Chips may replace corn for harvesting solar fuels Free

In JCAP’s conceptual two-stage cell, a light-absorbing semiconductor coated with a catalyst oxidizes water into protons and oxygen. Reducing protons to hydrogen will occur in a second semiconductor layer containing a different catalyst. Although platinum and iridium oxide will do the job, they are much too rare and expensive to be scaled up into a meaningful energy source. As JCAP’s scientific director Nathan Lewis explains, the best Earth-abundant water-oxidation catalysts work by oxidizing hydroxide, plentiful only in alkaline solutions. But the best catalysts for proton reduction work most efficiently in an acidic environment. A semipermeable material capable of separating hydrogen from oxygen while allowing ions to pass to the reduction stage is a safety requirement with its own pH restrictions. “There are no membranes that can do this at pH 7,” says Lewis. “If the membranes only work in acid or base, then the catalysts also need to work either in acid or in base for the system as a whole to work.”

Candidate materials for light absorbers also are pH limited, notes Lewis. Silicon, the most widely used light absorber for photovoltaics (PVs), dissolves in basic solutions. Other potential materials, including titanium dioxide, other metal oxides, and existing PVs, either dissolve or corrode in acid. “This is like building an airplane. Just having an engine doesn’t mean the plane flies. You need wings, you’ve got to have avionics, got to have a fuselage, and the thing has to fly,” Lewis says.

There are other, less obvious problems. Although hydrogen bubbles indicate success, they also refract light away from the catalytic structures. “We need to mitigate the effects and exploit the optical properties of these bubbles,” Lewis says, “or we are going to get creamed on a real device the instant we bring it outside for more than 10 seconds.”

Unique to JCAP is a high-throughput facility for synthesizing and screening potential catalyst compounds. Using large-format inkjet printers, researchers can make and reproduce 1-mm² samples of any mixture containing up to 8 of the 20 elements for which the center has created inks. Then, to rapidly measure the mixtures for their quantitative properties, the researchers use a variety of techniques, including x-ray diffraction, large-format x-ray photoelectron spectroscopy, and video recordings of bubble formation. “Basically, it’s an analytical pipeline to give us the structures, compositions, and activities in a mineable, searchable database,” Lewis says. He adds that the capability will accelerate progress for the community as a whole.

Discoveries so far have been “evolutionary, not revolutionary,” Lewis says. Among them are new compositions of iron–nickel catalysts, with cerium and cobalt, that are 20–50% more efficient than existing state-of-the-art iron–nickel compounds.

Smaller AP centers are operating elsewhere around the globe. The Israel Solar Fuels Consortium is devoting about $12 million over five years to developing photocatalysts. That effort focuses on metal oxides, iron oxide in particular. “We know it will never be as efficient as silicon, but we are trying to increase its efficiency so it can work together with silicon,” says Avner Rothschild, a professor at Technion–Israel Institute of Technology, who heads the consortium’s water-splitting work.

The biggest obstacle to the use of iron oxide has been its poor electrical transport properties. Researchers have struggled with the tradeoff between light absorption and the separation and collection of photogenerated charge carriers before they die out by recombination, he says. In December 2012 the group announced what it called a major advance: By combining resonant light trapping in quarter-wave films and photon retrapping in V-shaped structures (see the photograph on page 22), films as thin as 25 nm have been demonstrated to absorb light. At the same time, the ultrathin films efficiently collect charge carriers.

The Swedish Consortium for Artificial Photosynthesis takes a different approach: finding molecular catalysts to mimic and improve on the Photosystem II protein complex, which captures light and splits water in plants. Based at Uppsala University in Sweden, the research involves 40–50 scientists, mostly from other countries. The Swedish Energy Agency is providing about $10 million over four years to the consortium, with another $7.6 million from the Knut and Alice Wallenberg Foundation.

Anders Hagfeldt, an Uppsala chemist who leads a group working with dye-sensitized solar cells, a low-cost thin-film PV, says the consortium has developed manganese and cobalt oxide complexes for water oxidation and iron oxide and ruthenium complexes for proton reduction. He predicts that molecular catalysts will make rapid gains in efficiency within the next two years.

In Japan, a consortium backed by major chemical companies and the Ministry of Economy, Trade, and Industry seeks to develop catalysts to reduce CO₂. The 10-year project focuses on producing olefins for chemical feedstocks, says Kazunari Domen, a University of Tokyo professor who heads the collaboration. Advances made by the Japanese consortium could also help improve the efficiency of turning the hydrogen produced from water splitting into more complex liquid fuels. “If we are successful, it’s possible that eventually we could produce solar fuels,” he explains.

For Lewis, liquid fuels from hydrogen should be considered a long-term goal for JCAP. “It’s very clear that the first fuel we’re going to produce is [hydrogen] from water splitting,” he says. That hydrogen might be used to upgrade biofuels to higher energy content or to reduce CO₂ to make synthetic fuels. The choice of fuel could be left to the chemical and refining industries, which know how to convert one fuel to another with minimal energy loss, he notes.

Looking ahead five years, Rothschild sees efficient, functioning AP arrays of perhaps 1 m². Those will need to be tested over thousands of hours for durability and energy conversion efficiency, he says. Life-cycle analyses will indicate whether AP could produce hydrogen at a competitive cost, which Rothschild estimates is around $3/kg in the US and €5/kg ($6.70/kg) in Europe.

“This technology has only been demonstrated in the lab; it’s very difficult to make projections of how it will work in the field over many years,” Rothschild says. AP also could provide a solution to the grid-leveling challenge that increasing PV electricity generation will present; using combined water splitting and PV arrays would produce a clean-burning fuel for nighttime power generation. He cautions that AP will have its environmental impacts. “It’s clear that this is not zero emission, because at a minimum you should count the CO₂ you use in making the devices. You also need to think about how much water we need and where we’ll get it.”

Frei and Lewis say they expect JCAP to have a working prototype by 2015, when the center’s five-year contract is up for renewal. To make a device with the targeted 5–10% efficiency could take another five years. Hagfeldt agrees that 10 years is a reasonable estimate for having AP technology in practical use. “It’s very important that you keep several options open. Whether you go for electricity or for fuels from solar energy is complementary. It’s not an either–or; you need both.”