Characteristic Behavior of Polymer Electrolyte Fuel Cells during Cold Start

Open Access
Author:
Chacko, Charles Christopher
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 05, 2008
Committee Members:
  • Matthew M Mench, Thesis Advisor
Keywords:
  • cold start
  • polymer electrolyte fuel cells
  • high frequency resistance
Abstract:
In this study, experimental constant-current cold starts were performed on a polymer electrolyte fuel cell (PEFC) from -10°C in order to characterize high frequency resistance behavior (HFR), water motion, and ice accumulation before, during, and after cold start. A new diagnostic method for rapid and repeatable cold starts was developed and verified. HFR is shown to be a key diagnostic for cold start behavior and performance. Cold start performance, as measured by the amount of water produced during the cold start, is found to be optimized when cell resistance is increasing prior to startup, which is indicative of polymer electrolyte membrane (PEM) dehydration. During cold start, cell resistance initially decreases due to PEM hydration by the product water. Interestingly, after a certain water uptake capacity of the PEM is reached, resistance increases due to ice formation in and around the cathode catalyst layer (CL) with evidence of super-cooled water flow at low currents. Utilizing lower startup currents apparently does not increase the PEM water storage capability, but does increase the total volume of ice in and around the CL before freeze-out. Lower startup currents were found to produce more total heat, but at a reduced rate compared to high current densities. Therefore, an acceptable current density range exists for a given stack design which ensures sufficient heat is produced to raise cell temperature above 0°C before freezing out occurs, which is the overall design goal.