Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M8/1009—Fuel cells with solid electrolytes with one of the reactants being liquid, solid or liquid-charged
  • H01M8/1011—Direct alcohol fuel cells [DAFC], e.g. direct methanol fuel cells [DMFC]
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M8/1009—Fuel cells with solid electrolytes with one of the reactants being liquid, solid or liquid-charged
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/24—Grouping of fuel cells, e.g. stacking of fuel cells
  • H01M8/2455—Grouping of fuel cells, e.g. stacking of fuel cells with liquid, solid or electrolyte-charged reactants
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• the present invention relates generally to fuel cells, and, more particularly, to air breathing direct methanol fuel cells.
• Methanol fuel cells are a promising technology for these types of battery replacement applications.
• Methanol as the fuel, has a high energy density and is easily obtained, stored and transported.
• Direct methanol fuel cells and stacks with forced airflow on the cathode side and forced methanol flow on the anode side have been under development at Los Alamos National Laboratory for the past 5 years, both for portable power and for transportation applications.
• a direct methanol fuel cell does not fit the requirements for the low power battery replacement applications.
• the key challenges are to provide acceptable power output, high energy conversion efficiency, and high energy density with the cell operated in convenient conditions to the user.
• the typical desired operating condition are, for example, an operating temperature near room temperature, no forced airflow, no re-circulation methanol pump, and no water recovery system.
• a direct methanol fuel cell is passive, i.e., operates under no forced air (i.e., air breathing) at near room temperature. This type of cell is referred to herein as an air breathing direct methanol fuel cell (air breathing DMFC).
• the present invention includes an air breathing direct methanol fuel cell having a membrane electrode assembly, a conductive cathode assembly permeable to air and directly open to atmospheric air, and a conductive anode assembly permeable to methanol and directly contacting a mixture of liquid methanol and water. Water loss from the cell is minimized by making the conductive cathode assembly hydrophobic and the conductive anode assembly hydrophilic.
• FIGURE 1 is an exploded view of a fuel cell unit according to one embodiment of the present invention.
• FIGURE 2 graphically depicts the room temperature performance of a single air breathing DMFC as shown in FIGURE 1.
• FIGURE 3 graphically depicts cell power outputs at 0.45 V with 16 mL methanol solutions of 1.0, 1.5, and 2.0 M at the start of a test run.
• FIGURE 4 graphically depicts cell power output at 0.45 V with 16 mL 1.0 M methanol solution at the start of the test where a different membrane electrode assembly (MEA) is used than for the performance shown in FIGURE 3.
• FIGURE 5 is an exploded view of an air breathing DMFC with two cells connected in series.
• FIGURE 6 graphically depicts the voltage and power performance of the unit shown in FIGURE 5 at room temperature with a 1.0 M methanol solution.
• FIGURE 7 graphically depicts cell power output of the unit shown in FIGURE 5 at 0.90 V with 10 mL of 1.0 M methanol solution at the start of the run.
• FIG. 1 depicts the cell components for an air breathing direct methanol fuel cell unit with only one cell shown in exploded view.
• An identical cell can be placed in mirror image within cell body IfJ (see, e.g., Figure 5) with an anode backing contacting methanol reservoir 12.
• Reservoir _ 2 may be a sponge, or the like, filled within container 3 . to retain the methanol solution within container 3J . if the cell is oriented adversely .
• Each cell consists of a membrane electrode assembly 14, anode backing 16 and cathode backing 1_8, metal current collectors 22, 24, compression cover plate 26, reinforcement bars 28 and 29, air side filter 32, methanol solution container _Y_ formed by the body of cell 10, methanol reservoir 12, and cover 34 with methanol solution injection and C0 2 ventilation port 36.
• MEA Membrane electrode assembly
• MEA 14 was formed by applying anode ink and cathode ink directly onto a polymer proton conducting membrane, such as a polymer electrolyte membrane, a National® 117 membrane in particular, over a vacuum table at 60°C.
• the anode ink was made from high surface area PtRu catalyst powder and National ionomer solution of 1200 E.W. (N1200 E.W.), and the cathode ink from high surface area Pt catalyst powder and N1200 E.W. ionomer solution.
• the dry anode and cathode inks preferably contain 15 ⁇ 10 wt. % and 10 ⁇ 5 wt. % Nation® components, respectively.
• E-tek 2.02 hydrophilic single-sided carbon cloth backing (only one side of the carbon cloth is coated with felt made from carbon powder and polytetrafluoroethylene (PTFE) polymer) contacted the anode side active area of MEA 14.
• PTFE polytetrafluoroethylene
• Cathode backing 18 E-tek double-sided carbon backing (both sides of the carbon cloth are coated with felt made from carbon powder and PTFE polymer) designated as NC/DS ⁇ /2 was used to contact the cathode side active area of MEA 14.
• the desired hydrophobicity of the cathode backing provided by the PTFE component is based on the operating principle of the gas diffusion electrode, which allows a ready access of air to the catalyst surface through the gas diffusion channels within the backing electrode.
• Current collectors 22, 24 were made from perforated metal sheets, which were corrugated, into folds of ridges and valleys, as described in U.S. Patent Application, Flow Channel Device for Electrochemical Cells, S.N. 09/472,388, filed December 23, 1999, and incorporated herein by reference.
• the corrugations give the metal current collectors mechanical strength against bending stress arising from compressing the MEA, and the perforations create the openness to allow the reactants (methanol and air) to reach the catalyst layers, and to allow the reaction product (C0 2 ) to leave the anode catalyst layer while uniformly distributing methanol and air over the facing surface areas of the corresponding conductive electrodes.
• the perforated area covers up to 50% of the total metal sheet area.
• the cathode metal current collector 22 compresses the cathode backing 18, MEA 14, and the anode backing 16 against the anode metal current collector 24.
• Metal current collectors 22, 24 are placed in an orthogonal relationship along their corrugation folds, which are indicated by solid lines in Figure 1.
• Compression reinforcement bar 28, 29 Metal current collectors 22, 24 are further compressed between compression plate 26 containing reinforcement bar 28 and a recess within cell body 10 that defines openings to a methanol volume 3_1 and that also has a reinforcement bar 29.
• the space between bars 28, 29 is more widely separated than the corrugation folds of the metal current collectors 22, 24.
• Reinforcement bars 28, 29 run perpendicular to the respective corrugation folds of metal current collectors.
• the assembly consisting of MEA 14 sandwiched by anode backing 16 and cathode backing 1_8, and metal current collectors 22, 24 is a unit cell assembly.
• Air side filter 32 The air side filter 32 is, e.g., a piece of porous polypropylene paper of 10 mil thickness, covering the openings in compression plate 26. Outside air passes through filter 32 to reach cathode backing 18 and the cathode catalyst layer of MEA 14.
• filter 32 is highly hydrophilic to allow air movement through the filter while minimizing water vapor loss from the cathode through filter 32 to the atmosphere.
• Filter 32 serves the purpose of keeping dust particles outside the unit cell assembly and retaining water moisture within the unit cell, while allowing the natural diffusion of air that contains the oxygen reactant to MEA 14.
• Methanol solution container 3J_ Methanol solution container 3J_:
• a methanol solution container 3_1 is defined by cell body 10 and cover 34.
• a methanol reservoir 12 is placed within the methanol solution container to retain methanol and is compressed to place the sponge and the methanol within container 3_1, while permitting methanol access to MEA 14 through anode current collector 24 and anode backing 16.
• an absorbing sponge of suitable porosity and chemical inertness toward the methanol solution, or the like is used to soak up methanol, where the sponge within container 3_1 is compressed to place the sponge and the methanol absorbed therein in contact with adjacent anode current collector 24 and anode backing 16.
• Methanol solution injection and C0 2 ventilation port 36 A small opening 36 through cell cover 34 is provided for both refilling the cell with a methanol solution and venting the C0 2 reaction product.
• test cell was a single cell with a circular shaped active electrode area of 11.4 cm 2 formed as described above.
• Air cathode 18 is operated by the natural diffusion of oxygen in the air to the cathode catalyst layer, and the methanol anode 16 directly contacts the methanol solution contained in methanol reservoir 12 with no active mechanical devices required for reactant supply.
• a methanol solution may be mechanically supplied to maintain the concentration of the methanol in the solution.
• the performance reported here was obtained at room temperature (22°C) and with 0.76 atm air, obtained at Los Alamos altitude of 7200 feet above sea level. Under prolonged operation, the cell temperature became stable at 27°C under these test conditions.
• Figure 2 shows the curves of cell voltage and power output vs. current, obtained at a cell voltage scan rate of 2 mV/s.
• Figure 3 shows the power output of this cell at 0.45 V over a period of time when operated at concentrations of methanol of 1.0 M, 1.5 M, and 2.0 M.
• the power output "spikes" appeared when the cell was briefly shaken during the test.
• Figure 4 shows the performance of a second cell for the power output at a cell voltage of 0.45 V under the same testing conditions as shown in Figure 3.
• the better cell performance was achieved by using a more activated MEA. It was found that the MEA can be more effectively activated by conditioning the MEA before operation with methanol: the MEA, sandwiched by anode and cathode backings in a compressed fuel cell hardware, was run at 80°C with humidified hydrogen feed at the anode and air feed at the cathode at 0.7 V for over 3 hours.
• Figure 5 illustrates a two-cell assembly 44, 46, each of which has a square shaped active electrode area of 5 cm 2 .
• These two unit cell assemblies 44, 46 located on the two major faces of cell body 40, have opposed anode surfaces 62, 64, each of which contacts the central methanol container 42 defined by cell body 40 and cover 56.
• Each cell assembly 44, 46 is clamped to cell body 40 by a compression plate 48, 52, respectively.
• Higher cell voltage output was achieved by connecting the anodes and cathodes of two electrode assemblies in series. Thus the positive terminal is connected to the first cell cathode, the first cell anode is connected to the second cell cathode and the second cell anode becomes the negative terminal.
• the air cathode is operated by the natural diffusion of oxygen from atmospheric air to the cathode catalyst layer, and the methanol anode side is exposed to the methanol solution contained in the methanol reservoir 54 in container 42.
• This configuration provides an entirely passive unit with no external devices required for reactant feed, although a supply system could be included to maintain the methanol concentration in the reservoir and .
• the two anodes share the same methanol supply from the reservoir.
• Figure 6 shows the curves of the cell voltage and power output vs. current, obtained at a cell voltage scan rate of 4 mV/s with a methanol solution of 1.0 M MeOH.
• Figure 7 shows the power output of this unit at 0.90 V with an initial fuel loading of 10 mL 1.0 MeOH placed within the methanol container 42.
• the unit cells described herein exhibited membrane resistance as high as 0.8 ⁇ cm 2 , which is acceptable, when the cell cathode is exposed to outside air in normal cell operating conditions. With water vapor saturated air, the membrane resistance decreased to 0.6 ⁇ cm 2 , and with liquid water contacting the cathode side, the membrane resistance was further decreased to 0.25 ⁇ cm 2 .
• the high membrane resistance observed indicates that the membrane was drying out under the operating conditions, especially at the low cell current density produced in this cell at 0.45 V.
• Water transport by diffusion is dominant when the cell is at an open circuit condition or is operated at a low cell current density such that water activity at the cathode side is less than that on the anode side, which is in contact with an aqueous methanol solution.
• water activity at the cathode side is less than that on the anode side, which is in contact with an aqueous methanol solution.
• water accumulates at the cathode from the oxygen electro- reduction process and, most importantly, from electro-osmotic drag.
• PFSA perfluorosulfonic acid
• Dow membranes for example.
• the water electro-osmotic drag coefficient for these membranes which is defined as the number of water molecules dragged across the membrane by one proton, is 2 to 3 at room temperature. For one methanol molecule that is electro-oxidized at the anode, six protons are produced.
• Each of these protons “drags" 2 to 3 molecules of water when they migrate from the anode to the cathode so that 12 to 18 molecules of water move from the anode to the cathode for each methanol molecule consumed. If the water that emerges on the cathode side is not efficiently recovered and returned to the anode side, the passive methanol fuel cell system would incur a large water imbalance and water loss that would limit the output energy density and applications for passive methanol fuel cell devices.
• water loss for the passive direct methanol fuel cells is minimized by using different anode and cathode electrode backings.
• a highly hydrophilic anode backing that tends to retain water and a highly hydrophobic cathode electrode backing that tends to exclude water water can be moved backward from the cell cathode to the cell anode by a hydrostatic force arising from the hydrophobicity gradient between the two electrode backings.
• Hydrophobicity and hydrophilicity characteristics can be enhanced by coating the cathode and anode backings with appropriate materials that are well known.
• the cathode is coated with PTFE to be hydrophobic and the anode is coated with a National N1200EW ionomer solution to be hydrophilic.
• PTFE to be hydrophobic
• anode is coated with a National N1200EW ionomer solution to be hydrophilic.
• the total capillary force that drives water from the cathode side to the anode side of the membrane can be as high as 3 atmospheres.

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• 2001
  • 2001-10-31 JP JP2002543732A patent/JP2004522257A/ja active Pending
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