Gaining molecular insight into the electrical double layer

The electrical double layer (EDL) – the structure that appears on an object’s surface when that object is exposed to a fluid – plays an important role in interfacial electrochemical processes such as electrocatalysis, energy storage and corrosion. To understand and control such processes, scientists need to learn more about the molecular structure of solid-liquid interfaces.

Aiming to obtain a better grasp of solid-liquid interfaces at the molecular level, researchers supported in part by the EU-funded HMST-PC, AMPERE and MITICAT projects studied platinum and gold nanoparticle-water interfaces using nano-impact electrochemistry. Their findings were published in the journal ‘Angewandte Chemie International Edition’.Nano-impact electrochemistry is a powerful new tool that enables scientists to obtain physico-chemical information about structural effects on the EDL capacitance of nanomaterials, without any artefacts resulting from film porosity or additives. EDL capacitance occurs when an electrode and a liquid solution come into contact, causing two layers of electric charges with opposite polarities to form and enabling electricity to be stored there. This new tool has opened the way to new possibilities regarding the characterisation of colloidal nanoparticles.

Thanks to their high surface-to-volume ratio, nanoparticles are advantageous for many applications. However, the explicit characterisation of EDL capacitance is complicated for nanoparticles. As reported in the study, nanoparticles “must be processed to complete electrodes for conventional electrochemical measurements, often including additives and resulting in ensemble effects and uncertainties about the electrochemical active surface area.” Nano-impact electrochemistry solves this issue.

“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,” reports study senior author Prof. Dr Kristina Tschulik of MITICAT project host Ruhr University Bochum, Germany, in a news release posted on the ‘EurekAlert!’ website. For their investigation, the research team used colloidal nanoparticle dispersions, in which individual particles finely dispersed in aqueous solution randomly collided with an ultramicroelectrode. Using computer-aided molecular dynamics simulations, the team was able to find similarities and differences in the voltage-dependent measured capacitive currents of different types of nanoparticle dispersions.

Calculations indicated that the strong interaction of metal with water molecules leads to water chemisorption and an unexpectedly high accumulation of ions, resulting in a higher interface charge storage ability. “The charge storage ability of the double-layer is found to increase by about one order of magnitude with respect to the predictions based on traditional mean-field models,” the authors write in the study. “The large capacitance measured for platinum and gold surfaces is proposed to arise from strong interactions between the metal surface and the water adlayer. These promote water chemisorption and ion accumulation at the interface. The lower capacitance value measured for gold is ascribed to weaker binding of the water adlayer to gold vs. platinum surfaces.”

The insights gained with support from the HMST-PC (Synthesis of Hybrid Metal-Semiconductor Tetrapod Photocatalysts for Improved Water Splitting), AMPERE (Accounting for Metallicity, Polarization of the Electrolyte, and Redox reactions in computational Electrochemistry), and MITICAT (Microfluidic Tuning of Individual Nanoparticles to Understand and Improve Electrocatalysis) projects could launch the active tuning of solid-solvent and solvent-solvent interactions formed by the water adlayer. This could lead to better performing and more sustainable energy conversion and storage technologies.

For more information, please see:

HMST-PC project

AMPERE project website

MITICAT project