The complex nature
of liquid water saturation of polymer
electrolyte
fuel cell (PEFC) catalyst layers (CLs) greatly affects the device
performance. To investigate this problem, we present a method to quantify
the presence of liquid water in a PEFC CL using small-angle X-ray
scattering (SAXS). This method leverages the differences in electron
densities between the solid catalyst matrix and the liquid water filled
pores of the CL under both dry and wet conditions. This approach is
validated using ex situ wetting experiments, which aid the study of
the transient saturation of a CL in a flow cell configuration in situ.
The azimuthally integrated scattering data are fitted using 3D morphology
models of the CL under dry conditions. Different wetting scenarios
are realized in silico, and the corresponding SAXS data are numerically
simulated by a direct 3D Fourier transformation. The simulated SAXS
profiles of the different wetting scenarios are used to interpret
the measured SAXS data which allows the derivation of the most probable
wetting mechanism within a flow cell electrode.