Exploring Phase-Change-Induced Flow in Fuel Cells through X-Ray Computed Tomography

Abstract

Effective liquid-water management is critical for optimal polymer-electrolyte fuel-cell (PEFC) operation. At lower operating temperatures and during startup, there is a need to remove liquid water from the cathode electrode to ensure reactant delivery1. At higher operating temperatures, large thermal gradients from catalyst layer to flow-field channel in the system promote water removal in the vapor phase due to the exponential dependence of water vapor pressure on temperature (i.e., phase-change-induced (PCI) flow). To achieve maximum water permeation and consequently higher current densities, it is necessary to understand the interplay between PCI flow, capillary-driven liquid water transport, and evaporation/condensation in PEFC porous diffusion media. PCI flow causes the liquid water to evaporate in hotter locations (electrode) and condense in cooler locations (PEFC’s land). Simultaneously, the heat redistribution occurs because heat is consumed during evaporation and released during condensation. Currently there is little understanding of how water is redistributed under PCI flow. Although water transport in diffusion media has been investigated with modeling and experiments,2-3 evaporation and PCI flow are poorly understood. The basic fundamental knowledge is lacking primarily due to a challenge of experimental measurements and visualization of the evaporating water front within these porous materials. In this talk, we will examine the water distribution and quantify local evaporation/condensation within diffusion media subjected to thermal gradients using synchrotron-based X-ray micro computed tomography (XCT). The novel experimental in-situ setup is presented that allows for precise temperature control and simultaneous water injection. Conditions studied include varying liquid pressure, thermal gradients, and GDL materials. Acknowledgments We thank Prof. Jeff Gostick for helpful insights and discussions. We thank Dr. Felix Büchi, Dr. Jens Eller and Dr. Adrien Lamibrac for experimental concept of evaporation experiment. This work was supported by EERE, Fuel-Cell Technologies Office, of the U.S. DOE under contract number DE-AC02-05CH11231. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.