Soowhan Kim

Investigation of Temperature-Driven Water Transport in Polymer Electrolyte Fuel Cell: Phase-Change-Induced Flow

The objective of this work is to investigate phase-change-induced water transport of polymer electrolyte fuel cell materials subjected to a temperature gradient. Contrary to thermo-osmotic flow in fuel cell membranes, a net flux of water was found to flow from the hot to the 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.

Characteristic Behavior of Polymer Electrolyte Fuel Cell Resistance during Cold Start

bHyundai Motor Corporation, Yongin, Korea In this study, experimental constant-current cold starts were performed on a polymer electrolyte fuel cell from �10°C to characterize high-frequency resistance behavior, water motion, and ice accumulation before, during, and after cold start. A diagnostic method for rapid and repeatable cold starts was developed and verified. Cold-start performance 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 some evidence of supercooled 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 formation in and around the CL. Lower startup currents were found to produce more total heat but at a reduced rate compared to high currents. Therefore, an acceptable current range exists for a given stack design which balances the total heat generation and time required to achieve a successful cold start.

Quantification of Temperature Driven Flow in a Polymer Electrolyte Fuel Cell Using High-Resolution Neutron Radiography

In this study, the effect of a controlled temperature gradient on water transport across a single fuel cell was quantitatively investigated using high-resolution neutron imaging. The direction of liquid water transport under isothermal and non-isothermal conditions was observed in both hydrophilic and hydrophobic diffusion media (DM). The change in distribution of liquid saturation with time revealed two different mechanisms of water transport; capillary driven flow and phase-change induced (PCI) flow, in which a water vapor concentration gradient is created by condensation at a colder location. This concentration gradient drives diffusion flow toward the colder location. A maximum liquid saturation plateau of ca. 30% was shown for all conditions tested, indicating a critical transition between pendular and funicular modes of liquid water storage was captured. Based on this, it is suggested that PCI-flow may be the main mode of liquid transport below this critical transition threshold, above which, capillary flow dominates. As expected, both average cell temperature and the magnitude of temperature gradient were shown to significantly affect the rate of condensation within the DM. Experimental results were compared with water saturation distribution model predictions from literature and show reasonable qualitative agreement. Finally, it was concluded that current available models significantly over predict vapor phase diffusive transport in saturated fuel cell media using a Bruggeman type model.