Date of Award

5-2013

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

Ahmet Turhan

Committee Members

Matthew M. Mench, Rao Arimilli

Abstract

Polymer electrolyte fuel cells (PEFCs) are a promising alternative energy source. One challenge preventing widespread use of this technology is water management. A balance must be reached between providing sufficient water for membrane ionic conductivity while maintaining low enough water content to mitigate the reduction of available reaction sites in the cathode catalyst layer due to liquid water build up. Much exploration of this area of fuel cell research has been conducted, but the details of water transport in an operating fuel cell are not yet fully understood. The motivation of this work was to elucidate mass transport phenomena occurring in an operational fuel cell by measuring the real-time net water drag (NWD) behavior under different operating conditions and material properties.

Water measurements were made by four relative humidity sensors placed in the anode and cathode inlet and exit lines. Relationships between NWD and current density, reactant flow rates, inlet gas relative humidity, and microporous layers (MPLs) were studied. The time required for net water drag to reach a quasi-steady state value varied with current and was on the order of 200 seconds or less. At high current densities, phase change induced-flow (PCI) was found to dominate the other modes of transport due to elevated temperature gradients across the cathode MPL.

Asymmetrical MPL configurations were tested with different MPL thicknesses, and NWD increases with current were found to be significantly higher than those measured with a symmetrical configuration, regardless of the location of thicker MPL. The increase in NWD at high currents for the cathode-side thick MPL case was attributed to the enhanced PCI-flow across the cathode MPL. With the anode-side thick MPL, the decreased temperature gradient across the membrane was suggested as the cause of the NWD increase. Though NWD increases regardless of the location of the thicker MPL, the increased PCI flow has a larger impact on NWD than the reduced vapor transport. Experiments of high current transients were performed also, and it was concluded that anode dry-out may be avoided by increasing the back pressure of the cathode during a sudden jump to a high power condition.

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