Scientists build and test efficient water-splitting catalyst predicted by theory

Hydrogen (H₂) is a promising fuel for reducing greenhouse gases, especially when produced using renewable energy to split water molecules (H₂O). But as simple as it may seem to split water into hydrogen and oxygen, the chemistry is complex.

Each of the two separate, simultaneous electrochemical reactions requires catalysts, chemical “deal makers” that help break and reform chemical bonds. Now, scientists at the US Department of Energy’s (DOE) Brookhaven National Laboratory and Columbia University say they have developed a new efficient catalyst for the more challenging part: the oxygen evolution reaction.

As described in an article just published in Journal of the American Chemical SocietyThe catalyst was designed “from the bottom up” based on theoretical calculations that sought to minimize the amount of iridium, a precious metal used as a catalytic material, and to maximize the stability of the catalyst under acidic conditions.

When the team created models of the catalyst and tested them in the lab, the results confirmed the predictions. The researchers then produced a powdered form of the catalyst, like those used in industrial applications, and showed that it could efficiently produce hydrogen in a water-splitting electrolyzer.

“In this real-world test, our catalyst is about four times better than the state-of-the-art commercially available iridium catalyst,” said Jingguang Chen, a chemical engineer at Columbia University with a joint appointment in Brookhaven’s chemistry division. The lab that conducted the research. In other words, the new catalyst requires four times less iridium to produce hydrogen at the same rate as the commercial variety—or produces hydrogen four times faster with the same amount of iridium.

Brookhaven Lab theoretical chemist Ping Liu, who led the calculations underlying the catalyst design, said: “This study shows how you can go from a theoretical understanding of what happens at the atomic level to designing a catalyst for practical use.” work allows us to better understand how this catalyst works and brings us closer to a real-world application.”

The remaining challenge is to scale up production.

“We only produce milligrams of catalyst per dose,” Chen said. “If you want to make megatons of green hydrogen, you’ll need kilograms or tons of catalyst. We can’t do it on such a large scale yet.”

Reduction of iridium

Iridium is the catalyst of choice for the oxygen evolution reaction that takes place at the anode of the electrolyzer. It provides electrically charged active sites that separate tightly bound hydrogen ions (H⁺) from oxygen (O). In addition to the liberation of H⁺ ions – which contribute to strongly acidic reaction conditions – the reaction produces oxygen gas (O₂) and electrons. These electrons are needed for the second, less demanding “hydrogen evolution” reaction: the pairing of hydrogen ions to form hydrogen gas at the cathode of the electrolyzer.

“Iridium is currently one of the only stable elements for the acid oxygen evolution reaction,” Chen said. That’s “unfortunate,” he noted, because “iridium is even rarer and more expensive than platinum.”

Hence the motivation to reduce the amount of iridium.

“In industrial catalysts made of nanoparticles, only atoms on the surface participate in the reaction,” Chen said. “This means that most of the iridium inside the particle is lost.

Perhaps instead of using a particle that is all iridium, the catalyst could be made of a less expensive material with iridium only on the surface, the team reasoned.

The team explored the use of elements commonly used on Earth, such as titanium. They found that combining titanium with nitrogen gave these “titanium nitrides” enough stability to survive acidic reaction conditions. Perhaps titanium nitride could serve as the core of the iridium-coated catalytic particles.

But how much iridium should be layered on top? This is where theoretical calculations come into play.

Calculation of the ideal structure

“We used ‘density functional theory’ calculations to model how different overlays of iridium on titanium nitride will affect the stability and activity of the catalyst under acidic oxygen evolution reaction conditions,” Liu said. She and her team used computing resources at Brookhaven Lab’s Center for Functional Nanomaterials (CFN) and the National Energy Research Scientific Computing Center (NERSC) at DOE’s Lawrence Berkeley National Laboratory to run the simulations.

Calculations predicted that a single layer of iridium would not be sufficient to control the oxygen evolution reaction, but that two or three layers would improve both performance and catalytic stability.

“These were kind of preliminary trial experiments,” Liu said. “We then took these screening results to the experimental team to make real catalysts and evaluate their catalytic activity.”

Verification of predictions

First, the team created thin films in which they could create carefully controlled layers that closely resembled the surfaces used in theoretical modeling calculations. They also created powder samples composed of small nanoparticles, in the form that a catalyst would have in industrial applications. They then studied thin films—including the interface between layers—and nanoparticles using a variety of techniques.

These included transmission electron microscopy at CFN and X-ray spectroscopy studies at the Quick X-ray Absorption and Scattering (QAS) beamline of the National Synchrotron Light Source II (NSLS-II), a bright X-ray source for deciphering the samples’ chemical and physical properties.

“Our hypothesis was that if iridium is bonded to titanium nitride, this bonding would stabilize the iridium and improve the reaction,” Chen said.

Characterization studies confirmed the predictions.

“The synchrotron studies revealed the oxidation states and local coordination environments of the iridium and titanium atoms under the reaction conditions,” Chen said. “They confirmed that iridium and titanium interact strongly.

“Nanoparticle element mapping in CFN confirmed particle sizes and composition, including the presence of surface iridium oxides on titanium nitride supports,” he added.

Liu emphasized that the characterization studies have informed scientists’ understanding of the catalyst.

“We found that the interaction between iridium and titanium is useful not only for the stability of the catalyst, but also for fine-tuning its activity,” she said. “The charges change the chemistry in a way that improves the reaction.

Specifically, the charges transferred from the titanium to the iridium surface change the electronic structure of the iridium active sites to optimize the binding of the reaction intermediates, she explained.

“By going from one to three layers of iridium, you greatly increase the charge transfer from the nitride to the top iridium,” Liu noted. But the difference between two and three layers was not too big. Two layers may be sufficient to ensure high stability, activity and low cost.

To make this catalyst ready for real-world use, the researchers pointed out that in addition to solving the problem of increasing production, there could also be improvements to optimize the consistency of the powders.

“When we make thin films, we can control the layers, but with powder synthesis we don’t have that control,” Chen said. “Our powder particles do not have a continuous iridium shell around them. But this study provides guidelines that industrial chemists could use to create real core-shell structures with a uniform thin layer of iridium,” he said.

Such catalysts could help reduce the cost of water splitting and bring scientists closer to producing large amounts of green hydrogen.