Prospectof the Pt/C Catalyst for Fuel Cells Prepared By a Nano Particles Formation Pulse Arc Plasma Source

Abstract

By the use of a coaxial pulsed vacuum arc discharge deposition method , we have developed the catalyst deposition system onto support powders or sheet under vacuum. In this letter, we introduce Pt catalysis processing for fuel cell and evaluation for the electrode property. 1. Introductio n Precious metals nanoparticles such as Pt are often used as fuel cell catalyst or automotive one. In industry, these nanoparticle catalyses are manufactured by a wet impregnation process. It is essential for these nanoparticles 1) to be size-controllable 2) to be size-uniformly, and using nanoparticles as catalysts, 3) to be less coagulating. On the other hand, we have developed direct deposition method of nanoparticles onto support powder by using a coaxial pulsed vacuum arc discharge deposition1) We are getting the results that nanoparticles prepared by APD meet the above requirements. In this letter, we introduce nanoparticle formation method and application to Pt catalysis for fuel cell. 2. Experimental The schematic of a coaxial pulsed vacuum arc discharge depositing method (APD) is shown in Fig.1. Columnar cathode (evaporation material) is allocated at the center and cylindrical anode is coaxially-mounted. The substrate is vertically-placed to the radial direction. After a chamber evacuation, a triggerinduces an arc discharge on the surface of target rod. Then highly ionized metal plasma is generated from the target rod without any discharge gases, and deposit onto the substrate to form nanoparticles. Catalyst support powders are contained in a rotating and mechanical stirring container at the substrate position as shown in Fig.2 , and coated with the nanoparticles. In order to support nanoparticles, we used the APD instrument (Model: APD-1H) made by ULVAC-RIKO, Inc. 3. Results and discussion Figure 3 shows atomic force microscope (AFM) images of Pt particles on a highly oriented pyrolytic graphite (HOPG) substrate prepared by electron beam deposition (EB) and APD methods.5) Substrate temperature was 400- 500 °C during depositions. In the case of EB method, large particles are observed along the step on the HOPG surface. However, in the case of APD, uniformly–sized nanoparticles are formed on its surface. From this result, we considered that because metal vapor in the APD plasma has large kinetic energy of a few 10 eV and is highly ionized, uniformly nucleation of nanoparticle occurs on the substrate and its adsorption to the substrate is superior. Figure 4 shows transmission electron microscope (TEM) image of APD Pt nanoparticles with the uniformly-sized 2-3 nm which are supported and distributed on carbon powder (Vulcan) surface. Figure 5 shows CV property at the cathode and anode electrodes measured by the convection voltammetry using rotating electrodes. The electrode has Pt 5 % support on carbon powder (Ketjen Black) by the APD process (Dry process). The results of Pt 46 % support on the carbon powder formed by the conventional wet impregnation process (purchase) are also listed in Fig.5 in order to be compared with the dry process. Pt mass, 1 mg used to measure CV property for the dry process was the same with that for the wet one. From this result, the electromotive force, 0.9 V indicated in the red arrow was the almost same with each other. Currents of oxidation (yellow) and hydrogen (red) electrodes by the dry process were larger than those by the wet one F ig.1. Coaxial pulsed vacuum arc discharge deposition method. Fig. ‚Q . Powder stirring system for APD. ‚e ig. 3 . AFM images of Pt particles deposited on HOPG substrate. Fig.5. Fuel cell CV curves of Pt catalysis supporting with carbon powder (Ketjen Black). Upper F Conventional ( wet process ) Down F APD ( dry process ) Fig 4 . TEM image of Pt particles supported with carbon powder (Vulcan). 4. Summary We have developed a supporting method of the nano-catalysis by a new dry processing combined of a coaxial pulsed vacuum arc discharge deposition and a powder stirring container. In the future, we will plan to study how to decrease the catalysis usage, how to extend the catalysis lifetime under the high temperature environment, and whether effective catalytic materials including alloys exist except for Pt. 5.