Nanoscale X-ray tomographic imaging of liquid water in fuel cell electrode materials

Efficient transport of water in a polymer electrolyte membrane fuel cell (PEMFC) is one of the key factors to achieve a high performance with balanced cell humidity and transport of gas reactants. Micro-scale 3D X-ray computed tomography (XCT) has shown great promise in understanding and visualizing liquid water distribution and transport; although, mainly within the macroscopic gas diffusion layer (GDL) substrate. The objective of the present work is to extend XCT based liquid water visualization to the nano-scale pores of the microporous and catalyst layers (MPLs and CLs) in PEMFCs. A custom methodology of water distribution visualization in MPL and CL at 100 % relative humidity (RH) condition is developed using a lab-based nano-resolution XCT (NXCT) system. The design of a custom-built X-ray transparent fixture for capturing water domains along with the imaging procedure, constraints, and challenges are discussed. Utilization of an in-house built fixture coupled with Zernike phase-contrast imaging mode, has allowed for the quantification and visualization of $\sim$ 18 % and $\sim$ 6 % of water volume fraction in wetted MPL and CL, respectively. The methodology discussed here is a step forward to understand the water distribution in nano-porous media and can be further modified to be translated to real working conditions.