Additives for fuel cell layers

The technical field to which the description of exemplary embodiments generally relates includes fuel cells and fuel cell components including an ionomer layer, membrane, catalyst, substrate, and the like, and methods of making and using the same.

Fuel cells using solid polyelectrolyte membranes and electrodes are known. Those skilled in the art are continually working on improvements and alternatives to existing fuel cells, fuels components, and methods of making and using the same.

A product comprising: a first layer comprising an ionomer and an additive. The additive comprises a metal oxide comprising an oxide of at least one of Ce, Mn, V, Pt, Ru, Zr, Ni, Cr, W, Co, Mo or Sn. The additive may be present in at least 0.1 weight percent of the ionomer.

Other exemplary embodiments of the invention will be apparent from the following brief description of the drawings, detailed description of exemplary embodiments, and appended claims and drawings.

The exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 illustrates a product according to one embodiment of the invention.

FIG. 2 illustrates a product according to another embodiment of the invention.

FIG. 3 illustrates a product according to another embodiment of the invention;

FIG. 4 illustrates a product according to another embodiment of the invention.

FIG. 5 illustrates a product according to another embodiment of the invention.

The following description of the following embodiment(s) is merely exemplary in nature and is in no way intended to limit the claimed invention, its application, or uses.

Referring now to FIG. 1 , one embodiment of the invention includes a fuel cell substrate 10 including an ionomer and an additive. Substrate 10 may be a membrane or an electrode, In various embodiments of the invention, the additive improves cell performance, membrane durability and/or proton conduction. In one embodiment of the invention, the additive includes an oxide of at least one of Ce, Mn, V, Pt, Ru, Zr, Ni, Cr, W, Co, Mo or Sn or derivates thereof. In still another embodiment of the invention, the additive is a metal oxide including, but is not limited to, MnO 2 , CeO 2 , PtO 2 , RuO 2 , VO, VO 2 , V 2 O 3 , V 2 O 5 , CeO 2 , CoO, Co 2 O 3 , NiO 2 , CrO 2 , WO 3 , SnO 2 , MoO 3 , MoO 2 , Mn 2 O 3 , and/or derivatives thereof, including but not limited to, tertiary and/or quaternary compounds. In one embodiment the additive includes nanoparticies of ceria (CeO 2 ), CeZrO 4 or Ce 1-x Zr x O 2 , where X is less than 1. In another embodiment of the invention, the additive includes a combination of metal oxides including, but not limited to, CeO 2 and MnO 2 (preferred), CeO 2 and CoO; CeO 2 and Co 2 O 3 ; CeO 2 and VO; CeO 2 and VO 2 ; CeO 2 and V 2 O 3 ; CeO 2 and V 2 O 5 ; CeO 2 , NiO 2 or CrO 2 in combination with at least one of CeO 2 , MoO 3 , MoO 2 , MnO 2 , Mn 2 O 3 , RuO 2 , WO 3 , PtO 2 or SnO 2 . In one embodiment of the invention, the additive may include nanoparticles. That is, particles having at least one dimension that is less than 200 nm. The use of nanoparticles was found to improve the performance of the membrane assemblies compared to assemblies using marcoparticles.

The ionomer may be a polymeric macromolecule including a portion of its constituent units having ionizable or ionic groups, or both. In one embodiment of the invention, the ionomer includes polymeric molecules having carbon-fluorine backbone chains with perfluoro side chains containing sulfonic groups (also known as perfluorosulfonic acid polymers.) Such ionomers are available from E. I. DuPont de Nemours & Company under the trade designation NAFION®. Other such ionomers are available from Asahi Glass & Asahi Chemical Company. In alternative embodiments, the ionomer may include perfluorinated cation exchange polymers, hydrocarbon-based cation exchange ionomers, as well as anion-exchange ionomers. In other alternative embodiments, the ionomer includes sulfonated hydrocarbon polymers including aromatic and non-aromatic sulfonic acid polymers, including sulfonated-polysulfones, -polyetheretherketone, -polyarylene ether ketones, -polyarylenethioether ketones, -polyarylene ether sulfones, -polyarylenethioether sulfones, -polybenzimidazoles, -polyimides, -polyphenylenes or -polyphenylenesulfide.

In various illustrative embodiments of the invention, the additive may be present in at least 0.1 weight percent (wt. %), about 1 to about 20 wt. %, about 5 to about 15 wt. %, or about 8 to about 12 wt. % of the dried ionomer 10 . The additive may be present in greater amounts, however, too high oxide loading in the membrane may result in brittle membranes that are easy to tear apart. The additive may be mixed with the ionomer in wet form, including water and alcohol. In the wet phase of the mixture, the water may be present in about 40 to about 60 wt. % and the alcohol present in about 40 to about 60 wt. %. In some embodiments of the invention, the additive is introduced into the mixture with a metal oxide purity ranging from about 90-100 percent or about 99.99-100 percent. For example, for an additive including MnO 2 having a metal oxide purity ranging from 90-100 percent pure, the metal oxide includes 90-100 wt. % MnO 2 and 0-10 wt. % impurities, such as impurities that are typically found in Mn ore or in the manufacture of MnO 2 . In various embodiments of the invention, the substrate 10 including the ionomer and additive may have a thickness, in the dried state, of less than 100 microns, from about 5-50 microns, and from about 10-40 microns or thicknesses therebetween. When MnO 2 at less than 5-micrometers in particle dimensions and a purity of 85 percent was added to a 20 wt. % Nafion 1000 (DE 2020) dispersion in 1-propanol-water (available from E. I. DuPont de Nemours and Company), the ionomer aggregated after 1 hour and the dispersion was no longer coatable. Hence, a dispersion of 85 wt. % MnO 2 in Nafion 1000 (at 10 wt. % loading of MnO 2 per ionomer solids) must be coated into films within 1 hour. This does not take place with high-purity MnO 2 . Referring now to FIG. 2 , in another embodiment of the invention, the substrate 10 may include a first layer 12 , a second layer 14 overlying the first layer 12 , and a third layer 16 underlying the first layer 12 . In one embodiment of the invention, the first layer 12 includes an ionomer but without any metal oxide. The second layer 14 and the third layer 16 each include an ionomer and an additive, such as a metal oxide. Each of the first layer 12 , second layer 14 and third layer 16 may have a thickness, in the dried state, ranging from 5-50 microns, 5-35 microns, 10-35 microns, or thicknesses therebetween. A cathode catalyst layer may overlie the first layer 12 and an anode catalyst layer may underlie the third layer 16 , and wherein both the second and third layers 14 and 16 have an additive therein that may be the same or different from each other.

Referring now to FIG. 3 , in another embodiment of the invention, a solid polyelectrolyte membrane 100 is provided and is substantially free of metal oxides. A first catalyst layer 18 is provided overlying the membrane and a second catalyst layer 18 ′ is provided underlying the membrane 100 . At least one of the first catalyst layer 18 or second catalyst layer 18 ′ includes an additive, such as a metal oxide, present in at least one weight percent of the dried ionomer in the catalyst layer 18 or 18 ′. The additive in layers 18 and 18 ′ may be the same or different from each other.

Referring now to FIG. 4 , one embodiment of the invention may include a bilayer membrane 10 including a first portion 12 and a second portion 14 . A first catalyst layer (for example an anode) may overlie the membrane 10 and a second catalyst layer 18 ′ may underlie the membrane 10 . The first portion 12 may include a first additive such as a first metal oxide and the second portion 14 may include a second additive such as a second metal oxide. The first additive may include a metal oxide that is different from the metal oxide of the second additive. For example, the first portion 12 (adjacent the cathode 18 ′) may include MnO 2 and the second portion 14 (adjacent the anode 18 ) may include CeO 2 . In another embodiment at least one of the catalyst layers 18 , 18 ′ may include the first additive and at least a portion of the membrane 10 may include the second additive, wherein the membrane 10 may be made from a single layer or multiple layers. For example, the anode layer 18 may include CeO 2 and the first portion 12 (adjacent the cathode) of the membrane 10 may include MnO 2 , and wherein the second portion 14 of the membrane 10 and the cathode 18 ′ may or may not include a metal oxide additive.

Referring now to FIG. 5 , one embodiment of the invention includes a product including a fuel cell substrate 10 which functions as a proton exchange membrane. The substrate 10 includes a first layer 12 including an ionomer without a metal oxide. A second layer 14 is provided over the first layer 12 and a third layer 16 is provided under the first layer 12 . Each of the second layer 14 and third layer 16 includes an ionomer and an additive such as a metal oxide or a mixture of metal oxides. As such, the substrate 10 , in this case a polyelectrolyte membrane, includes a central portion that is free of the metal oxide additive. A first catalyst layer 18 is provided over the second layer 14 of the substrate 10 . A first gas diffusion media substrate 22 may be provided over the first catalyst layer 18 , and optionally, a microporous layer 20 may be interposed between the diffusion media substrate 22 and the first catalyst layer 18 . Diffusion media substrate 22 may include, but is not limited to, a carbon paper substrate. The microporous layer 20 may be coated on the diffusion media substrate 22 from a mixture of carbon and fluoropolymer such as FEP, PVDF, HFP, PTFE, etc. The first catalyst layer 18 preferably includes a group of finally divided catalyst support particles, such as carbon, supporting finally divided catalyst particles and an ion conductive material, intermingled with the support and catalyst particles. The ion conductive material in the first catalyst layer 18 may be an ionomer such as perfluorinated sulfonic acid polymer. Catalyst materials may include, but are not limited to, metals such as platinum, palladium and mixtures of metals such as platinum and molybdenum, platinum and cobalt, platinum and ruthenium, platinum and nickel, and platinum and tin, or platinum transition-metal alloys. A first bipolar plate 24 may be provided having gas flow field channels 28 and lands 26 formed in one face, and cooling channels 30 formed in another face. Likewise, a second catalyst layer 18 ′, a second microporous layer 20 ′, a second gas diffusion media substrate 22 ′ and second bipolar plate 24 ′ may be provided on the opposite side of the fuel cell substrate 10 in a similar fashion to form a fuel cell assembly 50 .

In one embodiment of the invention, a slurry is prepared including an ionomer, additive and a solvent. The solvent may include at least one of 1-propanol, ethanol, 1-butanol, methanol, or the like or water. The slurry may be milled, for example, using glass beads and thereafter filtered, for example, using a mesh having openings therein less than 90 micron. The filtered slurry may be cast and dried as a single layer or multiple layers may be cast and dried to form, in one embodiment of the invention, a multi-layer membrane wherein at least one of the layers includes the additive.

Processes other than ball-milling can be used to form the metal oxide ionomer dispersions. Other known wet mixing methods and melt processes can be used including: attritors, homogenizers, mixers, polytron heads, melt extruders, Banbury roll mills, and other such mixing equipment can be used. Illustrative examples of making membranes are also set forth in Examples 8 and 9 below.

Fuel cell test were perform for membrane electrode assemblies using a catalyst coated diffusion media and a membrane of NAFION (N1000) and an additive including an oxide of Mo 6+ , V 5+ , V 3+ , Cr/Zr, alone or with ceria, and the membrane assemblies were found to run every condition. The conditions for which these membranes were evaluated with catalyst coated diffusion media was as follows: Membranes were screened in fuel cells and performance were summarized in polarization curves where cell voltage (in volts) is plotted versus current density (in Amps/cm 2 ) under the following conditions: 150% relative humidity (R.H.) out: 2/2 (A/C) stoic; 100/50% (A/C) inlet R.H.; 80° C.; 170 kPa gauge; 110% relative humidity (R.H.) out: 2/2 (A/C) stoic; 100/50% (A/C) inlet R.H.; 80° C.; 50 kPa gauge; 85% relative humidity (R.H.) out: 3/3 (A/C) stoic; 50/50% (A/C) inlet R.H.; 80° C.; 75 kPa gauge; 80% relative humidity (R.H.) out: 2/2 (A/C) stoic; 35/35% (A/C) inlet R.H.; 80° C.; 50 kPa gauge; 63% relative humidity (R.H.) out: 3/3 (A/C) stoic; 32/32% (A/C) inlet R.H.; 80° C.; 50 kPa gauge; where (A/C) refers to anode/cathode. When polarization curves are obtained where the current density runs out to 1.2 A/cm 2 with reasonable voltage (usually greater than 0.4V), the membranes are said to “run every condition.”

When the terms “over”, “overlying”, “overlies”, or “under”, “underlying”, “underlies” are used herein with respect to the relative position of one component or layer with respect to a second component or layer, such shall mean that the first component or layer is in direct contact with the second component or layer, or that additional layers or components may be interposed between the first component or layer and the second component or layer.

Comparitive Example 1

As a control sample, Nafion® solution DE2020 at 20 wt. % polymer solids of Nafion® PFSA polymer with 1000 equivalent weight in 40-wt. % aqueous, 1-propanol (available from E.I. DuPont de Nemours, Co) was cast onto a glass plate with a 3-mil gap Bird applicator, and then the coated glass plate was oven-dried at 80° C. for 30 minutes. A second coating was applied to the first film with the 3-mil gap Bird applicator and the polymer-coated glass plate was heated at 120° C. for 16 hours. The coated glass plate, after cooling to 25° C., was immersed in a de-ionized water bath until the polymer film released from the glass. The polymer film was allowed to air dry on a paper towel, and the resultant dry film was 25-micrometers in thickness. The polymer film was cut into a 4-inch by 4-inch square and was assembled in a fuel cell (50-cm 2 hardware) with catalyst coated diffusion media. The performance was measured under dry, intermediate and wet conditions. A durability test was determined under an accelerated 5-minute wet, 5-minute dry protocol, and the life of the membrane was 392 hours before a 40 sccm cross-over leak was detected. The fluoride release during the test was 10 −5 g F − per cm 2 of membrane area per hour. A platinum line had formed on the cathode side where platinum from the catalyst layer had dissolved and diffused into the membrane where presumably the platinum ions then became reduced by diffusing hydrogen gas from the anode.