Chemists observed that just before oxygen is released during the reaction, water molecules unexpectedly flip their orientation. This flipping step, the team discovered, significantly contributes to the additional energy demand of water splitting. They further determined that raising the pH of water reduces the energy needed for this reorientation, suggesting a pathway to improve overall efficiency.
The study, published April 15 in Nature Communications, reveals that this molecular flip is a major bottleneck in the oxygen evolution reaction (OER), one of the two essential half-reactions in water splitting. "When you split water, two half-reactions occur," explained Franz Geiger, the study's lead author. "One produces hydrogen, the other oxygen. The oxygen-producing step is particularly challenging and requires more voltage than predicted. We argue that the energy required to flip the water is a significant contributor to this excess."
Geiger, a professor at Northwestern's Weinberg College of Arts and Sciences, collaborated with Raiden Speelman and Ezra J. Marker from his lab, as well as researchers from Argonne and Pacific Northwest National Laboratories. The team focused on hematite, a widely available iron oxide mineral, as a model electrode.
Water splitting holds promise for both clean energy and future space missions, such as oxygen generation on Mars. But materials like iridium, the most effective catalyst for OER, are rare and costly. "Iridium comes from meteoric events and is extremely limited in supply," noted Geiger. "That's why researchers are working with cheaper alternatives like nickel and iron."
To better understand why inexpensive materials underperform, the team employed phase-resolved second harmonic generation (PR-SHG), a light-based technique developed in Geiger's lab. By analyzing how water molecules interact with the hematite electrode under laser light, they could observe their reorientation in real time.
"Our method is like noise-canceling headphones but for photons," Geiger said. "It lets us isolate signal phases to see how many water molecules point toward or away from the electrode." They found that before voltage is applied, water molecules are randomly arranged. As voltage increases, molecules realign with their oxygen atoms facing the electrode, a necessary configuration for electron transfer.
The researchers quantified the energy needed for this molecular flip and found it comparable to the cohesive energy of liquid water. Importantly, they showed that water's pH affects this behavior. Lower pH levels raise the energy required for flipping, inhibiting current flow, while higher pH levels ease the process.
"Below a pH of nine, very little current flows," Geiger said. "The molecules still flip, but it takes so much energy that the reaction stalls."
These findings also reinforce conclusions from a March study by Geiger's team, which documented similar behavior on nickel electrodes. "Now we know water flipping occurs on both metal and semiconductor surfaces," he added. "This suggests it's a general requirement for OER, and we can start designing catalysts to accommodate it."
With materials like hematite offering potential as photoanodes for solar water splitting, understanding water flipping could accelerate the development of affordable, efficient systems that combine sunlight with electrocatalysis. As Geiger explained, "Our goal is to minimize voltage requirements. If solar energy can activate catalytic sites, we can lower costs and make hydrogen fuel more viable."
Research Report:Water flipping and the oxygen evolution reaction on Fe2O3 nanolayers
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