Ultrafast electrical charging of liquids
How quickly liquids become electrically charged through ionic movement – prediction confirmed
Charged surfaces that come into contact with liquids—such as biological cell walls or battery electrodes—attract oppositely charged ions from the liquid. This creates two distinctly different charged regions: the surface itself and an oppositely charged region within the liquid: the so-called electric double layer. Although crucial for energy storage, the rate of its formation was previously unknown. A team of researchers has now developed a light-based technique to observe this ultrafast process. The results confirm previous models and expand their applicability to various systems, from biological membranes to next-generation energy storage.
Whether in electric car batteries, where charge carriers are separated during charging to provide energy for the drive, in electrolytic capacitors found in almost every electronic device, or in electrolysis, in which water is split into its components hydrogen and oxygen: In all of these technological processes, charge carriers in liquids must move to an interface. Such processes are also 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 battery terminals, at the capacitor plates, at the electrodes during electrolysis, or at the cell membrane. While one side – e.g., the electrode – is negatively charged, the corresponding positive charge is located in the form of mobile ions 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 disturbance is important for understanding how quickly an energy storage device can absorb and release electrical energy, for example, for applications such as battery charging.
For a small number of mobile charge carriers, theoretical models and measurements have long predicted this dynamic and can describe the movement of ions in this double layer well. However, when the number of charge carriers is increased, as is the case in biological systems and is also necessary for batteries, the assumptions of these models break down. Therefore, it remains a mystery how exactly the electrical double layers form.
“Until now, it hasn’t been possible to study the precise processes involved in bilayer formation,” says Mischa Bonn, Director at the MPI for Polymer Research. “It’s simply not possible to use electronic circuits to study processes as fast as the movement of ions, because the circuits themselves can only offer limited temporal resolution. We use ultrafast spectroscopy to circumvent this limitation.”
The team from the Max Planck Institute for Polymer Research and the University of Vienna therefore used an optical measurement method to study the formation of the double layer. To do this, they added acid to water, which caused the formation of positive ions (H 3 O + ). 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 H 3 O + from the surface and disrupting the double layer. By probing the surface with further laser pulses after a certain 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.
Combining their experimental results with computer simulations, they were able to demonstrate that the formation of the double layer, even at high concentrations, is primarily caused by electric fields.
The new methodology, which they have now published in the journal Science, opens up new possibilities for studying such processes at interfaces in a wide variety of chemical and biological systems. Furthermore, the team found that even complex interfacial systems can be described with relatively simple physical models. They confirm that the existing theoretical models describe the formation of the bilayer with remarkable accuracy.
Source: Max-Planck-Institute for Polymer
