Surface structure for enhancing catalyst reactivity and method of manufacturing thereof

#The technical field relates to surface structures that enhance catalyst reactivity, and in particular, to catalyst-coated oxide substrate with hillock structures. The catalyst-coated oxide substrate may be used in electrochemical devices, including fuel cells, batteries, sensors, electrolyzers and the like.

#A desirable feature for electrochemical systems employing a catalysis process, such as fuel cells, batteries, sensors, and electrolyzers, is the ability to deliver the highest catalyst reactivity within the size and weight limit of the system. In typical fuel cells employing liquid fuel, such as methanol, and an oxygen-containing oxidant, such as air or pure oxygen, the methanol is oxidized at an anode catalyst layer to produce protons and carbon dioxide. The protons migrate through an electrolyte from the anode to a cathode. At a cathode catalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of direct methanol fuel cell are shown in the following equations: 1

#Net reaction: CH3OH+3/2 O2→2H2O+CO2   III

#The detailed description will refer to the following drawings, in which like numerals refer to like elements, and in which:

#FIG. 1 depicts an image of a Pt nanoparticle array.

#FIG. 2 depicts an image of Pt nano hillocks formed on a SiO2 substrate after annealing at a high temperature.

#There is ample evidence to show that electrocatalysts can differ in their activity due to preparation methods. For example, a Pt:Ru alloy prepared by sputtering a bulk alloy, annealing a bulk alloy, or depositing a submonolayer of Ru on Pt all yield fundamentally different catalytic properties. Recently, Baldelli, S. et al. reported that Pt nanoparticle arrays showed significantly enhanced catalytic reactivity compared to Pt thin film (Baldelli, S.,et al. “Surface enhanced sum frequency generation of carbon monoxide adsorbed on platinum nanoparticle arrays,” Journal of Chemical Physics, 113:5432-8, 2000). FIG. 1 shows an image of Pt nanoparticle array 101 where particles 103 have a diameter of 40 nm and are spaced 150 mn from each other (Supra). The Pt nanoparticle array 101 is fabricated using electron beam lithography (Supra). Briefly, a Si (100) wafer with 5 nm thick SiO2 on the surface is spin-coated with a layer of photoresist polymethyl methacryllate (PMMA, MW 950,000). Computer-designed patterns are then “written” into the photoresist layer with a highly collimated electron beam generated by a field emission source. The exposed photoresist is dissolved and a 15 nm thick film of Pt is deposited on the Si (100) wafer by electron beam evaporation. Finally, the remaining photoresist is removed by dissolution with organic solvent and the metal particles of the prescribed pattern remained on the Si (100) wafer (Supra). Using sum frequency generation (SFG) vibrational spectroscopy, Baldelli et al. found that the SFG signal of CO adsorbed to the Pt particles exhibited a significant enhancement compared to CO adsorbed on a smooth Pt film. The enhanced reactivity of Pt is attributed to the presence of the interface between Pt and the oxide substrate in the Pt nanoparticle array 101. The elaborated electron beam lithography and subsequent photolithography, however, would be difficult to perform on a porous substrate.

#According to the present invention, a catalyst film hillock structure similar to the Pt nanoparticle array 101 described above is generated by annealing a thin catalyst film at high temperatures to a suitable substrate. In an embodiment, a thin layer of Pt catalyst is deposited onto the surface of a substrate, such as SiO2, using electron beam evaporation. The catalyst layer is then annealed to the substrate at a high temperature. During the annealing process, a compressive stress builds up in the catalyst film plane as temperature rises. When the compressive stress exceeds the buckling point of the catalyst film, the film relieves the stress by pushing a portion of the film out of the plane to form a hillock (“Pt Hillock Formation and Decay,” by Summerfelt, S. R. et al. in “Ferroelectric Thin Films IV” Tuttle, B. A. et al., eds., Materials Research Society Symp. Proc., 361:257-26, 1995). This technique makes it possible to produce hillock structure on non-flat surfaces, such as on the surfaces of porous particles, without the need of photolithography. FIG. 2 shows an image of Pt hillock structure 105 formed on a SiO2 substrate after annealing at a temperature of 800-900° C. for 30 to 60 minutes.

#In fuel cell applications, the substrate for the hillock catalyst structure 105 is a conductive material so that the electrons/protons generated on the catalytic surface can be collected by an electrode. In one embodiment, the substrate is conductive oxygen deficient silicon oxide. Conductive oxygen deficient silicon oxide is a silicon oxide that is stoichiometrically deficient for oxygen. While normal silicon oxide is represented by the formula SiO2, the oxygen deficient silicon oxide is represented by the formula SiOx, where x is greater than zero but smaller than 2. SiOx is conductive because the oxygen deficiency provides conduction paths for electrons. Generally, good conductivity is obtained when x is smaller than 1.7. SiOx can be produced by reactive sputtering from a silicon target in an oxygen environment. Other conductive oxides, such as RuO2, ZnO2, SnO2 ZnInO and SrRuO, are also suitable substrates for the hillock structure 105.