In Situ Fuel Cell Water Metrology at the NIST Neutron Imaging Facility

Abstract

Neutron imaging has been demonstrated to be a powerful tool to measure the in situ water content of commercial proton exchange membrane fuel cells (PEMFCs) in two and three dimensions. The National Institute of Standards and Technology neutron imaging facility was designed to produce a high intensity, highly collimated neutron imaging beam to measure the water content of operating fuel cells. The details of the neutron optics and neutron detection are discussed in terms of the random uncertainty in measuring the liquid water thickness that is typical of operating PEMFCs.

Durability of Polymer Electrolyte Membrane Fuel Cells Operated at Subfreezing Temperatures

“…Initially the water over the lands increases at a rate of 0.0146 ± 0.001 mg cm −2 s −1 and decreases to 0.0113 ± 0.001 mg cm −2 s −1 after ≈1150 s. 53 For example, the dark blue solid line in Figure 5 shows a sharp decrease in the rate of water/ice accumulation over the lands at approximately ≈1150 s and a corresponding increase in the rate of water/ice accumulation over the channels. This transition where more ice/water starts to form under the channels, corresponds to the start of the plateau region in the HFR in Figure 4.…”

“…Neutron image analysis for both low-and high-resolution imaging was performed using NIST developed analysis code and a dry reference image as described by Hussey et al and Hickner et al 53,55 The intensity of the wet images (attenuated image, I) were divided by the reference image (dry image, I 0 at the same temperature) in order to obtain the water density images using Equation 1, where the attenuation coefficient of water is μ w = 0.3708 mm −1 (effects of beam hardening are neglected). Water thickness t (mm) is then obtained as using Beer-Lambert's law as reported previously.…”

“…The neutron imaging facility at NIST and technique to measure water content in operating fuel cells are detailed elsewhere. 53,54 Low-resolution neutron imaging measurements were conducted using a beam with collimation ratio (L/D) of 450 and a fluence rate of 2×10 7 neutrons cm −2 sec −1 . The imaging detector was an amorphous silicon detector with a resolution of approximately 300 μm, a field of view of 20 cm × 25 cm and 1 s exposure time.…”

“…53 Masks (to scale) were made to represent the lands, channels, and active area of the cells for image analysis. The land masks and channel masks were superimposed on the dry image to ensure that the correct area of the image was being analyzed.…”

Impact of Interfacial Water Transport in PEMFCs on Cell Performance

“…On the other hand, the neutron beam has high sensitivity only to hydrogenous materials (e.g. water) while metals and carbon are relatively transparent to neutrons [15], providing an advantage for NRG in in-situ quantification of liquid water inside PEMFC despite lower spatial and temporal resolution. A number of researchers have successfully used NRG to determine the effect of flow field on liquid water distribution in PEMFCs [13,14].…”