Liquid Transport in Superhydrophobic Walled Minichannels for Polymer-Electrolyte Fuel Cell Flow-Fields

Abstract

Liquid water byproduct management continues to be a challenge as polymer-electrolyte fuel cell (PEFC) designs reach higher power densities. At length scales where surface tension is dominant, the wettability of flow-field channels can significantly impact the distribution of water on gas diffusion media. This work explores the impact of a super-hydrophobic coating on liquid build up and pressure drop in PEFC minichannels which are of interest because prior work has shown improved electrochemical performance at hydraulic diameters approaching 0.4 mm. Using ex-situ experimental methods and optical imaging, time series liquid water distributions were captured along with the two-phase pressure drop in a specially designed transparent test channel. The sidewalls of the minichannel were coated to achieve a contact angle of >150⁰. The liquid was injected into the channel through the back of the gas-diffusion layer (GDL) through a small pore to simulate actual PEFC conditions where liquid forms in the catalyst layer and percolates through a microporous layer. Two minichannel geometries with hydraulic diameters of 0.46 mm and 0.30 mm were tested with flow velocities ranging from ~3.0 m/s to ~15 m/s and liquid injection rates ranging from 150 µL/hr to 300 µL/hr. A digital camera collected images of the liquid water distribution at 5 Hz which was synchronized with the pressure drop data collection. Images were post processed to capture the projected area of liquid coverage on the GDL surface. For each test, periodic purge cycles were included to understand the impact these coatings may have on startup and drying scenarios. Overall, data shows that super-hydrophobic coatings lead to mixed results in reducing pressure drop for steady flow conditions compared to untreated channels. Super-hydrophobic coatings did, however, lead to consistently lower channel saturations during purge/dry cycles. A discussion of the normalized data in terms of channel saturation, two-phase pressure amplification, and Weber number is made for each case.