Date of Award

11-8-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Tansel Karabacak

Abstract

Core-shell nanorod array geometry can offer enhanced durability for polymer electrolyte membrane (PEM) fuel cell electrodes if a catalyst shell is conformally coated around the nanorod support base. Uniform catalyst shell coating can minimize the interaction of acidic fuel cell medium with the nanorod base and can improve the overall electrocatalyst electrode stability. However, fabrication of the conformal shell layers on nanorod arrays using physical vapor deposition (PVD) methods (e.g. sputtering, thermal evaporation) has been a formidable task. In order to assess the PVD conditions for the production of conformal shell coatings, Monte Carlo simulation methods (MC) can provide useful guidance as well as helping to investigate fundamental growth dynamics during core-shell nanorod array fabrication. Our MC simulation results showed that an oblique flux with a small angle between the incident flux of the particles and the base nanorod surface can generate a fairly conformal shell coating. Therefore, the primary goal of this project was to investigate the effect of different sputter deposition methods on the conformality of thin film catalysts in core-shell nanostructures by theoretical and experimental fundamental studies, with the aim of achieving durable electrocatalysts in PEM Fuel Cells. Towards this goal, we fabricated core-shell nanorod arrays of Pt/Ni (Pt shell, Ni nanorod core) and Pt/WC (Pt shell, WC nanorod core) using a glancing angle deposition (GLAD) for the support nanorods as well as different sputter deposition approaches for the catalyst shell growth. Similarly, we developed MC computer simulation codes for the sputter deposition of the shell layer around GLAD nanorods. In our experimental and simulation studies, we investigated the conformality of shell coatings produced by small angle deposition (SAD), high pressure sputtering (HIPS), SAD-HIPS, and conventional normal angle deposition (NAD). We also investigated the effects of base nanorod length and aspect ratio on the conformality of the catalyst layer. Conformality of the shell layers fabricated by different sputter deposition methods were investigated through material characterization methods such as scanning electron microscopy, transmission electron microscopy, and cross-sectional energy dispersive spectroscopy. Catalyst durability for PEM fuel cell applications, and therefore the quality of the shell conformality, were studied by electrochemical methods including cyclic voltammetry measurements.

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