Wolter optics promise to radically transform neutron imaging by improving the time resolution of the method by at least a factor 1,000 while providing spatial resolution of order a few micrometers [1]. These optics will enable neutron tomography of operating fuel cells with a time resolution of about a few minutes, which will provide unique insight into the water transport phenomena in membrane electrode assemblies of fuel cells and electrolyzers.

Neutron imaging has played a critical role in understanding water transport in proton exchange membrane fuel cells and electrolyzers [2]. The primary limitation of the method is the inherent neutron source strength, which is about a billion times weaker than X-ray synchrotron sources. Additionally, neutrons are difficult to focus using refractive lenses so that neutron imaging beamlines heretofore can be described as pinhole cameras. In a pinhole camera, the best achievable spatial resolution (λ_g) is determined by the diameter of the pinhole (D ~1 cm), the pinhole-to-detector distance (L ~10 m), and the sample-to-detector distance (Z ~1 cm):

λ_g = Z D / (L – Z) ~ Z D / L

Returning to the source strength, the neutron fluence rate at the sample position scales as (D / L)². This means that higher spatial resolution for finite sized objects requires sacrificing neutron intensity and incurring long exposure times. Typical measurements require about 20 minutes for a single two-dimension image of the water content in fuel cell for a spatial resolution of about 10 µm. Such time requirements preclude the use of tomography, which is required to understand the water content in real fuel cell components since the MEA cannot be considered to be planar at the few micrometer level.

We will describe Wolter optics, and the optical system that will be first available in 2022. After the NCNR cold source upgrade is anticipated to yield a time resolution gain of about 10,000 compared to a pinhole camera. We will discuss our expected image acquisition and analysis procedures guided by ray tracing simulations.

Figure 1. Proof of concept Wolter optics image of a fuel cell test section.

[1] D. S. Hussey et al., “Design of a neutron microscope based on Wolter mirrors,” Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip., vol. 987, no. October 2020, p. 164813, 2021.

[2] T. A. Trabold, J. P. Owejan, J. J. Gagliardo, D. L. Jacobson, D. S. Hussey, and M. Arif, “Use of neutron imaging for proton exchange membrane fuel cell (PEMFC) performance analysis and design,” Handb. fuel cells, vol. 6, no. Table 1, 2010.