WO2005104279A2 - Membrane free fuel cell - Google Patents

Abstract

The invention is directed to a fuel cell comprising an anode (170) and a cathode (180) immersed in an electrolyte in the presence of a reductant and oxidant, wherein the distinguishing feature is that all of the elements of the fuel cell communicate freely with each other and there is no membrane (160) or other separator between the anode and the cathode.

Description

TITLE MEMBRANE FREE FUEL CELL This patent application claims the benefit of United States Provisional Application 60/508424, filed October 3, 2003. FIELD OF INVENTION The invention is directed to a fuel cell. Specifically, the fuel cell lacks a membrane or other barrier between the anode and cathode, thereby permitting mixing of the oxidant and reductant and non-restricted communication between all elements of the fuel cell. BACKGROUND There is much interest in renewable power generation, particularly fuel cells. A fuel cell is an energy conversion device that invariably comprises two electrodes, i.e., a cathode and an anode, upon which electrochemical reactions occur to enable the transformation of stored chemical energy into electrical energy. Fuel (e.g., hydrogen, methanol) is oxidized at the anode to release electrons that are then routed through an external circuit, while protons are transported through a proton exchange membrane to the cathode. The circuit is completed inside the fuel cell when the electrons are received back again from the external circuit at the cathode, where they combine with protons and oxygen atoms in a reduction reaction to produce water. The output of the fuel cell then is electrical energy and heat, produced by the production of water from protons and oxygen. Typically, catalysts are incorporated into the anode and cathode electrodes to facilitate fuel oxidation and oxygen reduction. Current preferred catalysts for fuel cells are particulate noble metals; however, these metals are expensive, inherently inefficient, non-renewable and not easily characterized. For these reasons, substitution of noble metals with homogeneous redox catalysts is a desirable goal; but, low current densities (which result in inadequate power and/or volume) have made this approach uneconomical with previously disclosed systems. Use of enzymatic catalysts permit the incorporation of the redox catalyst into fluidic microdomains and thereby makes higher current densities possible due to-: 1.) locally high concentrations of catalyst (since the catalyst is not confined to one monolayer density); 2.) high electron diffusion coefficients; and, 3.) opportunities for convective transport. Addtionally a redox catalyst that is enzymatic can easily use bio-available energy sources such as glucose. The redox enzyme in an enzymatic biofuel cell participates in an electron transfer chain at the anode by oxidizing the fuel. However, redox enzymes are incapable of direct contact with the electrode since their redox centers are insulated from the conductive support by their protein matrices (Katz et al., "Biochemical fuel cells", In Handbook of Fuel Cells - Fundamentals, Technology and Applications, 1 , Ch. 21 (2003)). In order to bring these enzymes into contact with the electrode and to improve the electron transfer rate, an initially oxidized electron transport redox mediator is used to reoxidize the enzyme. The electrons are then transferred to the anode and the electron transport mediator is once again oxidized. A similar process occurs at the cathode. Numerous examples of enzymatic biofuel cells are described in the literature for use in sensor applications where it is desirable to use electrodes in vivo at low power. However, immobilization of the redox catalyst and/or electron transport mediator and restricted access of the electrochemical reagents to the redox catalyst immobilized inside the electrode limit the current densities that can be obtained, limiting the commercial practicablity of these biofuel cells. Additionally, the majority of these systems require the presence of a membrane or other barrier, separating the anode and cathode, which compartmentalizes the reductant and oxidant. Such membranes add cost to the fuel cell and limit miniaturization. In co-pending U.S. Patent Application No. 60/508424, the

Applicants teach that the low current densities of a biofuel cell may be overcome by constructing a fuel cell comprising an electrode consisting of a nriicroporous current collector that incorporates a multitude of domains, wherein each domain contains a soluble redox catalyst. Neither the catalyst nor the electron transport mediator are immobilized in any fashion within the domain but are instead in fluid association with one another. The fluid association of these elements of the electrode is unique and responsible for a more efficient system than heretofore described. In spite of the advance in the art provided by this system, preferred electrodes of this type still require a membrane or presence of a membrane or other barrier to separate the anode and cathode. It is well accepted in the art of fuel cell design and manufacture that a membrane or other barrier is required to physically separate the anode and cathode. As such, a tremendous volume of literature exists that describe preferred materials for proton exchange and details of each material's proton transport and conductivity, etc. Recent work by Mano, N. et al. (J Am Chem Soc. 2002 Nov 6; 124(44): 12962-12963; Chem Commun (Camb). 2003 Feb 21 ; (4):518-519; J Am Chem Soc. 2003 May 28; 125(21 ):6588-94; Proceedings, Electrochemical Society 2003, 2002-25 (Micropower and Micro-devices):176-182; and WO 03/106966 A2; see also, Heller, A. "Miniature Biofuel Cells" (Review) Phys. Chem. Chem. Phys. 2004, 6:209-216) has suggested that modifying the conformation of the fuel cell elements may mitigate the need for a membrane per se. Specifically, this group describes miniature membrane-less biofuel cells, that operate at up to 0.78 V under physiological conditions. The anodic electrocatalyst was comprised of a glucose oxidase and an anode redox polymer mixture, crosslinked onto the surface of a carbon fiber in the form of a swollen hydrogel; likewise, the cathodic electrocatalyst was created by a hydrogel of bilirubin oxidase and a cathode redox copolymer crosslinked onto the surface of a second carbon fiber. Although no membrane is used in this system per se, immobilization of the cathode and anode in the hydrogel effectively produced a barrier, separating these elements of the cell. The work of S. Topcagic and S. Minteer (Abstracts of Papers, = 227th ACS National Meeting, Anaheim, CA, March 28-April 1 , 2004; American Chemical Society, Washington, D. C.) teach a membraneless ethanol/oxygen biofuel cell. Specifically, these authors have utilized enzymes at the cathode and anode. However, the enzymes are immobilized and may not be freely accessible to the fuel. A need exists therefore for the development of a membrane-free biological fuel cell having redox catalyst- or enzymatic redox catalyst- based electrodes that are capable of generating useful current densities. Applicants have solved the stated problem by the design of a fuel cell electrode that comprises the redox catalyst and substrate in fluid association with each other within a microdomain of the electrode; and when these electrodes are used in a fuel cell, no membrane is required for the separation of the anode and the cathode and requisite components. SUMMARY OF THE INVENTION The invention is directed to a fuel cell lacking a membrane for the separation of the cathode and anode and the attendant redox reactants associated with the cell. Accordingly it is within the context of the present invention to provide a fuel cell comprising: (i) an anode configured and arranged for electro-oxidizing an anode reductant; (ii) a cathode configured and arranged for electro-reducing a cathode oxidant; and (iii) an electrolyte in solution contained within a single compartment; wherein the anode and cathode are spaced apart and reside in the single compartment containing the electrolyte; and wherein the electrolyte, reductant and oxidant freely communicate with the anode and the cathode. Fuel cells of the invention will, in preferred embodiments, comprise fuel cell electrodes comprising a porous current collector comprising an electrical conducting material in contact with a multitude of domains, each domain containing: a) a redox catalyst; and b) an electron transport mediator; . wherein the redox catalyst and electron transport mediator are in fluid association with each other within the domain and wherein the redox catalyst and the electron transport mediator are bounded in the domain by the porous current collector and a containment material. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a typical fuel cell comprising an anode compartment and a cathode compartment, separated by a membrane or other barrier. Figure 2A illustrates a preferred electrode for use in the membrane- less fuel cell of the invention. Figure 2B illustrates the preferred electrode, with domains formed by the association of particulate current collectors. DETAILED DESCRIPTION OF THE INVENTION The invention provides a fuel cell that lacks a membrane or other barrier between the anode and cathode, thereby permitting mixing of the oxidant and reductant and non-restricted communication between all elements of the fuel cell. The present invention advances the art in that the elimination of the membrane reduces the cost and contributes to the miniaturization of the fuel cell. Addtionally the present fuel cell is unique in that it is characterized by unique electrodes that consist of a microporous current collector that incorporates a multitude of domains, wherein each domain contains a soluble redox catalyst. Neither the catalyst nor the electron transport mediator are immobilized in any fashion within the domain, but are instead in fluid association with one another; this unique association overcomes problems associated with low current densities and thereby makes the fuel cell herein much more efficient than previously described systems. The fuel cell herein comprises electrodes consisting of a microporous current collector incorporating a multitude of domains, wherein each domain contains a homogeneous redox catalyst and an electron transport mediator in fluid association with each other. The electron transport mediator is useful for electron transport between the redox catalyst and electrode surface. Preferably each domain is no more than about 100 μm across. The electrode has the advantage of being free of noble metals, improving the performance of the present fuel cell as compared with the art. The present electrode is designed to operate in a membrane-less fuel cell capable of using compounds from biological systems as fuel; e.g., fuel cells can be made that oxidize biochemicals available in the body of an animal, a plant or in plant residue. Definitions The following abbreviations and definitions may be used for the interpretation of the specification and the claims: "Electron transport mediator" is abbreviated as ETM. "2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate)" is abbreviated as

ABTS. "Redox reaction" or "reduction-oxidation reaction" means a reaction in which electrons are transferred between species or in which atoms change oxidation number. The electron-donating molecule is the "reducing agent" or "reductant"; the electron-accepting molecule is the "oxidizing agent" or "oxidant". Reducing and oxidizing agents function as conjugate reductant-oxidant pairs or redox pairs; thus, they undergo the reaction: Oxidant + n e^" → Reductant; and Reductant → Oxidant + n e^". The term "substrate molecules" will refer to reductant and/or oxidant. "Redox catalyst" means a catalyst that catalyzes a redox reaction. "Electrode" refers to an electric conductor through which an electric current enters or leaves a medium, whether it be an electrolytic solution, solid, molten mass, gas or vacuum. "Cathode" means the electrode to which electrons migrate from the outside circuit and is the electrode where reduction occurs. "Anode" means the electrode from which electrons migrate from the outside circuit and is the electrode where oxidation occurs. "Current collector" refers to the object at which current is available in an electrode. More specifically, the term is used to describe the conductive material in a fuel cell that collects electrons (i.e., on the anode side) or disburses electrons (i.e., on the cathode side). The current collectors are porous or microporous (to allow for fluid flow through them) and lie in between the catalyst/electrolyte surfaces and the bipolar plates. "Electrolyte" refers to a non-metallic electrical conductor in which current is carried by the movement of ions. The electrolyte may also contain the electron transport mediator (ETM) or the ETM may be present in a domain matrix. "Electron transport mediator" (ETM) means a substance that provides transport between the redox catalyst and electrode surface. "Membrane" or "barrier" refers to a separating layer in a tranditional fuel cell that acts as electrolyte (i.e., an ion-exchanger) as well as a barrier film separating the substrates in the anode and cathode compartments. "Free communication", as used with respect to the communication that may occur between an electrolyte, reductant and oxidant and the anode and cathode of a fuel cell as described herein, refers to non- restricted communication between all elements of the fuel cell. Specifically, the fuel cell described herein lacks a membrane or other barrier between the anode and cathode, thereby permitting mixing of the oxidant and reductant. As used herein, the terms "domain", "microdomain" and "domain matrix" will be used interchangeably and refer to an area within the electrode of the invention defined by the boundary of the current collector and either the containment material or the surface of another current collector. Typically domains are roughly spherical and have diameters of about 10 to about 100 μm. The term "fluid association" as applied to the ETM and the redox catalyst means that neither the ETM nor the catalyst are immobilized in any fashion and are free to move in fluid fashion within the domain. Membrane-Less Fuel Cell

17 Typical fuel cells are represented by the schematic shown in Figure

1. Referring to Figure 1 , the fuel cell consists of: 1.) a cathode compartment (140) containing a cathode (180); and 2.) an anode compartment (150) containing an anode (170). These two compartments are traditionally separated by a membrane (e.g., an ion exchange membrane), shown herein by a dotted line, (160) or other barrier between the anode and cathode that prevents mixing of the oxidant and reductant. The anode and cathode are connected though an external conductor which can also pass through a load (190) to produce useful work. Generally, each compartment contains an electrolyte into which the electrodes are immersed and in intimate contact; or,^' alternatively, the electrodes are in physical contact with the membrane, obviating the need for electrolyte. In the present invention, the fuel cell is distinguished in part by the absence of a membrane (160) or other barrier between the anode and cathode, thereby permitting mixing of the oxidant and reductant and non- restricted communication between all elements of the fuel cell. In preferred embodiments of the invention, the membrane-less fuel cell comprises an anode that may consist of a porous current collector containing microdomains, wherein the anode microdomains contain a redox catalyst and associated ETM for oxidation of an oxidizable substrate. Similarly, the cathode may consist of a porous current collector containing the microdomains, wherein the cathode microdomains contain a redox catalyst and associated ETM for reduction of a reducible substrate. Substrates (i.e., oxidizable and reducible) may be present in the electrolyte or added continuously. Oxidizable and Reducible Substrates As one of skill in the art is clearly aware, the particular substrates selected for use in a particular fuel cell are dependent on the configuration of the anode and cathode. More specifically, the particular substrate selected for use is dependent on the redox catalyst that the electrode contains. In preferred embodiments, the membrane-less fuel cell comprises a cathode that is configured and arranged for electro-reducing oxygen and an anode that is configured and arranged for electro-oxidizing hydrogen, alcohols (e.g., methanol), carbohydrates (e.g., glucose), carboxylic acids (e.g., formic acid) or carboxylic esters (e.g., methyl formate). Electrolytes The function of the electrolyte in a fuel cell, in which protons are generated on the anode, is to expedite transportation of those protons to the cathode where reaction with an oxidant takes place. In a membrane containing fuel cell, a proton exchange membrane serves to separate the anode from the cathode and also serves to conduct protons from the anode to the cathode. In a membrane free fuel cell, common electrolytes facilitate the movement of protons to the cathode. Examples of electrolytes include, but are not limited to: salts, acids and bases. The compounds may be in the form of dissolved salts, acids, or bases or may be in the form of polymeric salts, acids or bases. Preferred are salts that are also buffers. Examples include, but are not limited to, those salts containing phosphates, citrates and acetates. Especially preferred are salt buffers in the pH range of about 2-7. '

Electrode Structure The electrode for use in the membrane-less fuel cell of the invention is described more fully in Figure 2A. Referring to Figure 2A, an electrode element (10) is comprised of a porous or micro-porous current collector (20), in contact with a multitude of domains (optionally created by a containment material (50)), each domain containing a redox catalyst (60) and an ETM (70). Each of these components will be discussed below in detail. 1. Current Collector The current collector may take a variety of forms and be made of a number of different materials, but the result is a porous or microporous object within the electrode wherein electrons are available. The current collector forms one boundary of a domain (40). Components of the collector can be, but are not limited to: conductive fibers, particles, films, or combinations thereof, wherein the components are arranged to form the collector so that the collector is conductive in its entirety. Preferably the current collector comprises an electrical conducting material (30); for example, the material may be carbon, noble metals, or any other conductive material that is inert to the reactive conditions at the electrodes and that is capable of being formed into the element. Thus, non-limiting examples of a suitable electrical conducting material are: carbon paste, 5 carbon nanotubes, carbon particles, conducting polymer particles, noble metal particles, conducting metal oxide particles and carbon paper. The size of the components of the collector may be varied; but, in a preferred embodiment, the volume will be minimized so as to maximize the surface area available for current collection. Furthermore, the size of 10 the components must be consistent with the preferred domain size. The macroscopic form of the current collector composed of these elements may be, but is not limited to: paper, cloth, porous monoliths, particle aggregates and arrays of these forms. In an alternate embodiment to that shown in Figure 2A, an

15 electrode having similar function and interaction for use in the membrane- ; less fuel cell of the invention herein: may be formed with no containment material present. Specifically, the electrode may be formed by the association of electrical conducting materials, since the conducting materials form a boundary by virtue^; of their association (e.g., the

20 juxtaposition of several nanoparticles that comprise current collectors). Referring to Figure 2B, an electrode element (10) is comprised of a porous or micro-porous current collector (20). Preferably the current collector comprises an electrical conducting material (30). The current collector may take a variety of forms. For example, the current collector

,25 may be a porous conducting film or a collection of smaller conducting particles (e.g., formed into a film or monolith or any other shape or array of conducting particles, formed by any printing or stamping technique known in the art). The current collector forms the boundary of a domain (40). In one embodiment the diameter of the domain (40) is less than about 100

30 μm, where less than about 10 μm is preferred. The entire electrode consists of a multitude of this structure in continuous contact. Optionally, the entire electrode containing the multiplicity of current collectors and domains may be constrained by a containment material. 2. Containment Material

35 Structurally, the containment material (50) may form a boundary of the microdomain (although it allows free flow of substrates and products to and from the microdomain). In a preferred embodiment, the distance from the wall of the current collector (30) to the containment material (50) is less than about 100 μm, where less than about 10 μm is preferred. The entire electrode consists of a multitude of this structure in continuous contact. The containment material must be capable of being formed into a size and shape consistent with the preferred size and shape of the microdomain. Functionally, the containment material prevents loss of the redox catalyst and ETM from the domain. Thus, one possible form of the containment material of the invention is a permeable film of such a thickness so as to be consistent with the preferred domain size and of such a permeability so as to prevent egress of the redox catalyst and ETM from the domain. In another embodiment, a precursor to the containment material may be deposited on the collector element in precursor form and formed via polymerization of the precursor in place. A number of materials will be useful as containment materials, e.g., polyelectrolyte polymers, permeable polymers, porous metal oxide-based coatings, organically modified metal oxides and fluorine-containing polymers. Preferred in the present invention are containment materials that take the form of films. The composition of the films may be inorganic (e.g., composed of silica, titania, alumina, etc.), organic (e.g., composed of polyesters, polyamides, polyimides, fluoropolymers, polyelectrolytes, etc.) or organic/inorganic (e.g., composed of organo-substituted silica, silicones, titania, etc.). Likewise, film precursors may be inorganic, organic or organic/inorganic in nature; non-limiting examples are tetraalkoxysilanes, organoalkoxy silanes, alkoxytitanates, and organic polymer precursors such as diamines, diacids, etc. Additionally the containment material may be silica or colloidial carbon. 3. Redox Catalysts The invention provides a membrane-less fuel cell electrode comprising a redox catalyst. Specifically, the redox catalyst (60) and an ETM (70) are found within a domain. The redox catalyst (60) and the ETM (70) are not immobilized, but instead are in fluid association with each other and are free to move within the domain. The function of the redox catalyst is to catalyze an electrochemical reaction of an anode reductant or cathode oxidant, respectively. Redox catalysts may be comprised of species capable of reversibly transferring electrons, including (but not limited to) enzymes and organometallic redox complexes. Preferred redox catalysts of the invention are enzymes. A variety of enzymes are useful on the cathode including, for example: laccase and cytochrome C oxidase for electroreduction of oxygen; and, peroxidases for electroreduction of hydrogen peroxide. Similarly, useful enzymes on the anode include: hydrogenases for the electrooxidation of hydrogen; oxidases and dehydrogenases for electrooxidation of methanol, other alcohols, glucose, lactate and other substrates; alcohol oxidase, formaldehyde dehydrogenase and formate dehydrogenase for electrooxidation of methanol; pyranose oxidase for electrooxidation of D-glucose, L-sorbose and D-xylose; and, glucose oxidase, oligosaccharide dehydrogenase and pyrroloquinoline quinone (PQQ) glucose dehydrogenase for electrooxidation of glucose. A non- limiting list of enzymes useful in the present invention is given in U.S. Patent No. 6,294,281 , hereby incorporated by reference. Preferred for use at the cathode and anode in the present invention are those enzymes selected from the oxido-reductase group, a group containing (but not limited to): laccase, ascorbate oxidase, cytochrome c oxidase, multi-copper oxidases, bilirubin oxidase, blue copper oxidases, alcohol oxidase, formaldehyde dehydrogenase and formate dehydrogenase, L-lactate dehydrogenase, malate dehydrogenase, glucose oxidase, microbial pyruvate oxidase, and catechol oxidase. Laccases are most preferred at the cathode in the invention herein. In general, laccase (polyphenol-oxidase [EC 1.10.3.2]) is a multicopper oxidase that couples the one-electron oxidation of four substrate molecules to the four-electron reduction of dioxygen to water. Thus, laccase is useful for the biocatalytic reduction of dioxygen to water in electrochemical applications such as biofuel cells and biosensors. Several genes that encode different isoforms of laccase have been isolated and sequenced (e.g., Trametes versicolor, T. pubescens, Coriolus hirsutus and Pleurotus ostreatus); and, much work has been performed to biochemically characterize these enzymes (Galhaup C, et al., Microbiology. 2002 Jul;148(Pt 7):2159-2169; Leitner C, et al., Appl Biochem Biotechnol. 2002 Spring;98-100:497-507; Galhaup C, et al., Appl Microbiol Biotechnol. 2001 Jul;56(1 -2):225-232; Gorbatova ON, et al., Prikl Biokhim Mikrobiol. 2000 May-Jun;36(3):272-277). 4. Electron Transport Mediators Electron transport mediators (ETMs) (70) are in fluid association with the redox catalyst (60) in the electrode's domains. More specifically, the ETM (70) and the redox catalyst (60) are not immobilized, but instead are in fluid association with each other and are free to move within the domain. ETMs serve to facilitate the transport of electrons from the current collector to the redox catalyst; and, their presence generally increases the efficiency of the electrode. One feature of the ETM is that its redox potential must be greater than (more positive than) or equal to the redox catalyst potential in the anode. Furthermore, the redox potential must be less than (more negative than) or equal to the redox catalyst potential in the cathode. Additionally, it is advantageous if the ETM is a molecule that can transfer electrons rapidly and reversibly with both the redox catalyst and the current collector. Another desirable feature is that the ETM not react directly with the substrate molecules and that it be stable in both the oxidized and reduced states. Additionally, it is preferred that the ETM have a large diffusion coefficient in fluid media. Examples of the general classes of molecules that can function as ETMs are: quinones, hydroquinones, metalloorganic complexes, conjugated unsaturated acyclic and cyclic molecules (including aromatic systems), metal ions with and without ligands and biological electron carriers. One ETM that is particularly suitable in the present invention is 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS). The Electrode's Functionality in a Membrane-less Fuel Cell When the electrode functions as a cathode in the fuel cell of the invention, current is available at the current collector. Specifically, the following series of reactions occur: 1. A freely diffusing oxidized ETM molecule interacts with the current collector; 2. During the interaction, an electron(s) is transferred to the ETM, whereupon it becomes more reduced; 3. The reduced ETM then encounters a freely diffusing oxidized redox catalyst, whereupon an electron(s) is transferred to the redox catalyst to thereby reduce it; 4. The reduced redox catalyst can interact with a consumable reducible substrate molecule (e.g., oxygen), whereupon an electron(s) is transferred to the substrate. This completes the catalytic action of the redox catalyst. In the case where the electrode acts as an anode, the flow of electrons is reversed and an oxidizable substrate (e.g., methanol) provides the electrons that are transferred to the current collector via the redox catalyst and ETM. In both the anode and cathode, substrate molecules enter the electrode from spaces bounding the electrodes via permeation through the containment element of the electrode. Similarly, reaction products from oxidation or reduction of the substrates egress through the permeable containment element into the interstitial spaces bounding the ■ electrodes. While substrate and product molecules are free to permeate through the containment element, the redox catalyst and ETM are not. Commerical Parameters of a Membrane-less Fuel Cell Fuel cell systems that are able to attain current densities of 50 mA or greater are particularly useful for commercial applications. An optimal system that would allow for a current density of greater than 50 mA would have: • a microdomain with about 4 x 10^-9 moles of a redox catalyst and an ETM in a concentration of about 5 x 10^-3 M; and • an effective volumetric surface area of the microporous current , collector in excess of about 100 cm²; wherein all of the redox catalyst resides within about 3 μm of the effective current collector surface and all of the effective current collector surface resides within about 10 μm of a reservoir of air-saturated aqueous buffer. Preferred systems will include cathodes where the ETM has a redox potential that is less than 0.3 V positive of the redox potential of the catalyst, where less than 0.1 V positive of the redox potential of the catalyst is preferred. Similarly, preferred anodes will contain an ETM having a redox potential that is less than 0.3 V negative of the redox potential of the catalyst, where less than 0.1 V negative of the redox potential of the catalyst is preferred. EXAMPLES The present invention is further defined in the following Examples. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. GENERAL METHODS The meaning of abbreviations is as follows: "h" means hour(s), "min" means minute(s), "sec" means second(s), "d" means day(s), "μm" means micron(s), "μL" means microliter(s), "mL" means milliliters, "L" means liters, "μM" means micromolar, "mM" means millimolar, "M" means molar, "mA" means milliampere(s) and "V" means volt(s). The redox convention used is that where the standard hydrogen electrode is 0 V. Preparation Of Laccase Concentrate Two grams of dry fermentation extract containing laccase from the white rot fungus Trametes versicolor (obtained from Wacker Chemie, Germany) was suspended in 40 mL of Bis-Tris propane buffer 20 mM pH 6. The solution was spun for 10 min at 20,000 rpm (50,000 g) in an SS-34 (Sorvall^®) rotor. The supernatant was poured off and the pellet resuspended in an additional 40 mL of the same buffer. The suspension was again spun at 20,000 rpm for 10 min in the same rotor. The supernatants were pooled and concentrated in an Amicon concentrator to 20 mL using a YM-30 (Millipore) membrane. The solution was diluted up to 200 mL with the Bis-Tris propane buffer and concentrated back down to 15-20 mL. This step was repeated once more to give a final volume of 20 mL. The enzyme solution was aliquotted into 0.25 mL volumes in Eppendorf tubes and quick frozen in liquid nitrogen. The samples were stored in an -80 °C freezer until use. For each experiment described below, a 60 nM solution of laccase in 20 mM pH 6 Bis -Tris propane buffer was used. Chronoamperometrv For The Determination Of Electrode Function Electrodes were evaluated via the electrochemical measuring technique of chronoamperometry. Specifically, a constant voltage was applied to an electrode and the resultant current was recorded as a function of time. In the case of a cathodic electrode, the measured current results from consumption of electrons at the cathode by substrate. The cathodes of the present invention consume electrons via laccase- catalyzed reduction of oxygen to water. Electrons are transported to the laccase via an electron transport mediator (ETM). The initial pH of the electrolyte was set at 3 for optimal laccase performance and because of the need to supply protons to complete the reduction of oxygen to water. The electrolyte may also contain the ETM or the ETM may be present in the domain matrix. EXAMPLE 1 Laccase Cathodes With Various Containment Materials This example demonstrates the construction of a variety of laccase electrodes, wherein the current collector was carbon paper and the containment material was silica or colloidial carbon. Silica As Containment Material Laccase solution (12 μl) was applied to the bottom portion of a strip of Toray carbon paper (360 μm thick, 6 mm wide and 7 cm long). After drying, 24 μl of 20 mM ABTS in 50 mM pH 3 phosphate buffer was added to the same area as the laccase and dried. The applied portion of the paper was dipped into a stirred mixture of 200 μl 50 mM pH 3 phosphate buffer and 2 mL of tetramethoxysilane. The dipped paper was cured in a chamber at nearly constant 100% humidity for 2 days. ■ The activity of the electrode was tested by immersing the electrode in 28 mL of 50 mM pH 3 phosphate buffer electrolyte and 1 mM ABTS. Also immersed were a SCE reference electrode and a Pt coil counter electrode. All of these components were connected to the leads of a potentiostat. A voltage of 0.2 V was applied to the cathode and current draw measured by the potentiostat versus time. A current of 1.6 mA/cm² of cathode surface was measured after 1800 sec in bubbling oxygen. To demonstrate electrode stability, the electrode was then removed from the ABTS electrolyte and immersed¹ in fresh ABTS electrolyte and the chronoamperometry procedure repeated. After another repeat of this procedure, the current was 80% of the original value. This demonstrated that the laccase catalyst was effectively contained within the domain matrix and retained activity. Colloidal Carbon As Containment Material Laccase solution (12 μl) was applied to the bottom portion of a strip of Toray carbon paper (360 μm thick, 6 mm wide and 7 cm long). Additionally, 24 μl 20 mM ABTS in 50 mM pH 3 phosphate buffer and 100 μl of RTU colloidal carbon (Alfa Aesar, Catalog #41774) was applied to the same area as the laccase. The paper was dried for 2 hrs. Electrode activity was tested in a similar method as described above. Specifically, the electrode displayed 2.5 mA/cm² current after 1200 sec; in a similar series of experiments to demonstrate the stability, the electrode retained 100% of its original value. EXAMPLE 2 (COMPARATIVE) Evaluation Of Inferior Laccase Cathodes This example describes the performance of cathodes that lacked contained and localized domains, containment material, or appropriate orientation of redox catalyst and containment material. As expected, the performance of each of the cathodes described below was inferior to those described in Example 1. Cathodes Without Contained And Localized Domains A strip of Toray carbon paper (360 μm thick, 6 mm wide and 7 cm long) was immersed in a mixture of 24 μl of laccase solution and 24 mL of 1 mM ABTS in 50 mM pH 3 phosphate electrolyte to a depth of 2 cm. Chronoamperometry was performed as in Example 1 above. The performance was 1 mA cm² initially/but decayed to 0.4 mA within 1 hr. Cathodes Without Containment Material A cathode was made by drying 12 μl laccase solution and 24 μl 20 mM ABTS in 50 mM pH 3 phosphate electrolyte sequentially on the bottom 2 cm of a strip of Toray carbon paper (360 μm thick, 6 mm wide and 7 cm long). Chronoamperometry was performed as in Example 1. Performance was 2.3 mA/cm2 in oxygen after 1100 sec; however, this value declined to 56% of original after the ABTS change-out procedure described in Example 1.

Cathodes With Redox Catalyst On Too Of Containment Material A cathode was made by drying 100 μl RTU colloidal carbon (Alfa Aesar, Catalog #41774) on a Toray carbon paper strip (360 μm thick, 6 mm wide and 7 cm long). This was immersed in a mixture of 24 μl laccase solution and 24 mL 1 mM ABTS in 50 mM pH 3 phosphate electrolyte to a depth of 2 cm. Chronoamperometry was performed as in Example 1 above. The initial current was only 0.5 mA/cm² after 1400 sec. EXAMPLE 3 (Prophetic) Optimal Current Density In order to construct a cathode that can produce sufficient current density, both the redox catalyst activity and the electrode microstructure should be optimized. Using predictive equations well known in the field (i.e., Ikeda et al., J. Electroanal. Chem. 2001 , 496:69-75 and Anal. Sci. 2000, 16:1013), it can be shown that a cathode in which a microporous current collector incorporates microdomains of laccase and ETM in fluid association can be constructed wherein the current collector is capable of producing in excess of 100 mA/cm² of external surface area. For the modeling, laccase of the type used in the above examples is used as redox catalyst, with values of k_cat, K_M and K_s as measured for this enzyme in homogeneous buffer solution. The electron diffusion coefficient for the ABTS ETM (Palmore et al., J. Electroanal. Chem. 1999, 464:110-117) is as measured in homogeneous buffer solution, as well as the standard diffusion coefficient for oxygen in water. Specifically, in order to produce the desired current density: • the microdomain volume must contain 4 x 10^-9 moles of laccase and ETM in a concentration of 5 x 10^-3 M; • the effective volumetric surface area of the microporous current collector must be in excess of 100 cm²; • all of the redox catalyst must reside within 3 μm of the effective current collector surface; and, • all of the effective current collector surface must reside within 10 μm of a reservoir of air-saturated aqueous buffer. EXAMPLE 4 Laccase Cathodes With Various Domain Matrices This example demonstrates the construction of laccase electrodes having various domain matrices (i.e., of gold nanoparticles, carbon particles and silver particles, respectively). Following preparation of each porous domain matrix (infra), the matrix was connected to a standard flat conducting working electrode using double-sided conducting carbon tape, where the tape completely covered the original electrode. Control experiments with the tape itself did not lead to significant catalytic currents. Electrochemistry was carried out in 50 mM pH 3 phosphate buffer using laccase enzyme (15 μl in 2 mL) and recrystaHized ABTS (0.25 mM) as the ETM. Oxygen was continually bubbled through the solution, to stir the solution and provide an oxygen-saturated solution. Catalytic current was measured; this current was produced due to the reduction of ABTS, which in turn reduced laccase, which then reduced oxygen at 0.2 V. Domain Matrix Of Gold Nanoparticles First, a gold seed solution was prepared as follows: A 0.25 mM solution of HAuCI was boiled (250 mL). To this, 7.5 mL of 1 wt % sodium citrate was added and the solution boiled for another 30 min. After cooling, this seed solution was used as described below to create a gold film. The seed solution (4.8 mL) was added to 100 mL of water, and 49.8 mL of a 10 mM HAuCI₄ solution added. Next, 200 mL of 4 mM ascorbic acid was added at 2 mL per min and then the gold solution was left to stir overnight. The water-based solution of gold nanoparticles (50 mL; about 100 nm in size) was filtered using a 0.2 μm sized alumina filter (about 1 inch diameter). The filtered gold on the filter was left to dry and a gold film resulted (about 2-3 μm thick) that could be peeled away from the filter, thus producing a free-standing gold foil. A typical scanning electron microsopy image of the particles revealed a porous film. The gold film was cut into about 2 x 2 mm sized pieces for electrochemical experiments. The porous gold film was connected to a standard flat conducting working electrode and electrochemistry was carried out (as described above). The catalytic current was about 0.38 mA/cm². For comparison, a commercially available bulk (nonporous) gold electrode was used in a similar experiment. The current was about 0.015 mA/cm², measured as the current at 0 V versus a silver/silver chloride electrode. Thus, in the case of the porous electrode, current was approximately 25 times that of the bulk metal. Domain Matrix Consisting Of Carbon Particles Carbon powder (Vulcan XC72; 0.1 g) was added to 250 g of water and 50 mL of this solution was filtered using a 0.2 μm sized alumina filter (about 1 inch diameter). The filter cake was dried at 50 °C overnight. A razor blade was used to cut a sample approximately 2 x 2 mm and the powder cake was connected to a standard flat conducting working electrode (as described above). Prior to electrochemistry, the electrode was dipped in ethanol first and then rinsed in water to ensure wetting. Using the porous carbon electrode, the current measured at 0 V was about 1.75 mA/cm².

Domain Matrix Consisting Of Silver Particles Silver-based electrodes were made by pressing 0.3 g of 1 μm sized silver spheres at 5000 psi (34.47 MPa) in a 1 cm die for 1 min. The sample was removed and heated under nitrogen for 1 hr at 230 °C. A sample ca. 1 mm by 1 mm was connected to a standard flat conducting working electrode (as described in this example). Prior to electrochemistry, the electrode was dipped in ethanol first and then rinsed in water to ensure wetting. Using the porous silver electrode, the current measured at 0 V was about 2.1 mA/cm². EXAMPLE 5 Laccase Cathodes Utilizing Channeled Domain Matrices This example demonstrates the construction of laccase electrodes having domain matrices with channels. Specifically, channels were formed in a domain matrix of gold nanoparticles by soft lithography and channels were formed in a domain matrix of silver particles by high pressure. Following preparation of each channeled domain matrix (infra), electrochemistry was carried out in 50 mM pH 3 phosphate buffer using laccase enzyme (15 μl in 2 mL) and recrystallized ABTS (0.25 mM) as the ETM. Oxygen was continually bubbled through the solution, to stir the solution and provide an oxygen-saturated solution. Catalytic current was measured. Gold Nanoparticles And Channels Formed By Soft Lithography A soft lithographic mask with channels about 2-3 μm deep was filled with the gold solution of Example 4. The solution was left to dry overnight and the process was repeated once. The dried material was transferred to double-sided carbon tape by pressing the tape onto the surface of the gold/mask, at which point the gold imprint became transferred onto the working electrode. A typical scanning electron microscope image showed that the electrode consisted of both porous particles and channels about 2-3 μm deep. The gold leaves were essentially an imprint of the 'soft mask'. Excellent registry of the original mask was obtained. The current measured at 0 V was about 0.5 mA/cm². Silver Particles And Channels Formed From A Metal Grid Highly textured silver based particulate electrodes were made by pressing the surface of powdered silver (0.3 g) with porous TEM grids at 5000 psi (34.47 MPa). After 5 min, the TEM grid was removed to reveal a highly textured imprint. The surface can best be described as having the appearance of porous pillars. Electron micrographs showed that the resulting pillars varied in size but typically had dimensions of 20 μm x 20 μm wide by ca. 5 μm deep. These pillars sat on a flat bed of porous silver particles. Electrodes were made by using a razor blade to cut out a section of about 2 x 2 mm, and the textured surface was heated at 230 °C for 1 hr. In the case of the porous and textured silver electrodes the current measured at 0 V was about 4 mA/cm², which was higher than current measured using the non- textured surface (i.e., 2.1 mA/cm²; see Example

4). ^• EXAMPLE 6 Methanol Fuel Cell , A direct methanol fuel cell was constructed consisting of a split H configuration electrolytic cell separated by a Nafion® 117 membrane. The capacity of each half cell was approximately 45 mL. The anode compartment contained the anode and 0.9 M methanol in 1.0 M H₂S0 . The cathode compartment contained the cathode, 1.0 mM ABTS in 50 mM pH 3 phosphate buffer and a gas bubbler. The anode and cathode were connected through a decade resistance box. In parallel with the resistance box was a voltmeter and in series with the box was an ammeter. The anode was prepared by application of Pt/Ru inks to 360 μm thick carbon paper. The inks were made by mixing E-TEK 40% 1 :1 Pt/Ru alloy on Vulcan XC-72 carbon with water and Nafion® emulsion in a high shear mixer. The mixture was spread onto the carbon paper and allowed to dry at room temperature. Loading was about 1 mg metals/cm²; the anode contained 5 cm² active area. Anodes were conditioned by repetitive voltage scans in 1.0 M H₂S0₄ from -0.1 to 1.0 V until the cyclic voltammogram became constant. Laccase (12 μl) was applied to the bottom portion of a strip of Toray carbon paper (360 μm thick, 6 mm wide and 7 cm long). Additionally, 24 μl 20 mM ABTS in 50 M pH 3 phosphate buffer and 100 μl of RTU colloidal carbon (Alfa Aesar, Catalog #41774} was applied to the same area as the laccase. The paper was dried for 2 hrs. Loading of the laccase was approximately 0.02 mg/cm² on the strip; the active cathode area represented 6 cm² geometric surface area. No conditioning was necessary for the laccase cathode. The cathode side of the fuel cell was purged with bubbling air. Loads across the cell were cascaded from 100k ohms down to 10 ohms. At each intermediate resistance, full cell voltage drop and current were recorded. At a constant load of 100 ohms, current from the fuel cell measured 1.2 mA at 0.2 V and was constant for 100 min. EXAMPLE 7 Membrane and Membrane-Less Methanol Fuel Cells This example demonstrates the construction of two fuel cells, the first having a membrane and the second having no membrane. A Fuel Cell With Membrane A direct methanol fuel cell was constructed, as described above in

Example 6; however, the electrolytic cell was separated by a Nafion^® 112 membrane. The anode compartment contained the anode (prepared as previously described) and 0.9 M methanol in 0.05 M Na₂S0₄ and 0.001 M

H₂S0₄. The cathode compartment contained the cathode (also prepared as described in Example 6), 1.0 mM ABTS in 0.05 M Na₂S0₄ and 0.001 M H₂S0 , and a gas bubbler. Neither the anode nor the cathode required conditioning. The cathode side of the fuel cell was purged with bubbling air. Loads across the cell were cascaded from 100k ohms down to 10 ohms and measured as described in Example 6. At a constant load of 1000 ohms, current from the fuel cell was measured as 0.540 mA at 0.59 V after running for 45 min. The fuel cell was disassembled for the remaining experimentation, as described below. A Membrane-Less Fuel Cell The cathode was washed to remove residual electrolyte and soluble ABTS. Then, the fuel cell was reassembled without the membrane. The common electrolyte for both anode and cathode was 0.9 M methanol in 0.001 M H₂S0₄ and 0.05 M Na₂S0₄. Air was bubbled in the cathode side. At a constant load of 1000 ohms, current from the membrane-less fuel cell measured 0.387 mA at 0.42 V after running for 190 min.

Claims

CLAIMS What is claimed is:

1. A fuel cell comprising: (i) an anode configured and arranged for electro-oxidizing an anode reductant; (ii) a cathode configured and arranged for electro-reducing a cathode oxidant; and (iii) an electrolyte in solution contained within a single compartment; wherein the anode and cathode are spaced apart and reside in the single compartment containing the electrolyte; and wherein the electrolyte, reductant and oxidant freely communicate with the anode and the cathode.

2. A fuel cell according to Claim 1 wherein: a) the cathode is configured and arranged for electro-reducing oxygen; and b) the anode is configured and arranged for electro-oxidizing a reductant selected from the group consisting of: hydrogen, alcohols, carbohydrates, carboxylic acids and carboxylic esters

3. A fuel cell according to Claim 2 wherein the alcohol is methanol.

4. A fuel cell according to Claim 2 wherein the carbohydrate is glucose.

5. A fuel cell according to Claim 2 wherein the carboxylic acid is formic acid.

6. A fuel cell according to Claim 2 wherein the carboxylic ester is methyl formate.

7. A fuel cell according to Claim 1 wherein the cathode comprises a porous current collector comprising an electrical conducting material in contact with a multitude of domains, each domain containing: a) a redox catalyst; and b) an electron transport mediator; wherein the redox catalyst and electron transport mediator are in fluid association with each other within the domain and wherein the redox catalyst and the electron transport mediator are bounded in the domain by the porous current collector and a containment material.

8. A fuel cell according to Claim 1 wherein the anode comprises a porous current collector comprising an electrical conducting material, in contact with a multitude of domains, each domain containing: a) a redox catalyst; and b) an electron transport mediator; wherein the redox catalyst and electron transport mediator are in fluid association with each other within the domain and wherein the redox catalyst and the electron transport mediator are bounded in the domain by the porous current collector and a containment material.

9. A fuel cell according to either of Claims 7 or 8 wherein the diameter ^•of the domain is less than 100 μm.

10. A fuel cell according to either of Claims 7 or 8 wherein the containment material is selected from the group consisting of: polyelectrolyte polymers, permeable polymers, porous metal oxide-based coatings, organically modified metal oxides, fluorine-containing polymers, and silica and colloidal carbon emulsions.

11. A fuel cell according to- either of Claims 7 or 8 wherein the containment material is a film.

12. A fuel cell according to Claim 11 wherein the film is comprised of materials selected from the group consisting of: silica, titania, alumina, polyesters, polyamides, polyimides, fluoropolymers, polyelectrolytes, organosubstituted silica, silicones, organosubstituted titania and organo metal oxides.

13. A fuel cell according to either of Claims 7 or 8 wherein the porous current collector comprises an electrical conducting material.

14. A fuel cell according to either of Claims 7 or 8 wherein the electrical conducting material is selected from the group consisting of: carbon paper, carbon paste, carbon nanotubes, carbon particles, conducting polymers, noble metal particles and conducting metal oxide particles.

15. A fuel cell according to either of Claims 7 or 8 wherein the redox catalyst is an enzyme.

16. A fuel cell according to Claim 15 wherein the enzyme is an oxido- reductase.

17. A fuel cell according to Claim 7 wherein the redox catalyst is laccase.

18. A fuel cell according to either of Claims 7 or 8 wherein the electron transport mediator is selected from the group consisting of: quinones, hydroquinones, metalloorganic complexes, conjugated unsaturated acyclic and cyclic molecules, metal ions and biological electron carriers.

19. A fuel cell according to either of Claims 7 or 8 wherein the electron transport mediator is 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate).

20. A portable electronic device comprising the fuel cell of any of Claims 1-8.