Fuel cells as green and sustainable energy sources are at the heart of future Hydrogen Economy. Current research is focused on creating highly active, stable and low content Pt catalysts to improve fuel cell performance up to standards suitable for commercialization. The development of bimetallic Pt structures is the most promising route for achieving this goal. Besides the lower-noble metal content, combination of Pt with other metal at the nano-scale .can result in enhancement of catalytic activity due to the combination of geometric and electronic effects. The main aim of this work is the development of a surface limited redox replacement (SLRR) protocol for the design of epitaxial Pt-films with atomic scale control of the structure. SLRR exploits underpotentially deposited (UPD) layer as sacrificial layer that is replaced by more-noble Pt through a surface-controlled limited red-ox (galvanic) reaction. Different from previously developed SLRR protocols this work explores the one-cell configuration setup as an alternative to improve the efficiency and quality of the growth. The conditions for growth have been optimized and monitored with automated control of the SLRR cycles. The successful growth of Pt films on Au has been demonstrated for SLRR growth via Pb UPD. The electrochemical characterisation showed that using Pb UPD as sacrificial layer produces epitaxial Pt films of high quality with no significant roughness evolution up to 10 layers. Scanning tunnelling microscopy (STM) examination of the morphology has shown that Pt was deposited in clusters of 5-10 nm size, homogeneously and uniformly distributed over the surface. Electrochemical quartz crystal microbalance (EQCM) showed high deposition , yield and the Pt(II):Pb replacement stoichiomentric ratio higher than expected 1 : 1, suggesting an extra reduction power present in the system. The compositional analysis of Pt layers grown by SLRR suggests incorporation of minimum 4 at% of Pb. The SLRR protocol for the homoepitaxial growth of Pt thin films using adsorbed H i.e. under potentially deposited H (H-UPD) has been developed. This work presents first application of the SLRR protocol using a nonmetal UPD system. EQCM experiments demonstrated steady displacement kinetics and a yield equal to the expected stoichiometric Pt(II):H exchange ratio (1 :2). Electrochemical and STM characterization of Pt films showed that the growth via SLRR of H-UPD results in increase of the surface roughness with the number of replacement steps. The roughness of SLRR deposited Pt films has been compared with films grown in the same solution at two constant overpotentials: with and without adsorbed H floating on the surface. The results showed clear advantages of using . the SLRR of H-UPD approach which generated films with two times lower roughness and better quality then the ones grown potentiostatically. The generality of the SLRR approach using H-UPD is validated by growth of Pt films on Pd ultrathin films on Au. The Pt films of well-defined thickness and structure grown by SLRR of Pb UPD have been used in a fundamental study of Pt dissolution during formic acid oxidation (FAO). A quantitative analysis of long term durability tests of Pt films has been conducted by potential cycling over an extended potential range. Direct proportionality between overall life and thickness of the catalyst has been observed. The characteristic stages of the activity decay were correlated with the characteristic electrochemical behaviour during FAO and the surface morphology examined by the atomic force microscopy. An average Pt dissolution rate of 1.90±O.33 ng.cm-2.cycle-1 has been determined during FAO which . was almost four times faster than the rate under the same conditions in the background solution. This study suggests that Pt dissolution mechanism during FAO is influenced in by reaction intermediates and processes on the surface.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:616569 |
| Date | January 2013 |
| Creators | Nutariya, Jeerapat |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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