Molecular Orientation of Water in Electrolysis: New Insights

Franz M. Geiger, Ezra J. Marker and Raiden Speelman, Northwestern University, Evanston, IL, USA, have directly observed how water molecules behave during electrolysis, revealing that they must reorient at a critical voltage for oxygen evolution to occur. This discovery helps explain why the oxygen evolution reaction (OER) requires more energy than theoretical predictions suggest.

The oxygen evolution half-reaction requires more energy than theoretically expected, needing 1.5–1.6 V instead of the predicted 1.23 V. Catalysts can reduce this energy barrier, but materials like iridium are rare and expensive. Scientists have long suspected that inefficient water molecule orientation at the electrode contributes to the energy barrier, but direct observation was difficult due to interfering molecular vibrations.

The team developed a spectroscopic technique to observe water molecules in real time during electrolysis. They bypassed the issue of interfering molecular vibrations by analyzing the second harmonic, a type of overtone from laser light reflected by the molecules. This method works like noise-canceling headphones: it creates interference patterns that suppress unwanted vibrational signals. This allows the researchers to clearly detect the orientation of individual H₂O molecules at the electrode surface, determining how many face the electrode and how many point away.

The team found that before electrolysis begins, water molecules are randomly oriented, with many hydrogen atoms facing the electrode, blocking electron transfer from oxygen. When the voltage exceeds a threshold, the molecules flip, aligning their oxygen atoms toward the electrode, enabling electron flow. Complete reorientation requires a potential difference of at least 0.8 V.

The researchers assume that the energy required for water molecule reorientation significantly contributes to the extra energy demand in electrolysis. Understanding this process offers new opportunities to optimize electrolysis, potentially enabling the development of catalysts that facilitate molecular flipping, making hydrogen production more efficient and cost-effective.