Nov 14 2017
Similar to a well-maintained greenhouse garden, a specialized type of hydrogen fuel cell – which shows potential as a clean, renewable next-generation power source for vehicles and other applications – requires precise moisture and temperature controls to work well. If the internal conditions are too wet or too dry, the fuel cell will not function properly.
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)
But viewing the inside of a working fuel cell at a minute scale applicable to a fuel cell’s chemistry and physics is challenging, so researchers used X-ray-based imaging methods at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Argonne National Laboratory to investigate the inner workings of fuel-cell components exposed to a range of moisture and temperature conditions.
The research team, headed 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 called as a synchrotron.
The ALS allows researchers image in 3D at high resolution very rapidly, allowing them to look within working fuel cells in real-world scenarios. The team built a test bed to imitate the temperature conditions of a working polymer-electrolyte fuel cell that is supplied oxygen and hydrogen gases and creates water as a byproduct.
“The water management and temperature are critical,” said Adam Weber, a staff scientist in the Energy Technologies Area at Berkeley Lab and deputy director for a multi-lab fuel cell research effort, the Fuel Cell Consortium for Performance and Durability (FC-PAD).
The research paper has been published online in the journal Electrochimica Acta.
The research aims to discover the appropriate balance of temperature and humidity within the cell, and how water moves out of the cell.
Regulating how and where water vapor condenses in a cell, for instance, is important so that it does not obstruct incoming gases that enable chemical reactions.
“Water, if you don’t remove it, can cover the catalyst and prevent oxygen from reaching the reaction sites,” Weber said. But there has to be certain amount of humidity to ensure that the main membrane in the cell can efficiently conduct ions.
The research team applied an X-ray method referred to as micro X-ray computed tomography to record 3D images of a sample fuel cell measuring around 3 to 4 mm in diameter.
The ALS lets us image in 3D at high resolution very quickly, allowing us to look inside working fuel cells in real-world conditions.
Dula Parkinson, a research scientist at the ALS who took part in the study.
The sample cell included thin carbon-fiber layers, called as gas-diffusion layers, which in a working cell sandwich a fundamental polymer-based membrane coated with catalyst layers on both sides. These gas-diffusion layers help to dispense the reactant chemicals and then remove the products from the reactions.
Weber said that the research used materials that are applicable to commercial fuel cells. Some earlier studies have investigated how water wicks through and is eliminated from fuel-cell materials, and the new study added precise temperature controls and measurements to shed light on how temperature and water interact in these materials.
Complimentary experiments at the ALS and at Argonne’s Advanced Photon Source, a synchrotron that concentrates in a different range of X-ray energies, provided comprehensive views of the water condensation, evaporation, and distribution in the cell during temperature variations.
It took the ALS to explore the physics of this, so we can compare this to theoretical models and eventually optimize the water management process and thus the cell performance,
Adam Weber, staff scientist in the Energy Technologies Area at Berkeley Lab and deputy director for a multi-lab fuel cell research effort (FC-PAD).
The experiments concentrated on average temperatures ranging from around 95 to 122 °F, with temperature differences of 60 to 80 degrees (hotter to colder) within the cell. Measurements were taken during a four-hour period. The results provided vital information to confirm water and heat models that detail fuel-cell working.
This test cell included a hot side intended to reveal how water evaporates at the site of the chemical reactions, and a cooler side to reveal how water vapor condenses and stimulates the bulk of the water movement in the cell.
While the thermal conductivity of the carbon-fiber layers – their ability to convey heat energy – decreased marginally as the moisture content decreased, the study discovered that even the smallest degree of saturation produced approximately double the thermal conductivity of a totally dry carbon-fiber layer. Water evaporation within the cell appears to intensely increase at around 120 °F, researchers found.
The experiments revealed water distribution with millionths-of-a-meter precision, and indicated that water transport is mainly driven by two processes: the working of the fuel cell and the removal of water from the cell.
The study discovered that larger water clusters evaporate more quickly than smaller clusters. It also found that the shape of water clusters in the fuel cell tend to look like flattened spheres, while voids imaged in the carbon-fiber layers tend to be slightly football-shaped.
There are also some continuing studies, Weber said, to apply the X-ray-based imaging method to observe inside a full subscale fuel cell one section at a time.
“There are ways to stitch together the imaging so that you get a much larger field of view,” he said. This process is being assessed as a way to discover the origin of failure sites in cells via imaging before and after testing. A regular working subscale fuel cell measures around 50 sq. cm, he added.
Other researchers who contributed to this study were from Tufts University, Argonne National Laboratory, and the Norwegian University of Science and Technology. The work was supported by the U.S. 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 U.S. and world.