WO2001045189A1

WO2001045189A1 - Passive air breathing direct methanol fuel cell

Info

Publication number: WO2001045189A1
Application number: PCT/US2000/032917
Authority: WO
Inventors: Xiaoming Ren; Shimshon Gottesfeld
Original assignee: The Regents Of The University Of California
Priority date: 1999-12-17
Filing date: 2000-12-04
Publication date: 2001-06-21
Also published as: AU2059901A

Abstract

A passive air breathing methanol fuel cell is provided with a membrane electrode assembly (14), a conductive cathode permeable to air and open to atmospheric air, and which includes a backing (18) and a current collector (22), a conductive anode assembly permeable to methanol, which inculdes a backing (16) and current collector (24), and which directly contacts a liquid methanol source, such as a sponge (12).

Description

PASSIVE AIR BREATHING DIRECT METHANOL FUEL CELL

STATEMENT REGARDING FEDERAL RIGHTS This invention was made with government support under Contract No. W- 7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION The present invention relates generally to fuel cells, and, more particularly, to passive air breathing direct methanol fuel cells.

BACKGROUND OF THE INVENTION

As portable consumer electronics become ever increasingly popular, there is a strong demand for long lasting portable power sources in the range of a few hundreds milliwatts to a few watts range. At present, these demands are largely meet by the various types of batteries. Often, these batteries are very expensive, short-lived, and all have disposal problems.

Methanol fuel cells are promising technologies 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 sides 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 transportation applications. Usually, this type of direct methanol fuel cell works at elevated temperature with various auxiliary components and a rather complicated control system, and does not fit the requirements for the low power battery replacement applications. For such applications using a direct methanol fuel cell, the key challenges are to demonstrate acceptable power output, high energy conversion efficiency and high energy density with the cell operated in very convenient conditions to the user. The typical operating condition are, for example, an operating temperature near room temperature, no forced air flow, no re-circulation methanol pump, and no water recovery system. In the present invention, 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 as an air breathing direct methanol fuel cell (air breathing DMFC) hereinafter.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a passive air breathing direct methanol fuel cell having a membrane electrode assembly, a conductive anode assembly permeable to air and directly open to atmospheric air, and a conductive cathode assembly permeable to methanol and directly contacting a liquid methanol source.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiment(s) of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIGURE 1 is an exploded view of a fuel cell power 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 a photograph of an air breathing DMFC with two cell 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 methanol solution at the start of the run.

DETAILED DESCRIPTION Figure 1 shows 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 (9) with a cathode backing contacting methanol absorbing sponge (10). Each cell consists of a membrane electrode assembly (1 ), anode backing (2) and cathode backing (3), metal current collectors (4,5), reinforcement bars (6,7), air side filter (8), methanol solution container formed by the body of cell (9), methanol absorbing sponge (10), and covers (11 ) with methanol solution injection and CO₂ ventilation port (12). Each of these components is further described in details below. Membrane electrode assembly (MEA) (1 ):

The MEA was made by painting anode (methanol) ink and cathode (air/oxygen) ink directly onto a polymer proton conducting membrane, such as a polymer electrolyte membrane, a Nafion® 117 membranes in particular, over a vacuum table at 60°C. The anode ink was made from PtRu catalyst and N1200 E.W. ionomer solution, and the cathode ink from Pt catalyst and N1200 E.W. ionomer solution. The dry anode and cathode inks contain 15 w% and 10 w% recast Nafion® components, respectively. Because of the low temperature and low cell current density involved in this type of application, membranes that are less expensive and that are less permeable to methanol can be used, even where the membrane has a higher resistivity than the exemplary membrane. Besides the methanol permeation rate and protonic conductivity, the electroosmotic drag characteristic of a membrane is also an important property that needs to be considered in order to minimize water loss from the cell. Anode backing (2): E-tek 2.02 hydrophilic single sided carbon cloth backing was used to contact the anode side active area of the MEA. The criteria for selecting a suitable anode backing as described in U.S. Patent Application, Enhanced Methanol Utilization in Direct Methanol Fuel Cells, Docket S-91 ,744, can be applied here to achieve a high fuel utilization efficiency without sacrificing the fuel cell performance. Cathode backing(3):

E-tek carbon backing designated as NC/DSΛ/2 was used to contact the cathode side active area of the MEA. The desired hydrophobicity of the cathode backing provided by the PTFE component will be discussed later in the content of keeping water loss from the fuel cell to minimum. Metal current collectors (4,5):

These current collectors were made from perforated metal sheets, which have been corrugated, into folds of ridges and valleys, as described in U.S. Patent Application, Flow Channel Device for Electrochemical Cells, Docket S-91 ,719. The corrugation gives the metal current collectors mechanical strength against bending stress arising from compressing the MEA, and the perforations create the needed openness to allow the reactants (methanol and air) to reach the catalyst layers, and to allow the reaction product (CO₂) to leave the anode catalyst layer while uniformly distributing methanol and air over the facing surface areas of the corresponding conductive sheets. In this application, the perforated area covers up to 50% of the total metal sheet area. In an assembled methanol fuel cell, the anode metal current collector (4) compresses the anode backing (2), MEA (1 ), the cathode backing (3) against the cathode metal current collector (5). The two metal current collectors are placed in an orthogonal relationship along their corrugation folds. Compression plates (6,7): The metal current collectors (4,5) will be further compressed between compression plate (6) containing reinforcement bar (7) and a recess within cell body (9) which defines openings to the methanol reservoir (10), which also have a reinforcement bar (7). The space between the bars is more widely separated than the corrugation folds of the metal current collectors (4,5). The reinforcement bars run perpendicular to the respective corrugation folds of the metal current collectors. The assembly consisting of MEA (1 ) sandwiched by anode backing (2) and cathode backing (3), metal current collector (4,5) and reinforcement bars (6,7) is a unit cell assembly.

Air side filter (8):

The air side filter is a piece of porous polypropylene paper of 10 mil thickness, covering the openings in compression plate (6). Outside air passes through this filter to reach the cathode backing and the cathode catalyst layer. The purpose of this filter is to keep dirt particles outside, and keep water moisture within, the cell while allowing the natural diffusion of air containing the oxygen reactant.

Methanol solution container formed by the body of cell (9): A methanol solution container is formed by the body of cell (9), unit fuel cells within cell body (9), and cell cover (1 1 ) on the top side.

Methanol absorbing sponge (10):

A methanol absorbing sponge (10) is placed within the methanol solution container to soak up methanol and is compressed to place the sponge and the methanol absorbed therein in contact with adjacent anode current collectors, e.g., collector (5). By constraining the free flow of methanol solution within the container, the methanol fuel cell can thus be operated in all orientations. Such a feature is highly desirable in a portable power device.

Methanol solution injection and CO₂ ventilation port (12): A small opening on the cell cover is provided for both refilling the cell with a methanol solution and venting the CO₂ reaction product.

One type of the test cells demonstrated here is a single cell with a circular shaped active electrode area of 11.4 cm² formed as described above. The air cathode is operated by the natural diffusion of oxygen in air to the cathode catalyst layer, and the methanol anode side directly contacts the methanol solution contained in the methanol reservoir with no active supply components. The performance reported here was obtained at room temperature and with 0.76 atm. air, obtained at Los Alamos altitude. 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.

Figure 4. shows the performance of a second cell for the power output at 0.45V 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 cell 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. Noticeable increases in anode and catalyst layer protonic conductivity (measured use an electrochemical impedance spectroscopic method) and the methanol electro-oxidation activity (anode polarization against a hydrogen evolution counter electrode in a humidified H₂ atmosphere) were observed after the activation conditioning.

A second type air breathing methanol fuel cell has also been demonstrated. Figure 5 is a photograph of a two cell assembly, each of which has a square shaped active electrode area of 5 cm². These two electrode assemblies, located on the two major faces of the cell body, have opposed anodes, each of which contacts the central methanol reservoir. Higher cell voltage output was achieved by connecting the anodes and cathodes of two electrode assemblies in series. The air cathode is operated by the natural diffusion of oxygen from air to the cathode catalyst layer, and the methanol anode side is exposed to the methanol solution contained in the methanol reservoir. This configuration provides an entirely passive unit with no external devices required for reactant feed. The two anodes share the same methanol supply from the reservoir. The performance reported here was obtained at room temperature and with 0.76 atm. air, obtained at Los Alamos altitude. With prolonged operation, the cell temperature became stable at 27°C under these test conditions. Figure 6 shows the curves of the cell voltage and power output vs. current, obtained with a cell voltage scan rate of 4 mV/s. Figure 7 shows the power output of this unit at 0.90 V. Membrane resistance:

The cell exhibited membrane resistance as high as 0.8 Ω cm², which is acceptable, when the cell cathode is exposed to outside air as in normal cell operating conditions. With water vapor saturated air, the membrane resistance decreased to 0.6 Ω cm², and with liquid water contacting the cathode side, the membrane resistance was further decreased to 0.25 Ω cm². 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. Some beneficial effects of a dry membrane are that both methanol crossover rate and water electroosmotic drag flux are reduced, compared to those of a fully hydrated membrane. Thus, at an acceptable IR loss from the high membrane resistance of a partially dried membrane, better fuel efficiency and less water loss are obtained. With total cell weight kept constant, these derived benefits result in higher cell energy efficiency and higher energy density.

The concentration of the methanol solution contacting the fuel cell anodes can be managed to optimize performance of the fuel cell:

1. Active methanol concentration management: The cell body fuel reservoir can be divided into three compartments. One of the compartments contains pure methanol, one contains pure water, and third contains the methanol solution which directly contacts the anodes of the electrode assemblies within the cell body with the cathode sides of the electrode assemblies exposed to open air. The proper methanol concentration in the methanol solution container can be managed by two active micro-valves that open and close the passages between the water container and methanol solution container and between methanol solution container and pure methanol container based on the signal of a methanol concentration sensor, which senses the methanol concentration in the methanol solution container. 2. Passive methanol concentration management. The cell body fuel reservoir can be divided into three compartments. One of the compartments contains pure methanol, one contains pure water, and third contains the methanol solution which can reach the anodes of the electrode assemblies fixed to the cell body with the cathode sides of the electrode assemblies exposed to open air. The proper methanol concentration in the methanol solution container can be managed by passive diffusion between the water container and methanol solution container and between the methanol solution container and methanol container through specialized membranes.

The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. An air breathing direct methanol fuel cell operated comprising: a liquid methanol reservoir; a membrane electrode assembly (MEA); a conductive cathode assembly contacting the MEA, permeable to air and directly open to atmospheric air; and a conductive anode assembly contacting the MEA, permeable to methanol and arranged to directly contact liquid methanol within the liquid methanol reservoir.

2. The air breathing direct methanol fuel cell according to Claim 1 , further including: a cathode current collector contacting the conductive cathode assembly for distributing air over the conductive cathode assembly; and a anode current collector contacting the conductive anode assembly for distributing methanol over the conductive anode assembly.

3. The air breathing direct methanol fuel cell according to Claim 1 , further including: a housing defining a container for the liquid methanol reservoir and further defining a first recess for accepting a first unit cell assembly of the MEA, conductive cathode and anode assemblies, and cathode and anode current collectors, where the recess has at least one opening therethrough to the container; and a cover plate compressing the first unit cell assembly within the first recess and further having at least one opening therethrough to the air.

4. The air breathing direct methanol fuel cell according to Claim 1 , wherein the anode and cathode current collectors are formed as perforated corrugated sheets.

5. The air breathing direct methanol fuel cell according to Claim 4, wherein corrugations forming the anode current collector are oriented orthogonal to corrugations forming the flexible cathode current collector.

6. The air breathing direct methanol fuel cell according to Claim 4, wherein the housing and the cover plate each define pressure rib members oriented orthogonal to corrugations on the anode current collector and the cathode current collector, respectively.

7. The air breathing direct methanol fuel cell according to Claim 3, where the cover plate further includes a filter for filtering air to the conductive cathode assembly.

8 The air breathing direct methanol fuel cell according to Claim 1 , wherein the liquid methanol reservoir is a sponge effective to absorb methanol.

9. The air breathing direct methanol fuel cell according to Claim 3, wherein the liquid methanol reservoir is a sponge effective to absorb methanol and the housing further includes a top plate for compressing the sponge within the container.

10. The air breathing direct methanol fuel cell according to Claim 3, wherein the housing further defines a second recess for accepting a second unit cell assembly.

11. The air breathing direct methanol fuel cell according to Claim 1 wherein the conductive cathode assembly is hydrophobic and the conductive anode assembly is hydrophilic.

12. The air breathing direct methanol fuel cell according to Claim 7, wherein the filter is hydrophilic.