Solar-Powered Reaction Now 100 Times Faster

solar catalyst

A strong new catalyst that performs a solar-powered reaction 100 times faster than current technology has been developed by researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University. The catalyst also stands up to acid and even performs better with time.

A key step in a sustainable, renewable route to produce hydrogen or carbon-based fuels is to use a process that works like photosynthesis to split water molecules by using sunlight. The new catalyst could reduce the cost of this process, as it requires less iridium, a rare and costly metal. The photosynthesis process can power a wide range of energy technologies.

The catalyst consists of a layer of iridium oxide only a few atoms thick on top of strontium iridium oxide. Three groups of experts were conducting an extensive search for a more effective way to speed up the oxygen evolution reaction (OER) when they discovered the catalyst. OER is one-half of the process that uses sunlight to split water.

Thomas Jaramillo, an associate professor at SLAC and Stanford and deputy director of the SUNCAT Center for Interface Science and Catalysis, explains that in acidic conditions, OER has been a real bottleneck in the process. Iridium is one of the rarest metals on Earth. It is also the only active catalyst known that is able to survive the harsh conditions associated with OER. To reduce the cost of the process for making fuels from renewable sources, catalyst materials that use little or no iridium and are more active need to be developed. This is the only way to use the process on a much larger scale.

Catalysts speed up chemical reactions without being used up in the process. SUNCAT theorists started by searching a computerized database to explore materials and find the ones with the most potential to do exactly what they needed. Over time, databases like the one the researchers used have become an important tool for designing catalysts to order. This is much more effective than testing thousands of materials in a time-consuming, trial-and-error approach.

solar catalysts
An illustration demonstrates one potential way that a highly active iridium oxide layer could form on the surface of a strontium iridium oxide catalyst. After strontium atoms, green spheres on the image, left the top layer through a corrosion process midst the catalyst’s first 2 hours of activity, the top layer reorganized itself and became much better at stimulating chemical reactions. (Image credit: C.F. Dickens/Stanford University)

Once the most likely candidates were selected, a team led by SLAC/Stanford Professor Harold Hwang and SLAC Staff Scientist Yasuyuki Hikita, both investigators with the Stanford Institute for Materials and Energy Sciences (SIMES), created strontium iridium oxide. The material’s properties were then investigated by Linsey Seitz, a PhD student in Jaramillo’s group and first author of the report.

To the team’s delight, the catalyst worked even better than what they expected. It also kept improving over the first few hours of operation. Experiments checking the surface of the material revealed that during this initial period, a corrosion process released strontium atoms into the surrounding fluid. The result was a thin film of iridium oxide just a few atomic layers thick. This new layer was significantly more active than the original material, and 100 times more efficient at promoting the OER than anything else known to date.

Jaramillo noted that surfaces are very dynamic and often change during the course of a reaction – this is quite common in various materials. In this case, the change in the catalyst resulted in superior performance in acid, something that was not expected. Normally, most materials are either poor catalysts or they completely fall apart under these conditions.

It is not yet known exactly why this new surface layer is so active. SUNCAT graduate students Colin Dickens and Charlotte Kirk have some theories on this, but these still have to be proven. The next step will be to take a closer look at the catalyst with X-ray beams at SLAC’s Stanford Synchrotron Radiation Lightsource. Jaramillo and his group hope to find out exactly why boosts the catalyst’s performance is boosted and how the atoms are rearranged on the surface.

Jens Nørskov, director of SUNCAT and a professor at SLAC and Stanford notes that the amount iridium in the material needs to be reduce even more to make a commercially viable catalyst. This result does however provide useful direction for future studies into different possibilities.

The full study was published in the journal Science.