Study of multiphase water transport and removal phenomena from fuel cell during gas purge

Open Access
Author:
Cho, Kyu Taek
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 04, 2010
Committee Members:
  • Clinton Matthew Mench, Dissertation Advisor
  • Matthew M Mench, Committee Chair
  • Jack Brenizer Jr., Committee Member
  • Seungjin Kim, Committee Member
  • Michael Anthony Hickner, Committee Member
  • Karen Ann Thole, Committee Member
Keywords:
  • polymer electrolyte fuel cell
  • water management
  • gas purge
  • evaporation
  • gas diffusion media
  • neutron radiography
Abstract:
The current state of fuel cell technology faces a number of technical challenges for automotive application that must be surmounted in order to compete against the internal combustion engine. Among those, control of water amount in the fuel cell is the major bottle neck for achieving durable operation from sub-freezing temperature. When the fuel cell is shutdown to a frozen state without proper removal of water, the pore structure of the porous media (diffusion media (DM) and catalyst layer) can be damaged due to volume expansion of the residual water, and catalyst layer can be delaminated from the membrane due to frost heave, leading to degradation of the fuel cell. To remove water from the cell after shutdown, a gas purge has generally been applied into the cell. However, the common method of gas purge is just applied without much fundamental consideration of water removal. Therefore, the conventional method (i.e., purge with nitrogen gas or air into both side of cell after cell shutdown) generally takes a relatively long time to remove water from the cell sufficiently, resulting in significant parasitic loss and reduced system efficiency. Additionally in some case the purges can be conducted without control of the drying distribution, which causes the MEA to be degraded at an accelerated rate. Requirements for adequate gas purge are as follows; it should be performed in short time without major parasitic loss, and it should selectively remove water from the porous media and channels without causing degradation of the membrane via over drying. These purge issues can be solved in part through fundamental understanding of water removal during purge. The purge/water removal relationship should be understood in actual system to do optimize design, and those behaviors should be interpreted with a fundamental understanding of evaporative water removal in individual fuel cell components. In the first part of this study, water removal behavior was investigated in a full sized fuel cell, and compared for various purge conditions. With concurrent utilization of neutron radiography and high frequency resistance, membrane dryness could be compared along with water removal from the cell during purge. The results showed gas purge could be performed while maintaining membrane durability by a controlled RH and flow rate condition for anode and cathode, respectively. In the second part of this study, the characteristic water removal of each fuel cell component and coupled effect of each component on evaporative water removal were investigated in a novel test cell. The DM is known to store a significant fraction of the total liquid water, but quantitative fundamental research examining water removal behavior from DM is not yet available in literature. Therefore, a fundamental study was conducted to characterize the water removal behavior of the DM with newly designed test rig. Effects of purge flow rate, DM material properties, and flow field structure on water removal rate were investigated. From this new fundamental understanding, an advanced purge method was proposed to achieve highly efficient and durable purge with reduced energy consumption. The proposed protocol was validated experimentally with high resolution neutron radiography. The fundamental and applied understanding in this study can be utilized to provide guidelines for an optimized purge protocol for actual systems.