Date of Award

Spring 1-1-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Alan W. Weimer

Second Advisor

James W. Medlin

Third Advisor

Adam Holewinski

Fourth Advisor

Hans Funke

Fifth Advisor

Michael Marshak

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are highly-efficient devices that convert the chemical energy stored in hydrogen and oxygen into electricity. The commercialization of this technology is hindered by the use of platinum metal to catalyze the oxygen reduction reaction (ORR) on the cathode. In this work, atomic layer deposition (ALD) was applied to the fabrication of PEMFC catalysts in order to improve the utilization of Pt in the ORR as well as the stability of the catalytic Pt nanoparticles over voltage cycling.

Nanostructures of TiO2 were added via ALD to carbon-supported Pt catalysts (Pt/C) in order to investigate their stability effects. The durability of the catalysts was enhanced by the addition of TiO2, with up to 70% retention in mass activity recorded over voltage cycling of the coated materials. A high-temperature treatment was found to produce Pt-Ti nanoparticles with high initial activity, but very poor durability. Atomic layer deposition was also used to add WN nanostructures to the catalysts. After a high-temperature treatment, highly stable W2N blocking structures were formed around the Pt nanoparticles, which produced a catalyst with high activity and durability. Additional testing of these materials in a PEMFC showed that the addition of ALD nanostructures had significant effects on catalyst layer hydrophobicity, which has implications for water management during fuel cell operations.

In order to improve the utilization of Pt in the ORR, extended thin film electrocatalyst structures (ETFECS) were designed through the application of Pt ALD to a Ni nanowire (NiNW) substrate. Following a thermal annealing process to compress the lattice of the deposited Pt, the ETFECS were found to be 4 times more active than nanoparticle Pt catalysts. Upon further testing in a PEMFC, the performance of these ALD materials surpassed that of previously-reported ETFECS as well as the DOE 2020 target for fuel cells. Additional research was conducted to develop a bimetallic Pt-Ni ALD process for ETFECS. It was observed that the quantity of deposited metal can be finely tuned by adjusting the ALD parameters, but further work will be required to improve the final performance of the materials produced through this method.

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