NREL Hydrogen Expert Sees Promise in New Discovery
Photoelectrochemical pioneer John Turner says nickel film cracks the door for tandem artificial photosynthesis, greater efficiency
January 8, 2014
Producing hydrogen directly from the sun -- and in a way that is commercially viable -- is more a reality, less a pipedream, thanks in part to a new discovery, renowned hydrogen water-splitting expert John Turner stated in a commentary in the journal Science.
Turner, a research fellow at the Energy Department’s National Renewable Energy Laboratory, demonstrated 15 years ago that he could use sunlight in a photoelectrochemical process to extract hydrogen from water at a remarkable 12.4% efficiency.
Ever since, researchers have been trying to make that process more stable and reliable, while driving down the cost and increasing the efficiency. Getting a consistent and reliable hydrogen conversion rate of 15% or greater from a sun-powered water-splitting process could be a game changer.
Turner, in his commentary for Science, said a new discovery by Stanford University scientists is an important step. The Stanford researchers demonstrated that the silicon semiconductor that converts the sun’s photons in the water-splitting process works better, is more stable, and generates a high voltage when it is layered with a super-thin -- two nanometer -- nickel film.
Nickel is inexpensive and abundant. More importantly, the Stanford researchers demonstrated, it acts as an oxygen evolution catalyst, provides stability against corrosion, and achieves a high voltage without the need of a silicon-oxide layer when it is applied to a so-called n-type silicon semiconductor.
The discovery by the Stanford researchers, while not yet ready to be integrated into viable water-splitting devices, “opens up some additional possibilities for a solar water-splitting system with efficiencies of 15% or greater,” Turner said.
There are several ways to produce hydrogen via water-splitting, but the greenest approach is direct photoelectrochemical (PEC) splitting of water into hydrogen and oxygen. The PEC approach combines a photovoltaic cell and an electrolyzer into a single device. The energy source is the sun, rather than natural gas or electricity.
Unfortunately, an otherwise reliable n-type silicon semiconductor -- the type in which electrical conduction is due chiefly to the movement of electrons -- tends to corrode when placed in the aqueous solution needed for the electrolysis. That deficiency seemed to dash the hope that a tandem cell -- one n-type with extra electrons, one p-type with extra holes -- could produce hydrogen at greater efficiency by harvesting photons from more places on the solar spectrum.
The discovery that there is a way to slow corrosion in n-type silicon re-opens the door to tandem configurations that can perform unassisted water-splitting. A separated arrangement of the two cells, such as presented by NREL research fellow Art Nozik, eliminates the need to match lattices of the two layers, and still benefits from the different band gaps that can harvest more photons than can a single band gap, Turner said. A tandem configuration, using an n-type cell and a p-type cell of copper, gallium and diselenide, has a maximum theoretical efficiency of greater than 25%, Turner said.
The world already uses large amounts of hydrogen because it is a necessary ingredient in petroleum refining and in producing ammonia, which is essential in fertilizers. Hydrogen also is needed for fuel cell electric vehicles (FCEVs), with automakers already announcing plans for commercial FCEVs in 2015.
The most common way to harvest hydrogen is via steam methane reforming of natural gas. The drawback is that for each kilogram of hydrogen produced that way, about 12 kilograms of the greenhouse gas carbon dioxide are produced. A process that could replace all that natural gas reforming with the sun and water can keep 100,000 trillion kilograms of carbon dioxide from reaching the atmosphere each year. “This is a significant result,” Turner said. “I was pleased and honored that the editors of Science asked me to write the commentary on it.”