Abstract
This thesis investigates the electro reduction of oxygen on platinum nanoparticles, which serve as catalyst in low temperature fuel cells. Kinetic studies on model catalysts as well as commercially used systems are presented in order to investigate the particle size effect, the particle proximity effect and anion adsorption on the performance of Pt based electrocatalysts. The anion adsorption is additionally studied by in situ electrochemical infrared spectroscopy during the oxygen reduction reaction (ORR). For this purpose an in situ FTIR setup in attenuated total refection (ATR) configuration equipped with a wall jet electrode, is developed and demonstrated within the thesis.
The particle size effect examined in KOH, H2SO4, and HClO4 electrolyte revealed a repaid decrease in specific activity (SA) in the order polycrystalline Pt > unsupported Pt black particles (∼30 nm) > high surface area carbon (HSAC) supported Pt nanoparticle (Pt/C) catalysts (of various size between 1 and 5 nm). The difference in SA between the individual Pt/C catalysts (1 to 5 nm) is very small and within the error of the measurements. The factor four of loss in SA when comparing platinum bulk and Pt/C can largely be compensated via a particle proximity effect as demonstrated on highly dispersed Pt nanoclusters deposited on a glassy carbon substrate. The Pt nanocluster samples were created by laser ablation, which allows precise control of both the cluster size and coverage, independently. Extraordinarily high ORR activities are reached especially in terms of mass-normalized activity. It is observed that the Pt cluster coverage, and hence the interparticle distance, decisively influence the observed catalytic activity and that closely packed assemblies of Pt clusters
approach the surface activity of bulk Pt.
The influence of the ion adsorption strength, which is observed in the “particle size studies” on the oxygen reduction rate on Pt/C catalysts, is further investigated under similar reaction conditions by infrared spectroscopy. The designed in situ electrochemical ATR-FTIR setup features a high level of instrument automation and online data treatment, and provides welldefined mass transport conditions enabling kinetic measurements. A modified electrochemical / spectroscopic interface is presented allowing the exclusive investigation of the Pt/C catalyst layer. Three types of potential dependent adsorption bands are observed on the Pt/C layer: bands arising from the functional groups of the carbon support, bands related to water and hydronium, and bands related to the sulfur anion interaction with the catalyst. The correlation of the anion absorption to the ORR current leads to the proposition that anion adsorption on Pt does not block the ORR directly. Instead, the onset of oxide formation with the concomitant conversion of the anion adsorbate layer is the decisive blocking mechanism.
The particle size effect examined in KOH, H2SO4, and HClO4 electrolyte revealed a repaid decrease in specific activity (SA) in the order polycrystalline Pt > unsupported Pt black particles (∼30 nm) > high surface area carbon (HSAC) supported Pt nanoparticle (Pt/C) catalysts (of various size between 1 and 5 nm). The difference in SA between the individual Pt/C catalysts (1 to 5 nm) is very small and within the error of the measurements. The factor four of loss in SA when comparing platinum bulk and Pt/C can largely be compensated via a particle proximity effect as demonstrated on highly dispersed Pt nanoclusters deposited on a glassy carbon substrate. The Pt nanocluster samples were created by laser ablation, which allows precise control of both the cluster size and coverage, independently. Extraordinarily high ORR activities are reached especially in terms of mass-normalized activity. It is observed that the Pt cluster coverage, and hence the interparticle distance, decisively influence the observed catalytic activity and that closely packed assemblies of Pt clusters
approach the surface activity of bulk Pt.
The influence of the ion adsorption strength, which is observed in the “particle size studies” on the oxygen reduction rate on Pt/C catalysts, is further investigated under similar reaction conditions by infrared spectroscopy. The designed in situ electrochemical ATR-FTIR setup features a high level of instrument automation and online data treatment, and provides welldefined mass transport conditions enabling kinetic measurements. A modified electrochemical / spectroscopic interface is presented allowing the exclusive investigation of the Pt/C catalyst layer. Three types of potential dependent adsorption bands are observed on the Pt/C layer: bands arising from the functional groups of the carbon support, bands related to water and hydronium, and bands related to the sulfur anion interaction with the catalyst. The correlation of the anion absorption to the ORR current leads to the proposition that anion adsorption on Pt does not block the ORR directly. Instead, the onset of oxide formation with the concomitant conversion of the anion adsorbate layer is the decisive blocking mechanism.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Number of pages | 107 |
| Publication status | Published - 2013 |