Ultra-fast optical technology reveals how electrical double layers are formed in liquids

A charged surface in contact with a liquid that is in contact with a liquid that attracts attractively charged ions from the liquid. This creates two differently charged regions. The surface itself and the charging area in the liquid: it is what is called an electric double layer. While crucial for energy storage devices, the rate of formation remains elusive.

A team of researchers has developed a light-based method for observing this ultra-fast process. The results validate previous models and extend their applicability to a wide range of systems, from biofilms to next-generation energy storage devices.

This work has been featured in Journal Science.

In these technical processes, the carrier of liquid must be charged: whether electrolytic capacitors that can be found in almost all electronic devices, or whether charge carriers are separated in electrolysis where water is decomposed into its components, hydrogen, oxygen, or whether liquid carriers are separated: in order for the charge carriers to be separated by the charge carriers that are separated during charging to provide the energy of driving. Such processes are also seen in biological processes in the human body and are used for energy storage.

What all processes have in common is that the so-called “electric double layer” is formed at the poles of the battery, the plates of the capacitors, the electrodes undergoing electrolysis, or the cell membrane, which is what is called “electric double layer.”

One side (the EG of the electrode) is negatively charged, but the corresponding positive charge in the form of mobile ions is on the liquid side. Only a few nanometers thick, these double layers are fast enough to respond quickly to perturbations to understand how quickly energy storage devices can take and release electrical energy, such as applications such as battery charging.

When the number of mobile charge carriers is small, theoretical models and measurements have long predicted these dynamics, and can well explain the movement of ions in this bilayer. However, when the number of charge carriers increases, as in biological systems, and batteries are required, the assumptions of these models collapse. Therefore, it remains a mystery how the electrical double layer is formed precisely.

“Until now, we have not been able to study the exact process involved in the formation of the double layer,” says Misabon, director of polymer research at MPI.

“It is simply impossible to study processes that occur as quickly as ions in electronic circuits as they can be, as the circuit itself can provide limited time resolution. We use ultra-fast optics to avoid that limitation.”

Therefore, the Max Planck Institute for Polymer Research and the University of Vienna team studied the formation of bilayers using optical measurement methods. For this purpose, they add acid to the water and form a positive ion (H3O+).

These ions place themselves preferentially on the surface of the water, which forms the electrical double layer. The surface was heated using strong laser pulses in the infrared range to remove H3O+ from the surface, thereby disturbing the bilayer. By further investigation of the surface with laser pulses after the time delay and detecting reflected light, we were able to quantify the way ions could move away from the surface and reach a new equilibrium.

We combined experimental results with computer simulations. This proved that even at high concentrations of double layer formation, it is mainly caused by electric fields.

New methodologies open new ways to study such processes in the interfaces of a wide range of chemical and biological systems. Furthermore, the team found that even complex interfaces could be explained using relatively simple physical models.

They confirm that existing theoretical frameworks describe the formation of bilayers significantly more accurately.