Bubbles on electrodes? Not a problem

Australian researchers say they have shown that the formation of bubbles on electrodes – usually thought to be a hindrance – can, in fact, be beneficial.

When deliberately added, bubbles or oil droplets can accelerate processes such as the removal of pollutants from contaminated water and the production of chlorine, they write in a paper in Nature Communications.

Simone Ciampi from Curtin University, which led the project, says many industrial processes are electrochemical, meaning the desired chemical reaction to create an end product is assisted by the flow of electrical currents.

“Electrodes assist chemists to achieve required electrochemical reactions, such as in the purification of alumina, and the technology used to produce chlorine for swimming pools,” he says.

“Often, over the course of their use, small bubbles of gas begin to form on these electrodes, blocking parts of their surface. These bubbles prevent fresh solution from reaching the electrodes and therefore slow down the necessary reactions.

“It was generally thought these bubbles essentially stopped the electrode from working properly and the appearance of the bubbles was a bad thing. However, our new research suggests otherwise.”

Using fluorescence microscopy, electrochemistry and multi-scale modelling, the team showed that in the vicinity of bubbles that stick to an electrode surface, valuable chemical reactions occur under conditions where normally such reactions would be considered impossible.

Curtin’s Yan Vogel says these “impossible” reactions occurring in the corona of bubbles piqued the team’s interest and warranted further attention.

“We revealed for the first time that the surrounding surface of an electrode bubble accumulates hydroxide anions, to surprisingly large concentrations,” he says.

“This population of negatively charged ions surrounding bubbles is unbalanced by ions of the opposite sign, which was quite unexpected. Usually charged chemical species in solution are generally balanced, so this finding showed us more about the chemical reactivity of bubbles.

“Basically we’ve learned that surface bubbles can actually speed up electrochemical reactions where small molecules are joined to form large networks of molecules in a polymer, like in camera films or display devices like glucose sensors for blood sugar monitoring.”

The project was a collaboration with Australian National University, University of New South Wales, and University of Western Australia.