PEFC Cathode Catalysts of ZrO₂/C Prepared By RF Magnetron Sputtering and Crosslinked Polymer Methods

Fuel cells are expected to be clean power generation devices in the future energy society. Various types of fuel cells, which are different in electrocatalysts, electrolytes, and operating temperatures, are available for practical use in various places depending on their characteristic. Among them polymer electrolyte fuel cells (PEFC) have been applied to fuel cell vehicles (FCV), household cogeneration systems and etc. PEFC uses large amounts of Pt for electrocatalysts, which is one of the serious problems for large scale commercialization. The worldwide production of Pt is about 180 ton/year, and about 60 g of platinum is practically used in one FCV. If all of the world production of Pt is used for FCV, only 3 million of FCV can be manufactured in a year, although the annual production of cars in the world is about 100 million. Therefore, the development of electrocatalysts with non-Pt materials is indispensable for the broad spread of PEFC. The cathode catalysts significantly affect to the cell voltage, current density, and durability, so that the developments of alternative cathode materials to conventional Pt-based catalysts are expected. Compounds of oxides of group 4 and 5 elements transition metals such as Zr, Ta, Nb, and Ti, chalcogenides such as Ru and Mo, and carbon materials with N-containing complexes of Fe and Co, carbon materials doped with B and N are intensively investigated in the past decade. In this study, ZrO2/C catalysts were investigated for oxygen reduction reaction (ORR) activity in acidic media. Two types of preparation methods have been examined; radio-frequency magnetron sputtering and crosslinked polymer methods. In magnetron sputtering method, carbon black (CB) powders were stirred in a rotary drum chamber with milling balls and cubes. A Zr metal target was attached to a sputtering gun placed in the stirring drum. Sputtering was performed in 10 Pa of Ar. In crosslinked polymer method, ammonium zirconium carbonate (AZC) was used as a crosslink reagent for polyvinyl alcohol (PVA). Addition of citric acid (CA) was found to be effective. Mixture of AZC, PVA, and CA were well stirred at 80ºC, and CB was immersed in the mixture. The mixture was dried at 300ºC in air and them heat treated 600~1200ºC in N2 or H2. The catalyst ink was obtained by ultrasonically dispersing 300 µL of isopropanol (IPA), 4 mg of catalyst and 12 µL of 5% Nafion-dispersed solution. The sample electrode was prepared by dropping 150 µL of the catalyst ink on glassy carbon (GC) and drying. ORR activity was measured in 0.1 M H2SO4 with O2 bubbling. ORR activity of ZrO2/C catalysts which were prepared by sputtering method was shown in the attached figure. This figure shows ORR properties of ZrO2/C catalysts prepared at various sputtering times in 0.1 M H2SO4. ZrO2/C was pretreated in H2 flow at 1000˚C for 3 h. ORR current was difference between current in O2 and N2 saturated solutions. ORR Current appeared around 0.8 VR HE for the catalysts deposited longer than 40 min. This result indicate ZrO2/C have obvious ORR activity. X ray diffraction (XRD) of ZrO2/C showed the formation of both monoclinic and tetragonal ZrO2 crystals. ZrO2 particles with diameter in several nm scale were observed by transmission electron microscopic (TEM) images, and formation of these particles were concluded to contribute the obvious ORR activity. On the other hand, ZrO2/C catalysts which were prepared by crosslinked polymer methods also showed similar ORR activity to those by sputtering method. The details were discussed in the presentation. This work was supported by ESICB, Kyoto University and New Energy and Industrial Technology Development Organization (NEDO) of Japan. Figure 1