Doctoral Dissertations

Date of Award

5-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Matthew M. Mench

Committee Members

Thomas A. Zawodzinski, Feng-Yuan Zhang, Dibyendu Mukherjee

Abstract

Fuel cells offer the potential for high efficiency energy conversion with only water and heat as significant products of the electrochemical reaction. For a cost-competitive product, fuel cell researchers are exploring the limits of the Pt catalyst loading in parallel with performance and durability trade-offs. A significant portion of the performance loss in low-cost PEMFCs is associated with the partial pressure of oxygen (for an air cathode) at the Pt surface. This dissertation explores the main components of oxygen transport resistance which are associated with diffusion through partially saturated porous media and the ionomer coating in the catalyst layer.

Under typical proton exchange membrane fuel cell (PEMFC) operating conditions, temperature gradients through the porous gas diffusion layer (GDL) can result in product water condensation. As a result, non-uniform partial saturation of the GDL changes the local effective porosity and tortuosity encountered by oxygen diffusing to the catalyst layer. This work establishes the impact of saturation on practical fuel cell system efficiency losses related to shutdown purge time and overall stack resistance. The transport resistance is further investigated in two-dimensions using limiting current experiments with simultaneous neutron imaging. The analysis of these data results in a diffusion coefficient vs. saturation relationship for two common GDL carbon fiber substrates.

A significant oxygen transport limitation also occurs near the Pt surface. This is investigated here with loading studies that fix electrode thickness and bulk properties. The impact of Pt dispersion is probed by varying the average distance between Pt particles. Results elucidate how the electrode structure impacts local transport loss. It is demonstrated that local transport loss is not fully captured with a normalized Pt area. Additional geometric considerations that account for ionomer surface area relative to the Pt particles are required to resolve performance loss at low Pt loading as electrode structure varies. Furthermore, within this ionomer layer an interfacial resistance at both the gas and Pt interfaces is required to account for performance trends observed. These results demonstrate that residual performance loss associated with low cathode Pt loading can be mitigated by minimizing oxygen flux through the gas/ionomer interface.

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