jp6b10658_si_001.pdf (1.76 MB)
Investigating Evaporation in Gas Diffusion Layers for Fuel Cells with X‑ray Computed Tomography
journal contribution
posted on 2016-11-21, 00:00 authored by Iryna V. Zenyuk, Adrien Lamibrac, Jens Eller, Dilworth Y. Parkinson, Federica Marone, Felix N. Büchi, Adam Z. WeberUnderstanding evaporation
in porous media and the associated water
distribution for a given saturation is critical for optimizing many
different technologies including polymer–electrolyte fuel cells.
In these devices, heat and mass-transport are coupled due to the two-phase
transport of water and operating temperatures from subzero to 80 °C.
Especially critical is understanding phase change in the mixed wettability,
carbon gas-diffusion layers (GDLs). While previous works have measured
evaporation rates empirically for a given saturation, there remains
a need to explore the mechanisms governing evaporation, which are
tied directly to the internal water distribution. In this article,
liquid-water evaporation rates in GDLs are measured in situ using synchrotron X-ray computed tomography (CT). X-ray CT allows
visualizing the evaporating water-front location and interfacial water/air
surface area, thereby enabling true surface-area based evaporation
rates. It is found that the overall specific evaporation rate is essentially
constant as a function of saturation and that the water/air interfacial
area scales almost linearly with saturation. To isolate transport
and kinetic contributions to the overall evaporation rate, we systematically
varied gas flow rate and composition. A three-dimensional mathematical
model with direct meshes of liquid-water evaporation fronts from the
X-ray CT studies allowed for the determination that the evaporation
is transport limited. The overall results provide insight into evaporation
phenomena in porous media.