Abstract:
One embodiment includes a method of forming a hydrophilic particle containing electrode including providing a catalyst; providing hydrophilic particles suspended in a liquid to form a liquid suspension; contacting said catalyst with said liquid suspension; and, drying said liquid suspension contacting said catalyst to leave said hydrophilic particles attached to said catalyst.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 12/636,993 filed Dec. 14, 2009 and claims the benefit of the filing date thereof. 
     
    
     TECHNICAL FIELD 
       [0002]    The field to which the disclosure generally relates includes thin film electrodes and more particularly to fuel cell electrodes including Proton Exchange Membrane (PEM) nano-structured electrodes. 
       BACKGROUND 
       [0003]    Fuel cells have been demonstrated to be viable for vehicle and stationary applications. However, several problems remain relating to the durability of electrode materials, especially in vehicle applications where proton exchange membrane (PEM) electrode materials are expected to operate under a wide variety of conditions, including relatively dry conditions that may adversely affect operation of the electrode as well as durability and lifetime. 
         [0004]    Referring to  FIG. 1 , is shown a schematic diagram of a conventional fuel cell  10 , such as in a vehicle, including an organic membrane  12  with an anode electrode  12 A and a cathode electrode  12 B. Hydrogen H 2  may be converted to 4 protons and 4 electrons at the anode, where the 4 protons may pass through the proton exchange membrane  12  and the 4 electrons may pass through an electrical circuit  14  to the cathode  12 B where the 4 protons and 4 electrons may react with oxygen O 2  to produce water. 
         [0005]    There are several types of electrode catalysts known in the art including catalyst particles (e.g., noble metals including Pt) which are typically supported on carbon substrates, or where a thin film of catalyst is coated as a shell onto a core of another material, including another particle or a whisker (e.g., an elongated rod-like shape). In a conventional electrode, catalyst is mixed with an ionic conduction material, such as perfluorosulfonic acid (PFSA) polymer, in order to conduct proton across the electrode. In an electrode which made from a core-shell catalyst where Pt surface of each particles are in contact with each other, an alternative proton conduction mechanism can be achieved where proton (or adsorbed species) can transport on the surface of Pt. In this case, an additional ionic conduction material (e.g. PFSA) is not required. However, the surface conduction mechanism on Pt is suspected to be highly dependent to the water activity compared to PFSA. (For further discussion of surface transport mechanisms on Pt surfaces see J., McBreen, Journal of Electrochemical Society, May 1985, pp. 1112-6) In consequence, this kind of electrode is likely to suffer in performance in a dry condition. 
         [0006]    In this invention, the electrode hydrophilicity was modified by incorporation of hydrophilic particles into the electrode. The higher hydrophilicity results in a higher water activity in the electrode which thus improves the proton (or adsorbed species) conduction via the Pt surface transport mechanism. The hydrophilic particle was design so that it modifies the local water activity close to the Pt surface, and allows sufficient gas transport of the reactant gases to the Pt. 
       SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
       [0007]    In an exemplary embodiment, an electrode is provided including a thin film catalyst; and a plurality of hydrophilic particles dispersed on the thin film catalyst. 
         [0008]    In another exemplary embodiment, a method of forming a hydrophilic particle containing electrode is provided including providing a thin film catalyst; providing hydrophilic particles suspended in a liquid to form a liquid suspension; contacting the thin film catalyst with the liquid suspension; and, drying the liquid suspension contacting the thin film catalyst to leave said hydrophilic particles attached to the thin film catalyst. 
         [0009]    Other exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0011]      FIG. 1  shows a schematic diagram of a PEM fuel cell, such as a vehicle fuel cell, according to the prior art. 
           [0012]      FIGS. 2A-2E  show schematic diagrams an exemplary hydrophilic particles and hydrophilic particles on thin film catalysts according to exemplary embodiments. 
           [0013]      FIGS. 3A-3B  show SEM micrographs of exemplary hydrophilic particles on whisker shaped thin film catalysts according to exemplary embodiments. 
           [0014]      FIG. 4  shows exemplary operational data of exemplary PEM electrode using exemplary hydrophilic particles on whisker shaped thin film catalysts under different humidity conditions. 
           [0015]      FIG. 5  shows an exemplary process flow to form a hydrophilic particle containing thin film catalyst electrode according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0016]    The following description of the embodiment(s) is merely exemplary (illustrative) in nature and is in no way intended to limit the invention, its application, or uses. 
         [0017]    In an exemplary embodiment, hydrophilic particles are provided dispersed on a catalyst where the catalyst forms at least a portion of an electrode. The catalyst may be supported or unsupported. The support may be any of a variety of materials including, but not limited to, carbon or metal oxide structures. The hydrophilic particles may include a hydrophilic surface that attracts and spreads water (allows water to move across the surface). In other exemplary embodiments, the hydrophilic particles may be dispersed as primary particles and/or agglomerates of hydrophilic particles on the thin film catalyst. In another exemplary embodiment, the electrode may be a proton exchange membrane (PEM) fuel cell electrode, such as a vehicle fuel cell as shown in  FIG. 1 . In another exemplary embodiment, the thin film catalyst may include one or more noble metal catalyst. In another exemplary embodiment, the noble metal may include Pt or an alloy thereof. 
         [0018]    In an exemplary embodiment, the hydrophilic particles may be provided in a range of particle sizes, but not limited thereto, which may vary over a primary particle size range of about 1 nm to about 100 nm, more preferably from about 5 nm to about 50 nm, even more preferably, from about 5 nm to about 30 nm. For example, the effective primary particle size may be expressed as volume average particle size or weight average particle size and may be determined by common methods including visually by a transmission electron microscope (TEM), measurement of surface area by gas e.g., N 2  adsorption, or x-ray spectroscopy. In an exemplary embodiment, the hydrophilic particles may have surface areas of about 20 M 2 /g to about 250 M 2 /g. 
         [0019]    In another exemplary embodiment, the hydrophilic particles may be provided having a primary particle size which may be within a narrow range, e.g., where the size variation among particles is less than about 20 percent, more preferably less than about 10 percent. Referring to  FIG. 2A , in other exemplary embodiments, the hydrophilic particles may be in any shape, including substantially spherical  12 A or oval shaped  12 B e.g., as well as polyhedral and/or flake-shaped, and may include agglomerates e.g.,  14 , as shown in  FIG. 2B , made up of a plurality of agglomerated smaller particles, e.g.,  14 A,  14 B,  14 C, where the smaller particles are either partially bonded or held together by attractive physical forces. 
         [0020]    In an exemplary embodiment, the hydrophilic particles may be an oxide, such as a metal oxide, including silicon oxide, titanium oxide, or derivative thereof. For example, the silicon dioxide hydrophilic particles may include hydroxyls (OH groups), e.g., both vicinal and isolated hydroxyls, bonded to the particle surface. The hydrophilic particles may be formed by any process, including chemical processes such as sol-gel processes and/or flame pyrolysis, and may be treated by known chemical processes to achieve desired hydrophilic properties including hydroxyls on the particle surface. 
         [0021]    Referring to  FIG. 2C , in another exemplary embodiment, the hydrophilic particles e.g.,  16  may be provided as a cladding e.g.,  16 A of hydrophilic material at least partially surrounding a core of one or more second materials e.g.,  16 B, which may be a metal, and oxide, or an organic material. The hydrophilic particles may be isolated or agglomerated as shown in  FIG. 2B . 
         [0022]    Referring to  FIG. 2D , in another exemplary embodiment, the hydrophilic particles e.g.,  18  may have any shape including substantially spherical or oval and including one or more of the shapes/configurations shown in  FIGS. 2A-2C  and may be provided dispersed on the surface of a thin film catalyst e.g.,  20 A that may be a thin film or cladding of the thin film catalyst material surrounding a core of a second material  20 B. In another exemplary embodiment the catalyst  24 A may make up the entire particle. In addition, in other exemplary embodiments, the shape of the catalyst particle  20  may be any shape, including substantially spherical or oval shaped e.g., as shown in  FIG. 2D , as well as polyhedral and/or flake-shaped as well as rod or whisker or rod shaped as shown below in  FIG. 2E . In some embodiments, the hydrophilic particles e.g.,  18  and/or the catalyst particles e.g.,  20  may further be agglomerated into larger particle agglomerates, and may have a range of particle sizes. For example, in an exemplary embodiment, the catalyst particles e.g.,  20  may have a primary particle size from of about 20 nm to about 1 micron and the hydrophilic particles e.g.,  18 , may have a primary particle size from of about 1 nm to about 100 nm, more preferably from about 1 nm to about 50 nm. 
         [0023]    Referring to  FIG. 2E , in an exemplary embodiment, the hydrophilic particles e.g.,  31 , are dispersed onto elongated or whisker shaped thin film catalyst  32 A which may be provided covering a whisker shaped support  32 B, which may be an organic material as is known in the art. In one exemplary embodiment, the thin film catalyst e.g.,  32 A may be a noble metal, such as a Pt or Pt alloy coated onto the whisker shaped substrate or support  32 B. In another an exemplary embodiment, the thin film catalyst may have a noble metal or noble metal alloy loading of about 0.01 mg/cm2 to about 1 mg/cm 2 . Other suitable catalyst may include, but are not limited to, Pd or its alloy. 
         [0024]    In an exemplary embodiment, a plurality of the whisker shaped carbon supported thin film catalyst e.g.,  32 A,  32 B may be attached to the surface of a substrate  30 , such as a membrane of a proton exchange membrane (PEM) electrode, including a PEM electrode of a fuel cell, such as a vehicle fuel cell shown in  FIG. 1 . In an exemplary embodiment, the PEM may be fluorinated, partially fluorinated or non-fluorinated. In another exemplary embodiment, the thickness of the PEM electrode including the thin film catalyst may be less that 1 micron. In another exemplary embodiment, the thin film catalysts may be high aspect ratio particles such as the whisker shaped supported thin film catalyst e.g.,  32 A,  32 B and may have a width of about 10 to about 100 nm and a length of about 200 to about 1000 nm. 
         [0025]    In an exemplary embodiment, the hydrophilic particles may be dispersed onto the thin film catalyst (e.g.,  32 A whiskers) by suspending the hydrophilic particles in a liquid, such as a polar or non-polar solvent, and contacting the electrode and/or thin film catalyst with the liquid suspension. In one exemplary embodiment, the liquid suspension of hydrophilic particles may be sprayed onto the thin film catalyst using conventional spray methods. In another exemplary embodiment, the liquid suspension of hydrophilic particles may be coated onto the thin film catalyst by contacting a rod covered with the suspension to the thin film catalyst surface. In another exemplary embodiment, liquid suspension of hydrophilic particles may be coated onto the thin film catalyst by forcing the liquid suspension through a slot positioned above and moved over the thin film catalyst surface. It will be appreciated that surface energy forces, including capillary forces may operate to draw the liquid suspension onto the thin film catalyst surface to thereby disperse the hydrophilic particles onto the thin film catalyst surface. 
         [0026]    In another exemplary embodiment, the liquid suspension of hydrophilic particles on the thin film catalyst may be allowed to dry to form the hydrophilic particles, e.g.,  31  dispersed on the thin film catalyst surface. In an exemplary embodiment, the thin film catalyst with the dispersed hydrophilic particles may be attached to the electrode substrate e.g., PEM membrane  30 , either before or following dispersing the hydrophilic particles onto the thin film catalyst surface. In another exemplary embodiment the thin film catalyst with the dispersed hydrophilic particles (e.g., whiskers  32 A,  32 B) may be hot-pressed onto the electrode substrate e.g., PEM membrane  30 , by conventional methods. 
         [0027]    In an exemplary embodiment, the thin film catalyst with the dispersed hydrophilic particles (e.g., whiskers  32 A,  32 B) may be hot-pressed onto the electrode substrate e.g., PEM membrane  30 , at a pressing pressure of about 100 psi to about 10000 psi and where the hot-pressing may be at a temperature of about 30 C to about 200 C. 
         [0028]    Referring to  FIGS. 3A and 3B , in an exemplary embodiment, are shown scanning electron microscope (SEM) micrographs of exemplary dispersed hydrophilic particles on a thin film whisker shaped catalyst. In  FIG. 3A  are shown SiO 2  “beads” having a diameter of about 10 nm in  FIG. 3A  and in  FIG. 3B  are shown SiO 2  “beads” having a diameter of about 30 nm. The whiskers shown are Nano-Structured Thin Film (NSTF) Pt containing catalyst (e.g., a PtMnCo alloy) available from 3M Company. The loading of the SiO 2  particles on the thin film whisker shaped catalysts shown in  FIGS. 3A and 3B  is about 15 micrograms/cm 2   MEA  where the area cm 2  is with respect to a membrane electrode area (MEA) to which the catalyst may be attached. In other exemplary embodiments, the SiO 2  particle loading on thin film shaped catalysts including whisker shaped catalysts may be from about 1 micrograms/cm 2   MEA , to about 30 micrograms/cm 2   MEA . 
         [0029]    In addition, a surface roughness (surface area) may be defined for the hydrophilic particles dispersed on a thin film catalyst with respect to the area of a membrane electrode area (MEA). In  FIGS. 3A and 3B  the surface roughness may respectively be about 53 cm 2 /cm 2   MEA  and about 26 cm 2 /cm 2   MEA . In other exemplary embodiments, the surface roughness (surface area) of hydrophilic particles (e.g., SiO 2  particles) on thin film shaped catalysts including whisker shaped catalysts may be from about 20 cm 2 /cm 2   MEA  to about 75 cm 2 /cm 2   MEA . 
         [0030]    Referring to  FIG. 4 , in an exemplary implementation of embodiments, is shown a measured normalized resistance (HFR)  50  or Voltage  52  vs. current density for membrane electrodes including SiO 2  loaded (15 micrograms/cm 2   MEA  on cathode only) and unloaded (without SiO 2 ) NSTF whisker shaped thin film Pt alloy catalysts under different relative humidity (RH) conditions at about 80° C. For example, it is seen that under drier conditions, e.g., 70% RH versus 100% RH, that a significant decrease in performance for the unloaded electrodes (without SiO 2  particles) occurs (e.g., lower Voltage at higher current density as indicated by arrow showing change from curve A to B). In contrast, the 10 nm SiO 2  loaded electrodes show significantly improved performance under similar drier conditions as indicated by arrow showing change from curve C to D. 
         [0031]    While not bound by any particular theory of operation, it is believed that the SiO 2  hydrophilic particles act to attract water and spread water to the thin film catalyst surface thereby improving proton (or adsorbed species) conduction via the surface transport mechanism, resulting in improved accessibility to and utilization of the catalyst surface under lower operating relative humidity. 
         [0032]    Among the advantages of several embodiments, is the improved performance of electrodes, e.g., PEM electrodes, such as used in fuel cells, including vehicle fuel cells where dry operating environments may be frequently encountered. 
         [0033]    Referring to  FIG. 5  is a process flow diagram including several exemplary embodiments. In step  501 , a thin film catalyst is provided. In step  503  a liquid with a suspension of hydrophilic particles is provided. In step  505  the thin film catalyst surface is contacted with the liquid suspension of hydrophilic particles. In step  507 , the liquid suspension contacting the thin film catalyst is dried to leave hydrophilic particles adhered to the surface of the thin film catalyst. In optional step  509 , the thin film catalyst and hydrophilic particles may be hot pressed including onto a substrate. 
         [0034]    The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.