Water distribution in the fuel cell made visible in 4D

The sequence shows how the water volume in the functional layers (green) develops over time during the start-up of the fuel cell. The water increases until a state of equilibrium between water accumulation and degradation is reached.

The sequence shows how the water volume in the functional layers (green) develops over time during the start-up of the fuel cell. The water increases until a state of equilibrium between water accumulation and degradation is reached. © HZB

The fuel cell (grey) rotates around its longitudinal axis during the tomography. The data shows how the water volume increases in the functional layers (green) and channels during cell operation. Less water is formed in the anode channels (red) than in the cathode (blue).

The fuel cell (grey) rotates around its longitudinal axis during the tomography. The data shows how the water volume increases in the functional layers (green) and channels during cell operation. Less water is formed in the anode channels (red) than in the cathode (blue). © HZB

The video shows the water build-up in the cathode channels during the first 600 s of the fuel cell start-up. The water starts to nucleate on the channel edges and corners. © HZB

10.03 s

Teams from Helmholtz-Zentrum Berlin (HZB) and University College London (UCL) have visualised the water distribution in a fuel cell in three dimensions and in real time for the first time by evaluating neutron data from the Berlin Experimental Reactor shut down in 2019. The analysis opens new possibilities for more efficient and thus more cost-effective fuel cells.

"In a fuel cell, hydrogen and oxygen are combined to form water. This produces electrical energy," explains Ralf Ziesche from the imaging group at HZB. "Probably the most important component inside the fuel cell is the membrane." It is only about 20 micrometres thick (half as wide as a human hair) and connected with various functional layers to form a separation area about 600 micrometres wide inside the fuel cell.

"The membrane composite snatches the electrons from the hydrogen atoms. Only the hydrogen nuclei - the protons - can pass through the membrane." The electrons, on the other hand, flow off via an electrical connection and are used as an electric current. Air is let in on the other side of the separating wall. The oxygen it contains reacts with the protons that come through the membrane and the electrons that flow back from the other side of the electric circuit. Pure water is produced.

The function of channels

"Some of the water is discharged. Another part must remain in the fuel cell, because the membrane must not dry out," Ralf Ziesche explains. "But if there is too much water in it, the protons can no longer penetrate the membrane. Dead areas develop at these points, and the reaction can no longer take place there. The efficiency of the entire fuel cell drops." To allow hydrogen, air and water to flow in and out, tiny channels are milled into metal plates on both sides of the membrane. These channels can be used to optimise fuel cells and increase efficiency. Hereby, the channel design is the key for a balanced cell wetting and optimal efficiency.

Neutrons for the detection of water

To do this, it is advantageous to have as accurate a picture as possible of the water distribution within the channels. This was the goal of a collaboration between the research group from the Electrochemical Innovation Lab (EIL) at University College London (UCL) and HZB. "In principle, we subjected the fuel cell to computed tomography, as it is used in medicine," explains Nikolay Kardjilov from the imaging group at HZB. But while X-rays are used for medical analyses, Nikolay Kardjilov and his team preferred to use neutron radiation. "Because X-rays provide far too low an image contrast between hydrogen and water on one side and the metal structure on the other. Neutrons, on the other hand, are ideal here."

Rotating fuel cell

This was quite tricky. Because in order to get a three-dimensional image, the radiation source has to go around the object to be imaged. In medicine, this is quite easy to solve. There, the radiation source and scanner rotate around the patient, who is resting on a table. "But our radiation source was the Berlin Experimental Reactor BER II, where we had set up our imaging station CONRAD. And we can't simply rotate it around our fuel cell sample," says Nikolay Kardjilov. But with an engineering trick, his team managed to move the fuel cell, including the supply lines for hydrogen and air, the discharge line for water and the electric cables, into the neutron beam. "Until now, neutron imaging has only been able to produce two-dimensional images from inside the fuel cell. Now, for the very first time, we have also made the water distribution visible in three dimensions and in real time," the physicist is pleased to report. The BER II is shut down since the end of 2019. But the work will be continued as part of the joint research group "NI-Matters" between HZB, the Institut Laue-Langevin (ILL, France) and the University of Grenoble (France).

Kai Dürfeld

You might also be interested in

  • Stability of perovskite solar cells reaches next milestone
    Science Highlight
    27.01.2023
    Stability of perovskite solar cells reaches next milestone
    Perovskite semiconductors promise highly efficient and low-cost solar cells. However, the semi-organic material is very sensitive to temperature differences, which can quickly lead to fatigue damage in normal outdoor use. Adding a dipolar polymer compound to the precursor perovskite solution helps to counteract this. This has now been shown in a study published in the journal Science by an international team led by Antonio Abate, HZB. The solar cells produced in this way achieve efficiencies of well above 24 %, which hardly drop under rapid temperature fluctuations between -60 and +80 Celsius over one hundred cycles. That corresponds to about one year of outdoor use.
  • Scientists Develop New Technique to Image Fluctuations in Materials
    Science Highlight
    18.01.2023
    Scientists Develop New Technique to Image Fluctuations in Materials
    A team of scientists, led by researchers from the Max Born Institute in Berlin and Helmholtz-Zentrum Berlin in Germany and from Brookhaven National Laboratory and the Massachusetts Institute of Technology in the United States has developed a revolutionary new method for capturing high-resolution images of fluctuations in materials at the nanoscale using powerful X-ray sources. The technique, which they call Coherent Correlation Imaging (CCI), allows for the creation of sharp, detailed movies without damaging the sample by excessive radiation. By using an algorithm to detect patterns in underexposed images, CCI opens paths to previously inaccessible information. The team demonstrated CCI on samples made of thin magnetic layers, and their results have been published in Nature.
  • Recommended reading: Bunsen magazine with focus on molecular water research
    News
    13.01.2023
    Recommended reading: Bunsen magazine with focus on molecular water research
    Water not only has some well-known anomalies, but is still full of surprises. The first issue 2023 of the Bunsen Magazine is dedicated to molecular water research, from the ocean to processes in electrolysis. The issue presents contributions from researchers cooperating within the framework of a European research initiative in the "Centre for Molecular Water Science" (CMWS). A team at HZB presents results from the synchrotron spectroscopy of water. Modern X-ray sources can be used to study molecular and electronic processes in water in detail.