Patent Publication Number: US-2006008696-A1

Title: Nanotubular solid oxide fuel cell

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      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. 
    
    
     FIELD OF THE INVENTION  
      This invention relates to membrane electrode assemblies for fuel cells.  
     BACKGROUND  
      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.  
     SUMMARY  
      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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1   a  and  1   b  show perspective and cross-section views, respectively, of a template suitable for fabricating an embodiment of the invention.  
       FIGS. 2   a - f  show a sequence of processing steps suitable for fabricating a preferred embodiment of the invention.  
       FIGS. 3   a - b  show exemplary MEA support structures suitable for use with the invention.  
       FIG. 4  shows a close up cross section view of an MEA according to an alternate embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
       FIGS. 1   a  and  1   b  show perspective and cross-section views, respectively, of a template  102  suitable for fabricating a preferred embodiment of the invention.  FIG. 1   b  shows a cross section view of template  102  along line  104  on  FIG. 1   a . A key feature of templates suitable for fabricating embodiments of the invention is that they include at least two closed-end tubes. As indicated in the following description, MEA fabrication on such a template leads to approximate replication of these closed-end tubes in the MEA. In the example of  FIGS. 1   a - b , the tubes are arranged on a hexagonal lattice and themselves have a hexagonal cross section. More generally, the tubes can be arranged on a periodic lattice (e.g., a square or rectangular lattice), a quasi-periodic arrangement or an aperiodic arrangement. The tube cross section can be any shape (e.g., square, rectangular, circular, elliptical, etc.). The tubes of the present invention are micron or sub-micron features. More specifically, the depth of the tubes is preferably between about 20 nm and about 10 μm, and the lateral extent of the tubes is preferably less than 10 μm and is more preferably between about 20 nm to about 2 μm.  
      Template  102  can be made from any material compatible with the MEA fabrication steps of  FIGS. 2   a - e . Suitable materials include silicon, silicon oxide, metal oxides (such as anodized alumina), and polymers. The closed-end tubes can be formed in template  102  by known microfabrication and/or nanofabrication techniques (e.g., lithography, anodization and/or self-assembly techniques).  
       FIGS. 2   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  FIG. 1   a ). These three layers together form an MEA which has the desired features (i.e., closed-end tubes). The first electrode layer can be the anode and the second electrode layer can be the cathode. Alternatively, the first electrode layer can be the cathode and the second electrode layer can be the anode. Optionally, a catalyst is disposed on the first and/or second electrode layers.  
       FIG. 2   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).  
       FIG. 2   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 ABO 3  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.  
       FIG. 2   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  FIGS. 2   a - f  shows deposition of cathode on top of electrolyte on top of anode. Alternatively, deposition of anode on top of electrolyte on top of cathode can be used to fabricate embodiments of the invention.  
       FIG. 2   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.  
       FIG. 2   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.  
       FIG. 2   f  shows optional deposition of an anode catalyst  210  on anode  202 . The description of cathode catalyst  208  in connection with  FIG. 2   d  is also applicable to anode catalyst  210 . Catalyst  210  preferably facilitates the incorporation of fuel into anode  202  in a form that can diffuse through the anode. The completed MEA structure  250  shown in  FIG. 2   f  has several important structural features. In particular, MEA  250  includes closed-end tubes which are replicas (or near replicas) of the closed end tubes of template  102 . Although MEA  250  is thereby patterned, its thickness is substantially uniform. More specifically, the separation between an anode surface  230  and a cathode surface  220  is substantially uniform within the MEA. This “folding” of an otherwise planar MEA advantageously increases the area to volume ratio of the MEA.  
      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  FIG. 1   a , where the geometry is similar to that of a honeycomb. Honeycomb type geometries tend to be effective for increasing mechanical strength. By increasing mechanical strength in this manner, the present invention facilitates further decrease of electrode and electrolyte layer thickness, which in turn can advantageously reduce fuel cell loss.  
      Membrane electrode assemblies according to the invention are preferably supported by mechanical support structures. Suitable support structures are known in the fuel cell art.  FIGS. 3   a - b  show two exemplary MEA support structures suitable for use with the invention.  FIG. 3   a  shows an MEA  250  of the present invention on a support structure  302 . Support structure  302  is preferably porous and electrically conductive, in order to facilitate reactant flow to MEA  250  and to provide electrical contact to MEA  250 . An alternative arrangement is shown on  FIG. 3   b , where a flow plate  304  includes channels for reactant flow to MEA  250 . Flow plate  304  need not be porous, since the channels provide a reactant flow path. However, flow plate  304  is preferably electrically conductive in order to provide electrical contact to MEA  250 . Although  FIGS. 3   a - b  show support on only one side of the MEA, it is preferred for both sides of the MEA to be in contact with suitable support structures.  
      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.  FIG. 4  shows a close up (i.e., on a smaller scale than the tubular patterning) cross section view of an MEA according to the invention and having such a structure. In this example, an electrolyte  406  is sandwiched between non-porous anode and cathode layers  404  and  408  respectively. A porous anode layer  402  is adjacent to non-porous anode layer  404 . A porous cathode layer  410  is adjacent to non-porous cathode layer  408 . Use of both porous and non-porous electrode layers provides additional design parameters for optimizing the combination of mechanical strength and fuel cell performance. Porous electrode layers  402  and  410  can be made of the same materials described above as suitable for non-porous electrode 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  FIG. 1   a ). A further variation of the invention is to include electrolyte materials in the anode or cathode composition. More specifically, materials described above in connection with electrolyte  204  can be included in anode  202  and/or in cathode  206 . The addition of electrolyte material to the electrodes can increase the ionic conductivity of the anode and/or cathode, as well as decrease the interfacial resistance at the electrolyte-anode interface and/or the electrolyte-cathode interface.