A simulation shows one possible way that a highly active iridium oxide layer could form on the surface of a strontium iridium oxide catalyst. Experiments by SLAC and Stanford researchers showed that strontium atoms (green spheres) left the top layer through a corrosion process during the catalyst’s first two hours of operation. The top layer then rearranged itself and became much better at accelerating chemical reactions. Follow-up X-ray studies at SLAC will examine these surface changes in more detail. (C.F. Dickens/Stanford University)
Researchers have developed a tough new catalyst that carries out a solar-powered reaction 100 times faster than ever before, works better as time goes on and stands up to acid. And because it requires less of the rare and costly metal iridium, it could bring down the cost of a process that mimics photosynthesis by using sunlight to split water molecules – a key step in a renewable, sustainable pathway to produce hydrogen or carbon-based fuels that can power a broad range of energy technologies.
The discovery of the catalyst – a very thin film of iridium oxide layered on top of strontium iridium oxide – as the result of an extensive search by three groups of experts for a more efficient way to accelerate the oxygen evolution reaction, or OER, which is half of a two-step process for splitting water with sunlight.
“The OER has been a real bottleneck, particularly in acidic conditions,” said A/Prof Thomas Jaramillo. “The only reasonably active catalysts we know that can survive those harsh conditions are based on iridium, which is one of the rarest metals on Earth. If we want to bring down the cost of such a pathway for making fuels from renewable sources and carry it out on a much larger scale, we need to develop catalyst materials that are more active and that use little or no iridium.”
Sample of a new catalyst material created by SLAC and Stanford researchers. It’s 100 times better than previous catalysts at accelerating the oxygen-evolving reaction in acid, a key step in a pathway for making sustainable fuels. (Linsey Seitz/Stanford University)
The search started with SUNCAT theorists, who used computers to explore a database of materials and find the ones with the most potential to do exactly what was needed. Catalysts accelerate chemical reactions without being used up in the process, and databases like this one have become an important tool for designing catalysts to order, rather than testing thousands of materials in a time-consuming, trial-and-error approach.
Based on the results, SLAC investigators with Stanford Institute for Materials and Energy Sciences (SIMES), synthesized one of the catalyst candidates, strontium iridium oxide. To the team’s surprise, this catalyst worked even better than expected, and kept improving over the first 2 hours of operation. Experiments probing the surface of the material indicated that a corrosion process released strontium atoms into the surrounding fluid during this initial period. This left a film of iridium oxide just a few atomic layers thick that was much more active than the original material, and 100 times more efficient at promoting the OER than any other acid-stable catalyst known to date.
Images made with an atomic force microscope show variations in the height of the catalyst’s surface before (left) and after its first 30 hours of operation. The observed changes in surface texture reflect strontium atoms leaving the top layers of the material during operation, forming a very catalytically active thin film of iridium oxide. (L. Seitz et. al., Science)
The researchers still don’t know exactly why this surface layer is so active, although the theorists, including SUNCAT graduate students Colin Dickens and Charlotte Kirk, have provided some ideas. Jaramillo’s group will be taking a closer look at the catalyst with X-ray beams at SLAC’s Stanford Synchrotron Radiation Lightsource to see how atoms on the surface rearrange themselves and why this boosts the catalyst’s performance.
“To make a commercially viable catalyst we will need to reduce the amount of iridium in the material even more,” said Prof Jens Nørskov “But there are many possibilities, and this gives us some very good leads.” https://www6.slac.stanford.edu/news/2016-09-01-slac-stanford-team-finds-tough-new-catalyst-use-renewable-fuels-production.aspx
Recent Comments