A Gleam Of Light For Solar EnergyEmily T. Smith
One day last July, while walking across the Princeton University campus, Lori Vermeulen noticed that the chalky white powder she was carrying turned blue in the baking sun. The chemistry graduate student knew that such compounds become brown, yellow, or red when they decompose--but blue was another matter. "I wasn't sure what was happening," she recalls.
No one was more amazed than Vermeulen and Mark E. Thompson, her adviser and an assistant chemistry professor, when they found what caused the color change: Sunlight excited electrons to jump from one molecule to another, creating a negative charge and trapping energy. Even more surprising, the material stayed blue for days, meaning that the energy remained stored. The researchers published their work in August in the journal Nature. And now they've started the arduous task of trying to transform this novelty into an energy source.
CATCHING RAYS. Indeed, Vermeulen's campus stroll may mark a leap forward in the 30-year search for materials that can capture the sun's energy and convert it into a clean source of electricity and heat. Photovoltaic systems, which absorb sunlight and transform it into electricity, don't store energy and are expensive. So researchers continue to look for inorganic materials that can mimic the way chlorophyll in plants captures, stores, and releases energy at the molecular level. Scientists have several prospects, but none stores energy as long as Vermeulen's and Thompson's. Their powder "raises the prospects for developing inexpensive alternatives to store and convert solar energy into electrical or chemical energy," says Prabir Dutta, professor of chemistry at Ohio State University.
Like many discoveries, this one came about as much through serendipity as skill. Thompson, with Vermeulen's help, was testing the theory of electrochemist Rudolph A. Marcus, who was the winner of this year's Nobel prize in chemistry. Marcus' theory helps predict the speed of chemical reactions, such as photosynthesis, that involve the exchange of electrons between molecules and that underpin the workings of batteries and fuel cells. Thompson was using a compound of common materials, zirconium phosphonate and viologen halide, to study the rate of those transfers. According to conventional chemistry, this concoction shouldn't store energy.
Yet it does. After being dissolved and heated, zirconium phosphonate molecules crystallize into layers, with the viologen molecules sandwiched between them. Chloride ions from the viologen halide dot the surface of the zirconium phosphonate. The researchers still don't know the precise reaction that stores the energy, but they have an idea (diagram). When light strikes the compound, it causes an electron orbiting a molecule--a chloride ion, they think--to jump out mf its orbit into the viologen layer. Then, the zirconium phosphonate layer gives up an electron to replace the one lost by the chloride. This electron transfer captures the sun's energy and causes the material to turn blue.
TO HAVE AND TO HOLD. What is interesting about the material is that the reaction doesn't reverse quickly. One of the big difficulties in finding materials that store the sun's energy has been keeping electrons from immediately returning to their original places and releasing stored energy as heat, the way a black surface does after sitting in the sun. Last year, researchers at Argonne National Laboratory developed a material that can store solar energy for 0.004 second--about 10 times as long as the best earlier systems. In January, Israeli chemists revealed a porous-glass system that keeps electrons separated for several hours. But in Vermeulen's and Thompson's compound, electrons stay separated for months. That gives you "the chance to do the follow-up chemistry" needed to release the energy, says Thomas E. Mallouk, chemistry professor at the University of Texas at Austin.
It will take fancy science to accomplish that, but Thompson and Vermeulen are plunging ahead. By tinkering with the structure of the material--substituting different metals for zirconium, for instance--they are trying to discover which molecules are giving up and which are accepting electrons. Once they've identified those, it will be easier to improve the properties of the material.
Boosting efficiency is key. Now, the material absorbs about 10% of the sunlight that hits it--vs. up to 26% for photovoltaic cells--because it absorbs only ultraviolet light. By doing minor tinkering and producing the material as thin films, the researchers think they can coax the compound to absorb visible light, where most of the sun's energy is concentrated. That could help boost the rate at which it converts sunlight into energy--now only about 3%. Photovoltaic cells do 10% to 12%, but then the new material "is far cheaper than those technologies," says Thompson.
BIG SPLASH. The most daunting challenge will be extracting the stored energy. The dense molecular structure of the material, which traps the energy, also keeps out chemicals that could set off reactions to harvest the transferred electrons. The researchers hope to make the material more porous by replacing some of the viologen with smaller molecules to create cavities that allow other chemicals to penetrate.
If this works, there could be a number of ways to convert the stored energy. For instance, says Thompson, water might be pumped through the material, where it would be catalyzed by a metal such as platinum into hydrogen, which could be captured and burned as a fuel. The scheme would cost less than breaking down water with photovoltaic cells, which solar energy enthusiasts have long advocated.
As to when this might be achieved, Thompson shrugs: "Science proceeds in leaps. You don't know when to expect them," he says. In the meantime, the 29-year-old Vermeulen, a third-year PhD student, is still getting used to the splash her dissertation project has made. Few of her peers can boast of publishing work in such a prestigious journal as Nature. "I still feel overwhelmed by things I don't know," she says.
What they might find out is intriguing others. Thompson described their work in October at an energy conference in Italy. Even in these days of tight funding, support for their research is likely. "There will be money chasing them," says one Energy Dept. official. Vermeulen and Thompson face long odds. But that's an occupational hazard when the goal is achieving leaps in knowledge.
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