PHYSICAL DAMAGE OF POLYMER ELECTROLYTE FUEL CELLS SUBJECT TO FREEZING

Open Access
Author:
Kim, Soowhan
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 30, 2008
Committee Members:
  • Matthew M Mench, Committee Chair
  • Fan Bill B Cheung, Committee Member
  • Stephen R Turns, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • Water transport
  • Freeze damage
  • Polymer electrolyte fuel cell
  • Thermo-osmosis
  • Phase-change induced flow
Abstract:
One of the remaining technical challenges for polymer electrolyte fuel cell (PEFC) commercialization is to achieve shutdown to a frozen state, and rapid start from frozen conditions without damage. PEFCs generate water as reaction product, so when PEFCs are subjected to sub-freezing environments without removal of residual water, they can experience irreversible damage. There exist conflicts in the literature regarding freeze-damage, and the particular damage modes are not clearly understood. This work is designed to elucidate freeze-damage modes of fuel cell materials, and to investigate temperature-gradient driven water transport phenomena as a potential to mitigate freeze-damage. Freeze-damage modes were identified through ex-situ and in-cell freeze/thaw (F/T) cycling experiment. Three primary damage modes were discovered: 1) interfacial delamination, 2) ice expansion damage, and 3) pore level damage. The effect of interfacial delamination, which was diagnosed as a main damage mode on fuel cell performance, was also investigated by developing a two-dimensional anisotropic current disruption model. Extensive ex-situ material F/T cycling testing was conducted under the worst case conditions of liquid water submersion to identify the key factors leading to physical damage. Specifically, the membrane electrode assembly, porous media, and electrolyte were found to be sources of water that can damage the catalyst layers under freeze/thaw conditions. Damage was found to occur almost exclusively under the channel, and not under the land. Conceptually, the best material to mitigate freeze damage in a PEFC was found to be a crack-free virgin catalyst layer, and a thin reinforced membrane with a stiff hydrophobically treated diffusion media. The in-cell study affirmed that significant damage can occur for a single cell with no purge, indicating that water removal from the cell, especially from the catalyst layer pores, during shutdown is critical for robust PEFC operation in frozen conditions. A two-dimensional anisotropic model was developed to investigate the impact of local delamination on PEFC performance. Localized interfacial delamination of the membrane  catalyst layer and catalyst layer  diffusion media were found to significantly increase ohmic resistance. The in-plane resistance and in-plane-to-through-plane resistance ratio of PEFC components adjacent to the delamination were determined to be the key controlling parameters for increase in ohmic resistance. Temperature-gradient induced water transport phenomena were explored in PEFC materials because of a potential for non-parasitic water drainage at shutdown. Two different modes were identified: 1) thermo-osmosis, and 2) phase-change induced flow. Direct thermo-osmotic experiments on different polymer membrane types (non-reinforced NafionĀ® and two different commercial reinforced membranes) show that thermo-osmotic flow was observed in all membranes, and the water flow direction in the membrane was determined to always flow from the cold to the hot side, as anticipated for a small pore hydrophilic porous medium. The water flux was found to be proportional to temperature gradient and to increase with average membrane temperature. The dependency of the thermo-osmotic diffusivity on average temperature showed predictable Arrhenius-type behavior. True interfacial temperatures of the membrane were estimated using a two-dimensional thermal model, and empirical relations for the thermo-osmotic diffusivity for the membrane types tested were developed. These can be of use to design engineers concerned about achieving optimal water balance during steady and transient operation. Contrary to thermo-osmotic flow in fuel cell membranes, a net flux of water was found to flow from the hot to cold side of the full membrane electrode assembly. The key to this is the existence of some gas phase in the catalyst layer or other porous media. This mode of transport is a result of phase-change induced flow. The measured water transport through the membrane electrode assembly is the net effect of mass diffusion as well as thermo-osmosis in the membrane, which moves counter to the direction of the phase-change induced flow. Arrhenius functions that are dependent on material set, temperature gradient, and average temperature across the materials were developed that describe the net flux. In addition to direct quantification, phase-change induced flow was visualized and confirmed using high resolution neutron radiography.