(453c) Membrane Hydration Regulation in a PEMFC

At the heart of a Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a Nafion membrane with a functionality that greatly depends on water content. When membrane hydration is low, fuel cell performance degrades due to low ionic conductivity. When membrane hydration exceeds a certain limit, flooding occurs at the cathode gas diffusion layer and oxygen is prevented from reaching reaction sites. While advanced flow channel design can provide passive improves, direct regulation of the membrane hydration profile has yet to be attempted.

PEMFC simulations indicate the existence of a non-uniform hydration profile within the membrane. Similar profiles have also been observed experimentally [1]. Toward the goal of maximum efficiency, the desired profile is one in which the water content is large at all points in the membrane (to improve ionic conductivity), but does not exceed the flooding limit at the cathode interface. In the envisioned controller, such a profile will serve as the set-point condition. Additional simulations, [2], indicate that manipulation of temperature and relative humidity at the anode and cathode inlets can be used to manipulate the average and slope of the hydration profile.

To close the loop, one will need independent measurements of the hydration profile. Electrochemical Impedance Spectroscopy (EIS) is commonly used to decompose cell resistance into its various components. Specifically, the high frequency components of EIS can be used during normal cell operation to measure ohmic resistance and thus infer the hydration profile average. In addition, humidity measurements between the inlet and exit gas streams can be used to infer the water flux between the membrane and each gas chamber. Such measurements can then be used to calculate membrane hydration at each interface. Using the above measurements, a state estimator has been developed to provide an optimally weighted combination of the measurements as well as provide predictive capabilities. Finally, under closed-loop conditions the hydration regulator is tested in the face of a time varying power load.

[1] R. J. Bellows, M. Y. Lin, M. Arif, A. K. Thompson and D. Jacobson ?Neutron Imaging Technique for In Situ Measurement of Water Transport Gradients within Nafion in Polymer Electrolyte Fuel Cells,? Journal of the Electrochemical Society, 146 (3) pp 1099-1103, 1999.