Doctoral Dissertations

Orcid ID

http://orcid.org/0000-0002-9616-4135

Date of Award

12-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

Thomas A. Zawodzinski

Committee Members

Gerd Duscher, Ramki Kalyanaraman, Gila Stein

Abstract

In this thesis, the microstructure and performance of Proton Exchange Membrane Fuel Cells (PEMFCs) with platinum (Pt)-metal-group-free and Pt-based electrodes are evaluated to analyze the effect of the catalyst composition and structure on the electrode properties. In PEMFC systems, the thickness, structure and morphology of the catalyst layer (CL) are integral to cell performance and are particularly significant for cathodes using a non-precious metal catalyst due to higher catalyst loadings and thicker catalyst layers that compensate for the relatively low catalytic activity. An iron (III) porphyrin framework material was synthesized, pyrolyzed and its activity for the oxygen reduction reaction evaluated as a function of the catalyst loading (electrode thickness) and oxygen partial pressure. Various polarization losses were decoupled to reveal limiting processes. The kinetic overpotential was the major contributor. Mass transport contributions to voltage loss increased with higher cathode catalyst loading. Observed performance is discussed in the context of CL structure and morphology, analyzed using microscopy and X-Ray Diffraction.For Pt-based electrodes, the effects of the carbon support and the CL composition were experimentally studied. Machine-prepared cathode CLs with different Pt to carbon (Pt:C) ratios on highly graphitized carbon were prepared using a fixed 3M ionomer to carbon ratio. These were characterized and their PEMFC performance was evaluated. The Pt:C ratio had a significant influence on the CL structure and transport properties. The lowest Pt:C ratio (30:70) exhibited a higher volume of secondary pores and higher proton conductivity over the whole relative humidity range, with higher performance in PEMFC. This correlated with a more homogeneous ionomer distribution throughout the CL, caused by a preferential ionomer/carbon affinity. A study of interactions between 3M ionomer adsorption on a series of carbon supports with various Pt:C ratios provided further evidence for higher affinity of ionomer to the carbon surface. The adsorption isotherm for ionomer on carbon was quantified with two methods. Preferential adsorption on amorphous/graphitic carbon structure boundaries was confirmed. Ionomer adsorption occurs mainly on the secondary pores surface, consistent with previous BET results. Pt nanoparticles exhibited a negative effect on the adsorption process, possibly by blocking ionomer adsorption preferential sites.

Comments

Portions of this document were previously published in the Journal of Materials Chemistry A (“Investigation of a Microporous Iron (iii) Porphyrin Framework Derived Cathode Catalyst in PEM Fuel Cells.” J. Mater. Chem. A 4, no. 40 (2016): 15621–30) and in the ECS Transactions (“19F NMR Studies of 3M Ionomer Adsorption on Carbon Supports and Pt/C Catalysts”. ECS Transactions Volume 80 (2017): 259–268)

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