Scientists explore energy storage capability in water and metal surfaces

A research team from Ruhr-Universitaet-Bochum (RUB) have investigated energy storage capability through observation of water meeting metal surfaces.

What happens when water meets metal surfaces?

Interfaces between metals and water are where crucial processes of energy technologies such as water splitting occur. However, minimal information is known regarding their structure and changes during such processes.

The scientific description of such interfaces has been based on the model known in the scientific community as the ‘electrochemical double layer’, for the past 100 years. This model asserts that charge carriers in an aqueous solution are gradually arranged in the boundary region to the metal, to compensate for excess electrical charges on the metal side.

Furthermore, during this process the opposing charges are separated by water molecules. Similar to a standard plate capacitor, this nanoscopic charge separation in the interface allows energy to be stored and released later.

Additionally, processes where the molecular structure of the electrochemical double layer changes are relevant to many green technologies, such as supercapacitors and fuel cells.

What was considered when investigating energy storage capability?

The research team considered nanoparticles, which are thousands of times smaller than the diameter of a strand of human hair, for the technical applications. Due to their advantageous ratio of process-relevant surface area to volume, they offer particularly good conditions for investigating energy storage capability.

“In order to track down the capacitance and the rearrangement processes in the electrochemical double layer 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,” commented Kristina Tschulik.

Thus, it would not be possible to differentiate the impacts related to the electrochemical double layer from effects caused by the interaction of neighbouring nanoparticles, since billions of them are present on a conventional electrode.

What was the role of nanoparticle dispersions?

Dr Mahnaz Azimzadeh Sani, whose research was funded by the German Academic Exchange Service (DAAD), utilised what is known as ‘colloidal nanoparticle dispersions.’ During this process, the nanoparticles are separated from each other and are finely dispersed in an aqueous solution, randomly striking a biased microelectrode on occasion.

With the help of computer-aided molecular dynamics simulations, it was possible to interpret similarities and differences in voltage-dependent measured capacitive currents of different types of nanoparticle dispersions.

The research team measured unexpectedly high capacitances, which were attributed to the increased accumulation of dissolved ions in regions between the compact water layer bound to platinum (and less trongl to gold), and an adjacent water layer of a different arrangement.

“Furthermore, water molecules are detached from the metal surface when more negative voltage is applied” explained Dr Julia Linnemann, team leader at Tschulik’s chair.

For future developments, the RUB scientists want to discover whether and why the double layer structure is different on large electrodes consisting of many nanoparticles, to make the findings utilisable for commercial applications.

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