The mysterious workings of a new catalyst could help produce fuels from water and improve fuel cells, scientists say.
Splitting water into its constituent hydrogen and oxygen elements is an important starting point for the development of clean renewable fuels. Producing hydrogen from water could also become a method to store excess renewable energy.
It’s a process plants have already mastered via photosynthesis and humans are now working to replicate.
“While photosynthesis is extremely good at oxidizing water, the truth is many man-made processes of doing these things are not that good,” said Thomas Jaramillo, a researcher at the SUNCAT Center for Interface Science and Catalysis in Stanford University’s Department of Chemical Engineering.
Many of the artificial methods of making hydrogen and oxygen from water require materials that are too expensive, require too much energy or break down too quickly in real-world conditions, like the acidic electrolytes in fuel cells.
But splitting water to generate hydrogen may be an important way to cut greenhouse gas emissions. Jaramillo observed that the world produces more than 50 billion kilograms of hydrogen each year and more than 95 percent of it comes from fossil fuels through processes like steam reforming methane.
Jaramillo and his collaborators sought to develop a catalyst for the oxygen evolution reaction, the notoriously slow half of the water-splitting process. A catalyst is a substance that speeds up a chemical reaction or lowers the energy required to get it started without getting used up itself. Making these materials last longer, work faster and use less energy would cut prices and improve efficiency in producing renewable hydrogen.
In a paper published last week in the journal Science, the research team presented an oxygen evolution catalyst that worked in harsh conditions and beat all of its competitors.
“The biggest achievement in this paper is that we were able to find a stable catalyst that works in acid,” said co-author Yasuyuki Hikita, a staff scientist at SLAC National Accelerator Laboratory. “The activity that we found has been record-breaking.”
In an oxygen evolution reaction catalyst, one of the key benchmarks is the overpotential, or the amount of electricity needed to drive the reaction. The past record was 320 millivolts, but the new catalysts only needed between 270 and 290 millivolts of overpotential to reach the same level of oxygen production.
Hikita said this is a huge improvement in efficiency and would drastically cut the energy needed in a potential hydrogen production plant that would run around the clock.
The catalyst in this case was a thin film crystal, grown to be as flat as possible, with one layer made from iridium oxide and another layer made from strontium iridium oxide. With a flat, thin crystal, researchers had a better standard of comparison for their simulations.
“Iridium oxide is the only known catalyst that works in acid,” Hikita said.
In their experiments, the researchers also found that the catalyst’s performance improved over time. Measurements showed that the surface of the catalyst changed, but the mechanism that’s sped up oxygen evolution is unclear. “Microscopically, we’re not sure why at this point,” Hikita said. “Part of the strontium goes out into the solution.”
The researchers are now working to make their oxygen evolution catalyst more efficient and less expensive, as well as teasing out the physics of what makes the material work so well. “Iridium is still a very expensive metal,” Hikita said. “How much can we reduce the amount of iridium to achieve the same results? For practical industrial applications, we need to go much, much lower.”
“There’s much further room ahead than there is behind in terms of developing a better catalyst,” Jaramillo said. “You can imagine a catalyst a million times better than the one we made.”
Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. E&E provides daily coverage of essential energy and environmental news at www.eenews.net. Click here for the original story.