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Charged surfaces in contact with liquids – such as biological cell walls or battery electrodes – attract oppositely charged ions from the liquid. This creates two distinct charged regions: the surface itself and a counter-charged region in the liquid: The so-called electrical double layer. While pivotal to energy storage devices, the speed of its formation has remained elusive. A team of researchers has now developed a light-based technique to observe this ultrafast process. The results validate previous models and extend their applicability to diverse systems, from biological membranes to next-generation energy storage devices.
Whether in the batteries of electric cars, where charge carriers are separated during charging to provide energy for driving, in electrolytic capacitors that can be found in almost every electronic device, or in electrolysis, where water is broken down into its components hydrogen and oxygen: in all these technological processes, charge carriers in liquids have to move towards an interface. Such processes can also be found in biological processes in the human body and are used for energy storage.
What all processes have in common is that a so-called “electrical double layer” forms at an interface – at the poles of the battery, at the plates of the capacitor, the electrodes in electrolysis, or at the cell membrane. While one side – e.g. the electrode – is negatively charged, the corresponding positive charge in the form of mobile ions is found on the liquid side. How quickly these double layers, which are only a few nanometers thick, can form or how quickly they react to a perturbation is important for understanding how quickly an energy storage device can take up and release the electrical energy, for example for applications like battery charging.
For a low number of mobile charge carriers, theoretical models and measurements have long predicted these dynamics and can describe the movement of ions in this double layer well. However, if the number of charge carriers is increased, as in biological systems and is necessary for batteries, the assumptions of these models break down. It has therefore remained a mystery how exactly the electrical double layers form.
“Until now, it has not been possible to study the exact processes involved in the formation of the double layer,” says Mischa Bonn, Director at the MPI for Polymer Research. ”It is simply not possible to study processes that take place as quickly as the movement of ions with electronic circuits, because the circuits themselves can only provide a limited temporal resolution. We use ultrafast optics to circumvent that limitation.”
Therefore, the team at the Max Planck Institute for Polymer Research and the University of Vienna used an optical measurement method to study the formation of the double layer. For this purpose, they added acid to water, which causes positive ions (H3O+) to form. These ions preferentially arrange themselves at the water surface, where they form an electrical double layer. A strong laser pulse in the infrared range was used to heat the surface, removing H3O+ from the surface, thereby perturbing the double layer. By investigating the surface with further laser pulses after a time delay and detecting the reflected light, they were able to quantify how the ions moved away from the surface to reach a new equilibrium.
They combined their experimental results with computer simulations. This enabled them to prove that the formation of the double layer is primarily caused by electric fields, even at high concentrations.
The new methodology, which they have now published in the journal Science, opens up new ways to study such processes at interfaces in a wide range of chemical and biological systems. In addition, the team found that even complex interfaces can be described using relatively simple physical models. They confirm that the existing theoretical frameworks describe the double layer formation remarkably accurately.
Prof. Dr. Mischa Bonn
< bonn@mpip-mainz.mpg.de>
Greco, A.; Imoto, S.; Backus, E.; Nagata, Y.; Hunger, J.; Bonn, M.
Ultrafast aqueous electric double layer dynamics
Science Journal
DOI: https://www.science.org/doi/10.1126/science.adu5781
https://www.mpip-mainz.mpg.de/1014485/PM2025-08
A strong laser pulse heats up a surface and disturbs the electrical double layer.
MPI-P
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A strong laser pulse heats up a surface and disturbs the electrical double layer.
MPI-P
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