Development of Nano-Structured Direct Methanol Fuel Cell Anodes
Date
2013-07-31
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Abstract
Direct methanol fuel cells (DMFCs) are a clean and highly efficient technology. They also have more than one order of magnitude higher energy density compared to advanced energy delivery devices, such as Li ion batteries, making them a very good choice for portable applications. Pt-Ru alloys are believed to be one of the best DMFC anode catalyst materials. However, Ru can dissolve under DMFC operating conditions, crossing over to the cathode, causing membrane degradation and poisoning the oxygen reduction reaction. Pt-Ir anodes may be able to overcome some of these challenges. Therefore, the primary focus of this thesis has been to synthesize, characterize and test a range of Pt-Ir anode catalyst forms (solid solution thin films on Au, alloy nanoparticles (NPs), and Ircore@Ptshell NPs) for their methanol oxidation reaction (MOR) activity, in comparison with Pt-Ru.
Thin Pt-Ir films were electrodeposited on Au, with the mechanism of Ir electrodeposition (ED) from IrCl62- solutions found to depend on adsorbed H, while the ED of Pt from PtCl62- is a simpler process. The high utilization of the precious metals in these films was found to result from competition between metal ED and the adsorption of Cl- ions. Based on these findings, Pt-Ir alloy films with a high utilization (25 %) were formed. While the bulk composition of the Pt-Ir films was homogeneous and mirrored the Pt+Ir solution concentrations exactly, H-induced Pt surface enrichment was observed. Therefore, Pt-Ir NPs were synthesized using the simple polyol method, giving the same bulk and surface compositions. In order to obtain a higher utilization of Pt and to better understand the Ir role in the enhanced activity of Pt-Ir materials, compared to pure Pt, Ircore@Ptshell NPs were synthesized with a controlled Ptshell coverage and thickness, as determined by CO stripping and H adsorption measurements. Based on the methanol oxidation results obtained at all of these Pt-Ir materials, it was concluded that Ir enhances the kinetics of this reaction by the bi-functional effect, i.e., by the removal of adsorbed CO from Pt with OH groups attached to Ir. At the Ircore@Ptshell NPs, a weak electronic effect of the Ir core on the Ptshell was also noted at high potentials, contributing to the removal of the CO poison.
The optimum Ir content of these Pt-Ir catalysts was found to be in the range of 30-60%. The results for Pt-Ir were compared with the standard DMFC anode material, Pt-Ru. As no systematic study has been reported on the effect of a Rucore on the methanol oxidation activity of Pt, Rucore@Ptshell NPs, with a Rucore size of 2 or 3 nm and a Ptshell coverage of up to two monolayers, were synthesized. A fingerprint of the surface composition of these Rucore@Ptshell NPs was obtained using several novel electrochemical approaches. The bi-functional effect of Ru on Pt was found to be negligible compared to the strain and electronic effects, with the compression of the Ptshell shown to accelerate the methanol adsorption/dehydrogenation step. Furthermore, at low potentials, the electronic effect of Ru increased the rate of CO oxidation, while at higher potentials, it inhibited the methanol adsorption/dehydrogenation step.
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Physical, Physical, Physical, Energy, Materials Science
Citation
El Sawy, E. N. (2013). Development of Nano-Structured Direct Methanol Fuel Cell Anodes (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/25683