(631g) The Impact of Hydration Dynamics on the Control of a Pem Fuel Cell

The Polymer Electrolyte Membrane Fuel Cell (PEMFC) has been projected to be the fuel cell of choice for future automotive applications. Among the most challenging aspect of this application is the occurrence of severe and frequent changes in power demand. From a dynamic perspective, it is frequently assumed that the thermal phenomenon possesses the largest time constant and determines response times. In a recent effort, [1], we analyzed PEMFC control issues using this thermal dominant perspective, and concluded that consideration of the faster time-scale phenomena (electrochemical and species flow) will be critical to successful controller design.

In the PEMFC literature, the subjects of water management and flooding are frequently stated as key issues in the design and operation of these systems. Recent studies on the dynamics of membrane hydration [2, 3] suggest that the time constant of this phenomenon is just a bit faster than that of the thermal characteristics. In this paper we will present two dynamic models central to understanding the water management / flooding issue. The first considers membrane hydration and shows how the interplay between electro-osmotic drag and back-diffusion will determine ionic conductivity. Additionally, we will illustrate how over-hydration of the membrane will lead to liquid water being ejected from the membrane to the Gas Diffusion Layer (GDL). The second model focuses on the GDL and illustrates how capillary forces govern the evolving distribution of liquid water in the GDL and how this eventually leads to a blocking of oxygen transfer through the GDL and to the electrochemical reaction site (i.e., flooding). Finally, this expanded dynamic model will be employed in an updated analysis of closed-loop performance.

[1] K.C. Lauzze and D.J. Chmielewski, ?Power Control of a Polymer Electrolyte Membrane Fuel Cell? Ind. Eng Chem Res., in press, 2006.

[2] Y. Wang and C-Y. Wang, ?Transient analysis of polymer electrolyte fuel cells,? Electrochimica Acta, vol 50, pp 1307-1315, 2005.

[3] D. Song, Q. Wang, Z-S. Liu and C. Huang, ?Transient analysis for the cathode gas diffusion layer of PEM fuel cells,? J. of Power Sources., in press, 2005.