INDIVIDUAL AND COUPLED EFFECTS OF FLOW FIELD GEOMETRY, INTERFACE AND MATERIAL PROPERTIES ON LIQUID WATER STORAGE AND FLOODING IN A POLYMER ELECTROLYTE FUEL CELL

Open Access
Author:
TURHAN, AHMET
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 19, 2009
Committee Members:
  • Clinton Matthew Mench, Dissertation Advisor
  • Matthew M Mench, Committee Chair
  • Jack Brenizer Jr., Committee Member
  • Fan Bill B Cheung, Committee Member
  • John Michael Regan, Committee Member
Keywords:
  • polymer electrolyte
  • fuel cell
  • neutron imaging
  • liquid storage
  • two phase flow
  • flow field
Abstract:
The existence of excess liquid water in the catalyst layer, diffusion media and flow channels of a polymer electrolyte fuel cell (PEFC), termed as “flooding”, results in major performance and durability limitations. Flooding, and the stored liquid water content, has adverse performance effects on operations at normal operating conditions as well as during start-up at sub-zero temperatures. In this study, the liquid water distribution, storage and flooding mechanisms inside a PEFC were thoroughly investigated using neutron imaging. For the first time, the individual effects of flow field design, land|channel interface and flow channel surface properties on liquid accumulation and distribution were analyzed. The flooded and non-flooded conditions were determined and results were used to develop a novel liquid filling and transport mechanism that better reflects the actual observed operation characteristics. In the first part of this work, the impact of flow field geometry on liquid storage and flooding was investigated through in-plane neutron imaging experiments performed on seven different flow field configurations with land|channel (L:C) ratios ranging from 1:3 to 2:1. The stored liquid amount in each configuration was analyzed and a preferred land/channel ratio is suggested in terms of obtaining minimal residual water content in the fuel cell. For the configurations having same L:C ratio, the number of land|channel interfaces was found to be have determining impact on liquid storage and flooding. The cell performance corresponding to each liquid storage value was also obtained for all configurations. The results revealed that at identical performance between two different cells, it is possible to significantly reduce the accumulated liquid water overhead by only tailoring the flow field design. This advance is important in terms of reducing the parasitic losses required to purge the liquid after shutdown, and eliminating degradation experienced under freeze/thaw conditions. In the second part of this work, the impact of land|channel interface surface energy on through-plane liquid accumulation, distribution and transport was analyzed with the use of high-resolution neutron imaging. Neutron images were taken with polytetrafluoroethylene (PTFE) coated and uncoated land|channel interface (flow channel walls). Anode to cathode liquid distribution was analyzed for each case at low and high current conditions over twenty minutes of operation. The form and amount of liquid water inside the channels and diffusion media (DM) were compared for hydrophobically coated channels and hydrophilic channels, and a primary liquid transport-flooding mechanism is suggested for each case. The location and value of maximum water storage in DM at low and high current operation were analyzed and slopes of water mass versus distance curve were calculated to compare the significance of capillary liquid flow and phase- change-induced flow within the diffusion media. A significant effect of CL|MPL and MPL|DM interfaces on liquid transport and flooding is found through the analysis of micro-porous layer (MPL) water content and saturation profile along the CL|MPL and MPL|DM interface region. In the final part of this study, the nature of flooding and dry-out was investigated. In-plane neutron imaging was used to determine flooded, non-flooded and dry-out conditions in a PEFC and results were analyzed to identify the liquid mass responsible for the performance loss under flooding or dry-out. Results clearly indicate that common assumptions for liquid filling used in most computational studies are inadequate to represent the actual case. Based on the results, a modified liquid pore filling mechanism is suggested that is more appropriate.