it passes.

[0045] To operate the fuel cell 10, the fuel port 120 can be fluidly connected to a fuel source. Similarly, the oxidizer port 220 can be fluidly connected to an oxidizer source and the electrolyte port 320 can be fluidly connected to an electrolyte source. [0046] The fuel, oxidizer, and electrolyte can be fed to their respective ports 120,

220, 320 by gravity, surface forces, such as surface tension or electroosmotic flow, or a mechanically driven force. In one aspect, suitable flow generators, such as pumps or pressurized sources, can be used to generate the flows of the fuel, oxidizer, and electrolyte through their respective ports 120, 220, 320 and into the respective fluid passageway.

[0047] The electrodes (i.e., anode 110 and cathode 210) can be made up of any electrically conductive material that is coated with a suitable catalyst. In one aspect, 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 and the like.

[0048] The electrical current can be carried from the anode 110, through the external load, and to the cathode 210 with wires or conductive traces that are patterned on to one or more of the fuel cell substrates. In one aspect, the substrate's circuitry can be configured to directly route electrons from an anode zone, to a cathode zone in a separate reaction zone, as opposed to extracting and conveying the electrons from each anode 110 and cathode 210 through external means, which in some exemplary aspects increases resistance losses. For example, as shown in Figure 3, the anode of reaction zone 4 sends electrons to the cathode of reaction zone 6, anode of reaction zone 2 to cathode of reaction zone 4, anode of reaction zone 1 to cathode of reaction zone 2, anode of reaction zone 3 to cathode of reaction zone 1, and so on. The remaining anode of reaction zone 6 and cathode of reaction zone 7 can be used to reduce Ohmic losses and reduce the system's circuit complexity, e.g. a hydrogen PEM fuel cell stack with bipolar graphite plates.

[0049] In another aspect, the fuel cell 10 can be a membraneless microfluidic fuel cell that reuses a reactant, in contrast to conventional multi-channel systems that employ common manifolds for inlets and outlets.

[0050] Without limitation and in addition to any fuel, oxidant, electrolyte, or catalyst material mentioned above, any of the following in various combinations can 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. [0051] Electrodes/Catalysts: Platinum, Platinum black, Platonized 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.

[0052] 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, Vandiurn(II)/Vanadium(III), Carbon Monoxide, Sodium Borohydride, Other Borohydrides {e.g., Potassium), and other metal redox systems e.g.: Iron/chromium, Nickel/cadmium.

[0053] Oxidants: Air, Oxygen gas, Dissolved Oxygen, Hydrogen Peroxide,

Potassium Permanganate, Vanadium(IV)/Vanadium(V) and Manganese Oxide.

[0054] 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®).

EXPERIMENTAL

[0055] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the multipass membraneless fuel cell are constructed, used, and evaluated, and are intended to be purely exemplary of the invention and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers {e.g., amounts, temperature, film thickness, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C (Celsius) or is at ambient temperature, and pressure is at or near atmospheric.

EXAMPLE 1

[0056] In this example, a membraneless micro fluidic fuel cell 10 was created that integrated two reaction zones 20 into a single fuel cell. In one aspect, each reaction zone can be treated as an individual electrochemical cell (i.e., Cell 1 and Cell 2) with its own porous anode 110 and cathode 210, and external contact for current sourcing. Figure 6 illustrates the flow pattern in the multipass micro fluidic fuel cell 10 of this example. The fuel stream 105 and the oxidant stream 205 were introduced through a porous electrocatalyst. At the reaction zone of Cell 1, an electrolyte stream 305 directs the fuel through the first fluid passageway 100 and the oxidant through the second fluid passageway 200 channels leading from Cell 1 to Cell 2. The flow pattern was repeated in Cell 2, and terminated with the fuel and oxidant flowing to independent outlets.

[0057] The membraneless micro fluidic fuel cell 10 of this example was comprised of three PMMA layers fabricated using a carbon dioxide laser ablation system (M360, Universal Laser Systems, Scottsdale, AZ). The bottom layer had holes cut out for inserting 0.127 mm sections of platinum wire (SPPL-010, Omega Engineering, Stamford, CT) that served as current collectors. The wires came in contact with the electrodes which were 1 mm tall and 8 mm long stacked sheets of Toray carbon paper (E-TEK, Somerset, NJ) housed in the middle layer. The spacing between the electrodes (where the electrolyte stream 305 is introduced) was 1 mm. The active top projected electrode area in the cell was 0.08 cm², and all absolute current and power numbers were normalized by this area. The top layer of the fuel cell sealed the assembly with holes cut out for fluidic access. Liquids were delivered to the cell using 1.5 mm Tygon™ tubing (EW-06418-02, Cole Parmer, Vernon Hills, IL) bonded to the ports with quick dry epoxy. The three PMMA layers were adhered using double sided adhesive Mylar (3M, St. Paul, MN).

[0058] The electrolyte 305 and both reactants 105, 205 were delivered to the fuel cell 10 by two independent programmable syringe pumps (KDS200, KD Scientific, Holliston, MA). Reactant flow rates ranged from 50 to 500 μl/min, and electrolyte flow rates ranged from 0 to 250 μl/min. The fuel cell leads were connected to a source meter (Model 2410, Keithley Instruments, Cleveland, OH) operating in galvanostatic mode. Polarization data for Cell 1 and Cell 2 was recorded with a source meter (Model 2410, Keithley Instruments, Cleveland, OH) and a potentiostat (VersaSTAT 4, Princeton Applied Research, Oak Ridge, TN) respectively. Cell 1 was held at a fixed current density, and Cell 2 was then completely polarized by galvanostatic steps. After every step, the voltage in Cell 2 required 10- 15 seconds to reach steady state. The steady state voltage was then time averaged and reported for 10 seconds. Then Cell 1 was held at another current density, and the polarization for Cell 2 was repeated.

[0059] Vanadium redox species in acidic media (V²⁺AV³⁺ at the anode 110 and

VO₂VvO²⁺ at the cathode 210) were used to characterize the fuel cell 10. Vanadium on bare carbon was chosen because of its relatively high activity and its relatively high open circuit potential. 50 mM V²⁺ and VO₂ ⁺ in 1 M sulfuric acid were prepared through electrolysis of VO²⁺.

[0060] Reactants for the cell were obtained by preparing 50 mM vanadium(IV) oxide sulfate hydrate (CAS 123334-20-3, Sigma Aldrich, St. Louis, MO) in sulfuric acid (CAS 7664-93-9, EMD Chemicals, Hibbstown, NJ) diluted to 1 M in 18.3 MΩ deionized water (Millipore, Billerica, MA). After mixing, a clear blue solution indicated the presence of the vanadium(IV) ion. An in-house electrolytic cell was fabricated using PMMA for the housing, Toray paper for the electrodes, and a Nafion membrane (NRE212, Fuel Cell Store, Boulder, CO) as the ion exchange medium. The electrolytic cell generated the oxidation states vanadium(II) and vanadium(V) from the stock vanadium(IV). At the cathode 210 (negative electrode) the reaction,

VO²⁺ + 2H⁺ + e^~ «→ V³⁺+H₂O E⁰ = 0.337 V vs. standard hydrogen electrode

("SHE") (1) occurred, followed by

V³⁺ + e^~ → V²⁺ E⁰ - -0.255 V vs. SHE (2)

At the anodic (positive) half cell the reaction,

VO²⁺ + H₂O → VO₂ ⁺ + 2H⁺ + e^" E⁰ = 0.991 V vs. SHE (3) occurred. To obtain V as the fuel, the charge balance required that the anodic half cell of the electrolysis setup be twice the volume of its cathodic counterpart. A nitrogen gas stream was constantly introduced to the cathodic half cell to maintain vanadium(II) stability.

At the fuel cell anode 110 the oxidation reaction

V³⁺ + e^" → V²⁺ E⁰ = -0.255 V vs. SHE (4) occurred and at the cathode the reduction

VO₂ ⁺ + 2H⁺ + e^" <→ VO²⁺ + H₂O E⁰ = 0.991 V vs. SHE (5) occurred.

[0061] Because two independent current sources were used, the performance of Cell

2 as a function of the operating conditions of Cell 1 condition was studied. As can be appreciated, cell to cell variations can be observed for stacked cells that reuse reactants. In this case, Cell 2 was downstream of Cell 1 and its potential was dependent on the local reactant concentration and flow conditions which can be modified by the operation of Cell 1.

[0062] Figures 7 A, 7B, and 7C show polarization data for the multi-pass fuel cell 10 operating at reactant/electrolyte flow rate ratios (in μl/min) of 50/25, 500/250, and 500/25, respectively. The open circuit potential (OCP) for Cell 1 was approximately 1.2 V for all flow rate cases. The OCP for Cell 2 decreased with increasing Cell 1 current density. This behavior can be more pronounced at low reactant and electrolyte flow rates. Additionally, interfaces with lower flow rates can be more susceptible to reactant crossover. Reactants at their counter electrodes can also reduce the average OCP of the cell, where the magnitude of this voltage loss can be dependent on the sourced current.

[0063] At higher reactant flow rates, an increase in potential from both Cell 1 and

Cell 2 was observed. At low reactant flow rates, the diffusion boundary layer at the electrocatalyst surface was thicker, resulting in poor transport of the reactant to the surface and thus greater potential losses relative to higher reactant flow rates.

[0064] Figures 7B and 7C show similar Cell 1 polarization at varying separating electrolyte flow rates (250 and 25 μl/min, respectively). As can be seen, the flow rate of the separating electrolyte had negligible effects on the polarization of Cell 1. For Cell 2, and at the same reactant flow rate, lower maximum current densities at higher separating electrolyte flow rates were observed. Again with reference to Figures 7B and 7C, higher electrolyte flow rates appear to be detrimental to the performance of Cell 2, and therefore the losses due to reactant dilution were more dominant.

[0065] While the primary interest is the power output of the overall stack, it is important to understand how the polarization of one cell in the stack influences that of downstream cells. In this example, the potential of Cell 2 decreased with increasing Cell 1 current density, regardless of the flow rate used. The magnitude of the potential loss of Cell 2 increased with decreasing reactant flow rate, increasing electrolyte flow rate, and increasing Cell 1 current density. These parameters respectively result in mass transport losses, reactant dilution, and reactant utilization as it flows through the stack. All three aforementioned results reduce the potential of Cell 2 at each current density. For example, Figure 7A shows that at the 50 μl/min reactant flow rate, when Cell 1 is at its highest current density, the highest current density of Cell 2 is approximately 25% of that value. As illustrated in Figure 7C, for example, at 500 reactant and 25 μl/min electrolyte flow rate, the highest current density of Cell 2 is approximately equal to that of Cell 1, regardless of the current density of Cell 1.

[0066] The multi-pass fuel cell 10 allows for on-chip reactant recycling and can be analyzed as a single microfluidic fuel cell. In one aspect, Cell 1 and Cell 2 were electrically connected in parallel, i.e. Cell 1 and Cell 2 had a common anode 110 and common cathode 210. When the reactants are subjected to two ion exchange zones the effective cross sectional area of the fuel cell doubles. This area increase results in an effective reduction in overall Ohmic losses since the fuel cell resistance can be approximated as R=g/σA, where g is the length between the electrodes, σ is the conductivity of the solution in the gap, and A is the cross sectional area of the ion exchange zone. Figure 8 compares polarization and power density curves between a single and the multi-pass microfluidic fuel cells electrically connected in parallel. In this case, the reactant and the separating electrolyte flow rates are 500 and 25 μl/min, respectively. The Ohmic loss differences were distinct between the two cases, where the slope of the linear region of the stacked cell was approximately half that of the single cell. Curvature in the polarization curves that would typically be associated with activation or mass transport losses was not readily observed. As vanadium redox species exhibit fast electrode kinetics on bare carbon, and at 500 μl/min mass transport losses were delayed, the majority of the polarization curve reflected the Ohmic losses.

[0067] The peak power density (and its respective current density) increased from

7.5 to 16 mW/cm² upon doubling the number of fuel cell passes. The maximum fuel utilization also increased from 6% to 11%, calculated as

η = (6) nFCQ where i is the maximum measured current density, A is the top projected electrode area, n is the number of electrons transferred, F is Faraday's constant, C is the concentration of the fuel, and Q is the fuel flow rate.

[0068] Equation (6) describes the fuel utilization calculated from the maximum current density from a single polarization curve. However, the overall fuel utilization given independent polarization curves from Cell 1 and Cell 2 is calculated by:

[0069] Figure 9 plots the overall fuel utilization using equation (7) for the flow rate cases of 50/25 and 500/25, shown in filled circle and square symbols respectively. The open symbols reflect the first term of equation (7) and are shown for comparison. In both flow rate cases, the maximum fuel utilization from solely using Cell 1 was approximately equal to the overall fuel utilization of Cell 1 and Cell 2 (ij_o) when z^'/=0. The value of η_o also increased when ij increased. However, the degree of this increase depended on the flow rate. For example, for the 50/25 flow rate case, the difference in fuel utilization was approximately 0.15 when /_/=0, and approximately 0.075 when ij =7.5 mA/cm². For the 500/25 flow rate case, the difference in fuel utilization was approximately 0.06 for both

mA/cm². At low reactant flow rates and high current densities from Cell 1, Cell 2 received a lower fraction of reactants which resulted in an overall decrease in reaction rates, and thus lower contributions to fuel utilization from Cell 2. Figure 9 also illustrates that low flow rate fuel utilization can be achieved by increasing the number of passes at high flow rates. In this configuration, both high fuel utilization and power output from the fuel cell 10 can be achieved.

[0070] Both the applied voltage and current density define the efficiency of the fuel cell,

where Vop is the fuel cell voltage, and V_R is the reversible voltage. The first term in equation (8) is the thermodynamic efficiency, and the losses due to this term are demonstrated by the shaded areas in Figures 1OA and 1OB. In a single fuel cell, χis maximized at peak fuel cell power, as shown in Figure 1OA. However, the overall efficiency remains at about 50%. To operate at high thermodynamic efficiency, like the case shown in Figure 1OB, fuel utilization must be sacrificed due to a low value of i_jli_max- The multipass fuel cell 10 offers a microfluidic solution to this compromise by extracting z, from each pass and therefore having the overall efficiency:

[0071] The summation in equation (9) is over the total number of passes in the design, each pass designated by the subscripty. The multi-pass architecture decouples the thermodynamic and Faradaic efficiencies. This allows the designer or engineer to independently set the operating fuel cell voltage and current by simply tuning the number of passes the reactants flow through.

[0072] Parallel flow based laminar flow fuel cells suffer from complications in mass transport boundary layer growth over flat plate electrodes and diffusive broadening at the reaction zone of fuel and oxidant. Previous microfluidic fuel cell designs used porous electrocatalysts to maximize reaction surface area and brief ionic reaction zone zones where advection occurs in the direction of reactant concentration gradients. The multi-pass fuel cell 10 of this application successfully recycles reactants from one cell to the other through the use of multiple reaction zones 20, which increases both the overall fuel cell power and efficiency of the fuel cell. The influence of one reaction zone on the next is prominent at low reactant flow rates and high current densities. When the two reaction zones are interconnected to form a single cell, double the peak power density and fuel utilization is achieved in comparison to just using one reaction zone. This design allows independent and uncoupled control over desired potentials and current densities through prescribing a specific number of reaction zones.

[0073] In one aspect, the multi-pass fuel cell 10 separates of the reactants throughout the device. In another aspect, a high conductivity ionic exchange reaction zone reduces reactant diffusive mixing. In still another aspect, porous electrocatalysts increase available reactions surface area. This multi-pass fuel cell results in an effective increase in the ionic exchange cross sectional area by repeating reaction zones exhibiting reduced diffusive mixing as opposed to a single extended diffusive reaction zone, reducing Ohmic losses in the fuel cell.

[0074] Although several embodiments have been disclosed in the foregoing specification, it is understood by those skilled in the art that many modifications and other embodiments of the invention will come to mind to which the invention pertains, having the benefit of the teaching presented in the foregoing description and associated drawings. It is thus understood that the invention is not limited to the specific embodiments disclosed herein above, and that many modifications and other embodiments are intended to be included within the scope of the appended claims.

[0075] Moreover, although specific terms are employed herein, as well as in the claims which follow, they are used only in a generic and descriptive sense, and not for the purposes of limiting the described invention, nor the claims which follow.

Claims

CLAIMSWhat is claimed is:

1. A fuel cell comprising: a first fluid passageway comprising at least one anode; a fuel port in fluid communication with the first fluid passageway for supplying a fuel stream to the first fluid passageway; a second fluid passageway comprising at least one cathode; an oxidizer port in fluid communication with the second fluid passageway for supplying an oxidizer stream to the second fluid passageway; an electrolyte passageway positioned at a first reaction zone between the first and second fluid passageways comprising an electrolyte stream, wherein the electrolyte stream is in fluid communication with the fuel stream and the oxidizer stream and creates and maintains a separated interface between the fuel stream and the oxidizer stream.

2. The fuel cell of claim 1, wherein the fuel cell further defines a plurality of reaction zones between the fuel stream, the oxidizer stream, and the electrolyte stream wherein in each reaction zone an electrolyte stream creates and maintains separated interfaces between the fuel stream and the oxidizer stream.

3. The fuel cell of claim 2, wherein the electrolyte stream is substantially laminar.

4. The fuel cell of claim 2, wherein each consecutive reaction zone is in a downstream direction with respect to the fuel stream.

5. The fuel cell of claim 4, wherein each consecutive reaction zone is in a downstream direction with respect to the oxidizer stream.

6. The fuel cell of claim 5, wherein each consecutive reaction zone comprises unreacted fuel or oxidizer from a previous consecutive reaction zone.

7. The fuel cell of claim 4, wherein the first fluid passageway comprises a plurality of anodes.

8. The fuel cell of claim 7, wherein at least one anode of the plurality of anodes is positioned substantially adjacent each reaction zone.

9. The fuel cell of claim 4, wherein the second fluid passageway comprises a plurality of cathodes.

10. The fuel cell of claim 9, wherein at least one cathode of the plurality of cathodes is positioned substantially adjacent each reaction zone.

11. The fuel cell of claim 1, wherein the electrolyte passageway comprises an electrically insulating material.

12. The fuel cell of claim 11, wherein the electrically insulating material is porous and is configured to allow the electrolyte to flow therethrough.

13. The fuel cell of claim 1, wherein at least a portion of each anode is porous and is configured to allow the fuel and electrolyte to flow therethrough.

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

15. The fuel cell of o claim 1, wherein each anode and each cathode comprises a catalyst.

16. The fuel cell of claim 1, wherein each anode and each cathode is electrically conductive.

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

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

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

20. A fuel cell comprising: a fuel stream in fluid communication with at least one anode; an oxidant stream in fluid communication with at least one cathode; an electrolyte stream in fluid communication with the fuel stream and the oxidizer stream; and means for maintaining a separated interface between the fuel stream and the electrolyte stream; wherein the at least one anode oxidizes fuel from the fuel stream and generate electrons for conduction to a load, and wherein the at least one cathode receives electrons from the load to reduce oxidant from the oxidant stream when the oxidant comes into contact with the at least one cathode.