Patent Application
20060008696
This invention relates to membrane electrode assemblies for fuel cells.
Fuel cells provide electrical power generated by an electrochemical reaction. The reactants are typically a fuel (e.g., hydrogen) and an oxidizer (e.g., atomic or molecular oxygen). The fuel cell reaction takes place in or near an electrolyte, and electrodes (e.g., an anode and a cathode) are connected to the electrolyte in order to collect fuel cell output electrical current. The electrolyte conducts ions, but does not conduct electrons. The following description relates to solid oxide fuel cells, which are fuel cells having a solid oxide electrolyte. A catalyst is usually present at or near at least one of the electrodes, to facilitate the fuel cell reaction. Fuel cells have been under extensive development for many years. Accordingly, various fuel cell configurations have been considered in the art, which often differ from each other in structural and/or geometrical details relating to the electrolyte and electrodes.
For example, a commonly employed fuel cell configuration includes an integrated membrane electrode assembly (MEA). The MEA is a three layer structure with an electrolyte sandwiched between the electrodes. The electrodes are usually porous (e.g., as in U.S. Pat. No. 6,645,656) in order to permit flow of the fuel and oxidant through the electrode layers to the electrolyte. Elaborations on the basic idea of porous electrodes have been investigated. For example, U.S. Pat. No. 6,361,892 considers an electrode having through channels with a selected cross-section, to controllably modify reactant flow.
A significant motivation for the MEA configuration is to increase available reaction area. More specifically, the large interfaces between electrodes and electrolyte in an MEA provide much more reaction area than structures with point or wire electrode contacts. This desirable MEA feature has been further developed in U.S. Pat. No. 6,835,488, where an MEA is patterned in a mesoscopic 3-D pattern to further increase reaction area. Another example of a patterned MEA is considered in U.S. Pat. No. 5,518,829.
Instead of using a patterned MEA, an alternative approach for increasing fuel cell reaction area includes nanotubes (e.g., porous carbon nanotubes) in the MEA. Such approaches are considered in U.S. 2004/0170884 and U.S. 2004/0224217. Nanotubes have also been used as part of a support structure/flow plate in contact with an MEA, as in U.S. Pat. No. 6,589,682. Another approach for increasing reaction area (or power density) is considered in U.S. Pat. No. 6,495,279, where film deposition techniques are employed to fabricate multiple MEAs on top of each other in a stacked manner.
A noteworthy trend in the development of fuel cell technology is scaling the MEA to smaller and smaller dimensions (e.g. by reducing electrode and electrolyte layer thickness). A significant motive for this scaling is reducing internal fuel cell loss (e.g., ohmic ionic loss in the electrolyte). Such scaling can lead to problems not encountered in larger structures. In particular, mechanical fragility is an increasingly significant issue as MEA layer thickness decreases. The porous layers typically employed for anode and cathode electrode layers in an MEA are particularly troublesome, since the presence of pores in these layers significantly reduces their mechanical strength. Furthermore, since the electrolyte layer is preferably thin (to reduce its ohmic loss), it cannot easily be used to provide mechanical support for the electrodes.
Accordingly, it would be an advance in the art to provide a fuel cell MEA having improved mechanical strength and thereby scalable to smaller layer thicknesses than known MEAs. A further advance in the art would be to provide such an MEA having enhanced reaction area and catalytic activity.
A membrane electrode assembly (MEA) having a nano-tubular patterned structure and having solid (instead of porous) electrode layers is provided. Increased mechanical strength is provided by the use of solid electrode layers. The electrode layers are sufficiently thin to permit the flow of reactants to the electrolyte. The nano-tubular pattern includes multiple closed-end tubes and increase the reaction area to volume ratio of the MEA. The nano-tubular pattern also serves to increase mechanical strength, especially in a preferred honey-comb like arrangement of the closed-end tubes. A catalyst is preferably disposed on the anode and cathode surfaces of the MEA, and is preferably in the form of separated catalyst islands in order to increase reaction area. MEAs according to the invention can be fabricated by layer deposition on a patterned template. Atomic layer deposition is a preferred deposition technique.
a and 1b show perspective and cross-section views, respectively, of a template suitable for fabricating an embodiment of the invention.
a-f show a sequence of processing steps suitable for fabricating a preferred embodiment of the invention.
a-b show exemplary MEA support structures suitable for use with the invention.
a and 1b show perspective and cross-section views, respectively, of a template 102 suitable for fabricating a preferred embodiment of the invention.
Template 102 can be made from any material compatible with the MEA fabrication steps of
a-f show a sequence of processing steps suitable for fabricating a preferred embodiment of the invention. In summary, a first electrode layer, an electrolyte layer, and a second electrode layer are deposited in succession on a suitably patterned template (e.g., 102 on
a shows deposition of a first electrode layer 202 on template 102. In this example, first electrode layer 202 is a fuel-permeable, non-porous anode 202. The thickness of anode 202 is preferably in a range from about 2 nm to about 500 nm. Since anode 202 is not porous (i.e., does not include any voids extending across the anode thickness), diffusion of the fuel (in atomic, molecular and/or ionic form) through the solid anode is required for the fuel to reach the electrolyte. Such diffusion proceeds more efficiently as the anode thickness decreases. However, anode mechanical strength decreases as anode thickness decreases. Therefore, specific MEA designs according to the invention will require these competing factors to be appropriately balanced. Such balancing is within the skill of an art worker.
Suitable materials for anode 202 include: platinum, nickel, palladium, silver, doped perovskites (e.g., manganites, cobaltites and ferrites), and mixtures thereof. Suitable dopants for these perovskites include lanthanum, strontium, barium, cobalt and mixtures thereof. In more general terms, the anode is preferably a mixed ionic conductor having high conductivity for both ions and electrons. Suitable techniques for depositing anode 202 include sputtering, chemical vapor deposition, pulsed laser deposition, molecular beam epitaxy, evaporation and atomic layer deposition. Atomic layer deposition (ALD) is a preferred deposition technique because it can provide precise layer thickness control even when growth is performed on a patterned template having high aspect ratio features (i.e., the tubes).
b shows deposition of a solid oxide electrolyte layer 204 on anode 202. Suitable materials for electrolyte 204 include metal oxides having fluorite structure (e.g., stabilized zirconia, doped ceria, and doped bismuth oxide) and perovskites. Fluorite structure oxides can be doped with yttrium, scandium, gadolinium, ytterbium and/or samarium. The above electrolyte perovskites can have an ABO3 composition where A is lanthanum, calcium, strontium, samarium, praseodymium, or neodymium and B is aluminum, gallium, titanium or zirconium. Suitable dopants for electrolyte perovskites include lanthanum, strontium, barium, cobalt, magnesium, aluminum, calcium and mixtures thereof. The thickness of electrolyte 204 is preferably in a range from about 5 nm to about 500 nm. The above-mentioned techniques for depositing anode 202 are also applicable to depositing electrolyte 204. ALD is a preferred technique for electrolyte deposition.
c shows deposition of a second electrode layer 206 on electrolyte 204. In this example, second electrode layer 206 is an oxidant-permeable, non-porous cathode 206. The thickness of cathode 206 is preferably in a range from about 2 nm to about 500 nm. Since cathode 206 is not porous (i.e., does not include any voids extending across the cathode thickness), diffusion of the oxidant (in atomic, molecular and/or ionic form) through the solid cathode is required for the oxidant to reach the electrolyte. Such diffusion proceeds more efficiently as the cathode thickness decreases. However, cathode mechanical strength decreases as cathode thickness decreases. Therefore, specific MEA designs according to the invention will require these competing factors to be appropriately balanced. Such balancing is within the skill of an art worker.
Suitable materials for cathode 206 include: platinum, nickel, palladium, silver, doped perovskites (e.g., manganites, cobaltites and ferrites), and mixtures thereof. Suitable dopants for these perovskites include lanthanum, strontium, barium, cobalt and mixtures thereof. In more general terms, the cathode is preferably a mixed ionic conductor. The above-mentioned techniques for depositing anode 202 are also applicable to depositing cathode 206. ALD is a preferred technique for cathode deposition. The exemplary fabrication sequence of
d shows optional deposition of a cathode catalyst 208 on cathode 206. Preferably catalyst 208 includes multiple sub-micron catalyst islands separated from each other (as shown), in order to increase the effective reaction area of the catalyst. It is preferable for some of these catalyst islands to be disposed inside the closed-end tubes, in order to exploit the increased surface area provided by the tubes. Suitable catalyst materials include platinum, nickel, palladium, silver, and mixtures or alloys thereof. Preferably, catalyst 208 is deposited via ALD in a growth parameter regime that inherently provides islanded growth (e.g., as considered in U.S. 2003/0194598). In this manner, catalyst islands can be deposited on the patterned cathode without requiring a separate catalyst patterning step. Catalyst 208 preferably facilitates the incorporation of oxidant into cathode 206 in a form that can diffuse through the cathode.
e shows removal of template 102 from the membrane electrode assembly including anode 202, electrolyte 204 and cathode 206. Such removal can be performed by any process (e.g., etching) that selectively removes template 102 while not degrading the MEA.
f shows optional deposition of an anode catalyst 210 on anode 202. The description of cathode catalyst 208 in connection with
The mechanical strength of MEA 250 is advantageously increased by two important structural features. First, the anode and cathode layers are solid layers, in contrast to conventional porous electrode layers. Such solid layers provide increased mechanical strength. Second, the tubular pattern of MEA 250 can act to increase mechanical strength, especially in the preferred configuration shown on
Membrane electrode assemblies according to the invention are preferably supported by mechanical support structures. Suitable support structures are known in the fuel cell art.
The preceding description has been by way of example as opposed to limitation. Many variations of the preceding examples also fall within the scope of the present invention. Foe example, the MEA anode and cathode regions can include both porous and non-porous layers.
Another variation is to alter the geometry so that the closed-end tubes extend inward from both the anode surface and the cathode surface, as opposed to extending inward from only one of the surfaces (as shown on
This application claims the benefit of U.S. provisional patent application 60/584,767, entitled “Thin-Film Solid Oxide Fuel Cell”, filed on Jun. 30, 2004, and incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60584767 | Jun 2004 | US |
