Neutron Imaging Studies of Liquid Water Transport in PEM Fuel Cells
by Dr. Michael Hickner
Senior Member, Technical Staff, Sandia National Laboratories
Date: Tuesday, November 7, 2006 Time: 7:30 PM – 8:30 PM Place: Pomegranate Mediterranean Cuisine, 1585 University Ave., Berkeley, CA 94703, http://shopinberkeley.com/p/pomegranate/. Dinner: 6:30 PM – 7:30 PM. Cost: $20 per person; $12 for students. (No cost to attend presentation only.) RSVP: Adam Weber (AZWeber@lbl.gov) by 5PM Friday November 3, 2006. Please indicate if you will attend both the dinner and the talk or only the talk and if you are a student. Directions: Pomegranate Mediterranean Cuisine, from Interstate 80/580, exit University Ave. Head east on University Ave. approx. 1.3 miles to 1585 University Ave. between Sacramento St. and California St.).
Membership in ECS is not a requirement for attendance.
Water plays a key role in the performance of a proton exchange membrane fuel cell device. A high performance cell is typically supplied with water in addition to that produced by the oxygen reduction reaction in order to keep the electrodes and membrane hydrated and ionically conductive. However, too much water in the cell can cause “flooding“ where the porous layers and gas channels fill up with liquid water which cuts off reactant flow and thus decreases cell output power. There are very few techniques that can directly monitor liquid water in an operating fuel cell. Neutron radiography is an extremely promising technique for gaining quantitative information on the water content of the fuel cell.
Recent neutron imaging investigations(1) have revealed a strong temperature and current density dependence on the liquid water content of a PEM fuel cell. A maximum in liquid water content is observed at intermediate current densities and then subsequently declines as the current density is increased even though the water production increases linearly with current. These results can be explained by considering the local heating of the cell due to the waste heat of the reaction. In addition, small changes in bulk cell temperature can greatly impact the water carrying capacity of the gas, especially at higher temperature due to the exponential increase in water vapor pressure with temperature.
Our neutron imaging investigations have also shown that the change in water content of the cell is dramatically slower than the electrical response. In current step experiments, when the current was instantaneously increased from 0 mA/cm2 to 1000 mA/cm2, the water content did not approach a constant value for at least 100 s. Upon ceasing of the current, the water content declined slowly over a period of 600 s. We believe this slow response of the water content of the cell is due to the slow liquid transport in the porous gas diffusion layer of the cell.
Neutron imaging has proven to be a valuable tool in obtaining experimental information on the liquid water content of a PEMFC. This tool will help to further the understanding of liquid water transport in fuel cells and will aid in modeling these complex, coupled phenomena and dynamics. These results will be detailed along with more recent unpublished data that seeks to further elucidate the interplay between liquid water content and waste heat in a PEMFC under various fuel cell operating conditions.
1. Hickner, M.A., N.P. Siegel, K.S. Chen, D.N. McBrayer, D.S. Hussey, D.L. Jacobson, M. Arif, J. Electrochem. Soc.2006, 153(5).
Speaker Biographical Sketch
Michael Hickner grew up predominantly in Northern Michigan. He received his B.S. in Chemical Engineering from Michigan Tech in 1999 and his Ph.D. in Chemical Engineering from Virginia Tech in 2003 under the direction of James E. McGrath. Michael's research in Dr. McGrath's lab focused on the transport properties of proton exchange membranes and their structure-property relationships. He has spent time at Los Alamos National Laboratory studying novel membranes in direct methanol fuel cells. He moved to Sandia National Laboratories in 2003 as a postdoc and is currently a Senior Member of the Technical Staff. His research interests include transport in ion-containing polymers, membranes for water treatment, transport and performance of electrochemical systems, and wetting and two-phase flow in porous media.