Patent Application
20100196793
The present disclosure is relevant to the field of fuel cells, in particular, to high-temperature proton exchange membrane fuel cells.
Proton exchange membrane (“PEM”) fuel cells are an established fuel cell technology being developed for many applications including transportation (e.g., in automobiles), portable power systems, or stationary systems (e.g., to power a home or building). High-temperature proton exchange membranes (“HTPEM”) are an emerging technology that allows a fuel cell to run at temperatures above 100° C., thus, among other benefits, providing improved tolerance of carbon monoxide and reducing the likelihood that water generated by chemical reactions in the fuel cell will collect in porous layers of the device.
The anode 16 and cathode 18 each comprise respective gas diffusion layers 20, 22, which are porous so as to permit the passage of oxygen (O2, which may be supplied in air) supplied at the cathode-side gas diffusion layer 22 and a proton-donating fuel (in this example, hydrogen (H2)) supplied at the anode-side gas diffusion layer 20. The gas diffusion layers 20, 22 are also electrically conductive so as to permit the flow of electrons (e).
The anode 16 and cathode 18 further comprise catalyst layers 24, 26 (shown in
At the catalyst layer 24 of the anode 16, hydrogen gas is catalytically converted to protons and electrons. The central membrane 14 allows passage of protons through the central membrane 14 to the cathode-side catalyst layer 26, where they react with oxygen to form water. Since the central membrane 14 is not electrically conductive, electrons do not pass through it, but pass through a external circuit 40 to the cathode 18, where they participate in the reaction between the protons and oxygen. The three species (O2, e−, and H+) must converge on a catalytic site (e.g., a platinum site 28) in order to complete the electrochemical reaction that generates electricity in the fuel cell 10.
As is apparent from
In one aspect, some embodiments of the present invention comprise membrane-electrode assembly for use in a fuel cell that includes an anode, a cathode and a nano-engineered polymeric membrane therebetween. The membrane is an electrical insulator, but permits the passage of protons therethrough. The membrane has a plurality of blind pores therein, at least some of which having an electrically-conductive coating arranged along their inner walls so as to allow electrical communication therewith from an external location. In some embodiments, the coating includes a catalyst, such as platinum, for promoting the liberation of a proton and an electron from a chemical molecule, such as hydrogen, and the coating is in electrical communication with either the anode or the cathode. In other embodiments, each of two opposing sides of the membrane has a plurality of such blind pores. In yet other embodiments, the coating in the blind pores at one surface of the membrane includes a catalyst for promoting the liberation of a proton and an electron from a chemical molecule and the coating in the blind pores at the other, opposed side of the membrane includes a catalyst for promoting the combination of a proton, an electron and another molecule, such as oxygen, to form yet another molecule, such as water. For each of the aforesaid embodiments, there are associated embodiments wherein the blind pores have diameters of less than 1000 nm and aspect ratios of at least 10. For each of the aforesaid embodiments, there are associated embodiments wherein the membrane and other components of the membrane-electrode assembly are adapted for use in a high-temperature proton-exchange membrane electrode assembly.
In another aspect, some embodiments of the present invention comprise a fuel cell that includes a membrane-electrode assembly according to one of the aforesaid embodiments of a membrane-electrode assembly. In some embodiments, the fuel cell is adapted for operation at temperatures greater than 150° C. In other embodiments, the fuel cell is adapted for operation at temperatures greater than 400° C.
In yet another aspect, some embodiments of the present invention comprise a nano-engineered polymeric membrane The membrane is an electrical insulator, but permits the passage of protons therethrough. The membrane has a plurality of blind pores therein, at least some of which having an electrically-conductive coating arranged along their inner walls so as to allow electrical communicating therewith from an external location. In some embodiments, the coating includes a catalyst, such as platinum, for promoting the liberation of a proton and an electron from a chemical molecule, such as hydrogen. In other embodiments, each of two opposing sides of the membrane has a plurality of such blind pores. In yet other embodiments, the coating in the blind pores at one surface of the membrane includes a catalyst for promoting the liberation of a proton and an electron from a chemical molecule and the coating in the blind pores at the other, opposed side of the membrane includes a catalyst for promoting the combination of a proton, an electron and another molecule, such as oxygen, to form yet another molecule, such as water. For each of the aforesaid embodiments, there are associated embodiments wherein the blind pores have diameters of less than 1000 nm and aspect ratios of at least 10. For each of the aforesaid embodiments, there are associated embodiments wherein the membrane material is selected to provide a membrane that is structurally stable, electrically-insulating and permissive to the flow of protons at temperatures of at least 150° C. and/or at temperatures of at least 400° C.
In a further aspect, some embodiments of the present invention comprise a method for making a nano-engineered polymeric membrane, including the steps of forming blind pores in a polymeric membrane and coating the inner walls of the blind pores with an electrically-conductive coating. In some of said embodiments, the step of forming the blind pores includes pressing a heated template against the polymeric membrane at a temperature near the glass transition temperature of the polymer, then removing the template. In some such embodiments, the template includes a plurality of substantially-aligned whiskers extending from a substrate, wherein some of the whiskers have diameters of less than 1000 nm. In some embodiments, the step of coating the inner walls of the blind pores is performed using a method of atomic layer deposition. In other embodiments, the step of coating the inner walls of the blind pores is performed using a method of physical vapor deposition, such as ionized physical vapor deposition.
For a better understanding of the present invention, reference is made to the following detailed description of the exemplary embodiments considered in conjunction with the accompanying drawings, in which:
The nanopores 62 of central membrane 46 increase the interfacial area of the central membrane 46, thus decreasing the effects of transport resistance across the interfaces of the central membrane 46 and the catalyst layers 52, 58. The catalyst coating 66 of the nanopores 62 increases the contact area available to the hydrogen or other proton-donating fuel at the anode 48, allowing the thickness of the catalytic layer 52 to be reduced. The array of nanopores 62 in the central membrane 46, created with tools and processes developed in the field of nanotechnology, may enhance this interfacial area by factors of greater than 100. The higher the densities of nanopores 62 per area of the central membrane 46, the greater the interfacial area that is available. The catalyst layers 52, 58 of the nano-engineered MEA 44 (as opposed to the catalyst coating 66 in the nanopores 62) are similar in composition and structure to conventional catalyst layers (such as the catalyst layers 24, 26 of
Polymers suitable for use as the central membrane 46 in a nano-engineered MEA 44 (e.g., PBI) can be patterned with nanopores 62 by adaptations of methods already known in the art, which include hot embossing, nanoimprinting, and soft lithography. One adaptation of nanoimprinting that is particularly well-suited for fabricating a nano-engineered MEA 44 according to an embodiment of the present invention, is discussed elsewhere herein.
Platinum and platinum-plated carbon are discussed herein as exemplary catalysts. Platinum is known to promote efficient conversion of hydrogen molecules to protons. Other catalysts currently known, or to be developed, may be used in place of, or along with, platinum catalysts, as long as they can facilitate the proton exchange reactions that produce electricity in a HTPEM fuel cell. Such known catalysts include, but are not limited to, platinum, platinum-ruthenium, and platinum-nickel catalysts. Other catalysts that may be used in PEM fuel cells are presently under development, and their utility in embodiments of the present invention will be recognized as the art continues to develop.
The ability to create a thin catalyst coating for the nanopore walls (e.g., catalyst coating 66 of nanopores 62 of
Ionomeric membranes were patterned with microscale depressions having dimensions of about 1-5 μm by soft lithography molding (Zhou, et al., “Molded, High Surface Area Polymer Electrolyte Membranes from Cured Liquid Precursors,” Journal of the American Chemical Society 128, 12963-12972 (2006), the disclosure of which is incorporated herein by reference in its entirety). The maximum dimensions achieved were limited by the microfabrication techniques used, but the interfacial surface areas of the patterned areas of the membranes were doubled. With reference to
In an assessment of a membrane according to an embodiment of the present invention, a finite element model was constructed of a single-pore structure in a small section of PBI membrane to determine the effect of the three-dimensional structure on membrane resistance (i.e., the inverse of membrane conductance). The modeled structure represented a single pore having a closed end, a diameter of 1 μm (i.e., 1000 nm) and a depth of 10 μm. Due to the symmetric geometry of the pore, the computational domain consisted of half of the membrane with cylindrical symmetry applied. The highest proton current density around the modeled pore occurred at the closed end of the pore. For a modeled pore having a diameter of 500 nm, increasing pore depth from 0 to 15 μm improved conductance by factors as great as 2.4. This model is simplistic in that it solves for current flow through a material of defined shape and ionic conductance with fixed voltage at the boundaries. The model simply represents the membrane and includes no mass transport external to the membrane or reaction kinetic effects. However, it strongly indicates an immediate benefit of the incorporation of a nanostructured three-dimensional interface in terms of improved conductance.
Increasing the interfacial area between the catalyst layer and the central membrane allows the use of thinner catalyst layers while maintaining the same catalyst loading (i.e., related to the number of catalytic sites), or while decreasing the catalyst loading. As discussed below, it has been shown that reducing catalyst layer thickness in conventional, low-temperature fuel cells can lead to improved fuel cell performance. The embodiments of a nano-engineered HTPEM fuel cell, such as HTPEM fuel cell 42 of
A detailed kinetic and mass-transport model was developed to perform a numerical optimization study of the critical MEA cathode parameters, including catalyst layer thickness, catalyst loading, and others (e.g., ionomer content and void fraction) (Song, et al., “Numerical optimization study of the catalyst layer of PEM fuel cell cathode,” J. Power Sources 126, 104-111 (2004), the disclosure of which is incorporated by reference herein in its entirety). For a two-parameter optimization of catalyst layer thickness and loading, it was observed that, for a constant catalyst loading, electrical current density increased dramatically as catalyst thickness decreased, up to a maximum current density, after which the current density dropped precipitously. For constant catalyst thicknesses, increasing catalyst loading had only a slight effect on current density. The tradeoff between catalyst loading and layer thickness can be expressed as follows: electrical current density increases with catalyst loading (reflecting increases in the number of active catalytic sites) and decreases with catalyst layer thickness (reflecting limits on diffusion). Although this simulation was specific to a conventional, low-temperature Nafion®-PEM fuel cell rather than a high-temperature PBI-HTPEM fuel cell, it may be seen that (1) at a given catalyst loading, the use of thinner catalyst layers provides significant benefits, and (2) higher current densities may be achieved at lower catalyst loading by using thinner catalyst layers.
In other studies (K. Karan, “Assessment of transport-limited catalyst utilization for engineering of ultra-low Pt loading polymer electrolyte fuel cell anode,” Electrochem. Comm. 9, 747-753 (2007), and K. Karan, Proceedings of 211th Meeting of the Electrochemical Society, May 6-10, 2007, Chicago, Ill., The Electrochemical Society, Pennington, N.J. (2007), both of which are incorporated herein by reference in their entireties) a numerical study of anode catalyst utilization in conventional low-temperature PEM fuel cells was performed using an agglomerate model. It was shown that a significant fraction of the anode catalyst layer was inactive and contributed to polarization loss by introduction of mass transport resistance. This suggests that, for typical PEM anode parameters, a significant fraction of the catalyst layer is unnecessary and could be eliminated without diminishing performance. It was also shown that, for the PEM anode, reduction in thickness did not simply preserve performance. Rather, a thinner anode catalyst layer, even at reduced catalyst loading, led to an increase in power density.
Forming Nanopores with Nanoimprint Lithography
Nanoimprint lithography (NIL) may be beneficially employed to generate a three-dimensional pattern in a membrane according to an embodiment of the present invention.
The aforementioned conditions derive from the viscoelastic thermal response of thermoplastic polymers as illustrated in
In an embodiment of a process for making a nano-engineered HTPEM fuel cell according to the present invention, it is important to prepare a die with the appropriate nanoscale geometry to be replicated in the central membrane 46. While such a die may be used repeatedly, it must first be constructed with the desired geometry and pattern. To create a die suitable for forming nanopores in a membrane, such as nanopores 62 in central membrane 46 of
Suitable porous filter media for use in forming nanowhiskers include those such as the Nuclepore® and Anopore® membranes (Whatman, Inc., Maidstone, Kent, UK). The aforementioned filter media comprise membranes of either ion-track etched polymer or anodic aluminum oxide with highly uniform nano to microscale pores (i.e., from about 10 nm to about 20 μm) penetrating the membrane.
Forming Nanoporous Membranes with Solvent Casting
Besides being used to imprint pores into membranes, nanowhisker dies may be used as molding masters for solvent casting raw polymers into membranes having nanopores. In a solvent casting process, a raw polymer is dissolved in a solvent and the solution is deposited onto the die. The solvent evaporates, leaving behind a polymer membrane having the inverse imprint of the nanowhiskers (i.e., nanopores) of the die. PBI material, for example, is available as high purity raw polymer, which is able to be solvent cast into high quality HTPEM membranes.
Coating Nanopores with Atomic Layer Deposition
Atomic layer deposition (“ALD”) is a surface-controlled thin-film manufacturing process allowing the formation of atomically-controlled surfaces. It is based on the saturation of individual, sequentially-performed surface reactions between the substrate and each of the reactants needed. The saturation mechanism of sequenced ALD results in inherent conformal coating characteristics, thickness uniformity, and homogeneity of the films thus produced. The ability to conformally coat high-aspect-ratio pores is illustrated by
Coating Nanopores with Ionized Physical Vapor Deposition
Physical vapor deposition of thin films by sputtering has been in common usage for decades. Sputtering is a relatively violent, atomic-scale process in which an energetic particle strikes a solid, resulting in the emission of one or more substrate atoms from the solid. On a macroscopic scale, sputtering is a nearly isotropic deposition process when used at short-throw distances with a wide-area cathode source. However, on an atomic scale, the sputtered atoms tend to travel in straight lines without in-flight collisions from the cathode to the sample at the pressures most commonly used. Since the sputtered atoms are virtually all neutral (i.e., not electrically charged), it is not possible to redirect their trajectories in flight.
A method for enhancing the net directionality of a metallic deposit is to ionize the majority of the sputtered atoms and form the film from metal ions (i-PVD). If the acceleration potential for the ions is significantly greater than their thermal energy, the ions will arrive at the wafer surface at angles of almost exactly 90° with a controllable energy. If a high percentage of the metal flux is ionized, deposition will be primarily directional and the utilization of the sputtered atoms from the cathode will be high.
In the late 1980s and early 1990s, sputtering deposition systems were specifically fabricated to optimize the relative ionization of metal particles prior to deposition. Early systems used electron cyclotron resonance (ECR) and were based on either sputtering or evaporation. Another primary direction has been the use of dense, inductively-coupled plasmas in conjunction with a metal-sputtering source. This latter technique has proven to be the most robust for semiconductor manufacturing applications.
The effectiveness of i-PVD relative to other sputtering approaches for depositing titanium (Ti) into a pore is shown in
It should be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications thereto without departing from the spirit and scope of the present invention. All such variations and modifications, including those discussed above, are intended to be included within the scope of the invention, which is described, in part, in the claims presented below.
This application claims the benefit of U.S. Provisional Patent Application No. 61/206,627, filed on Feb. 2, 2009, the disclosure of which is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61206627 | Feb 2009 | US |
