PEM-fuel cell catalysts from nanoscopic precursors : a comparative study
1. Minimising the amount of stabiliser In an systematic study it was found that the amount of applied (Octyl)4N-protecting agent can be reduced to 60% from the amount usually applied for the preparation of Pt/Ru organosol using a combination of (Octyl)4N[Bet3H] and Li[Bet3H] as reductants. Further, it was found that the particle size can be controlled during this synthesis by the "reverse" addition of suspended metal salts to the diluted (Octyl)4N[Bet3H] solution. Using this optimisation the Pt/Ru organosol (ca. 2nm) with narrow size distribution is obtained. And moreover this optimisation eases the process of removing the protecting shell by "conditioning" at 300°C. 2. Comparing different reducing agents A comparison of different reducing agents for the synthesis of nanoscopic precursors was followed. The above mentioned (Octyl)4N[Bet3H] method gives a homogeneous alloyed Pt/Ru colloid (2nm), which is well redispersible. Formaldehyde reduction yields a mixture of bimetallic, alloyed Pt/Ru nanopowder(1.5nm) with big agglomerated grains of 15-20nm. In another case, use of stoichiometric excess of Li[Bet3H] and LiBH4 as the reducing agents yield a redispersible Pt/Ru colloid which is not long time stable. The stabilization around these particles is weak. 3. Seed mediated size and shape control An interesting result obtained during this work is the size and shape controlled synthesis of Pt nanoparticles with "seeding method". It was found that using diffusional growth induced by slow reduction under Hydrogen, truncated octahedral Pt nanoparticles can be prepared with different sizes by seeding the Pt2+: NaPA (1:1) solutions with pre-prepared Pt seeds stabilized with REWO.. In the similar manner tetrahedral particles with different sizes were obtained using PVP as stabiliser. 4. Electrochemical investigations of W-doped Pt- and Co-doped Pt/Ru catalysts "Surface doping" was applied as a method for tuning fuel cell catalysts. It was found that W(0) doping on the Pt/Vulcan catalysts in small amounts activates the catalysts shifting the onset potentials in potentio-dynamic CO-oxidation studies. 1 wt.% doping was found to be optimum. Which achieves 70-80% H2-oxidation activity in presence of 2 vol. % CO. In methanol oxidation studies it was found that Co doping (Pt:Ru:Co = 1:1:1) activates the catalysts in all cases. Also Hydrogen reduction mode of the catalyst preparation by metal salt reduction at 120°C, was found to be a very promising method for preparing clean and cost effective catalysts with narrow particle sizes.
info:eu-repo/classification/ddc/540, Membranbrennstoffzelle, Polyelektrolyt, Kolloides Metall, Nanostrukturiertes Material, Elektrokatalyse, Chemie, Colloidal Nanoparticles, Size and shape selective synthesis, PEMFC, Catalysts
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