Near ambient, unhumidified solid polymer fuel cell

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hydrogen and oxygen, that pass by the membrane on

NEAR AMBIENT, UNHUMIDIFIED SOLID each side of the membrane/electrodes assembly. The POLYMERFUEL CELL accessories required for humidification add instrumen tation and hence weigth to the fuel cell.

BACKGROUND OF THE INVENTION 5 The proton exchange membrane used in a solid poly mer fuel cell acts as the electrolyte as well as a barrier

This invention relates to a simplified fuel cell system. for

According to one aspect, the invention relates to a fuel preventing the mixing of the reactant gases. An cell using a solid polymer electrolyte. example of a suitable membrane is a copolymeric per Fuel cells have been used as a source of primary fluorocarbon material containing a basic unit of fluori power in remote areas where the usual sources of power 0 nated carbon chain and a sulphonic acid group. There may be variations in the molecular configurations of this are unavailable and where service and maintenance of equipment would be limited. Examples in this area are solid membrane. One membrane commonly used as a fuel cell unattended communications repeaters, navigational electrolyte is a well known perflourocarbon mate aids, and weather and oceanographic stations. For such 15 rial sold by E. I. DuPont de Nemours under the trade applications, the power system must be self-contained mark "Nafion.' Dow Chemicals Company has also and the reliability of the system is of maximum impor the developed proton exchange membranes that are still in tance. experimental stage. Excellent performances are Generally, a fuel cell is a device which converts the obtained using these membranes if the fuel cells are energy of a chemical reaction into electricity. It differs operated saturated under fully hydrated, essentially water conditions. As such, the membrane must be from a battery in that the fuel and oxidant are stored 20 external to the cell, which can generate power as long continuously humidified.

as the fuel and oxidant are supplied. A fuel cell produces In addition to the humidification problem, the mem an electromotive force by bringing the fuel and oxidant brane contributes to the ohmic resistance within the cell itself and, in turn, lowers fuel cell performance. If the in contact with two suitable electrodes and an electro lyte. A fuel (for example, hydrogen gas) is introduced at 25 fuel cell must perform at a lower temperature and pres sure when the reaction kinetics of the two halfcell reac one electrode where it reacts electrochemically with the electrolyte to impart electrons to the fuel electrode. tions as the shown above are slower, the use of less membrane electrolyte becomes imperative.

Protons are formed by dissociation of the hydrogen at the first electrode and pass through the electrolyte to 30 Cooling and humidification requirements increase the the second electrode. Simultaneously, an oxidant (for an cost and complexity of the fuel cell, reducing its use as example, oxygen gas or air) is introduced to the second alternative energy supply in many applications. Ac electrode where it reacts electrochemically with the cordingly, there is a need for a fuel cell with no require electrolyte to consume the electrons and the protons at ment for humidification which operates at near ambient conditions.

the second electrode. The halfcell reactions at the two electrodes are, respectively, as follows: 35 SUMMARY OF THE INVENTION H2-2H +2e According to one embodiment of the present inven tion, there is provided a membrane which does not need

O2+2H +2e.--H2O the humidification of the prior art, thus allowing for less 40 complicated and lighter modules.

Connecting the two electrodes through an external One embodiment of the invention provides a fuel cell circuit causes an electrical current to flow in the circuit for generating electricity from a reaction between a fuel and withdraws electrical power from the cell. The source and an oxidant source, the fuel cell comprising: overall fuel cell reaction produces electrical energy a first porous gas diffusion electrode; a second porous which is the sum of the separate halfcell reactions writ- 45 gas diffusion electrode, the second electrode defining an ten above. A byproduct of the reaction is formed as well electric field with the first electrode; a first electrolyte as some heat. deposit placed on the first electrode; a second electro In practice, fuel cells are not operated as single units; lyte deposit placed on the second electrode; and an rather, they are connected in a series, stacked one on electrolyte membrane positioned between and in top of the other, or placed side by side. A series of fuel 50 contact with the first and second electrolyte deposit, the cells, referred to as a fuel cell stack, is normally en membrane having a central hole.

closed in a housing. The fuel and oxidant are directed According to a more specific embodiment, the first with manifolds to the electrodes, and cooling is pro and second electrolyte deposit comprises a perfluoro vided either by the reactants or by a cooling medium. carbon copolymer proton conducting material. Also within the stack are current collectors, cell-to-cell 55 According to a further embodiment, the first and seals, insulation, piping, and instrumentation. The stack, second electrolyte deposit in the dry state comprises housing, and associated hardware make up the fuel cell about 10 to 20 mg per about 5 cm2 electrode area of module. about 5% solution of Nafion. Fuel cells can be classified by the type of electrolyte, According to yet another embodiment of the inven i.e., liquid or solid that they contain. The present inven- 60 tion, the electrolyte membrane comprises a perfluoro tion improves upon fuel cells in which the electrolyte is carbon copolymer proton conducting material. a solid (for example, a solid polymer also known as a According to still a further embodiment of the inven proton exchange membrane). This type of fuel cell op tion, there is provided a fuel cell comprising: a fuel erates best when the electrolyte membrane is kept moist electrode and an oxidant electrode; an electrolyte nem with water because the membrane will not operate 65 ber between and in contact with the fuel electrode and efficiently when it is dry. The membrane requires con the oxidant electrode, the electrolyte member defining a stant humidification during operation of the fuel cell, first and second electrolyte deposit on opposing sur normally by adding water to the reactant gases, usually faces of the electrodes and a electrolyte membrane

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sheet, having a central hole, positioned between and on DETAILED DESCRIPTION OF THE the periphery of the deposits; a fuel distribution member INVENTION in flow communication with the fuel electrode for sup plying fuel thereto; and an oxidant distribution member Referring to FIGS. 1 and 2, there is shown in sche in flow communication with the oxidant electrode for 5 matic cross-section a fuel cell having features of the supplying an oxidant thereto. present invention. Fuel cell assembly (5) includes gase According to yet another embodiment of the inven ous reactants which include a fuel source (10) and an tion, there is provided a fuel cell comprising: an oxidiz oxidizer anode source (12). The gases (10) and (12) diffuse ing electrode and a reducing electrode; electrolyte O through layer (16), backing layer (14) and cathode backing respectively, to porous electrodes forming means positioned therebetween, the electrolyte means defining a first and second electrolyte deposit on oppos anode (18) and cathode (20). The electrodes (18) and (20) have deposits of solid electrolyte (22) and (24).

ing surfaces of the electrodes and a electrolyte mem These electrolyte deposits (22) and (24), together with brane sheet, having a central hole, positioned between an oversized and on the periphery of the deposits; means for supply 5 with a hole (32),membrane (30) positioned therebetween ing a gaseous fuel to the oxidizing electrode; and means (20). Anode connection (42)anode separate the (18) and the cathode for supplying a gaseous oxidant to the reducing elec (44) are used to interconnect with an externalconnection and cathode circuit or trode. with other fuel cell assemblies. The membrane/elec According to still a further embodiment, there is trodes assembly was prepared by putting the compo further provided a method for assembling a fuel cell nents together and pressing for about 90 seconds at a comprising the steps of: placing a first porous gas diffu pressure of about 1,000 psig and at a temperature of sion electrode parallel to a second porous gas diffusion about 130' C.

electrode, the first and second electrode having two Suitable fuel sources (10) that are consumed by as sides, a backing layer side and a catalyst side, wherein semblies made in accordance with the invention in the catalyst side of the first and second electrode face 25 order to produce electrical energy are hydrogen-con each other; first attaching a first catalyst layer on the taining materials (for example, water, methane, and catalyst side of the first electrode; second attaching a methanol). According to some embodiments, the fuels second catalyst layer on the catalyst side of the second are supplied to the assemblies in liquid form, while according to alternative embodiments, fuels are sup electrode; first depositing a first electrolyte on the first 3O plied catalyst layer; second depositing a second electrolyte on in gaseous form. According to still further em bodiments, the second catalyst layer; and positioning an electrolyte hydride formers, hydrogen is obtained from reversible metal membrane between and in contact with the first and (for example, LaNis, FeTi and second electrolyte, the membrane having a central hole. others).MmNi4.5Fe0.85, where Mm is a mischmetal, among A small amount of electrolyte deposit on an electrode 3 5 Furthermore, many suitable oxidizer sources (12) or allows for operation under much reduced ohmic resis oxidizer species are available for combining with the tance. A reduced ohmic resistance compensates for the fuel to provide a mixture suitable for powering the fuel kinetic advantage that is obtainable for the above two cell assemblies described herein. In practice, the most halfcell reactions at a higher temperature and pressure. common oxidizer is gaseous oxygen or air. Thus, the fuel cell operation becomes feasible at a lower The anode (18) and cathode (20) halfcell hydrogen temperature and pressure. and oxygen reactions require catalysts to proceed at The above conditions allow the electrolyte to retain useful rates. As such, thin catalyst layers (36) and (38) sufficient water that is formed by the electrochemical are placed on the first electrode catalyst side and the reaction during fuel cell operation allowing a continu second electrode catalyst side. Various materials are ous cell performance without membrane failure. 45 suitable for forming such catalyst layers. These materi

BRIEF DESCRIPTION OF THE DRAWINGS

als include iridium, platinum, palladium, gold, nickel, and various alloys of these materials. Other suitable

The accompanying drawings, which are incorpo catalytic materials include non-metals, (for example, rated in and form a part of the specification, illustrate electronically conducting mixed oxides with a spinel or perovskite structure). According to a more specific the embodiments of the present invention, and, together O embodiment, with the description, serve to explain the principals of the hydrogen electrode catalyst is plati the invention. In the drawings: num, and the oxygen electrode catalyst is either plati FIG. 1 is a schematic cross-section of an embodiment num or another oxygen reducing catalyst (for example, of the invention in which a fuel cell is provided having 55 alyst macrocyclic chelate compound). The amount of cata used in one embodiment is about 1 mg platinum per two electrodes and a membrane with a central hole.

FIG. 2 is a schematic cross-section of the membrane cm2 of electrode area. It is foreseen that increasing the and electrodes assembly seen in FIG. 1. platinum loading or the efficiency of platinum utiliza FIG. 3 is a graph of cell potential versus current tion in the electrodes will improve the fuel cell perfor aCe.

density comparing a fuel cell at various temperatures According to one embodiment of the invention, a and with various oxidants. perfluorinated sulfonic acid copolymer known under FIG. 4 is a graph of cell potential versus current the trademark Nafion and available from E. I. DuPont density comparing a fuel cell at various temperatures de Nemours was used as the electrolyte deposits (22) and pressures. and (24) on the electrode. Nonetheless, any perfluoro It is to be noted that the drawings illustrate only 65 carbon copolymer that could be used as an electrolyte typical embodiments of the invention and are therefore membrane in a solid polymer fuel cell, such as those not be to considered limiting of its scope, for the inven made by Dow Chemicals Company, would be equally tion will admit to other equally effective embodiments. suitable in this invention. The electrolyte deposit can be

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made of a material that is characterized by a high con width of the gasket is kept the same (0.37 cm) for a ductivity for hydrogen ions (H+) or hydronium ions bigger electrode fuel cell, the effective electrode area (H3O+). Additionally, each electrolyte deposit is per would be greater for that electrode. It can be calculated meable to the fuel, to the oxidizer, and to the products that for a 25 cm2 electrode, the ratio is 73%, and for a of the reaction between the fuel and the oxidizer. Fur 100 cm2 electrode, 86%. Therefore, it is expected that ther, each electrolyte deposit constitutes an electron for a larger electrode area, the fuel cell performance insulator, with a resistivity of at least about 106 ohm will improve.

centimeters. Moreover, the solid electrolyte is capable As is usual in fuel cell experiments involving solid of being made in very thin layers, often below one mi polymer electrolytes, an initial period of conditioning crometer in thickness. Selected hydrated aluminum O for the fuel cell is allowed. The initial performance for oxides (for example, pseudoboehmite), which possesses the fuel cell is low; however, the voltage and current of the required hydrogen ion conductivity, electronic re the cell gradually increase upon running the cell for sistivity, and permeability to fuel, oxidizer, and prod some period of time. This conditioning can be done at a ucts of the reaction between them, are also acceptable. temperature of about 30" to 40 C., polarizing the cell The amount of electrolyte deposit that is to be made 15 for a period of about 1 to 5 days. on an electrode should be sufficient to prevent reactant The practice of the invention requires no additional cross-overs through the electrodes. According to one humidification of the electrolyte or the reactants. embodiment, the amount of electrolyte deposit is in the Therefore, the water generated by the fuel cell reaction range of about 10 mg to 20 mg per about 5 cm2 elec is not carried away from the electrolyte by a high flow trode area. The exact thickness of the deposit was not of reactants through the fuel cell or through operation determined; however, the deposit is thicker at the cen of the cell at high temperature. The preferable condi tral part of the electrode than it is around the electrode tions for fuel cell operation are thus near stoichiometric edges. According to the same embodiment, the electro flow of the reactants and substantially ambient tempera lyte deposit was made from a 5% solution of Nafion ture of the fuel cell.

117, in lower aliphatic alcohols and 10% water, sold by 25 The following examples are introduced to illustrate Aldrich Chemical of Milwaukee, Wis. Nafion 117 be further the novelty and utility of the present invention comes soluble in lower aliphatic alcohols (for example, but not with the intention of unduly limiting the same. ethanol and isopropanol) in a closed container heated in EXAMPLE the temperature range of about 200-300' C. for a per iod of about two to five hours. The solution is prefera 30 The graph in FIG. 3 shows the performance of a fuel bly applied to the catalyst layer of the electrodes by cell in the form of a current-potential plot for a fuel cell successive coatings made with a small brush. After each using electrodes obtained from E-Tek, Inc. of Framing coating is applied, the electrode is heated in an oven of ham, Mass. The catalyst is 20% platinum-on-carbon about 80 C. for about fifteen to twenty minutes and having a loading of 1 mg platinum per cm2 of electrode. weighed until a required amount of solid electrolyte is 35 The electrode area for the anode and the cathode is 5 deposited onto the electrode surface. The deposited cm2 each. The amount of electrolyte in the dry state layer of electrolyte thus attaches to the catalyst layer of deposited on each electrode was 15 mg. The fuel cell as the electrode. The use of Nafion as a solid polymer described above was conditioned for three days before electrolyte membrane is more particularly described in collection of the performance data. The gas flow for the U.S. Pat. No. 4,469,579, the disclosure of which is in hydrogen and oxygen was maintained near stoichiomet corporated herein by reference. ric to the current drawn-approximately 1.1 to the Embodiments of the present invention also include a stoichiometric current. No humidification was provided slightly oversized solid electrolyte membrane (30), posi either for cell conditioning or for data collection. The tioned between and in contact with the electrolyte de results are presented at three temperatures, 30' C., 40 posits (22) and (24), with the central part cut out. The 45 C. and 50° C. Both hydrogen and oxygen entered sepa purpose of the hole (32) in the membrane is to avoid an rate channels through the bottom of the cell and the additional electrolyte on top of the deposited layer and unreacted excess exited through channels at the top. hence to avoid an unnecessary increase in the electro Approximately 4 inches of water back pressure was lyte resistance. The purpose of the oversized membrane maintained for both gases. As expected, the cell perfor is for gasketting the perimeter of the electrodes and 50 mance increased with increasing temperature. At 50 C. preventing reactant cross-overs around the electrodes. and 300 mA/cm2 of current, the cell voltage was 0.655 Examples of suitable membranes include Nafion and V, while at 30 C. and under similar conditions of cur any other proton exchange membrane suitable for solid rent, the cell voltage was 0.615 V. polymer fuel cell work. Moreover, any gasketting mate This fuel cell was also run with air as the oxidant at rial that can effectively attach to the electrode perime 55 30' C. To compensate for lower oxygen content, the air ter and act as a barrier between the hydrogen and oxy flow volume was maintained five times that for the gen gases would be suitable instead of a proton ex stoichiometric flow of oxygen alone. The air perfor change membrane. mance is also shown in FIG. 3. Since a high flow of a It is believed that the fuel cell performance will vary reactant tends to evaporate the generated water in the proportionately with the ratio of the area of the cutout fuel cell quickly, the use of air as an oxidant at a higher portion of the membrane to the total electrode area. temperature is not preferred at atmospheric pressure. This ratio determines the area of electrode having the However, it is foreseen that the use of air is feasible, if least resistive electrolyte pathway. For a small elec the cell is operating at a lower temperature and higher trode fuel cell having a cutout portion area of 2.25 cm2 pressure.

and a total electrode area of 5 cm2, this ratio is 45%. 65 EXAMPLE 2 This would mean that the area of the electrode covered by the gasketting membrane was 2.75 cm2, or a gasket Referring now to FIG. 4, there is a graph which 0.37 cm wide all around the electrode perimeter. If the shows temperature and pressure effects on fuel cell

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performance of a cell assembled with electrodes having an oxidant distribution member in flow communica electrolyte deposits of 19 and 20 mg, respectively. tion with the oxidant electrode for supplying an Other conditions for cell assembly and operation were oxidant thereto.

similar to that described above. Curves 1 and 2 show 6. A fuel cell comprising: effect of increasing pressure from 0 to 20 psig for both 5 an oxidizing electrode and a reducing electrode; hydrogen and oxygen gases and the cell temperature of electrolyte means positioned therebetween, the elec 30° C. With increasing pressure of 20 psig, the cell trolyte means defining a first and second electro performance at 300 mA/cm2 increases from 0.59 V to lyte deposit on opposing surfaces of the electrodes 0.64 V. Curves 2, 3, and 4 show temperature effect at 10 and a electrolyte membrane sheet, having a central hole, positioned between and on the periphery of the high pressure of 20 psig. At 50 C., and 300 the deposits;

mA/cm2 current density, the cell voltage was 0.68 means for supplying a gaseous fuel to the oxidizing volts, an improvement of 25 millivolts over the 0 psig electrode; and data of FIG. 3. means for supplying a gaseous oxidant to the reduc According to a further embodiment of the invention, 15 ing electrode.

10 mg of electrolyte was deposited on each electrode. 7. A method for assembling a fuel cell comprising the The cell performance at 30' C. and 300 mA/cm2 current steps of:

density was 0.59 V and at 50 C. and the same current as placing a first porous gas diffusion electrode parallel above, the cell performance was 0.655 V. to a second porous gas diffusion electrode, the first The above embodiments are given by way of example 20 and second electrode having two sides, a backing and are not intended as limitations as further embodi layer side and a catalyst side, wherein the catalyst ments and advances will occur to those of skill in the art side of the first and second electrode face each which practice the present invention. other;

What is claimed is:

first attaching a first catalyst layer on the catalyst side 1. A fuel cell for generating electricity from a reac of the first electrode;

second attaching a second catalyst layer on the cata tion between a fuel source and an oxidant source, the lyst side of the second electrode; fuel cell comprising: first depositing a first electrolyte on the first catalyst a first porous gas diffusion electrode; layer;

a second porous gas diffusion electrode, the second 30 second depositing a second electrolyte on the second electrode defining an electric field with the first catalyst layer; and electrode; positioning an electrolyte membrane between and in a first electrolyte deposit placed on the first elec contact with the first and second electrolyte, the trode; membrane having a central hole.

a second electrolyte deposit placed on the second 35 8. A fuel cell as in claim 1, wherein the electrolyte electrode; and membrane comprises an electrolyte material which attaches to the electrode perimeter and acts as a barrier an electrolyte membrane positioned between and in between the fuel source and the oxidant source. contact with the first and second electrolyte de 9. A fuel cell for generating electricity from a reac posit, the membrane having a central hole. 40 tion between a fuel source and an oxidant source, the 2. A fuel cell as in claim 1, wherein the first and sec fuel cell comprising:

ond electrolyte deposit comprises a perfluorocarbon a first porous gas diffusion electrode; copolymer proton conducting material. a second porous gas diffusion electrode, the second 3. A fuel cell as in claim 2, wherein the first and sec electrode defining an electric field with the first ond electrolyte deposit in the dry state comprises about 45 electrode;

10 to 20 mg per about 5 cm2 electrode area of about 5% a first electrolyte deposit placed on the first elec solution of Nafion. trode;

4. A fuel cell as in claim 1, wherein the electrolyte a second electrolyte deposit placed on the second membrane comprises a perfluorocarbon copolymer electrode;

proton conducting material. SO an electrolyte membrane positioned between the first 5. A fuel cell comprising: and second electrolyte deposit, the membrane hav a fuel electrode and an oxidant electrode; ing a central hole; and an electrolyte member between and in contact with wherein said first and second electrodes, said first and the fuel electrode and the oxidant electrode, the 55 second electrolyte deposits and said electrolyte electrolyte member defining a first and second membrane are assembled into operative connection electrolyte deposit on opposing surfaces of the with one another by pressing them together for a sufficient period of time at a high pressure and at an electrodes and a electrolyte membrane-sheet, hav elevated temperature.

ing a central hole, positioned between and on the 10. A fuel cell as in claim 9, wherein said sufficient periphery of the deposits; 60 period of time is approximately 90 seconds, said high a fuel distribution member in flow communication pressure is approximately 1,000 psig and wherein said with the fuel electrode for supplying fuel thereto; elevated temperature is approximately 130 C. and s k k

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UNITED STATES PATENT AND TRADEMARK OFFICE

CERTIFICATE OF CORRECTION

PATENT NO. : 5,242,764 Page l of 2

it is certified that error appears in the above-identified patent and that said Letters Patent is hereby corrected as shown below:

Title Page:

In the Abstract, line 5, please replace "fuel to" with --fuel cell

In col. 2, line 4, please replace "weigth" with --weight. In col. 5, 1ine 29, please replace "C." with --C--. In col. 5, line 34, please replace "C." with --C--. In col. 6, line 14, please replace "C." with --C--. In col. 6, line 44, please replace "C." with --C--. In col. 6, line 45, at the first occurrence, please replace "C."

In col. 6, line 50, please replace "C." with --C--. In col. 6, line 52, please replace "C." with --C--. In col. 7, line 11, please replace "C." with --C--. In coli. 7, line 17, please replace "C." with --C--.

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UNITED STATES PATENT AND TRADEMARK OFFICE

CERTIFICATE OF CORRECTION

PATENT NO. : 5,242,764 Page 2 of 2

INVENTOR(S): Hari P. Dhar it is certified that error appears in the above-indentified patent and that said Letters Patent is hereby Corrected as shown below:

In col. 7, line 18, please replace "C." with --C--.

Signed and Sealed this

Fifth Day of April, 1994

BRUCELEHMAN

Attesting Officer Commissioner of Patents and Trademarks