Ion Beam Modification of Pt Electrocatalyst Nanoparticles for Polymer Electrolyte Membrane Fuel Cells
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1217-Y08-26
Ion Beam Modification of Pt Electrocatalyst Nanoparticles for Polymer Electrolyte Membrane Fuel Cells Tetsuya Yamaki, Shunya Yamamoto, Teruyuki Hakoda and Hiroshi Koshikawa Quantum Beam Science Directorate, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan
ABSTRACT Platinum (Pt) nanoparticles were prepared on a glassy carbon plate by a sputtering method and then irradiated with proton (H+) beams at energies of 0.38 and 10 MeV at room temperature. Cyclic voltammetry in an aqueous 0.5 mol/dm3 H2SO4 solution suggested that the lower-energy beam irradiation enhanced the active surface area of the Pt nanoparticles based on the calculated coulombic charge for hydrogen desorption. Thus, the nanoparticles would be modified by H+ beaminduced electronic excitation so that they have a higher surface activity. The mechanism of this irradiation effect seems to be rather complicated and is still unclear at present, but it can be discussed in relation to a change in the interfacial crystal structure during the irradiation.
INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFCs) have been receiving attention because of their attractive properties as a power source for portable, stationary, and automobile applications. While one of the major obstacles that should be overcome for the commercialization of PEMFCs is to find more efficient catalysts, nanoparticles of Pt and the Pt-group metals are still widely employed as the main catalyst [1]. Naturally, the reduction of such noble metal catalysts is a matter of extreme urgency in order to significantly cut production costs. To this end, researchers have already taken many technical approaches that have included: (i) alloying non-noble metal element(s), (ii) optimization of the particle geometry and configuration in an electrode, (iii) the use of support materials, and (iv) modification of the crystal surface states, etc. Despite these increasing efforts, we have not yet obtained the best way to produce reliable and affordable PEMFCs. This undoubtedly demonstrates how difficult it is to address the catalyst issues. High-energy ion beams, defined as a stream of electromagnetically-accelerated ions within a vacuum, have been used extensively for the surface treatment of bulk materials as well as for elemental or structural analysis, because they can focus a huge energy on the target materials in a concentrated form compared to the other ionizing beams. For example, materials have been modified through displacement mixing, radiation-enhanced diffusion, segregation, short range ordering or clustering, and self-diffusion [2]. In fact, one of the authors previously found for the first time that proton (H+)-beam irradiation at an energy of 1 MeV eliminated the localized electronic states of semiconductor fine particles due to their surface irregularity [3,4]. This study is concerned with our preliminary attempt to improve the catalytic properties of Pt nanoparticles by modification with two different H+ beams at energies of 0.38 and 10 MeV. We exp
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