Water distribution in the fuel cell made visible in 4D.
For the first time, teams from Helmholtz-Zentrum Berlin (HZB) and University College London (UCL) have visualized the water distribution in a fuel cell three-dimensionally and in real time.
For this purpose, they evaluated measurement data that were still obtained at the neutron source BER II at HZB. The analysis opens up new possibilities for more efficient and thus more cost-effective fuel cells.
Ralf Ziesche from HZB’s imaging group, said:
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In a fuel cell, hydrogen and oxygen are combined to form water.
“This generates electrical energy. Probably the most important component within the fuel cell is a membrane.”
At about 20 micrometers, this membrane is about half as thick as a human hair, it is connected to various functional layers to form a separation area about 600 micrometers wide within the fuel cell.
“The membrane composite snatches the electrons from the hydrogen atoms. Only the hydrogen nuclei, i.e. 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. On the other side of the partition wall, air is let in. 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 circuit. The result is pure water.
Channels are crucial
“Some of the water is discharged. Another part must remain in the fuel cell, because the membrane must not dry out,” explains Ralf Ziesche.
“But if there is too much water in it, the protons can no longer penetrate the membrane. Dead areas are created at these points, and the reaction can no longer take place there. The efficiency of the entire fuel cell decreases.”
In order for hydrogen, air and water to flow in and out, the tiniest channels are milled into metal plates on both sides of the membrane. These channels can be optimized to increase the efficiency of the fuel cells.
Neutrons instead of X-ray light
For this, it is advantageous to have as accurate a picture as possible of the water distribution within the canals. This was the aim of a collaboration between the research group from the Electrochemical Innovation Lab (EIL) of University College London (UCL) and HZB.
Nikolay Kardjilov, also from hzB’s imaging group.
In principle, we subjected the fuel cell to computed tomography, as it is also used in medicine.
But while X-ray light is used for medical analyses, Nikolay Kardjilov and his team preferred to use neutron radiation. “X-rays provide far too low image contrast between hydrogen and water on one side and the metal structure on the other. Neutrons, on the other hand, are ideal here.”
3D imaging by rotation of the fuel cell
However, they faced some challenges. In order to obtain a three-dimensional image, the radiation source must circle the object to be imaged. In medicine, this is quite easy to solve. The radiation source and scanner revolve around the patient resting on a table.
“But our radiation source was the Berlin neutron source BER II, where we had set up our CONRAD tomography station. And we can’t just turn this neutron source around the fuel cell,” says Nikolay Kardjilov.
However, his team has managed to rotate the fuel cell in the neutron beam, including supply lines for hydrogen and air, discharges for water and electrical cables.
“Until now, neutron imaging had only succeeded in producing two-dimensional images from inside the fuel cell. Now, for the very first time, we have also made the water distribution visible three-dimensionally and in real time, i.e. in a total of 4D,” says the physicist happily.
Since the end of 2019, the neutron source BER II has been shut down as planned. The work is now being continued within the framework of the joint research group “NI-Matters” between HZB, the Institut Laue-Langevin and the University of Grenoble (France).
Water distribution in the fuel cell made visible in 4D, June 2, 2022
