When an electrode makes contact with an electrolyte, an electrochemical double layer forms. All interactions between the charged surface of the electrode and the charged ions in the salt solution take place at this interface. Alter the electric voltage, and the structure within the double layer changes accordingly. The Gouy-Chapman theory describes this structure.

Or rather, that is its intention. Marc Koper, Professor of Catalysis and Electrochemistry at Leiden University, has shown that the structure described by the law does not correspond to reality for a platinum electrode. Together with his Leiden colleagues Katharina Doblhoff-Dier and Kasinath Ojha, he produced an update of the Gouy-Chapman theory; they published their work at the end of January in the journal PNAS.

‘There is something going on that is not chemical in nature’

The key factor in the research is the potential of zero charge. At this electrical voltage, the charge of the electrode surface is precisely zero. The ions from the electrolyte therefore do not accumulate in the double layer. The Gouy-Chapman theory predicts where the potential of zero charge lies.

Unresolved mystery

In the 1940s and 1950s, the American chemist David Grahame verified the Gouy-Chapman theory using a mercury electrode. Measurements with solid metals were more difficult, so everyone simply assumed that the law also applied to platinum, gold, and other commonly used materials. Since then, nobody has tested the theory against other electrodes. ‘In the mid-1990s, I was working as a postdoc in Germany,’ recalls Koper. ‘My colleague Tamás Pajkossy was attempting at the time to find the potential of zero charge of a platinum electrode. You do this by reducing the salt concentration to a very low level. Ultimately, you need to find a minimum in your graph. That is the potential of zero charge. But Pajkossy could not find that point anywhere in his results.’

The mystery remained unsolved at the time, but Koper could not let it go. When the opportunity arose, Kasinath Ojha, a postdoc in Koper’s research team and first author of the paper, set to work on it once again. ‘No matter how low I made the concentration of the salt solution, I too was unable to find the potential of zero charge,’ says Ojha. The postdoc decided to reduce the concentration by a further factor of ten. ‘Nobody had ever done that before, and I didn’t think the solution would lie there, but it turned out to be a brilliant move,’ says Koper. The eureka moment had arrived: the potential of zero charge appeared in Ojha’s results.

‘We suspect that the interaction has something to do with the structure of the water in the double layer with platinum’

Not chemical

The fact that extremely low concentrations are required tells Koper that far more ions are present in the double layer than the theory predicts. ‘Something is causing this quantity that the theory does not account for.’

The researchers have since gathered a few clues that may (partly) explain the remarkable result. Koper: ‘We know that the nature of the ion does not affect the quantity of ions at the interface, but the difference in size between the anion and the cation does matter. The smaller ion can get closer to the electrode than the larger one, and this creates an asymmetry in the measurements.’ Ojha also noted that the unknown interaction is sensitive to changes in temperature. ‘And that should not be the case if we are dealing with chemical interactions. So something else is going on — an explanation that is not chemical in nature.’

The researchers are provisionally calling the unknown interaction an ‘effective electrostatic interaction’. Thanks to the €2.5 million European grant that Koper received in April 2021, the research team is able to continue piecing together the puzzle. Koper: ‘We suspect that the interaction has something to do with the structure of the water in the double layer with platinum. It is possible that ions are thereby drawn more strongly towards the electrode, which could explain the large quantity.’ With liquid mercury, no such special water structure is present. ‘It may explain why the Gouy-Chapman theory works so well for mercury, but not for solid metals,’ adds Ojha.

Lectures

On the basis of Ojha’s measurements, the team produced a revised Gouy-Chapman theory. This now describes and predicts reality considerably more accurately for platinum. ‘We hope to verify the law for other metals as well, such as gold and perhaps silver,’ says Ojha. The researchers also hope that their findings will feature in the next generation of textbooks. Koper: ‘The next series of lectures I give on this subject will certainly look very different.’