It is well known that X-rays have wavelengths comparable with interatomic distances and can be utilized for atomic-scale structural determination. In addition, X-rays can penetrate through opaque objects and show the internal structure without destroying the object. It is for these reasons that X-rays are widely used for atomic-scale structural analysis of various kinds of crystals including small molecules and large protein molecules, and in some cases providing accurate electron density information. In addition, small-angle and ultra small-angle X-ray scattering (SAXS, USAXS) is an exceptionally useful technique for evaluating the size, shape, and morphology of higher-order structures in the range of a few nanometers to sub-micrometers. Recently, the resolution of X-ray microscopes has been pushed down to the sub-micrometer level. These devices are also capable of recording computed tomography (CT) data. This is a powerful technique for visualizing the internal structure of various specimens in three dimensions (3D) at the micrometer scale. By utilizing these X-ray techniques, we have the opportunity to investigate a very extended range of multi-scale structures from the atomic-scale (10⁻¹⁰ m) to macroscopic-scale (10⁻² m), continuously for various kinds of materials without destroying the specimens.
In this paper, we will investigate multi-scale structures of a polymer electrolyte membrane (PEM) fuel cell, which generates electricity with catalyst nanoparticles and its support, a proton exchange membrane, a micro-porous layer, a gas diffusion layer, and channels on separator. The sizes of those constituents are spread over nanometers to millimeters and providing the possibility to investigate using state-of-the-art X-ray analysis techniques