DAVID EHRLICH, cleantech.com, February 18, 2008
Researchers at Pennsylvania State University have come up with a way to get hydrogen from water that tries to copy the system that plants use.
“It’s a conceptual advance, I think. Kind of a proof of principle that you can make something that uses molecules to sort of do what photosynthesis does,” Professor Thomas Mallouk told Cleantech.com.
Mallouk is the DuPont professor of materials chemistry and physics at Penn State.
The researchers worked in collaboration with Arizona State University and with backing from the U.S. Department of Energy.
The group developed a catalyst system that, combined with a dye, mimics the electron transfer and water oxidation processes that occur in plants during photosynthesis.
But don’t look for a commercial application to come out anytime soon. The technology right now is very inefficient, although Mallouk said plants aren’t doing much better.
“Photosynthesis is basically a failed system. Since it’s had a billion-plus years to evolve and has only got to 1 to 3 percent efficient,” he said.
“So we need to do better than that in whatever man-made systems we make, whether they’re molecular systems like ours or semiconductor systems like conventional solar cells.”
The researchers have so far achieved an efficiency of only about 0.3 percent. They reported the results of their experiments at the annual meeting of the American Association for the Advancement of Science over the weekend in Boston.
Current catalytic systems can make hydrogen using a so-called sacrificial reducing agent that provides the electrons needed, with the agent consumed in the process.
“What we wanted to do was do that without cheating, using water as the electron donor and make hydrogen on the reducing side and oxygen on the oxidizing side.”
The process uses a cluster of molecules about 2 nanometers in diameter with a center catalyst of iridium oxide molecules surrounded by orange-red dye molecules.
The university said the researchers picked orange-red dye because it absorbs sunlight in the blue range, which has the most energy.
When the dye is hit with visible light, the energy excites electrons in the dye, which, with the help of the catalyst, can split the water molecule.
Attempts at a similar process by other researchers have run into the problem of the hydrogen and oxygen recombining.
“If you’re driving a reaction uphill, it wants to go back downhill,” said Mallouk. “Any catalyst that is a good catalyst for doing that uphill reaction, is also a good catalyst for sending it the other way.”
“So you have a real problem. You have to somehow separate the products physically or you have to do very tricky catalyst design that will only catalyze a reaction in one direction and not the other.”
The researchers impregnated a titanium dioxide electrode with the catalyst complex for the anode and used a platinum cathode, immersing the electrodes in a salt solution, but separating them from each other to avoid the problem of the hydrogen and oxygen recombining.
While the system is a step forward in making a process that can split water without using a reducing agent, it’s still well behind current, commercially available technology.
“I don’t know if this kind of water photolysis would ever catch up with power generating solar cells hooked to electrolyzer systems,” said Mallouk.
“Those systems are pretty good and getting better, but the costs are still pretty high.”
Companies like New Jersey-based Renewable Energy International are working on a solar cell-to-electrolyzer system for residential use.
Scaling up the the Penn State system, with its iridium oxide catalyst, isn’t likely to be a cheaper proposition.
“Iridium occupies a unique position of being the most expensive element in the periodic table,” said Mallouk. “Can’t get any worse than that.”
But he did point out that nature can achieve its oxygen evolution reaction 50 times faster than they can, using manganese, which is a cheap element.
“So it’s not hopeless to make this kind of thing out of cheap materials, but it would be a long, tough road to make a fully molecular water splitting system that was really efficient and really economical.”
He said the molecular systems are interesting, from a fundamental science point of view, but that the smart money is probably on semiconductor-based nanosystems for making really efficient solar cells.
“You could picture a microscopic system that develops the voltage you need to split water and then the components of the electrolyzer right on that particle.”