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SLAC, Stanford team develops new catalyst for water-splitting for renewable fuels production; 100x more efficient than other acid-stable catalysts

Researchers at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have developed a new highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction (OER).

The new catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte. Because it requires less of the rare and costly metal iridium, the new catalyst could bring down the cost of artifical photosynthetic processes that use 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 team published their results in the journal Science.

The OER (water oxidation) plays a key role in water splitting by providing the protons and electrons needed for these processes; improving the efficiency and durability of OER catalysts has a direct impact on device efficiency and cost effectiveness, the researchers noted in their paper.

Decades of research investigating catalysts for the OER have led to substantial advancements in the field and the development of active catalysts. However, even the best currently known catalysts require >320 mV overpotential to reach 10 mA/cm2oxide, a measure of intrinsic activity acquired by normalizing the OER current to the catalyst surface area. To partially overcome the low intrinsic activity of known OER catalysts, the activity of an OER electrode can be improved on a geometric-area basis by in- creasing catalyst loadings or by improving catalyst morphologies to expose large surface areas. Such approaches, however, can ultimately become limited by mass transport and/or catalyst conductivity. Another major concern is the cost and scarcity of precious metal–based OER catalysts, particularly in acidic electrolyte.

Whereas there are a plethora of precious and nonprecious metal catalysts that exhibit activity and stability in alkaline electrolyte, IrOx and RuOx are currently the only known materials that can reach ~5 mA/cm2oxide with overpotentials less than 750 mV in acidic electrolyte. New OER catalysts for use in acidic environments could have a direct impact on improving polymer electrolyte membrane–based water electrolyzers, a recent technology that overcomes many of the disadvantages of conventional alkaline electrolyzers but in which OER catalysts are primarily restricted to IrOx.

The recent development of techniques for high-quality growth of epitaxial complex oxide thin films and heterostructures provides an intriguing opportunity for the conception and fundamental study of OER catalysts. Using such an approach, we developed a catalyst IrOx/SrIrO3, which is stable in acidic electrolyte and substantially outperforms known IrOx catalysts.

Image
An illustration shows one possible way that a highly active iridium oxide layer could form on the surface of a strontium iridium oxide catalyst. After strontium atoms (green spheres) left the top layer through a corrosion process during the catalyst’s first two hours of operation, the top layer rearranged itself and became much better at accelerating chemical reactions. (C.F. Dickens/Stanford University) Click to enlarge.

The discovery of the catalyst—a very thin film of iridium oxide layered on top of strontium iridium oxide—was 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 search started with SUNCAT (Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford) theorists, who used computers to explore a database of materials and find the ones with the most potential to do exactly what was needed.

Based on the results, a team led by SLAC Staff Scientist Yasuyuki Hikita and SLAC/Stanford Professor Harold Hwang, both investigators with the Stanford Institute for Materials and Energy Sciences (SIMES), synthesized one of the catalyst candidates, strontium iridium oxide. Linsey Seitz, a PhD student in Jaramillo’s group and first author of the report, investigated the material’s properties.

To the team’s surprise, this catalyst worked even better than expected, and kept improving over the first two 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.

A lot of materials do this type of thing – surfaces can be very dynamic, changing during the course of a reaction – but in this case the catalyst changes in a way that gives you excellent performance in acid. This is unusual, because under these conditions most materials are either poor catalysts or they completely fall apart, or both.

—Thomas Jaramillo

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, a DOE Office of Science User Facility, to determine exactly how the 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. But there are many possibilities, and this gives us some very good leads.

—Jens Nørskov, director of SUNCAT and a professor at SLAC and Stanford

SUNCAT and SIMES are joint institutes of Stanford and SLAC. Major funding for the project came from the DOE Office of Science.

Resources

  • Linsey C. Seitz, Colin F. Dickens, Kazunori Nishio, Yasuyuki Hikita, Joseph Montoya, Andrew Doyle, Charlotte Kirk, Aleksandra Vojvodic, Harold Y. Hwang, Jens K. Norskov, Thomas F. Jaramillo (2016) “A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction” Science 353, Issue 6303, pp. 1011-1014 doi: 10.1126/science.aaf5050

Comments

gorr

The future is not bevs or hydrogen but low cost synthetic gasoline made with this process. Put that for sale near where i live and also invent a more compact gasoline engine with better efficiency so im gonna save a maximum of money. Im eagerly awaiting this advent.

HarveyD

A micro-ICE developed in a UK University (700 times more efficient than batteries) can power your watch, tablet, laptop for up to 2 years with a squirt of lighter fuel. A few dozens could recharge you BEV batteries while cruising on highways?

With 100X increased in efficiency, H2 could be created at much lower cost and make FCEVs operation much cheaper.

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