Fueled by the sun
Chemist Allen Bard looks to harness the power of sunlight to produce fuels that can substitute for oil
Jan. 24, 2011
Al Bard has a dream. It’s an area the size of New Mexico blanketed in panels of iron oxide — rust — that have been doped with just the right mix of other elements. When the panels are hit by hot southwestern sunlight, they help split water into its constituent atoms, hydrogen and oxygen. The oxygen bubbles up into the air cleanly. The hydrogen is captured, and made available as fuel.
Play this dream out a decade or two and newer, even more revolutionary materials start covering the landscape. These new panels mimic photosynthesis itself. Like plants, they use the sun’s energy to feed on carbon dioxide, breaking it apart and transforming it into sources of energy. Instead of fueling leaf growth, however, the panels produce fuels that can substitute for oil and transform the transportation industry.
The benefit of such fuels, if they emerge, will be breathtaking. They’ll be renewable into the far future, needing for their creation only water, air and modest amounts of metals, such as iron or titanium, we already have or can make in abundance. They’ll be vastly cleaner than the fuels we’re burning now, either emitting no carbon dioxide (in the case of hydrogen) or capturing as much carbon dioxide out of the air in the process of their creation as they emit when they’re burned. It’s not an exaggeration to say that such fuels, if they can be developed, will change the world forever.
“We’re not talking about 50 years or 20 years,” says Ray Orbach, director of The University of Texas at Austin’s Energy Institute and former under secretary for science at the U.S. Department of Energy. “We don’t know how long it will take, but my guess is that if Al Bard can’t do it in five years, it’s not doable.”
What stands between us and this particular vision of a more sustainable future, however, is a kind of gauntlet of photoelectrochemical challenges that Bard, one of the most decorated chemists on the planet, has been trying to navigate his way through for the last decade.
“We do combinatorial chemistry,” says Bard, professor of chemistry. “In combinatorial chemistry, we say: I admit I have no idea what the best material will be. I have a kind of intuition based on past experience. I know what the colors of the materials are. I know what the spectral response to these things is. I know which are stable and not so stable. But really, the trick is to do it fast, to synthesize and screen quickly and get hits. The faster you can try them, and the more things you can try, the better you start to understand what you need. So that’s what we’re doing.”
Bard needs to find materials that can use the radiant energy of the sun to pop free electrons (and the positively charged “holes” they leave behind) that can react with water and/or carbon dioxide, which are molecules that are fairly unreactive. These sunlight produced electron-hole pairs need to do their chemistry before the electrons return home, otherwise the absorbed photon is wasted.
Among other properties, says Bard, this new material will have to absorb as much as possible of the right regions of the solar spectrum. It will have to be efficient in its use of the energy it does absorb. It will have to be decently conductive. It will have to be sturdy. And, perhaps most important, it will have to be cheap to make and cheap to lay down over tens of thousands of square miles of solar fuel farms. Because unless we can someday buy solar fuels at the pump for a price comparable to what we pay for all that petroleum, coal and natural gas we dig up from the ground, then it won’t matter all that much. They won’t be bought, and therefore it won’t help repair the atmosphere or liberate us from our dependence on foreign oil.
“There’s no magic bullet,” says Bard, “but we can do it. The basic theory is pretty well understood. We can do it now, with lousy efficiency and at high cost, but you need a material that’s much better than anything that’s out there. We need a new material that’s cheap, sturdy and efficient. That’s been the history of chemistry for 200 years.”
What Bard brings to the task of discovery, along with the intuition and expertise he’s accumulated over more than 50 years of doing chemistry, is a system for rapidly synthesizing and screening new materials. Based on his pioneering Scanning Electrochemical Microscope, this system is simple in its conception, but extraordinarily advanced in its capacity to speed up the process of discovery.
At one end is a robot that dispenses droplets of different materials in different combinations into an array on a plate. The samples are then heated up to the point where they oxidize. The plate is then fed into Bard’s Scanning Photoelectrochemical Microscope, which shines a bright light on the compounds, one at a time, and then looks to see which one produces the highest current.
Bard then takes the good candidates, and hands them over to Buddie Mullins, his colleague in the Chemistry Department, who manipulates them at the nano-level.
“Al has this terrific experimental strategy for trying to identify materials of promising compositions,” says Mullins, who’s a professor of chemical engineering in the Cockrell School of Engineering, “but his process doesn’t give you a lot of control over the morphology or structure of the material. His group drips a liquid onto a substrate and heats it up, but we have a lot more control over its morphology.”
Mullins, given a lead by Bard, experiments with the new material’s structure. He looks for nanoformations that, for example, increase the utility of a given compound, allowing it to more efficiently use the charge carriers (the electrons and holes) that are created in the material once a photon is absorbed.
Mullins feeds his discoveries back to Bard who then refines his production process. The best of the best compounds are integrated into a prototype solar fuel system to see if they can do the job. And so on. With each iteration, the researchers creep closer to the day when commercially viable solar fuels are available. They also, perhaps, contribute to the kind of big leap forward that occasionally happens in the materials development field.
“Think about high-temperature superconductors,” says Mullins. “Until the late 1980s, people thought that the highest temp superconductor we were ever going to get would be about 20 or so Kelvin. Really cold. Then some guys at IBM in Zurich came up with a material that was a superconductor at 30 Kelvin, and that just turned that field on its ear. By 1988, they had superconductors up to 125 Kelvin and just blew everything apart. They got many times higher than what they’d thought possible.
“We use that field as an example because those high temperature superconductors are complicated materials. They’re four or five or six elements, and we’re thinking that the right material for photoelectrochemistry, to do this job, is going to be something really complicated like that. It’ll be several different elements, with a unique structure that’s never been synthesized before.”
For Bard, the hunt for a material to split water has focused recently on metal oxides, which are appealing for a few reasons. They’re cheap. They’re sturdy. They’re easy to find or make. And though they tend not to be very good on their own at doing many of the things that a successful “photomaterial” would have to do, they’re very familiar to chemists, and there’s a lot of accumulated experience of how to manipulate them.
“We know how to mess around,” says Bard. “Our winning material right now is bismuth vanadium oxide, doped with some tungsten. It works pretty well. It’s reasonably efficient, and it’s very stable and pretty inexpensive. If we can get that up to 15 percent efficiency, with a material that’s stable for 10 years and costs a couple dollars for a square meter, then we’re competitive.”
The hunt for materials to help make solar fuels from carbon dioxide and light is at an earlier stage, says Bard, because the chemistry to reduce it to a useful fuel is much more complex. In both cases, though, he’s relatively optimistic that workable systems will be found before the world runs out of fossil fuels in the next century or two.
The question that remains, however, is whether such solutions will be found in time to avert the major effects of climate change. The answer to that question, Bard believes, depends more on the political will society can summon to the task of cutting emissions and funding research than it does on the skill of scientists like him.
“I think we will probably have enough fossil fuels to last through the technological development of things like solar, wind and biofuels,” say Bard. “But we may not be able to have a system that will sustain nine billion people in this world. We barely have it now. But those problems, I think, will be solved. On the other hand, I’m not at all optimistic we’ll do this in time to avoid the major problems of climate change, which have nothing to do with running out of the fuels. They have to do with using them.”
For Orbach, whose Energy Institute is committed to broad-based thinking about the future, the greatest danger is passivity. And the costs of passivity will be high. Animal species will go extinct and coastlines will be lost. Climate instability will lead to political instability.
“What we’re talking about,” he says, “is the cumulative effect of all the carbon dioxide emitted since the beginning of the Industrial Revolution. It’ll be catastrophic even for the wealthy countries.”
Many of the effects of climate change are already visible, of course, but it’s not yet too late, says Orbach, to tilt the momentum back in the other direction. Solutions, like those Bard and Mullins are pursuing, can be found soon if the resources of The University of Texas at Austin and of other places like it are brought to bear with the intelligence and on the scale that problems of such magnitude deserve.
“I don’t want to wait until 2030 or 2050,” Orbach says. “I want to do something now.”
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