Jozsef Speder

The particle proximity effect: from model to high surface area fuel cell catalysts

In this work, Pt nanoparticles prepared by a colloidal method are supported on high surface area carbons. The electrocatalysts synthesized by this method have well-separated, size-controlled nanoparticles with tunable interparticle distance, and thus enable the examination of the particle proximity effect on the oxygen reduction reaction (ORR). The particle proximity effect proposes that the activity of fuel cell catalysts depends on the distance between the catalyst particles and is here for the first time demonstrated for high surface area catalysts; i.e. catalysts which can be used in fuel cells. Based on rotating disk electrode (RDE) experiments, we show that the kinetic current density of ORR depends on the distance between the neighboring nanoparticles, i.e. the ORR activity increases with decreasing interparticle distance.

Electrochemical Stability and Postmortem Studies of Pt/SiC Catalysts for Polymer Electrolyte Membrane Fuel Cells

In the presented work, the electrochemical stability of platinized silicon carbide is studied. Postmortem transmission electron microscopy and X-ray photoelectron spectroscopy were used to document the change in the morphology and structure upon potential cycling of Pt/SiC catalysts. Two different potential cycle aging tests were used in order to accelerate the support corrosion, simulating start-up/shutdown and load cycling. On the basis of the results, we draw two main conclusions. First, platinized silicon carbide exhibits improved electrochemical stability over platinized active carbons. Second, silicon carbide undergoes at least mild oxidation if not even silicon leaching.

Gold nanoparticles assembled with dithiocarbamate-anchored molecular wires.

A protocol for the bottom-up self-assembly of nanogaps is developed through molecular linking of gold nanoparticles (AuNPs). Two π-conjugated oligo(phenylene ethynylene) molecules (OPE) with dithiocarbamate anchoring groups are used as ligands for the AuNPs. OPE-4S with a dithiocarbamate in each end of the molecule and a reference molecule OPE-2S with only a single dithiocarbamate end group. The linking mechanism of OPE-4S is investigated by using a combination of TEM, UV-Vis absorption and surface enhanced Raman spectroscopy (SERS) as well as studying the effect of varying the OPE-4S to AuNP concentration ratio. UV-Vis absorption confirms the formation of AuNP aggregates by the appearance of an extended plasmon band (EPB) for which the red shift and intensity depend on the OPE-4S:AuNP ratio. SERS confirms the presence of OPE-4S and shows a gradual increase of the signal intensity with increasing OPE-4S:AuNP ratios up to a ratio of about 4000, after which the SERS intensity does not increase significantly. For OPE-2S, no linking is observed below full coverage of the AuNPs indicating that the observed aggregate formation at high OPE-2S:AuNP ratios, above full AuNP coverage, is most likely of a physical nature (van der Waals forces or π-π interactions).