Patent Publication Number: US-2016226077-A1

Title: Non-carbon catalyst support particles for use in fuel cell electrodes

Description:
TECHNICAL FIELD 
     This disclosure relates to non-carbon mixed material electrocatalyst support structures, and in particular, to a high surface area metal oxide support doped with a conductive metal used to produce electrocatalysts for hydrogen fuel cell vehicles having active catalyst particles deposited thereon. 
     BACKGROUND 
     Carbon has traditionally been the most common material of choice for polymer electrolyte fuel cell (PEFC) electrocatalyst supports due to its low cost, high abundance, high electronic conductivity, and high Brunauer, Emmett, and Teller (BET) surface area, which permits good dispersion of platinum (Pt) active catalyst particles. However, the instability of the carbon-supported platinum electrocatalyst due at least in part to carbon corrosion is a key issue that currently precludes widespread commercialization of PEFCs for automotive applications. 
     The adverse consequences of carbon corrosion include (i) platinum nanoparticle agglomeration/detachment; (ii) macroscopic electrode thinning/loss of porosity in the electrode; and (iii) enhanced hydrophilicity of the remaining support surface. The first results in loss of catalyst active surface area and lower mass activity resulting from reduced platinum utilization, whereas the second and third result in a lower capacity to hold water and enhanced flooding, leading to severe condensed-phase mass transport limitations. Clearly, both consequences directly impact PEFC cost and performance, especially in the context of automotive stacks. 
     To address the issues with carbon-based catalyst, non-carbon alternatives are being investigated, such as metal oxides. However, some metal oxides alternatives are cost-prohibitive, and dissolution, agglomeration and corrosion of the metal oxide alternatives can still occur. 
     SUMMARY 
     Non-carbon support particles are disclosed for use in electrocatalyst comprising a first metal oxide having a high surface area doped with an electrically conductive transition metal. An example of non-carbon support particle for use in electrocatalyst as disclosed herein comprises titanium oxide particles doped with ruthenium. 
     These and other aspects of the present disclosure are disclosed in the following detailed description of the embodiments, the appended claims and the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features, advantages and other uses of the present apparatus will become more apparent by referring to the following detailed description and drawing in which: 
         FIG. 1  is a schematic of an electrode using an embodiment of the improved non-carbon catalyst support particles as disclosed herein; 
         FIG. 2  is a schematic of an electrode using another embodiment of the improved non-carbon catalyst support particles as disclosed herein; and 
         FIG. 3  is a schematic of a fuel cell using the electrode of  FIG. 1  or  FIG. 2  as disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     One example of a non-carbon metal oxide catalyst support consists essentially of a non-conductive metal oxide having a high surface area. A non-limiting example of such a metal oxide is titanium dioxide. Titanium dioxide (TiO 2 ) has very good chemical stability in acidic and oxidative environments. However, titanium dioxide is a semiconductor and its electron conductivity is very low. 
     To overcome the deficiencies of the non-conductive metal oxide alone, a non-carbon metal oxide support having both a non-conductive oxide and a conductive metal have been developed. Disclosed herein are non-carbon support particles for use in electrocatalyst comprising a metal oxide having a high surface area doped with an electrically conductive transition metal. Doping the high surface area metal oxide with a conductive transition metal provides the requisite electron conductivity. Doping the conductive transition metal can also reduce or eliminate dissolution and agglomeration of the metal that can arise when one particle is deposited on another particle, as doping chemically bonds the conductive metal to the metal oxide support. The doped support particle provides greater stability than support particles comprised of a conductive metal deposited on a non-conductive, high surface area metal oxide. Doping the metal oxide with the conductive metal also maintains the high surface area of the metal oxide support on which the active catalyst particles are deposited. 
       FIG. 1  illustrates an electrode  10  for a fuel cell using one embodiment of a non-carbon support particle for use in electrocatalyst as disclosed herein. A catalyst layer  16  is positioned between a membrane  12  and a gas diffusion layer  14 . The catalyst layer  16  comprises catalyst support particles  18  consisting essentially of a high surface area metal oxide doped with a conductive metal. Active catalyst particles  20  are supported on the catalyst support particles  18 . The catalyst layer  16  can further include an ionomer and a binder. 
     The metal oxide in the catalyst support particles  18  is a high surface area metal oxide with low electron conductivity. As used herein, “low electron conductivity” refers to those metal oxides having insufficient electron conductivity to be used solely as the electron conductor in fuel cell catalyst and include metal oxides that do not conduct electrons. The metal oxide can be one or more metal oxides prepared with varying ratios of metal oxides and various particle sizes depending on the metal oxides used. As non-limiting examples, the metal oxide in the catalyst support particles  18  can be titanium dioxide. 
     The metal oxide of the catalyst support particles  18  is doped with a conductive metal, preferably a conductive transition metal. As a non-limiting example, the transition metal can be ruthenium. The metal oxide will have a larger particle size than the conductive transition metal and be doped with the conductive transition metal, making the catalyst support particle  18  electron conductive while maintaining the high surface area. 
     Active catalyst particles  20  are deposited onto the catalyst support particles  18 . The active catalyst particles  20  can include one or a combination of precious metals such as platinum, gold, rhodium, ruthenium, palladium and iridium, and/or transition metals such as cobalt and nickel. The precious metal can be in various forms, such as alloys, nanowires, nanoparticles and coreshells, which are bimetallic catalysts that possess a base metal core surrounded by a precious metal shell. 
       FIG. 2  illustrates an electrode  100  for a fuel cell using another embodiment of a non-carbon support particle for use in electrocatalyst as disclosed herein. A catalyst layer  160  is positioned between a membrane  12  and a gas diffusion layer  14 . The catalyst layer  160  comprises catalyst support particles  180  consisting essentially of a high surface area metal oxide doped with a conductive metal, such as a conductive transition metal. In this embodiment, the catalyst support particles  180  further include a conductive metal oxide  22  deposited onto the doped high surface area metal oxide. The addition of the conductive metal oxide  22  to the catalyst support particles  180  improves oxygen evolution reaction (OER) activity. Active catalyst particles  20  are supported on the catalyst support particles  180 . The catalyst layer  160  can further include an ionomer and a binder. 
     The conductive metal oxide can be an oxide of the conductive transition metal with which the high surface area metal oxide is doped. For example, the conductive transition metal can be ruthenium and the conductive metal oxide can be ruthenium dioxide. Alternatively, the conductive metal oxide can be an oxide of a different metal than the conductive transition metal. For example, the conductive transition metal can be ruthenium and the conductive metal oxide can be iridium oxide. The high surface area metal oxide can have a particle size greater than the particle size of the conductive metal oxide. 
       FIG. 3  illustrates the use of catalyst support particles,  18 ,  180  disclosed herein.  FIG. 3  is a schematic of a fuel cell  70 , a plurality of which makes a fuel cell stack. The fuel cell  70  is comprised of a single membrane electrode assembly  20 . The membrane electrode assembly  20  has a membrane  12  coated with the catalyst layer  16 ,  160  with a gas diffusion layer  14  on opposing sides of the membrane  12 . The membrane  12  has catalyst layers  16 ,  160  formed on opposing surfaces of the membrane  12 , such that when assembled, the catalyst layers  16 ,  160  are each between the membrane  12  and a gas diffusion layer  14 . Alternatively, a gas diffusion electrode is made by forming a catalyst layer  16 ,  160  on a surface of a gas diffusion layer  14  and layering the membrane  12  on the catalyst layer  16 ,  160 . In  FIG. 3 , the membrane  12  is sandwiched between two gas diffusion layers  14  such that the catalyst layers  16 ,  160  contact the membrane  12 . When fuel, such as hydrogen gas (shown as H 2 ), is introduced into the fuel cell  70 , the catalyst layer  16 ,  160  splits hydrogen gas molecules into protons and electrons. The protons pass through the membrane  12  to react with the oxidant (shown as O 2 ), such as oxygen or air, forming water (H 2 O). The electrons (e − ), which cannot pass through the membrane  12 , must travel around it, thus creating the source of electrical energy. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.