Date of Award

2-2-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Systems Engineering

First Advisor

Tansel Karabacak

Abstract

Polymer electrolyte membrane (PEM) fuel cells are electrochemical energy conversion devices which have demonstrated great promise as future energy sources for electric vehicles, as they convert chemical energy to electrical energy with a significantly higher efficiency and lower environmental impact than in standard combustion processes. However, the commercialization of PEM fuel cells for transportation applications has been hindered by several factors such as high cost of Pt, low Pt utilization, poor long-term durability of the conventional PEM fuel cell catalyst (Pt nanoparticels supported on carbon black, Pt/C), and poor thermal and chemical stability of the electrocatalyst supports (carbon black). The goal of this research was to fabricate high performance, durable, carbon-free, controllable porosity, and low cost (low Pt loading) sputtered-nanostructured electrocatalysts and investigate their morphologies, crystal properties, and electrocatalytic activities. First, the electrocatalytic oxygen reduction reaction (ORR) activity of vertically-aligned solid Pt nanorods was evaluated. A glancing angle deposition (GLAD) technique was used to fabricate Pt nanorod arrays directly on glassy carbon (GC) electrodes. It was found that Pt-nanorod electrocatalysts exhibit higher area-specific activity, greater electrochemical stability, higher electron-transfer rate constant, and comparable activation energy for ORR than those of Pt/C due to their larger crystallite size, single-crystal property, and dominance of the preferred crystal orientations (Pt[110]) for ORR. However, Pt nanorods show lower mass specific activity than that of Pt/C electrocatalyst due to the large diameter of nanorods. Second, to further enhance the mass-specific activity of solid GLAD Pt nanorods, the GLAD chromium (Cr) nanorods were used as low-cost catalyst supports for conformal Pt thin film coating achieved by a small angle deposition (SAD) technique as a potential catalyst electrode for oxygen reduction reaction (ORR) in PEM fuel cells. We also prepared two electrodes consisting of GLAD Cr nanorods coated Pt thin film deposited at conventional normal incidence (è=0o) and large angle of 60o for comparison. It was demonstrated that SAD Pt(30o)/GLAD-Cr nanorods have greater electrochemical stability, higher electrochemical active surface area (ECSA), and higher specific activities than Pt(0o)/GLAD-Cr nanorods and Pt(60o)/GLAD-Cr nanorods electrodes. The improved activity might be attributed to a better Pt conformality and a preferential exposure of certain crystal facets. Although a significant mass-specific activity of Pt/Cr nanorod catalyst was expected compared to solid Pt nanorods due to the fact that only Pt on the outer surfaces contributes effectively to the electrochemical reaction in PEM fuel cells, the Pt/Cr nanorod catalyst exhibited a slight mass-specific activity enhancement compared to solid Pt nanorods. This might be attributed to the incomplete coating of GLAD Cr nanorods by SAD Pt thin film, especially at the bottom of the nanorods. In addition, there is a possibility that the crystallographic structure of the nanorods sidewall of Pt(SAD)/GLAD-Cr nanorods is different than that of solid GLAD Pt nanorods where the ORR activity is dominated by Pt(110) which is the most active surface area for ORR. In order to investigate the effect of electrode porosity on the electrocatalytic activity of the nanostrutured electrocatalysts, the ORR electrocatalytic activity of periodic and well separated solid Pt nanorods produced by GLAD technique on patterned glassy carbon electrodes fabricated by modified-nanosphere lithography (m-NSL) technique was studied. Compared to GLAD Pt nanorods grown on flat substrates, the GLAD Pt nanorods on patterned substrates exhibited higher area-specific activity at high current density region in the polarization curve due to the effective oxygen mass transport within the wider pores for the periodic and well-separated GLAD Pt nanorods. Finally, hydrophilic nature of the Pt nanoparticles may lead to catalyst flooding and resultant degradation in the electrode performance in an operating PEM fuel cell. We also demonstrated that GLAD Pt nanorods are highly hydrophilic due to the enhanced surface roughness. Therefore, a hydrophobic property to vertically aligned hydrophilic metallic GLAD nanorods (Pt nanorods) was introduced by sputtering an ultra thin layer of Teflon (nanopatch) at a glancing angle è=85o on the tips of the nanorods.

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