Scientists are excited about using water splitting to create clean hydrogen fuel, but the process takes more energy than it theoretically should.
A team at Northwestern University has discovered why water splitting is so energy-intensive: right before releasing oxygen, water molecules “flip” surprisingly, which uses a lot of extra energy.
After observing the water molecules flip, the team quantified the precise energy cost associated with that critical step. They discovered the acrobatic act is a major contributor to water splitting’s efficiency bottleneck.
However, in yet another discovery, they found that increasing the pH of water lowers the energy cost and thereby contributes to making the process more efficient.
“When you split water, two half-reactions occur. One half-reaction produces hydrogen, and the other produces oxygen,” explained Northwestern’s Franz Geiger, who led the study.
“The half-reaction that produces oxygen is really difficult to perform because everything has to be aligned just right and ends up taking more energy than calculated.”
Benefits and challenges of water splitting
As the climate continues to warm, scientists have become increasingly interested in water splitting as a way to produce clean hydrogen energy as an alternative to fossil fuels.
To perform the process, scientists add water to an electrode and then apply a voltage. This electricity splits water molecules into two components — hydrogen and oxygen — without any unwanted byproducts. From there, researchers can collect hydrogen for fuel or repurpose the hydrogen and oxygen into energy-efficient fuel cells.
While water splitting could play a significant role in a future clean-energy economy, it faces several challenges. The main issue is that the oxygen part of the reaction, called the oxygen evolution reaction (OER), can be difficult and inefficient.
Although it’s most efficient when iridium is used as the electrode, Geiger said scientists need more affordable alternatives.
“Iridium only comes to Earth from meteoric impact, like the famous iridium anomaly at the Cretaceous-Paleogene boundary, so there’s a limited amount,” he said.
“It’s very expensive and certainly not going to help solve the energy crisis any time soon. Researchers are looking at alternatives, like nickel and iron, and we’re hoping to find ways to make these materials just as efficient.”
Using light to watch molecules in real time
To study this hidden process, the team used a cutting-edge laser technique called phase-resolved second harmonic generation (PR-SHG). PR-SHG allows researchers to observe water molecules at the electrode interface as reactions unfold.
In their experiment, the team used hematite, a low-cost iron-based semiconductor, as the electrode. When a laser was directed at the hematite in water, the scientists monitored how water molecules aligned themselves as voltage was applied.
Initially disordered, the molecules flipped to position their oxygen atoms toward the surface, enabling the crucial electron transfer needed for OER.
Measuring the energy barrier
The researchers quantified both the number of molecules flipping and the energy needed for this reorientation.
They found that the energy requirement closely matched the energy that holds water molecules together in liquid form. This shows that flipping isn’t just incidental — it’s a major part of why water splitting uses more energy than theory predicts.
When quantifying the amount of energy used, Geiger and his team discovered that the energy required to align the water molecules closely matches the energy that holds liquid water together.
They also found that the water’s pH level influences the orientation of water molecules. While low pH levels required more energy to flip the water molecules into the correct alignment, higher pH levels, by contrast, made the process more efficient.
Towards cheaper, scalable hydrogen production
This study builds on earlier work from Geiger’s lab, which observed the same flipping behaviour using nickel electrodes.
The fact that both metals and semiconductors show this trait suggests that water flipping is a general feature of water splitting, not an isolated phenomenon.
Designing catalysts that facilitate water flipping could significantly reduce energy demands. By tailoring electrode surfaces to support this molecular motion, researchers may finally unlock practical, cost-effective hydrogen fuel production — a major step toward a clean energy future.
