The performance of polymer-electrolyte fuel cells (PEFCs) and other multiphase flow technologies is significantly dependent on liquid-water management. This is particularly true for PEFCs at low operating temperatures and during startup operations due to hindered reactant delivery by water in the cathode. Because of the exothermic oxygen reduction reaction (ORR) at the cathode, a thermal gradient develops during operation in the through-plane direction, with the hottest location in the catalyst layer (CL). At higher temperatures (~80 ˚C), this thermal gradient, in combination with the dependence of vapor pressure on temperature, promotes removal of water in a vapor form. Water vapor within the CL travels through the gas diffusion layer (GDL) to the gas channels (GCs), where it condenses due to the decrease in temperature. This type of flow, which is due to the evaporation and condensation of water, is known as phase-change-induced (PCI) flow.

… PEFCs experience two-phase water flow and, consequently, substantially coupled heat and mass transport. As such, effective water management requires an understanding of the interaction between pressure-driven, capillary-driven, and PCI water transport. Phase change is not a drive potential or force like pressure and capillary forces, however, the term “PCI” has become the common name in literature for heat-driven mass transport of water by evaporation and condensation in a temperature gradient.

… In this study, a novel X-ray CT technique to explore PCI flow within a PEFC is presented. Coupled measurements of temperature, thermal gradients, and thermal conductivity are combined with visualizations of GDL morphology and water distribution. The overall results of this study contribute to the general understanding of evaporation phenomena in porous media pertaining to PEFCs.

—Shum et al.

This animated 3-D rendering, generated by an X-ray-based imaging technique at Berkeley Lab’s Advanced Light Source, shows tiny pockets of water (blue) in a fibrous sample. The X-ray experiments showed how moisture and temperature can affect hydrogen fuel-cell performance. (Credit: Berkeley Lab)

The research team, led by Iryna Zenyuk, a former Berkeley Lab postdoctoral researcher now at Tufts University, included scientists from Berkeley Lab’s Energy Storage and Distributed Resources Division and the Advanced Light Source (ALS), an X-ray source known as a synchrotron.

The ALS lets researchers image in 3-D at high resolution very quickly, allowing them to look inside working fuel cells in real-world conditions. The team created a test bed to mimic the temperature conditions of a working polymer-electrolyte fuel cell that is fed hydrogen and oxygen gases and produces water as a byproduct.

The research aims to find the right balance of humidity and temperature within the cell, and how water moves out of the cell. The research team used micro X-ray computed tomography to record 3-D images of a sample fuel cell measuring about 3 to 4 millimeters in diameter.

The sample cell included thin carbon-fiber layers, known as gas-diffusion layers, which in a working cell sandwich a central polymer-based membrane coated with catalyst layers on both sides. These gas-diffusion layers help to distribute the reactant chemicals and then remove the products from the reactions.

The study used materials that are relevant to commercial fuel cells. Some previous studies have explored how water wicks through and is shed from fuel-cell materials, and the new study added precise temperature controls and measurements to provide new insight on how water and temperature interact in these materials.

Complimentary experiments at the ALS and at Argonne’s Advanced Photon Source, a synchrotron that specializes in a different range of X-ray energies, provided detailed views of the water evaporation, condensation, and distribution in the cell during temperature changes.

The experiments focused on average temperatures ranging from about 95 to 122 degrees Fahrenheit, with temperature variations of 60 to 80 degrees (hotter to colder) within the cell. Measurements were taken over the course of about four hours. The results provided key information to validate water and heat models that detail fuel-cell function.

Water clusters in sample fuel-cell components shrink over time in this sequence of images, produced by a 3-D imaging technique known as micro X-ray computed tomography. The water clusters were contained in a fibrous membrane that was exposed to different temperatures. The mean temperature began at about 104 degrees Fahrenheit and was gradually increased to about 131 degrees Fahrenheit. The top side of the images was the hotter side of the sample, and the bottom of the images was the colder side. (Credit: Berkeley Lab). Click to enlarge.

This test cell included a hot side designed to show how water evaporates at the site of the chemical reactions, and a cooler side to show how water vapor condenses and drives the bulk of the water movement in the cell.

While the thermal conductivity of the carbon-fiber layers decreased slightly as the moisture content declined, the study found that even the slightest degree of saturation produced nearly double the thermal conductivity of a completely dry carbon-fiber layer. Water evaporation within the cell appears to increase significantly at about 120 degrees Fahrenheit, researchers found.

The experiments showed water distribution with millionths-of-a-meter precision, and suggested that water transport is largely driven by two processes: the operation of the fuel cell and the purging of water from the cell.

The study found that larger water clusters evaporate more rapidly than smaller clusters. The study also found that the shape of water clusters in the fuel cell tend to resemble flattened spheres, while voids imaged in the carbon-fiber layers tend to be somewhat football-shaped.

There are also some ongoing studies to use the X-ray-based imaging technique to look inside a full subscale fuel cell one section at a time. A typical working subscale fuel cell measures around 50 square centimeters.

The work was supported by the US Department of Energy’s Fuel Cell Technologies Office and Office of Energy Efficiency and Renewable Energy, and the National Science Foundation.

The Advanced Light Source and the Advanced Photon Source are DOE Office of Science User Facilities that are open to visiting scientists from around the US and world.

Resources

• Andrew D. Shum, Dilworth Y. Parkinson, Xianghui Xiao, Adam Z. Weber, Odne S. Burheim, Iryna V. Zenyuk (2017) “Investigating Phase‐Change‐Induced Flow in Gas Diffusion Layers in Fuel Cells with X‐ray Computed Tomography,” Electrochimica Acta, Volume 256, Pages 279-290 doi: 10.1016/j.electacta.2017.10.012