Although interfaces between metals and water are the local areas where crucial processes of energy technologies like water splitting take place, little is known so far about their structure and changes during such processes. For more than 100 years, the scientific description of such interfaces has been dominated by the model of the so-called electrochemical double layer. It states that charge carriers of an aqueous solution are increasingly arranged in the boundary region to the metal in order to compensate for excess electrical charges on the metal side. In the process, the opposing charges are separated by water molecules. Similar to a technical plate capacitor, this nanoscopic charge separation in the interface allows energy to be stored and retrieved later. Processes in which the molecular structure of the electrochemical double layer changes are relevant to many green technologies, such as supercapacitors and fuel cells.
A thousand times smaller than the diameter of a hair
Nanoparticles that are a thousand times smaller than the diameter of a human hair are increasingly being studied for such technical applications. Because of their advantageous ratio of process-relevant surface area to volume, they offer particularly good conditions for this. “In order to get to the bottom of the capacitance and rearrangement processes in the electrochemical bilayer on platinum and gold nanoparticles, it was crucial to develop a method with which precise discharge currents can be measured on individual nanoparticles in solution,” reports Kristina Tschulik. Otherwise, it would not be possible to distinguish effects related to the double layer from effects caused by the interaction of nanoparticles, such as billions of them present on a conventional electrode.
The Iranian scientist Dr. Mahnaz Azimzadeh Sani, who was funded by the German Academic Exchange Service, used so-called colloidal nanoparticle dispersions. There, nanoparticles are separated from each other and finely distributed in aqueous solution and randomly strike a microelectrode under voltage every now and then. With the help of computer-aided molecular dynamics simulations, on which researchers from the RUB and the Université Paris-Saclay and Sorbonne Université in Paris worked, it was possible to interpret similarities and differences in voltage-dependent measured capacitive currents of different types of nanoparticle dispersions.
The unexpectedly high capacitances are attributed to dissolved charged particles that increasingly accumulate in interstices of a compact water layer bound to platinum (and more weakly to gold) and an adjacent water layer of a different arrangement.In the future, the RUB scientists want to find out whether and why the double-layer structure is different at large electrodes consisting of many nanoparticles, in order to make the findings technically useful.
“Furthermore, water molecules are detached from the metal surface when more negative voltage is applied.”
- Dr. Julia Linnemann, team leader
Translated with www.DeepL.com/Translator (free version)
