Scientists are excited to use water splitting to create clean hydrogen fuels, but the process, in theory, requires more energy.
A team from Northwestern University discovered why water splitting is energy-intensive. Just before releasing oxygen, the water molecules surprisingly “invert” and use a lot of extra energy.
After observing the water molecule flip, the team quantified the exact energy costs associated with its critical steps. They found that acrobatic acts are key contributors to the bottleneck of efficiency of water splitting.
Yet another finding found that increasing the pH of water reduces energy costs, which contributes to making the process more efficient.
“When you divide water, you’ll get two semi-reactions. One semi-reaction produces hydrogen and the other produces oxygen,” explained Franz Geiger of Northwestern, who led the study.
“Half the reactions that produce oxygen are extremely difficult to perform because everything needs to be properly aligned and takes more energy than calculations.”
Benefits and challenges of water splitting
As the climate continues to warm, scientists are increasingly interested in moisture cracks as an alternative to fossil fuels and as a way to produce clean hydrogen energy.
To carry out the process, scientists add water to the electrodes and apply voltage. This electricity divides water molecules into two components: hydrogen and oxygen, without unwanted byproducts. From there, researchers can collect hydrogen for fuel and reuse hydrogen and oxygen in energy-efficient fuel cells.
The division of water may play a key role in the future clean energy economy, but it faces several challenges. The main problem is that the oxygen portion of a reaction called the oxygen evolution reaction (OER) can be difficult and inefficient.
While iridium is most efficient when used as an electrode, Geiger said scientists need a more affordable alternative.
“Iridiums come to Earth from the effects of meteors, like the anomaly of the iridium, famous for its Cretaceous and phylogenetic boundaries,” he said.
“It’s very expensive and certainly not going to help resolve the energy crisis any time soon. Researchers are looking at alternatives like nickel and iron. They want to find ways to make these materials efficient.”
Use light to watch molecules in real time
To study this hidden process, the team used state-of-the-art laser technology called phase-resolved second harmonic generation (PR-SHG). PR-SHG allows researchers to observe water molecules at the electrode interface as the reaction unfolds.
In the experiment, the team used hematite, a low-cost iron-based semiconductor, as the electrode. When the laser was directed at hematite in the water, scientists monitored how the water molecules align when the voltage was applied.
Initially, the molecules inverted to direct the oxygen atoms towards the surface, allowing for the critical electron transfer required for the OER.
Measurement of energy barriers
The researchers quantified both the number of molecular inversions and the energy required for this redirection.
They found that energy requirements are closely matched with the energy that holds water molecules together in liquid form. This indicates that flipping is not accidental. This is a major part of why water partitioning uses more energy than theory predicts.
When quantifying the amount of energy used, Geiger and his team discovered that the energy needed to align the water molecules closely matches the energy that holds liquid water together.
They also found that the pH level of water affects the direction of the water molecules. Lower pH levels required more energy to reverse the water molecules into the correct alignment, while higher pH levels made the process more efficient, in contrast.
Towards cheaper and scalable hydrogen production
This study is based on previous work from Geiger’s lab, which observed the same inversion behavior using nickel electrodes.
The fact that both metals and semiconductors exhibit this property suggests that water inversion is not an isolated phenomenon, but a general feature of water cracking.
Designing catalysts that promote water inversion can significantly reduce energy demand. By tuning the electrode surface to support this molecular motion, researchers can ultimately unlock the practical and cost-effective hydrogen fuel production, a major step into the future of clean energy.
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