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Daniel Nocera - 'Artifical leaf' moves closer to realtiy

MIT researchers develop a device that combines a solar cell with a catalyst to split water molecules and generate energy

David L. Chandler, MIT New Office
June 9, 2011

An important step toward realizing the dream of an inexpensive and simple "artifical leaf," a device to harness solar energy by splitting water molecules, has been accomplished by two seperate teams of researchers at MIT. Both teams produced devices that combined a standard silicon solar cell with a catalyst developed three years ago by professor Daniel Nocera. When submerged in water and exposed to sunlight, the device causes bubbles of oxygen to seperate out of the water.

The net step to producing a full, usable artificial leaf, explains Nocera, the Henery Dreyfus Professor of Energy and professor of chemisty, will be to integrate the final ingredient: an additional catalyst to bubble out the water's hydrogen atoms. In the current devices, hydrogen atoms are simpy dissociated into the solution as loose protons and electrons. If a catalyst could produce fully formed hydrogen molecules (H2), the molecules could be used to generate electricity or to make fule for vehicles. Realization of that step, Nocera says, will be the subject of a forthcoming paper.

The reports by the two teams were published in the journals Energy & Environmental Science on May 12, and the Proceedings of the National Academy of Sciences on June 6. Nocera encouraged two different teams to work on the project so that each would bring their special expertise to addressing the problem, and says the fact that both suceeded "speaks to the versatility of the catalyst system."

Ultimately, Nocera wants to produce a low–cost device that could be used where electricity is unavailable or unrealiable. It would consits of a glass container full of water, with a solar cell with the catalysts on its two sides attached to a divider seperating the container into two sections. When exposed to the sun, the electrified catalyst would produce two streams of bubbles — hydrogen on one side, oxygen on the other — which could be collected into two tanks, and later recombined through a fuel cell or other device to generate electricity when needed.

"These papers are really important, to show that the catalyst works" when bonded to silcon to make a single device, Nocera says, thus enabling a unit that combines the functions of collecting sunlight and converting it to storable fuel. Silicon is an Earth–abundant and reatively inexpensive material that is widely used and well understood, and the materials used for the catalyst — cobalt and phosphorus — are also abundant and inexpensive.

Putting it together

Marrying the technologies of silicon solar cells with the catalyst material — dubbed Co–PI for cobalt phosphate — was no trivial matter, explains Tonio Buonassisi, the SMA Assistant Professor of Mechanical Engineering and Manufacturing, who was a co–author of the PNAS paper. That's becuase the splitting of water by the catalyst creates a "very agressive" chemical environment that would tend to rapidly degrade the silicon, destroying the device as it operates, he says.

In order to overcome this, both teams had to find a way to protect the silicon surface, while at the same time allowing it to receive the incoming sunlight and to interact with the catalyst.

Professor of Electrical Engineering Vladimir Bulović, who lead the other team, says his team's approach was to form the Co–Pi material on the surface of the silicon cell, by first evaporating a layer of pure cobalt metal onto the cell electrode, and then exposing it to a phosphate buffer solution under an electrical charge to transform it into the CO–Pi catalyst. By using the layer of Co–Pi, now firmly bonded to the surface, "we were able to passivate the surface," says Elizabeth Young, a postdoc who was the lead author of the E&ES paper — in other words, it acts as a protective barrier that keeps the silicon from degrading in water.

"Most people have been staying away from silicon for water oxidation, becasue it forms silicon dioxide" when exposed to water, which is an insulator that would hinder the electrical conductivity of the material, says Ronny Costi, a postdoc on Bulović's team. "We had to find a way of solving that problem," which they did by using the cobalt coating.

Buonassisi's team used a dfferent appraoch, coating the silicon with a protective layer. "We did it by putting a think film of indium tin oxide on top," explains Joep Pijpers, a postdoc who was the lead author of the PNAS paper. Using its expertise in the design of silicon devices, that team then concentrated on matching the current output of the solar cell as closely as possible to the current consumption by the (catalyzed) water–splitting reaction. The system still needs to be optimized, Pjipers says, to improve the efficiency by a factor of 10 to bring it to a range comparable to conventional solar cells.

"It's really not trivial, integrating a low–cost, high–performance silicon device with the Co–Pi," Buonassisi says. "There's a substantial amoung of innovation in both devices processing and architecture."

Both teams had to add an extra power source to the system, because the voltage produced by a single–junction silcon cell is not high enough to use for powering the water–splitting catalyst. In later versions, two or three silcon solar cells will be used in series to provide the needed voltage without the need for any extra power source, the researchers say.

One interesting aspect of these collaborations, says postdoc Mark Winkler, who worked with Buonassisi's team, was that "material scientists and chemists had to learn to talk to each other." That's tricker that it may sound, he explains, becuase the two diciplines, even when talking about the same phenomena, tend to use different terminology and even different ways of measuring and displaying certain characteristics.

Portable power?

Nocera's ultimate goal is to produce an "artifical leaf" so simple and so inexpensive that is could be made widely available to the billions of people in the world who lack access to adequate, reliable sources of electricity. What's needed to accomplish that, in addition to stepping up the voltage, is the addition of a second catalyst material to the other side of the silicon cell, Nocera says.

Although the two approaches to bonding the catalyst with a silicon ell appear to produce functioning, stable devices, so far they have only been tested over periods of a few days. The expectation is that they will be stable for long periods, but accelerated aging tests will need to be performed to confirm this.

Rajeshwar Krishnan, Distinguished University Professor of Chemistry and Biochemistry at the University of Texas at Arlington, says it remains to be seen "whether this 'self–healing' catalyst would hold up to several hours of current flow...under rather harsh oxidative conditions." But he adds that these papers "certainly move the science forward. The state of the science in water photo–oxidation uses rather expensive noble matal oxides," whereas this work uses Earth–abundant, low–cost materials. He adds that while there is still no good storage or distribution system in place for hydrogen, "it is likely that the solar photon–to–hydrogen technology will ultimately see the light of day — for transportation applications — with the hydrogen internal combustion engine."

Meanwhile, Nocera has founded a company called Sun Catalytix, which will initially be producing a first generation system based on the Co–Pi catalyst material, connected by wires to conventional, seperate solar cells.

The "leaf" system, by contrast, is "still a science project," Nocera says. "We haven't even gotten to waht I would call an engineering design." He hopes, however, that the artifical leaf could become a reality within three years.

Bulović's team was funded partly by the Chesonis Family Foundation and the National Science Foundation. Buonassisi's team had support from the Netherlands Organization for Scientific Research (NOW–FOM), the National Science Foundation and the Chesonis Family Foundation. Nocera's work was funded by the Chesonis Family Foundation, the Air Force Office of Scientific Research and the National Science Foundation.