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

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

Two distinct simultaneous electrochemical reactions each require catalysts, chemical “negotiators” that help break and remake chemical bonds. Now, scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and Columbia University say they have developed an effective new catalyst for the hard part: the oxygen evolution reaction.

As described in an article just published in the Journal of the American Chemical Societythe catalyst was designed “from the bottom up” based on theoretical calculations aimed at minimizing the amount of iridium, an expensive metal used as a catalytic material, and maximizing the stability of the catalyst in acidic conditions.

When the team created models of the catalyst and tested them in the lab, the results validated the predictions. Next, the scientists made the catalyst in powder form, 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 the chemistry division. in Brookhaven. Laboratory 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 model, or produces hydrogen four times faster for the same amount of iridium.

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

The remaining challenge is to increase production.

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

Reduce Iridium

Iridium is the catalyst of choice for the oxygen evolution reaction, which takes place at the anode of an electrolyzer. It provides the electrically charged active sites that separate tightly bound hydrogen ions (H⁺) from oxygen (O). In addition to releasing the H⁺ ions – which contribute to the extremely acidic reaction conditions – the reaction produces oxygen gas (O₂) and electrons. These electrons are needed for the second, less difficult, “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 oxygen evolution reaction in acid,” Chen said. This is “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 nanoscale particles, only the atoms present on the surface participate in the reaction,” Chen said. “This means that most of the iridium inside the particle is wasted.”

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

The team had explored the use of elements found in abundance on Earth, such as titanium. They found that the combination of titanium and nitrogen provided enough stability for these “titanium nitrides” to survive acidic reaction conditions. Perhaps titanium nitride could serve as the core of iridium-coated catalytic particles.

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

Calculation of an ideal structure

“We used ‘density functional theory’ calculations to model how different layers of iridium on titanium nitride would affect the stability and activity of the catalyst under acidic, oxygen-evolving 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 enough to drive the oxygen evolution reaction, but that two or three layers would improve both performance and catalytic stability.

“These were sort of pre-screening experiments,” Liu said. “Then we passed these screening results to the experimental team to make real catalysts and evaluate their catalytic activity.”

Validate 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 powdered samples composed of small nanoscale particles, the form the catalyst would take in industrial applications. Next, they studied thin films, including interfaces between layers, and nanoparticles using various techniques.

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

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

Characterization studies confirmed the predictions.

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

“Element mapping of the nanoparticles at CFN confirmed the size and composition of the particles, including the presence of iridium oxides on the surface of the titanium nitride supports,” he added.

Liu emphasized that the characterization studies have shed light on scientists’ understanding of the catalyst.

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

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

“By going from one to three layers of iridium, you significantly increase the charge transfer from the nitride to the higher iridium,” Liu noted. But the difference between two and three layers was not very big. Two layers could be enough to enable high stability, activity and low cost.

To make this catalyst ready for real-world use, the scientists highlighted that in addition to addressing the challenge of increasing production, improvements could also be made 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 kind of control,” Chen said. “Our powder particles are not surrounded by a continuous iridium shell. But this study provides guidelines that industrial chemists could use to create true core-shell structures with a thin, uniform layer of iridium,” he said. he declared.

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