Sun Shines on Old Idea to Make Hydrogen

NREL Research Fellow John Turner tests a photoelectrochemical water-splitting system in NREL's hydrogen lab. Turner set a world record for efficiency for the process in 1998, and the record still stands today. Credit: Dennis Schroeder

Three hundred articles and news reports followed, there were predictions of the next big thing, and then … almost nothing happened.

Energy companies went back to their gasoline and their diesel, or their attempts to extract fuel from biofuels, or to extract hydrogen via algae or coal-powered electricity.

Now, with gasoline at $3.89 per gallon, and still the dominant transportation fuel, there is renewed interest in Turner's work.

Last month, the Japan Society of Coordination Chemistry awarded Turner with its first Lectureship Award for his pioneering work in the fields of solar hydrogen and fuel cells and for being an international spokesman for hydrogen production via photoelectrochemical water splitting. "It was quite an honor to be recognized that way," he said.

"It's what I've been doing for 30 years, pushing this technology into new areas that are more fruitful," Turner added. "People are looking at climate change, looking at solar fuels. The tsunami has people in Japan rethinking nuclear, plus they import 90% of their oil. Countries are looking at technologies that give them greater energy independence."

Momentum Grows for Solar-Fueled Water Splitting

At the recent Sustainable Mobility Seminar at NREL, reporters toured the parking garage where hydrogen fuel cell vehicles and electric plug-in vehicles awaited test drives. The car in the forefront can be resupplied with hydrogen at NREL's National Wind Technology Center 15 miles north of the main campus. Credit: Dennis Schroeder

As often happens in science, researchers are turning back to a breakthrough that had lost momentum.

Japan, South Korea, and Singapore are starting advanced artificial photosynthesis centers. This month, DOE announced the availability of $1 million in grants to evaluate technology pathways for cost-competitive hydrogen fuel.

In 2010, DOE established the Joint Center for Artificial Photosynthesis at the California Institute of Technology (Caltech) as one of its Energy Innovation Hubs, with the aim to find a cost-effective method to produce fuels using only sunlight, water, and carbon dioxide as inputs.

It's photosynthesis without the green stuff. Turner uses multijunction solar cells made mostly from elements in the third and fifth columns of the periodic table, such as gallium, arsenic, and indium. These so-called III-V solar cells still work the best.

NREL also funds research through its Laboratory Directed Research and Development Program aimed at trying to make alloys that combine the benefits of the III-V elements with the strengths of elements such as cobalt, manganese, phosphorous, or titanium. The trick is to combine the materials in such a way that they are not only stable and robust but also have the right band gaps to generate enough voltage to split water.

Turner, who has been with NREL since 1979, says the cleanest way to produce hydrogen is by using sunlight to directly split water into hydrogen and oxygen.

The amount of greenhouse gases such a method can save is almost unfathomable. Consider that in the United States alone, 9 million tons of hydrogen are made each year, much of it in petroleum refineries and primarily via a process called steam reforming of natural gas.

For each kilogram of hydrogen produced that way, about 12 kilograms of carbon dioxide are produced. A process that could replace all that natural gas reforming with the sun and water can save 100,000 trillion kilograms of carbon dioxide from reaching the atmosphere each year. Hydrogen produced from water and sunlight can replace natural gas reforming if it is made near the refinery and used in place of the hydrogen typically made from reforming.

It won't be easy. In fact, Turner says that the reason his initial breakthrough lost momentum is that improving the technique "was hard work, and people don't like to do hard work."

Better technology, the cost of gasoline, and worries about nuclear power in the post-tsunami era have combined to make researchers believe water splitting may be worth the hard work. Turner's pioneering work — and the renewed enthusiasm for it — is leading researchers in several directions.

New, More Robust Materials Show Promise

NREL researchers work with a flask during a probe experiment in NREL's hydrogen lab. Credit: Dennis Schroeder

Turner's NREL colleagues Dan Ruddy and Nate Neale are researching new materials that can duplicate the elegance and efficiency of the 1998 breakthrough, but are robust enough to remain stable hour after hour, month after month.

They're getting help from the Center for Inverse Design, a DOE Energy Frontier Research Center that uses theory and supercomputing to first come up with the ideal properties of a new material, then find the mix of chemicals that holds those properties.

The aim is materials that can regenerate even as they disintegrate. It's something the human body can already do, from replacing plasma every 48 hours to regenerating a new stomach lining every five days.

Some metal oxides look particularly promising, Ruddy and Neale said. They're plenty stable, but typically have large band gaps — meaning the gap between the band that can conduct energy and the band that cannot is too wide to allow for efficient conversion of photons into electrons.

Ruddy and Neale are taking hints from some previous work at NREL that combines several layers of semiconducting material into a solar cell that has multiple junctions and thus has ideal band gaps to capture the sun's energy morning, midday, and afternoon.

Instead of a junction where two layers meet, as in the case of a conventional solar cell, the water-splitting cell has a junction where a layer hits an electrolyte.

"We're seeing a lot of promise in some of these new materials," Neale said.

Researchers at MIT are bonding a solar cell to a newly developed catalyst to try to efficiently use the sun to split water into hydrogen and oxygen. The catalyst of the so-called artificial leaf is made of earth-abundant, inexpensive materials, mostly silicon, cobalt, zinc, nickel, and molybdenum. Like the NREL approach, MIT's needs no external wires or control circuits to operate.

"John Turner's work was seminal," said MIT and Harvard chemist Daniel Nocera. "He set a new path for photoelectrochemical cells. Before Turner, the path of photoelectrochemistry was to have materials that absorbed light and did the catalysis of water splitting. Turner separated the function of light collection and charge separation from catalysis in his now-called 'Turner cell.'