A better understanding of the lightning-bolt-like double layer of charge that spontaneously forms when a solid is placed in a liquid, has the potential to improve electrochemical processing.
Researchers from CSIRO Minerals and the Ian Wark Research Institute (the Wark) at the University of South Australia (UniSA), are learning more about the double-layer structure.
Double layer critical in electrochemical processes
Because it controls charge transfer across an interface and therefore the electrochemical processes that occur there, the double layer is critical. In aqueous solutions it can create electrical fields as strong as 109 volts per metre – fields that are so strong that they drastically alter materials’ basic physical properties.
The structure of double layers in aqueous solutions has been studied since the 1920s and sophisticated models exist to describe its behaviour. However, its structure in ionic liquids is unknown and researchers think it could be very different – and this is what they want to find out.
Ionic liquids
“Ionic liquids have the potential to be solvents and electrolytes with properties that can be tuned to suit the application.”
Mr Mike Horne, CSIRO Minerals researcher
Ionic liquids are electrolytes that are liquid at room temperature. A form of molten salts, their useful properties include:
'They are a new class of substances that have yet to be explored,' says CSIRO and UniSA-sponsored Research Associate Vera Lockett, who works at the Wark in a team led by Professor John Ralston.
CSIRO's Mr Mike Horne works with Dr Lockett, Dr Rossen Sedev and Professor John Ralston. He says there are almost an infinite number of ionic liquids (1018) and all perform differently.
'So ionic liquids have the potential to be solvents and electrolytes with properties that can be tuned to suit the application.'
Electrical double layers in ionic liquids
The Wark team is investigating electrical double layers to develop new models and ionic liquid theories. Many chemical processes occur at the interface, and because electrical double-layers’ structure influences kinetics and the mechanisms of these processes, it is important to understand this structure.
A better understanding of electrical double layers in ionic liquids could have ramifications for fuel cells, energy storage, electrosynthesis and, perhaps most importantly, in the minerals processing world, for metal electrodeposition. Ionic liquids could also be used as electrolytes in lithium batteries, replacing the more reactive and difficult solvents now used.
The research sees Dr Lockett making careful measurements of the interface’s differential capacitance. 'Data obtained provides information on the double-layer structure and the adsorption at the interface, as well as how these parameters are changed with electrical potential,' she says.
Metal electrodeposition
Dr Lockett says that metal electrodeposition depends on the interface structure and which ions are adsorbed at the electrode.
'The nucleation process, the structure of the metal deposit and the deposition speed are all a function of the double-layer structure,' she says. 'So a more fundamental understanding could lead to the effective control of all these processes by consciously varying parameters such as ionic liquids’ composition, specific additives, the nature of the electrodes and temperature.'
The beauty of ionic liquids is their low melting point. For example, conventional aluminium electrodeposition sees alumina dissolved in a molten cryolite bath at 1000 °C and an electric current applied to reduce aluminium ions to the metal state. The high temperature is needed to keep the cryolite molten. By contrast, the same process might possibly be achieved at temperatures below 100 °C using an ionic liquid.
Collaboration key to research success
Mr Horne says underlying a better understanding of double layers in ionic liquids is collaboration. 'CSIRO would not be able to do this work without collaboration. And our collaboration with the Wark has been particularly successful, not just because of the direct outcome, but also because of the links established, which are already being used to benefit other work.'
Dr Lockett says the Wark’s great experience at researching the interfacial phenomena works well with CSIRO’s minerals processing knowledge. 'For CSIRO, a better knowledge of the double-layer structure is very useful. But from the Wark side, CSIRO can advise on what is practical or not to investigate.'
So far the work is still in the early stages. However, Mr Horne says a fundamental understanding of processes in ionic liquids – a new and growing research field – could help create improved minerals processing techniques.
Read more articles in Process magazine (Oct 08).
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