Cheaper Platinum Alternatives for Splitting Water Could be Almost as Effective
The expense of platinum is a significant obstacle for its use in hydrogen fuel cells for electric vehicles. Now a research team led by Bruce Koel, a Princeton University professor of chemical and biological engineering, has initiated a search for far cheaper but somewhat less effective alternatives. (See the image above, from left, the research team included Bruce Koel, professor of chemical and biological engineering, and co-researchers Xiaofang Yang, a visiting researcher, Fang Zhao, a postdoctoral researcher, and Nan Yao, a senior research scholar and director of the Imaging and Analysis Center at Princeton's materials institute--Photo by Frank Wojciechowski).
In a paper published in the journal Nature Communications on April 4, the researchers reported that a hafnium-based chemical compound worked about 60% as effectively as platinum-related materials.
However, hafnium is about one-fifth the cost of platinum, the researchers noted.
"We hope to find something that is more abundant and cheaper to catalyze reactions," said Xiaofang Yang, principal scientist at HiT Nano Inc. and visiting collaborator at Princeton who is working with Koel on the project.
Fuel cells, which have been around for some time, convert the energy stored in hydrogen atoms directly into electricity. NASA has long employed fuel cells to power satellites and other space missions. Now, they're beginning to be utilized for electric cars and buses.
Hydrogen is the simplest and most abundant element on this planet and in the known universe. A catalyst material such as platinum is required for fuel cell operation. Catalysts are also employed in reactions that generate the hydrogen gas that serves as fuel for the fuel cell.
In the most advantageous, fossil-fuel independent case, renewable electrical energy can split water molecules into two hydrogen atoms (hydrogen gas) and one oxygen atom in the presence of a catalyst. Pairs of oxygen atoms then combine to form oxygen gas.
The more efficient a catalyst is, the less energy is required to split the water. Some advanced fuel cells, which are referred to as regenerative fuel cells, combine both reactions.
However, most current fuel cells rely on hydrogen created from separate systems that is sold as fuel.
Right now, platinum group metals are considered the best catalysts for both reactions. The researchers don't think this will change because "platinum is almost perfect," Koel said.
With platinum group metals, the electrochemical reactions to draw out the hydrogen are quick and efficient, and the metals can withstand the harsh acidic conditions currently required for such reactions.
The issue, though, is that platinum is rare and costly. "You can't really imagine replacing the transportation infrastructure with fuel cells based on platinum," Koel said. "It's too rare and too expensive to use at that scale."
For such applications, platinum's perfection may not be required. One sufficiently effective substitute, the researchers found, is hafnium oxyhydroxide that has been treated with a nitrogen plasma to incorporate nitrogen atoms into the material. Plasma is an ionized gas and is a state of matter in fluorescent lights and the sun.
Previously, numerous materials were overlooked for electrochemistry applications because they are non-conducting.
However, the researchers discovered that processing hafnium oxide with the nitrogen plasma forms a thin-film of material that functions as a highly active catalyst that also survives in strong acid conditions. While this hafnium-based film is just about two-thirds as effective as platinum, hafnium is much cheaper than platinum.
Although these could be useful in fuel cells, Yang and Koel believe that the biggest potential for these types of materials are for systems that use a catalyst to electrochemically split water to produce hydrogen for use as fuel.
"The future renewable economy heavily depends on how we can efficiently split water to generate hydrogen," Yang said. "This step is pretty important."
Yang and Koel emphasize that the discovery isn't going result in a rush of new affordable technologies just yet or even in the near future. Currently, the method of producing the material is complex and confined to the lab.
While they've confirmed the film's performance, they also need to find a way to make it practically on a large scale. Instead, the researchers say that this discovery opens the door to finding materials that may be able to replace platinum.
"We still don't understand why this particular material is so special, but we're confident about the properties that we've measured," Koel said. "The material is complicated, so we have a lot of work to do."
Next, the researchers plan to test zirconium, which is even cheaper.
Support for the project came in part from the Princeton Center for Complex Materials, the National Science Foundation, the Simons Foundation, and the John Templeton Foundation.
