Faster charging thanks to invisible mini-vortices
Publication by TU researchers in ‘Proceedings of the National Academy of Sciences of the USA’
2025/12/09
Tiny cavities in energy storage devices form small vortices that help with charging, according to a research team led by TU Darmstadt. This previously unknown phenomenon could advance the development of faster storage devices.
Solar and wind are the energy sources of the future, but they are subject to significant natural fluctuations. Storage solutions are therefore particularly important for a successful energy transition. Rechargeable batteries achieve very high energy densities by storing energy chemically. However, this high energy density comes at the price of long charging times and a dependence on precious raw materials such as cobalt.
In contrast to rechargeable batteries, so-called supercapacitors store energy in electric double layers: a voltage is applied between two electrodes. They are immersed in a liquid in which tiny charged particles, ions, float. The positive and negative ions move in opposite directions and accumulate in charged, nanometre-thick layers, the electric double layers, on the surfaces of the electrodes. In order to provide as much surface area as possible for the accumulation of ions, supercapacitors use porous electrodes that have many tiny pores, like a sponge.
The diameter of the pores in such electrodes is only a few nanometres. Although the energy densities are (still) slightly lower than in batteries, no precious metals are required. And because there are no chemical reactions that determine the charging time in batteries, supercapacitors can be charged in just a few seconds or minutes. The charging time is limited only by the transport of ions in the pores of the electrodes.
Surprising discovery
This is where the work of an international research team led by TU Darmstadt comes in, the results of which have now been published in the renowned journal ‘Proceedings of the National Academy of Sciences of the USA’. Using complex computer simulations, the team around Dr Aaron Ratschow and Alexander Wagner from the Institute for Nano- and Microfluidics (Faculty of Mechanical Engineering) under the direction of Professor Steffen Hardt investigated the charging process of a single pore and made a surprising discovery. Until now, it was assumed that ions in pores were transported by diffusion, i.e. random molecular motion, and by electromigration, the movement of charged particles due to electric fields. The team has now discovered that convection, i.e. a flow that carries particles along with it, also plays a decisive role in charge transport in pores.
Convection can be observed, for example, when cooking, where hot air rises above a pot and carries water vapour upwards with the flow. Something similar happens during the charging of tiny pores. The charged layer of ions first forms at the pore entrance and then grows into the pore. During this process, electric forces act on the liquid, causing it to flow into the pore along the pore wall. Because the end of the pore is closed, a countercurrent flows out of the pore at its centre. This recirculating flow now carries ions with it, thus accelerating the charging process. The analysis shows a clear influence: if convection is neglected, errors of up to 90 per cent occur in the prediction of charging times.
Important approach for faster charging
In addition to the sometimes very time-consuming computer simulations, the researchers also present a mathematical model that predicts the flow and ion transport by convection very accurately – without any lengthy simulations. The model provides insight into the underlying physical mechanisms and expands our understanding of charge transport in pores.
But this is only the beginning. While the present work focuses on a single pore, a real porous electrode consists of a multitude of pores that can interact with each other. The results lay the foundation for research into convection during pore charging and show how faster charging processes in supercapacitors can be achieved through the targeted selection of pore geometry, materials and operating voltage.
The publication
Ratschow, Aaron et al.: “Convection Can Enhance the Capacitive Charging of Porous Electrodes”, in: „Proceedings of the National Academy of Sciences of the USA“, (Issue: Vol. 122 Iss.50), DOI: 10.1073/pnas.2504322122.
