A recent study by Raiden Speelman, Ezra J. Marker and Franz M. Geiger from Northwestern University in Evanston, Illinois, USA, sheds light on a long-standing mystery of electrolysis: Why do higher voltages have to be applied during oxygen evolution than theory previously suggested? Using high-precision optical second-harmonic generation (SHG) measurements on nickel electrodes in alkaline media, the researchers were able to directly observe the critical reorientation process of the water molecules in the star layer immediately adjacent to the electrode surface for the first time.
Until now, it remained unclear why oxygen evolution (OER) was inefficient despite theoretically sufficient potentials. The new study shows that before the Faraday current occurs, a significant transformation already takes place in the star layer: Under increasing applied potential, almost a complete monolayer (approx. 1.1 × 10¹⁵ molecules/cm²) of water molecules rearrange themselves so that their electron-rich oxygen atoms point towards the electrode. This "water flipping" process requires an energy input of around 80 kJ/mol - an extra effort that explains the additional voltage requirement.
The experimental results, underpinned by a two-dimensional Ising model, thus provide a crucial building block for understanding the molecular mechanisms at electrode surfaces. They not only provide important benchmarks for theoretical models of the electrical double layer, but also open up new perspectives for optimizing electrode materials and increasing the efficiency of water splitting.
