Water Metrology in Operating PEM Fuel Cells: Simultaneous Neutron and Optical Imaging.

Performance and lifetime of a fuel cell are influenced by both the water content and the water location within the cell (cathode vs. anode side; membrane, CL, GDL, channels). It is therefore especially important to quantify and separately evaluate the effects of liquid water residing in different cell compartments.

Neutron imaging has been widely used in water management studies. The main benefit of this technique is the quantitative information about water content within an operating fuel cell. However, neutron imaging suffers from several notable limitations. The main disadvantage is the inability to distinguish the water location along the neutron beam direction (i.e. between different compartments through the cell thickness). Neutron imaging studies are therefore often unable to fully explain the correlations between the water content, operating conditions and the cell performance. Another challenge is to independently verify the water content measured by neutron imaging, since the reported systematic error contributions were as large as 30%.

To overcome some of the above limitations, we introduced an approach of simultaneous neutron imaging and optical visualization, providing two independent sets of data in an operating PEM fuel cell. Images of liquid water dynamics in the flow field were recorded optically through a transparent window while simultaneously measuring the through-thickness water content at each planar location of the cell with the neutron imaging technique. Neutron images were made semi-transparent, and overlaid on the optical images to correlate the neutron signal with the water residing in the channels.

The concurrent images provide complementary data that allow one to separate the water distribution in the flow field from the remainder of the cell thickness, which has been a great challenge in water management studies. The combined technique therefore enables improved correlations between the water dynamics and cell performance. Further, higher spatial and temporal resolutions of the direct visualization enable improved interpretation of the smeared neutron signal in situations with highly dynamic water behavior. Optical images may also be used for in-situ calibration of the attenuation coefficient of water, which is necessary for accurate processing of neutron images. This novel experimental approach offers a potential to validate the neutron data, and to mitigate the error contributions associated with neutron imaging.

Figure 1.Schematic of an experimental setup for simultaneous measurement of water transport with direct visualization and neutron imaging
Figure 2. Simultaneous neutron and cathode optical images in an operating cell with interdigitated channels.
(a) Processed neutron image (water thickness denoted by the scale).
(b) Optical image of the cathode flow field.
(c) Neutron image (a), with opacity 40%, overlaid on the concurrent optical image (b).
Spernjak D., Advani S.G., and Prasad A.K., "Simultaneous neutron and optical imaging in PEM fuel cells," Journal of the Electrochemical Society, Vol. 156, pp. B109-B117, November 10, 2008. doi:10.1149/1.3009585
Spernjak D., Prasad A.K., and Advani S.G., "In situ comparison of water content and dynamics in parallel, single-serpentine, and interdigitated flow fields of polymer electrolyte membrane fuel cells," Journal of Power Sources, Vol. 195, pp. 3553-3568, June 1, 2010. doi:10.1016/j.jpowsour.2009.12.031