Patent Application
20070065702
This invention relates generally to fuel cells, and more specifically to water management in fuel cells.
Fuel cell technology has been the subject of much recent research and development activity due to the environmental and long-term fuel supply concerns associated with fossil fuel burning engines and burners. Fuel cell technology generally promises a cleaner source of energy that is sufficiently compact and lightweight to enable use in vehicles. In addition, fuel cells may be located close to the point of energy use in stationary applications so as to greatly reduce the inefficiency associated with energy transmission over long distances.
Although many different fuels and materials may be used for fuel cells, all fuel cells generally have an anode and an opposing cathode separated by electrolyte. The anode and cathode are generally porous so that fuel may be introduced into the cell through one of them, generally the cathode, and oxidant introduced through the other, generally the anode. The fuel oxidizes in the cell, producing direct current electricity with water and heat as by-products. Each cell generally produces an electrical potential of about one volt, but any number of cells may be connected in series and separated by separator plates in order to produce a fuel cell stack providing any desired value of electrical potential. In modern fuel cell design, the anode, cathode, and electrolyte are often combined in a membrane electrode assembly, and the separator plates and current collectors are often combined in a “bipolar plate.” Details of fuel cell design and operation are further explained in “Fuel Cell Handbook, 5th Edition”, published by the U.S. Department of Energy, National Energy Technology Laboratory, Morgantown, West Virginia, October, 2000, hereby incorporated fully herein by reference. Various fuel cell components, including membrane electrode assemblies and bipolar plates, are further described in U.S. Pat. Nos. 4,988,583; 5,733,678; 5,798,188; 5,858,569; 6,071,635; 6,251,308; 6,436,568; and U.S. Published Patent Application Serial No. 2002/0155333, each of which is hereby fully incorporated herein by reference.
A persistent challenge in the design of fuel cells is that of managing water in the cell. Fuel cells produce water as a reaction product. Under some conditions, water is evolved very quickly within the cell. This water is generally produced on the cathode side of the cell, and if allowed to accumulate, may restrict or block the flow of fuel into the cell. Such a condition is known in the art as “cathode flooding”. In addition, the temperature differences between the cell and ambient environment may be large so that condensation of water vapor may be caused at times as air moves in and out of the cell during operation.
Typically, the surface of bipolar plates is provided with drainage channels so that water is directed through the channels to a collection area to be drained from the cell. In addition, the bipolar plates are often made from material having relatively low surface energy so water drains from the bipolar plate more easily. Neither of these measures has been entirely successful in eliminating cathode flooding and water management problems in fuel cells, however. In particular, even where low surface energy materials such as PTFE are used in fuel cells, water droplets may cling to bi-polar plates and other surfaces in the cell rather than draining away as desired. What is needed in the industry is a fuel cell with components facilitating improved water drainage within the cell.
The invention substantially satisfies the aforementioned need of the industry. The invention includes a fuel cell stack apparatus with components having directionally biased wetting surfaces, also referred to as anisotropic wetting surfaces, at selected locations where condensation of water may occur so as to improve water drainage within the apparatus. The anisotropic wetting qualities substantially inhibits any tendency of water droplets to flow in undesired directions, thereby significantly improving water drainage within the cell.
The creation of asymmetric asperities can directionally bias the retentiveness of a surface. This approach can be applied to flat surfaces as well as curved surfaces such as tubes or troughs. Directionally biased fluid retention can be incorporated into conventionally wetting surfaces as well as ultraphobic surfaces. The asymmetric features can be random or periodic in design. Periodic asperities may vary in two dimensions such as structured stripes, ridges, troughs or furrows. Periodic asperities may also vary in three dimensions such as posts, pyramids, cones or holes. The size, shape, spacing and angles of the asperities can be tailored to achieve a desired anisotropic wetting behavior.
Generally, anisotropic wetting qualities are effective with droplets on surfaces and slugs ithin tubes, troughs or channels. Surfaces having anisotropic wetting qualities can be used to nsure that small droplets of liquid drain fully from the surface or, alternately, can be used to elp ensure that droplets are retained so that there is less risk of dripping into an undesired location.
The asperities may be formed in or on the substrate material itself or in one or more layers of material disposed on the surface of the substrate. The asperities may be any regularly or irregularly shaped three dimensional solid or cavity and may be disposed in any regular geometric pattern.
Microscale asperities according to the invention may be formed using known molding and stamping methods by texturing the tooling of the mold or stamp used in the process. The processes could include injection molding, extrusion with a textured calendar roll, compression molding tool, or any other known tool or method that may be suitable for forming microscale asperities.
Smaller scale asperities may be formed using photolithography, or using nanomachining, microstamping, microcontact printing, self-assembling metal colloid monolayers, atomic force microscopy nanomachining, sol-gel molding, self-assembled monolayer directed patterning, chemical etching, sol-gel stamping, printing with colloidal inks, or by disposing a layer of carbon nanotubes on the substrate.
The invention is a fluid handling device having a normophobic or ultraphobic surface that has anisotropic wetting qualities. That is, fluids will demonstrate a variable resistance to flow across the surface depending on the direction in which they flow. The anisotropic wetting surface generally includes a substrate portion with a multiplicity of projecting asymmetrical regularly shaped microscale or nanoscale asperities.
The asperities may be formed in or on the substrate material itself or in one or more layers of material disposed on the surface of the substrate. The asperities may be any regularly or irregularly shaped three dimensional solid or cavity and may be disposed in any regular geometric pattern or randomly.
For the purposes of this application, the term “fuel cell” means any electrochemical fuel cell device or apparatus of any type, including but not limited to proton exchange membrane fuel cells (PEMFC), alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), and solid oxide fuel cells (SOFC). The term “fuel cell stack apparatus” refers to an apparatus including at least one fuel cell and any and all components thereof, along with any and all of the separate components related to the functioning of the fuel cell, including but not limited to, enclosures, insulation, manifolds, piping, and electrical components.
A portion of an embodiment of a fuel cell stack apparatus 100 according to the present invention is depicted in simplified cross section in
Bipolar plates 104 and end plates 106 are typically made from electrically conductive, corrosion and heat resistant material such as metal or carbon filled polymer. Surfaces 114 of bipolar plates 104 and the inwardly facing surfaces 116 of end plates 106 typically have channels 118 for conveying fuel and oxidant to membrane electrode assemblies 102 and to drain away water which is a product of the reaction. Heat transfer portions 120 of bipolar plates 104 and end plates 106 may provide additional surface area to remove heat from the cells.
According to the invention, all or any desired portions of the outer surfaces of bipolar plates 104 or end plates 106 may be anisotropic wetting surfaces. As depicted in
As depicted in
An enlarged view of exemplary directionally biased wetting surfaces 30 is depicted in
Each asperity 34 in this example protrudes from substrate 32. Asperities 34 may also be indentations into substrate 32.
Referring to
Hysteresis can be defined as:
Δθ=θa−θr
Hysteresis is caused by molecular interactions, surface impurities, heterogeneities and surface roughness.
In order to better understand the present invention, it is helpful to consider the following cases: Retention of sessile drops by flat surfaces; retention of a liquid slug by a cylindrical tube; and wetted rough surfaces which demonstrate increased liquid-solid adhesion. Wetted rough surfaces include surfaces having symmetric roughness which generally demonstrate isotropic wetting and surfaces demonstrating asymmetric roughness which demonstrate directionally biased wetting.
For Sessile drops, body forces, annotated F, are considered to be the forces acting on the Sessile drops tending to cause it to move along a surface. Body forces may arise from gravity, centrifugal forces, pressure differences or other forces.
Referring to
F=ρgV·sin β
Referring to
F=ρgV.
Referring to
F=ρVΩ2d,
Referring to
f=kγR·Δ cos θ,
Referring to
F=ρgV·sin α,
Referring to
F=AΔP=πR2ΔP,
Referring to
F=ρgV·sin β+πR2ΔP.
Now, referring to
f=kγR·Δ cos θ,
To summarize, retention force
f=kγR·Δ cos θ
For rough surfaces one can consider the geometric interaction of the droplet with the asperities 34 in the following equations.
θa=θa,0+ω,
θr=θr,0−ω.
Thus, for smooth surfaces, the retention force
fs=kγR(cos θr,0−cos θa,0).
For rough surfaces, the retention force
fr=kγR[ cos(θr,0−ω)−cos(θa,0+ω)].
Referring to
k=4/π, γ=72 mN/m,
2R=2 mm,
θa,0=110°,
θr,0=90°
and we will consider the variation in roughness (ω). Referring to
Thus, symmetric roughness leads to isotropic wetting because the value of fr is equal in symmetric directions.
Referring to
f1−f2=kγR[ cos(θr,0−ω1)−cos(θa,0+ω1)−cos(θr,0−ω1)+cos(θa,0+ω1)].
Thus, it is possible to calculate a retentive force ratio (f1/f2) caused by asymmetric roughness.
f1/f2=sin(ω1+1/2Δθ0)/sin(ω2+1/2Δθ0),
Thus, it is possible to compare the retentive forces on drops caused by asymmetric roughness. For this example we will assume a small sessile water drop on a PFA or PTFE surface. In this case k=4/π, y=72 mN/m, 2R=2 mm, θa,0=100°, θr,0=90° and we will vary the values of ω1 and ω2. The results of this calculation can be found in a table at
Referring to
It is also possible to compare the retentive forces related to slugs in a cylindrical tube. For this example we will assume a small water slug in PFA tube wherein
k=π,
γ=72 mN/m,
2R=10 μm,
θa,0=100°,
θr,0=90°.
When we vary the values of ω1 and ω2. The results of this calculation can be seen in the table depicted in
When these results are graphed, referring to
In addition, referring to
Generally, the substrate material may be any material upon which micro or nano scale asperities may be suitably formed. The asperities may be formed directly in the substrate material itself, or in one or more layers of other material deposited on the substrate material, by photolithography or any of a variety of suitable methods. Microscale asperities according to the invention may be formed using known molding and stamping methods by texturing the tooling of the mold or stamp used in the process. The processes could include injection molding, extrusion with a textured calendar roll, compression molding tool, or any other known tool or method that may be suitable for forming microscale asperities. Direct extrusion may be used to form asperities in the form of parallel ridges. Such parallel ridges are most desirably oriented transverse to the direction fluid flow. Features in flow channels of bipolar plates according to the invention may be formed with a compression molding tool having microscale asperities built into the molding surfaces for the flow channels.
Other methods that may be suitable for forming smaller scale asperities of the desired shape and spacing include nanomachining as disclosed in U.S. Patent Application Publication No. 2002/00334879, microstamping as disclosed in U.S. Pat. No. 5,725,788, microcontact printing as disclosed in U.S. Pat. No. 5,900,160, self-assembled metal colloid monolayers, as disclosed in U.S. Pat. No. 5,609,907, microstamping as disclosed in U.S. Pat. No. 6,444,254, atomic force microscopy nanomachining as disclosed in U.S. Pat. No. 5,252,835, nanomachining as disclosed in U.S. Pat. No. 6,403,388, sol-gel molding as disclosed in U.S. Pat. No. 6,530,554, self-assembled monolayer directed patterning of surfaces, as disclosed in U.S. Pat. No. 6,518,168, chemical etching as disclosed in U.S. Pat. No. 6,541,389, or sol-gel stamping as disclosed in U.S. Patent Application Publication No. 2003/0047822, all of which are hereby fully incorporated herein by reference. Carbon nanotube structures may also be usable to form the desired asperity geometries. Examples of carbon nanotube structures are disclosed in U.S. Patent Application Publication Nos. 2002/0098135 and 2002/0136683, also hereby fully incorporated herein by reference. Also, suitable asperity structures may be formed using known methods of printing with colloidal inks. A photolithography method that may be suitable for forming micro/nanoscale asperities is disclosed in PCT Patent Application Publication WO 02/084340, hereby fully incorporated herein by reference.
It is anticipated that fuel cell components having anisotropic wetting surfaces will exhibit greatly improved drainability due to the tendency of the surface to facilitate fluid flow in a desired direction, causing them to roll freely by gravity in the direction of surface slope. In addition, it is anticipated that an anisotropic wetting surface according to the present invention may improve heat transfer from the surface due to the increased surface area created by the presence of asperities on the surface.
The present invention may be embodied in other specific forms without departing from the central attributes thereof, therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than the foregoing description to indicate the scope of the invention.
