Patent Application
20090104476
The present invention relates to fuel cells and more particularly to fuel cells that have different diffusion media on the anode and cathode sides of the cell.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. Proton exchange membrane (PEM) type fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. As such these MEAs are relatively expensive to manufacture and require certain conditions, including proper water management and humidification and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
The electrically conductive plates sandwiching the MEAs may contain a reactant flow field for distributing the fuel cell's gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels.
Interposed between the reactant flow fields and the MEA is a diffusion media serving several functions. One of these functions is the diffusion of reactant gases from the various flow channels to the major face of the MEA and the respective catalyst layer. Another is to diffuse reaction products, such as water, across the fuel cell. A third function is to adequately support the MEA between the various lands across the flow channels. In order to properly perform these functions, the diffusion media must be sufficiently porous while maintaining certain mechanical properties. The porosity is required to ensure proper reactant distribution across the face of the MEA. The mechanical properties are required to maintain sufficient contact between MEA and the diffusion media over the channel region and also to prevent the MEA from damage when assembled within the fuel cell stack.
The flow fields are carefully sized so that at a certain flow rate of a reactant a specified pressure drop between the flow field inlet and the flow field outlet is obtained. At higher flow rates, a higher pressure drop is obtained while at lower flow rates, a lower pressure drop is obtained.
It is desirable to have some compressibility in the diffusion media to account for plate variation. However, when a force acts on a compressible diffusion media, portions of the diffusion media may intrude into the channels of the bipolar plate. This intrusion results in a pressure drop which may be undesirable. Likewise, non uniform intrusion into different cells will cause uneven flow distribution into different cells. The effect of diffusion media intrusion is greater on the anode side and less on the cathode side since anode hydrogen fuel has a much lower flow rate and usually has a lower stoichiometry.
Other situations also exist where differing material characteristics between anode and cathode sides of a fuel cell may be beneficial. A few examples of these characteristics include porosity, permeability, surface free energy and microporous layer thickness. It would be beneficial therefore to have different diffusion media for the anode and cathode sides of a fuel cell.
The present invention provides a fuel cell having a first diffusion media and a second diffusion media having a membrane electrode assembly disposed therebetween. The first diffusion media includes a first set of material characteristics and the second diffusion media includes a second set of material characteristics. The first set of material characteristics has at least one material characteristic substantially different from at least one material characteristic of the second set of material characteristics. The difference in material characteristics provides for enhanced fuel cell/stack performance.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
With reference to
The plates 18, 20 may be formed of carbon, graphite, coated plates or corrosion resistant metals. The MEA 12 and unipolar plates 18, 20 are clamped together between end plates (not shown). The unipolar plates 18, 20 each contain a plurality of flow channels 22, 24 respectively that form a flow field for distributing reactant gases (i.e. H2 and O2) to opposing faces of the MEA 12. In the case of a multi-cell fuel cell stack, a flow field is formed on either side of a bipolar plate, one for H2 and one for O2. Nonconductive gaskets 26, 28 provide seals and electrical insulation between the several components of the fuel cell 10.
With particular reference to
The anode and cathode DM 14, 16 may each include a microporous layer (MPL) 36, 38 located on the side of the anode or cathode DM 14, 16 proximate the respective catalyst layer 32, 34. The MPL 36, 38 has a thickness that may include both a layer extending above the surface of the DM 14, 16 and a portion penetrating the surface of the DM 14, 16. For illustration, the MPL 36, 38 is shown by broken line in
In operation, the H2-containing reformate stream or pure H2 stream (fuel feed stream) flows into an inlet side of the anode side flow field through channel 40 and concurrently, the air or pure O2 stream (oxidant feed stream) flows into an inlet side of the cathode side flow field through channel 42. The fuel feed stream flows through anode DM 14 and the presence of the anode catalyst 32 causes the H2 to be oxidized into hydrogen ions, or protons (H+), with each giving up two electrons. The electrons travel from the anode side to an electric circuit (not shown), enabling work to be performed (i.e. rotation of an electric motor). The membrane layer 30 enables protons to flow through while preventing electron flow therethrough. Thus, the protons flow directly through the membrane to the cathode catalyst 34. On the cathode side, the protons combine with the oxidant feed stream and electrons, thereby forming water.
Still referring to
Changing the characteristics of the DM 14, 16 based on whether it functions as an anode DM 14 or a cathode DM 16 has been found to improve fuel cell 10 system performance. Specifically, it has been determined that the mechanical characteristics, structural characteristics, thermal resistance and surface free energy of the DM 14, 16 impact on the performance of a fuel cell 10. The mechanical characteristics may include compressibility and bending stiffness. The structural characteristics may include thickness, porosity, gas permeability, gas diffusivity and MPL thickness.
For example, having an anode side DM 14 that is stiffer than a cathode side DM 16 allows the anode channels to be least affected by the DM intrusion variation and thus improve performance while still allowing the cathode side DM 16 to account for plate variation. The compressibility of a DM may be characterized as the deflection of the media as a function of a compressive force. Depending on the thickness and compressibility of the DM, the DM may partially intrude into the flow channels, such as illustrated in by DM 16 intruding into channel 42, thereby effectively reducing the flow area in
Air is normally used as the oxidant in the cathode side, which contains 21% O2 and 78% N2. The N2 is not consumed in the fuel cell and the cathode is normally operated at relatively high stoichiometry in comparison to the anode side. As a result, the cathode side can accommodate greater cell to cell flow variation without impacting cell performance. This allows the cathode side to be less sensitive to differences in cell to cell DM channel intrusion. Therefore, the cathode side DM 16 may be less stiff than the anode side DM 14.
In another example, the product water is produced at the cathode side of the fuel cell. Water is transported from the anode side to the cathode side through osmotic drag. At high current density operating conditions, this results in a much higher water concentration in the cathode side than the anode side, and thus causes uneven membrane hydration across the proton conductive membrane and lowers the membrane proton conductivity. It has been found that using a DM without MPL and with a lower thermal resistance on the anode side is beneficial for high current density operations. On the other hand, very often fuel cells might be operated at dryer operating conditions and it is especially favorable for automotive application. Using a DM on the anode side with a lower water vapor diffusivity will help maintaining the membrane hydration.
Other parameters may be altered as well, such as the surface free energy of the DM. Providing a greater surface free energy on the anode side DM 14 than the cathode side DM 16 has proven beneficial. Surface free energy can be used to characterize the hydrophobicity of a DM. Surface free energy defines the work required to enlarge the surface area of matter. A liquid completely wets a solid when the contact angle of the liquid on the surface of the solid is 0° and can be considered to be resistant to wetting when the contact angle is larger than 90°. Therefore, having a greater surface free energy typically implies a greater hydrophilicity.
The anode side DM 14 may also have a less open pore structure and a thicker MPL coating 36 to maintain a desirable hydration level for the proton conductive membrane under dry operating conditions. The less open pore structure may include a decreased porosity and/or permeability relative to the cathode DM 16. The porosity is a function of the bulk density of the DM, which can be calculated from a real mass and thickness. The permeability may be a liquid or gas permeability. A variety of methods may be used to characterize the permeability of a DM. For gas permeability, a gas flow rate may be defined through a given sample area at a given pressure drop. For low flow materials, such as those with a MPL 36, 38, this may be expressed as the time required to pass a certain volume of flow through a given sample size at a given pressure drop. Liquid permeability may be characterized as the liquid flow rate through a DM at a given pressure drop. A liquid permeability test may be used. In this method, a column of liquid is put on the top of a porous media, and a pressure is then applied to force the liquid through the sample. This less open pore structure DM 14 structure on the anode side may naturally result in a stiffer substrate with less intrusion into the channels and thus reduce uneven reactant gas flow distribution from cell to cell.
The cathode side may further include an optimized MPL coating 38 having deeper penetration into the DM 16 for better cathode side water management. This feature has been found to be effective in removing product water by preventing the formation of a continuous water film inside of the DM 16 substrate, thereby reducing the cathode mass transport loss.
Sample 1 was a control cell with a symmetric anode DM and cathode DM (i.e., with the same properties). Samples 2 and 3 were test cells with different anode DMs such that the anode and cathode DM are asymmetric. Specifically, the relative properties of the anode DM for the samples are set forth in Table 1 below.
Data plots 100, 102 and 104 represent the incremental voltage potential (V) generated by Samples 1, 2 and 3, respectively over a range of current densities. Data plots 200, 202 and 204 represent the resistance (Ω/cm2) across Samples 1, 2 and 3, respectively.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
