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Determining Water Content and Distribution in PEMFCs to Predict Aging While in Storage

Wednesday, 4 October 2017: 12:00
National Harbor 3 (Gaylord National Resort and Convention Center)
S. Stariha, M. S. Wilson (Los Alamos National Laboratory), J. M. LaManna (National Institute of Standards and Technology), D. L. Jacobson, D. S. Hussey (NIST), N. Macauley, J. Rau, and R. L. Borup (Los Alamos National Laboratory)
Proton Exchange Membrane Fuel Cells (PEMFCs) known for their use as portable energy-conversion devices have become increasingly more popular as backup power sources. Some backup power source applications require PEMFCs to startup within seconds after being stored for many years without any external aid. The ability to reach maximum power with a quick startup time in our design comes from the PEMFC being stored with water saturated membranes and catalyst layers. For the lowest overall resistance needed for an immediate startup, the water content of the membrane and catalyst layer ionomer should be maximized, which requires direct contact with liquid water i.e. Schroeder’s paradox. [1] However, any water collecting in the flow-fields or gas diffusion layers (GDLs) is detrimental, as it can obstruct reactant access. Consequently, the water distribution throughout the PEMFC during long-term storage is of critical interest for quick startup times.

Previous studies have shown that water content and its distribution within a PEMFC is a good indicator of performance during both normal operation and accelerated stress testing. These studies have been mainly done using neutron imaging. [2,3] The measurements and analysis can be applied to PEMFCs hermetically sealed in storage. Before a PEMFC is stored, neutron imaging can be used to measure the water content and its distribution within the cell. It can then be measured and mapped for a series of identical PEMFCs stored for different lengths of time.

Storage experiments have been performed by humidifying and hermetically sealing two identical single cells. Both cells were humidified by controlling the current using dry H2/O2 until a power of 2 W was reached and maintained for 60 seconds. It should be noted that for future experiments the cells will be stored in N2 to prevent any membrane permeation. Cell 1 was stored over night while cell 2 was stored for 14 days. For these cells, the goal was instant startup of at least 1.25 W at 0.5 V with 50 psig H2/O2 and maintaining that power output for at least 60 seconds. Figure 1 shows the results of both startups after storage. Cell 1 (solid grey line) after being stored overnight reached a peak power of 2 W in 1 second and maintained a power of 1.75 W for 120 seconds. Cell 2 (solid blue line) after being stored for 14 days reached a peak power of 3.25 W in 1 second however its power dropped rapidly and after 120 seconds reached the minimum power output of 1.25 W. Water movement within the cell during storage contributed to the different startup behaviors observed. With the use of high resolution neutron imaging in the z-direction (See Figure 2) the water content and distribution can be imaged through all components. The goal of this work is to store identical cells for varying lengths of time, compare the water content distribution in each cell and how it relates to the startup performance, and extrapolate how a cell will startup after multiple years in storage.

Acknowledgments: This work is supported by the NNSA Engineering Campaign with long term funding from the Fuel Cell Technologies Office

References

  1.  T.A. Zawodzinski et al. Journal of The Electrochemical Society, 1993. 140 (4) 1041-1047.
  2. Spernjak, D. et al. Journal of The Electrochemical Society, 2009. 156 (1) B109-B117.
  3. Macauley, N. et al. Journal of The Electrochemical Society, 2016. 163 (13) F1317-F1329.